CN111321322A

CN111321322A - Ni-Cr-Nb-Fe alloy having excellent internal quality and hot workability, and method for producing same

Info

Publication number: CN111321322A
Application number: CN201911281795.7A
Authority: CN
Inventors: 轰秀和
Original assignee: Nippon Yakin Kogyo Co Ltd
Current assignee: Nippon Yakin Kogyo Co Ltd
Priority date: 2018-12-14
Filing date: 2019-12-13
Publication date: 2020-06-23
Also published as: JP6526307B1; JP2020094251A

Abstract

A sound Ni-Cr-Nb-Fe alloy having excellent hot workability and preventing the adverse effect of large-sized nonmetallic inclusions on the internal quality is provided by properly controlling the slag composition with respect to the minor components of Al, Mg, Ca, O, etc. The alloy consists of C: 0.005-0.04%, Si: 0.05-0.5%, Mn: 0.05-0.5%, P: 0.02% or less, S: 0.0015% or less, Cr: 14-17%, Nb: 2-4%, Fe: 6-8% of Al: 0.01-0.2%, Mg: 0.003 to 0.03%, Ca: 0.0001-0.005%, B: 0.0005 to 0.01%, N: 0.005-0.02%, O: 0.0001 to 0.005% and Mo: 0.02% or more, optionallyThe balance of Ni and unavoidable impurities, and the total content of Mo, Cu and Ti is 0.5% or less. The manufacturing method comprises the following steps: melting raw materials in an electric furnace, decarbonizing in VOD, adding lime, fluorite and Al, and using a mixture of CaO: 40 to 70% of SiO₂: 10% or less, MgO: 3 to 35% of Al₂O₃: 5-30%, F: 5 to 15% of CaO-Al₂O₃‑MgO‑SiO₂And (3) carrying out Cr reduction, deoxidation and desulfurization while stirring with Ar, adding a Nb source to adjust the components, making a steel ingot by conventional ingot casting, forging the steel ingot, and carrying out hot rolling.

Description

Ni-Cr-Nb-Fe alloy having excellent internal quality and hot workability, and method for producing same

Technical Field

The present invention relates to a Ni-Cr-Nb-Fe alloy having excellent intergranular corrosion resistance and stress corrosion cracking resistance, and more particularly to a Ni-Cr-Nb-Fe alloy having improved internal quality and hot workability.

Background

The Ni-Cr-Nb-Fe alloy has excellent corrosion resistance and heat resistance. Therefore, it is a material used in an extremely severe environment. Due to such characteristics, the material in the pressure vessel, which is an atomic energy material, may be used in a particularly severe environment.

Since the amount of solid solution of C is small, the Ni-based alloy is originally likely to cause intergranular sensitization and intergranular stress corrosion cracking in high-temperature water. To overcome this, a technique of adding Nb while controlling C to an appropriate content has been developed. This successfully improves the intergranular corrosion resistance while maintaining the mechanical properties within an appropriate range (see, for example, patent documents 1 to 4).

The Ni — Cr — Nb — Fe alloy thus developed has excellent corrosion resistance particularly in high-temperature water, and is used in large amounts in severe environments. However, there is still another problem in hot workability, and techniques have been developed to reduce the S concentration and the oxygen concentration and to reduce the factors that deteriorate workability. In addition, techniques for adjusting the grain size, improving workability, and preventing surface cracking have also been developed. Further, techniques have been developed to optimize the heating temperature and the number of passes during hot rolling and improve the workability (see, for example, patent documents 5 to 9).

As described above, it is known from the development of Ni — Cr-Nb-Fe-based alloys having excellent corrosion resistance that all quality improvements have not been achieved, although development for improving hot workability has been made. That is, the alloy is deoxidized and desulfurized by slag refining, and various components such as Si and Mn are controlled, and in this process, it is necessary to control trace components such as Al, Mg, Ca and O. In some cases, these elements cause not only the inhibition of hot workability but also the aggregation and coarsening of the nonmetallic inclusions, which adversely affects the internal quality.

As a method for refining Ni-based alloys, a technique for improving hot workability by controlling a small amount of Mg, Ca, or the like is disclosed (see, for example, patent document 10).

However, this technique is difficult to apply to Ni — Cr — Nb — Fe alloys containing 2% or more of Nb, and further development is expected. Further, Nb has an oxidizing power equivalent to that of Si, and therefore, depending on the deoxidation state, Nb is transferred to slag as an Nb oxide, which causes a problem that it is difficult to effectively use expensive Nb.

Patent document 1: japanese laid-open patent publication No. 59-56555

Patent document 2: japanese laid-open patent publication No. 59-56556

Patent document 3: japanese laid-open patent publication No. 59-56557

Patent document 4: japanese laid-open patent publication No. 62-44546

Patent document 5: japanese patent No. 4683712

Patent document 6: japanese patent No. 4993328

Patent document 7: japanese patent No. 4993327

Patent document 8: japanese patent No. 4615120

Patent document 9: japanese patent No. 4414588

Patent document 10: japanese patent laid-open No. 2009-114544.

Disclosure of Invention

In view of the above problems, an object of the present invention is to provide a robust Ni — Cr — Nb-Fe alloy having excellent hot workability and preventing adverse effects of large-sized nonmetallic inclusions on the internal quality by controlling the slag composition to an appropriate composition with respect to minor components of the Ni — Cr — Nb-Fe alloy, particularly minor components such as Al, Mg, Ca, and O. Further, the present invention provides a manufacturing method for realizing the same.

The inventors have conducted intensive studies to solve the above problems. That is, the quality of the thick plate manufactured by the actual machine was compared with the chemical composition, and the product having a problem was subjected to internal observation and measurement. The present invention has been completed by intensive studies on a thick plate manufactured by an actual machine. The analysis process is described below.

First, a method for producing a steel ingot will be described. Melting raw materials in a 60t electric furnace, decarbonizing in VOD, and addingLime, fluorite, Al to form CaO-Al₂O₃-MgO-SiO₂-F-based slag. At the same time, Cr reduction, which is an operation to reduce Cr oxides transferred to the slag, is performed. Then, deoxidation and desulfurization are performed while stirring with Ar, and a Nb source such as pure Nb or Ni — Nb is added to adjust the composition. Finally, casting is performed through a conventional ingot, thereby manufacturing a steel ingot. Subsequently, the steel slab is forged, followed by hot rolling, thereby manufacturing a thick plate.

Regarding hot workability of the thick plate, the presence or absence of surface cracking was judged from the appearance, and the internal quality was examined by UT test (ultrasonic flaw detection test) to obtain the results. Further, these results were compared with the chemical components and slag components, and the optimum conditions were determined.

Namely, the alloy of the present invention is as follows.

The chemical composition of the Ni-Cr-Nb-Fe alloy is required to contain, in mass%: c as an essential component: 0.005-0.04%, Si: 0.05-0.5%, Mn: 0.05-0.5%, P: 0.02% or less, S: 0.0015% or less, Cr: 14-17%, Nb: 2-4%, Fe: 6-8% of Al: 0.01-0.2%, Mg: 0.003 to 0.03%, Ca: 0.0001-0.005%, B: 0.0005 to 0.01%, N: 0.005-0.02%, O: 0.0001-0.005%, and the balance of Ni and unavoidable impurities.

The alloy of the present invention contains 0.02% or more of Mo as an essential component, and 0.5% or less of Cu and Ti in total as optional components.

Further, as for the method for producing the alloy of the present invention, the following method is applied: the raw materials were melted in an electric furnace, then decarbonized in VOD, and then lime, fluorite, and Al were added to the mixture, using a mixture of CaO: 40 to 70% of SiO₂: 10% or less, MgO: 3 to 35% of Al₂O₃: 5-30%, F: 5 to 15% of CaO-Al₂O₃-MgO-SiO₂the-F-based slag is subjected to Cr reduction, deoxidation, and desulfurization while stirring with Ar, a Nb source is added, the composition is adjusted, and then, a steel ingot is produced by casting in a conventional ingot, and the steel ingot is subsequently forged and subsequently hot rolled.

Detailed Description

First, the reason for limiting the chemical composition of the stainless steel sheet of the present invention will be described. In the following description, "%" represents "% mass" ("mass%").

C：0.005~0.04%

C is added for the purpose of improving the strength of the alloy. However, if the content is large, Nb bonds to form carbide, and the content of Nb must be added so as to exceed 4% which is the upper limit of the range of the present invention, which may deteriorate hot workability. Therefore, the C content is set to 0.005-0.04%. The content is preferably 0.01 to 0.025%, more preferably 0.01 to 0.02%.

Si：0.05~0.5%

Si is effective as a deoxidizing element. However, if the Si concentration is too high, the intergranular corrosion resistance is lowered. Although Si is an effective element for deoxidation, since it contains Nb having an oxidizing power comparable to that of Si, in the present invention, as will be described later, deoxidation is effectively performed using Al having a higher oxidizing power than Si. Therefore, the Si content is set to 0.05 to 0.5%. Preferably 0.1 to 0.4%, more preferably 0.12 to 0.3%.

Mn：0.05~0.5%

Mn is an element effective for deoxidation as Si. On the contrary, if it exceeds 0.5%, the intergranular corrosion resistance is lowered. Therefore, the Mn content is set to 0.05 to 0.5%. Preferably 0.1 to 0.3%. More preferably 0.12 to 0.2%.

P: less than 0.02%

P is an element that deteriorates intergranular corrosion resistance and weldability. Therefore, the content is defined to be 0.02% or less. Preferably 0.015% or less, more preferably 0.01% or less.

S: less than 0.0015%

S is a harmful element because it reduces hot workability. Therefore, the S content is defined to be 0.0015% or less. Preferably 0.001% or less, more preferably 0.0005% or less.

Cr：14~17%

Cr is an important element because it secures corrosion resistance such as acid resistance, stress corrosion cracking resistance, interstitial corrosion resistance, and pitting corrosion resistance, and therefore needs to be contained in 14% or more. However, if the Cr content is too high, the generation of the σ phase is promoted, and the base material is embrittled. Therefore, the Cr content is set to 14 to 17%. Preferably 15 to 17%, more preferably 16 to 16.8%.

Nb：2~4%

Nb is a very important element because it improves resistance to intergranular corrosion. Therefore, it is required to contain 2% at minimum. However, if it is too high, hot workability may be deteriorated. Therefore, it is specified to be 2 to 4%. The content is preferably 2 to 3.5%, more preferably 2.2 to 3%.

Fe：6~8%

Fe has an effect of improving toughness. Further, the alloy element is inexpensive, and if it is contained, the cost can be reduced. However, if it is too high, the corrosion resistance is lowered. Therefore, the content is regulated to 6 to 8%. The content is preferably 6.1 to 7%, and more preferably 6.2 to 6.9%.

Al：0.01~0.2%

Al is an extremely important element in the invention of the present application. Al is an element very effective for deoxidation, and the oxygen concentration is reduced by carrying out deoxidation mainly with Al, and the purity can be improved, so that 0.01% or more is required. However, if the amount exceeds 0.2%, the quality of the weld portion is deteriorated in the welding application. Further, CaO-Al is reduced by the reactions of the following formulae (1) and (2)₂O₃-MgO-SiO₂CaO and MgO in the F-based slag, whereby 0.0001% or more of Ca and 0.003% or more of Mg are supplied to the molten alloy. Based on this, 0.01% or more of Al is also added.

3(MgO)+2Al=(Al₂O₃)+3Mg…(1)

3(CaO)+2Al=(Al₂O₃)+3Ca…(2)

Here, the parenthesis indicates the components in the slag, and the lower line indicates the components in the molten alloy.

Here, it should be noted that excessive Ca content promotes the generation of CaO inclusions which adversely affect the internal quality. That is, CaO inclusions are harmful inclusions which cause defects in the thick plate because they are easily aggregated and enlarged. In addition, the excessive Mg content makes the steel ingotIn which Mg bubbles are formed, and, in addition, due to Ni₂The formation of low-melting intermetallic compounds such as Mg deteriorates hot workability, and also deteriorates the internal quality of the thick plate. The excess supply of Ca and Mg (Ca)>0.005%、Mg>0.03%) was caused by excessive addition of Al. Therefore, it is necessary to suppress the Al concentration to 0.2%.

Al also plays an important role in stably adding Nb to the molten alloy. That is, Nb is oxidized without decreasing it and transferred to slag, and the utilization rate is determined by the following reaction equilibrium relationship.

3(NbO)+2Al=(Al₂O₃)+3Nb…(3)

If Al is less than 0.01% and too little, the oxygen potential becomes high and NbO is transferred to the slag. In some cases, the Nb concentration is less than 2%. Conversely, if Al exceeds 0.2% and is too high, Nb added according to the set utilization rate shows a high utilization rate, exceeding 4%. Based on this, it is also extremely important to control the Al concentration. Therefore, the Al content is set to 0.01 to 0.2%. Preferably 0.03 to 0.15%. More preferably 0.04 to 0.1%.

Mg：0.003~0.03%

Mg is an extremely important element in the invention of the present application. Mg has an effect of fixing S and is an element for improving hot workability. Further, the present invention is effective for controlling the composition of the non-metallic inclusions in the molten alloy to be MgO inclusions which do not adversely affect the internal quality. This is because MgO has a property of not being agglomerated and becoming large in size. The effect cannot be obtained when the content is less than 0.003%.

On the other hand, if it exceeds 0.03%, Ni is formed₂An intermetallic compound having a low melting point such as Mg deteriorates hot workability. Further, the final product is internally defective due to the formation of Mg bubbles in the steel ingot. Therefore, the Mg content is set to 0.003 to 0.03%. Preferably 0.005 to 0.025%. More preferably 0.008 to 0.02%.

In order to efficiently add Mg to the molten alloy, the reaction of the above formula (1) is preferably used. Namely, the Al content is 0.01-0.2%, and the composition of the slag is controlled to be CaO: 40 to 70% of SiO₂: 10% or less, MgO:3~35%、Al₂O₃: 5-30%, F: 5 to 15%, and thus can be controlled to fall within the scope of the present invention. In addition, at low times, it may be supplemented with an Mg source such as NiMg.

Ca：0.0001~0.005%

Ca is an extremely important element in the invention of the present application. Since S is fixed, Ca is an element for improving hot workability. However, if the content is too high, nonmetallic inclusions of CaO alone are formed, and they are agglomerated to increase the size, thereby causing internal defects in the thick plate product. Therefore, the Ca content is set to 0.0001 to 0.005%. Preferably 0.0002 to 0.003%. More preferably 0.0003 to 0.002%.

In order to efficiently add Ca to the molten alloy, it is preferable to supply Ca by the reaction of the above formula (2). Namely, the Al content is 0.01-0.2%, and the composition of the slag is controlled to be CaO: 40 to 70% of SiO₂: 10% or less, MgO: 3 to 35% of Al₂O₃: 5-30%, F: 5 to 15%, and thus can be controlled to fall within the scope of the present invention.

B：0.0005~0.01%

B is an element for improving hot workability. However, the high concentration content rather shifts the expression of the high temperature strength to the low temperature side, which is the opposite effect. Therefore, the B content is set to 0.0005 to 0.01%. In order to control B to such an appropriate range, in addition to positively adding the B compound, SiO may be used as shown in the following formula (4)₂The amount is controlled. (4) Left of formula if SiO₂In a large amount, B is SiO₂Oxidation and reduction of the amount of B.

3(SiO₂)+4B=2(B₂O₃)+3Si…(4)

Further, as shown in the formula (5), the amount of Al can be controlled, and if the amount of Al is large, B is controlled₂O₃Reduced by Al and increased B content.

(B₂O₃)+2Al=(Al₂O₃)+2B…(5)

The B content is preferably 0.0008 to 0.005%, more preferably 0.001 to 0.003%.

N：0.005~0.02%

N is an indispensable element in the present invention. Not only can high strength be maintained but also intergranular corrosion resistance and stress corrosion cracking resistance are improved, and therefore, it is effective. However, if N is too high, the solid solubility limit of the alloy is exceeded, and pores are formed. Therefore, N is set to 0.005 to 0.02%. Preferably 0.007 to 0.018%, and more preferably 0.008 to 0.016%.

O：0.0001~0.005%

O is an important element in the present invention as well as Al. If the amount of the catalyst is more than 0.005% by weight in the molten alloy, the reactions (1), (2) and (3) do not proceed to the right. That is, the supply of Mg and Ca is hindered, and the oxidation loss and the yield of Nb are reduced. On the other hand, if less than 0.0001%, the oxygen potential is too low, and the reactions (1), (2), and (3) proceed to the right excessively. That is, the supply of Mg and Ca is excessive and exceeds the scope of the present invention. Further, although the utilization rate of Nb is high, this is preferable, but it may exceed 4% at a higher utilization rate than the set utilization rate.

Therefore, the O content is set to 0.0001 to 0.005%. To achieve this range, the Al content is set to 0.01 to 0.2%, and the slag composition is controlled to be CaO: 40 to 70% of SiO₂: 10% or less, MgO: 3 to 35% of Al₂O₃: 5-30%, F: 5 to 15%, and thus can be controlled to fall within the scope of the present invention. The preferable range of the O content is 0.0002 to 0.003%, and more preferably 0.0003 to 0.002%.

The above is an essential constitution of the alloy of the present invention, and in addition, any 1 or 2 or more of Mo, Cu and Ti may be contained in a total amount of 0.5% or less. These elements are mixed in due to the use of inexpensive scrap. If the total content of the inclusions is suppressed to 0.5% or less, the corrosion resistance, hot workability, and inclusion composition are not affected. Therefore, the total content of 1 or 2 or more of Mo, Cu and Ti may be 0.5% or less.

Next, a method for producing the alloy of the present invention will be described.

In the step of melting the raw materials in an electric furnace, the raw materials such as scrap of Ni-based alloy, Ni, FeCr, and the like are controlled to the target compositions. After melting, the molten alloy is collected into a ladle lined with any one of magnesia carbon, magnesia chrome and dolomite bricks, and placed in vod (vacuum Oxygen decarburization) after deslagging.

In VOD, the mixture is first pumped to a reduced pressure and oxygen is blown by a top lance to conduct decarburization. After decarburization, lime, fluorite and Al are added to form CaO-Al₂O₃-MgO-SiO₂-F-based slag. At the same time, Cr reduction, which is an operation of reducing oxidized Cr, is performed while stirring with Ar. At the same time, deoxidation and desulfurization are carried out. In the present invention, AOD (argon oxygen decarbonylation) may also be used. Further, a combination of AOD and VOD refining may be used.

Next, an Nb source is added, the composition is adjusted, and then, casting is performed by a conventional ingot to produce a steel ingot, and then the steel ingot is forged, followed by hot rolling.

Important in this manufacturing step are: al is controlled to be Al within the scope of the invention of the present application: 0.01 to 0.2%, and the composition of the slag is controlled to the following composition. That is, the amount of CaO: 40 to 70% of SiO₂: 10% or less, MgO: 3 to 35% of Al₂O₃: 5-30%, F: 5 to 15% of CaO-Al₂O₃-MgO-SiO₂-F-based slag. The reason for this will be described.

CaO：40~70%

CaO is formed by adding limestone. If the content is low, Al is not used and deoxidation is impossible. Conversely, if it is high, Al tends to be high, resulting in excessive supply of Mg and Ca. Therefore, the content of the organic solvent is 40 to 70%. Preferably 50 to 70%.

SiO₂: less than 10%

SiO₂It increases the oxygen potential and is therefore harmful. That is, since a reaction between Al and the following formula (6) is initiated, Al in the molten alloy is reduced.

3(SiO₂)+4Al=2(Al₂O₃)+3Si…(6)

That is, the Al concentration is reduced to less than 0.01%, which hinders the supply of Ca and Mg. Further, oxidation loss of Nb also occurs. Therefore, SiO is specified₂The concentration is below 10%. Preferably 8% or less, more preferably 6% or less.

MgO：3~35%

Is an essential component for controlling the Mg concentration in the molten alloy within the scope of the present invention. The MgO is preferably supplied by adding waste bricks containing MgO to the slag. Therefore, MgO is set to 3 to 35%. Preferably 5 to 20%.

Al₂O₃：5~30%

It is essential to control the Al concentration in the molten alloy to 0.01 to 0.2% in the range of the present invention. Therefore, the content of the organic solvent is 5 to 30%.

F：5~15%

F is essential to maintain the molten slag and to ensure fluidity. This is achieved by adding fluorite (CaF)₂) To ensure. More than 5% is necessary, and if it exceeds 15%, the fluidity is too good, and the refractory is melted and damaged. Therefore, the F concentration is set to 5 to 15%.

In the present invention, Cr is used as a slag component, although not particularly limited₂O₃The concentrations of FeO, NbO and the like are preferably controlled as follows.

Cr₂O₃: 2% or less

Cr₂O₃Is an element that causes the following reaction and reduces the utilization rate of Al.

(Cr₂O₃)+2Al=(Al₂O₃)+2Cr…(7)

Therefore, 2% or less is preferable. More preferably 1% or less.

FeO: 2% or less

FeO is an element that causes the following reaction to reduce the utilization rate of Al.

3(FeO)+2Al=(Al₂O₃)+3Fe…(8)

Therefore, 2% or less is preferable. More preferably 1% or less.

NbO: less than 3%

According to the formula (3), the utilization rate of Al is reduced. Therefore, 3% or less is preferable. It is more preferably 2% or less.

Examples

Hereinafter, the present invention will be described more specifically by way of examples and comparative examples. First, the raw materials were melted in a 60-ton electric furnace. The raw material used is Ni-based alloy scrap, Ni, FeCr, or the like. After melting, the molten alloy is stored in a ladle, and placed in vod (vacuum Oxygen decarburization) after deslagging.

In VOD, the pressure is first reduced and decarburization is carried out by blowing oxygen from a top lance. After decarburization, lime, fluorite and Al are added to form CaO-Al₂O₃-MgO-SiO₂-F-based slag. At the same time, stirring with Ar is performed, and Cr reduction, which is an operation for reducing oxidized Cr, is performed. At the same time, deoxidation and desulfurization are performed, and then a Nb source such as pure Nb or NiNb is added to adjust the composition. Then, a steel ingot is produced by casting through a conventional ingot casting, and then the steel ingot is forged to produce a slab. Finally, hot rolling was carried out to produce a 80mmt thick plate.

The evaluation methods of the respective items are as follows:

(1) the chemical composition and slag composition of the alloy are as follows: quantitative analysis was performed using a fluorescent X-ray analyzer, and the oxygen and nitrogen concentrations of the alloy were quantitatively analyzed by an inert gas pulse melting infrared absorption method.

(2) Hot workability, namely, cracking after forging and cracking were evaluated as ×

(3) The internal quality of the thick plate was evaluated as × in the case of UT test (ultrasonic flaw detection test) and in the case of no test.

(4) Non-metallic inclusion composition: when the UT test indicated that the test was performed, the test piece was cut so as to include an internal defect, and observed and measured using SEM-EDS after polishing.

(5) Overall rating one × was also rated ×.

Examples are shown in table 1. The total amount of the slag compositions in the table is not 100%, and is 99.4 to 99.9%. This is because the composition contains trace elements, oxides of S, P, Ni, B, and the like in addition to the components shown in the table. In addition, "-" is indicated below the analytical limit, and parentheses indicating numerical values are indicated outside the scope of the present invention.

Since the conditions of nos. 1 to 10 of the invention example and the reference example both satisfy the scope of the invention of the present application, the hot workability is also excellent and no problem is found in the internal quality.

The comparative examples are explained below.

As for No.11, CaO in the slag is low, SiO₂High, out of range, so Al is low, 0.008%. Therefore, the oxygen concentration is high and the Mg concentration is low, which are out of the range. As a result, cracking occurs after forging.

In No.12, SiO in slag₂Since the concentration is high and out of the range, Al is low at 0.002%. Therefore, deoxidation does not progress, the S concentration also increases, Ca is not supplied, and the hot workability is deteriorated. Therefore, cracking occurs after forging. Since F in the slag is also low, slag cannot be removed sufficiently.

In No.13, since the CaO concentration in the slag became high, Al was 0.268% high. Therefore, desulfurization proceeds smoothly and is reduced to below the detection limit. Therefore, the Mg and Ca concentrations become high, and the hot workability becomes poor. Therefore, cracking occurs after forging, with the generation of internal defects. Large aggregates of CaO inclusions were observed inside. It is noted that F in the slag is high and hence the ladle bricks are significantly melted down.

In No.14, the CaO concentration in the slag was high, so that Al became high and was 0.321%. Therefore, the deoxidation and the desulfurization proceed smoothly, and the amount is reduced to the detection limit or less. Therefore, the Mg concentration is high, and B is also high, so that the hot workability is deteriorated if not in the range. Therefore, cracking occurs after forging. The Ca concentration also becomes too high, resulting in internal defects. Large aggregates of CaO inclusions were observed inside. Note that, since F in the slag becomes high, the brick of the ladle is significantly melted down.

In No.15, the B concentration was high, the hot workability was poor, and cracking was observed after forging. N is also reduced and the strength is insufficient.

In No.16, the CaO concentration in the slag became high, the Al utilization rate became excessive, and the deoxidation and the desulfurization proceeded smoothly. However, on the contrary, Mg and Ca are supplied in excess. Therefore, cracking and internal defects after forging occur. Large aggregates of CaO inclusions were observed inside. Further, Nb was also excessively high to be 4.22%. It is noted that the ladle brick is significantly damaged due to the increased F in the slag.

In No.17, SiO in the slag₂、Cr₂O₃Since the FeO concentration is increased, Al is decreased, deoxidation and desulfurization are not progressed, and Si is high and out of the range. Therefore, Mg and Ca were not supplied, and B was also decreased, and therefore, cracking was observed after forging. In addition, Nb was also oxidized and transferred to slag, and the utilization rate was reduced to 1.92%, which was out of the range. Further, N also decreases, and the strength decreases.

In No.18, SiO in slag₂The concentration is high, the utilization rate of Al is reduced to 0.007%. As a result, deoxidation and desulfurization were not smoothly performed, the supply of Mg and Ca was reduced, and cracking occurred after forging.

In No.19, SiO remained in the slag₂High concentration, low Al utilization rate (as low as 0.002%). As a result, deoxidation and desulfurization were not smoothly performed, the supply of Mg and Ca was reduced, and cracking occurred after forging.

In No.20, SiO in slag₂、Cr₂O₃Since the concentration of FeO is increased, Al is decreased, and deoxidation and desulfurization are not progressed. Therefore, Mg and Ca were not supplied, and B was also decreased, so that cracking was observed after forging. In addition, Nb was also oxidized and transferred to slag, and the utilization rate was reduced to 1.58%, which was out of the range.

According to the present invention, a Ni-Cr-Nb-Fe alloy having excellent intergranular corrosion resistance can be provided at a low cost with a high yield. The material can be used in an extremely severe environment in a pressure vessel in the nuclear power generation industry.

Claims

An Ni-Cr-Nb-Fe alloy, characterized by comprising, in mass%: c as an essential component: 0.005-0.04%, Si: 0.05-0.5%, Mn: 0.05-0.5%, P: 0.02% or less, S: 0.0015% or less, Cr: 14-17%, Nb: 2-4%, Fe: 6-8% of Al: 0.01-0.2%, Mg: 0.003 to 0.03%, Ca: 0.0001-0.005%, B: 0.0005 to 0.01%, N: 0.005-0.02%, O: 0.0001 to 0.005% and Mo: more than 0.02 percent of,

Cu and Ti as optional components,

The remainder of the Ni and unavoidable impurities,

the total of Mo, Cu and Ti is 0.5% or less.
2. The method for producing a Ni-Cr-Nb-Fe alloy according to claim 1, wherein the raw materials are melted in an electric furnace, decarburized in VOD, and then lime, fluorite, and Al are added to the molten raw materials, and the mixture is cooled by a cooling method using a cooling medium consisting of CaO: 40 to 70% of SiO₂: 10% or less, MgO: 3 to 35% of Al₂O₃: 5-30%, F: 5 to 15% of CaO-Al₂O₃-MgO-SiO₂an-F-based slag is subjected to Cr reduction, deoxidation and desulfurization while being stirred with Ar, a Nb source is added to adjust the composition, and then a steel ingot is produced by casting through a conventional ingot, followed by forging and hot rolling.
A method for producing a Ni-Cr-Nb-Fe alloy, which comprises the steps of: 0.005-0.04%, Si: 0.05-0.5%, Mn: 0.05-0.5%, P: 0.02% or less, S: 0.0015% or less, Cr: 14-17%, Nb: 2-4%, Fe: 6-8% of Al: 0.01-0.2%, Mg: 0.003 to 0.03%, Ca: 0.0001-0.005%, B: 0.0005 to 0.01%, N: 0.005-0.02%, O: 0.0001 to 0.005% and the balance of Ni and unavoidable impurities, characterized in that the raw materials are melted in an electric furnace, then decarburization is performed in VOD, lime, fluorite and Al are added, and a Ni-Cr-Nb-Fe system alloy consisting of CaO: 40 to 70% of SiO₂: 10% or less, MgO: 3 to 35% of Al₂O₃: 5-30%, F: 5 to 15% of CaO-Al₂O₃-MgO-SiO₂an-F-based slag is subjected to Cr reduction, deoxidation and desulfurization while being stirred with Ar, a Nb source is added to adjust the composition, and then a steel ingot is produced by casting through a conventional ingot, followed by forging and hot rolling.