Cold-rolled steel sheet and method for manufacturing same

Abstract

The objective of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing same, the cold-rolled steel sheet having, when a steel sheet for a vehicle is manufactured by recycling iron scrap, high strength and high formability even if containing residual elements.

Description

Cold rolled steel sheet and manufacturing method thereof

[0001] The present invention relates to a steel sheet suitable as a material for automobiles, and more specifically, to a cold-rolled steel sheet having high strength and high formability and a method for manufacturing the same.

[0002] To address the global climate crisis, automakers are demanding steelmakers supply automotive steel with reduced CO2 emissions, aiming to achieve carbon neutrality by 2050. For this reason, steelmakers are currently developing automotive steel with reduced CO2 emissions through the recycling of scrap iron, utilizing existing blast furnace-converter furnace or electric arc furnace steelmaking processes.

[0003] When using steel scrap as a raw material, it is difficult to remove residual elements (tramp elements) such as Cr, Cu, Ni, and Sn contained in the steel scrap during refining, so they end up being included in the steel after refining. These residual elements have the problem of lowering the physical properties of the steel or deteriorating the surface quality, so until now, high-grade thin plate products such as automotive steel sheets have been manufactured using molten iron as the main raw material and using a general blast furnace-converter process to greatly reduce the C and N in the steel while extremely controlling the content of residual elements.

[0004] Meanwhile, a technology for manufacturing automobile steel sheets with excellent formability (processability) from electric furnace steel containing residual elements using an electric furnace steelmaking method was proposed as follows.

[0005] Patent Documents 1 and 2 describe technologies for manufacturing cold-rolled steel sheets with excellent workability, and disclose cold-rolled steel sheets with excellent cold workability even when the steel contains a large amount of residual elements. These technologies mention the relationship between the tensile strength and elongation of the steel sheet (TS × El), and the relationship between the Lankford value (r-value) and elongation (r-value × El). However, these technologies only limit the inclusion of Cu and Ni as residual elements in response to the problem of the Lankford value (r-value) decreasing depending on the content of residual elements in the steel composition.

[0006] That is, in the case of steel sheets generally used as automobile exterior panels, there are only reports that the Lankford value (r-value) decreases as the residual elements increase, resulting in poor formability, and a decrease in the Lankford value (r-value) may mean that the formability is inferior.

[0007] While technologies for manufacturing automotive steel sheets containing residual elements such as impurities have been proposed, automotive steel sheets that maintain a certain level of strength and ductility while also exhibiting excellent formability, despite the presence of residual elements, have yet to be developed. In particular, cold-rolled steel sheets suitable for automotive exterior panels, which possess high strength while maintaining ductility and a Lankford value (r-value) at a high formability level despite containing residual elements, have yet to be developed.

[0008] (Patent Document 1) Japanese Patent Publication No. 1995-118795

[0009] (Patent Document 2) Japanese Patent Publication No. 1998-025541

[0010] One aspect of the present invention is to provide a cold rolled steel sheet having high strength and high formability while containing residual elements in manufacturing automotive steel sheets by recycling iron scrap, and a method for manufacturing the same.

[0011] The objectives of the present invention are not limited to the above-described content. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall description of the present invention.

[0012] According to one aspect of the present invention, in weight %, carbon (C): more than 0% to 0.0500% or less, silicon (Si): more than 0% to 0.080% or less, manganese (Mn): more than 0% to 0.500% or less, aluminum (Al): more than 0% to 0.080% or less, phosphorus (P): 0 to 0.080%, sulfur (S): 0 to 0.0500%, nitrogen (N): 0 to 0.0300%, titanium (Ti): 0.001 to 0.050%, niobium (Nb): 0.001 to 0.050%, copper (Cu): 0 to 0.700%, nickel (Ni): 0 to 0.700%, chromium (Cr): 0 to 0.700%, molybdenum (Mo): 0 to 0.700%, boron (B): We provide cold rolled steel sheets containing 0~0.0200%, antimony (Sb): 0~0.5000%, tin (Sn): 0~0.5000%, the remainder Fe, and other unavoidable impurities.

[0013] A cold rolled steel sheet according to one embodiment of the present invention can satisfy an A value defined in the following relational expression 1 of 0.40 to 1.40.

[0014] [Relationship 1]

[0015] A = [Mn] / (10×([Ti]+[Nb]+[N]))

[0016] (In equation 1, [Mn], [Ti], [Nb], and [N] are the weight contents of each element.)

[0017] In one embodiment of the present invention, the cold rolled steel sheet may have a sum of Cu, Ni, Cr, and Sn contents (Cu+Ni+Cr+Sn) of 0.7000 wt% or less.

[0018] The cold-rolled steel sheet of the present invention can secure excellent formability along with high strength even if it contains a certain amount of residual elements.

[0019] As an example, a cold rolled steel sheet according to one embodiment of the present invention may have a Lankford value (r-value) of 1.6 or more, and a T value defined in the following relational expression 2 may be 0.90 to 1.20.

[0020] [Relationship 2]

[0021] T = R / ((0.4×([Cu]+[Ni]+[Cr]+[Sn]))+1.6)

[0022] (In equation 2, R is the Lankford value (r-value), and [Cu], [Cr], [Ni], and [Sn] are the weight contents of each element.)

[0023] A cold-rolled steel sheet according to one embodiment of the present invention may include a microstructure of ferrite with an area fraction of 95% or more and a residual structure.

[0024] According to one embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet can be provided, comprising the steps of: preparing a steel slab; reheating the steel slab in a temperature range of 900 to 1300°C; finish-rolling the reheated steel slab in an austenite region higher than the Ar3 transformation point or in a ferrite region lower than the Ar3 transformation point to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature of 500°C or higher; cold-rolling the hot-rolled steel sheet after the coiling at a reduction ratio of 40% or higher to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet at 600°C or higher for 10 seconds or longer.

[0025] In one embodiment of the present invention, the steel slab may have the above-described alloy composition and may satisfy relational expression 1.

[0026] Cold rolled steel sheets manufactured through a series of processes such as this can have high strength and excellent formability, and as an example, can satisfy the above-mentioned relationship 2.

[0027] According to the present invention, a cold-rolled steel sheet having improved formability and high strength while containing a certain amount of residual elements can be provided, and such a cold-rolled steel sheet can be suitably applied as an automobile material.

[0028] In addition, the steel plate of the present invention is manufactured by recycling iron scrap, and has the advantage of excellent CO2 reduction effect when applied as a material for automobile bodies, etc.

[0029] 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.

[0030] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the invention. As used herein, the singular forms also include the plural forms, unless the context clearly dictates otherwise.

[0031] The term "comprising" as used in the specification means specifying a particular characteristic, region, integer, step, operation, element and / or component, but does not exclude the presence or addition of other particular characteristics, regions, integers, steps, operations, elements, components and / or groups.

[0032] 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.

[0033] Hereinafter, a cold-rolled steel sheet according to one aspect of the present invention, particularly a cold-rolled steel sheet with high strength and excellent formability, will be described in detail. It should be noted that, unless otherwise specified, the content of each element in the present invention refers to weight percent. Furthermore, unless otherwise specified, the ratio of crystals or structures is based on area.

[0034] A cold rolled steel sheet according to one aspect of the present invention comprises, in wt%, carbon (C): more than 0% to 0.0500% or less, silicon (Si): more than 0% to 0.080% or less, manganese (Mn): more than 0% to 0.500% or less, aluminum (Al): more than 0% to 0.080% or less, phosphorus (P): 0 to 0.080%, sulfur (S): 0 to 0.0500%, nitrogen (N): 0 to 0.0300%, titanium (Ti): 0.001 to 0.050%, niobium (Nb): 0.001 to 0.050%, copper (Cu): 0 to 0.700%, nickel (Ni): 0 to 0.700%, chromium (Cr): 0 to 0.700%, molybdenum (Mo): 0 to 0.700%, boron (B): It may contain 0~0.0200%, antimony (Sb): 0~0.5000%, tin (Sn): 0~0.5000%.

[0035] Below, the reason for limiting the alloy composition of the cold-rolled steel sheet provided in the present invention as described above is explained in detail.

[0036] Carbon (C): 0% to 0.0500%

[0037] Carbon (C) is an element that is effective in securing the strength of steel.

[0038] In one embodiment of the present invention, if the C content exceeds 0.0500%, it is difficult to secure the target level of elongation and r-value. Therefore, the C content may be 0.0500% or less. In another embodiment of the present invention, the C content may be 0.0400% or less.

[0039] Meanwhile, C is an essential element in steel manufacturing, and its content is greater than 0%. However, in one embodiment of the present invention, if the content is less than 0.0003%, the effect of reducing C cannot be obtained, and rather, it causes an increase in steel manufacturing cost, which is not preferable. Therefore, the C may be included in an amount of 0.0003% or more. In another embodiment of the present invention, the C may be included in an amount of 0.0010% or more, or 0.0015% or more.

[0040] Silicon (Si): 0% to 0.080%

[0041] Silicon (Si) is an element that enhances strength through solid solution strengthening, and is useful for strengthening ferrite and uniforming its structure, thereby improving workability. Furthermore, Si is an element necessary for deoxidation during steelmaking.

[0042] In one embodiment of the present invention, if the Si content exceeds 0.080%, there is a problem of causing plating defects such as underplating during plating and hindering the weldability of the steel sheet. Therefore, the Si content may be 0.080% or less. In another embodiment of the present invention, the Si content may be 0.070% or less.

[0043] Meanwhile, in one embodiment of the present invention, Si may be included in an amount exceeding 0%, and the content may be 0.001% or more in consideration of manufacturing costs.

[0044] Manganese (Mn): 0% to 0.500%

[0045] Manganese (Mn) is a useful element for simultaneously improving the strength and ductility of steel.

[0046] In one embodiment of the present invention, if the content of Mn exceeds 0.500%, processability deteriorates, which is not preferable. In another embodiment of the present invention, the content of Mn may be 0.400% or less.

[0047] Meanwhile, in one embodiment of the present invention, Mn may be included in an amount exceeding 0%, and the content may be 0.001% or more in consideration of manufacturing costs.

[0048] Aluminum (Al): 0% to 0.080%

[0049] Aluminum (Al) is an element that combines with oxygen (O) in steel and acts as a deoxidizer. Similarly to Si, it is an element that strengthens ferrite, homogenizes the structure, and improves workability.

[0050] In one embodiment of the present invention, if the Al content exceeds 0.080%, there is a problem that the processability is reduced. Therefore, the Al may be contained at 0.080% or less. In another embodiment of the present invention, the Al may be contained at 0.060% or less.

[0051] Meanwhile, in one embodiment of the present invention, Al may be included in an amount exceeding 0%. However, since controlling the Al content to less than 0.001% increases manufacturing costs, the content may be limited to 0.001% or more in consideration of this.

[0052] Phosphorus (P): 0.080% or less

[0053] Phosphorus (P) is an element added to improve the strength of steel. In one embodiment of the present invention, if the P content exceeds 0.080%, there is a problem of hindering the workability of the steel and causing brittleness. Therefore, the P may be included in an amount of 0.080% or less. In another embodiment of the present invention, the P may be included in an amount of 0.070% or less.

[0054] Meanwhile, in one embodiment of the present invention, P may be included in an amount exceeding 0%, and its content may be limited to 0.001% or more in consideration of manufacturing costs.

[0055] Sulfur (S): 0.0500% or less

[0056] Sulfur (S) is an element that is inevitably added during the steel manufacturing process, and it combines with manganese (Mn) in the steel to form MnS inclusions, thereby inhibiting the ductility of the steel. Therefore, in one embodiment of the present invention, the S content may be limited to 0.0500% or less. In another embodiment of the present invention, S may be included at 0.0400% or less.

[0057] Meanwhile, since S can inevitably be contained in steel, its content can exceed 0%. However, in one embodiment of the present invention, there is a problem that manufacturing costs increase significantly in order to control the S content to less than 0.0010%. Therefore, the S content can be limited to 0.0010% or more.

[0058] Nitrogen (N): 0.0300% or less

[0059] Nitrogen (N) is an element that is inevitably added during the steel manufacturing process, and is an element that causes cracks in slabs by forming nitrides during the continuous casting process. Therefore, in one embodiment of the present invention, N may be limited to 0.0300% or less. In another embodiment of the present invention, N may be included at 0.0200% or less.

[0060] As mentioned above, N may inevitably be contained in steel, and thus its content may exceed 0%. However, since controlling the N content to less than 0.0010% significantly increases manufacturing costs, the N content may be limited to 0.0010% or more.

[0061] Titanium (Ti): 0.001–0.050% and niobium (Nb): 0.001–0.050%

[0062] Titanium (Ti) and niobium (Nb) are elements that form precipitates in steel, and the formation of these precipitates can improve the strength and impact toughness of the steel. In particular, Ti and Nb can prevent deterioration of workability caused by the dissolved carbon and dissolved nitrogen in the steel by precipitating them as carbides, nitrides, etc.

[0063] In one embodiment of the present invention, the above elements may be included in an amount of 0.001% or more, thereby obtaining the aforementioned effects. However, if the content of each element exceeds 0.050%, the additive effect becomes saturated and the manufacturing cost increases, so the upper limit of each element may be limited to 0.050%. According to another embodiment of the present invention, the above elements may be included in an amount of 0.040% or less.

[0064] Copper (Cu): 0~0.700% and Nickel (Ni): 0~0.700%

[0065] Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. Furthermore, Cu and Ni are useful elements for suppressing hydrogen-delayed fracture by concentrating on the surface of steel plates and preventing the intrusion of hydrogen into the steel.

[0066] In one embodiment of the present invention, since processability becomes poor when the Cu and Ni contents are excessive, the contents may be limited to 0.700% or less. In another embodiment of the present invention, the Cu and Ni may be included in amounts of 0.600% or less.

[0067] Meanwhile, in one embodiment of the present invention, Cu and Ni may be included in amounts exceeding 0%, and may be included in amounts of 0.001% or more each in order to sufficiently obtain the effects of these elements.

[0068] Chromium (Cr): 0~0.700% and molybdenum (Mo): 0~0.700%

[0069] Chromium (Cr) and molybdenum (Mo) are elements that suppress the decomposition of austenite during alloying treatment of steel plates and, similar to Mn, stabilize austenite.

[0070] In one embodiment of the present invention, since excessive contents of Cr and Mo deteriorate processability, taking this into consideration, each may be limited to 0.700% or less. In another embodiment of the present invention, Cr and Mo may be included at 0.600% or less.

[0071] Meanwhile, in one embodiment of the present invention, Cr and Mo may be included in amounts exceeding 0%, and in order to sufficiently obtain the effects of these elements, they may be included in amounts of 0.001% or more, respectively.

[0072] Boron (B): 0~0.0200%

[0073] Boron (B) is an element that improves the hardenability of steel, thereby increasing its strength, and inhibits nucleation at grain boundaries.

[0074] In one embodiment of the present invention, if the content of B exceeds 0.0200%, there is a concern that the deep drawability of the steel may deteriorate. Therefore, the B may be included in an amount of 0.0200% or less. According to another embodiment, the B may be included in an amount of 0.0100% or less. Meanwhile, in one embodiment of the present invention, in order to obtain the effect of adding B, the B may be included in an amount of 0.0001% or more.

[0075] Antimony (Sb): 0~0.5000% and tin (Sn): 0~0.5000%

[0076] Antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of steel. In one embodiment of the present invention, if the content of each of the elements exceeds 0.5000%, the brittleness of the steel increases, which causes cracks during hot working or cold working, and therefore the content of each element may be limited to 0.5000% or less. According to another embodiment of the present invention, each of Sb and Sn may be included at 0.4000% or less. Meanwhile, in order to sufficiently obtain the effect of adding these elements, each of them may be included at 0.0005% or more.

[0077] In one embodiment of the present invention, the steel sheet may contain iron and other unavoidable impurities in addition to the aforementioned components. However, since unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during the typical manufacturing process, they cannot be completely excluded. Since these impurities are readily apparent to anyone skilled in the art, not all of them are specifically mentioned herein. Furthermore, the addition of additional effective components other than the aforementioned components is not completely excluded.

[0078] According to one embodiment of the present invention, the total content of Cu, Ni, Cr, and Sn in the cold-rolled steel sheet may be 0.7000 wt% or less (Cu+Ni+Cr+Sn).

[0079] A cold-rolled steel sheet according to one embodiment of the present invention uses iron scrap containing various impurity elements as a material, and may include elements such as Cu, Ni, Cr, and Sn as alloy components constituting the cold-rolled steel sheet. As mentioned above, if the content of these is excessive, the physical properties intended in the present invention cannot be obtained. Therefore, in one embodiment of the present invention, the sum of the contents of Cu, Ni, Cr, and Sn in the composition of the cold-rolled steel sheet may be limited to 0.7000% or less.

[0080] Meanwhile, there is no particular limitation on the lower limit of the content of the aforementioned elements, but it may be contained at 0.001% or more.

[0081] A cold rolled steel sheet according to one embodiment of the present invention can satisfy an A value of 0.40 to 1.40, where the content relationship of specific elements is defined by the following relational expression 1.

[0082] [Relationship 1]

[0083] A = [Mn] / (10×([Ti]+[Nb]+[N]))

[0084] (In equation 1, [Mn], [Ti], [Nb], and [N] are the weight contents of each element.)

[0085] When manufacturing cold rolled steel sheets using iron scrap, residual elements contained in the iron scrap may be mixed into the cold rolled steel sheets being manufactured, and generally, the higher the content of the residual elements, the poorer the formability of the cold rolled steel sheets.

[0086] Considering these points, the inventors of the present invention conducted in-depth research to manufacture a cold-rolled steel sheet with excellent formability even if it contains a certain amount of residual elements, and derived relational expression 1. That is, in one embodiment of the present invention, by controlling the relationship between the content of Mn and the content of Ti, Nb, and N that form precipitates according to relational expression 1, it is possible to secure not only the strength but also the target formability. If the value of A according to relational expression 1 is less than 0.40, the content of Mn, which is advantageous for securing strength, becomes excessively low, making it impossible to secure the target strength. On the other hand, if the value exceeds 1.40, precipitates are not properly formed in the steel, and as a result, excessive solid solution C, solid solution N, etc. remain in the steel, resulting in poor formability.

[0087] A cold-rolled steel sheet according to one embodiment of the present invention may include a ferrite phase as its main microstructure, and as an example, may include a ferrite phase with an area fraction of 95% or more. In this case, a single ferrite phase may also be used.

[0088] However, in terms of improving strength, the cold-rolled steel sheet according to one embodiment of the present invention may include a residual structure in addition to ferrite at 5% or less. The residual structure is not particularly limited, but as an example, the residual structure may be a hard phase such as martensite or bainite, or may be a pearlite phase. These residual structures may be collectively referred to as a low-temperature transformation phase.

[0089] A cold rolled steel sheet according to one embodiment of the present invention may have excellent formability characteristics.

[0090] Specifically, according to one embodiment of the present invention, the cold-rolled steel sheet may have a Lankford value (r-value) of 1.6 or more. As such, the cold-rolled steel sheet according to one embodiment of the present invention may have excellent formability with an r-value of 1.6 or more.

[0091] The cold rolled steel sheet according to one embodiment of the present invention can have the target formability even if the content of residual elements contained in the steel, particularly Cu, Ni, Cr and Sn, is relatively high, and this is defined by the following relational expression 2.

[0092] [Relationship 2]

[0093] T = R / ((0.4×([Cu]+[Ni]+[Cr]+[Sn]))+1.6)

[0094] (In equation 2, R is the Lankford value (r-value), and [Cu], [Cr], [Ni], and [Sn] are the weight contents of each element.)

[0095] That is, the cold-rolled steel sheet according to one embodiment of the present invention can satisfy the T value defined in the following relational expression 2 of 0.90 to 1.20, and such a characteristic means that even if the content of the residual elements increases, the r-value does not decrease but rather tends to increase. This ultimately means that according to one embodiment of the present invention, not only can the content of the residual elements contained in the steel be secured within a certain range, but also the formability of the cold-rolled steel sheet can be secured within that range.

[0096] If the value of relational expression 2 according to one embodiment of the present invention is less than 0.90, the r-value may decrease as the content of residual elements in the alloy composition of the steel increases, which may cause problems in securing formability. On the other hand, if the value exceeds 1.20, there is no problem in securing formability, but there is a concern that the strength may be significantly reduced.

[0097] Meanwhile, a cold-rolled steel sheet according to one embodiment of the present invention can secure high strength and high ductility. As an example, the cold-rolled steel sheet can have a tensile strength of 390 MPa or more and an elongation of 32% or more.

[0098] Furthermore, the cold-rolled steel sheet can satisfy a product of the tensile strength and elongation (TS×El) of 13.0 to 16.0 GPa·%, and a product of the Lankford value (r-value) and elongation (r-value×El) of 55.0 to 75.0%.

[0099] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to another aspect of the present invention will be described in detail.

[0100] In one embodiment of the present invention, a cold-rolled steel sheet can be obtained by undergoing a series of processes, for example, [steel slab heating - hot rolling - coiling - cooling - cold rolling - annealing]. Each process step is described in detail below, and it is to be noted that the following manufacturing process is an example for manufacturing the cold-rolled steel sheet of the present invention.

[0101] [Heating of steel slabs]

[0102] After preparing the steel slab, it can be heated. At this time, the steel slab may be a steel ingot. In one embodiment of the present invention, the steel slab may have the alloy composition described above, and thus, the alloy composition of the steel slab is replaced with the above-described content.

[0103] In one embodiment of the present invention, the heating temperature of the prepared steel slab or ingot may be in the range of 900 to 1300°C. If the heating temperature exceeds 1300°C, the steel may reach its melting point and melt, and the workability and plating adhesion of the steel may also be poor. On the other hand, if the temperature is lower than 900°C, there is a concern that the rolling load may increase during subsequent hot rolling, thereby reducing the stability of the hot rolling. In another embodiment of the present invention, the heating may be performed at 1160°C or lower.

[0104] According to one embodiment of the present invention, steel slabs or ingots can be manufactured through an electric furnace or a new blast furnace-converter process, and the raw materials used in the electric furnace or blast furnace-converter process include pig iron together with iron scrap. Here, pig iron refers to molten iron obtained from the blast furnace-converter process, or its cold material (Corrugate) or HBI (Hot Briquette Iron).

[0105] In the case of electric furnaces, desulfurization can be performed through ladle refining after the electric furnace discharge, and desulfurization and subsequent vacuum degassing can be performed. Furthermore, steel obtained from an electric furnace can be adjusted to achieve the desired final alloy composition by adding alloying elements during degassing. Common vacuum degassing methods include the RH and DH methods, but oxygen injection into the degassing tank can also be used. One such method is the oxygen injection method using a top blowing lance.

[0106] [Hot rolling]

[0107] The above heated steel slab or ingot can be hot rolled to obtain a hot rolled steel sheet.

[0108] In one embodiment of the present invention, in order to improve the workability and plating adhesion of the steel sheet, the finishing temperature during hot rolling may be performed in the austenite region above the Ar3 transformation point or in the ferrite region below the Ar3 transformation point. However, if the finishing temperature is too low, the rolling load during hot rolling increases, so the lower limit temperature may be limited to 800°C or higher. In another embodiment of the present invention, the finishing temperature may be 880°C or higher.

[0109] [Winding]

[0110] The hot-rolled steel sheet obtained by the above hot rolling can be wound into a coil shape.

[0111] In one embodiment of the present invention, in order to improve the high strength and high formability of the steel sheet, the coiling process may be performed at a temperature of 500°C or higher. However, if the temperature exceeds 800°C, excessively thick scale may form on the surface of the hot-rolled coil. In another embodiment of the present invention, the coiling temperature may be performed at 600°C or higher.

[0112] [Cold rolling]

[0113] The hot-rolled coil obtained through the above-described coiling process can be cold-rolled to obtain a cold-rolled steel sheet. At this time, before performing the cold rolling, the hot-rolled coil is cooled to room temperature (air-cooled), and cold rolling can be performed while uncoiling the hot-rolled coil cooled in this manner.

[0114] In one embodiment of the present invention, cold rolling may be performed at a cold reduction ratio of 40% or more. If the cold reduction ratio is less than 40%, it may be difficult to secure the target thickness, and the workability of the steel sheet may be poor. According to one embodiment of the present invention, there is no particular limitation on the upper limit of the cold reduction ratio during cold rolling, but if it is too excessive, a cold rolling load may be induced, and therefore, taking this into consideration, it may be limited to 90% or less.

[0115] [Sodun]

[0116] The cold rolled steel sheet manufactured above can be annealed and heat treated.

[0117] In one embodiment of the present invention, the annealing heat treatment process may be performed under the usual conditions applied to manufacturing cold-rolled steel sheets, and not only the continuous annealing method but also the box-type annealing method may be applied.

[0118] As an example, in the case of continuous annealing, annealing heat treatment can be performed by raising the temperature of a cold-rolled steel sheet to 600°C or higher and maintaining the temperature for 10 seconds or longer. If the temperature is lower than 600°C or the annealing time is shorter than 10 seconds during the annealing heat treatment, recrystallization will not sufficiently occur during the annealing process, making it difficult to ensure the workability of the steel.

[0119] According to one embodiment of the present invention, the upper limits of temperature and time during the annealing heat treatment are not particularly limited. However, considering the risk of equipment trouble due to high-temperature annealing and the inferiority of powdering properties, the upper limits may be set at 960°C and 15 minutes, respectively. That is, if the temperature during the annealing heat treatment is too high or the heat treatment is performed for a long time, there is a risk that the powdering properties of the plated steel sheet obtained through the subsequent plating process may be inferior.

[0120] According to another embodiment of the present invention, the annealing heat treatment process of the cold rolled steel sheet may be performed at 800°C or higher. According to another embodiment, the annealing heat treatment process may be performed at 900°C or lower.

[0121] The cold rolled steel sheet manufactured through the above-described series of processes can have the strength and formability targeted in the present invention.

[0122] In particular, the cold-rolled steel sheet according to one embodiment of the present invention may have a Lankford value (r-value) of 1.6 or more, a tensile strength of 390 MPa or more, and an elongation of 32% or more.

[0123] In addition, the cold-rolled steel sheet may have a product of tensile strength (TS) and elongation (El) (TS×El) of 13.4 to 15.6 GPa·%, and a product of Lankford value (r-value) and elongation (r-value×El) of 55.4 to 70.0%.

[0124] Meanwhile, a surface treatment process may be further performed on a cold-rolled steel sheet according to an embodiment of the present invention, i.e., a cold-rolled steel sheet manufactured through a series of processes, as needed. In this case, the surface treatment process may be performed after annealing heat treatment.

[0125] In one embodiment of the present invention, the surface treatment process may be a process such as plating or enamel, and the double plating may be zinc plating, tin plating, or the like.

[0126] In addition, cold-rolled steel sheets subjected to a surface treatment process according to one embodiment of the present invention may be subjected to further special treatment as needed. For example, special treatment may be performed after zinc plating. In this case, the special treatment may be performed to improve the steel sheet's chemical processability, weldability, press forming, corrosion resistance, and other properties.

[0127] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended solely to illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0128] (Example)

[0129] Steel slabs having the alloy compositions shown in Table 1 below were prepared. Hot-rolled steel sheets were manufactured from the steel slabs through the [reheating - hot rolling - coiling - cooling] process according to the respective conditions shown in Table 2 below. Thereafter, cold-rolled steel sheets were obtained by performing a series of subsequent processes (cold rolling - annealing heat treatment) shown in Table 2 below for each hot-rolled steel sheet.

[0130] The above steel slab was obtained through the following process. First, iron scrap mixed with molten iron from a conventional blast furnace was introduced into an electric furnace to obtain molten steel. This molten steel was then transferred to a ladle and vacuum degassing was performed in an RH degassing furnace (0.1 torr). At the same time, alloying components were added to produce molten steel having the desired composition. Thereafter, the molten steel was manufactured into a slab by continuous casting.

[0131] The microstructure and mechanical properties of the manufactured steel plates were measured and evaluated, and all results are shown in Table 3 below.

[0132] First, the microstructure of each cold-rolled steel sheet was determined by taking a specimen at t / 4 in the thickness (t, mm) direction, polishing it, and then observing the cross-section of the polished specimen using a scanning electron microscope (SEM) after nital etching. After nital etching, the structure without irregularities on the specimen surface was determined to be ferrite, and the structure with a spherical or needle-shaped structure was determined to be cementite.

[0133] In addition, a tensile test was conducted to evaluate the physical properties of each cold-rolled steel sheet. The tensile test was evaluated using specimens collected according to the JIS No. 5 standard at a 90-degree angle to the rolling direction of the rolled plate. At this time, after obtaining the tensile strength and ductility results, the product of the tensile strength (TS) and the elongation (El) (TS × El) was calculated. In addition, the Lankford value (r-value) for the same specimen was measured using the three-point method after 15% tensile prestrain, and the average values ​​in the rolling direction (L direction), the direction perpendicular to the rolling direction (C direction), and the direction at 45 degrees to the rolling direction (D direction) were calculated using the following formula.

[0134] r-value = (r L + 2r D + r C ) / 4

[0135] Steel alloy composition system (weight %)CSiMnPSAlNTiNbCuCrNiSn(Cu+Cr+Ni+Sn) relationship 1A0.00330.0320.2290.04490.00510.03240.00580.02120.02090.070 0.0690.0400.01600.19500.48B0.00290.0310.2270.04470.00480.03 200.00590.02110.02100.0670.0650.0370.01600.18500.47C0.00280 .0320.2270.04450.00480.03240.00560.02110.02070.0660.0640.03 70.01500.18200.48D0.00280.0300.2250.04460.00470.03220.00570 .02100.02060.0660.0650.0380.01600.18500.48E0.00260.0310.226 0.04450.00460.03230.00550.02120.02070.0680.0660.0380.01500. 18700.48F0.00210.0300.2260.04460.00470.03230.00540.02110.02 080.0680.0660.0380.01500.18700.48XA0.00160.0280.1600.01900. 00800.03900.00070.00100.00500.0750.0390.0290.00120.14422.39 XB0.00130.0280.1500.01900.00800.03900.00090.00100.00400.103 0.0540.0410.00130.19932.54XC0.00150.0270.1500.02000.00800.0 3700.00100.00100.00500.1400.0720.0530.00110.26612.14XD0.001 50.0290.1600.01900.00800.03900.00090.00100.00500.1800.0880. 0670.00120.33622.32XE0.00140.0300.1500.01900.00800.03400.00 100.00100.00500.2200.1040.0780.00110.40312.14XF0.00140.0300.1500.01900.00800.03800.00100.00100.00500.3800.1720.1300.00130.68332.14XG0.002 00.0030.1180.01080.00310.0230.00310.00370.00960.0660.0420.0300.0060.14400.24.

[0136]

[0137] Steel grade reheating temperature (℃) Finish rolling temperature (℃) Coiling temperature (℃) Cold reduction ratio (%) Annealing temperature (℃) Annealing time (s) Classification A12209407007581035 Invention example 1 B12209407007581035 Invention example 2 C12209407007581035 Invention example 3 D12209407007581035 Invention example 4 E12209407007581035 Invention example 5 F12209407007581035 Invention example 6 A12009206507583033 Invention example 7 B12009206507583033 Invention example 8C12009206507583033 Invention Example 9D12009206507583033 Invention Example 10E12009206507583033 Invention Example 11F12009206507583033 Invention Example 12A11809006007585030 Invention Example 13B11809006507585030 Invention Example 14C11809006507585030 Invention Example 15D11809006507585030 Invention Example 16E11809006507585030 Invention Example 17F11809006507585030 Invention Example 18XA12009206507583035Comparison Example 1XB12009206507583035Comparison Example 2XC12009206507583035Comparison Example 3XD12009206507583035Comparison Example 4XE12009206507583035Comparison Example 5XF12009206507583053Comparison Example 6XG11809207007585030Comparison Example 7

[0138]

[0139] Classification YS (MPa) TS (MPa) El (%) TS × El (GPa · %) r-valuer-value × El (%) Relationship 2 (T value) Invention example 1 3 30 40 8 35 14.31.6 256.7 0.97 Invention example 2 29 339 434 13.41.6 355.40.97 Invention example 3 29 239 335 13.81.6 557.80.99 Invention example 4 29 139 236 14.11.6 8 6 0.51.00 Invention example 5 29 039 036 14.01.67 6 0.11.00 Invention example 6 289 39 037 14.41.6 8 6 2.21.00 Invention example Invention example 73104003614.41.6459.00.98 Invention example 83013933714.51.6561.10.99 Invention example 93113993714.81.6460.70.98 Invention example 103063983714.71.6661.40.99 Invention example 113004043614.51.6760.11.00 Invention example 122984023815.31.6763.51.00 Invention example 133014003815.21.6964.21.01 Invention example 142963953915.41.7166.71.02 Invention example 152873923915.31.7267.11.03 Invention example 162903943815.01.7365.71.03 Invention example 172943904015.61.7469.61.04 Invention example 182923914015.61.7570.01.04 Comparative example 119730747.414.61.6678.71.00 Comparative example 220631245.214.11.5369.20.91 Comparative example 321131544.314.01.3961.60.81 Comparative example 421231945.314.51.3762.10.79 Comparative example 521432344.314.31.3057.60.74Comparative example 621532544.614.51.2555.80.67Comparative example 716130445.013.72.0190.51.21The r-value of the invention examples is r L < r D < r C The results were presented in order of size. It was confirmed that the microstructures of the invention examples all had more than 95% ferrite and the remainder structure. Meanwhile, there were also cases where the ferrite single phase (100%) existed.

[0140]

[0141] As shown in Tables 1 to 3 above, in the case of Invention Examples 1 to 18 that satisfy the alloy composition and manufacturing conditions according to one embodiment of the present invention, it can be confirmed that they have the desired properties, i.e., high strength, high ductility, and high formability. In particular, by controlling the content between specific elements, it was possible to secure formability even when a certain amount of residual elements was contained.

[0142] On the other hand, Comparative Examples 1 to 7, which did not satisfy the alloy composition system according to an embodiment of the present invention, had inferior properties in at least one or more properties. In particular, Comparative Examples 1 to 7 are examples that deviate from the value of Relationship 1 according to an embodiment of the present invention. Among these, Comparative Examples 1 to 6 could not secure the target strength because the effect of adding precipitate-forming elements was not exerted. In addition, Comparative Examples 3 to 6 could not satisfy Relationship 2 due to the relatively large content of residual elements, and showed a tendency for further inferior formability. Meanwhile, Comparative Example 7 had lower strength due to the relatively low content of Mn.

Claims

1. In weight%, carbon (C): more than 0% but less than 0.0500%, silicon (Si): more than 0% but less than 0.080%, manganese (Mn): more than 0% but less than 0.500%, aluminum (Al): more than 0% but less than 0.080%, phosphorus (P): 0 to 0.080%, sulfur (S): 0 to 0.0500%, nitrogen (N): 0 to 0.0300%, titanium (Ti): 0.001 to 0.050%, niobium (Nb): 0.001 to 0.050%, copper (Cu): 0 to 0.700%, nickel (Ni): 0 to 0.700%, chromium (Cr): 0 to 0.700%, molybdenum (Mo): 0 to 0.700%, boron (B): 0 to 0.0200%, Contains antimony (Sb): 0~0.5000%, tin (Sn): 0~0.5000%, the remainder Fe and other unavoidable impurities. Cold rolled steel sheet having an A value of 0.40 to 1.40 as defined in the following relational expression 1. [Relationship 1] A = [Mn] / (10×([Ti]+[Nb]+[N])) (In equation 1, [Mn], [Ti], [Nb], and [N] represent the weight contents of each element.) 2. In paragraph 1, The above cold rolled steel sheet is a cold rolled steel sheet having a total content of Cu, Ni, Cr, and Sn (Cu+Ni+Cr+Sn) of 0.7000 wt% or less.

3. In paragraph 1, The above cold rolled steel sheet has a Lankford value (r-value) of 1.6 or higher, Cold rolled steel sheet having a T value of 0.90 to 1.20 as defined in the following relational expression 2. [Relationship 2] T = R / ((0.4×([Cu]+[Ni]+[Cr]+[Sn]))+1.6) (In equation 2, R is the Lankford value (r-value), and [Cu], [Cr], [Ni], and [Sn] are the weight contents of each element.) 4. In paragraph 1, The above cold rolled steel sheet is a cold rolled steel sheet having a microstructure including a ferrite area fraction of 95% or more and a residual structure.

5. In paragraph 1, The above cold rolled steel sheet has a tensile strength of 390 MPa or more and an elongation of 32% or more. Cold rolled steel sheet having a product of the above tensile strength and elongation (TS × El) of 13.0 to 16.0 GPa·%.

6. In paragraph 1, A cold rolled steel sheet having a product of the Lankford value (r-value) and elongation (r-value × El) of 55.0 to 75.0%.

7. In weight%, carbon (C): more than 0% but less than 0.0500%, silicon (Si): more than 0% but less than 0.080%, manganese (Mn): more than 0% but less than 0.500%, aluminum (Al): more than 0% but less than 0.080%, phosphorus (P): 0 to 0.080%, sulfur (S): 0 to 0.0500%, nitrogen (N): 0 to 0.0300%, titanium (Ti): 0.001 to 0.050%, niobium (Nb): 0.001 to 0.050%, copper (Cu): 0 to 0.700%, nickel (Ni): 0 to 0.700%, chromium (Cr): 0 to 0.700%, molybdenum (Mo): 0 to 0.700%, boron (B): 0 to 0.0200%, A step for preparing a steel slab containing antimony (Sb): 0 to 0.5000%, tin (Sn): 0 to 0.5000%, the remainder Fe and other unavoidable impurities, and having an A value of 0.40 to 1.40 defined in the following relational expression 1; A step of reheating the above steel slab at a temperature range of 900 to 1300℃; A step of obtaining a hot rolled steel sheet by finish rolling the reheated steel slab in the austenite range above the Ar3 transformation point or in the ferrite range below the Ar3 transformation point; The above hot-rolled steel plate is coiled at a temperature of 500℃ or higher; A step of cold rolling the hot rolled steel sheet after the above-mentioned coiling at a reduction ratio of 40% or more to obtain a cold rolled steel sheet; and A method for manufacturing a cold rolled steel sheet, comprising a step of annealing the cold rolled steel sheet at 600°C or higher for 10 seconds or longer. [Relationship 1] A = [Mn] / (10×([Ti]+[Nb]+[N])) (In equation 1, [Mn], [Ti], [Nb], and [N] represent the weight contents of each element.) 8. In paragraph 7, The above steel slab is a method for manufacturing a cold rolled steel sheet having a total content of Cu, Ni, Cr and Sn (Cu+Ni+Cr+Sn) of 0.7000 wt% or less.

9. In paragraph 7, After the above annealing heat treatment, the cold rolled steel sheet has a Lankford value (r-value) of 1.6 or higher. A method for manufacturing cold rolled steel sheet having a T value of 0.90 to 1.20 as defined in the following relational expression 2. [Relationship 2] T = R / ((0.4×([Cu]+[Ni]+[Cr]+[Sn]))+1.6) (In equation 2, R is the Lankford value (r-value), and [Cu], [Cr], [Ni], and [Sn] are the weight contents of each element.)