JP2008195997A - Steel for low strain vacuum carburizing gas quenching, and low strain carburized component produced therefrom - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for low strain vacuum carburizing gas quenching capable of obtaining a carburized component having reduced strain, in case hardening steel used for producing a machine component used after being subjected to carburizing such as a gear. <P>SOLUTION: The steel has an alloy composition comprising, by weight, 0.10 to 0.30% C, 0.50 to 3.00% Si, 0.30 to 3.00% Mn, ≤0.03% P, ≤0.03% S, 0.01 to 1.00% Cu, 0.01 to 3.00% Ni and 0.30 to 1.00% Cr, and further comprising one or more kinds selected from ≤0.20% Al, ≤0.20% Nb, ≤0.20% Ti, ≤0.05% N and ≤0.01% B, and the balance Fe with inevitable impurities, and satisfying the condition of [Si%]+[Ni%]+[Cu%]-[Cr%]>0.30. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a steel for low strain vacuum carburizing gas quenching that can obtain a product with low distortion by cooling with high pressure gas and quenching instead of conventional oil quenching in the manufacture of carburized parts. The invention also relates to a method for producing low strain carburized parts from this steel and the resulting low strain carburized parts.

When steel is formed into machine parts, such as gears, and carburized into products, in order to ensure the impact strength of the products, the depth of the grain boundary oxide layer during carburizing should be reduced and temper softening resistance It is desirable to increase. From such a point of view, a case hardening steel is disclosed in which a relatively large amount (0.40 to 1.50%) of Si is selected and specific amounts of Ti, B and N are added (Patent Document 1). Regarding temper softening resistance, there is a proposal of an alloy composition in which the value of Si + 0.2Cr + 0.01Mo exceeds 1.3% (Patent Document 2).
JP 2004-300550 A JP2003-231943

As a carburizing method, a vacuum carburizing method has recently been adopted in place of the conventionally used gas carburizing method. The vacuum carburizing method has the following advantages over the gas carburizing method:
1) Since the material is not oxidized because the treatment is performed in a vacuum, grain boundary oxidation, which is likely to occur in the gas carburizing method, is avoided, which contributes to improvement in strength.
2) Due to the structure of the carburizing device, it is easy to perform high-temperature carburizing, which enables quick carburizing.
3) The carburizing gas used is small, and the running cost is low.
It is bought and used to manufacture various gears and shafts. However, vacuum carburization has a drawback in that there is a large difference in the degree of carburization depending on the parts. Specifically, the edge-shaped portion tends to have a higher carbon concentration than the planar portion, and as a result, the edge portion protrudes.

The carburizing operation is followed by quenching to quench the carburized product. Conventionally, this quenching has been based on oil quenching which is exclusively put into the cooling oil. In recent years, gas quenching, in which quenching is performed by blowing high-pressure gas, is becoming popular. For gas quenching,
1) Since the cooling rate can be adjusted by changing the pressure of the cooling gas and the wind speed to be blown, the range in which the cooling rate can be controlled is wide, and the optimum cooling condition for each component can be selected.
2) No oil tank or pit is required, and there are few restrictions on the installation location of equipment.
3) Since quenching oil, which is a dangerous material, is not used, it is not necessary to manage dangerous materials.
4) The working environment is improved.
There are advantages such as.

However, gas quenching has a disadvantage that distortion caused by quenching is larger than oil quenching. In addition, the edge is large in how the distortion occurs. The main reason is the difference in the retained austenite phase after quenching. As is well known, the austenite phase has a small expansion. In the case of oil quenching, the reached temperature of the parts after cooling is about 100 ° C., and since there is a large amount of residual austenite phase, no significant expansion occurs even in parts with high carbon concentration, whereas when gas quenching is performed, Since it is cooled to room temperature and the residual amount of the austenite phase is small, the high carbon portion expands significantly and the strain increases.

For this reason, vacuum carburization and gas quenching are both advantageous processing technologies, but performing them in combination becomes a bottleneck because the distortion of the product after carburizing and quenching becomes significant. It was not done. The inventor attempts to break this bottleneck, and as a measure, the inventors believe that preventing excessive carburization that occurs in the edge-shaped part of the component will be a fundamental solution, and an alloy composition that is unlikely to cause excessive carburization. Pursued. As a result, it is known that Cr, which tends to form carbides, is a major cause of excessive carburization, whereas Si, Ni, and Cu have an action to prevent excessive carburization, and in particular, there are specific We found that the relevant alloy composition should be selected.

The object of the present invention is to utilize the above-described knowledge of the inventor, and even if gas quenching is performed following vacuum carburization, the distortion generated in the components can be reduced, and as a result, the two operations can be combined. It is to provide a steel for strained vacuum carburizing gas quenching. It is also included in the object of the present invention to provide a low strain carburized part using this steel as a material and to provide the carburized part.

The steel for low-distortion vacuum carburizing gas quenching of the present invention is, by weight, C: 0.10 to 0.30%, Si: 0.50 to 3.00%, Mn: 0.30 to 3.00%, P: 0.03% or less, S: 0.03% or less, Cu: 0.01 to 1.00%, Ni: 0.01 to 3.00% and Cr: 0.30 to 1.00% Furthermore, one or more of Al: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less, N: 0.05% or less, and B: 0.01% or less And the balance consists of Fe and inevitable impurities,
[Si%] + [Ni%] + [Cu%]-[Cr%]> 0.30
This steel has an alloy composition that satisfies the following conditions.

If carburized parts are manufactured using the low-distortion vacuum carburizing gas quenching steel of the present invention, the advantages inherent in the vacuum carburization described above, that is, avoidance of grain boundary oxidation, rapid carburizing and low cost are obtained. In addition, the advantages of gas quenching, that is, the optimal cooling conditions for each part, the few restrictions on equipment, the management of dangerous goods is unnecessary, and the working environment is improved. You can enjoy it.

In addition to the basic alloy components described above, the low-distortion vacuum carburizing gas quenching steel of the present invention can further contain Mo: 2.00% or less.

As long as the carburized parts of the present invention are manufactured by vacuum carburizing, various hydrocarbon gases such as acetylene, ethylene, and propane can be used as the carburizing gas. The carburization pattern is also arbitrary. Those skilled in the art will be able to easily determine appropriate vacuum carburizing conditions with reference to the examples described below. Gas quenching can also be performed according to techniques known in the art.

Hereinafter, the alloy composition of the carburizing steel used as a material in the present invention will be described in the order of essential components and optional components.
C: 0.10 to 0.30%
This is an appropriate range for obtaining the strength required for machine parts. When the amount of C is less than the lower limit of 0.10%, ferrite is generated at the core of the component, and the strength is lowered. On the other hand, if it exceeds the upper limit of 0.30%, the workability, particularly the machinability, is lowered, which is disadvantageous for the molding of parts.

Mn: 0.30 to 3.00%
Mn is added as a deoxidizer during the melting of steel, but since it does not significantly affect the formation of carbides, the amount can be selected from a wide range. However, if a small amount does not reach 0.30%, ferrite forms in the core and the strength decreases, and if it exceeds 3.00%, the workability, particularly machinability, decreases. The same as the reason for limiting the amount.

P: 0.03% or less, S: 0.03% or less These are impurities and are undesirable components for the mechanical properties of the parts, such as causing embrittlement. Therefore, the amount is preferably low. Both of the above values are acceptable limits.

Si: 0.50 to 3.00%, Ni: 0.01 to 3.00%, Cu: 0.01 to 1.00%
As described above, these elements are components that suppress the formation of carbides, each of which is not less than the above lower limit value, and a value obtained by subtracting the amount of Cr from the total of these amounts is 0.30. , Preferably above 0.50. However, a large amount of addition all decreases hot workability. Si deteriorates workability, particularly machinability, and if Ni and Cu are too large, the amount of carbide deposited becomes too low, resulting in insufficient strength of the part. From these viewpoints, the above upper limit is set for each.

Cr: 0.30 to 1.00%
Since Cr is a component that promotes the formation of carbides, it cannot be present in large quantities in the carburizing steel of the present invention. 1.00% is the upper limit of the amount of Cr that can be achieved when Si, Ni, and Cu, which are components that suppress the formation of carbides, are abundant. A Cr amount higher than 1.00% cannot be added from the viewpoint of workability, particularly machinability. However, if it is reduced too much, the hardenability becomes low and the mechanical properties of the product become unsatisfactory, so 0.30% was set as the lower limit.

Al: 0.20% or less There is a function of suppressing the coarsening of crystal grains, and when it is desired to obtain the effect, 0.005% or more is added. Excessive addition must be avoided because alumina is formed in the steel, resulting in a decrease in strength. Also, the formation of alumina is not preferable from the viewpoint of impairing workability. For this reason, an Al addition amount of up to 0.20% is selected.

Nb: 0.20% or less, Ti: 0.20% or less These components are effective for the purpose of suppressing the growth of crystal grains generated during carburizing and maintaining a sized structure. Excessive addition has an adverse effect on processability, so the addition amount is limited to the above limit.

N: 0.05% or less When N is present, there is an effect of preventing the coarsening of crystal grains. Therefore, it is preferable that at least 0.001% be present. Since this effect is saturated at about 0.05%, it does not make it exist beyond this limit.

B: 0.01% or less B is effective in improving hardenability, so is added as desired. Since the presence of a large amount is harmful to processability, an addition amount of 0.01% or less is selected.

Mo: 2.00% or less It can be added to improve hardenability and increase temper softening resistance. When the amount is too large, the workability of steel deteriorates, so an appropriate amount of addition of 2.00% or less should be selected.

Case-hardened steels having the alloy compositions shown in Table 1 (Examples) and Table 2 (Comparative Examples) were melted. The table also shows the value of [Si%] + [Ni%] + [Cu%]-[Cr%]. Each case-hardened steel was machined into the shape of the following test helical gear.
Module: 2.5
Number of teeth: 26
Pitch circle diameter: 71.7mm
Teeth width: 20mm

Gas quenching is performed by performing vacuum carburization at 950 ° C. for 120 minutes, followed by a diffusion step at 850 ° C. for 30 minutes, and cooling by blowing N ₂ gas of 1 × 10 ³ KPa at a wind speed of 1 m / second. It was. As shown in Tables 1 and 2, the gas used for vacuum carburization is propane (P) or acetylene (A). The distortion of the tooth surface of the test gear, which was the product of the obtained carburizing gas quenching, was measured with an Arasa meter. The results are shown in Table 1 and Table 2 together.

Comparative Examples 1 to 4 do not satisfy the condition of [Si%] + [Ni%] + [Cu%] − [Cr%]> 0.30. Although Comparative Examples 5 and 6 satisfy this condition, the former has an excessive amount of Cr and the latter has an insufficient amount of Si, both of which are outside the range of the alloy composition of the present invention. In the comparative example, the strain amount reached 30 to 70 μm, whereas in the examples of the present invention, the strain amount was not much higher than 20 μm, and carburization with small strain was achieved even when using a combination of vacuum carburization and gas quenching. Parts were obtained.

Claims (3)

% By weight, C: 0.10 to 0.30%, Si: 0.50 to 3.00%, Mn: 0.30 to 3.00%, P: 0.03% or less, S: 0.03 %: Cu: 0.01 to 1.00%, Ni: 0.01 to 3.00% and Cr: 0.30 to 1.00%, Al: 0.20% or less, Nb : 0.20% or less, Ti: 0.20% or less, N: 0.05% or less, and B: 0.01% or less, containing one or more, the balance being Fe and inevitable impurities ,
[Si%] + [Ni%] + [Cu%]-[Cr%]> 0.30
Steel for low strained vacuum carburizing gas quenching having an alloy composition that satisfies the following conditions. Furthermore, the steel for low distortion vacuum carburizing gas quenching of Claim 1 containing Mo: 2.00% or less. A method for producing a low-distortion carburized part comprising forming the steel according to claim 1 or 2 into a part shape, performing vacuum carburizing, and then performing gas quenching.