Cold-rolled steel sheet and method for manufacturing the same

Abstract

An aspect of the present invention is to provide a cold-rolled steel sheet and a manufacturing method thereof. A preferred aspect of the present invention is to provide a cold-rolled steel sheet having excellent low-temperature bake hardenability and room-temperature aging resistance and a manufacturing method thereof.

Description

Technical Field

[0001] This invention relates to a cold-rolled steel sheet and its manufacturing method. Background Technology

[0002] In recent years, the pursuit of lightweighting to improve automotive fuel efficiency has led to a continuous demand for reducing steel thickness through increased steel sheet strength. Bake-hardening steel is considered the most suitable material for exterior panels due to these properties. Bake-hardening refers to the phenomenon where activated dissolved carbon and nitrogen adhere to dislocations generated during stamping during the painting and baking process, thereby increasing yield strength. Steels with excellent bake-hardening properties are easier to form before painting and baking, resulting in improved dent resistance in the final product, thus ensuring both excellent formability and strength.

[0003] However, bake-hardening steel may experience aging degradation such as yield point elongation when kept at room temperature for a long time due to the solid solution elements in the steel. Therefore, it needs to have room temperature aging resistance to ensure that aging can be guaranteed for a period of time above a certain level.

[0004] Typically, cold-rolled steel sheets known to have bake hardening properties are produced by applying a low-carbon, aluminum-killed steel with added phosphorus (P) to a bell-type annealing process at 400-500°C, resulting in a bake hardening amount of approximately 40-50 MPa. This is because bell-type annealing makes it easier to achieve both formability and bake hardening. In the case of phosphorus-containing aluminum-killed steel using a continuous annealing process, bake hardening is easily ensured due to the relatively fast cooling rate; however, on the other hand, rapid heating and short annealing lead to a deterioration in formability, thus limiting its application to automotive exterior panels where high machinability is not required.

[0005] In recent years, with the rapid development of steelmaking technology, it has become possible to control the amount of appropriate solid solution elements in steel. Furthermore, with the use of aluminum-killed steel sheets with added strong carbonitride forming elements such as Ti or Nb, it is possible to manufacture bake-hardening cold-rolled steel sheets with excellent formability. The use of this type of cold-rolled steel sheet, as a dent-resistant material for automotive exterior panels, continues to increase.

[0006] Typically, automotive baking processes are carried out at 170°C, and steel sheets suitable for this temperature have been manufactured to ensure bake hardening (BH) values.

[0007] However, in recent years, some automakers have been analyzing ways to reduce the bake-hardening temperature after painting as a cost-saving and CO2 emission reduction solution. Specifically, for the purpose of vehicle lightweighting, more and more automakers are using non-ferrous lightweight materials such as Al or plastic (or CFRP) to replace steel in the outer panel materials. Currently, steel and non-ferrous materials are stamped, painted, and baked separately, and then the two materials are assembled in the final stage. However, in order to improve production efficiency, reduce energy costs, and protect the environment, there is an increasing trend of assembling steel and non-ferrous materials after stamping and baking them at the same temperature. This low-temperature baking process leads to a sharp decrease in the bake-hardening value (BH value) previously obtained at 170°C, resulting in the inability to achieve the intended purpose of using bake-hardened steel—namely, ensuring adequate dent resistance.

[0008] Generally, lowering the bake-hardening temperature of steel sheets delays the deposition of dissolved carbon and nitrogen, and the time required for deposition, leading to reduced bake-hardening properties. For bake-hardening, a pre-strain of several percent is typically applied, followed by coating, and then heat treatment at 170°C for 20 minutes, requiring a hardening amount of at least 30 MPa. Therefore, to ensure adequate bake-hardening properties while lowering the bake-hardening temperature, bake-hardening properties need to be maximized at high temperatures. However, if the bake-hardening properties of the steel sheet increase beyond a certain level, it can actually lead to a deterioration in the steel sheet's aging resistance, increasing the likelihood of surface defects during component processing. Therefore, it is preferable to keep the difference between the bake-hardening amount obtained under conventional baking treatment (170°C heat treatment) and the BH value obtained under low-temperature (i.e., 140°C baking heat treatment) low. However, considering the exponential relationship between dissolved elements in steel and temperature, reducing the difference in BH values ​​between the low temperature of 140°C and the high temperature of 170°C is very difficult.

[0009] In other words, in order to ensure proper bake hardening at low temperatures, a manufacturing technology is needed that can simultaneously ensure superior bake hardening and corresponding aging resistance under normal conditions of 170°C.

[0010] [Existing Technical Documents] (Patent Document 1) Japanese Patent Publication No. 7-75803 (Patent Document 2) Japanese Patent Publication No. 2001-140038 Summary of the Invention

[0011] (a) Technical problems to be solved One aspect of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the same.

[0012] A preferred aspect of the present invention is to provide a cold-rolled steel sheet with excellent low-temperature baking hardening properties and room-temperature aging resistance, and a method thereof for manufacturing the same.

[0013] (II) Technical Solution One embodiment of the present invention provides a cold-rolled steel sheet, which, by weight percent, comprises: C: 0.010-0.020%, Si: 0.0050-0.10%, Mn: 0.50-1.450%, Cr: 0.30-1.50%, P: less than 0.030% (excluding 0%), S: less than 0.010% (excluding 0%), N: 0.0020-0.010%, Al: 0.010-0.040%, and consists of the balance Fe and other unavoidable impurities. The fine microstructure comprises a main phase ferrite and the balance phase transformation microstructure, wherein the phase transformation microstructure comprises at least one of martensite, bainite, and ferritic bainite, and the phase transformation microstructure is less than 1.0 area percent (excluding 0 area percent), and the average grain size is less than 1000 nm.

[0014] The tensile strength of the cold-rolled steel sheet can be above 350 MPa, the yield strength can be 200-300 MPa, the elongation can be above 30%, the bake hardening amount after heat treatment at 140℃ for 20 minutes can be above 30 MPa, the difference between the bake hardening amount after heat treatment at 140℃ for 20 minutes and the bake hardening amount after heat treatment at 170℃ for 20 minutes can be below 20 MPa, and the yield point elongation (AI) after heat treatment at 100℃ for 1 hour can be below 0.2%.

[0015] The cold-rolled steel sheet may have a hot-dip galvanized coating or an alloyed hot-dip galvanized coating formed on at least one side.

[0016] Another embodiment of the present invention provides a method for manufacturing a cold-rolled steel sheet, comprising the following steps: reheating a slab, wherein the slab, by weight percent, comprises: C: 0.010-0.020%, Si: 0.0050-0.10%, Mn: 0.50-1.450%, Cr: 0.30-1.50%, P: less than 0.030% (excluding 0%), S: less than 0.010% (excluding 0%), N: 0.0020-0.010%, Al: 0.010-0.040%, and the balance being Fe and other unavoidable impurities. The process involves: hot finishing rolling the reheated slab to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to 500-700°C and then coiling it; pickling the coiled hot-rolled steel sheet and then cold rolling it to obtain a cold-rolled steel sheet; heating the cold-rolled steel sheet once at 760-830°C; cooling the once-heated cold-rolled steel sheet once at an average cooling rate of 20°C / second or higher to a cooling termination temperature of 200-400°C; and heating the once-cooled cold-rolled steel sheet a second time at a heating rate of 5°C / second or lower to 400-600°C.

[0017] The slab can be reheated at 1100-1250℃.

[0018] The hot finishing rolling can be carried out at temperatures above 880°C.

[0019] The cold rolling can be carried out at a reduction rate of 60-90%.

[0020] The heating process can last for more than 30 seconds.

[0021] It may further include the step of leveling the cold-rolled steel sheet that has been heated twice using a skin pass roll with a surface roughness (Ra) of 1.0-2.2 μm at a reduction rate of 0.5-2%.

[0022] After the secondary heating, the process may further include immersing the secondary heated cold-rolled steel sheet in a galvanizing bath at 440-500°C.

[0023] After immersion in the galvanizing bath, the process may further include a step of leveling the cold-rolled steel sheet using leveling rolls with a roughness (Ra) of 1.0-2.2 μm at a reduction rate of 0.5-2%.

[0024] The process may then further include an alloying step of the cold-rolled steel sheet at 450-540°C.

[0025] (III) Beneficial Effects According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same may be provided.

[0026] According to a preferred aspect of the present invention, a cold-rolled steel sheet with excellent low-temperature baking hardening properties and room-temperature aging resistance, and a method thereof, can be provided. Best practice

[0027] The following describes a cold-rolled steel sheet according to one embodiment of the present invention. First, the alloy composition will be described. Unless otherwise specified, all alloy composition percentages described below refer to weight%.

[0028] C: 0.010-0.020% Carbon (C), as an interstitial solid solution element, effectively helps ensure the strength of steel. Furthermore, carbon is an important element for increasing the hardenability of steel and ensuring the martensite fraction; therefore, to ensure the desired amount of phase transformation structure in this invention, a certain level of carbon (C) needs to be added. When the C content is less than 0.010%, it is difficult to fully obtain the above-mentioned effects. When the C content exceeds 0.020%, excessive phase transformation structure forms, leading to increased strength and decreased elongation, increasing the likelihood of bending defects on the product surface during component processing. Therefore, the C content is preferably in the range of 0.010-0.020%. The lower limit of the C content is more advantageously 0.0110%, more advantageously 0.0120%, and most advantageously 0.0130%. The upper limit of the C content is more advantageously 0.0190%, more advantageously 0.0180%, and most advantageously 0.0170%.

[0029] Si: 0.0050-0.10% Silicon (Si) is typically an element that contributes to the increased strength of steel through solid solution strengthening. When the Si content is less than 0.0050%, it is difficult to fully achieve the aforementioned effects. However, in this invention, the lower limit of the Si content can be met even without intentional addition of Si. When the Si content exceeds 0.10%, there is a problem of deterioration in the surface properties of the coating. Therefore, the Si content is preferably in the range of 0.0050-0.10%. The lower limit of the Si content is more advantageously 0.00550%, more advantageously 0.0070%, and most advantageously 0.010%. The upper limit of the Si content is more advantageously 0.090%, more advantageously 0.070%, and most advantageously 0.050%.

[0030] Mn: 0.50-1.450% Manganese (Mn), as a solid solution strengthening element, not only helps improve the strength of steel but also precipitates sulfur (S) in the steel as MnS. In this invention, Mn, along with C and Cr, increases the hardenability of the steel, thereby helping to ensure the desired fraction of phase transformation structure in the steel of this invention. When the Mn content is less than 0.50%, it may be difficult to ensure an appropriate level of phase transformation structure fraction and the resulting low-temperature bake hardening and room-temperature aging resistance. When the Mn content exceeds 1.450%, excessive phase transformation structure formation may occur, making it difficult to ensure low-temperature bake hardening. Furthermore, since excessive addition of manganese (Mn) can form annealing oxides, problems may occur on the surface of the plated product, and elongation may decrease, thus potentially leading to deterioration in workability. Therefore, the Mn content is preferably in the range of 0.50-1.450%. The lower limit of the Mn content is more advantageously 0.60%, more advantageously 0.70%, and most advantageously 0.80%. The upper limit of the Mn content is more favorable at 1.40%, more favorable at 1.350%, and most favorable at 1.30%.

[0031] Cr: 0.30-1.50% Chromium (Cr), as a solid solution strengthening element, is one of the most important elements in this invention, along with C, Mn, and N mentioned above. Cr increases the hardenability of steel and effectively contributes to the formation of martensite. Furthermore, Cr forms Cr2 during hot rolling. 23 Coarse Cr-based carbides such as C6 control the amount of dissolved C in steel below an appropriate level, thereby suppressing the formation of yield elongation (YPel) and thus enabling the manufacture of composite steels with low yield strength ratios. Furthermore, Cr effectively helps ensure the elongation of composite steels by minimizing the decrease in elongation accompanying the increase in strength. When the Cr content is less than 0.30%, the hardenability of the steel decreases, making it difficult to ensure the desired fraction of phase transformation structure. This leads to a decrease in the BH140 value, and the BH170-BH140 value (the difference between the bake hardening amount after 20 minutes of heat treatment at 170°C and the bake hardening amount after 20 minutes of heat treatment at 140°C) cannot meet the requirement of less than 20 MPa. When the Cr content exceeds 1.50%, excessive martensite formation may exceed the phase transformation structure fraction proposed in this invention, resulting in a decrease in the BH140 value and potentially a decrease in elongation. Therefore, the Cr content is preferably in the range of 0.30-1.50%. The lower limit of the Cr content is more advantageously 0.40%, more advantageously 0.50%, and most advantageously 0.60%. The upper limit of the Cr content is more advantageously 1.40%, more advantageously 1.30%, and most advantageously 1.20%.

[0032] P: Below 0.030% (excluding 0%) Phosphorus (P) is an impurity present in steel and is an unavoidable element. However, even trace amounts do not significantly impair deep-drawing properties and are effective in ensuring the strength of steel through solid solution strengthening. Furthermore, when the P content exceeds 0.030%, the possibility of brittle fracture increases, potentially leading to slab breakage during hot rolling and significantly reducing the surface properties of coated steel sheets. Therefore, the P content is preferably in the range of 0.030% or less (excluding 0%). More advantageously, the P content is 0.020% or less; even more advantageously, 0.010% or less; and most advantageously, 0.0050% or less. Additionally, the lower limit of the P content is not particularly limited in this invention; as an example, it can be 0.0010%.

[0033] S: Below 0.010% (excluding 0%) Sulfur (S) is an impurity present in steel and is an unavoidable element. To ensure excellent weldability, its content is preferably controlled to a low level. In particular, S can cause red-hot brittleness, so the S content is preferably in the range of 0.010% or less (except 0%). More advantageously, the S content is 0.0080% or less, more advantageously, 0.0060% or less, and most advantageously, 0.0050% or less. Furthermore, the lower limit of the S content is not particularly limited in this invention; as an example, it can be 0.0010%.

[0034] N: 0.0020-0.010% Nitrogen (N) is an impurity present in steel and is an unavoidable element. Therefore, it is generally preferable to keep its content as low as possible. However, even trace amounts are crucial for ensuring both low-temperature bake hardening and room-temperature aging resistance in this invention. However, N diffuses very rapidly and can simultaneously cause bake hardening and aging degradation. Therefore, to suppress aging degradation caused by N, a certain amount of phase transformation microstructure fraction needs to be ensured through an appropriate combination of C, Mn, and Cr. A certain amount of N is required to achieve low-temperature bake hardening at 100°C in steel containing this phase transformation microstructure. When the N content is less than 0.0020%, it may be difficult to fully achieve the above-mentioned effect. When the N content exceeds 0.010%, it is difficult to simultaneously ensure low-temperature bake hardening and room-temperature aging resistance. Therefore, the N content is preferably in the range of 0.0020-0.010%. The lower limit of the N content is more advantageously 0.0030%, more advantageously 0.0040%, and most advantageously 0.0050%. The upper limit of the N content is more favorable at 0.0090%, more favorable at 0.0080%, and most favorable at 0.0070%.

[0035] Al: 0.010-0.040% Aluminum (Al) is added to refine the grain size and deoxidize steel. In order to manufacture stable aluminum-killed steel, the lower limit of the Al content can be limited to 0.010%. When the Al content exceeds 0.040%, although strength is improved through grain refinement, excessive inclusions are formed during continuous casting, which may not only degrade the surface quality of the steel plate but also increase manufacturing costs. Therefore, the Al content is preferably in the range of 0.010-0.040%. The lower limit of the Al content is more advantageously 0.0150%, and more advantageously 0.020%. The upper limit of the Al content is more advantageously 0.0350%, and more advantageously 0.030%.

[0036] In addition to the above-described composition, the cold-rolled steel sheet of the present invention may contain a balance of iron (Fe) and unavoidable impurities. Unavoidable impurities may be undesirably introduced during normal manufacturing processes, and therefore cannot be eliminated. Such impurities are well known to those skilled in the art of steel manufacturing, and therefore their contents are not specifically described in this specification.

[0037] The microstructure of the cold-rolled steel sheet of the present invention preferably comprises a main phase ferrite and a remainder of phase transformation structure. In this case, the phase transformation structure may comprise at least one of martensite, bainite, and ferritic bainite, and the phase transformation structure is less than 1.0 area % (except 0 area %), with an average grain size preferably less than 1000 nm. The present invention is characterized by utilizing the phase transformation structure in the microstructure to ensure an appropriate level of low-temperature bake hardening. The aforementioned C, Mn, and Cr are conducive to the formation of phase transformation structures, and a large number of mobile dislocations exist around the phase transformation structure, thus hindering the movement of interstitial elements such as C and N. That is, by appropriately utilizing N and C and the phase transformation structure, aging does not occur at room temperature, and bake hardening (BH) can be obtained even at low baking temperatures. When the phase transformation structure is absent, sufficient mobile dislocations cannot be obtained, and the BH140 value cannot meet the level proposed in this invention. Furthermore, the added carbon affects not only bake hardening (BH) properties but also aging properties, potentially increasing the BH170 value. This leads not only to an increase in the BH170-BH140 value but also to a deterioration in room-temperature aging resistance. When the fraction of the phase transformation structure exceeds 1.0 area%, excessive phase transformation, while improving aging resistance, results in a decrease in the BH140 value. Therefore, the fraction of the phase transformation structure is preferably in the range of 1.0 area% or less (excluding 0 area%). The lower limit of the phase transformation structure fraction is more advantageously 0.010%, more advantageously 0.0150%, and most advantageously 0.020%. The upper limit of the phase transformation structure fraction is more advantageously 0.090%, more advantageously 0.080%, and most advantageously 0.070%. Additionally, when the average grain size of the phase transformation structure exceeds 1000 nm, there may be a disadvantage of increased strength but decreased formability. Furthermore, the lower limit of the average grain size of the phase transformation structure is not particularly limited in this invention; as an example, it can be 100 nm.

[0038] The cold-rolled steel sheet of the present invention, as described above, has a tensile strength of 350 MPa or higher, a yield strength of 200-300 MPa, an elongation of 30% or higher, a bake hardening amount (BH140) of 30 MPa or higher after heat treatment at 140°C for 20 minutes, a difference between the bake hardening amount (BH170) after heat treatment at 170°C for 20 minutes and the bake hardening amount (BH140) after heat treatment at 140°C for 20 minutes is 20 MPa or less, and a yield point elongation (AI) of 0.2% or less after heat treatment at 100°C for 1 hour. Therefore, excellent low-temperature bake hardening and room-temperature aging resistance can be ensured. Higher values ​​of tensile strength, elongation, and BH140 are more advantageous in this invention; therefore, there is no particular upper limit. As an example, the tensile strength, elongation, and BH140 values ​​are unlikely to exceed 400 MPa, 40%, and 50 MPa, respectively. Furthermore, in this invention, lower BH170-BH140 values ​​and yield point elongation after 1 hour of heat treatment at 100°C are more advantageous; therefore, there is no particular limitation on their lower limits. As an example, the lower limits of the BH170-BH140 values ​​and yield point elongation after 1 hour of heat treatment at 100°C can be 0 MPa and 0%, respectively. Additionally, the BH value refers to the amount of hardening at a specific temperature, calculated as the increase in yield strength relative to room temperature after baking for a certain time at a specific temperature, based on the flow stress after 2% pre-strain.

[0039] Furthermore, the cold-rolled steel sheet of the present invention may have a hot-dip galvanized coating or an alloyed hot-dip galvanized coating formed on at least one side.

[0040] The following describes a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention.

[0041] First, the slab meeting the above alloy composition is reheated. This reheating step is performed to ensure smooth subsequent hot rolling and to fully obtain the target steel sheet's properties. The slab can be reheated at 1100-1250°C. When the reheating temperature is below 1100°C, inclusions and other contaminants may not dissolve sufficiently, potentially leading to material deviations or surface defects after hot rolling. When the reheating temperature exceeds 1250°C, excessive austenite grain growth reduces strength, and excessive oxide scale formation may degrade the steel sheet's surface quality. Therefore, the slab's reheating temperature can be between 1100-1250°C. The lower limit of the reheating temperature is more advantageously 1110°C, more advantageously 1120°C, and most advantageously 1130°C. The upper limit of the reheating temperature is more advantageously 1240°C, more advantageously 1230°C, and most advantageously 1220°C.

[0042] The reheated slab is then hot-finished to obtain a hot-rolled steel sheet. When the hot finishing is performed in the austenite single-phase region, it forms pancake-shaped austenite grains and deformation bands, thus being advantageous in terms of refining the final microstructure. The hot finishing can be performed at or above 880°C. When the hot finishing temperature is below 880°C, rolling occurs in the two-phase region of austenite and ferrite, causing material inhomogeneity and potentially leading to excessive rolling load. Therefore, to complete hot rolling in the austenite single-phase region, the hot finishing temperature can be controlled to be above 880°C. More advantageously, the hot finishing temperature is above 890°C, more advantageously above 900°C, and most advantageously above 910°C. Furthermore, the present invention does not specifically limit the upper limit of the hot finishing temperature. However, as an example, the hot finishing temperature can be below 950°C.

[0043] The hot-rolled steel sheet is then cooled to 500-700°C and coiled. When the coiling temperature is below 500°C, the shape of the steel sheet may become undesirable, and a large amount of low-temperature phase transformation phases such as martensite or bainite may form, leading to an excessive increase in the steel sheet's strength. When the coiling temperature exceeds 700°C, coarse ferrite grains form, and coarse carbides and nitrides are easily formed, potentially leading to steel material deterioration. Furthermore, due to the high coiling temperature, oxides such as Mn and Si increase in the hot-rolled sheet. During the pickling process, even if some oxides remain or are completely removed, deposits will form on the surface of the steel sheet, potentially causing surface defects during plating. Therefore, the coiling temperature is preferably in the range of 500-700°C. The lower limit of the coiling temperature is more advantageous at 510°C, more advantageous at 520°C, and most advantageous at 550°C. The upper limit of the winding temperature is more favorable at 690°C, more favorable at 680°C, and most favorable at 670°C.

[0044] Afterwards, the hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet. After coiling, before the subsequent cold rolling process, a pickling process to remove surface oxide scale can be performed. The conditions for the pickling process are not particularly limited in this invention, and all conditions commonly used in this technical field can be applied. The cold rolling can be performed with a reduction rate of 60-90%. When the reduction rate during cold rolling is less than 60%, the recrystallization driving force from cold rolling is insufficient, which may result in the inability to complete the recrystallization of ferrite and the retention of unrecrystallized ferrite. When the reduction rate during cold rolling exceeds 90%, the roll load is very high during operation, which may lead to a deterioration in the shape of the steel sheet. In particular, cracks may occur at the edges of the steel sheet, which may cause additional load during cold rolling. Therefore, the reduction rate during cold rolling can be 60-90%. A lower limit of 65% and a more favorable lower limit of 70% are preferred. The upper limit of the reduction rate during cold rolling is more favorable at 85%, and more favorable at 80%.

[0045] Subsequently, the cold-rolled steel sheet is heated once at 760-830°C. When the initial heating temperature is below 760°C, recrystallization is not fully completed, and an unrecrystallized structure may occur. Furthermore, annealing in the ferrite single-phase region presents the problem of not obtaining the fine microstructure desired by this invention. When the initial heating temperature exceeds 830°C, the austenite fraction becomes excessively high, the stability of austenite in the two-phase region decreases, and therefore a reverse phase transformation to ferrite occurs during cooling. Consequently, it is difficult to ensure the target martensite fraction in the final microstructure, making sufficient bake hardening and room-temperature aging resistance unattainable. Therefore, the initial heating temperature is preferably in the range of 760-830°C. The lower limit of the initial heating temperature is more advantageously 770°C, and more advantageously 780°C. The upper limit of the initial heating temperature is more advantageously 820°C, and more advantageously 810°C. The initial heating can be performed for more than 30 seconds. When the initial heating time is less than 30 seconds, recrystallization and growth do not occur sufficiently, and the microstructure may be uneven. The present invention does not specifically limit the upper limit of the heating time for a single heating cycle. As an example, the upper limit can be 50 seconds.

[0046] The cold-rolled steel sheet, after the initial heating, is then cooled to a cooling termination temperature of 200-400°C at an average cooling rate of 20°C / second or higher. This initial cooling is for the formation of martensite. When the initial cooling termination temperature is below 200°C, subsequent reheating requires a large amount of energy to raise the temperature, potentially leading to reduced productivity. When the initial cooling termination temperature exceeds 400°C, the Ms temperature is not reached, and the martensite required by this invention may not form. Therefore, the initial cooling termination temperature preferably has a range of 200-400°C. The lower limit of the initial cooling termination temperature is more advantageously 210°C, more advantageously 220°C, and most advantageously 230°C. The upper limit of the initial cooling termination temperature is more advantageously 390°C, more advantageously 380°C, and most advantageously 370°C. When the initial cooling rate is less than 20°C / second, martensite may not form. This invention does not specifically limit the upper limit of the initial cooling rate; as an example, the upper limit could be 100°C / second.

[0047] Subsequently, the cold-rolled steel sheet, after the initial cooling, is heated to 400-600°C at a heating rate of less than 5°C / second. When the secondary heating temperature is below 400°C, it may be difficult to ensure the bath induction temperature. When the secondary heating temperature exceeds 600°C, the ensured martensitic phase is tempered during the heat treatment process, which may result in unsatisfactory physical properties. Therefore, the secondary heating temperature is preferably in the range of 400-600°C. The lower limit of the secondary heating temperature is more advantageously 410°C, more advantageously 420°C, and most advantageously 430°C. The upper limit of the secondary heating temperature is more advantageously 590°C, more advantageously 580°C, and most advantageously 570°C. When the secondary heating rate exceeds 5°C / second, insufficient heat homogenization may occur. The lower limit of the secondary heating rate is not particularly limited in this invention; as an example, it can be 3°C / second.

[0048] Following the secondary heating process, the process may further include a step of leveling the secondary-heated cold-rolled steel sheet using a leveling roll with a surface roughness (Ra) of 1.0-2.2 μm at a reduction rate of 0.5-2%. When the surface roughness (Ra) of the leveling roll is less than 1.0 μm, the surface roughness (Ra) of the final product becomes lower, which may result in reduced stamping formability. When the surface roughness (Ra) of the leveling roll exceeds 2.2 μm, the surface sharpness of the product may decrease. Therefore, the surface roughness (Ra) of the leveling roll can be 1.0-2.2 μm. The lower limit of the surface roughness (Ra) of the leveling roll is more advantageously 1.1 μm, more advantageously 1.2 μm, and most advantageously 1.3 μm. The upper limit of the surface roughness (Ra) of the leveling roll is more advantageously 2.1 μm, more advantageously 2.0 μm, and most advantageously 1.9 μm. When the reduction rate during leveling rolling is less than 0.5%, sufficient dislocations may not be formed, which is detrimental to the plate shape and may cause surface defects during coating. Furthermore, it may also negatively impact aging resistance. When the reduction rate during leveling rolling exceeds 2%, not only may excessive dislocation density in the surface layer lead to material degradation, but also, due to equipment limitations, side effects such as plate breakage may occur. Therefore, the reduction rate during leveling rolling can be between 0.5% and 2%. The lower limit of the reduction rate during leveling rolling is more advantageously 0.6%, more advantageously 0.7%, and most advantageously 0.8%. The upper limit of the reduction rate during leveling rolling is more advantageously 1.9%, more advantageously 1.8%, and most advantageously 1.7%.

[0049] In addition, after the secondary heating process or the leveling rolling process, a plating process can be performed according to the desired purpose.

[0050] That is, the cold-rolled steel sheet that has undergone the secondary heating can be immersed in a galvanizing bath. The temperature of the galvanizing bath is not particularly limited in this invention, and a temperature range commonly used in this technical field can be applied. As an example, the temperature of the galvanizing bath can be in the range of 440-500°C.

[0051] Furthermore, after immersion in the galvanizing bath, the process may further include a step of leveling the cold-rolled steel sheet using leveling rolls with a surface roughness (Ra) of 1.0-2.2 μm at a reduction rate of 0.5-2%. Alternatively, as an example, the leveling rolling process can be performed without leveling rolling after the secondary heating process.

[0052] The process may further include an alloying step of the cold-rolled steel sheet. The alloying temperature is not particularly limited in this invention; a temperature range commonly used in this art can be applied. As an example, the alloying temperature can be in the range of 450-540°C. Detailed Implementation

[0053] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are only for illustrating the present invention in a more detailed manner and are not intended to limit the scope of the present invention.

[0054] (Example) The slabs having the alloy compositions described in Table 1 were subjected to reheating, hot finishing rolling, coiling, and cold rolling under the conditions described in Table 2. Then, they were subjected to a single heating, a single cooling, and a second heating under the conditions described in Table 2. After furnace cooling, they were immersed in a hot-dip galvanizing bath at 460°C for hot-dip galvanizing, and then leveled using leveling rolls with a surface roughness (Ra) of 2.0 μm at a reduction rate of 1.2%. In Comparative Examples 1 and 2, furnace cooling was performed after the single heating.

[0055] The microstructure and mechanical-physical properties of the steel plates manufactured as described above were measured, and the results are shown in Table 3 below.

[0056] The microstructure was measured using an optical microscope at a position 1 / 4 of the plate thickness after the steel plate was etched using Le Pera.

[0057] The tensile strength, yield strength, and elongation of the mechanical and physical properties are measured by tensile testing using ASTM-L specifications.

[0058] The bake hardening amount in the mechanical and physical properties refers to the increase in yield strength relative to room temperature after applying a 2% pre-strain to specimens of the same specifications used in the tensile test, followed by heat treatment at 140℃ and 170℃ for 20 minutes respectively.

[0059] In mechanical and physical properties, elongation at yield point (AI), which represents the resistance to aging at room temperature, is measured by performing a tensile test on the specimen after heat treatment at 100°C for 1 hour.

[0060] [Table 1] [Table 2] [Table 3] As can be seen from Tables 1 to 3 above, in the case of Invention Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions of the present invention, excellent mechanical and physical properties can be ensured by ensuring the fine microstructure of the objective of the present invention.

[0061] In Comparative Example 1, which did not meet the primary heating temperature and did not undergo primary cooling and secondary heating, the fine microstructure required for the present invention was not ensured, resulting in a low BH140 and a relatively high BH170-BH140.

[0062] In Comparative Example 2, which did not undergo a first cooling and second heating, the fine microstructure required for the present invention was not ensured, and therefore the BH140 level was lower.

[0063] In Comparative Example 3, which does not meet the C content requirement, the yield strength is high and the elongation is relatively low.

[0064] In Comparative Example 4, which did not meet the requirements for C and Mn content, the fine structure required to achieve the objectives of this invention was not ensured. Therefore, BH140 was low, while BH170-BH140 and Al were at relatively high levels.

[0065] In Comparative Example 5, which did not meet the Cr content requirement, the elongation was low, with BH170-BH140 being relatively high.

Claims

1. A cold-rolled steel sheet, comprising, by weight percent: C: 0.010-0.020%, Si: 0.0050-0.10%, Mn: 0.50-1.450%, Cr: 0.30-1.50%, P: less than 0.030% and excluding 0%, S: less than 0.010% and excluding 0%, N: 0.0020-0.010%, Al: 0.010-0.040%, and the balance being Fe and other unavoidable impurities. The microstructure consists of the main phase ferrite and the remaining phase transformation structure. The phase transformation structure includes at least one of martensite, bainite, and ferritic bainite. The phase transformation structure is less than 1.0% area and excluding 0% area, and the average grain size is less than 1000nm.

2. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet has a tensile strength of ≥350MPa, a yield strength of 200-300MPa, an elongation of ≥30%, a bake hardening amount of ≥30MPa after heat treatment at 140℃ for 20 minutes, a bake hardening amount of ≥30MPa after heat treatment at 140℃ for 20 minutes and a bake hardening amount of ≥20MPa after heat treatment at 170℃ for 20 minutes, and a yield point elongation (AI) of ≤0.2% after heat treatment at 100℃ for 1 hour.

3. The cold-rolled steel sheet according to claim 1, wherein, A hot-dip galvanized coating or an alloyed hot-dip galvanized coating is formed on at least one side of the cold-rolled steel sheet.

4. A method for manufacturing cold-rolled steel sheet, comprising the following steps: The slab is reheated, and the slab, by weight percent, comprises: C: 0.010-0.020%, Si: 0.0050-0.10%, Mn: 0.50-1.450%, Cr: 0.30-1.50%, P: less than 0.030% and excluding 0%, S: less than 0.010% and excluding 0%, N: 0.0020-0.010%, Al: 0.010-0.040%, and consists of the balance Fe and other unavoidable impurities; The reheated slab is then hot-rolled to obtain a hot-rolled steel plate; The hot-rolled steel sheet is cooled to 500-700℃ and then coiled. The hot-rolled steel sheet that has been coiled is pickled and then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is heated once at 760-830℃; The cold-rolled steel sheet, after being heated once, is cooled once at an average cooling rate of 20°C / second or higher, until it reaches a cooling termination temperature of 200-400°C; and The cold-rolled steel sheet that has undergone the first cooling process is then reheated at a heating rate of less than 5°C / second to 400-600°C.

5. The method for manufacturing cold-rolled steel sheet according to claim 4, wherein, The slab is reheated at 1100-1250°C.

6. The method for manufacturing cold-rolled steel sheet according to claim 4, wherein, The hot finishing rolling is carried out at a temperature above 880°C.

7. The method for manufacturing cold-rolled steel sheet according to claim 4, wherein, The cold rolling is carried out with a reduction rate of 60-90%.

8. The method for manufacturing cold-rolled steel sheet according to claim 4, wherein, Each heating cycle lasts for more than 30 seconds.

9. The method for manufacturing cold-rolled steel sheet according to claim 4, wherein, The manufacturing method further includes the step of leveling and rolling the cold-rolled steel sheet that has undergone secondary heating using leveling rolls with a surface roughness (Ra) of 1.0-2.2 μm at a reduction rate of 0.5-2%.

10. The method for manufacturing cold-rolled steel sheet according to claim 4, wherein, After the secondary heating, the process further includes immersing the cold-rolled steel sheet that has undergone the secondary heating in a galvanizing bath at 440-500°C.

11. The method for manufacturing cold-rolled steel sheet according to claim 9, further comprising, after immersion in the galvanizing bath, a step of leveling the cold-rolled steel sheet using a leveling roll having a roughness (Ra) of 1.0-2.2 μm at a reduction rate of 0.5-2%.

12. The method for manufacturing cold-rolled steel sheet according to claim 10 or 11, wherein, The manufacturing method further includes the step of alloying the cold-rolled steel sheet at 450-540°C.

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