KR102632877B1 - Ultra high strength galva-annealed steel sheet having excellent surface properties and method of manufacturing the same - Google Patents

Abstract

Provides ultra-high strength hot-dip galvanized steel with excellent surface properties and a manufacturing method thereof. According to one embodiment of the present invention, the ultra-high strength hot-dip galvanized steel has, in weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, and aluminum (Al): 0.5%. % or less (including 0%), Chromium (Cr): 1.0% or less (including 0%), Manganese (Mn): 1.0% to 3.0%, Phosphorus (P): More than 0% to 0.02%, Sulfur (S): steel base material exceeding 0% to 0.01%, and the balance containing iron (Fe) and other inevitable impurities; An iron coating layer formed on the surface of the steel base material; and a hot-dip galvanized layer formed on the surface of the iron coating layer, by forming an internal oxide at the interface between the steel base material and the iron coating layer, thereby suppressing the formation of oxide at the interface between the iron coating layer and the hot-dip galvanized layer. I order it.

Description

Ultra high strength galva-annealed steel sheet having excellent surface properties and method of manufacturing the same}

The technical idea of the present invention relates to steel materials, and more specifically, to ultra-high-strength hot-dip galvanized steel materials with excellent surface properties that can prevent unplated defects or plating peeling, and to a method of manufacturing the same.

Lightening vehicles is a global issue to respond to the global environment and improve vehicle fuel efficiency. There are also ways to reduce the weight of a vehicle by applying lightweight materials or reducing the coefficient of friction by changing vehicle design or materials. Meanwhile, in order to reduce vehicle weight and avoid collision safety regulations, the application of ultra-high strength steel is essential from a material perspective. These ultra-high-strength steels contain a variety of alloying elements compared to regular steels, such as manganese (Mn), silicon (Si), aluminum (Al), chromium (Cr), and boron (B), which have a higher oxidation tendency than iron. A lot of elements are added.

In hot dip galvanizing, the plating quality is determined by the surface condition of the annealed steel sheet just before plating. Manganese (Mn), silicon (Si), aluminum (Al), and chromium are added to secure the physical properties of the steel sheet during annealing. There is a problem in that elements such as (Cr) and boron (B) form surface oxides, thereby weakening plating quality.

Korean Patent Application No. 10-2016-0077463

The technical problem to be achieved by the technical idea of the present invention is to provide an ultra-high-strength hot-dip galvanized steel material with excellent surface properties that can prevent non-plating defects or plating peeling by suppressing the formation of surface oxides, and a method for manufacturing the same.

However, these tasks are illustrative, and the technical idea of the present invention is not limited thereto.

According to one aspect of the present invention, an ultra-high strength hot-dip galvanized steel material having excellent surface properties and a method for manufacturing the same are provided.

According to one embodiment of the present invention, the ultra-high strength hot-dip galvanized steel has, in weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, and aluminum (Al): 0.5%. % or less (including 0%), Chromium (Cr): 1.0% or less (including 0%), Manganese (Mn): 1.0% to 3.0%, Phosphorus (P): More than 0% to 0.02%, Sulfur (S): steel base material exceeding 0% to 0.01%, and the balance containing iron (Fe) and other inevitable impurities; An iron coating layer formed on the surface of the steel base material; and a hot-dip galvanized layer formed on the surface of the iron coating layer, by forming an internal oxide at the interface between the steel base material and the iron coating layer, thereby suppressing the formation of oxide at the interface between the iron coating layer and the hot-dip galvanized layer. You can do it.

According to one embodiment of the present invention, the ultra-high strength hot-dip galvanized steel has, in weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, and aluminum (Al): 0.5%. % or less (including 0%), Chromium (Cr): 1.0% or less (including 0%), Manganese (Mn): 1.0% to 3.0%, Phosphorus (P): More than 0% to 0.02%, Sulfur (S): steel base material exceeding 0% to 0.01%, and the balance containing iron (Fe) and other inevitable impurities; An iron coating layer formed on the surface of the steel base material; and an alloyed hot-dip galvanized layer formed on the surface of the iron coating layer, wherein an internal oxide is formed at the interface between the steel base material and the iron coating layer, thereby preventing the formation of an oxide at the interface between the iron coating layer and the hot-dip galvanized layer. It can be suppressed.

According to one embodiment of the present invention, the steel base material may include a total of 3.0% by weight or less of silicon (Si), aluminum (Al), and chromium (Cr).

According to one embodiment of the present invention, the steel base material, in weight percent, titanium (Ti): 0.5% or less (including 0%), molybdenum (Mo): 0.5% or less (including 0%), niobium (Nb) : 0.5% or less (including 0%), and boron (B): 0.05% or less (including 0%).

According to one embodiment of the present invention, the iron coating layer may have a thickness of 300 nm to 3 μm, and a clean area in which no internal oxide is formed with a thickness of 100 nm to 3 μm is formed from the interface with the hot-dip galvanized layer. You can have it.

According to one embodiment of the present invention, the iron coating layer is formed with an oxygen concentration of 5% by weight to 15% by weight, and after the internal oxide is formed, the iron coating layer has an oxygen concentration of 0% by weight to 5% by weight. You can have it.

According to an embodiment of the present invention, the method of manufacturing the ultra-high strength hot-dip galvanized steel is, in weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, aluminum (Al) ): 0.5% or less (including 0%), chromium (Cr): 1.0% or less (including 0%), manganese (Mn): 1.0% to 3.0%, phosphorus (P): more than 0% to 0.02%, sulfur ( S): manufacturing a hot-rolled steel sheet using steel containing more than 0% to 0.01%, and the remainder containing iron (Fe) and other unavoidable impurities; Manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; Forming an iron coating layer on the surface of the cold rolled steel sheet; Annealing heat treatment of the cold rolled steel sheet by maintaining it at a temperature of 780°C to 900°C for 30 to 150 seconds; and manufacturing a hot-dip galvanized steel sheet by immersing the annealed heat-treated cold-rolled steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the iron coating layer. In the annealing heat treatment step, the steel By forming an internal oxide at the interface between the base material and the iron coating layer, the formation of oxide at the interface between the iron coating layer and the hot-dip galvanized layer can be suppressed.

According to one embodiment of the present invention, the annealing heat treatment step may be performed by heating at a temperature increase rate of 1°C/sec to 5°C/sec.

According to one embodiment of the present invention, the iron coating layer is formed with an oxygen concentration of 5% by weight to 15% by weight, and after the internal oxide is formed, the iron coating layer has an oxygen concentration of 0% by weight to 5% by weight. You can have it.

According to one embodiment of the present invention, the iron coating layer may have a thickness of 300 nm to 3 μm, and may have a clean area in which no internal oxide is formed with a thickness of 100 nm to 3 μm from the interface with the hot-dip galvanized layer. there is.

According to an embodiment of the present invention, the method may further include manufacturing an alloyed hot-dip galvanized steel sheet by alloying and heat-treating the hot-dip galvanized steel sheet.

According to the technical idea of the present invention, ultra-high strength hot-dip galvanized steel can control the diffusion rate of oxidizing elements by performing annealing heat treatment at a low temperature rise temperature, and induce reduction of iron with sufficient annealing heat treatment temperature and time. Thus, the formation of surface oxide that causes plating defects can be prevented. The ultra-high strength hot-dip galvanized steel has 1) a silicon, aluminum, and chromium content of 3% or less, 2) an oxygen concentration in the iron coating layer of 5% to 15% by weight, and 3) a thickness of the iron coating layer of 300 nm to 300 nm. 3 μm, 4) By controlling the temperature increase temperature for annealing heat treatment from 1°C/sec to 5°C/sec and the annealing heat treatment from 30 seconds to 150 seconds at a temperature of 780°C to 900°C, silicon, aluminum, and chromium By preventing the diffusion of oxide-forming substances to the surface to be plated and by forming and depleting internal oxides inside the base steel or the iron coating layer, the formation of surface oxides can be prevented and excellent plating quality can be provided.

The effects of the present invention described above have been described as examples, and the scope of the present invention is not limited by these effects.

Figure 1 is a schematic diagram explaining the function of the iron coating layer of ultra-high strength hot-dip galvanized steel according to an embodiment of the present invention.
Figure 2 is a process flow chart showing a method of manufacturing ultra-high strength hot-dip galvanized steel according to an embodiment of the present invention.
Figure 3 is a photograph illustrating the plating quality of ultra-high strength hot-dip galvanized steel according to an embodiment of the present invention.
Figure 4 is a scanning electron microscope photograph showing internal oxides of ultra-high strength hot-dip galvanized steel according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those skilled in the art, and the following examples may be modified into various other forms, and the present invention The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the technical idea of the present invention to those skilled in the art. In this specification, like symbols refer to like elements throughout. Furthermore, various elements and areas in the drawings are schematically drawn. Accordingly, the technical idea of the present invention is not limited by the relative sizes or spacing drawn in the attached drawings.

The technical idea of the present invention is to provide an ultra-high-strength hot-dip galvanized steel material with excellent surface properties that can prevent non-plating defects or plating peeling by suppressing the formation of surface oxides, and a method for manufacturing the same.

In the process of forming hot-dip galvanized steel, the annealing conditions are such that the dew point in the furnace is -30°C or lower, and when a large amount of silicon is contained, silicon oxide is formed on the surface of the steel, thereby reducing zinc wettability on the surface of the steel. If zinc wettability is low in this way, there is a high possibility that plating defects such as non-plating or plating peeling will occur after zinc plating.

Various studies have been conducted to prevent silicon oxide from forming on the surface of steel.

The first is an oxidation/reduction heat treatment method, which provides a high oxidizing atmosphere to generate iron oxide on the surface of the steel to prevent silicon from diffusing to the surface of the steel, and then reduces the iron oxide through a reduction process to leave a clean surface layer. Forms an iron layer. However, a separate chamber for an oxidizing atmosphere is required, and if there is not enough iron oxide, the silicon diffusion inhibition effect is reduced, and furthermore, if the iron oxide is not completely reduced, zinc wettability is reduced.

The second is a furnace dew point control method, which increases the furnace dew point during the annealing heat treatment process and causes silicon to adhere to the inside of the steel as an oxide, suppressing surface diffusion of silicon. However, aging due to accelerated oxidation of the annealed heat treatment structure is likely to occur due to the increase in dew point, and steel materials with high manganese content may cause surface defects in the steel materials due to the formation of manganese oxides.

The third method of forming a nickel layer is to electroplate a small amount of nickel on the surface of the steel material before annealing heat treatment to suppress surface diffusion of silicon during annealing heat treatment. However, the production cost increases due to the use of expensive nickel, and nickel regulations in Europe and China are becoming more severe due to nickel's harmfulness to the human body, so there is a limit to the application of additional nickel to steel materials.

Ultra-high-strength hot-dip galvanized steel according to the technical idea of the present invention forms a thin iron coating layer on the surface of the steel using electroplating or the like before annealing heat treatment to suppress surface diffusion of silicon during annealing heat treatment. In this case, by controlling the content of elements that tend to form oxides, such as silicon, aluminum, and chromium, and controlling the process conditions for annealing heat treatment, silicon oxide, aluminum oxide, or chromium oxide is formed limited to the inside of the steel material. , the plating quality of steel materials can be increased by suppressing the diffusion of zinc to the surface being plated.

Figure 1 is a schematic diagram explaining the function of the iron coating layer of ultra-high strength hot-dip galvanized steel according to an embodiment of the present invention.

Referring to FIG. 1, an iron coating layer 20 is formed on a steel base material 10 using flash plating or the like. The steel base material 10 may include at least one of silicon, aluminum, and chromium to secure desired physical properties. The iron coating layer 20 may be composed of pure iron and may also contain oxygen.

Next, the steel base material 10 on which the iron coating layer 20 is formed is annealed and heat treated. By annealing heat treatment of the iron coating layer 20, the oxygen contained in the iron coating layer 20 preferentially reacts with plating-interfering elements such as silicon, aluminum, and chromium contained in the steel base material 10 to form internal oxides ( 30) is formed. As the diffusion of the plating interfering element is controlled, internal oxide 30 is formed intensively at the interface between the steel base material 10 and the iron coating layer 20, and the plating interfering element diffuses to the surface of the iron coating layer 20. suppress it from happening. The iron coating layer 20 forms a clean zone on the surface where iron can be reduced and the purity of iron can be further increased as the oxygen forms the internal oxide 30.

Next, a hot-dip galvanized layer 40 is formed on the iron coating layer 20. Since the internal oxide 30 is formed inside the iron coating layer 20 and is not exposed to the surface of the iron coating layer 20, the plating quality of the iron coating layer 20 and the hot-dip galvanized layer 40 is excellent. In other words, non-plating defects, plating peeling defects, or poor alloying defects can be prevented.

In the case of annealing cold-rolled steel sheets in a conventional internal oxidation method of an annealing furnace, oxygen in the annealing furnace is introduced through the surface of the annealing material to form internal oxides. In the case of internal oxidation by externally supplied oxygen, internal oxides are mainly formed in a linear form along the grain boundaries. However, in the present invention, since the iron coating layer already contains a certain amount of oxygen, it is internally oxidized by oxygen present inside the annealed material. In this case, the internal oxide is present within the crystal grains rather than along the grain boundaries, has a spherical or dot-shaped shape rather than a line, and may have an average diameter of 300 nm or less. In addition, the internal oxidation layer formed by the oxygen-containing iron coating layer is formed within a depth of 3㎛ from the final surface of the annealed base steel sheet.

Hereinafter, an ultra-high-strength hot-dip galvanized steel material having excellent surface properties that can prevent non-plating defects or plating peeling by suppressing the formation of surface oxides, which is an aspect of the present invention, will be described.

Ultra-high strength hot-dip galvanized steel

The ultra-high strength hot-dip galvanized steel material, which is one aspect of the present invention, has, in weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, aluminum (Al): 0.5% or less (0 % included), Chromium (Cr): 1.0% or less (0% included), Manganese (Mn): 1.0% ~ 3.0%, Phosphorus (P): Exceeding 0% ~ 0.02%, Sulfur (S): Exceeding 0% ~ 0.01%, and the balance is a steel base material containing iron (Fe) and other inevitable impurities; An iron coating layer formed on the surface of the steel base material; and a hot-dip galvanized layer formed on the surface of the iron coating layer. Additionally, by forming an internal oxide at the interface between the steel base material and the iron coating layer, the formation of an oxide at the interface between the iron coating layer and the hot-dip galvanized layer can be suppressed.

In addition, ultra-high strength hot-dip galvanized steel contains, in weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, aluminum (Al): 0.5% or less (including 0%), Chromium (Cr): 1.0% or less (including 0%), Manganese (Mn): 1.0% to 3.0%, Phosphorus (P): More than 0% to 0.02%, Sulfur (S): More than 0% to 0.01%, and The remainder is a steel base material containing iron (Fe) and other inevitable impurities; An iron coating layer formed on the surface of the steel base material; and an alloyed hot-dip galvanized layer formed on the surface of the iron coating layer. Additionally, by forming an internal oxide at the interface between the steel base material and the iron coating layer, the formation of oxide at the interface between the iron coating layer and the alloyed hot-dip galvanized layer can be suppressed.

In order to ensure plating quality, the ultra-high strength hot-dip galvanized steel contains a total of 3.0% by weight or less of silicon (Si), aluminum (Al), and chromium (Cr), for example, 0.5% by weight to 3.0% by weight. It can be included as a %. The silicone may contain a minimum of 0.5% by weight and a maximum of 2.5% by weight. The aluminum may not be included or may contain more than 0% by weight to 0.5% by weight. The chromium may not be included, or may include more than 0% by weight to 1.0% by weight.

The steel base material of the ultra-high strength hot-dip galvanized steel is, in weight percent, titanium (Ti): 0.5% or less (including 0%), molybdenum (Mo): 0.5% or less (including 0%), niobium (Nb): It may further include one or more of 0.5% or less (including 0%), and boron (B): 0.05% or less (including 0%).

The steel base material may be a cold rolled steel sheet formed by cold rolling a hot rolled steel sheet.

The iron coating layer may be formed to have an oxygen concentration of 5% by weight to 15% by weight. After the internal oxide is formed, oxygen in the iron coating layer is consumed and the oxygen concentration in the iron coating layer may decrease. For example, it may have an oxygen concentration of 0 wt% to 5 wt%.

The iron coating layer may have a total thickness of 300 nm to 3 μm. Additionally, the iron coating layer may have a clean area in which no internal oxide is formed with a thickness of 100 nm to 3 μm from the interface with the hot-dip galvanized layer or the alloyed hot-dip galvanized layer.

The iron coating layer may have a single-phase ferrite structure.

The hot dip galvanized layer and the alloyed hot dip galvanized layer may contain zinc (Zn) in an amount of 50% by weight or more. Additionally, aluminum (Al) and unavoidable impurities may be included, and since the content of aluminum (Al) in the hot-dip galvanized layer varies depending on the steel type and plating bath composition, the present invention may not limit it to a specific range.

Hereinafter, the role and content of each component included in the ultra-high strength hot-dip galvanized steel material according to the present invention will be described as follows. At this time, the content of the component elements all means weight%.

Carbon (C): 0.05% to 0.3%

Carbon is added to stabilize the retained austenite and ensure strength. If the carbon content is less than 0.05%, there may be difficulties in securing strength. If the carbon content exceeds 0.3%, weldability may decrease. Therefore, it is desirable to add carbon in an amount of 0.05% to 0.3% of the total weight of the steel.

Silicon (Si): 0.5% to 2.5%

Silicon is added as a deoxidizing agent to remove oxygen in steel materials during steelmaking, and is an element that suppresses precipitation of carbides in ferrite and promotes diffusion of carbon in ferrite into austenite, which can contribute to stabilizing retained austenite. If the silicon content is less than 0.5%, the effect of adding silicon is insufficient. If the silicon content exceeds 2.5%, the rolling load during rolling increases, red scale formation becomes excessive after hot rolling, and plating quality may deteriorate. Therefore, it is desirable to add silicon in an amount of 0.5% to 2.5% of the total weight of the steel.

Aluminum (Al): 0.5% or less (including 0%)

Aluminum contributes to the stabilization of retained austenite by suppressing the formation of carbides in ferrite. If the aluminum content exceeds 0.5%, it is difficult to manufacture a sound slab through reaction with mold flux during casting, and surface oxide may be formed, reducing plating properties. Therefore, it is desirable to add aluminum in an amount of 0.5% or less (including 0%) of the total weight of the steel.

Chromium (Cr): 1.0% or less (including 0%)

Chromium is an element with high hardenability and is added to suppress the formation of ferrite and increase strength through transformation strengthening. If the chromium content exceeds 1.0%, toughness may decrease, surface oxide may be formed to reduce plating properties, and cost may increase. Therefore, it is desirable to add chromium in an amount of 1.0% or less (including 0%) of the total weight of the steel.

Manganese (Mn): 1.0% ~ 3.0%

Manganese is added to form and stabilize retained austenite, suppress ferrite transformation upon cooling, and secure sufficient austenite to ensure strength and ductility. However, since manganese combines with sulfur to form MnS precipitates, it is necessary to control the content to optimize the manganese segregation zone. If the manganese content is less than 1.0%, the desired strength and ductility may not be provided due to insufficient austenite. If the manganese content exceeds 3.0%, band formation due to segregation induced in the slab and hot rolling processes may be excessive, which may reduce physical properties and, in particular, formability. Therefore, it is desirable to add manganese in an amount of 1.0% to 3.0% of the total weight of the steel.

Titanium (Ti): 0.5% or less (including 0%)

Titanium precipitates as TiN and suppresses austenite grain growth during reheating, thereby achieving high strength and excellent impact toughness. If the titanium content exceeds 0.5%, the carbon concentration and strength of martensite may decrease due to carbide precipitation, and the precipitates or crystals may coarsen, leading to a decrease in elongation, low-temperature toughness, and bendability characteristics. You can. Therefore, it is desirable to add titanium in an amount of 0.5% or less (including 0%) of the total weight of the steel.

Molybdenum (Mo): 0.5% or less (including 0%)

Molybdenum is effective in securing strength because it has a significant contribution to strength improvement and does not reduce the wettability of molten metals such as zinc. If the molybdenum content exceeds 0.5%, the effect is no longer significantly increased and costs may increase. Therefore, it is desirable to add molybdenum in an amount of 0.5% or less (including 0%) of the total weight of the steel.

Niobium (Nb): 0.5% or less (including 0%)

Niobium is segregated in the form of carbides at austenite grain boundaries and can increase strength by suppressing coarsening of austenite grains during annealing heat treatment. If the niobium content exceeds 0.5%, the effect is no longer significantly increased and costs may increase. Therefore, it is desirable to add niobium in an amount of 0.5% or less (including 0%) of the total weight of the steel.

Boron (B): 0.05% or less (including 0%)

Boron can secure low-temperature phase structures such as martensite and bainite, and secure high hardenability and hardenability. If the content of boron exceeds 0.05%, grain boundary embrittlement of the hard phase occurs, high toughness and bendability may not be secured, and it concentrates on the surface of the annealed material, greatly deteriorating the surface quality. Therefore, it is desirable to add boron in an amount of 0.05% or less (including 0%) of the total weight of the steel.

Phosphorus (P): >0% ~ 0.02%

Phosphorus partially contributes to improving strength and spot weldability, but can have a negative effect on the material by lowering toughness and forming micro-segregation as well as central segregation. Therefore, the lower the phosphorus content, the better. Therefore, it is desirable to limit phosphorus to more than 0% to 0.02% of the total weight of the steel.

Sulfur (S): >0% ~ 0.01%

Sulfur, along with phosphorus, is an element that is inevitably contained in the production of steel. It controls cleanliness and elongation inclusions, forms non-metallic inclusions such as MnS, and, as a low melting point element, has a high possibility of grain boundary segregation, reducing toughness. If the sulfur content exceeds 0.01%, the toughness of the steel base material and weld zone may be significantly reduced, and excessive MnS may be formed. Therefore, it is desirable to limit sulfur to more than 0% to 0.01% of the total weight of the steel.

The remaining component of the steel is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be introduced from raw materials or the surrounding environment, so this cannot be ruled out. Since these impurities are known to anyone skilled in the normal manufacturing process, all of them are not specifically mentioned in this specification.

The ultra-high-strength hot-dip galvanized steel material formed by controlling the specific components and content ranges of the above-described alloy composition and manufacturing method according to the technical idea of the present invention described later has a yield strength (YS): 580 MPa ~ 1470 MPa, Tensile strength (TS): 980 MPa ~ 1700 MPa, and elongation (EL): 3% ~ 25% can be satisfied.

Hereinafter, a method for manufacturing ultra-high strength hot-dip galvanized steel according to the technical idea of the present invention will be described.

Manufacturing method of ultra-high strength hot-dip galvanized steel

Figure 2 is a process flow chart showing a method of manufacturing ultra-high strength hot-dip galvanized steel according to an embodiment of the present invention.

In the method of manufacturing ultra-high strength hot-dip galvanized steel according to the present invention, the semi-finished product subject to the hot rolling process may be, for example, a slab. Slabs in a semi-finished state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

Referring to Figure 2, the method of manufacturing the ultra-high strength hot-dip galvanized steel includes a hot-rolled steel sheet manufacturing step (S10); Cold rolled steel sheet manufacturing step (S20); Iron coating layer forming step (S30); Annealing heat treatment step (S40); Hot dip galvanizing step (S50); and an alloying heat treatment step (S60).

Specifically, the method of manufacturing the ultra-high strength hot-dip galvanized steel is, in weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, aluminum (Al): 0.5% or less ( 0% included), chromium (Cr): 1.0% or less (0% included), manganese (Mn): 1.0% to 3.0%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% Manufacturing a hot-rolled steel sheet by hot-rolling steel containing ~ 0.01%, and the balance being iron (Fe) and other unavoidable impurities (S10); Manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet (S20); Forming an iron coating layer on the surface of the cold rolled steel sheet (S30); Annealing and heat treating the cold rolled steel sheet (S40); and manufacturing a hot-dip galvanized steel sheet by immersing the annealed heat-treated cold-rolled steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the iron coating layer (S50).

In addition, the method of manufacturing the ultra-high strength hot-dip galvanized steel may further include a step (S60) of manufacturing an alloyed hot-dip galvanized steel sheet by alloying and heat-treating the hot-dip galvanized steel sheet.

The steel material contains, in weight percent, titanium (Ti): 0.5% or less (including 0%), molybdenum (Mo): 0.5% or less (including 0%), niobium (Nb): 0.5% or less (including 0%), and boron (B): 0.05% or less (including 0%).

Hereinafter, the method for manufacturing ultra-high strength hot-dip galvanized steel according to the present invention will be described in detail step by step.

Hot rolled steel sheet manufacturing stage (S10)

The hot rolled steel sheet manufacturing step (S10) may be performed as follows. A steel material having the alloy composition is prepared, and the steel material is reheated at a reheating temperature (Slab Reheating Temperature, SRT) of, for example, 1,100°C to 1,300°C. Through this reheating, re-dissolution of components segregated during casting and re-dissolution of precipitates may occur. If the reheating temperature is less than 1,100°C, the hot rolling load may rapidly increase, and the precipitates may not be sufficiently re-dissolved, resulting in a decrease in precipitates in the process after hot rolling, and homogenization of the alloy elements may not be achieved. If the reheating temperature exceeds 1,300°C, the amount of surface scale increases, which may lead to loss of material and wasted energy.

After the reheating, hot rolling is performed using a conventional method. For example, a hot rolled steel sheet can be manufactured by performing hot finishing rolling at a finish delivery temperature (FDT) of 800°C to 1000°C. Uniform dispersion of precipitates can be improved by the hot rolling. If the finishing rolling temperature is lower than 800°C, it is difficult to secure the machinability of the steel sheet due to the occurrence of a mixed structure due to abnormal region rolling, and the machinability may be reduced due to microstructure unevenness. If the finish rolling temperature exceeds 1000°C, there is a risk of deterioration in the quality of the steel sheet due to increased brittleness of the steel sheet and generation of surface scale of the steel sheet along with a large amount of cooling.

Next, the hot-rolled steel sheet is cooled to a coiling temperature of, for example, 500°C or higher. The cooling can be done by either air cooling or water cooling, and can be performed by shear quenching. The cooling may be performed at a cooling rate of, for example, 1°C/sec to 50°C/sec. The cooling is preferably carried out to the coiling temperature.

Next, the hot rolled steel is wound. The winding step is performed at a temperature of 500°C or higher, for example, at a coiling temperature (CT) of 500°C to 700°C. If the coiling temperature is less than 500°C, a bainite or martensite structure is formed, making subsequent cold rolling difficult, and workability may be reduced due to increased strength and decreased ductility due to grain refinement. If the coiling temperature exceeds 700°C, excessive oxides may be generated on the surface of the steel material, which may cause defects, precipitates may become coarse, and the final microstructure may become coarse, reducing strength and workability.

Additionally, additional heat treatment may be optionally performed on the coiled hot rolled steel sheet to ensure cold rolling properties. At this time, heat treatment is generally performed in the form of a wound coil, and to prevent further oxidation, heat treatment can be performed in an atmosphere containing nitrogen and hydrogen, or in an atmosphere containing only nitrogen or hydrogen.

Cold rolled steel sheet manufacturing stage (S20)

The cold rolled steel sheet manufacturing step (S20) may be performed as follows. Pickling treatment is performed by cleaning the hot rolled steel sheet with acid. Next, the pickled hot-rolled steel sheet is cold-rolled at an average reduction ratio of, for example, 30% to 70% to form a cold-rolled steel sheet. The higher the average reduction ratio, the higher the formability due to the tissue refinement effect. If the average reduction ratio is less than 30%, it is difficult to obtain a uniform microstructure. If the average reduction ratio exceeds 70%, the rolling roll force may increase and the process load may increase.

Iron coating layer formation step (S30)

In the iron coating layer forming step (S30), an iron coating layer is formed on the surface of the cold rolled steel sheet. Formation of the iron coating layer may be performed using flash plating. The flash plating refers to thin plating performed in an extremely short time.

The iron coating layer may contain oxygen, and the oxygen may be introduced into the iron coating layer simultaneously when forming the iron coating layer without going through a separate process, for example, an oxidation heating step. Since this method of introducing oxygen may vary depending on the coating method, it is not particularly limited in the present invention. However, as a non-limiting example, when forming the iron coating layer using electroplating, the concentration of Fe ²⁺ ions in the iron electroplating solution is 1 g/L to 80 g/L and the current density is 10 ASD to 10 ASD. By performing this at 100 ASD, the oxygen content in the iron coating layer can be controlled. The iron coating layer may contain 5% to 15% by weight of oxygen. If the oxygen content in the iron coating layer is less than 5% by weight, oxygen resources may not be sufficient to form internal oxidation during annealing. On the other hand, if the oxygen in the iron coating layer exceeds 15% by weight, the brittleness of the iron coating layer increases and peeling of the iron coating layer may occur during rolling in an annealing furnace.

The iron coating layer may have various thicknesses, for example, 300 nm to 3 μm. When the thickness of the iron coating layer is less than 300 nm, the iron coating layer does not completely cover the surface of the steel base material, so some uncoated areas may be formed. If the thickness of the iron coating layer exceeds 3 μm, adhesion between the iron coating layer and the steel base material may become weak, and process costs may become excessive.

The coating method for forming the iron coating layer is not limited, but compared to other coating methods, the electroplating method can precisely control the thickness of the coating layer and uniformly coat the entire width of the steel sheet. When forming the iron coating layer using the electroplating method, the iron adhesion amount can be set to 0.5 g/m ² to 5 g/m ² .

Annealing heat treatment step (S40)

In the annealing heat treatment step (S40), the cold rolled steel sheet on which the iron coating layer is formed is annealed heat treated in a continuous annealing furnace with a normal slow cooling section. In the annealing heat treatment step, by forming an internal oxide at the interface between the steel base material and the iron coating layer, the formation of oxide at the interface between the iron coating layer and the hot-dip galvanized layer can be suppressed.

The annealing heat treatment may be performed by heating at a temperature increase rate of, for example, 1°C/sec to 5°C/sec and maintaining the temperature at a temperature of, for example, 780°C to 900°C for 30 to 150 seconds. The dew point in the annealing heat treatment may range from -65°C to 0°C. In the annealing heat treatment, the hydrogen concentration may be 3% to 20%, and the remainder may be in a nitrogen gas atmosphere.

If the temperature increase rate is less than 1°C/sec, the annealing heat treatment process may not be performed because it does not reach the line speed at which the steel is transported. If the temperature increase rate exceeds 5°C/sec, diffusion of highly oxidizing silicon, aluminum, and chromium may not be controlled.

If the annealing heat treatment temperature is less than 780°C, recrystallization may not occur sufficiently and the desired physical properties may not be obtained. If the annealing heat treatment temperature exceeds 900°C, deterioration of the heat-resistant equipment of the annealing heat treatment furnace may occur.

If the annealing heat treatment time is less than 30 seconds, recrystallization may not occur sufficiently and the desired physical properties may not be obtained. If the annealing heat treatment time exceeds 150 seconds, excessive cost increases may result, and diffusion of highly oxidizing silicon, aluminum, and chromium may not be controlled.

If the dew point is less than -65°C, the annealing heat treatment process may not be performed because it is outside the realistic process control range. If the dew point exceeds 0°C, there is a risk that oxidation may occur in the annealing heat treatment furnace, causing premature aging of the equipment.

If the hydrogen concentration is less than 3%, there is a risk that excessive oxidation of the iron coating layer may occur. If the hydrogen concentration exceeds 20%, there is a risk of explosion due to excess hydrogen.

After completing the annealing heat treatment, it is cooled to a temperature of 400°C to 500°C at a cooling rate of 1°C/sec to 30°C/sec. The above temperature range may be required to maintain the temperature of the plating bath in the subsequent process. Additionally, the cooling may be performed to room temperature, for example, 0°C to 40°C.

Hot dip galvanizing step (S50)

In the hot-dip galvanizing step (S50), a hot-dip galvanized steel sheet is manufactured by immersing the annealed cold-rolled steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the iron coating layer. The temperature of the hot-dip galvanizing bath may vary depending on the type and ratio of alloy elements for forming the plating layer and the composition of the steel base material (cold-rolled steel sheet), and may be, for example, 430°C to 480°C. Under the hot-dip galvanizing bath conditions, a hot-dip galvanized layer can be easily formed on the surface of the iron coating layer, and the adhesion of the plating layer can be excellent. Subsequently, the hot-dip galvanized layer can be wiped using nitrogen gas to control the amount of plating bath attached to the surface. The thickness of the hot-dip galvanized layer may be, for example, an average of 5 μm to 20 μm on one side. For example, the hot-dip galvanized layer may be formed with an adhesion amount of 35 g/m ² to 150 g/m ² on one side. Within the range of the above deposition amount, rust prevention performance can be secured.

If necessary, the hot-dip galvanized steel sheet can be completed by cooling to room temperature at a cooling rate of 1°C/sec to 30°C/sec, for example, 10°C to 40°C to solidify the hot-dip galvanized layer.

Alloying heat treatment step (S60)

In the alloying heat treatment step (S60), the hot-dip galvanized steel sheet on which the hot-dip galvanized layer is formed is maintained at a temperature of, for example, 500 ° C. to 600 ° C., for example, for 10 seconds to 60 seconds to perform alloying heat treatment, Manufactures alloyed hot-dip galvanized steel sheets. The alloying heat treatment step can be performed continuously without cooling after performing the previous hot-dip galvanizing step. During alloying heat treatment under the above temperature conditions, the hot-dip galvanized layer grows stably, and the adhesion of the plated layer can be excellent. If the alloying heat treatment temperature is less than 500°C, alloying may not proceed sufficiently and the soundness of the hot-dip galvanized layer may deteriorate. When the alloying heat treatment temperature exceeds 600°C, changes in material may occur as the temperature goes into the ideal temperature range. Subsequently, the alloyed hot-dip galvanized steel sheet can be manufactured by cooling to room temperature.

Experiment example

Below, preferred experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are only intended to aid understanding of the present invention, and the present invention is not limited by the following experimental examples.

Steel having the composition (unit: weight %) shown in Table 1 below was prepared, and predetermined steps as described above were performed to prepare ultra-high strength hot-dip galvanized steel. The following comparative examples and examples were manufactured using the ultra-high strength hot-dip galvanized steel. The plating quality of the ultra-high-strength hot-dip galvanized steel was examined after hot-dip galvanizing and after alloying. The remainder in Table 1 is iron (Fe) and other inevitable impurities.

element C Si Mn content 0.2 1.8 2.8

Table 2 is a table showing the characteristics of the ultra-high strength hot-dip galvanized steel according to an embodiment of the present invention. In all cases, the furnace dew point was set at -50°C.

classification iron coating layer
thickness
(nm) iron coating layer
oxygen concentration
(weight%) temperature rise
speed
(℃/sec) Plating quality analysis Example 1 300 5 4 Good Example 2 500 5 4 Good Comparative Example 1 0 0 4 Not plated Comparative example 2 100 2 4 Not plated Comparative Example 3 100 5 4 poor alloying Comparative example 4 300 2 4 poor alloying Comparative Example 5 500 2 4 poor alloying Comparative Example 6 300 5 6 poor alloying Comparative example 7 500 5 6 poor alloying

Figure 3 is a photograph illustrating the plating quality of ultra-high strength hot-dip galvanized steel according to an embodiment of the present invention. Figure 3 (a) shows a good plating state, (b) shows a non-plated state, and (c) shows a poor alloying state.

Referring to Table 2, Examples 1 and 2 showed good plating quality and alloying quality, as shown in Figure 3 (a). In Examples 1 and 2, the thickness of the iron coating layer was 300 nm and 500 nm, respectively, the oxygen concentration of the iron coating layer was 5% by weight, and the temperature increase rate was 4 °C/sec.

Comparative Example 1 is a case in which an iron coating layer was not formed, and a hot-dip galvanized layer was not formed as shown in Figure 3 (b). This is analyzed as a result of a decrease in wettability with the zinc plating layer as the silicon oxide is exposed to the surface.

Comparative Examples 2 and 3 are cases where the thickness of the iron coating layer was reduced to 100 nm compared to the examples, and the hot-dip galvanized layer was not formed as shown in (b) of Figure 3, or (c) of Figure 3 and Likewise, alloying defects occurred. From these results, it is analyzed that the thickness of the iron coating layer of 100 nm is insufficient to suppress the surface diffusion of silicon oxide.

Comparative Examples 4 and 5 were cases in which the oxygen concentration in the iron coating layer was reduced to 2% compared to the Examples, and alloying defects occurred, as shown in (c) of FIG. 3. From these results, it is analyzed that the oxygen concentration in the iron coating layer of 2% is insufficient to suppress the surface diffusion of silicon oxide.

Comparative Examples 6 and 7 were cases in which the temperature increase rate for annealing heat treatment was increased to 6°C/sec compared to the Examples, and alloying defects occurred, as shown in (c) of FIG. 3. From these results, it is analyzed that the temperature increase rate of 6 °C/sec is too fast to suppress the surface diffusion of silicon oxide.

Figure 4 is a scanning electron microscope photograph showing internal oxides of ultra-high strength hot-dip galvanized steel according to an embodiment of the present invention.

Referring to FIG. 4, it can be seen that internal oxides composed of silicon or manganese are formed inside the steel base material and at the interface between the steel base material and the iron coating layer. On the other hand, such internal oxides were hardly visible inside the iron coating layer, and the internal oxides were not observed on the surface of the iron coating layer, that is, at the interface between the iron coating layer and the hot-dip galvanized layer. In addition, it was confirmed that the iron coating layer had a clean region in which no internal oxide was formed with a thickness of about 100 nm to 300 nm from the interface with the hot-dip galvanized layer.

Therefore, the ultra-high strength hot-dip galvanized steel according to the technical idea of the present invention is,

1) The content of silicon, aluminum, and chromium is less than 3%,

2) The oxygen concentration in the iron coating layer is 5% by weight to 15% by weight,

3) The thickness of the iron coating layer is 300 nm ~ 3 μm,

4) The temperature increase temperature for annealing heat treatment is 1℃/sec to 5℃/sec, and

5) By controlling the annealing heat treatment at a temperature of 780°C to 900°C for 30 to 150 seconds, diffusion of oxide-forming materials such as silicon, aluminum, and chromium to the surface to be plated is prevented and the inside or inside of the base steel material is prevented. By forming and depleting internal oxides inside the iron coating layer, excellent plating quality can be provided by preventing the formation of surface oxides on the surface to which the hot-dip galvanized layer is attached.

The technical idea of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and various substitutions, modifications, and changes are possible without departing from the technical idea of the present invention. It will be clear to those skilled in the art.

Claims (11)

By weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, aluminum (Al): 0.5% or less (including 0%), chromium (Cr): 1.0% or less (0 % included), manganese (Mn): 1.0% to 3.0%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, and the balance is iron (Fe) and other inevitable impurities. Steel base material containing;
An iron coating layer formed on the surface of the steel base material; and
It includes a hot-dip galvanized layer formed on the surface of the iron coating layer,
By forming an internal oxide at the interface between the steel base material and the iron coating layer, the formation of oxide at the interface between the iron coating layer and the hot-dip galvanized layer is suppressed,
As the annealing heat treatment is performed by heating at a temperature increase rate of 1℃/sec to 4℃/sec and maintaining the temperature at a dew point temperature of -50℃~-65℃ and a temperature of 780℃~900℃ for 30 seconds to 150 seconds, Oxygen contained in the iron coating layer forms an internal oxide with the silicon, aluminum, or chromium,
The iron coating layer has a thickness of 300 nm to 3 μm,
The iron coating layer has a clean area in which the internal oxide is not formed with a thickness of 100 nm to 3 μm from the surface,
Ultra-high strength hot-dip galvanized steel. In weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, aluminum (Al): 0.5% or less (including 0%), chromium (Cr): 1.0% or less (0 % included), manganese (Mn): 1.0% to 3.0%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, and the balance is iron (Fe) and other inevitable impurities. Steel base material containing;
An iron coating layer formed on the surface of the steel base material; and
It includes an alloyed hot-dip galvanized layer formed on the surface of the iron coating layer,
By forming an internal oxide at the interface between the steel base material and the iron coating layer, the formation of oxide at the interface between the iron coating layer and the hot-dip galvanized layer is suppressed,
As the annealing heat treatment is performed by heating at a temperature increase rate of 1℃/sec to 4℃/sec and maintaining the temperature at a dew point temperature of -50℃~-65℃ and a temperature of 780℃~900℃ for 30 seconds to 150 seconds, Oxygen contained in the iron coating layer forms an internal oxide with the silicon, aluminum, or chromium,
The iron coating layer has a thickness of 300 nm to 3 μm,
The iron coating layer has a clean area in which the internal oxide is not formed with a thickness of 100 nm to 3 μm from the surface,
Ultra-high strength hot-dip galvanized steel. The method of claim 1 or 2,
The steel base material contains a total of 3.0% by weight or less of silicon (Si), aluminum (Al), and chromium (Cr),
Ultra-high strength hot-dip galvanized steel. The method of claim 1 or 2,
The steel base material is, in weight percent, titanium (Ti): 0.5% or less (including 0%), molybdenum (Mo): 0.5% or less (including 0%), niobium (Nb): 0.5% or less (including 0%) , and boron (B): 0.05% or less (including 0%),
Ultra-high strength hot-dip galvanized steel. The method of claim 1 or 2,
The clean area has a thickness of 100 nm to 300 nm,
Ultra-high strength hot-dip galvanized steel. The method of claim 1 or 2,
The iron coating layer is formed with an oxygen concentration of 5 wt% to 15 wt%, and after the internal oxide is formed, the iron coating layer has an oxygen concentration of 0 wt% to 5 wt%.
Ultra-high strength hot-dip galvanized steel. In weight percent, carbon (C): 0.05% to 0.3%, silicon (Si): 0.5% to 2.5%, aluminum (Al): 0.5% or less (including 0%), chromium (Cr): 1.0% or less (0 % included), manganese (Mn): 1.0% to 3.0%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, and the balance is iron (Fe) and other inevitable impurities. Manufacturing a hot rolled steel sheet using steel materials containing;
Manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet;
Forming an iron coating layer on the surface of the cold rolled steel sheet;
Annealing heat treatment of the cold-rolled steel sheet on which the iron coating layer is formed by maintaining it at a temperature of 780°C to 900°C for 30 seconds to 150 seconds; and
Manufacturing a hot-dip galvanized steel sheet by immersing the annealed heat-treated cold-rolled steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the iron coating layer,
In the annealing heat treatment step, by forming an internal oxide at the interface between the steel base material and the iron coating layer, the formation of oxide at the interface between the iron coating layer and the hot-dip galvanized layer is suppressed,
The annealing heat treatment step is heated at a temperature increase rate of 1°C/sec to 4°C/sec and is performed at a dew point temperature of -50°C to -65°C,
As the annealing heat treatment is performed, oxygen contained in the iron coating layer forms an internal oxide with the silicon, aluminum, or chromium,
The iron coating layer has a thickness of 300 nm to 3 μm,
The iron coating layer has a clean area in which the internal oxide is not formed with a thickness of 100 nm to 3 μm from the surface,
Manufacturing method of ultra-high strength hot-dip galvanized steel. delete According to claim 7,
The iron coating layer is formed with an oxygen concentration of 5 wt% to 15 wt%, and after the internal oxide is formed, the iron coating layer has an oxygen concentration of 0 wt% to 5 wt%.
Manufacturing method of ultra-high strength hot-dip galvanized steel. According to claim 7,
The clean area has a thickness of 100 nm to 300 nm,
Manufacturing method of ultra-high strength hot-dip galvanized steel. According to claim 7,
Further comprising: manufacturing an alloyed hot-dip galvanized steel sheet by alloying and heat-treating the hot-dip galvanized steel sheet,
Manufacturing method of ultra-high strength hot-dip galvanized steel.

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