High-strength thick steel plate with excellent impact toughness after deformation and method for manufacturing the same

Abstract

The present invention relates to a thick steel plate and a manufacturing method thereof, and more particularly to a thick steel plate having high strength and excellent impact toughness after deformation, and a manufacturing method thereof.

Description

Technical Field

[0001] The present invention relates to a high-strength thick steel plate having excellent impact toughness after deformation and a method for manufacturing the same. More specifically, the present invention relates to a high-strength thick steel plate having excellent impact toughness after deformation and a method for manufacturing the same, which has high strength and excellent impact toughness after deformation.

Background Art

[0002] Since the 2000s, attention has been focused on new renewable energy for environmental issues and greenhouse gas reduction. New renewable energy is a term that combines new energy (such as hydrogen and fuel cells) and renewable energy (such as solar heat, wind power, and biomass). Among these, wind power generation is an environmentally friendly power generation method without waste generation and pollution, and is attracting attention as a next-generation energy source. Among wind power generation, onshore wind power installed on land has recently seen a rapid growth in offshore wind power built in the sea, mainly in Europe, due to reasons such as noise and limited optimal wind formation space.

[0003] Although such offshore wind power has been activated later than onshore wind power, with the development of technical levels due to various advantages such as lower concerns about steel plate wind speed and noise generation and the ability to secure a large area, the relative advantage of offshore wind power over onshore wind power has become increasingly prominent. In particular, there is a great advantage that the power generation capacity per wind turbine can be increased compared to onshore wind power. That is, the average power generation capacity is about twice that of onshore wind power, and the average capacity of newly installed turbines per unit in Europe has increased rapidly from 4 MW in 2015 to 7.2 MW in 2019, and is expected to reach 10 MW or more within two to three years.

[0004] As a result, the strength of the steel plates used has gradually increased, with most being 325 MPa class with a thickness of 80 mm. However, since the 1920s, yield strengths of 380 and 410 MPa class have been applied. The substructures of such offshore wind turbines are broadly classified into monopile and jacket types, and jacket-type substructures are further divided into pinpile or suction bucket types depending on the method of fixing them to the seabed.

[0005] In the case of a monopile substructure, it is divided into the monopile section that is embedded in the seabed and the transition piece section that connects the monopile and the tower section. In such a structure, the load is highest, and high-strength steel can be mainly applied to the connection between the monopile and the transition piece, which is the joint section. In the critical support parts of such offshore wind turbine substructures, not only high strength but also extremely thick steel plates that can guarantee low-temperature toughness are used, and in cylindrical pipe structures, it is important to ensure impact toughness after deformation during pipe construction. [Overview of the project] [Problems that the invention aims to solve]

[0006] The present invention aims to provide a thick steel plate that possesses high strength while also exhibiting excellent impact toughness after deformation, 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 entirety of this specification. [Means for solving the problem]

[0008] The present invention comprises, by weight percent, C: 0.04~0.08%, Si: 0.1~0.35%, Mn: 1.4~1.8%, Sol.Al: 0.01~0.035%, Ni: 0.2~0.5%, Cr: 0.1~0.3%, Mo: 0.05~0.15%, Nb: 0.015~0.035%, Ti: 0.005~0.02%, N: 0.002~0.006%, P: 0.01% or less, S: 0.003% or less, with the remainder being iron (Fe) and other unavoidable impurities. The R value defined by the following relational equation 1 is between 0.85 and 1.35. The microstructure at the 1 / 4 thickness point contains 40-60 area percent acicular ferrite, 40-60 area percent bainite, and the remaining cementite and MA (martensite) totaling less than 3 area percent. The average size of the crystal grains of acicular ferrite at the 1 / 4 thickness point is 20 μm or less. This method provides a steel sheet in which the hard phase accounts for 5% or less of the microstructure in the 8mm portion directly below the surface. [Relationship 1] R = [Ni] + 3[Mo] + 2[Cr] (Here, [Ni], [Mo], and [Cr] are weight percent of each element.)

[0009] The hard phase described above can be the MA phase.

[0010] The above steel plate can have an average grain size of acicular ferrite at the 1 / 4 thickness point of the sheet being 10 to 20 μm.

[0011] The steel plate mentioned above can have a thickness of 50 to 100 mm.

[0012] The above steel plate may have a yield strength of 460 MPa or more and a tensile strength of 580 MPa or more.

[0013] The above steel plate may have an impact toughness of 100 J or more at -50°C at the deformed surface and at the 1 / 4 thickness point (where deformation is performed under tension, and after tension is stopped when the deformation reaches 5%, aging treatment is performed at 250°C).

[0014] Another aspect of the present invention is the step of reheating a steel slab having the following composition by weight %, C: 0.04~0.08%, Si: 0.1~0.35%, Mn: 1.4~1.8%, Sol.Al: 0.01~0.035%, Ni: 0.2~0.5%, Cr: 0.1~0.3%, Mo: 0.05~0.15%, Nb: 0.015~0.035%, Ti: 0.005~0.02%, N: 0.002~0.006%, P: 0.01% or less, S: 0.003% or less, with the remainder being iron (Fe) and other unavoidable impurities, and having an R value of 0.85~1.35 as defined by the following relational formula 1. The above reheated steel slab is subjected to recrystallization zone rolling in a temperature range of 900°C or higher with a final pass reduction of 15-25 mm. The above steps involve rolling the recrystallized steel sheet in the non-recrystallized region at a rolling completion temperature of Ar3+40 to Ar3+80, and The present invention provides a method for manufacturing a steel sheet, which includes a step of cooling the steel sheet rolled in the non-recrystallized region described above to a temperature range of 400°C or less at a cooling rate of 5 to 10°C / s, with the 1 / 4 thickness point as the reference point. [Relationship 1] R = [Ni] + 3[Mo] + 2[Cr] (Here, [Ni], [Mo], and [Cr] are weight percent of each element.)

[0015] During the reheating process described above, the temperature range should be 1020-1100°C. During the rolling process in the non-recrystallized region described above, the cumulative reduction ratio can be 30-50%.

[0016] The steel plate mentioned above can have a thickness of 50 to 100 mm. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a thick steel plate that has high strength while also having excellent impact toughness after deformation, and a method for manufacturing the same.

[0018] According to the present invention, there is provided a thick steel plate excellent in strength and low-temperature impact toughness, applicable as a steel material for extremely thick offshore wind power, and also usable as a structural steel material for infrastructure industries such as construction and bridges, and a method for manufacturing the same.

Brief Description of the Drawings

[0019] [Figure 1] This is a photograph of the microstructure of an inventive example of the present invention observed at a magnification of 500 using an optical microscope. (a) is a microstructure photograph at a position 8 mm below the surface, and (b) is a microstructure photograph at a position 1 / 4 of the thickness. [Figure 2] This is a photograph of the microstructure of Comparative Example 4 of the present invention observed at a magnification of 500 using an optical microscope.

Modes for Carrying Out the Invention

[0020] Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. This embodiment is provided to explain the present invention in more detail to an ordinary technician in the technical field to which the invention belongs.

[0021] Hereinafter, the present invention will be described in detail.

[0022] Hereinafter, the steel composition of the present invention will be described in detail.

[0023] Unless otherwise specified in the present invention, the % representing the content of each element is based on weight.

[0024] The steel of the present invention may contain, by weight percent, C: 0.04-0.08%, Si: 0.1-0.35%, Mn: 1.4-1.8%, Sol.Al: 0.01-0.035%, Ni: 0.2-0.5%, Cr: 0.1-0.3%, Mo: 0.05-0.15%, Nb: 0.015-0.035%, Ti: 0.005-0.02%, N: 0.002-0.006%, P: 0.01% or less, S: 0.003% or less, with the remainder being iron (Fe) and other unavoidable impurities.

[0025] Carbon (C): 0.04~0.08% Carbon (C) is an element that causes solid solution strengthening and exists as carbonitrides with Nb, etc., to ensure tensile strength, and its content can be limited to 0.04% or more. On the other hand, if the carbon (C) content exceeds 0.08%, it not only promotes the formation of MA but can also generate pearlite, which degrades the impact properties at low temperatures and may worsen the weld properties during structural welding. A more preferable upper limit for carbon (C) can be 0.07%.

[0026] Silicon (Si): 0.1-0.35% Silicon (Si) plays a role in deoxidizing molten steel in conjunction with Al and is an essential element for ensuring yield strength and tensile strength. Its content can be 0.1% or more. However, if its content exceeds 0.35%, there is a possibility of problems such as hindering C diffusion and promoting MA formation. Preferably, the silicon (Si) content can be 0.15% or more, and more preferably 0.25% or less.

[0027] Manganese (Mn): 1.4~1.8% Manganese (Mn) is preferable to be added at a concentration of 1.4% or more due to its significant effect in increasing strength through solid solution strengthening. On the other hand, if the content is excessive, it may cause a decrease in toughness due to the formation of MnS inclusions and segregation in the center, so the upper limit can be restricted to 1.8%. A more preferable lower limit can be 1.5%.

[0028] Aluminum (Sol.Al): 0.01~0.035% Aluminum (Sol.Al) is a major deoxidizing agent for steel, and it is preferable to add 0.01% or more to obtain its effect. However, if the content exceeds 0.035%, it may cause a decrease in low-temperature toughness due to an increase in the fraction and size of Al2O3 inclusions. Also, similar to Si, it may promote the formation of MA in the base material and heat-affected zone of the weld, potentially reducing the low-temperature toughness properties. Preferably, it can be contained at a concentration of 0.015% or more, and more preferably at a concentration of 0.03% or less.

[0029] Nickel (Ni): 0.2-0.5% Nickel (Ni) is an element that improves strength without reducing impact toughness, and it can increase strength by promoting the formation of an appropriate amount of acicular ferrite, so it is preferable to add 0.2% or more. On the other hand, if the content exceeds 0.5%, it may lower the Ar3 temperature and cause bainite to form, which may reduce the impact toughness of extremely thick materials. A more preferable lower limit can be 0.3%.

[0030] Chromium (Cr): 0.1-0.3% Chromium (Cr) is a favorable element for ensuring strength as a carbide-forming element, but in extremely thick steel materials, it can form coarse carbides depending on the cooling rate of the steel, thereby hindering impact toughness. Therefore, its content can be limited to 0.1-0.3%. A more preferable lower limit for its content can be 0.15%.

[0031] Molybdenum (Mo): 0.05~0.15% Molybdenum (Mo) is an element that effectively increases strength with small additions, and it is preferable to add 0.05% or more to improve strength by forming Mo-C series precipitates. However, since excessive addition of molybdenum (Mo) may cause coarsening of the precipitates, the upper limit can be restricted to 0.15%. The lower limit of the preferred content can be 0.08%, and the upper limit of the more preferred content can be 0.12%.

[0032] Niobium (Nb): 0.015~0.035% Niobium (Nb) can be added in amounts of 0.015% or more as an element that suppresses recrystallization during rolling or cooling by solid solution or carbonitride precipitation, thereby refining the microstructure and increasing strength. However, because carbon affinity can cause carbon concentration and promote MA formation, which can reduce toughness and fracture properties at low temperatures, its upper limit can be restricted to 0.035%. A preferred lower limit can be 0.02%, and a more preferred upper limit can be 0.03%.

[0033] Titanium (Ti): 0.005~0.02% Titanium (Ti) can combine with oxygen or nitrogen to form precipitates. Such precipitates contribute to refinement by suppressing coarsening of the structure and improve toughness, so it is preferable to add 0.001% or more. However, if the content exceeds 0.02%, the coarsening of the precipitates may cause fracture. A more preferable lower limit can be 0.01%, and a more preferable upper limit can be 0.018%.

[0034] Nitrogen (N): 0.002~0.006% Nitrogen (N), along with Ti, Nb, and Al, can form precipitates and refine the austenite structure during reheating, contributing to improved strength and toughness. However, excessive nitrogen content can induce surface cracks and precipitate formation at high temperatures, and residual nitrogen (N) in its atomic state may reduce toughness; therefore, its content can be limited to 0.002-0.006%.

[0035] Phosphorus (P): 0.01% or less Phosphorus (P) is an element that causes grain boundary segregation and can lead to steel embrittlement; therefore, its upper limit can be restricted to 0.01%. However, considering the level that is inevitably added to steel, 0% can be excluded.

[0036] Sulfur (S): 0.003% or less Sulfur (S) can primarily combine with Mn to form MnS inclusions, which can inhibit low-temperature toughness. Therefore, to ensure low-temperature toughness and low-temperature fatigue properties, its upper limit can be restricted to 0.003%. However, considering the level that is inevitably added to steel, 0% can be excluded.

[0037] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. These unavoidable impurities cannot be eliminated because they may be unintentionally introduced during the normal manufacturing process. Since such impurities are known to any engineer in the field of ordinary steelmaking, their full details are not specifically mentioned herein.

[0038] In particular, copper (Cu) may be present as an impurity, but in this invention, the copper (Cu) content can be limited to less than 0.05%.

[0039] According to one aspect of the present invention, the steel can have an R value of 0.85 to 1.35, as defined by the following relational expression 1.

[0040] In this invention, relational formula 1 is proposed to ensure both strength and low-temperature toughness at -50°C. Relational formula 1 relates to the component formula for ensuring strength and toughness, and strength and low-temperature toughness can be ensured by controlling the R value of relational formula 1. If the R value of relational formula 1 is less than 0.85, there is a problem that solid solution strengthening, precipitation strengthening, and hardening ability are insufficient, making it impossible to ensure the desired yield strength. If the value exceeds 1.35, hard structures such as MA and bainite may be formed, potentially leading to a deterioration of impact toughness. [Relationship 1] R = [Ni] + 3[Mo] + 2[Cr] (Here, [Ni], [Mo], and [Cr] are weight percent of each element.)

[0041] The steel microstructure of the present invention will be described in detail below.

[0042] In this invention, unless otherwise specified, the percentage (%) used to express the fraction of microstructure is based on area.

[0043] The steel of the present invention may contain 40-60 area percent of acicular ferrite, 40-60 area percent of bainite, and a total of 3% or less of the remaining cementite and MA as the microstructure at the 1 / 4 thickness point.

[0044] In this invention, in order to achieve impact toughness at -50°C at the 1 / 4 thickness point, the size and potential density of acicular ferrite are important, and it is preferable to minimize cementite and MA. More preferably, the total amount of cementite and MA can be 2% or less. In this invention, the 1 / 4 thickness point means t / 4, where t means the thickness of the steel plate.

[0045] The steel of the present invention can have an average grain size of acicular ferrite at the 1 / 4 thickness point of the steel being 20 μm or less.

[0046] In this invention, in order to ensure low-temperature impact toughness, the average size of the crystal grains of acicular ferrite can be limited to 20 μm or less. If the size exceeds 20 μm, there is a problem in that the impact toughness decreases. On the other hand, due to the characteristics of the thick steel plate with a thickness of 50 mm or more that is the target of this invention, there is a limit to the refinement of the crystal grains, so the lower limit of the size can be limited to 10 μm.

[0047] The steel of the present invention can have a hard phase of 5 area % or less in the microstructure of the 8 mm portion directly below the surface.

[0048] In this invention, the fraction of the hard phase in the microstructure near the surface can be limited to 5% or less in order to ensure the desired impact toughness after deformation. If the fraction of the hard phase exceeds 5%, it may act as the starting point for impact toughness fracture, potentially leading to a deterioration of impact toughness. In this invention, the MA phase may be a combination of martensite and austenite as the hard phase.

[0049] The steel manufacturing method of the present invention will be described in detail below.

[0050] The steel of the present invention can be manufactured by reheating, rolling, and cooling a steel slab that satisfies the alloy composition described above.

[0051] reheating A steel slab satisfying the alloy composition of the present invention can be reheated in a temperature range of 1020 to 1100°C.

[0052] If the reheating temperature exceeds 1100°C, the austenite grains may coarseen, and the increased hardening ability may lead to the formation of a bainite structure, reducing toughness. On the other hand, if the temperature is below 1020°C, Ti, Nb, etc. may not dissolve sufficiently, resulting in a decrease in strength.

[0053] Recrystallization zone rolling The reheated steel slab described above can be subjected to recrystallization zone rolling at a temperature range of 900°C or higher with a final pass reduction of 15 to 25 mm.

[0054] The recrystallization region rolling step in this invention is for the purpose of completely recrystallizing austenite and suppressing the refinement and growth of austenite. Recrystallization region rolling is preferably performed in a temperature range of 900°C or higher for complete austenite recrystallization, and the final pass reduction amount can be 15 to 25 mm for initial austenite refinement. If the final pass reduction amount is less than 15 mm, it is difficult to ensure the desired level of refinement. On the other hand, during rolling, the upper limit can be restricted to 25 mm, taking into account productivity based on equipment specifications.

[0055] Rolling in non-recrystallized area The steel sheet rolled in the recrystallized region described above can be rolled in the non-recrystallized region at a rolling completion temperature of Ar3+40 to Ar3+80 and a cumulative reduction ratio of 30 to 50%.

[0056] In this invention, the rolling termination temperature can be sufficiently increased to prevent the surface temperature before cooling begins after rolling from dropping below Ar3. If the rolling termination temperature is less than Ar3+40, the surface temperature before cooling will drop below the Ar3 temperature, entering the two-phase region. This can cause a concentration of C grain boundaries, leading to a transformation into a harder phase upon cooling, which can severely negatively affect the impact toughness after deformation. On the other hand, if the temperature exceeds Ar3+80, grain refinement at the 1 / 4 thickness position becomes difficult, potentially reducing the impact toughness at -50°C.

[0057] Furthermore, for thick materials with a thickness of 50 mm or more, a cumulative reduction ratio of 30 to 50% is preferable. If the cumulative reduction ratio is less than 30%, the amount of rolling in the unrecrystallized region decreases, which can lead to problems such as pancaking and difficulty in refining the microstructure. If it exceeds 50%, the amount of rolling in the recrystallized region becomes insufficient, which may lead to deterioration of low-temperature impact toughness. [formula] Ar3=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo] (Here, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are weight percent of each element.)

[0058] cooling The steel sheet rolled in the non-recrystallized region described above can be cooled to a temperature range of 400°C or less at a cooling rate of 5 to 10°C / s based on the 1 / 4 thickness point.

[0059] In the thick steel plate targeted by this invention, it is preferable to ensure impact toughness around the 1 / 4 thickness point. Therefore, the cooling rate limited in this invention can be based on the 1 / 4 thickness point. If the cooling completion temperature exceeds 400°C or the cooling rate exceeds 10°C / s, MA formation may be accelerated, and impact toughness may deteriorate. On the other hand, if the cooling rate is less than 5°C / s, it is difficult to ensure the desired level of strength. In this invention, water cooling can be used as the cooling method.

[0060] The steel of the present invention manufactured in this manner has a thickness of 50 to 100 mm, a yield strength of 460 MPa or more, a tensile strength of 580 MPa or more, and an impact toughness of 100 J or more at -50°C at the deformed surface and at the 1 / 4 thickness point, thus possessing high strength while also exhibiting excellent impact toughness after deformation.

[0061] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are merely illustrative and intended to explain the present invention in more detail, and are not intended to limit the scope of the rights of the present invention.

[0062] (Examples) Molten steel having the alloy composition shown in Table 1 was prepared, and slabs were manufactured using continuous casting. Steel plates were manufactured by reheating, rolling, and cooling the slabs under the conditions shown in Table 2. All recrystallization region rolling temperatures not disclosed in Table 2 were applied similarly at temperatures above 900°C, and the cumulative reduction ratio during non-recrystallization region rolling was applied similarly within the range of 30-50%. Table 1 also shows the calculated Ar3 temperature and R value of relational equation 1 according to the alloy composition of each steel type.

[0063] [Table 1] [Relationship 1] R = [Ni] + 3[Mo] + 2[Cr] (Here, [Ni], [Mo], and [Cr] are weight percent of each element.) [formula] Ar3=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo] (Here, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are weight percent of each element.)

[0064] [Table 2]

[0065] Table 3 below shows the microstructure of the manufactured steel sheet, as measured. The hard phase fraction just below the surface was observed in the 8 mm portion just below the surface, and the fraction of MA was measured as the hard phase. At the 1 / 4 thickness point, the fractions of acicular ferrite (AF), cementite, and MA are shown, respectively, and the remaining fraction was observed as bainite. The grain size of acicular ferrite at the 1 / 4 thickness point of the steel is also shown. The microstructure was measured by analyzing it under 500x magnification using an optical microscope.

[0066] Table 3 shows the measured physical properties for each test specimen. Yield strength (YS), tensile strength (TS), and elongation (El) were evaluated by tensile testing according to the EN-ISO 6892-1 standard. Annular specimens were taken at the 1 / 4 thickness position in the direction perpendicular to the rolling direction, and the average of two tests was measured. Impact toughness at -50°C was also measured. Impact toughness was measured according to the EN ISO 148-1 standard, with specimens taken at the 1 / 4 thickness position in the direction parallel to the rolling direction, and the average of three tests was measured. For the deformation impact test, a 450 mm long tensile test specimen was prepared. After the deformation reached 5%, the tensile test was stopped, and the specimen was aged at 250°C. After this, the impact test specimen was measured in the same manner as the base material impact test described above. [Table 3]

[0067] 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 microstructural characteristics proposed in the present invention are met, and the physical properties targeted in the present invention are secured.

[0068] Figure 1 shows photographs of the microstructure of an example of the present invention, observed at 500x magnification using an optical microscope. (a) is a microstructure photograph taken 8 mm below the surface, and (b) is a microstructure photograph taken 1 / 4 of the way through the thickness.

[0069] Figure 2 is a photograph of the microstructure of Comparative Example 4 of the present invention, observed at 500x magnification using an optical microscope.

[0070] On the other hand, Comparative Examples 1 and 2 are examples of alloy compositions that do not meet the requirements. In Comparative Example 1, the value of relational equation 1 was insufficient, resulting in a decrease in strength due to a reduction in the hardening ability of the base material. In Comparative Example 2, the value of relational equation 1 was exceeded, promoting the formation of MA, which led to a deterioration of the impact properties after deformation due to the increase in MA fraction, and the hardening ability of the surface increased, resulting in the appearance of a hard phase and a deterioration of the impact toughness after surface deformation.

[0071] In Comparative Example 3, the rolling temperature in the non-recrystallized region was excessive, resulting in insufficient grain refinement, which led to deterioration of impact toughness at the 1 / 4 thickness point and impact toughness after deformation.

[0072] In Comparative Example 4, the rolling temperature in the non-recrystallized region was not reached. Although the impact toughness at the 1 / 4 thickness point was excellent due to the grain refinement effect, the distribution of the hard phase after cooling increased significantly due to localized two-phase region entry before the surface cooled. As a result, the impact toughness properties after deformation of the surface deteriorated considerably.

[0073] In Comparative Example 5, the cooling rate exceeded the range of the present invention, and the impact toughness after surface deformation deteriorated due to an increase in the hard phase caused by overcooling of the surface.

[0074] In Comparative Example 6, the cooling termination temperature was too high, resulting in a rapid increase in MA formation at the 1 / 4 thickness point, and a deterioration in impact toughness after deformation.

[0075] In Comparative Example 7, the reduction amount during recrystallization rolling was insufficient, resulting in insufficient initial austenite refinement. Consequently, the final ferrite size was not refined, and the formation of a hard phase from coarse austenite became easier, leading to a decrease in yield strength and impact toughness.

[0076] Comparative Example 8 satisfies the alloy component content requirements proposed in the present invention, but does not satisfy the value of relational formula 1. Although all the presented operating conditions and temperature range are met and the strength is sufficiently satisfied, it was confirmed that the impact toughness at -50°C and the impact toughness after surface deformation rapidly decrease due to the decrease in acicular ferrite, the increase in bainite, and the increase in the hard phase of the surface.

[0077] Although the present invention has been described in detail through the examples above, other forms of embodiments are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. In weight percent, it contains C: 0.04-0.08%, Si: 0.1-0.35%, Mn: 1.4-1.8%, Sol. Al: 0.01-0.035%, Ni: 0.2-0.5%, Cr: 0.1-0.3%, Mo: 0.05-0.15%, Nb: 0.015-0.035%, Ti: 0.005-0.02%, N: 0.002-0.006%, P: 0.01% or less, S: 0.003% or less, with the remainder being iron (Fe) and other unavoidable impurities. The R value defined by the following relational equation 1 is between 0.85 and 1.

35. The microstructure at the 1 / 4 thickness point contains 40-60 area percent acicular ferrite, 40-60 area percent bainite, and the remaining cementite and MA totaling 3 area percent or less. The average size of the crystal grains of acicular ferrite at the 1 / 4 thickness point is 20 μm or less. In the microstructure of the 8 mm portion directly below the surface, the hard phase accounts for 5 area percentage or less. A high-strength, thick steel plate exhibiting excellent impact toughness after deformation, characterized in that the aforementioned hard phase is an MA phase. [Relationship 1] R=[Ni]+3[Mo]+2[Cr] (Here, [Ni], [Mo], and [Cr] are weight percent of each element.)

2. The steel plate is characterized in that the average size of the crystal grains of acicular ferrite at the 1 / 4 thickness point is 10 to 20 μm, making it a high-strength thick steel plate with excellent impact toughness after deformation, as described in claim 1.

3. The high-strength thick steel plate with excellent impact toughness after deformation, as described in claim 1, is characterized in that the steel plate has a thickness of 50 to 100 mm.

4. The steel plate is characterized in that it has a yield strength of 460 MPa or more and a tensile strength of 580 MPa or more, making it a high-strength thick steel plate with excellent impact toughness after deformation, as described in claim 1.

5. The steel plate is characterized in that the impact toughness at -50°C is 100 J or more at the deformed surface and at the 1 / 4 thickness point (where deformation is performed under tension, and after the amount of deformation reaches 5%, tension is stopped and then aging treatment is performed at 250°C), as described in claim 1, which is a high-strength thick steel plate with excellent impact toughness after deformation.

6. A method for manufacturing a high-strength thick steel plate with excellent impact toughness after deformation, as described in Claim 1, The process involves reheating a steel slab containing, by weight percent, C: 0.04-0.08%, Si: 0.1-0.35%, Mn: 1.4-1.8%, Sol. Al: 0.01-0.035%, Ni: 0.2-0.5%, Cr: 0.1-0.3%, Mo: 0.05-0.15%, Nb: 0.015-0.035%, Ti: 0.005-0.02%, N: 0.002-0.006%, P: 0.01% or less, and S: 0.003% or less, with the remainder being iron (Fe) and other unavoidable impurities, and having an R value of 0.85-1.35 as defined by the following relational formula 1. The reheated steel slab is subjected to recrystallization zone rolling at a temperature range of 900°C or higher with a final pass reduction of 15 to 25 mm. The steps include: rolling the steel sheet that has been rolled in the recrystallized region at a rolling completion temperature of Ar3+40 to Ar3+80 in the non-recrystallized region; and A method for manufacturing a high-strength thick steel sheet with excellent impact toughness after deformation, characterized by including a step of cooling the steel sheet rolled in the unrecrystallized region to a temperature range of 400°C or less at a cooling rate of 5 to 10°C / s with respect to the 1 / 4 thickness point. [Relationship 1] R=[Ni]+3[Mo]+2[Cr] (Here, [Ni], [Mo], and [Cr] are weight percent of each element.)

7. During the aforementioned reheating, the temperature range is 1020 to 1100°C. The method for manufacturing a high-strength thick steel sheet with excellent impact toughness after deformation, as described in claim 6, characterized in that the cumulative reduction rate during rolling in the non-recrystallized region is 30 to 50%.

8. The method for manufacturing a high-strength thick steel plate with excellent impact toughness after deformation, as described in claim 6, characterized in that the steel plate has a thickness of 50 to 100 mm.

Images