KR20190035142A - Cold rolled steel sheet having excellent yield strength and drawing-workability and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a cold rolled steel sheet having excellent yield strength and drawing formability and a manufacturing method therefor. An embodiment of the present invention provides: a cold rolled steel sheet having excellent yield strength and drawing formability, the cold rolled steel sheet containing, in wt%, 0.3-0.8% of C, 13-25% of Mn, 0.2-0.8% of V, 0.03% or less of V, 0.03% or less of S, 0.04% or less of N, and the balance Fe and inevitable impurities, the cold rolled steel sheet comprising 30 area% or more of recrystallized microstructures having a size of 3 μm or smaller and 30-70 area% of non-recrystallized microstructures; and a manufacturing method therefor.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolled steel sheet excellent in yield strength and drawability and a method of manufacturing the cold rolled steel sheet.

The present invention relates to a cold-rolled steel sheet excellent in yield strength and draw-formability and a method for producing the same.

In recent years, due to the regulation of carbon dioxide to reduce global warming, there has been a strong demand for lightening of automobiles. At the same time, the strength of automotive steel sheets has been continuously increased to improve the crash stability of automobiles.

Structural members such as side impact beams, B-pillar, and door impact beams of automobile parts should have excellent yield strength in order to ensure passenger safety in case of vehicle collision or rollover. On the other hand, since the automobile parts are manufactured by cold press forming, drawing moldability should be excellent.

Therefore, a steel sheet used as an automotive structural member should have a high yield strength to protect passengers, and is preferably excellent in drawability for smooth manufacture.

Since drawing formability is often not directly proportional to the uniform elongation obtained in the unilateral tensile test, it is evaluated through cup molding. Generally, after preparing steel sheet blanks, a punch having a diameter of 50 mm is used to judge whether the cup is completely formed. The forming ratio refers to the diameter of the blank divided by the diameter of the punch, and the limiting forming ratio refers to the highest forming ratio at which the cup can be fully molded. When the limit forming ratio is 1.8 or more, the moldability capable of press molding almost all automotive structural members is secured.

Generally, low-temperature transformed structures are used for producing steel sheets for automobile parts. However, it is difficult to obtain a critical forming ratio of 1.8 or higher at a yield strength of 800 MPa or more in the case of using a low-temperature transformed structure to secure a high strength. Therefore, it is difficult to apply to cold- There was a problem that the free part design was difficult.

In order to solve the above problem, as in Patent Document 1, a large amount of austenite stabilizing elements such as carbon (C) and manganese (Mn) are added to maintain the steel structure as austenite single phase, And moldability at the same time.

However, the conventional high manganese steel only considered the tensile strength and the elongation, but did not mention the improvement of the yield strength to ensure the safety of the automobile at the time of collision.

Therefore, there is a need to develop a steel sheet for automobiles that is excellent in strength and drawing moldability and can secure a high yield strength.

Korean Patent Publication No. 2007-0023831

An aspect of the present invention is to provide a cold-rolled steel sheet excellent in yield strength and draw-forming property and a method of manufacturing the same.

An embodiment of the present invention relates to a steel sheet comprising, by weight%, 0.3 to 0.8% of C, 13 to 25% of Mn, 0.2 to 0.8% of V, 0.03% or less of P, 0.03% or less of S, A cold-rolled steel sheet containing 30% by area or more of recrystallized microstructure having a size of 3 탆 or less and containing 30 to 70% by area of unrecrystallized microstructure and having excellent drawability and drawing formability, including remaining Fe and unavoidable impurities, to provide.

Another embodiment of the present invention is a steel sheet comprising, by weight%, 0.3 to 0.8% of C, 13 to 25% of Mn, 0.2 to 0.8% of V, 0.03% or less of P, 0.03% or less of S, Heating the steel slab containing the remaining Fe and unavoidable impurities to 1050 to 1250 占 폚; Subjecting the heated slab to a finish hot rolling at 750 to 1050 占 폚 to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet at 50 to 700 ° C; Hot rolling the hot rolled steel sheet at a cold reduction rate of 30 to 85% after pickling to obtain a cold rolled steel sheet; And a step of annealing the cold-rolled steel sheet at 550 to 900 ° C, wherein the cold-reduction rate and the annealing temperature satisfy the following relational expression (1), and provide a method of manufacturing a cold-rolled steel sheet excellent in yield strength and drawability.

[Relation 1] 1140-10.5 × cold rolling reduction (%) ≤ annealing temperature (캜) ≤ 750 + 1.6 × cold rolling reduction (%)

According to one aspect of the present invention, there is provided an effect of providing a cold-rolled steel sheet having a high yield strength and excellent draw-formability, and a method of manufacturing the same.

1 is a graph showing yield strengths according to cold reduction rates of Comparative Examples 3 and 4 and Inventive Examples 3 to 7 according to an embodiment of the present invention.
FIG. 2 is a graph showing the critical forming ratio of non-recrystallized microstructure fractions of Comparative Examples 2 to 4 and Inventive Examples 1 to 7 according to an embodiment of the present invention.
3 (a) is a scanning electron microscope (SEM) image of Comparative Example 1 according to an embodiment of the present invention, and FIG. 3 (b) is a scanning electron microscope image of Inventive Example 6 according to an embodiment of the present invention.
4 is a photograph of a cup molding specimen evaluated in Comparative Example 3, Comparative Example 4, and Inventive Example 1 according to an embodiment of the present invention at a molding ratio of 1.8.

Hereinafter, the present invention will be described in detail. First, the alloy composition of the present invention will be described. The content of the alloy composition of the present invention described below means weight%.

C: 0.3 to 0.8%

Carbon is an element contributing to the stabilization of the austenite phase, and as the content thereof increases, there is an advantage in securing the austenite phase. Carbon also increases the energy of lamination defects in the steel, thereby increasing both tensile strength and elongation. If the content of carbon is less than 0.3%, there is a problem that the α '(alpha re-) -martensite phase is formed on the surface layer due to decarburization at the time of high-temperature processing of the steel sheet, and the delayed fracture and fatigue performance become weak. There is a problem that is difficult to secure. On the other hand, if the content exceeds 0.8%, the electrical resistivity increases and the weldability may decrease. Therefore, in the present invention, the content of C is preferably limited to 0.3 to 0.8%.

Mn: 13-25%

Manganese is an element which stabilizes the austenite phase together with carbon. When the content is less than 13%, it is difficult to secure a stable austenite phase due to the formation of α '(alpha re-) martensite phase during deformation, There is a problem that the further improvement with respect to the increase of the strength, which is a concern of the present invention, does not occur substantially and the manufacturing cost rises. Therefore, the content of Mn in the present invention is preferably limited to 13 to 25%.

V: 0.2 to 0.8%

Vanadium is usually added to form a precipitate to increase the yield strength of the steel by precipitation, but is added in the present invention to control the recrystallization behavior. Vanadium is segregated in the grain boundaries during recrystallization and slows the grain boundary moving speed, thereby increasing the recrystallization temperature of the steel. When the content of V is less than 0.2%, the effect of segregation at the grain boundaries is small, and the recrystallization temperature is not largely changed. On the other hand, when the content of V is more than 0.8%, coarse precipitates are formed at the time of solidification, and even if a reheating process is performed, the V remains in the steel sheet and may cause cracking during rolling. Therefore, the content of vanadium in the present invention is preferably 0.2 to 0.8%.

P: not more than 0.03%

The phosphorus is an impurity which is inevitably contained and is an element that causes a deterioration in the workability of the steel due to segregation. Therefore, it is preferable to control the content as low as possible. Theoretically, it is preferable to limit the phosphorus content to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is controlled to 0.03% by weight.

S: not more than 0.03%

Sulfur is inevitably contained as impurities, which forms a coarse manganese sulfide (MnS) to generate defects such as flange cracks and greatly reduces the hole expandability of the steel sheet. Therefore, it is desirable to control the content as low as possible. Theoretically, it is advantageous to limit the content of sulfur to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is controlled to 0.03% by weight.

N: 0.04% or less

It reacts with Al during the solidification process in the nitrogen (N) austenite crystal grains to precipitate fine nitrides to promote the generation of twin, thereby improving the strength and ductility of the steel sheet during molding. However, when the content exceeds 0.04%, excessive nitrides are precipitated and the hot workability and elongation can be lowered. Therefore, in the present invention, the nitrogen content is preferably limited to 0.04% or less.

The cold-rolled steel sheet of the present invention may contain, in addition to the above alloy composition, 0.03 to 2.0% of Si, 0.3 to 2.5% of Al, 0.01 to 0.5% of Ti, 0.05 to 0.5% of Nb, 0.05 to 1.0% And 0.0005 to 0.005%.

Si: 0.03 to 2.0%

Silicon is a component that can be added to improve the yield strength and tensile strength of steel by solid solution strengthening. Since silicon is used as a deoxidizer, it can be contained in 0.03% or more of the steel in general. When the content of silicon exceeds 2.0%, a large amount of silicon oxide is formed on the surface during hot rolling to decrease acidity and increase electrical resistivity There is a problem that the weldability is lowered. Therefore, the content of silicon is preferably limited to 0.03 to 2.0%.

Al: 0.3 to 2.5%

Aluminum is usually added for deoxidation of steel, but the present invention enhances the ductility and delayed fracture characteristics of steel by suppressing the formation of ε (entrance run) -martensite by increasing the stacking fault energy. When the aluminum content is less than 0.3%, there is a problem that the ductility of the steel is lowered due to the rapid work hardening phenomenon and the delayed fracture resistance is inferior. On the other hand, when the aluminum content exceeds 2.5% There is a problem that the surface quality is deteriorated due to deepening of oxidation of the steel surface during hot rolling. Therefore, in the present invention, the aluminum content is preferably limited to 0.3 to 2.5%.

Ti: 0.01 to 0.5%

Titanium reacts with nitrogen in the steel to precipitate nitrides, which improves the formability of hot rolling. In addition, the titanium reacts with carbon in some steel to form precipitation phases, thereby increasing the strength. For this purpose, titanium is preferably contained in an amount of 0.01% or more, but if it exceeds 0.5%, precipitates are formed excessively to deteriorate the fatigue characteristics of the parts. Accordingly, the titanium content is preferably 0.01 to 0.5%.

Nb: 0.05 to 0.5%

Niobium is an element that reacts with carbon or nitrogen to form a carbonitride. It is a component that can be added to increase the yield strength by micronization of crystal grains and precipitation strengthening. In order to obtain such an effect, the content of niobium is preferably 0.05% or more. On the other hand, when the content of niobium exceeds 0.5%, coarse carbonitrides are formed at a high temperature, thereby deteriorating hot workability. Therefore, the content of Nb in the present invention is preferably limited to 0.05 to 0.5%.

Mo: 0.05 to 1.0%

Molybdenum is an element having a high solubility in carbonitride, and slows the growth rate of carbonitride, thereby making the size of the carbonitride small, thereby enhancing the strengthening effect by the precipitate. However, if the content of molybdenum is less than 0.05%, the above effect is not sufficiently exhibited. If the content of molybdenum exceeds 1.0%, further improvement in performance can not be expected and the cost is increased. Therefore, the content of molybdenum is preferably 0.05 to 1.0%.

B: 0.0005 to 0.005%

When boron is added in a small amount, the grain boundary of the cast steel is strengthened to improve the hot rolling property. However, if the content of boron is less than 0.0005%, the above effect is not sufficiently exhibited, and if the content of boron exceeds 0.005%, further performance improvement can not be expected and the cost is increased. Therefore, the content of boron is preferably 0.0005 to 0.005%.

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 cold-rolled steel sheet of the present invention preferably contains 30% by area or more of recrystallized microstructure having a size of 3 mu m or less and 30 to 70% by area of unrecrystallized microstructure. Potential in the microstructure acts as an obstacle to the movement of other dislocations, thereby increasing the strength of the steel. A large amount of dislocations can be introduced into the steel sheet by the cold rolling, and thereafter, the recovery step of decreasing the density of the dislocations as the temperature of the steel sheet rises in the annealing step, and the density of the dislocations is theoretically 0 Crystal grains near the crystal grains are generated. On the other hand, since the accumulation of the electric potential in the steel sheet by cold rolling can be different depending on the crystal orientation of the texture in the hot rolled state, the temperature at which the recrystallization starts in the tissue due to the difference in the dislocation density in the initial crystal orientation differs . Therefore, the initial dislocation density is high at the temperature zone where recrystallization progresses, so that recrystallized microstructure of recrystallized microstructure can be produced in which recrystallization proceeds preferentially and microcrystalline microstructure showing only recovery phenomenon due to low initial dislocation density. This binary structure is characterized by high yield strength by non - recrystallized structure and excellent moldability by recrystallized structure.

On the other hand, in the case of heat treatment at the same annealing temperature, the fraction of the non-recrystallized structure in the structure may be different due to the difference in the cold reduction ratio. In general, when the cold reduction rate is high, since the initial dislocation density accumulated in the cold rolled steel sheet is high, it is a favorable condition to start recrystallization. Therefore, in the case of annealing at the same temperature, a high fraction of the non-recrystallized region can be included in the steel sheet when the cold rolling reduction rate is low, and thus a high yield strength can be realized.

On the other hand, when the non-recrystallized microstructure exceeds 70% by area, the formability of the steel is lowered and the critical forming ratio of 1.8 or more can not be secured. On the other hand, when the area is less than 30% by area, the yield strength may be lowered. Therefore, the fraction of the non-recrystallized microstructure preferably ranges from 30 to 70% by area.

If the grain size is finely controlled, the yield strength can be ensured without significantly deteriorating the formability of the steel. At the same annealing temperature, the higher the cold reduction ratio, the finer the grain size can be controlled. As the cold reduction ratio increases, the yield strength of the steel sheet increases, and the dislocation density distributed in the steel sheet during cold deformation at a high reduction rate can be maintained at a high level. This is because the recrystallization progresses simultaneously in several places and the growth of the crystal grains is mutually restricted. The fine grain can increase the yield strength of the steel without significantly deteriorating the formability. However, in order to maintain a fine grain size with a uniform particle size distribution and to secure a high fraction, the cold rolling reduction ratio must be high, so that a load may be applied to the rolling mill to shorten the lifetime. In the present invention, To achieve the intended yield strength without deterioration. In the present invention, when a non-recrystallized structure is present, a yield strength of 800 MPa or more can be ensured by including 30% by area or more of a recrystallized structure having a size of 3 μm or less. The higher the area fraction of the recrystallized structure having a size of 3 탆 or less, the more advantageous it is to increase the yield strength. Therefore, in the present invention, the upper limit of the recrystallized structure fraction is not particularly limited. However, in order to obtain such an excessive amount, a cold rolling reduction rate of a high level is required to cause a rolling load. Therefore, in the present invention, the fraction of the non-recrystallized structure is appropriately adjusted so that drawability is not impaired, So that the moldability can be easily ensured.

On the other hand, it is preferable that the cold-rolled steel sheet of the present invention contains austenite having a microstructure of 95% by area or more. This is to secure the tensile strength and elongation at the same time. The microstructure of the cold-rolled steel sheet of the present invention is more preferably an austenite single phase. The austenite single phase means that all the microstructures except carbide are made of austenite, and some unavoidable impurity structure may be included.

The cold-rolled steel sheet of the present invention, which is provided as described above, can secure both a yield strength of 800 MPa or higher and a draw forming capability of 1.8 or more of a critical forming ratio, and can be preferably applied to structural members of automobiles.

Hereinafter, a method of manufacturing the hot-rolled steel sheet of the present invention will be described.

First, a steel slab having the above-described alloy composition is prepared, and then the steel slab is heated to 1050 to 1250 캜. When the slab heating temperature is lower than 1050 DEG C, it is difficult to ensure the finish rolling temperature during hot rolling, and the rolling load due to the temperature decrease increases, so that it is difficult to sufficiently roll to a predetermined thickness. On the other hand, when the slab heating temperature exceeds 1250 deg. C, crystal grain size increases and surface oxidation tends to occur to decrease the strength or surface disadvantage. In addition, since the liquid phase film is formed on the columnar phase boundary of the performance slab, there is a fear that cracks may occur during the subsequent hot rolling.

The heated slab is hot rolled at a temperature of 750 to 1050 占 폚 to obtain a hot-rolled steel sheet. Hot rolling may be performed by multi-stage rolling. When the finishing rolling temperature exceeds 1050 占 폚, a thick secondary scale is generated on the surface of the steel sheet in the cooling step up to winding, and the acidity may be lowered. On the other hand, when the finishing rolling temperature is less than 750 캜, the deformation resistance of the steel sheet becomes high, so that the rolling mill can be overloaded.

The hot-rolled steel sheet is wound at 50 to 700 ° C. When the coiling temperature exceeds 700 캜, vanadium carbonitrides can be generated in the wound steel sheet, and in this case, the content of vanadium employed for delaying the recrystallization behavior in the annealing step is reduced. On the other hand, the lower limit of the coiling temperature need not be specified, but if the coiling temperature is less than 50 캜, the retained water may remain between the steel sheets, resulting in surface defects due to scale.

The picked hot-rolled steel sheet is cold-rolled at a cold rolling reduction of 30 to 85% after pickling to obtain a cold-rolled steel sheet. When the cold rolling reduction is less than 30%, the fraction of the non-recrystallized structure is high and the formability is poor. On the other hand, if it exceeds 85, the lifetime of the rolling mill may be shortened by increasing the rolling load.

The cold-rolled steel sheet is annealed at 550 to 900 ° C. When the annealing temperature is less than 550 ° C., the non-recrystallized structure can not be maintained at 70% or less. When the annealing temperature is higher than 900 ° C., the fraction of the non-recrystallized structure is decreased to improve the formability, , It is impossible to contain at least 30% by area of the recrystallized microstructure having a size of 3 탆 or less by crystal grain coarsening, so that the yield strength of 800 MPa or more can not be secured.

In the present invention, it is preferable that the cold rolling reduction and the annealing temperature satisfy the following relational expression (1). When the annealing temperature is lower than 1140-10.5 x cold rolling reduction rate, the non-recrystallized microstructure exceeds 70% and the drawing processability can not be ensured. When the annealing temperature exceeds 750 + 1.6 x cold rolling reduction rate, And can not contain more than 30% by area of the recrystallized microstructure having a size of 3 mu m or less.

[Relation 1] 1140-10.5 × cold rolling reduction (%) ≤ annealing temperature (캜) ≤ 750 + 1.6 × cold rolling reduction (%)

Hereinafter, the present invention will be described in more detail with reference to 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)

A steel slab having the alloy composition shown in the following Table 1 was heated to 1200 DEG C and then rolled at a finish rolling temperature of 900 DEG C to produce a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was rolled at a coiling temperature of 400 캜, pickled, and cold-rolled at the cold rolling reduction described in the following Table 2 to produce a cold-rolled steel sheet. Then, after annealing at the annealing temperature described in Table 2, continuous annealing heat treatment was performed by setting the over-heat effect temperature at 400 占 폚. The microstructure was observed on the steel sheet thus produced, and the area fraction and grain size of the non-recrystallized steel sheet were measured. The results are shown in Table 2. The tensile test was carried out using a universal tensile tester, and the results are shown in Table 2. The cup forming test was conducted under the conditions of a molding ratio of 1.5 to 2.2, and the critical forming ratio is shown in Table 2 below.

division Alloy composition (% by weight) C Mn V P S N Si Al Ti Nb Mo B Comparative steel 0.65 17.5 - 0.012 0.002 0.0004 - 1.6 0.06 - - 0.0025 Invention river 0.7 16.5 0.28 0.008 0.002 0.004 - 1.3 0.06 - 0.3 0.002

division Steel grade
No. Cold
Reduction rate
(%) Annealing
Temperature
(° C) surrender
burglar
(MPa) Seal
burglar
(MPa) Elongation
(%) Limit
Molding ratio Unrecognition
Tissue fraction
(area%) Recrystallized structure fraction (area%) of 3 탆 or less Relationship 1
Satisfaction Comparative Example 1 Comparative steel 45 780 520 980 62 2.1 5 17 ○ Comparative Example 2 Invention river 40 820 777 1191 43 1.9 23 42 × Inventory 1 Invention river 40 791 872 1245 37 1.8 55 32 ○ Inventory 2 Invention river 40 778 832 1220 40 1.8 48 43 ○ Comparative Example 3 Invention river 33 770 914 1276 34 1.6 82 4 × Comparative Example 4 Invention river 35 770 889 1278 37 1.7 72 13 × Inventory 3 Invention river 38 770 836 1212 39 1.8 51 30 ○ Honorable 4 Invention river 43 770 839 1217 41 1.8 55 32 ○ Inventory 5 Invention river 49 770 856 1228 42 1.8 48 37 ○ Inventory 6 Invention river 53 770 874 1223 39 1.8 45 54 ○ Honorable 7 Invention river 58 770 890 1235 52 1.8 42 53 ○ [Relation 1] 1140-10.5 × cold rolling reduction (%) ≤ annealing temperature (캜) ≤ 750 + 1.6 × cold rolling reduction (%)

As can be seen from Tables 1 to 3, in the case of Inventive Examples 1 to 7, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, the microcrystalline microcrystalline structure was obtained in an amount of 30 to 70% Of the recrystallized microstructure having a yield strength of 800 MPa or more and a critical forming ratio of 1.8 or more.

However, in the case of Comparative Example 1, since the alloy composition proposed by the present invention was not satisfied, the recrystallization rapidly occurred and the crystal grains were coarsened even when the production conditions of the present invention were satisfied. As a result, The strength is low.

In Comparative Example 2, the alloy composition as shown in the present invention was satisfied. However, since the annealing temperature exceeded 750 + 1.6 x cold rolling reduction (%) of Relative Formula 1 and the recrystallized grain structure of 3 탆 or less could not be secured sufficiently, It can be understood that the strength is not secured.

In Comparative Examples 3 to 4, the alloy composition was satisfied, but the annealing temperature was lower than 750 + 1.6 x cold rolling reduction (%) of the relational expression 1, and the fraction of the non-recrystallized structure became excessively high. Could not be secured.

FIG. 1 is a graph showing the yield strength according to the cold rolling reduction of inventive steels (Comparative Examples 3 and 4 and Inventive Examples 3 to 7) annealed at 770 ° C. As can be seen from FIG. 1, the yield strength decreases with an increase in the cold reduction ratio, and the yield strength increases with an increase in the cold reduction ratio from the vicinity of the cold reduction ratio of about 40%. This is because the area fraction of the non-recrystallized region decreases as the cold rolling reduction increases as indicated by the black dashed line in the region where the cold rolling reduction is low. In the region where the cold rolling reduction is high, The yield strength is increased.

2 is a graph showing the critical forming ratio of the inventive steel (Comparative Examples 2 to 4 and Inventive Examples 1 to 7) according to the microcrystalline fraction of microcrystalline structure. As can be seen from FIG. 2, it can be seen that when the non-recrystallized fraction exceeds 70%, the critical forming ratio of 1.8 or more can not be ensured.

FIG. 3 (a) is a scanning electron microscope photograph of Comparative Example 1, and FIG. 3 (b) is a scanning electron microscopic photograph of Inventive Example 6. It is indicated by blue, green, yellow, orange and red depending on the orientation distribution in the crystal grain. The higher the numerical value of the orientation distribution, the higher the dislocation density in the structure. In the present invention, when the numerical value of the azimuth distribution has a color other than blue, it is classified as a non-recrystallized structure. In Comparative Example 1, the fraction of the non-recrystallized structure was low and the crystal grains were coarse, while in Inventive Example 6, the non-recrystallized structure was high and the grain size was very fine, and the yield strength could be secured.

4 is a photograph of a cup molding specimen evaluated in Comparative Example 3, Comparative Example 4, and Inventive Example 1 under the condition that the molding ratio is 1.8. In Comparative Example 3 and Comparative Example 4, the non-recrystallization fraction in the structure was high, so that the drawability was degraded due to the fracture in the cup forming. In Inventive Example 1 which satisfied the manufacturing conditions of the present invention, No breakage occurred.

Claims (6)

0.3 to 0.8% of C, 13 to 25% of Mn, 0.2 to 0.8% of V, 0.03% or less of P, 0.03% or less of S and 0.04% or less of N and the balance of Fe and unavoidable impurities In addition,
A cold rolled steel sheet comprising 30% by area or more of recrystallized microstructure having a size of 3 μm or less and excellent in yield strength and drawing moldability including 30 to 70% by area of uncrystallized microstructure.
The method according to claim 1,
Wherein the cold-rolled steel sheet comprises at least one selected from the group consisting of 0.03 to 2.0% of Si, 0.3 to 2.5% of Al, 0.01 to 0.5% of Ti, 0.05 to 0.5% of Nb, 0.05 to 1.0% of Mo and 0.0005 to 0.005% of B And further has excellent yield strength and drawability.
The method according to claim 1,
Wherein the cold-rolled steel sheet contains austenite of 95% by area or more and has excellent yield strength and drawability.
The method according to claim 1,
Wherein the cold-rolled steel sheet has a yield strength of 800 MPa or more, and a yield strength and drawing moldability excellent in a critical forming ratio of 1.8 or more.
0.3 to 0.8% of C, 13 to 25% of Mn, 0.2 to 0.8% of V, 0.03% or less of P, 0.03% or less of S and 0.04% or less of N and the balance of Fe and unavoidable impurities Heating the steel slab to 1050 to 1250 캜;
Subjecting the heated slab to a finish hot rolling at 750 to 1050 占 폚 to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet at 50 to 700 ° C;
Hot rolling the hot rolled steel sheet at a cold reduction rate of 30 to 85% after pickling to obtain a cold rolled steel sheet; And
And annealing the cold-rolled steel sheet at 550 to 900 占 폚,
Wherein the cold rolling reduction ratio and the annealing temperature satisfy the following relational expression (1): " (1) "
[Relation 1] 1140-10.5 × cold rolling reduction (%) ≤ annealing temperature (캜) ≤ 750 + 1.6 × cold rolling reduction (%)
The method of claim 5,
Wherein the steel slab comprises at least one member selected from the group consisting of 0.03 to 2.0% of Si, 0.3 to 2.5% of Al, 0.01 to 0.5% of Ti, 0.05 to 0.5% of Nb, 0.05 to 1.0% of Mo and 0.0005 to 0.005% of B Wherein the rolled steel sheet further has a yield strength and a drawing moldability.

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