JPH01129952A - Steel for rotating parts having longer service life and exellent chip treatability - Google Patents

Abstract

PURPOSE:To improve the chip treatability of the title steel without lowering the characteristics of its rotating fatigue service life by regulating O and Ti in a steel to low level and specifying the contents of P, S, Al, N and Bi. CONSTITUTION:The compsn. ot the steel for rotating parts having longer service life is formed with, by weight, 0.07-1.20% C, 0.03-1.50% Si, 0.10-2.00% Mn, 0.015-0.050% Al, <=0.02% P, <=0.02% S, <=0.005% Ti, <=0.0015% O, 0.003-0.020% N, 0.001-0.040% Bi and the balance Fe with inevitable impurities. If required, one or more kinds among <=5.00% Ni, <=2.00% Cr, <=1.00% Mo, <=0.2% V and <=0.2% Nb are incorporated thereto. By this method, the chips treatability of the steel can be improved without deteriorating its mechanical characteristics, rotating and bending fatigue strength, impact fatigue service life, rotating fatigue service life, etc.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to steel for long-life rolling parts with good chip disposal properties, mainly used in various gears, shafts, pins, bearings, etc. for automobiles and construction machinery. .

(Conventional technology and problems) In Japan, free-cutting steel is used in various fields, including the automobile industry. However, there are few materials that satisfy both mechanical properties and transfer fatigue life characteristics as well as machinability.
Low-oxygen, low-lead free-cutting steel is used for parts that require good machinability, low anisotropy, and somewhat good fatigue resistance. -However, since lead is a substance harmful to the human body, it is not uncommon for customers to dislike the use of low-acid, low-lead free-cutting steel.

(Means for Solving the Problems) The present invention has been made to solve the problems of the prior art described above, and the mechanical properties and rolling fatigue life characteristics are the same as those without the addition of free-cutting elements such as lead and sulfur. The object of the present invention is to provide a steel for transfer parts that is equivalent to or better than steel, has particularly good chip disposal properties, is harmless to the human body, has high toughness, and has a long life.

In order to achieve this objective, the present inventors have conducted numerous improvement studies on carburized, quenched and tempered case hardened steel, induction quenched and tempered medium carbon steel, and quenched and tempered high carbon steel.

As a result, among the trace impurities contained in the steel, especially the amount of oxygen was reduced to 0.0015% or less, Ti was reduced to an extremely low level of 0°005% or less, P was reduced to 0.02% or less, and S was reduced to 0.02%. %
At the same time, 0.015 to 0.050% Al, N
By controlling the amount of Bi to 0.003-0.020% and further adding 0.001-0.040% Bi, it is possible to maintain good mechanical properties, maintain good rolling fatigue life characteristics, and have excellent chip disposability. Discovered that steel for moving parts could be obtained,
This invention has been made.

The first invention steel in the present invention has e O,07~ in weight%
1.20%, Si 0.03-1.50%, Mn Q-
10-2.00%, 20102% or less, SO, 02%
Hereinafter, Al O,015~o, oso%, NO,00
3 to 0.020%, 0.0015% or less, Ti 0.
005% or less, Bi 0.001~0.040%, balance Fe and inevitably contained impurities.The second invention steel in the present invention is CQ, 07~ 1.2Q%, Si 0.03-1.5
0%, MnO, 10-2゜00%, P 0.02% or less, SO, 02% or less, AI 0.015-0.05
0%, NO,003-0.020%, OO,0015
% or less, TiO,005% or less, BiO,001~
0.040%, and if necessary, Ni 5.00% or less, Cr 2.00% or less, Mo 1.00% or less, V
It is a steel for rolling parts that has good chip disposal properties and contains one or more of O, 2% or less, Nb, O, 2% or less, and the balance is Fe and impurities that are inevitably included.

Next, the reason for limiting the composition range of the steel of the present invention as described above will be described below. In addition, unless otherwise specified, (%) in this specification is weight %.

C (Carbon) Various gears, shafts, pins, and bearings for automobiles and construction machinery are made of carbon steel that has been subjected to carburizing and quenching, carbonitriding and quenching, induction quenching and tempering, or ordinary quenching and tempering.
Low alloy steel is used.

Steel for carburizing requires a core hardness of HRC25 or more, and for this purpose, approximately 0.07% or more of C is required, so the lower limit of the amount of C is 0.07%. Since the upper limit of the C content is 1.20%, the lower limit of the C content of the steel of the present invention is set to 0.07% and the upper limit is set to 1°20%.

Si (Silicon) Si in steel is advantageous in improving solid solution hardening and temper softening resistance, but if it exceeds 1.50%, machinability deteriorates compared to improving driving force. Since the effect becomes significant, the upper limit was set at 1°50%, and the lower limit was set at 0.03%, since the effect is lost below 0.03%.

Mn (Manganese) Mn in steel plays a major role in adjusting the hardenability, and is particularly preferred for improving the hardenability of carbon steel because it is inexpensive. The effect of addition becomes significant at 0.10% or more.2.
If it exceeds 00%, it becomes small, so the lower limit was set to 0.10%, and the upper limit was set to 2.0θ%.

P (phosphorus) P in steel reduces toughness and is a harmful element. It is desirable that the amount of P be as low as possible. The effect of improving toughness by reducing the amount of P increases below 0.020%, so the upper limit is set to 0.02%.
It was set to 0%.

S (Sulfur) S in steel mainly exists in the form of sulfides. Sulfides and bismuth bond very easily and often form large complexes that reduce the effectiveness of bismuth, which will be described later, so they must be reduced as much as possible. The above-mentioned S reduction effect becomes noticeable below 0.02%, so the upper limit is set at 0.02%.
And so.

Al (Aluminum) AI effectively acts to adjust the oxygen level, the precipitation form of sulfides, and the crystal grain size. Such an effect is 0.015
It becomes noticeable from around %. On the other hand, if A1 is added in large amounts to O, OS% or higher, reoxidation of molten steel, segregation within the steel ingot, and coarsening of crystal grain size occur. Therefore, the lower limit was set to 0.015%, and the upper limit was set to o, oso%.

N (Nitrogen) N contained in steel mainly exists in the form of AIM and has the effect of refining crystal grains. This effect is 0°003%
It becomes noticeable above 0.020% and becomes saturated, so the lower limit was set to 0.003% and the upper limit was set to 0.020%.

Ti (Titanium): Titanium forms carbonitrides and significantly reduces the transfer fatigue life characteristics of steel. This effect becomes noticeable when the content exceeds 0.005%, so the upper limit was set at 0.005%.

- O (Oxygen) When oxygen exceeds 0.0015%, the amount of oxides increases significantly, significantly reducing transfer fatigue life. Furthermore, since oxides are extremely easy to combine with bismuth, if the amount of oxides is large, the effects of bismuth, which will be described later, cannot be fully exploited. Therefore, the upper limit was set at 0.0015%.

Bi (Bismuth) When bismuth is added to steel, cutting resistance is reduced due to the notch effect and lubrication effect of bismuth grains generated in the steel during cutting, and tool life and chip crushability are improved. P, S, O
In steel with a reduced Ti content, this effect appears from 0.001%, so the lower limit was set at 0.001%.

On the other hand, if the content exceeds 0.040%, the deterioration of the transfer fatigue life in particular becomes significant, so the upper limit was set at 0.040%.

The purpose of adding Ni to steel is to provide the necessary hardenability and improve mechanical properties after quenching and tempering.The steel of the present invention has good hardenability and excellent mechanical properties. Add Ni if necessary. On the other hand, if the amount of Ni is too large, residual austenite will be excessive and the surface hardness will decrease, making it impossible to meet the specifications required for the part. In such a case, it is preferable to improve the hardenability with Cr or Ha, which will be described later. Therefore, the upper limit was set at 5.00%.

Cr (Chromium) The purpose of adding Cr to steel is to impart hardenability and facilitate heat treatment operations such as carburizing. Shigashi, 2.00%
If it exceeds this value, double carbides will be formed during carburizing, and on the contrary, hardenability will deteriorate. In such a case, the above Or and the below I)
It is preferable to improve hardenability with o. Therefore, the upper limit was set at 2.00%.

Ha (Molybdenum) The purpose of adding MO to steel is to provide necessary hardenability and improve mechanical properties. However, like Cr, when the amount added increases, the hardenability deteriorates. In such a case, it is preferable to improve the hardenability with the aforementioned Ni and Cr. Therefore, considering economic efficiency, the upper limit was set at 1.00%.

V (vanadium) (2) dissolves into steel, strengthens ferrite, refines crystal grains, suppresses their growth, and improves the properties of alloying elements. The upper limit was set at 0.20% from the viewpoint of addition effect and economy.

(Nb) Nb combines with carbon, nitrogen, etc. in the steel to form fine precipitates, refine the crystal grains, and increase the strength at room and high temperatures. The upper limit is set to 0.0 from the viewpoint of addition effect and economy.
It was set at 20%.

(Examples and effects) Next, the present invention will be further explained with reference to Examples and Comparative Examples.

Table 1 shows the chemical components of a total of 48 heats of the comparative fp (Nos., A1 to A28) and the steels of the present invention (Nos., 81 to 828) tested.

All heats were melted in a 100 kg vacuum melting furnace, and lead and bismuth were added after melting the basic components and returning the inside of the vacuum chamber to 1 atmosphere with Ar gas.

The obtained steel ingot was forged to φ65, normalized and subjected to various investigations.

First, the cleanliness of non-metallic inclusions in φ65 forged and drawn material was determined by JISGO.
Table 2 shows the results measured by the nasal drop method specified in 555. There are almost no B or C-based inclusions in the heat, where the oxygen content is regulated to an extremely low value of 0.0015% or less.

Next, Tables 3.4 and 5 show the mechanical properties, Ono rotary bending fatigue test results, and pine impact fatigue test results of the comparative steel and the invention steel. These results show that bismuth has almost no effect on mechanical properties, rotating bending fatigue strength, and impact fatigue life as long as bismuth is 0.04% or less.

Table 6 shows the results of the thrust type rolling life test. The test piece has an outer diameter of φ65 and a thickness of 7 mm+.The surface hardness of the case hardened steel is carburized and quenched, the medium carbon steel is induction quenched and tempered, and the high carbon steel is quenched and tempered. It was subjected to a test after being adjusted to IRC62 or higher.

The number of stress cycles until flaking occurred was measured for 20 specimens in each heat, and the obtained 10% life (Table 4. Rotating bending fatigue strength specimen of specimen: parallel part φ8x30 Table 5. Test specimen Impact energy of material: 20kgfc+++ 36 times/min Specimen: Notch part φ10,5R Table 6. Maximum contact stress in Hertz of transfer fatigue life of sample material Pmx = 500kgf/mm2
Stress repetition rate 1800ro+n810), 5
0% life (B50) was compared. It is found that heat with a high content of oxygen, lead, and titanium has an extremely short life, and that oxygen, lead, and titanium are harmful elements for the life of thrust type rolling. Furthermore, in heats with low oxygen, uncomplexed and low titanium content, the lifetime gradually decreases with increasing bismuth addition. However, if the amount of bismuth added is 0.04% or less, the decrease in life is very slight, and a rapid decrease in life occurs when the amount of bismuth is added in excess of 0.04%. In the steel of the present invention, reduction in life could be prevented by regulating the amount of O to 0.0015% or less, the Ti content to extremely low values of 0, oos% or less, and the bismuth content to 0.04% or less.

Table 7 summarizes the processing properties (fragility) of chips obtained in cutting tests using cemented carbide tools. In the steel of the present invention, chips were cut finely due to the presence of fine bismuth particles, and moreover, they were rounded into small pieces due to the lubricating action of the cutting edge, exhibiting superior properties compared to comparative steels that do not contain bismuth. This is thought to be because the free-cutting effect of bismuth is maintained without loss even when a small amount of bismuth is added, since the amount of ○ is extremely low.

(Effects of the Invention) As detailed above, the steel of the present invention does not deteriorate properties such as mechanical properties, rotary bending fatigue strength, WJ impact fatigue life, and rolling fatigue life, and at the same time has particularly good machinability. It can be seen that the chip disposability (fractureability), which is a problem, is improved compared to the basic steel. When the steel of the present invention is used in important safety parts (gears, bearings, etc.) for automobiles, construction machinery, etc., machinability can be improved compared to existing materials, and cutting costs can be reduced.

Claims (2)

[Claims] (1) C: 0.07-1.20% Si: 0.03-1.50% Mn: 0.10-2.00% Al: 0.015-0.050% P: 0. 02% or less S: 0.02% or less Ti: 0.005% or less O: 0.0015% or less N: 0.003 to 0.020% Bi: 0.001 to 0.040% The remainder Fe and necessarily Steel for long-life rolling parts that contains impurities. (2) In weight%, C: 0.07-1.20% Si: 0.03-1.50% Mn: 0.10-2.00% Al: 0.015-0.050% P: 0. 02% or less S: 0.02% or less Ti: 0.005% or less O: 0.0015% or less N: 0.003 to 0.020% Bi: 0.001 to 0.040% Furthermore, Ni: 5.00 % or less Cr: 2.00% or less Mo: 1.00% or less Contains one or more of the following: V: 0.2% or less and Nb: 0.2% or less, with the remainder containing Fe and inevitably Steel for long-life rolling parts made of impurities.