KR100346307B1 - A Low Alloy Steel added Al and N for High Tough Nuclear Reactor Pressure Vessel - Google Patents

Abstract

The present invention is to remove and remove the non-metallic inclusions in the oxidation refining, ASEA-SKF secondary ladle refining furnace using an electric furnace, degassing after reduction refining by adding Al and Si, and then vacuum degassing Low alloy steel produced by ingot, forging, soaking, tempering, rough quenching and tempering heat treatment, and the chemical composition is 0.1% to 0.20% of carbon (C), 1.35 to 1.45% of manganese (Mn), Silicon (Si) 0.15 to 0.25%, phosphorus (P) up to 0.010%, sulfur (S) up to 0.010%, nickel (Ni) 0.80 to 0.90%, chromium (Cr) 0.17 to 0.20%, molybdenum (Mo) 0.45 to 0.55 %, Up to 0.10% of copper (Cu), 0.020 to 0.030% of aluminum (Al), and 70 to 90 ppm of nitrogen (N), the ratio of nitrogen to aluminum is 0.23 to 0.45, and the balance is iron (Fe) and steelmaking The present invention relates to a low-alloy steel for high toughness reactors having excellent impact toughness composed of inevitable impurity elements which cannot be removed by the present invention.

Description

A Low Alloy Steel added Al and N for High Tough Nuclear Reactor Pressure Vessel}

The present invention relates to a low-alloy steel for high toughness reactors in which aluminum and nitrogen are added, and in particular, deoxidized with silicon, and aluminum (Al) and nitrogen (N) are added, so that high temperature tensile strength is high, high toughness and lead-free transition temperature. The present invention relates to a low alloy steel, which is a kind of steel material that can be used in a temperature range of at least 280 ° C. or more, which is manufactured for use as a reactor for reactor pressure (Neclear Reactor Pressure Vessel).

Reactor pressure vessels are manufactured on the basis of the chemical composition specified in ASME SA508 class.3. This material is produced by various deoxidation methods in steelmaking to increase the required high temperature fracture toughness and cleanliness, homogeneity of chemical composition and mechanical properties. This has been applied.

Typically, steel manufactured by Vacuum Carbon Deoxidation (VCD) has a high value of Reference Nil-Ductility Transition Temprrature (ASTM E 208: RT _NDT ) by drop weight test and is unstable. In the case of steel deoxidized with Al in the VCD process, RT _NDT could be stably produced at -23 ° C, but the tensile strength was low.

Recently, the material prepared by deoxidation with Si is stable in tensile strength, but there is a big problem in changing RT _NDT value. In particular, in the construction of next-generation nuclear power plants that extend the design life from 40 years to 60 years by improving stability and economic efficiency and improving neutron irradiation embrittlement resistance, steel with an RT _NDT value of -29 ° C or lower should be stably manufactured.

The present invention has been made to solve the problems of the conventional steels as described above, and the low temperature fracture toughness is significantly improved by the addition of aluminum and nitrogen has a stable RT _NDT value of less than -29 ℃, high tensile strength The purpose is to provide a low alloy steel having.

1 is a graph showing the change in physical properties of the conventional nuclear pressure vessel produced by the steelmaking process,

2 is a graph showing the change of the reference non-flammable transition temperature according to the aluminum content,

3 is a manufacturing process diagram of the reactor steel with improved toughness by adding silicon deoxidation and aluminum, nitrogen,

4 is a graph showing the change of the non-flammable transition temperature according to the nitrogen / aluminum ratio.

Pressure vessels for nuclear power plants that are commonly used are exposed to radiation embrittlement by neutrons generated during nuclear fission, thermal fatigue generated during operation, and high temperature, resulting in material degradation.

Therefore, it is essential to ensure sufficient initial toughness before use of the reactor pressure vessel material in order to secure the tensile strength that can withstand high temperature and high pressure and the stability against sudden breakdown.

In order to secure sufficient toughness in steel materials, there are methods of adjusting alloy components and changing heat treatment conditions. However, in the latter case, it is difficult to make a substantial and substantial improvement because of limitations of manufacturing facilities and a small range of performance improvement.

Table 1 shows the impact toughness and tensile properties of the materials manufactured under various steelmaking conditions, and the aluminum was added to the VCD method rather than the aluminum, which was manufactured by vacuum carbon deoxidation (VCD). It can be seen that the impact toughness of the material is improved by (VCD + Al) and silicon de-oxidation (Si-killing), and FIG. 1 shows this trend together with the reference lead-free transition temperature.

As shown in FIG. 2, it can be seen that the RT _NDT is remarkably improved when the aluminum content is 0.01% or more from the relationship between the aluminum content and the reference lead-free transition temperature.

The improvement of toughness according to the aluminum content is combined with nitrogen in the steel by the addition of aluminum to form a fine AlN (aluminum nitride), thereby miniaturizing the austenite grain size by inhibiting the growth of grains during austenizing .

In particular, the reason why the toughness is improved is that lath boundary is well developed when aluminum is added during phase transformation to bainite, needle-shaped Mo ₂ C carbides precipitate in lath, and round carbides are 0.05 µm or less in size. It was thought to be because it was refined.

However, in general steelmaking, the nitrogen content is 0.0040 to 0.005%, so that it cannot maintain 0.23 to 0.45, which is the N / Al ratio required to control grain size. For this reason, there is a problem in that stable tensile properties and impact toughness cannot be obtained.

Therefore, by appropriately adjusting the content of Al and N it is possible to manufacture an improved reactor material having a stable tensile strength value and excellent toughness.

For the above reason, the low alloy steel according to the present invention aims at a material having the following alloy components, and the chemical composition of the low alloy steel according to the present invention is shown in Table 2 below.

Carbon (C)

Carbon is the most basic element that determines the strength and toughness, which is the basic property of steel, and it is necessary to increase the tensile strength, improve the bearing strength, and improve the hardenability to secure homogeneous mechanical properties to the center of thick steel.

If it is less than 0.1%, it cannot be secured to a value requiring high temperature strength, and if it exceeds 0.20%, weldability is lowered, making it difficult to manufacture a pressure vessel. Therefore, in order to maintain the required strength and ensure weldability, it is desirable to limit the amount to 0.19 to 0.20 wt%.

Manganese (Mn)

Manganese plays an important role as an alloying element to improve the hardenability during the maintenance of strength and heat treatment, but as the content of manganese increases, the ductility-brittle transition temperature increases.

In addition, by combining with sulfur (S) to form a non-metallic inclusions to reduce the toughness, the manganese remaining in the steel promotes temper embrittlement, such as silicon, so it is a better element to decrease. Manganese affects neutron irradiation embrittlement when it contains more than 1%, but the upper limit is set up to 1.5wt% to improve the toughness before irradiation, and the content is 1.35 ~ 1.45wt% to minimize the adverse effect of manganese. Regulate.

Silicon (Si)

Silicon acts as a deoxidizer to remove oxygen from molten steel during steelmaking, and if the content is low, deoxidation may be insufficient.When it is more than 0.3wt%, it promotes coarsening of carbides to reduce high temperature strength or creep for a long time. In order to minimize the degradation of the properties or macro segregation of large ingots, the low alloyed steel according to the present invention regulates the content to 0.15 to 0.25 wt% while employing a silicon deoxidation process to minimize the adverse effects of silicon.

Nickel (Ni)

Nickel is the only element that improves strength without compromising toughness in steel, and improves hardening depth to improve hardenability, but it inhibits high temperature strength. In particular, when nickel is 2.0 wt% or more, the neutron irradiation embrittlement of the steel may adversely affect the sensitivity, so it is preferable to control the amount to 0.80 to 0.90 wt%.

Molybdenum (Mo)

As an element forming carbon and carbide, it precipitates finely in a matrix to give an effect of increasing strength at low and high temperatures, and also inhibits tempering embrittlement. If the added amount is less than 0.45 wt%, this effect is not sufficient, and if 0.6 wt% is exceeded, the cracking susceptibility of the weld heat affected zone is increased. Therefore, it is specified in the range of 0.45 to 0.55 wt% to prevent the decrease in high temperature strength and toughness. .

Chrome (Cr)

It is added to improve high temperature strength and toughness, but the low alloy steel of the present invention reduces the Cr and adds manganese to give the same effect, thereby improving welding restraint stress and improving the underclad crack resistance, so that the maximum 0.17 to 0.20wt% Regulate as far as

Vanadium (V)

It is expected to refine the austenite grain size, but if it is too large, there is a risk of lowering the hardenability. Excessive precipitation hardening effects are encouraged to produce high temperature cracks during welding, so they are regulated to a maximum of 0.01wt%.

Phosphorus (P)

Phosphorus is an element that is inevitably incorporated in steelmaking, which plays a role in causing temper embrittlement during heat treatment or use at high temperatures and promotes irradiation embrittlement, so it is preferable to control it to the lowest possible amount in consideration of economic costs in the manufacturing process. Is limited to 0.010 wt%.

Sulfur (S)

Sulfur contains a small amount in a large mass, so non-metallic inclusions such as MnS are formed in the steel, thereby degrading the quality of the steel. This effect is because fine cleanliness is improved by the absence of sulfur. For economic reasons in the manufacturing process, the sulfur content is limited to 0.010 wt% or less.

Copper (Cu)

Copper is an element that promotes irradiation embrittlement by synergy with phosphorus. Nickel of 0.4% or more increases the adverse effect of copper. When copper is low, the effect of phosphorus is noticeable, but when copper is high, phosphorus is less. The limit is 0.1 wt% or less.

Aluminum (Al)

Al has a high affinity with oxygen, so it is added to remove oxygen in the form of aluminum oxide by combining with oxygen in molten steel, but a part of the remaining amount reacts with nitrogen to form fine nitride, which contributes to grain refinement and solid solution. Reduce nitrogen to improve toughness. It is defined as 0.020 to 0.030 wt% because it acts as a source of nonmetallic inclusions that inhibits high temperature creep ductility with nitrogen.

Nitrogen (N)

Nitrogen forms AlN (aluminum nitride) when contained in an appropriate ratio with aluminum to improve impact toughness and mechanical properties due to miniaturization of grains during heat treatment. Therefore, the content is added in an amount of 70 to 90 ppm so that the ratio of aluminum and nitrogen necessary for stably securing mechanical properties and low temperature impact toughness is 0.23 to 0.45.

Hereinafter, the manufacturing process according to the present invention will be described in detail step by step.

1) Steelmaking and Ingot

3 shows the main manufacturing process used for the production of pressure vessel steel for the reactor, as shown,

The steelmaking process goes through the process of Electric Arc Melting Furnace-Secondary Refining Furnace (ASEA-SKF LRF) -Vacuum Ingot Injection (VSD). The content of phosphorus (P) is controlled to%, and the silicon (Si) content is oxidized to 0.02% or less and then taken to the secondary refining furnace.

An important purpose of oxidative refining is to control silicon (Si) and phosphorus (P) below the target content, and to carry out the primary deoxidation treatment by adding manganese so that the amount of carbon (C) is easily added to the reducer. After oxidizing and refining, molten steel completely removes the slack of the oxidizer to prevent the increase of P content by reducing P ₂ O ₅ contained in the slack of the oxidizer during refining at high temperature.

In the refining refiner, a strong basic atmosphere slag is prepared and degassed in a vacuum atmosphere so as to be 0.002% or less of sulfur (S). In this way, the ASEA-SKF secondary refining furnace removes and removes non-metallic inclusions, reduces the uniform chemical composition and temperature, reduces refining, and vacuum degassing to minimize harmful gas content (hydrogen, oxygen, etc.). Suppress elemental content as much as possible.

Nitrogen-containing ferroalloy was added to the molten steel after the degassing treatment to adjust the nitrogen content to 0.012% or more and to prepare the ingot by vacuum drop method (VSD), resulting in a final nitrogen content of 0.007 to 0.009%, high cleanliness, component segregation and Defects harmful to mechanical properties can be reduced to the maximum.

2) forging

In order to manufacture the pressure vessel without homogeneity and internal defect of the product, the steel ingot is heated to 1250 ℃, cogging at 1.20S forging ratio, and then upsetting to 1 / 2.3U, and punching in the center After punching, finish with hole forging 3.59E and finish forging to match shape and dimension.

3) heat treatment

Low alloy steel according to the present invention, after obtaining a property of excellent resistance to high temperature strength, impact toughness and temper brittleness, after forging, the normal heat treatment (Normalizing) in order to improve the workability before roughing, 6.5 ~ 880 ~ 910 ℃ After cooling for 10 hours, air cooling was performed, followed by tempering heat treatment to maintain air cooling at 620 to 650 ° C. for 6.5 to 10 hours.

Quality heat treatment is maintained for 5.25 to 9 hours at 870-900 ° C., which is the austenitizing temperature, and then water-cooled, followed by tempering heat treatment at 655-660 ° C. for 9-11 hours. Post Weld Heat Treatment, which is performed to suppress the occurrence of defects on the welded part, is air-cooled after maintaining 30.5 to 42 hours at 600 to 625 ° C.

Hereinafter, the characteristics of the low alloy steel according to the present invention through the experimental example.

Forging at 1250 ° C using ASME 508 Class.3 material, which was used as a steel for reactor pressure vessel materials showing conventional non-uniform toughness, and a steel ingot manufactured in vacuum induction furnace (VIM) for the present invention, and then applied to a real product After heat treatment under the same temperature conditions as the heat treatment conditions, the properties of the materials were compared.

The following Table 3 shows the chemical composition of the test pieces prepared for comparative evaluation of the properties of the low alloy steel according to the present invention, and the conventional steels 1 and 2 were manufactured and evaluated for comparison.

In Table 4 below, as a result of the mechanical evaluation test for the steels of Table 3, it can be seen that it has excellent fracture toughness compared to the conventional steel.

Figure 4 shows the change in the non- _combustible transition temperature (T _NDT ) according to the nitrogen / aluminum ratio. As shown, it can be seen that the ratio of nitrogen / aluminum shows the best toughness in the range of 0.3 to 0.4. Here, the measurement of the reference lead-free transition temperature is a lead-free transition temperature converted by the conversion formula using the ductile embrittlement transition temperature (FATT) results obtained in the impact test, which is a method recommended by ASTM STP919 instead of the drop weight test. (T _NDT ) value.

According to the present invention having the above objects and configurations, the low temperature fracture toughness is remarkably improved to have a stable RT _NDT value of -29 ° C. or lower, and to produce a low alloy steel having high tensile strength, thereby providing excellent toughness. It is possible to manufacture an improved reactor material.

Claims (2)

The chemical composition is 0.1% to 0.20% of carbon (C), 1.35 to 1.45% of manganese (Mn), 0.80 to 0.90% of nickel (Ni), 0.15 to 0.25% of silicon (Si), 0.010% of phosphorus (P), Sulfur (S) up to 0.010%, chromium (Cr) 0.17 to 0.20%, molybdenum (Mo) 0.45 to 0.55%, copper (Cu) up to 0.10%, aluminum (Al) 0.020 to 0.030%, content of nitrogen (N) 70 to 90 ppm, and the ratio of nitrogen to aluminum (N / Al) is 0.23 to 0.45, and the balance is composed of aluminum and nitrogen, which are composed of inevitable impurity elements that cannot be removed by iron (Fe) and steelmaking refining. Low alloy steel for high toughness reactors. delete