JP2000282184A - High hardness steel for induction hardening excellent in machinability and corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high hardness steel good in corrosion resistance and cold workability, capable of obtaining hardness of >=55 HRC by induction hardening and having excellent machinability. SOLUTION: This steel is the one having a compsn. contg., by mass, 0.30 to 0.60% C, <=0.25% Si, <=1.0% Mn, 0.005 to 0.2% S, 4.0 to 11.0% Cr, 0.001 to 0.1% Al, 0.0005 to 0.02% Ca, 0.0005 to 0.01% O, and the balance substantial Fe and contg. sulfides of >=5 μm diameter equivalent to a circle contg. 0.1 to 10% Ca by >=5 pieces per 3.3 mm2. For covering its corrosion resistance, for refining the crystal grains of Mo and N, and for improving its machinability, Pb, Bi, Se and The can be contained therein.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention provides a good machinability and cold workability, a high induction hardening property, and an excellent corrosion resistance.
Accordingly, the present invention relates to a high-hardness steel suitable as a material for various bearing components.

[0002]

2. Description of the Related Art OA equipment parts such as printer shafts and paper feed rollers, AV equipment parts such as VTR motor shafts, bearing parts in various precision machines,
Alternatively, as a material for manufacturing an electronic device component such as a fuel injection device of an automobile engine, a hardness of HRC55 or more and a certain degree of corrosion resistance are required, and therefore, a conventional material is used.
S SUJ2 (C: 0.95 to 1.10%, Si: 0.
15 to 0.35%, Mn: 0.5% or less, P: 0.02
5% or less, S: 0.030% or less, Cr: 1.30 or more
A bearing steel containing 1.60% and substantially the remainder Fe or the like was used, and nickel-plated or the like was applied to improve corrosion resistance to obtain a product. However, the improvement in corrosion resistance by nickel plating is not a highly reliable method because there is a large variation in product quality and the plating may peel off.

[0003] Therefore, JIS SUS440C of martensitic stainless steel (C: 0.95 to 1.20%,
Si: 1.00% or less, Mn: 1.00% or less, P:
0.040% or less, S: 0.030% or less, Cr: 1
6.0 to 18.00%, with the balance being substantially Fe)
High-C stainless steel is used. Thereby, the requirements of hardness and corrosion resistance are satisfied, and the problems associated with plating are solved, but the material is expensive, and the cold workability is poor, so that cold forging or cold deformed drawing cannot be carried out industrially. There was a problem. In addition, this type of steel has poor machinability,
When precision cutting or grinding is performed, there is also a problem that a processed surface is rough and dimensional accuracy is not good.

JIS SUS4 instead of SUS440C
20J2 (C: 0.26 to 0.40%, Si: 1.00
%, Mn: 1.00% or less, P: 0.040% or less, S: 0.030% or less, Cr: 12.00-14.
By using 00% and substantially the balance of Fe), the steel has good cold workability and can be cold forged or deformed.

By the way, according to the so-called “sub hardening”, quenching of a long object such as a shaft is usually performed, in which the whole is heated in a heating furnace and quenched at a stretch.
Since warpage and quenching of parts often occur unevenly,
It is desirable to use induction hardening in which such a problem hardly occurs. When induction hardening is performed on a product using SUS420J2, there is a disadvantage that the quenching hardness does not reach HRC55, and the required characteristics of the product cannot be satisfied.

[0006] The cold workability is close to that of structural steel, and when induction hardening is performed, the quench hardness is set to HRC55.
The present inventor has previously proposed the technology disclosed in Japanese Patent Application No. 10-259717 as a high-hardness steel having a corrosion resistance that does not rust in a normal indoor use environment. This technology uses C: 0.3-0.6%, S
i: 0.25%, Mn: 1.0% or less, Cr: 4.0 to
11.0%, Mo: 1.0% or less, N:
0.02-0.2%, V: 0.05-0.5%, Nb:
About steel containing 0.05 to 0.5%,
According to this, the above SUS440C or SUS420J
The problem in 2 is avoided. However, parts requiring strict dimensional accuracy, such as VTR motors, have a problem that a long time is required for finishing even in a spheroidized annealing state, and strict dimensional accuracy of the VTR motors, etc. is required. A new challenge has been raised: the need for a material that is compatible with the part and that can be finished in a short time.

[0007]

DISCLOSURE OF THE INVENTION The present invention has a corrosion resistance of such a degree that it does not rust in a normal indoor use environment,
An object of the present invention is to provide a high-hardness steel that has good cold workability, can have a hardness of HRC 55 or more when subjected to induction hardening, and has excellent machinability.

[0008]

The high-hardness steel for induction hardening having excellent machinability and corrosion resistance according to the present invention, which achieves the above objects, has the following features. (1) By mass% (the same applies hereinafter), C: 0.30 ~ 0.6
0%, Si: 0.25% or less, Mn: 1.0% or less, S
: 0.005 to 0.2%, Cr: 4.0 to 11.0
%, Al: 0.001 to 0.1%, Ca: 0.0005
-0.02%, O: 0.0005-0.01%, the balance being substantially composed of Fe, and 0.1-10%
% Sulfide having a circle equivalent diameter of 5 μm or more containing 3% Ca.
It is characterized by containing 5 or more per 3 mm ² .

(2) The chemical component described in the above (1) further contains Mo: 0.2 to 1.0%, the balance being substantially Fe and 0.1 to 10%. It is characterized in that 5 or more sulfides containing Ca and having a circle equivalent diameter of 5 μm or more are contained per 3.3 mm ² .

(3) In addition to the chemical components described in any one of (1) and (2) above, V: 0.05-0.
5%, Nb: contains 0.05% to 0.5% of one or two kinds, and the balance substantially consists of Fe; and
It is characterized by containing at least 5 sulfides having a circle equivalent diameter of 5 μm or more containing 0.1 to 10% of Ca per 3.3 mm ² .

(4) The chemical component according to any one of (1) to (3), further comprising: Pb: 0.4% or less;
i: 0.4% or less, Se: 0.5% or less, Te: 0.1
% Or less, the balance being substantially Fe, and containing 0.1 to 10% of Ca and having a circle-equivalent diameter of 5 μm or more, which is 3.3 mm ^2. It is characterized by containing 5 or more per unit.

(5) The chemical component according to any one of (1) to (4) further contains N: 0.04 to 0.2%, and the balance substantially consists of Fe; And 0.1 ~
It is characterized in that 5 or more sulfides having a circle equivalent diameter of 5 μm or more containing 10% of Ca are contained per 3.3 mm ² .

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The components of the high hardness steel excellent in machinability and corrosion resistance according to the present invention and the reason for limiting the content thereof will be described. C: 0.30 to 0.60% C is an element contained to increase the hardness of steel and improve wear resistance. To obtain a hardness of 55 or more, the content of C is 0.30% or more. There is a need to. However, when the C content exceeds 0.60%, coarse carbides are formed, the cold workability is reduced, the corrosion resistance of the steel is impaired, the machinability is deteriorated, and quenching cracks are easily generated during induction hardening,
And it is difficult to form a solid solution.
%.

Si: 0.25% or less Si is added as a deoxidizing agent during the production of steel. In steel, the ferrite phase, which is a matrix in a spheroidized structure, is solid-solution strengthened to lower cold workability. Therefore, in consideration of these effects and the cost at the time of manufacturing, the upper limit of the content is set to 0.25%.

Mn: 1.0% or less Mn is an element to be added as a deoxidizing agent, but preferably contains an appropriate amount to enhance the hardenability of steel. However, an excessive content of Mn promotes work hardening and impairs cold workability. Therefore, the upper limit of the content is set to 1.0%.

S: 0.005 to 0.2% S is an element contained for improving the machinability of steel. If the content is less than 0.005%, the machinability is not improved.
If it exceeds 0.2%, the toughness of the steel is significantly reduced, so the content is made 0.005 to 0.2%.

Cr: 4.0-11.0% Cr is an element contained to improve the corrosion resistance of steel and to give induction hardening hardness. In order to obtain corrosion resistance to a degree that does not rust in a relatively mild environment, Cr must be added to 4.
0% or more, preferably 5.0% or more.
% Or more. However, when a large amount of Cr is contained, the corrosion resistance is improved, but the amount of carbide formed increases, and 11.0%
%, Coarse carbides are formed, which lowers the corrosion resistance, cold workability, and machinability of the steel. In addition, the solid solution of the carbides during induction hardening becomes insufficient, and it is difficult to secure quenching hardness. Therefore, the upper limit of the content is set to 11.0%. Preferably it is 9.0% or less.

Al: 0.001 to 0.1% Al is an element added for deoxidizing steel.
If it is less than 01%, the effect cannot be obtained, and if it exceeds 0.1%, hard alumina clusters are formed and the machinability of the steel is reduced. I do.

Ca: 0.0005-0.02% Ca is contained in the sulfide to form a protective film on the surface of the cutting tool during steel cutting and to extend the life of the tool. If the effect is less than 0.0005%, the effect cannot be obtained, and if it exceeds 0.02%, excessive Ca forms CaS having a high melting point and causes obstacles such as clogging of a nozzle in a casting process of molten steel. Therefore, the content of Ca is set to 0.0005 to 0.02%.

O: 0.0005 to 0.01% O is an oxide (CaO, Al ₂ ) serving as a nucleus for crystallization of sulfide.
O _3, etc.).
If it is less than 0005%, a large amount of high melting point Ca sulfide is generated to reduce the pouring property during the production of steel ingots.
If the content exceeds 0.01%, a large amount of hard oxide is formed to lower the machinability of the steel, and at the same time, all Ca forms an oxide and no sulfide is formed. Is set to 0.0005 to 0.01%. Preferably it is 0.001 to 0.004%.

Mo: 0.2 to 1.0% Mo is contained together with Cr to improve the corrosion resistance of steel. In order to obtain the effect, it is necessary to contain 0.2% or more. However, if the content exceeds 1.0%, the cold workability of the steel is reduced and the cost is increased.
o The upper limit of the content is set to 1.0%.

N: 0.04 to 0.2% N can be contained in order to improve the corrosion resistance of steel. If the N content is less than 0.04%, the effect cannot be obtained. If the N content is more than 0.2%, the nitride does not completely form a solid solution, thereby reducing the workability of the steel. 04 to 0.2%. Note that the N content can be controlled by selecting a scouring atmosphere, bubbling N ₂ gas, introducing a nitride alloy, and the like.

V: 0.05-0.5%, Nb 0.05-
0.5% V and Nb are elements that both form stable fine carbides and refine crystal grains to improve the strength and toughness of steel. To obtain the effect, 0.05% or more is contained. However, if the content exceeds 0.5%, not only does the cold workability of the steel deteriorate, but also the ductility decreases, so the content is made 0.05 to 0.5%. In the case of using two kinds in combination, it is preferable that the total content does not exceed 0.6%.

Pb: 0.4% or less Pb is contained to improve the machinability of steel. It exists alone or in a form that adheres to the outer periphery of the sulfide in steel, and has an effect of improving machinability by itself. However, when the content exceeds 0.4%, the solubility in steel is exceeded, and due to its large specific gravity, the excess is agglomerated and precipitated as a single substance and becomes a defect in the steel. 4% or less.

Bi: 0.4% or less Bi has properties similar to Pb and is contained in order to improve the machinability of steel. However, when the content exceeds 0.4%, the solubility in steel is exceeded, and due to its large specific gravity, the excess is agglomerated and precipitated as a single substance and becomes a defect in the steel. 4% or less.

Se: 0.5% or less Se is contained in order to improve the machinability of steel. However, if the content exceeds 0.5%, the hot workability of the steel is reduced and cracks frequently occur during hot working. Therefore, the upper limit of the content is set to 0.5%.

Te: 0.1% or less Te is contained in order to improve the machinability of steel. However, if the content exceeds 0.1%, the hot workability of the steel is reduced and cracks occur frequently, so the upper limit of the content is 0.1%.
And

Next, the form of the sulfide in the high hardness steel for induction hardening having excellent machinability and corrosion resistance according to the present invention will be described. Generally, the main component of sulfide contained in steel is
MnS, but in the steel of the present invention to which Ca is added, the M
A part of Mn in nS is replaced by Ca. The properties of MnS vary depending on the degree to which a part of Mn is substituted by Ca. A sulfide having a Ca content in the range of 0.1 to 10% in the sulfide forms a film that protects the tool during cutting, but Ca in the sulfide is less than 0.1% of the entire sulfide. In this case, the above-mentioned tool protection film is not formed, so that a tool life improving effect cannot be obtained.
If it exceeds 0%, CaS having a high melting point is excessively generated, so that a tool protective film is not formed and the productivity of steel is deteriorated.

In addition, sulfides having an equivalent circle diameter of 5 μm or more are used.
If not more than 5 per 3 mm ^{2, the} amount of the formed tool protective film is small, so that the front surface of the tool is not completely covered and the effect of improving the tool life is small. The “equivalent circle diameter” in the present invention is the diameter of a circle assuming a circle having the same area as the cross-sectional area.

When smelting alloy steel, Ni is often mixed from the raw material scrap. Ni has an effect of slightly improving the ductility of the martensite phase, but if it is contained excessively, it becomes difficult to soften and anneal. Therefore, the Ni content is preferably 0.6% or less. Also, P
Is an element that lowers the toughness of steel, so it is preferable to reduce it as much as possible.
Since the effect on toughness and the like is small, the upper limit of the content is set to 0.
It is preferably set to 03%.

[0031]

EXAMPLES The steels shown in Table 1 were melted and cast into steel ingots using a high-frequency induction furnace, and then the steel ingots were divided by hot forging. This was made into a wire having a diameter of 20 mm by hot rolling. The wire is heated to 850 ° C. for 3 hours, cooled to 600 ° C. at a cooling rate of 15 ° C./hour, then subjected to spheroidizing annealing by air cooling, and further cold drawn to a diameter of 18 m.
m, and then subjected to annealing at 850 ° C. to be used as test materials, which were subjected to the following tests.

[0032]

[Table 1]

Sulfide measurement: the number of sulfides having a circle-equivalent diameter of 5 μm or more in 3.3 mm ² (corresponding to about 30 visual fields at a microscope magnification of 100) on a longitudinal section including the central axis of the test material. Was measured. Table 1 shows the results as the number of sulfides.
Shown in The components were analyzed for sulfides arbitrarily selected from the measured sulfides, and Ca was 0.1 to 10%.
It was confirmed that it was in the range.

Machinability test: Tool life was measured by turning the test material under the following conditions, and the tool life ratio was calculated by setting the tool life obtained in Comparative Example 1 to 1. The ratio was used as an index for evaluating the machinability of the test material.

Tool: super high P10 Speed: 200 m / min Feed: 0.2 mm / rev Depth of cut: 2.0 mm Cutting oil: none Tool life judgment standard: average flank wear = 0.2 mm

Upsetting test: A cylindrical test piece having a diameter of 6 mm and a height of 12 mm was prepared from the test material by machining, and a load was applied in the axial direction of the test piece to perform an upsetting test. In the test, the quotient obtained by dividing the load when the true strain reached 1 by the initial sectional area of the test piece was defined as the deformation resistance.
Further, the rolling reduction at the time when a crack was recognized in the test piece for the first time was obtained and defined as the critical rolling reduction at which cracking occurred. The values of the deformation resistance and the critical draft of crack initiation were used as indices for evaluating cold workability.

Hardness test: The hardness of the test material was measured on a B scale using a Rockwell hardness tester for the cross section of the test material, and the hardness is shown in Table 2 as an annealing hardness. The test material was induction hardened under the following conditions, and then tempered by heating and quenching at 150 ° C. for 1 hour. Frequency 100 KHz Coil moving speed 13 mm / sec Output 30 KW The hardness of the surface of the test piece subjected to the induction hardening and tempering was measured on a C scale using a Rockwell hardness tester, and is shown in Table 2 as the induction hardening hardness.

Corrosion test: 15 mm in diameter from the test material
A test piece material having a length of 105 mm was cut out, quenched under the above-mentioned induction hardening conditions, and then tempered by heating at 150 ° C. for 1 hour and air cooling. The whole surface of the induction hardened and tempered test piece was polished to obtain a corrosion test piece. The corrosion test piece was alternately repeated four times between a wet state (temperature of 50 ° C., relative humidity of 98%, holding time of 4 hours) and a dry state (temperature of 70 ° C., holding time of 2 hours), and then visually observed. The presence of food was checked. The presence or absence of pitting was used as an index for evaluating corrosion resistance.

[0039]

[Table 2]

As is clear from Table 2, the embodiment of the present invention shows a high hardness of HRC 56 or more in the induction hardening / tempering state. Further, the example of the present invention is the same as that of Comparative Example 13 (SUJ
Deformation resistance and the critical rolling reduction rate of crack initiation are comparable to those of Comparative Example 15 (SUS420J2 equivalent steel), but the deformation resistance is smaller than Comparative Example 15 (SUS420J2 equivalent steel), and Comparative Example 14 (SUS440C). (Equivalent steel), the deformation resistance is remarkably low, and the critical reduction in crack generation is remarkably high. Furthermore, the examples of the present invention show a significantly larger tool life ratio than the comparative examples, and it is clear that the examples of the present invention have excellent machinability.

[0041]

As described above, according to the present invention, it has a corrosion resistance of such a degree that it does not rust in a normal indoor use environment, has good cold workability, and has a high quality when subjected to induction hardening. Hardness can be made HRC55 or more, and a high hardness steel for induction hardening having excellent machinability can be provided.

Claims (5)

[Claims] 1. In mass% (the same applies hereinafter), C: 0.30 to 0.60%, Si: 0.25% or less, Mn: 1.0% or less, S: 0.005 to 0.2% , Cr: 4.0 to 11.0%, Al: 0.001 to 0.1%, Ca: 0.0005 to 0.02%, O: 0.0005 to 0.01%, with the balance being Consisting essentially of Fe and 0.1
High-hardness steel for induction hardening excellent in machinability and corrosion resistance, characterized in that it contains 5 or more sulfides having a circle equivalent diameter of 5 μm or more containing 10% of Ca and having a diameter of 5 μm or more per 3.3 mm ² . 2. The high-hardness steel for induction hardening excellent in machinability and corrosion resistance according to claim 1, wherein said chemical component further contains Mo: 0.2 to 1.0%. 3. The chemical component described above further comprises one or two of V: 0.05 to 0.5% and Nb: 0.05 to 0.5%. The high hardness steel for induction hardening excellent in machinability and corrosion resistance according to any one of claims 1 and 2. 4. The chemical component described above, further comprising any one of the following: Pb: 0.4% or less, Bi: 0.4% or less, Se: 0.5% or less, Te: 0.1% or less The high-hardness steel for induction hardening excellent in machinability and corrosion resistance according to any one of claims 1 to 3, which contains two or more types. 5. The machinability and corrosion resistance according to claim 1, wherein said chemical component further contains N: 0.04 to 0.2%. Excellent hardened steel for induction hardening.