JPH10226847A - V-non added non-refined steel for hot forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a V-non added non-refined steel for hot forging capable of obtaining high proof stress and fatigue strength even if V is not added and refining treatment such as quenching and tempering is eliminated after hot forging. SOLUTION: This steel is the one having a compsn. contg., by weight, 0.30 to 0.60% C, 0.05 to 2.00% Si, 0.90 to 1.80% Mn, 0.10 to 1.00% Cr, 0.010 to 0.045% s-Al and 0.005 to 0.025% N, 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, in which hardness after hot-forging is regulated to <=30HRC, the structure is composed of ferrite + pearlite, the lamellar interval of pearlite is regulated to <=0.80μm, and furthermore, the area ratio of primary crystal ferrite is regulated to <=30%.

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 a sufficient strength even without a heat treatment such as quenching and tempering after hot forging without adding expensive V.

[0002]

2. Description of the Related Art In order to omit tempering treatments such as quenching and tempering after hot working, a medium carbon steel (C content 0.3 to
Ferrite + pearlite type non-heat treated steel to which about 0.1% of V is added to 0.5% by weight is widely used for parts for machine structural use. The uses of steel are expanding further. However, in recent years, the need for further cost savings has increased, and the development of an inexpensive ferrite + pearlite type non-heat treated steel that does not contain an expensive additive element such as V is desired.

[0003]

Conventionally, in order to obtain a desired strength without performing heat treatment such as quenching and tempering in non-heat treated steel, it has been conventionally added about 0.1% of V. Have been. However, V is a very expensive additive element, and although the heat treatment cost can be reduced by non-tempering, the material cost is greatly increased as compared with the conventional carbon steel. For this reason, the conventional V-added ferrite + pearlite type non-heat treated steel could not sufficiently meet the needs for cost saving.
Therefore, the present invention provides a relatively inexpensive additive element C, Mn, Cr, Si, S-Al, N without adding expensive V.
It is an object of the present invention to provide a V-free hot-forged non-heat-treated steel which can adjust the content of the steel to obtain a strength equivalent to that of a conventional non-heat-treated steel.

[0004]

In order to achieve the above object, the non-heat treated steel for hot forging of the present invention has a C:
0.30 to 0.60%, Si: 0.05 to 2.00%,
Mn: 0.90 to 1.80%, Cr: 0.10 to 1.0
0%, s-Al: 0.010 to 0.045%, N: 0.
005 to 0.025% and if necessary 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, and the balance Fe The hardness after hot forging is 30 HRC or less, the structure is ferrite + pearlite, the pearlite lamella spacing is 0.80 μm or less, and the proeutectoid ferrite area ratio is 30% or less. I do.

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: 0.30 to 0.60% C is an element effective for securing the strength, and in order to obtain such an effect, it is necessary to contain 0.30% or more. However, if the content is too large, the hardness becomes too high and the machinability deteriorates. Therefore, the content needs to be 0.60% or less.

Si: 0.05 to 2.00% Si has a deoxidizing effect when steel is melted, and also forms a solid solution in ferrite to effectively improve the strength of ferrite. This has the function of improving proof stress and fatigue strength as an alternative to V. In order to obtain such an effect, it is necessary to contain 0.05% or more. However,
If the content is too high, the machinability is deteriorated, or the surface is significantly decarburized by heating during hot forging, so 2.00%
It is necessary to:

Mn: 0.90 to 1.80%, Cr: 0.
10 to 1.00% Mn and Cr reduce the lamella spacing in the pearlite portion, and effectively improve the proof stress and fatigue strength. However, when added in a large amount, bainite is generated even in air cooling and the machinability is remarkably reduced, so that 0.90 to 1.80% and 0.10% respectively.
0 to 1.00%.

S-Al: 0.010-0.045% s-Al forms nitrides with N in steel and is finely dispersed in steel to suppress the growth of crystal grains during hot forging. In order to obtain such an effect, it is necessary to add 0.010% or more. However, the effect is saturated even if added in a large amount, so the upper limit was made 0.045% or less.

N: 0.005 to 0.025% N forms a nitride with Al, and suppresses crystal grain growth during hot forging by fine precipitation of the nitride. To obtain such an effect, 0.005% or more must be added. However, even if a large amount is added, the effect is saturated, so the upper limit is made 0.025% 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 the properties are required, one or more selected from these may be added in an appropriate amount as needed.

However, if the amount of addition is too large, the hot workability and the fatigue limit are reduced.
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.

[0012] In addition, non-heat treated steel with no added V generally has poor proof stress and fatigue limit. In the ferrite + pearlite structure, the proof stress and the fatigue limit decrease as the pearlite lamella spacing increases and the area ratio of proeutectoid ferrite increases. For this reason, in the present invention, the lamella spacing of pearlite having a ferrite + pearlite structure is 0.8 μm or less, and the ferrite area ratio is 30% or less.

[0013] A general mechanical structure component is often used with a hardness of 30 HRC or less, and a hardness of more than 30 HRC significantly lowers machinability. Therefore, the hardness after forging is 30H
Limited to RC or less.

[0014]

EXAMPLES The steels of the present invention and the comparative steels shown in Table 1 were melted and hot forged to form a forged material of 50 mm square, which was heated and maintained at 1200 ° C. for 60 minutes, and then formed into a round bar having a diameter of 22 mm. Hot forging was performed, and it was allowed to cool to room temperature at appropriate intervals. A test piece was cut out from the round bar having a diameter of 22 mm and subjected to a test. In addition, the drilling efficiency was measured for some of the test materials to evaluate the machinability.

Among these, the hardness was measured by a Rockwell hardness tester at the center of each forged product, the proof stress was measured by 0.2% proof stress by a tensile test, and the fatigue strength was measured by a rotary bending fatigue test. Was measured by The pearlite lamella spacing (ILS) was 30% for each forged product by SEM.
Observation was performed for 10 visual fields at a magnification of 00, and the area where the pearlite lamella was narrowest in each visual field was used as the true lamella interval, and the average of the 10 visual fields was used. Proeutectoid ferrite area ratio (fα)
Was measured by image analysis. Table 1 shows the results. Further, the tool life was measured by performing a drill test under the conditions shown in Table 2 and was determined for the invention steel no. Table 3 shows the relative value when 2 is set to 100 as the drilling efficiency.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

[Table 3]

The following can be seen from Table 1 above. In A, the C content is low, and the proeutectoid ferrite area ratio is outside the claimed range, so that sufficient strength cannot be obtained. No. B has too high a hardness because the C content is too high. C content is within the claims,
No. 1, 2, and 3 have high yield strength and fatigue limit in an appropriate hardness range, and must have a C content of 0.30 to 0.60% and a proeutectoid ferrite area ratio of 30% or less. I understand.

No. C has a high Si content.
Like B, the hardness is too high.

No. For D and E, the contents of Mn and Cr are too high, so that bainite is generated even in air cooling after forging, and the hardness is significantly increased.

No. The alloy component of F is within the claimed range, but the pearlite lamellar spacing is too large, so that the proof stress and fatigue limit are greatly reduced as compared with the inventive steel having the same hardness.

No. Since G and H have low contents of s-Al and N, respectively, the crystal grains become coarse at the time of hot forging, and the proof stress and fatigue limit are lower than that of the inventive steel having the same hardness.

In comparison with the comparative steels so far, no. 1 to N
o. It can be seen that the invention steel No. 10 has high proof stress and fatigue strength at the same hardness within an appropriate hardness range. In addition, No. Nos. 10, 11, and 12 are elements to which machinability improving elements are added. It can be seen that the drilling efficiency is improved as compared with No. 2.

No. 3 containing excessive amounts of Pb and S were added. Nos. I and J have the same level of other alloy elements. Although the drilling efficiency is improved as compared with No. 2, the fatigue limit is remarkably reduced, and it can be seen that excessive addition of elements that improve machinability, such as P, S, Te, Ca, and Bi, is not preferable.

[0026]

According to the present invention, in order to achieve the object, the non-heat treated steel for hot forging of the present invention has a C: 0.30 to 0.6 on a weight basis.
0%, Si: 0.05 to 2.00%, Mn: 0.90
1.80%, Cr: 0.10-1.00%, s-Al:
0.010-0.045%, N: 0.005-0.02
5% and if necessary 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
Containing at least one species, the balance being Fe and impurities, the hardness after hot forging is 30 HRC or less, the structure is ferrite + pearlite, and the pearlite lamellar spacing is 0.
80 μm or less, and the proeutectoid ferrite area ratio is 30% or less. Therefore, even if V is not added, even if tempering treatment such as quenching and tempering is omitted after hot forging, high yield strength is obtained. And fatigue strength can be obtained.

Claims (2)

[Claims] 1. C: 0.30 to 0.60% Si: 0.05 to 2.00% Mn: 0.90 to 1.80% Cr: 0.10 to 1.00% s- Al: 0.010 to 0.045% N: 0.005 to 0.025% The balance consists of Fe and impurities and has a hardness of 3 after hot forging.
The structure is ferrite + pearlite under 0HRC,
Non-heat treated steel for hot forging having a pearlite lamella spacing of 0.80 μm or less and a proeutectoid ferrite area ratio of 30% or less. 2. C: 0.30 to 0.60% Si: 0.05 to 2.00% Mn: 0.90 to 1.80% Cr: 0.10 to 1.00% s- Al: 0.010 to 0.045% 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 group consisting of
e and impurities, hardness after hot forging is 30 HR
A non-heat treated steel for hot forging having a structure of ferrite + pearlite at C or less, a pearlite lamella spacing of 0.80 μm or less, and a proeutectoid ferrite area ratio of 30% or less.