Steel sheet and method for manufacturing the same

Abstract

This invention relates to steel plates and methods for manufacturing the same, and more specifically, to steel plates with excellent resistance to ultra-low temperature brittle crack initiation and methods for manufacturing the same.

Description

Technical Field

[0001] This disclosure relates to steel plates and methods of manufacturing the same, and more specifically, to steel plates with excellent resistance to low-temperature brittle cracking and methods of manufacturing the same. Background Technology

[0002] Recently, with the strengthening of environmental regulations, the demand for liquefied natural gas (LNG), an environmentally friendly fuel, has increased, leading to an increase in the construction of LNG carriers and LNG-propelled vessels using LNG as fuel. In the case of LNG and LNG / ethane, high-Ni steel with a high Ni content is typically used due to their extremely low liquefaction temperatures. To reduce the construction costs of vessels transporting LPG and liquefied gases with liquefaction temperatures of -60°C or lower (such as ammonia or CO2), carbon steel with enhanced low-temperature toughness is increasingly being used.

[0003] For existing LNG carrier storage tanks, the large, unpressurized Type A tank design is unlikely to be used primarily, thus eliminating the need for high-strength steel or thick materials. However, high-pressure CO2 and ammonia fuel tanks can be designed as Type C tanks, potentially requiring high-strength steel and thick materials. Furthermore, when constructing Type C tanks, the mandatory application of post-weld heat treatment (PWHT) required by IGC specifications may necessitate ensuring the steel's properties before and after PWHT.

[0004] Because post-weld heat treatment (PWHT) is a high-temperature heat treatment, the strength may decrease after PWHT, and the required strength may not be achieved. To overcome this, when using precipitation strengthening elements, there may be a problem of reduced impact toughness, so it may be difficult to simultaneously guarantee the strength and toughness before and after PWHT.

[0005] Furthermore, to maximize efficiency in transporting liquefied gas, it may be necessary to increase the size of the storage tanks. In this case, after the storage tanks are built, it may be necessary to construct an extra-large heat treatment furnace for PWHT heat treatment.

[0006] However, the above-mentioned situations may incur significant costs.

[0007] Alternatively, PWHT heat treatment can be performed by attaching a heat treatment pad to the weld zone instead of a large furnace. However, attaching a heat treatment pad to a large tank and performing long-term heat treatment can be significantly costly and time-consuming. Therefore, shipbuilders may seek to eliminate PWHT heat treatment where possible.

[0008] To exempt Type C tanks from PWHT heat treatment, an engineering critical assessment (ECA) may be necessary to demonstrate that no design problems exist without stress relief through heat treatment. When determining whether an ECA can be used to ensure compliance, the crack tip opening displacement (CTOD) value of the steel used in the tank is likely the most critical factor. Furthermore, for steel used in liquefied CO2, it may be necessary to ensure the CTOD value of the coarse-grained heat-affected zone (CGHAZ) at the design temperature of -55°C.

[0009] However, since there is almost no history of developing carbon steel that guarantees CTOD properties at such extremely low temperatures, it may be crucial to develop steels that maximize toughness by optimizing microstructure and alloy composition. Summary of the Invention

[0010] Technical issues

[0011] One embodiment of this disclosure aims to provide a steel plate and a method for manufacturing the same.

[0012] One embodiment of this disclosure aims to provide a steel plate with excellent resistance to low-temperature brittle crack initiation and a method for manufacturing the same.

[0013] The subject matter of this disclosure is not limited to the foregoing. Other subjects of this disclosure will be readily apparent to those skilled in the art upon review of the entire contents of this specification.

[0014] Technical solution

[0015] According to one embodiment of this disclosure, the steel plate comprises, by weight percent: C: 0.060% to 0.080%, Mn: 1.40% to 1.60%, Si: 0.10% to 0.20%, Al: 0.01% to 0.04%, Ni: 0.30% to 0.50%, Ti: 0.008% to 0.016%, Nb: 0.010% to 0.025%, P: 0.008% or less, S: 0.002% or less, and the balance being Fe and unavoidable impurities.

[0016] In the weld heat-affected zone welded with a heat input of 1.5 kJ / mm, the microstructure in the region from the fusion line (FL) to FL+0.2 mm contains 5.0% or less of the MA phase by area %.

[0017] The R value for the steel plate, as defined in the following relational expression 1, is 0.38 or less:

[0018] [Relational Expression 1]

[0019]

[0020] Where [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the weight of each element.

[0021] In the weld heat-affected zone, the effective grain size at the center of the thickness in the region from the fusion line (FL) to FL+0.2 mm is 100 μm or smaller.

[0022] The steel plate has a yield strength of 355 MPa or greater and an impact toughness of 100 J or greater at -80°C.

[0023] The crack tip opening displacement (CTOD) of the steel plate at -60°C is 0.20 mm or greater.

[0024] According to one embodiment of this disclosure, a method for manufacturing a steel plate includes: heating a steel billet, the steel billet comprising, by weight %, C: 0.060% to 0.080%, Mn: 1.40% to 1.60%, Si: 0.10% to 0.20%, Al: 0.01% to 0.04%, Ni: 0.30% to 0.50%, Ti: 0.008% to 0.016%, Nb: 0.010% to 0.025%, P: 0.008% or less, S: 0.002% or less, and the balance being Fe and unavoidable impurities;

[0025] The heated steel billet is rough rolled;

[0026] The rough-rolled steel plate is then subjected to precision hot rolling; and

[0027] Using a 1 / 4 point in the direction from the surface to the center of the thickness as a reference, the hot-rolled steel sheet is cooled to a temperature range of 400°C to 700°C at a cooling rate of 10°C / second or higher.

[0028] The R value for the steel plate, as defined in the following relational expression 1, is 0.38 or less:

[0029] [Relational Expression 1]

[0030]

[0031] Where [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the weight of each element.

[0032] Heating is carried out in a temperature range of 1050℃ to 1160℃.

[0033] Rough rolling is performed at a cumulative reduction rate of 40% or greater in a temperature range of 900°C or higher, and

[0034] Hot rolling is carried out at a cumulative reduction rate of 50% or greater in a temperature range of 800°C or higher.

[0035] Beneficial effects

[0036] According to one embodiment of this disclosure, a steel plate and a method for manufacturing the same may be provided.

[0037] According to one embodiment of this disclosure, a steel plate with excellent resistance to low-temperature brittle cracking and a method thereof can be provided.

[0038] According to one embodiment of this disclosure, a steel plate and its manufacturing method can be provided that have excellent strength and toughness, as well as excellent resistance to brittle crack initiation at low temperatures, and is therefore suitable for various applications, such as liquefied gas tanks, ship hulls and structures in cryogenic environments.

[0039] The various advantages and effects of this disclosure are not limited to those described above, and will be more readily understood in the process of describing specific implementations of this disclosure. Detailed Implementation

[0040] Preferred embodiments of this disclosure will be described below. Embodiments of this disclosure may be modified in various ways, and the scope of this disclosure should not be construed as limited to the embodiments described below. These embodiments are provided to further describe this disclosure to those skilled in the art.

[0041] The contents of this disclosure will be described in detail below.

[0042] The steel composition of this disclosure will be described in detail below.

[0043] Unless otherwise stated in this disclosure, percentages (%) of each element are based on weight.

[0044] The steel plate according to one embodiment of this disclosure may contain, by weight %: C: 0.060% to 0.080%, Mn: 1.40% to 1.60%, Si: 0.10% to 0.20%, Al: 0.01% to 0.04%, Ni: 0.30% to 0.50%, Ti: 0.008% to 0.016%, Nb: 0.010% to 0.025%, P: 0.008% or less, S: 0.002% or less, and the balance Fe and unavoidable impurities. Furthermore, in the weld heat-affected zone welded at a heat input of 1.5 kJ / mm, the microstructure of the region from the fusion line (FL) to FL+0.2 mm contains 5.0% or less of the MA phase by area %

[0045] Carbon (C): 0.060% to 0.080%

[0046] Carbon (C) is arguably the most important element for ensuring the basic strength of the weld heat-affected zone. Therefore, it may be necessary to include carbon (C) in steel within an appropriate range. When the carbon (C) content exceeds 0.080%, martensite-austenite (MA), a coarse second phase, may form, and toughness may decrease. When the carbon content is below 0.060%, strength may decrease.

[0047] Manganese (Mn): 1.40% to 1.60%

[0048] Manganese (Mn) can be a useful element for improving strength through solid solution strengthening and for enhancing hardenability by forming low-temperature transformation phases. Therefore, to ensure the target properties described in this disclosure, 1.40% or more of manganese (Mn) may be included. According to one embodiment of this disclosure, 1.41% or more may be included. However, when the manganese (Mn) content exceeds 1.60%, hardenability may be excessively increased, potentially leading to the formation of coarse bainite in the substrate and a significant reduction in toughness. According to one embodiment of this disclosure, 1.59% or less of manganese (Mn) may be included.

[0049] Silicon (Si): 0.10% to 0.20%

[0050] Silicon (Si) can be an essential alloying element for deoxidation during steelmaking and casting processes by releasing dissolved oxygen from molten steel in the form of slag. When steel is manufactured using a converter, it can contain 0.10% or more silicon (Si). When contained in large quantities, it may coarsely form Si / Al composite oxides, or it may form large amounts of MA phase (hard phase) in the microstructure of the weld heat-affected zone. Therefore, the upper limit for silicon (Si) content can be limited to 0.20%.

[0051] Aluminum (Al): 0.01% to 0.04%

[0052] Aluminum (Al) is an essential alloying element used in steelmaking and casting processes to deoxidize molten steel by releasing dissolved oxygen in the form of slag. When steel is manufactured using a converter, it can contain 0.01% or more aluminum (Al). When large amounts of aluminum (Al) are present, coarse Si / Al composite oxides may form, or large amounts of the MA phase (hard phase) may form in the microstructure of the weld heat-affected zone. Therefore, the upper limit for aluminum (Al) content can be limited to 0.04%.

[0053] Nickel (Ni): 0.30% to 0.50%

[0054] Nickel (Ni) can be an important element that promotes dislocation cross-slip at low temperatures, thereby enhancing impact toughness and hardenability. To improve the low-temperature impact toughness of the substrate, nickel (Ni) can be included at 0.30% or more. However, when the content exceeds 0.50%, hardenability may be excessively increased, potentially leading to the formation of a large amount of bainite, and toughness may decrease. Manufacturing costs may also increase.

[0055] Titanium (Ti): 0.008% to 0.016%

[0056] Titanium (Ti) precipitates as TiN during reheating and can suppress grain growth in the weld heat-affected zone, thus significantly improving low-temperature toughness. Therefore, to effectively precipitate TiN, titanium (Ti) can be included at 0.008% or more. However, when this content exceeds 0.016%, problems such as nozzle clogging or reduced low-temperature toughness due to central crystallization may occur. In addition, the Ti / N ratio may decrease, so the TiN precipitates may become coarse, and resistance to brittle crack initiation may decrease.

[0057] Niobium (Nb): 0.010% to 0.025%

[0058] Niobium (Nb) can precipitate as NbC or NbCN and can improve strength. Furthermore, Nb dissolved during high-temperature reheating can precipitate relatively finely as NbC during rolling, thereby inhibiting austenite recrystallization. Therefore, the niobium (Nb) content can be 0.010% or more. When excessive niobium (Nb) is added, brittle cracking may occur at the edges of the steel, and a large amount of MA phase (hard phase) may form in the microstructure of the weld heat-affected zone, thus potentially reducing resistance to brittle crack initiation. According to one embodiment of this disclosure, the content can be 0.025% or less.

[0059] Phosphorus (P): 0.008% or less

[0060] Phosphorus (P) can cause grain boundary embrittlement or the formation of coarse inclusions, thus leading to embrittlement. To improve resistance to brittle crack initiation, its content can be controlled at 0.008% or less.

[0061] Sulfur (S): 0.002% or less

[0062] Sulfur (S) can cause grain boundary embrittlement or the formation of coarse inclusions, thus leading to embrittlement. To improve resistance to brittle crack initiation, its content can be controlled at 0.002% or less.

[0063] In addition to the components described above, the steel disclosed herein may also contain iron (Fe) and unavoidable impurities. Unavoidable impurities may be unintentionally included during the general manufacturing process and therefore may not be excluded. These impurities may be obvious to those skilled in the art of steel manufacturing, and therefore, they are not described herein.

[0064] The steel sheet according to one embodiment of this disclosure may contain Cu, Cr and Ni as impurities, but these elements may not be intentionally added.

[0065] According to one embodiment of this disclosure, the R value of the steel plate, as defined by the following relational expression 1, can be 0.38 or less:

[0066] [Relational Expression 1]

[0067]

[0068] (In the relational expression, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] can represent the weight percentage of each element.)

[0069] When the R value defined in relational expression 1 exceeds 0.38, there may be concerns that the target impact energy may not be adequately guaranteed. According to one embodiment of this disclosure, this value can be 0.37 or less. According to one embodiment of this disclosure, this value can be 0.36 or less. Meanwhile, there is no particular limitation on the lower limit of the R value in relational expression 1, but to properly ensure strength and impact energy, according to one embodiment of this disclosure, this value can be 0.31 or greater. According to one embodiment of this disclosure, this value can be 0.32 or greater.

[0070] The steel microstructure of this disclosure will be described in detail in the following description.

[0071] Unless otherwise stated in this disclosure, the percentage of a fraction of a microstructure may be based on area.

[0072] The microstructure of the steel plate according to one embodiment of this disclosure may include one or more of ferrite, acicular ferrite, and pearlite as the main phase. Because acicular ferrite and pearlite are not easily distinguishable, a mixture of these phases may be included according to one embodiment of this disclosure. According to one embodiment of this disclosure, the main phase may be 70% or more by area %. According to one embodiment of this disclosure, the main phase may be 80% or more by area %.

[0073] In a steel plate according to one embodiment of the present disclosure, in the weld heat-affected zone welded with a heat input of 1.5 kJ / mm, the MA phase may be 5.0% or less in the microstructure of the region from the fusion line (FL) to FL+0.2 mm, by area %.

[0074] In this disclosure, to improve resistance to brittle crack initiation, the microstructure fraction in the FL to FL+0.2 mm region can be restricted to a more stringent range. In other words, the FL to FL+0.2 mm region in the weld heat-affected zone is likely closest to FL and may have the weakest CTOD characteristics, and the MA phase may form in this region. The MA phase can act as a brittle crack initiation point, thereby deteriorating resistance to brittle crack initiation.

[0075] Simultaneously, during welding, the cooling rate may increase as heat input decreases. Therefore, the MA phase may readily form in the weld heat-affected zone (HAZ), and the CTOD characteristics of the HAZ may be weakened. The increased cooling rate as heat input decreases in the HAZ may further degrade its physical properties.

[0076] Therefore, even in the weld heat-affected zone welded with a low heat input of 1.5 kJ / mm, the steel plate according to one embodiment of this disclosure can control the MA phase fraction at a level of 5.0% or lower, thereby ensuring the target CTOD characteristics.

[0077] Meanwhile, the remaining microstructure in the weld heat-affected zone, besides the MA phase, may include one or more of upper bainite and granular bainite. According to one embodiment of this disclosure, one or more of upper bainite and granular bainite may be 90.0% or more.

[0078] According to one embodiment of this disclosure, the microstructure at the center of the thickness of a steel plate can be measured. According to one embodiment of this disclosure, the center of the thickness can be the midpoint in the thickness direction of the steel plate.

[0079] According to one embodiment of this disclosure, in a steel plate, in the weld heat-affected zone welded with a heat input of 1.5 kJ / mm, the effective grain size at the center of the thickness of the region from the fusion line (FL) to FL+0.2 mm can be 100 μm or less.

[0080] According to one embodiment of this disclosure, the effective grain size can indicate an average value.

[0081] In this disclosure, when the effective grain size at the center of the thickness in the region from FL to FL+0.2 mm is too large, the increased hardenability may promote the formation of low-temperature transformation phases such as bainite, thus potentially reducing the toughness of the weld heat-affected zone and decreasing resistance to brittle crack initiation. Therefore, in this disclosure, the grain size can be limited to 100 μm or less. According to one embodiment of this disclosure, this size can be 95 μm or less.

[0082] Furthermore, as described above, according to one embodiment of this disclosure, by strictly controlling the content of alloying elements, the target properties can be ensured even under harsh welding conditions. That is, according to one embodiment of this disclosure, the effective grain size in the FL to FL+0.2 mm region can be 100 μm or less.

[0083] In particular, in the weld heat-affected zone, the effective grain size may become coarser as the distance to the fusion line (FL) decreases. Therefore, by limiting the effective grain size in the region closest to the FL, from FL to FL+0.2 mm, the target characteristics can be effectively ensured.

[0084] Furthermore, as mentioned above, as heat input decreases, the cooling rate may increase, which could reduce the characteristics of the weld heat-affected zone. However, according to one embodiment of this disclosure, even in a weld heat-affected zone welded at a low heat input of 1.5 kJ / mm, the effective grain size can be 100 μm or smaller.

[0085] The steel plate according to one embodiment of this disclosure may have a yield strength of 355 MPa or greater, an impact toughness of 100 J or greater at -80°C, and a full-thickness crack tip opening displacement (CTOD) of 0.20 mm or greater at -60°C according to ISO 12135.

[0086] The following description will detail a method for manufacturing steel according to this disclosure.

[0087] According to one embodiment of this disclosure, a steel plate can be manufactured by heating, rough rolling, hot finishing and cooling a steel billet that meets the above alloy composition.

[0088] [heating]

[0089] A steel billet satisfying the alloy composition described in this disclosure can be heated at a temperature in the range of 1050°C to 1160°C.

[0090] The heating temperature can be 1050°C or higher, allowing the Ti and / or Nb carbonitrides formed during casting to dissolve and precipitate finely during subsequent rolling or PWHT. According to one embodiment of this disclosure, heating to 1100°C or higher can be performed to adequately dissolve the Ti and / or Nb carbonitrides. However, heating to excessively high temperatures may cause austenite coarsening, and therefore, the heating temperature can be limited to 1160°C or lower.

[0091] [Rough rolling]

[0092] Heated steel billets can be rough rolled at temperatures of 900°C or higher.

[0093] Rough rolling can be performed according to one embodiment of this disclosure to adjust the shape of the reheated billet.

[0094] During the rough rolling process, the temperature can be determined to be the temperature at which austenite recrystallization stops (Tnr) or higher. According to one embodiment of this disclosure, the temperature can be 900°C or higher. Through the rough rolling process, a reduction in grain size can be achieved by recrystallizing coarse austenite and breaking down the casting structure (e.g., dendrites formed during casting).

[0095] According to one embodiment of this disclosure, in order to induce sufficient recrystallization and refine the microstructure, the total cumulative reduction can be controlled at 40% or more during rough rolling.

[0096] [Hot Rolled]

[0097] Roughly rolled steel plates can be hot-rolled at 800°C or higher.

[0098] A finishing hot rolling process according to one embodiment of this disclosure can be performed to introduce an inhomogeneous microstructure into the austenitic structure of a rough-rolled steel sheet. To maximize strain in the microstructure, finishing hot rolling can be performed at 800°C or higher. When the finishing hot rolling temperature is below 800°C, the ferrite grain size may be inhomogeneous due to abnormal rolling, and coarse ferrite may form.

[0099] According to one embodiment of this disclosure, in order to maximize the formation of a finer microstructure, the cumulative reduction rate can be controlled at 50% or greater during finishing rolling.

[0100] [cool down]

[0101] Hot-rolled steel sheets can be cooled to a temperature range of 400°C to 700°C at a cooling rate of 10°C / second or higher, based on a point 1 / 4 of the way from the surface toward the center of the thickness.

[0102] When the cooling rate is less than 10°C / second, it may be difficult to ensure the target characteristics described in this disclosure. There is no specific upper limit to the cooling rate, but according to one embodiment of this disclosure, the upper limit can be 100°C / second.

[0103] When the cooling termination temperature is below 400°C, a large amount of hard phase may be generated, which may reduce the impact toughness of the substrate. When the temperature exceeds 700°C, a microstructure based on coarse ferrite and pearlite may be formed, which may reduce strength.

[0104] Invention Embodiments

[0105] The present disclosure is described in more detail below through embodiments. However, it should be noted that the following embodiments are intended only to illustrate and describe the present disclosure in more detail and are not intended to limit the scope of the present disclosure.

[0106] (Implementation Plan)

[0107] A steel billet with a thickness of 300 mm and the composition shown in Table 1 below is heated to a temperature of 1128°C to 1145°C and continuously rough-rolled at a cumulative reduction rate of 40% or greater, with the rough rolling completed at 950°C or higher. Subsequently, a finish hot-rolled steel sheet is produced at 820°C to 835°C with a cumulative reduction rate of 50% or greater to produce a steel plate with a thickness of 50 mm, and then cooled from the surface to 522°C to 543°C at a cooling rate of 10°C / s to 12°C / s, with a reference point at 1 / 4 of the thickness center.

[0108] [Table 1]

[0109]

[0110] *The unit may be ppm.

[0111] [Relational Expression 1]

[0112]

[0113] (In the relational expression, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] can represent the weight percentage of each element.)

[0114] Table 2 below lists the strength and impact toughness of the manufactured steel. Furthermore, the manufactured steel was subjected to multi-layer welding with a heat input of 1.5 kJ / cm, and the microstructure characteristics and CTOD values ​​of the weld heat-affected zone (FL) to FL+0.2 mm region were measured.

[0115] Yield strength was measured by room temperature tensile test according to JIS-5. Furthermore, impact absorbed energy was measured by Charpy impact test at -80°C at the center of the steel plate (halfway along the thickness direction), and the results are presented.

[0116] The microstructure fraction of the weld heat-affected zone was measured by observing the center position of each sample using an optical microscope after nital etching. In this case, one or more of upper bainite and granular bainite were observed as the remaining microstructure in addition to the MA phase.

[0117] The effective grain size of the weld heat-affected zone was measured five times at the thickness center of the steel plate using electron backscatter diffraction (EBSD) with a boundary angle of 15° or higher, and the average value is presented.

[0118] According to ISO 15653, the CTOD value was evaluated by performing a full-thickness CTOD test in the CGHAZ section at -60°C, which assessed the resistance to brittle crack initiation, and the results are listed in Table 2.

[0119] [Table 2]

[0120]

[0121] As shown in Table 3, the inventive examples that satisfy the alloy composition and manufacturing conditions of this disclosure satisfy the microstructure characteristics suggested in this disclosure and also ensure the material properties desired in this disclosure.

[0122] On the other hand, Comparative Example 1 can be an example where the carbon content exceeds that suggested in this disclosure. Due to the high hardenability, a large number of low-temperature transformation phases are formed in the microstructure, and therefore the substrate has excessively high yield strength and deteriorated impact energy. Furthermore, due to the high carbon content, an excessive amount of MA phase is formed in the weld heat-affected zone, and the target CTOD value cannot be guaranteed.

[0123] Comparative Example 2 can be an example where the manganese content exceeds that recommended in this disclosure. Due to the high hardenability, a large number of low-temperature transformation phases are formed in the microstructure, resulting in excessively high yield strength of the substrate and deterioration of impact energy. Furthermore, due to the high manganese content, coarse bainite is formed in the weld heat-affected zone, thus the effective grain size exceeds the target level, and the target CTOD value cannot be guaranteed.

[0124] Comparative Example 3 can be an example in which the Nb content exceeds the Nb content recommended in this disclosure and the Ti content is insufficient. Due to the addition of a large amount of Nb, the MA phase is excessively formed in the weld heat-affected zone, and TiN precipitates are not sufficiently formed. As a result, the microstructure in the FL to FL+0.2 mm region becomes coarse, and the target CTOD value cannot be guaranteed.

[0125] Comparative Example 4 can be an example where the Ni content does not meet the Ni content recommended in this disclosure. Therefore, toughness deteriorates, making it impossible to ensure the target CTOD value.

[0126] While this disclosure has been described in detail through embodiments, other embodiments are possible. Therefore, the technical spirit and scope of the appended claims are not limited to the embodiments.

Claims

1. A steel plate comprising: By weight percent, C: 0.060% to 0.080%, Mn: 1.40% to 1.60%, Si: 0.10% to 0.20%, Al: 0.01% to 0.04%, Ni: 0.30% to 0.50%, Ti: 0.008% to 0.016%, Nb: 0.010% to 0.025%, P: 0.008% or less, S: 0.002% or less, and the balance being Fe and unavoidable impurities. in, In the heat-affected zone of a weld where welding is performed with a heat input of 1.5 kJ / mm, the microstructure in the region from the fusion line (FL) to FL+0.2 mm contains 5.0% or less of the MA phase by area %.

2. The steel plate according to claim 1, wherein the R value of the steel plate as defined in the following relational expression 1 is 0.38 or less: [Relational Expression 1] Where [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the weight of each element.

3. The steel plate according to claim 1, wherein, In the weld heat-affected zone, the effective grain size at the center of the thickness in the region from the fusion line (FL) to FL+0.2 mm is 100 μm or smaller.

4. The steel plate according to claim 1, wherein the steel plate has a yield strength of 355 MPa or greater and an impact toughness of 100 J or greater at -80°C.

5. The steel plate according to claim 1, wherein the full-thickness crack tip opening displacement (CTOD) value of the steel plate in the coarse-grained heat-affected zone (CGHAZ) at -60°C is 0.20 mm or greater.

6. A method for manufacturing a steel plate, the method comprising: The steel billet is heated, the steel billet comprising, by weight percent: C: 0.060% to 0.080%, Mn: 1.40% to 1.60%, Si: 0.10% to 0.20%, Al: 0.01% to 0.04%, Ni: 0.30% to 0.50%, Ti: 0.008% to 0.016%, Nb: 0.010% to 0.025%, P: 0.008% or less, S: 0.002% or less, and the balance Fe and unavoidable impurities; The heated steel billet is rough rolled; The rough-rolled steel plate is then subjected to precision hot rolling; and Using a 1 / 4 point in the direction from the surface to the center of the thickness as a reference, the hot-rolled steel sheet is cooled to a temperature range of 400°C to 700°C at a cooling rate of 10°C / second or higher.

7. The method of claim 6, wherein the R value of the steel plate, as defined in the following relational expression 1, is 0.38 or less: [Relational Expression 1] Where [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the weight of each element.

8. The method according to claim 6, The heating is performed in a temperature range of 1050°C to 1160°C. The rough rolling is performed at a cumulative reduction rate of 40% or greater in a temperature range of 900°C or higher. The hot rolling is carried out at a cumulative reduction rate of 50% or more in a temperature range of 800°C or higher.