JP2595609B2 - Free cutting steel for carburizing and quenching - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the invention]

(Field of Industrial Application) The present invention relates to steel for machine structural use which is suitably used as a material for components requiring both surface hardness and overall toughness of gears, shafts, retainers, rods, pins and the like. In particular, the present invention relates to a free-cutting steel for carburizing and quenching, which exhibits excellent machinability when carburizing and quenching is performed and a carburized and quenched portion of a surface layer which has been conventionally finished by grinding is finished by cutting. (Prior art) Conventionally, parts such as gears, shafts, retainers, rods, and pins that require both surface hardness and overall toughness are subjected to carburizing and quenching into alloy steel for machine structure called case hardening steel. It is often used after being applied (for example, JIS G 4051 S20C
K, JIS G 4102 SNC415,815, JIS G 4103 SNCM2
20,415,420). In this case, the carburized and quenched part is Vickers hardness (Hv) 800
It has extremely high hardness, and it is extremely difficult to perform cutting with a carbide tool or a high-speed tool. Conventionally, finishing is performed by grinding. (Problems to be Solved by the Invention) However, the grinding process has a disadvantage that the processing efficiency is remarkably lower than the cutting process and the production cost is high due to poor productivity. On the other hand, new tools capable of cutting high-hardness materials, such as cBN tools and ceramic tools, are being developed, and cutting (turning) of high-hardness carburized and quenched portions is becoming possible. However, conventional free-cutting steels, such as sulfur free-cutting steel and lead free-cutting steel, show excellent machinability when cutting with carbide tools or high-speed tools, but are not sufficient for cutting of carburized and quenched parts. No significant improvement in machinability was observed, and the emergence of a new free-cutting steel was an issue. (Object of the Invention) The present invention has been made in view of such conventional problems, and it is possible to perform a good cutting process of a carburized and quenched portion using a new tool such as a cBN tool or a ceramic tool. It is an object of the present invention to provide a free-cutting steel for carburizing and quenching having excellent cutting workability of a carburizing and quenching part.

Configuration of the Invention

(Means for Solving the Problems) The free-cutting steel for carburizing and quenching according to the present invention contains C:
10 to 0.30%, Si: 1.0% or less, Mn: 3.0% or less, Cr: 8.
0% or less, Ni: 5.0% or less, Mo: 6.0% or less, Al: 2.0% or less.
004 to 0.020%, N: 0.005 to 0.050% and N / B: 0.5 to 4.0, and if necessary, Nb: 0.5% or less, V: 0.3% or less. , Also as needed
Ca: 0.008% or less, Pb: 0.10% or less, S: 0.15% or less, Bi: 0.10
% Or less, one or two selected from among Te: 0.05% or less
At least 0.15% or less of O: 0.0015% and total amount of Ti, Zr, REM: 0.01% or less, with the balance being Fe and impurities, and having excellent cutting workability of the carburized and quenched part Thus, the present invention is characterized in that the means for solving the conventional problem described above is provided. The present inventors have conducted basic research on various elements that affect the machinability of the carburized and quenched portion, and have found that BN inclusions are extremely effective, especially in cutting of the carburized and quenched portion with a ceramic tool. In this case, as a method of adding BN to steel, a method of directly adding a BN compound to molten steel and a method of separately adding B and N to molten steel to precipitate in a subsequent process such as solidification or heat treatment. There is a method to make it possible. However, the former method of directly adding a BN compound to molten steel is extremely difficult due to a difference in specific gravity between the BN compound and the molten steel. On the other hand, in the latter method of adding B and N separately to molten steel, the problem of specific gravity difference does not occur as much as the BN compound,
In this case, the contents of B and N are important, especially the ratio is important, and the contents of O (oxygen) and elements having a high degree of nitride formation such as Ti, Zr and REM are to be reduced. Is important from various experiments and studies. From the viewpoint of improving the machinability, it is desirable that the N / B ratio is in the range of 0.5 to 4.0, and further, from the viewpoint of the strength of the carburized and quenched material, the N / B ratio is set to make the crystal grains fine. 1.3-4.0
It is clear that it is desirable to set the range. Furthermore, it has been found that when carburizing and quenching is performed at a high temperature, it is effective to add carbide-forming elements such as Nb and V when it is desired to prevent coarsening of crystal grains. Furthermore, when cutting as roughing before carburizing and quenching, the effect of improving machinability with conventional free-cutting elements is expected, so that a small amount of carburizing and quenching is not adversely affected. It is effective to add a conventional machinability improving element such as Ca, Pb, S, Bi, and Te. The free-cutting steel for carburizing and quenching according to the present invention has been developed as a result of the above-described basic research by the present inventors, and the reasons for limiting the chemical composition (% by weight) will be described in further detail. C: 0.10 to 0.30% C is an element necessary for securing the strength of a machine structural component using the free-cutting steel for carburizing and quenching according to the present invention as a material. For this purpose, 0.10% or more must be contained. It is. However, if contained in a large amount, the overall toughness is reduced, so the upper limit is made 0.30%. Si: 1.0% or less Si is an element effective as a deoxidizing agent, and is an element effective for preventing the occurrence of surface defects in a steel ingot. Therefore, it is more preferable to contain 0.03% or more. However, if contained in a large amount, the toughness of the whole is reduced.
%. Mn: 3.0% or less Mn is an effective element as a deoxidizing agent and a desulfurizing agent. In addition to enhancing the hardenability of steel, it also has the effect of preventing sulfides such as MnS from causing hot embrittlement due to S. Therefore, when it is added, it is more desirable to contain 0.1% or more. However, if it is contained in a large amount, the machinability is deteriorated. Therefore, even if it is added, the upper limit is set to 3.0%. Cr: 8.0% or less Cr is an element effective for improving the hardenability of steel and the strength after quenching and tempering, and may be positively added as appropriate according to the required characteristics of machine structural parts. In order to obtain the above-mentioned effects, it is more desirable to contain 0.1% or more. However, if contained in a large amount, hardenability and machinability are impaired, so even if added, the upper limit is made 8.0%. Ni: 5.0% or less Ni is an element effective for improving the hardenability of steel and the toughness after quenching and tempering, and may be added as appropriate according to the required characteristics of mechanical structural parts. In order to obtain the effect described above, it is more desirable to contain 0.3% or more. However, the machinability is impaired if a large amount is contained, so even if added, the upper limit is made 5.0% or less. Mo: 6.0% or less Mo is an element effective for improving the hardenability of steel and the strength and toughness after quenching and tempering, and may be added as appropriate according to the required characteristics of mechanical structural parts. In order to obtain the above-mentioned effects, more preferably 0.05%
It is preferable to contain the above. However, if it is contained in a large amount, double carbides are formed, and the hardenability is lowered and the machinability is also lowered. Therefore, even if added, the upper limit is made 6.0% or less. Al: 2.0% or less Al acts as a deoxidizing element and is an element effective in reducing the amount of oxygen in the steel and at the same time improving the nitriding properties. In order to obtain the above-mentioned effects, the content is more desirably 0.005% or more. However, if contained in a large amount, the toughness is impaired. Therefore, even if added, the upper limit is made 2.0% or less. B: 0.004 to 0.020%, N: 0.005 to 0.050%, and N / B: 0.5 to 4.0 B and N form BN inclusions in the steel and cut the carburized and quenched part, especially for cBN and ceramic tools. This has the effect of significantly improving the machinability in cutting using a new tool. And, in order to improve such machinability, B is 0.004-0.020%, and N is
It is desirable to make 0.005 to 0.050% and N / B to be in the range of 0.5 to 4.0. Further, from the viewpoint of the strength of the carburized and quenched material, in order to make the crystal grains fine, N should be slightly large. Therefore, it is desirable that N / B be in the range of 1.3 to 4.0. Further, by leaving undissolved B (insol.B) in the steel, the hardenability can be improved, and from such a viewpoint, the amount of undissolved B in the steel can be improved. Is desirably adjusted to be about 0.0006 to 0.0100% if necessary. But in steel
If the BN content is too large, the hot workability of the steel is impaired, so it is necessary to set the above ranges of B, N and N / B. Nb: 0.5% or less, V: 0.3% or less One or two types selected from the group consisting of Nb and V are carbide-forming elements. Carburizing and quenching at high temperatures, for example, shortens carburizing time. In such a case, if it is desired to prevent coarsening of the crystal grains, it is more desirable to use Nb which is a carbide forming element.
0.01% or more, V is more preferably 0.01% or more, and one or two of these may be added as necessary.
However, if the content is too large, coarse carbides are likely to be generated and the toughness is reduced.
It is desirable that V is not more than 0.5% and V is not more than 0.3%. Ca: 0.008% or less, Pb: 0.10% or less, S: 0.15% or less, Bi: 0.10
% Or less, one or two selected from among Te: 0.05% or less
When cutting as roughing before carburizing and quenching, the effect of improving machinability can be expected by adding conventional free-cutting elements, so that the strength characteristics of the carburized and quenched part are not adversely affected. It is also desirable if necessary to add to the steel, and in order to improve the machinability during such roughing, more preferably 0.001% or more of Ca, 0.01% or more of Pb, and 0.05% of S
As described above, the content of Bi is 0.01% or more, the content of Te is 0.005% or more, and one or more of these may be added as necessary.
However, if it is too large, a large amount of inclusions tends to be generated,
If added, the content of Ca is 0.008% or less,
Is 0.10% or less, S is 0.15% or less, Bi is 0.10% or less, Te is
It is desirable to make it 0.05% or less. O: 0.0015% or less If the amount of O contained in the steel is too large, this combines with B
Since B ₂ O ₃ is formed and it is difficult to obtain the effect of generating BN inclusions in the steel and improving the machinability of the carburized and quenched part, the oxygen content in the steel is 0.0015% or less. There is a need to. Total amount of Ti, Zr, REM: 0.01% or less When a large amount of an element having a high degree of nitride formation, such as Ti, Zr, REM (one or more of rare earth elements), is included, it combines with N to form a nitride. The effect of improving the machinability of the carburized and quenched part by forming BN inclusions in the steel and making it difficult to obtain a sufficient effect becomes difficult, so Ti, Zr, REM The total amount must be 0.01% or less. (Example) The present invention steel and comparative steel were melted using a vacuum induction melting furnace and cast into ingots, respectively. Table 1 shows the chemical components of each ingot. Next, a steel ingot having the chemical composition shown in Table 1 was cast into a round bar having a diameter of 25 mm in a hot state, subjected to normalizing treatment, and then a part thereof was further carburized and quenched. Next, a cutting test by drilling was performed on the normalized material under the conditions shown in Table 2. Further, a cutting test by turning was performed on the carburized and quenched material under the conditions shown in Table 3. In the drilling test, the cutting speed was such that the tool life was 5000 mm. In the turning, the tool life at a cutting speed of 100 m / min was used, and the comparative steel 5 was 100. The machinability of each test steel was evaluated at the ratio of Table 4 shows the results. As is clear from the results shown in Table 4, the steel contains BN, the O content is reduced to 0.0015% or less, and Ti + Zr +
In any of the steels of the present invention in which the total amount of REM was 0.01% or less, B
Compared to comparative steels No. 3, 7, 12, 16 and 17, which do not contain N in steel, both drilling workability with high-speed tools and turning workability with carbide tools are considerably better,
It is clear that the cutting workability is excellent before and after carburizing and quenching, in particular. On the other hand, although the BN is contained in the steel, the O content and the total content of Ti + Zr + REM are too large.
It was confirmed that the comparative steel No. 9 in which the amount was too large was inferior in both drilling workability and turning workability.

【The invention's effect】

As described above, the free-cutting steel for carburizing and quenching according to the present invention is, by weight%, C: 0.10 to 0.30%, Si: 1.0% or less,
And Mn: 3.0% or less, Cr: 8.0% or less, Ni: 5.0% or less, Mo: 6.
One or two selected from 0% or less, Al: 2.0% or less
More than species, B: 0.004-0.020%, N: 0.005-0.0
50% and N / B: 0.5 to 4.0, if necessary Nb: 0.5%
Below, V: contains one or two selected from 0.3% or less, and, if necessary, Ca: 0.008% or less, Pb: 0.10% or less, S: 0.15% or less, Bi: 0.10% or less, Te : One or more selected from 0.05% or less, O: 0.0015% or less, and total amount of Ti, Zr, REM: regulated to 0.01% or less, the balance
Since it is composed of Fe and impurities, it shows excellent machinability when carburizing and quenching is performed using a new tool such as a cBN tool or ceramics tool after carburizing and quenching. It is possible to significantly improve the processing efficiency as compared with grinding, and it is necessary to use gears, shafts,
This provides an excellent effect that productivity of the retainer, rod, pin, and the like can be significantly improved.

Claims (4)

(57) [Claims] (1) In terms of% by weight, C: 0.10 to 0.30%, Si: 1.0% or less, Mn: 3.0% or less, Cr: 8.0% or less, Ni: 5.0% or less, M
o: At least one selected from 6.0% or less, Al: 2.0% or less, B: 0.004 to 0.020%, N: 0.005
-0.050% and N / B: 0.5-4.0, O: 0.0015% or less, and total amount of Ti, Zr, REM: 0.01% or less. Free-cutting steel for carburizing and quenching characterized by excellent workability. 2. C. by weight%: C: 0.10 to 0.30%, Si: 1.0% or less, Mn: 3.0% or less, Cr: 8.0% or less, Ni: 5.0% or less, M
o: At least one selected from 6.0% or less, Al: 2.0% or less, B: 0.004 to 0.020%, N: 0.005
0.00.050% and N / B: 0.5-4.0, and Nb: 0.
One or two selected from 5% or less, V: 0.3% or less
Including seeds, O: 0.0015% or less, and total amount of Ti, Zr, REM: 0.
A free-cutting steel for carburizing and quenching characterized by being regulated to 01% or less, with the balance being Fe and impurities, and having excellent machinability in the carburized and quenched part. (3) In weight%, C: 0.10 to 0.30%, Si: 1.0% or less, Mn: 3.0% or less, Cr: 8.0% or less, Ni: 5.0% or less, M
o: At least one selected from 6.0% or less, Al: 2.0% or less, B: 0.004 to 0.020%, N: 0.005
~ 0.50% and N / B: 0.5 ~ 4.0, and Ca: 0.
008% or less, Pb: 0.10% or less, S: 0.15% or less, Bi: 0.10% or less, Te: One or more selected from 0.05% or less, O: 0.0015% or less, and Ti, Total amount of Zr, REM: 0.
A free-cutting steel for carburizing and quenching characterized by being regulated to 01% or less, with the balance being Fe and impurities, and having excellent machinability in the carburized and quenched part. 4. In% by weight, C: 0.10 to 0.30%, Si: 1.0% or less, Mn: 3.0% or less, Cr: 8.0% or less, Ni: 5.0% or less, M
o: At least one selected from 6.0% or less, Al: 2.0% or less, B: 0.004 to 0.020%, N: 0.005
0.00.050% and N / B: 0.5-4.0, and Nb: 0.
One or two selected from 5% or less, V: 0.3% or less
Including seeds, Ca: 0.008% or less, Pb: 0.10% or less,
S: 0.15% or less, Bi: 0.10% or less, Te: 0.05% or less, including one or more selected from O, 0.0015% or less,
And the total amount of Ti, Zr and REM: 0.01% or less, respectively.
A free-cutting steel for carburizing and quenching, characterized by the balance of Fe and impurities, and excellent cutting workability of the carburized and quenched part.