JPH1129838A - Non-heat treated steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heat treated steel having fatigue resistance equal to that of a material prepared by applying refining treatment and soft-nitriding treatment to, e. g. a medium-C carbon steel for machine structural use and suitable for use as a steel for shafts. SOLUTION: This steel has a composition consisting of, by weight, 0.20-0.50% C, 0.05-0.70% Si, >0.60-1.00% Mn, 0.01-0.07% S, 0.02-0.50% V, 0-0.05% Ti, 0-0.05% Zr, 0.002-0.03% N, 0-0.050% P, 0-0.30% Cu, 0-0.30% Ni, 0-1.00% Cr, 0-0.30% Mo, 0-0.50% W, 0-0.05% Nb, 0-0.050% Al, 0-0.30% Pb, 0-0.010% Ca, 0-0.10% Te, 0-0.10% Bi, and the balance Fe with inevitable impurities, satisfying Ti+0.5Zr=0.005 to 0.06%, and also satisfying fn1=C+0.1Si+(Mn/6)+(Cr/3)+5N +1.65 V>0.60% and (C/fn1)<=0.60, where each symbol of element represents its content by weight percentage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention has good fatigue resistance without being subjected to tempering and nitrocarburizing after hot working, and is suitable as a material for shafts such as crankshafts of automobile engine parts. Non-heat treated steel.

[0002]

2. Description of the Related Art Conventionally, shafts such as crankshafts and pinion shafts of machine structural parts, especially automobile engine parts, have been processed into a predetermined shape by hot working or machining, and then are subjected to quenching and tempering. After that, it was often manufactured by performing a nitrocarburizing treatment for the purpose of improving fatigue resistance. That is, conventionally, JIS S45C and S50C, carbon steels for machine structural use, or steels to which free-cutting elements such as S, Pb, and Ca are added, processed into a required shape, and then subjected to a tempering treatment and a soft nitriding treatment. In many cases.

However, the heat treatment requires a lot of energy and cost. Therefore, in recent years, first, from the viewpoint of energy saving and cost reduction, carbide and carbonitride are formed on a non-heat-treated steel having the same characteristics as a heat-treated steel in a hot-worked state, especially a medium carbon steel. V, Nb and Ti
Steels to which such elements have been added have been developed, and attempts have been made to produce shafts by subjecting them to nitrocarburizing treatment.

[0004] However, the non-heat treated steel proposed under the above background is also subjected to a soft nitriding treatment at 530 to 570 ° C for 5 to 8 hours as described above in order to enhance the fatigue resistance. And there was a problem in terms of economics.

[0005] Alternatively, in order to increase the fatigue strength of non-heat treated steel without nitrocarburizing treatment, measures have been taken to increase the tensile strength. However, increasing the tensile strength leads to deterioration in machinability. Not preferred.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has been developed in such a manner that after both the refining treatment and the soft nitriding treatment are performed, It is an object of the present invention to provide a non-heat treated steel suitable as a material for various shafts capable of obtaining a tensile strength of 600 MPa or more and a fatigue strength of 320 MPa or more, which are equivalent to the case of performing a soft nitriding treatment.

[0007]

The gist of the present invention resides in the following non-heat treated steel.

That is, “in weight%, C: 0.20
0.50%, Si: 0.05 to 0.70%, Mn: 0.
More than 60% and 1.00% or less, S: 0.01 to 0.07
%, V: 0.02 to 0.50%, Ti: 0 to 0.05%
And Zr: 0 to 0.05% and Ti + 0.5 Zr: 0.0
05 to 0.06%, N: 0.002 to 0.03%, P:
0 to 05%, Cu: 0 to 0.30%, Ni: 0 to 0.3
0%, Cr: 0 to 1.00%, Mo: 0 to 0.30%,
W: 0 to 0.50%, Nb: 0 to 0.05%, Al: 0
~ 0.05%, Pb: 0 ~ 0.30%, Ca: 0 ~ 0.
010%, Te: 0 to 0.10%, Bi: 0 to 0.10
%, The balance consists of Fe and unavoidable impurities, and fn1 represented by the following formula is expressed as fn1 ≧ 0.60%, where the symbol of the element in the formula is the content in weight% of the element.
And fn2 represented by the following formula: fn2 ≦ 0.60.

Fn1 = C + 0.1Si + (Mn / 6) + 1.65V + 5N + (Cr / 3)..., Fn2 = (C / fn1).

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted various studies to solve the above-mentioned problems, and have obtained the following findings.

(A) N not only increases the static strength (tensile strength) of the non-heat treated steel, but also has an extremely large effect on the improvement of the fatigue strength.

(B) The chemical composition of the steel is% by weight, and C: 0.
20 to 0.50%, Si: 0.05% or more, Mn: 0.
In the case of more than 60%, Cr: 1.00% or less, V: 0.02% or more, and N: 0.002% or more, the fatigue strength in the state as it is hot worked can be arranged by the above formula. However, it is limited to the case where the value of the above equation is 0.60 or less.

Fn1 represented by the equation (c) is 0.60%
As described above, if fn2 represented by the formula is 0.60 or less, it is possible to obtain a fatigue strength of 320 MPa or more, which is equivalent to the case where a conventional carbon steel for machine structural use is subjected to a tempering treatment and then subjected to a soft nitriding treatment. it can.

The present invention has been completed based on the above findings.

Hereinafter, the reason for limiting the chemical composition of steel in the present invention as described above will be described. In addition, “%” of the component content means “% by weight”.

C: C is an element necessary for imparting a desired static strength to steel, but decreases the machinability, and
It is also an element that lowers the fatigue strength if it exceeds a certain amount.
Static strength (the tensile strength is 600 MPa in the present invention)
In order to obtain the above, the content needs to be 0.20% or more. On the other hand, if the content exceeds 0.50%,
Machinability decreases and fatigue strength (fatigue limit, σw)
Causes a decrease in Therefore, the content of C is set to 0.20 to
0.50%.

Si: Si has the effect of promoting deoxidation and strengthening ferrite by forming a solid solution in ferrite, thereby increasing static strength and fatigue strength. However, if the content is less than 0.05%, the desired effect cannot be obtained, while, on the other hand, the content of 0.1%.
If the content exceeds 70%, the machinability may deteriorate, so the content is set to 0.05 to 0.70%.

Mn: Mn has a function of improving static strength. However, when the content is 0.60% or less, the effect of addition is poor. On the other hand, if the content exceeds 1.00%, the effect is saturated, and only the cost is increased and the economic efficiency is impaired. Therefore, the content of Mn exceeds 0.60% and 1.
Up to 00%.

S: S has the effect of enhancing machinability. In order to exert its effect sufficiently, a content of 0.01% or more is required. On the other hand, if the content exceeds 0.07%, the fatigue strength may be deteriorated. Therefore, S
Was made 0.01 to 0.07%.

V: V has the effect of increasing static strength and fatigue strength. However, if the content is less than 0.02%, the effect of addition is poor, and if the content exceeds 0.50%, the effect is saturated, and only the cost rises and the economic efficiency is impaired. The content was set to 0.02 to 0.50%.

Ti and Zr: Ti and Zr, when added alone or in combination, have the effect of increasing strength and toughness and improving the strength-toughness balance. To secure the effect, the sum of Ti and 0.5Zr is 0.005%.
The above content is required. However, if Ti and Zr are contained in excess of 0.05%, respectively, and if the sum of Ti and 0.5Zr exceeds 0.06%, the toughness is rather reduced. Therefore, Ti and Z
As for r, the Ti content is 0 to 0.05% and the Zr content is 0 to 0.05%, and the content represented by the sum of Ti and 0.5Zr is 0.005 to 0.06%. did.

N: N not only increases the static strength of the non-heat treated steel, but also has an extremely large effect on the improvement of the fatigue strength. In order to fully demonstrate these effects,
N must be contained at 0.002% or more. On the other hand, if the content exceeds 0.03%, the effect is not only saturated, but also causes deterioration in hot workability. Therefore, the content of N is set to 0.002 to 0.03%.
In addition, from a comprehensive aspect, the content of N is 0.005 to 0.0.
Preferably, it is 22%.

P: P may not be contained. If contained, it has the effect of increasing the fatigue strength. In order to ensure this effect, it is preferable that the content of P be 0.005% or more. However, if the content exceeds 0.05%, the toughness is greatly deteriorated, and the fatigue strength is rather deteriorated. Therefore, the content is set to 0 to 0.05%.

Cu: Cu need not be added. If added, it has the effect of increasing the hardenability. To ensure this effect, it is preferable that the content of Cu be 0.01% or more. However, if the content exceeds 0.30%, deterioration of hot workability is caused. Therefore, the content of Cu is set to 0 to 0.30%.

Ni: Ni may not be added. When added, it has the effect of improving hardenability and improving toughness. To ensure this effect, Ni should be 0.0
The content is preferably 1% or more. However, if the content exceeds 0.30%, the machinability deteriorates, which is disadvantageous in terms of economy. Therefore,
The content of Ni was set to 0 to 0.30%.

Cr: Cr may not be added. When added, it has the effect of increasing hardenability and improving fatigue strength. To ensure these effects, it is preferable that the content of Cr be 0.02% or more. However, even if the content exceeds 1.00%, the effect is saturated, and only the cost increases and the economic efficiency is impaired. Therefore, the content of Cr is set to 0 to 1.00%.

Mo: Mo may not be added. When added, it has the effect of improving hardenability and improving toughness. To ensure this effect, Mo should be 0.0
The content is preferably 1% or more. However, if the content exceeds 0.30%, the effect is saturated and the economy is impaired. Therefore, the content of Mo is set to 0
０．３0.30%.

W: 0 to 0.50% W may not be added. When added, it has the effect of increasing the hardenability and toughness, similarly to Mo. In order to surely obtain this effect, the content of W is preferably set to 0.01% or more. However, even if the content exceeds 0.50%, the above effect is saturated, so that only the cost is increased, which is disadvantageous in terms of economy. Therefore, the content of W is set to 0 to 0.50.
%.

Nb: 0 to 0.05% Nb may not be added. Addition has the effect of increasing toughness. To ensure this effect, Nb should be 0.00
Preferably, the content is 5% or more. However, 0.
Even if the content exceeds 0.05%, the effect is saturated, and only the cost is increased and the economy is impaired. Therefore,
The content of Nb was set to 0 to 0.05%.

Al: Al may not be added. If added, it has the effect of stabilizing and homogenizing steel deoxidation. To ensure this effect, it is desirable that the content of Al be 0.001% or more. However, the content is 0.05
%, Oxide inclusions increase and the tool life is shortened during cutting. Therefore, the content of Al is 0 to
0.05%. In order to improve machinability, steel
When adding b, Ca, Te, and Bi, it is preferable to regulate the upper limit of the Al content to 0.010%.

Pb: Pb may not be added. If added, it has the effect of enhancing machinability. In order to surely obtain this effect, the content of Pb is preferably set to 0.01% or more. However, when the content exceeds 0.30%, the fatigue resistance is deteriorated. Therefore, Pb
Was set to 0 to 0.30%.

Ca: Ca may not be added. If added, it has the effect of enhancing machinability. In order to surely obtain this effect, the content of Ca is preferably set to 0.0003% or more. However, even if the content exceeds 0.010%, the effect is saturated, and economic efficiency is impaired. Therefore, the content of Ca is set to 0 to 0.010%.

Te: Te need not be added. If added, it has the effect of enhancing machinability. To ensure this effect, the content of Te is preferably set to 0.01% or more. However, even if the content exceeds 0.10%, the effect is saturated, and economic efficiency is impaired. Therefore,
The content of Te was set to 0 to 0.10%.

Bi: Bi may not be added. If added, it has the effect of enhancing machinability. To ensure this effect, the content of Bi is preferably 0.005% or more. However, even if the content exceeds 0.10%, the effect is saturated, and economic efficiency is impaired. Therefore, the content of Bi is set to 0 to 0.10%.

Fn1: The chemical composition of the steel is% by weight, and C:
0.20 to 0.50%, Si: 0.05% or more, Mn:
Over 0.60%, Cr: 1.00% or less, V: 0.02%
In the case of above and N: 0.002% or more, the fatigue strength in the state of hot working can be arranged by fn1 in the above equation. When this value is 0.60% or more and fn2 in the above formula is 0.60 or less, 320 MPa equivalent to the case where a conventional carbon steel for machine structural use is subjected to a tempering treatment and then subjected to a soft nitriding treatment. The above fatigue strength can be obtained.
Therefore, fn1 is set to 0.60% or more.

Fn2: When fn2 in the above formula exceeds 0.60, the amount of elements for strengthening ferrite is insufficient even when the above-mentioned fn1 is 0.60% or more. Will deteriorate. Therefore, fn2 is set to 0.60 or less.

The steel having the above-mentioned chemical composition is melted by an ordinary method, then subjected to hot rolling and forging, for example, by an ordinary method, and is further machined as necessary to form a shaft having a predetermined shape. Finished.

[0038]

EXAMPLES Steel having the chemical composition shown in Tables 1 to 3 was vacuum-melted in a conventional manner in a test furnace using a test furnace. Table 1,
Steels 1 to 15 in No. 2 are steels of the present invention, and Steels 16 to 30 in Tables 2 and 3 are comparative steels in which one of the components is out of the range specified in the present invention.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

Next, these steels were formed into billets by a usual method, and then heated to 1250 ° C.
Hot forging into a round bar with a diameter of 20 mm at a temperature of ~ 950 ° C,
Thereafter, it was air-cooled to room temperature.

From the round bar thus obtained, the diameter of the parallel portion was 8 m.
m Ono-type rotating bending fatigue test piece was cut out and subjected to a fatigue test at room temperature and in the air at 3000 rpm. or,
A JIS No. 4 tensile test piece was cut out and subjected to a tensile test at room temperature.

For reference, the above-mentioned 20 mm
The round bar was heated to 870 ° C., water-quenched, and tempered at 600 ° C., and then an Ono-type rotary bending fatigue test piece having a parallel part diameter of 8 mm and a JIS No. 4 tensile test piece were cut out and 530 were cut into these pieces.
A soft nitriding treatment was performed at 6 ° C. for 6 hours, and a fatigue test and a tensile test were performed under the above conditions.

Table 4 shows the test results.

[0046]

[Table 4]

Regarding the steels 1 to 15 of the present invention, the desired tensile strength of 600 MPa or more and 320 MPa
a fatigue strength of not less than a.

On the other hand, among the comparative steels in which one of the components is out of the range of the content specified in the present invention, the C content,
The steel 16, steel 18, steel 19, steel 21, steel 22, steel 23, and steel 29 in which the Si amount, Mn amount, N amount, V amount, and fn1 were respectively lower than their lower values did not reach the fatigue strength of 320 MPa. Further, among the above steels, steel 16, steel 18, steel 19, steel 2
In the case of No. 2 and steel 29, the tensile strength has not yet reached 600 MPa.

Further, in steel 17, steel 20, steel 24-28 and steel 30 in which the amounts of C, P, Pb and fn2 are respectively higher, the tensile strength exceeds 600 MPa, but the fatigue strength is higher. It has not reached 320 MPa.

As shown in Table 4 as a reference example, in the case of the conventional type in which the steel 25 was subjected to quenching and tempering and then nitrocarburizing, both the tensile strength and the fatigue strength were the target values (tensile strength). : 600MPa, fatigue strength: 320M
Pa).

[0051]

The use of the non-heat treated steel according to the present invention is equivalent to the case where the carbon steel for machine structure and the like is subjected to the tempering treatment and then to the nitrocarburizing treatment without performing both the tempering treatment and the soft nitriding treatment. Since the fatigue resistance of the shaft can be imparted to the shafts, the industrial effect is great.

Claims (1)

[Claims] C. 0.20 to 0.50% by weight, S
i: 0.05-0.70%, Mn: more than 0.60% and 1.00% or less, S: 0.01-0.07%, V: 0.
02 to 0.50%, Ti: 0 to 0.05%, and Zr: 0
Ti + 0.5Zr at 0.005 to 0.05%: 0.005 to 0.0
6%, N: 0.002 to 0.03%, P: 0 to 05%,
Cu: 0 to 0.30%, Ni: 0 to 0.30%, Cr:
0 to 1.00%, Mo: 0 to 0.30%, W: 0 to 0.
50%, Nb: 0 to 0.05%, Al: 0 to 0.05
%, Pb: 0 to 0.30%, Ca: 0 to 0.010%,
Te: 0 to 0.10%, Bi: 0 to 0.10%, the balance being Fe and unavoidable impurities, and fn1 represented by the following formula is fn1 ≧ 0.60%, and Non-heat treated steel characterized in that fn2 represented is fn2 ≦ 0.60. fn1 = C + 0.1Si + (Mn / 6) + 1.65V + 5N + (Cr / 3)..., fn2 = (C / fn1)... Represents the content of