Cold rolled steel sheet and method of manufacturing same

Abstract

The purpose of one aspect of the present invention is to provide a cold rolled steel sheet and a method of manufacturing same. The purpose of a preferred aspect of the present invention is to provide an ultra-high strength cold rolled steel sheet having excellent formability due to high elongation and hole expandability, and a method of manufacturing same.

Description

Cold rolled steel sheet and its manufacturing method

[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 crash and structural members.

[0002] Recently, automotive steel sheets have been required to have higher strength to improve fuel efficiency and durability due to various environmental and energy consumption regulations. In particular, with the recent expansion of automobile impact safety regulations, high-strength steels with superior yield strength are being adopted for structural members such as members, seat rails, and pillars to enhance the impact resistance of the body. These structural members have the characteristic of having a higher yield strength relative to the tensile strength, i.e., a higher yield ratio (yield strength / tensile strength), which is advantageous in terms of impact energy absorption. However, as the strength of steel sheets generally increases, elongation decreases, resulting in poor formability. Therefore, the development of materials that can compensate for this problem is urgently needed.

[0003] Conventional methods for strengthening steel include solid solution strengthening, precipitation strengthening, grain refinement strengthening, and transformation strengthening. However, among the above methods, solid solution strengthening and grain refinement strengthening have the disadvantage of making it very difficult to produce high-strength steel with a tensile strength of 490 MPa or higher.

[0004] Precipitation-strengthened high-strength steel is a technology that secures strength by precipitating carbon and nitride-forming elements such as Cu, Nb, Ti, and V to strengthen the steel sheet, or by refining the grains by suppressing grain growth due to fine precipitates. This technology has the advantage of easily obtaining high strength at a low manufacturing cost, but has the disadvantage of requiring high-temperature annealing to ensure sufficient recrystallization and ductility because the recrystallization temperature rises rapidly due to the fine precipitates. In addition, precipitation-strengthened steel, which strengthens by precipitating carbon and nitrides in a ferrite matrix, has the problem that it is difficult to obtain high-strength steel of the 600 MPa class or higher.

[0005] Various transformation-strengthened high-strength steels have been developed, including dual-phase ferrite-martensite steels that incorporate hard martensite into a ferrite matrix, TRIP (Transformation Induced Plasticity) steels that utilize the transformation-induced plasticity of retained austenite, and CP (Complexed Phase) steels composed of ferrite and hard bainite or martensite structures. However, when the tensile strength that can be achieved in these advanced high-strength steels is 1500 MPa, the elongation is limited to about 8%. In addition, for structural members to ensure crash safety, hot press forming steels that secure the final strength through rapid cooling through direct contact with a die that is formed at high temperatures and then cooled by water are attracting attention. However, the high investment in facilities and the high costs of heat treatment and processing have hindered their widespread application.

[0006] Recently, automotive seat components are being manufactured with both high strength and lightweight construction to further improve passenger safety in the event of a collision. These components are manufactured using both roll forming and press forming. Seat components, as the connecting element between the passenger and the vehicle body, must support the passenger with high stress to prevent them from being ejected during a collision. This requires high yield strength and yield ratio. Furthermore, most machined components require stretch flangeability, necessitating the use of steels with excellent hole expandability.

[0007] Meanwhile, Patent Document 1 discloses a high-strength cold-rolled steel sheet having a tensile strength of 880 to 1170 MPa, controlled to a martensite single-phase structure by optimizing the alloy composition and heat treatment conditions. Patent Document 2 discloses a method for manufacturing a high-strength steel sheet, in which a steel sheet in which a volume ratio of a low-temperature transformation phase composed of martensite and retained austenite accounts for 90% or more of the total metal structure is heated and maintained in a two-phase region, thereby controlling the steel sheet to a structure of fine ferrite and austenite including laths of a low-temperature transformation phase, and then cooling the steel sheet to form a metal structure in which ferrite and a low-temperature transformation phase are finely dispersed in a lath phase.

[0008] However, although the above patent documents 1 and 2 claim that high yield strength can be obtained without water cooling treatment, there are disadvantages such as very low ductility or low elongation flangeability due to the large amount of austenite generated in the steel.

[0009] Therefore, there is a need to develop ultra-high strength cold rolled steel sheets with high elongation and hole expandability and thus excellent formability.

[0010] [Prior Art Literature]

[0011] (Patent Document 1) Japanese Patent Publication No. 3729108

[0012] (Patent Document 2) Japanese Patent Publication No. 2005-272954

[0013] One aspect of the present invention is to provide a cold rolled steel sheet and a method for manufacturing the same.

[0014] A preferred aspect of the present invention is to provide an ultra-high strength cold rolled steel sheet having excellent formability due to high elongation and hole expandability, and a method for manufacturing the same.

[0015] One embodiment of the present invention provides a cold-rolled steel sheet comprising, in wt%, C: 0.050 to 0.40%, Mn: 1.0 to 3.0%, Cr: 0.010 to 1.20%, P: 0.0010 to 0.10%, S: 0.010% or less (excluding 0%), Si+Sol.Al: 0.10 to 3.0%, Mo: 0.020 to 0.20%, B: 0.0010 to 0.0050%, the remainder being Fe and other inevitable impurities, satisfying the following relational expression 1, and a microstructure including, in area%, at least one of bainite and tempered martensite: 70 to 95%, retained austenite: 3 to 15%, and the remainder structure, wherein the bainite and tempered martensite have an average grain size of 3 ㎛ or less.

[0016] [Relationship 1] 2.40 ≤ X = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0

[0017] (However, in the above relational expression 1, the content of each alloy element means weight%.)

[0018] The above cold rolled steel sheet may additionally contain at least one of N: 0.0010 to 0.010%, Ti: 0.010 to 0.120%, and Nb: 0.0010 to 0.050%.

[0019] The content of the above Si is 2.50% or less (excluding 0%), and the content of the above Sol.Al may be 0.010 to 0.10%.

[0020] The above residual structure may include at least one of fresh martensite: 10% or less (including 0%) and ferrite: 5% or less (including 0%).

[0021] The above cold rolled steel sheet may have a yield strength (YS): 1100 MPa or more, a tensile strength (TS): 1450 MPa or more, an elongation (El): 9% or more, and a hole expansion ratio (HER): 30% or more.

[0022] The above cold rolled steel sheet may have a tensile strength (TS) × elongation (El): 13050 MPa% or more.

[0023] Another embodiment of the present invention comprises the steps of: heating a slab comprising, in wt%, C: 0.050 to 0.40%, Mn: 1.0 to 3.0%, Cr: 0.010 to 1.20%, P: 0.0010 to 0.10%, S: 0.010% or less (excluding 0%), Si+Sol.Al: 0.10 to 3.0%, Mo: 0.020 to 0.20%, B: 0.0010 to 0.0050%, the remainder being Fe and other inevitable impurities, and satisfying the following relational expression 1; performing a finish hot rolling of the heated slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at a continuous annealing temperature (SS) of 750 to 900°C; The present invention provides a method for manufacturing a cold rolled steel sheet, comprising: a step of first cooling the continuously annealed cold rolled steel sheet; a step of second cooling the first-cooled cold rolled steel sheet to a second cooling end temperature (Tq) of 150 to 350°C; and a step of reheating the second-cooled cold rolled steel sheet at a reheating temperature (Tp) of 200 to 450°C, and satisfying the following relational expression 2.

[0024] [Relationship 1] 2.40 ≤ X = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0

[0025] [Relationship 2] 175 ≤ Y = 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52X ≤ 216

[0026] [Formula 2] A1(℃) = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr

[0027] [Formula 3] Ms(℃) = 539-423C-30.4Mn-12.1Cr-7.5Mo

[0028] (However, in the above relational expression 1, the content of each alloy element is weight%, and in the above relational expression 2, SS means continuous annealing temperature, A1 means eutectoid transformation temperature, Ms means martensite transformation initiation temperature, Tq means secondary cooling end temperature, and Tp means reheating temperature. A1 is expressed by formula 2, Ms is expressed by formula 3, and X means 7C + (1.3Si + Mn) / 6 + (Cr + 1.2Mo) / 5 + 100B.)

[0029] The above slab heating can be performed at 1000 to 1350°C.

[0030] The above finishing hot rolling can be performed at Ar3~1000℃.

[0031] The above winding can be performed at 450 to 750°C.

[0032] The above cold rolling can be performed at a cold rolling reduction ratio of 20 to 90%.

[0033] The above primary cooling can be performed up to a primary cooling end temperature of 550 to 750°C.

[0034] The above primary cooling can be performed at a primary cooling rate of 1 to 10°C / s.

[0035] The above secondary cooling can be performed at a secondary cooling rate of 5 to 60°C / s.

[0036] After the above reheating, a step of temper rolling the cold rolled steel sheet at an elongation of 0.010 to 1.0% may be additionally included.

[0037] According to one aspect of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0038] According to a preferred aspect of the present invention, an ultra-high strength cold rolled steel sheet having high elongation and hole expandability and thus excellent formability and a method for manufacturing the same can be provided.

[0039] Figure 1 is a photograph of Invention Example 1 according to one embodiment of the present invention observed using a scanning electron microscope (SEM).

[0040] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include plural forms as well, unless the context clearly dictates otherwise. The word "comprising," as used herein, specifies a particular feature, region, integer, step, operation, element, and / or component, but does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, components, and / or groups.

[0041] Although not defined otherwise, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or overly formal unless otherwise defined.

[0042] 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. However, unless otherwise specified, the content of the alloy composition described below refers to weight percent.

[0043] C: 0.050~0.40%

[0044] Carbon (C) is a very important element added to strengthen the transformation structure. C promotes high strength and promotes the formation of martensite in transformation structure steel. As the C content increases, the amount of martensite in the steel increases. However, when the C content exceeds 0.40%, the strength of martensite increases, but the difference in strength with ferrite, which has a low carbon concentration, becomes large. This difference in strength easily causes fracture at the interphase interface when stress is applied, thereby reducing the stretch flangeability. In addition, the weldability is reduced, resulting in welding defects during component processing at the customer site. On the other hand, when the C content is less than 0.050%, it is very difficult to secure the martensite strength desired in the present invention. Therefore, the C content is preferably in the range of 0.050 to 0.40%. The lower limit of the C content is more advantageously 0.080%, and 0.10% is even more advantageous. The upper limit of the above C content is more advantageously set at 0.350%, and even more advantageously at 0.30%.

[0045] Mn: 1.0~3.0%

[0046] Manganese (Mn) is an element that refines particles without damaging ductility, completely precipitates sulfur in steel as MnS, prevents hot embrittlement caused by the formation of FeS, and strengthens steel. In addition, it plays a role in lowering the critical cooling rate at which martensite phase is formed, so that martensite can be formed more easily. If the Mn content is less than 1.0%, it may be difficult to sufficiently secure the above-mentioned effect. If the Mn content exceeds 3.0%, problems such as weldability and hot rollability are likely to occur. Therefore, the Mn content is preferably in the range of 1.0 to 3.0%. The lower limit of the Mn content is more advantageously 1.20%, even more advantageously 1.50%, and most advantageously 1.70%. The upper limit of the above Mn content is more advantageous when it is 2.90%, more advantageous when it is 2.70%, and most advantageous when it is 2.50%.

[0047] Cr: 0.010~1.20%

[0048] Chromium (Cr) is an element added to improve the hardenability of steel and secure high strength, and is an effective element in forming martensite, a low-temperature transformation phase. When the Cr content is less than 0.010%, it is difficult to sufficiently obtain the above-described effect. When the Cr content exceeds 1.20%, not only is the effect saturated, but the strength of the hot-rolled steel sheet may increase excessively, thereby deteriorating the cold-rollability. Therefore, the Cr content is preferably in the range of 0.010 to 1.20%. The lower limit of the Cr content is more advantageously 0.050%, more advantageously 0.10%, and most advantageously 0.30%. The upper limit of the Cr content is more advantageously 1.0%, more advantageously 0.80%, and most advantageously 0.70%.

[0049] P: 0.0010~0.10%

[0050] Phosphorus (P) is a substitutional alloying element with the greatest strengthening effect, improving in-plane anisotropy and enhancing strength. When the P content is less than 0.0010%, it may be difficult to sufficiently secure the effect and may cause manufacturing cost problems. When the P content exceeds 0.10%, press formability deteriorates and steel may become brittle. Therefore, the P content is preferably in the range of 0.0010 to 0.10%. The lower limit of the P content is more advantageously 0.0050%, more advantageously 0.0080%, and most advantageously 0.010%. The upper limit of the P content is more advantageously 0.080%, more advantageously 0.060%, and most advantageously 0.040%.

[0051] S: 0.010% or less (excluding 0%)

[0052] Sulfur (S) is an impurity element in steel that impairs the ductility and weldability of steel sheets. When the content of S exceeds 0.010%, there is a high possibility that the ductility and weldability of the steel sheet will be impaired. Therefore, in the present invention, it is preferable to limit the content of S to 0.010% or less. The content of S is more advantageously 0.0050% or less, more advantageously 0.0010% or less, and most advantageously 0.0009% or less.

[0053] Si+Sol.Al: 0.10~3.0%

[0054] When the content of Si+Sol.Al is less than 0.10%, elongation may decrease. When the content of Si+Sol.Al exceeds 3.0%, weldability may deteriorate. Therefore, the content of Si+Sol.Al is preferably in the range of 0.10 to 3.0%. The lower limit of the Si+Sol.Al content is more advantageously 0.50%, more advantageously 0.80%, and most advantageously 1.0%. The upper limit of the Si+Sol.Al content is more advantageously 2.80%, more advantageously 2.60%, and most advantageously 2.50%.

[0055] The content of Si 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, thereby hindering the increase in strength of martensite. In addition, it is preferable to limit the addition as much as possible because it not only causes surface scale defects but also reduces chemical treatment properties in relation to surface properties. Accordingly, in the present invention, the content of Si may be controlled to 2.50% or less. The content of Si is more advantageously 2.30% or less, more advantageously 2.10% or less, and most advantageously 2.0% or less.

[0056] The content of the above Sol.Al may be 0.010 to 0.10%. Fusible aluminum (Sol.Al) is an effective element that not only performs a deoxidizing effect by combining with oxygen in steel, but also distributes carbon in ferrite to austenite, thereby improving the hardenability of martensite. If the content of the above Sol.Al is less than 0.010%, it may be difficult to sufficiently secure the above effect. If the content of the above Sol.Al exceeds 0.10%, not only may the above effect be saturated, but also the manufacturing cost may increase. Therefore, the content of the above Sol.Al may have a range of 0.010 to 0.10%. The lower limit of the above Sol.Al content is more advantageously 0.0150%, and 0.020% is even more advantageous. The upper limit of the above Sol.Al content is more advantageously 0.090%, and 0.080% is even more advantageous, and 0.070% is most advantageous.

[0057] Mo: 0.020~0.20%

[0058] Molybdenum (Mo) is an element that is advantageous in securing strength through improving hardenability and forming Mo-based precipitates. If the Mo content is less than 0.020%, it may be difficult to sufficiently secure the above effect. If the Mo content exceeds 0.20%, there may be a disadvantage in that coarse carbides are formed and the elongation is reduced. Therefore, the Mo content is preferably in the range of 0.020 to 0.20%. The lower limit of the Mo content is more advantageously 0.030%, more advantageously 0.040%, and most advantageously 0.050%. The upper limit of the Mo content is more advantageously 0.190%, more advantageously 0.180%, and most advantageously 0.170%.

[0059] B: 0.0010~0.0050%

[0060] Boron (B) is an element that delays the transformation of austenite into pearlite during the cooling process after annealing, and is an element that suppresses ferrite formation and promotes martensite formation. If the B content is less than 0.0010%, it may be difficult to sufficiently secure the above effect. If the B content exceeds 0.0050%, there may be a disadvantage in that the manufacturing cost increases due to excess alloy iron. Therefore, the B content is preferably in the range of 0.0010 to 0.0050%. The lower limit of the B content is more advantageously 0.00120%, more advantageously 0.00140%, and most advantageously 0.00150%. The upper limit of the B content is more advantageously 0.00450%, more advantageously 0.0040%, and most advantageously 0.00350%.

[0061] The remaining component is iron (Fe). However, during the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0062] The cold rolled steel sheet of the present invention may additionally include at least one of N: 0.0010 to 0.010%, Ti: 0.010 to 0.120%, and Nb: 0.0010 to 0.050%.

[0063] N: 0.0010~0.010%

[0064] Nitrogen (N) is an element that effectively stabilizes austenite. If the N content is less than 0.0010%, it is difficult to sufficiently obtain the above-described effect. If the N content exceeds 0.010%, the risk of cracks occurring during casting due to AlN formation, etc. greatly increases. Therefore, the N content is preferably in the range of 0.0010 to 0.010%. The lower limit of the N content is more advantageously 0.0020%, and 0.0030% is even more advantageous. The upper limit of the N content is more advantageously 0.0090%, and 0.0080% is even more advantageous, and 0.0070% is most advantageous.

[0065] Ti: 0.010~0.120%

[0066] Titanium (Ti) is an effective element for increasing the strength of steel sheets and for refining grains by precipitating nano-precipitates through bonding with carbon. These nano-precipitates also strengthen the matrix structure and reduce the hardness difference between phases. If the Ti content is less than 0.010%, it may be difficult to sufficiently secure the above effect. If the Ti content exceeds 0.120%, ductility may decrease due to the formation of coarse precipitates. Therefore, the Ti content is preferably in the range of 0.010 to 0.120%. The lower limit of the Ti content is more advantageously 0.020%, and 0.030% is even more advantageous. The upper limit of the Ti content is more advantageously 0.110%, and 0.10% is even more advantageous.

[0067] Nb: 0.0010~0.050%

[0068] Niobium (Nb) is an effective element for increasing the strength of steel sheets and refining grains by precipitating nano-precipitates through bonding with carbon. These nano-precipitates also strengthen the matrix structure and reduce the hardness difference between phases. If the Nb content is less than 0.0010%, it may be difficult to sufficiently secure the above effect. If the Nb content exceeds 0.050%, ductility may decrease due to the formation of coarse precipitates. Therefore, the Nb content is preferably in the range of 0.0010 to 0.050%. The lower limit of the Nb content is more advantageously 0.00150%, and 0.0020% is even more advantageous. The upper limit of the Nb content is more advantageously 0.0450%, and 0.040% is even more advantageous.

[0069] The cold-rolled steel sheet of the present invention can satisfy the above-described alloy composition and at the same time satisfy the following relational expression 1. The relational expression 1 below is for securing the high strength aimed at in the present invention while also securing the hole expansion ratio (HER) and elongation. When the below X value is less than 2.40, the tensile strength targeted at the present invention cannot be obtained. When the below X value exceeds 3.0, not only the tensile strength targeted at the present invention but also the elongation cannot be obtained. Therefore, it is advantageous that the below X value has a range of 2.40 to 3.0. The lower limit of the below X value is more advantageously 2.420, more advantageously 2.440, and most advantageously 2.460. The upper limit of the below X value is more advantageously 2.90, more advantageously 2.80, and most advantageously 2.70.

[0070] [Relationship 1] 2.40 ≤ X = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0

[0071] (However, in the above relational expression 1, the content of each alloy element means weight%.)

[0072] The microstructure of the cold-rolled steel sheet of the present invention may include, in area %, at least one of bainite and tempered martensite: 70 to 95%, retained austenite: 3 to 15%, and the remainder. The bainite and tempered martensite are structures advantageous for securing tensile strength. If the fraction of at least one of the bainite and tempered martensite is less than 70%, the tensile strength targeted by the present invention cannot be obtained. If the fraction of at least one of the bainite and tempered martensite exceeds 95%, the elongation targeted by the present invention cannot be obtained. The retained austenite is a structure advantageous for securing elongation. If the fraction of the retained austenite is less than 3%, the elongation targeted by the present invention cannot be obtained. If the fraction of the retained austenite exceeds 15%, the tensile strength targeted by the present invention cannot be obtained. The above residual structure is an impurity structure that may be inevitably formed during the manufacturing process. The above residual structure may include at least one of fresh martensite: 10% or less (including 0%) and ferrite: 5% or less (including 0%). The fresh martensite is a structure that is disadvantageous for securing formability. When the fraction of the fresh martensite exceeds 10%, it may be difficult to secure the formability targeted by the present invention. The ferrite is a structure that is disadvantageous for securing formability. When the fraction of the ferrite exceeds 5%, the interphase hardness difference may increase, thereby reducing hole expandability.

[0073] The above bainite and tempered martensite may have an average grain size of 3 μm or less. If the average grain size of the above bainite and tempered martensite exceeds 3 μm, it may be detrimental to securing formability. Meanwhile, as an example, the average grain size of the above bainite and tempered martensite can be measured using backscatter electron diffraction pattern analysis (EBSD).

[0074] The above cold-rolled steel sheet may have a yield strength (YS): 1100 MPa or more, a tensile strength (TS): 1450 MPa or more, an elongation (El): 9% or more, and a hole expansion ratio (HER): 30% or more. The tensile strength (TS) is more advantageously 1470 MPa or more, and the elongation (El) is more advantageously 10% or more. Meanwhile, in the present invention, the yield strength, tensile strength, elongation, and hole expansion ratio are not particularly limited in their upper limits, but for example, the upper limit of the yield strength may be 1470 MPa, the upper limit of the tensile strength may be 1700 MPa, the upper limit of the elongation may be 20%, and the upper limit of the hole expansion ratio may be 40%. In addition, the cold rolled steel sheet according to one embodiment of the present invention may have a tensile strength (TS) × elongation (El): 13050 MPa% or more. It is more advantageous that the tensile strength (TS) × elongation (El) is 14700 MPa% or more. In the present invention, there is no particular limitation on the upper limit of the tensile strength (TS) × elongation (El), but for example, the upper limit of the tensile strength (TS) × elongation (El) may be 34000 MPa%.

[0075] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0076] First, a slab satisfying the aforementioned alloy composition and relational expression 1 is heated. The slab heating can be performed at 1000 to 1350°C. If the heating temperature of the slab is lower than 1000°C, there is a possibility that the slab 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 the slab will reach the melting point of the steel and melt. Therefore, the slab heating can be performed at 1000 to 1350°C. The lower limit of the heating temperature of the slab is more advantageously 1100°C, more advantageously 1130°C, and most advantageously 1150°C. The upper limit of the heating temperature of the slab is more advantageously 1280°C, more advantageously 1260°C, and most advantageously 1250°C.

[0077] Thereafter, the heated slab is subjected to a finish hot rolling to obtain a hot-rolled steel sheet. The finish hot rolling can be performed at Ar3 to 1000°C. If the finish hot rolling temperature is lower than Ar3, there is a high possibility that the hot deformation resistance will increase rapidly, and there is a possibility that problems will occur in the manufacturing process. If the finish hot rolling temperature exceeds 1000°C, not only may an excessively thick oxide scale be generated, but there is also a high possibility that the microstructure of the steel sheet will coarsen. Therefore, the finish hot rolling can be performed at Ar3 to 1000°C. The lower limit of the finish hot rolling temperature is more advantageously 800°C, more advantageously 850°C, and most advantageously 870°C. The upper limit of the exit temperature of the finish rolling mill during the finish hot rolling is more advantageously 980°C, more advantageously 970°C, and most advantageously 960°C. Meanwhile, the above Ar3 can be obtained through the following [Formula 1].

[0078] [Formula 1] Ar3(℃) = 910 - 203√C - 30Mn + 44.7Si - 11Cr + 31.5Mo - 15.2Ni

[0079] Thereafter, the hot-rolled steel sheet is coiled. The coiling can be performed at 450 to 750°C. If the coiling temperature is lower than 450°C, excessive martensite or bainite may be generated, resulting in an excessive increase in the strength of the hot-rolled steel sheet, which may cause problems such as shape defects due to load during cold rolling. If the coiling temperature exceeds 750°C, the pickling property may deteriorate due to an increase in surface scale. Therefore, the coiling temperature is advantageously in the range of 450 to 750°C. The lower limit of the coiling temperature is more advantageously 465°C, more advantageously 480°C, and most advantageously 495°C. The upper limit of the coiling temperature is more advantageously 700°C, more advantageously 650°C, and most advantageously 600°C.

[0080] Thereafter, 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 particularly limited, and all conditions used in the relevant technical field can be used. However, as an example, the cold rolling may be performed at a cold reduction ratio of 20 to 90%. If the cold reduction ratio is less than 20%, it is difficult to secure the target thickness precision and shape correction of the steel sheet may become difficult. If the cold reduction ratio 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 reduction ratio may be 20 to 90%. The lower limit of the cold reduction ratio is more advantageously 23%, more advantageously 25%, and most advantageously 30%. The upper limit of the above cold rolling reduction ratio is more advantageous when it is 80%, more advantageous when it is 70%, and most advantageous when it is 50%.

[0081] Thereafter, the cold-rolled steel sheet is continuously annealed at a continuous annealing temperature (SS) of 750 to 900°C. If the continuous annealing temperature is less than 750°C, a large amount of ferrite may be 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 austenite may increase, thereby increasing the packet size of martensite formed during cooling. Therefore, it is advantageous that the continuous annealing temperature is in the range of 750 to 900°C. The lower limit of the continuous annealing temperature is more advantageously 760°C, more advantageously 770°C, and most advantageously 780°C. The upper limit of the continuous annealing temperature is more advantageously 890°C, more advantageously 880°C, and most advantageously 870°C.

[0082] Thereafter, the continuously annealed cold rolled steel sheet is first cooled. The first cooling is to suppress ferrite transformation and transform most of the austenite into martensite during the second cooling. The first cooling may be performed up to a first cooling end temperature of 550 to 750°C. If the first cooling end temperature is lower than 550°C or higher than 700°C, productivity may decrease. Therefore, it is advantageous for the first cooling end temperature to be in the range of 550 to 700°C. The lower limit of the first cooling end temperature is more advantageously 575°C, and more advantageously 600°C. The upper limit of the first cooling end temperature is more advantageously 690°C, and more advantageously 680°C. In addition, the first cooling may be performed at a first cooling rate of 1 to 10°C / s. If the primary cooling rate is less than 1°C / s, a ferrite phase is formed during cooling, making it difficult to secure high strength. If the primary cooling rate exceeds 10°C / s, there may be a disadvantage in that the cooling amount in the secondary cooling increases, which may increase the final temperature deviation and material deviation. Therefore, it is advantageous for the primary cooling rate to be in the range of 1 to 10°C / s. The lower limit of the primary cooling rate is more advantageously 2°C / s, more advantageously 3°C / s, and most advantageously 4°C / s. The upper limit of the primary cooling rate is more advantageously 8°C / s, more advantageously 7°C / s, and most advantageously 6°C / s.

[0083] Afterwards, the first-cooled cold-rolled steel sheet is cooled a second time to a second cooling end temperature (Tq) of 150 to 350°C. The second cooling is to secure the widthwise and lengthwise shapes of the coil as well as to secure a high yield ratio and hole expandability. If the second cooling end temperature is less than 150°C, the yield strength and tensile strength may increase simultaneously and the ductility may be significantly reduced due to the excessive increase of martensite during the overaging heat treatment. In particular, shape deterioration due to rapid cooling is expected to occur, which may deteriorate workability when processing automobile parts. If the second cooling end temperature exceeds 350°C, the austenite generated during annealing cannot transform into martensite, but high-temperature transformation phases such as bainite and granular bainite are generated, which may rapidly lower the yield strength. The occurrence of such structures may cause a decrease in the yield ratio as well as a decrease in hole expandability. Therefore, it is advantageous that the secondary cooling end temperature is in the range of 150 to 350°C. The lower limit of the secondary cooling end temperature is more advantageously 170°C, more advantageously 190°C, and most advantageously 200°C. The upper limit of the secondary cooling end temperature is more advantageously 340°C, more advantageously 330°C, and most advantageously 320°C. In addition, the secondary cooling may be performed at a secondary cooling rate of 5 to 60°C / s. When the secondary cooling rate is less than 5°C / s, a high-temperature phase such as upper bainite is mixed in during cooling, making it impossible to obtain the target tempered martensite fraction and high strength. When the secondary cooling rate exceeds 60°C / s, there may be a disadvantage that the shape of the product becomes inferior. Therefore, it is advantageous for the secondary cooling rate to be in the range of 5 to 60°C / s. The lower limit of the secondary cooling rate is more advantageously 6°C / s, more advantageously 7°C / s, and most advantageously 10°C / s.The upper limit of the above secondary cooling rate is more advantageously 50°C / s, more advantageously 40°C / s, and most advantageously 30°C / s.

[0084] Thereafter, the second-cooled cold-rolled steel sheet is reheated at a reheating temperature (Tp) of 200 to 450°C. The reheating is for interphase carbon distribution and additional bainite phase transformation necessary for stabilizing retained austenite. If the reheating temperature is lower than 200°C, there may be disadvantages such as excessively high strength and poor formability. If the reheating temperature exceeds 450°C, it is difficult to obtain the strength desired in the present invention. Therefore, the reheating temperature is advantageously in the range of 200 to 450°C. The lower limit of the reheating temperature is more advantageously 210°C, more advantageously 230°C, and most advantageously 250°C. The upper limit of the reheating temperature is more advantageously 440°C, more advantageously 430°C, and most advantageously 420°C.

[0085] The method for manufacturing the cold-rolled steel sheet of the present invention can satisfy the following relational expression 2. The relational expression 2 below is for simultaneously securing strength, elongation, and hole expandability. When the Y value below is less than 175, the target tensile strength cannot be obtained. When the Y value below exceeds 216, the target elongation cannot be obtained. Therefore, it is advantageous for the Y value below to have a range of 175 to 216. The lower limit of the Y value below is more advantageously 181, more advantageously 183, and most advantageously 185. The upper limit of the Y value below is more advantageously 215, more advantageously 210, and most advantageously 208.

[0086] [Relationship 2] 175 ≤ Y = 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52X ≤ 216

[0087] [Formula 2] A1(℃) = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr

[0088] [Formula 3] Ms(℃) = 539-423C-30.4Mn-12.1Cr-7.5Mo

[0089] (However, in the above equation 2, SS means continuous annealing temperature, A1 means eutectoid transformation temperature, Ms means martensite transformation initiation temperature, Tq means secondary cooling end temperature, and Tp means reheating temperature. A1 is expressed by equation 2, Ms is expressed by equation 3, and X means 7C + (1.3Si + Mn) / 6 + (Cr + 1.2Mo) / 5 + 100B.)

[0090] Meanwhile, after reheating, a step of temper rolling the cold rolled steel sheet at an elongation of 0.010 to 1.0% may be additionally included. Typically, when temper rolling a transformed structure steel, the yield strength increases by 50 MPa or more with almost no increase in tensile strength. If the elongation is less than 0.010%, shape control in ultra-high strength steel such as the present invention may become very difficult. If the elongation exceeds 1.0%, the high elongation operation may significantly destabilize the operability.

[0091] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0092] (Example)

[0093] A slab having the alloy composition described in Table 1 below was heated at 1200°C for 1 hour, and then hot-rolled, coiled, cold-rolled, first cooled, second cooled, and reheated under the conditions described in Table 2 below to produce a cold-rolled steel sheet. At this time, the reduction ratio during cold rolling was 40%. Thereafter, the cold-rolled steel sheet was subjected to temper rolling at an elongation of 0.2%.

[0094] The microstructure and mechanical properties of the cold-rolled steel sheet manufactured in this manner were measured, and the results are shown in Tables 3 and 4 below, respectively.

[0095] The types and fractions of microstructures were measured using the Point Counting method from photographs observed using a scanning electron microscope (SEM).

[0096] The average grain size of bainite and tempered martensite was measured using backscatter electron diffraction pattern analysis (EBSD).

[0097] Yield strength (YS), tensile strength (TS), and elongation (El) were measured by producing JIS No. 5 tensile test specimens and performing a tensile test.

[0098] The hole expandability (HER) was measured by forming a 10mmØ punching hole (die inner diameter 10.3mm, clearance 12.5%) by pressing and expanding a conical punch with a 60° apex angle at 20mm / min in the direction in which the burr of the punching hole becomes external.

[0099] Hole expandability (HER) (%) = {(D - D0) / D0} × 100

[0100] (Note that D: refers to the hole diameter (mm) when the crack penetrates the steel plate, and D0: refers to the initial hole diameter (mm).)

[0101] Steel grade No. Alloy composition (weight %) CSi+Sol.AlSiMnCrTiNbMoBSol.AlPSNX10.2450.6310.612.480.320.040.020.0480.00140.0210.0090.0030.0042.4820.2221.1411.122.680.630.100.010.0900.00210.0210.0110.0040.0052.6030.2521.49 81.482.400.050.050.040.1000.00180.0180.0110.0040.0052.7040.3310.4050.382.690.530.020.010.1200. 00220.0250.0110.0030.0053.2050.1800.6750.652.450.210.030.020.0700.00170.0250.0120.0030.0052.04X = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B

[0102] Classification Steel grade No. Ar3 (℃) Finish rolling temperature (℃) Coiling temperature (℃) Continuous annealing temperature (SS) (℃) 1st cooling end temperature (℃) 1st cooling rate (℃ / s) 2nd cooling end temperature (Tq) (℃) 2nd cooling rate (℃ / s) Reheating temperature (Tp) (℃) A1 (℃) Ms (℃) Y Invention example 117609125128516123202 32279720356205Invention Example 21760899523849632320132302720356176Comparative Example 11760918505852711221239358720356113Comparative Example 2176090252185369221983241372035641Invention Example 31760923 503846682224734302720356192Comparative Example 31760915506850693225034358720356126Comparative Example 41760897506853642315638253720356220Comparative Example 51760912512853662314939349720356 100 Invention Example 42780913523851632320433299738355182 Comparative Example 62780915505848672320137348738355120 Invention Example 52780916521853612324429305738355190 Comparative Example 727809185038956903.224934353738355145Comparative Example 82780906506852670314839302738355158Invention Example 63805915506851693220038300741358186Invention Example 738058975438506423250303007413582 04 Invention Example 8 38 0 5 9 1 2 5 3 2 8 4 7 6 6 2 3 2 4 8 3 2 3 1 5 7 4 1 3 5 8 1 8 4 Comparative Example 9 38 0 5 9 1 3 5 2 1 8 4 3 6 3 2 3 2 4 9 3 0 3 5 0 7 4 1 3 5 8 1 4 0 Comparative Example 10 38 0 5 9 1 5 5 2 6 8 5 6 6 7 2 3 2 4 7 3 1 4 1 1 7 4 1 3 5 8 6 9 Comparative Example 11 38 0 5 8 9 7 5 6 2 853612315036300741358168Comparative Example 124727912532845682220037300714310177Comparative Example 134727913514857693220038350714310120Comparative Example 1447279155428476723250333 00714310196Comparative Example 154727916516852612315036300714310161Comparative Example 165779905853853603319536307719307171Comparative Example 1738059155068116931.820038300741358173Y = 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52XAr3(℃) = 910 - 203√C - 30Mn + 44.7Si - 11Cr + 31.5Mo - 15.2NiA1(℃) = 723-10.7Mn-16.9Ni+29.1Si+16.9CrMs(℃) = 539-423C-30.4Mn-12.1Cr-7.5MoX = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B.

[0103] Classification MicrostructureTM + B 1 or more (area%) Residual γ (area%) FM (area%) F (area%) B Average particle size (㎛) TM Average Particle size (㎛) Invention example 1916212.21.2 Invention example 29071.61.42.41.6 Comparative example 18511312.31.5 Comparative example 28710302.11.6 Invention example 3889301.91.8 Comparative example 38611302.31.4 Comparative example 4962202.81.7 Comparative example 5962202.41.6 Invention example 4888312.31.7 Comparative example 6889212.31.5 Invention example 5887322.21.6 Comparative example 7859423.53.2 Comparative example 8962112.61.8 Invention example 6917. 81.202.31.6 Invention Example 7869.83.30.92.41.5 Invention Example 88611.81.40.81.91.1 Comparative Example 98811.10.901.81.1 Comparative Example 108612.10.911.81.0 Comparative Example 11962112.31.3 Comparative Example 12906312.51.4 Comparative Example 13898212.71.5 Comparative Example 148610312.61.6 Comparative Example 15963102.81.8 Comparative Example 16898212.91.7 Comparative Example 176989.413.62.62.1B: Bainite, TM: tempered martensite, γ: austenite, FM: fresh martensite, F: ferrite

[0104] Yield strength (YS) (MPa) Tensile strength (TS) (MPa) Elongation (El) (%) Hole expandability (HER) (%) TS × El (MPa%) Invention example 1 1 1 4 1 1 5 3 1 1 0.33 2 1 5 7 6 9 Invention example 2 1 1 9 2 1 4 9 3 1 0.63 5 1 5 8 2 6 Comparative example 1 1 2 2 1 1 4 2 1 9.73 8 1 3 7 8 4 Comparative example 2 1 2 2 1 1 3 4 8 1 2.33 4 1 6 5 8 0 Invention example 3 1151151010.73416157Comparative Example 31211143410.62915200Comparative Example 41224152482612192Comparative Example 5128314358.23611767Invention Example 41182151110.43115714Comparative Example 6122114329.82914034Invention Example 51142149310.63415826Comparative Example 712 3514218.92912647Comparative Example 8130115258.12112353Invention Example 61186150611.23616867Invention Example 71127152311.83217971Invention Example 81154150111.13116661Comparative Example 9124214269.53813547Comparative Example 101185138912.53617363Comparative Example 1 1128515788.12912782Comparative Example 12124115328.92313635Comparative Example 13128214559.53213823Comparative Example 14118515329.62814707Comparative Example 15122115119.52114355Comparative Example 161052143511.23616072Comparative Example 17912158212.42119612

[0105] As can be seen from Tables 1 to 4 above, in the case of Invention Examples 1 to 8 that satisfy the alloy composition and manufacturing conditions proposed by the present invention, it can be seen that the yield strength, tensile strength, elongation, hole expandability, and TS×El are excellent as the microstructure and average grain size of bainite and tempered martensite that the present invention seeks to obtain are secured.

[0106] In the case of Comparative Example 1, which does not satisfy the Y value, it can be seen that the average grain size of the bainite of the present invention is not secured, and the tensile strength, elongation, and TS×El are at insufficient levels.

[0107] In the case of Comparative Example 2, which does not satisfy the Y value, it can be seen that the average particle size of the bainite of the present invention is not secured and the tensile strength is at an insufficient level.

[0108] In the case of Comparative Example 3, which does not satisfy the Y value, it can be seen that the average particle size of the bainite of the present invention is not secured, and the tensile strength and hole expandability are at an insufficient level.

[0109] In the case of Comparative Example 4, which does not satisfy the Y value, it can be seen that the microstructure of the present invention is not secured, and the elongation, hole expandability, and TS×El are at insufficient levels.

[0110] In the case of Comparative Example 5, where the secondary cooling end temperature and Y value are not satisfied, it can be seen that the microstructure of the present invention is not secured, and the tensile strength, elongation, and TS×El are at insufficient levels.

[0111] In the case of Comparative Example 6, which does not satisfy the Y value, it can be seen that the average grain size of the bainite of the present invention is not secured, and the tensile strength, elongation, hole expandability, and TS×El are at insufficient levels.

[0112] In the case of Comparative Example 7, which does not satisfy the Y value, it can be seen that the average grain size of the bainite and tempered martensite of the present invention is not secured, and the tensile strength, elongation, hole expandability, and TS×El are at insufficient levels.

[0113] In the case of Comparative Example 8, where the secondary cooling end temperature and Y value are not satisfied, the microstructure of the present invention is not secured, and it can be seen that the elongation, hole expandability, and TS×El are at insufficient levels.

[0114] In the case of Comparative Example 9, which does not satisfy the Y value, it can be seen that the average grain size of the bainite of the present invention is not secured, and the tensile strength, elongation, and TS×El are at insufficient levels.

[0115] In the case of Comparative Example 10, which does not satisfy the Y value, it can be seen that the average particle size of the bainite of the present invention is not secured and the tensile strength is at an insufficient level.

[0116] In the case of Comparative Example 11, which does not satisfy the Y value, it can be seen that the microstructure of the present invention is not secured, and the elongation, hole expandability, and TS×El are at insufficient levels.

[0117] In the case of comparative example 12, which does not satisfy the X value, it can be seen that the elongation, hole expandability, and TS×El are at insufficient levels.

[0118] In the case of comparative example 13, which does not satisfy the X and Y values, it can be seen that the tensile strength, elongation, and TS×El are at insufficient levels.

[0119] In the case of comparative example 14, which does not satisfy the X value, it can be seen that the elongation and hole expandability are at an insufficient level.

[0120] In the case of Comparative Example 15, which does not satisfy the X and Y values, it can be seen that the microstructure of the present invention is not secured, and the elongation, pore expandability, and TS×El are at insufficient levels.

[0121] In the case of comparative example 16, which does not satisfy the X and Y values, it can be seen that the tensile strength and elongation are at insufficient levels.

[0122] In the case of Comparative Example 17, which does not satisfy the Y value, it can be seen that the microstructure of the present invention is not secured, and the yield strength and pore expandability are at an insufficient level.

[0123] Fig. 1 is a photograph of Invention Example 1 of the present invention observed using a scanning electron microscope (SEM). As can be seen from Fig. 1, Invention Example 1 includes bainite and tempered martensite as the main phases, which are the objectives of the present invention, and includes an appropriate fraction of retained austenite, and trace amounts of fresh martensite and ferrite are formed.

Claims

1. Contains, in wt%, C: 0.050~0.40%, Mn: 1.0~3.0%, Cr: 0.010~1.20%, P: 0.0010~0.10%, S: 0.010% or less (excluding 0%), Si+Sol.Al: 0.10~3.0%, Mo: 0.020~0.20%, B: 0.0010~0.0050%, and the remainder is composed of Fe and other unavoidable impurities. Satisfies the following relation 1, Microstructure is in area %, at least one of bainite and tempered martensite: 70 to 95%, retained austenite: 3 to 15%, and residual structure. The above bainite and tempered martensite are cold rolled steel sheets having an average grain size of 3㎛ or less. [Relationship 1] 2.40 ≤ X = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0 (However, in the above relational expression 1, the content of each alloy element means weight%.) 2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet additionally containing at least one of N: 0.0010 to 0.010%, Ti: 0.010 to 0.120%, and Nb: 0.0010 to 0.050%.

3. In claim 1, A cold rolled steel sheet having the above Si content of 2.50% or less (excluding 0%) and the Sol.Al content of 0.010 to 0.10%.

4. In claim 1, The above residual structure is a cold rolled steel sheet containing at least one of fresh martensite: 10% or less (including 0%) and ferrite: 5% or less (including 0%).

5. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a yield strength (YS): 1100 MPa or more, a tensile strength (TS): 1450 MPa or more, an elongation (El): 9% or more, and a hole expansion ratio (HER): 30% or more.

6. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a tensile strength (TS) × elongation (El): 13050 MPa% or more.

7. A step of heating a slab comprising, by weight%, C: 0.050 to 0.40%, Mn: 1.0 to 3.0%, Cr: 0.010 to 1.20%, P: 0.0010 to 0.10%, S: 0.010% or less (excluding 0%), Si+Sol.Al: 0.10 to 3.0%, Mo: 0.020 to 0.20%, B: 0.0010 to 0.0050%, the remainder being Fe and other unavoidable impurities, and satisfying the following relationship 1; A step of obtaining a hot-rolled steel sheet by final hot-rolling the above heated slab; A step of coiling the above hot-rolled steel plate; A step of cold rolling the above-mentioned hot-rolled steel sheet to obtain a cold-rolled steel sheet; A step of continuously annealing the above cold rolled steel sheet at a continuous annealing temperature (SS) of 750 to 900°C; A step of first cooling the continuously annealed cold rolled steel sheet; A step of secondarily cooling the first-cooled cold-rolled steel sheet to a second cooling end temperature (Tq) of 150 to 350°C; and A step of reheating the secondarily cooled cold rolled steel sheet at a reheating temperature (Tp) of 200 to 450°C; A method for manufacturing a cold rolled steel sheet satisfying the following relational expression 2. [Relationship 1] 2.40 ≤ X = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0 [Relationship 2] 175 ≤ Y = 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52X ≤ 216 [Formula 2] A1(℃) = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr [Formula 3] Ms(℃) = 539-423C-30.4Mn-12.1Cr-7.5Mo (However, in the above equation 1, the content of each alloy element is in wt%, and in the above equation 2, SS means continuous annealing temperature, A1 means eutectoid transformation temperature, Ms means martensite transformation initiation temperature, Tq means secondary cooling end temperature, and Tp means reheating temperature. A1 is expressed by equation 2, Ms is expressed by equation 3, and X means 7C + (1.3Si + Mn) / 6 + (Cr + 1.2Mo) / 5 + 100B.) 8. In claim 5, The above slab heating is a method for manufacturing cold rolled steel sheets, which is performed at 1000 to 1350°C.

9. In claim 5, The above finishing hot rolling is a method for manufacturing cold rolled steel sheets performed at Ar3 to 1000℃.

10. In claim 5, The above method for manufacturing cold rolled steel sheets is performed at 450 to 750°C.

11. In claim 5, The above cold rolling is a method for manufacturing cold rolled steel sheets, which is performed at a cold rolling reduction ratio of 20 to 90%.

12. In claim 5, A method for manufacturing a cold rolled steel sheet, wherein the above primary cooling is performed to a primary cooling completion temperature of 550 to 750°C.

13. In claim 5, A method for manufacturing a cold rolled steel sheet, wherein the above primary cooling is performed at a primary cooling rate of 1 to 10°C / s.

14. In claim 5, A method for manufacturing a cold rolled steel sheet, wherein the above secondary cooling is performed at a secondary cooling rate of 5 to 60°C / s.

15. In claim 5, A method for manufacturing a cold rolled steel sheet, further comprising the step of subjecting the cold rolled steel sheet to temper rolling at an elongation of 0.010 to 1.0% after the reheating.

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