Cold-rolled steel sheet and method for manufacturing the same

Abstract

The present invention relates to a cold-rolled steel sheet and a method of manufacturing the same, and more particularly, to a cold-rolled steel sheet having excellent surface quality and a method of manufacturing the same.

Description

Technical Field

[0001] This invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to a cold-rolled steel sheet with excellent surface quality and a method for manufacturing the same. Background Technology

[0002] Recently, automotive steel has been developed with a focus on achieving lightweighting through increased strength. To increase the strength of steel, various alloying elements are added to utilize precipitation strengthening and solid solution strengthening. Advanced high-strength steel (AHSS) that induces phase transformation during annealing is also being actively developed.

[0003] Elements such as Mn, Si, Cr, and B are representative examples that can be added to increase the strength of steel. However, due to their high oxidation tendency, they diffuse to the surface to combine with oxygen in the atmosphere during annealing, forming surface oxides. These surface oxides reduce the surface reactivity of the steel sheet, becoming a cause of deterioration in chemical treatment and plating properties.

[0004] Furthermore, with the increasing strength of steel, cracks in the weld heat-affected zone (HAZ) caused by liquid metal embrittlement (LME) have emerged during spot welding. While hot-dip galvanized steel sheets have primarily been a problem, recently, with the increasing emphasis on strength, dissimilar welding processes involving overlapping cold-rolled steel sheets and hot-dip galvanized steel sheets have also arisen, where the coating of the hot-dip galvanized steel sheet comes into contact with the cold-rolled steel sheet in a liquid state, leading to LME issues.

[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 because the decarburized layer on the surface can suppress the formation of cracks caused by tensile stress during spot welding.

[0006] Various techniques have been proposed to suppress the formation of surface oxides 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 to form iron oxides containing individual or combined oxides of Si, Mn, or Al at 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, when a certain amount of Si is added to a steel grade, Si accumulates directly beneath the iron oxide during the reduction process, forming banded Si oxides. This banded oxide then becomes the starting point for surface peeling. In other words, peeling occurs at the interface between the reduced iron and the base iron, which may make it difficult to ensure sealant adhesion and coating adhesion.

[0008] Patent document 2 discloses a method that improves plating properties by maintaining a high dew point in the annealing furnace, allowing easily oxidized alloy components such as Mn, Si, and Al to undergo internal oxidation inside the steel, thereby reducing the oxides oxidized on the external surface of the steel plate after annealing.

[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. Furthermore, there are limitations to the reduction of surface oxidation of Si, Mn, etc., and the formation of a 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 cold-rolled 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 cold-rolled steel sheet with excellent surface quality and a method for manufacturing the same.

[0013] The technical problems of this invention are not limited to those described above. Those skilled in the art can readily understand the additional technical problems of this invention from the entirety of this specification.

[0014] (II) Technical Solution According to one embodiment of the present invention, a cold-rolled steel sheet may be provided, by weight percent, comprising: 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 the Si surface enrichment being less than 0.0100.

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

[0016] With reference to the direction from the surface of the steel plate towards the center of thickness, the depth of the internal oxide layer of the cold-rolled steel plate, which is composed of one or more oxides of Mn, Si, Cr and B, can be 3-15 μm.

[0017] With reference to the direction from the surface of the steel plate towards the center of the thickness, the depth of the decarburized layer of the cold-rolled steel plate can be 20-150μm.

[0018] After the cold-rolled steel sheet is immersed in a phosphate treatment solution at 35°C for 40 seconds, the phosphate film coverage can be above 80%.

[0019] The maximum length of the LME crack in the cold-rolled steel sheet can be less than 10 μm.

[0020] According to one embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet may include the following steps: preparing a cold-rolled steel sheet, wherein the cold-rolled steel sheet comprises, by weight %,: 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; subjecting the cold-rolled steel sheet to a first annealing at a temperature range of 600-900°C and a dew point temperature of -10°C to 30°C; subjecting the first annealed cold-rolled steel sheet to a first pickling; subjecting the first pickled cold-rolled steel sheet to a second annealing at a temperature range of 700-900°C; and subjecting the second annealed cold-rolled steel sheet to a second pickling.

[0021] The cold-rolled steel sheet may further contain less than 1.2% of one or more of Ti, Mo and Nb by weight.

[0022] The first annealing step can be held for 50-600 seconds, and the second annealing step can be carried out at a dew point temperature of -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] It may further include a metal plating step following either the primary pickling step or the secondary pickling step.

[0025] The metal plating step can be Fe or Ni plating.

[0026] (III) Beneficial Effects According to one embodiment of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0027] According to one embodiment of the present invention, a cold-rolled steel sheet with excellent surface quality and a method for manufacturing the same can be provided.

[0028] According to one embodiment of the present invention, a cold-rolled steel sheet with excellent phosphate treatment properties and suitable for use in automotive steel, and a method thereof for manufacturing the same, can be provided.

[0029] According to one embodiment of the present invention, a cold-rolled steel sheet with excellent resistance to LME (Low Metal Melting) during welding with galvanized steel sheet and a method thereof can be provided.

[0030] 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

[0031] Figure 1 A method for calculating the Mn surface enrichment of a steel plate according to an embodiment of the present invention is shown.

[0032] Figure 2 This is a cross-sectional SEM image of Example 1 of the present invention, representing a method for measuring the depth of the internal oxide layer.

[0033] Figure 3 The method for determining the decarburized layer depth of a steel plate according to one embodiment of the present invention is shown by analyzing it with a glow discharge spectrometer (GDS) and displaying a graph of carbon concentration based on the depth of the steel plate.

[0034] Figure 4 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

[0035] The following describes preferred embodiments of the present invention. Various modifications can be made to the specific embodiments described below, and the scope of the present invention should not be construed as limited to the specific embodiments described. These specific embodiments are provided to illustrate the present invention in more detail to those skilled in the art.

[0036] The present invention will now be described in detail.

[0037] The steel composition of the present invention will be described in detail below.

[0038] In this invention, unless otherwise specified, the percentage of each element content is based on weight.

[0039] According to one embodiment of the present invention, the cold-rolled steel sheet may contain, by weight percent: 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%.

[0040] Carbon (C): 0.10-0.25% Carbon (C) is an important element that can be added to stabilize residual austenite, and to ensure the desired 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%, weldability deterioration may occur. According to one embodiment of the invention, the carbon (C) content can be 0.23% or lower.

[0041] Manganese (Mn): 1.5-5.0% Besides contributing to the formation and stabilization of retained austenite, manganese (Mn) also inhibits ferrite transformation during cooling, making it an essential element in transformation-structured steels. Furthermore, to ensure sufficient austenite and guarantee strength and ductility, manganese (Mn) content of 1.5% or more can be included. According to one embodiment of the invention, the manganese (Mn) content can be 2.0% or more. On the other hand, when the manganese (Mn) content exceeds 5.0%, band formation caused by segregation induced during slab heating and hot rolling becomes excessive, 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.

[0042] 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.

[0043] 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 chromium (Cr) content is too high, the amount of alloy iron used becomes 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%.

[0044] 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 a concentration of 0.005% or more. According to one embodiment of the invention, the aluminum (Al) content can be 0.010% or more. However, when the aluminum (Al) content exceeds 0.100%, it may be difficult to produce a sound slab during casting due to reaction with the protective slag. Furthermore, the formation of surface oxides may impair hot-dip plating properties; therefore, the upper limit of the aluminum (Al) content is limited to 0.100%. According to one embodiment of the invention, it can be 0.080% or less.

[0045] Phosphorus (P): less than 0.10% Phosphorus (P) is a solid solution strengthening element, but when the phosphorus (P) content exceeds 0.10%, weldability decreases and the risk of steel becoming brittle increases. Therefore, its upper limit is set at 0.10%.

[0046] Sulfur (S): below 0.020% Sulfur (S), as an impurity element, impairs the ductility and weldability of steel sheets. Therefore, a higher sulfur (S) content increases the likelihood of impairing the ductility and weldability of the steel sheet; thus, its upper limit is set at 0.020%.

[0047] 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 accumulates on the surface of the annealed material, significantly reducing the surface quality. Therefore, its content is limited to below 0.0050%.

[0048] The steel of the present invention may contain additional iron (Fe) and unavoidable impurities in addition to the above-described components. Unavoidable impurities may be unintentionally introduced during ordinary manufacturing processes and therefore cannot be excluded. These impurities are well known to those skilled in the art of steel manufacturing and therefore are not specifically mentioned in this specification.

[0049] According to one embodiment of the invention, the cold-rolled steel sheet may further contain less than 1.2% by weight of one or more of Ti, Mo and Nb.

[0050] One or more of titanium (Ti), molybdenum (Mo), and niobium (Nb): less than 1.2% Titanium (Ti) forms nitrides, which can reduce 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.

[0051] Molybdenum (Mo) can contribute to increased strength. In particular, its ability to maintain wettability with molten metals such as zinc ensures strength.

[0052] Niobium (Nb) segregates at the austenite grain boundaries in the form of carbides. During annealing heat treatment, it can suppress the coarsening of austenite grains and increase strength. However, excessive addition will increase costs.

[0053] With this in mind, it may contain more than 1.2% of one or more of the aforementioned titanium (Ti), molybdenum (Mo), and niobium (Nb).

[0054] The following describes in detail the steel microstructure of the present invention.

[0055] According to one embodiment of the present invention, the depth of the internal oxide layer of the cold-rolled steel sheet, which is composed of one or more oxides of Mn, Si, Cr and B, can be 3-15 μm, based on the direction from the surface of the steel sheet towards the center of the thickness.

[0056] 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. However, according to one embodiment of the invention, the depth of the internal oxide layer can be based on the internal oxide layer at the grain boundary.

[0057] When the depth of the internal oxide layer is less than 3 μm, the internal oxidation is insufficient, resulting in a relatively large amount of surface oxide diffused to the surface, which may not adequately improve the 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 becomes excessively long in order to form a deep internal oxide layer, which may lead to a decrease in economic efficiency.

[0058] Based on the direction from the surface of the steel plate towards the center of the thickness, the depth of the decarburized layer of the cold-rolled steel plate according to one embodiment of the present invention can be 20-150 μm.

[0059] When the depth of the decarburized layer is less than 20 μm, the thickness of the soft layer is insufficient, and the improvement effect on tensile strength (LME) may be inadequate. According to one embodiment of the present invention, the depth of the decarburized layer can be 30 μm or more. According to one embodiment of the present invention, the depth of the decarburized layer can be 35 μm or more. According to one embodiment of the present invention, the depth of the decarburized layer can be 40 μm or more. According to one embodiment of the present invention, the depth of the decarburized layer can be 50 μm or more. On the other hand, when the depth of the decarburized layer exceeds 150 μm, a decrease in tensile strength may occur due to the unnecessary thickness of the soft layer.

[0060] According to one embodiment of the present invention, the Si surface enrichment of the cold-rolled steel sheet can be less than 0.0100.

[0061] According to one embodiment of the present invention, the Si surface enrichment of the steel plate can refer to the amount of Si inside the steel that diffuses to the surface and combines with oxygen to form surface Si oxides. Furthermore, according to another embodiment of the present invention, the Si surface enrichment can refer to the maximum Si enrichment in a region extending from the surface of the steel plate towards the thickness center up to 0.1 μm.

[0062] When the Si surface enrichment exceeds 0.0100, the Fe exposure area on the steel plate surface that can react with the phosphate solution is insufficient, making it difficult to ensure excellent phosphate coverage. According to one embodiment of the present invention, the Si enrichment can be below 0.0070.

[0063] According to one embodiment of the present invention, after the cold-rolled steel sheet is immersed in a phosphate treatment solution at 35°C for 40 seconds, the phosphate film coverage is more than 80%, and the maximum length of LME cracks can be less than 10 μm.

[0064] According to one embodiment of the present invention, the maximum length of the LME crack is evaluated according to SEP 1220-2 specification. The cold-rolled steel sheet of the present invention and a hot-dip galvanized steel sheet without a decarburized layer are overlapped and spot-welded. Then, the welded cold-rolled steel sheet is cut at 0°, 45°, and 90° for each specimen, and the LME crack length is measured using an optical microscope (OM). At this time, only type B cracks in the heat-affected zone are measured, representing the maximum crack length.

[0065] The steel manufacturing method of the present invention will be described in detail below.

[0066] According to one embodiment of the present invention, a cold-rolled steel sheet can be manufactured by subjecting a cold-rolled steel sheet satisfying the above-mentioned alloy composition to a first annealing, a first pickling, a second annealing, and a second pickling.

[0067] Preparation of cold-rolled steel sheets Cold-rolled steel sheets that meet the alloy composition requirements of one embodiment of the present invention can be prepared. The method for manufacturing the cold-rolled steel sheet according to one embodiment of the present invention is not particularly limited and can be based on common conditions applicable in the same technical field.

[0068] First annealing The cold-rolled steel sheet can be annealed once by holding it at a temperature range of 600-900℃ and a dew point temperature of -10℃ to 30℃ for 50-600 seconds.

[0069] 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 above the surface of the steel plate, the dissolved Si, Mn, Cr, B, etc., in the surface layer of the steel plate can be depleted.

[0070] During a single annealing process, Si exhibits a very high oxidation tendency, allowing it to oxidize internally and preventing the formation of surface oxides. Conversely, Mn, with a relatively low oxidation tendency compared to Si, can form a large amount of surface oxides. These Mn-induced surface oxides can be removed by a single acid pickling process following the initial annealing.

[0071] An internal oxide layer is formed through a single annealing process, reducing the surface Si oxide layer. Subsequent acid washing removes the Mn oxide layer, resulting in a clean surface. When this clean surface is subjected to a second annealing, the exposed Fe area increases during phosphate treatment, promoting the phosphate reaction.

[0072] Furthermore, because the surface becomes cleaner after the first annealing, oxygen in the atmosphere is 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 high phosphate handleability, and by forming a sufficient decarburized layer, liquid metal embrittlement (LME) during welding can be reduced.

[0073] During a single annealing process, when the annealing temperature is below 600℃, the diffusion of alloying elements such as Si and Mn, which are necessary to form an internal oxide layer, to the grain boundaries of the surface layer is not smooth, and the diffusion of carbon, which is necessary 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 increases due to the excessively high temperature, which is uneconomical.

[0074] During a single annealing process, when the dew point temperature is below -10°C, the oxygen partial pressure in the atmosphere is insufficient, making it difficult for oxygen to penetrate 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 iron is also oxidized. According to one embodiment of the invention, to prevent iron from being oxidized and to more effectively oxidize the interior of Si, the dew point temperature can be limited to below 20°C.

[0075] During a single annealing process, if the holding time is less than 50 seconds, the formation of the internal oxide and decarburized layers may be insufficient. 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.

[0076] 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 an explosion hazard when the gas flows out, and costs increase.

[0077] According to one embodiment of the present invention, in order to obtain the desired fine microstructure after holding during a single annealing process, two or more cooling stages may be performed. The cooling rate during a single annealing process 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.

[0078] When the cooling rate is low, gas cooling using nitrogen gas containing a certain amount of hydrogen can be used. To increase the cooling rate, water mist cooling, water quenching, or water jet cooling can be used. During wet cooling with a large amount of water, the steel plate is in direct contact with the water, and the dew point temperature caused by water vapor may 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.

[0079] Alternatively, the cooling atmosphere can be a nitrogen atmosphere containing 1-80% by volume of hydrogen.

[0080] Furthermore, according to one embodiment of the present invention, the heating rate can be 1-50°C / second during a single annealing process.

[0081] One pickling The cold-rolled steel sheet that has undergone the first annealing process can be pickled once.

[0082] According to one embodiment of the present invention, oxides formed on the surface of the steel plate during a single pickling process can be removed by a single pickling process.

[0083] During a single pickling operation, the type of acid is not particularly limited; however, 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.

[0084] 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 in the same technical field. According to one embodiment of the present invention, when plating Fe, the adhesion amount can be 0.1-3 g / m³. 2 According to one embodiment of the present invention, the adhesion amount during Ni plating 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 the annealing line can be used.

[0085] Secondary annealing The cold-rolled steel sheet that has undergone the first pickling process can be subjected to a second annealing process at a temperature range of 700-900℃ and a dew point temperature of -60℃ to 30℃.

[0086] 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 with a significantly reduced depleted layer of Si, Mn, Cr, B, etc., in a solid solution state extending from the steel plate surface to a certain depth. 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.

[0087] That is, 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, resulting in increased surface reactivity and thus improved phosphate treatability. The phosphate treatability is further enhanced by the lower the number of oxides on the surface of the annealed steel sheet, because phosphate solution readily reacts with the Fe in the steel sheet. Various surface oxides that negatively affect phosphate treatability include Mn, Si, Cr, and B, but Mn and Si are representative examples.

[0088] During secondary annealing, if the annealing temperature is below 700°C, the desired fine microstructure may not be obtained sufficiently. On the other hand, when the annealing temperature exceeds 900°C, the economic efficiency due to energy consumption decreases.

[0089] During secondary annealing, when the dew point temperature is below -60°C, maintaining the atmosphere in large-scale production equipment presents a significant practical challenge. 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.

[0090] According to one embodiment of the present invention, the holding time during the second annealing can be 30-300 seconds.

[0091] 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 an explosion hazard when the gas flows out, and costs increase.

[0092] During secondary annealing, the cooling conditions are not particularly limited. According to one embodiment of the invention, during secondary annealing, the temperature can be slowly cooled to 650°C after holding, followed by rapid cooling according to the desired physical properties. According to one embodiment of the invention, the cooling conditions are not particularly limited, but different conditions can be applied to achieve the desired physical properties. Furthermore, the formation of surface oxides, internal oxide layers, and decarburized layers mostly occur in the high-temperature region; therefore, the cooling conditions are not particularly limited in this invention.

[0093] In addition, according to one embodiment of the present invention, during the second annealing, gas cooling and wet cooling using water can be performed after holding, just as during the first annealing.

[0094] According to one embodiment of the invention, during cooling, a reducing atmosphere can be applied, at least for iron, to prevent oxidation. 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.

[0095] Furthermore, according to one embodiment of the present invention, the rapidly cooled steel plate can be reheated to a certain temperature for tempering and then cooled to room temperature as needed.

[0096] Secondary pickling The cold-rolled steel sheet that has undergone the second annealing process can be subjected to a second pickling process.

[0097] According to one embodiment of the present invention, surface oxides formed during secondary annealing can be removed by secondary pickling. According to another embodiment of the present invention, the secondary pickling can be performed using the same method as the primary pickling.

[0098] During the secondary pickling, 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.

[0099] According to one embodiment of the present invention, metal plating can be performed after a secondary pickling. By performing the metal plating process after the secondary pickling, the surface conditioner can be well adsorbed onto the steel plate surface during the surface conditioning stage of phosphate treatment, which helps to improve phosphate treatability. The metal plating conditions after the secondary pickling can be applied in the same way as the metal plating process after the primary pickling described above. Detailed Implementation

[0100] 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 and are not intended to limit the scope of the invention.

[0101] (Example) Cold-rolled steel sheets with the compositions shown in Table 1 were prepared. Two types of steel sheets were subjected to a single annealing and a single pickling under the conditions shown in Table 2. During the single annealing, the atmosphere was nitrogen containing 5% hydrogen, and the process was carried out under a reducing atmosphere. Furthermore, during the single annealing, the temperature was increased to 800°C at a rate of 3°C / second and held for 150 seconds. After cooling, the sheets were immersed in a 5% (v / v) hydrochloric acid solution at 50°C for 5 seconds for a single pickling.

[0102] [Table 1] Furthermore, Table 2 below measures and illustrates the surface enrichment of Si and Mn before and after one annealing and one pickling, and analyzes and illustrates the depth of the internal oxide layer and decarburized layer. The surface enrichment of Si and Mn before and after pickling can be estimated from the amount of Si and Mn oxides remaining on the steel plate surface. The surface enrichment of Si and Mn is due to their diffusion to the surface during annealing and their combination with oxygen in the atmosphere, and can also be regarded as the amount of oxidation.

[0103] The specific Si and Mn surface enrichment amounts can be calculated using GDS analysis. In the GDS data, the main components of each steel element—Fe, Mn, Si, Cr, and B—are retained, with the sum of the weight percentages of the main components set to 100%. Fe is the main component of the steel sheet and is therefore included, excluding oxygen. Furthermore, although components not mentioned above that are present in the steel sheet can be added during processing, the amount of these components enriched on the surface during annealing is limited and extremely small, therefore they are omitted.

[0104] Figure 1 A method for calculating the Mn surface enrichment of a steel plate according to one 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 is plotted using the depth of the processed data as the x-axis and the weight percentage of each component as the y-axis. After limiting the depth to 0.1 μm for each component, the minimum weight percentage of each component within 0.1 μm is identified, and this value is estimated as the amount of the solid solution component. After finding the solid solution component amount, data at deeper depths are deleted based on this location. The solid solution component amount is subtracted from the weight percentage of each component from the surface to the solid solution component amount location. This process is to extract only oxides from the GDS data, excluding the values ​​of dissolved Mn or Si for calculation. Then, using the data after subtracting the solid solution component value, the integral value from the surface to the solid solution component value location is estimated as the enrichment amount. When the minimum weight percentage value of each component occurs at the outermost point between depths of 0-0.01 μm, it can be interpreted as almost no surface enrichment of that component.

[0105] 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, which recorded the internal oxide depth of the grain boundaries. Figure 2 This is a cross-sectional SEM image of Example 1 of the present invention, illustrating a method for measuring the depth of the internal oxide layer. (Example:) Figure 2 As shown, three points can be randomly measured in a 3000x magnification image, and the average value can be derived.

[0106] The specific decarburized layer depth can be derived by analyzing the carbon content at depth using glow discharge spectroscopy (GDS). There are two methods. The first is when the carbon content curve shows a horizontal interval within the measurable depth of GDS, meaning the GDS analysis has exceeded the decarburized layer depth and reached the matrix structure. The decarburized layer depth in this case is as follows: Figure 3 That's how you find it. Figure 3A method for determining the decarburized layer depth of a steel sheet according to an embodiment of the present invention is shown, illustrating a graph of carbon concentration based on the depth of the steel sheet during glow discharge spectroscopy (GDS) analysis. A graph is plotted using carbon content, and the average of the 10 deepest carbon content data points is calculated. This average of the 10 deepest carbon content data points is referred to as the depth value. Then, from the carbon content data moving towards the surface from the deepest position, the depth representing the first carbon content decrease of more than 5% from the depth value is identified. This position represents the deepest decarburized layer depth. The total decarburized layer depth of the cold-rolled steel sheet extends from the surface to the deepest decarburized depth.

[0107] The second scenario involves a very deep decarburized layer, exceeding the measurable range of GDS. Similar to the first scenario, analysis is performed using the carbon content values ​​at the deepest depth as a benchmark. Whether the carbon content at the deepest depth in the GDS data reaches a deep level 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 deeper. When the carbon content at the deepest depth is lower than the OES carbon content, it has not yet reached a deep level, 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 (containing 2-5 vol% ethanol or methanol containing nitric acid). Figure 4 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. For example... Figure 4 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 be considered the depth of the decarburized layer.

[0108] [Table 2] In Table 2 above, test pieces numbered 1 and 3 represent samples with a dew point temperature of -40°C, which do not meet the conditions of this invention. Test pieces numbered 2 and 4 represent samples with a dew point temperature that meets the conditions of this invention. It can be confirmed that, compared with test pieces numbered 1 and 3, the Si and Mn surface enrichment of test pieces numbered 2 and 4 is significantly reduced.

[0109] 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 internal oxidation enrichment before acid washing. Therefore, it can be confirmed that the enrichment after acid washing is lower than that of test pieces 1 and 3.

[0110] The surface enrichment of Mn also decreased, and the reduction was more significant compared to that of Si. After acid washing, the surface enrichment of Mn was significantly reduced compared to before acid washing. This is because Mn oxides are easily dissolved by acid washing. Before acid washing, in samples 2 and 4 where the dew point temperature met the conditions of this invention, the surface enrichment of Mn was lower than that in samples 1 and 3 due to the formation of an internal oxide layer, and this trend was also observed after acid washing.

[0111] In cases involving test pieces number 1 and 3, no internal oxide layer was formed, and the decarburized layer was less than 7 μm, indicating that it was almost non-existent. This can be confirmed.

[0112] In samples 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, forming an internal oxide layer. Therefore, it can be confirmed that the Si surface enrichment is higher in samples 1 and 3 than in samples 2 and 4.

[0113] In cases involving 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 pickling. It can be confirmed that Mn, which is more soluble in hydrochloric acid than Si, is removed after pickling. However, when secondary annealing is performed with a large amount of Si oxides on the surface, the adsorption and diffusion of oxygen from the atmosphere into the steel plate are hindered due to the Si surface oxides, which also negatively impacts the decarburization reaction where carbon from inside the steel diffuses to the surface and combines with adsorbed oxygen.

[0114] Furthermore, using cold-rolled steel sheets with the composition shown in Table 1 above, a first annealing, a first pickling, a second annealing, and a second pickling 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 rate of 3°C / second and held for 150 seconds. After cooling, the sheets were immersed in a 5% (v / v) hydrochloric acid solution at 50°C for 5 seconds for a first pickling. 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 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 sheets were finally cooled to room temperature at 2°C / second, thus producing the annealed steel sheet. The steel plate after final annealing is immersed in a 5% (v / v) hydrochloric acid solution at 50°C for 5 seconds for a second pickling.

[0115] For the manufactured steel plates, the surface enrichment of Si and Mn and the depth of the internal oxide layer and decarburized layer were measured using the above method, and are shown in Table 3 below.

[0116] In addition, to evaluate phosphate treatability, the phosphate film coverage was measured and is shown in Table 3 below. For phosphate treatment, the steel sheet that had undergone secondary pickling was subjected to degreasing, surface conditioning, and phosphate treatment stages. Degreasing was performed using an alkaline degreasing agent at 45°C for 120 seconds, and surface conditioning was performed at room temperature for 30 seconds. Afterwards, the sheet was immersed in a phosphate treatment solution at 35°C for 40 seconds to form a film.

[0117] Furthermore, the phosphate treatment according to one embodiment of the present invention is a method for relative evaluation between test pieces. Although it differs from that used by automobile companies, it is unlikely to be significantly different when evaluated using other solutions and methods.

[0118] The phosphate film coverage measurements in Table 3 below were obtained by observing the phosphate film at 500x magnification using SEM after phosphate treatment, and the coverage was recorded through image analysis. Coverage is the percentage of the steel plate surface covered by the phosphate film.

[0119] In addition, LME resistance was evaluated, and the results are shown in Table 3 below. The LME evaluation was conducted according to SEP 1220-2 specifications, by overlapping and spot-welding manufactured cold-rolled steel sheets and hot-dip galvanized steel sheets without a decarburized layer. LME cracks were determined by cutting the welded cold-rolled steel sheets at 0°, 45°, and 90° angles, observing the cracks using an optical microscope (OM), and measuring their lengths. Only type B cracks in the heat-affected zone were measured, and the maximum crack length was indicated by ○, △, and ×.

[0120] ○: The maximum length of LME cracks is less than 10 μm. △: The maximum length of LME crack is greater than 10 μm and less than 30 μm. ×: The maximum length of the LME crack is greater than 30 μm. [Table 3] As shown in Table 3, all inventive examples that satisfy the alloy composition and manufacturing method proposed in this invention ensure the target properties.

[0121] In particular, Examples 1 and 2 are examples of double annealing with internal oxidation occurring during the first annealing. In Example 1, even without internal oxidation during the second annealing, an internal oxide layer of considerable depth was formed, exhibiting excellent phosphate coverage characteristics. 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 surface Mn oxides were removed by pickling, resulting in a clean second annealing state. With a clean surface after the first annealing, the steel plate surface has almost no oxides 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 is at a very low level. Example 2, by also performing internal oxidation during the second annealing, exhibits an even cleaner surface state. This is because the clean steel plate surface, with almost no surface oxides after the first annealing and pickling, allows oxygen to penetrate more easily and deeply into the steel plate during the second annealing, thus resulting in almost no alloying elements diffusing to the surface during the second annealing.

[0122] Invention Examples 3 and 4 also confirm a tendency similar to that of Invention Examples 1 and 2.

[0123] On the other hand, Comparative Examples 1 and 2 are examples of annealing performed only twice, without a first annealing. No internal oxide layer was observed in Comparative Example 1, and the phosphate coverage was also very poor. The Si surface enrichment was relatively high, meaning that a large amount of Si oxide formed on the surface. Furthermore, the decarburized layer depth was also observed to be shallow. In Comparative Example 2, the internal oxide layer depth was 2.6 μm, surface oxidation was suppressed to a certain extent, and the phosphate coverage was improved. Furthermore, the decarburized layer was also formed at 18 μm, but this is insufficient for the expected improvement in LME (Liquid Metal Erosion).

[0124] 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 acid washing, the Si surface oxide remained intact. Secondary annealing in this state resulted in further accumulation of surface oxides and very poor phosphate coverage. In Comparative Example 4, a large amount of surface oxide formed during the single annealing, and Si surface oxide remained after acid washing. However, internal oxidation occurred during the secondary annealing, relatively reducing the additional surface oxides caused by the secondary annealing. However, the target level of phosphate coverage was not ensured.

[0125] Comparative Examples 2, 4, and 1 all involve a single internal oxidation process. Furthermore, in this invention, the single annealing for internal oxidation involves a longer holding time compared to a double annealing process, while the formation of the internal oxide layer and decarburized layer is governed 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.

[0126] Comparative Examples 5 to 8 also confirm a similar tendency to that of Comparative Examples 1 to 4.

[0127] 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 cold-rolled steel sheet, by weight percent, comprising: 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%, the balance being Fe and other unavoidable impurities, and the Si surface enrichment being less than 0.0100.

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

3. The cold-rolled steel sheet according to claim 1, wherein, With reference to the direction from the surface of the steel plate towards the center of thickness, the depth of the internal oxide layer of the cold-rolled steel plate, which is composed of one or more oxides of Mn, Si, Cr and B, is 3-15 μm.

4. The cold-rolled steel sheet according to claim 1, wherein, With reference to the direction from the surface of the steel plate towards the center of the thickness, the depth of the decarburized layer of the cold-rolled steel plate is 20-150μm.

5. The cold-rolled steel sheet according to claim 1, wherein, After the cold-rolled steel sheet is immersed in a phosphate treatment solution at 35°C for 40 seconds, the phosphate film coverage rate is more than 80%.

6. The cold-rolled steel sheet according to claim 1, wherein, The maximum length of the liquid metal embrittlement (LME) crack in the cold-rolled steel sheet is less than 10 μm.

7. A method for manufacturing cold-rolled steel sheet, comprising the following steps: Prepare cold-rolled steel sheets, which, by weight percent, comprise: 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 cold-rolled steel sheet is annealed once at a temperature range of 600-900℃ and a dew point temperature of -10℃ to 30℃. The cold-rolled steel sheet that has undergone the first annealing process is then subjected to a pickling process. The cold-rolled steel sheet that has undergone the first pickling process is then subjected to a second annealing at a temperature range of 700-900℃; and The cold-rolled steel sheet that has undergone the second annealing process is then subjected to a second pickling process.

8. The method for manufacturing cold-rolled steel sheet according to claim 7, wherein, The cold-rolled steel sheet further comprises less than 1.2% by weight of one or more of Ti, Mo and Nb.

9. The method for manufacturing cold-rolled steel sheet according to claim 7, 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 cold-rolled steel sheet according to claim 7, 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 cold-rolled steel sheet according to claim 7, wherein, The manufacturing method further includes a metal plating step following either the primary pickling step or the secondary pickling step.

12. The method for manufacturing cold-rolled steel sheet according to claim 11, wherein, The metal plating step is Fe or Ni plating.

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