KR101709201B1

KR101709201B1 - Ferritic lightweight steel sheet having excellent strength and ductility and method for manufacturing the same

Info

Publication number: KR101709201B1
Application number: KR1020150094345A
Authority: KR
Inventors: 김한수; 김낙준; 알리레자 자르가란; 손창영; 구민서
Original assignee: 주식회사 포스코; 포항공과대학교 산학협력단
Priority date: 2015-07-01
Filing date: 2015-07-01
Publication date: 2017-02-22
Also published as: KR20170005251A; US20170002436A1

Abstract

The ferritic steel sheet according to one embodiment of the present invention comprises 0.01 to 0.3% by weight of C, 0.5 to 8% by weight of Mn, 5 to 12% by weight of Al and 5 to 12% by weight of Nb, based on 100% To 0.2% by weight, and the remainder may contain Fe and impurities.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a high strength and high ductility lightweight steel sheet,

To a high strength and high ductility ferritic lightweight steel sheet and a manufacturing method thereof.

Lightweight steels containing a large amount of aluminum in steel are attracting attention as advanced structural materials such as automobile parts because of their high specific strength. Lightweight steels can be divided into ferritic lightweight steels, austenitic lightweight steels, and ferrite-austenite two phase (duplex) lightweight steels.

Ferritic lightweight steels are economical in terms of alloy cost compared to other types of lightweight steels, since the addition of alloying elements is not required for austenite stabilization.

However, it has been reported that when the Al content exceeds 8 wt%, the ductility of the ferritic lightweight steel is greatly lowered. This is because the binding energy of Fe and Al is high, and when the content of Al is high, a regular structure is formed, and the solubility enhancement effect becomes too high, and the activity of dislocation is greatly suppressed.

On the other hand, C is the most effective element for improving the strength of steel. However, when C is added to ferritic lightweight steel, κ-carbide ((Fe, Mn) ₃ AlC) is formed in grain boundaries to cause brittle fracture, Is controlled to a very low level to inhibit the formation of 虜 -carbides.

It is generally known that the size of grain in the structural material has an important influence on the mechanical properties. However, in the study of ferritic lightweight steel, attempts to improve the physical properties by controlling the grain size have been lacking. Most of the ferritic lightweight steels investigated in the past have a grain size of 40 to 90 袖 m which is very coarse and has a disadvantage in that strength and ductility are lower than those of carbon steels of the same grade.

One embodiment of the present invention provides a ferritic lightweight steel sheet having high strength and high ductility.

Another embodiment of the present invention provides a method for producing a ferritic lightweight steel sheet having high strength and high ductility.

The high strength and high ductility ferritic steel sheet according to one embodiment of the present invention comprises 0.01 to 0.3% by weight of C, 0.5 to 8% by weight of Mn, 5 to 12% by weight of Al, 0.015 to 0.2 wt% of Nb, and the remainder may contain Fe and impurities.

The average grain size of the crystal grains present in the steel sheet may be 30 탆 or less. More specifically, the average grain size of the crystal grains may be 15 占 퐉 or less.

The ferritic steel sheet comprises 0.04 to 2.0% by weight of Si, 2.0% by weight or less of Cr (not including 0%) and 1.0% by weight or less of Mo (not including 0%) based on 100% , Ni: not more than 1.0% by weight (not including 0%), Ti: not more than 0.1% by weight (not including 0%), V: not more than 0.2% % Or less (does not include 0%), Zr: 0.2% or less (does not include 0%), or a combination thereof.

The ferrite-based steel sheet may contain a 虜 -carbide in the shape of a sphere, an ellipse, an acicular or a band existing in the steel sheet.

The? -Carbide may be 1 to 10% by volume based on 100 volume% of the total volume of the steel sheet.

The particle size of the 虜 -carbide is 20 nm to 10 탆, and the 虜 -carbide is 5 × 10 ³ to 1 × 10 ⁶ per unit area (mm ² ) Can exist.

The ferritic steel sheet may include an NbC compound present in the steel sheet.

The NbC compound may be contained in an amount of 0.01 to 1% by volume based on 100 volume% of the total volume of the steel sheet.

The NbC compound has a particle diameter of 10 nm to 1 占 퐉, and the NbC compound has a particle size of 5 × 10 ⁴ to 3 × 10 ⁵ per unit area (mm ² ) Can exist.

In the ferritic steel sheet, the content of Al may be 10 to 12 wt%.

The method for producing a ferritic steel sheet according to one embodiment of the present invention comprises 0.01 to 0.3% by weight of C, 0.5 to 8% by weight of Mn, 5 to 12% by weight of Al, 0.0 > Nb: < / RTI > 0.025 to 0.2 weight percent, the remainder comprising Fe and impurities; Hot rolling the heated slab; Cold rolling the hot rolled steel sheet; And annealing the cold-rolled steel sheet after the cold-rolling to a cold-rolled sheet.

The heating temperature in the step of heating the slab may be 1000 to 1250 캜.

In the step of hot rough rolling the slab, the hot rolling temperature may be 700 to 1250 ° C.

And hot rolling at 600 to 850 ° C after the hot rough rolling.

And intermediate annealing at 700 to 900 < 0 > C after the warm rolling step.

The intermediate annealing may further include warm rolling at 600 to 850 < 0 > C.

And hot rolling at 1000 to 1250 캜 after the hot rough rolling.

After the hot rolling, intermediate annealing may be further performed at 700 to 900 ° C.

In the step of annealing the cold rolled sheet, the annealing temperature of the cold rolled sheet may be 650 to 900 캜.

The slab may contain 0.04 to 2.0% by weight of Si, not more than 2.0% by weight of Cr (not including 0%), not more than 1.0% by weight of Mo (not including 0%), Ni: not more than 1.0 wt% (not including 0%), Ti: not more than 0.1 wt% (not including 0 wt%), V: not more than 0.2 wt% (Not including 0%), Zr: not more than 0.2 wt% (not including 0%), or a combination thereof.

According to one embodiment of the present invention, a ferrite-based steel sheet having a low specific gravity, a high tensile strength and a high elongation rate can be provided.

FIG. 1 is an optical microscope photograph of an inventive steel 1 in which hot rolling has been completed.
2 is an optical microscope photograph of the comparative steel 2 in which hot rolling is completed.
FIG. 3 is a scanning electron microscope (SEM) photograph of the invention steel 1 in which hot rolling has been completed.
4 is a transmission electron microscope (TEM) photograph of the invention steel 1 in which hot rolling has been completed.
5 is an SEM photograph of the invention steel 1 in which warm rolling is completed.
6 is an SEM photograph of an inventive steel 1 having completed intermediate annealing.
FIG. 7 is an electron backscattering diffraction (EBSD) analysis image of the inventive steel 1 before annealing of the cold-rolled steel sheet subjected to cold rolling.
8 is an EBSD analysis image obtained by cold-rolling an inventive steel 1 at 700 ° C for 5 minutes.
9 is an EBSD analysis image obtained by annealing cold rolled steel sheet 1 at 700 ° C for 15 minutes.
10 is an SEM photograph of Inventive Steel 1 after annealing of the cold rolled sheet is completed.
11 is an SEM photograph of the comparative steel 2 in which the cold-rolled sheet annealing is completed.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims. Like reference numerals refer to like elements throughout the specification.

Thus, in some embodiments, well-known techniques are not specifically described to avoid an undesirable interpretation of the present invention. Unless defined otherwise, all terms (including technical and scientific terms) used herein may be used in a sense commonly understood by one of ordinary skill in the art to which this invention belongs. Whenever a component is referred to as "including" an element throughout the specification, it is to be understood that the element may include other elements, not the exclusion of any other element, unless the context clearly dictates otherwise. Also, singular forms include plural forms unless the context clearly dictates otherwise.

In the drawings, RD denotes the rolling direction and ND denotes the direction perpendicular to the surface of the steel sheet.

The ferritic steel sheet contains 0.04 to 2.0 wt% of Si, 2.0 wt% or less of Cr (does not include 0%), 1.0 wt% or less of Mo (does not include 0% ), Ni: not more than 1.0% by weight (not including 0%), Ti: not more than 0.1% by weight (not including 0%), V: not more than 0.2% (Not including 0%), Zr: not more than 0.2% (not including 0%), or a combination thereof.

First, the reason for the composition limitation will be described.

C plays an important role in enhancing the strength relative to the specific gravity of the steel sheet due to the strengthening effect of the steel and is an essential element for refining the crystal grains of the final product by forming NbC carbide and kappa carbide (κ-carbide, (Fe, Mn) ₃ AlC) . In order to obtain such effects in the present invention, it is preferable that the addition amount of carbon is 0.01 wt% or more. When the addition amount of carbon is more than 0.3% by weight, the hot workability and the cold workability of the steel sheet are significantly deteriorated by inducing high-temperature precipitation of? -Carbides. Therefore, in the present invention, the carbon content is preferably limited to 0.01 to 0.3% by weight. More specifically, the inventive steel may contain 0.03 wt% to 0.25 wt% of C.

Mn interferes with the ordering of Fe and Al to suppress embrittlement due to the formation of an intermetallic compound and also forms MnS by binding with S which is inevitably contained in the steel manufacturing process, It suppresses embrittlement. If the addition amount of Mn is less than 0.5% by weight, it is difficult to obtain the above-mentioned effect. When the addition amount exceeds 8% by weight, austenite is formed and a homogeneous ferrite structure can not be obtained. More specifically, Mn may include 1 to 6% by weight.

Al is an element that lowers the specific gravity of the steel sheet. When the addition amount is less than 5% by weight, the specific gravity reduction effect is insignificant. When the addition amount exceeds 12% by weight, the ordering of the steel sheet is promoted and the ductility of the steel sheet may be lowered . More specifically, Al may include 7 to 12 wt%. Also, in one embodiment of the present invention, Al may be added in an amount of 10 to 12% by weight in one embodiment of the present invention because addition of Al in a high content may compensate for the decrease in ductility.

Nb is a carbonitride-forming element, which improves the strength and formability of the steel and enhances the toughness of the steel by grain refinement. In order to obtain the above-mentioned effect, the addition amount thereof is preferably 0.015% by weight or more. When the content is more than 0.2% by weight, excessive carbide precipitation deteriorates the physical properties of steel making and steel. More specifically, Nb may include 0.02 wt% to 0.15 wt%.

Si improves the strength of the steel sheet by solid solution strengthening and is an element effective for improving the noble strength of the steel sheet due to its low specific gravity. When the addition amount is less than 0.04 wt%, it may be difficult to obtain the above effect. When the addition amount is more than 2.0 wt%, not only the hot workability is lowered but also the formation of intermetallic compounds is accelerated.

Cr serves not only to improve the strength-ductility balance of the steel but also to suppress excessive precipitation of 虜 -carbides. When the addition amount exceeds 2.0% by weight, the ductility and toughness of the steel are deteriorated, and precipitation of carbides such as cementite is promoted at a high temperature, so that hot workability and cold workability of the steel can be greatly lowered.

Mo serves to improve the strength and toughness of the steel. If the addition amount exceeds 1.0% by weight, excessive formation of a hard phase or a precipitate is promoted, thereby deteriorating the physical properties of the steel making and the steel.

Ni plays a role of suppressing excessive precipitation of 虜 -carbides and improving strength and toughness. When the addition amount exceeds 1.0% by weight, the formation of an intermetallic compound is promoted to deteriorate the physical properties of the steel.

Ti is a carbonitride-forming element, which improves the strength and moldability of the steel and suppresses grain coarsening, but toughness may be lowered when the addition amount exceeds 0.1 wt%.

V has the effect of forming fine carbonitride and suppressing crystal grain coarsening, but when the addition amount is more than 0.2% by weight, the toughness may be lowered.

B is added in a small amount to improve the toughness and promote the formation of the hard second phase. If the addition amount exceeds 0.01% by weight, hot workability, ductility and toughness may be lowered.

Zr is an effective element that suppresses hot workability and deterioration of toughness due to segregation of S, but toughness may be lowered when the addition amount exceeds 0.2 wt%.

The mean grain size of the crystal grains of the ferrite-based steel sheet may be 30 占 퐉 or less. If the grain size of the crystal grains exceeds 30 탆, tensile strength and ductility may be lowered. More specifically, the average grain size of the crystal grains may be 15 탆 or less.

The ferritic steel sheet may contain? -Carbides. The 虜 -carbide is a compound represented by the formula (Fe, Mn) ₃ AlC and having a perovskite structure.

The? -carbide may be from 0.5 to 10% by volume based on 100% by volume of the total volume of the steel sheet. When the content of 虜 -carbides is less than 0.5% by volume, the ferrite grain refinement effect is not exhibited, while when it exceeds 10% by volume, workability and ductility are deteriorated.

The particle diameter of the 虜 -carbide is 20 nm to 10 탆, and is 5 × 10 ³ to 1 × 10 ⁶ per unit area (mm ² ) Can exist.

The 虜 -carbides may be in the form of spheres, ellipses, beds, or bands. More specifically, the kappa-carbide may be spherical or oval. If it is in the form of an acicular or band, the 虜 -carbides may cause brittle fracture. In this case, the spherical or elliptical means an aspect ratio of less than 4, and the 虜 -carbide in the form of an acicular or band means an aspect ratio of 4 or more.

In addition, the ferrite-based steel sheet may further include an NbC compound present in the steel sheet.

The NbC compound may be 0.01 to 1% by volume based on 100 volume% of the total volume of the steel sheet. When the content of the NbC compound is less than 0.01 vol%, the effect of suppressing coarsening of the ferrite grains is small, and when it exceeds 1 vol%, the ductility is deteriorated due to excessive carbide formation.

The NbC compound has a particle diameter of 10 nm to 1 占 퐉 and has a surface area of 5 × 10 ⁴ to 3 × 10 ⁵ per unit area (mm ² ) Can exist. It is possible to improve the effect of suppressing the coarsening of the ferrite grains in the above-mentioned range.

The ferritic steel sheet may have a tensile strength of 450 MPa or more. More specifically, it may be 650 MPa or more.

Further, the ferritic steel sheet may have a weight reduction ratio of 7% or more.

Hereinafter, a method for manufacturing a ferritic steel sheet according to one embodiment of the present invention will be described.

First, 0.01 to 0.3% by weight of C, 0.5 to 8% by weight of Mn, 5 to 12% by weight of Al and 0.015 to 0.2% by weight of Nb are contained based on 100% And the slab containing impurities are heated (S100).

The temperature at which the slab is heated may be 1000 to 1250 캜. If the temperature at which the slab is heated is less than 1000 ° C, the rolling property may be lowered. If the temperature for heating the slab is more than 1250 DEG C, the partial melting of Al may cause the occurrence of liquid metal brittleness. More specifically, it can be heated to 1100 ° C to 1200 ° C.

Thereafter, the heated slab is hot rolled (S200). The hot rough rolling temperature may be 700 to 1250 ° C. If the hot rough rolling temperature is lower than 700 ° C, the rolling property may be lowered. If the hot rough rolling temperature is higher than 1250 ° C, grain boundary embrittlement may occur due to Al having a lower melting point, and the rolling property may be lowered.

After the hot rough rolling is completed, warm rolling may be performed (S210).

The warm rolling temperature may be 600 to 850 캜. If the warm rolling temperature is lower than 600 ° C, the rolling roll may be overloaded, and when the temperature is higher than 850 ° C, recrystallization may occur in the steel sheet, making it difficult to control the final microstructure.

After the warm rolling is completed, the steel sheet may be subjected to intermediate annealing (S211). The intermediate annealing temperature may be 700 to 900 占 폚. The intermediate annealing temperature is less than 700 deg. C, and the annealing time becomes long, and the productivity may be lowered. If the intermediate annealing temperature is higher than 900 DEG C, the kappa carbide is decomposed to fail to function as a ferrite recrystallization nucleus generation destination, and the grain refinement effect can be reduced. More specifically, the intermediate annealing temperature may be 800 to 900 占 폚.

After intermediate annealing is completed, hot rolling may be performed again (S211-1).

After the hot rough rolling is completed, hot rolling may be performed (S220).

The hot rolling temperature may be 1000 to 1250 캜. If the hot rolling temperature is lower than 1000 캜, the rolling property may be lowered. If the hot rolling temperature is higher than 1250 캜, grain boundary embrittlement may occur due to Al having a lower melting point, and the rolling property may be lowered.

After completion of the hot rolling, the steel sheet may be subjected to intermediate annealing (S221). The intermediate annealing temperature may be 700 to 900 占 폚. The intermediate annealing temperature is less than 700 deg. C, and the annealing time becomes long, and the productivity may be lowered. If the intermediate annealing temperature is higher than 900 DEG C, the kappa carbide is decomposed to fail to function as a ferrite recrystallization nucleus generation destination, and the grain refinement effect can be reduced. More specifically, the intermediate annealing temperature may be 800 to 900 占 폚.

Thereafter, the steel sheet is cold-rolled (S300). The reduction ratio in the cold rolling step may be 30 to 90%. However, the reduction rate is (the thickness of the steel sheet before rolling-the thickness of the steel sheet after rolling) / (the thickness of the steel sheet before rolling).

Then, the cold-rolled steel sheet after cold-rolling is annealed (S400). The temperature for annealing the cold rolled steel sheet (cold rolled steel sheet) may be 650 to 900 占 폚. If the annealing temperature of the cold-rolled sheet is less than 650 ° C, the recrystallization is delayed and the productivity deteriorates. If the annealing temperature of the cold-rolled sheet is higher than 900 ° C, the 虜 -carbide is decomposed to form austenite and homogeneous microstructure can not be obtained. And the physical properties may deteriorate.

As shown in Fig. 3, 虜 -carbides exist in the form of an acicular shape in the steel sheet after hot rough rolling. These κ-carbides cause brittle fracture. However, in the method for producing a ferrite-based steel sheet according to an embodiment of the present invention, 虜 -carbons are segmented during warm rolling and cold rolling, so that the needle-shaped 虜 -carbide is deformed into spherical or elliptical shapes as shown in Figs. 6 and 10, And is distributed evenly and evenly in the interior of the container.

When external forces are applied, stress is concentrated at the interface between the κ-carbide and the base, so deformation concentrates around the κ-carbide during rolling. Therefore, κ-carbide acts as a nucleation site of new BCC crystal grains during cold-rolled sheet annealing, and the grain size of the final microstructure can be miniaturized.

Further, NbC compounds are present in the steel sheet after the hot rolling is completed, as shown in Fig. These NbC compounds inhibit dynamic recrystallization during hot rolling and inhibit grain growth during annealing of cold rolled steel sheet. Therefore, the size of the crystal grains can be miniaturized.

Hereinafter, the embodiment will be described in detail. The following examples are illustrative of the present invention only and are not intended to limit the scope of the present invention.

&Lt; Example 1 >

[Slab heating and hot rolling]

A slab having the composition shown in Table 1, the balance being Fe and impurities was prepared. The thickness of the slab was 70 mm and the width was 140 mm. The proportion of some invented and comparative steels is shown in Table 1. The slab was heated at 1200 ° C for 1 hour and 30 minutes. Thereafter, the slab was subjected to hot rolling at a reduction rate of 25% per pass at 1100 DEG C, followed by finish rolling at 1000 deg. The thickness of the steel sheet after hot rough rolling was 20 mm. FIG. 1 is an optical microscope photograph of an inventive steel 1 in which hot rolling has been completed. 2 is an optical microscope photograph of the comparative steel 2 in which hot rolling is completed. Referring to FIGS. 1 and 2, it can be seen that the grain size of the grain of the invention steel 1 is finer than that of the comparative steel 2.

3 is an SEM photograph of the invention steel 1 in which hot rolling has been completed. Referring to FIG. 3, it can be seen that an acicular type 虜 -carbide exists in the matrix.

4 is a TEM photograph of the invention steel 1 in which the hot rolling has been completed. Referring to FIG. 4, it can be seen that .kappa. -Carbides and NbC compounds are present in the matrix.

Steel No. Al
(weight%) Mn
(weight%) Nb
(weight%) Si
(weight%) C
(weight%) importance
(g / cc) Inventive Steel 1 8.3 4.9 0.095 - 0.094 6.96 Invention river 2 6.78 5.08 0.095 0.006 0.107 7.11 Invention steel 3 7.68 5.04 0.095 - 0.107 7.02 Inventive Steel 4 8.49 4.45 0.101 0.07 0.114 6.93 Invention steel 5 9.53 4.18 0.08 0.05 0.104 6.84 Invention steel 6 10.45 4.13 0.08 0.05 0.091 6.78 Invention steel 7 11.8 4.25 0.08 0.06 0.089 6.72 Inventive Steel 8 7.86 4.43 0.096 0.04 0.0045 7.00 Invention river 9 8.02 4.30 0.089 - 0.0227 6.99 Invented Steel 10 7.33 5.27 0.074 - 0.047 7.05 Invention steel 11 8.30 4.66 0.032 - 0.088 6.96 Invention steel 12 8.28 4.42 0.034 0.06 0.204 6.96 Invention steel 13 11.00 4.51 0.040 0.007 0.210 6.75 Invented Steel 14 6.68 4.62 0.09 1.16 0.098 7.04 Invented Steel 15 8.84 4.40 0.041 0.254 0.216 6.90 Invented Steel 16 8.91 4.64 0.041 0.541 0.213 6.88 Comparative River 1 8.06 - - - 0.0096 7.00 Comparative River 2 7.89 4.42 - - 0.0170 6.99

&Lt; Example 1-1 >

[Warm rolling, intermediate annealing, warm rolling, cold rolling and cold rolled sheet annealing]

In Example 1, the hot-rolled steel sheet was hot-rolled at 650 캜 to a thickness of 3 mm. The reduction rate per pass was 30% and reheating was carried out for 5 minutes between passes. After the hot rolling was completed, intermediate annealing was carried out at 850 ° C for 15 minutes, and then the furnace was air-cooled to room temperature.

5 is an SEM photograph of the invention steel 1 in which warm rolling is completed. 6 is an SEM photograph of an inventive steel 1 having completed intermediate annealing.

Referring to FIG. 5 and FIG. 6, it can be seen that the needle-like 虜 -carbons are segmented into spherical or elliptical shapes.

The 3 mm-thick steel sheet was reheated at 650 ° C and then warm-rolled again to a thickness of 1.5 mm. The hot rolled plate was pickled with hydrochloric acid and then cold rolled to a thickness of 1 mm. Thereafter, cold rolled steel sheets were annealed at 750 ° C for 1 hour, and then air-cooled.

Figs. 7 to 9 are EBSD analysis photographs of a steel sheet before and after the annealing of the cold-rolled sheet, the annealing of the cold-rolled sheet for 5 minutes, and the annealing of the cold-rolled sheet for 15 minutes. Referring to the position of the arrow in Fig. 8, it can be seen that a new grain was generated near the spherical 虜 -carbide. It can be seen that spherical κ-carbides act as nucleation sites.

10 is an SEM photograph of Inventive Steel 1 after annealing of the cold rolled steel sheet is completed.

Referring to FIG. 10, it is distributed along the spherical--carbide crystal grain diameter. It can be seen that spherical κ-carbide acts as a grain growth inhibitor.

11 is an SEM photograph of the comparative steel 2 in which the cold-rolled sheet annealing is completed. 10 and 11, it can be seen that the crystal grains of the invention steel 1 are finer than the comparative steel 2.

The grain size of the final material was measured and subjected to a tensile test. The results are shown in Table 2 below.

Steel No. Yield strength
(MPa) Maximum tensile strength
(MPa) Uniform elongation
(%) Overall elongation
(%) Grain size
(탆) Invention Steel 1 583.5 673.1 15.5 30.4 9.9 Inventive Steel 5-1 634.1 731.5 14.0 26.7 9.5 Invention Steel 6-1 698.6 806.1 11.8 23.6 9.8 Inventive Steel 7-1 809.8 935.1 7.2 7.2 11.5 Inventive Steel 8-1 529.6 637.0 14.0 30.4 20 Inventive Steel 9-1 506.4 614.8 17.6 31.7 14 Inventive Steel 10-1 551.7 640.4 17.1 33.6 14 Comparative Steel 1 413.3 523.7 14.65 31.5 59 Comparative Steel 2-1 483.5 595.5 14.2 32.1 36

&Lt; Example 1-2 >

[Hot Rolling, Cold Rolling and Cold Rolled Sheet Annealing]

The steel sheet having a thickness of 20 mm which had been hot rolled in Example 1 was reheated at 1200 ° C for one hour, hot-rolled at a temperature of 1100 ° C to a thickness of 3 mm, and then air-cooled. The reduction rate per pass during hot rolling was 25%. The hot-rolled sheet was pickled with hydrochloric acid, and then cold-rolled to a thickness of 1 mm. Thereafter, cold rolled steel sheets were annealed at 750 ° C for 1 hour, and then air-cooled.

The grain size of the final material having the composition of Inventive Steel 10 (see Table 1) was measured and subjected to a tensile test. The results are shown in Table 3 below as Inventive Steel 10-2.

[Hot rolling, intermediate annealing, cold rolling and cold rolling annealing]

The steel sheet having a thickness of 20 mm which had been hot rolled in Example 1 was reheated at 1200 ° C for one hour, hot-rolled at a temperature of 1100 ° C to a thickness of 3 mm, and then air-cooled. The reduction rate per pass during hot rolling was 25%. After completion of the hot rolling, intermediate annealing was performed at 850 ° C for 15 minutes, and then the resultant was air-cooled to room temperature. The intermediate annealed sheet was subjected to pickling with hydrochloric acid, followed by cold rolling to a thickness of 1 mm. Thereafter, cold rolled steel sheets were annealed at 750 ° C for 1 hour, and then air-cooled.

The final material having the composition of Inventive Steel 10 (see Table 1) was subjected to a tensile test and the results are shown in Table 3 as Inventive Steel 10-2-1.

Steel No. Yield strength
(MPa) Maximum tensile strength
(MPa) Uniform elongation
(%) Overall elongation
(%) Grain size
(탆) Inventive Steel 10-2 529.4 616.1 15.0 28.9 19.8 Inventive Steel 10-2-1 543.6 629.4 15.4 28.7 18.6

&Lt; Example 1-3 >

[Warm rolling, intermediate annealing, cold rolling and cold rolled sheet annealing]

In Example 1, the hot-rolled steel sheet was hot-rolled at 650 캜 to a thickness of 3 mm. The reduction rate per pass was 30% and reheating was carried out for 5 minutes between passes. After the hot rolling was completed, intermediate annealing was carried out at 850 ° C for 15 minutes, and then the furnace was air-cooled to room temperature. The intermediate annealed sheet was pickled with hydrochloric acid and then cold rolled to a thickness of 1 mm. Thereafter, cold rolled steel sheet was annealed at 650 ° C for 1 hour, and then cooled.

The grain size of the final material was measured and subjected to a tensile test. The results are shown in Table 4.

&Lt; Example 1-3-1 >

A cold-rolled sheet having the composition of Inventive Steel 2 (see Table 1) was carried out in the same manner as in Example 1-3, and the cold-rolled sheet was annealed at 700 ° C for 1 hour, followed by air cooling. The grain size of the final material was measured and subjected to a tensile test. The results are shown in Table 4 as Inventive Steel 2-3-1.

&Lt; Example 1-3-2 >

A cold-rolled sheet having the composition of Invention Steel 4 (see Table 1) was carried out in the same manner as in Example 1-3, followed by annealing at 850 ° C for 1 minute for cold-rolled sheet, followed by air-cooling. The final material was subjected to a tensile test. The results are shown in Table 4 as 4-3-2 invention steel.

&Lt; Example 1-4 >

[Warm rolling, cold rolling and cold rolled sheet annealing]

In Example 1, the hot-rolled steel sheet was hot-rolled at 650 캜 to a thickness of 3 mm. The reduction rate per pass was 30% and reheating was carried out for 5 minutes between passes. The hot rolled plate was pickled with hydrochloric acid and then cold rolled to a thickness of 1 mm. Thereafter, cold rolled steel sheet was annealed at 650 ° C for 1 hour, and then cooled.

The final material having the composition of invention steel 4 (see Table 1) was subjected to a tensile test and the results are shown in Table 4 as invention steel 4-4.

Steel No. Yield strength
(MPa) Maximum tensile strength
(MPa) Uniform elongation
(%) Overall elongation
(%) Grain size
(탆) Invention Steel 1-3 599.6 694.5 16.5 30.7 9.8 Invention river 2-3 551.8 683.7 13.7 23.3 18 Invention steel 3-3 530.7 660.8 16.4 31.5 8.0 Invention steel 4-3 586.0 707.7 17.9 32.5 7.2 Invention steel 5-3 645.2 755.6 17.2 32.1 7.6 Inventive Steel 6-3 701.4 816.4 14.4 25.1 8.4 Invention river 7-3 822.4 946.6 9.7 9.9 8.2 Inventive Steel 10-3 548.6 629.6 16.3 30.1 12.3 Inventive Steel 11-3 581.1 670.9 15.9 29.7 7.2 Inventive Steel 12-3 641.6 756.7 16.9 31.0 6.0 Invention river 13-3 728.6 843.2 13.8 22.8 7.1 Invention river 14-3 698.1 809.6 15.8 29.1 8.4 Inventive Steel 15-3 690.7 804.2 15.4 28.8 7.3 Invention Steel 16-3 785.7 888.3 12.5 23.7 6.1 Invention river 2-3-1 551.8 683.7 13.7 23.3 18 Invention river 4-3-2 598.5 702.5 15.5 28.3 9.7 Inventive Steel 4-4 618.3 729.6 13.8 24.5 9.2 Comparative Steel 1-3 402.4 510.1 17.4 41.2 50 Comparative river 2-3 484.0 592.2 16.9 35.9 32

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims

delete

Wherein the composition contains 0.01 to 0.3% by weight of C, 0.5 to 8% by weight of Mn, 5 to 12% by weight of Al, 0.015 to 0.2% by weight of Nb and 0.1% %, The remainder comprising Fe and a slab containing impurities;
Hot rolling the heated slab;
Subjecting the hot-rolled steel sheet to primary warm rolling at 600 to 850 ° C;
Intermediate annealing the hot-rolled steel sheet at 700 to 900 ° C;
Subjecting the intermediate annealed steel sheet to a secondary warm rolling at 600 to 850 캜;
Cold rolling the steel sheet after the second warm rolling step; And
And annealing the cold-rolled steel sheet after the cold-rolling to cold-rolled steel sheet.

12. The method of claim 11,
Wherein the heating temperature in the step of heating the slab is 1000 to 1250 占 폚.

12. The method of claim 11,
Wherein the slab is hot rolled at a temperature of 700 to 1250 占 폚.

delete

12. The method of claim 11,
Further comprising a step of subjecting the hot rolled steel sheet to hot rolling at 1000 to 1250 占 폚.

18. The method of claim 17,
Further comprising the step of intermediate annealing at 700 to 900 占 폚 after the hot rolling.

12. The method of claim 11,
Wherein the annealing temperature of the cold rolled sheet in the step of annealing the cold rolled sheet is 650 to 900 占 폚.

12. The method of claim 11,
The slab may contain 0.04 to 2.0% by weight of Si, not more than 2.0% by weight of Cr (not including 0%), not more than 1.0% by weight of Mo (not including 0%), Ni: not more than 1.0 wt% (not including 0%), V: not more than 0.2 wt% (not including 0 wt%), B: not more than 0.01 wt% (not including 0 wt%), Or less (not including 0%), or a combination thereof.