Steel plate and its manufacturing method

Abstract

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

Description

[Technical Field]

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

[0002] In recent years, 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 collision energy can be efficiently absorbed 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 consisting of ferrite as the primary phase and hard microstructures as the secondary phase, is a typical example of a steel material that combines both strength and formability. However, DP steel has a low yield ratio because the primary phase is soft ferrite and the secondary phase is made up of hard microstructures such as bainite, martensite, or tempered martensite. Therefore, there are limitations to the applicability of DP steel to automotive parts that absorb collision energy while suppressing deformation.

[0004] On the other hand, Patent Document 1 proposes a steel sheet that has a structure that is composed of unrecrystallized ferrite and a hard second phase by preventing the recrystallization of ferrite. However, when excessive unrecrystallized ferrite is present, although the strength and yield ratio are high, there is a problem in that the formability is insufficient due to the low elongation rate.

[0005] To address such problems, Patent Documents 2 to 4 propose steel sheets having a structure composed of ferrite and pearlite by means of grain refinement, precipitation strengthening, or a reduction in the amount of solute C in ferrite, and steel sheets that aim to achieve both high strength and improved stretch flangeability. However, all of the proposed steel materials have a tensile strength of 500 MPa or less, and it has been difficult to achieve a strength exceeding 500 MPa.

[0006] Meanwhile, 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, these patents contain a minimum of 5% or more unrecrystallized ferrite, which causes directional variations in the material quality and, most importantly, uneven material quality along the length of the coil when manufacturing the coil.

[0007] That is, steel material composed of a recrystallized structure undergoes almost no change in its material properties even during the cooling process for coil manufacturing, whereas steel material containing a non-recrystallized structure has a problem in that the non-recrystallized fraction varies depending on the cooling conditions during cooling, resulting in significant variations in material properties depending on the position. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 53-005018 [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]

[0009] An embodiment of the present invention provides a high-strength steel plate having little variation in strength along the length of the steel plate, high yield strength, and excellent impact resistance, and a method for manufacturing the same.

[0010] 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 technical field 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]

[0011] One embodiment of the present invention is a composition containing, by weight, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, boron (B): 0.005% or less (excluding 0%), the balance being Fe and unavoidable impurities, The steel plate has a material variation of 60 MPa or less based on the yield strength over the length of the steel plate.

[0012] The microstructure of the steel sheet may contain, in area %, 80 to 95% ferrite, the remainder being pearlite and other unavoidable structures, and the ferrite may contain 20 to 50% unrecrystallized ferrite.

[0013] The aspect ratio of the ferrite may be 5 to 15.

[0014] The total amount of Ti and Nb may be 0.1% or less.

[0015] The steel sheet may satisfy the following relational expression 1.

[0016] [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)

[0017] The steel sheet may further include a hot-dip galvanized layer on the surface.

[0018] Another embodiment of the present invention is a method for producing a steel slab containing, by weight, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, boron (B): 0.005% or less (excluding 0%), 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 at a temperature range of 770 to 820°C. The method for producing a steel sheet includes heating a front end of the hot-rolled steel sheet to a range of a coiling temperature (T)+30°C to T+100°C and heating a rear end of the hot-rolled steel sheet to T+80°C to T+150°C before coiling the hot-rolled steel sheet.

[0019] The production method may satisfy the condition of the following [Relational Formula 2].

[0020] [Equation 2] K≦10 (K=621×[C]+222×[Ti]+1183×[Nb]-0.694×X-0.726×Y) However, if K<0, processing is performed with K=0.

[0021] Here, [C], [Ti], and [Nb] mean the amounts of C, Ti, and Nb added in wt.%, respectively. X means the temperature increase (°C) at the front end of the hot-rolled steel sheet relative to the coiling temperature, and Y means the temperature increase (°C) at the rear end of the hot-rolled steel sheet relative to the coiling temperature.

[0022] The production method may satisfy the conditions of the following [Relational Formula 3] and [Relational Formula 4].

[0023] [Equation 3] 2566+2.1×0.192×CR-1.79×SS-5.64×LS≧520

[0024] [Equation 4] 2438+1.9×0.192×CR-1.79×SS-5.64×LS≦700

[0025] Here, CR means cold reduction (%), SS means annealing temperature (°C), and LS means line speed (mpm) during continuous annealing.

[0026] The method may further include the step of hot-dip galvanizing the continuously annealed steel sheet. [Effects of the Invention]

[0027] 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.

[0028] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0029] [Figure 1] As an example, this is a graph showing temperature changes at the front end (inner winding portion) and the rear end (outer winding portion) of a hot-rolled steel sheet during coiling after hot rolling. [Figure 2] 1 shows the difference in yield strength between Inventive Steel 1 and Inventive Steel 2 according to the length of hot rolled coil among the examples of the present invention. [Figure 3] 1 shows the microstructures of the inventive steel 1 and the comparative steel 1 finally produced in the examples of the present invention, according to the length of the coil. DETAILED DESCRIPTION OF THE INVENTION

[0030] The terms used herein are for the purpose of describing the invention and are not intended to be limiting of the invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the relevant definition clearly dictates otherwise.

[0031] As used herein, "comprises" embodies features and does not exclude the presence or addition of other features.

[0032] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one 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.

[0033] It is not a common idea to leave some ferrite in the microstructure of cold-rolled steel sheets in an unrecrystallized state. Unrecrystallized ferrite refers to ferrite that has been elongated in the rolling direction by cold rolling, but in which recrystallization has not been completed and dislocations within the particles have been restored. If such an unrecrystallized ferrite structure is present in steel, the material properties of cold-rolled steel sheets will vary greatly in the width and length directions, and even a slight difference in the unrecrystallized fraction will result in very uneven material properties. Therefore, it is recognized that the best approach is to minimize the unrecrystallized structure as much as possible.

[0034] When a significant amount of titanium (Ti) or niobium (Nb) is added to steel that has been added with 0.05 wt% or more of carbon (C) to ensure strength, it becomes very difficult to obtain a fully recrystallized structure during 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, inhibiting recrystallization and suppressing grain growth.

[0035] Therefore, to obtain a recrystallized structure, a very high annealing temperature or a special process to suppress the precipitation of TiC, NbC, etc. is required. Here, the special process refers to extremely rapid quenching or rapid temperature increase to provide insufficient time for the precipitation of TiC, NbC, etc. However, this process cannot be achieved in normal operating processes and requires special equipment. On the other hand, to prevent the suppression of recrystallization by TiC, NbC, etc. by controlling the annealing temperature high, a temperature of approximately 900°C or higher must be maintained. Such high-temperature annealing can cause problems such as coil meandering and increased manufacturing costs. Furthermore, even if the annealing temperature is raised above 900°C, the formation of a recrystallized structure softens the material, resulting in a decrease in yield strength and making it impossible to achieve the high yield ratio required by the present invention.

[0036] Therefore, the present inventors have conducted extensive research to produce steel sheets 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 have succeeded in deriving a method for ensuring an excellent yield ratio by using unrecrystallized ferrite and also imparting excellent formability with an elongation of 10% or more.

[0037] However, at the same time, it was found that when manufacturing steel sheets containing fine precipitates such as TiC and NbC and unrecrystallized ferrite, there is a problem in that the material properties change significantly depending on the fraction of unrecrystallized ferrite. This is due to differences in the microstructure, and it has been recognized that such differences in structure are sensitively dependent on the coiling process after hot rolling in the manufacturing process.

[0038] Specifically, as shown in Figure 1, during the coiling process of a hot-rolled steel sheet after hot rolling, the front end (up to 50 mm from the front end of the steel sheet) is located on the inner winding of the coil and forms the inner winding section, while the rear end (up to 100 mm from the rear end of the steel sheet) is located on the outer winding of the coil and forms the outer winding section. Here, as shown in Figure 1, the inner winding section and the outer winding section have different cooling behaviors. That is, compared to the inner winding section, the outer winding section of the coil is exposed to external air and cools faster. This results in a problem of low-temperature transformation occurring in a shorter time, resulting in a relative increase in strength. To solve this problem, one method is to reduce the coiling temperature to reduce temperature unevenness throughout the coil. However, even with this method, temperature differences still exist between the inner winding section and the outer winding section of the coil, which causes material variations due to differences in the structure at different positions.

[0039] Therefore, the present inventors have conducted extensive research into a method for fundamentally solving such problems, and have arrived at the present invention.

[0040] 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, in the present invention, unless otherwise specified, the contents of the alloy composition are based on weight percent.

[0041] The steel plate contains carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, and boron (B): 0.005% or less (excluding 0%).

[0042] 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 provide 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, and if it exceeds 0.12%, formability and weldability deteriorate. Therefore, it is effective for the C content to be 0.05 to 0.12%. The C content may be 0.050 to 0.120%.

[0043] 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, together with Si, contributes to solid-solution strengthening and is also effective in increasing strength. From this perspective, a Mn content of 1.0% or more is effective. 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%. The Mn content may be 1.00 to 1.80%.

[0044] Silicon (Si): 0.6% or less (excluding 0%) Silicon (Si) is a deoxidizing and solid-solution strengthening element, and is effective in increasing strength. However, if the Si content exceeds 0.6%, Ac1 becomes too high, necessitating a higher annealing temperature, which accelerates transformation and makes it difficult to maintain unrecrystallized ferrite. Therefore, it is effective to control the Si content to 0.6% or less. Furthermore, excessive Si addition can cause problems such as reduced plating adhesion due to oxides during hot-dip galvanizing. However, 0% is excluded, taking into account the amount that is inevitably added during manufacturing. The Si content may be 0.60% or less.

[0045] Phosphorus (P): 0.03% or less (excluding 0%) Phosphorus (P) is an impurity that segregates at grain boundaries, causing a decrease in the toughness of steel sheets and a deterioration in weldability. It also significantly slows down the alloying reaction during hot-dip galvanizing, reducing productivity, so it is effective to limit P to 0.03% or less.

[0046] Sulfur (S): 0.01% or less (excluding 0%) Sulfur (S) is an impurity that is inevitably contained in steel, and it is preferable to control its content as low as possible. Therefore, the S content in steel does not include 0%, taking into account cases where it is unavoidably contained. In particular, since S in steel increases the possibility of causing red shortness, it is effective to control its content to 0.01% or less.

[0047] Nitrogen (N): 0.01% or less (excluding 0%) Nitrogen (N) is an impurity that is inevitably contained in steel, and it is important to control the N content as low as possible. Therefore, the N content in steel is set to a value other than 0% (i.e., greater than 0%), taking into account the case where it is inevitably contained. 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 within the range possible under operational conditions. The N content may be 0.010% or less.

[0048] Acid-soluble aluminum (sol.Al): 0.01-0.08% Acid-soluble aluminum (sol. Al) is an element added for grain refinement and deoxidation. If the sol. Al content is less than 0.01%, aluminum-killed steel cannot be produced under normal stable conditions. On the other hand, if the sol. Al content exceeds 0.08%, although it is advantageous for increasing strength due to the grain refinement effect, excessive inclusions are formed during continuous casting operations, increasing the likelihood of surface defects in the galvanized steel sheet. Furthermore, this can lead to a significant increase in production costs. Therefore, it is preferable to control the sol. Al content to 0.01 to 0.08%. The sol. Al content may be 0.010 to 0.080%.

[0049] Titanium (Ti): 0.02-0.06% and Niobium (Nb): 0.02-0.06% The Ti and Nb elements inhibit the recrystallization of ferrite during the annealing process of deformed ferrite generated by cold rolling, thereby promoting the retention of unrecrystallized ferrite. To achieve this effect, it is preferable to add at least 0.02% of Nb and Ti, respectively. 0.020% or more is more preferable. However, excessive addition of Ti and Ti increases the amount of unrecrystallized ferrite due to the formation of carbides such as TiC and NbC, excessively increasing the yield strength and yield ratio, and increasing manufacturing costs due to the excessive addition of alloying elements. Furthermore, it is more effective to control the total content of Ti and Nb (Ti + Nb) to 0.1% or less.

[0050] Boron (B): 0.005% or less (excluding 0%) 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.

[0051] The remainder includes Fe and unavoidable impurities. The addition of other effective components in addition to the above composition is not excluded. In other words, the unavoidable impurities include all impurities that are unintentionally mixed in during the normal manufacturing process of cold-rolled steel sheets (and plated steel sheets). Since the meaning of this term can be easily understood by anyone skilled in the art, it is not particularly limited here.

[0052] The steel plate of the present invention may have a longitudinal variation in material properties of 60 MPa or less based on yield strength. Specifically, the variation in yield strength at the head, middle, and tail portions along the length of the steel plate may be 60 MPa or less. The steel plate refers to the final steel plate, as opposed to a hot-rolled steel plate during the manufacturing process. The head and tail portions usually refer to the portion from the end of the steel plate (coil) up to 30 m, and the remaining portion can be referred to as the middle portion.

[0053] In the hot-rolled steel sheet, the front end (the inner winding portion when coiled) becomes the tail side after cold rolling and annealing, and the rear end (the outer winding portion when coiled) becomes the head side after cold rolling and annealing, because the outer winding portion of the hot-rolled steel sheet is unwound first and then cold rolled and annealed.

[0054] Next, the microstructure of the steel sheet will be described in detail. The microstructure of the steel sheet contains, in area %, 80 to 95% ferrite, and the remainder contains pearlite and other unavoidable structures. Here, the unavoidable structures include, but are not limited to, cementite, carbides, etc.

[0055] 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.

[0056] The fraction of unrecrystallized ferrite can be determined by interpreting crystal orientation measurement data from electron backscatter diffraction (EBSD) using the kernel average misorientation method (KAM). Because the KAM method can quantitatively indicate 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 area percentage of unrecrystallized ferrite is less than 20%, sufficient yield strength and yield ratio cannot be obtained. If the area percentage 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 area percentage of unrecrystallized ferrite to be 20 to 50%.

[0057] It is effective that the fraction of all ferrite including the unrecrystallized ferrite is 80 to 95%.

[0058] On the other hand, the steel sheet of the present invention contains pearlite and unavoidable structures in addition to ferrite.

[0059] It is effective that the microstructure of the steel sheet, particularly the aspect ratio (A / R) of the ferrite grains, is 5-15.

[0060] Here, the aspect ratio of the crystal grains was calculated by etching the microstructure with 5% nital etching solution, observing it under a scanning electron microscope (SEM) at 500x magnification, and then analyzing the image using an image analyzer program to determine the lengths of the major and minor axes of the crystal grains. The aspect ratio was calculated by dividing the length of the major axis of the crystal grain by the length of the minor axis of the ellipse. The average aspect ratio of each ferrite obtained using this method was defined as the aspect ratio of the crystal grains.

[0061] Here, an A / R of more than 15 means that the elongated grains are very large in the rolling direction, which means that the cold-rolled structure is barely recovered or recrystallized. The formation of such excessively elongated grains leads to an excessive increase in yield strength, exceeding the yield strength and yield ratio required in the present invention. On the other hand, an A / R of less than 5 means that a considerable amount of recrystallization has occurred, which means that the steel is softened, the desired yield strength is not achieved, and the yield ratio is lower than the level required for the steel of the present invention.

[0062] It is effective that the texture of the steel sheet satisfies the condition of the following relational expression 1.

[0063] [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)

[0064] 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. An X-ray diffraction integrated intensity ratio calculated using the above Relational Formula 1 exceeding 2 means that the fraction of the (222) texture, i.e., the recrystallized texture, increases, which 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.

[0065] Meanwhile, the steel sheet of the present invention may include a plating layer for improving corrosion resistance. In the present invention, the plating layer is not particularly limited, and it is sufficient if it is of the type and method of plating used in the technical field to which the present invention pertains. A preferred example is a hot-dip galvanized layer.

[0066] 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.

[0067] The steel sheet of the present invention can be produced by subjecting a steel slab having the above-mentioned composition to reheating, hot rolling, coiling, cold rolling and annealing. Each step will be described in detail below.

[0068] 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 and the like will not be sufficiently remelted, which may cause variations in material properties and surface defects after hot rolling. On the other hand, if the reheating temperature of the slab exceeds 1250°C, excessive growth of austenite grains may occur, resulting in a problem of reduced strength.

[0069] 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, resulting in the formation of an elongated structure, which can cause problems such as deterioration of anisotropy and deterioration of cold rolling ability. Therefore, it is effective to carry out the hot-rolling at a temperature of 880°C or higher.

[0070] Winding: 500-600℃ 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 becomes poor and a transformed structure such as acicular ferrite is generated, resulting in an excessive increase in strength and a decrease in ductility of the steel sheet after annealing. However, if the coiling temperature exceeds 600°C, coarse ferrite grains are formed, and coarse carbides and nitrides are likely to be formed, resulting in deterioration of the steel's properties. In addition, problems such as buckling due to high-temperature coiling occur, leading to problems such as deterioration of cold rolling properties.

[0071] In the present invention, in order to minimize the variation in the material properties of the produced steel sheet, before the hot-rolled steel sheet obtained by the hot rolling is coiled, the front end of the hot-rolled steel sheet (the portion from the front end of the hot-rolled steel sheet to 50 m) can be heated to a range of coiling temperature (T)+30°C to T+100°C, and the rear end of the hot-rolled steel sheet (the portion from the rear end of the hot-rolled steel sheet to 100 m) can be heated to T+80°C to T+150°C.

[0072] In addition, the manufacturing method may satisfy the following relational expression 2:

[0073] [Equation 2] K≦10 (K=621×[C]+222×[Ti]+1183×[Nb]-0.694×X-0.726×Y) However, if K<0, processing is performed with K=0.

[0074] Here, [C], [Ti], and [Nb] mean the amounts of C, Ti, and Nb added in wt.%, respectively. X means the temperature increase (°C) at the front end of the hot-rolled steel sheet relative to the coiling temperature, and Y means the temperature increase (°C) at the rear end of the hot-rolled steel sheet relative to the coiling temperature.

[0075] Analysis of the cooling rate and phase transformation behavior at each position after coiling of a hot-rolled steel sheet revealed that in order to prevent the formation of low-temperature transformation phases such as bainite at the cooling rates at the front and rear ends, the sheet must be coiled under the above temperature conditions to ensure sufficient completion of ferrite transformation through cooling. If the temperature rise is controlled below the above conditions, there will not be enough time for ferrite to be sufficiently formed, resulting in the formation of low-temperature transformation phases such as bainite, resulting in an excessive increase in strength and increased strength variation. If the temperature rise is too large, the burden on equipment will be excessively large.

[0076] High-strength steels with a high yield ratio and a predetermined amount of unrecrystallized ferrite are highly likely to have variations in material properties over the length of the coil depending on the added elements and heat treatment conditions, which are correlated with the fraction of unrecrystallized ferrite in the steel. Therefore, in the present invention, the amounts of added [C], [Ti], and [Nb], which affect the fraction of unrecrystallized ferrite, as well as the heating of the front and rear ends of the hot-rolled steel sheet (hot-rolled coil) during the hot-rolling coiling stage, which has the greatest impact on the variations in material properties over the length of the coil, are considered to satisfy Relational Formula 2.

[0077] If the above conditions are met, after final annealing, the variation in the material properties of the steel sheet (coil) along its length is extremely small, within 60 MPa based on the yield strength, and the steel sheet has a high yield ratio and excellent formability.

[0078] Cold rolling: cold reduction rate 45-70% After coiling and pickling, the hot-rolled steel sheet 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 small, resulting in the formation of excessive unrecrystallized ferrite, making it difficult to ensure the strength required in the present invention. On the other hand, if the cold reduction ratio exceeds 70%, the driving force for recrystallization becomes very large, facilitating the recrystallization of ferrite even at low annealing temperatures, making it difficult to produce a high-strength steel with a yield ratio of 0.8 to 0.9.

[0079] Continuous annealing: 770~820℃ It is effective to continuously anneal the cold-rolled steel sheet in a temperature range of 770 to 820°C. If the annealing temperature is less than 770°C, an excessive proportion of unrecrystallized ferrite is formed, resulting in high yield strength and poor ductility. On the other hand, if the annealing temperature exceeds 820°C, the proportion of unrecrystallized ferrite is too small, making it difficult to ensure the high yield ratio required in the present invention.

[0080] In the present invention, in order to ensure a predetermined 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].

[0081] [Equation 3] 2566+2.1×0.192×CR-1.79×SS-5.64×LS≧520

[0082] [Equation 4] 2438+1.9×0.192×CR-1.79×SS-5.64×LS≦700

[0083] Here, CR means cold reduction (%), SS means annealing temperature (°C), and LS means line speed (mpm) during continuous annealing.

[0084] The above [Relational Formula 3] and [Relational Formula 4] are operational factors that control the recrystallization driving force, and include the annealing temperature, cold reduction, and transfer speed (line speed) during annealing. In the present invention, it is effective that the transfer speed is 90 to 150 mpm. The transfer speed is controlled differently depending on the thickness of the steel sheet. That is, it is preferable to lower the transfer speed (line speed) for thick steel sheets and operate at a high speed for thin steel sheets, and to control both the cold reduction and the annealing temperature in accordance with these conditions.

[0085] In the present invention, subsequent to the continuous annealing, plating can be further carried out.

[0086] 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 same technical field. Through hot-dip galvanizing, the steel sheet according to one 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. Furthermore, if necessary, after the hot-dip galvanizing step, the steel sheet may be subjected to an alloying heat treatment. In one embodiment, the hot-dip galvanized steel sheet may be subjected to an alloying heat treatment in a temperature range of 460 to 530°C, followed by cooling to room temperature. Through the alloying heat treatment, the steel sheet may include an alloying hot-dip galvanized layer on its surface.

[0087] 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 rate is less than 0.1%, it is difficult to control the sheet shape. On the other hand, if it exceeds 1.0%, the material quality deteriorates due to an excessive increase in dislocation density in the surface layer, and side effects such as sheet breakage occur due to limitations in equipment capacity. [Example]

[0088] The following examples of the present invention will be described. It goes without saying that those skilled in the art can make various modifications to the following examples 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 appended claims and their equivalents.

[0089] (Example) A steel slab having the alloy composition (unit: weight %, the remainder being unavoidable impurities) shown in Table 1 below was 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.

[0090] The heating temperatures of the front end (position from the front end to 50 m) and the rear end (position from the rear end to 100 m) of the hot-rolled steel sheet produced after the hot rolling before the coiling were increased relative to the coiling temperature (CT) by X and Y, respectively, and are shown in Table 2.

[0091] The steel sheets were coiled into coils, pickled with hydrochloric acid, and then cold-rolled at a cold-rolling reduction of 60%. The cold-rolled steel sheets were continuously annealed under the conditions shown in Table 2 below, and then cooled by furnace cooling. To produce hot-dip galvanized steel sheets, the cold-rolled steel sheets were immersed in a hot-dip galvanizing bath maintained at a temperature of approximately 460°C, which is the usual condition, to perform hot-dip galvanizing. The galvanized steel sheets that had completed the hot-dip galvanizing were then subjected to a temper rolling reduction of 0.5% as shown in Table 2 below to produce final hot-dip galvanized steel sheets.

[0092] [Table 1]

[0093] [Table 2]

[0094] In Table 2 above, CT is the coiling temperature after hot rolling, X is the temperature rise at the front end of the hot-rolled steel sheet, and Y is the temperature rise at the rear end. SS is the continuous annealing temperature (°C), LS is the transfer speed (line speed, mpm), and SPM El is the temper rolling reduction.

[0095] On the other hand, K is derived from the following relational expression 2.

[0096] [Equation 2] K≦10 (K=621×[C]+222×[Ti]+1183×[Nb]-0.694×X-0.726×Y) However, if K<0, processing is performed with K=0.

[0097] Here, [C], [Ti], and [Nb] mean the amounts of C, Ti, and Nb added in wt.%, respectively. X means the temperature increase (°C) at the front end of the hot-rolled steel sheet relative to the coiling temperature, and Y means the temperature increase (°C) at the rear end of the hot-rolled steel sheet relative to the coiling temperature.

[0098] The steel sheets manufactured as described above were subjected to tensile tests in the rolling direction according to the DIN-L standard. The yield strength (YP), tensile strength (TS), and elongation (El.) of the steel sheets were measured five or more times, and the average results are shown in Table 3. To check for variations in the material properties of the coils along their length, the yield strength was measured at the head, middle, and tail of the annealed steel sheet coils. The tensile strength and elongation were measured at the middle of the coil. The yield ratio (YS) was also calculated using the ratio of the yield strength to the tensile strength at the middle of the coil. Meanwhile, the aspect ratio was measured using the microstructure measured using SEM and EBSD as described above.

[0099] [Table 3]

[0100] As shown in Table 3, Examples 1 to 8 satisfying the requirements of the present invention have yield strengths of 485 to 523 MPa, tensile strengths of 585 to 655 MPa, elongations of 17 to 21%, and yield ratios (YR) of 0.80 to 0.85, based on the middle section of the coil. The material variations in the head, middle, and tail sections of the coil are 42 to 59 MPa, and the variation in yield strength over length is within 60 MPa, satisfying the mechanical properties required for the steel of the present invention. Furthermore, steels with excellent physical properties, such as the steel of the present invention, have aspect ratios of 6.2 to 12.1, fully satisfying the requirement of an aspect ratio of 5 to 15 required for the steel of the present invention. Meanwhile, the K value of the steel of the present invention, as determined by Relation 2, is within the range of 0 to 8.8, satisfying the requirement of K≦10 required for the steel of the present invention.

[0101] Figure 2 shows the difference in yield strength according to the length of hot rolled coils of Inventive Steel 1 and Inventive Steel 2. When hot rolling and coiling the steel of the present invention, the front end was at a target coiling temperature (T) + 50°C and the rear end was at a target coiling temperature (T) + 100°C. As a result, the variation in yield strength according to the length of the inventive steel was extremely excellent, within 15 MPa.

[0102] For comparative steels 1 to 3 and 9, the temperatures at the front and rear ends of the hot-rolled coil were low during hot-rolling and coiling. As a result, the K value was very high and the variation in yield strength according to the length of the annealed coil was very high, at over 100 MPa.

[0103] Figure 3 shows the microstructures of steel sheets manufactured by the invention steel 1 and the comparative steel 1 along the length of the coil. Inventive steel 1, in which the temperature was raised by 50°C at the front end and 100°C at the rear end during hot rolling and the K value of Relation 2 satisfies the condition set forth for the invention steel, shows almost no difference in microstructure along the length. However, in the case of comparative steel 1, in which the temperature was raised by 10°C at the front end and 20°C at the rear end during hot rolling, the head and tail portions of the annealed coil have a much higher concentration of unrecrystallized structure than the middle portion, showing a non-uniform microstructure distribution along the length of the coil.

[0104] Comparative steel 4 was produced at a very high coiling temperature of 700°C. The chemical composition and the temperature conditions at the front and rear ends during hot rolling coiling met the requirements of the steel of the present invention, and the annealed sheet had low variation in the quality of the coils along their length. However, the high coiling temperature resulted in a very low yield strength, and the steel did not meet the physical properties of a high yield ratio steel as required by the steel of the present invention.

[0105] Comparative steels 5 to 7 contain Ti and Nb at lower contents than those used in the steels of the present invention. Comparative steel 7, in particular, contains low Ti and Nb, and the temperature at the front and rear ends of the coil is not increased during hot rolling and coiling. The low Ti and Nb contents lead to a lack of precipitates in the steel, which makes recrystallization more likely to occur during annealing. As a result, the yield strength of the annealed steel sheet is low, and the aspect ratio does not meet the requirements of the present invention. In particular, comparative steel 7, in which the temperature conditions during coiling were not appropriately controlled, also exhibited a very large variation in the yield strength of the annealed steel sheet.

[0106] Comparative Steel 8 has chemical compositions and temperature conditions during hot rolling and coiling that satisfy the K value specified for the steel of the present invention, and the annealed sheet has little variation in quality depending on the length, but the annealing temperature is very high at 850°C. The high-temperature annealing increases the ferrite recrystallization fraction, and the yield strength and yield ratio are low, so the results for the aspect ratio and other parameters deviate from the conditions for the steel of the present invention.

[0107] Comparative Steel 10 was annealed at a very low temperature of 720°C. Even if other conditions met the standards of the steel of the present invention, the annealing temperature was so low that the time required to secure the ferrite recrystallization fraction required for the steel of the present invention was insufficient, resulting in problems such as a deterioration in elongation due to an excessive increase in yield strength and yield ratio.

[0108] Comparative Steel 11 had a carbon content of 0.14%, which was outside the range of the composition of the steel of the present invention. Although other conditions met the standards of the steel of the present invention, the excessive carbon content increased the amount of carbides in the steel, which resulted in problems such as an increased yield ratio and a decrease in elongation. In addition, the addition of excessive carbon deteriorated weldability.

[0109] Comparative steel 12 had Ti and Nb contents of 0.07% and 0.08%, respectively, which exceeded the standards for the steel of the present invention. Such an increase in carbonitride-forming elements led to excessive precipitation of TiC and NbC, which in turn caused 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 (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol. Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, boron (B): 0.005% or less (excluding 0%), the balance being Fe and unavoidable impurities, A steel plate in which the variation in material properties over the length of the steel plate is 60 MPa or less based on the yield strength.

2. The steel sheet according to claim 1, wherein the microstructure of the steel sheet contains, in area %, 80 to 95% ferrite, the remainder containing pearlite and other unavoidable structures, and unrecrystallized ferrite accounts for 20 to 50% of the ferrite.

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

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

5. 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 plane at a depth position of ¼ of the thickness of the cold-rolled steel sheet)

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

7. heating a steel slab containing, by weight, 0.05-0.12% carbon (C), 1.0-1.8% 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), 0.01-0.08% aluminum (sol. Al), 0.02-0.06% titanium (Ti), 0.02-0.06% niobium (Nb), 0.005% or less (excluding 0%) boron (B), the balance being 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%; and continuously annealing the cold-rolled steel sheet at a temperature in the range of 770 to 820°C; before coiling the hot-rolled steel sheet, a front end of the hot-rolled steel sheet is heated to a range of a coiling temperature (T) + 30°C to T + 100°C, and a rear end of the hot-rolled steel sheet is heated to a range of T + 80°C to T + 150°C.

8. The method for producing a steel sheet according to claim 7 , wherein the method satisfies the condition of the following [Relational Expression 2]. [Relationship 2] K≦10 (K=621×[C]+222×[Ti]+1183×[Nb]-0.694×X-0.726×Y) However, if K<0, processing is performed with K=0. Here, [C], [Ti], and [Nb] mean the amounts of C, Ti, and Nb added in wt. %, respectively. X means the temperature increase (°C) at the front end of the hot-rolled steel sheet relative to the coiling temperature, and Y means the temperature increase (°C) at the rear end of the hot-rolled steel sheet relative to the coiling temperature.

9. The method for producing a steel sheet according to claim 7 , wherein the method satisfies the following conditions of [Relational Expression 3] and [Relational Expression 4]. [Relationship 3] 2566 + 2.1 x 0.192 x CR - 1.79 x SS - 5.64 x LS ≥ 520 [Relationship 4] 2438 + 1.9 x 0.192 x CR - 1.79 x SS - 5.64 x LS ≦ 700 Here, CR means cold reduction (%), SS means annealing temperature (°C), and LS means line speed (mpm) during continuous annealing.

10. 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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