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

Abstract

The present invention relates to an ultra-high strength steel plate with a high yield ratio and a manufacturing method thereof, and more particularly to a steel plate having high strength, a high yield ratio and excellent bending properties and a manufacturing method thereof.

Description

Technical Field

[0001] The present invention relates to a high yield ratio ultra-high strength steel sheet having excellent bending characteristics and a method for manufacturing the same.

Background Art

[0002] In recent years, in the automotive field, in developed countries led by Europe, research has been actively conducted to reduce the weight of vehicle bodies due to fuel consumption regulations and performance improvements. In the case of steel, in order to meet the requirements of such automotive manufacturers for weight reduction, efforts have been made to increase the strength at the same grade and further reduce the thickness of steel sheets compared to competing materials (Mg, Al, CFRP, etc.). In addition to weight reduction, due to the strengthening of safety regulations for vehicle passengers and pedestrians, there is a tendency to also require the stability and high strength of vehicle body materials.

[0003] On the other hand, in order to improve the stability and impact characteristics of vehicle bodies, the adoption of high-strength steel with excellent yield strength in BIW (Body-in-white) structural members has been increasing. Such structural members have the characteristic that the higher the yield ratio (yield strength / tensile strength), that is, the ratio of the yield strength to the tensile strength, the more advantageous it is for absorbing impact energy.

[0004] As a typical manufacturing method for increasing the yield strength, there is a method that utilizes water cooling during continuous annealing. After annealing a cold-rolled steel sheet in the two-phase region or single-phase region and then rapidly cooling it to room temperature level, an ultra-high strength steel can be manufactured by a tempering method. However, in such a case, although the yield ratio is very high, there is a problem that the shape quality of the coil deteriorates due to temperature deviation in the width direction and length direction, and problems such as material defects corresponding to the部位 and a decrease in workability may occur during the processing of roll forming parts. In addition, generally, as the strength of the steel sheet increases, the elongation rate decreases and the formability decreases, so the application as a material for cold stamping is limited.

[0005] To overcome the above problems, hot press forming (HPF method) has been developed, which involves forming the material at a relatively high temperature where forming is easier, and then securing the required strength by water cooling between the die and the material. Because it can secure higher strength for the same thickness, the HPF method is widely used in the manufacture of parts. However, excessive capital investment costs and increased process costs present problems in its application, and there is a need for the development of materials for cold stamping. Therefore, there is a need for the development of cold-rolled steel sheets that are suitable for use as materials for cold stamping, have high strength and a high yield ratio to ensure impact performance, and have excellent bending properties. [Overview of the project] [Problems that the invention aims to solve]

[0006] One embodiment of the present invention aims to provide a high yield ratio ultra-high strength steel sheet with excellent bending properties and a method for manufacturing the same.

[0007] The problems that the present invention addresses are not limited to those described above. A person of ordinary skill should have no difficulty understanding further problems that the present invention addresses from the overall content of this specification. [Means for solving the problem]

[0008] According to one embodiment of the present invention, the material contains, by weight percent, carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01-0.5%, with the remainder being Fe and unavoidable impurities. The R-value defined by the following relational equation 1 is between 0.12 and 0.27. 1 μm 2 The average number of carbides per unit area is 40 or less, and the average length of the long axis of the carbides is 300 nm or less. We can provide steel plates with a yield ratio exceeding 0.73.

[0009]

number

[0010] The above steel sheet may further contain two or more of the following: chromium (Cr): 0.01-0.2%, molybdenum (Mo): 0.01-0.2%, and boron (B): 0.005% or less (excluding 0%).

[0011] The above steel sheet may further contain one or more of the following: titanium (Ti): 0.1% or less (excluding 0%), niobium (Nb): 0.1% or less (excluding 0%).

[0012] The above steel sheet may contain 99% or more of martensite or tempered martensite as its microstructure.

[0013] The above-mentioned steel plate may have a tensile strength of 1300 MPa or more and a bending characteristic (R / t) of less than 4 (where R is the minimum bending radius at which no cracks occur in the bent portion after a 90° bending test, and t is the thickness of the steel plate).

[0014] According to another embodiment of the present invention, the process involves preparing a cold-rolled steel sheet containing, by weight, carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01-0.5%, the remainder being Fe and unavoidable impurities, and having an R value of 0.12-0.27 as defined by the following relational formula 1, The above cold-rolled steel sheet is heat-treated at a temperature of Ac3 or higher for 30 seconds or more. After the above heat treatment, the process involves a primary cooling step to a temperature range of 500-750°C at an average cooling rate of 1-10°C / s, The above-mentioned first-cooled steel plate is second-cooled to a temperature of Ms-190°C or lower at an average cooling rate of 20-80°C / s, It is possible to provide a method for manufacturing a steel sheet including a reheating and overaging step of heating the above-mentioned secondarily cooled steel sheet to a temperature range exceeding the secondary cooling end temperature + 30°C and less than 270°C and holding for 1 to 20 minutes.

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

[0016] The cold-rolled steel sheet can further contain two or more of the following: chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2%, boron (B): 0.005% or less (excluding 0%).

[0017] The cold-rolled steel sheet can further contain one or more of the following: titanium (Ti): 0.1% or less (excluding 0%), niobium (Nb): 0.1% or less (excluding 0%). [[ID=IS]]<00OO081>

[0018] The step of preparing the cold-rolled steel sheet includes a step of reheating the steel slab to a temperature range of 1100 to 1300°C, a step of hot-rolling the reheated steel slab at a finishing hot-rolling temperature of Ar3 or higher, a step of cooling and coiling the hot-rolled steel sheet in a temperature range of 700°C or lower, [[ID=ZS]] a step of cold-rolling the cooled and coiled steel sheet at a reduction rate of 30 to 80%, and can include. !D=31]]

[0019] The step of pickling the cooled and coiled steel sheet with hydrochloric acid can further be included. [Advantages of the Invention]

[0020] According to an embodiment of the present invention, it is possible to provide a steel sheet having high strength and a high yield ratio and excellent bending characteristics, and a method for manufacturing the same.

[0021] According to another embodiment of the present invention, a steel plate applicable to BIW (Body-in-white) structural members and a method for manufacturing the same can be provided. [Brief explanation of the drawing]

[0022] [Figure 1] (a) and (b) are SEM microstructure images (x10.000) of Example 15 and Comparative Example 21 according to the embodiment of the present invention. [Modes for carrying out the invention]

[0023] Preferred embodiments of the present invention are described below. Embodiments of the present invention can be modified in various ways, and the scope of the invention should not be construed as being limited to the embodiments described below. These embodiments are provided to further illustrate the present invention to those ordinary in the art in which the invention pertains.

[0024] The present invention will be described in detail below.

[0025] In this invention, the alloy composition and process conditions were optimized to provide a steel sheet with high strength, a high yield ratio, and excellent bending properties. In particular, the inventors confirmed that bending properties and high strength could be ensured while ensuring basic welding properties by strictly controlling the content of constituent elements such as C, Mn, Si, P, and S, and by optimizing the conditions of the secondary cooling and reheating of continuous annealing and the overaging process, thus completing the present invention.

[0026] The steel composition of the present invention will be described in detail below.

[0027] Unless otherwise specified in this invention, the percentages representing the content of each element are based on weight.

[0028] The steel according to one embodiment of the present invention 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): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01-0.5%, with the remainder being Fe and unavoidable impurities.

[0029] Carbon (C): 0.1~0.3% Carbon (C) is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. It is an essential element that must be added to ensure the strength of martensitic steel. In order to obtain ultra-high-strength steel that satisfies the target yield ratio and tensile strength in this invention, it is preferable to add 0.1% or more of carbon (C), and more preferably 0.12% or more. However, if the content exceeds 0.3%, the strength of the martensite may increase, but carbide formation is easy during the continuous annealing process, leading to coarsening and potentially poor bending properties along with a decrease in ductility. Furthermore, since increasing the carbon (C) content has the problem of impeding weldability, it is preferable to limit the upper limit to 0.3%. More preferably, the upper limit may be 0.28%.

[0030] Manganese (Mn): 1.0~2.3% Manganese (Mn) is an element that facilitates the formation of final martensite in composite steel by suppressing ferrite formation and promoting austenite formation. However, if its content exceeds 2.3%, manganese (Mn) segregates in the thickness direction, easily forming manganese bands within the slab, which increases the occurrence of defects during the rolling process, along with continuous casting cracks. Therefore, it is preferable to include 2.1% or less. On the other hand, if the content is less than 1.0%, it is difficult to ensure strength in ultra-high-strength steel, so the lower limit can be restricted to 1.0%. A more preferable lower limit may be 1.4%.

[0031] Silicon (Si): 0.05~1.0% In martensitic steel, silicon (Si) plays a role in suppressing carbide formation and controlling carbide size during the reheating and overaging stages after cooling, so its lower limit can be restricted to 0.05%. More preferably, it can be contained at 0.09% or more. However, silicon (Si) is a ferrite-stabilizing element, and if its content exceeds 1.0%, ferrite may be formed during cooling in a continuous annealing furnace, weakening the strength. Furthermore, since Si-based oxides may form in the heating furnace, causing surface oxidation problems, its upper limit can be restricted to 1.0%. More preferably, its upper limit can be restricted to 0.6%.

[0032] Phosphorus (P): 0.1% or less Phosphorus (P) is an impurity element contained in steel, and considering that it may be inevitably included during the manufacturing process, a content of 0% is excluded. However, since a phosphorus (P) content exceeding 0.1% may worsen weldability and cause brittleness of the steel, the upper limit can be restricted to 0.1%. A more preferable upper limit may be 0.03%.

[0033] Sulfur (S): 0.03% or less Sulfur (S), like phosphorus (P), is an unavoidable impurity in steel and inhibits the ductility and weldability of steel sheets. Therefore, it is preferable to control its content as low as possible, and thus it is preferable to limit the sulfur (S) content to 0.03% or less. More preferably, it can be limited to 0.005% or less. Note that 0% is excluded to account for cases where sulfur is unavoidably present during the manufacturing process.

[0034] Aluminum (Al): 0.01-0.5% Aluminum (Al) can be added to remove oxygen from molten steel and, like Si, is an element that stabilizes ferrite. Furthermore, it is a component that can increase the carbon content in austenite, thereby improving the hardening ability of the final martensitic steel; therefore, it is preferable to add at least 0.01% of Al. However, if the content exceeds 0.5%, ferrite may form during cooling in a continuous annealing furnace, weakening the strength. Additionally, the formation of AlN can induce slab cracks, hindering hot rolling, so the upper limit can be restricted to 0.5%.

[0035] The steel of the present invention may contain, in addition to the composition described above, the remaining iron (Fe) and unavoidable impurities. These unavoidable impurities cannot be eliminated because they may be unintentionally introduced during the normal manufacturing process. Since such impurities are easily recognizable to any engineer in the field of ordinary steelmaking, their full nature will not be mentioned in this specification.

[0036] The steel according to one embodiment of the present invention may further contain two or more of the following: chromium (Cr): 0.01-0.2%, molybdenum (Mo): 0.01-0.2%, and boron (B): 0.005% or less (excluding 0%).

[0037] Chromium (Cr): 0.01-0.2% Chromium (Cr) is an additive used to improve the hardening ability of steel and ensure high strength. It is useful in producing ultra-high-strength steel with pure martensite by suppressing the formation of bainite. Therefore, to ensure the aforementioned effects, it is preferable to add 0.01% or more of chromium (Cr). However, since excessive chromium content leads to increased ferroalloy costs, the upper limit can be restricted to 0.2%, and more preferably to 0.1%.

[0038] Molybdenum (Mo): 0.01-0.2% Molybdenum (Mo), like Cr, is an element that improves the hardening ability of steel, and it is preferable to add 0.01% or more to obtain the hardening effect. However, if the content exceeds 0.2%, the amount of alloy added becomes excessive, which raises the cost of ferroalloys. Therefore, it is preferable to limit the upper limit to 0.2%, and more preferably to 0.1%.

[0039] Boron (B): 0.005% or less (excluding 0%) Boron (B) is an element that suppresses the transformation of austenite to ferrite during the continuous annealing process, and even in very small amounts, it is an effective element that improves the hardening ability of martensite, similar to Cr and Mo. However, if the boron (B) content exceeds 0.005%, Fe 23 Since the (B,C)6 precipitate phase promotes ferrite formation by precipitation at the austenite grain boundaries, it is preferable to limit its upper limit to 0.005%.

[0040] The steel according to one embodiment of the present invention may further contain one or more of the following: titanium (Ti): 0.1% or less (excluding 0%), niobium (Nb): 0.1% or less (excluding 0%).

[0041] Titanium (Ti): 0.1% or less (excluding 0%) Titanium (Ti) is a fine carbide-forming element that contributes to securing yield strength and tensile strength. Furthermore, titanium (Ti) is used in scavenging by precipitating nitrogen (N) in steel as TiN. To achieve this, it is preferable to add 48 / 14*[N] or more in chemical equivalents, and when adding B, it is preferable to add titanium (Ti) to maximize its effect. However, if the content exceeds 0.1%, coarse carbides will precipitate, which may reduce the carbon content in the steel, leading to a decrease in strength and elongation. This may also cause nozzle clogging during continuous casting, so it is preferable to limit the upper limit to 0.1%.

[0042] Niobium (Nb): 0.1% or less (excluding 0%) Niobium (Nb) is an element that segregates at austenite grain boundaries and, during annealing heat treatment, suppresses the coarsening of austenite grains, forming fine carbides and contributing to increased strength. However, if the niobium (Nb) content exceeds 0.1%, the precipitation of coarse carbonitrides increases, which may reduce the strength and elongation due to a decrease in the carbon content in the steel, potentially leading to problems such as reduced workability of the base material and increased manufacturing costs. Therefore, it is preferable to limit the upper limit to 0.1%.

[0043] The steel according to one embodiment of the present invention may have an R value of 0.12 to 0.27 as defined by the following relational expression 1.

[0044] Relational expression 1 is a composite relational expression between Ceq1 and Ceq2 that shows the welding properties according to the content of each element, and when the R value of relational expression 1 is between 0.12 and 0.27, the physical properties targeted in the present invention, including welding properties, can be secured.

[0045] If the R value defined by relational expression 1 is less than 0.12, it is difficult to secure the strength required by the present invention, while if the R value exceeds 0.27, the physical properties, particularly the welding properties, may deteriorate. In the present invention, a more preferable lower limit for the R value can be 0.17, a more preferable upper limit for the R value can be 0.25, and more preferably 0.20.

[0046]

number

[0047] The microstructure of the steel according to the present invention will be described in detail below.

[0048] In this invention, unless otherwise specified, the percentage representing the fraction of microstructure is based on area.

[0049] The steel according to one embodiment of the present invention may contain 99% or more of martensite or tempered martensite as its microstructure, and 1 μm 2 The number of carbides per unit area can be 40 or less, and the average length of the long axis of the carbides can be 300 nm or less.

[0050] In the present invention, martensite or tempered martensite can be included as a microstructure to ensure a cold-rolled steel sheet with high strength and a high yield ratio, and it is preferable that it be included at 99% or more to ensure a high strength level of 1.3G or higher.

[0051] Furthermore, in order to ensure excellent bending properties, it is preferable to control the number of carbides to 40 or less, and more preferably to 35 or less.

[0052] Furthermore, in order to more effectively ensure the above effects, the average length of the long axis of the carbide is preferably 300 nm or less, and more preferably 200 nm or less.

[0053] In this invention, the number of carbides is 1 μm in an SEM image at x10,000. 2 This shows the average number of carbides in a region (average of 10 regions), and the length of the long axis of the carbides was measured and shown in TEM bright-field images at x30,000 to x100,000.

[0054] The steel manufacturing method of the present invention will be described in detail below.

[0055] The steel according to one embodiment of the present invention can be manufactured by heat-treating, primary cooling, secondary cooling, reheating, and overaging a cold-rolled steel sheet that satisfies the above-described alloy composition.

[0056] Preparation of cold-rolled steel sheets Cold-rolled steel sheets satisfying the alloy composition of the present invention can be prepared.

[0057] The cold-rolled steel sheet of the present invention can be manufactured under normal process conditions, and preferably, it can be manufactured by reheating a steel slab, hot-rolling, cooling, coiling, and cold-rolling it under the conditions described below.

[0058] reheating A steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1100 to 1300°C.

[0059] Reheating can be performed to ensure a smooth subsequent hot rolling process and to adequately secure the target material properties. If the reheating temperature is below 1100°C, the hot rolling load increases rapidly, and if the temperature exceeds 1300°C, the amount of surface scale increases, reducing material yield and potentially causing surface defects that negatively impact the final quality.

[0060] Hot rolling The reheated steel slab described above can be hot-rolled at a finishing hot-rolling temperature of Ar3 or higher.

[0061] In this invention, the finishing hot rolling temperature can be limited to Ar3 (the temperature at which ferrite begins to appear when austenite cools), because below Ar3, rolling occurs in a two-phase region of ferrite and austenite, or in the ferrite region, which may result in the formation of a mixed grain structure, and there is a risk of errors due to fluctuations in the hot rolling load.

[0062] Cooling and winding The hot-rolled steel sheet described above can be wound up after being cooled to a temperature range of 700°C or less.

[0063] If the winding temperature exceeds 700°C, an excessive oxide film may form on the surface of the steel sheet, potentially inducing defects. Lower winding temperatures result in higher strength of the hot-rolled steel sheet, which has the disadvantage of requiring higher rolling loads in the subsequent cold-rolling process. However, this is not a factor that makes actual production impossible, so the present invention does not particularly limit the lower limit.

[0064] Furthermore, in this invention, the oxide layer formed on the surface of the wound steel sheet can be removed by pickling before the subsequent cold rolling process.

[0065] cold rolling The cooled and wound steel sheet described above can be cold-rolled with a reduction ratio of 30-80%.

[0066] If the reduction ratio in cold rolling is less than 30%, it is not only difficult to achieve the target thickness, but the remaining hot-rolled grains may affect the formation of austenite and the final physical properties during annealing heat treatment. On the other hand, if the reduction ratio exceeds 80%, work hardening that occurs during cold rolling can lead to uneven reduction in the length and width directions, potentially causing variations in the material properties of the final steel sheet, and the rolling load may make it difficult to achieve the target thickness.

[0067] heat treatment The above cold-rolled steel sheet can be heat-treated at a temperature of Ac3 or higher for 30 seconds or more.

[0068] In this invention, heat treatment can be performed to ensure 100% austenite fraction by annealing the austenite single-phase region. By ensuring 100% austenite fraction through the above heat treatment, it is possible to prevent a decrease in strength due to ferrite formation during annealing.

[0069] Ac3=910-203√([C])-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W] (Here, [C], [Ni], [Si], [V], [Mo], and [W] are weight percent of each element.)

[0070] Primary cooling After the above heat treatment, primary cooling can be performed to a temperature range of 500-750°C at an average cooling rate of 1-10°C / s.

[0071] During primary cooling, if the cooling rate is less than 1°C / s, it may become difficult to achieve the target strength due to the formation of ferrite during cooling. On the other hand, if the rate exceeds 10°C / s, during secondary cooling, the average cooling rate decreases, increasing the proportion of other low-temperature transformation phases besides martensite, which may ultimately make it difficult to achieve the target strength.

[0072] If the temperature is below 500°C during primary cooling, phases such as ferrite and bainite may form, potentially reducing strength. If the temperature exceeds 750°C, problems may arise in actual production lines.

[0073] Secondary cooling The steel plate that has been cooled in the primary stage can be further cooled to a temperature of Ms-190°C or lower at an average cooling rate of 20-80°C / s.

[0074] In this invention, in order to ensure 99% or more martensite or tempered martensite, it is preferable to cool rapidly to below the martensite transformation finish temperature (Mf) during secondary cooling. Specifically, in this invention, it is preferable to cool to a temperature of Ms-190°C or lower. In this invention, it is possible to form a sufficiently hard martensite structure, and in order to ensure the effect of increasing yield strength due to carbide precipitation during subsequent tempering, the secondary cooling finish temperature is limited to Ms-190°C or lower. Furthermore, since bending properties may be poor if the tempering temperature is high, by limiting the above secondary cooling finish temperature, sufficient tempering can be achieved without raising the tempering temperature too much, thereby ensuring bending properties. If the cooling finish temperature exceeds Ms-190°C, the fraction of martensite or tempered martensite is not sufficiently secured, making it difficult to secure the desired physical properties.

[0075] On the other hand, if the average cooling rate during secondary cooling is less than 20°C / s, some bainite structures may form between the primary and secondary cooling phases. If the rate exceeds 80°C / s, the rapid martensitic transformation rate at the secondary cooling stage can lead to deterioration of the steel sheet's surface shape and variations in material properties in the width direction.

[0076] 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] (Here, [C], [Mn], [Si], [P], [Al], [Ni], [Cr], and [Mo] are weight percent of each element.)

[0077] Reheating and over-aging The steel plate that has been secondarily cooled can be reheated and overaged by heating it to a temperature range of over 30°C above the secondarily cooling completion temperature but less than 270°C and holding it for 1 to 20 minutes.

[0078] In this invention, the toughness of hard martensite with a high dislocation density formed during secondary cooling is improved by transforming it into tempered martensite through reheating and overaging. In this invention, in order to ensure a sufficient tempering effect, the lower limit of the reheating temperature is restricted to a temperature of 30°C or higher relative to the secondary cooling completion temperature. At this time, the yield strength increases due to the formation of fine carbides, but if the reheating and overaging temperature is less than the secondary cooling completion temperature + 30°C, it is difficult to obtain the above-mentioned effect. On the other hand, if the temperature is 270°C or higher, there is a problem that the carbides become coarser and the bending properties are inferior.

[0079] On the other hand, if the holding time is less than 1 minute, the martensite does not sufficiently transform into tempered martensite, making it difficult to ensure sufficient toughness. If the holding time exceeds 20 minutes, overaging occurs, and the resulting carbides may become coarse, potentially adversely affecting the bending properties and material properties.

[0080] The steel of the present invention manufactured in this manner has a tensile strength of 1300 MPa or more, a yield ratio exceeding 0.73, and a bending characteristic (R / t) of less than 4 (where R is the bending radius at which no cracks occur in the bent portion after a 90° bending test, and t is the thickness of the steel plate). Thus, it can possess excellent bending characteristics while having a high yield ratio.

[0081] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are for illustrative purposes to illustrate the present invention in more detail and are not intended to limit the scope of the rights of the present invention. [Examples]

[0082] (Examples) Steel slabs having the composition shown in Table 1 were heated to 1100-1300°C, finished hot-rolled at 850-950°C (a temperature above Ar3), wound up in the temperature range of 400-700°C, and cold-rolled steel sheets were produced by applying a cold reduction ratio of 45-65%. Next, after heat treatment at the temperature range of 800-900°C for 100-400 seconds, primary and secondary cooling were performed under the conditions described in Table 2. At this time, the primary cooling rate was 2-4°C / s and the secondary cooling rate was 25-60°C / s. Then, the slabs were reheated under the conditions in Table 2 and overaged for 1-20 minutes to produce steel sheets.

[0083] Table 1 below shows the calculated values ​​for Ac3, Ms temperature, and relational equation 1 according to the content of each element.

[0084] [Table 1]

[0085] Ac3=910-203√([C])-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W] (Here, [C], [Ni], [Si], [V], [Mo], and [W] are weight percent of each element.)

[0086] 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] (Here, [C], [Mn], [Si], [P], [Al], [Ni], [Cr], and [Mo] are weight percent of each element.)

[0087]

number

[0088] [Table 2]

[0089] Table 3 below shows the microstructure of each specimen, observed, and its physical properties measured. The microstructure was confirmed using SEM imaging, and the number of carbides was 1 μm in the x10,000 SEM image. 2 The average number of carbides in each region (average of 10 regions) is shown, and the length of the long axis of the carbides was measured and shown using TEM bright-field images at x30,000 to x100,000. In addition, the yield strength (YS), tensile strength (TS), yield ratio (YS / TS), total elongation (T-El), and uniform elongation (U-El) values ​​were measured by performing tensile tests at a test speed of 28 mm / min on cold-rolled steel sheets that had undergone continuous annealing and been processed to JIS standards (gauge length width × length: 25 × 50 mm, total length of test piece: 200 to 260 mm). Furthermore, the bending properties (R / t) were determined by processing the same cold-rolled steel sheet into a test specimen measuring 100 mm wide x 30 mm long, then performing a 90° bending test at a test speed of 100 mm / min. After that, cracks in the bent area were checked using a microscope, and the R / t value was calculated by dividing the minimum bending radius (R) at which no cracks occurred by the thickness (t) of the test specimen. A value less than 4 was represented as O, and a value of 4 or more was represented as X.

[0090] [Table 3] *M: Martensite, TM: Tempered Martensite

[0091] As shown in Table 3, Examples 1 to 25 of the invention, which satisfy the alloy composition and manufacturing conditions of the present invention, satisfy the microstructure and carbide characteristics proposed in the present invention and ensure the physical properties targeted in the present invention.

[0092] On the other hand, Comparative Examples 1, 2, 4, 5, 7, and 8, whose secondary cooling completion temperature did not meet the condition of the present invention of Ms-190°C or lower, did not meet the target yield ratio and bending characteristics of the present invention, and their tensile strength also failed to reach the target level.

[0093] In particular, Comparative Examples 1 to 9 are examples that do not include a reheating step. While the present invention includes quenching and tempering as essential steps, the above examples are cases where aging was performed at the cooling temperature without reheating. That is, the above examples may result in a decrease in martensitic hardening ability, and because there is no tempering step, the yield strength is very poor, and the desired strength cannot be obtained.

[0094] Furthermore, Comparative Examples 10 to 21, which did not meet the upper or lower limit conditions proposed in the present invention during reheating and overaging, exhibited inferior yield ratio and bending properties as intended in the present invention. In particular, when the lower limit was not met, a sufficient increase in yield strength was not achieved, and in examples where the upper temperature condition for reheating and overaging, which is below 270°C, was not met, bending properties could not be secured due to the formation of coarse carbides.

[0095] Comparative Examples 22 and 23 are examples that satisfy all the manufacturing conditions proposed in the present invention, but do not satisfy the alloy composition proposed in the present invention. Therefore, these examples not only fail to satisfy the desired microstructure fraction, but also fail to ensure the desired strength.

[0096] Figures 1(a) and 1(b) are SEM microstructure images (x10.000) of Invention Example 15 and Comparative Example 21 according to one embodiment of the present invention. Both Figures 1(a) and 1(b) show tempered martensite as the microstructure, and it can be confirmed that rice-grain-shaped carbides have been formed on the microstructure. On the other hand, in the case of (b), it can be confirmed that the amount of carbides per unit area formed on the microstructure exceeds the range proposed in the present invention, and that their size has also been formed to be excessively large.

[0097] As described above, the present invention has been explained in detail through the examples, but other forms of embodiments are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. A material comprising, by mass%, carbon (C): 0.1-0.3%, manganese (Mn): 1.0-2.3%, silicon (Si): 0.05-1.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01-0.5%, with the remainder being Fe and unavoidable impurities. The R value defined by the following relational equation 1 is between 0.12 and 0.

27. In a 10,000x SEM image, 1 μm ２ The average number of carbides per unit area is 40 or less, and the average length of the long axis of the carbides is 300 nm or less. Steel plate with a yield ratio exceeding 0.

73. [Math 1] (Here, [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu], and [Ni] are the mass percent of each element.)

2. The steel sheet according to claim 1, further comprising two or more of the following: chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2%, and boron (B): 0.005% or less (excluding 0%).

3. The steel sheet according to claim 1, further comprising one or more of titanium (Ti): 0.1% or less (excluding 0%) and niobium (Nb): 0.1% or less (excluding 0%).

4. The steel sheet according to claim 1, wherein the steel sheet contains 99 area percent or more of martensite or tempered martensite as a microstructure.

5. The steel plate according to claim 1, wherein the steel plate has a tensile strength of 1300 MPa or more and a bending characteristic (R / t) of less than 4 (where R is the minimum bending radius (mm) at which no cracks occur in the bent portion after a 90° bending test, and t is the thickness (mm) of the steel plate).

6. A step of preparing a cold-rolled steel sheet comprising, by mass%, carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.5%, with the remainder being Fe and unavoidable impurities, and having an R value of 0.12 to 0.27 as defined by the following relational formula 1, The steps include: heat-treating the cold-rolled steel sheet at a temperature of Ac3 or higher for 30 seconds or more; The process involves a first cooling step after the heat treatment, where the temperature is reduced to a range of 500 to 750°C at an average cooling rate of 1 to 10°C / s. The first step is to second-cool the first-cooled steel plate to a temperature of Ms-190°C or lower at an average cooling rate of 20 to 80°C / s, The process includes a reheating and overaging step in which the secondary cooled steel plate is heated to a temperature range of over 30°C above the secondary cooling completion temperature and less than 270°C, and held for 1 to 20 minutes. A method for manufacturing a steel sheet, wherein the reheated and overaged cold-rolled steel sheet has an average number of carbides per 1 μm² area of ​​40 or less, an average length of the long axis of the carbides of 300 nm or less, and a yield ratio exceeding 0.73 in a 10,000 x 10 SEM image. [Math 2] (Here, [C], [Mn], [Si], [P], [S], [Cr], [Mo], [V], [Nb], [Cu], and [Ni] are the mass percent of each element.)

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

8. The method for manufacturing a steel sheet according to claim 6, wherein the cold-rolled steel sheet further comprises one or more of titanium (Ti): 0.1% or less (excluding 0%) and niobium (Nb): 0.1% or less (excluding 0%).

9. The step of preparing the cold-rolled steel sheet is as follows: The process involves reheating the steel slab to a temperature range of 1100 to 1300°C, The steps include: hot rolling the reheated steel slab at a finish hot rolling temperature of Ar3 or higher; The steps include cooling and winding the hot-rolled steel sheet in a temperature range of 700°C or less, The method for manufacturing a steel sheet according to claim 6, comprising the step of cold-rolling the cooled and wound steel sheet at a reduction ratio of 30 to 80%.

10. The method for manufacturing a steel sheet according to claim 9, further comprising the step of pickling the cooled and wound steel sheet with hydrochloric acid.

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