Ultra-high strength steel plate with excellent hole expandability and its manufacturing method

Abstract

The present invention relates to a steel sheet used in automobiles, etc., and to an ultra-high strength steel sheet excellent in hole expandability and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to an automotive steel sheet used in automobiles and the like, and more specifically to an ultra-high strength cold-rolled steel sheet (and plated steel sheet) that has excellent hole expandability, excellent formability during part processing, and excellent crash resistance characteristics during a collision, and a method for producing the same. [Background technology]

[0002] Recently, there has been an increasing demand for higher strength steel sheets used in automotive frame parts, and the use of ultra-high strength steel sheets with tensile strengths exceeding 1500 MPa is being promoted for some parts.

[0003] To achieve such ultra-high strength, it is effective to use a steel structure mainly composed of hard structures such as martensite and bainite. However, such steel structures have a lower elongation rate than dual-phase steels of ferrite and martensite, and cracks occur during forming, limiting their use to parts that can be formed by bending into relatively simple shapes.

[0004] In order to improve the strength of steel sheets, the above-mentioned hardened structure is necessary, but not only does it cause cracks during part formation, but it also reduces impact properties during a collision, limiting its application as a satisfactory automotive part. Therefore, in order to achieve high strength in automotive parts using a martensite-based steel structure, it is necessary to develop a steel material that not only ensures the desired strength but also has better hole expandability and fatigue properties, etc., and only when it can ensure crash resistance during an automobile collision can it be widely used as a high-strength steel material for automotive parts.

[0005] Meanwhile, HPF steel is a material with superior part forming and impact properties. The HPF (Hot Press Forming) forming technique involves forming the material at high temperatures and then press-forming it in a hot state, ensuring the required strength through water cooling between the die and the material. This not only ensures high strength for the same thickness, but it also excels in formability, making it widely used. However, due to issues with application due to excessive capital investment costs and increased process costs, active research has recently been conducted into the development of cold stamping materials with tensile strengths of 1500 MPa or higher.

[0006] However, Patent Document 1 (JP 2010-215958 A), which is a patent that ensures strength by adding hardening elements such as C and Mn to ensure strength, and then performing normal annealing and then water cooling to ensure a martensite phase at room temperature, and Patent Document 2 (JP 2013-227657 A), which ensures high-strength steel by optimizing the cooling conditions during cooling, basically ensures strength by using the martensite phase, so in addition to the limitations of formability, they do not exceed the limitations of excellent crash resistance and fatigue properties. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-215958 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-227657 Summary of the Invention [Problem to be solved by the invention]

[0008] One aspect of the present invention is to overcome the limitations of the conventional technology described above, and has an object to provide a steel sheet having an ultra-high strength of 1500 MPa class tensile strength with excellent hole expandability and fatigue properties by optimizing the steel composition and manufacturing process.

[0009] 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 further object of the present invention from the contents described in the specification of the present invention. [Means for solving the problem]

[0010] 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): 1.0 to 3.0%, boron (B): 0.0005 to 0.003%, phosphorus (P): 0.01% 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%, the balance being Fe and other unavoidable impurities. Provided is a steel sheet in which the average area ratio of cementite precipitates present at the grain boundaries of one or more types of crystals selected from the group consisting of ferrite and bainite within 10 µm from the surface is 60% or less (excluding 0%) of the area of ​​cementite precipitates present at the grain boundaries of martensite at a ¼t point from the surface (where t is the total thickness of the steel sheet).

[0011] Yet another embodiment of the present invention is a step of reheating a steel slab containing, by weight, carbon (C): 0.1 to 0.3%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.0 to 3.0%, boron (B): 0.0005 to 0.003%, phosphorus (P): 0.01% 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%, the balance being Fe and other unavoidable impurities, at a temperature of 1100 to 1300°C; hot rolling the reheated slab to produce a hot rolled steel sheet; The hot-rolled steel sheet is coiled at 400-600°C; cold rolling the coiled hot-rolled steel sheet at a reduction ratio of 30 to 80%; and Annealing the cold-rolled steel sheet by heat treating it at a temperature of Ac3+10°C to Ac3+80°C; The annealing step controls the dew point temperature to -40°C or less at a temperature of Ac3+10°C or less, and controls the dew point temperature to -30°C to +30°C at a temperature of Ac3+10°C to Ac3+80°C. [Effects of the Invention]

[0012] According to the present invention, it is possible to overcome the limitations of conventional techniques and provide a steel sheet having an ultra-high strength of tensile strength of 1500 MPa or more, excellent hole expandability and fatigue properties.

[0013] 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. DETAILED DESCRIPTION OF THE INVENTION

[0014] The terminology used herein is for the purpose of describing the invention and is not intended to limit the invention. Furthermore, as used herein, the singular forms "a," "an," and "the" also include the plural forms unless the relevant definition clearly indicates otherwise.

[0015] As used herein, the meaning of "comprises" embodies features and does not exclude the presence or addition of other features.

[0016] 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 to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0017] Therefore, in conventional technologies, hardening elements such as C and Mn are added to ensure strength, and the martensite phase is maintained at room temperature through water cooling after normal annealing to ensure strength, or high-strength steel is achieved by optimizing the cooling conditions during cooling. However, since strength is basically achieved using the martensite phase, the limits of formability, crash resistance, fatigue properties, etc. are not exceeded.

[0018] The present inventors have conducted extensive research to solve the above-mentioned problems of the prior art, and as a result, have confirmed through experiments that when the chemical compositions and annealing operation conditions satisfy a specific relationship, the desired physical properties can be secured, and have completed the present invention.

[0019] Therefore, the present invention provides a method for producing ultra-high strength cold-rolled steel sheets (and plated steel sheets) with a tensile strength of 1500 MPa or more by optimizing the components and operating conditions, and in particular by developing and applying an operating technique that differentiates the phase structure between the surface layer and the center of the steel sheet to on-site production. This enables the production of steel sheets with superior properties.

[0020] 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.

[0021] The alloy composition of the steel sheet of the present invention includes, in weight percent, 0.1 to 0.3% carbon (C), 0.5% or less (excluding 0%) silicon (Si), 1.0 to 3.0% manganese (Mn), 0.0005 to 0.003% boron (B), 0.01% 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), with the balance being Fe and other unavoidable impurities. Unless otherwise specified in the present invention, the content of each element is based on weight percent.

[0022] Carbon (C): 0.1~0.3% Carbon (C) is necessary to improve hardenability and obtain a steel structure with a martensite area ratio of 95% or more at the quarter thickness position. Furthermore, C is necessary to increase martensite strength and ensure TS≧15000 MPa. If the C content is less than 0.1%, the desired strength cannot be obtained. Therefore, the C content can be 0.1% or more, and more preferably 0.15% or more. On the other hand, if the C content exceeds 0.3%, it becomes difficult to obtain good weldability and delayed fracture resistance. Therefore, the C content can be 0.30% or less, and more preferably the lower limit of the C content can be 0.25% or less. On the other hand, the C content can be 0.10 to 0.30%.

[0023] Silicon (Si): 0.5% or less (excluding 0%) Silicon (Si) is a strengthening element through solid solution strengthening, and is added to improve bendability by suppressing the formation of film-like carbides during high-temperature tempering. While a higher Si content is more advantageous in terms of achieving the above effects, if the Si content exceeds a certain standard content, weldability and LME (Liquid Metal Embrittlement) properties deteriorate. Therefore, the upper limit is set to 0.5% or less, and a preferred range is 0.32% or less. However, there is no particular lower limit for the Si content, and 0% is excluded because Si is added as an impurity element during the manufacturing process. On the other hand, a more preferred lower limit for the Si content may be 0.001% or more. On the other hand, the Si content may be 0.500% or less.

[0024] Manganese (Mn): 1.0-3.0% Manganese (Mn) contributes to increasing the martensite area ratio through increased hardenability and to improving strength through solid solution strengthening. Mn is also included to fix S in steel as MnS and reduce hot brittleness. If the lower limit of the Mn content is less than 1.0%, the likelihood of ferrite and bainite forming rather than martensite formation during cooling increases. Therefore, the lower limit is set to 1.0% or more, and a more preferred lower limit of the Mn content is 1.3% or more. From the viewpoint of welding stability, the Mn content is set to 3.0% or less. The Mn content is preferably 2.6% or less, more preferably 2.0% or less.

[0025] Boron (B): 0.0005-0.003% Boron (B) is an element that inhibits ferrite formation, which has the advantage of inhibiting the formation of ferrite during cooling after annealing in the present invention. 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%, not only will there be no hardening effect, the target strength cannot be ensured, but ferrite will form in the surface layer, tending to deteriorate bendability. Therefore, the B content is set to 0.0005 to 0.003%. On the other hand, the B content is more preferably in the range of 0.002% or less. On the other hand, the B content may be 0.0005 to 0.0030%.

[0026] Phosphorus (P): 0.01% or less (excluding 0%) Phosphorus (P) is an impurity element contained in steel, and the less added to steel, the better. However, the content is not limited to 0% in consideration of cases where it is unavoidable during the manufacturing process. However, if the content exceeds 0.01%, weldability may deteriorate and the steel may become brittle, so the upper limit can be limited to 0.01% or less. A more preferable upper limit is 0.003% or less. Meanwhile, the P content can be 0.010% or less.

[0027] Sulfur (S): 0.01% or less (excluding 0%) Sulfur (S), like P, is an impurity inevitably contained in steel and inhibits the ductility and weldability of steel sheets. Therefore, it is preferable to control the content as low as possible. Therefore, in the present invention, the sulfur content is preferably limited to 0.01% or less. It can be more preferably limited to 0.005% or less. It is even more preferable to control it to 0.002% or less to minimize MnS precipitates in the steel and further contribute to improving bendability. Note that 0% is excluded to take into account cases where sulfur is inevitably contained during the manufacturing process. Meanwhile, the above S content can be 0.010% or less.

[0028] Nitrogen (N): 0.01% or less (excluding 0%) Nitrogen (N) is an impurity element, and if its content exceeds 0.01%, it significantly increases the risk of cracks occurring during continuous casting due to the formation of AlN, etc., so it is preferable to limit its upper limit to 0.01%. The N content is more preferably 0.008% or less, and most preferably 0.006% or less. Note that 0% is excluded in consideration of cases where N is unavoidably contained during the manufacturing process. On the other hand, the N content can be 0.010% or less.

[0029] Aluminum (Al): 0.01 to 0.1% Aluminum (Al) can be added to remove oxygen from molten steel and, like Si, is an element that is effective in stabilizing retained austenite by suppressing 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 problems in production and equipment. More preferably, the upper limit of the Al content can be limited to 0.05% or less. The Al content can be 0.010 to 0.100%.

[0030] The remainder contains iron (Fe), and it is not possible to exclude unintended impurities from raw materials or the surrounding environment during normal manufacturing processes. These impurities are obvious to anyone skilled in the art during manufacturing processes, and therefore not all of them are specifically mentioned in this specification.

[0031] The steel sheet may further contain, by weight percent, one or more selected from the group consisting of chromium (Cr): 0.2% or less (excluding 0%), molybdenum (Mo): 0.01 to 0.1%, niobium (Nb): 0.01 to 0.05%, titanium (Ti): 0.01 to 0.05%, and antimony (Sb): 0.001 to 0.1%. The reasons for adding each component and the reasons for limiting the content will be specifically explained below.

[0032] Chromium (Cr): 0.2% or less (excluding 0%) Chromium (Cr) can be added to improve the quenchability of steel. However, because alloying elements have a higher cost than other hardening elements, it is advantageous to control the content as low as possible. While there is no particular lower limit, 0% is excluded in consideration of the level of impurities inevitably involved. Furthermore, if the Cr content exceeds 0.2%, the rate of solid solution of cementite during annealing is slowed, causing undissolved cementite to remain, thereby deteriorating bendability. Therefore, the Cr content should be controlled to 0.2% or less, with a more preferable upper limit of 0.1% or less. The Cr content may be 0.200% or less.

[0033] Molybdenum (Mo): 0.01 to 0.1% Molybdenum (Mo) has the effect of improving the quenching properties of steel, the effect of generating fine Mo-containing carbides 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, experimental results have 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, it is more preferable that the upper limit of the Mo content be 0.05%. On the other hand, the Mo content may be 0.010 to 0.100%.

[0034] Niobium (Nb): 0.01-0.05% Niobium (Nb) is an element that segregates at austenite grain boundaries, inhibits 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. 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%. On the other hand, from the viewpoint of improving the above-mentioned effects, the lower limit of the Nb content may be 0.02% or more, or the upper limit of the Nb content may be 0.04% or less. On the other hand, the Nb content may be 0.010 to 0.050%.

[0035] Titanium (Ti): 0.01 to 0.05% Titanium (Ti) is a nitride-forming element that scavenges N in steel by precipitating TiN. Without Ti, cracks may occur during continuous casting due to the formation of AlN. However, if the Ti content exceeds 0.05%, the strength of martensite may decrease due to the precipitation of additional carbides in addition to the removal of solute N. Furthermore, the formation of carbonitrides such as TiC and TiN may impair hole expandability and bending workability. Furthermore, if the Ti content is less than 0.01%, Ti, like Nb, does not contribute to increasing strength at all. Therefore, the lower limit of the Ti content in the present invention is set to 0.01% or more. More preferably, the lower limit of the Ti content may be 0.02% or more, or the upper limit of the Ti content may be 0.04% or less. The Ti content may be 0.010 to 0.050%.

[0036] Antimony (Sb): 0.001-0.1% Antimony (Sb) induces a uniform oxidation reaction in the surface layer region of the steel sheet, selectively suppressing the formation of C or B oxides in the surface layer portion, thereby improving fatigue properties. From the viewpoint of achieving this effect, the Sb content is set to 0.001% or more. On the other hand, the Sb content may be preferably 0.002% or more, more preferably 0.005% or more. However, if the Sb content exceeds 0.1%, castability deteriorates and Sb segregates at prior γ grain boundaries, deteriorating bendability, so the Sb content is set to 0.1% or less. On the other hand, the Sb content may be 0.001 to 0.100%.

[0037] The steel structure of the steel plate according to the present invention will be described below.

[0038] The average area ratio (C) of cementite precipitates present at one or more grain boundaries selected from the group consisting of ferrite and bainite within 10 μm from the surface of the steel sheet is 60% or more (excluding 0%) of the area of ​​cementite precipitates present at the grain boundaries of martensite at a point 1 / 4t from the surface (where t is the total thickness of the steel sheet).

[0039] According to the present invention, hole expandability can be improved by controlling the cementite precipitates in the extreme surface layer at a low level relative to the 1 / 4t point through control of the steel sheet composition and operating conditions (particularly the dew point during annealing). Specifically, if the average area fraction (C) of the cementite precipitates exceeds 60%, excessive fine cementite is formed, resulting in a decrease in fatigue strength. Therefore, the upper limit can be set to 60%, more preferably 50%. On the other hand, the lower limit of the average area fraction (C) of the cementite precipitates is not particularly limited, but it is preferable to control it as low as possible within the range possible from the equipment configuration. Therefore, it can be set to more than 0%, more preferably 10%.

[0040] Meanwhile, in the present invention, the total thickness t of the steel sheet may be in the range of 0.5 to 2.5 mm, and the total thickness of the steel sheet may be measured in a direction perpendicular to the rolling direction of the steel sheet.

[0041] The cementite precipitates can be measured based on the circle equivalent diameter of 2 nm or more and 30 nm or less.

[0042] According to one embodiment of the present invention, the steel sheet may have an area fraction (T1) of 15 to 60% of one or more phases selected from the group consisting of ferrite and bainite within 30 μm from the surface. In the above-described surface layer within 30 μm from the surface, a hard martensite structure may not provide the hole expandability required in the present invention and may also reduce fatigue strength. Therefore, in the present invention, the steel sheet may have an area fraction (T1) of 15 to 60% of one or more phases selected from the group consisting of ferrite and bainite. In this case, although not particularly limited, the remainder may be martensite in addition to the one or more phases selected from the group consisting of ferrite and bainite within 30 μm from the surface.

[0043] Specifically, according to one embodiment of the present invention, if the area ratio (T1) is less than 15%, fatigue strength and hole expandability can be deteriorated, so it can be set to 15% or more. On the other hand, if the area ratio (T1) exceeds 60%, the surface layer of the steel sheet softens, which is disadvantageous in ensuring overall strength, and fatigue properties in particular can be deteriorated. On the other hand, more preferably, the lower limit of the area ratio (T1) may be 18%, or the upper limit of the area ratio (T1) may be 52%.

[0044] According to one embodiment of the present invention, the area ratio (T2) of one or more phases selected from the group consisting of ferrite and bainite within 10 μm from the surface of the steel sheet may be 50 to 80%. That is, the present invention is characterized in that the area ratio of one or more phases selected from the group consisting of ferrite and bainite is optimized for each thickness of the surface layer portion.

[0045] According to one embodiment of the present invention, if the area ratio (T2) is less than 50%, the hole expandability contribution effect may be reduced, and if the area ratio (T2) exceeds 80%, the strength may be reduced. On the other hand, in order to further improve the above-mentioned effects, the lower limit of the area ratio (T2) may be 55%, or the upper limit of the area ratio (T2) may be 71%.

[0046] 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 zinc-based coating, aluminum-based coating, or the coating method may be hot-dip coating, electroplating, or the like. In other words, it is sufficient if it can be used in the technical field to which the present invention pertains. However, in a preferred example of the present invention, the coating layer may be a zinc-based coating layer.

[0047] Next, a preferred method for producing a steel sheet according to the present invention will be described.

[0048] 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, and if the reheating temperature exceeds 1300°C, the amount of surface scale increases, reducing the material yield. Therefore, the reheating temperature is limited.

[0049] The reheated slab is hot-rolled to obtain a hot-rolled steel sheet. At this time, the hot rolling can be performed at Ar3 to 1000°C. The reason why the hot-rolling temperature (finish rolling temperature) of the reheated slab is set to Ar3 or higher (the temperature at which ferrite begins to appear when austenite is cooled) is that below Ar3, rolling may occur in the two-phase region of ferrite and austenite or in the ferrite region, resulting in the formation of a duplex structure, and there is a concern that malfunction may occur due to fluctuations in the hot-rolling load.

[0050] 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 induce defects and deteriorate the surface properties of the plated material, and therefore the upper limit of the coiling temperature is limited. Furthermore, it is preferable to form the structure of the hot-rolled steel sheet as a single-phase structure rather than a composite structure as much as possible, and to maintain a low coiling temperature in order to ensure material uniformity across the entire length and width. However, as the coiling temperature decreases, the strength of the hot-rolled steel sheet increases, which has the disadvantage of increasing the rolling load in the subsequent cold rolling process. However, this can make actual production impossible, so the lower limit is limited to 400°C or higher. More preferably, the lower limit of the coiling temperature may be 420°C, or the upper limit may be 500°C, and water cooling can be performed after coiling.

[0051] Next, the oxide layer formed on the surface of the coiled hot-rolled steel sheet is removed in a pickling process, and then the 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 grains may affect austenite formation and final properties during annealing. Furthermore, if the reduction in the cold rolling exceeds 80%, there is a problem that work hardening occurs during cold rolling, resulting in uneven reduction in the length and width directions, which may cause variations in the material properties of the final steel sheet, and the rolling load may make it difficult to achieve the target thickness.

[0052] After cold rolling, the cold-rolled steel sheet is heat-treated within an annealing temperature range of Ac3+10°C to Ac3+80°C. The Ac3 temperature varies depending on the composition and is therefore determined by the following formula 1. If the annealing temperature is less than Ac3+10°C, a duplex grain structure will be formed throughout the entire length of the coil due to annealing in the two-phase region rather than the single-phase region, which will have a negative impact on the material properties, so the lower limit of the annealing temperature is set to Ac3+10°C. On the other hand, if the annealing temperature exceeds Ac3+80°C, the annealing furnace may be overloaded, causing equipment problems, so the upper limit of the annealing temperature is set to Ac3+80°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.)

[0053] According to one embodiment of the present invention, the heat treatment during annealing may be performed for 30 seconds or more at 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, fine carbides formed during hot rolling may not be remelted, resulting in poor bendability. Furthermore, the reverse transformation to austenite single phase may not be sufficient during annealing, resulting in the presence of a mixed phase, which may prevent the formation of a martensite phase during final cooling, potentially preventing the tensile strength from reaching the standard range. Therefore, a lower limit is set for the heat treatment time. Meanwhile, there is no particular upper limit.

[0054] Meanwhile, dew point control in the above-mentioned annealing process is important in the present invention. The annealing process is broadly divided into a preheating zone, a heating zone, and a soaking zone. At temperatures below Ac3+10°C, which are the temperature-raising and preheating stages in the preheating zone and the heating zone, the dew point temperature is controlled to −40°C or below. If the dew point temperature at temperatures below Ac3+10°C exceeds −40°C, Mn- and Si-based oxides dissolve into the surface layer, and although decarburization actually occurs in the soaking zone, soft ferrite is not formed in the surface layer, which is not useful for improving fatigue strength. Meanwhile, at temperatures below Ac3+10°C, which are the temperature-raising and preheating stages in the preheating zone and the heating zone, the lower limit of the dew point temperature is not particularly limited, and may be −60°C, for example.

[0055] However, in the above-mentioned annealing step, the lower temperature range is not limited to a large extent in the present invention, and may be lowered within the range permitted by the equipment.

[0056] Meanwhile, the dew-point temperature in the soaking zone temperature range of Ac3+10°C to Ac3+80°C is controlled between -30°C and +30°C. This allows for the removal of initially formed surface carbides or oxides using decarburization techniques, thereby removing as much of the carbides or oxides present in the surface as possible and reducing the [C] content, ensuring hole expandability and fatigue properties. If the dew-point temperature in the range of Ac3+10°C to Ac3+80°C is below -30°C, the decarburization reaction does not proceed satisfactorily, and the oxides formed in the preheating zone and heating zone cannot be effectively removed, resulting in the failure to achieve the benefits of the present invention. On the other hand, if the dew-point temperature in the range of Ac3+10°C to Ac3+80°C exceeds +30°C, excessive decarburization may actually reduce fatigue strength. Therefore, the dew-point temperature range is set to +30°C or below. On the other hand, according to one embodiment of the present invention, more preferably, the lower limit of the dew-point temperature in the range of Ac3+10°C to Ac3+80°C may be -20°C, or the upper limit of the dew-point temperature in the range of Ac3+10°C to Ac3+80°C may be +20°C.

[0057] According to one embodiment of the present invention, during the annealing, the dew point can be controlled so as to satisfy the following relational expression 1. [Equation 1] D2≦-0.4×D1 (In the above relational expression 1, D1 represents the dew point temperature (°C) at temperatures below Ac3+10°C, which is the temperature rise and preheating stage of the preheating zone and heating zone, and D2 represents the dew point temperature (°C) in the temperature range of Ac3+10°C to Ac3+80°C, which is the temperature range of the soaking zone.)

[0058] Next, according to one embodiment of the present invention, 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 750°C. If the primary cooling end temperature is less than 680°C, the bendability (R / t) evaluation value exceeds 3.7, deteriorating formability. On the other hand, if the primary cooling end temperature exceeds 750°C, not only is it impossible to reproduce due to the equipment configuration, but the structure becomes coarse and strength deteriorates. Therefore, the primary cooling end temperature is limited to 680 to 750°C. However, it is more advantageous if the primary cooling end temperature is 700 to 730°C.

[0059] Furthermore, if the average cooling rate during the primary cooling exceeds 10°C / s, problems may occur with the shape of the sheet. On the other hand, the lower limit of the average cooling rate during the primary cooling is not particularly limited, as any rate is possible as long as it is within the range possible due to the equipment configuration. However, as an example, the lower limit of the average cooling rate during the primary cooling may be 1°C / s.

[0060] Next, according to one embodiment of the present invention, the steel sheet after the primary cooling is subjected to secondary cooling at an average cooling rate of 60 to 160°C / s to a temperature of 100°C to Mf. Mf refers to the martensitic transformation finish temperature (Finish Temperature (Mf)), and can be determined by a method commonly known in the art, so it is not specifically defined herein and can be measured using a dilatometer.

[0061] According to one embodiment of the present invention, it is advantageous to maintain the above-described rapid cooling conditions for the secondary cooling in order to ensure the required level of strength. If the average cooling rate during the secondary cooling is less than 60°C / s, a bainite structure may be formed in part 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, a rapid martensite transformation rate may occur during the secondary cooling, resulting in shape deterioration of the steel sheet and variations in material properties across the width. Preferably, the lower limit of the average cooling rate during the secondary cooling may be 70°C / s, or the upper limit of the average cooling rate during the secondary cooling may be 150°C / s.

[0062] According to one embodiment of the present invention, if the cooling end temperature during 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. Meanwhile, if the cooling end temperature during the secondary cooling is less than 100°C, this too low temperature not only has disadvantages in terms of shape but also exceeds the range of manufacturing processes due to equipment limitations. Therefore, the lower limit of the cooling end temperature during the secondary cooling can be set to 100°C. Meanwhile, the lower limit of the cooling end temperature during the secondary cooling can be more preferably 90°C, and even more preferably 80°C. Similarly, the upper limit of the cooling end temperature during the secondary cooling can be more preferably 140°C, and even more preferably 120°C.

[0063] Next, according to one embodiment of the present invention, the second-cooled steel sheet may be reheated and over-aged at 150 to 240° C. The reheating and over-aging treatment transforms the martensite obtained by the rapid cooling process by the second cooling into tempered martensite, thereby increasing the yield strength.

[0064] If the overaging heat treatment temperature is less than 150°C, tempering is insufficient, resulting in low yield strength and insufficient toughness, whereas if the overaging heat treatment temperature exceeds 240°C, a large amount of carbides precipitate and become coarse, resulting in poor bending workability.

[0065] Meanwhile, the lower limit of the reheating and overaging heat treatment temperature is more advantageous in terms of bendability, but it can be set to 150°C or higher in consideration of equipment characteristics. However, the lower limit of the reheating and overaging heat treatment temperature is more advantageously 160°C, and may be even more advantageously 170°C. Similarly, the upper limit of the reheating and overaging heat treatment temperature is more advantageously 200°C, and may be even more advantageously 180°C.

[0066] On the other hand, the overaging heat treatment may be for 200 seconds or more. If the overaging heat treatment time is less than 200 seconds, tempering may not be performed sufficiently, which may result in a problem of low yield strength.

[0067] However, according to one embodiment of the present invention, the lower limit of the overaging heat treatment time may be 300 seconds, and more preferably 400 seconds.

[0068] Meanwhile, in the present invention, there is no particular upper limit to the overaging heat treatment time, but since it is difficult to exceed 1000 seconds due to the characteristics of continuous annealing equipment, it may be 1000 seconds or less.

[0069] However, according to one embodiment of the present invention, the upper limit of the overaging heat treatment time may be 900 seconds, or 800 seconds.

[0070] If necessary, the plate may then go through temper rolling or tension leveling to improve the shape.

[0071] If necessary, the method may further include forming a plating layer on the surface of the steel sheet. The plating may be performed using a hot-dip plating method in which a plating bath is provided and the steel sheet is immersed 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.

[0072] According to the above-mentioned manufacturing method, the tensile strength is 1500 MPa or more, the hole expansion capacity (HER) is 50% or more, and the maximum expansion capacity is 10 at a frequency of 20 Hz. 7 This effectively produces a steel plate that exhibits excellent fatigue properties and does not develop cracks under repeated tests. [Example]

[0073] Examples of the present invention will now be described. It goes without saying that various modifications of the following examples are possible within the scope of the present invention, as understood by those skilled in the art. 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.

[0074] (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, held there for 1 hour, and then finish hot rolled at 900°C. The slab was then charged into a heated furnace set under various conditions, held there for 1 hour, and then furnace cooled 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), and overaging treatment under the conditions shown in Table 2 below, and then electrogalvanized under conventional conditions.

[0075] For the tensile test, a JIS No. 5 tensile test piece was cut out from the steel plate at a position one-quarter of the plate width so that the direction perpendicular to the rolling direction of the steel plate surface was the length direction, and a tensile test (JIS Z2241) was performed. From the tensile test, the yield strength (YS), tensile strength (TS), and elongation (El) were determined.

[0076] Bending tests were performed using a jig with a 90-degree interior angle at the tip of a 100mm rectangular test piece cut from a quarter of the steel plate width, perpendicular to the rolling direction of the steel plate surface and 35mm in the rolling direction. The radius of curvature of the jig's interior angle was varied to determine the smallest interior angle at the tip of the jig at which no cracks were observed on the surface of the test piece. The critical bending radius (R / t) was calculated by dividing the resulting radius (R) by the plate thickness (t). The smaller this value, the better the bendability. Cracks were assessed using a stereomicroscope at a maximum magnification of 20x, and the length of the crack was measured. Because microcracks less than 0.1mm are difficult to distinguish from surface irregularities using a stereomicroscope, cracks of 0.1mm or larger were considered to be failures.

[0077] The hole expansion ability (HER) was evaluated by punching a hole in the center of the plate to determine the time when a crack occurred during forming, and then measuring the ratio of the inner diameter at which the crack occurred to the initial inner diameter.

[0078] The fatigue strength characteristics were evaluated by a pulsating tensile fatigue test. The direction perpendicular to the rolling direction of the steel plate surface was set as the length direction, and the test was performed at a frequency of 20 Hz for a maximum of 10 7 The conditions under which no cracks occurred after repeated cycles were judged to be excellent.

[0079] The steel sheet of the present invention has a tensile strength of 1500 MPa or more, a hole expansion rate (HER) of 50% or more, and a maximum of 10% at a frequency of 20 Hz. 7 No cracks were generated under repeated tests and the fatigue properties were excellent, but the steel sheets of the comparative examples did not satisfy at least one of these conditions.

[0080] 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. In Table 3, operating conditions that fall outside the ranges of the present invention for the steels of the present invention and the comparative steels are marked with an *, and also when the physical properties are outside the target range.

[0081] [Table 1]

[0082] [Table 2]

[0083] [Table 3]

[0084] In Table 2 above, (1) to (4) represent the following values. (1): Average area ratio (%) of one or more selected from the group consisting of ferrite and bainite within 30 μm from the surface (2): Average area ratio (%) of one or more selected from the group consisting of ferrite and bainite within 10 μm from the surface (3): The average area ratio (%) of cementite precipitates present in the grain boundaries of one or more types of ferrite and bainite within 10 μm from the surface to the area of ​​cementite precipitates present in the grain boundaries of martensite at a point 1 / 4 t from the surface (where t is the total thickness of the steel sheet). = [(Area of ​​cementite at (F+B) grain boundaries within 10 μm from the surface) / (Area of ​​cementite at martensite grain boundaries at 1 / 4 t from the surface) × 100] (4): Maximum 10 at 20Hz 7 This expresses the fatigue properties of the material, i.e., the ability to prevent cracks from occurring under repeated tests, and is displayed as excellent (○) or poor (×).

[0085] At this time, to measure the "average area fraction (%) of one or more types selected from the group consisting of ferrite and bainite within 30 μm from the surface" in (1) above, a cross section parallel to the rolling direction was mechanically polished and nital etched, and the ferrite and bainite in the region from the surface to 30 μm of the steel sheet were observed in four fields of view using a scanning electron microscope (SEM) (i.e., measurements were made at four points in the thickness direction in the region from the surface to 30 μm). That is, the area fraction of each structure was determined by image analysis of the SEM image at a magnification of 2000 times, and the average area fraction was determined by calculating the arithmetic mean of the area fractions determined in each of the four fields of view.

[0086] In addition, in order to measure the "average area ratio (%) of one or more species selected from the group consisting of ferrite and bainite within 10 μm from the surface" in (2) above, the area ratio was analyzed using an image analyzer after surface polishing and etching followed by SEM observation.

[0087] Furthermore, when measuring the "average area ratio (%) of cementite precipitates present at one or more grain boundaries selected from the group consisting of ferrite and bainite within 10 μm from the surface relative to the area of ​​cementite precipitates present at the grain boundaries of martensite at a point 1 / 4t from the surface (where t is the total thickness of the steel sheet)" in (3) above, in order to analyze the abundance ratio of cementite precipitates with a circle equivalent diameter of 30 nm or less present at the grain boundaries at each position, 10 fields of view were analyzed via 2000x SEM images, and the abundance ratio of cementite present at the grain boundaries in the circle equivalent diameter range of 2 to 30 nm was targeted, and the arithmetic average of the results was used to define a representative value.

[0088] As can be seen from Table 1 above, the examples of the present invention satisfy the tensile strength of 1500 MPa or more, the hole expandability (HER) of 50% or more, and the maximum expansion rate of 10 at a frequency of 20 Hz. 7 It was confirmed that no cracks occurred under repeated tests, demonstrating excellent fatigue properties.

[0089] 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 were inferior in one or more of the strength, hole expandability, and fatigue properties mentioned above.

[0090] 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 targeted excellent hole expandability and fatigue properties, as in the case of the invention examples.

Claims

1. In weight percent, carbon (C): 0.1 to 0.3%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.0 to 3.0%, boron (B): 0.0005 to 0.003%, phosphorus (P): 0.01% 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%, with the balance being Fe and other unavoidable impurities, A steel sheet in which an average area ratio of cementite precipitates present at grain boundaries of one or more types of crystal selected from the group consisting of ferrite and bainite within 10 μm from the surface is 60% or less (excluding 0%) of the area of ​​cementite precipitates present at grain boundaries of martensite at a point ¼t from the surface (where t is the total thickness of the steel sheet).

2. 2. The steel plate according to claim 1, further comprising, by weight, one or more selected from the group consisting of chromium (Cr): 0.2% or less (excluding 0%), molybdenum (Mo): 0.01 to 0.1%, niobium (Nb): 0.01 to 0.05%, titanium (Ti): 0.01 to 0.05%, and antimony (Sb): 0.001 to 0.1%.

3. The steel plate according to claim 1, wherein an average area ratio of one or more types selected from the group consisting of ferrite and bainite within 30 μm from the surface is 15 to 60%.

4. The steel plate according to claim 1, wherein an average area ratio of one or more types selected from the group consisting of ferrite and bainite within 10 μm from the surface is 50 to 80%.

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

6. 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), 1.0 to 3.0% manganese (Mn), 0.0005 to 0.003% boron (B), 0.01% 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), the balance being Fe and other unavoidable impurities, at a temperature of 1100 to 1300°C; hot rolling the reheated slab to produce a hot rolled steel sheet; Coiling the hot-rolled steel sheet at 400 to 600°C; cold rolling the coiled hot-rolled steel sheet at a reduction ratio of 30 to 80%; and Annealing the cold-rolled steel sheet by heat treating it at a temperature of Ac3+10°C to Ac3+80°C; The annealing step controls the dew point temperature to -40°C or less at a temperature of Ac3+10°C or less, and controls the dew point temperature to -30°C to +30°C at a temperature of Ac3+10°C to Ac3+80°C.

7. The method for manufacturing a steel sheet according to claim 6, wherein the annealing step comprises heat treatment at Ac3+10°C to Ac3+80°C for 30 seconds or more.

8. The method for manufacturing a steel sheet according to claim 6, further comprising the step of primarily cooling the annealed steel sheet to a temperature range of 680 to 750°C, which is an end temperature of the primary cooling, at an average cooling rate of 10°C / s or less.

9. The method for manufacturing a steel sheet according to claim 8, further comprising the 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.

10. The method of claim 9, further comprising the step of reheating the second-cooled steel sheet to 150 to 240°C and overaging the steel sheet.

11. The method of claim 10, wherein the overaging heat treatment is performed for 200 to 1000 seconds.

12. The steel slab further contains, by weight%, one or more selected from the group consisting of chromium (Cr): 0.2% or less (excluding 0%), molybdenum (Mo): 0.01 to 0.1%, niobium (Nb): 0.01 to 0.05%, titanium (Ti): 0.01 to 0.05%, and antimony (Sb): 0.001 to 0.1%. The method for producing a steel plate according to claim 6.

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