KR20160077571A - High-strength galvannealed sheet steels having excellent surface qualities and powdering resistance and method for manufacturing the same - Google Patents

Abstract

The present invention provides a high-strength alloyed hot-dip galvanized steel sheet with excellent surface quality and powdering resistance which forms a cold rolled steel sheet comprising: 0.1-0.3 wt% of C, 1-2.5 wt% of Si, 2.5-10 wt% of Mn, 0.001-0.5 wt% of sol.Al, 0.04 wt% or less of P, 0.015 wt% or less of S, 0.02 wt% or less of N (excluding 0 wt%), 0.1-0.7 wt% of Cr, 0.1 wt% or less of Mo, (48/14)*[N]-0.1 wt% of Ti, 0.005-0.5 wt% of Ni, 0.01-0.07 wt% of Sb, 0.1 wt% or less of Nb, 0.005 wt% or less of B; and the remaining consisting of Fe and inevitable impurities, and an alloy galvanized steel sheet by performing galvanized plating and alloying on the cold rolled sheet. Standard deviation of an iron (Fe) content at a 1/2 spot in a thickness direction of the alloy-plated layer is within 3%.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength alloyed hot-dip galvanized steel sheet excellent in surface quality and resistance to powdering, and a method for manufacturing the same. BACKGROUND ART [0002]

The present invention relates to a high-strength hot-dip galvanized steel sheet which can be used for an automobile body structural member and the like, and more particularly to a steel sheet having a high tensile strength of 1000 MPa or more and excellent press formability, High strength alloyed hot-dip galvanized steel sheet and a manufacturing method thereof.

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 satisfy these demands, recently, a high strength steel sheet of 1000 MPa or more has been developed and applied to automobiles. As a method for increasing the strength of a steel sheet, a steel sheet having a high strength can be easily manufactured by increasing the amount of reinforcing components of the steel including carbon. However, in the case of a steel sheet for automobile steel sheet, The elongation of the steel sheet must be secured at the same time.

In order to secure both the strength and ductility of steel sheets for automobiles, Mn, Si, Al, Cr, and Ti are mainly added to the steel, and by appropriately controlling the addition amount thereof and controlling the manufacturing process conditions, Can be produced.

Further, in order to prolong the life of automobiles, steel sheets used for automobiles need to be improved not only in strength and ductility but also in corrosion resistance. In order to improve corrosion resistance, hot-dip galvanized steel sheets and galvannealed galvanized steel sheets (hereinafter referred to as GA) have. To produce the GA steel sheet, after hot dip galvanizing, the steel sheet is heat-treated without cooling, and the Fe-Zn plating layer is thermally diffused to make a Zn-Fe alloyed plating layer. The GA steel sheet having an iron content of 8 to 12% by weight is widely used as a steel sheet for automobiles because it has excellent corrosion resistance and weldability and paintability as compared with a galvannealed steel sheet (GI) which is not alloyed.

In the case of a high strength steel sheet for automobiles having a strength of 1000 MPa or more, it is general to add a component such as Si, Mn or Al to the steel to secure a desired strength and elongation. However, a high-strength steel sheet containing Si, Mn and Al which is easily oxidized in steel reacts with a trace amount of oxygen or water vapor present in the annealing furnace to form Si, Mn or Al alone or a composite oxide on the surface of the steel sheet, So that the surface of the coated steel sheet is locally or entirely uncoated with no zinc adhered thereto, thereby greatly reducing the quality of the surface of the coated steel sheet. Also, in the heat treatment process for GA, single or complex oxides of Si, Mn and Al existing in the plating layer / substrate interface inhibit diffusion between Fe and Zn. Therefore, at the sites where the oxides are thin or absent, the diffusion is rapid and the alloying occurs. On the contrary, the diffusion is suppressed at the sites where the oxides are thick, and the alloying occurs. As a result, in the case of a steel sheet having a thick surface oxide after annealing, it is necessary to increase the heating temperature in order to obtain a Zn-Fe alloy layer having a certain amount of iron content. In this case, partial alloying can not be avoided. When the iron content in the plating layer is increased by this alloying, the weldability and paintability of the steel sheet are excellent, but the hardness of the alloying plating layer is increased, and the occurrence of the powdering ring in which the plating layer falls off during the forming of the steel sheet becomes severe. If the occurrence of powder ring in such a plating layer becomes severe, the cleaning cycle becomes faster due to contamination of the molding die during the molding process, and the corrosion resistance of the steel sheet is heated by the removal of the plating layer, so that the powdering of the plating layer needs to be minimized.

In order to solve these problems, various technologies have been proposed. In the patent document 1, air and fuel are controlled to have an air-fuel ratio of 0.80 to 0.95 in the annealing process, and the steel sheet is heated in a direct flame furnace in an oxidizing atmosphere. Oxidation is carried out to form an iron oxide containing Si, Mn and Al alone or a composite oxide to a predetermined depth in the steel sheet and then reduced and annealed in a reducing atmosphere to reduce the iron oxide and then subjected to hot dip galvanizing Hot-dip galvanized or alloyed hot-dip galvanized steel sheets. In this annealing process, when the oxidation-reduction method is used, the diffusion of the components having high affinity to oxygen such as Si, Mn, and Al into the surface layer is suppressed to a certain depth from the surface layer of the steel sheet. Al alone or in a composite oxide is reduced, so that the wettability with zinc in the plating bath is improved and the plating can be reduced. That is, if heated under a high oxygen partial pressure that can oxidize iron, the iron is oxidized 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, in the method of Patent Document 1, there is an internal oxide layer composed of Si, Mn and / or Al existing under the iron oxide layer formed in the oxidation process, and these internal oxide layers are also formed in the heat treatment process for alloying after plating, , It is necessary to increase the alloying temperature in order to produce a GA steel sheet having an iron content of about 8 to 12%. As a result, there is a partially overalloyed portion, which results in a problem of low resistance to powdering during molding.

Korea Patent Publication No. 2010-0030627

One aspect of the present invention is to provide a high strength alloyed hot-dip galvanized steel sheet having a high tensile strength of 1000 MPa or more and excellent surface quality and powderiness in a plating layer.

Another aspect of the present invention is to provide a method for producing a high strength alloyed hot dip galvanized steel sheet having a high tensile strength of 1000 MPa or more and excellent surface quality and powdering property in a plating layer.

An aspect of the present invention is a method of manufacturing a semiconductor device, comprising: 0.1 to 0.3% of C, 1 to 2.5% of Si, 2.5 to 10% of Mn, 0.001 to 0.5% of sol.Al, 0.04% N: not more than 0.02% (excluding 0%), Cr: 0.1 to 0.7%, Mo: not more than 0.1%, Ti: (48/14) * [N] to 0.1%, Ni: 0.005 to 0.5% 0.01 to 0.07%, Nb: not more than 0.1, B: not more than 0.005%, balance Fe and other unavoidable impurities, and an alloyed plated layer formed on the cold-rolled steel sheet, The present invention provides a high-strength alloyed hot-dip galvanized steel sheet excellent in surface quality and powderiness in a plating layer with a standard deviation of iron (Fe) content within 3%.

Another aspect of the present invention provides a method of manufacturing a semiconductor device, comprising: 0.1 to 0.3% of C, 1 to 2.5% of Si, 2.5 to 10% of Mn, 0.001 to 0.5% of sol.Al, 0.1 to 0.7%, Mo: 0.1% or less, Ti: (48/14) * [N] to 0.1%, Ni: 0.005 to 0.5% 0.01 to 0.07% Sb, 0.1 or less Nb, 0.005% or less B, the balance Fe and other unavoidable impurities; Reheating the steel slab to a temperature of 1100 to 1300 ° C; Finishing hot-rolling the reheated steel slab at a temperature equal to or greater than Ar ₃ ; Rolling the hot-rolled steel sheet at a temperature of 700 ° C or lower; Rolling and cold rolling the rolled steel sheet; recrystallizing and annealing the cold-rolled cold-rolled steel sheet at a dew point temperature of -60 to -20 占 폚 and a temperature of 750 to 950 占 폚 for 5 to 120 seconds; Cooling the annealed cold rolled steel sheet to 200 to 600 ° C at an average cooling rate of 2 to 150 ° C / sec; Reheating or cooling the cooled cold rolled steel sheet to a temperature of (plating bath temperature -20 ° C) to (plating bath temperature + 100 ° C); Immersing the reheated or cooled cold rolled steel sheet in a zinc plating bath maintained at a temperature of 450 to 500 ° C to form a zinc plated layer on the cold rolled steel sheet; And a step of alloying and cooling the zinc plated layer by heating the steel sheet having the zinc plated layer formed thereon to a temperature of 480 to 600 캜 and then cooling the surface of the galvanized layer and cooling the galvanized layer to obtain a galvannealed steel sheet having excellent powdering properties.

By producing the galvannealed steel sheet according to the present invention, it is possible to produce a galvannealed steel sheet which has a tensile strength (Mpa) x elongation (%) of 15000 or more, a tensile strength of 1000 MPa or more and a high strength An alloyed hot-dip galvanized steel sheet can be provided.

The present invention relates to a high strength alloyed hot-dip galvanized steel sheet having a high tensile strength of 1000 MPa or more and excellent moldability, and excellent in surface quality and powdering property in a plating layer, and a method for producing the same.

Hereinafter, the high-strength alloyed hot-dip galvanized steel sheet excellent in surface quality and powderiness in the plating layer of the present invention will be described in detail.

The high-strength alloyed hot-dip galvanized steel sheet excellent in surface quality and powderiness in the plating layer according to the present invention is characterized by containing 0.1 to 0.3% of C, 1 to 2.5% of Si, 2.5 to 10% of Mn, N: not more than 0.02% (excluding 0%), Cr: 0.1 to 0.7%, Mo: not more than 0.1%, Ti: (48/14) * [N] , The balance Fe and other unavoidable impurities, and the alloyed plated layer formed on the cold-rolled steel sheet, which are formed on the cold-rolled steel sheet, in an amount of 0.01 to 0.1%, Ni: 0.005 to 0.5%, Sb: 0.01 to 0.07%, Nb: 0.1 or less and B: , And the standard deviation of the iron (Fe) content at 1/2 point in the thickness direction of the alloying plating layer is within 3%.

Hereinafter, the reasons for limiting the composition of the steel material will be described in detail. (All the composition of the following components means weight% unless otherwise specified.)

Carbon (C): 0.1 to 0.3%

C is required for securing the strength of martensite, it should be added in an amount of 0.1% or more, but if it exceeds 0.3%, the ductility, bending workability and weldability are decreased to deteriorate press forming and roll workability. % Is preferable.

Silicon (Si): 1 to 2.5%

Si improves the yield strength of the steel and stabilizes the ferrite and the residual austenite at room temperature, so that Si preferably contains 1% or more. In addition, Si inhibits precipitation of cementite during cooling from austenite, and significantly inhibits the growth of carbides, thereby contributing to stabilization of a sufficient amount of residual austenite in case of TRIP (Tranformation Induced Plasticity) steel. Therefore, as in the present invention, it is necessary to ensure a tensile strength of 1000 MPa or more and a tensile strength (MPa) x elongation (%) of 15,000 or more. On the other hand, if too much is added, the hot rolling load is increased to cause hot cracking, and even if the other components and the manufacturing method meet the range of the present invention, the amount of Si concentration on the surface increases after annealing, .

Manganese (Mn): 2.5 to 10

The content is preferably 2.5 to 10. Mn in steel is well known as an element for increasing hardenability which suppresses ferrite formation and stabilizes austenite. In order to secure the tensile strength of the steel sheet to 1000Mpa or more, Mn of 2.5% or more is required. As the Mn content increases, the strength can be secured easily, but it is difficult to secure the plating ability by the manufacturing method of the present invention due to the increase of the surface oxidation amount of Mn in the annealing process.

Aluminum (sol.Al): 0.001-0.5%

Al is an element added for deoxidation in the steelmaking process and is a carbonitride-forming element. Al is an alloy element that expands the ferrite phase and has an advantage of lowering the Ac1 transformation point to reduce the annealing cost, and therefore, it is necessary to add at least 0.001%. If the Al content exceeds 1%, the weldability deteriorates, and it is difficult to secure the plating ability by the manufacturing method of the present invention due to an increase in the surface oxidation amount of Al in the annealing process, and the alloying variation increases. The content of sol.Al is preferably 0.001 to 0.5%.

Phosphorus (P): not more than 0.04%

If P is an impurity element and the content thereof exceeds 0.04%, the weldability is lowered, the risk of brittleness of steel is increased, and the possibility of occurrence of dent defect becomes higher, so that the upper limit is preferably limited to 0.04%.

Sulfur (S): not more than 0.015%

S, like P, is an impurity element that inhibits ductility and weldability of a steel sheet. If the content exceeds 0.015%, the ductility and weldability of the steel sheet are likely to be deteriorated. Therefore, the upper limit is preferably limited to 0.015%.

Nitrogen (N): 0.02% or less (excluding 0%)

If N is more than 0.02%, the risk of cracking during performance is greatly increased by the formation of AlN, so that the upper limit is preferably limited to 0.02%.

Cr (Cr): 0.1 to 0.7%

Cr is an element for increasing hardenability and has an advantage of suppressing the formation of ferrite. Therefore, it is preferable to add at least 0.1% in order to secure 5 to 25% of retained austenite. If it exceeds 0.7% , The content of Cr is preferably 0.1 to 0.7%.

Molybdenum (Mo): not more than 0.1%

Mo is optionally added and the content is preferably 0.1% or less, and more preferably 0.001 to 0.1%. Mo, like Cr, has a large effect of contributing to the improvement of strength, but it is economically disadvantageous if it exceeds 0.1% as a relatively expensive component.

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

Ti is an element which forms nitrides and has an effect of reducing the concentration of N in steel. To do this, it is necessary to add (48/14) * [N] or more in terms of chemical equivalent. There is a risk of occurrence of hot rolled crack due to AlN formation when Ti is not added. If it exceeds 0.1%, the carbon concentration and the strength of the martensite decrease due to the precipitation of additional carbides in addition to the removal of the solid solution N. Therefore, the content of Ti is preferably (48/14) * [N] to 0.1%.

Nickel (Ni): 0.005-0.5%

Ni is added in an amount of at least 0.005% in order to improve the strength because the Ni is not substantially concentrated on the surface during the annealing process. Therefore, when the Ni content exceeds 0.5%, the pickling of the hot-rolled steel sheet becomes uneven, And alloying, the content of Ni is preferably 0.005 to 0.5%.

Antimony (Sb): 0.01 to 0.07%

Sb is an important component added inevitably to reduce surface quality and incorporation variation in the present invention. In order to produce a steel sheet having high strength and elongation as described above, a large amount of Si, Al and Mn is added. When this steel sheet is annealed by reduction and recrystallization, Si, Al and Mn in the steel are diffused into the steel surface, To form a composite oxide. In this case, most of the annealed surface is covered with the oxide, so that when the steel sheet is immersed in the zinc plating bath, the wettability of the zinc is greatly lowered, so that uneven plating which does not adhere to zinc is not only caused but also occurs between the steel sheet and the zinc plated layer The oxide is partially present and prevents diffusion of the base Fe into the plating layer at the site where the oxide is present in the subsequent alloying process, thereby hindering the alloying of the plating layer. Therefore, in order to produce a GA steel sheet having 8 to 12% iron (Fe) content in the plated layer, the alloying temperature has to be increased, and in this case, it is partially alloyed and powdering is likely to occur during molding.

However, when Sb is added to the steel in an amount of 0.01 to 0.07% in the present invention and reduction annealing is performed by keeping the dew point inside the annealing furnace at -60 to -20 占 폚 in the present invention, Sb is concentrated within 0.2 占 퐉 from the surface layer of the steel sheet or the base steel, To suppress the surface diffusion of Si, Mn and Al, thereby reducing the amount of surface oxide composed of Si, Mn and Al. In this case, since the wettability with zinc is good at the portion where the oxide is not present, the plating ability is improved as a whole. Further, in the process of alloying after plating, the alloying of the base steel and the Zn plating layer is promoted by the effect of reducing the surface oxide, and the degree of alloying of the alloy plating layer is also reduced, thereby improving the powdering property in the plating layer.

However, when the dew point is lower than -60 ° C, Mn is a partially reduced dew point, so that the surface diffusion rate decreases and the diffusion rate to the surface of Si or Al increases instead, and the oxide of the surface oxide forms an Al- and Si- . Since the surface oxides based on Al or Si greatly reduce the wettability of zinc compared to the surface oxides based on Mn and also have an effect of inhibiting alloying, the effect of improving the plating property and reducing the alloying deviation are reduced even when Sb is added.

The Sb is preferably added in an amount of 0.01 to 0.07%. When the addition amount is less than 0.01%, the effect of inhibiting the surface concentration of Si, Mn, Al and the like is weak. When the addition amount is more than 0.07%, the brittleness of the steel sheet is increased and the elongation is likely to be decreased. .

Niobium (Nb): not more than 0.1

Nb is optionally added. Nb is segregated in the form of carbide in the austenite grain boundaries to suppress the coarsening of the austenite grains during annealing and increase the strength. When the Nb content exceeds 0.1%, the cost of the ferroalloy due to the excess amount of alloy is increased. The content is preferably 0.1% or less.

Boron (B): not more than 0.005%

B can be added selectively to secure strength. If the content of B exceeds 0.005%, it may be concentrated on the annealed surface to deteriorate the plating ability to a great extent, so that the content of B is preferably 0.005% or less.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. Mg, Zn, Co, Ca, Na, V, Ga, and Ge, which are impurities generated by the input of a certain amount of iron scrap, can be detected by anyone skilled in the art. , As, Se, In, Ag, W, Pb, and Cd may each contain less than 0.1%, but this does not impair the effect of the present invention.

The high-strength alloyed hot-dip galvanized steel sheet of the present invention comprises a hot-dip galvanized and alloyed galvannealed layer on a cold-rolled steel sheet, wherein a standard deviation of the iron (Fe) content at a half point in the thickness direction of the galvannealed layer is within 3% . The alloying degree (iron content) of the alloying plating layer of the present invention is preferably set to 8 to 12% in consideration of the powdering property, corrosion resistance, weldability, paintability and the like in the plating layer. Even if the average iron content is within 8 to 12% It is important to reduce the degree of alloying variation because the plating layer powdering occurs severely when the degree of alloying variation is large due to the presence of the surface oxide or the difference in the thickness and composition of the oxide. Therefore, the standard deviation of the iron content should be within 3% at the intermediate point on the section of the alloyed plated layer of the galvannealed steel sheet produced by the present invention, and in this case, the powdering property in the plated layer is excellent.

Here, the alloying plating layer may contain, by weight%, Al: 0.08 to 0.35%; At least one component selected from the group consisting of Ni, Cr, Mn, Mg, Si, P, S, Co, Sn, Bi, Sb and Cu, Fe: 8 to 12%, the remainder Zn and other unavoidable impurities. If the content of Al is less than 0.08%, formation of the Fe-Al alloy phase formed at the interface between the base steel and the plating layer may be inhibited. If the content of Al exceeds 0.35%, the content of Al in the plating layer increases, It is preferable to set the Al content in the alloying plating layer to 0.08 to 0.35% by weight. In addition, when a steel sheet is immersed in a plating bath while plating various steel species in a galvanizing bath, a portion of the steel sheet may be dissolved in the plating bath, and when plating is performed in such a plating bath, it is naturally included in the plating layer. In addition, during the alloying process, some of the components contained in the steel are incorporated into the plated layer while the base iron diffuses into the zinc plated layer. In the present invention, even if the content of the above-mentioned component contained in the plating layer is 0.5% or less due to these two factors, the same effect is obtained.

The microstructure of the high strength alloyed hot-dip galvanized steel sheet according to the present invention may include ferrite, bainite, martensite and austenite. Particularly, the residual austenite has an area fraction of 5 to 25% Tensile strength and tensile strength (Mpa) x elongation (%) ≥ 16000 can be obtained.

Hereinafter, a method for producing a high-strength alloyed hot-dip galvanized steel sheet excellent in surface quality and powderiness in a plating layer of the present invention will be described in detail.

A method for producing a high strength alloyed hot dip galvanized steel sheet excellent in surface quality and powderiness in a plating layer according to the present invention is characterized by comprising 0.1 to 0.3% of C, 1 to 2.5% of Si, 2.5 to 10% of Mn, : 0.001 to 0.5%, P: not more than 0.04%, S: not more than 0.015%, N: not more than 0.02% (excluding 0%), Providing a steel slab comprising: [N] to 0.1%; Ni: 0.005 to 0.5%; Sb: 0.01 to 0.07%; Nb: 0.1 or less; B: 0.005% or less; balance Fe and other unavoidable impurities; Reheating the steel slab to a temperature of 1100 to 1300 ° C; Finishing hot-rolling the reheated steel slab at a temperature equal to or greater than Ar ₃ ; Rolling the hot-rolled steel sheet at a temperature of 700 ° C or lower; Rolling and cold rolling the rolled steel sheet; recrystallizing and annealing the cold-rolled cold-rolled steel sheet at a dew point temperature of -60 to -20 占 폚 and a temperature of 750 to 950 占 폚 for 5 to 120 seconds; Cooling the annealed cold rolled steel sheet to 200 to 600 ° C at an average cooling rate of 2 to 150 ° C / sec; Reheating or cooling the cooled cold rolled steel sheet to a temperature of (plating bath temperature -20 ° C) to (plating bath temperature + 100 ° C); Immersing the reheated or cooled cold rolled steel sheet in a zinc plating bath maintained at a temperature of 450 to 500 ° C to form a zinc plated layer on the cold rolled steel sheet; And heating the steel sheet on which the zinc-plated layer is formed to 480 to 600 ° C to alloy the zinc-plated layer and then cooling the zinc-plated layer.

The present invention reheats slabs satisfying the above composition to a temperature range of 1100 to 1300 ° C. If the reheating temperature is lower than 1100 ° C, there is a problem that the hot rolling load sharply increases. When the reheating temperature is higher than 1300 ° C, reheating is performed at a temperature range of 1100-1300 ° C because the reheating cost increases and the surface scale amount increases.

The finishing hot rolling temperature of the reheated slab is limited to not less than Ar ₃ (the temperature at which ferrite starts to appear when the austenite is cooled), which is less than Ar ₃ , and the biaxial or ferrite reverse rolling of ferrite + austenite is performed Since the mixed grain structure is formed and a malfunction due to fluctuation of the hot rolling load may occur, finishing hot rolling is carried out above Ar ₃ .

After the hot rolling, the steel sheet is wound at a temperature of 700 DEG C or less. When the coiling temperature exceeds 700 캜, an oxide film on the surface of the steel sheet may be excessively generated and cause defects. Therefore, the steel sheet is wound at a temperature of 700 캜 or less.

After the rolled steel sheet is pickled and cold-rolled, the cold-rolled steel sheet is subjected to recrystallization annealing at a dew point temperature of -60 to -20 占 폚 for 5 to 120 seconds at a temperature of 750 to 950 占 폚. If the dew point of the atmospheric gas in the annealing furnace is lower than -60 캜, the diffusion rate of Si and Al on the surface of the steel becomes higher than the diffusion rate of Mn, so that the complex oxide containing Si, Mn and Al as main components formed on the surface of the steel sheet after annealing The Si and Al contents are greatly increased compared with Mn, and as the Si or Al content of the surface composite oxide is larger than Mn, the plating ability is weakened. Therefore, even in the case of the steel sheet having the constituent composition of the present invention, the wettability of zinc is insufficient, When the temperature is higher than 20 ° C, some of the Si, Mn and Al components are oxidized in the grain boundaries and in the grain boundaries inside the surface layer of the steel sheet to exist as internal oxides, so that the internal oxides during alloying after galvanization inhibit alloying, And the rate of diffusion of the base metal into the plating layer according to the distribution of internal oxides influences the alloying degree of the alloying plating layer to be large So that the powdering property in the plating layer is deteriorated. Therefore, the dew point of the atmosphere gas in the annealing furnace is preferably -60 to -20 ° C. When the annealing temperature is 750 ° C or higher, recrystallization sufficiently occurs. If the annealing temperature exceeds 950 ° C, the lifetime of the annealing furnace is reduced, and therefore, it is preferably 750 to 950 ° C. The annealing time is preferably at least 5 seconds in order to obtain a uniform recrystallized structure and within 120 seconds from the viewpoint of economy.

Here, the recrystallization annealing is preferably carried out in an annealing furnace in a H ₂ -N ₂ gas atmosphere. The hydrogen content in the atmospheric gas in the annealing furnace is preferably 3 to 70% by volume. When the hydrogen content is less than 3%, reduction of the iron oxide present on the surface of the steel sheet is insufficient, and when the hydrogen content exceeds 70%, the reduction effect of the iron oxide on the surface of the steel sheet is excellent, but is preferably limited to 30% in view of economical efficiency.

Preferably, the step of plating a plating amount of 0.01 to 2 g / m ² with at least one component selected from the group consisting of Fe, Ni, Co and Sn on the surface of the annealed cold rolled steel sheet before the recrystallization annealing is additionally carried out And recrystallization annealing can be performed. Such pre-plating is very effective in controlling the dew point in the annealing furnace to the target range.

After the recrystallization annealing, cooling may be performed at an average cooling rate of 2 to 150 占 폚 / sec to 200 to 600 占 폚 according to the microstructure to be obtained in accordance with the strength and elongation to be obtained. Preferably, the cooling can be divided into first and second cooling, wherein the second cooling rate is greater than the first cooling rate, and more preferably, the first cooling is 400 to 740 Lt; 0 > C to 200 < 0 > C to 600 < 0 > C in the second cooling.

In order to prevent transformation of austenite into pearlite in the ferritic and austenitic two-phase regions by recrystallization annealing, an average cooling rate of at least 2 캜 is required. On the other hand, if the cooling rate exceeds 150 DEG C / second, the temperature deviation in the width direction of the steel sheet becomes large due to quenching, and the shape of the steel sheet is not good.

The cooled steel sheet is subjected to reheating or cooling at a temperature of (plating bath temperature -20 ° C) to (plating bath temperature + 100 ° C) according to the temperature of the cooled steel sheet. If the cooling temperature of the steel sheet of the cooled steel sheet is lower than (plating bath temperature -20 ° C), the wettability of zinc is lowered. If the temperature is higher than (plating bath temperature + 100 ° C), the plating bath temperature is locally raised, There is a drawback that it is difficult.

The reheated or cooled steel sheet is immersed in a plating bath maintained at a temperature of 450 to 500 ° C to perform plating. If the temperature of the plating bath is less than 440 캜, the viscosity of the zinc increases and the rollability of the roll in the plating bath lowers. If the temperature exceeds 500 캜, the evaporation of zinc increases.

Here, the zinc plating bath, by weight%, Al: 0.05 ~ containing 0.14% and, Fe, Ni, Cr, Mn, Mg, Si, P, S, Co, Sn, from the group consisting of Bi, Sb, and Cu Preferably at least 0.5% of the selected at least one component, and the balance Zn and other unavoidable impurities. If the content of Al in the zinc plating bath is less than 0.05%, the lead iron is eluted in the plating bath, and the iron content in the plating bath is increased to form a dross of Zn-Fe compound to exist in the plating bath, And when the content of Al exceeds 0.14%, the Fe-Al compound is formed at the interface between the base steel and the plating layer to suppress alloying, so that the upper limit is preferably limited to 0.14%.

The plated steel sheet is heated to 480 to 600 ° C to alloy the galvanized layer produced by the plating with the cold rolled steel sheet and then cooled to produce the high strength alloyed hot-dip galvanized steel sheet of the present invention. The alloying temperature is required to be maintained at 480 to 600 ° C. in order to secure the iron content in the alloyed plated layer at 8 to 12%. When the alloying temperature is low, it is not economical because it takes a long time to secure the target iron content. The higher the alloying temperature is, the more easily the iron content of the alloy layer is secured. However, if the temperature is higher than 600 ° C, the evaporation of zinc becomes more severe on the surface of the steel sheet to decrease the amount of plating adhered, and the brittleness of the alloyed plating layer increases, It becomes.

As described above, the microstructure of the cold-rolled steel sheet produced by the manufacturing method of the present invention may contain ferrite, bainite, martensite and austenite, and in particular, the residual austenite has an area fraction of 5 to 25% , A tensile strength of 1000 MPa or more and a value of tensile strength (Mpa) x elongation (%)? 15000 can be obtained.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

After the steel having the composition shown in Table 1 below was melted, a slab was prepared. The slab was maintained at a temperature of 1200 ° C for 1 hour, then cooled to 650 ° C after finishing rolling at 900 ° C, maintained at 650 ° C for 1 hour, and then cooled.

The hot-rolled steel sheet after cooling was visually observed for the occurrence of hot cracks and was pickled for 30 seconds at 60 ° C with 17 vol% HCl solution to dissolve iron oxide on the surface of the steel sheet. Some specimens were subjected to an additional 20 seconds if the acidity was insufficient for 30 seconds and if the acidity of the surface iron oxide was present for a total of 50 seconds, the acidity was indicated as poor.

The cold rolled steel sheet was subjected to cold rolling at a reduction rate of 55%. After the cold rolled steel sheet was subjected to the pretreatment to remove foreign matter, the steel sheet was subjected to annealing in the heating and cooling conditions shown in Table 2, , And an air knife was used to adjust the adhesion amount of one-side reference plating to 60 g / m ^{2. The} alloy was subjected to alloying treatment under the conditions shown in Table 2, and then cooled to produce an alloyed hot-dip coated steel sheet.

The surface of the galvannealed steel sheet which had been plated and alloyed as described above was evaluated for surface quality by visually confirming whether or not an unplated portion existed and uniformity of surface color. In addition, to evaluate the powdering property of the steel sheet, a transparent tape was attached to the bent portion after the V-bending at 60 ° after the original was bent, and the powdering property was evaluated as the degree of the plating layer that was peeled off and peeled off from the tape. Respectively. In Table 3, the evaluation of unplated is expressed as " O: no uncoated portion, DELTA: uncoated having a diameter of 1 mm or less, X: uncoated with a diameter exceeding 1 mm " (Good): Powder ring width was less than 4 times the thickness of the steel sheet,? (Poor), and X: unevenness ", and the powder ring in the plating layer was evaluated as" ): Powder ring width is less than 6 times of steel sheet thickness, X (very poor): powder ring width is more than 6 times of steel sheet thickness ".

The tensile strength and elongation of the steel sheet were measured, and the results are shown in Table 3 in terms of tensile strength and tensile strength (Mpa) x elongation (%).

In order to confirm the alloying degree of the galvannealed steel sheet, the iron content was analyzed by dissolving 50 mm of the plated layer diameter in the acid and quantitatively analyzing it by ICP (ion coupled plasma spectrometer). In order to confirm the deviation of the alloying degree by site, the cross-section of the plating layer was polished and then photographed with SEM of 500 times, and the iron content at 1/2 point in the thickness direction of the plating layer in 10 places in parallel with the steel sheet was analyzed using EDX And the standard deviation is shown in Table 3.

As shown in Tables 1 to 3, Inventive Examples 3, 6, 8, 10 to 13, and 15 of the present invention were obtained by using the steel material having the composition range defined in the present invention, Galvannealed galvanized steel sheet. Hot cracking did not occur in the hot rolling process and the acidity was good. In addition, the tensile strength of the produced steel sheet was 1000 MPa or more, and the TS x El value was as high as 15000 or more. In addition, after the annealing, the amount of surface oxides to be concentrated on the surface of the steel sheet was reduced, unplating did not occur, and the surface color was uniform. In addition, the standard deviation of Fe content of the alloy layer was 3 or less, and the powdering property in the plating layer was also excellent or good.

In the case of Comparative Example 1, Sb was not added in the steel component, and the material properties satisfied the range defined in the present invention. However, since the thick Si, Mn, Al composite surface oxide on the surface of the steel sheet after annealing, And the standard deviation of Fe content of the alloy layer exceeded 3 as defined in the present invention and the powdering property in the plating layer was very poor.

In the case of Comparative Example 2, when the Mn and Cr contents in the steel component were lower than the range defined in the present invention, the tensile strength was lower than the range defined in the present invention, and Sb was not added to the steel, The wettability was worse and there was an unplated metal of more than 1 mm in diameter, and the standard deviation of Fe content of the alloy layer exceeded 3 as defined in the present invention, and the powdering property in the plating layer was very poor.

In the case of Comparative Examples 4 and 17, the steel component satisfies the range defined in the present invention, but when the dew point in the annealing furnace is higher than the range defined by the present invention, Si, Mn, But the Si, Mn and Al components are oxidized in the crystal grain boundaries and in the grain of the steel immediately below the surface of the steel sheet and exist as an internal oxide, so that in the alloying process, And thus the color of the surface of the steel sheet becomes ununiform. In addition, the standard deviation of the Fe content of the alloy layer exceeds 3, which is defined in the present invention, so that the powdering property in the alloyed plated layer Poor.

In the case of Comparative Example 5, the Si addition amount exceeded the present invention and Sb was not added. As a result, the edge of the hot-rolled steel sheet was cracked due to the Si overburden addition, Si was high and Sb was not added A thick surface oxide was formed on the surface of the steel sheet after annealing to deteriorate the wettability of zinc and there exist unplated diameters exceeding 1 mm, and the standard deviation of the alloy layer Fe content exceeded 3 as defined in the present invention, and the powdering property in the plating layer was very poor Respectively.

In the case of the comparative example 7, the steel component satisfies the range of the nominal value, but the annealing temperature is lower than the range defined in the present invention, and sufficient recrystallization can not be carried out and the strength is high but the elongation is low. Was lower than one range. However, the addition amount of Sb and other production conditions were satisfactory according to the present invention, and the amount of surface oxides to be reduced on the surface of the steel sheet after annealing did not occur and the surface color was uniform. In addition, the standard deviation of the Fe content of the alloy layer was 3 or less, which was excellent in the powdering property in the plating layer.

In the case of Comparative Example 9, when the dew point in the annealing furnace is lower than the range defined in the present invention, the Si, Mn and Al formed on the surface of the steel sheet in the annealing process Since the content of Si and Al in the composite oxide as the main component greatly increases compared to Mn, even in the case of the steel sheet having the component composition of the present invention, there is insufficient to secure the wettability of zinc, The standard deviation of the Fe content exceeded 3 as defined in the present invention and the powdering property in the plating layer was poor.

In Comparative Example 14, the Si and Mn contents in the steels were lower than the range defined in the present invention and Sb was not added, and the tensile strength was as low as 847 and the TS x El value was lower than the range defined in the present invention. In addition, the addition amount of the components forming the surface oxides such as Si, Mn and Al is low, but the Si-based oxide is formed on the surface because the Sb is not added and the dew point in the annealing furnace is lower than the range defined in the present invention, Or less. However, since the Si and Mn contents are low, the surface oxide is thin and the standard deviation of the Fe content of the alloy layer is less than 3, which is defined in the present invention, so that the powdering property in the plating layer is good.

In the case of Comparative Example 15, the occurrence of hot cracking due to the formation of AlN occurred when Ti and Sb were not added to the steel, and a thick surface oxide was formed on the surface of the steel sheet after annealing due to the absence of Sb, And the standard deviation of Fe content of the alloy layer exceeded 3 as defined in the present invention and the powdering property in the plating layer was very poor.

In the case of Comparative Example 18, the steel component is within the range defined by the present invention, the other manufacturing conditions are within the range of the present invention and the material properties are excellent. However, when the steel plate is lower in temperature than the range defined in the present invention, There was no plating less than 1mm in diameter because the wetting force of steel sheet and zinc was reduced. However, since the steel component and other manufacturing conditions satisfied the range of the present invention, the thickness of the surface oxide was made thin and the standard deviation of the alloy layer Fe content was smaller than 3 defined in the present invention, so that the powdering property in the plating layer was good.

In Comparative Example 19, the steel component was within the range defined in the present invention, but the cooling rate after annealing was slower than the range defined in the present invention, so that the austenite phase during cooling was transformed into a part of pearlite, The range was limited.

In the case of Comparative Example 20, the steel component is within the range defined by the present invention, the other manufacturing conditions are within the range of the present invention and the material properties are excellent. However, when the Al content in the plating bath is higher than the range defined in the present invention, Alloying temperature for obtaining an alloy layer having an iron content of 8 to 12% exceeds the range defined in the present invention because the after-plating layer / base alloy Fe-Al alloy phase is thickly formed and then inhibited the alloying in the alloying process. The evaporation of the plated zinc in the course of the process resulted in nonuniform surface color. In addition, due to the high alloying temperature, the thickness of the plating layer is uneven, and even if the standard deviation of the alloy layer Fe content is within the range defined in the present invention, the powdering property in the plating layer is extremely poor.

In case of Comparative Example 21, the Ni content of the steel exceeded the range of the present invention. As a result, the pickling ability of the hot-rolled steel sheet due to the high Ni deteriorated and some oxides precipitated on the surface of the hot-rolled steel sheet after the pickling were partially present. After the alloying of the three oxides remained in the steel sheet, there was a part of the uncoated metal having a diameter of 1 mm or less after the alloying and the surface color was partially uneven. However, the standard deviation of the alloy layer Fe content satisfied the range defined in the present invention, Sex was good.

In the case of Comparative Example 22, when the Sb content in the steel component is lower than the range defined in the present invention, the material characteristics satisfy the range defined in the present invention. However, after the annealing, a thick Si, Mn, The wettability of zinc was poor and there was an unplated metal of more than 1 mm in diameter, and the standard deviation of Fe content of the alloy layer exceeded 3 as defined in the present invention and the powdering property in the plating layer was poor.

 In the case of Comparative Example 23, the Mn content in the steel component exceeded the range defined by the present invention. Even if the other components and the manufacturing conditions satisfied the present invention, the oxide formed on the surface after annealing was thick, .

Claims (8)

Al: 0.001 to 0.5%, P: 0.04% or less, S: 0.015% or less, N: 0.02% or less, 0.1 to 0.7%, Mo: 0.1% or less, Ti: (48/14) * [N] to 0.1%, Ni: 0.005 to 0.5%, Sb: 0.01 to 0.07%, Nb : Not more than 0.1, B: not more than 0.005%, the balance Fe and other unavoidable impurities, and an alloyed plating layer formed on the cold-rolled steel sheet, wherein the iron (Fe) High-strength alloyed hot-dip galvanized steel sheet with excellent surface quality and powderiness within the plating layer with a standard deviation of 3% or less.
The high-strength alloyed hot-dip galvanized steel sheet according to claim 1, wherein the microstructure of the cold-rolled steel sheet is excellent in surface quality and powdering property in a plating layer, including residual austenite in an area fraction of 5 to 25%.
The high strength alloyed hot-dip galvanized steel sheet according to claim 1, wherein the cold-rolled steel sheet has a tensile strength of 1000 MPa or more, a tensile strength (Mpa) x elongation (%) of 15,000 or more, and excellent surface quality and powderability in the coating layer.
The method of claim 1, wherein the alloyed plated layer comprises, by weight percent, 0.08-0.35% Al, selected from the group consisting of Ni, Cr, Mn, Mg, Si, P, S, Co, Sn, Bi, A high strength alloyed hot-dip galvanized steel sheet comprising at least one component in an amount of 0.5% or less and excellent in surface quality and powdering property in a plating layer, comprising 8 to 12% of Fe, residual Zn and other unavoidable impurities.
Al: 0.001 to 0.5%, P: 0.04% or less, S: 0.015% or less, N: 0.02% or less, 0.1 to 0.7%, Mo: 0.1% or less, Ti: (48/14) * [N] to 0.1%, Ni: 0.005 to 0.5%, Sb: 0.01 to 0.07%, Nb : Not more than 0.1, B: not more than 0.005%, the balance Fe and other unavoidable impurities;
Reheating the steel slab to a temperature of 1100 to 1300 ° C;
Finishing hot-rolling the reheated steel slab at a temperature equal to or greater than Ar ₃ ;
Rolling the hot-rolled steel sheet at a temperature of 700 ° C or lower;
Cold rolling the picked-up steel sheet after pickling:
Subjecting the cold-rolled cold-rolled steel sheet to recrystallization annealing at a dew point temperature of -60 to -20 ° C and a temperature of 750 to 950 ° C for 5 to 120 seconds;
Cooling the annealed cold rolled steel sheet to 200 to 600 ° C at an average cooling rate of 2 to 150 ° C / sec;
Reheating or cooling the cooled cold rolled steel sheet to a temperature of (plating bath temperature -20 ° C) to (plating bath temperature + 100 ° C);
Immersing the reheated or cooled cold rolled steel sheet in a zinc plating bath maintained at a temperature of 450 to 500 ° C to form a zinc plated layer on the cold rolled steel sheet; And
And heating the steel sheet having the zinc-plated layer formed thereon to a temperature of 480 to 600 ° C to alloy the zinc-plated layer and then cooling the surface of the galvanized layer, and cooling the zinc-plated layer.
The method of manufacturing a high strength alloyed hot-dip galvanized steel sheet according to claim 5, wherein the recrystallization annealing is performed in an atmosphere of H ₂ -N ₂ gas and is excellent in surface quality and powdering property in a plating layer.
6. The method of claim 5, further comprising plating the surface of the cold-rolled steel sheet with a plating amount of 0.01 to 2 g / m ² as at least one component selected from the group consisting of Fe, Ni, Co, and Sn before the reheating annealing step A method of manufacturing a high strength alloyed hot dip galvanized steel sheet, further comprising surface quality and powdering resistance in a plating layer.
6. The zinc plating bath according to claim 5, wherein the zinc plating bath comprises Al: 0.05 to 0.14% by weight; At least one component selected from the group consisting of Fe, Ni, Cr, Mn, Mg, Si, P, S, Co, Sn, Bi, Sb and Cu; A method of manufacturing a high strength alloyed hot dip galvanized steel sheet excellent in surface quality and powderiness in a plating layer including a residual Zn and other unavoidable impurities.