KR20210000029A - Hot-dip galvanized steel sheets having good plating quality and method of manufacturing thereof - Google Patents

Abstract

The present invention provides a hot-dip galvanized steel sheet and a method for manufacturing the same. According to the present invention, the hot-dip galvanized steel sheet comprises: a plated steel sheet containing 0.15 wt% or less of C, 1.5 wt% or less of Mn (except for 0%), 0.5 or less of Si (except for 0%), 0.01 wt% or less of B (except for 0%), the balance Fe, and unavoidable impurities with respect to 100 wt% of the total; and a hot-dip galvanized layer provided on the plated steel sheet. An internal oxide composed of one or two or more of Mn, Si, and B is formed within the crystal grains within 3 μm in depth from the surface of the plated steel sheet, wherein the internal oxide had a spherical shape with a diameter of 0.5 μm or less.

Description

Hot-dip galvanized steel sheet with excellent plating quality and its manufacturing method {HOT-DIP GALVANIZED STEEL SHEETS HAVING GOOD PLATING QUALITY AND METHOD OF MANUFACTURING THEREOF}

The present invention relates to a hot-dip galvanized steel sheet having excellent plating quality and a method of manufacturing the same.

In recent years, in the automotive industry, a high-strength steel plate has been applied as a steel material for automobiles, thereby improving safety and reducing thickness by reducing weight. Here, in order to apply as a material for a high-strength automobile outer plate, it is possible to use the bake hardening property or use solid solution strengthening.

The bake hardening phenomenon is a phenomenon in which the activated solid solution carbon and nitrogen are adhered to the dislocation generated during the press, and the yield strength increases. Steel with excellent bake hardening is easy to form before painting and baking. As it has characteristics of improving dent resistance, it is very ideal as a material for exterior panels of automobiles.

In addition, solid solution strengthening by adding alloy components is also used. In this case, it is common to add alloy elements such as Mn, Si, and B in order to secure the high strength of the steel sheet used for the exterior of the vehicle.

The hot-dip galvanizing property of steel sheets for automobile outer plates is determined according to the surface condition of the annealed steel sheet just before plating is carried out.It is caused by elements such as Mn, Si, and B added to secure the properties of the steel sheet. Plating properties deteriorate due to the formation of surface oxides during annealing. In particular, even with a very small amount of B, it is difficult to secure the adhesion of the hot-dip galvanized layer. That is, in the annealing process, by reacting with a trace amount of oxygen or water vapor present in the annealing furnace to form a single or complex oxide of the above elements on the surface of the steel sheet, the reactivity of the surface decreases.

The annealed surface, which is less reactive, interferes with the wettability of the hot-dip galvanizing bath, causing unplated plating in which the plating metal does not adhere locally or entirely on the surface of the plated steel sheet.In addition, alloying necessary to secure the adhesion of the plating layer during the hot-dip plating process due to these oxides Insufficient formation of the suppression layer (Fe ₂ Al ₅ ) causes the plating layer to be peeled off.

Various techniques have been proposed to improve the plating quality of hot-dip galvanized steel sheets. Patent Document 1 controls the air-fuel ratio of air and fuel to 0.80 to 0.95 in the annealing process, and oxidizes the steel sheet in a direct flame furnace in an oxidizing atmosphere, so that Si, Mn, or Al alone or A technology for providing hot-dip galvanized or alloyed hot-dip galvanized steel sheets with excellent plating quality by forming iron oxides including composite oxides and then performing hot-dip galvanizing after reducing iron oxides in a reducing atmosphere is proposed.

When the oxidation and reduction method is used in the annealing process as in Patent Document 1, components having a high affinity for oxygen such as Si, Mn, and Al are internally oxidized from the surface layer of the steel sheet to a certain depth and diffusion to the surface layer is suppressed. Si, Mn, or Al alone or composite oxides are reduced, so that the wettability with zinc is improved and unplated can be reduced. However, in the case of Si-added steel, during the reduction process, Si is concentrated directly under the iron oxide to form a band-shaped Si oxide, which is why it is separated from the surface layer including the plating layer, that is, between the reduced iron and the underlying iron. There is a problem in that it is difficult to secure adhesion of the plating layer due to peeling at the interface.

On the other hand, as another method for improving the plating properties of hot-dip galvanized steel sheets, Patent Document 2 states that alloy components such as Mn, Si, and Al, which are easily oxidized by maintaining high dew point in the annealing furnace, are internally oxidized. A method of improving plating properties by reducing oxides externally oxidized on the surface of a steel sheet after annealing has been proposed. However, the method according to Patent Document 2 can solve the problem of plating properties due to external oxidation of Si, which is easy to internally oxidize, but when a large amount of Mn, which is relatively difficult to internally oxidize, is added, the effect is insignificant. .

As another conventional technique, there is a method of suppressing diffusion of alloying elements to the surface during annealing by performing Ni pre-annealing. However, although this method is also effective in inhibiting the diffusion of Mn, there is a problem that the diffusion of Si cannot be sufficiently inhibited.

Korean Patent Publication No. 2010-0030627 Korean Patent Publication No. 2009-0006881

An object of the present invention is to provide a hot-dip galvanized steel sheet having excellent plating quality in which non-plating does not occur and the plating layer is not peeled off, and a method of manufacturing the same.

The subject of the present invention is not limited to the above description. Those of ordinary skill in the art to which the present invention pertains will not have any difficulty in understanding the additional subject of the present invention from the general details of the present specification.

One aspect of the present invention is by weight %, carbon (C): 0.15% or less (excluding 0%), manganese (Mn): less than 1.5% (excluding 0%), silicon (Si): 0.5% or less (excluding 0%) ), boron (B): 0.01% or less (excluding 0%), the balance of Fe and the holding steel sheet containing inevitable impurities; And a hot-dip galvanizing layer provided on the holding steel sheet, wherein an internal oxide consisting of one or two or more of Mn, Si, and B is contained within the crystal grains within 3 μm depth in the thickness direction from the surface of the holding steel sheet. It is formed, and the internal oxide is a hot-dip galvanized steel sheet having a spherical shape of 0.5 μm or less in diameter.

The hot-dip galvanizing layer may be a hot-dip galvanizing layer alloyed with Fe of the holding steel sheet.

In the base steel sheet before the hot-dip galvanization layer formation, the base steel sheet before the hot-dip galvanization layer formation is an annealed cold-rolled steel sheet, and a value obtained by integrating B content from the surface of the annealed cold-rolled steel sheet to a depth of 0.03 μm in the thickness direction is 0.005. It can be below.

Another aspect of the present invention is by weight%, carbon (C): 0.15% or less (excluding 0%), manganese (Mn): less than 1.5% (excluding 0%), silicon (Si): 0.5% or less (0% Except), boron (B): preparing a cold-rolled steel sheet containing 0.01% or less (excluding 0%), the balance Fe and inevitable impurities; Forming an Fe coating layer containing 2 to 30% by weight of oxygen on the surface of the cold rolled steel sheet to a thickness of 0.02 to 1 μm to obtain an Fe-coated cold rolled steel sheet; The Fe-coated cold rolled steel sheet has a dew point temperature of -60°C to 10°C, and 1 to 70% of H ₂ and Annealing by maintaining for 5 to 120 seconds in a temperature range of 600 to 950°C based on the temperature of the steel sheet in an annealing furnace in an atmosphere consisting of the remaining N ₂ ; Cooling the annealed cold-rolled steel sheet; And hot-dip galvanizing the cooled cold-rolled steel sheet.

The step of preparing the cold-rolled steel sheet, in weight%, carbon (C): 0.15% or less (excluding 0%), manganese (Mn): less than 1.5% (excluding 0%), silicon (Si): 0.5% or less ( 0% excluded), boron (B): 0.01% or less (excluding 0%), preparing a steel slab containing the balance Fe and inevitable impurities; Reheating the steel slab at 1160 to 1250°C; Obtaining a hot-rolled steel sheet by hot rolling the reheated steel slab at 850 to 1150°C; Cooling the hot-rolled steel sheet to a cooling end temperature of 500 to 750°C at an average cooling rate of 10 to 70°C/sec, and winding in a temperature range of 500 to 750°C; And pickling and cold rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet.

The cooling of the annealed cold-rolled steel sheet may be performed at an average cooling rate of 1 to 50°C/sec to a cooling end temperature of 250 to 550°C.

The step of cooling the annealed cold-rolled steel sheet is divided into primary cooling and secondary cooling, and the cooling end temperature of the primary cooling is 600 to 700°C, and the cooling end temperature of the secondary cooling is 250 to 550°C. And, the average cooling rate in the secondary cooling may be greater than the average cooling rate in the primary cooling.

The step of hot-dip galvanizing the cooled cold-rolled steel sheet is, in weight%, Al: 0.1 to 0.3%, the remaining Zn and inevitable impurities, and can be plated with a zinc plating bath maintained in a temperature range of 440 to 500°C. have.

It may further include an alloying heat treatment step of alloying the hot-dip galvanized layer formed by the hot-dip galvanizing.

According to the present invention, it is possible to provide a hot-dip galvanized steel sheet having excellent adhesion to a plating layer without causing unplated by suppressing surface oxidation of alloy elements by annealing on the surface of a base steel sheet before plating.

In particular, it is possible to control the concentration of B low at the interface between the holding steel sheet and the hot-dip galvanizing layer, thereby effectively suppressing the peeling of the plating layer.

The various and beneficial advantages and effects of the present invention are not limited to the above description, and will be more easily understood in the course of describing specific embodiments of the present invention.

1 is a conceptual diagram of a conventional cold-rolled steel sheet used as a holding steel sheet of a hot-dip galvanized steel sheet, (a) is a schematic cross-sectional view of a cold-rolled steel sheet before annealing, and (b) is a cold-rolled steel sheet with surface oxide formed after annealing. It is a schematic cross-sectional view.
2 is a conceptual diagram of a cold-rolled steel sheet used as a holding steel sheet for a hot-dip galvanized steel sheet according to an embodiment of the present invention, (a) is a schematic cross-sectional view showing that an Fe coating layer is formed on the surface of the cold-rolled steel sheet before annealing. , (b) is a schematic cross-sectional view showing the formation of internal oxides and surface oxides after annealing.
3 is a schematic cross-sectional view of a hot-dip galvanized steel sheet according to the present invention.

The terminology used herein is for reference only to specific embodiments and is not intended to limit the invention. Singular forms as used herein also include plural forms unless the phrases clearly indicate the opposite.

As used herein, the meaning of "comprising" specifies a specific characteristic, region, integer, step, action, element and/or component, and other specific characteristic, region, integer, step, action, element, component and/or It does not exclude the presence or addition of the military.

Although not defined differently, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Terms defined in a commonly used dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal meaning unless defined.

The meaning of “hot-dip galvanized steel sheet” as used herein refers to all steel sheets formed with a hot-dip galvanized layer made of zinc as a main component by the hot-dip galvanizing method, and alloying as well as hot-dip galvanized steel sheet (GI) in the general sense It is used in a comprehensive sense to include hot-dip galvanized steel sheet (GA).

The inventors of the present invention believe that the peeling of the plating layer in the hot-dip galvanized steel sheet is caused by reduced surface reactivity due to oxides of Mn, Si, and B (hereinafter referred to simply as "plating barrier elements") formed at the interface between the holding steel sheet and the plating layer. It was found that the main cause is that the bonding of the plating layer is not smooth.

Accordingly, the present inventors discovered that the adhesion of the plating layer can be improved by reducing the total amount of oxides of the plating interfering elements between the holding steel sheet and the plating layer by preventing plating-blocking elements from diffusing to the surface of the holding steel sheet to form oxides on the surface. And completed the present invention.

Hereinafter, a hot-dip galvanized steel sheet having excellent plating quality according to an aspect of the present invention completed through the research of the present inventors will be described in detail. In the present invention, when expressing the content of each element, it is necessary to note that it means weight% unless otherwise specified. In addition, the ratio of crystals or tissues is based on area unless otherwise indicated. In addition, the content of gas is based on volume unless otherwise indicated.

The hot-dip galvanized steel sheet having excellent plating quality according to an aspect of the present invention is weight %, carbon (C): 0.15% or less (excluding 0%), manganese (Mn): less than 1.5% (excluding 0%), silicon ( Si): 0.5% or less (excluding 0%), boron (B): 0.01% or less (excluding 0%), a holding steel sheet containing the balance iron and inevitable impurities, and a hot-dip galvanized layer provided on the holding steel sheet .

First, the reason why the component system of the holding steel sheet was limited as above will be briefly explained.

Carbon (C): 0.15% or less (excluding 0%)

Carbon (C) is an interstitial solid solution element, so it is dissolved in the steel sheet during cold rolling and annealing and interacts with the dislocation formed by temper rolling, thereby exerting the bake hardening ability. Therefore, the higher the C content, the better the bake hardening ability. Is improved. However, if too much solid-solution carbon is present in the material, it can lead to poor aging, which causes a defect called orange peel on the surface during part molding.

In the present invention, when the C content exceeds 0.15%, it is disadvantageous in terms of moldability, and room temperature aging resistance is significantly inferior, so there may be a limit to the application of parts. Meanwhile, the lower limit of the C content may not be particularly limited, but 0% may be excluded because it is difficult to completely exclude the C content in the manufacturing process. The C content may be 0.15% or less (excluding 0%), and more preferably less than 0.05% (excluding 0%).

Manganese (Mn): less than 1.5% (excluding 0%)

Manganese (Mn) is a solid solution strengthening element that not only contributes to the strength increase, but also serves to precipitate S in the steel as MnS. However, when the content is 1.5% or more, even if the yield strength is increased, there is a problem that the drawability is deteriorated due to excessive solid solution of Mn, and thus the content of Mn may be limited to less than 1.5%.

Silicon (Si): 0.5% or less (excluding 0%)

Silicon (Si) contributes to an increase in the strength of the steel sheet by solid solution strengthening, but it is not intentionally added in the present invention, and even if Si is not added, there is no significant problem in terms of securing physical properties. However, 0% is excluded in consideration of the amount added unavoidably for manufacturing. On the other hand, when the silicon content exceeds 0.5%, a problem of inferior plating surface properties may occur. Therefore, the Si content may be 0.5% or less (excluding 0%), and more preferably less than 0.1% (excluding 0%).

Boron (B): 0.01% or less (excluding 0%)

Boron (B) is necessary for securing room temperature aging resistance to exhibit bake hardening properties. If B is segregated at the grain boundary during annealing and stabilized at room temperature, most of B remains at the grain boundary at low aging evaluation temperatures (about 100°C) and diffusion into the grain is suppressed, thereby securing room temperature aging resistance. In addition, it has the characteristics showing excellent solid solution strengthening performance.

As such, B has positive aspects such as solid solution reinforcement and aging resistance, but if it is excessively contained, it adversely affects the plating property and may cause grain boundary brittleness due to excessive segregation at the grain boundary. In particular, B makes it difficult to secure the adhesion of the hot-dip galvanized layer even in a very small amount. Therefore, in the present invention, in consideration of the adverse effect of B, it may be limited to 0.01% or less.

In the present invention, in addition to the above-described steel composition, the remainder may contain Fe and unavoidable impurities. Unavoidable impurities may be unintentionally incorporated in a conventional steel manufacturing process, and cannot be completely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. In addition, the present invention does not entirely exclude addition of a composition other than the aforementioned steel composition.

A hot-dip galvanized layer may be provided on the holding steel sheet. In the present invention, the hot-dip galvanizing layer may be formed by any method as long as it is a hot-dip galvanizing method known in the art, and thus the configuration may not be particularly limited.

However, as a non-limiting example, the hot-dip galvanized layer may contain 50% by weight or more of Zn, and if the alloyed hot-dip galvanized layer is alloyed with the holding steel sheet, 50% by weight or more of Zn is included in the remaining components except Fe. can do. In addition, Al and inevitable impurities may be included, and the content of Al in the hot-dip galvanizing layer varies depending on the steel type and the plating bath composition of the holding steel sheet, and thus the present invention may not limit it to a specific range.

In the hot-dip galvanized steel sheet according to an aspect of the present invention, an internal oxide consisting of one or two or more of Mn, Si, and B is formed in crystal grains within 3 μm depth in the thickness direction from the surface of the holding steel sheet, and the The internal oxide may have a spherical shape, and its diameter may be 0.5 μm or less.

In the present invention, an Fe coating layer is formed on the surface of the holding steel sheet before annealing of the holding steel sheet. When Fe coating is applied to the surface of the holding steel sheet and then annealing is performed, when plating-blocking elements diffuse to the surface, it first binds with oxygen in the Fe coating layer before reaching the final surface of the holding steel sheet including the Fe coating layer. An internal oxide composed of one single or two or more types of complex oxides among the plating-blocking elements is formed in the steel sheet region, that is, the interface between the Fe coating layer and the holding steel sheet and inside the Fe coating layer to form an internal oxide layer. Plating-blocking elements changed into oxides in this way cannot be diffused to the surface of the holding steel sheet any more. And during the annealing process, the Fe coating layer begins to be reduced from the surface in contact with the atmosphere in the annealing furnace, and the entire Fe coating layer is reduced when the annealing is completed.

In addition, the internal oxide is present in the crystal grains and is characterized in that it has a spherical shape having a diameter of 0.5 μm or less.

In the case of annealing of a cold-rolled steel sheet in a conventional conventional annealing furnace internal oxidation method, oxygen in the annealing furnace is introduced through the surface of the annealing material to form internal oxide. When internally oxidized by oxygen supplied from the outside, internal oxides are mainly formed in a linear form along the grain boundary.

On the other hand, in the present invention, since the Fe coating layer already contains a certain amount or more of oxygen, it is internally oxidized by oxygen present in the annealing material. In this case, the internal oxide is not formed along the grain boundaries, but exists in the grains, has a diameter of 0.5 μm or less, and has a shape of a sphere or a point instead of a linear shape. In addition, the internal oxide layer formed by the Fe coating containing oxygen is formed within a depth of 3 μm from the final surface of the annealed steel sheet. However, since the introduction of oxygen in the annealing furnace into the annealing material during annealing is not blocked, some internal oxides may also be formed at the grain boundaries, so the present invention does not exclude the case where the internal oxides exist at the grain boundaries.

The holding steel sheet of the present invention contains boron (B) among the plating-blocking elements, and B is also diffused to the surface of the holding steel sheet by annealing and is concentrated on the surface to form oxides. When a steel sheet in which an oxide is concentrated on the surface is immersed in a plating bath, the surface reactivity decreases, resulting in a problem that the formation of the alloying suppression layer (Fe ₂ Al ₅ ) is not uniform. In particular, it is very important to control the concentration of B at the interface between the base steel sheet and the hot-dip galvanized layer because B makes it difficult to secure the adhesion of the hot-dip galvanized layer even with a very small amount of 10 ppm.

However, oxides (surface concentrations) caused by plating-blocking elements such as B, Si, and Mn are concentrated on the surface of the holding steel sheet after annealing and before plating, but when the holding steel sheet is immersed in a plating bath and plated, the above Most of the surface oxides are removed by melting in the plating bath. If some of the alloying suppression layers are not formed well, it is easy to check the concentration at the interface between the plating layer and the base iron even after plating, but in all cases where the adhesion of the plating layer is good as in the preferred embodiment of the present invention, the final product, hot-dip galvanized steel sheet, It can be difficult to confirm the B concentration at the interface.

Therefore, it is effective to check the B concentration at the interface on the surface of the base steel sheet (or cold rolled steel sheet) annealed before plating. Accordingly, in the present invention, the amount of B concentration in the state of the annealed cold-rolled steel sheet before plating is limited, and the integral value of the B content from the surface of the annealed cold-rolled steel sheet to a depth of 0.03 μm in the thickness direction can be limited to 0.005 or less. .

The hot-dip galvanized steel sheet according to the present invention does not generate unplated even for hardly plated steel types and has excellent adhesion to the plating layer.

In addition, the strength of the hot-dip galvanized steel sheet according to the present invention may be less than or equal to 490 MPa based on a tensile strength.

Hereinafter, a method of manufacturing a hot-dip galvanized steel sheet having excellent plating quality according to an aspect of the present invention will be described in detail. However, the manufacturing method described below is only one of all possible embodiments, and does not mean that the hot-dip galvanized steel sheet of the present invention must be manufactured by the following manufacturing method.

A method of manufacturing a hot-dip galvanized steel sheet according to an aspect of the present invention includes preparing a cold-rolled steel sheet having the aforementioned component system as a base steel sheet, forming an Fe coating layer on the base steel sheet, and annealing the steel sheet having the Fe coating layer formed thereon. It includes cooling and hot dip galvanizing.

First, a cold-rolled steel sheet having the above-described alloy composition is prepared as a holding steel sheet. The cold-rolled steel sheet is weight percent, carbon (C): 0.15% or less (excluding 0%), manganese (Mn): less than 1.5% (excluding 0%), silicon (Si): 0.5% or less (excluding 0%), Boron (B): 0.01% or less (excluding 0%), the balance may contain Fe and inevitable impurities.

If the cold-rolled steel sheet having the above-described alloy composition can be applied without limitation as the holding steel sheet of the hot-dip galvanized steel sheet according to the present invention, the method of manufacturing the cold-rolled steel sheet may not be specifically limited.

However, as a non-limiting example, the cold rolled steel sheet may be manufactured according to the following manufacturing method.

First, in wt%, carbon (C): 0.15% or less (excluding 0%), manganese (Mn): less than 1.5% (excluding 0%), silicon (Si): 0.5% or less (excluding 0%), boron (B ): 0.01% or less (excluding 0%), a steel slab containing the balance Fe and inevitable impurities is prepared, and the steel slab is reheated at a temperature of 1160 to 1250°C. The reheating process is performed in order to smoothly perform the subsequent hot rolling process and to obtain sufficient properties of the target steel sheet.

At this time, if the reheating temperature is less than 1160°C, the inclusions in the steel slab are not sufficiently re-dissolved, resulting in material variation after hot rolling, and surface defects due to the inclusions may occur. On the other hand, when the reheating temperature exceeds 1250°C, the strength may decrease due to abnormal grain growth of austenite grains. Therefore, the reheating temperature is preferably limited to 1160 ~ 1250 ℃.

Next, the reheated steel slab is hot-rolled at a temperature in the range of 850 to 1150°C to obtain a hot-rolled steel sheet. When hot rolling is initiated at a temperature higher than 1150℃, the temperature of the hot-rolled steel sheet increases, resulting in coarse grain size and poor surface quality of the hot-rolled steel sheet. On the other hand, if hot rolling is completed at a temperature lower than 850°C, the development of elongated grains and high yield ratio are obtained due to excessive recrystallization delay, resulting in poor cold rolling and poor shearing.

Thereafter, the hot-rolled steel sheet can be cooled at an average cooling rate of 10 to 70°C/sec to a cooling end temperature in the range of 500 to 750°C, and wound at a winding temperature in the range of 500 to 750°C.

At this time, if the coil is cooled to a cooling end temperature of less than 500°C and wound, the shape of the steel sheet may be deteriorated due to the too low coiling temperature. On the other hand, when it is cooled and wound at a cooling end temperature exceeding 750°C, coarse ferrite grains are formed, and coarse carbides and nitrides are easily formed, and the material of the steel may be inferior.

During cooling, if the average cooling rate is less than 10℃/sec, coarse ferrite grains are formed and the microstructure may become uneven, and if the average cooling rate exceeds 70℃/sec, not only distortion of the plate shape occurs, but also the thickness of the plate. The microstructure in the direction may also become non-uniform, which may lead to poor shear workability of the steel.

Cold rolled steel sheet can be obtained by pickling and cold rolling the wound hot rolled steel sheet.

Pickling is a process for removing scale formed on the surface of a steel sheet, and if it is a general pickling method practiced in the art, it can be preferably applied to the present invention, and thus the pickling method may not be specifically limited in the present invention. However, as a non-limiting example, pickling may be performed by immersing in a solution having 10 to 20 vol% of hydrochloric acid at a temperature of about 50 to 80°C.

After that, cold rolling is performed on the pickled hot rolled steel sheet. In this case, as a non-limiting example, cold rolling may be performed at a cold reduction ratio of 50 to 90%. If the cold rolling reduction ratio is less than 50% during cold rolling, it may be difficult to secure the target thickness, and it may be difficult to correct the shape of the steel plate. On the other hand, when the cold reduction ratio exceeds 90%, cracks may occur at the edge of the steel sheet, and a cold rolling load may be caused.

In addition, it is preferable that the initial stand reduction ratio is 20 to 40% during cold rolling. Typically, a rolling mill consisting of 5 to 6 stands is used for cold rolling, but when the initial stand reduction ratio is less than 20%, there may be a limit to the shape control of the hot-rolled steel sheet due to the low reduction ratio. On the other hand, if the initial stent reduction ratio exceeds 40%, equipment load may occur due to an increase in the initial stent reduction ratio. Therefore, in the present invention, the initial stand reduction rate during cold rolling may be limited to 20 to 40%, and in some cases, may be limited to 25 to 35%.

An Fe coating layer is formed on the prepared cold-rolled steel sheet. The holding steel sheet of the present invention, that is, the Fe coating layer formed on the surface of the cold-rolled steel sheet may contain 2 to 30% by weight of oxygen, and the thickness may be 0.02 to 1 μm.

When oxygen in the Fe coating layer is less than 2% by weight, oxygen resources for forming internal oxidation during annealing may not be sufficient. On the other hand, when the oxygen concentration in the Fe coating layer exceeds 30% by weight, the brittleness of the Fe coating layer is increased, so that the coating layer may be peeled off during the plate in the annealing furnace.

In the present invention, oxygen in the Fe coating layer may be introduced into the Fe coating layer together at the time of Fe coating without going through a separate process, for example, an oxidation heating step. Since the introduction method may vary depending on the coating method, the present invention is not particularly limited. However, as a non-limiting example, when forming the Fe coating layer by the electroplating method, Fe ²⁺ ions in the Fe electroplating solution are at a concentration of 1 to 80 g/L and the current density is 10 to 100 ASD. The oxygen content can be controlled from 2 to 30%.

In addition, if the thickness of the Fe coating layer is less than 0.02μm, the Fe coating layer may not completely cover the surface of the holding steel sheet, and some uncoated portions may occur.On the other hand, if the thickness exceeds 1μm, the adhesion between the holding steel sheet and the Fe coating layer will be weak. Because it can be used and a large amount of Fe coating is required, the process cost may be excessive.

In the present invention, since the Fe coating layer contains 2 to 30% by weight of oxygen and satisfies the condition of forming a thickness of 0.02 to 1 μm, it is not necessary to specifically limit the coating method. However, in the case of the electroplating method, compared to other coating methods, the thickness of the coating layer can be precisely controlled and the entire width of the steel sheet can be uniformly coated, so it is preferable to form the Fe coating layer by the electroplating method, but is not limited thereto. . If the Fe coating layer is formed using the electroplating method, the amount of Fe adhesion may be set to 0.3~3g/m ² to achieve an appropriate thickness.

The cold rolled steel sheet on which the Fe coating layer is formed is annealed and cooled. At this time, the annealing conditions are preferably set to an annealing atmosphere, an annealing temperature, and a holding time in which the plating disturbing elements such as Mn, Si, and B are oxidized while Fe of the steel sheet is not oxidized. In a preferred implementation the cases annealing is 1 ~ 70% H ₂ controlled to a dew point temperature of -60 ~ 10 ℃ - may comprise a step of holding for 5 to 120 seconds in the annealing of the rest N ₂ gas atmosphere at 600 ~ 950 ℃ However, it is not limited thereto.

In the present invention, it is necessary to control the atmosphere and temperature at the time of annealing so as to prevent oxidation of Fe in the base steel sheet and to cause an oxidation reaction of plating-blocking elements such as Mn, Si, and B. To this end, the dew point temperature can be kept below 10℃. On the other hand, it is preferable to limit the dew point temperature to -60°C or higher when considering the ability to maintain the atmosphere in the production line.

In addition, in order to prevent oxidation of the base steel sheet and the Fe coating layer during annealing, the hydrogen content in the atmospheric gas during annealing may be set to 1% or more in volume %. On the other hand, even if the hydrogen content is increased, since no special technical problem occurs, the upper limit may not be particularly limited, but the hydrogen content may be set to 70% or less in consideration of economy.

When annealing, it can be annealed in the temperature range of 600~950℃. In the present invention, the temperature during annealing is based on the temperature of the steel sheet. For sufficient recrystallization during annealing, the temperature of the steel sheet during annealing needs to be 600℃ or higher. However, the upper limit of the temperature of the steel sheet during annealing may not be particularly limited, but may be set to 950°C or less in consideration of the life of the annealing furnace.

In addition, the holding time after reaching the target temperature during annealing can be limited to 5 to 120 seconds. For sufficient recrystallization during annealing, it needs to be maintained for 5 seconds or more. On the other hand, even if the annealing holding time is prolonged, the upper limit may not be specifically limited as no special technical problem occurs.However, if the holding time is too long, excessive cost increase may be caused, so it may be limited to 120 seconds or less in consideration of this. have.

After the annealing step, the annealed cold-rolled steel sheet may be cooled. Since the cooling conditions in the cooling step after the annealing step do not significantly affect the surface quality of the final product, that is, the plating quality, there is no need to specifically limit the cooling conditions in the present invention.

However, as a non-limiting example, the microstructure, strength, and elongation of the steel sheet can be controlled by cooling at an average cooling rate of 1 to 50°C/sec to a cooling stop temperature of 250 to 550°C. If the cooling stop temperature is high or the cooling rate is too low, the strength may be insufficient. Conversely, if the cooling stop temperature is too low or the cooling rate is too high, the elongation may deteriorate. In particular, if the average cooling rate is less than 1°C/sec, the cooling time takes too long to deteriorate the efficiency, and if the average cooling rate exceeds 50°C/sec, uniform cooling of the steel sheet may be difficult, resulting in poor shape.

If rapid cooling is performed at one time at an annealing temperature, the shape of the steel sheet may be poor. Accordingly, as another non-limiting example, the cooling may be performed by dividing into primary cooling and secondary cooling. In this case, the first cooling may be performed up to a cooling end temperature of 600 to 700°C, and the secondary cooling may be performed up to a cooling end temperature of 250 to 550°C.

In addition, the average cooling rate in the secondary cooling may be greater than the average cooling rate in the primary cooling. In the case of high-strength steel using phase transformation, it must reach the cooling end temperature through rapid cooling, because slow cooling takes precedence due to the characteristics of general facilities, and then the rapid cooling section continues.

After the annealing and cooling steps, the cooled cold-rolled steel sheet may be hot-dip galvanized to form a hot-dip galvanized layer. In the present invention, the hot-dip galvanizing method may not be particularly limited. However, as a non-limiting example, by weight%, Al: 0.1 to 0.3%, consisting of the remaining Zn and inevitable impurities, immersing the steel sheet in a zinc plating bath maintained in a temperature range of 440 to 500°C, then taking out and plating After adjusting the adhesion amount, it can be cooled to manufacture a hot-dip galvanized steel sheet.

When the Al content is less than 0.1% in the plating bath of the non-limiting embodiment, the formation of the Fe-Al alloy phase formed at the interface between the holding steel sheet and the plating layer may be suppressed, and thus plating peeling may occur. On the other hand, when the Al content exceeds 0.3%, the Al content in the plating layer increases, and thus weldability may be inferior.

On the other hand, selectively, prior to final cooling, an alloying heat treatment may be performed on the hot-dip galvanized steel sheet to obtain an alloyed hot-dip galvanized steel sheet. In the present invention, the alloying heat treatment process conditions are not particularly limited, and can be applied to the present invention as long as the process conditions are commonly performed in the art. However, as a non-limiting example, the alloying heat treatment process may be performed in a temperature range of 500 to 550°C.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for exemplifying the present invention and not for limiting the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

(Example)

First, a cold rolled steel sheet having the composition of Table 1 was prepared. And for the steel type A, steel type B and steel type C of Table 1, Fe electroplating was performed at an adhesion amount of 0 to 3.0 g/m ² under the conditions shown in Table 2 below to form an Fe coating layer. At this time, after performing Fe coating, it was confirmed that 2 to 30% by weight of oxygen and the remaining Fe were detected by analyzing the components of the Fe coating layer, and the thickness was measured and shown together in Table 2.

Steel grade C Si Mn B Balance Steel grade A 0.002 0.06 0.5 0.001 Fe and inevitable impurities Steel grade B 0.002 0.06 0.5 0.003 Fe and inevitable impurities Steel grade C 0.002 0.06 0.5 0.005 Fe and inevitable impurities

division Steel grade Fe coating adhesion amount
(g/m ² ) Fe coating layer thickness
(㎛) Comparative Example 1 Steel grade A - - Invention Example 1 Steel grade A 0.5 0.07 Inventive Example 2 Steel grade A 1.0 0.13 Comparative Example 2 Steel grade B - - Invention Example 3 Steel grade B 0.5 0.05 Comparative Example 3 Steel grade C - - Invention Example 4 Steel grade C 0.3 0.05 Invention Example 5 Steel grade C 0.5 0.07 Invention Example 6 Steel grade C 1.0 0.15 Invention Example 7 Steel grade C 2.0 0.31 Invention Example 8 Steel grade C 3.0 0.60

Thereafter, for each Inventive Example and Comparative Example, annealing and cooling were performed under the conditions of a dew point temperature of -50°C, an annealing temperature of 800°C, and a holding time of 64 seconds in a 3%H ₂ -N ₂ atmosphere.

For each Inventive Example and Comparative Example for which the annealing was completed, the amount of surface concentration, the depth of internal oxidation, and the average size of the internal oxide present in the crystal grains were measured, and the results are shown in Table 3 below.

Specifically, the concentration of B on the surface of the holding steel plate was measured by measuring the B content from the surface of the holding steel plate to a depth of 0.03㎛ in the thickness direction using GDS (Glow Discharge Spectrometer), and expressed as an integral value.

The internal oxidation depth was measured using STEM (Scanning Transmission Electron Microscopy) to measure the internal oxidation depth of Si and Mn, the presence of internal oxides in the grains, and their size. The amount of surface oxidation, the depth of internal oxidation, and the size of the internal oxide were all analyzed at the sample location subjected to heat treatment under the same conditions without plating.

division Steel grade Internal oxidation depth
(max. ㎛) Internal oxide
The presence or absence Internal oxide
Average diameter (㎛) B concentration from surface to depth of 0.03㎛ Comparative Example 1 Steel grade A 0 X - 0.0051 Invention Example 1 Steel grade A 1.0 O 0.07 0.0010 Inventive Example 2 Steel grade A 1.25 O 0.05 0.0007 Comparative Example 2 Steel grade B 0 X - 0.0079 Invention Example 3 Steel grade B 0.75 O 0.02 0.0009 Comparative Example 3 Steel grade C 0 X - 0.0158 Invention Example 4 Steel grade C 0.50 O 0.02 0.0035 Invention Example 5 Steel grade C 0.60 O 0.03 0.0009 Invention Example 6 Steel grade C 1.30 O 0.03 0.0005 Invention Example 7 Steel grade C 1.90 O 0.05 0.0002 Invention Example 8 Steel grade C 2.50 O 0.07 0.0002

As a result of checking the location and size of the internal oxides of Inventive Examples 1 to 8, most of the internal oxides were located within the crystal grains, and all of the diameters satisfies the range of 0.5 μm or less, and the average diameter was 0.02 to 0.07 μm. Confirmed.

In addition, looking at the results of Table 3, it can be seen that the amount of B concentrated on the surface decreases as the amount of Fe adhesion increases in all steel types A, B, and C. In addition, it can be seen that the internal oxidation depth increases according to the amount of Fe adhesion.

Thereafter, the steel sheet was reheated to 480°C close to the plating bath temperature, and hot-dip galvanization was performed using a plating bath with 0.22% Al content and a plating bath temperature of 460°C, and the results are shown in Table 4. Done. In the evaluation of plating properties, adhesion between unplated and plated layers was evaluated according to the following evaluation method.

The unplated was evaluated by observing the surface of the hot-dip galvanized steel sheet with the naked eye and an optical microscope to observe the area where the hot-dip galvanized steel sheet was not covered and the base steel sheet was exposed on the surface.

The adhesion of the plating layer was evaluated by the Sealer Bending Test. In the evaluation of sealer bending, a sample was prepared by bonding and curing the automotive structural sealer to the surface of the plating material, and then bending the sample to forcibly separate the sealer and the sample. This is a method of evaluating this as a poor adhesion to the plating layer.

division Steel grade Unplated evaluation Plating layer adhesion evaluation Comparative Example 1 Steel grade A Non-plating occurs Partial peeling Invention Example 1 Steel grade A Good Good Inventive Example 2 Steel grade A Good Good Comparative Example 2 Steel grade B Good Partial peeling Invention Example 3 Steel grade B Good Good Comparative Example 3 Steel grade C Non-plating occurs Partial peeling Invention Example 4 Steel grade C Good Good Invention Example 5 Steel grade C Good Good Invention Example 6 Steel grade C Good Good Invention Example 7 Steel grade C Good Good Invention Example 8 Steel grade C Good Good

In the case of steel type A, as in Comparative Example 1, when the Fe coating layer was not formed, spot plating occurred, and partially peeling occurred when evaluating the adhesion of the plating layer. On the other hand, it can be seen that, as in Inventive Examples 1 and 2, when an Fe coating layer having a thickness of 0.02 μm or more is formed before annealing, it is possible to secure adhesion of unplated and plated layers.

On the other hand, in the case of steel type B, when the Fe coating layer is not formed, the unplated is good, but the plating layer peeling occurs. However, it can be seen that when the Fe coating layer is formed before annealing, adhesion of the plating layer can be secured.

In addition, in the case of steel grade C, when the Fe coating layer was not formed as in steel grade A, viscous plating occurred and partial peeling occurred when evaluating the adhesion of the plated layer. You can see that is possible.

In the case of Comparative Examples 1 to 3, the integral value of the B content at the surface of the annealed base steel sheet, that is, at the interface, exceeded 0.005, and as a result, the adhesion of the plating layer was deteriorated and the plating layer was peeled off.

As can be seen from the above plating property evaluation result, it may be difficult to secure the plating quality of the hot-dip galvanized steel sheet depending on the alloy component of the holding steel sheet, but excellent plating regardless of the alloy component of the holding steel sheet by forming an Fe coating layer of an appropriate thickness before annealing. It can be seen that quality can be secured.

Although it has been described with reference to the above embodiments, it will be understood that a person skilled in the art can variously modify and change the present invention without departing from the spirit and scope of the present invention described in the following claims. I will be able to.

Claims (9)

By weight, carbon (C): 0.15% or less (excluding 0%), manganese (Mn): less than 1.5% (excluding 0%), silicon (Si): 0.5% or less (excluding 0%), boron (B) : Less than 0.01% (excluding 0%), the balance of Fe and the holding steel sheet containing inevitable impurities; And
A hot-dip galvanized layer provided on the base steel sheet;
Including,
An internal oxide composed of one or two or more of Mn, Si, and B is formed within the crystal grains within 3 μm depth in the thickness direction from the surface of the holding steel sheet,
The internal oxide is a hot-dip galvanized steel sheet having a spherical shape of 0.5 μm or less in diameter.
The method of claim 1,
The hot-dip galvanized steel sheet, characterized in that the hot-dip galvanizing layer is a hot-dip galvanized layer alloyed with Fe of the holding steel sheet.
The method of claim 1,
In the base steel sheet before the hot-dip galvanizing layer is formed,
The base steel sheet before the hot-dip galvanizing layer is formed is an annealed cold-rolled steel sheet,
Hot-dip galvanized steel sheet, characterized in that the integral value of the B content from the surface of the annealed cold rolled steel sheet to a depth of 0.03 μm in the thickness direction is 0.005 or less.
By weight, carbon (C): 0.15% or less (excluding 0%), manganese (Mn): less than 1.5% (excluding 0%), silicon (Si): 0.5% or less (excluding 0%), boron (B) : Preparing a cold rolled steel sheet containing 0.01% or less (excluding 0%), the balance Fe and inevitable impurities;
Forming an Fe coating layer containing 2 to 30% by weight of oxygen on the surface of the cold rolled steel sheet to a thickness of 0.02 to 1 μm to obtain an Fe-coated cold rolled steel sheet;
The Fe-coated cold rolled steel sheet has a dew point temperature of -60°C to 10°C, and 1 to 70% of H ₂ and Annealing by maintaining for 5 to 120 seconds in a temperature range of 600 to 950°C based on the temperature of the steel sheet in an annealing furnace in an atmosphere consisting of the remaining N ₂ ;
Cooling the annealed cold-rolled steel sheet; And
Hot-dip galvanizing the cooled cold-rolled steel sheet;
Method for producing a hot-dip galvanized steel sheet comprising a.
The method of claim 4,
Preparing the cold rolled steel sheet,
By weight, carbon (C): 0.15% or less (excluding 0%), manganese (Mn): less than 1.5% (excluding 0%), silicon (Si): 0.5% or less (excluding 0%), boron (B) : Preparing a steel slab containing 0.01% or less (excluding 0%), the balance Fe and inevitable impurities;
Reheating the steel slab at 1160 to 1250°C;
Obtaining a hot rolled steel sheet by hot rolling the reheated steel slab at 850-1150°C;
Cooling the hot-rolled steel sheet to a cooling end temperature of 500 to 750°C at an average cooling rate of 10 to 70°C/sec, and winding in a temperature range of 500 to 750°C; And
Pickling and cold rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet;
Method for producing a hot-dip galvanized steel sheet comprising a.
The method of claim 4,
The step of cooling the annealed cold rolled steel sheet,
A method of manufacturing a hot-dip galvanized steel sheet, characterized in that cooling to a cooling end temperature of 250 to 550°C at an average cooling rate of 1 to 50°C/sec.
The method of claim 4,
The step of cooling the annealed cold-rolled steel sheet is divided into primary cooling and secondary cooling,
The cooling end temperature of the primary cooling is 600 ~ 700 ℃,
The cooling end temperature of the secondary cooling is 250 ~ 550 ℃,
The method of manufacturing a hot-dip galvanized steel sheet, wherein the average cooling rate in the secondary cooling is greater than the average cooling rate in the primary cooling.
The method of claim 4,
The step of hot-dip galvanizing the cooled cold-rolled steel sheet,
A method of manufacturing a hot-dip galvanized steel sheet, comprising plating with a zinc plating bath maintained in a temperature range of 440 to 500°C, consisting of Al: 0.1 to 0.3%, remaining Zn and unavoidable impurities.
The method of claim 4,
The method of manufacturing a hot-dip galvanized steel sheet, further comprising an alloying heat treatment step of alloying the hot-dip galvanized layer formed by the hot-dip galvanizing.

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