JPH07188858A - Steel for cold forging - Google Patents

Abstract

PURPOSE:To produce automobile drive mechanism parts, etc., by cold forging. CONSTITUTION:A steel, having a composition consisting of 0.25-0.39% C, 0.03-0.10% Si, 0.60-1.00% Mn, <=0.05% Ni, <=0.30% Cr, <=0.03% Mo, <=0.05% Cu, 0.010-0.030% Al, 0.010-0.050% Ti, 0.0003-0.0050% B, 0.0050-0.0100% N, <=0.0015% O, <=0.020% P, 0.005-0.020% S, and the balance Fe, is used. After hot rolling is finished, the steel is cooled at <=0.7 deg.C/sec cooling rate and cold- forged in this state or in an as-normalized state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a drive system component for an automobile by cold forging.

[0002]

2. Description of the Related Art Shafts and yokes, which are driving force transmission parts for automobiles, are S40C to S45C in terms of price and strength.
It is manufactured by hot forging using such a medium carbon steel as a raw material, and then subjected to quenching, tempering treatment, and further mechanical processing such as cutting in order to obtain necessary mechanical strength. Alternatively, the medium carbon steel is manufactured by subjecting it to a softening heat treatment such as a spheroidizing annealing treatment, followed by forming by cold or warm forging, followed by quenching, tempering treatment and finishing. However, in recent years, in order to achieve energy saving and process saving for the purpose of reducing the manufacturing cost, these mechanical parts have been cold forged by omitting hot forging and reducing machining cost. However, in order to carry out cold forging, medium carbon steel that has been used as a conventional material has been inevitably subjected to softening heat treatment such as spheroidizing annealing as a pretreatment.

[0003]

Therefore, in the present invention, the relationship between the chemical composition of steel and the cold forgeability is investigated so that cold forging can be performed as hot rolling or as normalizing. Research, without compromising cold forgeability,
The chemical composition range to secure the strength of conventional steel was studied. In addition, we studied the influence of chemical components on the depth of the hardened layer and the surface hardness so that it is also suitable for surface hardening by induction hardening, and thus worked by cold forging without the need for softening heat treatment such as spheroidization. However, it is intended to obtain an excellent processed material.

[0004]

The present invention, in weight percent, comprises:
C: 0.25 to 0.39%, Si: 0.03 to 0.10.
%, Mn: 0.60 to 1.00%, Ni: 0.05% or less, Cr: 0.30% or less, Mo: 0.03% or less, C
u: 0.05% or less, Al: 0.010 to 0.030
%, Ti: 0.010 to 0.050%, B: 0.000
3 to 0.0050%, N: 0.0050 to 0.0100
%, O: 0.0015% or less, P: 0.020% or less,
S: 0.005 to 0.020% is contained, and the balance is Fe and inevitable impurity elements. A steel for cold forging and a steel having the above composition are used as raw materials. It is a steel for cold forging characterized by being cooled at a cooling rate of 7 ° C./sec or less and cold forged as it is or after being subjected to a normalizing treatment. The present inventors investigated the chemical composition and cold forgeability of steel so that cold forging can be performed as hot rolling or as normalizing treatment, without impairing cold forgeability. , Studied the chemical composition range to secure the strength of conventional steel. In addition, the effect of chemical components on the depth of the hardened layer and the surface hardness was investigated so that it is suitable for surface hardening by induction hardening.

First, the steel of the present invention and S43, which is the current steel, are used.
Normalizing treatment (S43C-N) under the conditions of "870 ° C x 1 hour holding and cooling" using C and "760 ° C x 20"
Comparative steel subjected to spheroidizing annealing treatment (S43C-SA) under the condition of "holding time and cooling at 10 ° C / h" was machined into cold compression test pieces shown in Fig. 1 and compressed and deformed at various compression rates. The relationship between the load and the chemical composition required for the deformations of 50% and 60% when tested was examined. The results are shown in Table 1.

[0006]

[Table 1]

Table 2 shows the results of tests carried out to find the relationship between the induction hardenability and the chemical composition. That is, the test material having various chemical components was tested at “870 ° C. × 1
After carrying out the normalizing treatment under the condition of "air cooling after holding time",
The test piece shown in FIG. 2 was produced, and induction hardening was carried out under the following conditions. Type: vacuum tube, capacity: 150KW, frequency: 25KH
z, voltage: 9.7 KV, current: 7.0 A, coil diameter: 3
5 mmφ, feed rate: 8.4 mm / S. After that, the hardness is measured, and the surface hardness and the hardened layer (Hv45
The result of having calculated | required the distance from the surface to 0) is shown.
From these results, the degree of inhibiting cold forgeability of chemical constituent elements in the relationship between the cold compressibility of 60% and the chemical composition is in the order of C>Mn>Si>Cr>Ni> Mo. On the other hand, for induction hardenability, Mo>Mn> C
It was revealed that they contribute to the increase in the depth of the hardened layer in the order of r>Si> C.

[0008]

[Table 2]

Therefore, based on the results of these studies, regarding the cold forgeability, the contents of C and Si should be suppressed, the contents of Ni, Cu and Mo should be made as low as possible, and the induction hardenability should be high. With respect to Mo, since it causes a cost increase and deteriorates the cold forgeability as described above, the chemical composition range is set to be secured by Mn, Cr, and B which are low cost elements. Defined as described above. And, such steel is 0.7 after the hot rolling is completed.
It is possible to cool at a cooling rate of ° C / sec or less and carry out cold forging as it is or after normalizing.

The reasons for limiting the components of the present invention are as follows. C: C is an essential element from the viewpoint of securing the strength after quenching and tempering treatment, and in the target parts to which the steel of the present invention is applied, at least 0 in consideration of securing the surface hardness by the induction quenching treatment. 25% is required. However, the addition of a large amount is not preferable because it impairs the cold forgeability. Therefore, the upper limit of C is 0.39%.

Si: Si is usually an element necessary as a deoxidizing material during steelmaking, and at least 0.03% is necessary to carry out sufficient deoxidation. However, if Si is added in a large amount, cold forgeability is impaired, so the upper limit is set to 0.
10%. Mn: Mn is also an element usually required as a deoxidizing material during steelmaking, and is also an essential element for ensuring the hardenability after quenching and tempering. In addition, it is an essential element for ensuring the depth of the hardened layer during induction hardening. Therefore, 0.6
If it is less than 0%, this effect cannot be expected, so 0.60% is made the lower limit. On the other hand, it impairs a large amount of added cold forgeability and reduces machinability during finishing. Therefore, the upper limit is set to 1.00%.

Cr: Cr is an element that improves the hardenability, and at the same time is an element that increases the hardness of the core portion as hot-rolled or as-normalized. Therefore, although it may be used for the purpose of adjusting hardenability as necessary,
If added in a large amount, the cold forgeability is impaired, so the upper limit is 0.30%. Ni: Ni is also an element that improves hardenability, but if added in a large amount, it is an element that increases the hardness of the core part as hot-rolled or as-normalized, and impairs cold forgeability, so the upper limit is Is 0.05%.

Mo: Mo is also an element for improving hardenability, but addition of a large amount is an element for increasing the hardness of the core as hot-rolled or as-normalized, which impairs cold forgeability. Therefore, the upper limit is made 0.03%. Cu: Cu is an unavoidable impurity element, but if contained in a large amount, it is an element that enhances the hardness of the core as hot-rolled or as-normalized, and impairs cold forgeability, so the upper limit is 0. 0.05%.

Al: Al is an indispensable element as a deoxidizing agent during steelmaking, and also produces AlN together with N to refine the austenite grain size of the steel. For that, 0.0
A content of 10% or more is necessary. Therefore, the lower limit is made 0.010%. On the other hand, a large amount of addition binds to oxygen in the atmosphere during molten steel and increases the amount of oxide-based non-metallic inclusions, which serves as the starting point and promotes the occurrence of cracks during cold forging.
Therefore, the upper limit is 0.030%.

Ti: Ti is an element that, like Al, combines with N and improves the hardenability of B. However, 0.010%
However, the effect is small, but on the other hand, when the content exceeds 0.050%, a large TiN inclusion is formed and becomes a starting point of cracking during cold forging. Therefore, 0.050% is set. B: B is an element that improves the hardenability by adding a trace amount,
If less than 0.0003%, the effect is not exhibited, so 0.0003% is made the lower limit. However, excessive addition lowers the toughness of the steel material, so the upper limit is 0.0050%.

N: N is preferably as low as possible in order to fully exert the effect of B on the hardenability. Therefore, the upper limit is set to 0.0100%. On the other hand, at least 0.0050% is required to combine with Al and achieve the refinement of the austenite crystal grains by the precipitate of AlN, so this is the lower limit. O: O forms oxide inclusions and becomes a starting point of cracks during cold forging, so it is desirable to be reduced as much as possible. Therefore, the upper limit is made 0.0015%.

P: P is concentrated in the austenite crystal grain boundaries and promotes cracking during quenching. Therefore, it is desired to be reduced as much as possible, and the upper limit is set to 0.020%. S: S is an element that hinders cold forgeability, so the upper limit is made 0.020%. However, if it is too low, the machinability is impaired, so the lower limit is made 0.005%. In the cold forging, the cooling rate after completion of hot rolling is set to 0.7 ° C./sec or less because the material becomes hard and the cold forgeability is impaired at a cooling rate higher than this.

[0018]

EXAMPLES Medium carbon steels of the present invention steel and conventional steels (S43
An example in which the characteristic evaluation of each of C) as the comparative steel was tested is shown below. The invention steels having the chemical composition shown in Table 3 are 2.1.
A steel ingot was melted, billet-rolled, then hot-rolled to a 25 mm round, and further held at 870 ° C. for 1 hour, and then air-cooled normalizing treatment was performed. And the conventional steel S43C-N and S4
3C-SA is the same material as in Table 1. Using this as a base material, the test pieces shown in FIGS. 1 and 2 were produced, and a cold compression deformation test and an induction hardening characteristic confirmation test were carried out. The results are also shown in Table 3.

[0019]

[Table 3]

As is clear from these results, it was confirmed that the steel of the present invention is superior in cold workability not only to the normalizing treated material of S43C, which is a conventional steel, but also to the spheroidized annealed material. In addition, with regard to induction hardenability, the depth of the hardened layer (hardness of Hv450) is deeper than that of conventional steel, and the surface hardness is also the minimum hardness HRC50 (Hv510) normally required for induction hardening. It was confirmed to have sufficient characteristics. Using the steel of the present invention having such characteristics, the test piece shown in FIG. 3 was prepared and subjected to a torsional fatigue test after induction hardening. The results are shown in FIG. 4, and it was confirmed that the fatigue properties were not inferior to the level of the conventional steel S43C.

[0021]

INDUSTRIAL APPLICABILITY The steel of the present invention is a medium carbon high-strength component which is conventionally used after the base material has been formed by hot forging and then finish-formed by machining and the surface is induction hardened. It became possible to perform cold forging without performing the spheroidizing annealing process that requires a long time as a pre-heat treatment.
Therefore, it greatly contributes to the improvement of workability, the improvement of productivity, and the reduction of cost as compared with the conventional art.

[Brief description of drawings]

FIG. 1 is an explanatory view of a cold compression test piece.

FIG. 2 is an explanatory diagram of an induction hardening characteristic test piece.

FIG. 3 is an explanatory diagram of a torsion fatigue test piece.

FIG. 4 is a graph showing the results of torsional fatigue test.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koho Kioi, No. 1, Nakashinri, Hashime-cho, Okazaki-shi, Aichi Prefecture Mitsubishi Motors Corp., Passenger Car Development Headquarters (72) Yoichi Taniguchi, Hashime-cho, Okazaki-shi, Aichi Nakashinkiri No.1 Passenger Car Development Headquarters, Mitsubishi Motors Corporation (72) Inventor Kinya Kato Hashime-cho, Okazaki City, Aichi Pref.

Claims (2)

[Claims] 1. C: 0.25 to 0.39% by weight,
Si: 0.03 to 0.10%, Mn: 0.60 to 1.0
0%, Ni: 0.05% or less, Cr: 0.30% or less,
Mo: 0.03% or less, Cu: 0.05% or less, Al:
0.010 to 0.030%, Ti: 0.010 to 0.0
50%, B: 0.0003 to 0.0050%, N: 0.
0050 to 0.0100%, O: 0.0015% or less,
P: 0.020% or less, S: 0.005-0.020%
A steel for cold forging, characterized by containing, and the balance being Fe and an unavoidable impurity element. 2. A steel having the composition according to claim 1 is used as a raw material, and after completion of hot rolling, it is cooled at a cooling rate of 0.7 ° C./sec or less and cold forged as it is or after normalizing. Steel for cold forging characterized by