Cold rolled steel sheet and method for manufacturing same

Abstract

One aspect of the present invention is to provide a cold rolled steel sheet and a method for manufacturing same. Another aspect of the present invention is to provide an ultra high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance, and a method for 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.

[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 components such as members, seat rails, and pillars to enhance the impact resistance of the body. These structural components 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, the elongation decreases, which leads to a problem of reduced formability. Therefore, the development of materials that can compensate for this problem is required.

[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] Meanwhile, 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 achieving high strength at low manufacturing costs. However, it has the disadvantage of requiring high-temperature annealing to ensure sufficient recrystallization and ductility because the recrystallization temperature rises rapidly due to 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 achieve a high strength of 600 MPa or higher.

[0005] Meanwhile, various transformation-strengthened high-strength steels have been developed, such as 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, although these advanced high-strength steels can achieve high strength, their elongation is limited to about 8% when the tensile strength is 1500 MPa. 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 equipment, heat treatment, and process costs limit their widespread application.

[0006] Recently, automotive seat components are being manufactured with both high strength and lightweight construction to further enhance passenger safety in the event of a collision. These components are manufactured using roll forming and press forming methods. Seat components, as components connecting the passenger to 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] Patent Document 1 discloses a high-strength cold-rolled steel sheet having a tensile strength of 880 to 1170 MPa and a microstructure of a single-phase martensite structure by controlling the alloy composition and heat treatment conditions. Patent Document 2 discloses a method for manufacturing a high-strength steel sheet, which comprises heating and maintaining a steel sheet in a two-phase region in which a volume fraction of a low-temperature transformation phase such as martensite or bainite accounts for 90% or more of the total microstructure, thereby controlling the structure to a fine ferrite and austenite structure that inherits the lath of the low-temperature transformation phase, and then obtaining a microstructure in which the ferrite and the low-temperature transformation phase are finely dispersed in a lath shape through subsequent cooling. These patents claim that a high yield strength can be obtained through the fine dispersion of the low-temperature transformation phase. However, there are disadvantages in that the difference in hardness between phases may increase due to the increase in the fraction of the low-temperature transformation phase created by cooling after heating in the two-phase region, and thus, the stretchability may be reduced.

[0008] Meanwhile, as disclosed in non-patent document 1, as the strength of steel sheets increases, the possibility of hydrogen embrittlement increases. When hydrogen penetrates into the steel sheet, the hydrogen is trapped at the austenite-ferrite interface, inclusions, and retained austenite. Therefore, as the fraction of these increases, the fraction of hydrogen concentration also increases, and the possibility of hydrogen embrittlement increases. In the case of TRIP steel, retained austenite is utilized to obtain high elongation. However, as the size of the retained austenite increases, the stability decreases. In addition, when forming a part, the retained austenite transforms into martensite, etc., and the amount of retained austenite decreases. As a result, hydrogen cannot be stably trapped, and the hydrogen that diffuses out causes hydrogen embrittlement. In addition, as the distance between retained austenite particles decreases, the content and concentration of local hydrogen trapped in the retained austenite increase, and therefore, the possibility of hydrogen embrittlement occurring after transformation to martensite increases.

[0009] Therefore, there is a need to develop a technology that can reduce the sensitivity to hydrogen embrittlement by appropriately controlling the stability of retained austenite and the distance between retained austenite.

[0010] [Prior Art Literature]

[0011] [Patent Document]

[0012] (Patent Document 1) Japanese Patent Publication No. 3729108

[0013] (Patent Document 2) Japanese Patent Application Laid-Open No. 2005-272954

[0014] [Non-patent literature]

[0015] (Non-patent Document 1) Hojo T, Kobayashi J, Kajiyama T, Sugimoto KI, Count A, "Effects of alloying elements on impact properties of ultra high-strength TRIP-aided bainitic ferrite steels", 2010, Vol. 52, pp. 9-16.

[0016] One aspect of the present invention is to provide a cold rolled steel sheet and a method for manufacturing the same.

[0017] Another aspect of the present invention is to provide an ultra-high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance and a method for manufacturing the same.

[0018] One embodiment of the present invention comprises, in wt%, C: 0.10 to 0.30%, Si: 2.50% or less (excluding 0%), Mn: 1.0 to 3.0%, Ti: 0.010 to 0.20%, Mo: 0.0010 to 0.50%, B: 0.0010 to 0.0050%, P: 0.0010 to 0.10%, S: 0.010% or less (excluding 0%), Sol.Al: 0.010 to 1.50%, N: 0.010% or less (excluding 0%), the remainder Fe and other inevitable impurities, and the microstructure is, in area%, the sum of tempered martensite and bainite: 70 to 95%, retained austenite: 3 to 15%, fresh martensite: 10% or less (including 0%), and ferrite: 5% or less (including 0%). A cold rolled steel sheet is provided that includes (including) and satisfies the following relational expression 2.

[0019] [Relationship 2] a / b ≤ 0.30

[0020] (However, in the above relational expression 2, a means the average size (㎛) of retained austenite, and b means the average distance (㎛) between the nearest neighbors of retained austenite.)

[0021] The above cold rolled steel sheet may additionally include at least one of Cr: 1.20% or less and Nb: 0.10% or less in weight %.

[0022] The above cold rolled steel sheet can satisfy the following relationship 1.

[0023] [Relationship 1] 2.40 ≤ X = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0

[0024] (However, in the above relational expression 1, the content of each alloy element is in weight%.)

[0025] The above tempered martensite may have an average size of the short axis of the lath of 3㎛ or less (excluding 0㎛).

[0026] The above cold rolled steel sheet may have a tensile strength of 1470 MPa or more, an elongation of 10% or more, and a hole expansion ratio (HER) of 25% or more.

[0027] Another embodiment of the present invention comprises the steps of: heating a slab containing, in wt%, C: 0.10 to 0.30%, Si: 2.50% or less (excluding 0%), Mn: 1.0 to 3.0%, Ti: 0.010 to 0.20%, Mo: 0.0010 to 0.50%, B: 0.0010 to 0.0050%, P: 0.0010 to 0.10%, S: 0.010% or less (excluding 0%), Sol.Al: 0.010 to 1.50%, N: 0.010% or less (excluding 0%), the remainder Fe, and other unavoidable impurities; subjecting the heated slab to a finish hot rolling so that the exit temperature of a finish rolling mill is Ar3 or higher to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at 450 to 750°C; The present invention provides a method for manufacturing a cold-rolled steel sheet, comprising: a step of pickling and cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; a step of continuously annealing the cold-rolled steel sheet at A1 to 900°C; a step of first cooling the continuously annealed cold-rolled steel sheet to 550 to 700°C at a cooling rate of 1 to 10°C / s; a step of second cooling the first-cooled cold-rolled steel sheet to 150 to 500°C at a cooling rate of 5 to 100°C / s; and a step of reheating and overaging the second-cooled cold-rolled steel sheet at 150 to 500°C; and satisfying the following relational expression 3.

[0028] [Relationship 3] 175 ≤ 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52X ≤ 216

[0029] (However, in the above equation 3, SS means continuous annealing temperature, A1 means eutectoid transformation temperature, Ms means martensite transformation initiation temperature, Tq means secondary cooling end temperature, Tp means reheating and overaging heat treatment temperature, A1 = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr, Ms = 539-423C-30.4Mn-12.1Cr-7.5Mo, and X means 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B.)

[0030] The above slab may additionally contain at least one of Cr: 1.20% or less and Nb: 0.10% or less in weight %.

[0031] The above slab can satisfy the following relationship 1.

[0032] [Relationship 1] 2.40 ≤ X = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0

[0033] (However, in the above relational expression 1, the content of each alloy element is in weight%.)

[0034] The heating temperature of the above slab can be 1000 to 1350°C.

[0035] The reduction ratio during the above cold rolling can be 20 to 90%.

[0036] After the above reheating and over-aging heat treatment, a step of temper rolling the cold rolled steel sheet at an elongation of 0.10 to 1.0% may be additionally included.

[0037] According to one aspect of the present invention, it is an object to provide a cold rolled steel sheet and a method for manufacturing the same.

[0038] According to another aspect of the present invention, it is an object to provide an ultra-high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance and a method for manufacturing the same.

[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: 0.10~0.30%

[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 increases. However, when the C content exceeds 0.30%, 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, 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 0.10%, it is very difficult to secure the strength of martensite desired in the present invention. Therefore, the C content is preferably in the range of 0.10 to 0.30%. The lower limit of the C content is more advantageously 0.130%, and even more advantageously 0.150%. It is more advantageous for the upper limit of the above C content to be 0.270%.

[0045] Si: 2.50% or less (excluding 0%)

[0046] Silicon (Si) promotes ferrite transformation and increases the carbon content in untransformed austenite, forming a composite structure of ferrite and martensite, thereby hindering the increase in strength of martensite. Furthermore, it is desirable to limit its addition as much as possible, as it not only causes surface scale defects but also reduces chemical processability. Accordingly, in the present invention, it is preferable to limit the Si content to 2.50% or less. More advantageously, the Si content is 2.30% or less.

[0047] Mn: 1.0~3.0%

[0048] 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 the 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 aforementioned 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%, 1.50% is more advantageously 1.70%, and the upper limit of the Mn content is more advantageously 2.70%.

[0049] Ti: 0.010~0.20%

[0050] Titanium (Ti) 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 Ti content is less than 0.010%, it may be difficult to sufficiently secure the above effect. If the Ti content exceeds 0.20%, castability may be impaired due to excessive formation of TiN inclusions, and recrystallization may be delayed due to local grain fixation, which may have the disadvantage of damaging the uniformity of the structure. Therefore, the Ti content is preferably in the range of 0.010 to 0.20%. The upper limit of the Ti content is more advantageously 0.170%, more advantageously 0.150%, and most advantageously 0.10%.

[0051] Mo: 0.0010~0.50%

[0052] Molybdenum (Mo) is an element that is advantageous in stabilizing Fe carbide. If the Mo content is less than 0.0010%, it may be difficult to sufficiently secure the above effect. If the Mo content exceeds 0.50%, phase transformation may be suppressed, making it difficult to introduce a bainite structure, and there may be a disadvantage in that the economic feasibility of the steel sheet deteriorates as it is an expensive element. Therefore, the Mo content is preferably in the range of 0.0010 to 0.50%. The lower limit of the Mo content is more advantageously 0.010%, more advantageously 0.050%, and most advantageously 0.070%. The upper limit of the Mo content is more advantageously 0.450%, more advantageously 0.40%, and most advantageously 0.350%.

[0053] B: 0.0010~0.0050%

[0054] 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%.

[0055] P: 0.0010~0.10%

[0056] 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%.

[0057] S: 0.010% or less (excluding 0%)

[0058] 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.

[0059] Sol.Al: 0.010~1.50%

[0060] Soluble 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 Sol.Al content is less than 0.010%, it may be difficult to sufficiently secure the above effect. If the Sol.Al content exceeds 1.50%, not only may the above effect be saturated, but also the manufacturing cost may increase. Therefore, the Sol.Al content is preferably in the range of 0.010 to 1.50%. The lower limit of the Sol.Al content is more advantageously 0.0150%, and 0.020% is even more advantageous. The upper limit of the Sol.Al content is more advantageously 1.30%, and 1.0% is even more advantageous, and 0.80% is most advantageous.

[0061] N: 0.010% or less (excluding 0%)

[0062] Nitrogen (N) is included in steel as an impurity, and it is advantageous to control its content as low as possible. If the content of N exceeds 0.010%, the risk of cracks occurring during casting due to AlN formation, etc. may significantly increase. Therefore, it is preferable that the content of N be in the range of 0.010% or less (excluding 0%). It is more advantageous when the content of N is 0.0080% or less, even more advantageous when it is 0.0050% or less, and most advantageous when it is 0.0030% or less.

[0063] 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.

[0064] Meanwhile, the cold-rolled steel sheet of the present invention may additionally include at least one of Cr: 1.20% or less and Nb: 0.10% or less in weight %.

[0065] Cr: 1.20% or less

[0066] 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. However, if the Cr content exceeds 1.20%, not only will the effect be saturated, but the strength of the hot-rolled steel sheet may increase excessively, thereby reducing the cold-rollability. Therefore, the Cr content is preferably in the range of 1.20% or less. The Cr content is more advantageously 1.0% or less, more advantageously 0.80% or less, and most advantageously 0.70% or less.

[0067] Nb: 0.10% or less

[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 exceeds 0.10%, there may be a disadvantage in that recrystallization is delayed due to local grain fixation, thereby hindering the uniformity of the microstructure. Therefore, the Nb content is preferably in the range of 0.10% or less. The Nb content is more advantageously 0.080% or less, more advantageously 0.050% or less, and most advantageously 0.040% or less.

[0069] The cold rolled steel sheet of the present invention can satisfy the above-described alloy composition and the following relational expression 1.

[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 above relational expression 1 is intended to secure high strength while also securing hole expansion ratio (HER) and elongation targeted by the present invention. When the X value is less than 2.40, the tensile strength targeted by the present invention cannot be obtained. When the X value exceeds 3.0, not only the tensile strength targeted by the present invention but also the elongation cannot be obtained. Therefore, the X value is preferably in the range of 2.40 to 3.0. The lower limit of the X value is more advantageously 2.420, more advantageously 2.440, and most advantageously 2.460. The upper limit of the X value is more advantageously 2.90, more advantageously 2.80, and most advantageously 2.70.

[0073] The microstructure of the cold-rolled steel sheet of the present invention preferably includes, in area %, a sum of tempered martensite and bainite: 70 to 95%, retained austenite: 3 to 15%, fresh martensite: 10% or less (including 0%), and ferrite: 5% or less (including 0%).

[0074] The above tempered martensite and bainite are structures advantageous for securing tensile strength. If the total fraction of the tempered martensite and bainite is less than 70%, the tensile strength targeted by the present invention cannot be obtained. If the total fraction of the tempered martensite and bainite exceeds 95%, the tensile strength may be excessively high. Meanwhile, the tempered martensite may have an average lath short axis size of 3 µm or less (excluding 0 µm). If the average size of the tempered martensite exceeds 3 µm, it may be disadvantageous for securing formability. 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 fresh martensite is a structure disadvantageous for securing formability. If the fraction of fresh martensite exceeds 10%, the tensile strength may become excessively high. The ferrite is a structure that is disadvantageous for securing formability. If the fraction of ferrite exceeds 5%, the interphase hardness difference may increase, reducing hole expandability.

[0075] It is preferable that the cold rolled steel sheet of the present invention satisfies the following relational expression 2.

[0076] [Relationship 2] a / b ≤ 0.30

[0077] (However, in the above relational expression 2, a means the average size (㎛) of retained austenite, and b means the average distance (㎛) between the nearest neighbors of retained austenite.)

[0078] The cold-rolled steel sheet of the present invention is an ultra-high-strength steel sheet having a tensile strength of 1470 MPa or more, and this high strength increases the possibility of hydrogen embrittlement. In addition, when hydrogen penetrates into the steel sheet, the hydrogen is trapped at the austenite-ferrite interface, inclusions, and retained austenite. Therefore, as the fraction of these increases, the fraction of hydrogen concentration also increases, and the possibility of hydrogen embrittlement increases. In the case of TRIP steel, retained austenite is utilized to obtain high elongation. However, as the size of the retained austenite increases, stability decreases. In addition, when forming a part, the retained austenite transforms into martensite, etc., and the amount of retained austenite decreases. As a result, hydrogen cannot be stably trapped, and the hydrogen that diffuses out causes hydrogen embrittlement. In addition, as the distance between retained austenite particles decreases, the content and concentration of localized hydrogen trapped in the retained austenite increase, and this increases the possibility of hydrogen embrittlement occurring after transformation into martensite is induced. Therefore, the sensitivity to hydrogen embrittlement can be reduced by increasing the stability of retained austenite and appropriately controlling the distance between retained austenite. When the value of the above relational expression 2 exceeds 0.30, the stability of retained austenite is reduced, and the nearest distance between retained austenite is close, so that the local hydrogen content and hydrogen concentration increase, which may cause hydrogen embrittlement. Meanwhile, a and b mentioned in relational expression 2 can be measured using the average size and average distance for the 10 nearest retained austenite from any one retained austenite as a starting point. In addition, the nearest distance between the retained austenite may mean the distance between the centers of austenite. However, for the ease of measuring the average size and average distance, the measurement target of the retained austenite may have a minimum size of 0.2 ㎛.

[0079] As described above, the cold-rolled steel sheet of the present invention may have a tensile strength of 1470 MPa or more, an elongation of 10% or more, and a hole expansion ratio (HER) of 25% or more. Meanwhile, in the present invention, the tensile strength, elongation, and hole expansion ratio are not particularly limited in their upper limits, but for example, 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 of the present invention has excellent hydrogen embrittlement resistance because, when a load is applied according to the ASTM G39 standard and it is immersed in a 0.1 N HCl solution, no cracks or fractures occur within 96 hours.

[0080] 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.

[0081] First, a slab satisfying the aforementioned alloy composition and equation 1 is heated. The heating temperature of the slab may be 1000 to 1350°C. If the heating temperature of the slab is lower than 1000°C, there is a possibility that the slab may 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 may reach the melting point of the steel and melt. Therefore, the heating temperature of the slab may be 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.

[0082] Thereafter, the heated slab is subjected to finish hot rolling so that the exit temperature of the finishing mill is Ar3 or higher to obtain a hot-rolled steel sheet. If the exit temperature of the finishing mill is lower than Ar3 during the finishing hot rolling, there is a high possibility that the hot deformation resistance will increase rapidly, and furthermore, the top, bottom (tail) and edge of the hot-rolled coil may become single-phase regions after coiling, which may increase in-plane anisotropy and deteriorate formability. Therefore, it is preferable that the exit temperature of the finishing mill be Ar3 or higher during the finishing hot rolling. The lower limit of the exit temperature of the finishing mill during the finishing hot rolling is more advantageously 800°C, more advantageously 850°C, and most advantageously 870°C. Meanwhile, the present invention does not specifically limit the upper limit of the exit temperature of the finishing mill during the finishing hot rolling. However, as an example, the exit temperature of the finishing mill during the finishing hot rolling may be 1000°C or lower. In the above finishing hot rolling, the upper limit of the exit temperature of the finishing rolling mill is preferably 980°C, more preferably 970°C, and most preferably 960°C. Meanwhile, the Ar3 can be obtained through the following [Equation 1].

[0083] [Formula 1] Ar3(℃) = 910 - 203√C - 30Mn + 44.7Si - 11Cr + 31.5Mo - 15.2Ni

[0084] Thereafter, the hot-rolled steel sheet is coiled at 450 to 750°C. If the coiling temperature is lower than 450°C, excessive martensite or bainite may be generated, which may result in excessive strength increase of the hot-rolled steel sheet, thereby causing 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 preferably 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 500°C. The upper limit of the coiling temperature is more advantageously 700°C, more advantageously 650°C, and most advantageously 600°C.

[0085] Thereafter, the coiled hot-rolled steel sheet is pickled and cold-rolled to obtain a cold-rolled steel sheet. In the present invention, the pickling and cold-rolling conditions are not particularly limited, and all conditions used in the relevant technical field can be used. However, as an example, the reduction ratio during the cold rolling may be 20 to 90%. If the cold reduction ratio is less than 20%, it is difficult to secure the target thickness precision and the shape of the steel sheet may also be difficult to correct. 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%.

[0086] Thereafter, the cold-rolled steel sheet is continuously annealed at A1 to 900°C. If the continuous annealing temperature is less than A1, a large amount of ferrite may be generated, making it difficult to obtain the yield strength and tensile strength desired in the present invention. If the continuous annealing temperature exceeds 900°C, the grain size of austenite increases, so that the packet size of martensite formed during cooling increases, making it difficult to obtain a fine grain size. Therefore, the continuous annealing temperature is preferably in the range of A1 to 900°C. The lower limit of the continuous annealing temperature is more advantageously 790°C, more advantageously 800°C, and most advantageously 820°C. The upper limit of the continuous annealing temperature is more advantageously 890°C, more advantageously 880°C, and most advantageously 870°C.

[0087] Thereafter, the continuously annealed cold rolled steel sheet is first cooled to 550 to 700°C at a cooling rate of 1 to 10°C / s. The purpose of the first cooling is to suppress ferrite transformation and transform most of the austenite into martensite during the second cooling. If the first cooling end temperature is lower than 550°C or higher than 700°C, productivity may decrease. Therefore, the first cooling end temperature is preferably in the range of 550 to 700°C. The lower limit of the first cooling end temperature is more advantageously 570°C, more advantageously 590°C, and most advantageously 610°C. The upper limit of the first cooling end temperature is more advantageously 680°C, more advantageously 670°C, and most advantageously 650°C. 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, the primary cooling rate is preferably 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.

[0088] Afterwards, the first-cooled cold-rolled steel sheet is cooled a second time to 150 to 500°C at a cooling rate of 5 to 100°C / s. The second cooling is to secure the shape of the coil in the width and length directions 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 an excessive increase in the amount 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 500°C, the austenite generated during annealing may not transform into martensite, but high-temperature transformation phases such as bainite and granular bainite may be 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 the hole expandability. Therefore, it is preferable that the secondary cooling end temperature is in the range of 150 to 500°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 480°C, more advantageously 450°C, and most advantageously 430°C. If the secondary cooling rate is less than 5°C / s, a high-temperature phase such as upper bainite is mixed during cooling, making it impossible to obtain the target tempered martensite fraction and high strength. Meanwhile, the present invention does not specifically limit the upper limit of the secondary cooling rate, but it is difficult to exceed 100°C / s due to equipment or process limitations. Therefore, it is preferable that the secondary cooling rate is in the range of 5 to 100°C / s. The lower limit of the above 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 30°C / s, and most advantageously 20°C / s.

[0089] Thereafter, the second-cooled cold-rolled steel sheet is reheated and over-aged at 150 to 500°C. The reheating and over-aging heat treatment is for interphase carbon distribution and additional bainite phase transformation necessary for stabilizing retained austenite. If the reheating and over-aging heat treatment temperature is lower than 150°C, there may be disadvantages such as excessively high strength and poor formability. If the reheating and over-aging heat treatment temperature exceeds 500°C, it is difficult to obtain the strength desired in the present invention. Therefore, the reheating and over-aging heat treatment temperature is preferably in the range of 150 to 500°C. The lower limit of the reheating and over-aging heat treatment temperature is more advantageously 170°C, more advantageously 190°C, and most advantageously 200°C. The upper limit of the above reheating and over-aging heat treatment temperature is more advantageously 480°C, more advantageously 460°C, and most advantageously 450°C.

[0090] It is preferable that the manufacturing method of the present invention satisfies the following relational expression 3.

[0091] [Relationship 3] 175 ≤ 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52X ≤ 216

[0092] (However, in the above equation 3, SS means continuous annealing temperature, A1 means eutectoid transformation temperature, Ms means martensite transformation initiation temperature, Tq means secondary cooling end temperature, Tp means reheating and overaging heat treatment temperature, A1 = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr, Ms = 539-423C-30.4Mn-12.1Cr-7.5Mo, and X means 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B.)

[0093] The above relational expression 3 is intended to secure strength, elongation, and hole expandability simultaneously. If the value of the above relational expression 3 is less than 175, the target tensile strength cannot be obtained. If the value of the above relational expression 3 exceeds 216, the target elongation cannot be obtained. Therefore, the value of the above relational expression 3 is preferably in the range of 175 to 216. The lower limit of the value of the above relational expression 3 is more advantageously 176, more advantageously 177, and most advantageously 180. The upper limit of the value of the above relational expression 3 is more advantageously 215, more advantageously 210, and most advantageously 208.

[0094] Meanwhile, after the over-aging heat treatment, a step of temper rolling the cold rolled steel sheet at an elongation of 0.10 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 during temper rolling is less than 0.10%, shape control may become very difficult in ultra-high strength steel such as the present invention. If the elongation during temper rolling exceeds 1.0%, the operability may become significantly unstable due to the high elongation operation.

[0095] 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.

[0096] (Example)

[0097] A slab having the alloy composition described in Table 1 below was heated at 1200°C for 1 hour, and then hot-rolled, coiled, pickled / cold rolled, first cooled, second cooled, and over-aging heat treated 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 temper-rolled at an elongation of 0.2%.

[0098] The microstructure and mechanical properties of the cold-rolled steel sheet manufactured in this manner were measured, and the results are shown in Table 3 below.

[0099] The types and fractions of microstructures were measured using the Point Counting method from photographs observed using a scanning electron microscope (SEM), and in particular, the fraction of retained austenite was measured using X-ray diffraction (XRD).

[0100] The average size and distance of retained austenite and the average size of the minor axis of tempered martensite laths were measured using backscatter electron diffraction pattern analysis (EBSD). At this time, the average size and average distance of retained austenite were measured by taking one retained austenite as the starting point and then measuring the average values ​​for the 10 most adjacent retained austenite. At this time, the closest distance between the retained austenite is the distance between the centers of the austenite, and the minimum size of the retained austenite to be measured was 0.2 ㎛.

[0101] Yield strength (YS), tensile strength (TS), and elongation (El) were measured by performing a tensile test on a JIS No. 5 tensile test specimen.

[0102] 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.

[0103] Hole expandability (HER) (%) = {(D - D0) / D0} × 100

[0104] (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).)

[0105] Hydrogen embrittlement resistance was evaluated by applying a load according to ASTM G39 standard, immersing in a 0.1 N HCl solution, and measuring the presence or absence of cracks after 96 hours. If cracks occurred, the specimen was evaluated as poor, and if no cracks occurred, the specimen was evaluated as good.

[0106] Steel grade No. Alloy composition (weight %) CSiMnCrTiNbMoBSol.AlPSNX10.2450.612.480.320.0150.0400.0480.00140.0210.0090.0030.0042.4820.2221.122.680.630.100.0450.0900.00210.0210.0110.0040.0052.6030.2521.482.400.050.0200.0200.1000.00180.0 180.0110.0040.0052.7040.3310.382.690.530.0170.0500.1200.00220.0250.0110.0030.0053.2050.1800.652.450.21 0.0220.0010.070.00170.0250.0120.0030.0052.0460.3451.952.430.030.0160.0430.00100.0250.0110.0040.0053.25X = 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B

[0107] Classification Steel grade No. Ar3 (℃) Finishing rolling mill exit temperature (℃) Coiling temperature (℃) A1 (℃) Continuous annealing temperature (SS) (℃) 1st cooling end temperature (℃) 1st cooling rate (℃ / s) 2nd cooling end temperature (Tq) (℃) 2nd cooling rate (℃ / s) Reheating / overheating treatment temperature (Tp) (℃) Ms (℃) Relationship equation 3 Invention Example 1 1760 90 352 1720 85 16 114.5 20 24 1.2 279 356 205 Invention Example 2 1760 92 45 187 20 84 9 6 0 14.6 20 14 0.3 30 235 6 ​​176 Comparative Example 1 1760 91 15 0 47 20 85 26 26 4.2 21 24 1.7 35 8 35 6 113 Comparative Example 2 1760 88 75 0 67 20 85 36 30 4.1 19 8 4 3.5 4 1 33 5 6 41 Invention Example 3 1760 87 6 5 13 720 84 6 5 8 3.5 2 4 7 41 430 235 6 ​​192 Comparative Example 3 1760 9165097208506463.825039.9358356126Comparative Example 417609145117208536184.415646.6253356221Comparative Example 517609015177208536224.314947.7349356101Invention Example 427809105207388516364.020443.5299355182Invention Example 527809005347388536394.024439.8305355190Comparative Example 62780921559738 8566284.224938.2353355134Invention Example 638058895227418516084.520041.1300358186Invention Example 738059065737418506403.925039.3300358204Invention Example 838058915457418476014.624835.6315358184Comparative Example 738059175757418566473.924740.341135869Comparative Example 838059065087418536354.1 15048.9312358161Comparative Example 947278945127148456284.020043.1300310177Comparative Example 1057798865297198576453.920044.835038559Comparative Example 1168058935517548476284.125038.1300319199Comparative Example 1268059165287548526194.315047.3300319163Comparative Example 1368058935077548536284.219543.6307319234Comparative Example 1468059025247548736464.220244.739431970[Relationship 3] 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52[Relationship 1].

[0108] MicrostructureMechanical propertiesTM+B(area%)Residualγ(area%)FM(area%)TM Rath Short-axis average Size (㎛) a (㎛) b (㎛) a / b YS (MPa) TS (MPa) El (%) HER (%) Hydrogen embrittlement resistance Invention example 192.4 4.33.30.17 0.34 2.70.13 114 115 3110 332 Good invention example 294.33.12.6 0.46 0.34 3.30.10 119 214 9 310.635 Good comparative example 192.15.8 2.10.66 0.31 0.94 0.33 122 114 219.738 Poor comparative example 290.6 7.22.20.25 0.37 1.190.31 122 1134 8 12.334 Poor invention example 394.23.62 .20.410.271.500.181151151010.734Good Comparison Example390.17.32.60.530.531.710.311211143410.629Poor Comparison Example489.84.95.30.620.250.570.44122415248.026Poor Comparison Example591.56.52.00.330.411.170.35128314358.236Poor Invention Example495.03.91.10.260.201.050.191182151110.431Good Invention Example594.53.42.10.450.44 2.660.171142149310.634Good Comparison Example 691.18.00.90.220.341.100.31123514218.929Poor Invention Example 694.34.11.60.190.271.330.201186150611.236Good Invention Example 793.73.72.60.460.446.930.061127152311.832Good Invention Example 893.44.91.70330.242.730.091154150111.129Good Comparison Example 792.95.71.40.270.371.170.3211 85138912.536Defective Comparison Example 892.54.53.00.340.310.900.34128515788.129Defective Comparison Example 993.45.31.30.840.330.780.42124115328.923Defective Comparison Example 1090.76.62.70.590.351.130.31128214559.532Defective Comparison Example 1179.614.95.50.540.812.610.31118515329.628Defective Comparison Example 1280.416.82.80.790.361.050.34122115119.521Defective Comparison Example 1379.615.54.90.660.942.940.321052143511.236Defective Comparison Example 1479.215.45.40.870.932.060.451180153917.19.9Defective TM: Tempered martensite, B: Bainite, γ: Austenite, FM: Fresh martensite, F: Ferrite, a: Average size of residual γ, b is the nearest average distance between residual γ.

[0109] As can be seen from Tables 1 to 3 above, in the case of invention examples 1 to 8 that satisfy the conditions proposed by the present invention, not only is the hydrogen embrittlement resistance good, but other mechanical properties are also excellent.

[0110] On the other hand, in the case of Comparative Examples 1 to 14, which do not satisfy the conditions proposed by the present invention, it can be seen that not only is the hydrogen embrittlement resistance poor, but other mechanical properties are also at a low level.

[0111] 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, it can be confirmed that Invention Example 1 has formed the microstructure targeted by the present invention.

Claims

1. Contains, in wt%, C: 0.10~0.30%, Si: 2.50% or less (excluding 0%), Mn: 1.0~3.0%, Ti: 0.010~0.20%, Mo: 0.0010~0.50%, B: 0.0010~0.0050%, P: 0.0010~0.10%, S: 0.010% or less (excluding 0%), Sol.Al: 0.010~1.50%, N: 0.010% or less (excluding 0%), the remainder being Fe and other unavoidable impurities. The microstructure is in area %, the sum of tempered martensite and bainite: 70 to 95%, retained austenite: 3 to 15%, fresh martensite: 10% or less (including 0%), and ferrite: 5% or less (including 0%). Cold rolled steel sheet satisfying the following relationship 2. [Relationship 2] a / b ≤ 0.30 (However, in the above relational expression 2, a means the average size of retained austenite (㎛), and b means the nearest average distance between retained austenite (㎛).) 2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet additionally containing at least one of Cr: 1.20% or less and Nb: 0.10% or less in weight %.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet that satisfies the following relationship 1. [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 is in weight%.) 4. In claim 1, The above tempered martensite is a cold rolled steel sheet having an average size of the short axis of the lath of 3㎛ or less (excluding 0㎛).

5. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a tensile strength of 1470 MPa or more, an elongation of 10% or more, and a hole expansion ratio (HER) of 25% or more.

6. A step of heating a slab containing, by weight%, C: 0.10 to 0.30%, Si: 2.50% or less (excluding 0%), Mn: 1.0 to 3.0%, Ti: 0.010 to 0.20%, Mo: 0.0010 to 0.50%, B: 0.0010 to 0.0050%, P: 0.0010 to 0.10%, S: 0.010% or less (excluding 0%), Sol.Al: 0.010 to 1.50%, N: 0.010% or less (excluding 0%), the remainder Fe and other unavoidable impurities; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated slab so that the exit temperature of the finishing mill is Ar3 or higher; A step of coiling the above hot-rolled steel plate at 450 to 750°C; A step of obtaining a cold rolled steel sheet by pickling and cold rolling the above-mentioned hot rolled steel sheet; A step of continuously annealing the above cold rolled steel sheet at A1 to 900℃; A step of first cooling the continuously annealed cold rolled steel sheet to 550 to 700°C at a cooling rate of 1 to 10°C / s; A step of second cooling the first-cooled cold-rolled steel sheet to 150 to 500°C at a cooling rate of 5 to 100°C / s; and A step of reheating and over-aging the second-cooled cold-rolled steel sheet at 150 to 500°C; A method for manufacturing a cold rolled steel sheet satisfying the following relational expression 3. [Relationship 3] 175 ≤ 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52X ≤ 216 (However, in the above equation 3, SS represents the continuous annealing temperature, A1 represents the eutectoid transformation temperature, Ms represents the martensite transformation initiation temperature, Tq represents the secondary cooling end temperature, and Tp represents the reheating and overaging heat treatment temperature. A1 = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr, Ms = 539-423C-30.4Mn-12.1Cr-7.5Mo, and X represents 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B.) 7. In claim 6, A method for manufacturing a cold rolled steel sheet, wherein the above slab additionally contains at least one of Cr: 1.20% or less and Nb: 0.10% or less in weight%.

8. In claim 6, The above slab is a method for manufacturing a cold rolled steel sheet satisfying the following relationship 1. [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 is in weight%.) 9. In claim 6, A method for manufacturing cold rolled steel sheets wherein the heating temperature of the above slab is 1000 to 1350℃.

10. In claim 6, A method for manufacturing a cold rolled steel sheet having a reduction ratio of 20 to 90% during the above cold rolling.

11. In claim 6, 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 0.10 to 1.0% after the reheating and overaging heat treatment.

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