Composite magnetic member excellent in corrosion resistance and method of producing the same

This is a composite magnetic member excellent in corrosion resistance having a chemical composition consisting essentially, by weight, of 0.30 to 0.80% C, more than 16.0% but not more than 25.0% Cr, 0.1 to 4.0% Ni, 0.1 to 0.06% N, at least one kind not more than 2.0% in total selected from the group consisting of Si, Mn and Al, and the balance Fe and impurities, and having a ferromagnetic portion and a non-magnetic portion.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a composite magnetic member combining a
 ferromagnetic portion and a non-magnetic portion suitable for use in an
 actuator which treats with automobile fuels and hydraulic operating fluids
 or the like (hereinafter referred to as an oil controlling device).
 2. Description of the Related Art
 An oil flow controlling device of an automobile conventionally has a
 structure in which an effective use of magnetic flux is made by providing
 a non-magnetic portion in a part of a stator, which stator is
 ferromagnetic (generally, soft magnetism), to cause magnetic flux to flow
 to a movable piece. Techniques such as the brazing and laser welding of a
 ferromagnetic part and a non-magnetic part have been employed to provide a
 non-magnetic portion in a part of the ferromagnetic portion. In contrast
 to these techniques of bonding dissimilar materials, the present authors
 propose the use of a single material as a composite magnetic material
 which is formed by providing a ferromagnetic portion and a non-magnetic
 portion by cold working or heat treatment. When such composite magnetic
 materials made of a single material are used, it is possible to obtain
 parts superior to those obtained by bonding a ferromagnetic portion and a
 non-magnetic portion with respect to ensuring airtightness and ensuring
 reliability, such as prevention of breakage by vibrations, etc.
 In Japanese Patent Unexamined Publication No. 9-157802 based on a proposal
 by the present inventors, for example, a martensitic stainless steel
 containing 0.5 to 4.0% Ni is disclosed as a composite magnetic member
 suitable for an oil controlling device of an automobile. This proposal is
 such that in a martensitic stainless steel composed of ferrite and
 carbides in an annealed condition, the austenite in a non-magnetic portion
 having a permeability (.mu.) of not more than 2, which portion is obtained
 by cooling a part of the martensitic stainless steel after heating, is
 stabilized by adding an appropriate amount of Ni to a C-Cr-Fe-base alloy
 from which ferromagnetic properties with a maximum permeability (.mu.m) of
 not less than 200 are obtained, whereby it is possible to lower the Ms
 point (temperature at which austenite begins to be transformed into
 martensite) to not more than -30.degree. C.
 Also, Japanese Patent Unexamined Publication No. 9-228004 based on a
 proposal by the present applicant discloses that in a composite magnetic
 material used in magnetic scales etc., by adding more than 2% but not more
 than 7% Mn and 0.01 to 0.05% N to a C-Cr-Fe-base alloy containing 10 to
 16% Cr and 0.35 to 0.75% C which alloy has ferromagnetic properties with a
 maximum permeability (.mu.m) of not less than 200, it is possible to
 stabilize the retained austenite with a permeability (.mu.) of not more
 than 2, which is obtained by cooling after heating, and to thereby lower
 the Ms point to not more than -10.degree. C. These proposals are excellent
 in the respect that a ferromagnetic portion with a maximum permeability
 (.mu.m) of not less than 200 and a stable non-magnetic portion with a
 permeability (.mu.) of not more than 2 and a low Ms point can be obtained
 in a single material.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide a composite magnetic
 member excellent in corrosion resistance, which combines a ferromagnetic
 portion and a non-ferromagnetic portion in a single material, in the
 member the corrosion resistance of the ferromagnetic portion being
 improved whose structure is mainly composed of ferrite and carbides, and
 to provide also a method of producing the composite magnetic member.
 The composite magnetic members disclosed in the above Japanese Patent
 Unexamined Publication No. 9-157802 and Japanese Patent Unexamined
 Publication No. 9-228004 have an advantage in being capable of combining a
 ferromagnetic portion with a maximum permeability (.mu.m) of not less than
 200 and a stable non-magnetic portion with a permeability (.mu.)of not
 more than 2. However, in these composite magnetic members, the corrosion
 resistance of the ferromagnetic portion mainly composed of ferrite and
 carbides is inferior to that of the non-magnetic portion mainly composed
 of austenite, with the result that rust is apt to be formed on the surface
 of the ferromagnetic portion. Thus, these composite magnetic members had
 such a serious problem as their surfaces corrode and deteriorate when they
 are used in oil controlling devices of automobiles, etc.
 The present inventors examined the microstructure of a ferromagnetic
 portion whose structure is mainly composed of ferrite and carbides in a
 composite magnetic material. As a result, they found out that the carbides
 are mainly composed of Cr carbides and that the formation of these Cr
 carbides causes Cr to be concentrated in the carbides, with the result
 that the Cr concentration is insufficient in the ferrite phase matrix near
 the carbides.
 As a result of a further examination, the present inventors also found out
 that the corrosion of the ferromagnetic portion starts from a layer of
 deficient Cr concentration near Cr carbides as the initiation point and
 that the corrosion resistance of the ferromagnetic portion and hence the
 corrosion resistance of the composite magnetic material can be
 substantially improved by increasing the amount of Cr contained in the
 composite magnetic material to more than 16.0% by weight, thereby
 increasing the Cr concentration of the ferrite phase matrix to not less
 than 12.0% by weight.
 In addition, the present inventors further examined the disclosure in
 Japanese Patent Unexamined Publication No. 9-228004 that it is difficult
 to form the austenite in the non-magnetic portion in a case of Cr
 concentrations exceeding 16.0% because the ferrite structure becomes
 stable at such high Cr concentrations.
 The present inventors previously considered that because Cr is a
 ferrite-forming element, the ferrite phase becomes stable when the Cr
 concentration exceeds 16.0% and, therefore, it is difficult to obtain the
 non-magnetic phase of austenite even when solution treatment is performed.
 This time, however, they found out that, surprisingly, an austenite phase
 with a permeability (.mu.)of not more than 2 is obtained when a material
 with a Cr concentration exceeding 16.0% was subjected to solution
 treatment at 1250.degree. C. for 10 minutes.
 As a consequence, the present inventors found out that when water cooling
 is performed after solution treatment is carried out at the temperature
 range of from 1050 to 1300.degree. C. in the manufacturing process of a
 composite magnetic member, austenitizing is possible, in other words,
 non-magnetic portion can be obtained.
 Furthermore, the present inventors found out that, by performing annealing
 at below the A3 transformation point after hot working, cold working and
 further annealing at below the A3 transformation point, it is possible to
 disperse carbides in the ferromagnetic portion having a maximum grain size
 range of 0.1 to 20 .mu.m, so that, corrosion resistance can be improved
 without the deterioration of the conventional magnetic properties even
 when Cr is added in amounts exceeding 16.0% if they are not more than
 25.0%.
 In the present invention there is provided a composite magnetic member
 excellent in corrosion resistance having a chemical composition consisting
 essentially, by weight, of 0.30 to 0.80% C, more than 16.0% but not more
 than 25.0% Cr, 0.1 to 4.0% Ni, 0.1 to 0.06% N, at least one kind not more
 than 2.0% in total selected from the group consisting of Si, Mn and Al,
 and the balance Fe and impurities, and having a ferromagnetic portion and
 a non-magnetic portion.
 The composite magnetic member of the present invention has such magnetic
 properties as the maximum permeability (.mu.m) of the ferromagnetic
 portion is not less than 200 and the permeability (.mu.) of the
 non-magnetic portion is not more than 2.
 The composite magnetic member of the present invention has a ferromagnetic
 portion with a maximum grain size of carbides controlled to the range of
 from 0.1 to 20 .mu.m.
 The maximum grain size of carbides in the ferromagnetic portion of the
 composite magnetic member of the present invention is preferably
 controlled to the range of from 5 to 20 .mu.m.
 A method of producing the composite magnetic member of the present
 invention comprises the steps of hot working a material for this composite
 magnetic member, annealing the material at a temperature below the A3
 transformation temperature, cold working it, and annealing it again at a
 temperature below the A3 transformation temperature to obtain a
 ferromagnetic body, and locally heating and cooling a part of the
 ferromagnetic body thus obtained to form a non-magnetic portion. A
 composite magnetic member excellent in corrosion resistance can be
 obtained by this method.
 In this method of producing a composite magnetic member excellent in
 corrosion resistance, the maximum grain size of carbides in the above
 ferromagnetic portion is controlled preferably to the range of from 0.1 to
 20 .mu.m, and more preferably to the range of from 5 to 20 .mu.m.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 As mentioned above, an important feature of the present invention resides
 in that in order to improve the corrosion resistance of a ferromagnetic
 portion of the composite magnetic member comprising ferrite and Cr
 carbides, the amount of Cr contained in the base material of the composite
 magnetic member is increased to concentrations of more than 16.0% by
 weight, whereby the Cr concentration of the ferrite phase matrix near the
 carbides is increased to not less than 12.0%.
 Reasons for the limited chemical composition of the present invention are
 described below.
 Cr is the most important element of the present invention that exists in
 the matrix in a solid solution state and partially becomes carbides,
 ensuring the mechanical properties and corrosion resistance of the present
 invention. The reason why the range of Cr concentration of the present
 invention is more than 16.0% but not more than 25.0% is that the Cr
 concentration of the ferrite phase matrix near Cr carbides becomes not
 more than 12.0% when the Cr concentration of the present invention is not
 more than 16.0%. This is also because ferromagnetism with a maximum
 permeability (.mu.m) of not less than 200 cannot be obtained when
 inversely the Cr concentration of the present invention exceeds 25.0%. The
 more preferred range of Cr concentration is more than 16.0% but not more
 than 20.0%.
 C is an important element that forms carbides and ensures the strength of a
 C-Ni-Cr-Fe-base alloy which is basic to the present invention. Also, C is
 an element that contributes to the stabilization of austenite. When the C
 concentration is less than 0.30%, it becomes difficult to obtain an
 austenite structure stable at a temperature below room temperature, when
 cooled after heating to above the austenite transformation temperature. On
 the other hand, at C concentration exceeding 0.80%, cold working becomes
 difficult because materials become too hard. For this reason, the range of
 C concentration specified in the present invention is 0.30 to 0.80%. The
 more preferred range of C concentration is 0.45 to 0.65%.
 Ni is an element that effectively lowers the Ms point of the non-magnetic
 portion. The reason why the range of Ni concentration of the present
 invention is 0.1 to 4.0% is that the Ms point of the non-magnetic portion
 does not easily decrease at Ni concentrations of less than 0.1%, whereas
 forming is difficult at Ni concentrations exceeding 4.0%, and it becomes
 difficult to obtain good soft magnetic properties.
 N is an element that has the same effect as Ni as an austenite-forming
 element. The reason why the range of N concentration of the present
 invention is 0.01 to 0.06% is that its effect on a decrease in the Ms
 point of the non-magnetic portion is small at N concentrations of less
 than 0.01%, whereas formability deteriorates because of excessive hardness
 at N concentrations exceeding 0.06%. Incidentally, the member of the
 present invention may include at least one kind selected from the group
 consisting of Si, Mn and Al as a deoxidizer in an amount of not more than
 2% in total so far as the magnetic properties are not deteriorated
 thereby.
 Next, reasons for the limited permeability of the present invention are
 described below.
 The member of the present invention is composed of a ferromagnetic portion
 and a non-magnetic portion and the reason why the maximum permeability
 (.mu.m) of the ferromagnetic portion of the present invention is not less
 than 200 is that this range is a necessary characteristic for the member
 of an oil controlling device, which is one of the applications of the
 composite magnetic member of the present invention.
 The reason why the permeability (.mu.) of the non-magnetic portion of the
 present invention is not more than 2 is that magnetic flux flows easily
 when this range is exceeded, with the result that the non-magnetic portion
 does not play its role as such.
 Next, reasons for the limited maximum grain size of carbides are described
 below.
 In the present invention it is preferable that the maximum grain size of
 carbides of ferromagnetic portion be controlled to the range of 0.1 to 20
 .mu.m. This is because the amount of C that exist in the ferrite phase
 matrix in a solid solution state becomes too much in a case of ranges less
 than 0.1 .mu.m, and it is impossible to obtain a maximum permeability
 (.mu.m) of not less than 200, which is necessary for the ferromagnetic
 portion. On the other hand, when the maximum grain size of carbides
 exceeds 20 .mu.m, formability deteriorates and, at the same time, the
 amount of C that exists in the ferrite phase matrix in a solid solution
 state becomes insufficient, with the result that a non-magnetic austenite
 phase cannot be easily obtained even when solution treatment is performed.
 The preferred range of maximum grain size of carbides is 5 to 20 .mu.m.
 When in the present invention, the maximum grain size of carbides of the
 above ferromagnetic portion in particular is controlled to the range of 5
 to 20 .mu.m, it is easy to obtain such magnetic properties as the maximum
 permeability (.mu.m) of the ferromagnetic portion is not less than 230.
 Therefore, this is especially preferred.
 Next, the reason for the limitations regarding the manufacturing process of
 the present invention is described below.
 In the present invention, hot working is an important process for
 controlling the maximum grain size of carbides and the heating temperature
 range is especially preferably from 900 to 1100.degree. C. This is because
 the amount of C that exists in the matrix in a solid solution state is
 small at heating temperatures less than 900.degree. C. and the maximum
 grain size of carbides exceeds 20 .mu.m, whereas the amount of C in a
 solid solution state becomes too much at temperatures exceeding
 1100.degree. and carbides with a maximum grain size of not less than 0.1
 .mu.m cannot be obtained.
 Furthermore, the reason why annealing is performed at a temperature not
 more than the A3 transformation point after hot working is that carbides
 are made to grow, thereby lowering the hardness of the member and
 facilitating the cold working after that. In other words, this is because
 the growth of carbides is not sufficient at temperatures more than the A3
 transformation point and hence the effect of annealing on a decrease in
 hardness is small.
 The A3 transformation point in this invention is a temperature at which the
 ferrite phase begins to be transformed into the austenite phase and this
 temperature varies in dependence upon a chemical composition of the
 material.
 The A3 transformation temperature decreases when the amount of added C, Ni,
 N, etc., which are austenite-forming elements, increases. On the other
 hand, the A3 transformation temperature rises when the amount of added Cr,
 which is a ferrite-forming element, increases. In the range of chemical
 composition of the material specified in the present invention, the A3
 transformation point is in the range of from 650 to 1000.degree. C.
 The reason why cold working is performed is that the strain-induced
 precipitation of carbides occurs by giving strains to the member and it is
 effective to adopt working ratios of from 40 to 90%.
 The reason why annealing is performed again at a temperature not more than
 the A3 transformation point after cold working is that the carbides which
 precipitate during cold working are made to grow, whereby the maximum
 grain size of carbides is stabilized in the range of 0.1 to 20 .mu.m.
 The more preferred range of annealing to be performed after hot working and
 cold working is from the A3 transformation point to a temperature less
 than the A3 transformation point by 200.degree. C.
 The grain size of carbides can be easily controlled to the range of from 5
 to 20 .mu.m by adopting the above method of the present invention.
 In the present invention, as a method of providing a non-magnetic portion
 in a part of the member made to be ferromagnetic by the above process, it
 is preferable that a part of the member be partially heated and subjected
 to solution treatment by high-frequency heating, laser heating, etc. and
 rapidly cooled after that. The solution treatment on this occasion is
 especially effective in the temperature range of from 1050 to 1300.degree.
 C. at which the austenite phase is obtained. Furthermore, as a cooling
 method, it is preferable to perform rapid cooling by water cooling, etc.
 immediately after heating.
 In the present invention, even when the amount of added Cr is increased,
 the above manufacturing process enables the non-magnetic portion to be
 easily formed in the ferromagnetic body without the deterioration of the
 magnetic properties and, at the same time, permits the corrosion
 resistance of the ferromagnetic portion to be improved.
 EXAMPLE 1
 Because the Cr content is important in the present invention, 10-kg ingots
 with various Cr contents were obtained by vacuum melting. These ingots
 were then forged, and hot rolling at 1000.degree. C. was performed to
 produce 4.0-mm thick plates. The material was annealed at 780.degree. C.
 below the A3 transformation temperature, oxide scale was removed, and
 sheets 1.5 mm in thickness were obtained by cold rolling. Table 1 shows
 the chemical compositions of the members tested.
 In the members Nos. 1 to 7, the amounts of added C, Si, Ni, Mn, etc., were
 almost the same and the amount of added Cr was varied. The amount of added
 Cr was lowered in the member No. 6 and increased in the member No. 7.
 The member No. 8 is the composite magnetic member described in
 JP-A-9-157802.
 TABLE 1
 No. C Si Cr Ni Mn Al N Fe Remarks
 1 0.54* 0.19 16.4 0.98 0.51 0.02 0.02 the the
 balance invention
 2 0.54 0.19 17.5 0.97 0.51 0.01 0.03 the the
 balance invention
 3 0.54 0.19 19.2 0.95 0.51 0.03 0.02 the the
 balance invention
 4 0.53 0.20 21.7 0.96 0.51 0.02 0.05 the the
 balance invention
 5 0.54 0.19 24.3 0.95 0.50 0.02 0.05 the the
 balance invention
 6 0.54 0.19 13.9 1.00 0.53 0.02 0.02 the comparative
 balance example
 7 0.54 0.19 25.8 0.98 0.54 0.02 0.04 the comparative
 balance example
 8 0.62 0.22 13.6 3.96 0.50 0.02 0.02 the comparative
 balance example
 *weight %
 This cold-rolled material was annealed at 780.degree. C. below the A3
 transformation point and was made ferromagnetic. A part of the sample
 obtained was heated by high-frequency heating and held at about
 1250.degree.C. for 10 minutes followed by water cooling. A sample which
 became partially non-magnetic was thus obtained. The surface of this
 sample was polished with paper and the salt spray testing was then carried
 out by the method described in JIS Z2371 to evaluate corrosion resistance
 from the rusting condition of sample surface. In the present invention,
 salt was sprayed on the sample for 100 hours as an index of corrosion
 resistance and corrosion resistance was judged by whether or not rust is
 observed on the surface of the member. The result of this judgment is
 shown in Table 2 by the marks .largecircle. and X.
 The Cr concentration of the ferrite phase near the carbides of
 ferromagnetic portion was measured with an X-ray microanalyzer and the
 size of Cr carbides was observed. As a result, it was observed that the CR
 carbides of all members have a maximum grain size of about 7 .mu.m. The
 microstructure of the ferromagnetic portion of the member No. 2 is shown
 in FIG. 1 as an example of observation of carbides.
 Furthermore, the maximum permeability (.mu.m) in portions other than the
 heat-affected zone obtained by high-frequency heating was seeked and the
 magnetic properties of the ferromagnetic portion was evaluated. On the
 other hand, it was ascertained by an X-ray diffraction analysis that a
 phase mainly composed of retained austenite is formed in the non-magnetic
 portion obtained by high-frequency heating and the permeability (.mu.) and
 Ms point of the non-magnetic portion were measured. A permeameter and a
 differential scanning type calorimeter were used for these measurements.
 The results of the measurement are shown in Table 2.
 TABLE 2
 Ferromagnetic Portion
 Cr Non-magnetic portion
 concentration corro- corro-
 of ferrite sion sion
 phase resist- resist- Ms
 No. (wt. %) ance .mu.m ance .mu. (.degree. C.) Remarks
 1 12.2 .largecircle. 680 .largecircle. 1.51 -42 the
 invention
 2 14.7 .largecircle. 537 .largecircle. 1.24 -38 the
 invention
 3 15.8 .largecircle. 416 .largecircle. 1.03 -39 the
 invention
 4 19.5 .largecircle. 325 .largecircle. 1.01 -39 the
 invention
 5 22.1 .largecircle. 211 .largecircle. 1.02 -39 the
 invention
 6 10.5 X 722 .largecircle. 1.40 -48
 comparative
 example
 7 23.7 .largecircle. 192 .largecircle. 1.39 -40
 comparative
 example
 8 10.1 X 260 .largecircle. 1.01 -48
 comparative
 example
 .largecircle.: no occurrence of rust
 X: occurrence of rust
 In the non-magnetic portion, no rust was observed on the sample surface of
 any member, as shown in Table 2. In the samples of the members of the
 present invention with a Cr content of more than 16.0% but not more than
 25.0%, the Cr concentration of the ferromagnetic ferrite phase was kept at
 levels of not less than 12.0%, rusting was not observed as in the
 non-magnetic portion, and good corrosion resistance was shown. It was
 ascertained that excellent ferromagnetic properties with a maximum
 permeability (.mu.m) of more than 200 were obtained in the ferromagnetic
 portion and that the permeability (.mu.) of the non-magnetic portion was
 not more than 2.
 In the samples of the member of the present invention, the permeability
 (.mu.) and Ms point in the non-magnetic portion are almost the same as
 those of the composite magnetic portion disclosed in JP-A-9-157802, i.e.,
 the member No. 8. Thus, it is apparent that in the member of the present
 invention, the characteristics of the non-magnetic portion necessary for a
 composite magnetic member can be maintained. On the other hand, in the
 members No. 6 and No. 8 with a Cr content not exceeding 16.0%, rust is
 observed in the ferromagnetic portion although excellent magnetic
 properties are obtained. Thus, it is apparent that the members No. 6 and
 No. 8 are inferior to the member of the present invention in corrosion
 resistance. It is apparent that in the sample No. 7 with a Cr content
 exceeding 25.0%, a maximum permeability (.mu.m) of 200 cannot be obtained
 in the ferromagnetic portion although excellent corrosion resistance is
 obtained.
 EXAMPLE 2
 The maximum grain size of carbides in the ferromagnetic portion is also
 important in the present invention. Therefore, for the member No. 2 shown
 in Table 1, which is one of the members of the present invention, the hot
 working temperature was varied in the range of from 850 to 1150.degree. C.
 and corrosion resistance and magnetic properties were investigated by
 measuring the maximum grain size of carbides in the ferromagnetic portion.
 After the mirror polishing of the member, chemically etched samples were
 observed under a scanning electron microscope in more than 10 fields of a
 magnification of 3000 and the maximum grain size of carbides observed. The
 member manufacturing process except the hot working temperature and the
 investigation methods of corrosion resistance and magnetic properties are
 the same as with Example 1. The results of the investigation are shown in
 Table 3.
 TABLE 3
 Ferromagnetic Portion Non-magnetic
 Hot Maximum portion
 working grain corro- corro-
 temper- size of sion sion
 ature carbide resist- resist- Ms
 No. (.degree. C.) (.mu.m) ance .mu.m ance .mu. (.degree. C.)
 Remarks
 11 900 12.70 .largecircle. 249 .largecircle. 1.23 -35
 the
 invention
 12 1000 7.10 .largecircle. 237 .largecircle. 1.03 -39
 the
 invention
 13 1050 0.61 .largecircle. 229 .largecircle. 1.16 -37
 the
 invention
 14 1100 0.15 .largecircle. 203 .largecircle. 1.34 -36
 the
 invention
 15 1150 0.06 .largecircle. 188 .largecircle. 1.21 -42
 comparative
 example
 16 850 21.10 .largecircle. 280 .largecircle. 2.24 -18
 comparative
 example
 .largecircle.: No rust occurred
 X: Rust occurred
 As shown in Table 3, it is apparent that the member No. 2 provides
 excellent corrosion resistance at all hot working temperatures.
 Furthermore, in the members Nos. 11 to 14 whose maximum grain size of
 carbides was controlled to the range of from 0.1 to 20 .mu.m, corrosion
 resistance is excellent and the requirements for magnetic properties,
 i.e., a maximum permeability (.mu.m) of not less than 200 in the
 ferromagnetic portion and a permeability (.mu.) of not more than 2 in the
 non-magnetic portion are met. Among others, the members Nos. 11 and 12 in
 which the maximum grain size of carbides was controlled to the range of
 from 5 to 20 .mu.m, have excellent magnetic properties with a maximum
 permeability (.mu.m) of not less than 230 in the ferromagnetic portion.
 On the other hand, in the member No. 15 whose maximum grain size of
 carbides is under 0.1 .mu.m, a maximum permeability (.mu.m) of not less
 than 200 in the ferromagnetic portion cannot be obtained although
 excellent corrosion resistance and non-magnetic properties are obtained.
 Inversely, it is apparent that in the member No. 16 whose maximum grain
 size of carbides exceeds 20 .mu.m, a non-magnetic portion with a
 permeability (.mu.) of not more than 2 cannot be obtained although
 excellent corrosion resistance and ferromagnetic properties are obtained.
 It is also apparent that hot working temperatures between 900 and
 1100.degree. C. are effective in controlling the maximum grain size of
 carbides to the range of from 0.1 to 20 .mu.m.
 According to the present invention, in a single material having a
 ferromagnetic portion and a non-magnetic portion, by increasing the Cr
 content of a C-Ni-Cr-Fe-base alloy to more than 16.0% but not more than
 25.0% and performing hot working and solution treatment in an appropriate
 temperature range, it is possible to dramatically improve the corrosion
 resistance of the ferromagnetic portion composed of ferrite and carbides
 and to obtain a stable non-magnetic portion having the same magnetic
 properties as conventionally. Thus, the present invention provides a
 technique that is indispensable for the application of a composite
 magnetic member to an oil controlling device of an automobile.