JPH027389B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to high Ni austenitic stainless steel used for seamless steel pipes such as line pipes, and in particular to high Ni austenitic stainless steel suitable for manufacturing seamless steel pipes through a piercing process using a piercing mill. It is. High Ni austenitic stainless steel is CO ₂
Since it exhibits excellent corrosion resistance and stress corrosion cracking resistance in corrosive environments containing H ₂ S and H 2 S, it has recently attracted attention as a material for oil country tubular goods, hot country tubular goods, line pipes, etc. By the way, seamless stainless steel pipes are generally manufactured by inclined rolling methods such as plug mill method and mandrel mill method, or hot extrusion methods such as Eugene Sejourne method and Erhardt push bench method, but they contain large amounts of Cr and Ni. The austenitic stainless steel contained in stainless steel has poor hot workability. Therefore, when manufacturing seamless steel pipes using inclined rolling methods such as plug mill method or mandrel mill method, the outer surface or Defects may occur on the inner surface. So high
For Ni austenitic stainless steel,
Seamless steel pipes have traditionally been manufactured using a hot extrusion method, such as the Eugene-Ségiurnet method. However, when applying the hot extrusion method to directly perforate a billet (direct perforation method), if the length of the billet is 5 to 7 times the diameter, the uneven thickness will increase, so long pipes cannot be used. is difficult to manufacture. In order to solve this problem, a conventional method has been to form a guide hole in the center of the billet by machining and expand the hole by pushing it out, that is, to manufacture a long tube using the so-called expansion method. It has been adopted. However, even with this expansion method, the length of the billet is limited to about 15 times the diameter. Furthermore, in the Eugene Sejourne method, which is a typical hot extrusion method,
Since a glass lubricant is used, it is necessary to peel off the glass lubricant after rolling, but there is a problem in that this peeling process requires considerable effort and cost. As mentioned above, in the hot extrusion method typified by the Eugene Sejourne method, the billet length is restricted, so productivity cannot be increased beyond a certain level, and the yield rate is also low due to the use of short billets. There is a problem in that it has to be low, which is disadvantageous in terms of cost. On the other hand, both the plug mill method and the mandrel mill method use a piercing mill that utilizes the Mannesmann effect, and these methods make long tubes more difficult to manufacture than hot extrusion methods such as the Eugene Sejourne method. It is known to have various advantages such as high productivity and cost advantages, but as mentioned above, it is expensive due to the problem of defects occurring during pipe making (drilling). The reality is that it has been considered difficult to apply to Ni austenitic stainless steel. This invention was made in view of the above circumstances, and can be applied to high Ni austenitic stainless steel, where it has been difficult to manufacture seamless steel pipes using conventional plug mill and mandrel mill methods due to the problem of defects during pipe manufacturing. The purpose of this project is to make the plug mill method and mandrel mill method practically applicable, thereby making it possible to manufacture seamless high-Ni austenitic stainless steel pipes with high productivity and at low cost. In other words, the present inventors have repeatedly investigated and researched the causes of defects when producing seamless high-Ni austenitic stainless steel pipes using the plug mill method or mandrel mill method. It was discovered that P and S as impurities in steel have a large effect on
It was discovered that seamless steel pipes can actually be manufactured by the plug mill method or the mandrel mill method without causing the above-mentioned defects by regulating B to 0.82% or less and adding B at the same time, and based on this invention. I arrived at the eggplant. Specifically, the first invention has C0.05% or less, Mn0.82
% or less, N0.01~0.25%, Ni20~40%, Cr20~25
%, Mo0.5~4%, Al0.005~0.1%, B0.001~
0.008%, P is regulated to 0.02% or less, S is regulated to 0.003% or less, and the balance is essentially Fe, and has excellent hot perforability. It provides Ni austenitic stainless steel. In addition to the above-mentioned components, the second invention also provides one or two of rare earth elements (hereinafter referred to as REM) and Ca.
Seeds, REM: 4 x (%S) ~ 20 x (%S), Ca:
Added within the range of 1x (%S) to 10x (%S),
The present invention provides a high Ni austenitic stainless steel for seamless steel pipes with excellent hot perforability. The high Ni austenitic stainless steel of the present invention will be explained in more detail below. First, to explain the reasons for limiting each component in the steel of this invention, C is an element necessary to ensure strength, but if it exceeds 0.05%, perforability deteriorates, so it was limited to 0.05% or less. Cr is an element that significantly improves corrosion resistance in a CO ₂ + H ₂ S environment, and must be added in an amount of 20% or more to prevent pitting corrosion. On the other hand, if the Cr content exceeds 25%, CO ₂
In order to reduce stress corrosion cracking resistance in +H ₂ S environment, it was set to 25% or less. N is an element necessary to improve pitting corrosion resistance and ensure strength in a CO ₂ + H ₂ S environment.
Below 0.01%, their effects are small, while 0.25
If it exceeds 0.01 to 0.25%, the perforability deteriorates.
The range of Ni is an element that improves corrosion resistance in a CO ₂ + H ₂ S environment, but if it is less than 20%, stress corrosion cracking resistance is not sufficient, while if it exceeds 40%, the effect is saturated and costs increase unnecessarily. We limited it to a range of 20% to 40%. Mo is an element that improves pitting corrosion resistance in a CO ₂ + H ₂ S environment, but if it is less than 0.5%, this effect cannot be obtained, while if it exceeds 4%, the effect is saturated.
Since this would only lead to an increase in costs, it was limited to a range of 0.5 to 4%. Al is a strong deoxidizing element, but if it is less than 0.005%, it has no effect, and if it exceeds 0.1%, the effect is saturated, so it was limited to a range of 0.005 to 0.1%. B improves hot workability when added in a small amount, but if it is less than 0.001%, it has no effect;
If B is added in excess of 0.008%, hot workability at temperatures above 1200°C will deteriorate, so B should be added from 0.001 to
It was limited to a range of 0.008%. Mn is an effective element for deoxidation, but if excessively contained, hot workability will be degraded and perforation performance will be reduced, so it is limited to 0.82% or less. S is normally contained in steel as an unavoidable impurity, but it significantly deteriorates hot workability in the steel types targeted by this invention. Its negative effects are
When making pipes using the plug mill method or mandrel mill method, this is especially noticeable when making holes with a piercing mill, and if the S content exceeds 0.003%, it will be difficult to make pipes without defects, so the S amount should be 0.003%.
% or less. P is also normally contained in steel as an unavoidable impurity, but it significantly deteriorates hot workability at high temperatures of 1200° C. or higher. In particular, P causes flaws that occur on the inner surface when making holes with a piercing mill. If the P content exceeds 0.02%, it becomes difficult to drill holes without causing scratches, so the P content must be regulated to 0.02% or less. Furthermore, in the second invention of the present application, one or both of REM and Ca are added. REM, Ca is a strong sulfide-forming element,
REM is effective in reducing the amount of solid solution S in steel and improving hot workability by forming Ca sulfides, but when REM is less than 4 times the amount of S,
Furthermore, when Ca is less than the equivalent amount of S, the effect is small, while when REM exceeds 20 times the S amount, or when Ca exceeds 10 times the S amount, the effect is saturated; Since oxides or sulfides may conversely increase surface defects, for REM, 4 x (%S) to 20 x (%S)
and for Ca, 1×(%S) ~ 10
It was limited within the range of x (%S). Table 1 shows the chemical components and defect occurrence conditions of Examples and Comparative Examples of the present invention. However, in Table 1, the defect occurrence situation is as follows: A billet with an outer diameter of 58 mm made of steel of each component was heated to 1250°C, and an outer diameter of 60.3 mm and a wall thickness of 5.3 mm were measured using an experimental piercing mill.
The inner and outer surfaces of the hole are observed, and four marks A, B, C, and D are made depending on the degree of defect occurrence.
The evaluation was divided into ranks. Here, A rank is one with almost no defects, B rank is one with defects only on the inner surface near the tube end,
Rank C indicates that defects occur not only in the area near the end of the tube but also at various locations on the inner surface, and rank D indicates that defects occur all over the inner and outer surfaces of the tube. In particular, for B rank, the distance L from the tube end to the defective part
I looked into it. Those with A rank and those with L within 30 mm of B rank are judged to be acceptable, and B
Among the ranks, those with L exceeding 30 mm, and those with C rank and D rank were determined to be defective products. The reason for determining pass/fail for B rank with L=30 mm as the boundary is as follows. In other words, in the production of normal seamless steel pipes, the leading and trailing ends of the pipe are cropped by approximately 100 mm (or approximately 1% of the length) per 10 m of pipe length due to poor shape. The length of the tube after perforation in is approximately 3 m, so 1% of 3 m or 30 mm
Defects occurring within this period are considered acceptable.
Defects exceeding 30 mm remained on the product even after crop cutting and were judged to be defective.

【table】

[Table] In Table 1, sample materials No. 5 to 10 are comparative examples.
The P content and S content of both of these products exceeded the upper limits of the present invention, and both of these were below the acceptance criteria and were determined to be defective products. In addition, sample materials No. 11 to 13 of the comparative example each had REM, Ca, and B exceeding the upper limit of the present invention, and these were also below the acceptance criteria and were determined to be defective. Furthermore, sample materials No. 14 and 15 of comparative examples have C and N exceeding the upper limit of this invention, and sample material No. 16 has C and N exceeding the upper limit of this invention.
Mn exceeds the upper limit of this invention,
These items also fell below the acceptance criteria and were determined to be defective. On the other hand, sample materials No. 1 to 4 of the examples of the present invention all exceeded the acceptance criteria and were determined to be non-defective products. Comparative materials 17 and 18 both have an S content of 0.002%, which is within the scope of this invention, but no B was added at all. Defects occur up to 20 to 25 mm from the hole, and there is a risk that the product will be defective under severe hot drilling conditions. On the other hand, sample materials No. 1 and 3 of the present invention example have the same S content.
B was added at 0.002%, and in this case, the occurrence of defects could be suppressed to an extremely slight level. As is clear from the above explanation, the high Ni austenitic stainless steel for seamless steel pipes of this invention is
Although it is a steel type that is difficult to manufacture seamless steel pipes using the industrial plug mill method or mandrel mill method, by regulating the amounts of C, N, P, S, and Mn and adding B, It has been able to improve hot workability, especially hot perforation, and therefore seamless steel pipes can be produced in the actual process without pipe manufacturing defects by plug mill or mandrel mill method instead of hot extrusion method. Therefore, productivity in manufacturing seamless steel pipes can be improved, yields can be improved, and costs can be reduced. In addition, the high Ni austenitic stainless steel of the second invention regulates the amounts of C, N, P, S, and Mn, and at the same time adds B, one or two types of REM, Ca, etc.
The hot perforability is further improved by adding seeds, and therefore, healthier seamless steel pipes can be manufactured using the mandrel mill method or the plug mill method.

Claims (1)

[Claims] 1 C0.05% (weight%, the same applies hereinafter) or less, Mn0.82
% or less, N0.01~0.25%, Ni20~40%, Cr20~25
%, Mo0.5~4%, Al0.005~0.1%, B0.001~
0.008%, P is regulated to 0.02% or less, S is regulated to 0.003% or less, and the balance is essentially Fe, and has excellent hot perforability. Ni austenitic stainless steel. 2 C0.05% or less, Mn0.82% or less, N0.01~0.25
%, Ni20~40%, Cr20~25%, Mo0.5~4%,
Contains Al0.005~0.1%, B0.001~0.008%, P is regulated to 0.02% or less, and S is 0.003%.
In addition, one of the rare earth elements and Ca is regulated as follows.
seeds or two species, REM is 4×(%S) to 20×(%
S), Ca is contained in the range of 1x (%S) to 10x (%S), and the balance is substantially Fe, and has excellent hot perforability. Ni austenitic stainless steel.