JP3485022B2 - Martensitic stainless steel with excellent hot workability - 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 material used as an oil well pipe, a line pipe, etc., mainly in a highly corrosive oil and gas environment, and is resistant to cracking during hot working such as rolling or forging. The present invention relates to a martensitic stainless steel material that does not generate surface defects such as flaws.

[0002]

2. Description of the Related Art Martensitic stainless steel materials are used in the fields of oil well pipes and line pipes.
With the sophistication of the market demand for material properties such as strength and toughness,
Steel types containing a large amount of alloying elements such as Ni, Mo, and Cu have been developed as disclosed in Japanese Patent Publication No. 3-2277 and Japanese Patent Laid-Open No. 2-247360. These martensitic stainless steels are generally referred to by the names Modified 13Cr for oil well pipes and Weldable 13Cr for line pipes, and have been put to practical use as products such as seamless steel pipes, electric resistance welded pipes and round steel. Has been done.

These various martensitic steels are formed by various hot rolling / forging methods, but when the alloying elements as described above are contained, the hot workability is deteriorated.
Surface defects such as cracks and flaws are likely to occur. Such surface defects need to be removed, and the defective portion is ground. For this reason, when a surface defect occurs, not only the appearance is bad, but also grinding cost occurs, and when the defect depth is large, there is a problem that the dimension becomes insufficient and the product cannot be manufactured. Particularly for seamless steel pipes, which are in high demand in the fields of oil country tubular goods and line pipes, the problem of surface defects is serious because the rolling conditions are severe.

As a main technique that has been adopted conventionally for this problem, as disclosed in Japanese Patent Publication No. 3-60904, impurity elements such as P and S, which are harmful to hot workability, are reduced as much as possible. A technique for limiting the metal structure during hot working to a δ ferrite fraction of 40% or less, and Japanese Patent Application Laid-Open No. 8-1203.
As disclosed in Japanese Patent Laid-Open No. 45-45, there is an alloy design technique for maintaining the metal structure in the γ single phase during hot working. However, even with the combination of these techniques, it has not been industrially stable to eradicate the above-mentioned surface defect problem, and there is still a problem of cost increase due to yield reduction.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to overcome the above-mentioned problems. That is, N
In order to prevent surface defects during hot working of various martensitic stainless steel materials such as seamless steel tubes containing a large amount of alloying elements such as i, Mo and Cu, which are useful for product performance, It is intended to provide technology for controlling material-side factors from different viewpoints.

[0006]

[Means for Solving the Problems] The inventors of the present invention have investigated in detail the actual state of cracks and flaws occurring in the manufacturing process of the seamless steel pipe of martensitic stainless steel, and as a result, the occurrence of cracks and flaws. It has been found that there is a good correlation between the frequency and the density of precipitates present in the product tube. In particular, T
The presence density of precipitates mainly composed of nitrides and oxides of i and Al, and those in which sulfides are compositely precipitated, and a pipe having a defect of 0.1 mm or more that must be ground in a product pipe. It was found that there is the relationship shown in FIG.

Further, as a result of investigating the adverse effects of these precipitates, voids starting from the precipitates illustrated in FIG. 2 and cracks connecting the voids were found, and this phenomenon is caused by the existence density of the precipitates. It was found that the higher the value, the more remarkable the appearance. Furthermore, the existence density of the above-mentioned precipitate is Ti, A
It has been found that it is necessary to control the content of these elements within a specific range in order to control the existing density, which is closely related to the contents of l and N, as shown in FIG. Further, the harmfulness of these precipitates also depends on the size, and although they are coarse, the harmfulness is high. In order to reduce this coarse precipitate, it is effective to control the material heating temperature before hot working.
It was found that it is desirable to set the temperature to 1350 ° C.

The present invention was made based on the above findings, and the gist thereof is as follows. (1) C: 0.005-0.05% by weight, Si: 0.1
~ 0.5%, Mn: 0.1-1.0%, P: 0.03%
Hereinafter, S: 0.005% or less, Cr: 10.0 to
14.0%, Ni: 6.11 to 8.0%, Mo: 0.5 to
3.0%, N: 0.005-0.05%, Al: 0.02-
0.15%, Ti: 0.003 to 0.050%, and the balance of Fe and unavoidable impurities in a martensitic stainless steel material, passing through the center line parallel to the rolling direction or the wrought axis. cut, the range from the surface in a cross section containing the center line from the surface to 5mm mirror polished as the test surface, multiple field-of-view as the test area of the total test surface at 400 magnification becomes 5mm ² or more The number of precipitates per unit area to be measured, which is obtained by randomly counting the number of precipitates and counting the number of precipitates, is 1
Martensitic stainless steel material with excellent hot workability, characterized in that it is less than 00 pieces / mm ² .

(2) A martensite system excellent in hot workability, characterized in that the steel having the composition described in (1) above further contains Cu: 0.3 to 0.76 % by weight. Stainless steel material. (3) In the steel of the composition described in (1) above, further weight% is added.
And heat containing one or more of Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005% and B: 0.0003 to 0.0180%. Martensitic stainless steel with excellent hot workability. (4) In addition to the steel of the composition described in (2) above, further weight%
And heat containing one or more of Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005% and B: 0.0003 to 0.0180%. Martensitic stainless steel with excellent hot workability.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. First, the reasons for limiting the components in the present invention will be described. The content of the components is% by weight. C: C is an element that deteriorates the corrosion resistance required for oil country tubular goods and line pipes. Further, it is an element that lowers the toughness of the welded portion required for the line pipe. For this reason, the lowest possible level is desirable, and it is set to 0.005 to 0.05% as a range that can be industrially and economically reached by the current refining technology.

Si: Si is added for deoxidation in the refining process, and has the effect of reducing oxygen in steel, which is detrimental to hot workability, while δ-ferrite is detrimental to hot workability. Since it is an element having a tendency to form, it should be the minimum content required for deoxidation, and the appropriate range was 0.1 to 0.5%.

Mn: Mn is an element that fixes S, which is harmful to hot workability, as a sulfide to render it harmless, and is an element that suppresses the formation of δ ferrite, but if it is contained too much, the effect is saturated. Therefore, the appropriate range is 0.1 to 1.0%.

P: P is an element which tends to deteriorate hot workability, but its harmfulness is not so remarkable,
Since there is almost no effect in the range of 0.03% or less, 0.0
The upper limit was 3%.

S: S is a typical element that deteriorates the hot workability and causes surface defects, so that the lowest possible level is desirable. The upper limit is set to 0.005% in consideration of refining costs, but a more preferable upper limit is 0.002%.

Cr: Cr is an element essential for ensuring the corrosion resistance, and it is necessary to contain 10.0% or more. However, if it is contained in a large amount, δ ferrite is formed and the hot workability is deteriorated. The upper limit was 0.0%.

Ni: Ni is an element effective in improving the corrosion resistance, and 6.11 from the viewpoint of preventing the formation of δ ferrite.
% Is included as a lower limit, but it is an expensive element, and if included in a large amount, hot deformation resistance is increased and workability is lowered, so the upper limit was made 8.0%.

Mo: Mo, like Cr, is an element effective in improving the corrosion resistance, and is contained at a lower limit of 0.5%.
In addition to being an expensive element like Ni, it is an element having a strong ability to form δ ferrite, and a large amount of δ ferrite forms δ ferrite to deteriorate the hot workability, so 3.0% was made the upper limit.

N, A1, Ti: N, Al, and Ti are constituent elements of the precipitate specified in the present invention, and the contents thereof are as follows in order to satisfy the necessary and sufficient conditions of the present invention concerning the density of the precipitate. Need to be controlled. N is an element having a strong δ-ferrite formation preventing effect like C, and is an element that should be positively contained because it hardly deteriorates the corrosion resistance as compared with C. However, in consideration of the Ti and Al contents described later, an excessive content deteriorates the hot workability, so the appropriate range is 0.005.
.About.0.05%.

Al, like Si, is an element necessary for deoxidation, and in order to promote desulfurization and stably secure the above S content, 0.02% is contained as a lower limit,
If it is contained excessively, in addition to a large amount of oxide-based inclusions, nitrides are also formed. If the existing density of these increases, the hot workability deteriorates.
It was set to 5%.

Similar to Mn, Ti is an element that fixes S, which is harmful to hot workability, as a sulfide and renders it harmless.
003% is contained as a lower limit. On the other hand, Ti is an element having a strong affinity for N and forms a nitride. However, if it is contained excessively and the existing density of the nitride becomes high, the hot workability is rather deteriorated, so the upper limit is 0.050%. And

Cu: Cu is an element effective for improving the corrosion resistance as well as Ni, and is an element having the effect of preventing the formation of δ ferrite. Therefore, if necessary, 0.3% is made the lower limit, but Cu is excessive. If it is contained in the alloy, the hot workability deteriorates, so the upper limit was made 0.76 %.

Ca, Mg, B: Ca, Mg and B are both
Is an element effective in improving hot workability, and if necessary, C
a: 0.0005 to 0.005%, Mg: 0.0005
~ 0.005%, B: 0.0003 to 0.0180%
One kind or two or more kinds are contained. B is 0.0180%
If it is contained in excess of 10%, the hot workability is rather deteriorated.
It

Next, the reasons for limiting the existence density of precipitates in the present invention will be described. As the precipitate in the present invention, Ti nitride or Ti nitride is mainly used, and one or two kinds of nitride of Al, oxide of Ti, Al or the like, or sulfide of Mn or the like using this as a nucleus. The above means a complex precipitate, and does not include granular oxides such as alumina, which are generally referred to as inclusions, clusters thereof, or sulfides such as MnS that viscously deforms by hot working or silicates. .

The Ti nitride-based precipitate is already formed at the time of casting and grows in the material heating step prior to the subsequent hot working. Due to the difference in deformability between the precipitate and the matrix during hot working, when a working strain is applied, a void is formed at the boundary as illustrated in FIG. The increased abundance of these precipitates creates voids throughout the matrix during hot working, and adjacent voids are connected by cracks, resulting in deep flaws. For this reason, it is desirable that the density of precipitates is as low as possible, but in the present invention, the density of precipitates is allowed up to 100 per 1 mm ^{2 of the} surface to be examined in the microscopic test from the data of FIG. The range was set.

Further, the coarser the precipitate, the easier it is to form voids at the precipitate / matrix boundary, which is detrimental to hot workability. Therefore, it is desirable to suppress the coarsening as much as possible, as well as the existence density of the precipitates.
For this purpose, it is most effective to limit the material heating conditions, and a desirable condition for achieving the effects of the present invention is, for example, a heating temperature of 1000 to 1350 ° C.

The reason why the upper limit of the desirable range is 1350 ° C. is that MnS is decomposed when heated and maintained at a temperature higher than 1350 ° C., and in the cooling process accompanying the progress of hot working thereafter, the nitride already existing at the time of heating. The reason for this is that the compound precipitates on the oxide to increase the size. Considering the temperature rise due to heat generation during hot working, the more preferable upper limit of the material heating temperature is 1300 ° C. Further, the lower limit of the desirable range is 1000 C, because A at a temperature lower than this is
This is to avoid the formation of a nitride.

The size of the precipitate which is a problem in the present invention is defined as 0.5 μm or more in the length in the major axis direction as the critical dimension that exerts the above-mentioned harmfulness. It should be noted that this size is a size that can be sufficiently viewed with an optical microscope at a magnification of 400 to 1000 times.

As a method for quantifying the density of precipitates, the precipitates are randomly observed over a plurality of visual fields at a magnification of 400 times or more so that the total test area is 5 mm ² or more. The precipitates are counted, and the count value is divided by the total test area to calculate. It is desirable that the total area to be tested necessary to give reliability to the calculated value of the existence density of precipitates is as wide as possible. However, the accuracy does not change at 5 mm ² or more, so the lower limit is 5 mm ² and the upper limit is not specified. did.

As for the surface to be inspected, as shown in FIG. 4, in a rolled or wrought steel material such as a pipe, plate or round bar, it is cut through the center line parallel to the rolling direction or the wrought axis. Then, in the section including the center line from the surface of the steel material, the range from the surface to 5 mm was defined as the test surface. In the case of pipes, it is customary to manufacture them by the Mannesmann rolling method, and since the processing conditions are different on the inner and outer surfaces,
Two locations, the inner surface side and the outer surface side, are to be the test surfaces.

[0030]

EXAMPLES The present invention will now be described in more detail based on examples. A bloom having the composition shown in Table 1 was used as a raw material, and the raw material was heated to various temperatures for 2 hours, and then a steel roll having an outer diameter of 178 mm and a wall thickness of 11.5 mm was formed by the Mannesmann rolling method of press roll perforation-plug mill system. After rolling and cooling, ultrasonic flaw detection and magnetic flaw detection are performed to evaluate the presence or absence of surface defects having a depth of more than 0.1 mm and the defect occurrence frequency with the number of surface defect occurrence pipes as a percentage of the number of rolled pipes being evaluated. A microscopic test for determining the precipitate density was performed on the test surface shown in FIG. Also,
The presence or absence of δ ferrite was also investigated. In addition, in the press roll perforation-plug mill method in the present example, since the pipe inner surface defect was not seen, the test results will be described only for the pipe outer surface defect, but this does not limit the working range of the present invention. Absent.

The results are shown in Table 2. As is clear from this, Comparative Example No. 3 , 4, and 12 are Ni and C, respectively
Since the content of the γ-stabilizing elements such as u and N is less than the range of the present invention, δ ferrite is generated and defects are generated. Comparative Example No. In Nos. 7 and 13 , the precipitate existing density is within the range of the present invention, but since the contents of ferrite stabilizing elements such as Mo and Cr are larger than the range of the present invention, δ ferrite is generated, which causes defects. Has occurred. Comparative Example No. Nos. 5 , 8 , 11 , and 14 directly affect the hot workability of Cu, B, Ti, and S, although the precipitate density is within the range of the present invention and no δ ferrite is observed. Satisfactory results have not been obtained because the content of the element to be included is outside the range of the present invention.

Comparative Example No. Nos. 6 , 9 , and 10 have no δ-ferrite and the contents of direct elements such as S are reasonable values, but the contents of N, Al, and Ti are too large, and the precipitate density is high. Since it exceeds the scope of the present invention, the degree of the defect problem is so serious that it is comparable to the defects caused by δ ferrite. In contrast to these comparative examples, the present invention No. For depths 1 and 2 , a depth of 0.1 is essential for grinding.
No defects exceeding mm were observed, and satisfactory surface properties were obtained.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

As described above, according to the present invention, a martensitic stainless steel material containing a large amount of alloy elements useful for product performance such as Ni, Mo, and Cu, which are likely to cause rolling defects, is cracked or flawed. It is possible to perform hot forming while highly preventing this.

[Brief description of drawings]

FIG. 1 is a chart showing the relationship between the existing density of precipitates and the number of defect generation pipes according to the present invention.

FIG. 2 is a diagram showing a typical example of the relationship between the existence density of precipitates and voids according to the present invention.

FIG. 3 is a diagram showing the density of precipitates and Ti, A according to the present invention.
The chart which shows the relationship with 1 and N content.

FIG. 4 is a diagram showing an example of a steel material test surface used for investigating the existence density of precipitates in the present invention, in which (a) is a steel pipe, (b) is a steel plate, and (c) is an example of round steel. .

Continuation of the front page (72) Inventor Naoji Sato 1-1 Tobata-cho, Tobata-ku, Kitakyushu 1-1 Nippon Steel Co., Ltd. Yawata Works (56) Reference JP-A-8-199236 (JP, A) JP-A-10 -25549 (JP, A) JP 10-237604 (JP, A) JP 10-110248 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00

Claims (4)

(57) [Claims] 1. By weight%, C: 0.005-0.05%, Si: 0.1-0.5%, Mn: 0.1-1.0%, P: 0.03% or less, S: 0.005% or less, Cr: 10.0 to 14.0%, Ni: 6.11 to 8.0%, Mo: 0.5 to 3.0%, N: 0.005 to 0.05 %, Al: 0.02 to 0.15%, Ti: 0.003 to 0.050%, with the balance being Fe and unavoidable impurities in the martensitic stainless steel material, parallel to the rolling direction or the forging axis. Then, cut through the center line, and in the cross section including the center line, mirror-polish the range from the surface to 5 mm from the surface as the test surface, and the test surface has a total test area of 5 mm at a magnification of 400 times. randomly microscopically observed across the field of view so that ^two or more, depending on the method of counting the number of precipitates The number of precipitates per unit test area sought to 1
Martensitic stainless steel material with excellent hot workability, characterized in that it is less than 00 pieces / mm ² . 2. In% by weight, C: 0.005-0.05%, Si: 0.1-0.5%, Mn: 0.1-1.0%, P: 0.03% or less, S: 0.005% or less, Cr: 10.0 to 14.0%, Ni: 6.11 to 8.0%, Mo: 0.5 to 3.0%, N: 0.005 to 0.05 %, Al: 0.02 to 0.15%, Ti: 0.003 to 0.050%, and Cu: 0.3 to 0.76 %, with the balance being Fe and inevitable impurities. In stainless steel, cut parallel to the rolling direction or wrought axis through the center line, and in the cross section including the center line, mirror-polish the surface to be inspected up to 5 mm from the surface, inspection area in total 400 times magnification is speculum randomly across the field of view to be 5 mm ² or more The number of precipitates per unit test area determined by the method of counting the number of precipitates 1
Martensitic stainless steel material with excellent hot workability, characterized in that it is less than 00 pieces / mm ² . 3. By weight%, C: 0.005-0.05%, Si: 0.1-0.5%, Mn: 0.1-1.0%, P: 0.03% or less, S: 0.005% or less, Cr: 10.0 to 14.0%, Ni: 6.11 to 8.0%, Mo: 0.5 to 3.0%, N: 0.005 to 0.05 %, Al: 0.02 to 0.15%, Ti: 0.003 to 0.050%, and Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005% and B: 0. 0.0003 to 0.0180% of 1 or 2
In a martensitic stainless steel material containing at least one species and the balance consisting of Fe and unavoidable impurities, cut through the center line parallel to the rolling direction or the wrought axis, and from the surface in the cross section including the center line Mirror-polish the area up to 5 mm as the surface to be inspected, and randomly inspect the surface to be inspected at a magnification of 400 times so that the total area to be inspected is 5 mm ² or more. The number of precipitates per unit test area obtained by the counting method is 1
Martensitic stainless steel material with excellent hot workability, characterized in that it is less than 00 pieces / mm ² . 4. By weight%, C: 0.005-0.05%, Si: 0.1-0.5%, Mn: 0.1-1.0%, P: 0.03% or less, S: 0.005% or less, Cr: 10.0 to 14.0%, Ni: 6.11 to 8.0%, Mo: 0.5 to 3.0%, N: 0.005 to 0.05 %, Al: 0.02~0.15%, Ti : 0.003~0.050%, Cu: 0.3~ 0.76%, and Ca: 0.0005~0.005%, Mg: 0 0.0005 to 0.005% and B: 0.0003 to 0.0180% of 1 or 2
In a martensitic stainless steel material containing at least one species and the balance consisting of Fe and unavoidable impurities, cut through the center line parallel to the rolling direction or the wrought axis, and from the surface in the cross section including the center line Mirror-polish the area up to 5 mm as the surface to be inspected, and randomly inspect the surface to be inspected at a magnification of 400 times so that the total area to be inspected is 5 mm ² or more. The number of precipitates per unit test area obtained by the counting method is 1
Martensitic stainless steel material with excellent hot workability, characterized in that it is less than 00 pieces / mm ² .