Ultrahigh-strength cold-rolled steel sheet and method for producing same

Abstract

A preferred aspect of the present invention provides an ultra-high strength cold-rolled steel sheet and a method for manufacturing the same, the ultra-high strength cold-rolled steel sheet comprising, in wt%, 0.25-0.4% of C, 0.5% or less (excluding 0) of Si, 3.0-4.0% of Mn, 0.03% or less (excluding 0) of P, 0.015% or less (excluding 0) of S, 0.1% or less (excluding 0) of Al, 1% or less (excluding 0) of Cr, 48 / 14 [N] to 0.1% or less of Ti, 0.1% or less (excluding 0) of Nb, 0.005% or less (excluding 0) of B, 0.01% or less (excluding 0) of N, and a remainder of Fe and other impurities, the microstructure contains 90% or more (including 100%) of martensite and 10% or less (including 0%) of ferrite and / or bainite.

Description

Cross-references to related applications

[0001] This application is a divisional application of Chinese Patent Application No. 2018800828901, filed on December 20, 2018, entitled "Ultra-high strength cold-rolled steel sheet and manufacturing method thereof". This application claims priority to KR10-2017-0178957 (December 24, 2017). Technical Field

[0002] This invention relates to a high-strength cold-rolled steel sheet for use in automotive collision components and structural components, and more specifically to an ultra-high-strength cold-rolled steel sheet with excellent shape quality and tensile strength, and a method for manufacturing the same. Background Technology

[0003] To protect the environment and address the conflicting goals of lightweight automotive steel and crash safety for passenger safety, various automotive steel types are being developed, including dual-phase (DP) steel, transformation-induced plasticity (TRIP) steel, and complexed-phase (CP) steel. However, the achievable tensile strength of these advanced high-strength steels is limited to approximately 1200 MPa. Hot-pressed steel has attracted considerable attention in structural components used to ensure crash safety. This type of steel achieves final strength through rapid cooling after high-temperature forming, followed by direct contact with a water-cooled die. However, its widespread adoption is hindered by excessively high equipment investment costs and high heat treatment and processing costs.

[0004] Compared to conventional stamping and hot press forming, the highly productive roll forming technology, which uses multi-stage roll forming to create complex shapes, is increasingly being applied to the forming of parts from ultra-high-strength materials with typically low elongation. It is primarily manufactured in continuous annealing furnaces equipped with water cooling systems, and the microstructure exhibits tempered martensite obtained by tempering martensite. However, due to temperature deviations in the width and length directions during water cooling, the shape quality is poor, resulting in poor operability and material variations at different locations when applied to roll forming. Therefore, there is a need to design an alternative rapid cooling method to water cooling.

[0005] As a technology for manufacturing ultra-high strength steel with excellent shape, Patent Document 1 proposes a method for manufacturing ultra-high strength cold-rolled steel sheets with strength exceeding 1 GPa and improved shape quality. Shape quality is ensured by controlling ΔT and alloy composition during rapid cooling in an annealing furnace. Furthermore, Patent Document 2 provides a method for manufacturing cold-rolled steel sheets that simultaneously achieves high strength and high ductility by utilizing tempered martensite, and the sheet shape after continuous annealing is also excellent. However, due to the high Si content, there is a possibility of indentations in the furnace.

[0006] In addition, in the case of Patent Document 3, a manufacturing method is provided to achieve a tensile strength of 1700 MPa using a water cooling method, but the thickness is limited to less than 1 mm, and it still suffers from the disadvantages of existing water-cooled martensitic steel, namely, poor shape quality and material deviation at different locations.

[0007] [Existing technical documents] (Patent Document 1) Korean Patent Publication No. 2012-0063198 (Patent Document 2) Japanese Patent Publication No. 2010-090432 (Patent Document 3) Korean Patent Publication No. 2017-7001783 Summary of the Invention

[0008] Technical problems to be solved A preferred aspect of the present invention aims to provide an ultra-high strength cold-rolled steel sheet with excellent shape quality and a method for manufacturing the same.

[0009] Another preferred aspect of the present invention aims to provide a method for manufacturing ultra-high strength cold-rolled steel sheets with excellent shape quality.

[0010] Technical solution In a preferred aspect of the present invention, an ultra-high strength cold-rolled steel sheet is provided, comprising, by weight percent: C: 0.25-0.4%, Si: less than 0.5% (excluding O), Mn: 3.0-4.0%, P: less than 0.03% (excluding O), S: less than 0.015% (excluding O), Al: less than 0.1% (excluding O), Cr: less than 1% (excluding O), and Ti: 48 / 14 [N] to less than 0.1%, Nb: less than 0.1% (excluding 0), B: less than 0.005% (excluding 0), N: less than 0.01% (excluding 0), balance Fe and other impurities, the microstructure contains more than 90% (including 100%) martensite and less than 10% (including 0%) ferrite and bainite, one or both.

[0011] Another preferred aspect of the present invention provides a method for manufacturing ultra-high strength cold-rolled steel sheet, the method comprising the following steps: heating a steel billet to a temperature of 1100-1300°C, wherein the steel billet, by weight%, comprises: C: 0.25-0.4%, Si: less than 0.5% (excluding O), Mn: 3.0-4.0%, P: less than 0.03% (excluding O), S: less than 0.015% (excluding O), Al: less than 0.1% (excluding O), Cr: less than 1% (excluding O), Ti: 48 / 14 [N] less than 0.1%, Nb less than 0.1% (excluding 0), B less than 0.005% (excluding 0), N less than 0.01% (excluding 0), balance Fe and other impurities; hot rolling the heated billet at a hot finishing temperature of Ar3 or higher to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature of 720°C or lower; cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature range of 780-880°C; cooling the annealed cold-rolled steel sheet as described above at a cooling rate of 5°C / second or lower to a first cooling termination temperature of 700-650°C; and cooling the first-cooled cold-rolled steel sheet as described above at a cooling rate of 5°C / second or higher to a second cooling termination temperature (RCS) of 320°C or higher, wherein C, Mn, and Cr and the second cooling termination temperature (RCS) satisfy the following relationship 1.

[0012] [Relation 1] 1200[C]+498.1[Mn]+204.8[Cr]-0.91[RCS]>1560 (Where, C, Mn, and Cr are expressed as % by weight, and RCS represents the secondary cooling termination temperature.) Beneficial effects A preferred aspect of the invention is to provide a cold-rolled steel sheet that, by utilizing a conventional continuous annealing furnace with a slow cooling zone, exhibits ultra-high strength of over 1700 MPa and superior shape quality compared to martensitic steel produced using water cooling. Attached Figure Description

[0013] Figure 1 This is a scanning electron microscope tissue photograph of Invention Example 1, which shows an example of a steel plate conforming to the present invention.

[0014] Figure 2 The image shows a scanning electron microscope tissue photograph of a steel plate, which is outside the scope of this invention, for comparative example 10.

[0015] Figure 3A schematic diagram illustrating the concept of wave height for measuring the shape quality of the present invention is shown. Detailed Implementation

[0016] One aspect of the present invention provides an ultra-high strength cold-rolled steel sheet and a method for manufacturing the same, wherein the ultra-high strength cold-rolled steel sheet does not exhibit the waveforms in the width and length directions caused by the rapid cooling of existing water-cooling equipment, and thus has excellent shape quality.

[0017] The following describes a preferred aspect of the ultra-high strength cold-rolled steel sheet of the present invention.

[0018] A preferred aspect of the present invention provides an ultra-high strength cold-rolled steel sheet comprising, by weight percent: C: 0.25-0.4%, Si: less than 0.5% (excluding O), Mn: 3.0-4.0%, P: less than 0.03% (excluding O), S: less than 0.015% (excluding O), Al: less than 0.1% (excluding O), Cr: less than 1% (excluding O), and Ti: 48 / 14 [N] to less than 0.1%, Nb: less than 0.1% (excluding 0), B: less than 0.005% (excluding 0), N: less than 0.01% (excluding 0), balance Fe and other impurities.

[0019] Carbon (C): 0.25-0.4% by weight (hereinafter also referred to as %) Carbon (C) is a necessary component to ensure the strength of martensite and should be added at least 0.25%. However, when the C content exceeds 0.4%, weldability deteriorates, so its upper limit is limited to 0.4%. Therefore, the C content is preferably 0.25-0.4%, more preferably 0.25-0.3%.

[0020] Silicon (Si): Less than 0.5% (excluding 0%) Silicon (Si) is a ferrite stabilizing element. In conventional continuous annealing furnaces with slow cooling zones, it promotes ferrite formation during slow cooling, thus reducing strength. For example, when a large amount of Mn is added to suppress phase transformation in this invention, there is a risk of pitting defects caused by Si surface enrichment and oxidation during annealing. Therefore, it is preferable to limit the Si content to 0.5% or less (excluding 0). More preferably, the Si content is 0.2% or less.

[0021] Manganese (Mn): 3.0-4.0% Manganese (Mn) in steel inhibits ferrite formation and facilitates austenite formation. When the Mn content is less than 3%, ferrite easily forms during slow cooling. When the Mn content exceeds 4%, banding occurs due to segregation, and excessive alloy input during converter operation increases the cost of alloy iron. Therefore, it is preferable to limit the Mn content to 3.0-4.0%. More preferably, the Mn content is 3.0-3.6%.

[0022] Phosphorus (P): less than 0.03% (excluding 0) Phosphorus (P) in steel is an impurity element. When the P content exceeds 0.03%, weldability decreases, the risk of steel brittleness increases, and the possibility of dent defects increases. Therefore, it is preferable to limit the upper limit of P content to 0.03%. More preferably, the P content is 0.02% or less.

[0023] Sulfur (S): less than 0.015% (excluding 0) Sulfur (S), like phosphorus (P), is an impurity element in steel, hindering the ductility and weldability of steel sheets. When the S content exceeds 0.015%, the likelihood of hindering the ductility and weldability of the steel sheet is high; therefore, it is preferable to limit the upper limit of the S content to 0.015%. More preferably, the S content is 0.01% or less.

[0024] Aluminum (Al): Less than 0.1% (excluding 0%) Aluminum (Al) is an alloying element that expands the ferrite region. As in this invention, when using a continuous annealing process with slow cooling, aluminum promotes the formation of ferrite. Due to the formation of AlN, the high-temperature hot rollability may decrease. Therefore, it is preferable to limit the aluminum (Al) content to 0.1% or less (excluding 0). The Al content is more preferably 0.05% or less.

[0025] Chromium (Cr): Less than 1% (except 0%) Chromium (Cr) is an alloying element that can easily ensure low-temperature phase transformation structure by suppressing ferrite phase transformation. As in this invention, when using a continuous annealing process with slow cooling, chromium has the advantage of suppressing the formation of ferrite. However, when the Cr content exceeds 1%, the amount of alloy added is too much, which leads to an increase in the cost of alloy iron. Therefore, it is preferable to limit the Cr content to less than 1% (except 0%).

[0026] Titanium (Ti): 48 / 14 [N] to 0.1% Titanium (Ti) is an element that forms nitrides. Ti scavenges nitrogen in steel by causing it to precipitate as TiN. Therefore, 48 / 14 stoichiometric amounts of titanium are required. Ti content of [N] or higher. Without Ti, cracks may occur during continuous casting due to the formation of AlN; therefore, Ti needs to be added. When the Ti content exceeds 0.1%, in addition to removing dissolved N, the martensitic strength decreases due to further precipitation of carbides. Therefore, it is preferable to limit the titanium (Ti) content to 48 / 14. [N] to 0.1%.

[0027] Niobium (Nb): Less than 0.1% (excluding 0%) Niobium (Nb) is an element that segregates at the austenite grain boundaries and inhibits the coarsening of austenite grains during annealing heat treatment, thus requiring its addition. However, when the Nb content exceeds 0.1%, the excessive addition of the alloy increases the cost of the alloy iron. Therefore, it is preferable to limit the niobium (Nb) content to 0.1% or less (excluding 0%). More preferably, the Nb content is 0.05% or less.

[0028] Boron (B): less than 0.005% (excluding 0) Boron (B) is a component that inhibits ferrite formation and has the advantage of suppressing ferrite formation during cooling after annealing. When the content of B exceeds 0.005%, the precipitation of Fe23(C,B)6 actually promotes ferrite formation; therefore, it is preferable to limit the boron (B) content to 0.005% or less (excluding 0). More preferably, the content of B is 0.003% or less.

[0029] Nitrogen (N): less than 0.01% (excluding 0) When nitrogen (N) exceeds 0.01%, the risk of cracking during continuous casting, such as through the formation of AlN, increases significantly. Therefore, it is preferable to limit the upper limit of N content to 0.01%.

[0030] The balance consists of Fe and unavoidable impurities.

[0031] In a preferred aspect of the present invention, the microstructure of the ultra-high strength cold-rolled steel sheet comprises more than 90% (including 100%) martensite and less than 10% (including 0%) ferrite and bainite, or one or both.

[0032] The martensite is a structure used to improve strength, and its fraction is preferably 90% or more. It can have a 100% martensitic structure.

[0033] Ferrite and bainite are unfavorable microstructures in terms of tensile strength. In the production of martensitic steel by delaying phase transformation using hardenability elements such as Mn and C, the likelihood of ferrite or bainite phases being incorporated during continuous annealing is high, rather than in processes using rapid cooling to produce martensitic steel. Therefore, in this invention, the proportion of one or both ferrite and bainite is limited to 10% or less. Ferrite and bainite may be omitted entirely.

[0034] In a preferred aspect of the present invention, the ultra-high strength cold-rolled steel sheet does not produce waveforms in the width and length directions, thus exhibiting excellent shape quality, and can have a tensile strength of over 1700 MPa.

[0035] After the steel plate is cut into 1000mm lengthwise, the wave height (ΔH) of the edge of the cold-rolled steel plate can be less than 3mm.

[0036] Hereinafter, a method for manufacturing ultra-high strength cold-rolled steel sheets according to another preferred aspect of the present invention will be described.

[0037] A preferred aspect of the invention describes a method for manufacturing ultra-high strength cold-rolled steel sheets, comprising the following steps: heating a steel billet to a temperature of 1100-1300°C, wherein the steel billet, by weight%, comprises: C: 0.25-0.4%, Si: less than 0.5% (excluding O), Mn: 3.0-4.0%, P: less than 0.03% (excluding O), S: less than 0.015% (excluding O), Al: less than 0.1% (excluding O), Cr: less than 1% (excluding O), and Ti: 48 / 14 [N] less than 0.1%, Nb less than 0.1% (excluding 0), B less than 0.005% (excluding 0), N less than 0.01% (excluding 0), balance Fe and other impurities; hot rolling the heated billet at a hot finishing temperature of Ar3 or higher to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature of 720°C or lower; cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature range of 780-880°C; cooling the annealed cold-rolled steel sheet as described above at a cooling rate of 5°C / second or lower to a first cooling termination temperature of 700-650°C; and cooling the first-cooled cold-rolled steel sheet as described above at a cooling rate of 5°C / second or higher to a second cooling termination temperature (RCS) of 320°C or higher, wherein C, Mn, and Cr and the second cooling termination temperature (RCS) satisfy the following relationship 1.

[0038] [Relation 1] 1200[C]+498.1[Mn]+204.8[Cr]-0.91[RCS]>1560 (Where, C, Mn, and Cr are expressed as % by weight, and RCS represents the secondary cooling termination temperature.) Slab heating steps First, the slab meeting the above composition is heated to a temperature range of 1100-1300°C. When the heating temperature is below 1100°C, the hot rolling load increases rapidly, and when the heating temperature exceeds 1300°C, the amount of surface oxide scale increases, which may lead to material loss. Therefore, it is preferable to limit the heating temperature of the slab to 1100-1300°C.

[0039] Steps to obtain hot-rolled steel sheets The heated steel billet is hot-rolled at a hot finishing temperature of Ar3 or higher to obtain hot-rolled steel sheet. Here, Ar3 represents the temperature at which ferrite begins to appear when austenite cools.

[0040] When the hot finishing temperature is below Ar3, rolling forms a two-phase region of ferrite + austenite or a ferrite region, resulting in a mixed-grain structure. Furthermore, variations in hot rolling load may lead to operational errors. Therefore, it is preferable to limit the hot finishing temperature to above Ar3. A preferred hot finishing temperature is 850-1000℃.

[0041] Rolling up steps The hot-rolled steel sheet is coiled at a temperature below 720°C.

[0042] When the coiling temperature exceeds 720°C, excessive oxide film formation on the steel plate surface may cause defects; therefore, the coiling temperature is limited to below 720°C. While there may be drawbacks such as higher strength in hot-rolled steel plates at lower coiling temperatures and increased rolling load in subsequent cold rolling processes, these drawbacks are not grounds for preventing actual production; therefore, the lower limit of the coiling temperature is not imposed. More preferably, the coiling temperature is below 600°C.

[0043] Steps to obtain cold-rolled steel sheets The hot-rolled steel sheet manufactured as described above is cold-rolled to obtain a cold-rolled steel sheet.

[0044] The reduction rate during cold rolling is preferably 40-70%.

[0045] Pickling can be performed before the cold rolling process.

[0046] Annealing heat treatment steps The cold-rolled steel sheet manufactured as described above is subjected to annealing heat treatment within a temperature range of 780-880℃.

[0047] The annealing heat treatment can be performed by continuous annealing.

[0048] When the annealing temperature is below 780°C, the strength decreases due to the formation of a large amount of ferrite. Furthermore, when connecting with other steel grades annealed at 800°C or above, a temperature gradient occurs between the top and bottom of the coil, potentially leading to material deviations. Additionally, when the annealing temperature exceeds 880°C, production may become difficult due to the reduced durability of the continuous annealing furnace.

[0049] Therefore, it is preferable to limit the annealing temperature to 780-880°C.

[0050] One cooling step (slow cooling zone cooling) The cold-rolled steel sheet that has undergone annealing heat treatment as described above is cooled once at a cooling rate of less than 5°C / second until the first cooling termination temperature of 700-650°C is reached.

[0051] Typically, in continuous annealing furnaces that include a slow cooling zone, there is a 100-200m slow cooling zone after annealing. Due to the slow cooling at high temperatures after annealing, soft phases such as ferrite undergo phase transformation, making it difficult to manufacture ultra-high strength steel. For example, when a 160m slow cooling zone exists in the continuous annealing furnace, and the sheet metal passes through at a speed of 160m per minute, the holding time in the slow cooling zone is 60 seconds. Furthermore, for example, when the annealing temperature is 830°C and the final temperature of the slow cooling zone is 650°C, the cooling rate of the slow cooling zone is very low, at 3°C ​​per second, thus the possibility of forming soft phases such as ferrite is very high. To ensure that the slow cooling rate after annealing is higher than 5°C / second, additional cooling equipment is required; therefore, it is preferable to limit the cooling rate to below 5°C / second.

[0052] Secondary cooling (rapid cooling zone cooling) steps The cold-rolled steel sheet that has undergone primary cooling as described above is subjected to secondary cooling at a cooling rate of 5°C / second or higher, until the secondary cooling termination temperature (RCS) reaches 320°C or higher.

[0053] When the secondary cooling termination temperature (RCS) is less than 320°C, the amount of martensite increases excessively during the over-aging process, resulting in a simultaneous increase in yield strength and tensile strength, and very poor ductility. In particular, the shape deteriorates due to rapid cooling, and there may be problems such as poor operability during roll forming. Therefore, it is preferable to limit the secondary cooling termination temperature (RCS) to above 320°C.

[0054] A more preferred secondary cooling termination temperature (RCS) is 320-460°C.

[0055] During the secondary cooling process, a cooling rate of less than 5°C / second is permissible, but to improve productivity, it is preferable to limit the cooling rate to more than 5°C / second.

[0056] A more preferred secondary cooling rate is 5-20°C / second.

[0057] The C, Mn, and Cr and the secondary cooling termination temperature (RCS) should satisfy the following relationship 1.

[0058] [Relation 1] 1200[C]+498.1[Mn]+204.8[Cr]-0.91[RCS]>1560 (Where, C, Mn, and Cr are expressed as % by weight, and RCS represents the secondary cooling termination temperature.) If the temperature is lower than the secondary cooling termination temperature (RCS) of the slow cooling range, bainite and other high-temperature phase transformation phases are formed, and the austenite formed during annealing cannot transform into martensite, resulting in a sharp decrease in tensile strength and yield strength.

[0059] In order to reduce the formation of ferrite and suppress the formation of bainite and other high-temperature phase transformation phases during cooling in a conventional continuous annealing furnace with the aforementioned slow cooling zone, thereby obtaining a tensile strength of 1700 MPa or more, the C, Mn, and Cr and the secondary cooling termination temperature (RCS) should satisfy the aforementioned relationship 1.

[0060] According to another preferred aspect of the present invention, a method for manufacturing ultra-high strength cold-rolled steel sheet can produce an ultra-high strength cold-rolled steel sheet that does not produce waveforms in the width and length directions, has excellent shape quality, and has a tensile strength of 1700 MPa or more.

[0061] After the steel plate is cut into 1000mm lengthwise, the wave height (ΔH) of the edge of the cold-rolled steel plate can be less than 3mm. Detailed Implementation

[0062] The present invention will now be described in more detail through embodiments. However, the present invention is not limited to the embodiments described below.

[0063] (Example) Steel with the composition shown in Table 1 was vacuum melted into 34 kg ingots, which were then hot-rolled into slabs by forming. The hot-rolled slab was held at 1200°C for 1 hour, then finished rolled at 900°C, placed in a furnace preheated to 680°C and held for 1 hour, followed by in-furnace cooling to simulate hot-rolled coiling. It was then pickled, cold-rolled with a 50% reduction, annealed at 800°C, slowly cooled to 650°C at a rate of 3°C / sec, and then cooled to the RSC temperature (secondary cooling termination temperature) in Table 2 at a normal cooling rate of 20°C / sec. Finally, it underwent over-aging heat treatment to produce steel sheets.

[0064] The mechanical properties and shape quality of the steel plate were measured, and the results are shown in Table 2 below.

[0065] Among them, shape quality is as follows Figure 3 As shown, the wave height (ΔH) of the edge is represented by measuring the edge after the steel plate is cut into 1000mm lengthwise.

[0066] In Table 2 below, RCS: secondary cooling termination temperature, M: martensite, TM: tempered martensite, B: bainite, F: ferrite, TS: tensile strength, YS: yield strength, and E1: elongation.

[0067] Furthermore, the fine structures of Invention Example 1 and Comparative Example 10 were observed. Figure 1 Example 1 of the invention is shown in the figure. Figure 2 Comparative Example 10 is shown in the figure.

[0068] [Table 1]

[0069] [Table 2]

[0070] As shown in Tables 1 and 2, it can be seen that Comparative Examples 2, 5, and 10 are steels with Mn content outside the scope of this invention, and their tensile strength is as low as 1700 MPa or less. In particular, Comparative steel 10, with a very low Mn content, exhibits a very low tensile strength of less than 1200 MPa. Especially in the case of Comparative Example 10, as... Figure 2 As shown, the proportions of ferrite and bainite are high.

[0071] On the other hand, Comparative Example 7 satisfies the composition and composition range of the present invention, but the secondary cooling termination temperature is 460°C, and it is a steel grade that does not satisfy Relationship 1 (1200[C]+498.1[Mn]+204.8[Cr]-0.91[RCS]>1560). As can be seen from Table 2, the tensile strength is 1700 MPa or less. In addition, in the case of Inventive Example 7, the secondary cooling termination temperature is 320°C, Relationship 1 is satisfied, and the tensile strength is 1700 MPa or more.

[0072] In the cases of invention examples (1, 3, 4, 6, 7, 8, 9), as shown in Table 2, it can be seen that the relation 1 (1200[C]+498.1[Mn]+204.8[Cr]-0.91[RCS]>1560) is satisfied, and under continuous annealing conditions including slow cooling, a tensile strength of more than 1700 MPa is also exhibited, and the wave height is as low as less than 3 mm, resulting in excellent shape quality.

[0073] like Figure 1 As shown, in the case of Invention Example 1, the main phase is martensite, and contains a small amount (less than 10%) of ferrite and bainite. It is believed that this second phase transformation occurs during the slow cooling and over-aging process that must be included in a conventional continuous annealing furnace.

Claims

1. An ultra-high strength cold-rolled steel sheet comprising, in mass%, C: 0.285-0.4%, Si: 0.5% or less except 0, Mn: 3.0-4.0%, P: 0.03% or less except 0, S: 0.015% or less except 0, Al: 0.1% or less except 0, Cr: 1% or less except 0, Ti: 0.1% or less except 0, Mo: 0.1% or less except 0, Ni: 0.1% or less except 0, Cu: 0.1% or less except 0, N: 0.01% or less except 0, and the balance of Fe and other impurities. [N] 0.1% or less, Nb: 0.1% or less except 0, B: 0.005% or less except 0, N: 0.01% or less except 0, and the balance of Fe and other impurities. The microstructure contains martensite and tempered martensite, The microstructure consists of: the sum of martensite and tempered martensite is 97% or more and includes 100%, one or both of ferrite and bainite is 3% or less and includes 0%.

2. The ultra-high strength cold rolled steel sheet according to claim 1, characterized by, The cold-rolled steel sheet has a tensile strength of 1700 MPa or more.

3. The ultra-high strength cold rolled steel sheet according to claim 1, wherein, After the steel sheet is cut in the length direction to a size of 1000 mm, a wave height (ΔH) of an edge portion of the cold-rolled steel sheet as shown is 3 mm or less.

4. A method of manufacturing an ultra-high strength cold-rolled steel sheet, comprising the steps of: A steel billet containing, in weight %, C: 0.285-0.4 %, Si: 0.5 % or less except 0, Mn: 3.0-4.0 %, P: 0.03 % or less except 0, S: 0.015 % or less except 0, Al: 0.1 % or less except 0, Cr: 1 % or less except 0, Ti: 48 / 14 [N] to 0.1 % or less, Nb: 0.1 % or less except 0, B: 0.005 % or less except 0, N: 0.01 % or less except 0, the balance of Fe and other impurities; hot-rolling the heated steel billet at a hot finish rolling temperature of Ar3 or more to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature of 720°C or less; cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; annealing heat-treating the cold-rolled steel sheet at a temperature range of 780-880°C; once-cooling the annealing heat-treated cold-rolled steel sheet as described above at a cooling rate of 5°C / sec or less to a once-cooling end temperature of 700-650°C; and secondary-cooling the once-cooled cold-rolled steel sheet as described above at a cooling rate of 5°C / sec or more to a secondary-cooling end temperature (RCS) of 320°C or more, the C, Mn, and Cr and the secondary-cooling end temperature (RCS) satisfy the following relation 1, The microstructure contains martensite and tempered martensite, the microstructure consists of: the sum of martensite and tempered martensite is 97% or more and includes 100%, one or both of ferrite and bainite is 3% or less and includes 0%, [Relation 1] 1200[C] + 498.1[Mn] + 204.8[Cr] - 0.91[RCS] > 1653.5 where C, Mn, and Cr are the contents of the respective components in weight%, and RCS represents the secondary-cooling end temperature.

5. The method of manufacturing a ultra-high strength cold rolled steel sheet according to claim 4, characterized by, The hot finish rolling temperature is 850-1000°C.

6. The method of manufacturing a ultra-high strength cold rolled steel sheet according to claim 4, characterized by, The cold-rolling reduction rate is 40-70%.

7. The method of manufacturing a ultra-high strength cold rolled steel sheet according to claim 4, characterized by, The secondary-cooling rate is 5-20°C / sec.

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