Cold-rolled steel sheet and manufacturing method therefor

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method therefor, the cold-rolled steel sheet being preferably applicable to automotive crash components, structural members and the like.

Description

Cold-rolled steel sheet and method of manufacturing the same

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to a cold-rolled steel sheet and a method for manufacturing the same that can be preferably applied to automobile collisions and structural members, etc.

[0002] The development of high-strength steel sheets has been continuously pursued to reduce vehicle weight and enhance safety. Recently, the importance of ultra-high-strength steel with a tensile strength of 1,270 MPa or higher has been growing to improve the driving range of electric vehicles and protect batteries. However, since existing MART steels lack formability, the development of ultra-high-strength steel sheets for cold forming that also possess formability is expected to have significant economic value. To improve the formability of steel materials and increase elongation, the method of introducing residual austenite to utilize the TRIP (Transformation Induced Plasticity) phenomenon is widely used. However, in the case of such TRIP steel sheets, the addition of Si and Al is required to introduce residual austenite, and a larger amount of residual austenite can be obtained when accompanied by bainite transformation. Nevertheless, because bainite transforms at relatively high temperatures, its tensile strength (TS) is low, and its yield strength (YS) is also relatively low for use as ultra-high-strength steel.

[0003] Therefore, the recent trend is to adopt the Quenching and Partitioning process to increase the strength of steel plates while utilizing the TRIP phenomenon. In the case of so-called Q&P steel, the main structure of the matrix is ​​tempered martensite, which has excellent yield strength and hole expansion ratio (HER), and if residual austenite is actively formed, an appropriate level of elongation can also be obtained.

[0004] Meanwhile, the importance of automotive safety is increasing day by day from the perspective of passenger protection. Although it is believed that the risk of vehicle collisions can be fundamentally eliminated through advancements in autonomous driving technology, there is a risk that the severity of accidents may actually increase during the transitional period leading up to technological maturity due to passengers' lack of awareness of the accident situation; consequently, regulations on vehicle crash tests have recently been strengthened. It is known that the bending characteristics of steel are crucial for reducing the risk of cracking in vehicle structural members during a collision. If steel possesses excellent bending properties, the material can fold rather than crack, absorbing more collision energy, while the remaining parts withstand the impact, thereby inducing the collapse of a stable structure.

[0005] However, applying the Q&P process alone is insufficient to obtain excellent bending formability, and additional ideas are needed to secure excellent bending formability along with high strength of 1270 MPa or more.

[0006] [Prior Art Literature]

[0007] [Patent Literature]

[0008] (Patent Document 1) Japanese Patent Publication No. JP 2005-272954

[0009] According to one embodiment of the present invention, an ultra-high strength cold-rolled steel sheet with excellent formability due to high elongation and hole expansion, and a method for manufacturing the same can be provided.

[0010] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0011] A cold-rolled steel sheet according to one embodiment of the present invention comprises, in weight%, C: 0.10~0.30%, Si: 2.50% or less, Mn: 1.0~3.0%, Cr: 0.010~1.20%, P: 0.0010~0.10%, S: 0.010% or less, Sol.Al: 0.010~0.10%, N: 0.0010~0.010%, Mo: 0.020~0.20%, B: 0.0010~0.0050%, Ti: 0.010~0.120%, Nb: 0.010~0.050%, the remainder being Fe and other unavoidable impurities, and satisfies the following Equation 1, wherein the microstructure of the steel sheet comprises, in area%, tempered martensite: 70~95%; Retained austenite: 3~15%; and one or more of fresh martensite: 10% or less (including 0%) and ferrite: 5% or less (including 0%), and the area occupied by the region with a GOS of 3° or more measured by EBSD at the 1 / 4t position in the thickness direction of the steel sheet may be 20% or less.

[0012] [Relationship 1]

[0013] 2.40 ≤ 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0

[0014] (In Equation 1, each element represents the weight percentage of the corresponding element)

[0015] The above cold-rolled steel sheet has a yield strength (YS) of 1100 MPa or more, tensile strength (TS) of 1270 MPa or more, elongation (El) of 10% or more, hole expansion ratio (HER) of 25% or more, and bending characteristic R min / t can have 3.0 or less.

[0016] A method for manufacturing a cold-rolled steel sheet according to another embodiment of the present invention comprises: a step of heating a steel slab satisfying the alloy composition described above and the relationship Equation 1 to a temperature range of 1100 to 1350°C; a step of manufacturing a hot-rolled steel sheet by finishing hot rolling the heated steel slab at a temperature range of Ar3 to 1000°C; a step of coiling the hot-rolled steel sheet at a temperature range of 450 to 650°C; a step of manufacturing a cold-rolled steel sheet by cold rolling the coiled hot-rolled steel sheet; a step of continuously annealing the manufactured cold-rolled steel sheet at a temperature range of 800 to 900°C; a step of first cooling the continuously annealed cold-rolled steel sheet to a temperature range of 550 to 750°C at an average cooling rate of 1 to 10°C / s; and a step of second cooling after the first cooling to a temperature range of 150°C to Ms at a cooling rate of 5 to 60°C / s. and after the second cooling above, the step of reheating to a temperature of 200℃ to Bs and overaging treatment for 6 to 40 minutes is included, and the following relationship 2 can be satisfied.

[0017] [Relationship 2]

[0018] 88 ≤ 0.2 × CT - 0.45 × (SS-Ac3) ≤ 99

[0019] (In Equation 2, CT represents the coiling temperature and SS represents the continuous annealing temperature)

[0020] After the above over-aging treatment, the method may additionally include a step of temper rolling the cold-rolled steel sheet with an elongation of 0.010 to 1.0%.

[0021] The present invention can effectively provide an ultra-high strength cold-rolled steel sheet with excellent formability having a uniform microstructure of 20% or less of the total area with a GOS of 3° or more as measured by EBSD at the 1 / 4t position in the thickness direction of the steel sheet by controlling the alloy composition and manufacturing process (such as the cooling process in continuous annealing).

[0022] Figure 1 is a scanning electron microscope (SEM) image of Invention Example 1 according to one embodiment of the present invention, showing a tissue with a GOS greater than 3° and a tissue with a GOS less than 3°.

[0023] FIG. 2 is a scanning electron microscope (SEM) image of Comparative Example 1 according to one embodiment of the present invention, showing a tissue with a GOS greater than 3° and a tissue with a GOS less than 3°.

[0024] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0025] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0026] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0027] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0028] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0029] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0030] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0031] It should be noted that, although not essential, the technical solution according to each aspect of the present invention may be usefully applied to other aspects of the technical solution. Furthermore, the composition and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to obtain advantageous effects.

[0032] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described.

[0033] C: 0.10~0.30%

[0034] Carbon (C) is a very important element added to strengthen the transformation structure. C promotes high strength and facilitates the formation of martensite in the transformed structure steel. As the C content increases, the amount of martensite in the steel increases. However, if the C content exceeds 0.30%, although the strength of the martensite increases, the strength difference with ferrite, which has a low carbon concentration, widens. Since this strength difference easily induces fracture at the interphase interface when stress is applied, the elongation flangeability is reduced. Furthermore, weldability is reduced, leading to welding defects during component processing at the customer's site. On the other hand, if the C content is less than 0.10%, it is difficult to secure the martensite strength intended for this invention. Therefore, it is desirable for the C content to be in the range of 0.10 to 0.30%. It is more preferable for the lower limit of the C content to be 0.12%, and even more preferable for it to be 0.14%. The upper limit of the above C content is more preferably 0.28%, and more preferably 0.26%.

[0035] Si: 2.50% or less

[0036] The above Si content may be 2.50% or less (excluding 0%). Silicon (Si) promotes ferrite transformation and increases the carbon content in untransformed austenite, thereby forming a composite structure of ferrite and martensite, which hinders the increase in strength of martensite. Furthermore, regarding surface characteristics, it not only causes surface scale defects but also reduces chemical treatment properties, so it is desirable to limit its addition as much as possible. Accordingly, in the present invention, the above Si content can be controlled to 2.50% or less. It is more preferable that the above Si content be 2.30% or less, and even more preferable that it be 2.10% or less.

[0037] Mn: 1.0~3.0%

[0038] Manganese (Mn) is an element that refines grains without compromising ductility and strengthens steel by completely precipitating sulfur in the steel as MnS, thereby preventing hot brittleness caused by the formation of FeS. Additionally, it lowers the critical cooling rate required to obtain the martensite phase, making it easier to form martensite. If the Mn content is less than 1.0%, it may be difficult to sufficiently secure the aforementioned effects. If the Mn content exceeds 3.0%, there is a high probability that problems such as weldability and hot rolling performance will occur. Therefore, it is desirable for the Mn content to be in the range of 1.0% to 3.0%. The lower limit of the Mn content is more preferably 1.20%, and more preferably 1.50%. The upper limit of the Mn content is more preferably 2.90%, and more preferably 2.70%.

[0039] Cr: 0.010~1.20%

[0040] Chromium (Cr) is a component added to improve the hardenability of steel and ensure high strength, and it is an effective element for forming martensite, a low-temperature transformation phase. If the Cr content is less than 0.010%, it is difficult to sufficiently obtain the aforementioned effects. If the Cr content exceeds 1.20%, not only are the effects saturated, but cold rolling performance may also deteriorate as the strength of the hot-rolled steel sheet increases excessively. Therefore, it is desirable for the Cr content to be in the range of 0.010 to 1.20%. It is more preferable for the lower limit of the Cr content to be 0.050%, and even more preferable for it to be 0.10%. It is more preferable for the upper limit of the Cr content to be 1.0%, and even more preferable for it to be 0.80%.

[0041] P: 0.0010~0.10%

[0042] Phosphorus (P) is a substitutional alloying element with the greatest solid solution strengthening effect, playing a role in improving in-plane anisotropy and enhancing strength. If the P content is less than 0.0010%, it may be difficult to sufficiently secure the above effects, and it may also cause problems with manufacturing costs. If the P content exceeds 0.10%, press formability is reduced, and brittleness of the steel may occur. Therefore, it is desirable for the P content to be in the range of 0.0010 to 0.10%. It is more desirable for the lower limit of the P content to be 0.0050%, and more desirable for it to be 0.0080%. It is more desirable for the upper limit of the P content to be 0.080%, and more desirable for it to be 0.060%.

[0043] S: 0.010% or less (excluding 0%)

[0044] Sulfur (S) is an impurity element in steel that impairs the ductility and weldability of steel sheets. Since there is a high possibility that the ductility and weldability of steel sheets will be impaired when the S content exceeds 0.010%, it is preferable in the present invention to limit the S content to 0.010% or less. It is more preferable that the S content be 0.0050% or less, and even more preferable that it be 0.0010% or less.

[0045] Sol.Al: 0.010~0.10%

[0046] The above-mentioned soluble aluminum (Sol.Al) is an effective element that not only performs deoxidation by combining with oxygen in the steel but also improves the hardenability of martensite by distributing carbon within ferrite to austenite. If the content of Sol.Al is less than 0.010%, it may be difficult to sufficiently secure the above effect. If the content of Sol.Al exceeds 0.10%, not only is the above effect saturated, but manufacturing costs may also increase. Therefore, the content of Sol.Al may have a range of 0.010 to 0.10%. It is more preferable that the lower limit of the Sol.Al content be 0.0150%, and more preferable that it be 0.020%. It is more preferable that the upper limit of the Sol.Al content be 0.090%, and more preferable that it be 0.080%.

[0047] N: 0.0010~0.010%

[0048] Nitrogen (N) is a component that acts effectively to stabilize austenite. If the content of N is less than 0.0010%, it is difficult to sufficiently obtain the aforementioned effect. If the content of N exceeds 0.010%, the risk of cracking during continuous casting increases significantly due to AlN formation, etc. Therefore, it is desirable for the content of N to be in the range of 0.0010% to 0.010%. It is more desirable for the lower limit of the N content to be 0.0020%, and more desirable for it to be 0.0030%. It is more desirable for the upper limit of the N content to be 0.0090%, and more desirable for it to be 0.0080%.

[0049] Mo: 0.020~0.20%

[0050] Molybdenum (Mo) is an element advantageous for securing strength through improved hardenability and the formation of Mo-based precipitates. If the Mo content is less than 0.020%, it may be difficult to sufficiently secure the above effects. If the Mo content exceeds 0.20%, coarse carbides may form, which may result in a disadvantage of reduced elongation. Therefore, it is desirable for the Mo content to be in the range of 0.020% to 0.20%. It is more desirable for the lower limit of the Mo content to be 0.030%, and more desirable for it to be 0.040%. It is more desirable for the upper limit of the Mo content to be 0.190%, and more desirable for it to be 0.180%.

[0051] B: 0.0010~0.0050%

[0052] Boron (B) is an element that delays the transformation of austenite into pearlite during the cooling process after annealing, inhibits the formation of ferrite, and promotes the formation of martensite. If the content of B is less than 0.0010%, it may be difficult to sufficiently secure the above effects. If the content of B exceeds 0.0050%, there may be a disadvantage in that manufacturing costs increase due to an excess of ferroalloy. Therefore, it is desirable for the content of B to have a range of 0.0010% to 0.0050%. It is more desirable for the lower limit of the B content to be 0.0012%, and more desirable for it to be 0.0014%. It is more desirable for the upper limit of the B content to be 0.0045%, and more desirable for it to be 0.0040%.

[0053] Ti: 0.010~0.120%

[0054] Titanium (Ti) is an element effective for increasing the strength of steel sheets and for grain refinement by forming nano-precipitates through bonding with carbon. These nano-precipitates also play a role in reducing the hardness difference between phases by strengthening the matrix structure. If the Ti content is less than 0.010%, it may be difficult to sufficiently secure the above effects. If the Ti content exceeds 0.120%, ductility may decrease due to the formation of coarse precipitates. Therefore, it is desirable for the Ti content to be in the range of 0.010 to 0.120%. It is more desirable for the lower limit of the Ti content to be 0.020%, and even more desirable for it to be 0.030%. It is more desirable for the upper limit of the Ti content to be 0.110%, and even more desirable for it to be 0.10%.

[0055] Nb: 0.0010~0.050%

[0056] Niobium (Nb) is an element effective for increasing the strength of steel sheets and for grain refinement by forming nano-precipitates through bonding with carbon. These nano-precipitates also play a role in reducing the hardness difference between phases by strengthening the matrix structure. If the Nb content is less than 0.0010%, it may be difficult to sufficiently secure the above effects. If the Nb content exceeds 0.050%, ductility may decrease due to the formation of coarse precipitates. Therefore, it is desirable for the Nb content to have a range of 0.0010% to 0.050%. It is more desirable for the lower limit of the Nb content to be 0.00150%, and even more desirable for it to be 0.0020%. It is more desirable for the upper limit of the Nb content to be 0.0450%, and even more desirable for it to be 0.040%.

[0057] Equation 1

[0058] [Relationship 1]

[0059] 2.40 ≤ 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0

[0060] (Note: In the above Equation 1, each alloy element refers to the weight percent of the corresponding element.)

[0061] If the value according to the above relationship 1 is less than 2.40, the strength cannot be satisfied, and if it exceeds 3.0, the weight of the additive element increases, and problems may occur in the usable properties, such as weldability becoming inferior.

[0062] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0063]

[0064] Meanwhile, the microstructure of the cold-rolled steel sheet of the present invention preferably comprises, in area %, one or more of tempered martensite: 70~95%; retained austenite: 3~15%; and fresh martensite: 10% or less (including 0%) and ferrite: 5% or less (including 0%).

[0065] The above-mentioned tempered martensite is a structure advantageous for securing tensile strength. If the fraction of the above-mentioned tempered martensite is less than 70%, the tensile strength targeted by the present invention cannot be obtained. If the fraction of the above-mentioned tempered martensite exceeds 95%, the tensile strength may become excessively high.

[0066] The above-mentioned retained austenite is a structure advantageous for securing elongation. If the fraction of the above-mentioned retained austenite is less than 3%, the elongation targeted by the present invention cannot be obtained. If the fraction of the above-mentioned retained austenite exceeds 15%, the tensile strength targeted by the present invention cannot be obtained.

[0067] The above fresh martensite is a structure that is disadvantageous for ensuring formability. If the fraction of the above fresh martensite exceeds 10%, the tensile strength may become excessively high. The above ferrite is a structure that is disadvantageous for ensuring formability. If the fraction of the above ferrite exceeds 5%, the difference in hardness between phases may increase, thereby reducing hole expansion.

[0068] In addition, the cold-rolled steel sheet of the present invention may have an area of ​​20% or less in which the region with a GOS of 3° or greater, as measured by EBSD, is occupied. GOS (Grain Orientation Spread, °) is an indicator representing the distribution of crystal orientation within the grains; the lower the GOS value, the higher the consistency of orientation within the grains, which implies that the microstructure of the material is uniform. Therefore, in the present invention, if the area occupied by the region with the measured GOS of 3° or greater exceeds 20%, non-uniformity increases, making it difficult to satisfy R / t and HER.

[0069] In addition, the cold-rolled steel sheet of the present invention may have an average diameter of the microstructure in the region of GOS 3° or higher as measured by EBSD in the range of 2 to 7 μm.

[0070] Meanwhile, in the present invention, the Grain Orientation Spread (GOS) was measured three times at 2000x magnification (Confidence Index (CI)≥0.3, measurement area: 45×45㎛, Step size: 80nm) at a 1 / 4 position in the thickness direction of the steel plate using an EBSD (Backscattered Electron Diffraction Pattern Analyzer), and calculated by quantifying it with OIM (Orientation Imaging Microscopy) Analysis software.

[0071] The cold-rolled steel sheet of the present invention having the alloy composition and steel sheet microstructure as described above has a yield strength (YS): 1100 MPa or more, a tensile strength (TS): 1270 MPa or more, an elongation (El): 10% or more, a hole expansion ratio (HER): 25% or more, and a bending characteristic R min / t can have 3.0 or less.

[0072]

[0073] Next, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0074] The method for manufacturing a cold-rolled steel sheet according to the present invention comprises: a step of heating a steel slab satisfying the alloy composition described above to a temperature range of 1100 to 1350°C; a step of manufacturing a hot-rolled steel sheet by finishing hot-rolling the heated steel slab at a temperature range of Ar3 to 1000°C; a step of coiling the hot-rolled steel sheet at a temperature range of 450 to 650°C; a step of manufacturing a cold-rolled steel sheet by cold-rolling the coiled hot-rolled steel sheet; a step of continuously annealing the manufactured cold-rolled steel sheet at a temperature range of 800 to 900°C; a step of first cooling the continuously annealed cold-rolled steel sheet to a temperature range of 550 to 750°C at an average cooling rate of 1 to 10°C / s; and a step of second cooling after the first cooling to a temperature range of 150°C to Ms at a cooling rate of 5 to 60°C / s. and may include a step of reheating to a temperature of 200℃ to Bs after the second cooling and overaging treatment for 6 to 40 minutes.

[0075] [Slab Heating]

[0076] First, in the present invention, a slab satisfying the aforementioned alloy composition is heated.

[0077] The steel slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0078] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0079] In the electric furnace process, molten steel is obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of the large amount of scrap included in this manner, elements not intended by the present invention may be present in the molten steel within permissible limits. Examples of such elements include Cr, Cu, Ni, Sn, and Mo.

[0080] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0081] The heating of the slab above may be performed at 1100 to 1350°C. If the heating temperature of the slab is less than 1100°C, there is a possibility that it will be hot-rolled in a region below the finishing hot-rolling temperature range. If the heating temperature of the slab exceeds 1350°C, there is a possibility that it will reach the melting point of the steel and melt. Therefore, it is preferable to perform the heating of the slab at 1100 to 1350°C.

[0082] [Hot Rolled]

[0083] Next, in the present invention, the heated slab is subsequently subjected to finish hot rolling to produce a hot-rolled steel sheet. The finish hot rolling can be performed at a temperature of Ar3 to 1000°C. If the finish hot rolling temperature is below Ar3, there is a high possibility that the resistance to hot deformation will increase rapidly, and there is a possibility that problems may occur during the manufacturing process. If the finish hot rolling temperature exceeds 1000°C, not only may an excessively thick oxide scale be formed, but there is also a high possibility that the microstructure of the steel sheet will become coarsened. Therefore, the finish hot rolling can be performed at a temperature of Ar3 to 1000°C. It is more preferable that the lower limit of the finish hot rolling temperature be 800°C, and even more preferable that it be 850°C. It is more preferable that the upper limit of the exit temperature of the finish rolling mill during the finish hot rolling be 980°C, and even more preferable that it be 970°C. Meanwhile, the Ar3 can be obtained through the following component relationship formula.

[0084] Ar3(℃) = 910 - 203√C - 30Mn + 44.7Si - 11Cr + 31.5Mo

[0085] [Record]

[0086] In addition, the hot-rolled steel sheet manufactured above is coiled in the present invention. The coiling may be performed at 450 to 650°C. If the coiling temperature is below 450°C, excessive martensite is generated, leading to an excessive increase in the strength of the hot-rolled steel sheet, which may cause problems such as shape defects due to the load during cold rolling. If the coiling temperature exceeds 650°C, pickling performance may be reduced due to an increase in surface scale. Therefore, it is preferable for the coiling temperature to have a range of 450 to 650°C. It is more preferable for the lower limit of the coiling temperature to be 465°C, and more preferable for it to be 480°C. It is more preferable for the upper limit of the coiling temperature to be 620°C, and more preferable for it to be 600°C.

[0087] [Cold Rolled]

[0088] Subsequently, in the present invention, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. In the present invention, the cold-rolling conditions are not specifically limited, and any conditions used in the relevant technical field may be utilized. However, as an example, the cold-rolling may be performed with a cold-rolling reduction rate of 20 to 90%. If the cold-rolling reduction rate is less than 20%, it may be difficult to secure the target thickness precision, and it may also be difficult to correct the shape of the steel sheet. If the cold-rolling reduction rate exceeds 90%, cracks may occur at the edge of the steel sheet, and the cold-rolling load may become excessively large in terms of productivity. Therefore, the cold-rolling reduction rate may be 20 to 90%. More preferably, it has a cold-rolling reduction rate of 40 to 70%.

[0089] [Continuous Annealing]

[0090] Next, in the present invention, the cold-rolled steel sheet is continuously annealed at a continuous annealing temperature (SS) of 800 to 900°C. If the continuous annealing temperature is below 800°C, a large amount of ferrite is generated, making it difficult to secure the yield strength and tensile strength targeted by the present invention. If the continuous annealing temperature exceeds 900°C, the grain size of the austenite increases, which may increase the packet size of the martensite formed during cooling. Therefore, it is preferable for the continuous annealing temperature to have a range of 800 to 900°C. It is more preferable for the lower limit of the continuous annealing temperature to be 820°C, and even more preferable for it to be 840°C. It is more preferable for the upper limit of the continuous annealing temperature to be 890°C, and even more preferable for it to be 880°C.

[0091] The manufacturing method of the present invention preferably satisfies the following relationship 2.

[0092] Equation 2

[0093] [Relationship 2]

[0094] 88 ≤ 0.2 × CT - 0.45 × (SS-Ac3) ≤ 99

[0095] (In Equation 2, CT is the coiling temperature, SS is the continuous annealing temperature, and Ac3 is calculated by the following)

[0096] Ac3(℃) = 910 - 203C + 44.7Si - 30.7Mn - 12.1Cr + 17.3Ni + 10.7Mo + 31.5V + 354Al

[0097] When the above coiling temperature (CT) is high, pearlite is formed in the hot rolling process, and the microstructure inhomogeneity in the final structure increases. Also, if the temperature gradient between the SS temperature and the Ac3 temperature increases, the microstructure inhomogeneity may increase due to the coarsening of PAGS (initial austenite size). On the other hand, if the above relationship 2 becomes less than 88, it is a case where the SS is low, and in this case, the microstructure inhomogeneity may increase due to abnormal annealing.

[0098] [1st Cooling]

[0099] In addition, the continuous annealed cold-rolled steel sheet is cooled in a first stage in the present invention. The purpose of the first stage cooling is to suppress ferrite transformation so that the austenite transforms into martensite during the subsequent second stage cooling. The first stage cooling may be performed until the first stage cooling end temperature is 550 to 750°C. If the first stage cooling end temperature is less than 550°C or exceeds 700°C, productivity may decrease. Therefore, in the present invention, it is preferable that the first stage cooling end temperature be in the range of 550 to 700°C.

[0100] Meanwhile, in the present invention, the first cooling may be performed at a cooling rate of 1 to 10°C / s. If the first cooling rate is less than 1°C / s, a ferrite phase is formed during cooling, making it difficult to secure high strength. If the first cooling rate exceeds 10°C / s, the amount of cooling in the second cooling increases, which may lead to an increase in the final temperature deviation and material deviation. Therefore, in the present invention, it is preferable to have the first cooling rate within the range of 1 to 10°C / s.

[0101] [Secondary Cooling]

[0102] Next, in the present invention, the firstly cooled cold-rolled steel sheet is secondarily cooled to a second cooling end temperature of 150°C to Ms. This second cooling is intended to secure the shape of the coil in the width and length directions, as well as to secure a high yield ratio and hole expansion capability. If the second cooling end temperature is below 150°C, the yield strength and tensile strength may increase simultaneously and ductility may be significantly reduced due to an excessive increase in martensite during over-aging heat treatment. In particular, shape deterioration due to rapid cooling is expected to occur, leading to a deterioration in workability during the processing of automotive parts. If the second cooling end temperature exceeds Ms(°C), the austenite generated during continuous annealing fails to transform into martensite, and high-temperature transformation phases such as bainite and granular bainite are formed, which may cause the yield strength to drop sharply. The occurrence of such a structure can lead to a decrease in hole expansion capability along with a decrease in the yield ratio. Accordingly, it is preferable that the above secondary cooling end temperature has a range of 150℃ to Ms. It is more preferable that the lower limit of the above secondary cooling end temperature be 170℃, and more preferable that it be 200℃. Meanwhile, the above Ms can be obtained through the following component relationship formula.

[0103] Ms(℃)= 539 - 423C - 30.4Mn - 7.5Si + 30Al - 12.1Cr - 7.5Mo

[0104] Meanwhile, the above secondary cooling can be performed at a secondary cooling rate of 5 to 60°C / s. If the above secondary cooling rate is less than 5°C / s, high-temperature phases such as upper bainite are incorporated during cooling, making it impossible to obtain the target tempered martensite fraction and high strength. On the other hand, if the above secondary cooling rate exceeds 60°C / s, there may be a disadvantage in that the shape of the product deteriorates. Therefore, it is desirable for the above secondary cooling rate to have a range of 5 to 60°C / s. More preferably, the above secondary cooling rate is limited to 7°C to 40°C / s.

[0105] [Reheating and Over-aging Treatment]

[0106] Subsequently, in the present invention, the secondary cooled cold-rolled steel sheet is reheated at a reheating temperature of 200°C to Bs and subjected to an over-aging treatment by maintaining it for 6 to 40 minutes. Through this process, interphase carbon distribution and additional phase transformation necessary for stabilizing the residual austenite are obtained. The reheating is intended for interphase carbon distribution necessary for stabilizing the residual austenite. If the reheating temperature is below 200°C, there may be disadvantages such as excessively high strength and poor formability. If the reheating temperature exceeds Bs, it is difficult to obtain the strength intended in the present invention. Therefore, it is preferable that the reheating temperature be in the range of 200°C to Bs. It is more preferable that the lower limit of the reheating temperature be 210°C, and even more preferable that it be 250°C. Meanwhile, Bs can be calculated through the following component relationship formula.

[0107] Bs(℃)=830-270C-90Mn-37Ni-70Cr-83Mo

[0108] Meanwhile, in the present invention, if the holding time is less than 6 minutes, the total amount of phase transformation is insufficient at the end of the holding step, so the residual austenite fraction increases and a large amount of fresh martensite is generated, which may lead to poor bending characteristics.

[0109] [Tough Rolling]

[0110] Furthermore, the present invention may optionally include a step of temper rolling the over-aged cold-rolled steel sheet with an elongation of 0.010 to 1.0% as needed. Typically, when temper rolling a transformed structure steel, an increase in yield strength of 50 MPa or more occurs with almost no increase in tensile strength. If the elongation is less than 0.010%, controlling the shape of the ultra-high strength steel, such as that of the present invention, may become very difficult. If the elongation exceeds 1.0%, the operability may become significantly unstable due to the high-elongation process.

[0111] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0112] (Example)

[0113] A cold-rolled steel sheet was manufactured by performing the processes of slab heating, hot rolling, coiling, cold rolling, annealing, primary cooling, secondary cooling, and overaging treatment on a slab having the alloy composition listed in Table 1 below under the conditions listed in Tables 2 and 3 below. Meanwhile, the conditions listed in Tables 2 and 3 below were based on the surface temperature of the steel sheet.

[0114] The microstructure and mechanical properties of the cold-rolled steel sheets manufactured in this manner were measured, and the results are shown in Tables 4 and 5 below.

[0115] The microstructure was measured using the Point Counting method from images observed with a scanning electron microscope (SEM), and in particular, the fraction of retained austenite was measured by XRD.

[0116] Then, the area occupied by regions with a GOS of 3° or greater measured by EBSD on the steel plate was measured, and the results are shown in Table 4 below. Specifically, the GOS (Grain Orientation Spread,°) was measured 10 times at 2000x magnification using EBSD (Backscattered Electron Diffraction Pattern Analyzer) at a 1 / 4 position in the thickness direction of the steel plate (Confidence Index (CI)≥0.3, measurement area: 45×45㎛, Step size: 80nm), and calculated by quantifying it using OIM (Orientation Imaging Microscopy) Analysis software.

[0117] Meanwhile, yield strength (YS), tensile strength (TS), and elongation (El) were measured by performing a tensile test on a JIS No. 5 tensile test specimen.

[0118] Hole expandability (HER) was measured by forming a 10 mmØ punching hole (die inner diameter 10.3 mm, clearance 12.5%) by applying a cone punch with an apex angle of 60° to the punching hole at a pressure of 20 mm / min in a direction where the burr of the punching hole becomes outward.

[0119] Hole Expandability (HER)(%) = {(D - D0) / D0} × 100

[0120] (Note: D: hole diameter when the crack penetrates the steel plate (mm), D0: initial hole diameter (mm).)

[0121] Bending characteristics are defined as the minimum radius (R) at which no cracks occur when the surface of a cold-rolled steel sheet is observed under a 1,000x magnification microscope after a 90-degree V-bending test. min The value of / t was calculated. At this time, a 90-degree V-bending test method was used in which a 90-degree angle punch with various different radii (R) was prepared, a steel plate of a constant thickness (t) was placed between 90-degree V-shaped dies, and the punch and die were brought into close contact.

[0122] Steel Grade No. Steel Composition (Weight%) Relationship Formula 1 Remarks CSI MnPS AlCrMoTiNbBN 10.24 11.3 12.48 0.00 90.00 30.00 210.3 20.048 0.02 0.02 0.00 14 0.00 42.60 Invention Steel 20.23 01.4 32.68 0.01 10.00 40.00 210.63 0.11 00.02 0.02 0.00 210.00 52.73 30.25 21.57 2.40 0.0110.0040.00180.050.1000.020.020.00180.0052.7240.3200.802.690.0110.0030.00300.530.1200.020.020.00220.0053.22Comparison50.1701.102.450.0120.0030.00250.210.0700.020.020.00170.0052.07

[0123] In Table 1, the remaining components are Fe and unavoidable impurities. And Equation 1 is 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B.

[0124] Classification Steel Grade Slab Heating Temperature (°C) Finishing Hot Rolling Temperature (FDT) (°C) Hot-rolled Steel Sheet Thickness (mm) Coiling Temperature (CT) (°C) Cold Reduction Rate (%) Cold-rolled Steel Sheet Thickness (mm) Continuous Annealing Temperature (°C) Relationship 2 Invention Example 1 112309202.3502391.486091.8 Invention Example 2 112309202.3493391.485492.7 Invention Example 3 112309202.3514391.486392.8 Invention Example 4 112309202.3534391.485998.6 Comparative Example 1 112309202.3505391.487187.4 Comparative Example 2 112309202.3513391.4835105.2 Comparative Example 3 112309202.3592391.4865107.5 Invention Example 5 2 12309202.3511391.486391.5 Invention Example 6 212309202.3521391.485298.5 Comparative Example 4 212309202.3565391.4851107.7 Invention Example 7 312309202.3512391.486897.2 Comparative Example 5 312309202.3582391.4866112.1 Comparative Example 6 312309202.3496391.488486.8 Comparative Example 7 312309202.3521391.4847108.5 Comparative Example 8 412309202.3513391.484579.7 Comparative Example 9 512309202.3502391.4856105.6

[0125] In Table 2, Equation 2 is 0.2 × CT - 0.45 × (SS-Ac3)(CT: coiling temperature, SS: continuous annealing temperature)

[0126] Classification Steel Grade 1st Cooling End Temperature (°C) 1st Average Cooling Rate (°C / s) 2nd Cooling End Temperature (°C) 2nd Average Cooling Rate (°C / s) Overaging Temperature (°C) Overaging Holding Time (Seconds) Invention Example 1 1650 222 3322 95540 Invention Example 2 1650 223 4322 88532 Invention Example 3 1650 222 5322 93552 Invention Example 4 1650 223 1322 87541 Comparative Example 1 1650 223 3322 76523 Comparative Example 2 1650 223 4322 88536 Comparative Example 3 1650 224 532 30 2546 Invention Example 5 2650 223 332 31 3605 Invention Example 62650224632315612 Comparative Example 42650225531316625 Invention Example 73650224531297588 Comparative Example 53650223632288596 Comparative Example 63650222133296578 Comparative Example 73650222332285566 Comparative Example 84650223132286588 Comparative Example 95650224132299575

[0127] Classification Microstructure Fraction (Area %) Area occupied by regions with GOS greater than 3° measured by EBSD (%) FT F M R A Invention Example 1 288 37 12.5 Invention Example 2 288 6.2 3.7 7.3 18.5 Invention Example 3 238 6.8 2.3 8.6 14.6 Invention Example 4 188 8.3 3.2 6.7 16.4 Comparative Example 1 1.6 90 3.6 4.8 23.3 Comparative Example 2 146 81 35 26.7 Comparative Example 3 388 9.3 3.4 3.5 31.1 Invention Example 5 22.1 89.7 1.1 7.1 13.1 Invention Example 61.790.41.66.313.4 Comparative Example 42.388.32.4724.8 Inventive Example 71.489.81.37.511.8 Comparative Example 52.190.12.45.424.6 Comparative Example 61.191.51.26.223.4 Comparative Example 78.682.12.56.826.3 Comparative Example 82.386.53.67.613.5 Comparative Example 91.689.67.61.214.8 F: Ferrite, TM: Tempered Martensite, FM: Fresh Martensite, RA: Retained Austenite

[0128] Classification YS(MPa)TS(MPa)El(%)HER(%)R minInvention Example 1 1135 1497 11.2322.1 Invention Example 2 1156 152 111.329 1.8 Invention Example 3 1123 1488 12.132 1.4 Invention Example 4 116 1152 110.727 2.1 Comparative Example 1 112 11496 9.624 2.9 Comparative Example 2 95 1158 212.419 3.2 Comparative Example 3 111415 119.724 2.9 Invention Example 5 1174152 411.2312 .1 Invention Example 6 11431488 11.633 1.8 Comparative Example 4 1111150 19.623 2.8 Invention Example 7 1177152 111.329 1.8 Comparative Example 5 1104153 49.424 2.5 Comparative Example 6 1135 14979.823 2.8 Comparative Example 79971573 12.119 3.2 Comparative Example 8 112114999 9.524 2.8 Comparative Example 9 1032 1224 12.521 2.1

[0129] As shown in Table 1-5 above, in the case of Examples 1-4 of the present invention satisfying the alloy composition and steel microstructure of the present invention, the internal structure is uniform, and the cold-rolled steel sheet has a yield strength (YS) of 1100 MPa or more, a tensile strength (TS) of 1270 MPa or more, an elongation (El) of 10% or more, a hole expansion ratio (HER) of 25% or more, and a bending characteristic R min It can be seen that / t satisfies 3.0 or less.

[0130] In contrast, in Comparative Examples 1-7, where the steel composition is within the scope of the present invention but the manufacturing process conditions deviate from the scope of the present invention, it can be seen that the area (%) occupied by the region with a GOS of 3° or higher measured by EBSD exceeds 20%, and consequently, the internal structure is not uniform, so the desired mechanical properties could not be obtained. In addition, it can be seen that Comparative Examples 8-9, which do not satisfy the steel composition relationship 1, have inferior elongation or yield strength.

[0131] Meanwhile, FIG. 1 is a scanning electron microscope (SEM) image of Inventive Example 1 according to an embodiment of the present invention, showing a distinction between a tissue with a GOS greater than 3° and a tissue with a GOS less than 3°, and FIG. 2 is a scanning electron microscope (SEM) image of Comparative Example 1 according to an embodiment of the present invention, showing a distinction between a tissue with a GOS greater than 3° and a tissue with a GOS less than 3°. As shown in FIG. 2, in Comparative Example 1, the area (%) occupied by a region with a GOS greater than 3° as measured by EBSD exceeds 20%, and accordingly, it can be seen that the internal tissue is not uniform.

Claims

1. In wt%, C: 0.10~0.30%, Si: 2.50% or less, Mn: 1.0~3.0%, Cr: 0.010~1.20%, P: 0.0010~0.10%, S: 0.010% or less, Sol.Al: 0.010~0.10%, N: 0.0010~0.010%, Mo: 0.020~0.20%, B: 0.0010~0.0050%, Ti: 0.010~0.120%, Nb: 0.010~0.050%, and the remainder being Fe and other unavoidable impurities, and the microstructure of the steel sheet, in area%, is tempered martensite: 70~95%; retained austenite: 3~15%; A cold-rolled steel sheet comprising at least one of fresh martensite: 10% or less (including 0%) and ferrite: 5% or less (including 0%), and having an area occupied by a region with a GOS of 3° or more measured by EBSD at the 1 / 4t position in the thickness direction of the steel sheet of 20% or less.

2. In Paragraph 1, Cold-rolled steel sheet satisfying the following relationship 1. [Relationship 1] 2.40 ≤ 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0 (In Equation 1, each element represents the weight percentage of the corresponding element) 3. In Paragraph 1, The above cold-rolled steel sheet has a yield strength (YS) of 1100 MPa or more, tensile strength (TS) of 1270 MPa or more, elongation (El) of 10% or more, hole expansion ratio (HER) of 25% or more, and bending characteristic R min Cold-rolled steel sheet having / t 3.0 or less.

4. In Paragraph 1, The above cold-rolled steel sheet is, Cold-rolled steel sheet having an average diameter of the microstructure in regions of GOS 3° or greater as measured by EBSD of 2 to 7 μm.

5. A step of heating a slab satisfying the following Equation 1, comprising, in wt%, C: 0.10~0.30%, Si: 2.50% or less, Mn: 1.0~3.0%, Cr: 0.010~1.20%, P: 0.0010~0.10%, S: 0.010% or less, Sol.Al: 0.010~0.10%, N: 0.0010~0.010%, Mo: 0.020~0.20%, B: 0.0010~0.0050%, Ti: 0.010~0.120%, Nb: 0.010~0.050%, and the remainder being Fe and other unavoidable impurities, to a temperature range of 1100~1350℃; A step of manufacturing a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab in a temperature range of Ar3 to 1000℃; A step of winding the above hot-rolled steel sheet at a temperature range of 450 to 650℃; A step of manufacturing a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet; A step of continuously annealing the above-mentioned cold-rolled steel sheet at a temperature range of 800 to 900°C; A step of first cooling the above continuously annealed cold-rolled steel sheet to a temperature range of 550 to 750℃ at an average cooling rate of 1 to 10℃ / s; A step of secondary cooling at a cooling rate of 5 to 60℃ / s to a temperature range of 150℃ to Ms after the above primary cooling; and A method for manufacturing a cold-rolled steel sheet comprising the step of reheating to a temperature of 200℃ to Bs after the above secondary cooling and overaging treatment for 6 to 40 minutes, satisfying the following equation 2. [Relationship 1] 2.40 ≤ 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0 (In Equation 1, each element represents the weight percentage of the corresponding element) [Relationship 2] 88 ≤ 0.2 × CT - 0.45 × (SS-Ac3) ≤ 99 (In Equation 2, CT represents the coiling temperature (°C) and SS represents the continuous annealing temperature (°C)) 6. A method for manufacturing a cold-rolled steel sheet according to claim 5, further comprising the step of temper-rolling the cold-rolled steel sheet with an elongation of 0.010 to 1.0% after the over-aging treatment.

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