JP2000265242A - Non-heattreated steel for hot forging, excellent in wear resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heattreated steel for hot forging, used as a material for manufacturing machine parts, such as automobile engine crankshaft, having wear resistance capable of withstanding use even if surface hardening treatment after working is omitted, also having hardness within limits not prejudicially to machinability, and usable in an as-hot-rolled state. SOLUTION: The steel has an alloy composition consisting of, by weight, 0.40-0.70% C, <=0.50% Si, 0.90-1.80% Mn, 0.05-1.00% Cr, 0.010-0.045% sol.Al, 0.005-0.025% N, and the balance Fe with impurities. Further, the structure of this steel after hot forging is composed of ferrite+pearlite, and the area ratio of proeutectoid ferrite is regulated to <=10%.

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, and more particularly, to a surface hardening treatment such as induction hardening or soft nitriding after forming into a part shape by hot forging and machining. Even if omitted, the present invention relates to a non-heat treated steel for hot forging that exhibits excellent wear resistance.

[0002]

2. Description of the Related Art In the production of machine parts, a small amount of medium carbon steel (C: 0.3 to 0.5% by weight) is intended to omit the tempering treatment of quenching and tempering performed after hot working. Ferrite + pearlite type non-heat treated steel to which V (about 0.1%) is added is widely used.

One application to which this type of non-heat treated steel is directed is mechanical parts such as crankshafts of automobile engines. Parts requiring wear resistance, such as crankshafts, need to be subjected to a surface hardening treatment such as induction hardening or soft nitriding after machining.

[0004] However, there is always a demand for cost reduction and lead time reduction, and in order to respond to these demands, it is desired to omit the surface hardening treatment. Thus, there is a demand for the appearance of a non-heat treated steel that exhibits sufficient wear resistance without performing a surface hardening treatment.

In general, it is effective to increase the hardness of steel to improve wear resistance. However, increasing the hardness decreases the machinability, so simply increasing the hardness does not solve the problem.

On the other hand, a crankshaft is also manufactured from cast iron. The cast iron crankshaft has a pearlite + graphite structure after casting, and has high wear resistance even without surface hardening. can get. Thus, the wear resistance also depends on the structure. However, cast iron has a drawback of low rigidity because it contains a large amount of graphite.

[0007] The inventors have examined the effect of texture on wear resistance alongside hardness, and have found that at the same hardness level, the pearlite structure is more wear resistant than other structures.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is based on the above findings obtained by the present inventors at a hardness level that does not impair machinability, and has high wear resistance, An object of the present invention is to provide a non-heat-treated steel for hot forging, which can be obtained as hot forged with a structure that can be used without performing surface hardening treatment.

[0009]

The non-heat treated steel for hot forging of the present invention, which achieves the above object and has excellent wear resistance, has a C: 0.40 to 0.70% by weight, Si : 0.50%
Hereinafter, Mn: 0.90 to 1.80%, Cr: 0.05 to
1.00%, s-Al: 0.010-0.045%, and N: 0.005-0.025%, with the balance being F
e and impurities, the structure after hot forging is ferrite + pearlite, and the proeutectoid ferrite area ratio is 10%.
It is characterized by the following.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The non-heat treated steel for hot forging according to the present invention comprises P as an optional additive element in addition to the above-mentioned essential alloy composition.
b: 0.030% or less, S: 0.20% or less, Te:
0.030% or less, Ca: 0.01% or less and Bi:
One or more selected from 0.30% or less can be contained.

Regarding the alloy composition, the functions of the essential components and optional additives and the reasons for limiting the composition range will be described below.

C: 0.40 to 0.70% C is an important element for securing the strength and wear resistance of steel. To obtain these effects, 0.40% or more is added. There is a need. If the amount is too large, the hardness becomes too high and the machinability deteriorates. Therefore, the upper limit of 0.70% was set.

Si: 0.50% or less Si acts as a deoxidizing agent at the time of melting and also forms a solid solution in a ferrite structure to increase the hardness. An increase in the hardness of the ferrite causes a decrease in machinability, so it is desired to suppress it. From this viewpoint, the upper limit of the amount of Si is set as low as 0.50%. In actual manufacturing techniques, about 0.01% will be the lower limit that can be realized without incurring economic disadvantages.

Mn: 0.90 to 1.80% Cr: 0.05 to 1.00% Both Mn and Cr narrow the lamellar spacing of the pearlite structure, thereby improving strength and wear resistance. In order to obtain this effect, it is necessary to add the above-mentioned lower limit Mn: 0.90% or more and Cr: 0.05% or more, respectively. The addition of a large amount leads to the formation of bainite, which cannot be avoided even if the air is cooled. Needless to say, bainite is hard and significantly reduces machinability. To prevent the adverse effects, Mn: 1.80% and Cr: 1.00, respectively.
Stop adding to the upper limit of%.

Sol-Al: 0.010-0.045% Al forms nitrides with N in the steel, which is finely dispersed in the steel and suppresses the growth of crystal grains during hot forging. I do. In order to obtain this effect, it is necessary to add 0.010% or more. Since the effect is saturated even if added in a large amount, 0.045% is set as a significant upper limit of the added amount.

N: 0.005 to 0.025% N combines with Al in the steel to form a nitride as described above, thereby preventing crystal grains from becoming coarse. To get this effect,
It is necessary to add 0.005% or more. Like Al, the effect saturates when added in large amounts. 0.025% of upper limit
The following additions are advantageous:

In non-heat-treated steel, V is generally added, but in the steel of the present invention, the pearlite structure having excellent wear resistance is intended to occupy an overwhelming part, whereby There is a considerable increase in hardness. It is not advisable in the present invention that machinability be respected to further increase the hardness by adding V. From such a viewpoint, the steel of the present invention does not add V, contrary to common sense of non-heat treated steel.

The functions of the optional elements and the reasons for limiting the composition range are as follows. Pb: 0.030% or less,
S: 0.20% or less, Te: 0.030% or less, Ca:
One or more elements selected from 0.01% or less and Bi: 0.30% or less These are elements that enhance machinability, and high machinability is required in machining forged products. In this case, it is desirable to add. However,
If the addition amount is too large, the hot workability decreases, and the fatigue limit decreases with an increase in the amount of inclusions. Therefore, the addition amount is selected within each of the above limits.

Utilizing the fact that the structure after hot forging is ferrite + pearlite and the area ratio of pro-eutectoid ferrite is 10% or less, the fact that pearlite has excellent wear resistance at the same hardness level. Therefore, it is a condition to be satisfied in order to make the pearlite structure dominant.
The structure of the steel having the above alloy composition becomes ferrite + pearlite after hot forging. The conditions for keeping the proeutectoid ferrite area ratio of the steel at 10% or less can be determined by combining the actual addition amounts of the respective alloy components within the above range of the alloy composition and experimentally finding an appropriate combination.

[0020]

EXAMPLE A steel having an alloy composition shown in Table 1 was melted and cast into an ingot to obtain a round bar-shaped material having a diameter of 50 mm by hot forging. Although not listed in Table 1, each steel is an impurity usually contained in ordinary steel, P: 0.03% or less, Cu: 0.30% or less, Ni: 0.20% or less, M
o: 0.10% or less and O: 0.003% or less.

Table 1 No. C Si Mn Cr sol-Al N Other Examples 1 0.41 0.12 1.75 0.10 0.013 0.006 2 0.55 0.03 1.23 0.35 0.025 0.015 3 0.68 0.02 0.95 0.08 0.021 0.018 4 0.44 0.48 0.80 0.98 0.043 0.023 5 0.54 0.06 1.20 0.31 0.025 0.016 Pb: 0.07 6 0.55 0.04 1.21 0.36 0.024 0.014 S: 0.05 Pb: 0.10 7 0.55 0.03 1.22 0.32 0.025 0.015 S: 0.05 Bi: 0.12 Ca: 0.0018 Comparative example A 0.35 0.13 1.25 0.50 0.025 0.015 B 0.45 0.70 1.21 0.30 0.024 0.019 C 0.56 0.15 2.05 0.31 0.023 0.018 D 0.55 0.15 2.05 0.31 0.025 0.018 E 0.68 0.32 0.50 0.35 0.052 0.027 F 0.50 0.12 0.95 0.15 0.021 0.019 G 0.55 0.05 1.21 0.31 0.026 0.014 Pb: 0.33 H 0.55 0.05 1.22 0.32 0.024 0.013 S: 0.25 Pb : 0.13

After the above forged material was heated and maintained at 1000 ° C. for 60 minutes, it was air-cooled to room temperature at a distance so as not to affect each other. The following tests were performed on each material after cooling.

[Area of proeutectoid ferrite] Using an optical microscope, five visual fields were photographed at a magnification of 100 times, and the photographs were subjected to image analysis. [Hardness] The hardness of the heated air-cooled material having a diameter of 50 mm at the (1/2) R portion was measured with a Rockwell hardness meter. [Fatigue limit] An Ono-type rotating bending test piece with a parallel part diameter of 8 mm
It was cut out from the (1/2) R portion of the heated air-cooled material and evaluated.

[Abrasion resistance] Diameter 3 from center of heated air-cooled material
A test piece of 0 mm x 8 mm length was sampled and evaluated using a Nishihara-type metal abrasion tester under the following conditions: Load: 80 kgf Counterpart material: SCM440 tempered steel Slip ratio: 3% Rotation speed: 400 rpm Lubrication: None

[Tool life] A drill test is performed under the following conditions. Tool: SKH51 (straight shank) Feed: 0.1 mm / rev. Hole depth: 10 mm Life judgment: Unable to cut When the total cutting length reaches 5000 mm The machinability was evaluated at a processing speed of, and the relative value based on the machinability of the steel of Example No. 2 as a reference (100) was expressed as "drilling efficiency".

Table 2 shows the results of the above tests.

[0027] Table 2 No. eutectoid Blow hardness fatigue limit wear amount drill site area ratio (%) (HRC) (MPa ) (g) machining efficiency Example 1 6.9 22.1 443 4.5 2 0. 0 26.3 441 2.2 100 3 0.0 25.3 459 2.6 4 0.0 27.3 384 1.8 5 0.0 25.1 434 2.3 212 26 0.0 26.3 439 2.1 248 7 0.0 26.0 440 2.1 362 Comparative Example A 9.5 19.5 367 10.2 B 6.1 24.9 412 4.7 C 0.0 45.2 539 1 0.1 D 0.0 47.3 476 0.9 E 0.0 25.5 377 3.6 F 13.5 18.1 390 20.3 G 0.0 25.7 394 1.9 218 H 0. 0 25.9 402 2.0 256

The steels of Examples 1 to 7 satisfy all the requirements of the present invention and have excellent wear resistance. The steels of Examples 5 to 7 to which an appropriate amount of the free-cutting element was added succeeded in improving the machinability without lowering the fatigue strength.

On the other hand, each steel of the comparative example has some disadvantages as follows. First, in Comparative Example A, the C content is low and the hardness is insufficient, so that the wear amount is large. Comparative Example B has too high a Si content, and therefore has a higher hardness than Example 1 showing the same level of performance with respect to the amount of wear, and is disadvantageous in terms of machinability. Comparative Examples C and D each have M
Since the contents of n and Cr are too high, bainite is generated, the hardness is significantly increased, and the machinability is poor. Since the wear resistance is low in spite of the high hardness, it is understood that the bainite structure has poor wear resistance. Comparative Example E is opposite to C in Mn.
The amount was too low, so that the lamella spacing of the pearlite became coarse, and the amount of wear was greater than that of Example 3 which was comparable in hardness. In Comparative Example F, the alloy composition was within the specified range, but since the Cr content was low, the area ratio of pro-eutectoid ferrite was high, the hardness was low, and the wear amount was large. Comparative Example G
Since H and H have excessive amounts of free-cutting elements, the fatigue strength is inferior when compared to Example 2 showing the same degree of hardness.

[0030]

The non-heat treated steel for hot forging of the present invention requires quenching and tempering after hot forging by appropriately selecting the alloy composition and regulating the area ratio of proeutectoid ferrite to a certain value or less. Non-heat treated steel without steel shows excellent abrasion resistance without being subjected to surface hardening treatment such as induction hardening or soft nitriding after forming by machining. The steel of the embodiment to which an appropriate amount of free-cutting elements is added has the advantages of good machinability and easy machining in addition to the above benefits. Accordingly, the steel of the present invention is an excellent material when it is desired to produce low cost mechanical parts such as crankshafts of automobile engines.

Claims (2)

[Claims] 1. C: 0.40 to 0.70 on a weight basis
%, Si: 0.50% or less, Mn: 0.90 to 1.80
%, Cr: 0.05 to 1.00%, s-Al: 0.01
0 to 0.045%, and N: 0.005 to 0.025
%, With the balance being Fe and impurities, the structure after hot forging being ferrite + pearlite, and having a proeutectoid ferrite area ratio of 10% or less. Non-heat treated steel for forging. 2. C: 0.40 to 0.70 on a weight basis
%, Si: 0.01 to 0.50%, Mn: 0.90 to
1.80%, Cr: 0.05 to 1.00%, s-Al:
0.010-0.045%, and N: 0.005-
0.025%, and in addition to these, Pb: 0.0
One or more selected from 30% or less, S: 0.20% or less, Te: 0.030% or less, Ca: 0.01% or less, and Bi: 0.30% or less, with the balance being Fe
Hot forging with excellent wear resistance, characterized in that the structure after hot forging is ferrite + pearlite, and the proeutectoid ferrite area ratio is 10% or less. Non-heat treated steel for forging.