Method for producing a heat transfer tube for steam generator using drawing, solution heat treatment, and straightening

Abstract

A method for producing a heat transfer tube for a steam generator comprises a step of providing a tube and then applying cold drawing to the tube by using a high-pressure lubricating oil of 40 MPa or more in pressure. After the step of applying cold drawing to the tube, a step of applying a solid solution heat treatment to the tube is conducted. After the step of applying a solid solution heat treatment to the tube, a step of straightening the tube by using a roll straightening machine is conducted. An offset amount of 5 mm or less is formed for at least successive three pairs of upper and lower straightening rolls of the roll straightening machine.

Claims

1. A method for producing a heat transfer tube for a steam generator, the method comprising: a step of providing a tube; a step of applying cold drawing to the tube, the step of applying cold drawing to the tube including using a high-pressure lubricating oil that has a pressure of 40 MPa or more; after the step of applying cold drawing to the tube, a step of applying a solid solution heat treatment to the tube; and after the step of applying a solid solution heat treatment to the tube, a step of straightening the tube by using a roll straightening machine, wherein the roll straightening machine includes at least five pairs of concave globoidal drum type rolls, each pair of rolls being arranged opposite to each other in a vertical direction and in a crossing manner such that directions of rotating shafts of each pair of rolls cross each other, a stand interval being set at 300 mm or less; in the step of straightening the tube, offsetting is formed by three points along a tube axial centerline that are crossing positions of at least three successive pairs of upper and lower straightening rolls of the roll straightening machine and offsetting makes expressed by Formula (1) described below to satisfy 0.910.sup.3 or more and ensures an offset amount of 5 mm or less:
=1/R(d/2)(1) where given that an outside diameter of the tube is d (mm) and a stand interval of the roll straightening machine is L (mm) and an offset amount is (mm), R=(.sup.2+L.sup.2)/2 is established.

2. The method for producing a heat transfer tube for a steam generator according to claim 1, wherein a chemical composition of the tube consists of, in mass %, C: 0.15% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 10.0 to 40.0%, Ni: 8.0 to 80.0%, Ti: 0.5% or less, Cu: 0.6% or less, Al: 0.5% or less, and N: 0.20% or less, the balance being Fe and impurities.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an example of a chart showing the result of an inner probe type eddy current flaw detection of an inner surface of a tube.

(2) FIG. 2 is an illustration depicting a roll alignment example of an inclined roll type straightening machine.

(3) FIG. 3 is a graph showing one example of a roughness measurement chart along a longitudinal direction of an inner surface of a tube produced by a production process including a cold working process, a solid solution heat treatment process, and a straightening process.

(4) FIG. 4 is a schematic illustration to depict an amount of dimensional variation along a longitudinal direction of the inner surface of the tube, which is discussed by the present invention.

DESCRIPTION OF EMBODIMENTS

(5) Hereinafter, a heat transfer tube for a steam generator and a method for producing the same will be described.

(6) [Heat Transfer Tube for Steam Generator]

(7) A heat transfer tube for a steam generator according to the present invention is characterized in that an amount of dimensional variation in a specific length of 50 mm taken from a roughness measurement chart, which is obtained by measuring a surface roughness of an inner surface of the tube along a longitudinal direction, is 4 m or less and that an amount of bend crookedness in a portion of a length of 1000 mm from a tube end is 1 mm or less.

(8) In the present invention, when the surface roughness of the inner surface of the tube is measured along a longitudinal direction, a dimensional variation along a longitudinal direction of the inner surface of the tube shall be measured by use of a detector having a contact probe of 0.8 mm in radius. As described above with reference to FIG. 4, this is because short-cycled variations having a little effect on an S/N ratio in an eddy current flaw detection are to be removed to thereby measure wave undulation with a long cycle. Further, in the present invention, an amount of dimensional variation means a difference between a maximum value and a minimum value in a specific length of 50 mm taken from the measured roughness chart.

(9) The dimensional variation along a longitudinal direction of the inner surface of the tube is generated and increased by Pilger rolling and drawing work in a cold working process or by straightening by a roll straightening machine in a straightening process. The dimensional variation generated and increased as such is known to have a cycle of 50 mm or less, so that an amount of dimensional variation is determined from a specific length of 50 mm taken from the measured surface roughness chart.

(10) When the amount of dimensional variation along a longitudinal direction of the inner surface of an SG tube is more than 4 m, the S/N ratio in the eddy current flaw detection is decreased to thereby impair the inspection efficiency. When the amount of dimensional variation along a longitudinal direction of the inner surface of the SG tube is 4 m or less, an inspection by the eddy current flaw detection can be performed with a high S/N ratio and hence the inspection efficiency can be improved.

(11) Further, in the case where an amount of bend crookedness in a portion of a length of 1000 mm from a tube end, that is, in a range of 1000 mm from the tube end is controlled to be 1 mm or less, in assembling tubes into a steam generator/heat exchanger, the interference of the tube with other parts attributable to such bend crookedness of the tube can be inhibited and hence an assembling operation can be readily performed.

(12) [Method for Producing Heat Transfer Tube for Steam Generator]

(13) A method for producing a heat transfer tube for a steam generator according to the present invention is characterized by the following: when a tube subjected to cold drawing by use of a high-pressure lubricating oil of 40 MPa or more in pressure and to solid solution heat treatment is straightened by use of a roll straightening machine in which at least five pairs of concave globoidal drum type straightening rolls are provided, each pair of rolls being arranged opposite to each other in a vertical direction and in a crossing manner where directions of rotating shafts cross each other, and in which a stand interval is set at 300 mm or less, the tube is subjected to offsetting that is formed by three points literally along a tube axial centerline as being crossing positions of at least successive three pairs of upper and lower straightening rolls of the roll straightening machine and that allows r, expressed by Formula (1) described below to satisfy 0.910.sup.3 or more and to ensure an offset amount of 5 mm or less
=1/R(d/2)(1)
where given that an outside diameter of the tube is d (mm), a stand interval of the roll straightening machine is L (mm) and an offset amount is 6 (mm), R=(.sup.2+L.sup.2)/2 is established.

(14) When the tube is subjected to the drawing work by a high-pressure drawing by e use of the high-pressure lubricating oil of 40 MPa or more in pressure in the cold working process, an amount of dimensional variation along a longitudinal direction generated on the inner surface of the tube after the cold working (before straightening) can be reduced as compared with the case where the tube is subjected to Pilger rolling or drawing work under a lubrication treatment by a chemical treatment lubricating coating.

(15) When the pressure of the lubricating oil used in the cold drawing by the high-pressure drawing is less than 40 MPa, a lubricating oil film having a sufficient thickness is not formed between tools and the tube and hence seizing and/or vibration/chattering is caused, which hence increases the amount of dimensional variation along a longitudinal direction generated on the inner surface of the tube. For this reason, the pressure of the lubricating oil is set at 40 MPa or more. It is preferable that the pressure of the lubricating oil is set at 50 MPa or more. Further, it is preferable that the pressure of the lubricating oil is set at 150 MPa or less. When the pressure of the lubricating oil is more than 150 MPa, there is a risk that part of the lubricating oil is trapped in a portion on the inner surface of the tube to form a recessed portion to thereby generate a defect referred to as an oil pit. The oil pit generated on the inner surface of the tube develops dimensional variations of a short cycle in a roughness measurement chart and hence has a small effect on the S/N ratio in the inspection by the eddy current flaw detection, but causes the roughness on the inner surface of the tube, referred to as an arithmetic average roughness, to be deteriorated.

(16) Various conventional methods can be employed as a solid solution heat treatment, and when the solid solution heat treatment is performed, a heating temperature and a retention time thereof for the tube can be adequately determined from the size and the chemical composition of the tube. The solid solution heat treatment can be applied to the tube, for example, at a heating temperature of 1000 to 1300 C. and for a retention time of 5 to 15 min.

(17) In the straightening process, the tube is straightened by use of the roll straightening machine which has at least five pairs of concave globoidal drum type straightening rolls, each pair of rolls being arranged opposite to each other in a vertical direction and in a crossing manner where directions of rotating shafts of paired rolls cross each other, and which has the stand interval of 300 mm or less. Since the roll straightening machine which has at least five pairs of concave globoidal drum type straightening rolls is used, the bends and the out-of-roundness of the tube can be straightened while an amount of work per a pair of straightening rolls is decreased as compared with a conventional (2-2-2-1) type straightening machine which has three pairs of straightening rolls. In the case where the stand interval is more than 300 mm, the bends of the tube cannot be straightened unless an offset amount is increased, but increasing the offset amount so as to straighten the bends of the tube should increase an amount of dimensional variation in the inner surface of the tube after straightening.

(18) When the q expressed by Formula (1) described above is in the range of 0.910.sup.3 or more, the out-of-roundness and the bends of the tube can be straightened. On the other hand, if the expressed by Formula (1) described above is less than 0.910.sup.3, the bends remain in the tube after being subjected to the straightening process, thus resulting in a defective product.

(19) When the offset amount applied to the tube is 5 mm or less, an amount of work per a pair of straightening rolls is decreased and hence the imposed deflection of the tube is decreased at the time of straightening, which can hence suppress an increase in an amount of dimensional variation along a longitudinal direction of the inner surface of the tube by the straightening. When the offset amount applied to the tube is more than 5 mm, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube by the straightening is noticeably increased.

(20) According to the method for producing a heat transfer tube for a steam generator in accordance with the present invention, the cold drawing is performed to the tube by use of the high-pressure lubricating oil of 40 MPa or more in pressure and then the tube is straightened with offsetting in which the expressed by Formula (1) described above is in the range of 0.910.sup.3 or more and in which an offset amount is 5 mm or less. In the heat transfer tube for a steam generator produced as such, an amount of dimensional variation along a longitudinal direction of the inner surface of the tube is 4 m or less and the amount of bend crookedness in a portion of a length of 1000 mm from a tube end is 1 mm or less, which hence makes it possible to inspect the tube by the eddy current flaw detection with a high S/N ratio and hence can improve the inspection efficiency.

(21) For example, in the case where the tube is straightened by use of a (2-2-2-2-2) type straightening machine having five pairs of straightening rolls, at least successive three pairs of straightening rolls in which and the offset amount are set within ranges specified by the present invention can be arranged either on an entrance side, or in the intermediate region excluding foremost and rearmost pairs of rolls, or on a delivery side.

(22) Further, a straightening roll cross angle and an amount of crushing that are setup conditions of the roll straightening machine can be selected adequately from the size and material grade of the tube to be straightened. It is preferable that in each pair of straightening rolls, the roll cross angle is set in a range from 28 to 31 and the amount of crushing is set in a range from 1.5 mm to 3.0 mm.

(23) [Chemical Composition of Tube]

(24) In the heat transfer tube for a steam generator according to the present invention and in the method for producing the same, it is preferable that the chemical composition of the tube consists of, in mass %, C: 0.15% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 10.0 to 40.0%, Ni: 8.0 to 80.0%: Ti 0.5% or less, Cu: 0.6% or less, Al: 0.5% or less, and N: 0.20% or less, the balance being Fe and impurities.

(25) Here, the impurities mean constituents which are mixed in the tube from ores and/or scraps when the tube is commercially produced and which are allowed in a range not having an adverse effect on the present invention. The reasons of limiting the contents of the respective elements are as follows. Here, in the following description. % of the content of the element means mass %.

(26) C: 0.15% or Less

(27) If a C content is more than 0.15%, it is likely that stress corrosion cracking resistance can be deteriorated. Thus, when C is contained, it is preferable that the content of C is 0.15% or less, more preferably, 0.06% or less. Here, C has an effect of increasing the grain boundary strength of an alloy. In order to acquire this effect, it is preferable that the content of C is 0.01% or more.

(28) Si: 1.00% or Less

(29) Si is used as a deoxidizer at the time of melting and remains as impurities in the alloy. At this time, it is preferable that the content of Si is limited to 1.00% or less. If the content of Si is more than 0.50%, the cleanliness of the alloy is lowered in some cases. Thus, it is more preferable that the content of Si is limited to 0.50% or less.

(30) Mn: 2.0% or Less

(31) Mn is an element that immobilizes S, an impurity element, as MnS to thereby improve hot workability and that is effective as a deoxidizer. If the content of Mn is more than 2.00%, the cleanliness of the alloy is lowered. Thus, it is preferable that the content of Mn is 2.0% or less, more preferably, 1.0% or less. Further, in the case of acquiring the effect of improving the hot workability by Mn, it is preferable that the content of Mn is 0.1% or more.

(32) P: 0.030% or Less

(33) P is an element that remains as impurity in the alloy and if the content of P is more than 0.030%, P has an adverse effect on a corrosion resistance in some cases. Thus, it is preferable that the content of P is limited to 0.030% or less.

(34) S: 0.030% or Less

(35) S is an element that remains as impurity in the alloy and when the content of S is more than 0.030%, S has an adverse effect on the corrosion resistance in some cases. Thus, it is preferable that the content of S is limited to 0.030% or less.

(36) Cr: 10.0 to 40.0%

(37) Cr is an element necessary for keeping the corrosion resistance of the alloy and hence it is preferable that the content of Cr is 10.0% or more. However, containing Cr by more than 40.0% means that the content of Ni gets relatively smaller and hence it is likely to lower the corrosion resistance and the hot workability of the alloy. Thus, it is preferable that the content of Cr is 10.0 to 40.0%. In particular, if the content of Cr is 14.0 to 1.7.0%, the alloy exhibits excellent corrosion resistance in the environment including chloride, and if the content of Cr is 27.0 to 31.0%, the alloy is excellent in the corrosion resistance even in the environment including pure water and alkali at high temperatures.

(38) Ni: 8.0 to 80.0%

(39) Ni is an element necessary for securing the corrosion resistance of the alloy and it is preferable that the content of Ni is 8.0% or more. On the other hand, since Ni is expensive, it is enough that a minimum content of Ni as needed is contained according to use and hence it is preferable that the content of Ni is 80.0% or less.

(40) Ti: 0.5% or Less

(41) If the content of Ti is more than 0.5%, it is likely that the cleanliness of the alloy is deteriorated. Thus, it is preferable that the content of Ti is 0.5% or less and, more preferably, 0.4% or less. However, from the viewpoint of improving workability of the alloy and of inhibiting a grain growth at the time of welding, it is preferable that the content of Ti is 0.1% or more.

(42) Cu: 0.6% or Less

(43) Cu is an element that remains as impurity in the alloy, and if the content of Cu is more than 0.6%, the corrosion resistance of the alloy is lowered in some cases. Thus, it is preferable that the content of Cu is limited to 0.6% or less.

(44) Al: 0.5% or Less

(45) Al is used as a deoxidizer at the time of steelmaking and remains as impurity in the alloy. The remaining Al becomes oxide-based inclusions in the alloy and lowers the cleanliness of the alloy. Hence, it is likely that Al has an adverse effect on the corrosion resistance and the mechanical property of the alloy. Thus, it is preferable that the content of Al is limited to 0.5% or less.

(46) N: 0.20% or Less

(47) N may not be added to the alloy but the alloy intended by the present invention typically contains about 0.01% of N as impurity. However, if N is positively added to the alloy, N can increase the strength of the alloy without impairing the corrosion resistance. However, when the content of N is more than 0.20%, the corrosion resistance is lowered. Thus, it is preferable that the upper limit of the content of N is 0.20%.

(48) In the heat transfer tube for a steam generator according to the present invention and in the method for producing the same, it is preferable that a Ni-based alloy having chemical composition consisting of C: 0.15% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 10.0 to 40.0%, Fe: 15.0% or less, Ti: 0.5% or less, Cu: 0.6% or less, Al: 0.5% or less, the balance being Ni and impurities because the Ni-based alloy is more excellent in the corrosion resistance.

(49) The typical Ni-based alloy having the above-mentioned chemical composition and preferably used for the tube will include two kinds of alloys described below.

(50) (a) Ni-based alloy consisting of C: 0.15% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 14.0 to 17.0%, Fe: 6.0 to 10.0%, Ti: 0.5% or less, Cu: 0.6% or less, Al: 0.5% or less, the balance being Ni and impurities.

(51) (b) Ni-based alloy consisting of C: 0.06% or less, Si: 1.00% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Cr: 27.0 to 31.0%, Fe: 7.0 to 11.0%, Ti: 0.5% or less, Cu: 0.6% or less, Al: 0.5% or less, the balance being Ni and impurities.

(52) The alloy (a) described above contains 14.0 to 17.0% of Cr and about 75% of Ni, so that the alloy (a) is excellent in the corrosion resistance in the environment including the chloride. In this alloy, it is preferable that the content of Fe is 6.0 to 10.0% from the viewpoint of the balance of the content of Ni and the content of Cr.

(53) The alloy (b) described above contains 27.0 to 31.0% of Cr and about 60% of Ni, so that the alloy (b) is excellent in the corrosion resistance not only in the environment including chloride but also in the environment including pure water and alkali at high temperatures. Also in this alloy, it is preferable that the content of Fe is 7.0 to 1.0% from the viewpoint of the balance of the content of Ni and the content of Cr.

EXAMPLES

(54) Tests for verifying effects of the heat transfer tube for a steam generator according to the present invention and the method for producing the same were conducted.

(55) [Test Procedure]

(56) A tube was acquired by a cold working process of finishing the tube into a predetermined size, a solid solution heat treatment process, and a straightening process using a roll straightening machine for straightening bends and the out-of-roundness of the tube. In the cold working process, the tube was finished into a predetermined size by Pilger rolling or drawing work (high-pressure drawing) using a high-pressure lubricating oil of 120 MPa in pressure. In the straightening process, a (2-2-2-1) type straightening machine having three pairs of straightening rolls or a (2-2-2-2-2) type straightening machine having five pairs of straightening rolls was used.

(57) Test conditions are as follows.

(58) Chemical Composition of Tube:

(59) Material grade: Ni-based alloy specified by ASME SB-163 UNS N06690

(60) Ni-based alloy consisting of, in mass %, C: 0.021%, Si: 0.33%, Mn: 027%, P: 0.013%, S: 0.0002%, Cr: 29.4%, Fe: 9.8%, Ti: 0.25%, Cu: 0.03%, and Al: 0.11%, the balance being Ni and impurities.

(61) Solid solution heat treatment was performed at 1100 C. for three minutes.

(62) Tube A and Tube B of Ni-based alloys, which had the chemical composition shown in the above test conditions and were different from each other in size, were tested. The Tube A had an outside diameter of 19.14 mm, a thickness of 1.125 mm, and a length of 10,000 mm (10 m). The Tube B had an outside diameter of 17.57 mm, a thickness of 1.05 mm, and a length of 10,000 mm (10 m).

(63) In Table 1 and Table 2, shown are test number, test category, tube tested, finishing method in cold working process, an amount of dimensional variation along a longitudinal direction of inner surface of tube after cold working process and before straightening, straightening conditions, and test results. As for the straightening conditions, the number of pairs of straightening rolls of the roll straightening machine, the stand interval, the offset amount set for successive three pairs of straightening rolls, and the value of calculated by Formula (1) described above are shown in Table 1 and Table 2. Here, in the tests using the (2-2-2-2-2) type straightening machine having five pairs of straightening rolls, the value of and the offset amount shown in Table 1 and Table 2 are set for successive three pairs of straightening rolls which were arranged in the intermediate region excluding foremost and rearmost pairs of rolls.

(64) TABLE-US-00001 TABLE 1 Test results (after straightening) Before straightening Amount of Amount of dimensional dimensional variation in variation in Straightening conditions longitudinal longitudinal Number direction of direction of inner Cold of inner pairs of Stand Offset surface Test Tube working surface of rolls interval amount Remaining of tube S/N Overall No. Category tested process tube (m) (pair) (mm) (mm) (10.sup.3) bends (m) ratio evaluation 1 Comparative A Pilger 7.5 3 380 4 0.53 x 7.5 12 x example rolling 2 Comparative A 6.5 3 380 5 0.66 x 6.5 15 x example 3 Comparative A 8.0 3 380 6 0.80 x 8.5 8 x example 4 Comparative A 7.5 3 380 9 1.19 8.5 9 x example 5 Comparative A 6.5 3 380 10 1.32 8.0 11 x example 6 Comparative A High- 1.0 3 380 4 0.53 x 2.0 88 x example pressure 7 Comparative A drawing 1.0 3 380 5 0.66 x 3.0 78 x example 8 Comparative A 1.0 3 380 6 0.80 x 3.5 75 x example 9 Comparative A 1.0 3 380 9 1.19 6.5 25 x example 10 Comparative A 1.0 3 380 10 1.32 7.0 21 x example 11 Comparative A Pilger 7.5 5 270 2 0.53 x 7.5 15 x example rolling 12 Comparative A 6.5 5 270 3 0.79 x 6.5 13 x example 13 Comparative A 8.0 5 270 4 1.05 8.0 10 x example 14 Comparative A 7.5 5 270 5 1.31 8.5 7 x example 15 Comparative A High- 1.0 5 270 2 0.53 x 1.5 95 x example pressure 16 Comparative A drawing 1.0 5 270 3 0.79 x 1.5 98 x example 17 Inventive A 1.0 5 270 4 1.05 2.5 75 example 18 Inventive B 1.0 5 270 4 0.96 2.0 81 example 19 Inventive A 1.0 5 270 5 1.31 2.5 65 example 20 Inventive B 1.0 5 270 5 1.20 2.0 79 example 21 Comparative A 1.0 5 270 6 1.57 5.0 34 x example

(65) TABLE-US-00002 TABLE 2 Test results (after straightening) Before straightening Amount of Amount of dimensional dimensional variation in variation in Straightening condition longitudinal longitudinal Number direction of direction of inner Cold of inner pairs of Stand Offset surface Test Tube working surface of rolls interval amount Remaining of tube S/N Overall No. Category tested process tube (m) (pair) (mm) (mm) (10.sup.3) bends (m) ratio evaluation 22 Comparative A Pilger 7.5 5 240 2 0.66 x 7.5 18 x example rolling 23 Comparative A 6.5 5 240 3 1.00 6.5 12 x example 24 Comparative A 8.0 5 240 4 1.33 8.5 7 x example 25 Comparative A 7.5 5 240 5 1.66 8.5 8 x example 26 Comparative A High- 1.0 5 240 2 0.66 x 1.5 88 x example pressure 27 Inventive A drawing 1.0 5 240 3 1.00 2.5 85 example 28 Inventive A 1.0 5 240 4 1.33 2.5 76 example 29 Inventive B 1.0 5 240 4 1.22 2.0 77 example 30 Inventive A 1.0 5 240 5 1.66 3.0 58 example 31 Inventive B 1.0 5 240 5 1.52 2.5 64 example 32 Comparative A 1.0 5 240 6 1.99 5.5 25 x example
[Evaluation Criterion]

(66) In each test, an amount of dimensional variation in the inner surface of the tube subjected to the cold working was measured before and after the tube was subjected to the straightening. Further, an S/N ratio of the tube subjected to the straightening was measured by the eddy current flaw detection and remaining bends of the tube was evaluated. Still further, an overall evaluation of the tube was made on the basis of the results of remaining bends, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube subjected to the straightening, and the S/N ratio.

(67) The amount of dimensional variation is a difference between a maximum value and a minimum value in a specific length of 50 m taken from a roughness measurement chart, which was obtained by measuring the surface roughness of the inner surface of the tube by use of a surface roughness measurement device (made by Tokyo Seimitsu Co., Ltd. Type: SURFCOM 1500SD3). When the surface roughness was measured, a detector having a contact probe of 0.8 mm in radius was used.

(68) The S/N ratio was determined in the following manner, the inner surface of the tube was inspected by use of the eddy current flaw detection under conditions of a frequency of 600 kHz and a type of detecting local differential by using a drilled through-hole having a diameter of 0.66 mm as a standard notch; to thereby obtain values of S/N ratio where the total length of tube is subdivided into one-foot-length portions and an individual value of S/N ratio is determined for each portion: and among obtained values of S/N ratio, a minimum value was regarded as the S/N ratio of the tube.

(69) As for remaining bends, particularly, the bend crookedness near an end of the tube (hereinafter also referred to as nose bend) was observed as the bend of the tube subjected to the straightening. The meanings of signs in the column of [Remaining bends] in Table 1 and Table 2 are as follows:

(70) : the amount of bend crookedness in a portion of a length of 1000 mm from a tube end was 1 mm or less and hence the bends of the tube are considered as being sufficiently straightened, and

(71) x: the amount of bend crookedness was more than 1 mm for the portion as above and hence the bends of the tube are considered as being insufficiently straightened.

(72) The meanings of signs in the column of [Overall evaluation] in Table 1 and Table 2 are as follows:

(73) : the evaluation of remaining bends of the tube subjected to the straightening was good (), the amount of dimensional variation along a longitudinal direction of the inner surface of the tube was 4 m or less, and the S/N ratio was 50 or more.

(74) x: any one of the following conditions was not satisfied: that is, (Condition 1) the evaluation of remaining bends of the tube subjected to the straightening was good (); (Condition 2) the amount of dimensional variation along a longitudinal direction of the inner surface of the tube was 4 m or less; and (Condition 3) the S/N ratio was 50 or more.
[Test Results]

(75) As shown in Table 1 and Table 2, in all of Test Nos. 1 to 5, 11 to 14, and 22 to 25, which are comparative examples, Pilger rolling was employed as the cold working process and the amount of dimensional variation along a longitudinal direction of the inner surface of the tube not yet subjected to the straightening was 4 m or more. For this reason, in all of Test Nos. 1 to 5, 11 to 14, and 22 to 25, irrespective of the straightening conditions including the number of pairs of straightening rolls and the stand interval of the straightening machine, the offset amount, and the value of , the amount of dimensional variation along a longitudinal direction of the inner surface of the tube subjected to the straightening was more than 4 m in any of the tests and hence the overall evaluations were all x.

(76) In Test Nos. 6 to 10 of comparative examples, the tube was subjected to the cold drawing by the high-pressure drawing using the lubricating oil of 40 MPa or more in pressure, and a (2-2-2-1) type straightening machine having three pairs of straightening rolls and having a stand interval set at 380 mm was used. In all of Test Nos. 6 to 10, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube before straightening was 1.0 m.

(77) Of these tests, in Test Nos. 9 and 10, the offset amount was set at 9 or 10 mm and was set at 1.1910.sup.3 or 1.3210.sup.3, whereby the amount of working per each pair of straightening rolls was increased as compared with the conditions specified by the present invention. As a result, remaining bends of the tube subjected to the straightening became but the amount of dimensional variation along a longitudinal direction of the inner surface of the tube increased and became more than 4 m, so that the overall evaluation became x. Further, in Test Nos 6 to 8, the offset amount was set at 4 to 6 mm and q was set at 0.5310.sup.3 to 0.8010.sup.3, whereby the amount of working per each pair of straightening rolls was decreased. As a result, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube subjected to the straightening became 4 m or less but remaining bends of the tube subjected to the straightening became x, so that the overall evaluation became x.

(78) In Test Nos. 15, 16, and 21, which are comparative examples, the tube was subjected to the cold drawing by the high-pressure drawing using the lubricating oil of 40 MPa or more in pressure and a (2-2-2-2-2) type straightening machine having five pairs of straightening rolls and having a stand interval set at 270 mm was used. In all of Test Nos. 15, 16, and 21, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube before straightening was 1.0 m.

(79) Of these tests, in Test Nos. 15 and 16, the offset amount was set at 2 or 3 mm, which is within the range specified by the present invention, but was set at 0.5310.sup.3 or 0.7910.sup.3, which is outside the range specified by the present invention. In these cases, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube subjected to the straightening became 4 m or less in both cases but the remaining bends evaluation became x, so that the overall evaluation became x. Further, in Test No 21, was set at 1.5710.sup.3, which is within the range specified by the present invention, but the offset amount was set at 6 mm, which is outside the range specified by the present invention. In this case, the remaining bend evaluation of the tube subjected to the straightening became but the amount of dimensional variation along a longitudinal direction of the inner surface of the tube became more than 4 m, so that the overall evaluation became x.

(80) In Test Nos. 26 and 32, which are comparative examples, the tube was subjected to the cold drawing by the high-pressure drawing using the lubricating oil of 40 MPa or more in pressure and the (2-2-2-2-2) type straightening machine having five pairs of straightening rolls and having a stand interval set at 240 mm was used. In both of Test Nos. 26, and 32, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube before straightening was 1.0 m.

(81) Of these tests, in Test No. 26, the offset amount was set at 2 mm, which is within the range specified by the present invention, but TI was set at 0.6610.sup.3, which is outside the range specified by the present invention. In this case, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube subjected to the straightening became 4 m or less but the remaining bend evaluation became x, so that the overall evaluation became x. Further, in Test No 32, was set at 1.9910.sup.3, which is within the range specified by the present invention but the offset amount was set at 6 mm, which is outside the range specified by the present invention. In this case, the remaining bend evaluation of the tube subjected to the straightening became but the amount of dimensional variation along a longitudinal direction of the inner surface of the tube became more than 4 m, so that the overall evaluation became x.

(82) On the other hand, in Test Nos. 17 to 20 and 27 to 31, which are inventive examples of the present invention, the tube was subjected to the cold drawing by the high-pressure drawing using the lubricating oil of 40 MPa or more in pressure. The (2-2-2-2-2) type straightening machine having five pairs of straightening rolls and having a stand interval set at 300 mm or less was used. The tube was straightened with set at 0.910.sup.3 or more and with the offset amount set at 5 mm or less. As a result, all of evaluations including the remaining bends of the tube subjected to the straightening, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube, and the S/N ratio became good, so that the overall evaluation became .

(83) From these tests, the following facts could be verified: when the tube subjected to the cold drawing by the high-pressure drawing using the lubricating oil of 40 MPa or more in pressure and to the solid solution heat treatment was straightened by the roll straightening machine having at least five pairs of straightening rolls and having a stand interval set at 300 mm or less with the value of r) set at 0.910.sup.3 or more and with the offset amount set at 5 mm or less, the bends of the tube could be straightened and the amount of dimensional variation along a longitudinal direction of the inner surface of the tube subjected to the straightening could be controlled to 4 m or less and the tube having an excellent S/N ratio could be produced. Thus, it was made clear that according to the method for producing a heat transfer tube for a steam generator according to the present invention, a heat transfer tube for a steam generator according to the present invention, in which the amount of dimensional variation along a longitudinal direction of the inner surface of the tube is 4 m or less, can be produced.

INDUSTRIAL APPLICABILITY

(84) In a heat transfer tube for a steam generator according to the present invention, the amount of dimensional variation along a longitudinal direction of the inner surface of the tube is 4 m or less, so that when the tube is produced, an inspection using an eddy current flaw detection can be conducted at a high S/N ratio and hence the inspection efficiency can be improved.

(85) The method for producing a heat transfer tube for a steam generator according to the present invention has the following remarkable effects.

(86) (1) The tube is subjected to cold drawing by use of the high-pressure lubricating oil of 40 MPa or more in pressure, so that an amount of dimensional variation along a longitudinal direction of the inner surface of the tube after the cold drawing and before straightening can be reduced.
(2) The tube is straightened by using the roll straightening machine in which at least five pairs of concave globoidal drum type straightening rolls are disposed and a stand interval is set at 300 mm or less, and by applying offsetting, which is formed by at least successive three pairs of straightening rolls of the roll straightening machine and has set at 0.910.sup.3 or more and has the offset amount set at 5 mm or less, to the tube. This can reduce an increase of the amount of dimensional variation along a longitudinal direction of the inner surface of the tube attributable to the straightening.
(3) The method for producing a heat transfer tube for a steam generator according to the present invention, from the effects (1) and (2) described above, can produce the tube in which the amount of dimensional variation along a longitudinal direction of the inner surface of the tube is 4 m or less and in which the amount of bend crookedness in a portion of a length of 1000 mm from a tube end is 1 mm or less.

(87) Therefore, the heat transfer tube for a steam generator according to the present invention and the tube produced by the method for producing the same can secure an excellent quality accuracy and hence can guarantee quality at high reliability.

REFERENCE SIGNS LIST

(88) 1: tube to be straightened R, Ra, and Rb: straightening roll