Steel sheet having high strength and excellent low-temperature impact toughness, and method for manufacturing same

Abstract

The invention relates to a steel sheet which can be suitably used in marine structures such as wind turbine vessels and marine installations. More specifically, the present invention relates to a steel sheet having high strength and excellent low temperature impact toughness and a method for manufacturing the same.

Description

Technical Field

[0001] The present disclosure relates to steel plates that can be suitably used for marine structures such as wind energy facility ships and marine equipment. More specifically, the present disclosure relates to steel plates having high strength and excellent low-temperature impact toughness, and a method for manufacturing the same. Background Art

[0002] Recently, the demand for marine structures has been increasing, and there is a trend toward larger marine structures. In addition, since the jack-up rig equipment used in marine equipment, wind energy facility ships, etc. is aging and the replacement period is approaching, the demand is also increasing.

[0003] As the sizes of these equipment structures increase, ultra-thick, high-strength steel plates with a thickness of 200 mm or more are required. As the use environment expands to extreme condition areas, excellent low-temperature impact toughness is required, and when the steel plates are subjected to severe processing during structure production, low-temperature strain aging impact toughness is also required.

[0004] When manufacturing ultra-thick steel plates, if relatively thin slabs are used, sufficient rolling force may not be applied to the central part thereof, and the types and fractions of the microstructures of the central part and the surface part of the manufactured steel plates are different according to the difference in the cooling rate, resulting in large differences in physical properties and making it difficult to ensure uniform strength in the thickness direction.

[0005] Meanwhile, to manufacture high-strength, ultra-thick steel plates, a method of improving the hardenability of steel and increasing the strength by adding appropriate amounts of hardenability improving elements such as Mn, Cr, and Mo to the steel is mainly used. In this case, the strength of the steel can be increased by cooling treatment such as tempering treatment to generate a large amount of low-temperature structures such as martensite or bainite inside the steel plate. However, if such hardenable elements are added in excess, the carbon equivalent increases, which causes problems such as an increase in the preheating temperature before welding and the occurrence of cracks, so it is necessary to control the alloy composition.

[0006] Meanwhile, Patent Document 1 proposes a technique for ensuring high strength and low-temperature impact toughness. However, although Patent Document 1 has an economic advantage by reducing the expensive Ni content, it is difficult to ensure sufficient strength and low-temperature impact toughness when manufacturing ultra-thick hot-rolled steel plates other than medium / thick steel plates, and a high content of Mn relative to Cr may significantly reduce its low-temperature impact toughness.

[0007] Therefore, there is a need to develop high-strength, ultra-thick steel plates having excellent low-temperature impact toughness that can be suitably used for marine structures such as wind energy facility ships and marine equipment.

[0008] (Patent Document 1) Korean Patent Publication No. 10-2020-0075964. Summary of the Invention

[0009] Technical Problem

[0010] One aspect of the present disclosure is to provide an ultra-thick steel plate having high strength and excellent low-temperature impact toughness, and a method for manufacturing the same.

[0011] The object of the present disclosure is not limited to the above description. The object 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 pertains will not have difficulty in understanding additional objects of the present disclosure.

[0012] Technical Solution

[0013] According to one aspect of the present disclosure, there is provided a steel plate comprising, by weight%: 0.14% to 0.18% of C, 0.2% to 0.5% of Si, 0.1% to 0.7% of Mn, 0.008% or less of P, 0.003% or less of S, 0.015% to 0.045% of Al, 1.0% to 2.0% of Cr, 3.0% to 4.5% of Ni, 0.25% or less of Cu, 0.4% to 0.6% of Mo, 0.002% to 0.01% of N, and the balance Fe and inevitable impurities,

[0014] wherein the steel plate satisfies the following Relational Expression 1 and Relational Expression 2, and

[0015] has a thickness greater than 133 mm and 300 mm or less.

[0016] [Relational Expression 1]

[0017] C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95

[0018] [Relational Expression 2]

[0019] 2.0 < Cr / Mn < 6.5

[0020] In the above Relational Expressions 1 and 2, each element refers to the content (weight%).

[0021] The microstructure observed at the t / 4 point and the t / 2 point (where t is the thickness of the steel plate) of the steel plate may include 20% to 70% of tempered martensite and the balance bainite phase by area fraction.

[0022] The average grain size of the carbides present within the bainite phase may be 5 μm or less.

[0023] The steel plate may have a yield strength of 690 MPa or greater, a tensile strength of 800 MPa or greater, and an average Charpy impact energy absorption value of 69 J evaluated in the rolling direction at -40°C.

[0024] The average reduction of area after a tensile test evaluated in the thickness direction of the steel plate may be 35% or greater.

[0025] According to another aspect of the present disclosure, there is provided a method for manufacturing a steel plate, the method comprising: preparing a steel slab that contains, by weight%, 0.14% to 0.18% C, 0.2% to 0.5% Si, 0.1% to 0.7% Mn, 0.008% or less P, 0.003% or less S, 0.015% to 0.045% Al, 1.0% to 2.0% Cr, 3.0% to 4.5% Ni, 0.25% or less Cu, 0.4% to 0.6% Mo, 0.002% to 0.01% N, and the balance Fe and inevitable impurities, wherein the following relational expressions 1 and 2 are satisfied;

[0026] Heating the steel slab at a temperature in the range of 1050°C to 1200°C;

[0027] Rough rolling the heated slab at a temperature of 1000°C or higher;

[0028] After rough rolling, finish rolling the heated slab at a temperature of Ar3 or higher based on the temperature in its central portion to manufacture a hot-rolled steel plate, and then air-cooling (primary cooling) the hot-rolled steel plate;

[0029] Reheating the cooled hot-rolled steel plate to a temperature in the range of 830°C to 930°C and performing heat treatment for 2.3t + 30 minutes (where t is the thickness (mm) of the steel) or longer, and then cooling (secondary cooling) the cooled hot-rolled steel plate to room temperature at a rate of 0.5°C / second or greater, and;

[0030] Performing tempering heat treatment on the cooled hot-rolled steel plate at a temperature in the range of 550°C to 650°C for 2.3t + 30 minutes or longer and 3.4t + 30 minutes or shorter, and then cooling (tertiary cooling) the cooled hot-rolled steel plate to room temperature.

[0031] [Relational expression 1]

[0032] C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95

[0033] [Relational expression 2]

[0034] 2.0 < Cr / Mn < 6.5

[0035] In the above relational expressions 1 and 2, each element refers to the content (wt%).

[0036] The operation of preparing the steel slab can be to manufacture a continuous casting slab by forging a continuous casting slab at a temperature of Ac3 or higher to a thickness of 50% to 85% of its initial thickness.

[0037] The reheating and cooling operations can be carried out two or more times.

[0038] Advantageous Effects

[0039] As described above, according to one aspect of the present disclosure, there is provided an ultra-thick steel material that can ensure high strength throughout the thickness and has excellent low-temperature impact toughness.

[0040] The various and beneficial advantages and effects of the present disclosure are not limited to the above, and can be more easily understood through the description of specific embodiments of the present disclosure. Brief Description of the Drawings

[0041] Figure 1 It is a photograph showing the microstructure at the t / 4 point of Invention Example 1 in one embodiment of the present disclosure.

[0042] Figure 2 It is a photograph showing the microstructure at the t / 4 point of Comparative Example 2 in one embodiment of the present disclosure. Detailed Description of the Embodiments

[0043] The terms used in this specification are used to describe the present disclosure and are not intended to limit the present disclosure. In addition, as used herein, unless the relevant definition clearly indicates the contrary, the singular form includes the plural form.

[0044] The meaning of "comprising" or "including" used in the specification designates a configuration and does not exclude the presence or addition of other configurations.

[0045] Unless otherwise defined, all terms including technical terms and scientific terms used in this specification have the same meaning as those generally understood by those skilled in the art to which the present disclosure belongs. Terms defined in the dictionary are interpreted to have a meaning consistent with the relevant technical literature and the present disclosure.

[0046] The inventors of the present disclosure have recognized the need to develop technologies for ensuring the properties required for materials due to the expansion of marine structures, etc. and their application to harsh condition areas. For this purpose, the inventors have conducted in-depth research on methods for ensuring that steel plates have excellent low-temperature impact toughness while ensuring high strength in the entire thickness direction. Therefore, the inventors have confirmed that by controlling the composition of components and the relationship between some components in alloy design and simultaneously optimizing manufacturing conditions, ultra-thick steel plates with target properties can be provided, and thus the present disclosure is provided.

[0047] First, the alloy composition of the steel plate according to one aspect of the present disclosure will be described in detail.

[0048] Unless otherwise specifically stated in the present disclosure, the content of each element is by weight and the ratio of the structure is by area.

[0049] Carbon (C): 0.14 wt% to 0.18 wt% (hereinafter referred to as %)

[0050] Carbon (C) is an effective element for increasing the strength of steel, and in order to fully obtain the above effect, it is effective that C is contained in an amount of 0.14% or more. However, when the C content exceeds 0.18%, the low-temperature impact toughness and strain aging impact toughness are significantly reduced. Therefore, the C content in the present disclosure is effectively 0.14% to 0.18%. The C content is preferably 0.140% to 0.180%.

[0051] Silicon (Si): 0.2% to 0.5%

[0052] Silicon (Si) is used as a deoxidizer and is an effective element for increasing strength and toughness. However, when the Si content exceeds 0.5%, the low-temperature toughness and weldability may deteriorate. On the other hand, when the Si content is less than 0.2%, the deoxidation effect may be insufficient. Therefore, it is effective that the Si content is 0.2% to 0.5%. The Si content is preferably 0.20% to 0.50%.

[0053] Manganese (Mn): 0.1% to 0.7%

[0054] Manganese (Mn) is an element that is beneficial for ensuring strength due to solid solution strengthening, and in order to obtain the above effect, Mn is preferably added in an amount of at least 0.1%. However, when the Mn content exceeds 0.7%, Mn combines with S to form MnS, which significantly reduces the room temperature elongation, low-temperature impact toughness and strain aging impact toughness. Therefore, it is effective that the Mn content is 0.1% to 0.7%. The Mn content is preferably 0.10% to 0.70%.

[0055] Phosphorus (P): 0.008% or less

[0056] Phosphorus (P) is an element that is beneficial for improving strength and corrosion resistance, but P may significantly impair impact toughness. Therefore, it is advantageous to keep the P content as low as possible. Thus, it is effective to set the upper limit of the P content to 0.008%, and more effective to set the upper limit of the P content to 0.005% or less.

[0057] Sulfur (S): 0.003% or less

[0058] Sulfur (S) is an element that significantly impairs impact toughness by forming MnS, etc. Therefore, it is advantageous to keep the S content as low as possible. Thus, it is effective to set the upper limit of the S content to 0.003%.

[0059] Aluminum (Al): 0.015% to 0.045%

[0060] Aluminum (Al) is an element that can deoxidize molten steel inexpensively, and in order to exhibit sufficient effects, it is effective to include Al in an amount of 0.015% or more. However, when the Al content exceeds 0.045%, nozzle clogging may occur during continuous casting, and the formation of Al-based oxides may significantly reduce low-temperature impact toughness and strain-ageing impact toughness. Therefore, it is effective that the Al content is 0.015% to 0.045%.

[0061] Chromium (Cr): 1.0% to 2.0%

[0062] Chromium (Cr) is an effective element for increasing hardenability, forming bainite or martensite, and ensuring strength when manufacturing ultra-thick steel plates. In order to exhibit sufficient effects, it is effective to add Cr in an amount of 1.0% or more. However, Cr not only has a negative impact on welding properties by significantly increasing the carbon equivalent, but also may reduce low-temperature impact toughness due to coarsening of carbides. Therefore, it is effective that the Cr content is 2.0% or less. The Cr content is preferably 1.00% to 2.00%.

[0063] Nickel (Ni): 3.0% to 4.5%

[0064] Nickel (Ni) is an element that can improve both the strength and low-temperature impact toughness of the base material. In order to achieve sufficient effects, it is effective to include Ni in an amount of 3.0% or more. However, if the Ni content exceeds 4.5%, there is a problem that during slab reheating, the surface properties may deteriorate significantly due to oxidation. Therefore, it is effective that the Ni content is 3.0% to 4.5%. The Ni content is preferably 3.00% to 4.50%.

[0065] Copper (Cu): 0.25% or less (including 0%)

[0066] Copper (Cu) is an element that can increase strength while minimizing the reduction of the toughness of the base material, so it is an effective element for increasing strength. However, excessive addition of Cu increases the carbon equivalent, which not only hinders weldability but also significantly deteriorates the surface quality of the product. Therefore, it is effective that the Cu content is 0.25% or less.

[0067] Molybdenum (Mo): 0.4% to 0.6%

[0068] Molybdenum (Mo) has the effect of significantly increasing hardenability, suppressing ferrite formation and inducing bainite or martensite formation, and can also significantly increase strength. Therefore, in order to manufacture high-strength and high-toughness steel plates, it is effective that Mo is included in an amount of 0.4% or more. However, since Mo is an expensive alloying element, when added in large amounts, Mo excessively increases the hardness, which may reduce toughness. Therefore, it is effective that the maximum content of Mo is 0.6%. The Mo content is preferably 0.40% to 0.60%.

[0069] Nitrogen (N): 20 ppm to 100 ppm (0.002% to 0.01%)

[0070] Nitrogen (N) is an element that inhibits grain growth by forming AlN when added simultaneously with Al. However, when a large amount of N is added or heating is not carried out at a sufficiently high temperature, N forms coarse AlN, which impairs low-temperature impact toughness. Therefore, it is effective that the maximum content of N is 100 ppm. However, since controlling the N content to less than 20 ppm not only increases the steelmaking load but also is insufficient to inhibit grain growth, it is effective that the lower limit of the N content is 20 ppm.

[0071] In addition to the above components, components such as vanadium (V), titanium (Ti), and boron (B) described below can be additionally included. In this case, the content of each component is as follows.

[0072] Vanadium (V): 0.03% or less (including 0%)

[0073] Vanadium (V) has a lower dissolution temperature than other alloying elements, and V forms VC during the air-cooling process after hot rolling, which greatly contributes to increasing strength, enabling the strength improvement effect to be obtained. However, when the V content exceeds 0.03%, there are problems such as too high hardness of the base phase and an increase in the fraction of hard phases such as MA, resulting in a significant reduction in low-temperature impact toughness. Therefore, V can be included in an amount of 0.03% or less.

[0074] Titanium (Ti): 0.005% or less (including 0%)

[0075] Titanium (Ti) forms TiN when added together with N, thereby reducing surface cracks due to the formation of AlN precipitates. However, when the Ti content exceeds 0.005%, coarse TiNb(C,N) is formed during the reheating of the steel slab, which acts as a factor inhibiting low-temperature impact toughness. Therefore, Ti can be included in an amount of 0.005% or less.

[0076] Boron (B): 0.0005% or less (including 0%)

[0077] Boron (B) is a low-cost alloying element that exhibits strong hardenability even when added in small amounts, but when the B content exceeds 0.0005%, B significantly reduces low-temperature impact toughness. Therefore, the B content is preferably 0.0005% or less.

[0078] The remaining component of the present disclosure is iron (Fe). However, since in the general manufacturing process, it may be inevitable to incorporate unexpected impurities from raw materials or the surrounding environment, this component may not be excluded. Since these impurities are known to any person skilled in the general manufacturing process, their entire content is not particularly mentioned in this specification.

[0079] Effectively, the steel plates of the present disclosure satisfy the following relational expressions 1 and 2. In this case, each element represents the content (wt%).

[0080] [Relational Expression 1]

[0081] C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95

[0082] [Relational Expression 2]

[0083] 2.0 < Cr / Mn < 6.5

[0084] The present disclosure aims to ensure high strength and excellent low-temperature impact toughness by adding a certain amount of elements beneficial to improving strength and hardenability and appropriately controlling their contents to ensure the target level of strength.

[0085] In particular, in the present disclosure, C, Mn, Cr, Mo, Cu, Ni, etc. are added to the steel, and when the contents of these elements are excessive, the carbon equivalent (Ceq) may increase, thereby causing problems such as an increase in the preheating temperature before welding or the initiation of cracks. Therefore, effectively, the added elements are such that the contents of the above elements satisfy the above relational expression 1.

[0086] In addition, the content of Cr relative to the content of Mn should be increased sufficiently to suppress the formation of coarse granular bainite as much as possible and obtain fine tempered bainite or martensite. For this purpose, when the Cr content is too high, coarse carbides may form during heat treatment, and when the Mn content is too low, it is difficult to ensure strength. Therefore, it is effective to satisfy the above relational expression 2 by considering the ratio of the two components.

[0087] The steel plate of the present disclosure that satisfies the above relational expressions 1 and 2 and the above alloy components is an ultra-thick steel plate with a thickness greater than 133 mm and 300 mm or less.

[0088] The steel plate of the present disclosure may include tempered martensite as the microstructure, and the remaining part may include tempered bainite.

[0089] More specifically, the steel plate includes 20% to 70% of a tempered martensite phase by area fraction throughout the thickness, and the remaining part includes tempered bainite. For example, at the t / 2 point and the t / 4 point of the steel plate (where t is the thickness of the steel plate (mm)), the steel plate includes 20% to 70% of a tempered martensite phase by area fraction, and the remaining part includes tempered bainite.

[0090] When the fraction of tempered martensite is less than 20%, it is difficult to satisfy the strength proposed in the present disclosure. On the contrary, when the fraction of tempered martensite exceeds 70%, the low-temperature impact toughness may be significantly reduced due to the excessive increase in strength. The fraction of tempered martensite may decrease toward the central part of the thickness.

[0091] At the same time, it is preferable that fine carbides are present in the tempered bainite, and the average particle size of the carbides is 5 μm or less. When the average particle size of the carbides in the tempered bainite exceeds 5 μm, it can act as a crack initiation point and cause intragranular fracture, which may significantly reduce the low-temperature impact toughness. In this case, the main carbide is Fe-C carbide (e.g., Fe3C) and it may contain Mn, Cr, Mo, etc.

[0092] The steel plate of the present disclosure may have a yield strength of 690 MPa or greater, a tensile strength of 800 MPa or greater, and an average Charpy impact energy absorption (CVN) of 69 J or greater at -40 °C throughout the thickness of the steel plate, for example, at the t / 4 point and the t / 2 point in the thickness direction (where t is the thickness of the steel plate (mm)), and may have high strength and excellent low-temperature impact toughness.

[0093] At the same time, the average impact energy absorption value of the steel plate of the present disclosure when subjected to an impact test at -40 °C after aging heat treatment at 5% strain and 250 °C for 1 hour is 69 J or greater, which indicates that the low-temperature impact toughness does not decrease during strain aging.

[0094] In addition, the steel plate of the present disclosure has an effect that the average reduction of area after a tensile test evaluated in the thickness direction is 35% or more.

[0095] Hereinafter, a method for manufacturing a steel plate according to another aspect of the present disclosure will be described in detail. The ultra-thick steel plate according to the present disclosure can be manufactured by a process of heating - hot rolling - cooling - reheating - cooling - tempering a steel slab that satisfies both the alloy components and the component relationship expressions proposed by the present disclosure.

[0096] Each process condition will be described in detail.

[0097] Steel slab heating

[0098] Preferably, a process of heating and homogenizing the steel slab is performed before hot rolling, and the heating process can be performed at a temperature in the range of 1050°C to 1200°C. When the heating temperature of the steel slab is lower than 1050°C, the precipitates (carbides and nitrides) formed in the slab are not sufficiently redissolved, so the formation of precipitates is reduced in the subsequent process after hot rolling. On the other hand, when the heating temperature exceeds 1200°C, there is a concern that the austenite grains may become coarser, which may deteriorate the properties of the steel.

[0099] Meanwhile, the steel slab can be a cast slab obtained by continuous casting, and the cast slab can be heated as it is, or the cast slab can be forged before heating to obtain a forged slab, and then the heating process can be performed. Specifically, before heating, it may further include heating the cast slab to a temperature of Ac3 or higher, and then forging the cast slab to a thickness of 50% to 85% of the initial thickness of the cast slab.

[0100] In this case, the Ar3 temperature can be obtained from the following relationship expression.

[0101] Ar3 = 910 - 310*C - 80*Mn - 20*Cu - 55*Ni - 80*Mo + 119*V + 124*Ti - 18*Nb + 179*Al

[0102] Each element represents the content (wt%).

[0103] The present disclosure ultimately aims to obtain thick steel plates with a thickness greater than 133 mm, and in order to obtain steel plates with a target thickness within a limited reduction ratio (3:1) during hot rolling, it is necessary to apply slab billets with a thickness of 400 mm or more. As described above, in the present disclosure, the cast slab billets are obtained by continuous casting, and when the thickness of the cast slab billets is about 600 mm to 700 mm, the thickness can be reduced by performing a forging process before heating the slab billets. In particular, through the above forging process, the thickness can be effectively reduced while minimizing the internal voids of the slab billets, and sufficient rolling force can be applied to the central part of the thickness during the subsequent process (hot rolling process).

[0104] Cooling after hot rolling (primary cooling)

[0105] The hot-rolled steel plates can be manufactured by hot rolling the steel slab billets heated as described above. In this case, the heated steel slab billets can be rough-rolled at a temperature of 1000 °C or higher, and then finish hot-rolled at Ar3 or higher.

[0106] When the temperature during rough rolling is lower than 1000 °C, there is a problem of temperature reduction during subsequent finish hot rolling. In addition, when the temperature during finish hot rolling is less than Ar3, the rolling load may increase, which may cause quality defects such as surface cracks.

[0107] The manufactured hot-rolled steel plates are air-cooled (primary cooling).

[0108] Reheating and cooling (secondary cooling)

[0109] The hot-rolled steel plates cooled in the primary cooling are reheated to a temperature range of 830 °C to 930 °C, heat-treated for 2.3t + 30 minutes or longer (where t means the thickness of the steel (mm)), and then cooled to room temperature at a cooling rate of 0.5 °C / second or higher (secondary cooling). In this case, water cooling is used as an example of secondary cooling. At the same time, the reheating and cooling (secondary cooling) process can be repeated two or more times.

[0110] The formation of fine austenite grains can be promoted through the reheating process, and low-temperature phases can be formed during the subsequent cooling. That is, the hot-rolled steel plates can be reheated to form austenite grains, but when the reheating temperature is lower than Ac3, the microstructure of the hot-rolled steel plates may become a duplex structure of ferrite and austenite. When the reheating and cooling process is repeated two or more times, an additional austenite refinement effect can be obtained.

[0111] Therefore, when reheating the hot-rolled steel sheet, the reheating can be carried out at a temperature of Ac3 or higher, preferably at a temperature in the range of 830°C to 930°C, and preferably the temperature is maintained for 2.3t + 30 minutes or longer (where t means the thickness (mm) of the steel), so that 100% austenite phase is fully formed up to the center part of the hot-rolled steel sheet.

[0112] When the hot-rolled steel sheet manufactured as described above is cooled to room temperature and then reheated at a temperature below Ac3, the hot-rolled steel sheet may have a duplex structure of ferrite and austenite. When the hot-rolled steel sheet is maintained for less than 2.3t + 30 minutes, there is not enough time to form 100% austenite. Therefore, it is preferable to maintain the steel sheet at a temperature in the range of 870°C to 930°C for 2.3t + 30 minutes or longer (where t is the thickness (mm) of the steel sheet). When the cooling rate during secondary cooling to room temperature is less than 0.5°C / second, the fraction of ferrite or coarse bainite increases, making it difficult to ensure strength. Therefore, the cooling rate is preferably 0.5°C / second or higher. However, considering the cooling equipment, the cooling can be carried out at a maximum cooling rate of 100°C / second. Here, the cooling rate is based on the t / 4 region in the thickness direction of the steel sheet.

[0113] Temper heat treatment and cooling (tertiary cooling)

[0114] After secondary cooling, a temper heat treatment process can be carried out to form a tempered structure. The steel sheet after secondary cooling may include a low-temperature tissue phase, preferably a martensite or bainite phase, as the microstructure. As described above, the steel sheet may include a low-temperature tissue, so that the steel sheet can have high strength but exhibit brittle characteristics. The steel sheet on which the low-temperature tissue is formed can be heated to a certain temperature and then maintained, and the strength of the steel can be slightly reduced while ensuring the impact toughness at low temperature.

[0115] Specifically, the steel sheet after secondary cooling is subjected to a temper heat treatment at a temperature range in the range of 550°C to 650°C for a time of 2.3t + 30 minutes or longer and 3.4t + 30 minutes or shorter (where t represents the thickness (mm) of the steel), and then cooled in air (tertiary cooling).

[0116] By performing temper heat treatment, tempered martensite phase and tempered bainite phase can be formed. When the temperature during the temper heat treatment is lower than 550 °C, a long period of heat treatment is required to fully ensure the temper heat treatment effect, which causes a problem of low economic efficiency. On the other hand, when the temperature exceeds 650 °C, not only may the strength reduction effect increase excessively, but there is also a risk that carbides may coarsen and the impact toughness may also decrease. In addition, when performing temper heat treatment within the above temperature range, when the time is less than 2.3t + 30 minutes, the tempering effect is insufficient, and when the time exceeds 3.4t + 30 minutes, there is a concern that the strength may decrease due to over-tempering.

[0117] The steel plate of the present disclosure air-cooled after quenching and temper heat treatment includes 20% to 70% of tempered martensite and the remaining part of tempered bainite, and effectively, the average size of carbides in the tempered bainite is 2 μm or less.

[0118] Embodiments of the invention

[0119] Hereinafter, the present disclosure will be specifically described through the following examples. However, it should be noted that the following examples are only used to describe the present disclosure by way of illustration and are not intended to limit the scope of rights of the present disclosure. The reason is that the scope of rights of the present disclosure is determined by what is described in the claims and what can be reasonably inferred therefrom.

[0120] (Example)

[0121] Molten steel having the alloy composition (in wt%, including the remaining part of Fe and inevitable impurities) shown in Table 1 below was continuously cast to produce a continuous casting slab. As shown in Table 2, a continuous casting slab with a thickness of 700 mm was produced. The continuous casting slab was heated to a temperature of Ac3 or higher at which a subsequent hot rolling process could be performed, and then forged to a thickness of 400 mm to produce a forged slab. The forged slab was heated to a temperature in the range of 1100 °C to 1132 °C, rough rolled, and then finish rolled and air-cooled at a temperature in the range of 1050 °C to 1070 °C to produce a steel plate with a thickness of 210 mm.

[0122] Thereafter, the steel plate was reheated to a temperature of 890 °C and held for 513 minutes, and then water-cooled to room temperature at a rate of about 0.6 °C / second based on the central part. Thereafter, the water-cooled steel plate was heated to a temperature in the range of 595 °C to 630 °C and held for 590 minutes to 774 minutes for temper heat treatment, and then air-cooled to room temperature to produce a final steel plate.

[0123] [Table 1]

[0124]

[0125] [Table 2]

[0126]

[0127] Observe the microstructure of the steel plates manufactured under the conditions of Table 1 and Table 2 above and show the results in Table 3. Observe the microstructure at the t / 4 point and t / 2 point in the thickness direction of the steel plates using an optical microscope and an electron microscope, visually distinguish martensite and bainite, and measure their area fractions. In addition, the average diameter of the carbides present in tempered bainite was indicated using an image analysis program.

[0128] [Table 3]

[0129]

[0130] Referring to Table 3 above, in the case of Invention Examples 1 to 3, at least 20 area% of tempered martensite is ensured at t / 4 and t / 2, and the average size of the carbides inside the tempered martensite all satisfies 2 μm or less.

[0131] On the other hand, in the case of Comparative Example 1, due to the high C content, the fraction of tempered martensite is outside the range proposed in the present disclosure, and in the case of Comparative Examples 2 and 3, it can be confirmed that due to the low C and Cr contents, the fraction of tempered martensite is outside the range values proposed in the present disclosure. In the case of Comparative Example 4, it can be confirmed that the size of the carbides in tempered bainite is coarse because the tempering heat treatment was maintained for a long period at a level outside the range proposed in the present disclosure. In the case of Comparative Example 5, the tempering temperature is outside the range proposed in the present disclosure, and it can be confirmed that due to the heat treatment at a high temperature, the size of the carbides in tempered bainite is coarse.

[0132] Meanwhile, Figure 1 is a photograph obtained by observing the microstructure at the t / 4 point of Invention Example 1 above, and Figure 2 is a photograph obtained by observing the microstructure at the t / 4 point of Comparative Example 2 above. In Invention Example 1, it can be confirmed that the fraction of tempered martensite is sufficiently high and the size of the carbides in the tempered martensite is fine, while in Comparative Example 2, it can be confirmed that not only tempered martensite is formed, but also the size of the carbides is coarse.

[0133] Meanwhile, for the manufactured steel plates, the tensile properties (yield strength, tensile strength, and elongation) at t / 4t and t / 2 and the Charpy impact energy absorption (CVN) at -40 °C were measured and the results are shown in Table 4. In addition, the aged impact absorption energy at -40 °C after aging heat treatment at 250 °C after 5% strain and the Z-RA value as the reduction of area ratio after the tensile test evaluated in the thickness direction are shown in Table 4.

[0134] [Table 4]

[0135]

[0136] In the cases of Invention Examples 1 to 3, all the properties within the scope recommended by the present disclosure are satisfied. On the other hand, in the case of Comparative Example 1, it can be confirmed that due to the high C content, the tensile properties satisfy the range recommended by the present disclosure, but the impact toughness at -40 °C does not satisfy the value recommended by the present disclosure. In the case of Comparative Example 2, due to the reduced hardenability caused by the low Cr and Ni contents, tempered martensite is not sufficiently formed, and the size of the carbides in tempered bainite is coarse, resulting in low strength and impact toughness. In the case of Comparative Example 3, it can also be confirmed that due to the low fraction of tempered martensite, it is difficult to ensure strength and impact toughness. In the case of Comparative Example 4, it can be confirmed that all the component ranges recommended by the present disclosure are satisfied, but strength can be sufficiently ensured, and due to the coarse carbides caused by the too long temper heat treatment time, it is difficult to ensure impact toughness. In the case of Comparative Example 5, it can be confirmed that the component conditions of the present disclosure are satisfied, but due to the too high temper heat treatment temperature, the strength is significantly reduced and the toughness is also significantly reduced.

[0137] Meanwhile, in the cases of Invention Examples 1 to 3, it can be confirmed that the aging impact absorption energy and the Z-RA value satisfy the values recommended by the present disclosure, but in the cases of Comparative Examples 1 to 4, the aging impact absorption energy values recommended by the present disclosure are not satisfied.

Claims

1. A steel plate, comprising by weight %: 0.14% to 0.18% of C, 0.2% to 0.5% of Si, 0.1% to 0.7% of Mn, 0.008% or less of P, 0.003% or less of S, 0.015% to 0.045% of Al, 1.0% to 2.0% of Cr, 3.0% to 4.5% of Ni, 0.25% or less of Cu, 0.4% to 0.6% of Mo, 0.002% to 0.01% of N, and the balance of Fe and inevitable impurities, wherein the steel plate satisfies the following relational expressions 1 and 2, and has a thickness greater than 133 mm and 300 mm or less, [Relational expression 1] C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95 [Relational expression 2] 2.0 < Cr / Mn < 6.5 In the above relational expressions 1 and 2, each element refers to the content (weight %).

2. The steel plate according to claim 1, wherein the microstructure observed at the t / 4 point and the t / 2 point of the steel plate comprises 20% to 70% of tempered martensite and the balance of bainite phase by area fraction, where t is the thickness of the steel plate.

3. The steel plate according to claim 2, wherein the average particle size of the carbides present in the bainite phase is 5 μm or less.

4. The steel plate according to claim 1, wherein the steel plate has a yield strength of 690 MPa or greater, a tensile strength of 800 MPa or greater, and an average Charpy impact energy absorption value evaluated in the rolling direction at -40°C of 69 J or greater.

5. The steel plate according to claim 1, wherein the average impact energy absorption value of the steel plate when subjected to an impact test at -40°C after aging heat treatment at 250°C for 1 hour after 5% strain is 69 J or greater.

6. The steel plate according to claim 1, wherein the average reduction of area after a tensile test evaluated in the thickness direction of the steel plate is 35% or greater.

7. A method for manufacturing a steel plate, comprising: Preparing a steel slab, the steel slab comprising by weight %: 0.14% to 0.18% of C, 0.2% to 0.5% of Si, 0.1% to 0.7% of Mn, 0.008% or less of P, 0.003% or less of S, 0.015% to 0.045% of Al, 1.0% to 2.0% of Cr, 3.0% to 4.5% of Ni, 0.25% or less of Cu, 0.4% to 0.6% of Mo, 0.002% to 0.01% of N, and the balance of Fe and inevitable impurities, which satisfies the following relational expressions 1 and 2; Heating the steel slab at a temperature in the range of 1050°C to 1200°C; Rough rolling the heated slab at a temperature of 1000°C or higher; After the rough rolling, the heated slab is finish-rolled at a temperature of Ar3 or higher based on the temperature in its central part to produce a hot-rolled steel sheet, and then the hot-rolled steel sheet is air-cooled (primary cooling); The cooled hot-rolled steel sheet is reheated to a temperature of 830°C to 930°C, and heat-treated for 2.3t + 30 minutes or longer, where t is the thickness (mm) of the steel, and then the hot-rolled steel sheet is cooled (secondary cooling) to room temperature at a rate of 0.5°C / second or greater, and The cooled hot-rolled steel sheet is subjected to tempering heat treatment at a temperature of 550°C to 650°C for 2.3t + 30 minutes or longer and 3.4t + 30 minutes or shorter, and then the hot-rolled steel sheet is cooled (tertiary cooling) to room temperature, [Relational expression 1] C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 < 0.95 [Relational expression 2] 2.0 < Cr / Mn < 6.5 In the above relational expressions 1 and 2, each element refers to the content (wt%).

8. The method for manufacturing a steel sheet according to claim 7, wherein the preparation of the steel slab further includes manufacturing a continuous casting slab by forging a continuous casting slab at a temperature of Ac3 or higher to a thickness of 50% to 85% of its initial thickness.

9. The method for manufacturing a steel sheet according to claim 7, wherein the reheating and cooling operations are performed two or more times.

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