KR102468037B1 - Cold rolled steel sheet and metal plated steel sheet having excellent bake hardenability and anti-aging properties and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a cold-rolled steel sheet, a coated steel sheet, and a manufacturing method thereof, and more particularly, a cold-rolled steel sheet, a coated steel sheet, and a method for manufacturing the same, which are excellent in baking hardenability and anti-aging properties after painting and baking, applicable as a material for automobile exterior panels. It is about.

Description

Cold-rolled steel sheet and plated steel sheet with excellent baking hardenability and aging resistance and their manufacturing method

The present invention relates to a cold-rolled steel sheet, a coated steel sheet, and a method for manufacturing them, and more particularly, to a cold-rolled steel sheet, a coated steel sheet, and a method for manufacturing them having excellent baking hardenability and anti-aging properties that can be applied as a material for automobile exterior panels.

Recently, a reduction in the thickness of a steel sheet is required as part of an active response to weight reduction and environmental problems for improving fuel efficiency of automobiles. In addition, in order to be applied as a material for automobile exterior panels, it is required to have a certain level of baking hardenability. The bake hardening phenomenon is a phenomenon in which carbon and nitrogen activated during paint baking are fixed to the dislocation generated during press to increase the yield strength. By having this improved characteristic, it is very ideal as a material for automobile exterior panels. In addition, in order to be applied as a material for automobile outer panels, it is required to have a certain level of room temperature aging resistance to guarantee aging for a certain period or more.

In general, as a method of manufacturing cold-rolled steel sheet having bake-hardenability, low-carbon P-added Al-killed steel is simply coiled at low temperature, that is, hot-rolled coiling temperature is in the range of 400 to 500 ° C. Steel of about 40~50MPa was mainly used. This was because coexistence of moldability and bake hardenability was easier by upper annealing. In the case of P-added Al-Killed steel by continuous annealing, it is easy to secure bake hardenability because it uses a relatively fast cooling rate, but there is a problem in that formability deteriorates due to rapid heating and short-time annealing, so it is not required for workability. is limited only to Thanks to the recent rapid development of steelmaking technology, it is possible to control the appropriate amount of dissolved elements in steel, and bake-hardenable cold-rolled steel sheet with excellent formability by using Al-Killed steel sheet to which strong carbonitride-forming elements such as Ti or Nb are added. It has been manufactured, and its use for automobile exterior panels requiring dent resistance is on the rise.

As Patent Document 1, C content 0.0005 ~ 0.015%, S + N content ≤ 0.005% of Ti addition or Ti and Nb complex addition Regarding ultra-low carbon cold-rolled steel sheet, or as Patent Document 2, C content of 0.010% or less Ti addition A method for manufacturing steel with a hardening capacity of about 40 MPa or more using steel is introduced. In this method, by controlling the addition amount of Ti and Nb or the cooling rate during annealing, the amount of solid solution elements in the steel is appropriately adjusted to prevent deterioration of the material while imparting bake hardenability. However, in the case of Ti or Ti, Nb composite added steel, since strict control of Ti, N, and S is required in the steelmaking process in order to secure an appropriate bake hardening amount, a problem of cost increase occurs. In addition, in the case of the Nb-added steel in the above patent, workability is deteriorated due to high temperature annealing and manufacturing cost increase due to the addition of special elements is expected.

On the other hand, in order to secure patentability through the addition of a new alloy element, Patent Document 3 and Patent Document 4 introduce a method of achieving both baking hardenability and aging resistance by adding Mo. Further, in Patent Document 5, formability improvement effect by Zr is achieved, and in Patent Document 6, formability is sought by minimizing high strength and deterioration of strain hardening index (N value) by addition of Cr.

However, the above patents simply focus on improving the bake hardenability or moldability, and there is no mention of the deterioration of the endoscopic effect due to the increase of the bake hardenability, or the interlocking of C, Nb, and Ti contents There is no mention of aging deterioration, so it is urgent to establish countermeasures against it.

Japanese Patent Laid-Open No. 1986-026757 Japanese Patent Laid-Open No. 1982-089437 Japanese Patent Laid-Open No. 1987-109927 Japanese Patent Laid-Open No. 1992-120217 Japanese Patent Laid-Open No. 1996-049038 Japanese Patent Laid-Open No. 1995-278654

One aspect of the present invention is to provide a cold-rolled steel sheet, a coated steel sheet, and a manufacturing method thereof having excellent bake hardenability and aging resistance.

Alternatively, another aspect of the present invention is to provide a cold-rolled steel sheet, a coated steel sheet, and a manufacturing method thereof that can be suitably used as a material for automobile exterior panels.

The object of the present invention is not limited to the foregoing. Anyone with ordinary knowledge in the technical field to which the present invention belongs will have no difficulty in understanding the additional objects of the present invention from the contents throughout the present specification.

One aspect of the present invention,

By weight, carbon (C): 0.005% or less, manganese (Mn): 0.1 to 1.0%, silicon (Si): 0.3% or less, phosphorus (P): 0.01 to 0.08%, sulfur (S): 0.01% or less , nitrogen (N): 0.01% or less, aluminum (sol.Al): 0.01 to 0.06%, niobium (Nb): 0.003 to 0.015%, boron (B): 0.0015 to 0.0045%, balance iron (Fe) and unavoidable impurities including,

As a microstructure, it contains 99 area% or more ferrite,

The average dislocation density of the surface layer is 5×10 ¹⁴ to 1×10 ¹⁶ /m ² ,

The residual stress of the surface layer is 250 ~ 350MPa,

The surface layer portion represents an area from the surface to 1/10t in the thickness direction, and t represents the overall average thickness of the steel sheet, providing a cold-rolled steel sheet.

In addition, another aspect of the present invention,

the aforementioned cold-rolled steel sheet; and

It provides a plated steel sheet comprising a zinc plating layer or a zinc alloy plating layer formed on at least one surface of the cold-rolled steel sheet.

In addition, another aspect of the present invention,

Reheating the steel slab having the above composition to 1100 ~ 1250 ℃;

Obtaining a hot-rolled steel sheet by hot-rolling the reheated steel slab at 850 to 980° C.;

Cooling and winding the hot-rolled steel sheet to 500 to 750° C. at an average cooling rate of 10 to 70° C./s;

Cold-rolling the rolled hot-rolled steel sheet at a reduction ratio of 60 to 90%;

Step of continuously annealing the cold-rolled steel sheet in a temperature range of 750 ~ 860 ℃; and

It provides a method for producing a cold-rolled steel sheet, comprising the step of temper rolling the continuously annealed steel sheet at a rolling ratio of 0.6 to 3.0%.

In addition, another aspect of the present invention,

Reheating the steel slab having the above composition to 1100 ~ 1250 ℃;

Obtaining a hot-rolled steel sheet by hot-rolling the reheated steel slab at 850 to 980° C.;

Cooling and winding the hot-rolled steel sheet to 500 to 750° C. at an average cooling rate of 10 to 70° C./s;

Cold-rolling the rolled hot-rolled steel sheet at a reduction ratio of 60 to 90%;

Step of continuously annealing the cold-rolled steel sheet in a temperature range of 750 ~ 860 ℃;

immersing the continuously annealed steel sheet in a galvanizing bath at 440 to 500° C. to obtain a steel sheet having a plating layer formed on its surface; and

It provides a method for manufacturing a coated steel sheet comprising the step of temper rolling the steel sheet on which the plating layer is formed on the surface at a rolling ratio of 0.6 to 3.0%.

According to one aspect of the present invention, it is possible to provide a cold-rolled steel sheet, a coated steel sheet, and a manufacturing method thereof having excellent bake hardenability and aging resistance.

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

1 shows a schematic diagram of constant speed rolling and double speed rolling and changes in stress distribution.
2 shows a comparison of dislocation density changes in the surface layer portion and the center portion of each steel sheet with respect to steel sheets subjected to constant speed rolling and double speed rolling during temper rolling, respectively.
Figure 3 is a graph showing the change in dislocation density of the surface layer according to the increase in the rolling ratio of the two speeds.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified in many different forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

As a result of intensive examination to solve the above-mentioned problems of the prior art, the inventors of the present invention have excellent bake hardenability (BH property) by applying double speed rolling in the temper rolling process in addition to strict control of steel components, and at the same time, at room temperature for a long time It was confirmed that it is possible to provide a cold-rolled steel sheet in which aging deterioration does not occur during storage, a coated steel sheet using the same, and a manufacturing method thereof. In other words, when the dual speed rolling ratio is managed in an appropriate range while applying the double speed rolling during the temper rolling process along with the control of the steel component, excellent room temperature aging resistance and bake hardenability can be achieved without excessive temper rolling elongation increase, In addition, it was found that cold-rolled steel sheets, coated steel sheets, and methods for manufacturing them can be provided with excellent endoscopic properties after painting and baking.

Hereinafter, the reason for adding the basic component and the reason for limitation in the cold-rolled steel sheet will be described. Unless otherwise specified in the present invention, the content of each element is based on weight%.

Carbon (C): 0.005% or less (excluding 0%)

Carbon (C) is an interstitial solid-solution element that is dissolved inside the steel sheet during cold rolling and annealing and interacts (locks) with dislocations formed by temper rolling to exhibit bake hardenability. Basically, the higher the C content, the better the baking Hardenability is improved. However, if too much dissolved carbon is present in the material, it may cause aging defects that cause a defect called orange peel on the surface during molding of the part. That is, when the C content exceeds 0.005%, it is disadvantageous in terms of moldability and greatly inferior in room temperature aging performance, limiting the application of parts, so in the present invention, the range of C content is limited to 0.005% or less. On the other hand, the lower limit of the C content is not particularly limited, but 0% is excluded (ie, greater than 0%) because a possible range is preferable in terms of the manufacturing process. However, more preferably, the lower limit of the C content may be 0.0010%, and the upper limit of the C content may be 0.0030%.

Manganese (Mn): 0.10 to 1.0%

Manganese (Mn) is a solid solution strengthening element that not only contributes to increase in strength, but also serves to precipitate S in steel as MnS. When the Mn content is less than 0.10% by weight, MnS cannot be effectively precipitated and drawability is deteriorated. On the other hand, when the Mn content exceeds 1.0% by weight, although the yield strength is increased, the Mn content is excessively dissolved and the drawability is deteriorated, so the Mn content is limited to 0.10 to 1.0% by weight. desirable. However, more preferably, the lower limit of the Mn content may be 0.20%, and the upper limit of the Mn content may be 0.80%.

Silicon (Si): 0.3% or less (excluding 0%)

Silicon (Si) is an element that contributes to the increase in strength of the steel sheet by solid solution strengthening. In the present invention, it is not intentionally added, and even if silicon is not added, there is no major obstacle in terms of securing physical properties. However, considering the amount unavoidably added in manufacturing, 0% is excluded. On the other hand, if the silicon content exceeds 0.3%, the plating surface properties deteriorate, so in the present invention, the silicon content is controlled to 0.3% or less. However, more preferably, the lower limit of the Si content may be 0.01%, and the upper limit of the Si content may be 0.20%.

Phosphorus (P): 0.01~0.08%

Phosphorus (P) is the most effective element for ensuring the strength of steel without greatly impairing drawability and having the most excellent solid solution effect. In particular, the P is easily segregated at grain boundaries to inhibit grain growth during annealing, and helps to improve room temperature endoscopic properties as grains are refined. When the P content is less than 0.01%, there is a problem in that it is impossible to secure the desired strength. On the other hand, when the P content exceeds 0.08%, too much dissolved P is segregated at the grain boundary, and the opportunity for grain boundary segregation of B and C required in the present invention is lost, securing the room temperature aging effect required in the present invention. will not be able to In addition, as the grain boundary P segregation increases, it may cause a problem of secondary processing brittleness, so the P content is preferably limited to 0.01 to 0.08% by weight. However, more preferably, the lower limit of the P content may be 0.02%, and the upper limit of the P content may be 0.06%.

Sulfur (S): 0.01% or less (excluding 0%)

Sulfur (S) is an impurity inevitably included in steel, and it is desirable to manage its content as low as possible. Therefore, the S content in the steel excludes 0% (ie, exceeds 0%) in view of the case where it is unavoidably included. In particular, S in steel increases the possibility of generating red-hot brittleness, and its content is controlled to 0.01% or less. However, more preferably, the lower limit of the S content may be 0.001%, and the upper limit of the S content may be 0.008%.

Nitrogen (N): 0.01% or less (excluding 0%)

Nitrogen (N) is an impurity inevitably included in steel, and it is important to manage the N content as low as possible. Therefore, the N content in the steel excludes 0% (ie, exceeds 0%) in view of the case where it is unavoidably included. However, in order to manage the N content in the steel very low, there is a problem in that the refining cost of the steel rises rapidly, so it is managed to 0.01% or less, which is a range where operating conditions are possible. However, more preferably, the lower limit of the N content may be 0.001%, and the upper limit of the N content may be 0.005%.

Acid soluble aluminum (sol.Al): 0.01~0.06%

Acid-soluble aluminum (sol.Al) is an element added for grain size refinement and deoxidation, and when the sol.Al content is less than 0.01%, aluminum-killed steel cannot be produced in a normal stable state. On the other hand, when the sol.Al content exceeds 0.06%, it is advantageous to increase the strength due to the crystal grain refinement effect, but the possibility of surface defects of the coated steel sheet increases due to excessive formation of inclusions during steel making casting operation. In addition, since there is a problem that causes a rapid increase in manufacturing cost, in the present invention, the sol.Al content is controlled to 0.01 to 0.06%. However, more preferably, the lower limit of the sol.Al content may be 0.015%, and the upper limit of the sol.Al content may be 0.055%.

Niobium (Nb): 0.003 to 0.015%

Niobium (Nb) combines with carbon in steel during hot rolling to precipitate as NbC, thereby reducing dissolved carbon and affecting bake hardenability and endoscopic effect. As the C content in the steel precipitated as NbC increases, the carbon content employed decreases, which reduces baking hardenability, although it is advantageous in terms of aging properties. In the present invention, controlling the carbon content as well as the boron content segregated at grain boundaries is very important. An appropriate level of control of solid solution carbon can obtain excellent bake hardenability on the premise of securing room temperature aging resistance, and an important element for controlling such solid solution carbon is Nb.

When the Nb content is less than 0.003%, there is almost no carbon precipitated as NbC, and most of the C in the steel remains as solid solution carbon, which is advantageous for baking hardenability, but there is a problem of room temperature endoscopic aging degradation. There is a limit to the application of parts. In addition, when the Nb content exceeds 0.015%, on the contrary, most of the C in the steel is precipitated as NbC, so that the solid solution C content is absolutely insufficient, even if the room temperature endoscopic effect is advantageous, the required lower BH value of 30 MPa or more (ie, bake hardenability) cannot obtain Therefore, in the present invention, it is preferable to manage the Nb content at 0.003 to 0.015%. However, more preferably, the lower limit of the Nb content may be 0.004%, and the upper limit of the Nb content may be 0.012%.

Boron (B): 0.0015 to 0.0045%

Boron (B) is an element added to prevent secondary processing brittleness due to grain boundary embrittlement in ultra-low carbon steel containing a large amount of P component. Typically, B has a higher grain boundary segregation tendency than other elements, and serves to prevent secondary processing brittleness by suppressing P segregation at grain boundaries by adding boron. However, in the present invention, the grain boundary segregation characteristics of B were used to improve the BH property.

That is, the present inventors discovered through various experiments that when a certain amount or more of B is segregated at the grain boundary during annealing, a Cottrell atmosphere is formed by interaction with dislocations in a mechanism similar to dissolved carbon or nitrogen, and the BH property increases. did To this end, the B content should be managed in the range of 0.0015 to 0.0045%.

When the B content is less than 0.0015%, it is not possible to secure an increase in bake hardenability required by the B content. In addition, when the B content exceeds 0.0045%, even if the bake hardenability is increased, material deterioration due to excessive grain boundary B segregation and peeling of the coated layer of the plated steel sheet may occur. However, more preferably, the lower limit of the B content may be 0.0015%, and the upper limit of the B content may be 0.004%.

In addition, according to one aspect of the present invention, although not particularly limited thereto, the above-described cold-rolled steel sheet may have an R _A value defined by the following relational expression 1-1 in the range of 1.0 to 2.5. By satisfying these requirements, excellent bake hardenability and material and plating deterioration can be prevented.

[Relationship 1-1]

R _A = [B]/[N]

(In the relational expression 1-1 above, [B] and [N] are the atomic % of the corresponding alloy element.)

That is, when the R _A value is less than 1, the added B is consumed as a BN precipitate, and the effect of securing bake hardenability by solid boron cannot be expected. In addition, when the R _A value exceeds 2.5, material and plating deterioration may occur due to an excessive increase in solute boron. However, more preferably, the lower limit of the R _A value may be 1.10, and the upper limit of the R _A value may be 2.20.

In addition, according to one aspect of the present invention, although not particularly limited, the cold-rolled steel sheet described above may have an R _B value defined by the following relational expression 1-2 in the range of 0.40 to 0.85.

[Relationship 1-2]

R _B = [Nb]/[C]

(In the relational expression 1-2, the [Nb] and [C] are the atomic % of the corresponding alloy element.)

If the R _B value is less than 0.40, a large amount of solid solution carbon that is not precipitated as NbC is distributed in the steel, which may cause a problem of deterioration of endoscopic properties. In addition, when the R _B value exceeds 0.85, most of the added carbon is precipitated as NbC, which may cause a problem in that the amount of solid carbon that secures the BH property is reduced.

In addition, the remaining Fe and unavoidable impurities are included. The addition of effective ingredients other than the above composition is not excluded. That is, all of the unavoidable impurities may be included as long as they can be unintentionally incorporated in a typical cold-rolled steel sheet (and coated steel sheet) manufacturing process. Since those skilled in the art can easily understand the meaning, it is not particularly limited here.

Meanwhile, according to one aspect of the present invention, the microstructure of the cold-rolled steel sheet may include 99% or more of ferrite in area fraction. At this time, the structure other than the ferrite may include one or more selected from pearlite, cementite, and bainite. When the microstructure of the cold-rolled steel sheet is less than 99%, the processability deteriorates, so the area fraction of ferrite is preferably 99% or more, more preferably the single-phase ferrite structure (ie, the area fraction of ferrite is 100%) may consist of

According to one aspect of the present invention, the average particle diameter of the ferrite may be in the range of 5 to 10 μm. If the average grain diameter of the ferrite is less than 5 μm, since the yield strength of the material increases, wrinkles called surface deformation may occur after press molding. On the other hand, when the average grain diameter of the ferrite exceeds 10 μm, dislocation density unevenness in the steel sheet increases, and endoscopic efficiency after molding or painting and baking may decrease.

The inventors of the present invention, in order to secure room temperature aging resistance and aging resistance after painting baking, the surface layer of the steel sheet, to be precise, the area from the surface of the steel sheet to 1/10t in the thickness direction (where t represents the overall average thickness of the steel sheet) It was found that controlling the dislocation density and residual stress in is a very important factor. In other words, by controlling the dislocation density and residual stress in the surface layer within a certain range, it is possible to secure excellent room temperature aging properties and aging characteristics after painting and baking.

That is, in order to secure excellent endoscopic properties, it is preferable to control the average dislocation density of the surface layer corresponding to the region from the surface of the cold-rolled steel sheet to 1/10 t in the thickness direction within the range of 5×10 ¹⁴ to 1×10 ¹⁶ /m ² do. At this time, the thickness direction means a direction perpendicular to the rolling direction of the steel sheet.

In the present invention, if the average dislocation density of the surface layer portion is less than 5 × 10 ¹⁴ , the dislocation density to suppress aging is insufficient, and endoscopic effect due to insufficient residual stress in the surface layer portion may not be sufficiently secured. On the other hand, when the average dislocation density of the surface layer exceeds 1×10 ¹⁶ /m ² , material deterioration such as increased yield strength may occur due to excessive dislocation density of the surface layer.

Further, according to one aspect of the present invention, in the aforementioned cold-rolled steel sheet, the residual stress of the surface layer portion may be in the range of 250 to 350 MPa. The residual stress of the surface layer portion has the effect of improving the endoscopic effect by suppressing the cottrell atmosphere of carbon and dislocation, but if the residual stress of the surface layer portion is less than 250 MPa, the effect of suppressing the cottrell atmosphere is not sufficiently secured. Problems can arise. In addition, when the residual stress of the surface layer portion exceeds 350 MPa, the endoscopic efficiency is improved due to the increase in the excessive stress field, but the formability may deteriorate due to the increase in strength and deterioration of ductility.

Further, according to one aspect of the present invention, in the above-described cold-rolled steel sheet, although not particularly limited, the average dislocation density of the entire steel sheet may be 5×10 ¹² /m ² or more. The average dislocation density of the entire steel plate is 5×10 ¹² If it is less than, after painting baking, the yield strength is lowered according to the change of time, that is, the room temperature endoscopic effect of the material may be lowered along with the deterioration of the dent property. In addition, when the dislocation density is relatively small with respect to the solid solution carbon, the moving dislocation, which is relatively easy to move the solid solution carbon during room temperature aging, is rapidly fixed, and the room temperature endoscopic property may be lowered. On the other hand, in terms of preventing the above-mentioned deterioration of dent property and deterioration of room temperature aging resistance, and further improving dent resistance and room temperature aging resistance, more preferably, the average dislocation density of the entire steel sheet is 4 × 10 ¹² to 7 × 10 ¹² /m ² can be in the range.

On the other hand, another aspect of the present invention, the aforementioned cold-rolled steel sheet; and a zinc plating layer or a zinc alloy plating layer formed on at least one surface of the cold-rolled steel sheet. At this time, in the plated steel sheet, the above description can be equally applied to the cold-rolled steel sheet. In addition, in the plated steel sheet, the same configuration of the plating layer commonly applied in the art may be applied to the zinc plating layer and the zinc alloy plating layer.

The above-mentioned cold-rolled steel sheet (and coated steel sheet) is not particularly limited, but a tensile strength (TS) of 340 MPa or more is preferable in order to reduce the weight of the vehicle body itself, to secure the safety of passengers, and to be suitably used as a steel material for automobile exterior panels. . In addition, the cold-rolled steel sheet (and coated steel sheet) may have a yield strength (YS) in the range of 180 to 250 MPa. If the yield strength is less than 180 MPa, it may be difficult to secure the strength of the desired steel for automobile outer plates, and if the yield strength exceeds 250 MPa, a problem of wrinkles called surface deformation after press forming may occur.

In addition, according to one aspect of the present invention, the cold-rolled steel sheet (and coated steel sheet) satisfies tensile strength: 340 MPa or more, yield strength: 180 to 250 MPa, and elongation: 35% or more, and at the same time, the lower baking hardening amount (lower BH ; when subjected to a tensile test after heat treatment at 170 ° C for 20 minutes) is 30 MPa or more, and by satisfying this, cold-rolled steel sheet (and coated steel sheet) with excellent baking hardenability and aging resistance can be suitably used as a steel material for automobile exterior panels.

In addition, the cold-rolled steel sheet (and coated steel sheet) according to one aspect of the present invention has the above-described tensile strength, yield strength and elongation, and at the same time, when subjected to a tensile test after heat treatment at 170 ° C. for 20 minutes, the lower baking hardening amount (lower BH; when subjected to a tensile test after heat treatment at 170 ° C for 20 minutes) is 30 MPa or more (more preferably 32.5 to 45.6 MPa), and the aging index (AI) when subjected to a tensile test after heat treatment at 100 ° C for 60 minutes is 0.2% The BH reduction amount measured from the amount of BH before heat treatment at 100 ° C. for 60 minutes and the amount of BH after heat treatment at 100 ° C. for 60 minutes is 10 MPa or less. can

Hereinafter, the manufacturing method of the aforementioned cold-rolled steel sheet will be described in detail. However, the cold-rolled steel sheet of the present invention does not necessarily mean that it must be manufactured by the following manufacturing method.

A method for manufacturing a cold-rolled steel sheet according to an aspect of the present invention includes reheating a steel slab having the above-described composition at 1100 to 1250 ° C. If the reheating temperature of the slab is less than 1100 ° C, slab inclusions and the like are not sufficiently re-dissolved, which may cause material deviation and surface defects after hot rolling. On the other hand, if the reheating temperature of the slab is higher than 1250° C., a problem in which strength is lowered due to excessive growth of austenite grains may occur. On the other hand, with respect to the composition of the steel slab, the description of the steel composition of the cold-rolled steel sheet described above can be equally applied.

A method of manufacturing a cold-rolled steel sheet according to an aspect of the present invention includes obtaining a hot-rolled steel sheet by hot-rolling the reheated steel slab at 850 to 980°C. If the temperature of the hot rolling is less than 850 ° C., ferrite transformation occurs during rolling to generate an elongated structure, and accordingly, problems such as anisotropic deterioration and cold rolling deterioration may occur. On the other hand, if the temperature of the hot rolling exceeds 980 ° C., the size of austenite crystal grains becomes coarse, and surface quality may deteriorate due to high-temperature work.

A method of manufacturing a cold-rolled steel sheet according to an aspect of the present invention includes cooling and winding the hot-rolled steel sheet to 500 to 750°C at an average cooling rate of 10 to 70°C/s. If the coiling temperature is less than 500 ° C., the shape of the steel sheet may be poor, and ductility may deteriorate due to the formation of fine crystal grains. In addition, when the coiling temperature exceeds 750 ° C., coarse ferrite crystal grains are formed, and coarse carbides and nitrides are easily formed, and the material of the steel may be deteriorated. In addition, if the cooling rate is less than 10 ° C / s, the size of the crystal grains increases, and there may be a problem that the endoscopic effect is lowered, and if the cooling rate is more than 70 ° C / s, fine crystal grains are generated due to rapid cooling, thereby yielding the material There may be a problem that defects occur after material press molding such as increased strength.

A method for manufacturing a cold-rolled steel sheet according to an aspect of the present invention includes obtaining a cold-rolled steel sheet by cold-rolling the rolled hot-rolled steel sheet at a reduction ratio of 60 to 90%. At this time, if the reduction ratio of cold rolling is less than 60%, it may be difficult to secure the target thickness and it may be difficult to correct the shape of the steel sheet. On the other hand, if the reduction ratio of cold rolling exceeds 90%, cracks may occur at the edge of the steel sheet, and a cold rolling load may be caused.

A method for manufacturing a cold-rolled steel sheet according to an aspect of the present invention includes continuously annealing the cold-rolled steel sheet in a temperature range of 750 to 860°C. At an annealing temperature of less than 750 ° C., recrystallization is not sufficiently completed, and there is a concern that a mixed structure may occur. In addition, when the annealing temperature exceeds 860 ° C., the field equipment trouble caused by high temperature annealing is very high, and the crystal grains are too coarse, so that the properties required in the present invention cannot be secured.

The manufacturing method of a cold-rolled steel sheet according to one aspect of the present invention may include temper rolling the continuously annealed cold-rolled steel sheet at a rolling ratio of 0.6 to 3.0% at different speeds. In order to secure the dislocation density and residual stress in the surface layer portion of the steel sheet as in the present invention, control of the temper rolling conditions is very important. In the present invention, in the temper rolling step, it was found that the dislocation density and residual stress of the surface layer, which affects the eunuch effect, can be controlled by applying double speed rolling.

In general, constant speed rolling means that the upper and lower rolling rolls have the same diameter and the upper and lower rolling speeds are the same. On the other hand, double-speed rolling means that the rolling speeds of the upper and lower parts of the rolling rolls are different, and at this time, the stress mode of the roll bite changes due to the difference in peripheral speed of the upper and lower rolls. 1 shows a schematic diagram of constant speed rolling and double speed rolling and a calculated change in stress distribution. As can be seen in FIG. 1, in the case of constant speed rolling, the peripheral speeds of the upper and lower rolls (V1 and V2) are the same (V1 = V2), and in the case of double speed rolling, the speed of one of the two upper and lower rolls is increased (for example, V1 < V2). In general, when rolling proceeds, shear stress is generated. When constant speed rolling is performed, most of the shear stress is concentrated on the surface layer, but in the case of double speed rolling, the shear stress is very large compared to constant speed rolling, and it is found that the distribution also penetrates into the inside of the steel sheet as well as the surface layer. That is, this increase in shear stress causes an increase in the dislocation density in the steel, and by controlling the residual stress with the dislocation density of the surface layer to an appropriate range, the endoscopic effect is improved.

On the other hand, FIG. 2 shows a comparison of the dislocation density changes in the surface layer portion and the center portion of each steel sheet with respect to steel sheets subjected to constant speed rolling and double speed rolling during temper rolling, respectively. That is, a) of FIG. 2 shows the case of performing constant speed rolling with a double speed rolling ratio of 0%, and b) of FIG. 2 shows a case of performing double speed rolling with a double speed rolling ratio of 1.2%. By comparing the experimental results of a) and b) of FIG. 2, it can be confirmed that the dislocation density of the steel sheet is very high when two-speed rolling is performed on both the surface layer and the center, despite the condition of giving the same temper rolling ratio of 1.5%. have.

Figure 3 is a graph showing the change in dislocation density of the surface layer according to the increase in the rolling ratio of the two speeds. As can be seen in Figure 3, it can be seen that there is almost no residual stress in a state where temper rolling is not applied at all. In addition, in the case of constant speed rolling with a double speed rolling ratio of 0%, a residual stress of about 100 MPa was present in the surface layer of the steel sheet. However, in the case of two-speed rolling, the residual stress rapidly increased, then showed a stable residual stress within a certain range, and then, when the rolling ratio increased again, the residual stress tended to increase rapidly.

As such, the region where the residual stress in the surface layer portion shows a stable state in the range of 250 to 350 MPa corresponds to the condition of the rolling ratio of 0.6 to 3.0%. That is, if the two-speed rolling ratio is less than 0.6%, the residual stress of the surface layer of 250 MPa or more, which is intended in the present invention, cannot be obtained, and accordingly, a problem of insufficient endoscopic effect may occur. On the other hand, if the rolling ratio of two speeds exceeds 3.0%, material deterioration may be caused by excessive stress increase in the surface layer portion. In addition, in terms of equipment, an excessive increase in shear stress causes a workload of the rolling equipment, which may cause failure and shortened lifespan of the equipment.

On the other hand, according to one aspect of the present invention, although not particularly limited, in order to secure the above-mentioned residual stress of the surface layer portion, the speed of temper rolling is also one of the very important factors in addition to the rolling ratio at different speeds. That is, even if the rolling ratio increases, if the working speed of temper rolling increases, the residual stress in the surface layer decreases and the residual stress in the surface layer required in the present invention cannot be obtained. Conversely, when the two-speed rolling ratio is reduced, the residual stress applied to the surface layer is reduced. Therefore, in order to secure the residual stress in the range of 250 to 350 MPa, which is intended in the present invention, the temper rolling speed must be lowered. Therefore, as a result of additional intensive examination, the inventors of the present invention performed temper rolling while controlling the speed of temper rolling and the rolling ratio at different speeds in an appropriate range to satisfy the following relational expression 2, so that cold-rolled steel sheet excellent in baking hardenability and aging effect after paint baking And it was confirmed that a coated steel sheet using the same can be provided. At this time, since the value of the relational expression 2 below is an empirical value, it is sufficient if Y satisfies the mpm unit and X satisfies % without particularly determining a unit.

[Relationship 2]

Y-10 ≤ -60×ln(X) + 117.7 ≤ Y+10

(In the relational expression 2, the Y represents the temper rolling speed, and the unit is mpm. In addition, the X is the double speed rolling ratio, and the unit is %.)

On the other hand, according to one aspect of the present invention, although not particularly limited, in the step of temper rolling, the speed of temper rolling can be managed in the range of 50 to 150 mpm. If the temper rolling speed is out of the above range, it may not be possible to secure the residual stress of the surface layer of the level desired in the present invention, which may lead to deterioration of eunuch efficiency due to a decrease in shear stress.

According to one aspect of the present invention, in the temper rolling step, since the dislocation density and residual stress in the surface layer of the steel sheet can be secured by performing double speed rolling under appropriate conditions, a temper rolling reduction rate similar to that of constant speed rolling is given. No need to. Therefore, in the present invention, a reduction ratio of 0.8 to 1.5% may be applied during temper rolling. That is, if the reduction ratio during the temper rolling is less than 0.8%, even if the two-speed rolling is sufficiently performed, sufficient surface layer dislocation desired in the present invention is not formed. On the other hand, when the rolling reduction ratio exceeds 1.5% during the temper rolling, side effects such as plate breakage may occur due to limitations in facility capability as well as material deterioration due to an excessive increase in dislocation density in the surface layer portion.

On the other hand, another aspect of the present invention, reheating the steel slab having the above composition to 1100 ~ 1250 ℃; Obtaining a hot-rolled steel sheet by hot-rolling the reheated steel slab at 850 to 980° C.; Cooling and winding the hot-rolled steel sheet to 500 to 750° C. at an average cooling rate of 10 to 70° C./s; Obtaining a cold-rolled steel sheet by cold-rolling the rolled hot-rolled steel sheet at a reduction ratio of 60 to 90%; continuously annealing the cold-rolled steel sheet in a temperature range of 750 to 860° C.; immersing the continuously annealed cold-rolled steel sheet in a galvanizing bath at 440 to 500° C. to obtain a steel sheet having a plating layer formed on its surface; and temper rolling the steel sheet on which the plating layer is formed on the surface at a rolling ratio of 0.6 to 3.0% at different speeds. At this time, the description of the cold-rolled steel sheet described above can be equally applied to each step other than the step of obtaining the steel sheet having the plating layer formed on the surface thereof. In particular, cooling after the continuous annealing step described above is performed under normal operating conditions.

In addition, in the case of a steel sheet having a plating layer formed on the surface (ie, plating material), GI hot-dip plating is performed under normal conditions in the range of 440 to 500 ° C., which is the temperature of the galvanizing bath. At this time, if the temperature of the plating bath is less than 440 ° C., plating is not performed, and if it exceeds 550 ° C., plating defects such as dross due to excessive plating bath temperature may occur.

In addition, even in the case of manufacturing an alloyed hot-dip galvanized steel sheet (GA), the alloying heat treatment process conditions are not particularly limited, and may be ordinary conditions. As an example, optionally, a step of alloying heat treatment of the steel sheet on which the plating layer is formed at 500 to 540° C. may be further included.

(Example)

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

(Experimental Example 1)

A steel slab having the alloy composition shown in Table 1 below was hot rolled at a reheating temperature of 1200 ° C, a hot rolling finish temperature of 920 ° C, which is higher than the Ar3 temperature, and a coiling temperature of 620 ° C. It was cold rolled at the rate. The cold-rolled steel sheet was continuously annealed under the conditions shown in Table 2, then cooled by furnace cooling, and temper rolling was performed under the conditions shown in Table 2 below. At this time, the dislocation density and residual stress of the surface layer portion described in Table 2 were measured centering on the area of 1/10t (here, t is the thickness of the steel sheet) excluding the extreme surface layer portion affected by decarburization.

When measuring the dislocation density of the surface layer, after measuring the number of intersections (N) of the orthogonal parallel lines of 5Х5 and the dislocation of the photograph measured at 100,000 times and 150,000 times magnification using a TEM (Transmission Electronic Microscope), ρ = The dislocation density was calculated by the 2N/(Lt) relational expression. Here, L is the total length of the cross line, and t is the sample thickness, which is 0.1 μm. In addition, the residual stress of the surface layer portion was evaluated using an XRD (X-ray Diffraction) machine.

steel grade C Mn Si P S N sol. Al Nb B B/N Nb/C One 0.0015 0.45 0.06 0.032 0.006 0.002 0.021 0.0078 0.003 2.10 0.67 2 0.0021 0.35 0.05 0.041 0.005 0.002 0.034 0.0081 0.0025 1.75 0.50 9 0.004 0.55 0.07 0.054 0.007 0.004 0.045 0 0.004 1.40 0.00

steel grade annealing temperature
[℃] Temper rolling conditions Surface layer dislocation density [/m ² ]
(surface ~ 1/10t) residual stress
[MPa] note reduction rate
[%] Lee Joo-sok
rolling ratio working speed [mpm] One 810 1.2 1.5 95 7x10 ¹⁴ 275.9 Invention Example 1 2 820 1.1 1.6 90 8x10 ¹⁴ 261.4 Invention example 2 9 810 1.2 One 115 7x10 ¹⁴ 255 Comparative Example 1

The physical properties of the cold-rolled steel sheets were measured under the conditions of Tables 1 and 2 and are shown in Table 3 below. In addition, a tensile test was conducted in the L direction (rolling direction of the steel sheet) using the ASTM standard for the temper-rolled steel, and the yield strength (YS), tensile strength (TS) and elongation (El) were determined according to VDA 239-100 It was measured based on the standard. In addition, the lower baking hardening amount (Low BH), aging index (AI), and BH reduction amount were measured in the C direction (direction perpendicular to the rolling direction of the steel sheet) using the same ASTM standard material (the following experimental conditions Reference).

steel grade Property evaluation note YS
[MPa] TS
[MPa] El
[%] Lower baking curing amount
(Lower BH*)
[MPa] AI*
[%] Amount of BH reduction*
[MPa] One 226 344 40 39.2 0 0 Invention Example 1 2 210 352 38 40.1 0 0 Invention example 2 9 261 355 32 59.1 1.6 35 Comparative Example 1

Lower BH (MPa)*: Measure the flow stress with 2% pre-straining, and measure the difference in lower yield strength (lower YS) by conducting a tensile test after heat treatment under baking conditions at 170 ° C for 20 minutes

AI*: Tensile test after heat treatment at 100°C for 60 minutes, YP-El measurement at yield point elongation (defective aging under the condition of YP-El>0.2%)

BH reduction*: Measure the BH reduction after heat treatment at 100 ° C for 60 minutes (1 hr) before heat treatment

As can be seen in Table 3, Inventive Examples 1 and 2 satisfying the alloy composition and manufacturing conditions of the present invention exhibited excellent material properties with a yield strength of 250 MPa or less, a tensile strength of 340 MPa or more, and an elongation of 35% or more. In addition, the average dislocation density in the surface layer (region from the surface to 1/10t in the thickness direction) satisfies the range of 5×10 ¹⁴ to 1×10 ¹⁶ /m ² , and the residual stress is also distributed in the range of 250 to 350 MPa. , AI (YP-El after holding for 100 ℃, 1 hr) is 0%, and at the same time, the lower BH reduction after heat treatment at 100 ℃ for 1 hour is 0, and bake hardening type hot-dip galvanized cold-rolled steel sheet having excellent endoscopic effect after painting and baking is manufactured I was able to confirm that it could be done. In addition, it was confirmed that the average dislocation density of the entire steel sheet prepared from Examples 1 and 2 was in the range of 4×10 ¹² to 7×10 ¹² /m ² , and the microstructure of the cold-rolled steel sheet had an area fraction of 99% or more ( That is, 100%), and it was confirmed that the average particle diameter of these ferrites was 5 to 10 μm.

On the other hand, Comparative Example 1 is a case in which Nb is not added at all and all the added carbon (0.004% of C) exists as a solid-solution element. Therefore, even if the temper rolling condition satisfies the scope of the present invention, the yield strength was high (YS: 261 MPa) and the elongation was low (El: 32%) due to the presence of excessive dissolved carbon. In particular, the aging index (AI) was 1.6%, and the lower BH reduction after heat treatment at 100 ° C. for 1 hour was 35 MPa, and the endoscopic effect was very poor.

(Experimental Example 2)

A steel slab having the alloy composition shown in Table 4 below was subjected to hot rolling at a reheating temperature of 1200 ° C, a hot rolling finish temperature of 920 ° C above Ar3 temperature, and a coiling temperature of 620 ° C. It was cold rolled at a rate. The cold-rolled steel sheet was continuously annealed under the conditions shown in Table 5, and then cooled by furnace cooling. In addition, the GI hot-dip plating temperature for manufacturing the hot-dip galvanized steel sheet was worked at around 470 ° C., and temper rolling was performed on the hot-dip galvanized steel sheet under the conditions shown in Table 5. At this time, the dislocation density and residual stress of the surface layer portion described in Table 2 were measured in the same manner as in Experimental Example 1 described above.

steel grade C Mn Si P S N sol. Al Nb B B/N Nb/C 3 0.0012 0.73 0.05 0.036 0.004 0.0015 0.045 0.0075 0.0023 2.15 0.81 4 0.0017 0.32 0.06 0.035 0.004 0.001 0.043 0.0083 0.0015 2.10 0.63 5 0.0017 0.22 0.04 0.041 0.006 0.0035 0.052 0.008 0.0032 1.28 0.61 6 0.002 0.55 0.05 0.05 0.004 0.0028 0.037 0.007 0.0035 1.75 0.45 7 0.0018 0.43 0.02 0.035 0.002 0.003 0.033 0.006 0.0025 1.17 0.43 8 0.0027 0.65 0.06 0.052 0.004 0.002 0.035 0.0091 0.0025 1.75 0.43 10 0.007 0.45 0.04 0.05 0.005 0.003 0.046 0.007 0.0025 1.17 0.13 11 0.0017 0.43 0.05 0.055 0.003 0.004 0.056 0.035 0 0.00 2.66 12 0.0015 0.34 0.03 0.038 0.004 0.002 0.048 0.006 0.003 2.10 0.52 13 0.004 0.55 0.04 0.035 0.005 0.0045 0.046 0.008 0.004 1.24 0.26 14 0.0029 0.5 0.02 0.049 0.006 0.0055 0.055 0.01 0.0044 1.12 0.44 15 0.0022 0.35 0.04 0.06 0.005 0.003 0.046 0.013 0.0031 1.45 0.76 16 0.0038 0.65 0.02 0.043 0.005 0.003 0.051 0.006 0.0034 1.59 0.20 17 0.0018 0.45 0.05 0.032 0.006 0.002 0.033 0.0071 0.0028 1.96 0.51

steel grade annealing temperature
[℃] Temper rolling conditions surface layer dislocation density
[/m ² ]
(surface ~ 1/10t) residual stress
[MPa] note reduction rate
[%] Lee Joo-sok
rolling ratio working speed [mpm] 3 800 1.3 1.2 105 2×10 ¹⁵ 262.1 Inventive example 3 4 800 1.1 2 75 8×10 ¹⁴ 301.8 Inventive Example 4 5 830 1.2 2 75 7×10 ¹⁴ 310.2 Inventive Example 5 6 810 One 2.5 65 3×10 ¹⁵ 288.5 Inventive Example 6 7 810 0.9 2.9 50 6×10 ¹⁴ 305.1 Inventive Example 7 8 810 1.5 0.7 140 7×10 ¹⁴ 265.3 Inventive Example 8 10 810 1.1 1.5 100 9×10 ¹⁴ 275.7 Comparative Example 2 11 810 1.1 1.5 105 8×10 ¹⁴ 285.7 Comparative Example 3 12 810 2.5 2.5 70 2×10 ¹⁶ 399.5 Comparative Example 4 13 810 0.5 One 115 4×10 ¹⁴ 175.6 Comparative Example 5 14 810 0.3 1.2 110 3×10 ¹⁴ 123.6 Comparative Example 6 15 810 One 0.3 180 1×10 ¹⁴ 102.1 Comparative Example 7 16 810 1.1 4.2 30 4×10 ¹⁶ 450 Comparative Example 8 17 800 1.2 1.5 200 2×10 ¹⁴ 105.3 Comparative Example 9

The physical properties of the steel sheets prepared under the conditions of Tables 4 and 5 were measured in the same manner as in Experimental Example 1, and are shown in Table 6 below.

steel grade Property evaluation note YS
[MPa] TS
[MPa] El
[%] Low BH*
[MPa] AI*
[%] Amount of BH reduction*
[MPa] 3 240 364 38 45.3 0 0 Inventive example 3 4 221 351 41 38.1 0 0 Inventive Example 4 5 232 350 40 38.2 0 0 Inventive Example 5 6 232 361 39 32.5 0 0 Inventive Example 6 7 215 366 38 33.3 0 0 Inventive Example 7 8 239 355 40 45.6 0 0 Inventive Example 8 10 277 406 31 76.3 4.2 40 Comparative Example 2 11 209 366 37 0 0 0 Comparative Example 3 12 266 363 31 29.1 0 0 Comparative Example 4 13 259 354 38 27.1 1.1 15 Comparative Example 5 14 266 348 36 25.1 1.9 30 Comparative Example 6 15 259 355 34 33.1 0.6 15 Comparative Example 7 16 277 377 34 40.5 0 0 Comparative Example 8 17 231 345 39 43.0 0.5 15 Comparative Example 9

As can be seen in Table 6, in the case of Inventive Examples 3 to 8 satisfying the alloy composition and manufacturing conditions according to the present invention, the yield strength is 215 to 240 MPa, the tensile strength is 340 MPa or more (within the range of 340 to 366 MPa), the elongation is 38 to 41%, and the lower BH value also showed excellent material properties of 30 MPa or more (ie, within the range of 32.5 to 45.6 MPa). In addition, the average dislocation density in the surface layer) satisfies 5 × 10 ¹⁴ to 1 × 10 ¹⁶ /m ² , and the residual stress is also distributed in the range of 250 to 350 MPa, so that the aging index (AI) is 0 and at the same time 100 ° C. The lower BH reduction after 1 hour heat treatment was 0%, and it was confirmed that it had excellent baking hardenability and endoscopic effect. In addition, in Inventive Examples 3 to 8, the average dislocation density of the entire steel sheet was 4×10 ¹² to 7×10 ¹² /m ² , and the microstructure of the cold-rolled steel sheet had an area fraction of 99% or more of ferrite (ie, , 100%), and it was confirmed that the average particle diameter of these ferrites was 5 to 10 μm.

On the other hand, in the case of Comparative Example 2, the C content was 0.007%, exceeding the C content range of the present invention, so that even if other component compositions and temper rolling conditions met the present invention range, the C content was too high, resulting in material deterioration and aging deterioration. occurred simultaneously.

In addition, in the case of Comparative Example 3, Nb is 0.035%, which exceeds the range of the present invention, and all of the added carbon is present as NbC precipitate, corresponding to the case where there is no solid solution carbon at all in the steel species. Therefore, in Comparative Example 3, the lower BH value was 0 MPa due to the high amount of Nb added, and thus the bake hardenability was inferior.

In addition, in the case of Comparative Example 4, the alloy composition satisfies the range of the present invention, but the temper rolling ratio during temper rolling is excessively high as 2.5%. Accordingly, the dislocation density of the surface layer portion of Comparative Example 4 exceeded the conditions presented in the present invention, and the residual stress of the surface layer portion was also out of the range of 250 to 350 MPa. Therefore, due to excessive temper rolling, material deterioration occurred in which the yield strength was as high as 266 MPa and the lower BH value did not satisfy 30 MPa or more, although the endoscopic effect was excellent.

In Comparative Examples 5 and 6, the alloy composition and the two-speed rolling ratio of the present invention were satisfied, but the temper reduction was very low, such as 0.5% or 0.3%, and the yield point elongation occurred. Due to the low temper rolling ratio, the dislocation density and residual stress in the surface layer did not satisfy the scope of the present patent. For this reason, the yield strength was high due to the elongation at the yield point, and the lower BH value did not satisfy 30 MPa. In addition, it showed very degraded results in terms of endoscopic efficacy.

In Comparative Examples 7 and 8, the alloy composition of the present invention was satisfied, but the rolling ratio at different speeds in temper rolling was out of the condition of the present invention steel. Comparative Example 7 had a low rolling ratio of 0.3%, so sufficient dislocations were not generated in the surface layer, and the residual stress was also low, resulting in poor eunuch efficiency.

In addition, in Comparative Example 8, the two-speed rolling ratio was very high at 4.2%, and the shear stress in the surface layer increased due to the excessive two-speed rolling ratio, which increased the residual stress to 450 MPa, which corresponds to the condition exceeding the standard of the present invention steel . Therefore, it was confirmed that although the BH property and room temperature aging performance were excellent, surface defects such as scratches on the surface of the steel sheet occurred along with the equipment load during rolling.

In Comparative Example 9, the alloy composition of the present invention, the rolling rate of temper rolling, and the rolling ratio at different speeds all satisfy the conditions presented in the present invention steel, but the temper rolling operation speed is 200 mpm, which is the range suggested by the present invention steel. This is the case outside of ~150 mpm. That is, even if the temper rolling ratio and the different speed rolling ratio satisfy the range of the present invention, the temper rolling speed is very fast and it is not possible to apply sufficient stress to the surface layer, and as a result, the residual stress of the surface layer is out of the range of the present invention steel. This lack of stress in the surface layer showed deterioration in terms of the AI value representing the endoscopic effect and the amount of BH reduction after heat treatment at 100 ° C / 1 hr.

Claims (14)

By weight, carbon (C): 0.005% or less (excluding 0%), manganese (Mn): 0.1 to 1.0%, silicon (Si): 0.3% or less (excluding 0%), phosphorus (P): 0.01 ~0.08%, Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01 to 0.06%, Niobium (Nb) : 0.003 to 0.015%, boron (B): 0.0015 to 0.0045%, the balance including iron (Fe) and unavoidable impurities,
As a microstructure, it contains 99 area% or more ferrite,
The average dislocation density of the surface layer is 5×10 ¹⁴ to 1×10 ¹⁶ /m ² ,
The residual stress of the surface layer is 250 to 350 MPa,
The surface layer portion represents an area from the surface to 1/10t in the thickness direction, and t represents the overall average thickness of the steel sheet.
The method of claim 1,
A cold-rolled steel sheet having an R _A value of 1 to 2.5 defined by the following relational expression 1-1.
[Relationship 1-1]
R _A = [B]/[N]
(In the relational expression 1-1 above, [B] and [N] are the atomic % of the corresponding alloy element.)
The method of claim 1,
A cold-rolled steel sheet having an R _B value of 0.40 to 0.85 defined by the following relational expression 1-2.
[Relationship 1-2]
R _B = [Nb]/[C]
(In the relational expression 1-2, the [Nb] and [C] are the atomic % of the corresponding alloy element.)
The method of claim 1,
The average dislocation density of the entire steel plate is 5×10 ¹² Lee Sang-in, cold-rolled steel sheet.
The method of claim 1,
The average particle diameter of the ferrite is 5 ~ 10㎛, cold-rolled steel sheet.
The method of claim 1,
The cold-rolled steel sheet has a yield strength of 180 to 250 MPa, a tensile strength of 340 MPa or more, and an elongation of 35% or more.
The method of claim 1,
A cold-rolled steel sheet having a lower baking hardening amount of 30 MPa or more, an aging index (AI) of 0.2% or less, and a BH reduction amount of 10 MPa or less before and after heat treatment at 100 ° C. for 60 minutes.
The cold-rolled steel sheet according to claim 1; and
A plated steel sheet comprising a galvanized layer or a zinc alloyed plating layer formed on at least one surface of the cold-rolled steel sheet.
By weight, carbon (C): 0.005% or less (excluding 0%), manganese (Mn): 0.1 to 1.0%, silicon (Si): 0.3% or less (excluding 0%), phosphorus (P): 0.01 ~0.08%, Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01 to 0.06%, Niobium (Nb) : 0.003 to 0.015%, boron (B): 0.0015 to 0.0045%, the balance iron (Fe) and reheating the steel slab containing unavoidable impurities to 1100 to 1250 ° C;
Obtaining a hot-rolled steel sheet by hot-rolling the reheated steel slab at 850 to 980° C.;
Cooling and winding the hot-rolled steel sheet to 500 to 750° C. at an average cooling rate of 10 to 70° C./s;
Cold-rolling the rolled hot-rolled steel sheet at a reduction ratio of 60 to 90%;
Step of continuously annealing the cold-rolled steel sheet in a temperature range of 750 ~ 860 ℃; and
Including the step of temper rolling the continuously annealed steel sheet at a double speed rolling ratio of 0.6 to 3.0%,
The temper rolling step is performed to satisfy the following relational expression 2, and is performed at a temper rolling speed in the range of 50 to 150 mpm and a reduction ratio of 0.8 to 1.5%, a method for manufacturing a cold-rolled steel sheet.
[Relationship 2]
Y-10 ≤ -60×ln(X) + 117.7 ≤ Y+10
(In the relational expression 2, the Y represents the temper rolling speed, and the unit is mpm. In addition, the X is the double speed rolling ratio, and the unit is %.)
delete delete delete By weight, carbon (C): 0.005% or less (excluding 0%), manganese (Mn): 0.1 to 1.0%, silicon (Si): 0.3% or less (excluding 0%), phosphorus (P): 0.01 ~0.08%, Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01 to 0.06%, Niobium (Nb) : 0.003 to 0.015%, boron (B): 0.0015 to 0.0045%, the balance iron (Fe) and reheating the steel slab containing unavoidable impurities to 1100 to 1250 ° C;
Obtaining a hot-rolled steel sheet by hot-rolling the reheated steel slab at 850 to 980° C.;
Cooling and winding the hot-rolled steel sheet to 500 to 750° C. at an average cooling rate of 10 to 70° C./s;
Cold-rolling the rolled hot-rolled steel sheet at a reduction ratio of 60 to 90%;
Step of continuously annealing the cold-rolled steel sheet in a temperature range of 750 ~ 860 ℃;
immersing the continuously annealed steel sheet in a galvanizing bath at 440 to 500° C. to obtain a steel sheet having a plating layer formed on its surface; and
Including the step of temper rolling the steel sheet on which the plating layer is formed on the surface at a rolling ratio of 0.6 to 3.0%,
The temper rolling step is performed to satisfy the following relational expression 2, and is performed at a temper rolling speed in the range of 50 to 150 mpm and a reduction ratio of 0.8 to 1.5%.
[Relationship 2]
Y-10 ≤ -60×ln(X) + 117.7 ≤ Y+10
(In the relational expression 2, the Y represents the temper rolling speed, and the unit is mpm. In addition, the X is the double speed rolling ratio, and the unit is %.)
The method of claim 13,
Further comprising the step of alloying heat treatment of the steel sheet on which the plating layer is formed at 500 ~ 540 ℃, the manufacturing method of the plated steel sheet.

Images