High-strength thin cold-rolled steel sheet, galvanized steel sheet, and manufacturing method thereof

Abstract

A cold-rolled steel sheet according to an embodiment of the present invention contains, in wt%, 0.15-0.22% of carbon (C), 0.75-1.2% of manganese (Mn), 0.005-0.02% of phosphorus (P), 0.005-0.07% of copper (Cu), and 0.015-0.05% of aluminum (Al), with the remainder comprising Fe and inevitable impurities, wherein the area fraction of ferrite in the steel sheet is at least 80%.

Description

High-strength cold-rolled steel sheets, galvanized steel sheets, and methods for manufacturing them

[0001] One embodiment of the present invention relates to high-strength cold-rolled steel sheets, galvanized steel sheets, and methods for manufacturing the same. More specifically, one embodiment of the present invention relates to high-strength cold-rolled steel sheets, galvanized steel sheets, and methods for manufacturing the same, which can be used in refrigerators, washing machines, air conditioner outdoor units, dishwashers, kitchen appliances, and the like.

[0002] With carbon emissions reduction emerging as a social issue, and ongoing efforts to reduce costs, customers are increasingly demanding thinner steel products. However, since steel is primarily used for rigid support purposes, particularly in home appliances, simple thinning can result in reduced rigidity. To achieve thinning without compromising product rigidity, high-strength materials, particularly those with higher yield strength compared to existing materials, are required. However, increased yield strength comes at the expense of formability, such as reduced elongation. Therefore, materials with both appropriate elongation and yield strength must be developed. CQ (Commercial Quality) and DQ (Drawing Quality) grades, primarily used in home appliance components, have yield strengths of 150 to 250 MPa and elongations of 35 to 55%. As a way to increase strength while minimizing the decrease in elongation compared to existing materials, the use of steel grades with a lower yield strength of 300 or 340 MPa is gradually increasing.

[0003] High-yield strength steel grades present various challenges in terms of application, but there are also many manufacturing considerations. In particular, if the final cold-rolled product is a thin, thin-walled steel, the thickness of the hot-rolled steel used as the raw material for the cold-rolled product must be further considered. If the hot-rolled steel is thickened for hot-rollability, the cold rolling reduction rate increases, leading to higher cold rolling loads. However, to achieve a lower cold rolling reduction rate, the hot-rolled steel must be hot-rolled into a thin-walled steel.

[0004] In addition, when utilizing elements that form precipitates, such as Ti and Nb, which were mainly used for high strength in the past, precipitates such as TiN, TiS, TiC, and Nb(C,N) are formed in a complex manner depending on the manufacturing conditions during the hot rolling process, and additional precipitates are formed during the annealing process after cold rolling. Accordingly, if the temperature within the manufacturing conditions is not strictly controlled, material deviations occur due to temperature deviations in the longitudinal and transverse directions of the final cold rolled steel sheet, so there is a problem that additional equipment must be utilized to control the hot rolling manufacturing conditions or the annealing conditions after cold rolling or to minimize temperature changes.

[0005] One embodiment of the present invention provides high-strength cold-rolled steel sheets, galvanized steel sheets, and methods for manufacturing the same. More specifically, one embodiment of the present invention provides high-strength cold-rolled steel sheets, galvanized steel sheets, and methods for manufacturing the same, which can be used in refrigerators, washing machines, air conditioner outdoor units, dishwashers, kitchen appliances, and the like.

[0006] A cold-rolled steel sheet according to one embodiment of the present invention contains, in wt%, carbon (C): 0.15 to 0.22%, manganese (Mn): 0.75 to 1.2%, phosphorus (P): 0.005 to 0.02%, copper (Cu): 0.005 to 0.07%, and aluminum (Al): 0.015 to 0.05%, with the remainder being Fe and unavoidable impurities, and the area fraction of ferrite in the steel sheet is 80% or more.

[0007] A cold rolled steel sheet according to one embodiment of the present invention can satisfy the following equation 1.

[0008] [Formula 1]

[0009] 571×[C] + 63×[Mn] ≥ 140

[0010] (In Equation 1, [C] and [Mn] are the contents (weight%) of C and Mn, respectively.)

[0011] A cold-rolled steel sheet according to one embodiment of the present invention may further include at least one of Ti: 0.01 wt% or less and Nb: 0.01 wt% or less.

[0012] A cold rolled steel sheet according to one embodiment of the present invention may have a sum of area fractions of bainite, martensite, and pearlite of 10% or less.

[0013] A cold rolled steel sheet according to one embodiment of the present invention may have a thickness of 1.0 mm or less.

[0014] According to one embodiment of the present invention, a cold-rolled steel sheet may not form cracks in a bending portion when the steel sheet is subjected to a bending test with a load of 10 tons in a V-bending die having a curvature of r=0.

[0015] A cold-rolled steel sheet according to one embodiment of the present invention may have a yield strength of 340 MPa or more, a tensile strength of 450 MPa or more, and an elongation of 20% or more.

[0016] A method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention includes the steps of: hot-rolling a slab containing, in wt%, carbon (C): 0.15 to 0.22%, manganese (Mn): 0.75 to 1.2%, phosphorus (P): 0.005 to 0.02%, copper (Cu): 0.005 to 0.07%, and aluminum (Al): 0.015 to 0.05%, with the remainder being iron and unavoidable impurities; cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and annealing the cold-rolled steel sheet.

[0017] In the process of manufacturing hot-rolled steel sheets, the rolling is completed at a temperature of 850°C or higher, and coiling can be performed at a temperature of 610°C or higher.

[0018] In the manufacturing stage of cold rolled steel sheets, the reduction ratio can be 70% or more.

[0019] In the annealing step, crack treatment can be performed at 750 to 830°C, and cooling can be performed from the cracking temperature to 550°C at a cooling rate of 35°C / sec or less.

[0020] Prior to the step of manufacturing the hot-rolled steel sheet, a step of heating the slab at 1100°C or higher for 150 minutes or longer may be further included.

[0021] When the slab is heated at a heating rate of 20°C / sec, maintained at 1200°C for 3 minutes, cooled at a cooling rate of 10°C / sec, and then subjected to a compression test at a temperature of 900°C to a deformation of 1.2, the maximum stress value may be 180 MPa or less.

[0022] In the process of manufacturing hot-rolled steel sheets, rough rolling can be completed at a temperature of 950℃ or higher.

[0023] After the step of manufacturing the hot-rolled steel sheet, the average grain size of the hot-rolled steel sheet may be 20㎛ or more.

[0024] After the step of manufacturing the hot-rolled steel sheet, the hot-rolled steel sheet may contain carbides in an amount of 20 area% or less.

[0025] According to one embodiment of the present invention, a galvanized steel sheet has a galvanized layer on the surface of the cold-rolled steel sheet described above.

[0026] The high-strength, thin-walled cold-rolled steel sheet according to one embodiment of the present invention can be utilized in various fields, and in particular, can be effectively utilized mainly for refrigerator doors and cavities in home appliance manufacturing.

[0027] When a high-strength, thin-walled cold-rolled steel sheet according to one embodiment of the present invention is used for home appliances, the weight of steel used in each home appliance is reduced due to thinning, and thus carbon emissions emitted in the production of individual products can be reduced, so there is an advantage of being environmentally friendly.

[0028] In addition, the high-strength, thin-walled cold-rolled steel sheet according to one embodiment of the present invention is economical because it can exclude elements such as Nb and Ti, and has the advantage of being environmentally friendly because it can reduce the emission of chemical impurities during the process of producing the elements.

[0029] Figure 1 is a photograph of the microstructure of a hot-rolled steel sheet manufactured in Invention Example 1 analyzed using an optical microscope.

[0030] Figure 2 is a photograph of the microstructure of the hot-rolled steel sheet manufactured in Comparative Example 8 analyzed using an optical microscope.

[0031] Figure 3 is a photograph of a cold-rolled steel sheet manufactured in Invention Example 1 after a V-bending experiment.

[0032] Figure 4 is a photograph of the cold-rolled steel sheet manufactured in Comparative Example 4 after a V-bending experiment.

[0033] The terms first, second, and third, etc., are used to describe, but are not limited to, various parts, components, regions, layers, and / or sections. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0034] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include plural forms as well, unless the context clearly dictates otherwise. The word "comprising," as used herein, specifies particular features, regions, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0035] When a part is referred to as being "on" or "over" another part, it can be directly on or over the other part, or there may be other parts intervening. Conversely, when a part is referred to as being "directly on" another part, there are no other parts intervening.

[0036] Although not defined otherwise, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or overly formal unless otherwise defined.

[0037] Also, unless otherwise stated, % means weight%, and 1 ppm is 0.0001 weight%.

[0038] In one embodiment of the present invention, the inclusion of additional elements means including the remaining iron (Fe) in an amount equivalent to the additional amount of the additional elements.

[0039] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0040] A cold-rolled steel sheet according to one embodiment of the present invention contains, in wt%, carbon (C): 0.15 to 0.22%, manganese (Mn): 0.75 to 1.2%, phosphorus (P): 0.005 to 0.02%, copper (Cu): 0.005 to 0.07%, and aluminum (Al): 0.015 to 0.05%, with the remainder being iron and unavoidable impurities.

[0041] Below, the reasons for limiting alloy components are explained.

[0042] Carbon (C): 0.15 to 0.22 wt%

[0043] Carbon (C) is an element that, when incorporated into steel, beneficially enhances strength. If too little C is present, it is difficult to achieve the desired strength characteristics. If too much C is present, a large amount of carbides and hard phases may form, leading to reduced weldability during the production process. More specifically, C may be present in an amount of 0.16 to 0.21 wt%.

[0044] Manganese (Mn): 0.75 to 1.20 wt%

[0045] Manganese (Mn) is dissolved in steel and plays a role in improving the yield strength of the steel. Insufficient Mn content makes it difficult to achieve the desired strength. Excessive Mn content can lead to excessive Mn band formation within the microstructure, which can cause cracks when bending in the rolling direction. More specifically, Mn can be included in an amount of 0.77 to 1.15 wt%.

[0046] A cold rolled steel sheet according to one embodiment of the present invention can satisfy the following equation 1.

[0047] [Formula 1]

[0048] 571×[C] + 63×[Mn] ≥ 140.0

[0049] When the C and Mn values ​​are adjusted to the values ​​of Equation 1, the A1 temperature, which is the completion temperature at which ferrite is formed, is lowered, carbide formation during the hot rolling stage is suppressed, and the hot rolling load during hot rolling can be controlled. More specifically, the value of the left side of Equation 1 can be 145.0 to 185.0.

[0050] Phosphorus (P): 0.005 to 0.020 wt%

[0051] Phosphorus is a representative solid-solution strengthening element that can enhance strength by being incorporated into steel. If phosphorus is not included sufficiently, it is difficult to properly perform the aforementioned role. Excessive phosphorus inclusion can cause room-temperature embrittlement. More specifically, phosphorus may be included in an amount of 0.010 to 0.015 wt%.

[0052] Copper (Cu): 0.005 to 0.070 wt%,

[0053] Copper has a low melting point, so when it is concentrated on the surface, it can penetrate into grain boundaries during the reheating process, causing defects in the hot-rolled slab. These defects can develop into edge scabs during hot rolling. More specifically, it can contain 0.010 to 0.050 wt% of Cu.

[0054] Aluminum (Al): 0.015 to 0.050 wt%

[0055] Aluminum is a representative deoxidizing element used to control oxygen in molten steel during the steelmaking process. High aluminum content can result in the formation of coarse aluminum oxide inclusions. These coarse oxides, when located at points of high deformation, can lead to crack formation. More specifically, aluminum can be included in amounts ranging from 0.020 to 0.035 wt%.

[0056] In one embodiment of the present invention, by excluding the addition of expensive Nb and Ti, material deviations in the longitudinal and transverse directions can be reduced. Specifically, the cold-rolled steel sheet according to one embodiment of the present invention may further include at least one of Ti: 0.01 wt% or less and Nb: 0.01 wt% or less. More specifically, it may further include at least one of Ti: 0.0001 to 0.01 wt% and Nb: 0.0001 to 0.01 wt%.

[0057] A cold-rolled steel sheet according to one embodiment of the present invention contains iron (Fe) as a remainder. It may also contain unavoidable impurities. Unavoidable impurities refer to impurities that are inevitably mixed in during the steelmaking and cold-rolled steel sheet manufacturing processes. Since unavoidable impurities are widely known, a detailed description thereof will be omitted. In one embodiment of the present invention, the addition of elements other than the aforementioned alloy components is not excluded, and various elements may be included within a scope that does not impair the technical spirit of the present invention. When additional elements are included, they are included in place of the remainder Fe.

[0058] According to one embodiment of the present invention, a cold-rolled steel sheet has an area fraction of ferrite within the steel sheet of 80% or more. In one embodiment of the present invention, by maximizing ferrite, workability can be further secured. Ferrite can be detected by image analysis using an optical microscope, and its area fraction can be determined by measuring a cross-section of a rolled specimen including the rolling direction using an optical microscope and an image analysis device. More specifically, the area fraction of ferrite can be 80 to 99 area%.

[0059] In addition to the aforementioned ferrite, other phases may include bainite, martensite, and pearlite. These are hard phases, and if they are included in large quantities, cracks may occur during forming. Therefore, the combined area fraction of bainite, martensite, and pearlite can be controlled to 10% or less. More specifically, it can be 1 to 5% by area.

[0060] A cold-rolled steel sheet according to one embodiment of the present invention exhibits excellent strength, and even when formed into a thin film, this strength remains high. Accordingly, the steel sheet may have a thickness of 1.0 mm or less. More specifically, the thickness may be 0.5 mm or less. More specifically, the thickness may be 0.1 to 0.4 mm.

[0061] According to one embodiment of the present invention, a cold-rolled steel sheet may not form cracks in the bending portion when subjected to a bending test with a load of 10 tons in a V-bending die having a curvature of r=0. Cracks may be detected with the naked eye, but in the present invention, a magnifying glass was used.

[0062] A cold-rolled steel sheet according to one embodiment of the present invention has excellent mechanical properties. Specifically, the yield strength may be 340 MPa or more, the tensile strength may be 450 MPa or more, and the elongation may be 20% or more. More specifically, the yield strength may be 340 to 430 MPa, the tensile strength may be 450 to 550 MPa, and the elongation may be 20 to 35%. Even more specifically, the yield strength may be 345 to 420 MPa, the tensile strength may be 480 to 545 MPa, and the elongation may be 21 to 33%. These properties may be measured by processing a cold-rolled steel sheet into a JIS13B standard specimen and then performing a uniaxial tensile test.

[0063] The cold-rolled steel sheet according to one embodiment of the present invention can be used as is, but can also be used after being galvanized. That is, the galvanized steel sheet according to one embodiment of the present invention has a galvanized layer on the surface of the aforementioned cold-rolled steel sheet. Since galvanizing is widely known, a detailed description thereof will be omitted.

[0064]

[0065] A method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention includes the steps of: hot-rolling a slab containing, in wt%, carbon (C): 0.15 to 0.22%, manganese (Mn): 0.75 to 1.2%, phosphorus (P): 0.005 to 0.02%, copper (Cu): 0.005 to 0.07%, and aluminum (Al): 0.015 to 0.05%, with the remainder being iron and unavoidable impurities; cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and annealing the cold-rolled steel sheet.

[0066] The alloy composition of the slab is identical to that of the cold-rolled steel sheet described above, so further explanation will be omitted. Since the alloy composition does not change during the cold-rolled steel sheet manufacturing process, the alloy compositions of the slab and the cold-rolled steel sheet are substantially identical.

[0067] Prior to the step of manufacturing a hot-rolled steel sheet, a step of heating the slab at a temperature of 1100°C or higher for 150 minutes or longer may be further included. Heating the slab facilitates the subsequent hot rolling process and homogenizes the slab. More specifically, the slab may be heated at a temperature of 1150 to 1300°C for 180 to 240 minutes.

[0068] When the slab is heated at a heating rate of 20℃ / sec, maintained at 1200℃ for 3 minutes, and then cooled at a cooling rate of 10℃ / sec, and a compression test is performed at 900℃ to a strain of 1.2, the maximum stress value may be 180 MPa or less. The aforementioned stress value is conveniently referred to as high-temperature compressive stress. The high-temperature compressive stress can be measured using a Gleeble3500 device, and a cylindrical compression test specimen with a diameter of 10 mm and a height of 15 mm is taken from the slab and a compression test is performed using the same. By performing the aforementioned compression test, a strain-stress curve is obtained, and the maximum stress value of the strain-stress curve is determined. If the high-temperature compressive stress is too large, the rolling load may increase, making it difficult to control the thickness during rolling. More specifically, the high-temperature compressive stress may be 120 to 175 MPa. In this way, the cold-rolled steel sheet according to one embodiment of the present invention has excellent manufacturing ease due to low deformation resistance during hot rolling, and while having such low deformation resistance, it can simultaneously secure high strength through an annealing heat treatment process after cold rolling.

[0069] Returning to the description of the manufacturing method for cold-rolled steel sheets, first, slabs are hot-rolled to produce hot-rolled steel sheets. Hot rolling can be broadly divided into three stages: rough rolling to create bars, finishing rolling to create the final shape, and coiling to coil the rolled steel sheets.

[0070] In one embodiment of the present invention, rough rolling can be completed at a temperature of 950°C or higher. By appropriately controlling the rough rolling finishing temperature, the subsequent finishing temperature of the hot rolling process can be controlled, contributing to softening during the hot rolling process. More specifically, the rough rolling finishing temperature can be between 950°C and 1050°C.

[0071] In one embodiment of the present invention, the finishing rolling can be completed at a temperature of 850°C or higher. In one embodiment of the present invention, by controlling the finishing rolling temperature (FDT) of the hot-rolled steel sheet, the microstructure and carbide fraction of the hot-rolled steel sheet can be controlled, thereby optimizing the strength of the hot-rolled steel sheet. More specifically, the finishing rolling temperature can be 860 to 1000°C.

[0072] In one embodiment of the present invention, coiling can be performed at a temperature of 610°C or higher. If the coiling temperature (CT) is too low, it may be difficult to control the hot-rolled thickness due to increased rolling load during the production of hot-rolled steel sheets, and cold rolling may become difficult due to increased initial strength. More specifically, the coiling temperature may be 615 to 700°C.

[0073] The thickness of the coiled hot-rolled steel sheet may be 2.0 to 5.0 mm. The manufactured hot-rolled steel sheet may be free from edge cracks.

[0074] As described above, by controlling the finishing rolling temperature (FDT) and coiling temperature (CT), the microstructure and carbide of the hot-rolled steel sheet are controlled.

[0075] After the step of manufacturing the hot-rolled steel sheet, the average grain size of the hot-rolled steel sheet may be 20㎛ or more. A large grain size is advantageous in preventing an increase in load during hot rolling for thin films. More specifically, the average grain size of the hot-rolled steel sheet may be 20 to 40㎛. The grain size can be obtained by dividing the number of grains by the area measured using an optical microscope based on a cross-section including the rolling direction. The grain size refers to the diameter of an imaginary circle having the same area as the grains.

[0076] After the step of manufacturing a hot-rolled steel sheet, the hot-rolled steel sheet may contain carbides up to 20 area%. If a large amount of carbides is formed, the hot-rolling load increases and formability is adversely affected. More specifically, the hot-rolled steel sheet may contain carbides up to 10 to 20 area%. The carbide area is expressed numerically by observing the cross-section including the rolling direction under an optical microscope at 500x magnification and quantifying the area occupied by carbides out of the total area.

[0077] Afterwards, the step of pickling the hot-rolled steel sheet may be further included.

[0078] Next, cold rolled steel sheets are manufactured by cold rolling the hot rolled steel sheets.

[0079] In the manufacturing process of cold rolled steel sheets, the reduction ratio is 70% or more. If the cold reduction ratio is increased, the microstructure can be refined during cold rolling of the hot rolled sheet, and the recrystallized grain size can be made fine during recrystallization during the subsequent annealing process. More specifically, the reduction ratio can be 70 to 90%. The reduction ratio can be obtained by (steel sheet thickness before rolling - steel sheet thickness after rolling) / steel sheet thickness before rolling × 100(%). After cold rolling, the thickness of the cold rolled steel sheet can be 1.0 mm or less. More specifically, it can be 0.5 mm or less. More specifically, it can be 0.1 to 0.4 mm.

[0080] Next, the cold-rolled steel sheet is annealed. The annealing temperature can range from 750 to 830°C. If the annealing temperature is too low, unrecrystallization can occur or the fraction of hard phases in the microstructure can increase, making it difficult to secure sufficient formability. If the temperature is too high, it can be difficult to achieve the desired strength characteristics. More specifically, the annealing temperature can range from 755 to 825°C. The annealing time can range from 40 to 100 seconds.

[0081] Cooling from the cracking temperature to 550°C can be performed at a cooling rate of 35°C / sec or less. If the cooling rate is too fast, hard phases such as bainite or pearlite may form in the remaining austenite region. If a hard phase is formed, the material strength increases, but formability decreases, and cracks may occur, especially during V-bending. More specifically, cooling can be performed at a cooling rate of 10 to 25°C / sec.

[0082] After that, a step of zinc plating the cold rolled steel sheet may be further included. The plating step may utilize various plating methods, such as, for example, hot-dip galvanizing or electrogalvanizing. More specifically, hot-dip galvanizing may be performed in a plating bath having a temperature range of 400 to 500°C, specifically, 420 to 480°C. If the hot-dip galvanizing temperature is lower than the lower limit of the above-mentioned temperature range, the zinc may not be sufficiently melted, resulting in uneven plating. If the temperature is higher than the upper limit of the above-mentioned temperature range, the plating bath may volatilize, and during cooling, the plating bath may sufficiently combine with the steel sheet, hindering sound tissue formation, and there is a problem of the steel sheet melting during the plating process.

[0083] Additionally, electrogalvanizing can be performed with an anti-fingerprint resin coating after plating. The coating can be performed at a temperature range of 100 to 200°C. For example, if the temperature is lower than the lower limit of the above-mentioned temperature range, there is a risk that the coated resin will dry unevenly. If the temperature is higher than the upper limit of the above-mentioned temperature range, there is a risk that aging will occur during the drying process, resulting in an increase in the yield point due to yield point elongation.

[0084]

[0085] Specific examples of the present invention are described below. However, the following examples are merely specific examples of the present invention, and the present invention is not limited to the following examples.

[0086]

[0087] Example 1

[0088] Slab steel was manufactured with the composition shown in Table 1 below, with the remainder containing Fe and unavoidable impurities, and the components are indicated as actual values.

[0089] The slab was heated at 1200℃ for 200 minutes and then hot-rolled to a thickness of 2.3 mm to produce a hot-rolled sheet. The rough rolling finishing temperature was 1000℃, the finishing rolling temperature (FDT) was 900℃, and coiling was performed at 650℃. A reduction ratio of 85% was applied to the hot-rolled steel sheet to produce a cold-rolled steel sheet, which was then annealed at 780℃ for 50 seconds and cooled to 550℃ at a cooling rate of 20℃ / sec.

[0090] The characteristics of slabs, hot-rolled steel sheets, and cold-rolled steel sheets were measured using the following methods and summarized in Tables 2 and 3 below.

[0091] Hot-rolled steel plate grain size: Based on the cross-section including the rolling direction, the number of grains was divided by the measured area using an optical microscope to obtain the average area, and the grain size was obtained from this.

[0092] Hot-rolled steel plate carbide: For the cross-section including the rolling direction, the area occupied by carbide among the total area was quantified after observing under an optical microscope at 500x.

[0093] Microstructure area fraction: The area fraction of ferrite and the area fraction of the remaining bainite, martensite, and pearlite hard phases were determined by measuring the optical microscope images.

[0094] V-bending test: The steel plate was subjected to a bending test with a load of 10 tons in a V-bending die with a curvature of r=0, and it was visually confirmed whether cracks occurred in the bending section.

[0095] Yield strength, tensile strength, and elongation: JIS13B standard specimens were collected and measured by performing a uniaxial tensile test.

[0096] Hot-rolled cracks were determined to be Euro when a crack of 2.5 mm or more was present using a steel plate magnifying glass.

[0097] (Wt%)CMnPAlCuFormula 1 Left side Invention example 10.200.900.010.0300.020170.90Invention example 20.160.900.010.0300.020148.06Invention example 30.210.900.010.0300.020176.61Invention example 40.200.780.010.0300.020163.34Invention example 50.201.120.010.0300.020184.76Comparative example 10.140.900.010.0300.020136.64Comparative example 20.250.900.010.0300.020199.45Comparative Example 30.200.600.010.0300.020152.00Comparative Example 40.201.300.010.0300.020196.10Comparative Example 50.200.900.030.0300.020170.90Comparative Example 60.200.900.010.0600.020170.90Comparative Example 70.200.900.010.0300.075170.90

[0098] Note Hot rolled Edge crack Hot rolled grain size (㎛) Hot rolled carbide (area%) Cold rolled ferrite (area%) Cold rolled hard phase (area%) Invention example 1 None 351582.03.0 Invention example 2 None 351285.32.7 Invention example 3 None 351781.03.2 Invention example 4 None 351582.22.8 Invention example 5 None 351581.93.1 Comparative example 1 None 351285.52.5 Comparative example 2 None 352274.63.4 Comparative example 3 None 351582.22.8 Comparative example 4 None 351581.43.6 Comparative example 5 None 281582.03.0 Comparative example 6 None 261582.03.0 Comparative example 7Yu251582.03.0

[0099] Note YP(MPa) TS(MPa) El(%) V-Bending Invention Example 138552025 No invention example 234549032 No invention example 339553522 No invention example 435550030 No invention example 540054021 No comparison example 131043037 No comparison example 243057018 No comparison example 332048036 No comparison example 444058017 No comparison example 539554023 No comparison example 639052524 No comparison example 738552326 No

[0100] As shown in Tables 1 to 3, it can be confirmed that the desired mechanical properties can be obtained when the steel composition is appropriately controlled. On the other hand, Comparative Example 1 has a low C content, and it can be confirmed that the yield strength and tensile strength are low. Comparative Example 2 has a high C content, and it can be confirmed that cracks occur during the V-bending test and formability is poor. Comparative Example 3 has a low Mn content, and it can be confirmed that the yield strength and tensile strength are low. Comparative Example 4 has a high Mn content, and it can be confirmed that cracks occur during the V-bending test and formability is poor. Comparative Example 5 has a high P content, and cracks occur during the V-bending test. Comparative Example 6 has a high Al content, and cracks occur during the V-bending test. Comparative Example 7 contains a large amount of Cu, and edge cracks occur in the hot-rolled steel sheet, and the part where the edge cracks occurred was cut out and cold-rolled, resulting in material loss.

[0101]

[0102] Example 2

[0103] Using the steel grades summarized in the above invention example 1, a cold rolled steel sheet was manufactured in the same manner as in Example 1, but some of the conditions were changed to the conditions summarized in Table 4 below.

[0104] High temperature compressive stress: The slab was heated at a heating rate of 20℃ / sec, maintained at 1200℃ for 3 minutes, and then cooled at a cooling rate of 10℃ / sec. A compression test was performed at 900℃ up to a strain of 1.2 to obtain a strain-stress curve, and the maximum stress value of the strain-stress curve was determined.

[0105]

[0106] NoteFDT(℃)CT(℃)Cold rolling reduction ratio(%)Cold rolling annealing temperature(℃)Cooling rate after annealing(℃ / sec.)Invention example 68606508578020Invention example 79006208578020Invention example 89006507078020Invention example 99006508576020Invention example 109006508582020Invention example 119006508578035Comparative example 88406508578020Comparative example 99006008578020Comparative example 109006506878020Comparative example 119006508573020Comparative example 129006508584020Comparative example 139006508578040

[0107] Note High temperature compressive stress (MPa) Hot rolled Edge crack Hot rolled grain size (㎛) Hot rolled carbide (area%) Cold rolled ferrite (area%) Cold rolled hard phase (area%) Invention example 6170 No 251497.03.0 Invention example 7170 No 222097.03.0 Invention example 8130 No 251599.01.0 Invention example 9130 No 251597.03.0 Invention example 10130 No 251597.03.0 Invention example 11130 No 251595.05.0 Comparative example 8220 No 152797.03.0 Comparative example 9200 No 182597.03.0 Comparative example 10130 No 251597.03.0 Comparative example 11130유251540.060.0Comparison Example 12130무251598.02.0Comparison Example 13130유251550.050.0

[0108] Note YP(MPa) TS(MPa) El(%) V-Bending Invention Example 641055023 No invention Example 741055023 No invention Example 835049027 No invention Example 941555521 No invention Example 1034248228 No invention Example 1142056020 No comparison Example 843057018 No comparison Example 943057017 No comparison Example 1032046130 No comparison Example 1148062014 No comparison Example 1232046130 No comparison Example 1345059016 Yes

[0109] As shown in Tables 4 to 6 above, it can be confirmed that the desired mechanical properties can be obtained when the process conditions are appropriately adjusted. On the other hand, Comparative Example 8 has a low finishing temperature for sand rolling, so it can be confirmed that the high-temperature compressive stress is high, and the elongation of the cold-rolled product after the final annealing is inferior. Comparative Example 9 has a low coiling temperature, so it has a high high-temperature compressive stress, and likewise, the fraction of carbides is high, so it can be confirmed that the elongation value after cold rolling decreases. Comparative Example 10 has a low cold rolling reduction ratio, and it cannot secure sufficient strength. Comparative Example 11 has a low annealing temperature, so the ferrite fraction is small, and the fraction of hard phases (bainite, martensite, and pearlite) is high, so it can be confirmed that the formability is poor. Comparative Example 12 has a high annealing temperature, so it cannot secure sufficient strength. Comparative Example 13 can be confirmed to have a fast cooling rate, a small ferrite fraction, and a high fraction of hard phases (bainite, martensite, and pearlite), resulting in poor formability. FIGS. 1 and 2 show the microstructure of the hot-rolled steel sheet manufactured in Inventive Example 1 and the microstructure of the hot-rolled steel sheet manufactured in Comparative Example 8, respectively. In Inventive Example 1, the hot-rolled steel sheet has coarse grain sizes and finely dispersed carbides, while in Comparative Example 8, the grain sizes are refined and the carbides are formed in clusters.

[0110] Figures 3 and 4 show photographs of the cold-rolled steel sheet manufactured in Inventive Example 1 after a V-bending test, and photographs of the cold-rolled steel sheet manufactured in Comparative Example 4 after a V-bending test, respectively. In Inventive Example 1, no cracks occurred after the V-bending test, but in Comparative Example 4, it can be confirmed that cracks occurred on the TD surface.

[0111]

[0112] The present invention is not limited to the above-described embodiments and / or examples, but can be manufactured in various different forms. Those skilled in the art will understand that the present invention can be implemented in other specific forms without altering the technical spirit or essential characteristics of the present invention. Therefore, the above-described embodiments and / or examples should be understood as illustrative in all respects and not restrictive.

Claims

Containing, in weight %, carbon (C): 0.15 to 0.22%, manganese (Mn): 0.75 to 1.2%, phosphorus (P): 0.005 to 0.02%, copper (Cu): 0.005 to 0.07%, and aluminum (Al): 0.015 to 0.05%, the remainder including iron and inevitable impurities, Cold rolled steel sheet having an area fraction of ferrite within the steel sheet of 80% or more. In the first paragraph, Cold rolled steel sheet satisfying the following equation 1. [Formula 1] 571×[C] + 63×[Mn] ≥ 140 (In Equation 1, [C] and [Mn] are the contents (weight%) of C and Mn, respectively.) In the first paragraph, Cold rolled steel sheet further containing at least one of Ti: 0.01 wt% or less and Nb: 0.01 wt% or less. In the first paragraph, Cold rolled steel sheet having a combined area fraction of bainite, martensite, and pearlite of 10% or less. In the first paragraph, Cold rolled steel sheet with a thickness of 1.0 mm or less. In the first paragraph, A cold rolled steel sheet that does not form cracks in the bending portion when subjected to a bending test with a load of 10 tons using a V-bending die having a curvature of r=0. In the first paragraph, Cold rolled steel sheet having a yield strength of 340 MPa or more, a tensile strength of 450 MPa or more, and an elongation of 20% or more. A step of manufacturing a hot-rolled steel sheet by hot-rolling a slab containing, in weight %, carbon (C): 0.15 to 0.22%, manganese (Mn): 0.75 to 1.2%, phosphorus (P): 0.005 to 0.02%, copper (Cu): 0.005 to 0.07%, and aluminum (Al): 0.015 to 0.05%, with the remainder being iron (Fe) and unavoidable impurities; A step of manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet; and Comprising a step of annealing the above cold rolled steel sheet, In the step of manufacturing the above hot rolled steel plate, the rolling is completed at a temperature of 850℃ or higher, and coiling is performed at a temperature of 610℃ or higher. In the step of manufacturing the above cold rolled steel sheet, the reduction ratio is 70% or more, A method for manufacturing a cold rolled steel sheet, comprising: performing a crack treatment at 750 to 830°C in the above-mentioned annealing step, and cooling from the cracking temperature to 550°C at a cooling rate of 35°C / sec or less. In Article 8, Before the step of manufacturing the above hot rolled steel plate, A method for manufacturing a cold rolled steel sheet, further comprising the step of heating the above slab at 1100°C or higher for 150 minutes or longer. In Article 8, A method for manufacturing a cold rolled steel sheet, wherein the above slab is heated at a heating rate of 20°C / sec, maintained at 1200°C for 3 minutes, cooled at a cooling rate of 10°C / sec, and then subjected to a compression test at a temperature of 900°C to a deformation amount of 1.2, and has a maximum stress value of 180 MPa or less. In Article 8, A method for manufacturing a cold rolled steel sheet, wherein, in the step of manufacturing the hot rolled steel sheet, rough rolling is completed at a temperature of 950°C or higher. In Article 8, A method for manufacturing a cold rolled steel sheet, wherein after the step of manufacturing the hot rolled steel sheet, the average grain size of the hot rolled steel sheet is 20㎛ or more. In Article 8, A method for manufacturing a cold rolled steel sheet, wherein after the step of manufacturing the hot rolled steel sheet, the hot rolled steel sheet contains carbides at 20 area% or less. A galvanized steel sheet having a galvanized layer on the surface of the cold rolled steel sheet according to any one of claims 1 to 7.

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