JP2005131688A - Manufacturing method of seamless steel tube for boiler and its piping - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of seamless steel tube for a boiler and its piping, which does not cause a rolling defect on a product, even when a Mannesmann type skew rolling process is adopted. <P>SOLUTION: A material consisting of C:≤0.15 mass%, Mn:0.30-0.60 mass%, Si:0.20-1.0 mass%, Cr:8.0-10.0 mass%, Mo:0.85-1.10 mass%, P:≤0.030 mass%, S:≤0.030 mass% and Fe and inevitable impurities as the balance is formed into a seamless steel tube by a Mannesmann type rolling process composed of various rolling mills. In the process, during rolling at each rolling mill, the material is rolled adjusting a strain rate of the material to a value within a designated range corresponding to a rolling temperature of the rolling mill. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing a boiler and its seamless pipe for piping, and in particular, even if so-called “9% Cr alloy steel” is used as a raw material, rolling wrinkles are generated on the inner and outer surfaces by Mannesmann's inclined rolling method. The present invention relates to a technology for stably producing seamless steel pipes.

  In recent years, high-temperature equipment used in thermal power generation or the chemical industry is required to withstand higher temperatures and pressures. Therefore, in order to improve the creep characteristics of steel materials used as equipment materials, steel materials containing 9 to 12% by mass of Cr have been developed and registered as standards in JIS and ASTM.

  In general, when the Cr content of the steel material increases, the creep characteristics are improved, but on the other hand, the workability is lowered. Therefore, in developing these high Cr steels, efforts have been made to improve the creep characteristics while exploring the limits of workability. However, even if an attempt is made to produce a seamless steel pipe by such an inclined rolling method such as a Mannesmann mill using such a steel material as a raw material, there is a problem that the rolling itself is difficult.

  As shown in FIG. 1, the seamless steel pipe is generally formed by a Mannesmann mandrel type or Mannesmann plug mill type rolling process. In other words, the raw material 2 that has passed through the heating furnace 1 is punched into a raw tube 3, and a piercer 4, a mandrel mill 5, an elongator 6, a plug mill 7, a reeler 8, a reducer 9, and a sizer 10 are used to expand, stretch, and finish the tube. In general, it is manufactured by a Mannesmann-mandrel method (upper-stage process in FIG. 1) or a Mannesmann-plug mill system (lower-stage process in FIG. 1) with an appropriate machine.

This Mannesmann tilt rolling is known as the processing method that gives the most severe load to steel materials during hot working of steel materials. When the steel materials are rolled by this method, rolling flaws are generated on the inner and outer surfaces of the product. There are many. And it is thought that generation | occurrence | production of this rolling wrinkle influences the following phenomena.
1) Since it contains a large amount of alloying elements such as Cr, brittle cracking occurs due to insufficient workability. 2) The material is inferior in hot workability due to excessive temperature rise in the heating furnace. Ferrite phase precipitates, and the grain boundary slip fracture occurs as the precipitate melts. For this phenomenon, Mannesmann rolling method uses a plurality of various rolling mills as shown in FIG. Because. That is, the rolling process is complicated and time consuming, so the temperature drop of the material during rolling is large, and in order to maintain the hot workability of the material, it is desired to roll at the highest possible temperature, but excessive temperature rise is As described above, since flaws are easily generated, the rolling temperature of the material in each rolling mill is uniformly determined except for the downstream side of the reheating furnace, and is just below the precipitation temperature of the δ-ferrite phase. This is because rolling starts.

  Countermeasures have been proposed for such cracking problems during rolling. For example, there is a technology that appropriately balances the addition amount of main alloy elements such as Cr, Ni, Mo, Cu, C, and N in order to make the metal structure of the material in the hot working temperature range into an austenite single phase (patent) Reference 1). It has also been proposed to limit the content of impure components harmful to hot workability, represented by S, to be particularly low (Patent Documents 2 and 3).

However, even if such countermeasures are applied to a steel material containing 9 to 12% by mass of Cr, the present situation is that the generation of wrinkles due to a decrease in hot workability cannot be solved. Further, measures to increase the amount of expensive alloy elements added and to reduce impurity components such as S are not desirable from an economic standpoint because they increase costs at the steelmaking stage.
Japanese Patent Application Laid-Open No. 5-263138 (page 3, lines 10 to 24 in the right column) Japanese Patent Publication No. 3-60904 (3 pages, 33 lines to 4 pages in the right column, 7 lines in the right column) JP 11-140594 A (all paragraphs [0011] on page 3)

  In view of such circumstances, an object of the present invention is to provide a boiler that does not cause rolling flaws in a product even when the Mannesmann tilt rolling method is employed, and a method for manufacturing a seamless steel pipe for piping.

  In order to achieve the above-mentioned object, the inventor conducted extensive research on the influence of the rolling temperature and strain rate on the hot workability of the material to be rolled, and strained according to the material temperature during rolling (hereinafter referred to as rolling temperature). It has been found that if the speed is adjusted, good hot workability can be maintained, and as a result, the rolling wrinkles of the product can be reduced, and the present invention has been completed.

That is, the present invention includes C: 0.15 mass% or less, Mn: 0.30 to 0.60 mass%, Si: 0.20 to 1.0 mass%, Cr: 8.0 to 10.0 mass% , Mo: 0.85 to 1.10% by mass, P: 0.030% by mass or less, S: 0.030% by mass or less, the balance of Fe and inevitable impurities, various rolling mills When making a seamless steel pipe in the mandrel type rolling process provided, at the time of rolling in each rolling mill, the strain rate of the material is adjusted to a value within a certain range according to the rolling temperature in the rolling mill, and the material is rolled It is the manufacturing method of the boiler and the seamless steel pipe for the piping characterized by doing. In this case, it is preferable that the strain rate during rolling of the material satisfies the following range.
1250 ° C. ≦ temperature range of rolling temperature: strain rate ≧ 1.0 / sec,
1200 ° C. ≦ rolling temperature <1250 ° C. temperature range: strain rate ≧ 0.6 / sec,
1150 ° C. ≦ rolling temperature <1200 ° C. temperature range: 3.5 / sec ≧ strain rate ≧ 0.3 / sec,
Rolling temperature <1150 ° C. temperature range: 1.4 / sec ≧ strain rate

  According to the present invention, the rolling defects of the boiler and its seamless pipe for piping can be greatly reduced. Moreover, it is not necessary to make the management range of components strict, and a reduction in material cost can be expected.

  Hereinafter, embodiments of the present invention will be described in detail.

First, a seamless steel pipe targeted by the present invention is manufactured from an alloy steel having the following composition. This is defined as JIS G3462-STBA26 and ASTM A213 / 213M-T9 and T91. The reason for limiting the content of each component as described above is as follows.
C: 0.15% by mass or less C is an element that forms Cr carbide and the like and deteriorates the corrosion resistance of steel, but is also an element necessary for exerting strength, so the lower limit is set to 0.15% by mass. .
Mn: 0.30 to 0.60 mass%
Mn is an austenite stabilizing element and is effective in suppressing precipitation of the δ ferrite phase during hot working, so 0.30% by mass is set as the lower limit. On the other hand, if added too much, the strength of the crystal grain boundaries is lowered and brittle deterioration of the steel is caused, so 0.60% by mass is made the upper limit.
Si: 0.20 to 1.00% by mass
Since Si is an element necessary as a deoxidizer in the steelmaking stage, the lower limit is set to 0.20% by mass. On the other hand, if it is contained too much, the toughness of the steel is deteriorated, so 1.00% by mass is set as the upper limit. To do.
Cr: 8.0-10.0 mass%
Cr is a basic element for improving corrosion resistance, and the lower limit is set to 8.0% by mass in order to obtain sufficient corrosion resistance. Moreover, it is also a stabilizing element of the ferrite phase, and if it is too much, the δ ferrite phase is precipitated during hot working and deteriorates hot workability and resistance to sulfide stress corrosion cracking. Therefore, the upper limit is made 10.0 mass%.
Mo: 0.85 to 1.10 mass%
Mo prevents corrosion between crystal grains, improves weldability, and increases the high-temperature strength of the steel sheet. However, excessive addition will promote secondary curing due to the precipitation of MoC2 and is not economical. Therefore, the upper and lower limits were set to 0.85 to 1.10% by mass.
P: 0.030% by mass or less P significantly deteriorates hot workability at a high temperature of 1200 ° C. or higher. In particular, when piercing a steel material with a piercer, it causes a flaw on the inner surface side of the hole, so 0.03% by mass is the upper limit.
S: 0.030% by mass or less S is an element that deteriorates hot workability. Since it contributes to generation | occurrence | production of an internal flaw especially at the time of the piercing | piercing with a piercer like P, 0.030 mass% is made an upper limit.

  Furthermore, in the present invention, in addition to the above components, an appropriate amount of Al, Ti, Nb, Ni, Cu, N, or the like may be added in order to obtain necessary strength and corrosion resistance.

  Next, the inventor decided to study the stable production of seamless steel pipes by using the above-described alloy steel as a raw material and suppressing the occurrence of wrinkles by the Mannesmann tilt rolling method. This is because rolling can be performed with good hot workability in order to suppress the generation of wrinkles. Then, as described above, as a factor affecting the hot workability of such a material, attention was paid to the rolling temperature and strain rate of the material. The reason why the strain rate of the material is selected in addition to the rolling temperature is that the strain rate of the material is considered to be an effective factor for controlling the hot working.

  Here, the strain rate of the material is defined as follows.

Strain rate = ln [(cross-sectional area before rolling of material / cross-sectional area after rolling of material) / time]
Or strain rate = ln [(wall thickness before rolling of material / wall thickness after rolling of material) / time]
In addition, the cross-sectional area of a raw material is a cross-sectional area of an actual meat part, when a raw material is a tubular shape.
Further, in order to specifically determine these strain rate values, the cross-sectional area or thickness may be measured by heating the material to a constant temperature and then applying a constant reduction. In addition, any of the values obtained by the above two formulas may be used.

  Accordingly, the inventor has focused on the fact that this strain rate is strongly influenced by the material temperature during rolling (hereinafter, rolling temperature), and has determined the relationship between the strain rate and the rolling temperature. And 9 mass% Cr steel (0.10C-0.25Si-0.45Mn-9.0Cr-1.0Mo) was used as a raw material, and the influence of rolling temperature and strain rate on its hot workability was investigated. . The investigation is that the material is heated from room temperature to 1250 ° C. in 10 seconds, held for 10 seconds, cooled to a temperature at which deformation (tensile) is performed at 10 ° C./sec, and held at that temperature for 10 seconds. This was performed by pulling at an arbitrary strain rate. That is, a value obtained by dividing the cross-sectional area of the fractured portion after the tension of the material by the cross-sectional area before the tension is defined as “drawing value”, and this “drawing value” is used as a scale indicating the hot workability of the material. Higher “drawing value” means better hot workability of the material.

  The investigation results are summarized in Table 1 as the relationship between the “drawing value” (unit%) with respect to the rolling temperature (vertical axis) and the strain rate (horizontal axis).

From the knowledge so far, it is known that if the “drawing value” is 75% or more, good hot workability is exhibited at that temperature. From Table 1, for example, in the rolling temperature range of 1250 ° C. The “aperture value” shows a high value in a relatively high strain rate region where the speed is 1.0 / sec or more, and the “aperture value” is 75% in a relatively low strain rate region where the strain rate is 0.6 / sec or less. It is clear that it shows a low value below. On the other hand, when focusing on the rolling temperature range of 1000 ° C., in the strain rate range of 0.1 to 10 / sec, all are lower than 75%, but the drawing value is higher when the strain rate is lowered. . In addition, the precipitation temperature (AC ₄ ) of the δ ferrite phase of 9.0 mass% Cr steel is 1260 ° C., and when considering the micro / macro segregation, the temperature during rolling should not be 1260 ° C. or higher. Omitted the strain rate investigation.

  This investigation result means that if the rolling temperature and strain rate of the material are appropriately selected, rolling can be performed with good hot workability. It was also found that in the conventional Mannesmann operation, the strain rate was not appropriate for the rolling temperature of the material in each rolling mill. Therefore, the present invention is to perform pipe making at a strain rate at which hot workability is most advantageous in each rolling mill.

Moreover, based on the results in Table 1, when the regions of the strain rate suitable for operating with good hot workability are arranged for each rolling temperature region,
1250 ° C. ≦ temperature range of rolling temperature: strain rate ≧ 1.0 / sec, especially appropriate strain rate = 10 / sec
1200 ° C. ≦ rolling temperature <1250 ° C. temperature range: strain rate ≧ 0.6 / sec, especially appropriate strain rate = 3.5 / sec
1150 ° C. ≦ rolling temperature <1200 ° C. temperature range: 3.5 / sec ≧ strain rate ≧ 0.3 / sec, especially appropriate strain rate = 1.8 / sec
Rolling temperature <1150 ° C. temperature range: 1.4 / sec ≧ strain rate, especially appropriate strain rate = 1.4 / sec
It becomes. Therefore, in the present invention, if the operation is performed by determining the rolling temperature and strain rate in each rolling mill according to this standard, a seamless steel pipe with fewer rolling mills than conventional can be manufactured.

  A seamless steel pipe was manufactured using 9 mass% Cr steel (0.10C-0.25Si-0.45Mn-9.0Cr-1.0Mo). The utilized process is the manufacturing process of the plug mill method shown on the upper side of FIG. The round billet of material 2 was an outer diameter of 260 mmφ, and the product steel pipe had an outer diameter of 323.9 mmφ and a wall thickness of 33.3 mm. In this production, the rolling temperature and strain rate in each rolling mill were determined according to the present invention. The rolling temperature at Piercer 4 and the rolling temperature and strain rate at each rolling mill at that time were set as shown in Table 2. In Table 2, Level 1 is the conventional example, and Level 2 is the example of the present invention.

  The results of the implementation are shown in FIG. 2, and the rate of occurrence of inner surface defects investigated with the product was 18.7% for the level 1 of the conventional example, whereas the level 2 of the present invention decreased to 2.4%, The outbreak of drought has dropped dramatically.

It is a flowchart which shows the manufacturing process of the seamless steel pipe by a Mannesmann system. It is the figure which compared the implementation result of this invention as the internal surface defect incidence rate with the past.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Material 3 Raw material pipe 4 Piercer 5 Mandrel mill 6 Elongator 7 Plug mill 8 Leela 9 Reducer 10 Sizer

Claims (2)

C: 0.15 mass% or less, Mn: 0.30 to 0.60 mass%, Si: 0.20 to 1.0 mass%, Cr: 8.0 to 10.0 mass%, Mo: 0.85 ~ 1.10 mass%, P: 0.030 mass% or less, S: 0.030 mass% or less, the balance consisting of Fe and inevitable impurities, the mandrel type rolling mill equipped with various rolling mills When making seamless steel pipes in the process,
At the time of rolling in each rolling mill, adjusting the strain rate of the raw material to a value within a certain range according to the rolling temperature in the rolling mill, and rolling the raw material, and a seamless steel pipe for its piping Production method. The method for producing a boiler and its seamless pipe for piping according to claim 1, wherein a strain rate during rolling of the material satisfies the following range.
1250 ° C. ≦ temperature range of rolling temperature: strain rate ≧ 1.0 / sec,
1200 ° C. ≦ rolling temperature <1250 ° C. temperature range: strain rate ≧ 0.6 / sec,
1150 ° C. ≦ rolling temperature <1200 ° C. temperature range: 3.5 / sec ≧ strain rate ≧ 0.3 / sec,
Rolling temperature <1150 ° C. temperature range: 1.4 / sec ≧ strain rate

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