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

Abstract

The present invention relates to a material used for automobile interior panels, reinforcing materials, etc., and relates to a high-yield-ratio high-strength steel sheet and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to a material used for inner panels and reinforcing materials of automobiles, and relates to a high-yield-ratio high-strength steel plate and a method for producing the same. [Background technology]

[0002] Recently, efforts to reduce the weight of automobile bodies have been progressing in order to improve fuel efficiency, and as a result, the use of high-strength steel sheets, which allow for a reduction in plate thickness, has been increasing in automobile parts. Furthermore, to ensure the safety of passengers, high-strength steel sheets are widely used in automobile bodies. To improve the collision performance of automobile bodies, efforts are being made to increase the yield strength of steel materials so that they can efficiently absorb collision energy even with a small amount of deformation. Therefore, steel sheets with a high yield ratio are required.

[0003] High-strength steel sheets for use in automobile bodies also require excellent formability. Dual-phase steel (DP steel), a composite structure with a ferrite primary phase and a hard secondary phase, is a typical example of a steel that combines both strength and formability. However, because DP steel has a soft ferrite primary phase and hard secondary phases such as bainite, martensite, and tempered martensite, it suffers from a low yield ratio. Therefore, there are limitations to the application of DP steel in automotive parts that absorb collision energy while suppressing deformation.

[0004] On the other hand, Patent Document 1 proposes a steel sheet that prevents ferrite recrystallization and has a structure consisting of unrecrystallized ferrite and a hard second phase. However, if excessive unrecrystallized ferrite is present, although the strength and yield ratio are increased, the elongation is low, resulting in a problem of insufficient formability.

[0005] To address this problem, Patent Documents 2 to 4 have proposed steel sheets that have a structure consisting of ferrite and pearlite and that achieve both high strength and improved stretch flangeability by refining crystal grains, precipitation strengthening, or reducing the amount of solute C in ferrite. However, all of the proposed steel materials had a tensile strength of 500 MPa or less, and it was difficult to achieve a strength exceeding 500 MPa.

[0006] On the other hand, Patent Documents 5 to 7 propose steel sheets that utilize unrecrystallized ferrite to improve stretch flangeability by using unrecrystallized ferrite with intermediate hardness between soft ferrite and a hard second phase. However, the steel sheets proposed in Patent Documents 5 and 6 have small amounts of Nb and Ti added, which means that the recrystallization suppression effect is small, and therefore require rapid heating during annealing. The steel sheet proposed in Patent Document 7 has small amounts of Nb and Ti added, and the heating rate during annealing is low at 10°C / s or less, which increases the time available for recrystallization, and therefore the effect of utilizing unrecrystallized ferrite is insufficient. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 1978-005018 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-138261 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-107099 [Patent Document 4] Japanese Patent Application Laid-Open No. 2001-152288 [Patent Document 5] Japanese Patent Application Laid-Open No. 2008-106351 [Patent Document 6] Japanese Patent Application Laid-Open No. 2008-106352 [Patent Document 7] Japanese Patent Application Laid-Open No. 2008-156680 Summary of the Invention [Problem to be solved by the invention]

[0008] One aspect of the present invention is to provide a steel sheet having a high yield ratio, high strength and excellent formability, and a method for producing the same.

[0009] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the general content of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0010] One aspect of the present invention is a steel sheet containing, by weight, 0.05 to 0.12% carbon (C), 1.0 to 1.8% manganese (Mn), 0.6% or less (excluding 0%) silicon (Si), 0.03% or less (excluding 0%) phosphorus (P), 0.01% or less (excluding 0%) sulfur (S), 0.01% or less (excluding 0%) nitrogen (N), 0.01 to 0.08% aluminum (sol.Al), 0.02 to 0.06% titanium (Ti), 0.02 to 0.06% niobium (Nb), 0.005% or less (excluding 0%) boron (B), with the balance being Fe and unavoidable impurities, The microstructure comprises, in area %, 80 to 99% ferrite, the remainder comprising pearlite and other unavoidable structures, and the ferrite contains 20 to 50% unrecrystallized ferrite, and the aspect ratio of the ferrite is 5 to 15. The total amount of Ti and Nb may be 0.1% or less. The cold-rolled steel sheet may satisfy the following relational expression 1. [Equation 1] X(222) / [X(200)+X(110)+X(112)]≦2 (Integrated intensity ratio of the {222}, {110}, {200}, and {112} X-ray diffraction planes parallel to the surface at a depth of 1 / 4 of the thickness of the cold-rolled steel sheet)

[0011] The steel sheet may further include a hot-dip galvanized layer on the surface.

[0012] The steel plate may have a yield strength of 460 MPa or more and a tensile strength of 520 MPa or more.

[0013] The steel plate may have a product of yield strength and elongation of 8600 or more.

[0014] Another aspect of the present invention is a method for producing a steel slab containing, by weight, 0.05 to 0.12% carbon (C), 1.0 to 1.8% manganese (Mn), 0.6% or less (excluding 0%) silicon (Si), 0.03% or less (excluding 0%) phosphorus (P), 0.01% or less (excluding 0%) sulfur (S), 0.01% or less (excluding 0%) nitrogen (N), 0.01 to 0.08% aluminum (sol. Al), 0.02 to 0.06% titanium (Ti), 0.02 to 0.06% niobium (Nb), 0.005% or less (excluding 0%) boron (B), the balance being Fe and unavoidable impurities, the steps of heating the steel slab to 1100 to 1250°C; hot rolling the heated steel slab at 880°C or higher to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to 500 to 600 ° C and coiling it; cold rolling the coiled hot-rolled steel sheet at a reduction rate of 45 to 70%; and continuously annealing the cold-rolled steel sheet in a temperature range of 770 to 820°C.

[0015] The method for manufacturing the steel sheet may satisfy the conditions of the following [Relational Formula 2] and [Relational Formula 3]. [Equation 2] 2566+2.1*0.192*CR-1.79*SS-5.64*LS≧520 [Equation 3] 2438+1.9*0.192*CR-1.79*SS-5.64*LS≦700 Here, CR is the cold reduction rate (%), SS is the annealing temperature (℃), and LS is the line speed during continuous annealing (mpm).

[0016] The method for manufacturing the steel sheet may further include a step of hot-dip galvanizing the continuously annealed steel sheet. [Effects of the Invention]

[0017] The steel sheet of the present invention has high strength and a high yield ratio, and therefore, when used as an interior panel material, a reinforcing material, etc., it has increased resistance to collision (crash resistance characteristics), which is advantageous in ensuring the safety of passengers. Furthermore, the present invention can provide a steel sheet with excellent formability.

[0018] The various advantageous and beneficial effects of the present invention are not limited to the above-mentioned contents, but will be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a photograph showing the microstructure of Inventive Steel 1 in the Examples. DETAILED DESCRIPTION OF THE INVENTION

[0020] The terminology used herein is for the purpose of describing the invention and is not intended to limit the invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the related definition clearly indicates otherwise.

[0021] The meaning of "comprises" as used in the specification is to specify features and does not exclude the presence or addition of other features.

[0022] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0023] It is not a common idea to leave some ferrite in an unrecrystallized state in the microstructure of cold-rolled steel sheets. Unrecrystallized ferrite is ferrite that has been elongated in the rolling direction by cold rolling, meaning that recrystallization is not complete and dislocations within the grains have been restored. If such an unrecrystallized ferrite structure is present in steel, the material properties of the cold-rolled steel sheet will vary greatly in the width and length directions, and even a slight difference in the unrecrystallized fraction can result in very uneven material properties. Therefore, it has been recognized that the best approach is to minimize the unrecrystallized structure as much as possible.

[0024] When a significant amount of titanium (Ti) or niobium (Nb) is added to steel containing 0.05 wt.% or more of carbon (C) to ensure strength, it becomes extremely difficult to obtain a fully recrystallized structure through the annealing process. Ti and Nb are known to form carbides such as TiC and NbC during the cooling process in the hot rolling stage and the temperature rise process in the annealing process, hindering recrystallization and suppressing grain growth.

[0025] Therefore, to obtain a recrystallized structure, either a very high annealing temperature is required, or a special process is required to suppress the precipitation of TiC, NbC, etc. Here, the special process involves extremely rapid quenching or rapid heating to insufficient time for TiC, NbC, etc. to precipitate. However, this is a process that cannot be achieved through normal operating processes and requires specialized equipment. Meanwhile, maintaining a high annealing temperature above approximately 900°C is also necessary to prevent recrystallization inhibition by TiC, NbC, etc. Such high-temperature annealing can cause problems such as coil meandering and increased manufacturing costs. Even if the annealing temperature is increased above 900°C, the formation of a recrystallized structure can soften the material, resulting in a decrease in yield strength and making it difficult to achieve the high yield ratio required in the present invention.

[0026] Therefore, the inventors conducted extensive research to produce a steel sheet with a high yield ratio, preferably a yield strength of 460 to 600 MPa, a tensile strength of 520 to 700 MPa, and a yield ratio of 0.8 to 0.9. As a result, they discovered a method that uses unrecrystallized ferrite to ensure the above-mentioned yield ratio while at the same time providing excellent formability with an elongation of 10% or more, and thus arrived at the present invention.

[0027] Hereinafter, one 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 specified in the present invention, the contents of the alloy composition are based on weight %.

[0028] The steel plate contains carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (0% excluded), phosphorus (P): 0.03% or less (0% excluded), sulfur (S): 0.01% or less (0% excluded), nitrogen (N): 0.01% or less (0% excluded), aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, and boron (B): 0.005% or less (0% excluded).

[0029] Carbon (C): 0.05~0.12% Carbon (C) is an element that contributes to increasing strength and forming pearlite, and is added in an appropriate amount to ensure the target strength. It is also an essential element for forming precipitates in the ferrite phase together with Ti, Nb, etc., to impart strength to the steel sheet. If the C content is less than 0.05%, it is difficult to ensure the strength required for the steel of the present invention. If the C content exceeds 0.12%, formability and weldability are deteriorated. Therefore, it is effective to set the C content to 0.05 to 0.12%.

[0030] Manganese (Mn): 1.0-1.8% 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 to form a single austenite phase. In other words, if the Mn content is low, the annealing temperature must be increased to promote the transformation, which makes it difficult to ensure the appropriate fraction of unrecrystallized ferrite required in the present invention. Furthermore, Mn, along with Si, contributes to solid-solution strengthening and is also effective in increasing strength. From this perspective, it is effective to have a Mn content of 1.0% or more. On the other hand, if the Mn content exceeds 1.8%, the hardenability increases, bainite and martensite are easily formed, and the yield ratio decreases. Therefore, it is effective not to exceed 1.8%.

[0031] Silicon (Si): 0.6% or less (0% excluded) Silicon (Si) is a deoxidizing element and a solid-solution strengthening element, which is effective in increasing strength. However, if the Si content exceeds 0.6%, Ac1 becomes too high, requiring a higher annealing temperature, which accelerates transformation and makes it difficult to maintain unrecrystallized ferrite. Therefore, it is effective to keep the Si content below 0.6%. Furthermore, excessive Si addition can cause problems such as reduced plating adhesion due to oxides during hot-dip galvanizing. However, 0% is excluded to take into account the amount that is unavoidably added during manufacturing.

[0032] Phosphorus (P): 0.03% or less (0% excluded) Phosphorus (P) is an impurity that segregates at grain boundaries, reducing the toughness of steel sheets and weldability. It also significantly slows down the alloying reaction during hot-dip galvanizing, reducing productivity, so it is effective to keep P content below 0.03%.

[0033] Sulfur (S): 0.01% or less (0% excluded) Sulfur (S) is an unavoidable impurity in steel, and it is preferable to keep its content as low as possible. Therefore, the S content in steel is set to 0% to take into account the possibility of unavoidable inclusion. In particular, because S in steel increases the likelihood of red shortness, it is effective to keep its content below 0.01%.

[0034] Nitrogen (N): 0.01% or less (0% excluded) Nitrogen (N) is an unavoidable impurity in steel, and it is important to keep the N content as low as possible. Therefore, the N content in steel is set at 0% (i.e., over 0%) to take into account cases where it is unavoidable. However, controlling the N content in steel to a very low level would result in a sharp increase in steel refining costs, so it is controlled to 0.01% or less, which is the range that is feasible under operating conditions.

[0035] Acid-soluble aluminum (sol.Al): 0.01-0.08% Acid-soluble aluminum (sol. Al) is an element added for grain refinement and deoxidation, and if the sol. Al content is less than 0.01%, it is not possible to produce aluminum-killed steel under normal stable conditions. On the other hand, if the sol. Al content exceeds 0.08%, it is advantageous for increasing strength due to the grain refinement effect, but it also causes problems such as excessive formation of inclusions during continuous steel casting operations, which increases the likelihood of surface defects in the plated steel sheet and leads to a sharp increase in production costs. Therefore, it is preferable to control the sol. Al content to 0.01 to 0.08%.

[0036] Titanium (Ti): 0.02-0.06% and Niobium (Nb): 0.02-0.06% The Ti and Nb elements inhibit the recrystallization of deformed ferrite generated by cold rolling during the annealing process, thereby promoting the retention of unrecrystallized ferrite. To achieve this effect, it is preferable to add at least 0.02% of each of Nb and Ti. However, excessive addition of Ti and Nb increases the amount of unrecrystallized ferrite due to the formation of carbides such as TiC and NbC, which may result in excessively high yield strength and yield ratio. The excessive addition of alloying elements may also increase manufacturing costs. Furthermore, it is more effective to control the total amount of Ti and Nb (Ti + Nb) to 0.1% or less.

[0037] Boron (B): 0.005% or less (0% excluded) Boron (B) is an element that improves hardenability, increases strength, and suppresses nucleation at grain boundaries. If the B content exceeds 0.005 wt%, not only will the effect be excessive, but it will also cause an increase in manufacturing costs. Therefore, it is preferable to control the B content to 0.005 wt% or less.

[0038] The remaining components include Fe and unavoidable impurities. The addition of other effective components other than the above composition is not excluded. In other words, the above unavoidable impurities may include all impurities that may be unintentionally mixed in during the normal manufacturing process of cold-rolled steel sheets (and plated steel sheets). Since the meaning of this is easily understood by those skilled in the art, no particular limitations are placed here.

[0039] Next, the microstructure of the steel sheet will be described in detail. The microstructure of the steel sheet contains 80 to 99% by area of ​​ferrite, with the remainder containing pearlite and other unavoidable structures. In this case, the unavoidable structures are not particularly limited, but may be cementite, carbides, etc. More specifically, the ferrite content may be 80 to 95%.

[0040] On the other hand, it is effective that the area % of the unrecrystallized ferrite in the ferrite is 20 to 50%. The area % of the unrecrystallized ferrite means the fraction relative to the entire microstructure.

[0041] The fraction of unrecrystallized ferrite can be determined by analyzing crystal orientation measurement data from electron backscattering diffraction (EBSD) images using the kernel average misorientation method (KAM). Because the KAM method can quantitatively represent the crystal orientation misorientation between adjacent pixels (measurement points), in the present invention, particles with an average crystal orientation misorientation between adjacent measurement points of within 1° are defined as unrecrystallized ferrite. To ensure sufficient yield strength and yield ratio, it is effective for the area percentage of unrecrystallized ferrite to be 20 to 50%. If the unrecrystallized ferrite is less than 20%, sufficient yield strength and yield ratio cannot be obtained. If it exceeds 50%, the high unrecrystallized structure results in excessive yield strength and yield ratio, and the aspect ratio of the crystal grains increases. Therefore, it is effective for the unrecrystallized ferrite to be 20 to 50%.

[0042] It is effective that the total ferrite fraction including the unrecrystallized ferrite is 80 to 99%. More specifically, it is even more effective that the total ferrite fraction is 80 to 95%.

[0043] On the other hand, the steel sheet of the present invention contains pearlite and unavoidable structures in addition to ferrite.

[0044] It is effective that the microstructure of the steel sheet, in particular the aspect ratio (A / R) of the ferrite crystal grains, is 5-15.

[0045] Here, the aspect ratio of the crystal grains was determined by etching the microstructure with 5% nital etching solution, observing it under a scanning electron microscope (SEM) at 500x magnification, and determining the long and short axis lengths of the crystal grains through image analysis processing using the Image Analyzer program. The aspect ratio was calculated by dividing the long axis length of the crystal grain by the short axis length of the ellipse. The average value of the aspect ratios of each ferrite calculated in this way was defined as the aspect ratio of the crystal grains.

[0046] If A / R exceeds 15, the elongated grains in the rolling direction are very large, which means that the cold-rolled structure is barely recovered or recrystallized. The formation of such excessively elongated grains results in an excessive increase in yield strength, exceeding the yield strength and yield ratio required in the present invention. However, if A / R is less than 5, it means that recrystallization has progressed significantly, which means that the steel is softened, resulting in insufficient yield strength and a yield ratio lower than the level required for the steel of the present invention.

[0047] It is effective that the texture of the steel sheet satisfies the condition of the following relational expression 1. [Equation 1] X(222) / [X(200)+X(110)+X(112)]≦2 (Integrated intensity ratio of the {222}, {110}, {200}, and {112} X-ray diffraction planes parallel to the surface at a depth of 1 / 4 of the plate thickness of cold-rolled steel plate)

[0048] Here, the X-ray diffraction integrated intensity ratio is a relative intensity when the X-ray diffraction integrated intensity of a non-oriented standard sample is used as a reference. X-ray diffraction can be performed using an X-ray diffractometer, such as an energy dispersive X-ray diffractometer, which is widely used in the technical field to which the present invention pertains. When the X-ray diffraction integrated intensity ratio calculated using the above Relational Formula 1 exceeds 2, it means that the fraction of the (222) texture, i.e., the recrystallized texture, increases. This means that the fraction of unrecrystallized ferrite in the steel is not formed within an appropriate range, making it impossible to ensure the aspect ratio.

[0049] A steel sheet satisfying the above alloy composition and microstructure may have a yield strength of 460 MPa or more, a tensile strength of 520 MPa or more, and a yield ratio of 0.8 to 0.9. At the same time, the steel sheet may have an elongation of 10% or more. More specifically, the yield strength of the steel sheet may be 460 to 600 MPa. The tensile strength of the steel sheet may be 520 to 700 MPa.

[0050] More specifically, the product of yield strength and elongation of the steel sheet may be 8500 or more. Even more specifically, the product of yield strength and elongation may be 8900 or more. Typically, the higher the yield strength, the lower the elongation. However, according to one embodiment of the present invention, since the product of yield strength and elongation is 8500 or more, it is possible to simultaneously improve the yield strength and elongation.

[0051] Meanwhile, the steel sheet of the present invention may include a coating layer for improving corrosion resistance. In the present invention, the coating layer is not particularly limited, and any coating type and coating method commonly used in the technical field to which the present invention pertains may be used. A preferred example is a hot-dip galvanized layer.

[0052] Next, one embodiment of a method for producing a steel sheet according to the present invention will be described in detail, although it does not necessarily mean that the steel sheet according to the present invention must be produced by the following production method.

[0053] To manufacture the steel sheet of the present invention, a steel slab satisfying the above-mentioned composition can be produced through the processes of reheating, hot rolling, coiling, cold rolling and annealing. Each process will be described in detail below.

[0054] Reheating: 1100~1250℃ It is effective to reheat a steel slab having the above-mentioned composition in a temperature range of 1100 to 1250°C. If the reheating temperature is less than 1100°C, slab inclusions may not be sufficiently remelted, which may cause material variations and surface defects after hot rolling. In contrast, if the slab reheating temperature exceeds 1250°C, excessive growth of austenite grains may occur, resulting in a decrease in strength.

[0055] Hot rolling: 880°C or higher The reheated steel slab is hot-rolled at a temperature of 880°C or higher to produce a hot-rolled steel sheet. If the hot-rolling temperature is lower than 880°C, ferrite transformation occurs during rolling, producing an elongated structure, which can lead to problems such as deterioration of anisotropy and reduced cold-rollability. Therefore, it is effective to perform the hot-rolling at a temperature of 880°C or higher.

[0056] Winding: 500 to 600°C It is effective to coil the hot-rolled steel sheet at a temperature in the range of 500 to 600°C. If the coiling temperature is lower than 500°C, the shape of the steel sheet may be poor, and a transformed structure such as acicular ferrite may be generated, which may result in an excessive increase in strength and a decrease in ductility of the steel sheet after annealing. However, if the coiling temperature exceeds 600°C, coarse ferrite grains may be formed, which may easily form coarse carbides and nitrides, resulting in a deterioration in the material properties of the steel. In addition, problems such as buckling may occur due to high-temperature coiling, and problems may arise in which cold rolling properties are deteriorated.

[0057] Cold rolling: cold reduction rate 45-70% The hot-rolled steel sheet after coiling and pickling is cold-rolled to produce a cold-rolled steel sheet. It is effective to perform the cold rolling at a reduction ratio (cold reduction ratio) of 45 to 70%. If the cold reduction ratio is less than 45%, the driving force for recrystallization is very low, resulting in the formation of excessive unrecrystallized ferrite, making it difficult to ensure the strength required by the present invention. On the other hand, if the cold reduction ratio exceeds 70%, the driving force for recrystallization is too high, facilitating ferrite recrystallization even at low annealing temperatures, making it difficult to produce a high-strength steel with a yield ratio of 0.8 to 0.9.

[0058] Continuous annealing: 770~820℃ It is effective to continuously anneal the cold-rolled steel sheet in the temperature range of 770 to 820°C. If the annealing temperature is less than 770°C, the fraction of unrecrystallized ferrite is excessively formed, resulting in high yield strength and poor ductility. On the other hand, if the annealing temperature exceeds 820°C, the fraction of unrecrystallized ferrite is too small, making it difficult to ensure the high yield ratio required in the present invention.

[0059] In the present invention, in order to ensure a certain amount of unrecrystallized structure, it is important to appropriately manage the operational factors that control the driving force for recrystallization. Therefore, it is effective to satisfy the conditions of the following [Relational Formula 2] and [Relational Formula 3]. [Equation 2] 2566+2.1*0.192*CR-1.79*SS-5.64*LS≧520 [Equation 3] 2438+1.9*0.192*CR-1.79*SS-5.64*LS≦700 Here, CR is the cold reduction (%), SS is the annealing temperature (°C), and LS is the line speed (mpm) during continuous annealing.

[0060] The above [Relationship 2] and [Relationship 3] are operational factors that control the driving force for recrystallization, and include the annealing temperature, cold reduction, and line speed during annealing. In the present invention, the effective line speed is 90 to 150 mpm. The conveying speed is controlled differently depending on the thickness of the steel sheet. That is, it is preferable to use a low line speed for thick steel sheets and a high line speed for thin steel sheets, and to control both the cold reduction and the annealing temperature in accordance with these conditions.

[0061] In the present invention, subsequent to the continuous annealing, plating can be further carried out.

[0062] The plating may be performed using a method commonly used in the technical field to which the present invention pertains, and the type and method of plating are not particularly limited. In the present invention, the hot-dip galvanizing conditions are not particularly limited, and hot-dip galvanizing may be performed under common conditions applicable in the technical field. Through hot-dip galvanizing, a steel sheet according to an embodiment of the present invention may include a hot-dip galvanized layer on its surface. As a preferred example, a hot-dip galvanized steel sheet is manufactured by immersing the steel sheet in a hot-dip galvanizing bath at 440 to 500°C. After the hot-dip galvanizing step, the steel sheet may be subjected to an alloying heat treatment, if necessary. As an embodiment, the hot-dip galvanized steel sheet may be subjected to an alloying heat treatment at a temperature in the range of 460 to 530°C, followed by cooling to room temperature. As a result of the alloying heat treatment, the steel sheet may include an alloying hot-dip galvanized layer on its surface.

[0063] After the hot dip galvanizing, temper rolling can be performed. Temper rolling can also be performed within the usual range of 0.1 to 1.0%. If the temper rolling elongation is less than 0.1%, it is difficult to control the sheet shape. On the other hand, if it exceeds 1.0%, there is a possibility that side effects such as deterioration of the material due to an excessive increase in dislocation density in the surface layer and the occurrence of sheet breakage due to limitations in equipment capacity may occur. [Example]

[0064] Examples of the present invention will now be described. It goes without saying that various modifications of the following examples are possible for those skilled in the art without departing from the scope of the present invention. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be defined by the claims below as well as equivalents thereof.

[0065] (Example) Steel slabs having the alloy compositions (units are weight %, the remainder being unavoidable impurities) shown in Table 1 below were hot rolled and coiled at a reheating temperature of 1200°C, a hot rolling finish temperature of 900°C, which is equal to or higher than the Ar3 temperature, and a coiling temperature of 560°C.

[0066] After the coiling operation, the steel sheets were pickled with hydrochloric acid, and then cold-rolled and annealed under the continuous annealing conditions shown in Table 2 to produce steel sheets.

[0067] [Table 1]

[0068] [Table 2]

[0069] In Table 2 above, CR is the cold rolling reduction (%), SS is the continuous annealing temperature (°C), and LS is the line speed (mpm). Meanwhile, Relational Formulas 2 and 3 are as follows: [Equation 2] 2566+2.1*0.192*CR-1.79*SS-5.64*LS≧520 [Equation 3] 2438+1.9*0.192*CR-1.79*SS-5.64*LS≦700

[0070] The steel sheets manufactured as described above were subjected to tensile tests in the rolling direction according to the DIN-L standard, and the yield strength (YP), tensile strength (TS), and elongation (El.) of the steel sheets were measured. The results are shown in Table 3. Furthermore, the microstructures measured using the aforementioned SEM (scanning electron microscope) and EBSD (electron backscatter diffraction) were used to measure the aspect ratio of ferrite grains and the fractions of unrecrystallized and recrystallized ferrite (area%). Meanwhile, the X-ray diffraction intensity values ​​of each texture component of the manufactured steel sheets were measured, and the X-ray diffraction intensity ratio was calculated using Equation 1. [Equation 1] X(222) / [X(200)+X(110)+X(112)]≦2 (These are the integrated intensity ratios of the {222}, {110}, {200}, and {112} X-ray diffraction planes parallel to the surface at a depth of 1 / 4 of the thickness of the cold-rolled steel sheet.)

[0071] [Table 3]

[0072] As can be seen from Table 3 above, Invention Steels 1 to 8, which satisfy the alloy composition and manufacturing conditions of the present invention, have yield strengths of 469 to 545 MPa, tensile strengths of 545 to 655 MPa, elongations of 18 to 21%, yield ratios (YR) of 0.82 to 0.86, and products of yield strength and elongation of 8900 or more, thereby satisfying the mechanical properties required for the steels of the present invention. Furthermore, the invention steels have grain aspect ratios of 5.9 to 10.2, which satisfy the grain aspect ratio requirement of 5 to 15 required for the present invention. Furthermore, the unrecrystallized ferrite fraction is 29 to 45%, which satisfies the requirement of 20 to 50% required for the present invention. The X-ray diffraction ratios of these steels are also 0.6 to 1.2, fully satisfying the requirements of the present invention.

[0073] Figure 1 shows an SEM photograph showing the microstructure of the annealed sheet of Inventive Steel 1. It was found that the microstructure contained unrecrystallized ferrite (symbol (1)) and recrystallized ferrite (symbol (2)), as well as some pearlite.

[0074] The cold reduction rates of Comparative Steels 1 and 3 were much lower than those recommended in the present invention. This caused insufficient ferrite recrystallization during annealing, resulting in very high yield strength and a yield ratio that exceeded the standard of the present invention. In addition, the insufficient recrystallization resulted in a very high aspect ratio of the grains.

[0075] In contrast to Comparative Steel 1, Comparative Steels 2 and 7 had a very high cold reduction of 80%. As the cold reduction increases, ferrite recrystallization readily occurs even at low annealing temperatures. This is because the strength decreases due to the increased recrystallization fraction, making it impossible to meet the yield strength and yield ratio requirements of the steels of the present invention. Furthermore, the grain aspect ratio and X-ray integrated intensity did not meet the standards of the present invention.

[0076] Comparative Steel 4 had very low Ti and Nb additions of 0.01% each. The lack of TiC and NbC precipitates promoted recrystallization, reducing the fraction of unrecrystallized ferrite after annealing. As a result, the yield strength, yield ratio, etc., fell outside the requirements of the present invention.

[0077] Comparative steels 5 and 8 were cases in which no Ti or Nb was added. Due to the lack of precipitates in the steel, recrystallization readily occurred during annealing, resulting in low yield strength of the steel sheet, and the grain aspect ratio and X-ray diffraction intensity ratio did not satisfy the requirements of the present invention.

[0078] Comparative steel 6 is a steel with the same chemical composition as comparative steel 5 but without the addition of Nb. The cold reduction rate was very low at 35% under conditions of insufficient precipitates, and the conditions of the present invention were not met even after high-temperature annealing at 860°C.

[0079] In Comparative Steel 9, the added elements all fulfilled the ranges of the steels of the present invention, but the annealing temperature was very high at 850°C. The increase in annealing temperature increased the ferrite recrystallization fraction, which resulted in low yield strength and yield ratio, and results such as the grain aspect ratio falling outside the conditions of the present invention.

[0080] Comparative Steel 10 has a Mn content outside the range of the present invention and is annealed at a very low temperature of 750° C. Such a low annealing temperature causes excessive insufficient ferrite recrystallization, which leads to excessive increases in yield strength and yield ratio, resulting in problems such as poor workability with elongation of 10% or less.

[0081] Comparative Steel 11 has a carbon content of 0.14%, which is outside the range of the composition of the steels of the present invention. The excessive carbon content increases the amount of carbides in the steel, which causes problems such as an increased yield ratio and a decrease in elongation. In addition, the addition of excessive carbon can cause deterioration of weldability.

[0082] Comparative steel 12 had a Ti content of 0.08%, which was outside the standards for the steels of the present invention, and the total amount of Ti and Nb was also outside the standards. Such an increase in carbonitride-forming elements caused excessive precipitation of TiC and NbC, which led to problems such as an increase in the yield ratio due to delayed recrystallization.

Claims

1. In weight percent, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (0% excluded), phosphorus (P): 0.03% or less (0% excluded), sulfur (S): 0.01% or less (0% excluded), nitrogen (N): 0.01% or less (0% excluded), aluminum (sol. Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, boron (B): 0.005% or less (0% excluded), the balance being Fe and unavoidable impurities, The microstructure contains, in area %, 80 to 99% ferrite, the remainder containing pearlite and other unavoidable structures, and the ferrite contains 20 to 50% unrecrystallized ferrite, The aspect ratio of the ferrite is 5 to 15.

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

3. The steel sheet according to claim 1, wherein the steel sheet satisfies the following relational expression 1: [Relationship 1] X(222) / [X(200)+X(110)+X(112)]≦2 (Ratio of integrated X-ray diffraction intensities of {222} plane, {110} plane, {200} plane, and {112} plane parallel to the surface at a depth position of ¼ of the thickness of the steel plate)

4. The steel sheet according to claim 1 , further comprising a hot-dip galvanized layer on a surface thereof.

5. The steel plate is 2. The steel plate according to claim 1, having a yield strength of 460 MPa or more and a tensile strength of 520 MPa or more.

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

7. heating a steel slab containing, by weight, 0.05-0.12% carbon (C), 1.0-1.8% manganese (Mn), 0.6% or less (0% excluded) silicon (Si), 0.03% or less phosphorus (P), 0.01% or less (0% excluded) sulfur (S), 0.01% or less (0% excluded) nitrogen (N), 0.01% or less (0% excluded) aluminum (sol. Al), 0.01-0.08% titanium (Ti), 0.02-0.06% niobium (Nb), 0.02-0.06% boron (B), and the balance Fe and unavoidable impurities to 1100-1250°C; hot rolling the heated steel slab at 880°C or higher to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to 500 to 600°C and coiling it; cold rolling the coiled hot-rolled steel sheet at a reduction ratio of 45 to 70%; continuous annealing the cold-rolled steel sheet at a temperature range of 770 to 820°C; A method for manufacturing a steel plate, comprising:

8. The method for producing a steel sheet according to claim 7 , wherein the method satisfies the following [Relational Expression 2] and [Relational Expression 3]. [Relationship 2] 2566+2.1*0.192*CR-1.79*SS-5.64*LS≧520 [Relationship 3] 2438+1.9*0.192*CR-1.79*SS-5.64*LS≦700 Here, CR is the cold reduction rate (%), SS is the annealing temperature (°C), and LS is the line speed during continuous annealing (mpm).

9. The method for manufacturing a steel sheet according to claim 7, further comprising the step of hot-dip galvanizing the continuously annealed steel sheet.

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