Coated steel sheet and its manufacturing method

Abstract

This invention relates to a clad steel sheet and a method for manufacturing the same. The clad steel sheet comprises: a base steel sheet, which, by weight percent, contains: C: 0.10-0.25%, Mn: 1.5-5.0%, Si: 0.5-2.5%, Cr: less than 1.5%, Al: 0.005-0.100%, P: less than 0.10%, S: less than 0.020%, B: less than 0.0050%, with the balance being Fe and other unavoidable impurities; and a coating formed on at least one side of the base steel sheet, wherein, with reference to the direction from the interface between the base steel sheet and the coating toward the thickness center, the depth of the internal oxide layer composed of one or more oxides of Mn, Si, Cr and B is 3-15 μm. More specifically, the invention relates to a clad steel sheet with excellent surface quality and weld LME resistance, and a method for manufacturing the same.

Description

Technical Field

[0001] This invention relates to a galvanized steel sheet and a method for manufacturing the same, and more specifically, to a galvanized steel sheet with excellent surface quality and weld LME resistance, and a method for manufacturing the same. Background Technology

[0002] In recent years, the technological development of automotive steel has focused on achieving lightweighting through increased strength. To achieve high strength in steel, various alloying elements are sometimes added to utilize precipitation strengthening and solid solution strengthening, and advanced high-strength steel (AHSS) that induces phase transformation during annealing is being actively developed.

[0003] Elements such as Mn, Si, Cr, and B are representative examples that can be added to achieve high strength in steel. 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 reduce the surface reactivity of the steel sheet and may contribute to deterioration of its processing or plating properties.

[0004] Furthermore, with the increasing strength of steel, a problem arises during spot welding: cracks form in the heat-affected zone due to liquid metal embrittlement (LME). This occurs during welding when the coating on the hot-dip galvanized steel sheet becomes liquid and penetrates along the grain boundaries on the steel sheet surface.

[0005] One representative method for reducing LME (Limited Metal Erosion) is to form a soft decarburized layer on the surface of the base steel plate. This is based on the effect of the surface decarburized layer in suppressing the occurrence of cracks caused by tensile stress generated during spot welding.

[0006] Various techniques have been proposed to suppress surface oxides formed on the steel sheet surface during annealing. Patent Document 1 controls the air-fuel ratio to 0.80-0.95 during annealing, oxidizing the steel sheet in a direct flame furnace with an oxidizing atmosphere, causing iron oxides containing individual or combined oxides of Si, Mn, or Al to form to a certain depth within the steel sheet. Subsequently, the iron oxides are reduced and annealed in a reducing atmosphere, followed by hot-dip galvanizing, providing a hot-dip galvanized or alloyed hot-dip galvanized steel sheet with excellent coating quality.

[0007] However, in steel grades with a certain or higher Si addition, Si concentrates beneath the iron oxide during reduction, forming banded Si oxides, which then cause peeling at the surface. That is, peeling occurs at the interface between the reduced iron and the base iron, potentially making it difficult to ensure sealant adhesion and coating adhesion. Furthermore, it is insufficient for the formation of a decarburized layer to improve LME resistance.

[0008] Patent document 2 discloses a method that maintains a high dew point in the annealing furnace, causing easily oxidized alloy components such as Mn, Si, and Al to undergo internal oxidation inside the steel, thereby reducing the oxides that oxidize on the outside of the steel plate surface after annealing and improving plating performance.

[0009] The method described in Patent Document 2 can solve the plating problem caused by the external oxidation of Si, which is easily oxidized internally. However, when a large amount of Mn, which is relatively difficult to oxidize internally, is added, its effect may be negligible. In addition, there are limitations to the surface oxidation reduction effect of Si, Mn, etc., and the formation of a sufficient decarburized layer that can be achieved through a single internal oxidation application.

[0010] (Patent Document 1) Korean Patent Publication No. 10-2010-0030627 (published on March 18, 2010) (Patent Document 2) Korean Patent Publication No. 10-2009-0006881 (Published on January 15, 2009) Summary of the Invention

[0011] (a) Technical problems to be solved According to one embodiment of the present invention, a galvanized steel sheet and a method for manufacturing the same are intended to be provided.

[0012] According to one embodiment of the present invention, it is intended to provide a coated steel sheet with excellent surface quality and weld LME resistance, and a method for manufacturing the same.

[0013] The technical problems addressed by this invention are not limited to those described above. Those skilled in the art will have no difficulty understanding the additional technical problems addressed by this invention from the overall content of this specification.

[0014] (II) Technical Solution According to one embodiment of the present invention, a coated steel sheet may be provided, comprising: a base steel sheet, which, by weight percent, comprises: C: 0.10-0.25%, Mn: 1.5-5.0%, Si: 0.5-2.5%, Cr: less than 1.5%, Al: 0.005-0.100%, P: less than 0.10%, S: less than 0.020%, B: less than 0.0050%, with the balance being Fe and other unavoidable impurities; and a coating formed on at least one side of the base steel sheet, wherein, with reference to the direction from the interface between the base steel sheet and the coating toward the thickness center, the depth of the internal oxide layer composed of one or more oxides of Mn, Si, Cr and B is 3-15 μm.

[0015] The base steel plate may further contain less than 1.2% of one or more of Ti, Mo and Nb by weight.

[0016] Based on the direction from the interface between the base steel plate and the coating towards the thickness center, the depth of the decarburized layer can be 20-150μm.

[0017] The maximum length of the LME crack in the galvanized steel sheet can be less than 10 μm.

[0018] The coating may be a zinc-based coating or a zinc alloy-based coating.

[0019] According to one embodiment of the present invention, a method for manufacturing a coated steel sheet includes the following steps: preparing a base steel sheet, which, by weight percent, comprises: C: 0.10-0.25%, Mn: 1.5-5.0%, Si: 0.5-2.5%, Cr: less than 1.5%, Al: 0.005-0.100%, P: less than 0.10%, S: less than 0.020%, B: less than 0.0050%, with the balance being Fe and other unavoidable impurities; annealing the base steel sheet once at a temperature range of 600-900°C and a dew point temperature of -10°C to 30°C; pickling the base steel sheet after the first annealing; annealing the base steel sheet after the first pickling once at a temperature range of 700-900°C; and coating at least one side of the base steel sheet after the second annealing.

[0020] The base steel plate may further contain less than 1.2% of one or more of Ti, Mo and Nb by weight.

[0021] The base steel plate can be a cold-rolled steel plate.

[0022] The annealing step is held for 50-600 seconds. The secondary annealing step can be carried out at a dew point temperature ranging from -60°C to 30°C.

[0023] The primary and secondary annealing steps can be performed in a nitrogen atmosphere containing 1-80% by volume of hydrogen.

[0024] The plating step can be zinc-based plating or zinc alloy-based plating.

[0025] It may further include a step of alloying heat treatment on the plated steel sheet.

[0026] It can be further included as a metal plating step after the first pickling step.

[0027] It can be further included as a step of metal plating before the secondary annealing step.

[0028] The metal plating step can be Fe plating or Ni plating.

[0029] (III) Beneficial Effects According to one embodiment of the present invention, a galvanized steel sheet and a method for manufacturing the same can be provided.

[0030] According to one embodiment of the present invention, a coated steel sheet with excellent surface quality and weld LME resistance and a method thereof can be provided.

[0031] According to one embodiment of the present invention, a coated steel sheet with excellent surface quality and weld LME resistance, which can be used for automotive steel, and a method thereof can be provided.

[0032] The various and beneficial advantages and effects of the present invention are not limited to those described above, and can be more easily understood in the process of describing specific embodiments of the present invention. Attached Figure Description

[0033] Figure 1 A method for calculating the Mn surface enrichment of a steel plate according to an embodiment of the present invention is shown.

[0034] Figure 2 This is a cross-sectional SEM image of Example 3 of the present invention, according to one embodiment of the present invention, showing a method for measuring the depth of the internal oxide layer.

[0035] Figure 3 The present invention illustrates a method for determining the decarburized layer depth of a steel plate according to an embodiment of the invention, which is based on a graph of carbon concentration at the depth of the steel plate during glow discharge spectrometer (GDS) analysis.

[0036] Figure 4A method is shown for determining the interface position between the base steel plate and the coating from GDS data to determine the decarburization layer depth of a steel plate according to an embodiment of the present invention.

[0037] Figure 5 This is a cross-sectional SEM image of a steel plate according to one embodiment of the present invention, illustrating a method for determining the decarburized layer depth of a steel plate etched with nitric acid alcohol. Best practice

[0038] The preferred embodiments of the present invention will be described below. The specific embodiments of the present invention can be modified in many ways, and the scope of the present invention should not be construed as limited to the specific embodiments described below. These specific embodiments are provided to illustrate the present invention in more detail to those skilled in the art.

[0039] The present invention will now be described in detail.

[0040] According to one embodiment of the present invention, a coated steel sheet may include: a base steel sheet; and a coating formed on at least one side of the base steel sheet.

[0041] The steel composition of the present invention will be described in detail below.

[0042] In this invention, unless otherwise specified, the percentage of each element content is based on weight.

[0043] By weight percent, the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention may contain: C: 0.10-0.25%, Mn: 1.5-5.0%, Si: 0.5-2.5%, Cr: less than 1.5%, Al: 0.005-0.100%, P: less than 0.10%, S: less than 0.020%, and B: less than 0.0050%.

[0044] Carbon (C): 0.10-0.25% Carbon (C) is an important element that can be added to stabilize retained austenite, and to ensure this effect, it can be added at a level of 0.10% or higher. According to one embodiment of the invention, carbon (C) can be added at a level of 0.15% or higher. On the other hand, when the carbon (C) content exceeds 0.25%, a problem of deteriorated weldability may occur. According to one embodiment of the invention, it can be 0.23% or lower.

[0045] Manganese (Mn): 1.5-5.0% Besides contributing to the formation and stabilization of retained austenite, manganese (Mn) also inhibits ferrite phase transformation during cooling, making it an essential element in transformation-structured steels. Furthermore, to ensure sufficient austenite to guarantee strength and ductility, manganese (Mn) content of 1.5% or more can be included. According to one embodiment of the invention, it can be 2.0% or more. On the other hand, when the Mn content exceeds 5.0%, excessive band formation due to segregation generated during slab heating and hot rolling processes may occur, potentially impairing physical properties. Therefore, according to one embodiment of the invention, the upper limit of the manganese (Mn) content can be limited to 5.0%. According to another embodiment of the invention, it can be 3.0% or less.

[0046] Silicon (Si): 0.5-2.5% Silicon (Si) is an element that inhibits the precipitation of carbides within ferrite and promotes the diffusion of carbon from ferrite to austenite, thus contributing to the stabilization of retained austenite. To achieve the effects described above, silicon (Si) can be added at 0.5% or more. According to one embodiment of the invention, it can be 0.7% or more. However, excessive addition may reduce surface reactivity; therefore, the upper limit of the silicon (Si) content is limited to 2.5%. According to one embodiment of the invention, it can be 2.0% or less.

[0047] Chromium (Cr): less than 1.5% Chromium (Cr), as a hardenability-enhancing element, plays a role in inhibiting ferrite formation. Therefore, to ensure adequate retained austenite, it can be added in small amounts as needed. According to one embodiment of the invention, the chromium (Cr) content can be 0%. However, when the Cr content is too high, the amount of alloy iron used will be excessive, leading to increased costs; therefore, the upper limit of the chromium (Cr) content is limited to 1.5%. According to one embodiment of the invention, it can be below 1.0%.

[0048] Aluminum (Al): 0.005-0.100% Aluminum (Al) is an element that helps stabilize retained austenite by inhibiting the formation of carbides within ferrite. To achieve the effects described above, it can be added at 0.005% or more. According to one embodiment of the invention, it can be 0.010% or more. However, when the Al content exceeds 0.100%, it is difficult to produce a sound slab during casting due to its reaction with the protective slag. Furthermore, surface oxides are formed, hindering hot-dip coating. Therefore, the upper limit of the aluminum (Al) content is limited to 0.100%. According to one embodiment of the invention, it can be 0.08% or less.

[0049] Phosphorus (P): less than 0.10% Phosphorus (P) is a solid solution strengthening element, but when the P content exceeds 0.10%, weldability decreases and the risk of steel becoming brittle increases. Therefore, its upper limit is set at 0.10%.

[0050] Sulfur (S): below 0.020% Sulfur (S), as an impurity element, hinders the ductility and weldability of steel sheets. Therefore, as the sulfur (S) content increases, the likelihood of hindering the ductility and weldability of steel sheets increases. With this in mind, its upper limit is set at 0.020%.

[0051] Boron (B): below 0.0050% Boron (B) is an element that can be added to ensure strength. However, when the boron (B) content exceeds 0.0050%, it will concentrate on the surface of the annealed material, significantly reducing the surface quality. Therefore, its content is limited to below 0.0050%.

[0052] In addition to the above-described composition, the steel of this invention may contain remaining iron (Fe) and unavoidable impurities. These unavoidable impurities may be unintentionally introduced during ordinary manufacturing processes and therefore cannot be eliminated. These impurities are well known to those skilled in the art of steel manufacturing, and therefore, not all details are specifically mentioned in this specification.

[0053] By weight percent, the base steel sheet of the coated steel sheet according to one embodiment of the present invention may further contain less than 1.2% of one or more of Ti, Mo and Nb.

[0054] One or more of titanium (Ti), molybdenum (Mo), and niobium (Nb): less than 1.2% Titanium (Ti) can form nitrides, reducing the N concentration in steel. On the other hand, excessive inclusion can lead to a decrease in the carbon concentration of martensite and a reduction in strength due to carbide precipitation.

[0055] Molybdenum (Mo) can contribute to increased strength. In particular, its ability to maintain wettability with molten metals such as zinc ensures strength.

[0056] Niobium (Nb) segregates at the austenite grain boundaries in the form of carbides, which can suppress the coarsening of austenite grains during annealing heat treatment and increase strength. However, excessive use can lead to increased costs.

[0057] With this in mind, one or more of the titanium (Ti), molybdenum (Mo), and niobium (Nb) can be included in less than 1.2%.

[0058] The steel microstructure of the present invention will be described in detail below.

[0059] According to one embodiment of the present invention, the depth of the internal oxide layer composed of one or more oxides of Mn, Si, Cr and B can be 3-15 μm, with reference to the direction from the interface between the base steel sheet and the coating towards the thickness center.

[0060] Although internal oxidation occurs simultaneously with the formation of an internal oxide layer along the grain boundary and an internal oxide layer formed within the grain, the depth of the internal oxide layer according to one embodiment of the present invention can be based on the internal oxide layer at the grain boundary.

[0061] When the depth of the internal oxide layer is less than 3 μm, the internal oxidation is insufficient, resulting in a relatively increased surface oxide layer formed by diffusion, which may lead to concerns about insufficient improvement in surface quality. According to one embodiment of the present invention, the depth of the internal oxide layer can be 4 μm or more. According to another embodiment of the present invention, the depth of the internal oxide layer can be 5 μm or more. On the other hand, when the depth of the internal oxide layer exceeds 15 μm, the heat treatment time required to form a deep internal oxide layer is too long, which may lead to a decrease in economic efficiency.

[0062] According to one embodiment of the present invention, the depth of the decarburized layer of the coated steel sheet can be 20-150 μm, with reference to the direction from the interface between the base steel sheet and the coating towards the thickness center.

[0063] When the depth of the decarburized layer is less than 20 μm, the thickness of the soft layer is insufficient, which may lead to inadequate improvement in tensile strength (LME). According to one embodiment of the present invention, the depth can be 30 μm or more. According to another embodiment of the present invention, the depth can be 35 μm or more. According to another embodiment of the present invention, the depth can be 40 μm or more. According to another embodiment of the present invention, the depth can be 50 μm or more. On the other hand, when the depth of the decarburized layer exceeds 150 μm, the problem of decreased tensile strength due to unnecessary soft layer thickness may exist.

[0064] According to one embodiment of the present invention, the coating of the plated steel sheet can be a zinc-based coating or a zinc alloy-based coating.

[0065] According to one embodiment of the present invention, the maximum length of the LME crack in the coated steel sheet can be less than 10 μm.

[0066] According to one embodiment of the present invention, the maximum length of the LME crack is evaluated according to SEP1220-2 specification. The coated steel sheet of the present invention and a coated steel sheet without a decarburized layer are overlapped and spot-welded. Then, the welded coated steel sheets are cut at 0°, 45°, and 90° for each specimen, and observed using an optical microscope (OM) to measure the LME crack length. At this time, only type B cracks in the heat-affected zone are measured, and the maximum crack length is shown.

[0067] The steel manufacturing method of the present invention will be described in detail below.

[0068] According to one embodiment of the present invention, the plated steel sheet can be manufactured by performing a first annealing, a first pickling, a second annealing, and a plated coating on a base steel sheet that satisfies the above alloy composition.

[0069] Preparation of foundation steel plates A base steel plate with an alloy composition that meets one embodiment of the present invention can be prepared.

[0070] According to one embodiment of the present invention, the base steel plate may be a cold-rolled steel plate. The manufacturing method of the cold-rolled steel plate according to one embodiment of the present invention is not particularly limited and may be any common conditions applicable in the same technical field.

[0071] First annealing The base steel plate can be annealed once at a temperature range of 600-900℃ and a dew point temperature of -10℃ to 30℃ for 50-600 seconds.

[0072] According to one embodiment of the present invention, an internal oxide layer and a decarburized layer can be effectively formed by performing a single annealing process. During a single annealing process, by forming an internal oxide layer composed of oxides of Si, Mn, Cr, B, etc., to a certain depth from the surface of the steel plate, the Si, Mn, Cr, B, etc., dissolved in the surface layer of the steel plate can be depleted.

[0073] During a single annealing process, Si has a very high oxidation tendency, allowing it to oxidize internally and preventing the formation of surface oxides. On the other hand, Mn, with a relatively low oxidation tendency compared to Si, forms a large amount of surface oxides. These Mn-induced surface oxides can be removed by a single acid pickling process following the initial annealing.

[0074] An internal oxide layer is formed by a single annealing process, which reduces the Si oxide on the surface. Then, the Mn oxide is removed by acid washing, so the surface can be kept clean.

[0075] Furthermore, because the surface becomes clean after the first annealing, oxygen in the atmosphere is more easily adsorbed onto the steel plate surface during the second annealing. Carbon in the steel diffuses to the surface, and the decarburization reaction, which combines with the adsorbed oxygen, can be promoted. This ensures excellent plating quality, and by forming a sufficient decarburized layer, liquid metal embrittlement (LME) during welding can be reduced.

[0076] During a single annealing process, if the annealing temperature is below 600℃, alloying elements such as Si and Mn, used to form the internal oxide layer, are difficult to diffuse to the grain boundaries of the surface layer. The diffusion of carbon used for decarburization of the surface layer to the surface may also be hindered. On the other hand, when the annealing temperature exceeds 900℃, although internal oxidation and decarburization can occur, the energy consumption for high-temperature holding is excessive, resulting in an uneconomical problem.

[0077] During a single annealing process, if the dew point temperature is below -10°C, the oxygen partial pressure in the atmosphere is insufficient, potentially hindering oxygen penetration into the steel plate. According to one embodiment of the invention, the dew point temperature can be above 0°C. On the other hand, when the dew point temperature exceeds 30°C, it may become an atmosphere where even iron is oxidized. According to one embodiment of the invention, to prevent iron oxidation and more effectively oxidize the interior of Si, the dew point temperature can be limited to below 20°C.

[0078] When annealing for a single time, a holding time of less than 50 seconds may not be sufficient to form an internal oxide layer and decarburized layer. On the other hand, when the holding time exceeds 600 seconds, the diffusible alloying elements in the surface layer have already fully reacted, making it difficult to expect any additional effects and also disadvantageous from an economic perspective.

[0079] According to one embodiment of the invention, during a single annealing process, the annealing atmosphere can be a nitrogen atmosphere containing 1-80% by volume of hydrogen. When the hydrogen content is less than 1% by volume, the iron in the steel sheet may be oxidized. On the other hand, when the hydrogen content exceeds 80% by volume, there is a risk of explosion in case of gas leakage, and the cost increases.

[0080] According to one embodiment of the present invention, during a single annealing process, in order to obtain the desired fine microstructure after holding, two or more cooling stages may be performed. During a single annealing process, the cooling rate is not particularly limited and can be any condition applicable in the same technical field. According to one embodiment of the present invention, during a single annealing process, cooling can be performed at a cooling rate of 1-100°C / second.

[0081] When the cooling rate is low, gas cooling with nitrogen containing a certain amount of hydrogen can be used. To increase the cooling rate, water spray cooling, water quenching, or water jet cooling can be employed. When wet cooling with a large amount of water is used, the steel plate is in direct contact with the water, and the dew point temperature caused by water vapor will rise sharply, potentially forming an iron oxide film on the surface. In this case, the iron oxide film needs to be removed by pickling after cooling.

[0082] Alternatively, the cooling atmosphere can be a nitrogen atmosphere containing 1-80% by volume of hydrogen.

[0083] Furthermore, according to one embodiment of the present invention, the heating rate can be 1-50°C / second during a single annealing process.

[0084] One pickling The base steel plate that has undergone the first annealing can be pickled once.

[0085] According to one embodiment of the present invention, oxides formed on the surface of a steel plate during a single pickling process can be removed by a single pickling process.

[0086] During a single pickling operation, the type of acid is not particularly limited, but according to one embodiment of the invention, a 3-20% by weight acid solution at 30-80°C can be used. According to one embodiment of the invention, pickling can be performed using 5% by weight hydrochloric acid at 50-60°C. According to one embodiment of the invention, when applying strong acid pickling, 18% by weight hydrochloric acid at 80°C can be used.

[0087] According to one embodiment of the present invention, metal plating can be performed after a single pickling process. According to one embodiment of the present invention, Fe, Ni, etc., can be plated as the metal plating. By performing the additional metal plating process, it is helpful to prevent the diffusion of Si, Mn, etc., to the surface during secondary annealing. The metal plating conditions are not particularly limited and can be conditions applicable within the same technical field. According to one embodiment of the present invention, in the case of Fe plating, the adhesion amount can be 0.1-3 g / m². 2 According to one embodiment of the present invention, in the case of Ni plating, the adhesion amount can be 5-700 mg / m². 2 According to one embodiment of the present invention, during metal plating, a device on the exit side of a primary annealing line can be used.

[0088] Secondary annealing The base steel plate that has undergone the first pickling can be subjected to a second annealing at a temperature range of 700-900℃ and a dew point temperature of -60-30℃.

[0089] According to one embodiment of the present invention, the tensile physical properties of the target can be ensured by performing secondary annealing. During secondary annealing, due to the internal oxide layer formed during the primary annealing process, annealing is performed in a state where the concentration of Si, Mn, Cr, B, etc., in a solid solution state at a certain depth from the steel plate surface is significantly reduced. Because of this depleted layer, the surface oxides of Si, Mn, Cr, and B that diffuse to the steel plate surface during secondary annealing can be significantly suppressed.

[0090] In other words, by performing one annealing and one pickling before the second annealing, the steel sheet after the second annealing can have a cleaner surface condition. A cleaner steel sheet has a larger exposed Fe area and improved surface reactivity, thus improving the coating quality. The less oxides on the surface of the annealed steel sheet, the better the hot-dip coating performance. This is because fewer surface oxides make it easier for the small amount of Al in the plating bath to react with the Fe in the steel sheet to form an inhibitory layer, and also improves the wettability of the liquid plating solution and the steel sheet. Various surface oxides that adversely affect coating performance include Mn, Si, Cr, and B, but Mn and Si are representative examples.

[0091] When performing secondary annealing, annealing temperatures below 700°C may not be sufficient to obtain the desired fine microstructure. On the other hand, when annealing temperatures exceed 900°C, the economic efficiency due to energy consumption decreases.

[0092] When secondary annealing occurs at dew point temperatures below -60°C, maintaining the atmosphere in large-scale production equipment presents significant practical difficulties. According to one embodiment of the invention, in addition to ensuring physical properties, the lower limit can be limited to -10°C to allow for deeper formation of the internal oxide and decarburized layers. According to another embodiment of the invention, it can be above 0°C. On the other hand, when the dew point temperature exceeds 30°C, iron is oxidized, making it difficult to attach and ensure the formation of the decarburized layer. According to one embodiment of the invention, it can be below 20°C.

[0093] According to one embodiment of the present invention, the holding time during secondary annealing can be 30-100 seconds. Furthermore, according to another embodiment of the present invention, the holding time for secondary annealing can be shorter than the holding time for primary annealing.

[0094] According to one embodiment of the invention, during secondary annealing, the annealing atmosphere can be a nitrogen atmosphere containing 1-80% by volume of hydrogen. When the hydrogen content is less than 1% by volume, the iron in the steel sheet may be oxidized. On the other hand, when the hydrogen content exceeds 80% by volume, there is a risk of explosion in case of gas leakage, and the cost increases.

[0095] During secondary annealing, the cooling conditions are not particularly limited. According to one embodiment of the present invention, after holding the temperature during secondary annealing, a slow cooling to 650°C can be performed, followed by rapid cooling according to the target physical properties. According to one embodiment of the present invention, the cooling conditions are not particularly limited, but different conditions can be applied to achieve the target physical properties. Furthermore, the formation of surface oxides, internal oxide layers, and decarburized layers mostly occurs in relatively high-temperature regions; therefore, cooling conditions are not particularly limited in the present invention.

[0096] In addition, according to one embodiment of the present invention, during the second annealing, the gas cooling and wet cooling using water can be performed in the same manner as during the first annealing.

[0097] According to one embodiment of the invention, in order to prevent oxidation of iron during cooling, a reducing atmosphere can be applied at least for iron. To maintain the reducing atmosphere, a nitrogen atmosphere containing 1-80% by volume of hydrogen can be used, similar to the atmosphere used during secondary annealing.

[0098] Furthermore, according to one embodiment of the present invention, the rapidly cooled steel plate can be reheated to a certain temperature as needed for tempering.

[0099] According to one embodiment of the present invention, metal plating can be performed additionally before secondary annealing. By performing the metal plating process before secondary annealing, it is helpful to prevent the diffusion of Si and Mn to the surface during secondary annealing. The metal plating conditions before secondary annealing can be applied in the same way as the metal plating process after primary pickling described above. According to one embodiment of the present invention, in the case of metal plating, a device on the inlet side of the secondary annealing line can be used.

[0100] Plating At least one side of the base steel plate that has undergone the secondary annealing can be plated.

[0101] According to one embodiment of the present invention, plating can be performed by reheating after secondary annealing. The type of plating is not particularly limited; according to one embodiment of the present invention, hot-dip plating can be performed. According to one embodiment of the present invention, a hot-dip galvanizing bath can be used for plating, and the conditions are not particularly limited during hot-dip plating. Furthermore, according to one embodiment of the present invention, alloying heat treatment can be performed as needed after plating. Detailed Implementation

[0102] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention for more detailed explanation and are not intended to limit the scope of the invention.

[0103] (Example) Prepare cold-rolled steel sheets with the compositions shown in Table 1 below. Using two types of steel sheets, perform one annealing and one pickling under the conditions shown in Table 2 below. During the first annealing, the atmosphere is nitrogen containing 5% hydrogen, used in a reducing atmosphere. Additionally, during the first annealing, heat to 800°C at a rate of 3°C / second and hold for 150 seconds, then cool and perform a first pickling by immersion in a 5% (v / v) hydrochloric acid solution at 50°C for 5 seconds.

[0104] [Table 1] Furthermore, Table 2 below shows the surface enrichment of Si and Mn before and after one annealing and one pickling, along with the analysis and display of the depth of the internal oxide and decarburized layers. The surface enrichment of Si and Mn before and after pickling can be estimated using the amount of Si and Mn oxides remaining on the steel plate surface. The surface enrichment of Si and Mn is the amount that diffuses to the surface during annealing, combines with oxygen in the atmosphere, and concentrates on the surface; it can be considered as an amount of oxidation.

[0105] The specific Si and Mn surface enrichment amounts can be calculated using GDS analysis. In the GDS data, the main components of each steel composition—Fe, Mn, Si, Cr, and B—are retained, with the total weight percentage of the main components representing 100% of the total weight. Here, Fe is the main component of the steel sheet and is therefore included, excluding oxygen. Furthermore, although components present in the steel sheet other than those mentioned above can be added for processing, the components concentrated to the surface during annealing are limited and present in very small quantities, and are therefore omitted.

[0106] Figure 1 A method for calculating the Mn surface enrichment of a steel plate according to an embodiment of the present invention is shown. The Si surface enrichment can also be calculated using the same method described later. Figure 1 As shown, a curve was plotted using the depth of the processed data as the x-axis and the weight percentage of each component as the y-axis. For each component, after limiting the depth to 0.1 μm, the minimum weight percentage of each component within 0.1 μm was found, and this value was estimated as the amount of the solid solution component. After finding the amount of the solid solution component, data at deeper depths were deleted based on this position. The amount of the solid solution component was subtracted from the weight percentage of each component from the position of the solid solution component from the surface. This process was performed to extract only oxides from the GDS data, excluding the values ​​of solid-solid Mn or Si. Then, using the data after subtracting the solid solution component value, the integral value from the position of the solid solution component value can be estimated as the enrichment amount. When the minimum weight percentage value of each component occurs at the outermost point between a depth of 0-0.01 μm, it can be interpreted as that the component has almost no surface concentration.

[0107] In addition, the internal oxide layer depth in Table 2 was recorded by observing the cross-section of the annealed steel plate at 3000x magnification using SEM, and the internal oxide depth of the grain boundaries was recorded. Figure 2 This is a cross-sectional SEM image of Example 3 of the invention, according to an embodiment of the present invention, illustrating a method for measuring the depth of the internal oxide layer. For example... Figure 2 As shown, three points can be randomly measured in a 3000x magnification image, and the average value can be derived.

[0108] The specific decarburized layer depth can be derived by analyzing the carbon content at different depths using glow discharge spectroscopy (GDS). There are two methods. The first is to analyze the region where the carbon content curve is horizontal within the measurable depth of GDS, i.e., when GDS analysis can exceed the decarburized layer depth and reach the matrix tissue. The decarburized layer depth in this case is as follows: Figure 3 That's how you find it. Figure 3 The present invention illustrates a method for determining the decarburized layer depth of a steel plate according to an embodiment of the present invention, which is based on a graph of carbon concentration at the depth of the steel plate during GDS analysis. Figure 4 This paper illustrates a method for determining the interface position between the base steel plate and the coating from GDS data to determine the decarburized layer depth of a steel plate according to an embodiment of the present invention. Firstly, as... Figure 4 As shown, the interface between the coating and the base steel sheet is located from the GDS data. This interface is the point where the zinc and iron curves intersect, and this point is presumed as the starting point for decarburization on the base steel sheet surface. Then, a carbon content curve is plotted, and the average of the 10 deepest carbon content data points is calculated. This average of the 10 deepest carbon content data points is called the depth value. Next, from the deepest point towards the surface, the first carbon content depth representing a decrease of more than 5% from the depth value is identified. This point represents the deepest decarburized layer depth. The total decarburized layer depth of the coated steel sheet extends from the zinc and iron intersection point to the deepest decarburization depth.

[0109] The second scenario involves a very deep decarburized layer, exceeding the measurable range of GDS. In this case, analysis is performed using the 10 carbon content values ​​at the deepest depth as a benchmark, similar to the first scenario. Whether the carbon content at the deepest depth in the GDS data reaches a deep value can be predicted using optical emission spectrometry (OES) analysis. That is, analysis up to the deepest depth measurable by GDS will yield analytical values ​​at least 50 μm deep. When the carbon content at the deepest depth is lower than the OES carbon content, it has not yet reached a deep layer, thus the decarburized layer depth can be estimated to be at least 50 μm. To determine the approximate decarburized layer depth, cross-sectional microstructure observation is performed using nitric acid alcohol etching (ethanol or methanol containing 2-5 vol% nitric acid). Figure 5 This is a cross-sectional SEM image of a steel plate according to an embodiment of the present invention, illustrating a method for determining the decarburized layer depth of a steel plate etched with nitric acid alcohol. Figure 5 As shown, SEM can be used for observation at magnifications ranging from 1000x to 3000x. The decarburized structure on the surface layer shows coarse grains dominated by ferrite, while the deeper (matrix) layer shows a fine structure containing austenite. The depth up to the point where the coarse decarburized structure is dominant can also be considered the depth of the decarburized layer.

[0110] [Table 2] In Table 2 above, test pieces 1 and 3 represent cases where the dew point temperature is -40°C, which does not meet the conditions of this invention. Test pieces 2 and 4 represent cases where the dew point temperature meets the conditions of this invention. It can be confirmed that, compared with test pieces 1 and 3, the Si and Mn surface enrichment of test pieces 2 and 4 is significantly reduced.

[0111] In addition, looking at the results after acid washing, it can be confirmed that the Si surface enrichment of test pieces 1 and 3 is reduced compared with that before acid washing. However, test pieces 2 and 4 also had a small amount of Si enrichment due to internal oxidation before acid washing, so the enrichment after acid washing is much lower than that of test pieces 1 and 3.

[0112] The surface enrichment of Mn also decreased, but the reduction was smaller compared to that of Si. After pickling, the surface enrichment of Mn was significantly reduced compared to before pickling. This is because Mn oxides are easily dissolved by acid. Before pickling, in the cases of specimens 2 and 4 where the dew point temperature met the conditions of this invention, the surface enrichment of Mn was lower than that of specimens 1 and 3 due to the formation of an internal oxide layer, and this trend was also observed after pickling.

[0113] It can be confirmed that in the cases of test piece number 1 and test piece number 3, no internal oxide layer was formed, and the decarburized layer was less than 7μm, which was almost non-existent.

[0114] In the cases of specimens 2 and 4, the Si with a high oxidation tendency did not diffuse to the surface. Due to the high dew point temperature in the atmosphere, it combined with oxygen that had penetrated into the steel plate to form an internal oxide layer. Therefore, it can be confirmed that the Si surface enrichment is higher in specimens 1 and 3 than in specimens 2 and 4.

[0115] In the cases of test pieces 1 and 3, when Si and Mn diffuse to the surface to form surface oxides instead of forming an internal oxide layer, the Si surface oxides are difficult to remove even after subsequent pickling. Mn, which is more soluble in hydrochloric acid than Si, can be confirmed to be removed after pickling. However, when secondary annealing is performed with a large amount of Si oxide formed on the surface, the Si surface oxides hinder the adsorption of oxygen in the atmosphere onto the steel plate and its diffusion inwards. This also impedes the decarburization reaction, where carbon from inside the steel diffuses to the surface and combines with the adsorbed oxygen.

[0116] Furthermore, using cold-rolled steel sheets with the composition shown in Table 1 above, a first annealing, a first pickling, a second annealing, and plating were performed under the conditions shown in Table 3 below. During the first annealing, the atmosphere was nitrogen containing 5% hydrogen, and the process was carried out in a reducing atmosphere. During the first annealing, the temperature was increased to 800°C at a heating rate of 3°C / second and held for 150 seconds, followed by cooling and a first pickling process of immersion in a 5% (v / v) hydrochloric acid solution at 50°C for 5 seconds. Next, during the second annealing, the atmosphere was nitrogen containing 5% hydrogen, and the process was carried out in a reducing atmosphere. During the second annealing, the temperature was increased to 810°C at a heating rate of 3.2°C / second and held for 50 seconds, followed by cooling. During cooling, a first cooling was performed at 3.1°C / second to 650°C, followed by a second cooling at 20°C / second to 350°C. After the second cooling, the temperature was reheated to 470°C for GA hot-dip plating. At this time, the GA alloying temperature was 500°C.

[0117] For the manufactured steel plates, the surface enrichment of Si and Mn, as well as the depth of the internal oxide layer and decarburized layer, were measured using the above method, as shown in Table 3 below.

[0118] In addition, in order to evaluate the quality of the coated surface, after alloying hot-dip galvanizing, the level of uncoated surface on the coated surface is visually evaluated and shown in the following grades.

[0119] 1: No spot-like uncoated areas were observed. 2: Some areas were observed to have uncoated spots. 3: Multiple spots of uncoated areas were observed. 4: Large areas of uncoated material render the product unusable. 5: The entire surface is uncoated and cannot be coated. In addition, to evaluate LME resistance, the maximum length of type B LME cracks was recorded. LME evaluation was conducted according to SEP1220-2 specifications, by overlapping and spot-welding manufactured galvanized steel sheets and galvanized steel sheets without a decarburized layer. LME cracks were measured by cutting the welded galvanized steel sheets at 0°, 45°, and 90° angles, observing the cracks using an optical microscope (OM), and measuring the lengths. Only type B cracks in the heat-affected zone were measured, and the maximum crack lengths are shown in Table 3 below.

[0120] [Table 3] As shown in Table 3 above, all inventive examples satisfying the alloy composition and manufacturing method proposed in this invention ensure the target characteristics. In particular, Inventive Examples 1 and 2 are examples of performing two annealing processes, with internal oxidation occurring in the first annealing. In Inventive Example 1, even without internal oxidation during the second annealing, an internal oxide layer of sufficient depth was formed, resulting in excellent plating quality and improved LME resistance. This is because internal oxidation was performed during the first annealing, forming a sufficiently high level of Mn and Si depletion layer. After annealing, the Mn surface oxides formed on the surface were removed by pickling, resulting in a clean second annealing state. When the surface of the steel plate is clean after the first annealing, there are almost no oxides on the steel plate surface during the second annealing. Even without internal oxidation during the second annealing, the amount of Si and Mn diffusing to the surface is limited due to the existing depletion layer. As a result, the final surface Si enrichment level is very low. Inventive Example 2, by also performing internal oxidation during the second annealing, shows an even cleaner surface state. After a single annealing and pickling, the clean steel sheet surface exhibits virtually no surface oxides. During a second annealing, oxygen can penetrate more easily and deeply into the steel sheet, resulting in minimal diffusion of alloying elements to the surface. Furthermore, the decarburization reaction, caused by carbon diffusion to the steel surface, can proceed actively during the second annealing process on the clean surface. Consequently, the plating quality is excellent, and no LME cracks occur.

[0121] Invention Examples 3 and 4 also confirm a tendency similar to that of Invention Examples 1 and 2.

[0122] On the other hand, Comparative Examples 1 and 2 are examples of projects that underwent only two annealing processes without a first annealing. No internal oxide layer was observed in Comparative Example 1, and the coating wettability was very poor. After the second annealing, the Si surface enrichment was relatively high, meaning that a large amount of Si oxide was formed on the surface. Furthermore, the decarburized layer depth was also observed to be shallow. As a result, LME resistance was poor. In Comparative Example 2, the internal oxide layer depth was 2.6 μm, surface oxidation was somewhat suppressed, and the Si surface enrichment was also higher than in Comparative Example 1, but many areas remained uncoated. Furthermore, the decarburized layer was also formed to 18 μm, but the surface quality and LME resistance at the target level of this invention were still not ensured.

[0123] Comparative Examples 3 and 4 are examples of single and double annealing, respectively, but without internal oxidation during the single annealing. In Comparative Example 3, a large amount of surface oxide formed during the single annealing. Although the Mn surface oxide was removed by pickling, the Si surface oxide remained. Secondary annealing under these conditions led to further accumulation of surface oxides, resulting in very poor plating surface quality. Furthermore, since almost no internal oxide layer or decarburized layer was observed, LME resistance was poor. In Comparative Example 4, a large amount of surface oxide formed during the single annealing. While Si surface oxide remained after pickling, internal oxidation occurred during the second annealing, thus relatively reducing the additional surface oxides resulting from the second annealing. However, the plating quality level targeted by the present invention was not ensured. The decarburized layer depth also did not reach the level proposed in this invention, resulting in poor LME resistance.

[0124] Comparative Examples 2, 4, and 1 all involve a single internal oxidation process. Furthermore, the holding time in the single annealing of this invention, used for internal oxidation, is longer than that in the double annealing, and the formation of the internal oxide layer and decarburized layer is dominated by the high-temperature holding time. Therefore, compared to Comparative Examples 2 and 4, where internal oxidation is performed in a double annealing process, the internal oxide layer and decarburized layer of Invention Example 1, where internal oxidation is performed in a single annealing process, are relatively deeper.

[0125] Comparative Examples 5 to 8 also confirm a similar tendency to that of Comparative Examples 1 to 4.

[0126] The present invention has been described in detail above through embodiments, but other embodiments are also possible. Therefore, the technical concept and scope of the claims set forth below are not limited to the embodiments.

Claims

1. A galvanized steel sheet, comprising: The base steel plate, by weight percent, comprises: C: 0.10-0.25%, Mn: 1.5-5.0%, Si: 0.5-2.5%, Cr: less than 1.5%, Al: 0.005-0.100%, P: less than 0.10%, S: less than 0.020%, B: less than 0.0050%, with the balance being Fe and other unavoidable impurities; and A coating, said coating being formed on at least one side of the base steel plate. Among them, taking the direction from the interface between the base steel plate and the coating towards the thickness center as a reference, the depth of the internal oxide layer composed of one or more oxides of Mn, Si, Cr and B is 3-15 μm.

2. The galvanized steel sheet according to claim 1, wherein, The base steel plate further comprises less than 1.2% of one or more of Ti, Mo and Nb by weight.

3. The galvanized steel sheet according to claim 1, wherein, Based on the direction from the interface between the base steel plate and the coating towards the thickness center, the depth of the decarburized layer is 20-150μm.

4. The galvanized steel sheet according to claim 1, wherein, The maximum length of the LME crack in the galvanized steel sheet is less than 10 μm.

5. The coated steel sheet according to claim 1, wherein, The coating is a zinc-based coating or a zinc alloy-based coating.

6. A method for manufacturing a galvanized steel sheet, comprising the following steps: Prepare a base steel plate, which, by weight percent, comprises: C: 0.10-0.25%, Mn: 1.5-5.0%, Si: 0.5-2.5%, Cr: less than 1.5%, Al: 0.005-0.100%, P: less than 0.10%, S: less than 0.020%, B: less than 0.0050%, with the balance being Fe and other unavoidable impurities; The base steel plate is annealed once at a temperature range of 600-900℃ and a dew point temperature of -10℃ to 30℃. The base steel plate that has undergone the first annealing process is then subjected to a pickling process. The base steel plate, after the initial pickling, is then subjected to a second annealing at a temperature range of 700-900℃; and At least one side of the base steel plate that has undergone the secondary annealing is plated.

7. The method for manufacturing galvanized steel sheet according to claim 6, wherein, The base steel plate further comprises less than 1.2% of one or more of Ti, Mo and Nb by weight.

8. The method for manufacturing galvanized steel sheet according to claim 6, wherein, The base steel plate is a cold-rolled steel plate.

9. The method for manufacturing galvanized steel sheet according to claim 6, wherein, The first annealing step is held for 50-600 seconds, and the second annealing step is carried out at a dew point temperature of -60°C to 30°C.

10. The method for manufacturing galvanized steel sheet according to claim 6, wherein, The primary and secondary annealing steps are performed in a nitrogen atmosphere containing 1-80% by volume of hydrogen.

11. The method for manufacturing galvanized steel sheet according to claim 6, wherein, The plating step involves zinc-based plating or zinc alloy-based plating.

12. The method for manufacturing galvanized steel sheet according to claim 6, wherein, The manufacturing method further includes a step of alloying heat treatment on the plated steel sheet.

13. The method for manufacturing galvanized steel sheet according to claim 6, wherein, The manufacturing method further includes a metal plating step after the first pickling step.

14. The method for manufacturing galvanized steel sheet according to claim 13, wherein, The manufacturing method further includes a metal plating step prior to the secondary annealing step.

15. The method for manufacturing galvanized steel sheet according to claim 13 or 14, wherein, The metal plating step is either Fe plating or Ni plating.

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