Precipitation-hardened steel sheet and manufacturing method therefor

Abstract

The present invention relates to a 0.04-0.06 wt% of C, 1.1-1.5 wt% of Mn, P in an amount greater than 0 wt% and equal to or less than 0.02 wt%, S in an amount greater than 0 wt% and equal to or less than 0.005 wt%, 0.1-0.4 wt% of Si, 0.015-0.06 wt% of Al, 0.02-0.05 wt% of Nb, 0.17-0.3 wt% of Ti, Mo in an amount greater than 0 wt% and equal to or less than 0.06 wt%, and the balance of Fe and other inevitable impurities, wherein the volume fraction of precipitates is 0.0045 to 0.0080.

Description

Precipitation-hardened steel sheet and method for manufacturing the same

[0001] The present invention relates to a precipitation-hardened steel plate and a method for manufacturing the same.

[0002] Cold-rolled precipitation-hardened steel utilizes a mechanism that increases yield strength through grain refinement and precipitation hardening effects by incorporating trace amounts of precipitation elements such as Ti, Nb, V, and Mo into a ferrite structure designed for low carbon (C) and low manganese as the basic matrix, thereby generating carbonitride (M(C, N)) precipitates during heat treatment.

[0003] Currently, looking at global standards for cold-rolled precipitation-hardened steel, the maximum strength grade in VDA standards is YP 460 MPa, while some automotive standards (such as TESLA and GM) set the maximum strength grade at YP 550 MPa. For applications requiring higher strength, hot-rolled precipitation-hardened steel is used (maximum YP grade of YP 700 MPa) or cold-rolled transformed structure steel is used.

[0004] This invention was developed to develop a cold-rolled precipitation-hardened steel of YP 800MPa grade and to apply it to parts requiring bending characteristics and high yield strength characteristics, such as seat rails.

[0005] The present invention aims to solve various problems, including those mentioned above, by providing a high-strength precipitation-hardened steel plate and a method for manufacturing the same. However, these problems are exemplary and do not limit the scope of the present invention.

[0006] According to one aspect of the present invention, carbon (C): 0.04 wt% or more and 0.06 wt% or less, manganese (Mn): 1.1 wt% or more and 1.5 wt% or less, phosphorus (P): greater than 0 wt% and 0.02 wt% or less, sulfur (S): greater than 0 wt% and 0.005 wt% or less, silicon (Si): 0.1 wt% or more and 0.4 wt% or less, aluminum (Al): 0.015 wt% or more and 0.06 wt% or less, niobium (Nb): 0.02 wt% or more and 0.05 wt% or less, titanium (Ti): 0.17 wt% or more and 0.3 wt% or less, molybdenum (Mo): greater than 0 wt% and 0.06 wt% or less, and the remainder being Fe and other unavoidable impurities, wherein the volume fraction of the precipitate is 0.0045 to A precipitation-hardened steel plate with a thickness of 0.0080 is provided.

[0007] According to the present embodiment, the precipitation water density of the steel plate is 9.92 x 10 14 / m 2 Up to 12.7x10 14 / m 2 It could be.

[0008] According to the present embodiment, the grain size of the steel plate may be 2.9㎛ to 3.9㎛.

[0009] According to the present embodiment, the yield strength of the steel plate may be 800 MPa or more.

[0010] According to the present embodiment, the yield ratio of the steel plate may be 90% or more.

[0011] According to the present embodiment, the elongation of the steel plate may be 8% or more.

[0012] According to another aspect of the present invention, a hot rolling step for manufacturing a hot-rolled plate by hot rolling a slab comprising carbon (C): 0.04 wt% or more and 0.06 wt% or less, manganese (Mn): 1.1 wt% or more and 1.5 wt% or less, phosphorus (P): greater than 0 wt% and 0.02 wt% or less, sulfur (S): greater than 0 wt% and 0.005 wt% or less, silicon (Si): 0.1 wt% or more and 0.4 wt% or less, aluminum (Al): 0.015 wt% or more and 0.06 wt% or less, niobium (Nb): 0.02 wt% or more and 0.05 wt% or less, titanium (Ti): 0.17 wt% or more and 0.3 wt% or less, molybdenum (Mo): greater than 0 wt% and 0.06 wt% or less, and the remainder being Fe and other unavoidable impurities; A method for manufacturing a precipitation-hardened steel plate is provided, comprising: a cold rolling step of cold rolling the hot-rolled plate to produce a cold-rolled plate; and a cold rolling annealing step of annealing the cold-rolled plate to produce a cold-rolled annealed plate; wherein the volume fraction of precipitates in the cold-rolled annealed plate is 0.0045 to 0.0080.

[0013] According to the present embodiment, the precipitation water density of the steel plate is 9.92 x 10 14 / m 2 Up to 12.7x10 14 / m 2 It could be.

[0014] According to the present embodiment, the grain size of the steel plate may be 2.9㎛ to 3.9㎛.

[0015] According to the present embodiment, the yield strength of the steel plate may be 800 MPa or more.

[0016] According to the present embodiment, the yield ratio of the steel plate may be 90% or more.

[0017] According to the present embodiment, the elongation of the steel plate may be 8% or more.

[0018] According to one embodiment of the present invention as described above, a high-strength precipitation-hardened steel plate and a method for manufacturing the same can be realized. Of course, the scope of the present invention is not limited by such effects.

[0019] FIG. 1 is a flowchart schematically illustrating a method for manufacturing a precipitation-hardened steel sheet according to one embodiment of the present invention.

[0020] Figure 2 schematically shows cross-sectional photographs showing the average diameter of precipitates of inventive materials 1 to 3.

[0021] Figure 3 schematically shows cross-sectional photographs showing the average diameter of precipitates of comparative material A and comparative material B.

[0022] Figure 4 schematically shows cross-sectional photographs showing the grain size of inventive materials 1 to 3.

[0023] Figure 5 schematically shows cross-sectional photographs showing the grain size of comparative material A and comparative material B.

[0024] The present invention is capable of various modifications and may have various embodiments; specific embodiments are illustrated in the drawings and described in detail in the detailed description. The effects and features of the present invention, and the methods for achieving them, will become clear by referring to the embodiments described below in detail together with the drawings. However, the present invention is not limited to the embodiments disclosed below but can be implemented in various forms.

[0025] In the following embodiments, terms such as first, second, etc. are used not in a limiting sense, but for the purpose of distinguishing one component from another component.

[0026] In the following examples, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0027] In the following embodiments, terms such as "include" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0028] In the drawings, the size of components may be exaggerated or reduced for convenience of explanation. For example, the size and thickness of each component shown in the drawings are depicted arbitrarily for convenience of explanation, so the present invention is not necessarily limited to what is illustrated.

[0029] Where an embodiment can be implemented differently, a specific process sequence may be performed differently from the order described. For example, two processes described consecutively may be performed substantially simultaneously or proceed in the reverse order of the description.

[0030] In this specification, "A and / or B" indicates the case where it is A, B, or both A and B. And, "at least one of A and B" indicates the case where it is A, B, or both A and B.

[0031] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. When describing with reference to the drawings, identical or corresponding components are given the same reference numerals, and redundant descriptions thereof will be omitted.

[0032]

[0033] FIG. 1 is a flowchart schematically illustrating a method for manufacturing a precipitation-hardened steel sheet according to one embodiment of the present invention.

[0034] Referring to FIG. 1, a method for manufacturing a precipitation-hardened steel sheet may include a hot rolling step (S100), a cold rolling step (S200), and a cold rolling annealing step (S300).

[0035] In the hot rolling step (S100), the slab may be reheated, then hot-rolled at a predetermined finishing rolling temperature, and then cooled and coiled. At this time, in the method for manufacturing a precipitation-hardening steel plate according to one embodiment of the present invention, the semi-finished product subject to hot rolling may be a slab. The slab in the semi-finished product state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0036] The slab may contain silicon (Si), manganese (Mn), aluminum (Al), carbon (C), phosphorus (P), sulfur (S), titanium (Ti), molybdenum (Mo), niobium (Nb), the remainder being iron (Fe) and other unavoidable impurities. Specifically, the slab may contain, in wt%, silicon (Si): 0.1 wt% or more and 0.4 wt% or less, manganese (Mn): 1.1 wt% or more and 1.5 wt% or less, aluminum (Al): 0.015 wt% or more and 0.06 wt% or less, carbon (C): 0.04 wt% or more and 0.06 wt% or less, phosphorus (P): greater than 0 wt% and 0.02 wt% or less, sulfur (S): greater than 0 wt% and 0.005 wt% or less, titanium (Ti): 0.17 wt% or more and 0.3 wt% or less, molybdenum (Mo): greater than 0 wt% and 0.06 wt% or less, niobium (Nb): 0.02 wt% or more and 0.05 wt% or less, and the remainder being iron (Fe) and unavoidable impurities.

[0037] Silicon (Si) not only contributes to strength increase as a solid solution strengthening element, but also acts as a ferrite stabilizing element, increasing the degree of supercooling during ferrite transformation to refine the grain size and suppress the formation of pearlite, thereby increasing the reactivity between solid solution carbon (C) and M (M=Nb, Ti). Since a silicon (Si) content exceeding 0.4 wt% is detrimental to the surface properties of the steel sheet, it may be desirable to add it in an amount of 0.1 wt% to 0.4 wt%.

[0038] Manganese (Mn) is a solid solution strengthening element that not only contributes to strength enhancement but also allows for the control of strength, toughness, and yield ratio depending on its content. However, adding large amounts can induce center segregation, thereby weakening the toughness of the steel. If the manganese (Mn) content is less than 1.1 wt%, it is difficult to achieve the target yield strength of 800 MPa; if it exceeds 1.5 wt%, while it is advantageous for achieving the target strength, it leads to the formation of inclusions or reduced machinability and resistance to delayed fracture due to segregation. Furthermore, it can increase the carbon (C) equivalent, thereby lowering weldability, which is a major advantage of precipitation hardening. Additionally, it can induce a transformation structure during annealing and cooling at high temperatures after cold rolling. Therefore, it may be desirable to add manganese (Mn) at a content of 1.1 wt% to 1.5 wt%.

[0039] Aluminum (Al) is used as a deoxidizer and can help clean ferrite. If the aluminum (Al) content is less than 0.015 wt%, the additive effect is insufficient, and if it exceeds 0.06 wt%, AlN is formed during slab manufacturing, which can cause cracks during casting or hot rolling. Therefore, it may be desirable to add aluminum (Al) with a content of 0.015 wt% to 0.06 wt%.

[0040] Carbon (C) can be added to control the microstructure and ensure the strength of the steel. If the carbon (C) content is less than 0.04 wt%, it is difficult to secure the desired yield strength because a sufficient precipitation effect cannot be obtained. Furthermore, problems may arise where MC (M=Nb, Ti)-based carbides become coarse, resulting in a reduced grain refinement effect. If the carbon (C) content exceeds 0.06 wt%, material properties may deteriorate as the fraction of phases such as pearlite increases. Additionally, as the carbon (C) content increases, the pearlite structure increases, and during high-temperature annealing after cold rolling, austenite transformation occurs in some pearlite structures, which may induce a two-phase structure. Therefore, it may be desirable to add carbon (C) in an amount between 0.04 wt% and 0.06 wt%.

[0041] Phosphorus (P) is the most advantageous element for securing strength, but if its content exceeds 0.02 wt%, the likelihood of brittle fracture may increase. Therefore, it may be desirable to limit the phosphorus (P) content to 0.02 wt% or less.

[0042] Since sulfur (S) is an element that is inevitably added as an impurity element in steel, it may be desirable to keep its content as low as possible. In addition, it can form non-metallic inclusions, which can reduce toughness and weldability. Therefore, it may be desirable to limit sulfur (S) to 0.005 wt% or less.

[0043] Titanium (Ti) is a powerful carbide-forming element that combines with carbon and nitrogen contained in steel during the hot rolling and annealing processes to form carbides or nitrides. By suppressing recrystallization and grain growth during the annealing process following cold rolling, these titanium-based carbides or nitrides can refine the grains, thereby improving both the strength and toughness of the steel. If the titanium (Ti) content is less than 0.17 wt%, it may be difficult to obtain sufficient recrystallization delay and precipitation strengthening effects. If it exceeds 0.3 wt%, the annealing time must be extended or the annealing temperature increased to secure the elongation of the steel due to the recrystallization delay effect of titanium (Ti). Therefore, it may be desirable to add the content in the range of 0.17 wt% to 0.3 wt%. In addition, since the price of titanium (Ti) is about 10% to 20% of that of niobium (Nb), it may be advantageous to increase the titanium (Ti) content as much as possible to secure cost competitiveness in ferroalloys.

[0044] Molybdenum (Mo) can be used to slow down the formation and growth rate of fine precipitates by acting when niobium (Nb) and titanium (Ti) components combine with carbon to form precipitates, thereby enhancing yield strength. However, due to the low cost competitiveness of the element molybdenum (Mo), it is necessary to limit its use as much as possible. For this reason, it may be desirable to manage its content to 0.06 wt% or less.

[0045] Niobium (Nb) is a powerful carbonitride-forming element that combines with carbon and nitrogen contained in steel during the hot rolling and annealing processes to form carbides or nitrides. By suppressing recrystallization and grain growth of these niobium (Nb)-based carbides or nitrides during the annealing process after cold rolling, the grain size is refined, thereby improving both the strength and toughness of the steel. If the niobium (Nb) content is less than 0.02 wt%, it may be difficult to obtain a precipitation strengthening effect, and if it exceeds 0.05 wt%, the annealing time must be extended or the annealing temperature increased to secure the elongation of the steel due to the recrystallization delay effect of Nb; therefore, it may be desirable to add the content in the range of 0.02 wt% to 0.05 wt%.

[0046]

[0047] In the hot rolling step (S100), the slab can be reheated, and then the reheated slab can be hot-rolled to produce a hot-rolled plate. For example, the slab on which the hot rolling step (S100) is performed can be called a hot-rolled plate.

[0048] In the hot rolling step (S100), the slab may be reheated. The slab reheating temperature (SRT) in the hot rolling step (S100) may be 1200°C to 1250°C. The reheating time at the slab reheating temperature (SRT) may be 40 minutes or more. The slab may be reheated to return the carbonitrides to a solid solution state by reheating the slab to a slab reheating temperature (SRT): 1200°C to 1250°C and a reheating time at the slab reheating temperature (SRT): 40 minutes or more. If the slab is reheated at a reheating temperature of less than 1200°C and a reheating time of 40 minutes or less, the carbonitrides may not dissolve and may become coarse, making it impossible to obtain material properties greater than the desired yield ratio.

[0049] The hot rolling step (S100) may finish rolling the slab at a predetermined finish rolling temperature. At this time, the finish rolling temperature may be higher than the Ar3 temperature. Additionally, the finish rolled slab may be cooled to a predetermined coiling temperature (CT) and coiled. At this time, the coiling temperature (CT) may be 500℃ to 550℃.

[0050] The hot rolling step (S100) is terminated at a temperature of Ar3 or higher, and coiling is performed at 500°C to 550°C to reduce the size of the precipitates and minimize the fraction of the precipitates. If the coiling temperature is below 500°C, it may be difficult to control the shape of the hot-rolled material, and if the coiling temperature is above 550°C, it may contribute to increasing the strength of the hot-rolled material by increasing the fraction of precipitates, but it may not be desirable from the perspective of the final cold-rolled material because the precipitates become coarse.

[0051] A cold rolling step (S200) may be performed after a hot rolling step (S100). A hot-rolled plate that has undergone a cold rolling step (S200) may be called a cold-rolled plate. In the cold rolling step (S200), the hot-rolled plate may be cold-rolled with a reduction rate of 50% or more.

[0052] A cold rolling annealing step (S300) may be performed after the cold rolling step (S200). A cold rolled plate that has undergone the cold rolling annealing step (S300) may be called a cold rolling annealing plate. The cold rolling annealing step (S300) may be performed under conditions of a holding temperature (e.g., cold rolling annealing temperature): 740°C to 800°C, a holding time (e.g., cold rolling annealing time): 30 s to 300 s, and a cooling rate: 5°C / s to 100°C / s. Specifically, in the cold rolling annealing step (S300), the cold rolled plate may be heated and held (e.g., annealed) at a holding temperature (e.g., cold rolling annealing temperature) of 740°C to 800°C for a holding time (e.g., cold rolling annealing time) of 30 s to 300 s, and cooled at a cooling rate of 5°C / s to 100°C / s. If annealed at less than 740℃, it may be difficult to secure elongation, and if annealed at more than 800℃, it may be difficult to secure yield strength and yield ratio.

[0053]

[0054] The precipitation-hardened steel sheet produced by the aforementioned manufacturing method may contain silicon (Si), manganese (Mn), aluminum (Al), carbon (C), phosphorus (P), sulfur (S), titanium (Ti), molybdenum (Mo), niobium (Nb), the remainder being iron (Fe) and other unavoidable impurities.

[0055] Specifically, the precipitation-hardened steel sheet may contain, in wt%, silicon (Si): 0.1 wt% or more and 0.4 wt% or less, manganese (Mn): 1.1 wt% or more and 1.5 wt% or less, aluminum (Al): 0.015 wt% or more and 0.06 wt% or less, carbon (C): 0.04 wt% or more and 0.06 wt% or less, phosphorus (P): greater than 0 wt% and 0.02 wt% or less, sulfur (S): greater than 0 wt% and 0.005 wt% or less, titanium (Ti): 0.17 wt% or more and 0.3 wt% or less, molybdenum (Mo): greater than 0 wt% and 0.06 wt% or less, niobium (Nb): 0.02 wt% or more and 0.05 wt% or less, and the remainder being iron (Fe) and unavoidable impurities.

[0056] In one embodiment of the present invention, the precipitation-hardened steel sheet may contain titanium (Ti) in an amount of 0.17 wt% to 0.3 wt%, and the fraction of TiC precipitates may increase to realize a steel sheet with high yield strength. When a large amount of nano precipitates are generated, the grain size may be reduced due to the pinning effect, and this may further contribute to an increase in yield strength.

[0057] The volume fraction of precipitates in the precipitation-hardened steel sheet may be 0.0045 to 0.0080, and the precipitation water density is 9.92 x 10⁻⁶ 14 / m 2 Up to 12.7x10 14 / m 2 It may be. The grain size of the precipitation-hardened steel sheet may be 2.9㎛ to 3.9㎛. The yield strength of the steel sheet may be 800 MPa or more, and the elongation may be 8% or more.

[0058]

[0059] Experimental Example

[0060] The present invention will be explained in more detail below through experimental examples. However, the following experimental examples are intended to explain the present invention more specifically, and the scope of the present invention is not limited by the following experimental examples. The following experimental examples may be appropriately modified or changed by those skilled in the art within the scope of the present invention.

[0061] Table 1 below shows the composition of the steel plate used in the experimental examples of the present invention. The steel plate in Table 1 contains the remainder of iron (Fe) and unavoidable impurities.

[0062]

[0063] Classification CsiMnPSAlNbTi Inventive Material 0.05 10.21.29 0.018 0.0007 0.03 10.03 20.24 Comparative Material A 0.07 60.19 0.86 0.014 0.003 60.03 20.039- Comparative Material B 0.07 30.19 1.07 0.014 0.004 00.037 0.051-

[0064]

[0065] Classification Winding Temperature (°C) Annealing Temperature (°C) Annealing Time (s) Grain Size (㎛) Precipitation Volume Fraction Average Precipitate Size (nm) Precipitation Number Density (x10 14 / m 2 Yield Strength (MPa) Yield Ratio (%) Elongation (%) Inventive Material 15 20 800 30 3.00.00 79 39.4 9.9 28 08 99.5 11.0 Inventive Material 25 20 770 100 3.90.00 59 88.01 0 35 82 49 8.2 9.4 Inventive Material 35 20 740 300 2.90.00 45 37.3 1.2 71 86 49 9.8 8.6 Comparison Material A5 80 780 60 6.00.000 23 3.6 6.5 44 52 84.5 24 Comparison Material B6 20 780 60 5.70.000 20 3.6 3.6 14 34 81.4 27

[0066]

[0067] Grain size measurement

[0068] Measurements were taken using an EBSD (electron backscattered diffraction) analysis instrument at a step size of 0.5 μm and a magnification of 500x. The raw data was analyzed using the Aztec Crystal program, and the average grain size was measured through the statistics of typically 20,000 to 25,000 grains.

[0069] Measurement of precipitation volume fraction

[0070] Carbon replica samples were used for TEM analysis. The results obtained from TEM analysis were provided as two-dimensional images, and based on this, the following procedures and assumptions were applied to calculate the three-dimensional volume fraction of the precipitate.

[0071] First, an image analysis program was used to identify precipitates within the total analysis area (A), and the number of precipitates (s) and the area (an) of each individual precipitate were calculated. Next, the radius (d) was calculated from the area (an) of each precipitate, and the volume (v) of the corresponding precipitate was calculated based on this. Subsequently, the volumes of all precipitates were summed to obtain the total volume of precipitates (V).

[0072] When calculating the volume fraction, considering that the analysis area (A) is two-dimensional data, representative thicknesses of the carbon replica samples were assumed to be 200 nm, 150 nm, 125 nm, and 100 nm, respectively. A xt was applied to calculate the total sample volume (P1, P2, P3) for each thickness (t1, t2, t3, t4). Subsequently, for each case, the precipitate volume fraction was calculated by dividing the total volume of the precipitate (V) by the sample volume (P1, P2, P3, P4). Finally, the average of the four volume fraction values ​​was derived to obtain the final average precipitate volume fraction (V / P).

[0073] Measurement of average precipitate size

[0074] The average size of the precipitates is the area of ​​each individual precipitate (a n It was calculated from ). To this end, the area of ​​each precipitate (a n Using ), the radius (or the radius of the equivalent disk) was derived through the following equation.

[0075]

[0076] Subsequently, the radius of each calculated precipitate (r n The average size of the precipitate was statistically derived by calculating the average value based on the total number(s) for ).

[0077]

[0078] Measurement of precipitation water density

[0079] The precipitation water density was calculated using the following formula.

[0080] Precipitation density = s / A

[0081] Here, s is the total number of precipitates and A is the total analysis area. Through this, the number of precipitates per unit area was calculated to derive the distribution density of the precipitates.

[0082] Yield strength measurement

[0083] The yield strength was determined by reading the stress value at the yield point from the stress-strain curve in the tensile test results. It was calculated using the 0.2% offset method, or, if there was a distinct lower yield point, measured based on the lower yield point.

[0084] Yield ratio measurement

[0085] The yield ratio is the ratio between yield strength and tensile strength (maximum stress), and was calculated using the following formula.

[0086] Yield ratio = Yield strength / Tensile strength elongation measurement

[0087] The change in length of the specimen before and after fracture was measured and calculated following the tensile test. The elongation is calculated using the following formula.

[0088]

[0089]

[0090] FIG. 2 schematically shows cross-sectional photographs showing the average diameter of precipitates of inventive materials 1 to 3. Specifically, FIG. 2 (a) is a cross-sectional photograph of inventive material 1, FIG. 2 (b) is a cross-sectional photograph of inventive material 2, and FIG. 2 (c) is a cross-sectional photograph of inventive material 3.

[0091] Figure 3 schematically shows cross-sectional photographs showing the average diameter of precipitates of comparative material A and comparative material B. Specifically, Figure 3 (a) is a cross-sectional photograph of comparative material A, and Figure 3 (b) is a cross-sectional photograph of comparative material B.

[0092] FIG. 4 schematically shows cross-sectional photographs showing the grain size of inventive materials 1 to 3. Specifically, FIG. 4 (a) is a cross-sectional photograph of inventive material 1, FIG. 4 (b) is a cross-sectional photograph of inventive material 2, and FIG. 4 (c) is a cross-sectional photograph of inventive material 3.

[0093] FIG. 5 schematically shows cross-sectional photographs showing the grain size of comparative material A and comparative material B. Specifically, FIG. 5 (a) is a cross-sectional photograph of comparative material A, and FIG. 5 (b) is a cross-sectional photograph of comparative material B.

[0094] Referring to Table 1, Table 2, FIG. 2(a), and FIG. 4(a), Inventive Material 1 may contain 0.24 wt% titanium (Ti). Inventive Material 1 may have a coiling temperature of 520°C during the hot rolling stage, an annealing temperature of 800°C during the hot rolling annealing stage, and an annealing time of 30 s. The grain size of Inventive Material 1 may be 3.0 μm, the precipitation volume fraction may be 0.00793, and the average size of the precipitates may be 9.4 nm. The yield strength of Inventive Material 1 may be 808 MPa, the yield ratio may be 99.5%, and the elongation may be 11%. The yield strength of Inventive Material 1 may be 800 MPa or higher.

[0095] Referring to Table 1, Table 2, FIG. 2(b), and FIG. 4(b), Inventive Material 2 may contain 0.24 wt% titanium (Ti). Inventive Material 2 may have a coiling temperature of 520°C during the hot rolling stage, an annealing temperature of 770°C during the hot rolling annealing stage, and an annealing time of 100 s. The grain size of Inventive Material 2 may be 3.9 μm, the precipitation volume fraction may be 0.00598, and the average size of the precipitates may be 8.0 nm. The yield strength of Inventive Material 2 may be 824 MPa, the yield ratio may be 98.2%, and the elongation may be 9.4%. The yield strength of Inventive Material 2 may be 800 MPa or higher.

[0096] Referring to Table 1, Table 2, Fig. 2(c), and Fig. 4(c), Inventive Material 3 may contain 0.24 wt% titanium (Ti). Inventive Material 3 may have a coiling temperature of 520°C during the hot rolling stage, an annealing temperature of 740°C during the hot rolling annealing stage, and an annealing time of 300 s. The grain size of Inventive Material 3 may be 2.9 μm, the precipitation volume fraction may be 0.00453, and the average size of the precipitates may be 7.3 nm. The yield strength of Inventive Material 3 may be 864 MPa, the yield ratio may be 99.8%, and the elongation may be 8.6%. The yield strength of Inventive Material 3 may be 800 MPa or higher.

[0097] Invention materials 1, 2, and 3 contain 0.24 wt% titanium (Ti), and can secure high yield strength as the fraction of TiC precipitates increases. In addition, the grain size is reduced due to the pinning effect, thereby enabling the securing of even higher yield strength.

[0098] Referring to Table 1, Table 2, Fig. 3(a), and Fig. 4(a), Comparative Material A may not contain titanium (Ti). For Comparative Material A, the coiling temperature during the hot rolling stage may be 580°C, and the annealing temperature during the hot rolling annealing stage may be 780°C and the annealing time may be 60 s. The grain size of Comparative Material A may be 6.0 μm, the precipitation volume fraction may be 0.00023, and the average size of the precipitates may be 3.6 nm. The yield strength of Comparative Material A may be 452 MPa, the yield ratio may be 84.5%, and the elongation may be 24%. Comparative Material A may have a low yield strength.

[0099] Referring to Table 1, Table 2, Fig. 3(b), and Fig. 4(b), Comparative Material B may have a coiling temperature of 620°C during the hot rolling stage, an annealing temperature of 780°C during the hot rolling annealing stage, and an annealing time of 60 s. The grain size of Comparative Material B is 5.7 μm, the precipitation volume fraction is 0.00020, and the average size of the precipitates is 3.6 nm. The yield strength of Comparative Material B may be 434 MPa, the yield ratio may be 81.4%, and the elongation may be 27%. Comparative Material B may have a low yield strength.

[0100]

[0101] The present invention has been described with reference to the embodiments illustrated in the drawings, but this is merely illustrative, and those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible therefrom. Accordingly, the true technical scope of protection of the present invention should be determined by the technical spirit of the appended claims.

Claims

1. Carbon (C): 0.04wt% or more and 0.06wt% or less, Manganese (Mn): 1.1wt% or more and 1.5wt% or less, Phosphorus (P): greater than 0wt% and 0.02wt% or less, Sulfur (S): greater than 0wt% and 0.005wt% or less, Silicon (Si): 0.1wt% or more and 0.4wt% or less, Aluminum (Al): 0.015wt% or more and 0.06wt% or less, Niobium (Nb): 0.02wt% or more and 0.05wt% or less, Titanium (Ti): 0.17wt% or more and 0.3wt% or less, Molybdenum (Mo): greater than 0wt% and 0.06wt% or less, the remainder being Fe and other unavoidable impurities, and Precipitation-hardened steel sheet having a volume fraction of precipitates of 0.0045 to 0.0080.

2. In Paragraph 1, The precipitation water density of the above steel plate is 9.92 x 10⁻⁶ 14 / m 2 Up to 12.7x10 14 / m 2 Phosphorus, precipitation-hardened steel plate.

3. In Paragraph 1, A precipitation-hardened steel plate having a grain size of 2.9㎛ to 3.9㎛.

4. In Paragraph 1, A precipitation-hardened steel plate having a yield strength of 800 MPa or more.

5. In Paragraph 1, A precipitation-hardened steel plate having a yield ratio of 90% or more.

6. In Paragraph 1, A precipitation-hardened steel plate having an elongation of 8% or more.

7. A hot rolling step for manufacturing a hot-rolled plate by hot rolling a slab containing carbon (C): 0.04wt% or more and 0.06wt% or less, manganese (Mn): 1.1wt% or more and 1.5wt% or less, phosphorus (P): greater than 0wt% and 0.02wt% or less, sulfur (S): greater than 0wt% and 0.005wt% or less, silicon (Si): 0.1wt% or more and 0.4wt% or less, aluminum (Al): 0.015wt% or more and 0.06wt% or less, niobium (Nb): 0.02wt% or more and 0.05wt% or less, titanium (Ti): 0.17wt% or more and 0.3wt% or less, molybdenum (Mo): greater than 0wt% and 0.06wt% or less, and the remainder being Fe and other unavoidable impurities; A cold rolling step for manufacturing a cold rolled plate by cold rolling the above hot rolled plate; and A cold rolling annealing step for manufacturing a cold rolling annealed plate by annealing the above cold rolling plate; is included, A method for manufacturing a precipitation-hardened steel sheet, wherein the volume fraction of precipitates in the above cold-rolled annealed sheet is 0.0045 to 0.0080.

8. In Paragraph 7, The precipitation water density of the above steel plate is 9.92 x 10⁻⁶ 14 / m 2 Up to 12.7x10 14 / m 2 Method for manufacturing phosphorus, precipitation-hardened steel sheet.

9. In Paragraph 7, A method for manufacturing a precipitation-hardened steel sheet, wherein the grain size of the steel sheet is 2.9㎛ to 3.9㎛.

10. In Paragraph 7, A method for manufacturing a precipitation-hardened steel plate having a yield strength of 800 MPa or more.

11. In Paragraph 7, A method for manufacturing a precipitation-hardened steel plate having a yield ratio of 90% or more of the above steel plate.

12. In Paragraph 7, A method for manufacturing a precipitation-hardening steel plate having an elongation of 8% or more of the above steel plate.

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