KR101726090B1

KR101726090B1 - High strength galvanized steel sheet having excellent surface property and coating adhesion and method for manufacturing the same

Info

Publication number: KR101726090B1
Application number: KR1020150184304A
Authority: KR
Inventors: 강기철; 김명수; 강대영
Original assignee: 주식회사 포스코
Priority date: 2015-12-22
Filing date: 2015-12-22
Publication date: 2017-04-12

Abstract

The present invention relates to a high-strength melted zinc galvanization steel plate having excellent surface quality and galvanization adhesion, wherein the high-strength melted zinc galvanization steel plate includes a base steel plate and a melted zinc plating layer formed on a surface of the base steel plate, wherein the base steel plate comprises: 0.05-0.3 wt% of C; 1.5-10 wt% of Mn; 0.3-1.5 wt% of Si; 0.001-2.0 wt% of Al; 0.04 wt% or less of P (excluding 0 wt%); 0.015 wt% or less of S (excluding 0 wt%); 0.02 wt% or less of N (excluding 0 wt%); and the remaining consisting of Fe and inevitable impurities. An Fe-Al alloy on an interlayer between the melted zinc plating layer and the base steel plate is formed to take 70% or more areas with respect to a surface area of the base steel plate.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet excellent in surface quality and plating adhesion, and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion, and a method for producing 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 satisfy these demands, a high strength steel sheet having a strength of 900 MPa or more has recently been developed and applied to automobiles.

As a method for increasing the strength of a steel sheet, a steel sheet with high strength can be easily manufactured by increasing the addition amount of reinforcing components such as carbon. However, in the case of a steel sheet for an automobile body, The elongation of the steel sheet must be secured at the same time.

In order to secure both strength and ductility of automotive steel sheets, components such as Mn, Si, Al, Cr, and Ti are mainly added to the steel. If the amount of these additives is properly controlled and manufacturing process conditions are controlled, high strength and 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.

In the case of high strength steel sheets for automobiles having a strength of 900 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 Si, Mn, and Al, which are easily oxidized in the steel, react with trace amounts of oxygen or water vapor present in the annealing furnace to form Si, Mn, or Al alone or complex oxides on the surface of the steel sheet to lower the wettability of zinc The surface of the coated steel sheet is uncoated with no zinc locally or locally, thereby significantly degrading the surface quality of the coated steel sheet. Also, when oxides are present on the surface of the steel sheet after annealing, when the steel sheet is immersed in a plating bath, the Fe-Al alloy phase formed by the reaction of Fe and Fe in the plating bath is not formed, A plating peeling phenomenon occurs in which the plating layer is dropped off. After the annealing, the Si, Mn, or Al alone or complex oxide formation becomes worse as the content of oxidizing components such as Si, Mn, and Al becomes larger, so that plating and plating peeling becomes severe in the case of a high strength steel sheet of 900 MPa or more.

Various techniques have been proposed to solve these problems. Among them, Patent Document 1 discloses that, in order to prevent Si and Mn from diffusing to the surface during the annealing process, the steel sheet is preliminarily subjected to reduction annealing at a deposition amount of 10 g / m 2 before the annealing, But it does not diffuse to the surface because there is no oxide on the surface, and the plating is excellent. In addition, since the Si and Mn oxide are discontinuously dispersed in the pre-plating layer, the plating adhesion is improved And the like. However, in order to prevent the oxidizing components such as Si and Mn from diffusing to the surface during reduction annealing, it is necessary to coat the plating amount as high as 10 g / m < 2 >. In this case, there is a problem in that an electroplating facility for forming a thick pre-plating layer becomes large, which leads to an increase in cost.

Further, in Patent Document 2, oxides which are externally oxidized on the surface of a steel sheet after annealing are reduced by internally oxidizing components such as Mn, Si, and Al, which are easily oxidized, while maintaining a high dew point in the annealing furnace, A method for enhancing the property is disclosed. Internal oxidation of the oxidizing component by this method can reduce the external oxidation and improve the plating ability. However, since water vapor should be injected into a specific section of the continuous hot dip annealing system for internal oxidation and the dew point of the steam should be precisely measured and controlled at the time of injecting steam, it is necessary to construct additional equipment and control the production process of the hot- And the productivity may be reduced. Further, when stress is applied to the steel sheet during press forming of the steel sheet, the internal oxide oxidized in the steel surface layer portion is vulnerable to external stress, so that fracture tends to occur.

On the other hand, in Patent Document 3, the air and fuel are controlled to have an air-fuel ratio of 0.80 to 0.95 in the annealing process to oxidize the steel sheet in a direct flame furnace in an oxidizing atmosphere, There is provided a hot-dip galvanized or alloyed hot-dip galvanized steel sheet having an excellent quality of plating by forming an iron oxide containing a composite oxide and then performing reduction annealing in a reducing atmosphere to reduce iron oxide and then performing 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 and diffusion is suppressed to the surface layer, Al alone or a composite oxide is suppressed and the wettability with zinc in the plating bath is improved, so that the plating can be reduced. However, in the case of using the direct flame method, the maximum temperature range that can be heated is about 650 ° C. and the content of Si, Al and Mn contained in the steel is often insufficient to cause surface oxidation of iron. Have limitations.

Japanese Patent Application Laid-Open No. 2002-322551 Korean Patent Publication No. 2009-0006881 Korean Patent Publication No. 2010-0030627

One aspect of the present invention is to provide a high-strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

One aspect of the present invention is a steel sheet comprising, by weight, 0.05 to 0.3% of C, 1.5 to 10% of Mn, 0.3 to 1.5% of Si, 0.001 to 2.0% of Al, 0.04% : 0.015% or less (excluding 0), N: 0.02% or less (excluding 0), the balance Fe and unavoidable impurities; And

And a hot-dip galvanized layer formed on the surface of the base steel sheet,

The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion which is formed such that the Fe-Al alloy phase occupies an area of 70% or more of the surface area of the base steel sheet at the interface between the hot-dip galvanized layer and the base steel sheet.

In another aspect of the present invention, there is provided a method for manufacturing a semiconductor device, comprising the steps of: 0.05 to 0.3% of C, 1.5 to 10% of Mn, 0.3 to 1.5% of Si, 0.001 to 2.0% of Al, ), S: not more than 0.015% (excluding 0), N: not more than 0.02% (excluding 0), the balance Fe and unavoidable impurities; An annealing step of heating the cold-rolled steel sheet after the oxidation of the cold-rolled steel sheet by oxidation, followed by reduction heating; Slowly cooling the annealed cold rolled steel sheet in a reducing atmosphere; Cooling and cooling the slowly cooled cold rolled steel sheet; And a step of dipping the heated cold-rolled steel sheet in a plating bath to perform hot-dip galvanizing, and a method of manufacturing a high-strength hot-dip galvanized steel sheet having excellent surface quality and plating adhesion.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to provide a hot-dip galvanized steel sheet excellent in tensile strength and excellent in surface quality and plating adhesion which can be suitably applied to an automobile itself. More specifically, a tensile strength of 900 MPa or more can be secured, and the Fe-Al alloy phase is formed so as to occupy an area of 70% or more of the surface area of the base steel sheet at the interface between the hot-dip galvanized layer and the base steel sheet. It is possible to provide an excellent hot-dip galvanized steel sheet.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

The present inventors have found that a certain amount of Mn, Si and Al must be added to the steel sheet in order to produce a steel sheet having excellent tensile strength and excellent elongation. When a general annealing method is applied to a steel sheet having such an alloy composition, The surface quality is poor, and the problem of the adhesion of the plating is generated. In order to solve this problem, a depth study has been conducted.

As a result, the Fe-Al alloy phase is formed so as to occupy an area of 70% or more of the surface area of the base steel sheet at the interface between the hot-dip galvanized layer and the base steel sheet by appropriately controlling the alloy composition and the manufacturing conditions, Galvanized steel sheet and a method of manufacturing the same. The present invention has been accomplished based on these findings.

Hereinafter, a hot-dip galvanized steel sheet having excellent surface quality and plating adhesion according to one aspect of the present invention will be described in detail.

A hot-dip galvanized steel sheet excellent in surface quality and plating adhesion according to one aspect of the present invention comprises 0.05 to 0.3% of C, 1.5 to 10% of Mn, 0.3 to 1.5% of Si, 0.001 to 2.0% of Al, , P: not more than 0.04% (excluding 0), S: not more than 0.015% (excluding 0), N: not more than 0.02% (excluding 0), Fe and unavoidable impurities; And a hot-dip galvanized layer formed on the surface of the base steel sheet, wherein an Fe-Al alloy phase is formed on an interface between the hot-dip galvanized layer and the base steel sheet to occupy an area of 70% or more of the surface area of the base steel sheet.

First, the alloy composition of the hot-dip galvanized steel sheet excellent in surface quality and plating adhesion according to one aspect of the present invention will be described in detail. Hereinafter, the unit of each element is% by weight.

C: 0.05 to 0.3%

C is added for strengthening solid solution in ferrite and austenite and securing martensite strength.

When the C content is less than 0.05%, it is difficult to secure sufficient high strength. On the other hand, when the C content is more than 0.3%, high strength can be secured, but ductility and bending workability are poor, and weldability due to an increase in carbon chemical equivalent is reduced, resulting in poor press formability and roll formability. Therefore, the C content is preferably 0.05 to 0.3%.

Mn: 1.5 to 10%

Mn has the role of increasing the hardenability by inhibiting the ferrite formation and stabilizing the austenite and enhancing the strength.

When the Mn content is less than 1.5%, it is difficult to secure sufficient high strength. On the other hand, when the Mn content is more than 10%, the strength can be secured easily, but the amount of surface oxidation of Mn is increased in the annealing process, which makes it difficult to secure the plating property. Therefore, the Mn content is preferably 1.5 to 10%.

Si: 0.3 to 1.5%

Si is an element that improves the yield strength of steel and stabilizes residual austenite and ferrite at room temperature. In addition, Si inhibits precipitation of cementite upon cooling from austenite, and significantly inhibits the growth of carbides, thereby contributing to stabilization of a sufficient amount of retained austenite in TRIP (Tranformation Induced Plasticity) steels.

When the Si content is less than 0.3%, it is difficult to secure tensile strength and elongation. On the other hand, when the Si content is more than 1.5%, the hot rolling load may increase, causing a hot crack, and the amount of Si concentrated in the lower part of the reducing Fe layer after annealing may be increased to impair the plating adhesion. Therefore, the Si content is preferably 0.3 to 1.5%.

Al: 0.001 to 2.0%

Al is added for deoxidation in the steelmaking process and solidified in the ferrite to generate solid solution strengthening and improve the strength.

When the Al content is less than 0.001%, the above-mentioned effect can not be sufficiently obtained. On the other hand, when the Al content is more than 2.0%, a film-like continuous oxide film is formed on the surface of the base steel sheet to lower the Zn wettability of the base steel sheet. The plating adhesion can be dulled. Therefore, the Al content is preferably 0.001 to 2.0%.

P: 0.04% or less (excluding 0)

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%.

S: 0.015% or less (excluding 0)

S, like P, is an impurity element in steel and is an element that hinders ductility and weldability of the 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%.

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%.

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. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

The hot dip galvanized steel sheet may contain 0.1 to 0.7% of Cr, 0.1% or less of Mo (excluding 0), Ti (48/14) * [N] to 0.1%, Ni of 0.005 to 0.5% %, Nb: not more than 0.1% (excluding 0) and B: not more than 0.005% (excluding 0).

Cr: 0.1-0.7%

Cr is an element for increasing hardenability, and is an element that plays a role of suppressing the formation of ferrite. In order to sufficiently obtain the above-mentioned effect, it is preferable to add 0.1% or more, but if the content exceeds 0.7%, the manufacturing cost is increased due to an excessive amount of alloy.

Mo: 0.1% or less (excluding 0)

Mo is an element capable of effectively improving strength without deteriorating the hot dip galvanizing property. However, when the content is more than 0.1%, the increase of the effect is slowed and the cost is increased.

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

Ti has an effect of reducing the concentration of N in the steel as a nitride-forming element. For this purpose, it is necessary to add at least (48/14) * [N] in chemical equivalent. On the other hand, when the Ti content exceeds 0.1%, the carbon concentration of the martensite can be reduced and the strength may be lowered due to the precipitation of additional carbides in addition to the removal of the solid solution N. In the above relational expression, [N] is a value indicating the N content by mass%.

Ni: 0.005-0.5%

Ni is an element that improves the strength without reducing the plating ability since it is hardly concentrated on the surface during annealing. For this purpose, it is preferable to add at least 0.005%, but if the content exceeds 0.5%, pickling of the steel sheet may become non-uniform.

Nb: 0.1% or less (excluding 0)

Nb is segregated as a carbide in the austenite grain boundaries, and acts to increase the strength by suppressing the coarsening of the austenite grains during the annealing heat treatment. However, if the content exceeds 0.1%, there is a problem that the production cost due to an excessive amount of alloy is increased.

B: 0.005% or less (excluding 0)

B is an element that is selectively added to secure strength. However, when the content of B exceeds 0.005%, the steel sheet is concentrated on the surface of the annealed steel, and the plating ability can be significantly lowered.

On the other hand, it is preferable that an Fe-Al alloy phase is formed on the interface between the base steel sheet and the plating layer at a ratio of 70% or more with respect to the steel sheet area. The Fe-Al alloy phase plays a role in improving the wettability of zinc in the molten state upon hot dip galvanizing. When the ratio of the area of the Fe-Al alloy phase is 70% or more, A steel sheet can be obtained.

The microstructure of the hot-dip galvanized steel sheet of the present invention may include at least one of ferrite, bainite, martensite and austenite.

Further, the hot-dip galvanized steel sheet has a tensile strength of 900 MPa or more and a tensile strength and an elongation multiplied by 16000 MPa% or more. By securing such physical properties, it can be suitably applied to automotive steel sheets and the like.

Hereinafter, a method for producing 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.

According to another aspect of the present invention, there is provided a method of manufacturing a high-strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion, comprising the steps of: preparing a cold-rolled steel sheet satisfying the alloy composition; An annealing step of heating the cold-rolled steel sheet after the oxidation of the cold-rolled steel sheet by oxidation, followed by reduction heating; Slowly cooling the annealed cold rolled steel sheet in a reducing atmosphere; Cooling and cooling the slowly cooled cold rolled steel sheet; And dipping the heated cold rolled steel sheet in a plating bath to perform hot dip galvanizing.

Step of preparing cold-rolled steel sheet

A cold-rolled steel sheet satisfying the alloy composition described above is prepared.

At this time, the step of preparing the cold-rolled steel sheet may include heating the steel slab satisfying the alloy composition described above to a temperature range of 1100 to 1300 ° C; Subjecting the heated steel slab to finish hot rolling at a temperature of the final hot rolling temperature Ar3 or higher to obtain a hot rolled steel sheet; Winding the hot-rolled steel sheet at a temperature of 700 ° C or lower; And hot rolling the hot rolled steel sheet after pickling to obtain a cold rolled steel sheet.

If the heating temperature of the steel slab is less than 1100 ° C, the hot rolling load may increase sharply. If the steel slab heating temperature exceeds 1300 ° C, the reheating cost may increase and the surface scale amount may increase.

When the final hot rolling temperature is lower than Ar3, the ferrite and the austenite are subjected to reverse biaxial rolling or ferrite reverse rolling to form a composite structure. The irregularities in the strength of each phase and the hot rolling due to the change in hot rolling load due to the phase transformation during hot rolling It is preferable that the final hot rolling temperature is Ar3 or higher.

In addition, when the coiling temperature exceeds 700 캜, an oxide film on the surface of the steel sheet is excessively generated, which may cause defects.

Annealing step

After the cold-rolled steel sheet is subjected to the oxidation-reduction treatment, it is heated by oxidation and subjected to reduction heating to be annealed.

When a general annealing method is applied to a steel sheet having the above-described alloy composition, unplated is generated, which results in poor surface quality and problems of adhesion of the plating.

More specifically, when annealing is performed in a reducing atmosphere, which is a general annealing method, oxygen is injected from oxygen and water vapor existing in the furnace to form a constant oxygen concentration gradient from the surface to the inside of the steel sheet, The oxygen concentration can be maintained at a level higher than the critical oxygen concentration. Since Mn, Si and Al are representative oxidizing elements and their energy stability due to the formation of oxides is relatively high compared with other elements, they rapidly diffuse to the surface during reduction annealing to form a depleted layer within a range of 0.1 μm or less on the surface of the steel sheet, A large amount of oxide is formed. The oxide interferes with the physical contact between the plating bath and the base steel sheet to significantly lower the wettability of the zinc, and the Fe-Al alloy phase is not formed at the interface between the base steel sheet and the zinc plated layer, resulting in poor plating adhesion.

Therefore, in the present invention, the non-oxidizing heating is set in the initial heating step so that the Fe oxide existing in the initial surface of the steel sheet is not reduced and the Fe oxide is maintained in an atmosphere in which no hydrogen gas is introduced, , Mn and Al and the like to the surface layer of the steel sheet. Then, a certain concentration of oxygen is injected into the annealing furnace in the oxidation heating so that Fe can be preferentially added to be oxidized to form Fe oxide, thereby suppressing oxidation of Si, Mn and Al, which are alloy components, do. Then, a high-temperature reducing atmosphere is formed in the reducing step to reduce the Fe oxide to pure Fe, and the thick pure Fe layer suppresses surface diffusion and oxidation of Si, Mn and Al to maintain pure Fe until the galvanizing bath is drawn, And the liquid zinc is improved so as to secure the plating property.

At this time, it is preferable that the annealing step is performed in a radiation tube furnace (RTF). This is because when the direct flame furnace is used, the maximum temperature range that can be heated is low, so that the Fe oxide can not be sufficiently generated depending on the contents of Al, Si and Mn contained in the steel. Also, by using RTF, the oxidation and reduction process can be performed at a high temperature of the heating zone and the crack zone during the annealing process, and the oxygen concentration in the temperature zone required for oxidation can be precisely supplied and controlled. Further, when the direct flame furnace is used in the annealing furnace, there may arise a problem that the iron oxide is unevenly oxidized.

The non-oxidizing heating in the annealing step may heat the cold-rolled steel sheet to 550 to 650 ° C in an N 2 gas atmosphere controlled at a dew point temperature of -50 to -30 ° C.

When the dew point temperature is lower than -50 ° C, Si and Al are included in the oxidation side to form surface oxides. When the dew point temperature is higher than -30 ° C, Mn is included in the oxidation side, It is because.

The atmosphere gas is supplied only with N ₂ gas to keep the surface Fe oxide formed in the cold rolling step without reducing it. When the temperature of the cold-rolled steel sheet due to the heat loss is less than 550 ° C, the oxidizing alloy elements such as Al, Si and Mn besides Fe may be preferentially oxidized in the subsequent oxidation heating step. If the temperature is higher than 650 ° C, The oxidizing alloy elements such as Al, Si and Mn can be oxidized.

In addition, the oxidizing heating can heat the cold-rolled steel sheet to 800 to 850 ° C while injecting O ₂ gas in an amount of 0.1 to 2 vol% in an N ₂ -2 to 8 vol% H ₂ gas atmosphere. Accordingly, the thickness of the Fe oxide layer formed on the surface of the cold-rolled steel sheet can be 300 nm or less by the oxidation heating.

If the concentration of the O ₂ gas is less than 0.1 vol%, oxidation of Fe may not occur sufficiently, and if it exceeds 2 vol%, formation of Fe oxide becomes excessive.

If the temperature of the cold-rolled steel sheet by oxidation heating is lower than 800 ° C, the Fe surface oxide may be insufficiently formed. If the temperature is higher than 850 ° C, the Fe oxide formation amount becomes excessive. Is not sufficiently reduced and the plating property and the plating adhesion property are weakened.

The reduction heating may be performed in a temperature range of 800 to 900 ° C in an N ₂ -2 to 8 vol% H ₂ reducing atmosphere controlled at a dew point temperature of -50 to -30 ° C, or may be maintained at an isothermal temperature. And the Fe oxide is reduced to pure Fe to secure the plating property and the plating adhesion. When the dew point temperature exceeds -30 ° C, the reduction zone is not formed sufficiently, and below -50 ° C is hardly controllable. If the H ₂ gas content is less than 2 vol%, the reducing atmosphere is not sufficiently formed and the Fe oxide is not reduced. If the H ₂ gas content is more than 8 vol%, the economical production cost increases. When the temperature of the cold-rolled steel sheet by reduction heating is less than 800 ° C., the reduction of the surface iron oxide may not occur sufficiently during the heating time and the surface Fe oxide may remain. When the temperature exceeds 900 ° C., There is a problem in that the cost is increased and the economical efficiency is lowered.

Slow cooling step

The annealed cold rolled steel sheet is slowly cooled in a reducing atmosphere. The Fe oxide is sufficiently reduced to improve the plating property and the plating adhesion property.

At this time, in the gradual cooling step, the annealed cold rolled steel sheet can be cooled in a N ₂ -5 to 20 vol% H ₂ reducing atmosphere to a slow cooling termination temperature of 600 to 650 ° C at a cooling rate of 2 to 15 ° C / s.

If the hydrogen content in the atmospheric gas is less than 5 vol%, a sufficient reducing atmosphere can not be formed, and if it exceeds 20 vol%, the cost increases.

In addition, when the cooling rate and the slow cooling end temperature are exceeded, there is a problem that the ferrite formation fraction during cooling becomes excessively low or high, and it is difficult to secure the material.

Cooling and heating stage

The slowly cooled cold rolled steel sheet is cooled and heated.

At this time, it can be cooled to a cooling rate of not less than 15 ° C / s and not more than 150 ° C / s, and can be heated to a temperature range of 400 to 500 ° C.

The cooling after the slow cooling needs to be higher than the slow cooling rate. A cooling rate of at least 15 ° C / s is required to prevent transformation of austenite into pearlite in the ferrite and austenite 2 phase by recrystallization annealing. On the other hand, if the cooling rate is higher than 150 ° C / s, 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.

Hot dip galvanizing step

The hot-rolled steel sheet is immersed in a plating bath and hot-dip galvanized.

At this time, it may further include heating or cooling the heated steel sheet to a temperature of (plating bath temperature -20 ° C) to (plating bath temperature + 100 ° C) before dipping in the plating bath. When the lead-in temperature of the steel sheet is lower than -20 캜, the wettability of zinc is lowered. When the temperature exceeds the plating bath temperature + 100 캜, it is difficult to control the temperature of the plating bath by increasing the plating bath temperature locally.

Further, the temperature of the plating bath may be 450 to 500 占 폚, and the composition of the plating bath may include Al: 0.1 to 0.3% by weight, the balance of Zn and unavoidable impurities.

When the plating bath temperature is lower than 450 캜, the viscosity of zinc increases and the rollability of the roll in the plating bath lowers. When the temperature exceeds 500 캜, the evaporation of zinc increases.

When the Al content of the plating bath is less than 0.1 wt%, formation of the Fe-Al alloy phase formed at the interface between the base iron and the plating layer can be suppressed. When the Al content exceeds 0.3 wt%, the Al content in the plating layer increases, There is a problem of dropping.

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 )

A steel slab having the composition shown in the following Table 1 was prepared and then heated to 1,200 ° C. and held for 1 hour. After finishing rolling at 900 ° C., the steel slab was cooled to 650 ° C. and maintained at 650 ° C. for 1 hour Froning was performed.

The cooled hot rolled steel sheet was visually observed for the occurrence of hot cracks and pickled for 30 seconds at 60 ° C and 17 vol% HCl solution to dissolve the iron oxide on the surface of the steel sheet. If the pickling was insufficient for 30 seconds in some specimens, additional pickling was performed for 20 seconds under the same conditions. Cold rolled steel sheet was obtained by cold rolling at 55% reduction rate of pickled steel.

The cold-rolled steel sheet was subjected to pretreatment to remove foreign matter on the surface thereof, followed by annealing under the conditions of non-oxidizing heating, oxidation heating and reduction heating shown in Table 2, followed by cooling. The temperature of the plating bath was 480 ° C, Plating was performed under a plating condition of an Al concentration of 0.23 wt% in a plating bath, and then adjusted to an adhesion amount of one side reference plating 60 g / m ² using an air knife and cooled to prepare a hot-dip galvanized steel sheet.

The tensile strength (TS), the product of the tensile strength and the elongation (TS * El) of the hot-dip galvanized steel sheet, the area of the Fe-Al alloy phase formed on the interface between the steel sheet and the plated layer, the surface quality and the plating adhesion were measured, Respectively.

The surface quality of the coated steel sheet after plating was evaluated visually by the naked eye for the presence and degree of the uncoated portion on the surface and evaluated by the following evaluation criteria.

○: No plating area

[Delta]: Unexplained presence of a diameter of 2 mm or less

X: Excess of 2mm or more in diameter

The area% of the Fe-Al alloy phase formed on the interface between the plated layer and the base steel sheet was obtained by dissolving and removing the plated layer, measuring the surface area using a scanning electron microscope (SEM) and image analysis software. In addition, in order to evaluate the coating adhesion of the steel sheet, an automotive structural adhesive was applied on the surface of the steel sheet, dried and bent to 90 ° after completion of solidification to separate the adhesive and the coated steel sheet, When there is no plating peeling, it is indicated by o, and when plating peeling occurs, it is indicated by X.

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

division Alloy composition (% by weight) C Mn Si Al P B S N A (invention steel) 0.22 1.8 1.15 0.0012 0.01 0.001 0.003 0.0045 B (invention steel) 0.25 2.2 1.45 0.0014 0.0085 0.002 0.0027 0.0062 C (invention steel) 0.24 4.1 0.3 0.003 0.01 0.001 0.0026 0.0036 D (invention steel) 0.17 3.9 1.423 0.0045 0.0095 0.001 0.003 0.0022 E (invention steel) 0.23 4.4 0.35 0.0041 0.008 0.001 0.003 0.0025 F (invention steel) 0.24 7.5 0.387 0.51 0.01 0.001 0.0026 0.0024 G (comparative steel) 0.25 8.3 2.45 0.0013 0.0095 - 0.003 0.0034 H (comparative steel) 0.24 7.1 0.041 2.7 0.01 - 0.0033 0.0035 J (invention steel) 0.21 4.5 0.35 0.03 0.012 - 0.0023 0.0054

Psalter
number Steel grade Annealing time Non-oxidizing heating Oxidation heating Reduction heating Slow cooling Annealing
Temperature dew point Annealing
Temperature Oxygen
density dew point Annealing
Temperature dew point Cooling
speed atmosphere
gas dew point s ℃ ℃ ℃ vol% ℃ ℃ ℃ ℃ / s vol% ℃ One
(Honorable Mention) A 45 551 -45 830 0.5 -45 850 -45 10 5H ₂ -N ₂ -45 2
(Comparative Example) D 78 587 -41 805 4.7 -48 851 -39 13 20H ₂ -N ₂ -48 3
(Comparative Example) G 34 554 -41 847 1.2 -41 847 -41 12 12H ₂ -N ₂ -41 4
(Comparative Example) C 60 584 -37 812 0.02 -37 851 -37 15 10H ₂ -N ₂ -37 5
(Honorable Mention) D 80 565 -48 812 0.97 -48 854 -48 11 5H ₂ -N ₂ -48 6
(Comparative Example) G 55 552 -37 837 1.15 -37 851 -37 14 15H ₂ -N ₂ -37 7
(Comparative Example) H 55 586 -36 838 1.56 -36 854 -36 15 12H ₂ -N ₂ -36 8
(Honorable Mention) E 42 556 -41 850 0.97 -41 870 -41 12 5H ₂ -N ₂ -41 9
(Comparative Example) F 59 562 -43 791 1.2 -43 792 -43 15 8H ₂ -N ₂ -43 10
(Comparative Example) F 94 571 -45 837 0.78 -49 851 -48 13 1H ₂ -N ₂ -48 11
(Comparative Example) D 56 551 -38 712 1.15 -38 852 -38 14 15H ₂ -N ₂ -38 12
(Honorable Mention) C 60 562 -42 849 1.1 -42 852 -42 10 5H ₂ -N ₂ -42 13
(Comparative Example) H 60 584 -45 819 1.25 -45 849 -45 14 8H ₂ -N ₂ -45 14
(Honorable Mention) B 60 562 -45 832 0.7 -45 850 -45 14 8H ₂ -N ₂ -45 15
(Honorable Mention) F 90 562 -48 832 0.15 -48 847 -48 15 7H ₂ -N ₂ -48 16
(Comparative Example) A 60 574 -15 827 0.2 -50 847 -50 11 15H ₂ -N ₂ -50 17
(Comparative Example) E 80 574 -39 923 1.2 -39 923 -39 11 12H ₂ -N ₂ -39 18
(Honorable Mention) J 78 561 -41 805 4.7 -48 851 -39 13 20H ₂ -N ₂ -48

In Table 2, all of the oxidation-free heating was performed in an N ₂ atmosphere, and both the oxidation heating and the source heating were performed in a 5H ₂ -N ₂ atmosphere.

Psalter
number Steel grade TS
(MPa) TSxEl
(MPa%) Steel plate surface
Fe-Al alloy phase area
(area%) surface
quality Plated
Adhesiveness division 1 (example) A 945 17808 87 ○ ○ Honor 2 (comparative example) D 1124 18963 56 X X Comparative Example 3 (comparative example) G 1180 19030 89 ○ X Comparative Example 4 (comparative example) C 940 18781 58 X X Comparative Example 5 (example) D 938 18963 91 ○ ○ Honor 6 (comparative example) G 1247 18760 78 ○ X Comparative Example 7 (comparative example) H 1023 21550 81 ○ X Comparative Example 8 (example) E 924 20050 89 ○ ○ Honor 9 (comparative example) F 1025 19800 65 X X Comparative Example 10 (comparative example) F 1047 18800 45 X X Comparative Example 11 (comparative example) D 943 19452 55 X X Comparative Example 12 (example) C 932 19417 87 ○ ○ Honor 13 (comparative example) H 1154 19452 79 ○ X Comparative Example 14 (example) B 912 19332 78 ○ ○ Honor 15 (example) F 1184 19874 85 ○ ○ Honor 16 (comparative example) A 952 21542 12 X X Comparative Example 17 (comparative example) E 856 15754 47 X X Comparative Example 18 (example) J 947 19452 84 ○ ○ Honor

Table 1 shows the composition of the steel sheet of the steel sheet on which the galvanized steel sheet is produced in the present invention. Table 2 shows the manufacturing method and evaluation characteristics of the galvanized steel sheet using the steel sheet according to the composition shown in Table 1. As shown in Table 2, the specimens Nos. 1, 5, 8, 12, 14, 15, and 18 of the present invention were prepared by using the steel materials having the composition ranges defined in the present invention, As a result, a hot-rolled steel sheet was not produced and the acidity was good. The tensile strength of the steel sheet was 900 MPa or more and TS (MPa) xEl (%) was 16000 or more. In addition, the reduction of the appropriate Fe oxide and Fe oxide in the annealing conditions of the steel sheet is suitably carried out to suppress the formation of surface oxides of Si, Al and Mn, whereby the Fe-Al alloy phase of the plating layer / And the plating adhesion was excellent.

In the case of Specimen Nos. 3, 6, 7, and 13, which are comparative examples, the manufacturing method satisfies the range of the present invention, but the range of Si and Al in the steel component exceeding the range suggested by the present invention, The wettability was obtained through the conditions as described in the present invention, and the plating surface was good, but the plating adhesion was poor due to excessive peeling of the plating due to excessive surface oxidation of the Fe surface reducing layer lower alloy component.

In the case of the comparative sample No. 2, since the oxygen concentration exceeded the range suggested by the present invention during the secondary oxidation heating, the Fe oxide was excessively generated due to the high oxygen concentration and was not sufficiently reduced during the subsequent reduction heating. And the wettability with hot dip galvanizing was lowered, and the surface grade and the plating adhesion were poor.

In the case of the comparative sample No. 4, since the oxygen concentration is less than the range suggested by the present invention during the secondary oxidation heating, the Fe oxide can not completely coat the surface due to insufficient oxygen concentration, and Si, Mn and Al Surface hardening and oxides were generated to lower the wettability with zinc, so that the Fe-Al alloy phase could not be sufficiently formed and the surface grade and adhesion were poor.

In Comparative Example No. 16, the surface oxide of the steel component Si and Mn was excessively formed in the primary anoxidized annealing section, the dew point of the primary anoxidizing heating section exceeded the range suggested by the present invention, The Fe oxide is not formed in the Si and Mn oxides formed, and the Si and Mn oxides are not reduced even during the reduction heating and the slow cooling periods. The Si and Mn oxides remain until the steel sheet comes into contact with the molten zinc to decrease the wettability with the molten zinc, The adhesion was poor.

In the case of Specimen No. 11 as a comparative example, since the surface iron oxide was sufficiently formed with the steel sheet heating temperature in the oxidation section being less than the range suggested in the present invention, the surface of the steel sheet could not be coated and the surface of the elements such as Si, Mn and Mn Thickening and oxides were generated, and the surface grade and adhesiveness were poor.

Specimen No. 17, which is a comparative example, is a temperature at which the steel sheet heating temperature in the oxidizing section exceeds the range suggested by the present invention. When excessive heating of the Fe oxide occurs after the reduction heating, And the wettability with molten zinc was lowered and the surface grade and adhesion were poor. In addition, tensile strength and tensile strength x elongation value were poor due to high annealing temperature.

In the case of the comparative example No. 9, when the annealing temperature in the third reduction heating is lower than the range suggested in the present invention, the surface Fe oxide generated in the secondary oxidation heating is not sufficiently reduced to pure Fe in the reducing heating section, And the surface of the plating and adhesion were poor.

In the case of the comparative sample No. 10, the hydrogen concentration in the slow cooling section was lower than the range suggested by the present invention, so that the surface Fe oxide formed in the oxidation heating section was not sufficiently reduced and the surface grade and adhesiveness were poor.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims

The steel sheet according to any one of claims 1 to 3, wherein the content of C is 0.05 to 0.3%, the content of Mn is 1.5 to 10%, the content of Si is 0.3 to 1.5%, the content of Al is 0.001 to 2.0%, the content of P is 0.04% , N: not more than 0.02% (excluding 0), Fe, and unavoidable impurities; And
And a hot-dip galvanized layer formed on the surface of the base steel sheet,
A high strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion, wherein an Fe-Al alloy phase is formed on an interface between the hot-dip galvanizing layer and the backing steel sheet so as to occupy an area of 70% or more of the surface area of the steel sheet.

The method according to claim 1,
The hot-dip galvanized steel sheet according to claim 1, wherein the molten zinc-plated steel sheet contains 0.1 to 0.7% of Cr, 0.1% or less of Mo (excluding 0), Ti (48/14) , And at least one of Nb: 0.1% or less (excluding 0) and B: 0.005% or less (excluding 0).

The method according to claim 1,
Wherein the microstructure of the hot-dip galvanized steel sheet comprises at least one of ferrite, bainite, martensite and austenite, and is excellent in surface quality and plating adhesion.

The method according to claim 1,
Wherein the hot-dip galvanized steel sheet has a tensile strength of 900 MPa or more and a tensile strength and an elongation multiplied by 16000 MPa% or more, which is excellent in surface quality and plating adhesion.

The steel sheet according to any one of claims 1 to 3, wherein the content of C is 0.05 to 0.3%, the content of Mn is 1.5 to 10%, the content of Si is 0.3 to 1.5%, the content of Al is 0.001 to 2.0%, the content of P is 0.04% , N: not more than 0.02% (excluding 0), the balance of Fe and unavoidable impurities;
Annealing the cold-rolled steel sheet in an N ₂ gas atmosphere controlled at a dew point temperature of -50 to -30 占 폚 to 550 to 650 占 폚 and then oxidizing and reducing and heating;
Slowly cooling the annealed cold rolled steel sheet in a reducing atmosphere;
Cooling and cooling the slowly cooled cold rolled steel sheet; And
Dipping the heated cold rolled steel sheet in a plating bath to perform hot dip galvanizing; Wherein the hot-dip galvanized steel sheet has excellent surface quality and plating adhesion.

6. The method of claim 5,
Wherein the annealing step is performed in a radiation tube furnace (RTF), wherein the annealing step is performed in a RTF. The method for manufacturing a high strength hot-dip galvanized steel sheet according to claim 1,

delete

6. The method of claim 5,
Wherein the oxidative heating is performed by heating the temperature of the cold rolled steel sheet to 800 to 850 캜 while injecting 0.1 to 2 vol% O ₂ gas in an N ₂ -2 to 8 vol% H ₂ gas atmosphere. A method for manufacturing a high strength hot dip galvanized steel sheet.

9. The method of claim 8,
Wherein the oxidation heating is performed so that the thickness of the Fe oxide layer formed on the surface of the cold-rolled steel sheet becomes 300 nm or less.

6. The method of claim 5,
The reduced heating is the surface quality and the coating adhesion, characterized in that for heating or maintaining an isothermal temperature in the range of 800 ~ 900 ℃ in dew point temperature -50 ~ -30 ℃ to control the N ₂ -2 ~ 8vol% H ₂ reducing atmosphere A method for manufacturing a high strength hot dip galvanized steel sheet.

6. The method of claim 5,
Wherein the annealing step comprises cooling the annealed cold rolled steel sheet at a cooling rate of 2 to 15 占 폚 / s in a N ₂ -5 to 20 vol% H ₂ reducing atmosphere to a slow cooling termination temperature of 600 to 650 ° C. And a method of producing a high strength hot-dip galvanized steel sheet excellent in plating adhesion.

6. The method of claim 5,
Wherein the step of cooling and heating the slowly cooled cold rolled steel sheet comprises cooling at a cooling rate of not less than 15 ° C / s and not more than 150 ° C / s, and heating to a temperature of 400 to 500 ° C. A method for producing a high strength hot dip galvanized steel sheet.

6. The method of claim 5,
Wherein the step of preparing the cold-rolled steel sheet comprises the steps of: C: 0.05 to 0.3%, Mn: 1.5 to 10%, Si: 0.3 to 1.5%, Al: 0.001 to 2.0%, P: 0.04% 0.015% or less (excluding 0) of S, 0.02% or less of N (excluding 0), the balance of Fe and unavoidable impurities to a temperature range of 1100 to 1300 캜;
Subjecting the heated steel slab to finish hot rolling at a temperature of the final hot rolling temperature Ar3 or higher to obtain a hot rolled steel sheet;
Winding the hot-rolled steel sheet at a temperature of 700 ° C or lower; And
And hot rolling the hot rolled steel sheet to obtain a cold rolled steel sheet. The method of manufacturing a high strength hot-dip galvanized steel sheet having excellent surface quality and plating adhesion.

6. The method of claim 5,
Further comprising the step of heating or cooling the heated steel sheet to a temperature of (plating bath temperature -20 DEG C) to (plating bath temperature + 100 DEG C) before immersing in the plating bath. A method of producing a high strength hot-dip galvanized steel sheet excellent in adhesion.

6. The method of claim 5,
Wherein the temperature of the plating bath is in a range of 450 to 500 ° C and the composition of the plating bath is 0.1 to 0.3% of Al, and the balance of Zn and unavoidable impurities. The production of a high strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion Way.

6. The method of claim 5,
The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet contains 0.1 to 0.7% of Cr, 0.1% or less of Mo (excluding 0), Ti (48/14) 0.1% or less (excluding 0), and B: 0.005% or less (excluding 0). The method for producing a high strength hot-dip galvanized steel sheet according to claim 1,