JP2000282183A - High hardness steel excellent in machinability and corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high hardness steel having corrosion resistance of a certain degree, good in cold workability, capable of controlling hardness at the time of quenching to >=58 HRC and moreover, having excellent machinability. SOLUTION: This steel is the one having a compsn. contg., by mass, 0.35 to 0.65% C, <=0.25% Si, <=0.25% 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, 0.02 to 0.2% N, and the balance substantial Fe and contg. sulfides of >=5 μm diameter equivalent to a circle contg. Ca of 0.1 to 10% by >=5 pieces per 3.3 mm2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material having good cold workability, high hardness, easy quenching, not only rusting in a moderately oxidizing environment but also corrosion resistance in a certain corrosive environment. High hardness steel with excellent heat resistance.

[0002]

2. Description of the Related Art Conventionally, OA equipment parts such as a printer shaft and a paper feed roller, AV equipment parts such as a VTR motor shaft, and bearing parts of various precision machines have a hardness of HRC50 or more and about HRC60.
Since some corrosion resistance is required, JIS SUJ
2 (C: 0.95 to 1.10%, Si: 0.15 to 0.
35%, Mn: 0.5% or less, P: 0.025% or less,
S: 0.030% or less, Cr: 1.30 to 1.60%, the remainder substantially using a bearing steel such as Fe),
Nickel plating or the like to improve corrosion resistance,
JIS SUS440 of martensitic stainless steel
C (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: 16.0 to 18.00
%, And the balance is substantially Fe),
The improvement in corrosion resistance by nickel plating of SUJ2 is not a highly reliable method because there is a large variation in product quality and the plating may peel off. In addition, SUS4
Although 40C is satisfactory in terms of hardness and corrosion resistance, it is difficult to perform cold forging or deformed drawing due to the expensive material and poor workability in cold and warm conditions.
When precision cutting or grinding is performed, there is also a problem that a processed surface is rough and dimensional accuracy is not good.

On the other hand, the inventor of the present invention has previously disclosed Japanese Patent Application No. 10-109706 as a material which provides good cold workability, excellent corrosion resistance, and high hardening hardness close to HRC60. The technology shown in No. was proposed. According to this technology, cold workability is excellent, corrosion resistance is excellent, and HRC is used.
A steel having a high quench hardness close to 60 and having the required performance was obtained. However, for parts requiring strict dimensional accuracy, such as motors for VTRs, there is a problem that it takes a long time for finishing even in the spheroidized annealing state.
A new problem has been raised in that a material that can be finished in a short time and that is compatible with parts requiring strict dimensional accuracy, such as the VTR motor, has been proposed.

[0004]

DISCLOSURE OF THE INVENTION The present invention has a certain degree of corrosion resistance, good cold workability, and can have a hardness of 58 or more when hardened.
An object of the present invention is to provide a high hardness steel having excellent machinability.

[0005]

In order to achieve the above object, the present inventor has a hardness of about 60 HRC, a cold workability close to that of structural steel, and rusts in a normal room. As a result of diligent research on steel having a corrosion resistance of a certain degree, in applications such as the above-mentioned OA equipment parts, AV equipment parts, bearing parts in various precision machines, the corrosion resistance is superior to SUJ2, and does not reach SUS440C. In order to do so, the Cr content is set to the above SUJ
More than 2 and less than SUS440C, to suppress the generation of coarse carbides that impair corrosion resistance and machinability, to minimize the C content as necessary, and to contain N for improved corrosion resistance. In addition, in order to stably improve the machinability of steel without impairing cold / warm forgeability, appropriate amounts of Ca, Al, O, S are contained, and their forms, especially It was found that it was necessary to control the morphology and distribution of sulfides. The present invention has been made based on the above findings.

That is, the high-hardness steel of the present invention having excellent machinability and corrosion resistance is as follows: (1) By mass% (the same applies hereinafter), C: 0.35 to 0.6
5%, Si: 0.25% or less, Mn: 0.25% 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 to 0.01%, N
: 0.002 to 0.2%, with the balance being substantially F
e and contains 5 or more sulfides having an equivalent circle diameter of 5 μm or more per 3.3 mm ² containing 0.1 to 10% of Ca.

(2) The chemical component described in the above (1) further contains Mo: 0.2 to 1.0%, and the balance is substantially F.
e and contains 5 or more sulfides having an equivalent circle diameter of 5 μm or more per 3.3 mm ² containing 0.1 to 10% of Ca.

(3) The chemical component described in any one of (1) and (2) above, further comprising: V: 0.05 to 0.5
%, Nb: any one of 0.05 to 0.5% or 2
Seeds, the balance consisting essentially of Fe;
It is characterized by containing 5 or more sulfides having a circle equivalent diameter of 5 μm or more containing 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.

[0010]

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: 0.35 to 0.65% C is an element contained to increase the hardness of the steel and improve the wear resistance. To obtain a hardness of 58 or more in HRC, 0.35% or more is contained. There is a need to. However, if the C content exceeds 0.65%, undissolved coarse carbides are formed, thereby deteriorating the cold workability and impairing the corrosion resistance of the steel.
Further, since the machinability is deteriorated, the upper limit of the C content is set to 0.
65%.

Si: 0.25% or less, Mn: 0.25%
In the following, Si and Mn are added as deoxidizers during the production of steel. However, in steel, the ferrite phase, which is a matrix, is solid-solution strengthened in a spheroidized structure to reduce cold workability. And the upper limit of the content rate is set to 0.25% in consideration of the manufacturing cost.

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 is significantly reduced, so the content is made 0.005 to 0.2%.

Cr: 4.0 to 11.0% Cr is an element contained for improving the corrosion resistance and quenching hardness of steel. In order to obtain corrosion resistance to the extent that it does not rust in a relatively mild environment, it is necessary to contain 4.0% or more of Cr. However, when a large amount of Cr is contained, the corrosion resistance increases, but the amount of carbide formed increases, and
If the content exceeds 1.0%, coarse carbides are formed, which lowers the cold workability and machinability of the steel. Therefore, the content is set to 4.0 to 11.0%.

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 to 0.02% Ca is contained in the sulfide to form a protective film on the surface of the cutting tool during steel cutting to extend the tool life. 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 )
If it is less than 0.0005%, a large amount of high-melting-point Ca sulfide is generated to reduce the pouring property in the production of steel ingots. As well as lowering the machinability of the alloy, all of the Ca forms oxides and no sulfides are generated.
The content is made 0.0005 to 0.01%. Preferably it is 0.001 to 0.004%.

N: 0.02 to 0.2% N is contained to improve the corrosion resistance of steel. If the N content is less than 0.02%, the effect cannot be obtained,
If it exceeds 0.2%, the workability of the steel is reduced, so the content is made 0.02 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.

Mo: 0.2-1.0% 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%.

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%.

Pb: 0.4% or less Pb is contained in order 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 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 of the martensitic stainless steel used in the present invention will be described. Generally, the main component of the sulfide contained in the steel is MnS, but in the steel of the present invention to which Ca is added, M
A part of n will be replaced by Ca. This M
The properties of nS vary depending on the degree to which a part of Mn is replaced by Ca. Ca content in sulfide is 0.1-1
The sulfide in the range of 0% forms a film that protects the tool at the time of cutting, but Ca in the sulfide is 0.1% of the entire sulfide.
If it is less than 1%, the above-mentioned tool protective film is not formed, so that the tool life improvement effect cannot be obtained. If it exceeds 10%, CaS having a high melting point is excessively generated, so that the tool protective film is not formed and steel Deteriorates the manufacturability.

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.

During the smelting of alloy steel, Ni is often mixed in from the raw material scrap. In the high hardness steel with excellent machinability and corrosion resistance of the present invention, Ni has an effect of slightly improving the ductility of the martensitic phase, but if it is contained excessively, it makes soft annealing difficult. , Ni is preferably 0.6% or less.

Since P is an element which lowers the toughness of steel, it is preferable to reduce P as much as possible.
If it is below, there is little effect on corrosion resistance, toughness and the like, so the upper limit of the content is preferably set to 0.03%.

[0028]

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.

[0029]

[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: The test material was turned under the following conditions and the tool life was measured. The tool life obtained in Comparative Example 6 was set to 1, and the respective tool life ratios were calculated. This was used as an index for evaluating the machinability of the test material. Tool: Carbide 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 an upsetting test was performed by applying a load in the axial direction of the test piece. 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:

A test piece material having a length of 50 mm was cut out from the test material, heated at 1050 ° C. for 30 minutes, quenched by oil cooling, and then tempered by heating and air cooling at 150 ° C. for 1 hour. Using a Rockwell hardness tester for the cross section at the center of the length of the quenched and tempered test specimen material,
The hardness was measured on a scale.

Corrosion test: 15 mm in diameter from the test material
A test piece material having a length of 105 mm was cut out, heated at 1050 ° C. for 30 minutes, oil-cooled and quenched, and then 150 ° C.
For 1 hour and then quenched. The entire surface of the quenched and tempered test specimen material was polished to obtain a corrosion test specimen. The corrosion test piece was placed in a wet state (temperature 50 ° C., relative humidity 9
8%, holding time 4 hours) and a dry state (temperature 70 ° C., holding time 2 hours) were alternately repeated four times, and then the presence or absence of pitting was visually observed. The presence or absence of pitting was used as an index for evaluating corrosion resistance.

Oxidation test: 15 mm in diameter from the test material
A test piece having a length of 20 mm was cut out, heated at 1050 ° C. for 30 minutes, oil-cooled and quenched, and then tempered by heating at 150 ° C. for 1 hour and rapidly cooling. The entire surface of the quenched and tempered test specimen material was polished to obtain an oxidation test specimen. First, after measuring the weight of the oxidized test piece, the test piece was heated to 500 ° C. in the air, kept for 50 hours and cooled, then the oxide scale was removed, and the weight was measured to determine the difference from the initial weight. The weight difference was divided by the total surface area of the original oxidized test piece to determine the oxidized weight loss. The oxidation loss was used as an index for evaluating oxidation resistance.
Table 2 shows the results of the above tests.

[0036]

[Table 2]

As is clear from Table 2, all of the examples of the present invention show a high hardness of 58 or more in the quenched and tempered state. Further, the example of the present invention is the same as that of Comparative Example 13 (SUJ
Comparative Example 14 (SUS420J2 equivalent steel), which has smaller deformation resistance, higher cracking reduction rate, superior corrosion resistance, and significantly reduced oxidation weight loss as compared with Comparative Example 14 (SUS420J2 equivalent steel).
And the deformation resistance was smaller than that of Comparative Example 15 (SUS4
Compared with Comparative Example 16 (SUH11), the deformation resistance is smaller and the crack generation reduction ratio is higher than that of Comparative Example 16 (SUH11). Furthermore, the examples of the present invention show a significantly larger tool life ratio than the comparative example. It is clear from this that the embodiment of the present invention has excellent machinability.

[0038]

As described above, the high-hardness steel of the present invention having excellent machinability and corrosion resistance is excellent in cold workability and machinability and hardened and tempered by adopting the above constitution. It has excellent properties such as high hardness and good corrosion resistance and oxidation resistance.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/60 C22C 38/60

Claims (4)

[Claims] 1. In mass% (the same applies hereinafter), C: 0.35 to 0.65%, Si: 0.25% or less, Mn: 0.25% 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%, N: 0.002 ~ 0.2%, the balance being substantially composed of Fe and 0.1%
High-hardness steel excellent in machinability and corrosion resistance, characterized by containing 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 excellent in machinability and corrosion resistance according to claim 1, wherein the chemical component further contains Mo: 0.2 to 1.0%. 3. The chemical component according to claim 1, further comprising one or two of V: 0.05 to 0.5% and Nb: 0.05 to 0.5%. Item 3. A high-hardness steel excellent in machinability and corrosion resistance according to any one of Items 1 or 2. 4. The above-mentioned chemical component further comprises at least one of Pb: 0.4% or less, Bi: 0.4% or less, Se: 0.5% or less, and Te: 0.1% or less. The high hardness steel excellent in machinability and corrosion resistance according to any one of claims 1 to 3, characterized by containing at least one kind.