KR101736635B1 - Cold rolled and galvanized steel sheet having excellent spot weldability and surface charateristics and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a high-strength cold rolled steel sheet and a molten galvanized steel sheet with excellent surface treatment characteristics and welding properties; and a manufacturing method thereof. According to the present invention, the cold rolled steel sheet comprises: 0.05-0.3 wt% of carbon (C); 0.6-2.5 wt% of silicon (Si); 0.01-0.5 wt% of aluminum (Al); 1.5-3.0 wt% of manganese (Mn); and the remainder consisting of Fe and inevitable impurities. A steel microstructure comprises: ferrite of 60 wt% or less; acicular bainite of 25 wt% or more; martensite of 5 wt% or more; and acicular remaining austenite of 5 wt% or more. The ferrite has an average diameter of 2 m or less; satisfies Fn2 defined by [Relational expression 1] is 89% or more; satifies Fa5 defined by [Relational expression 2] is 70% or less; and an Ni or Fe plating layer is formed on a surface thereof by the attachment amount of 5-40 mg/m^2.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength cold-rolled steel sheet, a hot-rolled steel sheet, a galvanized steel sheet, and a method of manufacturing the same.

More particularly, the present invention relates to a high strength cold rolled steel sheet excellent in hole expandability and elongation, excellent in press formability and excellent in phosphate treatment and spot weldability, A steel sheet and a manufacturing method of the same.

In order to reduce the weight of automobiles, many efforts have been made to increase the strength and reduce the thickness of the steel sheet used as the structural member. However, when the strength of the steel sheet is increased, there is a problem that the press formability is relatively lowered. In order to improve the press formability, a high hole expandability is required in addition to the elongation of steel, and a transformed structure steel utilizing residual austenite phase together with martensite and bainite which are low temperature structures have been developed and applied. However, a large amount of alloying element is added, and in particular, Si or Al is added to the steel in order to secure retained austenite, so that a Si enriched or oxide is formed on the surface. Therefore, the cold-rolled steel sheet has a poor phosphate treatment property, and the hot-dip galvanized steel sheet has a problem of deterioration of plating quality and cracking in the spot welded portion.

In order to solve the above problem, after the Ni or the like that workability ensure good tissue by the composition of the alloy lowers twice annealing heat treatment On the other hand, after the annealing, the surface of the steel sheet a coating weight 5 ~ 70mg / m ² adhesion, a method of reduction annealing (JP2002-47535A). However, there is a problem that plating failure occurs partially due to non-uniform plating and water defects during metal plating such as Ni after the first annealing because the plate shape is poor at a cooling rate of 30 DEG C / second or more during the first annealing .

On the contrary. (KR1998-7002926A), there has been proposed a method of securing the quality of hot dip galvanizing by reducing the amount of Si and Mn concentrated to the surface by internal oxidation during annealing (KR1998-7002926A). However, And there is a problem that the amount of the alloy for securing the retained austenite increases.

Also, since the surface oxides of Si and Mn formed during annealing inhibit the phosphate treatment of the cold-rolled steel sheet, the adhesion of the electrodeposition coating layer is lowered to cause corrosion of the electrodeposited coating layer due to chipping or the like, thereby deteriorating the durability of automobile parts .

SUMMARY OF THE INVENTION Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and it is an object of the present invention to provide a method and apparatus for forming a unique structure by utilizing a reverse transformation phenomenon, A hot-rolled steel sheet, a hot-dip galvanized steel sheet and a galvannealed hot-dip galvanized steel sheet which can improve not only press formability but also corrosion resistance and surface quality of assembled parts by improving phosphate treatment, plating layer adhesion and plating quality do.

It is another object of the present invention to provide a method of manufacturing the steel sheet.

Further, the technical problems to be solved by the present invention are not limited to the technical problems mentioned above, and other technical problems which are not mentioned can be understood from the following description in order to clearly understand those skilled in the art to which the present invention belongs .

According to an aspect of the present invention,

(Si): 0.6 to 2.5%, aluminum (Al): 0.01 to 0.5%, manganese (Mn): 1.5 to 3.0%, the balance Fe and unavoidable impurities Including,

Wherein the steel microstructure contains ferrite of 60% or less, acicular bainite of 25% or more, martensite of 5% or more and acicular retained austenite of 5% or more in an area fraction,

The ferrite has an average diameter of 2 占 퐉 or less,

The ferrite satisfies that Fn2 defined by the following [Relation 1] is not less than 89%, and Fa5 defined by the following [Relation 2] is not more than 70%, and

And a Ni or Fe plated layer is formed on the surface thereof with an adhesion amount of 5 to 40 mg / m < ^{2 &} gt ;. The present invention relates to a high strength cold rolled steel sheet excellent in surface treatment characteristics and weldability.

[Relation 1]

Fn2 = [number of ferrite grains of 2 mu m or less / number of total ferrite grains] x 100

[Relation 2]

Fa5 = [area of ferrite grains larger than 5 mu m / area of entire ferrite grains] x 100

In the present invention, the total content of Cr, Ni, and Mo may be one or more of 2% or less (here, 0% is not included).

Further, Ti may be added in an amount of 0.05% or less (here, 0% is not included), and B is 0.003% or less (0% is excluded).

The present invention also provides a hot-dip galvanized steel sheet in which a hot-dip galvanized layer is formed on the surface of a cold-rolled steel sheet,

(Si): 0.6 to 2.5%, aluminum (Al): 0.01 to 0.5%, manganese (Mn): 1.5 to 3.0%, the balance Fe and unavoidable impurities Including,

Wherein the microstructure contains an area fraction of not more than 60% of ferrite, not less than 25% of acicular bainite, not less than 5% of martensite and not less than 5% of austenite retained austenite,

The ferrite has an average diameter of 2 占 퐉 or less,

The ferrite satisfies that Fn2 defined by the above-mentioned [relational expression 1] is 89% or more and Fa5 defined by the [relational expression 2] is 70% or less, and

And a Ni or Fe plated layer is formed between the cold-rolled steel sheet and the hot-dip galvanized layer in an amount of 100 mg / m ² or more deposited thereon. The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in surface treatment characteristics and weldability.

Further, in the present invention, it is also possible to provide a galvannealed steel sheet obtained by alloying heat treatment on the hot-dip galvanized steel sheet.

Further, according to the present invention,

(Si): 0.6 to 2.5%, aluminum (Al): 0.01 to 0.5%, manganese (Mn): 1.5 to 3.0%, the balance Fe and unavoidable impurities Preparing a steel slab containing the steel slab, and reheating the steel slab;

Rolling the reheated steel slab under normal hot rolling conditions and then winding it in a temperature range of 750 to 550 ° C;

A step of cold-rolling the wound hot-rolled steel sheet to produce a cold-rolled steel sheet;

A primary annealing step in which the cold-rolled steel sheet is heated to a temperature equal to or higher than Ac3 and then cooled to 350 DEG C or less at a cooling rate of less than 20 DEG C / s;

After the first annealing, the steel sheet is heated and maintained at a temperature in a range of Ac1 to Ac3, cooled to a temperature range of Ms to Bs at a cooling rate of less than 20 deg. C / s, Annealing step; And

And forming a Ni or Fe plating layer on the surface of the secondary annealed steel sheet with an adhesion amount of 5 to 40 mg / m < ² >. The present invention also relates to a method of manufacturing a high strength cold rolled steel sheet having excellent surface treatment characteristics and weldability.

In the present invention, a Ni or Fe plating layer may be formed on the surface of the steel sheet with an adhesion amount of 5 to 40 mg / m < ² > before the primary annealing and after the secondary annealing.

In the present invention, it is preferable that the cold-rolled steel sheet has a microstructure before the second annealing step in an area fraction of 20% or less of ferrite and a residual low-temperature transformation texture.

The present invention also relates to a hot-dip galvanized steel sheet obtained by subjecting the surface of a steel sheet after primary annealing to Ni or Fe plating with an adhesion amount of 100 mg / m ² or more and then subjected to hot-dip galvanizing, and a hot- Thereby providing a plated steel sheet.

According to the present invention, compared with Q & P steel which has been subjected to heat treatment of high ductility transformation textural steel such as DP steel or TRIP steel and Q & P (Quenching & Partitioning) heat treatment, it has excellent tensile strength 980 MPa Cold-rolled steel sheets, hot-dip galvanized steel sheets, and galvannealed galvanized steel sheets. Particularly, after coating the Ni and Fe after the first and second annealing processes, the cold-rolled steel sheet is excellent in the phosphate treatment property and excellent in the adhesion of the electrodeposition coating layer, and Ni and Fe are coated before the secondary annealing to improve plating adhesion and non- It is possible to manufacture a hot-dip galvanized steel sheet excellent in moldability and corrosion resistance and excellent in spot weldability, which is advantageous in that the safety and life of automobile parts are prolonged.

Further, the cold-rolled steel sheet of the present invention is advantageously used in industrial fields such as building members, automotive steel sheets, and the like.

Fig. 1 is a photograph illustrating the structure of the microstructure of the steel and the influence of the geometrical structure on the hole expandability and the elongation, with examples of the embodiments and comparative examples.
FIG. 2 is a photograph of a tissue showing cracks in hole expansion in the tissue photograph of FIG. 1; FIG.
FIG. 3 is a view showing an example of the annealing heat treatment process according to the present invention (the dotted line in FIG. 1 (b) shows the thermal history at the time of melting alloy plating).
Fig. 4 is a photograph of microstructure observed in order to compare the difference in structure between the inventive example and the comparative example. Fig.
FIG. 5 is a graph showing differences in observation frequency of ferrite grain sizes according to the inventive example and comparative example.
6 is a graph showing the influence of the amount of Ni plating on the phosphate treatment.
7 is a photograph showing a comparison of the unplated defects of the hot-dip galvanized steel sheet according to the Ni plating amount.
Fig. 8 is a graph showing the degree of cracking of spot welded portions according to Ni plating amount.

Hereinafter, the present invention will be described.

The hole expandability in the steel utilizing residual austenite to improve the conventional elongation was not good. In order to improve the hole expandability and elongation simultaneously, the microfabrication method using reverse transformation requires a cooling rate of 20 ° C / s or more in order to obtain martensite structure in the first heat treatment step, The plate is twisted due to the uneven cooling, and the plate shape is not good, which causes a problem in press forming.

The inventors of the present invention have confirmed through experiments and experiments that fine lath ferrite obtained by reverse transformation heat treatment, bainite and retained austenite structure are important means for ensuring both hole expandability and elongation. It is also confirmed that the particle size distribution of ferrite plays an important role.

In order to obtain an excellent plate shape, a range of steel composition components that can obtain such microstructure is found even under a condition that the cooling rate is much lower than the conventional one, and a phosphate composition film forming the most frequently encountered problem in the conventional high- The present invention has been accomplished by finding means for solving the problem of defective, partial unplated, weld crack.

The high strength cold rolled steel sheet excellent in surface treatment characteristics and weldability according to the present invention comprises 0.05 to 0.3% of carbon (C), 0.6 to 2.5% of silicon (Si), 0.01 to 0.5% of aluminum (Al) (Mn): 1.5 to 3.0%, the balance Fe and unavoidable impurities.

Hereinafter, the composition of the alloy component of the cold-rolled steel sheet of the present invention and the reasons for the limitation thereof will be described in detail. Here, the content of each component means weight% unless otherwise specified.

C: 0.05 to 0.3%

Carbon (C) is an effective element for strengthening the steel. In the present invention, it is an important element added for stabilization of the retained austenite and strength. In order to obtain the above-mentioned effect, it is preferable that the content is 0.05% or more. However, when the content exceeds 0.3%, there is an increased risk of occurrence of the casting defects. In addition, the weldability may be greatly lowered, and further, there is a problem because the steel is cooled to a lower temperature in order to obtain a martensite structure during the primary annealing. Therefore, the content of C in the present invention is preferably limited to 0.05 to 0.3%.

Si: 0.6 to 2.5%

Silicon (Si) is an element that inhibits the precipitation of carbides in ferrite and promotes the diffusion of carbon in ferrite into austenite, and consequently contributes to the stabilization of retained austenite. In order to obtain the above-mentioned effect, it is preferable to add at least 0.6%. If the content exceeds 2.5%, however, the hot and cold rolling properties are extremely poor and oxides are formed on the surface of the steel, have. Therefore, the Si content in the present invention is preferably limited to 0.6 to 2.5%.

Al: 0.01 to 0.5%

Aluminum (Al) is an element that bonds with oxygen in the steel and acts as a deoxidizing agent. For this purpose, it is desirable that the content of aluminum is maintained at 0.01% or more. Al also contributes to the stabilization of retained austenite by suppressing the formation of carbide in ferrite like Si. If the content of Al exceeds 0.5%, it is difficult to produce a sound slab through a reaction with the mold flux during casting, and also the surface oxide is formed to deteriorate the plating property. Therefore, the content of Al in the present invention is preferably limited to 0.01 to 0.5%.

Mn: 1.5 to 3.0%

Manganese (Mn) is an element effective for forming and stabilizing retained austenite while controlling the transformation of ferrite. If the content of Mn is less than 1.5%, a large amount of ferrite transformation occurs and it becomes difficult to secure the desired strength. On the other hand, when the Mn content exceeds 3.0%, the phase transformation in the second annealing heat treatment step of the present invention is delayed too much As the martensite structure is formed in large quantities, there is a problem that it is difficult to secure the intended ductility. Therefore, in the present invention, the content of Mn is preferably limited to 1.5 to 3.0%.

As an impurity element of the steel of the present invention

P is preferably 0.03% or less, and if it exceeds 0.03%, there is a problem that the weldability is lowered and the risk of brittleness of steel is increased.

S is preferably 0.015% or less. Sulfur (S) is an impurity element inevitably contained in the steel, and its content is preferably suppressed to the maximum. In theory, it is advantageous to limit the content of S to 0%, but it is important to manage the upper limit because it is inevitably contained in the manufacturing process inevitably. If the content exceeds 0.015%, the possibility of inhibiting the ductility and weldability of the steel sheet high.

N is preferably 0.02% or less. Nitrogen (N) is an element effective for stabilizing austenite. However, when the content exceeds 0.02%, the risk of brittleness of steel increases, and AlN is excessively precipitated by reaction with Al, There is a problem of deterioration.

The cold-rolled steel sheet of the present invention may further include at least one of Cr, Ni, Mo, Ti, and B in addition to the above-mentioned components for the purpose of strength improvement and the like.

That is, in the present invention, the total content of one or more of Cr, Ni and Mo: 2% or less (here, 0% is not included) may be further included. The molybdenum (Mo), nickel (Ni) and chromium (Cr) contribute to the stabilization of the retained austenite. These elements act together with C, Si, Mn, Al and the like to contribute to the stabilization of austenite. If the content of these elements exceeds 2.0% in the case of Mo, Ni and Cr, there is a problem that the production cost is excessively increased. Therefore, it is preferable to control so as not to exceed the above content.

In the present invention, Ti may be added in an amount of not more than 0.05% (here, 0% is not included) and B is not more than 0.003% (where 0% is not included).

In the present invention, it is preferable that Ti is added in an amount of not less than 0.05% or not more than 0.05% when Al is added or B is added. Ti is an element which forms TiN and needs to precipitate at a higher temperature than B or Al, so it is effective when it is put in a lot, but there is a problem of clogging of nozzles and cost increase during performance. In the upper limit of the amounts of Al and B added according to the present invention, when Ti is added in the range of 0.05%, AlN or BN can not be formed and can act as a solid solution element, so that the upper limit is set to 0.05%.

B (boron) has an effect of suppressing soft ferrite transformation at a high temperature by improving the incombustibility by the combined effect with Mn, Cr and the like. However, if the content exceeds 0.003%, excess B is concentrated on the surface of the steel during plating, which may lead to deterioration of the plating adhesion, as well as inhibiting bainite transformation to lower the hole expandability and elongation, .

The remainder of the present invention is iron (Fe). However, in the ordinary steel 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 steel making.

Further, the high strength cold-rolled steel sheet of the present invention comprises steel microstructure in an area fraction of not more than 60% ferrite, not less than 25% needle-shaped bainite, not less than 5% martensite and not less than 5% needle-like retained austenite. That is, the cold-rolled steel sheet of the present invention includes steel microstructure, ferrite, and lath-type bainite include martensite and needle-like retained austenite. These structures are steel sheet main structures of the present invention which are advantageous for hole expanding, ductility and strength, and among them, the martensite structure is partially contained in the steel structure due to the heat treatment in the manufacturing process described later.

Among the above microstructures, the ferrite includes coarse polygonal ferrite and needle-like ferrite, and the area percentage of the microstructure is 60% or less. If the ferrite structure exceeds 60%, the strength is lowered and the proportion of coarse polygonal ferrite is increased. In addition, the difference in the content of the elements of redistribution (partitioning, partitioning) There is a problem that the hole expandability is deteriorated because the crack easily occurs.

Most of the bainite structure exists in the form of a needle, and forms a boundary with surrounding ferrite, martensite and retained austenite. Since bainite has intermediate strength between ferrite and two phase structure (martensite and retained austenite), bainite is required to be at least 25% in order to alleviate interfacial separation between phases during hole expansion to improve hole expandability. In the present invention, Respectively.

The martensite structure is formed when the chemically unstable austenite is cooled to room temperature during the final cooling, thereby lowering the elongation of the steel. However, in the present invention, martensite structure is used as a means for lowering the alloy element and improving the strength. If the martensite structure is small, there is a problem of cost increase because more alloying elements are added. Thus, the lower limit of the martensite area ratio was set to 5%.

In the present invention, the retained austenite is an important structure for securing ductility and ensuring hole expandability. Therefore, there is a problem that a large amount of the austenite stabilizing alloy element such as carbon is added, and the cost rise and weldability are lowered. Particularly, when the needle-like retained austenite is formed as in the present invention, the stability of austenite is remarkably increased even in the same chemical component, so that it is not necessary to include a large amount as in the conventional method. However, in order to make both ductility and hole expandability more than 20%, a minimum of 5% is required and the lower limit is set to 5%.

In the present invention, it is important to control the fraction and the size of the structure of the ferrite. As shown in FIGS. 1 and 2, cracks propagate easily in the bound polygonal ferrite along the boundary of the neighboring second phase at the time of hole expansion. However, when acicular ferrite is dispersed, crack propagation is suppressed, It can be understood that the scalability is improved. Therefore, the present invention is characterized in that the fraction and size of ferrite are controlled by using the heat treatment method described below.

Specifically, the ferrite is characterized in that the average diameter is 2 占 퐉 or less, Fn2 defined by the following [Relation 1] is 89% or more, and Fa5 defined by the following [Relation 2] is 70% or less .

[Relation 1]

Fn2 = [number of ferrite grains of 2 mu m or less / number of total ferrite grains] x 100

[Relation 2]

Fa5 = [area of ferrite grains larger than 5 mu m / area of entire ferrite grains] x 100

In the present invention, the length ratio of the long-side ferrite to the long-side ferrite is 4 or more. The size of the ferrite is evaluated by an image analyzer incorporating an analysis program that assumes that several hexagons are connected (ASTM E112 crystal grain measurement method). As a result, the size and number of grains as shown in FIG. 5 were measured. Based on this, the size and distribution of ferrite grains were determined which are excellent in elongation and hole expandability.

Specifically, when the ferrite has an acicular ferrite structure having an average size of 2 탆 or less and a distribution satisfying the relational expression 1-2, the hole expandability is excellent at 28% or more and the elongation at 20% or more And to present this technology configuration.

The cold-rolled steel sheet of the present invention, which satisfies the above-described microstructure and ferrite size and distribution, has a tensile strength of 980 MPa or more and is superior to conventional TRIP steel manufacturing method, Q & P heat treatment method, And ductility can be secured at the same time.

In addition, the cold-rolled steel sheet having excellent surface treatment characteristics and weldability of the present invention includes a Ni or Fe plating layer formed on the surface thereof, wherein the coating adhesion amount is preferably 5 to 40 mg / m ² . If the plating amount is less than 5 mg / m ² , as shown in FIG. 5, Mn or Si oxide is easily formed on the surface due to the minute oxidation during annealing or annealing, and as a result, the phosphate coating is not formed, This is because the adhesion is deteriorated. On the other hand, if the plating amount of Ni or Fe is more than 40 mg / m ² , the phosphate crystals are coarsened and the fine phosphate unevenness is reduced, so that the adhesion is lowered.

Further, the present invention is not limited to the cold-rolled steel sheet having the above-mentioned composition and texture, and can provide a hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on the surface of the cold-rolled steel sheet. At this time, it is preferable that Ni or Fe plating layer is formed between the cold-rolled steel sheet and the hot-dip galvanized layer at an amount of 100 mg / m ² or more.

Further, it is also possible to provide an alloyed hot-dip galvanized steel sheet comprising a galvannealed hot-dip galvannealed layer which has undergone alloying heat treatment on the hot-dip galvanized steel sheet.

Next, a method of manufacturing the cold-rolled steel sheet of the present invention will be described in detail.

The cold-rolled steel sheet according to the present invention can be produced by subjecting a steel slab satisfying the composition of the present invention to the reheating-hot rolling-winding-cold rolling-annealing process. Hereinafter, Will be described in detail.

[Steel slab reheating process]

In the present invention, it is preferable to carry out a step of reheating a steel slab having the above-mentioned composition components and homogenizing the steel slab prior to the hot rolling, and it is more preferably performed in a temperature range of 1000 to 1300 캜 .

If the temperature during the reheating is less than 1000 ° C, there is a problem that the rolling load sharply increases. On the other hand, when the temperature exceeds 1300 ° C, the energy cost increases and the amount of the surface scale becomes excessive. Therefore, in the present invention, the reheating step is preferably performed at 1000 to 1300 ° C.

[Hot rolling process]

In the present invention, the reheated steel slab is hot-rolled to produce a hot-rolled steel sheet, wherein the hot-rolling is preferably performed at a temperature of 800 to 1000 ° C. under normal conditions.

When the rolling temperature is lower than 800 ° C, the rolling load is increased significantly and the rolling becomes difficult. On the other hand, when the hot rolling temperature exceeds 1000 ° C, the thermal fatigue of the rolling roll is greatly increased, It causes. Therefore, in the present invention, the hot rolling temperature during hot rolling is preferably limited to 800 to 1000 ° C.

[Winding Process]

Next, in the present invention, the hot-rolled steel sheet produced according to the above is wound, and the coiling temperature is preferably in the range of 750 to 550 ° C.

If the coiling temperature is too high at the time of winding, the scale is excessively invented on the surface of the hot-rolled steel sheet, causing surface defects and deteriorating the plating ability. Therefore, the winding step is preferably carried out at 750 DEG C or lower. At this time, the lower limit of the coiling temperature is not particularly limited, but the lower limit of 550 占 폚 is taken into account in consideration of the difficulty of subsequent cold rolling as the strength of the hot rolled sheet due to the formation of martensite becomes excessively high.

[Cold Rolling Process]

Then, the cold rolled steel sheet is preferably produced by subjecting the rolled hot rolled steel sheet to a pickling treatment by a usual method to remove the oxide layer, and then cold rolling to match the shape and thickness of the steel sheet.

Normally, cold rolling is carried out in order to secure the thickness required by the customer. At this time, there is no restriction on the reduction rate, but in order to suppress generation of coarse ferrite grains during recrystallization in the subsequent annealing process, .

[Annealing Process]

The present invention relates to a process for producing a cold-rolled steel sheet comprising a needle-like ferrite and needle-like retained austenite phase having a major axis and a minor axis ratio of not less than 4 as a main phase. In order to obtain such a cold-rolled steel sheet, It is important. Particularly, in the present invention, in order to secure a desired microstructure from the partitioning of elements such as carbon and manganese during annealing, the present invention is not a continuous annealing process after ordinary cold rolling, And a partitioning heat treatment for securing the acicular ferrite and the retained austenite at the time of the secondary annealing is performed.

Primary Annealing

First, the produced cold-rolled steel sheet is annealed at a temperature equal to or higher than Ac3, and then subjected to a first annealing process in which the steel sheet is cooled to a temperature of 350 DEG C or lower at a cooling rate of less than 20 DEG C / s (see FIG.

This is to obtain the main phase of the microstructure of the cold-rolled steel sheet subjected to the first annealing at an area fraction of 20% or less and the remaining low-temperature transformed structure (bainite and martensite). This is to secure the strength and ductility of the cold-rolled steel sheet manufactured through the final secondary annealing step. If the ferrite fraction exceeds 20% due to the cooling after the primary annealing, The cold-rolled steel sheet of the present invention comprising ferrite, retained austenite, and low-temperature structure can not be obtained.

That is, if the annealing temperature is less than Ac3 or the cooling rate is too slow, a large amount of soft polygonal ferrite is formed, and the polygonal ferrite formed at the time of reverse annealing of the ferrite / austenite during the subsequent second annealing heat treatment This is because the ferrite area ratio of 5 탆 or more increases.

In addition, it is important not only for the annealing temperature but also for the cooling rate to obtain the structure through the primary annealing. When the cooling rate is 20 ° C / s or higher, the steel is inflated by the low-temperature transformed structure formed unevenly, and the plate is twisted and a wave is generated. In order to suppress this, the cooling rate is preferably set to less than 20 DEG C, and the lower limit is only required to obtain ferrite having the above-mentioned area fraction of 20% or less and the remaining low-temperature transformed structure. It is preferable that the cooling end temperature or the quenching start temperature after cooling is 350 ° C or lower, and if it is higher than that, carbide precipitation increases in bainite, and needle-shaped microstructure due to reverse transformation can not be obtained.

In the present invention, Ni or Fe plating may be applied to the surface of the steel sheet after the first annealing and before the subsequent secondary annealing, and the coating adhesion amount is preferably in the range of 5 to 40 mg / m ² . Ni or Fe plated on the surface of the steel sheet may be diffused into the steel sheet during the subsequent secondary annealing and may be annihilated, but Ni or the like diffused on the surface is preferable because it acts to suppress the oxidation of the steel sheet.

Secondary Annealing

In the present invention, after completion of the primary annealing, the steel sheet is heated and held in the range of Ac1 to Ac3, cooled to a temperature range of Ms to Bs at a cooling rate of less than 20 deg. C / Annealing heat treatment is performed (see Fig. 2 (b)).

In the present invention, heating in the range of Ac1 to Ac3 is intended to form fine ferrite and austenite which are maintained in an acicular structure by reverse transformation as the anodic transformation obtained in the first annealing is abnormally heated. And to ensure the stability of austenite through alloying element distribution to austenite during annealing to secure retained austenite in the final structure at room temperature.

The heating and holding at the temperature after the heating is intended to induce redistribution of alloying elements such as carbon and manganese together with the reverse transformation of the formed low-temperature structure (bainite and martensite) after the primary annealing heat treatment. The redistribution at this time is called the primary redistribution.

On the other hand, the maintenance for primary redistribution of the alloying elements is not particularly limited because the alloying elements are sufficiently diffused to the austenite side. However, if the holding time is excessively excessive, the productivity may be deteriorated and the redistribution effect is also saturated. Therefore, it is preferable to carry out the holding time within 2 minutes.

After the primary redistribution of the alloying elements is completed, the alloy is cooled to a temperature range of Ms (martensite transformation start temperature) to Bs (bainite transformation start temperature) at a cooling rate of less than 20 ° C / s, After maintaining the temperature at a constant temperature, it is cooled to room temperature. In the process of keeping the temperature constant, redistribution of alloying elements is performed once again, and redistribution at this time is referred to as secondary redistribution.

The average cooling rate during the cooling is preferably less than 20 DEG C / s, which is also intended to uniform the shape of the plate. By the primary redistribution, even if the austenite is sufficiently stabilized and slowly cooled, polygonal ferrite is not formed at the time of cooling. However, since the productivity is lowered when cooling is slow, a cooling rate of 5 DEG C / s or more is preferable.

The cooling termination temperature is preferably in the range of Ms to Bs because the degree of supersaturation is less than Bs so that secondary partitioning does not occur and the diffusion is very slow at temperatures below Ms and the time required for partitioning increases significantly. In the component system satisfying the composition of the present invention, the partitioning time of 30 seconds or more is sufficient in the Ms to Bs interval.

On the other hand, the cooling section can be passed immediately after annealing in order to suppress skewing of the steel sheet during cooling after annealing. In the present invention, the cooling rate means an average temperature from the temperature after the heat treatment to the cooling to the cooling end temperature.

In the case of producing the cold-rolled steel sheet after the secondary annealing, Ni or Fe plating may be applied to the surface of the steel sheet after the secondary annealing, and the coating adhesion amount is preferably in the range of 5 to 40 mg / m ² . The Ni or Fe plating layer thus formed has improved phosphate treatment properties to improve the electrodeposition performance and the welding characteristics.

As described above, in the present invention, after the primary annealing step, the formed low-temperature structure is heated and maintained in a range of Ac1 to Ac3 to induce primary reallocation of alloying elements such as carbon and manganese together with rapid reverse transformation, By reheating and inducing secondary redistribution, a fine needle-like microstructure as shown in Fig. 3 is obtained in comparison with the structure obtained by the conventional method, and excellent hole expandability and elongation can be secured at the same time.

[Plating process]

As the secondary annealing step, the cold-rolled steel sheet subjected to the primary annealing treatment may be plated using a hot-dip coating process or an alloying hot-dip plating process, and the plating layer formed therefrom is preferably zinc-based.

In the case of using the above-mentioned hot-dip coating method, it is possible to produce a hot-dip coated steel sheet by immersing it in a galvanizing bath, and in the case of the alloying hot-dip coating method, a galvannealed hot-dip galvanized steel sheet can be produced.

On the other hand, in the present invention, it is preferable that the surface of the steel sheet after the primary annealing is plated with Ni or Fe at an adhesion amount of 100 mg / m ² or more and then subjected to hot-dip galvanizing treatment. This is to prevent generation of Mn or Si oxides formed on the surface and surface enrichment of these elements by plating Ni or Fe more intense on the surface of the cold-rolled steel sheet. As a result, it is possible to manufacture a hot-dip galvanized steel sheet free from uncoated steel sheets by virtue of increased wettability of a ground steel sheet with little surface oxidation layer and hot-dip galvanizing. If the Ni or Fe plating adhesion amount is less than 100 mg / m ² , as shown in FIG. 7, unplated occurs, and intensive corrosion occurs on the unplated surface later. Further, there is a problem that a welding crack occurs in the spot welding portion and the fatigue life is lowered.

Hereinafter, the present invention will be described more specifically by way of examples.

(Example)

Molten metal having the composition shown in Table 1 below was prepared by vacuum melting as an ingot having a thickness of 90 mm and a width of 175 mm. Subsequently, the steel sheet was reheated at 1200 ° C. for 1 hour to homogenize the steel sheet, and then hot-rolled at 900 ° C. or higher, which is a temperature higher than Ar 3, to produce a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was cooled and then charged into a furnace heated to 600 ° C in advance, maintained for 1 hour, and then subjected to hot rolling to simulate hot rolling. Then, the hot-rolled plate was cold-rolled at a cold-reduction rate of 50 to 60%, and then subjected to annealing under the conditions shown in Table 2 to prepare a final cold-rolled steel sheet.

Steel number C Si Mn P S Al Cr Ni Mo Ti B N division One 0.08 0.7 1.5 0.008 0.003 0.02 0.5 0.02 0.002 0.003 Invention river 2 0.14 1.5 2 0.012 0.005 0.14 0.02 0.02 0.05 0.004 Invention river 3 0.22 1.5 1.8 0.011 0.006 0.48 0.01 0.11 0.025 0.0017 0.004 Invention river 4 0.18 1.8 2.5 0.008 0.004 0.03 0.5 0.02 0.023 0.0015 0.006 Invention river 5 0.07 0.3 1.4 0.011 0.006 0.04 0.02 0.02 0.004 Comparative steel 6 0.35 One 1.2 0.009 0.006 0.8 0.01 0.01 0.003 Comparative steel 7 0.2 0.8 3.5 0.008 0.004 0.02 0.02 0.02 0.004 Comparative steel

Steel Nos. 1 to 4 in Table 1 satisfy the steel composition range of the present invention, and Comparative Steels 5 to 7 show cases where the contents of C, Si and Mn are out of the range of the present invention. Specifically, in the comparative steel 5, Si and Mn are all out of the lower limit, and the comparative steel 6 has a carbon content higher than the claimed range and a very high Al content. The Mn content of the comparative steel 7 is 3.5%, which is outside the claimed range of 3%.

Subsequently, the cold-rolled steel sheet having the above composition was subjected to annealing treatment under the heat treatment conditions shown in Table 2 below. Ms and Bs at this time were calculated and shown in Table 2 below. Here, the chemical element means the weight percentage of the added element, Bs means the bainite transformation start temperature Ms means the martensitic transformation start temperature. Here, Ms and Bs are calculated by the following equations

Ms = 539-423C% -30.4Mn% -16.1Si% -59.9P% + 43.6Al% -17.1Ni% -12.1Cr% + 7.5Mo%

Bs = 830-270C% -90Mn% -37Ni% -70Cr% -83Mo%

division Steel number Annealing conditions (캜) Ms
(° C) Bs
(° C) Properties Primary Secondary crack Cooling shutdown CR
(° C / s) F
(%) crack Cooling shutdown YS
(MPa) TS
(MPa) Hand
(%) HER
(%) Inventory 1 One 850 330 18 12 830 400 442 638 567 983 26.5 37 Inventory 2 2 840 350 15 7 820 420 400 607 590 1003 24.9 39 Inventory 3 3 830 310 14 5 810 390 385 605 633 1089 27.8 31 Honorable 4 4 840 300 12 2 820 400 353 521 685 1214 20.3 28 Comparative Example 5 5 850 320 20 64 820 400 463 685 608 925 19.4 33 Comparative Example 6 6 825 280 14 3 810 400 373 628 703 1151 21.3 18 Comparative Example 7 7 830 300 5 0 800 390 336 461 722 1445 8.2 43 Comparative Example 8 One 810 450 15 83 - - 442 638 350 683 31.7 56 Comparative Example 9 2 820 420 16 74 - - 400 607 422 760 25.2 24 Comparative Example 10 2 840 350 5 42 820 420 400 607 453 840 26.1 22 Comparative Example 11 3 830 440 18 67 - - 385 605 521 923 24.6 6 Comparative Example 12 3 830 310 5 31 810 390 385 605 580 1054 26.5 13 Comparative Example 13 4 810 400 17 66 - - 353 521 511 962 20.8 8 Comparative Example 14 4 840 300 5 28 820 400 353 521 536 997 21.9 16

 * In Table 2, CR denotes the cooling rate, and F denotes the ferrite area fraction of the steel after the first annealing.

In the secondary annealing, the cooling rate was all 12 ° C / s and the holding time at the cooling end temperature was 120 seconds except for Comparative Example 7. In Comparative Example 7, since the Mn content was high, the temperature was kept at 300 ° C for 300 seconds to sufficiently induce bainite transformation. The yield strength, tensile strength, elongation and hole expandability (HER) were measured on the cold-rolled steel sheet after the second annealing, and the results are also shown in Table 2 above. At this time, the tensile test specimen of JIS No. 5 was used, and the HER was evaluated as 120 x 150 mm. Specifically, in Table 2, HER is a hole expanding property and a hole is machined by a punch of 10 mm in a clear lance condition of 12%. Then, a crack is seen on the machined surface by a cone of 60 degrees from the bottom And the value obtained by the following equation (3).

[Relation 3]

HER (%) = (hole diameter after machining - hole diameter before machining, 10 mm) / hole diameter before machining

On the other hand, the second heat treated specimens were analyzed by back scattering electron diffraction (EBSD) for ferrite, bainite, retained austenite and martensite, where the ferrite and retained austenite and bainite were analyzed for the IQ distribution of EBSD Gaussian distribution is assumed to be the sum of three curves, and the kernel mean misorientation is taken at the inflection point and phase separation is performed. The grain size of ferrite was also evaluated by using an image analyzer with embedded analysis program that assumes that several hexagons are connected (ASTM E112 crystal grain measurement method). Table 3 shows differences in tissue analysis between the inventive and comparative examples.

division F B
Area fraction (%) M
Area fraction (%) G
Area fraction (%) GS (μm) Area fraction (%) Fa5 (%) Fn2 (%) Inventory 1 1.3 52.1 68.4 91.5 28.1 8.7 11.1 Inventory 2 One 36.7 22.4 91 43.8 8.6 10.9 Inventory 3 1.2 48.1 65.9 93.8 30.6 9.5 11.8 Honorable 4 1.2 46.1 51.7 92.9 32.2 11.3 10.4 Comparative Example 5 1.4 20 52.1 81.7 54.3 20.3 5.4 Comparative Example 6 1.3 10.6 38.7 79.7 62.9 18.6 7.9 Comparative Example 7 1.2 26.5 71.3 72.8 55.7 14.7 3.1 Comparative Example 8 4.2 73.1 94.6 45.2 14.2 2.1 10.6 Comparative Example 9 3.3 68.9 87.5 58.1 19.5 5.3 6.3 Comparative Example 10 2.7 62.2 83.8 77.1 24.4 3.8 9.6 Comparative Example 11 2.2 64.6 83.4 62.3 17.3 9.9 8.2 Comparative Example 12 1.9 57.3 80.1 84.9 23.2 8.3 11.2 Comparative Example 13 2.3 61.8 82.2 66.7 20.1 10.1 8 Comparative Example 14 1.8 55.3 79.9 85.8 26.5 8.7 9.5

In Table 3, F means ferrite, B means bainite, M means martensite, and G means retained austenite. GS is an average crystal grain size of ferrite, Fn2 is the above-described Relation 1, and Fa5 is Relation 2.

As shown in Table 2-3, in the case of Comparative Example 5-7 which does not satisfy the composition range suggested in the present invention, it is understood that the tensile strength, the elongation, or the HER are low even when the reverse transformation heat treatment is performed. In Comparative Example 5 in which Si or Mn is low, tensile strength and HER are both low. In Comparative Examples 6 and 7 where C, Al, and Mn were very high, the HER or elongation was low, only with a very high strength.

On the other hand, Comparative Examples 8, 9, 11, and 13, to which the present invention was applied but satisfied the ordinary annealing method, did not show high strength. That is, Comparative Example 8-9 in which carbon, Si and Mn are low exhibited excellent elongation and HER, but could not obtain a tensile strength of 980 MPa or more. In Comparative Examples 11 and 13 in which alloying elements were added in a large amount, tensile strength Although slightly lower, the HER was significantly lowered. As shown in Table 3 and Table 2, in Comparative Examples 11 and 13, the area fraction of the ferrite grains having a size of 5 탆 or more occupies 80 to 95% of the entire ferrite. When the strength is high, the strength of the second phase is very high The HER decreased sharply. This is because, in the conventional heat treatment method in which the single heat treatment is performed, primary partitioning is performed in the temperature range of coexistence of ferrite and austenite during the cracking and then secondary heat treatment is performed in the bainite transformation temperature region to perform secondary partitioning, But because of the formation of coarse polygonal ferrite and austenite during cracking.

In Table 2, Comparative Examples 10, 12 and 14 satisfied all of the first and second annealing conditions, but the coarse ferrite was formed during the cooling process because the cooling rate after the cracking of the first annealing was as low as 5 캜 / , The area fraction of the ferrite grains having an area of ferrite exceeding 60% or a size of 5 탆 or more was about 80% or more, so that the tensile strength and HER were not high.

On the other hand, an important fact discovered by the present inventors is that ferrite crystal grains are fine and, particularly, having a needle-like structure can increase both mechanical strengths which have high strength and incompatibility of hole expansion and elongation.

Fig. 1 is a photograph of the tissue showing the influence of the structure and the geometrical structure on the hole expansion and elongation. 1 (a) corresponds to Comparative Example 11 and is annealed by the conventional heat treatment method. After the above reverse annealing, it was cooled and kept at 440 캜 at which the bainite transformation took place. The coarse ferrite is formed by the formation of polygonal ferrite and austenite under anomalous reverse annealing. After cooling, the bainite transformation is performed in the austenite, and the retained austenite is stabilized at the same time. It is.

1 (b), Example 1 in which carbon, Mn, and Si are not high, but a sufficient amount of low-temperature transformed structure was formed in the primary annealing. During the secondary annealing, due to the reverse transformation of these transformed structures, martensite, The austenite and ferrite structure of the needle-like structure is obtained because the primary partitioning occurs at the interface at the appearance of austenite between the laths. After cooling again, the bainite is annealed in the bainite region, and the bainite appears from the acicular austenite, and the secondary partitioning is carried out. As a result, the austenite becomes more stable phase and remains at room temperature.

In Comparative Example 7 of FIG. 1 (c), ferrite was not formed much at a low cooling rate of the first annealing. As a result of maintaining the temperature at a low temperature for 300 seconds during the second annealing, most of the austenite And transformed into bainite.

This organizational difference affects strength, HER and elongation. As shown in Fig. 2, in the coarse polygonal ferrite and the second phase structure (a: Comparative Example 11), the crack propagates along the boundary between the ferrite and the second phase, so that the HER is very low. On the other hand, in the case where ferrite is isolated (b: Inventive Example 1) and (c: Comparative Example 7), the crack propagates while breaking the hard phase 2, On the other hand, the elongation is greatly influenced by the fraction of retained austenite. As can be seen from the EBSD results shown in Fig. 1, (a) and (b) contain 8% and 11% of retained austenite, respectively, resulting in elongations of 24.6 and 26.5%, respectively. Particularly, the fine structure 1 (b) has a high strength and an excellent elongation. It can be seen from the photograph of FIG. 4 that the needle-like ferrite and the polygonal ferrite having a length ratio of the short side to the long side of 4 are remarkably developed compared to the conventional manufacturing method.

Particularly, in order to quantitate the structural characteristics of the ferrite, an image analyzer having an analysis program of a crystal grain size assuming that several hexagons are connected (ASTM E112 crystal grain measuring method) was evaluated. The number distribution of crystal grains is very different as shown in Fig. In Example 2, fine needle-like ferrite having a particle size of about 1 mu m is distributed at a very high density, whereas in Comparative Example 12, polygonal ferrite grains having a size of 1 to 3 mu m are large and grains having a size of 3 to 5 mu m are relatively high in frequency .

Table 3 shows the analysis of the structural characteristics of the test specimens subjected to the tempering conditions of Table 1 and the heat treatment conditions of Table 2. As shown in Table 3 and Table 2, the ferrite has an average diameter of 2 占 퐉 or less, Fn2 defined by the relational expression 1 of the ferrite is 89% or more, and Fa5 defined by the relational expression 2 satisfies 70% When very fine needle-shaped ferrite is developed, it can be found that both HER and ductility and strength are excellent.

Fig. 6 shows the influence of the Ni plating amount on the phosphate treatment. After the first and second annealing, the Ni plated amount was changed to 50 mg / m < ² > Nickel was used for the Ni plating solution, and the plating amount was changed by adjusting the current at a constant PH condition. Following, 45 ℃ surface component after forming the 150 seconds film in the phosphate solution was washed with water and dried, to observe the film determined by secondary electron microscope Meanwhile, GDS analysis on a specimen of Ni coating weight 3mg / m ² and 30mg / m ² Respectively.

As shown in Fig. 6 (a), as the amount of Ni plating increases, crystals of phosphate are precipitated. This is because the rate of growth is faster than the rate of nucleation. On the other hand, in the specimen with Ni plating amount of 3 mg / m ² , phosphate nucleation is difficult due to the influence of surface oxide,

Fig. 6 (b) shows the results of GDS analysis for Ni plating amounts of 3 mg / m ² and 30 mg / m ² . As described above, in the specimen having low Ni plating, surface oxides and internal oxides were present on the surface of the base steel sheet, and concentration of Si and Mn was large, and the concentration of oxygen was high on the surface. On the other hand, the specimen with the Ni plating amount of 30 mg / m ² had a low concentration of oxygen due to the oxygen blocking action of the surface Ni, and as a result, the amount of surface concentrated Si and Mn was not high.

Fig. 7 shows the result of Ni plating after 10, 150 mg / m < ² > before hot-dip galvanizing annealing after primary annealing, and hot-dip galvanizing. In the specimen of 10 mg / m ² , some oxide was present on the surface during the secondary annealing, and an unplated layer was observed. In the specimen of 150 mg / m ² , the plating surface was fine and no unplated defects were observed. This is because the formation of Mn or Si oxides formed on the surface and the surface enrichment of these elements are prevented by plating more Ni on the surface.

Fig. 8 shows the results of spot welding after 10 to 300 mg / m < ² > of Ni after the first annealing and before the second hot dip galvanizing annealing, and observing cracks in the weld cross section. For spot welding, the pressing force was 4 kN and the welding current was 7 kN. As a result, weld cracks did not occur in the Ni-plated specimen of 100 mg / m ² . This is because Ni is diffused into the surface and the plating layer of the steel and melts to increase the melting temperature of the plating layer. Therefore, welding crack occurs when the molten zinc penetrates into the grain boundary of the steel sheet under stress, Which increases the penetration temperature of liquid zinc.

As a result, the cold-rolled steel sheet produced according to the present invention has a tensile strength of 980 MPa or more and an excellent elongation, as well as excellent phosphate treatment and plating adhesion. Accordingly, the corrosion resistance of the parts is improved, welding cracks are not generated, and the fatigue life of the assembled parts is extremely excellent, so that cold forming for application to the structural member is facilitated compared with the steel produced through the conventional Q & P heat treatment process And the durability of the parts can be remarkably improved.

While the present invention has been particularly shown and described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, Of course, this is possible. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined by the equivalents as well as the claims that follow.

Claims (12)

(Si): 0.6 to 2.5%, aluminum (Al): 0.01 to 0.5%, manganese (Mn): 1.5 to 3.0%, the balance Fe and unavoidable impurities Including,
Wherein the steel microstructure contains ferrite of 60% or less, acicular bainite of 25% or more, martensite of 5% or more and acicular retained austenite of 5% or more in an area fraction,
The ferrite has an average diameter of 2 占 퐉 or less,
The ferrite satisfies that Fn2 defined by the following [Relation 1] is not less than 89%, and Fa5 defined by the following [Relation 2] is not more than 70%, and
And an Ni or Fe plated layer is formed on the surface thereof with an adhesion amount of 5 to 40 mg / m < ² >. The high strength cold rolled steel sheet excellent in surface treatment characteristics and weldability.
[Relation 1]
Fn2 = [number of ferrite grains of 2 mu m or less / number of total ferrite grains] x 100
[Relation 2]
Fa5 = [area of ferrite grains larger than 5 mu m / area of entire ferrite grains] x 100
The high-strength cold-rolled steel sheet according to any one of claims 1 to 3, further comprising at least one of Cr, Ni and Mo in an amount of not more than 2% (here, 0% is not included) .
The steel sheet according to claim 1, further comprising Ti of not more than 0.05% (excluding 0%) and B of not more than 0.003% (wherein 0% is not included). Cold rolled steel sheets.
A hot-dip galvanized steel sheet in which a hot-dip galvanized layer is formed on a surface of a cold-rolled steel sheet,
(Si): 0.6 to 2.5%, aluminum (Al): 0.01 to 0.5%, manganese (Mn): 1.5 to 3.0%, the balance Fe and unavoidable impurities Including,
Wherein the microstructure contains an area fraction of not more than 60% of ferrite, not less than 25% of acicular bainite, not less than 5% of martensite and not less than 5% of austenite retained austenite,
The ferrite has an average diameter of 2 占 퐉 or less,
The ferrite satisfies that Fn2 defined by the following [Relation 1] is not less than 89%, and Fa5 defined by the following [Relation 2] is not more than 70%, and
And a Ni or Fe plating layer is formed between the cold-rolled steel sheet and the hot-dip galvanized layer in an amount of 100 mg / m ² or more. The high-strength hot-dip galvanized steel sheet is excellent in surface treatment characteristics and weldability.
[Relation 1]
Fn2 = [number of ferrite grains of 2 mu m or less / number of total ferrite grains] x 100
[Relation 2]
Fa5 = [area of ferrite grains larger than 5 mu m / area of entire ferrite grains] x 100
A galvannealed steel sheet obtained by subjecting the hot-dip galvanized steel sheet of claim 4 to an alloying heat treatment.
(Si): 0.6 to 2.5%, aluminum (Al): 0.01 to 0.5%, manganese (Mn): 1.5 to 3.0%, the balance Fe and unavoidable impurities Preparing a steel slab containing the steel slab, and reheating the steel slab;
Rolling the reheated steel slab under normal hot rolling conditions and then winding it in a temperature range of 750 to 550 ° C;
A step of cold-rolling the wound hot-rolled steel sheet to produce a cold-rolled steel sheet;
A primary annealing step in which the cold-rolled steel sheet is heated to a temperature equal to or higher than Ac3 and then cooled to 350 DEG C or less at a cooling rate of less than 20 DEG C / s;
After the first annealing, the steel sheet is heated and maintained at a temperature in a range of Ac1 to Ac3, cooled to a temperature range of Ms to Bs at a cooling rate of less than 20 deg. C / s, Annealing step; And
And forming a Ni or Fe plating layer on the surface of the secondary annealed steel sheet with an adhesion amount of 5 to 40 mg / m < ² >. The present invention also relates to a method of manufacturing a high strength cold rolled steel sheet having excellent surface treatment characteristics and weldability.
The high-strength cold-rolled steel sheet according to claim 6, further comprising at least one of Cr, Ni and Mo in an amount of 2% or less (here, 0% is not included) Gt;
The steel sheet according to claim 6, further comprising Ti of not more than 0.05% (excluding 0%) and B of not more than 0.003% (wherein 0% is not included). Cold rolled steel sheet manufacturing method.
The high-strength cold-rolled steel sheet according to claim 6, wherein a Ni or Fe plating layer is formed on the surface of the steel sheet with an adhesion amount of 5 to 40 mg / m ² after the primary annealing and after the secondary annealing. ≪ / RTI >
7. The cold-rolled steel sheet according to claim 6, wherein the cold-rolled steel sheet has a microstructure in an area fraction of 20% or less of ferrite and a residual low-temperature transformation texture before the second annealing step. Gt;
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