KR101968001B1 - Giga grade ultra high strength cold rolled steel sheet having excellent elongation and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a super high strength cold rolled steel sheet having excellent elongation and a manufacturing method thereof.
An embodiment of the present invention is a steel sheet comprising, by weight, 0.15 to 0.25% of C, 7 to 11% of Mn, 1 to 2.5% of Al, 2.5% or less of Si (including 0% P: not more than 0.03%, N: not more than 0.01%, and the balance Fe and other unavoidable impurities. The microstructure has an area fraction of 20 to 35% of austenite, 20% Martensite, and the content of C in the austenite is 0.20 to 1.25% by weight, and a process for producing the same.

Description

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

The present invention relates to a super high strength cold rolled steel sheet having excellent elongation and a manufacturing method thereof.

One of the most important factors in energy saving and environmentally friendly automobile development is the weight reduction of the vehicle body. To this end, automobile companies and steel makers in advanced countries are studying the development of high strength steel materials. Among these advanced high-strength steels, the market for automotive high-grade steels is divided into cold-rolled high-strength plates and hot press forming steels (hereinafter referred to as HPF steels).

BACKGROUND ART Automobile plate materials are classified into a dual phase steel (hereinafter referred to as DP steel), a complex phase steel (hereinafter referred to as CP steel), a Transformation Induced Plasticity Steel Steel), and martensitic steel (hereinafter referred to as MART steel). At present, DP steel produces 980MPa class, CP steel, and TRIP steel produces 1180MPa strength products. In the 1470MPa class, only MART steel and HPF steel are used. The MART steel is made of light martensite with solid carbon, so it has high strength enough to meet the 1470 MPa class, but it can be processed only by roll forming due to the low elongation of about 5% Of the parts.

However, as a measure to reduce the weight of the vehicle, the demand for plate materials having a strength of 1470 MPa has increased, and a high temperature molding method such as HPF steel has been developed to be applied to various parts. In the current situation where high temperature molding is the only method to use ultra high strength cold rolled sheet of 1470MPa grade, HPF steel market share is expanding, but the energy consumption by heating process, productivity by process time, and zinc plating There is a problem such as occurrence of micro cracks accompanying with the above. Therefore, when cold-working of 1470MPa super high strength cold rolled steel sheet is possible, it is expected to reduce the process cost and improve the productivity, so it is possible to apply 1470MPa grade plate to more parts of the body. In order to develop a high-strength cold-rolled steel sheet of 1470 MPa in order to achieve this, the US Department of Energy is developing a steel sheet having a tensile strength and an elongation of 1200 MPa / 30% and 1500 MPa / 25%, respectively (see Patent Document 1 ).

Recently, a technique for forming a martensite and austenite microstructure by utilizing Q & P (Quenching & Partitioning) annealing as an approach for manufacturing such an ultra-high strength cold rolled steel sheet has been disclosed in Patent Document 1. However, And there is a problem due to the relatively low strength increase effect compared to the existing TRIP steel.

In order to compensate for this increase in strength, Patent Document 2 discloses a technique for securing excellent ductility with a finer structure than conventional Q & P annealing by applying a two-step high temperature heat treatment (high temperature annealing in one step and annealing in high temperature Q & P in two steps) . However, the above problem has not been solved by adding a new annealing process while keeping the existing Q & P annealing problem intact.

U.S. Patent Application Publication No. 2006-0011274 Korean Patent Laid-Open Publication No. 2015-0130612

 Proceedings of International Symposium on New Developments in Advanced High-Strength Steels, 2013, p. 341-349

One aspect of the present invention is to provide a gigantic ultra high strength cold rolled steel sheet having excellent elongation and a manufacturing method thereof.

An embodiment of the present invention is a steel sheet comprising, by weight, 0.15 to 0.25% of C, 7 to 11% of Mn, 1 to 2.5% of Al, 2.5% or less of Si (including 0% P: not more than 0.03%, N: not more than 0.01%, and the balance Fe and other unavoidable impurities. The microstructure has an area fraction of 20 to 35% of austenite, 20% Martensite, and the content of C in the austenite is 0.20 to 1.25% by weight.

In another embodiment of the present invention, there is provided a steel sheet comprising, by weight%, 0.15 to 0.25% of C, 7 to 11% of Mn, 1 to 2.5% of Al, 0.03% or less of P, 0.01% or less of N, the balance Fe and other unavoidable impurities at 1050 to 1250 占 폚; Hot-rolling the reheated steel slab at 850 to 950 ° C to obtain a hot-rolled steel sheet; Annealing the hot-rolled steel sheet at a temperature of 550 to 750 ° C; A step of cold-rolling the hot-rolled steel sheet annealed to obtain a cold-rolled steel sheet; Annealing and cold-rolling the cold-rolled steel sheet at 750 to 950 ° C; And a step of carbon-distributing and annealing the cold-rolled sheet annealed and accelerated-cooled cold-rolled sheet at 150 to 300 ° C.

According to an aspect of the present invention, it is possible to produce a super high strength cold rolled steel sheet having a yield strength of 800 MPa or higher, a tensile strength of 1400 MPa or higher, and an elongation of 15% or higher.

Hereinafter, the present invention will be described in detail. First, the alloy composition of the present invention will be described.

C: 0.15 to 0.25 wt%

C is an austenite stabilizing element which controls the austenite single phase region temperature (Ae3 temperature) and martensite formation starting temperature (Ms temperature) according to the amount of C, and asymmetric distortion in the lattice structure on the martensite And it is very effective in securing strong strength. In order to obtain such effects, it is preferable to add 0.15% by weight or more. However, when the content exceeds 0.25% by weight, there is a disadvantage that the ductility is reduced due to the formation of carbides and weldability is deteriorated. Therefore, C is preferably in the range of 0.15 to 0.25% by weight.

Mn: 7 to 11 wt%

Mn is known as an element which increases hardenability and stabilizes austenite. Thus lowering the Ae3 temperature to serve to stabilize the austenite phase at low temperatures and contribute to lowering the Ms temperature. If the Mn content is less than 7% by weight, the above-mentioned effect is insufficient and there is a problem in securing an austenite phase at room temperature. On the other hand, when the Mn content exceeds 11 wt%, the production cost increases and the stability of the austenite increases excessively, which makes it difficult to secure the martensite phase contributing to the improvement of the strength. Therefore, it is preferable that the Mn has a range of 7 to 11% by weight.

Al: 1 to 2.5 wt%

Al is an element that enlarges the ferrite region and increases the stacking defect energy and contributes to the transformation mechanism of the austenite phase to the formation of martensite transformation or twinning. Therefore, the formation of ε (epsilon) -martensite, which has little effect on the improvement of strength, is inhibited and induces the formation of α '(alpha prime) -martensite phase having a large strength enhancement effect. It is also very effective in increasing the Ae3 temperature, which is lowered by the addition of Mn, which enables a short time heat treatment at a high temperature due to the increased Ae3 temperature. When the content of Al is 1 wt% or more, the above-mentioned effect can be sufficiently obtained. When the Al content is more than 2.5 wt%, the Ae3 temperature is excessively increased, and the change of the deformation mechanism due to the increase of the stacking defect energy, do. Therefore, it is preferable that the above-mentioned Al has a range of 1 to 2.5 wt%.

Si: 2.5 wt% or less (including 0%)

Si is added in order to enhance strength with the goal of solid solution strengthening effect. In addition, the present invention plays a role in suppressing the formation of carbide, and it plays a role of suppressing the formation of carbide during the carbon distribution annealing and distributing the carbon on the martensite continuously to the austenite phase. However, as Si is a ferrite stabilizing element, when the content exceeds 2.5% by weight, Ae3 temperature rise and cracking occur during cold rolling. In order to suppress this, it is preferable that the Si content does not exceed 2.5% by weight.

S: not more than 0.015% by weight

When S is an impurity element and the content thereof exceeds 0.015% by weight, the ductility and weldability of steel deteriorate. Therefore, it is preferable to control the content to 0.015 wt% or less.

P: not more than 0.03% by weight

P is an element for enhancing solubility, but if it is more than 0.03% by weight as an impurity element like S, brittleness occurs in the steel and weldability is deteriorated. Therefore, it should be limited to 0.03% by weight or less.

N: not more than 0.01% by weight

N is known as an austenite stabilizing element, but when it exceeds 0.01% by weight, AlN is formed to increase the risk of cracking during casting. Therefore, it is preferable to control the content to 0.01 wt% or less.

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.

In the cold-rolled steel sheet of the present invention, the microstructure preferably contains 20 to 35% of austenite and 20% or less of ferrite and the balance martensite in an area fraction. The base structure of the present invention is preferably martensite, and the asymmetric lattice distortion of the carbon dissolved in the martensite contributes to the yield strength and the tensile strength, which are high in the strong solid solution strengthening effect. The cold-rolled steel sheet of the present invention preferably contains 20 to 35% by area of austenite. If the austenite is less than 20% by area, the effect of the work hardening due to the austenite transformation is limited, so that the tensile strength and the elongation can be reduced. On the other hand, when the austenite is more than 35% by area, The strength and tensile strength may decrease. On the other hand, the microstructure of the cold-rolled steel sheet of the present invention is preferably composed of austenite: 20 to 35% by area and the remainder martensite, but it may include ferrite which is inevitably formed in the manufacturing process. % Or less. When the content of the ferrite exceeds 20% by area, there may arise a problem that ferrite as a soft phase hinders improvement in yield strength and tensile strength. The ferrite is more preferably 10 percent by area or less, more preferably 5 percent by area or less.

In the cold-rolled steel sheet of the present invention, the content of C in the austenite is preferably 0.20 to 1.25% by weight. C in the austenite has an unstable state after accelerated cooling and maintains a stable state due to carbon moving during carbon distribution annealing and contributes to high yield strength and elongation. When the content of C in the austenite is less than 0.20 wt%, the stability of austenite can not be sufficiently secured. When the amount of C is more than 1.25 wt%, the effect of martensite transformation during deformation due to excessive stability can not be obtained. It also causes problems in securing strength. Therefore, the C content in the austenite is preferably in the range of 0.20 to 1.25 wt%.

In the cold-rolled steel sheet of the present invention, the C content in the post-austenite before and after the carbon distribution annealing preferably satisfies the following relational expression (1). The supersaturated carbon inside the martensite diffuses into the austenite during carbon distribution annealing. Through the diffusion of carbon through the carbon distribution annealing, the mechanical properties desired to be obtained by the present invention can be secured. For the above effect, it is preferable that the carbon content in the austenite after the carbon distribution annealing is higher than that before the carbon distribution annealing by 0.05 wt% or more. If it is less than 0.05% by weight, the effect of increasing the yield strength due to an increase in the stability of austenite may not be secured. In the present invention, the upper limit to the C content difference in the post-austenite before and after the carbon distribution annealing is not particularly limited. However, when the C content difference between the austenite and the post-austenite before and after the carbon distribution annealing is more than 1.25% by weight, it may be difficult to secure the employment strengthening effect due to the carbon depletion in the martensite.

[Relation 1] (C content in austenite after carbon distribution annealing) - (C content in austenite before carbon distribution annealing)? 0.05 wt%

On the other hand, in order to obtain the above-described microstructure, the alloy composition proposed by the present invention, as well as Mf (占 폚) expressed by the following relational expression 3 and satisfying the condition of Ms (占 폚) Preferably satisfies the condition of -79 to 71 占 폚. When Ms (占 폚) and Mf (占 폚) do not satisfy the following relational expressions 2 and 3, it is difficult to obtain the target microstructure of the present invention, and it may be difficult to secure excellent strength and elongation. In the present invention, the addition of the alloying element other than the above-mentioned alloying element is not excluded, but it is preferable to include the additional alloying element so as to satisfy the following relational expressions 2 and 3.

[Relation 2] Ms (占 폚) = 563 - 817C - 28.5Mn - 5.9Si + 16.6Al

[Relation 3] Mf (占 폚) = Ms-209

[Wherein C, Mn, Si and Al are values indicating the content of each element in weight%, and Mf is a temperature at which the fraction of martensite reaches 90%

The cold-rolled steel sheet of the present invention provided as described above can have a yield strength of 800 MPa or more, a tensile strength of 1400 MPa or more, and an elongation of 15% or more.

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

The steel slab having the above-described alloy composition is reheated at 1050 to 1250 占 폚. If the reheating temperature is less than 1050 ° C, the rolling load may excessively take over. If the reheating temperature is higher than 1250 ° C, internal oxidation and scale may be generated severely. Therefore, the reheating temperature may range from 1050 to 1250 ° C desirable.

The reheated steel slab is hot-rolled at 850 to 950 ° C to obtain a hot-rolled steel sheet. When the hot rolling temperature is higher than 950 ° C, the rolling load is small and the rolling is easy, but the surface quality deterioration due to the scale problem occurs. On the other hand, when the hot rolling temperature is lower than 850 ° C, Lt; RTI ID = 0.0 > 950 C. < / RTI >

The hot-rolled steel sheet is hot-rolled at 550 to 750 ° C. The hot-rolled steel sheet of the present invention has a high hardenability and is composed of 30% or less of an austenite phase and 70% or more of martensite phase in an area fraction. Therefore, due to the high strength, the rolling load increases during cold rolling and causes an edge crack. Annealing heat treatment is performed to prevent this. When the annealing temperature of the hot-rolled sheet exceeds 750 ° C, the formation of the austenite phase is excessive and there is a possibility of forming a martensite phase upon cooling. When the annealing temperature is less than 550 ° C, the austenite phase is insufficient, The hot-rolled sheet annealing temperature preferably has a temperature range of 550 to 750 ° C.

The hot-rolled steel sheet annealed in the hot-rolled sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio in cold rolling is preferably 50% or less. If the reduction rate exceeds 50%, it may cause edge cracking of the cold-rolled steel sheet. On the other hand, before the cold rolling, a pickling process for removing the oxide scale can be additionally performed.

The cold-rolled steel sheet is subjected to cold-rolled sheet annealing and accelerated cooling at 750 to 950 ° C. The annealing temperature of the cold-rolled sheet is preferably 750 ° C or higher at which the martensite phase can be generated upon cooling after annealing. When the annealing temperature is too high, the amount of martensite phase is excessively generated, . The cooling rate during the accelerated cooling of the cold-rolled steel sheet annealed in the cold-rolled steel sheet is preferably 10 ° C / s or more for securing the tensile strength through the formation of a stable martensite phase. On the other hand, when the cooling rate exceeds 100 ° C / s, excessive stress may be caused to the accelerated cooling facility. It is preferable that the termination temperature at the time of cooling is 25 DEG C or less which is the standard temperature.

The cold-rolled sheet annealed and accelerated cooled is subjected to carbon distribution annealing at 150 to 300 ° C. The above-mentioned carbon distribution annealing is a main purpose of carbon distribution. The carbon distribution on the martensite phase is shifted to the austenite phase through the carbon distribution annealing to increase the stability of the austenite phase during tensile deformation, thereby improving the yield strength and rapidly producing the modified martensite. So that the effect of improving the elongation can be secured. If the carbon distribution annealing temperature is lower than 150 ° C, the carbon diffusion rate is not sufficiently secured and the effect of increasing the elongation can not be obtained. If the temperature is higher than 300 ° C, the carbon concentration decreases due to the carbide formation inside the martensite phase and carbon distribution Strength can not be assured.

On the other hand, the annealing of the cold rolled sheet is preferably performed for 1 to 60 minutes. If the annealing time of the cold-rolled sheet is less than 1 minute, a uniform temperature distribution in the specimen can not be obtained and the structure may be uneven. If it exceeds 60 minutes, the austenite fraction may be secured There may be severe drawbacks. The carbon distribution annealing is preferably performed for 10 to 2,880 minutes. If the carbon distribution annealing time is less than 10 minutes, there is a disadvantage that the carbon diffusion is not properly performed. If the carbon distribution annealing time is more than 2880 minutes, there may be a disadvantage such as a decrease in elongation due to excessive carbide formation.

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)

The steel slab having the alloy composition shown in the following Table 1 was reheated at 1100 캜 for 2 hours, and then the steel slab was hot-rolled at 900 캜 to obtain a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was hot-rolled and annealed at 650 ° C. Thereafter, cold rolled steel sheets were obtained by cold rolling at a pickling and reduction ratio of 50%, and then cold rolled sheets were annealed under the conditions shown in Table 2 below. Then, carbon distribution annealing was further carried out under the conditions shown in the following Table 2, and then the carbon content and mechanical properties of the austenite before and after the microstructure, carbon distribution annealing and the mechanical properties were measured and the results are shown in Table 3 below.

The mechanical properties were measured using a tensile tester at a crosshead speed of 0.1 cm / min by making a tensile specimen having a gage length of 2.5 cm and a gauge width of 0.6 cm using the ASTM E8M standard. The yield strength was measured with an offset 0.2% method and the tensile strength was measured with a tensile tester. The elongation was measured using a 2.5-cm elongation meter. Microstructure was measured using X-ray diffraction analysis.

division
Alloy composition (% by weight) Ms (占 폚) Mf (占 폚) Thickness (mm) C Mn Al Si S P N Inventive Steel 1 0.21 10.2 1.94 - 0.007 0.0001 0.0003 132.93 -76.07 0.2 Invention river 2 0.2 9.888 2.06 2.01 0.006 0.0001 0.0001 140.36 -68.64 0.2 Invention steel 3 0.18 8.03 2.18 1.08 0.002 0.0001 0.0001 216.9 7.90 0.15 Comparative River 1 0.2 10.33 2.06 2.01 0.003 0.0001 0.0001 127.53 -81.47 0.2 Comparative River 2 0.18 5.25 2.05 1.03 0.002 0.0001 0.0003 294.27 85.27 0.15 Ms (占 폚) = 563 - 817C - 28.5Mn - 5.9Si + 16.6Al, Mf (占 폚) = Ms - 209

division Grade Nr. Cold-rolled sheet annealing Accelerated Cooling Rate
(° C / s) Cooling end temperature
(° C) Carbon distribution annealing Temperature (℃) Time (minutes) Temperature (℃) Time (minutes) Inventory 1 Inventive Steel 1

750 60 50 25 200 20 Inventory 2 850 3 20 25 200 20 Inventory 3 900 3 20 25 200 20 Honorable 4 950 3 20 25 200 20 Inventory 5 Invention river 2
900 3 20 25 300 20 Inventory 6 900 3 20 25 300 720 Honorable 7 900 3 20 25 300 1440 Honors 8 950 3 20 25 300 20 Proposition 9 Invention steel 3 800 30 50 25 200 20 Comparative Example 1 Inventive Steel 1
650 60 50 25 - - Comparative Example 2 700 60 50 25 - - Comparative Example 3 725 60 50 25 200 20 Comparative Example 4 750 60 50 25 - - Comparative Example 5 850 3 20 25 - - Comparative Example 6 900 3 20 25 - - Comparative Example 7 950 3 20 25 - - Comparative Example 8 Invention river 2 850 3 20 25 - - Comparative Example 9 900 3 20 25 - - Comparative Example 10 950 3 20 25 - - Comparative Example 11 725 60 50 25 300 20 Comparative Example 12 Comparative River 1 850 3 20 25 300 20 Comparative Example 13 900 3 20 25 300 20 Comparative Example 14 950 3 20 25 300 20 Comparative Example 15 Comparative River 2 1100 5 50 25 100 30 Comparative Example 16 1100 5 50 25 200 30

division Microstructure (area%) Mechanical properties C content in austenite (% by weight) Oster
Night Marten
site ferrite Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) carbon
Distribution
Before annealing carbon
Distribution
After annealing Inventory 1 23 74 3 997 1515 19.5 0.19 0.31 Inventory 2 32 61 7 870 1499 20.2 0.21 0.3 Inventory 3 24 74 2 833 1473 19 0.21 0.35 Honorable 4 26 74 0 846 1455 18.5 0.21 0.33 Inventory 5 28 71 One 981 1498 19.4 0.2 0.42 Inventory 6 31 68 One 946 1468 24.5 0.2 0.5 Honorable 7 29 70 One 999 1453 28.5 0.2 0.55 Honors 8 22 78 0 984 1500 21.5 0.2 0.37 Proposition 9 26 57 17 873 1400 15 0.18 0.28 Comparative Example 1 60 0 40 909 1011 56.5 0.36 0.36 Comparative Example 2 75 0 25 542 1335 16.3 0.25 0.25 Comparative Example 3 35 44 21 715 1532 21.6 0.23 0.27 Comparative Example 4 23 74 3 849 1685 8 0.19 0.19 Comparative Example 5 32 61 7 566 1324 3 0.21 0.21 Comparative Example 6 24 74 2 576 1396 3.2 0.21 0.21 Comparative Example 7 26 74 0 599 1308 2.7 0.21 0.21 Comparative Example 8 47 29 24 283 1547 8.6 0.2 0.2 Comparative Example 9 28 71 One 489 1212 2.3 0.2 0.2 Comparative Example 10 22 78 0 577 1266 2.2 0.2 0.2 Comparative Example 11 55 21 24 584 1470 16.6 0.2 0.21 Comparative Example 12 53 21 26 446 1672 22.4 0.2 0.21 Comparative Example 13 40 54 6 787 1549 22 0.2 0.24 Comparative Example 14 36 64 0 750 1485 20.7 0.2 0.24 Comparative Example 15 One 99 0 944 1374 10.7 0.18 0.18 Comparative Example 16 One 99 0 921 1304 11.3 0.18 0.18

As can be seen from Tables 1 to 3, in Examples 1 to 9, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, the microstructure, the C content in the austenite of the present invention and the content of austenite (C content in the austenite) - (C content in the austenite before carbon distribution)? 0.05 wt.%, It is understood that the yield strength is 800 MPa or more, the tensile strength is 1400 MPa or more and the elongation is 15% or more.

On the other hand, the alloy composition of the present invention is satisfactory, but in the case of Comparative Examples 1 to 3 which do not satisfy the cold-rolled sheet annealing temperature conditions of the present invention, the microstructure fraction proposed by the present invention is not satisfied, The yield strength or the tensile strength is low.

Comparative Examples 4 to 7 are the cases where the alloy composition of the present invention and the cold-rolled sheet annealing temperature conditions are satisfied but the carbon distribution annealing treatment is not carried out. Even though the microstructure proposed by the present invention is secured, the C content in the austenite and (C content in the austenite after the carbon distribution) - (C content in the austenite before the carbon distribution)? 0.05 wt%, it is found that the mechanical properties desired to be obtained by the present invention are not obtained.

Comparative Example 8 is a case where the alloy composition of the present invention and the cold-rolled sheet annealing temperature condition are satisfied but the carbon distribution annealing treatment is not performed, and the microstructure fraction, the C content in the austenite and the austenite content C content) - (C content in austenite before carbon distribution) ≥ 0.05 wt%, it is understood that the mechanical properties desired to be obtained by the present invention are not obtained.

Comparative Examples 9 and 10 are the cases where the alloy composition of the present invention and the cold-rolled sheet annealing temperature conditions are satisfied but the carbon distribution annealing treatment is not carried out. Even though the microstructure suggested by the present invention is secured, the C content in the austenite and (C content in the austenite after the carbon distribution) - (C content in the austenite before the carbon distribution)? 0.05 wt%, it is found that the mechanical properties desired to be obtained by the present invention are not obtained.

In Comparative Example 11, the alloy composition of the present invention is satisfied but the annealing temperature condition of the cold-rolled sheet is not satisfied. Since the microstructure fraction proposed by the present invention is not satisfied, the mechanical properties desired to be obtained by the present invention are not obtained Able to know.

In the case of Comparative Examples 12 to 14, the production conditions of the present invention are satisfied, but the microstructure fraction, the C content in the austenite, and the C content in the austenite after carbon distribution, proposed by the present invention because the alloy composition of the present invention is not satisfied, (C content in the austenite before carbon distribution) ≥ 0.05 wt%, the yield strength is low.

In Comparative Examples 15 and 16, the microstructure fraction, the C content in the austenite, and the C content in the austenite after the carbon distribution, proposed by the present invention, as well as the alloy composition of the present invention, The C content in the kneading) ≥ 0.05% by weight, the tensile strength and elongation are low.

Claims (10)

(%), S: not more than 0.015%, P: not more than 0.03%, N: not more than 0.03%, C: not more than 0.15% : 0.01% or less, the balance Fe and other unavoidable impurities,
The microstructure has an area fraction of 20 to 35% of austenite, 20% or less of ferrite (including 0%) and residual martensite,
The C content in the austenite is 0.20 to 1.25 wt%
The C content in the post-austenite before and after the carbon distribution annealing satisfies the following relational expression 1,
A super high strength cold rolled steel sheet excellent in elongation satisfying Ms (占 폚) of 130 to 280 占 폚 expressed by the following relational formula 2 and having Mf (占 폚) expressed by the following relational expression 3 satisfies -79 to 71 占 폚.
[Relation 1] (C content in austenite after carbon distribution annealing) - (C content in austenite before carbon distribution annealing)? 0.05 wt%
[Relation 2] Ms (占 폚) = 563 - 817C - 28.5Mn - 5.9Si + 16.6Al
[Relation 3] Mf (占 폚) = Ms-209
[Wherein C, Mn, Si and Al are values indicating the content of each element in weight%, and Mf is a temperature at which the fraction of martensite reaches 90%
delete delete The method according to claim 1,
The cold-rolled steel sheet has excellent elongation with a yield strength of 800 MPa or more, a tensile strength of 1400 MPa or more, and an elongation of 15% or more.
(%), S: not more than 0.015%, P: not more than 0.03%, N: not more than 0.03%, C: not more than 0.15% : 0.01% or less, remainder Fe and other unavoidable impurities at 1050 to 1250 캜;
Hot-rolling the reheated steel slab at 850 to 950 ° C to obtain a hot-rolled steel sheet;
Annealing the hot-rolled steel sheet at a temperature of 550 to 750 ° C;
A step of cold-rolling the hot-rolled steel sheet annealed to obtain a cold-rolled steel sheet;
Annealing and cold-rolling the cold-rolled steel sheet at 750 to 950 ° C; And
And subjecting the cold-rolled steel sheet annealed and accelerated to cold carbon annealing at 150 to 300 ° C,
In the cold-rolled steel sheet subjected to the carbon distribution annealing, the C content in the post-austenite before and after the carbon distribution annealing satisfies the following relational expression 1,
A method for producing a super high strength cold rolled steel sheet excellent in elongation satisfying Ms (占 폚) expressed by the following relational formula 2 satisfying 130 to 280 占 폚 and Mf (占 폚) expressed by the following relational expression 3 satisfying -79 to 71 占 폚 .
[Relation 1] (C content in austenite after carbon distribution annealing) - (C content in austenite before carbon distribution annealing)? 0.05 wt%
[Relation 2] Ms (占 폚) = 563 - 817C - 28.5Mn - 5.9Si + 16.6Al
[Relation 3] Mf (占 폚) = Ms-209
[Wherein C, Mn, Si and Al are values indicating the content of each element in weight%, and Mf is a temperature at which the fraction of martensite reaches 90%
The method of claim 5,
Wherein the cold rolling is performed at a reduction ratio of 50% or less.
The method of claim 5,
Wherein the accelerated cooling is performed at a cooling rate of 10 DEG C / s or more.
The method of claim 5,
Wherein the cold-rolled sheet annealing is performed for 1 to 60 minutes, and the elongation rate is excellent.
The method of claim 5,
Wherein the cooling end temperature during the accelerated cooling is 25 占 폚 or less.
The method of claim 5,
Wherein the carbon-distributed annealing is performed for 10 to 2,880 minutes, and the elongation rate is excellent.