Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
  • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

• the present invention relates to a martensitic stainless steel seamless pipe, such as a pipe for an oil well, which ensures no generation of cracks resulting from a delayed fracture.
• the present invention also relates to a method for manufacturing such a martensitic stainless steel pipe without any generation of inner surface defects such as internal scabs.
• Japanese Patent Unexamined Publication No. H8- 120415 discloses a martensitic stainless steel having a restricted N content.
• this patent specification only the improvement of toughness after a heat treatment is described.
• neither the relationship between the N content and a delayed fracture in the impact-worked portions of an as-rolled steel pipe nor the measures for suppressing such inner surface defects as internal scabs due to poor hot workability resulting from the decreased N content is described. It is not practical to manufacture a seamless steel pipe without any measures to suppress internal scabs.
• Japanese Patent Unexamined Publication No. H6-306551 discloses an invention, in which the hydrogen content is restricted to improve the toughness in the heat affected zone by welding of a martensitic stainless steel pipe having low carbon content. Furthermore, Japanese Patent Unexamined Publication No. H5-255734 describes an invention of dehydrogenating a martensitic stainless steel having low carbon content in order to prevent a delayed fracture. These inventions deal with a martensitic stainless steel having low carbon content. However, no description is given regarding the relationship between the hydrogen content and a delayed fracture in the impact-worked portions of an as-rolled martensitic stainless steel pipe containing such high C of about 0.2%.
• the present inventors have attained the first objective by restricting the correlation of the contents of C (carbon), H (hydrogen), N (nitrogen) and S (sulfur) in addition to specifying the contents of various elements in steel properly.
• the present inventors have attained the second object by specifying the condition to roll a steel pipe.
• the present invention is characterized by the following martensitic stainless steel (A) and the following method (B) for manufacturing martensitic stainless steel.
• “%” implies “mass %” regarding a content of each element.
• “as-rolled pipe” means a pipe which is formed by a hot rolling and to which a heat treatment has not been applied yet.
• a martensitic stainless steel seamless pipe characterized by consisting of, 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%, P: 0.020% or less, S: 0.010% or less, N: 0.05% or less, O (Oxygen): 0.0060% or less, Al: o to 0.1%, Ni: 0 to 0.5%, Cu: 0 to 0.25%, Ca: 0 to 0.0050% and at least one alloying element selected from at least one group of those mentioned below (totally 0.005 to 0.200 mass % in case of including two or more kinds of these alloying elements), and the balance Fe and impurities:
• the first group V, Nb and Ti of 0.005 to 0.200 mass %, respectively
• the second group B of 0.0005 to 0.0100 mass %, and is also characterized by satisfying either of the following inequalities (l), (2), (4) and (5) or the following inequalities (l), (3), (4) and (5): C * + ION * ⁇ 0.45, (1) HI ⁇ -0.003(C * + ION * ) + 0.0016, (2)
• N * N - [14 ⁇ (V/51) + (Nb/93) ⁇ /10]
• the steel pipe wherein has a C content of 0.18 to 0.21%, a Si content of 0.20 to 0.35%, a Cr content of 12.40 to 13.10%, a S content of 0.003% or less, and a N content of 0.035% or less.
• C * is an effective solute carbon content (mass %) defined by the following equation (6)
• N * is an effective solute nitrogen content (mass %) defined by equation (7)
• Cr * is a Cr equivalent defined by equation (8)
• CA. and F.A. in inequality (9) express a toe angle and a feed angle, respectively, a symbol of an element in each equation or inequality represents a content (mass %) of the respective element:
• N * N - [14 ⁇ (V/51) + (Nb/93) ⁇ /10]
• the steel pipe wherein has a C content of
• FIG. 1 is a diagram showing the relationship between a crack resulting from a delayed fracture and two parameters: the effective solute carbon content (C * ) and the effective solute nitrogen content (N * ).
• Fig. 2 is a diagram showing the relationship between the amount of residual hydrogen in an as-rolled steel pipe (Hi) and that in a heat-treated (H2).
• Fig. 3 is a diagram showing the relationship between a crack resulting from a delayed fracture and two parameters: "C * + ION * " and the amount of residual hydrogen in an as-rolled steel pipe (Hi).
• Fig. 4 is a diagram showing the relationship between a crack resulting from a delayed fracture and two parameters, "C * + ION * " and the amount of residual hydrogen in a heat-treated steel pipe (H2).
• Fig. 5 is a diagram of occurrence of internal scabs in a correlation of effective solute nitrogen content (N * ) and sulfur content.
• Fig. 6 is a diagram of occurrence of both internal scabs and external defects in correlation of "toe angle (CA.) + feed angle (F.A.)” and Cr equivalent (Cr * ).
• Sensitivity of a delayed fracture in the impact-worked portions of an as-rolled steel pipe depends upon the amount of both solute C and solute N, and especially upon that of solute N.
• the chemical composition of the martensitic stainless steel pipe according to the invention is determined as follows.
• C provides a solid- solution hardening of an as-rolled steel pipe together with N.
• the content of C should be 0.22% or less, and is preferably 0.21% or less, in order to suppress the delayed fracture of the impact-worked portions by the solid- solution hardening.
• a reduced C content makes it difficult to attain the aimed mechanical strength after a heat treatment.
• an excessive reduction in the C content causes internal scabs generated after making a steel pipe due to ⁇ -ferrite since C is an austenite-generating element.
• the content of C should be 0.15% or more, and the content of effective solute C should satisfy the inequality (l) above. The reason for this will be explained later. It is preferable that the C content is 0.18% or more.
• Si is added as deoxidant during steel making.
• a content of less than 0.1% provides no effect on deoxygenating whereas more than 1.0% causes a low toughness. Accordingly, the content should be 0.1 tol.0%.
• a preferable content is
• Cr is a basic element for enhancing a corrosion resistance of steel.
• a content of more than 12.00% improves a corrosion resistance for a pitting, and further greatly enhances a corrosion resistance under a CO2 environment.
• Cr is a ferrite -generating element
• the Cr content of more than 14.00% is apt to generate ⁇ ferrite in the process at a high temperature, causing a reduced hot workability .
• an excessive Cr content results in high cost of production. Accordingly, the content should be 12.00% to 14.00%, and is more preferably 12.40% to 13.10%.
• P is an impurity contained in steel.
• An excessive P content causes a low toughness of products after a heat treatment.
• An allowable upper limit of the P content should be 0.020%. It is preferable to minimize the P content as small as possible.
• S is an impurity that decreases a hot workability
• the S content should be minimized.
• An allowable upper limit of the S content is 0.010%.
• the S content should satisfy the inequality (5) above. It is preferable that the S content is 0.003% or less.
• N is an austenite -stabilizing element that improves the hot workability of steel.
• N causes a delayed fracture in the impact-worked portions of an as-rolled steel pipe.
• the upper limit of the N content should be 0.05%.
• the reduction in a hot workability resulting from a decreased N content is compensated by other elements, so that the N content should be minimized. It is preferable that the N content is 0.035% or less.
• O oxygen
• These elements combine with N to form nitrides.
• An inclusion of more than one selected from these elements provides a reduced the number of solute N solubility as if N content is decreased.
• an excessive N content causes extremely high hardness by the nitrides formed after a heat treatment and results in a reduction of a corrosion resistance and toughness.
• the V, Ti or Nb content should be 0.005 to 0.200%, respectively, and the B content should be 0.0005 to 0.0100%.
• the total content of these elements should be 0.005 to 0.200% in case of including two or more kinds of these alloying elements.
• Al can be added when deoxygenating during the steel making process and is effective for suppressing an external scab in a steel pipe.
• an excessive Al content causes a reduced cleanness of steel and also causes clogging of an immersion nozzle in the process of a continuous casting. Accordingly, it is preferable that the Al content is 0 to 0.1%.
• Ni is an austenite-stabilizing element and improves the hot workability of steel.
• an excessive Ni content causes a reduced sulfide stress corrosion cracking resistance. Accordingly, it is preferable that the Ni content is 0 to 0.5%.
• Cu is effective for enhancing corrosion resistance and is an austenite-stabilizing element to improve the hot workability of steel.
• Cu has a low melting point, and an excessive Cu content causes a reduced hot workability. Accordingly, it is preferable that the Cu content is 0 to 0.25%.
• Ca combines with S in steel and prevents a sulfur segregation in grain boundaries, which caused a reduced hot workability.
• an excessive Ca content causes macro-streak-flaws. Accordingly, it is preferable that the Ca content is 0 to 0.0050%.
• the present inventors studied the effect of C and N on a delayed fracture in the impact-worked portions of an as-rolled APT 13% Cr steel pipe.
• a delayed fracture test an impact load was applied to the steel pipes whose conditions will be described in "EXAMPLES”.
• the results are shown in Fig. 1 and Tables 1 to 4, in which an effective solute carbon content (C * ) and an effective solute nitrogen content (N * ) were used. The reason for using C * and N * is described below.
• C atoms combine with Cr atoms to form carbides.
• the content of C, acting as an interstitial element can be obtained by subtracting the content of C in the carbide from the total content of C Accordingly, an effective solute carbon content (C*) is defined by the equation (6).
• N atoms combine with V, Nb, Ti, B and Al atoms to form nitrides.
• the content of N, acting as an interstitial element can be obtained by subtracting the content of N in the nitride from the total content of N. Accordingly, an effective solute nitrogen content (N*) is defined by the equation (7).
• N* an effective solute nitrogen content
• Both C and N are interstitial elements in steel. If they have the same content, they provide approximately the same influence on the mechanical strength and the hardness. However, the content of C is restricted within a range of 0.18 to 0.21% in a 13% Cr martensitic stainless steel seamless pipe specified in the APTL80 grade, which is used for oil well. On the contrary, if the content of N is restricted only by "0.1% or less", then the content of N is widely selective. Usually, the N content is 0.01 to 0.05%, which is one tenth smaller than the C content. Therefore, the properties of steel were investigated on the relationship of the effective solute carbon content (C * ) and ten times of the effective solute nitrogen content (N * ).
• An interstitial element such as C and N influences on the work hardening due to a cold working when a steel pipe is subjected to the impact work.
• N provides pining of dislocations in order to increase the work hardening. From the experimental results, the inventors found that the work hardening and the delayed fracture due to hydrogen were suppressed remarkably when the amount of "C * + 10 N * " is restricted to 0.45 or less.
• the delayed fracture of the impact-worked portions is influenced by the hydrogen amount and the hardness of the portions. It is necessary to reduce the effective solute carbon content (C * > and the effective solute nitrogen content (N * > and thereby reduce hardness in order to suppress the generation of cracks.
• C * > effective solute carbon content
• N * > effective solute nitrogen content
• the amount of residual hydrogen in an as-rolled steel pipe is different from that in a heat-treated steel pipe.
• a heat treatment temperature is substantially fixed.
• the quenching temperature is 920 to 980 °C and the tempering temperature is 650 to 750°C.
• Fig. 2 is a diagram showing the relationship of the amount of residual hydrogen between HI (as-rolled) and H2 (after heat-treated) regarding the 13% Cr steel pipe used in the EXAMPLES below. For instance, at a point of the sign of O marked by "a", the amount of residual hydrogen (Hi) in an as-rolled steel pipe was approximately 3 ppm, and the amount of residual hydrogen (H2) after a heat treatment was approximately 2 ppm.
• Fig. 3 shows the result which is obtained by investigating a delayed fracture sensitivity of the impact-worked portions for an as-rolled steel pipe of a 13% Cr martensitic stainless steel having the C content of 0.19% and plotting the results on the correlation of "C * + ION * " and HI.
• Fig. 4 shows a result of a similar investigation and plots on the correlation of "C * + ION * " and H2 after a heat treatment.
• the inequalities (4) and (5) below represent the ranges of the Cr and S contents effective for suppressing an inner surface defect, which is called an internal scab.
• the satisfaction of the inequalities (2) and (3) above makes it possible to suppress a delayed fracture in the impact-worked proportions for an as-rolled steel pipe and after a heat treatment. Nevertheless, there is a possibility that an internal scab could be generated in the process of manufacturing a steel pipe.
• a generation of an internal scab results from a shear deformation in a circumferential direction in the process of pierce rolling with a piercing mill.
• the shear strain causes cracks on such a portion that has a different deformation resistance in a billet as ferrite/austenite grain boundaries, segregations of sulfur and inclusions. These cracks deform and cause internal scabs in the course of rolling.
• a sulfur-segregated portion also becomes an origin of generating a crack.
• S content should be 0.010% or less, and it is preferable that S content is 0.003% or less. It is preferable that the content of oxygen (O) is 0.0060% or less in order to reduce inclusions in steel, macro-streak-flaw and the S content during steel making.
• Fig. 5 illustrates a diagram of occurrence of internal scabs less than 2% (shown by a sign of O) or not less than 2% (shown by a sign of X ) in the correlation of N * in abscissa and S content in ordinate.
• This diagram leads to a recognition that restricting S content by the following inequality (5) suppresses an internal scab.
• the criteria line is decided to be 2 % of an internal scab generation from the viewpoint of work efficiency without interrupting manufacturing. S ⁇ 0.088 N * + 0.00056. (5)
• the steel having the above-mentioned chemical composition and satisfying the inequalities (l), (4) and (5) is pierce-rolled under conditions restricted by the inequality (9) with the aid of a cross roller type piercing mill.
• an increase in both a feed angle and a toe angle reduces the additional shear deformation in the process of pierce rolling, and makes it possible to roll the steel without generating cracks even if it has a poor hot workability.
• feed angle and toe angle cannot always be easily increased. In order to attain an increase in these angles, the replace of a main motor is required, and even a replace of the mill may be required. If the steel has a proper hot workability during rolling, it would be possible to choose a relatively small feed and toe angles.
• the relationship between an index regarding a hot workability during rolling and an index suppressing an internal scab i.e. an additional shear deformation can lead to a possible optimal manufacturing conditions of design of material of steel and conditions for pierce rolling from the viewpoint of economy in the manufacturing.
• the present inventors researched the past experimental data to investigate the influence of feed and toe angles on the additional shear deformation, and further studied the relationship between the Cr * and the sum of "CA. (toe angle) + F.A. (feed angle)". As a result, an explicit correlation between Cr * and "CA. + F.A.” was found on the basis that both of feed and toe angles contribute to the same extent to an additional shear stress
• Fig. 6 illustrates a diagram of the occurrence of both an internal scab and an external defect less than 2% (shown by O) or not less than 2% (shown by a sign of •) in a correlation of "CA. + F.A.” in abscissa and Cr * in ordinate.
• This map leads to the recognition that a boundary line of whether both an internal scab and an external defect are less than 2% (shown by O) or not (shown by a sign of •) can be expressed by the cubic curve.
• a condition satisfying the following inequality (9) leads to a suppressed generation of internal scabs.
• Cr * ⁇ 0.00009 (CA. + FA.) 3 - 0.0035 (CA. + FA.) 2
• a manufacturing method may include a process of re-heating before finishing rolling wherein a stretch reducer is used. It is preferable, in this case, that soaking is held at a temperature of 920°C or more during re-heating.
• a decreased soaking temperature during re-heating causes a reduced toughness of an as-rolled steel in T direction, which is perpendicular to a rolling direction, because of the incomplete recrystallization of flat grains, formed during working.
• C and N enriched areas are generated around Nb and/or V carbides/nitrides because of the incomplete solid solution or diffusion of the carbides and/or nitrides. Then, a hardening and a brittleness take place in the areas, which cause a delayed fracture.
• the lower limit of a soaking temperature during re-heating is 920°C, or more preferable 1000°C, and it is preferable that the upper limit of a soaking temperature is 1100°C or so.
• Drop test pieces having a 250 mm length were prepared from as-rolled steel pipes.
• a weight test element having 150 kg weight and a 90 mm curvature at its tip, was dropped from a 0.2 m height onto a test piece, which is deformed under an impact load (294J). After one week each piece was inspected as to whether or not cracks were generated. An inspection of cracks was carried out by a visual check and also by an ultrasonic test (UST). The results are listed in Tables 3 and 4.
• Fig. 1 is a diagram showing the relationship between the generated cracks and both effective solute carbon content (C * ) and effective solute nitrogen content (N * ).
• C * effective solute carbon content
• N * effective solute nitrogen content
• a straight line “a” implies a boundary of generating cracks.
• HI and H2 The amount of residual hydrogen of an as-rolled steel pipe and the amount of the same after a heat treatment were measured using an analyzing method specified in JIS Z2614. In the heat treatment, a test piece was water-quenched at the temperature of 950 °C and then tempered at 700 °C . The results of measurement are listed in Tables 3 and 4.
• Chemica composition (the balance: Fe and impurities, mass °/o)
• No .1, 2,4 -7,9,11,23 and 25 Present Invention o.3, 8,10,12- 22 and 24: -.omparative ⁇ : whether the inequality (1 ) is satisfied(O) or not( x ) (2): calculated value of the right side in the inequality (2) D: whether the inequality (2) is satisfied(O) or not( ) @: calculated value of the right side in the inequality(3) ⁇ : whether the inequality (3) is satisfied(O) or not( ) ): whether the inequality (4) is satisfied(O) or not( x ) Q): calculated value of the right side in the inequality (5) ®: whether the inequality (5) is satisfied(O) or not( x )
• Table 5 shows a relationship between an internal scab generation and two parameters, Cr * and "CA. + F.A.”
• a sign of O indicates that both an internal scab and an external scab are less than 2%
• a sign of # indicates that either an internal scab or an external scab is not less than 2%.
• Fig. 6 is a diagram of the results in Table 5 using the parameters, "CA. + F.A.” and Cr * .
• a cubic line in the diagram is expressed by the following equation (9)-l. Accordingly, the condition of suppressing an internal scab generation is to satisfy the inequality (9) above.
• Cr * 0.00009 (CA. + F.A.) 3 - 0.0035 (CA. + F.A.) 2
• a 13% Cr martensitic steel seamless pipe according to the invention prevents a delayed fracture generation when it is subjected to an impact cold working during handling after manufacturing the pipe.
• This steel pipe has an excellent corrosion resistance and is particularly available for oil well.
• a 13% Cr martensitic seamless steel pipe can be produced without an internal scab generation according to a manufacturing method of the invention.

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