Cold-rolled steel sheet and method for producing same

Abstract

The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to a cold-rolled steel sheet which can be preferably applied to a collision energy absorbing member such as a body-in-white (BIW) structural member, and a method for manufacturing the same. The purpose of one aspect of the present invention is to provide: a cold-rolled steel sheet having excellent strength, elongation, and hole expandability; and a method for producing the cold-rolled steel sheet.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to a cold-rolled steel sheet and a method for manufacturing the same that can be preferably applied to components for absorbing collision energy such as body-in-white (BIW) structural components. Background Art

[0002] In recent years, in the automotive field, developed countries led by Europe are actively researching to reduce the weight of vehicle bodies for reasons such as fuel efficiency regulations and performance improvement. In the case of the steel industry, in order to meet the lightweight requirements of these automakers, compared with competing materials (Mg, Al, CFRP, etc.), efforts are being made to increase the strength and further reduce the steel sheet thickness at the same grade. In addition to lightweight, due to the strengthening of CO2 emission regulations and the rapid transition to the electric vehicle era, as safety regulations for vehicle passengers and pedestrians are strengthened, there is a tendency to require the stability and high strength of vehicle body materials. In particular, the demand for high-strength steel in the 980-1180 Mpa grade is increasing. However, in the case of 980 Mpa grade steel, in order to be used as a component for absorbing collision energy, it requires a high elongation rate to be formed into a complex shape, and fracture caused by axial deformation should not occur, so it should also have excellent hole expansion properties.

[0003] Such structural components need to have high yield strength and hole expansion properties to facilitate the absorption of impact energy. A representative manufacturing method for increasing the yield strength is the method of using water cooling during continuous annealing. A representative technique of this method is Patent Document 1. Patent Document 1 relates to manufacturing a steel with a martensite volume fraction of 80-97% and the balance being ferrite by continuously annealing a steel containing 0.18-0.3% C, then water cooling to room temperature, and then performing an overaging treatment at a temperature of 120-300°C for 1-15 minutes. Ultra-high strength steel can be manufactured by annealing a cold-rolled steel sheet in a two-phase region or a single-phase region and then rapidly cooling to room temperature level and then tempering. In this case, it has excellent yield strength and hole expansion properties, but due to temperature deviations in the width direction and length direction, problems such as poor shape quality of the coiled sheet occur, and when processing roll-formed components, material defects according to the location and reduced workability may occur.

[0004] In addition, generally, as the strength of the steel sheet increases, the elongation rate decreases, resulting in a problem of reduced formability, so its application as a material for cold stamping is limited. In order to form steel into a complex shape, the elongation rate must be high. However, as a representative method for increasing the elongation rate, as shown in Patent Document 2, a method using the TRIP phenomenon by introducing retained austenite is widely used. However, as described in Patent Document 2, when a large amount of ferrite is introduced in addition to retained austenite to ensure additional elongation, the yield strength and hole expansion property may be poor.

[0005] Therefore, in order to solve the above problems, it is necessary to develop an ultra-high strength steel sheet with excellent elongation rate and hole expansion property and a tensile strength of 980 MPa or more.

[0006] [Prior Art Documents]

[0007] (Patent Document 1) Japanese Unexamined Patent Publication No. 1992-289120

[0008] (Patent Document 2) Japanese Unexamined Patent Publication No. 2002-382250 Summary of the Invention

[0009] (I) Technical Problems to be Solved

[0010] One aspect of the present invention aims to provide a cold-rolled steel sheet and a manufacturing method thereof.

[0011] A preferred aspect of the present invention aims to provide a cold-rolled steel sheet with excellent strength, elongation rate, and hole expansion property and a manufacturing method thereof.

[0012] (II) Technical Solutions

[0013] One embodiment of the present invention provides a cold-rolled steel sheet which, by weight%, comprises: carbon (C): 0.15 - 0.25%, manganese (Mn): 1.5 - 2.5%, silicon (Si): 1.0 - 2.0%, phosphorus (P): 0.1% or less (except 0%), sulfur (S): 0.03% or less (except 0%), aluminum (Al): 0.01 - 0.1%, molybdenum (Mo): 0.01% or less (except 0%), boron (B): 0.001% or less (except 0%), the balance being Fe and other inevitable impurities. By area%, the fine structure comprises: ferrite: more than 10% and 45% or less, retained austenite: 7 - 15%, fresh martensite: 10% or less (including 0%), and the sum of tempered martensite and bainite: 40 - 80%. The cold-rolled steel sheet includes a soft layer having a predetermined depth (t) from the surface along the thickness direction of the steel sheet, and the soft layer satisfies the following relational expressions 1 and 2.

[0014] [Relational Expression 1][Ct / 5 / [C M <0.3

[0015] [Relationship 2][C 3t / 5 / [C M <0.6

[0016] (wherein, in the said Relationship 1 and the said Relationship 2, [C t / 5 refers to the average C content of the region at the 1 / 5 position from the surface to the said predetermined depth (t) along the thickness direction of the steel plate, [C 3t / 5 refers to the average C content of the region from the 1 / 5 position of the said predetermined depth (t) to the 3 / 5 position of the said predetermined depth (t) along the thickness direction of the steel plate, [C M refers to the average C content of the steel plate.)

[0017] The fraction of the tempered martensite can be 40% or more.

[0018] The said predetermined depth (t) can be 50 - 100 μm.

[0019] On at least one surface of the said cold-rolled steel plate, one of a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), and an electro-galvanized layer (EG) can be formed.

[0020] The said cold-rolled steel plate can be 30000 MPa% ≤ X = yield strength × [total elongation + (2 × hole expansion property)] ≤ 70000 MPa%.

[0021] Another embodiment of the present invention provides a method for manufacturing a cold-rolled steel sheet, the manufacturing method comprising the following steps: heating a slab at 1100 - 1300 °C, wherein, by weight %, the slab comprises: carbon (C): 0.15 - 0.25%, manganese (Mn): 1.5 - 2.5%, silicon (Si): 1.0 - 2.0%, phosphorus (P): less than 0.1% (except 0%), sulfur (S): less than 0.03% (except 0%), aluminum (Al): 0.01 - 0.1%, molybdenum (Mo): less than 0.01% (except 0%), boron (B): less than 0.001% (except 0%), the balance being Fe and other inevitable impurities; hot finish rolling the heated slab above Ar3 to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature below 700 °C; 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 above 780 °C and below Ac3 - 10 °C for more than 30 seconds under an atmosphere condition with a dew point temperature of 0 - 30 °C; cooling the heated cold-rolled steel sheet at an average cooling rate of 1 - 10 °C / second to 600 - 750 °C; cooling the cold-rolled steel sheet after the first cooling at an average cooling rate of 10 - 45 °C / second to 150 °C to Ms; and subjecting the cold-rolled steel sheet after the second cooling to a second heating at Ms to 480 °C and then performing overaging treatment for 1 - 30 minutes.

[0022] The cold rolling can be carried out with a cold reduction rate of 30 - 80%.

[0023] After the second heating and holding, it may further include the step of immersing the cold-rolled steel sheet in a molten zinc plating bath at 440 - 480 °C to form a hot-dip galvanized layer.

[0024] After forming the hot-dip galvanized layer, it may further include the step of performing alloying heat treatment on the cold-rolled steel sheet at 450 - 520 °C.

[0025] After the second heating and holding, it may further include the step of forming an electro-galvanized layer.

[0026] (III) Beneficial effects

[0027] According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0028] According to a preferred aspect of the present invention, a cold-rolled steel sheet with excellent strength, elongation and hole expansion property and a method for manufacturing the same can be provided. Description of the drawings

[0029] Figure 1 A microstructural photograph of Invention Example 1 according to an embodiment of the present invention for observation with a SEM microscope. Best Mode for Carrying Out the Invention

[0030] The technical terms used in this specification are only for explaining specific embodiments and do not limit the present invention. As long as the opposite meaning is not clearly indicated in the context, the singular form used in this specification also includes the plural form. The meaning of "comprising" or "including" used in the specification is to specify specific characteristics, fields, integers, steps, actions, elements, and / or components, and does not exclude the existence or addition of other specific characteristics, fields, integers, steps, actions, elements, components, and / or groups.

[0031] Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as commonly understood by those skilled in the art. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with the relevant technical literature and the currently disclosed content, and should not be interpreted as ideal or overly formal meanings unless otherwise defined.

[0032] Hereinafter, a cold-rolled steel sheet according to an embodiment of the present invention will be described. First, the alloy composition will be described. Unless otherwise specified, the content of the following alloy composition refers to weight %.

[0033] Carbon (C): 0.15 - 0.25%

[0034] As an interstitial solid solution element, C is the most effective and important element for increasing the strength of steel. When the content of C is less than 0.15%, it may be difficult to obtain the yield ratio and tensile strength desired by the present invention. When the content of C exceeds 0.25%, due to the increase in hardenability, excessive martensite is formed during the cooling process, the strength increases rapidly, the elongation may deteriorate, and the weldability may decrease. Therefore, the content of C preferably has a range of 0.15 - 0.25%. The lower limit of the content of C is more preferably 0.18%, and further preferably 0.2%. The upper limit of the content of C is more preferably 0.24%.

[0035] Manganese (Mn): 1.5 - 2.5%

[0036] Mn is an element added to ensure strength. When the content of Mn is less than 1.5%, it may be difficult to ensure the strength at the level expected in the present invention. When the content of Mn exceeds 2.5%, the Ms temperature decreases during cooling after annealing, and it is difficult to smoothly ensure the initial martensite phase. This means that due to the decrease in the fraction of tempered martensite in the Quenching&Partitioning (Q&P) process, it may be difficult to simultaneously ensure the strength, elongation, and hole expansion property expected in the present invention. In addition, Mn segregates in the thickness direction and is likely to form an Mn band in the slab, increasing the possibility of defects during the rolling process and also causing continuous casting cracks. Therefore, the content of Mn preferably has a range of 1.5 - 2.5%. The lower limit of the content of Mn is more preferably 1.8%, and further preferably 2.0%. The upper limit of the content of Mn is more preferably 2.4%.

[0037] Silicon (Si): 1.0 - 2.0%

[0038] Si is a key element in Transformation Induced Plasticity (TRIP) steel, which plays a role in ensuring an appropriate level of retained austenite fraction and improving elongation by suppressing the precipitation of cementite. When the content of Si is less than 1.0%, the control of cementite precipitation in the reheating and overaging steps is not smooth, and the fraction of retained austenite finally obtained may decrease or its stability may decrease, so the elongation may be poor. On the other hand, when the content of Si exceeds 2.0%, due to the generation of Liquid Metal Embrittlement (LME) cracks, the physical properties of the welded part deteriorate, and the surface characteristics and plating properties of the steel also deteriorate. Therefore, the content of Si preferably has a range of 1.0 - 2.0%. The lower limit of the Si content is more preferably 1.2%. The upper limit of the content of Si is more preferably 1.8%.

[0039] Phosphorus (P): 0.1% or less (except 0%)

[0040] P is an impurity element contained in steel. When the content of P exceeds 0.1%, the weldability deteriorates, and brittleness of the steel may occur. The lower the P, the more beneficial it is, but considering the inevitable situation in the manufacturing process, 0% is excluded. Therefore, the content of P preferably has a range of 0.1% or less (except 0%). The content of P is more preferably 0.03% or less.

[0041] Sulfur (S): 0.03% or less (except 0%)

[0042] Like P, S is an impurity inevitably contained in steel. When the content of S exceeds 0.03%, the ductility and weldability may decrease. The lower the S, the more beneficial it is, but considering the inevitable situation in the manufacturing process, 0% is excluded. Therefore, the content of S preferably has a range of 0.03% or less (excluding 0%). The content of S is more preferably 0.005% or less.

[0043] Aluminum (Al): 0.01 - 0.1%

[0044] Al is an element added to remove oxygen in molten steel. Like Si, it has the effect of suppressing the precipitation of cementite to stabilize retained austenite in the reheating and overaging steps. When the content of Al is less than 0.01%, the deoxidation effect may not be fully obtained, so the cleanliness of the steel may be impaired. When the content of Al exceeds 0.1%, not only the castability of the slab becomes poor, but also the temperature required for heating in the single-phase region during annealing increases, and there may be production and equipment problems. Therefore, the content of Al preferably has a range of 0.01 - 0.1%. The upper limit of the content of Al is more preferably 0.05%.

[0045] Molybdenum (Mo): 0.01% or less (excluding 0%)

[0046] Mo is a representative element that can improve hardenability. However, in the present invention, since the balance of strength, elongation, and hole expansion is important and it is a steel with a tensile strength of 980 MPa grade, there is no need to add Mo for the purpose of improving hardenability and promoting the formation of martensite. When Mo is added, there is a problem of increased manufacturing cost. Therefore, in the present invention, it is preferably not intentionally added. Therefore, in the present invention, the content of Mo can be limited to 0.01% or less. Additionally, considering the inevitable situation in the manufacturing process, the lower limit of the content of Mo can be 0.001%.

[0047] Boron (B): 0.001% or less (excluding 0%)

[0048] B is a representative element that can improve hardenability. However, in the present invention, since the balance of strength, elongation, and hole expansion is important and it is a steel with a tensile strength of 980 MPa grade, there is no need to add B for the purpose of improving hardenability and promoting the formation of martensite. Therefore, in the present invention, it is preferably not added. Therefore, in the present invention, the content of B can be limited to 0.001% or less. Additionally, considering the inevitable situation in the manufacturing process, the lower limit of the content of B can be 0.0001%.

[0049] The remaining component is iron (Fe). However, in the usual manufacturing process, it is inevitable to mix in unwanted impurities from the raw materials or the surrounding environment, so it is impossible to exclude these impurities. These impurities are well-known to those skilled in the usual manufacturing process, so all of their details will not be specifically described in this specification.

[0050] In terms of area%, the fine structure of the cold-rolled steel sheet of the present invention preferably includes: ferrite: more than 10% and 45% or less, retained austenite: 7 - 15%, fresh martensite: 10% or less (including 0%), and the sum of tempered martensite and bainite: 40 - 80%. The ferrite is a structure that is beneficial for ensuring elongation. When the fraction of the ferrite is 10% or less, it may be difficult to ensure the elongation desired in the present invention. When the fraction of the ferrite exceeds 45%, it may be difficult to ensure the strength and hole expansion properties desired in the present invention. The retained austenite, together with the ferrite formed during the annealing process, is a necessary structure for ensuring elongation. When the fraction of the retained austenite is less than 7%, it may be difficult to ensure the elongation at the desired level in the present invention. When the fraction of the retained austenite exceeds 15%, the stability of the retained austenite is insufficient, and it may be difficult to ensure the desired elongation. When the fraction of the fresh martensite exceeds 10%, it may be difficult to obtain a steel material with excellent strength, elongation, and hole expansion properties. The tempered martensite and bainite are necessary structures for ensuring strength and hole expansion. When the phase transformation occurs to include the fraction within the above range, finally, it can contain the retained austenite that is stable at room temperature in the range of 7 - 15%. When the fraction of the tempered martensite and bainite is less than 40%, the total phase change amount is insufficient, and finally, it is difficult to ensure the fraction of the retained austenite desired in the present invention. Since 10% or more of fresh martensite is ensured, it may be difficult to ensure the strength, elongation, and hole expansion properties desired in the present invention. When the fraction of the tempered martensite and bainite exceeds 80%, strength and hole expansion can be ensured, but the fractions of ferrite and retained austenite are insufficient, and it may be difficult to ensure the high ductility desired in the present invention. Additionally, the fraction of the tempered martensite is more preferably 40% or more.

[0051] The cold-rolled steel sheet of the present invention includes a soft layer having a predetermined depth (t) along the thickness direction of the steel sheet from the surface. The soft layer preferably satisfies the following relational expressions 1 and 2. By forming the soft layer, liquid metal embrittlement (LME) cracks can be prevented. The predetermined depth (t) can be 50 - 100 μm. More specifically, the predetermined depth (t) can be 50 - 80 μm. At this time, the surface refers to the surface of the base steel sheet, excluding the coating layer that may be formed on the surface of the base steel sheet.

[0052] The soft layer preferably satisfies the following relational expressions 1 and 2.

[0053] [Relational expression 1][Ct / 5 / [C M <0.3

[0054] [Relationship 2][C 3t / 5 / [C M <0.6

[0055] (wherein, in the said Relationship 1 and the said Relationship 2, [C t / 5 refers to the average C content of the region at the 1 / 5 position from the surface to the said predetermined depth (t) along the thickness direction of the steel plate, [C 3t / 5 refers to the average C content of the region from the 1 / 5 position of the said predetermined depth (t) to the 3 / 5 position of the said predetermined depth (t) along the thickness direction of the steel plate, [C M refers to the average C content of the steel plate.)

[0056] When the conditions of the said Relationship 1 and the said Relationship 2 are not satisfied, it may be difficult to obtain good LME characteristics.

[0057] In addition, for the said [C 1 / 5t , by area%, the microstructure may contain 80% or more ferrite, and the balance bainite and tempered martensite.

[0058] The cold-rolled steel plate of the present invention may form one of a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), and an electro-galvanized layer (EG) on at least one surface. In the present invention, there are no particular limitations on the specific conditions for the said hot-dip galvanized layer (GI), alloyed hot-dip galvanized layer (GA), or electro-galvanized layer (EG), and all types commonly used in the technical field can be utilized.

[0059] The yield strength (YS) of the cold-rolled steel plate of the present invention may be 600 MPa or more, the tensile strength (TS) may be 980 MPa or more, the total elongation (T-El) may be 21% or more, and the hole expansion ratio (HER) may be 20 - 40%. The higher the yield strength, tensile strength, total elongation, and uniform elongation, the more advantageous. Therefore, in the present invention, there are no particular limitations on the upper limits of the said yield strength, tensile strength, and total elongation.

[0060] The cold-rolled steel sheet of the present invention may satisfy 30,000 MPa% ≤ X = yield strength × [total elongation + (2 × hole expansion property)] ≤ 70,000 MPa%. The purpose of controlling the X value is to ensure a yield strength of more than 600 MPa as expected in the present invention while ensuring excellent elongation and hole expansion property. When the value of X is less than 30,000 MPa% or exceeds 70,000 MPa%, one or more of the strength, elongation, and hole expansion property expected in the present invention deteriorates, and it may be difficult to be used as a component for absorbing collision energy. The lower limit of the X value is more preferably 35,000 MPa%. The upper limit of the X value is more preferably 65,000 MPa%, and further preferably 60,000 MPa%.

[0061] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention will be described.

[0062] First, the slab is heated at 1100 - 1300°C. The slab heating is performed to smoothly carry out the subsequent hot rolling process and obtain the desired physical properties of the steel sheet. When the slab heating temperature is lower than 1100°C, a problem of a rapid increase in hot rolling load may occur. When the slab heating temperature exceeds 1300°C, the amount of surface scale increases, and thus productivity may decrease.

[0063] After that, the heated slab is hot finish-rolled above Ar3 to obtain a hot-rolled steel sheet. When the hot finish-rolling temperature is lower than Ar3, a mixed grain structure is produced by rolling in the ferrite + austenite two-phase region or the ferrite region, and equipment failure may occur due to fluctuations in hot rolling load. In addition, Ar3 can be obtained by the following formula 1.

[0064] [Formula 1] Ar3 (°C) = 910 - 203√[C] + 44.7[Si] + 31.5[Mo] - 30[Mn] - 11[Cr] + 700[P] + 400[Al] + 400[Ti]

[0065] After that, the hot-rolled steel sheet is coiled at 700°C or lower. When the coiling temperature exceeds 700°C, too much oxide film is formed on the steel sheet surface, which may cause defects. The coiling temperature is more preferably 650°C or lower. In addition, the lower the coiling temperature, the higher the strength of the hot-rolled steel sheet, and there is a disadvantage that the rolling load of subsequent cold rolling as a post-treatment increases, but it is not a reason that leads to impossible actual production. Therefore, in the present invention, there is no particular limitation on its lower limit. However, as an example, the lower limit of the coiling temperature may be 300°C.

[0066] After that, the hot-rolled steel sheet wound as described above is pickled and then cold-rolled to obtain a cold-rolled steel sheet. The pickling is a process for removing the oxide layer formed on the surface of the hot-rolled steel sheet wound as described above. The cold rolling can be performed at a cold reduction rate of 30 - 80%. When the cold reduction rate is less than 30%, it is difficult to ensure the desired thickness, and due to the remaining grains formed during hot rolling, it may affect the formation of austenite and the ensuring of physical properties during annealing heat treatment. When the cold reduction rate exceeds 80%, due to work hardening occurring during cold rolling, the reduction amounts rolled in the length and width directions become uneven, material deviation may occur, and it may be difficult to ensure the desired thickness due to the rolling load.

[0067] After that, under an atmosphere condition with a dew point temperature of 0 - 30°C, the cold-rolled steel sheet is heated once at 780°C or higher and lower than Ac3 - 10°C for 30 seconds or longer. The purpose of the once heating is to form partial annealed ferrite in addition to retained austenite to ensure an elongation rate of 21% or higher. When the dew point temperature is lower than 0°C, the soft layer desired in the present invention cannot be sufficiently formed on the steel sheet surface. When the dew point temperature exceeds 30°C, there are problems of reduced equipment life and productivity. The lower limit of the dew point temperature is more preferably 2°C. The upper limit of the dew point temperature is more preferably 25°C. When the once heating temperature is lower than 780°C, excessive annealed ferrite is formed, and it may be difficult to ensure strength and hole expansion properties. When the once heating temperature is Ac3 - 10°C or higher, heating in the single-phase region level results in insufficient fraction of annealed ferrite, and as a result, the elongation rate may deteriorate. The lower limit of the once heating temperature is more preferably 790°C. The upper limit of the once heating temperature is more preferably Ac3 - 15°C. When the once heating time is less than 30 seconds, there is a disadvantage that a sufficient annealing effect cannot be obtained. In addition, the longer the once heating time, the more advantageous it is, so in the present invention, there is no particular limitation on its lower limit. However, as an example, the upper limit of the once heating time can be 500 seconds. In addition, the above Ac3 can be obtained by the following formula 2.

[0068] [Formula 2] Ac3 (°C) = 910 - 203√[C] - 15.2[Ni] + 44.7[Si] + 104[V] + 31.5[Mo] + 13.1[W]

[0069] Thereafter, the hot-rolled cold-rolled steel sheet is subjected to a first cooling at an average cooling rate of 1-10°C / second until it is cooled to 600-750°C. When the termination temperature of the first cooling is lower than 600°C, phases such as ferrite or bainite are formed, and the strength may decrease. When the termination temperature of the first cooling exceeds 750°C, problems may occur in the actual production line. The lower limit of the termination temperature of the first cooling is more preferably 610°C, and further preferably 630°C. The upper limit of the termination temperature of the first cooling is more preferably 740°C, and further preferably 730°C. When the average cooling rate of the first cooling is less than 1°C / second, ferrite is formed during cooling, and it may be difficult to ensure the desired strength. When the average cooling rate of the first cooling exceeds 10°C / second, the average cooling rate during the second cooling decreases, and it is difficult to ensure sufficient martensite, which will quickly lead to a decrease in the fraction of tempered martensite, and thus it may be difficult to ensure both strength and hole expansion at the same time. The upper limit of the average cooling rate of the first cooling is more preferably 6°C / second.

[0070] Thereafter, the cold-rolled steel sheet after the first cooling is subjected to a second cooling at an average cooling rate of 10-45°C / second until it is cooled to 150°C to Ms. In order to ensure a tempered martensite structure of more than 40% required by the present invention, it is necessary to cool during the martensite transformation start temperature to the martensite transformation termination temperature (Martensite Start (Ms)-Finish Temperature (Mf)) during the second cooling. That is, for this purpose, the termination temperature of the second cooling preferably has a range of 150°C to Ms. When the termination temperature of the second cooling is lower than 150°C, the fraction of tempered martensite is too high and the fraction of retained austenite decreases, so the elongation may deteriorate. When the termination temperature of the second cooling exceeds Ms, it is difficult to form a tempered martensite structure, and the strength and hole expansion may deteriorate. The lower limit of the termination temperature of the second cooling is more preferably 180°C. When the average cooling rate of the second cooling is less than 10°C / second, partial bainite structure may be formed from the first cooling section to the second cooling. When the average cooling rate of the second cooling exceeds 45°C / second, due to the rapid martensite transformation rate during the second cooling, the shape of the surface of the steel sheet deteriorates, and material deviation problems along the width direction may occur. The lower limit of the average cooling rate of the second cooling is more preferably 12°C / second. The upper limit of the average cooling rate of the second cooling is more preferably 42°C / second. In addition, the above Ms can be obtained by the following formula 3.

[0071] [Formula 3] Ms (°C) = 539 - 423[C] - 30.4[Mn] - 7.5[Si] + 30[Al] - 12.1[Cr] - 17.7[Ni] - 7.5[Mo]

[0072] Thereafter, the cold-rolled steel sheet after the secondary cooling is secondarily heated at a temperature from Ms to 480°C and then subjected to overaging treatment for 1 to 30 minutes. The purpose of the secondary heating and overaging treatment is to improve the toughness by transforming the martensite with a high dislocation density and hardness formed during the secondary cooling into tempered martensite. In addition, by ensuring a sufficient amount of tempered martensite and bainite during the secondary heating and overaging treatment, C is enriched (partitioned) from the remaining austenite starting from the annealing process. During this process, the martensite start temperature (Ms) of the C-enriched austenite is lowered below room temperature, and finally a large amount of retained austenite is formed, thereby ensuring the physical properties desired in the present invention. When the secondary heating temperature is lower than Ms or exceeds 480°C, it is difficult to ensure the fraction of the fine microstructure desired in the present invention. The lower limit of the secondary heating temperature is more preferably 360°C. The upper limit of the secondary heating temperature is more preferably 460°C. When the overaging treatment time is less than 1 minute, since sufficient phase transformation does not occur, it is difficult to obtain the partitioning effect. When the overaging treatment time exceeds 30 minutes, the secondary heating and overaging treatment intervals must be very long, and the productivity decreases, so it may be difficult to apply to an actual production line.

[0073] Alternatively, after the secondary heating and holding, the cold-rolled steel sheet may be immersed in a molten zinc plating bath at 440 - 480°C to form a hot-dip galvanized layer. When the temperature of the molten zinc plating bath is lower than 440°C, it may be difficult to control the molten zinc plating bath. When the temperature of the molten zinc plating bath exceeds 480°C, the final elongation may decrease.

[0074] In addition, after forming the hot-dip galvanized layer, the cold-rolled steel sheet having the hot-dip galvanized layer formed thereon may be subjected to an alloying heat treatment at 450 - 520°C. When the alloying heat treatment temperature is lower than 450°C, it may be difficult to form a sufficient Fe-Zn alloyed coating. When the alloying heat treatment temperature exceeds 520°C, due to the decomposition of the retained austenite formed in the previous step, the final elongation may be poor.

[0075] Alternatively, an electrogalvanized layer may be formed after the secondary heating and holding. Detailed Description of the Invention

[0076] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only for illustrating the present invention for more detailed description and are not used to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the content recorded in the claims and the content reasonably deduced therefrom.

[0077] (Examples)

[0078] A slab having the alloy composition shown in Table 1 below is heated at 1100 - 1300 °C and then hot finish rolled at 900 - 1000 °C to produce a hot rolled steel sheet. The hot rolled steel sheet is coiled at 350 - 650 °C, pickled, and then cold rolled with a cold reduction rate of 45 - 65%, and then the conditions shown in Table 2 below are applied to produce a cold rolled steel sheet. In addition, the conditions shown in Table 2 below are based on the surface temperature of the steel sheet. Thereafter, the obtained cold rolled steel sheet is hot dip galvanized or subjected to hot dip galvanizing and alloying heat treatment under the conditions shown in Table 2 below.

[0079] Measure the microstructure and mechanical and physical properties of the cold rolled steel sheet manufactured as described above, and then show the results in Table 3 and Table 4 below.

[0080] Measure the phase fraction of the microstructure at t / 4 (t: thickness of the steel) of the cold rolled steel sheet using XRD and EBSD.

[0081] Use GDS to measure whether a soft layer with a predetermined depth (t) is formed. For the average content of C [C t / 5 in the region from the surface to the 1 / 5 position of the predetermined depth (t) along the thickness direction of the steel sheet and the average content of C [C 3t / 5 in the region from the 1 / 5 position of the predetermined depth (t) to the 3 / 5 position of the predetermined depth (t) along the thickness direction of the steel sheet, calculate the average value of the content of C measured by GDS respectively, [C M is measured by using the OES and ICPC component analysis results of the base metal.

[0082] The yield strength (YS), tensile strength (TS), total elongation (T-El), and uniform elongation (U-El) are measured by processing the cold rolled steel sheet into a specimen with JIS standard (gauge length width × length: 25 × 50 mm, total length of the specimen: 200 - 260 mm), and then conducting a tensile test under the test speed condition of 28 mm / minute.

[0083] The hole expansion ratio (HER) is measured according to the ISO 16330 standard. The hole is sheared using a punch with a diameter of 10 mm with a clearance of 12%.

[0084] The LME is evaluated based on ISO 18278 - 2. When a type C crack occurs, it is marked as ○, and when a type C crack does not occur, it is marked as ×.

[0085] [Table 1]

[0086]

[0087] [Table 2]

[0088]

[0089]

[0090] [Table 3]

[0091]

[0092] [Table 4]

[0093]

[0094] As can be seen from Table 1 to Table 4 above, Invention Examples 1 to 9 that satisfy the alloy composition and manufacturing conditions proposed in the present invention ensure excellent physical properties by ensuring the fine microstructure and soft layer desired in the present invention.

[0095] In the case of Comparative Examples 1 to 4, it can be seen that the strength, elongation, and hole expansion properties required by the present invention are satisfied, but the dew point temperature is not satisfied in the manufacturing conditions. Therefore, the soft layer cannot be ensured, resulting in poor LME characteristics.

[0096] In the case of Comparative Example 5, it can be seen that the strength, elongation, and hole expansion properties required by the present invention are satisfied, but the dew point temperature cannot be satisfied in the manufacturing conditions. Therefore, decarburization occurs weakly and the relational expression 1 cannot be satisfied, resulting in poor LME characteristics.

[0097] In the case of Comparative Examples 6 to 16, it can be seen that since the manufacturing conditions of the present invention are not satisfied, the fine microstructure desired in the present invention cannot be ensured, and thus one or more of the strength, elongation, and hole expansion properties required by the present invention cannot be satisfied.

[0098] In the case of Comparative Examples 17 and 18, since the alloy composition or the alloy composition and manufacturing conditions of the present invention are not satisfied, the fine microstructure desired in the present invention cannot be ensured, and thus one or more of the strength, elongation, and hole expansion properties required by the present invention cannot be satisfied.

[0099] Figure 1 For observing the microstructural photograph of Invention Example 1 with a SEM microscope. Through Figure 1 it can be seen that Invention Example 1 ensures an appropriate fraction of the fine microstructure desired in the present invention.

Claims

1. A cold-rolled steel sheet, by weight %, the cold-rolled steel sheet contains: carbon (C): 0.15 - 0.25%, manganese (Mn): 1.5 - 2.5%, silicon (Si): 1.0 - 2.0%, phosphorus (P): less than 0.1% and excluding 0%, sulfur (S): less than 0.03% and excluding 0%, aluminum (Al): 0.01 - 0.1%, molybdenum (Mo): less than 0.01% and excluding 0%, boron (B): less than 0.001% and excluding 0%, the balance of Fe and other inevitable impurities, by area %, the microstructure contains: ferrite: more than 10% and 45% or less, retained austenite: 7 - 15%, fresh martensite: 10% or less and including 0%, the sum of tempered martensite and bainite: 40 - 80%, the cold-rolled steel sheet includes a soft layer having a predetermined depth (t) from the surface along the thickness direction of the steel sheet, the soft layer satisfies the following relational expression 1 and relational expression 2, [Relationship 1][C t / 5 / [C M <0.3 [Relationship 2][C 3t / 5 / [C M <0.6 Among them, In the relation 1 and the relation 2, [C t / 5 refers to the average C content in the region at the 1 / 5 position from the surface to the predetermined depth (t) along the thickness direction of the steel plate, [C 3t / 5 refers to the average C content in the region from the 1 / 5 position to the 3 / 5 position of the predetermined depth (t) along the thickness direction of the steel plate, and [C M refers to the average C content of the steel plate.

2. The cold-rolled steel sheet according to claim 1, wherein, the fraction of the tempered martensite is 40% or more.

3. The cold-rolled steel sheet according to claim 1, wherein, the predetermined depth (t) is 50 - 100 μm.

4. The cold-rolled steel sheet according to claim 1, wherein, one of a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), and an electro-galvanized layer (EG) is formed on at least one surface of the cold-rolled steel sheet.

5. The cold-rolled steel sheet according to claim 1, wherein, the cold-rolled steel sheet is 30000 MPa% ≤ X = yield strength × [total elongation + (2 × hole expansion)] ≤ 70000 MPa%.

6. A manufacturing method of a cold-rolled steel sheet, which includes the following steps: heating a slab at 1100 - 1300 °C, by weight %, the slab contains: carbon (C): 0.15 - 0.25%, manganese (Mn): 1.5 - 2.5%, silicon (Si): 1.0 - 2.0%, phosphorus (P): less than 0.1% and excluding 0%, sulfur (S): less than 0.03% and excluding 0%, aluminum (Al): 0.01 - 0.1%, molybdenum (Mo): less than 0.01% and excluding 0%, boron (B): less than 0.001% and excluding 0%, the balance of Fe and other inevitable impurities; hot finish rolling the heated slab above Ar3 to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet below 700 °C; pickling the coiled hot-rolled steel sheet and then cold rolling it to obtain a cold-rolled steel sheet; under an atmosphere condition with a dew point temperature of 0 - 30 °C, heating the cold-rolled steel sheet above 780 °C and below Ac3 - 10 °C for 30 seconds or more; cooling the heated cold-rolled steel sheet at an average cooling rate of 1 - 10 °C / second for the first cooling, cooling to 600 - 750 °C; cooling the cold-rolled steel sheet after the first cooling at an average cooling rate of 10 - 45 °C / second for the second cooling, cooling to 150 °C to Ms; and after the second cooling, heating the cold-rolled steel sheet at Ms to 480 °C and then performing over-aging treatment for 1 - 30 minutes.

7. The manufacturing method of the cold-rolled steel sheet according to claim 6, wherein, the cold rolling is performed at a cold reduction rate of 30 - 80%.

8. The manufacturing method of the cold-rolled steel sheet according to claim 6, wherein, after the second heating and holding, it further includes the step of dipping the cold-rolled steel sheet into a molten zinc plating bath at 440 - 480 °C to form a hot-dip galvanized layer.

9. The manufacturing method of the cold-rolled steel sheet according to claim 8, wherein, After forming the hot-dip galvanized layer, it further includes the step of subjecting the cold-rolled steel sheet to an alloying heat treatment at 450 - 520 °C.

10. The manufacturing method of the cold-rolled steel sheet according to claim 6, wherein, After the secondary heating and holding, it further includes the step of forming an electrogalvanized layer.

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