Ultra-thick steel plate with excellent low-temperature impact toughness and manufacturing method thereof

Abstract

The present invention relates to structural steel that can be used as a material for, for example, marine, bridge, and construction applications, and more particularly, to an ultra-thick steel plate having excellent low-temperature impact toughness and a method for manufacturing the same.

Description

Technical Field

[0001] The present disclosure relates to a structural steel material that can be used as a material for, for example, marine, bridge, and construction, and more particularly, to an ultra-thick steel plate having excellent low-temperature impact toughness and a method for manufacturing the same. Background Art

[0002] An ultra-thick steel plate having a certain thickness or more may be manufactured by a thick plate process, and in this case, the rolling method may be classified into general rolling, normalizing rolling, and thermo-mechanical controlled rolling (TMCP), etc. In addition, a heat treatment process may be performed after rolling, and in this case, the heat treatment process includes a normalizing heat treatment process, a quenching heat treatment process, a quenching-tempering heat treatment, etc.

[0003] Among the above-mentioned rolling processes, general rolling is a method of performing rolling without controlling the rolling temperature, which can be mainly applied to general steel that does not require impact toughness.

[0004] In contrast, TMCP uses temperature control to perform rolling in both the recrystallization and non-recrystallization zones, and can ensure strength and impact toughness through cooling as needed. However, when manufacturing ultra-thick materials using this TMCP process, long waiting times are required to adjust the rolling temperature, resulting in a significant reduction in productivity.

[0005] Normalizing rolling is done at relatively high temperatures, so strength and toughness may decrease due to grain growth during air cooling.

[0006] Therefore, when ultra-thick steel plates are manufactured through the TMCP process, the normalizing rolling process, or the heat treatment process after rolling, a high carbon component system containing 0.12% or more C needs to be applied to ensure strength, but since the toughness is severely deteriorated, the impact toughness can be guaranteed at room temperature and 0°C, and there is a problem of increased cost caused by heat treatment.

[0007] At the same time, the ultra-thick steel plates can be applied to various structural industries, such as infrastructure industries (such as ships); various frames of offshore structures, bridges, constructions, etc.; and wind power generation infrastructure, etc.

[0008] Recently, in most fields such as infrastructure industry, energy industry, etc., there is a trend toward larger structures due to minimization of installation costs and deterioration of installation environment, and it is expected that among structural steel plates used in various industrial fields, demand for ultra-thick steel plates with a thickness of 100 mm or more will increase along with the trend toward larger structures.

[0009] However, the metallurgical disadvantage of ultra-thick steel plates is that it is difficult to achieve strength and ensure toughness due to the reduced rolling amount and the limitation of the cooling process.

[0010] Due to the limitations of the rolling and cooling processes when manufacturing such ultra-thick steel plates, there is a tendency to excessively add alloy components to achieve steel plate strength, which can lead to problems such as increased costs and rapid deterioration of the toughness of the steel plates.

[0011] Furthermore, in the case where alloy components that adversely affect toughness are removed to ensure the toughness of the ultra-thick steel plate, a decrease in strength results.

[0012] Therefore, it is necessary to develop a technology that can achieve both strength and toughness of ultra-thick steel plates.

[0013] (Patent Document 1) Korean Patent Publication No. 10-2014-0003010 Summary of the Invention

[0014] Technical issues

[0015] One aspect of the present disclosure is to provide an ultra-thick steel plate having excellent strength and low-temperature impact toughness and a method of manufacturing the same by overcoming metallurgical disadvantages of existing ultra-thick steel plates.

[0016] The purpose of the present disclosure is not limited to the above description. The purpose of the present disclosure will be understood from the entire content of this specification, and those skilled in the art to which the present disclosure belongs will have no difficulty in understanding the other purposes of the present disclosure.

[0017] Technical Solution

[0018] According to one aspect of the present disclosure, there is provided an ultra-thick steel plate having excellent low-temperature impact toughness, the ultra-thick steel plate comprising, by weight, 0.06% to 0.1% carbon (C), 0.3% to 0.5% silicon (Si), 1.35% to 1.65% manganese (Mn), 0.015% to 0.04% aluminum (soluble Al), 0.015% to 0.04% niobium (Nb), 0.005% to 0.02% titanium (Ti), 0.15% to 0.4% chromium (Cr), 0.3% to 0.5% nickel (Ni), 0.002% to 0.008% nitrogen (N), 0.01% or less (excluding 0%) of phosphorus (P), 0.003% or less (excluding 0%) of sulfur (S), and iron (Fe) and unavoidable impurities in the balance, the ultra-thick steel plate satisfying the following Relationship 1:

[0019] The ultra-thick steel plate comprises, by area fraction, 80% to 90% ferrite and the remainder pearlite as the microstructure.

[0020] [Equation 1]

[0021] Mn+5(Ni+Ct)>3.6

[0022] Herein, each element refers to the weight content.

[0023] According to another aspect of the present disclosure, there is provided a method for manufacturing an ultra-thick steel plate having excellent low-temperature impact toughness, the method comprising the following operations: preparing a steel billet satisfying the above-mentioned alloy composition and Relationship 1; heating the steel billet at a temperature in the range of 1020° C. to 1150° C.; subjecting the heated steel billet to rough rolling at 1000° C. or higher; after the rough rolling, finishing hot rolling the steel billet at a temperature just above the non-recrystallization temperature (Tnr) or in the range of Tnr to A3; and air cooling it after the finish hot rolling.

[0024] Beneficial effects

[0025] As described above, according to the present disclosure, an ultra-thick steel plate having excellent strength and low-temperature impact toughness for an ultra-thick steel plate having a thickness of 100 mm to 200 mm can be provided.

[0026] As a structural material, the ultra-thick steel plate of the present disclosure can be used in various fields, such as infrastructure industries (eg, ships); various frames of marine structures, bridges, constructions, etc.; and wind power generation infrastructure, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A photograph showing the microstructure of an ultra-thick steel plate according to one embodiment of the present disclosure. DETAILED DESCRIPTION

[0028] In providing an ultra-thick steel plate having a thickness of 100 mm or more (100 mm to 200 mm) suitable for structural steel, the inventors of the present disclosure have intensively studied a method for ensuring excellent strength and low-temperature impact toughness.

[0029] As a result thereof, it was confirmed that an ultra-thick steel plate having target physical properties can be provided by optimizing the alloy composition system and rolling process of the ultra-thick steel plate, and thus the present disclosure is provided.

[0030] In particular, the technical significance of the present disclosure lies in that it can solve the productivity problems of existing TMCP steels, the problem of ensuring the physical properties of general rolled materials and heat-treated materials, the problem of the cost of heat-treated materials, etc.

[0031] Hereinafter, the present disclosure will be described in detail.

[0032] According to one aspect of the present disclosure, an ultra-thick steel plate having excellent low-temperature impact toughness may include, by weight, 0.06% to 0.1% carbon (C), 0.3% to 0.5% silicon (Si), 1.35% to 1.65% manganese (Mn), 0.015% to 0.04% aluminum (soluble Al), 0.015% to 0.04% niobium (Nb), 0.005% to 0.02% titanium (Ti), 0.15% to 0.4% chromium (Cr), 0.3% to 0.5% nickel (Ni), 0.002% to 0.008% nitrogen (N), 0.01% or less (excluding 0%) of phosphorus (P), and 0.003% or less (excluding 0%) of sulfur (S).

[0033] Hereinafter, the reasons for limiting the alloy composition of the steel sheet provided in the present disclosure as above will be described in detail.

[0034] Meanwhile, in the present disclosure, unless otherwise specified, the content of each element is based on weight, and the ratio of tissue is based on area.

[0035] Carbon (C): 0.06% to 0.1%

[0036] Carbon (C) is an element that causes solid solution strengthening and combines with Nb and the like in steel to form carbonitrides, and contributes to ensuring the strength of the steel.

[0037] In order to fully obtain the strength effect of C, C can be contained in an amount of 0.06% or more. However, when the C content exceeds 0.1%, the pearlite phase is excessively formed as a microstructure, so there is a problem of deterioration of impact properties and fatigue properties at low temperatures. In addition, as the solid solution C content increases, the impact properties decrease.

[0038] Therefore, C may be contained in an amount of 0.06% to 0.1%, and more advantageously, in an amount of 0.07% or more and 0.09% or less.

[0039] Silicon (Si): 0.3% to 0.5%

[0040] Silicon (Si) is used together with aluminum (Al) to deoxidize molten steel. Si has the effect of improving strength, but when the Si content is too high, the impact properties and fatigue properties at low temperatures may be impaired. Therefore, Si must be added in an appropriate amount.

[0041] When the Si content is less than 0.3%, sufficient strength may not be ensured. On the other hand, when the Si content exceeds 0.5%, diffusion of C is inhibited, and there is a problem in that the formation of an MA phase (martensite-austenite mixed structure) is promoted.

[0042] Therefore, Si may be contained in an amount of 0.3% to 0.5%.

[0043] Manganese (Mn): 1.35% to 1.65%

[0044] Manganese (Mn) is an element that has a large effect on improving strength through solid solution strengthening and may be contained in an amount of 1.35% or more. However, when the Mn content is too high, there is a problem that toughness may deteriorate due to the formation of MnS inclusions and segregation in the center portion. Therefore, considering this point, Mn may be contained in an amount of 1.65% or less.

[0045] Aluminum (soluble Al): 0.015% to 0.04%

[0046] Aluminum (soluble Al) is the primary deoxidizer in steel and contributes to the fixation of nitrogen (N) in steel. For this reason, Al inclusion in an amount of 0.015% or more is advantageous. However, when the Al content exceeds 0.04%, the fraction and size of Al2O3 inclusions increase, impairing low-temperature toughness. Furthermore, similar to Si, there is a problem of deteriorating low-temperature toughness and low-temperature fatigue properties due to the accelerated formation of MA phase in the base material and weld heat-affected zone.

[0047] Therefore, Al may be included in an amount of 0.015% to 0.04%.

[0048] Niobium (Nb): 0.015% to 0.04%

[0049] Niobium (Nb) has a solid solution strengthening effect and contributes to improving strength by forming carbonitrides to finely form the structure and suppress recrystallization during rolling or cooling.

[0050] In order to fully obtain the above effects, Nb may be contained in an amount of 0.015% or more. On the other hand, when the Nb content is too high, carbon aggregation occurs due to carbon affinity, so that the formation of the MA phase is promoted, and there is a problem of impairing toughness and fatigue properties at low temperatures. Therefore, considering this point, the Nb content can be limited to 0.04% or less.

[0051] Therefore, Nb may be included in an amount of 0.015% to 0.04%, and more advantageously, Nb may be included in an amount of 0.02% or more.

[0052] Titanium (Ti): 0.005% to 0.02%

[0053] Titanium (Ti) combines with nitrogen (N), which may deteriorate the impact properties and surface quality of steel, to form Ti-based nitrides (TiN), which serve to reduce the content of dissolved N. Ti-based precipitates contribute to refinement by suppressing coarsening of the structure and are useful for improving toughness.

[0054] In order to fully obtain the above-mentioned effects, Ti may be contained in an amount of 0.005% or more, but when the Ti content exceeds 0.02%, damage is caused due to coarsening of precipitates, and the remaining dissolved Ti forms Ti-based carbides (TiC) after combining with N, so there is a problem of impairing the toughness of the base material and the weld zone.

[0055] Therefore, Ti may be contained in an amount of 0.005% to 0.02%, and more advantageously, Ti may be contained in an amount of 0.01% or more.

[0056] Chromium (Cr): 0.15% to 0.4%

[0057] Chromium (Cr) is an element that contributes to improving the strength by increasing the hardenability of steel.

[0058] In order to fully obtain the above-mentioned effects, Cr may be contained in an amount of 0.15% or more, but when the content of Cr exceeds 0.4%, not only weldability is deteriorated but also there is a problem in that it is an expensive element causing an increase in manufacturing cost.

[0059] Therefore, Cr may be contained in an amount of 0.15% to 0.4%.

[0060] Nickel (Ni): 0.3% to 0.5%

[0061] Nickel (Ni) is an element that can improve both the strength and toughness of steel.

[0062] In particular, in order to fully obtain the effect of improving strength and toughness in the rolling process according to the present disclosure, Ni may be contained in an amount of 0.3% or more. However, when the Ni content exceeds 0.5%, the above effect is saturated, but there is a problem of increased manufacturing cost.

[0063] Therefore, Ni may be contained in an amount of 0.3% to 0.5%.

[0064] Nitrogen (N): 0.002% to 0.008%

[0065] Nitrogen (N) combines with Ti, Nb, Al, etc. in steel to form precipitates, and these precipitates are effective in improving strength and toughness by forming a fine austenite structure during reheating.

[0066] In order to fully obtain the above effects, it is advantageous to add 0.002% or more of N, but when the N content exceeds 0.008%, surface cracks are caused at high temperatures, and the remaining N exists in an atomic state after forming precipitates, resulting in deterioration of the toughness of the steel.

[0067] Therefore, N may be contained in an amount of 0.002% to 0.008%.

[0068] Phosphorus (P): 0.01% or less (excluding 0%)

[0069] Phosphorus (P) is an element that causes grain boundary segregation, which may cause embrittlement of steel. Therefore, the P content should be controlled to be as low as possible.

[0070] In the present disclosure, even when P is included in a maximum amount of 0.01%, there is no problem in ensuring the expected physical properties, so the P content can be limited to 0.01% or less. However, considering the inevitable addition level, 0% may not be included.

[0071] Sulfur (S): 0.003% or less (excluding 0%)

[0072] Sulfur (S) mainly combines with Mn in steel to form MnS inclusions, which are a factor that impairs low-temperature toughness.

[0073] Therefore, in order to ensure the low temperature toughness and low temperature fatigue properties desired in the present disclosure, the S content should be controlled as low as possible and may be preferably limited to 0.003% or less. However, considering the unavoidable addition level, 0% may not be included.

[0074] The remainder of this disclosure may be iron (Fe). However, in a typical manufacturing process, unavoidable impurities may be added from the raw materials or the surrounding environment, and thus impurities may not be eliminated. Those skilled in the art of typical manufacturing processes are aware of such impurities, and therefore, a description of the impurities may not be provided in this disclosure.

[0075] It is preferable that, in the steel plate of the present disclosure satisfying the above-mentioned alloy composition, the relationship among Mn, Ni, and Cr in the steel satisfy the following Relationship 1.

[0076] [Equation 1]

[0077] Mn+5(Ni+Cr)≥3.6

[0078] Herein, each element refers to the weight content.

[0079] In the present disclosure, in order to improve the low-temperature toughness of ultra-thick steel plates with a thickness of 100 mm to 200 mm, the C content can be limited to 0.10% or less. In the present disclosure, the relationship between Mn, Ni, and Cr in the steel is controlled by Relationship 1, so that even when the C content is relatively reduced, the ensured strength is not adversely affected.

[0080] Specifically, when the content relationship among Mn, Ni and Cr in the alloy composition proposed in the present disclosure does not satisfy the above-mentioned Relationship 1, that is, when the value of Relationship 1 is less than 3.6, the strength of the ultra-thick steel plate with a maximum thickness of 200 mm may not be obtained.

[0081] The ultra-thick steel plate of the present disclosure satisfying the above-described alloy composition and Relational Formula 1 may have a microstructure composed of a composite structure of ferrite and pearlite.

[0082] Specifically, it is preferred that the ultra-thick steel plate of the present disclosure contains, by area fraction, 80% to 90% of ferrite and the remainder of pearlite.

[0083] When the ferrite fraction is less than 80%, it is difficult to ensure the low-temperature toughness of the ultra-thick steel plate. On the other hand, when the ferrite fraction exceeds 90%, the pearlite fraction is insufficient, making it impossible to ensure the target level of strength.

[0084] Furthermore, the ultra-thick steel plate of the present disclosure has a fine structure because the average grain size of ferrite is 50 μm or less.

[0085] Here, it should be noted that the average grain size is based on the equivalent circle diameter.

[0086] As described above, the present disclosure has the effect of being able to simultaneously secure excellent strength and low-temperature toughness by finely realizing the structure of an ultra-thick steel plate.

[0087] Specifically, the ultra-thick steel plate of the present disclosure may have a yield strength of 300 MPa or greater and an impact toughness of 200 J or greater at -20°C, exhibiting high strength and excellent low-temperature impact toughness.

[0088] Hereinafter, a method for manufacturing an ultra-thick steel plate having excellent low-temperature impact toughness according to another aspect of the present disclosure will be described in detail.

[0089] In short, a steel plate can be manufactured by preparing a steel slab that satisfies the alloy composition and Relational Formula 1 proposed in the present disclosure, and then subjecting the steel slab to a process of [heating-rolling-air cooling]. In particular, the present disclosure has a technical significance in that the rolling process is performed in a normalizing heat treatment zone, rather than performing a separate heat treatment after the rolling process is completed.

[0090] Each process condition will be described in detail below.

[0091] [Bill heating]

[0092] In the present disclosure, it is preferred to perform a process of heating and homogenizing the steel slab before performing the rolling process, and in this case, the heating process may be performed at a temperature range of 1020°C to 1150°C.

[0093] When the heating temperature of the steel slab is lower than 1020°C, Ti, Nb, etc. may not be fully dissolved, resulting in a decrease in strength. On the other hand, when the heating temperature is higher than 1150°C, the austenite grains coarsen, which may cause the toughness of the steel to deteriorate.

[0094] The steel billet may have a thickness of 400 mm or less to ensure a sufficient rolling amount, thereby ensuring strength and toughness, while having a maximum thickness of 200 mm through the subsequent rolling process.

[0095] [Rolling process]

[0096] The steel slab heated as described above may be hot rolled to produce a hot-rolled steel sheet.

[0097] In the present disclosure, hot rolling is preferably performed in an operation of [recrystallization region rolling (rough rolling) - non-recrystallization region rolling (finish rolling)].

[0098] Rough rolling may be performed at 1000° C. or higher so that austenite can be completely recrystallized.

[0099] Thereafter, finish rolling can be performed in the austenite single-phase region at a temperature just above the non-recrystallization temperature (Tnr) or at a temperature in the range from Tnr to A3. In this case, to further promote the grain refinement effect, it is advantageous to perform finish rolling at a temperature close to A3, but to achieve the normalizing effect, it is advantageous to perform finish rolling at a temperature just above Tnr. The temperature just above Tnr can be expressed as a temperature range greater than Tnr to Tnr+50°C.

[0100] Tnr and A3 temperature can be obtained by the following formula, wherein each element means the weight content.

[0101] Tnr=887+464C+(6445Nb-644√NB)+(732V-230√V)+890Ti+363Al-357Si

[0102] A3=910-203√C-15.2Ni+44.7Si+104V+31.5Mo-30Mn+11Cr+20Cu-700P-400Al-400Ti

[0103] When the temperature during finish rolling is lower than A3, two-phase region rolling is performed, and the normalizing effect is insufficient, so that there may be a problem that an additional heat treatment process is required.

[0104] More preferably, the finish rolling may be completed at a temperature ranging from 820°C to 900°C.

[0105] Since the present disclosure aims to obtain an ultra-thick steel plate having a maximum thickness of 200 mm by performing the above-mentioned rolling process, it is necessary to consider the distribution of reduction ratios during rough rolling and finish rolling in the rolling process.

[0106] In the present disclosure, it is preferred to control the remaining rolling reduction immediately after rough rolling to 25% to 35%. If the remaining rolling reduction is less than 25%, there is a problem of prolonged rough rolling and reduced productivity. On the other hand, if the remaining rolling reduction exceeds 35%, there is a problem of not being able to achieve good rolling due to the load on the rolling mill during the finishing rolling after rough rolling.

[0107] Here, it should be noted that the remaining rolling reduction refers to the amount of finishing rolling remaining after rough rolling with respect to a target thickness.

[0108] [Air cooling]

[0109] The hot-rolled steel sheet obtained by completing the rolling process according to the above may be cooled, and in this case, air cooling is preferably performed to achieve a normalizing effect.

[0110] According to the present disclosure, by performing air cooling after completing the rolling process, not only the effect of grain refinement can be achieved, but also the effect of obtaining an ultra-thick steel plate with excellent strength and toughness without performing a subsequent heat treatment process can be achieved.

[0111] More specifically, when the intended microstructure is formed in the ultra-thick steel plate of the present disclosure, both excellent strength and toughness characteristics can be ensured for ultra-thick steel having a thickness of 100 mm to 200 mm.

[0112] To ensure strength, steel plates manufactured through conventional normalizing heat treatment have a higher carbon content than TMCP steels manufactured through controlled rolling and cooling. This results in steels manufactured through conventional normalizing heat treatment tending to have poor impact toughness even after heat treatment. Furthermore, when the heat treatment temperature is too high or the heat treatment time is too long, the strength of the steel plate may decrease due to grain growth compared to the as-rolled steel plate before heat treatment.

[0113] In the case of manufacturing an ultra-thick steel plate through the TMCP process, since a waiting time of several minutes for air cooling is required for temperature control, productivity is reduced and costs due to water treatment are required, which is economically disadvantageous.

[0114] The present disclosure proposes a manufacturing method capable of overcoming the disadvantages of ultra-thick plates produced by the above process, and can provide ultra-thick plates having excellent strength and low-temperature toughness characteristics by optimizing the rolling and cooling conditions of slabs having a specific alloy component system.

[0115] Hereinafter, the present disclosure will be described in more detail by the following examples. However, it should be noted that the following examples are only used to describe the present disclosure in detail by way of illustration and are not intended to limit the scope of the present disclosure. The reason is that the scope of the present disclosure is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0116] Embodiments of the invention

[0117] (Example)

[0118] Steel billets were prepared having the alloy compositions shown in Table 1. In this case, the content of the alloy composition is % by weight, and the remainder thereof contains Fe and inevitable impurities.

[0119] The prepared steel slabs were subjected to heating, hot rolling (rough rolling and finish rolling), and cooling (air cooling) under the conditions shown in Table 2, thereby producing respective hot-rolled steel sheets. In this case, the rough rolling was performed at 1000° C. or higher.

[0120] [Table 1]

[0121]

[0122] [Table 2]

[0123]

[0124] The microstructure and mechanical properties of each hot-rolled steel sheet manufactured as above were measured, and the results are shown in Table 3.

[0125] In the microstructure of each hot-rolled steel plate, a sample collected at a 1 / 4t point (where t means thickness (mm)) was observed with an optical microscope (OM), and the same sample was subjected to a Charpy impact test at -20°C to evaluate impact toughness.

[0126] In addition, the tensile strength, yield strength, and elongation of the specimens collected according to JIS No. 5 standard were measured using a universal tensile testing machine.

[0127] [Table 3]

[0128]

[0129] As shown in Tables 1 to 3, in Inventive Examples 1 to 3 that satisfy all the alloy compositions, Relationship 1, and manufacturing conditions proposed in the present disclosure, it can be confirmed that the steel plates have a yield strength of 300 MPa or more and an impact toughness of 200 J or more at -20°C, which is high strength and excellent low-temperature impact toughness.

[0130] On the other hand, in the case of Comparative Example 1 which satisfies the alloy composition system proposed in the present disclosure but has an excessively high finish temperature during finish rolling, coarse ferrite is formed, resulting in inferior strength and toughness.

[0131] Furthermore, in Comparative Example 2 in which the C content in the steel was excessive, pearlite was excessively formed, and while strength was maintained, toughness was significantly deteriorated.

[0132] In Comparative Example 3, which deviates from Relational Formula 1 proposed in the present disclosure, it was confirmed that even when the microstructure desired in the present disclosure was formed, the strength decreased. This demonstrates that it is difficult to ensure the target strength when the content of the hardenable element in the steel is not optimized according to Relational Formula 1 of the present disclosure.

[0133] Figure 1 This is a photograph of the microstructure of Inventive Example 3, and it can be confirmed that a composite structure having pearlite and a fine ferrite phase as a main phase is formed.

Claims

1. An ultra-thick steel plate having excellent low-temperature impact toughness, comprising, by weight, 0.06% to 0.1% carbon (C), 0.3% to 0.5% silicon (Si), 1.35% to 1.65% manganese (Mn), 0.015% to 0.04% soluble Al, 0.015% to 0.04% niobium (Nb), 0.005% to 0.02% titanium (Ti), 0.15% to 0.4% chromium (Cr), 0.3% to 0.5% nickel (Ni), 0.002% to 0.008% nitrogen (N), 0.01% or less and excluding 0% phosphorus (P), 0.003% or less and excluding 0% sulfur (S), and the balance being iron (Fe) and unavoidable impurities, the ultra-thick steel plate satisfying the following Relationship 1: The ultra-thick steel plate comprises, by area fraction, 80% to 90% ferrite and the remainder pearlite as a microstructure, [Equation 1] Mn+5(Ni+Cr)≥3.6 in, Each element refers to the weight content, The thickness of the steel plate is 100 mm to 200 mm. 2 . The ultra-thick steel plate having excellent low-temperature impact toughness according to claim 1 , wherein the average grain size of the ferrite is 50 μm or less.

3. The ultra-thick steel plate having excellent low-temperature impact toughness according to claim 1, wherein the steel plate has a yield strength of 300 MPa or more and an impact toughness of 200 J or more at -20°C.

4. A method for manufacturing an ultra-thick steel plate having excellent low-temperature impact toughness, comprising the following operations: A steel slab is prepared, the steel slab comprising, by weight, 0.06% to 0.1% carbon (C), 0.3% to 0.5% silicon (Si), 1.35% to 1.65% manganese (Mn), 0.015% to 0.04% soluble Al, 0.015% to 0.04% niobium (Nb), 0.005% to 0.02% titanium (Ti), 0.15% to 0.4% chromium (Cr), 0.3% to 0.5% nickel (Ni), 0.002% to 0.008% nitrogen (N), 0.01% or less and excluding 0% phosphorus (P), 0.003% or less and excluding 0% sulfur (S), and the balance being iron (Fe) and unavoidable impurities, the steel slab satisfying the following Relationship 1: heating the steel billet at a temperature in the range of 1020° C. to 1150° C.; subjecting the heated steel billet to rough rolling at a temperature of 1000° C. or higher; After the rough rolling, the steel slab is finish hot rolled at a temperature just above the non-recrystallization temperature Tnr or at a temperature in the range of Tnr to A3; and After the finish hot rolling, it is air-cooled. [Equation 1] Mn+5(Ni+Cr)≥3.6 in, Each element refers to the weight content, The thickness of the steel plate is 100 mm to 200 mm. 5 . The method according to claim 4 , wherein the finish hot rolling is completed at a temperature ranging from 820° C. to 900° C. The method according to claim 4 , wherein the remaining rolling reduction after the rough rolling is 25% to 35%.

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