JPH0116286B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field of the Invention) The present invention relates to a method for producing a duplex stainless steel sheet, in particular a method for manufacturing a duplex stainless steel sheet having a mixed structure consisting of two phases, a ferrite (α) phase and an austenite (γ) phase at around room temperature.
The present invention relates to a method for manufacturing a duplex stainless steel sheet, which is a duplex stainless steel whose main components are Fe, Cr, and Ni, and which is used as a material for superplastic deformation that is unlikely to cause plastic anisotropy. (Prior Art) Conventionally, duplex stainless steel belongs to the class of so-called difficult-to-work materials, and therefore various proposals have been made to improve workability. As a result, for example, by reducing S and O, which are harmful to hot workability, it has become possible to manufacture objects with simple shapes such as plates and tubes, and to manufacture forged products with relatively simple shapes. Ta. It has also been found that it is possible to apply large deformation processing methods that utilize superplastic deformation to the manufacture of products with complex shapes by creating microstructures by combining processing and heat treatment. However, when performing such large deformation processing, it is especially important that the material has almost no plastic anisotropy, but conventional methods cannot obtain the desired properties regarding plastic anisotropy, so a useful method has been developed. was desired. (Object of the Invention) Thus, the object of the present invention is to provide a method for manufacturing a duplex stainless steel sheet as a processing material that is unlikely to cause plastic anisotropy during large deformation processing using superplastic deformation at 700 to 1100°C. The goal is to provide the following. Another object of the present invention is to provide a method that makes it possible to produce a duplex stainless steel sheet with improved plastic anisotropy without impairing its performance as a superplastic material. Furthermore, it is an object of the present invention to provide a method for manufacturing a duplex stainless steel sheet that can significantly reduce costs by significantly increasing product yield. (Summary of the Invention) In order to achieve the above object, the inventors hereby provide the following:
Focusing on the fact that in conventional methods, rolling is performed only in one direction when manufacturing duplex stainless steel sheets, repeated experiments revealed that there is a certain correlation between the rolling direction and the plastic anisotropy during superplastic deformation. It was also discovered that by heat-treating to create a microstructure and then cold rolling by so-called cross rolling, the plastic anisotropy can be eliminated and the performance as a superplastic material can be greatly improved. Thus, the present invention was completed. That is, the gist of the present invention is that Fe,
After heating a duplex stainless steel whose main components are Cr and Ni and which has a mixed structure consisting of a ferrite phase and an austenite phase at room temperature to a temperature of (T - 150) °C or higher [where T is the duplex stainless steel temperature (°C) at which the steel becomes a single phase of ferrite], cooled to 500°C or less by water cooling or forced cooling, and then cross-rolled at 200°C or less with a total reduction of 20% or more. This is the manufacturing method. (Aspects of the Invention) Next, specific aspects of the present invention will be described in detail. In the present invention, the main component of duplex stainless steel is
The reason for limiting Fe, Cr, and Ni is that although a two-phase mixed structure of α phase and γ phase can be obtained by combining other elements, when considering the properties such as corrosion resistance of the material and cost, This is because it is more advantageous to use Fe, Cr, and Ni3 elements as the base, and in addition to these alloy components, the duplex stainless steel may, if necessary, be expressed in weight%; Mo of 5% or less, 1 % or less Cu, 0.5% or less Ti, 0.5% or less Nb, 0.5% or less Zr, 0.5% or less V, 1% or less W, 0.1% or less C, 0.2% or less N, when melted Si and Mn as deoxidizers, respectively
It may contain one or more of 2.5% or less, 2.0% or less.
Note that, of course, materials containing small amounts of Re, La, Ca, Ce, or other unavoidable impurities are also included, and the present invention is not limited thereto. Examples of suitable compositions include the following.

[Table] According to the present invention, when the temperature at which the two-phase stainless steel becomes α-phase single phase is T (°C), the heating temperature is set to (T-150)°C or higher, and then water cooling or forced cooling is performed. Therefore, it is cooled to below 500℃ and rolled with a total reduction rate of 20% or more at 200℃ or below. This is to obtain a fine mixed structure of α phase and γ phase when heated at °C.
These processing conditions are the conditions for realizing the superplastic phenomenon. The heating temperature is preferably higher, and is preferably a temperature in the α phase single phase region, that is, T°C or higher,
It may be slightly lower than this. However, if the heating temperature is too low, the γ phase, which has aggregated and coarsened into islands in the α phase soil, will remain and have an adverse effect on superplasticity, so the lower limit of the heating temperature was set at (T-150)°C. T is generally
In the range of 1050-1250℃. The cooling rate after heating is preferably as high as possible in order to prevent agglomeration and coarsening of the γ phase as much as possible, and water cooling is preferred, but forced cooling such as spray cooling may also be used. In this case, rapid cooling to below 500℃ is
This is because if cooling is stopped at a temperature higher than this, the γ phase will become coarse. Preferably 200
Rapidly cool down to below ℃. The reason why the rolling process is then performed at 200°C or lower is to obtain a fine structure by heating to 700 to 1100°C prior to hot working using superplastic deformation. This is because if the rolling temperature at this time exceeds 200°C, recovery of the α phase occurs during or after rolling, and the dislocation density, which becomes the nucleus of fine precipitation of the γ phase, decreases. Preferably,
The rolling temperature is 50°C or less. Next, according to the present invention, cross rolling is performed in the above-mentioned rolling at 200° C. or lower. The purpose is to uniformly disperse the finely precipitated γ phase in the direction of nucleation growth. Here, cross rolling refers to rolling in which the rolling reduction ratio of each pass is made as equal as possible, the total number of passes is 2 or more, and the rolling direction is changed for each pass. Preferably, it is a rolling method in which the rolling direction is changed by approximately 30 degrees or more for each pass, and more precisely, the rolling direction is rotated by 45 degrees or an integral multiple of 45 degrees, and the rolling direction is changed between the first and final passes. This refers to rolling where the angle formed is 90 degrees or an integral multiple of 90 degrees. Preferably, said cross rolling consists of repeating said rolling at least twice, changing the rolling direction by the same angle in each pass.
The rolling reduction rate for each pass may be 2/3 to 4/3 of the average rolling reduction rate. Although cross rolling has been carried out in the past, its purpose is to eliminate anisotropy in mechanical properties by controlling the shape of inclusions.
In many cases, it is only carried out as hot rolling. According to the present invention, by such cross rolling at 200°C or less, a structure without anisotropy in the direction of precipitation nucleus growth can be obtained, and plastic anisotropy during superplastic deformation can be eliminated, and furthermore, it can be used as a superplastic material. The performance is also significantly improved due to the uniform microstructure. This can be said to be an unexpected effect compared to the conventional cross rolling, which aims to control the shape of inclusions as described above. By reheating the steel sheet manufactured in this way to 700 to 1100°C prior to superplastic deformation, it is possible to obtain a uniform microstructure with no anisotropy. Therefore, even when performing large deformation processing using superplasticity, the workability is good and no plastic anisotropy is observed, so that efficient forming processing with extremely high yield can be performed. Next, the present invention will be further explained by examples. Example A steel plate with a thickness of 4 mm obtained by melting steel (T = 1300°C) having the composition shown in Table 1 and forging and hot rolling was heated to 1300°C, which is the α phase - single phase region. After heating for a period of time, the material was cooled to room temperature with water, and then pickled to obtain a cold rolled material.

[Table] These were cold rolled at room temperature under the conditions shown in Table 2. A disk with a radius of 300 mm was cut out from the obtained cold rolled material and used as a test piece for plastic anisotropy evaluation. The attached drawing is an explanatory diagram showing a plastic anisotropy test method in a schematic cross-sectional view. As can be seen from the figure, the plastic anisotropy of the specimen was evaluated by deep drawing a disk-shaped specimen. Processing of disk-shaped specimen 1 was performed at 1000℃.
Move a jig 3 with a diameter of 40 mm and a radius of curvature of 5 mm at the end to a 50 mm mold 2 in the direction of the arrow in the figure to increase the strain rate ~
It went at 10 ^-3 s ^-1 . In this way, cup-shaped specimens with a depth of 200 mm were manufactured from disc-shaped specimens from each sample material, and plastic anisotropy was evaluated based on the difference in length of the remaining ear portion depending on location. The results are summarized and shown in Table 2. Although almost no plastic anisotropy occurred in the steel sheets manufactured by the method according to the present invention, in the case of comparative examples where the conditions differ from those of the conventional method or the method of the present invention. In this case, significant anisotropy occurred.

【table】

【table】 [Brief explanation of drawings]

The attached drawing is an explanatory diagram of a test method for evaluating plastic anisotropy shown in a schematic cross-sectional view. 1: disk-shaped test piece, 2: mold, 3: jig.

Claims (1)

[Claims] 1. Heating a duplex stainless steel whose main components are Fe, Cr, and Ni and which has a mixed structure consisting of a ferrite phase and an austenite phase at room temperature to a temperature of (T-150)°C or higher. [However, T is the temperature (°C) at which the two-phase stainless steel becomes a single ferrite phase]; This heated steel is cooled to 500°C or less by water cooling or forced cooling; and this cooled steel plate is heated to 200°C. The total reduction rate is below
A method for producing a duplex stainless steel sheet, characterized by subjecting it to cross rolling of 20% or more.