High-strength, high-toughness steel plate and method for manufacturing the same

Abstract

The present invention relates to a high-strength, high-toughness steel plate and a manufacturing method thereof, and more particularly to a high-strength, high-toughness steel plate usable for automobile safety belt springs and the like, and a manufacturing method thereof.

Description

Technical Field

[0001] The present invention relates to a high-strength and high-toughness steel sheet and a method for manufacturing the same, and more particularly, to a high-strength and high-toughness steel sheet that can be used for automotive safety belt springs and the like, and a method for manufacturing the same.

Background Art

[0002] Generally, the material used for automotive safety belt springs is very thin, with a final thickness of about 0.1 to 0.3 mm and a width of about 3 to 25 mm, and is used in a spring shape. Therefore, high toughness is required. In addition, it is necessary to have excellent return performance, which is an important characteristic of the spring. In order to ensure the target restoring force and torque for each product, the tensile strength of the final cold-rolled steel sheet needs to be high.

[0003] In order to ensure the characteristics of thin and high-strength as described above, the most widely used is high-carbon steel containing carbon more than eutectoid steel. By controlling the elongated pearlite tissue form obtained after cold rolling by utilizing the pearlite tissue possessed by hypereutectoid high-carbon steel, high toughness and strength can be ensured. This is more economical than using expensive alloying elements or utilizing low-temperature transformation tissues such as bainite and tempered martensite by additional heat treatment processes.

[0004] In order to be used without breakage, damage, etc. during the use of the spring and to have a return number of more than 300,000 times, the fraction of uniform pearlite (fibrous pearlite) in the microstructure of the final cold-rolled material of about 0.2 t must be high.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] One embodiment of the present invention aims to provide a high-strength, high-toughness steel sheet 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] One embodiment of the present invention contains, by weight percent, carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): 0.01% or less, aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, with the remainder being iron (Fe) and other unavoidable impurities. It has a microstructure consisting of a pearlite structure as the main phase and a remaining grain boundary vryopropyl cementite making up less than 4 area. The above-described pearlite structure can provide a steel sheet composed of 40% or more of fibrous pearlite, 50% or less of bent pearlite, and 10% or less of non-uniform pearlite, by area percentage.

[0009] When observing the cross-section of the microstructure in the thickness direction of the steel plate, the average thickness of the uniform pearlite may be 2.5 μm or less.

[0010] The above steel plate may have an A value of 1.2 or less in the following relational equation 1.

[0011] [Relationship 1] A = [Mn] + [Cr] + [V] (Here, [Mn], [Cr], and [V] are weight percent of each element.)

[0012] The above-mentioned steel plate may have a tensile strength of 2100 MPa or more, an elongation of 2% or more, and a bending characteristic (R / t) of 3.0 or less (where R is the bending radius at which no cracks occur in the bent portion after a 180° bending test, and t is the thickness of the steel plate).

[0013] The steel plate described above may have a tensile strength of 2200 to 2350 MPa.

[0014] The thickness of the steel plate may be 0.1 to 0.6 mm.

[0015] Another embodiment of the present invention involves reheating a steel slab containing, by weight %, carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): 0.01% or less, aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, with the remainder being iron (Fe) and other unavoidable impurities. The above step involves roughly rolling the reheated steel slab, The above-mentioned rough-rolled steel sheet is finished-rolled to obtain a hot-rolled steel sheet, The above hot-rolled steel sheet is cooled to a temperature range of 540-660°C at a cooling rate of 5-50°C / s and then wound up. The process involves heating the cooled and wound steel sheet to a temperature range of 850-1050°C and holding it for 5-20 minutes, then cooling it to a temperature range of 520-590°C at a cooling rate of 50-150°C / s, and holding it for 30-120 seconds. A method for manufacturing a steel sheet can be provided, which includes the step of cold-rolling the heat-treated steel sheet at a cumulative reduction rate of 80-96%.

[0016] The above steel slab may have an A value of 1.2 or less in the following relational equation 1.

[0017] [Relationship 1] A = [Mn] + [Cr] + [V] (Here, [Mn], [Cr], and [V] are the weight percentages of each element.)

[0018] The above reheating is carried out in the temperature range of 1100 - 1300 °C, The above rough rolling is carried out in the temperature range of 1000 - 1100 °C, The above finish rolling can be carried out in the temperature range of 860 - 940 °C.

[0019] After the above coiling, the steel plate may further include a step of pickling in the temperature range of 200 °C or lower.

[0020] After the above heat treatment, the steel plate may further include a step of air cooling.

[0021] The microstructure of the heat-treated steel plate may include a pearlite structure as the main phase and grain boundary primary cementite of 4% or less of the remaining area.

[0022] After the above finish rolling, the thickness of the hot-rolled steel plate may be 1.5 - 2.6 mm.

[0023] After the above cold rolling, the thickness of the cold-rolled steel plate may be 0.1 - 0.6 mm.

Effect of the Invention

[0024] According to an embodiment of the present invention, a high-strength and high-toughness steel plate and a method for manufacturing the same can be provided.

[0025] An embodiment of the present invention relates to a high-strength and high-toughness steel plate and a method for manufacturing the same, which can be used for high-grade industries / tools and safety belt springs of automobiles, etc.

Brief Description of the Drawings

[0026] [Figure 1] It is a photograph of the form of homogeneous pearlite (fibrous pearlite) observed with a scanning electron microscope (x20,000). [Figure 2] It is a photograph showing a method for calculating the fraction of homogeneous pearlite (fibrous pearlite) in Invention Example 2. [Modes for carrying out the invention]

[0027] 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 explain the present invention in more detail to those ordinary people skilled in the art to which the invention pertains.

[0028] As mentioned above, the reel spring materials manufactured to date suffer from reduced durability and quality variations between materials due to an insufficient fraction of uniform pearlite (fibrous pearlite), making it difficult to ensure a stable and uniform pearlite (fibrous pearlite) structure. Furthermore, due to the compositional system and process characteristics resulting from cold rolling of a single-phase pearlite structure superior to that of eutectoid steel, there is a quality degradation due to protracted cementite, and improvement is needed to address this.

[0029] The inventors have diligently conducted research to produce cold-rolled steel sheets with excellent strength and toughness by controlling the steel composition and manufacturing process.

[0030] As a result, we confirmed that the above physical properties can be ensured by optimizing the alloy composition and manufacturing conditions to control the grain boundary vryoprecipitation cementite of the steel sheet before cold rolling, and by strictly controlling the uniform pearlite (fibrous pearlite) structure of the final steel sheet, thus completing the present invention.

[0031] The present invention will be described in detail below.

[0032] The steel composition of the present invention will be described in detail below.

[0033] Unless otherwise specified in this invention, the percentages representing the content of each element are based on weight.

[0034] A steel sheet according to one embodiment of the present invention may contain, by weight percent, carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): 0.01% or less, aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, with the remainder being iron (Fe) and other unavoidable impurities.

[0035] Carbon (C): 0.70~1.20% Carbon (C) is an element that greatly affects the strength and toughness of the pearlite structure, and it is preferable to add 0.70% or more of carbon (C) to ensure that 40% or more of uniform pearlite (fibrous pearlite) is obtained after cold rolling. However, if the carbon (C) content exceeds 1.20%, the fraction of grain boundary proeposition cementite after heat treatment increases, and the toughness decreases. The lower limit of the carbon (C) content is preferably 0.75%, more preferably 0.76%, even more preferably 0.77%, and most preferably 0.78%. The upper limit of the carbon (C) content is preferably 0.90%, more preferably 0.88%, even more preferably 0.87%, and most preferably 0.85%.

[0036] Manganese (Mn): 0.2-0.6% Manganese (Mn) can be added in amounts of 0.2% or more to improve strength through solid solution strengthening. However, if added in excess, there is a risk of reduced toughness due to carbide formation and a risk of brittleness due to the low-temperature structure of the segregated area due to central segregation; therefore, the upper limit of its content can be limited to 0.6%. The lower limit of the above manganese (Mn) content is preferably 0.22%, more preferably 0.24%, and even more preferably 0.25%. The upper limit of the above manganese (Mn) content is preferably 0.5%, more preferably 0.48%, even more preferably 0.46%, and most preferably 0.45%.

[0037] Silicon (Si): 0.01-0.4% Silicon (Si) can be added in amounts of 0.01% or more to strengthen the ferrite structure within pearlite using solid solution. However, if added excessively, it can excessively form primary scale generated in the heating furnace, inducing red scale defects, hindering heat treatment and processability, and posing a risk of inducing brittleness due to residual cementite. Therefore, its content can be limited to 0.4% or less. The lower limit of the silicon (Si) content is preferably 0.05%, more preferably 0.06%, even more preferably 0.08%, and most preferably 0.1%. The upper limit of the silicon (Si) content is preferably 0.3%, more preferably 0.28%, even more preferably 0.26%, and most preferably 0.25%.

[0038] Phosphorus (P): 0.005~0.02% If the phosphorus (P) content exceeds 0.02%, there is a risk of brittleness due to segregation. Therefore, it is preferable that the phosphorus (P) content be 0.02% or less. The upper limit of the phosphorus (P) content is preferably 0.015%, more preferably 0.014%, even more preferably 0.013%, and most preferably 0.012%. On the other hand, considering cases where phosphorus (P) is inevitably included during the manufacturing process, the lower limit can be restricted to 0.005%.

[0039] Sulfur (S): 0.01% or less Sulfur (S) is an element that forms nonmetallic inclusions and reduces toughness, so it is necessary to control its content to be as low as possible. Therefore, it is preferable that the sulfur (S) content be 0.01% or less. On the other hand, in the present invention, the lower the sulfur (S) content, the less the risk of brittleness due to segregation / inclusions is reduced, which is advantageous for ensuring toughness, so the lower limit is not particularly restricted. The sulfur (S) content is preferably 0.008% or less, more preferably 0.006% or less, and even more preferably 0.005% or less.

[0040] Aluminum (Al): 0.01-0.1% Aluminum (Al) is an element that refines austenite grains by forming AlN, and can be added to refine the pearlite structure. If the aluminum (Al) content is less than 0.01%, the above effect may not be sufficiently obtained. On the other hand, if the content exceeds 0.1%, there is a risk of brittleness due to inclusions formed by oxide formation. The lower limit of the aluminum (Al) content is preferably 0.012%, more preferably 0.014%, and even more preferably 0.015%. The upper limit of the aluminum (Al) content is preferably 0.06%, more preferably 0.05%, even more preferably 0.04%, and most preferably 0.03%.

[0041] Chromium (Cr): 0.1-0.8% Chromium (Cr) is preferably added in an amount of 0.1% or more to ensure strength and to refine the spacing between pearlite layers. On the other hand, if the content exceeds 0.8%, there is a risk of reduced toughness due to the formation of excessive carbides. The lower limit of the chromium (Cr) content is preferably 0.12%, more preferably 0.14%, and even more preferably 0.15%. The upper limit of the chromium (Cr) content is preferably 0.4%, more preferably 0.35%, even more preferably 0.33%, and most preferably 0.30%.

[0042] Vanadium (V): 0.02-0.25% Vanadium(V) is an element necessary for refining pearlite crystal grains and ensuring strength through work hardening after cold rolling. To ensure the above effect, the present invention allows for the addition of 0.02% or more of vanadium(V). On the other hand, if the content is excessive, there is a risk of brittleness due to the formation of coarse carbon / nitride, so the upper limit can be limited to 0.25%. The lower limit of vanadium(V) is preferably 0.03%, more preferably 0.04%, and even more preferably 0.05%. The upper limit of vanadium(V) is preferably 0.22%, more preferably 0.20%, and even more preferably 0.18%.

[0043] Cobalt (Co): 0.01-0.2% Cobalt (Co) is an element necessary for promoting the formation of uniform pearlite, increasing the degree of orientation of pearlite, and ensuring uniform pearlite (fibrous pearlite) after cold rolling, and can be present in amounts of 0.01% or more. On the other hand, if the content is excessive, it can reduce the curing ability and require a faster cooling rate, thus risking a decrease in heat treatability. Therefore, the upper limit of the cobalt (Co) content can be limited to 0.2%. The lower limit of the cobalt (Co) is preferably 0.02%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the cobalt (Co) is preferably 0.18%, more preferably 0.16%, and even more preferably 0.15%.

[0044] The steel sheet of the present invention may contain, in addition to the above-described composition, the remaining iron (Fe) and unavoidable impurities. Since unavoidable impurities can be unintentionally introduced during the normal manufacturing process, they cannot be eliminated. Such impurities are known to any engineer in the field of ordinary steel manufacturing, and therefore, their full details are not specifically mentioned herein.

[0045] In one embodiment of the present invention, the steel plate may have an A value of 1.2 or less in the following relational expression 1.

[0046] In this invention, the reduction in bendability due to segregation and the formation of excessive carbides are prevented by the following relational equation 1. When Mn, Cr, and V are added in excess, they can induce macro and micro segregation during the continuous casting process, and during the heat treatment process, they can form a large amount of carbides, potentially reducing the toughness and bendability of the final product. Therefore, in this invention, the A value can be controlled to 1.2 or less in order to prevent the above problems. The lower limit of the A value can be the sum of the lower limits of the elemental content of Mn, Cr, and V.

[0047] [Relationship 1] A = [Mn] + [Cr] + [V] (Here, [Mn], [Cr], and [V] are weight percent of each element.)

[0048] The microstructure of the steel according to the present invention will be described in detail below.

[0049] In this invention, unless otherwise specified, the percentage representing the fraction of microstructure is based on area.

[0050] A steel sheet according to one embodiment of the present invention may have a microstructure comprising a pearlite structure as the main phase and grain boundary prodetic cementite in an area percentage of 4% or less. The pearlite is composed of 40% or more of uniform pearlite (fibrous pearlite), 50% or less of zigzag pearlite (bent pearlite), and 10% or less of heterogeneous pearlite by area percentage, and the average thickness of the uniform pearlite may be 2.5 μm or less.

[0051] A sheet material with a pearlite structure before cold rolling undergoes compressive deformation in the thickness direction during cold rolling, ultimately resulting in three forms of pearlite structure. Fibrous pearlite is formed when the layered structure is stretched with the layers parallel to the rolling direction, exhibiting a morphology like the central part of Figure 1. Bent pearlite is formed when one or more folds occur in the direction perpendicular to the rolling, causing the layered structure of the pearlite to exhibit a zigzag pattern. Heterogeneous pearlite is formed when the layered structure of the pearlite folds, bends, and cracks at intervals of several micrometers after cold rolling, resulting in a morphology where fibrous or bent pearlite is clearly difficult to observe. The proportion of these final pearlite structure morphologies after cold rolling can vary depending on the composition and manufacturing conditions.

[0052] On the other hand, if the fraction of grain boundary progenitate cementite exceeds 4 area percent in the entire microstructure, a problem of brittle fracture due to grain boundary progenitate cementite may occur.

[0053] The present invention is characterized by controlling the fraction of each type of perlite formed when the perlite structure before cold rolling is formed into uniform perlite (fibrous perlite), zigzag perlite (bent perlite), and heterogeneous perlite by cold rolling, in order to ensure high strength and high toughness. Specifically, the present invention is characterized in that, after cold rolling, the perlite is composed of 40% or more of uniform perlite (fibrous perlite), 50% or less of zigzag perlite (bent perlite), and 10% or less of heterogeneous perlite by area percentage. It is preferable that fibrous perlite be present in a proportion of 40% or more by area percentage in order to ensure bendability for high toughness, and it is preferable that bent perlite and heterogeneous perlite be limited to 50% or less and 10% or less, respectively, in order to ensure the physical properties targeted in the present invention. More preferably, it can contain 50% or more of uniform perlite. In the present invention, it is possible to contain 100% fibrous perlite by fraction, and the fractions of bent perlite and heterogeneous perlite can each be 0%. On the other hand, in the present invention, the fraction of the pearlite phase can be calculated and expressed as an average of the fractions of the microstructure measured when observing any 10 to 15 points in the cross-section in the thickness direction of the entire steel sheet, and the thickness of the fibrous pearlite can also be calculated and expressed as an average.

[0054] If the average thickness of the above-mentioned uniform pearlite exceeds 2.5 μm, coarse uniform pearlite is formed, following the same principle as the strength decreases as the crystal grain size increases. As a result, the desired level of strength cannot be secured, brittleness increases, and flexibility cannot be ensured.

[0055] The method for manufacturing steel sheets according to the present invention will be described in detail below.

[0056] A steel sheet according to one embodiment of the present invention can be manufactured by reheating, rolling, cooling, winding, heat treating, and cold rolling a steel slab that satisfies the above-described alloy composition.

[0057] reheating A steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1100 to 1300°C.

[0058] If the reheating temperature is below 1100°C, it may not be possible to secure a slab temperature sufficient for sheet metal passage. On the other hand, if the temperature exceeds 1300°C, abnormal austenite growth and surface defects due to excessive scaling may occur.

[0059] Rough rolling The reheated steel slab described above can be roughly rolled in a temperature range of 1000 to 1100°C.

[0060] If the rough rolling temperature is below 1000°C, there is a potential drawback of increased rolling load and reduced passability. On the other hand, if the temperature exceeds 1100°C, there is a potential drawback of excessive scale formation and a significant decrease in surface quality.

[0061] Finishing rolling The roughly rolled steel sheet described above can be finished-rolled at a temperature range of 860 to 940°C to obtain a hot-rolled steel sheet.

[0062] If the finish rolling temperature is below 860°C, excessive rolling load may significantly reduce the hot rollability. On the other hand, if the temperature exceeds 940°C, the austenite grain size becomes very coarse, posing a risk of brittleness. In this invention, the thickness of the hot-rolled steel sheet after finish rolling may be 1.5 to 2.6 mm. A more preferable upper limit for the thickness of the hot-rolled steel sheet is 2.5 mm, and a more preferable lower limit may be 1.6 mm.

[0063] Cooling and winding The above hot-rolled steel sheet can be cooled to a temperature range of 540-680°C at a cooling rate of 5-50°C / s and then wound up.

[0064] During the above cooling process, if the cooling rate is less than 5°C / s, the pearlite structure becomes coarse, posing a risk of brittleness. On the other hand, if the cooling rate exceeds 50°C / s, the shape may deteriorate due to material variation in the width direction caused by overcooling of the edges in the width direction, potentially making winding difficult.

[0065] If the winding temperature is below 540°C, a bainite or martensitic structure, which is a low-temperature transformation structure, may be formed, making it difficult to obtain a uniform hot-rolled structure. On the other hand, the upper limit of the winding temperature can be limited to 680°C. However, since this may induce surface defects by forming an internal oxide layer and a decarburized layer on the surface, it is more preferable to limit it to 660°C or below to prevent this.

[0066] After the winding process described above, the present invention may further include a step of pickling the hot-rolled steel sheet. This pickling can be performed after the wound steel sheet has been naturally cooled to 200°C or below, and the pickling can remove scale formed on the surface of the steel sheet.

[0067] heat treatment The cooled and wound steel sheet can be heated to a temperature range of 850-1050°C and held for 5-20 minutes, then cooled to a temperature range of 500-650°C at a cooling rate of 50-250°C / s, and held for 30-180 seconds. More preferably, the upper limit of the cooling rate is 150°C / s, the lower limit of the more preferable cooling temperature range is 520°C, and the upper limit may be 590°C. A further preferable upper limit of the holding time may be 120 seconds.

[0068] If the heating temperature, i.e., the austenizing heating temperature, is below 850°C, insufficient austenizing may result in the retention of undissolved carbides, potentially inducing brittleness. On the other hand, if the temperature exceeds 1050°C, the austenite grains may become coarser, reducing toughness and potentially decreasing the work-hardening ability of the pearlite structure, making it difficult to secure a uniform pearlite (fibrous pearlite) structure thereafter. While the present invention does not particularly limit the heating method, methods such as high-frequency induction heating or a box-type heating furnace can be used.

[0069] If the holding time after heating is less than 5 minutes, complete austenizing may be difficult, and if it exceeds 20 minutes, the crystal grains may become excessively coarse.

[0070] During cooling after heating and holding, if the cooling rate is less than 50°C / s, the proportion of grain boundary proelicit cementite may increase excessively, making it difficult to induce brittleness and form a uniform pearlite (fibrous pearlite) structure. On the other hand, the upper limit of the cooling rate can be limited to 250°C / s. However, since controlling the cooling rate is not easy and there is a risk of forming low-temperature structures other than pearlite, a more preferable upper limit for the cooling rate may be 150°C / s.

[0071] The lower limit of the cooling termination temperature may be 500°C. However, to prevent the risk of low-temperature structures other than pearlite, such as bainite, forming, a more preferable lower limit may be 520°C. Furthermore, the upper limit of the cooling termination temperature may be 650°C. However, considering that the crystal grains of the structure may become coarser and it may be difficult to form uniform pearlite (fibrous pearlite) after cold rolling, a more preferable upper limit of the cooling termination temperature may be 590°C.

[0072] If the holding time after cooling is less than 30 seconds, the pearlite structure may not be sufficiently formed, and the upper limit of this time can be limited to 180 seconds. On the other hand, due to a decrease in strength, it may be difficult to secure sufficient strength through work hardening after cold rolling, so a more preferable upper limit of the holding time may be 120 seconds. The heat treatment method in the present invention can use hydrogen gas, a salt bath, a lead bath, etc., but is not particularly limited. Furthermore, in the present invention, the steel sheet can be air-cooled after heat treatment.

[0073] In the present invention, it is preferable that the microstructure of the steel sheet after heat treatment includes a pearlite structure as the main phase and grain boundary progenitate cementite in an area of ​​4% or less. In this case, by appropriately controlling the fraction of grain boundary progenitate cementite, the cementite with very high strength can be minimized, making it easier to stretch when cold rolling the pearlite, and the thickness of the uniform pearlite can be made 2.5 μm or less. On the other hand, if the fraction of grain boundary progenitate cementite exceeds 4% in area, there is a possibility that brittle fracture due to grain boundary progenitate cementite may occur during cold rolling.

[0074] Cold pressure The heat-treated steel sheet described above can be cold-rolled with a cumulative reduction ratio of 75-96%. A more preferable lower limit for the cumulative reduction ratio is 80%, and a more preferable upper limit can be 95%.

[0075] In this invention, cold rolling can be performed by applying a constant reduction ratio to produce a cold-rolled steel sheet of a desired thickness. The lower limit of the reduction ratio can be limited to 75%. However, a more preferable lower limit may be 80% because it may become difficult to ensure a uniform fraction of fibrous pearlite. On the other hand, if the reduction ratio exceeds 96%, there is a risk of cracking due to excessive work hardening. A more preferable lower limit of the reduction ratio may be 95%. Detailed rolling pass conditions such as the reduction ratio and speed per individual pass, and the width size, vary depending on the equipment and application, and are therefore not specified in this invention. In this invention, the thickness of the cold-rolled steel sheet is preferably 0.1 to 0.6 mm, and more preferably 0.3 mm or less.

[0076] As described above, this cold rolling process allows the pearlite structure, which is the main phase forming the microstructure of the sheet material, to ultimately have three forms of pearlite structure due to compressive deformation in the thickness direction.

[0077] Therefore, the steel sheet of the present invention can have a fine structure comprising a pearlite structure as the main phase and grain boundary protereminate cementite in an area percentage of 4% or less, and through the cold rolling described above, the pearlite structure can be formed with an area percentage comprising 40% or more of uniform pearlite (fibrous pearlite), 50% or less of zigzag pearlite (bent pearlite), and 10% or less of heterogeneous pearlite.

[0078] The steel sheet of the present invention manufactured in this manner has a thickness of 0.1 to 0.6 mm, a tensile strength of 2100 MPa or more, an elongation of 2% or more, and a bending characteristic (R / t) of 3.0 or less (R is the bending radius at which no cracks occur in the bent portion after a 180° bending test, and t is the thickness of the steel sheet), and can possess excellent toughness characteristics while having high strength. The upper limit of the preferred thickness of the steel sheet may be 0.3 mm. More preferably, the tensile strength value is 2200 MPa or more, and the upper limit of the tensile strength value may be 2350 MPa.

[0079] The present invention will be described in more detail below with reference to 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]

[0080] (Examples) A steel slab having the alloy composition shown in Table 1 was heated to 1200°C for 2 hours, and then cold-rolled steel sheets were produced under the conditions shown in Table 2. The rough rolling temperature was 1080°C, and the finish rolling temperature was 900°C. After hot rolling, the cooling rate until coiling was 20°C / s, and the sheets were coiled under the coiling temperature conditions shown in Table 2. The produced hot-rolled steel sheets were pickled, heated to 950°C for 10 minutes, cooled at a cooling rate of 70°C / s, and then cold-rolled under the conditions shown in Table 2.

[0081] [Table 1]

[0082] [Table 2]

[0083] Table 3 below shows the microstructure and physical properties of the manufactured steel sheets, as measured. The microstructure was observed and shown after heat treatment and after cold rolling. First, before cold rolling, the area fraction of grain boundary progenitate cementite was measured and shown using x3000x electron microscope images of the heat-treated steel sheet. In the microstructure of the steel sheet before cold rolling shown in Table 3 below, all fractions other than grain boundary progenitate cementite include pearlite. For the steel sheet after cold rolling, multiple images of the cross-section in the thickness direction of the steel sheet were taken using a x4300x electron microscope, with approximately 10 to 15 images taken. After measuring the length of the thickness occupied by the microstructure, the thickness was expressed as a percentage, and the average value was expressed as the microstructure fraction. Furthermore, for the uniform pearlite (fibrous pearlite) structure, after measuring each thickness, the average value is shown in Table 3 below. In this case, the fractions of uniform pearlite, zigzag pearlite, and heterogeneous pearlite represent the fraction relative to the total pearlite fraction.

[0084] Furthermore, tensile and bending tests were performed on the manufactured cold-rolled steel sheets to indicate their physical properties and the presence or absence of cracks. Tensile tests were conducted at room temperature according to JIS No. 5 standard, and the tensile strength and elongation were measured and indicated. The presence or absence of cracks was indicated by an O if the R / t ratio was 3.0 or less after a 180° bending test (R is the bending radius at which no cracks occur in the bent portion after the 180° bending test, and t is the thickness of the steel sheet), and an X otherwise.

[0085] [Table 3]

[0086] As shown in Table 3, in the case of the inventive example that satisfies the alloy composition and manufacturing conditions of the present invention, the characteristics of the microstructure proposed in the present invention are met, and the physical properties targeted in the present invention are secured.

[0087] Figure 2 shows a photographic example of the microstructure fraction and method for calculating the thickness of uniform perlite (fibrous perlite) in Invention Example 2. When a microstructure photograph is taken in the thickness direction of the steel plate, uniform perlite (fibrous perlite) appears as a layered structure without bends or segmentation, and can be represented by a dashed line as shown in Figure 2. Zigzag perlite (bent perlite) is characterized by a layered structure that is bent in a zigzag shape by folding one or more times, and appears as a solid line in Figure 2. The thickness can be measured separately from uniform perlite (fibrous perlite) by a mixture of zigzag and wavy shapes. The parts of Figure 2 excluding the solid and dashed lines represent non-uniform perlite. After measuring the thickness of each microstructure, their sum can be calculated and expressed as a fraction, and the thickness of uniform perlite (fibrous perlite) can be expressed as the average of the measured thickness values.

[0088] On the other hand, while Comparative Example 1 satisfied the alloy composition of the present invention, the winding temperature was too low, resulting in insufficient strength assurance through work hardening during cold rolling due to the formation of a low-temperature structure, and the tensile strength did not meet the level targeted by the present invention.

[0089] Comparative Example 2, while satisfying the alloy composition of the present invention, had an excessively high winding temperature, resulting in the formation of a coarse pearlite structure. This coarse pearlite structure hindered the formation of a uniform pearlite (fibrous pearlite) structure during cold rolling, and consequently, the uniform pearlite (fibrous fraction) did not meet the level targeted by the present invention. As a result, the target was not met. Bending properties We were unable to secure it.

[0090] Comparative Example 3 satisfies the alloy composition of the present invention, but the heat treatment temperature was too low, resulting in the partial formation of a low-temperature structure. Growth rate However, it did not meet the level intended by this invention.

[0091] Comparative Example 4, while satisfying the alloy composition of the present invention, had an excessively high heat treatment temperature, resulting in the formation of a coarse pearlite structure. Consequently, the fraction of uniform pearlite (fibrous pearlite) did not meet the level targeted by the present invention. As a result, its strength was reduced.

[0092] Comparative Example 5 shows that during heat treatment, the holding time after cooling was not within the range of the present invention, resulting in insufficient time for the formation of sufficient uniform perlite (fibrous perlite), and consequently, the increase in strength due to work hardening during cold rolling was insufficient.

[0093] In Comparative Example 6, during heat treatment, the holding time after cooling exceeded the range of the present invention, resulting in insufficient formation of uniform perlite (fibrous perlite). After perlite formation, the material softened, reducing its strength and failing to meet the strength level targeted by the present invention.

[0094] Comparative Example 7 shows a case where the reduction ratio during cold rolling falls outside the range of the present invention, resulting in a low reduction ratio and failure to secure the desired fraction of uniform perlite (fibrous perlite) and tensile strength.

[0095] Comparative Example 8 is a case where the cold rolling reduction ratio is excessive. Although the strength increases , Growth rate However, it did not meet the scope intended by the present invention.

[0096] Comparative Example 9 showed that the C content did not reach the range of the present invention, and due to the formation of coarse perlite, the fraction of uniform perlite (fibrous perlite) did not reach the target range, and the strength also did not reach the range targeted by the present invention.

[0097] Comparative Example 10 showed that the Mn content did not reach the range of the present invention, and the fraction of uniform perlite (fibrous perlite) did not meet the level targeted by the present invention, making it difficult to secure the desired level of strength.

[0098] Comparative Example 11 is a case where the Mn content exceeds the range of the present invention, and the strength is Although it will increase, The objective of this invention is The growth rate was not met. .

[0099] As described above, the present invention has been explained in detail through the examples, but other forms of examples are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. In mass percent, it contains carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): 0.01% or less, aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, with the remainder being iron (Fe) and other unavoidable impurities. It has a microstructure that includes a pearlite structure as the main phase and grain boundary prophylaxis cementite making up less than 4 area percent. The aforementioned perlite structure is composed of, by area percentage, 40% or more of homogeneous perlite (fibrous perlite), 50% or less of zigzag perlite (bent perlite), and 10% or less of heterogeneous perlite. A steel plate having a tensile strength of 2100 MPa or more, an elongation of 2% or more, and a bending characteristic (R / t) of 3.0 or less (where R is the bending radius at which no cracks occur in the bent portion after a 180° bending test, and t is the thickness of the steel plate).

2. The steel plate according to claim 1, wherein, when the cross-section of the microstructure in the thickness direction is observed, the average thickness of the uniform pearlite is 2.5 μm or less.

3. The steel plate according to claim 1, wherein the A value in the following relational expression 1 is 1.2 or less. [Relationship 1] A=[Mn]+[Cr]+[V] (Here, [Mn], [Cr], and [V] are mass percent of each element.)

4. The steel plate according to claim 1, wherein the steel plate has a tensile strength of 2200 to 2350 MPa.

5. The steel plate according to claim 1, wherein the thickness of the steel plate is 0.1 to 0.6 mm.

6. The process involves reheating a steel slab containing, by mass percent, carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): 0.01% or less, aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, with the remainder being iron (Fe) and other unavoidable impurities. The steps include: a step of roughly rolling the reheated steel slab, The step of finishing-rolling the roughly-rolled steel sheet to obtain a hot-rolled steel sheet, The steps include: cooling the hot-rolled steel sheet to a temperature range of 540 to 660°C at a cooling rate of 5 to 50°C / s, and then winding it up; The steps include: heating the cooled and wound steel plate to a temperature range of 850 to 1050°C and holding it for 5 to 20 minutes, then cooling it to a temperature range of 520 to 590°C at a cooling rate of 50 to 150°C / s and holding it for 30 to 120 seconds; The process includes the step of cold-rolling the heat-treated steel sheet at a cumulative reduction rate of 80-96%, A method for manufacturing a steel sheet having a tensile strength of 2100 MPa or more, an elongation of 2% or more, and a bending characteristic (R / t) of 3.0 or less (where R is the bending radius at which no cracks occur in the bent portion after a 180° bending test, and t is the thickness of the steel sheet).

7. The steel slab is a steel plate manufacturing method according to claim 6, wherein the A value in the following relational expression 1 is 1.2 or less. [Relationship 1] A=[Mn]+[Cr]+[V] (Here, [Mn], [Cr], and [V] are mass percent of each element.)

8. The aforementioned reheating is performed in a temperature range of 1100 to 1300°C. The rough rolling is carried out in a temperature range of 1000 to 1100°C. The method for manufacturing a steel sheet according to claim 6, wherein the finish rolling is performed in a temperature range of 860 to 940°C.

9. The method for manufacturing a steel sheet according to claim 6, further comprising the step of pickling the steel sheet in a temperature range of 200°C or less after the winding.

10. The method for manufacturing a steel sheet according to claim 6, further comprising the step of air-cooling the steel sheet after the heat treatment.

11. The method for manufacturing a steel sheet according to claim 6, wherein the microstructure of the heat-treated steel sheet comprises a pearlite structure as the main phase and grain boundary prodromal cementite in an area percentage of 4% or less.

12. The method for manufacturing a steel sheet according to claim 6, wherein, after the finish rolling, the thickness of the hot-rolled steel sheet is 1.5 to 2.6 mm.

13. The method for manufacturing a steel sheet according to claim 6, wherein, after cold rolling, the thickness of the cold-rolled steel sheet is 0.1 to 0.6 mm.

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