EP4357476A1

EP4357476A1 - Ultra high strength steel sheet having high yield ratio and excellent bendability and method of manufacturing same

Info

Publication number: EP4357476A1
Application number: EP22825376.1A
Authority: EP
Inventors: Sang-Hyun Kim; Min-Seo KOO; Eun-Young Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-06-18
Filing date: 2022-06-17
Publication date: 2024-04-24
Also published as: US20240141454A1; WO2022265453A1; KR20220169497A; JP2024526116A; CN117500951A

Abstract

The present invention relates to an ultra high strength steel sheet having a high yield ratio, and a method for manufacturing same and, more specifically, to a steel sheet having ultra high strength, a high yield ratio and excellent bendability, and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to an ultra-high strength steel sheet having a high yield ratio, and excellent bendability, and a method of manufacturing the same.

Background Art

In recent years, in the field of automobiles, research to reduce a weight of a vehicle body is being actively conducted in developed countries, led by Europe, for reasons of fuel economy regulations and performance improvements. In addition, in addition to weight reduction, stability and high strength of a material of the vehicle body are also required due to the strengthening of safety regulations for automobile passengers and pedestrians.
Meanwhile, in order to improve stability and impact characteristics of the vehicle body, adoption of high-strength steel having excellent yield strength for a body-in-white (BIW) structural member is increasing, and in other words, the higher a yield ratio (yield strength/tensile strength), the more advantageous it is absorb impact energy.
As a representative manufacturing method for increasing yield strength, there is a method of utilizing water cooling during continuous annealing. After a cold-rolled steel sheet is quenched to room temperature after being annealed in a two phase region or single phase region, an ultra-high strength steel sheet may be manufactured by a tempering method, and in this case, the yield ratio is very high, but a problem in which shape quality of a coil deteriorates may occur due to temperature deviation thereof in width and length directions, and a problem such as material defects, workability deterioration, and the like, depending on parts, when processing roll-forming parts, may occur. In addition, since elongation of the steel sheet generally decreases as the strength of the steel sheet increases, there may be a problem in that forming processability deteriorates, the application thereof as a material for cold stamping may be limited.
In order to overcome the above-described problems, a hot press forming (HPF) method, in which a material is formed at a high temperature at which forming is relatively easily performed, and then required strength is secured through water cooling between a die and the material is being developed. Since it is possible to secure high strength compared to the same thickness, the HPF method is widely used in manufacturing parts, but there is a problem in application thereof due to excessive equipment investment and increase in process costs, so it is necessary to develop a material for cold stamping. Therefore, it is required to develop a cold-rolled steel sheet suitable for use as a material for cold stamping, having high strength and a high yield ratio, and excellent bending properties in order to secure good crash performance.

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide an ultra-high strength steel sheet having a high yield ratio having excellent bending properties and a method of manufacturing the same.
The object of the present disclosure is not limited to the above. A person skilled in the art will have no difficulty in understanding the further subject matter of the present invention from the general content of this specification.

Solution to Problem

According to an aspect of the present disclosure, provided is a steel sheet, the steel sheet including, by weight: carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3°, silicon (Si): 0.05 to 1.0%, phosphorous (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.5%, with a balance of Fe and inevitable impurities,
wherein an R value defined by the following Relational Expression 1 is 0.12 to 0.27,

an average number of carbides per 1µm² area is 40 or less, and an average length of a major axis of carbides is 300 nm or less, and
a yield ratio is greater than 0.73.
$\begin{array}{l} R = \frac{\{{(Ceq 1)}^{2} + {(Ceq 2)}^{2}\}}{2} \\ Ceq 1 = [C] + \frac{[Mn]}{20} + \frac{[Si]}{30} + 2 [P] + 4 [S] \\ Ceq 2 = [C] + \frac{[Mn]}{6} + \frac{[Si]}{30} + \frac{([Cr] + [Mo] + [V] + [Nb])}{5} + \frac{([Cu] + [Ni])}{15} \end{array}$
where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu] and [Ni] are weight percent (%) of respective elements.

The steel sheet may further include two or more of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2%, and boron (B): 0.005% or less (excluding 0%).
The steel sheet may further include one or more of titanium (Ti): 0.1% or less (excluding 0%) and niobium (Nb): 0.1% or less (excluding 0%).
The steel sheet may include 99 area% or more of martensite or tempered martensite as a microstructure.
The steel sheet may have a tensile strength of 1300 MPa or more, and a bending property (R/t) of less than 4, where R is a minimum bending radius at which cracks do not occur in a bent portion after a 90° bending test, and t is a thickness of the steel sheet.
According to another aspect of the present disclosure, provided is a method of manufacturing a steel sheet, the method including operations of: preparing a cold-rolled steel sheet including, by weight: carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorous (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.5%, with a balance of Fe and inevitable impurities, wherein an R value defined by the following Relational Expression 1 is 0.12 to 0.27;

heat treating the cold-rolled steel sheet at a temperature of Ac3 or higher for 30 seconds or more;
primarily cooling the cold-rolled steel sheet to a temperature within a range of 500 to 750°C at an average cooling rate of 1 to 10°C/s after the heat treatment;
secondarily cooling the primarily-cooled steel sheet to a temperature of Ms-190°C or lower at an average cooling rate of 20 to 80°C/sec; and
reheating and overaging by heating the secondarily-cooled steel sheet to a temperature within a range of greater than secondary cooling end temperature+30°C and less than 270°C, and holding the same for 1 to 20 minutes.
$\begin{array}{l} R = \frac{\{{(Ceq 1)}^{2} + {(Ceq 2)}^{2}\}}{2} \\ Ceq 1 = [C] + \frac{[Mn]}{20} + \frac{[Si]}{30} + 2 [P] + 4 [S] \\ Ceq 2 = [C] + \frac{[Mn]}{6} + \frac{[Si]}{30} + \frac{([Cr] + [Mo] + [V] + [Nb])}{5} + \frac{([Cu] + [Ni])}{15} \end{array}$
where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu] and [Ni] are weight percent (%) of respective elements.

The cold-rolled steel sheet may further include two or more of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2%, and boron (B): 0.005% or less (excluding 0%).
The cold-rolled steel sheet may further include one or more of titanium (Ti): 0.1% or less (excluding 0%) and niobium (Nb): 0.1% or less (excluding 0%).
The operation of preparing the cold-rolled steel sheet may include operations of:

reheating a steel slab to a temperature within a range of 1100 to 1300°C;
hot rolling the reheated steel slab at a finish hot rolling temperature of Ar3 or higher;
cooling and winding the hot-rolled steel sheet to a temperature within a range of 700 °C or lower; and
cold rolling the cooled and wound steel sheet at a reduction ratio of 30 to 80%.

An operation of pickling the cooled and wound steel sheet with hydrochloric acid may be further included.

Advantageous Effects of Invention

As set forth above, according to an aspect of the present disclosure, a steel sheet having high strength and a high yield ratio, and excellent bending properties and a method of manufacturing the same may be provided.
According to another aspect of the present disclosure, a steel sheet that can be applied as a body-in-white (BIW) structural member and a method of manufacturing the same may be provided.

Brief description of drawings

FIG. 1 (a) and (b) are SEM microstructure pictures (x10.000) of Inventive Example 15 and Comparative Example 21 according to an embodiment of the present disclosure.

Best Mode for Invention

Hereinafter, preferred embodiments of the present disclosure will be described. 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. The present embodiments are provided to those skilled in the art to further elaborate the present disclosure.
Hereinafter, the present disclosure will be described in detail.
In the present disclosure, in order to provide a steel sheet having high strength and a high yield ratio and excellent bendability, an alloy composition and processing conditions were optimized. In particular, the present inventor has confirmed that a content of component elements such as C, Mn, Si, P, and S was strictly controlled, conditions of secondary cooling and reheating and overaging processes during continuous annealing were optimized, so that bending properties and high strength may be secured while securing basic welding properties, thereby completing the present disclosure.
Hereinafter, a steel composition of the present disclosure will be described in detail.
In the present disclosure, unless otherwise specified, % indicating a content of each element is based on weight.
According to an aspect of the present disclosure, steel may include by weight, carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.5%, with a balance of Fe and inevitable impurities.

Carbon (C): 0.1 to 0.3%

Carbon (C) is an interstitial solid-solution element, and is the most effective and important element in improving strength of steel, and is an element that should be added in order to secure strength of martensitic steel. In order to obtain an ultra-high strength steel satisfying a yield ratio and tensile strength, targeted by the present disclosure, carbon (C) is preferably added in an amount of 0.1% or more, more preferably 0.12% or more. However, when a content of C exceeds 0.3%, the martensite strength may be increased, but carbides may be easily generated and coarsened during a continuous annealing process, so that ductility may be reduced and bending properties may be inferior. In addition, since an increase in the content of carbon (C) has a problem of impairing weldability, it is preferable to limit an upper limit thereof to 0.3%. More preferably, the upper limit thereof may be 0.28%.

Manganese (Mn): 1.0 to 2.3%

Manganese (Mn) is an element that is easy to secure final martensite by inhibiting ferrite formation and promoting austenite formation in a composite structure steel. However, a content of manganese (Mn) exceeds 2.3%, manganese (Mn) is segregated in a thickness direction and it is easy to form a manganese (Mn) band in a slab, so there is a problem in that occurrence of defects increases during a rolling process along with continuous casting cracks. Therefore, manganese (Mn) may be included more preferably in an amount of 2.1% or less. On the other hand, when a content of manganese (Mn) is less than 1.0%, it is difficult to secure strength in ultra-high strength steel, so a lower limit thereof may be limited to 1.0%. A more preferable lower limit thereof may be 1.4%.

Silicon (Si): 0.05 to 1.0%

Since silicon (Si) serves to suppress carbide generation and control a size of carbides in reheating and overaging operations after cooling in a martensitic steel, a lower limit of silicon (Si) may be limited to 0.05%. More preferably, silicon (Si) may be included in an amount of 0.09% or more. However, silicon (Si) is a ferrite stabilizing element, and when a content of silicon (Si) exceeds 1.0%, ferrite may be generated during cooling in a continuous annealing furnace, which may weaken the strength. In addition, since Si-based oxides may be formed in a heating furnace and there may be a problem of surface oxidation, an upper limit of silicon (Si) may be limited to 1.0%. More preferably, the upper limit thereof may be limited to 0.6%.

Phosphorus (P): 0.1% or less

Phosphorus (P) is an impurity element included in steel, and a content of 0% is excluded in consideration of a case where P is inevitably included during a manufacturing process. However, when the content of Phosphorus (P) exceeds 0.1%, weldability deteriorates and there may be a concern that brittleness of steel occurs, so an upper limit of P may be limited to 0.1%. A more preferable upper limit of P may be 0.03%.

Sulfur (S): 0.03% or less

Sulfur (S), like P, is an impurity which is unavoidably included in steel, and is an element impairing ductility and weldability of a steel sheet, so it is preferable to keep a content of S as low as possible. Therefore, it is preferable to limit the content of S to 0.03% or less. More preferably, the content of S may be limited to 0.005% or less. Meanwhile, 0% is excluded in consideration of a case inevitably included during a manufacturing process.

Aluminum (Al): 0.01 to 0.5%

Aluminum (Al) may be added to remove oxygen, and like Si, is an element stabilizing ferrite. In addition, since Al is a component capable of improving hardenability of final martensitic steel by increasing a content of C in austenite, it is preferable to add 0.01% or more of a content of Al. However, when the content of Al exceeds 0.5%, ferrite may be generated during cooling in a continuous annealing furnace, which may weaken the strength. In addition, AlN formation may cause casting cracks in a slab, and there is a problem of inhibiting hot rolling properties, and an upper limit of Al may be limited to 0.5%.
The steel of the present disclosure may include remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in a common manufacturing process, 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 aspect of the present disclosure, steel may further include two or more of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo) : 0.01 to 0.2%, and boron (B) : 0.005% or less (excluding 0%).

Chromium (Cr): 0.01 to 0.2%

Chromium (Cr) is a component added to improve hardenability and secure high strength of steel, and is useful in manufacturing ultra-high strength steel having pure martensite by suppressing bainite generation. Therefore, it is preferable to add chromium (Cr) in an amount of 0.01% or more in order to secure the above-described effect. However, when a content of Cr is excessive, there is a problem in that a cost of ferroalloy increases, so an upper limit of Cr may be limited to 0.2%, more preferably 0.1%.

Molybdenum (Mo): 0.01 to 0.2%

Molybdenum (Mo), like Cr, is an element improving hardenability of steel, and is preferably added in an amount of 0.01% or more to obtain a hardenability effect. However, when a content of Mo exceeds 0.2%, an amount of alloy input is excessive and there is a problem of increasing a cost of ferroalloy, so it is preferable to limit an upper limit of Mo to 0.2%, and more preferably to 0.1%.

Boron (B): 0.005% or less (excluding 0%)

Boron (B) is an element which suppresses transformation of austenite into ferrite during a continuous annealing process, and is an element which is effective in improving hardenability of martensite, such as Cr, and Mo, even when added in a very small amount thereof. However, when a content of boron (B) exceeds 0.005%, an Fe₂₃(B,C)₆ precipitated phase precipitates at an austenite grain boundary, thereby promoting ferrite formation, so it is preferable to limit an upper limit of B to 0.005%.
Steel according to an aspect of the present disclosure may further include one or more of titanium (Ti): 0.1% or less (excluding 0%), niobium (Nb): 0.1% or less (excluding 0%).

Titanium (Ti): 0.1% or less (excluding 0%)

Titanium (Ti) is an element for forming fine carbides, thereby contributing to securing yield strength and tensile strength. In addition, titanium (Ti) is scavenged by precipitating N in steel as TiN, and to this end, it is preferable to add 48/14*[N] or more in a chemical equivalent, and when B is added, to maximize an addition effect thereof, it is preferable to add titanium (Ti). However, when a content of titanium (Ti) exceeds 0.1%, coarse carbides may be precipitated, strength and elongation may be reduced by reducing an amount of carbon in steel, and nozzle clogging may be caused during casting, so it is preferable to limit an upper limit of Ti to 0.1%.

Niobium (Nb): 0.1% or less (excluding 0%)

Niobium (Nb) is an element which is segregated at austenite grain boundaries to suppress coarsening of austenite crystal grains during an annealing heat treatment, and to contribute to increase strength by forming fine carbides. However, when a content of niobium (Nb) exceeds 0.1%, precipitation of coarse carbonitrides may increase, and there may be a concern that strength and elongation may decrease due to reduction in an amount of carbon in steel, and there may be a problem in which processibility of a base material decreases and manufacturing costs increase. Therefore, an upper limit of Nb may be preferably limited to 0.1%.
According to an aspect of the present disclosure, steel may have an R value, defined in the following Relational Expression 1 may be 0.12 to 0.27.
Relational Expression 1 is a complex relational expression of Ceq1 and Ceq2 representing welding properties according to the content of respective elements, and when the R value of Relational Expression 1 is 0.12 to 0.27, physical properties including welding properties, targeted by the present disclosure may be secured.

When the R value defined in Relational Expression 1 is less than 0.12, it may be difficult to secure the strength, targeted by the present disclosure. On the other hand, when the R value exceeds 0.27, among physical properties, particularly, welding properties may be deteriorated. In the present disclosure, a lower limit of the more preferable R value may be 0.17, an upper limit of the more preferable R value may be 0.25, and more preferably 0.20.
$\begin{array}{l} R = \frac{\{{(Ceq 1)}^{2} + {(Ceq 2)}^{2}\}}{2} \\ Ceq 1 = [C] + \frac{[Mn]}{20} + \frac{[Si]}{30} + 2 [P] + 4 [S] \\ Ceq 2 = [C] + \frac{[Mn]}{6} + \frac{[Si]}{30} + \frac{([Cr] + [Mo] + [V] + [Nb])}{5} + \frac{([Cu] + [Ni])}{15} \end{array}$
where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu] and [Ni] are weight percent (%) of respective elements.

Hereinafter, a microstructure of steel of the present disclosure will be described in detail.
In the present disclosure, % representing a fraction of the microstructure is based on an area unless otherwise specified.
The steel according to an aspect of the present disclosure may include martensite or tempered martensite by an amount of 99 area% or more as a microstructure, and the number of carbides per 1µm² area may be 40 or less, and an average length of a major axis of the carbide may be 300 nm or less.
In the present disclosure, martensite or tempered martensite may be included as a microstructure in order to secure a cold-rolled steel sheet having high strength and a high yield ratio, and it is preferable to add the same by an amount of 99% or more to secure a high strength level of 1.3G-level or higher.
In addition, in order to secure excellent bending properties, it is preferable to control the number of carbides to 40 or less, more preferably 35 or less.
In addition, in order to more effectively secure the above-described effect, an average length of a major axis of the carbide may be preferably 300 nm or less, more preferably 200 nm or less.
The number of carbides of the present disclosure represents a n average of the number of carbides in a 1µm² region (average of 10 regions) in a x10,000 SEM image, and a length of the major axis of is shown by measuring x30,000 to x100,000 images on a TEM bright field.
Hereinafter, a method of manufacturing steel of the present disclosure will be described in detail.
Steel according to an aspect of the present disclosure may be manufactured by heat treatment, primary cooling, secondary cooling, and reheating and overaging of a cold-rolled steel sheet satisfying the alloy composition described above.

Cold-rolled steel sheet preparation

A cold-rolled steel sheet satisfying the alloy composition of the present disclosure may be prepared.
The cold-rolled steel sheet of the present disclosure may be manufactured under common processing conditions, and may be manufactured by reheating, hot rolling, cooling, winding, and cold rolling a steel slab, preferably under conditions described below.

Reheating

A steel slab satisfying the above-described alloy composition of the present disclosure may be reheated to a temperature within a range of 1100 to 1300°C.
Reheating may be performed to smoothly perform a subsequent hot rolling process, and may be performed to sufficient secure physical properties, targeted by the present disclosure. When a reheating temperature is lower than 1100°C, there may be a problem in that hot rolling load increases rapidly. When the reheating temperature is higher than 1300°C, an amount of surface scales increases, reducing yield of a material and causing surface defects, which may adversely affect the final quality.

Hot rolling

The reheated steel slab may be hot rolled to a finish hot rolling temperature of Ar3 or higher.
In the present disclosure, when the finish hot rolling temperature may be limited to Ar3 (a temperature at which ferrite begins to appear during austenite cooling) or higher, which is because ferrite and austenite two-phase or ferrite reverse rolling may be performed at a temperature of Ar3 or lower to form a mixed structure, and there is a concern of malfunction due to fluctuations in hot rolling load.

Cooling and winding

The hot-rolled steel sheet may be cooled to a temperature within a range of 700°C or lower, and then wound.
When a winding temperature exceeds 700°C, an oxide may be excessively formed on a surface of the steel sheet, which may cause defects. The lower the winding temperature, the higher strength of the hot-rolled steel sheet, and there is a disadvantage that rolling load of cold rolling, which is a subsequent process, increases, but since it is not a factor making actual production impossible, in the present disclosure, a lower limit thereof is not particularly limited.
In addition, in the present disclosure, an oxide layer formed on a surface of the wound steel sheet may be removed by a pickling process prior to cold rolling, which is a subsequent process.

Cold rolling

The cooled and wound steel sheet may be cold rolled at a reduction ratio of 30 to 80%.
When the reduction ratio of cold rolling is less than 30%, it may be difficult to secure a target thickness, and there may be a concern in that austenite formation and final physical properties may be affected during annealing heat treatment due to remaining hot-rolled crystal grains. On the other hand, when the reduction ratio exceeds 80%, there may be a problem in which material deviation of the final steel sheet due to an uneven rolling reduction rate in length and width directions from work hardening, and it may be difficult to secure a target thickness due to a rolling load.

Heat treatment

The cold-rolled steel sheet may be heat treated at a temperature of Ac3 or higher for 30 seconds or more.
In the present disclosure, heat treatment may be performed to secure an austenite fraction of 100% through austenite single phase annealing. By securing the austenite fraction by 100% through the heat treatment, it is possible to prevent a decrease in strength due to ferrite formation during annealing. $\begin{matrix} Ac 3 = 910 - 203 \sqrt ([C]) - \\ 15.2 [Ni] + 44.7 [Si] + 104 [V] + 31.5 [Mo] + 13.1 [W] \end{matrix}$

where [C], [Ni], [Si], [V], [Mo], and [W] are weight percent (%) of respective elements.

Primary cooling

After the heat treatment, primary cooling may be performed at an average cooling rate of 1 to 10°C/s to a temperature within a range of 500 to 750°C.
During the primary cooling, when a cooling rate is less than 1°C/s or less, it may be difficult to secure a target strength due to formation of ferrite during cooling. On the other hand, when the cooling rate exceeds 10°C/s, during secondary cooling, the average cooling rate may be deteriorated and a fraction of other low-temperature transformation phases, other than martensite may increase, making it difficult to finally secure the target strength.
During the primary cooling, when the temperature is less than 500°C, phases such as ferrite, bainite, or the like, may be formed and there may be a concern that the strength is deteriorated, and when the temperature exceeds 750°C, there may be a problem in an actual production line.

Secondary cooling

The primarily-cooled steel sheet may be secondarily cooled at an average cooling rate of 20 to 80°C/s to a temperature of Ms-190°C or lower.
In the present disclosure, in order to secure 99% or more of martensite or temperature martensite, during secondary cooling, it is preferable to be rapidly cooled below a martensite transformation finish temperature (Mf). In the present disclosure, it is preferable to specifically be cooled at a temperature of Ms-190°C or lower. In the present disclosure, it is possible to form a martensitic structure, which is sufficiently hard, and a secondary cooling end temperature is limited to a temperature of Ms-190°C or lower in order to secure an effect of increasing yield strength by carbide precipitation during subsequent tempering. In addition, when a tempering temperature is increased, bendability may be deteriorated, it is intended to secure bending properties by limiting the secondary cooling end temperature to enable sufficient tempering without raising the tempering temperature too much. When the tempering temperature exceeds a temperature of Ms-190°C, it may be difficult to realize desired physical properties since a fraction of martensite or martensite is not sufficiently secured.
Meanwhile, during the secondary cooling, when the average cooling rate is less than 20°C/s, some bainite structure may be formed during secondary cooling from a primary cooling section, and when the average cooling rate exceeds 80°C/s, problems such as poor surface shape of the steel sheet and material deviation in a width direction may occur due to a rapid martensitic transformation rate at the time of the secondary cooling. $\begin{array}{l} Ms = 539 - 423 [C] - 30.4 [Mn] - 16.1 [Si] - 59.9 [P] + 43.6 [Al] - \\ 17.1 [Ni] - 12.1 [Cr] + 7.5 [Mo] \end{array}$

where [C], [Mn], [Si], [P], [Al], [Ni], [Cr] and [Mo] are weight percent(%) of respective elements.

Reheating and overaging

The secondarily-cooled steel sheet may be reheated and overaged by heating the steel sheet to a temperature within a range of greater than secondary cooling end temperature+30°C and less than 270°C and holding the same for 1 to 20 minutes.
In the present disclosure, it is intended to improve toughness by changing hard martensite having high dislocation density formed during secondary cooling to tempered martensite through reheating and overaging. In the present disclosure, in order to sufficiently secure the tempering effect, a lower limit of the reheating temperature is limited to a temperature of 30°C or higher, compared to a secondary cooling end temperature. In this case, yield strength increases due to formed fine carbides, but when a reheating and overaging temperature is less than the secondary cooling end temperature + 30°C, it is difficult to obtain the desired effect. On the other hand, when the temperature is higher than 270°C, there may be a problem in that bending properties may be inferior due to coarsening of carbides.
Meanwhile, when a holding time is less than 1 minute, martensite may not be sufficiently changed to tempered martensite, making it difficult to sufficiently secure toughness, and when the holding time exceeds 20 minutes, carbides generated by overaging may become coarse, which may adversely affect bending properties and materials.
The steel of the present disclosure manufactured as described above may have a tensile strength of 1300MPa or more, a yield ratio of exceeding 0.73, and a bending property (R/t) of less than 4, where R is a bending radius at which cracks do not occur in a bent portion after a 90° bending test, and t is a thickness of the steel sheet, and have excellent bending properties while having a high yield ratio.
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.

Mode for Invention

(Example)

A steel slab having the composition shown in Table 1 below was heated at 1100 to 1300 °C, finish hot-rolled at 850 to 950° C, which is a temperature of Ar3 or higher, wound at a temperature within a range of 400 to 700 °C, and a cold reduction rate of 45 to 65% was applied to manufacture a cold-rolled steel sheet. Subsequently, after heat treatment for 100 to 400 seconds in a temperature within a range of 800 to 900 °C, primary and secondary cooling were performed under the conditions illustrated in Table 2 below. In this case, a first cooling rate was applied at 2 to 4 °C/ s, and a second cooling rate was applied at 25 to 60 °C/ s. Next, it was reheated under the conditions of Table 2 and overaged for 1 to 20 minutes to manufacture a steel sheet.

In addition, in Table 1 below, Ac3, Ms temperature and values of relational expression 1 according to the content of respective elements were calculated and illustrated.

[Table 1]

STE EL TYP E	ALLOY COMPOSITION (wt%)											Ac 3	Ms	RELATIO NAL EXPRESS ION 1
STE EL TYP E	C	Si	Mn	P	S	Cr	Mo	Al	Ti	B	Nb	Ac 3	Ms	RELATIO NAL EXPRESS ION 1
A	0.1 5	0. 5	2. 0	0.0 1	0.0 02	0.0 5	-	0.0 25	0.0 25	0.0 02	0.0 4	85 4	40 7	0.18
B	0.1 7	0. 5	2. 0	0.0 1	0.0 02	-	0.0 5	0.0 25	0.0 25	0.0 02	0.0 4	85 0	39 9	0.19
C	0.1 7	0. 1	1 . 9	0.0 1	0.0 02	-	0.0 5	0.0 25	0.0 25	0.0 02	0.0 4	83 2	40 9	0.17
D	0.20	0.1	1.9	0.01	0.003	-	0.05	0.025	0.025	0.002	0.04	825	396	0.20
E	0.1 8	0. 2	1 . 7	0.0 1	0.0 03	0.0 5	-	0.0 25	0.0 25	0.0 02	0.0 4	83 3	40 8	0.17
F	0.1 5	0. 5	3. 5	0.0 1	0.0 02	-	0.0 5	0.0 25	0.0 25	0.0 02	0.0 4	85 5	36 2	0.36
G	0.1 7	0. 1	3. 0	0.0 1	0.0 02	0.0 5	-	0.0 25	0.0 25	0.0 02	0.0 4	83 1	37 4	0.30
H	0.0 9	0. 1	1. 9	0.0 1	0.0 02	-	0.0 5	0.0 25	0.0 25	0.0 02	0.0 4	85 5	44 2	0.11
I	0.1 7	1. 5	2. 5	0.0 1	0.0 02	-	0.0 5	0.0 25	0.0 25	0.0 02	0.0 4	89 5	36 8	0.28
J	0.2 2	0. 1	0. 6	0.0 1	0.0 02	0.0 5	-	0.0 25	0.0 25	0.0 02	0.0 4	81 9	42 6	0.10

\begin{matrix} Ac 3 = 910 - 203 \sqrt ([C]) - \\ 15.2 [Ni] + 44.7 [Si] + 104 [V] + 31.5 [Mo] + 13.1 [W] \end{matrix}

where [C], [Ni], [Si], [V], [Mo], and [W] are a weight percent(%) of respective elements. $\begin{array}{l} Ms = 539 - 423 [C] - 30.4 [Mn] - 16.1 [Si] - 59.9 [P] + 43.6 [Al] - \\ 17.1 [Ni] - 12.1 [Cr] + 7.5 [Mo] \end{array}$
where [C], [Mn], [Si], [P], [Al], [Ni], [Cr], and [Mo] are weight percent(%) of respective elements. $R = \frac{\{{(Ceq 1)}^{2} + {(Ceq 2)}^{2}\}}{2}$
$Ceq 1 = [C] + \frac{[Mn]}{20} + \frac{[Si]}{30} + 2 [P] + 4 [S]$
$Ceq 2 = [C] + \frac{[Mn]}{6} + \frac{[Si]}{30} + \frac{([Cr] + [Mo] + [V] + [Nb])}{5} + \frac{([Cu] + [Ni])}{15}$
where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu], and [Ni] are weight percent(%) of respective elements.

[Table 2]

SPECI MEN NO.	STE EL TYP E	COOLING			REHEATING AND OVERAGING
SPECI MEN NO.	STE EL TYP E	PRIMARY COOLING END TEMPERA TURE (°C)	SECONDA RY COOLING END TEMPERA TURE (°C)	WHETHER SECONDA RY COOLING CONDITI ONS ARE SATISFI ED	REHEATI NG TEMPERA TURE (°C)	OVERAGI NG TEMPERA TURE (°C)	WHETHER REHEATI NG AND OVERAGI NG CONDITI ONS ARE SATISFI ED
1	B	700	300	X	-	180	X
2	B	700	250	X	-	180	X
3	B	700	200	O	-	180	X
4	B	650	300	X	-	180	X
5	B	650	250	X	-	180	X
6	B	650	200	O	-	180	X
7	B	600	300	X	-	180	X
8	B	600	250	X	-	180	X
9	B	600	200	O	-	180	X
10	B	700	100	O	210	210	O
11	B	700	130	O	210	210	O
12	B	700	150	O	210	210	O
13	B	700	100	O	230	230	O
14	B	700	130	O	230	230	O
15	B	700	150	O	230	230	O
16	B	700	150	O	250	250	O
17	B	700	180	O	230	230	O
18	B	700	200	O	230	230	X
19	B	700	180	O	250	250	O
20	B	700	200	O	250	250	O
21	B	700	220	X	250	250	X
22	B	700	240	X	250	250	X
23	B	700	200	O	270	270	X
24	B	700	220	X	270	270	X
25	B	700	240	X	270	270	X
26	B	700	100	O	180	180	O
27	B	700	100	O	250	250	O
28	B	700	100	O	270	270	X
29	B	700	150	O	270	270	X
30	B	700	150	O	300	300	X
31	C	700	100	O	210	210	O
32	C	700	130	O	210	210	O
33	C	700	150	O	210	210	O
34	C	700	100	O	230	230	O
35	C	700	130	O	230	230	O
36	C	700	150	O	230	230	O
37	C	700	100	O	180	180	O
38	C	700	100	O	270	270	X
39	C	700	150	O	270	270	X
40	C	700	150	O	300	300	X
41	D	700	100	O	210	210	O
42	D	700	130	O	210	210	O
43	D	700	150	O	210	210	O
44	D	700	100	O	230	230	O
45	D	700	130	O	230	230	O
46	D	700	150	O	230	230	O
47	I	700	130	O	210	210	O
48	I	700	150	O	250	250	O

In Table 3 below, a microstructure of each specimen was observed and physical properties were measured and illustrated. The microstructure was confirmed through an SEM photograph, and the number of carbides is represented as an average of the number of carbides in a 1µm² region (average of 10 regions) in a x10,000 SEM image, and a length of a major axis of the carbides was measured from x30,000 to x100,000 images on a TEM bright field and illustrated. In addition, values of yield strength (YS), tensile strength (TS), yield ratio (YS/TS), total elongation (T-El), and uniform elongation (U-El) were measured by processing a cold-rolled steel sheet in which continuous annealing is completed based on JIS standards (gauge length: width x length: 25x50mm, specimen total length: 200 to 260mm), and then measured by performing a tensile test under a condition of a test speed of 28mm/m. In addition, the bending properties (R/t) were measured by specimen-processing the same cold-rolled steel sheet into a width of 100 mm x length of 30 mm, and then performing a 90° bending test under a condition of a test speed of 100 mm/min, and then cracks in a bent portion were confirmed using a microscope, so that an R/t value was obtained by dividing a minimum bending radius (R) at which cracks did not occur by a thickness (t) of a test piece, and when the value thereof was less than 4, it was represented as 0, and when the value thereof was greater than or equal to 4, it was represented as X.

[Table 3]

SPEC IMEN NO.	STEEL TYPE	MICROSTRUCTURE			PHYSICAL PROPERTIES						DIVISION
SPEC IMEN NO.	STEEL TYPE	FRACTION OF MOR TM (ARE A%)	THE NUMBER OF CARB IDES (NUM BER)	AVERAGE LEN GTH OF MAJ OR AXI S (nm )	YIELD STRENGTH (MPa )	TENSILE STRENGTH (MPa )	YIELD RA TI O	TOTAL ELONGATION (%)	UNIFORMELONGATION (%)	BENDING PROPERTIES	DIVISION
1	B	99	-	-	937	1303	0. 72	7.9	4.8	X	COMPA RATIV E EXAMP LE 1
2	B	99	-	-	926	1342	0. 69	8.6	5.6	X	COMPA RATIV E EXAMP LE 2
3	B	99	-	-	1015	1390	0. 73	8.0	5.0	X	COMPA RATIV E EXAMP LE 3
4	B	95	-	-	777	1223	0.64	9.4	6.0	X	COMPARATIV E EXAMP LE 4
5	B	99	-	-	828	1277	0. 65	8.5	5.5	X	COMPA RATIV E EXAMP LE 5
6	B	99	-	-	834	1275	0. 65	8.7	5.6	X	COMPA RATIV E EXAMP LE 6
7	B	95	-	-	749	1214	0. 62	11.2	7.4	X	COMPA RATIV E EXAMP LE 7
8	B	95	-	-	785	1237	0. 63	9.1	6.1	X	COMPA RATIV E EXAMP LE 8
9	B	95	-	-	755	1238	0.61	9.7	6.5	X	COMPARATIV E EXAMP LE 9
10	B	99	19	85	1054	1362	0. 77	6.9	3.8	O	INVEN TIVE EXAMP LE 1
11	B	99	23	97	1103	1406	0. 78	8.1	4.8	O	INVEN TIVE EXAMP LE 2
12	B	99	21	91	1085	1401	0. 77	7.7	4.5	O	INVEN TIVE EXAMP LE 3
13	B	99	21	129	1091	1355	0. 80	6.8	3.7	O	INVEN TIVE EXAMP LE 4
14	B	99	22	146	1078	1359	0. 79	7.0	4.1	O	INVEN TIVE EXAMP LE 5
15	B	99	20	115	1066	1370	0.78	7.8	4.7	O	INVENTIVE EXAMP LE 6
16	B	99	24	170	1156	1388	0. 83	7.3	3.9	O	INVEN TIVE EXAMP LE 7
17	B	99	22	134	1042	1354	0. 77	8.6	5.2	O	INVEN TIVE EXAMP LE 8
18	B	99	19	152	964	1326	0. 73	9.5	5.9	X	COMPA RATIV E EXAMP LE 10
19	B	99	25	189	1066	1339	0. 80	7.8	4.7	O	INVEN TIVE EXAMP LE 9
20	B	99	27	195	1007	1305	0. 77	6.9	4.2	O	INVEN TIVE EXAMP LE 10
21	B	99	25	301	1000	1310	0.76	7.6	4.6	X	COMPARATIV E EXAMP LE 11
22	B	99	21	254	952	1313	0. 73	9.5	6.1	X	COMPA RATIV E EXAMP LE 12
23	B	99	31	302	1004	1263	0. 79	7.5	4.2	X	COMPA RATIV E EXAMP LE 13
24	B	99	33	313	1017	1271	0. 80	7.3	4.1	X	COMPA RATIV E EXAMP LE 14
25	B	99	28	331	1070	1322	0. 81	8.2	5.0	X	COMPA RATIV E EXAMP LE 15
26	B	99	17	78	1093	1405	0.78	8.6	4.9	O	INVENTIVE EXAMP LE 11
27	B	99	22	162	1183	1397	0. 85	6.6	3.7	O	INVEN TIVE EXAMP LE 12
28	B	99	45	309	1136	1340	0. 85	7.6	4.2	X	COMPA RATIV E EXAMP LE 16
29	B	99	39	322	1181	1368	0. 86	6.3	3.6	X	COMPA RATIV E EXAMP LE 17
30	B	99	43	357	1170	1306	0. 90	6.3	3.0	X	COMPA RATIV E EXAMP LE 18
31	C	99	25	172	1079	1345	0. 80	7.5	4.5	O	INVEN TIVE EXAMPLE 13
32	C	99	27	189	1072	1344	0. 80	7.4	4.3	O	INVEN TIVE EXAMP LE 14
33	C	99	30	187	1060	1335	0. 79	8.4	4.7	O	INVEN TIVE EXAMP LE 15
34	C	99	28	173	1112	1349	0. 82	7.5	4.5	O	INVEN TIVE EXAMP LE 16
35	C	99	29	168	1105	1347	0. 82	7.7	4.4	O	INVEN TIVE EXAMP LE 17
36	C	99	27	174	1098	1345	0. 82	7.2	4.1	O	INVEN TIVE EXAMP LE 18
37	C	99	30	191	1061	1364	0. 78	7.0	4.3	O	INVEN TIVE EXAMP LE 19
38	C	99	52	399	1188	1325	0. 90	4.7	2.5	X	COMPA RATIV E EXAMP LE 19
39	C	99	49	407	1174	1319	0. 89	5.8	2.7	X	COMPA RATIV E EXAMP LE 20
40	C	99	48	462	1203	1288	0. 93	4.7	2.2	X	COMPA RATIV E EXAMP LE 21
41	D	99	30	184	1160	1453	0. 80	7.0	4.0	O	INVEN TIVE EXAMP LE 20
42	D	99	31	193	1151	1454	0. 79	6.9	3.9	O	INVEN TIVE EXAMP LE 21
43	D	99	26	178	1126	1439	0. 78	6.2	3.8	O	INVEN TIVE EXAMP LE 22
44	D	99	32	171	1187	1446	0. 82	6.5	4.3	O	INVEN TIVE EXAMP LE 23
45	D	99	30	189	1172	1437	0. 82	6.6	3.5	O	INVEN TIVE EXAMP LE 24
46	D	99	31	177	1165	1436	0. 81	6.3	3.6	O	INVEN TIVE EXAMP LE 25
47	I	80	-	-	998	1192	0. 84	9.3	7.1	-	COMPA RATIV E EXAMP LE 22
48	I	80	-	-	990	1201	0. 82	10.8	7.6	-	COMPA RATIV E EXAMP LE 23

* M: Martensite, TM: Tempered Martensite

As illustrated in Table 3, in Inventive Examples 1 to 25 satisfying the alloy composition and manufacturing conditions of the present disclosure, the microstructure and carbide characteristics proposed in the present disclosure were satisfied, and the desired physical properties in the present disclosure were secured.
Meanwhile, in Comparative Examples 1, 2, 4, 5, 7 and 8, in which a secondary end cooling temperature does not satisfy Ms to 190°C or lower, which is a condition of the present disclosure, a yield ratio and bending properties, targeted by the present disclosure, was not satisfied, and tensile strength did not reach the target.
In particular, Comparative Examples 1 to 9 illustrates examples in which a reheating step is not included, and quenching and tempering are included as essential processes in the present disclosure, but the above-described examples are examples in which aging is performed at a temperature during cooling without reheating. That is, in the above-described examples, martensitic hardenability may be deteriorated, and since there is no tempering process, the yield strength was very inferior, so that the desired strength may not be obtained.
In addition, during reheating and overaging, Comparative Examples 10 to 21, not satisfying upper limit or lower limit conditions proposed in the present disclosure, were inferior in a yield ratio and bending properties, targeted by the present disclosure. In particular, when the lower limit thereof was not satisfied, the yield strength cannot be sufficiently increased, and in examples not satisfying the upper limit temperature condition of less than 270°C, bending properties were not secured due to formation of coarse carbides.
Comparative Examples 22 and 23 illustrate examples satisfying all of the manufacturing conditions proposed in the present disclosure, but not satisfying the alloy composition proposed in the present invention. Therefore, in the above-described examples, not only did not satisfy the desired microstructure fraction, but also failed to secure the desired strength.
FIGS. 1 (a) and (b) are SEM microstructure pictures (x10.000) of Inventive Example 15 and Comparative Example 21 according to an embodiment of the present disclosure. Both (a) and (b) of FIG. 1 illustrate tempered martensite as a microstructure, and it can be confirmed that a carbide in a form of rice grains was formed on the microstructure. Meanwhile, in the case of (b), it can be confirmed that the carbide per unit area was formed on the microstructure in excess of the range proposed in the present disclosure, and a size thereof was also excessively large.
While example embodiments have been illustrated and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.

Claims

A steel sheet comprising, by weight:
carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorous (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.5%, with a balance of Fe and inevitable impurities,

wherein an R value defined by the following Relational Expression 1 is 0.12 to 0.27,

an average number of carbides per 1µm² area is 40 or less, and an average length of a major axis of carbides is 300 nm or less, and

a yield ratio is greater than 0.73, $\begin{array}{l} R = \frac{\{{(Ceq 1)}^{2} + {(Ceq 2)}^{2}\}}{2} \\ Ceq 1 = [C] + \frac{[Mn]}{20} + \frac{[Si]}{30} + 2 [P] + 4 [S] \\ Ceq 2 = [C] + \frac{[Mn]}{6} + \frac{[Si]}{30} + \frac{([Cr] + [Mo] + [V] + [Nb])}{5} + \frac{([Cu] + [Ni])}{15} \end{array}$

where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu] and [Ni] are weight % of respective elements.
The steel sheet of claim 1, wherein the steel sheet further comprises two or more of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo) : 0.01 to 0.2%, and boron (B) : 0.005% or less (excluding 0%).
The steel sheet of claim 1, wherein the steel sheet further comprises one or more of titanium (Ti): 0.1% or less (excluding 0%), and niobium (Nb): 0.1% or less (excluding 0%).
The steel sheet of claim 1, wherein the steel sheet comprises 99 area% or more of martensite or tempered martensite as a microstructure.
The steel sheet of claim 1, wherein the steel sheet has a tensile strength (TS) of 1300 MPa or more, a bending property (R/t) of less than 4, where R is a minimum bending radius at which cracks do not occur in a bent portion after a 90° bending test, and t is a thickness of the steel sheet.
A method for manufacturing a steel sheet, comprising operations of:
preparing a cold-rolled steel sheet including, by weight: carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorous (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.5%, with a balance of Fe and inevitable impurities, wherein an R value defined by the following Relational Expression 1 is 0.12 to 0.27,

heat treating the cold-rolled steel sheet at a temperature of Ac3 or higher for 30 seconds or more;

primarily cooling the cold-rolled steel sheet to a temperature within a range of 500 to 750°C at an average cooling rate of 1 to 10°C/s after the heat treatment;

secondarily cooling the primarily-cooled steel sheet to a temperature of Ms-190°C or lower at an average cooling rate of 20 to 80°C/sec; and

reheating and overaging by heating the secondarily-cooled steel sheet to a temperature within a range of greater than secondary cooling end temperature+30°C and less than 270°C, and holding the same for 1 to 20 minutes, $\begin{array}{l} R = \frac{\{{(Ceq 1)}^{2} + {(Ceq 2)}^{2}\}}{2} \\ Ceq 1 = [C] + \frac{[Mn]}{20} + \frac{[Si]}{30} + 2 [P] + 4 [S] \\ Ceq 2 = [C] + \frac{[Mn]}{6} + \frac{[Si]}{30} + \frac{([Cr] + [Mo] + [V] + [Nb])}{5} + \frac{([Cu] + [Ni])}{15} \end{array}$

where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu] and [Ni] are weight percent (%) of respective elements.
The steel sheet of claim 6, wherein the cold-rolled steel sheet further comprises two or more of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2%, and boron (B): 0.005% or less (excluding 0%).
The steel sheet of claim 6, wherein the cold-rolled steel sheet further comprises one or more of titanium (Ti): 0.1% or less (excluding 0%), and niobium (Nb): 0.1% or less (excluding 0%).
The method for manufacturing a steel sheet of claim 6, wherein the operation of preparing the cold-rolled steel sheet comprises operations of:
reheating a steel slab to a temperature within a range of 1100 to 1300°C;

hot rolling the reheated steel slab at a finish hot rolling temperature of Ar3 or higher;

cooling and winding the hot-rolled steel sheet to a temperature within a range of 700 °C or lower; and

cold rolling the cooled and wound steel sheet at a reduction ratio of 30 to 80%.
The method for manufacturing a steel sheet of claim 9, further comprising
pickling the cooled and wound steel sheet with hydrochloric acid.