KR101758563B1 - Ultra high strength steel sheet having excellent elongation, and method for manufacturing the same - Google Patents

Abstract

The present invention relates to an ultra high strength steel sheet for automobiles, and more particularly to an ultra high strength steel sheet having excellent elongation and a method for producing the same.

Description

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

The present invention relates to an ultra high strength steel sheet for automobiles, and more particularly to an ultra high strength steel sheet having excellent elongation and a method for producing the same.

Currently, automakers are continuously demanding the weight reduction of automobiles in order to regulate CO ₂ emissions and improve fuel efficiency of vehicles that are being strengthened. In order to accomplish this, the thickness of the steel sheet must be lowered. However, in order to secure the collision stability and the like, the thickness of the steel sheet must be increased.

In order to overcome this problem, it is necessary to increase the moldability while increasing the strength of the material. This is because the dual phase steel (hereinafter referred to as "DP steel") well known as AHSS (Advanced High Strength Steel) Transformation Induced Plasticity Steel (hereinafter referred to as "TRIP steel") and Complex Phase Steel (hereinafter referred to as "CP steel").

Although the strength or strength of the high strength steel sheet can be increased by increasing the amount of carbon or alloy, the tensile strength that can be achieved through the heat treatment is limited to the level of about 1200 MPa, and it is disadvantageous in that cold drawing is difficult due to relatively elongation.

On the other hand, TWIP (Twinning-Induced Plasticity) steel, which can secure both elongation and strength through the formation of twin during plastic deformation, has an advantage in that it can be molded into a complicated shape component because of its excellent elongation, Due to the strength, there are limitations when it is used as collision structural member of automobile.

For example, Patent Document 1 proposes a steel sheet having a tensile strength of 700 to 900 MPa with a high level of ductility by controlling alloy components such as C, Si, Mn, Al, Ni, Mo, P, , The steel sheet has a low yield strength and has a disadvantage that its application to a crash structure member for an automobile is limited in order to open a collision characteristic.

Patent Document 2 also includes Cr, Mo, Ni, Cu, Ti, Nb, V and the like while controlling the alloying components of C, Si, Mn, Al, P, S, , The steel sheet having a high tensile strength is presented. However, there is a disadvantage in that no consideration is given to the high deformation resistance, that is, the yield strength directly related to the impact characteristics.

Therefore, it is required to develop a steel sheet that is suitable for cold press forming and has excellent tensile strength and ductility as well as yield strength.

International Patent Publication No. WO2011-122237 U.S. Patent Application Publication No. 2015-0078954

An aspect of the present invention is to provide an ultrahigh strength steel sheet for cold press forming which has high ultrahigh strength and high ductility by controlling steel alloy components and manufacturing conditions and has high yield strength and excellent impact properties and a method for producing the same .

One aspect of the present invention is a method of manufacturing a silicon carbide semiconductor device comprising, by weight%, more than 0.7% to less than 0.9% carbon (C), less than 2.0% (excluding 0%) silicon (Si) P: not more than 0.020%, sulfur (S): not more than 0.010%, aluminum (Al): not more than 0.030%, nitrogen (N): not more than 0.060%, vanadium (V) Strength steel sheet containing an austenite having an area fraction of not less than 95% and a residual carbide and having an elongation satisfying the following relational expression (1), and a balance of Fe and other unavoidable impurities.

[Relation 1]

0.284 + 0.693C + 0.116Si - 0.016Mn - 0.011Al + 0.745V + 0.017X > 1.05

(C, Si, Mn, Al and V in the above relational expression 1 means the content by weight of each element, and X means the reduction ratio (%) during cold rolling).

According to another aspect of the present invention, there is provided a method of manufacturing a steel slab, comprising: preparing a steel slab satisfying the above-described composition; Reheating the steel slab in a temperature range of 1000 to 1250 占 폚; Subjecting the reheated steel slab to finish hot rolling at a temperature of 500 to 950 캜 to produce a hot-rolled steel sheet; Winding the hot-rolled steel sheet at a temperature of 750 ° C or lower; Picking up the hot rolled steel sheet and cold rolling the hot rolled steel sheet at a cold reduction rate of 30 to 80% to produce a cold rolled steel sheet; Annealing the cold-rolled steel sheet in a temperature range of 750 to 950 ° C; And cold-rolling the annealed heat-treated cold-rolled steel sheet at a reduction ratio of 5 to 50%, the method comprising the steps of:

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a steel sheet having excellent yield strength in addition to an elongation, which can be suitably applied to cold forming, .

By replacing the expensive hot-press-forming steel sheet with the low-cost cold-press-forming steel sheet in this way, the generation of CO ₂ caused at the time of high-temperature molding can be suppressed, which is advantageous in terms of production cost and environment.

FIG. 1 shows the result of observing the grain orientation spread according to the reduction ratio (%) during cold rolling of Invention Steel 2 in an embodiment of the present invention ((a) 0%, (b) 5%, (c) 12%, (d) 17%, (e) legend in the drawing).

The present inventors have intensively studied to develop a steel sheet for cold press forming capable of replacing a conventional hot-press forming steel sheet, having a mechanical property equal to or higher than that of the steel sheet, and capable of reducing the manufacturing cost. As a result, it has been found that an ultra-high strength steel sheet excellent in mechanical properties and elongation having microstructure suitable for cold press forming can be provided by optimizing the steel component composition and the manufacturing conditions, thereby completing the present invention.

Hereinafter, the present invention will be described in detail.

(C): not less than 0.7% to not more than 0.9%, silicon (Si): not more than 2.0% (excluding 0%), manganese (Mn) (P): 0.020% or less, S: not more than 0.010%, aluminum (Al): 0.01 to 3.0%, nitrogen (N): not more than 0.060% ): 0.5% or less (excluding 0%).

Hereinafter, the reason for controlling the alloy component of the ultra-high strength steel sheet having excellent elongation provided by the present invention will be described in detail. Herein, unless otherwise specified, the content of each component means weight%.

C: more than 0.7% to less than 0.9%

Carbon (C) contributes to the stabilization of the retained austenite as well as the strength of the steel. When the content of C is 0.7% or less, the austenite single phase structure can be secured during the final heat treatment, There is a problem that the yield strength can not be secured. In addition, there is a problem that decarburization occurs on the surface during heat treatment at a high temperature, and martensite structure is formed on the surface at the time of transformation at room temperature, so that it is vulnerable to delayed fracture. On the other hand, if the content exceeds 0.9%, the electrical resistivity increases and the weldability may be lowered.

Therefore, in the present invention, it is preferable to limit the content of C to more than 0.7% to 0.9%. , More preferably 0.75 to 0.9%, and even more preferably 0.8 to 0.9%.

Si: 2.0% or less (excluding 0%)

Silicon (Si) is typically used as a deoxidizing agent for steel, and is an element that not only inhibits the precipitation of carbides in steel but also contributes to the stabilization of retained austenite.

If the content of Si exceeds 2.0%, a large amount of silicon oxide is formed on the surface during hot rolling to deteriorate acidity and increase electrical resistivity, resulting in poor weldability.

Therefore, in the present invention, it is preferable to limit the Si content to 2.0% or less, and 0% is excluded. More preferably, the lower limit of the Si content is limited to 0.1%.

Mn: 12 to 20%

Manganese (Mn) is an effective element for the formation and stabilization of retained austenite. This Mn is known to be an element widely used in the transformational organic plastic steels and is usually added in an amount of about 3% for TRIP steels and about 12% for TWIP steels of austenite single phase steels. The reason for limiting the content in this manner is that when Mn is contained in the middle range of the above-mentioned content, a large amount of martensite is formed and the elongation rate is lowered.

In the present invention, when the content of Mn is less than 12%, a martensite phase is formed during deformation, making it difficult to secure a stable austenite phase, and there is a problem that the delayed fracture resistance after deformation becomes poor. On the other hand, when the content exceeds 20%, the effect obtained by Mn is saturated, which causes a problem of raising the production cost.

Therefore, in the present invention, the content of Mn is preferably limited to 12 to 20%.

P: not more than 0.020%

Phosphorus (P) is an element that is inevitably added in the steel. When the content exceeds 0.020%, the weldability is lowered and the risk of causing low-temperature embrittlement of the steel is greatly increased. Therefore, it is preferable to limit the upper limit to 0.020%.

S: not more than 0.010%

Sulfur (S) is also an inevitably added element in the steel, and if it exceeds 0.010%, the ductility and weldability of the steel are likely to be deteriorated. Therefore, it is preferable to limit the upper limit to 0.010%.

Al: 0.01 to 3.0%

Aluminum (Al) is an element which bonds with oxygen and acts as a deoxidizing agent. In order to stably secure the deoxidation effect, it is preferable that Al (Al) is contained at 0.01% or more. However, Al is a representative oxidizing element together with Si, and when the content exceeds 3.0%, the high temperature ductility is reduced and the main composition is not only dulled but also oxidized on the steel surface during hot rolling, have.

Therefore, in the present invention, the content of Al is preferably limited to 0.01 to 3.0%.

N: 0.060% or less (excluding 0%)

Nitrogen (N) is an element effective for stabilizing austenite. However, if the content exceeds 0.060%, the risk of brittleness increases greatly. Therefore, it is preferable to limit the upper limit to 0.060%.

V: 0.5% or less (excluding 0%)

Vanadium (V) is an element which contributes to the improvement of steel strength by forming a fine carbide by bonding with C, but when the content exceeds 0.5%, the above-mentioned effect is saturated and the production cost is increased as an expensive element .

Accordingly, in the present invention, the content of V is preferably limited to 0.5% or less, and 0% is excluded.

The present invention may further comprise the following components in addition to the above-mentioned components.

Specifically, the present invention may further include at least one of titanium (Ti): 0.005 to 0.3%, niobium (Nb): 0.005 to 0.3%, and molybdem (Mo): 0.05 to 0.3%.

The titanium (Ti) and niobium (Nb) are effective elements for increasing the strength of the steel sheet and refining the grain. If the contents of Ti and Nb are less than 0.005% each, it is difficult to sufficiently secure the above-mentioned effects. On the other hand, when the contents of Ti and Nb are more than 0.3%, not only the production cost increases but also the ductility is excessively decreased, have. Therefore, it is preferable to limit the content of Ti and Nb to 0.005 to 0.3%, respectively.

The molybdenum (Mo) is an element contributing to the improvement of the strength of the steel by forming a minute carbide together with the vanadium mentioned above. If the content of Mo is less than 0.05%, the above-mentioned effect can not be sufficiently ensured. If Mo is also an expensive element and the content thereof exceeds 0.3%, there is a problem that the manufacturing cost greatly increases. Therefore, it is preferable that the content of Mo is limited to 0.05 to 0.3%.

The remainder of the present invention is iron (Fe). However, in the ordinary 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 manufacturing.

The ultra-high strength steel sheet of the present invention satisfying the above-mentioned composition is preferably a microstructure and contains an austenite phase as a main phase.

More preferably, the steel sheet of the present invention contains an austenite phase with an area fraction of 95% or more, and preferably contains carbide as the remainder. However, austenite single phase may be used.

If the fraction of the austenite phase is less than 95%, it is difficult to secure a desired level of physical properties, particularly elongation, in the present invention, and it becomes difficult to secure a steel sheet suitable for cold press forming.

In addition, it is preferable that the ultra-high strength steel sheet of the present invention having the above-mentioned microstructure includes the strain grains after the cold rolling at an area fraction of 10% or more. If the fraction of the deformed crystal grains after the cold rolling is less than 10%, there is a problem that it is difficult to obtain a desired ultra high strength in the present invention.

The 'strain grains' are defined as 'deformed grains' that have a difference in crystal orientation in the grains of 3 ° or more during cold re-rolling. Grain Orientation Spread (Grain Orientation Spread) obtained through EBSD (Electron Back Sactter Diffraction) , GOS) map.

On the other hand, it is preferable that the steel sheet of the present invention having the aforementioned composition and having the microstructure described above satisfies the following relational expression 1 in order to secure a desired ultra high strength.

The following relational expression 1 shows the relationship between the components (C, Si, Mn, Al, V) influencing the strength of the steel and the reduction rate during re-rolling to improve the strength of the steel. That is, when the relationship between the content of the components and the reduction rate in re-rolling exceeds 1.05, the yield strength of the target level, that is, the yield strength of 1000 MPa or more is secured .

[Relation 1]

0.284 + 0.693C + 0.116Si - 0.016Mn - 0.011Al + 0.745V + 0.017X > 1.05

(C, Si, Mn, Al and V in the above relational expression 1 means the content by weight of each element, and X means the reduction ratio (%) during cold rolling).

As described above, the ultra-high strength steel sheet of the present invention contains an austenite phase that is stable with a microstructure and satisfies the proposed composition, and contains crystal grains deformed by satisfying the above-mentioned Relation 1 at 10% or more, A yield strength and a tensile strength of 1400 MPa or more can be secured. Further, a total elongation of not less than 15% and a yield ratio of 0.7 or more can be ensured.

That is, the ultra-high strength steel sheet of the present invention is not only suitable as a steel sheet for cold forming, but also has excellent impact characteristics and can be advantageously applied as a crash structure member for an automobile.

On the other hand, the steel sheet referred to in the present invention may be not only a cold rolled steel sheet but also a hot-dip galvanized steel sheet or a galvannealed hot-dip galvanized steel sheet obtained by plating the cold rolled steel sheet.

Hereinafter, a method of manufacturing an ultra-high strength steel sheet having an excellent elongation, which is another aspect of the present invention, will be described in detail.

First, a method of manufacturing a cold-rolled steel sheet according to the present invention will be described in detail below.

The cold-rolled steel sheet according to the present invention can be prepared by preparing a steel slab satisfying the above-mentioned composition and then subjecting it to reheating-hot rolling-winding-cold rolling-annealing heat treatment-cold rolling, Will be described in detail.

Steel slab reheating process

In the present invention, it is preferable that the steel slab prepared before the hot rolling is reheated and homogenized, and it is preferable to perform the reheating process at 1000 to 1250 ° C.

If the reheating temperature is less than 1000 캜, there is a problem that the load during the subsequent hot rolling increases sharply. On the other hand, when the reheating temperature is higher than 1250 캜, the energy cost increases and the amount of surface scale increases.

Therefore, it is preferable that the steel slab is heated in the temperature range of 1000 to 1250 deg.

Hot rolling process

Preferably, the reheated steel slab is hot-rolled to produce a hot-rolled steel sheet, and the hot-rolled steel sheet is preferably subjected to finish hot rolling in a temperature range of 500 to 950 ° C.

If the finish hot rolling temperature is less than 500 ° C, there is a problem that the rolling load increases greatly. If the temperature exceeds 950 ° C, the thermal fatigue of the roll is greatly increased, which shortens the service life.

Therefore, it is preferable that the heat treatment is carried out in the temperature range of 500 to 950 占 폚 in the final hot rolling.

Winding process

It is preferable that the hot-rolled steel sheet produced according to the above is rolled up at a temperature of 750 ° C or lower.

If the coiling temperature exceeds 750 캜, the scale of the surface of the steel sheet is excessively formed to cause defects, which causes deterioration of the plating ability. The lower limit temperature at the time of winding is not particularly limited.

Pickling and cold rolling processes

The hot rolled steel sheet wound in accordance with the above is preferably subjected to cold rolling in order to remove the oxide layer through ordinary pickling treatment and secure the shape of the steel sheet and the thickness required by the customer.

The cold rolling is preferably carried out at a cold reduction of 30 to 80%. If the cold rolling reduction is less than 30%, there is a high possibility that recrystallization may not be sufficiently performed due to insufficient accumulation energy for recrystallization in the subsequent annealing heat treatment, On the other hand, if it exceeds 80%, not only the rolling operation becomes unstable but also the power cost increases greatly.

Therefore, it is preferable that the cold rolling is performed at a cold reduction rate of 30 to 80%.

Annealing heat treatment process

It is preferable that the cold-rolled steel sheet produced according to the above-mentioned process is annealed at 750 to 950 캜.

If the temperature is lower than 750 ° C., the recrystallization may not occur sufficiently. On the other hand, if the temperature is higher than 950 ° C., not only the process cost increases due to the high temperature but also the plating ability is reduced due to the formation of surface oxides.

In the meantime, according to the present invention, the cold-rolled steel sheet subjected to the annealing heat treatment may be plated to produce a coated steel sheet.

At this time, an electroplating method, a hot-dip coating method, or an alloying hot-dip plating method can be used. Specifically, a hot-dip galvanized steel sheet can be manufactured by immersing the cold-rolled steel sheet in a zinc plating bath. Further, the hot-dip galvanized steel sheet may be subjected to an alloying heat treatment to produce a galvannealed hot-dip galvanized steel sheet.

The conditions for the plating treatment are not particularly limited, and the plating treatment can be carried out under the general conditions.

In addition, it is preferable that the cold-rolled steel sheet, the hot-dip galvanized steel sheet or the galvannealed steel sheet produced according to the present invention is subjected to a cold rolling process.

The cold rolling process is intended to increase the strength of the steel sheet. It is preferable to re-roll the steel sheet at a reduction ratio of 5% or more. If the rolling reduction is less than 5% at the re-rolling, the yield strength and tensile strength of the steel sheet are insufficient due to insufficient fraction of the cold-deformed grains. On the other hand, when the reduction ratio exceeds 50%

Therefore, it is preferable that the cold rolling is performed at a reduction rate of 5 to 50%.

Further, the present invention preferably satisfies the following relational expression (1) in the cold rolling performed as described above.

As described above, it is possible to manufacture a steel sheet having an ultrahigh strength, that is, a yield strength of 1000 MPa or more, by satisfying the following relational expression (1).

[Relation 1]

0.284 + 0.693C + 0.116Si - 0.016Mn - 0.011Al + 0.745V + 0.017X > 1.05

(C, Si, Mn, Al and V in the above relational expression 1 means the content by weight of each element, and X means the reduction ratio (%) during cold rolling).

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

The steel having the composition shown in the following Table 1 was vacuum-melted with an ingot of 30 kg, and then maintained at a temperature of 1200 캜 for 1 hour. Thereafter, the hot-rolled steel sheet was produced by subjecting the hot-rolled steel sheet to a finish rolling at 900 ° C, then hot rolled by heating the hot-rolled steel sheet at 600 ° C in a furnace heated for 1 hour and then cooling. Thereafter, each specimen was cooled to room temperature, pickled and cold rolled to produce a cold-rolled steel sheet. The cold rolling was carried out at a cold reduction of 50%. Each of the cold-rolled steel sheets prepared as described above was subjected to annealing at 800 ° C and cold-rolled under the conditions shown in Table 2 below.

Then, the mechanical properties of each specimen were measured, and the results are shown in Table 2 below. In addition, when the inventive steel was used, the fraction of the deformed crystal grains after the cold rolling was measured and shown in Table 2 below.

The tensile test specimens were subjected to a tensile test using a universal tensile tester.

Steel grade Component composition (% by weight) C Si Mn P S Al N V Comparative River 1 0.6 0.04 25 0.01 0.01 0.05 0.006 0 Comparative River 2 0.35 0.01 12 0.01 0.01 1.5 0.006 0 Comparative Steel 3 0.6 0.05 17 0.01 0.01 1.2 0.005 0 Comparative Steel 4 0.7 1.0 20 0.01 0.01 1.5 0.002 0 Invention river 2 0.8 0.5 20 0.01 0.01 0.02 0.003 0.5 Invention steel 3 0.85 0.5 16 0.01 0.01 0.02 0.002 0.5

Steel grade Reduction rate (%) relation
Equation 1 Mechanical properties transform
Grain (%) division YS (MPa) TS (MPa) TE (%) YR Comparative steel
One 0 0.30 353 772 45 0.46 - Comparative Example 1 22 0.68 652 1112 27 0.59 - Comparative Example 2 33 0.86 800 1280 18 0.63 - Comparative Example 3 40 0.98 954 1455 7 0.66 - Comparative Example 4 Comparative steel
2 0 0.32 353 737 58 0.48 - Comparative Example 5 21 0.68 655 1080 40 0.61 - Comparative Example 6 31 0.85 802 1250 31 0.64 - Comparative Example 7 Comparative steel
3 0 0.42 471 940 60 0.50 - Comparative Example 8 10.2 0.59 751 1047 45 0.72 Comparative Example 9 20.5 0.77 931 1210 22 0.77 - Comparative Example 10 30.1 0.93 1088 1371 13 0.79 - Comparative Example 11 Comparative steel
4 0 0.55 567 833 54 0.68 1.2 Comparative Example 12 11 0.74 743 1013 44 0.73 24.5 Comparative Example 13 20 0.89 873 1146 37 0.76 62.3 Comparative Example 14 Invention river
2 0 0.95 854 1332 33 0.64 0 Comparative Example 15 3.1 1.00 990 1444 33 0.69 3.7 Comparative Example 16 12 1.15 1220 1526 18 0.80 23.3 Inventory 3 17 1.24 1266 1628 17 0.78 51.5 Honorable 4 Invention river
3 0 1.05 866 1375 38 0.63 2.4 Comparative Example 17 5 1.10 1067 1476 31 0.72 11.3 Inventory 5 11 1.20 1210 1581 20 0.77 49.5 Inventory 6 13 1.23 1368 1682 19 0.81 54.7 Honorable 7

(YS: yield strength, TS: tensile strength, TE: total elongation, YR: yield ratio (YS / TS) in Table 2)

As shown in Tables 1 and 2, Inventive Examples 3 to 7, which satisfy both the steel component composition and the manufacturing conditions controlled by the present invention and the relational expression 1, are secured at 10% or more in the fraction of the strain grains after cold rolling , A yield strength of not less than 1000 MPa, a tensile strength of not less than 1400 MPa, a total elongation of not less than 15%, and a yield ratio of 0.7 or more.

Therefore, it can be confirmed that the ultra-high strength steel sheet according to the present invention is very suitable as a steel sheet for cold press forming which can replace the existing steel sheet for hot press forming.

On the other hand, in Comparative Examples 1, 5 and 8 using comparative steels 2 and 3 in which the content of Mn was too high and the content of Comparative Steels 1 and C was inadequate, it was difficult to obtain ultra- The yield ratio was less than 0.7. In the case of Comparative Examples 2-3, 6-7, and 9-10, the value of the relational expression 1 did not satisfy the present invention even though cold rolling was performed, and thus it was impossible to secure a yield strength of 1000 MPa or more. In Comparative Example 4, it was possible to secure a tensile strength of 1400 MPa or more. In Comparative Example 11, it was possible to secure a yield strength of 1000 MPa or more. However, in all of them, the total elongation was less than 15%.

In the case of Comparative Examples 12, 15 and 17 in which re-rolling was not performed and in Comparative Example 16 in which the re-rolling rate did not satisfy the present invention, the area fraction of the strain grains was less than 10% And thus it was difficult to secure a yield strength of 1000 MPa or more.

In the case of Comparative Example 13-14 which does not satisfy the relational expression 1, the strength improving effect can not be obtained and it is difficult to secure a yield strength of 1000 MPa or more.

FIG. 1 shows the result of observation of Grain Orientation Spread according to the cold rolling reduction ratio of Inventive Steel 2 using EBSD (Electron Back Xcatter Diffraction) GOS map analysis. In FIG. 1, the strain grains having an average crystal orientation difference of 3 or more in the crystal grains are shown in orange and red.

As shown in FIG. 1, it can be seen that the higher the cold reduction ratio, the higher the area fraction of the deformed grains having a difference of the average crystal orientation in the crystal grains of 3 degrees or more, which contributes to the increase in strength of the steel.

Claims (9)

(C): not less than 0.7% to not more than 0.9%, silicon (Si): not more than 2.0% (excluding 0%), manganese (Mn): 12 to 20%, phosphorus (P) (Except 0%), vanadium (V): not more than 0.5% (excluding 0%), the balance Fe and And further contains an unavoidable impurity and contains austenite having an area fraction of 95% or more and a residual carbide as a microstructure, and having an excellent elongation satisfying the following relational expression (1).

[Relation 1]
0.284 + 0.693C + 0.116Si - 0.016Mn - 0.011Al + 0.745V + 0.017X > 1.05
(C, Si, Mn, Al and V in the above relational expression 1 means the content by weight of each element, and X means the reduction ratio (%) during cold rolling).
The method according to claim 1,
Wherein the steel sheet further comprises at least one of 0.005 to 0.3% of titanium (Ti), 0.005 to 0.3% of niobium (Nb), and 0.05 to 0.3% of molybdenum (Mo).
The method according to claim 1,
Wherein the steel sheet contains deformation grains after cold rolling at an area fraction of 10% or more.
The method according to claim 1,
Wherein the steel sheet is one of a cold rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet.
(C): not less than 0.7% to not more than 0.9%, silicon (Si): not more than 2.0% (excluding 0%), manganese (Mn): 12 to 20%, phosphorus (P) (Except 0%), vanadium (V): not more than 0.5% (excluding 0%), the balance Fe and Preparing a steel slab containing other unavoidable impurities;
Reheating the steel slab in a temperature range of 1000 to 1250 占 폚;
Subjecting the reheated steel slab to finish hot rolling at a temperature of 500 to 950 캜 to produce a hot-rolled steel sheet;
Winding the hot-rolled steel sheet at a temperature of 750 ° C or lower;
Picking up the hot rolled steel sheet and cold rolling the hot rolled steel sheet at a cold reduction rate of 30 to 80% to produce a cold rolled steel sheet;
Annealing the cold-rolled steel sheet in a temperature range of 750 to 950 ° C; And
Cold rolling the annealed heat-treated cold rolled steel sheet at a reduction ratio of 5 to 50%
, And satisfies the following relational expression (1): " (1) "

[Relation 1]
0.284 + 0.693C + 0.116Si - 0.016Mn - 0.011Al + 0.745V + 0.017X > 1.05
(C, Si, Mn, Al and V in the above relational expression 1 means the content by weight of each element, and X means the reduction ratio (%) during cold rolling).
6. The method of claim 5,
Wherein the steel slab comprises an ultra-high strength steel sheet having excellent elongation, further comprising at least one of 0.005 to 0.3% of titanium (Ti), 0.005 to 0.3% of niobium (Nb), and 0.05 to 0.3% of molybdenum Gt;
6. The method of claim 5,
Characterized in that after the cold rolling, the deformed grains have an area fraction of 10% or more.
6. The method of claim 5,
Further comprising a step of immersing the cold-rolled steel sheet in the zinc plating bath before the cold rolling to produce a hot-dip galvanized steel sheet.
9. The method of claim 8,
Further comprising the step of subjecting the hot-dip galvanized steel sheet to an alloying heat treatment to produce a galvannealed hot-dip galvanized steel sheet.

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