KR20210062892A - Steel sheet with excellent low temperature toughness and its manufacturing method - Google Patents

Abstract

The present invention relates to a steel sheet with an excellent low temperature toughness and a manufacturing method thereof. According to a perspective of the present invention, the manufacturing method of the steel sheet with the excellent low temperature toughness comprises: a step of reheating a slab including 0.05-0.08 wt% of carbon (C), 0.15-0.25 wt% of silicone (Si), 1.5-1.7 wt% of manganese (Mn), more than 0 and 0.018 wt% or less of phosphorous (P), more than 0 and 0.003 wt% or less of sulfur (S), 0.01-0.05 wt% of aluminum (Al), 0.045-0.055 wt% of niobium (Nb), 0.035-0.045 wt% of vanadium (V), 0.015-0.025 wt% of titanium (Ti), 0.2-0.3 wt% of chrome (Cr), 0.03-0.10 wt% of molybdenum (Mo), 0.006 wt% or less of nitrogen (N), and remaining iron (Fe) and other inevitable impurities, at a temperature of 1,180-1,250℃; a step of rough-rolling the heated slab at 850-980℃ of RDT; a step of finishing-milling the rough-rolled steel sheet at 760-800℃ of FDT and obtaining a rolled steel sheet; and a step of cooling and winding the rolled steel sheet.

Description

Steel sheet with excellent cryogenic toughness and manufacturing method thereof {STEEL SHEET WITH EXCELLENT LOW TEMPERATURE TOUGHNESS AND ITS MANUFACTURING METHOD}

The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a high-strength steel sheet having excellent cryogenic toughness and a method for manufacturing the same.

Structural steels for ships and offshore structures, etc., and thick steel plates for multi-purpose tanks in which various types of liquefied gases such as carbon dioxide, ammonia, and LNG are mixed, have a very harsh usage environment. Therefore, strength is very important, and in these steel materials for marine structures or tanks, not only strength but also toughness at low temperatures are very important. Due to the depletion of resources in warm regions, the use environment for marine structural steel is gradually shifting to cold regions such as the Arctic, where marine oil and gas resources are abundant. is getting difficult

In the past, safety was secured only by guaranteeing toughness at 0°C, but as the area where new oil pipelines are installed is changed to an extreme area, it is required to guarantee toughness at cryogenic temperatures below 0°C. This is a more severe condition than the API (American Petroleum Institute) standard, which was the original main guarantee, of 85% or more of ductile fracture after drop weight tear test (DWTT) at 0°C. In general, the thicker the material for oil pipelines, the more severe it is. Because toughness is reduced, it is a very difficult technology to guarantee cryogenic toughness in ultra-thick materials. It is known that evaluating the low-temperature toughness of oil pipelines with DWTT, which has a longer fracture path than the CVN (Charpy V-notch) test, is suitable for indicating fracture propagation patterns.

In general, it is known that steel has excellent low-temperature toughness when the grains are fine or have an acicular ferrite structure. Therefore, as a method of improving the low-temperature toughness of hot-rolled steel sheets for oil pipelines, fine alloying elements such as niobium (Nb) and molybdenum (Mo) are added to refine crystal grains, and the production of needle-shaped ferrite is promoted.

The microstructure control mentioned above is very important to secure the toughness of steel at cryogenic temperatures below 0°C. As the hot-rolled steel sheet thickens, the low-temperature toughness property deteriorates during production under the same conditions. It is necessary to control the organization. However, alloying elements such as niobium (Nb) and molybdenum (Mo) are expensive elements, and even when a small amount is added, the price of the product increases. As raw material prices are rising recently and hot-rolled steel sheet prices are falling, an increase in ferroalloy costs lowers the competitiveness of hot-rolled steel sheets, so inventory is needed.

As a technology related thereto, there is Korean Patent Application Laid-Open No. 22014-0098903 (published on Aug. 11, 2014, high-strength steel sheet and its manufacturing method).

The problem to be solved by the present invention is to provide a cost-saving high-strength steel sheet capable of securing price competitiveness by securing low-temperature toughness without using a large amount of expensive alloying elements and a method for manufacturing the same.

High-strength ultra-thick steel sheet according to an aspect of the present invention, by weight, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.5% to 1.7%, Phosphorus (P): greater than 0 and less than or equal to 0.018%, sulfur (S): greater than zero and less than or equal to 0.003%, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.045% to 0.055%, vanadium (V): 0.035 % to 0.045%, titanium (Ti): 0.015% to 0.025%, chromium (Cr): 0.2% to 0.3%, molybdenum (Mo): 0.03% to 0.10%, nitrogen (N): 0.006% or less, It contains the remaining iron (Fe) and other unavoidable impurities, and is an ultra-thick steel sheet with a thickness of 20t or more, yield strength (YP): 485 to 605 MPa, tensile strength (TS): 570 to 760 MPa, elongation (EL): 25% or more, Yield ratio (YR): Characterized in that it has properties of 0.9 or less.

In the present invention, the steel sheet has a needle-shaped and polygonal ferrite structure, and exhibits a value of 85% or more in DWTT performed at a cryogenic temperature of -20°C.

The method of manufacturing a high-strength ultra-thick steel sheet according to another aspect of the present invention is, by weight, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.5% to 1.7%, phosphorus (P): more than 0 and less than 0.018%, sulfur (S): more than 0 and less than or equal to 0.003%, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.045% to 0.055%, vanadium (V) ): 0.035% to 0.045%, titanium (Ti): 0.015% to 0.025%, chromium (Cr): 0.2% to 0.3%, molybdenum (Mo): 0.03% to 0.10%, nitrogen (N): 0.006% or less Reheating the slab containing the remaining iron (Fe) and other unavoidable impurities at 1,180 ~ 1,250 ℃; RDT of the heated slab: rough rolling at 850 ~ 980 ℃; obtaining a hot-rolled steel sheet by finishing rolling the rough-rolled steel sheet at FDT: 760 to 800°C; and cooling and winding the hot-rolled steel sheet.

In the present invention, in the step of cooling and winding, it is preferable to cool to an average cooling rate of 10 to 20 °C/s, and a coiling temperature: 450 to 500 °C.

After the winding step, the steel sheet has a needle-shaped and polygonal ferrite structure, yield strength (YP): 485 to 605 MPa, tensile strength (TS): 570 to 760 MPa, elongation (EL): 25% or more, yield ratio ( YR): 90% or less, and may represent a value of 85% or more in DWTT performed at a cryogenic temperature of -20°C.

According to the present invention, a high-strength steel sheet having excellent toughness can be secured at a cryogenic temperature of -20°C while reducing costs by limiting the amount of expensive metal added and maintaining high strength.

1 and 2 are graphs showing the ductile fracture factor and room temperature tensile test results after DWTT at -20°C for specimens of Examples and Comparative Examples, respectively.
3 to 5 are photographs showing the fracture surface after DWTT at -20 ℃ for the specimens of Examples and Comparative Examples.
6 to 8 are microscopic photographs of observing the microstructures of Examples and Comparative Examples 1 and 2.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art to which the present invention pertains can easily practice it. The present invention may be embodied in several different forms, and is not limited to the embodiments described herein. The same reference numerals are assigned to the same or similar components throughout this specification. In addition, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the gist of the present invention will be omitted.

In the case of conventional hot-rolled steel sheets for oil pipelines, alloying elements such as Nb and Mo are added to secure the cryogenic toughness below 0°C by refining grains and promoting the formation of needle-shaped ferrite. However, in the case of niobium (Nb), 0.075% by weight or less, and in the case of molybdenum (Mo), the alloy is high, such as 0.25% by weight or less, so the cost of ferroalloy is high. In recent years, with raw material prices rising rapidly and the price of hot-rolled steel sheets falling, the competitiveness of high-priced hot-rolled steel sheets is declining. In the present invention, the addition amount of niobium (Nb) is fixed at 0.045% to 0.055%, and the addition amount of molybdenum (Mo) is reduced to 0.10% by weight or less, thereby providing a low-cost novel component system that can secure toughness even at a cryogenic temperature of -20°C. . In addition, the present invention proposes an optimized hot rolling condition because it is essential to control the microstructure in order to secure the cryogenic toughness in the novel component system. That is, the present invention provides a thick steel sheet having excellent toughness at a cryogenic temperature of -20°C and a method for manufacturing the same while reducing the cost by limiting the amount of expensive metal added and maintaining high strength.

High-strength steel sheet with excellent cryogenic toughness

High-strength steel sheet excellent in cryogenic toughness according to an aspect of the present invention, by weight, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.5% to 1.7 %, phosphorus (P): more than 0 and less than 0.018%, sulfur (S): more than 0 and less than or equal to 0.003%, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.045% to 0.055%, vanadium (V) : 0.035% to 0.045%, titanium (Ti): 0.015% to 0.025%, chromium (Cr): 0.2% to 0.3%, molybdenum (Mo): 0.03% to 0.10%, nitrogen (N): 0.006% or less do.

The rest other than the above components consists of iron (Fe) and impurities inevitably contained in the steelmaking process.

Hereinafter, the role and content of each component included in the high-strength steel sheet according to the present invention will be described as follows.

Carbon (C): 0.05 to 0.08 wt%

Carbon (C) is the most economical and effective alloying component to secure the strength of steel. In the present invention, by controlling the content of carbon (C) to 0.05 ~ 0.08% by weight to suppress the pearlite generation and to generate needle-shaped ferrite to secure low-temperature toughness. When carbon (C) is added in an amount of 0.05% by weight or less, it may be good in terms of toughness, but since the effect of strengthening the steel by combining with Nb, V or Ti is very small, it is necessary to add more than 0.05% by weight to secure strength have. On the other hand, when the content of carbon (C) exceeds 0.08% by weight, there is a problem in that the center segregation that reduces the DWTT toughness increases.

Silicon (Si): 0.15 to 0.25 wt%

Silicon (Si) as a ferrite stabilizing element increases the degree of supercooling during ferrite transformation to make crystal grains finer and suppress carbide formation. However, if the amount of addition is increased, it may cause deterioration of weldability, deterioration of plating quality, and problems of surface quality. On the other hand, in the case of steel to which 1.2 wt% or more of manganese (Mn) is added, the Mn/Si ratio should be in the range of 5 to 10 during ERW welding for steel pipe production. This is because Mn-Si-O oxides (Mn2SiO4 or MnSiO3) generated during welding are formed in that region and have the lowest melting temperature, which facilitates the discharge of oxides during welding. In order to obtain the above effect, it is preferable to add 0.15 wt% or more of silicon (Si). However, when the content of silicon (Si) exceeds 0.25% by weight, there is a problem in that a large amount of oxide is formed on the surface of the steel sheet, thereby impairing the plating characteristics of the steel sheet and reducing weldability.

Manganese (Mn): 1.5 to 1.7 wt%

Manganese (Mn) as an austenite stabilizing element is very effective in solid solution strengthening and has a great influence on the increase in hardenability of steel. The strength, toughness, and yield ratio of steel can be controlled according to the content of manganese (Mn), but when a large amount is added, MnS inclusions are formed and center segregation occurs during casting, thereby reducing the toughness of the steel. Considering the above effects, the appropriate manganese (Mn) content is 1.5 to 1.7 wt%.

Phosphorus (P): greater than 0 0.018% by weight or less

Phosphorus (P) has a problem in that weldability is deteriorated and corrosion resistance is lowered by segregation of the center of the slab. In addition, since segregation at the austenite grain boundary deteriorates toughness, it is preferable to limit the content to 0.018% by weight or less.

Sulfur (S): greater than 0 0.003% by weight or less

(S) is a component that greatly reduces brittleness by reacting with Mn in steel to form MnS, and when it exceeds 0.003% by weight, it greatly reduces low-temperature DWTT resistance. Therefore, the content of S is preferably controlled to 0.003% by weight or less.

Aluminum (Al): 0.01 to 0.05 wt%

Aluminum (Al) is a component that deoxidizes together with silicon (Si), and is preferably added in an amount of 0.01 to 0.05 wt%. When the content of aluminum (Al) is less than 0.01% by weight, sufficient deoxidation effect cannot be obtained. Conversely, when the content of aluminum (Al) exceeds 0.05 wt %, there is a problem of inhibiting weldability.

Niobium (Nb): 0.045 to 0.055 wt%,

Niobium (Nb) can promote grain refinement by delaying recrystallization during hot rolling. Solid solution niobium (Nb) during hot rolling is known to delay nucleation and growth of recrystallization, and since such recrystallization delay does not consume defect sites such as dislocations, it promotes nucleation during phase transformation to make grains finer. In addition, since the deformed organic precipitated carbide serves as a nucleation site for ferrite during phase transformation, it can promote phase transformation and refine crystal grains. Conventionally, Nb was added up to 0.075% by weight or less, but in the present invention, the content of niobium (Nb) was limited to 0.045 to 0.055% by weight in order to reduce the cost and maintain the effect of refining the grains at the same time.

Vanadium (V): 0.035 to 0.045 wt%

Vanadium (V) is a component that exhibits a precipitation strengthening effect by adding a small amount like niobium (Nb). In the carbon (C) range of the present invention, when it exceeds 0.035 wt %, low-temperature toughness and weldability may be reduced due to a large amount of precipitates. It is preferable to control the content of vanadium (V) from 0.035 to 0.045% by weight.

Titanium (Ti): 0.015 to 0.025 wt%

Titanium (Ti) has the effect of improving the toughness and strength of hot-rolled products by producing Ti(C,N) precipitates with high high-temperature stability, thereby preventing austenite grain growth during welding and refining the weld structure. However, when a large amount is added, the corrosion resistance of steel can be reduced by generating coarse TiN precipitates formed at high temperature, so the content of titanium (Ti) is limited to 0.015 to 0.025 wt%, which can sufficiently remove dissolved nitrogen.

Chromium (Cr): 0.2 to 0.3 wt%

Chromium (Cr), like manganese (Mn), lowers the equilibrium temperature, so it can affect the strength and yield ratio of steel. When a large amount of chromium (Cr) is added, it may combine with carbon (C) to form coarse carbide, which slightly increases strength, but is weak in toughness, so adding a large amount should be avoided. Therefore, in order to obtain the effect of phase transformation of steel, solid solution strengthening and fine precipitate formation, the chromium (Cr) content is limited to 0.2 to 0.3 wt% in the present invention.

Molybdenum (Mo): 0.03 to 0.10 wt%

Molybdenum (Mo), like manganese (Mn), lowers the equilibrium temperature, so it can affect the strength and yield ratio of steel. In addition, since molybdenum (Mo) improves hardenability when not precipitated as carbide, it is possible to appropriately control the secondary phase after ferrite transformation during phase transformation of steel. In order to suppress pearlite production and promote needle-shaped ferrite production, it was conventionally added in a large amount at 0.15 to 0.25 wt%, but in the present invention, the content of molybdenum (Mo) was obtained through an experiment to reduce the cost and control the microstructure, and the content of molybdenum (Mo) was 0.03~ It was limited to 0.10% by weight.

Nitrogen (N): greater than 0 0.0006% by weight or less

Nitrogen (N) combines with Nb, Ti, V, etc. to form carbonitrides to refine crystal grains, but when added in a large amount, dissolved nitrogen increases, lowering the impact properties and elongation of steel, and greatly impairing weld toughness, so its upper limit is 0.006 It is preferable to limit it to weight % or less.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all details are not specifically mentioned.

The steel sheet according to the present invention is an ultra-thick steel sheet having a thickness of 20t or more, having a needle-shaped and polygonal ferrite structure, yield strength (YP): 485 to 605 MPa, tensile strength (TS): 570 to 760 MPa, elongation (EL): 25 % or more, yield ratio (YR): It has physical properties of 90% or less, and exhibits a value of 85% or more in DWTT performed at a cryogenic temperature of -20°C, and is a high strength steel sheet excellent in cryogenic toughness.

The high-strength steel sheet excellent in cryogenic toughness of the present invention may be manufactured by the following manufacturing process. Hereinafter, a method for manufacturing a high-strength steel sheet having excellent cryogenic toughness according to another preferred aspect of the present invention will be described.

According to another preferred aspect of the present invention, the manufacturing method of high-strength steel having excellent DWTT ductile fracture ratio is by weight, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn) : 1.5% to 1.7%, phosphorus (P): more than 0 and less than 0.018%, sulfur (S): more than 0 and less than 0.003%, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.045% to 0.055% , Vanadium (V): 0.035% to 0.045%, Titanium (Ti): 0.015% to 0.025%, Chromium (Cr): 0.2% to 0.3%, Molybdenum (Mo): 0.03% to 0.10%, Nitrogen (N): Reheating the slab containing 0.006% or less and containing the remaining iron (Fe) and other unavoidable impurities, hot rolling the heated slab to obtain a hot-rolled steel sheet, and cooling and winding the hot-rolled steel sheet. .

Slab reheating stage

The steel slab composed as described above is heated at 1,180 to 1,250° C. for 2 hours or more. The heating temperature is determined by the solid solution temperature of the Nb-based precipitate, and the total solid solution of Nb is possible at 1,180° C. or higher in the component range of the present invention. If the reheating temperature is less than 1,180° C., there is a problem in that the solid solution of niobium carbide is not sufficient, and the components segregated during casting are not sufficiently evenly distributed. If the reheating temperature is 1,250° C. or higher, very coarse austenite grains are formed, making it difficult to secure strength. In addition, as the reheating temperature increases, it is not preferable because it causes an increase in manufacturing cost and a decrease in productivity due to additional extraction time required to match the heating cost and the rolling temperature.

hot rolling step

A hot-rolled steel sheet is obtained by performing hot rolling on the heated slab as described above. The hot rolling may be performed by rough rolling and finishing rolling.

The temperature of rough rolling needs to be controlled for grain refinement. When molybdenum (Mo) is added in addition to the fine carbide forming elements such as niobium (Nb), recrystallization during rolling is delayed, so that the grains can be refined. The temperature at which recrystallization is stopped is called Tnr, and rolling must be performed below this temperature to have the effect of grain refinement. Refining the austenite grains increases the nucleation rate of ferrite, which leads to the refinement of the final microstructure, thereby improving low-temperature toughness. Since the above effect increases when the rolling reduction ratio is greater than 80% in the non-recrystallized region, it must be controlled through the Rough Mill Delivery Temperature (RDT). Therefore, the proper RDT of the component system of the present invention is 850 ~ 890 ℃.

In order to obtain acicular ferrite, finishing rolling is completed at an appropriate temperature. If the Finishing Delivery Temperature (FDT) is less than 760°C, fine austenite grains can be secured, but rapid phase transformation occurs during cooling to form a low-temperature transformation phase such as bainite, which may reduce the toughness of the steel. . In addition, if the FDT temperature is too low, it may cause deviation of the electric field material of the hot rolled coil. If the FDT is too high (over 800°C), the austenite grains become coarse and it may be difficult to secure strength, and polygonal ferrite is formed due to slow phase transformation during cooling, which may reduce toughness.

Cooling and winding stage

The hot-rolled steel sheet obtained through the hot-rolling process is cooled and wound.

In order to secure the toughness of steel, it is necessary to obtain fine needle-shaped ferrite and suppress the formation of pearlite, so control of the cooling process is important. The cooling rate at which polygonal ferrite/perlite can be produced through the continuous cooling curve is 15°C/s or less. Since the formation of polygonal ferrite/pearlite should be suppressed and phase transformation should occur intensively during winding, the faster cooling after rolling is better, but if it is too fast, a large amount of bainite may be generated, so the cooling rate is 10 ~ 20℃/s is appropriate Therefore, the coiling temperature is appropriate 550 ~ 590 ℃.

According to the method for manufacturing a high-strength thick steel sheet having excellent cryogenic toughness according to another preferred aspect of the present invention, an ultra-thick steel sheet having a thickness of 20t or more, having needle-shaped and polygonal ferrite structures, yield strength (YP): 485 to 605 MPa, tensile strength (TS): 570 to 760 MPa, elongation (EL): 25% or more, yield ratio (YR): 90% or less A method for manufacturing a high-strength steel sheet is provided.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only examples for explaining the present invention in more detail, and do not limit the scope of the present invention.

Example

A slab having an alloy composition as shown in Table 1 below, a hot-rolled steel sheet was manufactured through the manufacturing conditions of Table 2 below. In addition, the yield strength (YP), tensile strength (TS), elongation (EL), yield ratio (YR), and low-temperature DWTT physical properties were measured and evaluated for the steel sheet prepared in this way, and the results are shown in Table 2, FIG. 1 and 2 are shown together.

Yield strength was measured through a room temperature tensile test, and the low-temperature DWTT physical properties were measured using a DWTT tester at -20°C to break the specimen while lowering the temperature using liquid nitrogen, then the ductile fracture factor was measured, and a cross-sectional photograph was shown. 3 to 5 are shown.

In addition, micrographs of the microstructures of Examples and Comparative Examples 1 and 2 were shown in FIGS. 6 to 8 .

division Ingredients (wt%) C Si Mn P S Al Nb V Ti Cr Mo N Example 0.067 0.18 1.55 0.0085 0.0013 0.034 0.055 0.038 0.019 0.230 0.04 0.0047 Comparative Example 1 0.060 0.17 1.57 0.0050 0.0011 0.034 0.070 0.038 0.020 0.030 0.06 0.0045 Comparative Example 2 0.066 0.21 1.60 0.0054 0.0012 0.037 0.071 0.041 0.017 0.017 0.19 0.0040

division size Hot rolling condition (℃) Properties Thickness (nm) FDT CT YP ts EL YR DWTT(-20℃) Example 17.8 780 570 519 613 44 0.85 100% Comparative Example 1 17.8 780 570 531 620 43 0.86 95% Comparative Example 2 19.1 780 570 550 660 40 0.83 95%

Referring to Table 1, Comparative Example 1 was different from the Example in order to examine the effect of the niobium (Nb) content, and the effect of the molybdenum (Mo) content was investigated through Comparative Example 1 and Comparative Example 2. In the case of Comparative Example 2, the conditions were similar to those of steels produced by the prior art, and a large amount of niobium (Nb) and molybdenum (Mo) were added.

Referring to Table 2, FIGS. 1 and 2 , in Comparative Example 2, crystal grain refinement was sufficiently achieved by adding a large amount of niobium (Nb) and molybdenum (Mo), and the majority of needle-shaped ferrite was secured to have high strength and excellent cryogenic temperature showed patience. In the case of Comparative Example 1, the content of molybdenum (Mo) was reduced by 1/3 compared to Comparative Example 2, and it was produced under the same hot rolling conditions. Although YS/TS was partially decreased compared to Comparative Example 2, the strength within the target specification was secured (API-X70 specification; yield strength: 485-635 MPa, tensile strength 570 MPa or more), and the ductile fracture surface after DWTT at -20℃ It was excellent at a rate of 95%.

In the case of Example, the content of niobium (Nb) was further reduced by 0.02% by weight compared to Comparative Example 1, and the content of molybdenum (Mo) was also reduced by 0.15% by weight, so that it was produced under the same hot rolling conditions. Due to the reduced Nb/Mo content, the YS/TS was partially reduced compared to Comparative Example 1, and the grain size was slightly larger than that of Comparative Example. However, the strength met the target specification.

Looking at the photos of FIGS. 3 to 5 observing the actual fracture state after DWTT at -20 ° C for the specimens of Examples and Comparative Examples 1 and 2 prepared with different contents of niobium (Nb) and molybdenum (Mo), It can be seen that the fracture surface of the specimen of Example (FIG. 3) is clean compared to the specimens of Comparative Example 1 (FIG. 4) and Comparative Example 2 (FIG. 5).

In addition, referring to FIGS. 6 to 8 observing the microstructures of Examples and Comparative Examples 1 and 2, in Comparative Examples 1 and 2, the higher the content of molybdenum (Mo), the more needle-shaped ferrite structures were formed, In the case of the embodiment, it can be seen that a number of needle-shaped ferrites and some polygonal ferrites were formed similarly even though molybdenum (Mo), an expensive alloying element, was reduced.

Through these results, it is possible to maintain the high strength of the hot-rolled steel sheet for ultra-thick oil pipelines and secure cryogenic toughness despite significantly reducing the content of niobium (Nb) and molybdenum (Mo) in steels produced under conventional hot rolling conditions. It was confirmed that it is possible. Compared to Comparative Example 2, the cost of the ferroalloy of the embodiment is expected to be reduced by about 50,000 to 60,000 won, which may vary depending on the price of the ferroalloy.

Although the above description has been focused on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. Such changes and modifications can be said to belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Claims (5)

In wt%, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.5% to 1.7%, phosphorus (P): greater than 0 and less than or equal to 0.018%, sulfur ( S): greater than 0 0.003% or less, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.045% to 0.055%, vanadium (V): 0.035% to 0.045%, titanium (Ti): 0.015% to 0.025%, chromium (Cr): 0.2% to 0.3%, molybdenum (Mo): 0.03% to 0.10%, nitrogen (N): contains less than 0.006%, and contains the remaining iron (Fe) and other unavoidable impurities,
As an ultra-thick steel sheet with a thickness of 20t or more,
Yield strength (YP): 485 to 605 MPa, tensile strength (TS): 570 to 760 MPa, elongation (EL): 25% or more, yield ratio (YR): 90% or less
Steel plate with excellent cryogenic toughness. The method of claim 1,
The steel sheet has a needle-shaped and polygonal ferrite structure,
Representing a value of 85% or more in DWTT conducted at a cryogenic temperature of -20 °C,
Steel plate with excellent cryogenic toughness. In wt%, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.5% to 1.7%, phosphorus (P): greater than 0 and less than or equal to 0.018%, sulfur ( S): greater than 0 0.003% or less, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.045% to 0.055%, vanadium (V): 0.035% to 0.045%, titanium (Ti): 0.015% to Slab containing 0.025%, chromium (Cr): 0.2% to 0.3%, molybdenum (Mo): 0.03% to 0.10%, nitrogen (N): 0.006% or less, and remaining iron (Fe) and other unavoidable impurities Reheating at 1,180 ~ 1,250 ℃;
RDT of the heated slab: rough rolling at 850 ~ 980 ℃;
obtaining a hot-rolled steel sheet by finishing rolling the rough-rolled steel sheet at FDT: 760 to 800°C; and
Including the step of cooling and winding the hot-rolled steel sheet,
A method for manufacturing a steel sheet with excellent cryogenic toughness. The method of claim 3,
The cooling and winding step,
Average cooling rate of 10~20℃/s, winding temperature: cooled to 450~500℃,
A method for manufacturing a steel sheet with excellent cryogenic toughness. The method of claim 3,
After the winding step, the steel plate,
It has needle-shaped and polygonal ferrite structures,
Yield strength (YP): 485 to 605 MPa, tensile strength (TS): 570 to 760 MPa, elongation (EL): 25% or more, yield ratio (YR): 90% or less,
Representing a value of 85% or more in DWTT conducted at a cryogenic temperature of -20 °C,
A method for manufacturing a steel sheet with excellent cryogenic toughness.

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