KR20220071035A - Ultra high strength cold rolled steel sheet treated by softening heat process and method of manufacturing the same - Google Patents

Abstract

The present invention relates to an ultra-high-strength cold-rolled steel sheet having high strength and elongation by controlling the microstructure, and a manufacturing method thereof. According to an embodiment of the present invention, an ultra-high-strength cold-rolled steel sheet comprises, by wt %, carbon (C): 0.25 to 0.40 %, silicon (Si): 1.0 to 2.5 %, manganese (Mn): 1.5 to 3.0 %, aluminum (Al): 0.01 to 0.05 %, chromium (Cr): > 0 % and < 1.0 %, molybdenum (Mo): > 0 % and < 0.5 %, niobium (Nb): > 0 % and < 0.05 %, titanium (Ti): > 0 % and < 0.05 %, vanadium (V): > 0 % and < 0.05 %, boron (B): > 0 % and < 0.005 %, phosphorus (P): > 0 % and < 0.02 %, sulfur (S): > 0 % and < 0.003 %, nitrogen (N): > 0 % and < 0.01%, and the balance of iron (Fe) and other inevitable impurities, and satisfies a yield strength (YP) of 1100 MPa or more, a tensile strength (TS) of 1400 MPa or more, an elongation (El) of 14 % or more, a yield ratio (YR) of 75 % or more, and a bendability (R/t) of 3.0 or less.

Description

Ultra high strength cold rolled steel sheet treated by softening heat process and method of manufacturing the same

The technical idea of the present invention relates to a cold-rolled steel sheet, and more particularly, to an ultra-high-strength cold-rolled steel sheet having high strength and elongation by controlling the microstructure, and a method for manufacturing the same.

For the purpose of collision safety and weight reduction of automobiles, structural materials of automobiles are required to have high strength and high formability. As a method to satisfy high strength and formability, dual phase steel composed of ferrite and martensitic structures and TRIP (Transformation induced plasticity steel) steel using the phase transformation effect during the deformation of retained austenite are used. have. Transformation induced plastic steel having a matrix structure of ferrite and bainite is disadvantageous in securing strength according to the mixing law. As a method of making martensite-based high-strength transformation organic plastic steel, it is possible to realize martensite or tempered martensite and retained austenite structures through quenching and reheating (QP) heat treatment.

High-strength and highly formable steel of 1.2 GPa or higher, especially 1.5 GPa or higher, requires high yield strength as well as tensile strength. . In the prior art, there is an insufficient limit to simultaneously secure a tensile strength of 1400 MPa or more and an elongation of 14% or more. In addition, when the microstructure is composed only of martensite and retained austenite, the tissue fraction is determined too sensitively to the quenching end point temperature. In particular, it is difficult to make a uniform microstructure and retained austenite uniformly because there is a difference in the martensite fraction according to the Ms temperature and the quenching temperature even with a fine component deviation that is difficult to avoid such as casting segregation.

In addition, in the prior art, martensite or tempered martensite was used as the main microstructure in order to secure high strength and formability, and elongation was secured through the retained austenite or ferrite structure. Characteristics of quenching and reheating heat treatment are that the tissue fraction of tempered martensite, martensite, and retained austenite varies according to the quenching end point temperature. In order to secure the target physical properties, the microstructure fraction is controlled by determining the optimum quenching end point temperature section according to the alloy composition. If the quenching end point temperature is too low, the size of retained austenite becomes fine, but the fraction becomes very small, If the quenching end point temperature is too high, the size of austenite is large and carbon concentration is insufficient, and it is transformed into a martensitic structure after final cooling or is unstable, thereby contributing little to securing elongation.

Therefore, in order to secure formability, it is necessary to secure an appropriate fraction of retained austenite, a fine shape, and stability through carbon concentration. On the other hand, since securing the elongation through ferrite in high-strength steel over 1400 MPa may lead to a decrease in yield strength or tensile strength, ferrite should be limited.

Korean Patent Application No. 10-2018-0047388

The technical problem to be achieved by the technical idea of the present invention is to provide an ultra-high strength cold-rolled steel sheet having high strength and elongation by controlling the microstructure and a method for manufacturing the same

However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, there is provided an ultra-high strength cold-rolled steel sheet having high strength and elongation by controlling the microstructure, and a method for manufacturing the same.

According to an embodiment of the present invention, the ultra-high-strength cold-rolled steel sheet is, by weight, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, Aluminum (Al): 0.01% to 0.05%, Chromium (Cr): >0% to 1.0%, Molybdenum (Mo): >0% to 0.5%, Niobium (Nb), Titanium (Ti) and Vanadium ( Sum of V): greater than 0% to 0.1%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.003%, nitrogen (N): greater than 0% to 0.01%, and Wealth contains iron (Fe) and other unavoidable impurities, yield strength (YP): 1100 MPa or more, tensile strength (TS): 1400 MPa or more, and elongation (El): 14% or more, yield ratio (YR): 75 % or more, and bendability (R/t): 3.0 or less may be satisfied.

According to an embodiment of the present invention, the ultra-high strength cold-rolled steel sheet may include a mixed structure in which ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed.

According to an embodiment of the present invention, the fraction of ferrite is in the range of more than 0% to 5%, the fraction of martensite is in the range of more than 0% to 20%, and the fraction of retained austenite is 10% to 30% range, the fraction of upper bainite is in the range of more than 0% to 30%, the fraction of lower bainite is in the range of more than 0% to 30%, and the fraction of tempered martensite may be included as the remaining fraction.

According to an embodiment of the present invention, the minimum value of the sum of the fraction of the upper bainite and the fraction of the lower bainite may be 10%.

According to an embodiment of the present invention, it includes a mixed structure in which tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed, and the fraction of martensite is greater than 0% to 20% range, wherein the fraction of retained austenite is in the range of 10% to 30%, the fraction of upper bainite is in the range of greater than 0% to 30%, and the fraction of lower bainite is in the range of greater than 0% to 30%, The fraction of tempered martensite may be included as the remaining fraction.

According to an embodiment of the present invention, the average diameter of the retained austenite may be 1.0 μm or less.

According to an embodiment of the present invention, the ultra-high-strength cold-rolled steel sheet is, by weight, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium (Cr): more than 0% to 1.0%, the sum of nickel (Ni) and copper (Cu): more than 0% to 1.0%, phosphorus (P): More than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.01%, and the balance contains iron (Fe) and other unavoidable impurities, the yield strength (YP) ): 1100 MPa or more, tensile strength (TS): 1400 MPa or more, and elongation (El): 14% or more, yield ratio (YR): 75% or more, and bendability (R/t): 3.0 or less have.

According to an embodiment of the present invention, the ultra-high-strength cold-rolled steel sheet may further include, by weight%, the total of niobium (Nb), titanium (Ti) and vanadium (V): more than 0% to 0.1%.

According to an embodiment of the present invention, the method for manufacturing the ultra-high strength cold-rolled steel sheet is, by weight, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, Aluminum (Al): 0.01% to 0.05%, Chromium (Cr): More than 0% to 1.0%, Molybdenum (Mo): More than 0% to 0.5%, Niobium (Nb), Titanium (Ti) and vanadium (V): greater than 0% to 0.1%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.003%, nitrogen (N): greater than 0% to 0.01% , and the remainder of manufacturing a hot-rolled steel sheet containing iron (Fe) and other unavoidable impurities; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature in the range of 810°C to 930°C; cooling the cold-rolled steel sheet in multiple stages; and partitioning and heat-treating the cooled cold-rolled steel sheet at a temperature in the range of 360°C to 500°C.

According to an embodiment of the present invention, the manufacturing of the hot-rolled steel sheet includes: preparing a steel slab having the alloy composition; reheating the steel slab in the range of 1,150° C. to 1,250° C.; preparing a hot-rolled steel sheet by hot finish rolling the reheated steel slab at a finish rolling end temperature in the range of 850°C to 970°C; and winding the hot-rolled steel sheet in the range of 400°C to 700°C.

According to an embodiment of the present invention, in the annealing heat treatment, the cold-rolled steel sheet is heated at a temperature increase rate in the range of 2° C./sec to 10° C./sec at a temperature in the range of 810° C. to 930° C. for 30 seconds to 120 This can be done by holding for a time in the range of seconds.

According to an embodiment of the present invention, in the step of multi-stage cooling, the cold-rolled steel sheet subjected to the annealing heat treatment is subjected to a primary cooling end temperature in the range of 650°C to 800°C at a cooling rate in the range of 2°C/sec to 15°C/sec. First cooling with a; and secondary cooling of the first cooled cold-rolled steel sheet to a secondary cooling termination temperature in the range of 180° C. to 300° C. at a cooling rate of 20° C./sec or more.

According to an embodiment of the present invention, after performing the secondary cooling step, the secondary cooling end temperature can be maintained for a time in the range of 5 seconds to 90 seconds.

According to an embodiment of the present invention, the partitioning heat treatment includes heating the cooled cold-rolled steel sheet at a temperature increase rate in the range of 3° C./sec to 20° C./sec at a temperature in the range of 360° C. to 500° C. for 20 seconds. This can be done by holding for a time ranging from ~500 seconds.

According to an embodiment of the present invention, the ultra-high-strength cold-rolled steel sheet manufactured by the method for manufacturing the ultra-high-strength cold-rolled steel sheet has a yield strength (YP): 1100 MPa or more, a tensile strength (TS): 1400 MPa or more, and an elongation ( El): 14% or more, yield ratio (YR): 75% or more, and bendability (R/t): 3.0 or less, ferrite, tempered martensite, martensite, retained austenite, upper bainite, and a mixed structure in which the lower bainite is mixed.

According to an embodiment of the present invention, the method for manufacturing the ultra-high strength cold-rolled steel sheet is, by weight, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium (Cr): more than 0% to 1.0%, the sum of nickel (Ni) and copper (Cu): more than 0% to 1.0%, phosphorus ( P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.01%, and the balance is a hot-rolled steel sheet containing iron (Fe) and other unavoidable impurities manufacturing a; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature in the range of 810°C to 930°C; cooling the cold-rolled steel sheet in multiple stages; and partitioning and heat-treating the cooled cold-rolled steel sheet at a temperature in the range of 360°C to 500°C.

According to an embodiment of the present invention, the method for manufacturing the ultra-high strength cold-rolled steel sheet is, by weight, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium (Cr): more than 0% to 1.0%, the sum of nickel (Ni) and copper (Cu): more than 0% to 1.0%, niobium ( Sum of Nb), titanium (Ti) and vanadium (V): greater than 0% to 0.1%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.003%, nitrogen (N) : Manufacturing a hot-rolled steel sheet containing more than 0% to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature in the range of 810°C to 930°C; cooling the cold-rolled steel sheet in multiple stages; and partitioning and heat-treating the cooled cold-rolled steel sheet at a temperature in the range of 360°C to 500°C.

According to the technical idea of the present invention, the ultra-high strength cold-rolled steel sheet is a transformation organic plastic steel formed through rapid cooling and reheating heat treatment. The ultra-high strength cold-rolled steel sheet has a high carbon content, so it has a yield strength of 1100 MPa or more, a high tensile strength of 1400 MPa or more, an elongation of 14% or more, a yield ratio of 75% or more, and a bending workability (R/t) of 3.0 or less based on 90 degree bending. can have The ultra-high-strength cold-rolled steel sheet can be stabilized by refining and stabilizing retained austenite using tempered martensite and upper bainite and lower bainite transformation structures as a microstructure, and stably providing yield strength and yield ratio.

In particular, in the process of quenching (secondary cooling), quenching maintenance, reheating, and partitioning, the phase transformations of martensite transformation (primary), lower bainite transformation (secondary), and upper bainite transformation (tertiary) are induced. , it can help to control the problem of tissue non-uniformity caused by casting segregation, which is inevitably present in steel, and can refine and stabilize retained austenite. When it is simply composed of a structure of martensite and retained austenite, the Ms point changes according to component non-uniformity such as casting segregation in the structure, and has different martensite and retained austenite fractions at the same quenching temperature. The ultra-high-strength cold-rolled steel sheet according to the present invention can solve this problem.

The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a process flowchart schematically illustrating a method of manufacturing an ultra-high strength cold rolled steel sheet according to an embodiment of the present invention.
2 is a time-temperature graph illustrating a heat treatment process of a cold-rolled steel sheet in the method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention.
3 is a scanning electron microscope photograph showing the microstructure of an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention.
4 is a scanning electron microscope EBSD distribution diagram showing the microstructure of an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention.
Referring to FIG. 5 , it was found that retained austenite in the cold rolled steel sheet of FIG. 4 had an average diameter of 0.78 μm.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

The technical idea of the present invention is that it has a yield strength of 1100 MPa or more, a tensile strength of 1400 MPa or more, an elongation of 14% or more, a yield ratio (yield strength / tensile strength) of 75% or more, and a 90 degree bending workability of 3.0 R/t It provides the following ultra-high strength cold-rolled steel sheet and a method for manufacturing the same.

Hereinafter, the ultra-high strength cold rolled steel sheet according to the technical idea of the present invention will be described in detail.

The ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention, by weight, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0% , Aluminum (Al): 0.01% to 0.05%, Chromium (Cr): >0% to 1.0%, Molybdenum (Mo): >0% to 0.5%, Niobium (Nb), Titanium (Ti) and Vanadium (V) Sum of: more than 0% to 0.1%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.01%, and the balance is iron (Fe) and other unavoidable impurities.

The ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention, by weight, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0% , Aluminum (Al): 0.01% to 0.05%, Chromium (Cr): More than 0% to 1.0%, Sum of Nickel (Ni) and Copper (Cu): More than 0% to 1.0%, Phosphorus (P): 0% Exceeds to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.01%, and the balance contains iron (Fe) and other unavoidable impurities.

The ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention, by weight, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0% , Aluminum (Al): 0.01% to 0.05%, Chromium (Cr): More than 0% to 1.0%, Total of Nickel (Ni) and Copper (Cu): More than 0% to 1.0%, Niobium (Nb), Titanium ( Sum of Ti) and Vanadium (V): greater than 0% to 0.1%, Phosphorus (P): greater than 0% to 0.02%, Sulfur (S): greater than 0% to 0.003%, Nitrogen (N): greater than 0% to 0.01%, and the balance contains iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the ultra-high strength cold rolled steel sheet according to the present invention will be described as follows. At this time, the content of the component elements all mean weight % with respect to the entire steel sheet.

Carbon (C): 0.25% to 0.40%

Carbon is added to secure the strength of steel, and in particular, increases the strength of the martensitic structure. In addition, sufficient carbon content is required because it is classified and stabilized austenite to secure elongation through transformation induced plasticity (TRIP) effect. When the carbon content is less than 0.25%, it may be difficult to obtain the target strength and elongation at the same time. When the carbon content exceeds 0.40%, weldability may be deteriorated and hydrogen embrittlement may occur. Therefore, it is preferable to add the carbon content in an amount of 0.25% to 0.40% of the total weight of the steel sheet.

Silicon (Si): 1.0% to 2.5%

Silicon, as a ferrite stabilizing element, delays the formation of carbides in ferrite and martensite, and has a solid solution strengthening effect. In particular, it is essential to differentiate carbon into austenite by delaying the formation of carbides in martensite. When the content of silicon is less than 1.0%, it may be difficult to sufficiently secure the stability of the retained austenite because the carbide formation inhibitory effect is small. When the content of silicon exceeds 2.5%, an oxide such as Mn ₂ SiO ₄ is formed during the manufacturing process, thereby inhibiting plating property and increasing carbon equivalent to reduce weldability. Therefore, it is preferable to add the content of silicon in an amount of 1.0% to 2.5% of the total weight of the steel sheet.

Manganese (Mn): 1.5% to 3.0%

Manganese has a solid solution strengthening effect and increases hardenability, thereby delaying the formation of ferrite and bainite during cooling. When the manganese content is less than 1.5%, the effect of adding manganese is not sufficient, so it may be difficult to secure hardenability. When the manganese content exceeds 3.0%, the transformation of bainite may be excessively delayed, and workability may deteriorate due to the formation or segregation of inclusions such as MnS, and the carbon equivalent may be increased to reduce weldability. Therefore, it is preferable to add the manganese content in an amount of 1.5% to 3.0% of the total weight of the steel sheet.

Aluminum (Al): 0.01% to 0.05%

Aluminum is used as a deoxidizer, and similar to silicon, it can help suppress carbide formation. When the content of aluminum is less than 0.01%, the deoxidation effect may be insufficient. When the content of the aluminum exceeds 0.05%, AlN may be formed during slab manufacturing to cause cracks during casting or hot rolling. Therefore, it is preferable to add the content of aluminum in an amount of 0.01% to 0.05% of the total weight of the steel sheet.

Chromium (Cr): >0% to 1.0%

Chromium has a solid solution strengthening effect and contributes to strength improvement by increasing hardenability, and works with C and Mn to refine martensite and bainite structures and contribute to stabilization of retained austenite. When the content of chromium exceeds 1.0%, the transformation of bainite may be excessively delayed, and the manufacturing cost of steel may be increased. Therefore, it is preferable to add chromium in an amount of more than 0% to 1.0% of the total weight of the steel sheet.

Nickel (Ni): >0% to 0.5%

Nickel stabilizes austenite and can also help increase the hardenability of steel. When the content of nickel exceeds 0.5%, it is not preferable to increase the manufacturing cost of the steel. Accordingly, it is preferable to add nickel in an amount of more than 0% to 0.5% of the total weight of the steel sheet.

Copper (Cu): >0% to 0.5%

Copper stabilizes austenite and can also help increase the hardenability of steel. When the copper content exceeds 0.5%, it is not preferable to increase the manufacturing cost of the steel. Accordingly, copper is preferably added in an amount of more than 0% to 0.5% of the total weight of the steel sheet.

In addition, it is preferable to add more than 0% to 1.0% of the total of nickel and copper.

Molybdenum (Mo): >0% to 0.5%

Molybdenum has a solid solution strengthening effect and contributes to strength improvement by increasing hardenability, and works with C and Mn to refine martensite and bainite structures and contribute to stabilization of retained austenite. When the content of molybdenum exceeds 0.5%, the transformation of bainite may be excessively delayed, and the manufacturing cost of steel may be increased. Therefore, it is preferable to add molybdenum in an amount of more than 0% to 0.5% of the total weight of the steel sheet.

Sum of niobium (Nb), titanium (Ti) and vanadium (V): >0% to 0.1%

Niobium, titanium, and vanadium are precipitate-forming elements, and strength can be increased by a precipitation strengthening effect, and a grain refining effect can also be obtained. When the total of niobium, titanium, and vanadium is added in excess of 0.1%, respectively, the manufacturing cost of the steel may be greatly increased, and due to many precipitation during rolling, the rolling load may be greatly increased, and the elongation may be reduced. . Therefore, it is preferable to add the total of niobium, titanium, and vanadium in an amount of more than 0% to 0.1% of the total weight of the steel sheet, respectively. In addition, each of niobium, titanium, and vanadium is preferably added in an amount of 0.05% or less of the total weight of the steel sheet. For example, it is preferable to add more than 0% to 0.05% of each of niobium, titanium, and vanadium.

Boron (B): >0% to 0.005%

Boron can improve hardenability like Mn, Cr, Mo, and the like. When the content of boron exceeds 0.005%, it may be concentrated on the surface and deterioration of quality such as plating adhesion may occur. Therefore, it is preferable to add boron in an amount of more than 0% to 0.005% of the total weight of the steel sheet.

Phosphorus (P): >0% to 0.02%

Phosphorus is an impurity included in the manufacturing process of steel, and although it can help improve strength by solid solution strengthening, low-temperature brittleness can occur when a large amount is contained. Therefore, it is preferable to limit the phosphorus content to more than 0% to 0.02% of the total weight of the steel sheet.

Sulfur (S): >0% to 0.003%

Sulfur is an impurity included in the manufacturing process of steel, and may reduce bendability, toughness, and weldability by forming non-metallic inclusions such as FeS and MnS. Therefore, it is preferable to limit the sulfur content to more than 0% to 0.003% of the total weight of the steel sheet.

Nitrogen (N): >0% to 0.01%

Nitrogen is an element that is unavoidably included in the manufacture of steel, and may help stabilize austenite, but may react with Al to form AlN, which may cause cracks during playing. Therefore, it is preferable to limit the nitrogen content to more than 0% to 0.01% of the total weight of the steel sheet.

The remaining component of the ultra-high strength cold-rolled steel sheet is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal steelmaking process, it cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The ultra-high strength cold-rolled steel sheet manufactured by controlling the specific components of the above alloy composition and their content ranges, and to be described later, is, for example, yield strength (YP): 1100 MPa or more, tensile strength (TS): 1400 MPa or more, and elongation (El): 14% or more, yield ratio (YR): 75% or more, and bendability (R/t): 3.0 or less can be satisfied. The ultra-high strength cold rolled steel sheet is, for example, yield strength (YS): 1100 MPa to 1500 MPa, tensile strength (TS): 1400 MPa to 1800 MPa, elongation (EL): 14% to 20%, yield ratio (YR) : 75% to 90%, and bendability: 1.0 to 3.0 may be satisfied.

The ultra-high strength cold-rolled steel sheet may have a mixed structure in which ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed.

The fraction of ferrite may be, for example, in the range of greater than 0% to 5%, the fraction of martensite may be, for example, in the range of greater than 0% to 20%, and the fraction of retained austenite is, for example, For example, it can be in the range of 10% to 30%, the fraction of upper bainite can be, for example, greater than 0% to 30%, and the fraction of lower bainite is, for example, greater than 0% to 30% %, the fraction of tempered martensite may be included as the remaining fraction, for example, may be in the range of 20% to 80%. The fraction means an area ratio derived from a microstructure photograph through an image analyzer.

In addition, the sum of the fraction of the upper bainite and the fraction of the lower bainite may be 10% to 60%. The minimum value of the sum of the fraction of the upper bainite and the fraction of the lower bainite may be 10%.

The ferrite may include polygonal ferrite.

In addition, the average diameter of the retained austenite may be, for example, less than or equal to 1.0 μm, for example, may be in the range of 0.1 μm to 1.0 μm.

Retained austenite is finely distributed in the laths and grain boundaries of the tempered martensite and the bainite, and accordingly, the retained austenite is stabilized, and strength and elongation can be stably secured.

In addition, the ultra-high strength cold-rolled steel sheet may not contain ferrite. In this case, the ultra-high strength cold rolled steel sheet includes a mixed structure in which tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed, and the fraction of martensite exceeds 0% ~ 20 %, the fraction of retained austenite is in the range of 10% to 30%, the fraction of upper bainite is in the range of greater than 0% to 30%, the fraction of lower bainite is in the range of greater than 0% to 30%, and , the fraction of tempered martensite may be included as the remaining fraction. In addition, the sum of the fraction of the upper bainite and the fraction of the lower bainite may be 10% to 60%. The minimum value of the sum of the fraction of the upper bainite and the fraction of the lower bainite may be 10%.

Hereinafter, a method for manufacturing an ultra-high strength cold rolled steel sheet according to the present invention will be described with reference to the accompanying drawings.

Manufacturing method of ultra-high-strength cold-rolled steel sheet

1 is a process flowchart schematically illustrating a method of manufacturing an ultra-high strength cold rolled steel sheet according to an embodiment of the present invention.

In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling process may be, for example, a slab. The semi-finished slab can be obtained through the continuous casting process after obtaining molten steel of a predetermined composition through the steelmaking process.

Referring to FIG. 1 , a method of manufacturing an ultra-high strength cold rolled steel sheet according to an embodiment of the present invention includes the steps of manufacturing a hot rolled steel sheet using the steel of the composition (S110); manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet (S120); annealing the cold-rolled steel sheet (S130); cooling the cold-rolled steel sheet in multiple stages (S140); and partitioning and heat-treating the cold-rolled steel sheet (S150).

2 is a time-temperature graph illustrating a heat treatment process of a cold-rolled steel sheet in a method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 2 , the cold-rolled steel sheet is heated to a temperature higher than Ac3 for annealing heat treatment and maintained for a certain period of time, and multi-stage cooling of two stages of slow cooling and rapid cooling is performed until the quenching end temperature. Then, after maintaining at the quench termination temperature for a certain period of time, the temperature is raised above the Ms temperature for the partitioning heat treatment, and after maintaining constant for the partitioning heat treatment time, the final cooling is performed below the Mf temperature.

Hot-rolled steel sheet manufacturing step (S110)

Prepare a steel slab having the alloy composition, and reheat the steel slab, for example, at a reheating temperature (Slab Reheating Temperature, SRT) in the range of 1,150° C. to 1,250° C. Through such reheating, re-dissolution of segregated components and re-dissolution of precipitates occurs during casting to homogenize and make hot rolling possible. When the reheating temperature is less than 1,150° C., re-solidification of segregation may be insufficient, and the hot rolling load may be increased. When the reheating temperature exceeds 1,250° C., the size of austenite grains may increase, and process costs may increase according to the temperature increase. The reheating time may be performed, for example, for 1 hour to 4 hours. If the reheating time is less than 1 hour, the homogenization of segregation may be insufficient. When the reheating time exceeds 4 hours, the size of the austenite grains may increase, and the process cost may increase according to the temperature increase.

After the reheating, hot rolling is performed by a conventional method, for example, hot finish rolling is performed at a finish delivery temperature (FDT) in the range of 850°C to 970°C to manufacture a hot-rolled steel sheet. When the finish rolling end temperature is less than 850°C, ferrite or pearlite may be produced. When the finish rolling end temperature exceeds 970° C., scale generation is increased and the grain size is coarsened, so that it may be difficult to fine-tune the structure.

Then, the hot-rolled steel sheet is cooled to a coiling temperature in the range of, for example, 400°C to 700°C. The cooling may be either air cooling or water cooling, for example, cooling at a cooling rate of 10° C./sec to 30° C./sec. A faster cooling rate is advantageous in reducing the average grain size. Preferably, the cooling is performed to a coiling temperature.

Then, the hot-rolled steel sheet is wound at a coiling temperature (CT) in the range of, for example, 400°C to 700°C. The range of the winding temperature may be selected from the viewpoint of cold rolling properties and surface properties. When the coiling temperature is less than 400 ° C., a hard phase such as martensite is excessively generated, and the material of the hot-rolled steel sheet is excessively increased, so that the rolling load during cold rolling can be significantly increased. If the coiling temperature exceeds 700 ℃, it may cause non-uniformity of the microstructure of the final product.

Cold-rolled steel sheet manufacturing step (S120)

The hot-rolled steel sheet is subjected to pickling treatment in which acid is washed to remove the surface scale layer. Then, the hot-rolled steel sheet is cold-rolled at an average reduction ratio of 40% to 70%, for example, to form a cold-rolled steel sheet. As the average reduction ratio is higher, there is an effect of increasing the formability due to the effect of refining the tissue. When the average reduction ratio is less than 40%, it is difficult to obtain a uniform microstructure. When the average reduction ratio exceeds 70%, the roll force is increased to increase the process load. It may have the thickness of the steel sheet finally produced by the cold rolling. The structure of the cold-rolled steel sheet may have a structure in which the structure of the hot-rolled steel sheet is stretched.

Annealing heat treatment step (S130)

The cold-rolled steel sheet is annealed and heat treated in a continuous annealing furnace having a normal slow cooling section. The annealing heat treatment is performed to form an austenite single-phase structure. The annealing heat treatment temperature and time affect the austenite grain size and, therefore, can greatly affect the strength of the cold rolled steel sheet.

The annealing heat treatment is, for example, a heating rate of 2°C/sec or more, for example, heating at a temperature increase rate in the range of 2°C/sec to 10°C/sec. If the temperature increase rate is less than 2 °C / sec, it takes a long time to reach the target annealing heat treatment temperature, the production efficiency may be reduced and the size of the crystal grains may increase.

The annealing heat treatment is, for example, at a temperature of Ae3 or higher, for example, at a temperature in the range of 810°C to 930°C, for example at a temperature in the range of 830°C to 900°C, for example in the range of 30 seconds to 120 seconds. It can be carried out by holding for a period of time. When the annealing heat treatment temperature is less than 810° C., an austenite single phase cannot be formed in order to make tempered martensite, which is a final structure. For reference, in order to form an austenite single phase, annealing heat treatment must be performed at an A3 temperature or higher. When the annealing heat treatment temperature exceeds 930° C., the austenite grains become coarse and the strength may be reduced.

As the annealing heat treatment time increases, the coarsening according to the austenite grain growth is affected similarly to the annealing heat treatment temperature, but the annealing heat treatment time has less effect than the annealing heat treatment temperature. If the annealing heat treatment time exceeds 120 seconds. The heat treatment efficiency may be reduced. When the annealing heat treatment time is less than 30 seconds, the annealing heat treatment effect may be insufficient.

Multi-stage cooling step (S140)

The cold-rolled steel sheet subjected to the annealing heat treatment is cooled in multiple stages. The cooling step may be performed in the following two steps.

First, the cold-rolled steel sheet subjected to the annealing heat treatment, for example, at a cooling rate in the range of 2 ℃ / sec ~ 15 ℃ / sec, for example, at a cooling rate in the range of 3 ℃ / sec ~ 10 ℃ / sec, ferrite transformation is suppressed The first cooling is performed by slow cooling to the temperature section to be used, for example, to the primary cooling end temperature in the range of 650°C to 800°C. When the primary cooling end temperature of the slow cooling is less than 650° C., ferrite transformation may occur in an undesired amount, and thus strength may be reduced. The fraction of ferrite produced by the ferrite transformation is preferably limited to 0% to less than 5%.

Then, the first cooled cold-rolled steel sheet, for example, at a cooling rate of 20 °C / sec or more, for example, at a cooling rate in the range of 20 °C / sec to 100 °C / sec, for example, at Ms temperature or less, for example For example, the secondary cooling is performed by rapid cooling to a temperature in the range of Ms-140 ℃ ~ Ms-30℃, for example, the secondary cooling end temperature in the range of 180 ℃ ~ 300 ℃. A part of austenite may be transformed into martensite by the secondary cooling. A fraction of the generated martensite may be 20% to 80%.

Then, the secondary cooled cold-rolled steel sheet is maintained at the secondary cooling termination temperature, for example, for a time in the range of 5 seconds to 90 seconds. In the holding time after such quenching, temperature homogenization of the steel may be initially performed. Subsequently, a portion of the retained austenite may be transformed into lower bainite or the like while maintaining the isothermal temperature at the secondary cooling termination temperature.

Partitioning heat treatment step (S150)

The multi-stage cooled cold-rolled steel sheet is reheated at a temperature increase rate in the range of, for example, 3°C/sec to 20°C/sec, and, for example, at a temperature in the range of 360°C to 500°C, for example, 360°C to 460°C in the range. The partitioning heat treatment is performed by maintaining at a temperature of, for example, a time in the range of 20 seconds to 500 seconds, for example, a time in the range of 30 seconds to 500 seconds.

When the partitioning heat treatment temperature is less than 360° C., the partitioning effect may be insufficient. When the partitioning heat treatment temperature exceeds 500° C., the size of the carbide may be coarsened, thereby reducing strength.

The partitioning heat treatment holding time has little effect compared to the partitioning temperature. When the partitioning heat treatment holding time is less than 20 seconds, it may be difficult to obtain a stable partitioning effect. When the partitioning heat treatment holding time exceeds 500 seconds, heat treatment efficiency is lowered, the size of the carbide is increased, and thus strength reduction may occur.

The partitioning heat treatment step (S150) may be performed immediately after performing the multi-stage cooling, or may be performed after maintaining it at room temperature for several minutes or more.

After the partitioning heat treatment step (S150) is finished, it is cooled to room temperature, for example, to a temperature in the range of 0°C to 40°C.

Microstructure change analysis

Hereinafter, changes in the microstructure of the ultra-high-strength cold-rolled steel sheet during the manufacturing method of the ultra-high-strength cold-rolled steel sheet according to the technical spirit of the present invention will be described in detail.

In the annealing heat treatment step (S130), the microstructure of the cold-rolled steel sheet is reversely transformed into austenite.

In the primary cooling of the multi-stage cooling step (S140), the fraction of ferrite generated by ferrite transformation is limited to less than 5%, and ferrite may not be generated. If 5% or more of ferrite is generated, the strength may be lowered and the target strength may not be realized.

In the secondary cooling of the multi-stage cooling step (S140), as the cold-rolled steel sheet is cooled at a fast cooling rate, ferrite transformation, pearlite transformation, and bainite transformation are suppressed, and a portion of the austenite is transformed into martensite. In this case, the fraction of martensite generated by martensite transformation may be limited to 20% to 80%. When the fraction of martensite generated during the secondary cooling exceeds 80%, it may be difficult to secure an appropriate amount of the fraction of retained austenite. If it is less than 20%, it is difficult to secure the stability of the retained austenite because the fraction of retained austenite after cooling is too high, and even if the bainite transformation structure is increased, the martensite fraction is decreased, thereby reducing strength. In addition, some martensitic structures increase the internal stress to increase the rate of bainite nucleation, so that the bainite transformation can be accelerated even at a low temperature below Ms.

In the secondary cooling of the multi-stage cooling step ( S140 ), a portion of the austenite is transformed into bainite while it is maintained at the secondary cooling termination temperature after rapid cooling, which may be mainly lower bainite. In addition, fine precipitates may be formed in the martensite produced in the previous step. In this case, the time maintained at the secondary cooling termination temperature may be in the range of 5 seconds to 90 seconds. If the holding time is less than 5 seconds, the lower bainite transformation may not sufficiently occur. When the holding time exceeds 90 seconds, the process cost may increase due to an excessively long heat treatment time.

In the partitioning heat treatment step ( S150 ), carbon is diffused and concentrated into the retained austenite, thereby stabilizing the retained austenite. In addition, a portion of the retained austenite may undergo bainite transformation. The bainite transformation may refine the shape of the retained austenite after rapid cooling, thereby contributing to stabilization of the retained austenite. For this operation, the sum of the fraction of the upper bainite and the fraction of the lower bainite may have 10% or more.

In the final cooling after performing the partitioning heat treatment step (S150), some unstable austenite may be transformed into martensite. In this case, if there is a lot of martensite produced, the final fraction of retained austenite may be reduced, which may adversely affect the formability. Therefore, it is preferable to control the produced martensite to less than 20%.

In order to suppress the martensite transformation in the final cooling, the retention time at the secondary cooling end temperature and the secondary cooling end temperature and the heat treatment without problems in the partitioning heat treatment step to refine and stabilize the retained austenite It is preferable to proceed.

There are cases where bainite transformation is not considered below the Ms point, but there are studies that bainite transformation is possible even below the Ms point, and there are studies that bainite nucleation increases than immediately above Ms below the Ms point.

Through this heat treatment process, the final microstructure is tempered martensite (20% ~ 80%), retained austenite (10% ~ 30%), and lower bainite (0% ~ 30%). It may contain upper bainite (0% to 30%), some ferrite (0% to 5%) or martensite (0% to 20%). The sum of the fraction of the upper bainite and the fraction of the lower bainite may be 10% or more. The average diameter of the retained austenite may be 1.0 μm or less.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

Steel having the composition (unit: wt%) of Tables 1 and 2 below was prepared, and cold rolled steel sheets according to Examples and Comparative Examples were prepared through predetermined hot and cold rolling processes and heat treatment processes. The balance is iron (Fe).

steel grade C Si Mn P S Al Cr A 0.352 1.78 2.02 0.014 0.0011 0.024 0.495 B 0.349 1.88 2.10 0.015 0.0009 0.013 0.507 C .0342 1.97 2.42 0.022 0.0024 0.041 0.0077 D 0.320 1.75 2.02 0.015 0.0012 0.016 0.296 E 0.349 1.71 2.01 0.015 0.0011 0.036 0.0052 F 0.229 1.65 2.56 0.008 0.0020 0.029 0 G 0.359 1.81 2.06 0.021 0.0046 0.025 0.515

steel grade Mo Ni Cu Nb Ti N A 0.104 0 0 0.0057 0 0.0055 B 0.103 0 0 0.049 0 0.0058 C 0 0.046 0.111 0 0 0.0045 D 0.072 0 0 0.034 0 0.0045 E 0 0 0.0029 0.0048 0 0.0053 F 0 0 0 0 0.020 0.0048 G 0.097 0 0.0087 0.041 0 0.0052

Referring to Tables 1 and 2, steel grade F and steel grade G are comparative examples outside the composition range of the present invention.

In the steel type F, the carbon content is low compared to the lower limit of the target composition, and in the steel grade G, the sulfur content is high compared to the upper limit of the target composition.

In addition, steel type C does not contain molybdenum (Mo) and niobium (Nb), but contains nickel (Ni) and copper (Cu).

Steel type E does not contain molybdenum (Mo) and nickel (Ni), but contains copper (Cu) and niobium (Nb).

Table 3 shows the Ae3 temperature, Ms temperature, Ms-140°C temperature, and Ms-30°C temperature for the above steel types. The unit is °C.

steel grade Ae3 temperature ms temperature Ms-140℃ temperature Ms-30℃ temperature A 800 322 182 292 B 802 321 181 291 C 799 320 180 290 D 809 338 198 308 E 804 330 190 300 F 815 365 225 335 G 801 318 178 288

Referring to Table 3, the Ae3 temperature was calculated using Thermo-Calc and TCFE9 database. The Ms temperature was calculated using the following empirical formula. In the following empirical formula, for example, "[C]" represents the weight percent of carbon.

Ms(℃) = 539 - 423 [C] - 30.4 [Mn] - 12.1 [Cr] - 17.7 [Ni] - 7.5 [Mo]

The slab of the above-described steel type was reheated at 1200°C and maintained for 3 hours, and after hot rolling to a thickness of 2.4 mm at the finish rolling end temperature of 950°C, it was wound at 600°C. Then, the wound hot-rolled steel sheet was pickled to remove scale on the surface, and cold-rolled to prepare a cold-rolled steel sheet having a thickness of 1.2 mm.

Then, heat treatment was performed under the process conditions of Table 4.

Table 4 shows the values of the heat treatment process conditions for manufacturing the cold rolled steel sheets of Comparative Examples and Examples.

steel grade division Annealing
temperature
(℃) Annealing
hour
(candle) Primary
Cooling
speed
(℃/sec) Primary
Cooling
temperature
(℃) Secondary
Cooling
speed
(℃/sec) Secondary
Cooling
temperature
(℃) quench
maintain
hour
(candle) reheating rate
(℃/sec) party
shunning
temperature
(℃) party
shunning
maintain
hour
(candle) A Example 1 850 60 -5 750 -69 206 19.5 11.7 400 240 A Comparative Example 1 850 60 -5 750 -98 167 22.5 3.6 400 240 B Example 2 850 60 -5 750 -67 209 30.0 6.9 400 240 B Example 3 850 60 -5 750 -68 215 60.6 6.5 400 240 B Comparative Example 2 850 60 -5 750 -68 163 19.4 3.7 400 240 B Comparative Example 3 850 60 -5 750 -52 305 15.2 3.3 370 480 C Example 4 870 60 -5 750 -47 223 19.2 11.4 400 240 C Comparative Example 4 870 60 -5 750 -52 161 23.2 3.7 400 240 D Example 5 900 180 -5 780 -78 261 61.2 14.8 430 240 D Example 6 900 180 -5 780 -79 234 59.5 14.7 430 240 D Comparative Example 5 900 180 -5 780 -76 257 0.9 15.1 430 240 D Comparative Example 6 870 60 -5 780 -75 266 1.2 15.0 400 240 E Example 7 900 180 -5 780 -68 243 58.7 3.1 400 240 F Comparative Example 7 850 60 -5 750 -68 279 23.2 3.4 430 60 F Comparative Example 8 850 60 -5 750 -68 318 24.1 3.5 430 60 G Comparative Example 9 850 60 -5 750 -67 202 22.5 11.4 400 240

Table 5 shows the physical and mechanical properties of the manufactured hot-rolled steel sheet and steel pipe, yield strength (YS), tensile strength (TS), and elongation (EL), yield ratio (YR), and 90 degree bendability (R) /t).

steel grade division yield strength
(MPa) The tensile strength
(MPa) elongation
(%) yield ratio
(%) 90 degrees
bendability A Example 1 1327 1585 14.6 83.8 2.00 A Comparative Example 1 1448 1638 8.9 90.9 1.83 B Example 2 1314 1543 15.7 85.2 2.17 B Example 3 1351 1580 16.9 85.5 2.00 B Comparative Example 2 1538 1663 8.6 92.5 2.00 B Comparative Example 3 840 1474 10.3 57.0 - C Example 4 1165 1421 15.4 82.0 2.83 C Comparative Example 4 1360 1546 12.6 88.0 - D Example 5 1151 1420 17.0 81.1 1.83 D Example 6 1282 1447 15.1 88.6 1.68 D Comparative Example 5 1019 1432 16.3 71.1 1.83 D Comparative Example 6 1054 1435 16.6 73.4 1.83 E Example 7 1228 1411 14.2 87.1 1.50 F Comparative Example 7 1162 1299 13.3 89.4 1.17 F Comparative Example 8 994 1284 15.1 77.4 1.33 G Comparative Example 9 1328 1629 17.0 81.5 4.33

Referring to Table 5, the Examples met the target ranges for yield strength (YS), tensile strength (TS), and elongation (EL), yield ratio (YR), and 90 degree bendability (R/t). .

Comparative Example 1 (steel type A), Comparative Example 2 (steel type B), and Comparative Example 4 (steel type C) had secondary cooling temperatures of 167°C, 163°C, and 161°C, respectively, compared to the lower limit of the target range of 180°C. As a low case, the target range of elongation of 14% or more was not satisfied. When the secondary cooling temperature is low, the amount of martensite transformation during cooling becomes too large, and the fraction of retained austenite may be insufficient.

In Comparative Example 3 (steel type B), the secondary cooling temperature was 305 ° C., which was higher than the upper limit of the target range of 300 ° C., and the yield strength, yield ratio, and elongation were respectively lower than the lower limit of the target range. If the secondary cooling temperature is too high, the amount of martensite transformation during cooling is too small, which may lead to a decrease in yield strength. In addition, it is analyzed that the elongation is lowered due to insufficient stability of retained austenite.

In Comparative Example 5 (steel type D) and Comparative Example 6 (steel type D), after performing secondary cooling, the rapid cooling holding times were 0.9 seconds and 1.2 seconds, respectively, shorter than the target ranges, and the yield strength and yield ratio were within the target ranges, respectively. was lower than the lower limit of This is analyzed to be due to insufficient time required to increase the yield strength due to the occurrence of lower bainite transformation during maintenance after secondary cooling or the formation of fine precipitates inside martensite.

Comparative Examples 7 and 8 were cases of steel grade F having a carbon content of 0.229 wt %, which was lower than the lower limit of the target composition, did not satisfy the tensile strength of 1400 MPa, and Comparative Example 7 had an elongation of 14% or more was also not satisfied.

Comparative Example 9 had a sulfur content of 0.0046 wt%, which was higher than the upper limit of the target composition, and was composed of steel grade G. Although the tensile strength and elongation met the target range, the 90 degree bendability (R/t) was 4.33, which was 3 or less. did not meet the target range of

3 is a scanning electron microscope photograph showing the microstructure of an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention.

3, (a) is Example 5, (b) is Comparative Example 2, (c) is Comparative Example 3.

3, in the case of Example 5, tempered martensite, upper bainite, lower bainite and retained austenite were observed.

In Comparative Example 2, martensite, upper bainite, and retained austenite were observed, and tempered martensite was not observed.

In Comparative Example 3, tempered martensite and retained austenite were observed. It can be seen that the amount of martensite transformation is too small and the stability of retained austenite is insufficient.

4 is a scanning electron microscope EBSD distribution diagram showing the microstructure of an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 4 , retained austenite as a microstructure of the cold-rolled steel sheet of Example 1 is shown in green. It can be seen that the retained austenite is distributed between the laths of martensite and bainite, and is also distributed at grain boundaries.

5 is a graph showing the size distribution of retained austenite in the microstructure of the ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 5 , it was found that retained austenite had an average diameter of 0.78 μm in the ultra-high strength cold-rolled steel sheet of Example 1 of FIG. 4 .

Referring to FIG. 5 , it can be seen that retained austenite has an average diameter of 0.78 μm in the ultra-high strength cold rolled steel sheet of Example 1 of FIG. 4 . In addition, it can be seen that the retained austenite structure of Example 1 has an average diameter of 0.78 µm, and the fraction of the structure having a diameter of 1.0 µm is 80% or less. Through this, fine retained austenite is distributed and stabilized, and strength and elongation can be stably secured.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible within the scope without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (15)

By weight%, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium ( Cr): greater than 0% to 1.0%, molybdenum (Mo): greater than 0% to 0.5%, sum of niobium (Nb), titanium (Ti) and vanadium (V): greater than 0% to 0.1%, phosphorus (P) : more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.01%, and the balance contains iron (Fe) and other unavoidable impurities,
Yield strength (YP): 1100 MPa or more, tensile strength (TS): 1400 MPa or more, and elongation (El): 14% or more, yield ratio (YR): 75% or more, and bendability (R/t): 3.0 satisfying the following,
High-strength cold-rolled steel sheet. The method of claim 1,
The ultra-high strength cold rolled steel sheet,
Containing a mixed structure in which ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed,
High-strength cold-rolled steel sheet. 3. The method of claim 2,
The fraction of ferrite ranges from greater than 0% to 5%,
The fraction of martensite is in the range of greater than 0% to 20%,
The fraction of retained austenite is in the range of 10% to 30%,
the fraction of upper bainite ranges from greater than 0% to 30%,
The fraction of lower bainite ranges from greater than 0% to 30%,
The fraction of tempered martensite is included as the remaining fraction,
High-strength cold-rolled steel sheet. 4. The method of claim 3,
The minimum value of the sum of the fraction of the upper bainite and the fraction of the lower bainite is 10%,
High-strength cold-rolled steel sheet. The method of claim 1,
The ultra-high strength cold rolled steel sheet,
It contains a mixed structure in which tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed,
The fraction of martensite is in the range of greater than 0% to 20%,
The fraction of retained austenite is in the range of 10% to 30%,
the fraction of upper bainite ranges from greater than 0% to 30%,
The fraction of lower bainite ranges from greater than 0% to 30%,
The fraction of tempered martensite is included as the remaining fraction,
High-strength cold-rolled steel sheet. 3. The method of claim 2,
The average diameter of the retained austenite is 1.0 μm or less,
High-strength cold-rolled steel sheet. By weight%, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium ( Cr): more than 0% to 1.0%, the sum of nickel (Ni) and copper (Cu): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.01%, and the balance contains iron (Fe) and other unavoidable impurities,
Yield strength (YP): 1100 MPa or more, tensile strength (TS): 1400 MPa or more, and elongation (El): 14% or more, yield ratio (YR): 75% or more, and bendability (R/t): 3.0 satisfying the following,
High-strength cold-rolled steel sheet. 8. The method of claim 7,
The ultra-high strength cold rolled steel sheet,
Further comprising, by weight %, the sum of niobium (Nb), titanium (Ti) and vanadium (V): greater than 0% to 0.1%;
High-strength cold-rolled steel sheet. By weight%, carbon (C): 0.25% to 0.40%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium ( Cr): greater than 0% to 1.0%, molybdenum (Mo): greater than 0% to 0.5%, sum of niobium (Nb), titanium (Ti) and vanadium (V): greater than 0% to 0.1%, phosphorus (P) : more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.01%, and the remainder to manufacture a hot-rolled steel sheet containing iron (Fe) and other unavoidable impurities to do;
manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet;
annealing the cold-rolled steel sheet at a temperature in the range of 810°C to 930°C;
cooling the cold-rolled steel sheet in multiple stages; and
Partitioning heat treatment of the cooled cold-rolled steel sheet at a temperature in the range of 360°C to 500°C; including,
A method for manufacturing an ultra-high strength cold rolled steel sheet. 10. The method of claim 9,
The step of manufacturing the hot-rolled steel sheet,
preparing a steel slab having the alloy composition;
reheating the steel slab in the range of 1,150° C. to 1,250° C.;
preparing a hot-rolled steel sheet by hot finish rolling the reheated steel slab at a finish rolling end temperature in the range of 850°C to 970°C; and
Including; winding the hot-rolled steel sheet in the range of 400 ℃ ~ 700 ℃;
A method for manufacturing an ultra-high strength cold rolled steel sheet. 10. The method of claim 9,
The annealing heat treatment step,
It is performed by heating the cold-rolled steel sheet at a temperature increase rate in the range of 2°C/sec to 10°C/sec and maintaining it at a temperature in the range of 810°C to 930°C for a time in the range of 30 seconds to 120 seconds,
A method for manufacturing an ultra-high strength cold rolled steel sheet. 10. The method of claim 9,
The step of the multi-stage cooling is,
first cooling the cold-rolled steel sheet subjected to the annealing heat treatment to a primary cooling end temperature in the range of 650° C. to 800° C. at a cooling rate in the range of 2° C./sec to 15° C./sec; and
Secondary cooling of the firstly cooled cold-rolled steel sheet to a secondary cooling termination temperature in the range of 180°C to 300°C at a cooling rate of 20°C/sec or more;
A method for manufacturing an ultra-high strength cold rolled steel sheet. 13. The method of claim 12,
After performing the secondary cooling step,
Maintained for a time in the range of 5 seconds to 90 seconds at the secondary cooling termination temperature,
A method for manufacturing an ultra-high strength cold rolled steel sheet. 10. The method of claim 9,
The partitioning heat treatment step is,
It is performed by heating the cooled cold-rolled steel sheet at a temperature increase rate in the range of 3°C/sec to 20°C/sec and maintaining it at a temperature in the range of 360°C to 500°C for a time in the range of 20 seconds to 500 seconds,
A method for manufacturing an ultra-high strength cold rolled steel sheet. 10. The method of claim 9,
The ultra-high-strength cold-rolled steel sheet manufactured by the method for manufacturing the ultra-high-strength cold-rolled steel sheet,
Yield strength (YP): 1100 MPa or more, tensile strength (TS): 1400 MPa or more, and elongation (El): 14% or more, yield ratio (YR): 75% or more, and bendability (R/t): 3.0 satisfies the following,
Containing a mixed structure in which ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed,
A method for manufacturing an ultra-high strength cold rolled steel sheet.

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