KR20150142791A - High strength cold rolled steel sheet excellent in shape freezability, and manufacturing method thereof - Google Patents

Abstract

According to one embodiment of the present invention, a cold-rolled steel sheet properly controls a composition and a manufacturing method of the steel sheet in order to provide the ultra-high strength cold-rolled steel sheet with excellent shape fixability and a method for manufacturing the same having an excellent shape quality, impact property, and weldability and having a yield strength of not less than 1000 MPa, a tensile strength of not less than 1300 MPa, and a yield ratio of not less than 0.77.

Description

TECHNICAL FIELD [0001] The present invention relates to an ultra-high strength cold-rolled steel sheet excellent in shape fixability and a method of manufacturing the same. BACKGROUND OF THE INVENTION [0002]

The present invention relates to an ultra high strength cold rolled steel sheet which can be used for structural members for automobiles such as a bumper beam, a side sill, a member, a seat rail, a pillar, .

Recently, steel plates for automobiles are required to have higher strength to improve fuel economy and durability by various environmental regulations and energy use regulations. Particularly, as the impact stability regulation of automobiles has been spreading recently, high-strength steels excellent in yield strength have been adopted as structural members such as members, seat rails and pillars in order to improve the impact resistance of the vehicle body.

In addition, ultra-high strength of a bumper beam part considering a frontal collision characteristic or a side sill part advantageous for side collision is progressing, and the higher the yield strength value against the tensile strength of the part, The higher the ratio (yield strength / tensile strength), the better the impact resistance.

Generally, methods of strengthening steel include solid solution strengthening, precipitation strengthening, strengthening by grain refinement, and transformation strengthening. However, the strengthening by solid solution strengthening and grain refinement in the above method is disadvantageous in that it is very difficult to produce a high strength steel having a tensile strength of 490 MPa or more.

On the other hand, in the precipitation strengthening steel sheet, by adding carbonitride forming elements such as Cu, Nb, Ti, V and the like, carbonitride is precipitated to reinforce the steel sheet, or crystal grains are inhibited by fine precipitates, It is a steel plate.

The precipitation hardening steel sheet has an advantage that a high strength can be easily obtained at a low manufacturing cost.

However, in the precipitation-strengthening steel sheet, the recrystallization temperature is rapidly increased due to the fine precipitates. Therefore, in order to ensure sufficient recrystallization and ductility, high temperature annealing must be performed. In addition, precipitation hardening steel in which carbonitride is precipitated and strengthened in a ferrite base has a problem that it is difficult to obtain a high strength steel of 600 MPa or higher.

On the other hand, the transformation-strengthening high-strength steel is a ferrite-martensite dual-phase steel containing a hard martensite in a ferrite base, TRIP (Transformation Induced Plasticity) steel using ferroelectricity of residual austenite, And complex phase steel (CP) steel composed of hard bainite or martensite structure have been developed.

However, the tensile strength that can be realized in such a transformation-strengthening high-strength steel is limited to a level of about 1200 MPa. In addition, the hot press forming steel that secures the final strength by quenching by direct contact with a die that is water-cooled after molding at a high temperature is being applied to structural members to secure collision safety, There is not a large increase in application due to excessive capital investment cost and high heat treatment and process cost.

In recent years, in order to further improve the stability of a passenger in the event of a collision, a bumper beam part considering a frontal collision characteristic in a vehicle or a side sill part advantageous for side collision has been intensified. These parts are mainly manufactured by using the roll forming method instead of the conventional press forming method. The roll forming method has a higher productivity than general press forming and hot press forming, and is a method for producing a complicated shape through multi-step roll forming. However, the application to ultra-high strength material having a low elongation rate is generally expanded have. Ultra high strength steels developed for roll forming are mainly produced in continuous annealing furnaces with water cooling equipment, and microstructures represent tempered martensite structures tempered with martensite. However, the ultrahigh strength steel for roll forming produced in a continuous annealing furnace with water cooling equipment has a tendency to deteriorate work quality due to workability deterioration and material deviation .

As a representative technology of super high strength steels developed for roll forming, there is the following Patent Document 1. These patents disclose a method for producing a steel sheet having a martensite volume ratio of 80 to 97% and a tensile strength of 1500 MPa or more by continuously annealing a steel material having a carbon content of at least 0.18%, cooling the steel sheet to a normal temperature and then subjecting the steel sheet to a temperature of 120 to 300 ° C for 1 to 15 minutes. To develop steel. In this way, when ultra-high strength steel is manufactured by the tempering method after water cooling, the yield ratio is very high, but the shape quality of the coil deteriorates due to the temperature deviation in the width direction and the length direction. Therefore, there are problems such as material defects and workability degradation depending on the parts during roll forming.

In addition, in the case of Patent Document 2, a method of manufacturing a cold-rolled steel sheet excellent in plate form after continuous annealing is obtained by simultaneously using tempering martensite to obtain high strength and high ductility, and the carbon content is as high as 0.2% There is concern about the possibility of inducing Rohna dent.

Patent Document 1: Japanese Laid-Open Patent Publication No. 1990-418479 Patent Document 2: JP-A-2010-90432

One aspect of the present invention is to provide a method of manufacturing a super high strength cold rolled steel sheet excellent in shape quality, impact characteristics, weldability and shape durability by securing the microstructure and martensite packet size of a desired steel sheet, And a manufacturing method thereof.

The object of the present invention is not limited to the above description. Those of ordinary skill in the art will appreciate that there is no difficulty in understanding the further subject of the invention from the description of the invention and the description in the drawings.

The ultra-high strength cold-rolled steel sheet excellent in shape-formability according to one aspect of the present invention comprises 0.12 to 0.17% of C, 0.3% or less of Si (excluding 0), 2.5 to 3.0% of Mn, 0.10% or less of S, 0.01 to 0.10% of Sol.Al, 0.5 to 1.0% of Cr, 0.0010 to 0.0050% of B, 0.01% or less of N, 0.01 to 0.05% of Nb, the balance Fe and other unavoidable impurities, and the microstructure contains martensite and tempered martensite at 90% by area or more.

A method for producing an ultra-high strength cold-rolled steel sheet excellent in shape-formability, which is another aspect of the present invention, comprises: 0.12 to 0.17% of C; 0.3% or less of Si (excluding 0) 0.001 to 0.10% of P, 0.01 to 0.10% of S, 0.01 to 0.10% of Sol.Al, 0.5 to 1.0% of Cr, 0.0010 to 0.0050% of B, 0.01% : 0.01-0.05%, Nb: 0.01-0.05%, the balance Fe and other unavoidable impurities; Hot-rolling the heated steel slab to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet; A step of cold-rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet at an annealing temperature of 770 ° C to 830 ° C; Cooling the continuously annealed cold rolled steel sheet to a temperature of 650 to 700 占 폚 at a cooling rate of 1 to 10 占 폚 / sec; Cooling the primary cold-rolled steel sheet to 250 to 340 ° C at a cooling rate of 5 to 20 ° C / sec and subjecting the cold- And satisfies the condition of the following relational expression (1).

The annealing temperature SS and the secondary cooling end temperature RCS are appropriately controlled according to the contents of C and Cr to satisfy the following relational expression 1. [ However, annealing temperature should be controlled in the range of 770 ~ 830 ℃ and secondary cooling end temperature (RCS) should be controlled in the range of 250 ~ 340 ℃.

[Relation 1] 9.9? 3.7 * C + 2.18 * Cr + 0.015 * SS-0.013 * RCS? 12.1

(Wherein C and Cr are respectively the contents (weight%) of the elements, SS is the annealing temperature (占 폚), and RCS is the secondary cooling end temperature (占 폚).

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages and effects of the present invention will become more fully understood with reference to the following specific embodiments.

According to one aspect of the present invention, it is possible to provide an ultra-high strength cold-rolled steel sheet excellent in shape quality, impact characteristics, weldability, and shape mobility by appropriately controlling the composition of the steel sheet and the manufacturing method thereof and a method of manufacturing the same.

The cold-rolled steel sheet according to one aspect of the present invention has a tensile strength of 1300 MPa or more, a yield ratio of 0.77 or more, and an elongation of 6% or more at a bumper beam part, a side sill part, a seat rail, , A pillar, and the like.

1 shows the microstructure of Invention steel 1 of the present invention.
Fig. 2 shows the microstructure of Comparative Steel 1. Fig.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below.

The cold rolled steel sheet according to an embodiment of the present invention may contain 0.12 to 0.17% of C, 0.3% or less of Si (excluding 0), 2.5 to 3.0% of Mn, 0.001 to 0.10% of P, 0.01 to 0.05% of N, 0.01 to 0.05% of Nb, 0.01 to 0.05% of Nb, 0.01 to 0.10% of Sol.Al, 0.5 to 1.0% of Cr, 0.0010 to 0.0050% , The balance Fe and other unavoidable impurities.

The reason for controlling the component range of the hot-rolled steel sheet as described above is as follows. It is to be noted that the content of each element is based on the weight% unless otherwise specified.

C: 0.12 to 0.17%

Carbon (C) is a very important element added to strengthen the metamorphosis. Carbon promotes high strength and promotes the formation of martensite in metamorphic steel. As the carbon content increases, the amount of martensite increases. However, if the carbon content is excessive, a welding defect occurs to open the weldability. When the carbon content is too small, the strength is lowered and it is difficult to obtain the desired strength. Therefore, according to one aspect of the present invention, it is preferable that the carbon content is 0.12 to 0.17%.

Si: 0.3% or less (excluding 0)

Silicon (Si) promotes ferrite transformation and increases the content of carbon in untransformed austenite to form a complex structure of ferrite and martensite, thereby interfering with the increase in the strength of martensite. In addition, it is desirable to limit possible additions, since it not only causes surface scale defects in terms of surface properties, but also deteriorates chemical conversion properties. Therefore, according to one aspect of the present invention, it is preferable that the Si content is 0.3% or less.

Mn: 2.5 to 3.0%

Manganese (Mn) fines particles without damaging ductility and precipitates sulfur into MnS in the steel to prevent hot brittleness due to the formation of FeS, as well as to strengthen the steel and to lower the critical cooling rate at which the martensite phase is obtained So that martensite can be formed more easily. However, if the Mn content is excessive, weldability and hot rolling property may be degraded. When the Mn content is too small, it is difficult to secure the desired strength in the present invention. Therefore, according to one aspect of the present invention, the Mn content is preferably included in the range of 2.5 to 3.0%.

P: 0.001 to 0.10%

Phosphorus (P) plays the role of improving the in-plane anisotropy and improving the strength as a substitutional alloying element having the largest effect of strengthening the solution. However, if the P content is excessive, press formability deteriorates and brittleness of steel may occur. When the P content is too small, it is difficult to obtain the intended strength in the present invention. Therefore, according to one aspect of the present invention, it is preferable that the P content is included in the range of 0.001 to 0.10%.

S: not more than 0.010%

Sulfur (S) is an impurity element in steel and is an element that hinders ductility and weldability of a steel sheet. If the S content is excessive, the ductility and weldability of the steel sheet are likely to be deteriorated. Therefore, according to one aspect of the present invention, the S content is preferably 0.010% or less.

Sol.Al: 0.01 to 0.10%

Soluble Al (Sol.Al) is a component effective for deoxidation in combination with oxygen in steel and for improving the martensitic hardenability by distributing carbon in ferrite to austenite like Si. However, if the Sol.Al content is excessive, the effect is not only saturated but also the manufacturing cost is increased. When the content of Sol.Al is too small, the effect of improving the hardenability can not be secured. Therefore, according to one aspect of the present invention, the Sol.Al content is preferably 0.01 to 0.10%.

Cr: 0.5 to 1.0%

Chromium (Cr) is a component added to improve hardenability of steel and ensure high strength. In the present invention, it is an element that plays a very important role in forming martensite, which is a low temperature transformation phase. When the Cr content is too small, it is difficult to secure the above effect. However, if the Cr content is excessive, the effect is not only saturated but economically disadvantageous. Therefore, according to one aspect of the present invention, it is preferable that the Cr content is included in the range of 0.5 to 1.0%.

B: 0.0010 to 0.0050%

The boron boron (B) was added as an element to retard the transformation of austenite into pearlite during the cooling process during annealing, to inhibit ferrite formation and promote the formation of martensite. When the B content is too small, it is difficult to secure the above effect. However, if the B content is excessive, the cost may be deteriorated due to the iron overfill. Therefore, according to one aspect of the present invention, the content of B is preferably 0.0010 to 0.0050%.

N: 0.01% or less (excluding 0)

Nitrogen (N) is a component effective to stabilize austenite. However, if the N content is excessive, the risk of cracking during performance through AlN formation is greatly increased. Therefore, according to one aspect of the present invention, the N content is preferably 0.01% or less (excluding 0).

0.01 to 0.05% of Ti, 0.01 to 0.05% of Nb,

Is an element effective for increasing the strength and grain size of titanium (Ti) and niobium (Nb) steel sheets. When the content of Ti and Nb is too small, it is difficult to secure the above effect. However, if the content of Ti and Nb is excessively high, ductility may be largely lowered due to an increase in manufacturing cost and excessive precipitates. Therefore, according to one aspect of the present invention, the content of Ti and Nb is preferably 0.01 to 0.05%, respectively.

The present invention may include residual Fe and other unavoidable impurities in addition to the above components. On the other hand, addition of an effective component other than the above-mentioned composition is not excluded.

Further, according to one aspect of the present invention, the microstructure of the cold-rolled steel sheet may contain 90% or more of martensite and tempered martensite.

If the sum of martensite and tempered martensite in the microstructure of the cold-rolled steel sheet is less than 90% by area, it is difficult to exhibit the high yield ratio of the present invention.

According to an aspect of the present invention, it is preferable that the microstructure contains at least 10% by area of at least one of ferrite and bainite in its total fraction.

If at least one of ferrite and bainite is contained in an amount exceeding 10% by area, the yield strength is low and it is difficult to show the yield strength and yield ratio of the present invention.

Further, according to one aspect of the present invention, the martensite packet size in the microstructure of the cold-rolled steel sheet may be 1.5 m or less.

A martensitic packet means a specific region where regions having similar crystal orientations gather together when the steel is transformed into martensite and transformation proceeds in the same direction.

As the martensite packet size increases, the yield strength decreases and the yield ratio decreases. Therefore, the size of the martensite packet is preferably 1.5 m or less.

At this time, according to one aspect of the present invention, the cold-rolled steel sheet may have a yield strength of 1000 MPa or more, a yield ratio of 0.77 or more, and a tensile strength of 1300 MPa or more.

At this time, according to one aspect of the present invention, the cold-rolled steel sheet may have an elongation of 6% or more.

Hereinafter, a method for manufacturing an ultra-high strength cold rolled steel sheet excellent in shape quality, impact characteristics, weldability and shape durability, which is another aspect of the present invention, will be described as a preferred example for producing the cold rolled steel sheet.

A method of producing an ultra-high strength cold rolled steel sheet excellent in shape quality, impact characteristics, weldability and shape durability, which is another aspect of the present invention, comprises 0.12 to 0.17% of C, 0.3% 0.01 to 0.10% of Al, 0.01 to 0.10% of Cr, 0.5 to 1.0% of Cr, 0.0010 to 0.0050% of B and 0.01% or less of N (%) of Mn, 0), Ti: 0.01-0.05%, Nb: 0.01-0.05%, the balance Fe and other unavoidable impurities; Hot-rolling the heated steel slab to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet; A step of cold-rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet at an annealing temperature of 770 ° C to 830 ° C; Cooling the continuously annealed cold rolled steel sheet to a temperature of 650 to 700 占 폚 at a cooling rate of 1 to 10 占 폚 / sec; Cooling the primary cold-rolled steel sheet to 250 to 340 ° C at a cooling rate of 5 to 20 ° C / sec and subjecting the cold- And satisfies the condition of the following relational expression (1).

The annealing temperature (SS) and the secondary cooling end temperature (RCS) are appropriately adjusted according to the contents of C and Cr to satisfy the following relational expression (1). However, the annealing temperature should be controlled in the range of 770 to 830 ° C, and the secondary cooling end temperature (RCS) should be controlled in the range of 250 to 340 ° C.

[Relation 1] 9.9? 3.7 * C + 2.18 * Cr + 0.015 * SS-0.013 * RCS? 12.1

(Wherein C and Cr are respectively the contents (weight%) of the elements, SS is the annealing temperature (占 폚), and RCS is the secondary cooling end temperature (占 폚).

The inventors of the present invention have studied to obtain a high strength steel sheet excellent in shape dynamics, shape quality and impact characteristics, and found that C content, Cr content, annealing temperature and secondary cooling end temperature play an important role in the production process In particular, it has been found that when the relationship 1 is satisfied as described above, the yield ratio is 0.77 or more and the yield strength is 1000 MPa or more.

In the above-mentioned relational expression 1, while controlling the annealing temperature to 770 to 830 캜 and the secondary cooling finishing temperature to a temperature range of 300 to 450 캜 under the condition that the content of carbon and Cr satisfies the composition range shown in the present invention, The annealing temperature and the secondary cooling end temperature are adjusted by using a correlation between the annealing temperature and the secondary cooling end temperature. If these conditions are not satisfied, the yield strength is low and the yield ratio of 0.77 or more is not satisfied.

Hereinafter, each step of the method for producing an ultra-high strength cold rolled steel sheet excellent in shape quality, impact characteristics, weldability and shape durability will be described in detail.

Step of obtaining hot-rolled steel sheet

0.1 to 0.10% of C, 0.3% or less of Si (excluding 0), 2.5 to 3.0% of Mn, 0.001 to 0.10% of P, , Fe: 0.5 to 1.0%, Cr: 0.0010 to 0.0050%, N: 0.01% or less (excluding 0), Ti: 0.01 to 0.05%, Nb: 0.01 to 0.05%, and the balance Fe and other unavoidable impurities After the slab is heated, it is hot-rolled to obtain a hot-rolled steel sheet.

At this time, the outlet side temperature during finish rolling may be 800 to 950 캜. When the temperature at the outlet side of the finish rolling is less than 800 ° C, the hot deformation resistance may increase rapidly, and the top, bottom, and edge of the hot-rolled coil become single- And the moldability is deteriorated. On the other hand, when the temperature is higher than 950 占 폚, there is a concern that a thick oxide scale may be generated on the surface of the hot-rolled steel sheet, and the microstructure of the steel sheet may be coarsened.

Winding  step

Thereafter, the hot-rolled steel sheet can be wound at 500 to 750 ° C. If the coiling temperature is less than 500 占 폚, excessive martensite or bainite is generated to cause a sudden increase in strength of the hot-rolled steel sheet, which may cause manufacturing problems such as defective shape due to load during cold rolling. On the other hand, when it exceeds 750 ° C, there arises a problem that the acidity is deteriorated due to an increase in the surface scale.

Step of Obtaining Cold Rolled Steel Sheet

Thereafter, the rolled hot-rolled steel sheet is cold-rolled at a reduction ratio of 40 to 70% to obtain a cold-rolled steel sheet. If the reduction rate is less than 40%, the recrystallization driving force is weakened and there is a large possibility of obtaining a good recrystallized grain, and the shape correction is very difficult. On the other hand, if the reduction rate exceeds 70%, there is a high possibility that cracks occur at the edge of the steel sheet, and the rolling load rapidly increases.

Continuous annealing  step

Thereafter, the cold-rolled steel sheet can be continuously annealed at an annealing temperature of 770 ° C to 830 ° C. At this time, the annealing temperature should be in a range satisfying the relational expression (1). Even if the annealing temperature satisfies the above-mentioned relational expression 1, when the annealing temperature is lower than 770 占 폚, a large amount of ferrite is produced and the yield strength is lowered. Therefore, it is impossible to produce a steel material having a high specific gravity of 0.77 or higher. The increase in the size of the osteon grains due to the high-temperature annealing increases the size of the martensite packet produced during cooling, and the tensile strength decreases, failing to satisfy the tensile strength values proposed in the present invention.

Cooling step

Thereafter, the continuously annealed cold rolled steel sheet can be firstly cooled to 650 to 700 占 폚 at a cooling rate of 1 to 10 占 폚 / sec. The primary cooling step is to inhibit ferrite transformation to transform most of the austenite to martensite.

Thereafter, the primary cooled cold rolled steel sheet is secondarily cooled to 250 to 340 占 폚 at a cooling rate of 5 to 20 占 폚 / sec to effect over-treatment. At this time, it is desirable to raise the tempered martensite phase in order to produce the ultra-high strength steel sheet having a high yield ratio. Therefore, the tempered martensite can be formed by tempering the martensite formed by cooling after annealing.

In addition, the secondary cooling termination temperature is intended to improve the shape quality of the coil in the width direction and the length direction, and is also a very important temperature condition for securing a high yield strength.

When the secondary cooling end temperature is lower than 250 ° C., the degree of integration of carbon in the martensite increases greatly during the over-treatment, and the excessive increase of the martensite strength increases the yield strength and tensile strength at the same time and greatly deteriorates the ductility. Shape deterioration due to rapid cooling may occur, resulting in workability deterioration during roll forming. Also

On the other hand, if the temperature is higher than 340 ° C, the austenite produced during annealing can not be transformed into martensite, and bainite, granular bainite, etc., which are high-temperature transformation phases, are produced and the yield strength is rapidly deteriorated Occurs. The occurrence of such a structure can not produce an ultrahigh-strength hot-rolled steel sheet having excellent shape-formability suggested by the present invention accompanied with a decrease in yield ratio.

At this time, according to one aspect of the present invention, the first and second cooling steps may be performed by mist cooling.

The mist cooling method is a method in which water is sprayed with a gas such as nitrogen and sprayed in the form of droplets to cool the steel sheet.

When the ultra high strength steel is manufactured by the tempering method after water cooling, the yield ratio is very high, but the shape quality of the coil deteriorates due to the temperature deviation in the width direction and the longitudinal direction. Therefore, there are problems such as material defects and workability degradation depending on the parts during roll forming.

Since the present invention is cooled by a mist cooling method instead of a water cooling method, the cooling speed is relatively low, so that the shape quality in the width direction and the longitudinal direction of the coil can be improved as compared with the water cooling treatment material.

Meanwhile, according to one embodiment of the present invention, skin pass rolling can be additionally performed on the cold-rolled steel sheet produced according to the above-described method at a reduction rate of 0.1 to 1.0%. This is for controlling the shape of the steel sheet. Generally, when skeletal rolling of a textured steel is performed, an increase in yield strength of at least 50 MPa or more occurs with little increase in tensile strength. When the elongation is less than 0.1%, it is very difficult to control the shape of the ultra high strength steel such as the steel of the present invention. When the work is performed at 1.0% or more, the workability is greatly unstable due to the high elongation work, and the value is limited to 0.1 to 1.0% .

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 from them.

( Example )

Steel slabs prepared as shown in Table 1 below were prepared and heated at a reheating temperature of 1200 캜 for 1 hour in a heating furnace, hot rolled, and then wound. At this time, the finish rolling in the hot rolling was performed such that the temperature at the outlet side was 880 캜, and the coiling temperature was set at 680 캜. The hot rolled steel sheet was pickled and subjected to cold rolling at a cold rolling reduction of 50%. The cold-rolled steel sheet was continuously annealed at the annealing temperature shown in Table 2. Thereafter, the continuously annealed cold rolled steel sheet was firstly cooled to 650 캜 at a cooling rate of 5 캜 / sec. Thereafter, the primary cooled cold-rolled steel sheet was secondarily cooled to a secondary cooling termination temperature of Table 2 at a cooling rate of 20 캜 / sec and subjected to over-activation treatment. Thereafter, skin pass rolling was performed at a reduction ratio of 0.2%.

In Table 1 and 2, C, Mn, Si, P, S, Al, Cr, Ti, Nb, B and N represent the contents (weight%) of each element, SS is the annealing temperature (≪ 0 > C).

Table 2 below shows the control factor of Relation 1 and the satisfaction of Relation 1.

In the following Table 3, JIS No. 5 tensile test specimens were prepared from inventive steels 1 to 9 and comparative steels 1 to 15, and the materials were measured. The microstructures were observed to determine the martensite packet size, martensite and tempered The area% (M + TM) of the martensite sum and the area% (F + B) of the ferrite and bainite sum are shown.

In Table 3, the unit of yield strength and tensile strength is MPa, elongation unit is%, and unit of M packet size (martensite packet size) is μm.

Steel number C Mn Si P S Al Cr Ti Nb B N SS RCS Remarks One 0.15 2.8 0.1 0.01 0.003 0.03 0.8 0.02 0.03 0.0025 0.007 800 300 Inventive Steel 1 800 400 Comparative River 1 2 0.17 2.6 0.1 0.011 0.003 0.025 0.6 0.04 0.02 0.0025 0.005 810 300 Invention river 2 3 0.17 2.8 0.1 0.012 0.003 0.033 0.6 0.02 0.02 0.002 0.007 820 300 Invention steel 3 4 0.19 2.5 0.1 0.01 0.005 0.03 0.5 0.02 0.02 0.002 0.006 800 270 Inventive Steel 4 5 0.14 2.9 0.1 0.012 0.003 0.041 0.9 0.03 0.02 0.002 0.005 810 270 Invention steel 5 820 420 Comparative River 2 6 0.15 2.7 0.1 0.01 0.006 0.041 0.7 0.04 0.02 0.002 0.004 810 270 Invention steel 6 7 0.13 2.7 0.1 0.011 0.003 0.051 0.9 0.03 0.03 0.0023 0.005 810 300 Invention steel 7 810 370 Comparative Steel 3 8 0.16 2.8 0.1 0.01 0.003 0.042 0.6 0.02 0.03 0.0025 0.003 800 300 Inventive Steel 8 800 350 Comparative Steel 4 800 340 Comparative Steel 5 9 0.14 2.7 0.1 0.013 0.003 0.029 0.7 0.02 0.03 0.0025 0.005 800 300 Invention river 9 790 450 Comparative Steel 6 750 300 Comparative Steel 7 800 340 Comparative Steel 8 780 300 Comparative Steel 9 10 0.1 2.7 0.1 0.01 0.002 0.03 0.9 0.03 0.03 0.0023 0.005 800 300 Comparative Steel 10 11 0.14 2.7 0.1 0.012 0.003 0.038 0.2 0.02 0.03 0.0025 0.006 800 300 Comparative Steel 11 12 0.2 2.7 0.1 0.01 0.004 0.033 0.7 0.04 0.02 0.002 0.007 810 310 Comparative Steel 12 13 0.16 2.2 0.1 0.011 0.005 0.044 0.9 0.04 0.02 0.002 0.005 810 310 Comparative Steel 13 14 0.14 3.3 0.1 0.01 0.003 0.035 0.6 0.04 0.02 0.002 0.006 810 310 Comparative Steel 14 15 0.15 2.9 0.1 0.01 0.003 0.03 1.5 0.04 0.02 0.002 0.007 800 310 Comparative Steel 15

Steel number C Cr SS RCS 3.7 * C + 2.18 * Cr + 0.015 * SS-0.013 * RCS Relationship 1 Remarks One 0.15 0.8 800 300 10.399 O Inventive Steel 1 800 400 9.099 X Comparative River 1 2 0.17 0.6 810 300 10.187 O Invention river 2 3 0.17 0.6 820 300 10.337 O Invention steel 3 4 0.19 0.5 800 270 10.283 O Inventive Steel 4 5 0.14 0.9 810 270 11.12 O Invention steel 5 820 420 9.32 X Comparative River 2 6 0.15 0.7 810 270 10.721 O Invention steel 6 7 0.13 0.9 810 300 10.693 O Invention steel 7 810 370 9.783 X Comparative Steel 3 8 0.16 0.6 800 300 10 O Inventive Steel 8 800 350 9.35 X Comparative Steel 4 800 340 9.48 X Comparative Steel 5 9 0.14 0.7 800 300 10.144 O Invention river 9 790 450 8.044 X Comparative Steel 6 750 300 9.394 X Comparative Steel 7 800 340 9.624 X Comparative Steel 8 780 300 9.844 X Comparative Steel 9 10 0.1 0.9 800 300 10.432 O Comparative Steel 10 11 0.14 0.2 800 300 9.054 X Comparative Steel 11 12 0.2 0.7 810 310 10.386 O Comparative Steel 12 13 0.16 0.9 810 310 10.674 O Comparative Steel 13 14 0.14 0.6 810 310 9.946 O Comparative Steel 14 15 0.15 1.5 800 310 11.795 O Comparative Steel 15

Steel number YS
(Yield strength) TS
(The tensile strength) Hand
(Elongation) YR
(Yield ratio) M
packet size M + TM F + B Remarks One 1081 1353 7.1 0.8 1.1 98 2 Inventive Steel 1 928 1331 10.5 0.7 2.5 89 11 Comparative River 1 2 1091 1389 7.5 0.79 1.2 97 3 Invention river 2 3 1099 1402 7.1 0.78 1.1 96 4 Invention steel 3 4 1079 1385 8.2 0.78 1.5 98 2 Inventive Steel 4 5 1106 1396 6.2 0.79 1.4 96 4 Invention steel 5 876 1325 10.7 0.66 2.7 88 12 Comparative River 2 6 1086 1369 6.9 0.79 1.2 97 3 Invention steel 6 7 1069 1358 7.2 0.79 1.2 98 2 Invention steel 7 952 1321 10.2 0.72 2.5 91 9 Comparative Steel 3 8 1094 1421 6.9 0.77 1.5 98 2 Inventive Steel 8 935 1352 8.9 0.69 2.4 92 8 Comparative Steel 4 950 1369 9.2 0.69 2.3 94 6 Comparative Steel 5 9 1067 1368 7.5 0.78 1.3 95 5 Invention river 9 831 1269 11.2 0.65 2.3 83 17 Comparative Steel 6 969 1317 8.9 0.74 2.3 90 10 Comparative Steel 7 965 1328 7.1 0.73 2.2 91 9 Comparative Steel 8 881 1298 10.3 0.68 2.5 84 16 Comparative Steel 9 10 969 1205 8.9 0.8 2.1 93 7 Comparative Steel 10 11 987 1264 7.8 0.78 2.6 93 7 Comparative Steel 11 12 969 1678 6.2 0.58 1.9 90 10 Comparative Steel 12 13 893 1305 7.1 0.68 2.3 87 13 Comparative Steel 13 14 1036 1596 3.1 0.65 2.2 95 5 Comparative Steel 14 15 1102 1496 3.2 0.74 1.8 96 4 Comparative Steel 15

As shown in Tables 1 to 3, in the inventive steels (1 to 9) satisfying the component range and the manufacturing conditions of the present invention, the annealing temperature and the secondary cooling end temperature It was possible to produce a high yield strength and high strength martensite steel having a yield strength of at least 1000 MPa, a tensile strength of 1300 MPa or more, a yield ratio of 0.77 or more, and an elongation of 6% or more.

Fig. 1 shows the microstructure of steel 1 according to the present invention in which steel of steel number 1 was produced under the conditions of an annealing temperature of 800 캜 and a secondary cooling end temperature (RCS) of 300 캜. The microstructure was martensite and tempered martensite (M + TM). ≪ / RTI > Such a structure is a very favorable condition for securing an ultrahigh strength steel having a yield strength of 1000 MPa or more and a yield ratio of 0.77 or more.

On the other hand, FIG. 2 shows the microstructure of comparative steel 1 in which the steel of steel number 1 was produced under conditions of an annealing temperature of 800 ° C. and a secondary cooling end temperature (RCS) of 400 ° C., and the microstructure was martensite and tempered martensite structure (M + TM) as well as granular bainite (GB), which is a high-temperature microstructure. Therefore, as shown in Table 2, the material can be a steel having a low yield ratio with a yield strength of 1000 MPa or less. Therefore, it is very important to control not only the chemical composition but also the annealing temperature and the secondary cooling end temperature in order to secure the material of the invention steel.

In other words, even if the composition conditions of the inventive steel are satisfied, when the annealing temperature and the secondary cooling end temperature do not satisfy the relational expression 1 shown in the present invention, the yield strength is as low as 1000 MPa or less, . This is because of the generation of ferrite in the steel or the formation of a high temperature transformation phase such as granular bainite.

In the case of comparative steels 5, 8 and 9, it satisfies the composition conditions of the present invention, the annealing temperature 770 to 830 캜 and the second cooling finishing temperature 250 to 340 캜, but does not satisfy the condition of the formula 1, And the yield ratio was low, so that the characteristics of the present invention were not satisfied.

In addition, the comparative steels 10 to 15 did not satisfy the component ranges proposed in the present invention, and even if the relation 1 was satisfied, the elongation rate decreased with an excessive increase in strength.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (11)

0.1 to 0.10% of C, 0.3% or less of Si (excluding 0), 2.5 to 3.0% of Mn, 0.001 to 0.10% of P, , Cr: 0.5-1.0%, B: 0.0010-0.0050%, N: 0.01% or less (excluding 0), Ti: 0.01-0.05%, Nb: 0.01-0.05%, the balance Fe and other unavoidable impurities,
The ultra-high strength cold-rolled steel sheet excellent in shape-formability including 90% by area or more of martensite and tempered martensite.
The method according to claim 1,
Wherein the microstructure includes at least one of ferrite and bainite in a total fraction of 10% by area or less.
The method according to claim 1,
A super-high strength cold-rolled steel sheet excellent in shape crystallinity having a martensite packet size of 1.5 m or less in the microstructure.
The method according to claim 1,
The cold-rolled steel sheet has excellent yield strength of 1000 MPa or more, yield ratio of 0.77 or more, and tensile strength of 1300 MPa or more.
The method according to claim 1,
Wherein said cold-rolled steel sheet has excellent elongation of not less than 6%.
0.1 to 0.10% of C, 0.3% or less of Si (excluding 0), 2.5 to 3.0% of Mn, 0.001 to 0.10% of P, , Fe: 0.5 to 1.0%, Cr: 0.0010 to 0.0050%, N: 0.01% or less (excluding 0), Ti: 0.01 to 0.05%, Nb: 0.01 to 0.05%, and the balance Fe and other unavoidable impurities Heating the slab;
Hot-rolling the heated steel slab to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet;
A step of cold-rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet;
Continuously annealing the cold-rolled steel sheet at an annealing temperature of 770 ° C to 830 ° C;
Cooling the continuously annealed cold rolled steel sheet to a temperature of 650 to 700 占 폚 at a cooling rate of 1 to 10 占 폚 / sec;
Cooling the primary cold-rolled steel sheet to 250 to 340 ° C at a cooling rate of 5 to 20 ° C / sec and subjecting the cold- Lt; / RTI >
A method for producing an ultra-high strength cold rolled steel sheet excellent in shape-formability satisfying the following condition (1).

[Relation 1] 9.9? 3.7 * C + 2.18 * Cr + 0.015 * SS-0.013 * RCS? 12.1
(Wherein C and Cr are respectively the contents (weight%) of the elements, SS is the annealing temperature (占 폚), and RCS is the secondary cooling end temperature (占 폚).
The method according to claim 6,
Wherein the step of obtaining the hot-rolled steel sheet is performed so that the temperature at the finish-rolling-exit side becomes 800 to 950 占 폚.
The method according to claim 6,
Wherein the winding step is performed at a temperature in the range of 500 to 750 占 폚.
The method according to claim 6,
Wherein the step of obtaining the cold-rolled steel sheet is a method of producing an ultra-high-strength cold-rolled steel sheet having excellent shape-formability at a reduction ratio of 40 to 70%.
The method according to claim 6,
Wherein the primary cooling and the secondary cooling are excellent in shape mobility performed by a mist cooling method.
The method according to claim 6,
And the skin pass rolling is performed at a reduction ratio of 0.1 to 1.0% after the above-mentioned overexposure treatment step.

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