WO2020111891A1 - High-strength steel plate having excellent low-temperature fracture toughness and elongation ratio, and manufacturing method therefor - Google Patents

Abstract

A high-strength steel plate having excellent fracture toughness and elongation ratio according an aspect of the present invention comprises, by weight%, 0.05-0.1% of carbon (C), 0.05-0.5% of silicon (Si), 1.4-2.0% of manganese (Mn), 0.01-0.05% of aluminum (Al), 0.005-0.02% of titanium (Ti), 0.002-0.01% of nitrogen (N), 0.04-0.07% of niobium (Nb), 0.05-0.3% of chromium (Cr), 0.05-0.4% of nickel (Ni), 0.02% or less of phosphorus (P), 0.005% or less of sulfur (S), 0.0005-0.004% of calcium (Ca), and the remainder of iron (Fe) and unavoidable impurities, wherein a microstructure includes 20-60 area% of ferrite and bainite, and the grain size of the upper 80% of the high-angle grain sizes based on 15 degrees in the centre of the steel plate may be 70 ㎛ or less.

Description

High-strength steel sheet with excellent low-temperature fracture toughness and elongation and its manufacturing method

The present invention relates to a high-strength steel sheet and a method for manufacturing the same, and in detail, it has a high-strength property by optimizing the steel composition, microstructure, and manufacturing process, and has excellent low-temperature fracture toughness and elongation, so that it can be stably used in harsh environments. It relates to a high-strength steel sheet and a method for manufacturing the same.

Recently, as oil fields have been developed mainly in cold regions such as Siberia and Alaska, where the climatic conditions are poor, projects to transport rich gas resources in oil fields to consumer regions through line pipes are actively underway. The steel materials to be used in these line pipe projects must not only have the pressure of transport gas, but also have to have durability against the deformation of pipes due to cryogenic temperatures and frost heave (a phenomenon in which the ground freezes during the change of season). It is required to have excellent DWTT (Drop Weight Tearing Test) ductility wavefront and high elongation characteristics while having a.

DWTT ductile fracture rate is a kind of indicator for determining whether line pipe steel has brittle fracture stopping properties for safe use at low temperatures. In general, the line pipe provided in the cold region is required to be provided with a DWTT ductile wavefront ratio of -20°C or higher at a level of 85% or more in a pipe state. In order to secure a DWTT ductile wave front index of at least 85% based on -20°C in a pipe state, the DWTT ductile wave front rate of a steel sheet provided for pipe manufacturing must satisfy a level of 85% or higher at -30°C.

In general, DWTT ductility is known to have a deep correlation with the effective grain size of a steel sheet. The effective grain size is defined as the size of grains having a high hard angle grain boundary, and as the effective grain size becomes finer, the propagation resistance of the crack increases. This is because the propagation path of the crack changes at the effective grain boundary when the crack starts and propagates.

In order to refine the effective grain size, a method of performing accelerated cooling immediately after rolling is widely used. Immediately after rolling, a mixed structure of acicular ferrite and bainite can be realized by accelerated cooling. However, the microstructure formed through normal accelerated cooling has a high hardness since carbon (C) is supersaturated in the crystal grains, and thus exhibits ductility for heat to a level of less than 9% uniform elongation and less than 20% total elongation. . As a result, the moldability is lowered during pipe construction, and local stress concentration is easily generated when external deformation is applied, and thus there is a problem that the stability of the pipe is significantly reduced.

Accordingly, in the production of the steel sheet for line pipe, the elongation deterioration of the steel sheet produced by accelerated cooling is improved, and the low temperature fracture toughness is excellent, but the uniform elongation of 9% or more and the total elongation of 28% or more have excellent ductility. There is a need for a method for manufacturing a line pipe steel sheet.

In the past, there have been studies on steel sheets having excellent elongation and low temperature fracture toughness. In this regard, Patent Document 1 is a non-recrystallized rolling of a steel material containing nickel (Ni), niobium (Nb), and molybdenum (Mo) in a rolling reduction of 65% or more, and Bs temperature at a cooling rate of 15 to 30°C/ Cooling to the first, and second cooling to a temperature range of 350 to 500°C at a cooling rate of 30 to 60°C/, finely equiaxing 30 to 60% equiaxed ferrite and 40 to 70% of bainite mixed structure It proposes a method of manufacturing a steel material included as a tissue.

However, Patent Document 1 is to perform low-temperature rolling on a steel sheet having a thickness of 20 mm or more, and there is a technical difficulty in applying the corresponding process conditions to a steel sheet having a thickness of less than 20 mm. This is because, in the case of a steel sheet having a thickness of less than 20 mm, it is difficult to secure the desired low temperature fracture toughness, strength and elongation in the entire longitudinal direction of the steel sheet, particularly in the rear end of the steel sheet, since the steel sheet is rapidly cooled after cold rolling.

(Advanced technical literature)

(Patent Document 1) Republic of Korea Patent Publication No. 10-2013-0073472 (2013.07.03. public)

According to one aspect of the present invention, a high-strength steel sheet having excellent low-temperature toughness and a method of manufacturing the same can be provided.

The subject of this invention is not limited to the above-mentioned content. Those skilled in the art will have no difficulty in understanding the additional subject matter of the present invention from the general contents of this specification.

High-strength steel sheet having excellent fracture toughness and elongation according to an aspect of the present invention, by weight, carbon (C): 0.05 to 0.1%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 1.4 to 2.0% , Aluminum (Al): 0.01 to 0.05%, Titanium (Ti): 0.005 to 0.02%, Nitrogen (N): 0.002 to 0.01%, Niobium (Nb): 0.04 to 0.07%, Chromium (Cr): 0.05 to 0.3% , Nickel (Ni): 0.05 to 0.4%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, Calcium (Ca): 0.0005 to 0.004%, containing the remaining iron (Fe) and unavoidable impurities , 20 ~ 60 area% of ferrite and bainite as a microstructure, the top 80% grain size of the high-angle crystal grain size of 15 degrees in the center of the steel sheet may be 70㎛ or less.

The steel sheet may further include molybdenum (Mo) of 0.3% by weight or less.

The fraction of bainite may be 35 to 75 area%.

The microstructure of the steel sheet may further include an island martensite of 5 area% or less.

The yield strength of the steel sheet may be 485 MPa or more.

The total elongation of the steel sheet is 28% or more, and the uniform elongation of the steel sheet in a perpendicular direction to rolling may be 9% or more.

The DWTT ductile wavefront at -30°C with respect to the perpendicular direction of rolling of the steel sheet may be 85% or more.

The thickness of the steel sheet may be less than 20mm.

High-strength steel sheet excellent in low temperature fracture toughness and elongation according to an aspect of the present invention, by weight, carbon (C): 0.05 to 0.1%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 1.4 to 2.0 %, Aluminum (Al): 0.01~0.05%, Titanium (Ti): 0.005~0.02%, Nitrogen (N): 0.002~0.01%, Niobium (Nb): 0.04~0.07%, Chromium (Cr): 0.05~0.3 %, Nickel (Ni): 0.05 to 0.4%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, Calcium (Ca): 0.0005 to 0.004%, including the remaining iron (Fe) and unavoidable impurities Reheating the slab to be maintained, holding and extracting the slab, re-reverse rolling of the retained and extracted slab at a temperature range of Tnr or higher, and rolling of the recrystallized rolled non-recrystallized station at a total rolling reduction of 30% or more. And, the non-recrystallized rolled steel sheet is manufactured by cooling to a temperature range of (Bs-80°C) to Bs, but the non-recrystallized rolling starts at a temperature range of Tnr or lower (Ar3+100°C) or higher. Can be terminated in.

The slab may further include molybdenum (Mo) of 0.3% by weight or less.

The reheating temperature range of the slab may be 1140 to 1200°C.

The maintenance and extraction temperature range of the slab may be 1140 to 1200°C.

The recrystallization rolling is performed in a plurality of passes, and the average rolling reduction of each pass may be 10% or more.

The recrystallized rolled rolling material may be cooled to a temperature range below Tnr by air cooling.

The unrecrystallized rolled steel sheet may be cooled at a cooling rate of 10 to 50°C/s.

Cooling of the non-recrystallized rolled steel sheet may be started at a temperature range of (Ar3+30°C) or higher.

The thickness of the steel sheet may be less than 20mm.

The solving means of the above problems does not list all the features of the present invention, and various features of the present invention and the advantages and effects thereof may be understood in more detail with reference to specific embodiments below.

According to one aspect of the present invention, it is possible to provide a steel sheet and a method of manufacturing the same, which are particularly suitable as a material for line pipes, having high strength characteristics and excellent low temperature fracture toughness and elongation.

1 is a photograph of the specimen 2 of the Example observed with an optical microscope.

2 is a result of measuring the grain size of the high-angle grain boundary based on 15 degrees of specimen 2 using EBSD.

3 is a photograph of the specimen 18 of the Example observed with an optical microscope.

4 is a result of measuring the grain size of the high-angle grain boundary based on 15 degrees of the specimen 18 using EBSD.

The present invention relates to a high-strength steel sheet having excellent low-temperature fracture toughness and elongation, and a method for manufacturing the same, which will be described below with reference to preferred embodiments of the present invention. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to those skilled in the art to further detail the present invention.

Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise indicated,% representing the content of each element is based on weight.

High-strength steel sheet excellent in low temperature fracture toughness and elongation according to an aspect of the present invention, by weight, carbon (C): 0.05 to 0.1%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 1.4 to 2.0 %, Aluminum (Al): 0.01~0.05%, Titanium (Ti): 0.005~0.02%, Nitrogen (N): 0.002~0.01%, Niobium (Nb): 0.04~0.07%, Chromium (Cr): 0.05~0.3 %, Nickel (Ni): 0.05 to 0.4%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, Calcium (Ca): 0.0005 to 0.004%, including the remaining iron (Fe) and unavoidable impurities do.

In addition, the steel sheet excellent in low temperature fracture toughness and elongation according to an aspect of the present invention may further include molybdenum (Mo): 0.3% or less in weight%.

Carbon (C): 0.05~0.1%

Carbon (C) is the most effective element for improving the strength of steel. In addition, when the amount of carbon (C) added is below a certain level, in order to secure the strength of steel, expensive alloying elements such as molybdenum (Mo) and nickel (Ni) must be added in large amounts, which is not preferable from an economical point of view. The present invention can limit the lower limit of the carbon (C) content to 0.05% to achieve this effect. However, when the carbon (C) is added in excess, the present invention can limit the upper limit of the carbon (C) content to 0.1% because it is not preferable in terms of weldability, formability, and toughness of the steel. Therefore, the carbon (C) content of the present invention may be in the range of 0.05 to 0.1%, and the more preferable carbon (C) content may be in the range of 0.05 to 0.095%.

Silicon (Si): 0.05~0.5%

Silicon (Si) is an element useful for deoxidation of molten steel, and is also an element contributing to the improvement of strength of steel by solid solution strengthening. The present invention can limit the lower limit of the silicon (Si) content to 0.05% to achieve this effect. The lower limit of the more preferable silicon (Si) content may be 0.1%. However, since silicon (Si) is an element having strong oxidizing properties, it is preferable to limit the upper limit of the silicon (Si) content to a certain range. That is, when an excessive amount of silicon (Si) is added, it causes red scale formation during hot rolling, which is undesirable in terms of surface quality, and also has an undesirable effect on the toughness of the welded portion. The upper limit can be limited to 0.5%. The upper limit of the more preferable silicon (Si) content may be 0.4%.

Manganese (Mn): 1.4-2.0%

It is an effective element for solid solution strengthening of manganese (Mn) steel. The present invention can limit the lower limit of the manganese (Mn) content to 1.4% in order to secure high strength properties of the steel. However, when manganese (Mn) is added in excess, a segregation portion may be formed over a wide range in the center of the thickness at the time of slab addressing in the steelmaking process. The upper limit of the content can be limited to 2.0%. The upper limit of the more preferable manganese (Mn) content may be 1.8%.

Aluminum (Al): 0.01~0.05%

Aluminum (Al) is a representative element added with silicon (Si) as a deoxidizer. In addition, aluminum (Al) is also an element that contributes to the improvement of the strength of the steel by solid solution strengthening. The present invention can limit the lower limit of the aluminum (Al) content to 0.01% to achieve this effect. The lower limit of the more preferable aluminum (Al) content may be 0.015%. However, when aluminum (Al) is excessively added, since it is not preferable in terms of impact toughness, the present invention may limit the upper limit of the aluminum content to 0.05%. The upper limit of the more preferable aluminum (Al) content may be 0.04%.

Titanium (Ti): 0.005~0.02%

Titanium (Ti) is an element that forms a TiN precipitate during the solidification process of steel, thereby suppressing austenite grain growth during slab heating and hot rolling, thereby minimizing the particle size of the final structure. The present invention can limit the lower limit of the titanium (Ti) content to 0.005% to achieve the effect of improving the toughness of the steel according to the refinement of the final tissue. The more preferable titanium (Ti) content may be 0.008%. However, when titanium (Ti) is excessively added, TiN is precipitated coarsely when the slab is heated, but rather is not suitable for refinement of the final structure. Therefore, the present invention may limit the upper limit of the titanium (Ti) content to 0.02%. . The upper limit of the more preferable titanium (Ti) content may be 0.018%.

Nitrogen (N): 0.002~0.01%

Nitrogen (N) is dissolved in steel and then precipitates, which serves to increase the strength of the steel, and it is known that such an effect of improving strength is much greater than that of carbon (C). In addition, the present invention is to form a TiN through the reaction of titanium (Ti) and nitrogen (N), and to suppress the grain growth during the reheating process, the lower limit of the nitrogen (N) content can be limited to 0.002% . However, when nitrogen (N) is excessively added, nitrogen (N) exists in the form of solid nitrogen (N) rather than TiN precipitate, and thus the toughness of the steel may be significantly reduced. ) The upper limit of the content can be limited to 0.01%. The upper limit of the preferred nitrogen (N) content may be 0.006%, and the upper limit of the more preferred nitrogen (N) content may be 0.005%.

Niobium (Nb): 0.04 to 0.07%

Niobium (Nb) is a very useful element for refining crystal grains, and is also an element that greatly contributes to the improvement of strength of steel by promoting the formation of acicular ferrite or bainite, which is a high-strength structure. In addition, hot rolling is unavoidable in the steel sheet having a thickness of less than 20 mm, which is the object of the present invention, so niobium (Nb), which has the greatest effect on increasing the recrystallization temperature, must be added in a certain amount or more. Therefore, the present invention can limit the lower limit of the niobium (Nb) content to 0.04%. However, when the niobium (Nb) is excessively added, the weldability of the steel may be deteriorated, and the present invention may limit the upper limit of the niobium (Nb) content to 0.07%. The upper limit of the preferred niobium (Nb) content may be 0.06%.

Chromium (Cr): 0.05~0.3%

Chromium (Cr) is an element that improves quenchability and is an effective element for increasing the strength of steel. In addition, chromium (Cr) is an element that contributes to the improvement of uniform elongation by promoting the formation of island martensite/austenite (MA) during accelerated cooling. The present invention can limit the lower limit of the chromium (Cr) content to 0.05% to achieve this effect. The lower limit of the more preferable chromium (Cr) content may be 0.08%. However, if chromium (Cr) is added excessively, it may cause a decrease in weldability, and thus the present invention may limit the upper limit of the chromium (Cr) content to 0.3%. The upper limit of the preferred chromium (Cr) content may be 0.25%, and the upper limit of the more preferred chromium (Cr) content may be 0.2%.

Nickel (Ni): 0.05 to 0.4%

Nickel (Ni) is an element that effectively contributes to improving the toughness and strength of steel. The present invention can limit the lower limit of the nickel (Ni) content to 0.05% to achieve this effect. However, nickel (Ni) is an expensive element, and excessive addition of nickel (Ni) is not desirable from the economical point of view, and the present invention may limit the upper limit of the nickel (Ni) content to 0.4%. The upper limit of the preferred nickel (Ni) content may be 0.3%, and the upper limit of the more preferred nickel (Ni) content may be 0.25%.

Phosphorus (P): 0.02% or less

Phosphorus (P) is a representative impurity element present in the steel, and is mainly segregated in the center of the steel sheet, causing deterioration of the toughness of the steel. However, the removal of phosphorus (P) from the steel requires excessive cost and time in the steelmaking process, which is undesirable in terms of economy, and thus the present invention can limit the content of phosphorus (P) to 0.02% or less. The more preferable content of phosphorus (P) may be 0.015% or less.

Sulfur (S): 0.005% or less

Sulfur (S) is also a representative impurity element present in the steel, and is combined with manganese (Mn) in the steel to form non-metallic inclusions such as MnS, and thus is a element that significantly damages the toughness and strength of the steel. It is desirable to do. However, the removal of sulfur (S) from the steel is excessively expensive and time-consuming in the steelmaking process, which is undesirable in economic terms, so the present invention can limit the content of sulfur (S) to 0.005% or less. The more preferable content of sulfur (S) may be 0.003% or less.

Calcium (Ca): 0.0005~0.004%,

Calcium (Ca) is an element effective in suppressing the formation of cracks around nonmetallic inclusions by spheroidizing nonmetallic inclusions such as MnS. The present invention can limit the lower limit of the calcium (Ca) content to 0.0005% to achieve this effect. However, when calcium (Ca) is excessively added, a large amount of CaO-based inclusions is formed, causing a decrease in impact toughness, and the present invention may limit the upper limit of the calcium (Ca) content to 0.004%. The upper limit of the preferred calcium (Ca) content may be 0.002%.

Molybdenum (Mo): 0.3% or less

Molybdenum (Mo) is an effective element to promote the formation of bainite, a low-temperature transformation structure, to simultaneously secure high strength and high toughness properties. Therefore, the present invention can selectively add molybdenum (Mo) to achieve this effect. However, since molybdenum (Mo) is an expensive element and is not desirable from an economic point of view when excessively added, the present invention may limit the upper limit of the molybdenum (Mo) content to 0.3%.

The present invention, in addition to the above-mentioned steel composition, the rest may include Fe and unavoidable impurities. The unavoidable impurities may be unintentionally incorporated in the ordinary steel manufacturing process, and cannot be completely excluded, and the meaning can be easily understood by those skilled in the ordinary steel manufacturing field. In addition, this invention does not exclude the addition of the composition other than the steel composition mentioned above entirely.

Hereinafter, the microstructure of the present invention will be described in more detail.

The steel sheet according to an aspect of the present invention includes ferrite and bainite as a microstructure, and in addition, may further include an island martensite. The fractions of ferrite and bainite may be 20-60 area% and 35-75 area%, respectively, and the fraction of island martensite may be 5 area% or less.

In the present invention, since ferrite having a fine high-angle grain boundary is included in 20% by area or more, it is possible to effectively secure low-temperature DWTT properties. In addition, since the present invention includes ferrite in an area of 60 area% or less, and bainite in an area of 35 area% or more, a yield strength of 485 MPa or more can be secured. However, the present invention limits the fraction of bainite to 75 area% or less in order to prevent the high-angle grain boundary from becoming too coarse, and thus can effectively secure low-temperature DWTT properties. In addition, since island martensite has an undesirable effect on low-temperature DWTT properties, it is desirable to suppress the fraction as much as possible. Therefore, the present invention can limit the fraction of island martensite to 5 area% or less.

In addition, the steel sheet according to an aspect of the present invention, the top 80% grain size of the high-angle crystal grain size of 15 degrees from the center of the steel sheet may be 70㎛ or less. That is, the present invention can refine the effective grain size by minimizing the high-angle crystal grain size, thereby effectively securing low-temperature DWTT characteristics. Here, the center of the steel sheet may be interpreted as a region including the point t/2, or may be interpreted as a region of the point t/4 to 3*t/4. (t: thickness of steel sheet, mm)

The steel sheet according to an aspect of the present invention may have a thickness of less than 20 mm, and a more preferred steel sheet may have a thickness of 16 mm or less. In addition, the steel sheet according to an aspect of the present invention may have a yield strength of 485 MPa or higher, a total elongation of 28% or higher, and a uniform elongation in a rolling perpendicular direction of 9% or higher, and DWTT of -30°C for a rolling perpendicular direction of the steel sheet. The ductile wavefront ratio may be 85% or more. Therefore, the present invention, while having a thickness of less than 20mm, can effectively provide strength, low temperature fracture toughness, and elongation, thereby providing a steel sheet particularly suitable as a material for line pipes.

Hereinafter, the manufacturing method of the present invention will be described in more detail.

High-strength steel sheet excellent in low temperature fracture toughness and elongation according to an aspect of the present invention, by weight, carbon (C): 0.05 to 0.1%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 1.4 to 2.0 %, Aluminum (Al): 0.01~0.05%, Titanium (Ti): 0.005~0.02%, Nitrogen (N): 0.002~0.01%, Niobium (Nb): 0.04~0.07%, Chromium (Cr): 0.05~0.3 %, Nickel (Ni): 0.05 to 0.4%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, Calcium (Ca): 0.0005 to 0.004%, including the remaining iron (Fe) and unavoidable impurities Reheating the slab to be maintained, holding and extracting the slab, re-reverse rolling of the retained and extracted slab at a temperature range of Tnr or higher, and rolling of the recrystallized rolled non-recrystallized station at a total rolling reduction of 30% or more. And, the non-recrystallized rolled steel sheet can be produced by cooling to a temperature range of (Bs-80°C) to Bs.

Slab reheating, holding and extraction

Since the slab of the present invention is provided with the same alloy composition as the alloy composition of the steel sheet described above, the description of the slab alloy composition of the present invention is replaced by the description of the steel sheet alloy composition described above.

Slab reheating is a process of smoothly performing the subsequent rolling process and heating the steel to secure the properties of the desired steel sheet, so the heating process must be performed within an appropriate temperature range suitable for the purpose. The lower limit of the slab reheating temperature should take into account whether or not the precipitated elements are sufficiently soluble in steel. Particularly, the present invention essentially includes niobium (Nb) in order to secure high-strength properties, so that the lower limit of the slab reheating temperature can be limited to 1140° C. in consideration of the temperature for reusing niobium (Nb). On the other hand, if the slab reheating temperature is excessively high, austenite grains may become too coarse to cause a problem that the grains of the final steel sheet are excessively increased, and the present invention can limit the upper limit of the slab reheating temperature to 1200°C. have.

The reheated slab may be subjected to a maintenance and extraction step as necessary, and for similar reasons to the slab reheating temperature, the maintenance and extraction temperature of the slab may be limited to a temperature range of 1140 to 1200°C.

Recrystallization rolling

Recrystallization rolling can be carried out in a temperature range above Tnr. In the present invention, Tnr means the lower limit of the temperature range in which austenite recrystallization occurs. That is, rolling in the recrystallization zone can be performed in the temperature range of the austenite recrystallization zone. Recrystallization rolling can be carried out in multiple passes, and rolling can be performed at an average rolling reduction of 10% or more per pass. This is because, if the average rolling reduction per pass is less than 10%, the grain size of the recrystallized austenite becomes coarse, which may lead to a decrease in toughness of the final steel sheet.

The recrystallized rolled rolled material can be cooled to a temperature range below Tnr under cooling conditions of air cooling. That is, without rolling the non-recrystallized station immediately on the rolled material having been recrystallized, it can be cooled to the temperature range of the non-recrystallized area by air cooling by waiting for a certain period of time. This is because partial recrystallization may occur when a pressing force is applied in a corresponding section, and brittle fracture due to coarse austenite particle size may occur.

Unrecrystallized station rolling

Unrecrystallized reverse rolling is performed on the recrystallized rolled rolled material. The initiation temperature of the non-recrystallized zone rolling is Tnr or less, and the end temperature of the non-recrystallized zone rolling may be (Ar3+100°C). Non-recrystallization rolling is a process to obtain a fine ferrite and bainite by elongating austenite produced by re-crystallization rolling and forming a deformed structure in the mouth. The stopping characteristics can be effectively improved.

The lower the end temperature of the non-recrystallized rolling, the higher the degree of deformation of austenite, which is effective in improving low-temperature fracture toughness. However, if the unrecrystallized rolling end temperature is excessively low, ferrite with low strength is generated, which is disadvantageous in securing strength. The present invention can limit the rolling end temperature of the non-recrystallized zone to (Ar3+50°C) or higher.

In addition, the rolling reduction amount of unrecrystallized reverse rolling is an important factor in securing low-temperature toughness of steel materials. The present invention can limit the rolling reduction amount of the unrecrystallized reverse rolling to 30% or more in order to secure the low-temperature DWTT ductile wavefront property according to the particle size refinement of the final steel. The larger the rolling height of the non-recrystallized zone rolling is, the more effective it is to improve low-temperature toughness, so the upper limit of the rolling amount of the non-recrystallized zone rolling may not be limited. However, when the rolling amount of non-recrystallized rolling exceeds a certain level, the effect of particle size refinement is saturated while the amount of rolling of recrystallized rolling decreases, so the present invention limits the rolling of unrecrystallized rolling to 90% or less. can do.

Cooling

The non-recrystallized rolled steel sheet can be cooled from a cooling start temperature of (Ar3+30°C) or higher to a cooling stop temperature of (Bs-80°C) to Bs. When the cooling start temperature is excessively low, a large amount of ferrite with low strength is produced, and accordingly, the strength of the steel sheet may be greatly reduced, so the present invention can start cooling in a temperature range of (Ar3+30°C) or higher.

In addition, since the final thickness of the steel sheet of the present invention is less than 20 mm, it is most preferable in terms of strength and elongation to stop cooling in the temperature range of (Bs-80°C) to Bs. When the cooling stop temperature is less than (Bs-80°C), a high hard angle grain boundary is coarsely formed, and a large amount of acicular ferrite and bainite having a low hard grain boundary may be formed, so that the elongation may be lowered, and the cooling stop temperature exceeds Bs. In this case, it is because the amount of bainite generated is small and the strength of the steel sheet cannot be secured. After cooling the steel sheet to a cooling stop temperature of (Bs-80°C) to Bs, the steel sheet can be cooled to room temperature by air cooling or cooling.

In addition, the cooling of the present invention can be carried out at a cooling rate of 10 ~ 100 ℃ / s. This is because when the cooling rate is less than 10°C/s, the fraction of equiaxed ferrite is greatly increased, so that high strength properties of the steel sheet cannot be effectively secured. In terms of equipment conditions and economics, the upper limit of the cooling rate may be limited to 100°C/s, and the upper limit of the more preferable cooling rate may be 50°C/s.

The steel sheet manufactured through the above manufacturing method includes ferrite and bainite as a microstructure, and in addition, may further include a martensite phase. The fractions of ferrite and bainite may be 20-60 area% and 35-75 area%, respectively, and the fraction of island martensite may be 5 area% or less. In addition, the steel sheet manufactured through the above-described manufacturing method may have a size of the upper 80% grain size of 70 µm or less of a 15° reference high-angle crystal grain at the center of the steel sheet.

Therefore, the steel sheet manufactured through the above-described manufacturing method is provided with a thickness of less than 20 mm, and can have a yield strength of 485 MPa or more, a total elongation of 28% or more, and a uniform elongation in a rolling perpendicular direction of 9% or more, and the rolling right angle of the steel sheet. DWTT ductile wavefront of -30 ℃ to the direction may be 85% or more. Therefore, according to the manufacturing method according to an aspect of the present invention, while having a thickness of less than 20 mm, it is possible to provide a steel sheet particularly suitable as a material for line pipes by effectively securing strength, low temperature fracture toughness, and elongation.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the embodiments described below are only intended to further illustrate the present invention and are not intended to limit the scope of the present invention.

(Example)

A slab having a thickness of 250 mm provided with the alloy composition of Table 1 was produced, and steel plate specimens having a thickness of 11 mm, 11.5 mm, and 22 mm, respectively, were manufactured by applying the process conditions of Table 3. At this time, the slab fabrication was applied to the process conditions used in the conventional slab fabrication, and recrystallization rolling was performed for all specimens by applying a condition of an average rolling reduction of 10% or more per pass over a temperature range of Tnr or higher. In addition, air cooling was applied to the temperature range of the non-recrystallized zone after rolling the recrystallized zone for all specimens. In Table 2, Tnr temperature, Ar3 temperature, and Bs temperature are calculated and calculated based on each alloy composition in Table 1, and calculation formulas used for calculating Tnr temperature, Ar3 temperature, and Bs temperature in Table 2 are separately described below Table 2. .

Steel	Alloy composition (% by weight)													division
	C	Si	Mn	P	S	Ni	Cr	Mo	Nb	Al	Ca	Ti	N
A	0.070	0.27	1.57	0.01	0.0020	0.10	0.10	-	0.049	0.023	0.0010	0.011	0.0032	Invention steel
B	0.074	0.25	1.66	0.013	0.0020	0.10	0.10	-	0.055	0.034	0.0011	0.014	0.0044
C	0.050	0.25	1.58	0.012	0.0019	0.10	0.11	0.08	0.048	0.023	0.0010	0.013	0.0043
D	0.060	0.24	1.55	0.011	0.0019	0.10	0.10	0.08	0.041	0.026	0.0013	0.014	0.0041
E	0.050	0.25	1.52	0.012	0.0018	0.10	0.10	0.08	0.041	0.024	0.0010	0.014	0.0044
F	0.09	0.34	1.45	0.010	0.0011	0.17	0.1	-	0.045	0.027	0.0010	0.016	0.0041
G	0.050	0.26	1.57	0.010	0.0020	0.10	0.11	0.08	0.028	0.028	0.0010	0.012	0.0046	Comparative steel
H	0.040	0.26	1.24	0.008	0.0010	0.14	0.2	0.07	0.04	0.030	0.0007	0.012	0.0046
I	0.060	0.25	2.10	0.0075	0.0015	0.3	0.15	0.10	0.050	0.030	0.0016	0.015	0.0050

Steel	Tnr (℃)	Ar3 (℃)	Ar3+30 (℃)	Ar3+100 (℃)	Bs (℃)	Bs-80 (℃)
A	1014	767	797	867	659	579
B	1060	760	790	860	650	570
C	1009	771	801	871	656	576
D	985	771	801	871	657	577
E	976	776	806	876	662	582
F	985	767	797	867	662	582
G	911	772	802	872	657	577
H	963	796	826	896	683	603
I	1028	718	748	818	595	515

Equation 1: Tnr(℃) = 887 + 464*[C] + 6445*[Nb]-644*[Nb] ^(1/2) + 732*[V]-230*[V] ^(1/2) + 890*[Ti] + 363*[Al]-357*[Si]

Equation 2: Ar3(℃) = 910-273*[C]-74*[Mn]-57*[Ni]-16*[Cr]-9*[Mo]-5[Cu]

Equation 3: Bs(℃) = 830-270*[C]-90*[Mn]-37*[Ni]-70*[Cr]-83*[Mo]

(In the above formulas 1 to 3, [C], [Si], [Mn], [Al], [Ti], [Nb], [V], [Cr], [Mo] and [Cu] are each It means the weight percent of alloy composition, and if the alloy composition is not included, it is calculated by substituting the value as 0.)

Psalter	Steel	Steel plate thickness (mm)	Slab heating temperature (℃)	Unrecrystallized reverse rolling start temperature (℃)	Cumulative rolling reduction (%)	Unrecrystallized reverse rolling end temperature (℃)	Cooling start temperature (℃)	Cooling stop temperature (℃)	Cooling rate (℃/s)	Remark
One	A	11.5	1150	1000	42.5	915	810	590	36	Inventive Example
2	A	11.5	1170	1000	42.5	915	820	640	36
3	A	11.5	1160	1000	50	875	800	580	43
4	B	11.5	1160	1035	56	915	805	615	32
5	B	11.5	1165	1035	56	915	820	610	35
6	B	11.5	1150	1025	56	875	800	625	35
7	C	11	1160	990	39	925	830	580	30
8	C	11	1150	995	39	920	825	635	30
9	D	11	1160	985	31	920	830	620	30
10	E	11	1150	975	31	920	825	590	30
11	E	11	1160	975	31	920	830	620	30
12	F	11	1150	981	39	875	805	620	35
13	C	11	1160	1000	50	875	795	460	42	Comparative example
14	D	11	1150	975	31	920	825	550	30
15	D	11	1150	970	31	920	825	510	30
16	D	11	1160	983	50	863	771	685	30
17	E	11	1150	975	31	920	820	530	30
18	G	11	1170	910	12	910	830	470	30
19	G	11	1150	910	12	910	830	580	30
20	H	11	1159	954	0	954	819	491	47
21	I	11	1160	1020	45	920	830	570	30
22	A	22	1150	945	98	835	775	610	23
23	D	22	1144	935	78	827	767	617	21

For each specimen in Table 3, the microstructure, yield strength and tensile strength, elongation and DWTT ductility at -30°C were measured and are shown in Table 4 below. The microstructure of each specimen was evaluated using optical microscopy and EBSD particle size distribution. Yield strength, tensile strength and elongation were evaluated by performing a room temperature tensile test on each specimen. The yield strength and tensile strength described in Table 4 refer to the measured values for each direction perpendicular to the rolling. In addition, the tensile properties and the ductility wavefront were evaluated by performing a DWTT test at -30°C for each specimen.

Psalter	Steel	Ferrite (area %)	Bainite (area %)	M/A (area %)	High hard angle grain size 80% particle size (㎛)	Yield strength (MPa)	Tensile strength (MPa)	Yield ratio (%)	Elongation (%)	Uniform elongation (%)	-30℃ DWTT(%)	Remark
One	A	25	71.0	4.0	52.0	524	667	79	33	9.0	100	Inventive Example
2	A	42	54.5	3.5	40.5	508	635	80	36	10.5	100
3	A	28	67.5	4.5	46.5	541	671	81	35	9.6	100
4	B	32	63.6	4.4	62.1	525	671	78	33	9.0	100
5	B	43	52.8	4.2	57.9	529	667	79	34	9.5	100
6	B	52	43.5	4.5	56.6	528	653	81	36	9.9	99
7	C	23	72.8	4.2	67.1	487	643	76	33	9.2	98
8	C	30	65.7	4.3	48.5	489	620	79	34	9.7	99
9	D	23	72.7	4.3	60.1	505	638	79	33	9.1	100
10	E	24	71.5	4.5	48.5	485	614	79	31	9.7	100
11	E	28	67.7	4.3	42.0	495	613	81	36	10.9	100
12	F	31	64.8	4.2	45.1	536	625	86	36	10.5	95
13	C	17	78.3	4.7	86.1	620	730	85	28	7.7	100	Comparative example
14	D	15	80.5	4.5	89.5	501	651	77	28	7.6	100
15	D	18	77.7	4.3	75.2	500	628	80	32	8.3	100
16	D	75	22.9	2.1	90.5	455	570	80	42	13.0	92
17	E	22	74.0	4.0	76.5	492	632	78	28	8.2	100
18	G	24	72	4.0	93	524	656	80	28	6.3	75
19	G	26	69.8	4.2	82.5	502	620	81	32	7.9	80
20	H	28	67.9	4.1	78.0	528	606	87	30	8.0	50
21	I	12	81.8	6.2	89.1	624	796	78	28	6.8	91
22	A	65	31.2	3.8	58.6	475	568	84	47	13.0	100
23	D	66	30.5	3.5	65.1	470	565	83	48	13.5	100

In the case of specimens 1 to 12 satisfying the alloy composition and process conditions of the present invention, the microstructure includes 20 to 60 area% ferrite, 35 to 75 area% bainite, and 5 area% or less island martensite, In the center of the steel sheet, the crystal grain size of the top 80% of the high-angle crystal grains based on 15 degrees is 70 µm or less, the yield strength is 485 MPa or more, the total elongation is 28% or more, the uniform elongation in the rolling right angle direction is 9% or more, and for the rolling right angle direction- Since it satisfies the DWTT ductile wavefront ratio of 85% or higher at 30°C, it can be confirmed that it has physical properties particularly suitable as a material for line pipes provided in a cryogenic environment.

Specimens 13 to 15 and 17 satisfy the alloy composition of the present invention, but are specimens when cooling is performed at a temperature range lower than the cooling start temperature or the cooling end temperature of the present invention. In the case of specimens 13 to 15 and 17, ferrite of less than 20 area% and bainite of more than 75 area% were formed, and since the grain size of the top 80% of the high-angle crystallinity based on 15 degrees in the center of the steel sheet exceeds 70 μm, It can be seen that the uniform elongation is less than 9%.

Specimen 16 satisfies the alloy composition of the present invention, but unrecrystallized reverse rolling is performed at a temperature range lower than the end temperature of the unrecrystallized reverse rolling of the present invention, and cooling is started at a temperature range lower than the cooling start temperature of the present invention. It is a specimen when cooling is finished in a temperature range higher than the cooling stop temperature of. In the case of the specimen 16, it can be confirmed that a ferrite of more than 60 area% was formed and the yield strength was less than 485 MPa.

Specimens 18 to 21 are specimens that do not satisfy the alloy composition and process conditions of the present invention, it can be confirmed that the present invention does not secure the desired microstructure and properties.

Specimens 22 and 23 satisfies the alloy composition of the present invention, but it can be confirmed that the thickness of the steel sheet exceeds 20 mm, so that ferrite is excessively formed.

1 is a photograph obtained by observing specimen 2 with an optical microscope, and FIG. 2 is a result of measuring the grain size of a 15 degree reference high-angle grain boundary of specimen 2 using EBSD. As shown in the graph of FIG. 2, it can be seen that the average size of the high-angle grain boundaries of the specimen 2 is 22,3 μm, and the grain size of the top 80% is 40.5 μm.

3 is a photograph obtained by observing the specimen 18 with an optical microscope, and FIG. 4 is a result of measuring the grain size of the standard 18 high-angle grain boundary of the specimen 18 using EBSD. As shown in the graph of FIG. 4, it can be seen that the average size of the high-angle grain boundaries of the specimen 18 is 38 µm, and the crystal size of the top 80% is 93 µm.

Accordingly, according to an aspect of the present invention, while having a thickness of less than 20 mm, a yield strength of 485 MPa or more, a total elongation of 28% or more, and a uniform elongation for a rolling perpendicular direction of 9% or more and a rolling perpendicular direction of a steel plate of 85% or more It has a DWTT ductile wavefront of -30℃, and it can provide a steel sheet and a manufacturing method particularly suitable for a line pipe material.

Although the present invention has been described in detail through the above embodiments, other types of embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (17)

1. In weight percent, carbon (C): 0.05 to 0.1%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 1.4 to 2.0%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 ~0.02%, Nitrogen (N): 0.002~0.01%, Niobium (Nb): 0.04~0.07%, Chromium (Cr): 0.05~0.3%, Nickel (Ni): 0.05~0.4%, Phosphorus (P): 0.02 % Or less, sulfur (S): 0.005% or less, calcium (Ca): 0.0005 to 0.004%, including the remaining iron (Fe) and unavoidable impurities,

   20~60 area% of ferrite and bainite as microstructure,

   High-strength steel sheet with excellent low-temperature fracture toughness and elongation at the top 80% of the grain size of the high-angle crystal grain at the center of the steel plate at 70 degrees or less.
2. According to claim 1,

   The steel sheet further comprises molybdenum (Mo) of 0.3% by weight or less, high strength steel sheet excellent in low temperature fracture toughness and elongation.
3. According to claim 1,

   The fraction of bainite is 35 to 75 area%, high-strength steel sheet excellent in low temperature fracture toughness and elongation.
4. According to claim 1,

   The microstructure of the steel sheet further comprises an island martensite of 5 area% or less, a high-strength steel sheet having excellent low-temperature fracture toughness and elongation.
5. According to claim 1,

   The steel sheet has a yield strength of 485 MPa or higher, a high-strength steel sheet having excellent low-temperature fracture toughness and elongation.
6. According to claim 1,

   The total elongation of the steel sheet is 28% or more,

   The steel sheet has a uniform elongation with respect to the perpendicular direction of rolling of at least 9%, a high-strength steel sheet having excellent low-temperature fracture toughness and elongation.
7. According to claim 1,

   A high-strength steel sheet having excellent low-temperature fracture toughness and elongation at a DWTT ductility of 85% or more at -30°C relative to the perpendicular direction of rolling of the steel sheet.
8. According to claim 1,

   The thickness of the steel sheet is less than 20mm, high strength steel sheet excellent in low temperature fracture toughness and elongation.
9. In weight percent, carbon (C): 0.05 to 0.1%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 1.4 to 2.0%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 ~0.02%, Nitrogen (N): 0.002~0.01%, Niobium (Nb): 0.04~0.07%, Chromium (Cr): 0.05~0.3%, Nickel (Ni): 0.05~0.4%, Phosphorus (P): 0.02 % Or less, sulfur (S): 0.005% or less, calcium (Ca): 0.0005 to 0.004%, reheat the slab containing the remaining iron (Fe) and unavoidable impurities,

   Maintaining and extracting the slab,

   The retained and extracted slabs are recrystallized in a temperature range above Tnr and rolled,

   The recrystallized rolled rolled material is unrecrystallized rolled at a total rolling reduction of 30% or more,

   The unrecrystallized reverse rolled steel sheet is cooled to a temperature range of (Bs-80°C) to Bs,

   The non-recrystallization rolling is initiated in a temperature range of Tnr or less (Ar3+100°C) and finished in a temperature range of (or higher), a method for manufacturing a high strength steel sheet having excellent low temperature fracture toughness and elongation.
10. The method of claim 9,

    The slab further comprises a molybdenum (Mo) of 0.3% by weight or less, low-temperature fracture toughness and a method for producing a high-strength steel sheet excellent in elongation.
11. The method of claim 9,

    The slab reheating temperature range of 1140 ~ 1200 ℃, low-temperature fracture toughness and elongation method of manufacturing a high-strength steel sheet excellent.
12. The method of claim 9,

    The slab maintenance and extraction temperature range is 1140 ~ 1200 ℃, low-temperature fracture toughness and elongation excellent method of manufacturing a high-strength steel sheet.
13. The method of claim 9,

    The recrystallization rolling is carried out in a plurality of passes,

    The average rolling reduction of each pass is 10% or more, low-temperature fracture toughness and elongation method of high-strength steel sheet excellent.
14. The method of claim 9,

    The recrystallized rolled rolling material is cooled to a temperature range below Tnr by air cooling, a method for manufacturing a high strength steel sheet having excellent low temperature fracture toughness and elongation.
15. The method of claim 9,

    The non-recrystallized rolled steel sheet is cooled at a cooling rate of 10 to 50°C/s, a method of manufacturing a high strength steel sheet having excellent low temperature fracture toughness and elongation.
16. The method of claim 9,

    The method of manufacturing a high-strength steel sheet having excellent low-temperature fracture toughness and elongation, which is initiated in a temperature range of (Ar3+30°C) or more, wherein cooling of the unrecrystallized rolled steel sheet.
17. The method of claim 9,

    The thickness of the steel sheet is less than 20mm, low-temperature fracture toughness and elongation method of high-strength steel sheet excellent.

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