KR20160074767A - Steel sheet having excellent strength and ductility and manufacturing method for the same - Google Patents

Abstract

The present invention relates to a steel sheet, and a manufacturing method thereof. The steel sheet comprises: 0.1-1.2 wt% of carbon (C); 2.0-10.0 wt% of manganese (Mn); 3.0-15 wt% of aluminum (Al); 0.02 wt% or less of phosphorus (P); 0.015 wt% or less of sulfur (S); 0.01 wt% or less (excluding 0 wt%) of nitrogen (N); 0.001-0.1 wt% of cerium (Ce); 0.06 wt% or less (excluding 0 wt%) of lanthanum (La); and the remaining consisting of iron (Fe) and inevitable impurities. The steel sheet satisfies the following expression: 0.2 <= lanthanum (La) / cerium (Ce) <= 0.8. As such, the present invention is capable of lightening a vehicle body.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having excellent strength and ductility,

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a steel plate used for automotive structural members, interior and exterior plates, and the like, and more particularly to a steel plate having excellent strength and ductility.

In recent years, as a new fuel vehicle (for example, an electric vehicle) appears, it is expected that the weight of an automobile fuel system such as a battery will be significantly increased as compared with the current internal combustion engine. Therefore, Development of materials is required. The use of aluminum or magnesium as a lightweight material has been discussed, but aluminum or magnesium has a problem of low strength and ductility and high cost. Therefore, it is still inevitable to use steel.

Steels are superior in strength and ductility to aluminum and magnesium, and their costs are also very low. Until now, the weight of the body has been reduced by thinning the thickness of the high-strength high-tensile steel. However, when the weight of the steel itself is high and the weight limit required for automobiles is not satisfied, it is inevitable to use a non-ferrous metal such as Al in the steel .

Therefore, the development of steels with lower specific gravity has been underway, mainly by adding Al as a raw material. As a production technology known so far, a ferrite-based steel material with 2.0 to 10.0 wt% of Al added to ultra-low carbon steel and an austenitic stainless steel containing 8 wt% of Al to ultra low carbon steel and 10 to 30 wt% There is a manufacturing technique of steel. These have a problem in that although the rigidity is secured and the ductility is secured to some extent by controlling the precipitates and the aggregate structure, the tensile strength is as low as 400 MPa and the elongation is only about 25%.

In order to solve this problem, a duplex lightweight steel sheet having excellent rigidity and ductility without ridging has been developed by including a large amount of retained austenite, causing transformation-induced plasticity, and controlling ferrite aggregate structure There is. However, in the case of the ideal structure lightweight steel sheet, if the slab is reheated to perform hot rolling or subjected to heat treatment to obtain mechanical properties, decarbonization is performed, and the strength and ductility are lowered due to the decrease of the austenite along with the disappearance of carbon There is a problem.

Disclosure of the Invention The present invention has been made to solve the above problems and it is an object of the present invention to provide a steel sheet capable of suppressing edge cracks generated during the cold rolling process of a duplex structure steel of ferrite and austenite and having excellent strength and ductility, .

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

In one aspect, the present invention provides a method of producing a carbon nanowire comprising: 0.1 to 1.2% by weight of carbon; 2.0 to 10.0% by weight of manganese; 3.0 to 15% by weight of aluminum; Sulfur (S): not more than 0.015 wt%, nitrogen (N): not more than 0.01 wt% (excluding 0 wt%), Ce (cerium): 0.001 to 0.1 wt%, La (lanthanum) (Fe) and other unavoidable impurities, and satisfies the following formula (1): &quot; (1) &quot;

(1): 0.2? Lanthanum (La) / cerium (Ce)? 0.8

On the other hand, the steel sheet preferably has a tensile strength of 700 Mpa or more and an elongation of 30% or more.

On the other hand, the microstructure of the steel sheet is preferably a ferrite-based structure containing residual austenite.

On the other hand, the steel sheet may be a hot-rolled steel sheet, a cold-rolled steel sheet, or a coated steel sheet.

In another aspect, the present invention provides a method for producing a carbon nanotube, which comprises 0.1 to 1.2% by weight of carbon (C), 2.0 to 10.0% by weight of manganese (Mn), 3.0 to 15% by weight of aluminum (Al) Sulfur (S): not more than 0.015 wt%, nitrogen (N): not more than 0.01 wt% (excluding 0 wt%), Ce (cerium): 0.001 to 0.1 wt%, La (lanthanum) Reheating the slab which satisfies the following formula (1), including the remaining iron (Fe) and other unavoidable impurities; And a step of hot-rolling the reheated slab to obtain a hot-rolled steel sheet.

(1): 0.2? Lanthanum (La) / cerium (Ce)? 0.8

At this time, the reheating step is preferably performed at a temperature of 1000 to 1200 ° C, and the hot rolling is preferably performed at a temperature of 700 ° C or more.

Meanwhile, the manufacturing method of the present invention may further include a step of cold-rolling the hot-rolled steel sheet.

At this time, it is preferable that the cold rolling is performed at a cold reduction ratio of 40% or more.

Meanwhile, the manufacturing method of the present invention may further include the step of annealing the cold-rolled steel sheet.

The annealing is preferably carried out at a heating rate of 1 to 20 ° C / s to a recrystallization temperature of up to 900 ° C, maintained for 10 to 180 seconds and then cooled to 400 ° C at a cooling rate of 1 to 100 ° C / s Do.

Meanwhile, the manufacturing method of the present invention may further comprise a step of forming a plated layer on at least one surface of the annealed cold rolled steel sheet to obtain a plated steel sheet.

At this time, the plating layer may be one selected from Zn, Zn-Fe, Zn-Al, Zn-Mg, Zn-Al-Mg, Al-Si and Al-Mg-Si.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

According to the present invention, it is possible to provide a hot-rolled steel sheet capable of suppressing edge cracks generated during a cold rolling process of a ferritic and austenitic duplex structure steel, as well as a hot-rolled steel sheet having excellent strength and ductility, Since the steel plate is provided, there is a remarkable effect in weight reduction of the vehicle body.

Fig. 1 is a photograph comparing edge qualities of cold-rolled steel sheets of Inventive Example 4 and Comparative Example 4. Fig.
Fig. 2 is a photograph comparing the structures of the hot-rolled steel sheets of Inventive Example 4 and Comparative Example 4. Fig.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

1. Steel plate

As a result of extensive research, the inventors of the present invention have found that the present inventors have found out that the present inventors have found that when carbon (C), manganese (Mn), phosphorus (P), sulfur (S), aluminum (Al), nitrogen (N) In the case of producing a steel sheet containing iron (Fe) and using a slab satisfying a specific composition ratio of Ce (cerium) and La (lanthanum), a cold rolled steel sheet having a duplex structure of ferrite and austenite Cracks can be effectively suppressed and the present invention has been completed.

Specifically, the steel sheet of the present invention contains 0.1 to 1.2 wt% of carbon (C), 2.0 to 10.0 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P) 3.0 to 15 wt% of aluminum (Al), 0.01 wt% or less (excluding 0 wt%) of Ce, 0.001 to 0.1 wt% of Ce, 0.06 wt% or less of La (lanthanum) 0% by weight), remaining Fe (Fe) and other unavoidable impurities, and satisfies the following formula (1).

(1): 0.2? Lanthanum (La) / cerium (Ce)? 0.8

The reason for adding each component constituting the steel composition of the present invention and the appropriate content range thereof are as follows.

Carbon (C): 0.1 to 1.2 wt%

Carbon not only stabilizes the austenite but also acts to strengthen the dispersion by the cementite. Particularly, since the main phase formed during the continuous casting is rapidly recrystallized and forms a coarse object during the hot rolling, the lower limit of the carbon content is set to 0.1 wt% in order to form a high temperature carbide to make the structure finer and to increase the strength. However, as the amount of carbon added increases, the cementite and kappa carbide increase, which contributes to the strength increase, but the ductility of the steel is significantly lowered. Particularly, in the steel to which aluminum is added, since the kappa carbide precipitates on the ferrite crystal grain boundaries to cause brittleness, the upper limit is set to 1.2 wt% or less. That is, an appropriate content range of carbon is 0.1 to 1.2% by weight, for example, 0.12 to 1.0% by weight.

Manganese (Mn): 2.0 to 10.0 wt%

In addition to carbon, manganese is an element that controls the characteristics of carbides in the present invention and contributes to the formation of austenite at high temperatures. Manganese coexists with carbon, thereby promoting high-temperature precipitation of carbides, thereby suppressing carbides in the grain boundaries, thereby suppressing hot brittleness. Which ultimately contributes to the improvement of the strength of the steel sheet. In addition, manganese decreases the density of steel by increasing the lattice constant of the steel. Therefore, the lower limit of manganese was set to 2.0 wt%, and if too much is added, the center segregation of manganese and excessive band structure in the hot-rolled steel sheet would result in lowering ductility, so that the upper limit was set to 10.0 wt%. That is, an appropriate content range of manganese is 2.0 to 10.0% by weight, for example, 2.2 to 9.9% by weight.

Aluminum (Al): 3.0 to 15.0 wt%

Aluminum is the most important element together with carbon and manganese in the present invention. The specific gravity of the steel is reduced through the addition of aluminum. That a large amount is added preferably to a weight reduction, however, was limited to a large amount when kappa carbide, and because increasing the intermetallic compounds such as FeAl and Fe ₃ Al to significantly degrade the river ductility of 3.0 wt% is the lower limit, adding the upper limit to 15.0% by weight . That is, an appropriate content range of aluminum is 3.0 to 15.0% by weight, for example, 3.0 to 12.0% by weight.

On the other hand, even if the content of carbon, manganese and aluminum is controlled as in the present invention, the constituent phase should contain 5% or more of austenite in the temperature range (700 to 1200 ° C) in which hot rolling is performed. When the austenite content is 5% or less, excellent strength and ductility can not be obtained with a tensile strength of 700 MPa or more and an elongation of 30% or more because the steel sheet can not have a duplex structure at room temperature after annealing.

Phosphorus (P): not more than 0.02% by weight

Phosphorus is an element which needs to be inhibited as much as possible in the present invention. Phosphorus segregates in grain boundaries and causes high-temperature brittleness and brittleness at room temperature, thereby significantly lowering the workability of the steel. Further, when a large amount of P is contained, the texture of the orientation is developed on the surface and the ridging is increased, so that the upper limit is set to 0.02 wt%. That is, an appropriate content range of phosphorus is 0.02 wt% or less, for example, 0 to 0.015 wt% or less.

Sulfur (S): 0.015 wt% or less

As with phosphorus, sulfur promotes high-temperature brittleness. In particular, since coarse MnS is formed to cause rupture of the rolled sheet during hot rolling and cold rolling, it is limited to 0.015 wt% or less. That is, an appropriate content range of sulfur is 0.015 wt% or less, for example, 0 to 0.01 wt% or less.

Nitrogen (N): not more than 0.01% by weight

When nitrogen is contained in a large quantity of aluminum as in the present invention, AlN crystallization is effective to miniaturize the main phase structure and to improve the equiaxed crystal ratio. However, the cost for increasing the nitrogen content is increased, and nozzle clogging and deterioration of ductility due to precipitation It causes. Accordingly, the upper limit of nitrogen was set to 0.01 wt%. That is, an appropriate content range of nitrogen is 0.01 wt% or less, for example, 0.005 wt% or less.

Cerium (Ce): 0.001 to 0.1 wt%

In the present invention, cerium plays an important role in crack suppression during cold rolling through refinement of hot-rolled structure. However, since cerium is an expensive element, it causes an increase in production cost and precipitates in the grain boundaries to lower the rolling property, so the upper limit is limited to 0.1 wt%. That is, an appropriate content range of cerium is 0.001 to 0.1% by weight, for example, 0.001 to 0.08% by weight.

Lantanum (La): not more than 0.06% by weight

Like lanthanum cerium, plays an important role in refinement of hot-rolled structure and suppression of cracks in cold rolling, but the upper limit is limited to 0.06% due to a problem of an increase in production cost and a decrease in rolling property due to grain boundary precipitation. That is, an appropriate content range of lanthanum is 0.06% by weight or less, for example, 0.05% by weight or less.

The remainder of the present invention is iron (Fe). However, in the ordinary steel manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of steel making.

On the other hand, it is preferable that lanthanum (La) and cerium (Ce) of the steel sheet of the present invention satisfy a specific composition ratio as shown in the formula (1). When the lanthanum (La) / cerium (Ce) is 0.2 to 0.8, the cost problem can be solved and the effect according to the present invention can be effectively realized. More preferably, the lanthanum La / cerium (Ce) may be in the range of 0.3 to 0.6.

On the other hand, the microstructure of the steel sheet of the present invention is preferably a ferrite structure containing residual austenite. For example, the microstructure of the steel sheet of the present invention may contain residual austenite in the ferrite matrix structure. As described above, the steel sheet of the present invention preferably has a duplex structure, and when it does not have such a structure, excellent strength and ductility can not be obtained with a tensile strength of 700 MPa or more and an elongation of 30% or more.

On the other hand, the steel sheet of the present invention satisfying the above-mentioned conditions has excellent mechanical properties because the retained austenite and carbide are dispersed in a ferrite matrix, tensile strength is 700 MPa or more, and elongation is 30% or more. As described above, the steel sheet of the present invention can secure both excellent strength and ductility, which are difficult to make compatible with each other.

The steel sheet of the present invention may be a hot-rolled steel sheet, a cold-rolled steel sheet, or a coated steel sheet. The coated steel sheet may be made of at least one of Zn, Zn-Fe, Zn-Al, Zn- Si, and Al-Mg-Si. The thickness of the plating layer is preferably about 10 to 200 mu m.

2. Method of manufacturing steel sheet

Next, a method of manufacturing the steel sheet of the present invention will be described in detail. However, the following manufacturing method is merely an example of producing the steel sheet of the present invention, and the manufacturing method of the steel sheet of the present invention is not limited to the following manufacturing method.

The steel sheet according to the present invention comprises 0.1 to 1.2% by weight of carbon (C), 2.0 to 10.0% by weight of manganese (Mn), 3.0 to 15% by weight of aluminum (Al) Sulfur (S): not more than 0.015 wt%, nitrogen (N): not more than 0.01 wt% (excluding 0 wt%), Ce (cerium): 0.001 to 0.1 wt%, La (lanthanum) Reheating the slab which satisfies the following formula (1), including the remaining iron (Fe) and other unavoidable impurities; And hot rolling the reheated slab.

(1): 0.2? Lanthanum (La) / cerium (Ce)? 0.8

First, a steel ingot or slab satisfying the composition and the formula (1) (hereinafter referred to as a slab) is prepared, and the slab is reheated to 1000 to 1200 ° C. The reheating temperature is preferably set to 1000 to 1200 ° C so as to secure a normal hot rolling temperature.

After the reheating, it is preferable that hot rolling is carried out by a usual method and finish rolling is performed at 700 캜 or higher. The finishing rolling temperature has a duplex structure at a high temperature and is a temperature at which rolling can be well performed by a ferrite having excellent ductility. The lower the finishing rolling temperature is, the more the rolling load is increased. Do. After the hot-rolling, the hot-rolled steel sheet can be produced by winding in a usual manner.

Duplex steels, on the other hand, have soft ferrite and hard austenitic anomalies, and most of the ferrite is deformed during hot rolling. This is because the recovery of low ferrite and recrystallization are very fast. As a result, there is a problem that a band-like structure in which carbide or austenite is arranged in layers in a ferrite base structure is formed. The band structure causes mechanical properties anisotropy of the steel, which deteriorates the workability and causes brittle fracture during cold rolling. In the present invention, by adding cerium (Ce) and lanthanum (La) in order to solve this problem, the band-shaped hot-rolled structure becomes finer and brittle fracture can be suppressed during cold rolling.

Meanwhile, the steel of the present invention is free from cracks due to the addition of cerium (Ce) and lanthanum (La), and therefore, there is no need for a separate heat treatment to reduce the austenite band structure and the carbide spheroidization in order to solve this problem. There is no loss advantage. As a result, it is possible to manufacture a low-cost, high ductile low specific gravity light-weight steel sheet.

On the other hand, the hot-rolled steel sheet can be cold-rolled to a cold-rolled steel sheet at a cold reduction ratio of 40% or more. Cold rolling is usually carried out after pickling, and if the cold rolling reduction is not less than 40%, the accumulation energy is secured by cold working, and a new recrystallized structure can be obtained.

On the other hand, the cold-rolled steel sheet can be produced by removing the rolling oil from the surface after cold rolling and annealing, wherein the annealing is performed at a heating rate of 1 to 20 ° C / s, Annealing for 180 seconds, and then cooling to 400 DEG C at a cooling rate of 1 to 100 DEG C / s. When the heating rate is less than 1 ° C / s, the productivity is lowered and exposed to a high temperature for too long, resulting in crystal grain coarsening and strength deterioration, resulting in deterioration of the material. When the heating rate exceeds 20 ° C / s, There is a problem that the amount of austenite formed decreases and the amount of retained austenite finally decreases, resulting in lower ductility. In addition, it is preferable to maintain at least 10 seconds or more for cracking so that sufficient recrystallization and grain growth can be performed at the recrystallization temperature to 900 ° C. If it exceeds 180 seconds, the productivity is lowered. In the subsequent plating process, The treatment time may be increased, and the corrosion resistance and surface properties may be deteriorated.

On the other hand, at least one surface of the cold-rolled steel sheet may be subjected to continuous hot dip galvanizing or the like to form a plating layer. The plating is not particularly limited, and zinc plating, aluminum-based or metal alloy plating is applied to secure corrosion resistance . A plating layer such as Zn, Zn-Fe, Zn-Al, Zn-Mg, Zn-Al-Mg, Al-Si or Al-Mg-Si can be formed. It is preferable that the plating layer is formed to have a thickness of 10 to 200 mu m per one side from the viewpoint of ensuring sufficient corrosion resistance.

Hereinafter, the present invention will be described more specifically by way of examples.

Example

Steel slabs prepared as shown in Table 1 below were prepared by vacuum induction melting and casting, heated at a temperature of about 1150 ° C and extracted, and then hot rolled at around 800 ° C. At this time, the thickness of the hot-rolled sheet was 3.0 mm.

division C Mn Al P S Ce La N La / Ce Inventory 1 0.12 9.9 3.3 0.01 0.003 0.0015 0.0009 0.004 0.6 Inventory 2 One 2.2 9.8 0.01 0.002 0.0042 0.0026 0.005 0.62 Inventory 3 0.5 6.1 6.1 0.014 0.003 0.016 0.0071 0.004 0.44 Honorable 4 0.338 3.58 5.79 0.012 0.004 0.032 0.0126 0.003 0.39 Inventory 5 0.31 8.2 4.8 0.012 0.006 0.048 0.0147 0.004 0.31 Inventory 6 0.6 2.5 7.6 0.014 0.003 0.068 0.0218 0.003 0.32 Comparative Example 1 0.004 0.24 10 0.02 0.006 0.0056 0.0031 0.004 0.55 Comparative Example 2 1.2 2.7 8.7 0.01 0.007 0.05 0.08 0.004 1.6 Comparative Example 3 0.5 7.2 5.8 0.01 0.002 0.25 0.04 0.003 0.16 Comparative Example 4 0.3 3.5 6.2 0.014 0.002 - - 0.004 0 * The content units of the above components are all% by weight

Examples 1 to 6 satisfied both the composition and the La / Ce ratio proposed in the present invention, Comparative Example 1 satisfied the La / Ce ratio, but had a ferrite single phase structure even at a high temperature, 0.0 &gt; La / Ce &lt; / RTI &gt;

The hot-rolled sheet was held at a temperature of about 600 ° C for one hour, and then was cooled to room temperature to prepare a cold-rolled sheet having a thickness of 1.0 mm. At this time, the cold reduction rate was about 50%. Next, the cold-rolled sheet was heated to 800 占 폚 at a rate of 5 占 폚 / sec and maintained for 60 seconds, then slowly cooled to about 650 占 폚, quenched to 400 占 폚 at a cooling rate of 20 占 sec / sec, To obtain a cold-rolled steel sheet. Thereafter, zinc was plated in a molten zinc bath at a temperature of about 450 DEG C after the constant temperature holding treatment, and then alloyed at about 550 DEG C and then cooled to room temperature to produce a galvannealed steel sheet. The mechanical properties of the galvanized steel sheet thus produced are shown in Table 2 below, and the measurement method is as follows.

1. Size and shape of tensile specimen: Specification JIS-13 (ASTM), specimen processing in which the tensile direction is parallel to the rolling direction, Cross Head Speed: 10 mm / min

2. Tensile strength (MPa): Maximum tensile load (N) / Specimen cross section (mm ² )

3. Elongation (%): (distance between the points after fracture - distance of gauge) / gauge distance

division Tensile Strength (MPa) Elongation (%) Cold crack Inventory 1 1064 31.3 Not occurring Inventory 2 998 38.4 Not occurring Inventory 3 884 35.8 Not occurring Honorable 4 798 32.1 Not occurring Inventory 5 837 34.6 Not occurring Inventory 6 881 37.5 Not occurring Comparative Example 1 - - Cold rolling is impossible due to hot crack occurrence Comparative Example 2 742 22.2 Occur Comparative Example 3 803 27.6 Occur Comparative Example 4 756 31.4 Occur

Inventive Examples 1 to 6 exhibited very good quality of less than 500 탆 in edge cracking of the cold-rolled sheet during cold rolling and exhibited excellent properties with an ultrahigh strength of 798 to 1064 MPa and an elongation of 31% or more. On the other hand, in Comparative Example 1, mechanical properties could not be confirmed due to cracking during hot rolling. In Comparative Examples 2 and 3, cracks occurred in the edge portion during cold rolling, and strength and elongation were lower than those of similar components.

Fig. 1 is a photograph comparing the edge quality of the cold-rolled steel sheet of Inventive Example 4 and Comparative Example 4. Fig. Specifically, the left side is the photograph showing the edge quality of the inventive fourth steel sheet, and the right side is the photograph showing the edge quality of the comparative example 4 steel sheet. Comparative Example 4 in which cerium (Ce) and lanthanum (La) were not added In the case of a steel sheet, fine cracks were generated in the edge portion after cold rolling, and cracks propagated as the reduction rate was increased. 4 shows good edge quality.

Fig. 2 is a photograph comparing the structures of the hot-rolled steel sheets of Inventive Example 4 and Comparative Example 4. Fig. Specifically, the left side is a photograph showing the structure of the fourth steel sheet and the right side is a photograph showing the structure of the steel sheet of Comparative Example 4. In the case of the inventive steel sheet, it can be seen that the addition of cerium (Ce) and lanthanum (La) makes the hot-rolled band structure composed of ferrite and carbapatite finer and effective in suppressing cracking during cold rolling. On the other hand, in the case of the steel sheet of Comparative Example 4, it can be seen that a coarse band structure is formed as compared with the inventive case.

As described above, according to the present invention, the hot-rolled band structure is made fine through addition of cerium (Ce) and lanthanum (La), and furthermore, the cracks of the cold rolled steel are suppressed without heat treatment for decomposing band structure, It is possible to manufacture.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, It will be obvious to those of ordinary skill in the art.

Claims (13)

(P): 0.02 wt.% Or less, sulfur (S): 0.015 wt.% Or less, (Excluding 0 wt.%), Ce (cerium): 0.001 to 0.1 wt.%, La (lanthanum): 0.06 wt.% Or less (Fe) and other unavoidable impurities, and is excellent in strength and ductility satisfying the following formula (1).
(1): 0.2? Lanthanum (La) / cerium (Ce)? 0.8
The method according to claim 1,
The steel sheet has a tensile strength of 700 MPa or more, and an elongation of 30% or more.
The method according to claim 1,
The microstructure of the steel sheet is a ferritic structure containing retained austenite, which is excellent in strength and ductility.
4. The method according to any one of claims 1 to 3,
The steel sheet is a hot-rolled steel sheet, a cold-rolled steel sheet, or a coated steel sheet having excellent strength and ductility.
(P): 0.02 wt.% Or less, sulfur (S): 0.015 wt.% Or less, (Excluding 0 wt.%), Ce (cerium): 0.001 to 0.1 wt.%, La (lanthanum): 0.06 wt.% Or less Reheating the slab containing iron (Fe) and other unavoidable impurities and satisfying the following formula (1); And
And hot-rolling the reheated slab to obtain a hot-rolled steel sheet.
(1): 0.2? Lanthanum (La) / cerium (Ce)? 0.8
6. The method of claim 5,
Wherein the reheating step is performed at a temperature of 1000 to 1200 DEG C and is excellent in strength and ductility.
6. The method of claim 5,
Wherein the finish rolling in the hot rolling is excellent in strength and ductility at a temperature of 700 캜 or higher.
6. The method of claim 5,
Further comprising a step of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet.
9. The method of claim 8,
Wherein the cold rolling is performed at a cold reduction rate of 40% or more, and is excellent in strength and ductility.
9. The method of claim 8,
And annealing the cold-rolled steel sheet.
11. The method of claim 10,
The annealing is carried out by heating to a recrystallization temperature of up to 900 DEG C at a heating rate of 1 to 20 DEG C / s, holding for 10 to 180 seconds, and then cooling at 400 DEG C to a cooling rate of 1 to 100 DEG C / A method for producing a steel sheet excellent in ductility.
11. The method of claim 10,
And a step of forming a plated layer on at least one surface of the annealed cold rolled steel sheet to obtain a plated steel sheet.
13. The method of claim 12,
Wherein the plating layer is a selected one of Zn, Zn-Fe, Zn-Al, Zn-Mg, Zn-Al-Mg, Al-Si and Al-Mg-Si and is excellent in strength and ductility.

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