High tensile strength steel sheet having excellent hot stability and method for manufacturing the same

Abstract

The present invention relates to an ultra-high strength steel sheet and a method of manufacturing the same, and more particularly, an object of the present invention is to provide a steel sheet and a method of manufacturing the same, the steel sheet of which has excellent thermal stability, and thus has a high yield ratio and ultra-high strength even after heat treatment at a relatively low temperature.

Description

Technical Field

[0001] This invention relates to an ultra-high strength steel plate and its manufacturing method, and more specifically, to an ultra-high strength steel plate with excellent thermal stability and a high yield strength ratio and its manufacturing method. Background Technology

[0002] Steel sheets used in heavy equipment booms, commercial vehicle frames and reinforcements, and structural components of building and machinery parts may be heated, either partially or entirely, during manufacturing and use for various purposes. As an example, commercial vehicle frames and reinforcements often require localized shape adjustments for integration with other components, necessitating localized heating and deformation of the steel. However, this heating process alters the steel's strength, leading to decreased durability. This is because carbon in the solid solution state is rearranged or aggregates at dislocations, grain boundaries, etc., forming carbides during heating, causing brittleness in the steel. Furthermore, the steel's strength changes rapidly due to alterations in its microstructure, including martensite, bainite, and retained austenite, also affecting formability and durability.

[0003] As mentioned above, the changes in the microstructure and physical properties of steel during the heating process vary depending on the initial steel composition and microstructure, and are highly dependent on heat treatment conditions such as heating temperature and holding time. So far, the focus has been solely on suppressing the reduction in strength at high temperatures above 600°C.

[0004] For example, Patent Documents 1 and 2 propose a technique that involves adding Cr, Mo, Nb, V, etc., as alloying components and using tempering after hot rolling to ensure high-temperature strength. However, this technique is only suitable for manufacturing thick steel plates for construction. Furthermore, for construction steel, considering unavoidable heating factors such as fires, adding large amounts of Cr, Mo, Nb, V, etc., alloying components to the steel can ensure a certain level of strength even after prolonged exposure to high-temperature environments above 600°C. However, this results in excessively high manufacturing costs, such as the need for tempering. In particular, short-term exposure to environments below 600°C leads to excessively high thermal stability during use.

[0005] Patent document 3 describes a technique for ensuring strength in the heat-affected zone of a weld by adding elements such as Ti, Nb, Cr, and Mo. This technique is suitable for suppressing softening in adjacent weld areas when welding structural components for automobiles. However, arc welding involves heating the area adjacent to the weld material that melts due to welding heat to temperatures exceeding 600°C, and sometimes even to temperatures above the austenitic region, thus presenting limitations.

[0006] Patent document 4 is a technology that ensures high-temperature strength by adding Cr, Mo, Ti, Nb, V and other materials. However, it also ensures strength when exposed to high temperatures above 600°C for a long time. But when manufactured with a given composition system and manufacturing conditions, it can only ensure a tensile strength (TS) of 530 MPa. Therefore, it is different from the gigabit-level ultra-high strength steel in terms of application and strength.

[0007] [Existing Technical Documents]

[0008] (Patent Document 1) Korean Patent Publication No. 10-0358939 (Published on October 16, 2002)

[0009] (Patent Document 2) Korean Patent Publication No. 10-1290382 (Published on July 22, 2013)

[0010] (Patent Document 3) Korean Patent Publication No. 10-0962745 (Published on June 3, 2010)

[0011] (Patent Document 4) Korean Patent Publication No. 10-1246390 (Published on March 21, 2013) Summary of the Invention

[0012] Technical problems to be solved

[0013] According to one aspect of the present invention, a steel plate and a method for manufacturing the same are provided. The steel plate has excellent thermal stability and therefore exhibits a high yield strength ratio and ultra-high strength even after heat treatment at relatively low temperatures.

[0014] The technical problem addressed by this invention is not limited to the above description. Those skilled in the art will readily understand the additional technical problems addressed by this invention from the entirety of this specification.

[0015] Technical solution

[0016] One aspect of the present invention provides a steel plate, comprising, by weight percent: C: 0.05-0.13%, Si: 0.01-0.5%, Mn: 0.8-2.0%, Cr: 0.005-1.2%, Mo: 0.001-0.5%, P: 0.001-0.02%, S: 0.001-0.01%, Al: 0.01-0.1%, N: 0.001-0.01%, and Ti: 0.01-0.05%. The steel sheet contains Nb: 0.001-0.03%, V: 0.001-0.2%, B: 0.0003-0.003%, with the balance being Fe and unavoidable impurities. The K value defined in Equation 1 below is -1.05 or higher, and the G value defined in Equation 2 below is 2 to 20. The microstructure comprises 60-90% martensite (including tempered martensite), 10-40% bainite, and less than 5% ferrite, and the yield strength ratio of the steel sheet is 0.8 or higher.

[0017] [Relation 1]

[0018] K=-0.6-1.42[C]+0.05[Si]-0.16[Mn]-0.08[Cr]-0.03[Mo]+0.09[Ti]+0.08[Nb] 2

[0019] (Where [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] are the weight percent of the corresponding alloying elements.)

[0020] [Relationship 2]

[0021] G=([Nb] / 93+[Mo] / 96+[V] / 51) / ([Ti] / 48)

[0022] (Where, [Nb], [Mo], [V], and [Ti] are the weight percent of the corresponding alloying elements.)

[0023] The tensile strength of the steel plate can be above 950 MPa.

[0024] The tensile strength of the steel plate after heat treatment at 400-600℃ can be more than 80% of the tensile strength before heat treatment.

[0025] Another aspect of the present invention provides a method for manufacturing a steel plate, comprising the steps of: reheating a steel billet, wherein the steel billet comprises, by weight percent: C: 0.05-0.13%, Si: 0.01-0.5%, Mn: 0.8-2.0%, Cr: 0.005-1.2%, Mo: 0.001-0.5%, P: 0.001-0.02%, S: 0.001-0.01%, Al: 0.01-0.1%, N: 0.001-0.01%, Ti: 0.01-0.05%, Nb: 0.00 1-0.03%, V: 0.001-0.2%, B: 0.0003-0.003%, balance Fe and unavoidable impurities, K value as defined in Equation 1 below is -1.05 or higher, G value as defined in Equation 2 below is 2 to 20; the reheated billet is hot-rolled; and the hot-rolled steel sheet is cooled once at a cooling rate of 60°C / second or higher to a temperature range of 300-500°C, and then cooled a second time at a cooling rate of 10-70°C / second to a temperature range of 50-200°C, and then coiled.

[0026] [Relation 1]

[0027] K=-0.6-1.42[C]+0.05[Si]-0.16[Mn]-0.08[Cr]-0.03[Mo]+0.09[Ti]+0.08[Nb] 2

[0028] (Where [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] are the weight percent of the corresponding alloying elements.)

[0029] [Relationship 2]

[0030] G=([Nb] / 93+[Mo] / 96+[V] / 51) / ([Ti] / 48)

[0031] (Where, [Nb], [Mo], [V], and [Ti] are the weight percent of the corresponding alloying elements.)

[0032] The reheating temperature in the reheating step can be 1150-1350℃, and the rolling termination temperature in the hot rolling step can be 850-1150℃.

[0033] The secondary cooling rate during cooling can be below 60°C / second.

[0034] Beneficial effects

[0035] According to one aspect of the present invention, a steel plate and a method for manufacturing the same may be provided, wherein the steel plate has excellent thermal stability and thus exhibits a high yield strength ratio and ultra-high strength even after heat treatment at relatively low temperatures.

[0036] According to another aspect of the present invention, an ultra-high strength steel plate and a method for manufacturing the same can be provided, wherein the ultra-high strength steel plate can be heat-treated at a relatively low temperature for a short time, thereby expanding its applications.

[0037] Best practice

[0038] The preferred embodiments of the present invention are described below. These embodiments can be modified in various ways and should not be construed as limiting the scope of the invention to the specific embodiments described below. These specific embodiments are provided to illustrate the invention in more detail to those skilled in the art.

[0039] In order to solve the problems of the prior art, the inventors measured the changes in room temperature tensile strength of steels with various compositions and microstructures after heat treatment in the temperature range of 400-600℃, and confirmed that the change in tensile strength depends on the slope of the dynamic strength value measured during the heating process of the steel.

[0040] Based on this result, the inventors were able to derive Equations 1 and 2 for optimizing the compositional contents of C, Mn, Si, Cr, Mo, Ti, Nb, and V, which are the main components of steel. At the same time, they confirmed that excellent thermal stability can be ensured by controlling the conditions of the manufacturing process, thus completing the present invention.

[0041] The present invention will now be described in detail.

[0042] The steel composition of the present invention will be described in detail below.

[0043] Unless otherwise specified in this invention, the percentages of each element are expressed by weight.

[0044] According to one aspect of the invention, the steel plate may contain, by weight percent: C: 0.05-0.13%, Si: 0.01-0.5%, Mn: 0.8-2.0%, Cr: 0.005-1.2%, Mo: 0.001-0.5%, P: 0.001-0.02%, S: 0.001-0.01%, Al: 0.01-0.1%, N: 0.001-0.01%, Ti: 0.01-0.05%, Nb: 0.001-0.03%, V: 0.001-0.2%, B: 0.0003-0.003%, with the balance being Fe and unavoidable impurities.

[0045] Carbon (C): 0.05-0.13%

[0046] Carbon (C) is the most economical and effective element for strengthening steel. As the amount added increases, tensile strength increases due to the increased fraction of martensite or bainite. When the carbon (C) content is less than 0.05%, it is difficult to fully achieve the above effect. When the carbon (C) content exceeds 0.13%, the strength of the martensite increases due to the excess carbon (C), but the solid solution strengthening effect of carbon (C) may be significantly reduced during heat treatment in the range of 400-600℃.

[0047] Therefore, the carbon (C) content can be 0.05-0.13%, more preferably the lower limit can be 0.07%, and more preferably the upper limit can be 0.11%.

[0048] Silicon (Si): 0.01-0.5%

[0049] Silicon (Si) deoxidizes molten steel and has a solid solution strengthening effect, delaying the formation of coarse carbides, thus it is an element that is beneficial to improving formability. When the silicon (Si) content is less than 0.01%, it is difficult to obtain the above effects, but when the silicon (Si) content exceeds 0.5%, a red oxide scale caused by silicon (Si) forms on the surface of the steel plate during hot rolling, which not only results in very poor surface quality of the steel plate, but also reduces weldability.

[0050] Therefore, the silicon (Si) content can be 0.01-0.5%, and more preferably, the upper limit can be 0.3%.

[0051] Manganese (Mn): 0.8-2.0%

[0052] Manganese (Mn), like silicon (Si), is an effective element for solid solution strengthening of steel. It increases the hardenability of steel and facilitates the formation of martensite and bainite during cooling after heat treatment. When the manganese (Mn) content is less than 0.8%, the above-mentioned effects cannot be obtained. When the manganese (Mn) content exceeds 2.0%, it helps to ensure initial strength, but the difference between initial strength and post-heat-treated strength may increase when heat-treated in the range of 400-600℃. In addition, during the casting of slabs in continuous casting processes, a large segregation zone develops in the center of the thickness, causing deviations and easily forming MnS, thus potentially reducing ductility.

[0053] Therefore, the manganese (Mn) content can be 0.8-2.0%, with a more preferred lower limit of 1.0% and a more preferred upper limit of 1.8%.

[0054] Chromium (Cr): 0.005-1.2%

[0055] Chromium (Cr) strengthens steel through solid solution treatment and delays the ferrite phase transformation during cooling, thus contributing to the formation of martensite and bainite. Furthermore, the precipitation of fine composite carbides such as Mo, Ti, and Ni contributes to the strength after heat treatment. When the chromium (Cr) content is less than 0.005%, the aforementioned effects are not achieved. When the chromium (Cr) content exceeds 1.2%, similar to Mn, segregation in the center of the thickness becomes significantly developed, the microstructure in the thickness direction becomes uneven, and it may also be disadvantageous in terms of alloy cost.

[0056] Therefore, the chromium (Cr) content can be 0.005-1.2%, and more preferably the lower limit can be 0.4%.

[0057] Molybdenum (Mo): 0.001-0.5%

[0058] Molybdenum (Mo) increases the hardenability of steel, thus facilitating the formation of martensite and bainite. Furthermore, during heat treatment, it forms Nb-Ti-Mo based fine carbides, thereby mitigating the reduction in strength. When the Mo content is less than 0.001%, the above-mentioned effects are not achieved, and when the Mo content exceeds 0.5%, it may be economically disadvantageous.

[0059] Therefore, the molybdenum (Mo) content can be 0.001-0.5%, with a more preferred lower limit of 0.05% and a more preferred upper limit of 0.3%.

[0060] Phosphorus (P): 0.001-0.02%

[0061] Phosphorus (P) has a solid solution strengthening effect, but it can cause brittleness due to grain boundary segregation. Manufacturing a phosphorus (P) content of 0.001% requires significant manufacturing costs, making it economically unfavorable and potentially insufficient in terms of strength. On the other hand, when the phosphorus (P) content exceeds 0.02%, brittleness arises due to grain boundary segregation, making it prone to microcracks during molding, and potentially significantly reducing ductility and impact resistance.

[0062] Therefore, the phosphorus (P) content can be 0.001-0.02%.

[0063] Sulfur (S): 0.001-0.01%

[0064] Sulfur (S) is an impurity present in steel. When the sulfur (S) content exceeds 0.01%, sulfur (S) combines with manganese (Mn) and other metals to form non-metallic inclusions. As a result, the steel is prone to developing micro-cracks during cutting and processing, and its impact resistance is significantly reduced. On the other hand, to reduce the sulfur (S) content to less than 0.001%, a significant amount of time is required during steelmaking operations, which may reduce productivity.

[0065] Therefore, the sulfur (S) content can be 0.001-0.01%.

[0066] Aluminum (Al): 0.01-0.1%

[0067] Aluminum (Al) is mainly added for deoxidation. When the aluminum (Al) content is less than 0.01%, the above-mentioned addition effect is insufficient. When the aluminum (Al) content exceeds 0.1%, Al combines with N to form AlN, which is prone to corner cracks in the slab during continuous casting and may also cause defects caused by the formation of inclusions.

[0068] Therefore, the aluminum (Al) content can be 0.01-0.1%, more preferably the lower limit can be 0.02%, and more preferably the upper limit can be 0.05%.

[0069] Nitrogen (N): 0.001-0.01%

[0070] Nitrogen (N), along with carbon (C), is a representative solid solution strengthening element. Nitrogen (N) forms coarse precipitates with elements such as titanium (Ti) and al. Generally, nitrogen (N) provides superior solid solution strengthening compared to carbon (C). However, as the amount of nitrogen (N) in steel increases, a significant decrease in toughness occurs; therefore, the upper limit for nitrogen (N) content is limited to 0.01%. On the other hand, achieving a nitrogen (N) content of less than 0.001% requires excessive time during steelmaking operations, thus reducing productivity.

[0071] Therefore, the nitrogen (N) content can be 0.001-0.01%.

[0072] Titanium (Ti): 0.01-0.05%

[0073] Titanium (Ti), along with Nb, Mo, and V, is a representative precipitation strengthening element, helping to mitigate the strength reduction caused by carbide formation after heat treatment. However, compared to other precipitation elements, the precipitation formation temperature is higher, thus reducing its effectiveness. Furthermore, titanium (Ti) has a strong affinity for N, resulting in the formation of coarse TiN. This TiN has the effect of inhibiting grain growth during the heating process for hot rolling and stabilizing the solid solution of N, thus facilitating the use of boron (B) added to improve hardenability. When the titanium (Ti) content is less than 0.01%, the above-mentioned effects are difficult to obtain; when the titanium (Ti) content exceeds 0.05%, the formation of coarse TiN and the coarsening of precipitates during heat treatment may lead to decreased impact resistance in the low-temperature region.

[0074] Therefore, the titanium (Ti) content can be 0.01-0.05%, and more preferably, the upper limit can be 0.03%.

[0075] Niobium (Nb): 0.001-0.03%

[0076] Niobium (Nb), along with Ti and V, is a representative precipitation strengthening element. During hot rolling, it forms carbides, which, due to delayed recrystallization, result in grain refinement, effectively improving the strength and impact toughness of steel. The formation of carbides reduces the carbon content in the steel, and during heat treatment in the 400-600℃ range, it mitigates the strength reduction caused by carbon. When the niobium (Nb) content is less than 0.001%, the above effects are not achieved. When the niobium (Nb) content exceeds 0.03%, the precipitates formed during rolling excessively delay recrystallization, potentially worsening the anisotropy of the steel.

[0077] Therefore, the niobium (Nb) content can be 0.001-0.03%, and more preferably, the upper limit can be 0.02%.

[0078] Vanadium (V): 0.001-0.2%

[0079] Vanadium (V) is a strong precipitation-curing element and is an active precipitation element within the reheating temperature range. Upon reheating, precipitates form, and the formation of precipitates can compensate for the strength reduction caused by martensitic annealing. The vanadium (V) content is preferably added at 0.001% or more, but when the vanadium (V) content exceeds 0.2%, it may be economically disadvantageous.

[0080] Therefore, the vanadium (V) content can be 0.001-0.2%.

[0081] Boron (B): 0.0003-0.003%

[0082] Boron (B) delays the ferrite phase transformation, thus facilitating the establishment of initial strength through bainite and martensite. When present in steel in a solid solution state, it stabilizes grain boundaries, improving the steel's brittleness in low-temperature regions. It forms BN with dissolved nitrogen, thereby suppressing the formation of coarse nitrides. When the boron (B) content is less than 0.0003%, these effects are difficult to achieve. When the boron (B) content exceeds 0.003%, it helps improve initial strength, but contributes little to the increase in strength after heat treatment; therefore, the decrease in strength after heat treatment may be increased.

[0083] Therefore, the boron (B) content can be 0.0003-0.003%.

[0084] In addition to the above-described components, the steel sheet of the present invention may contain a balance of iron (Fe) and unavoidable impurities. Unavoidable impurities may be undesirably introduced during ordinary manufacturing processes, and therefore cannot be excluded. Such impurities are well known in the field of ordinary iron and steel manufacturing, and therefore not all of them are specifically described in this specification.

[0085] The K value of the steel plate of the present invention, as defined in the following Equation 1, can be -1.05 or higher.

[0086] The thermal stability of steel, related to the K value in Equation 1, is based on the steel's resistance to deformation under external forces applied at a given temperature. As an example, in high-temperature compression or tensile testing of steel, an external force is applied at a constant deformation rate while the material is heated at a constant rate, and the force applied per unit area is measured. As mentioned above, the slope of the measured stress-temperature curve is called thermal stability, and this is an inherent characteristic of the steel.

[0087] In this invention, a high-temperature compression test method is used for measurement, wherein the steel is heated to 600°C at a heating rate of 1°C / second while a deformation of 30% is applied at a deformation rate of 0.005 / second. The slope K of the stress-temperature curve obtained at this time is calculated for various types of steel, thereby obtaining Equation 1.

[0088] When the K value of Equation 1 is less than -1.05, the thermal stability is insufficient, and the strength change before and after heat treatment at 100-600℃ may increase. In particular, when the change in yield strength before and after heat treatment as described above simultaneously satisfies Equation 2, a more stable tendency can be exhibited. A K value is more preferably -1.03 or higher.

[0089] [Relation 1]

[0090] K=-0.6-1.42[C]+0.05[Si]-0.16[Mn]-0.08[Cr]-0.03[Mo]+0.09[Ti]+0.08[Nb] 2

[0091] (Where [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] are the weight percent of the corresponding alloying elements.)

[0092] The G value of the steel plate of the present invention, as defined in the following relation 2, can be from 2 to 20.

[0093] When both Equation 1 and Equation 2 are satisfied, the reduction in strength after heat treatment is mitigated, thus ensuring thermal stability.

[0094] Equation 2 below, expressed in composition, illustrates the strength after heat treatment based on the precipitates, relating to the formation of precipitates within the fine grains generated during heat treatment. Precipitates have the effect of compensating for the reduction in strengthening caused by dislocations and dissolved carbon. However, when the G value is less than 2, the formation of precipitates in the heat-treated steel sheet is insufficient, or the formation of coarse precipitates in the initial steel sheet increases, while the formation of precipitates within the fine grains generated during heat treatment decreases, thus potentially leading to insufficient thermal stability. On the other hand, when the G value exceeds 20, the effect of further improving thermal stability decreases, and a large amount of high-priced alloying elements needs to be added, which may be economically disadvantageous. A G value of 3 or higher is more preferable, and a G value of 17 or lower is even more preferable.

[0095] [Relationship 2]

[0096] G=([Nb] / 93+[Mo] / 96+[V] / 51) / ([Ti] / 48)

[0097] (Where, [Nb], [Mo], [V], and [Ti] are the weight percent of the corresponding alloying elements.)

[0098] The microstructure of the steel of the present invention will be described in detail below.

[0099] Unless otherwise specified in this invention, the percentage of fine tissue is expressed as a percentage of area.

[0100] The microstructure of the steel plate according to one aspect of the invention may, in area percent, contain 60-90% martensite (including tempered martensite), 10-40% bainite and less than 5% ferrite.

[0101] Martensite is a microstructure that is unfavorable for ensuring thermal stability but is essential for ensuring initial strength. Strength can be ensured through solid solution with C and lattice distortion, but these effects disappear during heat treatment, thus potentially leading to very large changes in strength.

[0102] When the martensite content exceeds 90%, the strength change after heat treatment is significant, failing to meet the required post-heat treatment strength. However, when the martensite content is less than 60%, the initial strength cannot be guaranteed. Compared to martensite, bainite is a microstructure that is less favorable for ensuring initial strength but more favorable for strength changes after heat treatment. In this invention, the martensite content is represented by the inclusion of tempered martensite.

[0103] When the bainite content exceeds 40%, initial strength cannot be guaranteed; when the bainite content is less than 10%, the strength change after heat treatment may increase. Furthermore, a fine microstructure can contain less than 5% ferrite, but when the ferrite content exceeds 5%, it may be detrimental to ensuring initial strength.

[0104] The method for manufacturing steel according to the present invention will be described in detail below.

[0105] According to one aspect of the invention, steel can be manufactured by reheating, hot rolling, cooling and coiling a steel billet that satisfies the above alloy composition.

[0106] Reheating of slab

[0107] The steel billet that meets the above alloy composition can be reheated within a temperature range of 1150-1350℃.

[0108] When the reheating temperature is below 1150℃, precipitate-forming elements such as Nb and Ti cannot be fully dissolved again. During the heat treatment of the manufactured steel plate, the formation of precipitates is reduced, and coarse TiN remains. During continuous casting, it may be difficult to eliminate the segregation formed by diffusion. On the other hand, when the reheating temperature exceeds 1350℃, abnormal grain growth of austenite grains may lead to a decrease in strength and uneven microstructure.

[0109] Hot rolling

[0110] The reheated steel billet can be hot-rolled at a rolling termination temperature of 850-1150°C.

[0111] When the rolling termination temperature exceeds 1150°C, the temperature of the hot-rolled steel sheet increases, resulting in coarser grain sizes and potentially uneven phase transformation microstructure. Conversely, when the rolling termination temperature is below 850°C, excessive recrystallization leads to elongated grains, increased anisotropy, and potentially poorer formability. In particular, the formation of Nb carbides through strain-induced precipitation may hinder the formation of fine carbides during heat treatment.

[0112] Cooling and winding

[0113] The hot-rolled steel sheet is cooled once at a cooling rate of 60°C / second or higher to a temperature range of 300-500°C, and then cooled a second time at a cooling rate of 10-70°C / second to a temperature range of 50-200°C, and then coiled.

[0114] In this invention, in order to ensure the desired physical properties and optimize the microstructure, the cooling process can be carried out in two steps to achieve this effect.

[0115] During primary cooling, when the cooling rate is less than 60°C / second, the strength of the manufactured steel sheet may deteriorate due to the formation of ferrite. Furthermore, when the primary cooling termination temperature exceeds 500°C, ferrite formation reduces the initial strength of the steel sheet. However, when the primary cooling termination temperature is below 300°C, bainite formation is difficult, which helps ensure initial strength, but the reduction in strength after heat treatment may increase.

[0116] When cooled to a temperature range of 50-200°C via secondary cooling, self-tempering occurs, resulting in the precipitation of fine carbides. This reduces the initial tensile strength but increases the yield strength, thus exhibiting a high yield-to-tensile ratio and mitigating strength reduction during heat treatment. During secondary cooling, if the cooling termination temperature is below 50°C, self-tempering does not occur, leading to increased strength reduction after heat treatment. If the cooling termination temperature exceeds 200°C, the excessive self-tempering effect coarsens the carbides and may increase the steel's brittleness, potentially affecting the fine precipitation of Nb and Ti at high temperatures. More preferably, the secondary cooling rate can be 10-60°C / second. When the secondary cooling rate exceeds 70°C / second, the yield-to-tensile ratio is low and the initial tensile strength is high due to the absence of self-tempering, potentially increasing strength reduction after heat treatment. On the other hand, when the cooling rate is below 10°C / second, the self-tempering effect becomes excessive.

[0117] The steel of the present invention manufactured as described above has a tensile strength of 950 MPa or more, a yield strength ratio of 0.8 or more, and a tensile strength after heat treatment at 400-600°C that is more than 80% of the tensile strength before heat treatment. It has excellent thermal stability and can have a high yield strength ratio and ultra-high strength characteristics.

[0118] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are only for illustrating the present invention in a more detailed manner and are not intended to limit the scope of the present invention. Detailed Implementation

[0119] Table 1 below shows the results of calculating Equations 1 and 2 based on the alloy composition of the steel grade. For each steel grade in Table 1, steel plates are manufactured under the conditions listed in Table 2. Table 2 shows the rolling termination temperature, primary cooling termination temperature, secondary cooling termination temperature, primary cooling rate, and secondary cooling rate. The reheating temperature, not shown in Table 2, is 1250°C, and the thickness of the hot-rolled steel is manufactured to be the same for all steel grades, with a thickness of 3 mm.

[0120] [Table 1]

[0121]

[0122] [Relation 1]

[0123] K=-0.6-1.42[C]+0.05[Si]-0.16[Mn]-0.08[Cr]-0.03[Mo]+0.09[Ti]+0.08[Nb] 2

[0124] (Where [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] are the weight percent of the corresponding alloying elements.)

[0125] [Relationship 2]

[0126] G=([Nb] / 93+[Mo] / 96+[V] / 51) / ([Ti] / 48)

[0127] (Where, [Nb], [Mo], [V], and [Ti] are the weight percent of the corresponding alloying elements.)

[0128] [Table 2]

[0129]

[0130] Table 3 below shows the microstructure and mechanical and physical properties of the manufactured steel sheet. The fractions of ferrite, bainite, and martensite are measured and shown, along with the tensile strength and yield strength ratio (yield strength / tensile strength) of the manufactured steel. Here, the martensite fraction includes the fraction obtained after tempering. Furthermore, the tensile strength is measured after heat treatment of the manufactured steel sheet, and the ratio of the tensile strength after heat treatment to that before heat treatment is shown. Heat treatment is performed by heating to 500°C and holding for 15 minutes. Tensile tests are conducted using JIS 5 standard specimens taken in a direction parallel to the rolling direction. The microstructure at 1 / 4 of the thickness for each steel grade is measured, and the results are analyzed using SEM at magnifications of ×3000 and ×5000.

[0131] [Table 3]

[0132]

[0133] As shown in Table 3, the inventive steels 1 to 6, which satisfy the alloy composition and manufacturing method proposed in this invention, all ensure the mechanical properties desired in this invention.

[0134] On the other hand, the carbon content of comparative steel 1 and comparative steel 2 exceeds the scope of the present invention. Comparative steel 1 does not reach the carbon content of the present invention, thus failing to ensure the fine microstructure desired in the present invention, and therefore has insufficient tensile strength. Comparative steel 2 exceeds the carbon content and exceeds the scope of Equation 1, therefore, the tensile strength ratio before and after heat treatment cannot be satisfied.

[0135] The Mn content of comparative steel 3 and comparative steel 4 exceeds the scope of this invention. The Mn content of comparative steel 3 exceeds that of this invention and does not satisfy Relationship 1. Therefore, a fine microstructure cannot be ensured, and the yield strength ratio is also poor. The Mn content of comparative steel 4 is insufficient, making it difficult to ensure the fine microstructure proposed in this invention, and therefore the tensile strength is also insufficient.

[0136] Comparative steel 5 and comparative steel 6 did not meet the cooling conditions during the first cooling. Comparative steel 5 exceeded the range of cooling termination temperature, and comparative steel 6 had insufficient cooling rate. Therefore, they could not meet the fine structure desired in this invention and had insufficient strength.

[0137] Compared to steel 7, which exceeds the secondary cooling rate, excessive martensite formation occurs, thus failing to achieve the required yield strength ratio. The tensile strength changes significantly before and after heat treatment, therefore the tensile strength ratio does not meet the scope of this invention.

[0138] Comparing steel 8, which does not satisfy relation 2, the tensile strength changes significantly before and after heat treatment, thus failing to meet the tensile strength ratio before and after heat treatment proposed in this invention.

[0139] Compared to steel 9, due to the excessively low initial cooling termination temperature, excessive martensite formation occurred, resulting in the failure to achieve the required yield strength ratio, and the tensile strength changed excessively before and after heat treatment.

[0140] The secondary cooling termination temperature of steel 10 is lower than the temperature range proposed in this invention, thus excessive self-tempering occurs, resulting in insufficient yield strength ratio and failure to meet the tensile strength ratio before and after heat treatment.

[0141] The present invention has been described in detail above through embodiments, but embodiments in different forms are also possible. Therefore, the technical concept and scope of the claims are not limited to the embodiments.

Claims

1. A steel plate, by weight percent, comprising: C: 0.05-0.13%, Si: 0.01-0.5%, Mn: 0.8-2.0%, Cr: 0.005-1.2%, Mo: 0.001-0.5%, P: 0.001-0.02%, S: 0.001-0.01%, Al: 0.01-0.1%, N: 0.001-0.01%, Ti: 0.01-0.05%, Nb: 0.001-0.03%, V: 0.001-0.2%, B: 0.0003-0.003%, with the balance being Fe and unavoidable impurities. In relation 1 below, the value of K is defined as -1.05 or higher. In relation 2 below, the value of G is defined as 2 to 20. By area percentage, the fine microstructure comprises 60-80% martensite, 20-40% bainite, and less than 5% ferrite. The martensite includes tempered martensite. The yield strength ratio of the steel plate is 0.8 or higher. [Relation 1] K=-0.6-1.42[C]+0.05[Si]-0.16[Mn]-0.08[Cr]-0.03[Mo]+0.09[Ti]+0.08[Nb] 2 Where [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] are the weight percentages of the corresponding alloying elements. [Relation 2] G=([Nb] / 93+[Mo] / 96+[V] / 51) / ([Ti] / 48) Where [Nb], [Mo], [V] and [Ti] are the weights of the corresponding alloying elements.

2. The steel plate according to claim 1, wherein, The tensile strength of the steel plate is above 950 MPa.

3. The steel plate according to claim 1, wherein, The tensile strength of the steel plate after heat treatment at 400-600℃ is more than 80% of the tensile strength before heat treatment.

4. A method for manufacturing steel plates, comprising the following steps: The steel billet is reheated, and the billet, by weight percent, contains: C: 0.05-0.13%, Si: 0.01-0.5%, Mn: 0.8-2.0%, Cr: 0.005-1.2%, Mo: 0.001-0.5%, P: 0.001-0.02%, S: 0.001-0.01%, Al: 0.01-0.1%, N: 0.001-0.01%, Ti: 0.01-0.05%, Nb: 0.001-0.03%, V: 0.001-0.2%, B: 0.0003-0.003%, the balance being Fe and unavoidable impurities, with K values ​​defined in Equation 1 below being -1.05 or higher, and G values ​​defined in Equation 2 below being 2 to 20; The reheated steel billet is then hot-rolled; and The hot-rolled steel sheet is first cooled at a cooling rate of 60°C / second or higher to a temperature range of 300-500°C, and then second cooled at a cooling rate of 10-70°C / second to a temperature range of 50-200°C, before being coiled. [Relation 1] K=-0.6-1.42[C]+0.05[Si]-0.16[Mn]-0.08[Cr]-0.03[Mo]+0.09[Ti]+0.08[Nb] 2 Where [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] are the weight percentages of the corresponding alloying elements. [Relation 2] G=([Nb] / 93+[Mo] / 96+[V] / 51) / ([Ti] / 48) Where [Nb], [Mo], [V] and [Ti] are the weights of the corresponding alloying elements.

5. The method for manufacturing steel plate according to claim 4, wherein, The reheating temperature in the reheating step is 1150-1350℃, and the rolling termination temperature in the hot rolling step is 850-1150℃.

6. The method for manufacturing steel plate according to claim 4, wherein, The secondary cooling rate during cooling is below 60°C / second.

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