JPS5842264B2 - High-temperature austenitic steel with excellent oxidation resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an austenitic stainless steel with excellent oxidation resistance.

Cr-containing steel generally has excellent oxidation resistance, and the reason for this is thought to be as follows.

That is, when Cr-containing steel is exposed to a high temperature environment containing oxygen,
A Cr-rich oxide scale forms on the surface, which acts as a protective film that prevents direct contact between the steel and oxygen and prevents F
Suppresses the diffusion of e and oxygen to prevent scale growth.

However, when the amount of Cr in the oxide scale is small and the amount of Fe is large, (Fe.

Cr) 30 In some cases, FeO is generated, and the diffusion of Fe or oxygen in the oxide scale is significantly accelerated, resulting in deterioration of oxidation resistance.

Therefore, it can be expected that steel with a high Cr content has excellent oxidation resistance, but the properties of the protective film are not necessarily Cr-rich.
It is not determined only by the content, but also the influence of other alloy components or trace amounts of impurities cannot be overlooked.

In particular, when subjected to repeated thermal cycles of high-temperature heating and cooling, cracking and exfoliation of oxide scale are likely to occur, and even at the same Cr level, oxidation resistance may vary greatly depending on the steel type.

Conventionally, increasing the amount of Cr and adding Si or Al have been known as conventional means for improving the oxidation resistance of steel, and addition of Y or rare earth elements has also been proposed.

These components form an oxide layer that bonds more strongly with oxygen than Cr in the oxide scale, suppressing the oxidation of Fe, or converting it into a dense Fe with fewer defects and less diffusion of oxygen. or by improving the density of the scale.

It has a considerable effect in improving oxidation resistance.

However, unnecessarily increasing the amount of Cr, Si, AI, etc. not only increases the price of steel, but also adversely affects mechanical properties, weldability, workability, etc.

Further, Y and rare earth elements need to be added in large amounts to obtain sufficient effects, and there are concerns that they may cause difficulties in manufacturing steel and deteriorate weldability.

The present inventor conducted a detailed study on the test results showing that there is a large difference in oxidation resistance in steels having the same content of Cr and other major components f and under the same test conditions.

As a result, we learned that the behavior of S (sulfur) in steel has an extremely large effect on oxidation resistance.

S in steel usually combines with Mn and exists as MnS.

However, when the sample after the oxidation test was examined in detail, an abnormal oxidation phenomenon occurred around the MnS existing on the surface of the steel.

Therefore, when we investigated the behavior of MnS, we found that when the temperature of MnS on the steel surface reaches 800°C or higher, MnS becomes
-Cr-0-8 and finally Cr-Mn-
It was found that the value changed to 0.

In the above process, S is separated from the inclusions, and conversely, Cr and O are concentrated in the inclusions.

As a result, a large Cr concentration gradient is generated near the inclusion, and a Cr-depleted layer is locally generated around the inclusion.

Furthermore, due to the release of S, this peripheral area becomes an S-rich atmosphere, and the oxide produced from the initial stage of oxidation is clearly different from that in other areas.

In other words, the area around the inclusion is Fe-C containing a large amount of Fe.
An oxide consisting of r -Mn -3i -0 is formed,
The other parts form an oxide made of Cr, which has high oxidation resistance.

Therefore, as time passes, the area around the inclusions develops into abnormal oxidation, and furthermore, the entire scale grows to include Fe, which significantly impairs oxidation resistance.

Note that at temperatures above 1175° C., Fe25 i04 melts and accelerates abnormal oxidation.

However, in the case of steel that does not contain MnS and has a significantly reduced amount of S, and steel that has been converted to Ca-based sulfide, oxides consisting of Cr are uniformly formed, and such abnormal oxidation does not occur.

Based on the above findings, the present inventors worked on improving the oxidation resistance of austenitic stainless steel from a completely different perspective than before.

In other words, rather than increasing the amount of main components such as Cr and Ni, or using special additives, we focus on inclusions in steel, especially MnS, and reduce MnS or Mn
This method involves replacing S with a sulfide that is stable even at high temperatures.

According to this method, the main components can be considered based on existing standardized steels, so mechanical properties and other properties can be considered to be equivalent to or superior to those of existing steels. .

In other words, there is no need to worry about deterioration of properties other than oxidation resistance due to addition of special components or increase in amount.

Of course, in order to ensure the essential properties of austenitic stainless steel, C, Si, Mn, Cr,
Ni and other components must be within specified ranges.

However, even if Cr1Ni etc. are in the same range as existing steel, the oxidation resistance of the steel of the present invention is far superior.

Austenitic stainless steels are classified into levels based mainly on the content of Cr and Ni, and when the conditions of use become severe, steel types with high Cr and Ni content and therefore high prices are used.

The steel of the present invention has much better oxidation resistance than existing steels with the same levels of Cr and Ni, so it can be used in place of higher grade steels, and is economical in this respect. The advantages are great.

Based on the above idea, the present invention particularly provides
Our direct goal is to obtain an inexpensive steel with low Cr and Ni contents that can withstand use at high temperatures up to This was done as a.

The gist of the present invention is the following austenitic steel.

C: 0.10% or less, Si: 5% or less, Mn: 2
% or less, Ni: 9 to 15%, Cr: 17.5 to 25%,
and Ti, Nb1Zr, Ta, Hf 1.5
% or less, the balance consists of Fe and impurities, Cr+Si
is 22.5% or more, s in impurities is 0.003% or less, and is substantially free of MnS.

Among the compositions of the above-mentioned steel of the present invention, Ni and Cr were set so as to maintain the basic characteristics of an austenitic steel used at high temperatures and to be able to manufacture the steel at the lowest possible cost.

9% of Ni is the minimum necessary amount to stably secure an austenite structure, and 15% is the upper limit to avoid a price increase.

17.5% of Cr is a total amount of 22.5% with Si, which will be described later.
% or more, and is the minimum necessary amount to improve oxidation resistance.

When Cr exceeds 25%, Ni must be added in excess of the above upper limit in order to stabilize the austenite phase.

One of the characteristics of the steel of the present invention is that the amount of Si is increased and Cr+Si is 22.5% or more.

It is already widely known that Si is effective in improving the oxidation resistance of steel, but the above-mentioned effect of reducing MnS is particularly
It is a new finding of the present inventors that this is remarkable in steel containing 1.5% or more of i.

By setting S i -+-Cr to 22.5% or more and reducing MnS in the steel, 25Cr-2ONi can be improved even at relatively low Ni and Cr contents as described above.
Oxidation resistance comparable to steel (SUS310S) can be obtained.

Seeds are effective as deoxidizing agents and as stabilizing components of the austenite phase.

However, if the amount exceeds 2%, hot workability deteriorates.

C is an undesirable component from the viewpoint of both strength and oxidation resistance.

o, io% is the upper limit permissible value. In order to reduce the undesirable influence of such C, Ti, Nb, Zr, T
a. One or more types of Hf are added.

These combine with C to form stable carbides, which particularly contribute to improving oxidation resistance.

The content of these components should be from 4 times the amount of C to 1.5%.

The reason why S is set to 0.003% or less is to minimize the amount of MnS in the steel.

If S is lowered to around o, ooi% or below, MnS
The amount of precipitation is significantly reduced.

However, if S exists in an amount close to the above upper limit, MnS
This is desirable because it changes S into a compound that is stable even at high temperatures in order to ensure prevention of precipitation.

Such compounds are Ca1Mg, ALY or compounds with rare earth elements, such as Ca-Al-0-8.

These compounds do not decompose even at temperatures of 800° C. or higher, and therefore do not cause abnormal oxidation due to the generation of free S.

Even if Ca, Mg, etc. are not particularly added, trace amounts of them may be mixed in from furnace materials or slag during refining, so if the S content is sufficiently low, they can be captured as stable compounds.

However, in order to ensure the effect, Ca, Mg
It is better to actively add 1Al and Y1 rare earth elements.

However, the amount remaining in the steel is preferably 0.010% or less.

This is because if the amount exceeds this amount, inclusions other than compounds with S will increase, impairing workability and weldability.

Note that since A1 acts as a deoxidizing agent and increases the addition yield of Ca, Mg, etc., it is desirable to combine it especially when adding these components.

EXAMPLE Steel having the chemical composition shown in Table 1 was melted, hot-rolled and cold-rolled, and then subjected to bath forming treatment to form a 1.2 mm thick thin plate for use in experiments.

The existence form of these inclusions in steel is Ca-
Mg-Al-0 or partially Ca-Mg-Al-
Most of them are 0-8 type, and no MnS type inclusions are detected.

On the other hand, in the case of comparative steel and commercially available steel, the amount of S is high, and a large amount of MnS-based compounds is observed.

The oxidation resistance test was conducted under the following conditions.

In other words, after repeating the heating and cooling cycle 400 times by holding the sample in a furnace heated to each temperature for 30 minutes and then leaving it in the air for 30 minutes, the weight was measured and the oxidation resistance was determined by the difference from the weight before the test. evaluated.

The comparative and commercial steels present in the form of MnS show a rapid increase in the amount of oxidation after a few hours.

Furthermore, Table 1 shows that when the test is repeated, the amount of oxidation increases significantly due to the peeling off of the oxide scale during cooling and heating, resulting in a large weight change and poor oxidation resistance.

On the other hand, the amount of S is reduced and the remaining S is converted into Ca-MgA1-O
In the case of the -8 series steel of the present invention, the oxidation follows the parabolic curve, and even when heated repeatedly, a uniform oxide scale is formed and does not peel off, so the weight gain due to oxidation is significantly reduced. The weight change is small, indicating excellent oxidation resistance.

Claims (1)

[Claims] IC: 0.10% or less, Si: 5% or less, Mn: 2
% or less, Ni: 9-15%, Cr: 17.5-25
%, and one or more of Ti, Nb, Zr, Ta, Hf1
．． 5% or less, the balance consists of Fe and impurities, Cr+S
A high-temperature austenitic steel with excellent oxidation resistance and substantially no MnS, with i of 22.5% or more and S in impurities of 0.003% or less.