KR101091306B1 - High Strength Steel Plate for Containment Vessel of Atomic Plant and Manufacturing Method Thereof - Google Patents

Abstract

In the present invention, by weight%, C: 0.03-0.20%, Si: 0.15-0.55%, Mn: 0.9-1.5%, Al: 0.001-0.05%, P: 0.030% or less, S: 0.030% or less, Cr: 0.30% or less, Mo: 0.2% or less, Ni: 0.6% or less, V: 0.07% or less, Nb: 0.04% or less, Ca: 5-50 ppm, Ti: 0.005-0.025%, N: 0.0020-0.0060%, B: Provided is a high strength steel sheet containing 0.0005 to 0.0020%, balance Fe and other unavoidable impurities. The steel sheet of the present invention may be made of tempered martensite, and used in a nuclear power plant by optimizing cooling and recrystallization controlled rolling conditions in order to control the average particle size of microstructure and the shape ratio of tissue grains, for example, a tension of 650 MPa or more. It is possible to provide an excellent high strength steel sheet and its manufacturing method which can be used as a reactor containment vessel in a nuclear power plant of 1000MW or more with strength and impact toughness at -50 ° C of 200J or more.

Tempered martensite, grain shape ratio, impact toughness, tensile strength, reactor containment vessel

Description

High Strength Steel Plate for Containment Vessel of Atomic Plant and Manufacturing Method Thereof}

The present invention relates to a high strength steel sheet excellent in tensile strength and impact toughness and a method for manufacturing the same, and more particularly, to a high strength steel sheet and a method for manufacturing the same, capable of securing tensile strength and impact toughness that can be used as a reactor containment vessel. It is about.

Globally, the reserves of fossil energy such as coal and oil are gradually depleted. This global energy problem has highlighted the importance of nuclear energy, and in fact, the share of nuclear energy is increasing day by day.

However, in order to stably produce nuclear energy, facilities and components that can guarantee the safety of nuclear power plants must be secured. In fact, in the event of an accident at a nuclear power plant due to natural disasters or other causes, a big problem can occur, which can result in inevitable damages in terms of environmental and cost.

Various materials are used for the material which comprises the structure, installation, etc. which are used for a nuclear power plant according to the kind, a use, safety, etc. In particular, a steel material is used in a reactor vessel, and a thick steel sheet material, in particular, A516-70 steel manufactured by a normalizing heat treatment method is mainly used.

However, since the A516-70 steel exhibits a tensile strength (500 Mpa level) which is somewhat low to guarantee the safety of nuclear power plants, the use range of the A516-70 steel is limited. This is because when using a material with low tensile strength, it may not be able to withstand the high pressure from the inside, which may cause a serious danger.

However, simply adding a large amount of expensive alloying elements or a separate heat treatment to the steel in order to improve the tensile strength is inevitable to increase the production cost, and may cause other incidental problems. Therefore, there is a need to develop a tensile strength 650MPa material that can be used in a nuclear power plant, for example, as a nuclear reactor containment vessel while maintaining existing characteristics.

The present invention is to provide a high-strength steel sheet and a method of manufacturing the same that can be used in a nuclear power plant of 1000MW or more by improving the tensile strength of the steel for reactor containment vessels used in the existing nuclear power plant.

In the present invention, by weight%, C: 0.03-0.20%, Si: 0.15-0.55%, Mn: 0.9-1.5%, Al: 0.001-0.05%, P: 0.030% or less, S: 0.030% or less, Cr: 0.30% or less, Mo: 0.2% or less, Ni: 0.6% or less, V: 0.07% or less, Nb: 0.04% or less, Ca: 5-50 ppm, Ti: 0.005-0.025%, N: 0.0020-0.0060%, B: 0.0005 to 0.0020%, balance Fe and other unavoidable impurities, including Cu + Ni + Cr + Mo: 1.5% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less and Ca / S: 1.0 or less It relates to a satisfactory high strength steel sheet. In this case, the microstructure of the steel sheet may be made of tempered martensite, in which case the average particle size of the microstructure may be 30 μm or less. In addition, the grain shape ratio (long axis / short axis) of the microstructure may be 1.1 to 2.5.

Furthermore, the present invention is to heat the steel slab of the above composition to 1050 ~ 1250 ℃, rolled at a temperature of Tnr ~ Tnr + 100 ℃ and finish rolling at 870 ~ 950 ℃, 1.3 * t + (10 ~ ~ 870 ~ 950 ℃ After 30 minutes austenizing heat treatment and quenching, to a tempering at 650 ~ 700 ℃ to produce a high strength steel sheet. In this case, the recrystallization rolling step can be rolled in the range of 50 ~ 90% cumulative reduction amount with a reduction ratio of 10% or more per each rolling pass, and the grain shape ratio (long axis / short axis) of the old austenite structure is 1.1 to 2.5 Can be controlled by

According to the present invention, it is possible to provide an excellent high strength steel sheet capable of being used as a reactor containment vessel in a nuclear power plant of 1000 MW or more with a tensile strength of 650 MPa or more and an impact toughness at -50 ° C of 200 J or more, and a method of manufacturing the same.

The present invention relates to a steel sheet having a tempered martensite structure, and having a tensile strength of 650 MPa level by controlling the refinement and shape ratio of grains by recrystallization controlled rolling.

Hereinafter, the component system which comprises this invention is demonstrated in detail. However,% of the component system below means weight%.

C: 0.03-0.20%

In the present invention, C is limited to 0.03 to 0.20% as an element for securing strength. If the content of C is less than 0.03%, the strength on the matrix may be lowered, which is not preferable. However, if the content of C is less than 0.20%, the toughness and weldability may be deteriorated, which is not suitable for use in nuclear power plants.

Si: 0.15 to 0.55%

Si is an alloying element added for the deoxidation effect, the solid solution strengthening effect, and the impact transition temperature raising effect, and at least 0.15% is added. However, if the content exceeds 0.55%, the weldability is lowered and the oxide film may be severely formed on the steel plate surface, so Si is added in an amount of 0.15 to 0.55%, preferably 0.15 to 0.40%.

Mn: 0.9-1.5%

Excessive addition of Mn forms MnS, which is a non-metallic inclusion drawn together with S, thereby lowering room temperature elongation and low temperature toughness, thereby managing Mn to 1.5% or less. However, when Mn is less than 0.9% due to the component properties of the present invention, it is difficult to secure appropriate strength, so the amount of Mn added is limited to 0.9 to 1.5%.

Al: 0.001-0.05%

Al, together with Si, is one of the strong deoxidizers in the steelmaking process to add 0.001% or more to achieve this effect. However, if the content is added in excess of 0.05%, the effect is saturated and the manufacturing cost is increased, so Al is limited to 0.001 to 0.05%.

P: 0.030% or less

Since P is an element that impairs low temperature toughness, it is better to manage it as low as possible, but to remove it excessively in the steelmaking process is expensive, so it is managed within 0.030% or less.

S: 0.030% or less

S is also an element that adversely affects low temperature toughness along with P, but like P, it may be excessively expensive to remove in the steelmaking process, so it is appropriate to manage it within 0.030% or less.

Cr: 0.30% or less (excluding 0%)

Cr is an alloying element that can increase the strength, but is an expensive element, and if it is added in excess of 0.30%, Cr causes an increase in production cost, so it is limited to within 0.30%.

Mo: 0.2% or less (except 0%)

Mo is an alloying element that is effective for improving strength, such as Cr, and is known as an element that prevents cracking caused by sulfides. However, Mo is also an expensive element, it is preferable to add in the range of 0.2% or less from the economical point of view.

Ni: 0.6% or less (except 0%)

Ni is added to the present invention as an element effective in improving low-temperature toughness, but Ni is also added in an amount of 0.6% or less in the present invention because the production cost increases when a large amount is added as an expensive element.

V: 0.07% or less (except 0%)

V is an effective element for improving strength, such as Cr and Mo, but is added at 0.07% or less due to its high cost.

Nb: 0.04% or less (except 0%)

Nb is dissolved in austenite to increase the hardenability of austenite, and precipitated as carbonitrides (Nb (C, N)) that match with matrix (Mab) together with Ti, and thus the tensile strength of 650 MPa or more pursued by the present invention. It acts as an important element to get. However, when Nb is added in an excessively large amount, it may appear as a coarse precipitate in the playing step to act as a site of hydrogen organic cracking (HIC), so the content of Nb in the present invention is limited to 0.04% or less.

Ca: 5 ~ 50ppm

Ca is produced by CaS serves to suppress the non-metallic inclusions of MnS, for this purpose is added 5ppm or more. However, if the amount is excessive, the upper limit is set to 50 ppm because it reacts with O contained in the steel to form CaO, which is a non-metallic inclusion, which is not good for physical properties.

Ti: 0.005-0.025%

The proper addition amount of Ti may be somewhat fluid depending on the content of N. If the amount of Ti added is relatively small compared to the amount of N, the amount of TiN produced decreases, which is not good for refining grains. On the other hand, when Ti is excessively added, TiN may be coarsened during the heating process, thereby reducing the grain growth inhibition effect. Therefore, the amount of Ti is generally limited to 0.005 to 0.025% in consideration of the N content (20 to 60ppm) contained.

N: 0.0020 to 0.0060% (20 to 60 ppm)

N is known to play a role of increasing the toughness of the base metal and impact toughness of the HAZ portion by forming a TiN precipitate with Ti to refine the grain of steel, and is an element that must be added to achieve the purpose of grain refinement in the present invention. To this end, the amount of N added is limited to 0.0020 to 0.0060% in consideration of the content of Ti. The addition of N exceeding 0.0060% may cause excessive increase in the amount of TiN produced and lower the low temperature toughness.

B: 0.0005 ~ 0.0020%

B is an alloying element that can increase the hardenability even if a small amount is added, thereby achieving high strength, and serves as an important element for securing tensile strength in the present invention. Therefore, in order to secure high tensile strength, it is necessary to add B or more than 0.0005%, but excessive addition of more than 0.0020% saturation of the effect is saturated, so B is added in 0.0005 ~ 0.0020%.

Cu + Ni + Cr + Mo: 1.5% or less

Cr + Mo: 0.4% or less

V + Nb: 0.1% or less

Ca / S: greater than 1.0

The relationship between Cu + Ni + Cr + Mo, Cr + Mo, and V + Nb is a value limited by the basic standard for steel for pressure vessels (ASTM A20), and accordingly, the Cu + Ni + Cr + Mo content is 1.5%. Hereinafter, the Cr + Mo content is limited to 0.4% or less, and the V + Nb content is limited to 0.1% or less. (However, alloy elements not included in the present invention such as Cu can be calculated as 0.)

The ratio of Ca / S is an essential component ratio for spheroidizing MnS inclusions to improve hydrogen organic cracking resistance. When the ratio is 1.0 or less, the effect is hardly expected, so the ratio is adjusted to be greater than 1.0.

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

Microstructure: Tempered Martensite Tissue

In the present invention, in order to secure sufficient strength, the martensite structure generated by the quenching treatment in the manufacturing step is used. It is because the 650 MPa grade steel plate aimed at by this invention can be manufactured by using the high tensile strength improvement effect of a martensitic structure.

However, martensite is basically known as a brittle structure, and because of the strong residual stress, it can be easily broken by an impact from the outside, which is not suitable for use as a reactor containment vessel. Therefore, the microstructure is formed into a tempered martensite structure by tempering to have a tensile strength of 650 MPa grade and impact toughness of 200 J or more at -50 ° C by relieving residual stress while utilizing the strength improving effect of martensite.

Grain shape ratio: 1.1≤long axis / short axis≤2.5

It is necessary to control the shape ratio of the microstructure grains through the recrystallization control rolling. In the present invention, the ratio of the major axis to the minor axis is controlled to 1.1 to 2.5. The shape ratio of the crystal grains is for obtaining high impact toughness-strength, because when the grain shape ratio becomes 1.1 or less, it is difficult to expect the refinement of the grains, and when the shape ratio is 2.5 or more, the impact toughness is feared.

If the aspect ratio is lower than 1.1, the shape of the crystal grains is rounded, so that the surface energy is small and it is difficult to secure sufficient strength and toughness. On the other hand, if the ratio exceeds 2.5, the rolling load is large to form the grains, which is not preferable.

Hereinafter, each step condition in the manufacturing method of manufacturing the steel sheet of the present invention will be described in detail.

The steel sheet of the present invention is produced through a series of steps of reheat-rolling-cooling-heat treating steel slabs. In this case, in the present invention, in order to form a tempered martensite structure, the production step is important in the cooling step including the quenching treatment, the heat treatment step including the tempering treatment and the recrystallization zone control rolling step for grain control of the old austenite structure. Requires a condition.

Reheating Temperature: 1050 ~ 1250 ℃

In the present invention, the slab having the composition described above is reheated at 1050 to 1250 ° C. If the reheating temperature is lower than 1050 ° C, solute atoms are difficult to be dissolved, whereas if the reheating temperature exceeds 1250 ° C, the austenite grain size becomes too coarse and the properties of the steel sheet are degraded.

Recrystallization-controlled rolling: 50 to 90% cumulative reduction at a temperature of Tnr to Tnr + 100 ° C and a rolling reduction of 10% or more per each rolling pass

The steel slabs heated to perform the rolling are hot rolled at a temperature range above the unrecrystallized zone. The non-recrystallization temperature T _nr can be calculated by Equation 1 below. However, in the formula, the unit of each alloy element represents weight%.

[Equation 1]

Tnr (° C) = 887-464 × C + 890 × Ti + 363 × Al-357 × Si + (6445 × Nb-644 × Nb ^1/2 ) + (732 × V-230 × V ^1/2 )

In order to obtain sufficient strength improvement, it is necessary to refine the average grain size (Austenite Grain Size) of the old austenite to 30 µm or less in the recrystallization zone controlled rolling process. If the old austenite average particle diameter exceeds 30 µm, the strength and toughness of the product cannot be sufficiently exhibited, and thus the level of safety that can be used as the reactor containment vessel cannot be guaranteed. To this end, in the present invention, rolling is carried out at a temperature range of T _nr to T _nr + 100 ° C.

In this case, in the section of rolling, rolling is applied in the range of 50 to 90% of the cumulative rolling amount by applying a rolling reduction rate of 10% or more per rolling pass. This amount of reduction is for controlling the average size (30 µm or less) and grain shape ratio (long axis / short axis) of the microstructure required by the present invention to 1.1 to 2.5. Therefore, when the cumulative reduction is less than 50%, it is difficult to expect such a result. On the other hand, if the cumulative reduction exceeds 90%, the load of the rolling mill is inevitably increased, which may be a process problem.

Cooling condition: quenching after austenizing heat treatment for 1.3 * t + (10 ~ 30 minutes) at 870 ~ 950 ℃

The cooling step is an important step for forming the tempered martensite structure, and it is necessary to strictly control the conditions for constructing the microstructure to secure tensile strength of 650 MPa or more and impact toughness of -50 ° C. of 200 J or more.

To this end, in the present invention, austenizing heat treatment is performed for a time of 1.3 * t + (10 to 30 minutes) (where t denotes the thickness (mm) of steel) in the range of 870 to 950 ° C. The austenizing heat treatment is a heat treatment for generating martensite structure by quenching the tissue after austenizing, and when the temperature of the heat treatment is lower than 870 ° C., it is difficult to re-use solid solution solutes, thereby making it difficult to secure strength. If the temperature is higher than 950 ° C, grains may grow and coarse grains may occur, thereby impairing low-temperature toughness.

In addition, the austenizing heat treatment time is achieved by heating and maintaining in the range of 1.3 * t + (10-30 minutes), while the treatment for too short time may not be enough to heat the austenizing effect and it may be difficult to homogenize the tissue. This is because, if the heat treatment is too long, the production time may be long and the productivity may be reduced. For reference, it is preferable to set 1.3t as a heating time, and when the target temperature is reached, 10 to 30 minutes as a holding time to perform an austenizing heat treatment.

After the austenizing is finished, the steel sheet is quenched, preferably water cooled, to transform the microstructure into martensite. The quench treatment step in the present invention does not particularly limit its requirements, and may be applied to any quench treatment method including conventional water cooling.

Tempering condition: 1.9 * t + (10-30 minutes) at 650 ~ 700 ℃

In the present invention, the tempered treatment is performed to remove residual stress of the produced martensite structure, thereby obtaining a tempered martensite structure. In this case, the tempering temperature is carried out at 650 ~ 700 ℃,

If the tempering temperature is lower than 650 ° C., precipitation of carbides or the like is not smooth. On the contrary, the strength of the steel may be lowered at a high temperature exceeding 700 ° C., so it is necessary to appropriately control the tempering temperature conditions.

In addition, the tempering treatment takes place in the range of 1.9 * t + (10-30 minutes) (where t means the thickness of the steel in mm) in order to obtain a sufficient effect, where 1.9t is the heating time and It is preferable to set 10-30 minutes as a holding time, and to perform tempering.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

To prepare a slab consisting of the inventive material and the comparative material having the alloying elements of Table 1.

Each slab prepared with the composition of the invention and the comparative material was subjected to recrystallization controlled rolling in a heating and recrystallization zone as in the conditions of Table 2 below. Conditions such as control rolling and heat treatment were performed as shown in Table 2, and then the strength and low temperature toughness were evaluated, and the results are shown in Table 2 below. However, in Table 2 below, the low temperature toughness is evaluated by the Charpy impact energy value obtained by performing a Charpy impact test on a specimen having a V notch at -50 ° C.

* Grain Shape Ratio: Long Grain / Short Grain

** The quench temperature of the comparative material represents the normalizing temperature.

*** Impact toughness: Impact toughness in the T direction (V-notch perpendicular to the rolling direction).

Looking at the results of Table 2, even when manufacturing the steel sheet using the invention material it can be seen that the failure to control the grain shape ratio by the recrystallization control rolling impact impact toughness is reduced to obtain the physical properties required by the present invention. In addition, the comparative materials c and d can be inferred that the grain refinement is difficult because B and Ti are not added basically in the component system. Accordingly, it is difficult to produce a microstructure of a level capable of securing sufficient strength and toughness, and physical properties were not properly secured. Therefore, it is required to satisfy all of the conditions of the present invention in order to satisfy sufficient strength-toughness conditions and obtain physical properties that can be used in the reactor containment vessel.

Claims (8)

delete By weight%, C: 0.03-0.20%, Si: 0.15-0.55%, Mn: 0.9-1.5%, Al: 0.001-0.05%, P: 0.030% or less, S: 0.030% or less, Cr: 0.30% or less ( 0% excluding), Mo: 0.2% or less (excluding 0%), Ni: 0.6% or less (excluding 0%), V: 0.07% or less (excluding 0%), Nb: 0.04% or less (0%) Ca: 5-50 ppm, Ti: 0.005-0.025%, N: 0.0020-0.0060%, B: 0.0005-0.0020%, balance Fe and other unavoidable impurities, Cu + Ni + Cr + Mo: 1.5% or less; Cr + Mo: 0.4% or less; V + Nb: 0.1% or less; And Ca / S: greater than 1.0 The relationship between the high strength steel sheet, characterized in that the microstructure of the steel sheet is made of a tempered martensite structure. The high strength steel sheet according to claim 2, wherein the grain shape ratio (long axis / short axis) of the microstructure is 1.1 to 2.5. 4. The high strength steel sheet according to claim 2 or 3, wherein the steel sheet has a tensile strength of 650 MPa or more and an impact toughness at -50 ° C of 200 J or more. By weight%, C: 0.03-0.20%, Si: 0.15-0.55%, Mn: 0.9-1.5%, Al: 0.001-0.05%, P: 0.030% or less, S: 0.030% or less, Cr: 0.30% or less ( 0% excluding), Mo: 0.2% or less (excluding 0%), Ni: 0.6% or less (excluding 0%), V: 0.07% or less (excluding 0%), Nb: 0.04% or less (0%) Ca: 5-50 ppm, Ti: 0.005-0.025%, N: 0.0020-0.0060%, B: 0.0005-0.0020%, balance Fe and other unavoidable impurities, Cu + Ni + Cr + Mo: 1.5% or less; Cr + Mo: 0.4% or less; V + Nb: 0.1% or less; And Ca / S: greater than 1.0 To meet the river slabs, Reheating step to heat to 1050 ~ 1250 ℃; A recrystallization zone controlled rolling step of rolling at a temperature of Tnr ˜Tnr + 100 ° C .; Quenching step of quenching after austenizing heat treatment at 870 to 950 ° C for 1.3 * t + (10 to 30 minutes); And Tempering step tempering at 1.9 * t + (10-30 minutes) at 650 ~ 700 ℃ Method for producing a high strength steel sheet comprising a. The method of claim 5, wherein the recrystallization rolling step is performed in a rolling range of 10% or more and a 50% to 90% cumulative reduction amount per rolling pass. The method for manufacturing a high strength steel sheet according to claim 5 or 6, wherein the recrystallization zone controlled rolling step controls the grain shape ratio (long axis / short axis) of the microstructure to 1.1 to 2.5. delete