JP2022152427A - Duplex stainless steel forging and its manufacturing method - Google Patents

Abstract

To provide a duplex stainless steel forging capable of suppressing forging cracks caused by a sigma phase seen in duplex stainless steel forgings, and because forging can be performed without reheating, being very excellent in productivity, capable of improving dimensional accuracy and reducing a polishing yield, and having sufficient corrosion resistance equivalent or better than SUS 316L including a welded portion.SOLUTION: PREN represented by the following formula 1 satisfies 28.0 to 35.0; an austenite amount is 30 area% or more and 70 area% or less; and a sigma phase precipitation temperature estimation formula TSIGMA represented by the following formula 2 calculated using each component distributed in a ferrite phase is 900°C or less. PREN=Cr+3.3Mo+16N [Formula 1] TSIGMA=12Crα+6Niα+54Moα+23Siα-15Mnα+465 [Formula 2].SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a duplex stainless steel forging material having good corrosion resistance and excellent formability in hot forging, and a method for producing the same.

Stainless steel, which has good corrosion resistance, is often used as a material for manufacturing equipment and storage tanks for foods and medicines, etc., in order to prevent metal from being mixed into the product. Since food often contains salt, SUS316L, which has good corrosion resistance among stainless steels, is often used.

On the other hand, recently, austenitic stainless steel, which is generally used, is being replaced by duplex stainless steel. Duplex stainless steel contains approximately equal amounts of ferrite and austenite phases. In addition to being corrosion resistant, it is stronger than other stainless steels and carbon steels, and can be made thinner and lighter. In addition, it uses less expensive nickel. There are merits, and it is becoming widely used, for example, for large structures such as water gates and dams.

There are six JIS grades of duplex stainless steel, namely SUS821L1, SUS323L, SUS329J1, SUS329J3L, SUS329J4L, and SUS327L. Of these, SUS821L1 is a steel grade developed as a substitute for SUS304, SUS323L as a substitute for SUS316L, and SUS329J3L, SUS329J4L, and SUS327L are high corrosion-resistant steel grades having corrosion resistance even in harsher environments.

By the way, hot forging is one of the techniques for working and molding steel. This method heats and strikes the raw material to shape it, making it relatively easy to form steel that is difficult to form due to its high strength by cold forging. method.
Flanges are used to connect pipes in food and drug manufacturing facilities. This flange often has a complicated shape, and hot forging is often used as a manufacturing method.

Precipitation of sigma phase is a problem when hot forging is applied to processing of duplex stainless steel. The sigma phase is an intermetallic compound in which Cr, Mo, etc., are concentrated in Fe. However, in addition to the occurrence of large cracks during forging, the toughness and corrosion resistance of the material are extremely reduced. Hot forging generally requires a longer working time than hot rolling, and the temperature easily falls below 900° C. during working.

In this case, it is necessary to reheat again in order to avoid hot tearing, which greatly impairs manufacturing efficiency. Furthermore, the higher the forging temperature, the softer the material becomes, making it easier to deform, and the amount of heat shrinkage up to room temperature increases, resulting in a decrease in dimensional accuracy. impair workability.
Among duplex stainless steel grades, SUS329J3L, SUS329J4L, and SUS327L have very excellent corrosion resistance, but all of them have the problem of precipitation of sigma phase when forged at a low temperature of 900° C. or less.

In response to this problem, in Patent Document 1, heat treatment is performed after forging. A technique for suppressing the precipitation of sigma phases by increasing the average grain size of ferrite phases to 50 μm or more by cooling is disclosed.
Further, Patent Document 2 discloses a technique for suppressing the precipitation of the sigma phase by minimizing Si and Mn to 0.005 to 0.1%.

Another property that should be considered for the flange material is the toughness and corrosion resistance of the weld zone. This is because flanges are often attached to pipes or the like by circumferential welding. N contained in the duplex stainless steel precipitates as Cr nitride by heating and cooling during welding. This nitride promotes the propagation of cracks to lower the toughness, and the precipitation consumes Cr to form a so-called Cr-deficient layer, thereby lowering the corrosion resistance.

Among duplex stainless steel grades, SUS323L has a corrosion resistance of the base material equal to or higher than that of SUS316L, and is a steel grade with almost no precipitation of sigma phase. Have. SUS821L1 also has less precipitation of sigma phase and less deterioration in corrosion resistance of welds, but it is not suitable for this application because it is a substitute steel for SUS304.

On the other hand, in Patent Document 3, as a duplex stainless steel material that exhibits excellent corrosion resistance in an environment with a high chloride ion concentration similar to brackish water or seawater and is highly economical, C: 0.001 to 0.03%, Si: 0.05 to 1.5%, Mn: 0.1 to less than 2.0%, Cr: 20.0 to 26.0%, Ni: 2.0 to 7.0%, Mo: 0.5 After adjusting to ~3.0%, N: 0.10 to 0.25%, Al: 0.003 to 0.05%, Nb is contained in a trace amount of 0.005 to 0.10% and extraction residue A duplex stainless steel is disclosed in which the contents of [Nb] and [Cr] are optimized.

JP-A-9-217149 JP-A-11-50199 WO2018/181990

As a result of examining the prior art, the inventor found that there is a problem to be solved. Patent document 1 is an improvement by heat treatment after forging, and does not contribute to avoiding forging cracks due to sigma phase precipitation during forging, improving workability associated therewith, and improving dimensional accuracy.

According to Patent Document 2, extremely low Si and Mn require high purification of the raw material, resulting in a large cost. SUS323L and SUS821L1 can avoid the problem of sigma phase precipitation, but cannot avoid the problem of corrosion resistance of the weld zone.

The steel of Patent Document 3 can clear the problem of corrosion resistance of welded parts, and it is possible that it can replace the flange of SUS316L in terms of corrosion resistance, but it is not clear whether it can clear the problem of sigma phase precipitation.

After all, there is no disclosure of a technique for avoiding the complication of forging and low dimensional accuracy while ensuring corrosion resistance equal to or higher than that of SUS316L including welded portions at low cost. An object of the present invention is to improve the dimensional accuracy of a forged product by reviewing the steel material composition and manufacturing conditions in duplex stainless steel forgings, suppressing the precipitation of sigma phase while ensuring corrosion resistance.

The sigma phase is generally considered to be likely to occur in high Cr, Mo, and Ni duplex stainless steels. Reducing these elements lowers the temperature at which the sigma phase precipitates. The inventors determined this sigma phase precipitation start temperature and found that when this is 900° C. or less, forging can be completed without being substantially adversely affected by the sigma phase during production. This is because Mo, which has a relatively slow _diffusion rate, needs to move for the formation of the _sigma phase. Because it is low, it takes a considerably long time for precipitation to occur at low temperatures where the diffusion rate slows down.

On the other hand, Cr and Mo are important elements for improving the corrosion resistance of steel. Therefore, the present inventors have made intensive studies on a composition range and a manufacturing method that minimize the deterioration of corrosion resistance and do not substantially adversely affect hot forging in the sigma phase, resulting in the present invention. .

Specifically, in addition to the composition of steel, PREN (pitting resistance index) is specified to ensure corrosion resistance, and the composition of the ferrite phase, which is the source of sigma phase precipitation in duplex stainless steel, is measured and its composition Based on the range, the sigma phase precipitation start temperature was set to 900°C or less.

Furthermore, as a manufacturing method, the inventors found a method of obtaining the composition of the ferrite phase by specifying the composition of the steel and the heating temperature of the raw material, which led to the present invention.
Based on these findings, the gist of the present invention is as follows.

[1]
The chemical composition, in mass %,
C: 0.001 to 0.050%,
Si: 0.05 to 0.80%,
Mn: 0.10% to 3.00%,
Cr: 21.5-26.0%,
Ni: 3.0 to 6.0%,
Mo: 0.5-2.5%,
N: contains 0.10 to 0.25%,
Furthermore, Al: 0 to 0.05%,
Nb: 0 to 0.15%,
Ti: 0 to 0.020%,
Ta: 0 to 0.20%,
Zr: 0 to 0.05%,
Hf: 0-0.08%,
Sn: 0-0.10%,
W: 0 to 1.0%,
Co: 0 to 1.0%,
Cu: 0-3.0%,
V: 0 to 0.30%,
B: 0 to 0.0050%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0-0.050%,
containing one or more of
the balance being Fe and impurities,
O among impurities: 0.006% or less,
P: 0.05% or less,
S: limited to 0.0050% or less,
PREN represented by the following formula 1 satisfies 28.0 to 35.0,
Austenite content is 30 area% or more and 70 area% or less,
A duplex stainless steel forging _PREN =Cr+3.3Mo+16N・... (Equation 1)
T _SIGMA =12Cr _α +6Ni _α
+54Mo _α +23Si _α −15Mn _α +465 (Formula 2)
However, the element symbol in Formula 1 represents the content (% by mass) of each element in the duplex stainless steel forging, and the element symbol with α in Formula 2 represents the duplex stainless steel forging. shows the content (% by mass) of each element distributed in the ferrite phase of
[2]
Furthermore, the chemical composition, in mass%,
Al: 0.003-0.05%,
Nb: 0.005 to 0.15%,
Ti: 0.003 to 0.020%,
Ta: 0.005 to 0.20%,
Zr: 0.001 to 0.05%,
Hf: 0.001 to 0.08%,
Sn: 0.005 to 0.10%,
W: 0.01 to 1.0%,
Co: 0.01 to 1.0%,
Cu: 0.01 to 3.0%,
V: 0.01 to 0.30%,
B: 0.0001 to 0.0050%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0050%,
REM: 0.005-0.050%,
The duplex stainless steel forging material according to [1], characterized by containing one or more of
[3]
A duplex stainless steel material having the chemical composition described in [1] or [2] and having a sigma phase precipitation prediction temperature T _SIGMA2 represented by the following formula 3 of 890° C. or less is heated to 1150° C. or more and 1350° C. or less. The method for producing a duplex stainless steel _forging material according to claim 1, wherein hot forging is performed after the
However, the symbol of each element in Formula 3 indicates the content (% by mass) of each element in the duplex stainless steel forging.
[4]
The method for producing a duplex stainless steel forged material according to [3], characterized in that in the hot forging, the final surface temperature of the duplex stainless steel material after hot forging is less than 900°C.
A flange using the duplex stainless steel forging material according to [1] or [2].
[6]
The flange according to [5] for use in food and drug manufacturing equipment.
[7]
A welded structure using the duplex stainless steel forging material according to the above [1] or [2].

The forged material obtained by the present invention has sufficient corrosion resistance, can achieve weight reduction due to high strength, can minimize the number of forgings, has excellent dimensional accuracy during forging, and can reduce the grinding yield. , to minimize costs.

[Composition of Duplex Stainless Steel]
First, the reasons for limiting the composition and structure of the duplex stainless steel forging material constituting the present invention will be described below. In addition, % regarding a component represents the mass % unless there is particular notice in this specification.

If the C content exceeds 0.050%, Cr carbides are formed during hot rolling, degrading the corrosion resistance. Therefore, the content should be 0.050% or less. It is preferably 0.030% or less, more preferably 0.025% or less.
On the other hand, although the lower limit is not particularly limited, the lower limit is set to 0.001% from the viewpoint of the cost of reducing the C content. Preferably, it should be 0.025% or 0.005%.

Si should be contained in an amount of 0.05% or more for deoxidation. It is preferably 0.10% or more, more preferably 0.20% or more.
On the other hand, an excessive content deteriorates the toughness and promotes precipitation of the sigma phase, so the content is preferably 0.80% or less. It is preferably 0.50% or less, more preferably 0.40% or less.

Mn has the effect of increasing the austenite phase and improving the toughness. Therefore, it is preferable to contain 0.10% or more. It is preferably 0.30% or more, more preferably 0.50% or more.
On the other hand, Mn is an element that lowers the corrosion resistance of stainless steel, so its content should be 3.00% or less. It is preferably 2.00% or less, more preferably 1.50% or less.

Cr should be contained in an amount of 21.50% or more in order to ensure the basic corrosion resistance of the steel of the present invention. It is preferably 23.00% or more, more preferably 23.40% or more.
On the other hand, if the Cr content exceeds 26.00%, it promotes precipitation of the sigma phase and increases T _SIGMA and T _SIGMA2 . In addition, the ferrite phase fraction increases and deteriorates toughness and corrosion resistance of the weld zone when welding is performed. Therefore, the Cr content should be 26.00% or less. It is preferably 25.00% or less, more preferably 24.50% or less.

Ni stabilizes the austenite structure, and is preferably contained in an amount of 3.0% or more in order to improve corrosion resistance to various acids and toughness. It is preferably 4.0% or more, more preferably 4.5% or more.
On the other hand, Ni promotes the precipitation of the sigma phase, increases T _SIGMA and T _SIGMA2 , and is expensive, so the Ni content should be 6.0% or less.

Mo is a very effective element for enhancing corrosion resistance, and should be contained in an amount of 0.5% or more to ensure corrosion resistance. It is preferably 1.0% or more, more preferably 1.2% or more.
On the other hand, Mo is an element that is expensive, contributes greatly to promoting the precipitation of the sigma phase, and greatly increases T _SIGMA and T _SIGMA2 , so it is desirable to suppress it as much as possible. Therefore, the Mo content should be 2.5% or less. It is preferably 2.0% or less, more preferably 1.8% or less, more preferably 1.6% or less.

N is a strong austenite-generating element, and is a very effective element that dissolves in the austenite phase and increases the strength and corrosion resistance of duplex stainless steel. Therefore, the content should be 0.10% or more. It is preferably 0.12% or more, more preferably 0.15% or more.
On the other hand, since the solid solubility limit of N increases with the Cr content, if the content exceeds 0.25% due to the relationship with the Cr content, Cr nitrides will precipitate and the toughness and corrosion resistance will be impaired. . Therefore, the N content should be 0.25% or less. It is preferably 0.22% or less, more preferably 0.20% or less.

O (oxygen) is an impurity and an element that impairs the hot workability, toughness and corrosion resistance of stainless steel. Therefore, the O content should be 0.006% or less. In addition, the lower limit of the O content is not particularly limited, but since extremely high refining costs are required to extremely reduce the oxygen content, the oxygen content may be 0.001% or more from the economic point of view. .

P is an element that is unavoidably mixed from raw materials, and deteriorates hot workability and toughness. Preferably, it should be 0.04% or less. Although the lower limit of the P content is not particularly limited, reducing the P content to an extremely low amount increases the cost during refining. Therefore, it is preferable to set the lower limit of the P amount to 0.01% from an economical point of view.

S is an element that is unavoidably mixed from the raw material, and inhibits hot workability, promotes cracking during forging, and deteriorates toughness and corrosion resistance. should be It is preferably 0.0020% or less, more preferably 0.0010% or less. Although the lower limit of the S content is not particularly limited, reducing the S content to an extremely low amount increases the cost during refining. Therefore, from an economical point of view, the lower limit of the S content should be 0.0001%.

The balance is Fe and unavoidable impurities. The unavoidable impurities are elements contained in raw materials, elements unintentionally contained during production, and the like.

Furthermore, as one embodiment of the present invention, one or more of the following elements may be contained instead of Fe. These elements do not have to be contained, and the content range includes 0%.

Al is an element that has a deoxidizing effect on steel, and may be contained together with Ca and Mg in order to control the composition of inclusions in the present steel. Al may be contained together with Si in order to reduce oxygen in the steel. Al also has the effect of controlling the composition of inclusions and increasing the pitting corrosion resistance, and may be contained in an amount of 0.003% or more in order to ensure this effect. Preferably, it should be 0.005% or more.
On the other hand, Al is an element that has a relatively high affinity with N, and if it is contained excessively, Al nitrides are produced, impairing the toughness of stainless steel. Although the extent depends on the N content, if the Al content exceeds 0.05%, the toughness decreases significantly, so the content should be 0.05% or less. It is preferably 0.04% or less, more preferably 0.03% or less.

Nb is an element that has a strong affinity with N and has the effect of further reducing the deposition rate of chromium nitride. Therefore, it may be contained in an amount of 0.005% or more. It is preferably 0.010% or more, more preferably 0.020% or more, and more preferably 0.030% or more.
On the other hand, if the Nb content is excessive, a large amount of Nb nitride precipitates, impairing the toughness. It is preferably 0.09% or less, more preferably 0.07% or less, and more preferably 0.05% or less.
Although Nb is an expensive element, the raw material cost for melting stainless steel can be reduced by actively using Nb contained in low-grade scrap. It is preferable to reduce the melting cost of Nb-containing steel by such a method.

Ti has a very strong affinity with N and may be included because it forms nitrides of Ti in steel. However, coarse nitrides are likely to be formed, and if the content exceeds 0.020%, the toughness is impaired by the nitrides of Ti. % or less, more preferably 0.010% or less. When Ti is contained, the content is preferably 0.003% or more, preferably 0.005% or more, and more preferably 0.006% or more, in order to obtain the effect more reliably.

Ta is an element that improves corrosion resistance by modifying inclusions and may be contained. However, when the Ta content exceeds 0.200%, the normal temperature ductility and toughness are lowered, so the Ta content is preferably 0.200% or less, more preferably 0.100% or less. be. When the effect is expressed with a small amount of Ta, the amount of Ta is preferably 0.050% or less.
When Ta is contained, the amount of Ta is preferably 0.005% or more in order to ensure its effect.

Zr and Hf are effective elements for improving hot workability and cleanliness of steel and improving oxidation resistance. Sn has the effect of improving the acid resistance, so it may be contained.
On the other hand, excessive inclusion of these elements promotes intergranular fracture due to lower intergranular strength, so it is necessary to set Zr to 0.05% or less, Hf to 0.08% or less, and Sn to 0.10% or less. be. In order to obtain these effects more reliably, it is preferable to contain Zr: 0.001% or more, Hf: 0.001% or more, and Sn: 0.005% or more.

W, like Mo, is an element that improves the corrosion resistance of stainless steel and may be contained. However, since it is an expensive element, it should be 1.00% or less. It is preferably 0.70% or less, more preferably 0.50% or less. When it is contained, the content is preferably 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more, in order to obtain the effect more reliably.

Co is an effective element for increasing the toughness and corrosion resistance of steel and may be contained. Since Co is an expensive element, it should be contained in an amount of 1.0% or less. The content is preferably 0.7% or less, more preferably 0.5% or less. When Co is contained, the content is preferably 0.01% or more, preferably 0.03% or more, and more preferably 0.10% or more, in order to obtain the effect more reliably.

Cu is an element that additionally increases the acid corrosion resistance of stainless steel and has the effect of improving the toughness, so it may be contained. If the Cu content exceeds 3.0%, εCu precipitates beyond the solid solubility during cooling after hot rolling, resulting in embrittlement. The content is preferably 1.70% or less, more preferably 1.5% or less. When Cu is contained, the content should be 0.01% or more, preferably 0.33% or more, and more preferably 0.45% or more.

V is an element that has an affinity for N and acts to reduce the deposition rate of chromium nitride. Therefore, it may be contained. However, if the content exceeds 0.30%, a large amount of nitrides of V precipitates and the toughness is impaired. Therefore, the V content is 0.30% or less, preferably 0.25% or less. , and more preferably 0.20% or less. When V is contained, the content is preferably 0.01% or more, preferably 0.03% or more, and more preferably 0.08% or more, in order to obtain the effect more reliably.

B is an element that improves the hot workability of steel. In addition, it is an element that has a very strong affinity with N, and if it is contained in a large amount, nitrides of B precipitate and impair the toughness. Therefore, its content should be 0.0050% or less, preferably 0.0040% or less, more preferably 0.0030% or less. When B is contained, it should be contained in an amount of 0.0001% or more, preferably 0.0005% or more, and more preferably 0.0014% or more, in order to obtain its effect more reliably.

Ca and Mg may be contained in order to control the composition of inclusions and improve pitting corrosion resistance and hot workability.

On the other hand, an excessive content of both Ca and Mg adversely lowers hot workability and toughness, so Ca should be 0.0050% or less, and Mg should be 0.0050% or less. Ca is preferably 0.0040% or less, Mg is 0.0025% or less, more preferably Ca is 0.0035% or less, and Mg is 0.0020% or less.
When Ca and Mg are contained, they are contained using molten raw materials together with 0.003% or more and 0.050% or less of Al, or the content is adjusted through deoxidation and desulfurization operations, and the content of Ca is 0.05%. 0005% or more, and the Mg content is preferably 0.0005% or more. Ca is preferably 0.0010% or more and Mg is 0.0010% or more, more preferably Ca is 0.0015% or more and Mg is 0.0015% or more.

REM is an element that improves the hot workability of steel, and may be contained for that purpose. On the other hand, excessive content adversely lowers hot workability and toughness, so the content should be 0.050% or less. It is preferably 0.040% or less, more preferably 0.030% or less. When REM is contained, it should be contained in an amount of 0.005% or more in order to ensure its effect. The content is preferably 0.010% or more, more preferably 0.020% or more.
Here, REM refers to scandium Sc, yttrium Y, and lanthanoid-based rare earth elements such as lanthanum La and cerium Ce, and the REM content is the total content of each element belonging to REM.

[Austenite content is 30 area % or more and 70 area % or less]
In general, it is desirable that the austenite content in duplex stainless steel is close to the ferrite content. Excessive ferrite content lowers the toughness and facilitates the precipitation of Cr nitrides. On the other hand, excessive austenite impairs hot workability, promotes cracking during forging, and makes stress corrosion cracking more likely to occur. Furthermore, in either case, the difference in composition between the ferrite phase and the austenite phase becomes severe, and the corrosion resistance of one of the phases decreases. Therefore, the austenite content should be 30 area % or more and 70 area % or less. Since it is preferable that the amount of austenite and the amount of ferrite be as equal as possible, the amount of austenite is preferably 35% or more, 40% or more, or 45% or more, and similarly 65% or less, 60% or less, or 55%. You should do the following.

The amount of austenite is obtained by measuring the austenite area ratio by performing etching treatment, observation with an optical microscope, and image analysis using a mirror-polished steel cross section. The cross section of the steel material may be any cross section as long as it is in the thickness direction of the steel material (perpendicular to the surface of the steel material). From the viewpoint of avoiding the influence of processing, it is desirable to measure near the center of the cross section.

[28.0≦PREN≦35.0]
In the case of duplex stainless steel, considering the decrease in corrosion resistance due to precipitation of Cr nitrides when welding is performed, it is often the case that PREN higher than that of austenitic stainless steel is secured when the same corrosion resistance is aimed. As a result of experiments, when PREN defined by the following formula 1, which is an index of pitting corrosion resistance, is less than 28.0, even if the austenite content of the duplex stainless steel base material is 30 to 70 area%, it can be used in a brackish water environment. The corrosion resistance at the weld heat affected zone was below that of SUS316L.

PREN=Cr+3.3Mo+16N (Formula 1)
However, the element symbol in Formula 1 indicates the content (% by mass) of each element.

In addition, if the contents of Cr and Mo are excessively increased in order to increase the PREN of the duplex stainless steel base material, T _SIGMA and T _SIGMA2 , which will be described later, are increased, and not only the sigma phase is likely to be generated, but also the alloy cost is reduced. increase and the formation of intermetallic compounds. On the other hand, if the N content is excessively high, adverse effects such as deterioration of toughness appear.

As described above, in the present invention, the austenite content should be 30 to 70 area %, and the PREN value defined by the above formula (1) of the duplex stainless steel base material should be 28.0 or more and 35.0 or less. The PREN value is preferably 29.0 or more, 30.0 or more, 30.5 or more, or 31.0 or more, and 34.5 or less, 34.0 or less, 33.5 or less, or 33.0. You should do the following.

[T _SIGMA ≤ 900°C]
In conventional duplex stainless steel, when forging takes a long time and the surface temperature drops below about 900°C, the sigma phase, which is an intermetallic compound, precipitates and significantly reduces hot ductility and toughness, resulting in material cracking. cause. For this reason, reheating was required when the surface temperature fell below 900°C. However, it was found that by optimizing the composition of the material and suppressing the precipitation of the sigma phase, it is possible to obtain a material that does not cause deterioration in properties due to the precipitation of the sigma phase during forging, even if it is a duplex stainless steel.

In duplex stainless steel, the sigma phase is brought about by the phase transformation of the ferrite phase. It has been found that a certain T _SIGMA can be derived from the content of components distributed in the ferrite phase, and based on this it has been found that a material can be obtained which can suppress sigma phase precipitation.

Specifically, for a duplex stainless steel forging material, the components of the ferrite phase are measured, and the contents of the ferrite phase components are substituted into the following equation 2 to calculate T _SIGMA , which is the equilibrium precipitation temperature of the sigma phase, It was found that the temperature should be 900° C. or lower. That is, T _SIGMA is the prediction temperature of the equilibrium precipitation temperature of the sigma phase, and Equation 2 is the prediction formula. Since it is predicted that the sigma phase will not precipitate unless the T _SIGMA becomes lower than T SIGMA, the forging time until reheating due to the temperature drop can be ensured by adjusting the composition so as to lower the T _SIGMA . In addition, when the temperature is lowered, the diffusion of elements necessary for sigma phase precipitation is lowered, so the precipitation rate itself is suppressed. Equation 2 is based on an equation obtained by equilibrium calculation using thermodynamic calculation software "Thermo-Calc" (registered trademark) of Thermo-Calc, Inc., and is further modified by experiments conducted by the present inventors.

T _SIGMA =12Cr _α +6Ni _α
+54Mo _α +23Si _α −15Mn _α +465 (Formula 2)
Element symbols with α in Formula 2 indicate contents (% by mass) of respective elements distributed in the ferrite phase.

The components of the ferrite phase can be clearly seen in high and low Cr and Mo portions corresponding to the ferrite phase and austenite phase by performing line analysis in the thickness direction using EPMA using a mirror-polished steel cross section. Therefore, it can be obtained by averaging the numerical data of the high part as the ferrite phase. Alternatively, after determining the ferrite phase in which Cr and Mo are concentrated, a plurality of points on the ferrite phase may be analyzed and averaged.
As with the measurement of the austenite content, the cross section of the steel material may be any cross section of the steel material, and the measurement should be made at the central portion of the cross section.

[Method for producing forged material of the present invention]
Next, a method for manufacturing a forged material according to the present invention will be described.
A steel ingot for forging is manufactured by melting the steel having the composition described above. There are no particular limitations on the steel melting method and steel ingot manufacturing method. A steel ingot for forging can be produced by an existing conventional method.
The above T _SIGMA is an expected temperature obtained from the component contents distributed in ferrite in the forging material (forged steel material). Therefore, one must predict the composition in the ferrite phase after forging. Therefore, the present inventors have come up with the idea of estimating the content of the ferrite phase in the forged material after forging, even at the stage of the forging steel ingot, and predicting the transition precipitation temperature to the sigma phase, and T _SIGMA2 . Found it.

[T _SIGMA2 ≤ 890°C]
Unlike T _SIGMA , T _SIGMA2 is a formula calculated from the element content of steel. Similar to T _SIGMA , T _SIGMA2 is the predicted temperature for the equilibrium precipitation temperature of the sigma phase and Equation 3 is its prediction formula. The present inventors have found that it is possible to predict the distribution of components to the ferrite phase and the austenite phase by combining the content of elements in the steel and the heating temperature. That is, unlike T _SIGMA , there is no need to measure the content of components distributed to the ferrite phase in the steel, and if the heating temperature of the heat treatment before hot forging is in the range of 1150 to 1350 ° C., the components in the steel The equilibrium precipitation temperature of the sigma phase is predicted from the content. Specifically, the steel components of the duplex stainless steel forging are measured, and the content of each element (if the component content is known in advance, the content is not measured) and is substituted into Equation 3. T _SIGMA2 , which is the equilibrium precipitation temperature of the sigma phase, is calculated by T _SIGMA2 is an index that can be handled in the same way as T _SIGMA . That is, since it is predicted that the sigma phase will not precipitate unless the temperature reaches T _SIGMA2 or lower, the forging time until reheating can be extended. In addition, when T _SIGMA2 is low, the diffusion of elements necessary for sigma phase precipitation decreases, so the precipitation rate itself is suppressed. Equation 3, like T _SIGMA , is based on an equation obtained from equilibrium calculations using the thermodynamic calculation software "Thermo-Calc" (registered trademark) of Thermocalc, Inc., and further modified by the experiments of the present inventors. It is.
The upper limit of T _SIGMA2 is set at 890°C, preferably at 880°C, 870°C, 860°C or 850°C, taking into account variations from the actual T _SIGMA .

T _SIGMA2 = 4Cr+28Ni+55Mo+5Si-6Mn-30N+560 (Formula 3)
However, the symbol of each element in Formula 3 indicates the content (% by mass) of each element in the steel.

[Material heating temperature: 1150-1350°C]
Furthermore, the heating temperature of the raw material at the time of manufacturing the forged material is defined as 1150° C. or higher and 1350° C. or lower. By raising the heating temperature, the segregation of Cr and Mo in the material is relaxed, and in addition, there is the effect of reducing the distribution of Cr and Mo to the ferrite phase, and as a result, the ferrite phase transforms into the sigma phase. Suppressed. To obtain this effect, the heating temperature should be higher than 1150°C. It is preferably higher than 1170°C, more preferably higher than 1200°C, further preferably higher than 1220°C. On the other hand, if the heating temperature is too high, the furnace will be severely damaged. It is preferably 1300° C. or less.

[Final temperature at hot forging surface temperature: less than 900 ° C.]
If T _SIGMA and T _SIGMA2 are 900° C. or less, even if the surface temperature becomes less than 900° C., there is no need to avoid hot cracking due to precipitation of the sigma phase, so reheating is unnecessary. As a result, the number of times of reheating can be greatly reduced, and manufacturability is greatly improved. Furthermore, by forging at a lower temperature, deformation during processing due to low strength and deformation due to thermal contraction during cooling can be suppressed, thereby improving dimensional accuracy. As a result, the man-hours for grinding and grinding loss can be reduced.

According to the knowledge of the present inventors, forging is possible without any problem even if the surface temperature is less than 800° C., and the dimensional accuracy can be further improved. On the other hand, if the surface temperature is less than 500° C., the hot strength becomes too high, making it difficult to work. Therefore, the surface temperature is preferably 500° C. or higher during hot forging.

The present invention will now be described with reference to examples.
Table 1 shows the composition of the sample duplex stainless steel. The tailored steel for each sample was melted in a MgO crucible in a laboratory 25-75 kg vacuum induction furnace and cast into square ingots approximately 100 mm thick and wide. In addition, in Table 1, a blank indicates that the corresponding component is not contained. About 20 kg of material for hot forging was processed from the main body of the steel ingot, heated to a temperature of 1250°C and held for 2 hours, and then hammer forged to a thickness of 20 mm to obtain a forged material (single forged material). . At that time, the surface temperature decreased to about 650°C.

Table 2 shows the evaluation results of the forged materials. First, the appearance of the forged material was observed, and x was given to those with cracks several mm deep, which could not be removed by grinding, and ◯ was given to those that did not. Also, the thickness of the four corners of the forged material was measured, and the maximum difference from the target value of 20 mm was taken as the dimensional error (mm).

Next, a sample for structural observation was taken from the surface layer of the forged material, and the cross section was mirror-polished and observed using EPMA. A line analysis of 1 mm length is performed at a pitch of 1 μm in the thickness direction of the observation sample. components were obtained.

In addition, a sample with a crack judgment of 0 was subjected to solution heat treatment by holding it at 1060° C. for 1 hour and then cooling with water, and a pitting corrosion test piece was taken from 1 mm below the surface layer of the material. As a corrosion resistance evaluation, the pitting potential of the solution heat-treated sample was measured in a 3.5% NaCl solution at 50° C. in accordance with the method specified in JIS G0577. For evaluation of toughness, a V-notch test piece was taken from 1 mm below the surface layer of the solution heat-treated sample according to JIS Z 2202, and a Charpy impact test was performed at a test temperature of -20°C.

Furthermore, in order to simulate insertion welding of pipes, the following welding specimens were created. After polishing the surface of the material, it was cut into 20 mm thick × 50 mm × 100 mm and 5 mm thick × 50 mm × 100 mm. A welded joint was produced by combining the surface and the latter's cut surface of 5 mm×100 mm at right angles so as to be on the same plane, and welding both sides of the contact portion by tungsten arc welding (GTAW). Welding conditions were as follows: SUS329J3L welding wire with a diameter of 1.2 mm was used, welding current: 200 A, arc voltage: 12 V, welding speed: 20 cm/min, and 100% Ar shielding gas flow rate: 15 liters/min.

As a corrosion resistance evaluation of the weld zone, a pitting test piece was taken from the cross section of the material so as to include all the weld heat affected zone and the weld metal, and the pitting potential was measured in a 3.5% NaCl solution at 50 ° C. The measurement was carried out according to the method specified in JIS G0577.

Table 2 shows the results of ferrite phase component analysis, austenite phase area ratio, presence or absence of cracks during forging, dimensional errors, base material corrosion resistance and toughness evaluation, and weld corrosion resistance evaluation results for each forged material.
None of the forged materials of the present invention causes large cracks, has a dimensional error of 1.0 mm or less, has a pitting potential of the base material of 0.3 V vs SSE or more at 50 ° C., and a pitting potential of the weld zone of 0. The impact value at −20° C. was 100 J/cm ² or more at 0.28 V vs SSE or more, showing good characteristics.

On the other hand, No. of the comparative example. In Nos. 14 and 16, Cr and Mo were excessive, respectively, so T _SIGMA exceeded 900° C. As a result, sigma phase was precipitated during forging and cracks occurred. No. T _SIGMA2 exceeded 900°C. 21 was the same. No. Nos. 9, 11, 13, 15 and 19 were poor in corrosion resistance due to excessive C, excessive Mn, insufficient Cr, insufficient Mo, and insufficient PREN, respectively. No. In No. 17, Cr nitride was precipitated at the weld zone due to insufficient N, and the corrosion resistance was poor. No. In No. 18, Cr nitride was precipitated due to excessive N, and both toughness and corrosion resistance were poor. No. Nos. 12 and 20 had poor toughness due to insufficient Ni and excessive ferrite, respectively.

Next, Table 3 shows the results of comparing a forged material that was forged at once without reheating in the middle (single forging material) and a forged material that was reheated in the middle (additional reheating material).
Similar to the above sample, No. Steel materials Nos. 5, 7 and 21 were heated to a temperature of 1250° C. and held for 2 hours, and then subjected to hammer forging to a thickness of 20 mm to form forging materials. At that time, when the temperature reached 900°C, a step of reheating at 1250°C was added twice to complete the forging above 900°C to obtain a forged material (reheated material). The appearance of the material was observed, and x was given to those with cracks several millimeters deep, which could not be removed by grinding, and ◯ was given to those that did not. Also, the thickness of the four corners of the forged material was measured, and the maximum difference from the target value of 20 mm was taken as the dimensional error (mm).

Table 3 shows the cracking conditions and dimensional accuracy of the reheated material and the single forged material. A, C, and E are the same as those in Table 2, respectively. No. When 5 and 7 were forged once, there was no problem of cracking and the dimensional accuracy was good. No. When No. 21 was forged once, a large crack occurred and a product could not be obtained. The three samples of the reheating addition material were forged at a minimum temperature of 900°C, so they can be manufactured without cracking, but the dimensional error exceeds 1.0 mm, and the grinding load in the post-process is extremely high. got bigger.

Furthermore, Table 4 shows the results of investigating the effect of heating temperature. Similar to the above sample, No. 5, 7, and 21 steel materials are similarly processed into forging materials, the heating temperature is changed between 1130 and 1320°C, and after holding at that temperature for 2 hours, the forged materials are hammer forged to a thickness of 20 mm (single forging materials ). At that time, the surface temperature at the completion of forging decreased to about 600 to 880°C. After heating and holding the material at 1060° C. for 1 hour, it was cooled with water and subjected to solution heat treatment for evaluation.

c, h and k in Table 4 are the same as in Table 2. Although T _sigma increased as the heating temperature decreased, it was found that cracks did not occur when T _sigma was 900°C or less. The boundary temperature varies depending on the difference in T _sigma2 calculated from the composition of the steel material. When T _sigma2 originally exceeds 890° C. as in 21, T _sigma does not exceed 900° C. if heating is performed at 1150° C. or higher.

The duplex stainless steel forging obtained by the present invention can prevent forging cracks without reheating by substantially suppressing the precipitation of the sigma phase during production, and has very good manufacturability. In addition, since the forging temperature can be lowered, the dimensional accuracy is improved and the grinding yield can be reduced. In addition, it has sufficient corrosion resistance equal to or higher than that of SUS316L, including the welded parts, and it is possible to achieve weight reduction due to its high strength.In addition, the cost required for forging can be minimized, resulting in significant cost reduction and high efficiency. It is possible to contribute, and the place where it contributes to the industrial aspect and the environmental aspect is extremely large.
For example, it is expected to be applied as an alternative material to the current SUS316L for furan materials such as manufacturing equipment for food and medicine, storage tanks, and the like.

Claims (7)

The chemical composition, in mass %,
C: 0.001 to 0.050%,
Si: 0.05 to 0.80%,
Mn: 0.10% to 3.00%,
Cr: 21.5-26.0%,
Ni: 3.0 to 6.0%,
Mo: 0.5-2.5%,
N: contains 0.10 to 0.25%,
Furthermore, Al: 0 to 0.05%,
Nb: 0 to 0.15%,
Ti: 0 to 0.020%,
Ta: 0 to 0.20%,
Zr: 0 to 0.05%,
Hf: 0-0.08%,
Sn: 0-0.10%,
W: 0 to 1.0%,
Co: 0 to 1.0%,
Cu: 0-3.0%,
V: 0 to 0.30%,
B: 0 to 0.0050%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0-0.050%,
containing one or more of
the balance being Fe and impurities,
O among impurities: 0.006% or less,
P: 0.05% or less,
S: limited to 0.0050% or less,
PREN represented by the following formula 1 satisfies 28.0 to 35.0,
Austenite content is 30 area% or more and 70 area% or less,
A duplex stainless steel forging, characterized in that the sigma phase precipitation prediction temperature T _SIGMA calculated using each component distributed in the ferrite phase is 900° C. or less.
PREN=Cr+3.3Mo+16N (Formula 1)
T _SIGMA =12Cr _α +6Ni _α
+54Mo _α +23Si _α −15Mn _α +465 (Formula 2)
However, the element symbol in Formula 1 represents the content (% by mass) of each element in the duplex stainless steel forging, and the element symbol with α in Formula 2 represents the duplex stainless steel forging. shows the content (% by mass) of each element distributed in the ferrite phase of Furthermore, the chemical composition, in mass%,
Al: 0.003-0.05%,
Nb: 0.005 to 0.15%,
Ti: 0.003 to 0.020%,
Ta: 0.005 to 0.20%,
Zr: 0.001 to 0.05%,
Hf: 0.001 to 0.08%,
Sn: 0.005 to 0.10%,
W: 0.01 to 1.0%,
Co: 0.01 to 1.0%,
Cu: 0.01 to 3.0%,
V: 0.01 to 0.30%,
B: 0.0001 to 0.0050%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0050%,
REM: 0.005-0.050%,
The duplex stainless steel forging material according to claim 1, characterized in that it contains one or more of A steel material having the chemical composition according to claim 1 or 2 and having a sigma phase precipitation prediction temperature T _SIGMA2 represented by the following formula 3 of 890 ° C. or lower is heated to 1150 ° C. or higher and 1350 ° C. or lower, and then hot forged. The method for producing a duplex stainless steel forging according to claim 1 or 2, characterized in that:
T _SIGMA2 = 4Cr+28Ni+55Mo+5Si-6Mn-30N+560 (Formula 3)
However, the symbol of each element in Formula 3 indicates the content (% by mass) of each element in the duplex stainless steel forging. 4. The method for producing a duplex stainless steel forged material according to claim 3, characterized in that in the hot forging, the final surface temperature of the duplex stainless steel material after hot forging is less than 900[deg.]C. A flange using the duplex stainless steel forging material according to claim 1 or 2. 6. A flange according to claim 5 for use in food and drug manufacturing equipment. A welded structure using the duplex stainless steel forging according to claim 1 or 2.