Steel plate and its manufacturing method

Abstract

The present invention relates to a high-strength steel plate having excellent formability and a high yield ratio and a manufacturing method thereof, and more specifically, to a high-strength steel plate having excellent formability and a high yield ratio that can be used for various applications including automobile parts, and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel sheet having excellent formability and a high yield ratio, and a method for manufacturing the same. More specifically, it relates to a high-strength steel sheet having excellent formability and a high yield ratio that can be used for various applications, including automotive parts, and a method for manufacturing the same. [Background technology]

[0002] Recently, the automotive industry has been focusing heavily on ensuring passenger safety and improving vehicle fuel efficiency. For these reasons, the use of high-strength steel sheets in vehicle bodies is increasing to meet the demands for both safety and weight reduction.

[0003] To improve the crash performance of automobile bodies, increasing the yield strength of steel allows for efficient absorption of collision energy even with small deformations. Methods for increasing yield strength include using solid solution-strengthened steel and precipitation-strengthened steel.

[0004] Solid solution strengthened steel is a steel sheet in which solid solution strengthening elements (such as Mn, Si, and Cr) are dissolved in a ferrite phase, which has excellent formability, to increase its yield strength. However, Si or Cr are elements that tend to form oxides on the surface of the steel sheet in continuous annealing lines or continuous hot-dip galvanizing lines. In addition, Mn is an element that promotes the formation of low-temperature transformation phases (bainite or martensite) which have the characteristic of lowering the yield strength. Therefore, solid solution strengthened steel with large amounts of Mn, Si, and Cr added is not suitable as a method to increase the yield ratio of high-strength steel sheets with a tensile strength of 610 MPa or more.

[0005] On the other hand, precipitation-strengthened steel, which utilizes Nb, Ti, V, etc., is a steel sheet that improves yield strength by precipitating fine carbides within the ferrite. Because precipitation-strengthened steel increases the yield ratio without degrading workability, it is a suitable strengthening mechanism for high-strength steel sheets with a tensile strength of 610 MPa or more, exhibiting excellent impact performance and workability.

[0006] Patent documents 1 and 2 disclose methods for improving the formability and yield ratio of steel sheets by utilizing the introduction of unrecrystallized ferrite and the addition of Ti or Nb. Precipitation strengthening using Ti or Nb and the addition of unrecrystallized ferrite directly strengthen the ferrite, making them effective in increasing yield strength without significantly increasing tensile strength.

[0007] However, Patent Documents 1 and 2 have the drawback that, due to the inclusion of a large amount of unrecrystallized ferrite, it is difficult to simultaneously secure excellent strength, elongation, formability, and a high yield ratio. Patent Document 3 is a technology that uses unrecrystallized ferrite instead of the transformation hard phase (martensite, bainite, etc.) of existing DP (Dual Phase) steel, and thus has the drawback that, because it contains only a ferrite structure, it is difficult to simultaneously secure excellent strength, elongation, formability, and a high yield ratio. Patent Document 4 has the drawback that, because it contains Mn in the range of 0.15 to 0.45%, it is difficult to simultaneously secure excellent strength, elongation, formability, and a high yield ratio. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2009-114523 [Patent Document 2] Japanese Patent Publication No. 2017-002333 [Patent Document 3] Japanese Patent Publication No. 2017-002332 [Patent Document 4] Japanese Patent Publication No. 2015-147965 [Overview of the initiative] [Problems that the invention aims to solve]

[0009] One aspect of the present invention is to provide a high-strength steel sheet having excellent formability and a high yield ratio, as well as a method for manufacturing the same. [Means for solving the problem]

[0010] One embodiment of the present invention provides a high-strength steel sheet with excellent formability and a high yield ratio, comprising, by weight %, C: 0.05-0.25%, Si: 0.7% or less (excluding 0%), Mn: 0.46-1.8%, Al: 0.7% or less (excluding 0%), P: 0.05% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), the total amount of one or more of Ti, Nb, and V: 0.22% or less, the remainder being Fe and other unavoidable impurities, and a microstructure comprising, by area %, unrecrystallized ferrite: 1-13%, recrystallized ferrite: 67-98%, and cementite: 1-20%.

[0011] Another embodiment of the present invention involves heating a steel ingot or slab containing, by weight %, C: 0.05~0.25%, Si: 0.7% or less (excluding 0%), Mn: 0.46~1.8%, Al: 0.7% or less (excluding 0%), P: 0.05% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), the total amount of one or more of Ti, Nb, and V: 0.22% or less, with the remainder being Fe and other unavoidable impurities, to 1000~1350°C; and hot rolling the heated steel ingot or slab at a finishing rolling temperature of 800~1000°C. The present invention provides a method for manufacturing a high-strength steel sheet having excellent formability and a high yield ratio, comprising the steps of: obtaining a hot-rolled steel sheet; winding the hot-rolled steel sheet at 300-600°C; heat-treating the wound hot-rolled steel sheet at 650-800°C for 600-1700 seconds; cold-rolling the heat-treated hot-rolled steel sheet at a cold reduction ratio of 30-90% to obtain a cold-rolled steel sheet; reheating the cold-rolled steel sheet at 720-860°C and maintaining it for 50 seconds or more; primary cooling the reheated and maintained cold-rolled steel sheet to 600-760°C at an average cooling rate of 1°C / s or more; secondary cooling the primary-cooled cold-rolled steel sheet to 450-550°C at an average cooling rate of 2°C / s or more and maintaining it for 50 seconds or more; and tertiary cooling the secondary-cooled and maintained cold-rolled steel sheet to room temperature. [Effects of the Invention]

[0012] According to one aspect of the present invention, it is possible to provide a high-strength steel sheet having excellent formability and a high yield ratio, as well as a method for manufacturing the same. [Modes for carrying out the invention]

[0013] The following describes a high-strength steel sheet having excellent formability and a high yield ratio according to one embodiment of the present invention. First, the alloy composition will be described. The content of the alloy composition described below is in weight percent.

[0014] C: 0.05~0.25% Carbon (C) is an element that forms precipitates in the ferrite phase together with Ti, Nb, or V to impart strength to the steel sheet. If the C content is less than 0.05%, it is difficult to secure a tensile strength of 610 MPa or more. On the other hand, if the C content exceeds 0.25%, it is difficult to secure sufficient weld strength. Therefore, it is preferable that the C content be in the range of 0.05 to 0.25%. The lower limit of the C content is more preferably 0.06%, and even more preferably 0.07%. The upper limit of the C content is more preferably 0.24%, and even more preferably 0.23%.

[0015] Si: 0.7% or less (excluding 0%) Si is an element that improves strength through solid solution strengthening, strengthens ferrite, homogenizes the microstructure, and improves workability. It is also an element necessary for deoxidation during steelmaking. If the Si content exceeds 0.7%, it causes plating defects such as unplated areas in the plating process and reduces the weldability of the steel sheet. Therefore, it is preferable that the Si content be in the range of 0.7% or less. The lower limit of the Si content is more preferably 0.001%, and even more preferably 0.002%. The upper limit of the Si content is more preferably 0.69%, and even more preferably 0.68%.

[0016] Mn: 0.46~1.8% Mn is an element useful for enhancing both strength and ductility. When the content of Mn is less than 0.46%, it is difficult to fully obtain the above effects. When it exceeds 1.8%, the formation of low-temperature transformation phases such as martensite or bainite in austenite is promoted, and the yield ratio of the steel sheet decreases. Therefore, the content of Mn preferably has a range of 0.46 - 1.8%. The lower limit of the Mn content is more preferably 0.47%, and even more preferably 0.48%. The upper limit of the Mn content is more preferably 1.79%, and even more preferably 1.78%.

[0017] Al: 0.7% or less (excluding 0%) Al is an element that combines with oxygen in steel to have a deoxidizing effect. Also, like Si, it is an element that strengthens ferrite, homogenizes the microstructure, and improves workability. When the content of Al exceeds 0.7%, plating defects such as non-plating occur in the plating process, and the weldability of the steel sheet decreases. Therefore, the content of Si preferably has a range of 0.7% or less. The lower limit of the Al content is more preferably 0.001%, and even more preferably 0.002%. The upper limit of the Al content is more preferably 0.69%, and even more preferably 0.68%.

[0018] P: 0.05% or less (excluding 0%) P is an element contained as an impurity that deteriorates impact toughness. Therefore, it is preferable to control the content of P to 0.05% or less. The P content is more preferably 0.04% or less, and even more preferably 0.03% or less.

[0019] S: 0.03% or less (excluding 0%) S is an element contained as an impurity that generates MnS in the steel sheet and deteriorates ductility. Therefore, it is preferable to control the content of S to 0.03% or less. The S content is more preferably 0.02% or less, and even more preferably 0.01% or less.

[0020] N: 0.03% or less (excluding 0%) N is an element that is present as an impurity and generates nitrides during continuous casting, causing cracks in the slab. Therefore, it is preferable to control the N content to 0.03% or less. It is more preferable that the N content be 0.02% or less, and even more preferable that be 0.01% or less.

[0021] Total amount of one or more of Ti, Nb, and V: 0.22% or less Ti, Nb, and V are important elements that form precipitates in steel sheets. They may be included to improve the strength and impact toughness of the steel sheet. If the total amount of one or more of the above Ti, Nb, and V exceeds 0.22%, the unrecrystallized ferrite fraction will exceed 13% due to excessive precipitate formation, which may make it difficult to obtain the physical properties to be obtained in this invention, as well as increase manufacturing costs. Therefore, it is preferable that the total amount of one or more of the above Ti, Nb, and V be in the range of 0.22% or less. The lower limit of the total amount of one or more of the above Ti, Nb, and V is more preferably 0.03%, and even more preferably 0.05%. The upper limit of the total amount of one or more of the above Ti, Nb, and V is more preferably 0.21%, and even more preferably 0.20%.

[0022] In addition to the steel composition described above, the remainder may include Fe and unavoidable impurities. Unavoidable impurities are those that can be unintentionally introduced during the normal steel manufacturing process and cannot be completely eliminated; the implications of this are easily understood by engineers in the field of normal steel manufacturing. Furthermore, the present invention does not completely exclude the addition of compositions other than the steel composition described above.

[0023] On the other hand, the steel sheet of the present invention may further contain a total amount of one or more of Cr and Mo of 0.8% or less.

[0024] Cr and Mo are elements that suppress the decomposition of austenite during alloying treatment and stabilize austenite, similar to Mn. If the total amount of one or more of Cr and Mo exceeds 0.8%, the formation of low-temperature transformation phases such as martensite or bainite is promoted, and the yield ratio of the steel sheet decreases. Therefore, it is preferable that the total amount of one or more of Cr and Mo be in the range of 0.8% or less. The lower limit of the total amount of one or more of Cr and Mo is more preferably 0.0001%, and even more preferably 0.001%. The upper limit of the total amount of one or more of Cr and Mo is more preferably 0.7%, even more preferably 0.6%, and most preferably 0.53%.

[0025] Furthermore, the steel sheet of the present invention may further contain a total amount of one or more of Cu and Ni of 0.8% or less.

[0026] Cu and Ni are elements that stabilize austenite and suppress corrosion. Furthermore, Cu and Ni also have the effect of preventing hydrogen intrusion, which concentrates on the surface of the steel sheet and moves into the steel sheet, thereby suppressing hydrogen delayed breakdown. If the total amount of one or more of Cu and Ni exceeds 0.8%, it may not only be difficult to obtain the physical properties to be obtained in this invention, but it may also cause an increase in manufacturing costs. Therefore, it is preferable that the total amount of one or more of Cu and Ni be in the range of 0.8% or less. The lower limit of the total amount of one or more of Cu and Ni is more preferably 0.0001%, and even more preferably 0.001%. The upper limit of the total amount of one or more of Cu and Ni is more preferably 0.7%, even more preferably 0.6%, and most preferably 0.54%.

[0027] Furthermore, the steel sheet of the present invention may further contain B: 0.005% or less.

[0028] B is an element that improves hardenability, increases strength, and suppresses nucleation at grain boundaries. If the content of B exceeds 0.005%, it may be difficult to obtain the physical properties desired by the present invention, and it may also cause an increase in manufacturing costs. Therefore, it is preferable that the content of B be in the range of 0.005% or less. The lower limit of the B content is more preferably 0.0001%, and even more preferably 0.0003%. The upper limit of the B content is more preferably 0.0045%, and even more preferably 0.004%.

[0029] Furthermore, the steel sheet of the present invention may further contain a total amount of 0.05% or less of one or more of Ca, REM (excluding Y), and Mg.

[0030] REM elements, excluding Ca, Mg, and Y, are elements that improve the ductility of steel sheets by spheroidizing sulfides. If the total amount of one or more of the above Ca, REM (excluding Y), and Mg exceeds 0.05%, it may become difficult to obtain the physical properties desired by the present invention, and it may also cause an increase in manufacturing costs. Therefore, it is preferable that the total amount of one or more of the above Ca, REM (excluding Y), and Mg is in the range of 0.05% or less. The lower limit of the total amount of one or more of the above Ca, REM (excluding Y), and Mg is more preferably 0.0001%, and even more preferably 0.0003%. The upper limit of the total amount of one or more of the above Ca, REM (excluding Y), and Mg is more preferably 0.04%, even more preferably 0.03%, and most preferably 0.02%. On the other hand, REM refers to 17 elements including Sc, Y, and lanthanides.

[0031] Furthermore, the steel sheet of the present invention may further contain a total amount of one or more of W and Zr of 0.5% or less.

[0032] W and Zr are elements that improve hardenability and increase the strength of steel sheets. If the total amount of one or more of W and Zr exceeds 0.5%, it may become difficult to obtain the physical properties that the present invention aims to achieve, and it may also cause an increase in manufacturing costs. Therefore, it is preferable that the total amount of one or more of W and Zr is in the range of 0.5% or less. The lower limit of the total amount of one or more of W and Zr is more preferably 0.0001%, even more preferably 0.001%, and most preferably 0.01%. The upper limit of the total amount of one or more of W and Zr is more preferably 0.4%, even more preferably 0.35%, and most preferably 0.3%.

[0033] Furthermore, the steel sheet of the present invention may further contain a total amount of one or more of Sb and Sn of 0.5% or less.

[0034] Sb and Sn are elements that improve the wettability and plating adhesion of steel sheets. If the total amount of one or more of the above Sb and Sn exceeds 0.5%, the brittleness of the steel sheet increases, and cracks may occur during hot working or cold working. Therefore, it is preferable that the total amount of one or more of the above Sb and Sn is in the range of 0.5% or less. The lower limit of the total amount of one or more of the above Sb and Sn is more preferably 0.0001%, even more preferably 0.001%, and most preferably 0.005%. The upper limit of the total amount of one or more of the above Sb and Sn is more preferably 0.4%, even more preferably 0.3%, and most preferably 0.2%.

[0035] Furthermore, the steel sheet of the present invention may further contain a total amount of one or more of Y and Hf: 0.2% or less.

[0036] Y and Hf are elements that improve the corrosion resistance of steel sheets. If the total amount of one or more of Y and Hf exceeds 0.2%, the ductility of the steel sheet may deteriorate. Therefore, it is preferable that the total amount of one or more of Y and Hf is within the range of 0.2% or less. The lower limit of the total amount of one or more of Y and Hf is more preferably 0.0001%, even more preferably 0.001%, and most preferably 0.005%. The upper limit of the total amount of one or more of Y and Hf is more preferably 0.15%, even more preferably 0.12%, and most preferably 0.1%.

[0037] The following describes the microstructure. The fractions of the microstructure described below represent area percentages.

[0038] Unrecrystallized ferrite: 1-13% Generally, unrecrystallized ferrite contains a large potential and exhibits low ductility and hole expansion properties. However, the inventors have confirmed that when the fraction of unrecrystallized ferrite is 1 to 13%, a high yield ratio can be secured without degrading the elongation and hole expansion properties. When the fraction of unrecrystallized ferrite is less than 1% or greater than 13%, the yield ratio, elongation, or hole expansion properties decrease. On the other hand, the above unrecrystallized ferrite can be defined as ferrite that has been processed in the cold rolling process and has not been transformed into austenite during annealing, but has been cooled and formed. The above unrecrystallized ferrite has a form that is stretched in the cold rolling direction.

[0039] Recrystallized ferrite: 67-98% Recrystallized ferrite can be defined as ferrite formed when ferrite processed in the cold rolling process is transformed into austenite during annealing, and then further transformed during cooling. It exhibits effects such as improved ductility and hole-expandability of steel sheets. If the fraction of the above-mentioned recrystallized ferrite is less than 67% or more than 98%, the yield ratio, elongation, or hole-expandability decreases. The above-mentioned recrystallized ferrite is ordinary polygonal ferrite.

[0040] Cementite: 1 to 20% Cementite exhibits the effect of increasing the strength and hardness of the steel plate. When the fraction of the cementite is less than 1%, it may be difficult to ensure the strength. On the other hand, when it exceeds 20%, the precipitation of Ti, Nb, or V carbides is suppressed, and the ferrite fraction that the present invention aims to obtain cannot be ensured, and it may be difficult to ensure the mechanical properties.

[0041] As described above, the steel plate of the present invention provided has a tensile strength (TS): 610 MPa or more, a yield ratio (YR): 0.8 to 0.95, and a tensile strength (TS)

[0044] , ×√elongation rate (EL) of 1.8×10 6 ~2.3×10 6 MPa 2 % 0.5 and a tensile strength (TS) 2 ×√hole expansion property (HER): 2.5×10 6 ~3.8×10 6 MPa 2 % 0.5 and has excellent strength, formability, and a high yield ratio.

[0042] On the other hand, the steel plate of the present invention may be a cold-rolled steel plate or a plated steel plate, and the plated steel plate may be hot-dip galvanized, electro-galvanized, or hot-dip aluminum plated, etc.

[0043] Hereinafter, a method for manufacturing a high-strength steel plate having excellent formability and a high yield ratio according to an embodiment of the present invention will be described.

[0044] First, a steel ingot or slab satisfying the above alloy composition is heated at 1000 to 1350°C. When the heating temperature is less than 1000°C, there is a risk of hot rolling in a state outside the finish rolling temperature range. On the other hand, when it exceeds 1350°C, there is a risk of reaching the melting point of the steel and melting. The lower limit of the heating temperature of the steel ingot or slab is more preferably 1025°C, and even more preferably 1050°C. The upper limit of the heating temperature of the steel ingot or slab is more preferably 1325°C, and even more preferably 1300°C.

[0045] Subsequently, the heated steel ingot or slab is hot-rolled at a finishing rolling temperature of 800 to 1000°C to obtain a hot-rolled steel sheet. If the finishing rolling temperature is less than 800°C, the high strength of the steel may place a heavy burden on the hot-rolling mill. On the other hand, if it exceeds 1000°C, the crystal grains of the steel sheet after hot-rolling may become coarse, potentially reducing its mechanical properties. The lower limit of the finishing rolling temperature is more preferably 815°C, and even more preferably 830°C. The upper limit of the finishing rolling temperature is more preferably 985°C, and even more preferably 970°C.

[0046] On the other hand, after the finish rolling described above, the hot-rolled steel sheet can be cooled to the coiling temperature described below at an average cooling rate of 10°C / s or more. The above cooling is for the purpose of refining the crystal grains, and if the average cooling rate is less than 10°C / s, it may be difficult to obtain the above crystal grain refinement effect sufficiently. Since the faster the average cooling rate, the more advantageous it is, the present invention does not particularly limit the upper limit of the average cooling rate, however, considering the limitations of the equipment, it is difficult to exceed 500°C / s.

[0047] Subsequently, the hot-rolled steel sheet is wound at 300 to 600°C. If the winding temperature is below 300°C, the main phase of the hot-rolled steel sheet may be composed of a high-strength low-temperature transformation phase, making winding difficult. On the other hand, if the temperature exceeds 600°C, the scale formed on the surface of the hot-rolled steel sheet may penetrate deeply into the interior of the sheet, potentially making pickling difficult. The lower limit of the winding temperature is more preferably 315°C, and even more preferably 330°C. The upper limit of the winding temperature is more preferably 585°C, and even more preferably 570°C.

[0048] Subsequently, the wound hot-rolled steel sheet is heat-treated at 650-800°C for 600-1700 seconds. This heat treatment is intended to promote precipitate formation in the hot-rolled steel sheet and improve the yield ratio of the final product. If the heat treatment temperature is less than 650°C or the heat treatment time is less than 600 seconds, it may not be easy to optimize the precipitates in the annealed hot-rolled steel sheet. On the other hand, if the heat treatment temperature exceeds 800°C or the heat treatment time exceeds 1700 seconds, it may not be easy to form precipitates in the annealed hot-rolled steel sheet. The lower limit of the heat treatment temperature is more preferably 660°C, and even more preferably 670°C. The upper limit of the heat treatment temperature is more preferably 790°C, and even more preferably 780°C. The lower limit of the heat treatment time is more preferably 700 seconds, and even more preferably 800 seconds. The upper limit of the heat treatment time is more preferably 1600 seconds, and even more preferably 1500 seconds.

[0049] On the other hand, after the heat treatment described above, a pickling process can be further carried out to remove scale formed on the surface of the steel sheet. However, the present invention does not particularly limit the pickling process, and any pickling process used in the art can be applied.

[0050] Subsequently, the heat-treated hot-rolled steel sheet is cold-rolled at a cold reduction ratio of 30-90% to obtain a cold-rolled steel sheet. If the cold reduction ratio is less than 30%, it is difficult to secure an appropriate shape for the cold-rolled steel sheet, and if it exceeds 90%, the high strength of the steel sheet may make it difficult to perform the cold rolling in a short time. The lower limit of the cold reduction ratio is more preferably 31%, and even more preferably 32%. The upper limit of the cold reduction ratio is more preferably 89%, and even more preferably 88%.

[0051] Subsequently, the cold-rolled steel sheet is reheated to 720-860°C and maintained for 50 seconds or more. If the reheating temperature is below 720°C, the unrecrystallized ferrite fraction will exceed 13%, making it difficult to obtain the mechanical properties desired by the present invention. If the reheating temperature exceeds 860°C, the unrecrystallized ferrite fraction will not be formed at a rate of 1% or more, making it difficult to obtain the mechanical properties desired by the present invention. If the maintenance time is less than 50 seconds, the heat treatment time will be insufficient, making it difficult to obtain the mechanical properties desired by the present invention. The lower limit of the reheating temperature is more preferably 730°C, and even more preferably 740°C. The upper limit of the reheating temperature is more preferably 850°C, and even more preferably 840°C. The maintenance time is more preferably 55 seconds or more, and even more preferably 60 seconds or more. On the other hand, in the present invention, the longer the maintenance time, the more advantageous it is, so there is no particular limit on its upper limit. However, from the standpoint of productivity, the maintenance time can be 600 seconds or less. Furthermore, the present invention does not particularly limit the average heating rate during reheating, and can be, for example, 1 to 100°C / s.

[0052] Subsequently, the reheated and maintained cold-rolled steel sheet is subjected to primary cooling to 600-760°C at an average cooling rate of 1°C / s or higher. If the primary cooling stop temperature is below 600°C, the cementite fraction will exceed 20%, making it difficult to obtain the mechanical properties desired by the present invention. If the primary cooling stop temperature exceeds 760°C, the cooling stop temperature is too high, making it difficult to obtain the mechanical properties desired by the present invention. The lower limit of the primary cooling stop temperature is more preferably 610°C, and even more preferably 620°C. The upper limit of the primary cooling stop temperature is more preferably 750°C, and even more preferably 740°C. The primary average cooling rate is more preferably 1.5°C / s or higher. On the other hand, the present invention does not particularly limit the upper limit of the primary average cooling rate.

[0053] Subsequently, the cold-rolled steel sheet that has been first cooled is secondarily cooled to 450-550°C at an average cooling rate of 2°C / s or more and maintained for 50 seconds or more. If the secondarily cooling stop temperature is less than 450°C, it is difficult to obtain the mechanical properties that the present invention aims to achieve due to the low heat treatment temperature. On the other hand, if the secondarily cooling stop temperature exceeds 550°C, the unrecrystallized ferrite fraction will exceed 13%, making it difficult to obtain the mechanical properties that the present invention aims to achieve. If the secondarily cooling rate is less than 2°C / s, the cementite fraction will exceed 20%, making it difficult to obtain the mechanical properties that the present invention aims to achieve. If the maintenance time is less than 50 seconds, the maintenance time is insufficient, making it difficult to obtain the mechanical properties that the present invention aims to achieve. The lower limit of the secondarily cooling stop temperature is more preferably 460°C, and even more preferably 470°C. The upper limit of the secondarily cooling stop temperature is more preferably 540°C, and even more preferably 530°C. The average secondary cooling rate is more preferably 3°C / s or more. On the other hand, the present invention does not particularly limit the upper limit of the secondary mean cooling rate. Furthermore, since the longer the maintenance time, the more advantageous it is, the present invention does not particularly limit its upper limit. However, from the standpoint of productivity, the maintenance time can be 1800 seconds or less.

[0054] Subsequently, the cold-rolled steel sheet, which has undergone the secondary cooling and maintained temperature, is subjected to tertiary cooling to room temperature. The average cooling rate during the tertiary cooling can be 0.5 to 50°C / s.

[0055] On the other hand, after the tertiary cooling described above, a plating process can be carried out further. In this invention, the plating process is not particularly limited, and any conventional process used in the art can be used. [Examples]

[0056] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are for illustrative purposes to explain the present invention in more detail and are not intended to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0057] (Examples) A 100 mm thick slab having the alloy composition described in Table 1 below was prepared. The slab was then heated to 1200°C and hot-rolled at a finishing rolling temperature of 900°C to produce a 3 mm thick hot-rolled steel sheet. The hot-rolled steel sheet was cooled at an average cooling rate of 30°C / s to the winding temperature described in Table 2 below, and then wound. Subsequently, the wound hot-rolled steel sheet was heat-treated under the conditions described in Table 2 below, pickled, and then cold-rolled to produce a 1.5 mm thick cold-rolled steel sheet. After that, it was reheated, primary, secondary, and tertiary cooled under the conditions described in Tables 2 and 3 below.

[0058] The microstructure and mechanical properties of the cold-rolled steel sheets produced in this manner were measured, and the results are shown in Table 4 below.

[0059] The microstructure was observed using a scanning electron microscope (SEM) after polishing and nital etching of cross-sections of test specimens taken from cold-rolled steel sheets. After nital etching, structures with no surface irregularities were identified as ferrite, while structures with spherical or lamellar structures were identified as cementite. Unrecrystallized ferrite containing a high potential generates crystal orientation differences within the particles. Therefore, after measuring the crystal orientation of the ferrite using FESEM-EBSD, the fractions of unrecrystallized and recrystallized ferrite in the ferrite were distinguished using the KAM (Kernel Average Misorientation) method and measured.

[0060] Mechanical properties were measured by tensile tests and hole expansion tests. For the tensile tests, test specimens were taken according to JIS No. 5 standard, with the direction 0° relative to the rolling direction of the cold-rolled steel sheet as the reference point. For the hole expansion tests, a 10 mmφ punching hole (die inner diameter 10.3 mm, clearance 12.5%) was formed by compressing and expanding it at 20 mm / min with a conical punch with a 60° apex angle in the direction that caused the burrs of the punching hole to face outwards.

[0061] Hole expansion rate: HER(%) = {(D-D0) / D0} × 100 D: Diameter of the hole (mm) when a crack penetrates the steel plate. D0: Initial hole diameter (mm)

[0062] [Table 1] TIFF0007859738000002.tif30170

[0063] [Table 2] TIFF0007859738000004.tif139170

[0064] [Table 3] TIFF0007859738000006.tif134170

[0065] [Table 4] TIFF0007859738000008.tif131170

[0066] As shown in Tables 1 to 4 above, in the cases of Invention Examples 1 to 18, the alloy composition and manufacturing conditions proposed by the present invention are met, ensuring an appropriate microstructure, and thus demonstrating excellent strength, formability, and a high yield ratio.

[0067] In contrast, in the case of Comparative Examples 1 to 23, the alloy composition or manufacturing conditions proposed by the present invention were not met, making it impossible to secure an appropriate microstructure, and as a result, the mechanical properties were inferior.

Claims

1. In weight percent, it contains C: 0.05-0.25%, Si: 0.7% or less (excluding 0%), Mn: 0.46-1.8%, Al: 0.7% or less (excluding 0%), P: 0.05% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), and the total amount of one or more of Ti, Nb, and V: 0.22% or less, with the remainder being Fe and other unavoidable impurities. The microstructure consists of, by area percentage, unrecrystallized ferrite: 1-13%, recrystallized ferrite: 67-98%, and cementite: 1-20%. A steel plate with a tensile strength (TS) of 2 × √Hole expandability (HER): 2.5 × 10⁶ to 3.8 × 10⁶ MPa, 2% 0.5%.

2. The steel sheet according to claim 1, further comprising a total amount of one or more of Cr and Mo: 0.8% or less.

3. The steel sheet according to claim 1, further comprising a total amount of one or more of Cu and Ni: 0.8% or less.

4. The steel plate according to claim 1, further comprising B: 0.005% or less.

5. The steel sheet according to claim 1, further comprising a total amount of one or more of Ca, REM (excluding Y), and Mg: 0.05% or less.

6. The steel plate according to claim 1, further comprising a total amount of one or more of W and Zr: 0.5% or less.

7. The steel plate according to claim 1, further comprising a total amount of one or more of Sb and Sn: 0.5% or less.

8. The steel plate according to claim 1, further comprising a total amount of one or more of Y and Hf: 0.2% or less.

9. The steel plate has a yield ratio (YR): 0.8 to 0.95 and a tensile strength (TS) ２ ×√Elongation rate (EL) is 1.8 × 10 ６ ~2.3 x 10 ６ MPa ２ The steel plate according to claim 1, wherein the % is 0.

5.

10. A step of heating a steel ingot or slab, consisting of, by weight percent, C: 0.05-0.25%, Si: 0.7% or less (excluding 0%), Mn: 0.46-1.8%, Al: 0.7% or less (excluding 0%), P: 0.05% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), and the total amount of one or more of Ti, Nb, and V: 0.22% or less, with the remainder being Fe and other unavoidable impurities, at 1000-1350°C; The heated steel ingot or slab is hot-rolled at a finishing rolling temperature of 800 to 1000°C. The stage of obtaining hot-rolled steel sheets; The step of winding the hot-rolled steel sheet at 300 to 600°C; The step of heat-treating the wound hot-rolled steel sheet at 650 to 800°C for 600 to 1700 seconds; The step of obtaining a cold-rolled steel sheet by cold-rolling the heat-treated hot-rolled steel sheet at a cold reduction ratio of 30 to 90%; The cold-rolled steel sheet is reheated to 720-860°C and maintained at that temperature for 50 seconds or more; The step of first cooling the reheated and maintained cold-rolled steel sheet to 600-760°C at an average cooling rate of 1°C / s or more; The first step is to second-cool the first-cooled cold-rolled steel sheet to 450-550°C at an average cooling rate of 2°C / s or more and maintain the temperature for 50 seconds or more; and The method for manufacturing a steel sheet according to claim 1, further comprising the step of tertiarily cooling the cold-rolled steel sheet that has undergone secondary cooling and maintenance down to room temperature.

11. The method for manufacturing a steel plate according to claim 10, wherein the steel ingot or slab further comprises a total amount of one or more of Cr and Mo: 0.8% or less.

12. The method for manufacturing a steel plate according to claim 10, wherein the steel ingot or slab further comprises a total amount of one or more of Cu and Ni: 0.8% or less.

13. The method for manufacturing a steel plate according to claim 10, wherein the steel ingot or slab further comprises B: 0.005% or less.

14. The method for manufacturing a steel sheet according to claim 10, wherein the steel ingot or slab further contains a total amount of 0.05% or less of one or more of Ca, REM (excluding Y), and Mg.

15. The method for manufacturing a steel plate according to claim 10, wherein the steel ingot or slab further comprises a total amount of one or more of W and Zr: 0.5% or less.

16. The method for manufacturing a steel plate according to claim 10, wherein the steel ingot or slab further comprises a total amount of one or more of Sb and Sn: 0.5% or less.

17. The method for manufacturing a steel plate according to claim 10, wherein the steel ingot or slab further comprises a total amount of one or more of Y and Hf: 0.2% or less.

18. The method for manufacturing a steel sheet according to claim 10, further comprising the step of cooling the hot-rolled steel sheet to a winding temperature at an average cooling rate of 10°C / s or more after the finish rolling.

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