Non-magnetic austenitic stainless cast steel and manufacturing method of the same

Abstract

A non-magnetic austenitic stainless cast steel having high yield strength at room temperature without losing ductility at liquid nitrogen temperature, permitting easy casting without requiring the solution heat treatment, containing (by mass %) 0.08 percent or less carbon, 0.1 to 1.5 percent silicon, 0.1 to 1.5 percent manganese, 13 to 15 percent nickel, 18 to 19 percent chromium, 2 to 2.5 percent molybdenum, 0.005 to 0.1 percent aluminum, and 0.12 to 0.2 percent nitrogen, with the remainder of the steel being iron and incidental impurities, and having an elongation at liquid nitrogen temperature having 30 percent or more, 0.2% yield strength at room temperature having 240 MPa or more and relative magnetic permeability having 1.10 or less.

Description

[0001] The present invention relates to a non-magnetic austenitic stainless cast steel for a member for an electromagnet used below liquid nitrogen temperature and to a method of manufacturing such a non-magnetic austenitic stainless cast steel.DESCRIPTION OF THE RELATED ART[0002] Recently, techniques have been developed rapidly that relate to nuclear fusion reactors or the like using large magnets. Many of these techniques conventionally employ the superconductivity, and because of increase of the cost particularly for cooling of the coil, techniques of achieving cost reduction have been developed. Thus, demands are increased for members for electromagnets used at liquid nitrogen temperature (77K).[0003] Such members are required to be in excellent in strength and ductility at low and room temperatures as for superconducting material, and should have non-magnetic properties.[0004] As materials to cope with above requirements, there are proposed SUS304LN stainless steels such as 18%...

AI Technical Summary

Problems solved by technology

However, any technique has not been disclosed that configures a desired form by casting.

However, when the solution heat treatment is processed on a member, deformation due to its own weight is inevitable in heating because of a shortage of thermal strength, while it is difficult to avoid deformation due to large stress caused by cooling.

However, when the steels are manufactured by casting, since deformation may occur as in the case of performing the solution heat treatment, it is necessary to avoid heating at high temperature and rapid cooling.

Carbon of more than 0.08 percent combines with chromium easily between dendrites, is precipitated as chromium carbide and thereby impairs the ductility.

In addition, carbon of less than 0.03 percent is not effective.

Silicon of less than 0.1 percent is not sufficient in deoxidizing effect and tends to cause defects in the casting, while silicon of more than 1.5 percent tends to cause casting blockage.

Manganese of less than 0.1 percent is not sufficient in deoxidizing effect and tends to cause defects in the casting, while manganese of more than 1.5 percent saturates the deoxidizing effect.

However, nickel of less than 13 percent is less effective, while nickel of more than 15 percent causes 0.2% yield strength at room temperature to decrease.

Chromium of less than 18 percent leads to a shortage of the yield strength at room temperature, while chromium of more than 19 percent tends to form a ferrite structure and does not permit to achieve non-magnetic properties.

Molybdenum of less than 2 percent exhibits less effects, while molybdenum of more than 2.5 percent tends to form a ferrite structure and does not permit to achieve non-magnetic properties, as well as Chromium.

Aluminum of less than 0.005 percent fails to deoxidize sufficiently and tends to cause defects in the casting.

Meanwhile, the excessive nitrogen content impairs the ductility at liquid nitrogen temperature, and thus N atoms are important.

Nitrogen of less than 0.12 percent makes it difficult to obtain the yield strength at room temperature having 240 MPa or more, while nitrogen of more than 0.2 percent impairs the ductility at liquid nitrogen temperature.

The heating temperature of less than 300.degree. C. requires a long time to relax the residual stress, and is not practical.

The heating temperature exceeding 550.degree. C. causes nitrogen atoms soluted in the matrix to react with other atoms and to form precipitation, and as a result, decreases the ductility at liquid nitrogen temperature.

Furthermore, the reason for employing an air-cooling or a slow-cooling as the cooling rate to below 200.degree. C. is that faster rates than this cooling rate deform products due to distortions caused by cooling.

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