JPH09165651A - High strength non-heat treated steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength non-heat treated steel improved in fatigue strength without deteriorating its machinability. SOLUTION: This steel has a compsn. contg., by weight, 0.05 to 0.30% C, 0.05 to 1.00% Si, 0.10 to 1.50% Mn, 0.10 to 1.00% Cr, 0.10 to 0.50% V and 0.005 to 0.025% N and furthermore contg., at need, one or >= two kinds selected from among <=0.30% Pb, <=0.20% S, <=0.30% Te, <=0.01% Ca and <=0.30% Bi, and the balance Fe with impurities, and in which the structure after hot forging is composed of ferrite+pearlite, and the hardness of the ferritic part is regulated to >=220HV.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated steel for hot forging which can obtain high fatigue strength without performing heat treatment such as quenching and tempering after hot forging.

[0002]

2. Description of the Related Art Medium carbon steel (C content of 0.3 to
Ferrite + pearlite type non-heat treated steel with a small amount of V added to 0.5 wt%) is widely used for machine structural parts. In recent years, the use of such non-heat treated steel has expanded further due to the need for cost savings, and the development of non-heat treated steel applicable to parts requiring higher strength is desired. In this case, the required strength is generally fatigue strength.

[0003]

Conventionally, in order to improve fatigue strength in non-heat treated steel, measures have been taken to increase hardness and tensile strength. However, the increase in hardness and tensile strength has a problem that machinability is significantly reduced.
Therefore, an object of the present invention is to provide a non-heat treated steel for hot forging that significantly increases hardness and tensile strength to improve fatigue strength without reducing machinability.

[0004]

In order to achieve the above object, in the non-heat treated steel for hot forging of the present invention, as described in claim 1, C: 0.05 to 0.30% by weight, Si:
0.05 to 1.00%, Mn: 0.10 to 1.50%,
Cr: 0.10 to 1.00%, V: 0.10 to 0.5
0%, N: 0.005 to 0.025% and, if necessary, Pb: 0.30% or less, S: 0.20% or less, T
e: 0.30% or less, Ca: 0.01% or less, Bi:
It contains one or more selected from 0.30% or less, consists of the balance Fe and impurities, the structure after hot forging is ferrite + pearlite, and the hardness of the ferrite part is 220 HV or more. It is characterized by being.

Next, the reasons for limiting the component range (weight basis) of the non-heat treated steel for hot forging according to the present invention will be described. C: 0.05 to 0.30% C is an element effective in securing strength, and it is necessary to contain 0.05% or more to obtain such an effect. However, if the amount is too large, the hardness becomes too high and the machinability deteriorates, so it is necessary to make it 0.30% or less.

Si: 0.50 to 1.00% Si has a deoxidizing action when steel is melted, and is solid-solved in ferrite to effectively improve the strength of ferrite, and to improve hardness and tensile strength. Increase fatigue strength without increasing thickness. In order to obtain such effects, it is necessary to contain 0.05% or more. However, if the content is too large, the machinability and hot workability are deteriorated, so it is necessary to set it to 1.00% or less.

Mn: 0.10 to 1.50%, Cr: 0.
10 to 1.00% Mn, Cr refines the lamellar spacing in the pearlite portion and effectively improves the fatigue strength. However, if added in a large amount, bainite is generated even in air cooling and machinability is significantly reduced. Therefore, 0.10 to 1.50% and 0.10%, respectively.
It was set to 1.00%.

V: 0.10 to 0.50% V is an element that strengthens ferrite like Si, and greatly improves fatigue strength. In order to obtain such an effect, it is necessary to add 0.10% or more. However, the addition of a large amount is economically disadvantageous, so it must be 0.50% or less.

N: 0.005-0.025% N is an element which forms a nitride with V and strengthens ferrite by fine precipitation of this nitride. Therefore, N is an element that effectively improves the fatigue strength, like V and Si.
In order to obtain such an effect, 0.005% or more, 0.
It should be 025% or less.

Pb: 0.30% or less, S: 0.20% or less, Te: 0.30% or less, Ca: 0.01% or less, B
i: One or more selected from 0.30% or less Pb, S, Te, Ca, and Bi are all effective elements for improving machinability. When it is required to further improve the properties, one or more selected from these may be added in an appropriate amount as needed.

However, if the addition amount is too large, the hot workability and the fatigue limit are deteriorated.
Pb is 0.30% or less, S is 0.20% or less, Te is 0.30% or less, Ca is 0.01% or less, and Bi is 0.3% or less.
It must be 0% or less.

In the present invention, the hardness of ferrite in the ferrite + pearlite structure is 220 HV or more. This is because, as described above, increasing the strength of ferrite increases fatigue strength without reducing machinability. In order to obtain this effect, the hardness of ferrite needs to be 220 HV or more.

[0013]

[Examples] The steels of the present invention and comparative steels shown in Table 1 were melted, and then ingots were hot forged into 50 mm square forging materials, which were heated and held at 1200 ° C for 60 minutes and then the diameter was 22 mm The round bars were hot forged and left to cool to room temperature at appropriate intervals. A test piece was cut out from this round bar material having a diameter of 22 mm and subjected to a test. In addition, the machinability was evaluated by measuring the drilling efficiency of some test materials.

Among these, the hardness was measured by measuring the hardness of the center of each forged product with a Rockwell hardness tester, and the fatigue strength was measured by a rotary bending fatigue test. In addition, the hardness of the ferrite part was measured with a micro-Vickers hardness tester under a load of 10
It was measured at 9. Further, the tool life was measured by performing a drill test under the conditions shown in Table 2, and the invention steel No. 2 to 10
Table 1 also shows the relative value of 0 as the drilling efficiency.

[0015]

[Table 1]

[0016]

[Table 2]

The following can be seen from Table 1 above. N with lower C content than 1,2,3, B
o. No. A is an invention steel. Hardness is low compared to 1 and 2, and fatigue limit is also low. No. In B, the hardness is too high because the C content is too high, and the drilling efficiency is lower than that of the invention steel. Therefore, it is understood that the C content needs to be in the range of 0.05 to 0.30%.

No. C has a high Si content.
No. 1 having higher hardness than C The drilling efficiency is lower than that of No. 3 steel, and it is understood that the Si content needs to be 0.05 to 1.00%.

No. Since Mn and Cr contents of D and E are too high, bainite is generated even after air cooling after forging, and the hardness is high. For this reason, the drilling efficiency is remarkably reduced, and the contents of Mn and Cr are each 0.10%-
It must be within 1.50% and 0.10% -1.00%.

No. Since F does not contain V, the ferrite hardness is low, and No. F having almost the same hardness is used. The fatigue limit is lower than in 2. Therefore, V must be contained in the range of 0.10% to 0.50%.

No. No. G has a low N content, and thus No. F
Ferrite hardness is low, the fatigue limit is lower than that of the invention steels having the same hardness, and the N content is 0.005% to 0.
The range was 025%.

Compared with the comparative steels so far, No. 1 to N
o. It can be seen that the developed steels of No. 10 have high fatigue strength and excellent machinability even with the same hardness. In addition, No.
Although Nos. 11, 12, and 13 were added with an element that improves machinability, Nos. It can be seen that the drilling efficiency is improved as compared with No. 2.

No. 3 with excessive addition of Pb and S. Nos. K and L are No. containing alloy elements of almost the same level. Although the drilling efficiency is improved as compared with 2, the fatigue limit is remarkably reduced, and it is understood that excessive addition of elements such as P, S, Te, Ca, Bi that improve machinability is not preferable. Therefore, the Pb content is 0.30% or less and the S content is 0.
20% or less, Te 0.30% or less, Ca 0.01%
Hereinafter, Bi is set to 0.30% or less.

[0024]

As described above, the high strength non-heat treated steel for hot forging according to the present invention has a weight ratio of C: 0.0.
5 to 0.30%, Si: 0.05 to 1.00%, Mn:
0.10 to 1.50%, Cr: 0.10 to 1.00%,
V: 0.10 to 0.50%, N: 0.005 to 0.
025% and optionally Pb: 0.30% or less, S:
0.20% or less, Te: 0.30% or less, Ca: 0.0
1% or less, Bi: 1 selected from among 0.30% or less
Since it is composed of at least one kind or two or more kinds, consisting of the balance Fe and impurities, the structure after hot forging is ferrite + pearlite, and the hardness of the ferrite part is 220 HV or more. , It is a non-heat treated steel for hot forging even though it has both excellent fatigue strength and machinability by improving fatigue strength without significantly reducing machinability by greatly increasing hardness and tensile strength.

Claims (2)

[Claims] 1. C: 0.05 to 0.30% Si: 0.05 to 1.00% Mn: 0.10 to 1.50% Cr: 0.10 to 1.00% V: by weight 0.10 to 0.50% N: 0.005 to 0.025% The balance Fe and impurities, the structure after hot forging is ferrite + pearlite, and the hardness of the ferrite part is 220 HV. The above is the non-heat treated steel for hot forging. 2. C: 0.05 to 0.30% Si: 0.05 to 1.00% Mn: 0.10 to 1.50% Cr: 0.10 to 1.00% V: by weight 0.10 to 0.50% N: 0.005 to 0.025% and Pb: 0.30% or less S: 0.20% or less Te: 0.30% or less Ca: 0.01% or less Bi: 0 30% or less, including one or more selected from the rest, and the balance F
A non-heat treated steel for hot forging, wherein the microstructure after hot forging is ferrite + pearlite, and the hardness of the ferrite part is 220 HV or more, which is characterized by comprising e and impurities.