JP4544977B2 - Free-cutting soft magnetic stainless steel - Google Patents

Description

  The present invention relates to a free-cutting soft magnetic stainless steel.

  In recent years, free-cutting steel containing S as a machinability improving element has been used as a material in order to improve the productivity of members manufactured through cutting. This is because MnS inclusions are mainly generated, and the machinability is enhanced by the stress concentration effect on the inclusions during chip formation and the lubricating action between the tool and the chips.

US PATENT No. 5769974 JP 2001-140034 A

  Among them, as a soft magnetic property material suitable for magnetic core materials such as solenoids, motors, and sensors of solenoid valves, 430F or the like in which about 0.3% of S is added and MnS-based or CrS-based inclusions are dispersed. The free-cutting stainless steel is conventionally used (see Patent Document 1). However, such a free-cutting soft magnetic stainless steel has a problem that magnetic properties and corrosion resistance are impaired due to the importance of improving machinability by adding S.

  The present invention has been made in view of the above points, and an object thereof is to provide a free-cutting soft magnetic stainless steel provided with excellent machinability while having good magnetic properties and corrosion resistance. To do.

Means for solving the problems / effects of the invention

In order to solve the above problems, the free-cutting soft magnetic stainless steel of the present invention is, in mass%, C: 0.02 to 0.15%, Si: 0.5 to 3.0%, Mn: 2.0% Hereinafter, S: 0.1 to 0.5%, Cr: 10 to 22%, Al: 0.01 to 4.0%, Ti: 0.5 to 1.5% , Ni: 1% or less, Cu: It contains 1% or less, Mo: 2% or less , and the balance consists of Fe and inevitable impurities.

  In the present invention, by including Ti, C, and S in the above composition range, it is possible to generate Ti-based inclusions containing these in the steel structure, thereby imparting good machinability. can do. Moreover, compared with the case where MnS or CrS is used as the main inclusion (see Patent Document 1 above), the machinability can be improved even with a small amount of S, so that the magnetic properties and corrosion resistance can be maintained well. In other words, according to the present invention, it is possible to realize a free-cutting soft magnetic stainless steel having excellent machinability while having good magnetic properties and corrosion resistance.

Here, the Ti-based inclusions can mainly contain inclusions represented by the composition formula Ti ₄ C ₂ S ₂ (hereinafter also referred to as TiCS). In TiCS, part or all of Ti may be replaced with Zr, and part or all of S may be replaced with Se or Te. Identification of Ti inclusions in steel can be performed by X-ray diffraction (for example, diffractometer method) or electron probe microanalysis (EPMA) method. For example, whether or not Ti ₄ C ₂ S ₂ exists in the inclusion can be confirmed by whether or not the corresponding inclusion peak appears in the measurement profile by the X-ray diffractometer method. The inclusion formation region in the structure can be identified by performing a surface analysis with EPMA on the cross-sectional structure of the steel material and comparing the two-dimensional mapping results of the characteristic X-ray intensities of Ti, C, and S.

  The present inventors have previously made an invention in which Ti-based inclusions are dispersedly formed in ferritic stainless steel (see Patent Document 2 above). Such Ti-based inclusions are used in the present invention. When applied to such soft magnetic stainless steel, it was expected that the magnetic properties would be significantly degraded by the components of the Ti inclusions. However, the present inventors have found that the machinability can be imparted without actually causing much deterioration of the magnetic properties, and have arrived at the present invention.

  Hereinafter, the reasons for limiting the composition of each component will be described.

(1) C: 0.02 to 0.15%
C is an element constituting a Ti-based inclusion that improves machinability. Further, by fixing to the Ti-based inclusions, the amount of solid solution C in the matrix phase is reduced, and the soft magnetic properties of the matrix phase are favorably maintained. In order to obtain such an effect, addition of 0.02% or more is necessary. Furthermore, addition of 0.03% or more is preferable. On the other hand, excessive addition increases carbides and solute C and degrades magnetic properties, so the addition is made 0.15% or less. Furthermore, addition of 0.06% or less is preferable.

(2) Si: 0.5 to 3.0%
Si is an element not only effective as a deoxidizer but also effective in improving soft magnetic properties such as reducing the coercive force and reducing loss in the magnetic circuit due to the effect of increasing electrical resistance. In order to obtain such an effect, addition of 0.5% or more is necessary. Furthermore, addition of 0.6% or more is preferable. On the other hand, excessive addition reduces the magnetic flux density, so 3.0% or less is added. Furthermore, addition of 1.5% or less is preferable.

(3) Mn: 2.0% or less Mn is an element that is effective as a deoxidizer in refining, but excessive addition causes a decrease in soft magnetic properties, so 2.0% or less is added. Furthermore, addition of 0.5% or less is preferable. In the present invention, as described above, the formation of MnS is suppressed because Ti inclusions are generated in order to improve machinability.

(4) S: 0.1 to 0.5%
S is an element constituting a Ti-based inclusion that improves machinability. In order to obtain such an effect, addition of 0.1% or more is necessary. Furthermore, addition of 0.15% or more is preferable. On the other hand, excessive addition degrades the soft magnetic properties, so 0.5% or less is added. Furthermore, addition of 0.30% or less is preferable. S can be replaced by Se or Te. Specifically, one or more of S, Se, and Te can be contained so that S + 0.41Se + 0.25Te is 0.1 to 0.5%.

(5) Cr: 10-22%
Cr is an element that contributes to improvement of corrosion resistance as a main alloy element of ferritic stainless steel. In order to obtain such an effect, addition of 10% or more is necessary. Furthermore, addition of 12% or more is preferable. On the other hand, excessive addition inhibits cold forgeability, so it should be 22% or less. Furthermore, addition of 20% or less is preferable.

(6) Al: 0.01 to 4.0%
Al, like Si, is an element effective for reducing the loss of the magnetic circuit due to the effect of increasing the electrical resistance. In order to obtain such an effect, addition of 0.01% is necessary. Furthermore, addition of 0.1% or more is preferable. On the other hand, excessive addition inhibits the magnetic properties, so 4.0% or less is added. Furthermore, addition of 1.0% or less is preferable.

(7) Ti: 0.5 to 1.5%
Ti is an element that constitutes a Ti-based inclusion that improves machinability, and contributes to improvement of magnetic properties by reducing solid solution C and S in the matrix phase. Moreover, since formation of MnS is suppressed, it contributes also to an improvement in corrosion resistance. In order to obtain such an effect, addition of 0.5% or more is necessary. Furthermore, addition of 0.6% or more is preferable. On the other hand, excessive addition reduces the magnetic properties, so 1.5% or less is added. Furthermore, addition of 1.3% or less is preferable. Ti can be replaced by Zr. Specifically, it is possible to contain one or two of Ti and Zr so that Ti + 0.52Zr is 0.5 to 1.5%.

Next, the free-cutting soft magnetic stainless steel of the present invention may further contain any one or more of Nb: 2% or less and V: 1% or less as a steel component. Ni, Cu, Mo, Nb and V are all effective in improving the corrosion resistance, and can be added as necessary. However, excessive addition may impair the soft magnetic properties, so the upper limit is preferably set as described above.

  Next, the free-cutting soft magnetic stainless steel of the present invention can further contain Pb: 0.15% or less as a steel component. Compared with S, Pb can improve machinability, in particular drill drillability, without reducing corrosion resistance and cold workability. Moreover, by adding Pb, Pb precipitates dispersively in the steel almost alone. Such precipitates are effective for improving the machinability. However, excessive addition reduces hot workability, so addition of 0.15% or less is preferable.

Next, the free-cutting soft magnetic stainless steel of the present invention preferably has a solid solution C content in the matrix phase of 0.1 ppm or less. As a result, the soft magnetic properties of the matrix phase are well maintained. Here, the amount of dissolved C in the matrix phase is estimated by a value obtained by subtracting the amount of C that forms inclusions by combining with Ti from the amount of added C. That is, for example, the amount of C fixed as Ti ₄ C ₂ S ₂ is obtained from the amount of Ti added (however, when S is insufficient, it is fixed as TiC), and the remaining C is dissolved in the matrix phase. Suppose that

  Next, the free-cutting soft magnetic stainless steel of the present invention has a coercive force of 1.9 A / cm or less at the same S: about 0.3% as the amount of S generally required for MnS free-cutting materials. With a sufficient S: about 0.2% for obtaining machinability equivalent to or higher than that of a MnS free-cutting material, a coercive force of 1.7 A / cm or less can be obtained.

  Next, in the free-cutting soft magnetic stainless steel of the present invention, it is preferable that the average diameter of the Ti inclusions dispersed and formed in the matrix phase is 2 to 5 μm. Ti-based inclusions are finely dispersed in the matrix phase, thereby improving the machinability of the steel. In order to enhance such an effect, the diameter of the Ti inclusions (for example, the maximum interval when a circumscribed parallel line is drawn on the outline of the Ti inclusions observed in the polished cross-sectional structure of the steel material can be expressed. ) Is preferably in the above range. Moreover, it is preferable that the area ratio in the structure | tissue of Ti type inclusions is about 0.1 to 0.6%, for example.

Hereinafter, tests conducted for confirming the effects of the present invention will be described.
Based on the chemical composition shown in Table 1, an ingot of 100 kg was made in an Ar atmosphere using a vacuum induction melting furnace, and after making 80 square steel pieces by hot forging, the wire diameter was 9.5 mm by hot rolling. As a wire rod, a steel bar having a diameter of 8 mm was obtained by annealing at 750 ° C. for 3 hours, cold drawing, annealing at 850 ° C. for 3 hours, straightening, and centerless polishing.
In addition, in the composition of the comparative steel in Table 1, those that deviate from the composition range defined in the present invention include a downward arrow (↓) when below the lower limit, and an upward arrow (↑) when exceeding the upper limit. ) Is attached.

  Next, the magnetic properties, machinability, and inclusion diameter described below were evaluated for the inventive steel and the comparative steel. The evaluation results are shown in Table 1 and FIGS. In the figure, the “invention steel 12% Cr system” means the invention steel (invention steel 2, 3, 6, 7, 10) having a Cr content of about 12% and “comparative steel 12% Cr system”. Is a comparative steel having a Cr content of around 12% (comparative steels 13, 14, 15, 18, 19, 21), and “invented steel 17% Cr-based” is an invention steel having a Cr content of around 17% (invention) Steel “1,4,5,8,11” and “Comparison Steel 17% Cr” are comparative steels with a Cr content of around 17% (Comparative Steels 12, 16, 17, 20, 22) “Steel MnS” refers to comparative steels (comparative steels 23 and 24) mainly having MnS inclusions.

<Magnetic properties>
The magnetic properties were measured with a B-H loop tracer after cutting each into a ring of 8 mm outer diameter × 3.6 mm inner diameter × 5 mm thickness, annealing at 950 ° C. × 3 hours in a vacuum furnace.

<Machinability>
Machinability is an automatic board, SKH51 drill with a nominal diameter of 4 mm and tip angle of 118 degrees, using water-soluble lubricant, after cutting 1500 times at a cutting speed of 15 m / min, feed 0.07 mm / rev, depth 10 mm. The amount of cutting blade wear was measured.

<Inclusion diameter>
The inclusion diameters were each mirror-polished and the maximum and average values in the field of view were taken for the major and minor diameters of the inclusions observed. At this time, the slightly observed square TiN (EPMA, confirmed by the characteristic X-ray map) is excluded from the observation target.

  1 shows the coercive force, FIG. 2 shows the maximum magnetic permeability, and FIG. 3 shows the evaluation results of the S concentration dependence of the magnetic flux density. According to the figure, the dependence becomes mild when the S concentration exceeds about 0.1%. In addition, according to the evaluation result of cutting edge wear described later (FIG. 4), since the machinability is most excellent when the S concentration is around 0.2%, the invention steel generally has S of about 0.3%. It is possible to make the magnetic properties better than the MnS free-cutting material to which is added.

  FIG. 4 shows an evaluation result of S concentration dependency of drill cutting edge wear. The machinability is good when the S concentration is 0.1 to 0.5%, and is particularly excellent around 0.2%.

  FIG. 5 shows the observation result of the S concentration dependence of the inclusion diameter. As described above, the magnetic properties (FIGS. 1 to 3) have a moderate dependency when the S concentration is around 0.1%, but this is an average when the S concentration is around 0.1% as shown in FIG. It is presumed that the domain wall pinning effect is weakened when the diameter exceeds 2 μm. In general, it is said that the pinning effect is strongest when the domain wall thickness is the same as the particle diameter.

  From the above, it can be seen that, in the steel of the present invention, Ti inclusions are generated in the steel structure by setting the above composition range, and as a result, have good magnetic properties and machinability.

The figure showing S concentration dependence of holding power The figure showing the S concentration dependence of the maximum permeability The figure showing the S concentration dependence of magnetic flux density Diagram showing S concentration dependence of drill cutting edge wear The figure showing S concentration dependence of inclusion particle size

Claims (5)

  In mass%, C: 0.02-0.15%, Si: 0.5-3.0%, Mn: 2.0% or less, S: 0.1-0.5%, Cr: 10-22 %, Al: 0.01 to 4.0%, Ti: 0.5 to 1.5%, Ni: 1% or less, Cu: 1% or less, Mo: 2% or less, the balance being Fe and inevitable Free-cutting soft magnetic stainless steel, characterized by comprising mechanical impurities.   The free-cutting soft magnetic stainless steel according to claim 1, further comprising one or more of Nb: 2% or less and V: 1% or less as a steel component. In place of S: 0.1 to 0.5%, one or more of S, Se and Te are used so that S + 0.41Se + 0.25Te is (0.1) to (0.5)%. The free-cutting soft magnetic stainless steel according to claim 1 , wherein the free-cutting soft magnetic stainless steel is contained. In place of Ti: 0.5 to 1.5%, Ti + 0.52Zr contains one or two of Ti and Zr so that (0.5) to (1.5)%. The free-cutting soft magnetic stainless steel according to any one of claims 1 to 3 . Inclusions represented by the composition formula Ti ₄ C ₂ S ₂ dispersedly formed in the matrix phase, or part or all of Ti in the inclusions is Zr, and part or all of S is Se or Te. The free-cutting soft magnetic stainless steel according to any one of claims 1 to 4 , wherein the inclusions having an average diameter of 2 to 5 µm are substituted.

Images