Cold-rolled steel sheet and method for manufacturing same

Abstract

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

Description

Cold rolled steel sheet and its manufacturing method

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

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

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

[0004] Additionally, as steel materials become more high-strength, cracks occur in the heat-affected zone of the weld due to liquid metal embrittlement (LME) during spot welding. Hot-dip galvanized steel sheets were the primary source of the problem, but recently, with the advancement of high-strength steel sheets, cold-rolled steel sheets have also been observed. When welding dissimilar materials by overlapping hot-dip galvanized steel sheets, the plating layer of the hot-dip galvanized steel sheet comes into contact with the cold-rolled steel sheet in a liquid state, creating the problem of LME.

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

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

[0007] However, in the case of steel grades with Si added above a certain level, Si becomes concentrated directly under the iron oxide during the reduction process, forming a band-shaped Si oxide, which then peels off from the surface. In other words, peeling occurs at the interface between the reduced iron and the base iron, making it difficult to secure sealer adhesion and plating layer adhesion.

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

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

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

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

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

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

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

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

[0016] It may be a cold-rolled steel sheet having a Si surface concentration of 0.0100 or less.

[0017] The above cold rolled steel sheet may further contain, in weight %, one or more of Ti, Mo, and Nb in an amount of 1.2% or less.

[0018] The cold rolled steel sheet above may have an internal oxide layer composed of one or more oxides of Mn, Si, Cr, and B, the depth of which may be 3 to 15 μm from the surface of the steel sheet in the direction of the center of thickness.

[0019] The depth of the decarburized layer of the above cold-rolled steel sheet may be 20 to 150 μm from the surface of the steel sheet in the direction of the center of thickness.

[0020] The above cold rolled steel sheet may have a phosphate film coverage of 80% or more after being immersed in a phosphate treatment solution at 35°C for 40 seconds.

[0021] The above cold rolled steel sheet may have a maximum LME crack length of 10 μm or less.

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

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

[0024] A step of first pickling the above first annealed cold rolled steel sheet;

[0025] A step of secondary annealing the above-mentioned primary cold-rolled steel sheet at a temperature range of 700 to 900°C; and

[0026] It may be a method for manufacturing a cold-rolled steel sheet, including a step of secondarily pickling the secondarily annealed cold-rolled steel sheet.

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

[0028] The above first annealing step is maintained for 50 to 600 seconds,

[0029] The above second annealing step can be performed at a dew point temperature of -60 to 30°C.

[0030] The above first annealing step and second annealing step may be a nitrogen gas atmosphere containing 1 to 80 vol% of hydrogen.

[0031] A metal plating step may be further included after either the first acid pickling step or the second acid pickling step.

[0032] The above metal plating step may be Fe or Ni plating.

[0033] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0034] According to one embodiment of the present invention, a cold-rolled steel sheet having excellent surface quality and a method for manufacturing the same can be provided.

[0035] According to one embodiment of the present invention, a cold-rolled steel sheet having excellent phosphate treatment properties and usable as steel for automobiles and a method for manufacturing the same can be provided.

[0036] According to one embodiment of the present invention, a cold-rolled steel sheet having excellent resistance to LME generated when welding with a plated steel sheet and a method for manufacturing the same can be provided.

[0037] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0038] Figure 1 illustrates a method for calculating the Mn surface concentration of a steel plate according to one embodiment of the present invention.

[0039] FIG. 2 is a cross-sectional SEM image of Invention Example 1 according to one embodiment of the present invention, illustrating a method for measuring the depth of an internal oxidation layer.

[0040] FIG. 3 is a graph showing a method for obtaining the depth of a decarburized layer of a steel plate according to an embodiment of the present invention, and showing the carbon concentration according to the depth of the steel plate when analyzed by GDS (Glow Discharge Spectrometer).

[0041] FIG. 4 is a cross-sectional SEM image of a steel plate according to one embodiment of the present invention, illustrating a method for obtaining the depth of a decarburized layer of a steel plate etched with nital.

[0042] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0043] Hereinafter, the present invention will be described in detail.

[0044] Below, the steel composition of the present invention is described in detail.

[0045] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0046] A cold-rolled steel sheet according to one embodiment of the present invention may include, in weight %, C: 0.10 to 0.25%, Mn: 1.5 to 5.0%, Si: 0.5 to 2.5%, Cr: 1.5% or less, Al: 0.005 to 0.100%, P: 0.10% or less, S: 0.020% or less, and B: 0.0050% or less.

[0047] Carbon (C): 0.10~0.25%

[0048] Carbon (C) is an important element that can be added to stabilize retained austenite. To achieve this effect, carbon (C) can be added in an amount of 0.10% or more. According to one embodiment of the present invention, carbon (C) can be added in an amount of 0.15% or more. On the other hand, if the carbon (C) content exceeds 0.25%, the problem of poor weldability may arise. According to one embodiment of the present invention, the carbon (C) content can be 0.23% or less.

[0049] Manganese (Mn): 1.5~5.0%

[0050] Manganese (Mn) is an essential element in transformed structure steel because it forms and stabilizes retained austenite and suppresses ferrite transformation during cooling. In addition, in order to secure sufficient austenite and thereby ensure strength and ductility, manganese (Mn) may be included in an amount of 1.5% or more. According to one embodiment of the present invention, it may be 2.0% or more. On the other hand, if the content exceeds 5.0%, band formation due to segregation induced during slab heating and hot rolling processes may be excessive, which may cause a problem of deteriorating physical properties. Therefore, according to one embodiment of the present invention, the upper limit of the manganese (Mn) content may be limited to 5.0%. According to one embodiment of the present invention, it may be 3.0% or less.

[0051] Silicon (Si): 0.5–2.5%

[0052] Silicon (Si) is an element that suppresses the precipitation of carbides within ferrite and promotes the diffusion of carbon within ferrite into austenite, thereby contributing to the stabilization of retained austenite. To achieve the above effects, silicon (Si) may be added in an amount of 0.5% or more. In one embodiment of the present invention, it may be 0.7% or more. However, if added excessively, surface reactivity may be reduced, and therefore, the upper limit of the silicon (Si) content may be limited to 2.5%. In one embodiment of the present invention, it may be 2.0% or less.

[0053] Chromium (Cr): 1.5% or less

[0054] Chromium (Cr) is an element that increases hardenability and plays a role in suppressing the formation of ferrite. Therefore, it can be added in small amounts as needed to secure appropriate residual austenite. According to one embodiment of the present invention, the chromium (Cr) content may be 0%. However, if the content is excessive, the amount of alloy iron input may be excessive, which may cause an increase in cost. Therefore, the upper limit of the chromium (Cr) content may be limited to 1.5%. According to one embodiment of the present invention, it may be 1.0% or less.

[0055] Aluminum (Al): 0.005~0.100%

[0056] Aluminum (Al) is an element that contributes to the stabilization of retained austenite by suppressing the formation of carbides within ferrite, and to achieve this effect, it can be added in an amount of 0.005% or more. According to one embodiment of the present invention, it can be 0.010% or more. However, if the content exceeds 0.100%, it may be difficult to manufacture a sound slab due to a reaction with the mold flux during casting. In addition, it may form surface oxides, thereby hindering the hot dip galvanization, and therefore, the upper limit of the aluminum (Al) content may be limited to 0.100%. According to one embodiment of the present invention, it may be 0.080% or less.

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

[0058] Phosphorus (P) is a reinforcing element, but if its content exceeds 0.10%, weldability deteriorates and the risk of steel becoming brittle increases, so its upper limit can be limited to 0.10%.

[0059] Sulfur (S): 0.020% or less

[0060] Sulfur (S) is an impurity element that impairs the ductility and weldability of steel plates. Therefore, as the sulfur (S) content increases, the likelihood of impairing the ductility and weldability of steel plates increases. Therefore, considering this, the upper limit can be set at 0.020%.

[0061] Boron (B): 0.0050% or less

[0062] Boron (B) is an element that can be added to enhance strength. However, if the boron (B) content exceeds 0.0050%, it can concentrate on the surface of the annealed material, significantly reducing surface quality. Therefore, the content can be limited to 0.0050% or less.

[0063] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0064] A cold-rolled steel sheet according to one embodiment of the present invention may further include, by weight %, one or more of Ti, Mo, and Nb, in an amount of 1.2% or less.

[0065] At least one of titanium (Ti), molybdenum (Mo), and niobium (Nb): 1.2% or less

[0066] Titanium (Ti) can form nitrides and reduce the concentration of nitrogen in steel. On the other hand, if it is included excessively, it can cause a decrease in the carbon concentration of martensite and a decrease in strength due to carbide precipitation.

[0067] Molybdenum (Mo) can contribute to increased strength. In particular, it has the effect of securing strength without reducing the wettability of molten metals such as zinc.

[0068] Niobium (Nb) is segregated in the form of carbides at austenite grain boundaries, and can increase strength by suppressing coarsening of austenite grains during annealing heat treatment; however, excessive input can lead to increased cost.

[0069] Taking this into consideration, one or more of the above titanium (Ti), molybdenum (Mo), and niobium (Nb) may be included at 1.2% or less.

[0070] Below, the steel microstructure of the present invention is described in detail.

[0071] According to one embodiment of the present invention, a cold-rolled steel sheet may have an internal oxidation layer formed of one or more oxides of Mn, Si, Cr, and B, the depth of which may be 3 to 15 μm from the surface of the steel sheet in the direction of the center of thickness.

[0072] Internal oxidation occurs simultaneously with an internal oxidation layer formed along the grain boundary and an internal oxidation layer formed within the grain, but the depth of the internal oxidation layer according to one embodiment of the present invention may be based on the grain boundary internal oxidation layer.

[0073] If the depth of the internal oxidation layer is less than 3 μm, the amount of internal oxidation is insufficient, so the amount of surface oxidation formed by diffusion to the surface is relatively large, which may lead to an insufficient effect on improving surface quality. According to one embodiment of the present invention, the depth of the internal oxidation layer may be 4 μm or more. According to one embodiment of the present invention, the depth of the internal oxidation layer may be 5 μm or more. On the other hand, if the depth of the internal oxidation layer exceeds 15 μm, the heat treatment time may be excessively long to form a deep internal oxidation layer, which may lead to a problem of poor economic efficiency.

[0074] According to one embodiment of the present invention, the cold rolled steel sheet may have a depth of a decarburized layer of 20 to 150 μm from the surface of the steel sheet in the direction of the center of thickness.

[0075] If the depth of the decarburization layer is less than 20 μm, the soft layer may be insufficiently thick, resulting in a problem of insufficient LME improvement. In one embodiment of the present invention, it may be 30 μm or more. In one embodiment of the present invention, it may be 35 μm or more. In one embodiment of the present invention, it may be 40 μm or more. In one embodiment of the present invention, it may be 50 μm or more. On the other hand, if the depth of the decarburization layer exceeds 150 μm, there may be a problem of the tensile strength decreasing due to an excessively thick soft layer.

[0076] According to one embodiment of the present invention, a cold-rolled steel sheet may have a Si surface concentration of 0.0100 or less.

[0077] According to one embodiment of the present invention, the amount of Si surface enrichment of the steel sheet may refer to the amount of surface Si oxide formed when Si inside the steel diffuses to the surface and combines with oxygen. In addition, according to one embodiment of the present invention, the amount of Si surface enrichment may refer to the maximum amount of Si enrichment in a region of up to 0.1 μm from the surface of the steel sheet in the direction of the center of thickness.

[0078] If the Si surface concentration exceeds 0.0100, the Fe exposure area on the steel plate surface for the phosphate solution to react with the steel plate is insufficient, making it difficult to secure excellent phosphate coverage. According to one embodiment of the present invention, the Si concentration may be 0.0070 or less.

[0079] According to one embodiment of the present invention, a cold-rolled steel sheet may have a phosphate film coverage of 80% or more and an LME crack maximum length of 10 μm or less after being immersed in a phosphate treatment solution at 35°C for 40 seconds.

[0080] According to one embodiment of the present invention, the maximum length of the LME crack was evaluated according to the SEP 1220-2 standard. The cold-rolled steel sheet of the present invention and the hot-dip galvanized steel sheet without a decarburized layer were overlapped and spot-welded, and then the welded cold-rolled steel sheet was sheared in the 0°, 45°, and 90° directions per specimen and observed using OM (Optical Microscopy) to measure the LME crack length. At this time, only the B type crack in the heat-affected zone was measured, and the maximum crack length was indicated.

[0081] Below, the steel manufacturing method of the present invention is described in detail.

[0082] A cold-rolled steel sheet according to one embodiment of the present invention can be manufactured by first annealing, first pickling, second annealing, and second pickling of a cold-rolled steel sheet satisfying the above-described alloy composition.

[0083] Cold rolled steel sheet preparation

[0084] A cold-rolled steel sheet satisfying the alloy composition according to 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 may be a typical condition applicable in the same technical field.

[0085] 1st annealing

[0086] The above cold rolled steel sheet can be subjected to primary annealing by maintaining it at a temperature range of 600 to 900°C and a dew point temperature of -10 to 30°C for 50 to 600 seconds.

[0087] According to one embodiment of the present invention, by performing primary annealing, an internal oxide layer and a decarburization layer can be effectively formed. During primary annealing, an internal oxide layer composed of oxides such as Si, Mn, Cr, and B is formed at a depth greater than a certain depth from the surface of the steel sheet, thereby depleting Si, Mn, Cr, and B, etc. dissolved in the surface layer of the steel sheet.

[0088] During primary annealing, Si has a very high oxidation tendency, so it can be oxidized internally and the formation of surface oxides can be prevented. On the other hand, Mn, which has a relatively low oxidation tendency compared to Si, can form a large amount of surface oxides. This surface oxide caused by Mn can be removed through primary pickling performed after primary annealing.

[0089] Through primary annealing, an internal oxide layer is formed, reducing surface Si oxide. Subsequent acid pickling removes Mn oxide, leaving the surface clean. If secondary annealing is performed with this clean surface, the exposed surface area of ​​Fe increases during phosphate treatment, promoting the phosphate reaction.

[0090] Furthermore, the surface, purified after primary annealing, facilitates the adsorption of oxygen in the atmosphere onto the steel surface during secondary annealing, promoting the decarburization reaction in which carbon in the steel diffuses to the surface and combines with the adsorbed oxygen. This ensures high phosphate treatability, and by forming a sufficient decarburization layer, LME (Liquid Metal Embrittlement) can be reduced during welding.

[0091] During primary annealing, if the annealing temperature is below 600℃, alloying elements such as Si and Mn, which form an internal oxide layer, will not readily diffuse to the surface grain boundaries, and carbon diffusion to the surface for surface decarburization may also be difficult. On the other hand, if the annealing temperature exceeds 900℃, internal oxidation and decarburization may occur, but the excessively high temperature requires significant energy consumption to maintain the high temperature, making it uneconomical.

[0092] During primary annealing, if the dew point temperature is below -10°C, the oxygen partial pressure in the atmosphere may be insufficient, making it difficult for oxygen to penetrate into the steel sheet. According to one embodiment of the present invention, it may be 0°C or higher. On the other hand, if the dew point temperature exceeds 30°C, there is a risk that the atmosphere will oxidize even the iron. According to one embodiment of the present invention, the dew point temperature may be limited to 20°C or lower to prevent iron from being oxidized and to more effectively internally oxidize the silicon.

[0093] During primary annealing, a holding time of less than 50 seconds may be insufficient to form an internal oxidation and decarburization layer. Conversely, a holding time exceeding 600 seconds may result in the diffusible alloying elements in the surface layer having fully reacted, making it difficult to expect additional benefits and potentially being economically disadvantageous.

[0094] According to one embodiment of the present invention, during the primary annealing process, the annealing atmosphere may be a nitrogen gas atmosphere containing 1 to 80 vol% hydrogen. If the hydrogen content is less than 1 vol%, the iron in the steel sheet may oxidize. On the other hand, if the hydrogen content exceeds 80 vol%, there is a risk of explosion in the event of a gas leak, which may increase costs.

[0095] According to one embodiment of the present invention, during the primary annealing, two or more cooling steps may be performed to obtain the desired microstructure after maintenance. The cooling rate during the primary annealing is not particularly limited and may be any condition applicable in the same technical field. According to one embodiment of the present invention, during the primary annealing, cooling may be performed at a cooling rate of 1 to 100°C / s.

[0096] If the cooling rate is not high, gas cooling using nitrogen gas containing a certain amount of hydrogen can be performed. To increase the cooling rate, mist cooling, water quenching, or water jet cooling can be performed. If wet cooling is performed with a large amount of water, the steel plate comes into direct contact with water, and the dew point temperature can rise rapidly due to water vapor, which can cause an iron oxide film to form on the surface. In such cases, the iron oxide film must be removed through pickling after cooling is complete.

[0097] Meanwhile, during cooling, the atmosphere may be a nitrogen gas atmosphere containing 1 to 80 vol% of hydrogen.

[0098] Additionally, according to one embodiment of the present invention, during the first annealing, the heating rate may be 1 to 50°C / s.

[0099] 1st mountain tax

[0100] The above first annealed cold rolled steel sheet can be subjected to first pickling.

[0101] According to one embodiment of the present invention, oxides formed on the surface of a steel sheet during primary annealing can be removed through primary pickling.

[0102] In the first pickling, the type of acid is not particularly limited, but according to one embodiment of the present invention, a 3 to 20 wt% acid solution at 30 to 80°C can be used. According to one embodiment of the present invention, pickling can be performed using 5 wt% hydrochloric acid at 50 to 60°C. According to one embodiment of the present invention, when applying strong pickling, 18 wt% hydrochloric acid at 80°C can be used.

[0103] According to one embodiment of the present invention, metal plating may be additionally performed after the primary acid wash. According to one embodiment of the present invention, Fe, Ni, etc. may be plated as the metal plating. By additionally performing the metal plating process, it may be helpful to prevent Si, Mn, etc. from diffusing to the surface during secondary annealing. The metal plating conditions are not particularly limited and may be conditions applicable in the same technical field. According to one embodiment of the present invention, when Fe is plated, the adhesion amount is 0.1 to 3 g / m. 2 According to one embodiment of the present invention, when Ni is plated, the adhesion amount is 5 to 700 mg / m 2 It may be. According to one embodiment of the present invention, when plating metal, a device at the exit side of the annealing line may be used.

[0104] Secondary annealing

[0105] The above-mentioned first-processed cold-rolled steel sheet can be annealed for the second time at a temperature range of 700 to 900°C and a dew point temperature of -60 to 30°C.

[0106] According to one embodiment of the present invention, the desired tensile properties can be secured by performing secondary annealing. During secondary annealing, annealing can be performed in a state where a deficiency layer is formed from the surface of the steel sheet to a certain depth, in which Si, Mn, Cr, B, etc. in a solid solution state are significantly reduced due to the internal oxidation layer formed during the primary annealing process. Due to this deficiency layer, surface oxides of Si, Mn, Cr, and B that diffuse to the surface of the steel sheet and are formed during secondary annealing can be significantly suppressed.

[0107] That is, by performing primary annealing and primary pickling prior to secondary annealing, the steel sheet after secondary annealing can have a cleaner surface condition. The surface of the steel sheet in a clean condition has a large Fe exposure area, which improves surface reactivity and thus can have the effect of improving phosphate treatability. The phosphate treatability is improved as the number of oxides on the surface of the annealed steel sheet decreases, because the phosphate solution can easily react with the Fe of the steel sheet. Surface oxides that adversely affect phosphate treatability include various substances such as Mn, Si, Cr, and B, but Mn and Si are representative of them.

[0108] During secondary annealing, an annealing temperature below 700°C may be insufficient to achieve the desired microstructure. Conversely, an annealing temperature exceeding 900°C may result in reduced economic efficiency due to increased energy consumption.

[0109] During secondary annealing, if the dew point temperature is below -60°C, it is realistically very difficult to maintain the atmosphere in large-scale production facilities. According to one embodiment of the present invention, in addition to securing physical properties, the lower limit may be limited to -10°C to form a deeper internal oxidation layer and decarburization layer. According to one embodiment of the present invention, the lower limit may be 0°C or higher. On the other hand, if the dew point temperature exceeds 30°C, the iron oxidizes, making it difficult to additionally secure a decarburization layer. According to one embodiment of the present invention, the lower limit may be 20°C or lower.

[0110] According to one embodiment of the present invention, during the secondary annealing, the holding time may be 30 to 300 seconds.

[0111] According to one embodiment of the present invention, during secondary annealing, the annealing atmosphere may be a nitrogen gas atmosphere containing 1 to 80 vol% hydrogen. If the hydrogen content is less than 1 vol%, the iron in the steel sheet may oxidize. On the other hand, if the hydrogen content exceeds 80 vol%, there is a risk of explosion in the event of a gas leak, which may increase costs.

[0112] During secondary annealing, cooling conditions are not particularly limited. According to one embodiment of the present invention, during secondary annealing, slow cooling is performed to 650°C after holding, and then rapid cooling can be performed depending on the target physical properties. According to one embodiment of the present invention, during cooling, conditions are not particularly limited, but conditions can be changed to achieve the target physical properties. In addition, since the formation of surface oxides, internal oxidation layers, and decarburization layers mostly occur in relatively high temperature regions, the present invention does not particularly limit the cooling conditions.

[0113] Meanwhile, according to one embodiment of the present invention, during secondary annealing, gas cooling and wet cooling using water can be performed in the same manner as during primary annealing after maintenance.

[0114] According to one embodiment of the present invention, during cooling, a reducing atmosphere may be applied to at least the iron to prevent oxidation of the iron. To maintain the reducing atmosphere, a nitrogen gas atmosphere containing 1 to 80 vol% hydrogen, similar to the atmosphere gas used during secondary annealing, may be applied.

[0115] 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, if necessary.

[0116] Second mountain

[0117] The above-mentioned second-annealed cold-rolled steel sheet can be subjected to second acid pickling.

[0118] According to one embodiment of the present invention, surface oxides formed during secondary annealing can be removed through secondary pickling. According to one embodiment of the present invention, during secondary pickling, pickling can be performed using the same method as the primary pickling.

[0119] In the secondary pickling, the type of acid is not particularly limited, but according to one embodiment of the present invention, a 3 to 20 wt% acid solution at 30 to 80°C can be used. According to one embodiment of the present invention, pickling can be performed using 5 wt% hydrochloric acid at 50 to 60°C. According to one embodiment of the present invention, when applying strong pickling, 18 wt% hydrochloric acid at 80°C can be used.

[0120] According to one embodiment of the present invention, metal plating may be additionally performed after secondary pickling. By additionally performing the metal plating process after secondary pickling, the surface conditioner can be effectively absorbed onto the steel sheet surface during the surface conditioning step during phosphate treatment, thereby contributing to improved phosphate treatment properties. After secondary pickling, the metal plating conditions can be applied in the same manner as the metal plating process after primary pickling described above.

[0121] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0122] (Example)

[0123] Cold-rolled steel sheets having the compositions shown in Table 1 below were prepared. Primary annealing and primary pickling were performed with two types of steel sheets under the conditions shown in Table 2 below. Primary annealing was performed under a reducing atmosphere of nitrogen gas containing 5% hydrogen. Further, primary annealing was performed by heating the steel sheets to 800°C at a heating rate of 3°C / s and maintaining the temperature for 150 seconds. After cooling, the steel sheets were immersed in a 5 vol% hydrochloric acid solution at 50°C for 5 seconds to perform primary pickling.

[0124] Steel alloy composition (weight %)CMnSiCrAlPSTiBA0.202.41.0-0.0300.010.0020.020.0018B0.192.71.60.50.0400.010.0010.020.0020

[0125] In addition, Table 2 below shows the results of performing the first annealing, measuring the surface enrichment of Si and Mn before and after the first pickling, and analyzing the depth of the internal oxidation layer and decarburization layer. The surface enrichment of Si and Mn before and after pickling can be estimated by the amount of Si and Mn oxide remaining on the surface of the steel sheet. The surface enrichment of Si and Mn is that they diffuse to the surface during annealing, combine with oxygen in the atmosphere, and enrich on the surface, so they can be viewed as the amount of oxidation.

[0126] The specific Si and Mn surface enrichment amounts can be calculated by conducting GDS analysis. The GDS data is processed so that the sum of the weight % of the main components of each steel component, Fe, Mn, Si, Cr, and B, is 100%. Here, Fe is included because it is the main component of the steel plate, and oxygen is not included. In addition to the above components, components contained in the steel plate can be processed by adding them, but the components that enrich on the surface during annealing are limited and the amount is extremely small, so they are omitted.

[0127] Figure 1 illustrates a method for calculating the Mn surface enrichment amount of a steel sheet according to an embodiment of the present invention. The Si surface enrichment amount can also be calculated using the same method described below. As shown in Figure 1, a graph is drawn with the depth of the processed data on the x-axis and the weight % of each component on the y-axis. After limiting the depth for each component to 0.1 μm, the minimum weight % point of each component within 0.1 μm is found, and that value is estimated as the dissolved component amount. Once the dissolved component amount is found, data at deeper depths are deleted based on that location. The dissolved component amount is subtracted from each component weight % from the surface to the dissolved component amount location. This process is to extract only oxides from the GDS data and to calculate by excluding the dissolved Mn or Si values. Thereafter, the integrated value from the surface to the dissolved component value location using the data from which the dissolved component value has been subtracted can be estimated as the enrichment amount. If the minimum weight% value of each component occurs at the outermost layer between depths of 0 and 0.01 μm, it can be interpreted that there is almost no surface concentration of the component.

[0128] In addition, the depth of the internal oxidation layer in Table 2 was recorded by observing the cross-section of the annealed steel plate with a SEM at 3000x magnification and entering the depth of the grain boundary internal oxidation. Fig. 2 is a cross-sectional SEM image of Invention Example 1 according to one embodiment of the present invention, and illustrates a method for measuring the depth of the internal oxidation layer. As in Fig. 2, three points can be measured randomly in the 3000x magnification image to derive an average value.

[0129] The specific depth of the decarburization layer can be derived by analyzing the carbon content by depth using a GDS (Glow Discharge Spectrometer). There are two methods. The first is when a section of the carbon content graph appears horizontal within the GDS measurable depth, and when GDS analysis is possible beyond the decarburization layer depth to the bulk structure. In this case, the decarburization layer depth is obtained as shown in FIG. 3. FIG. 3 illustrates a method for obtaining the depth of the decarburization layer of a steel sheet according to an embodiment of the present invention, and is a graph showing the carbon concentration according to the depth of the steel sheet during GDS (Glow Discharge Spectrometer) analysis. A graph is drawn with carbon content, and the average of the 10 deepest carbon content data values ​​in the carbon content graph is obtained. The average value of these 10 carbon content data at the deepest depth is called the depth value. Then, among the carbon content data from the deepest location toward the surface, the depth that shows the first carbon content that is 5% or more lower than the depth value is found. This point is the deepest decarburization layer depth. The total decarburization depth of cold rolled steel sheet is from the surface to the deepest decarburization depth.

[0130] The second case is when the depth of the decarburization layer is very deep and beyond the GDS measurable range. In this case, as in the first case, the analysis is conducted based on the 10 carbon content values ​​of the deepest depth. Whether the carbon content of the deepest depth of the GDS data has reached the deep value can be estimated using the OES (Optical Emission Spectrometry) analysis value. That is, when analyzing to the deepest depth measurable by GDS, the analysis value will be at least 50 μm deep. If the carbon content of the deepest depth is lower than the OES carbon content, it means that the depth has not been reached yet, and the decarburization depth can be estimated to be at least 50 μm. To determine the approximate depth of the decarburization layer, cross-sectional tissue observation is performed through nital etching (ethanol or methanol containing 2-5 vol% nitric acid). FIG. 4 is a cross-sectional SEM image of a steel plate according to an embodiment of the present invention, illustrating a method for determining the depth of a decarburized layer of a nital-etched steel plate. As shown in FIG. 4, observation can be made with an SEM, and the magnification can be from 1000x to 3000x. The decarburized structure of the surface layer shows coarse grains mainly composed of ferrite, and the deep (bulk) structure shows a microstructure including austenite. The depth composed mainly of coarse decarburized structure can be regarded as the depth of the decarburized layer.

[0131] Specimen numberSteel grade1st annealingAfter 1st annealingAfter 1st annealing and 1st picklingDew point temperature(℃)Si surface enrichmentMn surface enrichmentSi surface enrichmentMn surface enrichmentInternal oxidation layer depth(μm)Decarburization layer depth(μm)1A-400.2080.3390.06720.0082072A50.0160.2120.00520.00484.4143B-400.2110.4220.06980.0095044B50.0130.2550.00440.00523.817

[0132] Specimen Nos. 1 and 3 in Table 2 above are cases where the dew point temperature is -40°C and does not satisfy the conditions of the present invention, and specimen Nos. 2 and 4 are cases where the dew point temperature satisfies the conditions of the present invention. It can be confirmed that the surface concentrations of Si and Mn of specimens 2 and 4 are significantly reduced compared to specimens 1 and 3.

[0133] Meanwhile, looking at the results after pickling, it can be confirmed that the amount of Si surface enrichment is lower for specimens No. 1 and 3 than before pickling, but for specimens No. 2 and 4, a small amount of enrichment was observed due to internal oxidation even before pickling, so it can be confirmed that the amount after pickling is significantly lower than for specimens No. 1 and 3.

[0134] The surface concentration of Mn also decreased, but the rate of decrease was small compared to the surface concentration of Si. The surface concentration of Mn after pickling was greatly reduced compared to before pickling. This is because the Mn oxide is easily dissolved by pickling. Before pickling, in the case of specimens No. 2 and 4, whose dew point temperatures satisfied the conditions of the present invention, the surface concentration of Mn was lower than that of specimens No. 1 and 3 because an internal oxide layer was formed, and this trend was the same after pickling.

[0135] In the case of specimens No. 1 and 3, it can be confirmed that no internal oxidation layer was formed, and the decarburization layer was also hardly formed, with a thickness of less than 7 μm.

[0136] For specimens Nos. 2 and 4, Si, which has a high oxidation tendency, was unable to diffuse to the surface and formed an internal oxidation layer by combining with oxygen that had penetrated into the steel plate due to the high dew point temperature in the high atmosphere. Therefore, it can be confirmed that the surface concentration of Si is higher for specimens Nos. 1 and 3 than for specimens Nos. 2 and 4.

[0137] In the case of specimen numbers 1 and 3, if Si and Mn do not form an internal oxide layer but diffuse to the surface to form surface oxide, it is difficult to remove the Si surface oxide even if pickling is performed thereafter. It can be confirmed that Mn, which is easily soluble in hydrochloric acid compared to Si, was removed after pickling. However, if secondary annealing is performed with a large amount of Si oxide formed on the surface, oxygen in the atmosphere is adsorbed to the steel sheet by the Si surface oxide and is prevented from diffusion into the interior, and the decarburization reaction in which carbon inside the steel sheet diffuses to the surface and combines with the adsorbed oxygen is also disadvantageous.

[0138] In addition, using the cold-rolled steel sheet having the composition of Table 1, first annealing, first pickling, second annealing, and second pickling were performed under the conditions shown in Table 3 below. At this time, during the first annealing, the atmosphere was nitrogen gas containing 5% hydrogen, which was a reducing atmosphere. In addition, during the first annealing, the temperature was increased to 800°C at a heating rate of 3°C / s, maintained for 150 seconds, and then cooled, followed by immersion in a 5 vol% hydrochloric acid solution at 50°C for 5 seconds to perform the first pickling. Subsequently, during the second annealing, the atmosphere was nitrogen gas containing 5% hydrogen, which was a reducing atmosphere, and during the second annealing, the temperature was increased to 810°C at a heating rate of 3.2°C / s, maintained for 50 seconds, and then cooled. During cooling, the steel sheet was first cooled at 3.1°C / s to 650°C, then cooled again at 20°C / s to 350°C. After the second cooling, the steel sheet was finally cooled at 2°C / s to room temperature to produce an annealed steel sheet. The final annealed steel sheet was immersed in a 5 vol% hydrochloric acid solution at 50°C for 5 seconds for a second pickling.

[0139] The surface concentrations of Si and Mn and the depths of the internal oxidation layer and decarburization layer were measured for the manufactured steel plate using the above-described method, and are shown in Table 3 below.

[0140] In addition, to evaluate the phosphate treatment property, the phosphate film coverage was measured and shown in Table 3 below. The steel plate that had completed the secondary pickling was subjected to the steps of degreasing, surface conditioning, and phosphate treatment to perform phosphate treatment. Degreasing was performed using an alkaline degreaser at 45°C for 120 seconds, and surface conditioning was performed at room temperature for 30 seconds. After that, the steel plate was immersed in a phosphate treatment solution at 35°C for 40 seconds to form a film.

[0141] Meanwhile, the phosphate treatment according to one embodiment of the present invention is a method for relative evaluation between specimens, and although it does not match the method used by automobile manufacturers, the tendency is not significantly different even when evaluated with other solutions and methods.

[0142] The phosphate film coverage measurement values ​​in Table 3 below were recorded by analyzing the image of the phosphate film observed at 500x magnification using an SEM after phosphate treatment. Here, the coverage is the area ratio that the phosphate film covers the steel plate surface.

[0143] In addition, LME resistance evaluation was performed and the results are shown in Table 3 below. The LME evaluation was performed according to the SEP 1220-2 standard, and spot welding was performed by overlapping the manufactured cold-rolled steel sheet and the hot-dip galvanized steel sheet without a decarburized layer. The LME crack length was measured by observing the welded cold-rolled steel sheet in the 0°, 45°, and 90° directions for each specimen using OM (Optical Microscopy). Only B type cracks in the heat-affected zone were measured, and the maximum crack length was indicated as ○, △, and ×.

[0144] ○: LME crack maximum length 10 μm or less

[0145] △: LME crack maximum length exceeds 10 μm and is less than 30 μm

[0146] ×: LME crack maximum length exceeds 30 μm

[0147] Specimen number Steel grade 1st annealing 2nd annealing 2nd annealing and 2nd pickling Phosphate film coverage (%) LME evaluation classification Dew point temperature (℃) Dew point temperature (℃) Si surface enrichment Mn surface enrichment Internal oxidation layer depth (μm) Decarburization layer depth (μm) 1A Not performed - 40 0.0586 0.00110538 × Comparative example 12A Not performed 50.0107 0.0068 2.61863 △ Comparative example 23A - 40 - 40 0.0854 0.00720235 Less than × Comparative Example 34A-4050.07100.00662.31650 △Comparative Example 45A5-400.00500.00324.42582 ○ Invention Example 16A550.00130.00246.25588 ○ Invention Example 27B Not implemented-400.06240.00070435 Less than × Comparative Example 58B Not implemented50.01180.00712.81558 △Comparative Example 69B-40-400.10440.00970535 Less than × Comparative Example 710B-4050.07670.00582.11447△Comparative Example 811B5-400.00530.00733.84881○Invention Example 312B550.00200.00186.77385○Invention Example 4

[0148] As shown in Table 3 above, the invention examples that satisfy the alloy composition and manufacturing method proposed in the present invention have secured all the desired characteristics.

[0149] In particular, Invention Examples 1 and 2 are examples in which internal oxidation was performed in the first annealing while performing two annealings. Invention Example 1, an internal oxide layer was formed to a certain depth or more even though internal oxidation was not performed during the second annealing, and the phosphate coverage characteristics were also excellent. This is the result of performing internal oxidation during the first annealing to form a sufficient level of Mn and Si deficiency layer, and performing the second annealing in a clean state by removing the Mn surface oxide formed on the surface due to pickling after annealing. If the surface of the steel sheet after the first annealing is clean, there will be almost no oxide on the surface of the steel sheet during the second annealing, and even if internal oxidation is not performed during the second annealing, the amount of Si and Mn diffusing to the surface may be limited due to the already existing deficiency layer. As a result, the amount of Si surface enrichment on the final surface is very low. Invention Example 2 shows an even cleaner surface state by performing internal oxidation even during the second annealing. The clean steel plate surface with almost no surface oxide after the first annealing and first pickling allows oxygen to penetrate more easily and deeply into the steel plate during the second annealing, so there are almost no alloying elements that diffuse to the surface during the second annealing.

[0150] Invention examples 3 and 4 also show similar trends to invention examples 1 and 2.

[0151] On the other hand, Comparative Examples 1 and 2 are examples in which only secondary annealing was performed without primary annealing. In Comparative Example 1, no internal oxidation layer was observed, and the phosphate coverage was also very poor. The Si surface concentration was relatively high, which means that a large amount of Si oxide was formed on the surface. In addition, the depth of the decarburization layer was also observed to be shallow. In Comparative Example 2, the internal oxidation layer was formed to a depth of 2.6 μm, which suppressed surface oxidation to a certain extent, and the phosphate coverage was also improved. In addition, the decarburization layer was also formed to a thickness of 18 μm, but it was not sufficient to expect an LME improvement effect.

[0152] Comparative Examples 3 and 4 are examples in which first and second annealing were performed, but internal oxidation was not performed during the first annealing. In Comparative Example 3, a large amount of surface oxide was formed during the first annealing, and the Mn surface oxide was removed through pickling, but the Si surface oxide remained. As the second annealing proceeded in this state, the surface oxide accumulated further, resulting in very poor phosphate coverage. In Comparative Example 4, a large amount of surface oxide was formed during the first annealing, and the Si surface oxide remained after pickling, but internal oxidation occurred during the second annealing, and the additional surface oxide due to the second annealing was relatively reduced. However, the phosphate coverage level targeted by the present invention was not secured.

[0153] Comparative Examples 2, 4, and Inventive Example 1 are all cases where internal oxidation was performed once. Meanwhile, the first annealing of the present invention for generating internal oxidation has a longer maintenance time than the second annealing, and the time maintained at high temperature is dominant in forming the internal oxidation layer and decarburization layer. Therefore, the depth of the internal oxidation layer and decarburization layer of Inventive Example 1, which performed internal oxidation in the first annealing, was relatively deeper than that of Comparative Examples 2 and 4, which performed internal oxidation in the second annealing.

[0154] Comparative examples 5 to 8 also show similar trends to comparative examples 1 to 4.

[0155] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Contains, in weight%, C: 0.10 to 0.25%, Mn: 1.5 to 5.0%, Si: 0.5 to 2.5%, Cr: 1.5% or less, Al: 0.005 to 0.100%, P: 0.10% or less, S: 0.020% or less, B: 0.0050% or less, the remainder being Fe and other unavoidable impurities. Cold rolled steel sheet with a Si surface concentration of 0.0100 or less.

2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further containing 1.2% or less of at least one of Ti, Mo, and Nb in weight %.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having an internal oxide layer composed of one or more oxides of Mn, Si, Cr, and B, the depth of which is 3 to 15 μm in the direction of the center of thickness from the surface of the steel sheet.

4. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a decarburization layer having a depth of 20 to 150 μm from the surface of the steel sheet in the direction of the center of thickness.

5. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a phosphate film coverage of 80% or more after being immersed in a phosphate treatment solution at 35°C for 40 seconds.

6. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet with an LME crack maximum length of 10 μm or less.

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

8. In claim 7, A method for manufacturing a cold rolled steel sheet, wherein the cold rolled steel sheet further contains, in weight %, 1.2% or less of at least one of Ti, Mo, and Nb.

9. In claim 7, The above first annealing step is maintained for 50 to 600 seconds, A method for manufacturing cold rolled steel sheets, wherein the above second annealing step is performed at a dew point temperature of -60 to 30°C.

10. In claim 7, A method for manufacturing a cold rolled steel sheet in which the first and second annealing steps are performed in a nitrogen gas atmosphere containing 1 to 80 vol% of hydrogen.

11. In claim 7, A method for manufacturing a cold rolled steel sheet, further comprising a step of metal plating after either the first pickling step or the second pickling step.

12. In claim 11, A method for manufacturing a cold rolled steel sheet, wherein the above metal plating step is Fe or Ni plating.

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