JPS5848613B2 - Heat treatment method for metal materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel heat treatment method for obtaining a uniform quality of a metal material. The process involves applying pressure to the material to delay the phase transition, and under that pressure, releasing the pressure just before the phase transition line to allow the phase transition to proceed.

Phase transformation is very widely used to improve various properties of metal materials, and for example, in Fe--C systems, it can be seen as paynite transformation, martensitic transformation, etc.

These general heat treatment methods involve once solution treatment at high temperature;
It rapidly cools and transforms into a metastable phase.

That is, when transformation occurs, the transformation temperatures during heating and cooling are generally different, and there is hysteresis.

This also indicates that the transformation during cooling is dependent on the cooling rate; if the material is cooled very rapidly, the high-temperature phase is frozen as it is.

When a new phase is transformed in this state, the rate of its formation or growth is determined by its transformation temperature. can get.

However, rapid cooling is a basic condition for obtaining these structures, but this condition depends on the mass of the material, and if the mass becomes large, rapid cooling becomes impossible.

In response to the generated thermal stress, extremely brittle materials may suffer from quench cracking, making rapid cooling even more difficult.

In such cases, applying pressure is extremely effective.

That is, if pressure is applied during the cooling process, it will be easily supercooled and the transformation start time will be extremely slow.

However, when the transformation is directly carried out under pressure, even though the mass effect is effective, the transformation product changes to a high-pressure phase, or its structure changes compared to when it is under normal pressure.

This changes the properties of the transformation product under normal pressure.

Furthermore, when the transformation is carried out under pressure in this way, the time required to complete the transformation is also significantly longer.

The present invention is characterized in that the pressure is applied, but the pressure is removed just before the start of transformation, and the transformation is carried out under normal pressure.Therefore, the structure after transformation is the structure obtained under normal pressure, and the structure obtained under normal pressure is It has an unobtainable uniformity.

Similar methods are also effective for precipitation treatments.

Normal precipitation treatment involves heating the material to a high temperature with a large amount of solid solution.
After sufficiently dissolving the precipitates, the material is cooled down to a low temperature with a small amount of solid solution, and the material is maintained at that temperature or higher to allow precipitation to proceed and obtain uniform and fine precipitates. As a result, the effective properties of the material can be improved.

Precipitation is similar to phase transformation in that it proceeds with the growth and growth of nuclei using supersaturation as a driving force.

The degree of supersaturation depends on the rate of cooling from high temperature, and there is a mass effect involved.

That is, when the material becomes large, it is difficult to obtain a uniform degree of supersaturation.

By applying pressure to such a cooling process, a sufficient degree of supersaturation can be obtained very easily.

The main idea of the present invention is to apply pressure during the cooling process, remove the pressure after reaching a predetermined temperature, and allow precipitation to proceed under normal pressure, thereby achieving a uniform precipitation state that cannot be obtained with normal precipitation processing. It is possible.

Next, the present invention will be explained in detail using specific examples.

Examples of patenting treatments for hard steel wire and piano wire are shown below.

Patenting treatment is carried out by solution-treating carbon steel at a high temperature range of A3 point or higher, rapidly supercooling it to a transformation temperature, and maintaining the temperature constant at that temperature.

In this case, the transformation temperature is as high as about 500° C., and a lead bath is currently used to rapidly supercool the wire.

However, even in this lead bath, as the wire diameter increases, it becomes difficult to uniformly and rapidly supercool the inside and outside of the wire.

That is, the surface of the wire is rapidly cooled, but as the wire diameter increases, the cooling rate at the center becomes slower.

On the other hand, as an indicator of the relationship between cooling rate and structure, TTT
, CCT diagram, but the temperature at the nose of this TTT curve is the transformation temperature of fine pearlite, and it is difficult to obtain a fine structure below this critical cooling rate.

It is well known that this TTT curve is a characteristic unique to the material and is greatly affected by the added elements.In order to deal with the above-mentioned problems, alloying elements have been added to improve the nose of the TTT curve. The large diameter wire is heat treated by moving it to the long time side.

However, the effect of additive elements is not only to slow down the critical cooling rate, but also to bring about structural changes, which often have a negative effect on deformability.

In the present invention, depending on the wire diameter in the supercooling stage after solution treatment at high temperature,
In addition, in the step of maintaining a constant temperature at the transformation temperature by applying pressure according to the cooling capacity of the device, the pressure is rapidly removed to use only the advantages of transformation under pressure and perform patenting.

According to this method, a very uniform and fine pearlite structure can be realized in a large diameter wire.

Table 1 shows the chemical composition of the tested sample, which is carbon steel with a nearly eutectoid composition.

A high-pressure device using a solid pressure medium was used for pressure.

The operating pressure is 25 Kb due to the cooling capacity of the device.

The wire used is a thick wire with a diameter of 12φ.

Figure j shows the cooling curve of the sample after austenite (in the figure
) and the pressure pattern (■ in the figure).

As shown in the figure, a sample heated to 900°C is subcooled to 500°C and kept at a constant temperature, and the pressure is 20 Kb after reaching 500°C.
The load was rapidly unloaded from to normal pressure.

Figure 2 shows the hardness distribution of the sample heat-treated under high pressure (I in the figure) and the hardness distribution of the lead-patented comparative material (chemical composition and wire diameter are the same) (Nakata in the figure). It is.

As can be seen from the hardness distribution shown in the same figure, compared to the case of simply lead patenting, the case of heat treatment under high pressure shows a very uniform and fine structure inside and outside the wire.

Although this example deals with eutectoid steel, the present invention is also effective for hypereutectoid steel. Normally, pro-eutectoid cementite inhibits ductility in large-diameter hypereutectoid steel, but the present invention is effective for hypereutectoid steel. This allows it to be uniformly dispersed within the grains.

In the present invention, as a method of applying pressure, hydrostatic pressure may be applied using multiaxial compression force, and uniaxial compression is ineffective.

The higher the pressure, the longer the phase transition time can be, but if the pressure is too high, it will be difficult to apply the load.

From the above explanation, the present invention is an outstanding heat treatment method for obtaining uniform quality of metal materials, especially metal materials with high mass effect.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the cooling curve and pressure pattern of eutectoid steel, and FIG. 2 is a diagram showing the hardness distribution of a cross section of the eutectoid steel wire.

Claims (1)

[Claims] 1. When a metal material is cooled from a high temperature to change its phase, pressure is applied to the material to delay the phase transition, and the pressure is released just before the phase transition under pressure to allow the phase transition to proceed. Heat treatment method for metal materials.