Steel sheet and method for manufacturing same

Abstract

The present invention relates to a material for automotive inner panels, reinforcements 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 for automotive inner panels, reinforcements, etc., and to a high yield ratio type high strength steel sheet and a method for manufacturing the same. Background Art

[0002] In recent years, in order to improve the fuel efficiency of automobiles, lightweighting 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, high strength steel sheets are widely used in automobile bodies to ensure the stability of passengers. In order to improve the collision performance of automobile bodies, attempts are made to effectively absorb collision energy even at low deformation amounts by increasing the yield strength of steel. For this purpose, steel sheets with a high yield ratio are required.

[0003] In order to apply high strength steel sheets to automobile bodies, excellent workability is also required. A representative steel that combines such strength and workability is dual phase steel (hereinafter simply 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 uses soft ferrite as the main phase and bainite, martensite, tempered martensite, etc., which are hard structures, as the second phase, there is a problem of low yield ratio. Therefore, DP steel has some limitations in applications for automobile components that suppress deformation while absorbing collision energy.

[0004] In addition, Patent Document 1 proposes a steel sheet that prevents recrystallization of ferrite and has a structure composed of unrecrystallized ferrite and a hard second phase. However, when there is too much unrecrystallized ferrite, the strength and yield ratio are increased, but there is a problem of insufficient formability due to low elongation.

[0005] In response to these problems, Patent Documents 2 to 4 propose steel sheets having a structure composed of ferrite and pearlite, and steel sheets that simultaneously achieve high strength and improvement of stretch flangeability, by means of grain refinement, precipitation strengthening, or reduction of the amount of dissolved C in ferrite. However, the tensile strength of the proposed steel materials is all below 500 MPa, and it is difficult to achieve high strength exceeding 500 MPa.

[0006] In addition, Patent Documents 5 to 7 propose a steel sheet that utilizes unrecrystallized ferrite and improves stretch flangeability by means of soft ferrite and unrecrystallized ferrite with an intermediate hardness having a hard second phase. However, these patents contain at least 5% or more of unrecrystallized ferrite. In addition to the deviation in the direction of the material, there is also a problem of non-uniformity of the material along the length of the coil during the manufacture of coil sheets.

[0007] That is, even during the cooling process of manufacturing coil plates, the steel material composed of recrystallized structure hardly changes, while for the steel material containing unrecrystallized structure, the unrecrystallized fraction changes according to the cooling conditions, and there is a problem of large deviation of the material quality with position.

[0008] (Patent Document 1) Japanese Patent Publication Gazette 1978 - 005018

[0009] (Patent Document 2) Japanese Patent Application Gazette 2007 - 138261

[0010] (Patent Document 3) Japanese Patent Publication Gazette 2007 - 107099

[0011] (Patent Document 4) Japanese Patent Publication Gazette 2001 - 152288

[0012] (Patent Document 5) Japanese Patent Publication Gazette 2008 - 106351

[0013] (Patent Document 6) Japanese Patent Publication Gazette 2008 - 106352

[0014] (Patent Document 7) Japanese Patent Publication Gazette 2008 - 156680 Summary of the Invention

[0015] (I) Technical Problem to be Solved

[0016] An object of one aspect of the present invention is to provide a high-strength steel plate and a manufacturing method thereof. Since the strength deviation of different lengths of the steel plate is small and the yield strength is high, the steel plate has excellent impact resistance.

[0017] The technical problem of the present invention is not limited to the above content. The additional technical problems of the present invention have been described in the overall content of the specification, and those of ordinary skill in the technical field to which the present invention pertains can easily understand the additional technical problems of the present invention from the content described in the specification.

[0018] (II) Technical Solution

[0019] One embodiment of the present invention relates to a steel plate which, by weight %, 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, and the material deviation according to the length of the steel plate is 60 MPa or less based on the yield strength.

[0020] By area %, the fine structure of the steel plate comprises 80 - 95% ferrite, and the remainder comprises pearlite and other inevitable structures, and the non-recrystallized ferrite in the ferrite may be 20 - 50%.

[0021] The aspect ratio of the ferrite may be 5 to 15.

[0022] The total amount of Ti and Nb may be 0.1% or less.

[0023] The steel plate may satisfy the following relational expression 1.

[0024] [Relational expression 1]

[0025] X(222) / [X(200)+X(110)+X(112)] ≤ 2

[0026] (Ratio of the X-ray diffraction integrated intensities of the {222}, {110}, {200}, and {112} planes parallel to the plane at a depth of 1 / 4 of the thickness of the cold-rolled steel plate)

[0027] The steel plate may further include a hot-dip galvanized layer on the surface.

[0028] Another embodiment of the present invention relates to 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 unavoidable impurities; hot-rolling the heated steel billet at a temperature above 880 °C 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 rate of 45 - 70%; and continuously annealing the cold-rolled steel plate in a temperature range of 770 - 820 °C, wherein, before coiling the hot-rolled steel plate, it includes: heating the front end portion of the hot-rolled steel plate to a range of coiling temperature (T) + 30 °C to T + 100 °C, and heating the rear end portion of the hot-rolled steel plate to T + 80 °C to T + 150 °C.

[0029] The manufacturing method can satisfy the conditions of the following [Relationship 2].

[0030] [Relationship 2]

[0031] K ≤ 10

[0032] (K = 621×[C] + 222×[Ti] + 1183×[Nb] - 0.694×X - 0.726×Y)

[0033] Wherein, when K < 0, it is treated as K = 0.

[0034] Wherein, [C], [Ti], [Nb] refer to the weight% addition amounts of C, Ti, Nb. In addition, X represents the temperature rise (°C) relative to the coiling temperature in the front end portion of the hot-rolled steel plate, and Y represents the temperature rise (°C) relative to the coiling temperature in the rear end portion of the hot-rolled steel plate.

[0035] The manufacturing method can satisfy the conditions of the following [Relationship 3] and [Relationship 4].

[0036] [Relationship 3]

[0037] 2566 + 2.1 0.192 CR - 1.79 SS - 5.64 LS ≥ 520

[0038] [Relationship 4]

[0039] 2438 + 1.9 0.192 CR - 1.79 SS - 5.64 LS ≤ 700

[0040] Among them, CR is the cold reduction ratio (%), SS is the annealing temperature (°C), and LS is the line speed (m / min (mpm)) during continuous annealing operation

[0041] It may further include the step of hot-dip galvanizing the continuously annealed steel sheet

[0042] (III) Beneficial effects

[0043] The steel sheet of the present invention has high strength and high yield ratio. Therefore, when used as an inner panel material, a reinforcement, etc., the collision resistance (anti-collision property) increases during a collision, which is beneficial to ensuring the stability of passengers. In addition, according to the present invention, a steel sheet with excellent formability can be provided

[0044] The multiple beneficial advantages and effects of the present invention are not limited to the above content and can be more easily understood during the description of specific embodiments of the present invention Description of the drawings

[0045] Figure 1 It is a schematic diagram showing the temperature change of the front end (inner coiled part) and the rear end (outer coiled part) of the hot-rolled steel sheet during coiling after hot rolling

[0046] Figure 2 It shows the difference in yield strength of different lengths of hot-rolled coils of Invention Steel 1 and Invention Steel 2 in the embodiments of the present invention

[0047] Figure 3 It shows the microstructure of different lengths of the coils of Invention Steel 1 and Comparative Steel 1 finally manufactured in the embodiments of the present invention Best mode

[0048] 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

[0049] The meaning of "comprising" used in the specification is for specifically explaining the composition and does not exclude the existence or addition of other compositions

[0050] Unless otherwise defined, all terms used in this specification, including technical and scientific terms, have the same meaning as commonly understood by those skilled in the art. Terms defined in a dictionary are interpreted to have a meaning consistent with the meaning in the relevant technical literature and the content currently disclosed.

[0051] It can be said that in the microstructure of cold-rolled steel sheets, the idea that a part of ferrite remains in an unrecrystallized state is not a common one. Unrecrystallized ferrite refers to ferrite elongated along the rolling direction by cold rolling, with incomplete recrystallization and dislocation recovery within the grains. If such unrecrystallized ferrite structure exists in the steel, the material of the cold-rolled steel sheet has a large deviation in the width direction and the length direction, and even a small difference in the unrecrystallized fraction will result in very uneven material. Therefore, it is considered that the best method is to minimize the unrecrystallized structure as much as possible.

[0052] 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 difficult to obtain a completely recrystallized structure through the annealing process. It is well 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.

[0053] Therefore, in order to obtain a recrystallized structure, it is necessary to control the annealing temperature at a very high level, or a special process for suppressing the precipitation of TiC, NbC, etc. is required. Among them, the special process means that by extremely fast rapid cooling or extremely fast heating, the time for precipitating TiC, NbC, etc. is insufficient. 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 higher level and prevent the inhibition of recrystallization by TiC, NbC, etc., the temperature needs to be maintained at nearly 900 °C or higher. Such high-temperature annealing may cause problems such as snaking of the coiled sheet and an increase in manufacturing cost. Even if the annealing temperature is raised above 900 °C, the high yield ratio required by the present invention cannot be ensured because the formation of the recrystallized structure will cause softening of the material, resulting in a decrease in yield strength, etc.

[0054] Therefore, the present inventors have conducted in-depth research on manufacturing steel sheets with a high yield ratio, preferably a yield strength of 460 - 600 MPa, a tensile strength of 520 - 700 MPa, and a yield ratio of 0.8 to 0.9. As a result, a method has been obtained that can ensure an excellent yield ratio while having excellent formability with an elongation of 10% or more by utilizing unrecrystallized ferrite.

[0055] However, at the same time, when manufacturing a steel plate containing fine precipitates such as TiC and NbC and containing unrecrystallized ferrite, it has been found that there is a problem that the material properties vary greatly depending on the fraction of unrecrystallized ferrite. This is due to the difference in the fine microstructure, and it has been recognized that this difference in microstructure is sensitively dependent on the process of coiling after hot rolling in the manufacturing process.

[0056] Specifically, as Figure 1 shown, during the process of coiling the hot-rolled steel plate after hot rolling, the front end portion (50 m from the front end of the steel plate) is located at the inner coiling position of the coiled sheet (coil) and becomes the inner coiled portion, and the rear end portion (100 mm from the rear end of the steel plate) is located at the outer coiling position of the coiled sheet and becomes the outer coiled portion. At this time, as Figure 1 shown, the cooling behaviors of the inner coiled portion and the outer coiled portion are different. That is, compared with the inner coiled portion, the outer coiled portion of the coiled sheet is exposed to the external air and has a faster cooling rate. Therefore, the low-temperature phase transformation occurs in a shorter time, resulting in a problem of relatively increased strength. To solve this problem, there is a method of reducing the coiling temperature to reduce the temperature deviation of the entire coiled sheet. However, even with this method, there is a temperature difference between the inner coiled portion and the outer coiled portion of the coiled sheet, which causes a material property deviation due to the difference in microstructure at different positions.

[0057] Therefore, the inventors of the present invention have conducted in-depth research on a method to fundamentally solve this problem and have finally completed the present invention.

[0058] Hereinafter, a specific embodiment of the steel plate of the present invention will be described in detail. First, the alloy composition of the steel plate will be described in detail. Hereinafter, unless otherwise clearly specified in the present invention, the content of the alloy composition is based on weight%.

[0059] The steel plate 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%).

[0060] Carbon (C): 0.05 - 0.12%

[0061] Carbon (C) is an element that contributes to the increase in strength and the formation of pearlite. An appropriate amount of carbon (C) is added to ensure the desired strength. In addition, carbon (C) forms precipitates with Ti, Nb, etc. in the ferrite phase and is an essential element for imparting strength to the steel sheet. When the content of carbon (C) is less than 0.05%, it is difficult to ensure the required strength in the steel of the present invention. When the content of carbon (C) exceeds 0.12%, the formability or weldability deteriorates. Therefore, it is effective when the content of C is 0.05 - 0.12%. The C content can be 0.050 - 0.120%.

[0062] Manganese (Mn): 1.0 - 1.8%

[0063] Manganese (Mn) is an element that lowers the Ac3 transformation temperature, and the Ac3 transformation temperature is the temperature at which the Ac1 and α - γ phase transformations are completed to become a single austenite phase. That is, when the amount of Mn is small, it is necessary to raise the annealing temperature to promote the phase transformation, which makes it difficult to ensure the appropriate fraction of unrecrystallized ferrite required in the present invention. In addition, Mn, together with Si, contributes to solid solution strengthening and is also effective in increasing strength. From this point of view, it is effective when the Mn content is 1.0% or more. On the other hand, when the Mn content exceeds 1.8%, the hardenability increases, bainite and martensite are easily formed, and the yield ratio decreases. Therefore, it is effective when the Mn content does not exceed 1.8%. The Mn content can be 1.00 - 1.80%.

[0064] Silicon (Si): 0.6% or less (except 0%)

[0065] Silicon (Si) is a deoxidizing element and a solid solution strengthening element and is effective in increasing strength. However, when the amount of Si exceeds 0.6%, the Ac1 becomes too high, so it is necessary to raise the annealing temperature, which makes it difficult to ensure unrecrystallized ferrite by promoting the phase transformation. Therefore, it is effective when the silicon (Si) content is controlled below 0.6%. 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 in manufacturing, 0% is excluded. The Si content can be 0.60% or less.

[0066] Phosphorus (P): 0.03% or less (except 0%)

[0067] Phosphorus (P) is an impurity that segregates at grain boundaries, thus causing a decrease in the toughness of the steel sheet or deterioration of weldability, etc. In addition, during hot - dip galvanizing, the alloying reaction is very slow, which reduces productivity. Therefore, it is effective when P is 0.03% or less.

[0068] Sulfur (S): 0.01% or less (except 0%)

[0069] Sulfur (S) is an impurity inevitably contained in steel, and it is preferable to control the S content at a low level as much as possible. Therefore, considering the inevitable inclusion of S in steel, 0% is excluded from the S content. In particular, S in steel has a relatively high possibility of causing red hot brittleness. Therefore, it is effective to control the S content below 0.01%.

[0070] Nitrogen (N): Below 0.01% (excluding 0%)

[0071] Nitrogen (N) is an impurity inevitably contained in steel, and it is important to control the N content at a low level as much as possible. Therefore, considering the inevitable inclusion of N in steel, 0% is excluded from the N content (i.e., more than 0%). 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 rapidly. Therefore, it is controlled below 0.01% within the range of operable conditions. The N can be below 0.010%.

[0072] Acid-soluble aluminum (sol.Al): 0.01 - 0.08%

[0073] Acid-soluble aluminum (sol.Al) is an element added for grain size refinement and deoxidation. When the acid-soluble aluminum content is less than 0.01%, aluminum-killed steel cannot be manufactured in a normal stable state. On the other hand, when the acid-soluble aluminum content exceeds 0.08%, due to the grain refinement effect, it is beneficial to increase strength. However, during the steelmaking continuous casting operation, excessive inclusions are formed, increasing the possibility of surface defects in the coated steel sheet. In addition, there is a problem of a sharp increase in manufacturing cost. Therefore, it is preferable to control the acid-soluble aluminum content at 0.01 - 0.08%. The acid-soluble aluminum content can be 0.010 - 0.080%.

[0074] Titanium (Ti): 0.02 - 0.06% and Niobium (Nb): 0.02 - 0.06%

[0075] The Ti and Nb are elements that promote the retention of unrecrystallized ferrite by suppressing the recrystallization of ferrite in the annealing process of deformation ferrite generated by cold rolling. To obtain this effect, it is preferable to add at least 0.02% or more of Nb and Ti respectively. More preferably, it is 0.020% or more. 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 be 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 of Ti and Nb (Ti + Nb) below 0.1%.

[0076] Boron (B): Below 0.005% (excluding 0%)

[0077] Boron (B) is an element that improves hardenability, increases strength, and inhibits nucleation at grain boundaries. When the content of B exceeds 0.005% by weight, not only is the effect excessive, but it also leads to an increase in manufacturing costs. Therefore, it is preferable to control the content of B below 0.005% by weight.

[0078] In addition, it contains the balance of Fe and inevitable impurities. In addition to the above composition, the addition of effective components is not excluded. That is, as long as the inevitable impurities are those that may be undesirably mixed in the manufacturing process of ordinary cold-rolled steel sheets (and plated steel sheets), they can all be included. As long as those skilled in the art can easily understand their meaning, no special limitation is made thereto.

[0079] Based on the yield strength, the material deviation according to the length of the steel sheet of the present invention can be 60 MPa or less. Specifically, the deviation of the yield strength of the head, middle, and tail parts along the length direction of the steel sheet can be 60 MPa or less. The steel sheet refers to the final steel sheet different from the hot-rolled steel sheet in the manufacturing process. The head and tail usually refer to within 30 m from the end of the steel sheet (coil), and the rest can be called the middle part.

[0080] For reference, the front end part (inner winding part during coiling) of the hot-rolled steel sheet becomes the tail part after cold rolling and annealing, and the rear end part (outer winding part during coiling) of the hot-rolled steel sheet becomes the head part after cold rolling and annealing. This is because the outer winding part of the hot-rolled steel sheet is first demolded and then cold rolled and annealed.

[0081] Next, the microstructure of the steel sheet will be described in detail. By area%, the microstructure of the steel sheet contains 80 - 95% ferrite, and the rest contains pearlite and other inevitable structures. At this time, the inevitable structure is not particularly limited, but can be cementite, carbide, etc.

[0082] In addition, it is effective when the non-recrystallized ferrite in the ferrite is 20 - 50% by area%. The area% of the non-recrystallized ferrite refers to the fraction relative to the entire microstructure.

[0083] Among them, the fraction of the non-recrystallized ferrite can be determined by interpreting the crystal orientation measurement data of electron backscatter diffraction (referred to as EBSD) using the Kernel Average Misorientation (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 within 1° from adjacent measurement points are defined as non-recrystallized ferrite. To ensure sufficient yield strength and yield ratio, it is effective when 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 content of non-recrystallized structure, the yield strength and yield ratio are too high, increasing the aspect ratio of the grains. Therefore, it is effective when the non-recrystallized ferrite is 20 - 50%.

[0084] It is effective when the total ferrite fraction including the non-recrystallized ferrite is 80 - 95%.

[0085] In addition, in addition to ferrite, the steel plate of the present invention contains pearlite and inevitable structures.

[0086] It is effective when the aspect ratio (A / R) of the fine structure of the steel plate, especially ferrite, is 5 to 15.

[0087] Among them, the aspect ratio of the grains is obtained by observing the fine structure etched with 5% nitric acid ethanol etching (Nital etching) solution under 500 times magnification with a scanning electron microscope (SEM) and performing image analysis processing using an Image Analyzer program to obtain the major axis length and minor axis length of the grains. The aspect ratio is obtained by the major axis length of the grains / minor axis length of the ellipse. The average value of the aspect ratios of each ferrite obtained by this technique is defined as the aspect ratio of the grains.

[0088] When A / R exceeds 15, it means that the elongated grains in the rolling direction are very large, which means that the cold-rolled structure has hardly recovered or recrystallized. The formation of such overly elongated grains leads to an excessive increase in yield strength, exceeding the yield strength and yield ratio required by the present invention. However, when A / R is less than 5, it means that recrystallization has occurred significantly, which means that due to the softening 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.

[0089] It is effective when the texture of the steel plate satisfies the conditions of the following relational expression 1.

[0090] [Relational expression 1]

[0091] X(222) / [X(200)+X(110)+X(112)] ≤ 2

[0092] (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 cold-rolled steel sheet)

[0093] 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 X-ray diffraction apparatus widely used in the technical field to which the present invention belongs, such as an energy dispersive type. 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.

[0094] In addition, the steel sheet of the present invention may include a plating layer to improve corrosion resistance. The plating layer described in the present invention is not particularly limited, and any plating type and plating method carried out in the technical field to which the present invention belongs can be used. As a preferred example, it may be a hot-dip galvanized layer.

[0095] Next, a specific 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.

[0096] In order to manufacture the steel sheet of the present invention, it can be manufactured through a process of reheating, hot rolling, coiling, cold rolling, and annealing a steel billet that satisfies the above composition. Hereinafter, each process will be described in detail.

[0097] Reheating: 1100 - 1250 °C

[0098] 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, slab inclusions and the like cannot be sufficiently redissolved, which may cause material deviation after hot rolling, surface defects, etc. In contrast, when the slab reheating temperature exceeds 1250 °C, problems such as a decrease in strength may occur due to excessive growth of austenite grains.

[0099] Hot rolling: 880 °C or higher

[0100] The reheated steel billet is hot rolled at a temperature of 880 °C or higher to manufacture a hot-rolled steel sheet. When the hot rolling temperature is lower than 880 °C, a ferrite phase transformation occurs during the rolling process, generating an elongated structure, so problems such as deteriorated anisotropy and reduced cold rollability may occur. Therefore, it is effective to carry out the hot rolling at 880 °C or higher.

[0101] Coiling: 500 - 600 °C

[0102] It is effective to coil the hot-rolled steel sheet within the temperature range of 500 - 600 °C. When the coiling temperature is lower than 500 °C, the shape of the steel sheet deteriorates, and transformation structures such as acicular ferrite are formed, which may lead to excessive increase in the strength of the steel sheet after annealing and reduction in ductility. However, when the coiling temperature exceeds 600 °C, coarse ferrite grains are formed, and coarse carbides and nitrides are easily formed, which may result in deterioration of the steel quality. In addition, due to high-temperature coiling, problems such as warping occur, resulting in poor cold rolling performance.

[0103] In the present invention, in order to minimize the material deviation of the manufactured steel sheet, before coiling the hot-rolled steel sheet obtained by the hot rolling, the front end portion (the portion 50 m away from the shearing of the hot-rolled steel sheet) of the hot-rolled steel sheet can be heated to the range of the coiling temperature (T) + 30 °C to T + 100 °C, and the rear end portion (the portion 100 m away from the rear end of the hot-rolled steel sheet) can be heated to T + 80 °C to T + 150 °C.

[0104] In addition, the manufacturing method can satisfy the following relational expression 2.

[0105] [Relational Expression 2]

[0106] K ≤ 10

[0107] (K = 621×[C] + 222×[Ti] + 1183×[Nb] - 0.694×X - 0.726×Y)

[0108] Wherein, when K < 0, it is processed as K = 0.

[0109] Wherein, [C], [Ti], and [Nb] refer to the weight % addition amounts of C, Ti, and Nb. In addition, X represents the temperature rise (°C) relative to the coiling temperature in the front end portion of the hot-rolled steel sheet, and Y represents the temperature rise (°C) relative to the coiling temperature in the rear end portion of the hot-rolled steel sheet.

[0110] As a result of analyzing the cooling rate and phase transformation behavior at each position after coiling the hot-rolled steel sheet, in order to avoid the formation of low-temperature phase transformations such as bainite at the cooling rates of the front end portion and the rear end portion, coiling should be performed under the above temperature conditions to fully complete the ferrite phase transformation while cooling. When the temperature rise is controlled below the above conditions, the time for sufficient formation of ferrite is insufficient, and low-temperature phase transformations such as bainite are generated, resulting in excessive increase in strength, which may lead to an increase in strength deviation. When the temperature rise is too high, the equipment burden may be too high.

[0111] High-strength steel containing a certain amount of unrecrystallized ferrite and having a high yield ratio has a very high possibility of material deviation of different lengths in the coiled sheet according to the added components and heat treatment conditions, which is related to the fraction of unrecrystallized ferrite in the steel. Therefore, in the present invention, in addition to the addition amounts of [C], [Ti], and [Nb] that affect the fraction of unrecrystallized ferrite, considering the heating of the front end and the rear end of the hot-rolled steel sheet (hot-rolled coiled sheet) in the hot-rolling coiling step that has the greatest influence on the material deviation of different lengths of the coiled sheet, the relational expression 2 can be satisfied.

[0112] When the above-mentioned conditions are satisfied, based on the yield strength, the material deviation in the different length directions of the steel sheet (coiled sheet) after final annealing is within 60 MPa and is very small, having a high yield ratio, and a steel sheet with excellent formability can be obtained.

[0113] Cold rolling: Cold reduction rate is 45 - 70%

[0114] The hot-rolled steel sheet after coiling and pickling is cold-rolled to manufacture a cold-rolled steel sheet. It is effective to carry out the cold reduction rate (cold reduction rate) at 45 - 70% during the cold rolling. When the cold reduction rate is less than 45%, the recrystallization driving force is very low, and too much 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 rate exceeds 70%, the recrystallization driving force is too high, and ferrite recrystallization is likely to occur even at a low annealing temperature, so it is difficult to manufacture high-strength steel with a yield ratio of 0.8 to 0.9.

[0115] Continuous annealing: 770 - 820 °C

[0116] It is effective to carry out continuous annealing of the cold-rolled steel sheet in the temperature range of 770 - 820 °C. When the annealing temperature is lower than 770 °C, the fraction of unrecrystallized ferrite formed 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, and it is difficult to ensure the high yield ratio required by the present invention.

[0117] In order to ensure a certain amount of unrecrystallized structure in the present invention, it is important to appropriately manage and control the operation factors that control the recrystallization driving force. Therefore, it is effective to satisfy the conditions of the following [Relational Expression 2] and [Relational Expression 3].

[0118] [Relational Expression 3]

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

[0120] [Relational Expression 4]

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

[0122] Wherein, CR is the cold reduction rate (%), SS is the annealing temperature (°C), and LS is the wire speed (m / min) during continuous annealing operation

[0123] The [Relationship 3] and [Relationship 4] are operation factors for controlling the recrystallization driving force, including the annealing temperature, cold reduction rate, and transfer speed (wire speed) during annealing. In the present invention, when the transfer speed is 90 - 150 m / min, it is effective. The transfer speed is controlled differently according to the thickness of the steel plate. That is, for thick materials, the transfer speed (wire speed) is lower, and for thin materials, high-speed operation is carried out. Preferably, the cold reduction rate and annealing temperature are controlled simultaneously according to such conditions

[0124] In the present invention, subsequently, plating can be further carried out after the continuous annealing

[0125] The plating can be carried out in a manner commonly carried out in the technical field to which the present invention belongs, and there are no particular limitations on the types and methods of plating. In the present invention, the hot-dip galvanizing conditions are not particularly limited, and hot-dip galvanizing can be carried out under the usual conditions applicable in the same technical field. By hot-dip galvanizing, the surface of the steel plate according to an embodiment of the present invention can include a hot-dip galvanized layer. As a preferred example, it is immersed in a molten zinc-based plating bath at 440 - 500 °C to manufacture a hot-dip galvanized steel sheet. In addition, if necessary, after the hot-dip galvanizing step, the steel plate can be subjected to alloying heat treatment. As an embodiment, the hot-dip galvanized steel plate can be cooled to room temperature after being subjected to alloying heat treatment in the temperature range of 460 - 530 °C. By alloying heat treatment, an alloyed hot-dip galvanized layer can also be included on the surface of the steel plate

[0126] Skin pass rolling can be carried out after the hot-dip galvanizing. Skin pass rolling can also be carried out within the range of 0.1 - 1.0% as a usual range. When the elongation rate of skin pass rolling is less than 0.1%, it is difficult to control the plate shape. On the other hand, when the elongation rate of 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 plate fracture may occur due to equipment capacity limitations Detailed implementation mode

[0127] Hereinafter, embodiments of the present invention will be described. Of course, those skilled in the art can make various modifications to the following embodiments without departing from the scope of the present invention. The following embodiments are intended to help understand the present invention, but the scope of rights of the present invention should not be limited to the following embodiments and should be determined by the claims and their equivalents.

[0128] (Example)

[0129] A steel billet having the alloy composition shown in Table 1 below (unit: wt%, the balance being inevitable impurities) is hot-rolled and coiled. Among them, the reheating temperature is 1200 °C, the hot finish rolling temperature is 900 °C which is above the Ar3 temperature, and the coiling temperature is 560 °C.

[0130] Before the coiling, the temperature rises of the front end portion (at a position 50 m from the front end) and the rear end portion (at a position 100 m from the rear end) of the hot-rolled steel sheet manufactured after hot rolling with respect to the coiling temperature (CT) are X and Y respectively, which are shown in Table 2.

[0131] After the coiling operation of the coiled sheet, pickling is performed using hydrochloric acid, and then cold rolling is performed under the condition of a cold rolling rate of 60%. The cold-rolled steel sheet is continuously annealed under the conditions shown in Table 2 below, and then cooled by furnace cooling. Then, in order to manufacture a hot-dip galvanized steel sheet, the cold-rolled steel sheet is immersed in a molten zinc plating bath maintained at a temperature of about 460 °C under normal conditions to perform hot-dip galvanizing. In addition, for the galvanized steel sheet after hot-dip galvanizing, as shown in Table 2, a skin pass rolling rate of 0.5% is given to manufacture the final hot-dip galvanized steel sheet.

[0132] [Table 1]

[0133]

[0134] [Table 2]

[0135]

[0136] In Table 2, CT is the coiling temperature after hot rolling, X is the temperature rise of the front end portion of the hot-rolled steel sheet, and Y is the temperature rise of the rear end portion. SS is the continuous annealing temperature (°C), LS is the transfer speed (linear speed, m / min). SPM El is the skin pass rolling rate.

[0137] In addition, K is derived from the following relational expression 2.

[0138] [Relational Expression 2]

[0139] K ≤ 10

[0140] (K = 621×[C] + 222×[Ti] + 1183×[Nb] - 0.694×X - 0.726×Y)

[0141] Among them, when K < 0, it is processed as K = 0,...

[0142] Among them, [C], [Ti], and [Nb] refer to the weight % addition amounts of C, Ti, and Nb. In addition, X represents the temperature rise (°C) relative to the coiling temperature at the front end of the hot-rolled steel sheet, and Y represents the temperature rise (°C) relative to the coiling temperature at the rear end of the hot-rolled steel sheet.

[0143] For the steel sheet manufactured as described above, a tensile test is carried out in the rolling direction using the DIN-L standard, and the yield strength (YP), tensile strength (TS), and elongation (El.) of the steel sheet are measured more than 5 times, and the average results are shown in Table 3 above. For the yield strength, in order to confirm the material deviation of different lengths of the coiled sheet, the yield strengths of the head, middle, and tail of the coiled sheet of the annealed steel sheet are measured, and the values of the middle of the coiled sheet are selected for the remaining tensile strength and elongation. The yield ratio (YS) is also calculated using the ratio of the yield strength to the tensile strength in the middle of the coiled sheet length. In addition, the aspect ratio is measured using the microstructure, and the microstructure is measured using the above-mentioned SEM and EBSD.

[0144] [Table 3]

[0145]

[0146] It can be seen from Table 3 above that in Invention Examples 1 to 8 that satisfy the conditions of the present invention, the yield strength based on the middle within the coiled sheet length is 485 - 523 MPa, the tensile strength is 585 - 655 MPa, the elongation is 17 - 21%, the yield ratio (YR) is 0.80 - 0.85, the material deviation of the head, middle, and tail within the coiled sheet length is 42 - 59 MPa, and the yield strength deviation of different lengths is within 60 MPa, meeting the mechanical and physical properties proposed in the steel of the present invention. In addition, for a steel material having excellent physical properties like the steel of the present invention, the aspect ratio is 6.2 to 12.1, fully meeting the condition that the aspect ratio proposed in the steel of the present invention is 5 to 15. In addition, regarding the K value of relational expression 2, the K value of the inventive steel is within 0 to 8.8, meeting the condition of K ≤ 10 proposed in the steel of the present invention.

[0147] Figure 2 Shows the yield strength differences of different lengths of the hot-rolled coiled sheets of Inventive Steel 1 and Inventive Steel 2. In the steel of the present invention, it is proposed that the front end during hot rolling coiling is operated under the condition of the target coiling temperature (T) + 50 °C, and the rear end is operated under the condition of the target coiling temperature (T) + 100 °C. As a result, the yield strength deviation of different lengths of the inventive steel is within 15 MPa and is very excellent.

[0148] Comparative Steel 1 to Comparative Steel 3 and Comparative Steel 9 are cases where the temperatures at the front and rear ends of the hot-rolled coil are relatively low during hot-rolling coiling. Therefore, the K value is very high, and the yield strength deviation of the annealed coil for different lengths is above 100 MPa and very high.

[0149] Figure 3 The microstructures of the steel sheet coils of Inventive Steel 1 and Comparative Steel 1 with different lengths are shown. It can be seen that during the hot-rolling coiling operation, the temperature at the front end is increased by 50 °C and the temperature at the rear end is increased by 100 °C. Inventive Steel 1, for which the K value of relational expression 2 meets the conditions proposed for the inventive steel, has almost no difference in the microstructure in the different length directions. In the case of Comparative Steel 1 where the temperature at the front end is increased by 10 °C and the temperature at the rear end is increased by 20 °C during the hot-rolling coiling operation, the unrecrystallized structure is much more in the head and tail of the annealed coil compared to the middle part, and an uneven tissue distribution for different coil lengths is shown.

[0150] Comparative Steel 4 is a case where the coiling temperature is as high as 700 °C. The composition and the temperature conditions at the front and rear ends during hot-rolling coiling meet the conditions proposed for the inventive steel. The material deviation for different coil lengths of the annealed sheet is relatively low, but due to the high coiling temperature, the yield strength is very low and cannot meet the physical properties of the high yield ratio steel proposed for the inventive steel.

[0151] Comparative Steel 5 to Comparative Steel 7 are cases where the added Ti and Nb contents are lower than the contents proposed for the inventive steel. In particular, Comparative Steel 7 is a case where the Ti and Nb contents are low and the temperatures at the front and rear ends of the coil during hot-rolling coiling are not increased. Due to the low Ti and Nb contents, the precipitates in the steel are insufficient, and recrystallization is not likely to occur easily during annealing. Therefore, the yield strength of the annealed sheet is low, and the aspect ratio does not meet the conditions of the inventive steel. In particular, the yield strength deviation of the annealed sheet of Comparative Steel 7 with improper temperature control during coiling is also very high.

[0152] Comparative Steel 8 is a case where, due to the composition and the temperature conditions during hot-rolling coiling meeting the K value proposed for the inventive steel, the material deviation for different lengths of the annealed sheet is relatively low, but the annealing temperature is 850 °C and very high. Due to the high-temperature annealing, the ferrite recrystallization fraction increases, and as a result, the yield strength and the yield ratio are relatively low, and the aspect ratio and other results deviate from the conditions of the inventive steel.

[0153] Comparative Steel 10 is a case where the annealing operation is carried out at a very low annealing temperature of 720 °C. Even if other conditions do not meet the standards of the inventive steel, the annealing temperature is very low, and there is insufficient time to ensure the ferrite recrystallization fraction required for the inventive steel. As a result, problems such as a deterioration in elongation occur due to an excessive increase in the yield strength and the yield ratio.

[0154] Comparative steel 11 has a carbon content of 0.14% and is outside the compositional range proposed for the steel of the present invention. Other conditions meet the standards of the steel of the present invention, but due to the excessive carbon content, carbides in the steel increase, resulting in problems such as an increase in the yield ratio and a deterioration in elongation. In addition, due to the excessive addition of carbon, the weldability deteriorates.

[0155] Comparative steel 12 has Ti and Nb contents of 0.07% and 0.08% respectively, exceeding the standards of the steel of the present invention. The increase in these carbide and nitride forming elements leads to the excessive precipitation of TiC and NbC, resulting in problems such as an increase in the yield ratio 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, Based on the yield strength, the material deviation according to the steel plate length is 60 MPa or less.

2. The steel plate according to claim 1, wherein, By area %, the microstructure of the steel plate comprises 80 - 95% ferrite, and the rest comprises pearlite and other inevitable structures, and the non-recrystallized ferrite in the ferrite is 20 - 50%.

3. The steel plate according to claim 2, wherein, The aspect ratio of the ferrite is 5 to 15.

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

5. The steel plate according to claim 1, wherein, The steel plate satisfies the following relational expression 1, [Relational expression 1] X(222) / [X(200)+X(110)+X(112)] ≤ 2 The X-ray diffraction integral 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 thickness of the cold-rolled steel plate.

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

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 above 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 with a reduction rate of 45 - 70%; And Continuously annealing the cold-rolled steel plate in the temperature range of 770 - 820 °C, wherein, before coiling the hot-rolled steel plate, the front end of the hot-rolled steel plate is heated to the range of coiling temperature (T) + 30 °C to T + 100 °C, and the rear end of the hot-rolled steel plate is heated to T + 80 °C to T + 150 °C.

8. The manufacturing method of the steel plate according to claim 7, wherein, The manufacturing method satisfies the conditions of the following [Relational expression 2], [Relational expression 2] K≤10 K = 621×[C] + 222×[Ti] + 1183×[Nb] - 0.694×X - 0.726×Y wherein, when K < 0, it is treated as K = 0, Wherein, [C], [Ti], and [Nb] refer to the weight % addition amounts of C, Ti, and Nb, X represents the temperature rise relative to the coiling temperature at the front end of the hot-rolled steel sheet, with the unit of °C, and Y represents the temperature rise relative to the coiling temperature at the rear end of the hot-rolled steel sheet, with the unit of °C.

9. The manufacturing method of the steel plate according to claim 7, wherein, The manufacturing method satisfies the conditions of the following [Relationship 3] and [Relationship 4], [Relationship 3] 2566 + 2.1 0.192 CR - 1.79 SS - 5.64 LS ≥ 520 [Relationship 4] 2438 + 1.9 0.192 CR - 1.79 SS - 5.64 LS ≤ 700 Wherein, CR is the cold reduction rate, with the unit of %, SS is the annealing temperature, with the unit of °C, and LS is the linear velocity during the continuous annealing operation, with the unit of m / min.

10. The manufacturing method of the steel plate according to claim 7, wherein, The manufacturing method further includes a step of hot-dip galvanizing the steel sheet that has undergone the continuous annealing.

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