CN116113721A

CN116113721A - Super-thick structural steel material excellent in surface NRL-DWT performance and method for producing same

Info

Publication number: CN116113721A
Application number: CN202180052244.2A
Authority: CN
Inventors: 李学哲
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2020-08-25
Filing date: 2021-08-23
Publication date: 2023-05-12
Also published as: KR102485117B1; WO2022045703A1; JP2023540044A; KR20220026194A

Abstract

The present disclosure relates to a steel material for an ultra-thick structure excellent in surface portion NRL-DWT performance and a method for producing the same, and more particularly to a steel material for an ultra-thick structure comprising, in weight%, C:0.05% -0.09%, si:0.1 to 0.4 percent of Al:0.01% -0.05%, mn:1.8 to 2.0 percent of Ni:0.3% -0.7%, nb:0.015% -0.040%, ti:0.005% -0.02%, cu: more than 0% and 0.05% or less, and the balance of Fe and other unavoidable impurities, and 0.1 or less microcracks having a length of 50 μm or more per square millimeter in a region of 5mm from the surface portion to just below the surface portion.

Description

Super-thick structural steel material excellent in surface NRL-DWT performance and method for producing same

Technical Field

The present disclosure relates to an ultra-thick steel material excellent in surface portion NRL-DWT performance and a method for manufacturing the same.

Background

In recent years, development of ultra-thick high-strength steel has been demanded in designing structures of ships and the like at home and abroad.

When the structure is designed using high-strength steel, the thickness of the plate material can be reduced in addition to economic benefits due to the weight reduction of the structure, so that the convenience of the processing and welding operations can be ensured at the same time.

In general, in the case of high-strength steel, the total reduction in the reduction ratio is reduced when an ultra-thick material is produced, and the entire structure is not sufficiently deformed, so that the microstructure becomes coarse.

In addition, when cooling rapidly to ensure strength, a difference in cooling rate occurs between the surface portion and the center portion due to the thick thickness of the super-thick material.

Therefore, coarse low-temperature transformation phases such as bainite are generated at the surface portion, and it is difficult to secure toughness of the ultra-thick material.

In particular, when applied to a main structure of a ship or the like, there is an increasing demand for ensuring brittle crack growth resistance representing structural stability.

In the case of an ultra-thick material, there is a great difficulty in ensuring the brittle crack propagation resistance due to the decrease in toughness caused by the difference in cooling rate between the surface portion and the center portion.

In practice, many classification companies and iron and steel companies have been conducting large-scale tensile tests that can accurately evaluate the actual brittle crack growth resistance to ensure brittle crack growth resistance.

However, in the case of a large-scale tensile test, a large amount of cost is incurred for the test, and thus it is difficult to ensure that the test is applied to mass production.

In order to improve such inconveniences, research into a small-scale substitution test that can substitute for a large-scale tensile test has been conducted steadily recently.

Of the small-scale substitution tests, the surface portion NRL-DWT (Naval Research Laboratory-Drop Weight Test ) Test of ASTM E208-06, which is the most powerful Test, is adopted by many classification companies and iron and steel companies.

The NRL-DWT test using the surface portion was based on the following study results: in the case of controlling the microstructure of the surface portion based on the conventional studies, the propagation speed of the crack is reduced at the time of brittle crack propagation, so that the brittle crack propagation resistance becomes excellent.

However, in the surface portion NRL-DWT test, when steel is collected from the sample surface, the original surface of the plate is used without chamfering (chamfer).

If surface cracks (crack) that easily cause brittle cracks are present in the specimen, the result of the NRL-DWT test, that is, the result of which the NDTT (Nil-Ductility Transition Temperature, zero ductile transition temperature) value is easily deteriorated, is obtained.

Therefore, a solution capable of suppressing surface portion cracks is required.

Disclosure of Invention

Technical problem

The present disclosure is directed to providing an ultra-thick steel material excellent in surface portion NRL-DWT performance and a method of manufacturing the same, which can solve the above-described problems of the prior art.

Specifically, the present disclosure aims to provide an ultra-thick steel material and a method for producing the same, which have excellent NRL-DWT performance by controlling the alloy composition to suppress surface portion microcracks of the ultra-thick steel material without containing expensive alloy elements in terms of composition.

Further, the present disclosure is directed to an ultra-thick steel material and a method of manufacturing the same, which suppresses surface portion microcracks by controlling rolling temperature and rolling reduction at the time of rolling, thereby having excellent surface portion NRL-DWT performance.

Further, the present disclosure is directed to provide a steel material for ultra-thick structures excellent in NRL-DWT performance, and more specifically, a steel material having a yield strength of 460MPa or more and a thickness of 80mm or more and 100mm or less, which has a NDTT (Nil-Ductility Transition Temperature) value of-70 ℃ or less based on an NRL-DWT test according to ASTM E208, by minimizing the amount of Cu added to cause surface cracks, wherein the number of fine cracks per square millimeter in a region from a plate surface portion to 5mm directly below is 50 μm or more, and a method for producing the same.

The objects of the present disclosure are not limited to the above objects, other objects and advantages of the present disclosure will be understood through the following description, and the objects and advantages of the present disclosure will be more clearly understood through the examples. Further, it is apparent that the objects and advantages of the present disclosure can be achieved by means described in the claims and combinations thereof.

Technical proposal

To achieve the above object, according to one embodiment of the present disclosure, which is embodied, a steel material for ultra-thick structures, the steel material comprising, in weight%, C:0.05% -0.09%, si:0.1 to 0.4 percent of Al:0.01% -0.05%, mn:1.8 to 2.0 percent of Ni:0.3% -0.7%, nb:0.015% -0.040%, ti:0.005% -0.02%, cu: more than 0% and 0.05% or less, the balance being Fe and other unavoidable impurities, and may have a microstructure in which microcracks of 50 μm or more per square millimeter in a region of 5mm from the surface portion to just below the surface portion are 0.1 or less.

Preferably, the NDTT (Nil-Ductility Transition Temperature) value of the surface portion NRL-DWT (Drop Weight Test) test based on ASTM E208-06 standard may be below-70 ℃.

Preferably, the plate thickness may be 80mm to 100mm, and the yield strength may be 460MPa or more.

A method for manufacturing a steel material for an ultra-thick structure according to an embodiment of the present disclosure to achieve the above object may be a manufacturing method including the steps of: the final finish rolling is carried out at a temperature of 740 ℃ or lower from the surface of the slab to the t/4 position.

In order to achieve the above object, a method for manufacturing a steel material for an ultra-thick structure according to an embodiment of the present disclosure may include the steps of: reheating a slab comprising, in weight%, C:0.05% -0.09%, si:0.1 to 0.4 percent of Al:0.01% -0.05%, mn:1.8 to 2.0 percent of Ni:0.3% -0.7%, nb:0.015% -0.040%, ti:0.005% -0.02%, cu: more than 0% and less than 0.05%, the balance being Fe and other unavoidable impurities; after rough rolling the reheated slab, performing final finish rolling at a temperature of 740 ℃ or lower from the surface of the slab to a t/4 position; and cooling the finish rolled steel.

Preferably, the slab reheating temperature may be 1000 ℃ to 1120 ℃.

Preferably, the rough rolling temperature may be 900 ℃ to 1100 ℃.

Preferably, the cumulative rolling reduction at finish rolling may be 50% or more.

Preferably, the cooling rate in the cooling step may be 3 ℃/sec or more.

Preferably, the cooling start temperature in the cooling step may be 720 ℃ or less, and the cooling end temperature may be 500 ℃ or less.

Effects of the invention

According to the present disclosure, an ultra-thick steel material excellent in surface portion NRL-DWT performance can be realized by controlling the composition and microstructure without excessively containing expensive alloy elements.

According to the present disclosure, by controlling the composition and composition ranges, finish rolling temperature, and cumulative rolling reduction so as to maximize the austenitic structure deformation amount at the surface portion and t/4, a method for producing an ultra-thick steel excellent in NRL-DWT performance of the surface portion can be realized, which has 0.1 or less microcracks of 50 μm or more per square millimeter in a region of 5mm from the surface portion to just below the surface portion.

According to the present disclosure, it is possible to realize an ultra-thick structural steel material excellent in NRL-DWT performance 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 ℃ or less based on an NRL-DWT test according to ASTM E208, and a method for producing the same.

In addition to the effects described above, specific effects of the present disclosure are described in describing specific contents for implementing the present disclosure.

Detailed Description

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings so as to be easily implemented by those of ordinary skill in the art to which the present disclosure pertains. The present disclosure can be implemented in a variety of different ways, not limited to the embodiments described herein.

For clarity of description of the present disclosure, parts irrelevant to the description are omitted, and the same reference numerals denote the same or similar constituent elements throughout the specification. Further, some embodiments of the present disclosure are described in detail with reference to the exemplary drawings. When reference numerals are given to constituent elements in the drawings, the same constituent elements are denoted by the same reference numerals even in different drawings. In addition, in describing the present disclosure, if it is determined that detailed description of related known configurations or functions may obscure the gist of the present disclosure, the detailed description may be omitted.

In describing the constituent elements of the present disclosure, terms such as first, second, A, B, (a), (b), and the like may be used. These terms are only used to distinguish one element from another element, and the nature, order or number of the elements is not limited by these terms. When a certain component is described as being "connected", "coupled" or "connected" to another component, the component may be directly connected or connected to the other component, but it is also understood that other components exist between the components, or that the components may be "connected", "coupled" or "connected" by the other components.

The present disclosure is directed to a steel material for ultra-thick structures excellent in NRL-DWT performance and a method for manufacturing the same, specifically, the steel material having a yield strength of 460MPa or more, a thickness of 80mm or more and 100mm or less, and a NDTT (Nil-Ductility Transition Temperature) value of-70 ℃ or less based on an NRL-DWT test according to ASTM E208, by minimizing an addition amount of Cu causing surface cracks, so that the number of microcracks of 50 μm or more per square millimeter in a region from a plate surface portion to 5mm directly below is 0.1 or less.

In order to satisfy the above-described characteristics, the steel material for ultra-thick structures according to one embodiment of the present disclosure may specifically contain the following alloying elements to satisfy the above-described characteristics of excellent NRL-DWT performance.

Unless otherwise indicated, the contents or component ranges of the following ingredients are in weight percent.

Carbon (C) is the most important element for ensuring basic strength in the steel material for ultra-thick structures of the present disclosure, and thus needs to be contained in the steel (or steel material) in a controlled range.

In the steel according to one embodiment of the present disclosure, the carbon content is in the range of 0.05% to 0.09% in weight% (hereinafter expressed as%).

If the carbon content in the steel of one embodiment of the present disclosure is less than 0.05%, the strength of the steel may be lowered, which has a problem in that it is difficult to achieve the strength target.

On the other hand, if the addition amount of carbon in the steel of one embodiment of the present disclosure is more than 0.09%, the excessive carbon improves hardenability, thereby generating a large amount of bulk martensite (massive martensite) and promoting the generation of a low-temperature transformation phase, with the result that there is a problem of reduced toughness of the steel.

Further, if the addition amount of carbon in the steel of one embodiment of the present disclosure is more than 0.09%, there is a problem in that the possibility of micro-cracks occurring on the surface of the steel material becomes high due to entering into the sub-peritectic zone (hypo-peritectic region) where surface cracks are liable to occur.

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

In particular, when a 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% in terms of weight% (hereinafter, expressed as%).

If the addition amounts of silicon and aluminum in 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 deoxidizing effect due to insufficient precipitation amounts of dissolved oxygen during the steelmaking and continuous casting processes.

On the other hand, if the addition amounts of silicon and aluminum in the steel of one embodiment of the present disclosure are more than 0.4% and 0.05%, respectively, there is a problem in that excessive silicon and aluminum may cause the formation of coarse Si, al composite oxides or massive martensite in a microstructure to be largely formed.

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

In the steel according to one embodiment of the present disclosure, the manganese content is in the range of 1.8% to 2.0% in weight% (hereinafter expressed as%).

If the addition amount of manganese in 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 460MPa or more of the steel.

On the other hand, if the addition amount of manganese in the steel of one embodiment of the present disclosure is more than 2.0%, the excessive manganese causes an excessive increase in hardenability, thereby causing promotion of upper bainitic (upper bainite) and martensitic generation, which has a problem in that impact toughness and surface portion NRL-DWT properties are greatly reduced.

Nickel (Ni) is an important element in the steel material for ultra-thick structures of the present disclosure that promotes cross slip (cross slip) of dislocations at low temperatures to improve impact toughness and hardenability to improve strength, and thus needs to be contained in the steel (or steel material) in a controlled range.

In the steel according to one embodiment of the present disclosure, the nickel content is in the range of 0.3% to 0.7% in weight% (hereinafter expressed as%).

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 in that it is difficult to improve impact toughness and brittle crack growth resistance in high strength steel having a yield strength of 460MPa or more.

On the other hand, if the addition amount of nickel in the steel of one embodiment of the present disclosure is more than 0.7%, there are problems in that excessive nickel excessively increases hardenability, resulting in generation of a low-temperature transformation phase to lower toughness, and in that manufacturing costs excessively increase.

Niobium (Nb) is precipitated as NbC or NbCN in the steel material for ultra-thick structure of the present disclosure to increase the strength of the base material, and Nb that is solid-dissolved at the time of reheating at high temperature is precipitated very finely as NbC during rolling to suppress recrystallization of austenite, so that the structure is refined, and thus needs to be contained in the steel (or steel material) in a controlled range.

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% (hereinafter expressed as%).

If the amount of niobium added to the steel according to one embodiment of the present disclosure is less than 0.015%, there is a problem in that fine structure refinement and strength enhancement are difficult to expect because the amount of precipitates in the form of NbC or NbCN is too small.

On the other hand, if the amount of niobium added in the steel of one embodiment of the present disclosure is more than 0.04%, the possibility that excessive niobium causes brittle cracks to occur at the edges of the steel becomes high, and there is a problem in that excessive precipitate formation may cause deterioration in toughness.

Titanium (Ti) precipitates as TiN upon reheating in the steel for ultra-thick structures 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 contained in the steel (or steel) in a controlled range.

In the steel according to one embodiment of the present disclosure, the titanium content is in the range of 0.005% to 0.02% in weight% (hereinafter expressed as%).

If the addition amount of titanium in 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 metal and the weld heat affected zone due to the excessively small precipitation amount of precipitates in the form of TiN.

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

Copper (Cu) is a main element that improves hardenability and strength of the steel by causing solid solution strengthening in the steel for ultra-thick structure of the present disclosure, and also is a main element that increases yield strength by generating epsilon-Cu precipitates at the time of tempering (tempering), and thus needs to be contained in the steel (or steel) in a controlled range.

In the steel according to one embodiment of the present disclosure, the copper content is in the range of 0.05% or less in weight% (hereinafter expressed as%).

If the amount of copper added to the steel of one embodiment of the present disclosure is more than 0.05%, there is a problem in that high temperature brittleness is caused or hot shortness (hot shortness) is likely to be generated in the steelmaking process to cause cracking of the slab.

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

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

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

In addition, the reference position for measuring the cooling rate at the time of water cooling is also a position t/4 (t: thickness of the steel sheet) in the thickness direction from the surface plate of the hot rolled steel sheet (slab).

And (3) slab reheating step: 1000-1120 DEG C

In the manufacturing method of the steel material according to one embodiment of the present disclosure, the slab reheating step is a process of solutionizing Ti and/or Nb carbide and/or carbonitride formed during casting without excessive coarsening of austenite grains and reducing a rheological stress (flow stress) to facilitate subsequent hot working.

In the manufacturing method of the steel material according to one embodiment of the present disclosure, the slab reheating temperature may be 1000 to 1120 ℃, more preferably 1050 to 1120 ℃.

If the slab reheating temperature is less than 1000 ℃, there is a fear that the Ti and/or Nb carbonitrides formed in casting will not be sufficiently solid-dissolved.

On the other hand, if the reheating temperature is higher than 1120 ℃, austenite forming a microstructure at the reheating temperature may coarsen.

Rough rolling: 900-1100 DEG C

In the method for manufacturing a steel material according to one embodiment of the present disclosure, the rough rolling step is a process for breaking up a cast structure such as dendrites formed during casting and reducing the grain size of coarse austenite grains by recrystallization.

Since dynamic recrystallization of austenite must occur during rough rolling (dynamic recrystallization), the rough rolling temperature is preferably at or above the temperature at which austenite recrystallization stops (Tnr).

Specifically, in the manufacturing method of the steel material according to one embodiment of the present disclosure, the rough rolling temperature is 900 to 1100 ℃.

If the rough rolling temperature is less than 900 ℃, dynamic recrystallization is difficult to occur during rough rolling, and there is a problem in that grain refinement is difficult.

On the other hand, if the rough rolling temperature is higher than 1100 ℃, there is a problem in that the grains cannot be effectively refined even by dynamic recrystallization because austenite grains in the slab are overgrown before the rough rolling starts.

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

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

Finish finishing temperature: 740 ℃ below

In the manufacturing method of the steel material according to one embodiment of the present disclosure, the finish rolling step is a process for introducing an uneven microstructure into an austenitic microstructure of the steel sheet after rough rolling.

In this case, the finish rolling finishing pass (pass) is preferably performed at a ferrite formation temperature of 740 ℃ or lower based on t/4.

The finish rolling end temperature range is set to a temperature range in which the grain size of the phase generated in the post-finish cooling process can be thinned by rolling in the vicinity of the polygonal ferrite generation temperature.

If the finishing pass is performed at a temperature higher than 740℃in terms of t/4, there is a problem that strength and toughness are lowered due to coarsening of the microstructure.

According to one embodiment of the present disclosure, the cumulative reduction in the finish rolling process is preferably at least 50% or more in order to maximize the formation of fine structures.

And (3) a cooling step after rolling: after cooling at a cooling rate of 3 ℃/sec or more at a temperature of 720 ℃ or less, cooling is terminated at a temperature of 500 ℃ or less.

In the method for manufacturing a steel product according to one embodiment of the present disclosure, the finish rolled steel sheet is preferably cooled from a temperature of 720 ℃ or less to a temperature of 500 ℃ or less at a cooling rate of 3 ℃/sec or more.

If the cooling start temperature is higher than 720 ℃, the generation of polygonal ferrite (polygonal ferrite) as a soft phase at the surface portion is not promoted, and there is a problem that the NDTT temperature may be-70 ℃ or higher.

If the cooling rate is less than 3 ℃/sec or the cooling end temperature is more than 500 ℃, the microstructure formed in the steel sheet by the phase transition during cooling may not be properly formed, and the final yield strength may be 460MPa or less.

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

An ultra-thick steel for a structure having excellent NRL-DWT performance of a surface portion according to an embodiment of the present disclosure can be manufactured by: after reheating the slab to a temperature of 1000 ℃ to 1120 ℃, rough rolling is performed at a temperature of 900 ℃ to 1100 ℃, the slab comprising, in weight percent, C:0.05% -0.09%, si:0.1 to 0.4 percent of Al:0.01% -0.05%, mn:1.8 to 2.0 percent of Ni:0.3% -0.7%, nb:0.015% -0.040%, ti:0.005% -0.02%, cu: more than 0% and less than 0.05%, the balance being Fe and other unavoidable impurities; air cooling the rolled rough rolled blank (Bar); finish rolling is carried out after air cooling is finished, and then finish rolling is carried out at a temperature below 740 ℃ with 1/4t as a standard; and cooling to a temperature of 500 ℃ or below at a cooling rate of 3 ℃/sec or more after the completion of the entire rolling.

At this time, according to the ultra-thick steel material of one embodiment of the present disclosure, by minimizing the addition amount of Cu causing surface cracks, it is possible to make 0.1 or less of micro cracks having a length of 50 μm or more per square millimeter in a region of 5mm from the surface of the plate material to just below the surface portion.

Thus, the above-described microstructure and thickness of the ultra-thick steel material according to one embodiment of the present disclosure can be achieved only by a combination of the composition and component ranges of the steel material and the technical characteristics of the manufacturing method being controlled.

Thus, in the present disclosure, it is possible to ensure an ultra-thick structural steel material having a yield strength of 460MPa or more and excellent NRL-DWT performance, which has an NDTT (Nil-Ductility Transition Temperature) value of-70 ℃ or less, based on the NRL-DWT test according to ASTM E208.

Hereinafter, the present disclosure will be described more specifically by examples. It should be noted, however, that the following examples are provided by way of example only to describe the present disclosure and are not intended to limit the scope of the claims of the present disclosure. Since the scope of the claims of the present disclosure depends on what is recited in the claims and what is reasonably derived therefrom.

Examples (example)

Billets having the compositions shown in table 1 were selected for reheating, rolling and cooling by the manufacturing method in the present disclosure.

Specifically, after reheating a billet having a thickness of 400mm and having the composition of table 1 below to a temperature of 1050 ℃ to 1070 ℃, rough rolling was continuously performed after starting rough rolling at a temperature of 1030 ℃ or less, and then rough rolling was ended at a temperature of 930 ℃ or more to produce a rough rolled billet.

After the rough rolling, finish rolling was performed at the cumulative rolling reduction shown in table 2 to obtain steel sheets having the thickness of table 2, and then cooled to a temperature ranging from 480 to 390 ℃ at a cooling rate of 3.4 to 5.3 ℃/sec.

TABLE 1

For the steels shown in table 1, the tensile characteristic evaluation results, the surface portion crack analysis results, and the yield strength of the steel sheets manufactured by the manufacturing method according to one embodiment of the present disclosure and the steels manufactured using conditions exceeding the manufacturing method according to one embodiment of the present disclosure are summarized in table 2.

Regarding the cracks, after observing 20 or more different positions in an area of 1mm x 1mm in a region of 5mm just below the surface of the steel sheet, the number of cracks was obtained from an average value of the number of cracks measured by 50 μm or more.

In addition, NDTT (Nil-Ductility Transition Temperature) based on NRL-DWT test according to ASTM E208 was measured on the steel sheet produced, and the results thereof are summarized in Table 2.

TABLE 2

In the case of comparative example 1, although the composition and composition ranges satisfy the conditions for the ultra-thick steel material according to one embodiment of the present disclosure, NDTT was measured to be-70 ℃ or higher because ferrite was not sufficiently generated at the surface portion during air cooling due to manufacturing at a temperature of the finish rolling end temperature or higher given in one embodiment of the present disclosure.

In the case of comparative examples 2 and 3, an amount higher than the upper limit of the Cu component range given in the ultra-thick steel material according to one embodiment of the present disclosure was added.

Therefore, comparative examples 2 and 3 generate a wide high temperature brittle region due to a high Cu content, and the possibility of thermal embrittlement (Hot shortness) becomes high, so that a large number of microcracks are generated just under the surface of the slab in the slab manufacturing process.

Since the generated microcracks elongated during rolling, comparative examples 2 and 3 produced 0.1 pieces/mm at the portion just below the surface of the steel material ² The above cracks having a length of 50 μm or more, and thus the NDTT was measured to be-70℃or more.

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 was added.

Therefore, comparative example 4 also produced a wide high-temperature brittle region due to the high C content, and also produced a large number of microcracks just below the slab surface in the slab manufacturing process.

Since the generated microcracks elongated during rolling, comparative example 4 also produced 0.1 pieces/mm in the portion immediately below the surface of the steel material ² The above cracks having a length of 50 μm or more, and thus the NDTT was measured to be-70℃or more.

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 was added.

Therefore, comparative example 5 produced a wide high temperature brittle region due to high Mn content, and a large number of microcracks were generated just below the surface of the slab in the slab manufacturing process.

As the generated microcrack was elongated during rolling, comparative example 5 produced 0.1 pieces/mm at a portion just below the surface of the steel material ² The above cracks having a length of 50 μm or more, and thus the NDTT was measured to be-70℃or more.

In the case of comparative example 6, lower amounts than the lower limits of the ranges of the C and Mn components given in the ultra-thick steel material according to one embodiment of the present disclosure were added.

Therefore, it was determined that comparative example 6 could not meet the yield strength of 460MPa given in the present disclosure due to low hardenability.

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 was added.

Therefore, the decrease in toughness is caused by the low Ni content, and the NDTT of comparative example 7 was measured to be at least-70 ℃.

In the case of comparative example 8, an amount higher than the upper limit of the Ti and Nb component ranges given in the ultra-thick steel material according to one embodiment of the present disclosure was added.

Therefore, comparative example 8 produced a wide high temperature brittle region due to high Ti and Nb contents, and a large number of microcracks were generated just below the slab surface in the slab manufacturing process.

Since the generated micro cracks were elongated during rolling, the cracks of 50 μm or more in length were generated in the portion just below the surface of the steel material in comparative example 8 by 0.1 pieces/mm 2 or more.

In comparative example 8, since the strength was increased by excessive precipitation and a large amount of high-strength tissue was formed on the surface, it was found that NDTT was not lower than-70 ℃.

On the other hand, as is clear from the above results, in the cases of inventive examples 1 to 4 produced by satisfying the composition ranges given in the present disclosure and performing finish rolling at a temperature of 740 ℃ or lower, the number of microcracks of 50 μm or more per square millimeter in the region from the surface portion to 5mm directly below the surface portion was 0.1 or less, the yield strength was 460MPa or more, and the NDTT (Nil-Ductility Transition Temperature) value based on the NRL-DWT test according to ASTM E208 standard was measured to be-70 ℃ or lower.

As described above, the present disclosure has been described with reference to the exemplary embodiments, but the present disclosure is not limited to the embodiments and drawings described in the present specification. It is apparent that various modifications can be made by the ordinary skilled person within the scope of the technical idea of the present disclosure. In addition, even if the operational effects of the technical features according to the present disclosure are not explicitly described when the embodiments of the present disclosure are described, effects that can be expected from the technical features should be confirmed.

Claims

1. A steel material for ultra-thick structures, wherein,

the steel comprises, in wt%, C:0.05% -0.09%, si:0.1 to 0.4 percent of Al:0.01% -0.05%, mn:1.8 to 2.0 percent of Ni:0.3% -0.7%, nb:0.015% -0.040%, ti:0.005% -0.02%, cu: more than 0% and less than 0.05%, and the balance of Fe and other unavoidable impurities,

and 0.1 or less microcracks having a length of 50 μm or more per square millimeter in a region of 5mm from the surface portion to just below the surface portion.

2. The steel material for ultra-thick structure according to claim 1, wherein,

the steel has an NDTT value of-70 ℃ or lower in a surface NRL-DWT test based on ASTM E208-06 standard.

3. The ultra-thick high-strength steel material according to claim 1 or 2, wherein,

the thickness of the steel is 80-100 mm, and the yield strength is above 460MPa.

4. A method for manufacturing an ultra-thick high strength steel material, comprising the steps of:

reheating a slab comprising, in weight%, C:0.05% -0.09%, si:0.1 to 0.4 percent of Al:0.01% -0.05%, mn:1.8 to 2.0 percent of Ni:0.3% -0.7%, nb:0.015% -0.040%, ti:0.005% -0.02%, cu: more than 0% and less than 0.05%, the balance being Fe and other unavoidable impurities;

after rough rolling the reheated slab, final finish rolling is performed at a temperature of 740 ℃ or less from the surface to the t/4 position;

and cooling the finish rolled steel.

5. The method for producing an ultra-thick high-strength steel material according to claim 4, wherein the reheating temperature of said slab is 1000 ℃ to 1120 ℃.

6. The method for producing an ultra-thick high-strength steel material according to claim 4, wherein,

the rough rolling temperature is 900-1100 ℃.

7. The method for producing an ultra-thick high-strength steel material according to claim 4, wherein,

the cumulative rolling reduction in the finish rolling step is 50% or more.

8. The method for producing an ultra-thick high-strength steel material according to claim 4, wherein,

the cooling rate in the cooling step is 3 ℃/sec or more.

9. The method for producing an ultra-thick high-strength steel material according to claim 8, wherein,

the cooling start temperature in the cooling step is 720 ℃ or lower, and the cooling end temperature is 500 ℃ or lower.