KR101412326B1 - High strength steel sheet and method for manufacturing the same - Google Patents

Abstract

A high strength cold rolled steel sheet production method which is excellent in moldability and bending workability while exhibiting an ultra high strength of tensile strength of 1180 MPa or more.
The method for producing a high strength cold rolled steel sheet according to the present invention comprises 0.1 to 0.2% of carbon (C), 1.0 to 2.0% of silicon (Si), 1.8 to 2.2% of manganese (Mn) (Fe) and unavoidable impurities is contained in an amount of at least one kind selected from the group consisting of 0.02 to 0.05% of niobium and 0.06 to 0.1% of vanadium, Reheating at 1300 ° C for 3 to 5 hours; Hot rolling the reheated plate at a finish rolling temperature of 840 to 880 ° C; Subjecting the hot-rolled plate to primary cooling to 580 to 620 占 폚 at an average cooling rate of 30 to 100 占 폚 / sec; Second cooling the primary cooled plate at an average cooling rate of 2 to 5 占 폚 / sec; Pickling and cold rolling the secondary cooled plate; Annealing the cold-rolled plate at an Al transformation point or higher; Thirdly cooling the annealed sheet material to Ms ~ Ms + 50 占 폚; Maintaining the third cooled plate at a constant temperature of Ms to Ms + 50 ° C; And cooling the quenched plate to the martensite temperature region.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength cold rolled steel sheet,

The present invention relates to a method for manufacturing a high strength cold rolled steel sheet, and more particularly to a method for manufacturing a high strength cold rolled steel sheet having a high strength of 1180 MPa or more and a tensile strength of 1180 MPa or more and excellent in formability and bending workability through controlling process conditions such as an alloy component and a slab reheating will be.

Recently, with the development of steel sheet manufacturing technology, ultra-high strength cold rolled steel sheets having a tensile strength of 1180 MPa or more have been produced.

Among the ultra-high strength cold-rolled steel sheets, in the case of a steel sheet excellent in bending property, it is usually prepared by performing a single-phase reverse heat treatment at an austenite A3 transformation point or higher and then cooling to Ms or lower and tempering treatment. The material properties of such cold-rolled steel sheets are excellent in bending properties due to the formation of high porosity, but there is a problem that they are blown in the molding process in the manufacture of parts requiring drawability.

Among the ultra-high strength cold-rolled steel sheets, the steel sheet which is required to have drawability is tempered after being subjected to an anomalous reverse heat treatment at the A1 transformation point to the A3 transformation point and tempered at a temperature below the Ms point. However, since the bending property is poor, there is a problem that an edge crack or a penetration crack occurs at the bending portion in the 90 ㅀ roll bending.

Therefore, there is a need for an ultra-high strength cold rolled steel sheet excellent in bending properties and moldability.

As a prior art related to the present invention, there is a method of manufacturing an ultra-high strength cold rolled steel sheet disclosed in Korean Patent Laid-Open Publication No. 10-2003-0055530 (published on Apr. 4, 2003).

An object of the present invention is to provide a high strength cold rolled steel sheet having a high strength of 980 MPa or higher in tensile strength and excellent in bending workability and a method for producing the same.

In order to achieve the above object, a method of manufacturing a high strength cold rolled steel sheet according to an embodiment of the present invention is characterized by comprising 0.1 to 0.2% of carbon (C), 1.0 to 2.0% of silicon (Si) (P): not less than 0% to not more than 0.05% and sulfur (S): not less than 0% to not more than 0.01%, further comprising at least one of 0.02 to 0.05% of niobium and 0.06 to 0.1% And reheating the slab plate made of the remaining iron (Fe) and unavoidable impurities at 1250 to 1300 캜 for 3 to 5 hours; Hot rolling the reheated plate at a finish rolling temperature of 840 to 880 ° C; Subjecting the hot-rolled plate to primary cooling to 580 to 620 占 폚 at an average cooling rate of 30 to 100 占 폚 / sec; Second cooling the primary cooled plate at an average cooling rate of 2 to 5 占 폚 / sec; Pickling and cold rolling the secondary cooled plate; Annealing the cold-rolled plate at an Al transformation point or higher; Thirdly cooling the annealed sheet material to Ms ~ Ms + 50 占 폚; Maintaining the third cooled plate at a constant temperature of Ms to Ms + 50 ° C; And cooling the quenched plate to the martensite temperature region.

At this time, the secondary cooling may be performed by a forced air cooling method using a cooling fan.

The annealing may be performed at 750 to 900 ° C for 60 to 150 seconds, the third cooling may be performed at an average cooling rate of 50 to 100 ° C / sec, and the constant temperature may be maintained for 200 to 400 seconds have.

In order to achieve the above object, a high strength cold rolled steel sheet according to an embodiment of the present invention comprises 0.1 to 0.2% of carbon (C), 1.0 to 2.0% of silicon (Si), 1.8 to 2.2% of manganese (Mn) (P): not less than 0% to not more than 0.05% and sulfur (S): not less than 0% to not more than 0.01%, further comprising at least one of 0.02 to 0.05% of niobium and 0.06 to 0.1% Wherein the microstructure consists of residual iron (Fe) and unavoidable impurities, the microstructure is composed of a composite structure of martensite as a main phase, ferrite as a second phase, and residual austenite as a third phase, Is contained at an area ratio of 10% or more.

At this time, the composite structure may contain 50 to 60% of the martensite, 30 to 40% of the ferrite, and 10 to 20% of the retained austenite in an area ratio.

In the composite structure, the average grain size of the martensite may be 1.0 to 3.0 탆, and the average grain size of the ferrite may be 1.0 to 4.0 탆.

In the composite structure, the average grain size of the martensite is 1.0 to 3.0 占 퐉, the average inter-phase distance of the martensite is 2.0 to 4.0 占 퐉, the average spheroidization ratio (length / width) 1.1.

The cold-rolled steel sheet may have a tensile strength of 1180 MPa or more and an elongation of 15% or more.

According to the method of manufacturing a high strength cold rolled steel sheet according to the present invention, it is possible to obtain a high strength cold rolled steel sheet having a tensile strength of 1180 MPa or more through control of alloy components such as silicon (Si), niobium (Nb), and vanadium (V), and slab reheating , An elongation of not less than 15%, and excellent moldability can be obtained.

Particularly, according to the manufacturing method according to the present invention, the steel sheet to be produced exhibits excellent strength in bending even though it exhibits ultra-high strength. This is because, in the manufacturing method according to the present invention, the grain size is minimized in the hot rolling step, a large amount of fine precipitates are formed after the primary cooling, and growth thereof is suppressed.

Through this, austenite nucleus generation is induced in the ferrite grain boundary micro-precipitates during the annealing treatment after the cold rolling, and the martensite dispersibility after the final cooling transformation can be improved by suppressing the ferrite and austenite growth.

1 is a flowchart schematically showing a hot rolling process in a method of manufacturing a high strength cold rolled steel sheet according to an embodiment of the present invention.
2 is a flowchart schematically illustrating a cold rolling process and a heat treatment process in a method of manufacturing a high strength cold rolled steel sheet according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method of manufacturing a high strength cold rolled steel sheet according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

The high strength cold rolled steel sheet according to the present invention comprises 0.1 to 0.2% of carbon (C), 1.0 to 2.0% of silicon (Si), 1.8 to 2.2% of manganese (Mn) To 0.05% and sulfur (S): 0% to 0.01% or less.

Further, the high-strength cold-rolled steel sheet according to the present invention further comprises at least one of 0.02 to 0.05% of niobium and 0.06 to 0.1% of vanadium.

The rest of the above components are composed of iron (Fe) and unavoidable impurities.

Hereinafter, the role and content of each component included in the high strength cold rolled steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) contributes to the improvement of martensite fraction and hardness in composite textured steel.

The carbon is preferably added in an amount of 0.1 to 0.2% by weight based on the total weight of the steel sheet. When the addition amount of carbon is less than 0.1% by weight, it is difficult to secure a tensile strength of 1180 MPa or more. On the other hand, when the carbon content exceeds 0.2% by weight, formation of carbide in the steel is promoted, and it is difficult to secure a desired elongation of 15% or more.

Silicon (Si)

Silicon (Si) serves as a deoxidizer, and in the present invention, it is added in an amount of 1.0 wt% or more to improve the elongation.

The silicon is preferably added in an amount of 1.0 to 2.0% by weight based on the total weight of the steel sheet. When the addition amount of silicon is less than 1.0% by weight, it is difficult to secure an elongation of 15% or more. On the other hand, when the addition amount of silicon exceeds 2.0 wt%, the performance is deteriorated and the plating property is deteriorated.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element that stabilizes austenite and lowers the annealing temperature, and martensite is easily produced even at a low cooling rate.

The manganese is preferably added in an amount of 1.8 to 2.2% by weight based on the total weight of the steel sheet. When the content of manganese is less than 1.8% by weight, the effect of the addition is insufficient. On the contrary, when the content of manganese exceeds 2.2% by weight, a manganese band develops in the center of the thickness direction of the material, and the elongation rate is lowered.

In (P)

Phosphorus (P) contributes to the improvement of the strength of the steel sheet by solid solution strengthening, and as an element effective in inhibiting the formation of carbide, it plays a role of preventing the elongation due to the formation of carbide in the constant temperature holding section after cooling. It is also effective for obtaining martensite by improving manganese equivalence. However, when phosphorus is added in excess, Fe3P stddite is formed, which causes hot brittleness.

In the present invention, the content of phosphorus is limited to not less than 0 wt% and not more than 0.05 wt% of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) inhibits toughness and weldability, and increases MnS nonmetallic inclusions, which hinders the effect of incombustibility of Mn and causes cracks. Also, if sulfur is contained excessively, the coarse inclusions are increased to deteriorate the fatigue characteristics.

Therefore, in the present invention, the content of sulfur is limited to more than 0% by weight to 0.01% by weight or less of the total weight of the steel sheet.

Niobium (Nb), vanadium (V)

Niobium (Nb) and vanadium (V) improve the strength of a steel sheet produced by forming a niobium precipitate in a steel or solid solution strengthening in Fe, and further improve grain bending and martensite dispersibility to improve bending workability .

In the case of niobium, 0.02 to 0.05% of the total weight of the steel sheet may be added, and in the case of vanadium, 0.06 to 0.1% of the total weight of the steel sheet may be added. When the addition amount of niobium is less than 0.02 wt% and the addition amount of vanadium is less than 0.06 wt%, it is difficult to secure the bending workability of the steel sheet. On the contrary, when the addition amount of niobium exceeds 0.02 wt% or the addition amount of vanadium exceeds 0.1 wt%, the formability is deteriorated.

In the case of these niobium and vanadium, only one kind may be added in the above-mentioned range, and both kinds may be added in the above-mentioned range. When any one of them is added in the above range, only one of them may be added in a small amount.

The high-strength cold-rolled steel sheet according to the present invention has a composite structure in which the final microstructure is composed of martensite as the main phase, ferrite as the second phase, and residual austenite as the third phase . At this time, the high-strength cold-rolled steel sheet according to the present invention contains the above-mentioned retained austenite at an area ratio of 10% or more, thereby greatly improving the elongation of the steel sheet. This is because the effect of delaying the diffusion of carbon is exhibited due to grain refinement due to grain refinement, the amount of carbon enrichment in austenite is increased, and the retained austenite can be retained in an area ratio of 10% or more even after the final cooling.

More specifically, the composite structure of the steel sheet according to the present invention may contain an area ratio of 50 to 60% of martensite, 30 to 40% of ferrite and 10 to 20% of retained austenite.

Further, in the composite structure of the steel sheet according to the present invention, the average grain size of the martensite is 1.0 to 3.0 탆, and the average grain size of the ferrite is 1.0 to 4.0 탆, which is very fine. These fine grains contribute to an increase in elongation.

In addition, in the composite structure of the steel sheet according to the present invention, the average distance between martensites was 2.0 to 4.0 占 퐉. In particular, the average spheroidization ratio (length / width) of martensite was 0.9 to 1.1, The results are shown. The high spheroidization ratio of the martensite contributes to the improvement of the bending workability by delaying the propagation of cracks formed therein.

The high-strength cold-rolled steel sheet according to the present invention may have a tensile strength of 1180 MPa or more and an elongation of 15% or more in terms of mechanical properties.

Hereinafter, a method for manufacturing a high strength cold rolled steel sheet according to the present invention having the above characteristics will be described.

1 and 2 are flowcharts schematically showing a method of manufacturing a high strength cold rolled steel sheet according to an embodiment of the present invention, in which FIG. 1 shows a hot rolling process, and FIG. 2 shows a cold rolling process and a heat treatment process.

Referring to FIGS. 1 and 2, the method for manufacturing a high strength cold rolled steel sheet according to the present invention includes a slab reheating step S110, a hot rolling step S120, a first cooling step S130, a second cooling step S140, A rolling step S150, an annealing step S160, a third cooling step S170, a constant temperature holding step S180, and a fourth cooling step S190.

In the slab reheating step S110, 0.1 to 0.2% of carbon (C), 1.0 to 2.0% of silicon (Si), 1.8 to 2.2% of manganese (Mn) : More than 0% to 0.05% and sulfur (S): more than 0% to 0.01%, further comprising at least one of 0.02 to 0.05% of niobium and 0.06 to 0.1% of vanadium, And unavoidable impurities are reheated to re-decompose the manganese segregation bed on the slab, and the niobium precipitate or the vanadium precipitate is redissolved.

In the present invention, the reheating of the slab plate is preferably performed at 1250 to 1300 ° C for 3 to 5 hours. If the reheating temperature of the slab plate is less than 1250 占 폚 or the reheating time is less than 3 hours, the efficiency of re-splitting of the manganese segregation zone may decrease and the bending workability of the cold-rolled steel sheet to be finally produced may deteriorate. On the other hand, if the reheating of the slab plate exceeds 1300 ° C or the reheating time exceeds 5 hours, the steel plate manufacturing cost can be increased without additional effect owing to excessive heating.

Next, in the hot rolling step (S120), the reheated plate is hot-rolled.

The hot rolling is preferably carried out at a finishing rolling temperature of 840 to 880 캜. If the finishing rolling temperature exceeds 880 DEG C, the strength and ductility may be reduced due to the increase in the ferrite grain size. On the other hand, if the finish rolling temperature is lower than 840 캜, a problem may arise such as the occurrence of blistering due to abnormal reverse rolling.

Next, in the primary cooling step (S130), the hot rolled plate is cooled to a predetermined temperature. If the winding is performed after the primary cooling, the termination temperature of the primary cooling may be the winding temperature.

The primary cooling is preferably carried out at 580 to 620 DEG C at an average cooling rate of 30 to 100 DEG C / sec. When the average cooling rate of the primary cooling is less than 30 DEG C / sec, it is difficult to suppress the formation and growth of the high-temperature precipitates. On the other hand, when the average cooling rate of the primary cooling exceeds 100 캜 / sec, the steel plate manufacturing cost can be raised only by excessive cooling without further effect. In addition, when the cooling termination temperature of the primary cooling exceeds 620 占 폚, precipitates can form and grow at a high temperature. On the contrary, when the cooling end temperature of the primary cooling is less than 580 占 폚, the low-temperature precipitate may not be formed properly.

Next, in the secondary cooling step (S140), the primary cooled plate is secondarily cooled at an average cooling rate of 2 to 5 DEG C / sec.

Usually, the cold rolled steel sheet is rolled after the first cooling and cooled to room temperature by air cooling after winding. At this time, the precipitate formed near the coiling temperature grows, and the elongation and bending workability of the steel sheet can be inhibited.

Accordingly, in the present invention, the primary cooled plate is secondarily cooled to approximately room temperature at an average cooling rate of 2 to 5 ° C / sec to suppress the growth of precipitates formed near the first cooling end temperature. Secondary cooling can be performed by a forced air cooling method using a cooling fan. When the average cooling rate of the secondary cooling is less than 2 캜 / sec, it is difficult to suppress the precipitate growth. However, when a cooling fan is used, the average cooling rate is less likely to exceed 5 캜 / sec in a plate material, particularly in a wound state.

Next, in the pickling / cold rolling step (S150), the secondary cooled plate is pickled to remove the scale on the surface of the steel sheet, and thereafter cold-rolled to a thickness of the steel sheet. In case of coiling after primary cooling, uncoil the steel sheet taken before pickling or pickling.

The reduction rate of the cold rolling can be set to about 50 to 70%, depending on the thickness of the plate after hot rolling and the final thickness of the target steel plate.

Next, in the annealing treatment step (S160), the cold-rolled sheet material is subjected to annealing at an A1 transformation point or higher. The austenite phase fraction can be controlled through annealing, thereby achieving the target strength and elongation through the third cooling, the constant temperature maintenance and the fourth cooling described later.

In the present invention, the annealing treatment is preferably performed at 750 to 900 DEG C for 60 to 150 seconds. When the annealing treatment temperature is less than 750 ° C or the annealing treatment time is less than 60 seconds, carbon in the austenite is not uniformly distributed, resulting in difficult distribution of martensite dispersion, and the bending workability of the finally produced steel sheet may be deteriorated. On the other hand, when the annealing treatment temperature exceeds 900 DEG C or the annealing treatment time exceeds 150 seconds, the austenite grain size increases greatly and the physical properties of the steel sheet such as strength may be deteriorated.

Next, in the third cooling step (S170), the annealed sheet material is thirdly cooled to Ms ~ Ms + 50 占 폚.

If the primary cooling end temperature exceeds Ms + 50 占 폚, it may become difficult to secure a sufficient tensile strength. If the primary cooling end temperature is lower than the Ms point, it may become difficult to secure an elongation of 15% or more.

It is more preferable that the tertiary cooling is carried out at an average cooling rate of 50 to 100 DEG C / sec. If the average cooling rate of the tertiary cooling is less than 50 ° C / sec, the austenite may transform into ferrite or pearlite during the cooling process, making it difficult to secure the final target martensite fraction. On the other hand, if the average cooling rate of the tertiary cooling is too high, exceeding 100 ° C / sec, the problem of material unevenness may occur.

Next, in the constant temperature holding step (S180), the third cooled plate is kept at a constant temperature for a predetermined time at Ms ~ Ms + 50 占 폚, and the carbon (C) concentration in the retained austenite is progressed in a short time, So that at least 10% of the retained austenite phase is formed at the area ratio in the structure. Here, the constant temperature maintenance includes not only keeping the temperature constant for a predetermined time, but also air cooling for a predetermined time.

It is more preferable that the constant temperature maintenance is performed for 200 to 400 seconds. If the constant temperature holding time is less than 200 seconds, the effect is insufficient, and if the constant temperature holding time exceeds 400 seconds, the productivity may be lowered without further effect.

Next, in the fourth cooling step (S190), the plate kept at a constant temperature is quenched to martensite temperature range, approximately from Ms point to Ms-150 deg. C to form a final microstructure. Quaternary cooling can be carried out at an average cooling rate of about 5-100 DEG C / sec.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

A slab plate having the composition shown in Table 1 was prepared.

[Table 1]

Thereafter, the slab plate was reheated at 1250 占 폚 for 3 hours, hot rolled at a finish rolling temperature of 860 占 폚, first cooled to 600 占 폚 at an average cooling rate of 40 占 sec and then rolled up, Lt; RTI ID = 0.0 > 3 C / sec. ≪ / RTI > Thereafter, the wound sheet was subjected to uncoiling and pickling, and then cold-rolled at a reduction ratio of 60%. Thereafter, the steel sheet was annealed at 800 ° C for 100 seconds, and then cooled to 350 ° C at an average cooling rate of 50 ° C / sec. Thereafter, the mixture was kept at 350 DEG C for 300 seconds, quenched to 200 DEG C at an average cooling rate of 20 DEG C / sec, and then cooled to room temperature to prepare final cold-rolled specimens 1 to 4.

For cold-rolled specimen 5, the composition and other conditions were the same as for cold-rolled specimen 1, but the slab reheating was carried out at 1200 ° C for 2 hours.

For the cold-rolled specimen 6, the composition and the other conditions were the same as those of the cold-rolled specimen 1, but after the winding, general air cooling was applied.

2. Evaluation of mechanical properties

Table 1 shows the tensile test and the bending test evaluation results of the cold-rolled specimens 1 to 6.

The bending test was carried out by using a 90 ° bending tester for each of bending radii 1R, 2R and 3R (where 1R, 2R and 3R mean bending radii corresponding to 1, 2 and 3 times the thickness of the steel sheet) And then judged by the following criteria.

◎ (Very good): No crack

O (Good): Micro crack

X (normal): Cracks visible to the naked eye

[Table 2]

Referring to Table 2, in the case of the cold-rolled specimens 1 to 3 satisfying the conditions of the present invention, the tensile strength was 1180 MPa or more and the elongation was 15% or more, and cracks were not visually observed even at a bending radius of 1R And showed excellent bending workability in general.

On the other hand, in the case of specimen 4 in which niobium and vanadium were not added, the strength was excellent, but the elongation and bending properties were poor.

In case of specimen 4, the yield strength is high and the elongation is low, so that the moldability is not excellent. The reason why the yield strength increases in specimen 4 is because the martensite of the coarse bed interferes with the dislocation movement and the reason for the lowering of the elongation rate is that the cold cooling transformation phase is accelerated during the same cooling transformation due to the increase of the Mn addition amount This is because the residual austenite phase is relatively decreased.

Also, in the case of specimen 5, in which the slab reheating temperature was relatively low, the strength and elongation were in line with the target values, but cracks occurred at a bending radius of 1R.

In addition, after the coiling, the specimen 6 to which the general air cooling was applied had the strength corresponding to the target value, but the elongation was 14%, which was slightly less than the target value. In particular, the bending property was lower than that of the specimens 1 to 3 .

Table 3 shows the results of microstructure analysis of the cold-rolled specimens 1 to 2 and 4.

In Table 3, F means ferrite, M means martensite, and R.A means retained austenite. Fraction means area ratio. The martensitic spheroidization ratio means the ratio of the length to the width.

[Table 3]

Referring to Table 3, it can be seen that the area ratio of retained austenite (R.A) is at least 10% for specimens 1 and 2, but the area ratio of retained austenite is only 5% for specimen 4. The elongation of specimens 1 and 2 is higher than that of specimen 4 due to the difference in retained austenite fraction. As can be seen from Table 3, this is an effect of grain refinement by addition of niobium or vanadium.

Further, in the case of specimens 1 and 2, it can be seen that the sphalerization rate of martensite (M) is significantly higher than that of specimen 4. The martensite close to the spherical shape is effective in retarding the propagation of cracks formed in the steel sheet, and it can be seen that excellent bending workability characteristics can be obtained even at a bending radius of 1R.

On the other hand, in the case of the specimen 4, as shown in Table 3, the ferrite grains are coarsened and the area fraction of the grain boundary martensite is almost the same, but the martensite is distributed in the irregular shape of the cluster type, The stress concentration occurs at the grain boundary of martensite, and the propagation of the crack rapidly proceeds along the grain boundaries of the ferrite and martensite.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Slab reheating step
S120: Hot rolling step
S130: primary cooling step
S140: Secondary cooling step
S150: pickling / cold rolling step
S160: Annealing step
S170: Third cooling step
S180:
S190: fourth cooling step

Claims (8)

(P): not less than 0% to not more than 0.05%, and sulfur (S): 0.1 to 0.2%, silicon (Si): 1.0 to 2.0%, manganese ): A slab plate containing more than 0% to 0.01% of at least one of 0.02 to 0.05% of niobium and 0.06 to 0.1% of vanadium, the remaining iron (Fe) and inevitable impurities, ≪ / RTI > for 3 to 5 hours;
Hot rolling the reheated plate at a finish rolling temperature of 840 to 880 ° C;
Subjecting the hot-rolled plate to primary cooling to 580 to 620 占 폚 at an average cooling rate of 30 to 100 占 폚 / sec;
Second cooling the primary cooled plate at an average cooling rate of 2 to 5 占 폚 / sec;
Pickling and cold rolling the secondary cooled plate;
Annealing the cold-rolled plate at an Al transformation point or higher;
Thirdly cooling the annealed sheet material to Ms ~ Ms + 50 占 폚;
Maintaining the third cooled plate at a constant temperature of Ms to Ms + 50 ° C; And
And cooling the quenched plate to quench the quenched to the martensite temperature region.
The method according to claim 1,
The secondary cooling
Wherein the cold rolling is performed by a forced air cooling method using a cooling fan.
The method according to claim 1,
The annealing treatment is performed at 750 to 900 DEG C for 60 to 150 seconds,
The tertiary cooling is carried out at an average cooling rate of 50-100 DEG C / sec,
Wherein the constant temperature holding is performed for 200 to 400 seconds.
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