JPS62222044A - Hot-working method for two-phase stainless steel powder - Google Patents

Abstract

PURPOSE:To easily hot-work a two-phase stainless steel powder in high yield, by subjecting an aggregate of atomized steel powder of two-phase stainless steel mainly composed of Fe, Cr, and Ni to deformation in a specific temp. range at a specific rate of strain. CONSTITUTION:The aggregate is formed of the atomized steel powder of two- phase stainless steel composed principally of Fe, Cr, and Ni, containing, if necessary, Mo and N, and showing a two-phase structure of ferritic and austenitic phases at the temp. close to the normal temp. As to the above steel composition, the one consisting of, by weight, 12.0-35.0% Cr, 3.0-18.0% Ni, and the balance Fe with accompanying impurities and further containing, if necessary, <=0.05% C, 0.05-5.0% Si, 0.05-8.0% Mn, <=5.0% Mo, 0.05-0.8% N, <=0.8% Cu, <=0.05% P, and <=0.005% S is suitably used. The above aggregate is subjected to deformation in a temp. region from 700 deg.C to (ferritic single phase-forming temp. -100) deg.C at 1X10<-6> 1/sec-1X10 deg. 1/sec rate of strain by means of hot isostatic pressing, etc., so as to be made dense.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is directed to a material containing Fe, Cr, and Ni as main components, which exhibits a two-phase structure of ferrite phase and austenite phase near room temperature, and optionally contains MolN. This invention relates to a method for hot superplastic working of duplex stainless steel atomized steel powder.

(Prior art) Duplex stainless steel is generally known as a high-strength, high-corrosion-resistant stainless steel that has excellent stress corrosion cracking resistance in a chloride environment and good weldability comparable to austenitic stainless steel. It is becoming widely used in the field of However, it is also known as a material that is difficult to machine due to σ embrittlement.

By the way, there are three types of stainless steel: martensitic, ferritic, duplex, and austenitic. Among these, ferritic stainless steel is characterized by being inexpensive and having excellent stress corrosion cracking resistance. The disadvantages are poor toughness and weldability. Austenitic stainless steel has excellent toughness and corrosion resistance, but
It has a drawback of being inferior in stress corrosion cracking resistance (hereinafter referred to as "SCC resistance").

Duplex stainless steel has excellent features that combine the advantages of both of these, and due to its high strength, its fields of use have been expanding in recent years.

Currently, 1BCr duplex stainless steel, 22Cr duplex stainless steel, 25Cr duplex stainless steel, etc. are commercially available, but one of the biggest manufacturing problems is poor hot workability. There is. Recently, through measures such as reducing S and oxygen, which are harmful to hot workability, it has become possible to manufacture forged products with simple shapes such as temporary pipes and relatively simple shapes. However, manufacturing parts with complex shapes, such as pipe joints and valves, is extremely difficult, and the current situation is that we still have to resort to machining, which has low yields and low efficiency.

Incidentally, the production of stainless steel by the powder method has been attracting attention in recent years along with the development of the gas atomization method. The establishment of a mass production method for high-quality stainless steel powder with low oxygen content in the steel is encouraging the development of new applications. For example, 1984
ASM International Conference
ence on New Develop + ments
In C1eas T.

As seen in the 8410-013 report reported by RNBERG, stainless steel powder produced by gas atomization is filled into a carbon steel capsule, the capsule is densified into a billet by cold isostatic pressure, and the capsule is made into a billet by hot extrusion. Seamless pipes are being used.

This method has also been applied to duplex stainless steels such as TP 329. However, in this case, processing is started after heating to a relatively high temperature in order to avoid processing in the σ phase generation region, so processing is performed in the ferrite single phase region at the initial stage of extrusion. In addition, since the pipe is made by hot extrusion and the extrusion speed is not controlled, the strain rate is 1.
xlO” 1/sec.

When stainless steel powder is used as a raw material, it is sintered, compacted, and bonded to increase its density and have the same functions and properties as conventional agglomerates. Component fluctuations caused by solidification segregation are not observed in steel materials made from stainless steel powder; (2) S%P in steel;
(3) What used to be difficult to manufacture due to poor workability at high temperatures can now be manufactured relatively easily. +41 V
For example, it is possible to manufacture composite materials containing IA powder.

On the other hand, many patents have already been filed regarding conventional techniques related to superplastic working, and the applicant of this patent has also filed patent applications, such as Japanese Patent Application No. 60-64163 and No. 60-84087.
It is proposed as a number.

In addition, as a general literature, “Tetsu to Hagane J, 70 (
.

1984) p. 2168.

All of these conventional methods hot-roll molten material. As for powder, the special publication 1986-10
There is No. 642, which gives a temperature cycle, Fe
This utilizes the transformation superplasticity of powder.

As described above, none of the conventional literature describes the use of atomized steel powder of duplex stainless steel for superplastic working, and there is nothing that shows the excellent superplastic workability at that time.

(Problems to be Solved by the Invention) The environment in which stainless steel is used is becoming more severe year by year, and on the other hand, there are constant demands from users to reduce costs. The use of duplex stainless steel as an inexpensive, highly corrosion-resistant, high-strength steel, and as a stainless steel with excellent SCC resistance is gradually expanding.

However, as is well known, in duplex stainless steel, problems such as poor workability typified by edge cracking during hot processing and toughness deterioration due to σ embrittlement are extremely important. In addition, in the case of a thin plate, the difference in tensile properties in the rolling direction and the T direction, that is, the large anisotropy, hinders the expansion of applications. Furthermore, duplex stainless steel cannot be said to have excellent cold workability.

(Purpose of the Invention) The present invention attempts to solve these problems all at once, and its first purpose is to produce a sintered body (a state in which the powder is diffusion bonded) using two-phase stainless atomized steel powder as a raw material. The object of the present invention is to provide a hot working method that allows easy processing into any shape, including (meaning .).

Another object of the present invention is to provide a hot working method that can easily work high Cr, high M0 duplex stainless steel, which is difficult to work in hot or cold due to σ phase precipitation. It is.

Still another object is to provide a hot working method that can easily process duplex stainless steel with good yield and low component segregation.

Yet another object is to provide a method for processing duplex stainless steel with less anisotropy.

Yet another object is to provide a hot working method for duplex stainless steel in which preliminary processing and preforming for preparing a raw material are unnecessary or easy.

(Means for Solving the Problems) In order to achieve this objective, the present inventors conducted various studies and focused on using stainless steel powder produced by the atomization method as the material instead of conventional melted material. However, after further investigation, the following points were discovered and the present invention was completed.

In other words, since the solidification rate of atomized steel powder is significantly higher than that of conventional ingot material, the segregation of components during solidification is extremely small, and the separation into α and γ two phases is not sufficient.
The aggregation of the dispersed phase also did not progress sufficiently. As a result, the dispersed phase of sintered duplex stainless steel produced from atomized steel powder has an extremely fine dispersed phase compared to that of ordinary ingot material. It is an extremely desirable state that the copper powder is finely dispersed, and such a duplex stainless steel atomized copper powder is extremely desirable as a material, and its superplastic effect is greatly superior to that of the melted material.

In addition, when atomized steel powder is used, although there are disadvantages such as a slightly high oxygen concentration in the steel after processing due to oxidized scale on the surface of the steel powder or adsorbed oxygen, the following advantages can be seen.

■Less component segregation.

■Good superplastic properties further reduce deformation resistance and improve formability.

■Product quality improves as processing at low temperatures becomes possible.

■It is also possible to use duplex stainless steel as a material, which is extremely difficult to work in hot or cold due to precipitation of the sigma phase.

■Good yield.

■Preliminary processing and preforming to make the material is unnecessary or easy.

(2) By performing N2 gas atomization or rapid cooling immediately after atomization in liquid nitrogen, it is possible to achieve a high N content, which is difficult to achieve with ingot material due to bubbling during solidification. N is α
, because diffusion is fast in the temperature range where γ two-phase separation progresses,
It has the effect of promoting α and γ distribution during superplastic deformation, and is desirable for superplastic deformation.

■There is no direction during processing.

Therefore, the gist of the present invention is to produce a two-phase stainless steel copper alloy which exhibits a two-phase structure of a ferrite phase and an austenite phase at around room temperature, which contains Fe, Cr and Ni as main components, and optionally contains Mo and N. The aggregate immediately after system atomization is heated to I
This is a hot working method for duplex stainless steel powder, which is characterized by deforming at a strain rate of 1/sec or more and 1 xlO' 1/sec or less.

According to another aspect of the present invention, the duplex stainless steel atomized steel powder is heated to the temperature range described above in a hot isostatic press apparatus, and deformed and densified at the strain rate range described above. A method for hot isostatic processing of duplex stainless steel atomized steel powder is provided.

The steel types targeted by the present invention are generally:
The main components are Fe, Cr and Ni, and M as necessary.
There is no particular restriction as long as the stainless steel contains o, N and exhibits a two-phase structure of a ferrite phase and an austenite phase at around room temperature, and superplastic working is performed under the coexistence of two phases.
The objective of the invention to achieve superplastic deformation is thus achieved. However, preferably such steel grades are
So, Cr: 12.0~35.0%, Ni: 3.0~
18.0% w4, balance consisting of Fe and incidental impurities
It has a composition. As described above, high Cr steel is known as a material that is difficult to machine.

More preferably, the duplex stainless steel contains, in weight percent, Cr: 12.0 to 35.0%, Ni: 3.0 to 18%.
．． 0%, C: 0.05% or less, Si: 0.05~
5.0%, Mn: 0.05-8.0%, Mo: 5.
0% or less, N: o, 05-0.8%, Cu: 0,
8% or less, P: 0.05% or less, S: 0.0
It has an am composition of 0.05% or less, the balance consisting of Fe and incidental impurities.

Note that the term "aggregate" herein refers to a green compact or a sintered compact, and also refers to a mixture of steel powder, such as a car. However, -i is a sintered body, and according to the present invention, it is forged,
It can be applied by extrusion, hot isostatic pressing, or a combination thereof.

Regardless of when a uniform forming force is applied to the aggregate as a whole, when it is uneven, the strain rate is the strain rate in the area where superplastic deformation is actually taking place, and the strain rate referred to in the present invention is shall be.

(Function) In the present invention, the reason why the w4 composition and processing conditions are limited as described above is as follows. Note that throughout this specification, "%" means "% by weight" unless otherwise specified.

Cr: Cr is a basic element that affects corrosion resistance, and the lower limit is set at 12.0% to ensure corrosion resistance. The higher the Cr concentration, the better the corrosion resistance, but on the other hand, it makes the steel brittle, so the upper limit was set at 35.0%.

Ni: Ni is an alloying element that affects corrosion resistance along with Cr and Mo, and is also an effective austenite forming element along with N. Crt! A degree 12.0-3
To form a two-phase structure at 5.0%, Ni must be 3.0~
18°0% is required.

Mo: Along with Cr and Ni, Mo is an element that affects corrosion resistance, and has an extremely large effect on improving corrosion resistance.

Mo is an expensive alloying element and is added as necessary.

However, the upper limit is 5.0 because it significantly promotes the precipitation of the σ phase.
%.

NUN, along with Ni and C, is an element that affects corrosion resistance, and is particularly effective for stabilizing austenite at high temperatures. N is 0.05-0.8% as necessary
Actively add within the range.

In addition to these elements, in the steel of the present invention, the following elements may be restricted or added as desired.

C: C produces chromium carbide and reduces effective Cr1l, which may impair corrosion resistance. Preferably 0.0
5% or less.

Si: Si is an effective deoxidizing element. It also works to improve oxidation resistance at high temperatures. Desirably 0.05~
Add within the range of 5.0%.

Mn: Mn is an effective element that has the function of fixing S in steel as MnS. Further, by coexisting with Si, it also has a deoxidizing effect. On the other hand, Mn is an effective austenite-forming element along with Ni, N, etc., and also has the function of increasing the solubility of N in steel. Preferably 0
．． It is added within the range of 0.05 to 8.0%. If the content is less than 0.05%, the effect of addition is lost, and if the content exceeds 8.0%, manufacturability deteriorates and the material becomes hard and brittle.

P: P is an impurity and is desirably limited to 0.05% or less.

SO3 is also an impurity, which deteriorates corrosion resistance and hot workability. The lower S is, the more desirable it is, and desirably it is less than o, oos%.

Cu: Cu has the function of improving corrosion resistance. However, if a large amount of Cu is added, the steel becomes brittle, so if Cu is added, it is preferably 0.8% or less.

Other impurity elements include too ppm or less of B,
Less than 0.1% AI, small amounts of rare earth elements, Ca, Mg
It may contain etc.

Here, more preferably hot working is performed.

Cr eq = Cr + Mo + 1.5SiNi so that the phase ratio of ferrite and austenite is almost equal around ℃.
eq = Ni + 0.5Mn + 30C + 2
It is preferable that the Cr eq expressed in 5N is about three times the Ni eq. This is important not only to make hot deformation favorable but also to ensure the properties of the product.

The reason why we set the deformation salinity range to be above 700℃ and below [(temperature at which ferrite becomes single phase) -100℃] is because below 700℃, heat such as precipitation of σ phase and recrystallization of γ phase, which are necessary for superplasticity, is set. This is because the activation process becomes insufficient, making it difficult to obtain superplasticity. On the other hand, the reason why the upper limit temperature was set to below [(temperature at which ferrite becomes single phase) -100°C] is because when this temperature is exceeded, the amount of γ phase decreases extremely and the γ phase as a second phase becomes dispersed and spheroidized. This is because the effect of promoting recrystallization of the α phase cannot be obtained.

Usually, the temperature at which α single phase is formed is about 1200 to 1350°C, and the more preferable range is about 800 to 1100°C.

In the present invention, when performing die forging or extrusion molding, an appropriate mold material is required, but such 1100 ° C.
In the following processing temperature range, austenitic stainless steels such as SUS 310, and even Ni-based high alloys have sufficient high temperature strength as mold materials and can be used, making it easier to implement the present invention. This is one major manufacturing advantage.

In the present invention, strain rate is also one of its constituent requirements, and a certain strain rate is specified in order to cause the superplastic phenomenon to occur. In the present invention, the strain rate (ε) during deformation is set to 1
The reason for setting the range to be 04 to '1/see is that outside this range, the above-mentioned structural change during deformation becomes difficult to occur during deformation, making it difficult to obtain superplasticity. Generally preferred in practical terms is 10-' to 10-' 1/see.

Next, the present invention will be explained in more detail with reference to Examples.

Two-phase stainless steel copper powder with the composition shown in Table 1 above is SO
5310 mold, heated to 1050℃,
After holding it under a hydrostatic pressure of ``kgf/w'' for 5 minutes, it was rapidly cooled to create a sintered body.The atmosphere was an Nt gas atmosphere.
The dew point was -45°C.

A round bar tensile test piece was prepared from the obtained sintered body, subjected to hot tensile deformation, and elongation was determined.

The results obtained are summarized in Table 2.

Duplex stainless steels 16 to 19 shown as comparative steels are melted materials having the same composition as the atomized steel powder used, and are ordinary forged and hot rolled materials.

The results shown in Table 2 also show that according to the method of the present invention, the duplex stainless steel atomized steel powder sintered bodies were all deformed at a high strain rate, but were all extremely good, exceeding 200%. It is clear that large deformations are easily possible under these conditions.

On the other hand, it is clear that none of the comparative methods in which the conditions marked with * in Table 2 are outside the scope of the present invention exhibit large elongation values.

Duplex stainless steel powders having various compositions shown in Table 1 above are compacted together with a wax binder in a mold with an inner diameter of 20 fins to form a compact with an outer diameter of 20 m and a height of 200 m. I got it. The density of this compact was approximately 75% of the theoretical density. All of the obtained compacts were sintered in a vacuum furnace at 1000°C for 20 minutes. The density of the sintered body was in the range of 75-85%. Using the obtained sintered body, free forging was performed in the apparatus shown in FIG. Molding was performed by keeping the lower mold 12 fixed and dropping the upper mold 14 along the surrounding guide 16. By appropriately providing heating elements surrounding the upper and lower molds, the illustrated material is constantly heated to a predetermined temperature.

The mold used was made of a Ni-based heat-resistant alloy, and a heating element was installed to enable processing under uniform conditions. The processing was performed in N2 + argon gas, and a glass-based lubricant was used for the material.

The results of processing at various temperatures and strain rates are shown in Table 3. Here, the strain rate ε is defined as ε=In
(v/h).

Note that the machining load in Table 3 is when the material height is half (
h, /2).

Figure 2 shows the relationship between rolling displacement and load in free forging of powder material obtained by compacting and sintering atomized powder. The initial height of the material is ho, the strain rate is 9xlo-'' (
1/5 ec). The material density increases in the region of
It can be seen that the load increases almost in proportion to the contact area after the density reaches almost 100%.

FIG. 3 is a drawing similar to FIG. 2 showing the case of free forging of ingot material, and the load increases rapidly with rolling reduction. In this case, the initial height of the material is ho, and the strain rate is 10 (1/5e
e).

As is clear from the results shown in Table 3, it can be seen that according to the present invention, processing can be performed at a relatively low temperature of 900 to 1050°C and with a significantly low load.

Table 1 (!!! Amount %) Table 2 *: Outside the scope of the present invention Table 3 (Effects of the invention) As explained above, the atomized steel powder of the duplex stainless steel according to the present invention is used. In the superplastic processing method, the load required for processing is significantly lower than that of conventional methods.
It is possible to process large products and complex-shaped products with a small load.

On the other hand, it is possible to lower the processing temperature, which not only reduces mold costs, but also prevents the growth of crystal grains, leading to improved product quality.

Furthermore, when we extruded duplex stainless steel powder under the same temperature range and strain rate as the above processing conditions, we were able to obtain a product equivalent to the conventional product under low load.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram of the free forging apparatus for sintered compacts used in the examples, FIG. 2 is a graph showing the relationship between rolling displacement and load in free forging of powder material according to the present invention, and FIG. The figure is a graph showing the relationship between rolling displacement and load in conventional free forging of ingot material.

Claims (6)

[Claims] (1) An aggregate of duplex stainless steel atomized steel powder that exhibits a two-phase structure of ferrite phase and austenite phase near room temperature, containing Fe, Cr, and Ni as main components, and containing Mo and N as necessary. , 1× in the temperature range of 700°C or higher and ((temperature at which ferrite becomes single phase) -100°C) or lower
10^-^61/sec or more, 1×10^01/sec
A method for hot working duplex stainless steel powder, characterized by deforming it at the following strain rate: (2) The duplex stainless steel has Cr: 12% by weight.
．． 0-35.0%, Ni: 3.0-18.0%, balance F
2. The method of claim 1, having a steel composition consisting of e and incidental impurities. (3) The duplex stainless steel has Cr: 12% by weight.
．． 0-35.0%, Ni: 3.0-18.0%, C: 0
．． 05% or less, Si: 0.05 to 5.0%, Mn: 0.
05~8.0%, Mo: 5.0% or less, N: 0.05~
0.8%, Cu: 0.8% or less, P: 0.05% or less,
The method according to claim 1, wherein the steel composition comprises S: 0.005% or less, the balance consisting of Fe and incidental impurities. (4) Duplex stainless steel atomized steel powder containing Fe, Cr, and Ni as main components and exhibiting a two-phase structure of ferrite and austenite phases at around room temperature was heated at 700°C or higher in a hot isostatic press machine. (Temperature at which ferrite becomes single phase) -100℃] In the temperature range below, 1 x 10^-^
A hot isostatic processing method for duplex stainless steel powder, characterized by deforming and densifying it at a strain rate of 61/sec or more and 1×10^01/sec or less. (5) The duplex stainless steel has Cr: 12% by weight.
．． 0-35.0%, Ni: 3.0-18.0%, balance F
5. The method of claim 4, having a steel composition consisting of e and incidental impurities. (6) The duplex stainless steel has Cr: 12% by weight.
．． 0-35.0%, Ni: 3.0-18.0%, C: 0
．． 05% or less, Si: 0.05 to 5.0%, Mn: 0.
05~8.0%, Mo: 5.0% or less, N: 0.05~
0.8%, Cu: 0.8% or less, P: 0.05% or less,
The method according to claim 4, wherein the steel composition comprises S: 0.005% or less, the balance being Fe and incidental impurities.