Super-thick steel material for structure having excellent performance of surface portion nrl-dwt and method for manufacturing the same

Abstract

The present disclosure relates to a steel material for an ultra-thick structure having excellent surface portion NRL-DWT performance and a manufacturing method thereof, and more particularly to a steel material for an ultra-thick structure and a manufacturing method thereof, the steel material comprising, in weight %, C: 0.05 to 0.09 %, Si: 0.1 to 0.4 %, Al: 0.01 to 0.05 %, Mn: 1.8 to 2.1 %, Ni: 0.3 to 1.0 %, Nb: 0.005 to 0.040 %, Ti: 0.005 to 0.03 %, Cu: 0.1 to 0.5 %, P: 100 ppm or less, S: 40 ppm or less, the balance of Fe and other inevitable impurities, a fraction of polygonal ferrite from a surface portion to a position 5 mm directly below the surface portion being 50 % or more, and having a fine structure with a grain size of 15 µm or less of a t / 4 fine structure having a large-angle grain boundary of 15 degrees or more measured by EBSD.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an ultra-thick steel material having excellent surface portion NRL-DWT performance and a method of manufacturing the same. BACKGROUND

[0002] In recent years, in designing structures such as ships, there is a demand for developing an ultra-thick high-strength steel material.

[0003] When a high-strength steel is used to design a structure, the weight of the structure is reduced, and in addition to economic benefits, the thickness of the plate can be thinned, thus ensuring the convenience of processing and welding operations.

[0004] Generally, in the case of a high-strength steel, since the total reduction rate is reduced when manufacturing an ultra-thick material, the overall structure does not sufficiently deform, and thus the fine structure becomes coarse.

[0005] In addition, when rapidly cooling to ensure strength, due to the thick thickness of the ultra-thick material, there is a difference in cooling speed between the surface portion and the center portion.

[0006] Therefore, coarse low-temperature transformation phases such as bainite are generated in the surface portion, and thus it is difficult for the ultra-thick material to ensure toughness.

[0007] In particular, when applied to a main structure of a ship or the like, there is an increasing demand for ensuring brittle crack propagation resistance, which indicates the stability of the structure.

[0008] In the case of an ultra-thick material, since the toughness is reduced due to the difference in cooling speed between the surface portion and the center portion, there is a great difficulty in ensuring the brittle crack propagation resistance.

[0009] In practice, many classification societies and steel companies have been implementing a large-scale tensile test, which can accurately evaluate the actual brittle crack propagation resistance to ensure the brittle crack propagation resistance.

[0010] However, in the case of a large-scale tensile test, a large amount of expenses will be incurred to implement the test, and thus it is difficult to apply it to mass production.

[0011] In order to improve such an unreasonable aspect, research on a small-scale alternative test that can replace the large-scale tensile test has been steadily conducted recently.

[0012] In the small-scale alternative test, as the most powerful test, the surface portion NRL-DWT (Drop Weight Test) test of the ASTM E208-06 standard is adopted by many classification societies and steel companies.

[0013] The surface portion NRL-DWT test is based on the following research results: on the basis of the existing research, the microstructure of the surface portion is controlled so that the crack propagation speed is reduced when brittle crack propagation occurs, and the resistance to brittle crack propagation becomes excellent.

[0014] In order to improve the performance of the NRL-DWT, various techniques for refining the grain size of the surface portion have been attempted.

[0015] Various techniques have been proposed, such as applying surface cooling during finish rolling or applying bending stress during rolling to adjust the grain size by increasing the amount of deformation.

[0016] However, the above attempts have the problem that the techniques themselves result in a significant reduction in productivity when applied to general mass production systems.

[0017] In addition, when a large amount of elements such as Ni that contribute to the improvement of toughness is added, the performance of the surface portion NRL-DWT can be improved.

[0018] However, since Ni is an expensive element, commercial application is difficult in terms of manufacturing costs.

[0019] At the same time, in order to strengthen the stability of ultra-large container ships, standards for strengthening the resistance to brittle crack propagation are coming into effect.

[0020] In order to ensure the existing BCA performance (brittle crack arrest), it is defined that the thickness is 80 t or less, Kca is 6000 or more, and for a thickness of 80 t or more, it is prescribed in the international ship classification standards to be negotiated with the classification society.

[0021] Since there were no actual structure test results for steel materials with a thickness of 80 t or more in the past, steel materials with a thickness of 80 t or more were also defined as Kca≥6000, as with 80 t or less.

[0022] However, as a result of recent actual structure tests in Japan, it was reported that the research results that only in the case of using steel materials with a thickness of 80 t or more in the hatch side coaming portion of the ship, Kca≥8000 is required for the crack to be arrested.

[0023] According to the above research results, the international ship classification standards are also to be changed, and thus new steel materials that ensure Kca≥8000 are required.

[0024] Since the Kca assurance value is increased, the possibility that the assurance temperature of the small-scale alternative test also reaches -70°C or less, which is more strengthened than the existing -60°C, is high, and thus steel materials that can ensure more assurance than the current are required to be developed. SUMMARY

[0025] Technical Problem

[0026] The present disclosure aims to provide an ultra-thick steel material having excellent surface portion NRL-DWT performance and a manufacturing method thereof, which can solve the problems of the prior art described above.

[0027] Specifically, the present disclosure aims to provide an ultra-thick steel material and a manufacturing method thereof, which, without including an expensive alloy element in terms of composition, has excellent surface portion NRL-DWT performance by controlling alloy composition to suppress the generation of coarse low-temperature transformation phases in the surface portion of the ultra-thick steel material.

[0028] Further, the present disclosure aims to an ultra-thick steel material and a manufacturing method thereof, which, by controlling the rolling temperature and the maximum reduction at rough rolling and finish rolling, gives the maximum deformation to the austenite structure of the surface portion, maximizes the fraction of polygonal ferrite from the surface portion to the position 5 mm below the surface portion, and maximizes the refinement of fine structures, thereby having excellent surface portion NRL-DWT performance.

[0029] In addition, the present disclosure aims to provide an ultra-thick structural steel material having excellent NRL-DWT performance and a manufacturing method thereof, more specifically, the steel material having a thickness of 80 mm or more and 100 mm or less, a fraction of polygonal ferrite from the surface portion to the position 5 mm below the surface portion of 50% or more, a grain size of t / 4 fine structure having a large-angle grain boundary of 15 degrees or more measured by EBSD of 15 μm or less, a yield strength of 460 MPa or more, an NDTT (Nil-Ductility Transition Temperature) value based on the NRL-DWT test according to the ASTM E208 standard of -70℃ or less, and a Kca value obtained by performing the ESSO test of 8000 or more.

[0030] The objects of the present disclosure are not limited to the above-described objects, and other objects and advantages of the present disclosure can be understood by the following description, and can be more clearly understood by the embodiments. Also, it will be obvious that the objects and advantages of the present disclosure can be achieved by the means described in the claims and combinations thereof.

[0031] Technical Solution

[0032] To achieve the above object, a steel material for an ultra-thick structure according to one embodiment of the present disclosure, which contains, in terms of mass%, C: 0.05% to 0.09%, Si: 0.1% to 0.4%, Al: 0.01% to 0.05%, Mn: 1.8% to 2.1%, Ni: 0.3% to 1.0%, Nb: 0.005% to 0.040%, Ti: 0.005% to 0.03%, Cu: 0.1% to 0.5%, P: 100 ppm or less, S: 40 ppm or less, the balance of Fe and other inevitable impurities, has a fraction of polygonal ferrite from the surface portion to a position 5 mm below the surface portion of 50% or more, and can have a fine structure with a grain size of t / 4 fine structure having a large-angle grain boundary of 15 degrees or more measured by EBSD of 15 μm or less.

[0033] Preferably, the NDTT (Nil-Ductility Transition Temperature) value based on the surface portion NRL-DWT (Drop Weight Test) test according to the ASTM E208-06 standard can be -70°C or lower.

[0034] Preferably, the Kca value based on the ESSO test can be 8000 or more.

[0035] Preferably, the plate thickness can be 80 mm to 100 mm, and the yield strength can be 460 MPa or more.

[0036] A method of manufacturing a steel material for an ultra-thick structure according to one embodiment of the present disclosure to achieve the above object can be a method of manufacturing including the step of performing finish rolling at a temperature range of 720°C to 740°C from the surface of a slab to a t / 4 position.

[0037] A method of manufacturing a steel material for an ultra-thick structure according to one embodiment of the present disclosure to achieve the above object, which is implemented according to the specific embodiment, can include the steps of: reheating a slab containing, in terms of mass%, C: 0.05% to 0.09%, Si: 0.1% to 0.4%, Al: 0.01% to 0.05%, Mn: 1.8% to 2.1%, Ni: 0.3% to 1.0%, Nb: 0.005% to 0.040%, Ti: 0.005% to 0.03%, Cu: 0.1% to 0.5%, P: 100 ppm or less, S: 40 ppm or less, the balance of Fe and other inevitable impurities; performing rough rolling on the reheated slab, and then performing finish rolling at a temperature range of 720°C to 740°C from the surface of the slab to a t / 4 position; and cooling the steel material after the finish rolling.

[0038] Preferably, the slab reheating temperature can be 1000°C to 1120°C.

[0039] Preferably, the rough rolling temperature can be 900°C to 1100°C.

[0040] Preferably, the cumulative reduction rate at the rough rolling can be 50% or more.

[0041] Preferably, the cooling rate in the cooling step can be 3°C / sec or more.

[0042] Preferably, the cooling end temperature in the cooling step can be 500°C or less.

[0043] Inventive Effects

[0044] According to the present disclosure, by controlling the composition and microstructure without unnecessarily containing an excessive amount of expensive alloying elements, an ultra-thick steel material having excellent surface portion NRL-DWT properties can be implemented.

[0045] According to the present disclosure, by controlling the finish rolling temperature and cumulative reduction amount, the surface portion and t / 4 austenite microstructure deformation amount is maximized to control the t / 4 microstructure grain size having a large angle grain boundary of 15 degrees or more measured by EBSD to be 15 μm or less, and the polygonal ferrite fraction from the surface portion to 5 mm below the surface portion is maximized, thereby a manufacturing method of an ultra-thick steel material having excellent surface portion NRL-DWT properties can be implemented.

[0046] According to the present disclosure, an ultra-thick structural steel material having a thickness of 80 mm or more and 100 mm or less, while having a yield strength of 460 MPa or more, an NDTT (Nil-Ductility Transition Temperature) value of -70°C or less based on the NRL-DWT test according to the ASTM E208 standard, and a Kca value of 8000 or more obtained by performing the ESSO test, and a manufacturing method thereof can be implemented.

[0047] In addition to the above effects, specific effects of the present disclosure are described when describing specific contents for implementing the present disclosure. DETAILED DESCRIPTION

[0048] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings so that those skilled in the art to which the present disclosure pertains can easily implement the present disclosure. The present disclosure can be implemented in various different ways, and is not limited to the embodiments described herein.

[0049] For the sake of clarity of the disclosure, portions unrelated to the description are omitted, and the same reference numerals refer to the same or similar constituent elements throughout the specification. Also, some embodiments of the disclosure are described in detail with reference to the exemplary drawings. When constituent elements in the drawings are added with reference numerals, the same constituent elements are denoted with the same reference numerals even in different drawings. Also, in describing the disclosure, if it is determined that a detailed description of related known configurations or functions can obscure the gist of the disclosure, the detailed description can be omitted.

[0050] In describing the constituent elements of the disclosure, terms such as first, second, A, B, (a), (b), etc. can be used. These terms are used only to distinguish the constituent elements from one another, and the nature, order, or number of the constituent elements is not limited by the terms. When a certain constituent element is described as being "connected," "coupled," or "communicated" to another constituent element, the constituent element can be directly connected or communicated to the other constituent element, but it can also be understood that there are other constituent elements therebetween, or the constituent elements can be "connected," "coupled," or "communicated" through the other constituent elements.

[0051] The disclosure aims to disclose a steel material for an ultra-thick structure having excellent NRL-DWT performance and a manufacturing method thereof, specifically, a fraction of polygonal ferrite from a surface portion to a position 5 mm below the surface portion is 50% or more, a grain size of 1 / 4t fine structure having a large-angle grain boundary of 15 degrees or more measured by EBSD is 15 μm or less, a thickness is 80 mm or more and 100 mm or less, an NDTT (Nil-Ductility Transition Temperature) value based on an NRL-DWT test according to ASTM E208 standard is -70℃ or less, and a Kca value obtained by performing an ESSO test is 8000 or more.

[0052] To satisfy the above-described characteristics, the steel material for an ultra-thick structure according to one embodiment of the disclosure can specifically include the following alloying elements to satisfy the above-described characteristics of excellent NRL-DWT performance.

[0053] Unless otherwise described, the content or composition range of each of the following components is measured in wt%.

[0054] Carbon (C) is the most important element in the steel material for an ultra-thick structure of the disclosure to secure basic strength, and thus needs to be included in the steel (or steel material) within a controlled range.

[0055] In the steel according to one embodiment of the disclosure, the carbon content is in the range of 0.05% to 0.09% in wt% (hereinafter, %).

[0056] If the amount of carbon added to the steel of one embodiment of the present disclosure is less than 0.05%, the strength of the steel decreases, and there is a problem in that it is difficult to achieve the strength target.

[0057] On the other hand, if the amount of carbon added to the steel of one embodiment of the present disclosure is more than 0.09%, the excess carbon increases the hardenability, thereby generating a large amount of massive martensite, and promotes the generation of a low-temperature transformation phase, and as a result, there is a problem in that the toughness of the steel decreases.

[0058] Silicon (Si) and aluminum (Al) are necessary alloying elements for a deoxidation operation by precipitating dissolved oxygen in molten steel in the form of a slag in a steelmaking and continuous casting process, and thus need to be contained in the steel (or steel material) within a controlled range.

[0059] In the steel according to one embodiment of the present disclosure, the silicon content is in the range of 0.1% to 0.4% and the aluminum content is in the range of 0.01% to 0.05% by weight (%).

[0060] If the amounts of silicon and aluminum added to the steel of one embodiment of the present disclosure are less than 0.1% and 0.01%, respectively, there is a problem in that it is difficult to expect a deoxidation effect due to insufficient precipitation of dissolved oxygen during the steelmaking and continuous casting process.

[0061] On the other hand, if the amounts of silicon and aluminum added to the steel of one embodiment of the present disclosure are more than 0.4% and 0.05%, respectively, there is a problem in that excess silicon and aluminum can cause the generation of coarse Si, Al complex oxides or a large amount of coarse massive martensite in a fine structure.

[0062] Manganese (Mn) is a useful element for improving the strength through solid solution strengthening and increasing the hardenability to generate a low-temperature transformation phase in the steel material for ultra-thick structures of the present disclosure, and thus needs to be contained in the steel (or steel material) within a controlled range.

[0063] In the steel according to one embodiment of the present disclosure, the manganese content is in the range of 1.8% to 2.1% by weight (%).

[0064] If the amount of manganese added to the steel of one embodiment of the present disclosure is less than 1.8%, there is a problem in that it is difficult to satisfy the yield strength of the steel of 460 MPa or more.

[0065] On the other hand, if the amount of manganese added to the steel of one embodiment of the present disclosure is more than 2.1%, the excess manganese causes excessive increase in the hardenability, thereby causing promotion of the generation of upper bainite and martensite, and there is a problem in that the impact toughness and surface portion NRL-DWT performance greatly decrease.

[0066] Nickel (Ni) is an important element for promoting cross slip of dislocations at low temperatures to improve impact toughness and improving hardenability to increase strength in the ultra-thick structural steel material of the present disclosure, and thus needs to be included in the steel (or steel material) within a controlled range.

[0067] In the steel according to one embodiment of the present disclosure, the nickel content is in the range of 0.3% to 1.0% in terms of weight % (hereinafter expressed as %).

[0068] If the addition amount of nickel in the steel of one embodiment of the present disclosure is less than 0.3%, there is a problem that it is difficult to improve impact toughness and brittle crack propagation resistance in a high-strength steel having a yield strength of 460 MPa or more.

[0069] On the other hand, if the addition amount of nickel in the steel of one embodiment of the present disclosure is more than 1.0%, there is a problem that excessive nickel causes excessive increase in hardenability, resulting in generation of a low-temperature transformation phase and reduction in toughness, and there is a problem that manufacturing cost excessively increases.

[0070] Niobium (Nb) is precipitated in the form of NbC or NbCN in the ultra-thick structural steel material of the present disclosure to improve base material strength, and Nb that is solid-solved at high temperatures is very finely precipitated in the form of NbC during rolling to suppress recrystallization of austenite, resulting in refinement of the structure, and thus needs to be included in the steel (or steel material) within a controlled range.

[0071] In the steel according to one embodiment of the present disclosure, the niobium content is in the range of 0.005% to 0.04% in terms of weight % (hereinafter expressed as %).

[0072] If the addition amount of niobium in the steel of one embodiment of the present disclosure is less than 0.005%, there is a problem that it is difficult to expect fine structure refinement and strength enhancement due to the insufficient amount of precipitates in the form of NbC or NbCN.

[0073] On the other hand, if the addition amount of niobium in the steel of one embodiment of the present disclosure is more than 0.04%, there is a problem that excessive niobium increases the possibility of generation of a brittle crack at the edge of the steel material.

[0074] Titanium (Ti) is precipitated as TiN when re-heated in the ultra-thick structural steel material of the present disclosure to suppress grain growth of the base material and the weld heat-affected zone, thereby greatly improving low-temperature toughness, and thus needs to be included in the steel (or steel material) within a controlled range.

[0075] In the steel according to one embodiment of the present disclosure, the titanium content is in the range of 0.005% to 0.03% in terms of weight % (hereinafter expressed as %).

[0076] If the amount of titanium added to the steel of one embodiment of the present disclosure is less than 0.005%, there is a problem in that it is difficult to expect grain refinement and improvement of toughness of the base material and the weld heat-affected zone due to too small amount of precipitates in the form of TiN.

[0077] On the other hand, if the amount of titanium added to the steel of one embodiment of the present disclosure is more than 0.03%, there is a problem in that the low-temperature toughness is reduced due to the clogging of the continuous casting nozzle or primary precipitation of excessive titanium.

[0078] Copper (Cu) is a main element for improving hardenability and increasing the strength of the steel material of the present disclosure by causing solid solution strengthening, and is also a main element for increasing the yield strength by generating ε-Cu precipitates at the time of tempering, and thus needs to be contained in the steel (or steel material) within a controlled range.

[0079] In the steel according to one embodiment of the present disclosure, the copper content is in the range of 0.1% to 0.5% by weight (%).

[0080] If the amount of copper added to the steel of one embodiment of the present disclosure is less than 0.1%, there is a problem in that it is difficult to expect improvement of hardenability at the time of cooling and improvement of strength.

[0081] On the other hand, if the amount of copper added to the steel of one embodiment of the present disclosure is more than 0.5%, there is a problem in that hot shortness can occur in the steelmaking process, resulting in a slab crack.

[0082] Phosphorus (P) and sulfur (S) are elements that cause grain boundary embrittlement or form coarse inclusions to cause embrittlement in the steel material for ultra-thick structures of the present disclosure, and thus it is necessary to minimize the content in the steel (or steel material) within a controlled range.

[0083] Therefore, in the steel according to one embodiment of the present disclosure, the contents of phosphorus and sulfur are limited to 100 ppm or less and 40 ppm or less, respectively, by weight (%).

[0084] Hereinafter, a method of manufacturing the steel material of the present disclosure as described above will be described in detail.

[0085] The manufacturing method of the steel material according to one embodiment of the present disclosure can include the processes of slab reheating-coarse rolling- finish rolling-cooling, and the detailed conditions of each process are as follows.

[0086] In the following description of the manufacturing method, unless otherwise described, the temperature of the hot-rolled steel sheet (slab) refers to the temperature at the position of t / 4 (t: thickness of the steel sheet) in the thickness direction from the surface plate of the hot-rolled steel sheet (slab).

[0087] In addition, in the case of water cooling, the reference position for measuring the cooling rate is also the t / 4 (t: thickness of the steel sheet) position from the surface sheet of the hot-rolled steel sheet (slab) in the thickness direction.

[0088] Slab reheating step:

[0089] In the steel manufacturing method according to one embodiment of the present disclosure, the slab reheating step is a process for solidifying the Ti and / or Nb carbide and / or carbonitride formed during casting and reducing the flow stress to facilitate subsequent hot working without excessively coarsening the austenite grains.

[0090] In the steel manufacturing method according to one embodiment of the present disclosure, the slab reheating temperature can be 1000°C to 1120°C, and more preferably 1050°C to 1120°C.

[0091] If the slab reheating temperature is lower than 1000°C, there is a concern that the Ti and / or Nb carbonitride formed during casting will not be sufficiently solidified.

[0092] On the other hand, if the reheating temperature is higher than 1120°C, there is a concern that the fine-grained austenite formed at the reheating temperature will be coarsened.

[0093] Rough rolling step:

[0094] In the steel manufacturing method according to one embodiment of the present disclosure, the rough rolling step is a process for destroying the casting structure such as dendrites formed during casting and reducing the grain size of the crystal grains through recrystallization of the coarse austenite.

[0095] Since dynamic recrystallization of the austenite must occur during rough rolling, the rough rolling temperature is preferably equal to or higher than the temperature at which austenite recrystallization stops (Tnr).

[0096] Specifically, in the steel manufacturing method according to one embodiment of the present disclosure, the rough rolling temperature is 900°C to 1100°C.

[0097] If the rough rolling temperature is lower than 900°C, there is a problem that it is difficult to refine the grains since dynamic recrystallization is difficult to occur during rough rolling.

[0098] On the other hand, if the rough rolling temperature is higher than 1100°C, there is a problem that the grains cannot be effectively refined even through dynamic recrystallization since the austenite grains in the slab before the start of rough rolling are excessively grown.

[0099] In addition, in order to induce recrystallization in the slab by rough rolling to refine the microstructure of the slab, it is necessary to apply a deformation amount sufficient to induce recrystallization to the slab during rough rolling.

[0100] According to one embodiment of the present disclosure, the cumulative reduction ratio in the rough rolling process is preferably 40% or more.

[0101] The finishing rolling step:

[0102] In the steel manufacturing method according to one embodiment of the present disclosure, the finishing rolling step is a process for introducing a non-uniform microstructure into the austenite microstructure of the steel plate after rough rolling.

[0103] At this time, the finishing rolling end temperature is preferably between 720°C and 740°C, inclusive, at t / 4.

[0104] The finishing rolling end temperature range is set to a temperature range in which fine air-cooled ferrite is promoted to be generated in the surface portion during the air-cooling step after the finishing rolling and water cooling by performing rolling near the polygonal ferrite generation temperature.

[0105] If the finishing rolling is performed at a temperature of 720°C or less, inclusive, at t / 4, elongated ferrite is generated before ferrite precipitation during the finishing rolling, and thus there is a problem in that the NDTT temperature increases.

[0106] On the other hand, if the finishing rolling is performed at a temperature of 740°C or more, inclusive, at t / 4, the grain size at t / 4 decreases due to insufficient deformation, which can cause a decrease in Kca performance, and there is a problem in that the NDTT temperature increases due to insufficient ferrite generation in the surface portion.

[0107] According to one embodiment of the present disclosure, the cumulative reduction ratio in the finishing rolling process is preferably 50% or more.

[0108] The cooling step:

[0109] In the steel manufacturing method according to one embodiment of the present disclosure, the steel plate after finishing rolling is preferably cooled to a temperature of 500°C or less at a cooling rate of 3°C / sec or more.

[0110] If the cooling rate is less than 3°C / sec or the cooling end temperature is 500°C or more, the microstructure formed in the steel plate during the cooling process due to phase transformation does not properly form, and there is a possibility that the final yield strength is 460 MPa or less.

[0111] The steel manufacturing method according to one embodiment of the present disclosure described above is summarized as follows.

[0112] The ultra-thick steel material for structures having excellent surface part NRL-DWT performance according to one embodiment of the present disclosure can be manufactured via the following steps: after reheating a slab to a temperature of 1000°C to 1120°C, coarsely rolling at a temperature of 900°C to 1100°C, the slab comprising, in terms of weight %, C: 0.05% to 0.09%, Si: 0.1% to 0.4%, Al: 0.01% to 0.05%, Mn: 1.8% to 2.1%, Ni: 0.3% to 1.0%, Nb: 0.005% to 0.040%, Ti: 0.005% to 0.03%, Cu: 0.1% to 0.5%, P: 100 ppm or less, S: 40 ppm or less, the balance of Fe and other inevitable impurities; finishing the rolled coarse billet at a temperature of 720°C to 740°C with a 1 / 4t gauge; and after the overall rolling is completed, cooling to a temperature of 500°C or less at a cooling rate of 3°C / sec or more.

[0113] At this time, the ultra-thick steel material according to one embodiment of the present disclosure has a polygonal ferrite fraction of 50% or more from the surface part to 5 mm below the surface part, and has a fine structure with a t / 4 fine structure grain size of 15 μm or less having a large angle grain boundary of 15 degrees or more measured by EBSD, and a thickness of 80 mm to 100 mm.

[0114] The polygonal ferrite fraction of 50% or more from the surface part to 5 mm below the surface part and the fine structure with a t / 4 fine structure grain size of 15 μm or less having a large angle grain boundary of 15 degrees or more measured by EBSD are achieved during the finishing process and the cooling process.

[0115] Therefore, for the fine structure and thickness of the ultra-thick steel material according to one embodiment of the present disclosure as described above, it can only be achieved by a combination of the composition and fine structure of the steel material and the technical features of the manufacturing method being controlled.

[0116] Thus, in the present disclosure, a steel material having a yield strength of 460 MPa or more, a surface part impact transition temperature of -40°C or less, an NDTT (Nil-Ductility Transition Temperature) value of -70°C or less based on the NRL-DWT test according to the ASTM E208 standard, and a Kca value of 8000 or more obtained by performing the ESSO test can be ensured.

[0117] Hereinafter, the present disclosure will be described more specifically through examples. However, it is to be noted that the following examples are only for describing the present disclosure by way of example, and are not intended to limit the scope of the rights of the present disclosure. Because the scope of the rights of the present disclosure depends on the content recited in the claims and what is reasonably derived therefrom.

[0118] [Example]

[0119] A steel slab having the components shown in Table 1 was selected, and reheating, rolling, and cooling were performed by the manufacturing method in the present disclosure.

[0120] Specifically, a steel slab having the components of Table 1 below and having a thickness of 400 mm was reheated to a temperature of 1070℃, and after starting rough rolling at a temperature of 1030℃ or less, rough rolling was continuously performed to complete rough rolling at a temperature of 930℃ or more to produce a rough rolled slab.

[0121] After the rough rolling, finish rolling was performed at the cumulative reduction ratio shown in Table 2 to obtain a steel sheet having the thickness of Table 2, and then cooling was performed at a cooling rate of 3.5℃ / sec to 5.2℃ / sec to a temperature in the range of 450℃ to 370℃.

[0122] [Table 1]

[0123]

[0124] For the steel materials shown in Table 1, the tensile property evaluation results of the steel materials manufactured by the manufacturing method according to one embodiment of the present disclosure and the steel materials manufactured using conditions deviating from the manufacturing method according to one embodiment of the present disclosure, the fine structure analysis results of the manufactured steel sheets, and the yield strength are summarized in Table 2.

[0125] In addition, for the manufactured steel sheets, NDTT (Nil-Ductility Transition Temperature) based on the NRL-DWT test according to the ASTM E208 standard and the Kca value at -10℃ based on the large-scale ESSO test were measured, and the results are summarized in Table 2.

[0126] Further, for the grain size analysis of the manufactured steel sheets, an electron backscatter diffraction (EBSD) widely used in the technical field to which the present disclosure pertains was used for measurement.

[0127] The EBSD is a method of analyzing the orientation of a material by detecting electrons reflected (i.e., backscattered) by a sample (i.e., a steel sheet) when an electron beam is injected into the sample.

[0128] In particular, the EBSD can perform phase analysis by analyzing the grain orientation of a material, and further, by crystal orientation mapping, it is possible to analyze fine texture and crystal grain size, etc.

[0129] [Table 2]

[0130]

[0131] In the case of Comparative Example 1, although the composition and the component range satisfy the conditions of the ultra-thick steel sheet according to one embodiment of the present disclosure, since finish rolling is performed at a lower cumulative reduction rate than 50% given in one embodiment of the present disclosure, the surface portion does not sufficiently form polygonal ferrite.

[0132] Further, since the finish rolling reduction rate is low, sufficient deformation is not applied at t / 4, and further the grain size is 15 μm or more, it is measured that the NDTT is -70°C or more and the Kca value is 8000 or less.

[0133] In the case of Comparative Example 2, although the composition and the component range satisfy the conditions of the ultra-thick steel sheet according to one embodiment of the present disclosure, since finish rolling is performed at a lower cumulative reduction rate than 50% given in one embodiment of the present disclosure, the surface portion does not sufficiently form polygonal ferrite.

[0134] In the case of Comparative Example 3, although the composition and the component range satisfy the conditions of the ultra-thick steel sheet according to one embodiment of the present disclosure, since finish rolling is performed at a lower cumulative reduction rate than 50% given in one embodiment of the present disclosure, the surface portion does not sufficiently form polygonal ferrite.

[0135] In the case of Comparative Example 4, the component range of C is higher than the upper limit of C of the ultra-thick steel sheet according to one embodiment of the present disclosure, and thus the phenomenon of excessively high strength resulting in a decrease in toughness occurs, and it is investigated that the NDTT is -70°C or more.

[0136] In the case of Comparative Example 5, the component range of Mn is higher than the upper limit of Mn of the ultra-thick steel sheet according to one embodiment of the present disclosure, and since the ferrite transformation temperature is lowered, sufficient ferrite is not generated during air cooling. It is finally known that the NDTT is -70°C or more and the Kca value is 8000 or less.

[0137] In the case of Comparative Example 5, the component range of Mn is higher than the upper limit of Mn of the ultra-thick steel sheet according to one embodiment of the present disclosure, and since the ferrite transformation temperature is lowered, sufficient ferrite is not generated during air cooling. It is finally known that the NDTT is -70°C or more and the Kca value is 8000 or less.

[0138] In the case of Comparative Example 6, the component ranges of C and Mn are lower than the lower limits of C and Mn of the ultra-thick steel sheet according to one embodiment of the present disclosure, so that a large amount of ferrite is generated in the surface portion, but since the hardenability is low, the yield strength of 460 MPa given in the present disclosure is not satisfied.

[0139] In the case of Comparative Example 7, the component ranges of Ni and Cu are lower than the lower limits of Ni and Cu of the ultra-thick steel sheet according to one embodiment of the present disclosure, so that a large amount of ferrite is generated in the surface portion, but since the hardenability is low, the yield strength of 460 MPa given in the present disclosure is not satisfied, and the toughness is lowered due to the low Ni content, so that the Kca value is also 8000 or less.

[0140] In the case of Comparative Example 8, the component ranges of Ti and Nb are higher than the upper limits of Ti and Nb of the ultra-thick steel sheet according to one embodiment of the present disclosure, so that the strength is increased due to excessive hardenability, and the NDTT is -70°C or more and the Kca value is 8000 or less due to the effect of lowering the toughness due to precipitation strengthening.

[0141] On the other hand, it is known from the above results that in the case of Inventive Examples 1 to 4 satisfying the component ranges given in the present disclosure and manufactured at a temperature of 740°C to 720°C at a cumulative reduction of 50% or more, the fraction of polygonal ferrite from the surface portion to the position 5 mm directly below the surface portion is 50% or more, the yield strength is 460 MPa or more, the NDTT (Nil-Ductility Transition Temperature) value based on the NRL-DWT test according to ASTM E208 standard is -70°C or less, and the Kca value obtained by performing the ESSO test is 8000 or more.

[0142] As described above, the present disclosure has been described with reference to exemplary embodiments, but the present disclosure is not limited to the embodiments and drawings described in the present specification. It is obvious that a person of ordinary skill in the art can make various modifications within the scope of the technical idea of the present disclosure. In addition, even if the effects of the technical features according to the present disclosure are not explicitly described when describing the embodiments of the present disclosure, it should be affirmed that the effects predictable from the technical features can be expected.

Claims

1. A type of ultra-thick structural steel, wherein, The steel, by weight percent, comprises: C: 0.05%–0.09%, Si: 0.1%–0.4%, Al: 0.01%–0.05%, Mn: 1.8%–2.1%, Ni: 0.3%–1.0%, Nb: 0.005%–0.040%, Ti: 0.005%–0.03%, Cu: 0.1%–0.5%, P: less than 100 ppm, S: less than 40 ppm, with the balance being Fe and other unavoidable impurities. The fraction of polygonal ferrite from the surface portion to 5 mm directly below the surface portion is 50% or more, and the microstructure with a grain size of less than 15 μm and large-angle grain boundaries of 15 degrees or more as measured by EBSD has a yield strength of 460 MPa or more.

2. The ultra-thick structural steel according to claim 1, wherein, The NDTT value of the surface part NRL-DWT test based on ASTM E208-06 standard is below -70°C.

3. The ultra-thick structural steel according to claim 1, wherein, The Kca value of the steel, based on the ESSO test, is above 8000.

4. The ultra-thick structural steel according to claim 1, wherein, The thickness of the steel plate is 80mm to 100mm.

5. A method for manufacturing ultra-thick structural steel, comprising the following steps: The slab is reheated, and the slab contains, by weight %, C: 0.05%–0.09%, Si: 0.1%–0.4%, Al: 0.01%–0.05%, Mn: 1.8%–2.1%, Ni: 0.3%–1.0%, Nb: 0.005%–0.040%, Ti: 0.005%–0.03%, Cu: 0.1%–0.5%, P: less than 100 ppm, S: less than 40 ppm, with the balance being Fe and other unavoidable impurities. After rough rolling the reheated slab, finish rolling is performed at a temperature range of 720℃ to 740℃ from the slab surface to the t / 4 position; The finished steel is cooled. in, The cumulative reduction rate during the finishing rolling process is 50% or more.

6. The method for manufacturing ultra-thick structural steel according to claim 5, wherein, The reheating temperature of the slab is 1000℃~1120℃.

7. The method for manufacturing ultra-thick structural steel according to claim 5, wherein, The temperature of the rough rolling is 900℃~1100℃.

8. The method for manufacturing ultra-thick structural steel according to claim 5, wherein, The cumulative reduction rate during rough rolling is 50% or more.

9. The method for manufacturing ultra-thick structural steel according to claim 5, wherein, The cooling rate during the cooling process is above 3°C / second.

10. The method for manufacturing ultra-thick structural steel according to claim 5, wherein, The cooling process ends at a temperature below 500°C.

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