JPS61177352A - Heat resistant cast steel having superior elongation characteristic at room temperature - Google Patents

Abstract

PURPOSE:To obtain a heat resistant austenitic cast steel having superior elongation characteristics at room temp. as well as superior characteristics at high temp. by restricting a composition consisting of C, Si, Mn, Cr, Ni, Al, Ti, B, Nb and Fe and by specifying the oxygen content and cleanliness. CONSTITUTION:This heat resistant cast steel having superior elongation characteristics at room temp. consists of 0.3-0.6% C, <=2.0% Si, <=2.0% Mn, 20.0-30.0% Cr, 30.0-40.0% Ni, 0.02-0.50% Al, 0.04-0.05% Ti, 0.0002-0.004% B, 0.3-1.5% Nb and the balance essentially Fe or further contains 0.5-5.0%, preferably 0.5-3.0% W combined optionally with 0.2-0.8% Mo and has <=50ppm oxygen content and <=0.05% cleanliness (d%) prescribed by JIS G 0555. The cast steel is suitable for use as a structural material used at >=about 1,000 deg.C high temp. The cast steel having the desired oxygen content and cleanliness can be manufactured by melting and degassing.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention provides a material having excellent high-temperature properties and room-temperature elongation properties, suitable as a structural material used at high temperatures of 1000°C or higher, such as reaction tubes for the petrochemical industry. Regarding austenitic heat-resistant cast steel.

[Prior art]

As a tube material for petroleum industrial reaction tubes, such as ethylene cranking tubes and reforming furnace paste tubes,
Conventionally, ASTM HK40 material (0,4C-25Cr
-2ON i -F e), HP40 material (0,4C
25Cr-35Ni-Fe), Nb, etc.
Improved materials have been used.

In recent years, demands for improving reaction efficiency have led to higher operating temperatures, and the conditions under which reaction tubes are used have become significantly more severe.

As a new material to deal with this, 25Cr35
A heat-resistant cast steel has been proposed in which the basic composition is Ni--Fe, and the three elements Al, Ti, and B, as well as elements such as Nb, Mo, and W are added thereto.

These heat-resistant cast steels have high-temperature properties that exceed those of conventional HP-improved materials, and are particularly excellent in creep rupture strength and thermal shock resistance in high-temperature ranges exceeding 1000°C, as well as good carburization resistance. .

[Problem to be solved]

However, although the heat-resistant cast steel has excellent creep rupture strength and thermal shock resistance at high temperatures, there remains room for improvement in room temperature elongation properties. In view of this point, the present invention provides 25Cr-35N i -Al-Ti-B
-(W-Mo-Nb)-Fe-based heat-resistant cast steel is intended to provide an austenitic heat-resistant cast steel having improved room temperature elongation properties as well as high-temperature properties.

[Technical means and effects]

The first heat-resistant cast steel according to the present invention has C: 0.3 to 0.6%.
, Si: 2.0% or less, Mn: 2.0% or less, Cr:
20.0-30.0%, Ni: 30.0-40
．． 0%, AIl: 0.02-0.50%, Ti: Q, Q
4-0.50%, B: 0.0002-0.004%
, Nb: 0.3 to 1.5%, the remainder being substantially Fe.

The second heat-resistant cast steel of the present invention has C: 0.3 to 0.6%, S
i: 2.0% or less, Mn: 2.0% or less, Cr: 20.
0-30.0%, Ni: 30.0-40.0%, Aj:
0.02-0.50%, Ti: 0.04-0.50%
, B: 0.0002-0.004%, W: 0.
5 to 5.0%, the remainder substantially Fe, the third heat-resistant cast steel of the present invention, C: 0.3 to 0.6%, Si: 2.0% or less,
Mn: 2.0% or less, Cr: 20.0-30.0%, N
i: 30.0-40.0%, An! :0.02~0.5
0%, Ti: 0.04-0.50%, B: 0.00
02-0.004%, Nb: 0.3-1.5%, W: 0
．． The fourth heat-resistant cast steel of the present invention contains C: 0.3 to 0.6%, S: 5 to 3.0%, and the remainder is substantially Fe.
i: 2.0% or less, Mn: 2.0% or less, Cr: 20.
0-30.0%, Ni:3Q, 0-40.0%, AIl
:0.02~0.50%, Ti:0.04~0.50%
, B: 0.0002-0.004%, Nb: 0.3
~1.5%, w: 0.5-3.0%, Mo:
The first to fourth heat-resistant cast steels each have a chemical composition of 0.2 to 0.8% and the remainder substantially Fe %, and the oxygen content of each of the first to fourth heat-resistant cast steels is 50 ppm or less, and the cleanliness is 0. It is characterized in that it is 0.05% or less. In addition, in this specification,
The cleanliness is the area percentage (d%) of inclusions determined according to the regulations of JISGO555 (however, number of visual fields for determination = 60, magnification: 400 times).

The heat-resistant cast steel of the present invention contains relatively large amounts of Al and Ti. As described later, AN and Ti are effective active elements for improving high-temperature properties, especially high-temperature creep rupture strength and thermal shock resistance, but on the other hand, they impair the cleanliness of steel and reduce room temperature elongation properties. cause it to happen.

That is, Aj7 and Ti both have a very high affinity for oxygen, so when normal levels of oxygen are present in steel, a significant amount of Aj and Ti added to it will form oxides. and turn into slag. A part of the generated slag remains in the steel as non-metallic inclusions without being floated and separated, impairing the cleanliness of the steel. Adding the loss of /l and Ti due to this oxidation, a large amount of Aj and Ti must be administered, which increases the amount of inclusions and significantly reduces the cleanliness. However, in the cast steel of the present invention, the effects of adding Al and Ti are fully exhibited, and as shown in the examples below,
It has both good high temperature properties and room temperature elongation properties such as yield strength, tensile strength, and total elongation.

Next, the reasons for limiting the components of the alloy of the present invention will be explained.

C: 0.3 to 0.6% C improves the castability of steel, and also contributes to Nb, Ti, and Cr described below.
It has the effect of increasing creep rupture strength by forming carbides in coexistence with other substances. At least 0.3% is required to achieve this effect. Creep rupture strength also improves as the amount of C increases, but on the other hand, the amount of precipitated secondary carbides increases, resulting in a significant decrease in toughness after use and a decrease in weldability. is the upper limit.

Si: 2.0% or less 54 has a role as a deoxidizer and is an element effective in improving carburization resistance. However, if it exceeds 2.0%,
Since weldability is significantly impaired, the upper limit is set at 2.0%.

Mn: 2.0% or less Mn has a role as a deoxidizing agent like Si, and
It has the effect of fixing and rendering harmless sulfur (S) in molten steel as M n S. However, if it exceeds 2.0%, the oxidation resistance will deteriorate, so the content should be 2.0% or less.

Cr: 20.0 to 30.0% Cr forms an austenite structure in coexistence with Ni, and improves high-temperature strength, oxidation resistance, etc.

In particular, in order to ensure strength and oxidation resistance at high temperatures of 1000° C. or higher, the content must be at least 20.0%. The effect increases as the content increases, but if the content is too large, the toughness after use will significantly deteriorate, so the upper limit is set at 30.0%.

Ni: 3Q, 0 to 40.0% Ni is an element that maintains an austenite structure in coexistence with Cr, and has the effect of increasing oxidation resistance, high-temperature strength, and structural stability. In particular, at least 30.0% is required to ensure oxidation resistance, strength, etc. in high-temperature applications of 1000° C. or higher. The effect increases as the content increases, but if it exceeds 40.0%, the habo effect becomes saturated, so 40.0% is the upper limit.

The cast steel of the present invention has the above-mentioned 25Cr 35Ni-Fe system as its basic component system, and three elements of AI, Ti, and B.

Ti forms compounds such as carbides, nitrides, and carbonitrides by combining with C and N in steel, and Al and B are
It has the function of finely dispersing these compounds and strengthening grain boundaries.

Ti: 0.04 to 0.50% Ti improves high temperature strength, thermal shock resistance, etc. by forming carbonitrides and the like. Moreover, it is effective in improving carburization resistance due to the mutual effect with Al. If the content is less than 0.04%, the effect will not be sufficient. The effect increases as the content increases, but when it exceeds 0.50%, the strength slightly decreases due to coarsening of precipitates and an increase in the amount of oxide inclusions. Therefore, it is set to 0.04 to 0.5Q%.

Al70.02-0.50% Af has the effect of improving creep rupture strength and the like, and as mentioned above, increases carburization resistance in coexistence with Ti. If the content is less than 0.02%, the effect is not sufficient. The effect increases as the content increases, but if it exceeds 0.50%, the strength will actually decrease. Therefore, 0.02-0.50
%.

B: 0.0002 to 0.004% B strengthens the grain boundaries of the matrix, prevents the coarsening of the Ti-based precipitates, contributes to fine precipitation, and further delays agglomeration and coarsening after precipitation. This increases creep rupture strength. In this case, if it is less than 0.0002%, the effect will be insufficient. Up to 0.004%, strength increases as the content increases, but beyond that, the effect is almost saturated.

In addition, weldability also deteriorates. Therefore, 0.0002 to 0.
004%.

In addition to the above-mentioned elements, the heat-resistant cast steel of the present invention further contains Nb, W
, , Mo. In this case, the first heat-resistant cast steel contains Nb, the second heat-resistant cast steel contains W,
The third heat-resistant cast steel contains Nb and W, and the fourth heat-resistant cast steel contains Nb, W, and Mo.

Nb: 0.3 to 1.5% Nb has the effect of increasing creep rupture strength and carburization resistance. If it is less than 0.3%, these characteristics cannot be satisfied at the same time, but if it exceeds 1.5%,
On the contrary, we see a decrease in creep rupture strength. Therefore, 0.3
~1.5%. Note that Nb usually accompanies Ta, which is an element with the same effect as Nb, so in that case, the total amount with Ta should be within the above range.

W: O, S ~ 5.0% W has the effect of increasing high temperature strength. The effect is recognized from a content of 0.5%. Although the effect increases as the content increases, it is not particularly necessary to exceed 5.0%, and since it is accompanied by a decrease in oxidation resistance, the upper limit is set at 5.0%. In addition, when either one or both of Nb or Mo is contained in combination, a mutual effect with these elements is added, so the W content does not need to exceed 3.0 (therefore, 0
．． 5 to 3.0%.

Mo: 0.2 to 0.8% Mo contributes to improving high temperature strength in combination with Nb and W. The effect is recognized from a content of 0.2%. However, if it exceeds 0.8%, oxidation resistance will deteriorate. Therefore, it is set at 0.2 to 0.8%.

The heat-resistant cast steel of the present invention can be produced by degassing melting. In degassing melting, dissolved oxygen in the steel is removed by reacting with C, as shown by the following formula: % formula % (). By sufficiently lowering the amount of oxygen in the steel, Af and T
By adding i, compared to the case of atmospheric dissolution, Al and T
The amount of oxidation consumption of i is small, and accordingly, the necessary addition amounts of Al and Ti are reduced accordingly, and the amount of non-metallic inclusions formed is reduced, resulting in an increase in the cleanliness of the steel.

To ensure the cleanliness (d%) specified in the present invention: 0.05% or less, the amount of oxygen in the steel must be reduced to 50p during degassing melting.
It is necessary to reduce the amount to below pm. The degassing treatment for this purpose is sufficiently achieved by setting the maximum degree of vacuum in the vacuum tank to 5.0 mHg or less and continuing this maximum degree of vacuum for about 20 minutes or more, and at most 30 minutes. This degassing and dissolution not only provides a deoxidizing effect but also a nitrogen gas reduction effect, thereby reducing the amount of nonmetallic inclusions produced by the reaction of Al and Ti with nitrogen, thereby further increasing the cleanliness.

In addition, the amount of gas absorbed by the molten metal once degassed by degassing and melting is extremely small and can be ignored during the period from the time of tapping to the completion of casting.
It has also been determined that there is no substantial difference in the quality of the resulting castings whether carried out in a degassed atmosphere, an argon atmosphere, or in air.

〔Example〕

A cast product was obtained by casting heat-resistant cast steel melted by the earth degassing melting method or the atmospheric melting method in the atmosphere.

Table 1 shows the chemical composition, oxygen content, and cleanliness (d%') (JIS G 0555) of the cast products.

Note that for Ti and Al, the amounts added (numerical values in parentheses) are also shown.

A room temperature tensile test was conducted on each cast product, and the results shown in Table 2 were obtained.

In the table, the vacuum degree in the "melting conditions" column is the highest vacuum degree achieved, and the duration time is 20 minutes in each case.

As shown in Table 1, in the example of the present invention using degas dissolution,
Since the amount of oxygen in the steel is low, the addition yield of AA'STi is high, and the required content can be adjusted by adding half the amount compared to the comparative example using atmospheric dissolution.

As a result, the cleanliness of the present invention example is 0.03 to 0.05.
% and the cleanliness of the comparative example due to atmospheric dissolution (approximately 0.14 to 0
．． 18%), this is extremely good.

Furthermore, as is clear from Table 2, the examples of the present invention have excellent tensile properties at room temperature, and when looking at the elongation in particular, the comparative examples have
While it remains in the 13% range at most, in the example of the present invention,
It exceeds 20%.

Table 2 [Effects of the Invention] The heat-resistant cast steel of the present invention has excellent high-temperature properties and good elongation properties at room temperature, so it is suitable as a pipe material for ethylene cracking tubes, Foma tubes, etc. In addition to being able to withstand high-temperature use, it has excellent resistance to damage to the crank, etc., even when starting up or cooling down due to interruptions in operation, enabling safe and smooth operation, and offering greater durability than conventional materials. Guarantee.

The heat-resistant cast steel of the present invention is useful not only as a reaction tube material for the petrochemical industry, but also as a structural material for hearth rolls, radiant tubes, and other high-temperature applications in steel-related equipment, and for temperatures up to about 1150 degrees. Sufficiently withstands operating temperatures.

Claims (1)

[Claims] 1) C: 0.3 to 0.6%, Si: 2.0% or less, Mn
: 2.0% or less, Cr: 20.0 to 30.0%, Ni:
30.0-40.0%, Al: 0.02-0.50%,
Ti: 0.04-0.50%, B: 0.0002-0.
004%, Nb: 0.3-1.5%, remainder substantially Fe
Oxygen content: 50 ppm or less, cleanliness (d%
) [JIS G0555]: Heat-resistant cast steel with excellent room temperature elongation properties of 0.05% or less. (2) C: 0.3 to 0.6%, Si: 2.0% or less, M
n: 2.0% or less, Cr: 20.0-30.0%, Ni
:30.0~40.0%, Al:0.02~0.50%
, Ti: 0.04-0.50%, B: 0.0002-0
．． 004%, W: 0.5-5.0%, remainder substantially Fe
Oxygen content: 50 ppm or less, cleanliness (d%
) [JIS G0555]: Heat-resistant cast steel with excellent room temperature elongation properties of 0.05% or less. (3) C: 0.3 to 0.6%, Si: 2.0% or less, M
n: 2.0% or less, Cr: 20.0-30.0%, Ni
:30.0~40.0%, Al:0.02~0.50%
, Ti: 0.04-0.50%, B: 0.0002-0
．． 004%, Nb: 0.3-1.5%, W: 0.5-3
．． 0%, the remainder essentially consists of Fe, oxygen content: 50
ppm or less, cleanliness (d%) [JIS G0555]:
Heat-resistant cast steel with excellent room temperature elongation properties of 0.05% or less. (4) C: 0.3 to 0.6%, Si: 2.0% or less, M
n: 2.0% or less, Cr: 20.0-30.0%, Ni
:30.0~40.0%, Al:0.02~0.50%
, Ti: 0.04-0.50%, B: 0.0002-0
．． 004%, Nb: 0.3-1.5%, W: 0.5-3
．． 0%, Mo: 0.2 to 0.8%, remainder substantially composed of Fe, oxygen content: 50 ppm or less, cleanliness (d%)
[JIS G0555]: Heat-resistant cast steel with excellent room temperature elongation properties of 0.05% or less.