KR20220026194A - Ultra thick steel plate having excellent surface part nrl-dwt property and manufacturing method thereof - Google Patents

Abstract

The present invention relates to an ultra-thick structural steel with excellent surface NWL-DWT physical property and a manufacturing method thereof. The ultra-thick structural steel comprises: 0.05-0.09 % of C, 0.1-0.4 % of Si, 0.01-0.05 % of Al, 1.8-2.0 % of Mn, 0.3-0.7 % of Ni, 0.015-0.040 % of Nb, 0.005-0.02 % of Ti, and 0.05 % or less (excluding 0 %) of Cu by weight, and the remaining of Fe and other inevitable impurities. The ultra-thick structural steel has less than 0.1 fine cracks having a length of 50 ㎛ or more per area of 1 mm^2 in an area from a surface part to 5 mm below the surface part. According to the present invention, the ultra-thick structural steel has excellent NRL-DWT physical property by suppressing micro cracks of the surface part without including expensive alloy elements.

Description

Structural ultra-thick steel material with excellent surface properties of NRL-DWT and its manufacturing method

The present invention relates to an ultra-thick steel material having excellent surface properties of NRL-DWT and a method for manufacturing the same.

Recently, in the design of structures such as domestic and foreign ships, the development of ultra-thick and high-strength steel is required.

When high-strength steel is used in designing a structure, the ease of processing and welding can be secured at the same time because the plate thickness can be reduced along with economic benefits due to the weight reduction of the structure.

In general, in the case of high-strength steel, the microstructure becomes coarse because the overall structure is not sufficiently deformed due to a decrease in the total reduction ratio during the manufacture of ultra-thick materials.

Also, during rapid cooling to secure strength, a difference in cooling rate between the surface and the center occurs due to the thick thickness of the ultra-thick material.

As a result, coarse low-temperature transformation phases such as bainite are generated on the surface, making it difficult to secure toughness for extremely thick materials.

In particular, in the case of brittle crack propagation resistance, which indicates the stability of the structure, the number of cases requiring guarantees when applied to major structures such as ships is increasing.

In the case of an extremely thick material, it is difficult to guarantee the brittle crack propagation resistance due to a decrease in toughness caused by a difference in cooling rate between the surface portion and the center portion.

In fact, many classification societies and steel companies are conducting large-scale tensile tests to accurately evaluate actual brittle crack propagation resistance to guarantee brittle crack propagation resistance.

However, in the case of a large-scale tensile test, it is difficult to guarantee when mass-produced because a large amount of cost is incurred to conduct the test.

In order to improve this absurdity, research on a small substitute test that can replace a large tensile test has been steadily progressing.

Among the small alternative tests, the Naval Research Laboratory-Drop Weight Test (NRL-DWT) test of the ASTM E208-06 standard is being adopted by many classification societies and steel companies as the most powerful test.

The surface part NRL-DWT test is being adopted based on the research result of improving the brittle crack propagation resistance by slowing the propagation speed of cracks during brittle crack propagation when the microstructure of the surface part is controlled based on existing research.

However, in the NRL-DWT test of the surface part, when the steel material is taken from the surface of the specimen, the surface of the blade plate is used without chamfering.

If a surface crack that easily induces a brittle crack exists in the specimen, the NDTT (Nil-Ductility Transition Temperature) value, which is the result of the NRL-DWT test, is easily deteriorated.

Therefore, there is a need for a solution to suppress surface cracks.

An object of the present invention is to provide an ultra-thick steel material having excellent physical properties of the surface portion NRL-DWT capable of solving the problems of the prior art and a method for manufacturing the same.

Specifically, the present invention suppresses microcracks on the surface of ultra-thick steel materials through alloy component control without including expensive alloying elements in terms of composition, thereby providing an ultra-thick steel material with excellent NRL-DWT physical properties and a method for manufacturing the same The purpose.

Another object of the present invention is to provide an ultra-thick steel material having excellent physical properties of the surface portion NRL-DWT and a method for manufacturing the same by suppressing microcracks on the surface portion by controlling the rolling temperature and reduction amount during rolling.

In addition, the present invention is more specifically, 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 length per 1 mm2 area in the area from the plate material surface to directly 5 mm is NRL-DWT (Nil-Ductility Transition Temperature) value of NDTT (Nil-Ductility Transition Temperature) of -70°C or lower by NRL-DWT test that conforms to ASTM E208 with 0.1 or less microcracks of 50㎛ or more is intended to provide

The objects of the present invention are not limited to the above-mentioned objects, and other objects and advantages of the present invention not mentioned may be understood by the following description, and will be more clearly understood by the examples of the present invention. It will also be readily apparent that the objects and advantages of the present invention may be realized by the means and combinations thereof indicated in the appended claims.

The steel for ultra-thick structure according to an embodiment of the present invention embodied for achieving the above object is in weight %, C: 0.05 to 0.09%, Si: 0.1 to 0.4%, Al: 0.01 to 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%) Including the remaining Fe and other unavoidable impurities, the surface part to the surface In an area up to 5 mm non-directly, the microstructure may have 0.1 or less microcracks having a length of 50 μm or more per 1 mm 2 area.

Preferably, the NDTT (Nil-Ductility Transition Temperature) value by the NRL-DWT (Drop Weight Test) test of the surface of the ASTM E208-06 standard may be -70° C. or less.

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

The manufacturing method of the ultra-thick structure steel material according to an embodiment of the present invention for achieving the above object may be a manufacturing method comprising the step of finish rolling at a temperature of 740 ° C. or less at a t/4 position from the slab surface. .

The manufacturing method of an ultra-thick structural steel material according to an embodiment of the present invention embodied for achieving the above object is, in weight %, C: 0.05 to 0.09%, Si: 0.1 to 0.4%, Al: 0.01 to 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%) reheating; After rough rolling the reheated slab, finishing rolling at a temperature of 740° C. or less at a t/4 position from the slab surface; It may include; cooling the finished-rolled steel material.

Preferably, the slab reheating temperature may be 1,000 ~ 1,120 ℃.

Preferably, the rough rolling temperature may be 900 ~ 1,100 ℃.

Preferably, the cumulative reduction ratio during the finishing rolling may be 50% or more.

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

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

According to the present invention, it is possible to implement an ultra-thick steel material with excellent surface NRL-DWT physical properties through composition and microstructure control without excessively including expensive alloying elements.

According to the present invention, the amount of deformation in the surface part and t/4 part austenite structure is maximized by controlling the component and composition range, finishing rolling temperature, and cumulative rolling reduction, so that 1 mm2 in the area from the surface part to 5 mm directly under the surface part It is possible to implement a method of manufacturing an ultra-thick steel material with excellent physical properties of the surface portion NRL-DWT having 0.1 or less microcracks with a length of 50 μm or more per area.

According to the present invention, the NDTT (Nil-Ductility Transition Temperature) value by the NRL-DWT test conforming to the ASTM E208 standard is -70 while having a thickness of 80 mm or more and 100 mm or less, which is the thickness of an ultra-thick steel material, and a yield strength of 460 MPa or more. It is possible to implement a steel material for ultra-thick structure with excellent physical properties of NRL-DWT below ℃ and a manufacturing method thereof.

In addition to the above-described effects, the specific effects of the present invention will be described together while describing specific details for carrying out the invention below.

Hereinafter, with reference to the drawings, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. The present invention may be embodied in many different forms and is not limited to the embodiments described herein.

In order to clearly explain the present invention, parts irrelevant to the description are omitted, and the same reference numerals are assigned to the same or similar components throughout the specification. Further, some embodiments of the present invention will be described in detail with reference to exemplary drawings. In adding reference numerals to components of each drawing, the same components may have the same reference numerals as much as possible even though they are indicated in different drawings. In addition, in describing the present invention, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present invention, the detailed description may be omitted.

In describing the components of the present invention, terms such as first, second, A, B, (a), (b), etc. may be used. These terms are only for distinguishing the elements from other elements, and the nature, order, order, or number of the elements are not limited by the terms. When it is described that a component is “connected”, “coupled” or “connected” to another component, the component may be directly connected or connected to the other component, but other components may be interposed between each component. It should be understood that each component may be “interposed” or “connected”, “coupled” or “connected” through another component.

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 length per 1 mm2 area is 50 μm in the region from the plate surface to directly 5 mm Invented a steel material for ultra-thick structure with excellent physical properties and a manufacturing method thereof, in which the number of fine cracks greater than or equal to 0.1 is less than or equal to 0.1, and the NRL-Ductility Transition Temperature (NDTT) value according to the NRL-DWT test conforming to the ASTM E208 standard is -70°C or less want to

The steel for ultra-thick structure according to an embodiment of the present invention for satisfying the above characteristics may specifically include the following alloy elements in order to satisfy the excellent properties of the NRL-DWT.

It should be noted in advance that the content or composition range of each component to be described below is based on weight% unless otherwise specified.

Since carbon (C) is the most important element for securing basic strength in the steel for ultra-thick structure of the present invention, it needs to be contained in the steel (or steel) within a controlled range.

In the steel according to an embodiment of the present invention, carbon is contained in the range of 0.05 to 0.09% by weight (hereinafter referred to as %).

If carbon is added to less than 0.05% in the steel of one embodiment of the present invention, there is a problem in that it causes a decrease in the strength of the steel, making it difficult to achieve the strength target.

On the other hand, when carbon is added more than 0.09% in the steel of one embodiment of the present invention, excess carbon improves hardenability to generate a large amount of massive martensite and promotes the formation of a low-temperature transformation phase, resulting in lowering the toughness of the steel. There is a problem with lowering.

In addition, when carbon is added more than 0.09% in the steel of one embodiment of the present invention, it enters into a hypo-peritectic region where surface cracks can easily occur. there is

Silicon (Si) and aluminum (Al) are essential alloying elements for deoxidation by precipitating dissolved oxygen in the molten steel in the form of slag during the steelmaking and casting process, so they need to be contained in the steel (or steel) within a controlled range. .

In particular, when manufacturing a steel material using a converter, silicon is contained in the range of 0.1 to 0.4% by weight (hereinafter referred to as %) in the steel according to an embodiment of the present invention, and aluminum is contained in the range of 0.01 to 0.05% .

If silicon and aluminum are added in less than 0.1% and 0.01%, respectively, in the steel of an embodiment of the present invention, the amount of precipitation of dissolved oxygen during the steelmaking and casting process is insufficient, so it is difficult to expect the deoxidation effect.

On the other hand, when silicon and aluminum are added more than 0.4% and 0.05%, respectively, in the steel of one embodiment of the present invention, excessive silicon and aluminum generate coarse Si and Al complex oxides or coarsely large amounts of martensite on the microstructure. There are problems that can be created.

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

In the steel according to an embodiment of the present invention, manganese is contained in the range of 1.8 to 2.0% by weight (hereinafter referred to as %).

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

On the other hand, when manganese is added more than 2.0% in the steel of one embodiment of the present invention, excessive manganese excessively increases hardenability and thereby promotes the formation of upper bainite and martensite to improve impact toughness and surface NRL - There is a problem of greatly reducing the physical properties of DWT.

Nickel (Ni) is an important element to improve impact toughness and hardenability by facilitating cross slip of dislocations at low temperatures in the steel for ultra-thick structure of the present invention. (or steel) needs to be contained.

Nickel in the steel according to an embodiment of the present invention is contained in the range of 0.3 to 0.7% by weight (hereinafter referred to as %).

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

On the other hand, when nickel is added more than 0.7% in the steel of one embodiment of the present invention, excessive nickel excessively increases hardenability to generate a low-temperature transformation phase, thereby reducing toughness and excessively increasing manufacturing cost.

Niobium (Nb) is precipitated in the form of NbC or NbCN in the steel for ultra-thick structure of the present invention to improve the strength of the base material, and Nb dissolved in reheating to a high temperature is very finely precipitated in the form of NbC during rolling to austenite Since it is an element that suppresses recrystallization of and refines the structure, it needs to be contained in steel (or steel) within a controlled range.

In the steel according to an embodiment of the present invention, niobium is contained in the range of 0.015 to 0.04% by weight (hereinafter referred to as %).

If niobium is added to less than 0.015% in the steel according to an embodiment of the present invention, the amount of precipitates in the form of NbC or NbCN is too small, making it difficult to expect microstructure refinement and strength enhancement.

On the other hand, when more than 0.04% of niobium is added in the steel of one embodiment of the present invention, excessive niobium is more likely to cause brittle cracks at the edge of the steel, and there is a problem that toughness may be deteriorated due to excessive formation of precipitates.

Titanium (Ti) is an element that significantly improves low-temperature toughness by inhibiting the growth of crystal grains in the base metal and the weld heat-affected zone by precipitating as TiN during reheating in the ultra-thick structural steel of the present invention. need to be included.

In the steel according to an embodiment of the present invention, titanium is contained in the range of 0.005 to 0.02% by weight (hereinafter referred to as %).

If titanium is added to less than 0.005% in the steel of one embodiment of the present invention, the amount of precipitates in the form of TiN is too small, so it is difficult to expect grain refinement and toughness improvement of the base material and the weld heat-affected zone.

On the other hand, if titanium is added more than 0.02% in the steel of one embodiment of the present invention, excessive titanium has a problem of reducing the low-temperature toughness due to clogging of the playing nozzle or central precipitation (primary precipitation).

Copper (Cu) is a major element in improving hardenability and solid solution strengthening in the steel for ultra-thick structure of the present invention to improve the strength of steel, and yield strength through the generation of epsilon (ε) Cu precipitates when tempering is applied Since it is a major element in raising

In the steel according to an embodiment of the present invention, copper is contained in a weight % (hereinafter referred to as %) in the range of 0.05% or less.

If copper is added more than 0.05% in the steel of one embodiment of the present invention, there is a problem that may cause high temperature brittleness in the steelmaking process or may cause cracking of the slab due to hot shortness.

Hereinafter, a method for manufacturing the steel of the present invention as described above will be described.

The method of manufacturing a steel material according to an embodiment of the present invention may include a process of slab reheating - rough rolling - finishing rolling - cooling, and detailed conditions for each process are as follows.

Unless otherwise stated in the description of the manufacturing method below, the temperature of the hot-rolled steel sheet (slab) means the temperature at the t/4 (t: thickness of the steel sheet) position from the surface of the hot-rolled steel sheet (slab) in the sheet thickness direction. do.

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

Slab reheating stage: 1,000~1,120℃

In the method of manufacturing a steel material according to an embodiment of the present invention, the reheating step of the slab employs carbides and/or carbonitrides of Ti and/or Nb formed during the casting process without excessive coarsening of the austenite grains and flows stress (flow stress). ) to facilitate subsequent hot working.

In the method for manufacturing a steel material according to an embodiment of the present invention, the slab reheating temperature may be 1,000 to 1,120 °C, more preferably 1,050 to 1,120 °C.

If the slab reheating temperature is less than 1,000 ℃ reheating temperature, there is a fear that Ti and / or Nb carbonitride formed during casting may not be sufficiently dissolved.

On the other hand, if the reheating temperature exceeds 1,120 ℃, there is a fear that austenite forming a microstructure at the reheating temperature is coarse.

Rough rolling stage: 900~1,100℃

In the method of manufacturing a steel material according to an embodiment of the present invention, the rough rolling step is a process for reducing the grain size of coarse austenite grains through recrystallization together with destruction of the cast structure such as dendrites formed during casting.

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

Specifically, in the method of manufacturing a steel material according to an embodiment of the present invention, the rough rolling temperature is 900 ~ 1,100 ℃.

If the rough rolling temperature is lower than 900 ° C., there is a problem in that it is difficult to achieve dynamic recrystallization during rough rolling, making it difficult to refine grains.

On the other hand, if the rough rolling temperature is higher than 1,100 ℃, there is a problem that the grain refinement is not effective even by dynamic recrystallization because the austenite grains in the slab grow excessively before the start of the rough rolling.

On the other hand, in order to cause recrystallization in the slab through rough rolling to refine the microstructure of the slab, a sufficient amount of deformation to cause recrystallization during the rough rolling process must be applied to the slab.

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

Finishing rolling finish temperature: 740℃ or less

In the method of manufacturing a steel material according to an embodiment of the present invention, the finishing rolling step is a process for introducing a non-uniform microstructure into the austenite microstructure of the rough-rolled steel sheet.

At this time, it is preferable to perform the finishing rolling pass (pass) at a ferrite generation temperature of 740° C. or less on the basis of t/4.

The finishing temperature range was set to a temperature range capable of making fine grain sizes of phases generated during cooling after finishing rolling by performing rolling around the polygonal ferrite generation temperature.

If the finishing pass is performed at a temperature higher than 740° C. on a t/4 basis, there is a problem in that strength and toughness are reduced as the microstructure is coarsened.

According to an embodiment of the present invention, the cumulative reduction ratio in the finishing rolling process is preferably at least 50% or more in order to form a fine structure to the maximum.

Cooling step after rolling: Cooling at a cooling rate of 3°C/s or higher at 720°C or lower and then cooling at 500°C or lower

In the method for manufacturing a steel material according to an embodiment of the present invention, the finished rolled steel sheet is preferably cooled at a cooling rate of 3° C./s or more from a temperature of 720° C. or less to a temperature of 500° C. or less.

If the cooling start temperature is carried out in excess of 720 ℃, there is a problem that the NDTT temperature can be -70 ℃ or higher because the formation of polygonal ferrite, which is a soft phase, is not promoted on the surface there is.

If the cooling rate is lower than 3°C/s or the cooling termination temperature exceeds 500°C, the microstructure formed on the steel sheet by the phase transformation during the cooling process is not properly formed, so that the final yield strength is 460 MPa or less is likely to become

The steel manufacturing method of the present invention according to an embodiment of the present invention is summarized as follows.

In weight %, C: 0.05 to 0.09%, Si: 0.1 to 0.4%, Al: 0.01 to 0.05%, Mn: 1.8 to 2.0%, Ni: 0.3 to 0.7%, Nb: 0.015 to 0.040%, Ti: 0.005 to 0.02%, Cu: 0.05% or less (excluding 0%) after reheating the slab containing the remaining Fe and other unavoidable impurities to a temperature of 1,000 to 1,120 ℃, rough rolling at a temperature of 900 to 1,100 ℃; After air cooling the rolled bar and after finishing the air cooling, finishing rolling is performed, and then finishing rolling at 740° C. or less based on 1/4t; After the entire rolling is finished, cooling to a temperature of 500° C. or less at a cooling rate of 3° C./s or more; a structural ultra-thick steel material having excellent physical properties of the surface portion NRL-DWT according to an embodiment of the present invention can be manufactured there is.

At this time, in the ultra-thick steel material according to an embodiment of the present invention, the amount of Cu added that causes surface cracks is minimized, so that in the area from the surface of the plate material to 5 mm directly below the surface, microcracks having a length of 50 µm or more per 1 mm2 area It can have 0.1 or less.

Therefore, the microstructure and thickness of the ultra-thick steel material according to an embodiment of the present invention can be implemented only by a controlled combination of the technical characteristics of the manufacturing method together with the composition and composition range of the steel material.

Through this, in the present invention, the NRL-DWT steel material with excellent physical properties is secured with a yield strength of 460 MPa or more and an NRL-DWT (Nil-Ductility Transition Temperature) value of NDTT (Nil-Ductility Transition Temperature) according to the NRL-DWT test conforming to ASTM E208 standard -70 ° C or lower. can do.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only for explaining the present invention by way of illustration and not for limiting the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[Example]

A steel slab having the composition shown in Table 1 was selected as the manufacturing method targeted in the present invention, and reheating, rolling and cooling were performed.

Specifically, after reheating a 400 mm thick steel slab having the composition shown in Table 1 to a temperature of 1,050 to 1,070 ℃, starting rough rolling at a temperature of 1030 ℃ or less, and then continuously performing rough rolling, rough rolling at 930 ℃ or higher Completed to make a bar.

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

[Table 1]

The results of evaluating the tensile properties of the steels manufactured by the manufacturing method according to an embodiment of the present invention and the conditions outside the manufacturing method according to an embodiment of the present invention for the steels disclosed in Table 1 were evaluated, and The surface crack analysis results and yield strength of the manufactured steel sheet are summarized in Table 2.

The cracks were derived from the average value of measuring the number of cracks of 50 μm or more after observing 20 or more different locations with an area of 1 mm * 1 mm in an area of 5 mm directly below the surface of the steel plate.

In addition, NDTT (Nil-Ductility Transition Temperature) by the NRL-DWT test conforming to ASTM E208 standard was measured for the manufactured steel sheet, and the results are summarized in Table 2.

[Table 2]

In the case of Comparative Example 1, although the components and composition ranges satisfy the conditions of the ultra-thick steel material according to an embodiment of the present invention, the surface during air cooling as it is manufactured at or above the finishing rolling finish temperature suggested in an embodiment of the present invention Since sufficient ferrite was not generated in the part, NDTT was measured to be -70°C or higher.

In Comparative Examples 2 and 3, an amount higher than the upper limit of the Cu composition range presented in the ultra-thick steel material according to an embodiment of the present invention was added.

Accordingly, due to the high Cu content, in Comparative Examples 2 and 3, a large area of high-temperature brittleness was generated, and the possibility of hot shortness was increased.

As the generated fine cracks were elongated during rolling, in Comparative Examples 2 and 3, cracks of 50 μm or more were generated in the portion directly below the surface of the steel at 0.1 pieces/mm 2 or more, and thus NDTT was measured to be -70° C. or more.

In the case of Comparative Example 4, an amount higher than the upper limit of the C composition range presented in the ultra-thick steel material according to an embodiment of the present invention was added.

Accordingly, due to the high C content, a high-temperature brittle region of Comparative Example 4 was also generated, and a large amount of microcracks occurred directly under the slab surface during the slab manufacturing process.

As the generated fine cracks were elongated during rolling, in Comparative Example 4, cracks of 50 μm or more were generated in the portion directly below the surface of the steel at 0.1 pieces/mm 2 or more, and thus NDTT was measured to be -70° C. or more.

In the case of Comparative Example 5, an amount higher than the upper limit of the Mn composition range presented in the ultra-thick steel material according to an embodiment of the present invention was added.

Accordingly, in Comparative Example 5 due to the high Mn content, a large area of high-temperature brittleness occurred, and a large amount of microcracks occurred directly under the slab surface during the slab manufacturing process.

As the generated fine cracks were elongated during rolling, in Comparative Example 5, cracks of 50 μm or more were generated in the portion directly below the surface of the steel at 0.1 pieces/mm 2 or more, and thus NDTT was measured to be -70° C. or more.

In the case of Comparative Example 6, an amount lower than the lower limit of the C and Mn composition ranges suggested in the ultra-thick steel material according to an embodiment of the present invention was added.

Accordingly, it was measured that Comparative Example 6 did not satisfy the yield strength of 460 MPa presented in the present invention due to low hardenability.

In Comparative Example 7, an amount lower than the lower limit of the Ni composition range presented in the ultra-thick steel material according to an embodiment of the present invention was added.

Accordingly, the NDTT of Comparative Example 7 was measured to be -70°C or higher due to the decrease in toughness due to the low Ni content.

In the case of Comparative Example 8, an amount higher than the upper limit of the Ti and Nb composition range presented in the ultra-thick steel material according to an embodiment of the present invention was added.

Accordingly, due to the high Ti and Nb content, Comparative Example 8 had a large area of high-temperature brittleness, and a large amount of microcracks occurred directly under the surface of the slab during the slab manufacturing process.

As the generated fine cracks were elongated during rolling, in Comparative Example 8, cracks of 50 μm or more were generated in the portion directly below the surface of the steel at 0.1 cracks/mm 2 or more.

Also, in Comparative Example 8, the NDTT was measured to be -70° C. or higher because a large amount of high-strength tissue was generated on the surface according to the increase in strength due to excessive precipitates.

On the other hand, as can be seen from the above results, in the case of Inventive Examples 1 to 4, which satisfy the component range presented in the present invention and were prepared by finish rolling at a temperature of 740° C. or less, 1 in the area from the surface portion to 5 mm directly below the surface portion It was measured that there are 0.1 or less fine cracks with a length of 50 μm or more per mm2, the yield strength is 460 MPa or more, and the NDTT (Nil-Ductility Transition Temperature) value by the NRL-DWT test conforming to ASTM E208 standard is -70 ° C. or less.

As described above, the present invention has been described with reference to the illustrated embodiments, but the present invention is not limited by the embodiments and drawings disclosed in the present specification. It is evident that various modifications may be made. In addition, although the effects according to the configuration of the present invention are not explicitly described and described while describing the embodiments of the present invention, it is natural that the effects predictable by the configuration should also be recognized.

Claims (9)

In % 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%), including the remaining Fe and other unavoidable impurities,
Steel for ultra-thick structure with 0.1 or less microcracks with a length of 50 µm or more per 1 mm2 area in the area from the surface part to 5 mm directly below the surface part.
The method of claim 1,
Steel for ultra-thick structure with NDTT (Nil-Ductility Transition Temperature) value of -70℃ or less by the surface part NRL-DWT (Drop Weight Test) test of ASTM E208-06 standard.
The method of claim 1,
The plate thickness is 80~100mm, and the yield strength is 460MPa or more.
In % 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%) reheating the slab containing the remaining Fe and other unavoidable impurities;
After rough rolling the reheated slab, finishing rolling at a temperature of 740° C. or less at a t/4 position from the surface;
cooling the finished rolled steel;
A method of manufacturing an ultra-thick high-strength steel comprising a.
5. The method of claim 4,
The slab reheating temperature is a method of manufacturing an ultra-thick high-strength steel of 1,000 ~ 1,120 ℃.
5. The method of claim 4,
The rough rolling temperature is 900 ~ 1,100 ℃ method of manufacturing an ultra-thick high-strength steel.
5. The method of claim 4,
A method of manufacturing an ultra-thick high-strength steel material, wherein the cumulative reduction ratio in the finishing rolling step is 50% or more.
5. The method of claim 4,
The cooling rate in the cooling step is 3 ℃ / sec or more method of manufacturing an ultra-thick high-strength steel.
9. The method of claim 8,
The cooling start temperature in the cooling step is 720 ℃ or less and the cooling end temperature is 500 ℃ or less of the ultra-thick high-strength steel manufacturing method.