Cold-rolled steel sheet and its manufacturing method

Abstract

The cold rolled steel sheet according to one embodiment of the present invention contains, by weight percent, C: 0.01 to 0.10%, Mn: 0.1 to 0.4%, Al: 0.01 to 0.10%, P: 0.003 to 0.020%, N: 0.001 to 0.006%, S: 0.015% or less, Cr: 0.1 to 0.4%, B: 0.0005 to 0.0035%, Ni: 0.04 to 0.10%, the balance being Fe and other unavoidable impurities, contains (Cr-B)(CN)-based composite precipitates having a size of 0.01 to 0.1 μm, and contains [I (222) +I (554) ] / I (200) The texture ratio is 5.0 to 10.0, the ear occurrence rate is less than 3.0%, and the alloying rate of the alloy layer is 5 to 15%.

Description

[Technical field]

[0001] The present invention relates to a steel sheet, and more particularly to a cold-rolled steel sheet and a method for producing the same. [Background technology]

[0002] In the case of circular cans used for battery cases of primary batteries, corrosion resistance is fundamentally required to withstand the alkaline properties of the battery contents, so nickel (Ni) plating is generally applied to the steel plate. Recently, cylindrical battery case materials are widely used not only for primary batteries, but also for mobile devices such as mobile phones, power tools, and secondary batteries such as electric vehicles.

[0003] As the environments in which battery case materials are used become more diverse, there is an increasing demand for improved characteristics and life span of battery cases. In addition, in order to increase the capacity of the charger and thereby improve battery performance, technology is being developed to reduce the thickness of the case body while ensuring safety.

[0004] As the application of steel battery cases expands to the automotive industry, there is an increasing demand for improved properties, especially high-temperature properties, to ensure the safety of the cans. Research is underway to apply steel sheets in a circular can shape to reduce costs and improve productivity for battery cases that have used materials such as aluminum in the electric and hybrid vehicle fields. As such, battery case products may be exposed to high temperatures of several hundred degrees Celsius in an instant in the usage environment, so they must have heat resistance properties that can withstand high-temperature conditions.

[0005] The heat resistance can be evaluated in various ways, for example, by filling a battery in an electric vehicle battery case and heating it to a temperature of about 600° C. to evaluate the stability of the cell, and therefore the heat resistance at a certain temperature is becoming an important control factor. In addition, since a local temperature rise during vehicle operation can deteriorate the battery cell portion and affect the vehicle's operation, deformation characteristics at high temperatures must also be strictly controlled to prevent this.

[0006] In addition, sagging resistance, which prevents the occurrence of sagging, a phenomenon in which materials sag due to changes in their properties caused by repeated exposure to high temperatures, is one of the important factors. When the sagging phenomenon occurs, it becomes difficult to maintain the shape of the formed part, and if thermal stress is concentrated in a specific area, the high temperature resistance decreases, causing the product to deform in shape, and in severe cases, it may cause breakage. In order to ensure the stability of parts by ensuring the shape stability of processed products, a cold-rolled steel sheet that has high yield strength at high temperatures and little sagging of the material is required for the application of battery cases for electric vehicles, as well as the management temperature of parts.

[0007] Conventionally, stainless steel sheets have been mainly used for heat-resistant applications. However, the stainless steel sheets are expensive to manufacture due to the addition of large amounts of expensive alloy elements such as chromium (Cr) and nickel (Ni). In addition, when used at high temperatures, chromium at the grain boundaries combines with carbon (C) and precipitates in the form of chromium carbide at the grain boundaries, forming a chromium-depleted zone, which causes intergranular corrosion and reduces corrosion resistance.

[0008] In the case of battery cases used in automobiles, identical processed products are installed in a stacked form in a limited space, and multi-stage processing such as drawing and stretching are required during molding, so in addition to the high-temperature properties, workability at room temperature is also an important factor. Steel case materials have been attracting attention for their superior properties compared to other materials in terms of productivity and stability, and since most cases are manufactured in a cylindrical shape, the desired case shape is obtained mainly through D&I molding, which involves the steps of drawing and ironing processing.

[0009] The processing methods of cold-rolled steel sheets include the stretching process, which is closely related to ductility, the deep drawing process, which is related to the Lankford value, and the bending process, which is closely related to the ratio of yield strength to tensile strength. Most cold-rolled steel sheets are formed by a complex processing mode, and the deep drawing method is generally similar to the processing method for manufacturing a cup shape and is related to the Lankford value (hereinafter referred to as r value), which is expressed as the deformation rate in the width direction relative to the deformation rate in the thickness direction of the material. The Δr value measured from the r values ​​measured in each direction with respect to the rolling direction is a representative value for easily evaluating the occurrence of ears in the formed cup, and is calculated according to the following formula 1.

[0010] Equation 1 △r=(r0+r 90 -2r 45 ) / 2

[0011] In the above formula 1, r i indicates the r value measured on a test piece taken in the i° direction from the rolling direction. The closer the Δr value is to 0, the more uniform the deformation rate in all directions is, indicating isotropic properties, and there is less earing during cup processing, minimizing loss due to cutting of the material after processing.

[0012] The ear occurrence phenomenon is caused by the difference in cup height at each processing location due to the anisotropy of the formed cup. Planar anisotropy indicates the degree to which the physical and / or mechanical properties of a material have directionality. Factors that induce the planar anisotropy include a combination of two-phase structures such as carbides and / or inclusions, surface friction effects, and local steel sheet thickness or property differences. The planar anisotropy may be fundamentally caused by the strong directionality of each plastically deformed ferrite grain.

[0013] If the crystal grains that have undergone processing such as the plastic deformation are random, these crystals will not have any orientation, and the problem of earing, which is closely related to in-plane anisotropy, will not occur. However, ferrite crystal grains that have undergone the hot rolling, soddus, and alc recrystallization annealing processes show strong orientation, which results in anisotropy of plastic behavior during the D&I processing.

[0014] The ears of the cup form peaks at 0° and 90°. As the rolling reduction increases, the peak of the cup transitions to 0°, 60°, and 45°, and this phenomenon occurs repeatedly. It is necessary to keep the ear occurrence rate during cup molding to less than 3%.

[0015] In addition, steel sheets used as materials for alkaline manganese dry cell and lithium battery cases are manufactured into two-piece circular cans, in which the lower part of the can, which has been formed into a cylindrical shape by press forming, and the body part are integrally processed. In this case, the material is punched into a circular blank, and at the same time, it is deep-drawn into a cylindrical shape, and the deep-drawn material is passed through a number of ironing dies to reduce the thickness and increase the height of the can through D&I processing. In this way, the two-piece circular can can be manufactured by ironing the case body part in the manufacturing process to make it thinner than a circular plate, and as a result, the final thickness of the case body is about 20 to 40% thinner than that of a circular plate.

[0016] In addition, Japanese Patent Publication No. 11-189841 proposed a method for improving the sealing performance of battery cans by using low- and medium-carbon steel and increasing the strength of the material through secondary rolling. However, this method does not solve the problems related to ensuring strength at high temperatures, e.g., 600°C, required for battery cases for electric vehicles and suppressing dynamic deformation aging, and also has a problem in that the addition of a secondary rolling process in the manufacturing process of the disks acts as a separate factor in increasing manufacturing costs. Summary of the Invention [Problem to be solved by the invention]

[0017] The technical problem to be solved by the present invention is to provide a cold-rolled steel sheet capable of ensuring room temperature and high temperature D&I workability that can be applied to fields such as battery cases.

[0018] Another technical problem to be solved by the present invention is to provide a method for producing a cold rolled steel sheet having the above-mentioned advantages. [Means for solving the problem]

[0019] In order to solve the above problems, a cold-rolled steel sheet according to one embodiment of the present invention contains, by weight%, C: 0.01 to 0.10%, Mn: 0.1 to 0.4%, Al: 0.01 to 0.10%, P: 0.003 to 0.020%, N: 0.001 to 0.006%, S: 0.015% or less, Cr: 0.1 to 0.4%, B: 0.0005 to 0.0035%, Ni: 0.04 to 0.10%, the balance being Fe and other unavoidable impurities, contains (Cr-B)(CN)-based composite precipitates having a size of 0.01 to 0.1 μm, and contains [I (222) +I (554) ] / I (200) The texture ratio is 5.0 to 10.0, the ear occurrence rate is less than 3.0%, and the alloying rate of the alloy layer is 5 to 15%. In one embodiment, the cold rolled steel sheet can satisfy the following formula 1.

[0020] <Expression 1> 0.0002≦([Cr] / 52)×1.6([B] / 11)) / (([C] / 12)+([N] / 14))≦0.0006 (In the above formula 1, [Cr], [B], [C], and [N] respectively represent the weights of Cr, B, C, and N.)

[0021] In one embodiment, no breakage occurs during cupping when the blank diameter / drawing die diameter during drawing is 1.85. In one embodiment, no red rust occurs within 12 hours in a salt spray test.

[0022] In one embodiment, the cold-rolled steel sheet may have a sagging degree of less than 3 mm after heating the cold-rolled steel sheet for 100 hours at 600° C. In one embodiment, the cold-rolled steel sheet may have a yield strength of 95 MPa or more at 600° C.

[0023] In order to solve the above problems, a method for producing a cold rolled steel sheet according to another embodiment of the present invention includes the steps of heating a steel slab containing, by weight percent, C: 0.01-0.10%, Mn: 0.1-0.4%, Al: 0.01-0.10%, P: 0.003-0.020%, N: 0.001-0.006%, S: 0.015% or less, Cr: 0.1-0.4%, B: 0.0005-0.0035%, Ni: 0.04-0.10%, the balance being Fe and other inevitable impurities; and The method may include the steps of finish hot rolling a steel slab at 860 to 930°C, coiling the finish hot rolled hot rolled steel sheet at 580 to 700°C, cold rolling the coiled hot rolled steel sheet at a rolling reduction of 78 to 92%, annealing the cold rolled cold rolled steel sheet at a temperature of 650 to 750°C, cooling the annealed cold rolled steel sheet at a cooling rate of 30 to 70°C / sec, electroplating the cold rolled steel sheet, and then subjecting it to an alloying heat treatment at 650 to 750°C. In one embodiment, the method for producing the cold rolled steel sheet may satisfy the following formula 1.

[0024] <Expression 1> 0.0002≦([Cr] / 52)×1.6([B] / 11)) / (([C] / 12)+([N] / 14))≦0.0006 (In the above formula 1, [Cr], [B], [C], and [N] respectively represent the weights of Cr, B, C, and N.) Effect of the Invention

[0025] According to an embodiment of the present invention, the cold rolled steel sheet has excellent room temperature workability even in a D&I process, which is a processing process such as ironing and drawing, by controlling the contents of chromium (Cr), boron (B), carbon (C), and nitrogen (N) among essential components, and therefore it is possible to manufacture a cold rolled steel sheet having excellent heat resistance and D&I processability, which has excellent high temperature strength and high temperature deformation properties even at high temperatures, ensures shape fixability at high temperatures, and has high heat resistance properties with high product safety.

[0026] A method for manufacturing a cold-rolled steel sheet according to another embodiment of the present invention can be provided using a cold-rolled steel sheet having the above-mentioned advantages. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Terms such as first, second and third are used to describe various parts, components, regions, layers and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Thus, 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.

[0028] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular form includes the plural form unless the text clearly indicates otherwise. The term "comprising" as used in the specification embodies certain features, regions, integers, steps, operations, elements and / or components and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements and / or components.

[0029] When an element is referred to as being "on" another element, it may mean that it is directly on top of the other element, or there may be other elements between them. In contrast, when an element is referred to as being "directly on" another element, there are no other elements between them.

[0030] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless defined.

[0031] Moreover, unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight.

[0032] In one embodiment of the present invention, the term "additionally contain an additional element" means that the remaining iron (Fe) is replaced by an additional amount of the additional element.

[0033] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to exemplary embodiments thereof, so that those skilled in the art can easily practice the present invention. However, the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein.

[0034] According to one embodiment of the present invention, the cold rolled steel sheet contains, by weight percent, C: 0.01-0.10%, Mn: 0.1-0.4%, Al: 0.01-0.10%, P: 0.003-0.020%, N: 0.001-0.006%, S: 0.015% or less, Cr: 0.1-0.4%, B: 0.0005-0.0035%, Ni: 0.04-0.10%, with the balance being Fe and other unavoidable impurities.

[0035] The reasons for limiting the alloy components will be explained below (hereinafter, weight percent will be abbreviated as %).

[0036] Carbon (C): 0.01~0.10% Carbon is an element added to improve the strength of steel sheet, and in the present invention, it is utilized for reacting with chromium (Cr) to form carbide-based precipitates. The carbon content may be 0.01 to 0.10%, specifically 0.01 to 0.10%, more specifically 0.015 to 0.09%.

[0037] As the carbon content increases, the tensile and yield strengths increase. If the carbon content is too high, the workability decreases, and if the carbon content is too low, the strengthening effect at high temperatures due to the (Cr-B)(CN)-based composite precipitates cannot be fully obtained, and the grain size increases, which may cause processing defects such as orange peel during molding.

[0038] Manganese (Mn): 0.1-0.4% Manganese is an austenite stabilizing element and a solid solution strengthening element that increases the strength of steel and prevents hot cracking of slabs by precipitating sulfur (S) in the form of MnS. The manganese content may be 0.1 to 0.4%, specifically 0.12 to 0.29%.

[0039] If the manganese content is too high, not only the ductility decreases and center segregation occurs, but also the corrosion resistance of the steel sheet and the plating adhesion during nickel (Ni) plating may decrease, whereas if the manganese content is too low, there is a problem of hot cracking of the slab.

[0040] Aluminum (Al): 0.01 to 0.10% Aluminum is an element added to promote deoxidation of molten steel and bonding with dissolved nitrogen in the steel to prevent deterioration of the material due to aging. The aluminum content may be 0.01 to 0.10%, specifically 0.015 to 0.085%.

[0041] If the aluminum content is too high, the deoxidizing effect is saturated, while the inclusions in the steel increase, which may cause surface defects, whereas if the aluminum content is too low, there is a problem that deterioration of the material due to aging may occur.

[0042] Phosphorus (P): 0.003 to 0.020% Phosphorus is an element added to steel to improve its strength and corrosion resistance. The phosphorus content may be 0.003 to 0.020%, specifically 0.004 to 0.019%.

[0043] If the phosphorus content is too high, it may segregate at the grain boundaries, causing grain boundary embrittlement, reducing workability, and reducing adhesion when nickel (Ni) is plated.If the phosphorus content is too low, it is difficult to ensure the strength and corrosion resistance of the steel.

[0044] Nitrogen (N): 0.001-0.006% Nitrogen is an element that is added to steel in a solid solution state to strengthen the material and to form nitrides to form an annealing texture and ensure physical properties at high temperatures. The nitrogen content may be 0.001 to 0.006%, specifically 0.0014 to 0.0056%.

[0045] If the nitrogen content is too high, it may cause aging due to an excess of solute elements, which may deteriorate formability. If the nitrogen content is too low, it may be difficult to form an annealing texture and ensure high-temperature properties.

[0046] Sulfur (S): 0.015% or less Sulfur combines with iron (Fe) in the steel to form nonmetallic inclusions that act as starting points for corrosion and cause red shortness, so it is preferable to reduce the content of sulfur. The content of sulfur may be 0.015%, specifically 0.014% or less.

[0047] Chromium (Cr): 0.1-0.4% Chromium is an element added to control the texture through solid solution strengthening and grain refinement, and to ensure the material properties of the product. The chromium content may be 0.1 to 0.4%, specifically 0.13 to 0.39%.

[0048] If the chromium content is too high, the excessive addition of an expensive element increases the manufacturing cost, which is uneconomical, and the recrystallization temperature increases, whereas if the chromium content is too low, it is difficult to achieve solid solution strengthening, texture control by grain refinement, and ensure the material properties of the product.

[0049] Boron (B): 0.0005 to 0.0035% Boron is an element that forms high-temperature nitrides, improves hardenability, and inhibits grain growth at high temperatures to improve high-temperature properties. The boron content may be 0.005 to 0.0035%, specifically 0.0005 to 0.0035, and more specifically 0.0007 to 0.0032%.

[0050] If the boron content is too high, recrystallization is delayed, which reduces annealing passability and workability, whereas if the boron content is too low, grain growth at high temperatures cannot be suppressed, which reduces high-temperature properties.

[0051] Nickel (Ni): 0.04-0.10% Nickel is an element added to improve high-temperature properties and corrosion resistance. The nickel content may be 0.04 to 0.10%, specifically 0.042 to 0.095%.

[0052] If the nickel content is too high, there is a problem of deterioration in operability such as rollability, whereas if the nickel content is too low, there is a problem of deterioration in high temperature properties and corrosion resistance.

[0053] The present invention can reduce the yield strength of the steel material used in press forming, increase the ductility, and ensure excellent press workability by eliminating expensive elements such as Nb, Ti, P, and Mn, and optimizing the content of alloy elements such as C, N, and S, which are relatively low-cost elements. Specifically, the steel material can contain Nb 0.002% or less, Ti 0.002% or less, Ni 0.03% or less, and Cu 0.03% or less.

[0054] The cold rolled steel sheet contains iron (Fe) as the balance. It may also contain unavoidable impurities. Inevitable impurities refer to impurities that are inevitably mixed in during the steelmaking and manufacturing process of the cold rolled steel sheet. Since unavoidable impurities are widely known, a detailed description will be omitted. In one embodiment of the present invention, the addition of elements other than the above-mentioned alloy components is not excluded, and various elements are included within a range that does not impair the technical idea of ​​the present invention. When an additional element is further included, it is included to replace the balance Fe.

[0055] In one embodiment, the cold-rolled steel sheet of the present invention may contain (Cr-B)(CN)-based composite precipitates, the size of which may be in the range of 0.01 to 0.10 μm.

[0056] If the size of the (Cr-B)(CN) based composite precipitates exceeds the upper limit of the above range, there is a problem that high temperature properties cannot be ensured.If the size of the (Cr-B)(CN) based composite precipitates is below the lower limit of the above range, grain growth can be suppressed, but there is a problem that annealing workability is deteriorated.

[0057] In one embodiment, the cold rolled steel sheet has a thickness of [I (222) +I (554) ] / I (200) The texture ratio may be in the range of 5.0 to 10.0. (222) is the intensity of the (222) plane in the plane intensity, I (554) is the intensity of the (554) plane, and I (200) means the strength of the (200) plane. The texture ratio may be in the range of 5.0 to 10.0, specifically 5.2 to 9.9. When the texture ratio satisfies the above range, D&I properties can be ensured.

[0058] When the texture ratio exceeds the upper limit of the range, it is suitable for increasing the depth of the formed cup, but there are problems in that the plastic deformation ratio in the direction at 45° to the rolling direction increases, increasing the occurrence rate of ears in the formed cup, causing short-cans, and the amount of trimming of the material increases, increasing material loss.When the texture ratio is below the lower limit of the range, there is a problem in that the (200) component, which is unfavorable to formability, develops, deteriorating workability.

[0059] In one embodiment, the cold-rolled steel sheet may have an ear occurrence rate of less than 3.0%. The ear occurrence rate is a value obtained by measuring the height of each portion of a formed cup and calculating {(maximum cup height)-(minimum cup height)}×100 / (maximum cup height). The ear occurrence rate of the cold-rolled steel sheet may be less than 3.0%, specifically, less than 2.8%. If the ear occurrence rate is 3.0% or more, there is a problem that the amount of trimming to cut out unnecessary formed portions increases due to the large height deviation of the formed cup, resulting in increased loss of material.

[0060] In one embodiment, the cold-rolled steel sheet may include an alloy layer, and the alloying ratio of the alloy layer may be in the range of 5 to 15%. The alloying ratio of the alloy layer may be a percentage of a region where iron (Fe) and nickel (Ni) coexist, and may be a percentage of an area where the iron and the nickel coexist, and the alloying ratio may have a close relationship with the corrosion resistance and surface hardness of the material.

[0061] When the alloying ratio of the alloy layer exceeds the upper limit, it is advantageous in terms of improving the life of the die, but there is a problem that the corrosion resistance of the surface layer is deteriorated, whereas when the alloying ratio of the alloy layer is below the lower limit, the surface material of the plated material is hardened due to the decrease in the alloying degree, which causes a problem of shortening the life of the die.

[0062] The cold rolled steel sheet according to one embodiment of the present invention satisfies the following formula 1.

[0063] <Expression 1> 0.0002≦([Cr] / 52)×1.6([B] / 11)) / (([C] / 12)+([N] / 14))≦0.0006 (In the above formula 1, [Cr], [B], [C], and [N] respectively represent the weights of Cr, B, C, and N.)

[0064] The above formula 1 means the effective atomic ratio of carbon (C) and nitrogen (N) to chromium (Cr) and boron (B), and by controlling the precipitation conditions of the (Cr-B)(CN)-based composite precipitates, it is possible to simultaneously ensure aging resistance, workability, and high-temperature deformation behavior.

[0065] In one embodiment, the range of the above formula 1 may be 0.0002 to 0.0006, specifically 0.00021 to 0.00058. If the range of the above formula 1 exceeds the upper limit, it is advantageous for ensuring strength, but there is a problem that the recrystallization temperature increases, the annealing passability and surface properties deteriorate, and the workability in the subsequent processes decreases. If the range of the above formula 1 is below the lower limit, there is a problem that the amount of solid solution elements in the steel increases, deteriorating the aging resistance and workability, and the amount of precipitation of the (Cr-B)(CN)-based composite precipitates decreases, making it difficult to ensure high-temperature strength.

[0066] In one embodiment, the cold-rolled steel sheet does not generate processing cracks. In one embodiment, the cold-rolled steel sheet may have excellent corrosion resistance. The corrosion resistance may be evaluated by a salt spray test (SST) based on whether red rust occurs within 12 hours. Thus, it can be confirmed that the cold-rolled steel sheet does not generate red rust within 12 hours when subjected to a salt spray test.

[0067] In one embodiment, the cold-rolled steel sheet may have a sag of less than 3 mm after heating at 600° C. for 100 hours. The sag of the cold-rolled steel sheet may refer to sag resistance, and the sag resistance is measured by measuring the sag of the steel sheet after heating at 600° C. for 100 hours. When the sag is less than 3 mm, the cold-rolled steel sheet may have excellent sag resistance.

[0068] In one embodiment, the cold-rolled steel sheet may have a yield strength of 95 MPa or more at 600° C. Measuring the yield strength after heating the cold-rolled steel sheet at 600° C. for 100 hours means high-temperature yield strength, and is intended to evaluate safety at high temperatures.

[0069] The high temperature yield strength may be 95 MPa or more, specifically 100 MPa. If the high temperature yield strength is 95 MPa or less, when the cold rolled steel sheet is applied to a battery, there is a problem that the safety at high temperatures is reduced and the possibility of fire or explosion is increased.

[0070] According to another embodiment of the present invention, a method for producing a cold rolled steel sheet includes the steps of heating a steel slab containing, by weight percent, 0.01-0.10% C, 0.1-0.4% Mn, 0.01-0.10% Al, 0.003-0.020% P, 0.001-0.006% N, 0.015% or less S, 0.1-0.4% Cr, 0.0005-0.0035% B, 0.04-0.10% Ni, the balance being Fe and other inevitable impurities, and heating the heated steel slab to 86°C. The method includes the steps of finish hot rolling at 0 to 930°C, coiling the finish hot rolled hot rolled steel sheet at 580 to 700°C, cold rolling the coiled hot rolled steel sheet at a rolling reduction of 78 to 92%, annealing the cold rolled cold rolled steel sheet at a temperature of 650 to 750°C, cooling the annealed cold rolled steel sheet at a cooling rate of 30 to 70°C / sec, and electroplating the cold rolled steel sheet and then subjecting it to an alloying heat treatment at 650 to 750°C. The alloy composition of the slab is the same as that of the above-mentioned cold rolled steel sheet, so a duplicated description will be omitted. Since the alloy composition does not change during the manufacturing process of the cold rolled steel sheet, the alloy composition of the slab and the cold rolled steel sheet is substantially the same.

[0071] The step of heating the steel slab containing the alloy composition is to heat the steel slab at a low temperature at which the (Cr-B)(CN)-based composite precipitates cannot be redissolved. In one embodiment, the step of reheating the slab can be performed at a temperature range of 1180 to 1280° C. By proceeding with the heating within the above temperature range, the temperature during hot rolling described below can be sufficiently ensured.

[0072] The step of finish hot rolling the heated steel slab at 860 to 930° C. involves reheating the steel slab and then hot rolling the steel slab at a temperature range of 860 to 930° C. The finish hot rolling step is performed at a temperature range of 860 to 930° C., specifically, 870 to 920° C.

[0073] If the temperature exceeds the upper limit of the above range, the grains become coarse, which reduces the impact toughness, whereas if the temperature is below the lower limit of the above range, the hot rolling is completed in a relatively low temperature region, which causes the final formed grains to become mixed, which reduces the workability and rollability.

[0074] The step of coiling the finish hot-rolled hot-rolled steel sheet at 580 to 700°C involves cooling the finish hot-rolled hot-rolled steel sheet on a run-out table and coiling it. The coiling step is performed at a temperature range of 580 to 700°C, specifically, 590 to 695°C.

[0075] If the temperature exceeds the upper limit of the above range, the crystal grains of the product become coarse, which deteriorates high-temperature strength and corrosion resistance, whereas if the temperature falls below the lower limit of the above range, the material of the hot-rolled material hardens, which increases the rolling load in the cold rolling step described below and increases the temperature non-uniformity in the width direction, which deteriorates material deviation and processability.

[0076] The step of cold rolling the coiled hot-rolled steel sheet at a rolling reduction of 78 to 92% is for producing a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet. The cold rolling is performed at a rolling reduction of 78 to 92%, specifically, at a rolling reduction in the range of 80 to 90%.

[0077] If the reduction exceeds the upper limit of the range, the load on the cold rolling mill increases, which reduces workability, whereas if it is below the lower limit of the range, it is difficult to form a texture suitable for planar anisotropy.

[0078] The step of annealing the cold-rolled steel sheet at a temperature of 650 to 750°C is for recrystallization and microstructural control, and is a soaking heat treatment. The annealing step may be performed at a temperature in the range of 650 to 750°C, specifically, 660 to 740°C.

[0079] If the temperature exceeds the upper limit of the above range, recrystallization is completed and the transformation driving force of the acicular ferrite structure can be secured, but defects such as heat buckle due to high temperature heat treatment may be induced, resulting in a problem of reduced annealing threadability.If the temperature is below the lower limit of the above range, the fraction of recrystallized crystal grains is low and strength is high, but ductility is reduced and workability cannot be secured.

[0080] The step of cooling the annealed cold-rolled steel sheet at a cooling rate of 30 to 70° C. / sec refers to cooling the soaked steel sheet at a temperature of 30 to 70° C. per second. The cooling rate per second may be in the range of 30 to 70° C. / sec, specifically, 35 to 65° C. / sec.

[0081] If the upper limit of the range is exceeded, problems may occur such as deterioration of workability due to an increase in material quality, and uneven cooling in the width direction may cause deviations in shape and material quality.If the lower limit of the range is exceeded, the fraction of acicular ferrite grains is reduced, causing grain growth at high temperatures, making it difficult to ensure high-temperature properties.

[0082] The step of electroplating the cold-rolled steel sheet and then performing an alloying heat treatment at 650 to 750°C is for forming a 5 to 15% alloy layer by electroplating the heat-treated steel sheet and performing an alloying heat treatment. The alloying heat treatment may be performed for a short soaking time of 3 seconds or less. The alloying heat treatment may be performed at a temperature in the range of 650 to 750°C, specifically 660 to 745°C.

[0083] If the temperature exceeds the upper limit of the above range, it is advantageous in terms of ensuring the alloying ratio of the alloy layer of the plating layer, but there is a problem that the surface crystal grains of the plating material grow abnormally, which may act as a factor of deteriorating workability and corrosion resistance.If the temperature is below the lower limit of the above range, it is not possible to ensure the target alloying ratio of the alloy layer, and there is a problem that the workability is reduced when the cold-rolled steel sheet is applied to a battery case.

[0084] Hereinafter, specific examples of the present invention will be described. However, the following examples are merely specific examples of the present invention, and the present invention is not limited to the following examples. EXAMPLES

[0085] Slab steel was manufactured having the composition shown in Table 1 below, with the balance being Fe and unavoidable impurities, and the components are shown as actual values. Formula 1 in Table 1 below means the value of 0.0002<(Cr weight% / 52)*1.6(B weight% / 11) / {(C weight% / 12)+(N weight% / 14)<0.0006.

[0086] [Table 1]

[0087] It can be seen that the values ​​of formula 1 for all of the inventive steels 1 to 4 having the compositions in Table 1 can be in the range of 0.0002 to 0.0006, while the values ​​of comparative steels 1 to 5 are not included in the range of formula 1. Tables 2 and 3 below show characteristics according to the differences in the manufacturing processes in Table 1, and the inventive steels 1 to 4 and comparative steels 1 to 5 in Table 1 are manufactured under process conditions controlled to the finish rolling temperature, coiling temperature, cold reduction, annealing temperature, cooling rate, and alloying temperature shown in Table 2 below to manufacture the inventive steels 1 to 4 and comparative steels 1 to 9, and then the properties of each of the steel sheets [I (222) +I (554) ] / I (200) The texture ratio, ear occurrence rate, the presence or absence of processing cracks, workability, precipitate size, alloying ratio of the alloy layer, corrosion resistance, sag resistance, and high temperature yield strength were evaluated and are shown in Table 3 below.

[0088] [I (222) +I (554) ] / I (200)The texture ratio is a value calculated from the relational formula after measuring the strength of the (222), (554), and (200) planes of each plane of the cold-rolled steel sheet by X-ray. The Ear occurrence rate is a value calculated by measuring the height of each part of the cup formed after cupping under the condition of a drawing ratio during drawing processing, that is, material blank diameter / drawing die diameter of 1.85, and calculating {(maximum cup height)-(minimum cup height)}×100 / (maximum cup height).

[0089] The evaluation of processing cracks was carried out by judging whether or not one or more of the cups formed after cupping under the condition of blank diameter / drawing die diameter of 1.85 by drawing for three test pieces broke, and if one or more breaks occurred, it was expressed as broken. The evaluation of processability was classified as poor (X) if the edge generation rate during drawing was 3.0% or more or processing cracks occurred, and as good (O) if the edge generation rate was less than 3.0 and processing cracks did not occur.

[0090] The alloying ratio is the percentage of the area where iron (Fe) and nickel (Ni) coexist, as measured by an energy dispersive X-ray spectroscopy (EDS) for the alloyed heat-treated material. Corrosion resistance was evaluated by a salt spray test (SST), and was classified as poor (X) if red rust was formed within 12 hours, and good (O) if no red rust was formed.

[0091] For sag resistance, a material with a total length of 250 mm and a width of 30 mm was heated at 600°C for 100 hours using heat treatment equipment, and the sag of the steel plate was measured, with the sag of the steel plate being categorized as good (O) if it was less than 3 mm, and poor (X) if it was 3 mm or more. For high-temperature yield strength, a cold-rolled steel plate was held at 600°C for 15 minutes, and then a high-temperature tensile test was performed, with the yield strength being categorized as good (O) if it was 95 MPa or more, and poor (X) if it was less than 95 MPa.

[0092] [Table 2]

[0093] [Table 3]

[0094] Looking at Tables 2 and 3, Examples 1 to 7, which satisfy the ranges of the steel components in Table 1, are (222) +I (554) ] / I (200) It can be confirmed that the texture ratio, ear occurrence rate, the presence or absence of processing cracks, workability, the size of (Cr-B)(CN)-based composite precipitates, alloying ratio, corrosion resistance, sag resistance, and high-temperature yield strength satisfy the target ranges of the present invention. This confirms that Examples 1 to 7 ensure excellent quality characteristics at room temperature and high temperature. In contrast, Comparative Examples 1 to 4, which satisfy the ranges of the steel components but have process conditions that fall outside the manufacturing range of the present invention, have sizes of (Cr-B)(CN)-based composite precipitates that exceed the conditions of the present invention or have [I (222) +I (554) ] / I (200) It can be seen that the texture ratio exceeds the range, and high temperature resistance and D&I processability are not satisfactory.

[0095] In the case of Comparative Examples 5 to 9, which are outside the range of the steel components, the size of the (Cr-B)(CN)-based composite precipitates, the texture ratio, the alloying rate, the processing cracks, the corrosion resistance, the sag resistance, and the high-temperature yield strength do not satisfy the ranges of the present invention, and therefore it can be confirmed that the heat resistance and the D&I processability are not satisfactory.

[0096] As a result, by satisfying the ranges of steel components disclosed in Table 1 of Examples 1 to 7 and the manufacturing process disclosed in Table 2, it is possible to secure properties such as the size, texture ratio, and alloying ratio of (Cr-B)(CN)-based composite precipitates targeted by the present invention, and thereby produce a cold-rolled steel sheet having excellent heat resistance and D&I workability that satisfies high-temperature properties, corrosion resistance, and workability at low cost.

[0097] The present invention is not limited to the above-described embodiments and / or examples, and can be manufactured in various different forms, and a person skilled in the art to which the present invention pertains will understand that the present invention can be embodied in other specific forms without changing the technical idea or essential features of the present invention. Therefore, the above-described embodiments and / or examples should be understood to be illustrative in all respects and not limiting.

Claims

1. The alloy contains, by weight, C: 0.01-0.10%, Mn: 0.1-0.4%, Al: 0.01-0.10%, P: 0.003-0.020%, N: 0.001-0.006%, S: 0.015% or less, Cr: 0.1-0.4%, B: 0.0005-0.0035%, Ni: 0.04-0.10%, the balance being Fe and other unavoidable impurities, Contains (Cr-B)(C-N) based composite precipitates having a size of 0.01 to 0.1 μm, [I （２２２） +I （５５４） ] / I （２００） The texture ratio is 5.0 to 10.0, Ear incidence is less than 3.0%; A cold-rolled steel sheet having an alloying ratio of 5 to 15% in the alloy layer.

2. The cold rolled steel sheet according to claim 1, which satisfies the following formula 1. <Formula 1> 0.0002≦([Cr] / 52)×1.6([B] / 11)) / (([C] / 12)+([N] / 14))≦0.0006 (In the above formula 1, [Cr], [B], [C], and [N] respectively represent the weights of Cr, B, C, and N.)

3. The cold rolled steel sheet according to claim 1, which does not break during cupping under a blank diameter / drawing die diameter ratio of 1.85 during drawing.

4. The cold-rolled steel sheet according to claim 1, which does not develop red rust within 12 hours in a salt spray experiment.

5. The cold rolled steel sheet according to claim 1, wherein the sagging of the cold rolled steel sheet is less than 3 mm after heating at 600°C for 100 hours.

6. The cold rolled steel sheet according to claim 1, having a yield strength of 95 MPa or more at 600°C.

7. heating a steel slab containing, by weight percent, C: 0.01-0.10%, Mn: 0.1-0.4%, Al: 0.01-0.10%, P: 0.003-0.020%, N: 0.001-0.006%, S: 0.015% or less, Cr: 0.1-0.4%, B: 0.0005-0.0035%, Ni: 0.04-0.10%, with the balance being Fe and other unavoidable impurities; finish hot rolling the heated steel slab at 860-930°C; A step of coiling the finish hot-rolled hot-rolled steel sheet at 580 to 700 ° C.; cold rolling the coiled hot-rolled steel sheet at a rolling reduction of 78 to 92%; Annealing the cold-rolled steel sheet at a temperature of 650 to 750°C; cooling the annealed cold-rolled steel sheet at a cooling rate of 30 to 70 ° C. / sec; and subjecting the cold-rolled steel sheet to an alloying heat treatment at 650 to 750° C. after electroplating the cold-rolled steel sheet.

8. The method for producing a cold-rolled steel sheet according to claim 7, which satisfies the following formula 1. <Formula 1> 0.0002≦([Cr] / 52)×1.6([B] / 11)) / (([C] / 12)+([N] / 14))≦0.0006 (In the above formula 1, [Cr], [B], [C], and [N] respectively represent the weights of Cr, B, C, and N.)

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