Steel sheet and manufacturing method therefor

Abstract

An embodiment of the present invention can provide a steel sheet and a manufacturing method therefor, wherein the steel sheet exhibits excellent hydrogen embrittlement resistance through the improvement of corrosion resistance and has ultra-high strength with a tensile strength of 1500 Mpa grade.

Description

Steel plate and method of manufacturing the same

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

[0002] Recently, the demand for high-strength steel sheets used in automotive frame parts is increasing. In addition, the application of ultra-high-strength steel sheets with a tensile strength of 1500 MPa or more is underway for some parts.

[0003] To achieve such ultra-high strength, it is effective to use a steel structure primarily composed of hard materials such as martensite or bainite. However, compared to steels with composite structures of ferrite and martensite, such steel structures have lower elongation and are prone to cracking during forming; consequently, they are limited to parts formed by bending processes that have relatively simple shapes.

[0004] Although the composition of the aforementioned hardened structure is necessary to improve the strength of steel sheets, its application as a suitable automotive component has been limited due to crack formation during part forming as well as degraded impact characteristics during collisions. Therefore, to achieve high strength in automotive components using a martensitic-based steel structure, it is necessary to develop steel materials that possess superior hole expansion capability, fatigue characteristics, and hydrogen embrittlement resistance—in addition to securing the desired strength—thereby ensuring collision resistance during vehicle crashes, which enables their widespread application as high-strength steel materials for automotive parts.

[0005] Patent Document 1 is a representative prior art of this method. Patent Document 1 relates to a steel having a single-phase martensitic structure containing C: 0.25~0.4%, Si: 1.0% or less, Mn: 1.5~2.5%, P: 0.02% or less, S: 0.003% or less, Al: 0.01~0.1%, N: 0.005% or less, B: 0.0005~0.005%, and also containing Ti: 0.005~0.1%, Nb: 0.005~0.1%, and a total of 0.005~0.1%. It discloses that the steel can be obtained by heating and holding the steel in a temperature range above the Ae3 transformation point and below 900°C, then rapidly cooling it to below 200°C at an average cooling rate of 300°C / s, and subsequently tempering it at below 250°C. However, in the case of Patent Document 1, there is a problem in that defects occur during molding because the shape (flatness) is inferior due to rapid cooling (water cooling).

[0006] Patent Document 2 relates to a thin steel sheet having a high-strength structure comprising C: 0.05% or more and 0.35% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.8% or more and 3.0% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less, and N: 0.0060% or less, with a ferrite area ratio of 0% or more and 90% or less, a bainite area ratio of 5% or less (including 0%), a martensite and tempered martensite area ratio of 10% or more (including 100%), and a retained austenite area ratio 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 the steel sheet when sheared at a length of 1 m of 10 mm or less. However, in the case of Patent Document 2, there is a problem that shape defects occur due to rapid cooling after annealing.

[0007] Meanwhile, to manufacture ultra-high-strength steel with a tensile strength of 1470 MPa or higher, it is essential to introduce martensite or some bainite. In this case, brittle fracture is prone to occur due to hydrogen remaining within the steel or introduced from the outside, a phenomenon referred to as hydrogen embrittlement. Hydrogen embrittlement manifests at a strength lower than the fracture strength, meaning the material can fracture due to hydrogen embrittlement even at applied stresses that are very small compared to the actual fracture strength of the material. In particular, this hydrogen embrittlement becomes more sensitive as the strength of the steel increases. Furthermore, since resistance to hydrogen embrittlement improves as bending characteristics are superior when the initial hydrogen content within the steel is the same, it is also necessary to improve bending characteristics.

[0008] Therefore, to solve the aforementioned problems, it is necessary to develop ultra-high-strength cold-rolled steel sheets with excellent bending characteristics, hydrogen embrittlement resistance, and corrosion resistance.

[0009] (Patent Document 1) Patent Document 1) Japanese Patent Publication No. JP 2010-248565

[0010] (Patent Document 2) Japanese Patent Publication No. JP 2020-019992

[0011] One embodiment of the present invention can provide a steel plate with excellent hydrogen embrittlement resistance by improving corrosion resistance and a method for manufacturing the same.

[0012] One embodiment of the present invention can provide a steel plate having ultra-high strength of 1500 MPa tensile strength by improving corrosion resistance and a method for manufacturing the same.

[0013] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0014] A steel sheet according to one embodiment of the present invention comprises, in weight percent, carbon (C): 0.1~0.3%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.0~3.0%, boron (B): 0.0005~0.003%, 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): 0.01~0.1%, copper (Cu): 0.03~0.2%, chromium (Cr): 0.005~0.2%, and the remainder being Fe and other unavoidable impurities, and comprises a decarburized layer comprising ferrite and bainite in the thickness direction from the surface, wherein the thickness of the decarburized layer is 15㎛~60㎛. At a point 1 / 4t from the surface (where t is the total thickness of the steel plate), the density of precipitates with a diameter of 100 nm or less is 30 particles / ㎛ 2 The above is as follows, and the above precipitate contains kappasulfide (CuS).

[0015] The above steel plate may further include at least one selected from the group consisting of calcium (Ca): 0.005~0.2%, niobium (Nb): 0.01~0.05%, and titanium (Ti) 0.01~0.05%.

[0016] The above precipitate may further include at least one selected from the group consisting of chromium carbide (CrC) and calcium carbide (CaC).

[0017] A method for manufacturing a steel plate according to one embodiment of the present invention comprises the steps of: reheating a steel slab at a temperature of 1100 to 1180°C, comprising, 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%, copper (Cu): 0.03 to 0.2%, chromium (Cr): 0.005 to 0.2%, and the remainder being Fe and other unavoidable impurities; and hot rolling the reheated steel slab. The method comprises the steps of: cooling and coiling the hot-rolled steel sheet; cold-rolling the coiled steel sheet; continuously annealing the cold-rolled steel sheet at a dew point temperature of -25 to 20℃ at Ac3+20℃ to Ac3+90℃ for 50 to 200 seconds; first cooling the continuously annealed cold-rolled steel sheet to a first cooling end temperature of 670 to 750℃; second cooling to a temperature range of 40 to 250℃ after the first cooling at an average cooling rate of 50 to 120℃ / s; and reheating to a temperature range of 130 to 300℃ after the second cooling for over-aging treatment.

[0018] The above steel slab may further include at least one selected from the group consisting of calcium (Ca): 0.005~0.2%, niobium (Nb): 0.01~0.05%, and titanium (Ti) 0.01~0.05%.

[0019] The above hot rolling step is performed in a temperature range of Ar3 to Ar3+120℃, the above coiling step is performed in a temperature range of Ms to 650℃, the above cold rolling step is performed with a reduction rate of 30 to 80%, the above first cooling step is performed at an average cooling rate of 1 to 6℃ / s, and the above overaging treatment step can be maintained for a time of 100 to 700 seconds.

[0020] A steel plate and a method for manufacturing the same, which is an embodiment of the present invention, can have excellent hydrogen embrittlement resistance by improving corrosion resistance.

[0021] A steel plate and a method for manufacturing the same, which is an embodiment of the present invention, can have ultra-high strength of 1500 MPa tensile strength by improving corrosion resistance.

[0022] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0023] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0024] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0025] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0026] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0027] Additionally, throughout the specification, when it is said that one part is 'connected' to another part, this includes not only cases where they are 'directly connected,' but also cases where they are 'indirectly connected' with other elements in between.

[0028] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0029] One embodiment of the present invention can provide a steel plate having excellent hydrogen embrittlement resistance by improving corrosion resistance and ultra-high strength of 1500 MPa tensile strength, and a method for manufacturing the same.

[0030] The present invention will be described in detail below.

[0031] Below, the steel plate composition of the present invention will be described in detail.

[0032] Unless otherwise specifically stated in the present invention, the % indicating the content of each element is based on weight.

[0033] A steel sheet according to one embodiment of the present invention comprises, in weight percent, carbon (C): 0.1~0.3%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.0~3.0%, boron (B): 0.0005~0.003%, 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): 0.01~0.1%, copper (Cu): 0.03~0.2%, chromium (Cr): 0.005~0.2%, and the remainder may include Fe and other unavoidable impurities.

[0034] Carbon (C): 0.1~0.3%

[0035] Carbon (C) is required to improve hardenability and obtain a steel structure in which the martensite area ratio at the 1 / 4 thickness position is 95% or more. Additionally, C is required to increase martensite strength and ensure a TS ≥ 15,000 MPa. If the C content is less than 0.1%, the desired strength cannot be obtained. Therefore, the C content should be 0.1% or more, and a more preferable C content may be 0.15% or more. On the other hand, if the C content exceeds 0.3%, it becomes difficult to obtain good weldability or delayed fracture resistance. Therefore, the C content should be 0.30% or less, and more preferably, the lower limit of the C content may be 0.25% or less.

[0036] Silicon (Si): 0.5% or less (excluding 0%)

[0037] Silicon (Si) is included as a strengthening element through solid solution strengthening, and also to improve bendability by suppressing the formation of film-like carbides during the tempering process at high temperatures. While a higher Si content is advantageous for obtaining the above effects, if the Si addition exceeds a certain standard content, weldability and Liquid Metal Embrittlement (LME) properties become inferior; therefore, the upper limit is set to 0.5% or less, and the preferred range is set to 0.32% or less. However, the lower limit of the Si content is not specifically limited, and 0% is excluded as it is added as an impurity element during the manufacturing process. Meanwhile, a more preferable lower limit of the Si content may be 0.001% or more.

[0038] Manganese (Mn): 1.0~3.0%

[0039] Manganese (Mn) contributes to strength improvement through solid solution strengthening or by increasing the martensite area ratio through increased hardenability. Additionally, Mn is included to reduce hot brittleness by fixing sulfur in the steel as MnS. If the lower limit of the Mn content is less than 1.0%, the likelihood of ferrite and bainite formation rather than martensite formation increases during cooling; therefore, the lower limit is restricted to 1.0% or more, and a more preferable lower limit of Mn content is 1.3% or more. Meanwhile, the Mn content is set to 3.0% or less from the perspective of weld stability. Furthermore, the Mn content is preferably 2.6% or less, and more preferably 2.0% or less.

[0040] Boron (B): 0.0005~0.003%

[0041] Boron (B) is an element that inhibits ferrite formation, and accordingly, the present invention has the advantage of inhibiting the formation of ferrite during cooling after annealing. However, if the content of B exceeds 0.003%, ductility may be significantly reduced. On the other hand, if the content of B is less than 0.0005%, there is no hardenability effect, so not only is the target strength not secured, but ferrite is formed on the surface layer, and bendability tends to be inferior; therefore, the content is set to 0.0005~0.003%. Meanwhile, it is more preferable for the content of B to have a range of 0.002% or less.

[0042] Phosphorus (P): 0.01% or less (excluding 0%)

[0043] Phosphorus (P) is an impurity element included in steel, and while it is advantageous to have a lower amount added to the steel, a content of 0% is excluded to account for cases where it is inevitably included during the manufacturing process. However, if the content exceeds 0.01%, weldability deteriorates and there is a risk of brittleness in the steel, so the upper limit may be restricted to 0.01% or less. A more desirable upper limit may be 0.003% or less.

[0044] Sulfur (S): 0.01% or less (excluding 0%)

[0045] Sulfur (S), like phosphorus, is an impurity inevitably contained in steel and is an element that impairs the ductility and weldability of steel sheets; therefore, it is desirable to manage its content as low as possible. Thus, in the present invention, it is desirable to limit the sulfur content to 0.01% or less. More preferably, it can be limited to 0.005% or less. Even more preferably, it is desirable to manage it to 0.002% or less to minimize MnS precipitates in the steel and contribute more to improving bendability. Meanwhile, 0% is excluded to account for cases where it is inevitably included during the manufacturing process.

[0046] Nitrogen (N): 0.01% or less (excluding 0%)

[0047] Nitrogen (N) is an impurity element, and if its content exceeds 0.01%, it significantly increases the risk of cracking during continuous casting due to AlN formation, etc. Therefore, it is desirable to limit its upper limit to 0.01%. It is more desirable that the N content be 0.008% or less, and most desirable that it be 0.006% or less. Meanwhile, 0% is excluded to account for cases where it is inevitably included during the manufacturing process.

[0048] Aluminum (Al): 0.01~0.1%

[0049] Aluminum (Al) can be added to remove oxygen from molten steel and, like Si, is an element effective in stabilizing residual austenite by suppressing the precipitation of cementite during the reheating and overaging stages. If the Al content is less than 0.01%, the steel is not sufficiently deoxidized, and the cleanliness of the steel is compromised. On the other hand, if the Al content exceeds 0.1%, not only is the castability of the slab deteriorating, but the temperature required for single-phase heating during annealing also increases, which may lead to production and equipment problems. More preferably, the upper limit of the Al content can be restricted to 0.05% or less.

[0050] Copper (Cu): 0.03~0.2%

[0051] Copper (Cu) is an element that improves corrosion resistance within steel sheets. Hydrogen introduced during the manufacturing process penetrates into grain boundaries, causing hydrogen embrittlement. However, if a large amount of CuS precipitates are present at the grain boundaries, it not only suppresses hydrogen penetration but also improves intergranular corrosion resistance, thereby suppressing pitting corrosion and significantly contributing to the improvement of hydrogen embrittlement. If the Cu content is less than 0.03%, the CuS precipitates in the steel do not form sufficiently and do not contribute to the improvement of corrosion resistance. If the Cu content exceeds 0.2%, the Cu precipitates melt into a liquid state during hot rolling reheating, embrittle the grain boundaries, which increases the risk of cracking and is also disadvantageous from an economic perspective. Preferably, the upper limit of the Cu content can be restricted to 0.015% or less. A more preferable upper limit of the Cu content is 0.012% or less.

[0052] Chrome (Cr): 0.005~0.2%

[0053] Chromium (Cr) is an element that improves corrosion resistance in steel sheets. In this invention, when Cu is added, if the reheating temperature during hot rolling is high, it liquefies, causing a defect known as a "star crack" during hot rolling operations. Nickel (Ni) is often added to suppress this, but it is economically disadvantageous due to the high cost of the Ni component. This invention proposes a patent by confirming that crack formation is suppressed by adding Cr instead of Ni, optionally adding Ca, and controlling the reheating temperature during hot rolling. During the process, Cr not only improves corrosion resistance by mainly forming CrC at grain boundaries but also promotes improved hydrogen embrittlement by preventing the liquefaction of Cu precipitates. If the Cr content is less than 0.005%, sufficient CrC precipitates are not formed in the steel, so it does not contribute to the improvement of corrosion resistance. If the Cr content exceeds 0.2%, there is a problem where not only does manufacturing costs increase, but strength decreases as the excessive CrC precipitation lowers the dissolved carbon content, hindering the formation of martensite. Preferably, the upper limit of the Cr content is 0.15%, and the upper limit of the Cu content is more preferably 0.1%.

[0054] The remainder contains iron (Fe), and since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the normal manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the art during the manufacturing process, all details thereof are not specifically mentioned in this specification.

[0055] In addition, the above steel plate may further contain one or more of, in mass%, calcium (Ca): 0.005~0.2%, niobium (Nb): 0.01~0.05%, and titanium (Ti): 0.01~0.05%. Below, the reasons for adding each component and limiting its content will be explained in detail.

[0056] Calcium (Ca): 0.005~0.2%

[0057] Calcium (Ca) can be effectively added in conjunction with Cr to prevent the liquefaction of Cu by promoting the spheroidization of CuS precipitates. In this invention, when Ca is added, if the heating temperature of the hot-rolled material is high, liquefaction occurs, causing a defect known as a "star crack" during hot-rolling operations. Nickel (Ni) is added to prevent this liquefaction, but since the cost of the Ni component is high, it is economically disadvantageous; therefore, Ca is added to substitute for this effect. During the process, Ca not only improves corrosion resistance by mainly forming CaC at grain boundaries but also promotes the spheroidization of Cu precipitates to prevent the liquefaction of Cu, thereby improving hydrogen embrittlement. If the Ca content is less than 0.005%, the CaC precipitates in the steel do not form sufficiently and do not contribute to the improvement of corrosion resistance. If the Ca content exceeds 0.2%, it is disadvantageous from an economic perspective, as it not only increases manufacturing costs but also causes a decrease in strength due to excessive CaC precipitation. The upper limit of the desirable Ca content is 0.12%, and the upper limit of the more desirable Ca content is 0.1%.

[0058] Niobium (Nb): 0.01~0.05%

[0059] Niobium (Nb) is an element that contributes to an increase in strength through a precipitation strengthening effect by segregating at the austenite grain boundaries and inhibiting the growth of austenite grains during annealing heat treatment. However, if the content of Nb exceeds 0.05%, the precipitation of carbides and nitrides increases, which lowers the workability of the base material and increases costs as the amount of alloy input becomes excessive. If it is less than 0.01%, it does not contribute at all to an increase in strength, so the lower limit is set to 0.01%. Meanwhile, in order to improve the aforementioned effect, the lower limit of the Nb content may be 0.02% or higher, or the upper limit of the Nb content may be 0.04% or lower.

[0060] Titanium (Ti): 0.01~0.05%

[0061] Titanium (Ti) is a nitride-forming element that scavenges N in steel by precipitating it as TiN. If the above-mentioned Ti is not added, there is a possibility that cracks may occur during continuous casting due to the formation of AlN. However, if the above-mentioned Ti content exceeds 0.05%, the strength of the martensite may be reduced due to additional carbide precipitation in addition to the removal of dissolved N, and hole expandability and bendability may be impaired due to the formation of carbides and nitrides such as TiC and TiN. Furthermore, if the above-mentioned Ti content is less than 0.01%, it does not contribute at all to the increase in strength, similar to the Nb element; therefore, the lower limit of the Ti content in the present invention is set to 0.01% or more. Meanwhile, more preferably, the lower limit of the above-mentioned Ti content may be 0.02% or more, or the upper limit of the above-mentioned Ti content may be 0.04% or less.

[0062] Hereinafter, the steel structure of the steel plate according to the present invention will be described.

[0063] A decarburization layer composed of ferrite and bainite exists in the thickness direction from the surface of the steel plate. It is preferable to include a decarburization layer having an average thickness of 15 to 60 μm in the thickness direction from the surface. This allows for improved bending characteristics and resistance to hydrogen embrittlement. If the average thickness of the decarburization layer is less than 15 μm, it is difficult to secure a sufficient soft phase, making it difficult to secure the target bendability (R / t). The lower limit of the average thickness of the decarburization layer is more preferably 17 μm, more preferably 19 μm, and most preferably 21 μm.

[0064] If the average thickness of the above decarburized layer exceeds 60㎛, it is difficult to secure the target strength due to the excessively thick decarburized layer, and fatigue characteristics may be degraded. The upper limit of the average thickness of the above decarburized layer is more preferably 55㎛, more preferably 50㎛, and most preferably 45㎛.

[0065] The microstructure of the decarburized layer may comprise, in area %, ferrite: 80% or more, and the remainder being one or more of bainite, martensite, and tempered martensite. If the fraction of ferrite is less than 80%, bending properties may deteriorate due to insufficient ductility. Therefore, it is preferable that the fraction of ferrite be 80% or more. It is more preferable that the fraction of ferrite be 85% or more.

[0066] In addition, at a point 1 / 4t from the surface (where t is the total thickness of the steel plate), the density of precipitates with a diameter of 100 nm or less is 30 particles / ㎛ 2 A steel plate comprising the above can be provided. The above precipitate may include copper sulfide (CuS). The density of the precipitate is 30 particles / ㎛ 2In cases below this level, there is insufficient precipitate, so the Cu precipitates added in this invention melt into a liquid state during hot rolling operations and exist at grain boundaries, making it ineffective in improving surface corrosion resistance (hereinafter referred to as pitting corrosion). Therefore, precipitates are formed as much as possible to strengthen grain boundaries and improve corrosion resistance, thereby bringing about an effect of improving hydrogen embrittlement. Preferably, the density of precipitates is 50 particles / ㎛ 2 The above is the case, and the upper limit of a more desirable precipitate density is not significantly limited in the present invention.

[0067] In addition, the above precipitates may further include at least one selected from the group consisting of chromium carbide (CrC) and calcium carbide (CaC). When the above precipitates further include chromium carbide (CrC) and / or calcium carbide (CaC) in addition to copper sulfide (CuS), the steel has excellent shear surface quality and superior hole expansion capacity (HER) compared to steel containing only copper sulfide (CuS). Steel containing additional chromium carbide (CrC) or calcium carbide (CaC) typically has a shear surface ratio of 10 to 15% or less during shearing, which is lower than the shear surface ratio (15 to 20%) of steel containing only copper sulfide (CuS), thus exhibiting excellent shear surface quality and superior characteristics when evaluating hole expansion capacity. This has the effect of improving hole expansion ability by reducing the variation in hardness within the grains, as the chromium carbide (CrC) or calcium carbide (CaC) precipitated in the steel not only suppresses the influx of external hydrogen but also reduces the variation in hardness within the grains.

[0068] When the above cold-rolled steel sheet undergoes 30 cycles of corrosion resistance evaluation, there may be zero holes with a diameter of 3 mm or more. One cycle of the above corrosion resistance evaluation can be performed, for example, as follows: Slat Spray (5% NaCl, neutral, 35℃, salt spray volume: 1~2 mL / hr, 6 hr) → Drying (50℃, 20~40%RH, 3 hr) → Wetting (50℃, 95%RH, 14 hr) → Cold air blasting (50%RH, 1 hr).

[0069] The thickness of the cold-rolled steel sheet of the present invention may be 0.6 to 2.3 mm. The lower limit of the thickness of the cold-rolled steel sheet is more preferably 0.7 mm, and more preferably 0.8 mm. The upper limit of the thickness of the cold-rolled steel sheet is more preferably 2.2 mm, and more preferably 2.1 mm.

[0070] The cold-rolled steel sheet of the present invention may have an electro-galvanized layer formed on at least one surface. The present invention does not specifically limit the type of electro-galvanized layer, and any type of electro-galvanized layer commonly used in the relevant technical field may be formed.

[0071] The steel sheet of the present invention can provide a 1500 MPa grade annealed and electro-galvanized steel sheet with excellent hole expansion ability and hydrogen embrittlement resistance.

[0072] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0073] A steel plate according to one embodiment of the present invention can be manufactured by reheating, hot rolling, coiling, cold rolling, continuous annealing, first cooling, second cooling, and overaging treatment of a steel slab satisfying the alloy composition described above.

[0074] First, a slab satisfying the aforementioned alloy composition is heated at 1100 to 1180°C. The above slab heating process is performed to facilitate the subsequent hot rolling process and to sufficiently obtain the target physical properties of the steel sheet. If the above slab heating temperature is below 1100°C, a problem arises in which the hot rolling load increases rapidly. If the above slab heating temperature exceeds 1180°C, the amount of surface scale increases, thereby lowering the yield of the material. In particular, in the case of steel with added Cu, Cu precipitates melt and degenerate into a liquid state, existing at the grain boundaries of the steel sheet and adversely affecting corrosion resistance and hydrogen embrittlement. Therefore, the heating temperature is preferably managed below 1180°C, and more preferably below 1160°C.

[0075] Subsequently, the heated slab is finished hot-rolled at Ar3 to Ar3+120℃ to obtain a hot-rolled steel sheet. If the finish hot-rolling temperature is below Ar3, rolling occurs in a two-phase region of ferrite + austenite or in a ferrite region, resulting in a mixed grain structure, and plate fracture may occur due to fluctuations in the hot-rolling load. If the finish hot-rolling temperature exceeds Ar3+120℃, a large amount of surface scale may form, which may degrade the surface quality. The lower limit of the finish hot-rolling temperature is more preferably Ar3+10℃, more preferably Ar3+20℃, and most preferably Ar3+30℃. The upper limit of the finish hot-rolling temperature is more preferably Ar3+110℃, more preferably Ar3+100℃, and most preferably Ar3+90℃. Meanwhile, the above Ar3 refers to the temperature at which austenite begins to transform into ferrite upon cooling, and can be calculated using the following Equation 1.

[0076] [Equation 1]

[0077] Ar3(℃) = 910 - 203√vC + 44.7Si + 31.5Mo

[0078] Subsequently, the hot-rolled steel sheet is coiled at Ms~650℃. If the coiling temperature (CT) exceeds 650℃, internal oxidation occurs on the surface of the steel sheet, causing the microstructure formed in the surface layer to become non-uniform, and consequently, the bending characteristics may deteriorate. Meanwhile, it is desirable to manage the coiling temperature at a low level to ensure material uniformity across the entire length and width by forming the microstructure of the hot-rolled steel sheet into a single-phase structure rather than a composite structure as much as possible. However, if the coiling temperature is below Ms, the strength of the hot-rolled steel sheet becomes excessively high, which may make actual production impossible due to the increased rolling load during the subsequent cold rolling process. It is more preferable that the lower limit of the coiling temperature be Ms+10℃. It is more preferable that the upper limit of the coiling temperature be 550℃. Ms refers to the temperature at which austenite begins to transform into martensite upon cooling, and can be calculated using Equation 2 below.

[0079] [Equation 2]

[0080] Ms(℃) = 521 - 379C - 15.1Si - 43.9Mn - 19.5Cr - 14.7Mo + 43.7Nb + 91.9Ti + 169B

[0081] Meanwhile, after the above coiling, the material may be cooled by air cooling or water cooling. In addition, after the above cooling, a pickling process may be performed to remove the oxide layer formed on the surface of the hot-rolled steel sheet. Subsequently, the material is rolled to a reduction ratio of 30 to 80%, which is the range in which cold rolling is typically possible. In this case, the reduction ratio range is not significantly relevant in the present invention. The operation should be performed in accordance with the usual required cold-rolled thickness.

[0082] Afterwards, the above cold-rolled steel sheet is continuously annealed at a dew point temperature of -25 to 20℃ at Ac3+20℃ to Ar3+90℃ for 50 to 200 seconds.

[0083] By controlling the dew point temperature in this manner, a decarburization layer can be formed on the surface of the steel sheet during the continuous annealing process. The dew point temperature in a typical continuous annealing furnace is typically between -40 and -50°C. However, as in the present invention, if the oxygen partial pressure is increased by raising the dew point temperature to -25°C or higher, carbon (C) in the steel sheet and oxygen (O) in the annealing furnace meet and are released as CO gas, causing decarburization to occur on the surface layer of the steel sheet. If the dew point temperature is below -25°C, a sufficient decarburization layer may not be formed on the surface of the steel sheet. If the dew point temperature exceeds 20°C, equipment lifespan and productivity may decrease. Therefore, it is desirable for the dew point temperature to have a range of -20 to 20°C. It is more preferable for the lower limit of the dew point temperature to be -20°C. It is more preferable for the upper limit of the dew point temperature to be 15°C. Meanwhile, the present invention does not specifically limit the method for controlling the dew point temperature, but as an example, the dew point temperature can be controlled using humid nitrogen (N2+H2O).

[0084] If the above continuous annealing temperature is less than Ac3+20℃, two-phase annealing rather than single-phase annealing occurs over the entire length of the steel sheet, which may result in the formation of a mixed-grain structure. Consequently, it is difficult to secure the physical properties targeted by the present invention, and in particular, the difference in hardness between phases becomes large, which may significantly reduce hole expandability. If the above continuous annealing temperature exceeds Ac3+90℃, equipment troubles may occur due to overloading of the annealing furnace. The lower limit of the above continuous annealing temperature is more preferably Ac3+25℃, and more preferably Ac3+30℃. The upper limit of the above continuous annealing temperature is more preferably Ac3+80℃, and more preferably Ac3+70℃. Meanwhile, the above Ac3 refers to the temperature at which austenite begins to appear upon heating and can be calculated using Equation 3 below. The above continuous annealing is preferably performed for 50 to 200 seconds.

[0085] If the above continuous annealing time is less than 50 seconds, it is difficult to secure a single-phase austenite structure, and as undissolved carbides remain and coarsen, bending characteristics and hydrogen embrittlement resistance may be reduced, and it may be difficult to sufficiently form a decarburized layer. If the above continuous annealing time exceeds 200 seconds, there is a disadvantage that the austenite size coarsen, making it difficult to secure strength. The lower limit of the above continuous annealing time is more preferably 60 seconds, and more preferably 70 seconds. The upper limit of the above continuous annealing time is more preferably 190 seconds, and more preferably 180 seconds.

[0086] [Equation 3]

[0087] Ac3(℃) = 900 - 206C + 26.2Si - 25Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0088] Subsequently, the continuously annealed cold-rolled steel sheet is first cooled at an average cooling rate of 1 to 6°C / s to a first cooling end temperature of 670 to 750°C. If the first cooling end temperature is less than 670°C, the bending characteristics may deteriorate as a large amount of soft ferrite and bainite, in addition to martensite, is formed during the cooling process. If the first cooling end temperature exceeds 750°C, the temperature difference between the first cooling end temperature and the second cooling end temperature becomes severe, causing a rapid phase transformation and potentially resulting in defective product shape. It is more preferable that the lower limit of the first cooling end temperature be 680°C. It is more preferable that the upper limit of the first cooling end temperature be 740°C. If the first average cooling rate is less than 1°C / s, ferrite is formed during cooling, making it impossible to secure the level of strength targeted by the present invention. If the above first average cooling rate exceeds 6℃ / s, the average cooling rate during the subsequent second cooling decreases, increasing the fraction of low-temperature transformation phases other than martensite, and thus the level of strength targeted by the present invention cannot be secured. It is more preferable that the lower limit of the above first average cooling rate is 2℃ / s. It is more preferable that the upper limit of the above first average cooling rate is 5℃ / s.

[0089] Subsequently, the firstly cooled cold-rolled steel sheet is secondarily cooled at an average cooling rate of 30 to 600°C / s to a second cooling end temperature of 40 to 250°C. The second cooling is intended to secure at least one of the main phases of the present invention, namely martensite and tempered martensite. If the second cooling end temperature is less than 40°C, shape defects are caused by rapid phase transformation, and there is a disadvantage that continuous production is difficult due to strip meandering problems. If the second cooling end temperature exceeds 250°C, it may be difficult to secure the strength targeted by the present invention. The lower limit of the second cooling end temperature is more preferably 50°C, more preferably 55°C, and most preferably 60°C. The upper limit of the second cooling end temperature is more preferably 240°C, more preferably 230°C, and most preferably 220°C. If the above secondary average cooling rate is less than 30℃, a soft ferrite transformation occurs during cooling, making it difficult to secure the target strength. If the above secondary average cooling rate exceeds 600℃ / s, the product shape may become defective due to rapid phase transformation. The lower limit of the above secondary average cooling rate is more preferably 35℃ / s, more preferably 40℃ / s, and most preferably 45℃ / s. The upper limit of the above secondary average cooling rate is more preferably 500℃ / s, more preferably 400℃, and most preferably 300℃ / s.

[0090] During the above second cooling, the temperature difference between Mf and the second cooling termination temperature is strictly controlled to be 20°C or more. If the above temperature difference is less than 20°C, the martensite transformation may not occur sufficiently, making it difficult to secure the target strength. It is more preferable that the above temperature difference be 30°C or more. Meanwhile, Mf refers to the temperature at which the martensite transformation is terminated during cooling, and can be calculated using Equation 4 below.

[0091] [Equation 4]

[0092] Mf(℃) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17Mo + 49.2Nb + 95Ti + 202B

[0093] Subsequently, the above-mentioned secondary cooled cold-rolled steel sheet is reheated to an over-aging treatment temperature of 130 to 300°C and then subjected to over-aging treatment. Through the above reheating and over-aging treatment, the martensite obtained by the aforementioned rapid cooling process is transformed into tempered martensite, thereby increasing the yield strength. If the above-mentioned reheating temperature and over-aging treatment temperature are below 130°C, there is a disadvantage in that tempering is not sufficiently performed, resulting in low yield strength and inability to secure sufficient toughness. If the above-mentioned reheating temperature and over-aging treatment temperature exceed 300°C, there is a disadvantage in that bendability is degraded due to the precipitation and coarsening of large amounts of carbides. The lower limit of the above-mentioned reheating temperature and over-aging treatment temperature is more preferably 140°C, more preferably 150°C, and most preferably 160°C. The upper limit of the above reheating temperature and over-aging treatment temperature is more preferably 280℃, more preferably 260℃, and most preferably 240℃.

[0094] Meanwhile, the above overaging treatment may be performed for 5 to 12 minutes. If the above overaging treatment time is less than 5 minutes, tempering is not sufficiently performed, and the yield strength may be lowered. If the above overaging treatment time exceeds 12 minutes, carbides may coarsen due to excessive tempering, and bending characteristics may deteriorate. The lower limit of the above overaging treatment time is more preferably 5.5 minutes, more preferably 6.0 minutes, and most preferably 6.5 minutes. The upper limit of the above overaging treatment time is more preferably 11.5 minutes, more preferably 11 minutes, and most preferably 10.5 minutes.

[0095] Subsequently, the above-mentioned over-aged cold-rolled steel sheet is subjected to temper rolling (SPM (Skin Pass Mill)) with a rolling force of 500 to 1,000 tons. This temper rolling enables the control of surface roughness (Rsk). When the rolling force during the temper rolling is less than 500 tons, the load is low, making it difficult to control the surface roughness (Rsk); conversely, when it exceeds 1,000 tons, severe work hardening of the surface may occur, potentially leading to inferior bending characteristics. It is more preferable that the lower limit of the rolling force during the temper rolling be 550 tons, and more preferable that it be 600 tons. It is more preferable that the upper limit of the rolling force during the temper rolling be 950 tons, and more preferable that it be 900 tons.

[0096] Meanwhile, after the tension leveling above, the step of forming an electro-galvanized layer on at least one surface of the cold-rolled steel sheet may be additionally included. The present invention does not specifically limit the method of forming the electro-galvanized layer, and any method commonly used in the relevant technical field may be used.

[0097] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0098] (Example)

[0099] A slab having the alloy composition listed in Table 1 below was heated to 1150°C, and then the heated slab was finished hot-rolled at 900°C to obtain a hot-rolled steel sheet. Subsequently, the hot-rolled steel sheet was coiled under the conditions listed in Table 2 below and then cold-rolled to obtain a cold-rolled steel sheet. Subsequently, a cold-rolled steel sheet with a thickness of 1.4 mm was manufactured by continuous annealing, first cooling, second cooling, reheating / overaging treatment, temper rolling, and tension leveling under the conditions listed in Table 2 below.

[0100] The microstructure, composition related to the decarburization layer, and mechanical properties of the cold-rolled steel sheet manufactured in this manner were measured, and the results are shown in Table 3 below.

[0101] The types and fractions of the microstructure of the center and decarburized layer of the steel plate were observed using a scanning electron microscope (SEM) and an optical microscope (OM) at a position t / 4 (t: thickness of the steel) in the thickness direction of the steel, and the fractions of each phase were analyzed three times through image analysis to calculate the average value.

[0102] The composition of the decarburized layer was measured using a GDS (Glow discharge spectrometer).

[0103] In addition, for the observation of precipitates, TEM was used to analyze 10 fields of view by counting the size and distribution of precipitates within an equivalent diameter of 10 μm at a point 1 / 4 t from the surface (where t is the total thickness of the steel plate), and the arithmetic mean was used to determine the representative value.

[0104] Yield strength, tensile strength, yield ratio, and total elongation were measured by processing cold-rolled steel sheets into specimens of JIS No. 5 and then performing a tensile test under conditions of a test speed of 28 mm / min.

[0105] The bending workability (R / t) was determined by processing a cold-rolled steel sheet into a specimen with a width of 30 mm x a length of 100 mm, performing a 90° bending test at a test speed of 800 mm / min, checking for the occurrence of cracks in the bending section using a stereoscopic microscope, and calculating the minimum bending radius (R value of the die) at which no cracks occurred by dividing it by the thickness (mm) of the specimen.

[0106] Hydrogen embrittlement resistance was evaluated as excellent (no cracks), average (1 or fewer), and poor (more than 1) by visually checking how many cracks of 3 mm or larger occurred when three specimens with a width of 30 mm x a length of 100 mm were extracted from cold-rolled steel sheets, 90° V-bending test was performed under conditions of R / t of 4, and immersed in a 0.1 N HCl solution for 120 hours.

[0107] The corrosion resistance of the sheet metal was measured by preparing a specimen with a width of 75 mm × a length of 150 mm, performing 30 cycles of a corrosion test in which one cycle consisted of Slat Spray (5% NaCl, neutral, 35℃, salt spray volume: 1~2 mL / hr, 6 hr) → Drying (50℃, 20~40%RH, 3 hr) → Wetting (50℃, 95%RH, 14 hr) → Cold air blasting (50%RH, 1 hr), and then visually checking how many holes with a diameter of 3 mm or more occurred. If the number of holes with a diameter of 3 mm or more was 0, it was evaluated as not occurring, and if there was 1 or more, it was evaluated as occurring.

[0108] Hole expansion ability (HER) was evaluated by punching with a diameter of 10 mm based on a clearance of 12% and expanding until a crack occurred at the edge, dividing the diameter until the crack occurred by the initial diameter.

[0109] Steel Grade CSiMnBPSNS.AlCuCrCaNbTi Invention Steel 10.240.121.80.0020.0070.0030.0030.0340.120.01-0.03 Invention Steel 20.240.121.820.0020.0050.0020.0040.0350.110.008-- Invention Steel 30.230.081.810.0010.0080.0010.0030.0350.08-0.0110.028 Invention Steel 40.240.111.780.0020.0060.0010.0030.0360.13--0.0320.02 Invention Steel 50.260.091.790.0020.0040.0010.0040.0370.15---0.02Invention Lecture 60.190.112.220.0020.0030.0020.0040.00420.16---0.02Comparison Lecture 10.220.121.950.0030.0070.0010.0030.0041----Comparison Lecture 20.250.131.960.0020.0060.0020.0040.0035----Comparison Lecture 30.230.101.890.0010.0070.0020.0030.0038---0.03

[0110]

[0111] Classification Steel Grade No. Reheating Temperature (°C) Coiling Temperature (°C) Annealing Temperature (°C) Average Dew Point Temperature (°C) 1st Cooling End Temperature (°C) 1st Average Cooling Rate (°C / s) 2nd Cooling End Temperature (°C) 2nd Average Cooling Rate (°C / s) Overaging Heat Treatment Temperature (°C) Overaging Heat Treatment Time (sec) Preheating Zone / Heating Zone ① Cracking Zone ② Invention Example 1 Invention Steel 11175455851-45107106.211082182451 Invention Example 2 Invention Steel 21168532850-43157125.511285173460 Invention Example 3 Invention Steel 31168454849-45107106.111081181459 Invention Example 4 Invention Steel 41171545850-42137135.8113103182473 Comparative Example 1 Invention 41171453851-20257135.8108107192463 Comparative Example 2 Invention 41171571861-42157326.228282173452 Invention Example 5 Invention 51168611861-43117105.310595182452 Comparative Example 3 Invention 51168455862-46-107116.228043350412 Invention Example 6 Invention 61173453845-42157055.611375182450Comparative Example 4 Invention Steel 61173482845-45156596.318082325450Comparative Example 5 Comparative Steel 11175452812-42-107023.526075280480Comparative Example 6 Comparative Steel 21174452832-45127254.311938182480Comparative Example 7 Comparative Steel 31210*482824-43167004.311082180450Comparative Example 8 Comparative Steel 31173481815-43107105.310585312450

[0112] Classification YS(MPa)TS(MPa)El(%) Decarburization layer F / B phase fraction(%) Precipitate density number / ㎛ 2)Flexibility R / t Hydrogen Embrittlement Resistance Sheet Corrosion Resistance Hole Expansion Ability (HER) Invention Example 1 123 215 107.28 335 2.8○○45 Invention Example 2 123 315 307.98 54 22.8○○44 Invention Example 3 118 515 427.39 14 12.5○○46 Invention Example 4 13 2216 426.98 75 32.8○○38 Comparative Example 1 128 215 75 6.14 332 4.2ΔΔ34 Comparative Example 2 109 815 327.98 55 3.8○○33 Invention Example 5 116 515 268.48 85 12.5○○36 Comparative Example 3 118 615 288.57 245 3.5ΔΔ35 Invention Example 6120115358.285422.8○○37 Comparative Example 4118515107.983413.5○○33 Comparative Example 5123215867.652102.8××34 Comparative Example 6118615067.4682.8××35 Comparative Example 7985127810.21783.2××34 Comparative Example 8965131210.3573.2××33

[0113] As can be seen in Table 3 above, the inventive examples of the present invention satisfy a tensile strength of 1500 MPa or more and a bendability (R / t) of 3.0 or less, and it was confirmed that they have excellent hydrogen embrittlement resistance and sheet corrosion resistance.

[0114] On the other hand, in the case of the comparative examples of the present invention, it was confirmed that one or more characteristics among the aforementioned strength, bendability (R / t), hydrogen embrittlement, sheet corrosion resistance, and hole expansion ability were inferior because they did not meet the conditions required by the present invention.

[0115] That is, by controlling the components and operating conditions required in the present invention, it is possible to manufacture a 1500 MPa-class annealed and electro-galvanized steel sheet with excellent hole expansion ability and hydrogen embrittlement resistance characteristics, as in the example of the invention.

Claims

1. In wt%, containing carbon (C): 0.1–0.3%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.0–3.0%, boron (B): 0.0005–0.003%, 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): 0.01–0.1%, copper (Cu): 0.03–0.2%, chromium (Cr): 0.005–0.2%, and the remainder being Fe and other unavoidable impurities, It includes a decarburized layer comprising ferrite and bainite in the thickness direction from the surface, wherein the thickness of the decarburized layer is 15㎛ to 60㎛. At a point 1 / 4t from the surface (where t is the total thickness of the steel plate), the density of precipitates with a diameter of 100 nm or less is 30 particles / ㎛ 2 That is all, The above precipitate is a steel plate containing copper sulfide (CuS).

2. In Paragraph 1, The steel plate further comprises at least one selected from the group consisting of calcium (Ca): 0.005~0.2%, niobium (Nb): 0.01~0.05%, and titanium (Ti) 0.01~0.05%.

3. In Paragraph 1, A steel plate comprising at least one selected from the group consisting of chromium carbide (CrC) and calcium carbide (CaC) as the precipitate.

4. A step of reheating a steel slab containing, in weight percent, carbon (C): 0.1~0.3%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.0~3.0%, boron (B): 0.0005~0.003%, 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): 0.01~0.1%, copper (Cu): 0.03~0.2%, chromium (Cr): 0.005~0.2%, and the remainder being Fe and other unavoidable impurities, at a temperature of 1100~1180℃; A step of hot rolling the above-mentioned reheated steel slab; A step of cooling and coiling the above hot-rolled steel plate; A step of cold rolling the above-mentioned wound steel plate; A step of continuously annealing the above cold-rolled steel sheet at a dew point temperature of -25 to 20℃ at Ac3+20℃ to Ac3+90℃ for 50 to 200 seconds; A step of first cooling the above continuously annealed cold-rolled steel sheet to a first cooling end temperature of 670~750℃; A step of secondary cooling at an average cooling rate of 50 to 120℃ / s to a temperature range of 40 to 250℃ after the above primary cooling; and A method for manufacturing a steel plate comprising the step of overaging treatment by reheating to a temperature in the range of 130 to 300℃ after the above secondary cooling.

5. In Paragraph 4, A method for manufacturing a steel plate, wherein the above steel slab further comprises at least one selected from the group consisting of calcium (Ca): 0.005~0.2%, niobium (Nb): 0.01~0.05%, and titanium (Ti) 0.01~0.05%.

6. In Paragraph 4, The above hot rolling step is performed in a temperature range of Ar3 to Ar3+120℃, and The above winding step is performed in a temperature range of Ms~650℃, and The above cold rolling step is performed with a reduction rate of 30 to 80%, and The above first cooling step is performed at an average cooling rate of 1 to 6℃ / s, and A method for manufacturing a steel plate, wherein the above-mentioned overaging treatment step is maintained for a time of 100 to 700 seconds.