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Products Specification

METHOD FOR MANUFACTURING MARTENSITIC STAINLESS STEEL PIPE OR TUBE

 

METHOD FOR MANUFACTURING MARTENSITIC STAINLESS STEEL PIPE OR TUBE

The present invention relates to a method for manufacturing a martensitic stainless steel pipe or tube. More particularly, the present invention relates to a method capable of manufacturing a martensitic stainless steel pipe or tube with high efficiency by reducing the time required for a heat treatment step. Hereinafter, "pipe or tube" is referred to as "pipe" when deemed appropriate.


   

BACKGROUND ART


   

Conventionally, a martensitic stainless steel pipe has been used widely for such applications as oil wells, and the like because it has high resistance to corrosion for CO2. On the other hand, as for the martensitic stainless steel pipe, quenching cracks develop easily if cooling for quenching in the heat treatment step is all performed by water cooling, because the material thereof has extremely high hardenability. Therefore, to quench the martensitic stainless steel pipe in the heat treatment step, an air cooling method requiring many hours has generally been used, which reduces the manufacturing efficiency.


   

To eliminate the above-described disadvantage of poor manufacturing efficiency as one purpose, a method described in, for example, WO 2005/035815 (Patent Document 1) has been proposed. In the method described in Patent Document 1, a water cooling method having a high cooling rate and an air cooling method are combined in the temperature range excluding the vicinity of Ms point (a temperature at which martensitic transformation of steel starts in cooling at the quenching time).


   



    DISCLOSURE OF THE INVENTION


   

As described above, Patent Document 1 only discloses that the cooling rate of water cooling before air cooling is merely set within the range of 1 to 40 C./sec. To enhance the heat treatment efficiency as far as possible, it is generally thought that the cooling rate is increased (in Patent Document 1, 40 C./sec) so that the cooling time of water cooling before air cooling is shortest.


   

However, as the result of earnest studies, the present inventors found that in the case where the cooling method in which water cooling, air cooling, and water cooling are performed in that order is used in the heat treatment step in the manufacturing process of the martensitic stainless steel pipe, as the cooling rate of water cooling before air cooling is increased, the time required for cooling the steel pipe to a predetermined temperature in the subsequent air cooling step is prolonged, and consequently the total cooling time is prolonged. That is to say, it was found that if the cooling rate of water cooling before air cooling is made too high, the cooling time in this water cooling step is reduced, whereas the total cooling time is inversely prolonged.


   

The present invention has been made in view of the above-described prior art, and accordingly an object thereof is to provide a method capable of manufacturing a martensitic stainless steel pipe or tube with high efficiency by reducing the time required for cooling for quenching in a heat treatment step.


   

To solve the above-described problem, the present inventors carried out studies earnestly, and consequently obtained a knowledge of the following items (A) to (C) concerning the case where a cooling method in which water cooling, air cooling, and water cooling are performed in that order is used in the heat treatment step in the manufacturing process of the martensitic stainless steel pipe or tube.