EP0408281B1

EP0408281B1 - Stainless steels

Info

Publication number: EP0408281B1
Application number: EP90307465A
Authority: EP
Inventors: Shinichiro Yahagi; Akihiko Saito; Tetsuya Shimizu
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 1989-07-11
Filing date: 1990-07-09
Publication date: 1996-05-01
Anticipated expiration: 2010-07-09
Also published as: JP2876627B2; DE69026763D1; EP0408281A1; DE69026763T2; JPH0344448A

Description

This invention relates to a stainless steel which is excellent in cold forgeability, corrosion resistance and electromagnetic properties, and suitable for use as materials of, for example, electromagnetic components requiring good corrosion resistance and electromagnetic properties as well as good workability at cold forging.
Heretofore, there has been used, for example, pure iron or the like generally as materials for electromagnetic components requiring good electromagnetic properties.( '9.6 Magnetic Materials' on pp. 1025∼ 1062 of "KINZOKU BINRAN 4th edition" issued on Dec. 20, 1982 by The Japan Instisute of Metals or '13.8 Magnetic Materials' on pp. 1021∼ 1037 of "KAGAKU BINRAN, Applied Chemistry Vol. II Materials" issued on Oct.15, 1986 by The Chemical Society of Japan may be referred to for example.)
However, aforementioned materials of pure iron or the like are liable to be corrode because of poor corrosion resistance thereof. Therefore, there is a problem since it is feared that the electromagnetic components may deteriorate in durability and reliability.
GB-1002909 discloses a ferritic stainless steel consisting of, inter alia, 11.5 to 19 wt.% Cr and 0 to 4 wt.% Si.
Werkstoffkunde Stahl, Band 2, 1985 (page 404) discloses Schaeffler's phase diagram which is used to estimate the amount of ferrite in a stainless steel by calculating an index number of austenite forming elements such as C, Mn, Ni and N and an index number of ferrite forming elements such as Cr, Mo, Si, Nb and Ti.
Therefore, this invention is made in view of the aforementioned problem of the prior art and aims to provide a stainless steel suitable for materials of electromagnetic components requiring good corrosion resistance and electromagnetic properties as well as good workability for manufacturing by cold forging and which reliably maintains such properties over a long period of time.
The present invention provides a stainless steel comprising: not more than 0.030 wt.% of C; not more than 1.00 wt.% of Si; not more than 0.50 wt.% of Mn; not more than 0.030 wt.% of P; not more than 0.030 wt.% of S; from 20.5 to 35.0 wt.% of Cr; from more than 0.03 wt.% to 1.89 wt.% of Al; not more than 0.030 wt.% of N; not more than 0.010 wt.% of 0; optionally at least one of not more than 1.5 wt.% of Nb, not more than 1.5 wt.% of Ta, not more than 1.5 wt.% of Ti, not more than 1.5 wt.% of Zr and not more than 1.5 wt.% of V, and/or at least one of not more than 2.0 wt.% of Cu, not more than 3.0 wt.% of Ni and not more than 5.0 wt.% of Mo, and/or at least one of from 0.03 to 0.30 wt.% of Pb, from 0.002 to 0.020 wt.% of Bi, from 0.002 to 0.020 wt.% of Ca, from 0.01 to 0.20 wt.% of Te and from 0.03 to 0.30 wt.% of Se; and the balance being Fe and incidental impurities.
The present invention will now be described in greater detail and by way of example only.
The reason why the chemical composition (by weight percentage) of the stainless steel according to this invention is limited to the above range will be described below.
C : not more than 0.030%
C is an element having a bad influence upon the electromagnetic properties, especially upon the coercive force in case of a stainless steel used for a material of electromagnetic components, and is also an element harmful to the toughness and the cold forgeability. Thus the content of C is not more than 0.030%.
Si : not more than 1.00%
Si is an element effective for improving the electromagnetic properties and the corrosion resistance, but harmful to the cold forgeability. Therefore, the upper limit of Si content shoud be 1.00% or 0.20% preferably in order to maintain good workablity of the components by cold forgeability which makes a contribution to improve the dimensional accuracy.
Mn : not more than 0.50%
If Mn is contained in excess, the cold forgeability is degraded, so that the upper limit of Mn content is defined as 0.50%.
P : not more than 0.030%
Containing P in excess causes deterioration of the cold forgeability. The content of P is limited to not more than 0.030% for such a reason.
S : not more than 0.030%
Containing S in excess invites deterioration of the cold forgeability. The content of S is limited to not more than 0.030% for such a reason.
Cr : 20,5 to 35%
Cr is an element effective for improving the corrosion resistance, many types of chromic stainless steels such as 13% Cr steel (type 405 stainless steel) and 17% Cr steel (type 430 stainless steel) have been developed for electromagnetic components so far. However, the requirement of the corrosion resistance of stainless steels of this kind used for electromagnetic components is now much stricter as compared with that of recent years, therefore it becomes impossible to satisfy the requirement of the corrosion resistance sufficiently by the aforementioned conventional stainless steels. Accordingly, in the stainless steel according to this invention, the lower limit of Cr content is defined as 20.5% so as to satisfy the corrosion resistance required more strictly. However, the saturation magnetic flux density decreases if the content of Cr increases in excess, and so the performance characteristics sometimes deteriorates at the case in which the stainless steel according to this invention is used for an electromagnetic valve and so on. Therefore, the content of Cr is limited to not more than 35%.
Al : more than 0.03% and not more than 1.89%
Al is an element effective for increasing the volume resistivity and effective for improving the electromagnetic properties owing to the reduction of the coercive force. Therefore Al is contained more than 0.03% in order to obtain the effects such as the increase of the volume resistivity and the improvement of the electromagnetic properties according to the reduction of the coercive force. However, the content of Al is limited to not more than 1.89% because the cold forgeability is degraded if Al is contained more than 1.89%.
N : not more than 0.030%
N is an element having a bad influence upon the electromagnetic properties, especially upon the coercive force similarly to C, in the case where the stainless steel according to this invention is used for the material of electromagnetic components, and is also an element harmful to the toughness and the cold forgeability. Therefore the content of N is limited to not more than 0.030%.
O : not more than 0.010%
O deteriorates the cold forgeability remarkably by forming inclusions consisting of oxides. The coercive force is reduced by lowering the content of O, and the electromagnetic properties improved, so that the content of O is limited to not more than 0.010%. Further, it is preferable to control the content of O within a range of not more than 0.006% for such a reason.
Nb : not more than 1.5%, Ta : not more than 1.5%, Ti : not more than 1.5%, Zr : not more than 1.5%, V : not more than 1.5%.
Since C and N are elements which cause the degradation of the coercive force as described above, it is also preferable to try to refine the grains by adding at least one of Nb, Ta, Ti, Zr and V which are carbide and nitride-forming elements according to demand in order to reduce the bad influence caused by said C and N. For example, when Ti is added in the stainless steel according to this invention in order to reduce the bad influence of C and N upon the coercive force, said Ti improves the electromagnetic properties, the cold forgeability and the toughness by forming TiC, TiN. A similar effect can be obtained by Nb, Ta Zr and V. However, the addition of these elements in excess damages the workability by deteriorating the cold forgeability or by degrading the machinability. Therefore, the upper limits of the respective elements are defined as 1.5% in case of addition.
Cu : not more than 2.0%, Ni : not more than 3.0%, Mo : not more than 5.0%
Cu, Ni and Mo are elements effective for improving the corrosion resistance, therefore one or more of these elements may be added according to demand. However, since the excessive addition of these elements deteriorates the cold forgeability, it is necessary to limit the contents within a range of not more than 2.0% of Cu, of not more than 3.0% of Ni and of not more than 5.0% of Mo in case of addition.
Pb : 0.03 to 0.30%, Bi : 0.002 to 0.020%, Ca : 0.002 to 0.020%, Te : 0.01 to 0.20%, Se : 0.03 to 0.30%
Pb, Bi, Ca, Te and Se are elements effective for improving the machinability, for example, the machinability in the case where a very small drill hole is made in the component after forming the external shape of said component by cold forging. Accordingly, one or more of these elements also may be added according to demand in order to obtain such an effect. However, because these elements degrade the cold forgeability and deteriorate the magnetic properties by the excessive addition, these elements should be added within an appropriate range so as not to harm said characteristics. These elements may be added according to demand within a range of 0.03 to 0.30% as to for Pb, a range of 0.002 to 0.020% as to for Bi, a range of 0.002 to 0.020% as to for Ca, a range of 0.01 to 0.20% as to for Te and a range of 0.03 to 0.30% of Se.
The stainless steel according to this invention has the aforementioned chemical composition, and remarkably exhibits excellent cold forgeability by the proper control of C, Si, Mn, P, S, N and O. The electromagnetic properties of the stainless steel are improved and the grain size is refined by the effect of the addition of Al, Nb, Ta,Ti, Zr, and V in the proper quantity and by the control of C, N and O. Furthermore the corrosion resistance is improved by the effect of the proper addition of Cr, Cu, Ni, and Mo, and the machinability of the stainless steel is also improved by the effect of the addition of Pb, Bi, Ca, Te and Se in the proper quantity.

EXAMPLE

Each of stainless steels having chemical compositions shown in Table 1 was melted. And then, the electric resistance, the magnetic flux density and the coercive force of the respective stainless steel were measured. The cold forgeability, corrosion resistance and machinability were investigated by the methods shown in Table 2 and were evaluated respectively according to the evaluating classifications as also shown in Table 2. The results are shown in Table 3.
As shown in Table 1 to Table 3, comparative steel No 6 containing excessive Si and Cr has a large coercive force and poor cold forgeability, and the machinability of the steel is not so good. Comparative steels Nos 7 and 8 containing insufficient Cr are poor in corrosion resistance. Further, comparative steel No. 9 containing C and N in excess is inferior in the magnetic properties because of the large coercive force thereof. Furthermore, comparative steel No 10 containing excessive O is poor in cold forgeability, and it is seen that the coercive force of comparative steel No 10 takes a high value and so the magnetic properties are also considerably inferior.
Contrary to above, the steels Nos. 1 to 5 according to this invention which contain C, Si, Mn, P, S, N and O limited to the proper contents and Cr and Al controlled within appropriate ranges are satisfactory in their machinability, corrosion resistance and cold forgeability as well as electric resistance, magnetic flux density and magnetic properties in all cases. And it is confirmed that it is possible to prevent the deterioration of the magnetic properties caused by the addition of the other alloying elements or to further improve the magnetic properties by adding at least one of Nb, Ta, Ti, Zr and V, and it is possible to prevent the degradation of the corrosion resistance caused by the addition of the other alloying elements or to further improve the corrosion resistance by adding one or more of Cu, Ni and Mo. Further, is possible to improve the machinability by adding at least one of Pb, Bi, Ca, Te and Se.
As described above, the stainless steel according to this invention consists essentially of not more than 0.030 wt.% of C, not more than 1.00 wt.% of Si, not more than 0.50 wt.% of Mn, not more than 0.030 wt.% of P, not more than 0.030 wt.% of S, 20.5 to 35 wt.% of Cr, from more than 0.03 wt.% to 1.89 wt.% Al, not more than 0.030 wt.% of N, not more than 0.010 wt.% of O, and at least one of not more than 1.5 wt% of Nb, not more than 1.5 wt.% of Ta, not more than 1.5 wt.% of Ti, not more than 1.5 wt.% of Zr and not more than 1.5 wt.% of V if necessary, and similarly at least one of not more than 2.0 wt.% of Cu, not more than 3.0 wt.% of Ni and not more than 5.0 wt.% of Mo if necessary, further at least one of 0.03 to 0.30 wt.% of Pb, 0.002 to 0.020 wt.% of Bi, 0.002 to 0.020 wt.% of Ca, 0.01 to 0.20wt.% of Te and 0.03 to 0.30 wt.% of Se if necessary, and the balance being Fe and inevitable impurities. Therefore, the stainless steel according to this invention exhibits favorable workability, particularly in the case of manufacture by cold forging which produces components excellent in dimensional accuracy and productivity. The stainless steel also exhibits satisfactory corrosion resistance and electromagnetic properties, and so is especially suitable for the raw material of electromagnetic components. A very excellent effect can be obtained since it is possible to maintain a high relability in electromagnetic components over a long period.

Claims

A stainless steel comprising: not more than 0.030 wt.% of C; not more than 1.00 wt.% of Si; not more than 0.50 wt.% of Mn; not more than 0.030 wt.% of P; not more than 0.030 wt.% of S; from 20.5 to 35.0 wt.% of Cr; from more than 0.03 wt.% to 1.89 wt.% of Al; not more than 0.030 wt.% of N; not more than 0.010 wt.% of O; optionally at least one of not more than 1.5 wt.% of Nb, not more than 1.5 wt.% of Ta, not more than 1.5 wt.% of Ti, not more than 1.5 wt.% of Zr and not more than 1.5 wt.% of V, and/or at least one of not more than 2.0 wt.% of Cu, not more than 3.0 wt.% of Ni and not more than 5.0 wt.% of Mo, and/or at least one of from 0.03 to 0.30 wt.% of Pb, from 0.002 to 0.020 wt.% of Bi, from 0.002 to 0.020 wt.% of Ca, from 0.01 to 0.20 wt.% of Te and from 0.03 to 0.30 wt.% of Se; and the balance being Fe and incidental impurities.