KR101899688B1 - High strength hot-rolled steel sheet having excellent continuously producing property, high strength gavanized steel sheet having excellent surface property and plating adhesion and method for manufacturing thereof - Google Patents

Abstract

0.1 to 0.3% of C, 1 to 2.0% of Si, 2.6 to 5% of Mn, 0.001 to 2% of sol.Al, Ti: (48/14) : Not more than 0.04% (excluding 0%), S: not more than 0.015% (excluding 0%), N: not more than 0.02% (excluding 0%), Sb, Bi, Sn, Wherein the inner surface of the hot-rolled steel sheet has an inner oxide containing at least one element selected from the group consisting of Si, Mn, Al and Fe, and the maximum depth of the inner oxide is 1 μm or less A high-strength hot-rolled steel sheet, and the like.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength hot-rolled steel sheet excellent in continuous productivity, a high-strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion, and a method of manufacturing the same. BACKGROUND OF THE INVENTION ADHESION AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a high-strength hot-rolled steel sheet excellent in continuous productivity, a high-strength hot-dip galvanized steel sheet excellent in surface quality and adhesion to plating, and a method of manufacturing the same.

Recently, there has been a continuing demand for strengthening of automotive steel sheets in order to improve the weight of automobiles and the stability of collision of automobiles due to the regulation of carbon dioxide for global environmental preservation.

In order to meet these demands, a high strength steel sheet having a strength of 950 MPa or more has recently been developed and applied to automobiles.

As a method of increasing the strength of a steel sheet, it is possible to easily produce a steel sheet of high strength by increasing the addition amount of reinforcing components of steel including carbon. However, in the case of steel sheet for automobile steel sheet, Therefore, the elongation of the steel sheet must be secured at the same time.

In order to secure the strength and ductility of steel sheets for automobiles, Mn, Si, Al, Cr, Ti, etc. are added as ingredients mainly added to the steel. If the amount of these additives is controlled and the manufacturing process conditions are controlled, And a steel sheet having ductility can be produced.

Generally, in order to extend the life of automobiles, steel sheets used in automobiles need to be improved in corrosion resistance, and hot-dip galvanized steel sheets are used for this purpose.

In the case of automotive high-strength steel plates with a strength of 950 MPa or more, components such as Si, Mn and Al are added to the steel to ensure the desired strength and elongation. However, high-strength steel sheets containing Mn, Si and / or Al which are easy to oxidize in steel react with trace amounts of oxygen or water vapor present in the annealing furnace to form Mn, Si, Al alone or composite oxides on the steel sheet surface, The wettability of zinc is interfered with, resulting in uneven plating which is not adhered locally or globally to the surface of the coated steel sheet, thereby significantly deteriorating the quality of the surface of the coated steel sheet.

A conventional continuous process for producing such a high strength galvanized steel sheet comprises melting the iron to make a slab through continuous casting, heating the slab to a high temperature to perform hot rolling, removing iron oxide on the surface of the hot rolled steel sheet in a pickling process, To prepare a steel sheet, and thereafter performing annealing in a hot-dip coating facility to perform hot-dip coating.

In the case of a high strength steel containing a large amount of Si, Mn and / or Al in the steel, when the coiling temperature for rolling the steel sheet after hot rolling is high, Mn, Si and Al are added to the main constituents of Fe, Si, Mn, If the cold rolling is performed, the oxides existing in the grain boundaries of the steel sheet surface are broken by the rolling, and the grains are weakened and are easily removed. In the annealing and plating process, The crystal grains dropped from the surface are adhered to various rolls and then transferred in a form stamped on the steel sheet, resulting in a defect called a dent.

In order to prevent internal oxidation in such a hot rolling process, it is possible to set the coiling temperature to a low temperature of 550 DEG C or less so that internal oxidation does not occur. However, in a high-strength steel, when the coiling temperature is low, the structure of the hot-rolled steel sheet includes bainite or martensite structure, which causes an increase in strength of the hot-rolled steel sheet, It is usually required to be higher than 600 DEG C and the higher the strength of the hot-rolled steel sheet is, the higher the coiling temperature is required to reduce the load applied to the roll during cold rolling, thereby enabling cold rolling. However, the higher the coiling temperature, the more the internal oxidation depth increases. Also, the higher the content of oxidizing components such as Mn, Si, and Al in the steel at the same coiling temperature, It has a big influence.

On the other hand, when the content of Mn, Si, Al, etc., especially Si, is low in the steel, internal oxidation hardly occurs even if the coiling temperature is increased. However, in order to obtain the desired strength and elongation in a product having a tensile strength of 950 MPa or more, Addition is essential.

In the steel sheet in which internal oxidation occurs, the internal oxides present in the grain boundaries are also partially dissolved by the acid in the pickling process, so that the grain boundaries are weakened. In the cold rolling process, the grain boundaries in which the internal oxidation occurs are destroyed and the crystal grains are excited. Inside the furnace, the crystal grains are removed by various rolls and attached to the rolls, causing a dent defect in the form of being imprinted on the steel sheet.

Therefore, dent defects do not occur when steel sheets are produced for the first time, but as production continues, the number of dents on the steel sheet increases, making production more difficult. On the other hand, the dent generated on the steel sheet is a problem due to a concave defect in the form of a shot, which is clearly visible during the painting process after machining into an automotive part.

As a result, in order to produce a hot-dip galvanized steel sheet having a high strength of 950 MPa or higher in tensile strength, which is used for automobile structural members and the like, and which can be continuously and stably produced without occurrence of dent defects, The invention has been proposed.

Various techniques have been proposed to improve the quality of high strength steel plating. Among them, Patent Document 1 discloses a method of oxidizing a steel sheet in a direct flame furnace in an oxidizing atmosphere by controlling air and fuel at an air-fuel ratio of 0.80 to 0.95 in an annealing process, Or a composite oxide, and then reducing and annealing the mixture in a reducing atmosphere to reduce the iron oxide, followed by hot-dip galvanizing. In the annealing process, if the oxidation-reduction method is used, the components having high affinity with oxygen such as Si, Mn, and Al at a predetermined depth from the surface layer of the steel sheet are internally oxidized to inhibit diffusion to the surface layer. So that the wettability with zinc in the plating bath can be improved and the plating can be reduced. That is, when heated under a high oxygen partial pressure that can oxidize iron, the iron is oxidized up to a certain depth of the surface layer to form an iron oxide layer. Elements that are more easily oxidized than iron are oxidized below the iron oxide layer and are no longer diffused to the surface because they are present as oxides. In the subsequent reduction process, the iron oxide is easily reduced to iron in an atmosphere containing a certain amount of hydrogen and is present as a reduced iron layer in the surface layer, so that the wettability with zinc is improved and the plating ability is improved. However, even if the plating ability is improved by the post-oxidation reduction method of the high-strength steel sheet, it is not possible to prevent the liquefied brittle crack at the time of spot welding in the post-molding assembly process using the steel sheet.

As another method for improving high-strength steel plating performance, Patent Document 2 discloses a method of oxidizing internal components of a steel such as Mn, Si, and Al which are easy to oxidize while maintaining a high dew point in the annealing furnace, The present invention provides a technique for improving the plating ability by reducing oxides which are externally oxidized in the process of the present invention. Internal oxidation of the oxidizing component by this method has a feature of reducing external oxidation and improving the plating ability. However, as in the case of the internal oxide in the hot rolling process, internal oxides formed in the grain boundary of the steel sheet surface layer during annealing are also adhered to various rolls in the annealing furnace There is a problem that a dent defect occurs.

Accordingly, the present invention relates to a steel sheet having a high strength steel containing a large amount of Mn, Si, and / or Al in a steel, and capable of suppressing the generation of internal oxides in the surface layer of the steel sheet after the hot rolling, Which is excellent in surface quality and plating adhesion, by suppressing the formation of single or complex oxides such as Mn, Si and Al on the surface of the steel sheet during the annealing process, and a method for producing the same.

Korean Patent Publication No. 10-2010-0030627 Korean Patent Laid-Open No. 10-2009-0006881

One of the objects of the present invention is to provide a high-strength hot-rolled steel sheet excellent in continuous productivity, a high-strength hot-dip galvanized steel sheet excellent in surface quality and adhesion to plating, and a method for producing the same.

In one aspect of the present invention, there is provided a ferritic stainless steel comprising 0.14 to 0.3% of C, 1 to 2.0% of Si, 2.6 to 5% of Mn, 0.001 to 2% of sol. S, 0.015% or less (excluding 0%), N: 0.02% or less (excluding 0%), at least one of Sb, Bi, Sn and Zn : 0.08 to 0.2% in total, the balance being Fe and inevitable impurities, wherein the hot-rolled steel sheet has an inner oxide containing at least one element selected from the group consisting of Si, Mn, Al and Fe in the surface layer portion, And a depth of 1 μm or less (including 0 μm).

In another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: 0.14 to 0.3% of C, 1 to 2.0% of Si, 2.6 to 5% of Mn, 0.001 to 2% of sol.Al, S, 0.015% or less (excluding 0%), N: 0.02% or less (excluding 0%), at least one of Sb, Bi, Sn and Zn : A total of 0.08 to 0.2%, a remainder Fe and inevitable impurities, and a hot-dip galvanized layer formed on the surface of the cold-rolled steel sheet, wherein at least one of Si, Mn, An inner oxide containing an element is present, and a maximum depth of the inner oxide is 0.2 탆 or less (excluding 0 탆).

Another aspect of the present invention is a ferritic stainless steel comprising 0.14 to 0.3% of C, 1 to 2.0% of Si, 2.6 to 5% of Mn, 0.001 to 2% of sol. S: not more than 0.015% (excluding 0%), N: not more than 0.02% (excluding 0%), Sb, Bi, Sn and Zn Or more and 0.08 to 0.2% in total, remainder of the slab containing Fe and inevitable impurities, and then subjecting the hot rolled steel sheet to a hot rolling under the condition of a finish hot rolling temperature Ar 3 캜 or higher, And cooling the rolled hot-rolled steel sheet to a temperature of 550 DEG C or lower at an average cooling rate of 2 DEG C / min or more. The present invention also provides a method of manufacturing a high-strength hot-rolled steel sheet.

Another aspect of the present invention is a ferritic stainless steel comprising 0.14 to 0.3% of C, 1 to 2.0% of Si, 2.6 to 5% of Mn, 0.001 to 2% of sol. S: not more than 0.015% (excluding 0%), N: not more than 0.02% (excluding 0%), Sb, Bi, Sn and Zn Or more and 0.08 to 0.2% in total, remainder of the slab containing Fe and inevitable impurities, and then subjecting the hot rolled steel sheet to a hot rolling under the condition of a finish hot rolling temperature Ar 3 캜 or higher, Cooling the rolled hot-rolled steel sheet to a temperature of 550 DEG C or lower at an average cooling rate of 2 DEG C / min or more, cold-rolling the cooled hot-rolled steel sheet to obtain a cold-rolled steel sheet, Heating the steel sheet to a temperature of 820 to 870 캜 under a temperature of -60 to -30 캜, and then performing recrystallization annealing by holding the steel sheet for 5 to 120 seconds; ℃ Holding the cooled and maintained cold-rolled steel sheet at a temperature of 460 to 500 ° C at a rate of 30 ° C / sec or more, and then immersing it in a zinc plating bath within 7 seconds, And a method of manufacturing a high-strength hot-dip galvanized steel sheet.

As one of various effects of the present invention, the high-strength hot-rolled steel sheet according to the present invention is advantageous in continuous productivity.

Further, the high strength hot-dip galvanized steel sheet according to the present invention has an advantage of excellent surface quality and plating adhesion.

The various and advantageous advantages and effects of the present invention are not limited to the above description, and can be more easily understood in the course of describing a specific embodiment of the present invention.

1 is a graphical representation of a result of analysis of a cold-rolled steel sheet according to Inventive Example 9 by 3D-AP.
2 (a) is an SEM image of a cross-section of a cold-rolled steel sheet according to Comparative Example 31;

Hereinafter, a high-strength hot-rolled steel sheet excellent in continuous productivity, which is one aspect of the present invention, will be described in detail.

First, the alloy component and the preferable content range of the high-strength hot-rolled steel sheet of the present invention will be described in detail. It is to be noted that the content of each component described below is based on weight unless otherwise specified.

C: 0.14 to 0.3%

C is an essential element for securing the strength of martensite. In order to obtain such an effect in the present invention, it is preferable that C is contained at 0.14% or more. However, if the content is excessive, ductility, bending workability and weldability are decreased to deteriorate press formability and roll workability, and the upper limit is preferably limited to 0.3%.

Si: 1 to 2.0%

Si improves the yield strength of steel and stabilizes ferrite and retained austenite at room temperature. Especially, TRIP (Tranformation Induced Plasticity) steel inhibits precipitation of cementite from austenite during cooling and inhibits growth of carbide Thereby contributing to stabilizing the retained austenite. Therefore, as in the present invention, it is an essential element for producing a steel sheet having a tensile strength of 950 MPa or more and excellent ductility. In order to obtain such an effect in the present invention, the content is preferably 1% or more, more preferably 1.1% or more. However, if the content is excessive, an inner oxide containing a large amount of Si is formed in the grain boundaries of the hot-rolled steel sheet and in the mouth. Due to the inner oxide, the surface layer crystals are removed during the annealing process after cold rolling, Type defects. In addition, since the Ar3 transformation temperature rises significantly during hot rolling, the strength of the hot-rolled steel sheet is greatly increased because the austenite and ferrite are rolled in the opposite direction, thereby causing cracks in cold rolling. Considering this, the upper limit of the Si content is preferably limited to 2.0%.

Mn: 2.6 to 5%

Mn is well known as an element for increasing hardenability which suppresses ferrite formation and stabilizes austenite. In particular, in order to secure the desired strength and ductility in the present invention, it is preferable that Mn is contained in an amount of 2.6% or more. However, if the content thereof is excessive, it is difficult to secure the plating ability by increasing the surface oxidation amount of Mn in the annealing process. Therefore, the upper limit is preferably limited to 5%.

sol.Al: 0.001 to 2%

sol.Al is an element added for deoxidation in the steelmaking process. In addition, sol.Al also helps in the formation of carbonitrides and also helps reduce annealing costs by reducing the Ac1 transformation point by enlarging the ferrite phase. In order to obtain such effects in the present invention, it is preferable that the content is 0.001% or more. However, if the content thereof is excessive, it is difficult to secure the plating ability by increasing the amount of surface oxidation of sol.Al in the annealing process, so that the upper limit is preferably limited to 2%.

Ti: (48/14) * [N] to 0.1%

Ti is a nitride-forming element, which serves to lower the content of solid solution N in the steel and to suppress the occurrence of hot rolling cracks due to the formation of AlN. In order to obtain this effect in the present invention, it is necessary to add (48/14) * [N] or more in terms of chemical equivalent. However, if the content is excessive, the carbon concentration and the strength of martensite may be reduced due to the precipitation of additional carbides other than the removal of the solid solution N. Therefore, the upper limit is preferably limited to 0.1%.

P: 0.04% or less (excluding 0%)

P is an impurity inevitably contained in steel. When the content is excessive, the weldability is lowered, the possibility of brittleness of steel is increased, and the possibility of dent defect occurrence is increased. In order to prevent this, the upper limit of the P content is preferably limited to 0.04%.

S: 0.015% or less (excluding 0%)

S, like P, is an impurity that is inevitably contained in the steel. If its content is excessive, the ductility and weldability of the steel deteriorate. In order to prevent this, the upper limit of the S content is preferably limited to 0.015%.

N: 0.02% or less (excluding 0%)

N is an impurity inevitably contained in the steel. When the content is excessive, the risk of cracking during performance is greatly increased due to the formation of AlN. In order to prevent this, the upper limit of the N content is preferably limited to 0.02%.

Sb, Bi, Sn and Zn: 0.08 to 0.2% in total

In the present invention, Sb, Sn, Bi and Zn are added for two reasons.

First, when these components are added to the steel, they are concentrated at the hot rolled steel sheet surface and surface layer at the high coiling temperature after hot rolling to prevent diffusion of oxygen into the steel, and the inside of the steel is composed of Si, Mn, Al and Fe There is an effect of inhibiting the formation of oxides.

Second, Sb, Sn, Bi and Zn components are first concentrated on the surface of steel sheet and surface layer during hot annealing process after pickling and cold rolling of hot-rolled steel sheet to prevent diffusion of oxidative components such as Si, Mn and Al in steel By suppressing the formation of oxides composed of Si, Mn and Al on the surface of the annealed steel sheet, it plays a role of facilitating the adhesion of zinc in the zinc plating bath.

Mn, and Al on the surface of the annealed steel sheet even when the amount of one or more of the Sb, Sn, Bi, and Zn components is less than 0.08%, zinc is easily adhered in the zinc plating bath, And a high-strength hot-dip galvanized steel sheet excellent in adhesion can be produced. However, since the internal oxidation of the hot-rolled steel sheet can not be sufficiently suppressed, dent defects are caused in the steel sheet due to the adhesion of the surface layer crystals to various rolls in the annealing furnace. As a result, Therefore, even if a high-strength hot-dip galvanized steel sheet having excellent plating quality and adhesion is manufactured, the first one to two coils can not be produced due to the occurrence of dent defects thereafter.

Therefore, if one or more of the Sb, Sn, Bi, and Zn components to be added to the steel should be at least 0.08% or more in order to suppress internal oxidation, the internal oxidation depth of the hot- , It is possible to prevent a dent defect in annealing process after cold rolling. As a result, the oxidation of the hot-rolled steel can be suppressed, so that continuous production is possible without dent occurrence in the subsequent annealing process. At the same time, the minimum addition amount to suppress the formation of annealed oxide on the steel sheet surface during annealing is limited to 0.08% .

However, if the contents of the steel such as Sb, Sn, Bi and Zn are excessive, the internal oxidation inhibiting effect is excellent, but the ductility is deteriorated. In the present invention, a specific heat treatment is performed in order to prevent the ductility from lowering due to the addition of these components. When the sum of these components exceeds 0.2%, even if the heat treatment proposed in the present invention is applied, the desired elongation The upper limit of the sum of the contents of at least one of Sb, Sn, Bi, and Zn is preferably limited to 0.2%.

The rest of the composition is Fe. However, it is not possible to exclude inevitable impurities that are not intended from the raw material or the surrounding environment in a conventional manufacturing process, since they may be inevitably incorporated. For example, Cu, Mg, Co, Ca, Na, V, Ga, and the like may be used as the impurities. The impurities may be any one of ordinary skill in the art, , Ge, As, Se, In, Ag, W, Pb and Cd. When the content of each of these elements is less than 0.1%, the effect of the present invention is not impaired.

On the other hand, addition of an effective component other than the above-mentioned composition is not excluded, and it may further include at least one selected from the group consisting of Cr, Mo, Nb and B, for example.

Cr: not more than 1.0%

Cr is an effective component for securing the strength of steel as an element for increasing hardenability, but there is no great problem in securing physical properties without adding Cr. On the other hand, if the Cr content is excessive, the manufacturing cost sharply increases, which is not desirable. Considering this, the upper limit of the Cr content is preferably limited to 1.0%.

Mo: not more than 0.2%

Mo is an effective component for improving the strength of steel without deteriorating the melt zinc wettability, but there is no problem in securing the physical properties without adding it. On the other hand, if the Mo content is excessive, the manufacturing cost sharply increases, which is not preferable. Considering this, the upper limit of the Mo content is preferably limited to 0.2%.

Nb: not more than 0.1%

Nb is segregated in the form of carbide in the austenite grain boundaries to inhibit coarsening of crystal grains of austenite during annealing and improve the strength of steel. However, even if Nb is not added, there is no problem in securing the physical properties. On the other hand, if the Nb content is excessive, the manufacturing cost sharply increases, which is not preferable. Considering this, the upper limit of the Nb content is preferably limited to 0.1%.

B: not more than 0.005%

B is an effective component for securing the strength of the steel, but there is no problem in terms of securing the physical properties without addition of B. However, if the B content is excessive, it may be concentrated on the surface of the annealed steel to greatly deteriorate the plating ability, and the upper limit of the content is preferably limited to 0.005%.

Hereinafter, the microstructure and the like of the high-strength hot-rolled steel sheet of the present invention will be described in detail.

In the surface layer portion of the hot-rolled steel sheet of the present invention, an inner oxide containing at least one element of Si, Mn, Al and Fe is present, and the maximum depth of the inner oxide is 1 μm or less (including 0 μm).

In the case of the hot-rolled steel sheet of the present invention, Si, Mn, Al and the like, which are superficial thickening elements, are added in large amounts, and thus inner oxide is unavoidably formed in the surface layer portion. As a result of research conducted by the inventors of the present invention, when the maximum depth of the internal oxide exceeds a certain range, dent defects are caused in the annealing process after cold rolling, thereby deteriorating the continuous productivity. In order to prevent this, it is necessary to suppress the maximum depth of the internal oxide to 1 μm or less (including 0 μm). Meaning that the maximum depth of the inner oxide includes 0 mu m means that the inner oxide does not exist at all.

On the other hand, in the present invention, the microstructure of the hot-rolled steel sheet is not particularly limited, and for example, one or more of ferrite, pearlite and bainite may be contained in an appropriate ratio. However, when the double bainite is excessively formed, the strength of the hot-rolled steel sheet is excessively increased, and cracks may be caused in the steel sheet during the cold rolling process. In order to prevent this, the upper limit of the area fraction of bainite is preferably controlled to 50%.

Hereinafter, a high-strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion, which is another aspect of the present invention, will be described in detail.

A high strength hot dip galvanized steel sheet according to another aspect of the present invention comprises a cold rolled steel sheet having the aforementioned component system and a hot dip galvanized layer formed on the surface of the cold rolled steel sheet, Wherein an inner oxide containing at least two kinds of elements is present, and a maximum depth of the inner oxide is 0.2 탆 or less (excluding 0 탆). If the maximum depth of the inner oxide of the cold-rolled steel sheet exceeds 0.2 탆, dent defects may be caused in the annealing process, thereby deteriorating the continuous productivity.

According to one example, the sum of the content of at least one of Sb, Bi, Sn and Zn in the region from the surface of the cold-rolled steel sheet to the position of 0.005 m in the thickness direction from the surface of the cold-rolled steel sheet, , The sum of the contents of at least one of Sb, Bi, Sn, and Zn in the region up to 3 to 15 times.

The concentration of Sb, Bi, Sn and Zn components in the surface layer of the cold-rolled steel sheet suppresses the surface diffusion of Mn, Si and / or Al in a high-temperature annealing process. In order to ensure the plating quality and plating adhesion, the average content of at least 0.01 .mu.m in the thickness direction of the base steel from the base steel surface to the average thickness of 0.005 .mu.m in the direction of the thickness of the base steel from the plating layer and the base of the base steel It is necessary to concentrate at least three times the contrast. However, when it exceeds 15 times, the upper limit is limited to 15 times because the adhesion is reduced by interfering with the formation of the alloying inhibition layer composed of Fe-Al-Zn in the plating bath.

On the other hand, in the present invention, the microstructure of the cold-rolled steel sheet is not particularly limited and may include, for example, at least one structure of ferrite, pearlite, bainite, martensite and retained austenite in an appropriate ratio. However, from the viewpoint of ensuring both strength and ductility, it is preferable to control the area fraction of the double retained austenite to 5 to 50%.

According to one example, the high strength hot dip galvanized steel sheet of the present invention may further comprise an anti-alloying layer formed at the interface between the cold-rolled steel sheet and the hot-dip galvanized layer. In this case, And 0.001 to 0.05% by weight in total of at least one of them. As described above, when at least one of Sb, Bi, Sn and Zn in the alloying suppressing layer is contained in a certain amount or more, the plating adhesion is improved. However, if the content thereof is excessively excessive, the alloying inhibition layer becomes coarse, and the adhesion of the plating can be lowered, and the total content thereof is controlled to 0.05 wt% or less.

The high-strength hot-dip galvanized steel sheet of the present invention has an advantage of extremely high strength and ductility. According to one example, the high-strength hot-dip galvanized steel sheet of the present invention has a tensile strength of 950 MPa or more and a product of tensile strength and elongation of 16000 MPa ·% .

The high-strength hot-rolled steel sheet and the high-strength hot-dip galvanized steel sheet of the present invention can be produced by various methods, and the production method thereof is not particularly limited. However, as a preferable example, it can be produced by the following method.

Hereinafter, a method for manufacturing a high-strength hot-rolled steel sheet excellent in continuous productivity and a method for manufacturing a high-strength hot-dip galvanized steel sheet having excellent surface quality and plating adhesion, which is another aspect of the present invention, will be described in detail.

First, the slab having the above-mentioned component system is reheated. At this time, the reheating temperature is preferably 1100 to 1300 ° C. If the reheating temperature is lower than 1100 ° C, the hot rolling load may increase sharply. If the reheating temperature is higher than 1300 ° C, the reheating cost may increase and the surface scale amount may excessively increase.

Next, the hot-rolled steel sheet is obtained by hot-rolling under conditions of a finish hot-rolling temperature Ar 3 ° C or higher. If the finishing rolling temperature is lower than Ar3 ° C, it is limited to a temperature of Ar3 ° C or higher, since a bimetal structure is formed by ferrite and austenite bifunctional or ferrite reverse rolling, and a malfunction due to fluctuations in hot rolling load may occur.

Next, the hot-rolled steel sheet is wound at a temperature of 600 to 800 ° C. If the coiling temperature is less than 600 ° C, the strength of the hot-rolled steel sheet is excessively high, so that the rolling roll may be broken in the cold rolling process, and the material deviation in the width direction of the steel sheet is so large that bending of the steel sheet after cold- On the other hand, when the coiling temperature exceeds 800 DEG C, the maximum depth of the inner oxide in the hot-rolled steel sheet exceeds 1 mu m, and the inner oxidation depth of the surface layer portion of the cold-rolled steel sheet in the subsequent annealing process exceeds 0.2 mu m, Defects can become worse.

The next hot rolled steel sheet is cooled to a temperature of 550 DEG C or lower at an average cooling rate of 2 DEG C / min or more. In general, after winding, natural cooling, that is, air cooling, is performed in air at room temperature. In the case of air cooling, since the cooling rate is slow, it takes a long time until the internal oxidation is lowered to 550 deg. And will proceed further. Therefore, it is necessary to perform forced cooling until at least the surface temperature of the hot rolled steel sheet becomes 550 ° C or less. In the case of the present invention, it is necessary to cool the steel sheet at an average cooling rate of 2 ° C / min or more. On the other hand, the higher the cooling rate after winding, the better the prevention of internal oxidation. In the present invention, the upper limit is not particularly limited, but if the cooling rate is excessively high, the strength of the hot-rolled steel sheet increases and the cold rolling load can be increased , Thereby making it difficult to perform cold rolling. Therefore, in consideration of this, the upper limit can be limited to 10 DEG C / min.

Next, optionally, the cooled hot-rolled steel sheet may be washed with water after pickling. In this case, pickling may be carried out in an aqueous hydrochloric acid solution of 15 to 20 vol% at a temperature of 60 to 80 ° C for 30 to 60 seconds. The pickling process removes the oxide scale existing on the surface of the hot-rolled steel sheet and dissolves a part of the surface layer of the hot-rolled steel sheet, thereby having a function of partially dissolving the inner oxide which may exist in the surface layer of 1 μm or less. Therefore, if the acid concentration is too high, localized corrosion may occur on the surface of the steel sheet, resulting in a small hole-type defect. Further, when the acid concentration is too high, When the temperature of the aqueous hydrochloric acid solution is high, the evaporation amount of hydrochloric acid is high, which may cause corrosion of peripheral equipment. The pickling time, on the other hand, requires a minimum of 30 seconds to remove the surface oxide scale, and too long to limit productivity to 60 seconds.

Next, the cooled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. At this time, the cold reduction ratio may be 30 to 60%. If the cold rolling reduction rate is less than 30%, the thickness of the hot rolled sheet must be made too thin, thereby making hot rolling difficult. On the other hand, when the cold rolling reduction rate exceeds 60%, the load applied to the cold rolling roll is greatly increased, .

Next, optionally, the surface of the cold-rolled steel sheet Fe, Ni, Co, and can form a good plating layer consisting of at least one element of Sn, In this case, the adhesion amount of the wire coating layer is from 0.01 per one side ~ 2g / m ² Can be controlled. As described above, when conducting the gold plating, it is very effective in controlling the dew point to the target range in the subsequent recrystallization annealing process.

Next, the cold-rolled steel sheet is heated to a temperature of 820 to 870 ° C under the condition of a dew point temperature of -60 to -30 ° C, and is then subjected to recrystallization annealing by holding for 5 to 120 seconds.

In the present invention, it is very important to heat the annealing temperature to a single phase of austenite in order to obtain a desired material. In the present invention, at least one of Sb, Bi, Sn and Zn is added for improving the internal oxidation and plating properties of hot rolled steel, and these additions lead to a decrease in elongation. Therefore, in the present invention, it is important to maximize the retained austenite after annealing and cooling in order to increase the elongation, and then to temper the some austenite through reheating to secure the elongation. Therefore, it is necessary to heat at least 820 ° C, which is the austenite single phase phase. However, when the temperature exceeds 870 ℃, the concentration of Sb, Bi, Sn, and Zn on the surface is excessively increased, and a large amount of Sb, Bi, and Sn are decreased in the plating quality and adhesion improving effect. Therefore, the annealing temperature is preferably limited to 820 to 870 캜.

The annealing time requires a minimum of 5 seconds to obtain a uniform recrystallized structure, and a productivity of less than 120 seconds.

If the dew point temperature is lower than -60 ° C, the diffusion rate of the Si and Al to the surface of the steel becomes higher than the diffusion rate of Mn, and Si and Al Since the plating ability is inferior as the content of Mn increases from the surface of the composite oxide and the content of Si or Al in the surface of the composite oxide is larger than Mn, the composition and the manufacturing conditions of the present invention are insufficient for securing the wettability of zinc, , The Si, Mn and Al components are partially oxidized in the grain boundary and the grain in the surface layer of the steel sheet to exist as an inner oxide in the annealing process to control the inner oxide depth of the hot-rolled steel sheet to be 1 μm or less, Even if the maximum depth of the inner oxide is less than 0.2 탆, further internal oxidation occurs again in the annealing process, and the internal oxidation depth of the steel sheet surface layer Beyond a 0.2μm limited in the invention because it can cause the furnace Dent (Dent) dew point of the atmosphere gas in the annealing it is preferably also limited to -60 ~ -30 ℃.

According to one example, the recrystallization annealing can be performed under a 3 to 70% by volume H ₂ -N ₂ gas atmosphere. When the hydrogen content is less than 3 vol%, reduction of the iron oxide present on the surface of the steel sheet may be insufficient. As the hydrogen content increases, the reducing effect is favorable in view of the reduction effect, .

Next, the cold-rolled steel sheet subjected to recrystallization annealing is cooled to a temperature of 250 to 350 占 폚 at a rate of 20 占 폚 / sec or more, and then held for 50 to 150 seconds.

In the present invention, the cooling process after recrystallization annealing is also an important process for securing the strength and ductility of the material. In order to maximize the retained austenite after cooling by minimizing the ferrite transformation in the cooling process after forming the austenite single phase by annealing heating and holding of the steel sheet according to the present invention, the higher the cooling rate, the better the average cooling rate It is necessary to cool it to 250 to 350 占 폚. If the average cooling rate is less than 20 ° C / sec, the desired strength and ductility can not be secured due to increased ferrite transformation during cooling. The cooling termination temperature is required to be at least 250 캜 in order to maximize the retained austenite. When the cooling temperature is lower than 250 캜, some martensite phase is formed and the strength is increased but the elongation is greatly reduced. If the cooling temperature exceeds 350 ° C, the amount of transformation of the austenite into bainite increases, which is disadvantageous for securing the desired strength and ductility.

On the other hand, the cooled steel sheet needs to be maintained for 50 to 150 seconds thereafter. The austenite stabilization and the transformation of some austenite to bainite are induced while maintaining the cooling temperature for at least 50 seconds. However, when the retention time exceeds 150 seconds, the bainite transformation amount increases and the ductility may be reduced to the final product.

Next, the cold-rolled steel sheet that has been cooled and held is heated to a temperature of 460 to 500 ° C at a rate of 30 ° C / sec or more, and then plated in a galvanizing bath within 7 seconds.

The steel sheet kept at the cooling temperature needs to be heated to immerse in the plating bath, which is the next step. During the heating process, a part of the retained austenite is tempered and the tensile strength is slightly lowered. However, the elongation can be secured by the addition of Sb, Bi, Sn, Zn and the like. However, when the heating rate is less than 30 ° C. per second, it takes a long time for heating and the tempering should proceed excessively, resulting in a decrease in tensile strength. Similarly, it is necessary to immerse in a plating bath within 7 seconds after heating to 460 to 500 ° C. Also, if the holding time after heating is increased, the tempering is increased and the strength is decreased.

According to one example, the zinc plating bath may contain 0.12-0.3 wt% of Al. If the galvannealed galvanized steel sheet is manufactured, it is preferable to control the Al content to 0.12 to 0.15%, and when manufacturing the galvanized galvanized steel sheet, it should be controlled to 0.15% to 0.3%.

According to one example, the temperature of the zinc plating bath may be between 450 and 500 ° C. Below 450 ° C, the viscosity of the zinc increases and the rollability of the roll in the zinc plating bath decreases. When the temperature exceeds 500 ° C, the evaporation of zinc increases, which is undesirable.

Next, optionally, alloying heat treatment can be performed at a temperature of 480 to 600 DEG C for at least 1 second. In this case, the hot-dip galvanized layer subjected to alloying treatment may contain 7 to 13% by weight of Fe.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the description of these embodiments is intended only to illustrate the practice of the present invention, but the present invention is not limited thereto. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example  One)

The steel slabs having the compositions shown in the following Tables 1 and 2 were reheated at a temperature of 1200 ° C for one hour and then subjected to finish rolling at 900 ° C and then cooled to the coiling temperature shown in Table 3 and then cooled to a temperature of 550 ° C at a rate of 3 ° C / lt; RTI ID = 0.0 > g / min, < / RTI > . The cold - rolled steel sheet was observed with a scanning electron microscope to observe the oxide inside the hot - rolled steel sheet. At this time, the maximum depth of the inner oxide of the hot-rolled steel sheet was measured at five places x5000 times, and the maximum depth was selected.

Next, the cooled hot-rolled steel sheet was pickled with 60 vol% HCl solution of 17 vol% for 40 seconds to dissolve the surface iron oxide, and then cold-rolled at a reduction ratio of 45%. The cross section of steel sheet was observed by scanning electron microscope to observe cold internal oxide. At this time, the maximum depth of the inner oxide of the cold-rolled steel sheet was measured at five places x5000 times, and the maximum depth was selected. In addition, the occurrence of cracks in the cold-rolled steel sheet was observed, and the results are shown in Table 3 below.

Next, after removing impurities adhering to the surface through the pretreatment, annealing was carried out under the heating and cooling conditions shown in Table 4, and the plating bath temperature was 455 to 460 DEG C, the Al content was 0.13 wt% (in the case of GA) or 0.22 wt% (In the case of GI), and then adjusted to an adhesion amount of one side reference plating 60 g / m ² using an air knife and cooled to obtain a plated steel sheet. At this time, some specimens were not plated to observe the surface and the surface layer enrichment after annealing. After that, some alloys were annealed at 550 ° C for 25 seconds. In Table 4, GA for specimens subjected to alloying heat treatment and GI for alloying heat treatment specimens were shown.

Then, in order to evaluate the surface quality of the coated steel sheet after plating, the existence and degree of the unplated portion of the surface was visually confirmed. To evaluate the coating adhesion, the structural adhesive for automobile was coated on the surface of the steel sheet, After the road was bent, it was confirmed whether or not the coating layer was adhered to the adhesive. The surface quality and the adhesion of the coating were evaluated according to the following criteria, and the results are shown in Table 5 below.

- surface quality; ?: No plating area,?: Existence of uncoated having a diameter of less than 2 mm, X: Existence of uncoated diameter exceeding 2 mm

- plating adhesion; ○: no plating peeling, partial peeling, X complete peeling

Thereafter, the tensile strength and the elongation of the coated steel sheet were measured by a tensile test according to JIS No. 5, and the results are shown in Table 5 below. In addition, the maximum depth of the inner oxide of the coated steel sheet was measured at five sites x5000 times, and the maximum depth was selected.

Next, in order to measure Sb, Bi, and Sn content in the galvannealing layer of the hot-dip galvanized layer and the cold-rolled steel sheet interface, only the zinc-plated layer was removed, and the galvannealing layer at the interface was dissolved and analyzed by ICP (Inductively Coupled Plasma) . In order to observe the thickening of the surface layer of the cold-rolled steel sheet of the coated steel sheet, the cross-section was processed with FIB (Focused Ion Beam), and through the composition profile of 3-D APT (Atom Probe Topography) The content of Sb, Bi, Sn and Zn in the surface layer portion was measured and the content of Sb, Bi, Sn and Zn at 0.02 μm from the surface layer portion of the cold-rolled steel sheet was measured at a depth of 0.02 μm And the surface layer was concentrated.

(Total wt% of Sb, Bi, Sn, and Zn components at the point of surface layer concentration = 0.001 μm)

(Wt%) of Sb, Bi, Sn and Zn components at a point of 0.02 μm /

As shown in Tables 1 to 5, the examples 1 to 2, 4 to 6, 9, 14 to 15, 20, and 22 to 24 of the present invention were obtained by using steel species having the component ranges defined in the present invention, A method for manufacturing a hot-rolled steel sheet, a cold-rolled steel sheet and a hot-dip galvanized steel sheet by a manufacturing method, wherein a maximum depth of the inner oxide of the hot- The maximum depth of internal oxide was found to be less than 0.2μm. The surface layer partial oxidation degree of Sb, Bi, Sn and Zn is 3 to 15, and 0.001 to 0.05% of one or more of Sb, Bi and Sn components in the zinc-plated layer and the cold- And the tensile strength (Mpa) x elongation (%) = 16000, with a tensile strength of 950 mpa or more and excellent surface quality and plating adhesion.

In Comparative Examples 3 and 13, the hot-rolled steel sheet was rolled at a temperature lower than 550 占 폚 in which the steel sheet coiling temperature was limited in the present invention, and the area fraction of bainite was 74% and 69% A crack occurred in the steel sheet during the cold rolling process.

On the other hand, in the case of Comparative Example 29, Si in the steel component was higher than the range defined in the present invention, and cracks occurred in the steel sheet during the cold rolling process.

In the case of Comparative Example 7, the steel component and the hot rolled coiling temperature satisfy the range defined in the present invention and the depth of the hot-rolling internal oxidation satisfies the range defined in the present invention. However, in the annealing process, The internal oxidation progresses further in the annealing furnace and the internal oxidation depth of the steel sheet after annealing exceeds the limit range of the present invention.

In the case of Comparative Example 8, when the annealing temperature was higher than the range defined in the present invention, the degree of surface layer thickening of Sb, Bi, Sn, and Zn exceeded the range defined by the present invention, resulting in partial plating detachment.

In the case of Comparative Example 10, the steel component and hot-rolled condition satisfy the range defined in the present invention, but when the cooling temperature in the annealing step is higher than the limited range of the present invention, the amount of transformation of austenite into bainite increases, Was lower than the range defined in the present invention.

In the case of Comparative Examples 11 and 25, when the hot rolled coiling temperature is higher than the range defined in the present invention, the content of Sb, Bi, Sn and Zn which are the internal oxidation inhibiting components satisfies the range defined in the present invention, Exceeded the range defined in the present invention, and the internal oxidation depth after the cold-rolled steel sheet and annealing exceeded the range defined in the present invention.

On the other hand, in the case of Comparative Example 12, the Sb, Bi, Sn and Zn components were not added to the steel components, and even if the hot rolled coiling temperature satisfied the range of the present invention, And the internal oxidation depth of annealed was deep to 5.8 탆. In the annealing process of cold-rolled steel sheet, the surface of the steel sheet was not plated with a diameter exceeding 2 mm due to the concentration of oxidizing components such as Si, Mn and Al, and the plating peeling occurred severely.

In the case of Comparative Example 13, the Sb, Bi, Sn, and Zn components were not added to the steel components. However, since the hot rolled coiling temperature was lower than the range of the present invention and the coiling temperature was low, However, cracking occurred during the cold rolling process because the coiling temperature was low and the hot rolling strength was greatly increased.

In Comparative Example 16, the steel component and hot-rolled condition satisfy the range defined in the present invention, but when the average cooling rate in the annealing step is lower than the range defined by the present invention, the austenite is transformed into ferrite in the cooling process, The austenite content was decreased and the TSxEL was lower than the range defined in the present invention.

In Comparative Example 17, the steel component and the hot rolling condition satisfied the range defined in the present invention. However, when the holding time at the cooling temperature was out of the limiting range of the present invention, the austenite stabilization did not occur, Respectively.

On the other hand, in Comparative Example 18, the steel component, the hot rolling condition, the annealing and the cooling conditions satisfied the range defined in the present invention, but the rate of reheating to 470 캜 after cooling was maintained is slower than the range defined in the present invention, Strength and ductility decreased at the same time.

In Comparative Example 19, the steel component, hot rolling condition, annealing, cooling and reheating conditions satisfied the range defined in the present invention. However, after reheating to 480 deg. C, the tempering was increased for a long time until being immersed in the plating bath, Low.

In the case of Comparative Example 21, when the annealing temperature was lower than the range defined in the present invention, the steel was annealed in the ferrite and austenite phase, and then the retained austenite content decreased through cooling, holding and reheating, and TSxEL was low.

Comparative Example 26 is a case where the annealing temperature is not the austenite single phase in the present invention and the cooling temperature after the annealing in the reverse direction is higher than the range defined in the present invention and the residual austenite content is low, Range.

In the case of Comparative Examples 27 and 28, when the Sb, Bi, Sn and Zn components in the steel components were added in a range lower than the range defined in the present invention, the depth of oxidation in the hot rolled steel exceeded the range limited by the present invention, And the internal oxidation depth in the annealed steel sheet was deeper than the range defined in the present invention. However, the Sb, Bi, Sn, and Zn components were effective in suppressing the formation of sintered oxides such as Si, Mn, and Al in the annealing process, so that the plating quality and adhesion were excellent.

Comparative Example 29 is a case in which the Si content in the steel component exceeds the range defined in the present invention. In the hot rolled steel sheet, a large amount of work hardened ferrite and cementite having no reverse recrystallization in the hot- So that the strength of the hot-rolled steel sheet greatly increased, causing plate breakage during the cold rolling operation.

Comparative Example 30 is a case in which the Mn content in the steel component exceeds the range defined by the present invention. Even if the added amount of the Sb, Bi, Sn and Zn components satisfies the range of the present invention, Si, Mn, The amount of annealed oxides such as Al was so large that point-like uncoated platelets of less than 2 mm occurred on the steel sheet and partly peeled off.

On the other hand, FIG. 1 is a graphical representation of the result of analyzing the cold-rolled steel sheet according to Inventive Example 9 with 3D-AP.

( Example  2)

In order to check the occurrence of dent defects while producing coiled products continuously, they were produced and analyzed in actual production facilities. After continuous casting of the components having the compositions of Tables 6 and 7 below, a steel slab was prepared. After being subjected to finish rolling at 900 ° C after being maintained at a temperature of 1200 ° C for 1 hour, the steel sheet was cooled to the winding temperature shown in Table 8 and wound into a coil, and then cooled to a temperature of 550 ° C at an average cooling rate of 3 ° C / And then air-cooled.

For the hot - rolled steel sheet which has been cooled to room temperature, the cross - section of the steel sheet was observed with a scanning electron microscope to observe the oxide inside the hot - rolled steel. At this time, the maximum depth of the inner oxide of the steel sheet was measured at five places at x5000 times, and the maximum depth was selected

The pickling of the hot-rolled steel sheet was carried out at 70 DEG C in a 17Vol% HCl solution for 30 to 50 seconds, followed by cold rolling immediately. The cold - rolled steel sheet was observed with a scanning electron microscope to observe the oxidation depth in the cross section. At this time, the maximum depth of the inner oxide of the steel sheet was measured at five places at x5000 times, and the maximum depth was selected

The cold-rolled steel sheet was subjected to a pretreatment to remove foreign matters on the surface thereof, and then subjected to annealing in the heating and cooling conditions shown in Table 8, plating was performed at a plating bath temperature of 456 ° C and an Al content of 0.22 wt% in a plating bath, And the coating amount was adjusted to 60 g / m < ² > and the coated steel sheet was prepared. In order to confirm the occurrence of dent defects in the annealing furnace according to the amount of production, the same steel was continuously produced in the same condition at a rate of 15 coils each, and the number of dent start coils was measured, Are shown in Table 9 below.

Then, the surface quality and the plating adhesion of the plated steel sheet were evaluated, and the tensile strength and elongation were measured. The results are shown in Table 9 below. The specific measurement and evaluation method is the same as that in Example 1. [

As shown in Tables 6 to 9, Example 36 of the present invention satisfies both the alloy composition and the manufacturing conditions proposed in the present invention, in which the maximum depth of the inner oxide is appropriately controlled and dents are generated in all 15 coils (Mpa) x elongation (%) = 16000, even when Sb, Bi, Sn and Zn components were added in an amount of 0.08% or more to prevent internal oxidation, It can be confirmed that a steel sheet can be provided.

In the case of Comparative Example 31, since there is no ingredient capable of inhibiting internal oxidation of steel, it is internally oxidized to a depth of 12 탆 from the surface of the steel of the hot-rolled steel sheet and is then oxidized to a depth of 5.2 탆 from the surface of the high- The internal oxidized grains of the surface layer were detached from the inner surface of the annealing furnace and adhered to the rolls in the annealing furnace, and a dent defect was observed from the second coil.

In the case of Comparative Example 32, 0.04% of Sb as a hot-rolled internal oxidation inhibiting component was added to steel, and the oxidation index in the hot-rolled steel sheet decreased to 5.2 μm as compared with Comparative Example 31, Dent was observed from the fourth production coil at a depth of 2.4 탆 which is deeper than the internal oxidation depth defined in the present invention in the cold-rolled steel sheet after pickling and cold rolling exceeding the limited 1 탆.

delete

2 (a) is an SEM image of the cross section of the cold-rolled steel sheet according to Comparative Example 31. FIG.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, It will be obvious to those of ordinary skill in the art.

Claims (20)

0.1 to 0.3% of C, 1 to 2.0% of Si, 2.6 to 5% of Mn, 0.001 to 2% of sol.Al, Ti: (48/14) : Not more than 0.04% (excluding 0%), S: not more than 0.015% (excluding 0%), N: not more than 0.02% (excluding 0%), Sb, Bi, Sn, Wherein the inner surface of the hot-rolled steel sheet has an inner oxide containing at least one element selected from the group consisting of Si, Mn, Al and Fe, and the maximum depth of the inner oxide is 1 μm or less High-strength hot-rolled steel sheet.
The method according to claim 1,
Wherein the hot-rolled steel sheet further comprises at least one member selected from the group consisting of Cr: not more than 1.0%, Mo: not more than 0.2%, Nb: not more than 0.1%, and B: not more than 0.005%.
The method according to claim 1,
The hot-rolled steel sheet further comprises at least one member selected from the group consisting of Cu, Mg, Co, Ca, Na, V, Ga, Ge, As, Se, In, Ag, W, Pb and Cd, High strength steel sheet with less than 0.1%.
The method according to claim 1,
A high-strength hot-rolled steel sheet containing bainite as its microstructure and having an area fraction of 50% or less.
0.1 to 0.3% of C, 1 to 2.0% of Si, 2.6 to 5% of Mn, 0.001 to 2% of sol.Al, Ti: (48/14) : Not more than 0.04% (excluding 0%), S: not more than 0.015% (excluding 0%), N: not more than 0.02% (excluding 0%), Sb, Bi, Sn, A cold rolled steel sheet containing the remainder Fe and unavoidable impurities; And a hot-dip galvanized layer formed on the surface of the cold-rolled steel sheet,
Wherein an inner oxide containing at least one element selected from the group consisting of Si, Mn, Al and Fe is present in the surface layer portion of the cold-rolled steel sheet, and a maximum depth of the inner oxide is 0.2 탆 or less (excluding 0 탆).
6. The method of claim 5,
The sum of the content of at least one of Sb, Bi, Sn and Zn in the region from the surface of the cold-rolled steel sheet to the position of 0.005 m in the thickness direction is within a range from the position of 0.01 m in the thickness direction to the center of the thickness direction Sb, Bi, Sn, and Zn is 3 to 15 times the sum of the content of at least one of Sb, Bi, Sn, and Zn.
6. The method of claim 5,
A high strength hot-dip galvanized steel sheet having a tensile strength of 950 MPa or more and a product of tensile strength and elongation of 16000 MPa ·% or more.
6. The method of claim 5,
A high strength hot-dip galvanized steel sheet containing residual austenite as its microstructure and having an area fraction of 5 to 50%.
6. The method of claim 5,
Wherein the alloying suppressing layer is formed on the interface between the cold-rolled steel sheet and the hot-dip galvanizing layer, and the alloying suppressing layer comprises 0.001 to 0.05% by weight in total of at least one of Sb, Bi, Sn and Zn, .
0.1 to 0.3% of C, 1 to 2.0% of Si, 2.6 to 5% of Mn, 0.001 to 2% of sol.Al, Ti: (48/14) : Not more than 0.04% (excluding 0%), S: not more than 0.015% (excluding 0%), N: not more than 0.02% (excluding 0%), Sb, Bi, Sn, After reheating the slab containing the remainder Fe and unavoidable impurities, hot rolling the steel sheet at a finish hot rolling temperature Ar3 DEG C or higher to obtain a hot rolled steel sheet;
Winding the hot-rolled steel sheet at a temperature of 600 to 800 ° C; And
Cooling the rolled hot rolled steel sheet to a temperature of 550 DEG C or lower at an average cooling rate of 2 DEG C / min or more;
Wherein the high-strength hot-rolled steel sheet comprises a high-strength hot-rolled steel sheet.
11. The method of claim 10,
And the reheating temperature of the slab is 1100 to 1300 ° C.
11. The method of claim 10,
And the cooling rate of the wound hot rolled steel sheet is 2 to 10 占 폚 / min.
0.1 to 0.3% of C, 1 to 2.0% of Si, 2.6 to 5% of Mn, 0.001 to 2% of sol.Al, Ti: (48/14) : Not more than 0.04% (excluding 0%), S: not more than 0.015% (excluding 0%), N: not more than 0.02% (excluding 0%), Sb, Bi, Sn, After reheating the slab containing the remainder Fe and unavoidable impurities, hot rolling the steel sheet at a finish hot rolling temperature Ar3 DEG C or higher to obtain a hot rolled steel sheet;
Winding the hot-rolled steel sheet at a temperature of 600 to 800 ° C;
Cooling the rolled hot rolled steel sheet to a temperature of 550 DEG C or lower at an average cooling rate of 2 DEG C / min or more;
Cold-rolling the cooled hot-rolled steel sheet to obtain a cold-rolled steel sheet;
Heating the cold-rolled steel sheet to a temperature of 820 to 870 캜 under a dew point temperature of -60 to -30 캜, and then performing recrystallization annealing by keeping the temperature for 5 to 120 seconds;
Cooling the recrystallized annealed cold rolled steel sheet to a temperature of 250 to 350 ° C at a rate of 20 ° C / sec or more, and then maintaining the temperature for 50 to 150 seconds; And
Heating and cooling the cooled and held cold-rolled steel sheet at a temperature of 460 to 500 ° C at a rate of 30 ° C / sec or more, and then plating the steel sheet in a zinc plating bath within 7 seconds;
Wherein the hot-dip galvanized steel sheet is a steel sheet.
14. The method of claim 13,
Further comprising the step of washing the rolled hot-rolled steel sheet after pickling the cold-rolled steel sheet after the winding,
Wherein the hot rolled steel sheet is pickled in an aqueous hydrochloric acid solution at a temperature of 60 to 80 占 폚 of 15 to 20 vol% for 30 to 60 seconds.
14. The method of claim 13,
Wherein the reduction rate in the cold rolling is 30 to 60%.
14. The method of claim 13,
The recrystallization annealing is 3 to 70 vol% H ₂ -N ₂ method for producing a high-strength hot-dip galvanized steel sheet performed with the gas atmosphere condition.
14. The method of claim 13,
Wherein the zinc plating bath contains 0.12 to 0.3 wt% of Al.
14. The method of claim 13,
Wherein the temperature of the zinc plating bath is 450 to 500 占 폚.
14. The method of claim 13,
Further comprising the step of forming a pre-plating layer composed of at least one element selected from the group consisting of Fe, Ni, Co and Sn on the surface of the cold-rolled steel sheet before the recrystallization annealing of the cold-rolled steel sheet, wherein the deposition amount of the pre- / m < ² >.
14. The method of claim 13,
Further comprising the step of performing an alloying heat treatment at a temperature of 480 to 600 占 폚 for 1 second or more after the plating.

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