KR100832487B1 - A stainless steel having high yield strength and excellent strain properties at cryogenic environment - Google Patents

Abstract

A high strength stainless steel having high yield strength and excellent elongation in a cryogenic environment is provided to manufacture a structure of the combustor head part easily and reduce weight of the structure by manufacturing a combustor head part of a liquid rocket engine from high strength stainless steel, and reduce size and weight of the structures by manufacturing structures such as low temperature tanks or valves from the high strength stainless steel. A high strength stainless steel having high yield strength and excellent elongation at cryogenic temperatures comprises 9.0 to 10.5 wt.% of nickel(Ni), 11.5 to 12.5 wt.% of chromium(Cr), 0.10 to 0.25 wt.% of titanium(Ti), 0.5 to 0.8 wt.% of molybdenum(Mo), and the balance of iron(Fe), and further comprises 0.01 to 0.03 wt.% of carbon(C), 0.001 to 0.01 wt.% of sulfur(S), 0.1 to 0.3 wt.% of manganese(Mn), 0.1 to 0.3 wt.% of silicon(Si), 0.001 to 0.01 wt.% of phosphorous(P), 0.1 to 0.2 wt.% of aluminum(Al), 0.01 to 0.1 wt.% of zirconium(Zr), 0.001 to 0.005 wt.% of boron(B), and 0.01 to 0.05 wt.% of calcium(Ca).

Description

Stainless steel having high yield strength and excellent strain properties at cryogenic environment

1 is a view showing a specimen after a tensile test of a conventional stainless steel.

2 is a view showing a specimen after the tensile test of the high strength stainless steel according to the present invention.

Figure 3 is a graph showing the results of the tensile test of the conventional stainless steel and the stainless steel of the present invention in the cryogenic state.

The present invention relates to high strength stainless steel, and more particularly, to high strength stainless steel having excellent yield strength and excellent elongation in an ultra low temperature environment.

Generally, liquid rocket combustors developed in Korea use liquid oxygen (LO _X ) and kerosene (Kerosene) as propellants.

The combustor head portion serves as a manifold for supplying the oxygen and kerosene as a propellant to the combustion chamber through an injector so that the propellant mixed with oxygen and kerosene can combust.

Therefore, such a combustor head part operates in a state where the operating environment is subjected to a very low temperature and a very large high pressure by liquid oxygen because of this operating characteristic.

In consideration of such operating conditions, the material used for the combustor head portion can reduce the combustor weight and increase the structural stability of the combustor head portion by using a material having a high material strength without cryogenic brittleness at an extremely low temperature.

At present, the production material of the combustor head part used in Korea is STS316L. Since general metal materials exhibit low temperature embrittlement in an ultra low temperature state, much care is required in selecting a material for fabricating a structure used in an ultra low temperature environment. However, the STS316L material is suitable as a material of the combustor head because of its low brittleness and good oxidation resistance at low temperatures.

In foreign countries, the head of the Vulcain engine combustor used for Space Shuttle Main Engine (SSME) and ARIANE 5 projectiles uses Inconel 718. Since the material is much stronger than stainless steel, the structural stability of the combustor can be improved, and the weight of the combustor can be reduced by reducing the weight of the combustor head under the same operating conditions, thereby increasing the projectile efficiency. .

However, STS316L material currently used in Korea is significantly lower in structural stability under the same operating conditions and shape because the strength of the material is significantly lower than that of Inconel 718.

Thus, an increase in the weight of the combustor head structure having the same operating conditions is required, thereby lowering the efficiency of the combustor and the projectile.

In terms of supply and process of materials, STS316L material is easy to supply and processable, while Inconel 718 material is very expensive, difficult to supply and very stiff. not.

For this reason, a structure manufactured using Inconel 718 material is cast in the shape of the structure to be produced by casting the material, and some post-processing is performed in parallel with the structure produced by casting.

Therefore, in consideration of the advantages and disadvantages of such materials, in order to develop and manufacture high-pressure, high-performance combustors in the future, the supply and demand of materials is easy (domestic production), low temperature brittleness, and material strength similar to that of Inconel 718. There is an urgent need for materials to have.

Accordingly, the present invention was devised to meet the conventional requirements as described above, to develop a high-strength stainless steel with excellent yield strength and excellent elongation in the cryogenic environment, to provide a high-strength stainless steel that facilitates supply of materials Its purpose is to.

The object as described above is achieved by high strength stainless steel having high yield strength and excellent elongation at very low temperatures of the present invention, the high strength stainless steel according to the present invention will be described in detail below.

The stainless steel of the present invention is formed to have high yield strength and excellent elongation at ultra low temperature, and its composition is 9.0 to 10.5 wt% of nickel (Ni), 11.5 to 12.5 wt% of chromium (Cr), and 0.10 to 0.25 of titanium (Ti). It is comprised by weight%, molybdenum (Mo) 0.5-0.8 weight%, and remainder iron (Fe).

Here, the selected element and content are based on the following reasons.

Nickel (Ni) is used to stabilize austenite structures in large amounts of alloys, prevent low temperature brittleness, improve corrosion resistance and oxidation resistance at high temperatures, and toughen tempered steels.

Therefore, when the nickel content is less than 9.0% by weight, the structure of the metallographic structure of the steel, that is, the austenite structure, becomes unstable, and when the content is more than 10.5% by weight, it promotes separation of nitrogen and hydrogen to bubble the steel. ) To form a loose (Loose), the nickel content is preferably 9.0 to 10.5% by weight.

In addition, chromium (Cr) increases hardenability, increases yield stress and hardness, but decreases ductility, improves corrosion resistance and stabilizes martensite.

If the content of chromium is less than 11.5% by weight, the hardness is weak. If the content is more than 12.5% by weight, it may be good in terms of oxidation resistance. However, the steel material is embrittled and the workability is poor due to the increase in hardness. . Therefore, the content of chromium is preferably 11.5 to 12.5% by weight.

In addition, titanium (Ti) is effective in preventing precipitation among stainless steels and hardening precipitation.

When the content of titanium is less than 0.10% by weight, good physical properties, that is, the effect of preventing corrosion, are inferior, and when the content of more than 0.25% by weight, the physical property of the steel, that is, precipitation hardening, is degraded and thus the strength of the steel is deteriorated. The content of is preferably 0.10 to 0.25% by weight.

Molybdenum (Mo) improves hardenability, creep characteristics, and tempering and softening resistance, and increases high temperature strength, impact toughness and wear resistance.

If the content of molybdenum is less than 0.5% by weight, the wear resistance is lowered. If the content of molybdenum is more than 0.8%, the wear resistance is improved, but the toughness of the stainless steel is lowered, so the content of molybdenum is preferably 0.5 to 0.8% by weight.

And, iron (Fe), as a main component of high strength stainless steel, may be composed of the remaining composition except the composition of the above-described element.

On the other hand, the stainless steel of the present invention configured as described above is configured to further contain carbon (C) in an amount of 0.01 to 0.03% by weight for improving the strength and hardenability of the stainless steel.

If the content of carbon (C) having the properties of these materials is less than 0.01% by weight, the strength of the steel is reduced and the corrosion resistance is increased. If the content of more than 0.03% by weight, the strength of the steel will be increased. Its disadvantage is that its toughness and corrosion resistance are reduced and its physical properties deteriorate.

Therefore, the content of carbon is preferably 0.01 to 0.03% by weight.

Moreover, the stainless steel of this invention is further comprised including sulfur (S) in content of 0.001 to 0.01 weight% in order to improve the workability of stainless steel.

When the content of sulfur (S) is less than 0.001% by weight, the workability of the steel is inferior, and when the content of the sulfur (S) is more than 0.01% by weight, the workability of the steel is improved, but the corrosion resistance is poor. Therefore, the content of sulfur is 0.001 to 0.01% by weight. desirable.

The stainless steel of the present invention further comprises manganese (Mn) in an amount of 0.1 to 0.3% by weight to improve the hardenability and viscosity of the stainless steel and to prevent brittleness due to sulfur (S). .

When the content of manganese (Mn) is less than 0.1% by weight, the strength of quenching and viscosity is lowered, and when the content of manganese (Mn) is more than 0.3% by weight, the viscosity of the austenite structure is too high. Therefore, the content of manganese is 0.1 to 0.3% by weight. % Is preferable.

And, in the stainless steel of the present invention, silicon (Si) of 0.1 to 0.3 for increasing the temper resistance of martensite, preventing acidification to promote corrosion of the steel, and improving the corrosion resistance of the steel at high concentrations of nitric acid. It is comprised by containing in the content of the weight% further.

When the content of silicon (Si) is less than 0.1% by weight, the corrosion resistance is weak. When the content of silicon (Si) is more than 0.3% by weight, the corrosion resistance is too high and it is difficult to deform the steel, so the content of silicon is 0.1 to 0.3% by weight. Is preferred.

And in order to improve the strength of stainless steel, the stainless steel of this invention is comprised by including phosphorus (P) further in the content of 0.001 to 0.01 weight%.

When the content of phosphorus (P) is less than 0.001% by weight, the strength of the steel becomes weak, and when the content of phosphorus (P) is more than 0.01% by weight, low temperature brittleness appears, so that the content of phosphorus is preferably 0.001 to 0.01% by weight.

In the stainless steel of the present invention, aluminum (Al), which is alloyed with titanium or nickel and precipitated and hardened, suppresses grain growth, improves corrosion resistance of the steel, and improves toughness and oxidation resistance of the steel at low temperatures. It is comprised by containing in content of 0.1 to 0.2 weight% further.

If the content of aluminum (Al) is less than 0.1% by weight, the corrosion resistance of the steel is insufficient. If the content of aluminum (Al) is more than 0.2% by weight, the corrosion resistance of the steel is improved. 0.1-0.2 weight% is preferable.

In addition, the stainless steel of the present invention is configured to further contain zirconium (Zr) in an amount of 0.01 to 0.1% by weight, which improves resistance to intercrystallization corrosion.

If the content of zirconium (Zr) is less than 0.01% by weight, the corrosion resistance is inferior, and if more than 0.1% by weight, there is a disadvantage that the physical properties of the steel is poor, the content of zirconium is preferably 0.01 to 0.1% by weight.

The stainless steel of the present invention further comprises boron (B) in an amount of 0.001 to 0.005% by weight as an element that improves the workability of stainless steel and suppresses cracking during multistage molding.

If the content of boron (B) is less than 0.001% by weight, the workability is poor and cracks are generated during multi-stage molding. If the content of boron (B) is more than 0.005% by weight, there is a disadvantage in that steel is poor in manufacturability, weldability, toughness and delayed fracture. As for silver, 0.001-0.005 weight% is preferable.

In addition, the stainless steel of the present invention further comprises calcium (Ca) in an amount of 0.01 to 0.05% by weight, which suppresses cracking during molding and improves coating property, and the content of calcium (Ca) is greater than 0.01% by weight. If there is little, there exists a disadvantage that crack suppression and coating property fall, and when it is more than 0.05 weight%, workability is inferior, and content of calcium is preferably 0.01 to 0.05 weight%.

EXAMPLE

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

First, a specimen was manufactured to test tensile strength and elongation among physical properties of a stainless steel having a composition according to the present invention.

On the other hand, the specimen is iron (Fe) 76.527% by weight, nickel (Ni) 10% by weight, chromium (Cr) 12% by weight, titanium (Ti) 0.2% by weight, molybdenum (Mo) 0.6% by weight, carbon (C) 0.02 % By weight, 0.01% by weight sulfur (S), 0.15% by weight manganese (Mn), 0.15% by weight silicon (Si), 0.01% by weight phosphorus (P), 0.2% by weight aluminum (Al), 0.1% by weight zirconium (Zr) It was produced to have a composition ratio of 0.003% by weight of boron (B) and 0.03% by weight of calcium (Ca).

In addition, the conventional stainless steel used in this tensile test can be divided into two types, 400 series (Fe-Cr-based) and 300 series (Fe-Cr-Ni-based).

Fe-Cr type has a ferrite (marite), martensite (martensite) system, Fe-Cr-Ni is austenite (austenite), an ideal (duplex), and precipitation hardening (precipitation hardening) system.

Nitrile-free payratt and martensite series, which increase the low temperature toughness of the material, are not used in structures operating in cryogenic or cryogenic environments because brittle fracture occurs at low temperatures.

The stainless steel materials that can be used at cryogenic or cryogenic temperatures are austenitic stainless steels, STS316L, STS304 and STS310, and these metals were selected and tested as specimens of conventional stainless steel in this tensile test. Figures 1 to 3 are shown.

As a result of the test, the conventional specimens, or metals, are not vulnerable at low temperatures (-196 ° C.) and have a high elongation compared to other metals, so they are used for the design and fabrication of structures that operate in low temperature environments. However, the 0.2% offset yield strength of these conventional specimens was about 330 MPa to 630 MPa through the literature and the cryogenic tensile test. STS630, a precipitation hardening system containing 3% to 5% of nickel (Ni), showed a very high yield strength of 1461 MPa, but the ultimate tensile strength of 1475 MPa was very small. , The elongation was small at about 18.0%.

In the design of the liquid rocket combustor head that operates at cryogenic and high pressure, using material with high material strength value and good elongation without low temperature brittleness, it is possible to secure the structural stability of the combustor and reduce the weight. It can increase the payload weight of the economically very effective.

In the cryogenic environment of the Inconel 718 material currently used in the combustor head of a foreign projectile, the yield strength is about 1282 MPa and the maximum tensile strength is about 1675 MPa, as shown in Table 1 below. The elongation of this material is 26%.

Inconel 718 Mechanical Property Values Metal Measuring temperature (℃) 0.2% Offset Yield Strength (0.2% Yield Strength (MPa)) Ultimate tensile strength (MPa) Elongation (%) Inconel 718 -196 ℃ 1282 1675 26

These mechanical properties have yield strength values of about 2.0 to 3.9 times that of conventional materials, STS316L, STS304, and STS310.

However, this Inconel 718 material, as mentioned above, is not easy to supply and very poor in workability, and thus, a lot of difficulties are involved in the fabrication of the seal structure.

Therefore, the high-strength stainless steel according to the present invention has a very good workability as stainless steel compared to Inconel 718 to compensate for this disadvantage, yield strength is about 1.5 to 3 times greater than STS304, STS316L, and STS310, the elongation is currently This material is superior to Inconel 718, which is used as the liquid rocket combustor head.

Table 2 below shows the results of fabricating and testing tensile specimens according to the ASTM E 8M standard for high strength stainless steel and conventional metals according to the present invention.

Comparison between Specimen of the Invention and Conventional Specimen Metal Measurement temperature (℃) 0.2% Offset Yield Strength (0.2% Yield Strength (MPa)) Ultimate tensile strength (MPa) Elongation (%) (Elongation)  High strength stainless steel of the present invention -196 975.1 1631 31.1 -196 880.1 1660 29.1 -196 913.8 1631 28.7 average 923.00 1640.67 29.6   STS304 -196 425.6 1525 39.8 -196 563.7 1562 41.4 -196 398.7 1518 40.8 -196 555.2 1564 38.8 average 485.80 1542.25 40.2   STS310 -196 622.5 916 59.2 -196 621.3 1111 68.0 -196 619.3 1113 64.4 -196 627.6 1114 66.0 average 622.67 1063.50 64.4   STS316L -196 278.8 602 75.2 -196 301.3 602 78.1 -196 281.6 601 77.4 average 287.2 601 76.9   STS630 -196 1461 1479 19.0 -196 1458 1470 - -196 1464 1476 17.8 -196 1462 1476 17.2 average 1461.25 1475.25 18.0

The high-strength stainless steel developed by the present invention is a low carbon steel having a carbon (C) content of less than 0.03%. The ferritic, martensitic and austenite contents of nickel (Ni) and chromium (Cr) are conventionally developed stainless steels. It is a material of a new composition which is not included in a system or an ideal system.

High strength stainless steel according to the present invention can be seen through Table 1 and Table 2, ultra-low temperature (-196 ℃) compared to Inconel 718, a high-strength special steel commonly used as a combustor head of the rocket engine in the United States or Europe The yield strength is 72%, the ultimate strength is 98%, while the material workability, which is important in terms of material utilization, is much better than Inconel 718, and the elongation at cryogenic is 14% higher than that of Inconel 718. It is

As described above, according to the high-strength stainless steel of the present invention, by providing a high-strength stainless steel excellent in yield strength and excellent elongation in the cryogenic environment, there is an advantage that the supply of the material is easy.

In addition, by using the high-strength stainless steel of the present invention as a combustor head of the liquid rocket engine, the combustor head structure that operates in an extreme environment of high pressure / cryogenic temperature can be easily manufactured, and the same structural stability can be ensured. In this case, there is an advantage that the weight can be lighter than the structure manufactured by using the existing stainless steel available.

In addition, the high-strength stainless steel of the present invention can be used in the production of structures that operate in an ultra low temperature environment, that is, a low temperature tank or a low temperature valve in the industry, and thus, there is an advantage of reducing the size and weight of the manufactured structure.

Claims (10)

9.0 to 10.5 weight percent nickel (Ni), 11.5 to 12.5 weight percent chromium (Cr), 0.10 to 0.25 weight percent titanium (Ti), 0.5 to 0.8 weight percent molybdenum (Mo) and the balance of iron (Fe) 0.01 to 0.03 wt% carbon (C), 0.001 to 0.01 wt% sulfur (S), 0.1 to 0.3 wt% manganese (Mn), 0.1 to 0.3 wt% silicon (Si), 0.001 to 0.01 wt% phosphorus (P), 0.1-0.2 wt% aluminum (Al), 0.01-0.1 wt% zirconium (Zr), 0.001-0.005 wt% boron (B), 0.01-0.05 wt% calcium ( High strength stainless steel having high yield strength and excellent elongation at ultra low temperature, characterized in that it further comprises Ca). delete delete delete delete delete delete delete delete delete

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