Precipitation-hardened soft magnetic ferritic stainless steels

Abstract

A precipitation-hardened soft magnetic ferritic stainless steel comprises C: not more than 0.2 mass %, Si: 0.01-3.0 mass %, Mn: not more than 0.5 mass %, S: not more than 0.3 mass %, Cr: 12.0-19.0 mass %, Ni: 1.0-4.0 mass % and Al: 0.2-4.0 mass % and further contains at least one of Ti: less than 0.5 mass % and Zr: less than 0.3 mass % and the remainder being inevitable impurities and Fe, and has substantially a microstructure of a ferrite phase after a solution treatment and an aging treatment.

Description

[0001] 1. Field of the Invention[0002] This invention relates to a precipitation-hardened soft magnetic ferritic stainless steel, and more particularly to a precipitation-hardened soft magnetic ferritic stainless steel having not only excellent magnetic properties and corrosion resistance but also a high hardness, which contributes to an improvement of a durability such as a wear resistance, a resistance to a buckling and the like in a movable part of a magnetic circuit apparatus, and a good cold workability. Especially, it is preferable to use such a ferritic stainless steel as a magnetic core material for various electromagnetic valves, an electronic control fuel injection device and others.[0003] 2. Description of Related Art[0004] As the magnetic core for the electromagnetic valve, the electronic control fuel injection device and the like, the soft magnetic ferritic stainless steels are practically and frequently used from demands to the magnetic properties and the corrosion res...

AI Technical Summary

Benefits of technology

[0010] The above Japanese Patent 1194892 is an epoch-making invention getting a head start on the precipitation-hardened soft magnetic ferritic stainless steel and discloses conditions for stabilizing the ferrite phase in detail. And also, the above Japanese Patent 1832191 is an invention further improving an appearance luster of the material.

[0036] Mo is a ferrite stabilizing element and is an element effective for improving the corrosion resistance. However, the excess addition of Mo exceeding 4.0% obstructs the cold workability and lowers the productivity, so that the Mo content is limited to not more than 4.0%.

Problems solved by technology

And now, when the parts made of the soft magnetic ferritic stainless steel are used in a sliding portion or an impacting portion of the apparatus in operation, it often becomes a problem that the deformation through wearing or buckling causes not only the change of a size but also the deterioration of magnetic circuit properties and airtightness to bring about the deterioration of a control accuracy.

However, such a countermeasure is not preferable because it brings about the deterioration of the magnetic properties as an important characteristic for the soft magnetic material and the increase of the production cost of the apparatus.

In both the patents, the hardness after the solution treatment and the aging treatment has a very high value of not less than 400 HV as a single phase material of the soft magnetic ferrite, but there are problems that the magnetic properties do not reach to the level required when using in recent electronic control apparatuses and further the cold workability is deteriorated due to the excessively high hardness to cause troubles in the mass productivity and the like.

C is an austenite stabilizing element obstructing the formation of steel microstructure based on the ferrite phase and also adversely affecting the magnetic properties.

However, when the Si content exceeds 3.0%, the cold workability is obstructed to brings about the lowering of the productivity.

However, since Mn is an austenite stabilizing element, when Mn is excessively added in an amount exceeding 0.5%, the ferrite phase is destabilized and further the magnetic properties and the corrosion resistance are deteriorated, so that the Mn content is limited to not more than 0.5%.

However, when Cr content is less than 12.0%, these effects are poor, while when Cr is added in an amount exceeding 19.0%, the magnetic properties are adversely affected.

However, the excess addition of Al exceeding 4.0% not only needs a special refining process but also obstructs the cold workability, the upper limit of Al content is 4.0%.

The Ti content of not less than 0.5% and / or the Zr content of not less than 0.3% contribute to the increase of the hardness, but obstruct the cold workability to lower the productivity, so that the Ti content is limited to less than 0.5% and the Zr content is limited to less than 0.3%.

However, the excess addition of Nb exceeding 1.0% rather obstructs the magnetic properties and the cold workability, so that the Nb content is limited to not more than 1.0%.

However, the excess addition of Mo exceeding 4.0% obstructs the cold workability and lowers the productivity, so that the Mo content is limited to not more than 4.0%.

However, the excess addition of Cu exceeding 2.0% brings about the embrittlement, and complicates the cold working such as cold drawing, straightening or the like, and lowers the productivity, so that the Cu content is limited to not more than 2.0%.

Both B and REM contribute to improve the cold workability, but when B and REM exceed 0.01% and 0.1%, respectively, they rather become a factor obstructing the cold workability.