KR20100076728A - High strength steel sheet with excellent low-temperature toughness for pressure vessel and manufacturing method thereof - Google Patents

Abstract

PURPOSE: A high strength steel plate for a pressure vessel with superior low temperature toughness and a manufacturing method thereof are provided to improve shock toughness and tensile strength by controlling cooling speed and performing a tampering process under proper conditions. CONSTITUTION: A high strength steel plate for a pressure vessel comprises C 0.03~0.20 weight%, Si 0.15~0.55 weight%, Mn 0.9~1.5 weight%, Al 0.001~0.05 weight%, P 0.030 weight% or less, S 0.030 weight% or less, Cr 0.30 weight% or less, Mo 0.2 weight% or less, Ni 0.6 weight% or less, V 0.07 weight% or less, Nb 0.04 weight% or less, Ca 5~50ppm, Ti 0.005~0.025 weight%, N 0.0020~0.0060 weight%, and the rest including Fe and inevitable impurities. Cu+Ni+Cr+Mo are included less than 1.5 weight%. Cr+Mo are included less than 0.4 weight%. V + Nb are included less than 0.1 weight%.

Description

High Strength Steel Sheet with Excellent Low-Temperature Toughness for Pressure Vessel and Manufacturing Method Thereof}

The present invention relates to a steel plate for tensile strength 700MPa pressure vessel that can be used in a nuclear reactor containment vessel for a nuclear power plant of 1000MW or more, and a method of manufacturing the same. The present invention relates to a steel plate for a pressure vessel that can be used to manufacture a reactor containment vessel for a nuclear power plant by optimizing a manufacturing process to improve tensile strength, and a method of manufacturing the same.

As fossil fuels are becoming increasingly depleted, the use of nuclear energy is now becoming an indispensable energy supply plan. The use of nuclear energy is always accompanied by the safety issues of nuclear power plants, and there is a constant research on plant materials that can safely operate them.

In particular, steel products, in particular steel plates, are used for the material for the containment vessels used in nuclear power plants. As a conventional reactor containment material, A516-70 steel manufactured by the normalizing heat treatment method was mainly used. However, since the A516-70 steel has a tensile strength of generally only 500 Mpa, the use range is limited and there is always a concern that safety problems may occur.

However, the development of new steel by the addition of expensive alloying elements or the addition of processing equipment can be costly, and there is always a concern that other problems will arise instead of improving the tensile strength. There is a need for the development of excellent reactor containment materials that can improve the tensile strength to 700MPa level while maintaining the characteristics.

The present invention provides an excellent steel sheet and a method of manufacturing the same which can be used to manufacture a reactor containment container for a large nuclear power plant of 1000MW or more because of excellent tensile strength but no problems with other physical properties.

The present invention, for this purpose, 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%, balance Fe And other unavoidable impurities, and satisfying the relationship of 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 Provides a high strength steel sheet for use in pressure vessels. The steel sheet of the present invention may be made of a tempered martensite structure, the tensile strength is 700MPa or more and the impact toughness at -50 ℃ is 300J or more.

Further, the present invention is heated to 1050 ~ 1250 ℃, rolled at a temperature of Tnr ~ Tnr + 100 ℃ for steel slab that satisfies the above component system conditions, finish rolling at 870 ~ 950 ℃, cooling of 5 ~ 50 ℃ / sec It provides a method for producing a high strength steel sheet for pressure vessels that can be used in a nuclear power plant directly quenched and tempered at 650 ~ 700 ℃ at a rate. In this case, the rolling may be carried out with a cumulative reduction of 50% or more at a rolling reduction rate of 10% or more per each rolling pass, and the tempering may be performed for 1.9 * t + (10 to 30 minutes).

According to the present invention, it is possible to provide an excellent pressure vessel steel sheet that can be used in a nuclear power plant having a high tensile strength of 700 MPa or more and a low temperature impact toughness at -50 ° C of 300 J or more, thus requiring considerable safety.

In order to optimize the component system and obtain sufficient tempered martensite structure that can secure tensile strength, the present invention can control the cooling rate, use direct quenching and tempering under appropriate conditions, and tensile strength of over 700 MPa and 300J. The steel plate for pressure vessels provided with impact toughness at -50 degreeC mentioned above is provided.

Hereinafter, the component system contained in the steel sheet of the present invention will be described in more detail. However,% of a component system 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 surface of the steel sheet, so Si is added at 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 0.6 to 1.2%.

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

Cr is an alloying element that can increase the strength, but since it is an expensive element, the addition of more than 0.30% causes an increase in manufacturing cost, so it is limited to within 0.30%.

Mo: 0.2% or less

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

Ni is added to the present invention as an element effective in improving low temperature toughness, but Ni is also added as an expensive element at an economical level of 0.6% or less.

V: 0.07% or less

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

Nb: 0.04% or less

Nb is dissolved in austenite to increase the hardenability of austenite, and also precipitates as carbonitride ((Nb, Ti) (C, N)) that matches the matrix with Ti to increase the strength of the steel. Acts as an important element 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, 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 appropriate amount of Ti may vary somewhat depending on the content of Nb and N. If the amount of Ti added is relatively small compared to the amount of N, the amount of (Nb, Ti) N is reduced, which is detrimental to the refinement of grains, whereas when the amount is added in excess, (Nb, Ti) N becomes coarse during the heating process. Thus, the effect of inhibiting grain growth is rather reduced. 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 forms TiN precipitates together with Nb and Ti to refine the grains of the steel to increase toughness of the base metal and impact toughness of the HAZ portion. To this end, in the present invention, the amount of N is limited to 0.0020 ~ 0.0060% range 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.

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

Cr + Mo: 0.4% or less

V + Nb: 0.1% or less

Ca / S: 1.0 or less

The relationship between Cu + Ni + Cr + Mo, Cr + Mo, and V + Nb is a value limited by the basic standard (ASTM A20) of steel for pressure vessels, and accordingly, Cu + Ni + Cr + Mo content is 1.5 Below%, Cr + Mo content is limited to 0.4% or less, and V + Nb content is limited to 0.1% or less.

And the ratio of Ca / S is an essential constituent ratio to improve the hydrogen organic crack resistance by spheroidizing the MnS inclusions, and since the effect is difficult to be expected at 1.0 or less, the ratio is adjusted to exceed 1.0.

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

Organization: Temper Martensite

In the present invention, it is quenched to form martensite structure during the cooling process through rolling. In the present invention, it is possible to obtain a tensile strength of 700 MPa or more due to the strong tensile strength improving effect of martensite. However, since martensite is brittle in itself, if used in a nuclear power plant as it is, the vessel may be damaged by an external shock, which causes a serious danger. Therefore, in the present invention, the tempered martensite structure is formed through the tempering process in the martensite structure to obtain impact toughness of 300J or more based on -50 ° C, so that the tensile strength can be greatly improved and sufficient toughness is obtained at the same time. do.

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

In the present invention, in order to form a structure consisting of tempered martensite which can obtain a tensile strength of 700MPa or more based on the above-described component system is subjected to the appropriate rolling and heat treatment process as follows.

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 ℃, it is difficult to solute the solute atoms, while if the reheating temperature exceeds 1250 ℃ austenite grain size becomes too coarse to reduce the properties of the steel sheet.

Recrystallization zone controlled rolling: Temperature of Tnr to Tnr + 100 ° C, rolling reduction at 50% or more at 870 ° C to 950 ° C with a cumulative reduction of 10% or more per rolling pass

In the present invention, in order to obtain a fine old austenite structure of less than about 30㎛ on average, it is subjected to a controlled rolling and controlled cooling process. In order to control the size of the old austenite average grain size to about 30 μm or less on average, recrystallization controlled rolling serves as the most important variable.

The recrystallized controlled rolling means hot rolling at a temperature higher than the _{uncrystallized crystal} , and the recrystallized temperature, T _nr , can be calculated through Equation 1 below. However, in the formula, the unit of each alloy element represents weight%. The rolling reduction rate and the cumulative rolling reduction per rolling pass are carried out under such rolling conditions for the structure refinement to the conditions necessary for securing a banding index value to be described later.

[Equation 1]

Tnr (℃) = 887-464ⅹC + 890ⅹTi + 363ⅹAl-357ⅹSi + (6445ⅹNb-644ⅹNb ^1/2 ) + (732ⅹV-230ⅹV ^1/2 )

In the present invention, the recrystallized controlled rolling is applied with a reduction ratio of 10% or more per rolling pass in a temperature range section of Tnr to Tnr + 100 ° C. to give a cumulative reduction of 50% or more. This is because when the cumulative reduction is less than 50%, a fine grain size cannot be expected.

And, the end of the rolling is in the range of 870 ~ 950 ℃, if the end of the rolling temperature is less than 870 ℃ quenching effect is insufficient, it is difficult to secure the strength and toughness even after the direct quenching process, whereas if it exceeds 950 ℃ Grain grows coarsely, leading to degradation of toughness.

Direct hardening: 5 ~ 50 ℃ / sec cooling rate

After rolling, direct hardening is performed at a cooling rate of 5 to 50 ° C / sec. If the cooling rate is less than 5 ℃ / sec, it is difficult to secure sufficient strength and toughness due to the lack of the hardening effect, while the cooling rate exceeding 50 ℃ / sec is difficult to obtain in the normal accelerated cooling process, so The range is limited to 5 to 50 ° C / sec.

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

Steel quenched by direct quenching includes martensite structure, and in order to alleviate brittleness of the martensite structure, the present invention performs a tempering process. In this case, the tempering temperature is carried out at 650 ~ 700 ℃, but if the tempering temperature is lower than 650 ℃, the precipitation of carbides, etc. is not smooth, so that the tempering effect of martensite is not sufficiently obtained, whereas at temperatures exceeding 700 ℃ The strength of the steel can be lowered.

In addition, the tempering time condition is preferably different depending on the thickness, in the present invention is carried out under the conditions of 1.9 * t + (10-30 minutes) (where t means the thickness (mm) of the steel).

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

(Example)

In this example, the invention and the comparative material were prepared using the chemical components shown in Table 1 below. The steel slabs of each chemical composition were heated at an appropriate temperature range, and recrystallization controlled rolling was carried out at a reduction ratio of 55 to 80% in the recrystallization zone to control the average austenite grain size of about 30 μm or less.

C Mn Si P S Al Ni Cr Mo V Nb B Ti N Ca foot
persons
ashes a 0.06 1.35 0.35 0.008 0.0013 0.02 0.05 0.03 0.15 0.003 0.013 0.0015 0.013 0.0028 0.0018 b 0.07 1.40 0.34 0.010 0.0014 0.01 0.15 0.05 0.10 0.005 0.012 0.0012 0.012 0.0035 0.0021 ratio
School
ashes c 0.17 1.05 0.28 0.010 0.0017 0.03 0.18 0.15 0.08 0.010 0.010 - - 0.0032 0.0025 d 0.14 1.15 0.29 0.012 0.0014 0.01 0.15 0.20 0.15 0.009 0.012 - - 0.0034 0.0023

Then, after performing conditions such as controlled rolling, direct quenching, tempering heat treatment, and the like as shown in Table 2 below, yield strength, tensile strength and low temperature toughness were evaluated, and the results are shown in Table 3 below.

However, low-temperature toughness is evaluated by the Charpy impact energy value obtained by carrying out the Charpy impact test of the specimen which has a V notch at -50 degreeC.

division Steel plate thickness
(mm) Slab
Reheating temperature
(℃) Recrystallization Control Rolling
Cumulative pressure drop
(%) Direct hardening
Temperature
(℃) Tempering
Temperature
(℃) foot
persons
ashes a 13 1200 65 900 670 25 1180 70 930 670 45 1120 60 910 670 50 1120 65 910 670 b 30 1100 80 910 670 60 1100 75 930 670 80 1100 65 900 670 ratio
School
ashes c 20 1200 20 900 * - 25 1150 30 900 * - d 50 1100 40 900 * -

** However, the direct quenching temperature of the comparative material represents the 'normalizing temperature'.

division YS (yield strength)
(Mpa) TS (tensile strength)
(Mpa) -50 ℃
Impact Toughness (J) Invention a 694 735 310 691 731 309 680 734 315 672 725 329 b 690 730 314 695 732 320 687 728 330 Comparative material c 370 539 186 365 530 175 d 358 520 190

As can be seen in Table 3, the invention materials a and b satisfying the component system and the production conditions of the present invention was excellent in yield strength and tensile strength, it can be seen that the low temperature toughness also improved.

Claims (5)

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% 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%, 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: 1.0 or less Satisfy the relationship of, Microstructure is a high strength steel sheet for pressure vessel, characterized in that the tempered martensite structure. The high strength steel sheet for pressure vessel according to claim 1, wherein the steel sheet has a tensile strength of 700 MPa or more and an impact toughness of -50 ° C. or more. 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% 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%, 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: 1.0 or less For steel slabs that satisfy the relationship of Reheating step to heat to 1050 ~ 1250 ℃; A recrystallization station rolling step of rolling at a temperature of Tnr ˜Tnr + 100 ° C. to finish rolling at 870 ° C. to 950 ° C .; Direct quenching step of cooling at a cooling rate of 5 ~ 50 ℃ / sec; And Tempering step to temper at 650 ~ 700 ℃ Method for producing a high strength steel sheet for pressure vessel comprising a. The method of claim 3, wherein the recrystallization rolling step is performed at a cumulative reduction of 50% or more at a reduction ratio of 10% or more per each rolling pass. The method of claim 3, wherein the tempering step is performed for 1.9 * t + (10 to 30 minutes).