JP2012153911A - Soft magnetic steel component for direct current - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a soft magnetic steel component for direct current having high strength without deteriorating magnetic characteristic in a direct current field and a soft magnetic steel component for direct current having excellent cold forging performance when forming in component shape.SOLUTION: In the steel component, a chemical component composition of a matrix phase comprises C: 0.002-0.20% (hereinafter% means mass%) Si: 1.2% or less (not including 0%), Mn: 0.05-2.6%, P: 0.050% or less (not including 0%), S: 0.05% or less (not including 0%), Cr: 4% or less (not including 0%), Al: 0.002-2.2%, N:0.01% or less (not including 0%), O: 0.03% or less (not including 0%), and a balance: iron and inevitable impurities. In the soft magnetic steel component for direct current, an Al diffuse layer containing 10-30 mass% of Al is formed on the surface of the soft magnetic steel component and the thickness of the Al diffuse layer is 10-80 μm.

Description

  The present invention relates to a soft magnetic steel part, and more particularly to a DC soft magnetic steel part used in a DC magnetic field.

  Many electrical components are used in automobiles to improve comfort. Steel parts (for example, electronic control parts and electromagnetic control parts) that make up magnetic circuits in automotive electrical parts and the like are easily magnetized in a low external magnetic field with a DC magnetic field (hereinafter referred to as DC magnetic characteristics). In addition, there is a demand for characteristics such as low coercive force and low power consumption. As a material of the steel part, a soft magnetic steel material in which the magnetic flux density inside the part easily responds to an external magnetic field is generally used. For example, a steel material having a C content of about 0.01% by mass or less is usually used.

  In addition, from the viewpoint of protecting the global environment, there is a demand for lower fuel consumption of automobiles, and studies have been made on reducing fuel consumption by increasing the strength, reducing the size and weight of the steel parts. In order to increase the strength of steel parts, it is effective to positively add C to the soft magnetic steel material. As the soft magnetic steel material, for example, a steel material having a C content of about 0.01% by mass is usually used. . However, it is known that when C is excessively contained in excess of 0.01% by mass, the direct current magnetic characteristics of the steel material are significantly lowered, the coercive force is increased, and the resistance of the magnetic circuit is increased. Therefore, it is required to increase the strength of steel parts while satisfying DC magnetic characteristics, low coercive force, and low power consumption.

  A technique for improving both the magnetic properties and strength of steel parts is proposed in Patent Document 1. This document discloses a technique for improving magnetic characteristics and strength by using Cu age hardening by adding Cu in the range of 0.3 to 2.0%. However, as a result of studies by the present inventor, the level of magnetic properties and strength improved by this technique is insufficient, and further improvement is required.

  As a technique for improving the magnetic properties of the core soft magnetic steel sheet, Patent Document 2 proposes a technique for increasing the {222} plane integration degree of the αFe phase near the steel sheet surface to 55% or more. In this document, a second phase composed of a metal mainly composed of one or both of Al and Si is attached to the surface of a base steel plate, cold-rolled, and then recrystallized by heat treatment {222} Increasing the degree of surface integration is disclosed. However, the magnetic characteristics improved in this document are characteristics that are easily magnetized by an alternating magnetic field, and are not direct current magnetic characteristics. Further, the strength and cold workability of the soft magnetic steel sheet were not considered.

  By the way, the soft magnetic steel material is also required to have good cold forgeability in order to be formed into a part shape. As cold forgeability, it is necessary that the deformation resistance is small and the deformability is high. Since the load during forging can be reduced by reducing the deformation resistance, the life of the mold used in cold forging can be improved. Further, by increasing the deformability and making it difficult for cracks to occur even when cold forged, it is possible to reduce the size of the soft magnetic steel part and to complicate the part shape.

  Patent Documents 3 and 4 are known as techniques for improving the cold workability of steel. Among these, in Patent Document 3, by adding B and Zr to steel having a mixed structure of ferrite and pearlite, the strengthening due to refinement of ferrite crystal grains is suppressed, the tensile strength is reduced, and deformation at room temperature is performed. A technique for improving cold workability by reducing an increase in resistance is disclosed. In Patent Document 4, the ferrite grain size number in the ferrite structure in the range from the center of the rolled material to the diameter / 8 and the ferrite grain size number in the ferrite structure in the outermost layer of the rolled material are respectively controlled within predetermined ranges. Thus, a technique for improving the cold workability by suppressing the strengthening due to the refinement of ferrite crystal grains to reduce the tensile strength during cold working and reducing the deformation resistance at room temperature is disclosed. However, the steels targeted in these documents are assumed to be used as machine parts such as bolts and nuts in the state of being cold forged or cut after cold forging, and are not soft magnetic steel materials. Therefore, no consideration was given to the DC magnetic characteristics.

Patent Document 5 discloses a magnetic circuit member for an electromagnetic valve for alternating current that has high electrical resistance, has excellent high-speed response, and can be mass-produced to reduce product cost. . In this document, it is possible to improve the machinability and cold forgeability by using electromagnetic soft iron or low carbon steel as the base material of the magnetic circuit member, and the electrical resistance is increased by containing Al in the magnetic circuit member. It is described that eddy current loss can be reduced. As a method for incorporating Al in the magnetic circuit member, a magnetic circuit member made of electromagnetic soft iron is embedded in a mixture of Al powder and Al ₂ O ₃ powder and NH ₄ Cl added, and 900 ° C. in a hydrogen stream. After performing the heat treatment for 3 hours, a method of further performing a diffusion treatment at 1000 ° C. for 20 hours in an Ar gas stream is adopted. It is described that an Al—Fe alloy having a concentration gradient of about 6% Al—Fe in the surface layer and 1% Al—Fe in the center is obtained.

JP 2007-46076 A JP 2009-256758 A JP 2001-303189 A JP 2001-342544 A JP-A-63-3318380

  As disclosed in Patent Document 5, in the method of embedding a magnetic circuit member in the mixed powder and diffusing and infiltrating Al into the surface of the magnetic circuit member, the film treatment and the diffusion treatment are performed at the same time. It was difficult to manage the Al concentration. Further, Patent Document 5 aims at improving the AC magnetic characteristics, and does not consider the improvement of the DC magnetic characteristics. With the Al diffusion amount described in Patent Document 5, the DC magnetic characteristics are improved. could not. Furthermore, in the said patent document 5, intensity | strength was not considered at all.

  This invention is made | formed in view of such a condition, It is providing the high intensity | strength soft magnetic steel part for direct current | flow without deteriorating the magnetic characteristic in a direct current magnetic field. Another object of the present invention is to provide a soft magnetic steel part for direct current with good cold forgeability when formed into a part shape.

  The direct-current soft magnetic steel part according to the present invention that has solved the above problems has a chemical composition composition of the parent phase of C: 0.002 to 0.20% (meaning mass%, the same applies hereinafter). Si: 1.2% or less (excluding 0%), Mn: 0.05 to 2.6%, P: 0.050% or less (not including 0%), S: 0.05% or less (0 %), Cr: 4% or less (not including 0%), Al: 0.002 to 2.2%, N: 0.01% or less (not including 0%), O: 0.03 % Or less (excluding 0%), balance: steel and steel parts that are inevitable impurities. And the Al diffusion layer containing 10-30 mass% of Al is formed in the surface layer part, and it has the summary in the point whose thickness of the said Al diffusion layer is 10-80 micrometers.

It is preferable that the chemical component composition further satisfies the following formula (1). In the following formula (1), [] indicates the content (% by mass) of each element.
13 × [C] + 2 × [Si] + [Mn] + [Cr] / 5 + [Al] ≦ 2.8 (1)
The steel part may further contain Cu: 0.5% or less (not including 0%) and / or Ni: 0.5% or less (not including 0%) as other elements. .

  According to the present invention, since the Al diffusion layer containing Al having a higher concentration than the Al amount contained in the base material is formed on the surface layer portion of the soft magnetic steel part with a predetermined thickness, the magnetic characteristics in a DC magnetic field The surface hardness can be increased and the strength of soft magnetic steel parts can be increased. Moreover, in the present invention, the deformation resistance of the steel material is reduced by appropriately adjusting the relationship among the amounts of C, Si, Mn, Cr and Al contained as alloy elements in the steel material as the material of the soft magnetic steel part. Since the performance can be improved, the cold forgeability when forming into a part shape can also be improved.

FIG. 1 is a graph showing the relationship between the thickness of the Al diffusion layer and the surface hardness of the test pieces used in the examples. FIG. 2 is a graph showing the relationship between the thickness of the Al diffusion layer and the ratio of magnetic flux density for the test pieces used in the examples. FIG. 3 is a graph showing the relationship between the value (Z value) on the left side of the equation (1) defined in the present invention and the deformation resistance for the test pieces used in the examples.

In order to increase the strength of soft magnetic steel parts used in a DC magnetic field, it is effective to add C, Si, Al or the like to the soft magnetic steel material and dissolve it in the steel to strengthen the solution. However, when these elements are dissolved to increase the strength of the entire steel material, the DC magnetic characteristics tend to deteriorate. Therefore, the present inventor can increase the strength of the soft magnetic steel part by increasing the hardness of the surface of the soft magnetic steel part, and further, since the hardness inside the soft magnetic steel part is not increased, the deterioration of the DC magnetic characteristics can be prevented. I thought that it might be, and have been examining it repeatedly. as a result,
(1) Utilizing Al among solid solution elements and forming Fe-Al based solid solutions and compounds on the surface layer of steel parts can increase the hardness of the steel part surface and increase the strength of the steel parts. What you can do,
(2) Specifically, if an Al diffusion layer containing 10 to 30% by mass of Al is formed on the surface layer part of the steel part with a thickness of 10 to 80 μm, the above-described strength improvement effect is exhibited.
(3) Since the Al diffusion layer is formed only in the surface layer portion of the steel part, a decrease in the magnetic moment inside the steel material is prevented, and the direct current magnetic characteristics of the entire steel part are hardly deteriorated.
(4) Since the Al easily diffuses into the steel material by heat treatment, the Al diffusion layer can be easily obtained by, for example, heat-treating the steel material having an Al film on the surface and processed into a part shape. That can be formed,
(5) Since the steel part can be strengthened by providing the Al diffusion layer, the amount of alloying elements (for example, C, Si, Mn, etc.) added to increase the strength of the steel material can be reduced, and the deformation resistance of the steel material It has been found that the cold forgeability when machining into a part shape can be improved.

  Hereinafter, the soft magnetic steel part of the present invention will be described in detail.

  In the soft magnetic steel part of the present invention, an Al diffusion layer containing 10 to 30% by mass of Al is formed in the surface layer portion. In the present invention, a region containing 10 to 30% by mass of Al is defined as an Al diffusion layer, and the surface hardness of the soft magnetic steel part is increased by forming this Al diffusion layer on the surface layer of the steel part. When Al is less than 10% by mass, the hardness of the surface layer portion cannot be increased. On the other hand, if the Al content exceeds 30% by mass, the ductility is lowered, so that cracks and the like are likely to occur.

  The said surface layer part means the surface vicinity including the outermost surface among soft-magnetic steel components, and specifically points out the area | region from the outermost surface to a depth of about 200 micrometers position.

  The Al diffusion layer preferably has an Al content decreasing from the outermost surface side toward the center. The DC magnetic characteristics can be effectively improved by inclining the Al concentration in the surface layer portion.

  The thickness of the Al diffusion layer is 10 to 80 μm. The hardness of the surface layer portion can be increased by setting the thickness of the Al diffusion layer to 10 μm or more. The thickness of the Al diffusion layer is preferably 15 μm or more, more preferably 20 μm or more. However, if the thickness of the Al diffusion layer becomes too large, the DC magnetic characteristics are deteriorated. Therefore, the thickness of the Al diffusion layer is 80 μm or less, preferably 70 μm or less, more preferably 60 μm or less.

  The Al concentration in the surface layer is the same distance in the depth direction in the region from the outermost surface of the steel part to the depth of 200 μm, for example, with an electron probe X-ray micro analyzer (EPMA). What is necessary is just to measure by (for example, several micrometer interval). The thickness of the Al diffusion layer can be calculated based on the measurement result by EPMA.

  Next, the component composition of the steel material (matrix) that is the material of the soft magnetic steel part according to the present invention will be described. Steel materials used in the present invention are: C: 0.002 to 0.20%, Si: 1.2% or less (excluding 0%), Mn: 0.05 to 2.6%, P: 0.050% Or less (excluding 0%), S: 0.05% or less (not including 0%), Cr: 4% or less (not including 0%), Al: 0.002 to 2.2%, N: It contains 0.01% or less (not including 0%), O: 0.03% or less (not including 0%), and the balance: iron and inevitable impurities. The reason for specifying such a range is as follows.

  C is an important element for ensuring the balance between strength and ductility of the steel material. However, if C exceeds 0.20%, the strength becomes too high, the deformation resistance increases, and the cold forgeability deteriorates. Further, due to C dissolved in the steel, strain aging occurs at the time of forming the part, and the DC magnetic characteristics are also deteriorated. Therefore, C is 0.20% or less, preferably 0.1% or less, more preferably 0.08% or less. The smaller the C, the lower the strength and the better the ductility, so the cold forgeability becomes better. However, if the amount of C is reduced too much, the strength of the steel part is reduced too much. Therefore, C is 0.002% or more, preferably 0.003% or more.

  Si is an element used as a deoxidizer at the time of melting, and is also an element that acts to improve magnetic properties. However, if it exceeds 1.2%, the deformation resistance increases and the cold forgeability deteriorates. Therefore, Si is 1.2% or less, preferably 1.0% or less, more preferably 0.8% or less. In particular, in order to reduce the deformation resistance of the steel material and improve the cold forgeability, Si is preferably 0.7% or less, more preferably 0.5% or less, still more preferably 0.1% or less. It is.

  Mn is an element used as a deoxidizer at the time of melting, and has an action of binding to S in steel and suppressing embrittlement due to S. Therefore, Mn is 0.05% or more, preferably 0.1% or more, more preferably 0.15% or more. However, if Mn exceeds 2.6%, the deformation resistance becomes too large and the cold forgeability deteriorates. Further, when Mn is excessive, the magnetic moment is lowered and the DC magnetic characteristics are deteriorated. Therefore, Mn is 2.6% or less, preferably 2% or less, more preferably 1% or less, and still more preferably 0.5% or less.

  P segregates at the grain boundaries to lower the deformability and cause cracks during cold forging. Moreover, when it contains excessively, a DC magnetic characteristic will also deteriorate. Therefore, P is 0.050% or less, preferably 0.02% or less, and more preferably 0.01% or less. It is desirable to reduce P as much as possible.

  S combines with Mn or the like to form a sulfide, and the sulfide is precipitated at the grain boundary, so that the deformability is lowered and the cold forgeability is deteriorated. Therefore, S is 0.05% or less, preferably 0.02% or less, more preferably 0.015% or less.

  Cr is an element that acts to increase the strength of steel parts. However, if it exceeds 4%, the hardness of the ferrite structure increases too much due to the solid solution of Cr, so the deformability decreases and cracking occurs during cold forging. To do. Therefore, Cr is 4% or less, preferably 2% or less, more preferably 1% or less, and still more preferably 0.5% or less.

  Al is an element having an action of fixing the solid solution N in the steel as AlN and reducing the deformation resistance to improve the cold forgeability. Therefore, Al is contained in an amount of 0.002% or more, preferably 0.003% or more. However, if the content exceeds 2.2%, the deformation resistance of the steel material becomes too large and the cold forgeability deteriorates. Therefore, Al is 2.2% or less, preferably 1% or less, more preferably 0.8% or less, still more preferably 0.5% or less, and particularly preferably 0.1% or less.

  N is an element that age hardens the steel, and if it exceeds 0.01%, the deformability of the steel decreases and causes cracking during cold forging. Therefore, N is 0.01% or less, preferably 0.008% or less, more preferably 0.005% or less. It is desirable to reduce N as much as possible.

  O (oxygen) is an element that forms an oxide in the steel, reduces the deformability of the steel material, and generates cracks during cold forging. Moreover, the oxide formed in steel becomes a cause of deteriorating DC magnetic characteristics. Therefore, O is 0.03% or less, preferably 0.01% or less, more preferably 0.005% or less. It is desirable to reduce O as much as possible.

  The balance of the steel material is iron and inevitable impurities. As inevitable impurities, elements mixed in depending on the situation of raw materials, materials, manufacturing equipment, etc. are allowed.

  The steel material may further contain Cu: 0.5% or less (not including 0%) and / or Ni: 0.5% or less (not including 0%) as other elements. Cu and Ni are inevitably mixed elements. However, if Cu or Ni is contained in excess of 0.5%, the magnetic moment is lowered and the DC magnetic characteristics are deteriorated. Therefore, the preferable upper limit of Cu and Ni is set to 0.5%. Cu is more preferably 0.1% or less, and Ni is more preferably 0.1% or less.

It is recommended that the steel material used in the present invention satisfies the following formula (1) as well as the chemical component composition satisfying the above range. The following formula (1) shows a relational expression defined based on the degree of influence of each element by extracting elements that affect the deformation resistance of the steel material from among the alloy elements contained in the steel material. When the value on the left side of the following formula (1) is the Z value, the deformation resistance of the steel material can be reduced by suppressing the Z value to preferably 2.8 or less, and the cold forgeability can be improved. In addition, in following formula (1), [] has shown content (mass%) of each element.
13 × [C] + 2 × [Si] + [Mn] + [Cr] / 5 + [Al] ≦ 2.8 (1)

  In other words, C, Si, Mn, Cr, and Al are elements that increase the strength of steel materials and contribute to increasing the strength of soft magnetic steel parts, but all of these elements are dissolved or precipitated in steel. It also acts to increase the strength of steel materials by forming objects and to increase the deformation resistance of steel materials. Therefore, the tendency for cold forgeability to deteriorate was recognized when content increased.

  On the other hand, in the present invention, as described above, the hardness of the surface layer portion is increased by forming an Al diffusion layer in the surface layer portion of the soft magnetic steel part, and the strength of the soft magnetic steel part is increased. , Mn, Cr, Al content can be reduced. Therefore, in this invention, the cold forgeability of steel materials can further be improved by restraining the Z value calculated based on the content of these elements to preferably 2.8 or less. The Z value is more preferably 2.5 or less, still more preferably 2 or less, and particularly preferably 1 or less.

  Next, a manufacturing method that can be suitably employed in manufacturing the soft magnetic steel part of the present invention will be described.

  The soft magnetic steel part can be manufactured by heat-treating a steel material having an Al film on the surface. That is, by forming an Al film on the surface of the steel material and heat-treating it, Al can be uniformly diffused and penetrated from the surface of the steel material, and the Al diffusion layer can be formed on the surface layer portion of the steel part. In the present invention, since Al is uniformly diffused and penetrated from the surface of the steel material, it is possible to prevent Al from locally concentrating on the surface layer portion of the soft magnetic steel part. Further, as described above, the steel material used in the present invention preferably suppresses the Z value calculated based on the alloy element amount of C, Si, Mn, Cr, and Al below a predetermined value. The effect that the cold forgeability can be further improved can be exhibited.

  The steel material before the heat treatment only needs to have an Al film on the surface and processed into a part shape, and the order of the process of forming the Al film on the surface of the steel material and the process of processing the steel material into a part shape is particularly limited. Not. That is, the Al film may be formed after the steel material is processed into a part shape, or the Al film may be formed after forming the Al film on the steel material. The processing to the part shape may be performed by cold forging, for example.

  The Al film may be a film containing Al element (Al-containing film), and the method for forming the Al film is not particularly limited. For example, chemical vapor deposition (CVD), physical vapor deposition ( PVD) method, plating method and the like. Examples of the plating method include a molten Al plating method and an electric Al plating method. Among these, it is preferable to manufacture by the hot Al plating method.

The above heat treatment for uniformly diffusing and infiltrating Al from the steel material surface is carried out so that the thickness of the Al diffusion layer is formed in the surface layer portion of the steel part by 10 to 80 μm. What is necessary is just to adjust Al coating amount etc. For example, it is preferable to adjust the heating temperature to 750 ° C. or more, the heating time to 1 hour or more, and the Al coating amount to be adjusted in the range of 30 g / m ² or more.

  The heating temperature is more preferably 800 ° C. or higher, and further preferably 850 ° C. or higher. However, if the heating temperature is too high, Al diffuses too deep into the steel material, making it difficult to form an Al diffusion layer with a desired thickness on the surface layer of the steel part. Therefore, the upper limit of the heating temperature is preferably 1000 ° C.

  The heating time is more preferably 2 hours or more, and further preferably 3 hours or more. However, if the heating time is too long, Al diffuses too deep into the steel material, making it difficult to form an Al diffusion layer with a desired thickness on the surface layer of the steel part. Therefore, the upper limit of the heating time is preferably 10 hours.

The adhesion amount of the Al film is more preferably 40 g / m ² or more, and further preferably 50 g / m ² or more. However, if the amount of Al coating deposited is too large, the Al diffusion layer becomes too thick and the DC magnetic characteristics deteriorate, so the upper limit is preferably set to 110 g / m ² , for example.

  What is necessary is just to let the temperature increase rate when heating to the said heating temperature be 100-400 degreeC / hour, for example. Moreover, what is necessary is just to let the temperature fall rate when cooling to room temperature after heat processing be 100-400 degreeC / hour, for example.

  The thus obtained soft magnetic steel parts according to the present invention are used, for example, as electrical cores, electromagnetic coils, and alternator iron cores that are driven through magnetic force among steel parts mounted on automobiles and industrial machines.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  In Experimental Example 1 below, the DC magnetic characteristics of the soft magnetic steel part were evaluated, and in Experimental Example 2 below, the cold forgeability of the steel material used as the material of the soft magnetic steel part was evaluated.

[Experimental Example 1]
Steel containing chemical components shown in Table 1 below (with the balance being iron and inevitable impurities) was vacuum-melted to prepare 150 kg of melted material. In Table 1 below, the value (Z value) on the left side of the above formula (1) is calculated and shown as a reference value.

  The obtained molten material is forged to produce a steel material having a diameter of 40 mm, a ring-shaped test piece is cut out from this steel material, and the DC magnetic characteristics when an Al diffusion layer is provided on the test piece are as follows. evaluated. Specifically, a ring-shaped test piece having an outer diameter of 38 mm, an inner diameter of 30 mm, and a thickness of 4 mm is cut out from the steel material (diameter 40 mm), an Al film is formed on the surface of the test piece by a molten Al plating method, and then heat-treated for testing. Al on one surface was diffused and penetrated into the test piece. In the comparative example, Al was diffused and penetrated from the surface of the test piece to the inside by a powder coating method. The detailed conditions are as follows.

In the hot-dip Al plating method, the ring-shaped test piece is used as a pure Al plating bath or a hot Al plating bath containing about 10% by mass of Si, and the bath temperature is set to 650 to 700 ° C. and immersed for 1 to 10 minutes. A film was formed. The adhesion amount of the Al film formed on the surface of the ring-shaped test piece was measured with EPMA (“JXA-8900RL (device name)” manufactured by JEOL Ltd.), and the adhesion amount of the Al film (g / m ² ) is shown in Table 2 below. Indicates. After the formation of the Al film, it was heated to a temperature shown in the following Table 2 at a heating rate of 300 ° C./hour in a hydrogen reducing atmosphere, and kept at this temperature for the time shown in the following Table 2 to heat-treat, and Al on the test piece surface was The sample was diffused and penetrated into the specimen. After the heat treatment, it was cooled to room temperature at a temperature drop rate of 300 ° C./hour.

In the powder coating method, a mixed powder obtained by mixing equal amounts of Al powder (500 g) and Al ₂ O ₃ powder (500 g) and 10 g of NH ₄ Cl is placed in a slushless case, and the above ring-shaped test piece is contained therein. After heating at a temperature shown in the following Table 2 in a hydrogen reduction atmosphere, heat treatment was performed at this temperature for the time shown in the following Table 2 to diffuse Al infiltrate from the surface of the test piece to the inside.

  In Table 2 below, No. 1 is a comparative example in which the formation of the Al film and the heat treatment are not performed.

  Next, the Al concentration in the surface layer portion of the ring-shaped test piece obtained by heat treatment was measured using EPMA (“JXA-8900RL (device name)” manufactured by JEOL Ltd.). In the measurement, the area from the Al film formation surface to a depth of 200 μm on the cut surface exposed by cutting so as to be parallel to the thickness direction (axial direction) of the ring-shaped test piece is the beam diameter of EPMA. The thickness of the Al diffusion layer containing 10 to 30% by mass of Al was measured at an interval of 1 μm. The measurement results are shown in Table 2 below. Note that the Al film formed on the surface of the ring-shaped test piece was diffused into the ring-shaped test piece by heat treatment. In addition, the surface layer portion of the ring-shaped test piece obtained by heat treatment had the largest amount of Al on the outermost surface, and the Al amount decreased toward the center portion, indicating that it had a gradient composition.

  Next, the surface hardness of the ring-shaped test piece obtained by heat treatment was measured.

<Measurement of surface hardness>
The surface hardness was measured on the basis of a micro Vickers hardness test specified in JIS Z2244 using a micro Vickers hardness tester with a test load of 0.3 N (30 gf) and a Vickers hardness (Hv). The measurement position was measured at a position where the depth from the Al film forming surface was 15 μm on the cut surface exposed by cutting so as to be parallel to the thickness direction (axial direction) of the ring-shaped test piece. The measurement place was made into four places, and the result was averaged. In addition, No. shown in Table 2 below. The Vickers hardness (Hv) of the test piece 1 was also measured under the same conditions. The measurement results are shown in Table 2 below. In addition, No. in Table 2 below. The amount of increase in Vickers hardness with respect to 1 (no Al diffusion layer) was calculated, and the results are also shown in Table 2 below. The case where the increase amount was Hv240 or higher was evaluated as high hardness (passed. Evaluation ○), and the case where it was less than Hv240 was evaluated as low hardness (failed. Evaluation x). The amount of increase and the evaluation results are shown in Table 2 below.

  FIG. 1 shows the relationship between the thickness of the Al diffusion layer and the surface hardness. In addition, in FIG. 1, it plotted about the result of the test piece (No. 2-10) which formed Al membrane | film | coat by the hot Al plating method.

  Next, the DC magnetic characteristics of the ring-shaped test piece obtained by heat treatment were evaluated.

<Evaluation of DC magnetic characteristics>
The DC magnetic characteristics were evaluated based on the magnetic flux density of the test piece. The method for measuring the magnetic flux density is as follows. A primary coil for magnetic field application and a secondary coil for magnetic flux detection are wound around the ring-shaped test piece, and an automatic magnetization measuring device [manufactured by Riken Denshi Co., Ltd., DC magnetization BH characteristic automatic recording device (BHS- 40)] was used to measure the BH curve to determine the magnetic flux density. In addition, No. shown in Table 2 below. No. 1 test piece (ring-shaped test piece cut out from the above steel type α) was subjected to magnetic annealing at 850 ° C. for 3 hours, and then Al film formation and heat treatment were not performed. 1 and a secondary coil for magnetic flux detection were wound (hereinafter sometimes referred to as an untreated product), and the BH curve was measured to determine the magnetic flux density. The measurement results are shown in Table 2 below.

  The ratio of the magnetic flux density of each ring-shaped test piece to the magnetic flux density of the untreated product (No. 1 in Table 2) (ring-shaped test piece / untreated product) was calculated, and the results are shown in Table 2 below. Further, based on the value of the magnetic flux density ratio, the DC magnetic characteristics are evaluated according to the following criteria, and the evaluation results are also shown in Table 2 below.

<Evaluation criteria>
○ (Pass): Ratio of magnetic flux density is 0.94 or more x (Fail): Ratio of magnetic flux density is less than 0.94 FIG. 2 shows the relationship between the thickness of the Al diffusion layer and the ratio of magnetic flux density. . In FIG. 2, it plotted about the result of the test piece (No. 2-10) which formed Al membrane | film | coat by the hot-dip Al plating method.

  It can be considered as follows from Table 2 below. No. No. 1 is a comparative example (untreated product) in which no Al film was formed and no heat treatment was performed, and since the Al diffusion layer was not formed, the surface hardness was as low as Hv80. No. 2 and 3 are examples in which the Al diffusion layer is too thin, and the surface hardness could not be improved. No. No. 10 is an example in which the Al diffusion layer is too thick, the magnetic flux density is small, and the DC magnetic characteristics are deteriorated.

  No. Nos. 4 to 9 are examples that satisfy the requirements defined in the present invention, and an Al diffusion layer having an appropriate thickness can be formed on the surface layer portion of the test piece. Compared with 1), the surface hardness can be increased without substantially deteriorating the DC magnetic characteristics.

  No. 11 to 13 are comparative examples in which an Al film formed by a powder coating method was heat-treated and diffused into the test piece. Of these, No. 11 and 12 are examples in which the Al diffusion layer is too thick, the magnetic flux density is small, and the DC magnetic characteristics are deteriorated. No. In Nos. 11 and 12, since Al diffuses and penetrates to the inside and the Al concentration in the surface layer portion is low, the surface hardness is also reduced. No. In No. 13, an Al diffusion layer containing 10 to 30% by mass of Al was not formed, and a region having an Al concentration of less than 10% by mass was formed over a wide range in the surface layer portion of the test piece. As a result of the diffusion of Al over a wide range, the DC magnetic characteristics deteriorated. Further, since the Al diffusion layer was not formed, the surface hardness could not be improved.

  No. When comparing 2 to 7, it can be seen that the Al diffusion layer becomes thicker as the heat treatment temperature is increased when the adhesion amount of the Al film is the same and the retention time is the same among the heat treatment conditions (holding for 3 hours).

  No. Comparing 7 to 10, it can be seen that when the heat treatment conditions are the same (held at 900 ° C. for 3 hours), the Al diffusion layer becomes thicker as the adhesion amount of the Al film increases.

  1 and 2 can be considered as follows. As is apparent from FIG. 1, it can be seen that the surface hardness tends to increase rapidly when the thickness of the Al diffusion layer is 30 μm or more. However, as is clear from FIG. 2, it can be seen that when the Al diffusion layer becomes thicker, the ratio of magnetic flux density becomes smaller and the DC magnetic characteristics tend to deteriorate.

[Experiment 2]
Steel containing chemical components shown in Table 3 below (with the balance being iron and inevitable impurities) was vacuum-melted to prepare 150 kg of melted material. In Table 3 below, the value (Z value) on the left side of the formula (1) is calculated and shown. In addition, No. shown in Table 3 below. The chemical composition of 21 is the same as the steel type α shown in Table 1 above.

  The obtained molten material was forged to produce a steel material having a diameter of 40 mm, and the cold forgeability was evaluated by the following procedure.

<Evaluation of cold forgeability>
The cold forgeability of the steel material was evaluated based on the deformation resistance when the test piece was compressed by 50% and the deformability when compressed. Specifically, the deformation resistance (N / mm ² ) was measured by cutting a test piece having a diameter of 16 mm and a height of 24 mm from the steel material and compressing the test piece so that the height of the test piece was 50%. The compression process was performed by constraining the end face at a strain rate of 10 / sec. The measured deformation resistance is shown in Table 3 below. In this invention, deformation resistance evaluated less than 580 N / mm < ² > as pass, and evaluated 580 N / mm < ² > or more as failure. FIG. 3 shows the relationship between the Z value and the measured deformation resistance.

  On the other hand, the deformability of the steel material was evaluated by performing compression processing under the above conditions and then observing the test piece with the naked eye and an optical microscope (observation magnification: 40 times) to check for the presence of cracks. The presence or absence of cracking is shown in Table 3 below. The case where the crack does not occur is passed, and the case where the crack is generated is rejected.

  In the present invention, the case where both the deformation resistance and the deformability are acceptable is evaluated as “excellent in cold forgeability”, and the case where at least one of the cases is unacceptable is “inferior in cold forgeability”. evaluated.

The following can be considered from Table 3 and FIG. No. 21, 22, 24, 26, 30, 32, 39, 40 to 42 are examples in which the chemical composition of the steel material satisfies the requirements defined in the present invention, the deformation resistance is less than 580 N / mm ² , and compression No cracks occur during processing, and it has excellent cold forgeability.

In contrast, no. 23, 25, 27 to 29, 31, 33 to 38 are examples in which the chemical composition of the steel material does not satisfy the requirements defined in the present invention, and the deformation resistance is 580 N / mm ² or more, or compression processing Since cracks sometimes occurred, cold forgeability is poor.

Specifically, no. 23, 25, and 27 are examples in which C, Si, and Mn respectively exceed the upper limit values defined in the present invention. Since the Z value is larger than 2.8, the deformation resistance is 580 N / mm ² or more. became. No. No. 28 is an example in which P exceeds the upper limit defined in the present invention. Since the grain boundary segregation amount of P increased, cracks occurred during compression processing. No. 29 is an example in which S exceeds the upper limit defined in the present invention. Since a large amount of sulfides precipitated at the grain boundaries, cracks occurred during compression processing. No. 31 is an example in which Cr exceeds the upper limit defined in the present invention. Since the hardness of the ferrite structure was excessively increased by the dissolved Cr, cracking occurred during compression processing. No. In No. 33, Al exceeded the upper limit defined in the present invention, so the deformation resistance was 580 N / mm ² or more. No. 34 is an example in which N exceeds the upper limit defined in the present invention. Age hardening was caused by excess N, and cracking occurred during compression processing. No. 35 is an example in which O (oxygen) exceeds the upper limit defined in the present invention. Since excessive oxide was generated in the steel due to excessive O, cracking occurred during compression processing.

No. In Nos. 36 to 38, the chemical composition of the steel material satisfies the range specified in the present invention, but the Z value exceeds 2.8, so the deformation resistance is 580 N / mm ² or more.

Claims (3)

The chemical composition of the parent phase is
C: 0.002 to 0.20% (meaning mass%, the same shall apply hereinafter),
Si: 1.2% or less (excluding 0%),
Mn: 0.05 to 2.6%,
P: 0.050% or less (excluding 0%),
S: 0.05% or less (excluding 0%),
Cr: 4% or less (excluding 0%),
Al: 0.002 to 2.2%,
N: 0.01% or less (excluding 0%),
O: 0.03% or less (excluding 0%),
The rest: steel parts that are iron and inevitable impurities,
A soft magnetic steel part for direct current, wherein an Al diffusion layer containing 10 to 30% by mass of Al is formed in a surface layer portion, and the thickness of the Al diffusion layer is 10 to 80 μm. The soft magnetic steel part according to claim 1, wherein the chemical component composition further satisfies the following formula (1).
13 × [C] + 2 × [Si] + [Mn] + [Cr] / 5 + [Al] ≦ 2.8 (1)
[In Formula (1), [] has shown content (mass%) of each element. ] The steel part further includes, as other elements,
The soft magnetic steel part according to claim 1 or 2, which contains Cu: 0.5% or less (excluding 0%) and / or Ni: 0.5% or less (excluding 0%). .