JPS6366365B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for hot working a duplex stainless steel mainly composed of Fe, Cr and Ni, which exhibits two phases, a ferrite phase and an austenite phase, at around room temperature. In general, duplex stainless steel is known not only to have excellent corrosion resistance but also to have excellent properties such as strength, toughness, and weldability, and is widely used in various fields. Although it is one of the materials that has become increasingly popular, it was also known as belonging to the category of so-called difficult-to-process materials. As a result of various studies and considerations to date, measures have been taken, such as reducing S and O, which are harmful to hot workability. Although it has become possible to manufacture forged products with relatively simple shapes, it is extremely difficult to manufacture parts with complex shapes, such as pipe fittings and valves, and still have low yields and poor machinability. The current situation is that we have no choice but to rely on inefficient machining. From the above-mentioned viewpoint, the present inventors aimed to improve the hot workability of duplex stainless steel, which has excellent properties including corrosion resistance. As a result of conducting research to provide a hot working method that can stably give any shape to duplex stainless steel, we systematically investigated the influence of the microstructure state and deformation conditions on the stainless steel. Duplex stainless steel material
They came to the knowledge that when deformation is applied under strict control of temperature and strain rate, the ductility of the material increases dramatically, resulting in so-called superplasticity. This invention was made based on the above knowledge, and relates to a method for processing duplex stainless steel using the superplastic phenomenon that enables large deformations that are normally unimaginable. This makes it possible to manufacture products with complex shapes that cannot be manufactured using conventional processing methods, and eliminates the need for cutting processes even for products that have already been manufactured with a cutting process. This is a two-phase stainless steel that is mainly composed of Fe, Cr, and Ni, and exhibits two phases: ferrite and austenite at room temperature. Heating the steel to over 1000℃, then heating it to 700℃
Add processing at a processing rate of 30% or more in a temperature range of ℃ or above, or add processing at a processing rate of 20% or more in a temperature range of at least 700℃, and then apply processing to 700℃.
~ [Temperature at which ferrite becomes single phase -200℃] is reheated to 1×10 ^-4 /sec or more 1×10 ^-1 /
The object of the present invention is to easily form an article into an arbitrary shape by deforming at a strain rate of less than sec. Next, the reason why the processing conditions are limited as described above in the method of the present invention will be explained in detail. The main components of duplex stainless steel are limited to Fe, Cr, and Ni, although it is possible to obtain a two-phase mixed structure of ferrite and austenite phases by combining other elements. Considering the properties and cost of Fe−
This is because it is more advantageous to use the Cr-Ni3 element as the base, and in addition to these components, Mo: 5% or less ( (Hereinafter, % representing the component ratio is expressed as weight %), Cu: 1% or less, Ti: 0.5% or less, Zr: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less, W: 1% or less, C: 0.1% or less, N: 0.2% or less, or further contains one or more of the following as a deoxidizing agent during dissolution: Si: 2.5% or less, Mn: 2.0% or less, Furthermore, it goes without saying that materials containing small amounts of Re, La, Ce, and Ca, or unavoidable impurities may also be present. Heating the steel to over 1000℃ as a pre-processing treatment is used to remove the processing strain accumulated in the previous process, and to remove the carbonitrides and σ that have been generated.
The purpose is to dissolve intermetallic compounds of the same phase into the matrix to facilitate subsequent processing, and after this state, hot working or warm working is applied, and then the deformation temperature is returned to the superplastic region. By heating, a two-phase mixed microstructure of ferrite and austenite, which is a condition for superplastic deformation, can be obtained. Machining to obtain a microstructure requires a processing rate of 30% or more in the temperature range of 700℃ or higher, and
In the temperature range below 700℃, 20% or more is sufficient.
Immediately after such processing,
Alternatively, after being cooled once, the material is reheated to a temperature at which it exhibits superplastic deformation, thereby deforming it. The temperature range for reheating and deformation was set to 700℃ ~ [temperature at which ferrite becomes single phase - 200℃] was set at 700℃.
At temperatures below ℃, it is difficult to obtain a fine mixed structure of ferrite and austenite through recrystallization;
This is because, if it exceeds 20%, the ferrite or austenite grains will become coarse, making it difficult to obtain a fine mixed structure. In this case, depending on the chemical composition, precipitation of the σ phase may occur during deformation, but the precipitation of the σ phase prevents the coarsening of austenite and ferrite and contributes to the refinement of the structure, so it is not harmful. It was also found that this is not a problem, but is rather favorable for superplastic deformation. The holding time in the specified temperature range immediately before deformation is
If it is a high temperature of 1000℃ or more, about 1 minute is enough.
In the low temperature range of around 700°C, it is preferable to extend the heating time to about 10 to 60 minutes because it is easier to obtain the above-mentioned fine mixed structure of ferrite and austenite. The strain rate during deformation is 1×10 ^-4 /sec to 1×10 ^-1 /
sec was determined because if the strain rate is 1×10 ^-1 /sec or more, large deformation due to superplasticity cannot be expected, whereas if the strain rate is less than 1×10 ^-4 /sec, ductility decreases. Not only that, but the work efficiency is also significantly lowered, which is undesirable. The deformation resistance in such a superplastic region is extremely low, and combined with the above-mentioned remarkable improvement in ductility, duplex stainless steel becomes extremely easy to undergo large deformations. Next, the present invention will be explained by examples and in comparison with comparative examples. Example First, duplex stainless steel having the composition shown in Table 1 was melted by a conventional method, decomposed forged, and hot rolled to form a plate material with a thickness of 30 mm. This plate material was rolled under the conditions shown in Table 2, reheated to the temperature also shown in Table 2, and then subjected to tensile deformation at a predetermined strain rate to determine the elongation and stress. -We evaluated whether large deformations due to superplastic phenomena are possible by determining the maximum stress at strain rate. The results are also shown in Table 2. From the results shown in Table 2, it is clear that methods 1 to 1 of the present invention
According to 10, each duplex stainless steel is

【table】

【table】

[Table] (Note) * indicates that the conditions of the present invention are not met.
It shows an extremely good elongation of over 300%, and the maximum stress, which is a measure of deformation resistance, is also low, and it is clear that large deformations are easily possible under these conditions. On the other hand, in Comparative Methods 11 to 16, in which the conditions marked with * in Table 2 are outside the scope of the present invention, the elongation is not large, and the maximum stress is not uniformly low. It's obvious. Among these methods, Comparative Methods 12 to 16 were unable to obtain the microstructure necessary for the occurrence of superplastic phenomena, but this was due to the fact that either the deformation temperature or the strain rate was inappropriate. It is caused by In Comparative Method 11, the heating temperature during pretreatment was as low as 900°C, and a large amount of σ phase precipitated, causing embrittlement, resulting in significant cracking during rolling, making subsequent tests impossible. It is something. As mentioned above, according to the present invention, it is possible to apply plastic processing to duplex stainless steel, which has been considered to be a difficult-to-process material despite its excellent properties such as corrosion resistance, and whose field of application has been somewhat limited. By chiseling, extremely complex shapes can be formed simply and easily, and the field of application can be further expanded, which brings about industrially useful effects.

Claims (1)

[Claims] 1. The main components are Fe, Cr and Ni, and exhibit two phases, a ferrite phase and an austenite phase, at around room temperature. 2.
After heating phase stainless steel to over 1000℃,
Processing rate: 30% or more in a temperature range of 700°C or higher, and then reheated to a temperature range of 700°C to [temperature at which ferrite becomes single phase - 200°C] to 1x
A method for hot working duplex stainless steel, characterized by deforming at a strain rate of 10 ^-4 /sec or more and less than 1×10 ^-1 /sec. 2 Mainly composed of Fe, Cr and Ni, it exhibits two phases: ferrite phase and austenite phase at around room temperature.
After heating phase stainless steel to 1000℃ or higher, processing is performed at a processing rate of 20% or higher in a temperature range of at least 700℃ or lower, and then from 700℃ to [temperature at which ferrite becomes single phase - 200℃]. A method for hot working duplex stainless steel, which comprises reheating to a temperature range and deforming at a strain rate of 1×10 ^-4 /sec or more and less than 1×10 ^-1 /sec.