Steel sheet and manufacturing method thereof

Abstract

The present invention relates to a steel sheet and a manufacturing method thereof and, more specifically, to a steel sheet having excellent resistance to the initiation of ultra-low temperature brittle cracking and a manufacturing method thereof.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel plate and a method for manufacturing the same, and more particularly, to a steel plate having excellent resistance to cryogenic brittle crack initiation and a method for manufacturing the same.

[0002] With recent tightening of environmental regulations, demand for liquefied gas (LNG), an environmentally friendly fuel, is increasing. This has led to an increase in the construction of LNG carriers and LNG-propelled ships that use LNG as fuel. For LNG and LNG / ETH, high-Ni steel with a high Ni content is typically used due to the extremely low liquefaction temperature. However, for LPG, ammonia, CO2, and other liquefied gases with a liquefaction temperature below -60°C, carbon steel with enhanced cryogenic toughness is increasingly being used to reduce shipbuilding costs.

[0003] Existing liquefied gas carrier tanks have not primarily used the large Type A tank design that does not require high pressure, so there has been no need for high strength or heavy materials. However, recently, CO2 tanks and ammonia fuel tanks that require high pressure during liquefaction are designed as Type C, requiring high strength and heavy materials. In addition, when building Type C tanks, the IGC code requires mandatory application of PWHT (Post Weld Heat Treatment), so the steel used needs to have its properties guaranteed before and after PWHT.

[0004] Because PWHT is a high-temperature heat treatment, strength can decrease after PWHT, falling short of the required strength. To overcome this, precipitation-strengthening elements can be used, but this also reduces impact toughness. Therefore, it is extremely difficult to simultaneously guarantee both strength and toughness before and after PWHT.

[0005] In addition, in order to maximize efficiency when transporting liquefied gas, it is necessary to enlarge the tank size. In this case, a separate, ultra-large heat treatment furnace must be constructed to perform PWHT heat treatment after tank drying is complete.

[0006] However, in the above case, there is a problem of high cost.

[0007] Meanwhile, instead of using a large treatment furnace, PWHT heat treatment can be performed by attaching a heat treatment pad to the weld, but since the process of attaching a heat treatment pad to a large tank and performing a long-term heat treatment requires a lot of cost and air, shipbuilders are hoping to be exempted from PWHT heat treatment if possible.

[0008] When manufacturing a Type C tank, in order to be exempted from PWHT heat treatment, an Engineering Critical Assessment (ECA) must be conducted to demonstrate that there are no design issues even without stress relief through heat treatment. The most important factor in determining whether or not the ECA can be guaranteed is the CTOD (Crack Tip Opening Displacement) value of the welded part of the tank steel used. In the case of steel for liquefied CO2 tanks, the CTOD value of the Coarse Grained Heat Affected Zone (CGHAZ) at the design temperature of -55℃ must be guaranteed.

[0009] However, since there is little history of carbon steels that guarantee CTOD properties at such extremely low temperatures, there is a need to develop steels that maximize toughness through optimization of microstructure and alloy composition.

[0010] According to one embodiment of the present invention, a steel plate and a method for manufacturing the same are provided.

[0011] According to one embodiment of the present invention, it is an object to provide a steel plate having excellent resistance to cryogenic brittle crack initiation and a method for manufacturing the same.

[0012] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0013] According to one embodiment of the present invention, it contains, in wt%, 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, the remainder being Fe and unavoidable impurities.

[0014] In the heat-affected zone welded with a heat input of 1.5 kJ / mm, the microstructure in the region of FL (Fusion Line) ~ FL+0.2 mm may be a steel plate in which the MA phase has an area% of 5.0% or less.

[0015] The above steel plate may have an R value defined in the following relational expression 1 of 0.38 or less.

[0016] [Relationship 1]

[0017] R = [C] + [Mn] / 6 + [Cu] / 15 + [Ni] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5

[0018] (In the formula, [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are the weight percent of each element.)

[0019] In the above welding heat-affected zone, the effective grain size at the center of the thickness in the FL (Fusion Line) ~ FL+0.2 mm region may be 100 μm or less.

[0020] The above steel plate may have a yield strength of 355 MPa or more and an impact toughness at -80°C of 100 J or more.

[0021] The above steel plate may have a full thickness CTOD (Crack Tip Opening Displacement) value of 0.20 mm or more at -60°C in the CGHAZ (Coarse Grained Heat Affected Zone).

[0022] According to one embodiment of the present invention, there is provided a step of heating a steel slab containing, in wt%, 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, the remainder being Fe and unavoidable impurities;

[0023] A step of rolling the above heated steel slab;

[0024] A step of finishing hot rolling the above-mentioned rolled steel sheet; and

[0025] It may be a steel plate manufacturing method including a step of cooling the above-mentioned hot-rolled steel plate at a cooling rate of 10°C / s or more to a temperature range of 400 to 700°C based on a 1 / 4 point from the surface toward the center of the thickness.

[0026] The above steel slab may have an R value of 0.38 or less as defined in the following relational expression 1.

[0027] [Relationship 1]

[0028] R = [C] + [Mn] / 6 + [Cu] / 15 + [Ni] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5

[0029] (In the formula, [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are the weight percent of each element.)

[0030] The above heating step is performed at a temperature range of 1050 to 1160°C,

[0031] The above rolling step is performed at a temperature range of 900℃ or higher and a cumulative rolling reduction ratio of 40% or higher.

[0032] The above finishing hot rolling step can be performed at a temperature range of 800°C or higher and a cumulative reduction ratio of 50% or higher.

[0033] According to one embodiment of the present invention, a steel plate and a method for manufacturing the same can be provided.

[0034] According to one embodiment of the present invention, a steel plate having excellent resistance to cryogenic brittle crack initiation and a method for manufacturing the same can be provided.

[0035] According to one embodiment of the present invention, a steel plate having excellent strength and toughness and excellent resistance to brittle crack initiation at extremely low temperatures, and applicable to various fields such as liquefied gas tanks, ship hulls, and structures in extremely low-temperature environments, and a method for manufacturing the same can be provided.

[0036] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0037] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0038] Hereinafter, the present invention will be described in detail.

[0039] Below, the steel composition of the present invention is described in detail.

[0040] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0041] A steel sheet according to one embodiment of the present invention may include, in wt%, 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, and S: 0.002% or less.

[0042] Carbon (C): 0.060~0.080%

[0043] Carbon (C) is the most important element for securing the basic strength of the weld heat-affected zone, and therefore must be contained within an appropriate range in steel. If the carbon (C) content exceeds 0.080%, a coarse secondary phase, the MA (Martensite-Austenite) phase, may form, resulting in reduced toughness. Conversely, if the carbon (C) content is less than 0.060%, a decrease in strength may occur.

[0044] Manganese (Mn): 1.40~1.60%

[0045] The above manganese (Mn) is a useful element that improves strength through solid solution strengthening and enhances hardenability to form a low-temperature transformation phase. Therefore, in order to secure the desired properties of the present invention, it may be included in an amount of 1.40% or more. According to one embodiment of the present invention, it may be included in an amount of 1.41% or more. However, when the manganese (Mn) content exceeds 1.60%, coarse bainite may be formed in the base material due to an excessive increase in hardenability, which may significantly reduce toughness. According to one embodiment of the present invention, it may be included in an amount of 1.59% or less.

[0046] Silicon (Si): 0.10~0.20%

[0047] The above silicon (Si) is an essential alloying element that deoxidizes dissolved oxygen in molten steel by precipitating it in the form of slag during the steelmaking and casting processes, and can be contained at 0.10% or more when manufacturing steel using a converter. However, if contained in large amounts, it is an alloying element that can coarsely form Si, Al composite oxides or form a large amount of hard MA phase in the microstructure of the weld heat-affected zone, so the upper limit of the silicon (Si) content can be limited to 0.20%.

[0048] Aluminum (Al): 0.01~0.04%

[0049] The above aluminum (Al) is an essential alloying element that deoxidizes dissolved oxygen in molten steel by precipitating it in the form of slag during the steelmaking and casting processes. It can be contained at 0.01% or more when manufacturing steel using a converter. However, if contained in large amounts, it is an alloying element that can coarsely form Si, Al composite oxides or form a large amount of hard MA phase in the microstructure of the weld heat-affected zone. Therefore, the upper limit of the aluminum (Al) content can be limited to 0.04%.

[0050] Nickel (Ni): 0.30~0.50%

[0051] The above nickel (Ni) is an important element that improves impact toughness by facilitating cross-slip of dislocations at low temperatures and increases strength by enhancing hardenability. To improve the low-temperature impact toughness of the base material, nickel (Ni) may be included in an amount of 0.30% or more. However, if the content exceeds 0.50%, there is a problem that the hardenability is excessively increased, which leads to the formation of a large amount of bainite, thereby reducing toughness, and there may also be a problem of increasing manufacturing costs.

[0052] Titanium (Ti): 0.008~0.016%

[0053] The above titanium (Ti) can significantly improve low-temperature toughness by precipitating as TiN during reheating, thereby suppressing the growth of grains in the heat-affected zone of the weld. Therefore, to effectively precipitate TiN, titanium (Ti) may be included in an amount of 0.008% or more. However, if the content exceeds 0.016%, problems such as clogging of the casting nozzle or a decrease in low-temperature toughness due to central crystallization may occur, and the problem of reduced brittle crack initiation resistance may occur as the Ti / N ratio is lowered and the TiN precipitates become coarser.

[0054] Niobium (Nb): 0.010~0.025%

[0055] The above niobium (Nb) improves strength by precipitating in the form of NbC or NbCN. In addition, when reheated at a high temperature, the dissolved Nb is very finely precipitated in the form of NbC during rolling, which has the effect of inhibiting recrystallization of austenite. Therefore, the niobium (Nb) may be included in an amount of 0.010% or more. However, if niobium (Nb) is excessively added, it may cause brittle cracks at the edges of the steel, and a large amount of hard MA phase may be generated in the microstructure of the weld heat-affected zone, which may cause a problem of reduced resistance to brittle crack initiation. According to one embodiment of the present invention, it may be included in an amount of 0.025% or less.

[0056] Phosphorus (P): 0.008% or less

[0057] The above phosphorus (P) is an element that causes grain boundary embrittlement or forms coarse inclusions to cause embrittlement, and can be controlled to 0.008% or less to improve resistance to brittle crack initiation.

[0058] Sulfur (S): 0.002% or less

[0059] The above sulfur (S) is an element that causes grain boundary embrittlement or forms coarse inclusions to cause embrittlement, and can be controlled to 0.002% or less to improve resistance to brittle crack initiation.

[0060] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0061] According to one embodiment of the present invention, the steel sheet may have Cu, Cr, and Ni added at an impurity level, but not intentionally added.

[0062] A steel plate according to one embodiment of the present invention may have an R value defined in the following relational expression 1 of 0.38 or less.

[0063] [Relationship 1]

[0064] R = [C] + [Mn] / 6 + [Cu] / 15 + [Ni] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5

[0065] (In the formula, [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are the weight percent of each element.)

[0066] If the R value defined in the above relational expression 1 exceeds 0.38, there is a concern that the desired impact energy may not be sufficiently secured. According to one embodiment of the present invention, it may be 0.37 or less. According to one embodiment of the present invention, it may be 0.36 or less. Meanwhile, the lower limit of the R value in the above relational expression 1 is not particularly limited, but in order to appropriately secure strength and impact energy, it may be 0.31 or more according to one embodiment of the present invention. According to one embodiment of the present invention, it may be 0.32 or more.

[0067] Below, the steel microstructure of the present invention is described in detail.

[0068] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0069] The microstructure of the steel sheet according to one embodiment of the present invention may include at least one of ferrite, aesculate ferrite, and pearlite as a main phase. Since aesculate ferrite and pearlite are not easily distinguished, one embodiment of the present invention may include a mixed phase thereof. According to one embodiment of the present invention, the main phase may be at least 70% in area %. According to one embodiment of the present invention, the main phase may be at least 80%.

[0070] According to one embodiment of the present invention, in a welded heat-affected zone of a steel plate welded with a heat input of 1.5 kJ / mm, the microstructure of the region of FL (Fusion Line) ~ FL+0.2 mm may be an MA phase with an area% of 5.0% or less.

[0071] In the present invention, the microstructure fraction in the FL to FL+0.2 mm region can be restricted to a more stringent range to improve brittle crack initiation resistance. In other words, the FL to FL+0.2 mm region in the weld heat-affected zone is the region closest to FL and is the part with the weakest CTOD characteristics, and the MA phase is likely to form in this region. The MA phase can act as a brittle crack initiation point and thus deteriorate the brittle crack initiation resistance.

[0072] Meanwhile, during welding, the lower the heat input, the faster the cooling rate, which can facilitate the formation of MA phases in the weld heat-affected zone and can lead to poor CTOD characteristics in the weld heat-affected zone. The lower the heat input in the weld heat-affected zone, the faster the cooling rate, which can lead to poor physical properties.

[0073] Accordingly, the steel plate according to one embodiment of the present invention can control the MA phase fraction to a level of 5.0% or less even in the weld heat affected zone welded with a low heat input of 1.5 kJ / mm, thereby securing the desired CTOD characteristics.

[0074] Meanwhile, the residual structure excluding the MA phase in the weld heat-affected zone may include at least one of upper bainite and granular bainite. According to one embodiment of the present invention, at least one of upper bainite and granular bainite may account for 90.0% or more.

[0075] According to a temporary embodiment of the present invention, the microstructure can be measured at the center of the thickness of the steel plate. According to a temporary embodiment of the present invention, the center of the thickness may be a point halfway in the thickness direction of the steel plate.

[0076] According to one embodiment of the present invention, a steel plate may have an effective grain size of 100 μm or less at the center of the thickness in a region of FL (Fusion Line) to FL+0.2 mm in a welded heat-affected zone welded with a heat input of 1.5 kJ / mm.

[0077] The effective crystal grain size according to one embodiment of the present invention may mean its average value.

[0078] In the present invention, if the effective grain size at the center of the thickness in the FL~FL+0.2mm region is excessively coarse, the formation of a low-temperature transformation phase such as bainite due to increased hardenability is promoted, which reduces the toughness of the weld heat-affected zone and reduces the brittle crack initiation resistance. Therefore, in the present invention, the size can be limited to 100μm or less. According to one embodiment of the present invention, the size can be 95μm or less.

[0079] Meanwhile, as previously described, according to one embodiment of the present invention, by strictly controlling the alloy element content, the desired properties can be secured even during welding under strict conditions. That is, according to one embodiment of the present invention, the effective crystal grain size can be 100 μm or less in the FL to FL+0.2 mm range.

[0080] In particular, in the weld heat-affected zone, the closer the distance to the FL (melting line), the larger the effective grain size may become. Therefore, by limiting the effective grain size in the region of FL ~ FL + 0.2 mm closest to the FL, the desired properties can be effectively secured.

[0081] In addition, as described above, the lower the heat input, the faster the cooling rate, which may lead to a deterioration in the properties of the weld heat-affected zone. However, according to one embodiment of the present invention, even in a weld heat-affected zone welded with a low heat input of 1.5 kJ / mm, the effective grain size may be 100 μm or less.

[0082] A steel plate according to one embodiment of the present invention may have a yield strength of 355 MPa or more, an impact toughness at -80°C of 100 J or more, and a full thickness CTOD (Crack Tip Opening Displacement) value of 0.20 mm or more at -60°C of CGHAZ in accordance with the ISO 12135 standard.

[0083] Below, the steel manufacturing method of the present invention is described in detail.

[0084] A steel plate according to one embodiment of the present invention can be manufactured by heating, rough rolling, finish hot rolling, and cooling a steel slab satisfying the above-described alloy composition.

[0085] [heating]

[0086] A steel slab satisfying the alloy composition of the present invention can be heated in a temperature range of 1050 to 1160°C.

[0087] By setting the heating temperature to 1050°C or higher, the carbonitrides of Ti and / or Nb formed during casting can be dissolved and finely precipitated during subsequent rolling or PWHT. According to one embodiment of the present invention, in order to sufficiently dissolve the carbonitrides of Ti and / or Nb, heating may be performed at 1100°C or higher. However, since there is a concern that austenite may coarsen if heated to an excessively high temperature, the heating temperature may be limited to 1160°C or lower.

[0088] [Rough rolling]

[0089] The above heated steel slab can be rolled at a temperature range of 900°C or higher.

[0090] According to one embodiment of the present invention, the pre-rolling process can be performed to adjust the shape of the reheated steel slab.

[0091] During the above rough rolling, the temperature may be set to be higher than the temperature (Tnr) at which austenite recrystallization stops. According to one embodiment of the present invention, the temperature may be 900°C or higher. The above rough rolling process can achieve the effect of reducing the grain size through recrystallization of coarse austenite along with destruction of cast structures such as dendrites formed during casting.

[0092] According to one embodiment of the present invention, in order to cause sufficient recrystallization and refine the structure, the total cumulative reduction ratio can be controlled to 40% or more during rough rolling.

[0093] [Finishing hot rolling]

[0094] The above-mentioned pre-rolled steel sheet can be finished by hot rolling at a temperature range of 800°C or higher.

[0095] According to one embodiment of the present invention, the finishing hot rolling may be performed to introduce a non-uniform microstructure into the austenite structure of the rough-rolled steel sheet. The finishing hot rolling may be performed at a temperature of 800°C or higher to induce maximum deformation within the structure. If the finishing hot rolling temperature is lower than 800°C, the ferrite grain size may not be uniform due to abnormal rolling, and coarse ferrite may be formed.

[0096] According to one embodiment of the present invention, in order to create the finest possible structure, the cumulative reduction ratio can be controlled to 50% or more during rolling.

[0097] [cooling]

[0098] The above hot-rolled steel sheet can be cooled at a cooling rate of 10°C / s or more to a temperature range of 400 to 700°C based on a point 1 / 4 of the thickness from the surface toward the center.

[0099] If the cooling rate is less than 10°C / s, it may be difficult to secure the desired properties of the present invention. The upper limit of the cooling rate is not particularly limited, but according to one embodiment of the present invention, the upper limit of the cooling rate may be 100°C / s.

[0100] If the above cooling end temperature is below 400℃, a large amount of hard phase may be generated, which may reduce the impact toughness of the base material. On the other hand, if the temperature exceeds 700℃, there may be a problem of reduced strength due to the generation of a microstructure based on coarse ferrite and pearlite.

[0101] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0102] (Example)

[0103] A steel slab having a thickness of 300 mm and a composition as shown in Table 1 below was heated to a temperature of 1128 to 1145°C, and then subjected to rough rolling at a cumulative reduction ratio of 40% or more continuously, and finished at 950°C or higher. Thereafter, a finishing hot rolling was performed at a cumulative reduction ratio of 50% or more at 820 to 835°C to produce a steel plate having a thickness of 50 mm, and then cooled to 522 to 543°C at a cooling rate of 10 to 12°C / s from the surface toward the center of the thickness at a 1 / 4 point.

[0104] Steel alloy composition (wt%) relationship 1CMnSiAlNiNbTiP*S*RA0.0711.470.150.020.460.0180.01438100.35B0.0641.510.170.030.370.0220.01641120.34C0.0761.420.120.030.420.0170.0135180.34D0.0691.530.110.020.490 .0190.01248120.36E0.0931.560.180.030.320.0240.01344110.37F0.0791.690.170.020.350.0280.0 124690.38G0.0681.460.160.020.370.0380.00651120.34H0.0671.480.110.020.180.0210.0145880.33

[0105] * Unit is ppm.

[0106] [Relationship 1]

[0107] R = [C] + [Mn] / 6 + [Cu] / 15 + [Ni] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5

[0108] (In the formula, [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are the weight percent of each element.)

[0109] Table 2 below shows the strength and impact toughness of the manufactured steel. In addition, the manufactured steel was multi-layer welded with a heat input of 1.5 kJ / cm, and the microstructural characteristics and CTOD values ​​in the FL (Fusion Line) ~ FL+0.2 mm region of the weld heat-affected zone were measured and shown.

[0110] Yield strength was measured by a room-temperature tensile test according to JIS-5 standard. In addition, the impact absorption energy was measured by a Charpy impact test performed at -80℃ at the center of the steel plate (1 / 2 point in the thickness direction), and the results are presented.

[0111] The microstructural fraction of the weld heat-affected zone was determined by observing the central location of each specimen using an optical microscope after nital etching. At this time, at least one type of upper bainite or granular bainite was observed as the residual structure excluding the MA phase.

[0112] The effective grain size of the weld heat-affected zone was measured five times using EBSD (Electron Backscatter Diffraction) at the center of the steel plate thickness direction for grains with a boundary angle of 15° or more, and the average value was expressed.

[0113] The CTOD value was evaluated for brittle crack initiation resistance by conducting a full-thickness CTOD test at -60℃ in the CGHAZ department in accordance with the ISO 15653 standard, and the results are shown in Table 2.

[0114]

[0115] Specimen numberSteel gradeParent materialWelding heat affected zoneRemarksYield strength(MPa)Center impact energy(-80℃,J)FL~FL+0.2mm areaCGHAZFailure thicknessCTOD(-60℃,mm)MAPhase fraction(area%)Effective grain Size (μm) 1A4152873.6890.36 Invention Example 12B4372432.8780.51 Invention Example 23C4412964.1930.36 Invention Example 34D4632653.8810.29 Invention Example 45E492896.2790.14 Comparative Example 16F524944.31050.16 Comparative Example 27G4872075.91320.09 Comparative Example 38H4361982.7870.18 Comparative Example 4

[0116] As shown in Table 3 above, in the case of an invention example that satisfies the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties targeted by the present invention were also secured.

[0117] On the other hand, Comparative Example 1 is an example in which the C content proposed in the present invention was exceeded. Due to the high hardenability, a large amount of low-temperature transformation phases were formed in the microstructure, resulting in the base material having an excessively high yield strength and poor impact energy. In addition, due to the high C content, an excessive MA phase was formed in the weld heat-affected zone, and the desired CTOD value could not be secured.

[0118] Comparative Example 2 is an example where the Mn content exceeded the proposed Mn content of the present invention. Due to the high hardenability, a large amount of low-temperature transformation phases were formed in the microstructure, resulting in an excessively high yield strength of the base material and poor impact energy resistance. Furthermore, the high Mn content resulted in the formation of coarse bainite in the weld heat-affected zone, resulting in an effective grain size exceeding the target level and failing to secure the target CTOD value.

[0119] Comparative Example 3 is an example in which the Nb content exceeds the Nb content proposed in the present invention and the Ti content is insufficient. Due to the large amount of Nb added, an excessive MA phase was formed in the weld heat-affected zone, and TiN precipitates were not sufficiently formed, resulting in a coarsening of the microstructure in the FL~FL+0.2 mm region, and the desired CTOD value could not be secured.

[0120] Comparative Example 4 is an example that falls short of the Ni content proposed in the present invention. As a result, the toughness is reduced, and the desired CTOD value cannot be secured.

[0121] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Contains, in wt%, 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, the remainder being Fe and unavoidable impurities. A steel plate in which the microstructure of the heat-affected zone welded with a heat input of 1.5 kJ / mm in the region of FL (Fusion Line) ~ FL+0.2 mm has an area% of MA phase of 5.0% or less.

2. In claim 1, The above steel plate is a steel plate having an R value of 0.38 or less as defined in the following relational expression 1. [Relationship 1] R = [C] + [Mn] / 6 + [Cu] / 15 + [Ni] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5 (In the formula, [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are the weight % of each element.) 3. In claim 1, A steel plate having an effective grain size of 100 μm or less at the center of the thickness in the region of FL (Fusion Line) ~ FL + 0.2 mm in the above-mentioned weld heat-affected zone.

4. In claim 1, The above steel plate is a steel plate having a yield strength of 355 MPa or more and an impact toughness at -80°C of 100 J or more.

5. In claim 1, The above steel plate is a steel plate having a full thickness CTOD (Crack Tip Opening Displacement) value of 0.20 mm or more at -60℃ in the CGHAZ (Coarse Grained Heat Affected Zone).

6. A step of heating a steel slab containing, 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, the remainder being Fe and unavoidable impurities; A step of rolling the above heated steel slab; A step of finishing hot rolling the above-mentioned pre-rolled steel sheet; and A method for manufacturing a steel sheet, comprising: a step of cooling the hot-rolled steel sheet, which has been finished above, from the surface toward the center of the thickness to a temperature range of 400 to 700°C at a cooling rate of 10°C / s or more.

7. In claim 6, The above steel slab is a method for manufacturing a steel plate having an R value of 0.38 or less as defined in the following relational expression 1. [Relationship 1] R = [C] + [Mn] / 6 + [Cu] / 15 + [Ni] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5 (In the formula, [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are the weight % of each element.) 8. In claim 6, The above heating step is performed at a temperature range of 1050 to 1160°C. The above rolling step is performed at a temperature range of 900℃ or higher and a cumulative reduction ratio of 40% or higher. A method for manufacturing a steel sheet, wherein the above finishing hot rolling step is performed at a temperature range of 800°C or higher and a cumulative reduction ratio of 50% or higher.