EP4640881A1

EP4640881A1 - Steel sheet and manufacturing method therefor

Info

Publication number: EP4640881A1
Application number: EP23907583.1A
Authority: EP
Inventors: Cheol-Seung HYUN; Yeon-Sang Ahn; Kang-Hyun Choi
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-21
Filing date: 2023-12-14
Publication date: 2025-10-29
Also published as: WO2024136297A1; EP4640881A4; JP2025541897A; KR20240098674A; CN120344692A; MX2025007284A

Abstract

The present invention relates to a steel sheet and a manufacturing method therefor, and, more specifically, to a high-strength cold-rolled steel sheet having excellent ductility and hole-expandability, and a manufacturing method therefor.

Description

Technical Field

The present disclosure relates to a steel sheet and a manufacturing method therefor, and, more specifically, to a high-strength cold-rolled steel sheet having excellent ductility and hole-expandability, and a manufacturing method therefor.

Background Art

Recently, as interest in carbon dioxide reduction and the corresponding regulatory standards have increased, automobile manufacturers are making great efforts to improve fuel efficiency through lightweight vehicle body reduction. In order to efficiently implement such lightweight vehicle body reduction, it is essential to adopt high-strength steel that reduces the thickness of a steel sheet to reduce the weight of a vehicle body while simultaneously ensuring passenger safety.
Recently, the application of high-strength steel to structural members such as members, seat rails, and A/B/C pillars, or the like has increased in order to improve the impact resistance of the vehicle body. Automobile structural members have characteristics that are advantageous in absorbing impact energy as yield strength as compared to tensile strength, that is, the yield ratio (yield strength/tensile strength), is higher.
However, as the strength increases, the formability decreases due to the decrease in ductility, and thus, there is a need to develop materials that have high yield ratio characteristics and improved formability at the same time. Accordingly, in order to meet these technical requirements, it is necessary to develop automobile steel sheets that are not only suitable as automobile materials but also have excellent ductility and hole-expandability.
Patent document 1 discloses a technology for manufacturing a steel sheet having a martensite phase having an area fraction of 80% or more by utilizing water cooling during continuous annealing and overaging treatment. When tempering is performed by immersing the steel sheet in water after soaking during the annealing process, a steel sheet having a tempered martensite structure may be manufactured. In this case, a yield ratio increases due to a tempering effect of martensite, but there may be a concern that workability may be reduced because the shape quality of the coil deteriorates due to temperature deviations in width and length directions of the steel sheet, or cracks may occur during forming due to material deviation problems.
On the other hand, Patent document 2 discloses a technology for manufacturing a composite structure steel sheet with excellent workability by utilizing a retained austenite phase. This technology may simultaneously secure strength and ductility required by automobile manufacturers by utilizing transformation-induced plasticity, but it is difficult to secure quality in steelmaking and continuous casting due to the large amount of Si and Al added to create retained austenite, and it may be difficult to obtain surface quality on a level of an outer plate.
Accordingly, development of a steel sheet that may simultaneously secure strength, ductility, and hole-expandability while solving the problems of the existing technologies described above is required.

[Prior Art Document]

[Patent Document]

(Patent Document 1) Japanese Laid-open Patent Publication No. 1992-289120
(Patent literature 2) Japanese Laid-open Patent Publication No. 2015-113504

Summary of Invention

Technical Problem

An embodiment of the present disclosure is to provide a steel sheet and a manufacturing method therefor.
An embodiment of the present disclosure is to provide a high-strength cold-rolled steel sheet having excellent ductility and hole-expandability, and a manufacturing method therefor.
The aspects of the present disclosure are not limited to the above-described contents. Those skilled in the art will have no difficulty in understanding additional aspects of the present disclosure from the overall contents of this specification.

Solution to Problem

According to an embodiment of the present disclosure, provided is a steel sheet including: by wt%, carbon (C): 0.05 to 0.20%, manganese (Mn): 2.3 to 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, and a balance of iron (Fe) and inevitable impurities,

a value R defined in the following relational expression 1 is 5.0 to 6.0,
a microstructure includes, in area%, 40% or more of tempered martensite, and includes a balance structure of ferrite, fresh martensite, bainite, and retained austenite, and
MR defined in the following relational expression 2 is 0.60 to 0.80,

$R = [C] + 1.3 [Si] + 1.5 [Mn] + 1.8 [Nb]$
(where [C], [Si], [Mn], and [Nb] are wt% of each element),

$MR = TM / (FM + TM)$
(where TM and FM are area% of tempered martensite and fresh martensite, respectively.)

The microstructure may include 30 to 50% of ferrite and 5 to 25% of fresh martensite.
The steel sheet may include at least one of Nb-based fine precipitates and Ti-based fine precipitates having an average diameter (circle equivalent diameter) of 50 nm or less, at a number of 10¹² or more per unit area (m²).
The steel sheet may have a yield strength of 700 MPa or more, a tensile strength of 1000 MPa or more, a yield ratio of 0.70 or more, and an elongation of 13% or more.
A product of a yield strength and an elongation may be 9100 MPa·% or more, a product of a tensile strength and an elongation may be 13000 MPa·% or more, and a hole expansion ratio (HER) may be 50% or more.
According to an embodiment of the present disclosure, provided is a manufacturing method for a steel sheet, including: reheating a steel slab including, by wt%, carbon (C): 0.05 to 0.20%, manganese (Mn): 2.3 to 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S) : 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, and a balance of iron (Fe) and inevitable impurities, and having a value R of 5.0 to 6.0 defined in the following relational expression 1;

hot-rolling the reheated steel slab;
cooling and coiling the hot-rolled steel sheet;
cold-rolling the coiled steel sheet;
continuously annealing by heating the cold-rolled steel sheet to a temperature within a temperature range of 760 to 830°C and holding the steel sheet for 30 to 300 seconds;
primarily cooling the continuously annealed steel sheet to a temperature within a temperature range of 500 to 700°C at an average cooling rate of 2.0 to 6.0°C/s;
secondarily cooling the steel sheet after the primary cooling to a temperature within a temperature range of 100 to 300°C at an average cooling rate of 35.0 to 60.0°C/s; and
subjecting the steel sheet to an overaging treatment by heating the steel sheet to a temperature within a temperature range of 200 to 450°C after the secondary cooling and holding the steel sheet for 100 to 500 seconds,
$R = [C] + 1.3 [Si] + 1.5 [Mn] + 1.8 [Nb]$
(where [C], [Si], [Mn], and [Nb] are wt% of each element.)

The reheating may be performed in a temperature within a temperature range of 1000 to 1350°C,
The hot-rolling may be performed at a finishing rolling temperature of Ar3 to Ar3+50°C,

the coiling may be performed in a temperature within a temperature range of 450 to 700°C, and
the cold-rolling may be performed at a reduction ratio of 35 to 65%.

The manufacturing method for a steel sheet may further include pickling the steel sheet after the coiling.
The manufacturing method for a steel sheet may further include temper rolling the steel sheet at a reduction ratio of 0.1 to 1.0% after the overaging treatment.

Advantageous Effects of Invention

An embodiment of the present disclosure is to provide a steel sheet and a manufacturing method therefor.
An embodiment of the present disclosure is to provide a high-strength cold-rolled steel sheet having excellent ductility and hole-expandability, and a manufacturing method therefor.
According to an embodiment of the present disclosure, provided is a cold-rolled steel sheet which may be used as a material for automobiles, especially for members requiring high formability, and which have excellent strength and excellent ductility and hole-expandability, and a manufacturing method therefor.

Brief Description of Drawings

FIG. 1 is a microstructure image of Inventive Example 1 according to an embodiment of the present disclosure.
FIG. 2 is a microstructure image of Comparative Example 4 deviating from an embodiment of the present disclosure.

Best Mode for Invention

Hereinafter, preferred embodiments of the present disclosure will be described. The embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present disclosure in more detail to those skilled in the art to which the present disclosure pertains.
The inventors of the present disclosure have conducted in-depth research to provide a steel sheet having improved ductility and hole-expandability so that the steel sheet has strength suitable for automobile materials and formability that may be processed into components requiring forming into complex shapes.
As a result, the inventors of the present disclosure have confirmed that targeted properties may be secured by optimizing an alloy composition and a microstructure of steel, from which it may be provide a steel sheet suitable for automobile structural members requiring processing into complex shapes, and thus has completed the present disclosure.
The present disclosure will be described in detail below.
The composition of the steel sheet of the present disclosure will be described in detail below.
In the present disclosure, unless otherwise specifically stated, % indicating the content of each element is based on weight.
According to an embodiment of the present disclosure, a steel sheet may include, by wt%, carbon (C): 0.05 to 0.20%, manganese (Mn): 2.3 to 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, and a balance of iron (Fe) and inevitable impurities.

Carbon (C): 0.05 to 0.20%

Carbon (C) is an element that is advantageous in forming martensite in steel to secure strength, and is an essential element in manufacturing high-strength steel. In general, the higher the content of the carbon (C), the easier it is to form martensite, which is advantageous for the formation of a composite structure required for manufacturing high-strength steel. Accordingly, in the present disclosure, in order to secure the target strength and form an appropriate level of martensite, carbon (C) may be included in the content of 0.05% or more. In an embodiment of the present disclosure, carbon (C) may be included in the content of 0.07% or more. However, in order to simultaneously control the intended strength and elongation, it is required to control the content thereof on an appropriate level. When the content of carbon (C) exceeds 0.20%, there is a problem that weldability and formability are deteriorated. According to an embodiment of the present disclosure, it may be included at 0.18% or less.

Manganese (Mn): 2.3-3.0%

Manganese (Mn) is an element that improves the hardenability of steel, and specifically plays an important role in forming martensite. Additionally, manganese (Mn) contributes to the increase in strength of steel by the effect of solid solution strengthening, and serves to suppress the occurrence of sheet fracture and high-temperature embrittlement phenomenon due to S during hot-rolling by precipitating S, which is inevitably added to the steel, as MnS. In order to sufficiently obtain the above-described effect, manganese (Mn) may be added in the content of 2.3% or more. In an embodiment of the present disclosure, manganese (Mn) may be included in the content of 2.5% or more. However, when the content thereof exceeds 3.0%, not only will the weldability be deteriorated, but martensite may be excessively formed, making the material unstable, and a band-shaped oxide band may be formed, which may increase the risk of processing cracks and sheet fracture. Additionally, there may be a concern that manganese oxide may be dissolved on a surface of a steel sheet during annealing, which may hinder the plating property. In an embodiment of the present disclosure, manganese (Mn) may be included in the content of 2.8% or less.

Silicon (Si): 1.5% or less

Silicon (Si) is a useful element that may secure strength without lowering the ductility of the steel sheet. Silicon (Si) is also advantageous in promoting the formation of ferrite and promoting the formation of martensite by promoting the enrichment of C into untransformed austenite. However, when the content of silicon (Si) exceeds 1.5%, since there may be a concern that hydrogen embrittlement and weldability deterioration may occur, the present disclosure may include silicon (Si) in the content of 1.5% or less. However, considering a level that is inevitably added to the steel, 0% may be excluded.

Aluminum (Al): 0.10% or less

Aluminum (Al) is an element added for deoxidation and grain refinement of steel. When the content of aluminum (Al) exceeds 0.10%, not only may the castability deteriorate during continuous casting, but there is also a problem that the possibility of causing material defects of an annealed material and surface defects of the plating material increases due to excessive formation of inclusions. Accordingly, in the present disclosure, aluminum (Al) may be included in the content of 0.10% or less. However, considering a level that is inevitably added to steel, 0% may be excluded. In an embodiment of the present disclosure, aluminum (Al) may be included in the content of 0.01% or more.

Phosphorus (P): 0.05% or less

Phosphorus (P) is the most advantageous element for securing the strength of steel without significantly damaging formability. However, when phosphorus (P) is excessively added, the possibility of brittle fracture greatly increases, and thus, phosphorus (P) is also an element increasing the possibility of sheet fracture of a slab during hot-rolling. Accordingly, the content of phosphorus (P) may be limited to 0.05% or less in the present disclosure. However, considering a level that is inevitably added to the steel, 0% may be excluded.

Sulfur (S): 0.010% or less

Sulfur (S) is an impurity element that is inevitably added to the steel, and it is preferable to manage the content of sulfur (S) as low as possible. Specifically, sulfur (S) in steel may increase the possibility of occurrence of red-hot embrittlement, and accordingly, in the present disclosure, the content thereof may be limited to 0.010% or less. However, considering a level that is inevitably added during steel manufacturing, 0% may be excluded.

Nitrogen (N): 0.010% or less

Nitrogen (N) is an impurity element that is inevitably added to the steel, and it is preferable to manage the content thereof as low as possible. However, for this purpose, there is a problem that refining costs of steel increases rapidly, and thus, nitrogen (N) may be controlled to be 0.010% or less, which belongs to range in which operating conditions are possible. However, considering a level that is inevitably added during steel manufacturing, 0% may be excluded.

Niobium (Nb): 0.10% or less

Niobium (Nb) is an element segregating at austenite grain boundaries and suppresses the coarsening of austenite grains during annealing heat treatment, as well as forming fine carbides to contribute to improving yield strength and tensile strength. However, when the content of niobium (Nb) is excessive, there may be a concern that the strength and elongation may decrease due to the precipitation of coarse carbides and the reduction of the carbon content in the steel, and manufacturing costs may increase, which may result in poor economic feasibility. Accordingly, in the present disclosure, during an addition of niobium (Nb), an upper limit thereof may be limited to 0.10%. In an embodiment of the present disclosure, the upper limit thereof may be limited to 0.05%. However, considering a level that is inevitably added during steel manufacturing, 0% may be excluded.

Titanium (Ti): 0.10% or less

Titanium (Ti) not only contributes to securing the yield strength and tensile strength by forming fine carbides, but also effectively reduces the risk of cracks occurring during casting by precipitating N in steel as TiN and suppressing the precipitation of AlN. However, when the content thereof is excessive, the strength and elongation may decrease due to the precipitation of coarse carbides and reduction of carbon content in steel, and nozzle clogging may also occur during continuous casting. Accordingly, in the present disclosure, when adding titanium (Ti), an upper limit thereof may be limited to 0.10%. In an embodiment of the present disclosure, the upper limit thereof may be limited to 0.05%. However, considering a level that is inevitably added during steel manufacturing, 0% may be excluded.

Boron (B): 0.003% or less

Boron (B) is effective in delaying the transformation of austenite into pearlite during the cooling process after continuous annealing. However, when the content of the boron (B) is excessive, boron (B) may be concentrated on a surface of the steel sheet, resulting in surface defects. Accordingly, in the present disclosure, when adding the boron (B), an upper limit thereof may be limited to 0.003%. However, considering a level that is inevitably added during steel manufacturing, 0% may be excluded.
The steel material of the present disclosure may include a balance of iron (Fe) and inevitable impurities in addition to the composition described above. Since inevitable impurities may be unintentionally mixed during a normal manufacturing process, the inevitable impurities may not be excluded. Since these impurities are known to anyone skilled in the field of normal steel manufacturing, not all of their contents are specifically mentioned in this specification.
The steel sheet according to an embodiment of the present disclosure may have a value R of 5.0 to 6.0 defined in the following relational expression 1. $R = [C] + 1.3 [Si] + 1.5 [Mn] + 1.8 [Nb]$
(In the relational expression, [C], [Si], [Mn], and [Nb] are wt% of each element.)
The relational expression 1 is obtained as an empirical numerical value for a content relationship of specific components to secure the basic material of the steel sheet intended for the present disclosure.
When the value R defined in the relational expression 1 is less than 5.0, the hardenability of the steel is reduced, and a phase fraction of a low-temperature transformation structure such as martensite is reduced, which may lead to the problem that the strength intended for the present disclosure may not be secured. In an embodiment of the present disclosure, the value R may be 5.2 or more. In an embodiment of the present disclosure, the value R may be 5.3 or more. On the other hand, when the value exceeds 6.0, the hardenability of the steel increases excessively, and a material having the formability desired in the present disclosure may not be obtained. In an embodiment of the present disclosure, an upper limit of the value R may be 5.8.
Hereinafter, the microstructure of the steel sheet of the present disclosure will be described in detail.
Unless otherwise specifically stated in the present disclosure, the % indicating a fraction of the microstructure is based on an area.
The microstructure of the steel sheet according to an embodiment of the present disclosure may include, in area%, 40% or more of tempered martensite. Additionally, the balance structure may include ferrite, fresh martensite, bainite, and retained austenite.
In the present disclosure, martensite may be divided into tempered martensite and fresh martensite. The martensite may be formed by transforming a portion of austenite during secondary cooling at a temperature equal to or lower than a martensite transformation initiation temperature (Ms) after a primary cooling section.
The tempered martensite may refer to a structure formed by tempering the transformed martensite during the secondary cooling while passing through an overaging treatment section. During an overaging treatment, a Cottrell atmosphere phenomenon may be caused in which carbons (C) that escaped during the martensite tempering are fixed to a surrounding dislocation, in which case the yield strength of the steel sheet may increase.
The fresh martensite may refer to a structure formed by diffusionless transformation when austenite remaining before the final cooling after the overaging treatment is cooled at room temperature.
When the tempered martensite is less than 40%, there is a problem in that the strength and ductility targeted by the present disclosure may not be properly secured. According to an embodiment of the present disclosure, an upper limit thereof may be 65%.
Generally, during the phase transformation of ferrite, bainite, or the like, since the carbon concentration in the surrounding austenite increases, the strength of the steel increases due to martensite having increased solid-solution carbon in a final structure. In this case, when the TRIP effect is accompanied by stabilization of retained austenite, an improvement in elongation may be expected at the same time. However, in the case of the present disclosure, the high elongation despite the relatively low fraction of retained austenite may be seen as a result of the homogeneous structure of tempered martensite and a large reduction in a hardness difference between phases. In other words, due to the high fraction of tempered martensite, along with a high tensile strength of 1000 MPa or more, ductility and hole-expandability may be secured due to the homogeneity of the structure and the reduction in the hardness difference between phases.
According to an embodiment of the present disclosure, in order to more effectively secure the strength and ductility proposed in the present disclosure, the steel sheet may include 30 to 50% of ferrite and 5 to 25% of fresh martensite, and may include a balance structure of bainite and retained austenite. According to an embodiment of the present disclosure, the steel sheet may include 30 to 45% of ferrite.
The steel sheet according to an embodiment of the present disclosure may have an MR defined in the following relational expression 2 of 0.60 to 0.80. $MR = TM / (FM + TM)$
(In the relational expression, TM and FM are area% of tempered martensite and fresh martensite, respectively.)
The relational expression 2 may refer to a fraction of tempered martensite with respect to the total martensite fraction. When the value MR defined in the relational expression 2 is 0.60 to 0.80, the desired strength may be secured, and the yield ratio may also be 0.70 or higher to secure formability.
When the value MR defined in relational expression 2 is less than 0.60, the fraction of fresh martensite in martensite can be improved, so the tensile strength can increase, but it may be difficult to satisfy a desired yield ratio and desired hole-expandability. On the other hand, when the value exceeds 0.80, tempered martensite in the martensite may be excessively formed, which may make it difficult to secure the desired tensile strength.
According to an embodiment of the present disclosure, a steel sheet may include at least one of Nb-based fine precipitates and Ti-based fine precipitates having an average diameter (circular equivalent diameter) of 50 nm or less, at a number of 10¹² or more per unit area (m²).
In the present disclosure, as described above, the fine precipitates may be included so that the desired high strength characteristics may be more advantageously secured. More specifically, the present disclosure may effectively secure not only bendability thereof but also hole expansion ratio (HER) by effectively reducing a hardness difference between phases from precipitation and by tempering of martensite.
According to an embodiment of the present disclosure, a steel sheet may have a yield strength of 700 MPa or more, a tensile strength of 1000 MPa or more, a yield ratio of 0.70 or more, an elongation of 13% or more, a product of yield strength and elongation of 9100 MPa·% or more, a product of tensile strength and elongation of 13000 MPa·% or more, and a hole expansion ratio (HER) of 50% or more, so that the steel sheet may have excellent strength while also having excellent ductility and hole-expandability.
Hereinafter, a method for manufacturing a steel sheet of the present disclosure will be described in detail.
According to an embodiment of the present disclosure, a steel sheet may be manufactured by reheating, hot-rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, and overaging a steel slab satisfying the alloy composition described above.

Reheating

A steel slab satisfying the alloy composition of the present disclosure may be reheated in a temperature within a temperature range of 1000 to 1350°C.
When a reheating temperature is less than 1000°C, there may be a concern that hot-rolling may be performed in a temperature range equal or to lower than an intended finishing rolling temperature. On the other hand, when the temperature exceeds 1350°C, there is a possibility that the steel may reach a melting point and melt.

Hot-rolling

The reheated steel slab may be hot-rolled at a finishing rolling temperature of Ar3 to Ar3+50°C.
When the finishing rolling temperature is less than Ar3, there is a high possibility that the hot deformation resistance may increase rapidly. On the other hand, when the temperature exceeds Ar3+50°C, not only may an excessively thick oxide scale occur, but there is also a concern that the grains of the hot-rolled steel sheet may be formed coarsely, which may cause a deterioration in the properties of a final steel sheet. Here, the finishing rolling temperature may refer to a temperature at an exit of a finishing rolling mill. $\begin{array}{l} Ar 3 = 910 - 95 * [C] - \\ 15.2 * [Ni] + 44.7 * [Si] + 104 * [V] + 31.5 * [Mo] - \\ (15 * [Mn] + 11 * [Cr] + 20 * [Cu] - 700 * [P] - 400 * [Al] - 400 * [Ti]) \end{array}$
(In the formula, [C], [Ni], [Si], [V], [Mo], [Mn], [Cr], [Cu], [P], [Al], and [Ti] are wt% of each element.)

Coiling

The hot-rolled steel sheet may be cooled and coiled at a temperature within a temperature range of 450 to 700°C.
When the coiling temperature is less than 450°C, martensite or bainite may be excessively generated, which may cause manufacturing problems such as shape defects due to load during a subsequent cold rolling process. On the other hand, when the temperature exceeds 700°C, there is a problem that the pickling property deteriorates due to an increase in surface scale. Meanwhile, cooling conditions from hot-rolling to the coiling temperature are not particularly limited, but may be cooling conditions applicable in the same technical field. In an embodiment of the present disclosure, air cooling may be used.

Cold Rolling

The coiled steel sheet may be cold-rolled at a reduction ratio of 35 to 65%.
During cold rolling, when the reduction ratio is less than 35%, recrystallization driving force is weakened, which may make it difficult to secure good recrystallized grains, and shape correction may be difficult. On the other hand, when the reduction ratio exceeds 65%, there is a high possibility that cracks may occur in an edge of the steel sheet, and there may be a concern that a rolling load may increase rapidly.
According to an embodiment of the present disclosure, before performing the cold rolling, a pickling process may be performed to remove scales formed on a surface of the steel sheet. The pickling process may be performed under normal conditions, and the conditions may not be particularly limited.

Continuous Annealing

The cold-rolled steel sheet may be continuously annealed by heating the steel sheet in a temperature within a temperature range of 760 to 830°C and holding the steel sheet for 30 to 300 seconds.
In the present disclosure, the foundation of the microstructure targeted by the present disclosure may be established through the continuous annealing.
During the continuous annealing, when the heating temperature is less than 760°C, a fraction of unrecrystallized ferrite may exist, which may cause defects in which a material deviation increases in a length or width direction of a coil. According to an embodiment of the present disclosure, the heating may be performed at 770°C or higher. On the other hand, when the temperature exceeds 830°C, there may be a concern that annealing oxides may be generated on the surface of the steel sheet. Additionally, it may be impossible to secure the target elongation due to the non-generation of ferrite during continuous annealing. According to an embodiment of the present disclosure, the heating may be performed at a temperature of 810°C or lower.
During the continuous annealing, when the holding time is less than 30 seconds, recrystallization is not sufficiently achieved, which may make it difficult to secure the elongation, and the possibility of material deviation occurring in a length or width direction of the steel sheet increases. On the other hand, when the holding time exceeds 300 seconds, the annealing effect is saturated, and there is a problem of reduced productivity.

Primary Cooling

The continuously annealed steel sheet may be primarily cooled at an average cooling rate of 2.0 to 6.0°C/s to a temperature within a temperature range of 500 to 700°C.
In the present disclosure, during the primary cooling, slow cooling may be performed as compared to a subsequent secondary cooling process, and a plate shape deterioration due to a temperature drop during the secondary cooling, which is a relatively rapid cooling section, may be suppressed.
During the primary cooling, when a cooling end temperature is less than 500°C or exceeds 700°C, the cooling end temperature may deviated from an appropriate temperature gradient range with a subsequent secondary cooling, and thus, it may be difficult to secure stable cooling performance.
During the primary cooling, when an average cooling rate exceeds 6.0°C/s, there may be a concern that C and Mn enrichment in austenite may not occur sufficiently. In the present disclosure, there is no particular limitation on a lower limit of the average cooling rate during the primary cooling, but cooling may be performed at a cooling rate of normal slow cooling. In the present disclosure, the lower limit may be 2.0°C/s.

Secondary Cooling

After the primary cooling, secondary cooling may be performed at an average cooling rate of 35.0 to 60.0°C/s up to a temperature within a temperature range of 100 to 300°C.
In the present disclosure, during the secondary cooling, the cooling rate and the cooling end temperature may be appropriately adjusted according to the width and thickness of the steel sheet to be obtained, thereby securing an optimal plate shape.
During the secondary cooling, when the cooling end temperature is less than 100°C or the average cooling rate exceeds 60.0°C/s, this may lead to a condition that is difficult to implement in a normal manufacturing process, which may cause a problem of reduced productivity. According to an embodiment of the present disclosure, an upper limit of the average cooling rate may be 58.0°C/s.
On the other hand, when the temperature exceeds 300°C or the average cooling rate is less than 35.0°C/s, the fraction of martensite transformed during the secondary cooling decreases, which may results in a decrease in the fraction of tempered martensite, and may cause a decrease in the yield strength.

Overaging Treatment

After the secondary cooling, an overaging treatment may be performed by heating the steel sheet to a temperature within a temperature range of 200 to 450°C and holding the steel sheet for 100 to 500 seconds.
During the overaging treatment, when the overaging temperature is less than 200°C, the amount of carbon (C) escaping from the martensite transformed during the secondary cooling decreases during the overaging treatment. Accordingly, since the degree of tempering decreases, the yield strength of the steel sheet may be inferior. On the other hand, when the temperature exceeds 450°C, the amount of carbon (C) escaping from the martensite transformed during the secondary cooling during the overaging treatment increases excessively during the overaging treatment. Accordingly, the degree of tempering exceeds an appropriate range, which may reduce the tensile strength of the steel sheet.
When the holding time during the overaging treatment is excessive and exceeds 500 seconds, there may be a concern that excessive bainite transformation may occur during the holding process, which may increase the fraction of bainite in a final structure. This may lead to a decrease in the fraction of martensite, which may prevent the desired strength from being effectively secured. On the other hand, when the time is less than 100 seconds, since a bainite nose may be avoided, the final structure may not include bainite, which may reduce the ductility of the steel.
The steel sheet that has been overaging as described above may be cooled to room temperature under normal conditions, and there are no particular limitations on the cooling process. However, it is obvious that the cooling may be replaced with known cooling methods such as water cooling, oil cooling, and furnace cooling.

Temper rolling

After the overaging treatment, temper rolling may be performed at a reduction ratio of 0.1 to 1.0%.
When temper rolling is usually performed on a steel sheet, the effect of increasing the yield strength may be obtained without an increase in the tensile strength. However, when the reduction ratio of the temper rolling is less than 0.1%, not only may the effect of increasing the yield strength be insignificant, but shape control may also be difficult. On the other hand, when the reduction ratio exceeds 1.0%, there may be a concern that the operability may be greatly inferior due to the high-elongation operation.

Mode for Invention

Hereinafter, the present disclosure will be described more specifically through examples. However, it should be noted that the examples below are only intended to illustrate the present disposed and explain the same in more detail, and are not intended to limit the scope of the rights of the present disclosure.

(Example)

A steel slab having the alloy composition described in Table 1 below (the balance of Fe and inevitable impurities) was vacuum-melted, reheated in a temperature within a temperature range of 1200°C, and then hot-rolled at a finishing rolling temperature of 880 to 920°C, which is a temperature equal to or higher than Ar3, and coiled at 600°C. Then, a surface scale was removed on the steel sheet by pickling, and then, the steel sheet was cold rolled at a cold reduction ratio of 50% to manufacture a cold-rolled steel sheet. Thereafter, the cold-rolled steel sheet was subjected to continuous annealing, stepwise cooling, and overaging under the conditions illustrated in Table 2 below.

[Table 1]

Steel Type	Alloy Composition (wt%)										Relati onal Expres sion 1
Steel Type	C	Si	Mn	Nb	B	Ti	Al	P	S	N	Relati onal Expres sion 1
A	0.10	1.0	2.6	0.03	0.002	0.03	0.03	0.01	0.002	0.003	5.4
B	0.25	1.0	2.6	0.03	0.002	0.03	0.03	0.01	0.002	0.003	5.5
C	0.10	0	2.6	0.03	0.002	0.03	0.03	0.01	0.002	0.003	4.1
D	0.10	1.7	2.6	0.03	0.002	0.03	0.03	0.01	0.002	0.003	6.3
E	0.10	1.0	2.0	0.03	0.002	0.03	0.03	0.01	0.002	0.003	4.5
F	0.10	1.0	3.3	0.03	0.002	0.03	0.03	0.01	0.002	0.003	6.4
G	0.10	1.0	2.6	0	0.002	0.03	0.03	0.01	0.002	0.003	5.3
H	0.10	1.0	2.6	0.15	0.002	0.03	0.03	0.01	0.002	0.003	5.6
I	0.15	0.7	2.6	0.03	0.002	0.03	0.03	0.01	0.002	0.003	5.2

R = [C] + 1.3 [Si] + 1.5 [Mn] + 1.8 [Nb]

(in the relational expression 1, [C], [Si], [Mn] °2 [Nb]are wt% of each element.)

[Table 2]

Specim en Number	Steel Type	Continuous Annealing		Primary Cooling		Secondary Cooling		Overaging Treatment
Specim en Number	Steel Type	Tempera ture (°C)	Holding Time (s)	End Tempera ture (°C)	Cooling Rate (°C/s)	End Tempera ture (°C)	Cooling Rate (°C/s)	Tempera ture (°C)	Holdin g Time (s)
1	A	790	122	600	4.1	150	52.3	350	320
2	A	850	122	600	5.4	150	52.3	350	320
3	A	790	122	400	8.4	150	29.1	350	320
4	A	790	122	600	4.1	50	64.0	350	320
5	A	790	122	600	4.1	250	40.7	350	320
6	A	790	122	600	4.1	350	29.1	350	320
7	A	790	122	600	4.1	450	17.4	450	320
8	A	790	122	600	4.1	150	52.3	150	320
9	B	790	122	600	4.1	150	52.3	350	320
10	C	790	122	600	4.1	150	52.3	350	320
11	D	790	122	600	4.1	150	52.3	350	320
12	E	790	122	600	4.1	150	52.3	350	320
13	F	790	122	600	4.1	150	52.3	350	320
14	G	790	122	600	4.1	150	52.3	350	320
15	H	790	122	600	4.1	150	52.3	350	320
16	I	790	122	600	4.1	150	52.3	350	320

In Table 3 below, the microstructure of the manufactured steel sheet was observed and the physical properties were measured and are shown.
Among the microstructures, tempered martensite (TM), fresh martensite (FM), ferrite (F), and bainite (B) were observed using a scanning electron microscope (SEM) after etching a polished specimen cross-section with nital, and the retained austenite (R-γ) was measured using XRD analysis.
The precipitates in the microstructure were observed using a transmission electron microscope (TEM). In this case, an image thereof was observed at a magnification of 30,000 times, and the number of precipitates per unit area was measured for precipitates with an average diameter (circle diameter) of 50 nm or less, and the average diameter of the precipitates was shown. The fine precipitates refer to at least one fine precipitate selected from the group consisting of Nb-based and Ti-based.
Additionally, the physical properties for each specimen were measured, and the results are described in Table 3 below. Yield strength (YS), tensile strength (TS), and elongation (El) were evaluated through tensile tests, and mechanical properties were measured by evaluating test samples collected in accordance with JIS-5 standards at 90° to a rolling direction. Then, a yield ratio (YR), a product of yield strength and elongation (YSxEl), and a product of tensile strength and elongation (TSxEl) were calculated and are expressed.
Additionally, hole expansion ratio (HER) was evaluated through hole expansion tests, and after forming a 10 mmΨ punching hole (die inner diameter of 10.3 mm, and clearance of 12.5%), a conical punch having a 60° apex angle was inserted into a punching hole so that a burr of a punching hole was an outer side, and a periphery of the punching hole was compressed and expanded at a moving speed of 12 mm/min, and then calculated using the following [formula]. $Hole expansion ratio (HER, %) = \{(D - D_{0}) / D_{0}\} \times 100$
(In the formula, D refers to a hole diameter (mm) when cracks penetrate through the steel sheet in a thickness direction, and D₀ refers to an initial hole diameter (mm).)

[Table 3]

Spec imen Numb er	Ste el Typ e	Microstructure								Mechanical properties							Division
		Fraction (area%)						Precipitat es
		TM	FM	F	B	R-γ	Relat ional Expre ssion 2	Densi ty (/m²)	Avera ge diame ter (nm)	YS (MP a)	TS (MPa)	El (%)	YR	YSXE1 (MPa · %)	TSXE1 (MPa · % )	HER (%)
1	A	42	16	32	7	3	0.72	10¹⁴	13	722	1014	13.7	0.71	9891	13892	53	Inventive Example 1
2	A	75	15	0	7	3	0.83	10¹³	16	104 1	1150	8.0	0.91	8328	9200	51	Comparati ve Example 1
3	A	37	18	32	11	2	0.67	10¹³	14	702	996	13.5	0.70	9477	13446	45	Comparati ve Example 2
4	A	61	8	30	0	1	0.88	10¹⁴	11	897	992	12.6	0.90	11302	12499	52	Comparati ve Example 3
5	A	40	22	30	6	2	0.65	10¹³	14	710	1021	13.2	0.70	9372	13477	51	Inventive Example 2
6	A	11	44	32	12	1	0.20	10¹³	14	681	1089	12.1	0.63	8240	13177	41	Comparati ve Example 4
7	A	0	47	32	20	1	0	10¹⁴	15	660	1114	11.8	0.59	7788	13145	36	Comparati ve Example 5
8	A	33	36	30	0	1	0.52	10¹⁴	12	674	1170	11.4	0.58	7684	13338	40	Comparati ve Example 6
9	B	30	31	27	10	2	0.49	10¹³	16	787	1201	8.0	0.66	6296	9608	31	Comparati ve Example 7
10	C	38	27	25	7	3	0.58	10¹⁴	13	690	983	12.0	0.70	8280	11796	38	Comparati ve Example 8
11	D	24	27	36	7	6	0.47	10¹³	13	735	1257	12. 6	0.58	9261	15838	45	Comparati ve Example 9
12	E	45	13	33	6	3	0.78	10¹⁴	12	696	992	13. 4	0.70	9326	13293	43	Comparati ve Example 10
13	F	29	32	28	9	2	0. 48	10¹³	14	775	1328	8.3	0.58	6433	11022	33	Comparati ve Example 11
14	G	44	14	32	7	3	0.76	10⁷	15	705	1013	13.0	0.70	9165	13169	40	Comparati ve Example 12
15	H	33	28	29	7	3	0.54	10¹⁷	13	747	1071	11.9	0.70	8889	12745	51	Comparati ve Example 13
16	I	49	14	30	6	2	0.78	10¹⁶	14	750	1042	13.1	0.72	9825	13650	55	Inventive Example 3

* TM: tempered martensite, FM: fresh martensite, F: ferrite, B: bainite, R-γ: retained austenite

MR = TM / (FM + TM)

(In the relational expression 2, TM and FM are area% of tempered martensite and fresh martensite, respectively.)
As illustrated in Table 3, in the case of the inventive examples satisfying the conditions of the present disclosure, the microstructure characteristics proposed by the present disclosure were satisfied, and the properties targeted by the present disclosure were also secured.
FIG. 1 is a microstructure image of Inventive Example 1 according to an embodiment of the present disclosure. As shown in FIG. 1, it may be confirmed that tempered martensite was formed in large quantities, and ferrite was formed in large quantities as an additional structure.
On the other hand, Comparative Example 1 is a case in which a continuous annealing temperature exceeded a temperature within a temperature range of the present disclosure, in which case the formation of soft ferrite during continuous annealing and cooling was insufficient, and tempered martensite was excessively formed as a final microstructure. As a result, a desired elongation may not be secured.
Comparative Example 2 is a case in which the primary cooling end temperature was below the temperature within a temperature range of the present disclosure, in which case it was difficult to secure stable cooling capacity as the average cooling rate exceeded the range presented in the present disclosure, and thus, a bainite fraction increased during cooling and then the formation of tempered martensite was insufficient, so that the desired strength may not be secured.
Comparative Example 3 is a case in which the second cooling end temperature was below the temperature within a temperature range of the present disclosure, and an average cooling rate was higher than the range presented in the present disclosure. Accordingly, most of the structure was transformed into martensite during cooling, and martensite was tempered during overaging treatment, so that the desired tensile strength may not be secured.
A secondary cooling end temperature of Comparative Example 4 belonged to a temperature range equal to or lower than a martensite transformation initiation temperature (Ms), identical to the inventive example, but as the temperature approached the martensite transformation initiation temperature (Ms), the fraction of martensite transformed during the secondary cooling decreased. As a result, the fraction of tempered martensite among the generated martensite decreased and the fraction of fresh martensite increased. Accordingly, the desired yield strength may not be obtained. FIG. 2 is a microstructure image of Comparative Example 4, which deviates from an embodiment of the present disclosure. As shown in FIG. 2, it may be confirmed that a large amount of fresh martensite was formed and that the formation of tempered martensite was insufficient.
A secondary cooling end temperature of Comparative Example 5 belonged to a temperature range equal or to higher than the martensite transformation initiation temperature (Ms), but in this case, since only fresh martensite transformation was induced without tempered martensite, a low yield ratio and low hole-expandability were exhibited accordingly.
Comparative Example 6 is a case in which an overaging treatment temperature fell below the temperature within a temperature range of the present disclosure. The amount of carbon (C) escaping from the martensite transformed during the secondary cooling during the overaging treatment decreased, and therefore, since a degree of tempering decreased, a desired yield strength may not be secured.
Comparative Example 7 is a case in which the martensite formed by excessive addition of C in steel was relatively harder, and a difference in hardness from the surrounding phases increased, so that the ductility and hole expandability were inferior.
Comparative Examples 8 and 9 are cases in which a value of the relational expression 1 did not satisfy the range of the present disclosure. Specifically, Comparative Example 8 is a case in which Si was not added, and there is no solid solution strengthening effect due to the addition of Si, and therefore, a value of the relational expression 1 was obtained as 4.1, whereas Comparative Example 9 is a case in which the content of Si in the steel was excessive, and it may be seen that a value of the relational expression 1 was 6.3, which exceeded 6.0. That is, it may be seen that Comparative Examples 8 and 9 did not secure the ductility and hole-expandability targeted by the present disclosure.
Comparative Examples 10 and 11 are examples in which the content of Mn in steel deviated from the scope of the present disclosure. Comparative Example 10 is a case in which the content of Mn in steel was insufficient, and thus the targeted strength and hole expandability were not secured. Specifically, Comparative Example 11 obtained a value of the relational expression 1 as 4.5, and thus, it may be understood that Comparative Example 10 did not secure the strength intended by the present disclosure due to the reduced hardenability of steel. Comparative Example 11 is a case in which the content of Mn in steel was excessive, and specifically, it may be seen that a value of the relational expression 1 was 6.4, which exceeded 6.0. In other words, Comparative Example 11 did not secure the ductility and hole-expandability targeted by the present disclosure due to the excessively high hardenability of the steel.
Comparative Examples 12 and 13 are examples in which the content of Nb deviated from the scope of the present disclosure. Comparative Example 12 is a case in which Nb was not added, in which the target hole-expandability may not be secured due to the inferiority of a hardness difference between phases caused by the failure to secure a target level of precipitation in the ferrite. Comparative Example 13 is a case in which the content of Nb in steel was excessive, in which case the ductility targeted by the present disclosure may not be secured due to the decrease in ductility caused by the excessive generation of precipitates in the ferrite.
Although the present disclosure has been described in detail through examples above, other forms of examples are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

A steel sheet, comprising: by wt%, carbon (C): 0.05 to 0.20%, manganese (Mn): 2.3 to 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, and a balance of iron (Fe) and inevitable impurities,
wherein a value R defined in the following relational expression 1 is 5.0 to 6.0,

a microstructure includes, in area%, 40% or more of tempered martensite, and includes a balance structure of ferrite, fresh martensite, bainite, and retained austenite, and

MR defined in the following relational expression 2 is 0.60 to 0.80, $R = [C] + 1.3 [Si] + 1.5 [Mn] + 1.8 [Nb]$

where [C], [Si], [Mn], and [Nb] are wt% of each element, $MR = TM / (FM + TM)$

where TM and FM are area% of tempered martensite and fresh martensite, respectively.
The steel sheet of claim 1, wherein the microstructure includes 30 to 50% of ferrite and 5 to 25% of fresh martensite.
The steel sheet of claim 1, wherein the steel sheet includes at least one of Nb-based fine precipitates and Ti-based fine precipitates having an average diameter (circle equivalent diameter) of 50 nm or less, at a number of 10¹² or more per unit area (m²).
The steel sheet of claim 1, wherein the steel sheet has a yield strength of 700 MPa or more, a tensile strength of 1000 MPa or more, a yield ratio of 0.70 or more, and an elongation of 13% or more.
The steel sheet of claim 1, wherein a product of a yield strength and an elongation is 9100 MPa·% or more, a product of a tensile strength and an elongation is 13000 MPa·% or more, and a hole expansion ratio (HER) is 50% or more.
A manufacturing method for a steel sheet, comprising:
reheating a steel slab including, by wt%, carbon (C): 0.05 to 0.20%, manganese (Mn): 2.3 to 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, and a balance of iron (Fe) and inevitable impurities, and having a value R of 5.0 to 6.0 defined in the following relational expression 1;

hot-rolling the reheated steel slab;

cooling and coiling the hot-rolled steel sheet;

cold-rolling the coiled steel sheet;

continuously annealing by heating the cold-rolled steel sheet to a temperature within a temperature range of 760 to 830°C and holding the steel sheet for 30 to 300 seconds;

primarily cooling the continuously annealed steel sheet to a temperature within a temperature range of 500 to 700°C at an average cooling rate of 2.0 to 6.0°C/s;

secondarily cooling the steel sheet after the primary cooling to a temperature within a temperature range of 100 to 300°C at an average cooling rate of 35.0 to 60.0°C/s; and

subjecting the steel sheet to an overaging treatment by heating the steel sheet to a temperature within a temperature range of 200 to 450°C after the secondary cooling and maintaining the steel sheet for 100 to 500 seconds, $R = [C] + 1.3 [Si] + 1.5 [Mn] + 1.8 [Nb]$

where [C], [Si], [Mn], and [Nb] are wt% of each element.
The manufacturing method for a steel sheet of claim 6,
wherein the reheating is performed in a temperature within a temperature range of 1000 to 1350°C,

the hot-rolling is performed at a finishing rolling temperature of Ar3 to Ar3+50°C,

the coiling is performed in a temperature within a temperature range of 450 to 700°C, and

the cold-rolling is performed at a reduction ratio of 35 to 65%.
The manufacturing method for a steel sheet of claim 6, further comprising:
pickling the steel sheet after the coiling.
The manufacturing method for a steel sheet of claim 6, further comprising:
temper rolling the steel sheet at a reduction ratio of 0.1 to 1.0% after the overaging treatment.