KR102409897B1 - Pressure vessel steel plate having excellent low-temperature impact toughness and method for manufacturing thereof - Google Patents

Abstract

Provided are a steel material for a pressure vessel having excellent low-temperature impact toughness and a method for manufacturing the same.
The steel material for a pressure vessel of the present invention is, by weight, carbon (C): 0.12 to 0.18%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.5%, phosphorus (P): 0.01% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.25% or less, molybdenum (Mo): 0.01 to 0.15%, Titanium (Ti): 0.005% or less (including 0%), Nitrogen (N): 0.002 to 0.01%, and Niobium (Nb): 0.02% or less (excluding 0%) and Vanadium (V): 0.05% or less (0%) NbC or VC carbide containing at least one of these, the remainder Fe and unavoidable impurities, having a mixed microstructure of ferrite and pearlite, the grain size of the ferrite is 15 μm or less, and the size in the ferrite structure is 40 nm or less this exists

Description

Steel material for pressure vessel with excellent low-temperature impact toughness and manufacturing method thereof

The present invention relates to a steel material that can be suitably used for a pressure vessel that can be used in petrochemical manufacturing facilities, storage tanks, etc., and more particularly, to a steel material having excellent low-temperature impact toughness and a method for manufacturing the same.

Recently, the demand for steel for pressure vessels used in industries such as mining, production, transport, storage, refining, and power generation of energy resources is gradually increasing. In addition, excellent low-temperature impact toughness is required as the use environment of these structures is extended to extreme cold regions.

In general, as the operating temperature of steel for pressure vessels is lowered, the impact toughness is lowered, and thus there is a problem in stability. In particular, as the thickness increases in steel of the same strength, the toughness of the internal structure decreases to a greater extent. Therefore, in the steel for pressure vessel applied to a region with a low temperature environment, the composition or microstructure must be appropriately controlled so that the impact toughness does not deteriorate even at low temperatures. In the case of steel materials for pressure vessels that are usually subjected to normalizing heat treatment, it is common to form a mixed structure of ferrite and pearlite.

The rolling process is one of the representative methods of grain refinement. When rolling is performed at a temperature at which recrystallization is possible, new austenite fine grains are generated by using the internal stress generated by the rolling force as a driving force.

On the other hand, rolling in the non-recrystallization region temperature region causes the grains to be stressed and a band structure is formed in the rolling direction. may cause

However, as the thickness of the steel increases, the reduction force that can be applied by rolling is limited, so it is difficult to form fine grains through rolling as the inner structure, particularly, the closer to the center of the steel material. Austenite grains tend to grow at higher temperatures than Ae3 and longer heating times. Some alloying elements have the effect of inhibiting the growth of austenite grains. Nb, V, Ti, Al, etc. are widely known austenite grain growth inhibitory elements, and are mainly dissolved in steel to act as an obstacle to grain growth. Therefore, in the case of thick steel, which is difficult to refine grains by rolling, the addition of such alloying elements should be considered for grain refinement.

On the other hand, in many cases, it is difficult to secure sufficiently small crystal grains only through slab reheating and rolling, which are processes in which austenite grain refinement occurs mainly. In particular, the higher the temperature of the rolled steel, the lower the deformation resistance during rolling, so for easy rolling, reheating of the slab is mainly carried out at a much higher temperature than the Ae3 temperature, at which time the austenite grains grow large. When the effect of refining grains through rolling is not sufficient, additional austenite grain refining effect can be expected through reheat treatment, which usually corresponds to normalizing heat treatment.

This heat treatment promotes austenite transformation by heating the steel cooled to room temperature after rolling to a temperature of Ac3 or higher, but it is common to minimize austenite grain growth due to temperature. According to the purpose, the reheated steel is transformed into fine ferrite and pearlite structures through air cooling.

According to Patent Document 1, in order to realize the high strength of the ultra-thick steel material, the old austenite grains are refined by repeating the reheat treatment process of heating up to the austenite generation temperature range, and quenching and tempering (quenching and tempering) It is disclosed that a high-strength, high-toughness hot-rolled steel sheet composed of tempered martensite, bainite, and some cementite structures can be obtained by heat treatment.

On the other hand, in the case of a steel having a mixed structure of ferrite and pearlite, in order to secure the cryogenic impact toughness at -60 ^o C level, it is necessary to appropriately control the alloying elements, as well as additional ferrite through rough rolling/finishing rolling condition control and heat treatment condition control. refinement is required. This prevents the grains of austenite from coarsening by increasing the rolling reduction during rough rolling, and it is possible to refine grains by performing rolling in the non-recrystallized region during finishing rolling. However, for more effective grain refinement, it is urgently required to develop a steel material for a pressure vessel that can be used even in extreme cold regions, such as the normalizing heat treatment control described above.

Korean Patent Publication No. 10-1677350

An object of the present invention is to provide a steel material for a pressure vessel excellent in low-temperature impact toughness by refining crystal grains and a method for manufacturing the same.

The subject of the present invention is not limited to the above. The subject of the present invention will be understood from the overall content of the present specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject of the present invention.

One aspect of the present invention is

By weight%, carbon (C): 0.12 to 0.18%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.5%, phosphorus (P): 0.01% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.25% or less, molybdenum (Mo): 0.01 to 0.15%, titanium (Ti): 0.005% or less ( 0% included), nitrogen (N): 0.002 to 0.01%, and niobium (Nb): 0.02% or less (excluding 0%) and vanadium (V): at least one of 0.05% or less (excluding 0%), the balance Fe and unavoidable impurities;

It has a mixed microstructure of ferrite and pearlite,

The grain size of the ferrite is 15 μm or less, and

Provided is a steel material for a pressure vessel having excellent low-temperature impact toughness in which NbC or VC carbide having a size of 40 nm or less in the ferrite structure is present.

The steel material preferably has a yield strength of 260 MPa or more, a tensile strength of 485 MPa or more, and an impact toughness of 150J or more, evaluated at -60°C.

Another aspect of the present invention is

Preparing a steel slab that satisfies the above-described alloy composition;

heating the steel slab to a temperature range of 1100 to 1200 °C;

rough rolling the heated steel slab at a temperature of 1050° C. or higher and a final pass reduction ratio of 10% or higher;

manufacturing a hot-rolled steel sheet by finishing hot rolling at a temperature in the range of Ar3 to Tnr after the rough rolling;

air-cooling the manufactured hot-rolled steel sheet to room temperature; and

The air-cooled hot-rolled steel sheet is reheated to a temperature of Ac3 or higher, heat-treated for (1.3t~30) minutes (here, t means the thickness of the steel (mm)) or more, and then the process of air-cooling to room temperature is repeated 2-3 times. It provides a method of manufacturing a steel material for a pressure vessel having excellent low-temperature impact toughness, including the step.

Furthermore, after repeating the above heat treatment process, it is preferable to perform PWHT (post-weld heat treatment) heat treatment for 1 hour or more per inch of steel thickness in a temperature range of 550 to 650°C.

According to the present invention, it is possible to provide a steel material for a pressure vessel excellent in strength and low-temperature impact toughness after repeated normalizing heat treatment and PWHT heat treatment.

1 is a microstructure photograph at a point 1/4t of steel according to an embodiment of the present invention, (a) is Comparative Example 1 in which normalizing is performed once, and (b) is Inventive Example in which normalizing is performed three times; 1 represents.

Hereinafter, the present invention will be described.

The present inventors have recognized that as the steel for pressure vessels used in petrochemicals, storage tanks, etc. is enlarged and the use environment is extended to extreme cold areas, it is necessary to develop a method to secure the properties required for the material.

In particular, in steel materials for pressure vessels having a thickness greater than or equal to a certain level, a method for securing high strength and low-temperature impact toughness was studied in depth. As a result, it was confirmed that it is possible to provide a steel material for a pressure vessel having target properties by controlling the relationship between the component composition and some components in the alloy design and optimizing the manufacturing conditions, and completed the present invention. .

That is, the high-strength ultra-thick steel material excellent in low-temperature impact toughness of the present invention, by weight, carbon (C): 0.12 to 0.18%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.5%, Phosphorus (P): 0.01% or less, Sulfur (S): 0.01% or less, Aluminum (Al): 0.01 to 0.05%, Nickel (Ni): 0.01 to 0.5%, Copper (Cu): 0.25% or less, Molybdenum (Mo) ): 0.01 to 0.15%, titanium (Ti): 0.005% or less (including 0%), nitrogen (N): 0.002 to 0.01%, and niobium (Nb): 0.02% or less (excluding 0%) and vanadium (V) : At least one of 0.05% or less (excluding 0%), the remainder contains Fe and unavoidable impurities, has a mixed microstructure of ferrite and pearlite, the grain size of the ferrite is 15 μm or less, and the size within the ferrite structure NbC or VC carbides with a <40 nm are present.

Hereinafter, first, the alloy composition of the steel sheet provided in the present invention and the reasons for limiting the composition will be described in detail. Meanwhile, unless otherwise specified in the present invention, the content of each element is based on weight %.

Carbon (C): 0.12 to 0.18%

Carbon (C) is an effective element for improving the strength of steel. In order to sufficiently obtain this effect, the C may be included in an amount of 0.12% or more. However, when the content exceeds 0.18%, there is a problem of greatly impairing low-temperature impact toughness, and when the content is less than 0.10%, it is not sufficient to secure strength.

Therefore, in the present invention, the C content is preferably limited to 0.12 to 0.18%, more preferably 0.12 to 0.17%, and most preferably 0.12 to 0.15%.

Silicon (Si): 0.1-0.5%

Silicon (Si) is not only used as a deoxidizer, but also is an element advantageous for improving strength and toughness of steel. In order to sufficiently obtain the above-described effect, the Si may be included in an amount of 0.1% or more. However, if the content exceeds 0.5%, there is a fear that the weldability and low-temperature toughness of steel may be inferior.

Therefore, in the present invention, it is preferable to limit the Si content in the range of 0.1 to 0.5%.

Manganese (Mn): 1.0-1.5%

Manganese (Mn) is an element advantageous for improving the strength of steel by a solid solution strengthening effect. In order to sufficiently obtain the effect, Mn may be included in an amount of 1.0% or more. However, when the content exceeds 1.5%, there is a problem in that room temperature elongation and low temperature toughness are greatly impaired by combining with sulfur (S) in steel to form MnS.

Therefore, in the present invention, it is preferable to limit the Mn content in the range of 1.0 to 1.5%, and more preferably to include it in the range of 1.2 to 1.5%.

Phosphorus (P): 0.01% or less

Phosphorus (P) is an element advantageous for improving the strength of steel and securing corrosion resistance, but since it can significantly impair the impact toughness of steel, it is preferable to limit the content to as low as possible.

In the present invention, even if the P is contained in a maximum of 0.01%, there is no difficulty in securing the target physical properties, so the content may be limited to 0.01% or less. However, 0% may be excluded in consideration of the unavoidably added level.

Sulfur (S): 0.01% or less

Sulfur (S) is an element that greatly inhibits the impact toughness of steel by combining with Mn in steel to form MnS and the like. Therefore, it is advantageous to limit the S to as low a content as possible.

In the present invention, even if the S is contained at a maximum of 0.01%, there is no difficulty in securing the target physical properties, so the content may be limited to 0.01% or less. However, 0% may be excluded in consideration of the unavoidably added level.

Aluminum (Al): 0.01~0.05%

Aluminum (Al) is an element capable of deoxidizing molten steel inexpensively, and in order to sufficiently obtain the above-described effects, Al may be included in an amount of 0.01% or more. However, if the content is excessive and exceeds 0.05%, continuous casting It is undesirable because it causes nozzle clogging.

Therefore, in the present invention, it is preferable to limit the Al content to 0.01 to 0.05%.

Nickel (Ni): 0.01 to 0.5%

Nickel (Ni) is an element capable of simultaneously improving the strength and low-temperature impact toughness of the base material, and in order to sufficiently obtain such an effect, it is necessary to add the Ni in an amount of 0.01% or more. However, Ni is an expensive element, and when its content exceeds 0.5%, there is a problem in that economic efficiency is greatly reduced.

Therefore, in the present invention, it is preferable to limit the Ni content to 0.01 to 0.5%.

Copper (Cu): 0.25% or less

Copper (Cu) is an element advantageous for improving strength while minimizing deterioration in toughness of the base material. When the content of Cu is excessive, there is a problem of not only impairing weldability by increasing the carbon equivalent, but also greatly deteriorating the surface quality of the product.

Therefore, in the present invention, when the Cu is added, it may be included in an amount of up to 0.25%. However, in the present invention, it is revealed that there is no difficulty in securing the target physical properties even if the Cu is not added.

Molybdenum (Mo): 0.01~0.15%

Molybdenum (Mo) is advantageous for greatly improving the strength by significantly improving the hardenability of steel. In order to sufficiently obtain such an effect, the Mo may be added in an amount of 0.01% or more. However, the Mo is an expensive element, and since there is a possibility of inhibiting the formation of ferrite and inhibiting the low-temperature impact toughness by forming bainite when excessively added, it may be limited to 0.15% or less in consideration of this.

Therefore, in the present invention, it is preferable to limit the Mo content in the range of 0.01 to 0.15%.

Titanium (Ti): 0.005% or less (including 0%)

When titanium (Ti) is added together with N, it forms TiN, thereby reducing the occurrence of surface cracks due to the formation of AlN precipitates. However, when the content exceeds 0.005%, coarse TiN is formed during reheating, normalizing, or PWHT heat treatment of the steel slab, which acts as a factor impairing low-temperature impact toughness. Therefore, in the present invention, it is preferable to limit the Ti content to 0.005% or less.

Nitrogen (N): 0.002 to 0.01%

When nitrogen (N) is added together with Ti, it forms TiN and is an element advantageous for suppressing grain growth due to heat effect during welding. In order to sufficiently obtain the above-described effects when Ti is added, it is necessary to add the N in an amount of 0.002% or more. However, when the content exceeds 0.01%, coarse TiN is formed, which is not preferable because low-temperature impact toughness is impaired. Therefore, in the present invention, it is preferable to control the N content to 0.002 to 0.01%.

At least one of niobium (Nb) and vanadium (V)

Niobium (Nb): 0.02% or less (excluding 0%)

Niobium (Nb) is precipitated in the form of NbC or Nb(C,N) to greatly improve the strength of the base material, and when reheated to a high temperature, the dissolved Nb inhibits the recrystallization of austenite and the transformation of ferrite or bainite, thereby refining the structure effect can be obtained. However, Nb may be a factor to inhibit low-temperature impact toughness due to coarsening of NbC or Nb(C,N) precipitates during excessive repeated normalizing heat treatment.

Therefore, in the present invention, it is preferable to limit the addition amount of Nb to 0.02% or less.

Vanadium (V): 0.05% or less (excluding 0%)

Vanadium (V) has a lower solid solution temperature than other alloying elements, and has an effect of preventing a decrease in strength by precipitating in the heat-affected zone during welding. When the strength after welding post-welding heat treatment (PWHT) for the steel material as in the present invention is not sufficiently secured, the strength improvement effect can be obtained by adding the V in an amount of 0.01% or more. However, when the content exceeds 0.05%, not only the fraction of hard phase such as MA increases, but also there is a problem in that low-temperature impact toughness is lowered due to coarse VC precipitation during multiple normalizing heat treatment.

Therefore, in the present invention, it is preferable to limit the amount of V added to 0.05% 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, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

In addition, the steel material for pressure vessels excellent in low-temperature impact toughness of the present invention has a mixed microstructure of ferrite and pearlite.

And it is preferable that the grain size of the ferrite is 15 μm or less. If the grain size exceeds 15㎛, there is a problem that it is difficult to guarantee the impact toughness at a low temperature of -60 ^o C.

In addition, in the present invention, NbC or VC carbide having a size of 40 nm or less may be present in the ferrite structure. As described above, the carbide present in the ferrite matrix serves to improve strength, and the size needs to be 40 nm or less. If the size exceeds 40 nm, there is a problem that not only the strength decreases after PWHT, but also the low-temperature impact toughness is greatly reduced.

The steel material for a pressure vessel of the present invention having the above composition and microstructure has a yield strength of 260 MPa or more, a tensile strength evaluated perpendicular to the rolling direction at the 1/4t point (here, t means the steel thickness (mm)) The strength is 485 MPa or more, and the average Charpy impact energy (CVN) value at -60 ° C is 150 J or more.

Next, a method for manufacturing a steel material for a pressure vessel having excellent low-temperature impact toughness of the present invention will be described in detail.

The manufacturing method of the steel material of the pressure vessel of the present invention comprises the steps of preparing a steel slab satisfying the above-described alloy composition; heating the steel slab to a temperature range of 1100 to 1200 °C; rough rolling the heated steel slab at a temperature of 1050° C. or higher and a final pass reduction ratio of 10% or higher; manufacturing a hot-rolled steel sheet by finishing hot rolling at a temperature in the range of Ar3 to Tnr after the rough rolling; air-cooling the manufactured hot-rolled steel sheet to room temperature; And after reheating the air-cooled hot-rolled steel sheet to a temperature of Ac3 or higher, heat-treating it for (1.3t-30) minutes (here, t means the thickness (mm) of the steel), and then air-cooling to room temperature 2 to 3 times. including;

[Heating of steel slabs]

First, in the present invention, the steps of preparing a steel slab satisfying the above-described alloy composition; The steel slab is heated to a temperature range of 1100 to 1200 °C.

In the present invention, prior to performing hot rolling to be described later, it is preferable to heat the steel slab to undergo a homogenization process, and at this time, it is preferable to perform the heating process in a temperature range of 1100 to 1200 ° C.

When the heating temperature of the steel slab is less than 1100° C., the precipitates (carbonitrides) formed in the slab are not sufficiently re-dissolved, so that the formation of precipitates in the process after hot rolling is reduced. On the other hand, when the temperature exceeds 1200 °C, the austenite grains are coarsened and there is a fear that the physical properties of the steel are impaired.

[Hot rolling]

Next, in the present invention, the heated steel slab is rough-rolled at a temperature of 1050° C. or higher and a final pass reduction ratio of 10% or higher.

If the temperature during the rough rolling is less than 1050 ℃, there is a problem that the temperature is lowered during the subsequent finish hot rolling. At this time, since it is important to prevent coarsening of the grains by providing a sufficient reduction force during the rough rolling, it is preferable to give a reduction ratio of 10% or more in the last pass of rough rolling. If the rolling force is not sufficient during rough rolling, there is a high possibility that the grains will be coarsened after rough rolling.

And in the present invention, the rough-rolled steel slab is finish hot-rolled at a temperature in the range of Ar3 to Tnr to manufacture a hot-rolled steel sheet.

If the finish hot rolling temperature exceeds Tnr, the austenite grain size increases during rolling, and there is a problem in securing strength and low-temperature impact toughness. There is a risk of quality defects. Meanwhile, in the present invention, Tnr and Ar3 can be represented as follows.

Tnr=887 + 464C + 363×(Al×0.8) - 357×(Si×0.8) + 6445×(Nb×0.8)-644×(Nb×0.8) ^1/2 + 732×(V×0.8) - 230 ×(V×0.8) ^1/2 (here, each element means weight content)

Ar3 = 910 - 310C - 80Mn - 20Cu - 55Ni - 80Mo + 119V + 124Ti - 18Nb + 179Al (wherein each element means the weight content)

[Cooling and reheating (normalizing)]

And, after the hot-rolled steel sheet manufactured according to the above is air-cooled to room temperature, the air-cooled hot-rolled steel sheet is reheated to a temperature of Ac3 or higher for (1.3t-30) minutes (here, t means the thickness of the steel (mm)) or longer heat treatment After that, the process of air cooling to room temperature is repeated 2-3 times.

In the present invention, it is possible to induce the generation of a fine austenite structure through the reheating process, and also contribute to the refinement of ferrite after air cooling.

In particular, in the present invention, by repeating the reheating/cooling process two or more times, it is possible to further refine the austenite structure to improve the low-temperature impact toughness, but considering the heat treatment load and the prevention of coarsening of carbides in the matrix 3 It is effective not to exceed the number of times.

On the other hand, an austenite structure may be formed by reheating of the hot-rolled steel sheet, but if the reheating temperature is less than Ac3, there is a fear that the structure of the hot-rolled steel sheet becomes a two-phase structure of ferrite and austenite. Therefore, in the present invention, it is preferable to perform reheating of the hot-rolled steel sheet at a temperature range of Ac3 or more, preferably 830 to 930°C.

In addition, it is preferable to maintain for (1.3t-30) minutes (here, t means the thickness of the steel (mm)) or more in the temperature range so that 100% of the austenite phase can be sufficiently formed up to the center of the hot-rolled steel sheet. .

Meanwhile, in the present invention, Ac3 can be represented as follows.

Ac3 = 937.2 - 436.5C + 56Si - 19.7Mn - 26.6Ni + 38.1Mo + 124.8V + 136.3Ti - 19.1Nb + 198.4Al (where each element means weight content)

The air-cooled hot-rolled steel sheet may include a microstructure of ferrite and pearlite phase.

[PWHT heat treatment]

On the other hand, in general, since the steel for pressure vessel is used by welding, it is common to perform PWHT heat treatment to overcome the deterioration of the toughness of the welded part.

Therefore, in the present invention, if necessary, it is desirable to stabilize the toughness of the steel after welding by heat-treating the air-cooled hot-rolled steel sheet in a temperature range of 550 to 650° C. for at least 1 hour per inch of steel thickness.

If the temperature during the PWHT heat treatment is less than 550° C., a long time heat treatment is required, thereby reducing economic efficiency. On the other hand, when the temperature exceeds 650 ℃, the strength reduction effect is excessively large, as well as the carbide is coarsened, there is a risk that the impact toughness is also reduced.

By air-cooling the hot-rolled steel sheet on which the PWHT heat treatment has been completed to room temperature, it is possible to effectively obtain a steel material composed of ferrite and pearlite phases.

Hereinafter, the present invention will be described in more detail through examples.

(Example)

A cast slab was manufactured by continuously casting molten steel having an alloy composition shown in Table 1 below. At this time, the playing slab was manufactured to a thickness of 300mm.

After heating the cast slab to 1120 ℃, rough rolling by applying a final pass reduction of 12% at 1050 ℃, and then finish hot rolling at 870 ℃ to obtain a 100mm thick hot-rolled steel sheet. After the hot-rolled steel sheet was air-cooled to room temperature, it was reheated to 890° C. and maintained for 160 minutes, followed by normalizing heat treatment in which the hot-rolled steel sheet was air cooled to room temperature again. At this time, the number of heat treatment was performed 1 to 4 times for each invention steel and comparative steel. Thereafter, the air-cooled hot-rolled steel sheet was heated to 615° C. and maintained for 240 minutes, subjected to PWHT (post-weld heat treatment) heat treatment, and then air-cooled to room temperature to prepare the final steel, detailed conditions are shown in Table 2.

C Si Mn P S Al Nb Ni Mo V N Tnr Invention lecture 1 0.145 0.20 1.40 0.008 0.002 0.0 0.017 0.35 0.10 - 0.0035 918 Invention lecture 2 0.145 0.20 1.40 0.008 0.002 0.03 0.009 0.35 0.10 - 0.0035 898 Invention lecture 3 0.145 0.20 1.40 0.008 0.002 0.03 - 0.35 0.10 0.009 0.0035 892 Invention lecture 4 0.145 0.20 1.40 0.008 0.002 0.03 - 0.35 0.10 0.017 0.0035 889 Comparative steel 5 0.145 0.20 1.40 0.008 0.002 0.03 - 0.35 0.10 - 0.0035 906 Comparative lecture 6 0.145 0.20 1.40 0.008 0.002 0.03 0.050 0.35 0.10 - 0.0035 1035 Comparative lecture 7 0.145 0.20 1.40 0.002 0.002 0.03 - 0.35 0.10 0.1 0.0035 899

division heating temperature
(℃) Rolling end temperature (℃) Heat treatment temperature (℃) heat treatment time
(min) number of heat treatment PWHT temperature (℃) PWHT time (min) note Invention lecture 1 1-1 1120 872 892 160 One 615 240 Comparative Example 1 1-2 1117 870 888 160 3 614 240 Invention Example 1 1-3 1121 869 891 160 4 615 240 Comparative Example 2 Invention lecture 2 2-1 1120 871 890 160 One 614 240 Comparative Example 3 2-2 1121 869 891 160 3 616 240 Invention example 2 2-3 1121 870 891 160 4 615 240 Comparative Example 4 Invention lecture 3 3-1 1122 873 889 160 One 614 240 Comparative Example 5 3-2 1121 872 888 160 3 615 240 Invention example 3 3-3 1120 872 889 160 4 614 240 Comparative Example 6 Invention lecture 4 4-1 1119 868 890 160 One 614 240 Comparative Example 7 4-2 1117 869 891 160 3 616 240 Invention Example 4 4-3 1120 870 891 160 4 615 240 Comparative Example 8 Comparative steel 5 5-1 1124 871 889 160 One 616 240 Comparative Example 9 5-2 1122 869 890 160 3 616 240 Comparative Example 10 5-3 1123 872 892 160 4 614 240 Comparative Example 11 Comparative lecture 6 6-1 1120 871 888 160 One 614 240 Comparative Example 12 6-2 1119 873 891 160 3 615 240 Comparative Example 13 6-3 1118 877 890 160 4 616 240 Comparative Example 14 Comparative lecture 7 7-1 1123 869 893 160 One 614 240 Comparative Example 15 7-2 1123 869 888 160 3 615 240 Comparative Example 16 7-3 1118 871 890 160 4 615 240 Comparative Example 17

Thereafter, the microstructure at the 1/4t point in the thickness direction of each of the manufactured steel materials was observed, and the results are shown in Table 3 below. Specifically, the steel microstructure was observed with an optical microscope, and then the diameter of the ferrite was measured using an analysis program. In addition, the carbide in the matrix was observed using a transmission electron microscope (TEM), and then the diameters of NbC and VC were measured and averaged.

In addition, the mechanical properties of each of the manufactured steels were evaluated, and the results are shown in Table 3 below. Specifically, the mechanical properties were evaluated at 1/4t point in the thickness direction of each steel material. In this case, for the tensile specimen, JIS No. 1 standard test specimen was taken from each thickness direction point in the direction perpendicular to the rolling direction, and the tensile strength (TS), yield Strength (YS) and elongation (El) were measured. And for the impact specimen, the average impact toughness (CVN) at -60 ℃ was measured by taking a JIS No. 4 standard test specimen from a point 1/4t in the thickness direction in the direction perpendicular to the rolling direction, and the results are shown in Table 3 below. .

division Ferrite diameter (㎛) NbC or VC
Carbide diameter (nm) YP(MPa) TS(MPa) -60℃ impact toughness (J) note Invention lecture 1 1-1 16 20 353 511 142 Comparative Example 1 1-2 13 24 367 522 188 Invention Example 1 1-3 12 28 380 523 172 Comparative Example 2 Invention lecture 2 2-1 18 27 351 506 136 Comparative Example 3 2-2 15 29 364 520 156 Invention example 3 2-3 13 35 388 519 158 Comparative Example 4 Invention lecture 3 3-1 17 17 351 517 120 Comparative Example 5 3-2 14 21 373 523 169 Invention Example 5 3-3 11 24 384 522 156 Comparative Example 6 Invention lecture 4 4-1 18 20 344 498 126 Comparative Example 7 4-2 15 22 355 503 151 Invention Example 7 4-3 14 25 341 497 155 Comparative Example 8 Comparative steel 5 5-1 24 - 305 477 94 Comparative Example 9 5-2 22 - 302 475 88 Comparative Example 10 5-3 21 - 315 478 96 Comparative Example 11 Comparative lecture 6 6-1 14 40 354 534 144 Comparative Example 12 6-2 13 47 364 541 136 Comparative Example 13 6-3 10 56 368 543 149 Comparative Example 14 Comparative lecture 7 7-1 16 42 381 528 148 Comparative Example 15 7-2 14 48 395 550 144 Comparative Example 16 7-3 14 55 394 559 132 Comparative Example 17

As shown in Table 1-3, Inventive Examples 1-4, which satisfy the alloy composition, component relationship, and manufacturing conditions proposed in the present invention, have fine grain size and carbide size of ferrite, so the yield strength and tensile strength of steel It can be seen that not only is excellent, but also the impact toughness of the steel is excellent.

On the other hand, although the steel alloy composition is within the scope of the present invention, Comparative Examples 1, 3, 5 and 7, which were subjected to normalizing heat treatment once, had a large ferrite grain size, so the yield strength and tensile strength of the steel were somewhat lower. It can be seen that not only falls, but also the impact toughness is not good.

In addition, although the steel alloy composition is within the range of the present invention, in Comparative Examples 2, 4, 6 and 8, in which normalizing heat treatment was performed four times, the grain size of ferrite decreased as the number of normalizing heat treatment increased, whereas the average of precipitates It can be seen that the size is increasing. That is, it can be seen that the yield strength is continuously improved as the number of normalizing heat treatments increases, whereas when the number of times is 4 times, the tensile strength and impact toughness are almost equal or slightly lower than that of the 3 heat treatment due to coarsening of the precipitates can be checked

In Comparative Examples 9-11, Nb and V were not added, and showed low strength and impact toughness regardless of the number of normalizing heat treatments.

In Comparative Examples 12-17 in which Nb and V were excessively added, the yield strength and tensile strength were improved as the number of normalizing heat treatment increased, but NbC to VC precipitates were coarsened, so that the improvement in impact toughness was not evident even after repeated heat treatment. Able to know.

On the other hand, Figure 1 is a microstructure photograph at 1/4t point of steel according to an embodiment of the present invention, (a) is Comparative Example 1 in which normalizing is performed once, and (b) is a foot in which normalizing is performed three times. Represents honor 1.

As described above, in the detailed description of the present invention, preferred embodiments of the present invention have been described, but those of ordinary skill in the art to which the present invention pertains may make various modifications without departing from the scope of the present invention. Of course, this is possible. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined by the claims to be described later as well as equivalents thereof.

Claims (4)

By weight%, carbon (C): 0.12 to 0.18%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.5%, phosphorus (P): 0.01% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.25% or less, molybdenum (Mo): 0.01 to 0.15%, titanium (Ti): 0.005% or less ( 0% included), nitrogen (N): 0.002 to 0.01%, and niobium (Nb): 0.02% or less (excluding 0%) and vanadium (V): at least one of 0.05% or less (excluding 0%), balance Fe and unavoidable impurities;
It has a mixed microstructure of ferrite and pearlite,
The grain size of the ferrite is 15 μm or less, and
A steel material for pressure vessels with excellent low-temperature impact toughness in which NbC or VC carbide having a size of 40 nm or less in the ferrite structure is present.
[Claim 2] The steel material for pressure vessels having excellent low-temperature impact toughness according to claim 1, wherein the steel material has a yield strength of 260 MPa or more, a tensile strength of 485 MPa or more, and an impact toughness of 150 J or more measured at -60°C.
By weight%, carbon (C): 0.12 to 0.18%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.5%, phosphorus (P): 0.01% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nickel (Ni): 0.01 to 0.5%, copper (Cu): 0.25% or less, molybdenum (Mo): 0.01 to 0.15%, titanium (Ti): 0.005% or less ( 0% included), nitrogen (N): 0.002 to 0.01%, and niobium (Nb): 0.02% or less (excluding 0%) and vanadium (V): at least one of 0.05% or less (excluding 0%), balance Fe and preparing a steel slab containing unavoidable impurities;
heating the steel slab to a temperature range of 1100 to 1200 °C;
rough-rolling the heated steel slab at a temperature of 1050° C. or higher and a final pass reduction ratio of 10% or higher;
manufacturing a hot-rolled steel sheet by finishing hot rolling at a temperature in the range of Ar3 to Tnr after the rough rolling;
air-cooling the manufactured hot-rolled steel sheet to room temperature; and
The air-cooled hot-rolled steel sheet is reheated to a temperature of Ac3 or higher, heat treated for (1.3t~30) minutes (here, t means the thickness of the steel (mm)) or more, and then the process of air cooling to room temperature is repeated 2-3 times A method of manufacturing a steel material for a pressure vessel having excellent low-temperature impact toughness, comprising the steps of:
4. The pressure vessel with excellent low-temperature impact toughness according to claim 3, wherein after repeating the heat treatment process, PWHT (post-weld heat treatment) heat treatment is performed for 1 hour or more per inch of steel thickness in a temperature range of 550 to 650 ° C. Method of manufacturing steel for use.

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