Steel material and manufacturing method therefor

Abstract

The present invention relates to a steel material which can be mainly used as a component in vehicles such as all-terrain cranes and excavators and thus can be suitably used for industrial machinery.

Description

Steel material and method of manufacturing the same

[0001] The present invention relates to a steel material suitable for use in industrial machinery, which can be used mainly as a part of a vehicle such as an All Terrain crane or excavator. More specifically, it relates to a high-strength steel material having a yield strength of 960 MPa or more, a tensile strength of 980 MPa or more, an elongation of 14% or more, and excellent impact toughness at -40℃, and a method for manufacturing the same.

[0002] The demand for high-strength and high-performance materials in modern industrial sectors is continuously increasing. In particular, the need for materials that possess both lightweight properties and high strength is becoming increasingly prominent in the industrial machinery, shipbuilding, construction, and energy industries. One material developed to meet these demands is High-Strength Steel, which possesses 2 to 3 times the strength of conventional structural steel, enabling innovative applications in various industrial fields.

[0003] In the development process of conventional high-strength steel, the strength-ductility trade-off, where increasing strength leads to a decrease in ductility, has been identified as a major problem. Consequently, it has been difficult to secure sufficient ductility and toughness in the ultra-high strength range of 900 MPa or higher, which has become a factor limiting the actual application range of the material. In addition, there have been problems such as reduced weldability due to increased strength, increased sensitivity to hydrogen embrittlement, and increased complexity of the production process.

[0004] There is an urgent need in industrial settings for the development of high-strength steel that simultaneously possesses higher strength, excellent formability, and weldability. In particular, the application of ultra-high-strength steel of 980 MPa or higher is expanding in the industrial machinery industry, such as AT cranes, to improve fuel efficiency and enhance safety through component weight reduction. Additionally, the need for high-performance materials capable of withstanding extreme environments is increasing in the shipbuilding and offshore structure sectors. Consequently, there is a need to develop innovative high-strength steel that can overcome the limitations of existing technologies and meet the demands of the industry.

[0005] Recent developments in high-strength steel aim to simultaneously improve strength and ductility through microstructure control technology, alloy design optimization, and heat treatment process optimization. Furthermore, the introduction of alloy design techniques utilizing artificial intelligence (AI) and big data has made it possible to more efficiently derive optimal alloy compositions and process conditions.

[0006] Grain refinement is effective for simultaneously securing high strength, impact toughness, and high ductility. For steels with a main martensitic structure, it is important not only to control temperature and loading time conditions to prevent grain size from increasing during the austenitizing process, but also to properly control the rolling pass and temperature.

[0007] Meanwhile, to manufacture high-strength steel, a widely used method involves enhancing strength by improving hardenability through the addition of high content of carbon and appropriate amounts of hardenability-enhancing elements such as Mn, Cr, Mo, and Ni. While manufacturing high-strength steel by rolling below the recrystallization temperature, followed by water cooling and tempering heat treatment to ensure impact toughness, offers the advantage of stable production with minimal material variation, controlling the amount of alloying elements within an appropriate range is crucial because adding large quantities of these elements significantly increases manufacturing costs.

[0008] Patent Document 1 presents a method for manufacturing steel used for industrial machinery. By utilizing a combination of components with high hardenability through the addition of a low content of C, an appropriate level of Mn, and B, it proposes a method that can simultaneously achieve high strength (high hardness) and impact toughness by securing over 97% martensite and the remainder bainite in terms of area fraction. In particular, it has the advantage of securing a higher level of strength by performing reheating and rapid cooling after rolling and omitting subsequent tempering heat treatment. However, low-temperature impact toughness may be compromised due to the excessive increase in strength and hardness, and since it contains a high content of Mn and lacks a process for removing dislocations through subsequent tempering heat treatment, the crack sensitivity in bending tests is inevitably very high, making it unsuitable for use as high-strength steel.

[0009] [Prior Art Literature]

[0010] [Patent Literature]

[0011] (Patent Document 1) KR10-1899687B1

[0012] According to one embodiment of the present invention, a high-strength steel with excellent impact toughness can be provided.

[0013] According to another embodiment of the present invention, a method for manufacturing high-strength steel with excellent impact toughness can be provided.

[0014] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0015] A steel according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.10~0.22%, silicon (Si): 0.1~0.5%, manganese (Mn): 0.1~0.7%, phosphorus (P): 0.012% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.01~0.05%, chromium (Cr): 0.5~2.0%, molybdenum (Mo): 0.3~0.7%, vanadium (V): 0.02~0.1%, nickel (Ni): 0.01~0.5%, titanium (Ti): 0.005% or less (including 0%), nitrogen (N): 0.001~0.007%, remainder Fe and unavoidable impurities, and has a steel microstructure comprising, in area%, 80% or more of tempered martensite and less than 20% of bainite. The average packet size of the tempered martensite is 80 μm or less, and the aspect ratio of the cementite present in the tempered martensite packet or lath satisfies 1.0 to 1.5.

[0016] In the present invention, the steel may have a yield strength of 960 MPa or more, a tensile strength of 980 MPa or more, an elongation of 14% or more, and an impact toughness of 27 J or more when evaluated at -40℃.

[0017] A method for manufacturing steel according to another embodiment of the present invention comprises the step of heating a steel slab, comprising, in weight percent, carbon (C): 0.10~0.22%, silicon (Si): 0.1~0.5%, manganese (Mn): 0.1~0.7%, phosphorus (P): 0.012% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.01~0.05%, chromium (Cr): 0.5~2.0%, molybdenum (Mo): 0.3~0.7%, vanadium (V): 0.02~0.1%, nickel (Ni): 0.01~0.5%, titanium (Ti): 0.005% or less (including 0%), nitrogen (N): 0.001~0.007%, remainder Fe, and unavoidable impurities, in a temperature range of 1100~1250℃; The method comprises the steps of: rough rolling the heated steel slab in a temperature range of 900 to 1100°C; finish rolling the rough rolled steel in a finish rolling temperature range defined by the following equation 1, and then water cooling it to a temperature range of room temperature to 400°C; and heating the water-cooled steel to a temperature of 550 to 700°C at a heating rate of 10°C / min. or more, and then heat treating it for a heat treatment time range of (1.9t+10 min) (where t is the thickness of the steel).

[0018] [Relationship 1]

[0019] 910 - (310 × C) - (80 × Mn) - (55 × Ni) - (80 × Mo) + (119 × V) - (18 × Nb) + (179 × Al) < Finish Rolling Temperature (°C) < 887 + (464 × C) + {(6445 × Nb) - (644 × Nb 0.5 )} + {(732 × V) - (230 × V 0.5 )} + (363 × Al) - (357 × Si)

[0020] According to the present invention, by applying rolling and Direct Quenching & Tempering (DQT) heat treatment, it is possible to provide a high-strength steel with excellent low-temperature impact toughness, having a yield strength of 960 MPa or more, a tensile strength of 980 MPa or more, an elongation of 14% or more, and an impact toughness of 27 J or more at -40°C.

[0021] FIG. 1 is an optical microscope image of the microstructure observed at the 1 / 4 t point of a 25t thick hot-rolled steel material used in an embodiment of the present invention, where (a) represents Invention Example 1 and (b) represents Comparative Example 3.

[0022] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0023] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0024] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0025] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0026] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0027] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0028] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0029] It should be noted that, although not essential, the technical solution according to each aspect of the present invention may be usefully applied to other aspects of the technical solution. Furthermore, the composition and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to obtain advantageous effects.

[0030] The inventors recognized the need to develop a method to secure the required physical properties of steel for industrial machinery, such as parts for high-altitude cranes, as high strength and low-temperature impact toughness are required.

[0031] In particular, regarding steel materials for industrial machinery such as AT cranes, we conducted in-depth research on methods to secure high strength, low-temperature impact toughness, and economic feasibility. As a result, we confirmed that high-strength steel with target properties can be provided by controlling the composition and the relationships between certain components in alloy design, while simultaneously optimizing manufacturing conditions, leading to the completion of the present invention.

[0032] Hereinafter, the alloy composition of the steel material provided in the present invention and the reasons for limiting its content will be explained in detail.

[0033] Carbon (C): 0.10~0.22%

[0034] Carbon (C) is an element effective in improving the strength of steel. In order to sufficiently obtain the effect of strength improvement through increased hardenability, the above C may be included in an amount of 0.10% or more. However, if the content exceeds 0.22%, it may be advantageous for securing strength, but there is a problem of significantly impairing low-temperature impact toughness, and if the content is less than 0.1%, it is unsuitable for securing strength because the martensite fraction cannot be sufficiently secured. Therefore, in the present invention, it is preferable to limit the above C content to a range of 0.10 to 0.22%. More preferably, the above C content is limited to a range of 0.12 to 0.20%, and most preferably, it is limited to 0.13 to 0.18%.

[0035] Silicon (Si): 0.1~0.5%

[0036] Silicon (Si) is not only used as a deoxidizer but is also an element advantageous for improving the strength of steel. To sufficiently obtain the aforementioned effects, the above-mentioned Si may be included in an amount of 0.1% or more. However, if the content exceeds 0.5%, there is a risk that the low-temperature impact toughness will be compromised due to the excessive formation of MA. Therefore, in the present invention, it is preferable to limit the above-mentioned Si content to a range of 0.1 to 0.5%. More preferably, the above-mentioned Si content is limited to a range of 0.2 to 0.4%.

[0037] Manganese (Mn): 0.1~0.7%

[0038] Manganese (Mn) is an element advantageous for improving the strength of steel through solid solution strengthening effects. To fully obtain this effect and to suppress the isolated formation of sulfur (S) in the steel, the above Mn may be included in an amount of 0.1% or more. However, if the content exceeds 0.7%, a Mn segregation zone is formed in the rolling direction, which may cause cracks when bending is applied, and there is a problem with internal quality or significantly impairing low-temperature impact toughness due to an excessive amount of MnS in the center of the hot-rolled steel sheet. Therefore, in the present invention, it is necessary to control the Mn content to be lower than that of a conventional hot-rolled steel sheet. Accordingly, in the present invention, it is preferable to limit the Mn content to 0.1~0.7%, and more preferably to 0.1~0.5%.

[0039] Phosphorus (P): 0.012% or less

[0040] Phosphorus (P) is an element beneficial for improving steel strength and ensuring corrosion resistance, but since it can significantly impair steel impact toughness, it is desirable to limit its content to the lowest possible level.

[0041] In the present invention, since there is no difficulty in securing the target physical properties even if the above P is contained at a maximum of 0.012%, the content can be limited to 0.012% or less. However, 0% may be excluded considering the level that is inevitably added.

[0042] Sulfur (S): 0.003% or less

[0043] Sulfur (S) is an element that significantly impairs low-temperature impact toughness by combining with Mn in steel to form MnS, etc. Therefore, it is advantageous to limit the content of the aforementioned S to the lowest possible level.

[0044] In the present invention, since there is no difficulty in securing the target physical properties even if the above S is contained at a maximum of 0.003%, the content can be limited to 0.003% or less. However, 0% may be excluded considering the level that is inevitably added.

[0045] Aluminum (Al): 0.01~0.05%

[0046] Aluminum (Al) is an element that can deoxidize molten steel at a low cost. To sufficiently obtain the aforementioned effect, the Al may be included in an amount of 0.01% or more. However, if the content is excessive and exceeds 0.05%, it is undesirable because it not only causes nozzle clogging during continuous casting but also significantly reduces impact toughness due to the formation of oxidative inclusions. Therefore, in the present invention, it is preferable to limit the Al content to 0.01 to 0.05%. More preferably, it is limited to 0.02 to 0.04%.

[0047] Chrome (Cr): 0.5~2.0%

[0048] Cr is an effective element for securing strength by increasing hardenability and forming low-temperature phases such as bainite or martensite, and it is desirable to include at least 0.5% to achieve sufficient effect. However, excessive addition of Cr significantly increases the carbon equivalent, which not only impairs weldability but also harms economic efficiency, so it is desirable to include 2.0% or less. Therefore, in the present invention, it is desirable to include the above-mentioned Cr in an amount of 0.5 to 2.0%, and more preferably, to include Cr in a range of 0.6 to 1.8%.

[0049] Molybdenum (Mo): 0.3~0.7%

[0050] Mo has the effect of significantly improving hardenability to suppress ferrite formation while simultaneously inducing the formation of bainite or martensite, and can also greatly improve strength; therefore, it is necessary to add at least 0.3% to manufacture high-strength steel. In addition, it is an effective element that can obtain additional strength improvement by causing secondary hardening during the tempering heat treatment process. However, since it is an expensive alloying element and significantly increases carbon equivalents, leading to a decrease in welding efficiency as the preheating temperature increases before welding, it is necessary to limit it to a maximum of 0.7%. Accordingly, in the present invention, it is preferable to have the content of Mo in the range of 0.3 to 0.7%, and more preferably in the range of 0.3 to 0.6%.

[0051] Niobium (Nb): 0.03% or less (including 0%)

[0052] Niobium (Nb) precipitates in the form of NbC or Nb(C,N), significantly improving the strength of the base material. Furthermore, upon reheating to high temperatures, dissolved Nb inhibits the recrystallization of austenite and the transformation of ferrite or bainite, thereby providing a microstructure refinement effect. Additionally, as an element that significantly raises the unrecrystallization temperature, it is advantageous for grain refinement even without excessively lowering the rolling temperature. However, if the content is excessive, undissolved Nb forms in the form of TiNb(C,N), which becomes a factor that impairs UT defects and low-temperature impact toughness; therefore, it is desirable to limit the upper limit of the Nb to 0.03%. Accordingly, in the present invention, the Nb may be optionally included in an amount of 0.03% or less, and even if it is not included, there is no significant difficulty in securing material properties if the rolling temperature is maintained low without considering the rolling load.

[0053] Vanadium (V): 0.02~0.1%

[0054] Vanadium (V) precipitates in the form of VC or VN, significantly improving the strength of the base material and effectively preventing a significant decrease in strength during high-temperature tempering heat treatment. However, if the content is excessive, the size of the precipitates becomes too coarse during the tempering heat treatment process, and since the strength of the base material is significantly increased, it may impair low-temperature impact toughness; therefore, it is desirable to limit the upper limit of V to 0.1%. Accordingly, in the present invention, it is desirable to limit the V content to 0.02~0.1%, and more preferably to 0.05~0.08%.

[0055] Nickel (Ni): 0.01~0.5%

[0056] Nickel (Ni) is an element capable of simultaneously improving the strength and low-temperature impact toughness of the base material, and in order to sufficiently obtain these effects, it is necessary to add at least 0.01% of the said Ni. However, since the said Ni is an expensive element, if its content exceeds 0.5%, there is a problem of significantly reduced economic feasibility. Therefore, in the present invention, it is preferable to limit the said Ni content to 0.01% to 0.5%.

[0057] Titanium (Ti): 0.005% or less

[0058] When titanium (Ti) is added together with N, it forms TiN, thereby reducing the occurrence of surface cracks caused by the formation of AlN precipitates. However, in the case of high-strength hot-rolled steel sheets undergoing QT heat treatment, if the content exceeds 0.005%, coarse TiN is formed during the reheating of the steel slab and during reheating for quenching and tempering heat treatment, which acts as a factor that impairs low-temperature impact toughness. Therefore, it is desirable that the above Ti content be 0.005% or less, and it is acceptable for the Ti content to be 0%.

[0059] Nitrogen (N): 0.001~0.007%

[0060] Nitrogen (N) is an element that is advantageous for suppressing grain growth due to heat effect during welding by forming TiN when added together with Ti. In order to sufficiently obtain the aforementioned effect when adding Ti, the above N may be included in an amount of 0.001% or more. However, if the content exceeds 0.007%, coarse TiN is formed, which impairs low-temperature impact toughness, so this is undesirable. Therefore, the content of N is preferably 0.001 to 0.007%, and more preferably 0.001 to 0.004%.

[0061] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0062] Next, the microstructure of a steel material according to one embodiment of the present invention will be described.

[0063] The steel of the present invention may have a microstructure comprising 80% or more of tempered martensite and less than 20% of bainite in area percent. If the fraction of tempered martensite is less than 80%, not only is the strength not secured to a sufficient level, but an upper bainite containing coarse carbides may be formed, which may cause problems that impair impact toughness.

[0064] In addition, in the present invention, it is preferable that the average packet size of the tempered martensite be 80 μm or less. However, if the average packet size exceeds 80 μm, there may be a problem in that the impact toughness is significantly reduced.

[0065] In addition, in the present invention, the aspect ratio of the cementite present inside the martensite packet or martensite lath may satisfy 1.0 to 1.5. If the aspect ratio exceeds 1.5, the cementite and the matrix may separate relatively easily, which may cause problems that impair strength and toughness.

[0066] The steel of the present invention having the composition and microstructure as described above can have excellent low-temperature impact toughness along with appropriate strength and elongation, such that at the 1 / 4 t point in the thickness direction (where t means steel thickness (mm)), the yield strength evaluated perpendicular to the rolling direction is 960 MPa or more, the tensile strength is 980 MPa or more, the elongation is 14% or more, and the Charpy impact energy (CVN) value evaluated perpendicular to the rolling direction at -40℃ is 27 J or more.

[0067]

[0068] Hereinafter, a method for manufacturing a high-strength steel with excellent low-temperature impact toughness according to another embodiment of the present invention will be described in detail.

[0069] The method for manufacturing a high-strength steel with excellent low-temperature impact toughness according to the present invention comprises the steps of: heating a steel slab having the above compositional components in a temperature range of 1100 to 1250°C; rough rolling the heated steel slab in a temperature range of 900 to 1100°C; finishing rolling the rough rolled steel in a finishing rolling temperature range defined by the following equation 1, and then water cooling it to a temperature range of room temperature to 400°C; and tempering heat treating the water-cooled steel by heating it to a temperature of 550 to 700°C at a heating rate of 10°C / min. or more.

[0070] [Heating Steel Slabs]

[0071] In the present invention, prior to performing the hot rolling described later, it is preferable to undergo a process of heating and homogenizing the steel slab, and at this time, it is preferable to perform the heating process in a temperature range of 1100 to 1250°C.

[0072] If the heating temperature of the above steel slab is less than 1100℃, the precipitates (carbonitrides) formed within the slab are not sufficiently re-dissolved, and thus the formation of precipitates decreases in the process after hot rolling. On the other hand, if the temperature exceeds 1250℃, the austenite grains may coarsen, which may impair the physical properties of the steel.

[0073] [Rough Rolling]

[0074] Next, in the present invention, the heated steel slab is rough rolled in a temperature range of 900 to 1100°C. If the temperature during rough rolling is below 900°C, there is a problem that the temperature becomes excessively low during subsequent finishing hot rolling, and if the temperature exceeds 1100°C, there is a risk that the austenite grains will coarsen, thereby impairing low-temperature impact toughness.

[0075] [Final Rolling and Cooling]

[0076] In addition, in the present invention, the above-mentioned rough-rolled steel is finished-rolled in the temperature range shown in the following equation 1 and water-cooled at a cooling rate of 25℃ / s or more to a temperature range of 400℃ to room temperature.

[0077] [Relationship 1]

[0078] 910 - (310 × C) - (80 × Mn) - (55 × Ni) - (80 × Mo) + (119 × V) - (18 × Nb) + (179 × Al) < Finish Rolling Temperature (°C) < 887 + (464 × C) + {(6445 × Nb) - (644 × Nb 0.5 )} + {(732 × V) - (230 × V 0.5 )} + (363 × Al) - (357 × Si)

[0079] (Here, each component element represents the weight percentage of the corresponding element)

[0080] In the present invention, the formation of a fine austenite structure is induced through the low-temperature rolling process, and can also contribute to the refinement of the structure after subsequent accelerated cooling and tempering heat treatment.

[0081] If the above finishing rolling temperature is higher than the upper limit of Equation 1, the grain refinement effect by recrystallization may not occur sufficiently. Conversely, if it is lower than the lower limit of Equation 1, the rolling load increases, which may lead to difficulties in securing the shape of the hot-rolled steel sheet and potential quality defects such as surface cracks. Furthermore, the subsequent accelerated cooling start temperature may become excessively low, which may result in a significant decrease in strength due to the formation of ferrite caused by abnormal cooling.

[0082] In the present invention, if the cooling rate during water cooling is less than 25℃ / s, an excessive amount of bainite is formed, which may impair strength and low-temperature impact toughness. Although no upper limit for the cooling rate is specified, it is preferable that it be 200℃ / s or less, which is the maximum level for the equipment.

[0083] [Heat Treatment]

[0084] Finally, the water-cooled hot-rolled steel sheet is heat-treated at a heating rate of 10°C / min or higher in a temperature range of 550 to 700°C for a duration of (1.9t + 10 min) (t is the thickness of the hot-rolled steel sheet). If the water-cooled hot-rolled steel sheet is heated at a rate of less than 10°C / min, relatively coarse film-shaped cementite may be distributed at the martensite lath boundary, whereas if heated at a sufficient rate, spherical fine cementite is evenly distributed at the martensite lath boundary and within, which is advantageous for improving toughness. Additionally, if the water-cooled hot-rolled steel sheet is heat-treated at a temperature below 550°C, it is advantageous for securing strength but difficult to secure low-temperature impact toughness, and if heat-treated at a temperature above 700°C, the strength may be significantly reduced due to the excessive removal of dislocations.

[0085] In addition, in the present invention, if the heat treatment time is less than (1.9t+10 min), a problem may occur in which the center of the hot-rolled steel sheet does not reach the target temperature. In the present invention, the upper limit of the heat treatment time is not restricted, but it is preferably within (2.7t+10 min), considering the economic efficiency of the equipment, it is desirable to keep it within (2.3t+10 min).

[0086] And the above heat-treated steel can then be air-cooled to room temperature.

[0087] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0088] (Example)

[0089] Steel Grade Composition (Weight%) CSI Mn PS Al Cr Mo VN i Nb Ti N 10.20 0.35 0.25 0.00 80.00 20.02 5 0.90 0.5 30.06 5 0.10 0.01 0.00 20.00 35 20.19 0.30 0.20 0.00 70.00 20.02 5 1.00 0.5 40 .0700.10-0.0020.003530.080.301.200.0070.0020.0251.000.550.0650.10-0.0020.003340.150.30.40.0080.0020.0250.30.550.070.10-0.0020.0035

[0090] A continuously cast slab was manufactured by continuously casting molten steel having the alloy composition shown in Table 1 above. At this time, the continuously cast slab was manufactured to a thickness of 250 mm. Steel grades 1 and 2 in Table 1 above represent cases where all component ranges intended for the present invention are satisfied, steel grade 3 represents cases where the C and Mn content deviates from the values ​​presented in the present invention, and steel grade 4 represents cases where the Cr content deviates from the range of the present invention. Subsequently, the continuously cast slab was heated and extracted under the conditions of Table 2 below and rough rolled at 900°C or higher. Then, the rough rolled slab was finished hot-rolled under the conditions of Table 2 below to manufacture a hot-rolled steel sheet with a thickness of 25 mm. Afterward, accelerated cooling was performed, and the final hot-rolled steel material was manufactured by accelerated cooling, heating, and tempering heat treatment under the conditions of Table 2 below. Specifically, except for Comparative Example 3, the others were 10°C / min. The temperature was raised to 620°C at the above heating rate, maintained for a certain period of time, and then air-cooled to room temperature. In Comparative Example 2, the finishing hot rolling temperature exceeded the upper limit of Equation 1 presented in the present invention. Comparative Example 3 is a case where the heating rate during tempering is too low. Comparative Example 4 is a case where the end temperature of accelerated cooling after the end of rolling is higher than the value presented in the present invention.

[0091] Steel Grade Manufacturing Process Condition Relationship 1 Satisfaction Remarks Heating Temperature (°C) Finish Rolling End Temperature (°C) Cooling Start Temperature (°C) Cooling End Temperature (°C) Tempering Heating Rate (°C / min) Tempering Temperature (°C) Tempering Time (min) 11 1208 10770 250 136 2068 Satisfactory Invention Example 1 2 1 1208 15775 250 136 2068 Satisfactory Invention Example 2 3 1 1208 15775 250 136 2068 Satisfactory Comparative Example 1 1 1208 908 50 300 136 2068 Unsatisfactory Comparative Example 2 1 1 1208 15775 250 56 2068 Satisfactory Comparative Example 3 1 1 1208 15775 450 136 2068 Satisfactory Comparative Example 4 4 ​​1 1208 15770 25 136 2068 Satisfactory Comparative Example 5 1 1 1208 15770 25 137 2068 Satisfactory Comparative Example 6 1 1 1208 15770 25 136 2038 Satisfactory Comparative Example 7

[0092] Subsequently, the microstructure of each manufactured steel was observed and its mechanical properties were evaluated. Specifically, microstructures such as tempered martensite and bainite were observed using an optical microscope, and then the boundaries of each phase were distinguished visually, after which the fractions of each were measured. The martensite packet diameter was determined using the Linear Intercept method, and the aspect ratio (the ratio of width to length of cementite inside the packet or lath) was determined by randomly selecting about 10 to 20 cementite samples, measuring their lengths in the x / y direction, and calculating the average value. At this time, the microstructure was measured at the 1 / 4t point in the thickness direction of each steel, and the results are shown in Table 3 below.

[0093] In addition, the mechanical properties of each manufactured steel were evaluated at the 1 / 4t point in the thickness direction. For the tensile test, JIS No. 13 annular specimens with a gauge length of 50 mm were taken perpendicular to the rolling direction to measure the tensile strength (TS), yield strength (YS), and elongation (El). For the impact test, JIS No. 4 specimens were taken perpendicular to the rolling direction at the 1 / 4t point in the thickness direction to measure the average impact toughness (CVN) at -40℃, and the results are shown in Table 4 below.

[0094] Steel Grade Tempered Martensite Fraction (Area %) Bainite Fraction (Area %) Average Packet Size (㎛) Cementite Aspect Ratio Remarks 190 104 51.3 Invention Example 1 290 105 51.3 Invention Example 2 35 54 54 01.7 Comparative Example 1 190 108 51.2 Comparative Example 2 190 104 52.1 Comparative Example 3 170 306 01.2 Comparative Example 4 46 535 6 51.3 Comparative Example 5 125 108 01.3 Comparative Example 6 190 105 51.3 Comparative Example 7

[0095] Steel Grade YP(MPa)TS(MPa)El.(%)CVN(J@-40℃)Remarks 1100 2107 4249 4Inventive Example 129 86 106 724 105Inventive Example 23 101 5108 817 16Comparative Example 118 94 102 313 24Comparative Example 219 68 104 514 19Comparative Example 318 79 100 41 512Comparative Example 448 729 79 22 24Comparative Example 516 9 78 43 271 15Comparative Example 61 100 91 13 211 10Comparative Example 7

[0096] As shown in Table 3-4 above, Invention Example 1-2, which satisfies all the component ranges and manufacturing processes presented in the present invention, satisfies all the fractions of tempered martensite and bainite, packet size, and cementite aspect ratio inside the packet or lath presented in the present invention, and it can be confirmed that it has excellent yield strength, tensile strength, and impact toughness.

[0097] On the other hand, Comparative Example 1 did not meet the values ​​for yield strength and tensile strength presented in the present invention due to the low C content. Unlike the expectation that impact toughness would be excellent when low C content is added, it was confirmed that the impact toughness was inferior despite exhibiting low strength, as the fraction of tempered martensite was insufficient and the fraction of coarse tempered bainite was excessively high. Furthermore, it was determined that the high Mn content caused additional adverse effects on impact toughness, as segregation zones formed in the rolling direction or inclusions such as MnS occurred as crack initiation points.

[0098] Comparative Example 2 can be seen to have a yield strength that deviates from the value presented in the present invention due to the coarse packet size caused by the high rolling temperature, and it can also be seen to have poor elongation and impact toughness.

[0099] Comparative Example 3 satisfies the strength, but it can be seen that due to the excessively low heating rate, coarse carbides precipitate in the form of a film at the lath or packet boundaries, resulting in inferior impact toughness.

[0100] And in the case of Comparative Example 4, a low level of impact toughness was exhibited due to the excessively high fraction of bainite, in which cementite is formed inside, caused by the high cooling end temperature.

[0101] In the case of Comparative Example 5, it can be seen that the hardenability is insufficient due to the low Cr content, and a low level of strength is exhibited due to the lack of martensite fraction.

[0102] In the case of Comparative Example 6, it can be seen that due to the excessively high tempering heat treatment temperature, a significant portion of the martensite structure transforms into ferrite, causing a significant decrease in fraction and a significant decrease in strength.

[0103] In the case of Comparative Example 7, the tempering heat treatment time is short, so unlike Comparative Example 6, the dislocations are not sufficiently loosened, and it can be confirmed that although the strength is very high, the impact toughness is inferior.

[0104] Meanwhile, FIG. 1 is an optical microscope image of the microstructure observed at the 1 / 4 t point of a hot-rolled steel material with a thickness of 25 t used in an embodiment of the present invention, where (a) represents Inventive Example 1 and (b) represents Comparative Example 4. As shown in FIG. 1, Inventive Example 1 was heated to the tempering temperature at a sufficiently high speed, so spherical cementite was formed inside the lath or packet, whereas Comparative Example 3 was heated at an insufficient speed, so relatively large, film-shaped cementite was formed, which can be judged to have adversely affected impact toughness.

Claims

1. A steel microstructure comprising, in weight%, carbon (C): 0.10–0.22%, silicon (Si): 0.1–0.5%, manganese (Mn): 0.1–0.7%, phosphorus (P): 0.012% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.01–0.05%, chromium (Cr): 0.5–2.0%, molybdenum (Mo): 0.3–0.7%, vanadium (V): 0.02–0.1%, nickel (Ni): 0.01–0.5%, titanium (Ti): 0.005% or less (including 0%), nitrogen (N): 0.001–0.007%, remainder Fe and unavoidable impurities, and in area%, comprising 80% or more of tempered martensite and less than 20% of bainite, wherein the above tempered A steel material having an average packet size of martensite of 80㎛ or less, and an aspect ratio of cementite present in the martensite packets or lattice of 1.0 to 1.

5.

2. In Paragraph 1, Niobium (Nb): Steel containing 0.03% or less additionally.

3. In Paragraph 1, The above steel has a yield strength of 960 MPa or more, a tensile strength of 980 MPa or more, an elongation of 14% or more, and an impact toughness of 27 J or more as evaluated at -40℃.

4. A step of heating a steel slab containing, in weight percent, carbon (C): 0.10~0.22%, silicon (Si): 0.1~0.5%, manganese (Mn): 0.1~0.7%, phosphorus (P): 0.012% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.01~0.05%, chromium (Cr): 0.5~2.0%, molybdenum (Mo): 0.3~0.7%, vanadium (V): 0.02~0.1%, nickel (Ni): 0.01~0.5%, titanium (Ti): 0.005% or less (including 0%), nitrogen (N): 0.001~0.007%, remainder Fe and unavoidable impurities, in a temperature range of 1100~1250℃; A step of rough rolling the above heated steel slab at a temperature range of 900 to 1100℃; A step of finishing rolling the above rough-rolled steel at a finishing rolling temperature range defined by the following Equation 1, and then water-cooling it to a temperature range of room temperature to 400℃; and A method for manufacturing steel comprising the step of heating the above water-cooled steel to a temperature of 550 to 700°C at a heating rate of 10°C / min. or more, and then heat treating for a heat treatment time range of (1.9t + 10 min) (t is the thickness of the steel). [Relationship 1] 910 - (310 × C) - (80 × Mn) - (55 × Ni) - (80 × Mo) + (119 × V) - (18 × Nb) + (179 × Al) < Finish Rolling Temperature (°C) < 887 + (464 × C) + {(6445 × Nb) - (644 × Nb 0.5 )} + {(732 × V) - (230 × V 0.5 )} + (363 × Al) - (357 × Si) (Here, each component element represents the weight percentage of the corresponding element) 5. In Paragraph 4, A method for manufacturing steel, wherein the heat treatment is performed within a heat treatment time range of (2.7t + 10 min) (t is the thickness of the steel) or less during the above heat treatment.

6. In Paragraph 4, The above steel slab is a method for manufacturing steel that additionally contains 0.03% or less of niobium (Nb).

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