JPS60135520A - Manufacturing method of cold forging material - Google Patents

Abstract

PURPOSE:To produce a blank material for cold forging having good formability by subjecting a blank steel material to hot forging, cooling, reheating, recooling and alternate repetitions of heating and cooling under specific conditions thereby decreasing the hardness thereof and spheroidizing the blank material. CONSTITUTION:The carbon steel represented by JIS S45C-S53C is used as a steel material. The billet of said material is heated to about 1,150 deg.C and is subjected to hot forging to form a blank material for a joint outer. The blank material is then cooled to <=600 deg.C and is then reheated to the temp. above the Ac3 transformation temp. (about 850 deg.C) to effect recrystallization. The material is recooled to <=600 deg.C to form the finer grains of the coarsened crystal grains by which the uniform structure consisting of ferrite and pearlite is formed. The blank material is further subjected alternately repeatedly about 3 times heating and cooling in the temp. range higher by about 30 deg.C from the Ac1 transformation temp. from said temp., for example, about 740-720 deg.C by which the blank material is subjected to a spheroidizing annealing treatment.

Description

[Detailed description of the invention] The present invention relates to a method of manufacturing a material for cold forging.

Conventionally, when manufacturing a constant velocity joint outer for an automobile, for example, a material for cold forging is obtained from a steel material by hot forging, and then the material is subjected to final forming by cold forging to obtain a joint outer. After the above-mentioned hot forging, the material is cooled to 600C or less, and then the temperature ranges from the AC1 transformation temperature to about 20 to 30C higher than that temperature.
For example, spheroidizing annealing is performed by alternately repeating heating and cooling at 720 to 740C, but since the crystal grains become coarse during hot forging and bainite precipitates in addition to ferrite and pearlite, only the spheroidizing annealing is performed. In some cases, the hardness of the material is difficult to reduce, resulting in severe wear and tear on the mold during the next cold forging process, resulting in a shortened lifespan.

Furthermore, in the spheroidizing annealing process, the temperature of the chamber for decomposing paynite is higher than that of the conventional annealing heating temperature, and the chamber is narrow, so temperature control becomes strict and mass productivity is impaired.

In view of the above, the present invention aims to provide a manufacturing method including a heat treatment step that reduces the hardness of a material for cold forging, is easy to control temperature, and is excellent in mass productivity. A step of forming a material for cold forging by performing inter-forging processing; cooling the material to 600C or less, and then
Recrystallization by reheating the material to a temperature below the transformation temperature, followed by a step of recooling the material to a temperature below 600C;
It is characterized by a step of performing a spheroidizing annealing treatment by alternately repeating heating and cooling in a temperature range from the transformation temperature to about 30 tZ' higher than the transformation temperature.

An example in which the present invention is applied to the manufacture of a joint outer material in a constant velocity joint will be described below.
Carbon steel specified by JIS 545C-553C is used as the steel material.

First, the billet made of carbon steel is heated to 11,500 ℃ (line a in FIG. 1).

The billet heated to the above temperature is subjected to hot forging to form a joint outer material (line h in the figure).

The above material is cooled to below 600C (line C in the figure), then reheated to 850C, which is above the material's Ac3 transformation temperature, for recrystallization (line d in the figure).
Re-cool to below 00C (line e in the figure).

By this recrystallization treatment, the crystal grains that have become coarse during hot forging can be made finer, and most of the bainite precipitated during the hot forging process can be decomposed to obtain a uniform structure of ferrite and pearlite.

For the above material, Ac, from the transformation temperature to about 30% above that temperature.
A spheroidizing annealing process is performed in which heating and cooling are alternately repeated three times in a high temperature range, for example 740 to 720C (first
Line f in the figure). In this case, the time t from the first heating to the next heating is 1 hour.

By combining the recrystallization treatment and the spheroidizing annealing treatment, the hardness of the material can be significantly reduced compared to the case where the recrystallization treatment is not performed.

Figure 2 shows the change in hardness with respect to the heating temperature during the spheroidization process, and shows the hardness change at point 1 at the heating temperature of 745+50. ~g4
If the area surrounded by recrystallization is performed, the area surrounded by
The area surrounded by 4 is the case where no recrystallization treatment was performed, and it is clear that the former has significantly lower hardness than the latter.

This is because the recrystallization process slows down the hardness change with respect to the heating temperature of the material. The cooling temperature in this case is 7
It is 20-725C.

When the joint actor was formed by cold forging the above material, the forming load was small, the wear and tear on the mold was small, and the joint outer was free from cracks and forging cracks. confirmed.

FIG.
The hardness changes under the same processing conditions as in the figure are shown, and points m,
If the area surrounded by m4 is recrystallized, point n
, ~n4 is the case where the recrystallization treatment was not performed, and it is clear that the hardness of the former is significantly lower than that of the latter, as described above.

As described above, according to the present invention, by subjecting the material to recrystallization treatment and spheroidizing annealing treatment after hot forging, the hardness of the material is reduced and the material is spheroidized. It is possible to obtain raw materials for forging, and it is possible to eliminate defects such as wear and tear on dies. In addition, since the temperature range of recrystallization treatment is wide, temperature control is easy and mass production is excellent. Furthermore, since the recrystallization treatment and the spheroidizing annealing treatment are performed in series, it is possible to combine these treatments in series with hot forging to improve production efficiency.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between material temperature and time in each process, Figure 2 is a graph showing the relationship between heating temperature and hardness degree in the spheroidizing annealing treatment of carbon steel expressed in JIS 545C, and Figure 3 is With JLS 553C
- is a graph showing the relationship between heating wire temperature and hardness in spheroidizing annealing treatment of carbon steel. Patent applicant Honda Motor Co., Ltd. = 7 = Cutting and cutting kidney field!

Claims (1)

[Claims] A step of hot forging a steel material to form a material for cold forging; cooling the material to 600C or less, then reheating the material to a temperature above A C3 transformation temperature to perform a recrystallization treatment, and then A step of re-cooling the material to below 600C;
The material has an A C1 transformation temperature of about 30% above that temperature.
A method for producing a material for cold forging, comprising: performing a spheroidizing annealing treatment by alternately repeating heating and cooling in a high temperature range.