WO2024128737A1 - High strength steel sheet having high yield ratio, and manufacturing method therefor - Google Patents

Abstract

The present invention relates to a material used for an automobile interior panel, a reinforcement and the like, and to a high strength steel sheet having a high yield ratio, and a manufacturing method therefor.

Description

High-yield ratio high-strength steel plate and manufacturing method thereof

The present invention relates to materials used for automobile interior panels, reinforcements, etc., and to a high-yield ratio high-strength steel plate and a method of manufacturing the same.

In order to improve automobile fuel efficiency, lightweighting of automobile bodies has recently been made, and to this end, the application of high-strength steel plates capable of reducing plate thickness is increasing in automobile parts. In addition, high-strength steel plates are widely used in automobile bodies to ensure the safety of occupants, and to improve the crash performance of automobile bodies, the yield strength of steel is increased to efficiently absorb collision energy even with low deformation. For this purpose, steel plates with a high yield ratio are required.

In order to apply high-strength steel sheets to automobile bodies, excellent processability is also required, and as a steel material that achieves both strength and processability, it is a dual phase steel (dual phase steel) that has a composite structure consisting of ferrite as the main phase and a hard structure as the second phase. Steel, hereinafter referred to as DP steel) is a representative example. However, DP steel has a problem of low yield ratio because its main phase is soft ferrite and hard structures such as bainite, martensite, and tempered martensite are used as secondary phases. Therefore, there are some limits to the application of DP steel for automobile parts that absorb impact energy while suppressing deformation.

On the other hand, Patent Document 1 proposed a steel sheet that prevents recrystallization of ferrite and has a structure composed of unrecrystallized ferrite and a hard second phase. However, if excessive unrecrystallized ferrite exists, strength and yield ratio increase, but there is a problem of insufficient formability due to low elongation.

In response to this problem, steel sheets with a structure composed of ferrite and pearlite by refining grains, precipitation strengthening, or reducing the amount of dissolved C in ferrite, and steel sheets that achieve both high strength and improved elongation flangeability are presented in Patent Documents 2 to 4. It has been done. However, all of the proposed steels had tensile strengths of 500 MPa or less, making it difficult to achieve high strength exceeding 500 MPa.

Meanwhile, Patent Documents 5 to 7 proposed a steel sheet with improved elongation flangeability by using unrecrystallized ferrite and using unrecrystallized ferrite having an intermediate hardness of soft ferrite and hard secondary phase. However, the steel sheets proposed in Patent Documents 5 and 6 have a small amount of Nb or Ti added, so the recrystallization inhibition effect is small, so rapid heating is necessary during annealing, and the steel sheets proposed in Patent Document 7 have a small amount of Nb or Ti added. Since the temperature increase rate during annealing is low, below 10°C/s, the time available for recrystallization increases, and the effect of using unrecrystallized ferrite is not sufficient.

(Patent Document 1) Japanese Patent Laid-open Publication 1978-005018

(Patent Document 2) Japanese Patent Application Publication 2007-138261

(Patent Document 3) Japanese Patent Laid-open Publication 2007-107099

(Patent Document 4) Japanese Patent Laid-open Publication 2001-152288

(Patent Document 5) Japanese Patent Laid-open Publication 2008-106351

(Patent Document 6) Japanese Patent Laid-open Publication 2008-106352

(Patent Document 7) Japanese Patent Laid-open Publication 2008-156680

One aspect of the present invention is to provide a steel plate with a high yield ratio and high strength, and excellent formability, and a method for manufacturing the same.

The object of the present invention is not limited to the above-mentioned matters. The additional problems of the present invention are described throughout the specification, and those skilled in the art will have no difficulty in understanding the additional problems of the present invention from the content described in the specification of the present invention.

One aspect of the present invention is weight percent, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03. % or less (excluding 0%), Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02~0.06%, Niobium (Nb): 0.02~0.06%, Boron (B): 0.005% or less (excluding 0%), including the remaining Fe and inevitable impurities,

The microstructure includes 80 to 99% of ferrite in terms of area percent, the remainder includes pearlite and other inevitable structures, and unrecrystallized ferrite is 20 to 50% of the ferrite,

A steel plate having an aspect ratio of 5 to 15 of the ferrite is provided.

The total amount of Ti and Nb may be 0.1% or less.

The cold rolled steel sheet may satisfy the following relational expression 1.

[Relational Expression 1]

X(222)/[X(200)+X(110)+X(112] ≤ 2

(X-ray diffraction integrated intensity ratio of the {222} plane, {110} plane, {200} plane, and {112} plane parallel to the plane at the depth of 1/4 the thickness of the cold rolled steel sheet)

The steel sheet may further include a hot-dip galvanized layer on the surface.

The steel plate may have a yield strength of 460 MPa or more and a tensile strength of 520 MPa or more.

In the steel plate, the product of yield strength and elongation may be 8600 or more.

Another embodiment of the present invention is by weight percentage, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01 to 0.08%, Ti (titanium): 0.02~0.06%, niobium (Nb): 0.02~0.06%, boron (B): 0.005% or less (excluding 0%), steel slabs containing the remaining Fe and inevitable impurities are stored at 1100~1250℃. Heating with;

Obtaining a hot rolled steel sheet by hot rolling the heated steel slab to 880°C or higher;

Cooling the hot rolled steel sheet to 500-600°C and winding it;

Cold rolling the coiled hot-rolled steel sheet at a reduction ratio of 45 to 70%; and

A method of manufacturing a steel sheet is provided including the step of continuously annealing the cold rolled steel sheet at a temperature range of 770 to 820°C.

The method of manufacturing the steel plate can satisfy the conditions of [Relational Expression 2] and [Relational Expression 3] below.

[Relational Expression 2]

2566 +2.1*0.192*CR - 1.79*SS - 5.64*LS ≥ 520

[Relational Expression 3]

2438 +1.9*0.192*CR - 1.79*SS - 5.64*LS ≤ 700

Here, CR is the cold rolling reduction rate (%), SS is the annealing temperature (℃), and LS is the line speed (mpm) during continuous annealing.

The method of manufacturing the steel sheet may further include hot-dip galvanizing the continuously annealed steel sheet.

The steel plate of the present invention has high strength and high yield ratio, so when used as an inner plate, reinforcement, etc., the resistance (collision resistance characteristics) increases during a collision, which is advantageous in ensuring the safety of passengers. Additionally, according to the present invention, a steel plate with excellent formability can be provided.

The various and beneficial advantages and effects of the present invention are not limited to the above-described content, and may be more easily understood through description of specific embodiments of the present invention.

Figure 1 is a photograph showing the microstructure of invention steel 1 in Examples.

The terms used in this specification are for describing the present invention and are not intended to limit the present invention. Additionally, as used herein, singular forms include plural forms unless the relevant definition clearly indicates the contrary.

The meaning of “including” used in the specification specifies a configuration and does not exclude the presence or addition of another configuration.

Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as commonly understood by a person of ordinary skill in the technical field to which the present invention pertains. Terms defined in the dictionary are interpreted to have meanings consistent with related technical literature and current disclosure.

In the microstructure of cold rolled steel sheets, it can be said that it is not a common idea to leave some ferrite in an unrecrystallized state. Non-recrystallized ferrite is ferrite stretched in the rolling direction by cold rolling, meaning that recrystallization has not been completed and dislocations within the particles have been recovered. When such an unrecrystallized ferrite structure exists in steel, the material of the cold rolled steel sheet has large deviations in the width and length directions, and even a slight difference in the unrecrystallized fraction may cause the material to appear very non-uniform. Therefore, it was recognized that the best method was to minimize the unredetermining tissue as much as possible.

If a significant amount of titanium (Ti) or niobium (Nb) is added to steel with more than 0.05% by weight of carbon (C) added to ensure strength, it is very difficult to obtain a complete recrystallized structure through an annealing process. The Ti and Nb form carbides such as TiC and NbC during the temperature increase process in the cooling process and annealing process in the hot rolling step, and are known as elements that interfere with recrystallization and suppress grain growth.

Therefore, in order to obtain a recrystallized structure, the annealing temperature must be managed very high or a special process is required to suppress precipitation of TiC, NbC, etc. Here, the special process is to shorten the time for TiC, NbC, etc. to precipitate through very rapid quenching or very rapid heating. However, this is a process that cannot be achieved during normal operation, and special equipment is required to achieve it. Meanwhile, in order to maintain a high annealing temperature and prevent inhibition of recrystallization by TiC, NbC, etc., it is necessary to maintain a temperature of almost 900°C or higher. Such high-temperature annealing can cause problems such as coil meandering and increased manufacturing costs. Even if the annealing temperature is raised to 900°C or higher, the yield strength decreases due to material softening due to the creation of a recrystallization structure, so it is difficult to secure the high yield ratio required in the present invention.

Accordingly, the present inventors conducted in-depth research to manufacture a steel plate with a high yield ratio, preferably a yield strength of 460 to 600 MPa, a tensile strength of 520 to 700 MPa, and a yield ratio of 0.8 to 0.9. As a result, a method was derived that could secure the above-mentioned yield ratio using unrecrystallized ferrite and at the same time have excellent formability with an elongation of 10% or more, leading to the present invention.

Hereinafter, an embodiment of the steel sheet of the present invention will be described in detail. First, the alloy composition of the steel sheet will be described in detail. Hereinafter, unless otherwise specifically stated in the present invention, the content of the alloy composition is based on weight%.

The steel sheet contains carbon (C): 0.05-0.12%, manganese (Mn): 1.0-1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%). , Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01 to 0.08%, Titanium (Ti): 0.02 to 0.06 %, niobium (Nb): 0.02~0.06%, boron (B): 0.005% or less (excluding 0%).

Carbon (C): 0.05~0.12%

Carbon (C) is an element that contributes to the increase in strength and the creation of pearlite, and is added in an appropriate amount to secure the target strength. In addition, it is an essential element to give strength to steel sheets by forming precipitates with Ti, Nb, etc. in the ferrite phase. If it is less than 0.05%, it is difficult to secure the strength required for the present invention steel, and if it exceeds 0.12%, formability or weldability will be deteriorated, so it is effective that the C content is 0.05 to 0.12%.

Manganese (Mn): 1.0~1.8%

Manganese (Mn) is an element that lowers the Ac3 transformation temperature, which is the temperature at which Ac1 and α-γ transformations are completed and austenite becomes a single phase. That is, if the amount of Mn is small, it is necessary to increase the annealing temperature to promote transformation, which makes it difficult to secure the appropriate fraction of unrecrystallized ferrite required by the present invention. Additionally, Mn is an element that contributes to solid solution strengthening along with Si and is also effective in increasing strength. From this point of view, a Mn content of 1.0% or more is effective. On the other hand, if the Mn content exceeds 1.8%, hardenability increases, bainite and martensite are easily formed, and the yield ratio is lowered, so it is effective not to exceed 1.8%.

Silicon (Si): 0.6% or less (excluding 0%)

Silicon (Si) is a deoxidizing element and is effective in increasing strength as a solid solution strengthening element. However, if the Si amount exceeds 0.6%, Ac1 becomes too high and it is necessary to increase the annealing temperature, which accelerates transformation and makes it difficult to secure unrecrystallized ferrite. Therefore, it is effective to manage it to 0.6% or less. Additionally, excessive addition of Si may cause problems such as deterioration of plating adhesion due to oxide during hot dip galvanizing. However, considering the amount inevitably added during manufacturing, 0% is excluded.

Phosphorus (P): 0.03% or less (excluding 0%)

Phosphorus (P) is an impurity and segregates at grain boundaries, causing a decrease in the toughness of the steel sheet and deterioration of weldability. In addition, since the alloying reaction is very slow during hot dip galvanizing and productivity decreases, it is effective for P to be 0.03% or less.

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

Sulfur (S) is an impurity that is inevitably included in steel, and it is desirable to keep its content as low as possible. Therefore, considering that the S content in steel is inevitably included, 0% is excluded. In particular, since S in steel increases the possibility of causing red heat embrittlement, it is effective to control its content to 0.01% or less.

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

Nitrogen (N) is an impurity that is inevitably included in steel, and it is important to keep the N content as low as possible. Therefore, considering the case where N content in steel is inevitably included, 0% is excluded (i.e., exceeds 0%). However, in order to manage the N content in the steel to a very low level, there is a problem that the refining cost of the steel increases rapidly, so it is managed to 0.01% or less, which is the range within which operating conditions are possible.

Aluminum for acid value (sol.Al): 0.01~0.08%

Acid-soluble aluminum (sol.Al) is an element added for particle size refinement and deoxidation. If the sol.Al content is less than 0.01%, aluminum killed (Al-killed) steel cannot be manufactured in a normal stable state. On the other hand, if the sol.Al content exceeds 0.08%, it is advantageous to increase strength due to the grain refinement effect, but the possibility of surface defects in the plated steel sheet increases due to excessive formation of inclusions during steelmaking operation. In addition, because there is a problem that causes a sharp increase in manufacturing cost, it is desirable to manage the sol.Al content at 0.01 to 0.08%.

Titanium (Ti): 0.02~0.06% and Niobium (Nb): 0.02~0.06%

The Ti and Nb are elements that promote the retention of unrecrystallized ferrite by suppressing recrystallization of ferrite during an annealing process for deformed ferrite generated by cold rolling. In order to achieve this effect, it is desirable to add at least 0.02% or more of Nb and Ti each. However, if added excessively, the amount of unrecrystallized ferrite increases due to the formation of carbides such as TiC and NbC, the yield strength and yield ratio may increase excessively, and excessive addition of alloy elements may cause an increase in manufacturing costs. Moreover, it is more effective to keep the total amount of Ti and Nb (Ti+Nb) below 0.1%.

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

Boron (B) is an element that improves hardenability, increases strength, and suppresses the nucleation of grain boundaries. If the content of B exceeds 0.005% by weight, the effect becomes excessive and causes an increase in manufacturing costs, so it is preferable to control the content of B to 0.005% by weight or less.

In addition, it includes the remaining Fe and inevitable impurities. The addition of effective ingredients other than the above composition is not excluded. In other words, the unavoidable impurities may be included as long as they can be unintentionally introduced during the manufacturing process of ordinary cold rolled steel sheets (and plated steel sheets). Since a person skilled in the art can easily understand the meaning, it is not particularly limited here.

Next, the microstructure of the steel plate will be described in detail. The microstructure of the steel sheet is expressed in terms of area percentage and includes 80 to 99% ferrite, and the remainder includes pearlite and other inevitable structures. At this time, the inevitable structure is not particularly limited, but may be cementite, carbide, etc. More specifically, ferrite may be included in 80 to 95%.

Meanwhile, it is effective for the unrecrystallized ferrite to be 20 to 50% by area among the ferrites. The area % of the non-nodulated ferrite refers to the fraction of the entire microstructure.

Here, the fraction of unrecrystallized ferrite can be determined by analyzing crystal orientation measurement data from electron back scattering diffraction (EBSD) using the Kernel Average Misorientation method (KAM method). Since the KAM method can quantitatively express the crystal orientation difference with adjacent pixels (measurement points), in the present invention, particles with an average crystal orientation difference with adjacent measurement points of less than 1° are defined as unrecrystallized ferrite. In order to secure sufficient yield strength and yield ratio, it is effective for the area percent of the unrecrystallized ferrite to be 20 to 50%. If the unrecrystallized ferrite is less than 20%, sufficient yield strength and yield ratio cannot be obtained, and if it exceeds 50%, the yield strength and yield ratio become excessive due to the high unrecrystallized structure, and the aspect ratio of the grains increases. . Therefore, it is effective to have 20 to 350% of unrecrystallized ferrite.

It is effective that the total ferrite fraction including the unrecrystallized ferrite is 80 to 99%. More specifically, a total ferrite fraction of 80-95% may be more effective.

Meanwhile, the steel sheet of the present invention contains pearlite and inevitable structures other than ferrite.

It is effective that the microstructure of the steel sheet, especially the grain aspect ratio (A/R) of ferrite, is 5 to 15.

Here, the grain aspect ratio is determined by etching the microstructure with 5% Nital etching solution, observing it at 500x with a scanning electron microscope (SEM), and analyzing the grains using the Image Analyzer program. The major axis length and minor axis length were obtained. The aspect ratio was obtained as the major axis length of the grain/ellipse minor axis length. The average aspect ratio of each ferrite obtained by this technique was defined as the grain aspect ratio.

If A/R exceeds 15, it means that the stretching grains in the rolling direction are very large, which means that the cold rolling structure has hardly been recovered or recrystallized. The formation of such excessive stretched grains causes an excessive increase in yield strength, exceeding the yield strength and yield ratio required by the present invention. However, if A/R is less than 5, it means that recrystallization has progressed to a significant extent, which means that due to softness of the steel, the yield strength is insufficient and the yield ratio is lower than the level required for the steel of the present invention.

It is effective that the texture of the steel sheet satisfies the conditions of equation 1 below.

[Relational Expression 1]

X(222)/[X(200)+X(110)+X(112] ≤ 2

(X-ray diffraction integrated intensity ratio of the {222} plane, {110} plane, {200} plane, and {112} plane parallel to the plane at the depth of 1/4 the thickness of the cold rolled steel sheet)

Here, the X-ray diffraction integrated intensity ratio is the relative intensity based on the X-ray diffraction integrated intensity of the non-oriented standard sample. X-ray diffraction can be performed using an X-ray diffraction device widely used in the technical field to which the present invention belongs, such as an energy dispersive type. If the X-ray diffraction integrated intensity ratio calculated in equation 1 above exceeds 2, it means that the (222) texture, that is, the fraction of recrystallized texture increases, which means that the fraction of unrecrystallized ferrite in the steel is not formed in an appropriate range. This means that the aspect ratio cannot be secured.

A steel sheet that satisfies the above alloy composition and microstructure may have a yield strength of 460 MPa or more, a tensile strength of 520 MPa or more, and a yield ratio of 0.8 to 0.9. At the same time, the steel sheet may have an elongation of 10% or more than 10%. More specifically, the yield strength of the steel plate may be 460 to 600 MPa. The tensile strength of the steel plate can be 520 to 700 MPa.

More specifically, the product of the yield strength and elongation of the steel plate may be 8500 or more. More specifically, the product of yield strength and elongation may be 8900 or more. Typically, as yield strength increases, elongation may decrease. However, according to one embodiment of the present invention, the product of yield strength and elongation is 8500 or more, so the yield strength and elongation can be improved simultaneously.

Meanwhile, the steel sheet of the present invention may include a plating layer to improve corrosion resistance. In the present invention, the plating layer is not particularly limited, and any plating type or plating method performed in the technical field to which the present invention belongs is sufficient. A preferred example may be a hot-dip galvanized layer.

Next, an embodiment of the method for manufacturing the steel sheet of the present invention will be described in detail. However, this does not mean that the steel sheet of the present invention must be manufactured by the following manufacturing method.

In order to manufacture the steel sheet of the present invention, a steel slab satisfying the above-described composition can be manufactured through the process of reheating, hot rolling, coiling, cold rolling, and annealing. Below, each process is described in detail.

Reheating: 1100~1250℃

It is effective to reheat the steel slab having the above-described composition to a temperature range of 1100 to 1250°C. If the reheating temperature is less than 1100°C, slab inclusions, etc. are not sufficiently re-dissolved, which may cause material deviation and surface defects after hot rolling. In contrast, if the slab reheating temperature exceeds 1250°C, strength may be reduced due to excessive growth of austenite grains.

Hot rolling: above 880℃

The reheated steel slab is hot rolled at a temperature of 880°C or higher to produce a hot rolled steel sheet. If the temperature of the hot rolling is less than 880°C, ferrite transformation occurs during rolling and an elongated structure is created, which may cause problems such as anisotropy deterioration and cold rolling resistance. Therefore, the hot rolling is performed at 880°C or higher. It is effective.

Winding: 500~600℃

It is effective to wind the hot rolled steel sheet at a temperature range of 500 to 600°C. If the coiling temperature is lower than 500°C, the shape of the steel sheet becomes poor and a transformation structure such as acicular ferrite is generated, which may result in an excessive increase in strength and a decrease in ductility of the steel sheet after annealing. However, if the coiling temperature exceeds 600°C, coarse ferrite grains are formed, and coarse carbides and nitrides are easily formed, which may deteriorate the steel material. In addition, problems such as buckling due to high-temperature coiling occur and cold rolling properties deteriorate.

Cold rolling: Cold rolling reduction rate 45~70%

Cold rolled steel sheets are manufactured by cold rolling the hot rolled steel sheets after coiling and pickling. It is effective to perform the cold rolling reduction rate (cold rolling reduction rate) at 45 to 70%. If the cold rolling reduction ratio is less than 45%, the recrystallization driving force is very low and excessive unrecrystallized ferrite is formed, making it difficult to secure the strength required in the present invention. On the other hand, when the cold rolling reduction ratio exceeds 70%, the recrystallization driving force becomes too high, making it easy to recrystallize ferrite even at low annealing temperatures, making it difficult to manufacture high-strength steel with a yield ratio of 0.8 to 0.9.

Continuous annealing: 770~820℃

It is effective to continuously anneale the cold rolled steel sheet at a temperature range of 770 to 820°C. If the annealing temperature is less than 770°C, the unrecrystallized ferrite fraction is excessively formed, resulting in high yield strength and deteriorated ductility. On the other hand, when the annealing temperature exceeds 820°C, the unrecrystallized ferrite fraction is too small, making it difficult to secure the high yield ratio required in the present invention.

In the present invention, in order to secure a certain amount of unrecrystallized tissue, it is important to properly manage the operation factor that controls the recrystallization driving force, so it is effective to satisfy the conditions of [Relational Expression 2] and [Relational Expression 3] below.

[Relational Expression 2]

2566 +2.1*0.192*CR - 1.79*SS - 5.64*LS ≥ 520

[Relational Expression 3]

2438 +1.9*0.192*CR - 1.79*SS - 5.64*LS ≤ 700

Here, CR is the cold rolling reduction rate (%), SS is the annealing temperature (℃), and LS is the line speed (mpm) during continuous annealing.

The above [Relational Expression 2] and [Relational Expression 3] are operation factors that control the recrystallization driving force and include annealing temperature, cold rolling reduction rate, and line speed during annealing. In the present invention, it is effective for the transfer speed to be 90 to 150 mpm. The transfer speed is controlled differently depending on the thickness of the steel plate. In other words, for thick materials, the line speed is low, and for thin materials, high-speed work is performed. It is desirable to manage the cold rolling reduction rate and annealing temperature together to suit these conditions.

In the present invention, plating may be additionally performed after the continuous annealing.

The plating may be performed in a manner commonly performed in the technical field to which the present invention pertains, and the type and method of plating are not particularly limited. In the present invention, hot dip galvanizing conditions are not particularly limited, and hot dip galvanizing can be performed under normal conditions that can be applied in the same technical field. Through hot dip galvanizing, the steel sheet according to an embodiment of the present invention may include a hot dip galvanizing layer on the surface. In a preferred example, a molten zinc-based galvanized steel sheet is manufactured by immersing the steel sheet in a molten zinc-based plating bath at 440 to 500°C. In addition, if necessary, after the hot-dip galvanizing step, the steel sheet may be subjected to alloying heat treatment. In one embodiment, the hot-dip galvanized steel sheet is subjected to alloying heat treatment at a temperature range of 460 to 530° C. and then cooled to room temperature. You can. Through alloying heat treatment, the steel sheet may include an alloyed hot-dip galvanized layer on the surface.

After the hot dip galvanizing, temper rolling can be performed. Temper rolling can also be performed within the normal range of 0.1 to 1.0%. If the temper rolling elongation is less than 0.1%, it is difficult to control the plate shape. On the other hand, if it exceeds 1.0%, material deterioration due to excessive increase in dislocation density in the surface layer may occur, and side effects such as plate fracture may occur due to limitations in facility capacity.

Hereinafter, embodiments of the present invention will be described. Of course, various modifications to the following examples can be made by those skilled in the art without departing from the scope of the present invention. The following examples are for understanding of the present invention, and the scope of the present invention should not be limited to the following examples, but should be determined by the claims described below as well as their equivalents.

(Example)

Steel slabs having the alloy composition shown in Table 1 below (unit is weight %, the remainder being inevitable impurities) were hot rolled and coiled at a reheating temperature of 1200°C, a hot rolling finishing temperature of 900°C above Ar3 temperature, and a coiling temperature of 560°C. It was carried out.

After winding the coil, it was pickled using hydrochloric acid, and annealed under the cold rolling and continuous annealing conditions listed in Table 2 to manufacture a steel sheet.

In Table 2, CR is the cold rolling reduction rate (%), SS is the continuous annealing temperature (°C), and LS is the feed speed (line speed, mpm). Meanwhile, relations 2 and 3 are as follows.

[Relational Expression 2]

2566 +2.1*0.192*CR - 1.79*SS - 5.64*LS ≥ 520

[Relational Expression 3]

2438 +1.9*0.192*CR - 1.79*SS - 5.64*LS ≤ 700

For the steel sheet manufactured as above, a tensile test was performed in the rolling direction using the DIN-L standard to measure the yield strength (YP), tensile strength (TS), and elongation (El.) of the steel sheet. It is shown in Table 3 above. In addition, the grain aspect ratio of ferrite and the fractions (area %) of unrecrystallized and recrystallized ferrite were measured using the microstructure measured using the previously described scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). Meanwhile, the X-ray diffraction intensity values for each texture component were measured for the manufactured steel sheet, and the X-ray diffraction intensity ratio was calculated using the equation in Equation 1.

[Relational Expression 1]

X(222)/[X(200)+X(110)+X(112] ≤ 2

(This is the X-ray diffraction integrated intensity ratio of the {222} plane, {110} plane, {200} plane, and {112} plane parallel to the plane at the depth of 1/4 the thickness of the cold rolled steel sheet.)

As can be seen in Table 3, invention steels 1 to 8 that meet the alloy composition and manufacturing conditions of the present invention have a yield strength of 469 to 545 MPa, a tensile strength of 545 to 655 MPa, an elongation of 18 to 21%, and a yield ratio (YR) of 0.82 to 0.86, and the product of yield strength and elongation is 8900 or more, satisfying the mechanical properties presented in the present invention steel. In addition, the inventive steel has a grain aspect ratio of 5.9 to 10.2, which satisfies the condition of 5 to 15 grain aspect ratio proposed in the present invention, and the unrecrystallized ferrite fraction is 29 to 45%, which is 29 to 45%. The suggested 20 to 50% conditions are met. The X-ray diffraction ratio of these steels is also 0.6 to 1.2, which sufficiently satisfies the conditions presented in the present invention.

Figure 1 shows an SEM photograph showing the microstructure of the annealed plate of invention steel 1 among the examples. The microstructure included unrecrystallized ferrite (code ①) and recrystallized ferrite (code ②), and some pearlite.

Comparative steels 1 and 3 had cold rolling reduction rates much lower than the conditions suggested in the present invention. This caused a lack of ferrite recrystallization during annealing, so the yield strength was very high and the yield ratio exceeded the standards of the present invention. Additionally, the grain aspect ratio value was very high due to lack of recrystallization.

Contrary to comparative steel 1, comparative steels 2 and 7 have a very high cold rolling reduction rate of 80%. When the cold rolling reduction rate is high, ferrite recrystallization easily occurs even at low annealing temperatures. This is due to a decrease in strength due to an increase in the recrystallization fraction, making it impossible to satisfy the yield strength and yield ratio conditions required for the steel of the present invention. Additionally, the grain aspect ratio and X-ray integrated intensity exceeded the standards of the present invention.

Comparative steel 4 had very low Ti and Nb addition amounts of 0.01% each. Recrystallization was promoted due to the lack of TiC and NbC precipitates, and the fraction of unrecrystallized ferrite after annealing was lowered, and as a result, the yield strength and yield ratio were outside the conditions of the present invention.

Comparative steels 5 and 8 are cases where no Ti or Nb is added. Due to the lack of precipitates in the steel, recrystallization easily occurred during annealing, and as a result, the yield strength of the steel sheet was low and the grain aspect ratio and X-ray diffraction intensity ratio did not meet the conditions of the present invention.

Comparative steel 6 has the same composition as comparative steel 5 and is a steel with no added Nb, and its cold rolling reduction rate was very low at 35% under conditions of insufficient precipitates, so it did not satisfy the conditions of the present invention even after high-temperature annealing at 860°C.

Comparative steel 9 satisfied all of the added components within the range of the present invention steel, but the annealing temperature was very high at 850°C. As the annealing temperature increases, the ferrite recrystallization fraction increases, which results in low yield strength and yield ratio, and results such as grain aspect ratio are outside the conditions of the present invention.

Comparative steel 10 has a Mn content outside the range of the present invention and an annealing temperature of 750°C, which is very low. This low annealing temperature causes excessive lack of ferrite recrystallization, which causes problems such as deterioration of workability when the elongation is below 10% due to excessive increase in yield strength and yield ratio.

Comparative steel 11 has a carbon content of 0.14%, which is outside the composition range of the present invention steel. Due to the excessive carbon content, the amount of carbide in the steel increased, which caused problems such as increased yield ratio and deterioration of elongation. Additionally, excessive addition of carbon causes deterioration of weldability.

Comparative steel 12 had a Ti content of 0.08%, which was outside the standard for the present invention steel, and the total amount of Ti+Nb was also outside the standard. This increase in carbonitride forming elements causes excessive TiC and NbC precipitation, which causes problems such as increased yield ratio due to delayed recrystallization.

Claims (9)

In weight percent, carbon (C): 0.05-0.12%, manganese (Mn): 1.0-1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%) ), Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01~0.08%, Titanium (Ti): 0.02~ 0.06%, Niobium (Nb): 0.02~0.06%, Boron (B): 0.005% or less (excluding 0%), including the remaining Fe and inevitable impurities, The microstructure includes 80 to 99% of ferrite in terms of area percent, the remainder includes pearlite and other inevitable structures, and unrecrystallized ferrite is 20 to 50% of the ferrite, A steel plate with an aspect ratio of the ferrite of 5 to 15. In claim 1, A steel plate containing a total amount of Ti and Nb of 0.1% or less. In claim 1, The steel sheet satisfies the following relational expression 1. [Relational Expression 1] X(222)/[X(200)+X(110)+X(112] ≤ 2 (X-ray diffraction integrated intensity ratio of the {222} plane, {110} plane, {200} plane, and {112} plane parallel to the plane at the depth of 1/4 of the steel plate thickness) In claim 1, The steel sheet further includes a hot-dip galvanized layer on the surface. In claim 1, The steel plate is, Steel plate with a yield strength of 460 MPa or more and a tensile strength of 520 MPa or more. According to paragraph 1, The steel plate is, Steel plate whose product of yield strength and elongation is 8600 or more. In weight percent, carbon (C): 0.05-0.12%, manganese (Mn): 1.0-1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%) ), Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01~0.08%, Ti (titanium): 0.02~ 0.06%, niobium (Nb): 0.02-0.06%, boron (B): 0.005% or less (excluding 0%), heating the steel slab containing the remaining Fe and inevitable impurities to 1100-1250°C; Obtaining a hot rolled steel sheet by hot rolling the heated steel slab to 880°C or higher; Cooling the hot rolled steel sheet to 500-600°C and winding it; Cold rolling the coiled hot-rolled steel sheet at a reduction ratio of 45 to 70%; and The step of continuously annealing the cold rolled steel sheet at a temperature range of 770 to 820 ° C. A method of manufacturing a steel plate comprising: In claim 7, The manufacturing method is a method of manufacturing a steel plate that satisfies the conditions of [Relational Expression 2] and [Relational Expression 3] below. [Relational Expression 2] 2566 +2.1*0.192*CR - 1.79*SS - 5.64*LS ≥ 520 [Relational Expression 3] 2438 +1.9*0.192*CR - 1.79*SS - 5.64*LS ≤ 700 Here, CR is the cold rolling reduction rate (%), SS is the annealing temperature (℃), and LS is the line speed (mpm) during continuous annealing. In claim 7, A method of manufacturing a steel sheet further comprising hot-dip galvanizing the continuously annealed steel sheet.

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