Cold-rolled steel sheet and its manufacturing method

Abstract

The present invention provides 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 method for producing the same. [Background technology]

[0002] In recent years, the automotive industry has been focusing on reducing the weight of vehicle bodies and ensuring collision stability in order to improve fuel efficiency and stability of vehicles, in addition to regulating greenhouse gas emissions associated with global warming. This has led to an increased demand for the development of manufacturing technologies for ultra-high strength steel sheets.

[0003] Automotive parts that use ultra-high strength steel sheets require not only strength but also excellent elongation and hole expandability for part forming, such as formability and weldability. Generally, as the strength of steel sheets increases, press formability deteriorates, but to overcome this, a method is being used that uses TRIPS (Transformation Induced Plasticity) steel that utilizes retained austenite. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Korean Patent Publication No. 2017-7015003 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of one embodiment of the present invention is to provide an ultra-high strength cold rolled steel sheet having excellent elongation and hole expandability, and a manufacturing method thereof.

[0006] Another embodiment of the present invention aims to provide an ultra-high strength cold rolled steel sheet that is excellent not only in elongation and hole expandability but also in hydrogen embrittlement resistance, and a method for manufacturing the same.

[0007] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the general content of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0008] One embodiment of the present invention comprises: In weight percent, C: 0.05 to 0.4%, Si: 0.1 to 3.0%, Al: 0.005 to 3.0%, Mn: 1.0 to 4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001 to 0.5%, B: 0.0001 to 0.003%, Nb: 0.001 to 0.05%, Ti: 0.001 to 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 unavoidable impurities. The microstructure includes, by area%, ferrite: 30% or less (including 0%), retained austenite: more than 10% and less than 25%, tempered martensite: more than 40% and less than 80%, bainite: 40% or less (including 0%), and fresh martensite: 5% or less (including 0%). Provided is a cold-rolled steel sheet in which the value defined by the following relational expression 1 is greater than 145 and less than 160. [Equation 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 above Relational Formula 1, the above [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb], and [B] represent the weight percent content of each element in parentheses.)

[0009] The cold-rolled steel sheet can satisfy the value defined by the following relational expression 2 of more than 155 and less than 175. [Equation 2] 630×[C]-23×[Si]+410×[P]-150×[Cr]-15×[Ni]+300×[B] (In the above Relational Formula 2, the above [C], [Si], [P], [Cr], and [Ni] represent the weight percent content of each element in parentheses.) The cold-rolled steel sheet can satisfy the value defined by the following relational expression 3 of 40 or more. [Equation 3] [P-El]2+[RA] (In the above relational expression 3, [P-El] represents the post-elongation value in the uniaxial tensile test, and [RA] represents the fraction of retained austenite (%).)

[0010] The cold-rolled steel sheet can satisfy the following relational expression 4. [Equation 4] 0.3 ppm or less IH2 (In the above relational expression 4, IH2 represents the critical hydrogen content for fracture of the cold-rolled steel sheet.)

[0011] Another embodiment of the present invention is reheating a steel slab containing, by weight percent, C: 0.05 to 0.4%, Si: 0.1 to 3.0%, Al: 0.005 to 3.0%, Mn: 1.0 to 4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001 to 0.5%, B: 0.0001 to 0.003%, Nb: 0.001 to 0.05%, Ti: 0.001 to 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 unavoidable impurities, and satisfying a value defined by the following relational expression 1 of more than 145 and less than 160; Finish hot rolling the reheated steel slab at 830 to 980 ° C to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet at 450 to 700 ° C; cold rolling the coiled hot-rolled steel sheet; Continuously annealing the cold-rolled steel sheet at a temperature of 800 to 900 ° C. so that the dew point temperature is −45 ° C. or less; The continuously annealed steel sheet is subjected to primary cooling at an average cooling rate of less than 10 ° C. / s to a primary cooling end temperature of 550 to 650 ° C.; Secondary cooling the primarily cooled steel sheet to a secondary cooling end temperature of 150 to 400 ° C. at an average cooling rate of 10 ° C. / s or more; The method for manufacturing a cold-rolled steel sheet includes a step of heat-treating the second-cooled steel sheet at a temperature in the range of 350 to 480°C. [Equation 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 above Relational Formula 1, the above [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb], and [B] represent the weight percent content of each element in parentheses.)

[0012] In the continuous annealing step, the atmosphere in the continuous annealing furnace can be controlled with a gas consisting of, by volume, 95% or more nitrogen and the remainder hydrogen. [Effects of the Invention]

[0013] According to one embodiment of the present invention, an ultra-high strength cold rolled steel sheet having excellent elongation and hole expandability and a manufacturing method thereof can be provided.

[0014] According to another embodiment of the present invention, it is possible to provide an ultra-high strength cold rolled steel sheet that is excellent not only in elongation and hole expandability but also in hydrogen embrittlement resistance, and a manufacturing method thereof.

[0015] The various advantageous and beneficial effects of the present invention are not limited to the above-mentioned contents, but will be more easily understood in the course of describing specific embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present invention will be described. It goes without saying that those skilled in the art can make various modifications to the following embodiments without departing from the scope of the present invention. The following embodiments are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following embodiments, but should be defined by the claims below as well as equivalents thereto.

[0017] However, the terms used herein are for the purpose of describing the present invention and are not intended to limit the present invention. Furthermore, the singular forms used herein also include the plural forms unless the relevant definition clearly indicates otherwise.

[0018] The meaning of "comprises" as used in the specification is to specify features and does not exclude the presence or addition of other features.

[0019] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0020] Prior art discloses TRIP steel sheets incorporating retained austenite to ensure excellent formability in ultra-high strength steels with tensile strengths of 1470 MPa or higher. However, achieving both ultra-high strength and high formability requires the addition of large amounts of Si and Al. The higher the Si content, the greater the likelihood of LME (Liquid Metal Embrittlement) occurring during spot welding, which increases manufacturing costs.

[0021] In particular, adding Al, which has a similar effect to Si, to prevent LME not only increases manufacturing costs but also raises the transformation temperature of the steel, increasing the load during hot rolling. Furthermore, because a high SS (Soaking Section) temperature is required for Q&P (Quenching & Partitioning) steel, which requires single-phase annealing by raising the Ac3 temperature, the invention cannot be realized with current equipment and devices. Changing all of the equipment and devices would be excessively costly, and new equipment and devices would also need to be developed.

[0022] Therefore, it is necessary to develop an ultra-high strength steel with a tensile strength of 1470 MPa that has excellent elongation and hole expandability under annealing heat treatment conditions at an operational level while limiting the amounts of C, Si, and Al added to the steel sheet, but no such steel has been reported to date.

[0023] To improve local formability, it is effective to reduce the hardness deviation between the microstructures that make up the steel. Hole Expansion Ratio (HER) measurement is a widely used test for evaluating industrial local formability. HER is measured by punching a 10mm diameter hole in a test specimen, fixing it to a die, and expanding the hole by pushing it upward with a conical punch. The diameter of the expanded hole is measured when a crack penetrating the entire thickness occurs, and a value is obtained as shown in the following relational expression A. Detailed evaluation criteria for hole expansion are in accordance with ISO 16630. [Relationship A] λ(HER)=(df-do) / do (In the above relational expression A, do represents the initial hole diameter, and df represents the hole diameter at thickness break.)

[0024] As a result, the inventors discovered that this problem could be solved by precisely controlling the alloy composition, structure fraction, and manufacturing conditions in order to manufacture steel under mass-producible conditions while ensuring ultra-high strength of tensile strength of 1470 MPa or more and excellent elongation and hole expandability, and thus completed the present invention.

[0025] Hereinafter, an ultra-high strength cold rolled steel sheet having excellent elongation and hole expandability and a manufacturing method thereof according to one embodiment of the present invention will be described.

[0026] First, the alloy composition of the cold-rolled steel sheet according to an embodiment of the present invention will be described. The contents of the alloy components mentioned below are in percent by weight.

[0027] C: 0.05 to 0.4% Carbon (C) is an element that ensures the strength of steel through solid solution strengthening and precipitation strengthening, and is an effective element for stabilizing retained austenite and ensuring high elongation. If the C content is less than 0.05%, a tensile strength of 1500 MPa cannot be obtained, and if it exceeds 0.4%, it is not possible to produce steel sheets by cold rolling. Therefore, the C content is preferably 0.05 to 0.4%. On the other hand, in order to further improve the above-mentioned effects, the lower limit of the C content may be 0.2%, or the upper limit of the C content may be 0.4%.

[0028] Si: 0.1 to 3.0% Silicon (Si) is a useful element for increasing the strength of steel sheets through solid-solution strengthening and precipitation hardening. It inhibits the formation of cementite and promotes the enrichment of C in austenite. It is an essential element for generating retained austenite after annealing, thereby increasing the strength and elongation of steel. If the Si content is less than 0.1%, no retained austenite remains, making it impossible to obtain a uniform elongation. On the other hand, if the Si content exceeds 3.0%, LME cracking occurs, deteriorating the physical properties of the weld, and the surface properties and platability of the steel deteriorate. Therefore, the Si content is preferably in the range of 0.1 to 3.0%. To further improve the aforementioned effects, the lower limit of the Si content may be 0.3%, 0.45%, or 0.5%. Similarly, to further improve the aforementioned effects, the upper limit of the Si content may be 2.5%.

[0029] Al: 0.005 to 3.0% Aluminum (Al) is an element that has a deoxidizing effect on molten steel, and like Si, it acts to improve the stability of austenite and is effective in increasing the elongation ratio. If the Al content is less than 0.005%, the steel material will not be sufficiently deoxidized, impairing the cleanliness of the steel material. On the other hand, if the Al content is excessive, such as exceeding 3.0%, the transformation temperature will rise significantly, the ferrite fraction will increase, and ultra-high strength will not be achieved. Therefore, in the present invention, the Al content is set to 0.005 to 3.0%. On the other hand, in order to further improve the above-mentioned effects, the lower limit of the Al content may be 0.01% or 0.02%. Similarly, in order to further improve the above-mentioned effects, the upper limit of the Al content may be 2.0% or 1.0%.

[0030] Mn: 1.0 to 4.0% Manganese (Mn) is an element added to ensure strength. If the Mn content is less than 1.0%, it becomes difficult to ensure strength. On the other hand, if the Mn content exceeds 4.0%, the bainite transformation rate slows, and too much fresh martensite is formed, making it difficult to obtain high hole expandability. In addition, band structures are formed due to Mn segregation, impairing the quality uniformity and formability of the material. Therefore, the Mn content is controlled to 1.0 to 4.0%. On the other hand, to further improve the above-mentioned effects, the lower limit of the Mn content may be 1.5%, or the upper limit of the Mn content may be 3.5%.

[0031] Cr: 1.5% or less (including 0%) Chromium (Cr) is an element effective in improving strength. It suppresses the formation of carbides and facilitates the retention of retained austenite. On the other hand, if the Cr content exceeds 1.5%, local corrosion resistance deteriorates and the phosphate treatability is impaired due to the formation of surface oxides. Therefore, the Cr content is controlled to 1.5% or less (including 0%). On the other hand, to further improve the above-mentioned effects, the upper limit of the Cr content may be 1.0%.

[0032] Mo: 0.001 to 0.5% Molybdenum (Mo) improves the stability of Fe carbides, and Mo precipitates improve hydrogen embrittlement resistance. To ensure the aforementioned effects, it is necessary to add 0.001% or more of Mo. On the other hand, if the Mo content exceeds 0.5%, phase transformation is suppressed, making it difficult to introduce a bainite structure, and Mo, being an expensive element, reduces the economic viability of the steel sheet. Therefore, in the present invention, the Mo content is set to a range of 0.001 to 0.5%. To further improve the aforementioned effects, the lower limit of the Mo content may be 0.07%, or the upper limit of the Mo content may be 0.495%.

[0033] B: 0.0001 to 0.003% Boron (B) strengthens grain boundaries and suppresses ferrite transformation during cooling after annealing. To achieve the above-mentioned effects, the B content is set to 0.0001% or more. On the other hand, if the B content exceeds 0.003%, hot rolling properties decrease and B accumulates excessively on the surface, hindering platability. Therefore, in the present invention, the B content is set to 0.0001 to 0.003%. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the B content may be 0.0005%, or the upper limit of the B content may be 0.0025%.

[0034] Nb: 0.001 to 0.05% Niobium (Nb) forms alloy carbides and contributes to improving strength through precipitation strengthening and microstructural refinement. To achieve the above-mentioned effects, the Nb content is set to 0.001% or more. On the other hand, if the Nb content exceeds 0.05%, localized grain fixation retards recrystallization and impairs the uniformity of the microstructure. Therefore, in the present invention, the Nb content is set to 0.001 to 0.05%. On the other hand, to further improve the above-mentioned effects, the lower limit of the Nb content may be 0.015%, or the upper limit of the Nb content may be 0.03%.

[0035] Ti: 0.001 to 0.05% Titanium (Ti) is an element that combines with C or N to form fine precipitates, refines prior austenite grains, and improves strength and hydrogen embrittlement resistance. If Ti is added in an amount less than 0.001%, it is difficult to obtain the effects of improving strength and refining the structure. On the other hand, if the Ti content exceeds 0.05%, castability is impaired due to excessive formation of TiN, and recrystallization is delayed due to localized grain fixation, resulting in a loss of structural uniformity. Therefore, the Ti content is preferably in the range of 0.001 to 0.05%. On the other hand, in order to further improve the above-mentioned effects, the lower limit of the Ti content may be 0.015%, or the upper limit of the Ti content may be 0.03%.

[0036] P: 0.04% or less (0% excluded) Phosphorus (P) is contained as an impurity and segregates at grain boundaries, reducing toughness. Therefore, it is preferable to control its content as low as possible. If P is added in excess, the toughness of the steel material deteriorates. To prevent this, the present invention preferably limits its upper limit to 0.04%. However, in consideration of the case where P is inevitably mixed in as an impurity during the manufacturing process, 0% is excluded from the P content. On the other hand, in order to further improve the above-mentioned effects, the lower limit of the P content may be 0.002%, or the upper limit of the P content may be 0.0173%.

[0037] S: 0.01% or less (0% excluded) Sulfur (S), like P, is contained in steel as an impurity. S combines with Mn to form inclusions, which can reduce hole expandability and potentially reduce weldability and hot rolling properties. Therefore, it is advantageous to control the S content as low as possible. Taking into account cases where S is unavoidably contained, it is preferable to exclude 0% and limit the S content to 0.01% or less. On the other hand, in order to further improve the aforementioned effects, the lower limit of the S content may be 0.0009%, or the upper limit of the S content may be 0.005%.

[0038] N: 0.01% or less (0% excluded) In the present invention, nitrogen (N) is contained in the steel as an impurity, and it is advantageous to control its content as low as possible. Therefore, the lower limit of the N content excludes 0% (i.e., exceeds 0%), taking into account cases where N is unavoidably contained. However, it is preferable to limit the upper limit of the N content to 0.01%. On the other hand, in order to further improve the above-mentioned effects, the lower limit of the N content may be 0.0005%. Similarly, in order to further improve the above-mentioned effects, the upper limit of the N content may be 0.007%, 0.006%, or 0.0052%.

[0039] The remainder of the steel composition may include Fe and unavoidable impurities. Unavoidable impurities may be unintentionally mixed in during normal steel manufacturing processes, and they cannot be completely eliminated. This is easily understood by engineers in the field of normal steel manufacturing. Furthermore, the present invention does not completely exclude the addition of other components than the steel composition described above.

[0040] According to one embodiment of the present invention, although not particularly limited, the cold-rolled steel sheet may further include at least one selected from the group consisting of Cu: 0.1% or less (including 0%) and Ni: 0.1% or less (including 0%).

[0041] Cu: 0.1% or less (including 0%), Ni: 0.1% or less (including 0%) Copper (Cu) and nickel (Ni) are elements that increase the strength of steel. While these elements increase the strength and hardenability of steel, adding excessively large amounts of them may result in the steel exceeding the target strength grade. Because these elements are expensive, it is preferable to limit their upper limits to 0.1% or less from an economical standpoint. Meanwhile, because Cu and Ni act as solid-solution strengthening elements, adding at least one of Cu and Ni at less than 0.03% may result in insignificant solid-solution strengthening effects. Therefore, adding at least 0.03% of each of these elements is preferable.

[0042] According to an embodiment of the present invention, the cold rolled steel sheet may further include, but is not limited to, V: 0.05% or less (including 0%).

[0043] V: 0.05% or less (including 0%) Although vanadium (V) can increase the strength of steel materials even with a small amount of addition, its effect on improving the elongation rate is not significant, so its content is preferably controlled to 0.05% or less. Taking the elongation rate into consideration, the V content is more preferably 0.04% or less, and even more preferably 0.03% or less.

[0044] The microstructure of a cold-rolled steel sheet according to one embodiment of the present invention can include, in area percentages, ferrite: 30% or less (including 0%), retained austenite: more than 10% and not more than 25%, tempered martensite: more than 40% and not more than 80%, bainite: 40% or less (including 0%), and fresh martensite: 5% or less (including 0%).

[0045] Although not particularly limited, according to one embodiment of the present invention, the cold-rolled steel sheet has an objective of ensuring excellent formability even with a high tensile strength of 1470 MPa or higher. To achieve particularly high local formability, it is necessary to control the additive elements and reduce the hardness difference between the microstructural phases constituting the steel sheet. In the present invention, by satisfying the above-described alloy composition under normal annealing heating conditions and controlling the alloy composition so that the value defined by the following Relational Formula 1 is greater than 145 and less than 160, a single austenite phase can be obtained and the ferrite fraction can be maintained at a low level of 30% or less. However, if the ferrite fraction exceeds 30%, the yield strength decreases and hole expandability deteriorates. On the other hand, from the viewpoint of ensuring high yield strength and excellent hole expandability, the upper limit of the ferrite fraction may more preferably be 10%, and even more preferably be 7%. [Equation 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 above Relational Formula 1, the above [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb], and [B] represent the weight percent content of each element in parentheses.)

[0046] By adjusting the value defined by the above relational expression 1 to be greater than 145 and less than 160, it is possible to avoid the excessive formation of a soft ferrite phase, but if bainite, which is the softest phase after ferrite, is not introduced sufficiently, it may be difficult to ensure the ductility of the steel material. On the other hand, from the perspective of further improving the above-mentioned effects, the lower limit of the value defined by the above relational expression 1 may be 146, or the upper limit of the value defined by the above relational expression 1 may be 159.

[0047] Furthermore, although not particularly limited, according to one embodiment of the present invention, the value defined by the following relational expression 2 may satisfy a value greater than 155 and less than 175. [Equation 2] 630×[C]-23×[Si]+410×[P]-150×[Cr]-15×[Ni]+300×[B] (In the above Relational Formula 2, the above [C], [Si], [P], [Cr], and [Ni] represent the weight percent contents of each element in parentheses.)

[0048] The cold-rolled steel sheet according to the present invention has a main matrix containing more than 40% and not more than 80% tempered martensite and more than 10% and not more than 25% retained austenite, and also contains 40% or less (including 0%) bainite and 5% or less (including 0%) fresh martensite. If the hardness difference between these main phases is large, hole expandability may deteriorate, and if the retained austenite is insufficient, the elongation rate may decrease. Therefore, a method is needed to reduce the hardness difference between the phases and ensure the retained austenite fraction.

[0049] As a result of extensive research, the present inventors have discovered that by controlling the composition of the added elements and the proportion of the main structure within appropriate ranges in the above Relational Formulas 1 and 2, it is possible to manufacture an ultra-high strength cold-rolled steel sheet that is excellent in elongation and hole expandability.

[0050] Specifically, the cold-rolled steel sheet according to the present invention preferably has a ferrite area fraction of 30% or less (including 0%). If the ferrite area fraction exceeds 30%, the interphase hardness between the soft ferrite and the hard tempered martensite and bainite of the main phase increases, making it difficult to ensure high hole expandability.

[0051] The retained austenite content is preferably more than 10% and not more than 25%. If it is 10% or less, it becomes difficult to ensure the elongation rate due to the lack of retained austenite. If it exceeds 25%, it is necessary to carry out phase transformation at high temperatures, which has the effect of further improving the elongation rate, but the target strength level (1470 MPa or more) cannot be obtained due to the insufficient tempered martensite fraction.

[0052] The bainite content is preferably 40% or less. If it exceeds 40%, the strength and elongation rate decrease due to a lack of tempered martensite and retained austenite. As the bainite fraction decreases, the retained austenite fraction decreases, making it difficult to obtain the desired elongation rate. In this case, the lower limit of the bainite fraction may include 0%, exceed 0%, or be 7.4%.

[0053] Furthermore, although not particularly limited, according to one embodiment of the present invention, the material properties and structure 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. [Equation 3] [P-El]2+[RA] (In the above relational expression 3, [P-El] represents the post-elongation value in the uniaxial tensile test, and [RA] represents the fraction of retained austenite (area %).)

[0054] P-El is the difference between T-El and U-El, and is a value that indicates local formability. The retained austenite fraction can be controlled to satisfy the correlation with elongation defined by Relational Equation 3, which is 40 or greater. This allows for the production of ultra-high strength steel sheets with excellent elongation and hole expandability, even under normal annealing conditions.

[0055] Furthermore, although not particularly limited, according to one embodiment of the present invention, the following relational expression 4 can be satisfied, thereby ensuring a cold-rolled steel sheet with excellent resistance to hydrogen embrittlement. [Equation 4] 0.3 ppm or less IH2 (In the above relational expression 4, IH2 represents the critical hydrogen content for fracture of the cold-rolled steel sheet.)

[0056] Meanwhile, the cold-rolled steel sheet of the present invention may have a hot-dip galvanized layer formed on at least one surface. The present invention is not particularly limited in terms of the configuration of the hot-dip galvanized layer, and any hot-dip galvanized layer commonly used in the relevant technical field may 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.

[0057] Hereinafter, a method for manufacturing an ultra-high strength cold-rolled steel sheet having excellent elongation and hole expandability according to one embodiment of the present invention will be described. However, this does not necessarily mean that the cold-rolled steel sheet of the present invention must be manufactured only by the following manufacturing method.

[0058] According to one embodiment of the present invention, the method for producing the cold-rolled steel sheet is carried out in the following order: reheating of steel having the above-mentioned alloy composition, hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, and heat treatment.

[0059] [Reheating of steel slabs] The method for producing the slab to be provided for hot rolling is not limited, and as an example, a continuously cast slab can be used, and as another example, a slab produced by a thin slab caster or the like can be used. Alternatively, the slab may be hot rolled immediately after continuous casting. When reheating the slab, the reheating temperature is preferably 1150 to 1250°C. If the heating temperature is less than 1150°C, the finish rolling temperature is likely to be less than 850°C, resulting in a high rolling load. From the viewpoint of production costs, the heating temperature is preferably less than 1250°C.

[0060] [Hot rolling] The reheated slab is then finish hot rolled at 830 to 980°C to obtain a hot-rolled steel sheet. If the finish hot rolling temperature (hereinafter also referred to as "FDT") is less than 830°C, the rolling load is large, resulting in an increase in shape defects and a decrease in productivity. On the other hand, if the finish hot rolling temperature exceeds 980°C, the surface quality deteriorates due to an increase in oxides caused by excessively high-temperature operation. Therefore, the finish hot rolling temperature is preferably in the range of 830 to 980°C. The lower limit of the finish hot rolling temperature is more preferably 880°C. The upper limit of the finish hot rolling temperature is more preferably 950°C, and even more preferably 930°C.

[0061] [Winding] The hot-rolled steel sheet obtained by the hot rolling is coiled at 450 to 700°C. If the coiling temperature (hereinafter also referred to as "CT") exceeds 700°C, thick internal oxidation of the hot-rolled steel sheet surface may occur, which may result in a decrease in pickling properties. In order to improve toughness by refining the effective grain size and to homogenize the retained austenite to improve hole expandability, the lower limit of the coiling temperature is set to 450°C. On the other hand, from the viewpoint of further improving the above-mentioned 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. On the other hand, although not particularly limited, after the finish hot rolling, cooling to the coiling temperature can be performed at an average cooling rate of 10 to 100°C / s. If the average cooling rate is less than 10°C / s, hot rolling productivity may decrease, and a cooling medium with reduced cooling capacity may have to be used during actual production. If the average cooling rate exceeds 100°C / s, the temperature deviation inside the steel sheet may become non-uniform, resulting in poor shape and excessive strength of the steel sheet.

[0062] [Cold rolling] The coiled hot-rolled steel sheet is cold-rolled. During the cold rolling, the cold reduction may be 30 to 60%. If the cold reduction is less than 30%, it may be difficult to ensure the target thickness accuracy and it may also be difficult to correct the shape of the steel sheet. On the other hand, if the cold reduction exceeds 60%, cracks may be more likely to occur at the edge of the steel sheet, and the cold rolling load may become excessively large. Therefore, it is preferable that the cold reduction is in the range of 30 to 60%.

[0063] [Continuous annealing] The cold-rolled steel sheet is subjected to continuous annealing at a temperature of 800 to 900°C, with a dew point temperature of -45°C or less. The purpose of this continuous annealing step is to heat the steel sheet to the austenite single-phase region, thereby forming nearly 100% austenite for subsequent phase transformation. If the continuous annealing temperature (hereinafter also referred to as "SS") is less than 800°C, sufficient recrystallization and austenite transformation do not occur, and the desired martensite and bainite fractions cannot be secured after annealing. On the other hand, if the continuous annealing temperature exceeds 900°C, productivity decreases, coarse austenite is formed, which may result in material degradation, and surface quality such as peeling of the plated material may deteriorate. The continuous annealing can be performed in a continuous alloying hot-dip galvanizing furnace.

[0064] Meanwhile, during the continuous annealing, it is preferable to control the atmosphere in the continuous annealing furnace with a gas consisting of, by volume, 95% or more nitrogen and the remainder hydrogen. If the nitrogen fraction is less than 95%, and the hydrogen fraction is not increased accordingly, an oxidizing atmosphere is formed in the furnace, and oxides are formed on the surface of the steel sheet, deteriorating the surface quality. If the hydrogen fraction is too high, process difficulties such as the possibility of explosion are aggravated.

[0065] [Primary cooling] Thereafter, the continuously annealed steel sheet is primarily cooled to a primary cooling finish temperature (hereinafter also referred to as "SCS") of 550 to 650°C at an average cooling rate of less than 10°C / s (more preferably, 1°C / s or more and less than 10°C / s).

[0066] The primary cooling end temperature can be defined as the point at which secondary cooling (quenching) begins, when additional quenching equipment not used in the primary cooling is added. By performing the cooling process stepwise, dividing it into primary and secondary cooling stages, the temperature distribution of the steel sheet can be uniform during the slow cooling stage, reducing the final temperature and material deviations and achieving the desired phase composition. If the primary cooling end temperature is below 550°C, the bainite fraction becomes excessively high, and it is difficult to cool the steel sheet to below 550°C at a cooling rate of less than 10°C / s due to the actual equipment length. In other words, cooling to below 550°C requires equipment modification or installation, which can result in additional costs. Furthermore, if the primary cooling end temperature exceeds 650°C, the required cooling temperature to reach the secondary cooling end temperature increases, resulting in poor steel sheet shape and a lower bainite fraction than the target level.

[0067] [Secondary cooling] The primarily cooled steel sheet is then secondarily cooled to a secondary cooling end temperature (hereinafter also referred to as "RCS") of 150 to 400°C at an average cooling rate of 10°C / s or more. The secondary cooling end temperature is set to be equal to or lower than the Ms temperature of the steel sheet, so that martensite transformation occurs during cooling. This martensite eventually becomes tempered martensite through a subsequent reheating step. If the secondary cooling end temperature is less than 150°C, the amount of initial martensite transformation is too large, resulting in an excessively high yield strength and poor formability. On the other hand, if the secondary cooling end temperature exceeds 400°C, martensite is not formed during cooling, and a large amount of fresh martensite is ultimately formed, which may exceed the appropriate tensile strength and make it difficult to achieve high yield strength and hole expandability. If the secondary cooling rate is less than 10°C / s, even if the target secondary cooling end temperature is reached, high-temperature phase transformation occurs during cooling, making it impossible to achieve the target martensite fraction and high strength. From the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the secondary cooling rate may be 20°C / s, and the upper limit of the secondary cooling rate may be 60°C / s.

[0068] As mentioned above, the secondary cooling can be performed using a quenching equipment that is not used in the primary cooling. The present invention does not particularly limit the type of quenching equipment, but a preferred example is a hydrogen quenching equipment. More specifically, the hydrogen quenching equipment can use a gas consisting of 50 to 80% hydrogen by volume and the remainder nitrogen. If the hydrogen fraction exceeds 80%, it may be difficult to manage the equipment, such as by controlling explosions. If it is less than 50%, it may be difficult to utilize the efficient heat transfer properties of hydrogen, a light element.

[0069] [Heat treatment] The secondarily cooled steel sheet is then reheated to 350-480°C. Through this process, interphase carbon partitioning and additional bainite transformation, necessary for stabilizing the retained austenite, are achieved. In the present invention, the end temperature of the heating section is conveniently referred to as the reheating temperature (hereinafter also referred to as "RHS"). If the reheating temperature is less than 350°C, the strength becomes excessively high and the elongation rate deteriorates. On the other hand, if the reheating temperature exceeds 480°C, the austenite phase remains without being transformed and transforms into fresh martensite during final cooling, impairing hole expandability and elongation rate. Meanwhile, the so-called nose temperature, where bainite transformation is most active, is approximately 400-420°C. Taking this into consideration, the lower limit of the reheating temperature is more preferably 410°C, and the upper limit of the reheating temperature is more preferably 440°C.

[0070] In addition, in one embodiment of the present invention, after the reheating step, the reheated steel sheet may further be subjected to hot-dip galvanizing, galvannealing, and skin pass rolling processes, if necessary. Specifically, the method may further include a step of galvanizing the reheated steel sheet in a galvanizing bath at 450 to 470°C.

[0071] Furthermore, one embodiment of the present invention may further include, if necessary, a step of subjecting the plated steel sheet to an alloying heat treatment at a temperature in the range of 470 to 550°C. The alloying heat treatment is performed to obtain an appropriate alloying level, and the temperature is determined depending on the surface condition of the steel sheet. By controlling the surface condition of the steel material, the alloying heat treatment temperature can be prevented from exceeding 550°C, thereby preventing softening of the steel sheet due to excessive tempering and loss of retained austenite. Meanwhile, in order to accelerate the alloying process, the alloying heat treatment temperature is preferably higher than the hot-dip galvanizing temperature, and the lower limit is controlled to 470°C. Furthermore, after the alloying heat treatment, the method may further include a step of cooling the alloying heat-treated steel sheet to room temperature and then temper rolling it at a reduction rate of less than 1% in order to correct the shape of the steel sheet and adjust its yield strength. [Example]

[0072] The present invention will be described in more detail through the following examples. However, it should be noted that the following examples are only intended to illustrate and embody the present invention, and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them.

[0073] (Example) Slabs having the alloy compositions shown in Tables 1 and 2 below were prepared, and then reheated at 1180 to 1220°C, followed by hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, and heat treatment to produce cold-rolled steel sheets under the conditions shown in Tables 3 and 4. During the continuous annealing, the atmosphere in the continuous annealing furnace was controlled with a gas consisting of, by volume, 95% nitrogen and the remainder hydrogen, and the dew point temperature at temperatures of 800 to 900°C was controlled to -45°C.

[0074] The evaluation results of the tensile properties, elongation, and hole expansion of the cold-rolled steel sheets manufactured in this way are shown in Table 5 below. Tensile strength (TS), yield strength (YS), and elongation (EL) were measured through tensile tests in the direction perpendicular to the rolling direction, using a gauge length of 50 mm and a tensile test specimen width of 25 mm. Hole expansion (HER) was measured according to the ISO 16330 standard, and the holes were sheared using a 10 mm diameter punch with 12% clearance.

[0075] To evaluate hydrogen embrittlement resistance, after four-point bending, the specimens were immersed in a 0.1N HCl solution for 120 hours under an applied stress of 80% of the tensile strength (TS), and the critical hydrogen content for fracture was measured for steel types that did not fracture after 120 hours of immersion, and the results are shown in Table 5. In this case, if fracture occurred after 120 hours of immersion, the specimen was rated as "X," meaning that the standard for hydrogen embrittlement resistance was not met, and if no fracture occurred, the specimen was rated as "O."

[0076] In addition, the results of measuring the microstructure of the cold-rolled steel sheet manufactured as above and the calculation results of Relational Formula 3 used in the present invention are shown in Table 5.

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] [Table 4]

[0081] [Table 5]

[0082] In the case of the invention examples that satisfy the alloy composition and manufacturing conditions of the present invention, it was confirmed that they have higher tensile strength (TS), yield strength (YS), hole expandability (HER) and elongation (El) than the comparative examples, and therefore have excellent elongation and hole expandability while also having ultra-high strength properties.

[0083] On the other hand, in the case of comparative examples that do not satisfy one or more of the alloy compositions and manufacturing conditions of the present invention, it was confirmed that one or more properties selected from tensile strength (TS), yield strength (YS), hole expandability (HER), and elongation (El) were inferior.

Claims

1. In weight percent, C: 0.05 to 0.4%, Si: 0.1 to 3.0%, Al: 0.005 to 3.0%, Mn: 1.0 to 4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001 to 0.5%, B: 0.0001 to 0.003%, Nb: 0.001 to 0.05%, Ti: 0.001 to 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 unavoidable impurities, The microstructure contains, by 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%), A cold-rolled steel sheet having a value defined by the following relational expression 1 that is greater than 145 and less than 160. [Relationship 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 above Relational Formula 1, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb], and [B] represent the weight percent contents of each element in parentheses.)

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. [Relationship 2] 630×[C]-23×[Si]+410×[P]-150×[Cr]-15×[Ni]+300×[B] (In the above-mentioned relational expression 2, the [C], [Si], [P], [Cr], and [Ni] represent the weight percent contents of each element in parentheses.)

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

4. The cold-rolled steel sheet according to claim 1, which satisfies the following relational expression 4: [Relationship 4] 0.3 ppm≦IH2 (In the relational expression 4, IH2 represents the critical hydrogen content for fracture of the cold-rolled steel sheet.)

5. a step of reheating a steel slab containing, by weight, C: 0.05 to 0.4%, Si: 0.1 to 3.0%, Al: 0.005 to 3.0%, Mn: 1.0 to 4.0%, Cr: 1.5% or less (including 0%), Mo: 0.001 to 0.5%, B: 0.0001 to 0.003%, Nb: 0.001 to 0.05%, Ti: 0.001 to 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 unavoidable impurities, and satisfying a value defined by the following relational expression 1 of more than 145 and less than 160; Finish hot rolling the reheated steel slab at 830 to 980°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 450 to 700°C; cold rolling the coiled hot-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature of 800 to 900°C so that the dew point temperature is -45°C or less; subjecting the continuously annealed steel sheet to a primary cooling end temperature of 550 to 650°C at an average cooling rate of less than 10°C / s; Secondarily cooling the primarily cooled steel sheet to a secondary cooling end temperature of 150 to 400°C at an average cooling rate of 10°C / s or more; and heat treating the second-cooled steel sheet at a temperature in the range of 350 to 480°C. [Relationship 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 above Relational Formula 1, the [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb], and [B] represent the weight percent contents of each element in parentheses.)

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

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