High yield ratio ultra-high strength steel sheet having excellent bending properties and method for manufacturing the same

Abstract

This invention relates to a high yield strength ratio ultra-high strength steel plate and its manufacturing method, and more specifically, to a steel plate with high strength and high yield strength ratio and excellent bending properties and its manufacturing method.

Description

Technical Field

[0001] This invention relates to a high yield strength ratio ultra-high strength steel plate with excellent bending properties and its manufacturing method. Background Technology

[0002] In recent years, in the automotive industry, developed countries, led by Europe, have been actively researching ways to reduce vehicle weight, citing fuel efficiency regulations and performance improvements. In the case of steel, to meet these lightweighting requirements from automakers, efforts are being made to achieve higher strength and further reduce steel sheet thickness at the same grade compared to competing materials (Mg, Al, CFRP, etc.). In addition to lightweighting, stricter safety regulations for vehicle passengers and pedestrians are also driving a trend towards requiring more stable and stronger body materials.

[0003] In addition, in order to improve the stability and impact characteristics of the vehicle body, the use of high-strength steel with excellent yield strength in body-in-white (BIW) structural components is increasing. These structural components are characterized by a higher yield strength to tensile strength ratio (yield strength / tensile strength), which is more conducive to absorbing impact energy.

[0004] A representative manufacturing method for improving yield strength is water cooling during continuous annealing. Ultra-high strength steel can be manufactured by rapidly cooling cold-rolled steel sheets to room temperature after two-phase or single-phase annealing, followed by tempering. In this case, the yield strength ratio is very high, but due to temperature deviations in the width and length directions, the shape quality of the coil deteriorates, and problems such as material defects and reduced machinability may occur in different areas when processing roll-formed parts. Furthermore, generally, as the strength of the steel sheet increases, the elongation decreases, resulting in reduced formability and thus limiting its application as a material for cold stamping.

[0005] To overcome the aforementioned problems, a hot press forming (HPF) process is being developed. This process involves forming the material at a relatively easy-to-form high temperature, followed by water cooling between the mold and the material to ensure the required strength. Because it ensures high strength compared to the same thickness, the HPF process is widely used in component manufacturing. However, its application is hampered by high equipment investment costs and increased process costs. Therefore, there is a need to develop a material suitable for cold stamping. Consequently, there is a need to develop a cold-rolled steel sheet suitable for use as a material for cold stamping, possessing high strength and a high yield strength ratio to ensure impact performance, and excellent bending properties. Summary of the Invention

[0006] Technical problems to be solved

[0007] According to one aspect of the present invention, the object is to provide a high yield strength ratio ultra-high strength steel plate with excellent bending properties and a method for manufacturing the same.

[0008] The technical problems of this invention are not limited to those described above. Those skilled in the art can readily understand additional technical problems of this invention from the entire contents of this specification.

[0009] Technical solution

[0010] One aspect of the present invention provides a steel plate, by weight percent, comprising: carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.03%, aluminum (Al): 0.01-0.5%, the balance being Fe and unavoidable impurities, wherein the R value of the steel plate, as defined in Equation 1 below, is 0.12 to 0.27, and the steel plate per 1 μm 2 The average number of carbides per area is less than 40, the average length of the long axis of the carbides is less than 300 nm, and the yield strength ratio of the steel plate exceeds 0.73.

[0011] [Relation 1]

[0012]

[0013]

[0014]

[0015] (Where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu], and [Ni] are weight percent of each element.)

[0016] The steel plate may further contain two or more of the following: 0.01-0.2% chromium (Cr), 0.01-0.2% molybdenum (Mo), and less than 0.005% (excluding 0%) boron (B).

[0017] The steel plate may further contain one or more of titanium (Ti) at less than 0.1% (excluding 0%) and niobium (Nb) at less than 0.1% (excluding 0%).

[0018] The steel plate may contain more than 99% by area martensite or tempered martensite as a fine microstructure.

[0019] The tensile strength of the steel plate can be above 1300MPa, and the bending characteristic (R / t) can be less than 4 (where R is the minimum bending radius at which no cracks are generated in the bent part after a 90° bending test, and t is the thickness of the steel plate).

[0020] Another aspect of the present invention provides a method for manufacturing a steel sheet, the method comprising the steps of: preparing a cold-rolled steel sheet, wherein the cold-rolled steel sheet comprises, by weight %,: carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.03%, aluminum (Al): 0.01-0.5%, the balance being Fe and unavoidable impurities, wherein the R value of the cold-rolled steel sheet as defined in the following relation 1 is 0.12 to 0.27; The cold-rolled steel sheet is heat-treated at a temperature above Ac3 for at least 30 seconds; after the heat treatment, it is cooled once at an average cooling rate of 1-10°C / second to a temperature range of 500-750°C; the steel sheet that has been cooled once is cooled a second time at an average cooling rate of 20-80°C / second to a temperature below Ms-190°C; and then a reheating and over-aging step is performed, in which the steel sheet that has been cooled twice is heated to a temperature range exceeding the second cooling termination temperature +30°C and below 270°C, and held for 1-20 minutes.

[0021] [Relation 1]

[0022]

[0023]

[0024]

[0025] (Where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu], and [Ni] are weight percent of each element.)

[0026] The cold-rolled steel sheet may further contain two or more of the following: 0.01-0.2% chromium (Cr), 0.01-0.2% molybdenum (Mo), and less than 0.005% (excluding 0%) boron (B).

[0027] The cold-rolled steel sheet may further contain one or more of titanium (Ti) at less than 0.1% (excluding 0%) and niobium (Nb) at less than 0.1% (excluding 0%).

[0028] The steps for preparing the cold-rolled steel sheet may include the following steps: reheating the billet to a temperature range of 1100-1300°C; hot rolling the reheated billet at a hot finishing temperature of Ar3 or higher; cooling the hot-rolled steel sheet to a temperature range of below 700°C and coiling it; and cold rolling the cooled and coiled steel sheet at a reduction rate of 30-80%.

[0029] The method may further include the step of pickling the cooled and coiled steel sheet with hydrochloric acid.

[0030] Invention Effects

[0031] According to one aspect of the present invention, a steel plate having high strength and high yield strength ratio and excellent bending properties, and a method thereof, can be provided.

[0032] According to another aspect of the present invention, a steel sheet that can be used as a structural component of a body-in-white (BIW) and a method thereof can be provided. Attached Figure Description

[0033] Figure 1 (a) and (b) are SEM images (×10.000) of fine tissues of Invention Example 15 and Comparative Example 21 according to an embodiment of the present invention.

[0034] Best practice

[0035] 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.

[0036] The present invention will now be described in detail.

[0037] In this invention, the alloy composition and process conditions were optimized to provide a steel sheet with high strength, high yield strength ratio, and excellent bending properties. Specifically, the inventors confirmed that by strictly controlling the content of constituent elements such as C, Mn, Si, P, and S, and optimizing the conditions of the secondary cooling, reheating, and over-aging processes during continuous annealing, it is possible to ensure both basic weldability and bending properties, thereby completing this invention.

[0038] The composition of the steel of the present invention will be described in detail below.

[0039] Unless otherwise specified, the percentage of each element in this invention is based on weight.

[0040] According to one aspect of the invention, the steel may contain, by weight percent: carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.03%, aluminum (Al): 0.01-0.5%, balance Fe and unavoidable impurities.

[0041] Carbon (C): 0.1-0.3%

[0042] Carbon (C) is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. It is also an element that must be added to ensure the strength of martensitic steel. To obtain ultra-high strength steel that meets the target yield strength ratio and tensile strength of the present invention, it is preferable to add 0.1% or more of carbon (C), more preferably 0.12% or more of carbon (C). However, when the carbon (C) content exceeds 0.3%, the strength of the martensite may increase, but carbides may easily form during continuous annealing, and the material may easily coarsen, thus potentially leading to reduced ductility and poor bending properties. Furthermore, increasing the carbon (C) content can impair weldability; therefore, it is preferable to limit the upper limit of the carbon (C) content to 0.3%. More preferably, the upper limit of the carbon (C) content may be 0.28%.

[0043] Manganese (Mn): 1.0-2.3%

[0044] Manganese (Mn) is an element that readily ensures martensite formation by suppressing ferrite formation and promoting austenite formation in multiphase steel. However, when the manganese (Mn) content exceeds 2.3%, manganese (Mn) segregates in the thickness direction, easily forming manganese bands in the slab, thus leading to problems such as increased continuous casting cracks and defects during the rolling process. Therefore, more preferably, the manganese (Mn) content can be 2.1% or less. On the other hand, when the manganese (Mn) content is below 1.0%, it is difficult to ensure the strength of ultra-high strength steel, so the lower limit of the manganese (Mn) content can be limited to 1.0%. More preferably, the lower limit of the manganese (Mn) content can be 1.4%.

[0045] Silicon (Si): 0.05-1.0%

[0046] Silicon (Si) in martensitic steel plays a role in suppressing carbide formation and controlling carbide size during reheating and overaging after cooling. Therefore, the lower limit of the silicon (Si) content can be limited to 0.05%. More preferably, the silicon (Si) content can be 0.09% or more. However, silicon (Si) is an element that stabilizes ferrite. When the silicon (Si) content exceeds 1.0%, ferrite may form during cooling in a continuous annealing furnace, which may reduce strength. In addition, since Si-based oxides may form in the heating furnace, surface oxidation may occur. Therefore, the upper limit of the silicon (Si) content can be limited to 1.0%. More preferably, the upper limit of the silicon (Si) content can be limited to 0.6%.

[0047] Phosphorus (P): less than 0.1%

[0048] Phosphorus (P) is an impurity element contained in steel, and considering its unavoidable presence during the manufacturing process, its content is excluded as 0%. However, when the phosphorus (P) content exceeds 0.1%, weldability may deteriorate and it may cause brittleness in the steel; therefore, the upper limit of the phosphorus (P) content can be limited to 0.1%. More preferably, the upper limit of the phosphorus (P) content can be 0.03%.

[0049] Sulfur (S): less than 0.03%

[0050] Like phosphorus (P), sulfur (S) is an unavoidable impurity in steel and an element that impairs the ductility and weldability of steel sheets. Therefore, it is preferable to control the sulfur (S) content to a low level as much as possible, and thus preferably to limit the sulfur (S) content to 0.03% or less. More preferably, the sulfur (S) content can be limited to 0.005% or less. Furthermore, considering the unavoidable inclusion during the manufacturing process, 0% is excluded.

[0051] Aluminum (Al): 0.01-0.5%

[0052] Aluminum (Al) can be added to remove oxygen from molten steel, and like Si, aluminum (Al) is an element that stabilizes ferrite. Furthermore, aluminum (Al) is a component that can improve the hardenability of the final martensitic steel by increasing the carbon content in austenite; therefore, the aluminum (Al) content is preferably 0.01% or more. However, when the aluminum (Al) content exceeds 0.5%, ferrite may form during cooling in a continuous annealing furnace, potentially reducing strength. In addition, the formation of AlN may cause slab cracking and impair hot rollability; therefore, the upper limit of the aluminum (Al) content can be limited to 0.5%.

[0053] In addition to the above-described composition, the steel of this invention may contain a balance of iron (Fe) and unavoidable impurities. Unavoidable impurities may be unintentionally introduced during conventional manufacturing processes, and therefore cannot be eliminated. These impurities are well known to those skilled in the art of conventional steel manufacturing, and therefore their contents are not specifically described herein.

[0054] According to one aspect of the invention, the steel may further contain two or more of 0.01-0.2% chromium (Cr), 0.01-0.2% molybdenum (Mo), and less than 0.005% (excluding 0%) boron (B).

[0055] Chromium (Cr): 0.01-0.2%

[0056] Chromium (Cr) is an additive component that improves the hardenability of steel and ensures high strength. Chromium (Cr) inhibits the formation of bainite, and is therefore useful for manufacturing ultra-high strength steel with pure martensite. Therefore, to ensure the above effects, it is preferable to add 0.01% or more of chromium (Cr). However, when the chromium (Cr) content is too high, there is a problem of increased cost of alloy iron; therefore, the upper limit of the chromium (Cr) content can be limited to 0.2%, and more preferably, the upper limit of the chromium (Cr) content can be limited to 0.1%.

[0057] Molybdenum (Mo): 0.01-0.2%

[0058] Like Cr, molybdenum (Mo) is an element that improves the hardenability of steel. To achieve this effect, it is preferable to add more than 0.01% molybdenum (Mo). However, when the molybdenum (Mo) content exceeds 0.2%, the amount of alloy added is excessive, leading to an increase in the cost of alloyed iron. Therefore, it is preferable to limit the upper limit of the molybdenum (Mo) content to 0.2%, and more preferably, to 0.1%.

[0059] Boron (B): less than 0.005% (except 0%)

[0060] Boron (B) is an element that inhibits the transformation of austenite to ferrite during continuous annealing, and like Cr and Mo, boron (B) is an element that can effectively improve the hardenability of martensite even when added in very small amounts. However, when the boron (B) content exceeds 0.005%, due to Fe... 23 The (B,C)6 precipitate precipitates at the austenite grain boundaries, thereby promoting the formation of ferrite. Therefore, it is preferable to limit the upper limit of the boron (B) content to 0.005%.

[0061] The steel according to one aspect of the invention may further contain one or more of titanium (Ti) at less than 0.1% (except 0%) and niobium (Nb) at less than 0.1% (except 0%).

[0062] Titanium (Ti): Less than 0.1% (except 0%)

[0063] Titanium (Ti) is a fine carbide-forming element and helps ensure yield strength and tensile strength. Furthermore, titanium (Ti) scavenges nitrogen in steel by causing it to precipitate as TiN. Therefore, it is preferable to add titanium (Ti) with a stoichiometric equivalent of 48 / 14 * [N] or higher. When adding boron (B), titanium (Ti) is preferred to maximize its effect. However, when the titanium (Ti) content exceeds 0.1%, coarse carbides precipitate, reducing the carbon content in the steel. This may decrease strength and elongation, and could cause nozzle clogging during continuous casting. Therefore, it is preferable to limit the titanium (Ti) content to an upper limit of 0.1%.

[0064] Niobium (Nb): Less than 0.1% (except 0%)

[0065] Niobium (Nb) segregates at austenite grain boundaries, suppressing austenite grain coarsening during annealing heat treatment and forming fine carbides, thus contributing to increased strength. However, when the niobium (Nb) content exceeds 0.1%, the precipitation of coarse carbonitrides increases, the carbon content in the steel decreases, and therefore strength and elongation may decrease. Furthermore, it may lead to reduced machinability of the base material and increased manufacturing costs. Therefore, it is preferable to limit the niobium (Nb) content to an upper limit of 0.1%.

[0066] According to one aspect of the invention, the value of R defined in the following relation 1 for steel can be from 0.12 to 0.27.

[0067] Equation 1 is a composite equation of Ceq1 and Ceq2 representing the welding characteristics based on the content of each element. When the R value of Equation 1 is between 0.12 and 0.27, the physical properties desired by the present invention, including welding characteristics, can be ensured.

[0068] When the R value defined in Equation 1 is less than 0.12, it is difficult to ensure the strength required by the present invention. On the other hand, when the R value exceeds 0.27, the physical properties, especially the weldability, may decrease. In the present invention, a lower limit of R value is more preferably 0.17, an upper limit of R value is more preferably 0.25, and even more preferably 0.20.

[0069] [Relation 1]

[0070]

[0071]

[0072]

[0073] (Where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu], and [Ni] are weight percent of each element.)

[0074] The microstructure of the steel of the present invention will be described in detail below.

[0075] Unless otherwise specified, the percentage of fine tissue in this invention is based on area.

[0076] According to one aspect of the invention, the steel may contain more than 99% by area martensite or tempered martensite as a fine microstructure, per 1 μm 2 The number of carbides in the area can be less than 40, and the average length of the long axis of the carbides can be less than 300 nm.

[0077] In this invention, in order to ensure that the cold-rolled steel sheet has high strength and high yield strength ratio, it may contain martensite or tempered martensite as a fine structure, and in order to ensure a high strength level of 1.3G or above, it is preferable to contain more than 99% martensite or tempered martensite.

[0078] In addition, to ensure excellent bending properties, the number of carbides is preferably controlled to be less than 40, and more preferably less than 35.

[0079] In addition, in order to more effectively ensure the above-mentioned effects, the average length of the long axis of the carbide is preferably less than 300 nm, and more preferably less than 200 nm.

[0080] The number of carbides in this invention is expressed as 1 μm in a ×10000 SEM image. 2 The average number of carbides in the region (average of 10 regions), and the length of the long axis of the carbides is the length of the long axis measured and shown in the TEM bright field image from ×30000 to ×100000.

[0081] The method for manufacturing steel according to the present invention will be described in detail below.

[0082] According to one aspect of the invention, steel can be manufactured by heat treatment, primary cooling, secondary cooling, reheating, and over-aging of cold-rolled steel sheets that satisfy the above-mentioned alloy composition.

[0083] Preparing cold-rolled steel sheets

[0084] Cold-rolled steel sheets with alloy compositions that satisfy the present invention can be prepared.

[0085] The cold-rolled steel sheet of the present invention can be manufactured under conventional process conditions, preferably by reheating, hot rolling, cooling, coiling and cold rolling of the steel billet under the conditions described below.

[0086] Reheating

[0087] The steel billet with the alloy composition that meets the requirements of this invention can be reheated to a temperature range of 1100-1300°C.

[0088] Reheating can be performed to ensure the smooth execution of subsequent hot rolling processes and to fully guarantee the desired physical properties. When the reheating temperature is below 1100°C, there may be a problem of rapid increase in hot rolling load. When the reheating temperature exceeds 1300°C, the amount of surface oxide scale increases, which may reduce the yield of the material and cause surface defects, thus potentially having an adverse effect on the final quality.

[0089] Hot rolling

[0090] The reheated steel billet can be hot rolled at a hot finishing temperature of Ar3 or higher.

[0091] In this invention, the hot finishing temperature can be limited to above Ar3 (the temperature at which ferrite begins to appear when austenite is cooled) because, at temperatures below Ar3, rolling is carried out in the two-phase region of ferrite and austenite or in the ferrite region, which may result in a mixed structure and may lead to mis-rolling due to variations in hot rolling load.

[0092] Cooling and winding

[0093] The hot-rolled steel sheet can be cooled to a temperature range below 700°C before being coiled.

[0094] When the coiling temperature exceeds 700°C, excessive oxide film may form on the surface of the steel sheet, potentially leading to defects. As the coiling temperature decreases, the strength of the hot-rolled steel sheet increases, resulting in an increased rolling load for the subsequent cold rolling process. However, this is not a factor that would prevent practical production, and therefore, no lower limit is specifically imposed in this invention.

[0095] In addition, in this invention, before cold rolling as a subsequent process, the oxide layer formed on the surface of the coiled steel sheet can be removed by pickling.

[0096] cold rolling

[0097] The cooled and coiled steel sheet can be cold rolled at a reduction rate of 30-80%.

[0098] When the cold rolling reduction is less than 30%, it is not only difficult to ensure the target thickness, but also the residual hot-rolled grains may affect the formation of austenite and the final physical properties during annealing heat treatment. On the other hand, when the reduction exceeds 80%, due to work hardening during cold rolling, the reduction amount rolled along the length and width directions is uneven, which may lead to material deviations in the final steel sheet, and the target thickness may be difficult to ensure due to the rolling load.

[0099] Heat treatment

[0100] The cold-rolled steel sheet can be heat-treated at a temperature above Ac3 for more than 30 seconds.

[0101] In this invention, heat treatment is performed to ensure a 100% austenite fraction through annealing in the austenite single-phase region. By ensuring a 100% austenite fraction through this heat treatment, the strength reduction caused by ferrite formation during annealing can be prevented.

[0102] Ac3=910-203√([C])-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]

[0103] (Where [C], [Ni], [Si], [V], [Mo], and [W] are weight percent of each element.)

[0104] One cooling

[0105] After the heat treatment, it can be cooled once at an average cooling rate of 1-10℃ / second to a temperature range of 500-750℃.

[0106] When the cooling rate during the first cooling is less than 1°C / second, it may be difficult to ensure the target strength because ferrite is generated during cooling. On the other hand, when the cooling rate during the first cooling exceeds 10°C / second, the average cooling rate during the second cooling decreases, thereby increasing the fraction of low-temperature phase transformation phases other than martensite, which may also make it difficult to ensure the final target strength.

[0107] When the temperature during primary cooling is below 500°C, phases such as ferrite and bainite are formed, which may reduce the strength. When the temperature during primary cooling exceeds 750°C, problems may occur in the actual production line.

[0108] Secondary cooling

[0109] The steel plate that has been cooled once can be cooled a second time at an average cooling rate of 20-80℃ / second to a temperature below Ms-190℃.

[0110] In this invention, to ensure a martensite or tempered martensite content of over 99%, the secondary cooling is preferably performed rapidly to below the martensite finish temperature (Mf). Specifically, this is preferably achieved to a temperature below Ms-190°C. In this invention, to ensure the formation of a sufficiently hard martensite structure and the resulting increase in yield strength due to carbide precipitation during tempering, the secondary cooling finish temperature is limited to below Ms-190°C. Furthermore, as the tempering temperature increases, the flexural properties may deteriorate; therefore, by limiting the secondary cooling finish temperature, sufficient tempering can be performed without significantly increasing the tempering temperature, thereby ensuring flexural properties. When the cooling finish temperature exceeds Ms-190°C, the martensite or tempered martensite fraction cannot be sufficiently ensured, making it difficult to guarantee the desired physical properties.

[0111] In addition, when the average cooling rate during secondary cooling is less than 20°C / second, some bainite structure may be formed when secondary cooling starts from the primary cooling zone. When the average cooling rate during secondary cooling exceeds 80°C / second, the rapid martensitic phase transformation rate during secondary cooling may cause problems such as deterioration of the surface shape of the steel plate and material deviation in the width direction.

[0112] Ms=539-423[C]-30.4[Mn]-16.1[Si]-59.9[P]+43.6[Al]-17.1[Ni]-12.1[Cr]+7.5[Mo]

[0113] (Where [C], [Mn], [Si], [P], [Al], [Ni], [Cr], and [Mo] are weight percent of each element.)

[0114] Reheating and aging

[0115] Reheating and over-aging can be performed, wherein the steel plate that has been cooled twice is heated to a temperature range exceeding the second cooling termination temperature by 30°C but below 270°C and held for 1-20 minutes.

[0116] This invention aims to improve toughness by transforming the hard martensite with high dislocation density formed during secondary cooling into tempered martensite through reheating and overaging. In this invention, to ensure sufficient tempering effect, the lower limit of the reheating temperature is limited to a temperature 30°C or higher than the secondary cooling termination temperature. At this temperature, the yield strength increases due to the formation of fine carbides, and the desired effect is difficult to achieve when the reheating and overaging temperatures are below 30°C. On the other hand, when the reheating and overaging temperatures are above 270°C, the bending characteristics deteriorate due to the coarsening of the carbides.

[0117] In addition, when the holding time is less than 1 minute, the martensite cannot be fully transformed into tempered martensite, so it is difficult to ensure toughness. When the holding time exceeds 20 minutes, the carbides generated due to over-aging may become coarse, which may have an adverse effect on bending properties and material properties.

[0118] The steel of the present invention manufactured as described above has a tensile strength of 1300 MPa or more, a yield strength ratio of more than 0.73, and a bending characteristic (R / t) of less than 4 (where R is the bending radius at which no crack is generated in the bent part after a 90° bending test, and t is the thickness of the steel plate). Therefore, it can have both a high yield strength ratio and excellent bending characteristics.

[0119] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention for more detailed explanation and do not limit the scope of the invention. Detailed Implementation

[0120] (Example)

[0121] A steel billet with the composition shown in Table 1 is heated to 1100-1300℃, hot-rolled at 850-950℃ (Ar3 or higher), coiled at 400-700℃, and cold-rolled into a sheet using a 45-65% cold-rolling reduction. Next, it is heat-treated at 800-900℃ for 100-400 seconds, followed by primary and secondary cooling under the conditions described in Table 2. The primary cooling rate is 2-4℃ / second, and the secondary cooling rate is 25-60℃ / second. Then, it is reheated under the conditions in Table 2 and aged for 1-20 minutes to produce the steel sheet.

[0122] Furthermore, in Table 1 below, the values ​​of Ac3, Ms, temperature, and Equation 1 are calculated and shown based on the content of each element.

[0123] [Table 1]

[0124]

[0125] Ac3=910-203√([C])-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]

[0126] (Where [C], [Ni], [Si], [V], [Mo], and [W] are weight percent of each element.)

[0127] Ms=539-423[C]-30.4[Mn]-16.1[Si]-59.9[P]+43.6[Al]-17.1[Ni]-12.1[Cr]+7.5[Mo]

[0128] (Where [C], [Mn], [Si], [P], [Al], [Ni], [Cr], and [Mo] are weight percent of each element.)

[0129] [Relation 1]

[0130]

[0131]

[0132]

[0133] (Where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu], and [Ni] are weight percent of each element.)

[0134] [Table 2]

[0135]

[0136] Table 3 below shows the microstructure of each specimen, along with the measured physical properties. The microstructure was confirmed using SEM images; the amount of carbides is expressed as per 1 μm in a ×10000 SEM image. 2 The average number of carbides in the region (average of 10 regions) and the length of the long axis of the carbides were measured and shown in TEM bright-field images ranging from ×30,000 to ×100,000. Furthermore, the values ​​of yield strength (YS), tensile strength (TS), yield ratio (YS / TS), total elongation (T-El), and uniform elongation (U-El) were measured by performing tensile tests on continuously annealed cold-rolled steel sheets at a test speed of 28 mm / min according to JIS standards (gauge length width × length: 25 × 50 mm, total specimen length: 200-260 mm). In addition, for bending characteristics (R / t), the same cold-rolled steel sheet was processed into specimens with a width of 100 mm and a length of 30 mm. A 90° bending test was performed at a test speed of 100 mm / min. Cracks in the bent part were then confirmed using a microscope. The R / t value was obtained by dividing the minimum bending radius (R) that does not produce cracks by the thickness (t) of the specimen. When the R / t value is less than 4, it is represented as 0, and when the R / t value is greater than 4, it is represented as X.

[0137] [Table 3]

[0138]

[0139]

[0140] *M: Martensite, TM: Tempered Martensite.

[0141] As shown in Table 3, Examples 1 to 25 of the invention, which satisfy the alloy composition and manufacturing conditions of the present invention, meet the characteristics of fine structure and carbides proposed in the present invention and ensure the target physical properties of the present invention.

[0142] On the other hand, Comparative Examples 1, 2, 4, 5, 7 and 8, whose secondary cooling termination temperature does not meet the conditions of the present invention (Ms-190°C or below), do not meet the target yield strength ratio and bending characteristics of the present invention, and their tensile strength does not reach the target tensile strength.

[0143] Specifically, Comparative Examples 1 to 9 are examples that do not include a reheating step. While quenching and tempering are necessary processes in this invention, these examples are examples of aging performed at the temperature during the cooling process without reheating. That is, in these examples, the martensitic hardenability may be reduced, and the yield strength is very poor due to the lack of a tempering process, thus failing to obtain the target strength.

[0144] Furthermore, in Comparative Examples 10 to 21, which did not meet the upper or lower limit conditions proposed in this invention during reheating and overaging, the target yield strength ratio and flexural characteristics of this invention were poor. In particular, when the lower limit was not met, a sufficient increase in yield strength could not be achieved. In examples below 270°C, which did not meet the upper limit temperature conditions for reheating and overaging, flexural characteristics could not be ensured due to the formation of coarse carbides.

[0145] Comparative Examples 22 and 23 are examples that satisfy all the manufacturing conditions proposed in this invention but do not satisfy the alloy composition proposed in this invention. Therefore, in these examples, not only is the target fineness fraction not met, but the target strength cannot be ensured as well.

[0146] Figure 1 (a) and (b) are SEM images (×10.000) of fine tissues of Invention Example 15 and Comparative Example 21 according to an embodiment of the present invention. Figure 1 Both (a) and (b) show tempered martensite as a fine structure, and it can be confirmed that rice-grain-shaped carbides are formed on the fine structure. On the other hand, in the case of (b), it can be confirmed that the number of carbides formed per unit area on the fine structure exceeds the scope proposed in this invention, and the size formed is also too large.

[0147] The present invention has been described in detail above through embodiments, but other embodiments 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: carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.03%, aluminum (Al): 0.01-0.5%, with the balance being Fe and unavoidable impurities. The R value defined in Equation 1 for the steel plate is between 0.12 and 0.

27. The average number of carbides per 1 μm 2 of area of the steel sheet is 40 or less, and the average length of the long axis of the carbides is 300 nm or less, The steel plate has a fine microstructure containing more than 99% martensite or tempered martensite by area. The yield strength ratio of the steel plate exceeds 0.

73. [Relation 1] in, [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu], and [Ni] represent the weights of each element.

2. The steel plate according to claim 1, wherein, The steel plate further comprises two or more of the following: 0.01-0.2% chromium (Cr), 0.01-0.2% molybdenum (Mo), and less than 0.005% but less than 0% boron (B).

3. The steel plate according to claim 1, wherein, The steel plate further comprises one or more of titanium (Ti) at less than 0.1% and excluding 0% and niobium (Nb) at less than 0.1% and excluding 0%.

4. The steel plate according to claim 1, wherein, The steel plate has a tensile strength of 1300 MPa or higher and a bending characteristic R / t of less than 4, where R is the minimum bending radius at which no cracks are generated in the bent part after a 90° bending test, and t is the thickness of the steel plate.

5. A method for manufacturing a steel plate, comprising the following steps: Prepare cold-rolled steel sheets, which, by weight percent, comprise: carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.03%, aluminum (Al): 0.01-0.5%, balance Fe and unavoidable impurities, wherein the R value defined in the following relationship 1 of the cold-rolled steel sheet is 0.12 to 0.27; The cold-rolled steel sheet is heat-treated at a temperature above Ac3 for at least 30 seconds. After the heat treatment, the material is cooled once at an average cooling rate of 1-10℃ / second to a temperature range of 500-750℃. The steel plate, which had been cooled once, was then subjected to a secondary cooling process at an average cooling rate of 20-80°C / second, cooling it to a temperature below Ms-190°C; and The reheating and over-aging steps involve heating the secondary-cooled steel plate to a temperature range exceeding the secondary cooling termination temperature by 30°C but below 270°C, and holding it for 1-20 minutes. [Relation 1] Where [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu], and [Ni] represent the weight of each element.

6. The method for manufacturing steel plate according to claim 5, wherein, The cold-rolled steel sheet further comprises two or more of the following: 0.01-0.2% chromium (Cr), 0.01-0.2% molybdenum (Mo), and less than 0.005% but less than 0% boron (B).

7. The method for manufacturing steel plate according to claim 5, wherein, The cold-rolled steel sheet further comprises one or more of titanium (Ti) at less than 0.1% and excluding 0% and niobium (Nb) at less than 0.1% and excluding 0%.

8. The method for manufacturing steel plate according to claim 5, wherein, The steps for preparing the cold-rolled steel sheet include the following: The steel billet is reheated to a temperature range of 1100-1300℃; The reheated steel billet is hot-rolled at a hot finishing temperature of Ar3 or higher; The hot-rolled steel sheet is cooled to a temperature range below 700°C and then coiled. as well as The cooled and coiled steel sheet is cold-rolled at a reduction rate of 30-80%.

9. The method for manufacturing a steel plate according to claim 8, wherein, The method further includes the step of pickling the cooled and coiled steel sheet with hydrochloric acid.

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