Plated steel sheet and method for manufacturing same

Abstract

The present invention relates to a plated steel sheet and a method for manufacturing same, the plated steel sheet comprising: a base steel sheet that contains, in wt%, 0.10-0.25% of C, 1.5-5.0% of Mn, 0.5-2.5% of Si, 1.5% or less of Cr, 0.005-0.100% of Al, 0.10% or less of P, 0.020% or less of S, and 0.0050% or less of B, with the remainder comprising Fe and inevitable impurities; and a plating layer formed on at least one surface of the base steel sheet, wherein the depth of an internal oxide layer composed of an oxide of at least one among Mn, Si, Cr, and B is 3-15 μm from the interface between the base steel sheet and the plating layer toward the middle of the plated steel sheet in the thickness direction. More specifically, the present invention relates to a plated steel sheet having excellent surface quality and welding LME resistance, and a method for manufacturing same.

Description

Galvanized steel sheet and its manufacturing method

[0001] The present invention relates to a plated steel sheet and a method for manufacturing the same, and more particularly, to a plated steel sheet having excellent surface quality and welding LME resistance and a method for manufacturing the same.

[0002] Recently, technological development in automotive steels has focused on lightweighting through high strength. To achieve this, various alloying elements are added to enhance strength, utilizing precipitation and solution strengthening. Active development is also underway for Advanced High Strength Steel (AHSS), which induces phase transformation during the annealing process.

[0003] Elements that can be added to increase the strength of steel include manganese, silicon, chromium, and boron. However, due to their high oxidation tendency, they diffuse to the surface during annealing to combine with oxygen in the atmosphere, forming surface oxides. These surface oxides can reduce the surface reactivity of the steel sheet, deteriorating chemical treatment properties and plating properties.

[0004] Additionally, as steel becomes more high-strength, cracks occur in the weld heat-affected zone during spot welding due to liquid metal embrittlement (LME). This occurs when the plating layer of the molten steel sheet transforms into a liquid state during welding and penetrates along the grain boundaries of the steel sheet surface.

[0005] A representative method for reducing LME is to form a soft decarburized layer on the surface of the steel plate. This is due to the effect of the decarburized layer on the surface that suppresses cracking caused by tensile stress during spot welding.

[0006] Various techniques have been proposed to suppress surface oxides formed on the surface of steel sheets during annealing. Patent Document 1 describes a process in which the air-fuel ratio of air to fuel is controlled to 0.80 to 0.95 during the annealing process, thereby oxidizing the steel sheet in a direct flame furnace in an oxidizing atmosphere, thereby forming iron oxides containing single or complex oxides of Si, Mn, or Al to a certain depth within the steel sheet. Subsequently, the iron oxides are reduced and annealed in a reducing atmosphere, and then hot-dip galvanized is performed to provide a hot-dip galvanized or alloyed hot-dip galvanized steel sheet with excellent plating quality.

[0007] However, in steel grades with Si added above a certain level, Si becomes concentrated directly beneath the iron oxide during the reduction process, forming a band-like Si oxide, which then peels off from the surface. In other words, peeling occurs at the interface between the reduced iron and the base iron, making it difficult to ensure sealer adhesion and plating layer adhesion. Furthermore, it is ineffective in forming a decarburized layer to improve LME resistance.

[0008] Patent Document 2 discloses a method for improving plating properties by maintaining a high dew point in an annealing furnace, thereby internally oxidizing alloy components such as Mn, Si, and Al that are easily oxidized, and thereby reducing oxides that are externally oxidized on the surface of a steel sheet after annealing.

[0009] The method of Patent Document 2 above can solve the problem of plating due to external oxidation of Si, which is easily oxidized internally. However, if a large amount of Mn, which is relatively difficult to oxidize internally, is added, the effect may be minimal. Furthermore, there are limitations in reducing surface oxidation of Si, Mn, etc. and forming a sufficient decarburization layer that can be achieved with a single application of internal oxidation.

[0010] (Patent Document 1) Korean Patent Publication No. 10-2010-0030627 (published on March 18, 2010)

[0011] (Patent Document 2) Korean Patent Publication No. 10-2009-0006881 (published on January 15, 2009)

[0012] According to one embodiment of the present invention, a plated steel sheet and a method for manufacturing the same are provided.

[0013] According to one embodiment of the present invention, it is an object to provide a plated steel sheet having excellent surface quality and welding LME resistance and a method for manufacturing the same.

[0014] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0015] According to one embodiment of the present invention, a steel sheet comprising, in wt%, C: 0.10 to 0.25%, Mn: 1.5 to 5.0%, Si: 0.5 to 2.5%, Cr: 1.5% or less, Al: 0.005 to 0.100%, P: 0.10% or less, S: 0.020% or less, B: 0.0050% or less, the remainder being Fe and other unavoidable impurities; and

[0016] Including a plating layer formed on at least one surface of the above steel plate;

[0017] It may be a plated steel sheet having an internal oxide layer composed of one or more oxides of Mn, Si, Cr, and B, the depth of which is 3 to 15 μm in the direction of the center of thickness from the interface between the base steel sheet and the plated layer.

[0018] The above steel plate may further contain, in weight %, one or more of Ti, Mo, and Nb in an amount of 1.2% or less.

[0019] The depth of the decarburized layer can be 20 to 150 μm in the direction of the center of thickness from the interface between the base steel plate and the plating layer.

[0020] The above-mentioned galvanized steel sheet may have a maximum LME crack length of 10 μm or less.

[0021] The above plating layer may be a zinc-based or zinc alloy-based plating layer.

[0022] According to one embodiment of the present invention, there is provided a step of preparing a steel sheet containing, in wt%, C: 0.10 to 0.25%, Mn: 1.5 to 5.0%, Si: 0.5 to 2.5%, Cr: 1.5% or less, Al: 0.005 to 0.100%, P: 0.10% or less, S: 0.020% or less, B: 0.0050% or less, the remainder being Fe and other unavoidable impurities;

[0023] A step of first annealing the above steel plate at a temperature range of 600 to 900°C and a dew point temperature of -10 to 30°C;

[0024] A step of first pickling the above first annealed steel plate;

[0025] A step of second annealing the first-treated steel plate at a temperature range of 700 to 900°C; and

[0026] It may be a method for manufacturing a plated steel sheet, including a step of plating at least one surface of the secondary annealed steel sheet.

[0027] The above steel plate may further contain, in weight %, one or more of Ti, Mo, and Nb in an amount of 1.2% or less.

[0028] The above steel plate may be a cold rolled steel plate.

[0029] The above first annealing step is maintained for 50 to 600 seconds,

[0030] The above second annealing step can be performed at a dew point temperature of -60 to 30°C.

[0031] The above first annealing step and second annealing step may be a nitrogen gas atmosphere containing 1 to 80 vol% of hydrogen.

[0032] The above plating step can be performed using zinc-based or zinc alloy-based plating.

[0033] A step of alloying heat treatment of the above-mentioned plated steel plate may be further included.

[0034] After the above first acid washing step, a metal plating step may be further included.

[0035] The step of plating metal before the above second annealing step may further be included.

[0036] The above metal plating step may be Fe or Ni plating.

[0037] According to one embodiment of the present invention, a plated steel sheet and a method for manufacturing the same can be provided.

[0038] According to one embodiment of the present invention, a plated steel sheet having excellent surface quality and welding LME resistance and a method for manufacturing the same can be provided.

[0039] According to one embodiment of the present invention, a plated steel sheet having excellent surface quality and weld LME resistance and being usable for automotive steel and a method for manufacturing the same can be provided.

[0040] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0041] Figure 1 illustrates a method for calculating the Mn surface concentration of a steel plate according to one embodiment of the present invention.

[0042] FIG. 2 is a cross-sectional SEM image of Invention Example 3 according to one embodiment of the present invention, illustrating a method for measuring the depth of an internal oxidation layer.

[0043] FIG. 3 is a graph showing a method for obtaining the depth of a decarburized layer of a steel plate according to an embodiment of the present invention, and showing the carbon concentration according to the depth of the steel plate when analyzed by GDS (Glow Discharge Spectrometer).

[0044] FIG. 4 illustrates a method for obtaining the position of the interface between the base steel sheet and the plating layer from GDS data to obtain the depth of the decarburization layer of the steel sheet according to one embodiment of the present invention.

[0045] FIG. 5 is a cross-sectional SEM image of a steel plate according to one embodiment of the present invention, illustrating a method for obtaining the depth of a decarburized layer of a steel plate etched with nital.

[0046] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0047] Hereinafter, the present invention will be described in detail.

[0048] A plated steel sheet according to one embodiment of the present invention may include a base steel sheet; and a plated layer formed on at least one surface of the base steel sheet.

[0049] Below, the steel composition of the present invention is described in detail.

[0050] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0051] The base steel sheet of the plated steel sheet according to one embodiment of the present invention may include, in weight %, C: 0.10 to 0.25%, Mn: 1.5 to 5.0%, Si: 0.5 to 2.5%, Cr: 1.5% or less, Al: 0.005 to 0.100%, P: 0.10% or less, S: 0.020% or less, and B: 0.0050% or less.

[0052] Carbon (C): 0.10~0.25%

[0053] Carbon (C) is an important element that can be added to stabilize retained austenite. To achieve this effect, carbon (C) can be added in an amount of 0.10% or more. According to one embodiment of the present invention, carbon (C) can be added in an amount of 0.15% or more. On the other hand, if the carbon (C) content exceeds 0.25%, the problem of poor weldability may arise. According to one embodiment of the present invention, the carbon (C) content can be 0.23% or less.

[0054] Manganese (Mn): 1.5~5.0%

[0055] Manganese (Mn) is an essential element in transformed structure steel because it forms and stabilizes retained austenite and suppresses ferrite transformation during cooling. In addition, in order to secure sufficient austenite and thereby ensure strength and ductility, manganese (Mn) may be included in an amount of 1.5% or more. According to one embodiment of the present invention, it may be 2.0% or more. On the other hand, if the content exceeds 5.0%, band formation due to segregation induced during slab heating and hot rolling processes may be excessive, which may cause a problem of deteriorating physical properties. Therefore, according to one embodiment of the present invention, the upper limit of the manganese (Mn) content may be limited to 5.0%. According to one embodiment of the present invention, it may be 3.0% or less.

[0056] Silicon (Si): 0.5–2.5%

[0057] Silicon (Si) is an element that suppresses the precipitation of carbides within ferrite and promotes the diffusion of carbon within ferrite into austenite, thereby contributing to the stabilization of retained austenite. To achieve the above effects, silicon (Si) may be added in an amount of 0.5% or more. In one embodiment of the present invention, it may be 0.7% or more. However, if added excessively, surface reactivity may be reduced, and therefore, the upper limit of the silicon (Si) content may be limited to 2.5%. In one embodiment of the present invention, it may be 2.0% or less.

[0058] Chromium (Cr): 1.5% or less

[0059] Chromium (Cr) is an element that increases hardenability and plays a role in suppressing the formation of ferrite. Therefore, it can be added in small amounts as needed to secure appropriate residual austenite. According to one embodiment of the present invention, the chromium (Cr) content may be 0%. However, if the content is excessive, the amount of alloy iron input may be excessive, which may cause an increase in cost. Therefore, the upper limit of the chromium (Cr) content may be limited to 1.5%. According to one embodiment of the present invention, it may be 1.0% or less.

[0060] Aluminum (Al): 0.005~0.100%

[0061] Aluminum (Al) is an element that contributes to the stabilization of retained austenite by suppressing the formation of carbides within ferrite, and to achieve this effect, it can be added in an amount of 0.005% or more. According to one embodiment of the present invention, it can be 0.010% or more. However, if the content exceeds 0.100%, it may be difficult to manufacture a sound slab due to a reaction with the mold flux during casting. In addition, it may form surface oxides, thereby inhibiting the melting plating property, and therefore, the upper limit of the aluminum (Al) content may be limited to 0.100%. According to one embodiment of the present invention, it may be 0.08% or less.

[0062] Phosphorus (P): 0.10% or less

[0063] Phosphorus (P) is a reinforcing element, but if its content exceeds 0.10%, weldability deteriorates and the risk of steel becoming brittle increases, so its upper limit can be limited to 0.10%.

[0064] Sulfur (S): 0.020% or less

[0065] Sulfur (S) is an impurity element that impairs the ductility and weldability of steel plates. Therefore, as the sulfur (S) content increases, the likelihood of impairing the ductility and weldability of steel plates increases. Therefore, considering this, the upper limit can be set at 0.020%.

[0066] Boron (B): 0.0050% or less

[0067] Boron (B) is an element that can be added to enhance strength. However, if the boron (B) content exceeds 0.0050%, it can concentrate on the surface of the annealed material, significantly reducing surface quality. Therefore, the content may be limited to 0.0050% or less.

[0068] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0069] According to one embodiment of the present invention, the base steel sheet of the plated steel sheet may further include, by weight %, one or more of Ti, Mo, and Nb in an amount of 1.2% or less.

[0070] At least one of titanium (Ti), molybdenum (Mo), and niobium (Nb): 1.2% or less

[0071] Titanium (Ti) can form nitrides and reduce the concentration of nitrogen in steel. On the other hand, if it is included excessively, it can cause a decrease in the carbon concentration of martensite and a decrease in strength due to carbide precipitation.

[0072] Molybdenum (Mo) can contribute to increased strength. In particular, it has the effect of securing strength without reducing the wettability of molten metals such as zinc.

[0073] Niobium (Nb) is segregated in the form of carbides at austenite grain boundaries, which can increase strength by suppressing coarsening of austenite grains during annealing heat treatment; however, excessive input can lead to increased cost.

[0074] Taking this into consideration, one or more of the above titanium (Ti), molybdenum (Mo), and niobium (Nb) may be included at 1.2% or less.

[0075] Below, the steel microstructure of the present invention is described in detail.

[0076] According to one embodiment of the present invention, the plated steel sheet may have an internal oxidation layer formed of one or more oxides of Mn, Si, Cr, and B, the depth of which may be 3 to 15 μm in the direction of the center of thickness from the interface between the base steel sheet and the plated layer.

[0077] Internal oxidation occurs simultaneously with an internal oxidation layer formed along the grain boundary and an internal oxidation layer formed within the grain, but the depth of the internal oxidation layer according to one embodiment of the present invention may be based on the grain boundary internal oxidation layer.

[0078] If the depth of the internal oxidation layer is less than 3 μm, the amount of internal oxidation is insufficient, so the amount of surface oxidation formed by diffusion to the surface is relatively large, which may lead to an insufficient effect on improving surface quality. According to one embodiment of the present invention, the depth of the internal oxidation layer may be 4 μm or more. According to one embodiment of the present invention, the depth of the internal oxidation layer may be 5 μm or more. On the other hand, if the depth of the internal oxidation layer exceeds 15 μm, the heat treatment time may be excessively long to form a deep internal oxidation layer, which may lead to a problem of poor economic efficiency.

[0079] According to one embodiment of the present invention, the depth of the decarburized layer of the plated steel sheet may be 20 to 150 μm in the direction of the center of thickness from the interface between the base steel sheet and the plated layer.

[0080] If the depth of the decarburization layer is less than 20 μm, the soft layer may be insufficiently thick, resulting in a problem of insufficient LME improvement. In one embodiment of the present invention, it may be 30 μm or more. In one embodiment of the present invention, it may be 35 μm or more. In one embodiment of the present invention, it may be 40 μm or more. In one embodiment of the present invention, it may be 50 μm or more. On the other hand, if the depth of the decarburization layer exceeds 150 μm, there may be a problem of the tensile strength decreasing due to an excessively thick soft layer.

[0081] The plating layer of the plated steel sheet according to one embodiment of the present invention may be a zinc-based or zinc alloy-based plating layer.

[0082] According to one embodiment of the present invention, the galvanized steel sheet may have a maximum LME crack length of 10 μm or less.

[0083] According to one embodiment of the present invention, the maximum length of the LME crack was evaluated according to the SEP 1220-2 standard. The plated steel sheet of the present invention and the plated steel sheet without a decarburized layer were overlapped and spot welded, and then the welded plated steel sheet was sheared in the 0°, 45°, and 90° directions per specimen and observed using OM (Optical Microscopy) to measure the LME crack length. At this time, only the B type crack in the heat-affected zone was measured, and the maximum crack length was indicated.

[0084] Below, the steel manufacturing method of the present invention is described in detail.

[0085] A plated steel sheet according to one embodiment of the present invention can be manufactured by first annealing, first pickling, second annealing, and plating a base steel sheet satisfying the above-described alloy composition.

[0086] Preparing the steel plate

[0087] A steel sheet satisfying the alloy composition according to one embodiment of the present invention can be prepared.

[0088] According to one embodiment of the present invention, the base steel sheet may be a cold-rolled steel sheet. The method for manufacturing the cold-rolled steel sheet according to one embodiment of the present invention is not particularly limited and may be a typical condition applicable in the same technical field.

[0089] 1st annealing

[0090] The above steel plate can be subjected to primary annealing by maintaining it at a temperature range of 600 to 900°C and a dew point temperature of -10 to 30°C for 50 to 600 seconds.

[0091] According to one embodiment of the present invention, by performing primary annealing, an internal oxide layer and a decarburization layer can be effectively formed. During primary annealing, an internal oxide layer composed of oxides such as Si, Mn, Cr, and B is formed at a depth greater than a certain depth from the surface of the steel sheet, thereby depleting Si, Mn, Cr, and B, etc. dissolved in the surface layer of the steel sheet.

[0092] During primary annealing, Si has a very high oxidation tendency, so it can be oxidized internally and the formation of surface oxides can be prevented. On the other hand, Mn, which has a relatively low oxidation tendency compared to Si, can form a large amount of surface oxides. This surface oxide caused by Mn can be removed through primary pickling performed after primary annealing.

[0093] Through primary annealing, an internal oxide layer is formed, reducing the Si oxide on the surface, and then through acid washing, the Mn oxide is removed, so the surface can be kept clean.

[0094] Furthermore, the surface, purified after primary annealing, facilitates the adsorption of oxygen in the atmosphere onto the steel plate surface during secondary annealing, promoting the decarburization reaction in which carbon in the steel diffuses to the surface and combines with the adsorbed oxygen. This ensures superior plating quality, and by forming a sufficient decarburization layer, LME (Liquid Metal Embrittlement) can be reduced during welding.

[0095] During primary annealing, if the annealing temperature is below 600℃, alloying elements such as Si and Mn, which form an internal oxide layer, will not readily diffuse to the surface grain boundaries, and carbon diffusion to the surface for surface decarburization may also be difficult. On the other hand, if the annealing temperature exceeds 900℃, internal oxidation and decarburization may occur, but the excessively high temperature requires significant energy consumption to maintain the high temperature, making it uneconomical.

[0096] During primary annealing, if the dew point temperature is below -10°C, the oxygen partial pressure in the atmosphere may be insufficient, making it difficult for oxygen to penetrate into the steel sheet. According to one embodiment of the present invention, it may be 0°C or higher. On the other hand, if the dew point temperature exceeds 30°C, there is a risk that the atmosphere will oxidize even the iron. According to one embodiment of the present invention, the dew point temperature may be limited to 20°C or lower to prevent iron from being oxidized and to more effectively internally oxidize the silicon.

[0097] During primary annealing, a holding time of less than 50 seconds may be insufficient to form an internal oxidation and decarburization layer. Conversely, a holding time exceeding 600 seconds may result in the diffusible alloying elements in the surface layer having fully reacted, making it difficult to expect additional benefits and potentially being economically disadvantageous.

[0098] According to one embodiment of the present invention, during the primary annealing process, the annealing atmosphere may be a nitrogen gas atmosphere containing 1 to 80 vol% hydrogen. If the hydrogen content is less than 1 vol%, the iron in the steel sheet may oxidize. On the other hand, if the hydrogen content exceeds 80 vol%, there is a risk of explosion in the event of a gas leak, which may increase costs.

[0099] According to one embodiment of the present invention, during the primary annealing, two or more cooling steps may be performed to obtain the desired microstructure after maintenance. The cooling rate during the primary annealing is not particularly limited and may be any condition applicable in the same technical field. According to one embodiment of the present invention, during the primary annealing, cooling may be performed at a cooling rate of 1 to 100°C / s.

[0100] If the cooling rate is not high, gas cooling using nitrogen gas containing a certain amount of hydrogen can be performed. To increase the cooling rate, mist cooling, water quenching, or water jet cooling can be performed. If wet cooling is performed with a large amount of water, the steel plate comes into direct contact with water, and the dew point temperature can rise rapidly due to water vapor, which can cause an iron oxide film to form on the surface. In such cases, the iron oxide film must be removed through pickling after cooling is complete.

[0101] Meanwhile, during cooling, the atmosphere may be a nitrogen gas atmosphere containing 1 to 80 vol% of hydrogen.

[0102] Additionally, according to one embodiment of the present invention, during the first annealing, the heating rate may be 1 to 50°C / s.

[0103] 1st mountain tax

[0104] The above first annealed steel plate can be subjected to first pickling.

[0105] According to one embodiment of the present invention, oxides formed on the surface of a steel sheet during primary annealing can be removed through primary pickling.

[0106] In the first pickling, the type of acid is not particularly limited, but according to one embodiment of the present invention, a 3 to 20 wt% acid solution at 30 to 80°C can be used. According to one embodiment of the present invention, pickling can be performed using 5 wt% hydrochloric acid at 50 to 60°C. According to one embodiment of the present invention, when applying strong pickling, 18 wt% hydrochloric acid at 80°C can be used.

[0107] According to one embodiment of the present invention, metal plating may be additionally performed after the primary acid wash. According to one embodiment of the present invention, Fe, Ni, etc. may be plated as the metal plating. By additionally performing the metal plating process, it may be helpful to prevent Si, Mn, etc. from diffusing to the surface during secondary annealing. The metal plating conditions are not particularly limited and may be conditions applicable in the same technical field. According to one embodiment of the present invention, when Fe is plated, the adhesion amount is 0.1 to 3 g / m. 2 According to one embodiment of the present invention, when Ni is plated, the adhesion amount is 5 to 700 mg / m 2 According to one embodiment of the present invention, when plating metal, a device at the exit side of the first annealing line can be used.

[0108] Secondary annealing

[0109] The above-mentioned primary annealed steel sheet can be subjected to secondary annealing at a temperature range of 700 to 900°C and a dew point temperature of -60 to 30°C.

[0110] According to one embodiment of the present invention, the desired tensile properties can be secured by performing secondary annealing. During secondary annealing, annealing can be performed in a state where a deficiency layer is formed from the surface of the steel sheet to a certain depth, in which Si, Mn, Cr, B, etc. in a solid solution state are significantly reduced due to the internal oxidation layer formed during the primary annealing process. Due to this deficiency layer, surface oxides of Si, Mn, Cr, and B that diffuse to the surface of the steel sheet and are formed during secondary annealing can be significantly suppressed.

[0111] That is, by performing primary annealing and primary pickling prior to secondary annealing, the steel sheet after secondary annealing can have a cleaner surface condition. The surface of the steel sheet in a clean condition has a large Fe exposure area, which improves surface reactivity and thus can have the effect of improving the plating quality. The hot dip galvanizing is improved when there are fewer oxides on the surface of the annealed steel sheet. This is because the smaller the oxides on the surface, the easier it is for a small amount of Al in the plating bath to react with the Fe of the steel sheet to form an inhibition layer, and the wettability of the liquid plating solution and the steel sheet is improved. There are various surface oxides that adversely affect the plating property, such as Mn, Si, Cr, and B, but Mn and Si are representative of them.

[0112] During secondary annealing, an annealing temperature below 700°C may be insufficient to achieve the desired microstructure. Conversely, an annealing temperature exceeding 900°C may result in reduced economic efficiency due to increased energy consumption.

[0113] During secondary annealing, if the dew point temperature is below -60°C, it is realistically very difficult to maintain the atmosphere in large-scale production facilities. According to one embodiment of the present invention, in addition to securing physical properties, the lower limit may be limited to -10°C to form a deeper internal oxidation layer and decarburization layer. According to one embodiment of the present invention, the lower limit may be 0°C or higher. On the other hand, if the dew point temperature exceeds 30°C, the iron oxidizes, making it difficult to additionally secure a decarburization layer. According to one embodiment of the present invention, the lower limit may be 20°C or lower.

[0114] According to one embodiment of the present invention, during the secondary annealing, the holding time may be 30 to 100 seconds. Furthermore, according to one embodiment of the present invention, during the secondary annealing, the holding time may be shorter than the holding time of the primary annealing.

[0115] According to one embodiment of the present invention, during secondary annealing, the annealing atmosphere may be a nitrogen gas atmosphere containing 1 to 80 vol% hydrogen. If the hydrogen content is less than 1 vol%, the iron in the steel sheet may oxidize. On the other hand, if the hydrogen content exceeds 80 vol%, there is a risk of explosion in the event of a gas leak, which may increase costs.

[0116] During secondary annealing, cooling conditions are not particularly limited. According to one embodiment of the present invention, during secondary annealing, slow cooling is performed to 650°C after holding, and then rapid cooling can be performed depending on the target physical properties. According to one embodiment of the present invention, during cooling, conditions are not particularly limited, but conditions can be changed to achieve the target physical properties. In addition, since the formation of surface oxides, internal oxidation layers, and decarburization layers mostly occur in relatively high temperature regions, the present invention does not particularly limit the cooling conditions.

[0117] Meanwhile, according to one embodiment of the present invention, during secondary annealing, gas cooling and wet cooling using water can be performed in the same manner as during primary annealing after maintenance.

[0118] According to one embodiment of the present invention, during cooling, a reducing atmosphere may be applied to at least the iron to prevent oxidation of the iron. To maintain the reducing atmosphere, a nitrogen gas atmosphere containing 1 to 80 vol% hydrogen, similar to the atmosphere gas used during secondary annealing, may be applied.

[0119] Furthermore, according to one embodiment of the present invention, the rapidly cooled steel plate can be reheated to a certain temperature for tempering, if necessary.

[0120] According to one embodiment of the present invention, metal plating may be additionally performed prior to secondary annealing. By additionally performing the metal plating process prior to secondary annealing, it may be helpful in preventing diffusion of Si, Mn, etc. to the surface during secondary annealing. The metal plating conditions prior to secondary annealing may be applied in the same manner as the metal plating process after primary pickling described above. According to one embodiment of the present invention, during metal plating, a device at the entrance of the secondary annealing line may be used.

[0121] plating

[0122] At least one side of the above-mentioned second-annealed steel plate can be plated.

[0123] According to one embodiment of the present invention, plating can be performed through reheating after secondary annealing. The type of plating is not particularly limited, and according to one embodiment of the present invention, hot dip plating can be performed. According to one embodiment of the present invention, plating can be performed using a molten zinc-based plating bath, and the conditions during hot dip plating are not particularly limited. Furthermore, according to one embodiment of the present invention, an alloying heat treatment can be additionally performed after plating, if necessary.

[0124] Hereinafter, the present invention will be described in more detail 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 present invention.

[0125] (Example)

[0126] Cold-rolled steel sheets having the compositions shown in Table 1 below were prepared. Primary annealing and primary pickling were performed with two types of steel sheets under the conditions shown in Table 2 below. Primary annealing was performed under a reducing atmosphere of nitrogen gas containing 5% hydrogen. Further, primary annealing was performed by heating the steel sheets to 800°C at a heating rate of 3°C / s and maintaining the temperature for 150 seconds. After cooling, the steel sheets were immersed in a 5 vol% hydrochloric acid solution at 50°C for 5 seconds to perform primary pickling.

[0127] Steel alloy composition (weight %)CMnSiCrAlPSTiBA0.202.41.0-0.0300.010.0020.020.0018B0.192.71.60.50.0400.010.0010.020.0020

[0128] In addition, Table 2 below shows the results of performing the first annealing, measuring the surface enrichment of Si and Mn before and after the first pickling, and analyzing the depth of the internal oxidation layer and decarburization layer. The surface enrichment of Si and Mn before and after pickling can be estimated by the amount of Si and Mn oxide remaining on the surface of the steel sheet. The surface enrichment of Si and Mn is that they diffuse to the surface during annealing, combine with oxygen in the atmosphere, and enrich on the surface, so they can be viewed as the amount of oxidation.

[0129] The specific Si and Mn surface enrichment amounts can be calculated by conducting GDS analysis. The GDS data is processed so that the sum of the weight % of the main components of each steel component, Fe, Mn, Si, Cr, and B, is 100%. Here, Fe is included because it is the main component of the steel plate, and oxygen is not included. In addition to the above components, components contained in the steel plate can be processed by adding them, but the components that enrich on the surface during annealing are limited and the amount is extremely small, so they are omitted.

[0130] Figure 1 illustrates a method for calculating the Mn surface enrichment amount of a steel sheet according to an embodiment of the present invention. The Si surface enrichment amount can also be calculated using the same method described below. As shown in Figure 1, a graph is drawn with the depth of the processed data on the x-axis and the weight % of each component on the y-axis. After limiting the depth for each component to 0.1 μm, the minimum weight % point of each component within 0.1 μm is found, and that value is estimated as the dissolved component amount. Once the dissolved component amount is found, data at deeper depths are deleted based on that location. The dissolved component amount is subtracted from each component weight % from the surface to the dissolved component amount location. This process is to extract only oxides from the GDS data and to calculate by excluding the dissolved Mn or Si values. Thereafter, the integrated value from the surface to the dissolved component value location using the data from which the dissolved component value has been subtracted can be estimated as the enrichment amount. If the minimum weight% value of each component occurs at the outermost layer between depths of 0 and 0.01 μm, it can be interpreted that there is almost no surface concentration of the component.

[0131] In addition, the depth of the internal oxidation layer in Table 2 was recorded by observing the cross-section of the annealed steel plate with a SEM at 3000x magnification and entering the depth of the grain boundary internal oxidation. Fig. 2 is a cross-sectional SEM image of Invention Example 3 according to one embodiment of the present invention, and illustrates a method for measuring the depth of the internal oxidation layer. As in Fig. 2, three points can be measured randomly in the 3000x magnification image to derive an average value.

[0132] The specific decarburization depth can be derived by analyzing the carbon content by depth using a GDS (Glow Discharge Spectrometer). There are two methods. The first is when a section of the carbon content graph appears horizontal within the GDS measurable depth, and when GDS analysis is possible beyond the decarburization depth to the bulk structure. In this case, the decarburization depth is obtained as shown in FIG. 3. FIG. 3 illustrates a method for obtaining the decarburization depth of a steel sheet according to an embodiment of the present invention, and is a graph showing the carbon concentration according to the steel sheet depth during GDS (Glow Discharge Spectrometer) analysis. FIG. 4 illustrates a method for obtaining the position of the interface between the base steel sheet and the plating layer from GDS data to obtain the decarburization depth of the steel sheet according to an embodiment of the present invention. First, as shown in FIG. 4, the position of the interface between the plating layer and the base steel sheet is found from the GDS data. The interface between the plating layer and the base steel sheet is the point where the graphs of zinc and iron intersect, and this point is estimated as the surface of the base steel sheet where decarburization begins. Then, a graph is drawn with carbon content, and the average of the 10 carbon content data values ​​at the deepest depths in the carbon content graph is calculated. The average value of these 10 carbon content data at the deepest depths is called the depth value. Then, among the carbon content data from the deepest position toward the surface, the depth that shows the first carbon content that is 5% or more lower than the depth value is found. This point is the depth of the deepest decarburization layer. The total decarburization depth of the plating steel sheet is from the point where the zinc and iron intersect to the deepest decarburization depth.

[0133] The second case is when the depth of the decarburization layer is very deep and exceeds the GDS measurable range. In this case, as in the first case, the analysis is conducted based on the 10 carbon content values ​​of the deepest depth. Whether the carbon content of the deepest depth of the GDS data has reached the deep value can be estimated using the OES (Optical Emission Spectrometry) analysis value. That is, when analyzing up to the deepest depth measurable by GDS, the analysis value is at least 50 μm deep or more. If the carbon content of the deepest depth is lower than the OES carbon content, it means that the deep layer has not been reached yet, and therefore the decarburization depth can be estimated to be at least 50 μm or more. In order to determine the approximate depth of the decarburization layer, cross-sectional tissue observation is performed through nital etching (ethanol or methanol containing 2 to 5 vol% nitric acid). Fig. 5 is a cross-sectional SEM image of a steel sheet according to an embodiment of the present invention, illustrating a method for obtaining the depth of the decarburization layer of a nital-etched steel sheet. As shown in Fig. 5, it can be observed with SEM at magnifications of 1000x to 3000x. The surface decarburized structure shows coarse grains mainly composed of ferrite, and the deep (bulk) structure shows a microstructure including austenite. The depth to which the coarse decarburized structure is composed mainly of decarburized structure can be considered the depth of the decarburized layer.

[0134] Specimen numberSteel grade1st annealingAfter 1st annealingAfter 1st annealing and 1st picklingDew point temperature(℃)Si surface enrichmentMn surface enrichmentSi surface enrichmentMn surface enrichmentInternal oxidation layer depth(μm)Decarburization layer depth(μm)1A-400.2080.3390.06720.0082072A50.0160.2120.00520.00484.4143B-400.2110.4220.06980.0095044B50.0130.2550.00440.00523.817

[0135] Specimen Nos. 1 and 3 in Table 2 above are cases where the dew point temperature is -40°C and does not satisfy the conditions of the present invention, and specimen Nos. 2 and 4 are cases where the dew point temperature satisfies the conditions of the present invention. It can be confirmed that the surface concentrations of Si and Mn of specimens 2 and 4 are significantly reduced compared to specimens 1 and 3.

[0136] Meanwhile, looking at the results after pickling, it can be confirmed that the amount of Si surface enrichment is lower for specimens No. 1 and 3 than before pickling, but for specimens No. 2 and 4, a small amount of enrichment was observed due to internal oxidation even before pickling, so it can be confirmed that the amount after pickling is significantly lower than for specimens No. 1 and 3.

[0137] The surface concentration of Mn also decreased, but the rate of decrease was small compared to the surface concentration of Si. The surface concentration of Mn after pickling was greatly reduced compared to before pickling. This is because the Mn oxide is easily dissolved by pickling. Before pickling, in the case of specimens No. 2 and 4, whose dew point temperatures satisfied the conditions of the present invention, the surface concentration of Mn was lower than that of specimens No. 1 and 3 because an internal oxide layer was formed, and this trend was the same after pickling.

[0138] In the case of specimens No. 1 and 3, it can be confirmed that no internal oxidation layer was formed, and the decarburization layer was also hardly formed, with a thickness of less than 7 μm.

[0139] For specimens Nos. 2 and 4, Si, which has a high oxidation tendency, was unable to diffuse to the surface and formed an internal oxidation layer by combining with oxygen that had penetrated into the steel plate due to the high dew point temperature in the high atmosphere. Therefore, it can be confirmed that the surface concentration of Si is higher for specimens Nos. 1 and 3 than for specimens Nos. 2 and 4.

[0140] In the case of specimen numbers 1 and 3, if Si and Mn do not form an internal oxide layer but diffuse to the surface to form surface oxide, it is difficult to remove the Si surface oxide even if pickling is performed thereafter. It can be confirmed that Mn, which is easily soluble in hydrochloric acid compared to Si, was removed after pickling. However, if secondary annealing is performed with a large amount of Si oxide formed on the surface, oxygen in the atmosphere is adsorbed to the steel sheet by the Si surface oxide and is prevented from diffusion into the interior, and the decarburization reaction in which carbon inside the steel sheet diffuses to the surface and combines with the adsorbed oxygen is also disadvantageous.

[0141] In addition, using the cold-rolled steel sheet having the composition of Table 1, first annealing, first pickling, second annealing, and plating were performed under the conditions shown in Table 3 below. At this time, during the first annealing, the atmosphere was a reducing atmosphere of nitrogen gas containing 5% hydrogen. In addition, during the first annealing, the temperature was increased to 800°C at a heating rate of 3°C / s, maintained for 150 seconds, and then cooled, followed by immersion in a 5 vol% hydrochloric acid solution at 50°C for 5 seconds to perform the first pickling. Subsequently, during the second annealing, the atmosphere was a reducing atmosphere of nitrogen gas containing 5% hydrogen, and during the second annealing, the temperature was increased to 810°C at a heating rate of 3.2°C / s, maintained for 50 seconds, and then cooled. During cooling, the first cooling was performed at 3.1°C / s to 650°C, and the second cooling was performed at 20°C / s to 350°C. After the second cooling, the temperature was reheated to 470°C to perform GA plating. At this time, the GA alloying temperature was 500°C.

[0142] The surface concentrations of Si and Mn and the depths of the internal oxidation layer and decarburization layer were measured for the manufactured steel plate using the above-described method, and are shown in Table 3 below.

[0143] In addition, in order to evaluate the plating surface quality, after performing alloy zinc plating, the level of unplated area on the plating surface was visually evaluated and displayed as the grade below.

[0144] 1: No plating observed

[0145] 2: Some plating was observed

[0146] 3: Multiple dot platings observed

[0147] 4: Product is unavailable due to large unplated defects.

[0148] 5: Plating is not possible due to non-plating on the front.

[0149] In addition, the maximum length of B type LME cracks was recorded to evaluate LME resistance. LME evaluation was performed according to the SEP 1220-2 standard, and spot welding was performed by overlapping the manufactured galvanized steel sheets and galvanized steel sheets without decarburization layer. LME crack lengths were measured by observing the welded galvanized steel sheet specimens by shearing them in the 0°, 45°, and 90° directions using OM (Optical Microscopy). Only B type cracks in the heat-affected zone were measured, and the maximum crack lengths are shown in Table 3 below.

[0150] Specimen number Steel grade 1st annealing 2nd annealing After 2nd annealing 2nd annealing and plating After plating Surface quality B type LME Maximum crack length (μm) Classification Dew point temperature (℃) Dew point temperature (℃) Si surface enrichment amount Mn surface enrichment amount Internal oxidation layer depth (μm) Decarburization layer depth (μm) 1A Not implemented - 400.2 100.3 470 5564 Comparative example 12A Not implemented 50.0 560.2 182 6183 42 Comparative example 23A - 40 - 400.3 031.2 190 2566 Comparative example 34A - 40 50.2 521.0 342 316 438 Comparative example 45A 5 - 400.0 490.1 054.4 2517 Invention example 16A 5 5 0.0 1 1 0.0 9 16.25 51 Unpublished invention example 27B Unimplemented - 400.2150.31804577 Comparative example 58B Unimplemented 50.0620.2132.815352 Comparative example 69B-40-400.3101.19805569 Comparative example 710B-4050.2561.1522.114444 Comparative example 811B5-400.0570.1283.8481 Unpublished invention example 312B550.0140.0876.7731 Unpublished invention example 4

[0151] As shown in Table 3 above, the invention examples that satisfy the alloy composition and manufacturing method proposed in the present invention have secured all the desired characteristics. In particular, invention examples 1 and 2 are examples in which internal oxidation was performed in the first annealing while performing two annealings. In invention example 1, even though internal oxidation was not performed during the second annealing, an internal oxidation layer was formed to a certain depth or more, and as a result, the plating quality was excellent and the LME resistance was also improved. This is the result of performing internal oxidation in the first annealing to form a sufficient level of Mn and Si deficiency layer, and performing the second annealing in a clean state after the Mn surface oxide formed on the surface due to pickling after annealing was removed. If the surface of the steel sheet after the first annealing is clean, there will be almost no oxide on the surface of the steel sheet during the second annealing, and even if internal oxidation is not performed during the second annealing, the amount of Si and Mn diffusing to the surface may be limited due to the already existing deficiency layer. As a result, the Si surface concentration of the final surface is very low. Inventive Example 2 shows a cleaner surface state by performing internal oxidation even during the second annealing. The clean steel sheet surface with almost no surface oxide after the first annealing and first pickling allows oxygen to penetrate more easily and deeply into the steel sheet during the second annealing, so that almost no alloying elements diffuse to the surface during the second annealing. In addition, during the second annealing in the clean surface state, the decarburization reaction caused by carbon diffusing to the steel sheet surface can actively occur. As a result, the plating quality is excellent, and LME cracking did not occur.

[0152] Invention examples 3 and 4 also show similar trends to invention examples 1 and 2.

[0153] On the other hand, Comparative Examples 1 and 2 are examples in which only the secondary annealing was performed without the primary annealing. In Comparative Example 1, no internal oxidation layer was observed, and the wettability of the plating layer was very poor. After the secondary annealing, the amount of Si surface enrichment was relatively high, which means that a large amount of Si oxide was formed on the surface. In addition, the depth of the decarburization layer was also observed to be shallow. As a result, the LME resistance was poor. In Comparative Example 2, the depth of the internal oxidation layer was formed to be 2.6 μm, so surface oxidation was suppressed to a certain level, and the amount of Si surface enrichment also increased compared to Comparative Example 1, but unplating occurred frequently. In addition, the decarburization layer was also formed to be 18 μm, but the surface quality and LME resistance targeted by the present invention were still not secured.

[0154] Comparative Examples 3 and 4 are examples in which first and second annealing were performed, but internal oxidation was not performed during the first annealing. In Comparative Example 3, a large amount of surface oxide was formed during the first annealing, and the Mn surface oxide was removed through pickling, but the Si surface oxide remained. As the second annealing proceeded in this state, the surface oxide accumulated further, resulting in a very poor plating surface quality. In addition, since the internal oxidation layer and decarburization layer were hardly observed, the LME resistance was poor. In Comparative Example 4, a large amount of surface oxide was formed during the first annealing, and the Si surface oxide remained after pickling, but during the second annealing, internal oxidation occurred, and the additional surface oxide due to the second annealing was relatively reduced. However, the plating quality level targeted by the present invention was not secured. The decarburization layer depth was also not formed to the level suggested by the present invention, resulting in poor LME resistance.

[0155] Comparative Examples 2, 4, and Inventive Example 1 are all cases where internal oxidation was performed once. Meanwhile, the first annealing of the present invention for generating internal oxidation has a longer maintenance time than the second annealing, and the time maintained at high temperature is dominant in forming the internal oxidation layer and decarburization layer. Therefore, the depth of the internal oxidation layer and decarburization layer of Inventive Example 1, which performed internal oxidation in the first annealing, was relatively deeper than that of Comparative Examples 2 and 4, which performed internal oxidation in the second annealing.

[0156] Comparative examples 5 to 8 also show similar trends to comparative examples 1 to 4.

[0157] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. A steel sheet containing, by weight%, C: 0.10 to 0.25%, Mn: 1.5 to 5.0%, Si: 0.5 to 2.5%, Cr: 1.5% or less, Al: 0.005 to 0.100%, P: 0.10% or less, S: 0.020% or less, B: 0.0050% or less, the remainder being Fe and other unavoidable impurities; and Including a plating layer formed on at least one surface of the above steel plate; A plated steel sheet having an internal oxide layer composed of one or more oxides of Mn, Si, Cr, and B, the depth of which is 3 to 15 μm in the direction of the center of thickness from the interface between the base steel sheet and the plated layer.

2. In claim 1, The above-mentioned steel sheet is a plated steel sheet further containing, by weight %, 1.2% or less of at least one of Ti, Mo, and Nb.

3. In claim 1, A plated steel sheet having a depth of decarburization of 20 to 150 μm in the direction of the center of thickness from the interface between the base steel sheet and the plated layer.

4. In claim 1, The above-mentioned galvanized steel sheet is a galvanized steel sheet having an LME crack maximum length of 10 μm or less.

5. In claim 1, The above plating layer is a zinc-based or zinc alloy-based plating layer of a plated steel sheet.

6. A step for preparing a base steel sheet containing, by weight%, C: 0.10 to 0.25%, Mn: 1.5 to 5.0%, Si: 0.5 to 2.5%, Cr: 1.5% or less, Al: 0.005 to 0.100%, P: 0.10% or less, S: 0.020% or less, B: 0.0050% or less, the remainder being Fe and other unavoidable impurities; A step of first annealing the above steel plate at a temperature range of 600 to 900°C and a dew point temperature of -10 to 30°C; A step of first pickling the above first annealed steel plate; A step of annealing the first-stage steel plate in a temperature range of 700 to 900°C; and A method for manufacturing a plated steel sheet, comprising: a step of plating at least one surface of the secondarily annealed steel sheet.

7. In claim 6, A method for manufacturing a plated steel sheet, wherein the above-mentioned steel sheet further contains, in weight %, one or more of Ti, Mo, and Nb, in an amount of 1.2% or less.

8. In claim 6, The above-mentioned steel sheet is a method for manufacturing a galvanized steel sheet, which is a cold-rolled steel sheet.

9. In claim 6, The above first annealing step is maintained for 50 to 600 seconds, A method for manufacturing a plated steel sheet, wherein the above-mentioned second annealing step is performed at a dew point temperature of -60 to 30°C.

10. In claim 6, A method for manufacturing a plated steel sheet in which the first and second annealing steps are performed in a nitrogen gas atmosphere containing 1 to 80 vol% of hydrogen.

11. In claim 6, The above plating step is a method for manufacturing a plated steel sheet by performing zinc-based or zinc alloy-based plating.

12. In claim 6, A method for manufacturing a plated steel sheet further comprising the step of performing alloying heat treatment on the plated steel sheet.

13. In claim 6, A method for manufacturing a plated steel sheet further comprising a step of metal plating after the first acid pickling step.

14. In claim 13, A method for manufacturing a plated steel sheet further comprising a step of metal plating before the above-mentioned secondary annealing step.

15. In claim 13 or 14, A method for manufacturing a plated steel sheet wherein the above metal plating step is Fe or Ni plating.

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