KR20200076795A - High strength cold rolled steel sheet and galvannealed steel sheet having excellent burring property, and method for manufacturing thereof - Google Patents

Abstract

According to one aspect of the present invention, a high-strength cold rolled steel sheet having excellent burring property comprises: 0.13-0.25 wt% of carbon (C); 1.0-2.0 wt% of silicon (Si); 1.5-3.0 wt% of manganese (Mn); 0.08-1.5 wt% of the sum of aluminum (Al), chromium (Cr), and molybdenum (Mo); 0.1 wt% or less of phosphorus (P); 0.1% wt% or less of sulfur (S); 0.01 wt% or less of nitrogen (N); and the balance Fe and inevitable impurities. On a cross-sectional surface of a t/4 point etched with a nital solution, the fraction of the embossed hard tissue is 40 area% or more, wherein t means the thickness of the steel sheet. In the hard tissues, a fraction of the acicular hard tissue having an aspect ratio (length in a long axis direction/length in a short axis direction) of 2 or more may be 70 area% or more.

Description

High strength cold rolled steel sheet and galvannealed steel sheet having excellent burring property, and method for manufacturing thereof

The present invention relates to a cold-rolled steel sheet and an alloyed hot-dip galvanized steel sheet and a method of manufacturing the same, and more particularly, to a cold-rolled steel sheet and an alloyed hot-dip galvanized steel sheet having a high-strength property and effectively improving burring properties. .

Automotive steel sheet is increasing the adoption of high-strength steel to secure passenger stability in case of accidents such as fuel consumption regulations and collisions to preserve the global environment. The grade of automotive steel is usually represented by the product of tensile strength and elongation (TS×EL), and is not limited to this, but is not limited to this, AHSS (Advanced High Strength Steel) with TS×EL less than 25,000 MPa·%, 50,000 MPa· UHSS (Ultra High Strength Steel) exceeding %, and X-AHSS (Extra-Advanced High Strength Steel) having a value between AHSS and UHSS may be presented as representative examples.

When the grade of the steel is determined, it is not easy to satisfy the tensile strength and elongation of the steel at the same time because the product of the tensile strength and the elongation is determined to be approximately constant. This is because the tensile strength and elongation are inversely proportional to each other.

As a steel material with a new concept to increase the product of the strength and elongation of the steel material, a steel material using a so-called TRIP (transformation induced plasticity) phenomenon capable of improving both workability and strength due to the presence of retained austenite in the steel material has been developed. TRIP steel has been mainly used to manufacture high-strength high-strength steel materials with improved elongation at the same strength.

Although such a conventional steel material can secure a tensile strength or elongation at a certain level, there are technical limitations in securing an elongation at a certain level or higher in a high-strength steel having a tensile strength of 1180 MPa or higher. This is because, as described above, it is a general property of steel materials that the tensile strength and elongation are inversely proportional to each other. Particularly, in the case of high-strength steel having a tensile strength of 1180 MPa or more, there is a problem in that it is not possible to secure a workability suitable for a steel for automobiles because it cannot secure a certain level of burring.

Burring property was widely used as a property for evaluating the hole expansion workability of steel materials, but recently, burring property is not necessarily interpreted as being limited to only a property for evaluating hole expansion workability of steel materials. That is, since it is difficult to prevent the damage of the steel material if the burring property is not sufficiently secured in the steel material subjected to severe processing, the burring property can be used as an index for confirming the break resistance of the steel material under extreme processing conditions. That is, in the case of automobile steel materials processed under extreme conditions such as cold press processing, not only high strength properties but also excellent burring properties are required to prevent damage to steel materials by processing.

Japanese Patent Application Publication No. 2014-019905 (2014.02.03. published)

According to one aspect of the present invention, a high-strength cold rolled steel sheet having excellent burring properties and an alloyed hot-dip galvanized steel sheet and a method of manufacturing the same can be provided.

The subject of this invention is not limited to the above-mentioned content. Those skilled in the art will have no difficulty in understanding the additional subject matter of the present invention from the general contents of this specification.

High-strength cold-rolled steel sheet excellent in burring property according to an aspect of the present invention, by weight, carbon (C): 0.13 to 0.25%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.5 to 3.0%, Aluminum (Al) + Chromium (Cr) + Molybdenum (Mo): 0.08 to 1.5%, Phosphorus (P): 0.1% or less, Sulfur (S): 0.01% or less, Nitrogen (N): 0.01% or less, the rest of Fe and The fraction of hard tissue that is embossed on the cross-sectional surface at the point t/4 where it contains unavoidable impurities and is etched with a Nital solution (where t denotes the thickness of the steel sheet) is 40% by area or more, Among the hard tissues, the proportion of acicular hard tissues having an aspect ratio (long axis direction/short axis length) of 2 or more may be 70 area% or more.

The hard tissue may be one of lath martensite, tempered martensite, upper bainite, lower bainite, retained austenite and carbide, or Two or more types may be included.

The cold rolled steel sheet may include less than 15 area% of ferrite.

The cold rolled steel sheet may further include at least one of boron (B): 0.001 to 0.005% and titanium (Ti): 0.005 to 0.04% by weight.

The aluminum (Al) may be included in the cold rolled steel sheet in an amount of 0.01 to 0.09% by weight.

The chromium (Cr) may be included in the cold rolled steel sheet in an amount of 0.01 to 0.7% by weight.

The chromium (Cr) may be included in the cold rolled steel sheet in an amount of 0.2 to 0.6% by weight.

The molybdenum (Mo) may be included in the cold rolled steel sheet in an amount of 0.02 to 0.08% by weight.

The cold rolled steel sheet may have a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a hole expansion ratio (HER) of 25% or more.

The alloyed hot-dip galvanized steel sheet according to one aspect of the present invention includes a steel sheet and an alloyed hot-dip galvanized layer formed on the surface of the steel sheet, and the steel sheet may be the cold rolled steel sheet.

High-strength cold-rolled steel sheet excellent in burring property according to an aspect of the present invention, by weight, carbon (C): 0.13 to 0.25%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.5 to 3.0%, Aluminum (Al) + Chromium (Cr) + Molybdenum (Mo): 0.08 to 1.5%, Phosphorus (P): 0.1% or less, Sulfur (S): 0.01% or less, Nitrogen (N): 0.01% or less, the rest of Fe and Cold rolling a steel material containing unavoidable impurities, heating the cold rolled steel to a temperature range of Ac ₃ or higher, and slowly cooling the heated steel at a cooling rate of 2 to 10°C/s to a temperature range of 580 to 620°C. And rapidly cooling the slow-cooled steel material at a cooling rate of 7 to 30°C/s to a temperature range of (Ms-120°C) to Ms, and exceeding Ms, and a temperature range of (Ms+120°C) or less It can be prepared by the distribution process to maintain for 300 to 600 seconds.

The cold-rolled steel is heated to a temperature range of Ac ₃ or higher by the first heating and the second heating, the heating rate of the first heating is 20 to 30°C/s, and the heating rate of the second heating is 2 to 6°C/s.

The heating stop temperature of the first heating is (Ac ₃ -120°C) to (Ac ₃ -50°C), and the second heating may be started at the heating stop temperature of the first heating and stopped at Ac ₃ or higher. .

The steel material may further include one or more of boron (B): 0.001 to 0.005% and titanium (Ti): 0.005 to 0.04% by weight.

The aluminum (Al) may be included in the steel material in an amount of 0.01 to 0.09% by weight.

The chromium (Cr) may be included in the steel material in an amount of 0.01 to 0.7% by weight.

The chromium (Cr) may be included in the steel material in an amount of 0.2 to 0.6% by weight.

The molybdenum (Mo) may be included in the steel material in an amount of 0.02 to 0.08% by weight.

High-strength alloyed hot-dip galvanized steel sheet having excellent burring properties according to an aspect of the present invention can be produced by forming a hot-dip galvanized layer on the surface of the cold-rolled steel sheet and alloying it.

The solving means of the above problem does not list all the features of the present invention, and various features of the present invention and the advantages and effects thereof may be understood in more detail with reference to specific embodiments below.

According to one aspect of the present invention, it is possible to provide a cold-rolled steel sheet and an alloyed hot-dip galvanized steel sheet which are particularly suitable for automobile steel sheets, while having high strength properties and excellent elongation properties and burring properties.

1 is a graph schematically showing a manufacturing process of the present invention using a change in temperature over time.
2 and 3 are photographs obtained by observing the microstructures of Inventive Example 1 and Comparative Example 1 with a scanning electron microscope, respectively.

The present invention relates to a cold rolled steel sheet and an alloyed hot-dip galvanized steel sheet having excellent burring properties, and a method of manufacturing the same, hereinafter, to describe preferred embodiments of the present invention. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to those skilled in the art to further detail the present invention.

Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise indicated, below, unless otherwise indicated, the percentage representing the content of each element is based on weight.

Cold rolled steel sheet in one aspect of the present invention, by weight, carbon (C): 0.13 to 0.25%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.5 to 3.0%, aluminum (Al) + chrome (Cr) + Molybdenum (Mo): 0.08~1.5%, Phosphorus (P): 0.1% or less, Sulfur (S): 0.01% or less, Nitrogen (N): 0.01% or less, remaining Fe and unavoidable impurities have. In addition, the cold-rolled steel sheet according to an aspect of the present invention may further include one or more of boron (B): 0.001 to 0.005% and titanium (Ti): 0.005 to 0.04% by weight. The aluminum (Al), chromium (Cr), and molybdenum (Mo) may be included in a content of 0.01% to 0.09%, 0.2 to 0.7%, and 0.02 to 0.08% by weight, respectively.

Carbon(C) 0.13~0.25%

Since carbon (C) is an important element that can secure strength economically, the present invention can limit the lower limit of the carbon (C) content to 0.13% to achieve this effect. However, when carbon (C) is excessively added, a problem of deterioration in weldability may occur, and thus, the present invention may limit the upper limit of the carbon (C) content to 0.25%. Therefore, the carbon (C) content of the present invention may range from 0.15 to 0.25%. The preferred carbon (C) content may range from 0.14 to 0.25%, and the more preferred carbon (C) content may range from 0.14 to 0.20%.

Silicon (Si): 1.0-2.0%

Since silicon (Si) is an element that can effectively improve the strength and elongation of steel, the present invention can limit the lower limit of the silicon (Si) content to 1.0% to achieve this effect. Since silicon (Si) not only causes surface scale defects, but also lowers the surface properties of the plated steel sheet and degrades chemical conversion, the content of silicon (Si) is usually limited to a range of 1.0% or less. Due to the development of plating technology and the like, it is possible to manufacture up to about 2.0% of the content in steel without any major problems, so the present invention can limit the upper limit of the silicon (Si) content to 2.0%. Therefore, the silicon (Si) content of the present invention may be in the range of 1.0 to 2.0%. The preferred silicon (Si) content may range from 1.2 to 2.0%, and the more preferred silicon (Si) content may range from 1.2 to 1.8%.

Manganese (Mn): 1.5-3.0%

Manganese (Mn) is an element that can play a large role in solid solution strengthening when present in a steel material, and is an element contributing to improvement of hardenability in metamorphic reinforced steel, so the present invention limits the lower limit of the manganese (Mn) content to 1.5%. Can. However, when manganese (Mn) is added excessively, there is a high possibility of problems such as weldability and cold-rolled load, and surface defects such as dent may be caused by the formation of annealed concentrate. The upper limit of the (Mn) content can be limited to 3.0%. Therefore, the manganese (Mn) content of the present invention may range from 1.5 to 3.0%. The preferred manganese (Mn) content may range from 2.0 to 3.0%, and the more preferred manganese (Mn) content may range from 2.2 to 2.9%.

The sum of aluminum (Al), chromium (Cr) and molybdenum (Mo): 0.08 to 1.5%

Aluminum (Al), chromium (Cr), and molybdenum (Mo) are elements that are useful to increase the strength and to secure the ferrite fraction as a ferrite backbone expansion element, so the present invention is the content of aluminum (Al), chromium (Cr), and molybdenum (Mo) The sum can be limited to 0.08% or more. However, when aluminum (Al), chromium (Cr), and molybdenum (Mo) are added excessively, the surface quality of the slab is lowered and the production cost is increased, so the present invention provides aluminum (Al), chromium (Cr), and The sum of the molybdenum (Mo) content may be limited to 1.5% or less. Therefore, the sum of the aluminum (Al), chromium (Cr), and molybdenum (Mo) contents of the present invention may range from 0.08 to 1.5%.

Aluminum (Al): 0.01~0.09%

Aluminum (Al) is an important element in improving the martensite hardenability by distributing carbon (C) in ferrite to austenite, such as silicon (Si), in combination with oxygen (O) in steel. In order to achieve such an effect, the lower limit of the aluminum (Al) content may be limited to 0.01%. However, when aluminum (Al) is added excessively, there is a possibility that nozzle clogging may occur during performance, and deterioration of burring properties may occur due to increased strength, so the present invention sets the upper limit of the aluminum (Al) content to 0.09%. Can be limited. Therefore, the aluminum (Al) content of the present invention may range from 0.01 to 0.09%. The preferred aluminum (Al) content may range from 0.02 to 0.09%, and the more preferred aluminum (Al) content may range from 0.02 to 0.08%. In the present invention, aluminum (Al) means acid-soluble Al (sol. Al).

Chromium (Cr): 0.01~0.7%

Since chromium (Cr) is an effective hardenability enhancing element, the present invention can limit the lower limit of the chromium (Cr) content to 0.01% in order to achieve the effect of improving strength. However, when the chromium (Cr) is excessively added, the oxidation of silicon (Si) is promoted to increase the red scale defect on the surface of the hot rolled material, and the surface quality of the final steel material is deteriorated. The upper limit of the content can be limited to 0.7%. Therefore, the chromium (Cr) content of the present invention may range from 0.01 to 0.7%. The preferred chromium (Cr) content may be in the range of 0.1 to 0.7%, and the more preferred chromium (Cr) content may be in the range of 0.2 to 0.6%.

Molybdenum (Mo): 0.02~0.08%

Molybdenum (Mo) is also an element that effectively contributes to the improvement of hardenability, so the present invention can limit the lower limit of the molybdenum (Mo) content to 0.02% in order to achieve the effect of improving strength. However, as molybdenum (Mo) is an expensive element, excessive addition is not preferable in terms of economy, and when molybdenum (Mo) is added excessively, the strength increases excessively, resulting in a problem of deterioration in burring property. The upper limit of the molybdenum (Mo) content may be limited to 0.08%. The preferred molybdenum (Mo) content may be in the range of 0.03 to 0.08%, and the more preferable molybdenum (Mo) content may be in the range of 0.03 to 0.07%.

Phosphorus (P): 0.1% or less

Phosphorus (P) is an element that is advantageous for securing strength without impairing the formability of steel, but when added excessively, the possibility of brittle fracture is greatly increased, which increases the likelihood of slab plate fracture during hot rolling and improves the surface properties of the plating. It can also act as an inhibitory element. Therefore, the present invention may limit the upper limit of the phosphorus (P) content to 0.1%, and the upper limit of the more preferable phosphorus (P) content may be 0.05%. However, considering the level inevitably added, 0% may be excluded.

Sulfur (S): 0.01% or less

Sulfur (S) is an element that is inevitably added as an impurity element in the steel, so it is desirable to manage its content as low as possible. In particular, sulfur (S) is an element that inhibits the ductility and weldability of the steel, and it is preferable to suppress the content as much as possible in the present invention. Therefore, the present invention may limit the upper limit of the sulfur (S) content to 0.01%, and the more preferable upper limit of the sulfur (S) content may be 0.005%. However, considering the level inevitably added, 0% may be excluded.

Nitrogen (N): 0.01% or less

Nitrogen (N) is an element that is inevitably added as an impurity element. It is important to manage nitrogen (N) as low as possible, but for this, there is a problem that the refining cost of steel rises rapidly. Therefore, the present invention can control the upper limit of the nitrogen (N) content in consideration of the possible range in the operating conditions to 0.01%, the upper limit of the more preferred nitrogen (N) content may be 0.005%. However, considering the level inevitably added, 0% may be excluded.

Boron (B): 0.001~0.005%

Boron (B) is an element that effectively contributes to the improvement of strength by solid solution, and is an effective element that can secure such an effect even by adding a small amount. Therefore, the present invention can limit the lower limit of the boron (B) content to 0.001% to achieve this effect. However, when the boron (B) is excessively added, the strength improvement effect is saturated, while the excessive boron (B) thickening layer may be formed on the surface, resulting in deterioration of the plating adhesion, and thus the present invention is boron (B) content The upper limit of can be limited to 0.005%. Therefore, the boron (B) content of the present invention may range from 0.001 to 0.005%. The preferred boron (B) content may be in the range of 0.001 to 0.004%, and the more preferred boron content may be in the range of 0.0013 to 0.0035%.

Titanium (Ti): 0.005~0.04%

Titanium (Ti) is an element effective for increasing the strength of steel and refining the particle size. In addition, titanium (Ti) is combined with nitrogen (N) to form a TiN precipitate, so boron (B) is combined with nitrogen (N) to effectively prevent the loss of the effect of adding boron (B). . Therefore, the present invention can limit the lower limit of the titanium (Ti) content to 0.005%. However, if titanium (Ti) is excessively added, it may cause nozzle clogging during performance or deteriorate the ductility of steel due to excessive precipitation, so the present invention limits the upper limit of the titanium (Ti) content to 0.04%. Can. Therefore, the titanium (Ti) content of the present invention may range from 0.005 to 0.04%. The preferred titanium (Ti) content may be in the range of 0.01 to 0.04%, and the more preferable titanium (Ti) content may be in the range of 0.01 to 0.03%.

The cold rolled steel sheet of the present invention may contain Fe and inevitable impurities other than the above-described steel composition. The unavoidable impurities can be unintentionally incorporated in the ordinary steel manufacturing process, and cannot be completely excluded, and the meaning can be easily understood by those skilled in the ordinary steel manufacturing field. In addition, this invention does not exclude the addition of the composition other than the steel composition mentioned above entirely.

Hereinafter, the microstructure of the present invention will be described in more detail. Hereinafter, unless otherwise indicated,% representing the proportion of the microstructure is based on the area.

The inventors of the present invention, while simultaneously securing the strength and elongation of the steel material, and also reviewing the conditions for having both the burring properties, as a result of controlling the composition and the type and fraction of the steel material appropriately to control the strength and elongation to an appropriate range, It has been confirmed that high burring properties cannot be obtained without appropriately controlling the shape of the tissue present in the steel, and the present invention has been reached.

The present invention includes ferrite, bainite, retained austenite, and martensite, but since it properly controls the fraction and shape of hard tissue, it is a TRIP steel material that effectively secures elongation and burring properties while securing high strength properties of the steel. do.

In general, TRIP steels can include ferrite, martensite and residual austenite. Ferrite may be included in a predetermined range in the steel material to ensure the elongation of the steel material, martensite may be included in a predetermined range in the steel material to ensure high strength. In addition, the retained austenite may be included in a predetermined range in the steel for transformation to martensite during the processing process, and the retained austenite may contribute to improving the workability of the steel through this transformation process.

The present invention is particularly intended to provide a TRIP steel material having a tensile strength of 1180 MPa or more, while simultaneously securing excellent elongation and burring properties. Therefore, the present invention may include a hard tissue of a certain level or more to secure the strength of the steel, and the proportion of the acicular hard tissue occupied by the needle-shaped hard tissue among the hard tissues formed in the steel to improve the burring property by suppressing crack propagation Can be controlled.

The hard tissue of the present invention can be defined as follows. When a surface of a steel material is etched for a period of time using a nitral solution containing nitric acid (HNO ₃ ), the degree of etching due to reaction with the nitral solution may be different depending on each tissue. . That is, when etching with a nitral solution, the soft tissue is etched in a relatively large amount compared to the hard tissue, so that the soft and hard tissues are respectively engraved and embossed on the surface of the etched steel. Therefore, the present invention is used to etch the surface of the steel material for 3 to 10 seconds using a Nital solution containing 3 to 5% nitric acid (HNO ₃ ), and then embossed on the surface of the etched steel material. The structure of the part to be defined is defined as a hard structure, and the structure of the part that is intaglio on the surface of the etched steel is defined as a soft structure. In the present invention, the microstructure is measured based on the cross section of the t/4 point. t means the thickness (mm) of the steel material of the steel material.

The hard tissue of the present invention is one of lath martensite, tempered martensite, upper bainite, lower bainite, residual austenite and carbide. Species or two or more species. Since the present invention is intended to provide a high-strength steel having a tensile strength of 1180 MPa or more, it is preferable that the hard structure of the present invention is included in the steel at a fraction of 40 area% or more relative to the area of the etched surface.

In addition, among the hard tissues of the present invention, acicular hard tissues having an aspect ratio (aspect ratio, long axis/short axis length) of 2 or more are 70 area% or more compared to the total area of the hard tissue that is embossed on the etched surface. It is preferably included in the steel material in a fraction. Needle-like hard tissue effectively suppresses the propagation of cracks by limiting the path of movement of cracks generated during the processing of the steel, and thus, as the proportion of the needle-like hard tissue among the total hard tissue increases, the tendency of the steel to increase is increased. Therefore, the present invention can limit the proportion of acicular hard tissues among all hard tissues to 70 area% or more in order to secure the desired burring property.

In addition, in order to secure the elongation in a conventional TRIP steel, it is common to control the proportion of the soft structure ferrite to 15 area% or more. However, the present invention is intended to achieve a tensile strength of 1180 MPa or more, so ferrite of 15 area% or more is not preferable in terms of securing strength. Therefore, the present invention limits the ratio of ferrite to less than 15 area%, and thus can effectively implement a tensile strength of 1180 MPa or more. The present invention does not specifically limit the lower limit of the ferrite ratio in terms of securing the tensile strength and yield strength, but from the viewpoint of not entirely excluding the ferrite formed inevitably, the lower limit of the preferred ferrite fraction may be 3 area%.

In addition, the present invention may include a hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on the above-described cold-rolled steel sheet, and may include an alloyed hot-dip galvanized steel sheet. The hot-dip galvanized layer may be provided with a composition commonly used to secure corrosion resistance, and may include additional elements such as aluminum (Al) and magnesium (Mg) in addition to zinc (Zn).

The cold rolled steel sheet and alloyed hot-dip galvanized steel sheet of the present invention satisfying these conditions may satisfy a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a hole expansion ratio (HER) of 25% or more. In terms of processability pictorial, a more preferable hole expansion device (HER) may be 30% or more.

Hereinafter, the manufacturing method of the present invention will be described in more detail.

After cold-rolling the steel of the above-described composition, the steel is heated so that the cold-rolled steel is completely transformed into austenite, and the heated steel is cooled to 2-10°C/s to a temperature range of 580-620°C. Slowly cooling at a speed, and rapidly cooling the slow-frozen steel at a cooling rate of 7 to 30°C to a temperature range of (Ms-120°C), exceeding Ms, and over 300 at a temperature range of (Ms+120°C) It can be kept for ~600 seconds for distribution. Process conditions after cold rolling using temperature changes over time are depicted in FIG. 1.

The steel material provided for the cold rolling of the present invention may be a hot rolled material, and such a hot rolled material may be a hot rolled material used for manufacturing conventional TRIP steel. The method of manufacturing the hot rolled material provided for the cold rolling of the present invention is not particularly limited, but the slab having the above-described composition is reheated at a temperature range of 1000 to 1300°C, and hot rolled at a finish rolling temperature range of 800 to 950°C. It can be produced by rolling and winding in a temperature range of 750°C or less. The cold rolling of the present invention can also be carried out under process conditions carried out in the manufacture of conventional TRIP steel. Cold rolling may be performed at an appropriate rolling reduction rate in order to secure the required thickness of the customer, but it is preferable to perform cold rolling at a cold rolling reduction of 30% or more in order to suppress generation of coarse ferrite in a subsequent annealing process.

Hereinafter, the process conditions of the present invention will be described in more detail.

After cold rolling, steel is heated to the austenite area

In order to transform all of the structure of the cold rolled steel into austenite, the steel is heated to the austenite temperature range (full austenite area). In the case of TRIP steels containing ferrite at a certain level, the steel material is often heated to a so-called abnormal temperature range in which austenite and ferrite coexist, but when heated in this way, the ferrite uniform distribution and particle size refinement effect intended in the present invention Not only is it very difficult to obtain, but the band structure formed in the hot rolling process remains as it is, which is disadvantageous in improving the burring property. Therefore, in the present invention, the cold rolled steel is heated to an austenite region of Ac ₃ or higher.

In the present invention, heating of the cold-rolled steel material may be performed in two stages of heating conditions including first heating and second heating. That is, the cold rolled steel is first heated to a temperature range of (Ac ₃ -120°C) to (Ac ₃ -50°C) at a heating rate of 20 to 30°C/s, and the first heated steel is 2 to 6 The second heating may be performed up to a temperature range of Ac ₃ or higher at a heating rate of ℃/s. In general, when heating a steel, it is common to apply a constant heating rate up to a target heating temperature. The amount of oxide formation increases as the heating time of the steel material increases, and thus the surface quality of the final steel material may be poorly implemented. Further, as the heating time of the steel material increases, the crystal grains of the initial austenite become coarse, so that the needle-like structure desired by the present invention cannot be sufficiently formed in the final steel material.

Therefore, the present invention is a relatively fast heating temperature of 20 ~ 30 ℃ / s (Ac ₃ -120 ℃) ~ (Ac ₃ -50 ℃) to the temperature range of cold-rolled steel first heating, the initial austenite structure It is possible to prevent the coarsening of and at the same time actively suppress the oxide formation. The present invention can be limited to the lower limit of the heating rate of the first heating to 20 ℃ / s to achieve a reduction effect of the heating time. However, excessive heat source supply is not desirable from the economical point of view, and may cause excessive oxide formation, so that the present invention can limit the heating rate of the first heating to 30°C/s or less. In addition, the present invention can limit the lower limit of the heating stop temperature of the first heating to (Ac ₃ -120°C) in order to achieve a reduction effect of the heating time. However, when the heating stop temperature of the first heating is excessively high, a material deviation between the steel surface side and the center side may be induced. In the present invention, the heating stop temperature of the first heating is (Ac ₃ -50°C) or less. Can be limited to range. In FIG. 1, x and y may mean 50°C and 120°C, respectively.

Further, in the present invention, the first heated steel material may be heated to a temperature range of Ac ₃ or higher at a heating rate of 2 to 6° C./s from a heating stop temperature of the first heating. The second heating is a heating process in consideration of reduction of material deviations on the surface side and the center side of the steel while preventing coarsening of the initial austenite and excessive formation of oxides when heating the cold rolled steel. When the heating rate of the second heating is less than a certain level, the heating time increases, so the second heating of the present invention may be performed at a heating rate of 2°C/s or more. In addition, when the heating rate of the second heating is excessive, it does not sufficiently provide time to alleviate the material deviation between the steel surface side and the center side, but may cause excessive oxide generation. In the present invention, the heating rate of the second heating is increased. It can be limited to 6°C/s or less. The steel material heated to a temperature of Ac ₃ or higher may be slowly cooled after being maintained at a corresponding temperature for a certain time.

Slowly cooling the heated steel to the region of 580~620℃

The present invention is intended to limit the fraction of ferrite to 15 area% or less in order to secure the strength of the final steel of ferrite, so that cooling of the heated steel to a slow cooling stop temperature of 620°C or less, which can avoid the nose of ferrite on the cooling curve, is performed. You can. In addition, since the present invention does not intend to form pearlite, the lower limit of the slow cooling stop temperature may be limited to 580°C in order to prevent the formation of pearlite. In order to suppress the formation of coarse ferrite, the lower limit of the cooling rate may be limited to 2°C/s, and the upper limit of the cooling rate may be limited to 10°C/s for uniformity and refinement of the final tissue. Therefore, the slow cooling of the present invention can cool the heated steel to a temperature range of 580 to 620°C at a cooling rate of 2 to 10°C/s.

Quenching slow-cooled steel to (Ms-120℃)~Ms

In order to obtain a hard tissue in a ratio intended in the present invention, a procedure for rapidly cooling the annealed steel to a temperature range of (Ms-120°C) to Ms may be subsequently performed. Here, Ms denotes the martensite transformation start temperature. Since the present invention intends to form a certain percentage of martensite in the final steel, the upper limit of the quenching end temperature can be limited to Ms. However, when the quenching end temperature is excessively low, martensite is excessively formed, so that a desired ratio of residual austenite cannot be formed, and thus the present invention limits the lower limit of the quenching end temperature to the range of (Ms-120°C). can do. Therefore, ferrite, martensite, and retained austenite may be included in the steel of the present invention as a microstructure, and some bainite may be included in the microstructure depending on cooling conditions. The preferred quenching cooling rate for forming the desired tissue of the present invention may be in the range of 7-30° C./s, and quenching may be used as one preferred means.

Partitioning of quenched steel

In the quenched structure, a large amount of carbon is contained in the martensite because austenite, which contains a large amount of carbon, is a non-diffusion transformation. In this case, the hardness of the tissue may be increased, but on the contrary, a problem that the toughness deteriorates rapidly may occur. In the normal case, a method of additionally tempering the steel sheet produced at a high temperature is used to allow carbon to precipitate as a carbide in martensite, but in the present invention, a method other than additional tempering is used to control the tissue with a unique texture. use.

In the present invention, the quenched steel material can be subjected to distribution treatment by maintaining the steel in a temperature range of greater than Ms and below (Ms+120°C) for 300 to 600 seconds. Here, Ms denotes the martensite transformation start temperature. The carbon present in the martensite may be partitioned into residual austenite due to the difference in high capacity through the distribution treatment in the corresponding temperature range. Since the stability of the retained austenite increases when the amount of carbon employed in the retained austenite increases, it is possible to effectively secure the retained austenite in a desired ratio.

In addition, the present invention can be effectively formed a lower bainite (lower bainite) to secure the fraction of the bed hard tissue, since the quenched steel is maintained at a temperature range above (Ms+120°C) below Ms and distributed. , Accordingly, it is possible to effectively form the microstructure intended in the present invention.

In order to obtain a sufficient distribution effect, the above-described holding time may be 300 seconds or more. However, when the holding time exceeds 600 seconds, it is difficult to expect an increase in the effect any more, and productivity may be lowered. Therefore, in one aspect of the present invention, the upper limit of the holding time described above may be limited to 600 seconds. .

The cold rolled steel sheet subjected to the above-described treatment can be plated by a known method. The plating treatment of the present invention may be any kind of plating treatment including zinc-based plating treatment, aluminum-based plating treatment, alloy plating treatment, alloying plating treatment, and the like.

The cold-rolled steel sheet manufactured by the above-described manufacturing method has a proportion of hard tissue present in an embossed surface on the surface etched with a Nital solution of 40 area% or more, and an aspect ratio among the hard tissue. The fraction of acicular hard tissue having a direction length/short axis length of 2 or more may be 70 area% or more.

In addition, the cold rolled steel sheet manufactured by the above manufacturing method may satisfy a tensile strength of 1180 MPa, an elongation of 14% or more, and a hole expansion ratio (HER) of 25% or more.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only intended to exemplify the present invention and are not intended to limit the scope of the present invention.

(Example)

Cold rolled steel sheets were prepared by treating the steel materials having the composition shown in Table 1 below under the conditions shown in Table 2. In Table 2, quenching was performed by spraying mist on the surface of the cold rolled steel sheet or by spraying nitrogen gas or nitrogen-hydrogen mixed gas. Comparative Example 1 is a case where the holding time after quenching is shorter than the conditions of the present invention, and Comparative Example 3 is a case where the second heating stop temperature is lower than the conditions of the present invention. Comparative Example 4 is a case where the slow cooling stop temperature is higher than the range of the present invention, and Comparative Example 5 is a case where the rapid cooling stop temperature is lower than the range of the present invention. Comparative Example 6 is a case where the heating rate of the first heating is lower than the range of the present invention. After quenching, the holding temperature satisfies the range of Ms in excess of (Ms+120°C) in all inventive and comparative examples.

division Steel composition (wt%) C Si Mn P S Al N Cr Mo Ti B Inventive Example 1 0.23 1.8 2.4 0.02 0.003 0.03 0.006 0.3 0.01 0.02 0.002 Inventive Example 2 0.2 1.7 2.6 0.006 0.005 0.21 0.004 0.01 0.03 0.02 0.002 Inventive Example 3 0.16 1.1 2.8 0.011 0.006 0.047 0.005 0.03 0.02 0.02 0.002 Comparative Example 1 0.22 1.2 2.5 0.008 0.005 0.39 0.006 0.05 0.05 0.02 0.002 Comparative Example 2 0.27 0.1 1.1 0.015 0.008 0.043 0.005 0.002 0.01 0.02 0.002 Comparative Example 3 0.16 1.4 2.2 0.01 0.005 0.03 0.006 0.008 0 0.02 0.002 Comparative Example 4 0.23 1.8 2.4 0.02 0.003 0.03 0.006 0.3 0.01 0.02 0.002 Comparative Example 5 0.23 1.8 2.4 0.02 0.003 0.03 0.006 0.3 0.01 0.02 0.002 Comparative Example 6 0.23 1.8 2.4 0.02 0.003 0.03 0.006 0.3 0.01 0.02 0.002

division The first
heating
Elevated temperature
speed
(℃/s) The first
heating
stop
Temperature
(℃) 2nd
heating
Elevated temperature
speed
(℃/s) 2nd
heating
stop
Temperature
(℃) After heating
maintain
time
(second) Slow cooling
stop
Temperature
(℃) Slow cooling
time
(second) Quenching
stop
Temperature
(℃) After quenching
maintain
Temperature
(℃) After quenching
maintain
time
(second) Plated
practice
Whether Inventive Example 1 25 750 3 870 60 600 40 300 400 500 practice Inventive Example 2 25 750 3 870 60 600 40 300 400 500 practice Inventive Example 3 25 750 3 850 60 600 40 300 400 500 practice Comparative Example 1 25 750 3 870 60 600 40 300 400 100 practice Comparative Example 2 25 750 3 870 60 600 40 300 400 500 Not implemented Comparative Example 3 25 750 3 810 60 600 40 300 400 500 practice Comparative Example 4 25 750 3 870 60 670 40 300 400 500 practice Comparative Example 5 25 750 3 870 60 600 40 100 400 500 practice Comparative Example 6 3 750 3 870 60 600 40 300 400 500 practice

Table 3 below shows the results of evaluating the microstructure and physical properties of the cold-rolled steel sheet prepared by the above-described process. The microstructure was etched and etched on the surface of each specimen using a 2% nitrogen solution, and then observed and evaluated using a scanning electron microscope (SEM). In Table 3, the ratio of hard tissue refers to the area fraction of hard tissue that is embossed on the etched surface. It refers to the area fraction of acicular hard tissue having a length in the minor axis direction of 2 or more. In addition, the ferrite fraction and residual austenite fraction of Table 3 refer to the area fraction of the corresponding tissue in the entire observation area. The strength and elongation were measured and evaluated through tensile tests. The evaluation of the plating property was performed only for the plated steel, and it was judged based on whether an unplated area exists on the surface of the steel (X) or not (O).

division reshuffle
group
Fraction
(area%) couch
reshuffle
group
Fraction
(area%) Ferrara
Yt
Fraction
(area%) Residual
Auste
Knight fraction
(area%) surrender
burglar
(MPa) Seal
burglar
(MPa) Elongation
(%) HER
(%) Plating property Inventive Example 1 47 88 10 13 1060 1282 18 40 O Inventive Example 2 42 83 11 11 1036 1270 16 38 O Inventive Example 3 41 79 14 10 983 1214 15 35 O Comparative Example 1 44 38 16 4 888 1363 9 17 O Comparative Example 2 66 85 8 3 1135 1445 6 31 - Comparative Example 3 31 35 28 10 643 1165 17 11 O Comparative Example 4 38 78 17 12 840 1265 19 23 O Comparative Example 5 78 89 9 2 1253 1402 5 45 O Comparative Example 6 44 68 8 12 948 1277 17 24 0

As can be seen from Table 3, Inventive Examples 1 to 3 satisfying the composition of the present invention and satisfying the manufacturing conditions of the present invention have a hard tissue fraction of 40 area% or more and an aspect ratio of 2 or more among these hard tissues. Since the fraction of the needle-shaped hard bathing satisfies 70 area% or more, it can be confirmed that while securing a tensile strength of 1180 MPa or more, high elongation and hole expandability (HER) are secured.

Comparative Example 1 is a case in which the retention time after quenching is shorter than the conditions of the present invention, and it is confirmed that the needle hard tissue was not formed at the desired level, and thus the hole expandability (HER) did not reach the desired level. have.

Comparative Example 2 is a case where the C content exceeds the range of the present invention and does not reach the range of the present invention of Si and Mn content, it can be confirmed that the elongation does not reach the desired level.

Comparative Example 3 is when the heating temperature is lower than the range of the present invention, ferrite is excessively formed, hard tissue and needle hard tissue is not formed to the desired level, strength and hole expandability (HER) is the desired level It can be confirmed that it does not reach.

Comparative Example 4 is a case where the slow cooling stop temperature is higher than the range of the present invention, ferrite is excessively formed, and the hard tissue fraction does not reach the range of the present invention, so the hole expandability (HER) does not reach the desired level. can confirm.

Comparative Example 5 is a case where the quenching stop temperature is lower than the range of the present invention, while the hard tissue and acicular hard tissue are formed in a large amount, the residual austenite does not reach the desired level, confirming that the elongation does not reach the desired level. Can.

Comparative Example 6 is a case where the heating rate of the first heating is lower than the range of the present invention, the hard tissue and acicular hard tissue is not formed to the desired level, the hole expandability (HER) does not reach the desired level Can be confirmed.

2 and 3 are photographs of the microstructures of Inventive Example 1 and Comparative Example 1 observed with a scanning electron microscope, respectively. In the case of Inventive Example 1, the hard tissue is not only dense, but a large amount of acicular hard tissue is formed. In the case of 1, it can be seen that it not only distributed over a relatively small area of the hard tissue, but also showed a round shape.

Therefore, according to one aspect of the present invention, a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a hole expandability (HER) of 25% or more are provided at the same time, thereby providing a particularly suitable cold rolled steel sheet for automobile steel sheets and a method for manufacturing the same. can confirm.

Although the present invention has been described in detail through the above embodiments, other types of embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (19)

In weight percent, carbon (C): 0.13 to 0.25%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.5 to 3.0%, aluminum (Al) + chromium (Cr) + molybdenum (Mo): 0.08 ~1.5%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, including the remaining Fe and unavoidable impurities,
The fraction of hard tissue present at an embossed surface at a cross section surface at the point t/4 (etched with t being the thickness of a steel sheet) etched with a Nital solution is 40% by area or more,
A high-strength cold-rolled steel sheet having excellent burring property in which the fraction of acicular hard tissue having an aspect ratio (aspect ratio, long axis/short axis length) of 2 or more among the hard tissue is 70 area% or more. According to claim 1,
The hard tissue may be one of lath martensite, tempered martensite, upper bainite, lower bainite, retained austenite and carbide, or High-strength cold-rolled steel sheet with excellent burring properties including two or more types. According to claim 1,
The cold-rolled steel sheet is a high-strength cold-rolled steel sheet having excellent burring properties including less than 15 area% of ferrite. According to claim 1,
The cold-rolled steel sheet, by weight, boron (B): 0.001 ~ 0.005% and titanium (Ti): 0.005 ~ 0.04% further comprising one or more of the high-strength cold-rolled steel sheet excellent in burring properties. According to claim 1,
The aluminum (Al) is contained in the cold-rolled steel sheet in an amount of 0.01 to 0.09% by weight, high-strength cold-rolled steel sheet having excellent burring properties. According to claim 1,
The chromium (Cr) is contained in the cold-rolled steel sheet in an amount of 0.01 to 0.7% by weight, high-strength cold-rolled steel sheet having excellent burring properties. The method of claim 6,
The chromium (Cr) is contained in the cold-rolled steel sheet in an amount of 0.2 to 0.6% by weight, high-strength cold-rolled steel sheet having excellent burring properties. According to claim 1,
The molybdenum (Mo) is contained in the cold rolled steel sheet in an amount of 0.02 to 0.08% by weight, high strength cold rolled steel sheet having excellent burring properties. According to claim 1,
The cold rolled steel sheet has a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a hole expansion ratio (HER) of 25% or more. It includes a steel sheet and an alloyed hot-dip galvanized layer formed on the surface of the steel sheet,
The holding steel sheet is a cold-rolled steel sheet of any one of claims 1 to 9, high strength alloyed hot-dip galvanized steel sheet having excellent burring properties. In weight percent, carbon (C): 0.13 to 0.25%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.5 to 3.0%, aluminum (Al) + chromium (Cr) + molybdenum (Mo): 0.08 ~1.5%, Phosphorus (P): 0.1% or less, Sulfur (S): 0.01% or less, Nitrogen (N): 0.01% or less, cold rolling the steel containing the remaining Fe and unavoidable impurities,
The cold-rolled steel is heated to a temperature range of Ac ₃ or higher,
Slowly cooling the heated steel material at a cooling rate of 2 to 10°C/s to a temperature range of 580 to 620°C,
The annealed steel is rapidly cooled to a temperature range of (Ms-120°C) to Ms at a cooling rate of 7 to 30°C/s,
A method of manufacturing a high strength cold rolled steel sheet having excellent burring property to maintain and distribute the quenched steel in a temperature range of greater than Ms and below (Ms+120°C) for 300 to 600 seconds. The method of claim 11,
The cold-rolled steel is heated to a temperature range of Ac ₃ or higher by first heating and second heating,
The heating rate of the first heating is 20 ~ 30 ℃ / s,
The method of manufacturing a high-strength cold rolled steel sheet having excellent burring properties with a heating rate of 2 to 6°C/s. The method of claim 11,
The heating stop temperature of the first heating is (Ac ₃ -120°C) to (Ac ₃ -50°C),
The second heating is a method of manufacturing a high-strength cold rolled steel sheet having excellent burring property that is started at the heating stop temperature of the first heating and stopped at Ac ₃ or higher. The method of claim 11,
The steel material, by weight, boron (B): 0.001 ~ 0.005% and titanium (Ti): 0.005 ~ 0.04% further comprising one or more of the method of manufacturing a high-strength cold rolled steel sheet having excellent burring properties. The method of claim 11,
The aluminum (Al) is contained in the steel material in an amount of 0.01 to 0.09% by weight, a method for manufacturing a high strength cold rolled steel sheet having excellent burring properties. The method of claim 11,
The chromium (Cr) is contained in the steel material in an amount of 0.01 to 0.7% by weight, a method for producing a high strength cold rolled steel sheet having excellent burring properties. The method of claim 16,
The chromium (Cr) is contained in the steel material in an amount of 0.2 to 0.6% by weight, a method for manufacturing a high strength cold rolled steel sheet having excellent burring properties. The method of claim 11,
The molybdenum (Mo) is contained in the steel material in an amount of 0.02 to 0.08% by weight, a method for producing a high strength cold rolled steel sheet having excellent burring properties. Forming a hot dip galvanized layer on the surface of the steel sheet and alloying it,
The holding steel sheet is a cold-rolled steel sheet manufactured by any one of the manufacturing methods of claims 11 to 18, a method for manufacturing a high-strength alloyed hot-dip galvanized steel sheet having excellent burring properties.

Images