Cold-rolled steel sheet and method for producing same

Abstract

Provided are: an ultra-high-strength cold-rolled steel sheet having excellent elongation and hole expandability; and a method for producing the same.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof. Background Art

[0002] In recent years, in the field of the automotive industry, in order to control greenhouse gas emissions caused by global warming and improve the fuel efficiency and stability of vehicles, increasing attention has been paid to ensuring vehicle body lightweight and collision stability. Therefore, the demand for manufacturing technologies for ensuring ultra-high strength steel sheets is increasing day by day.

[0003] For component forming such as formability or weldability, automotive components using ultra-high strength steel sheets require not only strength but also excellent elongation and hole expansion properties. Generally, as the strength of the steel sheet increases, the stamping formability deteriorates. To overcome this problem, a method using transformation induced plasticity (TRIP) steel utilizing retained austenite is used.

[0004] (Patent Document 1) Patent Publication Gazette No. 2017-7015003 Summary of the Invention

[0005] (I) Technical Problems to be Solved

[0006] An object of one aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet having excellent elongation and hole expansion properties and a manufacturing method thereof.

[0007] Another object of the present invention is to provide an ultra-high strength cold-rolled steel sheet having excellent elongation, hole expansion properties, and hydrogen embrittlement resistance and a manufacturing method thereof.

[0008] The technical problems of the present invention are not limited to the above. Additional technical problems of the present invention are described in the entire content of the specification, and those skilled in the technical field to which the present invention pertains can easily understand the additional technical problems of the present invention based on the content described in the specification of the present invention.

[0009] (II) Technical Solutions

[0010] One embodiment of the present invention provides a cold-rolled steel sheet which, by weight %, comprises: C: 0.05 - 0.4%, Si: 0.1 - 3.0%, Al: 0.005 - 3.0%, Mn: 1.0 - 4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001 - 0.5%, B: 0.0001 - 0.003%, Nb: 0.001 - 0.05%, Ti: 0.001 - 0.05%, P: 0.04% or less (except 0%), S: 0.01% or less (except 0%), N: 0.01% or less (except 0%), the balance being Fe and other inevitable impurities, and which, by area %, comprises as the fine structure: ferrite: 30% or less (including 0%), retained austenite: more than 10% and 25% or less, tempered martensite: more than 40% and 80% or less, bainite: 40% or less (including 0%), fresh martensite: 5% or less (including 0%), and the value defined by the following relational expression 1 satisfies more than 145 and less than 160.

[0011] [Relational expression 1]

[0012] 573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73×[Ti] - 68×[Nb] + 100×[B]

[0013] (In the relational expression 1, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B] represent the weight % contents of the respective elements in parentheses.)

[0014] The value defined by the following relational expression 2 for the cold-rolled steel sheet can satisfy more than 155 and less than 175.

[0015] [Relational expression 2]

[0016] 630×[C] - 23×[Si] + 410×[P] - 150×[Cr] - 15×[Ni] + 300×[B]

[0017] (In the relational expression 2, the [C], [Si], [P], [Cr] and [Ni] represent the weight % contents of the respective elements in parentheses.)

[0018] The value defined by the following relational expression 3 for the cold-rolled steel sheet can satisfy 40 or more.

[0019] [Relational expression 3]

[0020] [P-El]2 + [RA]

[0021] (In the relation 3, [P-El] represents the Post-Elongation value of the uniaxial tensile test, and [RA] represents the percentage fraction of retained austenite.)

[0022] The cold-rolled steel sheet can satisfy the following relation 4.

[0023] [Relation 4]

[0024] 0.3 ppm ≤ IH2

[0025] (In the relation 4, IH2 represents the critical hydrogen amount for fracture of the cold-rolled steel sheet.)

[0026] In addition, another aspect of the present invention provides a method for manufacturing a cold-rolled steel sheet, which includes the following steps: reheating a steel slab, by weight%, the steel slab contains: C: 0.05 - 0.4%, Si: 0.1 - 3.0%, Al: 0.005 - 3.0%, Mn: 1.0 - 4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001 - 0.5%, B: 0.0001 - 0.003%, Nb: 0.001 - 0.05%, Ti: 0.001 - 0.05%, P: 0.04% or less (except 0%), S: 0.01% or less (except 0%), N: 0.01% or less (except 0%), the balance of Fe and other inevitable impurities, and the value defined by the following relation 1 satisfies being more than 145 and less than 160; hot finish rolling the reheated steel slab at 830 - 980 °C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 450 - 700 °C; cold rolling the coiled hot-rolled steel sheet; continuously annealing the cold-rolled steel sheet at a temperature of 800 - 900 °C so that the dew point temperature satisfies -45 °C or less; performing primary cooling on the continuously annealed steel sheet at an average cooling rate of less than 10 °C / second until the primary cooling termination temperature of 550 - 650 °C; performing secondary cooling on the steel sheet after the primary cooling at an average cooling rate of 10 °C / second or more until the secondary cooling termination temperature of 150 - 400 °C; and performing heat treatment on the steel sheet after the secondary cooling within the range of 350 - 480 °C.)

[0027] [Relation 1]

[0028] 573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73×[Ti] - 68×[Nb] + 100×[B]

[0029] (In the relational expression 1, [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb], and [B] represent the weight % contents of the respective elements within the brackets.)

[0030] The continuous annealing step can control the atmosphere in the continuous annealing furnace with a gas composed of more than 95% nitrogen and the balance hydrogen by volume.

[0031] (III) Beneficial effects

[0032] According to one aspect of the present invention, an ultra-high strength cold-rolled steel sheet excellent in elongation and hole expansion property and a manufacturing method thereof can be provided.

[0033] According to another aspect of the present invention, an ultra-high strength cold-rolled steel sheet excellent in elongation, hole expansion property, and hydrogen embrittlement resistance and a manufacturing method thereof can be provided.

[0034] The various beneficial advantages and effects of the present invention are not limited to the above, and can be more easily understood during the process of describing the specific embodiments of the present invention. Detailed embodiments

[0035] Hereinafter, embodiments of the present invention will be described. It will be obvious to those skilled in the art that various modifications can be made to the following embodiments without departing from the scope of the present invention. The following embodiments are for understanding the present invention, and the scope of rights of the present invention is not limited to the following embodiments, and should be determined by the scope of rights and their equivalents.

[0036] In addition, the terms used in this specification are for describing the present invention and are not for limiting the present invention. Furthermore, unless clearly indicated to the contrary in the relevant definitions, the singular forms used in this specification also include the plural forms.

[0037] The meaning of "comprising" or "including" used in the specification is to make the composition specific and does not exclude the existence or addition of other compositions.

[0038] Unless otherwise defined differently, all terms including technical terms and scientific terms used in this specification have the same meaning as commonly understood by those skilled in the art. Terms defined in the dictionary should be interpreted as having a meaning consistent with the relevant technical literature and the currently disclosed content.

[0039] In the prior art, a TRIP steel sheet introducing retained austenite is disclosed to ensure excellent formability of ultra-high strength steel with a tensile strength of 1470 MPa or more. However, in order to meet high formability in ultra-high strength, a large amount of Si and Al need to be added. As the content of Si increases, the possibility of liquid metal embrittlement (LME) occurring during spot welding increases, and there is a problem of increased manufacturing cost.

[0040] In particular, when Al having an effect similar to that of Si is added instead to prevent the occurrence of LME, in addition to the increased manufacturing cost, the transformation temperature of the steel is increased, thereby increasing the load during hot rolling. In addition, since a high soaking section (SS) temperature is required in quenching and partitioning (Q&P) steel that requires single-phase region annealing by increasing the Ac3 temperature, the present invention cannot be achieved by current equipment and devices. When all the equipment and devices are changed, it requires too high a cost, and there is also a problem that the equipment or device must be newly developed.

[0041] Therefore, it is necessary to develop an ultra-high strength steel with a tensile strength of 1470 MPa grade that has excellent elongation and hole expansion property under annealing heat treatment conditions at an operable level while restricting the addition amounts of C, Si, and Al in the steel sheet, but there has been no report so far.

[0042] To improve local formability, it is effective to reduce the hardness deviation between the fine structures constituting the steel. As an evaluation of industrial local formability, the widely conducted test is the measurement of the hole expansion ratio (HER). For the hole expansion ratio (HER), a test piece with a 10-mm diameter hole punched out by a punch is fixed on a die, and a conical punch is used to push up and expand the hole. The diameter of the expanded hole is measured at the moment when a crack penetrating the entire thickness occurs, thereby obtaining a value as shown in the following relational expression A. The detailed evaluation standard for the hole expansion ratio is based on ISO 16630.

[0043] [Relational expression A]

[0044] λ(HER)=(df - do) / do

[0045] (In the relational expression A, do represents the diameter of the initial hole, and df represents the diameter of the hole at the time of thickness fracture.)

[0046] Therefore, in order to manufacture a steel that ensures an ultra-high strength with a tensile strength of 1470 MPa or more, while ensuring excellent elongation and hole expansion properties and being mass-producible, the inventor of the present invention found that the above problems can be solved by precisely controlling the alloy composition, tissue fraction, and manufacturing conditions, thereby completing the present invention.

[0047] Hereinafter, a method for manufacturing an ultra-high strength cold-rolled steel sheet with excellent elongation and hole expansion properties according to an embodiment of the present invention will be described.

[0048] First, the alloy composition of the cold-rolled steel sheet according to an embodiment of the present invention will be described. The content of the alloy composition mentioned below refers to wt%.

[0049] C: 0.05 - 0.4%

[0050] Carbon (C) is an element that ensures the strength of steel through solid solution strengthening and precipitation strengthening, and is an effective element for ensuring high ductility by stabilizing retained austenite. When the content of C is less than 0.05%, a tensile strength of 1500 MPa cannot be obtained. When the content of C exceeds 0.4%, a steel sheet cannot be manufactured by cold rolling. Therefore, the content of C is appropriately 0.05 - 0.4%. Additionally, in terms of further improving the above effects, the lower limit of the content of C may be 0.2%, or the upper limit of the content of C may be 0.4%.

[0051] Si: 0.1 - 3.0%

[0052] Silicon (Si) is a useful element for increasing the strength of the steel sheet through solid solution strengthening and precipitation hardening. Since Si suppresses the formation of cementite, Si has the effect of promoting the enrichment of C in austenite, and retained austenite is generated after annealing, which is an essential element for increasing the strength and elongation of steel. When the content of Si is less than 0.1%, uniform elongation cannot be obtained due to the absence of retained austenite. On the other hand, when the content of Si exceeds 3.0%, the physical properties of the welded part deteriorate due to LME cracks, and the surface characteristics and plating properties of the steel material deteriorate. Therefore, the content of Si preferably has a range of 0.1 - 3.0%. In terms of further improving the above effects, the lower limit of the content of Si may be 0.3% or 0.45%, or may be 0.5%. Similarly, in terms of further improving the above effects, the upper limit of the content of Si may be 2.5%.

[0053] Al: 0.005 - 3.0%

[0054] Aluminum (Al) is an element that deoxidizes molten steel. Similar to Si, it has the effect of increasing the stability of austenite and is effective for increasing elongation.

[0055] When the content of Al is less than 0.005%, the deoxidation of the steel cannot be carried out sufficiently, and the cleanliness of the steel is impaired. On the other hand, when the content of Al exceeds 3.0%, due to the excessive content of Al, the transformation temperature rises significantly, and the fraction of ferrite increases, so it is impossible to achieve ultra-high strength. Therefore, in the present invention, the content of Al is set to 0.005 - 3.0%. Additionally, in terms of further improving the above effects, the lower limit of the content of Al can be 0.01%, or can be 0.02%. Similarly, in terms of further improving the above effects, the upper limit of the content of Al can be 2.0%, or can be 1.0%.

[0056] Mn: 1.0 - 4.0%

[0057] Manganese (Mn) is an element added to ensure strength. When the content of Mn is less than 1.0%, it is difficult to ensure strength. On the other hand, when the content of Mn exceeds 4.0%, the bainite transformation rate becomes slow, resulting in the formation of too much fresh martensite, so it is difficult to obtain a high hole expansion rate. In addition, banded structures caused by the segregation of Mn are formed, thus impairing the material uniformity and formability of the material. Therefore, the content of Mn is controlled within 1.0 - 4.0%. Additionally, in terms of further improving the above effects, the lower limit of the content of Mn can be 1.5%, or the upper limit of the content of Mn can be 3.5%.

[0058] Cr: 1.5% or less (including 0%)

[0059] Chromium (Cr) is an effective element for improving strength. The chromium (Cr) inhibits the formation of carbides, thus contributing to ensuring retained austenite. Additionally, when the content of Cr exceeds 1.5%, the local corrosion resistance becomes poor, and surface oxides are formed, so the phosphate treatability is impaired. Therefore, the content of Cr is controlled within 1.5% or less (including 0%). Additionally, in terms of further improving the above effects, the upper limit of the content of Cr can be 1.0%.

[0060] Mo: 0.001 - 0.5%

[0061] Molybdenum (Mo) improves the stability of Fe carbides, and the precipitates caused by Mo improve the hydrogen embrittlement resistance characteristics. In order to ensure the above effects, it is necessary to add more than 0.001% of Mo. On the other hand, when the content of Mo exceeds 0.5%, the transformation is inhibited, resulting in difficulty in introducing bainite structure, and Mo is a high-cost element, so the economy of the steel plate becomes poor. Therefore, in the present invention, the content of Mo is set in the range of 0.001 - 0.5%. Additionally, in terms of further improving the above effects, the lower limit of the content of Mo can be 0.07%, or the upper limit of the content of Mo can be 0.495%.

[0062] B: 0.0001 - 0.003%

[0063] Boron (B) strengthens the grain boundaries and inhibits the ferrite phase transformation during the cooling process after annealing. To achieve the above effects, the content of B is 0.0001%, and the addition amount is set to be above 0.0001%. On the other hand, when the content of the B exceeds 0.003%, the hot rollability decreases, and B accumulates excessively on the surface, thus hindering the plating property. Therefore, in the present invention, the content of the B is set to 0.0001 - 0.003%. Additionally, in terms of further improving the above effects, the lower limit of the content of the B can be 0.0005%, or the upper limit of the content of the B can be 0.0025%.

[0064] Nb: 0.001 - 0.05%

[0065] Niobium (Nb) forms alloy carbides and contributes to improving the strength through precipitation strengthening and microstructure refinement. To achieve the above effects, the content of Nb is set to be above 0.001%. On the other hand, when the content of Nb exceeds 0.05%, due to local grain fixation, the recrystallization is delayed, thus impairing the tissue uniformity. Therefore, in the present invention, the content of the Nb is set to 0.001 - 0.05%. Additionally, in terms of further improving the above effects, the lower limit of the content of the Nb can be 0.015%, or the upper limit of the content of the Nb can be 0.03%.

[0066] Ti: 0.001 - 0.05%

[0067] Titanium (Ti) is an element that combines with C or N to form fine precipitates and refines the old austenite grains to improve the strength and hydrogen embrittlement resistance characteristics.

[0068] When adding less than 0.001% of the Ti, it is difficult to obtain the effects of improving the strength and making the microstructure refined. On the other hand, when the content of the Ti exceeds 0.05%, due to the formation of excessive TiN, the castability is impaired, and due to local grain fixation, the recrystallization is delayed, thus impairing the tissue uniformity. Therefore, the content of the Ti preferably has a range of 0.001 - 0.05%. Additionally, in terms of further improving the above effects, the lower limit of the content of the Ti can be 0.015%, or the upper limit of the content of the Ti can be 0.03%.

[0069] P: 0.04% or less (except 0%)

[0070] Phosphorus (P) is included as an impurity and segregates at grain boundaries, thereby reducing toughness. Therefore, it is preferable to control the content of phosphorus at as low a level as possible. When too much of the said P is added, the toughness of the steel deteriorates. Thus, in the present invention, to prevent such a problem, the upper limit of the said P content is preferably limited to 0.04%. However, considering the inevitable inclusion as an impurity during the manufacturing process, 0% is excluded from the content of the said P. Additionally, in terms of further improving the above effects, the lower limit of the content of the said P can be 0.002%, or the upper limit of the content of the said P can be 0.0173%.

[0071] S: 0.01% or less (excluding 0%)

[0072] Sulfur (S) is included as an impurity in the steel in the same manner as the said P. S combines with Mn to form inclusions and reduces the hole expansion property, and can also reduce the weldability and hot rolling property. Therefore, it is advantageous to control it at as low a level as possible. Considering the inevitable inclusion, 0% is excluded from the content of S, but the upper limit of the content of the said S is limited to 0.01% or less. Additionally, in terms of further improving the above effects, the lower limit of the content of the said S can be 0.0009%, or the upper limit of the content of the said S can be 0.005%.

[0073] N: 0.01% or less (excluding 0%)

[0074] In the present invention, nitrogen (N) is included as an impurity in the steel, and it is advantageous to control the content of nitrogen at as low a level as possible. Therefore, for the lower limit of the N content, considering the inevitable inclusion of N, 0% is excluded (i.e., more than 0%). However, the upper limit of the N content is preferably limited to 0.01%. Additionally, in terms of further improving the above effects, the lower limit of the content of the said N can be 0.0005%. Similarly, in terms of further improving the above effects, the upper limit of the content of the said N can be 0.007%, or can be 0.006% or 0.0052%.

[0075] In addition to the above steel composition, the balance of Fe and inevitable impurities may be included. Unwanted impurities inevitably mix in during conventional steel manufacturing processes, so these impurities cannot be completely excluded, and those skilled in the art of conventional steel manufacturing can easily understand their meaning. Furthermore, in the present invention, the addition of other compositions in addition to the above steel composition is not completely excluded.

[0076] According to one embodiment of the present invention, although not particularly limited, the cold-rolled steel sheet may selectively further include one or more selected from Cu: 0.1% or less (including 0%), Ni: 0.1% or less (including 0%).

[0077] Cu: 0.1% or less (including 0%), Ni: 0.1% or less (including 0%)

[0078] The copper (Cu) and nickel (Ni) are elements that improve the strength of steel. These elements are those that improve the strength and hardenability of steel, but when excessive amounts of Cu and Ni are added, it may exceed the desired strength grade, and since Cu and Ni are high-cost elements, economically, it is preferred to limit the upper limits of the contents of Cu and Ni to 0.1% or less respectively. In addition, the Cu and Ni act as solution strengthening elements, so when one or more of Cu and Ni less than 0.03% are added, the solution strengthening effect may be negligible, so it is preferred to add 0.03% or more respectively.

[0079] According to an embodiment of the present invention, although not particularly limited, the cold-rolled steel sheet may selectively further contain V: 0.05% or less (including 0%).

[0080] V: 0.05% or less (including 0%)

[0081] Vanadium (V) can also improve the strength of steel even when added in trace amounts, but has little effect on improving the elongation, so it is preferred to control the content of the V to 0.05% or less. Considering the elongation, the content of the V is more preferably 0.04% or less, and further preferably 0.03% or less.

[0082] The microstructure of the cold-rolled steel sheet according to an embodiment of the present invention may contain, in area %, ferrite: 30% or less (including 0%), retained austenite: more than 10% and 25% or less, tempered martensite: more than 40% and 80% or less, bainite: 40% or less (including 0%), and fresh martensite: 5% or less (including 0%).

[0083] Although not particularly limited, according to an embodiment of the present invention, the purpose of the cold-rolled steel sheet is to ensure excellent formability at a high tensile strength of 1470 MPa grade. In particular, in order to obtain high local formability, it is necessary to control the added elements and reduce the hardness difference between the phases constituting the microstructure of the steel sheet. In the present invention, by satisfying the above alloy composition under conventional annealing heating conditions and controlling the alloy composition so that the value defined by the following relational expression 1 satisfies more than 145 and less than 160, austenite single phase can be obtained and the ferrite fraction can be maintained at 30% or less. However, when the fraction of the ferrite exceeds 30%, the yield strength decreases and the hole expansion property deteriorates. In addition, in terms of ensuring high yield strength and excellent hole expansion property, more preferably, the upper limit of the fraction of the ferrite can be 10%, and further preferably, the upper limit of the fraction of the ferrite can be 7%.

[0084] [Relational Expression 1]

[0085] 573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73×[Ti] - 68×[Nb] + 100×[B]

[0086] (In the relation 1, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb], and [B] represent the weight % content of each element in the brackets.)

[0087] By adjusting the value defined by the relation 1 to be greater than 145 and less than 160, excessive formation of the soft ferrite phase can be avoided. However, when bainite, which is a soft phase second only to ferrite, is not sufficiently introduced, it may be difficult to ensure the ductility of the steel. Additionally, in terms of further improving the above effects, the lower limit of the value defined by the relation 1 can also be 146, or the upper limit of the value defined by the relation 1 can also be 159.

[0088] In addition, although there is no particular limitation, according to one embodiment of the present invention, the value defined by the following relation 2 can satisfy being greater than 155 and less than 175.

[0089] [Relation 2]

[0090] 630×[C] - 23×[Si] + 410×[P] - 150×[Cr] - 15×[Ni] + 300×[B]

[0091] (In the relation 2, the [C], [Si], [P], [Cr], and [Ni] represent the weight % content of each element in the brackets.)

[0092] In the cold-rolled steel sheet according to the present invention, in the main matrix containing more than 40% and 80% or less of tempered martensite and more than 10% and 25% or less of retained austenite, it contains: bainite: 40% or less (including 0%), newly formed martensite: 5% or less (including 0%). When the hardness difference between these main phases is large, the hole expansion property deteriorates, and when the retained austenite is insufficient, the elongation may decrease. Therefore, a solution for reducing the hardness difference between phases and ensuring the fraction of retained austenite is required.

[0093] Therefore, as a result of careful research by the present inventors, it has been found that in the relation 1 and relation 2, by controlling the composition of the additive elements and controlling the proportion of the main microstructure within an appropriate range, an ultra-high strength cold-rolled steel sheet with excellent elongation and hole expansion property can be manufactured.

[0094] Specifically, in the cold-rolled steel sheet according to the present invention, ferrite is preferably 30% or less (including 0%). When the area fraction of ferrite exceeds 30%, as the hardness difference between the soft ferrite and the hard main phases tempered martensite and bainite increases, it is difficult to ensure high hole expansion performance.

[0095] Retained austenite is preferably more than 10% and 25% or less. When the retained austenite is 10% or less, the amount of retained austenite is insufficient, making it difficult to ensure elongation. When the retained austenite exceeds 25%, phase transformation must be carried out at a high temperature, which has the effect of further improving elongation, but the fraction of tempered martensite is insufficient, so the strength at the target level (1470 MPa or more) cannot be obtained.

[0096] Bainite is preferably 40% or less. When bainite exceeds 40%, due to the insufficient amount of tempered martensite and retained austenite, the strength and elongation decrease. As the fraction of bainite decreases, the fraction of retained austenite decreases, so the desired elongation cannot be obtained. At this time, the lower limit of the bainite fraction may include 0%, or may exceed 0% or may be 7.4%.

[0097] In addition, although there is no particular limitation, according to an embodiment of the present invention, the material and tissue fraction of the cold-rolled steel sheet can be controlled so that the value defined by the following relational expression 3 satisfies 40 or more.

[0098] [Relational expression 3]

[0099] [P-El]2 + [RA]

[0100] (In the relational expression 3, the [P-El] represents the post-elongation value of the uniaxial tensile test, and [RA] represents the retained austenite fraction (area %).)

[0101] P-El represents the difference value between T-El and U-El and is a value representing local formability. In addition, the retained austenite fraction can be controlled so that the value defined by the relational expression 3 related to elongation satisfies 40 or more. Therefore, under normal annealing conditions, an ultra-high strength steel sheet with excellent elongation and hole expansion performance can be manufactured.

[0102] In addition, although there is no particular limitation, according to an embodiment of the present invention, the following relational expression 4 can be satisfied, whereby a cold-rolled steel sheet with excellent hydrogen embrittlement resistance can be ensured.

[0103] [Relational expression 4]

[0104] 0.3 ppm ≤ IH2

[0105] (In the relational expression 4, the IH2 represents the fracture critical hydrogen amount of the cold-rolled steel sheet.)

[0106] In addition, a hot-dip galvanized layer may be formed on at least one surface of the cold-rolled steel sheet of the present invention. In the present invention, the composition of the hot-dip galvanized layer is not particularly limited, and as long as it is a hot-dip galvanized layer commonly used in the technical field, it can be preferably applied to the present invention. In addition, the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer alloyed with some alloy components of the steel sheet.

[0107] Hereinafter, a method for manufacturing an ultra-high strength cold-rolled steel sheet excellent in elongation and hole expansion property according to one aspect of the present invention will be described. However, it does not mean that the cold-rolled steel sheet of the present invention must be produced only by the following manufacturing method.

[0108] According to an embodiment of the present invention, the manufacturing method of the cold-rolled steel sheet may be carried out in the order of reheating, hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, heat treatment, etc. of the steel having the above alloy composition.

[0109] [Reheating of the steel billet]

[0110] There is no limitation on the manufacturing method of the slab for hot rolling. As an example, a continuous casting slab can be used, and as other examples, slabs manufactured by a thin slab caster or the like can also be used. In addition, hot rolling can also be carried out immediately after continuous casting. When reheating the slab, the reheating temperature is preferably 1150 - 1250 °C. When the heating temperature is lower than 1150 °C, the finish rolling temperature is likely to be lower than 850 °C, and the rolling load increases. Considering the manufacturing cost, the heating temperature is preferably set to be lower than 1250 °C.

[0111] [Hot rolling]

[0112] After that, at 830 - 980 °C, the reheated slab is hot finish rolled to obtain a hot-rolled steel sheet. When the hot finish rolling temperature (hereinafter, also referred to as "FDT") is lower than 830 °C, the rolling load is large and the shape defects increase, resulting in poor productivity. On the other hand, when the hot finish rolling temperature exceeds 980 °C, excessive high-temperature operation leads to an increase in oxides, so the surface quality will deteriorate. Therefore, the hot finish rolling temperature preferably has a range of 830 - 980 °C. The lower limit of the hot finish rolling temperature is more preferably 880 °C. The upper limit of the hot finish rolling temperature is more preferably 950 °C, and further preferably 930 °C.

[0113] [Coiling]

[0114] The hot-rolled steel sheet obtained by the above hot rolling is coiled at 450 - 700°C. When the coiling temperature (hereinafter, also referred to as "CT") exceeds 700°C, thick internal oxidation on the steel sheet surface is caused, and the pickling property may also decrease. In order to improve toughness by refining the diameter of effective grains and to improve the hole expansion property by homogenizing retained austenite, the lower limit of the coiling temperature is set at 450°C. Additionally, in terms of further improving the above effects, the lower limit of the coiling temperature is more preferably 480°C, and even more preferably 500°C. Similarly, the upper limit of the coiling temperature is more preferably 670°C, and even more preferably 640°C. Additionally, although not particularly limited, after the hot finish rolling, it can be cooled to the coiling temperature at an average cooling rate of 10 - 100°C / second. When the average cooling rate is less than 10°C / second, the hot rolling productivity decreases, and there may be a problem that a cooling medium with reduced cooling capacity must be deliberately used during actual production. When the average cooling rate exceeds 100°C / second, the temperature deviation inside the steel sheet is uneven, the shape deteriorates, and there may be a problem that the strength of the steel sheet becomes too high.

[0115] [Cold rolling]

[0116] The coiled hot-rolled steel sheet is cold-rolled. During the cold rolling, the cold reduction rate can be 30 - 60%. When the cold reduction rate is less than 30%, it is difficult to ensure the desired thickness accuracy, and it may also be difficult to correct the shape of the steel sheet. On the other hand, when the cold reduction rate exceeds 60%, the possibility of cracks occurring in the edge part of the steel sheet increases, and the cold rolling load may become too large. Therefore, the cold reduction rate preferably has a range of 30 - 60%.

[0117] [Continuous annealing]

[0118] The cold-rolled steel sheet is continuously annealed within a temperature range of 800 - 900°C so that the dew point temperature satisfies -45°C or lower. The continuous annealing step is to heat the steel sheet to the austenite single-phase region to form nearly 100% austenite for subsequent phase transformation. When the continuous annealing temperature (hereinafter, also referred to as "SS") is lower than 800°C, sufficient recrystallization and austenite phase transformation cannot be achieved, so the desired fractions of martensite and bainite cannot be ensured after annealing. On the other hand, when the continuous annealing temperature exceeds 900°C, the productivity decreases, and coarse austenite is formed, so the material properties may deteriorate, and the surface quality will deteriorate, such as peeling of plating materials, etc. Additionally, the continuous annealing can be carried out in a continuous alloying hot-dip galvanizing continuous furnace.

[0119] In addition, during the continuous annealing, it is preferable to use a gas composed of 95% or more nitrogen and the balance hydrogen by volume% to control the atmosphere in the continuous annealing furnace. When the fraction of the nitrogen is less than 95% and the proportion of hydrogen does not increase correspondingly, an oxidizing atmosphere is formed in the furnace, resulting in the formation of oxides on the steel plate surface and deteriorating the surface quality. When the proportion of hydrogen increases, it will increase the process difficulties such as explosion prevention.

[0120] [Primary cooling]

[0121] After that, the steel plate that has undergone the continuous annealing is subjected to primary cooling at an average cooling rate of less than 10 °C / second (more preferably, 1 °C / second or more and less than 10 °C / second) and cooled to a primary cooling termination temperature of 550 - 650 °C (hereinafter, also referred to as "SCS").

[0122] The primary cooling termination temperature can be defined as the starting point for additionally applying a rapid cooling device not applied in the primary cooling and starting the secondary cooling (rapid cooling). When the cooling process is divided into primary cooling and secondary cooling for segmented processing, the temperature distribution of the steel plate is made uniform in the slow cooling step. Therefore, the deviation of the final temperature and material can be reduced, and the desired phase composition can be obtained. When the primary cooling termination temperature is lower than 550 °C, the bainite fraction becomes too high, and due to the length of the actual equipment, it is difficult to cool to below 550 °C at a cooling rate of less than 10 °C / second. That is, in order to cool to below 550 °C, it may be necessary to change the equipment or add additional settings, and there may be a problem of additional costs. In addition, when the primary cooling termination temperature exceeds 650 °C, the necessary cooling temperature until the secondary cooling termination temperature becomes higher, resulting in poor steel plate shape, and the bainite fraction may be lower than the target level.

[0123] [Secondary cooling]

[0124] Thereafter, the steel sheet that has undergone the primary cooling is subjected to secondary cooling at an average cooling rate of 10°C / second or more, and cooled to a secondary cooling termination temperature of 150 - 400°C (hereinafter, also referred to as "RCS"). The secondary cooling termination temperature is set to be below the Ms temperature of the steel sheet, such that martensite phase transformation occurs during the cooling process, and this martensite undergoes a reheat step as a subsequent process and finally becomes tempered martensite phase. When the secondary cooling termination temperature is lower than 150°C, the initial martensite phase transformation amount is excessive, so the yield strength increases excessively and the formability deteriorates. On the other hand, when the secondary cooling termination temperature exceeds 400°C, martensite is not generated during the cooling process, so a large amount of fresh martensite is finally generated, which may exceed the appropriate tensile strength, so it is difficult to obtain a high yield strength and a high hole expansion ratio. When the secondary cooling rate is less than 10°C / second, even when the desired secondary cooling termination temperature is reached, high-temperature phase transformation occurs during the cooling process, so the desired martensite fraction and high strength cannot be obtained. In terms of enhancing the above effects, more preferably, the lower limit of the secondary cooling rate can be 20°C / second, and the upper limit of the secondary cooling rate can be 60°C / second.

[0125] As described above, a rapid cooling device that was not applied in the primary cooling can be additionally applied during the secondary cooling. In the present invention, the type of the rapid cooling device is not particularly limited, but as a preferred example, a hydrogen rapid cooling device can be used. More specifically, the hydrogen rapid cooling device can use a gas composed of 50 - 80% hydrogen and the balance nitrogen by volume%. When the fraction of hydrogen exceeds 80%, there may be a disadvantage that management such as explosion control of the device becomes difficult. When the fraction of hydrogen is less than 50%, there may be a disadvantage that it is difficult to utilize the effective heat transfer characteristics of hydrogen as a light element.

[0126] [Heat treatment]

[0127] Thereafter, the steel sheet that has undergone the secondary cooling is reheated to 350 - 480°C. Through this process, interphase carbon partitioning required for obtaining stable retained austenite and additional bainite phase transformation are achieved. In the present invention, for convenience, the end temperature of the heating range is referred to as the reheat temperature (hereinafter, also referred to as "RHS"). When the reheat temperature is lower than 350°C, the strength becomes too high and the elongation deteriorates. On the other hand, when the reheat temperature exceeds 480°C, the austenite phase cannot transform and remains, and finally becomes fresh martensite during the cooling process, and the hole expansion ratio and elongation are impaired. Additionally, the so-called nose temperature at which bainite phase transformation is most active is at a level of about 400 - 420°C. Considering this, the lower limit of the reheat temperature is more preferably 410°C, or the upper limit of the reheat temperature is more preferably 440°C.

[0128] In addition, according to an embodiment of the present invention, after the reheating step, if necessary, a process of hot-dip galvanizing, alloyed hot-dip galvanizing, and skin pass rolling of the reheated steel sheet may be further performed. Specifically, it may further include a step of plating the reheated steel sheet in a zinc plating bath at 450 - 470 °C.

[0129] In addition, according to an embodiment of the present invention, if necessary, it may further include a step of subjecting the plated steel sheet to an alloying heat treatment at a temperature in the range of 470 - 550 °C. The alloying heat treatment is to obtain an appropriate alloying level, and its temperature depends on the surface state of the steel sheet. By controlling the surface state of the steel, the alloying heat treatment temperature must not exceed 550 °C to prevent softening of the steel sheet and disappearance of retained austenite due to over-tempering. Additionally, to perform alloying quickly, the alloying heat treatment temperature is preferably higher than the hot-dip galvanizing temperature, so the lower limit of the alloying heat treatment temperature is controlled at 470 °C. And after the alloying heat treatment, to correct the shape of the steel sheet and adjust the yield strength, it may further include a step of skin pass rolling the alloying heat-treated steel sheet after cooling it to room temperature at a reduction rate of less than 1%. Detailed implementation mode

[0130] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention for more specific description and are not used to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the content recorded in the claims and the content reasonably deduced therefrom.

[0131] (Example)

[0132] After preparing a slab having the alloy compositions described in Tables 1 to 2 below, it is reheated at 1180 - 1220 °C, and hot rolled, coiled, cold rolled, continuously annealed, first cooled, second cooled, and heat treated under the conditions described in Tables 3 to 4 below to manufacture a cold rolled steel sheet. Additionally, during the continuous annealing, the atmosphere of the continuous annealing furnace is controlled to be a gas composed of 95% nitrogen and the balance hydrogen by volume%, and the dew point temperature at a temperature of 800 - 900 °C is controlled at -45 °C.

[0133] The evaluation results of the tensile properties, elongation, and hole expansion of the cold-rolled steel sheets manufactured as described above are shown in Table 5 below. The tensile strength (TS), yield strength (YS), and elongation (EL) are measured by a tensile test in the direction perpendicular to rolling, using a specimen standard with a gauge length of 50 mm and a width of 25 mm for the tensile specimen. The hole expansion (HER) is measured according to the ISO 16330 standard. For the hole, a punch with a diameter of 10 mm is used for shearing with a clearance of 12%.

[0134] In addition, to evaluate hydrogen embrittlement resistance, an experiment is conducted by impregnating in a 0.1 N HCl solution for 120 hours under an applied stress of 80% of the tensile strength (TS) after four-point bending. The critical hydrogen amount for fracture is measured among the steel grades that do not fracture after 120 hours of impregnation and is shown in Table 5 below. At this time, if fracture occurs after 120 hours of impregnation, it indicates that the hydrogen embrittlement resistance standard is not met, that is, it is evaluated as "X", and when no fracture occurs, it is evaluated as "O".

[0135] In addition, for the manufactured cold-rolled steel sheets, the results of measuring the microstructure and the calculation results of relational expression 3 used in the present invention are shown in Table 5.

[0136] [Table 1]

[0137]

[0138] [Table 2]

[0139]

[0140] [Table 3]

[0141]

[0142] [Table 4]

[0143]

[0144] [Table 5]

[0145]

[0146] IH2 : Critical hydrogen amount for fracture

[0147] F: Area ratio of ferrite

[0148] γ: Area ratio of retained austenite

[0149] TM: Area ratio of tempered martensite

[0150] FM: Area ratio of fresh martensite

[0151] B: Area ratio of bainite

[0152] In the case of the inventive examples satisfying the alloy composition and manufacturing conditions of the present invention, it was confirmed that, compared with the comparative examples, they had higher tensile strength (TS), yield strength (YS), hole expansion ratio (HER), and elongation (El), and had excellent elongation and hole expansion ratio as well as ultra-high strength characteristics.

[0153] On the other hand, in the case of the comparative examples not satisfying one or more of the alloy composition and manufacturing conditions of the present invention, it was confirmed that one or more of the properties selected from tensile strength (TS), yield strength (YS), hole expansion ratio (HER), and elongation (El) deteriorated.

Claims

1. A cold-rolled steel sheet, by weight %, the cold-rolled steel sheet contains: C: 0.05 - 0.4%, Si: 0.1 - 3.0%, Al: 0.005 - 3.0%, Mn: 1.0 - 4.0%, Cr: 1.5% or less including 0%, Mo: 0.001 - 0.5%, B: 0.0001 - 0.003%, Nb: 0.001 - 0.05%, Ti: 0.001 - 0.05%, P: 0.04% or less excluding 0%, S: 0.01% or less excluding 0%, N: 0.01% or less excluding 0%, the balance of Fe and other inevitable impurities, by area %, as the fine structure contains: ferrite: 30% or less including 0%, retained austenite: more than 10% and 25% or less, tempered martensite: more than 40% and 80% or less, bainite: 40% or less including 0%, fresh martensite 5% or less including 0%, the value defined by the following relational expression 1 satisfies more than 145 and less than 160, [Relational expression 1] 573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73×[Ti] - 68×[Nb] + 100×[B] In the relational expression 1, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B] represent the weight % content of each element in the brackets.

2. The cold-rolled steel sheet according to claim 1, wherein, the value defined by the following relational expression 2 satisfies more than 155 and less than 175, [Relational expression 2] 630×[C] - 23×[Si] + 410×[P] - 150×[Cr] - 15×[Ni] + 300×[B] In the relational expression 2, the [C], [Si], [P], [Cr] and [Ni] represent the weight % content of each element in the brackets.

3. The cold-rolled steel sheet according to claim 1, wherein, the value defined by the following relational expression 3 satisfies 40 or more, [Relational expression 3] [P-El]2 + [RA] In the relational expression 3, the [P-El] represents the post-elongation value of the uniaxial tensile test, and [RA] represents the fraction % of retained austenite.

4. The cold-rolled steel sheet according to claim 1, wherein, the cold-rolled steel sheet satisfies the following relational expression 4, [Relational expression 4] 0.3 ppm ≤ IH2 In the relational expression 4, the IH2 represents the fracture critical hydrogen amount of the cold-rolled steel sheet.

5. A method for manufacturing a cold-rolled steel sheet, which includes the following steps: The steel billet is reheated. By weight percentage, the steel billet contains: C: 0.05 - 0.4%, Si: 0.1 - 3.0%, Al: 0.005 - 3.0%, Mn: 1.0 - 4.0%, Cr: 1.5% or less including 0%, Mo: 0.001 - 0.5%, B: 0.0001 - 0.003%, Nb: 0.001 - 0.05%, Ti: 0.001 - 0.05%, P: 0.04% or less excluding 0%, S: 0.01% or less excluding 0%, N: 0.01% or less excluding 0%, the balance being Fe and other inevitable impurities, and the value defined by the following relational expression 1 satisfies being more than 145 and less than 160; At 830 - 980 °C, the reheated steel billet is hot finish rolled to obtain a hot rolled steel plate; At 450 - 700 °C, the hot rolled steel plate is coiled; The coiled hot rolled steel plate is cold rolled; At a temperature of 800 - 900 °C, the cold rolled steel plate is continuously annealed so that the dew point temperature satisfies -45 °C or less; The continuously annealed steel plate is first cooled at an average cooling rate of less than 10 °C / second to a first cooling termination temperature of 550 - 650 °C; The first cooled steel plate is second cooled at an average cooling rate of 10 °C / second or more to a second cooling termination temperature of 150 - 400 °C; and In the range of 350 - 480 °C, the second cooled steel plate is heat treated, [Relational Expression 1] 573×[C] - 45×[Si] + 25×[Mn] - 95×[Al] + 318×[Cr] - 59×[Ni] - 83×[Mo] - 5×[Cu] - 73×[Ti] - 68×[Nb] + 100×[B] In the relational expression 1, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B] represent the weight percentage contents of the respective elements in the brackets.

6. The manufacturing method of the cold-rolled steel sheet according to claim 5, wherein, The continuous annealing step controls the atmosphere in the continuous annealing furnace with a gas composed of 95% or more of nitrogen and the balance of hydrogen by volume percentage.

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