Steel sheet and method for manufacturing same

Abstract

According to exemplary embodiments of the present invention, provided may be a steel sheet capable of improving plating wettability and plating adhesion of a plated steel sheet and a method for manufacturing same.

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] Steel sheets can be exposed to various environments from the storage stage after manufacturing to the point where they are applied to and formed into products and used. Due to these external environments, oxidation and corrosion may occur, leading to a deterioration in the quality of the steel sheets. To prevent this, a plating layer can be formed on the surface of the steel sheet. This plating layer can provide a physical barrier against external oxygen, moisture, and salt, and the materials within the plating layer can act as an electrochemical sacrificial anode to prevent corrosion or oxidation of the steel sheet.

[0003] Various studies are being conducted to improve the quality of such galvanized steel sheets. In particular, there is a need for technological development to improve plating wettability and adhesion in order to enhance plating quality.

[0004] The problem that the technical concept of the present invention aims to solve is to provide a steel sheet capable of improving plating wettability and plating adhesion of a plated steel sheet, and a method for manufacturing the same.

[0005] 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 details of the specification.

[0006] According to exemplary embodiments for solving the problem of the present invention, a steel sheet is provided. The steel sheet comprises, in weight percent, C: 0.04~0.2%, Si+Ti: 0.2% or less, Mn: 1.5~3.0%, Cr: 0.8% or less (excluding 0%), N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), the remainder being Fe and unavoidable impurities, and may satisfy the following Equations 1 and 2.

[0007] [Relationship 1]

[0008] 1.6*[C]+0.3*[Mn]+0.7*[Cr]+57.5*[B] > 0.8

[0009] [Relationship 2]

[0010] D > 35 nm

[0011] The above steel plate may include a plating layer disposed on the surface of the steel plate.

[0012] The above Si may be included in an amount of 0 to 0.03% or less.

[0013] The above Ti may be included in an amount of 0 to 0.05% or less.

[0014] The above steel plate may include, in its microstructure, ferrite with an area fraction of 50 to 95% and the remainder being a secondary phase.

[0015] The above steel plate may further include one or more of, in weight%, P: 0.1% or less, S: 0.01% or less, Nb: 0.05% or less, and Al: 0.01~0.06%.

[0016] The above steel plate may have a tensile strength of 590 MPa or more.

[0017] The above plating layer may be one or more of a Zn-based plating layer, a Zn-Mg-based plating layer, a Zn-Mg-Al-based plating layer, a Zn-Mg-Al-Si-based plating layer, an Al-based plating layer, and a combination of these plating layers.

[0018] According to other exemplary embodiments, a method for manufacturing a steel sheet is provided. The method for manufacturing the steel sheet comprises the steps of: hot rolling a steel slab comprising, in weight percent, C: 0.06–0.15%, Si: less than 0.15%, Mn: 1.5–2.3%, Cr: 0.5% or less (excluding 0%), Ti: 0.05% or less, N: 0.01% or less (excluding 0%), B: 0.003% or less (excluding 0%), the remainder being Fe and unavoidable impurities to provide a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to provide a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a dew point of -10°C or higher and at an Ac1 transformation point (°C) or higher; and cooling the annealed cold-rolled steel sheet to a cooling end temperature in the range of 400–700°C. and a step of heat treating a cold-rolled steel sheet cooled at a temperature (°C) below the cooling end temperature; wherein the steel slab can satisfy the following relationship 1.

[0019] [Relationship 1]

[0020] 1.6*[C]+0.3*[Mn]+0.7*[Cr]+57.5*[B] > 0.8

[0021] The method for manufacturing the above steel plate can further satisfy the following relationship 3.

[0022] [Relationship 3]

[0023] 2*Ts < To

[0024] The method for manufacturing the above steel plate may further include the step of immersing the heat-treated cold-rolled steel plate in a plating bath to perform plating treatment.

[0025] The above cold rolling can be performed with a reduction rate of 40 to 80 percent.

[0026] The above annealing step can be performed in the range of 750 to 850°C.

[0027] The average cooling rate of the above cooling step may be 5℃ / s or higher.

[0028] According to exemplary embodiments of the present invention, a steel sheet capable of improving plating wettability and plating adhesion of a plated steel sheet and a method for manufacturing the same can be provided.

[0029] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0030] Terms and words used in this specification and claims shall not be interpreted as being limited to their ordinary or dictionary meanings, but shall be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0031] In the following embodiments, the singular expression includes the plural expression unless the context clearly indicates otherwise.

[0032] In the following embodiments, terms such as "include" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0033] Where an embodiment can be implemented differently, a specific process sequence may be performed differently from the order described. For example, two processes described consecutively may be performed substantially simultaneously or proceed in the reverse order of the description.

[0034] In addition, in describing the present invention, if it is determined that a detailed description of related known components or functions may obscure the essence of the invention, such detailed description is omitted.

[0035] The present invention will be described in detail below through each embodiment. It should be noted that each embodiment described in this specification is not limited to a single embodiment but may also be combined with other embodiments. 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.

[0036] Unless otherwise specifically stated in the present invention, the content of each element is based on weight, and the ratio of the structure is based on area. At this time, the phase fraction of the microstructure constituting the steel plate may be the result measured with respect to the total thickness (t, mm) of the steel plate, and as one example, the result measured at the t / 4 point in the thickness direction may be represented.

[0037] The present invention will be described in detail below through examples. However, it should be noted that the following examples 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.

[0038] [Steel Plate]

[0039] According to exemplary embodiments, the steel plate may have a tensile strength of 590 MPa or more. In this way, by securing sufficient tensile strength, it can be used as an automotive structural material or the like where high strength characteristics are required.

[0040] The steel sheet may contain, in weight%, C: 0.04~0.2%, Si+Ti: 0.2% or less, Mn: 1.5~3.0%, Cr: 0.8% or less (excluding 0%), N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), the remainder being Fe and unavoidable impurities.

[0041] Carbon (C): 0.04~0.2%

[0042] Carbon (C) is an element that plays a role in improving the strength of the steel sheet. If the content of C is less than 0.04%, it may be difficult to secure the level of strength targeted by the present invention. If the content of C exceeds 0.2%, the strength of the steel sheet may increase, but there is a risk that processability may be reduced by causing a decrease in elongation. In this regard, C may be 0.04 to 0.2%. More specifically, C may be 0.04 to 0.15%. More specifically, C may be 0.06 to 0.15%.

[0043] Manganese (Mn): 1.5~3.0%

[0044] Manganese (Mn) can contribute to increasing the strength of steel by promoting the formation of austenite, thereby contributing to securing a fraction of martensite, which is one of the hard structures. If the Mn content is less than 1.5%, the fraction of martensite decreases, making it difficult to secure strength. If the Mn content exceeds 3.0%, an excessive amount of oxide is generated on the surface of the steel sheet during the manufacturing process, which may reduce plating performance during plating treatment. Additionally, there is a concern that workability may be reduced by causing a decrease in elongation. In this regard, Mn may be 1.5 to 3.0%. More specifically, Mn may be 1.7 to 3.0%.

[0045] Chrome (Cr): 0.8% or less (excluding 0%)

[0046] Chromium (Cr) can contribute to improving the hardenability of steel and securing high strength. In addition, Cr can effectively contribute to securing the elongation of composite microstructure steel by minimizing the decrease in elongation relative to the increase in strength. If the Cr content exceeds 0.8%, not only are the above effects saturated, but corrosion resistance may also decrease. In this regard, the Cr content may be 0.8% or less (excluding 0%). More specifically, the Cr content may be 0.10 to 0.8%.

[0047] Boron (B): 0.005% or less (excluding 0%)

[0048] Boron (B) is a hardenability-enhancing element that can contribute to strength improvement by promoting the formation of martensite. In particular, in composite structure steel, it is a component that significantly increases hardenability with only a small amount of addition and effectively contributes to the transformation of martensite during cooling. In this regard, the content of B may be 0.0003% or more. According to exemplary embodiments, a higher content of B is preferable. However, if the content of B increases excessively, it leads to a decrease in elongation, and plating wettability may decrease due to surface concentration of B oxides. In this regard, the content of B may be 0.010% or less. In summary, the content of B may be 0.0003% to 0.010%. More specifically, the content of B may be 0.0005% to 0.010%.

[0049] Silicon (Si) + Titanium (Ti): 0.2% or less

[0050] Silicon (Si) can contribute to improving strength through solid solution strengthening. However, if the Si content in steel is excessive, plating wettability decreases, which can lead to a deterioration in plating performance. Therefore, to prevent incomplete plating, it is necessary to reduce the Si content.

[0051] According to exemplary embodiments, by securing a relatively high content of B, the reduction in strength resulting from the suppression of Mn or Si content can be offset. Therefore, it is important to secure an effective B content that can contribute to strength improvement in steel. However, as B combines with N in steel and precipitates in the form of BN, the strength improvement effect may be inferior compared to the B content in steel. To prevent this, according to exemplary embodiments, a nitride in the form of TiN is formed through Ti to reduce the concentration of N in steel and increase the effective B content. In this way, by controlling the Si content and Ti content together, a sufficient effective B content can be secured to contribute to the strength improvement of the steel sheet. Accordingly, the Si content and Ti content can be controlled within the ranges described above, and more specifically, Si+Ti can be controlled to 0.15% or less (excluding 0%). Even more specifically, Si+Ti can be controlled to 0.12% or less (excluding 0%).

[0052] The respective ranges of Si and Ti are not particularly limited as long as they are controlled to the ranges described above. As a non-limiting example, Si may be included in an amount of 0 to 0.3%. More specifically, Si may be included in an amount of 0 to 0.15%. As a non-limiting example, Ti may be included in an amount of 0 to 0.05%. More specifically, Ti may be included in an amount of 0 to 0.03%.

[0053] Nitrogen (N): 0.01% or less (excluding 0%)

[0054] Nitrogen (N) is an impurity that is inevitably introduced during the steel manufacturing process. As the N content increases, uncontrolled, non-uniform nitrides may precipitate in the steel sheet, forming BN and potentially lowering the effective B content in the steel. Therefore, the upper limit of N can be controlled to 0.01%. While a lower N content is preferable, considering that it is an element that may inevitably be added during the manufacturing process, the lower limit may exceed 0%. In summary, the N content may be 0.01% or less. More specifically, the N content may be greater than 0% and less than or equal to 0.01%.

[0055] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the conventional steel manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the conventional manufacturing process, all details thereof are not specifically mentioned in this specification.

[0056] Optionally, according to exemplary embodiments, the steel sheet may further contain, in weight percent, one or more of P: 0.10% or less, S: 0.01% or less, Nb: 0.05% or less, and Al: 0.01 to 0.06%.

[0057] Phosphorus (P): 0.10% or less

[0058] Phosphorus (P) is a solid solution strengthening element, but if its content exceeds 0.10%, weldability may be reduced and the risk of steel brittleness may increase. Therefore, the P content can be controlled to 0.10%. More specifically, the P content may be greater than 0% and less than or equal to 0.10%.

[0059] Sulfur (S): 0.01% or less

[0060] Sulfur (S) is an impurity element that can impair the ductility and weldability of steel sheets. Therefore, as the S content increases, the ductility and weldability of steel sheets may become inferior, so the upper limit may be limited to 0.010% in consideration of this. More specifically, the S content may be greater than 0% and less than or equal to 0.020%.

[0061] Niobium (Nb): 0.05% or less

[0062] Niobium (Nb) can contribute to an increase in strength by precipitating in the form of carbides at austenite grain boundaries and inhibiting the growth of austenite grain size during annealing. However, if the Nb content exceeds 0.05%, it may increase manufacturing costs. In this regard, the Nb content may be 0.05% or less. More specifically, the Nb content may be 0.0010% to 0.040%. More specifically, the Nb content may be 0.001% to 0.030%.

[0063] Aluminum (Al): 0.01~0.06%

[0064] Aluminum (Al) can be added to remove oxygen from molten steel and can contribute to improving austenite refinement and hardenability by combining with nitrogen in the steel to form AlN. Additionally, it can be utilized as a substitute element for Si as it can suppress some carbides. If the Al content is less than 0.01%, it may be difficult to sufficiently secure the aforementioned effects. If the Al content exceeds 0.06%, the likelihood of surface defects in the steel sheet increases due to the excessive formation of inclusions during steelmaking / continuous casting, and manufacturing costs may rise. In this regard, the Al content may be 0.01% to 0.06%.

[0065] According to exemplary embodiments, the steel plate can satisfy the following relationship 1.

[0066] [Relationship 1]

[0067] 1.6*[C]+0.3*[Mn]+0.7*[Cr]+57.5*[B] > 0.8

[0068] In the above Equation 1, [C], [Mn], [Cr], and [B] represent the content (weight%) of C, Mn, Cr, and B in the steel, respectively. By satisfying the above Equation 1, the steel plate can secure the hardenability of the steel and provide sufficient strength. The value calculated through the above Equation 1 may be greater than 0.8. More specifically, the value calculated through the above Equation 1 may be 0.85 or higher.

[0069] According to exemplary embodiments, the steel plate can satisfy the following relationship 2.

[0070] [Relationship 2]

[0071] D > 35 nm

[0072] In the above Equation 2, D represents the depth (nm) at which the B content calculated as ([B]max-[B]p) / 3 appears in the first region from a depth of Dmax(nm) to a depth of 100 nm, based on the surface of the steel plate; [B]max represents the maximum value of the B content in the second region from a depth of 0.1 to 100 nm; [B]p represents the B content at a depth of 100 nm; and Dmax represents the depth at which [B]max appears. As an example, the average depth at which [B]max appears measured at 5 points can be set as Dmax. The first region may overlap with the second region. More specifically, the first region may be included in the second region.

[0073] [B]max and [B]p can be determined based on the GDS profile of the B component measured from the surface of the steel plate toward the center of the thickness. More specifically, among the GDS profile of the B component, the maximum value of the B component appearing in the second region may be [B]max, and the point (depth) where [B]max appears may be Dmax. Among the GDS profile of the B component, the B content appearing at the 100nm point may be [B]p. Among the GDS profile of the B component, the point where the B value calculated through the above-described formula ([B]max-[B]p) / 3) using the values ​​of [B]max and [B]p in the first region defined by Dmax appears may be D(nm).

[0074] B can be concentrated in the surface layer of the steel sheet during the manufacturing process. As a result, during the plating treatment of the steel sheet, the plating adhesion may be reduced, and consequently, plating peeling may occur. However, according to exemplary embodiments, the steel sheet can satisfy the distribution of the B component according to the above-described Equation 1. As a result, the plating adhesion of the steel sheet can be improved. More specifically, the value of D in Equation 2 may be 40 nm or more. More specifically, the value of D in Equation 2 may be 60 nm or more. The upper limit of the value of D in Equation 2 may be 100 nm or less.

[0075] According to exemplary embodiments, the steel sheet may include an internal oxide layer having a thickness in the thickness direction from the interface with the plating layer to a depth range of 0.01 to 6.0 μm. According to exemplary embodiments, the internal oxide layer may include one or more oxides of Mn, Si, Al, Fe, B, and their alloying elements. This allows for more effective prevention of the aforementioned alloying elements forming oxides on the surface, thereby reducing plating performance. In particular, additional segregation of B in the extreme surface layer of the steel sheet can be prevented, thereby further improving plating adhesion. The average thickness of the internal oxide layer can be determined by calculating the depth using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) on a cross-section of the steel sheet. If the thickness of the internal oxide layer is less than 0.01 μm, it may be difficult to sufficiently secure the aforementioned effects.

[0076] According to exemplary embodiments, the steel sheet may include, in its microstructure, ferrite with an area fraction of 50 to 95% and the remainder being a secondary phase. The secondary phase may include one or more of a martensite phase, a pearlite phase, and a bainite phase, which can be classified as hard phases. In this way, by including ferrite as the main phase and appropriately controlling the fraction of the hard phase, it is possible to secure strength above a certain level while simultaneously reducing the possibility of defects occurring during processing. However, the present invention is not limited thereto and may further include impurities that do not alter the concept of the present invention, such as eutectic transformation structures like the pearlite phase and precipitates that inevitably occur during the process.

[0077] According to exemplary embodiments, the steel sheet may include a plating layer disposed on the surface of the steel sheet. The steel sheet can improve plating wettability and plating adhesion through the control of the B distribution as described above. As a result, according to exemplary embodiments, a plated steel sheet having excellent corrosion resistance and improved plating quality can be provided.

[0078] The plating layer can be disposed on one side of the steel sheet, as one example. As another example, the plating layer can be disposed on both sides of the steel sheet.

[0079] The plating layer may be one or more of the following, but is not particularly limited: a Zn-based plating layer, a Zn-Mg-based plating layer, a Zn-Mg-Al-based plating layer, a Zn-Mg-Al-Si-based plating layer, an Al-based plating layer, and a combination thereof.

[0080] When the steel plate according to the exemplary embodiments is provided in the form of a plated steel plate including a plating layer, the surface of the steel plate described above may refer to the interface between the steel plate and the plating layer. Meanwhile, in Equation 2, when performing GDS analysis of component B, the surface of the steel plate can be identified by observing the cross-section of the plated steel plate using an optical microscope (OM), a scanning electron microscope (SEM), or a transmission electron microscope (TEM). In this case, the surface of the steel plate in Equation 2 may refer to the interface between the plating layer and the steel plate (substrate steel plate). Alternatively, GDS analysis may be performed on the steel plate after removing the plating layer with an etching solution. Since the GDS analysis method is a general analysis method known in the art, a detailed description is omitted.

[0081] [Method for manufacturing steel plates]

[0082] A method for manufacturing a steel sheet may include the steps of providing a hot-rolled steel sheet, providing a cold-rolled steel sheet, annealing, cooling, and heat treating. Additionally, according to exemplary embodiments, a method for manufacturing a steel sheet may include the steps of plating.

[0083] The step of providing hot-rolled steel sheets is not particularly limited as long as the steel slab can be hot-rolled to provide hot-rolled steel sheets. As a non-limiting example, the steel slab can be heat-treated at 1100 to 1300°C for a time of 30 minutes or more, and then hot-rolled.

[0084] As a non-limiting example, a steel slab may be hot-rolled such that the finishing rolling temperature is above the Ar3 transformation point (°C). More specifically, the finishing rolling temperature may be from Ar3 to 1000°C. If the finishing rolling is performed at a temperature below the Ar3 transformation point, two-phase rolling of ferrite and austenite may occur, potentially leading to material non-uniformity. If the finishing rolling temperature exceeds 1000°C, there is a risk of material non-uniformity occurring due to the formation of abnormal aggregate zones caused by high-temperature rolling.

[0085] The composition of the steel slab may be described by referring to the description of the steel plate above. The steel slab may contain C: 0.04~0.2%, Si+Ti: 0.2% or less, Mn: 1.5~3.0%, Cr: 0.8% or less (excluding 0%), N: 0.01% or less (excluding 0%), B:, the remainder being Fe and unavoidable impurities. Optionally, the steel slab may further contain, in weight%, one or more of P: 0.1% or less, S: 0.01% or less, Nb: 0.05% or less, and Al: 0.01~0.06%. Additionally, the steel slab may satisfy the above-described Equation 1.

[0086] Optionally, the steps of coiling the hot-rolled steel sheet and pickling may be further included. The coiling and pickling conditions are not particularly limited, and conditions commonly used in the relevant technical field may be applied without restriction.

[0087] The step of providing a cold-rolled steel sheet may be a step of cold-rolling a hot-rolled steel sheet. According to exemplary embodiments, cold rolling may be performed with a reduction rate of 40 to 80%. If the reduction rate of cold rolling is less than 40%, the recrystallization driving force by cold rolling is insufficient, so the recrystallization of ferrite is not completed, and unrecrystallized ferrite structure may remain. If the reduction rate of cold rolling exceeds 80%, an excessive load may be applied to the rolling rolls.

[0088] The annealing step may be a step of heat-treating a cold-rolled steel sheet at a dew point of -5°C or higher and an Ac1 transformation point (°C or higher).

[0089] At this time, the Ac1 temperature can be determined by the following Equation 1.

[0090] [Equation 1]

[0091] Ac1 = 723-10.7*[Mn]-16.9*[Ni]+29.1*[Si]+16.9*[Cr]+290*[As]+6.38*[W]

[0092] In the above Equation 1, [Mn], [Ni], [Si], [Cr], [As], and [W] represent the content (wt%) of each element in the steel grade.

[0093] According to exemplary embodiments, by performing annealing under dew point conditions of -10°C or higher, surface oxides can be reduced and the distribution of oxidizing alloy components in the extreme surface layer (second region) of the steel sheet can be controlled. In particular, by controlling the distribution of component B in the extreme surface layer, it can contribute to improving plating quality. When the dew point temperature is below -10°C, the partial pressure of oxygen in the atmosphere is insufficient, making it difficult for oxygen to penetrate into the steel sheet, and thus making it difficult to reduce surface oxides.

[0094] The upper limit of the dew point temperature is not specifically restricted, but since there is a risk of oxidation of Fe in the steel sheet if it exceeds 30℃, it may be controlled to 30℃ or lower. In this regard, the dew point temperature may be performed in the range of -10 to 30℃. More specifically, the dew point temperature may be performed in the range of 0 to 30℃. Even more specifically, the dew point temperature may be performed in the range of 2 to 30℃. Even more specifically, the dew point temperature may be performed in the range of -5 to 25℃. Even more specifically, the dew point temperature may be performed in the range of 0 to 25℃.

[0095] The annealing heat treatment temperature can be performed at the Ac1 transformation point (°C) or higher. In this way, by performing annealing heat treatment in a two-phase region where ferrite and austenite coexist at the Ac1 transformation point or higher, it is possible to form ferrite as well as austenite simultaneously with the recrystallization of the microstructure, while distributing carbon.

[0096] According to exemplary embodiments, the annealing step can be performed in the range of 750 to 850°C. If the annealing heat treatment temperature is below 750°C, not only is recrystallization not sufficiently achieved, but the austenite phase is also not sufficiently formed, making it impossible to secure the target microstructure phase composition after continuous annealing, for example, a composite structure of ferrite and secondary phases. On the other hand, if the temperature exceeds 850°C, the surface concentration of elements that impede plating wettability in the alloy composition (e.g., B, etc.) becomes severe, which may degrade the plating surface quality.

[0097] Furthermore, the continuous annealing process is not particularly limited, and conditions commonly used in the relevant technical field may be applied without restriction.

[0098] Optionally, according to exemplary embodiments, the holding time of the continuous annealing step may be 30 to 200 s. If the holding time of the continuous annealing step is excessively short, the control of the internal oxide layer may not be sufficient. If the holding time of the continuous annealing step is excessively long, the alloy elements dispersed from the surface and capable of diffusing into the surface layer may have already fully reacted, making it difficult to expect additional effects. Above all, the process time may be excessively extended, leading to an excessive increase in process costs. Therefore, the holding time of the continuous annealing step may be 30 to 2000 s. More specifically, the holding time of the continuous annealing step may be 40 to 150 s.

[0099] The cooling step may be a step of cooling an annealed cold-rolled steel sheet to a cooling end temperature in the range of 400 to 700°C. If the cooling end temperature is lower than 400°C, an excessive hard phase may be formed. If the cooling end temperature exceeds 700°C, the surface of the steel sheet may oxidize, and it may be difficult to control oxidizing alloy components in the extreme surface layer of the steel sheet.

[0100] According to exemplary embodiments, the average cooling rate of the cooling step may be 5°C / s or higher. In this way, the microstructure of the steel sheet can be appropriately controlled by performing rapid cooling to the cooling end temperature described above after annealing. The upper limit of the average cooling rate is not particularly limited, but may be 100°C or lower.

[0101] The cooling method is not particularly limited, and conditions commonly used in the relevant technical field, such as cooling gas injection and mist injection, may be applied without restriction.

[0102] The heat treatment step may be a step of holding a cold-rolled steel sheet cooled at a temperature (°C) below the cooling end temperature. This allows the structure of the steel sheet to be stabilized. The holding temperature of the heat treatment step is not particularly limited if it is performed at the cooling end temperature or at a temperature lower than the cooling end temperature. As a non-limiting example, the holding temperature of the heat treatment step may be 300 to 600°C.

[0103] The holding time of the heat treatment step may be 200 to 600 seconds. If the holding time of the heat treatment step is excessively long, productivity may decrease, and if it is excessively short, there is a concern that the structural stabilization of the steel sheet may not be sufficiently achieved.

[0104] According to exemplary embodiments, the method for manufacturing a steel plate may further satisfy the following relationship 3.

[0105] [Relationship 3]

[0106] 2*Ts < To

[0107] In the above Equation 3, To represents the holding time of the heat treatment step, and Ts represents the holding time of the annealing step. In this way, sufficient strength can be secured by satisfying the above-described Equation 3 for the holding time of the heat treatment step.

[0108] The plating step may be a step of immersing a heat-treated cold-rolled steel sheet in a plating bath. As a result, a plating layer is formed on the surface of the cold-rolled steel sheet to provide a plated steel sheet. The type of plating bath is not particularly limited, but may be one or more of a Zn-based plating bath, a Zn-Mg-based plating bath, a Zn-Mg-Al-based plating bath, a Zn-Mg-Al-Si-based plating bath, an Al-based plating bath, and a combination thereof.

[0109] Other plating conditions may be applied without limitation to conditions commonly used in the relevant technical field. As one example, it may be performed in a temperature range of 430 to 500°C. As another example, it may be performed in a temperature range of 600 to 680°C.

[0110] [Example]

[0111] The present invention will be explained in more detail below through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the rights of the present invention.

[0112] Steel slabs as shown in Table 1 below were prepared, and galvanized steel sheets were manufactured under the conditions shown in Table 2 below. In all examples, the steel slabs were heat-treated at 1220°C for 1 hour, followed by hot rolling; after coiling and pickling at 570°C, cold rolling was performed with a reduction rate of 60%. Additionally, all examples satisfied the heat treatment holding time condition according to Equation 3. The plating quality (plating wettability and plating adhesion) of the manufactured galvanized steel sheets was evaluated and is shown in Table 2.

[0113] Tensile strength was measured by taking tensile specimens conforming to the JIS-C standard from each plated steel sheet and performing a tensile test.

[0114] Plating wettability was evaluated by determining the presence or absence of visible unplated areas after plating. If there were no unplated areas, it was rated as "Good," while if visible dots or full unplated areas were present, it was unconditionally rated as "Unplated."

[0115] Plating adhesion was evaluated by applying an automotive structural adhesive to the surface of a plated steel sheet, drying and allowing solidification to complete, and then bending the sheet at a 90-degree angle to separate the adhesive from the plated steel sheet. Cases where the plating layer did not peel off and did not adhere to the adhesive were classified as "Good." Cases where the plating layer peeled off and adhered to the adhesive were classified according to the degree of peeling: full peeling occurred if the entire layer peeled off, and partial peeling occurred if peeling occurred in parts or spots. Cases where the plating wettability was poor and the plating layer was not formed normally were classified as "Unable to Evaluate" as evaluation was not possible.

[0116] Steel Grade C(wt%)Mn(%)Si(%)Cr(%)Ti(%)B (ppm)N (ppm) Relationship 1 Comparative Example 1 0.12 1.6000.02 10500.73 Comparative Example 2 0.12 1.6000.02 20500.79 Comparative Example 3 0.08 1.8000.02 10500.73 Comparative Example 4 0.08 1.8000.02 20500.78 Example 1 0.08 1.70.12 0.3505500.91 Comparative Example 5 0.13 200.20 .0210501.01 Example 20.13200.20.0210501.01 Comparative Example 60.08200.20.0210500.93 Example 30.08200.20.0210500.93 Comparative Example 70.081.800.20.0210500.87 Example 40.081.800.20.0210500.87 Relationship 1: 1.6*[C]+0.3*[Mn]+0.7*[Cr]+57.5*[B]

[0117] Material Ac1 Transformation Point (°C) Dew Point (°C) Annealing Heat Treatment Temperature (°C) Cooling Temperature (°C) Heat Treatment Temperature After Cooling (°C) D Plating Wetness Plating Adhesion Tensile Strength (MPa) Comparative Example 1 706-508 105 305 00 14 Good Full Surface Peeling 520 Comparative Example 2 706-508 105 305 00 5 Good Full Surface Peeling 520 Comparative Example 3 704-508 105 305 00 21 Good Full Surface Peeling 508 Comparative Example 4 704-508 105 305 00 7 Good Full Surface Peeling 508 Example 1 714 58 105 305 00 62 Good Good 621 Comparative Example 5 705-508 105 305 00 5 Unplated Evaluation Unsuitable 709 Example 2 70558 1053050098 Good Good 709 Comparative Example 6 705-508 1053050011 Unplated Evaluation Unsuitable 737 Example 3 70558 1053050095 Good Good 737 Comparative Example 7 707-508 105505207 Good Partial Delamination 639 Example 4 70758 1055052064 Good Good 639 D: Depth (nm) where the B content calculated as ([B]max-[B]p) / 3 appears in the first region from a depth of Dmax(nm) to a depth of 100nm, based on the surface of the steel plate

[0118] Referring to Tables 1 and 2, it was confirmed that the embodiments satisfying the conditions proposed in the present invention have excellent plating wettability and plating adhesion, while also having the target tensile strength.

[0119] However, it was confirmed that Comparative Examples 1 to 7, which deviated from the conditions proposed in the present invention, had inferior plating quality due to incomplete plating or plating peeling, and failed to secure the target tensile strength.

[0120] Although the invention has been described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as described in the following claims.

Claims

1. In wt%, C: 0.04~0.2%, Si+Ti: 0.2% or less, Mn: 1.5~3.0%, Cr: 0.8% or less (excluding 0%), N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), the remainder being Fe and unavoidable impurities, and A steel plate satisfying the following Equations 1 and 2. [Relationship 1] 1.6*[C]+0.3*[Mn]+0.7*[Cr]+57.5*[B] > 0.8 [Relationship 2] D > 35 nm (In the above Equation 1, [C], [Mn], [Cr], and [B] represent the content (weight%) of C, Mn, Cr, and B in the steel, respectively. In the above Equation 2, D represents the depth (nm) at which the B content calculated as ([B]max-[B]p) / 3 appears in the first region from a depth of Dmax(nm) to a depth of 100nm, based on the surface of the steel plate; [B]max represents the maximum value of the B content in the second region from a depth of 0.1 to 100nm; [B]p represents the B content at a depth of 100nm; and Dmax represents the depth at which [B]max appears.) 2. In Paragraph 1, A steel plate comprising a plating layer disposed on the surface of the above steel plate.

3. In Paragraph 1, The above steel plate containing 0 to 0.03% or less of Si.

4. In Paragraph 1, The above steel plate containing 0 to 0.05% or less of Ti.

5. In Paragraph 1, The above steel plate is, A steel sheet having a microstructure comprising an area fraction of 50–95% ferrite and the remainder being a secondary phase.

6. In Paragraph 1, The above steel plate comprises, in weight%, P: 0.1% or less, S: 0.01% or less, Nb: 0.05% or less, and Steel plate containing one or more of Al: 0.01~0.06%.

7. In Paragraph 1, The above steel plate is a steel plate having a tensile strength of 590 MPa or higher.

8. In Paragraph 2, The above plating layer is a Zn-based plating layer, a Zn-Mg-based plating layer, a Zn-Mg-Al-based plating layer, a Zn-Mg- A steel sheet having one or more of an Al-Si-based plating layer, an Al-based plating layer, and a combination of the same.

9. In wt%, C: 0.06~0.15%, Si: less than 0.15%, Mn: 1.5~2.3%, Cr: 0.5% or less (0% A step of providing a hot-rolled steel sheet by hot-rolling a steel slab containing Ti: 0.05% or less, N: 0.01% or less (excluding 0%), B: 0.003% or less (excluding 0%), the remainder being Fe and unavoidable impurities; A step of providing a cold-rolled steel sheet by cold-rolling the above hot-rolled steel sheet; A step of annealing the cold-rolled steel sheet at a dew point of -10℃ or higher and at an Ac1 transformation point (℃) or higher; A step of cooling annealed cold-rolled steel sheet to a cooling end temperature in the range of 400 to 700℃; and The step of heat treating a cold-rolled steel sheet cooled at a temperature (°C) below the above cooling end temperature; is included, The above steel slab is a method for manufacturing a steel plate satisfying the following relationship 1. [Relationship 1] 1.6*[C]+0.3*[Mn]+0.7*[Cr]+57.5*[B] > 0.8 (In the above Equation 1, [C], [Mn], [Cr], and [B] represent the content (weight%) of C, Mn, Cr, and B in the steel, respectively.) 10. In Paragraph 9, A method for manufacturing a steel sheet comprising the additional step of immersing a heat-treated cold-rolled steel sheet in a plating bath to perform plating treatment.

11. In Paragraph 9, A method for manufacturing a steel plate that further satisfies the following relationship 3. [Relationship 3] 2*Ts < To (In the above Equation 3, To represents the holding time (sec) of the heat treatment step, and Ts represents the holding time (sec) of the annealing step.) 12. In Paragraph 9, The above cold rolling is a method for manufacturing steel sheets performed with a reduction rate of 40 to 80 percent.

13. In Paragraph 9, A method for manufacturing a steel plate in which the above annealing step is performed in the range of 750 to 850℃.

14. In Paragraph 9, A method for manufacturing a steel plate in which the average cooling rate of the above cooling step is 5℃ / s or higher.