High-strength ultra-thick steel sheet having excellent low-temperature impact properties and method for manufacturing same

Abstract

The purpose of an aspect of the present invention is to provide: a high-strength ultra-thick steel sheet having excellent low-temperature impact properties; and a method for manufacturing same.

Description

High-strength, ultra-thick steel sheet with excellent low-temperature impact properties and method for manufacturing the same

[0001] The present invention relates to a high-strength, ultra-thick steel plate with excellent low-temperature impact characteristics and a method for manufacturing the same. More specifically, the present invention relates to a high-strength, ultra-thick steel plate with excellent low-temperature impact characteristics and a method for manufacturing it, applicable as steel for offshore wind power monopiles, jackets, offshore platforms for gas and crude oil extraction, and infrastructure industries such as construction and bridges.

[0002] Recently, the installation of offshore wind power generation facilities has been rapidly increasing worldwide. In particular, offshore wind substations, which are essential for the efficient operation of offshore wind farms, play a crucial role. Offshore wind substations are core facilities that collect electricity generated by offshore wind turbines, convert it to high voltage, and transmit it to land; generally, one to two units are installed per 300–400 MW class facility. Since these facilities include expensive electronic equipment and office space, structural stability and durability are essential.

[0003] To ensure the stability of an offshore wind farm, the design of the substructures and transition pieces installed on the seabed, as well as the selection of steel materials, are critical. The substructure serves to firmly support the turbines and towers and is generally classified into monopile and jacket structures. The monopile structure, in the form of a single cylinder, is suitable for shallow waters, whereas the jacket structure is suitable for water depths of approximately 30 to 60 meters and provides high stability on soft seabeds.

[0004] In addition, the transition piece connecting the substructure and the tower plays a crucial role in maintaining load transfer and balance between the turbine and the substructure.

[0005] As offshore wind power generation becomes larger and the application of super-large turbines increases, the steel used in these substructures and transition pieces must not only possess high strength and stability but also excellent resistance performance against external loads such as wind, waves, and currents. To achieve this, the thickness and strength of the steel are often increased, but increased thickness and high strength can lead to a decrease in impact toughness.

[0006] Impact toughness refers to the amount of energy a material can absorb through plastic deformation before fracture. It is an important indicator representing the resistance to crack initiation and propagation, as well as the allowable degree of plastic deformation up to the point of failure. For large structures used in offshore wind power generation, impact toughness is considered a particularly important material property.

[0007] Currently, active research is underway on ultra-high-strength, ultra-thick steel materials capable of simultaneously securing high strength and excellent impact toughness to address these issues.

[0008] According to one embodiment of the present invention, a high-strength, extremely thick steel plate with excellent low-temperature impact properties and a method for manufacturing the same can be provided.

[0009] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0010] A steel plate according to one embodiment of the present invention comprises, in weight%, C: 0.040~0.080%, Si: 0.10~0.35%, Mn: 1.50~1.90%, Sol.Al: 0.010~0.035%, Ni: 0.20~0.50%, Cr: 0.10~0.25%, Nb: 0.010~0.040%, Ti: 0.001~0.020%, N: 0.0020~0.0060%, P: 0.0100% or less, and S: 0.0030% or less, and the remainder is Fe and unavoidable impurities, satisfying the following equation 1, and the microstructure at the center of the thickness comprises, in area%, 95% or more of acicular ferrite, and the yield strength may be 420 MPa or more.

[0011] [Relation 1] 2.520 ≤ [Mn] + 3 x [Ni] + 4.2 x [Cr] ≤ 4.250

[0012] In the above equation 1, [Mn], [Ni], and [Cr] represent the content (weight%) of each element, respectively.

[0013] The steel plate described above can satisfy the following relationships 2 and 3.

[0014] [Relation 2] |G 1 / 2T - G 1 / 4T | ≤ 7.0,

[0015] G in the above relationship 2 1 / 2T represents the average grain size (μm) at a point 1 / 2T in the thickness direction from the surface of the steel plate, and G 1 / 4T represents the average grain size (μm) at a point 1 / 4T in the thickness direction from the surface of the steel plate.

[0016] [Relation 3] |G 1 / 4T - G 표면부 | ≤ 4.5

[0017] In the above relationship 3, G 1 / 4T represents the average grain size (μm) at a point 1 / 4T in the thickness direction from the surface of the steel plate, and G 표면부represents the average grain size (μm) at the surface of the steel plate.

[0018] The above-described acicular ferrite can have an average grain size of 25 μm or less.

[0019] The steel plate described above may include at least one of martensite, cementite, and carbide in addition to the acicular ferrite described above.

[0020] The steel plate described above may have a thickness of 80 to 120 mm.

[0021] The above-described steel plate has a tensile strength of 500 MPa or more, and may have an impact toughness of -40°C of 200 J or more measured at 1 / 2 T in the thickness direction from the surface of the steel plate, and an impact toughness of -50°C of 200 J or more measured at 1 / 4 T.

[0022] A method for manufacturing a steel sheet according to another embodiment of the present invention comprises the steps of: reheating a slab satisfying the following equation 1, wherein the slab comprises, in weight percent, C: 0.040~0.080%, Si: 0.10~0.35%, Mn: 1.50~1.90%, Sol.Al: 0.010~0.035%, Ni: 0.20~0.50%, Cr: 0.10~0.25%, Nb: 0.010~0.040%, Ti: 0.001~0.020%, N: 0.0020~0.0060%, P: 0.0100% or less, and S: 0.0030% or less, and the remainder being Fe and unavoidable impurities; the step of recrystallizing the slab; and the step of obtaining a hot-rolled steel sheet by rolling the slab in an unrecrystallizing zone. The method may include a step of first cooling the hot-rolled steel plate from a cooling start temperature of 780 to 800°C to a cooling end temperature of 550°C to 680°C at a cooling rate of 4 to 10°C / sec; and a step of second cooling the hot-rolled steel plate at a cooling rate of 1.5°C / sec or more and a cooling end temperature of 200°C or more and 400°C or less.

[0023] [Relation 1] 2.520 ≤ [Mn] + 3 x [Ni] + 4.2 x [Cr] ≤ 4.250

[0024] In the above equation 1, [Mn], [Ni], and [Cr] represent the content (weight%) of each element, respectively.

[0025] The cooling rate in the aforementioned secondary cooling step may be 4℃ / sec or less.

[0026] The aforementioned secondary cooling step may be oscillation cooling.

[0027] The reheating temperature in the aforementioned reheating step is 1020 to 1120°C, and the aforementioned recrystallization rolling step may be performed at 900°C or higher with a reduction rate of 15 to 20% or more for each of the last two passes.

[0028] The aforementioned rolling of the unrecrystallized zone may have a finishing temperature of (Ar3+20) to (Ar3+80)℃ and a cumulative reduction rate of 30 to 50%.

[0029] The present invention can provide a high-strength, extremely thick steel plate with excellent low-temperature impact characteristics and a method for manufacturing the same.

[0030] Accordingly, it is possible to provide an ultra-thick steel plate and a method for manufacturing the same, which can secure structural stability and improve resistance to deformation and destruction of the structure caused by continuous impacts from waves, fish, currents, ships, etc., making it applicable to the offshore wind power industry, the marine platform industry, etc.

[0031] Figure 1 is an optical microscope image showing the microstructure of Example 1.

[0032] Figure 2 is an optical microscope image showing the microstructures of (A) Example 4 and (B) Example 5.

[0033] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0034] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0035] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0036] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0037] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0038] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0039] For extremely thick steel sheets of about 80mm to 120mm, the main challenge is to achieve both strength and impact toughness simultaneously.

[0040] To increase the strength of ultra-thick steel sheets, hardenable elements such as carbon must be added, but in this case, the impact toughness of the steel sheet may decrease.

[0041] Accordingly, the inventors derived the present invention by recognizing that high strength characteristics and excellent impact toughness of the steel sheet can be simultaneously secured by controlling the relative content among the aforementioned hardenable elements.

[0042] More specifically, the inventors have found that by lowering the carbon content and instead adding appropriate amounts of alloying elements such as Mn, Cr, and Ni, it is possible to secure high strength characteristics while maintaining excellent impact toughness of the steel sheet.

[0043] That is, a steel plate according to one embodiment of the present invention comprises, in weight%, C: 0.040~0.080%, Si: 0.10~0.35%, Mn: 1.50~1.90%, Sol.Al: 0.010~0.035%, Ni: 0.20~0.50%, Cr: 0.10~0.25%, Nb: 0.010~0.040%, Ti: 0.001~0.020%, N: 0.0020~0.0060%, P: 0.0100% or less, and S: 0.0030% or less, and the remainder being Fe and unavoidable impurities, satisfying the following equation 1, and the microstructure at the center of the thickness comprises 95% or more of acicular ferrite in area%, and the yield strength may be 420 MPa or more.

[0044] [Relation 1] 2.5200 ≤ [Mn] + 3 x [Ni] + 4.2 x [Cr] ≤ 4.2500

[0045] In the above equation 1, [Mn], [Ni], and [Cr] represent the content (weight%) of each element, respectively.

[0046] Below, the alloy composition of the steel plate of the present invention is described in detail.

[0047] C: 0.040~0.080%

[0048] C is an element that causes solid solution strengthening and exists as a carbonitride with Nb, etc., to secure tensile strength. For high strength, the present invention may add C at a concentration of 0.040% or more. However, if C is added in a concentration exceeding 0.080%, it not only promotes the formation of MA but also generates pearlite, which can degrade impact properties at low temperatures and worsen welding properties during welding of structures. Therefore, the present invention may include C at a concentration of 0.080% or less. As another example, C may be added in a concentration of 0.045~0.075% or 0.050~0.070%.

[0049] Si: 0.10~0.35%

[0050] Si plays a role in deoxidizing molten steel by assisting Al and is an element necessary for securing yield and tensile strength. To this end, the present invention may add Si at a level of 0.10% or more. However, if Si is added in an amount exceeding 0.35%, it may hinder the diffusion of C and promote MA formation; therefore, the present invention may set the upper limit of Si to 0.35% to secure impact characteristics at low temperatures. As another example, Si may be added in an amount of 0.13~0.30% or 0.15~0.25%.

[0051] Mn: 1.50~1.90%

[0052] Mn has a significant effect on increasing strength through solid solution strengthening. To this end, the present invention may add Mn at 1.50% or more. However, if added excessively, MnS inclusions may be formed, and toughness may be reduced due to segregation in the center. Therefore, the present invention may add Mn at 1.90% or less. As another example, Mn may be added at 1.55~1.85% or 1.60~1.80%.

[0053] Sol.Al: 0.010~0.035%

[0054] In the present invention, Al serves as a major deoxidizer for steel and needs to be added in an amount of 0.010% or more. However, if the Al is added in an amount exceeding 0.035%, the fraction and / or size of Al2O3 inclusions may increase, thereby reducing low-temperature toughness. Additionally, since Al, similar to Si, can promote the formation of MA phases in the base metal and weld heat-affected zone, thereby reducing low-temperature toughness properties, the present invention may limit the Al content to 0.010 to 0.035%. As another example, the Al may be added in an amount of 0.015 to 0.030% or 0.020 to 0.027%.

[0055] Ni: 0.20~0.50%

[0056] Ni is an element that improves strength without reducing impact toughness. The present invention can increase strength by adding 0.20% or more of Ni to form an appropriate amount of acicular ferrite. However, since a Ni content exceeding 0.50% can lower the Ar3 temperature and form bainite, the present invention may limit the upper limit to 0.50%. In this case, bainite can reduce impact toughness in extremely thick materials. As another example, Ni may be added in an amount of 0.25~0.47% or 0.30~0.45%.

[0057] Cr: 0.10~0.25%

[0058] Cr is a carbide-forming element that can easily increase strength, but in extremely thick steel, it can form coarse carbides depending on the cooling rate of the plate, which can impair impact toughness. Therefore, the present invention may add the above-mentioned Cr in a range of 0.10 to 0.25%. As another example, the above-mentioned Cr may be added in a range of 0.15 to 0.25%.

[0059] Nb: 0.010~0.040%

[0060] Nb is an element that refines the microstructure and increases strength by suppressing recrystallization during rolling or cooling through the precipitation of solid solution or carbonitrides, but due to its affinity for carbon, carbon may concentrate locally, thereby promoting the formation of MA phases. In this case, toughness and fracture characteristics at low temperatures may be reduced, so the present invention may add the Nb in the range of 0.010 to 0.040%. As another example, the Nb may be added in the range of 0.015 to 0.035% or 0.020 to 0.030%.

[0061] Ti: 0.001~0.020%

[0062] Ti combines with oxygen or nitrogen to form precipitates. Since these precipitates can contribute to refinement by inhibiting the coarsening of the structure and improve toughness, the present invention may add 0.001% or more of Ti. However, since a Ti content exceeding 0.020% may cause fracture due to the coarsening of precipitates, the present invention may set the upper limit of Ti to 0.020%. As another example, the Ti content may be 0.005~0.017% or 0.010~0.015%.

[0063] N: 0.0020~0.0060%

[0064] N can improve strength and toughness by forming precipitates together with Ti, Nb, and Al, thereby refining the austenite structure upon reheating. However, if the N content is excessive, it may cause surface cracks at high temperatures, and since the N remaining after forming precipitates exists in an atomic state and may reduce toughness, the present invention may limit the N content to a range of 0.0020 to 0.0060%. As another example, the N content may be 0.0025 to 0.0055% or 0.0030 to 0.0050%.

[0065] P: 0.0100% or less

[0066] Since P is an impurity element that causes grain boundary segregation and can cause steel to become brittle, the present invention may set the upper limit of P to 0.0100%.

[0067] S: 0.0030% or less

[0068] S mainly combines with Mn to form MnS inclusions, and these can be a factor that impairs low-temperature toughness. Therefore, in order to secure low-temperature toughness and low-temperature fatigue characteristics, the present invention may set the upper limit of S to 0.0030%.

[0069] The steel sheet of the present invention may contain the remainder of iron (Fe) and unavoidable impurities in addition to the composition described above. Unavoidable impurities may be unintentionally incorporated during the conventional manufacturing process, and such impurities and their content are widely known to those skilled in the art and are therefore not described separately in this specification.

[0070] A steel plate according to one embodiment of the present invention can satisfy the following relationship 1.

[0071] [Relation 1] 2.520 ≤ [Mn] + 3 x [Ni] + 4.2 x [Cr] ≤ 4.250

[0072] In the above equation 1, [Mn], [Ni], and [Cr] represent the content (weight%) of each element, respectively.

[0073] If the value derived by the above-mentioned Equation 1 is less than 2.520, it may be difficult to secure the desired level of high strength within the carbon range of the present invention. On the other hand, if the value derived by the above-mentioned Equation 1 is greater than 4.250, impact toughness may be reduced as a large amount of hardenable elements are included in the steel plate. As another example, the value derived by the above-mentioned Equation 1 may be 2.700 to 4.200 or 3.000 to 4.100.

[0074] Below, the microstructure of the steel plate of the present invention is described.

[0075] A steel plate according to one embodiment of the present invention may contain 95% or more of acicular ferrite in terms of area % as a microstructure at the center of the thickness. At this time, the center of the thickness may refer to a point 1 / 2T thick in the thickness direction from the surface of the steel plate.

[0076] That is, the present invention can improve the impact toughness of the inner coil by including the acicular ferrite in an area of ​​95% or more. As another example, the acicular ferrite may be included in an area of ​​96% or more. Since the more the acicular ferrite is included, the more advantageous it is for achieving the purpose of the present invention, the present invention does not separately limit the upper limit of the acicular ferrite. However, as an example, the acicular ferrite may be included in an area of ​​100% or less.

[0077] In addition to the acicular ferrite, the present invention may have a remainder microstructure comprising at least one of martensite, cementite, and carbide. Since it is advantageous for achieving the objectives of the present invention as the amount of such martensite, cementite, and carbide formed decreases, the present invention may not include the aforementioned remainder microstructure at all.

[0078] The above acicular ferrite may have an average grain size of 25 μm or less.

[0079] If the grain size of the above acicular ferrite exceeds 25 μm, the strength and impact toughness of the steel sheet may be poor due to the coarse grain size. As another example, the above acicular ferrite may be 23 μm or less or 21 μm or less.

[0080] Meanwhile, the steel plate of the present invention may be an extremely thick steel plate with a thickness of 80 to 120 mm.

[0081] In addition, the ultra-thick steel sheet of the present invention having the aforementioned alloy composition and microstructure can have excellent low-temperature impact toughness along with high strength characteristics.

[0082] As an example, the steel plate of the present invention may have a yield strength of 420 MPa or more, a tensile strength of 500 MPa or more, and an impact toughness of -50°C measured at 1 / 4 T of 200 J or more. Such high-strength, ultra-thick steel can be used as structural steel in various fields.

[0083] In particular, the steel plate of the present invention has the advantage of being able to secure a uniform grain size in the thickness direction on the surface of the steel plate, even though it is an extremely thick steel plate.

[0084] That is, a steel plate according to one example of the present invention can satisfy the following relationships 2 and 3.

[0085] [Relation 2] |G 1 / 2T - G 1 / 4T | ≤ 7.0,

[0086] G in the above relationship 2 1 / 2T represents the average grain size at a point 1 / 2T in the thickness direction from the surface of the steel plate, and G 1 / 4T represents the average grain size at a point 1 / 4T in the thickness direction from the surface of the steel plate.

[0087] [Relation 3] |G 1 / 4T - G 표면부 | ≤ 4.5

[0088] In the above relationship 3, G 1 / 4T represents the average grain size at a point 1 / 4T in the thickness direction from the surface of the steel plate, and G 표면부 represents the average grain size at the surface of the steel plate.

[0089] At this time, in the above relationship 3, the steel plate surface portion may refer to a point within 1 / 10T (T: steel plate thickness) in the thickness direction from the surface of the steel plate.

[0090] That is, the ultra-thick steel plate of the present invention can maintain a uniform grain size from the surface to the center of the steel plate by satisfying the above-mentioned equations 2 and 3, despite its thick thickness. As a result, variations in mechanical properties such as strength and low-temperature toughness appearing in the thickness direction within the steel plate can be reduced. As an example, the steel plate may exhibit excellent impact toughness not only at the 1 / 4T point based on thickness but also at the center of the thickness. More specifically, the impact toughness at -40℃ measured at the 1 / 2T point in the thickness direction from the surface of the steel plate may be 200J or more.

[0091] In addition, the present invention can reduce local strength differences through the above-mentioned equations 2 and 3 to suppress the initiation or growth of cracks that may occur in extreme environments such as seabed environments.

[0092] As another example, the value derived by the above relationship 2 may be 5.0 or less, and the value derived by the above relationship 3 may be 4.0 or less.

[0093] Below, the method for manufacturing the steel plate of the present invention will be described in detail.

[0094] In the case of ultra-thick steel sheets, residual stress can occur during manufacturing processes such as rolling and cooling, which may reduce impact toughness.

[0095] Such residual stress can be removed through heat treatment such as tempering, but the method for manufacturing a steel plate according to the present invention may be characterized by manufacturing the steel plate through a series of processes including reheating, rolling, and controlled cooling (first cooling and second cooling) without such a separate heat treatment process. As a result, the method for manufacturing a steel plate according to the present invention can reduce the manufacturing cost of the steel plate.

[0096] Furthermore, as the thickness of the steel plate increases, the reduction rate per pass during rolling becomes insufficient, and it is difficult to secure a sufficient cooling rate. In this case, the probability of defects (pores, segregation, microcracks) existing within the extremely thick steel plate—which act as the initiation point for fracture when impact loads are applied—increases, or the grain size may become coarser.

[0097] Accordingly, in the present invention, a fine acicular ferrite matrix structure is realized through the optimization of rolling and cooling conditions, thereby securing impact toughness in the inner coil despite being an ultra-thick steel sheet.

[0098] Each step is explained in detail below.

[0099] A method for manufacturing a steel plate according to one embodiment of the present invention may include the step of reheating a slab satisfying the following equation 1, comprising, in weight%, C: 0.040~0.080%, Si: 0.10~0.35%, Mn: 1.50~1.90%, Sol.Al: 0.010~0.035%, Ni: 0.20~0.50%, Cr: 0.10~0.25%, Nb: 0.010~0.040%, Ti: 0.001~0.020%, N: 0.0020~0.0060%, P: 0.0100% or less, and S: 0.0030% or less, with the remainder being Fe and unavoidable impurities.

[0100] [Relation 1] 2.520 ≤ [Mn] + 3 x [Ni] + 4.2 x [Cr] ≤ 4.250

[0101] In the above equation 1, [Mn], [Ni], and [Cr] represent the content (weight%) of each element, respectively.

[0102] As the alloy composition of the slab has been mentioned above in relation to the steel plate, it will be omitted.

[0103] The reheating temperature in the above reheating step may be 1020 to 1120℃.

[0104] If the heating temperature exceeds 1120℃, the hardenability increases due to the coarsening of austenite grains, and a bainite structure may develop. In this case, toughness may decrease. On the other hand, if the heating temperature is below 1020℃, Ti and Nb may not be sufficiently dissolved, which may result in a decrease in strength.

[0105] After the reheating described above, the method for manufacturing a steel plate according to one embodiment of the present invention may include a step of rolling the slab in a recrystallization zone. This is to refine the grain size by completely recrystallizing the austenite and suppressing the growth of the austenite.

[0106] The temperature and reduction rate during the recrystallization rolling described above can be appropriately designed within the range where the aforementioned purpose can be achieved, but as an example, the step of rolling the recrystallization rolling may be performed at 900°C or higher with a reduction rate of 15-20% or higher for each of the last two passes.

[0107] After performing recrystallization rolling in this manner, the method for manufacturing a steel plate according to one embodiment of the present invention may include the step of rolling the slab in an unrecrystallization zone to obtain a hot-rolled steel plate.

[0108] The finishing temperature range during rolling in the unrecrystallized zone mentioned above may be a range commonly practiced in the relevant technical field. However, as an example, the present invention may control the finishing rolling temperature just above the Ar3 temperature to refine the grain size of acicular ferrite, and to this end, the present invention may set the finishing temperature to (Ar3+20) to (Ar3+80)°C. At this time, the Ar3 temperature may be calculated by the following equation (Ouchi eq.).

[0109] Ar3(℃)=910-310xC-80xMn-20xCu-15xCr-55xNi-80xMo+0.35x(Thick-8)

[0110] In addition, the cumulative reduction rate during rolling in the unrecrystallized zone is not particularly limited within a range that allows for obtaining a hot-rolled steel sheet having an appropriate thickness. However, as an example, the present invention may set the cumulative reduction rate to 30 to 50%.

[0111] The rolled material in this manner can have the microstructure and physical properties described above through controlled cooling, which controls the cooling temperature, cooling rate, and cooling pattern, as described below.

[0112] That is, a method for manufacturing a steel plate according to one embodiment of the present invention may include a step of first cooling the hot-rolled steel plate from a cooling start temperature of 780 to 800°C to a cooling end temperature of 550°C to 680°C at a cooling rate of 4 to 10°C / sec. By doing so, the present invention can simultaneously secure a desired level of strength and impact toughness by refining the crystal grains of the inner portion.

[0113] If the cooling start temperature in the above first cooling step is less than 780°C, a problem may arise in which abnormal cooling below the Ar3 temperature occurs due to a drop in the cooling start temperature of the surface. In particular, if such abnormal cooling of the surface occurs, it may lead to a rapid decrease in impact toughness due to the formation of hard phases caused by coarse air-cooled ferrite and carbon concentration, and it may be difficult to achieve the target microstructure of the surface and inner layers due to excessive flow rate. On the other hand, if the above cooling start temperature exceeds 800°C, a problem may arise in which the cooling rate increases because an excessive flow rate is required to achieve the final cooling temperature. Therefore, the present invention may set the cooling start temperature in the above first cooling step to 780~800°C.

[0114] In addition, the present invention can secure high strength characteristics by preventing the excessive formation of coarse polygonal ferrite by setting the cooling rate in the first cooling step to 4℃ / sec or higher. Furthermore, if the cooling rate exceeds 10℃ / sec, bainite may be formed, which may result in poor impact toughness. Therefore, the present invention may set the upper limit of the cooling rate to 10℃ / sec. As another example, the cooling rate may be 5~9℃ / sec or 5~8℃ / sec.

[0115] If the cooling end temperature in the first cooling step is less than 550°C, the formation of a bainite structure is easy, which may cause a problem of reduced impact toughness. On the other hand, if the cooling end temperature is greater than 680°C, the slow cooling rate after the first cooling may cause the formation of coarse ferrite to be easy, which may cause a problem of reduced strength. Therefore, the present invention may set the cooling start temperature in the first cooling step to 550 to 680°C.

[0116] Although not limited to this, such rapid cooling can be achieved through water cooling.

[0117] Then, a method for manufacturing a steel plate according to one embodiment of the present invention may include a step of secondarily cooling the hot-rolled steel plate at a cooling rate of 1.5℃ / sec or more, with the cooling end temperature being 200℃ or higher and 400℃ or lower.

[0118] The present invention can prevent the formation of a large amount of hard phases, such as the MA phase, by setting the cooling end temperature in the second cooling step to 200°C or higher. Additionally, if the cooling end temperature exceeds 400°C, the crystal grains may coarsen, which may lower strength and toughness; therefore, the present invention may set the upper limit of the cooling end temperature to 400°C.

[0119] In addition, if the cooling rate in the second cooling step is less than 1.5℃ / sec, the grain size may become coarsened, making it difficult to secure physical properties such as strength and toughness; therefore, the present invention may set the cooling rate in the second cooling step to 1.5℃ / sec or more.

[0120] Meanwhile, the second cooling step of the present invention may be oscillation cooling.

[0121] Oscillation cooling refers to a cooling method in which a steel sheet remains in a cooling zone for a certain period of time and is cooled in multiple passes. This type of oscillation cooling is characterized by its ability to reduce variations in mechanical properties between the leading and trailing ends of the steel sheet. This oscillation cooling is distinguished from accelerated cooling, in which medium-thick materials are cooled by passing through a cooling zone.

[0122] The steel plate of the present invention can achieve a cooling end temperature of the inner coil and secure a uniform microstructure by adopting such oscillation cooling in the second cooling step.

[0123] The cooling rate in the second cooling step, implemented through the aforementioned oscillation cooling, may be 4℃ / sec or less.

[0124] On the other hand, if the cooling rate exceeds 4.0℃ / sec, it may be difficult to secure a uniform microstructure because cooling between the surface and the center of the steel plate is not uniform. As a result, the strength and low-temperature toughness of the center may be compromised, and cracks may occur due to localized strength differences.

[0125] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0126] First, molten steel having the composition shown in Table 1 below was prepared, and then a slab was manufactured using continuous casting.

[0127] The above slab was manufactured into a steel plate through a process of reheating, recrystallization zone rolling, non-recrystallization zone rolling, first cooling, and second cooling under the conditions of Table 2 below. The recrystallization zone rolling was performed at 900°C or higher with a reduction rate of 15-20% or more for each of the last two passes. In addition, the cumulative reduction rate during the non-recrystallization rolling was 30-50%, and the first cooling was performed by setting the cooling start temperature to 780-800°C and the cooling end temperature to 550°C-680°C, followed by cooling according to the average cooling rate shown in Table 2 below. The cooling rates during the first and second cooling shown in Table 2 are based on the 1 / 4T point in the thickness direction from the surface of the steel plate (where T is the thickness of the steel plate in mm).

[0128] Steel gradeCSiMnPSAlNiCrTiNbNMn+3xNi+4.2xCrA0.0580.171.740.00740.00180.0230.380.170.0120.0230.00383.594B0.0620.181.660.00 720.00180.0240.420.240.0130.0250.00363.928C0.0650.211.780.00680.00190.0250.370.220.0110.0280.00403.814D0.0620.161 .350.00690.00150.0250.060.080.0140.0290.00361.866E0.0590.221.960.00800.00180.0240.570.280.0120.0240.00324.846F0.0 610.191.670.00720.00180.0240.47-0.0130.0220.00373.080G0.0630.181.700.00770.00150.0260.450.320.0120.0240.00334.394

[0129] Classification Steel Type Reheating Temperature (°C) Unrecrystallized Zone Rolling Primary Cooling Secondary Cooling Rolling Start Temperature (°C) Rolling End Temperature (°C) Cooling Rate (°C / s) Secondary Cooling Finish Temperature (°C) Secondary Cooling Rate (°C / s) Oscillation Cooling Status Example 1A 10908107876.23453.2O Example 2B 11028087865.33622.8O Example 3C 10988127882.33352.5O Example 4D 10887957816.43503.4O Example 5E 10868047975.83892.9O Example 6F 10988117835.93662.4O Example 7G 11088047815.43942.1O Example 8A11028077845.63651.3O Example 9A10928037855.43614.5X

[0130] The microstructure and mechanical properties of each steel plate were measured and listed in Tables 3 and 4 below.

[0131] Below, each measurement method is explained.

[0132] (Microstructure)

[0133] To analyze the microstructure of the steel, a microstructure specimen was taken at the 1 / 2T thickness reference point of the manufactured steel, the specimen was polished, and then etched with Nital solution. The area fraction of acicular ferrite on the etched specimen was analyzed using an image analyzer connected to an optical microscope and is shown in Table 3 below.

[0134] (Average grain size of acicular ferrite)

[0135] Table 3 below shows the measured average grain size of acicular ferrite at the surface, at the 1 / 4T thickness point, and at the 1 / 2T thickness point. For the average grain size, an Electron Backscatter Diffraction (EDD) pattern analyzer was used to measure the effective grain size of acicular ferrite at each point relative to the steel thickness. Here, the effective grain was defined as a grain consisting of a boundary where the orientation difference between the microstructure crystals measured by the EDD pattern analyzer is 15 degrees or more. The measured results are shown in Table 3 below.

[0136] (Mechanical properties)

[0137] Regarding mechanical properties, annular tensile specimens were taken from the manufactured steel at a thickness of 1 / 4T so that the length of the specimen was perpendicular to the rolling direction, processed, and then subjected to a tensile test at room temperature using the EN ISO 6892-1 test method. In the tensile test, if a yield point phenomenon occurred, the upper yield point was defined as the yield strength, and if no yield point phenomenon occurred, the 0.2% offset yield point was defined as the yield strength.

[0138] After processing the same specimen into a V-notch specimen, three impact tests were performed at -50℃ using the test method of EN ISO 148-1, and the average of the values ​​is shown in Table 4 below.

[0139] Additionally, a specimen was taken at the 1 / 2T thickness reference point, processed into a V-notch specimen, and then three impact tests were performed at -40℃ using the test method of EN ISO 148-1, and the average of the values ​​is shown in Table 4 below.

[0140] Classification Steel Grade AF Fraction (%) Average Grain Size (μm) Equation 2 Equation 3 Surface 1 / 4T Point 1 / 2T Point Example 1A97 15.6 19.4 22.5 3.1 3.8 Example 2B96 16.8 18.6 21.5 2.9 1.8 Example 3C75 22.5 29.6 38.5 8.9 7.1 Example 4D65 18.6 20.6 23.8 3.22 Example 5E72 17.3 19.6 23.1 3.5 2.3 Example 6F68 16.9 20.7 24.6 3.9 3.8 Example 7G74 18.4 22.6 23.9 1.3 4.2 Example 8A96 17.6 24.3 36.5 12.2 6.7 Example 9A8818.423.432.38.95.0

[0141] Classification 1 / 4T Yield Strength (MPa) 1 / 4T Tensile Strength (MPa) 1 / 2T Impact (-40℃, J) 1 / 4T Impact (-50℃, J) Example 1 446535254265 Example 2 452541246278 Example 3 412504214245 Example 4 395485225234 Example 5 4755622675 Example 6 388478132152 Example 7 4485453471 Example 8 3794725258 Example 9 42650242210

[0142] Example 3 satisfied the component range presented in the present invention, but due to the low primary cooling rate, polygonal ferrite increased and the grain size also increased, so high strength characteristics could not be secured.

[0143] In Example 4, although the manufacturing conditions were satisfied by the low value of Equation 1, the acicular ferrite fraction was less than 95 area%, and as a result, the strength decreased rapidly.

[0144] In Example 5, the area fraction of the bainite structure increased due to the high value of Equation 1, so the strength increased sharply, but the impact toughness of the inner coil decreased.

[0145] Example 6 is a case where Cr is not added, and it can be seen that although it falls within the range of values ​​in Equation 1, it falls short of the target strength.

[0146] Example 7 is a case where Mn and Ni were added within the normal range but Cr was added beyond the range, and it can be seen that although the strength value increased due to the formation of a bainite structure, the toughness of the inner part was inferior.

[0147] Example 8 satisfied the alloy composition presented by the present invention, but the cooling rate during secondary cooling was less than 1.5℃ / s, so the strength and toughness were poor.

[0148] In Example 9, a general accelerated cooling method was adopted instead of oscillation cooling during the secondary cooling, so the cooling rate at the 1 / 4T point exceeded 4℃ / s, but cooling was slow in the center. As a result, Equations 2 and 3 were not satisfied; thus, while high strength and high toughness could be secured at the 1 / 4T point, toughness was poor at the 1 / 2T point due to grain coarsening. In this case, the possibility of crack formation may increase due to the difference in hardness between soft coarse ferrite and pearlite.

[0149] On the other hand, Examples 1 and 2 satisfied the alloy composition and manufacturing conditions presented in the present invention, and as a result, the yield strength was 420 MPa or higher, the tensile strength was 500 MPa or higher, the impact toughness at -40°C measured at 1 / 2 T in the thickness direction from the surface of the steel plate was 200 J or higher, and the impact toughness at -50°C measured at 1 / 4 T was also 200 J or higher.

[0150] Meanwhile, FIG. 1 is an optical microscope image of the microstructure of Example 1, and FIG. 2 is an optical microscope image of the microstructures of (A) Example 4 and (B) Example 5.

[0151] Looking at Figures 1 and 2 above, it can be seen that in Example 1, fine needle-shaped ferrite is formed, whereas in Example 4, where the value of Equation 1 falls short of the range intended by the present invention, a large amount of coarse polygonal ferrite is formed, and in Example 5, where the value of Equation 1 exceeds the range intended by the present invention, the bainite area fraction is increased.

Claims

1. In wt%, comprising C: 0.040~0.080%, Si: 0.10~0.35%, Mn: 1.50~1.90%, Sol.Al: 0.010~0.035%, Ni: 0.20~0.50%, Cr: 0.10~0.25%, Nb: 0.010~0.040%, Ti: 0.001~0.020%, N: 0.0020~0.0060%, P: 0.0100% or less, and S: 0.0030% or less, and the remainder being Fe and unavoidable impurities, satisfying the following Equation 1, and The microstructure at the center of the thickness contains more than 95% acicular ferrite in area %, and Steel plate with a yield strength of 420 MPa or higher. [Relation 1] 2.520 ≤ [Mn] + 3 x [Ni] + 4.2 x [Cr] ≤ 4.250 In the above equation 1, [Mn], [Ni], and [Cr] represent the content (weight%) of each element, respectively.

2. In Paragraph 1, A steel plate satisfying the following equations 2 and 3. [Relation 2] |G 1 / 2T - G 1 / 4T | ≤ 7.0, G in the above relationship 2 1 / 2T represents the average grain size (μm) at a point 1 / 2T in the thickness direction from the surface of the steel plate, and G 1 / 4T represents the average grain size (μm) at a point 1 / 4T in the thickness direction from the surface of the steel plate. [Relation 3] |G 1 / 4T - G 표면부 | ≤ 4.5 In the above relationship 3, G 1 / 4T represents the average grain size (μm) at a point 1 / 4T in the thickness direction from the surface of the steel plate, and G 표면부 represents the average grain size (μm) at the surface of the steel plate.

3. In Paragraph 1, The above needle-shaped ferrite is a steel plate having an average grain size of 25 μm or less.

4. In Paragraph 1, A steel plate comprising, in addition to the above-mentioned acicular ferrite, at least one of martensite, cementite, and carbide.

5. In Paragraph 1, Steel plate with a thickness of 80 to 120 mm.

6. In Paragraph 1, A steel plate having a tensile strength of 500 MPa or more, an impact toughness of -40°C of 200 J or more measured at 1 / 2 T in the thickness direction from the surface of the steel plate, and an impact toughness of -50°C of 200 J or more measured at 1 / 4 T.

7. A step of reheating a slab comprising, in wt%, C: 0.040~0.080%, Si: 0.10~0.35%, Mn: 1.50~1.90%, Sol.Al: 0.010~0.035%, Ni: 0.20~0.50%, Cr: 0.10~0.25%, Nb: 0.010~0.040%, Ti: 0.001~0.020%, N: 0.0020~0.0060%, P: 0.0100% or less, and S: 0.0030% or less, with the remainder being Fe and unavoidable impurities, satisfying the following Equation 1; A step of recrystallizing and rolling the above slab; A step of obtaining a hot-rolled steel sheet by rolling the above slab in an unrecrystallized zone; A step of first cooling the above hot-rolled steel plate from a cooling start temperature of 780~800℃ to a cooling end temperature of 550℃~680℃ at a cooling rate of 4~10℃ / sec; A method for manufacturing a steel plate, comprising the step of secondarily cooling the hot-rolled steel plate at a cooling rate of 1.5℃ / sec or higher and a cooling end temperature of 200℃ or higher and 400℃ or lower. [Relation 1] 2.520 ≤ [Mn] + 3 x [Ni] + 4.2 x [Cr] ≤ 4.250 In the above equation 1, [Mn], [Ni], and [Cr] represent the content (weight%) of each element, respectively.

8. In Paragraph 7, A method for manufacturing a steel plate, wherein the cooling rate in the above-mentioned second cooling step is 4℃ / sec or less.

9. In Paragraph 7, A method for manufacturing a steel plate, wherein the above secondary cooling step is oscillation cooling.

10. In Paragraph 7, The reheating temperature in the above reheating step is 1020~1120℃, and A method for manufacturing a steel sheet, wherein the above-mentioned step of rolling in the recrystallization zone is to roll at a temperature of 900°C or higher with a reduction rate of 15-20% or more for each of the last two passes.

11. In Paragraph 7, The above unrecrystallized zone rolling has a finishing temperature of (Ar3+20) to (Ar3+80)℃, and A method for manufacturing a steel plate having a cumulative reduction rate of 30 to 50%.

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