JPS5929645B2 - Manufacturing method for steel materials for cold forging - Google Patents

Description

[Detailed Description of the Invention] The present invention is annealing a steel material having an intermediate stage structure at a temperature below the Al transformation point with scale attached in a process such as hot rolling of copper for cold forging. The present invention relates to a method for producing steel for cold forging with excellent productivity, which spheroidizes carbides in the steel without promoting decarburization on the surface of the steel.

Cold forging is a processing technology that has developed rapidly in recent years, but when using medium carbon steel or low alloy steel, which is hard and has poor workability,
In order to improve cold workability, it is necessary to perform annealing to spheroidize the carbides in the steel (hereinafter referred to as spheroidizing annealing).

In order to facilitate the spheroidization of carbides, it is common to apply drawing to break up the carbides in the steel or apply internal strain, or heat treat the steel to create a martensitic structure, followed by spheroidizing annealing. However, in this case, a drawing process and a heat treatment process are required, which increases the manufacturing cost.

When these pre-annealing steps are omitted and hot-rolled steel is subjected to direct spheroidizing annealing, the temperature is higher than the Al transformation point of the steel, and As
This is carried out by holding the temperature at a temperature below the transformation point for several hours and then holding the temperature at a temperature below the Al transformation point for several hours.

However, in this method, the spheroidizing annealing treatment takes a long time, and since the temperature is maintained at a temperature higher than the Al transformation point to form a steel-like austenite structure, carbon easily diffuses and moves from the inside of the steel material to the surface.

Furthermore, if oxidizing components such as moisture, oxygen, and scale attached to the steel material are present in the annealing furnace, these components and the carbon of the steel material quickly react with each other on the surface of the steel material, resulting in the development of a decarburized layer.
However, the presence of a decarburized layer causes inconveniences such as deterioration of cold forging properties and mechanical properties, and failure of the product surface to reach the desired hardness after forging, so no agreement is made regarding the allowable depth of the decarburized layer. This is usually one of the important control items for spheroidizing annealing technology.

JIS has also prepared draft decarburized layer tolerance standards shown in Table 1, and the decarburized layer depth of spheroidized annealed material must be below these values.

Table 1: Permissible decarburization depth for cold forging steel (JIS draft) Therefore, when conventional hot-rolled steel is subjected to direct spheroidizing annealing, the atmospheric gas in the annealing furnace does not contain carburizing or decarburizing. It was necessary to use reducing gases such as CO and 02 that were strictly controlled so that they would not occur, and to remove scales attached to the steel material serving as the oxygen source in advance by pickling or the like.

These measures are not only very expensive but also CO,
The use of reducing gases such as H2 is difficult to control and there is a risk of explosion.

Also, when pickling the sump to remove scale, careful attention must be paid to pollution control. In order to solve these problems, the present invention has developed a steel material containing 0.20 to 0.50% of carbon and 2.00% or less of manganese, and 0.5 to 0.5% of chromium added as necessary.
In hot rolling of cold forging steel materials containing 1.5% and/or molybdenum of 0.5% or less, after the steel material passes through the finishing mill, the temperature range is 550°C or less and above the Ms point.
By cooling rapidly at a cooling rate of ~100°C/second or more and then holding the same temperature range for 1 to 30 minutes or more, the structure of the steel material becomes an intermediate stage structure, which naturally forms on the surface due to the hot rolling etc. The generated scale was directly charged into an annealing furnace with the scale attached to the surface of the steel material, and heated to 650
℃ or more A? Spheroidizing annealing is performed in a temperature range of below 100° C., which spheroidizes the carbides in the steel without developing a decarburized layer on the surface of the steel, producing steel for cold forging with excellent cold workability. It provides a method for manufacturing the product. In the spheroidizing annealing of the present invention, annealing is performed at a temperature below the 80 transformation point at which austenite does not occur, so the rate at which carbon inside the steel material diffuses to the surface is extremely slow (and the depth of the decarburized layer hardly progresses). .

FIG. 1 shows the progress of the decarburized layer during the spheroidizing annealing conducted by the inventors. Table 2 shows the components and manufacturing conditions of the sample materials. According to test AI corresponding to the present invention,
Testing labs that remove scale in advance by pickling and use reducing gases such as CO and H2 in the annealing furnace atmosphere.
It is at the same level as No. 3, and the depth of the decarburized layer hardly progresses, fully satisfying the JIS proposal regarding the depth of the decarburized layer.

Next, the reasons for the limitations of the present invention will be described. In the first invention, when the carbon content is less than 0.20%, the cold workability is inherently good (no spheroidizing annealing is required), and when the carbon content exceeds 0.50%, the strength is high and it is not suitable for cold forging steel. I can't do it.

Manganese is an element normally contained in steel for cold forging, but if it exceeds 2.00%, the strength will be too high and it will not be suitable for cold forging, and the cost will unnecessarily increase.
It was limited to 0% or less.

If a steel material containing the above components is cooled at a rate of 100°C/second or less after hot rolling, or if the temperature range for rapid cooling exceeds 550°C after hot rolling, the steel material will be ferrite/pearlite. In order to spheroidize the carbides in the steel without drawing or heat treating it, it is necessary to annealing it to a temperature above the transformation point.
In a state where scale is attached, a decarburized layer progresses.

When rapidly cooling to a temperature below the Ms point or 550
If the time maintained in the temperature range from °C to Ms point is 1 minute or less, martensitic structure will be mixed into the steel material. The martensitic structure is very hard and difficult to handle, and there are many practical problems such as the possibility of quench cracking. For this reason, in the present invention, after hot rolling, the steel sheet is cooled at a rate of 100° C./second or more to a temperature range of 550° C. or higher than the Ms point, and then maintained in the same temperature range for 1 minute or more to obtain an intermediate stage structure. When the spheroidizing annealing temperature exceeds the A1 transformation point, the steel material becomes an austenitic structure, and decarburization progresses rapidly, and in order to spheroidize the carbides, it is necessary to continue annealing at a temperature below the A1 transformation point for several hours. Since the present invention forms an intermediate stage structure as described above, the spheroidization of the carbide progresses sufficiently in a short time even during annealing at a temperature below the A1 transformation point.

When the spheroidizing annealing temperature is less than 650° C., the movement of carbides becomes extremely slow and the spheroidizing process takes a long time.

The spheroidizing annealing temperature of this invention is A? 65 below the metamorphosis point
The temperature was 0°C or higher. Furthermore, even if annealing is performed for a long time, it will not impair the properties of the material of the present invention, but from the viewpoint of productivity,
The atmosphere gas during annealing is not particularly limited, but CO or H2, which is expensive and has the risk of explosion, is suitable. There is no need to use reducing gases. N2 gas is most preferable for industrial production, but air atmosphere may also be used. In the second invention, carbon and manganese are the first
It is the same as the invention of

0.50% to 1.50% chromium and 0.5% or less molybdenum are added individually or in combination to facilitate quenching after cold forging and to increase heat treatment effects such as preventing strength loss due to tempering. The upper limit is set from the viewpoint of increasing the strength before cold forging. After hot rolling, the steel material containing the above components is rolled at a speed of 25°C/second or more to 550°C to M
It is possible to obtain a hot rolled steel material with an intermediate stage structure by rapidly cooling it to the temperature range of the s point and then keeping it in the same temperature range for 30 minutes or more, but under these conditions, ferrite/pearlite cannot be obtained. structure or martensitic structure, and cannot be adopted for the same reason as the first invention.

The hot-rolled steel material with the intermediate stage structure obtained in this way is annealed with scale attached under the same conditions as in the first invention, that is, in the temperature range of 650° C. or higher below the A1 transformation point. is based on the same reason as the first invention. This makes it possible to spheroidize the carbides in the steel without progressing decarburization. Next, examples of the present invention will be described.

Prepare a steel material having the components shown in Table 3, and from this, wire diameter 14
Hot rolling of TILmφ wire rod is carried out at normal temperature, and a cooling device installed between the finishing rolling mill and the winding device is used to
After cooling with water under the conditions shown in Table 3, it was wound into a coil.

The weight of the coil is about 1 ton, and the cooling rate during air cooling is relatively slow due to the latent heat of the coil.

For this reason, carbon steels that require a short holding time from 550°C to the Ms point were air cooled as they were, but steel materials containing chromium that needed to be held in the same range for more than 30 minutes were cooled for 100 to 400°C after rolling. It was kept warm at ℃ and then passed through a slow cooling tunnel. This hot-rolled wire rod was directly charged into an annealing furnace with the scale attached without going through a pickling process, and spheroidizing annealing was performed under the conditions shown in Table 4.

Table 5 shows the depth of decarburization, mechanical properties, and cold workability after spheroidizing annealing. In the material of the present invention, the decarburization layer has hardly progressed, and the carbide is well spheroidized, so it is clear that the material has much better cold workability than the comparative material. As explained above, in the present invention, a hot rolled steel material for cold forging having an intermediate stage structure is directly charged into an annealing furnace with scale attached, and heated to 655°C.
By performing annealing at a temperature below the A1 transformation point,
This method spheroidizes carbide without substantially developing a decarburized layer, and has various advantages as described below and is extremely useful industrially. Advantage 1: There is no need to remove scale attached to the hot rolled steel material before spheroidizing annealing, and the pickling process can be omitted. Advantage 2: There is no need to use a reducing gas such as CO or H2, which is expensive and has a risk of explosion, which has been conventionally used in the annealing atmosphere.

Advantage 3: It is possible to obtain a spheroidized annealed material with better cold forgeability than conventional methods by annealing in a short time, and productivity is very high.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the relationship between the depth of decarburization progress and annealing time during spheroidizing annealing.

Claims (1)

[Scope of Claims] 1 Containing 0.20 to 0.50% carbon and 2.00% or less manganese, cooled at a rate of 100°C/second or more to a temperature range of 550°C to Ms point after hot finish rolling. Then, the steel material with scale on the surface and an intermediate stage structure is annealed in the temperature range of 650℃ or higher and lower than the A_1 transformation point by holding the same temperature range for 1 minute or more and with the scale attached. , A method for producing a steel material for cold forging, characterized in that carbides in the steel are spheroidized without increasing the depth of the decarburized layer on the surface of the steel material. 2 Contains 0.20 to 0.50% carbon, 2.00% or less manganese, 0.5 to 1.5% chromium, and/or 0.5% or less molybdenum, 550°C to M after hot finish rolling.
Cooling to the temperature range of point s at a rate of 25°C/second or more and then maintaining the same temperature range for 30 minutes or more to produce a steel material with scale on the surface and an intermediate stage structure inside, with the scale attached. A method for producing a steel material for cold forging, characterized in that the carbides in the steel are spheroidized without increasing the depth of the decarburized layer on the surface of the steel material by annealing the steel material in a temperature range of 650° C. or higher and below the A_1 transformation point.