KR101736634B1

KR101736634B1 - Cold-rolled steel sheet and galvanized steel sheet having excellent hole expansion and ductility and method for manufacturing thereof

Info

Publication number: KR101736634B1
Application number: KR1020150185458A
Authority: KR
Inventors: 곽재현; 조항식; 신동석
Original assignee: 주식회사 포스코
Priority date: 2015-12-23
Filing date: 2015-12-23
Publication date: 2017-05-17

Abstract

The present invention provides a high strength cold-rolled steel sheet having excellent ductility and excellent hole processing performance, a galvanized steel sheet, and a manufacturing method for the same. According to the present invention, the cold-rolled steel sheet comprises: 0.05-00.3 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 remaining consisting of Fe and inevitable impurities. A steel fine tissue contains 60% or less of ferrite, 25% or greater of lath type bainite, 5% or greater of martensite, and 5% or greater of lath type residual austenite. The ferrite has an average diameter of 2 m or less; and the ferrite satisfies Fn2 defined by equation 1 of being 89% or greater, and Fa5 defined by equation 2 of being 70% or less.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet, a hot-rolled steel sheet, and a method of manufacturing the same,

More particularly, the present invention relates to a high-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet and a method of manufacturing the same, which are excellent in ductility and hole expandability and excellent in press formability .

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 ductility and hole expandability are relatively lowered. When ductility and hole expandability are lowered, breakage easily occurs in the press-molded article, and the degree of freedom in the shape of the molded article is remarkably reduced.

Therefore, many studies have been made to improve the relationship between strength, ductility and hole expandability, and as a result, a transformed structure steel utilizing residual austenite phase together with martensite and bainite, which are low temperature structures, have been developed and applied.

As is well known, strength and ductility are inversely related. The higher the elongation ratio in an ultrahigh strength steel having a tensile strength of 980 MPa or more, the deeper the depth of the press molded product can be, and the more complicated the shape can be molded. The hole expandability is inversely proportional to the strength as in ductility, but the steel sheet having the same strength is excellent in ductility, so that the hole expanding performance is not necessarily excellent. Hole expandability is an indicator of workability when flange forming to extend the machined hole. The hole expansion is performed by punching the hole before expanding. In this case, a lot of fine holes or cracks are generated in the hole machining part, and when extended processing, micropores or cracks grow to break.

If the hole expandability is low, there is a tendency that not only the hole expandability of the component but also cracks easily occur from the flange portion of the blank during press forming, so efforts are made to simultaneously secure the hole expanding performance and elongation [JP2010-038035, JP2012-159387, PCT-IB2013-001708].

In order to ensure ductility and hole expandability at the same time, these methods utilize tempered martensite and make the hardness ratio of ferrite and tempered martensite 3 or less, add Bi, anneal, A method of tempering the martensite is used. However, in the case of JP2010-038035 mentioned above, since the coiling temperature after hot rolling is set to a low temperature of 300 to 550 占 폚, when bainite or martensite is introduced by low temperature coiling, the strength difference in the hot and cold width direction is large, There is a problem that plate breakage easily occurs. In the case of JP2012-159387, Bi is added for uniform elongation and yield ratio control. However, there is a problem in that Bi addition causes cracks during performance and hot rolling due to loss due to volatilization during steelmaking and segregation of Bi having a low melting point. In the case of PCT-IB2013-001708, tempering is performed at a temperature range of 150 to 500 ° C in an annealing furnace after galvannealed hot dip galvanizing in the usual manner to ensure hole expandability. However, brittle fracture due to P segregation is likely to occur, There is a problem that the manufacturing cost is increased.

Accordingly, it is an object of the present invention to provide a cold-rolled steel sheet having excellent ductility and hole expandability compared to conventional methods, while using a conventional alloy component by constructing a unique structure utilizing reverse- , A hot-dip galvanized steel sheet and a galvannealed hot-dip galvanized steel sheet.

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 the following conditions: 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 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).

Further, the present invention can provide a hot-dip galvanized steel sheet which is subjected to hot-dip galvanizing treatment on the surface of the cold-rolled steel sheet.

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; And

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, And an annealing step of producing a high-strength cold-rolled steel sheet excellent in ductility and pitting ability.

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.

According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more and excellent ductility and hole expandability, as compared with Q & P steel subjected to high-ductile transformation texture steel such as DP steel or TRIP steel and Q & A galvanized steel sheet and a galvannealed galvanized steel sheet can be effectively provided.

Therefore, the cold-rolled steel sheet of the present invention has an advantage of being highly applicable to 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.

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.

The present invention has been accomplished on the basis of finding a steel composition range capable of obtaining the above-mentioned microstructure even under a condition where the cooling rate is much lower than the conventional one to obtain an excellent plate shape.

The high-strength cold-rolled steel sheet excellent in ductility and hole expanding performance of 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 inhibition of bainite transformation to decrease 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.

In the high-strength cold-rolled steel sheet excellent in ductility and hole expanding performance of the present invention, the steel microstructure has an area fraction of not more than 60% of ferrite, not less than 25% of needle-shaped bainite, not less than 5% of martensite, % Or more. That is, in the cold-rolled steel sheet of the present invention, the steel microstructure includes ferrite, lath-type bainite, 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 by cooling from a chemically unstable austenite 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]

[Relation 2]

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.

Furthermore, the present invention is not limited to the cold-rolled steel sheet having the above-mentioned composition and structure, 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.

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 is for producing a cold-rolled steel sheet containing a needle-like ferrite and needle-like retained austenite phase as main phases, which is a final microstructure having four or more major axes and minor axes. In order to obtain such a cold-rolled steel sheet, Do. 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.

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. 3 (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 first redistribution of the alloying elements is completed, the alloy is cooled to a temperature range of Ms (martensitic transformation start temperature) to Bs (bainite transformation tempering 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.

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 inducing secondary redistribution by reheating, it is possible to obtain fine needle-shaped microstructure as shown in Fig. 4 which is finer than the tissue obtained by the conventional method, and thus it is possible to secure both excellent hole expandability and elongation.

[Plating process]

The cold-rolled steel sheet subjected to the first annealing may be subjected to a plating process using a hot-dip coating process or an alloying hot-dip process in a secondary annealing 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.

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 in 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 a 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 reverse inverse 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.

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

(Al): 0.01 to 0.5%, manganese (Mn): 1.5 to 3.0%, and at least one selected from the group consisting of Cr, Ni, Mo Of the total of at least one species or two or more species: 2% or less (0% is excluded), the balance Fe and unavoidable impurities,
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 the following conditions: 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

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The high-strength cold-rolled steel sheet according to any one of claims 1 to 3, further comprising not more than 0.05% Ti (not including 0%) and B not more than 0.003% (wherein 0% is not included) Steel plate.

A hot-dip galvanized steel sheet obtained by hot-dip galvanizing the surface of a cold-rolled steel sheet according to any one of claims 1 to 3.

A galvannealed steel sheet obtained by subjecting the hot-dip galvanized steel sheet of claim 4 to an alloying heat treatment.

(Al): 0.01 to 0.5%, manganese (Mn): 1.5 to 3.0%, and at least one selected from the group consisting of Cr, Ni, Mo (2)% or less (here, 0% is not included), the balance Fe and inevitable impurities, 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; And
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, And annealing the steel sheet. The method of producing a high-strength cold-rolled steel sheet excellent in ductility and pitting ability.

delete

The high-strength cold-rolled 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) Steel plate manufacturing method.

7. The method of manufacturing a high-strength 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 ferrite and a residual low-temperature transformation texture before the second annealing step .