JP5011770B2 - Method for producing martensitic stainless steel pipe - Google Patents

Description

  The present invention relates to a method for manufacturing a martensitic stainless steel pipe for an oil well pipe or the like that suppresses the occurrence of cracking due to delayed fracture and prevents the occurrence of internal flaws.

  Since martensitic stainless steel pipes such as API standard 13% Cr containing about 13% by mass of chromium (Cr) are required to have a yield strength of 80 ksi (552 MPa) or more and hot workability is usually required. Contains about 0.2% by mass of C. Thus, since there is much Cr content and C content, if it is hot pipe making, it will become a high-hardness martensitic structure, and toughness will be low. Therefore, when manufactured by the conventional steel component composition and manufacturing method, a portion subjected to processing by impact load or static load (hereinafter also referred to as “impact processed portion”) before pipe-forming heat treatment is performed. Cracks may occur due to delayed fracture. For this reason, when transporting and storing steel pipes, it is necessary to take measures such as limiting the stacking height of the steel pipes, and to shorten the waiting time until heat treatment after pipe forming.

  The above-mentioned restrictions on the transportation and storage of steel pipes require a large space due to the stacking height restrictions, and in order to perform heat treatment within the time limit after pipe making, the process from pipe making to heat treatment is required. Many problems occur throughout the steel pipe manufacturing process.

  Patent Document 1 stipulates the amount of effective solute carbon, the amount of effective solute nitrogen, the Cr equivalent and the S content, thereby making it difficult for delayed fracture of the impacted part to occur and preventing the occurrence of internal defects. A martensitic stainless steel seamless pipe and a method for producing the same are disclosed. However, even in the technique disclosed in this document, it is still necessary to shorten the time until the heat treatment after pipe making. In particular, the problem of laying cracks after one week or more before the heat treatment after hot pipe forming has not been solved.

JP 2004-43935 A (claims, paragraphs [0009] to [0016])

  The present invention has been made in view of the above problems, and the problem is that a martensitic stainless steel pipe having a C content of about 0.2% by mass is left for one week or longer until the heat treatment after hot pipe making. Another object of the present invention is to provide a method of manufacturing a steel pipe that does not cause a crack due to delayed fracture and can prevent the occurrence of internal flaws.

  In order to solve the above-mentioned problems, the present inventor prevents the occurrence of cracking due to delayed fracture of the impacted part, and the appropriate range of the steel component composition of the billet for suppressing the occurrence of inner surface flaws, and Appropriate conditions for the soaking temperature of the pierced and rolled element pipe and the cross-section working degree in the final rolling were studied, and the following findings (a) to (c) were obtained to complete the present invention.

  (A) In order to prevent delayed fracture of the impacted part due to solid solution strengthening after the production of the martensitic stainless steel pipe, the C content is 0.22% or less in mass%, and the N content Must be 0.05% or less. To suppress the occurrence of internal flaws after pipe making due to δ ferrite, the C content should be 0.15% or more, and to improve hot workability and prevent the formation of internal flaws, austenite stabilization It is necessary to make the N content of the chemical element 0.01% or more. The Mn content needs to be 0.10 to 1.00% by mass in order to ensure the strength, deoxidation action and hot workability of the steel.

  (B) Further, the steel component was selected from mass%, V: 0.200% or less, Nb: 0.200% or less, Ti: 0.200% or less, and B: 0.0100% or less. It is preferable to contain one or more types since the effect of preventing delayed fracture of the impacted part is exhibited.

(C) In order to prevent the occurrence of cracking due to delayed fracture of the impacted part and to suppress the occurrence of internal flaws, the billet having the steel component composition of the above (a) and (b) is pierced and rolled. It is necessary to adjust the reheating temperature T (° C.) during soaking of the obtained tube and the cross-section working degree R (%) in the final rolling so as to satisfy the relationship represented by the following expression (1). However, the upper limit of the reheating temperature T is 1020 ° C.

T> 44.4 × ln (R) +821 (1)
The present invention has been completed based on the above findings, and the gist of the present invention resides in a method for manufacturing a martensitic stainless steel pipe as shown in the following (1) to (4).

(1) By mass%, C: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10 to 1.00%, Cr: 12.00 to 14.00% N: 0.01-0.05%, Al: 0.001-0.1% , P: 0.020% or less and S: 0.010% or less, and V: 0.005-0 200%, Nb: 0.005 to 0.200%, Ti: 0.005 to 0.200%, and B: 0.0005 to 0.0100%, with the balance being Fe and A billet made of impurities is pierced and rolled, and further stretched and rolled to form a raw pipe. After the soaking so as to satisfy the relationship represented by the following formula (1), the final rolling is performed. Manufacturing method of onsite stainless steel pipe.

T> 44.4 × ln (R) +821 (1)
However, the upper limit of the reheating temperature T is 1020 ° C.

(2) In the method for manufacturing a martensitic stainless steel tube described in (1), wherein the billet is further, instead of part of Fe, by mass%, Ni: 0.001~ 0.5% Contact and Cu: One or more of 0.001 to 0.25 % may be contained.

(3) In the method for producing a martensitic stainless steel pipe according to the above (1) or (2) , the billet is further replaced by a part of Fe and is in mass%, and C a: 0.0050% or less It may contain.

  The seamless manufacturing method for seamless steel pipes includes a piercing and rolling process for drilling a hole in the center of a solid billet, a drawing and rolling process mainly for the wall thickness processing of the raw pipe obtained by piercing and rolling, And a constant diameter rolling process in which the diameter is reduced to finish the target dimension.

  Drawing 1 is a figure explaining an example of the manufacturing process of the Mannesmann pipe manufacturing method which manufactures a seamless steel pipe hot. In this pipe making method, a solid billet 1 heated to a predetermined temperature is fed to a piercing and rolling machine 3 and pierced and rolled to produce a raw pipe 2. Next, the raw tube 2 is fed to a subsequent mandrel mill (stretching mill) 4 and stretched and rolled. When the mandrel mill 4 is drawn and rolled, the blank 2 is cooled simultaneously with the drawn and rolled by the inserted mandrel bar 4b and a rolling roll 4r that regulates the outer surface of the blank. For this reason, the raw tube 2 is then charged into the reheating furnace 5 and soaked at a predetermined reheating temperature. Thereafter, the raw tube 2 is sent to the stretch reducer 6, and becomes a product through final rolling such as sizing, shape correction, and polishing tube by the rolling roll 6r.

  In the present invention, “final rolling” means constant diameter rolling with a stretch reducer, sizer, or the like.

  In addition, “cross section processing degree R (%) in final rolling” means a value calculated by the following equation (2).

R (%) = {(Sin−Sout) / Sin} × 100 (2)
Here, Sin represents the cross-sectional area of the steel pipe before the final rolling, and Sout represents the cross-sectional area of the steel pipe after the final rolling.

  In the following description, “mass%” for the component content is also simply denoted as “%”.

  According to the method of manufacturing a steel pipe of the present invention, the steel component composition of the billet and the relation between the soaking temperature of the pierced and rolled raw pipe and the cross-sectional processing degree in the final rolling are optimized, so that 13% Cr or the like It is possible to prevent the occurrence of cracks due to delayed fracture of the impacted part of the martensitic stainless steel pipe and to suppress the occurrence of internal flaws. Therefore, the present invention is useful as a method capable of producing martensitic stainless steel pipes for oil well pipes and the like without being restricted in the process from transportation and storage of steel pipes and from pipe making to heat treatment.

As described above, the present invention is, in mass%, C: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10 to 1.00%, Cr: 12.00. -14.00%, N: 0.01-0.05%, Al: 0.001-0.1% , P: 0.020% or less and S: 0.010% or less, and V: One or more of 0.005 to 0.200%, Nb: 0.005 to 0.200%, Ti: 0.005 to 0.200% and B: 0.0005 to 0.0100% are contained. The remaining billet is formed by piercing and rolling a billet made of Fe and impurities, and further stretch-rolled to form a raw pipe. The raw pipe is subjected to reheating temperature T (° C.) in soaking before final rolling and cross-sectional work degree in final rolling. After soaking so that R (%) satisfies the relationship represented by the formula (1), it is finally rolled. A method for manufacturing a martensitic stainless steel pipe characterized by and. Hereinafter, the method of the present invention will be described in more detail.

(1) Chemical component composition of steel pipe The reason why the chemical component composition of the martensitic stainless steel pipe of the present invention is specified as described above and the preferred range will be described.

C:
C is an element that brings about solid solution strengthening of the steel pipe after the pipe making together with N, that is, the steel pipe that has not been heat-treated while being piped by hot rolling. In order to prevent delayed fracture of the impacted part due to solid solution strengthening, the content needs to be 0.22% or less. However, if the C content is too low, it becomes impossible to maintain an appropriate strength after the heat treatment. Further, since C is an austenite-generating element, if the C content is excessively reduced, burrs are generated after pipe making due to the generation of δ ferrite. For the above reasons, the C content needs to be 0.15% or more. A more preferable range of the C content is 0.18% to 0.21%.

Si:
Si is an element having a deoxidizing action of steel. In order to obtain the effect, the Si content needs to be 0.1% or more. On the other hand, if the content exceeds 1.0%, the toughness deteriorates. For the above reason, the appropriate range of the Si content is set to 0.1 to 1.0%. When priority is given to ensuring toughness, the content is preferably 0.75% or less. A more preferable range of the content is 0.20 to 0.35%.

Mn:
Mn is an element that has an effect of improving the strength of steel and also has a deoxidizing action similar to Si. Furthermore, it has the effect | action which fixes S in steel as MnS and improves hot workability. In order to acquire those effects, it is necessary to contain 0.10% or more. However, when the content exceeds 1.00%, the toughness deteriorates. For the above reason, the appropriate range of Mn content is 0.10 to 1.00%.

Cr:
Cr is a basic component having an effect of improving the corrosion resistance of steel. In particular, by setting the Cr content to 12.00% or more, the pitting corrosion resistance and crevice corrosion resistance are improved, and the corrosion resistance in a CO ₂ environment is remarkably improved. On the other hand, since Cr is a ferrite-forming element, if its content exceeds 14.00%, δ-ferrite tends to be generated during high-temperature processing, and hot workability is impaired. Further, excessive addition of Cr causes an increase in manufacturing cost. For the above reason, the appropriate range of Cr content is 12.00 to 14.00%. A more preferable range of the content is 12.40 to 13.10%.

N:
N is an austenite stabilizing element and has an action of improving the hot workability of steel and preventing the occurrence of internal flaws. In order to obtain the effect, it is necessary to contain 0.01% or more. However, if the content exceeds 0.05%, delayed fracture of the impacted part is caused. Therefore, the proper range of N content is set to 0.01 to 0.05%. More preferably, the content is 0.02 to 0.035%.

P:
P is an impurity element in steel. When the content is large, the toughness of the product after the heat treatment is lowered, so the allowable upper limit of the content is set to 0.020% or less. The P content is preferably as small as possible.

S:
Since S is an impurity element that decreases the hot workability of steel as the content increases, the allowable upper limit of the content is set to 0.010%. The smaller the S content, the better, and even more preferably 0.003% or less.

One or more of V, Ti, Nb and B:
If you ask these elements are free chromatic exhibits the effect of preventing delayed fracture of the impact processing unit. For this purpose, to contain one or more of these elements, the content thereof, V, 0.005% or more for Ti and Nb, also, for B you 0.0005% or more. On the other hand, if the content of these elements is too large, the increase in hardness resulting from the formation of nitride after the heat treatment causes a deterioration in corrosion resistance and a decrease in toughness, and causes a fluctuation in strength. Therefore, V, about Ti and Nb are their contents and the 0.200% or less, also for the B shall be the 0.0100% or less.

One or more of Ni and Cu:
These elements may or may not be contained, but if one or more elements are contained, they have an effect of improving the hot workability and corrosion resistance of steel.

  Ni: Ni is an austenite stabilizing element. Therefore, when Ni is contained, it has the effect of improving the hot workability of steel. In order to acquire the effect, it is preferable to contain 0.001% or more. On the other hand, when the content is excessively large, the resistance to sulfide stress corrosion cracking is lowered. Therefore, the content is preferably 0.5% or less.

  Cu: Cu is an element that improves the corrosion resistance of steel, and since it is an austenite stabilizing element, if contained, it has the effect of improving the hot workability of steel. In order to acquire the effect, containing 0.001% or more is preferable. On the other hand, Cu is a low melting point metal, and when the content is excessive, the hot workability is reduced. Therefore, the content is preferably 0.25% or less.

Al: When Al is contained, it effectively acts as a deoxidizing agent for steel, and also has an effect of preventing the occurrence of outer shell flaws in the steel pipe. To obtain the effect, Ru is contained more than 0.001%. On the other hand, if the content is too large, the degree of cleaning of the steel is lowered, and the immersion nozzle is clogged during continuous casting. Therefore, Al content is you 0.1% or less.

  When Ca: Ca is contained, it has an action of combining with S in the steel and preventing a decrease in hot workability due to segregation of S at the grain boundary. In order to acquire the effect, containing 0.0001% or more is preferable. On the other hand, when Ca is contained excessively, it causes ground generation, so the content is preferably 0.0050% or less.

(2) Relationship between reheating temperature in soaking and cross-sectional working degree in final rolling After piercing and rolling a billet having an outer diameter of 190 mm having a steel component composition in the above-mentioned appropriate range, it is drawn and rolled into a raw pipe. After soaking at various reheating temperatures T (° C), the final rolling is performed with the cross-section processing degree R (%) changed, and the occurrence of cracks due to delayed fracture of the steel pipe after pipe making and the inner surface defect A basic test was conducted to investigate the occurrence. In some basic tests, tests using billets in which the N content or Mn content in the steel was outside the proper range were also conducted.

  For piercing and rolling, a two-roll type inclined rolling mill having a cone type roll was used, and the roll inclination angle was set to 12 to 15 ° and the crossing angle was set to 10 °.

  Table 1 shows the steel component composition of the billet, the manufacturing conditions of the steel pipe, and the test results.

  In the same table, the cross-section work degree R (%) means the cross-section work degree in the final rolling, and shows a calculated value by the following equation (2).

R (%) = {(Sin−Sout) / Sin} × 100 (2)
However, Sin represents the cross-sectional area of the steel pipe before final rolling, and Sout represents the cross-sectional area of the steel pipe after final rolling.

  The test results were evaluated based on the following evaluation criteria, and the evaluation results are shown in the evaluation column in the table.

Evaluation of inner surface defects:
50 or more steel pipes having a length of 10 to 12.5 m were manufactured, and the occurrence state of inner surface flaws was examined by visual inspection and an ultrasonic test (UST). The case where the occurrence rate of internal flaws was 3% or less was evaluated by ○ mark, and the case where the occurrence rate of internal flaws exceeded 3% was evaluated by X mark.

Crack evaluation:
A test piece for drop weight test having a length of 250 mm was collected from the steel pipe after pipe making, and a weight having a radius of curvature of 90 mm at the tip and a weight of 150 kg was dropped from the height of 0.2 m on this test piece (that is, The sample was deformed by applying an impact load of 294J), and the presence or absence of cracks was examined by visual inspection and UST. The case where cracks did not occur after 1 week was evaluated by a mark ◯, the case where cracks did not occur after 3 days was evaluated by a mark Δ, and the case where cracks occurred after 3 days were evaluated by a mark X.

Comprehensive evaluation:
Of the evaluation results of the inner surface defects and the evaluation results of cracks, the lower evaluation result was taken as the comprehensive evaluation result.

  Further, among the test results shown in Table 1 above, with respect to the test in which the N and Mn contents in the steel satisfy the appropriate range specified in claim 1, the degree of cross-sectional work and the effect on the comprehensive evaluation of inner surface flaws and cracks The effect of heating temperature was arranged.

  FIG. 2 is a diagram showing the influence of the cross-section working degree and the reheating temperature on the overall evaluation of cracks due to inner surface defects and delayed fracture of a steel pipe.

  From the results shown in Table 1 and FIG. 2, the boundary line between the case where the overall evaluation is ○ evaluation and the case where the evaluation is Δ evaluation or × evaluation is the cross-sectional work degree R (%) in final rolling and soaking. It calculated | required as a relational expression with reheating temperature T (degreeC), and obtained the following (3) Formula.

T = 44.4 × Ln (R) +821 (3)
From the relationship of the above formula (3), in order to prevent the occurrence of cracking due to delayed fracture of the impacted part and to suppress the occurrence of inner surface flaws, It has been found that it is necessary to adjust the reheating temperature T (° C.) and the cross-section working degree R (%) in the final rolling so as to satisfy the relationship represented by the formula (1).

  In addition, in test numbers A31 and A35 in which the N content in the steel deviates from the proper range and in test numbers A36 and A40 in which the Mn content deviates from the proper range, internal flaws or cracks occurred, and the overall evaluation was Δ evaluation or × evaluation Met.

  In order to confirm the effect of the manufacturing method of the martensitic stainless steel pipe of the present invention, a steel pipe manufacturing test described below was conducted and the result was evaluated.

  In the same manner as in the basic test described above, billets having an outer diameter of 190 mm having various steel composition compositions were pierced and rolled to produce raw tubes, and the raw tubes were soaked at various reheating temperatures T (° C.). After that, final rolling was performed while changing the cross-section working degree R (%), and the occurrence of cracks and the occurrence of internal flaws were investigated due to delayed fracture of the steel pipe after pipe making.

  For piercing and rolling, a two-roll type inclined rolling mill having a cone-type roll was used, and the roll inclination angle was 12 to 15 ° and the crossing angle was 10 °.

  Tables 2 and 3 show the steel component composition of the billet, the manufacturing conditions of the steel pipe, and the test results.

  The test results were evaluated according to the same criteria as those used in the basic test.

Test number B 9, B10, B14, B15 , B19, B20, B24, B25, B29, B30, B34, B35, B39 and B40 are examples of the present invention which satisfies the scope of the present invention, respectively. Test numbers B5 to B8 are comparative examples in which the N content or Mn content in the steel components does not satisfy the scope of the present invention, and the test numbers B11 to B13, B16 to B18, B21 to B23, and B26 to B28. , B31 to B33 and B36 to B38 are comparative examples in which the relationship between the cross-section processing degree R and the reheating temperature T does not satisfy the relationship of the expression (1).

  In the above examples of the present invention satisfying the scope of the present invention, neither cracking due to delayed fracture after pipe production nor internal flaws occurred, and a steel pipe of good quality was obtained. In particular, test numbers B9, B10 of the present invention containing one or more of V, Ti, Nb and B, or one or more of Ni and Cu, or one or more of Al and Ca, B14, B15, B19, B20, B24, B25, B29, B30, B34, B35, B39, and B40 each have further excellent delayed fracture resistance, hot workability, and the like.

  On the other hand, in the test numbers B5 to B8 of comparative examples in which the N content or the Mn content in the steel component does not satisfy the scope of the present invention, the occurrence of internal flaws due to a decrease in hot workability, delayed fracture resistance The occurrence of cracks due to the deterioration of the steel and the occurrence of cracks due to the decrease in the toughness were observed, resulting in a steel pipe of inferior quality. Moreover, test numbers B11 to B13, B16 to B18, B21 to B23, B26 to B28, B31 to B33, and B36 to Comparative Examples in which the cross-section processing degree R and the reheating temperature T do not satisfy the relationship of the expression (1). Each of B38 was a steel pipe with inferior performance due to cracks and internal flaws caused by delayed fracture.

  According to the method of manufacturing a steel pipe of the present invention, the steel component composition of the billet and the relation between the soaking temperature of the pierced and rolled raw pipe and the cross-sectional processing degree in the final rolling are optimized, so that 13% Cr or the like It is possible to prevent the occurrence of cracks due to delayed fracture of the impacted part of the martensitic stainless steel pipe and to suppress the occurrence of internal flaws. Therefore, the method of the present invention is a process for producing martensitic stainless steel pipes for oil well pipes and the like as a process capable of producing high-quality steel pipes without being restricted in the process from transportation and storage of steel pipes and from pipe making to heat treatment. Widely applicable in the field.

It is a figure explaining an example of the manufacturing process of the Mannesmann pipe manufacturing method which manufactures a seamless steel pipe hot. It is a figure which shows the influence of the cross-section work degree and the reheating temperature which give to the evaluation of the crack by internal flaw and delayed fracture of a steel pipe.

Explanation of symbols

1: solid billet, 2: blank tube, 3: piercing and rolling mill, 4: mandrel mill,
4b: mandrel bar, 4r: rolling roll, 5: reheating furnace,
6: Stretch reducer, 6r: Rolling roll

Claims (3)

In mass%, C: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10 to 1.00%, Cr: 12.00 to 14.00%, N: 0.01 to 0.05%, Al: 0.001 to 0.1% , P: 0.020% or less and S: 0.010% or less, and V: 0.005 to 0.200% Nb: 0.005 to 0.200%, Ti: 0.005 to 0.200% and B: 0.0005 to 0.0100%, and the balance is Fe and impurities. The billet is pierced and rolled, and further stretched and rolled to form a raw pipe. The raw pipe has a reheating temperature T (° C.) in soaking before the final rolling and a cross-section processing degree R (%) in the final rolling (1) The martenser is characterized in that it is soaked so as to satisfy the relationship represented by the formula, followed by final rolling. Method of manufacturing a door stainless steel pipe.
T> 44.4 × ln (R) +821 (1)
However, the upper limit of the reheating temperature T is 1020 ° C. Claims containing one or more of from 0.001 to 0.25 percent: the billet, further, instead of part of Fe, by mass%, Ni: 0.001 to 0.5% Contact and Cu Item 2. A method for producing a martensitic stainless steel pipe according to Item 1 . The billet is further, instead of part of Fe, at mass%, C a: method for manufacturing a martensitic stainless steel tube according to 0.0050% claim 1 or 2 containing hereinafter.

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