JPS5854169B2 - Heat-resistant cast steel with improved weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides improved weldability of 0,3
The present invention relates to a method for manufacturing a C-24Cr-24Ni-1,5Nb type heat-resistant cast steel product.

Although the conventional 0.3C-24Cr-24Ni-1,5Nb type heat-resistant cast steel has excellent high-temperature strength, it has a major drawback of poor weldability.

The factors that have traditionally been cited as causes of this low weldability are:
The heat-resistant cast steel mentioned above is the widely used HK-40 material (AST
Compared to M A 351.0.4C25Cr2ONi type cast steel, it is a fully austenitic steel with low C, so there are fewer precipitated carbides, so the casting microstructure becomes dog-like, and the grain boundary area decreases due to coarsening of the crystal grains. It promotes grain boundary concentration of low melting point compounds such as oxides, making it easier to generate hot cracks, and it has a low carbon content. Because it contains CNb, it absorbs oxygen and other gases more easily than HK-40 materials and contains more low-melting point compounds such as oxides during melting. 3 Therefore, in the past, 0.
3-24 In order to ensure good weldability in heat-resistant cast copper of the Cr-24Ni1,5Nb type, it was thought that it was necessary to forcibly deoxidize the molten metal to reduce the oxygen content of the base metal. Therefore, in order to improve weldability by forced deoxidation, it is necessary to perform particularly strong forced deoxidation and to limit the amount of Si to 0.5φ or less in order to prevent grain boundary concentration of Si-based compounds. An effective weldability improvement effect was recognized.

However, even with improved weldability in this way, weld cracking is still sensitive, and automatic TIG with stable arc and less heat shock to the base metal is still a problem.
Although a good welded joint can be obtained by welding, the weldability is not stable with manual TIG@welding due to variations in arc voltage, and the components and welding method are manufactured under optically controlled conditions. However, with regard to weldability, reproducibility was poor and there was a problem in industrial production.

In other words, the fact that even after sufficiently removing oxygen, which was thought to have an extremely negative effect on weldability, and severely limiting the amount of Si, the above-mentioned drawback still exists indicates that there is a factor other than oxygen that significantly deteriorates weldability. As a result of pursuing this, the present inventors discovered that hydrogen remaining in the heat-resistant cast steel has the most adverse effect on its weldability, and the present invention is based on this knowledge. , was made with the purpose of providing a method for manufacturing 0.3C-24Cr-24Ni-15Nb type heat-resistant cast steel products that can always and stably obtain excellent welding results even by manual TIG welding, and its characteristics: Where it is a heavy steamed bun, C: 0.25 ~
0.35%, Si: 1.5% or less, Mn: 1.5% or less, Cr: 23-26ol), Ni: 23-26ol, N
b:1. When obtaining a cast product of a heat-resistant cast steel material consisting of O~2.0 mm and the remainder substantially Fe, at least the Nb-added material among the melted materials is used that has been dehydrogenated in a solid state. .

In describing the present invention in detail below, the weldability deterioration effect due to hydrogen, which was discovered by the present inventors, will be explained first. As mentioned above, the present invention material contains Nb', and Nb+
The solubility of hydrogen in this is particularly high, and the pure Nb or FeNb alloy used as the melting material already contains a considerable amount of hydrogen. Therefore, the molten metal using such Nb-added materials has a high hydrogen concentration, and the same Although some of the dissolved hydrogen is released when the molten metal solidifies, most of it remains in the defects in the structure, and this residual hydrogen gasifies at the grain boundaries during welding, causing intergranular cracking. be.

From the above, the present invention is based on 0.3 C-24Cr -24
The aim is to improve weldability by reducing the hydrogen contained in Ni-1,5Nb type heat-resistant cast steel, and as a means to do so, at least the Nb-added material in the molten material is dehydrogenated, and it is also dehydrogenated in the solid state. Of course, it is also possible to dehydrogenate and use a dissolved material such as a Si additive material (pure 5itFe Si) in a solid state.

The reason why at least the Nb-added material must be dehydrogenated among the melted materials is that, as already mentioned, the Nb-added material contains the most hydrogen, and the This is because without dehydrogenation, no effect of improving weldability can be obtained, and sufficiently satisfactory weldability can be obtained by using only the Nb-added material after dehydrogenation.

Furthermore, the reason why the dehydrogenation treatment was carried out in the solid state was because the heating temperature was lower and the treatment was easier compared to dehydrogenation by vacuum melting or the like.

In other words, the same treatment can be easily achieved, for example, by maintaining a dehydrogenating temperature and degree of vacuum for an appropriate time in a high-frequency induction vacuum degassing furnace.
The treatment of tFe-8i in a high-frequency induction vacuum degassing furnace was carried out at a vacuum degree of 600 to 9000 C1 and maintained at a vacuum of 0.3 to 10 torr for about 1 hour to obtain a sufficiently satisfactory dehydrogenation effect.

Next, examples of the present invention are shown in the table below.

Points 3 and 4 in the table are examples of the present invention, &1 and 2 are Ti? This is a conventional material that performs particularly strong forced deoxidation and has a Si content of 0.5% or less.

As is clear from the table above, the conventional material still has good weldability even after strong deoxidation with Ti and with consideration given to grain boundary concentration of Si-based compounds by keeping the Si content at all times below 0.5%. On the other hand, the material of the present invention shows good weldability and the reproducibility of the good results has been confirmed. , A4 (this shows that even in cast steels with a Si content exceeding 0.5%, good weldability could be ensured by using both Nb-added materials and Si-added materials after solid-state dehydrogenation treatment.

This means that removing the harmful effects of hydrogen by dehydrogenating the dissolved material has the effect of reducing the harmful effects of grain boundary concentration of Si-based compounds.

However, if the Si content exceeds 15, the above effect is lost and the weldability deteriorates rapidly (sigma phase is also likely to be generated, which tends to cause embrittlement of the material. The reason why it was set below (5i15cl) is here (
There it is.

However, the range of components other than Si is the same as the conventional material, and from a compositional standpoint, it is a conventional material.

As described above, the heat-resistant cast steel obtained by the present invention has a 0.3
The low weldability of C-24Cr-24Ni-1,5Nb type heat-resistant cast steel has been greatly improved to improve manual TIG with variations in arc voltage.
Good welding can be done stably by welding,
It has the advantage that its melting process is easier than in the past, and fluidity of the molten steel is easily ensured due to the increased Si range.Furthermore, the hydrogen reduction method in the cast steel is based on dehydrogenation of the molten material in its solid state. In addition to the fact that the molten material itself is easy to dehydrogenate, it is also much easier to dehydrogenate molten metal, for example, compared to dehydrogenation by vacuum melting. Moreover, compared to the method of adding a dehydrogenating agent to the molten metal, it is superior in that there is no contamination of the molten metal by the dehydrogenating agent, and from a resource saving perspective, there is no need for a dehydrogenating agent. It is extremely useful as parts for welded structures that require high-temperature strength, such as reaction tubes, centrifugal casting tubes for steel heat treatment, and cast casting products.
Furthermore, the melted material dehydrogenation means used in the present invention can be applied to the production of other alloys in which low hydrogen is desired, and the present invention has great industrial value.

Claims (1)

[Claims] 1. Weight φte, C: 0.25-0935, Si:
1.5% or less, Mn: 1.5% or less, Cr:
23-26%. When obtaining a cast product of heat-resistant cast steel material consisting of Ni: 23 to 26%, Nb: 1.0 to 2.0%, and the remainder substantially Fe, at least the Nc-added material among the melted materials is dehydrated in a solid state. A method for manufacturing a heat-resistant cast steel product with improved weldability, characterized by using a material that has undergone raw treatment.