JPH01191764A - Free cutting steel for carburization quenching - Google Patents

Abstract

PURPOSE:To improve the machinability of the carburization quenched part of the title steel having specific compsn. by a new tool such as cBN tool, ceramic tool, etc., by specifying the contents of B and O and the ratio of N/B in said steel. CONSTITUTION:The free cutting steel for carburization quenching contains, by weight, one or more kinds among 0.10-0.30% C, <=1.0% Si, <=3.0% Mn, <=8.0% Cr, <=5.0% Ni, <=6.0% Mo and <=2.0% Al, furthermore contains 0.004-0.020% B and 0.005-0.050% N as well as 0.5-4.0 N/B, contains <=0.0015% O, in which the total amounts of elements having high generated degree of nitride such as Ti, Zr and REM is regulated to <=0.01 and the balance consisting of Fe. In the steel, B and N forms BN inclusions and the machinability at the time of machining the carburization quenched part, particularly at the time of machining in the use of said new tool is drastically improved. The contents of B and N and N/B are preferably regulated to the above range in order to improve its machinability.

Description

[Detailed description of the invention] [Purpose of the invention]

(Industrial Application Field) The present invention relates to mechanical structural steel that is suitably used as a material for parts such as gears, shafts, retainers, rods, and pins that require both surface hardness and overall toughness. In particular, it relates to a free-cutting steel for carburizing and quenching that exhibits excellent machinability when carburizing and quenching and finishing the carburized and quenched portion of the surface layer, which was conventionally finished by grinding, by cutting. (Prior technology) Conventionally, parts such as gears, shafts, retainers, doors, and bottles that require both surface hardness and overall toughness are made of carburized machine structural alloy steel called case hardening steel. It is often used after being hardened (for example, JIS G 405
1 520CK, JIS G 4102 5NC4
15,815, JIS G 4103 SN0M
2jO. 415.420 etc.). In this case, the carburized and quenched part has a Vickers hardness (HV) of 8
The hardness is as high as 0.0000000000000000000000000000000000000000000000000000000000000000, it is extremely difficult to cut with carbide tools or high-speed steel tools. Conventionally, finishing was done by grinding. (Problems to be Solved by the Invention) However, grinding has a drawback that processing efficiency is significantly lower than that of cutting, and production costs are high due to inferior productivity. On the other hand, new tools such as cBN tools and ceramic tools that are capable of cutting high-hardness materials are being developed, and cutting (turning) of high-hardness carburized and quenched parts is also becoming possible. However, although conventional free-cutting steels such as sulfur free-cutting steel and lead free-cutting steel exhibit excellent machinability when cutting with carbide tools or high-speed steel tools, they are insufficient when cutting carburized and hardened parts. No significant improvement in machinability was observed, and the emergence of a new free-cutting steel was a challenge. (Object of the Invention) The present invention has been made in view of such conventional problems, and it is possible to perform cutting of carburized and quenched parts well using a new tool made of a cBN tool, a ceramic tool, etc. The purpose of the present invention is to provide a free-cutting steel for carburizing and quenching that has excellent machinability in carburized and quenched parts.

[Structure of the invention]

(Means for Solving the Problems) The free-cutting steel for carburizing and quenching according to the present invention has C:0 in weight%.
．． 10 to 0.30%, Si: 1.0% or less, and M
n: 3, 0% or less, Cr: 8.0% or less, Ni
: 5.0% or less, Mo: 6.0% or less, AJI: 2.0
Contains one or more types selected from % or less,
Furthermore, B: 0.004-0.020%, N: 0.005
~0.050% and includes N/B: 0, 5 to 4.0, Nb: 0.5% or less, V: 0.3% as necessary
Including one or two types selected from the following (7)'),
Similarly, if necessary, Ca: 0.008% or less, Pb: 0
．． 10% or less, S: O, 15% or less, Bi: 0.10
% or less, Te: 0.05% or less, including 0:0.0015% or less, and the total amount of elements with a high degree of nitride formation such as Ti, Zr, and REM. : 0.01% or less, the remainder consists of Fe and impurities, and the carburized and quenched part has excellent machinability, thereby solving the above-mentioned conventional problems. It is a feature. The present inventors have carried out basic research on various elements that affect the rigidity of carburized and quenched parts, particularly when cutting carburized and quenched parts with ceramic tools. It has been found that BN inclusions are extremely effective. In this case, there are two methods for adding BN into steel: one is to add the BN compound directly to molten steel, and the other is to add B and N separately to molten steel so that they precipitate during the solidification process or subsequent processes such as heat treatment. There are ways to do this. However, the former method of directly adding a BN compound to molten steel is extremely difficult due to the difference in specific gravity between the BN compound and molten steel. On the other hand, the latter method of adding B and N separately to molten steel does not cause the problem of the difference in specific gravity of the BN compound, but
In this case, the content of B and N is important, especially their ratio, and the content of elements with high nitride formation such as Ti, Zr, and REM and O (oxygen) must be kept low. It has become clear from various experiments and studies that this is important. From the point of view of improving machinability, the N/B ratio is 0.5 to 4.
It is desirable that the N/B ratio be in the range of 0, and furthermore, from the point of view of the strength of the carburized and quenched material, it is clear that it is desirable that the N/B ratio be in the range of 1.3 to 4.0 in order to make the crystal grains fine. It became. Furthermore, when it is desired to prevent coarsening of crystal grains when performing carburizing and quenching at high temperatures, Nb,
It has been found that it is effective to add a carbide-forming element such as V. Furthermore, when rough cutting is performed before immersion hardening, conventional free-cutting elements are expected to have the effect of improving machinability. Trace amount c7) Ca, Pb, S, Bf, Te
It is effective to add conventional machinability improving elements such as. The free-cutting steel for carburizing and quenching according to the present invention was developed as a result of the above-mentioned basic research by the present inventors, and the reasons for limiting its chemical composition (wt%) will be explained in more detail. C: 0.10 to 0.30% C is an element necessary to ensure the strength of mechanical structural parts made of free-cutting steel for carburizing and quenching according to the present invention,
For this purpose, it is necessary to contain 0.10% or more. However, if it is contained in a large amount, the overall toughness is reduced, so the upper limit is set at 0.30%. Si: 1.0% or less Si is an effective element as a deoxidizing agent and an effective element for preventing the occurrence of surface defects in steel ingots, so it is more desirable to contain it at 0.03% or more. However, if it is contained in a large amount, the overall toughness is reduced, so the upper limit is set at 1.0%. Mn: 3.0% or less Mn is an effective element as a deoxidizing agent and desulfurizing agent, which not only improves the hardenability of steel, but also prevents hot embrittlement caused by S by forming sulfides such as MnS. Since it is effective, it is more preferable to add 0. ．． It is good to add 1% or more, but if it is added in a large amount, machinability deteriorates, so even if it is added, the upper limit is 3.0%.
%. Cr: 8.0% or less Cr is an effective element for improving the hardenability of steel and its strength after quenching and tempering, and it may be actively added as appropriate depending on the required characteristics of mechanical structural parts. In order to obtain the above-mentioned effects, the content is more preferably 0.1% or more. However, if it is contained in a large amount, hardenability and machinability will be impaired, so even if it is added, the upper limit is set at 8.0%. Ni: 5.0% or less Ni is an effective element for improving the hardenability of steel and the toughness after quenching and tempering, and it is recommended to actively add it as appropriate depending on the required characteristics of mechanical structural parts. In order to obtain the above-mentioned effects, it is more desirable to contain 0.3% or more.However, machinability will be impaired if the content is large, so even if added, the upper limit should be 5.0% or less. do. Mo: 6.0% or less Mo is an effective element for improving the hardenability of steel and the strength and toughness after quenching and tempering, and should be actively added as appropriate according to the required characteristics of mechanical structural parts. and more preferably 0.05 in order to obtain the above-mentioned effect.
However, if it is added in a large amount, double carbides are formed and the hardenability and machinability are reduced, so even if it is added, the upper limit is 6.0%.
% or less. AjL: 2.0% or less Since A acts as a deoxidizing element and is an effective element for reducing the amount of oxygen in steel and improving nitriding properties, it should be actively added as necessary. In order to obtain the above-mentioned effects, it is more desirable to contain o, oos% or more. However, if it is added in a large amount, the toughness will be impaired, so even if it is added, the upper limit should be 2.0%. The following shall apply. B: 0.004-0.020%, N: 0.005-0.050%, and N/B: 0. 5-4.0 B and N form BN inclusions in steel, which significantly improves the machinability of carburized and quenched parts, especially when cutting using new tools such as cBN and ceramic tools. It has a positive effect. In order to improve such machinability, B should be adjusted to 0.004 to 0.0.
20%, N 0.005 to 0.050%, and N/B in the range of 0.5 to 4.0. Furthermore, from the viewpoint of the strength of the carburized and quenched material, the crystal grains are In order to make it fine, N/B should be set to 1 so that N becomes slightly threaded.
．． It is desirable that the value be in the range of 3 to 4.0.
In addition, by leaving undissolved B (insofL, B) in the steel, it is possible to improve the hardenability.
If necessary, it is desirable to adjust the amount so that the amount is about o,oos~0.0100%.However, if the amount of BN in the steel is too large, it will impair the hot workability of the steel. Therefore, it is necessary to keep the above-mentioned ranges of B, N, and N/B. Nb: 0.5% or less, V: 0.3% or less, one or two selected from Nautica Nb and V are carbide-forming elements, and by performing carburizing and quenching at particularly high temperatures, for example, carburizing time can be shortened. If you want to prevent the coarsening of crystal grains when measuring the
It is also good to add one or two of these as necessary, but if the amount is too large, coarse carbides are likely to be formed and the toughness will be reduced, so even if Nb is added, 0 .5% or less, and ■ is preferably 0.3% or less. Ca: 0.008% or less, Pb: 0.10% or less, S:
0.15% or less, B i: 0, lQ% or less, Te: 0.05% or less, when cutting is performed as rough processing before carburizing and quenching. Since the addition of conventional free-cutting elements can be expected to improve machinability, it is desirable to add them to an extent that does not adversely affect the strength characteristics of the carburized and quenched part.
In order to improve machinability during such rough machining, more preferably Ca is 0.001% or more, Pb is 0.01% or more, S is 0.05% or more, and Bi is 0.01% or more. However, it is also possible to add 0.005% or more of Te and one or more of these as necessary. If the amount is too large, inclusions tend to form in large quantities, reducing toughness and manufacturability of the steel.
is 0.10% or less, S is 0.15% or less, Bi is 0.1
It is desirable that the content of Te is 0% or less, and Te is 0.05% or less. 0: 0.0015% or less If the amount of O contained in the steel is too large, it combines with B and becomes B.
The oxygen content in the steel is set at 0.0015% because 2O3 is formed and the effect of generating BN inclusions in the steel and improving the machinability of the carburized and quenched part becomes difficult to obtain.
It is necessary to do the following. Total amount of elements with a high degree of nitride formation such as Ti, Zr, REM: o, oi% or less A large amount of elements with a high degree of nitride formation such as Ti, Zr, REM (one or more rare earth elements) If included,
It is difficult to sufficiently obtain the effect of combining with N to form nitrides, producing BN inclusions in the steel, and improving the machinability of carburized and hardened parts. The total amount must be 0.01% or less. (Example) Inventive steel and comparative steel were melted using a vacuum induction melting furnace,
Each was cast into a steel ingot, and Table 1 shows the chemical composition of each steel ingot. Next, the steel ingots having the chemical composition shown in Table 1 above were hot forged into round bars with a diameter of 25 mm, normalized, and then partially carburized and quenched. Next, the normalized materials were subjected to a cutting test by drilling under the conditions shown in Table 2. Table 2 Furthermore, the carburized and quenched materials were subjected to a cutting test by turning under the conditions shown in Table 3. Table 3: In the drilling test, a cutting speed that would give a tool life of 5000 mm was used, and in the turning process, a cutting speed of 100 m/min was used.
Comparative steel 5 and 100 respectively using the tool life of
The stiffness of each test steel was evaluated based on the ratio when These results are shown in Table 4. Table 4 As is clear from the results shown in Table 4, when BN is contained in the steel and the ON is 0.0015% or less, T
i+Zr+; In all of the steels of the present invention in which the total amount of REM is 0.01% or less, the comparative fi in which BN is not contained in the steel
No. 3, 7. 12. Compared to 16 and 17, both the drilling workability with a high-speed steel tool and the turning workability with a carbide tool are considerably superior.It has excellent cutting workability before carburizing and quenching and especially after carburizing and quenching. It is clear that there are. On the other hand, although BN is contained in the steel, 0 amount and the total amount of Ti + Zr + REM are too large.
It was confirmed that comparative steels No. 9 and No. 9, which had an even larger amount of , 8 and 0, were inferior in both drilling workability and turning workability.

【Effect of the invention】

As explained above, the free-cutting steel for carburizing and quenching according to the present invention has C:0. lO ~ 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, Ai: 2.0% or less, and further includes B: 0.004 to 0.0.
20%, N: 0.005-0.050% and N/B:
0.5 to 4.0, and one or two selected from Nb: 0.5% or less and V: 0.3% or less, if necessary.
Including seeds, and also as necessary 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, O: '0.0015
% or less, and the total amount of elements with a high degree of nitride formation such as Tf, Zr, and REM: 0.01% or less, and the balance is Fe.
Therefore, when cutting the carburized and quenched part using a new tool such as a cBN tool or a ceramic tool after the carburizing and quenching process, it shows excellent machinability and is better than the conventional grinding process. It is possible to significantly improve machining efficiency compared to machining, and it can be used for gears, shafts, etc. that require surface hardness and overall toughness.
This has an excellent effect in that the productivity of retainers, rods, bottles, etc. can be significantly improved. Patent applicant: Daido Steel Co., Ltd. Representative patent attorney: Yutaka Oshio

Claims (4)

[Claims] (1) In weight%, C: 0.10-0.30%, Si: 1
．． 0% or less, and Mn: 3.0% or less, Cr: 8.0
% or less, Ni: 5.0% or less, Mo: 6.0% or less, A
Contains one or more selected from l: 2.0% or less, further B: 0.004 to 0.020%, N:
0.005-0.050% and N/B: 0.5-4.
0:0.0015% or less, and Ti, Zr
, Total amount of elements with high nitride formation such as REM: 0.0
1. A free-cutting steel for carburizing and quenching, characterized in that it is regulated to 1% or less, the balance is Fe and impurities, and it has excellent machinability in carburized and quenched parts. (2) In weight%, C: 0.10-0.30%, Si: 1
．． 0% or less, and Mn: 3.0% or less, Cr: 8.0
% or less, Ni: 5.0% or less, Mo: 6.0% or less, A
Contains one or more selected from l: 2.0% or less, further B: 0.004 to 0.020%, N:
0.005-0.050% and N/B: 0.5-4.
0, further Nb: 0.5% or less, V: 0.3
% or less, including one or two selected from O:
The total amount of elements with a high degree of nitride formation such as Ti, Zr, and REM is regulated to 0.0015% or less, and the total amount of elements with a high degree of nitride formation such as Ti, Zr, and REM is regulated to 0.01% or less, with the remainder consisting of Fe and impurities, and has excellent machinability in carburized and quenched parts. Free-cutting steel for carburizing and quenching. (3) In weight%, C: 0.10-0.30%, Si: 1
．． 0% or less, and Mn: 3.0% or less, Cr: 8.0
% or less, Ni: 5.0% or less, Mo: 6.0% or less, A
Contains one or more selected from l: 2.0% or less, further B: 0.004 to 0.020%, N:
0.005-0.050% and N/B: 0.5-4.
0, and also 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, O: 0.0015% or less, and the total amount of elements with a high degree of nitride formation such as Ti, Zr, and REM: A free-cutting steel for carburizing and quenching, characterized in that it is regulated to 0.01% or less, the remainder is Fe and impurities, and that the carburized and quenched part has excellent machinability. (4) In weight%, C: 0.10-0.30%, Si: 1
．． 0% or less, and Mn: 3.0% or less, Cr: 8.0
% or less, Ni: 5.0% or less, Mo: 6.0% or less, A
Contains one or more selected from l: 2.0% or less, further B: 0.004 to 0.020%, N:
0.005-0.050% and N/B: 0.5-4.
0, further Nb: 0.5% or less, V: 0.3
% or less, and furthermore Ca: 0.008% or less, Pb: 0.10% or less, S: 0.15% or less, Bi: 0.10% or less, Te :
Contains one or more selected from 0.05% or less, O: 0.0015% or less, and Ti, Zr,
Total amount of elements with high nitride formation such as REM: 0.01
A free-cutting steel for carburizing and quenching, characterized in that the balance is Fe and impurities, and has excellent machinability in carburized and quenched parts.