JP3417275B2 - Martensitic stainless steel seamless steel pipe with excellent hot workability and sulfide stress cracking resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel excellent in hot workability and sulfide stress cracking resistance.

[0002]

2. Description of the Related Art Martensitic stainless steel is AI
As represented by SI420 steel, it has been used as an oil country tubular good since around 1980 due to its excellent strength, CO ₂ corrosion resistance, and relatively low price, but in recent years, it has been used at high temperatures and in large quantities in addition to CO ₂ and In order to be adaptable to an oil well environment containing H ₂ S, a low C-Ni-Mo-added steel, which has corrosion resistance superior to that of 420 steel, as seen in JP-A-3-120337, or the like, or Kaihei 2-2473
Low C-Cu-Ni- as seen in Japanese Patent Publication No. 60, etc.
Steel types such as Mo-added steels (so-called Modified 13Cr steels) have been developed. Generally, as the amount of alloy increases, the corrosion resistance improves, but the workability deteriorates. Therefore, in the development of these steels, efforts have been made to maximize corrosion resistance to the extent that workability is not significantly reduced.

The above-mentioned steel oil country tubular goods are usually manufactured as seamless tubes by a rolling method of the Mannesmann system. Conventionally, Mannesmann rolling is known as the most severe working method among the hot working methods.
Since a large amount of alloying elements such as i, Mo, and Cu are contained, rolling defects may occur when pipes are manufactured by the Mannesmann rolling method. For such a cracking problem during rolling, as seen in JP-A-5-263138, Cr, Ni, Mo, Cu, in order to control the structure in the hot working temperature region to be an austenite single phase, A method for adjusting the balance of addition of major alloying elements such as C, N, etc.
As disclosed in Japanese Patent Laid-Open No. 60904 and the like, a technique for limiting the content of impurities typified by S that are harmful to hot workability to a particularly low level has been proposed. However, even with these measures, the problem of defects associated with hot working has not been solved yet.

When the above steel is produced into a seamless pipe by the Mannesmann rolling method, a slab having a large cross section or a bloom-shaped slab is used as a pipe forming material having a small cross section (rectangular cross section bloom or Round section billet, below,
It was customary to slab-roll into pipes).

However, from the viewpoint of improving the productivity, it is preferable to omit the slabbing and produce a small-section slab directly into a seamless pipe by the rolling method of the Mannesmann system. In recent years, there has been a trend toward a seamless pipe manufacturing process that omits such slabbing, but if the Mannesmann rolling method is applied directly from a slab to a difficult-to-process material as described above, the pipe surface is rolled. There was a problem that defects occurred.

[0006] While the pipe material that has undergone slabbing does not have a problem of rolling flaws during pipe manufacturing, the problem of rolling flaws when directly casting a slab is due to the difference between the structures of the two.
That is, the pipe material that has undergone the slabbing process has a rolling recrystallized structure, and since impurities are diffused during heating, hot workability is high. On the other hand, the slab is accompanied by microsegregation, has a coarse grain size, and has low hot workability. For this,
When producing a slab, rolling flaws are much more likely to occur than when producing a pipe material that has undergone the slabbing process. As a result, the fact is that the high efficiency production of the seamless steel pipe itself is hindered.

As described above, it is difficult to solve the problem of rolling flaws that occur when the above-mentioned steel slab is directly pipe-formed into a seamless pipe by the rolling method of the Mannesmann system. It was

Further, in the above conventional steel, as described above, in order to control the structure in the hot working temperature region to be an austenite single phase, the balance of addition amounts of major alloying elements such as Cr, Ni, Mo, Cu, C, N, etc. Has been adopted, but due to this limitation, the amount of alloys such as Cr and Mo that are effective for corrosion resistance was limited, so that the function as an oil country tubular good reached its ceiling and it responded to market needs. New seeds need to be developed for.

[0009]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and a direct Mannesmann system hot rolling is performed on a slab having a small cross section by omitting the slabbing process. The purpose of the present invention is to provide a martensitic stainless steel seamless steel pipe that can be manufactured by the method without rolling defects and that has superior sulfide stress cracking resistance to conventional Modified 13Cr steel seamless steel pipe. .

[0010]

Means for Solving the Problems The inventors of the present invention have conducted extensive research on hot workability and sulfide stress corrosion cracking resistance with respect to various materials having different components, and as a result, S
Is 0.001% or less, it is possible to directly produce a cast slab into a seamless steel pipe by the hot rolling method of the Mannesmann method without causing a problem of rolling flaw, and Al is added to 0.06%. It was found that the sulfide stress cracking resistance is improved by adding more than 0.3%.

The present invention is constructed on the basis of such findings, and the gist of the present invention is as follows.
% By weight, C: 0.05% or less, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.001% or less, Cr: 10-14% , Ni: 4.0 to 7.0%, Al: 0.06% to 0.3% or less, N: 0.08% or less, Mo: 1.0 to 3.0%, Cu: 1. 0 to 2.0%, and if necessary, Ca: 0.001 to
A cast slab containing 0.01%, Ti: 0.5S to 0.05%, one or two, and the balance consisting of Fe and unavoidable impurities is directly subjected to a Mannesmann hot rolling process. A seamless martensitic stainless steel pipe excellent in hot workability and sulfide stress cracking resistance.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. Modified 13Cr steel (0.02C-0.02N-1.5Cu-12.2Cr-
Figure 1 shows the effect of deformation temperature on the hot workability of as-cast and rolled 5.8Ni-2.0Mo). As-cast material and rolled material have exactly the same composition, containing 0.0029% S,
Only the organization is different. Here, the as-cast material is 250
The slab is a continuously cast slab having a section of mm × 650 mm, and the slab is slab rolled into a bloom having a section of 217 mm × 217 mm. A hot tensile test was performed using these materials under the conditions shown in FIG. That is, 125
After heating to 0 ° C and holding for 1 minute, up to the deformation temperature (T1 ° C) 10
After cooling at ℃ / sec and holding at that temperature for 1 minute, 1.4 /
A tensile test was conducted at a strain rate of sec. The value obtained by dividing the cross-sectional area of the fractured part after the test by the cross-sectional area before the test is defined as the aperture value.

The ordinate and the abscissa of FIG. 1 represent the aperture value and the deformation temperature T1, respectively. The higher the drawing value, the better the hot workability. FIG. 1 also shows the processing temperature range of the piercer and elongator mill, which are the maximum reduction mills in the seamless pipe making by the Mannesmann hot rolling method. From the knowledge obtained so far, it is known that if the drawing value is 75% or more, good hot workability is exhibited at that temperature. From FIG. 1, the hot workability of the as-cast material is significantly worse than that of the rolled material, and even if the S value is as low as 0.0029%, it does not show good hot workability in the processing temperature range of the piercer and elongator mill. I understand.

The steel of the present invention is formed into a seamless pipe mainly by the Mannesmann hot rolling. The mannesmann method rolling method here is a normal hot rolling method for producing seamless steel pipes, using a tubular material with a rectangular cross section or a round cross section, and after punching by the press roll perforation method or the Mannesmann perforation method, it is necessary. Depending on the situation, it is drawn by a slant rolling machine (elongator), the wall thickness is adjusted by a plug mill or mandrel mill, the tube is ground, and the roundness is adjusted by a final finishing rolling machine (sizer mill) to make a pipe. It is a series of processes.

Next, FIG. 3 shows the effect of the S content on the surface flaw generation state of the pipe when the cast slab is formed into a seamless pipe by the Mannesmann hot rolling method. The number attached to each data in FIG. 3 means the code of steel in the examples described later. In the data of FIG. 3, the base component is almost the same and S
It is the result of investigating the state of occurrence of surface flaws on the pipe after the completion of rolling, by continuously casting cast slabs with a 217 mm x 217 mm cross section having different compositions only by the Mannesmann hot rolling method. . From Figure 3, S is 0.001%
It can be seen that rolling defects cannot be prevented unless the following restrictions are applied.

Furthermore, the inventors have found that if the remaining solid solution S is fixed in the ultra low sulfur steel, the hot workability is further improved, and as a result of repeated research, Ca and Ti alone or It has been found that the hot workability is dramatically improved by the combined addition. However, the addition amount of Ti is S by weight%.
If less than 0.5 times the addition amount of
Even if added in excess of 0.05%, the effect will be saturated, and on the contrary, coarse nitrides will precipitate and the toughness will decrease, so 0.05
% Or less. The amount of Ca added is 0.0
If less than 01%, the effect is not exhibited, and if added over 0.01%, Ca-based inclusions increase and the sulfide stress cracking resistance deteriorates. Therefore, the optimum addition amount is 0.001% to 0.
It was set to 01%.

Next, the sulfide stress cracking resistance will be described. Continuously cast slabs of 217 mm x 217 mm cross-section with almost the same base composition but different Al contents were made into seamless pipes by hot rolling in the Mannesmann system so that they would have the same strength level. A typical corrosive environment (0.01MP
a H ₂ S, pH = 3.0, 5% NaCl, additional stress σ
ap = 90% of yield strength YS, 720 hr, 24 ° C.) FIG. 4 shows the effect of Al content on the fracture time when a constant load SSC (Sulfide Stress Cracking) test is performed. The ∘ mark indicates that the sample did not break even after 720 hours, and the ● mark indicates that the sample did not break within 720 hours. Figure 4
The numbers attached to the respective data mean the symbols of steel in the examples described later.

From FIG. 4, it is clear that the sulfide stress cracking resistance is improved by setting the Al addition amount to more than 0.06% and 0.3% or less. The addition of Al in excess of 0.06% improves the sulfide stress cracking resistance because the passive film is strengthened and the pitting corrosion resistance is improved. Also,
When Al is added in excess of 0.3%, the sulfide stress cracking resistance deteriorates because coarse Al-based inclusions are generated and serve as a starting point of pitting corrosion or a crack propagation path, which deteriorates SSC resistance. This is because.

Further, the high Al content is also effective for improving the desulfurization efficiency in the steelmaking stage, and is a secondary factor which enables stable and economical industrial production of extremely low-sulfur steel for maintaining good hot workability. It also has an effect. As described above, by setting the addition amount of Al to more than 0.06% and 0.3% or less, the sulfide stress cracking resistance can be improved, and the efficiency is low and the cost is extremely low. It is possible to sulphide.

The reasons for limiting the components of the martensitic stainless steel in the present invention are as follows. C: C is Cr
It is an element that forms carbides and deteriorates corrosion resistance. In addition, the strength is increased and the sulfide cracking resistance required when used as an oil country tubular good is deteriorated.
It was set to 5% or less.

Si: Si is added as a decarburizing agent in the steel making process. If the content exceeds 0.5%, the toughness deteriorates, so the content was made 0.5% or less.

Mn: Mn is an austenite stabilizing element and is effective in preventing rolling flaws by suppressing the precipitation of the δ phase during hot working, but if added in excess of 1.5%, the grain boundary strength decreases. As a result, the toughness deteriorates, so the content was made 1.5% or less.

P: P is an impurity element that segregates at the grain boundaries to lower the grain boundary strength and deteriorates the toughness. It is desirable that the level is as low as possible, but in consideration of the attainable level and cost of the present refining technology. Was set to 0.03% or less.

S: S is an impurity element that deteriorates hot workability. If it exceeds 0.001%, cracking occurs during hot rolling, so S was made 0.001% or less.

Cr: Cr is a basic element for improving corrosion resistance, and it is necessary to add 10% or more in order to obtain sufficient corrosion resistance, but it is also a ferrite stabilizing element, and if too much, the δ phase is formed during hot working. Since it precipitates and deteriorates hot workability and sulfide stress cracking resistance, it was set to 14% or less.

Ni: Ni is effective for improving corrosion resistance and toughness. It is also an austenite stabilizing element,
Inhibits the formation of the δ phase, which leads to rolling defects. Since these effects are small when the added amount is less than 4%, it is set to 4% or more. In addition, the effect is saturated even if added in excess of 7% and A
c1 It lowers the transformation point and makes strength refining difficult.
It was set to 7% or less.

Al: Added to improve sulfide stress cracking resistance and accelerate deoxidation and desulfurization. The effect of improving the resistance to sulfide stress cracking is not exhibited at 0.06% or less, but if it is added in excess of 0.3%, coarse Al will be adversely affected.
Since system inclusions are generated and become the starting point of pitting corrosion or the crack propagation path, the sulfide stress cracking resistance deteriorates.
The optimum addition range was set to more than 0.06% and 0.3% or less. Further, within this addition range, sufficient deoxidation and desulfurization are possible.

N: N is a strong austenite stabilizing element similar to Mn and Ni and is effective in preventing rolling flaws, but it increases strength like C and is required when used as an oil country tubular good. In order to prevent deterioration of the stress corrosion cracking resistance, the content is made 0.08% or less. The lower the lower limit, the better.

Mo: Mo is an essential element for improving pitting corrosion resistance and sulfide stress corrosion cracking resistance. These effects are small when added in an amount of less than 1.0%, so 1.
It was set to 0% or more. Further, it is a strong ferrite stabilizing element, and the addition of more than 3% facilitates the formation of the δ phase, so the content was made 3.0% or less.

Cu: Cu, like Ni, is an element effective in improving the corrosion resistance, is an austenite stabilizing element, suppresses the formation of the δ phase, and is effective in preventing rolling flaws, so Cu is added if necessary. However, if less than 1.0%, these effects cannot be sufficiently obtained, so the content was made 1.0% or more. Further, if added in excess of 2.0%, it is excessively segregated at the grain boundaries to lower the grain boundary strength, resulting in a marked deterioration in hot workability, so the content was made 2.0% or less.

Ti: Ti suppresses the deterioration of hot workability due to S, and is added as necessary. However, if it is less than 0.5 times the addition amount of S in weight%, the effect is not exhibited.
Even if added in excess of 0.05%, the effect is saturated and, conversely, coarse nitrides are precipitated and the toughness is reduced. Therefore, the optimum addition range is 0.5 times the addition amount of S or more than 0. It was set to 05% or less.

Ca: Ca suppresses the deterioration of hot workability due to S, and is added as necessary.
If less than 1%, the effect is not exhibited, and if added over 0.01%, Ca-based inclusions increase and the sulfide stress cracking resistance deteriorates. Therefore, the optimum addition amount is 0.001% or more,
It was set to 0.01% or less.

[0033]

Example Continuously cast slabs having a composition of 217 mm × 217 mm in cross section as shown in Table 1 were formed into a seamless pipe by the hot rolling method of the Mannesmann system as cast. After completion of rolling, the state of occurrence of surface defects on the pipe was investigated. The results are also shown in Table 1.

[0034]

[Table 1]

Focusing on steels (Nos. 1, 2, 3, 12, 13, and 14) having almost the same base composition but different S contents, the influence of the S content on the occurrence of surface defects during pipe manufacturing FIG. 3 shows the above. Examples of the present invention (No. 1, 2,
In 3), surface flaws did not occur during pipe manufacturing. On the other hand, a comparative example in which the S content exceeds the component limitation range of the present invention (No.
12, 13, 14), surface flaws occurred during pipe manufacturing, and the surface flaws became more severe as the S content increased.

After conditioning the pipes to the same strength level, a constant load SSC test was conducted in a typical corrosive environment where sulfide stress cracking (SSC) could occur.
Regarding the test conditions, the steels shown in Table 1 were divided into two groups according to the difference in the base composition, and group 1 (No.
The steels of Nos. 6, 12 to 19) are condition A (0.01 MPa H ₂
S, pH = 3.0, 5% NaCl, additional stress σap = 90% of yield stress YS, 720 hr, 24 ° C.), and the steel of group B (No. 7 to 11,20 to 22) is condition B (0. ．
003 MPa H ₂ S, pH = 3.2, 5% NaCl,
Additional stress σap = 90% of yield stress YS, 720 hr, 2
A constant load SSC test was performed at 4 ° C. The results are also shown in Table 1.

Focusing on steels (Nos. 1, 4, 5, 6, 15, 16, 17) having almost the same base composition but different Al contents, the effect of A on the breaking time in the constant load SSC test was examined.
FIG. 4 described above shows the influence of the 1 content. In the present invention example (No. 1, 4, 5, 6), 720 under the above test conditions.
No sulfide stress cracking occurred even after the lapse of time. on the other hand,
In the comparative examples (Nos. 15, 16, 17) in which the Al content exceeds the component limiting range of the present invention, sulfide stress cracking occurs.

From Table 1, FIG. 3 and FIG. 4, if the S content and the Al content are within the limits of the present invention, even if the slab is directly piped by the Mannesmann hot rolling method, a rolling defect will occur. It is clear that no sulfide stress cracking occurs and that good sulfide stress cracking resistance is obtained.

[0039]

As described above, according to the present invention, it is possible to obtain a martensitic stainless steel which is free from cracks during slab rolling and is excellent in hot workability and sulfide stress cracking resistance.

[Brief description of drawings]

[Figure 1] Modified 13Cr steel (low C-low N-1.5Cu-12.2Cr-5.8
2 is a table showing the effect of deformation temperature on the hot workability of as-cast material and rolled material of Ni-2.0Mo).

FIG. 2 is a chart showing conditions of a hot tensile test.

FIG. 3 is a chart showing the effect of S content on the maximum crack depth during slabbing.

FIG. 4 is a table showing the relationship between the rupture time and the Al content when a constant load SSC test is performed in a typical corrosive environment where sulfide stress cracking occurs.

Claims (2)

(57) [Claims] 1. By weight%, C: 0.05% or less, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.001% or less, Cr : 10 to 14%, Ni: 4.0 to 7.0%, Al: more than 0.06% and 0.3% or less, N: 0.08% or less, Mo: 1.0 to 3.0% , Cu: 1.0 to 2.0%, and a slab containing the balance Fe and unavoidable impurities, is directly subjected to hot rolling in a Mannesmann system, and is manufactured. And martensitic stainless steel seamless steel pipe with excellent sulfide stress cracking resistance. 2. In% by weight, C: 0.05% or less, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.001% or less, Cr : 10 to 14%, Ni: 4.0 to 7.0%, Al: more than 0.06% and 0.3% or less, N: 0.08% or less, Mo: 1.0 to 3.0% , Cu: 1.0 to 2.0%, further, Ca: 0.001 to 0.01%, Ti: 0.5S to 0.05%, one or two kinds, and the balance. A martensitic stainless steel seam excellent in hot workability and sulfide stress cracking resistance, characterized in that it is produced by directly subjecting a slab composed of Fe and unavoidable impurities to hot rolling in a Mannesmann system. No steel pipe.