KR20190076789A - Steel sheet having ultra high strength and superior ductility and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a steel sheet having an ultrahigh strength and a superior ductility and a method of manufacturing the same. According to an embodiment of the present invention, the steel sheet having an ultrahigh strength and a superior ductility comprises: 0.1-0.4 wt% of C; 2.0 wt% or less of Si (including 0 wt%); 2.0 wt% or less of Al (excluding 0 wt%); 3.5-7.5 wt% of Mn; 0.05 wt% or less of P (excluding 0 wt%); 0.02 wt% or less of S (excluding 0 wt%); and 0.02 wt% or less of N (excluding 0 wt%). The Si and the Al satisfy the conditions of the below formula 1. A micro-structure includes: 5 area% or more of delta ferrite; 10 area% or more of residual austenite; 40 area% or less of annealed martensite; and 20 area% or less of alpha martensite and epsilon martensite. [Formula 1] (Si + 5 Al) >= 4.5 (But, the content of the Si and Al is wt%.)

Description

TECHNICAL FIELD [0001] The present invention relates to a super high strength high ductility steel sheet and a method of manufacturing the same,

The present invention relates to an ultra-high strength high ductility steel sheet and a manufacturing method thereof.

In order to reduce the weight of automotive steel sheet, the thickness of the steel sheet must be lowered. On the other hand, there is an inconsistent aspect that the thickness of the steel sheet must be increased to secure collision safety. In order to solve this problem, it is necessary to increase the moldability while increasing the strength of the material. This is because the dual phase steel (hereinafter referred to as "DP steel") known as AHSS (Advanced High Strength Steel) Inducted Plasticity Steel (hereinafter referred to as "TRIP steel"), and Complex Phase Steel (hereinafter referred to as "CP steel"). The strength of the advanced high strength steel can be increased by increasing the amount of carbon or alloy, but in this case, the elongation required for forming the steel is lowered.

As techniques for solving such problems, Patent Documents 1 and 2 are known. Patent Document 1 is a steel containing 3.5 to 9% of Mn and shows a very good physical property with a product of a tensile strength and a total elongation of 30,000 MPa or more. However, in order to realize the above properties, heat treatment must be performed for 1 hour or more It is not easy to apply to the continuous annealing process. Patent Document 2 describes a steel containing 2 to 9% of Mn and a method of realizing a high yield strength and a tensile strength. However, since the physical properties must be realized through warm-forming, the process cost may increase.

CN 101638749B CN 103060678A

An aspect of the present invention is to provide an ultra-high strength high ductility steel sheet having an ultra high strength and a good product of a tensile strength and an elongation, and a method for manufacturing the same.

An embodiment of the present invention is a steel sheet comprising, by weight%, 0.1 to 0.4% of C, 2.0% or less of Si (including 0%), 2.0% or less of Al (excluding 0%), 3.5 to 7.5% of Mn, , Si: not more than 0.02% (excluding 0%), N: not more than 0.02% (excluding 0%), Si and Al satisfy the following condition (1) Provided is an ultra high strength high ductility steel sheet comprising an area fraction of 5% or more of delta ferrite, 10% or more of retained austenite, 40% or less of annealed martensite, and 20% or less of alpha martensite and epsilon martensite.

[Relation 1] (Si + 5Al)? 4.5 (provided that the content of Si and Al is% by weight)

In another embodiment of the present invention, there is provided a steel sheet comprising, by weight%, 0.1 to 0.4% of C, 2.0% or less of Si (including 0%), 2.0% (Excluding 0%), S: not more than 0.02% (excluding 0%), N: not more than 0.02% (excluding 0%), and Si and Al satisfy the following condition Reheating to a temperature of ~ 1300 ° C; Hot-rolling the reheated steel slab to a temperature of Ar 3 to 1000 ° C or less to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet at a temperature of 720 占 폚 or lower; A step of cold-rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet; Subjecting the cold-rolled steel sheet to a first heat treatment at a temperature of Ac3 or higher; Slowly cooling the cold-rolled steel sheet subjected to the first heat treatment to room temperature; And subjecting the slowly cooled cold-rolled steel sheet to a secondary heat treatment at a temperature of 580 to 900 DEG C for 10 seconds to 15 minutes.

[Relation 1] (Si + 5Al)? 4.5 (provided that the content of Si and Al is% by weight)

According to one aspect of the present invention, there can be provided an ultra-high strength high-ductility steel sheet having an ultra-high strength and a good product of tensile strength and elongation, and a method of manufacturing the same.

1 is a photograph of Inventive Example 1 and Comparative Example 1 according to an embodiment of the present invention.
2 is a photograph of microstructure of Examples 1 to 8 according to an embodiment of the present invention.
3 is a microstructure photograph of Comparative Examples 4 to 9 according to an embodiment of the present invention.

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

C: 0.1 to 0.4%

C is an important element added for the stabilization of the retained austenite, and it should be added by 0.1% or more. However, if the content exceeds 0.4%, sufficient δ-ferrite can not be secured and the elongation ratio is increased, so it is desirable to limit the upper limit to 0.4%. It is more preferable that the content of C is in the range of 0.1 to 0.35% from the viewpoint of securing line homogeneous weldability which is important for continuous productivity.

Si: 2.0% or less (including 0%)

Si contributes to stabilization of retained austenite as an element which suppresses the precipitation of carbide in ferrite and promotes diffusion of carbon in ferrite into austenite. Here, when the amount of Si exceeds 2%, the hot and cold rolling properties are extremely poor, and Si is an element which inhibits the molten metal through the formation of surface oxides, so it is preferable to limit the upper limit. The reason for not including the lower limit of Si but including 0% is that in the steel containing a large amount of Mn as in the present invention, it is easy to secure the stability of the retained austenite without addition of Si.

Al: 2.0% or less (excluding 0%)

Al also contributes to the stabilization of the retained austenite through inhibition of the formation of carbide in the ferrite. Further, by increasing the Ac1 and Ac3 temperatures, annealing is possible at a normal annealing temperature. However, when the content of the slab is increased, it is difficult to produce a sound slab through reaction with the mold flux during casting, and it is an element which inhibits the molten conversion through the formation of the surface oxide. Therefore, the content of Al is limited to 2.0% or less.

Mn: 3.5.0 to 7.5%

Mn is the most widely used element in the steel of the transformed structure for formation and stabilization of retained austenite and suppression of ferrite transformation upon cooling. When manganese is added in an amount of less than 3.5%, ferrite transformation is likely to occur and austenite is insufficiently secured, so that high mechanical properties can not be ensured. When the manganese content is more than 7.5% It is difficult to stably secure the productivity such as the temperature drop of the molten steel, so that it is preferable to limit the upper limit and the lower limit.

P: 0.05% or less (excluding 0%)

P is an employment hardening element, however, when the content exceeds 0.05%, the weldability is lowered and the risk of brittleness of steel is increased. Therefore, the upper limit is preferably limited to 0.05%. And more preferably 0.02%.

S: 0.02% or less (excluding 0%)

S, like P, is an impurity element in steel and is an element that hinders ductility and weldability of a steel sheet. When the content is more than 0.02%, it is highly likely to deteriorate the ductility and weldability of the steel sheet, so that the upper limit is preferably limited to 0.02%.

N: 0.02% or less (excluding 0%)

N is an effective component for stabilizing austenite. However, if it exceeds 0.02%, there is a high risk of brittleness and deterioration of performance due to over-precipitation of AlN, so that the upper limit is preferably limited to 0.02%.

On the other hand, the steel sheet of the present invention may further include at least one selected from the group consisting of not more than 1.5% of Cr, not more than 0.15% of Ti, not more than 0.5% of Nb, not more than 0.5% of V and not more than 0.5% of Mo .

Cr: 1.5% or less

Cr is an element contributing to stabilization of retained austenite, and works in combination with C, Si, Mn and the like to contribute to stabilization of austenite. However, if Cr exceeds 1.5%, the increase of the manufacturing cost is excessive and the upper limit is limited.

Ti: 0.15% or less

Ti is a fine carbide-forming element and contributes to securing the strength of the present invention. Further, Ti precipitates N, which forms AlN, as a nitride-forming element into TiN. As a result, precipitation of AlN can be suppressed, thereby reducing the risk of cracking during performance. However, when the content exceeds 0.15%, the strength can be reduced by coarse carbide precipitation and reduction of the amount of carbon in the steel. Meanwhile, in one embodiment of the present invention, the Ti may be added in a chemical equivalent of 48/14 * [N] or more.

Nb: not more than 0.5%

Nb is an element that segregates in the austenite grain boundaries and inhibits the coarsening of austenite grains during annealing and increases the strength through formation of fine carbides. Therefore, when Nb exceeds 0.5%, coarse carbide precipitation and carbon in the steel Reductions can result in reduction in strength and are limited to increases in ferrous iron costs due to excess alloying inputs.

V: not more than 0.5%

V is required to be added because it contributes to the increase in strength by forming a low-temperature precipitate. If it exceeds 0.5%, strength can be reduced by coarse carbide precipitation and carbon reduction in the steel, do.

Mo: 0.5% or less

Mo has an advantage of increasing the hardenability and suppressing formation of ferrite, which is advantageous in suppressing the formation of ferrite upon cooling after annealing. In addition, it is preferable to add since it contributes greatly to the increase in strength through formation of fine carbides. However, if it exceeds 0.5%, it is limited to the increase of alloy iron cost due to excessive alloy input.

In addition, the steel sheet of the present invention may further comprise at least one of Zr: 0.001 to 0.1% and W: 0.001 to 0.5%. Zr and W are effective elements for precipitation strengthening and grain refinement of a steel sheet like Ti, Nb, V and Mo. When the content of Zr and W is less than 0.001%, it is difficult to secure the above effect. When the content of Zr exceeds 0.1% or the content of W exceeds 0.5%, the above effect is not increased, And ductility can be lowered due to excessive precipitates.

Further, the steel sheet of the present invention may further include at least one of Ni: 1% or less and Cu: 0.5% or less. The Ni and Cu are elements contributing to stabilization of the retained austenite, and function together with C, Si, Mn, Al and the like described above to contribute to stabilization of austenite. However, when Ni exceeds 1% or Cu exceeds 0.5%, an increase in manufacturing cost is excessive, thereby limiting the upper limit. Here, in the case of Cu, it may cause brittleness during hot rolling. Therefore, when Cu is added, it is more preferable to add Ni together.

The steel sheet of the present invention may further comprise at least one selected from the group consisting of Sb: 0.1% or less, Ca: 0.01% or less, and B: 0.01% or less. Sb has an effect of inhibiting the movement of surface oxidation elements such as Si and Al through grain boundary segregation to improve the quality of the surface of the plating. When it exceeds 0.1%, the alloying of the zinc plating layer is delayed. Ca is an element effective for improving workability by controlling the form of sulfide, and when it exceeds 0.01%, the effect is saturated. In addition, B has an advantage of suppressing soft ferrite transformation upon cooling at a high temperature by improving the ingotability by the combined effect with Mn, Cr and the like, but if it exceeds 0.01%, excess B is concentrated on the surface during production of the coated steel sheet, It is preferable to limit the upper limit thereof.

The rest of the alloy composition of the present invention is Fe, and other impurities that are inevitably included in the manufacturing process may be included.

In the steel sheet of the present invention, it is preferable that Si and Al satisfy the following conditional expression (1). The following relational expression 1 is intended to ensure sufficient delta ferrite to be obtained in the present invention. If it is less than 4.5%, it is difficult to secure sufficient delta ferrite and there is a problem that the tensile strength is lowered after annealing.

[Relation 1] (Si + 5Al)? 4.5 (provided that the content of Si and Al is% by weight)

On the other hand, it is preferable that the microstructure of the inventive steel sheet contains at least 5% delta ferrite, at least 10% retained austenite, at most 40% annealed martensite and at most 20% alpha martensite and epsilon martensite Do. Further, it is preferable that the microstructures have a lath shape, and the present invention can secure excellent strength, elongation and yield ratio by controlling the microstructure. On the other hand, since the measurement method of the three-dimensional concept is not easy in the measurement of the microstructure fraction, it is replaced with the area measurement by the cross-sectional observation used in the normal microstructure observation.

The steel sheet of the present invention provided as described above may have a tensile strength of 1000 MPa or more and a total elongation of 30% or more. That is, the product of tensile strength and elongation can be 30,000 MPa% or more. The steel sheet of the present invention may have a yield ratio of 0.8 or more.

In addition, the present invention is not particularly limited to the kind of steel sheet. For example, the steel sheet may be a cold-rolled steel sheet, a zinc-based hot-dip galvanized steel sheet, a zinc-base galvanized hot-dip galvanized steel sheet, a galvanized electrolytic- Based hot-dip galvanized steel sheet. Meanwhile, the zinc-based PVD-coated steel sheet can be manufactured by EML-PVD (Electro Magnetic Levitation-Physical Vapor Deposition) method.

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

First, the steel slab having the above-described alloy composition is reheated to a temperature of 1100 to 1300 占 폚. If the reheating temperature is less than 1100 ° C, there is a problem that the hot rolling load sharply increases. When the reheating temperature is higher than 1300 ° C, the amount of surface scale is increased to lead to the loss of the material. Therefore, we restrict this.

The reheated steel slab is hot rolled to a temperature of Ar3 or more and 1000 占 폚 or less to obtain a hot-rolled steel sheet. This is because when the temperature is lower than Ar3 (the temperature at which ferrite starts to appear when the austenite is cooled), a bimetallic structure of ferrite + austenite or a ferrite reverse rolled is formed and a malfunction due to fluctuation of the hot rolling load is feared Limit. If the temperature is higher than 1000 ° C, a thick oxide layer may be formed on the surface of the steel slab, which may cause problems in acid pickling at the time of cold rolling.

The hot-rolled steel sheet is wound at a temperature of 720 占 폚 or lower. If the coiling temperature exceeds 720 占 폚, the oxide film on the surface of the steel sheet may be excessively generated to cause surface defects. The lower the coiling temperature is, the higher the strength of the hot-rolled steel sheet is, and the lower the rolling load of the cold rolling, which is a post-process, is increased. However, the lower limit is not particularly limited since heat treatment is not performed before the cold rolling. However, the rolling ability of the cold rolling mill is excellent, or in the case of the reversing mill, heat treatment before cold rolling may be unnecessary.

The heat treatment after the winding can be performed for at least 30 minutes in the temperature range of Ac1 to Ac1 + (Ac3-Ac1) / 2. For example, when the hot-rolled steel sheet has a strength of 1200 Mpa or more, more desirable effects can be obtained. Ac1 is a temperature at which austenite starts to appear when the temperature is raised at a low temperature, and Ac3 means a temperature at which austenite becomes 100% at a temperature rise. Through the heat treatment, the rolling load during cold rolling can be reduced.

Thereafter, the rolled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. It is preferable that the reduction ratio in the cold rolling is in the range of 10 to 80%. When the reduction rate is less than 10%, recrystallization does not occur during annealing and the final properties are poor. On the other hand, when the reduction rate exceeds 80%, there arises a problem in cold rolling property, which may cause a problem of deteriorating productivity.

Thereafter, the cold-rolled steel sheet is first heat-treated at a temperature of Ac3 or higher. When the primary heat treatment is performed, martensite is transformed into austenite. However, if the steel sheet is cooled after the first heat treatment, the austenite is transformed into martensite. That is, the primary heat treatment is a process for transforming austenite into martensite in order to accelerate the recrystallization behavior in the secondary heat treatment (final annealing). On the other hand, when the primary heat treatment temperature is less than Ac3, martensite can not be secured at room temperature during cooling after annealing, and thus the final annealed structure may have a crystal grain of clobber. Such a grain size of clobar has a disadvantage of lowering the yield ratio after final annealing. The primary heat treatment is preferably performed for 5 to 300 seconds. When the primary annealing time, that is, the primary annealing is less than 5 seconds, it is difficult to obtain the above-mentioned effect, and when it exceeds 300 seconds, the process cost can be greatly increased.

Thereafter, the cold-rolled steel sheet subjected to the first heat treatment is slowly cooled. The cold-rolled steel sheet having the alloy composition of the present invention is transformed into martensite even if it is subjected to gradual cooling instead of ordinary quenching.

Thereafter, the slowly cooled cold rolled steel sheet is subjected to a secondary heat treatment at a temperature of 580 to 900 DEG C for 10 seconds to 15 minutes. The secondary heat treatment is a two-phase annealing heat treatment, whereby high strength and ductility and good shape quality can be ensured at the same time through the secondary heat treatment. If the secondary heat treatment temperature is lower than 580 占 폚, a sufficient amount of retained austenite can not be ensured and a total elongation after the final annealing may be lowered. If the secondary annealing temperature exceeds 900 占 폚, So that it is easy to secure a high tensile strength, but a problem that the elongation rate is lowered may occur. If the secondary heat treatment time is less than 10 seconds, it is difficult to obtain the above-mentioned effect. If the secondary heat treatment time exceeds 15 minutes, the process cost is greatly increased and the stability of the retained austenite is poor and sufficient elongation is not obtained .

On the other hand, the secondary heat treatment can be performed through a continuous annealing process, thereby improving the productivity. Of course, steel materials containing a large amount of Mn and the like may have a product of a high tensile strength and an elongation rate when the final annealing is performed for a long time for 30 minutes or longer, but a practical method of performing the heat treatment for a long time is a batch ) Type annealing, and when batch annealing is used, there is a disadvantage that the steel sheet after the heat treatment is bent in the rolling longitudinal direction.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

(Example)

The steel having the alloy composition shown in the following Table 1 was vacuum-melted with a 30 kg ingot and maintained at a temperature of 1200 ° C for 1 hour and then subjected to hot rolling to finish finish rolling at 900 ° C, And maintained for 1 hour and then subjected to hot rolling by low cooling. The specimens cooled to room temperature were heat-treated at 600 ° C for 10 hours, pickled, and then cold-rolled at a reduction ratio of 50% to obtain cold-rolled steel sheets. The cold-rolled steel sheet thus obtained was subjected to primary and secondary heat treatments under the conditions shown in Table 2, and then the microstructure and mechanical properties thereof were measured, and the results are shown in Tables 2 and 3 below. The Ac1 and Ac3 temperatures of the steel can be obtained through dilatometer experiments. When the specimen is heated to the desired heating rate by induction heating, the molar volume of the austenite is lower than that of the ferrite when the ferrite is changed to the austenite. The specimen shrinks due to the lowering of the austenite. Ac3 can be defined as the point at which contraction continues to occur and the length of the specimen increases again.

division Alloy composition (% by weight) C Si Mn Al Ti Nb P S N Relationship 1 Inventive Steel 1 0.39 0.23 3.66 0.91 - - 0.01 0.0056 0.0044 4.78 Invention river 2 0.35 - 4.1 1.33 0.11 0.19 0.011 0.002 0.0058 6.65 Invention steel 3 0.15 0.5 6.15 1.52 - - 0.009 0.0062 0.0065 8.10 Inventive Steel 4 0.15 One 6.05 1.51 - - 0.0069 0.0057 0.0054 8.55 Invention steel 5 0.15 1.5 6 1.49 - - 0.0057 0.0045 0.0045 8.95 Invention steel 6 0.15 One 5.1 1.48 - - 0.0085 0.0065 0.0063 8.40 Invention steel 7 0.15 One 6.98 1.53 - - 0.0061 0.0071 0.0045 8.65 Inventive Steel 8 0.12 One 7.05 1.5 - - 0.0064 0.0055 0.0047 8.50 Comparative River 1 0.161 1.07 6.2 0.045 - - 0.0085 0.0068 0.0051 1.30 Comparative River 2 0.157 1.02 4.02 0.039 - - 0.0059 0.0041 0.0016 1.22 Comparative Steel 3 0.158 1.06 5.12 0.044 - - 0.0064 0.004 0.0046 1.28 Comparative Steel 4 0.14 0.5 7.09 0.045 0.03 0.04 0.0056 0.0065 0.0049 0.20 Comparative Steel 5 0.1 - 5.11 0.036 - - 0.0066 0.0063 0.0044 0.18 Comparative Steel 6 0.136 - 6.02 0.04 - - 0.0063 0.0051 0.004 0.73 However, the relational expression 1 is (Si + 5Al)? 4.5.

division Grade Nr. Primary heat treatment 2nd heat treatment Delta ferrite (area%) Temperature (℃) Time (seconds) Temperature (℃) Time (seconds) Inventory 1 Inventive Steel 1 800 60 720 122 9 Comparative Example 1 - - 720 122 - Inventory 2 Invention river 2 800 60 690 122 12 Comparative Example 2 - - 690 122 - Inventory 3 Invention steel 3 840 122 700 122 18 Comparative Example 3 - - 700 122 - Honorable 4 Inventive Steel 4 840 122 720 122 20 Inventory 5 Invention steel 5 840 122 680 122 26 Inventory 6 Invention steel 6 840 122 680 122 22 Honorable 7 Invention steel 7 840 122 720 122 21 Honors 8 Inventive Steel 8 840 122 700 122 19 Comparative Example 4 Comparative River 1 760 61 660 122 2 Comparative Example 5 Comparative River 2 760 61 680 122 One Comparative Example 6 Comparative Steel 3 760 61 660 122 1.5 Comparative Example 7 Comparative Steel 4 820 70 640 122 - Comparative Example 8 Comparative Steel 5 810 61 660 122 - Comparative Example 9 Comparative Steel 6 800 122 620 122 2.2

division The tensile strength
(TS) (MPa) Total elongation
(TE) (%) TS × TE
(MPa%) Yield strength
(YS) (MPa) Yield ratio
(YS / TS) Inventory 1 1005 38 38190 858 0.85 Comparative Example 1 1050 20 21000 600 0.57 Inventory 2 1200 39 46800 1011 0.84 Comparative Example 2 1190 21 24990 762 0.64 Inventory 3 1037 30.5 31629 969 0.93 Comparative Example 3 1060 22.1 23426 726 0.68 Honorable 4 1167 30.5 35594 1011 0.87 Inventory 5 1126 35 39410 998 0.89 Inventory 6 1043 31.5 32855 1013 0.97 Honorable 7 1064 30.2 32133 919 0.86 Honors 8 1170 30 35100 1026 0.88 Comparative Example 4 1241 28.41 35257 1083 0.87 Comparative Example 5 1024 22.46 22999 863 0.84 Comparative Example 6 1095 26.15 28634 978 0.89 Comparative Example 7 1085 23 24955 1045 0.96 Comparative Example 8 865 21.9 18944 770 0.89 Comparative Example 9 915 21.3 19490 865 0.95

As can be seen from Tables 1 to 3, in the case of Inventive Examples 1 to 8 satisfying the alloy composition, the relational expression and the manufacturing conditions proposed by the present invention, satisfying the condition that the area fraction of delta ferrite is 5% or more, And the elongation rate are secured.

In the case of Comparative Examples 1 to 4, the alloying composition proposed by the present invention and the manufacturing conditions of Relation 1 are satisfied, but it can be seen that the total elongation is considerably low when the first heat treatment is not performed.

In Comparative Examples 4 to 9, the alloy composition and the manufacturing conditions proposed by the present invention were satisfied, but the relation 1 was less than 4.5, and the fraction of delta ferrite was not ensured to be 5% or more, and thus the elongation was low, It can be understood that the product of tensile strength and elongation of 30,000Mpa% or more is not satisfied.

1 is a photograph of Inventive Example 1 and Comparative Example 1. Fig. As can be seen from FIG. 1, it can be confirmed that the microstructure of Inventive Example 1 has lasse-like crystal grains, whereas in Comparative Example 1, it can be seen that the crystal grains of clobular crystals are formed.

2 is a photograph of the microstructure of Inventive Examples 1 to 8, and Fig. 3 is a microstructure photograph of Comparative Examples 4 to 9. Fig. As can be seen from FIGS. 2 and 3, it can be seen that sufficient delta ferrite is formed in Examples 1 to 8, whereas in Comparative Examples 4 to 9, no or small amount of delta ferrite is formed Can be confirmed.

Claims (20)

(Excluding 0%), Al: not more than 2.0% (excluding 0%), Mn: 3.5 to 7.5%, P: not more than 0.05% , S: not more than 0.02% (excluding 0%), N: not more than 0.02% (excluding 0%),
Wherein Si and Al satisfy the following condition (1)
The ultra-high strength high ductility steel sheet according to claim 1, wherein the microstructure comprises at least 5% delta ferrite, at least 10% retained austenite, at most 40% annealed martensite, and at most 20% alpha martensite and epsilon martensite.
[Relation 1] (Si + 5Al)? 4.5 (provided that the content of Si and Al is% by weight)
The method according to claim 1,
Wherein the steel sheet further comprises at least one selected from the group consisting of Cr: at most 1.5%, Ti: at most 0.15%, Nb: at most 0.5%, V: at most 0.5%, and Mo: .
The method according to claim 1,
Wherein the steel sheet further comprises at least one of 0.001 to 0.1% of Zr and 0.001 to 0.5% of W.
The method according to claim 1,
Wherein the steel sheet further comprises at least one of Ni: 1% or less and Cu: 0.5% or less.
The method according to claim 1,
Wherein the steel sheet further comprises at least one member selected from the group consisting of Sb: 0.1% or less, Ca: 0.01% or less, and B: 0.01% or less.
The method according to claim 1,
The steel sheet has a tensile strength of 1000 MPa or more, a total elongation of 30% or more, and a yield ratio of 0.8 or more. The method according to claim 1,
Wherein the steel sheet is one selected from the group consisting of cold-rolled steel sheets, zinc-based hot-dip galvanized steel sheets, zinc-based galvannealed steel sheets, zinc-based electrolytic galvanized steel sheets, zinc-based PVD-coated steel sheets, and aluminum-based hot-dip galvanized steel sheets.
(Excluding 0%), Al: not more than 2.0% (excluding 0%), Mn: 3.5 to 7.5%, P: not more than 0.05% , S: not more than 0.02% (excluding 0%), N: not more than 0.02% (excluding 0%), and Si and Al satisfy the following relational expression 1: ;
Hot-rolling the reheated steel slab to a temperature of Ar 3 to 1000 ° C or less to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet at a temperature of 720 占 폚 or lower;
A step of cold-rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet;
Subjecting the cold-rolled steel sheet to a first heat treatment at a temperature of Ac3 or higher;
Slowly cooling the cold-rolled steel sheet subjected to the first heat treatment to room temperature; And
And subjecting the slowly cooled cold-rolled steel sheet to a secondary heat treatment at a temperature of 580 to 900 占 폚 for 10 seconds to 15 minutes.
[Relation 1] (Si + 5Al)? 4.5 (provided that the content of Si and Al is% by weight)
The method of claim 8,
Wherein the steel slab further comprises at least one member selected from the group consisting of not more than 1.5% of Cr, not more than 0.15% of Ti, not more than 0.5% of Nb, not more than 0.5% of V and not more than 0.5% of Mo, A method of manufacturing a steel sheet.
The method of claim 8,
Wherein the steel slab further comprises at least one of Zr: 0.001 to 0.1% and W: 0.001 to 0.5%.
The method of claim 8,
Wherein the steel slab further comprises at least one of Ni: 1% or less and Cu: 0.5% or less.
The method of claim 8,
Wherein the steel slab further comprises at least one selected from the group consisting of Sb: 0.1% or less, Ca: 0.01% or less, and B: 0.01% or less.
The method of claim 8,
Further comprising the step of heat-treating the hot-rolled steel sheet after the rewinding at a temperature range of Ac1 to Ac1 + (Ac3-Ac1) / 2 for 30 minutes or more.
The method of claim 8,
Wherein the cold rolling is performed at a reduction ratio of 10 to 80%.
The method of claim 8,
Wherein the primary heat treatment is performed for 5 to 300 seconds.
The method of claim 8,
Further comprising a step of plating the secondary heat-treated cold-rolled steel sheet after the secondary heat-treating step.
18. The method of claim 16,
Wherein the plating is one selected from the group consisting of zinc-based hot-dip plating, zinc-based electrolytic plating, zinc-based PVD plating, and aluminum-based hot-dip plating.
18. The method of claim 17,
Wherein the zinc-based hot-dip coating is one selected from the group consisting of Zn-based, Zn-Al-based and Zn-Mg-Al based.
18. The method of claim 17,
Wherein the aluminum-based hot-dip coating is one of Al-Si-based and Al-Si-Mg-based high-strength high-ductility steel sheets.
18. The method of claim 16,
Further comprising the step of alloying the plated cold-rolled steel sheet after the plating step.

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