KR102010077B1 - High strength galvanized steel sheet having excellent surface property and coating adhesion and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a high strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion and a method of manufacturing the same.
Preferred aspects of the invention are by weight, C: 0.05-0.3%, Mn: 1.5-20%, Si: 0.3-3.0%, Al: 0.001-1.5%, P 0.04% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.02% or less (excluding 0), Ti: (48/14) * [N] ~ 0.1%, cold rolled steel and this cold rolled steel sheet containing residual Fe and other unavoidable impurities A Fe electroplating layer of 1,000 to 3,000 mg / m ² formed thereon, a Fe-Al alloy phase formed on the Fe electroplating layer, and a hot dip galvanized layer formed on the Fe-Al alloy phase, wherein the Fe-Al alloy phase is the cold rolled steel. It is to provide a high strength hot-dip galvanized steel sheet having a surface area of 70% by weight or more with respect to the surface area of the steel sheet and having an excellent surface quality and plating adhesion with a surface coating area ratio (%) of 50% or less (including 0%) and a method of providing the same. .
According to the present invention, it is possible to provide a high strength hot dip galvanized steel sheet having a tensile strength of 900 MPa or more, a tensile strength × elongation of 15,000 MPa% or more, and excellent surface quality and plating adhesion.

Description

High-strength hot-dip galvanized steel sheet with excellent surface quality and plated adhesion and manufacturing method thereof

The present invention relates to a hot-dip galvanized steel sheet used in automotive steel plates and the like, and more particularly, to a high-strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion and a method of manufacturing the same.

Recently, there is a continuous demand for increasing the strength of automobile steel sheets to reduce the weight of automobiles and improve the collision stability of automobiles due to the regulation of carbon dioxide for the preservation of the global environment.

In order to satisfy these demands, high strength steel sheets of 900 MPa or more have been recently developed and applied to automobiles.

In order to increase the strength of the steel sheet, a method of increasing the addition amount of reinforcing components of steel such as carbon may be used. However, in the case of the steel sheet for automobile body, cracks should not occur during forming into the vehicle body. It must be secured.

In order to secure the strength and ductility of the steel sheet for automobiles at the same time, components such as Mn, Si, Al, Cr, Ti, etc. are mainly added to the steel. The steel plate which has can be manufactured. In particular, in the case of a high strength steel sheet for automobiles having a strength of 900 MPa or more, a large amount of components such as Si, Mn, and Al are added to the steel to secure target strength and elongation.

In general, steel sheets used in automobiles need to improve corrosion resistance for extending the life of automobiles, and hot-dip galvanized steel sheets are used for this purpose. However, since Si, Mn, and Al mentioned above are susceptible to oxidation, the high-strength steel sheet containing these elements reacts with trace amounts of oxygen or water vapor in the annealing furnace to form Si, Mn, Al alone or composite oxide on the surface of the steel sheet. The wettability of zinc is lowered by forming a so-called unplating, in which zinc is not attached locally or entirely to the surface of the plated steel sheet, thereby greatly reducing the surface quality of the plated steel sheet.

In addition, when an oxide is present on the surface of the steel sheet after annealing, the Fe-Al alloy phase formed by reacting Al in the plating bath with Fe in the plating bath is not formed when immersed in the plating bath, so that the adhesion between the plating layer and the base iron is weak. The so-called plating peeling phenomenon occurs that the plating layer is dropped during the molding process. After annealing, the formation of Si, Mn or Al alone or composite oxide becomes more severe as the content of oxidizing components such as Si, Mn, Al increases, so that in the case of high strength steel sheet of 900 MPa or more, unplating and plating peeling become more severe.

In order to solve this problem, Patent Literature 1 discloses 2 to 8 vol% of hydrogen in the annealing heating section (650 to 750) in order to prevent the oxidized alloy elements Mn, Si, etc. from diffusing into the surface during oxidation annealing. In the reducing atmosphere, the steel sheet is heated as quickly as possible to suppress the surface oxidation of Si and Mn, and the FeO oxide is formed by heating for 1 to 10 seconds while supplying 0.01 to 1 vol% of oxygen in an isothermal state in the cracking zone, and then in the heating section. Reduction of FeO oxide to Fe by reducing heating in hydrogen atmosphere of 2-8 vol% to 900, and cooling in a reducing atmosphere until plating secured the plating property of high strength steel. However, even in such a method, it is possible to secure the plating property by forming reduced Fe on the surface of the steel sheet, but under the reduced Fe layer, Si and Mn are concentrated in parallel with the Fe layer to form oxide bands. There is a problem in that the brittleness is high and the plating layer is peeled off while being broken by stress applied in a bending environment such as processing and forming a plated steel sheet.

Patent Document 2 proposed as another method for improving the plating properties of high strength steels maintains the dew point in the annealing furnace higher than the general range, so that the oxygen partial pressure is up to several micrometers inside the ferrous iron. Disclosed is a method of forming higher than the oxidation-producing critical oxygen partial pressure, such as, to form an oxide mainly in grain boundaries inside the ferrous iron, thereby suppressing surface oxide formation to improve the plating property. Internal oxidation of the oxidizing component by this method reduces the external oxidation, thereby improving the plating property. However, as the surface of the steel sheet and the internal oxygen partial pressure increase, decarburization occurs due to oxidation of carbon (C), which is a solid element in the ferrous iron, resulting in thermal deterioration of the surface material, and water vapor in a specific section of the continuous hot dip annealing facility for internal oxidation. In the injection and steam injection, the dew point inside the furnace must be precisely measured and controlled, which requires the construction of additional equipment, and the production process of the hot-dip galvanized steel sheet becomes difficult, resulting in a decrease in productivity. In addition, when stress is applied to the steel sheet during press forming of the steel sheet, the internal oxide internally oxidized to the steel surface layer is vulnerable to external stress, so that fracture of the steel sheet tends to occur.

US Patent Publication No. 2008-0308191 Japanese Laid-Open Patent Publication No. 2013-501959

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

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

According to a preferred aspect of the present invention, in weight%, C: 0.05-0.3%, Mn: 1.5-20%, Si: 0.3-3.0%, Al: 0.001-1.5%, P 0.04% or less (excluding 0) S: 0.015% or less (excluding 0), N: 0.02% or less (excluding 0), Ti: (48/14) * [N] ~ 0.1%, cold rolled steel sheet containing residual Fe and other unavoidable impurities The Fe-Al alloy comprising 1,000 to 3,000 mg / m ² of Fe electroplating layer formed on the cold rolled steel sheet, a Fe-Al alloy phase formed on the Fe electroplating layer, and a hot dip galvanized layer formed on the Fe-Al alloy phase. The phase is formed of more than 70 area% of the surface area of the cold-rolled steel sheet, and the high-strength hot-dip zinc having excellent surface quality and plating adhesion, wherein the surface coating area ratio (%) of the oxide formed on the Fe electroplating layer is 50% or less (including 0%). Plated steel sheets are provided.

The cold rolled steel sheet may further include one or more selected from the group consisting of Cr: 0.1 to 1.0%, Mo: 0.1% or less, Ni: 0.005 to 0.5%, Nb: 0.1% or less, and B: 0.005% or less.

According to another preferred aspect of the present invention, in weight%, C: 0.05-0.3%, Mn: 1.5-20%, Si: 0.3-3.0%, Al: 0.001-1.5%, P 0.04% or less (excluding 0) ), S: 0.015% or less (excluding 0), N: 0.02% or less (excluding 0), Ti: (48/14) * [N] ~ 0.1%, balance Fe and other unavoidable impurities Preparing;

Electroplating the cold rolled steel sheet to form a Fe electroplating layer of 1,000 to 3,000 mg / m ² on a single side of a steel sheet surface;

Annealing the cold rolled steel sheet on which the Fe electroplating layer is formed as described above in an annealing furnace in a reducing atmosphere having a dew point temperature of -80 to -45 ° C; And

After cooling the annealed cold rolled steel sheet, immersing it in a zinc plating bath containing 0.1 to 0.3% by weight of Al and the balance Zn and other unavoidable impurities, wherein the electroplating comprises soluble Fe electrodes. It is carried out by using, the annealing is provided for producing a high strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion is carried out for 5 to 120 seconds at a temperature of 750 ~ 950 ℃.

The cold rolled steel sheet may further include one or more selected from the group consisting of Cr: 0.1 to 1.0%, Mo: 0.1% or less, Ni: 0.005 to 0.5%, Nb: 0.1% or less, and B: 0.005% or less.

According to a preferred aspect of the present invention, it is possible to provide a high strength hot dip galvanized steel sheet excellent in surface quality and plating adhesion.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

First, a high strength hot dip galvanized steel sheet excellent in surface quality and plating adhesion according to a preferred aspect of the present invention will be described.

High-strength hot-dip galvanized steel sheet having excellent surface quality and plating adhesion according to a preferred aspect of the present invention is by weight%, C: 0.05 ~ 0.3%, Mn: 1.5 ~ 20%, Si: 0.3 ~ 3.0%, Al: 0.001 ~ 1.5%, P 0.04% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.02% or less (excluding 0), Ti: (48/14) * [N]-0.1%, A cold rolled steel sheet containing remainder Fe and other unavoidable impurities, and a Fe electroplating layer of 1,000 to 3,000 mg / m ² formed on the cold rolled steel sheet, a Fe-Al alloy phase formed on the Fe electroplating layer, and a Fe-Al alloy phase formed on the Including a hot-dip galvanized layer, the Fe-Al alloy phase is formed more than 70 area% with respect to the surface area of the cold-rolled steel sheet, the surface coating area ratio (%) of the oxide formed on the Fe electroplating layer 50% or less (including 0%) )to be.

C: 0.05-0.3 wt% (hereinafter also referred to as "%")

C is preferably added at least 0.05% as an element necessary for solid solution strengthening in ferrite and austenite and securing martensite strength. However, if the content exceeds 0.3%, the ferrite and austenite strengths, martensite fractions, and strength increases are excessive, resulting in poor ductility and bending workability, and decrease in weldability due to carbon chemical equivalent increase, resulting in poor press forming and roll workability. There are disadvantages. Therefore, the C content is preferably limited to 0.05 to 0.3%.

Mn: 1.5 ~ 20%

Mn is a hardenability increasing element which suppresses ferrite formation and stabilizes austenite. In addition, it is preferable that 1.5% or more is included as an advantageous element for improving the strength of the steel sheet in order to secure the tensile strength of the steel sheet to 900 MPa or more. As Mn content increases, it is easy to secure strength and stabilize residual austenite, but when it exceeds 20%, there is an economic problem that it becomes difficult to secure plating property and increase production cost by increasing surface oxidation content of Mn during annealing. . Therefore, the Mn content is preferably limited to 1.5 to 20%.

Si: 0.3 ~ 3.0%

Si is an element that improves the yield strength of steel and stabilizes residual austenite and ferrite at room temperature. Si contributes to stabilizing a sufficient amount of retained austenite in TRIP (Triformation Induced Plasticity) steel by inhibiting the cementite precipitation from austenite upon cooling and significantly inhibiting the growth of carbides. Therefore, as in the present invention, it is essential to secure a tensile strength × elongation of 900 MPa or more and 15,000 or more. For the above effect, it is preferable that Si is included in 0.3% or more. However, in the case of exceeding 3.0%, the hot rolling load increases to cause hot cracking, and even though other alloying components and manufacturing methods satisfy the scope of the present invention, the Si concentration of the steel sheet surface is increased after annealing, resulting in inferior plating adhesion. There are disadvantages. Therefore, the Si content is preferably limited to 0.3 ~ 3.0%.

Al: 0.001-1.5%

Al is added for deoxidation in the steelmaking process and is an element added to improve the strength by solid-solution in ferrite to generate solid solution strengthening, and it is preferable to include 0.001% or more for this effect. However, in the case of exceeding 1.5%, a continuous oxide film in the form of a film is formed on the surface of the steel sheet during the annealing of the cold rolled material to reduce the Zn wettability of the base iron, and the amount of Al enriched on the surface of the steel sheet increases, resulting in poor plating adhesion. have. Therefore, the Al content is preferably limited to 0.001 to 1.5%.

P: 0.04% or less (excluding 0)

The content of P is preferably 0.04% or less (excluding 0). P is an impurity element in steel, and if its content exceeds 0.04%, the weldability is lowered, the risk of brittleness of steel is increased, and the likelihood of dent defects is increased. Therefore, the upper limit is preferably limited to 0.04%. .

S: 0.015% or less (excluding 0)

The content of S is preferably 0.015% or less (excluding 0). S, like P, is an impurity element in steel and is an element that inhibits the ductility and weldability of a steel sheet. If the content exceeds 0.015%, the possibility of inhibiting the ductility and weldability of the steel sheet is increased, so it is preferable to limit the upper limit to 0.015%.

N: 0.02% or less (excluding 0)

The content of N is preferably 0.02% or less (excluding 0). When N exceeds 0.02%, the risk of cracking during playing due to the formation of AlN greatly increases, so it is preferable to limit the upper limit to 0.02%.

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

The content of Ti is preferably (48/14) * [N] to 0.1%. Ti has an effect of reducing the concentration of N in the steel as a nitride forming element, it is necessary to add more than (48/14) * [N] in chemical equivalent. However, if it exceeds 0.1%, it is preferable to control the upper limit to 0.1% because carbon concentration and strength of martensite are reduced by additional carbide precipitation in addition to removal of solid solution N.

The cold rolled steel sheet of the present invention may further include one or more selected from the group consisting of Cr: 0.1 to 1.0%, Mo: 0.1% or less, Ni: 0.005 to 0.5%, Nb: 0.1% or less, and B: 0.005% or less. have.

Cr: 0.1-1.0%

The content of Cr is preferably 0.1 to 1.0%. Cr has an advantage of suppressing the formation of ferrite as an element of increasing hardenability, and is preferably added at least 0.1% for the above effect. However, if it exceeds 1.0%, it is preferable to control the upper limit to 1.0% due to the increase in cost due to excessive alloy input.

Mo: 0.1% or less

Mo content is preferably 0.1% or less. Mo, like Cr, is effective not only in contributing to the improvement of strength but also in securing the strength because it does not reduce molten zinc wettability. If it exceeds 0.1%, there is no problem, but it is economically undesirable.

Ni: 0.005-0.5%

The content of Ni is preferably 0.005 to 0.5%. Ni is added to improve the strength of the steel sheet, it is preferable to contain 0.005% or more for the effect. In addition, since Ni hardly concentrates on the surface during annealing, the plating property is not degraded. However, when Ni exceeds 0.5%, the pickling of the steel sheet becomes nonuniform, so the upper limit is preferably controlled to 0.5%.

Nb: 0.1% or less

The content of Nb is preferably 0.1% or less. Nb segregates in the form of carbides in the austenite grain boundary and is an element that is advantageous for increasing the strength by inhibiting coarsening of austenite grains during annealing heat treatment. However, when exceeding 0.1%, it is preferable to control the upper limit to 0.1% by the cost increase by excessive alloy input amount.

B: 0.005% or less

The content of B is preferably 0.005% or less. Steel B can be added to ensure strength. However, when the content of B exceeds 0.005%, since the annealing is concentrated on the surface of the product to greatly reduce the plating property, it is preferable to control the upper limit to 0.005%.

The remaining components of the steel sheet of the present invention are Fe, and impurities caused by injecting impurities and iron scrap in a predetermined amount due to components inevitably introduced for deoxidation and decarburization, for example, Cu, Mg, Zn, Co, Ca, Na Even if V, Ga, Ge, As, Se, In, Ag, W, Pb, Cd and the like are contained in less than 0.03%, respectively, the effect of the present invention is not impaired.

Hot-dip galvanized steel sheet of the present invention is a cold-rolled steel sheet having the alloy composition as described above and 1,000 ~ 3000mg / m ² Fe electroplating layer formed on the cold-rolled steel sheet, Fe-Al alloy phase formed on the Fe electroplating layer, and the Fe- A hot-dip galvanized layer formed on the Al alloy phase, wherein the Fe-Al alloy phase is formed more than 70 area% with respect to the surface area of the cold rolled steel sheet, the surface coating area ratio (%) of the oxide formed on the Fe electroplating layer is 50% It is below (including 0%).

The Fe-Al alloy phase serves to improve the wettability of the zinc in the molten state during hot dip galvanizing, and when the Fe-Al alloy phase is formed at 70 area% or more, good plating property may be imparted to the entire surface of the steel sheet.

In the hot-dip galvanized steel sheet of the present invention, the surface coating area ratio (%) of the oxide formed on the Fe electroplating layer is 50% or less (including 0%).

In the case of Si and Mn-added steels, in an ordinary annealing atmosphere of a reducing atmosphere, in addition to oxides composed of a combination of oxygen and one element such as SiO ₂ and MnO, MnSiO ₃ and Mn ₂ SiO ₄ and (Fe, Mn) ₂ SiO ₄ and the like It constitutes a composite oxide. When alloy elements with high oxygen affinity, such as Al and Cr, are added, these elements form a composite oxide in the form of addition to the composite oxide. The characteristic of the composite oxide composed of these elements is that the surface energy is significantly lower than liquid zinc. When the composite oxide is excessively formed on the surface because the surface energy is lower than the liquid zinc, there is a problem that the liquid zinc does not generate wettability with the surface oxide, thereby preventing plating and plating adhesion. Therefore, when the surface coating area ratio (%) of the oxide exceeds 50%, there is a fear that unplated and plating peeling may appear. The surface coating area ratio (%) of the preferred oxide is 20% or less.

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

Hot-dip galvanized steel sheet of the present invention may have a value of tensile strength of 900MPa or more and tensile strength x elongation of 15,000MPa% or more.

Hereinafter, a method of manufacturing a high strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion according to another preferred aspect of the present invention.

According to another preferred aspect of the present invention, the method for producing a high-strength hot-dip galvanized steel sheet having excellent surface quality and plating adhesion is in weight%, C: 0.05 to 0.3%, Mn: 1.5 to 20%, Si: 0.3 to 3.0%, Al: 0.001-1.5%, P 0.04% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.02% or less (excluding 0), Ti: (48/14) * [N] Preparing a cold rolled steel sheet including ˜0.1%, balance Fe and other unavoidable impurities;

Electroplating the cold rolled steel sheet to form a Fe electroplating layer of 1,000 to 3,000 mg / m ² on a single side of a steel sheet surface;

Annealing the cold rolled steel sheet on which the Fe electroplating layer is formed as described above in an annealing furnace in a reducing atmosphere having a dew point temperature of -80 to -45 ° C; And

After cooling the annealed cold rolled steel sheet, immersing it in a zinc plating bath containing 0.1 to 0.3% by weight of Al and the balance Zn and other unavoidable impurities, wherein the electroplating comprises soluble Fe electrodes. It is carried out using, the annealing is carried out for 5 to 120 seconds at a temperature of 750 ~ 950 ℃.

Steps to prepare cold rolled steel sheet

First, a cold rolled steel sheet having the alloy composition described above is prepared.

Preparation of the cold rolled steel sheet is not particularly limited, and, for example, reheating a steel slab satisfying the alloy composition of the present invention at a temperature of 1100 to 1300 ° C .; Mapping the reheated steel slab to a finish hot rolling temperature Ar ₃ or more to obtain a hot rolled steel sheet; And pickling the hot rolled steel sheet and then cold rolling to obtain a cold rolled steel sheet.

The reheating step is preferably made in a temperature range of 1100 ~ 1300 ℃. If the reheating temperature is less than 1100 ℃ may cause a problem that the hot rolling load is sharply increased to infer the hot rolling operation, if the reheating temperature is higher than 1300 ℃ may cause a problem of an increase in the reheating cost and the amount of surface scale increases. have.

It is preferable to obtain a hot rolled steel sheet by finishing rolling the reheated steel slab to a temperature of the final hot rolling temperature Ar ₃ or more. This is less than Ar3 is made of a two-phase or ferritic reverse rolling of ferrite + austenite, the composite structure is made, it is preferable to control as described above because there is a fear of malfunction due to the variation of the hot rolling load.

After obtaining the hot rolled steel sheet, it may further comprise the step of winding the hot rolled steel sheet at a temperature of 700 ℃ or less. When the winding temperature exceeds 700 ℃ it is preferable not to exceed the temperature because the oxide film on the surface of the steel sheet may be excessively generated to cause defects. As mentioned above, if the temperature is 700 degrees C or less, the minimum will not be specifically limited. However, when the temperature is less than 300 ° C., it may cause a problem of increasing the cold rolling load. Therefore, the winding temperature is more preferably 300 ° C. or more.

Thereafter, the hot rolled steel sheet is pickled and cold rolled to obtain a cold rolled steel sheet.

Fe electricity Plating layer  Formation stage

Electroplating the cold-rolled steel sheet to form a Fe electroplating layer of 1,000 ~ 3,000mg / m ² on one side of the steel sheet surface. The electroplating is carried out using a soluble Fe electrode.

As in the present invention, a large amount of Mn, Si, and Al are added to produce a steel sheet having high strength and elongation. When the steel sheet is subjected to the general annealing process, the plating property and the plating adhesion are deteriorated. When the steel sheet to which a large amount of Si, Al, and Mn is added is cold rolled and annealed in a reducing atmosphere, oxygen generated by the equilibrium reaction of oxygen and water vapor in the furnace is present, and thus the oxygen is uniform from the surface of the steel sheet to the inside. Form a concentration gradient. At this time, the oxygen concentration is higher than the critical oxygen concentration required for the alloying elements to be oxidized, and the alloying element is oxidized when the oxidation reaction is lowered and the energy of the Gibbs free energy becomes lower than that of the alloying elements independently. To produce an oxide. Energy stabilization according to the oxide formation provides a driving force for the alloy element to diffuse in the surface direction, the degree of energy stabilization is different depending on the alloy element. Mn, Si, and Al are representative oxidative elements, and the energy stabilization according to oxide formation is relatively higher than that of other elements. Therefore, it rapidly diffuses to the surface during reduction annealing to form a depleted layer in the region within 0.1 μm of the steel plate surface and forms a large amount of oxide on the surface. At this time, the surface of the reduced-annealed steel sheet is mostly covered with oxide, and when the steel sheet is immersed in the galvanizing bath, the physical contact between the plating bath and the base iron is blocked so that the wettability of the zinc is greatly reduced, so that the zinc is not attached. In addition to this, even when plating, the Fe-Al alloy phase is not formed at the interface between the steel sheet and the galvanized layer, and thus the adhesion between the galvanized layer and the base iron is reduced, causing plating peeling.

In order to solve this problem, in the present invention, the electroplating using a soluble Fe electrode for the cold rolled steel sheet before the annealing of the cold rolled steel sheet to the surface of the cold-rolled steel sheet of 1,000 ~ 3,000mg / m ² Fe plating layer on both sides Form.

As described above, when the Fe plated layer is formed before annealing, the amount of surface oxides annealed according to the amount of Fe plating by acting as a shielding film to diffuse to the surface when the oxides are formed by the steel components Si, Mn, and Al are concentrated to the surface during annealing. It is possible to secure the plating property and plating adhesion of the non-plated high strength steel sheet by securing the wettability of the molten zinc during the introduction of the molten plating bath of the steel sheet after the annealing.

When the Fe adhesion amount is less than 1,000 mg / m ² during the Fe electroplating, it is not possible to sufficiently block diffusion of the oxidative alloy component during the annealing process, and when the Fe content exceeds 3,000 mg / m ² , oxide formation may be suppressed, but the electroplating operation process As the cost rises and economic problems occur, the amount of Fe deposited is preferably 1,000 to 3,000 mg / m ^{2 on a} single side.

The form of the soluble Fe anode during the Fe electroplating is not limited, and the component is preferably a steel grade having a high Fe content and a low content of other alloy components. If the soluble Fe electrode contains a large amount of alloying components, when the voltage is turned over during the electroplating operation, the alloying components are also dissolved in the process of Fe being ionized and dissolved in the electrolyte solution. It forms an oxide and increases the alloying component in the electrolyte solution, which reduces the electrolyte solution life. When using soluble and insoluble electrodes such as the electrode instead of iridium oxide as Fe oxide as Fe ^{+ 3} ions in the water within the Fe ^{+ 2} ions during the electroplating operation proceeds to form an Fe (OH) _3. Fe (OH) ₃ is a solid precipitate, which is not reduced in solution, thereby reducing the life of the solution, consuming Fe ^{+ 2} ions, decreasing electroplating efficiency, and thus decreasing continuous operation. Therefore, it is preferable to use a soluble Fe electrode.

The electrolytic solution during the electroplating is not particularly limited. For example, the electrolyte solution is composed of FeSO ₄ 7H ₂ O, Na ₂ SO ₄ and citric anhydride, and an acid such as sulfuric acid or hydrochloric acid is added to adjust the pH. FeSO ₄ 7H ₂ O serves to supply Fe ^{+ 2} ions in electrolyte solution together with soluble Fe electrode, and Na ₂ SO ₄ is a conduction aid to improve the electroplating efficiency by improving the electrical conductivity in electrolyte solution. .

The anhydrous citric acid is complexed to the ion Fe ^{+ 2} Fe ^{+ 3} serves to suppress the ions oxidize and when added in excess to suppress the reduction of the Fe ^{+ 2} ion suppresses the electrical plating efficiency turned to less than 1.5g / L It is desirable to.

When the pH of the electrolyte solution is high, the electroplating efficiency is lowered due to the decrease in the concentration of hydrogen ions in the solution, and the steel plate and the Fe plating are dissolved by the electrolyte solution, which lowers the electroplating efficiency and makes it difficult to maintain the solution pH. It is desirable to keep it in the 2.5 range.

Annealing Step

As described above, the cold rolled steel sheet on which the Fe electroplating layer is formed is annealed in an annealing furnace in a reducing atmosphere having a dew point temperature controlled at -80 to -45 ° C. At this time, the annealing is carried out for 5 to 120 seconds at a temperature of 750 ~ 950 ℃.

When the dew point temperature is higher than -45 ° C, the concentration of oxygen is higher than the critical oxygen concentration required to form oxides of the steel components Mn and Si, and Mn and Si are concentrated on the surface of the steel sheet during heating in the annealing furnace to form oxides, thereby inferring plating properties. There is a drawback to this. On the other hand, since it is technically difficult to control the dew point temperature lower than -80 ° C, in the present invention, it is preferable to control the dew point temperature range in the range of -45 ~ -80 ° C.

On the other hand, the annealing is preferably made for 5 to 120 seconds at a temperature of 750 ~ 950 ℃. In order to secure sufficient recrystallization structure of the steel sheet, it is preferable to control the annealing temperature to 750 ° C. or higher, but when the temperature exceeds 950 ° C., the life of the annealing furnace is reduced. Therefore, the annealing temperature is controlled to 750 ° C. to 950 ° C. It is desirable to. In addition, the annealing time is required for at least 5 seconds to obtain a uniform recrystallized structure, if the time exceeds 120 seconds crack heating time is excessive to give sufficient time for the steel component to diffuse to the surface and react with oxygen to give a steel sheet surface An oxide is formed in the molten zinc plating bath so that the Fe-Al alloy phase cannot be sufficiently formed by the chemical reaction between Al and Fe in the hot dip galvanizing bath. There is a disadvantage of being inferior.

The annealing atmosphere may be an atmosphere containing 2 to 10% of H ₂ and the remaining nitrogen (N ₂ ).

Hot dip galvanizing step

After cooling the annealed cold-rolled steel sheet, it is immersed in a zinc plating bath and hot-dip galvanized. The zinc plating bath contains 0.1 to 0.3 wt% Al and the balance Zn and other unavoidable impurities.

The hot dip galvanizing process is not particularly limited. For example, 0.1 to 0.3% by weight of Al after reheating or recooling the cooled cold rolled steel sheet at a plating bath temperature of -20 ° C to a plating bath temperature of + 100 ° C. It is preferable that it consists of immersion in the plating bath of 450-500 degreeC containing an excess of remainder Zn and other unavoidable impurities.

When the reheating or recooling temperature, that is, the plating bath inlet temperature is lower than the plating bath temperature -20 ° C, the wettability of zinc is lowered, and when the plating bath temperature exceeds + 100 ° C, the plating bath temperature is locally increased. There is a disadvantage that the temperature management is difficult. In addition, when the Al content in the plating bath is less than 0.1% by weight, there is a disadvantage in that the formation of Fe-Al alloy phase formed at the interface between the base iron and the plating layer is suppressed. When the content of Al exceeds 0.3% by weight, the Al content in the plating layer is increased. There is a problem of deteriorating the weldability. In addition, when the plating bath temperature is less than 450 ° C, the viscosity of the zinc is increased to deteriorate the driveability of the roll in the plating bath, it is not preferable because the evaporation of zinc increases when it exceeds 500 ° C.

According to a method of manufacturing a high strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion according to another preferred aspect of the present invention, the Fe electroplating layer of 1,000 ~ 3,000mg / m ² formed on the cold-rolled steel sheet, Fe formed on the Fe electroplating layer An Al alloy phase and a hot dip galvanized layer formed on the Fe-Al alloy phase, wherein the Fe-Al alloy phase is formed in an area of at least 70 area% with respect to the surface area of the cold rolled steel sheet, High strength hot dip galvanized steel sheet having excellent surface quality and plating adhesion of 50% or less (including 0%) can be produced.

The hot-dip galvanized steel sheet may have a value of tensile strength of 900 MPa or more and tensile strength × elongation of 15,000 MPa% or more.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following examples.

(Example)

After melting the steel having the alloy composition of Table 1, a slab was prepared. After using the prepared steel slab was maintained for 1 hour at a temperature of 1200 ℃, finish rolling at 900 ℃ and then cooled to 650 ℃ to maintain for 1 hour in a thermal furnace maintained at 650 ℃ and then subjected to cooling. After cooling, the hot rolled steel sheet was visually observed for hot rolled cracks and pickled for 30 seconds with 60 and 17% by volume of HCl solution to dissolve iron oxide on the surface of the steel sheet. In some specimens, when pickling for 30 seconds was insufficient, additional pickling was performed for 20 seconds under the same conditions. Cold rolling was performed at a 55% rolling rate of the hot-rolled steel sheet.

Steel grade Chemical composition (% by weight) C Mn Si Al P S N Cr Mo Ti Ni Nb One 0.215 2.5 1.71 0.024 0.025 0.01 0.018 0.45 - 0.07 0.45 - 2 0.185 3.4 1.54 1.74 0.018 0.009 0.015 0.12 0.08 0.05 0.12 0.08 3 0.154 23.2 3.5 0.024 0.025 0.008 0.014 0.11 - 0.05 0.04 0.07 4 0.127 8.1 2.54 0.021 0.018 0.007 0.019 0.15 0.08 0.07 0.024 0.05 5 0.214 18.2 1.46 5.74 0.009 0.005 0.015 0.15 - 0.05 0.3 - 6 0.245 19.2 1.1 0.013 0.014 0.004 0.014 0.23 0.045 0.05 0.025 0.08 7 0.175 24.1 1.24 0.023 0.021 0.012 0.012 0.34 0.07 0.04 0.12 - 8 0.145 2.5 4.05 3.21 0.032 0.011 0.018 0.12 0.065 0.06 0.063 0.07 9 0.151 2.3 2.02 0.021 0.021 0.008 0.014 0.45 - 0.05 0.041 - 10 0.123 21.5 1.24 5.42 0.018 0.013 0.011 0.5 0.056 0.04 0.045 - 11 0.245 2.7 1.23 0.032 0.024 0.012 0.008 0.42 0.024 0.03 - 0.045 12 0.265 2.5 1.45 1.13 0.021 0.011 0.012 - - 0.04 - -

The cold rolled steel sheet thus obtained was subjected to pre-treatment to remove foreign matters on the surface, and to perform Fe electroplating and annealing under the conditions shown in Table 2 below. After the plating was performed under the plating condition of the weight%, the hot-dip galvanized steel sheet was prepared by adjusting and cooling it to 60g / m ² of one-side standard coating deposition amount using an air knife.

Example
No. Steel grade Fe electroplating condition Soft loss annealing conditions electrode
type precipitate
Formation Fe adhesion amount
(mg / m ² ) Annealing Temperature
(℃) Annealing time
(s) Atmosphere gas Dew point temperature
(℃) Inventive Example 1 One Availability Not Occurred 1,200 874 121 5H ₂ -N ₂ -57 Comparative Example 1 One Availability Not Occurred 1,200 745 120 5H ₂ -N ₂ -54 Comparative Example 2 One Availability Not Occurred 200 865 118 5H ₂ -N ₂ -54 Comparative Example 3 2 Availability Not Occurred 2,500 876 107 7H ₂ -N ₂ -47 Comparative Example 4 2 Availability Not Occurred 3,000 842 105 7H ₂ -N ₂ -47 Comparative Example 5 3 Availability Not Occurred 2,500 817 114 7H ₂ -N ₂ -50 Comparative Example 6 4 Availability Not Occurred 1,200 987 162 5H ₂ -N ₂ -51 Inventive Example 2 4 Availability Not Occurred 1,000 845 120 7H ₂ -N ₂ -48 Comparative Example 7 5 Availability Not Occurred 1,800 834 117 7H ₂ -N ₂ -54 Comparative Example 8 6 Availability Not Occurred 2,000 842 113 5H ₂ -N ₂ -15 Comparative Example 9 6 Availability Not Occurred 300 862 110 5H ₂ -N ₂ -51 Comparative Example 10 7 Availability Not Occurred 2,800 874 117 5H ₂ -N ₂ -48 Comparative Example 11 8 Availability Not Occurred 2,500 857 121 7H ₂ -N ₂ -47 Comparative Example 12 9 Availability Not Occurred 1,200 845 113 1H ₂ -N ₂ -57 Comparative Example 13 10 Availability Not Occurred 2,700 856 118 5H ₂ -N ₂ -47 Comparative Example 14 11 Availability Not Occurred 1,100 842 115 1H ₂ -N ₂ -51 Inventive Example 3 11 Availability Not Occurred 1,500 847 117 5H ₂ -N ₂ -57 Inventive Example 4 12 Availability Not Occurred 2,800 856 123 7H ₂ -N ₂ -47 Comparative Example 15 One Insoluble Occur Unformed 812 115 4H ₂ -N ₂ -47 Comparative Example 16 6 Insoluble Occur Unformed 811 127 7H ₂ -N ₂ -51 Comparative Example 17 11 Insoluble Occur Unformed 812 117 5H ₂ -N ₂ -57

The presence of the unplated portion and the degree (surface quality) of the surface of the obtained plated steel sheet were visually confirmed, and the surface quality thereof was evaluated and shown in Table 3 below. In Table 3, ○ represents a case where there is no unplated portion, Δ represents a case where an unplated size of 2 mm or less in diameter is present, and × denotes a case where an unplated size of more than 2 mm in diameter exists.

In addition, after dissolving and removing the hot-dip galvanized layer, the surface was photographed by SEM (Scanning Electron Microscope), and image analysis software was used to measure the area% of the Fe-Al alloy phase formed at the interface between the hot-dip galvanized layer and the base steel sheet. It is shown in Table 3 below.

In addition, the plating adhesion to each steel sheet was evaluated, and the results are shown in Table 3. The plating adhesion evaluation was performed by applying the automotive structural adhesive on the surface of the steel sheet, dried to complete the solidification and then bent at 90 degrees to separate the adhesive and the plated steel sheet, and to check whether the plating layer is peeled off and adhered to the adhesive plating Adhesiveness was evaluated and the result is shown in Table 3 by (circle) and x. In Table 3, (circle) shows the case where a plating layer peels and an adhesive does not adhere, and plating adhesion is favorable, and x represents a case where a plating layer peels and an adhesive sticks and poor plating adhesiveness.

Tensile tests were performed on the coated steel sheet in JIS 5 to measure the tensile strength and elongation of the steel sheet. The tensile strength and tensile strength (MPa) × elongation (%) were converted into the form, and the values are shown in Table 3 below. .

Example
No. Steel grade The tensile strength
(MPa) The tensile strength
Elongation (MPa%) Fe-Al
Alloy phase fraction (area%) Oxide surface
Area ratio (%) Surface quality Plating adhesion Inventive Example 1 One 1,192 17,880 89 9 ○ ○ Comparative Example 1 One 840 13,421 90 7 ○ ○ Comparative Example 2 One 1,182 17,254 21 61 × × Comparative Example 3 2 1,254 21,354 17 70 △ × Comparative Example 4 2 1,245 22,534 12 69 × × Comparative Example 5 3 1,452 5,621 9 72 △ × Comparative Example 6 4 1,184 28,421 7 71 × × Inventive Example 2 4 1,184 28,416 94 7 ○ ○ Comparative Example 7 5 1,321 5,123 11 81 × × Comparative Example 8 6 1,354 41,974 35 74 × × Comparative Example 9 6 1,324 56,321 21 78 × × Comparative Example 10 7 1,402 54,211 20 69 × × Comparative Example 11 8 1,241 21,097 12 83 × × Comparative Example 12 9 1,245 19,920 34 81 × × Comparative Example 13 10 1,394 52,142 27 69 ○ × Comparative Example 14 11 981 21,450 51 63 × × Inventive Example 3 11 1,002 20,895 91 12 ○ ○ Inventive Example 4 12 992 20,185 87 13 ○ ○ Comparative Example 15 One 1,092 17,745 5 84 × × Comparative Example 16 6 1,321 41,424 7 90 × × Comparative Example 17 11 962 20,830 3 91 × ×

As can be seen from Tables 1 to 3, Inventive Examples 1, 4, and 11 to 14 satisfying the alloy composition and manufacturing conditions proposed by the present invention, tensile strength is 900 MPa or more, and tensile strength x elongation is 15,000. It can be seen that excellent mechanical properties are secured as MPa% or more.

In addition, the Fe-Al alloy phase is formed in the interface between the cold-rolled steel sheet and the hot-dip galvanized layer is not less than 70 area% not only has excellent surface quality without the occurrence of unplated, it can also be seen that the plating adhesion is also excellent.

In addition, since the oxide surface surface area ratio (%) of 50% or less, it can be seen that the plating surface quality and plating adhesion is excellent.

On the other hand, Comparative Example 1 is a case where the alloy component proposed by the present invention is satisfied, but the annealing temperature is low, the tensile strength was not secured and the material was inferior because the tensile strength x elongation was also insufficient.

In the case of Comparative Example 6, the alloy composition and Fe electroplating deposition amount suggested by the present invention were satisfied, but the amount of the alloy component and the surface oxide diffused on the surface through the Fe plating during the annealing process exceeded the annealing temperature. Excessive Fe-Al alloy phase fraction was inferior to the unplated, the surface quality was inferior, plating peeling occurred.

In the case of Comparative Example 8, the alloy composition proposed by the present invention and the Fe electrical conductivity were also satisfied, but the dew point in the furnace exceeded the proposed range during the annealing process. When the dew point in the furnace rises, the amount of oxygen formed through the steam-oxygen equilibrium reaction increases, thereby increasing the oxygen concentration in the furnace. When the oxygen concentration is increased, excessive surface oxides are formed, so that the Fe-Al alloy phase fraction is inferior, unplating occurs, and plating peeling occurs.

In Comparative Examples 12 and 14, the alloy composition and Fe electroplating adhesion amount proposed by the present invention were satisfied, but the reducing atmosphere was not formed because the hydrogen partial pressure of the gas was low during the annealing process. When annealing proceeds in the absence of a reducing atmosphere, the surface oxide is excessively formed, so that the Fe-Al alloy phase fraction is inferior, unplating occurs, and plating peeling occurs.

In Comparative Examples 2 and 9, the alloy composition and annealing conditions proposed by the present invention were satisfied, but the Fe electroplating deposition amount was less than the suggested range, and oxidative alloy components such as Mn, Si, and Al were diffused during the annealing process. Excessive oxide was formed on the surface through the plating layer, so that sufficient Fe-Al alloy phase could not be formed on the surface during hot dip plating. Plating peeling occurred and plating adhesion was inferior.

In Comparative Examples 3, 4, 5, 7, 10, 11, and 13, all of the Fe electroplating deposition amount and annealing conditions of the present invention were satisfied, but the range of the content of Mn, Si, and Al in the alloying components was suggested. Exceeded. If the content of the alloying components exceeds the proposed range, the Fe plating layer is formed to a sufficient thickness during annealing, and then the alloying components diffuse into the interface between the Fe plating layer and the surface of the steel sheet to form an oxide while forming a band along the interface. . In this case, when the amount of oxide formed on the surface of the Fe plated layer is small, the level of plating property to which a degree of unplating can be secured, but the interfacial oxide between the Fe plated and the surface of the steel sheet is brittle and the steel sheet is bent during processing such as molding. Peeling of the plating layer may occur in the process of losing, and even in this embodiment, plating peeling occurred, thereby inferring plating adhesion.

On the other hand, in Comparative Examples 15, 16 and 17, although the alloy component proposed by the present invention was satisfied, an insoluble electrode was used for Fe electroplating. In case of electroplating using insoluble electrode, Fe ^{+ 3} series sediment was generated excessively in the process of forming Fe electroplating, so continuous electroplating was not possible and Fe electroplating was not formed. Since Fe electroplating was not formed, even when plating was performed under the annealing and plating conditions presented in the present invention, excessive surface oxides were formed during annealing, so that the Fe-Al alloy phase was not sufficiently formed during the annealing process, and inferior plating and plating adhesion. .

Claims (7)

By weight%, C: 0.05-0.3%, Mn: 1.5-20%, Si: 0.3-3.0%, Al: 0.001-1.5%, P 0.04% or less (excluding 0), S: 0.015% or less (0 is N: 0.02% or less (excluding 0), Ti: (48/14) * [N] ~ 0.1%, cold rolled steel sheet containing residual Fe and other unavoidable impurities and 1,000 to 3,000 mg formed on this cold rolled steel sheet / m ² Fe electroplating layer, a Fe-Al alloy phase formed on the Fe electroplating layer, and a hot dip galvanized layer formed on the Fe-Al alloy phase, the Fe-Al alloy phase is 70 to the surface area of the cold rolled steel sheet A high strength hot-dip galvanized steel sheet having an excellent surface quality and plating adhesion, having an area% or more and a surface coating area ratio (%) of an oxide formed on the Fe electroplating layer of 50% or less (including 0%).
The method of claim 1, wherein the cold-rolled steel sheet is at least one selected from the group consisting of 0.1: 1.0% Cr: 0.1% or less Mo, Ni: 0.005% ~ 0.5%, Nb: 0.1% or less and B: 0.005% or less High-strength hot-dip galvanized steel sheet further comprising excellent surface quality and plating adhesion.
The hot-dip galvanized steel sheet according to claim 1, wherein the hot-dip galvanized steel sheet has a tensile strength of 900 MPa or more and a tensile strength x elongation value of 15,000 MPa% or more.
By weight%, C: 0.05-0.3%, Mn: 1.5-20%, Si: 0.3-3.0%, Al: 0.001-1.5%, P 0.04% or less (excluding 0), S: 0.015% or less (0 is N): 0.02% or less (excluding 0), Ti: (48/14) * [N] -0.1%, preparing a cold rolled steel sheet containing remainder Fe and other unavoidable impurities;
Electroplating the cold rolled steel sheet to form a Fe electroplating layer of 1,000 to 3,000 mg / m ² on a single side of a steel sheet surface;
Annealing the cold rolled steel sheet on which the Fe electroplating layer is formed as described above in an annealing furnace in a reducing atmosphere having a dew point temperature of -80 to -45 ° C; And
After cooling the annealed cold rolled steel sheet, immersing it in a zinc plating bath containing 0.1 to 0.3% by weight of Al and the balance Zn and other unavoidable impurities, wherein the electroplating comprises soluble Fe electrodes. It is carried out by using, the annealing is carried out for 5 to 120 seconds at a temperature of 750 ~ 950 ℃ high quality hot dip galvanized steel sheet excellent production method.
The method of claim 4, wherein the cold-rolled steel sheet is at least one selected from the group consisting of Cr: 0.1 to 1.0%, Mo: 0.1% or less, Ni: 0.005 to 0.5%, Nb: 0.1% or less, and B: 0.005% or less. Method for producing a high-strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion, characterized in that it further comprises.
The method of claim 4, wherein the preparing of the cold rolled steel sheet comprises:
Reheating the steel slab at a temperature of 1100-1300 ° C .;
Mapping the reheated steel slab to a finish hot rolling temperature Ar ₃ or more to obtain a hot rolled steel sheet; And
After pickling the hot rolled steel sheet and cold rolled to obtain a cold rolled steel sheet manufacturing method of high strength hot-dip galvanized steel sheet excellent in surface quality and plating adhesion.
5. The method of claim 4, wherein the hot dip galvanizing is performed by reheating the cooled cold rolled steel sheet at a plating bath temperature of -20 ° C. to a plating bath temperature of + 100 ° C., or recooling to a zinc plating bath having a temperature of 450 ° C. to 500 ° C. 6. A method for producing a high strength hot dip galvanized steel sheet excellent in surface quality and plating adhesion by dipping.