EP4640884A1

EP4640884A1 - Cold rolled steel sheet and method for manufacturing same

Info

Publication number: EP4640884A1
Application number: EP23907614.4A
Authority: EP
Inventors: Ki-Taek Jung; Young-Roc Im; Chang-Hyo Seo; Sang-Hyun Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-19
Filing date: 2023-12-15
Publication date: 2025-10-29
Also published as: JP2025538971A; MX2025005273A; EP4640884A4; KR20240095963A; WO2024136328A1; CN120225713A

Abstract

The present invention provides: an ultra-high-strength cold-rolled steel sheet having excellent elongation and hole expansion properties; and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a cold-rolled steel sheet and a method for manufacturing the same.

Background Art

Recently, the automobile industry is focusing on reducing the weight of a vehicle body and securing collision stability to improve fuel efficiency and stability of a vehicle along with regulations on greenhouse gas emissions due to global warming. Accordingly, demand for securing manufacturing of a technology for an ultra-high strength steel sheet is increasing.
Automotive parts to which an ultra-high steel sheet is applied require not only strength, but also excellent elongation and hole expandability for forming parts having formability, weldability, or the like. In general, as the strength of the steel sheet increases, press formability deteriorates, and to overcome this, a method using Transformation Induced Plasticity (TRIP) steel utilizing retained austenite is being used.
(Patent Document 1) Patent Publication No. 2017-7015003

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide an ultra-high strength cold-rolled steel sheet having excellent elongation and hole expandability and a method for manufacturing the same.
Another aspect of the present disclosure is to provide an ultra-high strength cold-rolled steel sheet having excellent elongation and hole expandability as well as excellent hydrogen embrittlement resistance and a method for manufacturing the same.
An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.

Solution to Problem

According to an aspect of the present disclosure, provided is a cold-rolled steel sheet,

the cold-rolled steel sheet including by weight %: 0.05 to 0.4% of carbon (C), 0.1 to 3.0% of silicon (Si), 0.005 to 3.0% of aluminum (Al), 1.0 to 4.0% of manganese (Mn), 1.5% or less (including 0%) of chromium (Cr), 0.001 to 0.5% of molybdenum (Mo), 0.0001 to 0. 003% of boron (B), 0.001 to 0. 05% of niobium (Nb), 0.001 to 0.05% of titanium (Ti), 0.04% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and a balance of Fe and other unavoidable impurities,
as a microstructure, includes by area%, 30% or less (including 0%) of ferrite, more than 10% to 25% or less of retained austenite, more than 40% to 80% or less of tempered martensite, 40% or less (including 0%) of bainite, and 5% or less (including 0%) of fresh martensite.
wherein a value defined by the following Relational Expression 1 satisfies a value of more than 145 to less than 160. 573X[C] - 45X[Si] + 25X[Mn] - 95X[Al] + 318X[Cr] - 59X[Ni] - 83X[Mo] - 5X[Cu] - 73X[Ti] - 68X[Nb] + 100X[B]

In Relational Expression 1 above, [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B] represent a weight% content for each element in parentheses.
The cold-rolled steel sheet may satisfy a value defined by the following Relational Expression 2 of more than 155 to less than 175. 630×[C] - 23×[Si] + 410×[P] - 150×[Cr]-15×[Ni] + 300×[B]
In Relational Expression 2 above, [C], [Si], [P], [Cr] and [Ni] represent a weight% content for each element in parentheses.
The cold-rolled steel sheet may satisfy a value defined by the following Relational Expression 3 of 40 or more. $[P - El] 2 + [RA]$
In Relational Expression 3, [P-El] represents a post-elongation value of a uniaxial tensile test, and [RA] represents a fraction % of retained austenite.
The cold-rolled steel sheet may satisfy the following Relational Expression 4. $0.3 ppm \leq IH 2$
In Relational Expression 4 above, IH2 represents a critical hydrogen amount for fracture of the cold-rolled steel sheet.
According to another aspect of the present disclosure, provided is a method for manufacturing a cold-rolled steel sheet,

the method including operations of: reheating a steel slab including by weight%, 0.05 to 0.4% of carbon (C), 0.1 to 3.0% of silicon (Si), 0.005 to 3.0% of aluminum (Al), 1.0 to 4.0% of manganese (Mn), 1.5% or less (including 0%) of chromium (Cr), 0.001 to 0.5% of molybdenum (Mo), 0.0001 to 0.003% of boron (B), 0.001 to 0.05% of niobium (Nb), 0.001 to 0.05% of titanium (Ti), 0.04% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and a balance of Fe and other unavoidable impurities, wherein a value defined by the following Relational Expression 1 satisfies more than 145 to less than 160;
finishing hot rolling the reheated steel slab at a temperature within a range of 830 to 980°C to obtain a hot-rolled steel sheet;
coiling the hot-rolled steel sheet at a temperature within a range of 450 to 700°C;
cold rolling the coiled hot-rolled steel sheet;
continuously annealing the cold-rolled steel sheet at a temperature within a range of 800 to 900°C to satisfy a dew point temperature of -45°C or lower;
primarily cooling the continuously annealed steel sheet to a primary cooling end temperature within a range of 550 to 650°C at an average cooling rate of less than 10°C/s;
Secondary cooling the primarily-cooled steel sheet to a secondary cooling end temperature within a range of 150 to 400°C at an average cooling rate of 10°C/s or more; and
heat treating the Secondary-cooled steel sheet at a temperature within a range of 350 to 480°C. 573X[C] - 45X[Si] + 25X[Mn] - 95X[Al] + 318X[Cr] - 59X[Ni] - 83X[Mo] - 5X[Cu] - 73X[Ti] - 68X[Nb] + 100X[B]

In Relational Expression 1 above, [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb], and [B] represent a weight% content for each element in parentheses.
Wherein, in the continuous annealing, an atmosphere within a continuous annealing furnace may be controlled in a gas including, by volume%, 95% or more of nitrogen, and a balance of hydrogen.

Advantageous Effects of Invention

As set forth above, according to an aspect of the present disclosure, an ultra-high strength cold-rolled steel sheet having excellent elongation and hole expandability and a method for manufacturing the same may be provided.
According to another aspect of the present disclosure, an ultra-high strength cold-rolled steel sheet having excellent elongation and hole expandability, as well as excellent hydrogen embrittlement resistance and a method for manufacturing the same may be provided.
The various and beneficial advantages and effects of the present disclosure are not limited to the above-described contents, and may be more easily understood through descriptions of specific embodiments of the present disclosure.

Best Mode for Invention

Hereinafter, embodiments of the present disclosure will be described. It is obvious that various modifications may be provided to the following examples by those skilled in the art to which the present disclosure pertains, without departing from the scope of the present disclosure. The following embodiments are provided to help understand the present disclosure, and the scope of the present disclosure should not be limited to the following examples, but should be defined not only by the claims described below but also by equivalents thereof.
The terms used herein are for the purpose of describing the present invention and are not intended to limit the present invention. In addition, the singular forms used herein include the plural forms unless the relevant definition clearly indicates a meaning contrary thereto.
The term "comprising" as used in the specification means specifying a configuration, and does not exclude the presence or addition of other configurations.
Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as would be commonly understood by a person of ordinary skill in the art to which the present invention belongs. Terms defined in the dictionary are interpreted to have a meaning consistent with the relevant technical literature and the present disclosure.
Prior art discloses a TRIP steel sheet which introduces retained austenite to secure excellent formability of ultra-high strength steel with a tensile strength of 1470MPa or more. However, in order to satisfy high formability at ultra-high strength, a large amount of Si and Al is required to be added, and as a content of Si increases, there is a problem in that the possibility of Liquid Metal Embrittlement (LME) occurring during spot welding increases and manufacturing costs increase.
In particular, when Al, having a similar effect to Si, is added as a substitute to prevent the occurrence of LME, a transformation temperature of steel was increased in addition to the increase in manufacturing costs, which increased a load during hot rolling. In addition, since a high soaking section (SS) temperature is required in Quenching & Partitioning (Q&P) steel which requires single-phase annealing by increasing an Ac3 temperature, the invention may not be implemented with the current facilities and devices, and when all the facilities and devices are changed, an excessive amount of costs is required, and furthermore, there is a problem that the facilities and devices also have to be newly developed.
Accordingly, there is a need to develop an ultra-high strength steel with a tensile strength of 1470 MPa and excellent elongation and hole expandability under annealing heat treatment conditions at an operable level, while limiting the amount of C, Si, and Al added to the steel sheet, but this has not been reported to date.
In order to improve local formability, it is effective to reduce a deviation in hardness between microstructures comprising a steel material. As an industrial local formability evaluation, a widely performed test is the hole expansion ratio (HER) measurement. For hole expandability (HER), a specimen having a hole with a diameter of 10 mm formed by punching is fixed to a die, and the hole is expanded by pushing up the hole with a conical punch, and when a crack penetrating through the total thickness of the hole occurs, a diameter of the expanded hole is measured, so that a value equal to that of in the following Relationship A is obtained. Detailed evaluation criteria for hole expandability are based on ISO 16630. $λ (HER) = (df - do) / do$
In Relational Expression A above, do is a diameter of an initial hole, and df is a diameter of the hole when the thickness is fractured.
Accordingly, the inventors of the present disclosure have discovered that this may be solved by precisely controlling an alloy composition, structure fraction, and manufacturing conditions to manufacture steel under conditions that can be mass-produced while securing ultra-high strength with a tensile strength of 1470 MPa or more, and excellent elongation and hole expandability, thereby completing the present disclosure.
Hereinafter, an ultra-high strength cold-rolled steel sheet having excellent elongation and hole expandability and a method for manufacturing the same according to an embodiment of the present disclosure will be described.
First, an alloy composition of the cold-rolled steel sheet according an embodiment of the present disclosure will be described. A content of an alloy composition described below is based on weight%.

Carbon (C): 0.05 to 0.4%

Carbon (C) is an element which secures the strength of a steel material through solid solution strengthening and precipitation strengthening, and is an element effective for securing high elongation by stabilizing retained austenite. When the content of C is less than 0.05%, a tensile strength of 1500 MPa may not be obtained, and when the content of C exceeds 0.4%, a steel sheet may not be manufactured by cold rolling. Therefore, the appropriate content of C is in the range of 0.05 to 0.4%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of C may be 0.2%, or an upper limit of the content of C may be 0.4%.

Silicon (Si): 0.1 to 3.0%

Silicon (Si) is a useful element for increasing the strength of a steel sheet through solid solution strengthening and precipitation hardening. Since Si suppresses the formation of cementite, Si has the effect of promoting C concentration in austenite, and is an essential element for increasing the strength and elongation of steel by generating retained austenite after annealing. When the content of Si is less than 0.1%, retained austenite does not remain, and thus uniform elongation may not be obtained. On the other hand, when the content of Si exceeds 3.0%, the weld properties deteriorate due to LME cracking, and the surface properties and plating properties of the steel material deteriorate. In order to further improve the above-described effect, a lower limit of the content of Si may be 0.3%, 0.45%, or 0.5%. Similarly, in order to further improve the above-described effect, an upper limit of the content of Si may be 2.5 .

Aluminum (Al): 0.005 to 3.0%

Aluminum (Al) is an element which has a deoxidizing effect on molten steel, and similarly to Si, and acts to improve the stability of austenite, and is effective in increasing elongation.
When the content of Al is less than 0.005%, the deoxidation of a steel material may not be sufficient, thereby impairing cleanliness of the steel material. On the other hand, when the content of Al is too excessive, exceeding 3.0%, a transformation temperature increases significantly, and a fraction of ferrite increases, so that ultra-high strength may not be achieved. Therefore, in the present disclosure, the content of Al is set to 0.005 to 3.0%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Al may be 0.01%, or 0.02%. Similarly, in terms of further improving the above-described effect, an upper limit of the content of Al may be 2.0%, or 1.0%.

Manganese (Mn): 1.0 to 4.0%

Manganese (Mn) is an element added to secure strength. When the content of Mn is less than 1.0%, it is difficult to secure the strength. On the other hand, when the content of Mn exceeds 4.0%, a bainite transformation speed is slowed, so that an excessive amount of fresh martensite is formed and thus it is difficult to obtain high hole expandability. In addition, a band structure is formed due to segregation of Mn, impairing material uniformity and formability of the material. Therefore, the content of Mn is controlled to be in the range of 1.0 to 4.0%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Mn may be 1.5%, or an upper limit of the content of Mn may be 3.5%.

Chromium (Cr): 1.5% or less (including 0%)

Chromium (Cr) is an element effective for improving strength. Cr suppresses the formation of carbides and makes it easier to secure retained austenite. Meanwhile, when the content of Cr exceeds 1.5%, local corrosion resistance deteriorates and surface oxides are formed, thereby impairing phosphate treatment properties. Therefore, the content of Cr is controlled to be 1.5% or less (including 0%). Meanwhile, in terms of further improving the above-described effect, an upper limit of the content of Cr may be 1.0%.

Molybdenum (Mo): 0.001 to 0.5%

Molybdenum (Mo) improves the stability of Fe carbides, and precipitation by Mo improves hydrogen embrittlement resistance properties. In order to secure the above-described effect, 0.001% or more of Mo is required to be added. On the other hand, when the content of Mo exceeds 0.5%, the phase transformation is suppressed, making it difficult to introduce a bainite structure, and Mo is an expensive element, thereby deteriorating the economic feasibility of the steel sheet. Therefore, in the present disclosure, the content of Mo is set to be in the range of 0.001 to 0.5%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Mo may be 0.07%, or an upper limit of the content of Mo may be 0.495%.

Boron (B): 0.0001 to 0.003%

Boron (B) strengthens grain boundaries and suppresses ferrite transformation during cooling after annealing. To obtain such an effect, a content of B added is set to be 0.0001% or more. On the other hand, when the content of B exceeds 0.003%, the hot-rolling properties deteriorate and B is excessively accumulated on the surface, thereby impairing plating properties. Therefore, the content of B is set to be in a range of 0.0001 to 0.003%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of B may be 0.0005%, or an upper limit of the content of B may be 0.0025%.

Niobium (Nb): 0.001 to 0.05%

Niobium (Nb) forms alloy carbides and contributes to strength improvement through precipitation strengthening and structural refinement. To obtain the above-described effect, a content of Nb is set to 0.001% or more. On the other hand, when the content of Nb exceeds 0.05%, recrystallization is delayed due to local grain fixation, which impairs the uniformity of the structure. Therefore, in the present disclosure, the content of Nb is set to be in the range of 0.001 to 0.05%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Nb may be 0.015%, or an upper limit of the content of Nb may be 0.03%.

Titanium (Ti): 0.001 to 0.05%

Titanium (Ti) is an element that combines with C or N to form fine precipitates and refine old austenite grains, thereby improving strength and hydrogen embrittlement resistance properties.
When the content of Ti is less than 0.001%, it is difficult to obtain the effects of strength improvement and structural refinement. On the other hand, when the content of Ti exceeds 0.05%, castability is impaired due to excessive TiN formation, and recrystallization is delayed due to local crystal grain fixation, which impairs the uniformity of the structure. Therefore, it is preferable that the content of Ti be in the range of 0.001 to 0.05%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Ti may be 0.015%, or an upper limit of the content of Ti may be 0.03%.

Phosphorous (P): 0.04% or less (excluding 0%)

Phosphorous (P) is contained as an impurity and segregates at grain boundaries, thereby lowering toughness. Therefore, it is preferable that a content of P is controlled to be as low as possible. Since the toughness of the steel material deteriorates when P is excessively added, in the present disclosure, it is preferable that an upper limit of the content of P is limited to 0.04%, to prevent this. However, considering the case in which P is inevitably incorporated as an impurity during the manufacturing process, 0% is excluded from the content of P. Meanwhile, in terms of the above-described effect, a lower limit of the content of P may be 0.002%, or an upper limit of the content of P may be 0.0173%.

Sulfur (S): 0.01% or less (excluding 0%)

Sulfur (S)is contained as an impurity in steel, like in the case of P. Sulfur (S) combines with Manganese (Mn) to form inclusions, which may reduce hole expandability, and may also reduce weldability and hot rolling properties, so it is advantageous to control a content of S as low as possible. Considering the case in which the content of S is inevitably included, an upper limit of the content of S is preferably controlled to 0.01% or less, excluding 0%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of S may be 0.0009%, or the upper limit of the content of S may be 0.005%.

Nitrogen (N): 0.01% or less (excluding 0%)

In the present disclosure, nitrogen (N) is an impurity and is included in a steel material, and it is advantageous to control a content of N as low as possible. Therefore, as a lower limit of the content of N, 0% is excluded (i.e., more than 0%), considering the case in which N is inevitably included. However, an upper limit of the content of N is preferably limited to 0.01% or less. Meanwhile, in terms of further improving the above-described effect, the lower limit of the content of N may be 0.0005%. Likewise, in terms of further improving the above-described effect, the upper limit of the content of N may be 0.007%, or 0.006%, or 0.0052%.
The remaining components of the present disclosure may be iron (Fe) and inevitable impurities. However, since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
According to an embodiment of the present disclosure, although not particularly limited, the cold-rolled steel sheet may optionally further include at least one selected from the group consisting of 0.1% or less (including 0%) of Cu and 0.1% or less (including 0%) of Ni.

Copper (Cu) : 0.1% or less (including 0%), Nickel (Ni): 0.1% or less (including 0%)

Copper (Cu) and Nickel (Ni) are elements which increase the strength of a steel material. The elements are elements increasing the strength and hardenability of a steel material, but when excessive amounts of Cu and Ni are added, the target level of strength may be exceeded, and since Cu and Ni are expensive elements, it is preferable to limit an upper limit each of contents of Cu and Ni to 0.1% or less, from an economic perspective. Meanwhile, since Cu and Ni act as solid solution strengthening elements, when adding at least one of Cu and Ni, when 0.03% or less of at least one of Cu and Ni is added, a solid solution strengthening effect may be insignificant, so it is preferable to 0.03% or more of at least one of Cu and Ni is added, respectively.
In an embodiment of the present disclosure, although not particularly limited, the cold-rolled steel sheet may optionally further include 0.05% or less (including 0%) of vanadium (V).

Vanadium (V): 0.05% or less (including 0%)

Vanadium (V) may increase the strength of steel even with a trace amount of addition, but the effect on improving elongation is not significant, so it is preferable to control a content of V to 0.05% or less. Considering the elongation, the content of V is more preferably 0.04% or less, and even more preferably 0.03% or less.
The microstructure of the cold-rolled steel sheet according to an embodiment of the present disclosure may include, by area %, 30% or less (including 0%) of ferrite, more than 10% to 25% or less of retained austenite, more than 40% to 80% or less of tempered martensite, 40% or less (including 0%) of bainite, and 5% or less (including 0%) of fresh martensite.
Although not particularly limited, according to an embodiment of the present disclosure, the cold-rolled steel sheet is intended to secure excellent formability even at a high tensile strength of 1470 MPa, and in particular, in order to obtain high local formability, it is necessary to control the added elements and reduce a difference in hardness between phases of the microstructure comprising the steel sheet. In the present disclosure, an alloy composition may be controlled so that the alloy composition described above is satisfied under the normal annealing heating conditions, and a value defined by the following Relational Expression is greater than 145 to less than 160, so that an austenite single phase may be obtained and a fraction of ferrite may be maintained low at 30% or less. However, when the fraction of ferrite exceeds 30%, the yield strength decreases and the hole expandability deteriorates. Meanwhile, in terms of securing high yield strength and excellent hole expandability, an upper limit of the fraction of ferrite may be 10%, and even more preferably, the upper limit of the fraction of ferrite may be 7%. 573X[C] - 45X[Si] + 25X[Mn] - 95X[Al] + 318X[Cr] - 59X[Ni] - 83X[Mo] - 5X[Cu] - 73X[Ti] - 68X[Nb] + 100X[B]
In Relational Expression 1 above, [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B] represent a weight% content for each element in parentheses.
The value defined by Relational Expression 1 above may be controlled to be more than 145 to less than 160, so that excessive formation of a soft ferrite phase may be avoided, but when bainite, a soft phase, next to ferrite, is not sufficiently introduced, it may be difficult to secure the ductility of the steel material. Meanwhile, in terms of further improving the above-described effect, a lower limit of the value defined by Relational Expression 1 above may be 146, or an upper limit of the value defined by Relational Expression 1 above may be 159.
In addition, although not particularly limited, according to an embodiment of the present disclosure, a value defined by Relational Expression 2 above may satisfy a value more than 155 to less than 175. 630×[C] - 23×[Si] + 410×[P] - 150×[Cr]-15×[Ni] + 300×[B]
In Relational Expression 2 above, [C], [Si], [P], [Cr] and [Ni] represent a weight% content for each element in parentheses.
The cold-rolled steel sheet according to the present disclosure includes a main base including more than 40% to 80% or less of tempered austenite and more than 10% to 25% or less of retained austenite, 40% or less (including 0%) of bainite, and 5% or less (including 0%) of fresh martensite. When the difference in hardness between the main phases is large, hole expandability may deteriorate, and when retained austenite is insufficient, elongation may decrease. Therefore, a method to reduce the difference in hardness between the phases and secure the fraction of retained austenite is necessary.
Accordingly, the inventors of the present disclosure have conducted in-depth studies, and then have found that it is possible to manufacture an ultra-high cold-rolled steel sheet having excellent elongation and hole expandability by controlling the components of the added elements and a ratio of the main structure in Relational Expressions 1 and 2 above to be within an appropriate range.
Specifically, the cold-rolled steel sheet according to the present invention preferably has 30% or less (including 0%) of ferrite. When the area fraction of ferrite exceeds 30%, it is difficult to secure high hole expandability as hardness increases between the phases of ferrite, which is a soft phase, tempered martensite, which is a hard main phase, and bainite.
Retained austenite is preferably more than 10% to 25% or less. When a fraction of retained austenite is 10% or less, it is difficult to secure elongation due to insufficient retained austenite. When the fraction of retained austenite exceeds 25%, phase transformation should be performed at a high temperature, and although it has the effect of further improving elongation, the target level of strength (1470 MPa or more) may not be obtained due to insufficient fraction of tempered martensite.
Bainite is preferably 40% or less. When a fraction of bainite exceeds 40%, the strength and elongation decrease due to a lack of tempered martensite and retained austenite, and as the fraction of bainite decreases, the desired elongation may not be obtained due to a decrease in the fraction of retained austenite. In this case, a lower limit of the fraction of bainite may include 0%, or exceed 0%, or be 7.4%.
In addition, although not particularly limited, according to an embodiment of the present disclosure, a material and structural fraction of the cold-rolled steel sheet may be controlled so that the value defined by Relational Expression 3 above satisfies a value of 40 or more. $[P - El] 2 + [RA]$
In Relational Expression 3 above, [P-El] represents a post-elongation value of a uniaxial tensile test, and [RA] represents a fraction (area%) of retained austenite.
P-El is a difference in between T-El and U-El, and is a value which represents local formability. In addition, the fraction of retained austenite may be controlled so that the value defined by Relational Expression 3 satisfies 40 or more as a correlation with elongation. As a result, an ultra-high strength steel sheet having excellent elongation and hole expandability may be manufactured even under normal annealing conditions.
In addition, although not particularly limited, according to an embodiment of the present disclosure, the following Relational Expression 4 may be satisfied, and thereby, a cold-rolled steel sheet with excellent hydrogen embrittlement resistance may be secured. $0.3 ppm \leq IH 2$
In Relational Expression 4, IH2 is a critical hydrogen amount for fracture of the cold-rolled steel sheet.
Meanwhile, the cold-rolled steel sheet of the present disclosure may have a hot-dip galvanized layer formed on at least one surface thereof. The present disclosure does not specifically limit the composition of the hot-dip galvanized layer, and any hot-dip galvanized layer commonly applied in the relevant technical field may be preferably applied to the present disclosure. In addition, the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer alloyed with some alloy components of the steel sheet.
Hereinafter, a method for manufacturing an ultra-high strength cold-rolled steel sheet having excellent elongation and hole expandability according to an aspect of the present disclosure will be described. However, this does not mean that the cold-rolled steel sheet of the present disclosure should be manufactured only by the following manufacturing method.
According to an embodiment of the present disclosure, the method for manufacturing the cold-rolled steel sheet is performed in an order of reheating, hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, and heat treatment of steel having the alloy composition described above.

[Reheating of steel slab]

A method for manufacturing a slab provided for hot rolling is not limited, and as an example, a continuous casting slab may be used, and as another example, a slab manufactured by a thin slab caster may be used. In addition, hot rolling may be performed immediately after continuous casting. When reheating the slab, the reheating temperature is preferably 1150 to 1250°C. When the heating temperature is lower than 1150°C, a finishing rolling temperature tends to be lower than 850°C, and a rolling load increases. From the perspective of manufacturing costs, the heating temperature is preferably lower than 1250°C.

[Hot rolling]

Thereafter, the reheated slab is finished hot rolled at a temperature within a range of 830 to 980°C to obtain a hot-rolled steel sheet. When the finishing hot rolling temperature (hereinafter, referred to as 'FDT') is lower than 830°C, the rolling load is high and shape defects increase, resulting in poor productivity. On the other hand, when the finishing hot rolling temperature exceeds 980°C, the surface quality deteriorates due to an increase in oxides caused by excessive high-temperature operation. Therefore, it is preferable that the finishing hot rolling temperature be in the range of 830 to 980°C. A lower limit of the finishing hot rolling temperature is more preferably 880°C. An upper limit of the finishing hot rolling is more preferably 950°C, and even more preferably 930°C.

[Coiling]

The hot-rolled steel sheet obtained by the hot rolling is coiled at a temperature within a range of 450 to 700°C. When the coiling temperature (hereinafter, referred to as 'CT') exceeds 700°C, thick hot-rolled internal oxidation occurs on a surface of the steel sheet, and the pickling property may be reduced. In order to improve toughness by making an effective grain diameter fine and to improve hole expandability by uniformizing retained austenite, a lower limit of the coiling temperature is set to 450°C. Meanwhile, in terms of further improving the above-described effect, the lower limit of the coiling temperature is more preferably 480°C, and more preferably 500°C. Likewise, an upper limit of the coiling temperature is more preferably 670°C, and more preferably 640°C. Meanwhile, although not particularly limited, after the finishing hot rolling, cooling may be performed at an average cooling rate of 10 to 100°C/s to the coiling temperature. When the average cooling speed is less than 10°C/s, the hot rolling productivity may decrease, and there may be a problem that a cooling medium with low cooling capacity should be deliberately adopted during actual production. When the average cooling speed exceeds 100°C/s, a deviation in temperatures inside the steel sheet may become uneven, which may cause problems such as poor shape and excessively high strength of the steel sheet.

[Cold rolling]

The coiled hot-rolled steel sheet is cold rolled. During the cold rolling, a cold reduction ratio may be 30 to 60%. When the cold reduction ratio is less than 30%, it is difficult to secure the target thickness precision, and the shape correction of the steel sheet can also be difficult. On the other hand, when the cold reduction ratio exceeds 60%, the possibility of cracks occurring at an edge of the steel sheet increases, and a cold rolling load may become excessively high. Therefore, it is preferable that the cold reduction ratio be in the range of 30 to 60%.

[Continuously Annealing]

The cold-rolled steel sheet is continuously annealed at a temperature within a range of 800 to 900°C to satisfy a dew point temperature of -45°C or lower. The continuous annealing operation is performed to heat the steel sheet to an austenite single-phase region to form austenite close to 100% and use the same for subsequent phase transformation. When the continuous annealing temperature (hereinafter, referred to as 'SS') is lower than 800°C, sufficient recrystallization and austenite transformation may not be performed, and thus the target fractions of martensite and bainite may not be secured after annealing. On the other hand, when the continuous annealing temperature is higher than 900°C, productivity may decrease, coarse austenite may be formed, and the material may deteriorate, and the surface quality may deteriorate, such as peeling of a plating material. In addition, the continuous annealing may be performed in a continuous alloying molten plating continuous furnace.
Meanwhile, during the continuous annealing, it is preferable to control an atmosphere within a continuous annealing furnace with a gas including by volume %, 95% or more and a balance of hydrogen. When the fraction of nitrogen is less than 95%, and the ratio of hydrogen is not increased accordingly, an oxidizing atmosphere is formed within the furnace, causing oxides to form on the surface of the steel sheet, resulting in poor surface quality. When the ratio of hydrogen increases, process difficulty such as explosion risk is aggravated.

[Primary cooling]

Thereafter, the continuously annealed steel sheet is primarily cooled to a primary cooling end temperature (hereinafter, referred to as 'SCS') within a range 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).
The primary cooling end temperature may be defined as a point in time at which secondary cooling begins by additionally applying quenching equipment which was not applied in the primary cooling. When the cooling process is divided into primary and secondary cooling processes and performed step by step, a temperature distribution of the steel sheet may be made uniform in a slow cooling stage, thereby reducing a final temperature and material deviation, and obtaining the required phase composition. When the primary cooling end temperature is lower than 550°C, the fraction of bainite becomes excessively high, and it is difficult that cooling is performed to below 550°C at a cooling rate of less than 10°C/s due to the length of actual equipment. That is, in order to cool to below 550°C, changes or additional installation of equipment may be required, which may result in additional costs. In addition, when the primary cooling end temperature exceeds 650°C, the required cooling temperature until the secondary cooling end temperature increases, so that the shape of the steel sheet may be poor and the fraction of bainite may be lower than the target level.

[Secondary Cooling]

Thereafter, the primarily-cooled steel sheet is secondarily cooled at an average cooling rate of 10°C/s or more to a secondary cooling end temperature (hereinafter, referred to as 'RCS') within a range of 150 to 400°C. The secondary cooling end temperature is set to be lower than a Ms temperature of the steel sheet so that martensite transformation occurs during cooling, and this martensite ultimately becomes a tempered martensite phase through a reheating operation, which is a post-process. When the secondary cooling end temperature is lower than 150°C, an initial martensite transformation amount is too large, the yield strength becomes excessively high, and the formability deteriorates. On the other hand, when the secondary cooling end temperature exceeds 400°C, martensite is not generated during cooling, so a large amount of fresh martensite is ultimately generated and an appropriate tensile strength may be exceeded, making it difficult to obtain high yield strength and hole expandability. When the secondary cooling rate is less than 10°C/s, even if the secondary cooling reaches the target secondary cooling end temperature, high-temperature phase transformation occurs during cooling, making it impossible to obtain the target martensite fraction and high strength. More preferably, in terms of improving the above-described effect, a lower limit of the secondary cooling rate may be 20°C/s, and an upper limit of the secondary cooling rate may be 60°C/s.
As mentioned above, the secondary cooling may be performed by additionally applying a quenching equipment which was not applied in the primary cooling, and in the present disclosure, the type of quenching equipment is not particularly limited, but as a preferred example, hydrogen quenching facility may be used. More specifically, the hydrogen quenching facility can use a gas including, by volume %, 50 to 80% of hydrogen, with a remainder of nitrogen. When the fraction of hydrogen exceeds 80%, there may be a disadvantage in that management, such as explosion control of the facility, becomes difficult, and when the fraction of hydrogen is less than 50%, there may be a disadvantage in that it becomes difficult to utilize the efficient heat transfer characteristics of hydrogen, which is a light element.

[Heat treatment]

Thereafter, the secondarily-cooled steel sheet is reheated to a temperature within a range of 350 to 480°C. Through the heat-treatment process, carbon distribution between phases required for stabilizing retained austenite and additional bainite phase transformation are obtained. In the present disclosure, the end point temperature of the heating section is conveniently referred to as a reheating temperature (hereinafter, also referred to as 'RHS'). When the reheating temperature is lower than 350°C, the strength becomes excessively high and the elongation becomes poor. On the other hand, when the reheating temperature exceeds 480°C, the austenite phase is not transformed and remains, and then becomes fresh martensite during the final cooling, which impairs hole expandability and elongation. Meanwhile, the so-called nose temperature, where the bainite transformation is most active, is around 400 to 420°C. Considering the same, it is more preferable that a lower limit of the reheating temperature is 410°C, or it is more preferable that an upper limit of the reheating temperature is 440°C.
In addition, in an embodiment of the present disclosure, after the reheating operation, a process of hot-dip galvanizing, alloying hot-dip galvanizing, and temper rolling on the reheated steel sheet may be further performed. Specifically, the process may further include an operation of plating the reheated steel sheet in a zinc plating bath at a temperature within a range of 450 to 470°C.
In addition, in an embodiment of the present disclosure, an operation of performing an alloying heat treatment of the plated steel sheet at a temperature within a range of 470 to 550°C may be further included. The alloying heat treatment is intended to obtain an appropriate alloying level, and an alloying heat treatment temperature is determined according to surface conditions of the steel sheet. By controlling the surface conditions of the steel sheet, the alloying heat treatment temperature should not exceed 550°C, thereby preventing softening of the steel sheet and loss of retained austenite due to excessive tempering. Meanwhile, in order to accelerate alloying, the alloying heat treatment temperature is preferably higher than a hot-dip galvanizing temperature, so a lower limit thereof is controlled to 470°C. In addition, after the alloying heat treatment, an operation of cooling the alloying heat-treated steel sheet to room temperature and then performing temper rolling at a reduction ratio of less than 1% may be further included in order to correct the shape of the steel sheet and adjust the yield strength.

Mode for Invention

Hereinafter, the present disclosure will be specifically described through the following Examples. However, it should be noted that the following examples are only for describing the present disclosure by illustration, and not intended to limit the right scope of the present disclosure. The reason is that the right scope of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.

(Example)

After preparing a slab having the alloy composition described in Tables 1 to 2 below, the slab was reheated at a temperature within a range of 1180 to 1220°C, and was subjected to hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, and heat treatment under the conditions described in Tables 3 to 4 below to manufacture a cold-rolled steel sheet. Meanwhile, during the continuous annealing, an atmosphere within a continuous annealing furnace was controlled with a gas including by volume%, 95% or nitrogen, and a remainder of hydrogen, and a dew point temperature at a temperature within a range of 800 to 900°C was controlled to -45°C.
The results of evaluating the tensile characteristics, elongation, and hole expandability of the cold-rolled steel sheet manufactured in this manner were shown in Table 5 below. A tensile strength (TS), yield strength (YS), and elongation (EL) were measured through a tensile test in a direction perpendicular to a rolling direction, and a tensile specimen having a gauge length of 50 mm and a width of 25 mm was used. Hole expandability (HER) was measured according to the ISO 16330 standard, and holes were sheared with a clearance of 12% using a 10 mm diameter punch.
In addition, in order to evaluate hydrogen embrittlement resistance, an experiment was conducted by immersing a sample in a 0.1N HCl solution for 120 hours under an applied stress of 80% of the tensile strength (TS) after 4-point banding, and a critical hydrogen amount for fracture in the type of steel that no fracture occurred after immersion for 120 hours was measured, and the results were shown in Table 5 below. In this case, when fracture occurred after 120 hours of immersion, it was evaluated as 'X', meaning that the standard for hydrogen embrittlement resistance was not met, and if no fracture occurred, it was evaluated as 'o'.

In addition, the results of measuring the microstructure of the cold rolled steel sheet manufactured above and the calculation results of Relational Expression 3 used in the present disclosure were shown in Table 5.

[Table 1]

Division	Alloy composition (weight %)
Steel type	C	Si	Mn	P	S	Al	Cr	Ni	Mo
A	0.313	2.02	2.48	0.010	0.0025	0.0320	0.00	0.00	0.001
B	0.329	1.96	2.47	0.0090	0.0026	0.0421	0.00	0.00	0.001
C	0.342	1.92	2.46	0.0084	0.0026	0.0480	0.00	0.096	0.001
D	0.334	1.97	2.47	0.0098	0.0025	0.0424	0.00	0.00	0.106
E	0.354	1.89	2.65	0.0108	0.0040	0.0461	0.00	0.00	0.001
F	0.347	1.89	2.65	0.0102	0.0042	0.0458	0.00	0.00	0.001
G	0.354	1.88	2.67	0.0097	0.0042	0.0434	0.00	0.00	0.494

[Table 2]

Divi sion	Alloy composition (weight %)
Steel type	Ti	Nb	Cu	B	N	Relational Expression1	Relational Expression2
A	0.02	0.000	0.000	0.0018	0.0044	146.05	155.37
B	0.0212	0.000	0.118	0.0021	0.0035	156.06	166.51
C	0.0221	0.000	0.000	0.0021	0.0025	159.36	173.93
D	0.0230	0.0182	0.000	0.0022	0.0037	148.96	169.79
E	0.0211	0.000	0.000	0.000	0.0051	178.04	183.98
F	0.0193	0.0221	0.000	0.000	0.0048	172.69	179.32
G	0.0206	0.000	0.000	0.000	0.0054	138.36	183.76

[Table 3]

Division	Steel type	Thickness of hot-rolled steel sheet [mm]	Thickness of cold-rolled steel sheet [mm]	Reduction ratio [%]	FDT [°C]	CT [°C]
Inventive Example 1	A	2.4	1.4	42	905	563
Inventive Example 2	B	2.4	1.4	42	899	572
Inventive Example 3	C	2.4	1.4	42	921	515
Inventive Example 4	D	2.4	1.4	42	864	603
Comparative Example 1	E	2.4	1.4	42	915	595
Comparative Example 2	F	2.4	1.4	42	896	612
Comparative Example 3	G	2.4	1.4	42	933	544

[Table 4]

Division	SS [°C]	SCS [°C]	RCS [°C]	RHS [°C]	OAS [°C]	FCS [°C]
Inventive Example 1	874	613	195	404	356	149
Inventive Example 2	873	605	203	410	360	152
Inventive Example 3	871	624	206	409	359	151
Inventive Example 4	894	601	224	405	355	150
Comparative Example 1	878	611	209	412	368	154
Comparative Example 2	877	613	205	408	352	150
Comparative Example 3	871	607	208	406	355	153

[Table 5]

Division	Microstructure (area %)					YS	TS	E1	PE1	HER	I_H2*	Hydrogen embrittlement resistance	Relational Expression 3
Division	F	γ	TM	FM	B	[MPa]	[MPa]	[%]	[%]	[%]	[ppm]	[○/X]	Relational Expression 3
Inventive Example 1	0	14.61	70.59	3.3	11.5	1257	1542	16.8	5.3	25.2	0.388	○	42.7
Inventive Example 2	0	17.74	70.76	3.3	8.2	1271	1543	17.7	5.6	22	0.324	○	49.1
Inventive Example 3	0	17.74	70.86	4.0	7.4	1263	1545	17.6	5.4	22.1	0.357	○	46.9
Inventive Example 4	0	18.9	69.88	3.5	7.9	1192	1535	18	5	20.9	0.316	○	43.9
Comparative Example 1	0	17.94	65.43	7.3	9.3	1193	1532	18	4.6	7.9	0.205	X	39.1
Comparative Example 2	0	17.96	63.44	8.1	10.5	1181	1538	17.4	4.2	7.1	0.233	X	35.6
Comparative Example 3	0	17.39	66.71	6.4	9.5	1184	1635	16	3.1	12.4	0.163	X	27

IH2*: Critical hydrogen amount for fracture
F: Area fraction of ferrite
γ: Area fraction of retained austenite
TM: Area fraction of tempered martensite
FM: Area fraction of fresh martensite
B: Area fraction of bainite

In Inventive Example satisfying the alloy composition and manufacturing conditions of the present disclosure, as compared to Comparative Examples, it was confirmed that Inventive Examples had higher tensile strength (TS), yield strength (YS), hole expandability (HER), and elongation (El), thereby having excellent elongation and hole expandability as well as ultra-high strength characteristics.
On the other hand, in Comparative Examples that did not satisfy one or more of the alloy composition and manufacturing conditions of the present disclosure, it was confirmed that one or more characteristics selected from among tensile strength (TS), yield strength (YS), hole expandability (HER), and elongation (El) were inferior.

Claims

A cold-rolled steel sheet, comprising by weight%:
0.05 to 0.4% of carbon (C), 0.1 to 3.0% of silicon (Si), 0.005 to 3.0% of aluminum (Al), 1.0 to 4.0% of manganese (Mn), 1.5% or less (including 0%) of chromium (Cr), 0.001 to 0.5% of molybdenum (Mo), 0.0001 to 0.003% of boron (B), 0.001 to 0.05% of niobium (Nb), 0.001 to 0.05% of titanium (Ti), 0.04% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and a balance of Fe and other unavoidable impurities,

as a microstructure, includes by area%, 30% or less (including 0%) of ferrite, more than 10% to 25% or less of retained austenite, more than 40% to 80% or less of tempered martensite, 40% or less (including 0%) of bainite, and 5% or less (including 0%) of fresh martensite,

wherein a value defined by the following Relational Expression 1 satisfies more than 145 to less than 160, 573X [C] - 45X[Si] + 25X[Mn] - 95X[Al] + 318X[Cr] - 59X[Ni] - 83X[Mo] - 5X[Cu] - 73X[Ti] - 68X[Nb] + 100X[B]

in Relational Expression 1 above, [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb] and [B] represent a weight% content for each element in parentheses.
The cold-rolled steel sheet of claim 1, wherein a value defined by the following Relational Expression 2 satisfies more than 155 to less than 175, 630×[C] - 23×[Si] + 410×[P] - 150×[Cr]-15×[Ni] + 300×[B] in Relational Expression 2 above, [C], [Si], [P], [Cr], and [Ni], represent a weight% content for each element in parentheses.
The cold-rolled steel sheet of claim 1, wherein a value defined by the following Relational Expression 3 satisfies 40 or more, $[P - El] 2 + [RA]$ in Relational Expression 3 above, [P-El] represent a post-elongation value of a uniaxial tensile test, and [RA] represents a fraction % of retained austenite.
The cold-rolled steel sheet of claim 1, wherein the following Relational Expression 4 is satisfied, $0.3 ppm \leq IH 2$ in Relational Expression 4 above, IH2 represents a critical hydrogen amount for fracture of the cold-rolled steel sheet.
A method for manufacturing a cold-rolled steel sheet, comprising:
reheating a steel slab including by weight%, 0.05 to 0.4% of carbon (C), 0.1 to 3.0% of silicon (Si), 0.005 to 3.0% of aluminum (Al), 1.0 to 4.0% of manganese (Mn), 1.5% or less (including 0%) of chromium (Cr), 0.001 to 0.5% of molybdenum (Mo), 0.0001 to 0.003% of boron (B), 0.001 to 0.05% of niobium (Nb), 0.001 to 0.05% of titanium (Ti), 0.04% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and a balance of Fe and other unavoidable impurities, wherein a value defined by the following Relational Expression 1 satisfies more than 145 to less than 160;

finishing hot rolling the reheated steel slab at a temperature within a range of 830 to 980°C to obtain a hot-rolled steel sheet;

coiling the hot-rolled steel sheet at a temperature within a range of 450 to 700°C;

cold rolling the coiled hot-rolled steel sheet;

continuously annealing the cold-rolled steel sheet at a temperature within a range of 800 to 900°C to satisfy a dew point temperature of -45°C or lower;

primarily cooling the continuously annealed steel sheet to a primary cooling end temperature within a range of 550 to 650°C at an average cooling rate of less than 10°C/s;

secondary cooling the primarily-cooled steel sheet to a secondary cooling end temperature within a range of 150 to 400°C at an average cooling rate of 10°C/s or more; and

heat treating the Secondary-cooled steel sheet at a temperature within a range of 350 to 480°C, 573X [C] - 45X[Si] + 25X[Mn] - 95X[Al] + 318X[Cr] - 59X[Ni] - 83X[Mo] - 5X[Cu] - 73X[Ti] - 68X[Nb] + 100X[B]

in Relational Expression 1 above, [C], [Si], [Mn], [Al], [Cr], [Ni], [Mo], [Cu], [Ti], [Nb], and [B]represent a weight% content for each element in parentheses.
The method for manufacturing a cold-rolled steel sheet of claim 5, wherein, in the continuously annealing,
an atmosphere within a continuous annealing furnace is controlled in a gas comprising by volume%, 95% or more of nitrogen, and a balance of hydrogen.