Extra-thick structural steel with excellent surface NRL-DWT properties and its manufacturing method

Abstract

The present invention provides an extra-heavy structural steel material that does not contain expensive alloying elements and that has excellent NRL-DWT physical properties by suppressing fine cracks on the surface of the extra-heavy steel material through controlling the alloying components, and a method for manufacturing the same. [Solution] The present invention relates to an extra-heavy structural steel product having excellent NRL-DWT physical properties in the surface region and a method for manufacturing the same, and more specifically, the product contains, by weight, 0.05-0.09% C, 0.1-0.4% Si, 0.01-0.05% Al, 1.8-2.0% Mn, 0.3-0.7% Ni, 0.015-0.040% Nb, 0.005-0.02% Ti, 0.05% or less Cu (excluding 0%), with the remainder consisting of Fe and unavoidable impurities, and the product has a surface area of ​​1 mm from the surface region to 5 mm directly below the surface region. 2 The number of microcracks per area having a length of 50 μm or more is 0.1 or less.

Description

[Technical Field]

[0001] The present invention relates to an extra-heavy structural steel material having excellent surface NRL-DWT physical properties and a manufacturing method thereof, and more particularly to an extra-heavy structural steel material having excellent surface NRL-DWT physical properties in which fine cracks on the surface of the extra-heavy structural steel material are suppressed by controlling alloy components, and a manufacturing method thereof. [Background technology]

[0002] Recently, when designing structures such as ships both domestically and internationally, there has been a demand for the development of extra-thick, high-strength steel materials. When high-strength steel is used in the design of a structure, economic benefits are obtained by reducing the weight of the structure, and the plate thickness can be reduced, which simultaneously ensures ease of processing and welding work. In the case of high-strength steel, when manufacturing extra-thick materials, the total reduction rate is usually reduced, and as a result, the microstructure does not deform sufficiently overall, resulting in coarsening of the microstructure. Furthermore, when rapid cooling is performed to ensure strength, the large thickness of the extra-thick material results in a difference in cooling rate between the surface and the center. This causes the formation of coarse low-temperature transformed phases such as bainite in the surface portion, making it difficult to ensure toughness in the extra-thick material. In particular, in the case of brittle crack propagation resistance, which indicates the stability of a structure, there are an increasing number of cases where guarantees are required when applying it to major structures such as ships.

[0003] In the case of extremely thick materials, it is very difficult to ensure the resistance to brittle crack propagation due to a decrease in toughness caused by the difference in cooling rate between the surface and center. In fact, many classification societies and steel manufacturers conduct large-scale tensile tests to ensure brittle crack propagation resistance, which can actually accurately evaluate brittle crack propagation resistance. However, in the case of large-scale tensile tests, the cost of conducting the tests is enormous, making it difficult to guarantee their effectiveness when applied to mass production. In order to overcome these disadvantages, research has been ongoing recently into small-scale alternative tests that can replace large-scale tensile tests. The most effective of these small-scale alternative tests is the surface NRL-DWT (Naval Research Laboratory-Drop Weight Test) test according to the ASTM E208-06 standard, which is currently adopted by many classification societies and steel manufacturers.

[0004] The surface NRL-DWT test is adopted based on existing research findings that controlling the microstructure of the surface can slow the crack propagation speed during brittle crack propagation and improve brittle crack propagation resistance. However, in the surface NRL-DWT test, when steel is taken from the surface of the specimen, chamfering is not performed and the surface of the plate is used as is. If a surface crack that is likely to cause brittle cracking is present in the specimen, the NDTT (Nil-Ductility Transition Temperature) value, which is the result of the NRL-DWT test, is likely to be poor. Therefore, a solution that can suppress surface cracks is needed. Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide an extra-heavy-gauge steel product having excellent surface NRL-DWT physical properties, which can solve the above-mentioned problems of the prior art, and a manufacturing method thereof. Specifically, the present invention aims to provide an extra-heavy steel product having excellent NRL-DWT physical properties, which does not contain expensive alloying elements and suppresses microcracks on the surface of the extra-heavy steel product by controlling the alloying components, and a method for manufacturing the same. Another object of the present invention is to provide an extra-heavy steel product having excellent NRL-DWT physical properties in the surface region by controlling the rolling temperature and reduction during rolling to suppress microcracks in the surface region, and a method for manufacturing the same.

[0006] More specifically, the present invention provides a sheet having a yield strength of 460 MPa or more, a thickness of 80 mm or more and 100 mm or less, and a thickness of 1 mm or less in a region from the surface of the sheet to 5 mm directly below by minimizing the amount of Cu added, which causes surface cracks. 2 The objective of this invention is to provide an extra-heavy structural steel material with excellent NRL-DWT physical properties, in which the number of microcracks per area having a length of 50 μm or more is 0.1 or less, and the NDTT (Nil-Ductility Transition Temperature) value in the NRL-DWT test according to the ASTM E208 standard is -70°C or less, and a manufacturing method thereof. The objects of the present invention are not limited to the objects mentioned above, and other objects and advantages of the present invention not mentioned above can be understood from the following description and can be more clearly understood from the examples of the present invention. Furthermore, it is clear that the objects and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims. [Means for solving the problem]

[0007] In order to achieve the above object, the extra-heavy structural steel material of the present invention contains, by 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: 0.05% or less (excluding 0%), and also contains Fe and unavoidable impurities, and 2 The steel sheet can have a microstructure in which the number of microcracks per area having a length of 50 μm or more is 0.1 or less.

[0008] Preferably, the Nil-Ductility Transition Temperature (NDTT) value in the surface NRL-DWT (Drop Weight Test) test according to ASTM E208-06 standard may be −70° C. or less. The plate thickness may be 80 to 100 mm, and the yield strength may be 460 MPa or more.

[0009] The method for producing an extra-heavy structural steel material of the present invention may be a production method including a step of finish rolling a slab at a temperature of 740°C or less at a t / 4 position from the surface. The method for manufacturing an extra-heavy structural steel material of the present invention, which has been embodied to achieve the above-mentioned object, includes the steps of reheating a slab containing, by weight, C: 0.05-0.09%, Si: 0.1-0.4%, Al: 0.01-0.05%, Mn: 1.8-2.0%, Ni: 0.3-0.7%, Nb: 0.015-0.040%, Ti: 0.005-0.02%, Cu: 0.05% or less (excluding 0%), with other elements being Fe and unavoidable impurities; rough rolling the reheated slab, and then finish rolling it at a temperature of 740°C or less at a t / 4 position from the surface of the slab; and cooling the finish-rolled steel material. Preferably, the reheating temperature of the slab may be 1,000 to 1,120 ° C., The rough rolling temperature may be 900 to 1,100 ° C., The cumulative reduction rate during the finish rolling may be 50% or more, The cooling rate in the cooling step may be 3°C / sec or more, The cooling step may start at a temperature of 720°C or less and finish at a temperature of 500°C or less. [Effects of the Invention]

[0010] According to the present invention, by controlling the components and microstructure without excessively containing expensive alloying elements, it is possible to realize an extra-heavy steel material having excellent NRL-DWT physical properties of the surface portion. In addition, by controlling the components and composition range, the finish rolling temperature and the cumulative reduction, the amount of deformation in the austenite structure of the surface and t / 4 section is maximized, and the area from the surface to 5 mm directly below the surface is reduced by 1 mm. 2 This allows for the realization of a method for manufacturing an extra-thick steel product with excellent surface NRL-DWT physical properties, in which the number of microcracks per area is 0.1 or less and the length of which is 50 μm or more. Furthermore, it is possible to provide an extra-heavy structural steel material with excellent NRL-DWT physical properties, having a thickness of 80 mm to 100 mm, a yield strength of 460 MPa or more, and an NDTT (Nil-Ductility Transition Temperature) value of -70°C or less in the NRL-DWT test according to ASTM E208 standard, and a manufacturing method thereof.

[0011] The above-mentioned effects and specific effects of the present invention will be described in conjunction with the following description of the preferred embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the accompanying drawings so that those skilled in the art can easily understand and practice the present invention. As those skilled in the art may realize, the present invention is not limited to the embodiments set forth herein. In order to clearly explain the present invention, parts that are not relevant to the explanation will be omitted, and the same or similar components throughout the specification will be designated by the same reference numerals. Some embodiments of the present invention will be described in detail with reference to the drawings. When designating components in each drawing with reference numerals, the same components may be designated by the same reference numerals as much as possible, even if they are depicted in different drawings. Furthermore, when describing the present invention, if a detailed description of related known structures or functions is deemed to obscure the gist of the present invention, such detailed description may be omitted. In describing components of the present invention, terms such as first, second, A, B, (a), (b), etc. may be used. These terms are used to distinguish the component from other components, and do not limit the nature, order, sequence, or number of the components. When a component is described as being "coupled," "coupled," or "connected" to another component, it should be understood that the component may be directly coupled or connected to the other component, but that other components may be "intervening" between the components, or that each component may be "coupled," "coupled," or "connected" via other components.

[0013] Specifically, in the present invention, the yield strength is 460 MPa or more, the thickness is 80 mm or more and 100 mm or less, and by minimizing the amount of Cu added that causes surface cracks, the area from the surface of the plate material to 5 mm directly below the surface is 1 mm 2 This paper describes an extra-thick structural steel material with excellent NRL-DWT physical properties, in which the number of microcracks per area that are 50 μm or more in length is 0.1 or less, and the NDTT (Nil-Ductility Transition Temperature) value in the NRL-DWT test according to the ASTM E208 standard is -70°C or less, and a manufacturing method thereof. In order to satisfy the above-mentioned properties, the extra-heavy structural steel material of the present invention may specifically contain the following alloying elements in order to satisfy the above-mentioned excellent NRL-DWT physical properties. It should be made clear in advance that the content or composition range of each component described below is based on weight percent unless otherwise specified.

[0014] Carbon (C) is the most important element for ensuring the basic strength of the extra-heavy structural steel material of the present invention, and therefore must be contained in the steel (or steel material) within a controlled range. The carbon content in the steel of the present invention is in the range of 0.05 to 0.09% by weight (hereinafter referred to as %). If carbon is added to the steel of the present invention in an amount of less than 0.05%, the strength of the steel will be reduced, making it difficult to achieve the target strength. On the other hand, if carbon is added in an amount greater than 0.09% in the steel of the present invention, the excessive carbon increases the hardenability, generates a large amount of martensite islands, and promotes the formation of low-temperature transformation phases, resulting in a problem of reducing the toughness of the steel. Furthermore, if carbon is added to the steel of the present invention in an amount greater than 0.09%, it may penetrate into the hypo-peritectic region where surface cracks are likely to occur, increasing the likelihood of microcracks occurring on the surface of the steel material.

[0015] Silicon (Si) and aluminum (Al) are essential alloying elements for deoxidizing molten steel by precipitating dissolved oxygen in the steel as slag during the steelmaking and continuous casting processes. Therefore, their content in steel (or steel products) must be controlled within certain limits. In particular, when steel is produced using a converter, the silicon content in the steel of the present invention is in the range of 0.1 to 0.4% by weight (hereinafter referred to as %), and the aluminum content is in the range of 0.01 to 0.05%. If silicon and aluminum are added to the steel of the present invention in amounts less than 0.1% and 0.01%, respectively, the amount of dissolved oxygen precipitated during the steelmaking and continuous casting processes is insufficient, making it difficult to expect a deoxidizing effect. On the other hand, if silicon and aluminum are added in amounts exceeding 0.4% and 0.05%, respectively, in the steel of the present invention, the excessive silicon and aluminum may cause the formation of coarse Si-Al composite oxides or the formation of large amounts of coarse island martensite in the microstructure.

[0016] Manganese (Mn) is a useful element that improves the strength of the extra-heavy structural steel material of the present invention through solid solution strengthening and improves the hardenability so that a low-temperature transformation phase is formed, and therefore it must be contained in the steel (or steel material) within a controlled range. The manganese content in the steel of the present invention is in the range of 1.8 to 2.0% by weight (hereinafter referred to as %). If manganese is added to the steel of the present invention in an amount less than 1.8%, it becomes difficult for the steel to meet the yield strength requirement of 460 MPa or more. On the other hand, if manganese is added to the steel of the present invention in an amount greater than 2.0%, the excessive manganese excessively increases the hardenability, thereby promoting the formation of upper bainite and martensite, resulting in a significant decrease in impact toughness and surface NRL-DWT properties.

[0017] Nickel (Ni) is an important element in the extra-heavy structural steel material of the present invention, as it facilitates cross-slip of dislocations at low temperatures to improve impact toughness, hardenability, and strength, and therefore must be contained in the steel (or steel material) within a controlled range. The nickel content in the steel of the present invention is in the range of 0.3 to 0.7% by weight (hereinafter referred to as %). If nickel is added to the steel of the present invention in an amount of less than 0.3%, it becomes difficult to improve the impact toughness and brittle crack propagation resistance of high strength steel having a yield strength of 460 MPa or more. On the other hand, if nickel is added to the steel of the present invention in an amount of more than 0.7%, the excessive nickel excessively increases the hardenability, generates a low-temperature transformation phase, and reduces toughness, which results in an excessive increase in manufacturing costs.

[0018] In the extra-thick structural steel material of the present invention, niobium (Nb) precipitates in the form of NbC or NbCN to improve the strength of the base material, and Nb, which is dissolved in solid solution during reheating at high temperatures, precipitates very finely in the form of NbC during rolling to suppress the recrystallization of austenite and refine the structure, so it is necessary for Nb to be contained in the steel (or steel material) within a controlled range. The niobium content in the steel of the present invention is in the range of 0.015 to 0.04% by weight (hereinafter referred to as %). If niobium is added to the steel of the present invention in an amount less than 0.015%, the amount of NbC or NbCN-like precipitates is too small, which makes it difficult to expect refinement of the microstructure and strengthening of the strength. On the other hand, if niobium is added in an amount greater than 0.04% in the steel of the present invention, the excessive niobium may increase the likelihood of brittle cracks at the corners of the steel, and excessive precipitates may form, reducing toughness.

[0019] Titanium (Ti) is an element that precipitates as TiN when the extra-heavy structural steel material of the present invention is reheated, suppresses the growth of crystal grains in the base material and the weld heat-affected zone, and significantly improves low-temperature toughness, so it must be contained in the steel (or steel material) within a controlled range. The titanium content in the steel of the present invention is in the range of 0.005 to 0.02% by weight (hereinafter referred to as %). If titanium is added to the steel of the present invention in an amount less than 0.005%, the amount of TiN-like precipitates is too small, which poses a problem that it is difficult to expect refinement of the crystal grains in the base metal and the weld heat-affected zone and improvement in toughness. On the other hand, if titanium is added in an amount of more than 0.02% to the steel of the present invention, the excessive titanium causes problems such as clogging of the continuous casting nozzle and reduction in low temperature toughness due to primary precipitation.

[0020] Copper (Cu) improves the hardening ability of the structural steel material of the present invention. Se, solid Melt strengthening occurs steel Cu is the main element that improves the strength of materials, and when tempering is applied, it is the main element that increases the yield strength by forming epsilon (ε) Cu precipitates, so it must be contained in steel (or steel products) within a controlled range. The copper content in the steel of the present invention is in the range of 0.05% or less by weight percent (hereinafter referred to as %). If copper is added in an amount greater than 0.05% in the steel of the present invention, there is a problem that it may cause high temperature embrittlement in the steelmaking process or cause cracks in the slab due to hot shortness.

[0021] A method for producing the steel material of the present invention will be described below. The method for manufacturing a steel material according to the present invention may include the steps of reheating a slab, rough rolling, finish rolling, and cooling, and the detailed conditions for each step are as follows. In the following description of the manufacturing method, unless otherwise specified, the temperature of the hot-rolled steel sheet (slab) means the temperature at a position t / 4 (t: thickness of the steel sheet) from the surface of the hot-rolled steel sheet (slab) in the sheet thickness direction. Furthermore, during water cooling, the position that serves as the reference for measuring the cooling rate is also t / 4 (t: thickness of the steel plate) from the surface of the hot-rolled steel plate (slab) in the plate thickness direction.

[0022] Slab reheating stage: 1,000 to 1,120°C In the method for producing a steel material according to the present invention, the reheating step of the slab is a process for dissolving carbides and / or carbonitrides of Ti and / or Nb formed during the casting process without excessively coarsening the austenite grains, thereby reducing flow stress and facilitating subsequent hot working. In the method for producing a steel material of the present invention, the reheating temperature of the slab may be 1,000 to 1,120°C, and more preferably 1,050 to 1,120°C. If the reheating temperature of the slab is less than 1,000°C, there is a risk that the Ti and / or Nb carbonitrides formed during casting will not be sufficiently solid-dissolved. On the other hand, if the reheating temperature exceeds 1,120°C, the austenite that forms the fine structure at the reheating temperature may become coarse.

[0023] Rough rolling stage: 900 to 1,100°C In the method for producing a steel material of the present invention, the rough rolling stage is a process for destroying cast structures such as dendrites formed during casting and reducing the grain size of crystal grains through coarse austenite recrystallization. Since dynamic recrystallization of austenite must occur during the rough rolling process, the rough rolling temperature is preferably equal to or higher than the temperature (Tnr) at which the recrystallization of austenite stops. Specifically, in the method for producing a steel material of the present invention, the rough rolling temperature is 900 to 1,100°C. If the rough rolling temperature is lower than 900°C, dynamic recrystallization is unlikely to occur during rough rolling, making it difficult to refine the crystal grains. On the other hand, if the rough rolling temperature is higher than 1,100°C, the austenite grains in the slab grow too much before the rough rolling starts, and the grain refinement by dynamic recrystallization is no longer effective. On the other hand, in order to cause recrystallization in the slab by rough rolling and refine the microstructure of the slab, a sufficient amount of deformation that causes recrystallization must be applied to the slab during the rough rolling process. The cumulative rolling reduction in the rough rolling step of the present invention is preferably 40% or more.

[0024] Finishing temperature for finishing rolling: 740°C or less In the method for producing a steel material of the present invention, the finish rolling step is a process for introducing a non-uniform microstructure into the austenitic microstructure of the roughly rolled steel plate. At this time, the finish pass of the finish rolling is preferably carried out at a ferrite formation temperature of 740° C. or less on a t / 4 basis. The finishing temperature range of the finish rolling was set to a temperature range in which rolling was performed around the polygonal ferrite formation temperature, and the grain size of the phase formed during cooling after the finish rolling could be refined. If the finishing pass of the finish rolling is carried out at a temperature higher than 740°C for the t / 4 standard, there is a problem that the strength and toughness decrease as the microstructure becomes coarse.

[0025] The cumulative rolling reduction in the finish rolling step of the present invention is preferably at least 50% or more in order to maximize the formation of a fine structure. Cooling stage after rolling: Cooling at a rate of 3°C / s or more to 720°C or less, then finishing at 500°C or less In the method for producing a steel material of the present invention, the finish-rolled steel plate is preferably cooled from a temperature of 720°C or lower to a temperature of 500°C or lower at a cooling rate of 3°C / s or higher. If the cooling start temperature exceeds 720°C, the formation of polygonal ferrite, a soft phase on the surface, is not promoted, resulting in a problem that the NDTT temperature may become -70°C or higher. If the cooling rate is lower than 3°C / s or the cooling end temperature exceeds 500°C, the microstructure formed in the steel sheet due to phase transformation during the cooling process will not be properly formed, and the final yield strength may become 460 MPa or less.

[0026] The above-described method for producing a steel material according to the present invention can be summarized as follows. A slab containing, by weight, C: 0.05-0.09%, Si: 0.1-0.4%, Al: 0.01-0.05%, Mn: 1.8-2.0%, Ni: 0.3-0.7%, Nb: 0.015-0.040%, Ti: 0.005-0.02%, Cu: 0.05% or less (excluding 0%), with the remainder consisting of Fe and unavoidable impurities, is reheated at a temperature of 1,000-1,120°C, and then subjected to a sintering treatment at 900°C. The extra heavy structural steel material of the present invention having excellent surface NRL-DWT physical properties may be manufactured through a process of rough rolling at a temperature of 1,100°C or less, air-cooling the rolled bar, and after air-cooling, performing finish rolling, followed by finish rolling at 740°C or less for a 1 / 4 ton standard, and after completing the entire rolling, cooling to a temperature of 500°C or less at a cooling rate of 3°C / s or more.

[0027] At this time, the amount of Cu added to the extra-thick steel material, which causes surface cracks, is minimized, and the area from the surface of the plate material to 5 mm directly below the surface is reduced to 1 mm. 2 The number of microcracks per area having a length of 50 μm or more may be 0.1 or less. Therefore, the above-described microstructure and thickness of the extra-heavy steel material of the present invention can only be realized by a controlled combination of the technical features of the manufacturing method as well as the components and composition range of the steel material. As a result, the present invention can secure an extra-heavy structural steel material with excellent NRL-DWT physical properties, such as a yield strength of 460 MPa or more and an NDTT (Nil-Ductility Transition Temperature) value of -70°C or less in the NRL-DWT test according to the ASTM E208 standard.

[0028] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention, as the scope of the present invention is defined by the matters set forth in the claims and matters that can be reasonably inferred therefrom. [Example]

[0029] According to the manufacturing method of the present invention, steel slabs having the compositions shown in Table 1 were selected and subjected to reheating, rolling and cooling. Specifically, a 400 mm thick steel slab having the composition shown in Table 1 below was reheated at a temperature of 1,050 to 1,070°C, and then rough rolling was started at a temperature of 1,030°C or less, followed by continuous rough rolling, which was then completed at a temperature of 930°C or more to produce a bar. After the rough rolling, finish rolling was performed at the cumulative reduction shown in Table 2 to obtain steel sheets having the thicknesses shown in Table 2, which were then cooled to a temperature range of 480 to 390°C at a cooling rate of 3.4 to 5.3°C / sec. [Table 1]

[0030] For the steel materials disclosed in Table 1, the results of evaluating the tensile properties of steel materials manufactured by a manufacturing method according to an embodiment of the present invention and steel materials manufactured under conditions other than those of the manufacturing method according to an embodiment of the present invention, as well as the results of analyzing surface cracks in the manufactured steel plates and the yield strength are summarized in Table 2. The cracks were determined by observing 20 or more different positions in an area of ​​1 mm x 1 mm in a region 5 mm directly below the surface of the steel sheet, and then calculating the average number of cracks of 50 μm or more. Furthermore, the NDTT (Nil-Ductility Transition Temperature) of the manufactured steel sheets was measured by the NRL-DWT test according to the ASTM E208 standard. The results are summarized in Table 2.

[0031] [Table 2]

[0032] In the case of Comparative Example 1, even though the elements and composition ranges satisfy the conditions for the structural extra-heavy steel material according to one embodiment of the present invention, since it was manufactured at a temperature higher than the finishing temperature of the finish rolling proposed in one embodiment of the present invention, ferrite was not sufficiently formed on the surface during air cooling, and the NDTT was measured to be -70°C or higher. In the case of Comparative Examples 2 and 3, the Cu composition range is lower than the upper limit of the Cu composition range proposed in the extra-heavy steel material according to an embodiment of the present invention. Many amount added. As a result, in Comparative Examples 2 and 3, a wide region of hot brittleness was generated due to the high Cu content, increasing the possibility of hot shortness, and numerous microcracks were generated just below the surface of the slab during the slab manufacturing process. The generated fine cracks were elongated during rolling, and in Comparative Examples 2 and 3, the cracks of 50 μm or more were 0.1 pieces / mm 2 The above was generated just below the surface of the steel material, and as a result, the NDTT was measured to be -70°C or higher.

[0033] In the case of Comparative Example 4, an amount of C added was higher than the upper limit of the range of the C composition proposed in the extra-heavy steel material according to an embodiment of the present invention. As a result, a wide area of ​​high-temperature brittleness was generated in Comparative Example 4 due to the high C content, and many fine cracks were generated just below the surface of the slab during the slab manufacturing process. The generated fine cracks elongate during rolling, and the number of cracks of 50 μm or more is 0.1 / mm in Comparative Example 4. 2 The above was generated just below the surface of the steel material, and as a result, the NDTT was measured to be -70°C or higher. In the case of Comparative Example 5, an amount higher than the upper limit of the Mn composition range proposed in the extra-heavy steel material according to an embodiment of the present invention was added. As a result, in Comparative Example 5, due to the high Mn content, a wide region of high-temperature brittleness occurred, and many fine cracks occurred just below the surface of the slab during the slab manufacturing process. The generated fine cracks were elongated during rolling, and in Comparative Example 5, the number of cracks of 50 μm or more was 0.1 / mm 2 The above was generated just below the surface of the steel material, and as a result, the NDTT was measured to be -70°C or higher.

[0034] In the case of Comparative Example 6, the amounts of C and Mn added were lower than the lower limit of the composition range of the extra heavy steel material according to an embodiment of the present invention. As a result, it was determined that Comparative Example 6 did not meet the yield strength of 460 MPa proposed in the present invention due to its low hardenability. In the case of Comparative Example 7, an amount of Ni added was lower than the lower limit of the Ni composition range proposed for the extra heavy steel material according to an embodiment of the present invention. As a result, the NDTT of Comparative Example 7 was measured to be -70°C or higher due to the decrease in toughness caused by the low Ni content.

[0035] In the case of Comparative Example 8, Ti and Nb were added in amounts higher than the upper limit of the composition range proposed for the extra heavy-gauge steel material according to an embodiment of the present invention. As a result, in Comparative Example 8, due to the high Ti and Nb contents, a wide region of high-temperature brittleness occurred, and many fine cracks occurred just below the surface of the slab during the slab manufacturing process. The generated fine cracks were elongated during rolling, and in Comparative Example 8, the number of cracks of 50 μm or more was 0.1 / mm 2 The above is generated just below the surface of the steel material. In addition, in Comparative Example 8, the NDTT was measured to be -70°C or higher due to the increase in strength caused by excessive precipitates, which resulted in the formation of a large amount of high-strength structure in the surface region. On the other hand, as can be seen from the above results, in the case of Examples 1 to 4, which satisfied the composition ranges proposed in the present invention and were produced by finish rolling at a temperature of 740°C or less, the thickness of the steel sheet was 1 mm or less in the region from the surface to 5 mm directly below the surface. 2 The number of microcracks with a length of 50 μm or more was measured to be less than 0.1, the yield strength was measured to be more than 460 MPa, and the NDTT (Nil-Ductility Transition Temperature) value was measured to be less than -70°C according to the NRL-DWT test in accordance with the ASTM E208 standard.

[0036] Although the present invention has been described above with reference to exemplary embodiments, it is obvious that the present invention is not limited to the embodiments and drawings disclosed herein, and that various modifications can be made by those skilled in the art within the scope of the technical concept of the present invention. Furthermore, even if the functions and effects of the configurations of the present invention are not explicitly described while describing the embodiments of the present invention, it is natural that the effects that can be predicted by the configurations should also be recognized.

Claims

1. The alloy contains, by 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: 0.05% or less (excluding 0%), with the remainder consisting of Fe and inevitable impurities, In the region from the surface to 5 mm directly below the surface, 20 or more different positions in an area of ​​1 mm x 1 mm were observed, and the average number of microcracks with a length of 50 μm or more was 0.1 or less, An extra-thick structural steel material characterized in that the NDTT (Nil-Ductility Transition Temperature) value of the surface portion according to the NRL-DWT (Drop Weight Test) test of ASTM E208-06 standard is -70°C or less.

2. 2. The extra-thick structural steel material according to claim 1, characterized in that the plate thickness is 80 to 100 mm and the yield strength is 460 MPa or more.

3. A method for manufacturing the extra-thick structural steel material of claim 1, comprising: a step of reheating a slab containing, by 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: 0.05% or less (excluding 0%), with the remainder consisting of Fe and unavoidable impurities; A step of rough rolling the reheated slab and then finish rolling it at a temperature of 740°C or less at a t / 4 position from the surface; cooling the finish-rolled steel material; A method for manufacturing an extra-thick structural steel material, comprising:

4. 4. The method for manufacturing an extra-thick structural steel material according to claim 3, wherein the reheating temperature of the slab is 1,000 to 1,120°C.

5. 5. The method for manufacturing an extra-thick structural steel material according to claim 3, wherein the rough rolling temperature is 900 to 1,100°C.

6. The method for producing an extra-heavy structural steel material according to any one of claims 3 to 5, characterized in that the cumulative reduction in the finish rolling stage is 50% or more.

7. 7. The method for manufacturing an extra-heavy structural steel material according to claim 3, wherein the cooling rate in the cooling step is 3° C. / sec or more.

8. The method for manufacturing an extra-thick structural steel material according to any one of claims 3 to 7, characterized in that the cooling start temperature in the cooling step is 720°C or less, and the cooling end temperature is 500°C or less.

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