Ferritic heat-resistant steel and method for producing it

Abstract

The invention provides a ferritic heat-resistant steel having excellent high-temperature oxidation resistance, especially excellent steam oxidation-resistant characteristics. In high-Cr ferritic heat-resistant steel, ultra-fine oxide particles having a size of not larger than 1 μm are formed just below the oxide films and formed on the steel base, whereby the adhesiveness between the films and the base is enhanced. The ferritic heat-resistant steel contains Cr in an amount of from 8.0 to 13.0% by weight, and at least one of Rh and Ir in a total amount of from 0.3 to 5.0% by weight.

Description

FIELD OF THE INVENTION [0001] The present invention relates to ferritic heat-resistant steel and to a method for producing it; More precisely, it relates to ferritic heat-resistant steel suitable for materials for apparatus that are used under high-temperature and high-pressure conditions, such as boilers, apparatus in chemical industry, etc., and to a method for producing it. Specifically, it relates to ferritic heat-resistant steel having excellent oxidation-resistance at high temperatures, especially steam oxidation-resistance which are not worsened even at high temperatures higher than 630° C., and having high creep strength which is comparable to that of ordinary steel, and relates to a method for producing it. BACKGROUND OF THE INVENTION [0002] In general, heat-resistant steel for use for high-temperature heat-resistant and pressure-resistant parts of boilers, atomic powered apparatus and other apparatus in chemical industry is required to have high-temperature strength, tough...

AI Technical Summary

Benefits of technology

[0040] In the invention, for example, Ti or Y oxides are formed through internal oxidation in steel, while the steel is to have a scale structure composed of an outer scale layer (of Fe oxides) (1) and an inner scale layer (of Fe—Cr oxides) (2) as formed on the surface of the steel base (3), as in FIG. 1, in which fine oxide particles (4) exist around the scale / base interface.

[0041] In ordinary steel, it is believed that the pores existing in the scale will aggregate in the interface between the scale and the steel base to give voids (5), as in FIG. 2(A), and those voids (5) will be linked to each other to cause the scale peeling. However, as in FIG. 2(B), if fine oxide particles (4) exist around the interface between the scale layers (1) (2) and the steel base (3), especially in the region just below the scale (2), they could be void-filling points and even could act as a barrier to the linking of the voids (5). In addition, the particles could further act to mechanically bond the scale and the steel base, whereby the scale is prevented from being swelling up or peeling away.

[0042] Existing oxide particles having a size of not larger than 1 micron, but preferably not larger than 0.5 microns in and / or around the interface between the oxide film and the steel base prevents the film from peeling, and is effective to attain the intended purpose. However, large particles having a size of 3 microns or larger, if existing in the interface, are not effective for the intended purpose, but rather promote the film peeling. <Steel Composition>

[0043] (1) Cr: In general, the oxide film formed on ferritic heat-resistant steel is composed of an outer layer consisting essentially of Fe oxides and an inner layer consisting essentially of Cr oxides or Fe—Cr oxides. Stabilizing the sound Cr2O3 film without peeling it is effective for improving the oxidation resistance of the steel. From this viewpoint, Cr is one essential alloying element in the invention. Regarding its amount to be added, Cr must be added to steel in an amount of not smaller than 8.0% in order to form a sound oxide film. However, if the amount of Cr added is larger than 13.0%, much Cr will promote the formation of d-ferrite, whereby the properties of the steel, including the toughness thereof, are much worsened. For these reasons, the Cr content of the steel of the invention preferably falls between 8.0 and 13.0%.

[0044] (2) Ti: Ti has high affinity for oxygen. When added to steel in a small amount, Ti forms fine oxide particles just below the oxide film formed on steel. Ti easily bonds to not only oxygen but also carbon and nitrogen. Therefore, Ti added in steel alloys well bonds to those elements to form its carbides, etc. If the amount of Ti added is smaller than 0.01%, all Ti will bond to carbon and other elements existing in steel alloys, and could no more form its oxides while the alloys are used. Therefore, it is desirable to add Ti to steel in an amount not smaller than 0.01%. On the other hand, however, if the amount of Ti added is too large, Ti oxides formed will be in the form of coarse and large particles, and have some negative influences on steel. For these reasons, the uppermost limit of the amount of Ti to be added may be 0.3%.

[0045] In addition, Ti traps oxygen. Therefore, adding Ti to steel prevents oxygen from diffusing into the inside of steel, whereby the oxidation speed in steel is greatly reduced.

Problems solved by technology

Given that situation, steel for boilers is being required to have extremely high performance, and conventional high-Cr ferrite steel could no more satisfy the requirements of high oxidation resistance and long-term creep strength, especially steam oxidation-resistance.

If the steam oxidation-resistance of boilers are poor, oxide films will be formed on the inner surfaces of steel pipes of boilers through which high-temperature steam passes.

After having grown to a certain thickness, the oxide films peel off due to thermal stress that may be caused by the temperature change in boilers, for example, when boilers being driven are stopped, by which pipes will be clogged.

However, austenitic stainless steel is expensive, and its use in commercial plants is limited because of the economic reasons.

In addition, because austenitic stainless steel has a large thermal expansion coefficient, its thermal stress to be caused by the temperature change in drive stopping or the like is large.

For these reasons, the use of austenitic stainless steel in plants is problematic because of the difficulties in designing and driving the plants using it.

Even though their high-temperature creep strength is increased as a large amount of W is added thereto, those types of steel are still problematic in that the decrease in their toughness is inevitable.

Therefore, this has poor steam oxidation resistance.

However, as containing a large amount of Ni and Cu, this steel is still defective, like the steel disclosed in JP-A Hei-5-311342, in that it changes the structure of oxides consisting essentially of Cr2O3 and that its steam oxidation resistance is poor.

As a result, the temper softening resistance of the steel is poor, and, in addition, carbides and nitrides in the steel rapidly aggregate to give large coarse grains therein.

Therefore, the long-term creep strength of the steel is low.

As mentioned hereinabove, known is no satisfactory ferritic heat-resistant steel having sufficient oxidation resistance and steam oxidation resistance for use in ultra-supercritical pressure conditions at high temperatures and high pressures.

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