KR102231345B1 - High-strength cold-rolled steel sheet having excellent surface property and method for manufacturing thereof - Google Patents

Abstract

The present invention is by weight %, C: 0.05 to 0.3%, Si: 0.1 to 2.0%, Sol.Al: 0.005 to 1.5%, Mn: 1.5 to 8.0%, P: 0.04% or less (excluding 0%), S : 0.015% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), remaining Fe and inevitable impurities, 3㎛ depth from the surface in the thickness direction An internal oxide consisting of one or two or more of Mn, Si, Al, Cr, and B is formed within the crystal grains, and the internal oxide is a spherical cold-rolled steel sheet having a diameter of 0.5 µm or less, and a method of manufacturing the same. Provides.

Description

High-strength cold-rolled steel sheet with excellent surface quality and its manufacturing method {HIGH-STRENGTH COLD-ROLLED STEEL SHEET HAVING EXCELLENT SURFACE PROPERTY AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a high-strength cold-rolled steel sheet having excellent surface quality and a method of manufacturing the same.

In recent years, in the automotive industry, by applying a high-strength steel plate as a steel material for automobiles, safety has been improved and weight reduction has been achieved by reducing the thickness. Precipitation reinforced steel and solid solution reinforced steel have been developed as steel materials that can be preferably applied as steel materials for automobiles, and DP steel (Dual Phase Steel) and CP steel (Complex Phase) using phase transformation to improve strength and elongation at the same time have been developed. Steel), TRIP steel (Transformation Induced Plasticity Steel) and TWIP steel (Twinning Induced Plasticity Steel) were developed. These high-strength steels are added with various alloying elements compared to general steels. In particular, many elements having a higher oxidation tendency compared to Fe, such as Mn, Si, Al, Cr, and B, are added.

However, as above, elements such as Mn, Si, Al, Cr, and B added to secure the properties of high-strength steel migrate to the surface of the steel sheet during annealing to form surface oxides. As a result, the phosphate treatment property of the cold-rolled steel sheet decreases, and problems such as discoloration (yellowing) of the surface of the steel sheet occur. That is, in the annealing process, the reactivity of the surface is degraded 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 elements on the surface of the steel sheet.

To solve this problem, surface reactivity is improved by removing surface oxides through pickling after annealing. However, since Si oxide is strong against acids, it is difficult to completely remove the surface oxide. In addition, there is a problem that the surface discoloration of the steel sheet becomes severe due to corrosion products caused by pickling.

Another method is to prevent diffusion of alloying elements to the surface during annealing by conducting Ni pre-annealing. However, although the Mn diffusion suppression is effective, there is a problem that the diffusion of Si cannot be sufficiently suppressed.

In addition, there is an annealing furnace internal oxidation method in which diffusion to the surface is suppressed by increasing the dew point temperature of the annealing furnace to oxidize alloy elements to the surface layer. However, compared to Si, the effect of suppressing the surface diffusion of Mn is not great, and there is a problem that the total amount of surface oxides cannot be sufficiently reduced.

An object of the present invention is to provide a high-strength cold-rolled steel sheet with improved phosphate treatment and reduced discoloration (yellowing) of the steel sheet surface, 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 %, C: 0.05 to 0.3%, Si: 0.1 to 2.0%, Sol.Al: 0.005 to 1.5%, Mn: 1.5 to 8.0%, P: 0.04% or less (excluding 0% ), S: 0.015% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), balance Fe and unavoidable impurities, from the surface to the thickness direction An internal oxide composed of one or two or more of Mn, Si, Al, Cr, and B is formed in the crystal grains within a depth of 3㎛, and the internal oxide is a cold rolled steel sheet having a spherical shape of 0.5㎛ or less in diameter. .

The cold-rolled steel sheet may have a carbon-deficient layer having a C content of 1 to 80% relative to the C content of the base material of the steel sheet within a depth of 3 μm in the thickness direction from the surface.

The cold-rolled steel sheet is in wt%, N: 0.02% or less (excluding 0%), Mo: 0.2% or less (including 0%), Ti: 0.2% or less (including 0%), Sb: 0.05% or less (including 0%) ) And Nb: 0.1% or less (including 0%).

Another aspect of the present invention is by weight%, C: 0.05 to 0.3%, Si: 0.1% to 2.0%, Sol.Al: 0.005 to 1.5%, Mn: 1.5 to 8.0%, P: 0.04% or less (0% ), S: 0.015% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), balance Fe and unavoidable impurities The step of doing; Reheating the steel slab at 1100 to 1300°C; Hot rolling the reheated steel slab at a temperature of Ar3 or higher (a temperature at which ferrite starts to appear when cooling austenite) to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet in a temperature range of 700°C or less after cooling; Pickling and cold rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet; 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; And cooling the annealed cold-rolled steel sheet.

In the step of cooling the annealed cold-rolled steel sheet, it may be cooled to a cooling end temperature of 250 to 550°C at an average cooling rate of 5 to 100°C/sec.

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 steel slab is, by weight, N: 0.02% or less (excluding 0%), Mo: 0.2% or less (including 0%), Ti: 0.2% or less (including 0%), Sb: 0.05% or less (0%) Including) and Nb: 0.1% or less (including 0%) may further include one or more.

According to the present invention, by reducing the total amount of surface oxides due to annealing on the surface of the steel sheet, it is possible to provide a cold-rolled steel sheet having excellent surface quality and a method of manufacturing the same, in which phosphate treatment is improved and the surface discoloration of the steel sheet is reduced.

Various and beneficial advantages and effects of the present invention are not limited to the above-described contents, and may be more easily understood in the course of describing specific embodiments of the present invention.

1 is a conceptual diagram of a conventional high-strength cold-rolled steel sheet, (a) is a cross-sectional schematic diagram of a cold-rolled steel sheet before annealing, and (b) is a cross-sectional schematic diagram of a cold-rolled steel sheet on which surface oxides are formed after annealing.
2 is a conceptual diagram of a high-strength cold-rolled steel sheet according to an embodiment of the present invention, (a) is a schematic cross-sectional view showing the formation of an Fe coating layer on the surface of the cold-rolled steel sheet before annealing, and (b) is an internal oxide after annealing It is a schematic cross-sectional view showing formation.

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

The meaning of "comprising" as used in the specification specifies a specific characteristic, region, integer, step, action, element and/or component, and other specific characteristic, region, integer, step, action, element, component and/or group It does not exclude the existence or addition of

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 inventors of the present invention studied the cause of poor phosphate treatment and surface discoloration when annealing a cold-rolled steel sheet, and the main cause is the formation of surface oxidation of alloy elements such as Mn, Si, Al, Cr, and B that occurs during annealing. I thought it would be one of them. As a result of in-depth research on this, in the case of phosphate treatment, the higher the ratio of the surface oxide covering the surface of the cold-rolled steel sheet, the lower the reactivity between the phosphate solution and the Fe in the holding steel sheet, resulting in lower processing properties. It was confirmed that the more Mn oxide was formed on the surface, the worse it was.

Accordingly, the present inventors discovered that excellent phosphate treatment properties and surface discoloration suppression effect can be obtained by reducing the total amount of oxides of the alloy elements formed on the surface of the cold-rolled steel sheet after annealing, and completed the present invention.

High strength cold rolled steel sheet with excellent surface quality

Hereinafter, a high-strength cold-rolled steel sheet having excellent surface 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, unless otherwise specified, it means weight %. In addition, the ratio of crystals or tissues is based on the area unless otherwise indicated. In addition, the content of gas is based on volume unless otherwise indicated.

The high-strength cold-rolled steel sheet according to the present invention is in wt%, C: 0.05 to 0.3%, Si: 0.1 to 2.0%, Sol.Al: 0.005 to 1.5%, Mn: 1.5 to 8.0%, P: 0.04% or less (0% Excluding), S: 0.015% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), balance Fe and inevitable impurities may be included. Here, the term “high strength” is used to include not only the case of having high strength after annealing but also the case of having high strength by heat treatment in a subsequent process or the like. In addition, in the present invention, high strength may mean more than 490 MPa based on tensile strength, but is not limited thereto.

First, the reason why the component system of the cold-rolled steel sheet is limited as described above will be briefly explained.

Carbon (C): 0.05~0.3%

Carbon (C) is an important element added for stabilizing residual austenite, and for this purpose, it is preferably added in an amount of 0.05% or more. On the other hand, when the C content exceeds 0.3%, a problem of poor weldability may occur. Therefore, the C content in the present invention can be limited to 0.05 ~ 0.3%.

Silicon (Si): 0.1~2.0%

Silicon (Si) is an element that suppresses precipitation of carbides in ferrite and promotes diffusion of carbon in ferrite to austenite, and contributes to stabilization of retained austenite. In order to obtain such an effect, it is necessary to add Si in an amount of 0.1% or more. However, if it is added too much, a problem of lowering the rollability may occur, so the upper limit of the content may be limited to 2.0%.

Solid solution aluminum (Sol.Al): 0.005~1.5%

Aluminum (Al) is an element that contributes to stabilization of retained austenite through suppression of formation of carbides in ferrite, and may be added in an amount of 0.005% or more to obtain such an effect. However, if the content exceeds 1.5%, it is difficult to manufacture a sound slab through reaction with the mold flux during casting, and it may impair the melt-plating property by forming a surface oxide. Therefore, the upper limit of the Al content is set to 1.5% or less. It can be limited.

Manganese (Mn): 1.5~8.0%

Manganese (Mn) is an essential element in transformed structure steel because it exhibits the effect of inhibiting ferrite transformation upon cooling along with the formation and stabilization of retained austenite. In addition, in order to sufficiently secure austenite to secure strength and ductility, Mn may be included in an amount of 1.5% or more. On the other hand, if the content exceeds 8.0%, the formation of bands due to segregation caused by the slab and hot rolling process becomes excessive, which impairs physical properties, so the upper limit of the Mn content may be limited to 8.0% or less.

Phosphorus (P): 0.04% or less (excluding 0%)

Phosphorus (P) is a solid solution strengthening element, but if its content exceeds 0.04%, the weldability decreases and the risk of occurrence of brittleness of the steel increases, so the upper limit thereof can be limited to 0.04%.

Sulfur (S): 0.015% or less (excluding 0%)

Sulfur (S) is an impurity element and is an element that impairs the ductility and weldability of the steel sheet. Therefore, when the S content is increased, the possibility of impairing the ductility and weldability of the steel sheet is increased, and the upper limit thereof is limited to 0.015% in consideration of this.

Chrome (Cr): 1.5% or less (including 0%)

Chromium (Cr) is an element for increasing hardenability and serves to suppress the formation of ferrite. Therefore, in order to secure 5-30% of retained austenite, a small amount can be added as needed. However, if the content is excessive, the amount of ferroalloy is excessive, which may cause an increase in cost, and thus the upper limit of the Cr content may be limited to 1.5% or less.

Boron (B): 0.005% or less (including 0%)

Boron (B) is an element that can be selectively added to secure strength, and when the content of B exceeds 0.005%, it is concentrated on the surface of the annealed material and greatly degrades the surface quality, so the content is preferably 0.005% or less. Do.

In addition to the above-described alloy composition, the holding steel sheet is, by weight, N: 0.02% or less (excluding 0%), Mo: 0.2% or less (including 0%), Ti: 0.2% or less (including 0%), Sb: 0.05 It may further include at least one of% or less (including 0%) and Nb: 0.1% or less (including 0%).

Nitrogen (N): 0.02% or less (excluding 0%)

Nitrogen (N) is a component that plays an effective role in stabilizing austenite, but if its content exceeds 0.02%, there is a high risk of brittleness, and when it is combined with Al, excessive precipitation of AlN may deteriorate the playing quality. Therefore, it is preferable to limit the N content to 0.02% or less in the present invention.

Molybdenum (Mo): 0.2% or less (including 0%)

Molybdenum (Mo) is an element that is optionally added, and when added, it may be added in an amount of 0.2% or less. Mo is effective in securing strength because it has a large effect contributing to strength improvement and does not degrade the wettability of molten metals such as zinc. However, even if it exceeds 0.2%, there is no problem, but since the effect is no longer increased and saturated, the upper limit can be limited to 0.2% or less.

Titanium (Ti): 0.2% or less (including 0%)

Titanium (Ti) is a nitride-forming element and has an effect of reducing the concentration of N in steel, so it can be added in a small amount if necessary. When the Ti content exceeds 0.2%, the carbon concentration and strength of martensite may decrease due to carbide precipitation in addition to the removal effect of solid solution N, and thus the upper limit of the Ti content may be limited to 0.2% or less.

Antimony (Sb): 0.05% or less (including 0%)

Antimony (Sb) is a component that is selectively added to improve surface quality by inhibiting surface oxidation and internal oxidation. When Sb is added, Sb is concentrated in the surface layer of the holding steel sheet, which relatively suppresses surface diffusion of Si, Mn, and Al, and prevents oxygen from penetrating into the steel sheet, which is effective in inhibiting surface oxidation and internal oxidation. However, if it exceeds 0.05%, it is preferable to limit it to 0.05% because the effect of suppressing the surface diffusion of Si, Mn, and Al during annealing decreases.

Niobium (Nb): 0.1% or less (including 0%),

Niobium (Nb) is an element that exhibits an effect of increasing strength by suppressing coarsening of austenite grains during annealing heat treatment by segregating in a carbide form at the austenite grain boundary, and may be selectively added for this effect. However, if the Nb content exceeds 0.1%, an increase in the cost of ferroalloy is caused by an excessive amount of alloy input, so the upper limit may be limited to 0.1%.

In addition to the above-described steel composition, the remainder may include 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 their meaning. In addition, the present invention does not entirely exclude addition of a composition other than the aforementioned steel composition.

In the high-strength cold-rolled steel sheet according to an aspect of the present invention, an internal oxide composed of one or two or more of Mn, Si, Al, Cr, and B is formed in crystal grains within a depth of 3 μm in the thickness direction from the surface, 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 cold-rolled steel sheet before annealing. When the surface of the cold-rolled steel sheet is coated with Fe and then annealing, when alloy elements such as Mn, Si, Al, Cr, and B are diffused to the surface, oxygen in the Fe-coated layer before reaching the final surface of the cold-rolled steel sheet including the Fe coating layer is The internal oxide formed of one single oxide or two or more composite oxides of the alloy elements is formed in the inner area close to the Fe coating layer, that is, the interface between the Fe coating layer and the cold-rolled steel sheet, and inside the Fe coating layer. Will be achieved. The alloy elements changed into oxides as described above cannot be diffused to the surface of the cold-rolled 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 particular, the internal oxides present in the high-strength cold-rolled steel sheet of the present invention are located in crystal grains and have a spherical shape having a maximum 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 oxides. When internally oxidized by oxygen supplied from the outside, internal oxides are mainly formed in a line shape 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, the diameter is less than 0.5㎛, and has a shape of a sphere or a point instead of a linear shape. 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.

In the high-strength cold-rolled steel sheet according to an aspect of the present invention, a carbon-deficient layer may be formed within a depth of 3 μm in the thickness direction from the surface. As described above, when the Fe coating layer is formed on the cold-rolled steel sheet, the carbon content inside the Fe coating layer is less than that of the base material. And this carbon-deficient layer is also observed in cold-rolled steel sheets after final annealing.In this carbon-depletion layer formation, not only the diffusion of carbon in the base material is slowed by the Fe coating during annealing, but also oxygen and carbon contained in the Fe coating layer are combined with CO or carbon. It is presumed to be because it is released as _{CO 2 gas.}

In this way, the thickness of the carbon-deficient layer of the surface layer formed on the surface of the cold-rolled steel sheet by Fe coating is proportional to the thickness of the Fe coating layer, and the carbon in the Fe coating layer can be formed at a level of 1 to 80% compared to the base material, and the depth is increased to the surface. It can be formed within 3㎛ in the thickness direction.

The high-strength cold-rolled steel sheet according to the present invention may have a phosphate coverage of 80% or more, and a phosphate adhesion amount of about 1.9 g/m ² or more when pickling is performed in an acid solution of 5% HCl and 50° C. for 4 seconds and then phosphate treatment. Can be improved to the level.

In addition, the high-strength cold-rolled steel sheet has the effect of reducing the amount of surface oxidation due to the Fe coating before annealing, so when the surface before and after pickling is observed, the color difference value decreases compared to the cold-rolled steel sheet without Fe coating, and the whiteness and glossiness are reduced. You can get an improved effect.

Manufacturing method of high-strength cold-rolled steel sheet with excellent surface quality

Hereinafter, a method of manufacturing a high-strength cold-rolled steel sheet having excellent surface quality according to another 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 cold-rolled steel sheet of the present invention must be manufactured by the following manufacturing method.

A method of manufacturing a high-strength cold-rolled steel sheet according to another aspect of the present invention comprises: preparing a steel slab having the above-described component system; Reheating and hot rolling the steel slab to obtain a hot-rolled steel sheet; Pickling and cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; Forming an Fe coating layer on the surface of the cold-rolled steel sheet, and annealing and cooling the cold-rolled steel sheet on which the Fe coating layer is formed.

First by weight%, C: 0.05~0.3%, Si: 0.1~2.0%, Sol.Al: 0.005~1.5%, Mn: 1.5~8.0%, P: 0.04% or less (excluding 0%), S: 0.015 % Or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), a steel slab containing the balance Fe and inevitable impurities was prepared, and the steel slab was 1100 Reheat at ~1300℃.

In addition, steel slabs are, by weight, N: 0.02% or less (excluding 0%), Mo: 0.2% or less (including 0%), Ti: 0.2% or less (including 0%), Sb: 0.05% or less (0%) Including) and Nb: 0.1% or less (including 0%) of at least one component may be further included.

If the reheating temperature is less than 1100°C, the hot-rolling load may rapidly increase, whereas if the reheating temperature exceeds 1300°C, the reheating cost may increase and the amount of surface scale may increase.

Next, the reheated steel slab is hot-rolled at a temperature of Ar3 or higher (the temperature at which ferrite starts to appear when cooling austenite) to obtain a hot-rolled steel sheet. In the present invention, the finish hot rolling temperature of the reheated slab is limited to Ar3 or higher, which is less than Ar3, two-phase reverse or ferrite reverse rolling of ferrite + austenite is performed to create a mixed grain structure. This is because there is a concern about problems such as shape defects or plate breakage.

After cooling the hot-rolled steel sheet, it is wound in a temperature range of 700℃ or less. At this time, if the coiling temperature exceeds 700°C, the oxide film on the surface of the steel sheet may be excessively generated and cause defects, so the coiling temperature may be limited to 700°C or less.

Then, the wound hot-rolled steel sheet may be pickled and cold-rolled to obtain a cold-rolled steel sheet, and the pickling and cold-rolling conditions may not be particularly limited.

Next, an Fe coating layer is formed on the surface of the cold rolled steel sheet. 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.

If 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, it is not particularly limited in the present invention, but as a non-limiting embodiment, when forming the Fe coating layer by the electroplating method, Fe ²⁺ ions in the Fe electroplating solution are 1 to 80 g/g/ By setting L concentration and current density to 10~100ASD, the oxygen content in the above Fe coating layer can be controlled to 2~30%.

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 cold rolled steel sheet and some uncoated portions may occur.On the other hand, if the thickness exceeds 1μm, the adhesion between the cold rolled steel sheet and the Fe coating layer will be weak. It can be done and a large amount of Fe coating is required, so 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 then cooled. At this time, the annealing conditions are preferably set to an annealing atmosphere, an annealing temperature, and a holding time in which the alloy elements such as Mn, Si, Al, Cr, and B are oxidized while Fe of the steel sheet is not oxidized. As a preferred embodiment, annealing may include maintaining a dew point temperature of 1 to 70%H ₂ controlled at a dew point temperature of -60 to 10°C for 5 to 120 seconds at 600 to 950°C in an annealing furnace with the remaining N _{2 gas atmosphere.} However, it is not limited thereto.

In the present invention, it is necessary to control the atmosphere and temperature during annealing so that the alloy elements can cause oxidation reactions while preventing the oxidation of Fe in the cold-rolled steel sheet. To this end, the dew point temperature can be maintained below 10°C. 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 atmosphere gas during annealing may be set to 1% by volume or more. 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 vol% or less in consideration of economical efficiency.

During 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 because no special technical problem occurs.However, if the holding time is too long, excessive cost increase may occur, so it may be limited to 120 seconds or less in consideration of this. have.

1 is a schematic view showing a cross section of a conventional high-strength cold-rolled steel sheet before (a) and after (b) annealing. Referring to FIG. 1 (a), the alloy components of the cold-rolled steel sheet before annealing are mostly dissolved and present in a matrix. However, when annealing is performed, alloy components having a high oxidation tendency are diffused to the surface by oxygen present in a small amount of oxygen in the annealing furnace at a high temperature and combined with oxygen in the annealing furnace to form surface oxides on the surface of the steel sheet (Fig. 1 (b)). Reference). That is, the total amount of the surface oxide and the shape of the surface oxide formed in this way are determined according to the content of the alloy component of the steel sheet.

In the case of annealing a conventional high-strength cold-rolled steel sheet, the total amount of surface oxide is proportional to the annealing temperature and time because there is no mechanism that can prevent the surface diffusion of alloy elements having a high oxidation tendency. In addition, in the case of a steel to which a large amount of Si is added, the shape of the surface oxide is formed like a band-shaped film when observing the cross section while the shape of the surface oxide is widely covered. The surface oxide in the form of a film causes a problem of lowering the surface reactivity by reducing the area where Fe is exposed on the surface of the holding steel sheet.

On the other hand, when Fe coating is applied to the surface of the cold-rolled steel sheet as shown in FIG. 2 (a), the Fe coating layer acts as a mechanism to prevent surface diffusion of alloy elements having a high oxidation tendency. That is, when the alloying components diffuse to the surface, the alloy is combined with oxygen in the Fe coating layer before reaching the final surface including the Fe coating layer, so that the alloy is in the inner area close to the Fe coating layer, that is, the interface between the Fe coating layer and the cold-rolled steel sheet, and the inside of the Fe coating layer. It forms single or complex internal oxides of one or more of the elements. Since the alloy components changed into oxides as described above cannot be further diffused, the total amount of surface oxides formed on the surface of the final annealed steel sheet is reduced. During the annealing process, the Fe coating layer begins to be reduced from the surface in contact with the atmosphere in the furnace, and the entire Fe coating layer may be reduced when the annealing is completed.

In addition, in the case of a high-strength steel such as the present invention, a large amount of Mn and Si are added as alloying components. In general, since an element with a high oxidation tendency forms internal oxidation by oxygen in the Fe coating layer more quickly, Si having a higher oxidation tendency than Mn is relatively As a result, it is more consumed in the formation of internal oxidation. As a result, the surface oxide formed on the final surface has a smaller Si content than that of the conventional surface oxide, and as a result, the shape of the surface oxide is changed to an island shape rather than a film shape. Since the surface oxide changed to the island shape exposes the Fe of the cold-rolled steel sheet in the meantime, the Fe exposed area on the entire surface is larger than that of the film-type surface oxide. Accordingly, a more reactive surface state can be obtained.

The inner oxide layer formed by the Fe coating may be formed within 3 μm depth from the final annealed surface. In addition, the internal oxide in the internal oxide layer has a spherical shape with a diameter of 0.5 μm or less and is mainly formed in the mouth.

Since the internal oxide layer formed by the Fe coating has a thin thickness, in some cases, it may not be observed in the final product because it is removed by post-treatment such as pickling and phosphate treatment after annealing in some cases. In addition, the surface oxide formed on the surface of the cold-rolled steel sheet may be removed by post-treatment in some cases and may not be observed in the final product.

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, there is no need to specifically limit the cooling conditions in the present invention.

However, as a non-limiting example, it is possible to control the microstructure, strength, and elongation of the steel sheet by performing cooling at an average cooling rate of 5 to 100°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.

If rapid cooling is performed at one time at an annealing temperature, the shape of the steel sheet may be poor. Therefore, as another non-limiting embodiment, 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 step, the step of pickling the steel sheet may be followed. Pickling may be performed by a conventional method, and the conditions may not be particularly limited.

In the high-strength cold-rolled steel sheet manufactured by the above-described steel components and manufacturing method, the Fe coating layer formed on the surface of the steel sheet is completely reduced, and within 3 μm in the depth direction from the surface, one or two or more of Mn, Si, Al, Cr, B Composite internal oxide is formed, and this internal oxide is mainly formed in the form of dots (spherical shape) within 0.5 μm in diameter.

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 steel slab having the composition of Table 1 was prepared, and after heating the steel slab to 1200°C, hot rolling was performed at a temperature of 1050°C. Then, it was wound up at 630°C to air-cool, and the scale was removed through pickling, and cold rolling was performed at a cold rolling reduction ratio of 60% to prepare a cold rolled steel sheet. And, for each steel type, Fe electroplating was performed in the thickness of the conditions shown in Table 2 below to form an Fe coating layer. At this time, after performing the Fe coating, the components of the Fe coating layer were analyzed to confirm that 2 to 30% by weight of oxygen and the remaining Fe were detected, and the thickness was measured and shown together in Table 2. Thereafter, the Fe-coated steels 1 and 2 were heat treated under the annealing conditions shown in Table 2 below.

Steel grade C Si sol.Al Mn P S Cr B Balance River 1 0.12 1.0 0.03 2.2 0.01 0.007 1.0 0.002 Fe River 2 0.15 1.5 0.05 2.3 0.02 0.005 0.5 0.002 Fe

division Steel grade Fe coating
Adhesion amount
(g/m ² ) Fe coating layer thickness
(㎛) Annealing process Annealing furnace dew point temperature
(℃) Annealing temperature
(℃) Holding time
(second) Cooling stop temperature
(℃) Average cooling speed
(℃ sec) Comparative Example 1 River 1 - - -50 850 114 450 25 Invention Example 1 River 1 0.3 0.03 -50 850 114 450 25 Inventive Example 2 River 1 0.5 0.08 -50 850 114 450 25 Invention Example 3 River 1 0.7 0.14 -50 850 114 450 25 Invention Example 4 River 1 1.0 0.24 -50 850 114 450 25 Inventive Example 5 River 1 1.5 0.30 -50 850 114 450 25 Comparative Example 2 River 2 - - -50 850 53 290 13 Invention Example 6 River 2 0.5 0.07 -50 850 53 290 13 Invention Example 7 River 2 0.7 0.13 -50 850 53 290 13 Invention Example 8 River 2 1.0 0.22 -50 850 53 290 13 Inventive Example 9 River 2 1.5 0.28 -50 850 53 290 13 Inventive Example 10 River 2 2.0 0.32 -50 850 53 290 13 Invention Example 11 River 2 2.5 0.42 -50 850 53 290 13 Inventive Example 12 River 2 3.0 0.62 -50 850 53 290 13

The amount of surface oxidation and the depth of internal oxidation were measured for each Inventive Example and Comparative Example for which annealing was completed, and the results are shown in Table 3 below.

The amount of surface concentration was measured using GDS (Glow Discharge Spectrometer) and the maximum weight% value of each element on the surface was used as an index. The presence or absence and the average size were measured. The amount of surface oxidation, the depth of internal oxidation, and the size of internal oxide were all measured on the cold-rolled steel sheet without post-treatment after annealing.

division Steel grade Fe coating layer thickness
(㎛) Si internal oxidation depth
(max. ㎛) Si average internal oxide
Diameter (㎛) Carbon deficient layer Surface concentration
(GDS max.% by weight) Depth (㎛) Deficiency level (%) Mn Si Al Comparative Example 1 River 1 - 0 - - - 13.68 16.64 0.31 Invention Example 1 River 1 0.03 0.06 0.05 0.25 65 13.30 14.54 0.29 Inventive Example 2 River 1 0.08 0.14 0.03 0.48 69 12.99 10.52 0.16 Invention Example 3 River 1 0.14 0.19 0.08 0.68 21 12.33 8.45 0.09 Invention Example 4 River 1 0.24 0.39 0.05 0.72 14 12.05 7.22 0.05 Inventive Example 5 River 1 0.30 0.31 0.05 0.94 15 10.61 6.64 0.03 Comparative Example 2 River 2 - 0 - - - 8.25 13.52 0.75 Invention Example 6 River 2 0.07 0.06 0.06 0.30 52 8.03 12.25 0.64 Invention Example 7 River 2 0.13 0.28 0.11 0.4 43 6.56 9.44 0.55 Invention Example 8 River 2 0.22 0.17 0.08 0.54 37 4.22 6.75 0.14 Inventive Example 9 River 2 0.28 0.36 0.15 0.81 13 4.01 4.84 0.08 Inventive Example 10 River 2 0.32 0.31 0.08 0.93 16 3.87 3.58 0.07 Invention Example 11 River 2 0.42 0.69 0.1 1.1 10 3.84 3.06 0.05 Inventive Example 12 River 2 0.62 0.50 0.12 One 20 3.55 2.98 0.02

It can be seen that the maximum weight% of the surface of Mn, Si, and Al formed on the surface decreases as the amount of Fe adhesion increases in both steel types. In addition, the internal oxidation depth of Si tends to increase according to the amount of Fe adhesion.

The depth of internal oxidation may be different for each component. In general, assuming the same content, elements with a high oxidation tendency tend to be formed relatively deeply, and among Mn, Si, and Al, Al is formed deeply. However, since Al was contained in a small amount in the steel component, the internal oxidation depth of Si, which greatly affects the surface quality, was representatively expressed.

In addition, the internal oxides observed in Inventive Examples 1 to 12 were mainly observed in crystal grains, were spherical, and had an average diameter of 0.15 μm or less, which was very small. In addition, it was confirmed that the maximum diameter of the internal oxide satisfies 0.5 μm or less for each of the inventive examples.

In addition, from the results of Inventive Examples 1 to 5 of Steel 1 and Inventive Examples 6 to 12 of Steel 2, it was confirmed that the depth of the carbon-deficient layer increases as the thickness of the Fe coating layer (Fe adhesion amount) increases, and the deficiency level compared to the amount of carbon in the base material is also It was confirmed that there was a tendency to increase in proportion to the amount of Fe adhesion.

For the example using the steel grade of steel 1 for the evaluation of phosphate treatment according to the reduction of surface oxide, the annealed material was pickled in an acid solution of 5% HCl and 50° C. for 4 seconds, followed by phosphate treatment, and the results are shown in the table. It is shown in 4. Phosphate coverage refers to the ratio of phosphate crystals formed on the entire surface, and the amount of phosphate adhesion is the weight of phosphate crystals per unit area, which can be an index to evaluate phosphate treatability.

division Steel grade Fe coating layer thickness
(㎛) Phosphate treatment Phosphate coverage (%) Phosphate adhesion amount (g/m ² ) Comparative Example 1 River 1 - 62.4 1.4 Invention Example 1 River 1 0.03 81.9 2.1 Inventive Example 2 River 1 0.08 80.3 2.0 Invention Example 3 River 1 0.14 83.3 2.0 Invention Example 4 River 1 0.24 86.9 1.9 Inventive Example 5 River 1 0.30 88.6 2.0

As can be seen from Table 4, when the Fe coating was not performed (Comparative Example 1), the phosphate coverage was 62.4%, and the adhesion amount was 1.4 g/m ^{2, which} was quite hot.

On the other hand, it can be seen that when the Fe coating layer is formed to a thickness of 0.03 μm or more as in Inventive Examples 1 to 5, the phosphate coverage is greatly increased to 80% or more, and the amount of phosphate adhesion is also improved to a level of ^{1.9 g/m 2 or more.}

Meanwhile, in order to evaluate the degree of surface discoloration after annealing of Steel 1, the annealing material was pickled in an acid solution of 5% HCl and 50° C. for 4 seconds to measure the color difference before and after pickling. In addition, the whiteness and glossiness after pickling were measured and shown in Table 5. Color difference and whiteness showed an average value after measuring twice, and glossiness showed an average value after measuring three times.

division Steel grade Fe coating layer thickness
(㎛) Evaluation of the degree of surface discoloration Color difference (△E) Whiteness (L) Glossiness (60°) Comparative Example 1 River 1 - 14.4 65.8 107 Invention Example 1 River 1 0.03 8.8 70.4 133 Inventive Example 2 River 1 0.08 8.7 70.4 134 Invention Example 3 River 1 0.14 8.4 70.7 143 Invention Example 4 River 1 0.24 8.5 70.3 138 Inventive Example 5 River 1 0.30 7.1 71.7 144

In the case of performing Fe coating, it was confirmed that the color difference value before and after pickling decreased by at least 5.6 or more due to the effect of reducing the amount of surface oxidation by the Fe coating before annealing.

In addition, it can be seen that the whiteness increased to 70 or more when the Fe coating was performed in 65.8 when the Fe coating was not carried out (Comparative Example 1). Glossiness may vary depending on the occurrence of irregularities on the surface due to the etching of Mn surface oxide by pickling, which may be one of the causes of discoloration. It was confirmed that the glossiness was also relatively low at 107 in the case of not implementing the Fe coating, but increased to more than 130 when performing the Fe coating.

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 (7)

By weight%, C: 0.05~0.3%, Si: 0.1~2.0%, Sol.Al: 0.005~1.5%, Mn: 1.5~8.0%, P: 0.04% or less (excluding 0%), S: 0.015% Or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), balance Fe and inevitable impurities,
An internal oxide composed of one or two or more of Mn, Si, Al, Cr, and B is formed in the crystal grains within 3㎛ depth in the thickness direction from the surface,
The internal oxide is a cold-rolled steel sheet having a spherical shape of 0.5 μm or less in diameter.
The method of claim 1,
A cold-rolled steel sheet, characterized in that a carbon-deficient layer having a C content of 1 to 80% relative to the C content of the base material of the steel sheet is formed within a depth of 3 μm in the thickness direction from the surface.
The method of claim 1,
In% by weight, N: 0.02% or less (excluding 0%), Mo: 0.2% or less (including 0%), Ti: 0.2% or less (including 0%), Sb: 0.05% or less (including 0%), and Nb: Cold rolled steel sheet, characterized in that it further comprises at least one of 0.1% or less (including 0%).
In% by weight, C: 0.05~0.3%, Si: 0.1%~2.0%, Sol.Al: 0.005~1.5%, Mn: 1.5~8.0%, P: 0.04% or less (excluding 0%), S: 0.015 % Or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), the balance of Fe and preparing a steel slab containing inevitable impurities;
Reheating the steel slab at 1100 to 1300°C;
Hot rolling the reheated steel slab at a temperature of Ar3 or higher (a temperature at which ferrite starts to appear when cooling austenite) to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet in a temperature range of 700° C. or less after cooling;
Pickling and cold rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet;
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; And
Cooling the annealed cold-rolled steel sheet;
A method of manufacturing a cold-rolled steel sheet comprising a.
The method of claim 4,
The step of cooling the annealed cold rolled steel sheet,
A method of manufacturing a cold-rolled steel sheet, characterized in that cooling to a cooling end temperature of 250 to 550°C at an average cooling rate of 5 to 100°C/sec.
The method of claim 4,
The step of cooling the annealed cold-rolled steel sheet is carried out by dividing 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 cold-rolled steel sheet, characterized in that 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 steel slab is, by weight, N: 0.02% or less (excluding 0%), Mo: 0.2% or less (including 0%), Ti: 0.2% or less (including 0%), Sb: 0.05% or less (0%) Including) and Nb: 0.1% or less (including 0%).

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