JP7372325B2 - High-strength steel plate with excellent low-temperature fracture toughness and elongation, and its manufacturing method - Google Patents

Description

The present invention relates to a high-strength steel plate and a method for manufacturing the same, and in particular, by optimizing the composition, microstructure, and manufacturing process of the steel, it has high strength properties and excellent low-temperature fracture toughness and elongation. , relates to a high-strength steel plate for line pipes that can be used stably even in harsh environments, and a method for manufacturing the same.

In recent years, oil field development has been carried out mainly in cold regions such as Siberia and Alaska with poor climatic conditions, and projects are actively underway to transport the rich gas resources of oil field regions to consumption regions via line pipes. It is being The steel provided for such linepipe projects is designed to withstand pipe deformation not only from the pressure of the transported gas, but also from cryogenic temperatures and frost heave (a phenomenon in which soil freezes and rises at the change of seasons). Although it is essential, it is required to have excellent DWTT (Drop Weight Tearing Test) ductile fracture ratio and high elongation properties while having high strength properties.
The DWTT ductile fracture ratio is a kind of index for determining whether a steel material for line pipes has brittle fracture arresting characteristics for safe use at low temperatures. In general, line pipes provided to cold regions are required to have a DWTT ductile fracture ratio of 85% or higher based on -20°C. In order to ensure a DWTT ductile fracture ratio of 85% or more based on -20°C in the pipe state, the DWTT ductile fracture ratio of the steel plate provided for pipe production must be at a level of 85% or more based on -30°C. must be met.

In general, it is known that the DWTT ductile fracture ratio has a deep relationship with the effective grain size of the steel sheet. Effective grain size is defined by the size of grains with high-angle grain boundaries, and as the effective grain size becomes finer, crack propagation resistance increases. This is because when a crack starts and propagates, the propagation path of the crack is changed at the effective grain boundary.
In order to refine the effective grain size, a method of performing accelerated cooling immediately after rolling is widely used. By accelerated cooling immediately after rolling, a mixed structure of acicular ferrite and bainite can be realized. However, the microstructure formed by normal accelerated cooling has high hardness because the crystal grains are supersaturated with carbon (C). This shows poor ductility with a uniform elongation of less than 9% and a total elongation of less than 20%. As a result, formability during pipe production is reduced, and local stress concentration tends to occur when external deformation is applied, resulting in a problem in that the stability of the pipe is greatly reduced.

Therefore, when manufacturing steel plates for line pipes, we improve the elongation deterioration of steel plates manufactured by accelerated cooling, and while having excellent low-temperature fracture toughness, we have to improve uniform elongation of 9% or more and total elongation of 28% or more. Therefore, there is a need for a method for manufacturing line pipe steel sheets with excellent ductility.
There has been research into steel sheets with excellent elongation and low-temperature fracture toughness. In relation to this, Patent Document 1 discloses that a steel material containing nickel (Ni), niobium (Nb), and molybdenum (Mo) is rolled in a non-recrystallized region with a reduction of 65% or more, and then cooled at 15 to 30°C. By first cooling to the Bs temperature at a cooling rate of 30 to 60°C and second cooling in a temperature range of 350 to 500°C at a cooling rate of 30 to 60°C, equiaxed ferrite with an area fraction of 30 to 60% and 40 to 70% % of bainitic mixed structure as a microstructure.

However, Patent Document 1 performs low-temperature rolling on a steel plate with a thickness of 20 mm or more, and there are technical difficulties in applying the process conditions to a steel plate with a thickness of less than 20 mm. This is because steel plates with a thickness of less than 20 mm are rapidly cooled after low-temperature rolling. This is because it is difficult to secure.

Korean Publication Patent No. 10-2013-0073472 (published on 2013.07.03)

According to one aspect of the present invention, it is an object to provide a high-strength steel plate with excellent low-temperature toughness and a method for manufacturing the same.
The object of the present invention is not limited to the above-mentioned contents. A person of ordinary skill in the art will have no difficulty in understanding the additional objects of the present invention from the overall content of this specification.

A high-strength steel sheet with excellent fracture toughness and elongation according to one aspect of the present invention includes, in weight percent, carbon (C): 0.05 to 0.1%, silicon (Si): 0.05 to 0.5%, Manganese (Mn): 1.4-2.0%, 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): 0.05~0.4%, Phosphorus ( P): 0.02% or less, sulfur (S): 0.005% or less, calcium (Ca): 0.0005 to 0.004%, the remainder consists of iron (Fe) and inevitable impurities, 20 to 60 area % of ferrite and bainite as a microstructure, and in the center of the steel sheet, the grain size of the top 80% of high angle grain sizes based on 15 degrees can be 70 μm or less.

The steel plate may further include 0.3% by weight or less of molybdenum (Mo).
The fraction of bainite may be 35 to 75 area %.
The microstructure of the above-mentioned steel plate can further include island-like martensite of 5 area % or less.
The steel plate may have a yield strength of 485 MPa or more.

The total elongation of the steel plate may be 28% or more, and the uniform elongation of the steel plate in the direction perpendicular to rolling may be 9% or more.
The steel plate may have a DWTT ductile fracture ratio of 85% or more at -30°C in the direction perpendicular to rolling.
The thickness of the steel plate may be less than 20 mm.

A high-strength steel sheet with excellent low-temperature fracture toughness and elongation according to one aspect of the present invention has carbon (C): 0.05 to 0.1% and silicon (Si): 0.05 to 0.5% by weight. , manganese (Mn): 1.4 to 2.0%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0. 002-0.01%, 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%, and the remainder is iron (Fe) and unavoidable impurities. Reheat the slab. Then, hold and take out the slab, roll the held and taken out slab in a recrystallization region at a temperature range of Tnr or higher, and unrecrystallize the rolled material rolled in the recrystallization region at a total reduction rate of 30% or more. produced by rolling the steel sheet in the non-recrystallized region and cooling the steel plate rolled in the non-recrystallized region to a temperature range of (Bs - 80 ° C.) to Bs, and the rolling in the non-recrystallized region is started in a temperature range below Tnr, The process can be completed in a temperature range of (Ar3+100°C) or higher.

The slab may further include 0.3% by weight or less of molybdenum (Mo).
The temperature range for reheating the slab may be 1140-1200°C.
The temperature range for holding and unloading the slab can be 1140-1200°C.

The recrystallization zone rolling may be performed in a plurality of passes, and the average rolling reduction of each pass may be 10% or more.
The rolled material rolled in the recrystallization zone can be cooled to a temperature range below Tnr by air cooling.
The steel sheet rolled in the non-recrystallized region can be cooled at a cooling rate of 10 to 50° C./s.

Cooling of the steel sheet rolled in the non-recrystallized region may be started in a temperature range of (Ar3+30° C.) or higher.
The thickness of the steel plate may be less than 20 mm.
The solution to the above problem does not list all the features of the present invention, and various features of the present invention and advantages and effects thereof can be understood in more detail with reference to the following specific embodiments. .

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

It is a photograph of Test Piece 2 of Example observed with an optical microscope. These are the results of measuring the grain size of high-angle grain boundaries based on 15 degrees in test piece 2 using EBSD. It is a photograph of test piece 18 of Example observed with an optical microscope. These are the results of measuring the grain size of high-angle grain boundaries based on 15 degrees in test piece 18 using EBSD.

The present invention relates to a high-strength steel plate with excellent low-temperature fracture toughness and elongation, and a method for manufacturing the same. Preferred embodiments of the present invention will be described below. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as limited to the embodiments described below. These embodiments are provided so that the present invention will be more fully understood and understood by those skilled in the art to which the invention pertains.

Hereinafter, the composition of the steel of the present invention will be explained in more detail. Hereinafter, unless otherwise indicated, percentages indicating the content of each element are based on weight.
A high-strength steel sheet with excellent low-temperature fracture toughness and elongation according to one aspect of the present invention has carbon (C): 0.05 to 0.1% and silicon (Si): 0.05 to 0.5% by weight. , manganese (Mn): 1.4 to 2.0%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0. 002-0.01%, 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%, and the remainder consists of iron (Fe) and inevitable impurities.
In addition, the steel sheet having excellent low-temperature fracture toughness and elongation according to one aspect of the present invention may further include molybdenum (Mo): 0.3% or less by weight.

Carbon (C): 0.05-0.1%
Carbon (C) is the most effective element for improving the strength of steel. Additionally, if the amount of carbon (C) added is below a certain level, large amounts of expensive alloying elements such as molybdenum (Mo) and nickel (Ni) must be added to ensure the strength of the steel. Therefore, it is not desirable from an economic point of view. In the present invention, in order to achieve such effects, the lower limit of the carbon (C) content can be limited to 0.05%. However, if an excessive amount of carbon (C) is added, it is unfavorable from the viewpoint of weldability, formability, and toughness of the steel, so in the present invention, the upper limit of the carbon (C) content is limited to 0.1%. can do. Therefore, the carbon (C) content of the present invention can be in the range of 0.05 to 0.1%, and the more preferable carbon (C) content is in the range of 0.05 to 0.095%. Something can happen.

Silicon (Si): 0.05-0.5%
Silicon (Si) is an element useful for deoxidizing molten steel, and is also an element that contributes to improving the strength of steel through solid solution strengthening. In the present invention, in order to achieve such effects, the lower limit of the silicon (Si) content can be limited to 0.05%. A more preferable lower limit of the silicon (Si) content may be 0.1%. However, since silicon (Si) is a strongly oxidizing element, it is preferable to limit the upper limit of the silicon (Si) content to a certain range. That is, when silicon (Si) is added in an excessive amount, it induces the formation of red scale during hot rolling, which is undesirable from the viewpoint of surface quality and also has an unfavorable effect on the toughness of the weld zone. Therefore, in the present invention, the upper limit of the silicon (Si) content can be limited to 0.5%. A more preferable upper limit of the silicon (Si) content may be 0.4%.

Manganese (Mn): 1.4-2.0%
Manganese (Mn) is an element effective in solid solution strengthening of steel. In the present invention, the lower limit of the manganese (Mn) content can be limited to 1.4% to ensure high strength properties of the steel. However, if an excessive amount of manganese (Mn) is added, a segregated area may be formed over a wide range at the center of the thickness during slab casting in the steelmaking process, which is unfavorable from the viewpoint of weldability of the final product. . Therefore, in the present invention, the upper limit of the manganese (Mn) content can be limited to 2.0%. A more preferable upper limit of the manganese (Mn) content may be 1.8%.

Aluminum (Al): 0.01-0.05%
Aluminum (Al) is a typical element added as a deoxidizing agent together with silicon (Si). Moreover, aluminum (Al) is also an element that contributes to improving the strength of steel through solid solution strengthening. In the present invention, in order to achieve such effects, the lower limit of the aluminum (Al) content can be limited to 0.01%. A more preferable lower limit of the aluminum (Al) content may be 0.015%. However, if too much aluminum (Al) is added, it is not preferable from the viewpoint of impact toughness. Therefore, in the present invention, the upper limit of the aluminum content can be limited to 0.05%. A more preferable upper limit of the aluminum (Al) content may be 0.04%.

Titanium (Ti): 0.005-0.02%
Titanium (Ti) is an element that refines the grain size of the final structure by forming TiN precipitates during the solidification process of steel and suppressing the growth of austenite crystal grains during the heating and hot rolling processes of the slab. In the present invention, the lower limit of the content of titanium (Ti) can be limited to 0.005% in order to achieve the effect of improving the toughness of steel through refinement of the final structure. A more preferable content of titanium (Ti) may be 0.008%. However, when too much titanium (Ti) is added, coarse TiN is precipitated when the slab is heated, which is rather unsuitable for refining the final structure. Therefore, in the present invention, the upper limit of the titanium (Ti) content can be limited to 0.02%. A more preferable upper limit of the content of titanium (Ti) may be 0.018%.

Nitrogen (N): 0.002-0.01%
Nitrogen (N) is dissolved in steel and then precipitated, and plays the role of increasing the strength of steel, and this strength-improving effect is known to be significantly greater than that of carbon (C). . In addition, in the present invention, in order to form TiN through the reaction of titanium (Ti) and nitrogen (N) and to suppress the growth of crystal grains during the reheating process, the lower limit of the nitrogen (N) content is set to 0.002%. can be limited to. However, when too much nitrogen (N) is added, nitrogen (N) exists in the form of solid solution nitrogen (N) rather than in the form of TiN precipitates, which may significantly reduce the toughness of the steel. Therefore, in the present invention, the upper limit of the nitrogen (N) content can be limited to 0.01%. A preferable upper limit of the nitrogen (N) content is 0.006%, and a more preferable upper limit of the nitrogen (N) content can be 0.005%.

Niobium (Nb): 0.04-0.07%
Niobium (Nb) is an extremely useful element for refining crystal grains, and is also an element that promotes the formation of acicular ferrite or bainite, which are high-strength structures, and greatly contributes to improving the strength of steel. Furthermore, since high-temperature rolling is unavoidable for steel sheets with a thickness of less than 20 mm, which is the object of the present invention, it is necessary to add a certain amount or more of niobium (Nb), which has the greatest effect on increasing the non-recrystallization temperature. Therefore, in the present invention, the lower limit of the niobium (Nb) content can be limited to 0.04%. However, if too much niobium (Nb) is added, the weldability of the steel may deteriorate. Therefore, in the present invention, the upper limit of the niobium (Nb) content can be limited to 0.07%. A preferable upper limit of the niobium (Nb) content may be 0.06%.

Chromium (Cr): 0.05-0.3%
Chromium (Cr) is an element that improves hardenability and is effective in improving the strength of steel. Further, chromium (Cr) is an element that promotes the formation of island martensite/austenite (MA) during accelerated cooling and contributes to improving uniform elongation. In the present invention, in order to achieve such effects, the lower limit of the chromium (Cr) content can be limited to 0.05%. A more preferable lower limit of the chromium (Cr) content may be 0.08%. However, if too much chromium (Cr) is added, there is a possibility that weldability will deteriorate. Therefore, in the present invention, the upper limit of the chromium (Cr) content can be limited to 0.3%. A preferable upper limit of the chromium (Cr) content is 0.25%, and a more preferable upper limit of the chromium (Cr) content may be 0.2%.

Nickel (Ni): 0.05-0.4%
Nickel (Ni) is an element that effectively contributes to improving the toughness and strength of steel. In the present invention, in order to achieve such effects, the lower limit of the nickel (Ni) content can be limited to 0.05%. However, nickel (Ni) is an expensive element, and adding too much nickel (Ni) is not preferable from the economic point of view. Therefore, in the present invention, the upper limit of the nickel (Ni) content can be limited to 0.4%. A preferable upper limit of the nickel (Ni) content is 0.3%, and a more preferable upper limit of the nickel (Ni) content may be 0.25%.

Phosphorus (P): 0.02% or less Phosphorus (P) is a typical impurity element that exists in steel, and is mainly segregated in the center of the steel plate and induces a decrease in the toughness of the steel. It is preferable to control it to the lowest possible level. However, completely removing phosphorus (P) from steel requires excessive cost and time in the steel manufacturing process, which is unfavorable from an economical point of view. Therefore, in the present invention, the content of phosphorus (P) can be limited to 0.02% or less. A more preferable phosphorus (P) content may be 0.015% or less.

Sulfur (S): 0.005% or less Sulfur (S) is also a typical impurity element present in steel, and combines with manganese (Mn) in steel to form nonmetallic inclusions such as MnS. Since it is an element that greatly damages the toughness and strength of steel, it is preferable to control it to the lowest possible level. However, completely removing sulfur (S) from steel requires excessive cost and time in the steel manufacturing process, which is undesirable from an economical point of view. Therefore, in the present invention, the content of sulfur (S) can be limited to 0.005% or less. More preferably, the content of sulfur (S) may be 0.003% or less.

Calcium (Ca): 0.0005-0.004%
Calcium (Ca) is an element effective in making nonmetallic inclusions such as MnS spheroidal and suppressing the formation of cracks around the nonmetallic inclusions. In the present invention, in order to achieve such effects, the lower limit of the calcium (Ca) content can be limited to 0.0005%. However, when too much calcium (Ca) is added, a large amount of CaO-based inclusions are formed, leading to a decrease in impact toughness. Therefore, in the present invention, the upper limit of the calcium (Ca) content can be limited to 0.004%. A preferable upper limit of the calcium (Ca) content may be 0.002%.

Molybdenum (Mo): 0.3% or less Molybdenum (Mo) is an element that promotes the formation of bainite, which is a low-temperature transformed structure, and is effective in ensuring high strength and high toughness characteristics at the same time. Therefore, in the present invention, molybdenum (Mo) can be selectively added to achieve such effects. However, molybdenum (Mo) is an expensive element, and when added in excess, it is not preferable from an economic point of view. Therefore, in the present invention, the upper limit of the molybdenum (Mo) content can be limited to 0.3%.

In the present invention, in addition to the above-described steel composition, the steel may contain Fe and unavoidable impurities. Unavoidable impurities can be accidentally mixed in during normal steel manufacturing processes, so they cannot be completely eliminated. Engineers in the field of ordinary steel manufacturing can easily understand its meaning. Furthermore, the present invention does not completely exclude the addition of other compositions than the above-mentioned steel compositions.

Hereinafter, the microstructure of the present invention will be explained in more detail.
A steel sheet according to one aspect of the present invention includes ferrite and bainite as a microstructure, and may further include island-shaped martensite. The fraction of ferrite and bainite can be 20 to 60 area % and 35 to 75 area %, respectively, and the fraction of island martensite can be 5 area % or less.
Since the present invention contains 20% by area or more of ferrite having fine high-angle grain boundaries, low-temperature DWTT characteristics can be effectively ensured. Moreover, since the present invention contains 60 area % or less of ferrite and 35 area % or more of bainite, it is possible to ensure a yield strength of 485 MPa or more. However, in the present invention, in order to prevent high-angle grain boundaries from becoming excessively coarse, the bainite fraction is limited to 75 area% or less, and thereby, low-temperature DWTT characteristics cannot be effectively ensured. can. Incidentally, since the island-like martensite has an unfavorable effect on the low-temperature DWTT characteristics, it is preferable to suppress its fraction as much as possible. Therefore, in the present invention, the fraction of island martensite can be limited to 5% by area or less.

Further, in the steel sheet according to one aspect of the present invention, the grain size of the top 80% of the high angle grain size based on 15 degrees can be 70 μm or less in the center of the steel sheet. That is, the present invention can refine the high-angle grain size to refine the effective grain size, thereby effectively ensuring low-temperature DWTT characteristics. Here, the center of the steel plate can be interpreted as an area including point t/2, or may be interpreted as an area from t/4 to 3*t/4 (t: steel plate thickness, mm).

A steel plate according to one aspect of the present invention may have a thickness of less than 20 mm, and a more preferred thickness of the steel plate may be 16 mm or less. Further, the steel plate according to one aspect of the present invention can have a yield strength of 485 MPa or more, a total elongation of 28% or more, and a uniform elongation in the direction perpendicular to rolling of the steel plate of -30 MPa or more in the direction perpendicular to rolling. The DWTT ductile fracture ratio at °C can be 85% or more. Therefore, the present invention can provide a steel plate that effectively ensures strength, low-temperature fracture toughness, and elongation even though it has a thickness of less than 20 mm, and is particularly suitable as a material for line pipes.

Hereinafter, the manufacturing method of the present invention will be explained in more detail.
A high-strength steel sheet with excellent low-temperature fracture toughness and elongation according to one aspect of the present invention has carbon (C): 0.05 to 0.1% and silicon (Si): 0.05 to 0.5% by weight. , manganese (Mn): 1.4 to 2.0%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0. 002-0.01%, 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%, and the remainder is iron (Fe) and unavoidable impurities. Reheat the slab. Then, hold and take out the slab, roll the held and taken out slab in a recrystallization region at a temperature range of Tnr or higher, and unrecrystallize the rolled material rolled in the recrystallization region at a total reduction rate of 30% or more. It can be manufactured by performing region rolling and cooling the non-recrystallized region rolled steel sheet to a temperature range of (Bs-80°C) to Bs.

Reheating, Holding, and Removal of Slabs Since the slab of the present invention is provided with the same alloy composition as that of the above-mentioned steel sheet, the description of the alloy composition of the slab of the present invention is not limited to the above-mentioned steel sheet alloy. Substitute with an explanation about the composition of
Reheating the slab is a process of heating the steel in order to smoothly perform the subsequent rolling process and to ensure the desired physical properties of the steel plate. Therefore, it is necessary to perform the heating step within an appropriate temperature range depending on the purpose. The lower limit of the reheating temperature of the slab needs to be determined in consideration of whether the temperature is such that the precipitated elements can be sufficiently dissolved in the steel. In particular, in the present invention, since niobium (Nb) is essential to ensure high strength properties, the lower limit of the reheating temperature of the slab is limited to 1140°C in consideration of the solid solution temperature of niobium (Nb). can do. On the other hand, if the reheating temperature of the slab is excessively high, the austenite crystal grains become excessively coarse, which may cause a problem that the crystal grains of the final steel sheet increase excessively. Therefore, in the present invention, the upper limit of the slab reheating temperature can be limited to 1200°C.
The reheated slab can optionally undergo holding and unloading steps, and for similar reasons to the reheating temperature of the slab, the holding and unloading temperature of the slab is limited to a temperature range of 1140-1200°C. be able to.

Recrystallization Zone Rolling Recrystallization zone rolling can be performed in a temperature range of Tnr or higher. In the present invention, Tnr means the lower limit of the temperature range in which austenite recrystallization occurs. That is, the recrystallization region rolling can be performed in the temperature range of the austenite recrystallization region. Recrystallization zone rolling can be performed in multiple passes, and rolling can be performed at an average rolling reduction of 10% or more for each pass. This is because if the average rolling reduction per pass is less than 10%, the grain size of recrystallized austenite becomes coarse, which may induce a decrease in the toughness of the final steel plate.
The rolled material rolled in the recrystallization zone can be cooled to a temperature range below Tnr under air cooling conditions. That is, the rolled material that has been rolled in the recrystallized region is not immediately rolled in the non-recrystallized region, but can be waited for a certain period of time and cooled to the temperature range of the non-recrystallized region by air cooling. This is because if a rolling force is applied in the relevant section, partial recrystallization may occur, and brittle fracture due to coarse austenite grain size may occur.

Non-recrystallized region rolling The rolled material that has been rolled in the recrystallized region is subjected to non-recrystallized region rolling. The starting temperature of rolling in the non-recrystallized region may be Tnr or lower, and the finishing temperature of rolling in the non-recrystallized region may be (Ar3+100° C.). Non-recrystallization zone rolling is a process for obtaining fine ferrite and bainite by elongating austenite produced by recrystallization zone rolling and forming a deformed structure within the grains. By rolling in the non-recrystallized region, the strength, elongation, and brittle fracture arresting properties of the steel sheet can be effectively improved.

The lower the end temperature of rolling in the non-recrystallized region, the more the degree of deformation of austenite increases, which is effective in improving low-temperature fracture toughness. However, if the end temperature of rolling in the non-recrystallized region is too low, the Ferrite is generated, which is disadvantageous in securing strength. Therefore, in the present invention, the end temperature of rolling in the non-recrystallized region can be limited to (Ar3+50°C) or higher.
Further, the amount of reduction in rolling in the non-recrystallized region is a factor that has an important influence on ensuring the low-temperature toughness of the steel material. In the present invention, in order to ensure low-temperature DWTT ductile fracture area characteristics due to grain size refinement of the final steel material, the reduction amount in non-recrystallization zone rolling can be limited to 30% or more. Since the larger the reduction amount in the non-recrystallized region rolling is, the more effective it is in improving the low-temperature toughness, there is no need to limit the upper limit of the reduction amount in the non-recrystallized region rolling. However, when the rolling reduction in the non-recrystallized area exceeds a certain level, the effect of grain size refinement is saturated, while the reduction in the recrystallized area relatively decreases. The amount of reduction in area rolling can be limited to 90% or less.

Cooling A steel plate rolled in the non-recrystallized region 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. If the cooling start temperature is too low, a large amount of low-strength ferrite is produced, which may significantly reduce the strength of the steel plate. Therefore, in the present invention, cooling can be started in a temperature range of (Ar3+30°C) or higher.
Further, since the steel plate of the present invention has a final thickness of less than 20 mm, it is most preferable to stop cooling in the temperature range of (Bs-80°C) to Bs from the viewpoint of strength and elongation. If the cooling stop temperature is less than (Bs - 80 ° C.), high-angle grain boundaries are coarsely formed, a large amount of acicular ferrite and bainite having low-angle grain boundaries is formed, and elongation may decrease. This is because when the cooling stop temperature exceeds Bs, the amount of bainite produced is small and the strength of the steel plate cannot be ensured. After rapidly cooling the steel plate to a cooling stop temperature of (Bs-80°C) to Bs, the steel plate can be cooled to room temperature by air cooling or natural cooling.

Further, cooling in the present invention can be performed at a cooling rate of 10 to 100° C./s. This is because if the cooling rate is less than 10° C./s, the fraction of equiaxed ferrite increases significantly, making it impossible to effectively ensure the high strength properties of the steel sheet. From the equipment conditions and economic point of view, the upper limit of the cooling rate can be limited to 100° C./s, and a more preferable upper limit of the cooling rate can be 50° C./s.
The steel plate manufactured through the above manufacturing method contains ferrite and bainite as a fine structure, and in addition to this, can further contain island-like martensite. The fraction of ferrite and bainite can be 20 to 60 area % and 35 to 75 area %, respectively, and the fraction of island martensite can be 5 area % or less. Further, in the steel sheet manufactured by the above manufacturing method, the grain size of the top 80% of the high angle grain size based on 15 degrees can be 70 μm or less in the center of the steel sheet.

Therefore, the steel plate manufactured through the above manufacturing method has a yield strength of 485 MPa or more, a total elongation of 28% or more, and a uniform elongation in the direction perpendicular to rolling of 9% or more, although it has a thickness of less than 20 mm. The DWTT ductile fracture ratio at −30° C. in the direction perpendicular to the rolling direction of the steel sheet can be 85% or more. Therefore, according to the manufacturing method according to one aspect of the present invention, the steel plate is particularly suitable as a material for line pipes by effectively ensuring strength, low-temperature fracture toughness, and elongation while having a thickness of less than 20 mm. can be provided.

Hereinafter, the present invention will be explained in more detail with reference to Examples. However, it should be noted that the Examples described below are merely for illustrating the present invention and embodying it in more detail, and are not intended to limit the scope of the present invention.
(Example)
A slab with a thickness of 250 mm having the alloy composition shown in Table 1 below was manufactured, and by applying the process conditions shown in Table 3, steel plate test pieces with thicknesses of 11 mm, 11.5 mm, and 22 mm were manufactured, respectively. At this time, the process conditions used for normal slab production were applied to the production of the slab, and the conditions of an average reduction rate of 10% or more per pass in a temperature range of Tnr or higher were applied to all test specimens. Recrystallization zone rolling was performed. In addition, air cooling was applied to all test specimens after rolling in the recrystallization region to a non-recrystallization temperature range. Table 2 shows the calculated Tnr temperature, Ar3 temperature, and Bs temperature based on each alloy composition in Table 1. The calculation formulas used to calculate the Tnr temperature, Ar3 temperature, and Bs temperature in Table 2 are separately described below Table 2.

Formula 1: Tnr (℃) = 887 + 464 * [C] + 6445 * [Nb] - 644 * [Nb] (1/2) + 732 * [V] - 230 * [V] (1/2) + 890 * [Ti] +363*[Al]-357*[Si]
Formula 2: Ar3 (°C) = 910-273*[C]-74*[Mn]-57*[Ni]-16*[Cr]-9*[Mo]-5[Cu]
Formula 3: Bs (°C) = 830-270*[C]-90*[Mn]-37*[Ni]-70*[Cr]-83*[Mo]
(In formulas 1 to 3 above, [C], [Si], [Mn], [Al], [Ti], [Nb], [V], [Cr], [Mo], and [Cu] are , means the weight percent of each alloy composition, and if the corresponding alloy composition is not included, calculate by substituting 0 as the value.)

For each test piece in Table 3, the microstructure, yield strength, tensile strength, elongation, and DWTT ductile fracture ratio at -30°C were measured and shown in Table 4 below. The microstructure of each test piece was evaluated using an optical microscope microstructure photograph and an EBSD particle size distribution map. The yield strength, tensile strength, and elongation were evaluated by performing a room temperature tensile test on each test piece. The yield strength and tensile strength listed in Table 4 each mean a value measured in the direction perpendicular to rolling. In addition, a DWTT test was conducted on each test piece at -30°C to evaluate the tensile properties and ductile fracture ratio.

Test specimens 1 to 12 that meet the alloy composition and process conditions of the present invention contain 20 to 60 area % ferrite, 35 to 75 area % bainite, and 5 area % or less island martensite as a microstructure, In the center of the steel plate, the grain size of the top 80% of high-angle grain sizes based on 15 degrees is 70 μm or less, yield strength is 485 MPa or more, total elongation is 28% or more, uniform elongation in the direction perpendicular to rolling is 9% or more, Since the material satisfies the DWTT ductile fracture area ratio of 85% or more at −30° C. with respect to the direction perpendicular to the rolling direction, it can be confirmed that the material has physical properties particularly suitable as a material for line pipes provided in cryogenic environments.
Test pieces 13 to 15 and 17 satisfy the alloy composition of the present invention, but were cooled in a temperature range lower than the cooling start temperature or cooling end temperature of the present invention. In test pieces 13 to 15 and 17, less than 20 area % of ferrite and more than 75 area % of bainite were formed, and in the center of the steel plate, the grain size of the top 80% of the high angle grain size based on 15 degrees was 70 μm. It can be confirmed that the uniform elongation is at a level of less than 9%.

Test piece 16 satisfies the alloy composition of the present invention, but was rolled in a non-recrystallized region at a temperature lower than the end temperature of the non-recrystallized region rolling of the present invention, and cooled in a temperature range lower than the cooling start temperature of the present invention. This is a test piece obtained by starting cooling and finishing cooling in a temperature range higher than the cooling stop temperature of the present invention. It can be confirmed that in the test piece 16, more than 60% by area of ferrite was formed, and the yield strength was less than 485 MPa.
Test pieces 18 to 21 are test pieces that do not satisfy the alloy composition and process conditions of the present invention, and it can be confirmed that the desired microstructure and physical properties of the present invention cannot be secured.
Although test pieces 22 and 23 satisfy the alloy composition of the present invention, it can be confirmed that the thickness of the steel plate exceeded 20 mm and excessive ferrite was formed.

FIG. 1 is a photograph of the test piece 2 observed with an optical microscope, and FIG. 2 is the result of measuring the high-angle grain boundary grain size of the test piece 2 using EBSD based on 15 degrees. As shown in the graph of Figure 2, the average size of the high-angle grain boundary grains in test specimen 2 is 22.3 μm, and it can be confirmed that the top 80% of these grains are 40.5 μm. .
FIG. 3 is a photograph of the test piece 18 observed with an optical microscope, and FIG. 4 is the result of measuring the high-angle grain boundary grain size of the test piece 18 based on 15 degrees using EBSD. As shown in the graph of FIG. 4, it can be confirmed that the average size of the high-angle grain boundary grains of the test piece 18 is 38 μm, and the top 80% of these grains are 93 μm.
Therefore, according to one aspect of the present invention, although having a thickness of less than 20 mm, it has a yield strength of 485 MPa or more, a total elongation of 28% or more, a uniform elongation in the direction perpendicular to rolling of 9% or more, and a thickness of 85% or more. Since the steel sheet has a DWTT ductile fracture area ratio of -30° C. with respect to the direction perpendicular to the rolling direction of the steel sheet, it is possible to provide a steel sheet particularly suitable as a material for line pipes and a method for manufacturing the same.

Although the present invention has been described above in detail with reference to the embodiments, embodiments in different forms are also possible. Therefore, the spirit and scope of the appended claims should not be limited to the embodiments.

Claims (11)

In 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% Below, calcium (Ca): 0.0005 to 0.004%, the balance consists of iron (Fe) and inevitable impurities,
The microstructure consists of 20 to 60 area% ferrite, 35 to 75 area% bainite, and 5 area% or less island martensite,
In the center of the steel plate, the top 80% grain size of the high angle grain size based on 15 degrees is 70 μm or less,
A high-strength steel plate with excellent low-temperature fracture toughness and elongation, characterized in that the thickness of the steel plate is less than 20 mm. The high-strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 1, wherein the steel plate further contains 0.3% by weight or less of molybdenum (Mo). The high-strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 1, wherein the steel plate has a yield strength of 485 MPa or more. The total elongation of the steel plate is 28% or more,
The high-strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 1, wherein the steel plate has a uniform elongation of 9% or more in a direction perpendicular to rolling. The high-strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 1, wherein the steel plate has a DWTT ductile fracture area ratio of 85% or more at -30°C in a direction perpendicular to rolling. In 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% Hereinafter, a slab consisting of calcium (Ca): 0.0005 to 0.004%, the remainder being iron (Fe) and unavoidable impurities is reheated in a temperature range of 1140 to 1200°C,
holding and removing the slab;
The held and taken out slab is rolled in a recrystallization region in a temperature range of Tnr or higher,
Rolling the rolled material rolled in the recrystallization zone at a total reduction rate of 30% or more in the non-recrystallization zone,
Cooling the steel plate rolled in the non-recrystallized region to a temperature range of (Bs-80°C) to Bs,
The rolling in the non-recrystallized region is started in a temperature range of Tnr or lower, and finished in a temperature range of (Ar3+100°C) or higher,
The recrystallization zone rolling is performed in multiple passes, and the average rolling reduction of each pass is 10% or more,
The steel plate has a microstructure of 20 to 60 area% ferrite, 35 to 75 area% bainite, and 5 area% or less island-like martensite , and in the center of the steel plate, it has a top 80 high-angle grain size based on 15 degrees. % grain size is 70 μm or less and the thickness is less than 20 mm. The method of manufacturing a high-strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 6 , wherein the slab further contains 0.3% by weight or less of molybdenum (Mo). 7. The method for manufacturing a high-strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 6 , wherein the temperature range for holding and taking out the slab is 1140 to 1200°C. 7. The method of manufacturing a high-strength steel sheet with excellent low-temperature fracture toughness and elongation according to claim 6 , wherein the rolled material rolled in the recrystallization zone is cooled to a temperature range below Tnr by air cooling. The method for producing a high-strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 6 , wherein the steel plate rolled in the non-recrystallized region is cooled at a cooling rate of 10 to 50° C./s. The method for manufacturing a high-strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 6 , wherein cooling of the steel plate rolled in the non-recrystallized region is started in a temperature range of (Ar3+30°C) or higher. .

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