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 2.0 %, Ni: 0.3 to 0.7 %, Nb: 0.015 to 0.040 %, Ti: 0.005 to 0.02 %, Cu: more than 0 % and 0.05 % or less, the balance of Fe and other inevitable impurities, and the number of micro cracks having a length of 50 μm or more per square millimeter in a region from a surface portion to 5 mm directly below the surface portion is 0.1 or less.

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, a difference in cooling speed occurs between the surface portion and the center portion due to the thick thickness of the ultra-thick material.

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

[0007] In particular, when applied to a main structure of a ship or the like, there is an increasing demand for ensuring the 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, which can replace the large-scale tensile test, has been steadily conducted recently.

[0012] Among the small-scale alternative tests, as the most powerful test, the surface portion NRL-DWT (Naval Research Laboratory-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 existing research, the microstructure of the surface portion is controlled, and the crack propagation speed is slowed down when brittle crack propagation occurs, so that the resistance to brittle crack propagation becomes excellent.

[0014] However, in the surface portion NRL-DWT test, when the steel material is collected from the surface of the test sample, the original surface of the plate is used without chamfering.

[0015] If a surface portion crack that easily causes brittle cracks exists in the test sample, the result of the NDTT (Nil-Ductility Transition Temperature) value of the NRL-DWT test is easily deteriorated.

[0016] Therefore, a solution that can suppress surface portion cracks is needed. SUMMARY

[0017] TECHNICAL PROBLEM

[0018] The present disclosure aims to provide an ultra-thick steel material having excellent surface portion NRL-DWT performance and a method of manufacturing the same, which can solve the problems of the prior art described above.

[0019] Specifically, the present disclosure aims to provide an ultra-thick steel material and a method of manufacturing the same, which suppresses surface portion microcracks of the ultra-thick steel material by controlling alloying components without including expensive alloying elements, thereby having excellent NRL-DWT performance.

[0020] In addition, the present disclosure aims to provide an ultra-thick steel material and a method of manufacturing the same, which suppresses surface portion microcracks by controlling the rolling temperature and the reduction amount at the time of rolling, thereby having excellent surface portion NRL-DWT performance.

[0021] In addition, the present disclosure aims to provide an ultra-thick structural steel material having excellent NRL-DWT performance and a method of manufacturing the same, more specifically, the steel material has a yield strength of 460 MPa or more, a thickness of 80 mm or more and 100 mm or less, and the number of microcracks having a length of 50 μm or more per square millimeter in a region 5 mm below the surface portion of the plate is 0.1 or less by minimizing the addition amount of Cu, which causes surface cracks, and the NDTT (Nil-Ductility Transition Temperature) value based on the NRL-DWT test according to the ASTM E208 standard is -70°C or less.

[0022] The objects of the present disclosure are not limited to the above-mentioned objects, and other objects and advantages of the present disclosure can be understood from the following description, and the objects and advantages of the present disclosure can be more clearly understood through embodiments. Also, it will be obvious that the objects and advantages of the present disclosure can be implemented by the means described in the claims and combinations thereof.

[0023] Technical Solution

[0024] To achieve the above object, a steel material for an ultra-thick structure according to one embodiment of the present disclosure can include, in weight %, C: 0.05% to 0.09%, Si: 0.1% to 0.4%, Al: 0.01% to 0.05%, Mn: 1.8% to 2.0%, Ni: 0.3% to 0.7%, Nb: 0.015% to 0.040%, Ti: 0.005% to 0.02%, Cu: more than 0% and 0.05% or less, the balance of Fe and other inevitable impurities, and can have a microstructure in which the number of micro-cracks having a length of 50 μm or more per square millimeter is 0.1 or less in a region from a surface portion to a region 5 mm directly below the surface portion.

[0025] 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 less.

[0026] Preferably, the plate thickness can be 80 mm to 100 mm, and the yield strength can be 460 MPa or more.

[0027] To achieve the above object, a method of manufacturing a steel material for an ultra-thick structure according to one embodiment of the present disclosure can be a method of manufacturing including the step of performing final finishing at a temperature of 740°C or less from a surface of a slab to a t / 4 position.

[0028] To achieve the above object, a method of manufacturing a steel material for an ultra-thick structure according to one embodiment of the present disclosure can include the steps of: reheating a slab including, in weight %, C: 0.05% to 0.09%, Si: 0.1% to 0.4%, Al: 0.01% to 0.05%, Mn: 1.8% to 2.0%, Ni: 0.3% to 0.7%, Nb: 0.015% to 0.040%, Ti: 0.005% to 0.02%, Cu: more than 0% and 0.05% or less, the balance of Fe and other inevitable impurities; performing rough rolling on the reheated slab, and performing final finishing at a temperature of 740°C or less from a surface of the slab to a t / 4 position; and cooling the steel material after the finishing.

[0029] Preferably, the slab reheating temperature can be 1000°C to 1120°C.

[0030] Preferably, the rough rolling temperature can be 900°C to 1100°C.

[0031] Preferably, the cumulative reduction rate at the finish rolling can be 50% or more.

[0032] Preferably, the cooling rate in the cooling step can be 3°C / sec or more.

[0033] Preferably, the cooling start temperature in the cooling step can be 720°C or less, and the cooling end temperature can be 500°C or less.

[0034] Inventive Effects

[0035] According to the present disclosure, by controlling the composition and microstructure without unnecessarily including an excessive amount of expensive alloying elements, an ultra-thick steel material having excellent surface portion NRL-DWT properties can be achieved.

[0036] According to the present disclosure, by controlling the composition and component range, finish rolling temperature, and cumulative reduction, so that the deformation amount of the austenite structure at the surface portion and at t / 4 is maximized, a manufacturing method of an ultra-thick steel material having excellent surface portion NRL-DWT properties can be achieved, the ultra-thick steel material having 0.1 or less of micro-cracks having a length of 50μm or more per square millimeter in a region from the surface portion to a region 5mm directly below the surface portion.

[0037] According to the present disclosure, an ultra-thick structural steel material having excellent NRL-DWT properties and a manufacturing method thereof can be achieved, the ultra-thick steel material having a thickness of 80mm or more and 100mm or less, a yield strength of 460MPa or more, and an NDTT (Nil-Ductility Transition Temperature) value of -70°C or less based on an NRL-DWT test according to the ASTM E208 standard.

[0038] In addition to the above effects, specific effects of the present disclosure will be described when describing specific contents for implementing the present disclosure. DETAILED DESCRIPTION

[0039] 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.

[0040] 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 respective 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 the related known configuration or function can obscure the gist of the disclosure, the detailed description can be omitted.

[0041] 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 is another constituent element between the constituent elements, or the constituent elements can be "connected," "coupled," or "communicated" through the other constituent element.

[0042] The disclosure aims to disclose a steel material for an ultra-thick structure having excellent NRL-DWT performance and a manufacturing method thereof, in particular, the steel material has a yield strength of 460 MPa or more, a thickness of 80 mm or more and 100 mm or less, and by minimizing the addition amount of Cu that causes surface cracks, the number of micro cracks having a length of 50 μm or more per square millimeter in the region 5 mm below the surface of the plate is 0.1 or less, and the NDTT (Nil-Ductility Transition Temperature) value based on the NRL-DWT test according to the ASTM E208 standard is -70℃ or less.

[0043] 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.

[0044] Unless otherwise described, the content or composition range of each of the following components is measured in wt%.

[0045] 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.

[0046] In the steel according to one embodiment of the disclosure, the content of carbon is in the range of 0.05% to 0.09% by weight (hereinafter, %).

[0047] 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.

[0048] 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.

[0049] In addition, if the amount of carbon added to the steel of one embodiment of the present disclosure is more than 0.09%, there is a problem in that the possibility of the generation of fine cracks on the surface of the steel increases since it enters the hypo-peritectic region where surface cracks are easily generated.

[0050] Silicon (Si) and aluminum (Al) are necessary alloying elements for deoxidation by precipitating dissolved oxygen in molten steel in the form of slag in the steelmaking and continuous casting processes, and thus need to be contained in the steel (or steel material) within a controlled range.

[0051] In particular, when the steel material is manufactured using a converter, 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 (%).

[0052] 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 the deoxidation effect since the amount of precipitated dissolved oxygen during the steelmaking and continuous casting processes is insufficient.

[0053] 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 the excess silicon and aluminum can cause the generation of coarse Si, Al complex oxides or a large amount of coarse massive martensite in the microstructure.

[0054] 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 ultra-thick structure steel material of the present disclosure, and thus needs to be contained in the steel (or steel material) within a controlled range.

[0055] In the steel according to one embodiment of the present disclosure, the manganese content is in the range of 1.8% to 2.0% by weight (%).

[0056] 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.

[0057] On the other hand, if the amount of manganese added to the steel of one embodiment of the present disclosure is more than 2.0%, the excess manganese causes the hardenability to excessively increase, thereby causing upper bainite and martensite to be generated, and there is a problem in that the impact toughness and surface portion NRL-DWT performance greatly decrease.

[0058] 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 steel material for ultra-thick structures of the present disclosure, and thus needs to be included in the steel (or steel material) within a controlled range.

[0059] In the steel according to one embodiment of the present disclosure, the nickel content is in the range of 0.3% to 0.7% by weight (%).

[0060] If the amount of nickel added to the steel of one embodiment of the present disclosure is less than 0.3%, there is a problem in that it is difficult to improve the impact toughness and brittle crack propagation resistance in a high-strength steel having a yield strength of 460 MPa or more.

[0061] On the other hand, if the amount of nickel added to the steel of one embodiment of the present disclosure is more than 0.7%, there is a problem in that the excess nickel causes the hardenability to excessively increase, resulting in the generation of low-temperature transformation phases and the decrease in toughness, and there is a problem in that the manufacturing cost excessively increases.

[0062] Niobium (Nb) is precipitated in the form of NbC or NbCN in the steel material for ultra-thick structures of the present disclosure to increase the base material strength, and the Nb solid-solved at high temperatures is very finely precipitated in the form of NbC during rolling to suppress the recrystallization of austenite, resulting in the refinement of the microstructure, and thus needs to be included in the steel (or steel material) within a controlled range.

[0063] In the steel according to one embodiment of the present disclosure, the niobium content is in the range of 0.015% to 0.04% by weight (%).

[0064] If the amount of niobium added to the steel of one embodiment of the present disclosure is less than 0.015%, there is a problem in that it is difficult to expect the refinement of the microstructure and the strengthening of the strength due to the insufficient amount of precipitates in the form of NbC or NbCN.

[0065] On the other hand, if the amount of niobium added to the steel of one embodiment of the present disclosure is more than 0.04%, the excess niobium causes the possibility of brittle cracks to occur at the edges of the steel material to be high, and there is a problem in that the generation of excess precipitates can cause the toughness to decrease.

[0066] Titanium (Ti) is precipitated as TiN when the steel material for a super-thick structure according to the present disclosure is reheated, thereby suppressing the grain growth of the base material and the weld heat-affected zone, greatly improving the low-temperature toughness, and thus needs to be contained in the steel (or steel material) within a controlled range.

[0067] In the steel according to one embodiment of the present disclosure, the titanium content is in the range of 0.005% to 0.02% by weight (%).

[0068] If the addition amount of titanium in the steel according to one embodiment of the present disclosure is less than 0.005%, there is a problem in that it is difficult to expect the grain refinement and the improvement of the toughness of the base material and the weld heat-affected zone due to the insufficient amount of precipitates in the form of TiN.

[0069] On the other hand, if the addition amount of titanium in the steel according to one embodiment of the present disclosure is more than 0.02%, there is a problem in that the low-temperature toughness is reduced due to the clogging of the continuous casting nozzle or the primary precipitation of excessive titanium.

[0070] Copper (Cu) is a main element for improving the hardenability and the strength of the steel material by causing solid solution strengthening in the steel material for a super-thick structure according to the present disclosure, and is also a main element for improving 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.

[0071] In the steel according to one embodiment of the present disclosure, the copper content is in the range of 0.05% or less by weight (%).

[0072] If the addition amount of copper in the steel according to one embodiment of the present disclosure is more than 0.05%, there is a problem in that the high-temperature brittleness or the slab cracks due to the possibility of hot shortness are caused in the steelmaking process.

[0073] Hereinafter, a method of manufacturing the steel material according to the present disclosure as described above will be described in detail.

[0074] The method of manufacturing the steel material according to one embodiment of the present disclosure can include the processes of slab reheating-coarse rolling-fine rolling-cooling, and the detailed conditions of each process are as follows.

[0075] In the following description regarding the manufacturing method, unless otherwise described, the temperature of the hot-rolled steel plate (slab) refers to the temperature at the position of t / 4 (t: thickness of the steel plate) from the surface plate to the thickness direction of the hot-rolled steel plate (slab).

[0076] In addition, in the water cooling, the reference position for measuring the cooling speed is also the position of t / 4 (t: thickness of the steel plate) from the surface plate to the thickness direction of the hot-rolled steel plate (slab).

[0077] slab reheating step: 1000 to 1120 °C

[0078] In the steel material manufacturing method according to one embodiment of the present disclosure, the slab reheating step is a process of solidifying the Ti and / or Nb carbide and / or carbonitride formed in the casting process and reducing the flow stress to facilitate subsequent hot working without excessively coarsening the austenite grains.

[0079] In the steel material manufacturing method according to one embodiment of the present disclosure, the slab reheating temperature can be 1000 to 1120 °C, and more preferably 1050 to 1120 °C.

[0080] If the slab reheating temperature is lower than 1000 °C, there is a concern that the Ti and / or Nb carbonitride formed in the casting process will not be sufficiently solidified.

[0081] 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.

[0082] rough rolling step: 900 to 1100 °C

[0083] In the steel material 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 in the casting process and reducing the size of the coarse austenite grains through recrystallization.

[0084] 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).

[0085] Specifically, in the steel material manufacturing method according to one embodiment of the present disclosure, the rough rolling temperature is 900 to 1100 °C.

[0086] 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.

[0087] 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 by dynamic recrystallization since the austenite grains in the slab are excessively grown before the start of rough rolling.

[0088] In addition, in order to induce recrystallization in the slab by rough rolling to refine the fine structure of the slab, it is necessary to apply a deformation amount sufficient to induce recrystallization to the slab during rough rolling.

[0089] According to one embodiment of the present disclosure, the cumulative reduction ratio in the rough rolling process is preferably 40% or more.

[0090] Finish rolling end temperature: 740°C or less

[0091] In the steel material manufacturing method according to one embodiment of the present disclosure, the finish rolling step is a process for introducing a heterogeneous microstructure into the austenite microstructure of the steel sheet after rough rolling.

[0092] At this time, the finish rolling end pass is preferably performed at a ferrite generation temperature of 740°C or less with t / 4 as a reference.

[0093] The finish rolling end temperature range is set to a temperature range in which the grain size of the phase generated during the post-rolling cooling process can be refined by performing rolling near the polygonal ferrite generation temperature.

[0094] If the finish rolling end pass is performed at a temperature higher than 740°C with t / 4 as a reference, there is a problem in that the strength and toughness decrease due to the coarsening of the microstructure.

[0095] According to one embodiment of the present disclosure, the cumulative reduction ratio in the finish rolling process is preferably at least 50% or more in order to maximize the formation of a microstructure.

[0096] Post-rolling cooling step: after cooling at a cooling rate of 3°C / sec or more at a temperature of 720°C or less, the cooling is ended at a temperature of 500°C or less.

[0097] In the steel material manufacturing method according to one embodiment of the present disclosure, the steel sheet after finish rolling is preferably cooled from a temperature of 720°C or less to a temperature of 500°C or less at a cooling rate of 3°C / sec or more.

[0098] If the cooling start temperature is higher than 720°C, there is a problem in that the NDTT temperature can be -70°C or more because the polygonal ferrite, which is a soft phase, is not promoted to be generated in the surface portion.

[0099] If the cooling rate is lower than 3°C / sec or the cooling end temperature is higher than 500°C, there is a possibility that the final yield strength is 460 MPa or less because the microstructure formed in the steel sheet during the cooling process is not properly formed due to the phase transition.

[0100] The above-described steel material manufacturing method according to one embodiment of the present disclosure is summarized as follows.

[0101] The ultra-thick steel material for structures having excellent surface portion 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 1,000°C to 1,120°C, performing rough rolling at a temperature of 900°C to 1,100°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 2.0%, Ni: 0.3% to 0.7%, Nb: 0.015% to 0.040%, Ti: 0.005% to 0.02%, Cu: more than 0% and 0.05% or less, the balance of Fe and other inevitable impurities; air-cooling the rough-rolled bar after rolling; performing finish rolling after the air-cooling is completed, and then performing finish rolling at a temperature of 740°C or less at a rate of 1 / 4t; 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.

[0102] At this time, the ultra-thick steel material according to one embodiment of the present disclosure can have 0.1 or less of fine cracks per square millimeter having a length of 50 μm or more in a region 5 mm below the surface portion from the surface of the steel material, by minimizing the amount of Cu added to cause surface cracks.

[0103] Therefore, the fine structure and thickness of the ultra-thick steel material according to one embodiment of the present disclosure as described above can be achieved only by the combination of the composition and component ranges of the steel material and the technical features of the manufacturing method controlled.

[0104] Accordingly, in the present disclosure, it is possible to secure an ultra-thick steel material for structures having excellent NRL-DWT performance with a yield strength of 460 MPa or more and an NDTT (Nil-Ductility Transition Temperature) value of -70°C or less based on the NRL-DWT test according to ASTM E208 standards.

[0105] Hereinafter, the present disclosure will be described more specifically through examples. However, it should be noted that the following examples are merely 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 deduced therefrom.

[0106] [Examples]

[0107] A steel billet having the components shown in Table 1 was selected, and reheating, rolling, and cooling were performed by the manufacturing method in the present disclosure.

[0108] Specifically, a slab having the components of Table 1 below with a thickness of 400 mm is reheated to a temperature of 1050°C to 1070°C, after which rough rolling is continuously performed starting at a temperature of 1030°C or less, and then ending at a temperature of 930°C or more, to manufacture a rough rolled slab.

[0109] After the rough rolling, finish rolling is performed at the cumulative reduction ratio shown in Table 2 to obtain a steel sheet having the thickness of Table 2, and then cooled at a cooling rate of 3.4°C / sec to 5.3°C / sec to a temperature in the range of 480°C to 390°C.

[0110] [Table 1]

[0111]

[0112] 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 disclosure and the steel materials manufactured using conditions deviating from the manufacturing method according to one embodiment of the disclosure, the surface portion crack analysis results of the manufactured steel sheets, and the yield strengths are summarized in Table 2.

[0113] For the cracks, 20 or more different locations are observed in an area of 1 mm*1 mm in size 5 mm below the surface of the steel sheet, and then the number of cracks is obtained from the average value of the number of cracks of 50 μm or more.

[0114] In addition, the NDTT (Nil-Ductility Transition Temperature) based on the NRL-DWT test according to the ASTM E208 standard is measured for the manufactured steel sheets, and the results are summarized in Table 2.

[0115] [Table 2]

[0116]

[0117] In the case of Comparative Example 1, although the composition and component ranges satisfy the conditions for the ultra-thick steel material according to one embodiment of the disclosure, since it is manufactured at a temperature above the finish rolling end temperature given in one embodiment of the disclosure, ferrite is not sufficiently generated in the surface portion during the air cooling process, and thus the NDTT was measured to be -70°C or more.

[0118] In the cases of Comparative Examples 2 and 3, a higher amount than the upper limit of the Cu component range given in the ultra-thick steel material according to one embodiment of the disclosure is added.

[0119] Therefore, since the Cu content is high, Comparative Examples 2 and 3 have a wider high-temperature brittle region, and the possibility of hot shortness increases, and thus a large amount of fine cracks are generated 5 mm below the surface of the slab in the slab manufacturing process.

[0120] Since the generated fine cracks are elongated during rolling, Comparative Example 2 and 3 generate 0.1 / mm or more of the fine cracks in the portion just below the surface of the steel material 2 The length of the above cracks is 50 μm or more, and thus the NDTT is measured to be -70°C or more.

[0121] In the case of Comparative Example 4, an amount higher than the upper limit of the C component range given in the ultra-thick steel material according to one embodiment of the present disclosure is added.

[0122] Thus, since the C content is high, Comparative Example 4 also generates a wide high-temperature embrittlement region, and a large amount of fine cracks are generated in the portion just below the surface of the slab during the slab manufacturing process.

[0123] Since the generated fine cracks are elongated during rolling, Comparative Example 4 also generates 0.1 / mm or more of the fine cracks in the portion just below the surface of the steel material 2 The length of the above cracks is 50 μm or more, and thus the NDTT is measured to be -70°C or more.

[0124] In the case of Comparative Example 5, an amount higher than the upper limit of the Mn component range given in the ultra-thick steel material according to one embodiment of the present disclosure is added.

[0125] Thus, since the Mn content is high, Comparative Example 5 generates a wide high-temperature embrittlement region, and a large amount of fine cracks are generated in the portion just below the surface of the slab during the slab manufacturing process.

[0126] Since the generated fine cracks are elongated during rolling, Comparative Example 5 generates 0.1 / mm or more of the fine cracks in the portion just below the surface of the steel material 2 The length of the above cracks is 50 μm or more, and thus the NDTT is measured to be -70°C or more.

[0127] In the case of Comparative Example 6, an amount lower than the lower limit of the C and Mn component ranges given in the ultra-thick steel material according to one embodiment of the present disclosure is added.

[0128] Thus, since the hardenability is low, Comparative Example 6 is measured to fail to satisfy the yield strength of 460 MPa given in the present disclosure.

[0129] In the case of Comparative Example 7, an amount lower than the lower limit of the Ni component range given in the ultra-thick steel material according to one embodiment of the present disclosure is added.

[0130] Thus, since the Ni content is low, Comparative Example 7 is measured to have a NDTT of -70°C or more due to a decrease in toughness.

[0131] In the case of Comparative Example 8, an amount higher than the upper limit of the Ti and Nb component range given in the ultra-thick steel material according to one embodiment of the present disclosure was added.

[0132] Therefore, Comparative Example 8 produced a wide high-temperature brittle region due to the high Ti and Nb contents, and a large amount of fine cracks were generated in the slab surface just below the slab surface during the slab manufacturing process.

[0133] Since the generated fine cracks were elongated during the rolling process, Comparative Example 8 produced cracks having a length of 50 μm or more at 0.1 or more per mm2in the portion just below the surface of the steel material.

[0134] In addition, Comparative Example 8 had a high strength due to the excess precipitates, and a high-strength structure was generated in a large amount in the surface portion, and thus the NDTT was measured to be -70°C or more.

[0135] On the other hand, from the above results, in the case of Inventive Examples 1 to 4 manufactured to satisfy the component range given in the present disclosure and to perform finish rolling at a temperature of 740°C or less, the fine cracks having a length of 50 μm or more per square millimeter in the region from the surface portion to 5 mm just below the surface portion were 0.1 or less, the yield strength was 460 MPa or more, and the NDTT (Nil-Ductility Transition Temperature) value based on the NRL-DWT test according to the ASTM E208 standard was measured to be -70°C or less.

[0136] 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, the effects predictable from the technical features should be affirmed.

Claims

1. A steel material for an ultra-thick structure, wherein the steel material comprises, 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 2.0%, Ni: 0.3 to 0.7%, Nb: 0.015 to 0.040%, Ti: 0.005 to 0.02%, Cu: more than 0% and 0.05% or less, the balance of Fe and other inevitable impurities, and the steel material has a surface portion NDTT value of -70°C or lower based on a surface portion NRL-DWT test according to ASTM E208-06 standard, and a micro crack length of 50 μm or more per 1 mm2 in a region from the surface portion to 5 mm below the surface portion is 0.1 or less.

2. The steel material for an ultra-thick structure according to claim 1, wherein the steel material comprises polygonal ferrite generated on the surface.

3. The steel material for an ultra-thick structure according to claim 1 or 2, wherein the steel material has a plate thickness of 80 to 100 mm and a yield strength of 460 MPa or more.

4. A method for manufacturing a steel material for an ultra-thick structure, comprising the steps of: reheating a slab comprising, 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 2.0%, Ni: 0.3 to 0.7%, Nb: 0.015 to 0.040%, Ti: 0.005 to 0.02%, Cu: more than 0% and 0.05% or less, the balance of Fe and other inevitable impurities; rough rolling the reheated slab; after the rough rolling, performing final finishing at a temperature of 740°C or lower in the vicinity of a polygonal ferrite generation temperature from the surface to a t / 4 position; and cooling the steel material after the finishing.

5. The method for manufacturing a steel material for an ultra-thick structure according to claim 4, wherein the reheating temperature of the slab is 1000°C to 1120°C.

6. The method for manufacturing a steel material for an ultra-thick structure according to claim 4, wherein the temperature of the rough rolling is 900°C to 1100°C.

7. The method for manufacturing a steel material for an ultra-thick structure according to claim 4, wherein the cumulative reduction in the finishing step is 50% or more.

8. The method for manufacturing a steel material for an ultra-thick structure according to claim 4, wherein the cooling rate in the cooling step is 3°C / sec or more.

9. The method for manufacturing a steel material for an ultra-thick structure according to claim 8, wherein the cooling start temperature in the cooling step is 720°C or lower and the cooling end temperature is 500°C or lower. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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