JP2957871B2 - Cold forging steel - Google Patents

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 transmitting parts for automobiles, are manufactured from S40C to S45C in view of cost and strength.
Are formed by hot forging using a medium carbon steel as described above, and thereafter, quenching and tempering are performed to obtain the required mechanical strength, and further, machining such as cutting is performed. Alternatively, the medium carbon steel is manufactured by subjecting it to a softening heat treatment such as a spheroidizing annealing treatment, then forming it by cold or warm forging, then quenching, tempering and finishing. However, in recent years, in order to achieve energy savings and process savings for the purpose of reducing manufacturing costs, these machine parts have been subjected to cold forging by omitting hot forging and reducing the machining allowance. However, medium carbon steel conventionally used as a raw material has had to be subjected to a softening heat treatment such as spheroidizing annealing as a pre-treatment in order to perform cold forging.

[0003]

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

[0004]

SUMMARY OF THE INVENTION The present invention provides, in weight percent,
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: containing 0.005 to 0.020%, the remainder of the set formed ing Fe and unavoidable impurity elements steels and materials,
After the completion of hot rolling, cooling is performed at a cooling rate of 0.7 ° C./second or less,
Cold forging steel characterized by being cold forged as it is or after normalizing. The present inventors have investigated the chemical composition and cold forgeability of steel so that cold forging can be performed while hot rolling or normalizing, without impairing cold forgeability. We studied the range of chemical components to ensure the strength of conventional steel. In addition, the effects of chemical components on the depth of the hardened layer and the surface hardness were studied so as to be suitable for surface hardening by induction hardening.

[0005] First, the steel of the present invention and the working steel S43 are used.
Using C, normalizing treatment under the conditions of “cooling after holding at 870 ° C. × 1 hour” (S43C-N) and “760 ° C. × 20
The comparative steel subjected to spheroidizing annealing treatment (S43C-SA) under the condition of “cooling at 10 ° C./h after holding for a time” was machined into cold-compressed test specimens shown in FIG. 1 and compression-deformed at various compression ratios. The relationship between the load required for the deformation of 50% and the load required for the deformation of 60% and the chemical composition was examined. Table 1 shows the results.

[0006]

[Table 1]

Table 2 shows the results of tests conducted to determine the relationship between induction hardening properties and chemical components. That is, the test materials having various chemical components are referred to as “870 ° C. × 1
After performing normalizing treatment under the condition of `` air cooling after holding for
The test piece shown in FIG. 2 was produced, and induction hardening was performed 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) are measured.
The figure shows the result of calculating the distance from the surface to 0).
From these results, the degree of inhibiting the cold forgeability of the chemical component element in the relationship between the cold compressibility of 60% and the chemical component may be in the order of C>Mn>Si>Cr>Ni> Mo. On the other hand, on the other hand, regarding the induction hardening property, Mo>Mn> C
It became clear that the order of r>Si> C contributed to the increase in the depth of the hardened layer.

[0008]

[Table 2]

Therefore, based on the results of these studies, regarding the cold forgeability, it is necessary to suppress the contents of C and Si, to reduce the contents of Ni, Cu, and Mo as much as possible, and to perform induction hardening. As for Mo, since it becomes a factor of cost increase and deteriorates cold forgeability as described above, the chemical composition range is set to secure with Mn, Cr, B which are inexpensive elements. Specified as described above. And after completion of hot rolling, the steel
It is possible to cool at a cooling rate of not more than ° C./sec and to perform 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 ensuring the strength after quenching and tempering, and in the target parts to which the steel of the present invention is applied, at least 0. 25% is required. However, the addition of a large amount is not preferable because it inhibits cold forgeability. Therefore, the upper limit of C is set to 0.39%.

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

Cr: Cr is an element that improves hardenability, and at the same time, is an element that increases the hardness of the core as it is hot-rolled or as it is after normalizing. Therefore, it may be used to adjust hardenability as necessary,
When added in a large amount, the upper limit is set to 0.30% because cold forgeability is impaired. Ni: Ni is also an element that improves the hardenability, but a large amount of addition is an element that increases the hardness of the core as it is hot-rolled or as-normalized, and impairs cold forgeability, so that the upper limit is added. Is set to 0.05%.

Mo: Mo is also an element which improves hardenability, but when added in a large amount, it is an element which increases the hardness of the core as it is hot-rolled or as-normalized, and impairs cold forgeability. Therefore, the upper limit is set to 0.03%. Cu: Cu is an unavoidable impurity element, but a large amount is an element that increases the hardness of the core as hot-rolled or as-normalized, and impairs cold forgeability. 0.05%.

Al: Al is an essential element as a deoxidizing agent during steel making and also forms AlN together with N to reduce the austenite grain size of the steel. For that, 0.0
A content of 10% or more is necessary. Therefore, the lower limit is set to 0.010%. On the other hand, a large amount of addition combines with oxygen in the atmosphere at the time of molten steel and increases the amount of oxide-based nonmetallic inclusions, which serves as a starting point and promotes the generation of cracks during cold forging.
Therefore, the upper limit is 0.030%.

Ti: Ti is an element that combines with N like Al and improves the hardenability of B. However, 0.010%
The effect is small, however, on the other hand, if added over 0.050%, large TiN inclusions are formed and serve as starting points for cracking during cold forging. Therefore, it is set to 0.050%. B: B is an element that improves hardenability by adding a small amount,
If less than 0.0003%, the effect is not exhibited, so the lower limit is 0.0003%. 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 sufficiently exert the effect of B on hardenability. Therefore, the upper limit is set to 0.0100%. On the other hand, since at least 0.0050% is required to achieve the refinement of austenite crystal grains by the AlN precipitate by combining with Al, the lower limit is set. O: O forms oxide inclusions and becomes a starting point of cracking during cold forging. Therefore, it is desirable that O be reduced as much as possible. Therefore, the upper limit is made 0.0015%.

P: P concentrates on austenite grain boundaries and promotes cracking during quenching. Therefore, it is desirable to reduce as much as possible, and the upper limit is made 0.020%. S: Since S is an element that inhibits cold forgeability, the upper limit is made 0.020%. However, if it is too low, the cutting property is impaired, so the lower limit is made 0.005%. The reason why the cooling rate after the completion of hot rolling in cold forging is set to 0.7 ° C./sec or less is that if the cooling rate exceeds this, the material becomes hard and the cold forgeability is impaired.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Medium carbon steel of the present invention and conventional steel (S43)
Examples in which the characteristics evaluation was tested for each of C) and Comparative Steel are shown below. Inventive steel having the chemical composition shown in Table 3
The ingot was melted with a steel ingot, billet-rolled, hot-rolled into a 25 mm round, further kept at 870 ° C. × 1 hour, and air-cooled normalizing treatment was performed. And S43C-N and S4 of the conventional steel
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 prepared, and a cold compression deformation test and an induction hardening characteristic confirmation test were performed. The results are also shown in Table 3.

[0019]

[Table 3]

As is clear from the results, it was confirmed that the steel of the present invention was superior not only to the normalizing material of the conventional steel S43C but also to the cold workability of the spheroidized annealed material. In addition, the depth of the hardened layer (hardness of Hv450) is deeper than that of conventional steel in terms of induction hardening, and the minimum hardness HRC50 (Hv510 which is usually required for induction hardening) is also required in terms of surface hardness. ), And thus it was confirmed that the film had sufficient characteristics. A test piece shown in FIG. 3 was prepared using the steel of the present invention having such characteristics, and a torsional fatigue test was performed after induction hardening. The results are shown in FIG. 4, and it was confirmed that the fatigue properties were not inferior to those of the conventional steel S43C.

[0021]

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

[Brief description of the 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 view of a torsional fatigue test piece.

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

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koho Shiroi 1-Chushinkiri, Hashime-cho, Okazaki-shi, Aichi Pref. No. 1 Chushinkiri, Mitsubishi Motors Corporation Passenger Car Development Headquarters (72) Inventor Kinya Kato 1 Chushinkiri, Hashime-cho, Okazaki-shi, Aichi Pref., Mitsubishi Motors Corporation Passenger Car Development Headquarters (56) References JP-A-4-285121 (JP, A) JP-A-3-17737 (JP, A) JP-A-5-105957 (JP, A) JP-A-63-216952 (JP, A) JP-A-63-216920 (JP, A) JP-A-62-23929 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00 301 C22C 38/54

Claims (1)

(57) [Claims] 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:
0.010-0.030%, Ti: 0.010-0.0
50%, B: 0.0003-0.0050%, N: 0.
0050 to 0.0100%, O: 0.0015% or less,
P: 0.020% or less, S: 0.005 to 0.020%
And the remainder is made of steel having a composition of Fe and unavoidable impurity elements.
Cooling at the lower cooling rate, as it is or normalizing
Cold forging steel characterized by being cold forged as it is.