KR101528008B1 - Galvanealed steel sheet with good surface quality and adhesion and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a hot-dip galvanized steel sheet used as a material for automobiles, home appliances and building materials, and a method of manufacturing the same.
More specifically, in the present invention, in the production of a hot-dip galvanized steel sheet, by controlling annealing heat treatment conditions, it is possible to obtain a hot-dip galvanized steel sheet that not only has surface quality but also excellent plating adhesion, .

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet having excellent surface quality and plating adhesion,

The present invention relates to a hot-dip galvanized steel sheet used as a material for automobiles, home appliances and building materials, and a method of manufacturing the same.

The hot-dip galvanized steel sheet has excellent corrosion resistance and is widely used in building materials, structures, household appliances, and automobile bodies. Hot-dip galvanized steel sheets (GI steel sheets) and galvannealed galvanized steel sheets (GA steel sheets) are the most frequently used hot-dip galvanized steel sheets. In particular, GI steel sheets are hot-dip galvanized steel sheets, And it is widely used as a material of the automobile itself.

In recent years, there has been a continuing demand for improvement of collision safety and fuel economy for steel sheets used as such automobile materials, and studies for increasing the strength of steel sheets applied thereto are increasing. However, when only the improvement of the strength of the steel sheet is sought, the ductility is relatively lowered, so it is necessary to improve the ductility while securing a certain strength.

In order to cope with the above, recently, it has been proposed to add Mn, Si and / or Al to the steel to form an ideal structure steel (dual phase steel, DP steel) having ferrite and martensite phase as a microstructure, ferrite, High strength steels such as complex phase steels (CP steels) and transformed organo-plastic steels (TRIP steels) have been actively developed. As described above, when Mn, Si and / or Al is added to steel, it is possible to manufacture a steel sheet with improved strength and improved ductility.

On the other hand, in the process of manufacturing a hot-dip galvanized steel sheet, cold-rolled cold-rolled steel sheet (aka Full Hard steel sheet) is subjected to degreasing to remove oils and foreign substances on the surface in a pre- Annealing is carried out, annealing is carried out, the steel is cooled in an appropriate temperature and then immersed in a hot-dip galvanizing bath to adhere zinc to the surface of the steel sheet, followed by control of the amount of coating with an air knife (air knife) .

In producing the hot-dip galvanized steel sheet by the above-described process, the atmosphere of the annealing furnace in the annealing step is set to a low dew point of -30 ° C or lower in a reducing atmosphere state. Do not.

However, a steel sheet containing elements which are easily oxidized in steel, that is, Si, Mn or Al, 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 steel sheet surface, The wettability of zinc during plating is disturbed, and an unpatched phenomenon occurs on the surface of the coated steel sheet locally or wholly without adherence of zinc, thereby significantly deteriorating the surface quality of the coated steel sheet.

Thus, as the measures to prevent their easy-to-oxidation of the steel elements from diffusing into the surface during the annealing process, the reduction annealing after the leading gold to iron (Fe) in the annealing before the steel sheet in Patent Document 1 as a coating weight of 10g / m ² The present inventors have proposed a method of manufacturing a steel sheet excellent in surface plating due to the fact that Si and Mn diffuse into the Fe-line plating layer in the ferrous iron but can not diffuse to the surface during normal annealing and annealing times.

In the case of Patent Document 1, the thick Fe line plating layer does not allow Si and Mn present in the base steel below the pre-plating layer to diffuse to the surface to form a surface oxide, and thus the plating wettability is excellent and the oxide is dispersed in the pre- However, in order to prevent the oxidizing elements such as Si and Mn from diffusing to the surface during reduction annealing, it is necessary to form a thicker pre-plating layer. To this end, an electroplating facility is required to be large, There is a problem.

In Patent Document 2, the steel sheet is oxidized in a direct flame furnace in an oxidizing atmosphere containing air and fuel at an air-fuel ratio of 0.80 to 0.95 in the annealing step, so that Si, Mn or Al alone Or a method of producing a hot-dip galvanized or alloyed hot-dip galvanized steel sheet excellent in plating quality by forming iron oxide containing a complex oxide and then performing reduction annealing in a reducing atmosphere to reduce iron oxide and then performing hot dip galvanizing Respectively.

When the method of performing the post-oxidation reduction step in the annealing step as described in Patent Document 2 is used, it is possible to suppress diffusion of elements such as Si, Mn and Al which are easily oxidized to the surface layer by the oxide formed to a certain depth from the steel sheet surface layer And the content of these oxides is relatively accumulated on the surface layer, so that the wettability with zinc in the plating bath is improved, and the plating can be reduced. That is, if an iron oxide layer is formed to a certain depth in the surface layer by heating under a high oxygen partial pressure that can oxidize iron, elements such as Si, Mn or Al existing in the iron oxide layer are also present as oxides, . However, the Si, Mn or Al existing in the iron oxide layer under the iron oxide layer diffuses to the surface layer of the iron oxide as the annealing time increases. At this time, diffusion is blocked by the iron oxide at the interface between the iron oxide and the iron oxide layer, As the elements react with the iron oxide at the interface, the iron oxide is reduced to iron, and Si, Mn or Al forms a single or composite oxide. Therefore, after the completion of the annealing, a reduced iron layer containing a single or complex oxide of Si, Mn, or Al exists in the outermost layer portion, and an oxide layer composed of Si, Mn, or Al alone or a complex oxide is formed thereunder .

As a result, in the case of Patent Document 2, since the surface layer is composed mainly of a reduced iron component after the completion of the annealing, it is advantageous in that the plating adhesion is excellent at the time of hot dipping to improve the unplated steel. However, There is a problem that the oxide layer existing under the iron layer is weak and the adhesion at the site is greatly deteriorated.

Japanese Patent Application Laid-Open No. 2002-322551 Korean Patent Publication No. 10-2010-0030627

One aspect of the present invention relates to a hot-dip galvanized steel sheet excellent in surface quality and excellent in plating adhesion by suppressing surface diffusion of elements easily oxidizable in the steel by controlling annealing heat treatment conditions in the production of a hot-dip galvanized steel sheet, To provide a way to

One aspect of the present invention includes a steel sheet comprising 3.0 to 10% manganese (Mn), 3.0% or less (including 0%) and 3.0% or less (including 0%) of aluminum At least one of Si and Al is limited to not less than 0.5%, and the balance of Fe and unavoidable impurities, wherein the sum of at least two elements among Mn, Si and Al is 3 to 15% and Mn ≥ Preparing a base steel sheet satisfying a relational expression of Si + Al;

The base steel sheet was heated to 740 to 850 ° C at a heating rate of 2 ° C / s or more in an annealing furnace of atmosphere gas containing hydrogen and residual nitrogen and inevitable impurities at a dew point temperature of -30 to 5 ° C and 1 to 5% Followed by annealing for 10 to 70 seconds;

Cooling the heated and held ground steel sheet to 600 DEG C or less under an atmospheric gas containing 1 to 70% by volume of hydrogen and the balance of nitrogen and unavoidable impurities; And

And a step of immersing the annealed and cooled ground steel sheet in a hot dip galvanizing bath at 440 to 480 캜 for plating, and a method for producing a hot dip galvanized steel sheet excellent in surface quality and plating adhesion.

According to another aspect of the present invention, And a zinc plating layer formed on the surface of the base steel sheet, wherein an inner portion made of Mn oxide, Si oxide, Al oxide, Fe oxide, or a composite oxide thereof at a depth of 0.3-5.0 袖 m from the interface between the base steel sheet and the zinc- Wherein the inner oxide satisfies Si + Al > 1/5 Mn in terms of% by weight, wherein the inner oxide satisfies Si + Al > / 5 Mn.

According to the present invention, when a hot-dip galvanized steel sheet is manufactured by using a steel sheet containing elements that form oxides on the surface of the steel sheet at a high temperature, Si or Al among the elements is diffused Therefore, it is possible to obtain a hot-dip galvanized steel sheet having excellent surface quality and excellent plating adhesion even after plating.

1 is a schematic cross-sectional view of a hot-dip galvanized steel sheet produced according to the present invention.

The inventors of the present invention have found that when using a base steel sheet containing easily oxidizable elements such as Si, Mn or Al to react with a trace amount of oxygen or water vapor existing in the annealing furnace in the process of producing a hot-dip galvanized steel sheet, Mn or Al alone or a complex oxide is formed on the surface of the steel sheet to prevent the wettability of zinc upon galvanizing to thereby cause local or total uncoated surface on the surface after plating to significantly reduce the surface quality of the coated steel sheet As a result of studying deeply, it was found that the diffusion of Si, Mn or Al into the surface of the steel was controlled by controlling the content of Mn, Si or Al and the content ratio of the steel components and controlling the annealing conditions, It is possible to produce a hot-dip galvanized steel sheet excellent in surface quality and plating adhesion and at the same time having high strength OK, it led to the present invention.

Hereinafter, a method of manufacturing a hot-dip galvanized steel sheet as one aspect of the present invention will be described in detail.

(Mn): 3.0 to 10%, silicon (Si): not more than 3.0% (inclusive of 0%) and aluminum (Al): not more than 3.0% by weight based on the total weight of the molten zinc- (Including 0%), wherein at least one of the silicon and aluminum is limited to not less than 0.5%, the balance Fe and inevitable impurities, and the sum of at least two of Mn, Si and Al is 3 To 15%.

At this time, if the content of one of the silicon and aluminum satisfies the lower limit value, the remaining steel may include 0%.

At this time, manganese (Mn) is an element contributing to stabilization of austenite by lowering the martensitic transformation starting temperature (Ms) by promoting the formation of austenite, improving hardenability and achieving high yield strength. In order to obtain the above-mentioned effect, it is necessary to contain Mn at not less than 3.0%. However, when it is added in an excessively large amount, Mn may be oxidized at a high temperature to cause cracks in the inside of the continuous casting or hot rolling, 10%. ≪ / RTI >

Silicon (Si) is an element that stabilizes ferrite and residual austenite at room temperature while improving the yield strength of the steel. That is, Si inhibits precipitation of cementite upon cooling from austenite, and contributes to stabilize a sufficient amount of the residual austenite to obtain a TRIP effect by significantly inhibiting the growth of carbide. In order to obtain the above effect, it is necessary to add Si at not less than 0.5%. However, if too much Si is added, the austenite temperature becomes high, so that hot rolling, which is usually carried out in the austenite region, becomes difficult. Therefore, it is preferable to limit the upper limit of the Si content to 3.0%.

Aluminum (Al), like Si, is an element that stabilizes ferrite during cooling of the steel sheet, accelerates the formation of ferrite, and refines the ferrite crystal. Therefore, it may be added instead of Si, or may be added together with Si. In order to obtain the above-mentioned effect, it is necessary to add at least 0.5%. However, if too much is added, internal cracks may occur in the slab during the continuous casting process, so it is preferable to limit the upper limit of the Al content to 3.0% Do.

As described above, when one component of Si and Al is added together with Mn, the present invention can be applied to the present invention without addition of the remaining elements, and even if added in a small amount, there is no problem in exhibiting the desired effect.

The above easily oxidizable components, that is, Mn, Si or Al diffuses to the surface of the steel sheet during the annealing step to form Mn, Si or Al alone or a composite oxide on the surface, It is difficult to produce a hot-dip galvanized steel sheet having excellent surface quality.

However, in the case of a high-strength steel, it is preferable to contain Mn, Si or Al in a certain amount in order to secure strength and ductility at the same time, and to contain these elements in a certain amount in order to maximize the effect of the present invention.

However, when the content of these elements is too large, it is difficult to obtain the effects of the present invention and it is difficult to obtain the effect of each element. Therefore, the total content of these elements is limited to 3-15 weight %, And it is preferable that the relationship of Mn (%)? Si (%) + Al (%) is satisfied. This is because, as will be described below, the external oxide formed on the surface of the steel sheet at the time of annealing is relatively formed of Mn oxide because a part of the Mn oxide is reduced by Al in the plating bath in the subsequent plating process .

The steel sheet used for producing the hot-dip galvanized steel sheet according to the present invention may contain, in addition to the above-mentioned components, 0.05 to 0.30% of carbon (C), 0.5% or less of molybdenum (Mo) (P): not more than 0.03%, titanium (Ti): not more than 0.2%, niobium (Nb): not more than 0.2%, nickel (Ni): not more than 1.0%, copper (Cu) 0.2% or less and boron (B): 0.1% or less.

At this time, carbon (C) is essential for obtaining good mechanical properties, and it is necessary to add C to not less than 0.05% in order to promote hardenability and to obtain sufficient yield strength and to form stabilized austenite. However, the higher the content of C is, the more the austenite phase formed at a high temperature when cooled to room temperature is not transformed into martensite so that the austenite phase remaining at room temperature is increased, which is advantageous for increasing ductility. However, It is preferable to limit the upper limit value to 0.30%.

Although molybdenum (Mo) is not essential, there is no problem in obtaining the desired mechanical properties, but Mo of 0.5% or less promotes the formation of martensite and has a beneficial effect in increasing corrosion resistance. However, a large amount of Mo accelerates the phenomenon of low-temperature cracking in the welding zone and can lower the toughness of the steel, resulting in an increase in manufacturing cost. Therefore, when Mo is added, its content is preferably limited to 0.5% or less.

Chromium (Cr) has the effect of forming fine carbides in the steel to inhibit the occurrence of cracks during bending. However, when added too much, there is a problem that the wettability of zinc is inhibited. Therefore, the content of Cr is limited to 1.0% or less .

Phosphorus (P) increases the stability of residual austenite by suppressing precipitation of carbides together with Si, but a large amount of P causes a problem of brittleness of steel. Therefore, it is preferable to limit the content to 0.03% or less.

Titanium (Ti) is an advantageous element for increasing the yield strength. However, in order to avoid decrease in toughness due to Ti addition, its content is preferably limited to 0.2% or less.

Niobium (Nb) is an element that improves the precipitation of carbonitride and increases the yield strength. However, if too much Nb is added, the weldability and high-temperature moldability of the steel deteriorate. Therefore, the content of Nb is preferably limited to 0.2%.

Nickel (Ni) is an element which is advantageous for raising the yield strength, but if it is added in an excessively large amount, the production cost rises. Therefore, it is preferable to limit the content to 1.0% or less.

Copper (Cu) is an element which is advantageous for increasing the yield strength together with Ti and Ni, and is an element which interferes with surface diffusion of oxidizing elements such as Si, Mn or Al during the annealing process. However, since it is an expensive element, It is preferably added in an amount of 1.0% or less.

Vanadium (V) is an element which is advantageous for improving the yield strength by grain refinement and increasing the wettability of the steel. However, if the content exceeds 0.4%, the toughness of the steel tends to deteriorate and cracks may occur in the welded portion, so that the upper limit value of V is preferably limited to 0.4%.

Boron (B) is an element that greatly improves the yield strength even in a small amount. However, when added in a large amount, low-temperature embrittlement may occur, and there is a problem that the wettability of zinc is also deteriorated. Limited is preferred.

The base steel sheet according to the present invention is composed of the remainder Fe and inevitable impurities in addition to the above-mentioned components. The base steel sheet containing such components may be a cold rolled steel sheet, and may be a cold rolled steel sheet subjected to degreasing by a conventional degreasing method . However, it is more preferable to apply the alkali degreasing method in consideration of cost.

After preparation of the base steel sheet having the above-mentioned composition is completed, the base steel sheet is subjected to pretreatment for washing and drying the base steel sheet, and then the steel sheet can be drawn into the annealing furnace for annealing and annealing can be performed.

In the present invention, when the dew point temperature inside the annealing furnace is set to -30 to 5 占 폚 in the annealing heat treatment, the base steel sheet is heated to a temperature of 740 to 850 占 폚 under an atmospheric gas containing 1 to 5% of hydrogen, residual nitrogen and inevitable impurities at a volume ratio Lt; 0 > C to 2 < 0 > C or more per second and then held for 10 to 70 seconds.

After the annealing under the above-mentioned conditions, the surface of the backing steel sheet may include an outer oxide composed of Mn oxide, Si oxide, Al oxide, Fe oxide, or composite oxide thereof, and the outer oxide may be Si + Al? 1/10 Mn.

Generally, in an annealing furnace in a reducing atmosphere, Mn, Si or Al diffused into the surface as the annealing temperature rises reacts with a trace amount of oxygen in the annealing furnace to form an oxide. Since the oxygen affinity of Si or Al is higher than that of Mn First, an oxide is formed. As a result, the composition of the external oxide formed on the surface after annealing is Si (%) + Al (%)> 1/10 Mn (%) composite oxide. Since the external oxide thus formed has a relatively high content of Si or Al, the wettability with zinc in the plating bath of the hot dip galvanizing process is lowered, resulting in non-plating.

However, when the steel sheet having the same steel component range as in the present invention is annealed in a nitrogen atmosphere gas containing 1 to 5% of hydrogen at a volume ratio by setting the dew point temperature inside the annealing furnace to -30 to 5 캜 , Oxygen is infiltrated into the interior of a certain depth from the surface of the base steel due to the high oxygen partial pressure in the annealing furnace, and Si or Al having a high affinity with oxygen forms oxides within the base steel sheet, while Mn is continuously formed on the surface of the base steel sheet To form an oxide on the outside. Therefore, the composition of the external oxide formed on the surface after the completion of the annealing forms a composite oxide having a higher content of Mn as compared with the case of the general reduction annealing. More specifically, the composition of the external oxide after the annealing is Si (%) + Al (% )? 1/10 Mn (%).

The external oxide satisfying the above relational expression has a relatively high content of Mn with a relatively good plating property, so that even when an external oxide is present, excellent plating ability can be ensured.

When the dew point temperature inside the annealing furnace is less than -30 ° C during annealing, elements easily oxidizable such as Mn, Si or Al in the steel are diffused to the surface of the steel to form these oxides on the surface. There is a problem that the wettability of zinc is lowered during the zinc plating process. Therefore, the higher the dew point temperature in the annealing furnace, the higher the oxygen partial pressure, which increases the amount of oxygen penetrating into the steel, and Mn, Si or Al in the steel reacts with the permeated oxygen to form oxides in the steel. However, if the dew point temperature is too high to exceed 5 ° C, Mn, Si or Al in the steel first diffuses to the surface to form oxides on the surface of the base steel before the oxygen penetration at the beginning of the annealing, So that the composition of the external oxide existing on the surface of the steel sheet does not satisfy Si + Al? 1/10 Mn.

More specifically, when annealing is performed in an annealing furnace having a dew point temperature of -30 to 5 ° C, oxygen permeates into the steel, and the permeated oxygen preferentially reacts with Si or Al to form an internal oxide, And Mn is oxidized only when the relative partial pressure of oxygen is higher than that of Si and Al. The Mn is not completely oxidized in the steel, and some of the Mn is diffused to the surface of the steel sheet to form Mn oxide in the surface layer of the steel sheet. As a result, an internal oxide composed of Mn oxide, Si oxide, Al oxide, Fe oxide, and composite oxide thereof is formed in a thickness of 0.3 to 5 μm from the surface of the base steel sheet after annealing. At this time, the composition of the internal oxides satisfies Si (%) + Al (%)? 1/5 Mn (%) within 0.3 to 5 占 퐉 from the surface of the base steel sheet in the thickness direction of the base steel sheet, The composition of Si (%) + Al (%)? 1/10 Mn (%) (see the schematic diagram of FIG. 1). Since the Mn oxide formed on the surface layer of the above-mentioned backed steel sheet is reduced and reacted with Al present in the plating bath during the subsequent zinc plating process, the content of Mn oxide is higher on the surface of the base steel sheet than in the case where the content of Si oxide or Al oxide is high When it is high, the wettability of zinc is better.

The atmospheric gas in the annealing furnace satisfying the above-described dew point temperature is preferably composed of a reducing atmosphere gas containing 1 to 5% by volume of hydrogen, residual nitrogen and unavoidable impurities.

If the content of hydrogen in the atmospheric gas is less than 1%, iron oxide formed on the surface of the steel sheet in the cold rolling step before the annealing can not be sufficiently reduced in the annealing process, and iron oxide may remain on the surface of the steel sheet . On the other hand, if hydrogen is contained in an amount exceeding 5%, there is no problem in sufficiently reducing the iron oxide, but it is preferable to limit the upper limit to 5% in consideration of economical efficiency.

In the annealing furnace satisfying the above-mentioned conditions, it is preferable to heat the base steel sheet to 740 to 850 占 폚 and hold it for 10 to 70 seconds.

When heating, it is preferable to heat from room temperature to the above-mentioned temperature range at an average heating rate of 2 ° C or more per second. If the heating rate is too slow, the time required for Mn, Si and Al in the steel to reach the temperature at which oxidation occurs in the steel (%) + Al (%) < / = 1/10 Mn (%) after the annealing is completed, since the amount of diffusion of the oxide to the surface of the steel sheet before the formation of the inner oxide increases, . Therefore, the faster the heating rate is, the better, but if it is at least 2 캜 / s, there is no problem in achieving the object of the present invention.

For the purpose of ensuring the optimum material after annealing, the heating temperature is preferably set at 740 to 850 占 폚, and the holding time at this temperature is preferably set to 10 to 70 seconds.

It is possible to cool the base steel sheet having undergone the annealing to 600 DEG C or less according to the above-described conditions.

The cooling process preferably uses a gas composed of 1 to 70% by volume of hydrogen and the balance of nitrogen and unavoidable impurities. When the content of hydrogen is 1% or more during the cooling, the oxidation of the iron does not occur. As the content of hydrogen increases It is preferable to limit the content to 70% in consideration of the stability and economical efficiency of the hydrogen gas.

The molten zinc plated layer may be formed by immersing the already-finished coated steel sheet directly in the hot dip galvanizing bath, and the method of forming the plated layer is not particularly limited.

For example, in the case of hot-dip galvanizing, hot-dip galvanizing can be carried out by immersing in 0.1 to 0.25 wt% of Al and the remainder of Zn in a hot-dip galvanizing bath at 460 to 480 캜 for 3 to 5 seconds.

However, in the present invention, a plating bath of 440 to 480 DEG C containing 0.18 to 1.0 wt% of aluminum (Al) in a hot dip galvanizing bath can be used. In the present invention, if the Al content in the plating bath is less than 0.18%, the effect of reducing the Mn oxide present on the surface of the base steel sheet is insignificant. Therefore, the higher the content of Al in the plating bath is, the more advantageous for reducing the Mn oxide, but if it is contained in an excessively large amount, the content of Al in the plating layer increases and the weldability is deteriorated. Therefore, it is preferable to limit the Al content in the plating bath to 0.18 to 1.0%. The temperature of the plating bath is preferably set to 440 to 480 캜 in consideration of the fluidity of the plating bath, evaporation of zinc, and dissolution of the steel sheet in the plating bath.

In the case of the present invention, in the production of a hot-dip galvanized steel sheet, elements which are easily oxidized by controlling the dew point temperature during annealing form oxides in the steel and sufficiently suppress diffusion to the surface of the steel sheet, And a hot-dip galvanized steel sheet excellent in plating adhesion can be produced.

Hereinafter, a hot-dip galvanized steel sheet according to another aspect of the present invention will be described in detail.

The hot-dip galvanized steel sheet according to the present invention comprises: A Si oxide, an Al oxide, a Fe oxide, or a composite oxide thereof at a depth of 0.3 to 5.0 μm from the interface between the base steel sheet and the zinc plated layer toward the base steel sheet, and a zinc oxide layer formed on the surface of the base steel sheet. Wherein the composition of the internal oxides satisfies Si (%) + Al (%) ≥ 1/5 Mn (%).

Generally, in the high strength hot-dip galvanized steel sheet produced by the conventional method, oxides are formed at the interface between the zinc plated layer and the base steel sheet. However, when the present invention is applied, oxides are formed up to 5 mu m in the direction of the base steel sheet at the interface between the base steel sheet and the zinc plated layer. On the other hand, in order to form oxides from the interface of the zinc plating layer to the depth of more than 5 탆 in the direction of the base steel sheet, the oxidizing atmosphere in the annealing process becomes too high, and a large amount of Fe oxide is formed on the surface of the steel sheet, which may lower the wettability of zinc. Therefore, when oxides are formed in the steel sheet, it is preferable to form up to 5 mu m as in the present invention.

The zinc plated layer of the hot-dip galvanized steel sheet according to the present invention preferably contains 0.18 to 1.0% by weight of aluminum (Al).

Generally, the Al content in the plating bath for producing the hot-dip galvanized steel sheet is 0.12 to 0.25%. In the present invention, in order to effectively reduce the Mn oxide among the external oxides existing on the surface of the base steel sheet in the plating bath, Is required. Therefore, in order to obtain the above effect, it is preferable to use a plating bath containing 0.18 to 1.0% of Al.

Since Al in the plating bath has a greater affinity for oxygen than Mn, the Mn oxide present on the surface of the base steel sheet can be reduced to Mn by Al in the plating bath.

The hot-dip galvanized steel sheet according to the present invention is characterized in that the diffusion of elements such as Mn, Si or Al into the steel sheet surface is suppressed and oxides are formed in the steel during the annealing process, And is excellent in the plating adhesion.

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 cold-rolled steel sheet having the composition shown in Table 1 and having a thickness of 1.2 mm was used as a base steel sheet.

The dew point temperature, the heating rate, the concentration of hydrogen in the atmospheric gas, the annealing temperature and the holding time in the heating and holding sections of the continuous annealing and hot dip galvanizing equipment were measured in the following Table 2 And then cooled by varying the hydrogen concentration and temperature of the reducing atmosphere gas in the cooling section, and then plated by dipping in a hot dip galvanizing bath controlled in Al content and temperature. At this time, the temperature immediately before pulling the respective annealed annealed cold rolled steel sheets into the plating bath was kept constant at 480 DEG C, and if the temperature just before the pulling was cooled to a temperature lower than 480 DEG C, Respectively.

After completion of the plating, the amount of plating adhered on one side was adjusted to 60 g / m ² by air wiping and then cooled to room temperature to finally produce a hot-dip galvanized steel sheet.

The inventors of the present invention observed the surface of each of the produced hot-dip galvanized steel sheets with naked eyes to determine the surface quality according to the presence or absence of the unplated and the degree of surface roughness.

At this time, the surface quality was determined according to the following criteria.

Very good (⊚): When there is no unplated over the whole plated steel sheet

Excellent (∘): When no dot plating of less than 0.5 mm is observed for two or less per 100 cm ²

Defective (△): When a large number of non-spotted plating of 0.5mm to 3mm size is observed

Very poor (X): When unplated size exceeding 3mm is observed

 In addition, a transparent tape was attached to the surface after 30 ° B-vending, and then the tape was peeled off. The width of the plating material was measured on the tape, and the plating adhesion was evaluated. The results are shown in Table 3 below .

At this time, the plating adhesion property was determined according to the following criteria.

A: No peeling at all

?: When peeling occurred in a small amount

X: When peeling was totally occurred

Then, a thin film was prepared by using FIB (Focused Ion Beam) on the cross section of each of the manufactured hot-dip galvanized steel sheets, and then a cross section was observed using a TEM (Transmission Electron Micrograph) and EDX (Energy Dispersive X-ray Spectrometer) , The distribution on the oxide present on the cross section, and the depth of the internal oxide.

In particular, the composition of the external oxides at the interface between the zinc plated layer and the base steel sheet was determined by analyzing the external oxide at a depth of 0.01 μm from the outer oxide surface toward the base steel sheet by EDX. At this time, 5 sites were measured and the average value was taken as a representative value.

The composition of the oxides (Mn, Si and / or Al) existing at the depth of 0.5 탆 is measured for the composition of the oxides existing in the range of 0.3 to 5.0 탆 from the interface toward the base steel sheet, Respectively. At this time, 10 sites were measured and the average value was taken as a representative value.

Steel grade C Mn Si Al Mo Cr P Ti Nb Ni Cu V B A 0.1 3 1.5 0.2 0.001 0.1 0.015 0.005 0.001 0.05 - 0.001 - B 0.1 6 1.5 2 0.02 0.05 0.015 0.001 0.001 0.05 0.05 0.001 0.0005 C 0.1 4 One 2 0.001 - 0.025 0.001 - - 0.01 - - D 0.2 8 1.5 3 - 0.001 0.015 - - 0.01 0.01 - 0.0002 E 0.1 2 2.5 2 0.001 0.05 0.020 - - 0.01 0.02 - -

division Steel grade Heating and maintenance section Cooling section Plating bath dew point
Temperature
(° C) Hydrogen
content
(vol%) heating
speed
(° C / s) Annealing
Temperature
(° C) maintain
time
(second) Hydrogen
content
(vol%) Temperature
(° C) Al content
(wt%) Temperature
(° C)


foot
persons
Yes One A -10 5 3 760 55 10 400 0.23 455 2 D 5 5 2.2 780 45 5 450 0.22 460 3 B -20 2 3.5 800 60 40 400 0.19 460 4 B -25 1.2 2.5 780 32 60 480 0.5 460 5 B -10 One 5 820 25 5 440 0.7 460 6 C -3 4 3 800 35 10 300 0.2 450 7 B -28 5 2.5 780 50 2 550 0.22 460 8 B -5 4 2.8 800 55 5 570 0.21 460 9 C -5 5 3.3 760 55 5 350 0.21 455

ratio
School
Yes One A -40 5 2.5 780 45 10 440 0.2 458 2 B -45 3 3 800 43 5 400 0.24 457 3 B 10 3 3 800 35 5 450 0.23 460 4 B -10 3 1.5 800 40 5 400 0.22 460 5 C -5 5 2.2 800 42 55 500 0.15 450 6 E -15 3 2.6 780 48 10 380 0.23 460

division Steel grade Internal oxide
Maximum depth
(μm) External oxide
Ingredient content
(wt%) Relationship 1
(Si + Al?
1/10 Mn) Internal oxide
Ingredient content
(wt%) Relation 2
(Si + Al?
1/5 Mn) surface
quality
evaluation Plated
Adhesiveness
evaluation 0.01μm depth 0.5μm depth Mn Si + Al (1/10) * Mn Satisfaction Mn Si + Al (1/5)
* Mn satisfied
Whether


foot
persons
Yes One A 0.9 86 0.8 8.6 ○ 45 9.4 9 ○ ◎ ◎ 2 D 4.1 86 0.2 8.6 ○ 38 36 7.6 ○ ○ ◎ 3 B 1.1 88 0.1 8.8 ○ 54 20 10.8 ○ ○ ◎ 4 B 0.8 83 0.6 8.3 ○ 49 28 9.8 ○ ○ ◎ 5 B 1.4 91 - 9.1 ○ 41 32 8.2 ○ ◎ ◎ 6 C 2.1 89 - 8.9 ○ 50 18 10 ○ ○ ◎ 7 B 0.6 81 2.2 8.1 ○ 33 35 6.6 ○ ○ ◎ 8 B 2.6 89 - 8.9 ○ 39 33 7.8 ○ ◎ ◎ 9 C 3.8 85 0.1 8.5 ○ 49 23 9.8 ○ ◎ ◎

ratio
School
Yes One A - 15 40.5 1.5 × - - - - × × 2 B - 7 26 0.7 × - - - - × × 3 B 6.8 31 19 3.1 × 75 8.5 15 × △ △ 4 B 1.7 56 7.4 5.6 × 53 17 10.6 ○ △ △ 5 C 2.5 83 - 8.3 ○ 51 16 10.2 ○ △ × 6 E 1.2 41 11.1 4.1 × 35 19 7 ○ × ×

As shown in Table 3, in the hot-dip galvanized steel sheets of Inventive Examples 1 to 9 satisfying the present invention, the inner oxide was present from the interface between the zinc plated layer and the base steel sheet in the thickness direction of the steel sheet from 0.6 μm to 4.1 μm And the composition of the oxides was observed through a cross section observation at a depth of 0.5 μm. As a result, it was confirmed that the relationship of Si (%) + Al (%) ≥ 1/5 Mn (%) was satisfied.

As a result of observing the cross section of the external oxide at a depth of 0.01 μm from the surface of the external oxide to the direction of the base steel sheet, the composition of these external oxides was found to be Si (%) + Al (%), Lt; = 1/10 Mn (%). Therefore, the hot-dip galvanized steel sheets according to the present invention are excellent in surface quality after plating, and do not peel off in the evaluation of coating adhesion, and are excellent in plating adhesion.

On the other hand, in Comparative Examples 1 and 2, the steel composition satisfies the present invention, but when the dew point temperature is lower than the range proposed in the present invention in the heating and holding sections during the annealing heat treatment, The surface was oxidized at the surface, and the oxide was formed on the surface. Thus, the surface quality was extremely poor, and the plating peeling occurred severely.

In the comparative example 3, the composition of the steel composition satisfies the requirements of the present invention, but when the dew point temperature in the heating and holding sections during the annealing heat treatment is higher than the range proposed in the present invention, the amount of oxygen penetrating into the steel during annealing is too high The inner oxide was formed to a depth exceeding 5 탆 and the oxide of Mn, Si or Al was formed in the surface layer before the internal oxide was formed at the initial stage of heating, and the surface quality remained on the surface even after the end of annealing, The plating peeling occurred.

In the comparative example 4, the steel composition component satisfies the present invention, but when the heating rate is less than 2 캜 / s during the annealing heat treatment, some Mn, Si , Al elements diffuse to the surface to form oxides on the surface, resulting in poor surface quality and a small amount of plating peeling.

In Comparative Example 5, both the steel composition and the annealing heat treatment conditions satisfied the present invention. However, when the Al content in the plating bath at the time of plating was lower than the range proposed in the present invention, the content of the inner oxide or the outer oxide, However, the proportion of the Mn oxide partially present on the surface was reduced by Al in the plating bath, resulting in poor surface quality, and the plating detachment due to the Mn oxide remaining on the surface was severely generated.

Comparative Example 6 is a case where a steel sheet whose composition of steel composition does not satisfy the requirements of the present invention is used. Even if the annealing heat treatment and the plating process both satisfy the present invention, the steel sheet is peeled from the interface between the steel sheet and the zinc- As the component content of the external oxide existing within 0.1 mu m did not satisfy the range defined in the present invention, the surface quality was extremely poor and the plating peeling occurred severely.

Claims (9)

(Si): 3.0% or less (including 0%) and aluminum (Al): 3.0% or less (including 0%), wherein the content of Si and Al At least one of them is limited to not less than 0.5% and the balance Fe and inevitable impurities, and the sum of at least two of Mn, Si and Al is 3 to 15%, satisfying the relationship of Mn ≥ Si + Al The method comprising: preparing a base steel sheet;
The base steel sheet was heated to 740 to 850 ° C at a heating rate of 2 ° C / s or more in an annealing furnace of atmosphere gas containing hydrogen and residual nitrogen and inevitable impurities at a dew point temperature of -30 to 5 ° C and 1 to 5% Followed by annealing for 10 to 70 seconds;
Cooling the heated and held ground steel sheet to 600 DEG C or less under an atmospheric gas containing 1 to 70% by volume of hydrogen and the balance of nitrogen and unavoidable impurities; And
And dipping the annealed and cooled ground steel sheet in a hot dip galvanizing bath at 440 to 480 캜 for plating,
After the annealing step, the base steel sheet includes an outer oxide composed of Mn oxide, Si oxide, Al oxide, Fe oxide, or composite oxide thereof on the surface, and the outer oxide contains Si + Al? Wherein the hot-dip galvanized steel sheet has a surface quality and a plating adhesion.
The method according to claim 1,
Wherein said base steel sheet comprises 0.05 to 0.30% carbon (C), 0.5% or less of molybdenum (Mo), 1.0% or less of chromium (Cr), 0.03% or less of phosphorus, (Cu): not more than 1.0%, vanadium (V): not more than 0.2%, and boron (B): not more than 0.1% And further comprising at least one component selected from the group consisting of zinc oxide and zinc oxide.
The method according to claim 1,
Wherein the hot-dip galvanizing bath comprises 0.18 to 1.0% by weight of aluminum (Al), and the balance of zinc (Zn) and inevitable impurities is excellent in surface quality and plating adhesion.
delete (Si): 3.0% or less (including 0%) and aluminum (Al): 3.0% or less (including 0%), wherein the content of Si and Al At least one of them is limited to not less than 0.5% and the balance Fe and inevitable impurities, and the sum of at least two of Mn, Si and Al is 3 to 15%, satisfying the relationship of Mn ≥ Si + Al Coated steel sheet; And a zinc plated layer formed on the surface of the base steel sheet,
An inner oxide consisting of Mn oxide, Si oxide, Al oxide, Fe oxide, or composite oxide thereof at a depth of 0.3-5.0 μm from the interface between the base steel sheet and the zinc plated layer toward the base steel sheet in the direction of the base steel sheet, Si + Al? 1/5 Mn,
And an outer oxide composed of Mn oxide, Si oxide, Al oxide, Fe oxide, or a composite oxide thereof at the interface between the backing steel sheet and the zinc plated layer, wherein the outer oxide satisfies Si + Al? Wherein the hot-dip galvanized steel sheet is excellent in surface quality and plating adhesion.
delete delete 6. The method of claim 5,
Wherein said base steel sheet comprises 0.05 to 0.30% carbon (C), 0.5% or less of molybdenum (Mo), 1.0% or less of chromium (Cr), 0.03% or less of phosphorus, (Cu): not more than 1.0%, vanadium (V): not more than 0.2%, and boron (B): not more than 0.1% Wherein the hot-dip galvanized steel sheet further comprises at least one component selected from the group consisting of zinc oxide and zinc oxide.
6. The method of claim 5,
A hot-dip galvanized steel sheet containing 0.18 to 1.0% by weight of aluminum (Al) in the zinc plated layer and having excellent surface quality and plating adhesion.

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