High-yield-ratio high-strength steel sheet and method for manufacturing same

Abstract

The present invention relates to a material for use as an automobile inner panel, a reinforcing material, and the like, and relates to a high-yield-ratio high-strength steel sheet and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a material used as an inner panel of an automobile, a reinforcing material, etc., and relates to a high yield ratio type high strength steel sheet and a method for manufacturing the same. Background Art

[0002] In order to improve the fuel efficiency of automobiles, in recent years, the weight reduction of automobile bodies has been carried out. For this purpose, the application of high strength steel sheets with reduced plate thickness in automobile components is increasing. In addition, in order to ensure the stability of passengers, high strength steel sheets are widely used in automobile bodies. In order to improve the collision performance of automobile bodies, an attempt is made to increase the yield strength of steel materials so that collision energy can be effectively absorbed even at low strains. For this purpose, a steel sheet with a high yield ratio is required.

[0003] In order to apply high strength steel sheets to automobile bodies, excellent workability is also required. As a steel material having both such strength and workability, a representative is dual phase steel (Dual Phase steel, hereinafter referred to as DP steel), which has a composite structure with ferrite as the main phase and a hard structure as the second phase. However, since DP steel has soft ferrite as the main phase and bainite, martensite, tempered martensite, etc. as the hard structure as the second phase, there is a problem of a low yield ratio. Therefore, the application of DP steel as an automobile component for absorbing collision energy while suppressing deformation has certain limitations.

[0004] In addition, Patent Document 1 proposes a steel sheet that prevents the recrystallization of ferrite and has a structure composed of unrecrystallized ferrite and a hard second phase. However, when there is an excessive amount of unrecrystallized ferrite, the strength and yield ratio increase, but the elongation is low, so there is a problem of insufficient formability.

[0005] Regarding this problem, Patent Documents 2 to 4 propose a steel sheet having a structure composed of ferrite and pearlite and a steel sheet having both high strength and improved stretch flangeability by grain refinement, precipitation strengthening, or reducing the amount of solid solution C in ferrite. However, the tensile strength of the proposed steel materials is all 500 MPa or less, and it is difficult to achieve high strength exceeding 500 MPa.

[0006] In addition, Patent Documents 5 to 7 propose a steel plate that improves the flange stretchability by using unrecrystallized ferrite with an intermediate hardness between soft ferrite and a hard second phase. However, in the steel plates proposed in Patent Documents 5 and 6, due to the small addition amounts of Nb or Ti, the recrystallization inhibition effect is small, so rapid heating is required during annealing. In the steel plate proposed in Patent Document 7, the addition amounts of Nb or Ti are small, and the heating rate during annealing is also as low as 10 °C / second or less. Therefore, the time for recrystallization increases, and the effect brought by using unrecrystallized ferrite is insufficient.

[0007] (Patent Document 1) Japanese Patent Publication Gazette 1978 - 005018

[0008] (Patent Document 2) Japanese Patent Application Gazette 2007 - 138261

[0009] (Patent Document 3) Japanese Patent Publication Gazette 2007 - 107099

[0010] (Patent Document 4) Japanese Patent Publication Gazette 2001 - 152288

[0011] (Patent Document 5) Japanese Patent Publication Gazette 2008 - 106351

[0012] (Patent Document 6) Japanese Patent Publication Gazette 2008 - 106352

[0013] (Patent Document 7) Japanese Patent Publication Gazette 2008 - 156680 Summary of the Invention

[0014] (I) Technical Problems to be Solved

[0015] An object of one aspect of the present invention is to provide a steel plate having a high yield ratio and high strength and excellent formability, and a manufacturing method thereof.

[0016] The technical problems of the present invention are not limited to the above. Additional technical problems of the present invention are described in the entire content of the specification, and those skilled in the art to which the present invention pertains can easily understand the additional technical problems of the present invention from the content described in the specification of the present invention.

[0017] (II) Technical Solutions

[0018] One embodiment of the present invention provides a steel plate which, by weight %, comprises: carbon (C): 0.05 - 0.12%, manganese (Mn): 1.0 - 1.8%, silicon (Si): below 0.6% (except 0%), phosphorus (P): below 0.03% (except 0%), sulfur (S): below 0.01% (except 0%), nitrogen (N): below 0.01% (except 0%), aluminum (sol.Al): 0.01 - 0.08%, titanium (Ti): 0.02 - 0.06%, niobium (Nb): 0.02 - 0.06%, boron (B): below 0.005% (except 0%), the balance being Fe and inevitable impurities. By area %, the microstructure comprises 80 - 99% ferrite, and the remainder comprises pearlite and other inevitable structures. The non-recrystallized ferrite in the ferrite is 20 - 50%, and the aspect ratio of the ferrite is 5 to 15.

[0019] The total amount of the Ti and the Nb may be below 0.1%.

[0020] The cold-rolled steel plate may satisfy the following relational expression 1.

[0021] [Relational expression 1]

[0022] X(222) / [X(200)+X(110)+X(112)] ≤ 2

[0023] (Ratio of the X-ray diffraction integrated intensities of the {222}, {110}, {200}, and {112} planes parallel to the plane at a depth position of 1 / 4 of the plate thickness of the cold-rolled steel plate)

[0024] The surface of the steel plate may further include a hot-dip galvanized layer.

[0025] The yield strength of the steel plate may be 460 MPa or more, and the tensile strength may be 520 MPa or more.

[0026] The product of the yield strength and the elongation of the steel plate may be 8600 or more.

[0027] Another embodiment of the present invention provides a method for manufacturing a steel plate, the manufacturing method comprising the following steps: heating a steel billet to 1100 - 1250 °C, wherein, by weight%, the steel billet comprises: carbon (C): 0.05 - 0.12%, manganese (Mn): 1.0 - 1.8%, silicon (Si): less than 0.6% (except 0%), phosphorus (P): less than 0.03% (except 0%), sulfur (S): less than 0.01% (except 0%), nitrogen (N): less than 0.01% (except 0%), aluminum (sol.Al): 0.01 - 0.08%, titanium (Ti): 0.02 - 0.06%, niobium (Nb): 0.02 - 0.06%, boron (B): less than 0.005% (except 0%), the balance being Fe and inevitable impurities;

[0028] Hot-rolling the heated steel billet at a temperature above 880 °C to obtain a hot-rolled steel plate;

[0029] Cooling the hot-rolled steel plate to 500 - 600 °C and coiling it;

[0030] Cold-rolling the coiled hot-rolled steel plate at a reduction ratio of 45 - 70%; and

[0031] Continuously annealing the cold-rolled steel plate in a temperature range of 770 - 820 °C.

[0032] The method for manufacturing the steel plate can meet the conditions of the following [Relationship 2] and [Relationship 3].

[0033] [Relationship 2]

[0034] 2566 + 2.1 0.192 CR - 1.79 SS - 5.64 LS ≥ 520

[0035] [Relationship 3]

[0036] 2438 + 1.9 0.192 CR - 1.79 SS - 5.64 LS ≤ 700

[0037] Wherein, CR is the cold reduction ratio (%), SS is the annealing temperature (°C), and LS is the line speed (linespeed) (meters per minute (mpm)) during the continuous annealing operation

[0038] The method for manufacturing the steel plate may further comprise the step of hot-dip galvanizing the continuously annealed steel plate.

[0039] (III) Advantageous Effects

[0040] The steel plate of the present invention has high strength and a high yield ratio. Therefore, when used as an inner plate material, a reinforcing material, etc., the collision resistance (collision resistance characteristics) increases during a collision, which is beneficial to ensuring the stability of passengers. In addition, according to the present invention, a steel plate with excellent formability can be provided.

[0041] The various beneficial advantages and effects of the present invention are not limited to the above content, and the various beneficial advantages and effects of the present invention will be more easily understood during the process of describing the specific embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 A photograph showing the microstructure of the inventive steel 1 in the embodiment. BEST MODE FOR CARRYING OUT THE INVENTION

[0043] The terms used in this specification are for explaining the present invention and are not intended to limit the present invention. In addition, unless the relevant definitions show a clearly opposite meaning, the singular forms used in this specification also include the plural forms.

[0044] The meaning of "comprising" or "including" used in the specification is for specifically explaining the constitution and does not exclude the existence or addition of other constitutions.

[0045] Unless otherwise defined, all terms, including technical terms and scientific terms, used in this specification have the same meaning as commonly understood by those skilled in the art. The terms defined in the dictionary are interpreted to conform to the meaning of the relevant technical literature and the currently disclosed content.

[0046] In the microstructure of a cold-rolled steel plate, it can be said that leaving a part of the ferrite in an unrecrystallized state is not a common idea. Unrecrystallized ferrite is ferrite that has been elongated in the rolling direction through cold rolling, which means that recrystallization is not complete and the dislocations in the grains have recovered. When such an unrecrystallized ferrite structure exists in the steel, the material of the cold-rolled steel plate varies greatly in the width direction and the length direction, and even due to a slight difference in the unrecrystallized fraction, the material appears very non-uniform. Therefore, it is considered that the best method is to minimize the unrecrystallized structure as much as possible.

[0047] In order to ensure strength, when a large amount of titanium (Ti) and niobium (Nb) are added to a steel containing 0.05 wt% or more of carbon (C), it is very difficult to obtain a completely recrystallized structure through an annealing process. It is known that the Ti and Nb form carbides such as TiC and NbC during the cooling process in the hot rolling step and the heating process in the annealing process, and are elements that hinder recrystallization and inhibit grain growth.

[0048] Therefore, in order to obtain a recrystallized structure, it is necessary to control the annealing temperature very high, or a special process for suppressing the precipitation of TiC, NbC, etc. is required. Among them, the special process means that through very fast rapid cooling or very fast heating, TiC, NbC, etc. hardly have time to precipitate. However, this is a process that cannot be achieved during normal operation and requires special equipment to implement. In addition, in order to control the annealing temperature at a relatively high level and prevent the inhibition of recrystallization by TiC, NbC, etc., it is necessary to maintain a temperature of almost 900 °C or higher. Such high-temperature annealing may cause problems such as the snake-like movement of the coiled sheet and an increase in manufacturing costs. Even when the annealing temperature is increased to above 900 °C, due to the generation of the recrystallized structure resulting in softening of the material and a decrease in yield strength, etc., it is difficult to ensure the high yield ratio required by the present invention.

[0049] Therefore, the present inventors conducted in-depth research to manufacture a steel sheet having a high yield ratio. Preferably, the yield strength is 460 - 600 MPa, the tensile strength is 520 - 700 MPa, and the yield ratio is 0.8 to 0.9. As a result, a method was obtained that can ensure the above-mentioned yield ratio by using unrecrystallized ferrite and can have excellent formability with an elongation of 10% or more, thereby completing the present invention.

[0050] Hereinafter, a specific 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, the content of the alloy composition in the present invention is based on weight %.

[0051] The steel sheet contains: carbon (C): 0.05 - 0.12%, manganese (Mn): 1.0 - 1.8%, silicon (Si): 0.6% or less (except 0%), phosphorus (P): 0.03% or less (except 0%), sulfur (S): 0.01% or less (except 0%), nitrogen (N): 0.01% or less (except 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 (except 0%).

[0052] Carbon (C): 0.05 - 0.12%

[0053] Carbon (C) is an element that contributes to the increase in strength and the formation of pearlite. In order to ensure the desired strength, an appropriate amount of carbon (C) is added. In addition, in the ferrite phase, carbon (C) forms precipitates together with Ti, Nb, etc. and is an essential element for imparting strength to the steel sheet. When the content of C is less than 0.05%, it is difficult to ensure the strength required for the steel of the present invention. When the content of C exceeds 0.12%, the formability or weldability deteriorates. Therefore, it is effective that the content of C is 0.05 - 0.12%.

[0054] Manganese (Mn): 1.0 - 1.8%

[0055] Manganese (Mn) is an element that lowers the Ac3 transformation temperature, which is the temperature at which the Ac1 and α-γ transformations are completed and the austenite becomes single-phase. That is, when the amount of Mn is small, in order to promote the transformation, it is necessary to increase the annealing temperature, which makes it difficult to ensure the appropriate fraction of unrecrystallized ferrite required for the present invention. In addition, Mn is an element that contributes to solid-solution strengthening together with Si and is also effective in increasing strength. From this perspective, it is effective for the content of Mn to be 1.0% or more. On the other hand, when the content of Mn exceeds 1.8%, the hardenability increases, bainite and martensite are easily formed, and the yield ratio decreases, so it is effective not to exceed 1.8%.

[0056] Silicon (Si): 0.6% or less (excluding 0%)

[0057] Silicon (Si) is a deoxidizing element and also a solid-solution strengthening element effective in increasing strength. However, when the amount of Si exceeds 0.6%, Ac1 becomes too high and it is necessary to increase the annealing temperature, making it difficult to ensure unrecrystallized ferrite by promoting the transformation. Therefore, it is effective to control the content of Si to 0.6% or less. In addition, when Si is added in excess, problems such as reduced plating adhesion due to oxides may occur during hot-dip galvanizing. However, considering the amount inevitably added during the manufacturing process, 0% is excluded.

[0058] Phosphorus (P): 0.03% or less (excluding 0%)

[0059] Phosphorus (P) is an impurity that segregates at grain boundaries, thus causing a decrease in the toughness of the steel sheet or a deterioration in weldability, etc. In addition, the alloying reaction during hot-dip galvanizing is very slow, resulting in reduced productivity, so it is effective for P to be 0.03% or less.

[0060] Sulfur (S): 0.01% or less (excluding 0%)

[0061] Sulfur (S) is an impurity inevitably contained in steel, and it is preferable to control the content of S at as low a level as possible. Therefore, considering the inevitable inclusion of S, 0% is excluded from the content of S in the steel. In particular, S in the steel has a high possibility of causing red-hot brittleness, so it is effective to control the content of S to 0.01% or less.

[0062] Nitrogen (N): 0.01% or less (excluding 0%)

[0063] Nitrogen (N) is an impurity inevitably contained in steel, and it is important to control the content of N at as low a level as possible. Therefore, considering the inevitable inclusion, 0% (i.e., more than 0%) is excluded from the N content in steel. However, in order to control the N content in steel at a very low level, there is a problem that the refining cost of steel increases sharply, so it is controlled below 0.01% which is within the operable condition range.

[0064] Acid-soluble aluminum (sol.Al): 0.01 - 0.08%

[0065] Acid-soluble aluminum (sol.Al) is an element added for grain refinement and deoxidation. When the content of acid-soluble aluminum is less than 0.01%, it is impossible to produce normal and stable aluminum-killed steel. On the other hand, when the content of acid-soluble aluminum exceeds 0.08%, due to the grain refinement effect, it is beneficial to increase strength, but excessive inclusions are formed during the steelmaking continuous casting operation, so the possibility of surface defects occurring in the coated steel sheet increases. In addition, there is a problem of a sharp increase in manufacturing cost, so it is preferable to control the content of the acid-soluble aluminum at 0.01 - 0.08%.

[0066] Titanium (Ti): 0.02 - 0.06% and Niobium (Nb): 0.02 - 0.06%

[0067] The Ti and the Nb are elements that promote the retention of unrecrystallized ferrite by suppressing the recrystallization of deformed ferrite generated by cold rolling during the annealing process. In order to obtain this effect, it is preferable to add at least 0.02% of Nb and Ti respectively. However, when added in excess, due to the formation of carbides such as TiC and NbC, the amount of unrecrystallized ferrite increases, and the yield strength and yield ratio may become too high. Due to the excessive addition of alloying elements, the manufacturing cost may increase. In addition, it is more effective to control the total amount (Ti + Nb) of the Ti and the Nb below 0.1%.

[0068] Boron (B): below 0.005% (except 0%)

[0069] Boron (B) is an element that increases strength by improving hardenability and suppresses nucleation at grain boundaries. When the content of the B exceeds 0.005% by weight, not only is the effect excessive, but it also leads to an increase in manufacturing cost, so the content of the B is preferably controlled below 0.005% by weight.

[0070] In addition, the balance of Fe and inevitable impurities are included. Except for the above composition, the addition of effective components is not excluded. That is, the inevitable impurities may include all impurities inadvertently mixed in during the manufacturing process of ordinary cold-rolled steel sheets (and coated steel sheets). Those skilled in the art can easily understand its meaning, so no special limitation is made thereto.

[0071] Next, the microstructure of the steel plate will be described in detail. In terms of area %, the microstructure of the steel plate contains 80-99% ferrite, and the rest contains pearlite and other inevitable structures. At this time, the inevitable structures are not particularly limited, but may be cementite, carbides, etc. More specifically, it may contain 80-95% ferrite.

[0072] In addition, in terms of area %, it is effective that the non-recrystallized ferrite in the ferrite is 20-50%. The area % of the non-recrystallized ferrite represents the fraction relative to the entire microstructure.

[0073] Among them, the fraction of the non-recrystallized ferrite can be determined by using the Kernel Average Misorientation (KAM) method to interpret the crystal orientation measurement data of Electron Backscattering Diffraction (referred to as EBSD). Since the KAM method can quantitatively represent the crystal orientation difference with adjacent pixels (measurement points), in the present invention, particles with an average crystal orientation difference within 1° from adjacent measurement points are defined as non-recrystallized ferrite. To ensure sufficient yield strength and yield ratio, it is effective that the area % of the non-recrystallized ferrite is 20-50%. When the non-recrystallized ferrite is less than 20%, sufficient yield strength and yield ratio cannot be obtained. When the non-recrystallized ferrite exceeds 50%, due to the high non-recrystallized structure, the yield strength and yield ratio are too high, and the aspect ratio of the grains increases. Therefore, it is effective that the non-recrystallized ferrite is 20-350%.

[0074] It is effective that the total fraction of ferrite containing the non-recrystallized ferrite is 80-99%. More specifically, a total ferrite fraction of 80-95% may be more effective.

[0075] In addition, in addition to ferrite, the steel plate of the present invention contains pearlite and inevitable structures.

[0076] It is effective that the aspect ratio (A / R) of the grains in the microstructure of the steel plate, especially ferrite, is 5 to 15.

[0077] Among them, for the aspect ratio of the grains, after etching the microstructure with 5% nitric acid etching (Nital etching) solution, it is observed with a scanning electron microscope (SEM) at 500 times magnification, and the major axis length and minor axis length of the grains are obtained through image analysis processing using an Image Analyzer program. The aspect ratio is obtained by the major axis length of the grain / minor axis length of the ellipse. The aspect ratio of the grains is defined by the average value of the aspect ratios of each ferrite obtained by this technique.

[0078] When the A / R exceeds 15, it means that the elongated grains in the rolling direction are very large, which means that the cold-rolled structure hardly recovers or recrystallizes. The excessive formation of such elongated grains leads to an excessive increase in the yield strength, exceeding the yield strength and yield ratio required by the present invention. However, when the A / R is less than 5, it means that recrystallization proceeds significantly, which means that due to the softening of the steel, the yield strength is not reached and the yield ratio is lower than the level required for the steel of the present invention.

[0079] It is effective that the texture of the steel plate satisfies the conditions of the following relational expression 1.

[0080] [Relational expression 1]

[0081] X(222) / [X(200)+X(110)+X(112)]≤2

[0082] (Ratio of the X-ray diffraction integrated intensities of the {222}, {110}, {200}, and {112} planes parallel to the plane at a depth position of 1 / 4 of the plate thickness of the cold-rolled steel plate)

[0083] Among them, the X-ray diffraction integrated intensity ratio is the relative intensity based on the X-ray diffraction integrated intensity of a non-oriented standard sample. X-ray diffraction can use an energy dispersive type or the like of X-ray diffraction apparatus widely used in the technical field to which the present invention pertains. When the X-ray diffraction integrated intensity ratio calculated in the relational expression 1 exceeds 2, it means that the fraction of the (222) texture, that is, the recrystallization texture, increases, which means that the fraction of unrecrystallized ferrite in the steel is not formed within an appropriate range and the aspect ratio cannot be ensured.

[0084] The yield strength of the steel plate satisfying the above alloy composition and microstructure can be 460 MPa or more, the tensile strength can be 520 MPa or more, and the yield ratio can be 0.8 to 0.9. At the same time, the steel plate can have an elongation of 10% or more. More specifically, the yield strength of the steel plate can be 460 - 600 MPa. The tensile strength of the steel plate can be 520 - 700 MPa.

[0085] More specifically, the product of the yield strength and the elongation of the steel plate can be 8500 or more. More specifically, the product of the yield strength and the elongation can be 8900 or more. Generally, as the yield strength increases, the elongation may decrease. However, according to a specific embodiment of the present invention, the product of the yield strength and the elongation is 8500 or more, so the yield strength and the elongation can be improved simultaneously.

[0086] In addition, the steel plate of the present invention may include a plating layer for improving corrosion resistance. In the present invention, the plating layer is not particularly limited as long as it is a plating type and plating method carried out in the technical field to which the present invention pertains. As a preferred example, it can be a hot-dip galvanized layer.

[0087] Next, a specific embodiment of the method for manufacturing the steel plate of the present invention will be described in detail. However, it does not mean that the steel plate of the present invention must be manufactured by the following manufacturing method.

[0088] In order to manufacture the steel plate of the present invention, it can be manufactured through the processes of reheating, hot rolling, coiling, cold rolling, and annealing of a steel billet satisfying the above composition. Hereinafter, each process will be described in detail.

[0089] Reheating: 1100 - 1250 °C

[0090] It is effective to reheat the steel billet having the above composition to a temperature range of 1100 - 1250 °C. When the reheating temperature is lower than 1100 °C, inclusions in the slab cannot be sufficiently remelted, which may lead to material deviation, surface defects, etc. after hot rolling. In contrast, when the slab reheating temperature exceeds 1250 °C, problems such as a decrease in strength due to excessive growth of austenite grains may occur.

[0091] Hot rolling: 880 °C or higher

[0092] The reheated steel billet is hot rolled at a temperature of 880 °C or higher to manufacture a hot rolled steel plate. When the temperature of the hot rolling is lower than 880 °C, since ferrite phase transformation occurs during the rolling process and an elongated structure is generated, problems such as deteriorated anisotropy and reduced cold rollability may occur. Therefore, it is effective to perform the hot rolling at 880 °C or higher.

[0093] Coiling: 500 - 600 °C

[0094] It is effective to coil the hot rolled steel plate within a temperature range of 500 - 600 °C. When the coiling temperature is lower than 500 °C, the shape of the steel plate deteriorates, and phase transformation structures such as acicular ferrite are generated, which may lead to an excessive increase in the strength of the steel plate after annealing and a decrease in ductility. However, when the coiling temperature exceeds 600 °C, coarse ferrite grains are formed, and coarse carbides and nitrides are easily formed, so the quality of the steel may deteriorate. In addition, due to coiling at a high temperature, problems such as bending occur, resulting in deteriorated cold rollability.

[0095] Cold rolling: Cold reduction ratio is 45 - 70%

[0096] The hot-rolled steel sheet after coiling and pickling is cold-rolled to produce a cold-rolled steel sheet. It is effective that the reduction ratio (cold reduction ratio) during the cold rolling is 45 - 70%. When the cold reduction ratio is less than 45%, the recrystallization driving force is very low, and excessive unrecrystallized ferrite is formed, so it is difficult to ensure the strength required by the present invention. On the other hand, when the cold reduction ratio exceeds 70%, the recrystallization driving force is too high, and ferrite is prone to recrystallize at a low annealing temperature, thus it is difficult to manufacture a high-strength steel with a yield ratio of 0.8 to 0.9.

[0097] Continuous annealing: 770 - 820 °C

[0098] It is effective to subject the cold-rolled steel sheet to continuous annealing in the temperature range of 770 - 820 °C. When the annealing temperature is lower than 770 °C, the fraction of unrecrystallized ferrite is too high, the yield strength is high, and the ductility becomes poor. On the other hand, when the annealing temperature exceeds 820 °C, the fraction of unrecrystallized ferrite is too small, so it is difficult to ensure the high yield ratio required by the present invention.

[0099] In the present invention, in order to ensure a certain amount of unrecrystallized structure, it is very important to appropriately control the operating factors for controlling the recrystallization driving force. Therefore, it is effective to satisfy the conditions of the following [Relationship 2] and [Relationship 3].

[0100] [Relationship 2]

[0101] 2566 + 2.1 0.192 CR - 1.79 SS - 5.64 LS ≥ 520

[0102] [Relationship 3]

[0103] 2438 + 1.9 0.192 CR - 1.79 SS - 5.64 LS ≤ 700

[0104] Wherein, CR is the cold reduction ratio (%), SS is the annealing temperature (°C), and LS is the line speed (m / min) during continuous annealing operation.

[0105] The [Relationship 2] and the [Relationship 3] are operating factors for controlling the recrystallization driving force, including the annealing temperature, the cold reduction ratio, and the transportation speed (line speed) during annealing. In the present invention, it is effective that the transportation speed is 90 - 150 m / min. The transportation speed is controlled differently according to the thickness of the steel sheet. That is, in the case of thick materials, the transportation speed (line speed) is low, and for thin materials, high-speed operation is carried out. It is preferable to control the cold reduction ratio and the annealing temperature simultaneously to adapt to this condition.

[0106] In the present invention, subsequently, plating may be further performed after the continuous annealing.

[0107] The plating may be carried out by a method commonly used in the technical field to which the present invention pertains, and there are no particular limitations on the type and method of plating. In the present invention, there are no particular limitations on the hot-dip galvanizing conditions, and hot-dip galvanizing may be carried out under the general conditions applicable in the same technical field. By hot-dip galvanizing, a hot-dip galvanized layer may be included on the surface of the steel sheet according to an embodiment of the present invention. As a preferred example, the steel sheet is immersed in a molten zinc-based plating bath at 440 - 500 °C to produce a hot-dip galvanized steel sheet. In addition, if necessary, after the hot-dip galvanizing step, the steel sheet may be subjected to an alloying heat treatment. As an embodiment, the hot-dip galvanized steel sheet may be cooled to room temperature after being subjected to an alloying heat treatment in the temperature range of 460 - 530 °C. By the alloying heat treatment, the steel sheet may also include an alloyed hot-dip galvanized layer on its surface.

[0108] Skin pass rolling may be carried out after the hot-dip galvanizing. The skin pass rolling may also be carried out within the normal range of 0.1 - 1.0%. When the elongation rate of the skin pass rolling is less than 0.1%, it is difficult to control the sheet shape. On the other hand, when the elongation rate of the skin pass rolling exceeds 1.0%, in addition to the deterioration of the material due to the excessive increase in the dislocation density of the surface layer, side effects such as the occurrence of sheet breakage may be caused due to the limitation of the equipment capacity. Detailed Embodiments

[0109] Hereinafter, embodiments of the present invention will be described. It will be obvious to those skilled in the technical field to which the present invention pertains that various modifications can be made to the following embodiments without departing from the scope of the present invention. The following embodiments are for understanding the present invention, and the scope of rights of the present invention should not be limited to the following embodiments, but should be determined not only by the scope of rights but also by its equivalents.

[0110] (Example)

[0111] At a reheating temperature of 1200 °C, a hot finishing rolling temperature of 900 °C above the Ar3 temperature, and a coiling temperature of 560 °C, a steel billet having the alloy composition (in weight %, the rest being inevitable impurities) shown in Table 1 below was hot-rolled and coiled.

[0112] After the coiling operation of the coiled sheet, pickling was carried out using hydrochloric acid, and annealing was carried out under the cold rolling and continuous annealing conditions shown in Table 2 to produce a steel sheet.

[0113] [Table 1]

[0114]

[0115] [Table 2]

[0116]

[0117] In Table 2, CR is the cold rolling reduction rate (%), SS is the continuous annealing temperature (°C), and LS is the transportation speed (linear speed, m / min). In addition, Relationship 2 and Relationship 3 are as follows.

[0118] [Relationship 2]

[0119] 2566 + 2.1 0.192 CR - 1.79 SS - 5.64 LS ≥ 520

[0120] [Relationship 3]

[0121] 2438 + 1.9 0.192 CR - 1.79 SS - 5.64 LS ≤ 700

[0122] For the steel plate manufactured as described above, a tensile test is carried out along the rolling direction using the DIN-L standard, and the yield strength (YP), tensile strength (TS), and elongation (El.) of the steel plate are measured. The results are shown in Table 3 above. In addition, using the microstructure measured by the above Scanning Electron Microscope (SEM) and electron backscatter diffraction (EBSD), the grain aspect ratio of ferrite and the unrecrystallized and recrystallized ferrite fractions (area %) are measured. In addition, for the manufactured steel plate, the X-ray diffraction intensity values of each texture component are measured, and the X-ray diffraction intensity ratio is calculated using the formula of Relationship 1.

[0123] [Relationship 1]

[0124] X(222) / [X(200) + X(110) + X(112)] ≤ 2

[0125] (Ratio of the X-ray diffraction integrated intensities of the {222}, {110}, {200}, and {112} planes parallel to the plane at a depth position of 1 / 4 of the thickness of the cold-rolled steel plate)

[0126] [Table 3]

[0127]

[0128] As can be seen from Table 3, the yield strengths of Invention Steels 1 to 8 that meet the alloy composition and manufacturing conditions of the present invention are 469 - 545 MPa, the tensile strengths are 545 - 655 MPa, the elongation rates are 18 - 21%, the yield ratio (YR) is 0.82 to 0.86, and the product of the yield strength and the elongation rate is 8900 or more, thus meeting the mechanical and physical properties required for the steel of the present invention. In addition, the grain aspect ratio of the invention steel is 5.9 to 10.2, the grain aspect ratio proposed by the present invention meets the condition of 5 to 15, and the non-recrystallized ferrite fraction is 29 - 45%, meeting the condition of 20 - 50% proposed by the present invention. The X-ray diffraction ratio of this steel is 0.6 to 1.2, fully meeting the conditions proposed by the present invention.

[0129] Figure 1 Fig. is a SEM photograph showing the microstructure of the annealed sheet of Invention Steel 1 in the examples. The microstructure includes non-recrystallized ferrite (reference numeral ①) and recrystallized ferrite (reference numeral ②), and includes a part of pearlite.

[0130] The cold rolling reduction ratios of Comparative Steel 1 and Comparative Steel 3 are very low compared with the conditions proposed by the present invention. This results in insufficient ferrite recrystallization during annealing, so the yield strength is very high and the yield ratio exceeds the standard of the present invention. In addition, due to insufficient recrystallization, the grain aspect ratio value is very high.

[0131] Comparative Steel 2 and Comparative Steel 7 are different from Comparative Steel 1, corresponding to the case where the cold rolling reduction ratio is 80% and very high. When the cold rolling reduction ratio is high, ferrite recrystallization easily occurs even at a low annealing temperature. This is because the recrystallization fraction increases, resulting in a decrease in strength and unable to meet the yield strength and yield ratio conditions required for the steel of the present invention. In addition, the grain aspect ratio and the X-ray integrated intensity deviate from the standard of the present invention.

[0132] The addition amounts of Ti and Nb in Comparative Steel 4 are 0.01% respectively and very low. Due to the insufficient precipitation of TiC and NbC, recrystallization is promoted, resulting in a decrease in the non-recrystallized ferrite fraction after annealing, so the yield strength, yield ratio, etc. deviate from the conditions of the present invention.

[0133] Comparative Steel 5 and Comparative Steel 8 are cases where Ti or Nb is not added at all. Due to the insufficient precipitates in the steel, recrystallization easily occurs during annealing, so the yield strength of the steel plate is low, and the grain aspect ratio and the X-ray diffraction intensity ratio do not meet the conditions of the present invention.

[0134] Comparative Steel 6 is a steel with the same composition system as Comparative Steel 5 and without Nb added. Under the condition of insufficient precipitates, the cold rolling reduction ratio is 35% and very low, and even high-temperature annealing at 860 °C cannot meet the conditions of the present invention.

[0135] In Comparative Steel 9, the added components all fall within the scope of the steel of the present invention, but the annealing temperature is 850 °C and is very high. As the annealing temperature increases, the ferrite recrystallization fraction increases, so the yield strength and yield ratio are low, and results such as the grain aspect ratio deviate from the conditions of the present invention.

[0136] Comparative Steel 10 corresponds to the case where the content of Mn deviates from the scope of the present invention and the annealing temperature is 750 °C and is very low. This low annealing temperature results in insufficient ferrite recrystallization, which causes problems such as an elongation of less than 10% and poor workability due to excessive increases in the yield strength and yield ratio.

[0137] Comparative Steel 11 is the case where the carbon content is 0.14% and deviates from the proposed range of the composition of the steel of the present invention. Due to the excessive carbon content, carbides in the steel increase, so problems such as an increase in the yield ratio and poor elongation occur. In addition, the addition of excessive carbon leads to poor weldability.

[0138] Comparative Steel 12 is the case where the content of Ti is 0.08% and deviates from the standard of the steel of the present invention, and the total amount of Ti + Nb also deviates from the standard. The increase in such carbide and nitride forming elements causes excessive precipitation of TiC and NbC, so problems such as an increase in the yield ratio occur due to delayed recrystallization.

Claims

1. A steel plate, by weight %, the steel plate comprises: carbon (C): 0.05 - 0.12%, manganese (Mn): 1.0 - 1.8%, silicon (Si): less than 0.6% and excluding 0%, phosphorus (P): less than 0.03% and excluding 0%, sulfur (S): less than 0.01% and excluding 0%, nitrogen (N): less than 0.01% and excluding 0%, aluminum (sol.Al): 0.01 - 0.08%, titanium (Ti): 0.02 - 0.06%, niobium (Nb): 0.02 - 0.06%, boron (B): less than 0.005% and excluding 0%, the balance of Fe and inevitable impurities, By area %, the microstructure comprises 80 - 99% ferrite, and the remainder comprises pearlite and other inevitable structures. The non-recrystallized ferrite in the ferrite is 20 - 50%, The aspect ratio of the ferrite is 5 to 15.

2. The steel plate according to claim 1, wherein, The total amount of the Ti and the Nb is less than 0.1%.

3. The steel plate according to claim 1, wherein, The steel plate satisfies the following relation 1, [Relation 1] X(222) / [X(200)+X(110)+X(112)]≤2 The X-ray diffraction integrated intensity ratios of the {222} plane, {110} plane, {200} plane, and {112} plane parallel to the plane at the depth position of 1 / 4 of the plate thickness of the steel plate.

4. The steel plate according to claim 1, wherein, The surface of the steel plate further comprises a hot-dip galvanized layer.

5. The steel plate according to claim 1, wherein, The yield strength of the steel plate is 460 MPa or more, and the tensile strength is 520 MPa or more.

6. The steel plate according to claim 1, wherein, The product of the yield strength and the elongation rate of the steel plate is 8600 or more.

7. A method for manufacturing a steel plate, which comprises the following steps: Heating a steel billet to 1100 - 1250 °C. By weight %, the steel billet comprises: carbon (C): 0.05 - 0.12%, manganese (Mn): 1.0 - 1.8%, silicon (Si): less than 0.6% and excluding 0%, phosphorus (P): less than 0.03% and excluding 0%, sulfur (S): less than 0.01% and excluding 0%, nitrogen (N): less than 0.01% and excluding 0%, aluminum (sol.Al): 0.01 - 0.08%, titanium (Ti): 0.02 - 0.06%, niobium (Nb): 0.02 - 0.06%, boron (B): less than 0.005% and excluding 0%, the balance of Fe and inevitable impurities; Hot-rolling the heated steel billet at 880 °C or higher to obtain a hot-rolled steel plate; Cooling the hot-rolled steel plate to 500 - 600 °C and coiling it; Cold-rolling the coiled hot-rolled steel plate at a reduction ratio of 45 - 70%; And Continuously annealing the cold-rolled steel plate in the temperature range of 770 - 820 °C.

8. The method for manufacturing a steel plate according to claim 7, wherein, The manufacturing method satisfies the conditions of the following [Relation 2] and [Relation 3], [Relation 2] 2566 + 2.1 0.192 CR - 1.79 SS - 5.64 LS ≥ 520 [Relation 3] 2438 + 1.9 0.192 CR - 1.79 SS - 5.64 LS ≤ 700 Wherein, CR is the cold reduction ratio, in %, SS is the annealing temperature, in °C, and LS is the line speed during the continuous annealing operation, in m / min.

9. The manufacturing method of the steel plate according to claim 7, wherein, The manufacturing method further comprises the step of hot-dip galvanizing the continuously annealed steel plate.

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