KR20240098839A - High strength steel shet having high yield ratio and method for manufacturing the same - Google Patents

Abstract

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.

Description

High yield ratio high strength steel sheet with excellent material uniformity and method of manufacturing the same {HIGH STRENGTH STEEL SHET HAVING HIGH YIELD RATIO AND METHOD FOR MANUFACTURING THE SAME}

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 plates with a structure composed of ferrite and pearlite by grain refinement, precipitation strengthening, or reduction of the amount of dissolved C in ferrite, and steel plates 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 a tensile strength 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, these patents contain at least 5% of unrecrystallized ferrite, and in addition to material variation in each direction, there is a problem with material non-uniformity according to coil length when manufacturing coils.

In other words, steel materials composed of a recrystallized structure show little change in material even through the cooling process for coil manufacturing, whereas in the case of steel materials containing a non-recrystallized structure, the non-recrystallized fraction varies depending on the cooling conditions during cooling, resulting in positional changes. There is a problem that there is a large variation in material.

Japanese Patent Publication No. 1978-005018 Japanese Patent Application Publication 2007-138261 Japanese Patent Publication No. 2007-107099 Japanese Patent Publication No. 2001-152288 Japanese Patent Publication No. 2008-106351 Japanese Patent Publication No. 2008-106352 Japanese Patent Publication No. 2008-156680

One aspect of the present invention is to provide a high-strength steel plate with excellent impact resistance and a method of manufacturing the same, with little variation in strength by length of the steel plate and high yield strength.

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,

This relates to a steel plate in which the material deviation along the length of the steel plate is 60 MPa or less based on yield strength.

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%, Titanium (Ti): 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

Continuously annealing the cold rolled steel sheet at a temperature range of 770 to 820°C,

Before winding the hot-rolled steel sheet, the front end of the hot-rolled steel sheet is heated to a coiling temperature (T) +30℃ to T+100℃, and the rear end of the hot-rolled steel sheet is heated to T+80℃ to T+150℃. It relates to a manufacturing method of a steel plate including.

The steel plate of the present invention has high strength and high yield ratio, so when used as an inner plate, reinforcement, etc., resistance to collision (collision resistance characteristics) increases, 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 an example, a graph showing temperature changes at the front end (inner winding part) and the rear end (outer winding part) of a hot rolled steel sheet during coiling after hot rolling.
Figure 2 shows the difference in yield strength by hot-rolled coil length between invention steel 1 and invention steel 2 in an embodiment of the present invention.
Figure 3 shows the microstructure by coil length of invention steel 1 and comparative steel 1 finally manufactured in an embodiment of the present invention.

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 in the cooling process and annealing process in the hot rolling step, and are known as elements that prevent 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, making it impossible 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, it was possible to derive a method that could secure an excellent yield ratio using non-recrystallized ferrite and at the same time have excellent formability with an elongation of 10% or more.

However, at the same time, when manufacturing a steel sheet containing fine precipitates such as TiC and NbC and also containing non-recrystallized ferrite, it was discovered that there is a problem in that the material changes greatly depending on the non-recrystallized ferrite fraction. This is due to differences in microstructure, and it has been recognized that these differences in structure are sensitively dependent on the winding process after hot rolling during the manufacturing process.

Specifically, as shown in Figure 1, in the process of winding a hot-rolled steel sheet after hot rolling, the front end (up to 50 m from the front end of the steel sheet) is located in the inner winding of the winding coil and becomes the inner winding part, and the rear end (at the rear end of the steel plate) is (up to 100 mm) is located on the outer winding of the winding coil and becomes the outer winding part. At this time, as shown in Figure 1, there is a difference in cooling behavior between the inner and outer coils. In other words, compared to the inner part, the outer part of the coil is exposed to the outside air and cools more quickly. As a result, low-temperature transformation occurs in a shorter time, causing a problem of relatively increased strength. To solve this problem, there is a method of lowering the temperature deviation of the entire coil by lowering the winding temperature. However, even with this method, there is a temperature difference between the inner and outer parts of the coil, which is due to differences in the structure of the material at each location. It can cause deviation.

Accordingly, the present inventors conducted in-depth research into a method that can fundamentally solve this problem and came up with 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. In addition, 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, taking into account cases 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 use (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 it is 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 the excessive addition of alloy elements may cause an increase in manufacturing costs. Furthermore, 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 cost, 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.

The material deviation of the steel plate of the present invention according to length may be 60 MPa or less based on yield strength. Specifically, the yield strength deviation for the head portion, middle portion, and tail portion in the longitudinal direction of the steel plate may be 60 MPa or less. The steel sheet refers to the final steel sheet, unlike the hot rolled steel sheet during the manufacturing process. The head part and tail part usually refer to the area up to 30 m from the end of the steel plate (coil), and the rest can be referred to as the middle part.

For reference, the front end (inner winding part when coiling) of a hot rolled steel sheet becomes the tail after cold rolling and annealing, and the rear end (outer winding part when coiling) of a hot rolled steel sheet becomes the head after cold rolling and annealing. It becomes a side. This is because the outer coil part of the hot rolled steel sheet is released first and then cold rolled and annealed.

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 95% of 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.

Meanwhile, it is effective for unrecrystallized ferrite to be 20 to 50% by area among the ferrites. The area percent 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 50% of unrecrystallized ferrite.

It is effective that the total ferrite fraction including the unrecrystallized ferrite is 80 to 95%.

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-directional 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.

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.

Reheat: 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.

In the present invention, in order to minimize the material deviation of the manufactured steel sheet, before winding the hot rolled steel sheet obtained through the hot rolling, the front end of the hot rolled steel sheet (the part up to 50 m from the front end of the hot rolled steel sheet) has a coiling temperature (T) + 30. It can be heated in the range of ℃ to T+100℃, and the rear end of the hot rolled steel sheet (part up to 100m from the rear end of the hot rolled steel sheet) can be heated to T+80℃ to T+150℃.

In addition, the above manufacturing method can satisfy the following relational equation 2.

[Relational Expression 2]

K ≤ 10

(K = 621×[C] + 222×[Ti] + 1183×[Nb] - 0.694×X - 0.726×Y)

However, if K < 0, it is treated as K = 0.

Here, [C], [Ti], and [Nb] mean the wt.% addition amounts of C, Ti, and Nb. In addition,

As a result of analyzing the cooling rate and phase transformation behavior at each location after coiling the hot-rolled steel sheet, it was found that in order to avoid generating low-temperature transformation phases such as bainite at the cooling rate at the front and rear ends, coiling must be done under the above temperature conditions to sufficiently form ferrite while cooling. The metamorphosis can be complete. If the temperature rise is managed lower than the above conditions, a low-temperature transformation phase such as bainite is created due to insufficient time for ferrite to be sufficiently formed, resulting in an excessive increase in strength, which may lead to an increase in strength deviation. If the temperature rise is too high, the equipment burden may become excessively high.

High-strength steels with a high yield ratio containing a certain amount of unrecrystallized ferrite are very likely to have material differences depending on the coil length depending on the added components and heat treatment conditions, and this is correlated with the fraction of unrecrystallized ferrite in the steel. Accordingly, in the present invention, in addition to the addition amount of [C], [Ti], and [Nb], which affects the unrecrystallized ferrite fraction, the shearing of the hot rolled steel sheet (hot rolled coil) in the hot rolling winding step has the greatest influence on the material deviation by coil length. Considering the heating of the front and rear ends, Equation 2 can be satisfied.

If the above conditions are satisfied, the material deviation in the longitudinal direction of the steel sheet (coil) after final annealing is very small (within 60 MPa based on yield strength), and a steel sheet with a high yield ratio and excellent formability can be obtained.

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 3]

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

[Relational Expression 4]

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 3] and [Relational Expression 4] 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%, side effects such as plate fracture may occur due to material deterioration due to excessive increase in dislocation density in the surface layer and 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.

Before performing the coiling, the heating temperature of the front end (located up to 50 m from the front end) and the rear end (located up to 100 m from the rear end) of the hot rolled steel sheet manufactured after hot rolling is measured by increasing the temperature rise compared to the coiling temperature (CT) by X and Y, respectively. It is shown in Table 2.

After winding the coil, it was pickled using hydrochloric acid, and then cold rolled at a cold rolling rate of 60%. The cold-rolled steel sheet was continuously annealed under the conditions shown in Table 2 below, and then cooled by furnace cooling. Then, for the production of hot-dip galvanized steel sheet, the cold-rolled steel sheet was maintained at a temperature of around 460°C, which is the normal condition. Hot-dip galvanizing was performed by immersing it in a hot-dip galvanizing bath. And for the galvanized steel sheet on which hot dip galvanization was completed, a temper rolling rate of 0.5% was given as shown in Table 2 below to manufacture the final hot dip galvanized steel sheet.

Steel grade C Si Mn P S S.A.L. Ti Nb B N Ti+Nb One 0.085 0.05 1.35 0.02 0.006 0.021 0.03 0.04 0.001 0.002 0.07 2 0.082 0.06 1.4 0.025 0.007 0.046 0.04 0.03 0.0008 0.002 0.07 3 0.09 0.1 1.45 0.01 0.004 0.045 0.045 0.035 0.001 0.002 0.08 4 0.083 0.09 1.4 0.025 0.004 0.043 0.04 0.04 0.0009 0.001 0.08 5 0.095 0.09 1.3 0.018 0.006 0.052 0.05 0.03 0.0009 0.0035 0.08 6 0.1 0.04 1.7 0.01 0.004 0.037 0.02 0.04 0.001 0.0028 0.06 7 0.105 0.1 1.2 0.012 0.007 0.033 0.03 0.03 0.001 0.0039 0.06 8 0.075 0.21 1.5 0.011 0.003 0.035 0.04 0.03 0.0007 0.002 0.07 9 0.091 0.08 1.65 0.025 0.007 0.045 0.01 0.01 0.003 0.004 0.02 10 0.08 0.06 1.75 0.01 0.005 0.046 0.04 0 0.001 0.003 0.04 11 0.081 0.21 1.55 0.009 0.003 0.056 0.05 0.04 0.0009 0.004 0.09 12 0.093 0.35 1.35 0.025 0.004 0.048 0.03 0.03 0.001 0.002 0.06 13 0.095 0.09 2.5 0.015 0.006 0.055 0.04 0.03 0.001 0.0044 0.07 14 0.14 0.12 1.3 0.013 0.005 0.045 0.06 0.025 0.0015 0.0035 0.09 15 0.088 0.15 1.4 0.015 0.005 0.044 0.07 0.08 0.0021 0.0034 0.15

Steel grade Manufacturing conditions K note CT(℃) X (℃) Y (℃) SS(℃) LS SPM El(%) One 560 50 100 810 100 0.5 0 Invention Lecture 1 10 30 78.0 Comparison lecture 1 2 560 50 100 810 100 0.5 0 Invention Lecture 2 50 10 53.3 Comparison lecture 2 3 560 50 100 790 100 0.5 0 Invention Lecture 3 0 70 56.5 Comparison lecture 3 4 560 50 100 800 100 0.5 0.4 Invention Lecture 4 700 50 100 0.5 0.4 Comparison lecture 4 5 560 50 100 820 100 0.5 0 Invention Lecture 5 6 560 50 100 780 100 0.5 6.6 Invention Lecture 6 7 560 50 100 790 100 0.5 0.1 Invention Lecture 7 8 560 50 100 790 100 0.5 0 Invention Lecture 8 9 560 50 100 830 100 0.5 0 Comparison lecture 5 10 560 50 100 800 100 0.5 0 Comparison lecture 6 560 0 0 800 100 0.5 58.6 Comparative lecture 7 12 560 50 100 850 100 0.5 1.4 Comparative Lecture 8 13 560 10 50 800 100 0.5 56.7 Comparison lecture 9 14 560 50 100 720 100 0.5 0 Comparison lecture 10 15 560 50 100 800 100 0.5 22.5 Comparison Lecture 11 16 560 50 100 800 100 0.5 57.5 Comparison Lecture 12

In Table 2, CT is the coiling temperature after hot rolling, SS is the continuous annealing temperature (℃), and LS is the feed speed (line speed, mpm). SPM El is the temper rolling rate.

Meanwhile, K is derived from the following relational equation 2.

[Relational Expression 2]

K ≤ 10

(K = 621×[C] + 222×[Ti] + 1183×[Nb] - 0.694×X - 0.726×Y)

However, if K < 0, it is treated as K = 0.

Here, [C], [Ti], and [Nb] mean the wt.% addition amounts of C, Ti, and Nb. In addition,

For the steel sheet manufactured as above, a tensile test was performed in the rolling direction using the DIN-L standard, and the yield strength (YP), tensile strength (TS), and elongation (El.) of the steel sheet were measured at least five times. The average results are shown in Table 3 above. The yield strength was measured at the head, middle, and tail of the annealed steel sheet coil to check the material deviation by coil length, and the remaining tensile strength and elongation were determined by the middle of the coil. The value of (middle) was selected. Yield ratio (YS) was also calculated using the ratio of yield strength and tensile strength for the middle part of the coil length. Meanwhile, the aspect ratio was measured using the microstructure measured using SEM and EBSD as previously described.

Steel grade mechanical properties aspect ratio
(Aspect ratio) note YS-Head
(MPa) YS-Middle
(MPa) YS-Tail
(MPa) TS-Middle
(MPa) EL-Middle
(%) YR-Middle YS deviation One 532 505 563 595 21 0.85 58 7.9 Invention Lecture 1 603 510 646 605 20 0.84 136 8.1 Comparison lecture 1 2 511 493 539 613 20 0.80 46 8.2 Invention Lecture 2 521 503 614 621 19 0.81 111 8.2 Comparison lecture 2 3 543 515 573 625 18 0.82 58 5.8 Invention Lecture 3 625 510 578 611 20 0.83 115 5.7 Comparison lecture 3 4 502 485 544 605 19 0.80 59 7.5 Invention Lecture 4 420 380 436 525 25 0.72 56 2.5 Comparison lecture 4 5 534 495 551 585 21 0.85 56 6.6 Invention Lecture 5 6 533 525 558 655 17 0.80 59 8.2 Invention Lecture 6 7 533 511 568 635 18 0.80 57 10.1 Invention Lecture 7 8 544 523 565 645 17 0.81 42 12.1 Invention Lecture 8 9 366 345 376 455 30 0.76 31 2.5 Comparison lecture 5 10 375 380 389 545 27 0.70 9 3.8 Comparison lecture 6 466 375 491 569 22 0.66 116 4.1 Comparative lecture 7 11 406 365 425 565 28 0.65 60 2.5 Comparative Lecture 8 12 610 535 650 615 20 0.87 115 4.8 Comparison lecture 9 13 710 690 711 785 6 0.88 54 21 Comparison lecture 10 14 751 700 758 825 5 0.85 58 25 Comparison Lecture 11 15 676 640 685 726 8 0.88 45 19 Comparison Lecture 12

As can be seen in Table 3, Invention Examples 1 to 8 that meet the conditions of the present invention have a yield strength of 485 to 523 MPa, a tensile strength of 585 to 655 MPa, an elongation of 17 to 21%, and a yield ratio (YR) based on the middle portion of the coil length. ) is 0.80 to 0.85, the material deviation of the head, middle, and tail parts within the coil length is 42 to 59 MPa, and the yield strength deviation by length is within 60 MPa, which is the mechanical deviation presented in the present invention steel. The physical properties are satisfactory. In addition, in the case of steels with excellent physical properties such as the present invention steel, the aspect ratio is 6.2 to 12.1, which sufficiently satisfies the condition of aspect ratio 5 to 15 suggested by the present invention steel. Meanwhile, looking at the K value in Equation 2, the inventive steels were within 0 to 8.8, satisfying the condition of K ≤ 10 suggested by the inventive steels.

Figure 2 shows the difference in yield strength of invention steel 1 and invention steel 2 by hot rolled coil length. As a result of working under the conditions of target coiling temperature (T) + 50°C at the front end and target coiling temperature (T) + 100°C at the rear end when hot-rolling the invention steel, the yield strength deviation by length of the invention steel is very much within 15 MPa. It was excellent.

Comparative steels 1 to 3 and 9 are cases where the temperature at the front and rear ends of the hot rolled coil is low during hot rolling. As a result, the K value was very high, and the yield strength deviation by length of the annealed coil was very high, exceeding 100 MPa.

Figure 3 shows the microstructure of the manufactured steel sheet coil length of invention steel 1 and comparative steel 1. Inventive steel 1, in which the front end was raised to 50℃ and the rear end to 100℃ during the hot-rolled coiling operation, and the K value in relational equation 2 satisfied the conditions presented in the present invention steel, has almost no difference in microstructure in the longitudinal direction, but the shear temperature during the hot-rolled coiling operation is In the case of comparative steel 1, where the temperature was raised by 10°C at the part and 20°C at the rear end, it can be seen that the head and tail parts of the annealed coil showed a non-uniform structure distribution by coil length, with much more unrecrystallized structure than the middle part.

Comparative steel 4 has a very high coiling temperature of 700°C. The composition and temperature conditions of the front and rear ends during hot rolling satisfy the conditions presented in the present invention steel, so the material deviation by coil length of the annealed plate is low, but the yield strength is very low due to the high coiling temperature, and the high yield ratio presented by the present invention steel The physical properties of the steel were not satisfactory.

Comparative steels 5 to 7 have lower Ti and Nb contents than those suggested in the present invention steel. In particular, comparative steel 7 has low Ti and Nb contents and does not increase the temperature of the front and rear ends of the coil during hot rolling. This is not the case. Due to the low Ti and Nb contents, recrystallization easily occurred during annealing due to the lack of precipitates in the steel. As a result, the yield strength of the annealed plate was low and the aspect ratio value did not satisfy the conditions of the present invention. In particular, comparative steel 7, where the temperature conditions were not properly controlled during coiling, had a very high yield strength deviation of the annealed plate.

In comparative steel 8, the composition and temperature conditions during hot rolling satisfied the K value presented in the present invention steel, so the material deviation by length of the annealed plate was low, but the annealing temperature was very high at 850°C. The ferrite recrystallization fraction increased due to high-temperature annealing, and the yield strength and yield ratio were low, so the results such as aspect ratio were outside the conditions of the present invention steel.

Comparative steel 10 was annealed at a very low temperature, with an annealing temperature of 720°C. Even if other conditions satisfied the standards for the present invention steel, the annealing temperature was very low, so there was not enough time to secure the ferrite recrystallization fraction required for the present invention steel, and the elongation was reduced due to excessive increases in yield strength and yield ratio. This causes problems such as deterioration.

Comparative steel 11 has a carbon content of 0.14%, which is outside the composition range of the present invention steel. Although other conditions satisfied the standards for the present invention steel, carbides in the steel increased due to excessive carbon content, 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 Ti and Nb contents of 0.07% and 0.08%, respectively, which exceeded the standards for the present invention steel. 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 (10)

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,
A steel plate whose material deviation along the length of the steel plate is less than 60 MPa based on yield strength.
In claim 1,
The microstructure of the steel sheet includes 80 to 95% of ferrite in terms of area percentage, the remainder includes pearlite and other inevitable structures, and 20 to 50% of unrecrystallized ferrite among the ferrite.
In claim 2,
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 the thickness of the cold rolled steel sheet)
In claim 1,
The steel sheet further includes a hot-dip galvanized layer on the surface.
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%), 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
Continuously annealing the cold rolled steel sheet at a temperature range of 770 to 820°C,
Before winding the hot-rolled steel sheet, the front end of the hot-rolled steel sheet is heated to a coiling temperature (T) +30℃ to T+100℃, and the rear end of the hot-rolled steel sheet is heated to T+80℃ to T+150℃. A method of manufacturing a steel plate comprising:
In claim 7,
The above manufacturing method is a method of manufacturing a steel plate that satisfies the conditions of [Relational Equation 2] below.
[Relational Expression 2]
K ≤ 10
(K = 621×[C] + 222×[Ti] + 1183×[Nb] - 0.694×X - 0.726×Y)
However, if K < 0, it is treated as K = 0.
Here, [C], [Ti], and [Nb] mean the wt.% addition amount of C, Ti, and Nb. In addition,
In claim 7,
The manufacturing method is a method of manufacturing a steel plate that satisfies the conditions of [Relational Expression 3] and [Relational Expression 4] below.
[Relational Expression 3]
2566 +2.1*0.192*CR - 1.79*SS - 5.64*LS ≥ 520
[Relational Expression 4]
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 the step of hot-dip galvanizing the continuously annealed steel sheet.

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