High strength steel plate and its manufacturing method

Abstract

The present invention relates to a high-strength steel sheet having improved platability and excellent surface quality, and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel sheet having improved platability and excellent surface quality, and a method for producing the same. [Background technology]

[0002] Transformation-induced plasticity (TRIP) steel, a giga-level high-formability steel, has the advantage of superior elongation compared to other giga-level steels by utilizing the austenite phase. However, it has the problem of liquid metal embrittlement (LME) caused by the Si added at 1.5 wt.% for high formability during welding.

[0003] LME is a phenomenon in which liquid zinc (Zn) penetrates the grain boundaries in the surface layer of the steel during spot welding, causing cracks and accelerating these cracks. It is greatly influenced by the amount of heat input and thermal stress during spot welding, as well as the proportion of C and Si in the steel.

[0004] To suppress this LME phenomenon, attempts have been made to improve the material's properties by applying an oxidation-reduction method in which the steel is oxidized at around 600°C during annealing heat treatment and then re-reduced at 700-800°C during the TRIP steel manufacturing process, or by adding antimony (Sb), tin (Sn), etc. to the steel to suppress the internal oxidation of oxidizing elements (Mn, Si, etc.).

[0005] However, the above method has the drawback that it does not significantly improve the LME phenomenon.

[0006] Therefore, there is a need for a method to significantly suppress the LME phenomenon in TRIP steels containing a certain amount of oxidizing elements, thereby improving the galvanic properties and surface quality. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Registration No. 10-1630976 Summary of the Invention [Problem to be solved by the invention]

[0008] One aspect of the present invention is to provide a high-strength steel sheet having excellent surface quality, which suppresses the LME cracking phenomenon by minimizing the surface concentration of Mn, Si, etc. present in the steel, and a manufacturing method thereof.

[0009] The object of the present invention is not limited to the above-mentioned content, and can be understood from the entire content of this specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention. [Means for solving the problem]

[0010] One aspect of the present invention is A base steel sheet, a ferrite layer formed on a surface layer portion of the base steel sheet, the ferrite layer has an internal oxide layer on top of which an Fe-Ni alloy layer is formed, The internal oxide layer is formed from the surface of the ferrite layer along the grain boundaries of the base structure of the base steel sheet to a depth of up to 3 μm in the thickness direction, and The Fe-Ni alloy layer is formed in the internal oxide layer from the surface of the ferrite layer along the grain boundaries of the base structure of the base steel sheet to a depth of up to 2 μm in the thickness direction thereof. This provides a high-strength steel sheet with excellent surface quality.

[0011] Another aspect of the present invention is a method for manufacturing a steel sheet, the method comprising the steps of: preparing a base steel sheet; forming a Ni+Fe / rGO composite coating layer on at least one surface of the base steel sheet; and annealing the base steel sheet on which the composite coating layer is formed, The annealing heat treatment is carried out in a temperature range of a maximum of 850°C and a dew point temperature of -10 to +5°C, thereby providing a method for producing a high-strength steel sheet with excellent surface quality. [Effects of the Invention]

[0012] The present invention can more effectively suppress the LME phenomenon in TRIP steel than conventional techniques for suppressing the LME phenomenon in TRIP steel, and in particular minimizes oxide formation near the steel surface, thereby providing a high-strength steel sheet with improved surface quality as well as improved galvanizability of the TRIP steel. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic diagram illustrating a cross section in the thickness direction of a high-strength steel plate according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] The inventors conducted extensive research into ways to effectively prevent the problem that TRIP steels containing a certain amount of oxidizing elements have excellent ductility and are suitable for high formability, but that the oxidizing elements concentrate on the surface during the welding process, causing defects such as LME.

[0015] As a result, the researchers found that when manufacturing TRIP steel, Ni coating is performed before annealing, and that by adding a specific substance during the Ni coating to form a composite coating layer and optimizing the subsequent annealing process, it is possible to fundamentally suppress the surface segregation of oxidizing elements, which led to the completion of the present invention.

[0016] The present invention will be described in detail below.

[0017] First, the high-strength steel sheet with excellent surface quality provided by the present invention includes a base steel sheet and a ferrite layer formed on a surface layer portion of the base steel sheet, and the ferrite layer may have an internal oxide layer thereon in which an Fe-Ni alloy layer is formed.

[0018] The base steel sheet is a high-strength TRIP steel, and its alloy composition is not particularly limited, but as an example, it may contain, by weight, carbon (C): 0.17 to 0.19%, silicon (Si): 1.3 to 1.7%, manganese (Mn): 2.4 to 2.7%, aluminum (Al): 0.01 to 0.7%, phosphorus (P): 0.01% or less, sulfur (S): 0.003% or less, the balance being Fe and other unavoidable impurities.

[0019] Carbon (C) is an important element added to ensure strength and stabilize retained austenite. To fully achieve the above-mentioned effects, 0.17% or more of C can be added. However, excessive C content can cause problems such as reduced weldability, so in consideration of this, the C content can be limited to 0.19% or less.

[0020] Silicon (Si) is an element that suppresses the precipitation of carbides in ferrite and promotes the diffusion of carbon in ferrite into austenite, thereby contributing to the stabilization of retained austenite. To fully achieve the above-mentioned effects, the Si content can be 1.3% or more. However, excessive Si content can cause problems such as reduced rollability and the formation of Si oxides on the steel sheet surface, which can impair galvanization. Therefore, the Si content can be limited to 1.7% or less.

[0021] Manganese (Mn) is an element that contributes to the formation and stabilization of retained austenite and is effective in ensuring strength and ductility. To fully achieve the above-mentioned effects, it is advantageous to include 2.4% or more of Mn. However, if the Mn content is excessive, segregation induced during the casting and hot rolling processes can deteriorate mechanical properties, so taking this into consideration, it can be limited to 2.7% or less.

[0022] Aluminum (Al) is an element added to deoxidize steel and is effective in stabilizing retained austenite by suppressing the precipitation of cementite. If the Al content is less than 0.01%, the deoxidizing effect becomes insufficient, resulting in a deterioration in the cleanliness of the steel. On the other hand, to enhance the stabilization effect of retained austenite, it is advantageous to add 0.1% or more of the above-mentioned Al. However, if the Al content exceeds 0.7%, there are problems such as a decrease in the castability and plating adhesion of the steel.

[0023] Phosphorus (P) is a solid solution strengthening element, but if its content is excessive, it may cause embrittlement of the steel, so the upper limit of P can be set to 0.01%.

[0024] Sulfur (S) is an impurity element in steel and may impair the ductility and weldability of steel, so its content can be limited to 0.003% or less.

[0025] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, there is a possibility that unintended impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, the contents of all of them will not be specifically mentioned in this specification.

[0026] That is, the present invention has a technical significance in that it targets steel containing a certain amount of oxidizing elements such as Mn and Si and minimizes the surface concentration of oxidizing elements such as Mn and Si present in the steel.

[0027] The high-strength steel sheet of the present invention is characterized in that it includes a ferrite layer formed on the surface layer of the base steel sheet, and the ferrite layer has an internal oxide layer on top of it, in which an Fe-Ni alloy layer is formed (Fig. 1).

[0028] First, the surface layer portion of the base steel sheet can be referred to as a region extending from the outermost surface of the ferrite layer to a maximum of 50 μm, more preferably a maximum of 30 μm, in the thickness direction of the base steel sheet. Thus, in the present invention, the ferrite layer can be present within the base steel sheet to a maximum of 50 μm, preferably a maximum of 30 μm, in the thickness direction of the base steel sheet. Within the ferrite layer, the internal oxide layer can be formed from the surface of the ferrite layer along the grain boundaries of the base steel sheet's matrix to a depth of a maximum of 3 μm in the thickness direction, and the Fe-Ni alloy layer can be formed within the internal oxide layer from the surface of the ferrite layer along the grain boundaries of the base steel sheet's matrix to a depth of a maximum of 2 μm in the thickness direction. The Fe-Ni alloy layer and the internal oxide layer can be present continuously along the grain boundaries to a depth of a maximum of 2 μm and a maximum of 3 μm, respectively, from the outermost surface of the ferrite layer, or can be present discontinuously at a certain distance. Here, the term "grain boundary" refers to the grain boundary of the base structure of the base steel sheet, and refers to not only ferrite grain boundaries but also austenite grain boundaries, bainite grain boundaries, and martensite grain boundaries, and it is clear that the grain boundary may exist in the grain boundary of at least one of the phases.

[0029] The base steel sheet may be a plated steel sheet having a plating layer formed on at least one surface thereof through a plating process, and in this case, a ferrite layer may be included immediately below the plating layer, i.e., at the interface between the base steel sheet and the plating layer. In this case, the surface immediately below the plating layer may be the outermost surface of the ferrite layer.

[0030] In the present invention, for example, in the case of a GA plated steel sheet, an Fe—Ni alloy layer may be formed inside the plated layer adjacent to the ferrite layer.

[0031] As will be described in detail below, the Fe-Ni alloy layer and the internal oxide layer on the ferrite layer can be formed by forming a Ni composite coating layer on the base steel sheet before subjecting it to annealing heat treatment, and then performing an annealing heat treatment step.

[0032] More specifically, the Ni composite coating layer may be formed from a mixed composition of Fe oxide and Ni compound coated with reduced graphene oxide, i.e., rGO. The Fe oxide in the coating layer thus formed diffuses into the base steel sheet during the subsequent annealing heat treatment process to suppress the surface diffusion of Mn, Si, etc. in the steel, while bonding with Ni, which diffuses faster into the base steel sheet than Fe, to form an Fe-Ni composite coating layer on the surface. alloy layer Form.

[0033] When the segregation energy of each element in Ni-X or Fe-X (where X is Si or Mn) crystal grains is examined, Fe-Si and Fe-Mn are both at the 10-90 kJ / mol level, with Ni-Si having a negative value and Ni-Mn having a positive value. In other words, conditions are favorable for Mn segregation at the Ni-Mn crystal grain boundaries, but the diffusion of Ni into the Fe makes it difficult for Mn to diffuse to the surface.

[0034] On the other hand, while surface diffusion of Si is easy during this process, rGO contained in the Ni composite coating layer also diffuses into the matrix and exists together with the Fe-Ni alloy layer. The high oxygen reactivity of the pyridinic and graphitic moieties present in the rGO effectively suppresses the surface diffusion of oxidizing elements in the steel, including Si.

[0035] In addition, in the annealing heat treatment process, moist nitrogen is introduced to increase the dew point, which causes Si, Mn, etc. to form an internal oxide layer within the surface layer.

[0036] Meanwhile, the ferrite layer present on the surface of the base steel sheet can be present inside the base steel sheet with a thickness of up to 50 μm in the thickness direction of the base steel sheet (FIG. 1).

[0037] In the present invention, the ferrite layer is formed by a reaction in which oxygen (O) atoms in the internal oxide layer formed during the annealing heat treatment process bond with carbon in the steel and are decarburized to carbon monoxide (CO).

[0038] The ferrite layer is soft, making it less susceptible to cracking and suppressing the LME phenomenon. To fully exert this effect, it is preferable that the ferrite layer contains a ferrite phase with an area fraction of 50% or more.

[0039] The base steel sheet in the present invention may be a cold-rolled steel sheet having the above-mentioned alloy composition, or may be a plated steel sheet having a plating layer on at least one surface of the above-mentioned cold-rolled steel sheet.

[0040] When the base steel sheet is a plated steel sheet, the plated layer is not particularly limited, but may generally be a zinc-based plated layer, and it is clarified that this plated layer may be formed on top of a ferrite layer present on the surface of the base steel sheet (cold-rolled steel sheet).

[0041] The method for producing a high strength steel sheet having excellent surface quality provided by the present invention will be described in detail below.

[0042] Briefly, the method may include the steps of preparing a base steel sheet, forming a composite coating layer containing Ni on at least one surface of the base steel sheet, and then subjecting the base steel sheet on which the composite coating layer is formed to an annealing heat treatment.

[0043] The conditions for each step are described in detail below.

[0044] First, as mentioned above, the base steel sheet is a TRIP steel, and although there are no particular limitations on its alloy composition, it may, for example, contain, by weight, carbon (C): 0.17 to 0.19%, silicon (Si): 1.3 to 1.7%, manganese (Mn): 2.4 to 2.7%, aluminum (Al): 0.01 to 0.7%, phosphorus (P): 0.01% or less, sulfur (S): 0.003% or less, with the balance being Fe and other unavoidable impurities.

[0045] The base steel sheet may be a cold-rolled steel sheet, and the description of each element may be substituted by the above-mentioned content.

[0046] A composite coating layer containing Ni, preferably a Ni+Fe / rGO composite coating layer, can be formed on at least one surface of the base steel sheet prepared as described above.

[0047] The Ni+Fe / rGO composite coating layer may be formed from a coating composition prepared by preparing a nickel (Ni) compound and an Fe / rGO aqueous solution, respectively, and then mixing them.

[0048] First, we prepare reduced graphene oxide (rGO), which is obtained by oxidizing graphite and reducing the graphene oxide (GO) that contains oxygen (O) atoms on the surface.

[0049] Specifically, rGO can be produced by adding 1-10 ml of hydrazine monohydrate to 200 ml of a solution of graphene oxide (GO) dispersed at 0.001-0.01 g per ml of distilled water, keeping the mixture at a high temperature, adding 50-100 ml of sulfuric acid, and then ultrasonically treating the mixture.

[0050] The high-temperature holding step can be performed at 70-90°C for 1-3 hours, and the ultrasonic treatment can be performed for 20-40 minutes. If the holding step is performed at a temperature above 90°C for more than 3 hours, excessive water will evaporate, making it difficult to obtain a solution of the appropriate level. Furthermore, if the ultrasonic treatment time is less than 20 minutes, it is difficult to obtain a uniform rGO. Furthermore, since the subsequent Fe coating process requires another ultrasonic treatment, the ultrasonic treatment time should be 40 minutes or less.

[0051] In the present invention, the rGO can be coated with Fe. The Fe is effective in forming an alloy phase with Ni in the composite coating layer, and the rGO is effective in suppressing the surface diffusion of oxidizing elements in the base steel sheet.

[0052] The process of coating the rGO with Fe can be carried out by mixing the rGO prepared as described above with an iron (Fe) oxide aqueous solution and then subjecting the mixed solution to ultrasonic treatment.

[0053] Specifically, based on 10 mL / L of rGO, 1–10 mg / L of an aqueous solution saturated with FeSO4 or FeCl3 hydrate and the rGO were mixed with 100–500 mL of pure water, followed by ultrasonic treatment for 60–600 minutes to obtain Fe / rGO containing up to 3 wt% nanosized Fe oxide. If the ultrasonic treatment time is less than 60 minutes, the amount of Fe coating is insufficient, making it difficult to form Fe-coated rGO. On the other hand, if the ultrasonic treatment time is longer than 600 minutes, the Fe coating becomes difficult.

[0054] By the ultrasonic process described above, the Fe oxide can be coated to a size of several tens of nanometers (nm), and preferably, the size may be 10 to 50 nm.

[0055] A coating composition for forming a composite coating layer can be prepared by mixing a nickel (Ni) compound with the Fe / rGO aqueous solution prepared as above.

[0056] Specifically, a Watts bath containing 1 to 1.5 M (mol) NiSO, 0.1 to 0.5 M NiCl, and 0.1 to 0.5 M HBO was prepared based on 10 ml of the Fe / rGO aqueous solution, and the Fe / rGO aqueous solution was added to the Watts bath to obtain a Ni+Fe / rGO coating composition.

[0057] The Ni+Fe / rGO coating composition may have a pH of 1 to 2. By adjusting the pH of the coating composition within the above range, the graphene (rGO) contained in the composition can be uniformly dispersed within the coating layer. By uniformly dispersing the rGO within the coating layer, the corrosion resistance, electrical properties, and physical properties of the steel sheet can be improved.

[0058] If the amount of Fe / rGO aqueous solution added to the Watts bath is excessive, a large amount of immersion occurs, making it difficult to ensure the stability of the solution.

[0059] In the present invention, the Ni+Fe / rGO coating composition prepared as described above can be coated onto at least one surface of a previously prepared base steel sheet, and in this case, a desired Ni+Fe / rGO composite coating layer can be formed by electroplating.

[0060] When forming the composite coating layer by electroplating, the Ni deposition amount is calculated based on the unit area (m 2 It is preferable to carry out the deposition in an amount of 200 to 800 mg per unit area. If the Ni deposition amount is less than 200 mg per unit area, it is not possible to effectively suppress the surface diffusion of oxidizing elements inside the steel. On the other hand, if it exceeds 800 mg, the effect saturates and it becomes economically disadvantageous. More advantageously, it is preferable to deposit Ni in an amount of 200 to 800 mg per unit area (m 2 ) can be applied with an adhesion amount of 400 mg or more per coating.

[0061] The higher the temperature of the solution during electroplating, the higher the electrical conductivity and the better the plating efficiency. However, if the temperature exceeds 60°C, the amount of evaporation of the solution increases significantly. Therefore, the plating can be carried out at a temperature below 60°C, and it is advantageous to carry out the plating at a temperature above 30°C to obtain a certain level of electrical conductivity.

[0062] After completing the electroplating, the base steel sheet having the Ni+Fe / rGO composite coating layer formed on at least one surface thereof is preferably subjected to an annealing heat treatment.

[0063] The annealing heat treatment is preferably carried out at a dew point temperature of -10 to +5°C, with a maximum temperature range of 850°C, in order to promote internal oxidation while suppressing surface diffusion of oxidizing elements in the base steel sheet.

[0064] If the dew point temperature during the annealing heat treatment exceeds +5°C, the base steel itself may be oxidized. However, if the temperature is too low, there is a problem that the coating performance deteriorates. Taking this into consideration, the lower limit of the dew point temperature can be limited to -10°C.

[0065] During annealing in an annealing furnace with a controlled atmosphere as described above, heat treatment can be performed at a maximum temperature of 850°C, preferably in the temperature range of 750 to 850°C. If the temperature during the heat treatment is less than 750°C, internal oxidation may not occur sufficiently. On the other hand, if the temperature exceeds 850°C, decarburization may become excessive, resulting in a deterioration in tensile properties.

[0066] Meanwhile, when heating for the annealing heat treatment, moist nitrogen can be introduced into the heating section, preferably when the temperature is raised to 700°C or higher. This is to induce internal oxidation and decarburization of oxidizing elements, and the nitrogen content is 50 to 200 m. 3 / h. In this case, the amount of moist nitrogen to be added is 50m 3 If the temperature is less than 200 m / h, the dew point is not sufficiently increased, and although an internal oxide layer can be partially formed, it is difficult to induce decarburization. 3 If the heating rate exceeds 1 / h, the dew point will exceed 5°C and become excessively high, which will cause the problem of the base iron itself being oxidized.

[0067] In the present invention, by performing annealing heat treatment under the above-mentioned conditions, the Fe oxides imparted to the rGO in the Ni+Fe / rGO composite coating layer formed on at least one surface of the base steel sheet are reduced to Fe in the surface layer by the reducing atmosphere in the annealing furnace, and some of the reduced Fe diffuses into the interior of the base steel sheet.

[0068] At this time, Ni in the composite coating layer diffuses into Fe, forming an Fe-Ni surface layer. alloy layer Form.

[0069] The oxidizing elements (Mn, Si, etc.) present inside the steel are alloy layer While surface segregation is suppressed by oxygen, the rGO is oxidized by water vapor in the nitrogen atmosphere of the annealing furnace or by the pyridinic and graphitic properties of the rGO, forming an internal oxide layer.

[0070] The above Fe-Ni alloy layer The internal oxide layer and the internal oxide layer can both be formed along the grain boundaries, and can be formed with sizes (lengths) of up to 2 μm and up to 3 μm, respectively.

[0071] During the annealing heat treatment, an internal oxide layer is formed in place of annealing concentrates, which subsequently causes water vapor to dissociate into O atoms at the surface of the base steel sheet, and these O atoms combine with carbon (C) in the steel to be decarburized to carbon monoxide (CO). As a result, a ferrite layer of a certain thickness is formed on the surface of the base steel sheet toward the interior of the base steel sheet, and the ferrite layer includes an internal oxide layer on top of which an Fe-Ni alloy layer is formed.

[0072] In this way, the above Fe-Ni alloy layer The presence of the ferrite layer having the internal oxide layer formed therein has the effect of minimizing the propagation of cracks, and as a result, has the effect of suppressing the occurrence of LME. [Example]

[0073] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0074] (Example) A 1.5 mm thick cold-rolled steel sheet (TRIP steel) composed by weight of 0.18%C, 1.5%Si, 2.5%Mn, 0.05%Al, 0.005%P, and 0.0015%S (the balance being Fe and unavoidable impurities) was prepared, and one side of the cold-rolled steel sheet was then coated.

[0075] The coating composition for the above coating treatment was prepared as follows.

[0076] [rGO production] 10 ml of hydrazine monohydrate was slowly added dropwise to a graphene oxide dispersion (200 ml) in which 0.01 g of graphene oxide (GO) was dispersed per 1 ml of distilled water, and the mixture was stirred at 80°C for 2 hours. Sulfuric acid was then added to the solution, which was then ultrasonicated for 30 minutes to obtain a solution in which graphene oxide was reduced and rGO was dispersed.

[0077] [Production of Coating Composition] 10 ml of the prepared rGO solution and 10 mg / L of a saturated aqueous solution of FeSO4 hydrate were added to 500 ml of pure water, mixed, and then ultrasonicated for 60 minutes to obtain an Fe / rGO aqueous solution containing 3 wt% Fe oxide particles with a particle size of 10 nm.

[0078] Then, 10 ml of the Fe / rGO aqueous solution was added to a Watts bath containing nickel sulfate (262.7 g, 1 M), nickel chloride (64.9 g, 0.5 M), and boric acid (30.4 g, 0.5 M), and the mixture was stirred for 1 hour to obtain a Ni+Fe / rGO coating composition with a pH of 1.

[0079] The Ni+Fe / rGo coating composition prepared as above is coated on one surface of the above-mentioned base steel sheet, and at this time, 200 to 800 mg / m 2 Electroplating was carried out at 50°C with a Ni deposition weight of 10 ...

[0080] The steel sheets coated with different Ni deposition amounts were then subjected to annealing heat treatment in an annealing furnace containing 3 to 5% by volume of nitrogen, with the temperature increased to 850°C. At this time, the dew point temperature was set to -50°C, -10°C, or +5°C, and 100m of humidified nitrogen was used in the 700°C range. 3 / h was introduced.

[0081] Table 1 below shows the results of GDS analysis of the Mn and Si contents from the outermost surface to 100 nm in the thickness direction of each test piece after electroplating and annealing. The Ni+Fe / rGO coating composition was used to compare the Ni coating weight during electroplating and the changes in the dew point temperature during annealing, as well as the results with and without Fe / rGO.

[0082] [Table 1]

[0083] As shown in Table 1 above, when the coating composition is a Ni+Fe / rGO coating composition compared to a Ni-only composition, it can be confirmed that the concentration of Mn and Si on the surface is significantly suppressed.

[0084] Furthermore, it can be seen that the higher the dew point temperature and the higher the Ni deposition amount, the greater the tendency to suppress the surface diffusion of oxidizing elements.

[0085] Table 2 below shows the results of measuring the depth (μm) of the internal oxide layer of each test specimen after electroplating and annealing. The Ni coating weight during electroplating using a Ni+Fe / rGO coating composition and the changes in the dew point temperature during annealing were compared, along with the results with and without Fe / rGO. The depth of the internal oxide layer was measured by cutting the test specimen perpendicular to the rolling direction and observing the cross section with an SEM.

[0086] [Table 2]

[0087] As shown in Table 2 above, when the coating composition was Ni alone, no internal oxide layer was observed. On the other hand, when the Ni+Fe / rGO coating composition was used, the Ni deposition amount was 400 mg / m at a dew point temperature of -50°C. 2 An internal oxide layer was observed when the dew point temperature was above 10°C, and at dew points of -10°C and +5°C, the Ni deposition amount increased and the thickness increased to a maximum of 2.5±1.2μm.

[0088] On the other hand, the adhesion amount is 800 mg / m 2 In the case of 2 It can be seen that the thickness of the internal oxide layer has decreased compared to the thickness of the coating layer. This is believed to be due to the fact that the coating layer is relatively thick, and some Ni remains without diffusing, forming a residual layer.

[0089] Table 3 below shows the results of measuring the ferrite fraction (area %) from the outermost surface to 50 μm in the thickness direction of each test piece after electroplating and annealing. The Ni coating weight during electroplating using a Ni+Fe / rGO coating composition and the changes due to the dew point temperature during annealing were also compared, along with the results with and without Fe / rGO.

[0090] [Table 3]

[0091] As shown in Table 3, when the coating composition was Ni alone, no decarburization occurred. Furthermore, it can be seen that the higher the dew point temperature and the Ni deposition amount, the more favorable the decarburization occurred, resulting in a higher ferrite fraction.

[0092] The following Table 4 shows the results of observing the surface quality of each test piece after electroplating, annealing, and subsequent galvannealing treatment. The galvannealing treatment was carried out by hot-dip galvanizing using a conventional galvanizing bath followed by galvannealing heat treatment at 480°C.

[0093] The Ni+Fe / rGO coating composition was used to measure the Ni deposition amount during electroplating, and the changes in the dew point temperature during annealing were compared with the results for the presence or absence of Fe / rGO. A surface microanalyzer was used to observe the presence or absence of unplated areas, and specimens for which no unplated areas were observed were rated as "good."

[0094] [Table 4]

[0095] As shown in Table 4 above, when the coating composition contained only Ni, either no plating occurred or the degree of alloying was poor regardless of the Ni coating amount, meaning that no surface improvement effect was observed.

[0096] On the other hand, when the Ni+Fe / rGo coating composition was used, the Ni deposition amount was 400 mg / m at a dew point temperature of -50°C. 2 At a dew point temperature of -10°C, the adhesion amount is 200mg / m 2 From these times onwards, the surface improved, and at +5°C the surface quality was good regardless of the amount of Ni attached.

Claims

1. A base steel sheet, a ferrite layer formed on a surface layer portion of the base steel sheet, the ferrite layer has an internal oxide layer formed thereon; the internal oxidation layer has an Fe—Ni alloy layer formed thereon; the Fe—Ni alloy layer comprises reduced graphene oxide (rGO); The internal oxidation layer is formed from the surface of the ferrite layer along the grain boundaries of the base structure of the surface layer portion of the base steel sheet to a depth of up to 3 μm in the thickness direction, and The Fe-Ni alloy layer is formed from the surface of the ferrite layer along the grain boundaries of the matrix structure of the surface layer portion of the base steel sheet to a depth of up to 2 μm in the thickness direction. High strength steel plate.

2. The high-strength steel sheet according to claim 1, wherein the ferrite layer is present inside the base steel sheet to a maximum thickness of 50 μm in the thickness direction of the base steel sheet.

3. The high-strength steel plate according to claim 2, wherein the ferrite layer contains a ferrite phase with an area fraction of 50% or more.

4. The high-strength steel sheet according to claim 1, wherein the ferrite layer is present inside the base steel sheet to a maximum thickness of 30 μm in the thickness direction of the base steel sheet.

5. 2. The high-strength steel sheet according to claim 1, wherein the base steel sheet is a cold-rolled steel sheet containing, by weight, carbon (C): 0.17 to 0.19%, silicon (Si): 1.3 to 1.7%, manganese (Mn): 2.4 to 2.7%, aluminum (Al): 0.01 to 0.7%, phosphorus (P): 0.01% or less, sulfur (S): 0.003% or less, the balance being Fe and other unavoidable impurities.

6. providing a base steel sheet; forming a composite coating layer of Ni and Fe / rGO on at least one surface of the base steel sheet; and subjecting the base steel sheet on which the composite coating layer is formed to an annealing heat treatment, The step of forming the composite coating layer comprises: i) producing rGO; ii) mixing the prepared rGO with an aqueous iron oxide solution and sonicating the aqueous solution; iii) mixing the ultrasonically treated aqueous solution with a nickel compound to form a coating composition; iv) electroplating the coating composition onto at least one surface of the base steel sheet; The annealing heat treatment is carried out at a temperature range of 750 to 850°C and a dew point temperature of -10 to +5°C.

7. The method for producing a high-strength steel plate according to claim 6, wherein the Fe / rGO composite coating layer is formed by coating a surface of rGO with Fe oxide.

8. The method for producing a high-strength steel plate according to claim 6, wherein the coating composition has a pH of 1 to 2.

9. The electroplating step is carried out based on the Ni deposition amount per unit area (m ２ 7. The method for producing a high strength steel plate according to claim 6, wherein the deposition amount is 200 to 800 mg per molten metal.

10. When heating for the annealing heat treatment, when the temperature rises to 700°C or more, the dew point is -10 to +5°C and the temperature is 50 to 200 m ３ 7. The method for producing a high strength steel plate according to claim 6, wherein moist nitrogen is introduced at a rate of 1 / hour.

11. 7. The method for producing a high-strength steel sheet according to claim 6, wherein the base steel sheet is a cold-rolled steel sheet containing, by weight, carbon (C): 0.17 to 0.19%, silicon (Si): 1.3 to 1.7%, manganese (Mn): 2.4 to 2.7%, aluminum (Al): 0.01 to 0.7%, phosphorus (P): 0.01% or less, sulfur (S): 0.003% or less, the balance being Fe and other unavoidable impurities.

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