JPH03215625A - Production of superplastic duplex stainless steel and hot working method therefor - Google Patents

Abstract

PURPOSE:To produce a duplex stainless steel capable of superplastic deformation at high strain rate by subjecting a stock, such as Cr-Ni stainless steel plate or bar, to solution treatment and then to working under specific conditions. CONSTITUTION:For example, a plate stock, a bar stock, etc., composed of a duplex stainless steel having a bar basic composition consisting of, by weight, 4-18% Ni, 15-35% Cr, and the balance Fe are heated up to (a temp. where the structure is formed into a ferritic single phase) - 100 deg.C or above, held for 30-60min, and cooled rapidly by means of water cooling, etc., to undergo solution treatment, or, a molten metal of this stainless steel is cast and then solidified from a temp. where the structure is practically formed into a ferritic single phase down to 700 deg.C at >=3 deg.C/sec cooling rate. A slab or cast slab of the above stainless steel is worked at <=700 deg.C at >=50% draft, by which a superplastic duplex stainless steel slab can be produced. By subjecting this steel slab to heating up to 900-1100 deg.C at >=0.5 deg.C/sec temp.-rise rate, to soaking treatment for <10min, and then to deformation at a strain rate of 10<-2>S<-1> to 10<2>S<-1>, superplastic working can be carried out at high strain rate.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing duplex stainless steel exhibiting excellent superplasticity and a method for hot working the same.

(Prior Art) In general, duplex stainless steel has a
It is used after being subjected to solution treatment in which it is heated to around 1100°C and then rapidly cooled, and in that state it exhibits two phases, an α phase and a T phase.

Duplex stainless steels are known to not only exhibit excellent corrosion resistance, but also have excellent properties such as strength, toughness, and weldability, and have recently been used in various fields. Demand is increasing. However, because it has a two-phase structure, it is also known to belong to so-called difficult-to-process materials, and its applications have been severely limited in terms of processing.

Therefore, based on the results of previous research seeking ways to mass-produce products with the above-mentioned properties of duplex stainless steel, measures have been taken to reduce impurities such as S and O, which are harmful to hot working. In recent years, it has become possible to manufacture simple shapes such as pipes and plates, as well as forged products with relatively simple shapes, but it is extremely difficult to manufacture parts with complex shapes, such as pipe fittings and valves. The current situation is that we still have to rely on machining and casting, which are difficult and have low efficiency and yield.

(Problem to be Solved by the Invention) By the way, as a method for processing such difficult-to-process materials into complex shapes, the application of a method using superplastic processing technology, whose research has made remarkable progress in recent years, has been proposed for duplex stainless steel. is also being studied. The inventor of the present invention found that if the component system and appropriate pretreatment are selected, processing deformation of more than 2000% is possible at a strain rate of 10-'s-', and even at a relatively high strain rate of 10-'s, processing deformation of about 200% can be achieved. It was revealed that superplastic deformation is possible (Tetsu to Hagane, vol.70, Nα15, p.2168-2
175). As a result, today, efforts are being made to actively apply duplex stainless steel to superplastic processing and superplastic solid phase joining.

However, conventional materials still have the disadvantage that their deformation speed is slow compared to normal hot working, making them unsuitable for mass production processes.

The present invention was made in order to solve the above-mentioned problems, and its main purpose is to develop a high-strain system capable of superplastic deformation at a high strain rate comparable to that of normal hot working of duplex stainless steel. The object of the present invention is to provide a method for producing a superplastic duplex stainless steel and a hot working method suitable for the steel obtained by the method.

(Means for Solving the Problems) The present inventor has conducted extensive research from the above-mentioned viewpoints, and has found that it is difficult to achieve the above-mentioned objectives with only fine-grained superplasticity due to a two-phase mixed structure, which has been conventionally said. However, if the transformation from δ-ferrite to austenite (γ) phase during processing deformation is simultaneously used, the superplastic phenomenon can be greatly accelerated, and furthermore, the above transformation rate can be accelerated by appropriate pretreatment, and The present invention was completed based on the finding that the above transformation progresses in a state where further refinement is possible and the superplastic phenomenon is further accelerated.

Thus, the gist of the present invention is that Pe, C',
Heating duplex stainless steel mainly composed of Ni to a temperature of -100°C or higher at which it becomes substantially single-phase ferrite,
This is a method for producing superplastic duplex stainless steel, which is characterized in that, after solution treatment, processing is performed by 50% or more at 700° C. or lower.

Moreover, from another aspect, the present invention provides Fe, Ni, Cr
After melting and solidifying duplex stainless steel whose main component is ferrite, the steel billet is cast at a cooling rate of 3°C/s or more from the temperature at which it becomes substantially single-phase ferrite to 700°C.
This is a method for producing superplastic duplex stainless steel that requires processing of 50% or more at temperatures below 00'C.

As described above, the superplastic duplex stainless steel manufactured according to the present invention effectively exhibits the superplastic phenomenon by deforming at a strain rate of 10-2 s-' to 102 s-' after heating to 900-1100°C. It becomes possible to make it come out. Also,
At that time, the temperature increase rate when heating to the hot working temperature was set to 0.
By setting the heating temperature to 5° C./S or more and setting the soaking time until deformation to less than 10 minutes, the superplastic phenomenon can be brought out even more effectively.

(effect) The structure and operation of the present invention will be explained.

The duplex stainless steel for exhibiting excellent properties according to the present invention may be any stainless steel as long as it has Fe, Cr, or Ni as its main components, but it is preferably in a thermal equilibrium state at around 1000°C, which is the superplastic deformation temperature. It is preferable that 30 to 50% (volume %) of T phase is mixed in the δ-ferrite ground.

The target rope is preferably . Ni: 4-18%, Cr
:15 to 35%, the balance being Fe. Furthermore, it may contain one or more of Si≦5% and Mn≦5%. In the following, unless otherwise specified, "r%" means "wt%".

If necessary, Mo≦6%, Cu≦1%, Ti≦0.
5%, Zr≦0.5%, Nb≦0.5%, ■≦0
．． 5%, W≦1%, C≦0.1%, N≦0.8%, and B≦0.01%, and in some cases, further contains Re and Ca. It may also contain a small amount.

Among these, solid N promotes δ→T transformation and accelerates superplasticity, so it is preferable to add 0.05 to 0.25%.On the contrary, TiXZr.
It is preferable not to add too much Nb and ■.

Next, each step of the method for producing superplastic duplex stainless steel according to the present invention will be explained.

First, the steel having the above composition is melted in a normal process, and then
It is made into an ingot by continuous casting method or ingonoto method, and made into steel plate, steel bar or pipe material by blooming rolling or hot rolling.

The material prepared in this way is then heated to a temperature above -100°C at which it becomes a single ferrite phase, which increases the δ-ferrite phase and reduces the amount of δ-T transformation during superplastic deformation. This is to ensure sufficient capacity. Therefore, this temperature is preferably in the δ single phase range, and may be -100''C or higher, at which the ferrite single phase with a small amount of γ remains.

The holding time at this temperature is not particularly limited, but usually
After holding for 30 to 60 minutes, rapidly cool to form a solution. After heating in this way, it is dissolved and then cooled with water, etc. for 7
The material is cooled to below 00°C, and processed by 50% or more at below 700°C. This is because by applying processing, T is finely precipitated within the 6-ferrite grains using dislocations introduced by processing during heating to the superplastic deformation temperature or during deformation, and the transformation rate is also increased, resulting in a high strain rate. This is to enable superplasticity.

The upper limit of the working temperature was set at 700°C because at temperatures exceeding this, not only the working strain is eliminated by recovery, but also the δ→T transformation progresses. Further, the higher the degree of processing, the better, and if it is less than 50%, there is no effect, so the lower limit was set at 50%.

Preferably, 50% or more cold working is performed at room temperature.

Here, the above-mentioned degree of processing is indicated by the amount of plate thickness reduction.

In the above case, the raw material is first processed into a processed material such as a plate, rod, or tube, and then heated and processed under the above-mentioned conditions in order to impart superplasticity according to the present invention.

On the other hand, according to another variant of the invention, a pretreatment method in combination with a casting process is also possible. In this case, it is important to control the cooling rate from the point where the steel becomes δ single phase after casting and solidification to 700℃, and to ensure a sufficient amount of ferrite, the cooling rate should be 3'C/s or more. For example, when using a continuous casting method, the balance between the cross-sectional shape of the slab and the cooling rate becomes important. The cooling rate is preferably 10°C/S or more.

The superplastic duplex stainless steel produced in this way is hot worked while developing superplasticity, and according to the present invention, the superplastic deformation temperature is set at 900 to 1100°C during hot working. This is because superplasticity is most easily obtained within this range, and on the other hand, the reason why the strain rate is set to 10<-2> to 10<2>S<-1> is that it is difficult to obtain superplasticity outside this range.

In the processing method according to the present invention, the heating rate to the superplastic deformation temperature and the holding time after heating and before deformation are also important. That is, if the heating rate is too slow or the holding time is too long, the δ→T transformation will progress during that time and the superplastic properties will deteriorate, so the lower and upper limits were set to 0.5°C/s and 10 minutes, respectively.
Preferably it is 1° C./s or more and 5 minutes or less.

In addition, superplastic processing in the present invention includes forging, bulge forming, wire drawing, extrusion, etc., and includes all processing performed under the above conditions, as well as processing including diffusion bonding using superplasticity. Of course, it is included within the scope of the present invention.

Although no particular post-treatment is required for the product processed according to the present invention, in some cases, pickling for scale removal or solution treatment may be necessary when σ phase is precipitated.

The structure of the products obtained in this way is significantly refined by superplastic processing, so they also have better mechanical properties and corrosion resistance than products manufactured using conventional processes.

Next, the present invention will be explained in more detail with reference to Examples. Example 1 The components shown in Table 1 were melted in the atmosphere in a high-frequency furnace in a laboratory to form ingots each weighing 50 kg.

Various steel plates with a thickness of 20 to 5 mm were processed by hot forging and hot rolling.

(Left below) First, a 5mm thick steel plate of A steel was heated to various temperatures for 30 minutes, then water-cooled to 50'C and subjected to solution treatment.
It was cold rolled to a thickness of 1+n+ (workability: 80%).

The cold rolling temperature was 30°C. Tensile test pieces were taken from the material thus obtained, and the relationship between heating temperature and superplastic elongation was examined. Superplastic working was performed by heating to 1000''C and strain rate t=IX10-'s-'.

The results are shown graphically in FIG.

As can be seen, excellent superplasticity is obtained within the scope of the method of the present invention. The superplastic deformation temperature was reached by raising the temperature to 3°C and holding it for 2 minutes before deformation.

Figure 2 shows a 5 m+s thick material of A steel heated to 1350°C, held for 30 minutes, water cooled to 50°C, and then cold rolled at 30°C to II. Test pieces were taken and the influence of superplastic deformation temperature was investigated, and an elongation of 200% or more was obtained in the range of 900 to 1100°C. For superplastic processing, the strain rate is t"7X10-'
I went with s-'. Note that I, which is the upper limit strain rate <1>
Superplastic elongation of over 200% was obtained for X102s-' Niite at 900 to 1100°C. The heating conditions to the superplastic deformation temperature were the same as in FIG.

FIG. 3 shows the effect of pre-processing conditions on the same solution treated material at <1350°C, and excellent results were obtained by rolling at 700°C or less after solution treatment. Pre-processing is 5II
Ifi → 2.5 mm, and the processing degree was 50%.

Superplastic processing is performed by heating to 1000℃ and strain rate t-1×10
I went with -'s-'. The superplastic deformation temperature was reached at a rate of 3° C./s and held for 2 minutes before deformation.

Example 2 In this example, the influence of the heating rate to the superplastic working temperature and the holding time before deformation was investigated using Steel A.

The pretreatment involved solution treatment at 1350°C, water cooling to room temperature, and cold working at that temperature from 51 to lIIIm. Superplastic processing is performed by heating to 1000℃ and strain rate t-2×10
I went with -'s-'. The superplastic deformation temperature was reached by raising the temperature to 3°C and holding it for 2 minutes before deformation.

The results are shown graphically in FIGS. 4 and 5, which clearly demonstrate the effectiveness of the method of the present invention.

Furthermore, Figure 6 shows a comparison of the results of working elongation conducted in a similar test by steel type. Example 3 Finally, all the same steel as A was melted and the internal volume thickness was 20+
It was cast into a split mold marked **, and immediately after solidification, the mold was removed and cooled with water. 1350 to 700 at the center of the slab at this time
The average cooling rate to 5°C was 8°C.

After removing 5111N of the black scale on the surface of the slab, 3
A test piece was collected after cold working to III1.

Each test piece was heated at a temperature of 1000″C and a strain rate of t = l.
When superplastic processing was performed with XIO1s-1, 800%
The superplastic elongation was as follows. Superplastic processing has a heating rate of 3℃
/S, held for 2 minutes before deformation.

On the other hand, those that were left to cool after casting (the cooling rate in the center was 0.
2° C./s), the superplastic elongation was only 150%.

(Effects of the Invention) By having the structure as explained above, the present invention has the remarkable effect of enabling superplastic deformation at a high strain rate comparable to that of normal hot working, and is suitable for industrial use. The profits are extremely large.

[Brief explanation of drawings]

Figures 1 to 6 are Obtained by the present invention This is a graph showing the characteristics of superplastic materials.

Claims (4)

[Claims] (1) Duplex stainless steel mainly composed of Fe, Cr, and Ni is heated to a temperature above -100°C at which it becomes essentially a single phase of ferrite, and after solutionization, 50% at below 700°C.
A method for producing superplastic duplex stainless steel, characterized by performing the above processing. (2) After melting and solidifying the duplex stainless steel whose main components are Fe, Ni, and Cr, the cooling rate from the temperature at which it becomes substantially single-phase ferrite to 700°C is 3°C/s or more. A method for producing superplastic duplex stainless steel, which comprises subjecting a cast piece of steel to 50% or more processing at a temperature of 700°C or lower. (3) After heating the superplastic duplex stainless steel manufactured by the method described in claim 1 or 2 to 900 to 1100°C, the strain is 10^-^2s^-^1 or more and 10^2s^-^1 or less. A hot working method for superplastic duplex stainless steel characterized by rapid deformation. (4) Temperature increase rate when heating to hot processing temperature is 0.5℃
4. The method of hot working a superplastic duplex stainless steel according to claim 3, wherein the soaking time is at least 10 minutes and the soaking time is less than 10 minutes.