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. A preferred aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet having excellent LME resistance and formability, 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, 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] Meanwhile, representative alloying elements added to manufacture ultra-high-strength cold-rolled steel sheets with a tensile strength of 1470 MPa or higher include Mn, Si, Cr, and B. However, since these elements have a high oxidation tendency, they diffuse to the surface to combine with oxygen during annealing, forming surface oxides. However, these surface oxides lower the surface reactivity of the steel sheet, which causes deterioration of chemical treatment properties and plating properties. In addition, as the strength of steel materials increases, there is a problem of cracks occurring in the weld heat-affected zone due to liquid metal embrittlement (LME) during spot welding.

[0010] Therefore, there is a need to develop ultra-high-strength cold-rolled steel sheets with excellent LME resistance and formability.

[0011] [Prior Art Literature]

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

[0013] (Patent Document 2) Japanese Patent Publication No. 2005-272954

[0014] One aspect of the present invention is to provide a cold rolled steel sheet and a method for manufacturing the same.

[0015] A preferred aspect of the present invention is to provide an ultra-high strength cold rolled steel sheet having excellent LME resistance and formability and a method for manufacturing the same.

[0016] 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%, Cr: 0.010 to 1.20%, P: 0.0010 to 0.10%, S: 0.010% or less (excluding 0%), Sol.Al: 0.010 to 0.10%, N: 0.0010 to 0.010%, Mo: 0.020 to 0.20%, B: 0.0010 to 0.0050%, Ti: 0.010 to 0.120%, Nb: 0.010 to 0.050%, the remainder being Fe and other inevitable impurities, and satisfying the following relational expression 1, and the microstructure is, in area%, tempered martensite: 70 to 95% and residual A cold-rolled steel sheet containing 3 to 15% austenite, wherein the tempered martensite has an average size of the short axis of the lath of 3 ㎛ or less and a decarburization depth of 30 to 100 ㎛ in the thickness direction from the surface is provided.

[0017] [Relationship 1] 2.40 ≤ 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0

[0018] (However, in the above relational expression 1, the content of each alloy element means weight%.)

[0019] The above microstructure may additionally include at least one of fresh martensite: 10% or less and ferrite: 5% or less.

[0020] The cold-rolled steel sheet may include a microstructure in an area of ​​5 ㎛ or less from the surface in the thickness direction, in terms of area%, of ferrite: 20 to 99% and the sum of at least one type of fresh martensite and tempered martensite: 1 to 80%.

[0021] The above cold rolled steel sheet may have a yield strength of 1100 MPa or more, a tensile strength of 1470 MPa or more, an elongation of 10% or more, and a hole expansion ratio (HER) of 25% or more.

[0022] Another embodiment of the present invention comprises a step of heating a slab comprising, in wt%, C: 0.10 to 0.30%, Si: 2.50% or less (excluding 0%), 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%), Sol.Al: 0.010 to 0.10%, N: 0.0010 to 0.010%, Mo: 0.020 to 0.20%, B: 0.0010 to 0.0050%, Ti: 0.010 to 0.120%, Nb: 0.010 to 0.050%, the remainder being Fe and other unavoidable impurities, and satisfying the following relational expression 1; The present invention provides a method for manufacturing a cold rolled steel sheet, comprising: a step of finishing hot rolling the heated slab to obtain a hot rolled steel sheet; a step of coiling the hot rolled steel sheet at 450 to 750°C; 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 750 to 900°C with a dew point temperature of 0 to 15°C; a step of first cooling the continuously annealed cold rolled steel sheet to 550 to 750°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 320°C at a cooling rate of 5 to 60°C / s; and a step of reheating and overaging the second-cooled cold rolled steel sheet at 200 to 400°C, and satisfying the following relational expression 2.

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

[0024] [Relationship 2] 180 ≤ 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52 [Relationship 1] ≤ 216

[0025] (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 and overaging heat treatment temperature, and A1 = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr, Ms = 539-423C-30.4Mn-12.1Cr-7.5Mo.)

[0026] The above slab heating can be performed at 1000 to 1350°C.

[0027] The above finishing hot rolling can be performed at Ar3~1000℃.

[0028] The above cold rolling can be performed at a cold rolling reduction ratio of 20 to 90%.

[0029] After the above reheating and over-aging heat treatment, a step of temper rolling the cold rolled steel sheet at an elongation of 0.010 to 1.0% may be additionally included.

[0030] According to one aspect of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0031] According to a preferred aspect of the present invention, an ultra-high strength cold rolled steel sheet having excellent LME resistance and formability and a method for manufacturing the same can be provided.

[0032] Figure 1 is a photograph of Invention Example 1 according to one embodiment of the present invention observed using a scanning electron microscope (SEM).

[0033] 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.

[0034] 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.

[0035] 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.

[0036] C: 0.10~0.30%

[0037] 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, if 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, the weldability is reduced, resulting in welding defects when processing parts at the customer site. On the other hand, if 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%. The upper limit of the above C content is more advantageously set at 0.290%, and even more advantageously at 0.270%.

[0038] Si: 2.50% or less (excluding 0%)

[0039] 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. In addition, it is preferable to limit the addition as much as possible because it not only causes surface scale defects in relation to surface properties but also reduces chemical treatment properties. Accordingly, in the present invention, it is preferable to limit the content of Si 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. Meanwhile, the present invention does not specifically limit the lower limit of the Si content, but as an example, the lower limit may be 0.0010%.

[0040] Mn: 1.0~3.0%

[0041] 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%.

[0042] Cr: 0.010~1.20%

[0043] 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%.

[0044] P: 0.0010~0.10%

[0045] 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%.

[0046] S: 0.010% or less (excluding 0%)

[0047] 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.

[0048] Sol.Al: 0.010~0.10%

[0049] 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 Sol.Al content is less than 0.010%, it may be difficult to sufficiently secure the above effect. If the Sol.Al content exceeds 0.10%, not only may the effect be saturated, but also the manufacturing cost may increase. Therefore, the Sol.Al content is preferably in the range of 0.010 to 0.10%. 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 0.090%, and 0.080% is even more advantageous, and 0.070% is most advantageous.

[0050] N: 0.0010~0.010%

[0051] 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.

[0052] Mo: 0.020~0.20%

[0053] 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%.

[0054] B: 0.0010~0.0050%

[0055] 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%.

[0056] Ti: 0.010~0.120%

[0057] 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.

[0058] Nb: 0.010~0.050%

[0059] 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 0.010%, 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 0.010 to 0.050%. The lower limit of the Nb content is more advantageously 0.0150%, and the upper limit is more advantageously 0.020%. The upper limit of the Nb content is more advantageously 0.0450%, and the upper limit is more advantageously 0.040%.

[0060] 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.

[0061] It is preferable that the cold-rolled steel sheet of the present invention satisfy the above-described alloy composition and also satisfy the following relational expression 1.

[0062] [Relationship 1] 2.40 ≤ 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0

[0063] (However, in the above relational expression 1, the content of each alloy element means weight%.)

[0064] 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. If the value of the above relational expression 1 is less than 2.40, the tensile strength targeted by the present invention cannot be obtained. If the value of the above relational expression 1 exceeds 3.0, not only the tensile strength targeted by the present invention but also the elongation cannot be obtained. Therefore, the value of the above relational expression 1 is preferably in the range of 2.40 to 3.0. The lower limit of the value of the above relational expression 1 is more advantageously 2.420, more advantageously 2.440, and most advantageously 2.460. The upper limit of the value of the above relational expression 1 is more advantageously 2.90, more advantageously 2.80, and most advantageously 2.70.

[0065] The microstructure of the cold-rolled steel sheet of the present invention preferably includes, in terms of area %, tempered martensite: 70 to 95% and retained austenite: 3 to 15%. In addition, the microstructure may additionally include at least one of fresh martensite: 10% or less and ferrite: 5% or less.

[0066] The above tempered martensite is a structure advantageous in securing tensile strength. If the fraction of the tempered martensite is less than 70%, the tensile strength targeted by the present invention cannot be obtained. If the fraction of the tempered martensite exceeds 95%, the tensile strength may be excessively high. The tempered martensite may have an average size of the minor axis of the lath of 3 µm or less (excluding 0 µm). If the average size of the tempered martensite exceeds 3 µm, it may be disadvantageous in securing formability. The above retained austenite is a structure advantageous in 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 fresh martensite is a structure disadvantageous in securing formability. If the fraction of the above fresh martensite exceeds 10%, the tensile strength may be excessively high. The above ferrite is a structure that is disadvantageous for securing formability. If the fraction of the above ferrite exceeds 5%, the interphase hardness difference may increase, reducing hole expandability.

[0067] The cold rolled steel sheet of the present invention preferably has a decarburization depth of 30 to 100 μm in the thickness direction from the surface. If the decarburization depth is less than 30 μm, LME cracks may occur during welding. If the decarburization depth exceeds 100 μm, the material may deteriorate. Therefore, the decarburization depth is preferably 30 to 100 μm. The lower limit of the decarburization depth is more advantageously 35 μm, more advantageously 40 μm, and most advantageously 50 μm. The lower limit of the decarburization depth is more advantageously 90 μm, more advantageously 80 μm, and most advantageously 70 μm. Meanwhile, the decarburization may be determined based on a reduction of 10% or more compared to the C content of the base material.

[0068] The cold-rolled steel sheet of the present invention may include, in terms of area %, a microstructure of ferrite: 20 to 99% and the sum of at least one of fresh martensite and tempered martensite: 1 to 80% in a region within 5 μm in the thickness direction from the surface. The cold-rolled steel sheet of the present invention increases the ferrite fraction in the surface layer by the decarburization described above. Accordingly, the cold-rolled steel sheet of the present invention may include, in terms of area %, a ferrite fraction of 20 to 99% in a microstructure of ferrite in a region within 5 μm in the thickness direction from the surface.

[0069] As described above, the cold rolled steel sheet of the present invention may have a yield strength of 1100 MPa or more, 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 yield strength, tensile strength, elongation, and hole expansion ratio are not particularly limited in terms of 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 of the present invention may not cause B type LME cracking after spot welding according to the SEP 1220-2 standard.

[0070] 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.

[0071] First, a slab satisfying the aforementioned alloy composition and equation 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.

[0072] 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].

[0073] [Formula 1] Ar3(℃) = 910 - 203√C - 30Mn + 44.7Si - 11Cr + 31.5Mo - 15.2Ni

[0074] 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 495°C. The upper limit of the coiling temperature is more advantageously 700°C, more advantageously 650°C, and most advantageously 600°C.

[0075] 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 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 also 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%.

[0076] Thereafter, the cold-rolled steel sheet is continuously annealed at 750 to 900°C with a dew point temperature of 0 to 15°C. If the dew point temperature is less than 0°C, it is difficult to secure the decarburization layer depth desired in the present invention, and LME cracking may occur. If the dew point temperature exceeds 15°C, the strength may be reduced due to over-decarburization. Therefore, the dew point temperature is preferably in the range of 0 to 15°C. The lower limit of the dew point temperature is more advantageously 1°C, more advantageously 2°C, and most advantageously 3°C. The upper limit of the dew point temperature is more advantageously 14°C, more advantageously 13°C, and most advantageously 12°C. If the continuous annealing temperature is less than 750°C, a large amount of ferrite is generated, making it difficult to secure the yield strength and tensile strength targeted by the present invention. When the continuous annealing temperature exceeds 900°C, the grain size of austenite increases, which may increase the packet size of martensite formed during cooling. Therefore, the continuous annealing temperature is preferably 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.

[0077] 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 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. If the first cooling rate is less than 1°C / s, a ferrite phase is formed during cooling, making it difficult to secure high strength. If the primary cooling rate exceeds 10°C / s, there may be a disadvantage in that the cooling amount in the secondary cooling increases, which increases 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.

[0078] Afterwards, the first-cooled cold-rolled steel sheet is cooled a second time to 150 to 320°C at a cooling rate of 5 to 60°C / s. 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 in 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 320°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 hole expandability. Therefore, it is preferable that the secondary cooling end temperature is in the range of 150 to 320°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 310°C, more advantageously 300°C, and most advantageously 290°C. When 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. When the secondary cooling rate exceeds 60°C / s, there may be a disadvantage in that the shape of the product becomes inferior. Therefore, it is preferable that the secondary cooling rate is in the range of 5 to 60°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 40°C / s, and most advantageously 30°C / s.

[0079] Thereafter, the second-cooled cold-rolled steel sheet is reheated and over-aged at 200 to 400°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 200°C, there may be disadvantages such as excessively high strength and poor formability. If the reheating and over-aging heat treatment temperature exceeds 400°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 200 to 400°C. The lower limit of the reheating and over-aging heat treatment temperature is more advantageously 210°C, more advantageously 230°C, and most advantageously 250°C. The upper limit of the above reheating and over-aging heat treatment temperature is more advantageously 390°C, more advantageously 380°C, and most advantageously 370°C.

[0080] Meanwhile, it is preferable that the manufacturing method of the present invention satisfies the following relational expression 2.

[0081] [Relationship 2] 180 ≤ 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52 [Relationship 1] ≤ 216

[0082] (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, Tp means reheating and overaging heat treatment temperature, and A1 = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr, Ms = 539-423C-30.4Mn-12.1Cr-7.5Mo.)

[0083] The above relational expression 2 is intended to secure strength, elongation, and hole expandability simultaneously. If the value of the above relational expression 2 is less than 180, the target tensile strength cannot be obtained. If the value of the above relational expression 2 exceeds 216, the target elongation cannot be obtained. Therefore, the value of the above relational expression 2 is preferably in the range of 180 to 216. The lower limit of the value of the above relational expression 2 is more advantageously 181, more advantageously 183, and most advantageously 185. The upper limit of the value of the above relational expression 2 is more advantageously 215, more advantageously 210, and most advantageously 208.

[0084] Meanwhile, after the reheating and overaging heat treatment, 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 may become very difficult in ultra-high strength steel such as the present invention. If the elongation exceeds 1.0%, the high elongation operation may significantly destabilize the operability.

[0085] 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.

[0086] (Example)

[0087] 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%.

[0088] The microstructure, decarburization depth, 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.

[0089] The types and fractions of microstructures were measured using the Point Counting method from photographs observed using a scanning electron microscope (SEM).

[0090] The average size of the short axis of the tempered martensite lath was measured using backscatter electron diffraction pattern analysis (EBSD).

[0091] The decarburization depth was measured using GDS equipment and SEM.

[0092] Yield strength (YS), tensile strength (TS), and elongation (El) were measured by producing JIS No. 5 tensile test specimens and performing a tensile test.

[0093] 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.

[0094] Hole expandability (HER) (%) = {(D - D0) / D0} × 100

[0095] (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).)

[0096] LME characteristics were evaluated by measuring the occurrence of type B LME cracks after spot welding according to the SEP 1220-2 standard. If no type B LME cracks occurred, the LME characteristics were evaluated as good, and if they did, the characteristics were evaluated as poor.

[0097] Steel grade No. Alloy composition (weight %) CSiMnCrTiNbMoBSol.AlPSN Relationship 110.2221.122.680.630.040.020.090.00210.0210.0110.0040.0052.620.2521.482.400.050.110.030.100.00180.0180.0110.0040.0052.730.3110.382.690.530.050.040.120.00220.0500.0110.0030.0053.2 [Relationship 1] 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B

[0098] Classification Steel grade No. Ar3 (℃) Finish rolling temperature (℃) Coiling temperature (℃) Continuous annealing temperature (SS) (℃) Dew point temperature (℃) First cooling end temperature (℃) First cooling rate (℃ / s) Second cooling end temperature (Tq) (℃) Second cooling rate (℃ / s) Reheating / over-aging heat treatment temperature (Tp) (℃) Ms (℃) Relationship equation 2 Invention example 1 17809 125 1285 15.26 12320 432299 355 182 Comparative example 1 17808995 2385 1-56 32320 433299 355 182 Comparative example 2 17809 185 05848-407 11220 139 348 355 120 Invention example 2 178090 25218532.6692224435305355190Comparative Example 31780923503856-38682224934353355134Invention Example 328059155068518.6693220038300358186Invention Example 428058975068507.5 642325030300358204Comparative Example 428059125438565.866232473041135869Comparative Example 52805913532856-4263232473041135869Comparative Example 62727915521853-42672315040300 358168Comparative Example 737279165268456.5612320032300310177Comparative Example 83727918562845-42690320038300310177Comparative Example 93727906577852-49670315039300310161[Relationship 2] 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52[Relationship 1]

[0099] Classification Microstructure Decarburization depth (㎛) Microstructure of area within 5 ㎛ in thickness direction from the surface TM (area %) Residual γ (area %) FM (area %) F (area %) TM Lath Short-axis Average size (㎛) TM (area %) Residual γ (area %) FM (area %) F (area %) Invention example 1888221.445510049 Comparative example 1887321.3285852 Comparative example 2887231.3187661 Invention example 2878322.154460054 Comparative example 3878321.62826102 Invention example 3878.831.21.263370063 Honor 48610.11.62.31.659410059Comparative Example 48411.11.63.33.462380062Comparative Example 58411323.628211.14.92Comparative Example 6952211.1295212Comparative Example 7906221.757430057Comparative Example 8906222.1289632Comparative Example 9917111.3192611TM: Tempered Martensite, γ: Austenite, FM: Fresh Martensite, F: Ferrite

[0100] Classification Yield strength (MPa) Tensile strength (MPa) Elongation (%) Hole expandability (%) LME characteristics Invention example 11152150210.234 Good Comparative example 11182151110.431 Poor Comparative example 2122114329.829 Poor invention example 21142149310.634 Good Comparative example 3123514218.929 Poor invention example 31186150611.236 Good invention example 41127 152311.832 Good Comparison Example 4 1168137412.938 Good Comparison Example 5 1185138912.536 Bad Comparison Example 6 128515788.129 Bad Comparison Example 7 123515129.824 Good Comparison Example 8 124115328.923 Bad Comparison Example 9 124115119.521 Bad

[0101] As can be seen from Tables 1 to 4 above, in the case of Invention Examples 1 to 4 that satisfy the alloy composition and manufacturing conditions of the present invention, it can be seen that not only the mechanical properties but also the LME resistance are excellent by securing the steel sheet microstructure, decarburization depth, and surface layer microstructure targeted by the present invention.

[0102] In the case of Comparative Example 1, which does not satisfy the dew point temperature, it can be seen that the LME resistance is insufficient because the decarburization depth and surface microstructure targeted by the present invention are not secured.

[0103] In the case of Comparative Examples 2 and 3, which do not satisfy the dew point temperature and relational equation 2, it can be seen that the tensile strength and elongation are low and the LME resistance is insufficient because the decarburization depth and surface microstructure targeted by the present invention are not secured.

[0104] In the case of Comparative Example 4, which does not satisfy Tp and Equation 2, it can be seen that the tensile strength is low because the average size of the TM lath shortening targeted by the present invention is not secured.

[0105] In the case of Comparative Example 5, which does not satisfy the dew point temperature, Tp, and relational expression 2, it can be seen that the tensile strength is low and the LME resistance is insufficient because the average size of the TM's lath shortening, decarburization depth, and surface microstructure targeted by the present invention are not secured.

[0106] In the case of Comparative Example 6, which does not satisfy the dew point temperature and relational equation 2, it can be seen that the elongation is low and the LME resistance is insufficient because the steel plate microstructure targeted by the present invention, the average size of the TM lath shortening, the decarburization depth, and the surface microstructure are not secured.

[0107] In the case of comparative example 7, which does not satisfy C content, relation 1, and relation 2, it can be seen that the elongation and hole expandability are at low levels.

[0108] In the case of comparative examples 8 and 9, which do not satisfy C content, relation 1, and dew point temperature, relation 2, it can be seen that the elongation and pore expandability are low and the LME resistance is insufficient due to the failure to secure the decarburization depth and the surface microstructure.

[0109] 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 to 0.30%, Si: 2.50% or less (excluding 0%), 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%), Sol.Al: 0.010 to 0.10%, N: 0.0010 to 0.010%, Mo: 0.020 to 0.20%, B: 0.0010 to 0.0050%, Ti: 0.010 to 0.120%, Nb: 0.010 to 0.050%, and the remainder is composed of Fe and other unavoidable impurities. Satisfies the following relation 1, The microstructure is composed of area % tempered martensite: 70–95% and retained austenite: 3–15%. The above tempered martensite has an average size of the short axis of the lath of 3㎛ or less, Cold rolled steel sheet with a decarburization depth of 30 to 100 ㎛ in the thickness direction from the surface. [Relationship 1] 2.40 ≤ 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 accordance with claim 1, The above microstructure is a cold rolled steel sheet additionally containing at least one of fresh martensite: 10% or less and ferrite: 5% or less.

3. In accordance with claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a microstructure in an area of ​​5 ㎛ or less from the surface in the thickness direction, in terms of area%, of ferrite: 20 to 99% and the sum of at least one of fresh martensite and tempered martensite: 1 to 80%.

4. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a yield strength of 1100 MPa or more, a tensile strength of 1470 MPa or more, an elongation of 10% or more, and a hole expansion ratio (HER) of 25% or more.

5. 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%, Cr: 0.010 to 1.20%, P: 0.0010 to 0.10%, S: 0.010% or less (excluding 0%), Sol.Al: 0.010 to 0.10%, N: 0.0010 to 0.010%, Mo: 0.020 to 0.20%, B: 0.0010 to 0.0050%, Ti: 0.010 to 0.120%, Nb: 0.010 to 0.050%, with 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 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 750 to 900°C with a dew point temperature of 0 to 15°C; A step of first cooling the continuously annealed cold rolled steel sheet to 550 to 750°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 320°C at a cooling rate of 5 to 60°C / s; and A step of reheating and over-aging the second-cooled cold-rolled steel sheet at 200 to 400°C; A method for manufacturing a cold rolled steel sheet satisfying the following relational expression 2. [Relationship 1] 2.40 ≤ 7C + (1.3Si+Mn) / 6 + (Cr+1.2Mo) / 5 + 100B ≤ 3.0 [Relationship 2] 180 ≤ 0.3(SS-A1) + 0.85(Ms-Tq) - 1.22(Tp-Tq) + 52[Relationship 1] ≤ 216 (However, in the above relational expression 1, the content of each alloy element is in 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 and overaging heat treatment temperature, and A1 = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr, Ms = 539-423C-30.4Mn-12.1Cr-7.5Mo.) 6. In claim 5, The above slab heating is a method for manufacturing cold rolled steel sheets, which is performed at 1000 to 1350°C.

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

8. 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%.

9. 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 and overaging heat treatment.

Images