High-strength steel material having excellent low-temperature toughness and manufacturing method therefor

Abstract

One aspect of the present invention is to provide a high-strength steel material having excellent low-temperature toughness, which is suitable for use in a submarine pressure hull, and a manufacturing method therefor.

Description

High-strength steel with excellent low-temperature toughness and method for manufacturing the same

[0001] The present invention relates to a high-strength steel with excellent low-temperature toughness and a method for manufacturing the same. More specifically, the present invention relates to a high-strength steel with excellent low-temperature toughness suitable for use in submarine pressure hulls and a method for manufacturing the same.

[0002] In the modern maritime defense industry, submarines have established themselves as a core element of national defense and strategic deterrence. In particular, the pressure hull, a key component of a submarine, is a critical structure that must withstand extreme pressures in the deep sea while maximizing submersion capability and the performance of onboard weaponry. Accordingly, the materials used for the pressure hull face high technical requirements, satisfying high strength, excellent weldability, durability, and corrosion resistance.

[0003] High-strength, low-alloy steels such as HY80 and HY100 have been primarily used for conventional submarine pressure hulls. However, due to increasingly stringent operational performance requirements for submarines, there is a growing need to develop steel with a yield strength of 180 ksi (approx. 1240 MPa) that exceeds the strength and reliability of these materials. This is an essential element for overcoming the performance limitations of steel, particularly in deep-sea environments.

[0004] Accordingly, it is an important task to secure manufacturing technology for high-strength and high-reliability steel and, through this, play a leading role in the development of steel for submarine pressure hulls.

[0005] According to one embodiment of the present invention, a high-strength steel with excellent low-temperature toughness and a method for manufacturing the same can be provided.

[0006] 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.

[0007] A steel according to one embodiment of the present invention comprises, in weight percent, C: 0.10~0.20%, Si: 0.50% or less, Mn: 0.05~0.50%, P: 0.010% or less, S: 0.006% or less, Ni: 8.0~12.0%, Cr: 1.0~3.0%, Mo: 0.5~2.5%, Co: 2.0~6.0%, and the remainder being Fe and unavoidable impurities, wherein the value of A derived by the following equation 1 is 0.10 or more and 0.70 or less, the value of B derived by the following equation 2 is 0.30 or more, the yield strength is 1240 MPa or more and 1340 MPa or less, and the Charpy impact toughness at -20℃ is 68 J or more.

[0008] [Relationship 1]

[0009] In the above equation 1, Cr, Mo, Co, and C represent the content (weight%) of each element.

[0010] [Relationship 2]

[0011] In the above equation 2, Ni and Co represent the content (weight%) of each element.

[0012] The aforementioned steel contains 5 to 20 (Mo,Cr)(C,N) precipitates / μm with a particle size of 5 to 20 nm within the matrix structure. 2 It can be distributed as.

[0013] The steel described above may contain 90% or more of tempered martensite in its microstructure.

[0014] The effective grain size of the tempered martensite described above may be 20 μm or less.

[0015] A method for manufacturing a steel material according to another embodiment of the present invention comprises the steps of: heating a steel material comprising, in weight percent, C: 0.10~0.20%, Si: 0.50% or less, Mn: 0.05~0.50%, P: 0.010% or less, S: 0.006% or less, Ni: 8.0~12.0%, Cr: 1.0~3.0%, Mo: 0.5~2.5%, Co: 2.0~6.0%, and the remainder being Fe and unavoidable impurities, wherein the value of A derived by the following Equation 1 is 0.10 or more and 0.70 or less, and the value of B derived by the following Equation 2 is 0.30 or more; the step of obtaining a hot-rolled steel sheet by hot-rolling the steel material at a temperature of 850°C or more and 950°C or less with a final pass reduction amount of 10% or more; and the step of cooling the hot-rolled steel sheet. The method may include the steps of: reheating and quenching the hot-rolled steel sheet at a temperature of 800°C or higher and 900°C or lower for a time of 1.6 * thickness (mm) + 20 minutes or more and 1.6 * thickness (mm) + 60 minutes or less; tempering the hot-rolled steel sheet at a tempering temperature of 500°C to 600°C for 3 hours or more and 10 hours or less; and pressing the surface of the hot-rolled steel sheet with a plasticity of 70 to 85%.

[0016] [Relationship 1]

[0017] In the above equation 1, Cr, Mo, Co, and C represent the content (weight%) of each element.

[0018] [Relationship 2]

[0019] In the above equation 2, Ni and Co represent the content (weight%) of each element.

[0020] The heating temperature in the heating step described above may be 900°C or higher and 1200°C or lower.

[0021] In the aforementioned reheating and quenching step, the quenching may be performed by cooling to 150°C or lower at a cooling rate of 10°C / s or more.

[0022] The present invention can provide a high-strength steel with excellent low-temperature toughness and a method for manufacturing the same.

[0023] Thus, the present invention can provide steel suitable for use in submarine pressure hulls.

[0024] Preferred embodiments of the present invention are described below. 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0029] 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.

[0030] Cobalt in steel is a key element that helps maintain the high strength of the steel by delaying dislocation recovery in the lath martensite matrix.

[0031] However, when cobalt is added for the aforementioned purpose, there was a problem that the manufacturing cost of the steel became excessive due to the addition of a large amount of expensive cobalt.

[0032] Accordingly, the inventors have found that the deterioration in the strength of steel due to the reduction of cobalt can be compensated by forming (Mo, Cr)-based precipitates while reducing the aforementioned cobalt content.

[0033] In particular, the inventors derived the present invention by recognizing that high strength and high strength characteristics of steel can be simultaneously secured by controlling the relative content between Cr, Mo, Co, and C, and the relative content between Ni and Co, while reducing the content of C, a hardenable element.

[0034] First, the alloy composition of the steel of the present invention and the reasons for its limitation will be explained in detail.

[0035] A steel material according to one embodiment of the present invention may contain, in weight%, C: 0.10~0.20%, Si: 0.50% or less, Mn: 0.05~0.50%, P: 0.010% or less, S: 0.006% or less, Ni: 8.0~12.0%, Cr: 1.0~3.0%, Mo: 0.5~2.5%, and Co: 2.0~6.0%.

[0036] C: 0.10~0.20%

[0037] Carbon (C) is the most economical and effective element for ensuring hardenability during heat treatment and hardness after heat treatment. To achieve these effects, it is desirable to include at least 0.10% of the carbon. On the other hand, if the content is excessive, the strength increases excessively, which leads to a problem of deterioration in low-temperature toughness. Therefore, it is desirable that the upper limit of the carbon content be 0.20%. As another example, the carbon content may be 0.11~0.19% or 0.12~0.18%.

[0038] Si: 0.50% or less

[0039] Silicon (Si) is an element that deoxidizes molten steel and has a solid solution strengthening effect. However, if the content is excessive, red scale is formed on the surface of the steel sheet, which not only significantly degrades the surface quality of the steel sheet but also causes an excessive increase in strength, which degrades the impact toughness of the final product and may have an adverse effect on temper brittleness. Therefore, it is desirable to limit the upper limit of the silicon content to 0.50%. As another example, the silicon content may be 0.01 to 0.20 or less.

[0040] Mn: 0.05~0.50%

[0041] Manganese (Mn) is an element that contributes to securing hardness after heat treatment by increasing hardenability. If the manganese content in the steel is excessively low, coarse FeS may form, which can degrade the impact toughness of the steel. Therefore, it is desirable that the lower limit of the manganese content be 0.05%. However, if the content is excessive, there is a problem where thickness-centered segregation develops significantly during the casting of steel slabs in the continuous casting process, and low-temperature toughness decreases. Therefore, it is desirable that the upper limit of the manganese content be 0.50%. As another example, the manganese content may be 0.10~0.40% or 0.15~0.30%.

[0042] P: 0.010% or less

[0043] Phosphorus (P) is an inevitably contained impurity that impairs the weldability of steel and is a major cause of increased temper brittleness due to segregation at grain boundaries; therefore, it is desirable to control its content to be as low as possible. Accordingly, it is desirable to limit the upper limit to 0.010%. Since it is theoretically advantageous to limit the phosphorus content to 0%, the present invention may set the lower limit to 0%.

[0044] S: 0.006% or less

[0045] Sulfur (S), like phosphorus (P) mentioned above, is an inevitably contained impurity that combines with Mn and others to form non-metallic inclusions, which significantly reduces the toughness of steel; therefore, it is desirable to suppress its content as much as possible. Accordingly, it is important to manage the upper limit, and in the present invention, it is preferable to limit the upper limit of the sulfur content to 0.006%. Meanwhile, since it is theoretically advantageous to limit the sulfur content to 0%, the lower limit of the present invention can be set to 0%.

[0046] Ni: 8.0~12.0%

[0047] Ni is an alloying element that improves low-temperature toughness and is limited to 8.0 to 12.0%, because if the content is less than 8.0%, the effect of improving low-temperature toughness as described above is insufficient, and if it exceeds 12.0%, it is uneconomical because Ni is expensive. As another example, the nickel content may be 9.0 to 11.0% or 9.5 to 10.5%.

[0048] Cr: 1.0~3.0%

[0049] Cr is an alloying element directly related to (Cr, Mo)-based precipitates, and its content needs to be 1.0% or more to improve strength. However, since excessive addition impairs weldability, the present invention may set the upper limit of the Cr content to 3.0%. As another example, the nickel content may be 1.5~2.8% or 2.0~2.6%.

[0050] Mo: 0.5~2.5%

[0051] Mo is an alloying element directly related to (Cr, Mo)-based precipitates, such as Cr, and must be added in an amount of 0.5% or more; however, since it is expensive, it can be kept below 2.5%. As another example, the molybdenum content may be 1.0~2.3% or 1.2~2.1%.

[0052] Co: 2.0~6.0%

[0053] Co is a very important alloying element in secondary hardening alloys and can delay the recovery of dislocations in the lath martensite matrix, thereby promoting the precipitation of fine carbides. As a result, the Co can play a significant role in improving strength. For this purpose, it can be added in an amount of 2.0% or more, but due to its high cost, the upper limit may be restricted to 6.0%. As another example, the cobalt content may be 3.0~5.5% or 3.5~5.0%.

[0054] 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. Since these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification. As an example, in addition to essential components such as C, Si, and Mn, the steel may additionally include Al, N, etc., as impurity elements inevitably incorporated during the steelmaking process.

[0055] Meanwhile, the steel according to one embodiment of the present invention satisfies the alloy composition described above, and at the same time, it is preferable that the value of A derived by the following relationship 1 is 0.10 or more and 0.70 or less.

[0056] [Relationship 1]

[0057] In the above equation 1, Cr, Mo, Co, and C represent the content (weight%) of each element.

[0058] Through the above relationship 1, the present invention can appropriately form fine precipitates by controlling the relative content of Cr, Mo, Co, and C, thereby simultaneously securing high toughness and high strength.

[0059] If the above A value is less than 0.10, it may be difficult to secure high strength due to insufficient precipitation of (Mo,Cr)(C,N) fine precipitates. On the other hand, if the above A value is greater than 0.70, low-temperature toughness may be reduced due to excessive precipitation of (Mo,Cr) carbonitrides. As another example, the above A value may be 0.15 to 0.60 or 0.20 to 0.50.

[0060] In addition, the steel according to one embodiment of the present invention may have a B value of 0.30 or higher derived by the following relationship 2.

[0061] [Relationship 2]

[0062] In the above equation 2, Ni and Co represent the content (weight%) of each element.

[0063] Thus, the present invention can secure excellent low-temperature toughness of the steel by limiting the addition of Co content while simultaneously adding Ni. As another example, the B value may be 0.50 or higher or 0.70 or higher. Since a higher B value is advantageous for achieving the objective of the present invention, the present invention does not separately limit the upper limit of the B value. However, as an example, the B value may be 2.00.

[0064] The microstructure of the steel of the present invention will be described below.

[0065] First, the steel according to one embodiment of the present invention may have a matrix structure of tempered martensite as its microstructure.

[0066] The above tempered martensite may be included in an area of ​​90% or more.

[0067] If the above tempered martensite is included in an area of ​​less than 90%, a problem may arise in that the desired level of strength cannot be secured.

[0068] The effective grain size of the tempered martensite may be 20 μm or less. That is, the present invention can improve low-temperature impact toughness through a matrix structure of fine tempered martensite. As another example, the effective grain size of the tempered martensite may be 17 μm or less or 15 μm or less. The smaller the effective grain size of the tempered martensite, the better. However, as an example, the lower limit of the effective grain size of the tempered martensite may be 5 μm.

[0069] However, in addition to the above-mentioned tempered martensite, a secondary phase may inevitably be formed during the manufacturing process. The secondary phase may be one or more of retained austenite, bainite, pearlite, and ferrite, and it is preferable that the fraction thereof be 10 area% or less in total. If the fraction of the secondary phase exceeds 10%, it may be disadvantageous for securing strength.

[0070] In addition, the steel of the present invention contains 5 to 20 (Mo,Cr)(C,N) precipitates / μm with a particle size of 5 to 20 nm within the matrix structure. 2 It can be distributed as follows. In this case, the particle size may represent the diameter equivalent to a circle.

[0071] The present invention enables high strength to be secured despite low C and Co content by distributing the aforementioned (Mo,Cr)(C,N) fine precipitates within the matrix structure. To this end, the present invention provides the above (Mo,Cr)(C,N) precipitates at 5 / μm 2 It can be distributed as described above. However, since the toughness of the steel may decrease if the aforementioned precipitates become excessive, the present invention [manifests] the (Mo,Cr)(C,N) precipitates at 20 particles / μm 2 It can be distributed as follows. As another example, the above (Mo,Cr)(C,N) precipitates in the matrix structure are 10 to 15 per μm 2 It can be distributed as.

[0072] The steel of the present invention described above has excellent low-temperature impact toughness along with high strength characteristics. Specifically, the steel of the present invention described above may have a yield strength of 1240 MPa or more and 1340 MPa or less, and a Charpy impact toughness of 68 J or more at -20℃.

[0073] Below, a method for manufacturing steel according to the present invention will be described.

[0074] First of all, according to a method for manufacturing steel according to one embodiment of the present invention, a steel material having the alloy components described above can be heated.

[0075] When heating the steel material described above, the heating temperature may be 900°C or higher and 1200°C or lower.

[0076] If the above heating temperature is less than 900℃, the deformation resistance of the steel increases, making it impossible to effectively perform the subsequent hot rolling process, whereas if it exceeds 1200℃, the austenite grains become coarse, and there is a risk that the toughness will deteriorate.

[0077] Next, a method for manufacturing steel according to one embodiment of the present invention may include the step of hot-rolling the steel material to obtain a hot-rolled steel sheet.

[0078] The finishing temperature during the hot rolling described above may be 850°C or higher and 950°C or lower.

[0079] If the above finishing temperature is less than 850°C, sufficient tempered martensite cannot be formed in the final product, which may result in a decrease in the strength of the steel. On the other hand, if the above finishing temperature exceeds 950°C, the grains may coarsen, and toughness may be compromised. As another example, the above finishing temperature may be 900°C or higher and 930°C or lower.

[0080] In addition, the final pass reduction amount during the aforementioned hot rolling can be 10% or more.

[0081] If the above final pass reduction amount is less than 10%, the degree of reduction is small, so the grain size may become coarse. As another example, the above final pass reduction amount may be 12% or more.

[0082] Next, the method for manufacturing steel according to one embodiment of the present invention may cool the hot-rolled steel sheet obtained as described above. Through this, recrystallization can be suppressed and grain growth can be limited. Although the cooling end temperature and cooling rate during the cooling according to the present invention may be designed in various ways within a range that does not impair the effects described above. As an example, the cooling may be an air cooling method.

[0083] After the cooling described above, the method for manufacturing steel according to one embodiment of the present invention may include the step of reheating and quenching the hot-rolled steel sheet.

[0084] The reheating described above may involve maintaining the hot-rolled steel sheet at a temperature of 800°C or higher and 900°C or lower for a time of 1.6 * thickness (mm) + 20 minutes or more and 1.6 * thickness (mm) + 60 minutes or less.

[0085] If the reheating temperature is less than 800°C or the reheating time is less than 1.6 * thickness (mm) + 20 minutes, it may be difficult to form a sufficient amount of tempered martensite even by the quenching and tempering processes described later. On the other hand, if the reheating temperature exceeds 900°C or the reheating time exceeds 1.6 * thickness (mm) + 60 minutes, the grain size may coarsen, and the toughness of the steel may decrease. As another example, the reheating may be performed by maintaining the hot-rolled steel sheet at a temperature of 810°C or higher and 890°C or lower for a duration of 1.6 * thickness (mm) + 30 minutes or longer and 1.6 * thickness (mm) + 50 minutes or less.

[0086] Then, the method for manufacturing steel according to the present invention can secure a high-strength martensite matrix structure by quenching the reheated hot-rolled steel sheet. Within the scope of achieving the above-described effect, the present invention does not separately limit the cooling end temperature and cooling rate during quenching, but as an example, cooling during quenching may be performed to a temperature of 150°C or lower at a cooling rate of 10°C / s or more.

[0087] Next, a method for manufacturing steel according to one embodiment of the present invention may include a step of tempering the hot-rolled steel sheet.

[0088] The tempering described above may be performed at a tempering temperature of 500 to 600°C for 3 hours or more and 10 hours or less.

[0089] If the above tempering temperature is less than 500°C or the tempering time is less than 3 hours, toughness may be reduced due to temper brittleness or strength may be reduced due to insufficient precipitation of fine precipitates. On the other hand, if the above tempering temperature exceeds 600°C or the tempering time exceeds 10 hours, coarse precipitates may be formed, and as a result, it may be difficult to secure high strength. As another example, the tempering described above may be performed at a tempering temperature of 510 to 570°C for 3.5 hours or more and 8 hours or less.

[0090] Finally, a method for manufacturing steel according to one embodiment of the present invention may include a step of pressing the surface of the hot-rolled steel sheet.

[0091] At this time, the firing rate may be 70~85%, and the firing rate can be derived by the following formula.

[0092]

[0093] In the above formula, t represents the thickness of the steel before rolling (mm), E represents the Young's modulus of the steel before rolling (MPa), R represents the amount of reduction of the rolling machine (mm), P represents the gap between the rollers of the rolling machine (mm), and YS represents the yield strength of the steel before rolling (MPa).

[0094] If the above plasticity is less than 70%, the strength may be poor due to insufficient work hardening, and if the above plasticity exceeds 85%, the strength may actually decrease due to the loss of yield point elongation. In particular, as described above, since tempered steel has high strength characteristics, it is not practically easy to exceed the above plasticity of 85%. As another example, the above plasticity may be 75~83%.

[0095] 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.

[0096] (Example)

[0097] First, a steel material having the compositions shown in Tables 1 and 2 was prepared. In Tables 1 and 2, each unit is weight%. The steel material was heated at a heating temperature of 900°C or higher and 1200°C or lower, and then a steel plate was manufactured by hot rolling, cooling, reheating and quenching, tempering, and surface pressing according to the conditions shown in Table 3. At this time, quenching after reheating was performed by cooling to 150°C or lower at a cooling rate of 10°C / s or higher. The thickness of the manufactured steel plate was 20 mm.

[0098] Classification CsiMnPSAl Invention Example 10.14 0.100.200.004 0.001 0.03 Invention Example 20.16 0.100.200.004 0.001 0.03 Comparative Example 10.19 0.100.200.004 0.001 0.03 Comparative Example 20.06 0.100.200.004 0.001 0.03 Comparative Example 30.15 0.100.200.0040.0010.03Comparative Example 40.150.100.200.0040.0010.03Comparative Example 50.150.100.200.0040.0010.03Comparative Example 60.150.100.200.0040.0010.03Comparative Example 70.150.100.200.0040.0010. 03Comparative Example 80.150.100.200.0040.0010.03Comparative Example 90.150.100.200.0040.0010.03Comparative Example 100.150.100.200.0040.0010.03Comparative Example 110.150.100.200.0040.0010.03Comparative Example 120.150.100. 200.0040.0010.03Comparative Example 130.150.100.200.0040.0010.03Comparative Example 140.150.100.200.0040.0010.03Comparative Example 150.150.100.200.0040.0010.03Comparative Example 160.150.100.200.0040.0010.03

[0099] Classification NiCrMoCoNA Value B Invention Example 1 102.5 2.0 40.00 30.2 11.02 Invention Example 2 102.5 1.5 40.00 30.1 71.02 Comparative Example 1 102.5 1.7 30.00 30.07 2.00 Comparative Example 2 102.8 2.3 50.00 30.8 10.69 Comparative Example 3 8.5 2.5 1.7 5.5 0.00 30.3 20.17 Comparative Example 4 102.5 1.7 50.00 30.2 70.69 Comparative Example 5 102.5 1.7 50.00 30.2 70.69 Comparative Example 6 102.5 1.7 50.00 30.2 70.69 Comparative Example 7 102.5 1.7 50.00 30.2 70.69 Comparative Example 8 102.5 1.750.0030.270.69 Comparative Example 9 102.5 1.750.0030.270.69 Comparative Example 10 102.5 1.750.0030.270.69 Comparative Example 11 102.5 1.750.0030.270.69 Comparative Example 12 102.5 1.750.0030.270.69 Comparative Example 13 102.5 1.750.0030.270.69 Comparative Example 14 102.5 1.750.0030.270.69 Comparative Example 15 102.5 1.750.0030.270.69 Comparative Example 16 102.5 1.750.0030.270.69

[0100] Classification Hot Rolled Material Heating Quenching Tempering Reduction Completion Temperature (°C) Final Pass Reduction Amount (%) Temperature (°C) Time (Min) Temperature (°C) Time (Hour) Plasticity Rate (%) Invention Example 19 18 148 28 60 54 248 2 Invention Example 28 9 8 18 8 31 60 53 8 68 1 Comparative Example 18 70 15 8 43 60 55 56 79 Comparative Example 29 21 19 8 6 160 51 27 76 Comparative Example 38 8 41 9 8 54 60 52 9 876 Comparative Example 4 100 41 78 19 60 54 0 479 Comparative Example 58 24 14 8 10 60 55 0 477 Comparative Example 69 0 77 8 40 60 51 76 83 Comparative Example 79131795060559479 Comparative Example 88971776860552780 Comparative Example 99091581215528381 Comparative Example 1087715839200511377 Comparative Example 118721487960620575 Comparative Example 129251083660480377 Comparative Example 1391912869605311273 Comparative Example 149081887860564277 Comparative Example 159001183860549565 Comparative Example 168831487760530887

[0101] Next, the microstructure and mechanical properties of the obtained steel sheets were measured and listed in Table 4 below. Each measurement method is as follows. (Area fraction and grain size of tempered martensite)

[0102] The area fraction of tempered martensite was measured by observing a specimen taken from the steel at a thickness of 1 / 4T using an electron microscope after Nital etching. In addition, the average effective grain size of tempered martensite was measured using EBSD based on a misorientation angle of 15° or greater.

[0103] ((Mo,Cr)(C,N) precipitate number density)

[0104] The water density of (Mo,Cr)(C,N)-based precipitates formed within tempered martensite grains was calculated by cutting a manufactured steel plate into a specimen of an arbitrary size, measuring five arbitrary areas at a magnification of 50,000 times (2 μm x 2 μm) on the polished surface of the cross-section in the rolling direction, and calculating the average value.

[0105] (Mechanical properties)

[0106] Among the above mechanical properties, the yield strength was measured using the ASTM E8M test method.

[0107] In addition, impact toughness was measured at a temperature of -20℃ using the ISO 148-1 test method.

[0108] Classification™ Area Fraction (%)™ Grain Size (㎛) Precipitate Number Density (pieces / ㎛) 2) Yield Strength (MPa) Impact Toughness (-20℃, J) Invention Example 19 214.212127090 Invention Example 29 210.214128689 Comparative Example 19 28.031203189 Comparative Example 29 112.033139249 Comparative Example 39 217.712129251 Comparative Example 49 127.39130044 Comparative Example 58 216.251156100 Comparative Example 69 9 29.07131033 Comparative Example 79 632.616126441 Comparative Example 88717.1111188143 Comparative Example 99722.69128346 Comparative Example 10837.3191181133 Comparative Example 119611.821145119 Comparative Example 129015.97130712 Comparative Example 139610.82120792 Comparative Example 14949.511094118 Comparative Example 15918.4191212138 Comparative Example 16947.814113586

[0109] In Comparative Example 1, the value of A derived by Equation 1 fell short of the range proposed in the present invention, resulting in insufficient precipitation of (Mo,Cr)(C,N) precipitates, and consequently, high strength characteristics could not be secured. In Comparative Example 2, the value of A exceeded the range proposed in the present invention, resulting in excessive formation of precipitates; as a result, high strength could be secured but toughness was reduced.

[0110] Comparative Example 3 could not secure the desired level of low-temperature toughness as the B value derived by Equation 2 was less than 0.3.

[0111] In Comparative Example 4, the finishing temperature during hot rolling exceeded 950°C, causing the grains to coarsen. As a result, it was difficult to secure high toughness.

[0112] In Comparative Example 5, the finishing temperature during hot rolling was less than 800°C, so the area fraction of tempered martensite was less than 90%. As a result, the strength was reduced.

[0113] Comparative Example 6 had a final pass reduction ratio of less than 68J, and as coarse grains were formed, the impact toughness at -20℃ was less than 68J.

[0114] Comparative Examples 7 and 9 had high reheating temperatures or excessive reheating times during reheat quenching, which resulted in coarsened tempered martensite grains and consequently inferior toughness.

[0115] In Comparative Examples 8 and 10, the reheating temperature was low or the reheating time was short during reheat quenching, so it was not possible to secure tempered martensite of 90% or more of the area. As a result, it was difficult to secure high strength.

[0116] Comparative Examples 11 and 13 could not secure a yield strength of 1240 MPa or higher because the temperature during tempering was high or the time was excessive, making it difficult to secure finely divided precipitates.

[0117] In Comparative Example 12, the temperature was low during tempering, resulting in temper brittleness, which made it difficult to secure high toughness.

[0118] In Comparative Example 14, the tempering time was short, resulting in insufficient precipitate formation. Consequently, the strength characteristics were inferior.

[0119] In Comparative Example 15, the plasticity rate in the process of pressing the surface of the steel plate after tempering fell short of the desired level, so a yield strength of 1240 MPa or higher could not be secured.

[0120] In Comparative Example 16, the plasticity was excessive during surface compression of the steel plate, and the strength was actually reduced due to the loss of yield point elongation.

[0121] On the other hand, Invention Examples 1 and 2 satisfied all the alloy compositions and manufacturing conditions presented in the present invention. Meanwhile, the steel used in Invention Examples 1 and 2 contained Al at a level of 0.03 wt% and N at a level of 0.003 wt% due to the deoxidizer added during the steelmaking process. These Al and N were not specifically controlled as impurity elements.

[0122] Accordingly, Invention Examples 1 and 2 were able to secure high strength and excellent low-temperature impact toughness. Specifically, Invention Examples 1 and 2 had a yield strength of 1240 MPa or more and 1340 MPa or less, and a Charpy impact toughness of 68 J or more at -20℃.

Claims

1. In wt%, it comprises C: 0.10~0.20%, Si: 0.50% or less, Mn: 0.05~0.50%, P: 0.010% or less, S: 0.006% or less, Ni: 8.0~12.0%, Cr: 1.0~3.0%, Mo: 0.5~2.5%, Co: 2.0~6.0%, and the remainder consists of Fe and unavoidable impurities, wherein the A value derived by the following Equation 1 is 0.10 or more and 0.70 or less, and The value of B derived by the following Equation 2 is 0.30 or greater, and Steel having a yield strength of 1240 MPa or more and 1340 MPa or less, and a Charpy impact toughness of 68 J or more at -20℃. [Relationship 1] In the above equation 1, Cr, Mo, Co, and C represent the content (weight%) of each element. [Relationship 2] In the above equation 2, Ni and Co represent the content (weight%) of each element.

2. In Paragraph 1, 5–20 (Mo,Cr)(C,N) precipitates with a particle size of 5–20 nm within the matrix structure / μm 2 Steel distributed as.

3. In Paragraph 1, Steel containing 90% or more of tempered martensite in its microstructure.

4. In Paragraph 3, Steel having an effective grain size of 20㎛ or less of the above-mentioned tempered martensite.

5. A step of heating a steel material comprising, in wt%, C: 0.10~0.20%, Si: 0.50% or less, Mn: 0.05~0.50%, P: 0.010% or less, S: 0.006% or less, Ni: 8.0~12.0%, Cr: 1.0~3.0%, Mo: 0.5~2.5%, Co: 2.0~6.0%, and the remainder being Fe and unavoidable impurities, wherein the A value derived by the following Equation 1 is 0.10 or more and 0.70 or less, and the B value derived by the following Equation 2 is 0.30 or more; A step of obtaining a hot-rolled steel sheet by hot-rolling the above steel material at a temperature of 850°C or higher and 950°C or lower with a final pass reduction of 10% or more; A step of cooling the above hot-rolled steel sheet; A step of reheating and quenching the above hot-rolled steel sheet at a temperature of 800℃ or higher and 900℃ or lower for a time of 1.6*thickness(mm) + 20 minutes or more and 1.6*thickness(mm) + 60 minutes or less; A step of tempering the above hot-rolled steel sheet at a tempering temperature of 500 to 600°C for 3 hours or more and 10 hours or less; and A method for manufacturing steel, comprising the step of pressing the surface of the hot-rolled steel sheet with a plasticity of 70~85%. [Relationship 1] In the above equation 1, Cr, Mo, Co, and C represent the content (weight%) of each element. [Relationship 2] In the above equation 2, Ni and Co represent the content (weight%) of each element.

6. In Paragraph 5, A method for manufacturing steel, wherein the heating temperature in the above heating step is 900℃ or higher and 1200℃ or lower.

7. In Paragraph 5, A method for manufacturing steel, wherein in the above-mentioned reheating and quenching step, the quenching is performed by cooling to 150°C or lower at a cooling rate of 10°C / s or more.

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