CN116096933A - Super-thick structural steel material excellent in surface NRL-DWT performance and method for producing same - Google Patents

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 2.1 percent of Ni:0.3% -1.0%, nb:0.005% -0.040%, ti:0.005% -0.03%, cu:0.1% -0.5%, P:100ppm or less, S: the fraction of polygonal ferrite from the surface portion to 5mm just below the surface portion is 50% or more, and the fraction has a microstructure having a t/4 microstructure having a large-angle grain boundary of 15 degrees or more and a grain size of 15 μm or less as measured by EBSD.

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 (Drop Weight Test) Test of ASTM E208-06 is adopted by many classification companies and iron and steel companies as the most powerful Test.

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.

In order to improve NRL-DWT performance, various techniques for surface portion granularity refinement have been tried.

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

However, the above-described attempts have a problem in that the productivity is greatly lowered when the technology itself is applied to a general mass production system.

In addition, when a large amount of an element such as Ni which contributes to improvement of toughness is added, the surface portion NRL-DWT performance can be improved.

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

At the same time, in order to enhance the stability of oversized container ships, the criteria for enhancing resistance to brittle crack growth are coming into effect.

Under the condition of ensuring the existing BCA performance (brittle crack arrest, brittle crack stopping performance), the thickness is defined as Kca being equal to or more than 6000 below 80t, and the thickness of 80t or more is regulated to be negotiated with a class agency in the international class standard.

Since the steel with the thickness of more than 80t has no practical structural test result in the past, the steel with the thickness of more than 80t is also defined as Kca not less than 6000 and is the same as the steel with the thickness of less than 80 t.

However, recently, as a result of performing an actual structural test in japan, the following study results have been reported: only when 80t or more steel is used in the hatch edge coaming (hatch side coaming) of the ship, kca is more than or equal to 8000, and the crack is prevented.

According to the above-mentioned research results, the international ship standard is about to be changed, so that a new steel product having Kca of not less than 8000 is required.

Since the Kca assurance value is increased and the probability that the assurance temperature of the small-scale substitution test reaches below-70 ℃ which is stronger than the existing-60 ℃ becomes high, it is necessary to develop a steel material which can ensure assurance which is stronger than the existing ones.

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 suppresses generation of coarse low-temperature transformation phases in the surface portion of the ultra-thick steel material by controlling the alloy composition without containing expensive alloy elements in terms of composition, thereby having excellent surface portion NRL-DWT properties.

Further, the present disclosure is directed to an ultra-thick steel material and a method of manufacturing the same, which have excellent surface portion NRL-DWT properties by imparting maximum deformation to a surface austenitic structure, maximizing polygonal ferrite fraction from the surface portion to 5mm directly below the surface portion, and maximizing refinement of the microstructure by controlling rolling temperature and maximum rolling reduction at rough rolling and finish rolling.

Further, the present disclosure is directed to provide a steel material for an ultra-thick structure excellent in NRL-DWT performance, more specifically, a steel material having a thickness of 80mm or more and 100mm or less, a fraction of polygonal ferrite from a surface portion to 5mm directly below the surface portion of 50% or more, a grain size (grain size) of t/4 microstructure having a large-angle grain boundary of 15 degrees or more as measured by EBSD of 15 μm or less, a yield strength of 460MPa or more, an NDTT (Nil-Ductility Transition Temperature) value of-70 ℃ or less based on an NRL-DWT test according to ASTM E208 standard, and a Kca value of 8000 or more as obtained by performing an esco test, 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 2.1 percent of Ni:0.3% -1.0%, nb:0.005% -0.040%, ti:0.005% -0.03%, cu:0.1% -0.5%, P:100ppm or less, S: the fraction of polygonal ferrite from the surface portion to 5mm just below the surface portion is 50% or more, and the Fe and other unavoidable impurities in the balance of 40ppm or less may have a microstructure having a grain size of 15 μm or less of t/4 microstructure having a large-angle grain boundary of 15 degrees or more as measured by EBSD.

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 Kca value based on the ESSO test may be 8000 or more.

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: and performing finish rolling at 720-740 ℃ 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 2.1 percent of Ni:0.3% -1.0%, nb:0.005% -0.040%, ti:0.005% -0.03%, cu:0.1% -0.5%, P:100ppm or less, S: less than 40ppm Fe and other unavoidable impurities in balance; after rough rolling is carried out on the reheated slab, finish rolling is carried out at the temperature ranging from 720 ℃ to 740 ℃ from the surface of the slab to the 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 the time of rough rolling may be 50% or more.

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

Preferably, the cooling end temperature in the cooling step 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 finish rolling temperature and the cumulative rolling reduction so that the deformation amount of the austenite structure at the surface portion and t/4 is maximized, the grain size of the t/4 microstructure having a large-angle grain boundary of 15 degrees or more as measured by EBSD is controlled to 15 μm or less, and the polygonal ferrite fraction from the surface portion to 5mm directly below the surface portion is maximized, whereby a method for producing an ultra-thick steel excellent in NRL-DWT performance of the surface portion can be realized.

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, an NDTT (Nil-Ductility Transition Temperature) value of-70 ℃ or less based on an NRL-DWT test according to ASTM E208, and a Kca value of 8000 or more obtained by performing an ESSO test, 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 producing the same, wherein the fraction of polygonal ferrite from a surface portion to a position 5mm below the surface portion is 50% or more, the grain size of 1/4t microstructure having a large-angle grain boundary of 15 degrees or more as measured by EBSD is 15 [ mu ] m or less, the thickness is 80mm or more and 100mm or less, the NDTT (Nil-Ductility Transition Temperature) value based on an NRL-DWT test according to ASTM E208 standard is-70 ℃ or less, and the Kca value obtained by performing an ESSO test is 8000 or more.

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.

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 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 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 2.1% 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 2.1%, the excessive manganese causes an excessive increase in hardenability, thereby causing promotion of upper bainitic (upper bainite) and martensitic generation, which has a problem 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 1.0% 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 1.0%, 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 0.005% to 0.04% in weight% (hereinafter expressed as%).

If the amount of niobium added to the steel according to one embodiment of the present disclosure is less than 0.005%, there is a problem 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%, excessive niobium causes a problem that the possibility of brittle cracks occurring at the edges of the steel becomes high.

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 0.03% 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 0.03%, 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 0.1% to 0.5% in weight% (hereinafter expressed as%).

If the addition amount of copper in the steel of one embodiment of the present disclosure is less than 0.1%, there is a problem in that it is difficult to expect an improvement in hardenability upon cooling and an improvement in strength.

On the other hand, if the addition amount of copper in the steel of one embodiment of the present disclosure is more than 0.5%, there is a problem in that hot shortness (hot shortness) may occur in the steelmaking process to cause slab cracking.

Phosphorus (P) and sulfur (S) are elements that cause brittleness due to grain boundary brittleness or coarse inclusion formation in the steel for ultra-thick structure of the present disclosure, and therefore it is necessary to minimize the content in the steel (or steel) within a controlled range.

Thus, in the steel according to one embodiment of the present disclosure, phosphorus and sulfur are limited to 100ppm or less and 40ppm or less, respectively, in weight% (hereinafter expressed as%).

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:

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:

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 grains by recrystallization of coarse austenite.

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.

And (3) finish rolling:

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 temperature is preferably set to be between 720 ℃ and 740 ℃ in terms of t/4.

The finish rolling finishing temperature range is set to a temperature range in which fine air-cooled ferrite can be promoted to be formed in the surface portion in the air-cooling step before water cooling after finish rolling by rolling in the vicinity of the polygonal ferrite forming temperature.

If finish rolling is performed at 720 ℃ or lower based on t/4, extended ferrite is generated before ferrite precipitation during finish rolling, and thus there is a problem in that the NDTT temperature increases.

On the other hand, if finish rolling is performed at a temperature of 740 ℃ or higher based on t/4, the grain size at t/4 may be decreased due to insufficient deformation, resulting in a decrease in Kca performance, and there is a problem in that the NDTT temperature increases due to insufficient ferrite generated at the surface portion.

According to one embodiment of the present disclosure, the cumulative reduction in the finish rolling process is preferably 50% or more.

And (3) a cooling step:

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

If the cooling rate is less than 3 ℃/sec or the cooling end temperature is 500 ℃ or more, the microstructure formed in the steel sheet due to phase transformation 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 2.1 percent of Ni:0.3% -1.0%, nb:0.005% -0.040%, ti:0.005% -0.03%, cu:0.1% -0.5%, P:100ppm or less, S: less than 40ppm Fe and other unavoidable impurities in balance; performing finish rolling on the rolled rough rolled blank at the temperature of 720-740 ℃ 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 an embodiment of the present disclosure, the fraction of polygonal ferrite from the surface portion of the steel material to a position 5mm below the surface portion is 50% or more, and has a microstructure having a grain size of 15 μm or less of t/4 microstructure having a large angle grain boundary of 15 degrees or more as measured by EBSD, and the thickness may be 80mm to 100mm.

The microstructure in which the fraction of polygonal ferrite from the surface portion to 5mm directly below the surface portion is 50% or more and the grain size of t/4 microstructure having a large-angle grain boundary of 15 degrees or more as measured by EBSD is 15 μm or less is realized during the finish rolling process and the cooling process.

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 microstructure of the steel material and the technical characteristics of the manufacturing method being controlled.

Thus, in the present disclosure, it is possible to ensure a steel product having a yield strength of 460MPa or more, a surface impact transition temperature of-40 ℃ or less, an NDTT (Nil-Ductility Transition Temperature) value of-70 ℃ or less based on an NRL-DWT test according to ASTM E208, and a Kca value of 8000 or more obtained by performing an ESSO test.

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 1070 ℃, rough rolling was continuously performed after starting rough rolling at a temperature of 1030 ℃ or less to finish rough rolling at a temperature of 930 ℃ or more to obtain 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 450 to 370 ℃ at a cooling rate of 3.5 to 5.2 ℃/sec.

TABLE 1

For the steels shown in table 1, the tensile characteristic evaluation results, the microstructure analysis results, and the yield strength of the manufactured steel sheet of the steels 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.

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

Further, for the grain size analysis of the manufactured steel sheet, measurement was performed using an electron back scattering diffractometer (electron backscatter diffraction, EBSD) widely used in the technical field to which the present disclosure pertains.

The EBSD is a method of analyzing the orientation of a material by detecting electrons reflected (i.e., backscattered) by a sample (i.e., a steel plate) when the electrons are injected into the sample.

In particular, the EBSD can be phase analyzed by analyzing the grain orientation of a material, and further can analyze a fine texture, a crystal grain size, and the like by a crystal orientation map analysis.

TABLE 2

In the case of comparative example 1, although the composition and composition ranges satisfy the conditions of the ultra-thick steel material according to one embodiment of the present disclosure, since finish rolling is performed at a lower cumulative rolling reduction than the 50% finish rolling cumulative rolling reduction given in one embodiment of the present disclosure, polygonal ferrite is not sufficiently formed in the surface portion.

Further, since the finish rolling reduction is low, sufficient deformation is not applied at t/4 and the grain size is 15 μm or more, NDTT is measured to be-70 ℃ or more and Kca value is 8000 or less.

In the case of comparative example 2, although the composition and composition ranges satisfy the conditions for the ultra-thick steel material according to one embodiment of the present disclosure, since finish rolling is performed at 772 to 780 ℃ higher than the finish rolling finish temperature given in one embodiment of the present disclosure, ferrite is not generated at all in the surface portion during air cooling, low-temperature transformation phases are generated in large amounts, and high finish rolling temperature does not impart sufficient deformation at t/4, the grain size is 15 μm or more. As a result of investigation, comparative example 2 was found to have a NDTT of-70℃or higher, and the finish rolling was carried out at a high temperature, and thus refinement of the microstructure (refinement) did not occur sufficiently, and thus the Kca value was also 8000 or lower.

In the case of comparative example 3, although the composition and composition ranges satisfy the conditions for the ultra-thick steel material according to one embodiment of the present disclosure, since finishing rolling is performed at 703 to 710 ℃ lower than the finishing rolling finish temperature given in one embodiment of the present disclosure, part of ferrite is first generated before polygonal ferrite is precipitated in the finishing rolling process, and the previously generated ferrite is extended in the form of elongated coarse ferrite in the finishing rolling process, coarse air-cooled ferrite is also generated at t/4, resulting in an increase in grain size.

The final investigation showed that comparative example 3 was a NDTT of-70℃or higher, although the fraction of polygonal ferrite was high.

In the case of comparative example 4, the component range of C is higher than the upper limit of C of the ultra-thick steel material according to one embodiment of the present disclosure, so that a phenomenon occurs in which too high strength causes a decrease in toughness, and investigation shows that NDTT is-70℃or higher.

In the case of comparative example 5, the composition range of Mn is higher than the upper limit of Mn of the ultra-thick steel material according to one embodiment of the present disclosure, and sufficient ferrite is not generated during air cooling due to the reduction of ferrite transformation temperature. As is finally found, the NDTT is at least-70℃and the Kca value is at most 8000.

In the case of comparative example 6, the composition ranges of C and Mn were lower than the lower limits of C and Mn of the ultra-thick steel material according to one embodiment of the present disclosure, so that a large amount of ferrite was generated at the surface portion, but the yield strength of 460MPa given in the present disclosure was not satisfied due to low hardenability.

In the case of comparative example 7, the composition ranges of Ni and Cu were lower than the lower limits of Ni and Cu of the ultra-thick steel according to one embodiment of the present disclosure, so that a large amount of ferrite was generated in the surface portion, but the yield strength of 460MPa given in the present disclosure was not satisfied due to low hardenability, and the Kca value was also 8000 or less due to low Ni content, resulting in reduced toughness.

In the case of comparative example 8, the composition ranges of Ti and Nb are higher than the upper limits of Ti and Nb of the ultra-thick steel material according to one embodiment of the present disclosure, so that strength is increased due to excessive hardenability, NDTT is-70 ℃ or higher, and Kca value is 8000 or lower due to the influence of the decrease in toughness caused by precipitation strengthening.

On the other hand, as is clear from the above results, in the cases of inventive examples 1 to 4, which satisfy the composition ranges given in the present disclosure and were produced at a cumulative rolling reduction of 50% or more in the temperature range of 740 to 720 ℃, the fraction of polygonal ferrite from the surface portion to 5mm directly below the surface portion was 50% or more, the yield strength was 460MPa or more, the NDTT (Nil-Ductility Transition Temperature) value based on the NRL-DWT test according to ASTM E208 standard was-70 ℃ or less, and the Kca value obtained by performing the esco test was 8000 or more.

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 (10)

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 2.1 percent of Ni:0.3% -1.0%, nb:0.005% -0.040%, ti:0.005% -0.03%, cu:0.1% -0.5%, P:100ppm or less, S: below 40ppm, the balance Fe and other unavoidable impurities,

the fraction of polygonal ferrite from the surface portion to 5mm directly below the surface portion is 50% or more, and the microstructure has a grain size of 15 μm or less in t/4 microstructure having a large-angle grain boundary of 15 degrees or more as measured by EBSD.

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

the NDTT value of the surface portion NRL-DWT test based on ASTM E208-06 standard is-70 ℃ or lower.

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

the Kca value of the steel material based on ESSO test is 8000 or more.

4. The ultra-thick high-strength steel material according to claim 1, wherein,

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

5. 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 2.1 percent of Ni:0.3% -1.0%, nb:0.005% -0.040%, ti:0.005% -0.03%, cu:0.1% -0.5%, P:100ppm or less, S: less than 40ppm Fe and other unavoidable impurities in balance;

after rough rolling is carried out on the reheated slab, finish rolling is carried out at the temperature ranging from 720 ℃ to 740 ℃ from the surface of the slab to the t/4 position;

and cooling the finish rolled steel.

6. The method for producing an ultra-thick high-strength steel material according to claim 5, wherein,

the reheating temperature of the slab is 1000-1120 ℃.

7. The method for producing an ultra-thick high-strength steel material according to claim 5, wherein,

the rough rolling temperature is 900-1100 ℃.

8. The method for producing an ultra-thick high-strength steel material according to claim 5, wherein,

the cumulative rolling reduction at the time of rough rolling is 50% or more.

9. The method for producing an ultra-thick high-strength steel material according to claim 5, wherein,

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

10. The method for producing an ultra-thick high-strength steel material according to claim 5, wherein,

the cooling end temperature in the cooling step is 500 ℃ or lower.