JPH0788556B2 - High yield strength and high corrosion resistance duplex stainless cast steel - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duplex stainless cast steel suitable as a structural material for a device member such as a rotor of a centrifuge, which requires high yield strength, high ductility and corrosion resistance. .

2. Description of the Related Art Conventional centrifuges having corrosion resistance to salt water and the like have been used in the food industry or oil / water separation devices for ships.

However, in recent years, to improve the performance of centrifuges,
Or, due to the increase in size of the device such as the use of a centrifuge for environmental applications such as soot and sludge separation, the conditions imposed on the material forming the rotating body are becoming more severe.
In particular, high specific strength (proof strength / material density) is an essential factor for centrifuges, and in recent years, 0.2% yield strength is 55 kgf.
Values above / mm ² are also required.

The materials for these centrifuges are conventional austenitic stainless steels such as SUS304 and SUS316, and ferrite phases such as SCS13, SCS14, and SCS16 considering the rotational balance, size, shape, and strength of the structure. 5 to 3
Duplex stainless steel containing 0% is often used. However, austenitic stainless steels such as SUS304 and SUS316 have inferior mechanical properties, and have the drawback that chloride stress corrosion cracking, pitting corrosion, or crevice corrosion are likely to occur, and SCS13 and SCS14 are not sufficient in the above properties. I can't say.

Furthermore, in recent years, various high chromium low nickel type duplex stainless steels having a ferrite phase increased to about 30 to 60% have been developed, and attempts have been made to improve strength and corrosion resistance.
In particular, duplex stainless steels containing Ni, Cr, Mo, which are the basic components of stainless steel, and Cu, N, W, Co, etc., which are effective in improving strength and corrosion resistance, are disclosed in JP-A-50-91516 and JP-B-59. -7347, JP-A-60-1653561 to 165362
No. 165363, 165364, and Japanese Patent Publication No. 60-33185.

However, the invention of the above publication does not contain Co, and the invention of the above publication and the invention of the above publication each contain W.
N and N are not added or are not essential components, and it is difficult to say that they are sufficient to obtain the high strength and high corrosion resistance targeted by the present invention. Further, the invention of the above-mentioned publication contains a large amount of P, and segregation of P occurs in the two-phase stainless cast steel of the present invention, which is often harmful.

Japanese Patent Application Laid-Open No. 59-107019 discloses a high-performance material that has excellent seawater resistance.
A method for producing a Cr low Ni-based duplex stainless cast steel product is disclosed. In the present invention, Cu is 0.3 when added together with Co.
%, The effect can be sufficiently exhibited, but Cu alone cannot achieve the desired effect unless it is 0.7% or more.

Further, JP-A-56-119721 discloses a solution heat treatment method for duplex stainless steel, and the major components such as Cr, Ni and Mo in the composition of the duplex stainless steel are specified. , C, Si, M as a component that is appropriately added as necessary
Although n, P, S, Cu, N, W, V, Ti and Nb are listed, it should be noted that the various components added as necessary and the main components are contained in any combination. Is not disclosed.

Means for Solving the Problems The present inventor succeeded in developing the high-strength, high-corrosion resistant duplex stainless cast steel of the present invention as a result of various studies and experiments for solving the above-mentioned various problems. The structural constitution is as specified in each claim, that is, it has a 0.2% proof stress of 55 kgf / mm ² or more at room temperature and an elongation of 25% or more, and has a stress corrosion cracking resistance in a chloride environment. sex,
The present invention provides a duplex stainless cast steel that is superior to conventional steel grades in pitting corrosion resistance and crevice corrosion resistance, and is also significantly superior to other conventional steel grades in general corrosion resistance.

The reasons for limiting the composition range of the duplex stainless cast steel of the present invention will be described below.

C: 0.1% or less: C acts as a strong austenite-forming element and has a significant effect as an interstitial solute element for improving strength, but if the content is too large, Cr carbides tend to precipitate, and ductility and In addition to impairing toughness, it causes deterioration of corrosion resistance. Therefore, the upper limit was made 0.1%.

Si: 0.2 to 2.0%: Si is necessary for deoxidation of molten steel and securing castability, and it also contributes to strengthening by solid-solutioning much in the ferrite phase. For this, at least 0.2% or more is necessary. However, if it exceeds 2.0%, the impact properties are deteriorated, and the σ phase also tends to precipitate, so the composition range was made 0.2 to 2.0%.

Mn: 0.2-2.0%: Like Si, Mn is contained in steel in an amount of about 0.2% or more due to the deoxidizing action of molten steel, and acts as an austenite forming element. However, if added in a large amount, the amount of austenite increases and it does not contribute to strengthening. Therefore, the composition range is 0.
It was set to 2 to 2.0%. Cr: 20.0 to 30.0%: Cr is a basic component for maintaining corrosion resistance. If the addition amount is less than 20.0%, sufficient corrosion resistance cannot be obtained and the optimum ferrite amount necessary for the stainless cast steel of the present invention cannot be obtained, and if it exceeds 30.0%, the ferrite amount becomes excessive and the tensile strength, ductility, and toughness decrease. Cause Therefore, the composition range is set to 20.0% to 30.0%.

Ni: 3.0 to 10.0%: Ni is an element that has a significant effect on the corrosion resistance to non-oxidizing raw acids,
Acts as an austenite forming element. The amount of addition thereof is required to be at least 3.0% or more in order to obtain the optimum amount of ferrite in consideration of the balance with the above required amount of Cr. But 10.0
If added in excess of%, the amount of austenite becomes excessive and the yield strength decreases. Therefore, the upper limit was made 10.0%.

Mo: 1.0 to 5.0%: Mo is an element that is particularly effective in improving pitting corrosion resistance and crevice corrosion resistance, and at least 1.0% for this purpose.
Is necessary. Further, even if added in excess of 5.0%, the effect of economically matching is not obtained, but rather the toughness is lowered. Therefore, the range is set to 1.0 to 5.0%.

Cu: 0.3-3.0%: Cu concentrates in the austenite phase as an austenite forming element and has the effect of improving tensile strength, while at the same time Ni
As well as improving the corrosion resistance to non-oxidizing acids. But,
If it is less than 0.7%, the effect is not remarkable, and if it is added in excess of 3.0%, segregation occurs during solidification by casting, and hot cracking easily occurs. Note that Co, which will be described later, has the same effect as Cu, and when Co is added together, Cu shows a sufficient effect even at 0.3%. Therefore, Cu was 0.7 to 3% when no Co was added, and 0.3 to 3.0% when Co was added.

Ni: 0.1-0.3%: N is a strong austenite-forming element, similar to C,
The yield strength and tensile strength are remarkably improved. It also has the effect of improving pitting corrosion resistance and crevice corrosion resistance. N is contained in about 0.05% even in a general steelmaking process, and at least 0.1% or more is necessary in order to bring out the above effects more than conventional steel types. However, addition of more than 0.3% causes excessive austenite and lowers toughness. Therefore, the upper limit was set to 0.3%.

W: 0.5 to 2.0%: W is an element that characterizes the present invention most. That is, W is a ferrite-forming element, and as a substitutional solute element is mainly concentrated in the ferrite phase to strengthen the ferrite phase, thereby improving the yield strength, which is the aim of the duplex stainless cast steel of the present invention. Bring an increase. See also W
Is a metal with a noble potential in itself, and also has an effect on corrosion resistance. The effect on the proof stress is particularly remarkable when 0.5% or more is added, so the lower limit was set to 0.5%. However, even if added over 2.0%, the effect which is economically commensurate cannot be obtained. Therefore, the upper limit was set to 2.0%.

Co: 0.3-2.0%: Co also has the same effect as W and Cu. However, Co is an austenite forming element, which is opposite to W, and is concentrated in the austenite phase to contribute to the improvement of tensile strength, and at the same time, it is effective in improving the yield strength. Its effect on tensile strength and proof stress is remarkable at 0.3% or more, while on the other hand, adding more than 2.0% does not have the effect of economically matching, so the addition range is 0.3-2.0%. did.

On the other hand, various conditions for heat treatment of the cast steel of the present invention are preferably as follows.

Most of the alloys standardized by JIS, ASTM, ACI, and other known alloys are solution-annealed in the temperature range of 1000 to 1200 ° C. In particular, the temperature of stainless steel added with several alloying elements in addition to Cr and Ni, such as Mo-containing system, is high, and it is common to perform solution treatment in the range of 1100 to 1200 ° C.

In this case, no problem occurs in the austenitic stainless cast steel and the duplex stainless cast steel having the ferrite content of 40% or less. However, like the cast steel of the present invention, a duplex stainless steel having a ferrite content of 40% or more, for example, SUS329J1 and SCS11 in JIS, and especially a duplex stainless steel in which N is added to it, for example, SUS329J2L and SCS10 have a total of about 1100.
It was found that the ductility and toughness of the material are significantly impaired when the solution is quenched from a solutionizing temperature of ℃ or more. In addition, it was found from the subsequent research that the above-mentioned reduction in ductility and toughness is a behavior common to the high temperature embrittlement observed in high Cr ferrite type stainless steels.

And at least 1100-12 to avoid this embrittlement
After solution annealing at a temperature of 00 ° C, it is gradually cooled to a temperature range that does not cause quenching and embrittlement by furnace cooling, that is, 1000 to 1080 ° C, and precipitation of carbides and σ phase is avoided from this temperature range. Therefore, a heat treatment method of quenching to room temperature is effective. The same effect can also be obtained by a heat treatment method in which solution annealing is performed at 1100 to 1200 ° C, followed by rapid cooling to room temperature, heating to 1000 to 1080 ° C again, and cooling to room temperature again.

The heat treatment conditions according to the present invention are as described above.
Solution temperature of ~ 1200 ℃, especially Mo and Cu such as the present invention steel
Σ phase present in the as-cast structure of the cast steel containing, and carbides, etc., is the temperature range required to completely form a solid solution, while the temperature range of 1000 to 1080 ° C causes rapid embrittlement. It is a temperature range that is difficult and does not cause carbides and the like to be deposited due to the soaking. By carrying out the heat treatment as described above, the cast steel of the present invention has properties having both optimum strength and toughness.

Examples Hereinafter, the present invention will be described in detail with reference to Examples.

Stainless cast steels having the respective component compositions shown in Table 1 were melted and cast, and their mechanical properties and corrosion resistance were investigated in relation to heat treatment conditions. Table 2 shows 1150 for each sample steel
It shows the mechanical properties when solution annealing is carried out at 0 ° C for 2 hours. Steel Nos. 3 to 11 and 17 are the cast steels of the present invention,
Steel Nos. 1 and 2, and 12-16 are comparative cast steels. Among the comparative examples, Steel No. 1 is a cast steel corresponding to ASTM A240 S 32550. Steel No. 2 is cast steel equivalent to JIS G5121 SCS11 and steel No. 12
JIS G5121 SCS10, DIN WNr.1.4463, J
Cast steel equivalent to IS G5121 SCS14A, SCS16, and Japan Stainless Steel Association standard SAS329LJ2.

Steel No. 18 is a material whose W and Co contents are in the range specified by the present invention or more.

In Table 2, steel Nos. 3 to 11, which are the cast steels of the present invention, have mechanical properties, especially 0.2% proof stress and tensile strength.
And 16 (the composition of components other than W and Co and the amount of ferrite are within the range specified in the present invention) and Steel No. 2
(W, Co, and N are beneficial additions, and the other component compositions are within the range defined by the present invention, but the amount of ferrite is outside the range defined by the present invention.) Further, in comparison with Steel Nos. 14 and 15 of Comparative Examples (the amount of ferrite is significantly lower than the range specified by the present invention, and W, Co, N are not added, and the component composition deviates from the specified range of the present invention), The cast steel of the present invention
It can be seen that the 0.2% proof stress reaches more than double these values.

From the above results, the object of the present invention is 55 kgf / mm ² or more 0.2
In order to obtain the% yield strength, it is essential that at least 40% or more of the ferrite phase is contained in the metallographic structure and N is added, and from the comparison of steel numbers 3 to 12 and 16,17, It can be seen that the 0.2 yield strength is further increased when W and Co are added. The strengthening effect of W and Co is
When N is 0.17% and the amount of ferrite is about 50%,
W is about 2 kgf / mm ² per 1% of the yield strength increasing effect, while Co is 1
A yield strength increasing effect of% per about 1.2 kgf / mm ^2, it is presumed to have the effect of increasing the tensile strength of about 1.5 kgf / mm ^2. However, as can be seen from the comparison between steel No. 9 and steel No. 18, W and Co
Even if 2% or more is added, the effect of increasing the yield strength is not so remarkable and tends to be saturated. Therefore, it is appropriate that the amounts of W and Co added be within the range specified in the present invention.

Table 3 shows examples of mechanical properties of each sample steel after solution annealing at 1150 ° C for 2 hours, furnace cooling to 1050 ° C, holding for 2 hours and then water cooling. . As is clear from the comparison between this result and the results in Table 2, by subjecting the cast steel of the present invention to the above-mentioned heat treatment results, materials such as steel Nos. 3 and 5, 7 having a ferrite content of more than 50% are obtained. -13 and the like, the elongation at break and the shear energy absorbed are remarkably improved regardless of the cast steel of the present invention and the comparative cast steel. In particular, the cast steel of the present invention has a target of 0.2% proof stress of 55 kgf / mm ² or more and elongation of 25% or more, and also has a Charpy absorbed energy of 130 J or more, and excellent mechanical properties due to such heat treatment. It can be seen that different materials can be obtained.

1 and 2 show the material of steel No. 10 which is the cast steel of the present invention, and after solution annealing at 1150 ° C. for 2 hours, respectively, 1050 ° C.
Of the effect of holding time at 1050 ℃ on yield strength and toughness when furnace-cooled at 1150 ℃, and after solution annealing at 1150 ℃ for 2 hours, quenching once to room temperature, then reheating to 1050 ℃
It shows the effect of holding time on the toughness and proof stress when the temperature is raised and held. As is clear from these figures, a heat treatment method of solution annealing in a temperature range of 1100 to 1200 ° C, followed by furnace cooling, holding in a temperature range of 1000 to 1080 ° C and rapid cooling, and a temperature range of 1100 to 1200 ° C. It can be seen that the two types of heat treatment methods of solution annealing, quenching once to room temperature once, quenching again to a temperature range of 1000 to 1080 ° C. and quenching again have almost the same effect in improving toughness. In addition, in each heat treatment method, 10 to improve toughness
Looking at the effect of the holding time in the temperature range of 100 to 1080 ° C., it is understood that the effect is sufficiently exhibited in about 1 hour in any heat treatment method. Therefore, the holding time should be appropriately set in consideration of the size and shape of the product and the soaking condition of the heat treatment furnace.

On the other hand, FIGS.
After solution annealing at 2 ℃ for 2 hours, quenching at various temperatures of 1000 to 1120 ℃ for 2 hours at uniform heating, and after solution annealing at 1150 ℃ for 2 hours. In the two types of heat treatment methods of quenching to room temperature, raising the temperature to various temperatures of 1000 to 1120 ° C again, holding the soaking for 2 hours, and quenching again, the purpose was to improve ductility and toughness. This is an examination of the influence of the holding temperature for the third time. After solution annealing at 1150 ℃ for 2 hours, elongation is not achieved by simply quenching.
It is about 14%, but if the soaking temperature after furnace cooling or the reheating after water cooling and the soaking temperature are in the range of 1000 to 1080 ° C, the elongation is 34%.
It turns out that it will be improved to%. However, this temperature is 1080 ℃
Above the above, the growth tends to decrease again.

Each sample steel was subjected to solution annealing at 1150 ° C for 2 hours and then simply quenched for corrosion resistance under various conditions.
In the table, meanwhile, after solution annealing at 1150 ℃ for 2 hours, 1050 ℃
No.5 for the corrosion resistance when the furnace is cooled down to
Shown in the table. The cast steels of the present invention, Steel Nos. 3 to 11 and 17, are remarkably superior in various corrosion resistances to Steel Nos. 2 and 12 to 16 of Comparative Examples. Also, the effect of heat treatment means
From the comparison of the results shown in Tables and Table 5, it can be seen that the heat treatment of the cast steel of the present invention also has an effect of improving the corrosion resistance.

FIG. 5 is a graph showing a comparison of chloride stress corrosion cracking resistance in 42% magnesium chloride boiling solution according to JIS G0576.

For example, the steel numbers 1 and 2 of the comparative example and the cast steel of the present invention 3
8 to 8, it is clear that the steel of the present invention, which has improved yield strength with the addition of W and Co, and has a ferrite phase of 40 to 60%, exhibits superior properties to these, and
When compared with steel numbers 14 and 15 in which the amount of ferrite is less than the requirements of the present invention, it is understood that the cast steel of the present invention has a critical stress of twice or more.

EFFECTS OF THE INVENTION According to the present invention, a duplex stainless cast steel having mechanical properties, particularly 0.2% proof stress and corrosion resistance, which are remarkably superior to conventional casting grades, and which is suitable as a centrifugal separator material and a seawater resistant material is provided. be able to.

[Brief description of drawings]

FIG. 1 shows the relationship between the holding time and toughness and proof stress when steel No. 10 of the present invention was solution annealed at 1150 ° C., then held in a furnace for cooling at 1050 ° C. for several hours and then rapidly cooled. Fig. 2 shows invention example steel No. 10
Solution annealing at 1150 ℃, quench once to room temperature,
Relationship between holding time and toughness and proof stress when the temperature was raised to 50 ° C, held for several hours, and then rapidly cooled again. 3 and 4 are 11
Relationship between furnace cooling temperature and elongation when solution annealed at 50 ° C, furnace cooled to various temperatures, soaked and then rapidly cooled. FIG. 5 is a graph showing a comparison of the chloride stress corrosion cracking characteristics of the cast steel of the present invention and the comparative cast steel in accordance with JIS G0576 in a 42% magnesium chloride solution.

Claims (2)

[Claims] 1. By weight%, C: 0.1% or less, Si: 0.2 to 2.0%,
Mn: 0.2-2.0%, Cr: 20.0-30.0%, Ni: 3.0- 10.0%, M
o: 1.0-5.0%, Cu0.7-3.0%, N: 0.1-0.3%, W: 0.5-
A high-strength, high-corrosion-resistant duplex cast stainless steel characterized in that it contains 2.0%, the balance consists essentially of Fe, and the area ratio of the ferrite phase in the metal structure is 40 to 70%. 2. In% by weight, C: 0.1% or less, Si: 0.2 to 2.0%,
Mn: 0.2-2.0%, Cr: 20.0-30.0%, Ni: 3.0- 10.0%, M
o: 1.0-5.0%, Cu: 0.3-3.0%, N: 0.1-0.3%, W: 0.5-
2.0%, Co: 0.3-2.0%, the balance consisting essentially of Fe, and the area ratio of the ferrite phase in the metal structure is 40-
High strength / corrosion resistant duplex stainless cast steel characterized by 70%.