JPH09287023A - Method for producing martensitic stainless steel seamless steel pipe - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a method for producing a seamless martensitic stainless steel pipe that has no anisotropy and is excellent in strength and corrosion resistance by performing heat treatment subsequent to pipe manufacturing in a pipe manufacturing line. A method for producing a seamless steel pipe using martensitic stainless steel, characterized in that the following steps (1) to (2) are sequentially performed. 40% or more of the total working ratio of both drawing and finishing processes applied to hollow shells, finishing temperature 800 ~
The process of making a seamless steel pipe at 1100 ° C, charging the above seamless steel pipe into a reheating furnace, and the temperature T at which the value of fn defined by the following equation (a) becomes a value between 22000 and 27000. (° C) and time t (hr) for supplementary heat treatment, at least 60
A process of cooling to 0 ° C at a cooling rate of 10 ° C / minute or more to 200 ° C or less, and then tempering at 500 to 780 ° C. fn = (T + 273) × (21 + logt) (a) However, T ≧ 800 (° C.).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a seamless steel pipe having high strength, excellent toughness and corrosion resistance, using stainless steel which has a substantially martensitic structure at room temperature.

[0002]

2. Description of the Related Art A seamless martensitic stainless steel pipe has hitherto been manufactured by a quenching-tempering heat treatment after the pipe is manufactured. In this method, it is necessary to reheat and quench the steel pipe once cooled after it has been manufactured.
There are many processes and energy consumption is large. Therefore, an attempt has been made to apply the so-called "direct quenching" used in the production of seamless steel pipes of ordinary steel and low alloy steel to the production of seamless steel pipes of martensitic stainless steel.

For example, Japanese Examined Patent Publication (Kokoku) No. 5-45651 discloses a method in which a steel pipe manufactured by a mandrel mill system is cooled to room temperature and then tempered under specific conditions.
Japanese Unexamined Patent Publication No. 5-98347 discloses that martensitic stainless steel (steel plate, steel pipe, etc.) immediately after hot working should be used as it is.
Each method of performing the stepwise cooling is disclosed.

However, in the steel pipes obtained by these methods, the formation of a texture is so great that it is superposed with the effect of chromium carbide precipitated at the grain boundaries, and mechanical properties such as toughness and sulfide stress crack resistance ( Anisotropy appears remarkably in corrosion resistance such as SSC resistance).

As a method for solving the above-mentioned problem of anisotropy, the present inventor has proposed a method in which hot working is performed under conditions of complete recrystallization after finish rolling and direct quenching is performed (Japanese Patent Laid-Open No. 4-104420). No. Gazette, see). However, since the recrystallization temperature of martensitic stainless steel is remarkably higher than that of low alloy steel, it may be difficult to completely recrystallize it in a conventional pipe mill of a seamless steel pipe depending on the product size. Further, even if the steel pipe is of a size that can be finished at a temperature higher than the recrystallization temperature, there are temperature variations depending on the portion of the steel pipe, which adversely affects the product.

[0006] In the process of manufacturing a seamless steel pipe, the hollow shell or the steel pipe after rolling does not come into uniform contact with the entire surface of the rolling roll, the beam of the conveying line or the like. Therefore, the cooling condition differs depending on the part of the one steel pipe (position in the longitudinal direction and the circumferential direction), and a considerable temperature difference occurs. If such a steel pipe is hardened as it is, depending on the part, it will be hardened without being recrystallized, and as a result, an anisotropic part and a part having different mechanical properties and corrosion resistance will occur in one steel pipe. Resulting in. That is, the product steel pipe becomes unusable for practical use with many variations in characteristics.

[0007]

DISCLOSURE OF THE INVENTION An object of the present invention is to perform conventional quenching which is carried out by reheating in a separate line after pipe making.
The tempering process is performed in the pipe making line following the pipe making,
Moreover, it is to provide a method for producing a seamless martensitic stainless steel pipe having no anisotropy and excellent in strength and corrosion resistance in all product sizes.

[0008]

The present invention is a method for producing a seamless steel pipe using a stainless steel that substantially has a martensitic structure at room temperature, characterized in that the following steps are sequentially performed. The manufacturing method of a seamless steel pipe is as follows.

A step of performing a drawing process and a finishing process on the hollow shell at a total working ratio of 40% or more and a finishing temperature of 800 to 1100 ° C. to obtain a seamless steel pipe, wherein the seamless steel pipe is a reheating furnace. The process of charging at the temperature T (° C) and time t (hr) at which the value of fn defined by the formula (a) below becomes a value between 22000 and 27000, At least 6 seamless steel pipes removed from the furnace
A process of cooling to 200 ° C or less at a cooling rate of 10 ° C / min or more up to 00 ° C, and then tempering at 500 to 780 ° C.

Fn = (T + 273) × (21 + logt) (a) However, T ≧ 800 (° C.).

The "stainless steel having a substantially martensitic structure at room temperature" which is the subject of the method of the present invention is a structure mainly containing martensite at room temperature (may contain less than 50% δ ferrite). It is stainless steel. There are no particular restrictions on the chemical composition, but typical components and their contents are as follows (% is% by weight).

C: 0.001 to 1.2%, Si: 1% or less, M
n: 2% or less, Cr: 8-17%, sol.Al: 0.005
~ 0.1%, P, S: 0.05% or less each, Mo: 0-3
%, Ni: 0 to 8%, Cu: 0 to 5%, N: 0.001 to 0.15
%, B: 0 to 0.01%, Ti, Nb, V: 0 to 0.5, respectively
%, Ca, Mg, Y, rare earth elements (La, Ce, etc.): 0 for each
~ 0.01%.

In addition to these alloy elements, a proper amount of other alloy elements may be contained.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Each step of the method of the present invention will be sequentially described below. The pipe-making material (billet) may be a billet manufactured by slab rolling or forging from an ingot or a continuously cast slab, bloom, or the like.
If a round billet is cast by continuous casting, it can be directly subjected to the punching process.

Hollow shell for drawing and rolling (hollow shell)
The manufacturing, i.e. drilling, can be done in any way. For example, a so-called piercer such as an inclined roll mill can be used. The perforation conditions may be basically the same as in the case of producing a seamless martensitic stainless steel seamless steel pipe. However, a large mill power is required to roll a thick hollow shell with a large degree of workability in the subsequent drawing and rolling. Therefore, in order to increase the workability of the rolling process in the next step, it is preferable to make the wall thickness as thin as possible in the punching step. For example, a method of making a piercer into a cone shape and using a piercer of a type capable of expanding and thinning a pipe with a roll having a crossed angle is recommended.

Stretching and finishing processes: There are various systems for performing this process, such as the Mannesmann-mandrel mill system and the Mannesmann-plug mill system. Either method can be adopted in the method of the present invention. For example, in the mandrel mill system, a drawing process is performed by a mandrel mill and a finishing process is performed by a sizer or reducer.

The stretching and finishing processes are low-temperature processes as compared with the piercing processes, and are important process steps for refining crystal grains. When the finishing temperature in these workings becomes lower than 800 ° C, coarse chromium carbides which do not form a solid solution sufficiently by the subsequent supplementary heat are precipitated, and the SSC resistance and toughness of the product steel pipe deteriorate. On the other hand, if the temperature exceeds 1100 ° C, the crystal grains become coarse, and the SSC resistance and toughness also deteriorate. Therefore, the finishing temperature must be 800-1100 ℃. It is desirable that the finishing temperature be as low as 800 to 900 ° C from the viewpoint of microstructure refinement.

The total processing degree of stretching and finishing is
If it is less than 40%, the grain refinement is not sufficient. The upper limit of the workability is not particularly limited, but if it exceeds 90%, the load on the tool will be heavy, so it is preferably in the range of 40 to 90%.

From the viewpoint of grain refinement, it is preferable that the interval between the drawing process and the finishing process be as short as possible.
That is, the effect is great if the finishing process is performed before the dislocations introduced during the stretching process are recovered, and the strain is sufficiently accumulated and then the grain size is reduced by recrystallization.

In the above-mentioned processing, the distance between the rolling mill (for example, a mandrel mill) for stretching and the rolling machine (for example, sizer or reducer) for finishing is determined by
It can be carried out using equipment installed at intervals shorter than the length of the hollow shell processed by the former. For example, a rolling process is preferred in which the finish rolling is immediately performed and the bar is pulled out from the hollow shell at the same time by using the extracting sizer.

Supplementary heating step: Supplementary heating is performed by charging the seamless steel pipe produced into a supplementary heating furnace provided in a pipe production line. This supplementary heat includes recrystallizing the steel pipe to which strain has been introduced in the previous work to form a fine structure, solidifying the chromium carbide precipitated during rolling, and heating the steel pipe evenly to disperse the characteristics. There are many purposes to reduce local anisotropy. The use of the supplementary heating furnace has an advantage that not only the temperature of the entire tube can be made uniform but also the temperature can be accurately adjusted, and the heat treatment conditions can be selected according to the characteristics desired for the product.

When the temperature of supplementary heat is lower than 800 ° C., precipitation and coarsening of chromium carbide are remarkable.

Therefore, the supplementary heat must be performed at 800 ° C. or higher. That is, T (° C) in the above equation (a) must be 800 or more.

When recrystallization is carried out at a high temperature, since the growth and coarsening of crystal grains start immediately after the recrystallization, supplementary heat must be made in a short time. Supplementary heat temperature T (° C) and time t
It is necessary to adjust the relationship of (hr) so that the value of fn in the equation (a) derived from the activation energy of recrystallization is 22000 to 27000. The recrystallization was not completed completely by supplementary heating under the condition that the value of fn was less than 22000.
Under the condition of more than 7,000, the crystal grains are remarkably coarsened, and the SSC resistance and toughness of the product steel pipe are deteriorated.

The finishing temperature in the above step may be higher than, equal to, or lower than the appropriate temperature in the reheating step. Therefore, the supplementary heat in the method of the present invention may be any of gradual cooling from the rolling finishing temperature, holding at a temperature substantially the same as the finishing temperature, and heating (heating) from the finishing temperature. There is no restriction on the heat pattern as long as the condition of the expression (a) is satisfied. In addition,
If the heat is supplemented under the condition that satisfies the expression (a), the temperature of the entire tube can be made uniform.

Cooling step: Since the martensitic stainless steel has a high hardenability, the cooling of the steel pipe exiting the auxiliary heating furnace is sufficient if the chromium carbide does not precipitate. Cool at a cooling rate of 10 ° C / min or more at least up to 600 ° C, which is a temperature range in which chromium carbide easily precipitates. As a result, the precipitation of carbides can be suppressed to such an extent that it does not pose a practical problem. If the temperature is lower than 600 ° C., it may be cooled at an arbitrary cooling rate to 200 ° C. or lower which is the substantial Mf point. However, it is preferable to completely cool to room temperature in order to reduce the retained austenite as much as possible.

The tempering is carried out by tempering the martensite structure produced by quenching to obtain the toughness and SSC resistance of the product steel pipe.
To improve the sex. If the temperature is lower than 500 ° C, the tempering effect is not sufficient, and if the temperature exceeds 780 ° C, the strength is lowered. The tempering time may be about 5 minutes to 1 hour.

[0028]

EXAMPLE Steels A to G shown in Table 1 were melted to prepare a billet having an outer diameter of 225 mm, which was rolled using a Mannesmann-mandrel mill to form a steel pipe having an outer diameter of 273.1 mm and a wall thickness of 9.3 mm. Manufactured. The manufacturing conditions are as shown in Table 2. In the heat treatment after supplementary heating, cooling in a temperature range lower than 600 ° C. was air cooling, and reheating was performed from the respective cooling end temperatures to the tempering temperature. Steel pipe strength changes with steel type, so
The tempering temperature was changed to adjust the yield strength (yield strength) to around 60 kgf / mm ² for all steel types.

The conventional example of Table 2 is the same as the above-mentioned JP-A-4-110420.
This is an example in which, according to the method disclosed in Japanese Patent Laid-Open Publication No. 2003-242242, the working is finished at a sufficiently high temperature and the recrystallized steel pipe is directly quenched and tempered.

[0030]

[Table 1]

[0031]

[Table 2]

With respect to the obtained steel pipe, a tensile test piece and a Charpy impact were applied in the pipe axial direction from 3 places at intervals of 3 m in the longitudinal direction from the pipe end and a total of 12 places obtained by equally dividing each of these positions in the circumferential direction. A test piece and a corrosion resistance test piece were sampled and subjected to the following tests to examine mechanical properties and corrosion resistance.

The tensile test was carried out using a round bar test piece having a diameter of 4 mm and a parallel portion of 34 mm. Charpy impact test is 5mm
The impact value at 0 ° C. was evaluated using a 2 mm V notch test piece of × 10 mm × 55 mm. Corrosion resistance (SSC resistance) is NACE TM 01
According to the constant load test specified in 77 METHOD-A, 45 kgf / mm
Applying the stress of ² , "30 atm. CO ₂ ＋ 0.01 atm. H ₂ S ＋ 5
% NaCl ", and evaluated for the presence or absence of cracks after 200 hours. The test results are shown in Table 3.

In Table 3, the yield strength and impact value are shown by the maximum value (M), the minimum value (m), the average value, and the variation (M-m) of the above 12 test pieces. The anisotropy was indicated by whether or not separation was generated on the fracture surface of the impact test piece. The SSC resistance was evaluated based on how many of the 12 test pieces passed (no cracking occurred).

[0035]

[Table 3]

The following facts are clear from Table 3.

1) The trial numbers 1 to 14, which are examples of the present invention,
Both strength and toughness are good, and the difference in these values depending on the part of the steel pipe is extremely small. That is, the variation is small.
Further, no separation, which is an index of anisotropy, is observed on the fracture surface of the impact test piece of the present invention example.

2) In the SSC resistance test, all 12 test pieces were free from cracks and had good SSC resistance.

3) In the conventional sample Nos. 22 to 28, the mechanical properties, particularly the strength, vary greatly depending on the part of the steel pipe.
In addition, even the SSC resistance test has not passed the total number.

4) Test Nos. 15 to 21 are comparative examples in which any of the conditions of pipe making and heat treatment do not satisfy the conditions defined in the present invention. Of these, trial No. 15 has a small total working ratio of stretching and finishing of 5%, and trial No. 16 has an excessively high finishing temperature, which results in coarse austenite grains, resulting in poor toughness and SSC resistance. Inferior.

5) In the sample No. 17, the supplementary heating temperature was too low, the carbides grew coarsely, and the ferrite transformation occurred, so the strength was low and the toughness and SSC resistance were poor.

6) In the sample No. 18, recrystallization was not sufficient and separation was observed because the value of fn was too small. That is, the anisotropy is large. On the other hand, in sample No. 19, the toughness and SSC resistance are inferior because the value of fn is too large and the austenite crystal grains become coarse.

7) Sample No. 20 has a low cooling rate up to 600 ° C. after supplementary heating, so that the toughness and SSC resistance are lowered due to the precipitation of coarse carbide. On the other hand, Sample No. 21 is tempered in a state where the martensite transformation is not completed because the cooling end temperature is too high, and has low strength and inferior toughness and SSC resistance.

[0044]

As is clear from the examples, the seamless steel pipe of the martensitic stainless steel produced by the method of the present invention has a characteristic variation which was a drawback of the steel pipe produced by the conventional direct quenching method. Almost none and no anisotropy. Since the method of the present invention can be continuously carried out online from pipe production to heat treatment, it greatly contributes to improvement of productivity and reduction of production cost in production of seamless steel pipe.

Claims (1)

[Claims] 1. A method for producing a seamless steel pipe using stainless steel having a substantially martensitic structure at room temperature, which is characterized in that the following steps (1) to (2) are sequentially performed. 40% or more of the total working ratio of both drawing and finishing processes applied to hollow shells, finishing temperature 800 ~
The process of making a seamless steel pipe at 1100 ° C, charging the above seamless steel pipe into a reheating furnace, and the temperature T at which the value of fn defined by the following equation (a) becomes a value between 22000 and 27000. (° C) and time t (hr) for supplementary heat treatment, at least 6
A process of cooling to 200 ° C or less at a cooling rate of 10 ° C / min or more up to 00 ° C, and then tempering at 500 to 780 ° C. fn = (T + 273) × (21 + logt) (a) However, T ≧ 800 (° C.).