JPH0152467B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention provides a high-performance free-cutting steel for gears that has excellent root strength and rolling contact fatigue strength, and also has good machinability, grindability, cold forgeability, and other mass production properties, and the like. Regarding the manufacturing method.

[Conventional technology]

Generally, case-hardened steel is used as gear steel, which can obtain high surface pressure strength through carburizing or nitriding. From the standpoint of mass production, there is a demand for materials with improved machinability, grindability, and cold forgeability. For some time now, gear steels with improved machinability or grindability by adding appropriate amounts of free-cutting elements such as S and Pb have been developed, and some of them are in practical use. It has not been adopted for use in heavy-load gears due to problems with dedendum strength and rolling contact fatigue strength, which are important properties for industrial steel. Therefore, the inventors aimed to develop a high-performance gear steel that satisfies the various properties required for gear steel and is also easy to mass-produce. As a result of a detailed study of the effects, it was found that in steel with a specific alloy composition, a material in which extremely fine nonmetallic inclusions with a mainly (Al, Ca, Mn) composition are uniformly dispersed is superior to conventional gear steel. Compared to
It has been found that it has good strength properties such as rolling contact fatigue strength, and has high mass production manufacturability such as machinability and cold forgeability.

[Problem to be solved by the invention]

Based on the above knowledge, an object of the present invention is to provide a high-performance free-cutting steel for gears that has strength characteristics suitable as a material for gears used under severe conditions and is highly manufacturable in mass production, and a method for manufacturing the same. It's about doing.

[Means to solve the problem]

The free-cutting steel for high-performance gears of the present invention has a C: 0.10~
0.55%, Si: 0.35% or less, Mn: 1.70% or less,
Pb: 0.02~0.15%, Al: 0.010~0.040%, Ca:
0.0002 to 0.0050%, S: 0.060% or less and N:
0.020% or less, O: 0.0030% or less, the remainder substantially consists of Fe, and the inclusions are mainly Al ₂ O ₃ -SiO ₂ -CaO-based oxides and (Mn, Ca).
It is characterized in that fine nonmetallic inclusions that are a composite type with S-based sulfides and have a major axis/minor axis ratio of ≦20 and a major axis of 10μ or less account for 80% or more of the total nonmetallic inclusions. do. The method for manufacturing the above-mentioned free-cutting steel for high-performance gears is C:
0.10-0.55%, Si: 0.35% or less, Mn: 1.70% or less, Pb: 0.02-0.15%, Al: 0.010-0.040%,
Contains Ca: 0.0002 to 0.0050%, S: 0.060% or less, N: 0.020% or less, O: 0.0030% or less, the remainder is substantially composed of Fe, and the inclusions are mainly Al ₂ O ₃ -SiO ₂ −CaO-based oxide and (Mn,
Fine nonmetallic inclusions that are a complex type with Ca) S-based sulfides, and have a major axis/minor axis ratio of ≦20 and a major axis of 10μ or less, accounting for 80% or more of the total nonmetallic inclusions. A method for producing Al, Ca
and Vacuum degassing treatment of molten steel containing the above alloy components other than Pb, during or after degassing.
Refining is performed by adding Al and Ca-Si, and at that time, non-oxidizing gas is introduced into the molten steel and forced stirring is carried out to float away large non-metallic inclusions, and then an appropriate amount of lead is stirred. It is characterized by being added to the molten steel flow and uniformly dispersed.

[Function] In the inclusions of the steel of the present invention, there is no Al ₂ O ₃ -based oxide that acts abrasively on the cutting tool and promotes chipping and wear of the cutting edge of the tool, and most of the inclusions are composed of the aforementioned Al. ₂ O ₃ -SiO ₂ -CaO-based oxide and (Mn,Ca)S-based sulfide composite type with a major axis/minor axis ratio of ≦20 and a major axis of 10μ or less are non-metallic intercalated. It accounts for more than 80% of things,
There are almost no giant oxides or elongated sulfides that are often found in conventional gear steels. Therefore, such inclusions do not cause a decrease in rolling contact fatigue strength or dedendum strength. It also causes tool wear and is harmful to cold forging properties.
Since Al ₂ C ₃ based oxides rarely form alone,
Good mass production efficiency. The reasons for limiting the composition of the steel of the present invention will be described below. C: Strength required for gear steel by carburizing or normal quenching (to ensure H _R C30-45, it is necessary to add at least 0.10%.
However, if the amount is too large, the hardness will increase and the impact resistance will decrease, so it was limited to 0.55% or less. Si: In addition to being added to deoxidize molten steel, it also contributes to increasing hardenability and temper softening resistance, but machinability will be impaired if added in large amounts.
Limited to 0.35% or less. Mn: It is an inexpensive element that is effective in improving hardenability, and it is desirable to actively add it.
If the amount is too large, hot workability will be impaired.
Limited to 1.70% or less. Pb: An effective element for improving machinability and grindability, and is added at least 0.02%.
However, since it is harmful to rolling fatigue strength, 0.15
% or less. Al: An element that is effective in adjusting oxygen levels and crystal grains, but if added in large amounts, _{Al2O3 -} based oxides that are harmful to machinability and cold forging properties are generated, so it should be added at 0.010 to 0.040%. The range of Ca: Al ₂ O ₃ −SiO ₂ − which is effective in deoxidizing molten steel and improves the composition of nonmetallic inclusions in steel, which is advantageous for machinability, cold forgeability, and rolling fatigue resistance.
It is an essential element for changing into a composite type of CaO-based oxide and (Mn,Ca)S-based sulfide, and in order to obtain the above composition, the content in the steel must be reduced to 0.0002
% or more. However, even if added in large amounts, no improvement in effectiveness can be expected; rather, it impairs the cleanliness of the steel and reduces rolling fatigue properties.
% or less. S: Mainly exists in the form of sulfide and serves as a main force for improving machinability. If added in large quantities, giant sulfides, such as MnS, will form on their own, causing a significant decrease in the tooth base strength of the gear, so 0.060
% or less. N: Mainly exists in the form of AlN and is useful for refining crystal grains, but since a large amount of N decreases hot workability and cold workability, it is limited to 0.020% or less. O: In order to prevent the formation of Al ₂ O ₃ -based oxides that are harmful to machinability and cold forgeability, the content was limited to 0.0030% or less. Composition and morphology of non-metallic inclusions: If a large amount of Al ₂ O ₃ -based oxide is distributed alone in steel, it has an abrasive effect on the cutting tool and promotes chipping of the cutting edge of the tool, so it is preferable. do not have. Since Al ₂ O ₃ -based oxides are also a source of cracks during cold forging, it is necessary to reduce them as much as possible. In the present invention, since an appropriate amount of Ca is added and the amounts of Al, S, and O are limited, Al ₂ O ₃ -based oxides are not generated alone, and most of them become composite inclusions as described above. , the problems mentioned above are minor. If a large amount of giant inclusions or elongated inclusions are present, rolling life characteristics and root strength will be impaired. Unless the condition that the inclusions account for 80% or more of the total amount of inclusions is met, the shape of the inclusions will have an adverse effect. When manufacturing the steel of the present invention, when degassing molten steel, appropriate amounts of Al and Ca-Si, which have strong deoxidizing power, are added, and forced stirring is performed by blowing non-oxidizing gas to float large non-metallic inclusions. Separate and then add appropriate amount of Pb
is added to the stirred molten steel stream and dispersed uniformly. By doing this, most of the Al-based oxides are combined with the Ca-Si added at the same time and separated by flotation, and most of them are removed as slag, and the Al ₂ O ₃ remaining in the steel is removed. , mostly combined with Ca−Si,
It becomes a fine Al ₂ O ₃ --SiO ₂ --CaO-based oxide, and further combines with the (Mn, Ca) S-based sulfide in the steel to form a composite inclusion. Therefore, there are almost no oxide inclusions of Al ₂ O ₃ alone in the steel.

[Example 1] Using SMn21 and S48C steel as basic steel types, inventive steel and comparative steel having the structures shown in Table 1 were produced. As comparative steels, conventionally used general steel (non-free-cutting steel) and lead-containing steel were used. All test steels were melted in a basic electric furnace, but
The steel of the present invention is obtained by adding elements such as Cr, Ni, and Mo, followed by vacuum degassing treatment, and then strong deoxidation with Al and Ca-Si. Much lower amounts of Al and O. Regarding the sample steels shown in Table 1, the amount of nonmetallic inclusions in the steel, their composition, and morphology were investigated. The results are also listed in Table 1. The steels of the present invention all have a lower amount of nonmetallic inclusions and higher cleanliness than the comparative steels. In addition, non-metallic inclusions in the steel of the present invention account for 80% of the non-metallic inclusions.
The above is the combination of Al ₂ O ₃ −SiO ₂ −CaO-based oxides and (Mn,
It is a composite type inclusion in which Ca)S-based sulfides are combined, and almost no Al ₂ O ₃ oxides, which are harmful to machinability and cold forgeability, are observed. Most of the composite inclusions have a shape close to a sphere with a major axis/minor axis ratio of 2 or less,
Moreover, it is characteristic that the long axis is minute, with a length of 10μ or less. Various characteristic values were investigated for the test steels shown in Table 1. (b) Gear cutting performance and turning workability The test steels in Table 1 were normalized at the temperatures shown in Table 2 to adjust their hardness, and then the gear cutting performance with a hob and the carbide tool were evaluated. The turning workability was investigated. The cutting conditions are as follows. Gear cutting tool SKH3, m=2, PA=20゜hob Axial feed 1.30mm/t.rev Cutting speed 30m/min Cutting method Climb cut Cutting oil Wet Work material Super gear Outer diameter: 92mm Gear width: 40mm Tool wear measurement Number of pieces cut: 20 Turning tool P10 (-5, -5, 5, 5, 30,
0,0.4) Feed 0.20mm/rev Depth of cut 20mm Cutting speed 150mm/min Cutting oil Dry Tool life judgment VB: 0.2mm Tool life is also shown in Table 2. As shown in the table, the steel of the present invention exhibits superior performance in both gear cutting and turning compared to the comparative steel. (b) Grindability A cylindrical specimen was taken from the test steel shown in Table 1,
SMn21 series steel B is subjected to gas carburizing and quenching-tempering to adjust the surface hardness to H _R C61 to 62.5, and S48C series steel E is induction hardened and tempered to adjust the surface hardness to H _R After adjusting the C62 to C63, it was subjected to a cylindrical grinding test. The grinding conditions are as follows. Grinding method: Traverse conical grinding Magnet: WA54K8V71, 405×30×15240, No. 1 Feed: 1300m/min Grinding speed: 2700m/min Depth of cut: 5μ/Pass Grinding oil: Wet Total grinding amount: 8.0cm ³ Test results It is shown in Table 3. From the same table, it can be seen that the steel of the present invention has better wear in the diametrical direction of the grinding wheel and surface roughness of the workpiece, and has better grindability than the comparative steel. As described above, the steel of the present invention has excellent gear cutting workability, turning workability, and grindability, all of which are important in the mass production of gears. The surface roughness of the workpiece was measured using a Kosaka type surface roughness meter. (c) Cold forgeability Specimens were taken from the test steels shown in Table 1, heat treated, and then subjected to an upset test. Test conditions were: test piece diameter: 8 mm x length: 16 mm, heat treatment: 760
°C x 6 hours → air cooling (spheroidizing annealing), lubricant:
MoS ₂ grease, repeated n=5. The test results are shown in Figure 1. The graph in the figure shows that the steel of the present invention has a lower limit strain amount (height before upsetting H/height after upsetting) than the comparative steel.
Ho) is high, indicating good cold forgeability. (d) Tooth root strength A gear with tooth profile: 20°, regular teeth, module: 3, and number of teeth: 26 was manufactured from the test steel shown in Table 1, and SMn21B
gas carburizing and quenching-tempering treatment to improve surface hardness to H R C60~62 and core hardness to H _R C60~62.
H _{R C} was adjusted to 40-42, and S48C series E was subjected to induction hardening and tempering treatment to adjust the surface layer hardness to H _R C 62-63 and the core hardness to H _R C 35-38. They were subjected to a power circulation gear testing machine to examine the tooth root strength of the gears. The results for SMn21 series B are shown in FIG.
As shown in the drawings, the steel of the present invention has a longer service life than the comparative steel, regardless of the magnitude of dynamic tooth root stress. This is due to the high dentinal strength. (e) Rolling fatigue properties A large number of test pieces with a diameter of 12 mm and a length of 22 mm were taken from the test steels shown in Table 1, and each steel was heat treated in the same manner as in the previous section to adjust its hardness. Tested. The test conditions were: surface pressure: 600Kg/mm ² ;
The number of rotations was 46.240 rpm, and the number of repetitions until flaking occurred was measured. The results are shown in FIGS. 3 and 4. As shown in the figure, the service life until flaking of the steel of the present invention is clearly longer than that of comparative steels, especially lead-added steel, and is comparable to or higher than that of general steel (non-free-cutting steel). .

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【Effect of the invention】

The high-performance free-cutting steel for gears of the present invention has improved machinability, grindability, and cold forgeability compared to conventional gear steels by carefully adjusting the composition and morphology of inclusions in the steel. We saw a significant improvement in the results. The strength properties required for gear steel, namely dedendum strength and rolling contact fatigue properties, are comparable to or better than conventional steel, and this steel can be used without problems as a material for a wide variety of gears. .

[Brief explanation of drawings]

FIG. 1 is a graph showing the upset test results of the steel of the present invention and comparative steel. FIG. 2 is a graph showing the root strength test results of the steel of the present invention and comparative steel.
FIG. 3 and FIG. 4 are graphs showing the results of rolling contact fatigue tests of the steel of the present invention and comparative steel.

Claims (1)

[Claims] 1 C: 0.10 to 0.55%, Si: 0.35% or less, Mn:
1.70% or less, Pb: 0.02~0.15%, Al: 0.010~
Contains 0.040%, Ca: 0.0002 to 0.0050%, S: 0.060% or less and N: 0.020% or less, O: 0.0030
% or less, the remainder substantially consisting of Fe,
The inclusions are mainly a composite type of Al ₂ O ₃ -SiO ₂ -CaO-based oxide and (Mn,Ca)S-based sulfide, and the major axis/minor axis ratio is ≦20 and the major axis is 10 μ or less. Fine nonmetallic inclusions account for 80% of total nonmetallic inclusions
Free-cutting steel for high-performance gears, which is characterized by the following: 2 C: 0.10-0.55%, Si: 0.35% or less, Mn:
1.70% or less, Pb: 0.02~0.15%, Al: 0.010~
Contains 0.040%, Ca: 0.0002 to 0.0050%, S: 0.060% or less and N: 0.020% or less, O: 0.0030
% or less, the remainder substantially consisting of Fe,
The inclusions are mainly a composite type of Al ₂ O ₃ -SiO ₂ -CaO-based oxide and (Mn,Ca)S-based sulfide, and the major axis/minor axis ratio is ≦20 and the major axis is 10 μ or less. Fine nonmetallic inclusions account for 80% of the total nonmetallic inclusions
This is a method for producing free-cutting steel for high-performance gears, which accounts for the above, in which molten steel containing the above-mentioned alloy components other than Al, Ca, and Pb is vacuum degassed, and Al and Ca- Refining is performed by adding Si, during which non-oxidizing gas is introduced into the molten steel and forced stirring is performed to float and separate large non-metallic inclusions, and then an appropriate amount of Pb is added to the stirred molten steel stream. A manufacturing method characterized by uniformly dispersing the liquid.