KR102381829B1 - Cold rolled steel sheet and metal plated steel sheet having excellent bake hardenability and anti-aging properties at room temperature and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a cold-rolled steel sheet, a plated steel sheet, and a manufacturing method thereof, and more particularly, to a cold-rolled steel sheet, a plated steel sheet, and a manufacturing method thereof having excellent bake hardenability and excellent room temperature aging resistance.

Description

COLD ROLLED STEEL SHEET AND METAL PLATED STEEL SHEET HAVING EXCELLENT BAKE HARDENABILITY AND ANTI-AGING PROPERTIES AT ROOM TEMPERATURE AND MANUFACTURING METHOD THEREOF}

The present invention relates to a cold-rolled steel sheet, a plated steel sheet, and a manufacturing method thereof, and more particularly, to a cold-rolled steel sheet, a plated steel sheet, and a manufacturing method thereof having excellent bake hardenability and excellent room temperature aging resistance.

Recently, as part of an active response to weight reduction and environmental problems for improving fuel efficiency of automobiles, a reduction in the thickness of the steel sheet is required.

In addition, in order to be applied as a material for automobile exterior panels, bake hardenability (BH) above a certain level is required. Bake hardening is a phenomenon in which the activated carbon and nitrogen are fixed to the dislocation generated during press and when painting is baked, the yield strength is increased. By having this improved characteristic, it is a very ideal material for automobile exterior panels.

In addition, in order to be applied as a material for automobile exterior panels, a certain level of aging resistance is required to guarantee aging over a certain period of time.

In general, cold-rolled steel sheet having bake hardenability is baked-hardened by simply winding low-carbon, P-added Al-Killed steel at a low temperature (low-temperature winding where the hot-rolling temperature is in the range of 400 to 500°C) and then applying the top annealing method. The method of manufacturing steel with a weight of about 40-50 MPa was mainly used. This was because coexistence of moldability and bake hardenability was easier by upper annealing. On the other hand, in the case of Al-Killed steel with P addition by continuous annealing, it is easy to secure bake hardenability because a relatively fast cooling rate is used, but there is a problem in that the formability is deteriorated due to rapid heating and short-time annealing. Its use is limited only to automobile exterior panels that do not require processability.

Thanks to the recent rapid development of steelmaking technology, it is possible to control the appropriate amount of dissolved elements, and the use of aluminum-killed steel sheet containing strong carbonitride forming elements such as Ti or Nb is used, which has excellent formability. Cold-rolled steel sheet is manufactured and its use is increasing for automotive exterior panels that require dent resistance.

In the case of Patent Document 1, C: 0.0005 to 0.015% and S + N ≤ 0.005% It relates to an ultra-low carbon cold-rolled steel sheet containing a composite addition of Ti, Ti, and Nb, and in the case of Patent Document 2, the content of C is 0.010% or less, Ti Disclosed is a method for producing steel having a bake hardening amount of about 40 MPa or more using an additive steel. In this method, by controlling the amount of Ti and Nb added or the cooling rate during annealing, the amount of dissolved elements in the steel is appropriately adjusted to prevent deterioration of the material while providing bake hardenability. However, in the case of Ti or Ti and Nb composite added steel, strict control of Ti, N, and S is required in the steelmaking process in order to secure an appropriate amount of bake hardening, so a problem of cost increase may occur. In addition, in the above literature, in the case of Nb-added steel, it is expected that the workability deteriorated due to high temperature annealing and the manufacturing cost increased due to the addition of special elements.

On the other hand, Patent Document 3 using the addition of a new alloying element discloses an increase in bake hardenability by adding Sn, and Patent Document 4 discloses an effect of improving ductility through stress concentration relaxation at grain boundaries by compounding V with Nb. In Patent Document 5, the moldability improvement effect by Zr, and in Patent Document 6, Cr is added to increase the strength and minimize the deterioration of the work hardening index, thereby promoting the formability.

However, the above documents focus only on improving bake hardenability or moldability, and there is no mention of deterioration of aging resistance due to increase in bake hardenability, so it is urgent to establish countermeasures.

Japanese Unexamined Patent Publication (Kokai) No. 61-026757 (published on June 6, 1986) Japanese Unexamined Patent Publication (Kokai) No. 57-089437 (published on June 3, 1982) Japanese Laid-Open Patent Publication No. 1994-306531 (published on January 1, 1994) Japanese Laid-Open Patent Publication No. 1997-249936 (published on September 22, 1997) Japanese Laid-Open Patent Publication No. 1996-049038 (published on February 20, 1996) Japanese Laid-Open Patent Publication No. 1995-278654 (published Oct. 24, 1995)

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

The subject of the present invention is not limited to the above. A person of ordinary skill in the art will have no difficulty in understanding the further problems of the present invention from the overall content of 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 containing iron (Fe) and unavoidable impurities;

R value defined in the following relation 1 is 1 to 2.5,

It contains 99 area% or more of ferrite as a microstructure, and the average particle diameter of the ferrite is 5 to 10 μm,

The average dislocation density of the region from the surface to 1/10t (here, t means the thickness of the steel sheet) is 5×10 ¹⁴ to 1×10 ¹⁶ /m ² , and the average dislocation density over the entire thickness is 5×10 ¹² /m It is possible to provide a cold-rolled steel sheet excellent in bake hardenability and room temperature aging resistance of ² or more.

[Relational Expression 1]

R = [B]/[N] (atomic ratio)

(Here, [B] and [N] are atomic % of the corresponding alloying element.)

The steel sheet may have a yield strength of 210 to 270 MPa, a tensile strength of 340 MPa or more, and an elongation of 35% or more.

The bake hardening amount (Lower BH, 170°C, tensile test after 20 minutes heat treatment) of the steel sheet is 30 MPa or more, and the aging index (AI, 100° C., tensile test after 60 minutes heat treatment) is 0.2% or less, and 1 at 100° C. After the time heat treatment, the BH reduction amount before the heat treatment may be 10 MPa or less.

Another aspect of the present invention, the above-described cold-rolled steel sheet; and

It is possible to provide a plated steel sheet excellent in bake hardenability and room temperature aging resistance including a plating layer or an alloy plating layer formed on at least one side of the cold-rolled steel sheet.

Another 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%, Re-heating the steel slab containing the remainder iron (Fe) and unavoidable impurities, and having an R value of 1 to 2.5 defined in the following relation 1 to a temperature range of 1160 to 1250° C.;

hot rolling the reheated steel slab to a temperature range of 850 to 980 °C;

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

cold rolling the cooled steel sheet at a reduction ratio of 70 to 90%;

continuously annealing the cold-rolled steel sheet in a temperature range of 750 to 860°C;

The continuously annealed steel sheet is temper-rolled at a reduction ratio of 1.0 to 2.0%, the line load (A) being 1×10 ⁶ ~ 5×10 ⁸ N/m ² , and the tension (B) being 1×10 ⁵ ~2 It is controlled to ×10 ⁶ N/m ² , and the ratio (A/B) of preload (A) to tension (B) is 50 to 150. there is.

[Relational Expression 1]

R = [B]/[N] (atomic ratio)

(Here, [B] and [N] are atomic % of the corresponding alloying element.)

Another aspect of the present invention, the step of immersing the above-described cold-rolled steel sheet in a hot-dip plating bath having a temperature in the range of 440 ~ 500 ℃ after the continuous annealing; and

Optionally, it is possible to provide a method of manufacturing a plated steel sheet excellent in bake hardenability and room temperature aging resistance, further comprising the step of alloying heat treatment at a temperature in the range of 500 ~ 540 ℃ the hot-dip plated steel sheet.

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

According to another aspect of the present invention, it is possible to provide a steel sheet that can be applied as a material for an automobile exterior panel and a method for manufacturing the same.

Hereinafter, preferred embodiments of the present invention will be described. 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 explain the present invention in more detail to those skilled in the art to which the present invention pertains.

In the present invention, the alloy composition and process conditions are optimized in order to manufacture a steel sheet having aging resistance at room temperature and having a bake hardenability of 30 MPa or more at a baking temperature (usually 170° C., 20 minutes). In particular, the present inventor confirmed that it was possible to secure both room temperature aging resistance and bake hardenability at the same time by appropriately controlling the addition amount of B and N, and it was confirmed that dislocation density has a large effect on room temperature aging resistance and aging characteristics after painting and baking did Accordingly, it was confirmed that aging resistance can be secured by controlling the average dislocation density of the entire steel sheet and the average dislocation density of the surface layer by optimizing the conditions during temper rolling, thereby completing the present invention.

Hereinafter, the present invention will be described in detail.

Hereinafter, the steel composition of the present invention will be described in detail.

In the present invention, unless otherwise specified, percentages indicating the content of each element are based on weight.

The steel sheet according to an aspect of the present invention is, 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% , the remainder may include iron (Fe) and unavoidable impurities.

Carbon (C): 0.005% or less

Carbon (C) is an interstitial solid solution element that is dissolved in the steel sheet during cold rolling and annealing and interacts (locking) with dislocations formed by temper rolling to exhibit bake hardenability. Basically, the higher the carbon (C) content, the more Bake hardening ability is improved. However, if excessive solid carbon is present in the material, it causes a defect called orange peel on the surface during part molding, which may lead to poor aging. If the content of carbon (C) exceeds 0.005%, it is disadvantageous in terms of formability, and the room temperature aging resistance is also greatly inferior, so there is a limit to the application of parts. The lower limit value of carbon (C) is not particularly limited, but 0% may be excluded because a possible range is preferable in the manufacturing process.

Accordingly, the content of carbon (C) may be more than 0% and 0.005% or less, and more preferably 0.0010 to 0.003%.

Manganese (Mn): 0.1~1.0%

Manganese (Mn) is a solid solution strengthening element, and not only contributes to an increase in strength, but also serves to precipitate S in steel as MnS. When the content of manganese (Mn) is less than 0.1%, MnS cannot be effectively precipitated, so that drawability may be deteriorated. On the other hand, when the content exceeds 1.0%, the yield strength is increased, but manganese (Mn) is dissolved in excess, which also causes a problem of lowering drawability.

Accordingly, the content of manganese (Mn) may be 0.1 to 1.0%, more preferably 0.2 to 0.9%.

Silicon (Si): 0.3% or less

Although silicon (Si) contributes to the increase in strength of steel by solid solution strengthening, it is not intentionally added in the present invention, and even if silicon (Si) is not added, there is no significant hindrance in securing physical properties. However, 0% is excluded in consideration of the range that is unavoidably included in manufacturing. On the other hand, when the content of silicon (Si) exceeds 0.3%, a problem occurs in that the plating surface properties are inferior.

Accordingly, the content of silicon (Si) may be greater than 0% and less than or equal to 0.3%, more preferably 0.005 to 0.2%.

Phosphorus (P): 0.01 to 0.08%

Phosphorus (P) is the most effective element for securing the strength of steel without significantly impairing the drawability and having the best solid solution effect. In particular, phosphorus (P) is easily segregated at the grain boundary, inhibiting grain growth during annealing, thereby reducing the grain size, which is advantageous in improving the aging resistance at room temperature. When the content of phosphorus (P) is less than 0.01%, it is impossible to secure the strength targeted in the present invention, whereas when the content exceeds 0.08%, an excess of the employed person (P) is segregated at the grain boundary, which is required in the present invention. It is impossible to secure the target room temperature aging resistance by losing the chance of grain boundary segregation of B and C. In addition, as the grain boundary segregation of phosphorus (P) increases, secondary processing embrittlement may occur.

Accordingly, the content of phosphorus (P) may be 0.01 to 0.08%, more preferably 0.015 to 0.075%.

Sulfur (S): 0.01% or less

Sulfur (S) is an impurity that is unavoidably included in steel, and it is desirable to manage its content as low as possible. In particular, since sulfur (S) is highly likely to cause red heat embrittlement, its content is limited to 0.01% or less.

Therefore, the content of sulfur (S) may be more than 0% and 0.01% or less, and more preferably 0.001 to 0.008%.

Nitrogen (N): 0.01% or less

Nitrogen (N) is an impurity that is unavoidably included in steel, and it is important to manage its content as low as possible.

Accordingly, the content of nitrogen (N) may be greater than 0% and less than or equal to 0.01%, more preferably greater than 0% and less than or equal to 0.005%.

Aluminum (sol.Al): 0.01~0.06%

Aluminum (sol.Al) is an element added for particle size refinement and deoxidation. When the content of aluminum (sol.Al) is less than 0.01%, it is impossible to manufacture ordinary aluminum-killed steel in a stable state. . On the other hand, when the content exceeds 0.06%, it is advantageous to increase the strength due to the effect of grain refinement, while inclusions are excessively formed during the steel making operation, which increases the possibility that the surface of the plated steel sheet becomes poor, and the manufacturing cost rises sharply. there is a problem with

Accordingly, the content of aluminum (sol.Al) may be 0.01 to 0.06%, more preferably 0.015 to 0.055%.

Niobium (Nb): 0.003 to 0.015%

Niobium (Nb) combines with C during hot rolling to precipitate NbC, thereby reducing solid solution carbon and affecting bake hardenability and aging resistance. As the C content precipitated as NbC increases, the dissolved C content decreases, which is advantageous in terms of aging resistance, but bake hardenability decreases. According to the present invention, it is very important to control the C content as well as the B content segregated at the grain boundary. Controlling an appropriate level of solid-solution carbon can obtain excellent bake hardenability on the premise of ensuring room temperature aging resistance, and an important element controlling such solid-solution carbon is niobium (Nb).

When the content of niobium (Nb) is less than 0.003%, there is almost no C precipitated as NbC, and most of the C in the steel remains as solid solution carbon, so it is advantageous for bake hardening, but room temperature aging resistance is poor there is On the other hand, when the content exceeds 0.015%, most of the C in the steel precipitates as NbC, and the dissolved carbon content becomes absolutely insufficient. cannot be secured

Accordingly, the content of niobium (Nb) may be 0.003 to 0.015%, more preferably 0.004 to 0.012%.

Boron (B): 0.0015 to 0.0045%

Originally, boron (B) is an element added to prevent secondary processing embrittlement due to grain boundary embrittlement in ultra-low carbon steel containing a large amount of P component. In general, boron (B) has a high tendency for grain boundary segregation compared to other elements, and the addition of boron (B) serves to suppress P segregation at grain boundaries to prevent secondary processing brittleness. In the present invention, many experiments were conducted on bake hardened steel having excellent room temperature aging resistance by using the grain boundary segregation characteristics of boron (B), and based on the results, the present invention was derived.

If boron (B) is segregated at the grain boundary during annealing and then stabilized at room temperature, most of the boron (B) remains at the grain boundary at a low aging evaluation temperature (about 100°C), and diffusion into the grain is suppressed to ensure room temperature aging resistance. can do. Aging and bake hardenability are similar mechanisms, and are generated by locking of solid solution elements (C, B, etc.) and dislocation. As the interaction increases, aging and bake hardenability increase simultaneously. Bake hardening steel used as an exterior plate material is advantageous as the bake hardenability is higher, the aging resistance is lower, that is, the better the aging resistance is. It is important. In the present invention, boron (B) segregates at the grain boundary to secure aging resistance by inhibiting locking of boron (B) with dislocations at room temperature, and increases the interaction between boron (B) and dislocations at high temperature. Controlling to ensure bake hardenability is the key.

If the content of boron (B) is less than 0.0015%, it is not possible to secure the target bake hardenability in the present invention, and if the content exceeds 0.0045%, even if the bake hardenability increases, it may entail inferiority in room temperature aging resistance. In addition, there is a fear that the plated layer of the plated steel sheet may be peeled off.

Accordingly, the content of boron (B) may be 0.0015 to 0.0045%.

The steel sheet of the present invention may include the remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in a normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the art of steel manufacturing, all of them are not specifically mentioned in the present specification.

The steel sheet of the present invention may have an R value of 1 to 2.5 defined in Relation 1 below.

Relation 1 below is for controlling the content of boron solid solution, and means the B/N atomic ratio. If the R value in Relation 1 is less than 1, the added B is consumed as BN precipitate, and the bake hardening effect by boron solid solution cannot be expected, whereas if the value exceeds 2.5, material and plating deteriorate due to excessive increase in boron solid solution. can occur

[Relational Expression 1]

R = [B]/[N] (atomic ratio)

(Here, [B] and [N] are atomic % of the corresponding alloying element.)

Hereinafter, the steel microstructure of the present invention will be described in detail.

In the present invention, unless otherwise specified, % indicating the fraction of microstructure is based on the area.

The steel sheet according to an aspect of the present invention may contain 99 area% or more of ferrite as a microstructure, and the average particle diameter of the ferrite may be 5 to 10 μm.

In addition to ferrite, pearlite, cementite, or bainite may be included as the microstructure, but if the ferrite content is less than 99%, workability may be reduced. Accordingly, the ferrite may contain 99% or more, and more preferably a ferrite single-phase structure.

If the average particle 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 forming. On the other hand, when the average particle diameter exceeds 10 μm, the non-uniformity of dislocation density in the steel sheet increases, and aging resistance after molding or painting and baking may be reduced.

In the steel sheet according to one aspect of the present invention, the average dislocation density of the region from the surface to 1/10t (here, t means the thickness of the steel sheet) is 5×10 ¹⁴ to 1×10 ¹⁶ /m ² , and The average dislocation density may be 5×10 ¹² /m ² or more.

The present inventors have discovered that the control of dislocation density is very important in order to secure room temperature aging resistance and aging characteristics after painting and baking. If the average dislocation density of the entire steel sheet is less than 5×10 ¹² /m ² , after painting and baking, the yield strength decreases with the change of time, that is, the room temperature aging resistance of the material may be deteriorated along with deterioration of dent properties. In addition, when the dislocation density is relatively low with respect to the solid-solution carbon, the movable dislocation, which is relatively easy to move the solid-solution carbon during aging at room temperature, may be rapidly fixed, and thus the room temperature aging resistance may be deteriorated.

The average dislocation density of the region from the surface to 1/10t (here, t means the thickness of the steel sheet) has a favorable effect on the room temperature aging resistance through the increase of the residual stress. If the average dislocation density of the region from the surface to 1/10t is less than 5×10 ¹⁴ /m ² , the dislocation density to suppress aging is not sufficient, and aging performance may not be sufficiently secured due to the lack of residual stress in the surface layer. On the other hand, if the average dislocation density of the region exceeds 1×10 ¹⁶ /m ² , material deterioration such as an increase in yield strength may occur due to excessive dislocation density.

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

The steel sheet according to an aspect of the present invention may be manufactured by reheating, hot rolling, cooling, cold rolling and continuous annealing of a steel slab satisfying the above-described alloy composition.

Slab reheating and hot rolling

The present invention can reheat and hot-roll a steel slab that satisfies the above-described alloy composition.

Since reheating and hot rolling are performed to sufficiently obtain physical properties of the steel sheet, the present invention does not specifically limit the reheating and hot rolling conditions. Therefore, the slab reheating and hot rolling of the present invention can be performed under normal conditions, for example, the slab reheating can be carried out in a temperature range of 1160 to 1250 ° C., and hot rolling can be performed in a temperature range of 850 to 980 ° C. .

Cooling

After cooling the hot-rolled steel sheet to a temperature range of 500 to 750° C. at a cooling rate of 10 to 70° C./s, it can be wound up.

If the cooling temperature and coiling temperature are less than 500℃, the shape of the steel sheet becomes poor, and ductility may be reduced due to the formation of fine grains. If the temperature exceeds 750℃, coarse ferrite grains are formed, and coarse carbides and nitrides It is easy to form and there exists a possibility that the material of steel may become inferior.

cold rolled

The cooled steel sheet may be cold rolled at a reduction ratio of 70 to 90%.

When the reduction ratio is less than 70%, it may be difficult to secure the target thickness of the steel sheet, and it may be difficult to correct the shape of the steel sheet. On the other hand, when the cold rolling reduction ratio exceeds 90%, cracks may occur in the edge portion of the steel sheet, and a load of cold rolling may be caused.

continuous annealing

The cold-rolled steel sheet may be continuously annealed in a temperature range of 750 to 860°C.

If the continuous annealing temperature is less than 750℃, recrystallization is not sufficiently completed and a mixed grain structure occurs, whereas if the temperature exceeds 860℃, there is a very high risk of problems with on-site equipment due to high-temperature annealing, and the crystal grains become coarse. It is not possible to secure the characteristics targeted by the invention. Cooling after continuous annealing may be performed under normal operating conditions.

If necessary, a plated steel sheet may be manufactured by applying a plating process to the cold rolled steel sheet of the present invention, and a hot-dip plated steel sheet may be manufactured by hot-dip plating and alloying heat treatment.

plating bath immersion

When manufacturing a hot-dip galvanized steel sheet (GI), the cold-rolled steel sheet manufactured through the above-described process may be immersed in a hot-dip zinc-based plating bath having a temperature range of 440 to 500°C.

alloy heat treatment

If necessary, when the alloyed hot-dip galvanized steel sheet (GA) is manufactured, it may be immersed in the plating bath and then heat treated for alloying in a temperature range of 500 to 540°C.

temper rolling

The steel sheet is temper-rolled at a reduction ratio of 1.0 to 2.0%, but the line load (A) is in the range of 1×10 ⁶ to 5×10 ⁸ N/m ² , and the tension (B) is 1×10 ⁵ to 2×10 It is controlled in the range of ⁶ N/m ² , and at this time, the ratio (A/B) of the line load (A) to the tension (B) may be 50 to 150.

During temper rolling, if the reduction ratio is less than 1.0%, sufficient dislocations are not formed, which is disadvantageous in terms of plate shape, and in particular, there is a possibility that plating surface defects may occur. On the other hand, if the reduction ratio exceeds 2.0%, material deterioration due to an excessive increase in dislocation density of the surface layer, and plate breakage due to equipment capacity limitations may occur.

When the line load (A) is less than 1×10 ⁶ N/m 2 , the amount of dislocation introduced into the surface layer part of the steel sheet (here, the surface layer means the area from the surface of the steel sheet to 1/10t) is small, and the yield strength is lowered when maintaining room temperature, that is, , the dent property may deteriorate, and the room temperature aging resistance of the material may be reduced. On the other hand, if it exceeds 5×10 ⁸ N/m 2 , since the average dislocation density increases, the ductility of the steel sheet is lowered and cracks occur during press forming, and there is a risk that bake hardenability may be lowered.

On the other hand, if the tension (B) is less than 1×10 ⁵ N/m 2 , the shape of the steel plate may be poor and use as an exterior plate for automobiles may be unsuitable. On the other hand, if it exceeds 2×10 ⁶ N/m 2 , there is a fear that plate breakage may occur, and productivity may decrease.

If the ratio (A/B) of the line load (A) to the tension (B) is less than 50, dislocations are not introduced to the center of the sheet thickness, and the average dislocation density of the entire steel sheet is 5×10 ¹² /m ² proposed in the present invention. The above conditions cannot be satisfied. On the other hand, when it exceeds 150, the average dislocation density of the surface layer may not satisfy the 5×10 ¹⁴ ~ 1×10 ¹⁶ /m ² proposed in the present invention due to excessive stress applied to the surface layer of the plate thickness.

The steel sheet of the present invention prepared in this way has a yield strength of 210 to 270 MPa, a tensile strength of 340 MPa or more, and an elongation of 35% or more, a Lower BH of 30 MPa or more to evaluate bake hardenability, and an AI index evaluating room temperature aging resistance of 0.2 % or less, and after 100 ° C. / 1 hr heat treatment, the BH decrease with respect to before the heat treatment is 10 MPa or less, can have bake hardenability and excellent room temperature aging resistance, and can have aging resistance after painting baking .

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention.

(Example)

After the slab was manufactured with the alloy composition shown in Table 1 below, reheating, hot rolling and cold rolling were performed using normal operating conditions. In the present invention, the slab reheating temperature was about 1200 ℃, the finish rolling temperature was 920 ℃ or higher Ar3 temperature, and the cooling end temperature and the coiling temperature were 620 ℃. It was cold-rolled in After cold rolling was completed, the steel was annealed under the annealing conditions shown in Table 2 below and then cooled under normal conditions. The GI hot-dip plating temperature for manufacturing the hot-dip galvanized steel sheet was limited to around 470° C., and the hot-dip galvanized steel sheet was subjected to temper rolling under the conditions described in Table 2 below.

In Table 2 below, the grain size of the manufactured steel sheet, the surface layer portion (here, the surface layer area from the surface to 1/4t, t means the thickness of the steel sheet) and the average dislocation density of the entire steel sheet were measured and shown. The grain size was measured using magnifications of 200 times and 500 times, centered on the area of 1/4t (here, t means the thickness of the steel plate) excluding the polar surface layer affected by decarburization. At this time, it was observed that all of the steel sheets in Table 2 contained ferrite in an area of 99% or more as a microstructure. In addition, the dislocation density was measured using a TEM (Transmission Electronic Microscope) at 100,000 times and 150,000 times the magnification of 5x5 orthogonal parallel lines and the number of intersections (N) of dislocations after measuring, ρ = 2N/( Lt) The dislocation density was calculated and expressed by the relational expression. Here, L is the total length of the intersecting line, and t is 0.1 μm as the sample thickness.

steel grade Alloy composition (wt%) R(B/N)
(Relation 1) Nb/C C Mn Si P S N sol. Al Nb B A 0.0015 0.45 0.06 0.032 0.006 0.002 0.021 0.0078 0.003 2.10 0.67 B 0.0021 0.35 0.05 0.041 0.005 0.002 0.034 0.0081 0.0025 1.75 0.50 C 0.0012 0.73 0.05 0.036 0.004 0.0015 0.045 0.0075 0.0023 2.15 0.81 D 0.0017 0.32 0.06 0.035 0.004 0.001 0.043 0.0083 0.0015 2.10 0.63 E 0.0017 0.22 0.04 0.041 0.006 0.0035 0.052 0.008 0.0032 1.28 0.61 F 0.002 0.55 0.05 0.05 0.004 0.0028 0.037 0.007 0.0035 1.75 0.45 G 0.0018 0.43 0.02 0.035 0.002 0.003 0.033 0.006 0.0025 1.17 0.43 H 0.0027 0.65 0.06 0.052 0.004 0.002 0.035 0.0091 0.0025 1.75 0.43 I 0.004 0.55 0.07 0.054 0.007 0.004 0.045 0 0.004 1.40 0.00 J 0.007 0.45 0.04 0.05 0.005 0.003 0.046 0.007 0.0025 1.17 0.13 K 0.0017 0.43 0.05 0.055 0.003 0.004 0.056 0.035 0 0.00 2.66 L 0.0015 0.34 0.03 0.038 0.004 0.002 0.048 0.006 0.003 2.10 0.52 M 0.004 0.55 0.04 0.035 0.005 0.0045 0.046 0.008 0.004 1.24 0.26 N 0.0029 0.5 0.02 0.049 0.006 0.0055 0.055 0.01 0.0044 1.12 0.44 O 0.0022 0.35 0.04 0.06 0.005 0.003 0.046 0.013 0.0031 1.45 0.76 P 0.0038 0.65 0.02 0.043 0.005 0.003 0.051 0.006 0.0034 1.59 0.20 Q 0.0021 0.71 0.03 0.055 0.005 0.003 0.049 0.008 0.0061 2.82 0.49 R 0.0025 0.43 0.02 0.045 0.006 0.003 0.042 0.009 0.0025 1.17 0.46 S 0.0018 0.38 0.03 0.044 0.005 0.0015 0.039 0.008 0.0042 3.92 0.57

[Relational Expression 1]

R = [B]/[N] (atomic ratio)

(Here, [B] and [N] are atomic percent of the corresponding alloying element.)

steel grade Annealing temperature
(℃) temper rolling (N/m ² ) microstructure Dislocation density (/m ² ) division Line load (A) Tension (B) Ratio (A/B) reduction ratio (%) grain
Size (㎛) superficial Average A 810 3x10 ⁷ 5x10 ⁵ 60 1.5 6 6x10 ¹⁴ 7x10 ¹² Invention lecture 1 B 820 4x10 ⁷ 4x10 ⁵ 100 1.5 6.5 7x10 ¹⁴ 8x10 ¹² Invention lecture 2 C 800 4x10 ₇ 4x10 ⁵ 100 1.5 5 7x10 ¹⁴ 8x10 ¹² Invention lecture 3 D 800 3x10 ₇ 5x10 ⁵ 60 1.5 5.2 6x10 ¹⁴ 7x10 ¹² Invention lecture 4 E 830 6x10 ⁷ 5x10 ⁵ 120 1.5 7.9 6x10 ¹⁴ 1x10 ¹³ Invention River 5 F 810 1x10 ⁷ 2x10 ⁵ 50 1.2 5.1 8x10 ¹⁴ 6x10 ¹² Invention lecture 6 G 810 9x10 ⁷ 1x10 ⁶ 90 1.8 4.9 5x10 ¹⁵ 2x10 ¹³ Invention Lesson 7 H 810 4x10 ⁷ 4x10 ⁵ 100 1.5 6.1 7x10 ¹⁴ 8x10 ¹² Invention lecture 8 I 810 3x10 ⁷ 5x10 ⁵ 60 1.5 5.5 6x10 ¹⁴ 7x10 ¹² Comparative lecture 1 J 810 3x10 ⁷ 5x10 ⁵ 60 1.5 5.1 6x10 ¹⁴ 7x10 ¹² Comparative lecture 2 K 810 4x10 ⁷ 4x10 ⁵ 100 1.5 5.3 7x10 ¹⁴ 8x10 ¹² Comparative lecture 3 L 810 4x10 ⁸ 1x10 ⁶ 400 2.5 5.9 2x10 ¹⁶ 3x10 ¹³ Comparative lecture 4 M 810 5x10 ⁴ 4x10 ⁵ 1.25 One 6.1 3x10 ¹⁴ 6x10 ¹² Comparative steel 5 N 810 4x10 ⁵ 5x10 ⁵ 0.8 1.1 6.6 2x10 ¹⁴ 7x10 ¹² Comparative lecture 6 O 810 3x10 ⁷ 3x10 ⁷ One 1.5 5.7 1x10 ¹⁴ 7x10 ¹² Comparative lecture 7 P 810 2x10 ⁶ 8x10 ⁷ 0.025 1.5 5.2 1x10 ¹⁴ 1x10 ¹³ Comparative steel 8 Q 810 3x10 ⁷ 4x10 ⁵ 75 1.6 3.2 7x10 ¹⁴ 4x10 ¹³ Comparative lecture 9 R 900 3x10 ⁷ 6x10 ⁵ 50 1.5 15.7 6x10 ¹⁴ 5x10 ¹² Comparative Steel 10 S 810 4x10 ⁷ 6x10 ⁵ 67 1.5 4.5 6x10 ¹⁴ 5x10 ¹² Comparative lecture 11

Table 3 below shows the mechanical properties of the manufactured steel sheet. A tensile test was conducted in the L direction (longitudinal direction) using the ASTM standard for the steel material on which the temper rolling was completed, and the yield strength (YS), tensile strength (TS) and elongation (El) were measured, and the bake hardening amount (BH) and The aging index was investigated in the C direction (direction perpendicular to the rolling direction) using the same ASTM standard material. Bake hardenability (Lower BH) is evaluated by the difference in lower yield strength by measuring the flow stress at 2% with 2% pre-strain, and performing a tensile test after heat-treating the same piece at 170°C and baking for 20 minutes. did The aging resistance (AI) was measured by measuring the elongation at the yield point (YP-El) during a tensile test after heat treatment at 100° C. for 60 minutes. In addition, the BH reduction amount represents the difference between the BH value after heat treatment at 100° C. for 1 hour and the BH value before the heat treatment.

steel grade mechanical properties division YS(MPa) TS(MPa) El (%) Low BH
(Mpa) AI (%) After 100℃/1hr heat treatment
BH reduction (Mpa) A 221 345 39 40.5 0 0 Invention lecture 1 B 216 355 38 39.1 0 0 Invention lecture 2 C 245 369 37 42.1 0 0 Invention lecture 3 D 225 354 40 39.3 0 0 Invention lecture 4 E 217 348 41 40.8 0 0 Invention River 5 F 231 364 38 33.5 0 0 Invention lecture 6 G 236 358 38 31.5 0 0 Invention Lesson 7 H 231 348 39 42 0 0 Invention lecture 8 I 271 354 34 65 2.2 30 Comparative lecture 1 J 287 399 34 85.1 3.5 35 Comparative lecture 2 K 210 352 39 0 0 0 Comparative lecture 3 L 275 363 32 34.1 0 0 Comparative lecture 4 M 240 354 38 35.1 0.5 15 Comparative Steel 5 N 230 348 38 36.3 1.1 25 Comparative lecture 6 O 220 355 37 35.5 0.8 20 Comparative lecture 7 P 245 365 37 40.5 2.2 25 Comparative steel 8 Q 271 401 32 37.1 0 0 Comparative lecture 9 R 240 365 36 45.3 0.5 15 Comparative Steel 10 S 269 385 32 39.5 0 0 Comparative Steel 11

As shown in Tables 2 and 3, Inventive Steels 1 to 8 satisfying the alloy composition, Relational Expression 1 and manufacturing conditions of the present invention satisfied the grain size and dislocation density suggested in the present invention, and the lower objective of the present invention After the BH value, AI index, and heat treatment, the BH reduction amount with respect to before the heat treatment was satisfied.

On the other hand, Comparative Steel 1 is a case in which Nb is not added at all, so that all of the added carbon is present as a solid solution element. Even if the temper rolling condition satisfies the conditions presented in the present invention, the yield strength is high and the elongation is low due to the presence of excessive solid solution carbon. At this 30 MPa, the aging resistance at room temperature was very poor.

Comparative Steel 2 has a carbon content of 0.007%, exceeding the carbon range of the present invention. Even if other components and temper rolling meet the conditions of the present invention, the carbon content is excessively high, so material deterioration and aging deterioration occur simultaneously did.

Comparative Steel 3 has an Nb content of 0.035%, exceeding the range of the present invention, and an Nb/C ratio of 2.7. An Nb/C ratio of 1.0 or more means that all of the added carbon is present as NbC precipitates, and there is no solid solution carbon in the steel grade. Due to this high addition of Nb, the lower BH value was 0 MPa.

Comparative Steel 4 satisfies all of the composition ranges of the present invention, but the preload/tension ratio (A/B) is 400 under the condition of temper rolling, which is out of the range suggested by the present invention. Due to the application of such an excessive line load, the average dislocation density of the region from the surface layer to 1/10t was out of the scope of the present invention, which resulted in an increase in yield strength and inferior elongation.

In Comparative Steels 5 to 8, the alloy composition satisfies all of the ranges suggested in the present invention, but the temper rolling condition is out of the condition of the present invention. That is, in Comparative Steels 5 and 6, the tension satisfies the conditions presented by the present invention, but the ratio of preload to tension (A/B) is out of 50 to 150 presented in the present invention. In other words, the tension value compared to the line load was relatively high, so that a sufficient dislocation was not generated in the region from the surface layer to 1/10t. As a result, the AI index exceeded 0.2%, which is the standard of the present invention, and L after heat treatment at 100°C for 1 hour -BH decrease was inferior to 15~25MPa. On the other hand, Comparative Steels 7 and 8 is a case in which the tension ratio (A/B) of the preload to the tension is low because the tension is higher than the range of the present invention during the temper rolling conditions. Due to this, the AI value and the amount of lower BH decrease after heat treatment at 100° C. for 1 hour did not meet the range suggested by the present invention.

Comparative Steel 9 is a case where other alloy components and manufacturing conditions except for the addition amount of boron satisfy the conditions of the present invention. The boron content was 0.0061%, which was out of the upper limit of the boron content presented in the present invention, and thus the B/N ratio was 2.82, which was out of the B/N range condition presented in the present invention. Even if the annealing temperature, temper rolling rate, and other operating conditions given during temper rolling were satisfied, the B content was very high, resulting in fine grains, and high strength and poor elongation due to excessive boron segregation at grain boundaries.

In Comparative Steel 10, the alloy component satisfies the conditions of the present invention, but the grain size is 15.7 μm due to high annealing temperature, which is out of the condition of the grain size suggested in the present invention. In particular, since the annealing temperature was high, a mixed grain structure occurred, which resulted in the AI value and the decrease in BH after heat treatment at 100° C./1 hr outside the range suggested in the present invention. The steel material to which B is added as in the present invention has a high possibility of generating a mixed structure when an excessive annealing temperature is increased, and this mixed structure is very disadvantageous in terms of material non-uniformity, particularly aging resistance. Therefore, it can be seen that the annealing temperature must be controlled within the range of the temperature suggested in the present invention, so that fine crystal grains can be manufactured.

Comparative Steel 11 is a case where the alloy composition, annealing, and temper rolling conditions are all included in the range suggested in the present invention, but the B/N ratio (atomic ratio) is 3.92, which is out of the B/N ratio conditions suggested in the present invention. That is, as in Comparative Steel 11, even if the content of B and N satisfies the conditions of the present invention, when the B/N ratio is excessively high, the boron remaining after forming BN precipitates is segregated at the grain boundaries, resulting in grain refinement as well as excessive segregation of grain boundaries. causes material deterioration. As shown in Table 3, BH and aging resistance were good, but due to excessive boron segregation at the grain boundary, the yield strength and tensile strength increased and the elongation deteriorated.

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

Claims (6)

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,
R value defined in the following relation 1 is 1 to 2.5,
It contains 99 area% or more of ferrite as a microstructure, and the average particle diameter of the ferrite is 5 to 10 μm,
The average dislocation density of the region from the surface to 1/10t (here, t means the thickness of the steel sheet) is 5×10 ¹⁴ to 1×10 ¹⁶ /m ² , and the average dislocation density over the entire thickness is 5×10 ¹² /m Cold-rolled steel sheet with excellent bake hardenability and room temperature aging resistance of ² or higher.
[Relational Expression 1]
R = [B]/[N] (atomic ratio)
(Here, [B] and [N] are atomic % of the corresponding alloying element.)
According to claim 1,
A cold-rolled steel sheet having a yield strength of 210 to 270 MPa, a tensile strength of 340 MPa or more, and an elongation of 35% or more, bake hardenability and room temperature aging resistance of the steel sheet.
According to claim 1,
The bake hardening amount (Lower BH, 170°C, tensile test after 20 minutes heat treatment) of the steel sheet is 30 MPa or more, and the aging index (AI, 100° C., tensile test after 60 minutes heat treatment) is 0.2% or less, and 1 at 100° C. A cold-rolled steel sheet having excellent bake hardenability and room temperature aging resistance with a BH decrease of 10 MPa or less after the time heat treatment compared to before the heat treatment.
The cold-rolled steel sheet of any one of claims 1 to 3; and
A plated steel sheet having excellent bake hardenability and room temperature aging resistance, comprising a plating layer or an alloy plating layer formed on at least one side of the cold-rolled steel sheet.
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 Reheating a steel slab having an R value of 1 to 2.5 defined in the following Relation 1 to a temperature range of 1160 to 1250° C.;
hot rolling the reheated steel slab to a temperature range of 850 to 980 °C;
winding the hot-rolled steel sheet after cooling at an average cooling rate of 10 to 70° C./s to a temperature range of 500 to 750° C.;
cold rolling the cooled steel sheet at a reduction ratio of 70 to 90%;
continuously annealing the cold-rolled steel sheet in a temperature range of 750 to 860°C;
The continuously annealed steel sheet is temper-rolled at a reduction ratio of 1.0 to 2.0%, but the preload (A) is 1×10 ⁶ ~ 5×10 ⁸ N/m ² , and the tension (B) is 1×10 ⁵ ~2 A method of manufacturing a cold-rolled steel sheet with excellent bake hardenability and room temperature aging resistance with a ratio (A/B) of 50 to 150 of preload (A) and tension (B), controlled by ×10 ⁶ N/m ² .
[Relational Expression 1]
R = [B]/[N] (atomic ratio)
(Here, [B] and [N] are atomic % of the corresponding alloying element.)
6. The method of claim 5,
immersing in a hot-dip plating bath having a temperature in the range of 440 to 500° C. after the continuous annealing; and
Optionally, the method of manufacturing a plated steel sheet excellent in bake hardenability and room temperature aging resistance further comprising the step of alloying heat treatment at a temperature in the range of 500 ~ 540 ℃ the hot-dip plated steel sheet.