JPS62207846A - Heat-resistant cast steel excellent in strength at high temperature and in ductility - Google Patents

Abstract

PURPOSE:To obtain a heat-resistant cast steel excellent in strenght at high temp. and ductility and standing long use as material for high temp. service, by providing a composition containing C, Si, Mn, Cr, Ni, Nb, Ti, and Al each in a prescribed percentage. CONSTITUTION:The titled heat-resistant steel has a composition consisting of, by weight ratio 0.3-0.6% C, <=2.0% Si, <=2.0% Mn, 20-30% Cr, 20-40% Ni, 0.5-1.5% Nb, 0.05-0.3% Ti, 0.0005-<0/02% Al, and the balance Fe with inevitable impurities. The above heat-resistant steel has high ductility as well as high creep rupture strength, so that is can be suitably used as reaction tubes for modification furnace and cracking furnace for use in petrochemical industry, production of reducing gas for direct reduction iron manufacture, fuel cell, etc. Further, said heat-resistant steel can be used for a long period as a material for high temp. service, such as hearth roll for various thermal treatment equipments in ironworks, etc.

Description

[Detailed Description of the Invention] [Industrial Application Field 1] The present invention relates to heat-resistant cast steel with excellent high-temperature strength and ductility, and more specifically, hydrogen for petroleum refining, ammonia, methanol, ethylene, reducing gas for direct reduction iron manufacturing, Regarding heat-resistant cast steel with excellent high-temperature strength and ductility used as various heat treatment furnace equipment such as reformer reaction tubes used in the production of hydrogen for fuel cells, cracking reaction tubes, hearth rolls in steel plants, radiant tubes, etc. .

[Prior art] Conventionally, ASTM standard HK- has been used as reforming furnace reaction tubes, cracking furnace reaction tubes in petrochemical plants, direct reduction steel plants, steel mill heat treatment furnace hearth rolls, and radiant tubes.
40 (JIS: 5CH22 equivalent, 25Cr-20N i
)) and ASTM standard HP (JIS: 5CH24 equivalent, 2
5Cr-35Ni) has been widely used and has a long history.

However, in recent years, efforts have been made to improve the yield by increasing the operating temperature and to improve thermal efficiency by reducing the wall thickness of the reaction tube, that is, to reduce fuel consumption.
Steel consisting of 3C24Cr 24N iwt, 5Nb-balance Fe, 0.4C-25Cr-35N i wtN
b- High carbon content such as steel with balance Fe, and N
Heat-resistant cast steel that has high creep rupture strength at high temperatures by containing b has been developed and is now in practical use. However, the trend toward higher operating conditions and reductions in fuel consumption is even stronger, and recently there has been a demand for materials with even higher high-temperature strength than the Nb-containing materials mentioned above. Various materials containing one or more of Ti%A1, Zr%N%B, Mitshu metal, etc. have been proposed. However, many of these proposals mainly focus on improving high-temperature strength (especially creep rupture strength), and there are few materials that have improved ductility as well as high-temperature strength. It is well known that the materials used in reforming furnace reaction tubes, cracking furnace reaction tubes, and various heat treatment equipment have excellent high-temperature strength and ductility, which leads to longer lifespans. Both of the required characteristics have not yet been fully satisfied technically.

Conventionally, in this type of heat-resistant cast steel, Ti forms carbides and precipitates in the matrix during use at high temperatures, and is included as an important element responsible for high-temperature strength. However, since it is an element that easily oxidizes or nitrides, Sometimes it acts as a deoxidizing agent and denitrifying agent, forming oxides and nitrides, which remain in the steel as inclusions, and these inclusions suppress the growth of crystal grains, resulting in a grain refinement effect. However, on the other hand, it impairs the cleanliness of the material.

In addition, Japanese Patent Publication No. 54-024366 discloses that in heat-resistant cast steel containing Nb, by actively utilizing the deoxidizing and denitrifying effects of Ti, the degassing effect is exhibited and the high-temperature ductility of the material is improved. Also, after deoxidizing with Ti and Nb, Mitshu metal is added to further exhibit the deoxidizing and desulfurizing effect and improving the high temperature ductility of the material (Japanese Patent Publication No. 49-023454). However, in either case, it is inevitable that inclusions will increase, and the effect of Nb5Ti, which is included to improve high-temperature strength, will decrease, so it is necessary to include extra Nb and Ti. .

[Problems to be Solved by the Invention] The present invention solves various problems of conventional heat-resistant cast steel as explained above. In heat-resistant cast steel whose basic composition is Ni and Nb, high creep rupture strength and large fracture ductility can be obtained by containing Ti and A1 as a composite metal or containing Ti%A1, Zr and B in a composite manner.
Ti, AI that further contains Zr and B has strength,
In addition to improving ductility, it was discovered that this technology has the effect of addressing and suppressing the poor oxidation resistance of Nb-containing heat-resistant cast steel, which tends to cause the oxide film to peel off when subjected to temperature fluctuations during high-temperature use. In addition, we have also developed a heat-resistant cast steel with excellent high-temperature strength and ductility according to the present invention.

[Means for solving the problem J Heat-resistant cast steel with excellent high-temperature strength and ductility according to the present invention (1)
C0.3-0.6wt%, Si 2.0wt% or less, M
n 2.0wt% or less, Or 20-30wt%, Ni
20-40wt%, Nb 0.5-1.5wt%, T
i 0.05"0.3wt%, AI 0.0005~0.02wt% (0.02wt%
The first invention is a heat-resistant cast steel with excellent high-temperature strength and ductility, which is characterized by containing Fe and unavoidable impurities, (2) C0.3 to 0.6 wt%, Si 2 .0wt%
Below, Mn 2.0wt% or less, Cr 20-30wt
%, Ni 20-40wt%, Nb 0.5-1.5w
t%, T i 0.05 to 0.3 wt%, A I 0.0005 to 0.02 wt% (0.02 wt
(% not included) Zr 0.0005-0.04wt%, B
The second invention is a heat-resistant cast steel with excellent high-temperature strength and ductility, which is characterized by containing 0.03 wt% of FeB and the remainder being FeB and unavoidable impurities.

The heat-resistant cast steel excellent in high-temperature strength and ductility according to the present invention will be described in detail below.

First, the heat-resistant cast steel with excellent high-temperature strength and ductility according to the present invention is made by adding Ca, Si, and Mg before adding Ti during melting.
By thoroughly deoxidizing and denitrifying the steel, etc., and then adding Ti, the fraction of Ti in the steel, that is, the significant increase in the amount of solid solution in the steel, can be effectively utilized. T
Similar to i, a small amount of Al is added after sufficient deoxidation with Ca, Si, Mg, etc., and most of it remains in the steel, and it is present together with Ti and strengthens the grain boundaries by utilizing the grain boundary degassing effect. It works to significantly improve strength and ductility at high temperatures. However, if a large amount of A1 is added and left in steel, it will segregate at the grain boundaries and seriously reduce the strength of the grain boundaries.
0.02wt% (not including 0.02wt%).Also, the inclusion of small amounts of Zr and B is also effective in strengthening the grain boundaries of *#, but the inclusion of small amounts of Ti and A1 High creep rupture strength and large ductility are obtained for the first time. Therefore, the content of Zr%B is
This effect cannot be obtained unless the effect of grain boundary strengthening due to gold inclusion is used, and the effect cannot be obtained even with Zr and B alone, and Zr.
Since r is also a deoxidizing and desulfurizing element, the grain boundary strengthening effect of Zr can only be obtained when the grain boundary degassing effect is obtained by Ti and AI, and the grain boundary strengthening effect of B is also obtained for the same reason. is obtained. Also in this case, the A1 content needs to be 0.0005 to 0.02 wt% (not including 0.02 u+t%) because excessive A1 content increases the segregation effect and causes a decrease in strength.

The purpose and effect of the inclusion of trace amounts of A1, Zr, and B in the heat-resistant cast steel with excellent high-temperature strength and ductility according to the present invention are based on a novel technical idea that is completely absent from the prior art. Special public license number 59-006907
This is proposed in Japanese Patent Publication No. 58-064359, etc.

A1 content in heat-resistant cast steel containing Nb is 0.02
A completely opposite effect to that of ~0.2 wt% can be obtained.

The components and component ratios of the heat-resistant cast steel having excellent high-temperature strength and ductility according to the present invention will be explained.

C is an element that is necessary to improve the creep rupture strength of heat-resistant cast steel and improves the fluidity of molten metal.If the content is less than 0.3 wt%, this effect will be small; As the content increases, the creep rupture strength increases, but this strength reaches saturation at 0.6 wt%, and if the content exceeds this level, the toughness decreases.

Therefore, the C content is set to 0.3 to 0.6 wt%.

Si is a necessary element as a deoxidizing agent during melting and to ensure the carburization resistance of I steel. This makes it easier to precipitate the sigma phase.

Therefore, the Si content is set to 2.0 wt% or less.

Like Si, Mn is an element necessary to act as a deoxidizing agent during melting and as a desulfurizing agent, and excessive content impairs oxidation resistance.

Therefore, the Mn content is set to 2.0 wt% or less.

Cr is important for imparting oxidation resistance and carburization resistance to heat-resistant cast steel, and is an essential element that combines with carbon to form carbides during use at high temperatures and improves high-temperature strength. In order to ensure sufficient oxidation resistance, carburization resistance, and high-temperature strength at operating temperatures around 1000°C, a minimum content of 20 wt% is required, and if it is contained in a large amount, the sigma phase may form during use. It precipitates and causes a decrease in strength and toughness. Therefore, the Cr content is 20~30u+t
%.

Ni is an essential element for forming an austenite structure, and is an important element that provides carburization resistance, oxidation resistance, and high-temperature strength, and is necessary to ensure sufficient high-temperature strength at operating temperatures of around 1000°C. The content must be 20 wt% or more, and as the content increases, high temperature strength, carburization resistance, and oxidation resistance gradually improve, but the content is 40 wt% or more.
These effects become saturated when the content exceeds 40 wt%, and the cost of cast steel becomes uneconomical. Therefore, the Ni content is set to 20 to 40 wt%.

Nb precipitates as a carbide and increases high-temperature strength, while at the same time IIf! It is an element effective in improving the carburization resistance of No. 4, and if the content is less than 0.5 wt%, these effects are small, and excessive content seriously impairs weldability and acts as a protective film. The upper limit was set at 1.5 wt% because surface oxides tend to peel off, resulting in poor oxidation resistance, and further, a decrease in creep strength during long-term use at high temperatures. Therefore, the Nb content is 0.5~
It is set to 1.5 wt%.

Ti is an element that precipitates as a carbide and increases creep rupture strength. If the content is less than 0.05 wt%, this effect is small, and as the content increases, the creep rupture strength increases, but if the content is large The upper limit is 0.3 because it increases nitride-based and oxide-based inclusions and causes a decrease in strength.
Let it be wL%. Therefore, the Ti content is 0.05 to 0.3
Let it be wt%.

As explained above, the content of AI must be at least 0.0005wt% for the grain boundary fixing effect, and if it is contained in a large amount, it will cause a decrease in strength due to grain boundary segregation, so the upper limit is It is set to 0.02wL%.

Therefore, the AI content is 0.0005 to 0.02 wt% (
(excluding 0.02 wt%).

Zr content is required to be 0.0005wt% or more in order to strengthen the grain boundaries by deoxidizing and desulfurizing the grain boundaries and prevent the peeling of oxide scale, and to improve oxidation resistance. In addition, since Zr is a carbide-forming element, it combines with C as a carbide, and Nb
The upper limit is set to 0.04 wt% because it reduces the amount of precipitated carbides of Ti and significantly lowers the creep rupture strength. Therefore, Zr content is 0.0005-0.04w
It is assumed to be t%.

In order to strengthen grain boundaries and improve ductility, B content must be 0.0005wt% or more, and the strength increases as the content increases, but the effect is saturated at 0.03wt%. Therefore, no increase in strength can be expected even if more than this is contained. Therefore, the B content is 0.0005 to 0.03
11t%.

In addition to the elements explained above, unavoidable impurities such as p, s, and o%N are allowed as long as they are within the range normally contained in this type of heat-resistant cast steel.

[Example 1] An example of heat-resistant IIW4 having excellent high-temperature strength and ductility according to the present invention will be described.

Example w & 1 Melt the steel with the ingredients and ingredient ratios shown in the table,
By centrifugal casting, the outer diameter is 150mm and the wall thickness is 18mm.

A steel pipe with a length of 3500m+i was manufactured.

A test piece was taken from each cast tube and subjected to a creep rupture test and an intermittent oxidation test to measure oxide scale loss by repeated intermittent heating and cooling in the atmosphere.

- Creep rupture test in accordance with JIS Z-2272, parallel part diameter 10,
Using V and S pieces with a distance of 50 wheels between parallel parts, the temperature is 10
A test was conducted under the conditions of 50°C, stress 3, and Okgf/agI2, and the time to break and elongation at break were measured.

The results are shown in Table 2.

- Intermittent oxidation test A plate was cut out from each cast pipe and processed to a width of 30 mm, thickness of 5 mm, and length of 50 IIll. This test piece was heated to a temperature of 1100° C., held for 30 minutes, and then left in the air to cool down, which was repeated. After repeating this operation 50 times, the weight of the test piece was measured, and the weight loss per unit area was measured.

The results are shown in Table 2.

The heat-resistant cast steel according to the present invention, which contains Ti and AI and has excellent high-temperature strength and ductility, is No.
, 1 to No. 4 (invention steel 1), also Ti, Al5Z
The steel according to claim 2 which contains all r1B is N
o, 5 to No. 8 (invention steel 2).

No. 9 to No. 30 are comparative steels, and No. 9-
No. 12 is a steel containing neither A1, Zr, nor B, and No. 11 is an HP steel containing Nb.

13 to No. 30 are A as described in the notes column of Table 1.
1. Ti%B and Zr are insufficient or excessive, and the steel is outside the range of heat-resistant cast steel with excellent high-temperature strength and ductility according to the present invention.

As is clear from Table 2, the invention steel No., 1 to No.
4 and No. 5- No. 8 have a significantly longer life, that is, higher creep rupture strength, compared to HPwI of comparison steel No. 11 containing Nb, which is conventionally considered to have excellent uphill creep rupture strength. In addition, compared to steels that do not contain, are deficient in, or contain excessive amounts of any of Ti, AI, Zr, and B, the steels of the present invention has excellent creep rupture strength and also shows a large value of creep rupture ductility, and inventive steels No. 2, No. 5 to No. 8, and No. 8 have excellent creep rupture ductility and oxidation resistance. [Advantageous Effects of the Invention 1] As explained above, the heat-resistant cast steel with excellent high-temperature strength and ductility according to the present invention has high creep rupture strength and high ductility due to the above structure. It is used as reforming furnace reaction tubes and cracking furnace reaction tubes used in the petrochemical industry, direct reduction steel production, fuel cells, etc., and is also used at high temperatures such as hearth rolls and radiant tubes in various heat treatment equipment at steel plants. It has the advantage that it can be used as a material for a long period of time.

Claims (2)

[Claims] (1) C0.3-0.6wt%, Si2.0wt or less,
Mn2.0wt% or less, Cr20-30wt%, Ni2
0-40wt%, Nb0.5-1.5wt%, Ti0.
05 to 0.3 wt%, Al 0.0005 to 0.02 wt% (excluding 0.02 wt%), and the remainder consists of Fe and inevitable impurities, and has excellent high-temperature strength and ductility. (2) C0.3-0.6wt%, Si2.0wt% or less, Mn2.0wt% or less, Cr20-30wt%, Ni
20-40wt%, Nb0.5-1.5wt%, Ti0
．． 05-0.3wt%, Al0.0005-0.02wt% (not including 0.02wt%), Zr0.0005-0.04wt%, B0.0005-0.03wt%, and the balance is Fe and unavoidable impurities. A heat-resistant cast steel with excellent high-temperature strength and ductility.