JP4517811B2 - Seamless steel pipe manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a seamless steel pipe using a Cr-containing steel or a martensitic stainless steel having a Cr content of 9% or more, and more specifically, piercing and rolling using the above-mentioned Cr-containing steel or stainless steel. It is related with the method of manufacturing a seamless steel pipe with few internal flaws by performing.

  As a method for producing a seamless steel pipe, a method of piercing and rolling a slab having a circular cross section using a piercing machine such as a Mannesmann mandrel mill method, a Mannesmann plug mill method, or a Mannesmann Assel mill method is widely used. The manufacturing method of seamless pipes by these Mannesmann methods is to pierce and roll a billet for pipe making heated to a predetermined temperature in a heating furnace with an inclined rolling piercing machine, and then use the obtained hollow shell pipe as a mandrel mill or a plug mill. The tube is expanded using a drawing and rolling machine, and after reducing the wall thickness, the outer diameter is reduced by a drawing mill such as a stretch reducer or sizer to finish the product into a steel pipe.

The manufacturing method of the seamless steel pipe by the Mannesmann method is to pierce the billet by using the “rotary forging effect” in which the center portion of the billet becomes brittle and a hole is formed in piercing and rolling using an inclined roll. However, depending on the type of material constituting the billet and its quality, there is a problem that wrinkles occur due to the embrittlement of the billet center.

  FIG. 1 is a schematic axial cross-sectional view of a billet for explaining the occurrence of an embrittlement region due to the rotary forging effect in inclined piercing rolling. The billet 1 which is a rolled material advances while being spirally moved in the rolling direction indicated by the arrow, and is fed into the gap between the rolling rolls 2 rotating around the rotation axes inclined in the opposite directions, and is rolled down by the rolling roll 2. After that, it is rolled by the plug 3 and the rolling roll 2. At this time, since the billet 1 having a circular cross section is subjected to rotational compression by the rolling roll 2, tensile stress and shear stress are generated at the center of the billet, and the billet material becomes brittle near the tip F of the plug 2. To do.

  At that time, since the solidification zone is long, a material that easily generates center segregation or porosity in the vicinity of the center of the cross section generates a δ-ferrite phase at the center segregation part of the billet during heating before pipe making. Due to the difference in strain at the grain boundaries based on the difference in strength from austenite, cracks occur during inclined piercing rolling, and rashes form on the inner surface of the steel pipe after inclined piercing rolling. Therefore, if Cr segregation in which the solute component is concentrated exists in the region F where the rotary forging effect is remarkable, it may cause an inner surface defect or an inner defect of the steel pipe.

  The following method is disclosed as a method for preventing the occurrence of internal surface defects or internal defects in the production of seamless steel pipes.

  In Patent Document 1, in a method of producing a seamless steel pipe by piercing and rolling a solid round billet obtained by continuous casting, an internal void generated during casting is within a range of 3% of the diameter from the center of the cross section. A method of manufacturing a seamless steel pipe using a round solid billet produced so as not to exist is disclosed. Patent Document 2 discloses a method of manufacturing a seamless steel pipe by a Mannesmann method using a seamless steel pipe rolling material made of round bar steel, and a microcrack is formed from the center of the material as a seamless steel pipe rolling material. A method of manufacturing a seamless steel pipe is disclosed in which a material having a distance up to a predetermined value calculated by the radius of the material and the outer diameter and inner diameter of the seamless steel pipe after piercing and rolling is pierced and rolled.

  However, the methods disclosed in Patent Documents 1 and 2 are drilling methods for preventing defects in seamless steel pipes caused by internal voids and microcracks in the billet, and the effects of component segregation in the billet are considered. Not.

  In Patent Document 3, in a method of producing a seamless steel pipe from a continuously cast round billet, a round billet is used by using a round billet in which the final solidification position of the slab is shifted from 1 to 3% of the slab diameter from the center of the slab. A method of manufacturing a seamless steel pipe is disclosed in which a hollow shell is manufactured by piercing and rolling with the center as a center. This method prevents the occurrence of inner surface flaws in the steel pipe by shifting the final solidified part in the billet, which has the most segregated components, from the center during drilling. Therefore, there is a risk that internal defects such as cracking of two sheets and medium rashes may occur during pipe production.

  Patent Document 4 discloses a method for producing a seamless steel pipe in which segregation in a round billet is divided in a diameter direction and pierced and rolled using a round billet having a degree of division of 20% or more. Discloses a method for producing a seamless steel pipe by piercing and rolling using a round billet having a segregation displacement degree of 5 to 40% from the center of the round billet. However, in any case, since the most prominent part of the component segregation is located inside the tube meat, there is a possibility that an internal defect such as a two-piece crack or a medium rash occurs in the pipe making process.

  As described above, the inclined piercing and rolling method that prevents the occurrence of internal flaws during the production of seamless steel pipes still has a number of problems that must still be solved.

Japanese Patent No. 3129064 (Claims and paragraph [0010]) Japanese Patent No. 3305604 (Claims and paragraphs [0006] and [0007]) Japanese Patent No. 3033446 (Claims and paragraphs [0014] and [0015]) JP 2002-361384 A (Claims and paragraphs [0009] and [0010]) JP 2002-224711 A (Claims and paragraph [0008])

  The present invention has been made in view of the above problems, and the problem is that tilt piercing rolling is performed using Cr-containing steel or martensitic stainless steel having a Cr content of 9% or more, which is likely to cause internal flaws. An object of the present invention is to provide a method for producing a seamless steel pipe that can produce a seamless steel pipe with little occurrence of internal flaws even when it is performed.

  In order to solve the above-mentioned problems, the present inventors are directed to high Cr-containing steels and martensitic stainless steels that are likely to generate δ-ferrite and are susceptible to internal defects, based on the conventional problems described above. The present inventors have completed the present invention by studying a method for producing a seamless steel pipe that prevents the occurrence of internal flaws, and obtaining the following knowledge (a) and (b).

  (A) In a round billet made of Cr-containing steel or martensitic stainless steel containing 9% or more of Cr by mass%, the width (r) of the center segregation zone and the diameter (D) of the round billet in the cross section By performing piercing and rolling using a billet having a ratio of (r / D) of 0.12 or more, occurrence of inner surface flaws in the seamless steel pipe can be reduced.

  (B) Even if the center segregation zone is a billet extending in the billet radial direction so as to satisfy the above condition (a), in the case of a billet where the final solidification position or the segregation zone has been divided into two or more, perforation Two-piece cracks are generated in the segregation part divided by the rolling in the double-sided inner surface flaw and the tube wall.

  FIG. 2 is a diagram schematically showing the distribution state of the center segregation zone in the cross section of the round billet described later. FIG. 2A shows the case where the center segregation zone is concentrated at the center of the billet. FIG. 2B shows the case where the central segregation zone is continuously distributed in the center of the billet, and FIG. 2C shows the case where the final solidification position and the central segregation zone are divided.

  In the case of a billet in which the final solidification position or segregation zone described in (b) is divided into two or more, as shown in FIG. Sheet breakage occurs.

The present invention has been completed on the basis of the above findings, and the gist of the present invention resides in the method for producing a seamless pipe shown in the following (1) and (2) .

(1) The steel billet containing 9% or more by mass of Cr steel or martensitic stainless steel having a rectangular cross section is heated by soaking and subjected to block rolling, and the obtained billet for pipe making is obtained. In the method for producing a seamless steel pipe by piercing and rolling, the soaking is performed under the condition that the value of the heating coefficient K calculated by the following formula (1) is 1580 or more, and in the transverse section of the billet, The center segregation zone and the final solidification position are not divided, and the ratio of the center segregation zone width r to the billet diameter D in the cross section of the billet, the value of (r / D) is not less than 0.12. The manufacturing method of the seamless steel pipe characterized by using the billet for pipe making which is.
K = A × √ (2 × 60 × T / B) (1)
Here, A represents a soaking temperature (° C.), T represents a soaking time (h), and B represents a slab thickness (mm).
(2) The method for producing a seamless steel pipe according to claim 1, wherein when the billet for pipe making is pierced and rolled, the draft rate Dr calculated by the following formula (2) is 7% or less.
Dr = {(DG) / D} × 100 (2)
Here, D represents the billet diameter (mm), and G represents the gorge interval (mm).

  In the present invention, the “martensitic stainless steel” means a stainless steel having a martensite structure as represented by a stainless steel having a Cr content of 13%, and the martensite structure is obtained by quenching from the austenite temperature range. It means stainless steel that forms and hardens.

  “Punching and rolling” refers to piercing and rolling while rolling the rolled material with a roll that rotates around an axis inclined with respect to the pass line of the rolled material. For example, Mannesmann mandrel mill method, Mannesmann plug mill method It means rolling using a drilling machine.

  Further, the “center segregation zone” means a structure in which the center segregation is distributed in a band shape or a line shape in the cross section of the billet.

  And, “without segregation of the central segregation zone and the final solidification position” means that the central segregation zone is distributed in a macro-continuous form, and the final solidification position is located near the center, Neither the central segregation zone nor the final solidification position is divided into two or more. That is, it does not become the C pattern shown in FIG.

  According to the method for producing a seamless steel pipe of the present invention, the center segregation in the cross section of the billet for pipe making is possible even if the Cr content is easily 9% or more and the martensitic stainless steel has a Cr content of 9% or more. Manufacturing seamless steel pipes with less internal flaws by piercing and tilting rolling using billets with a central segregation zone width of 12% or more of the billet diameter without breaking the zone and final solidification position can do.

  As described above, the method for producing a seamless steel pipe according to the present invention is a product obtained by using a Cr steel or a martensitic stainless steel containing 9% or more of Cr, and by rolling a material having a rectangular cross section. In the cross section of the billet for pipes, the ratio of the center segregation band width r and the billet diameter D in the cross section of the billet without being separated from the center segregation zone and the final solidification position, (r / D) This is a method for producing a seamless steel pipe in which piercing and rolling is performed using a billet for pipe making having a value of 0.12 or more.

  In the following, the method of the present invention will be described in more detail, and the reason for limiting the scope of the present invention as described above, the preferred range, and the like will be described.

  A slab cast by continuous casting or the like is subjected to segment rolling with various perforated rolls according to the purpose using a segment rolling mill and processed into a round billet having a circular cross section.

  FIG. 2 is a diagram schematically showing the distribution state of the center segregation band in the cross section of the round billet as described above.

  FIG. 6A shows a case where the center segregation zone 5 in the cross section of the billet 1 is concentrated at the center of the billet (hereinafter referred to as “segregation pattern A-1”). In this case, an inner surface flaw such as a rash occurs on the inner surface of the hollow shell during the piercing and rolling of the billet from the central segregation zone concentrated in the center.

  In the case where the center segregation zone 5 in the cross section of the billet 1 is continuously distributed in the billet radial direction as shown in FIG. 5 (b) (hereinafter referred to as “segregation pattern A-2”), If the segregation zone is dispersed so that the end 51 in the billet radial direction of the segregation zone reaches a certain distance or more from the center of the cross section of the billet 1, the billet is pierced and rolled to irritate the inner surface of the hollow shell. There will be no internal flaws. This point will be described in more detail with reference to FIGS. 3 and 4 described later.

  On the other hand, as shown in FIG. 5C, the final solidification position 7 in the cross section of the billet 1 is divided, and the central segregation zone 5 is also divided and divided into positions that are largely separated from the center of the cross section. When the segregation bands 6 and 6 are present (hereinafter referred to as “segregation pattern B”), when the billet 1 is pierced and rolled, the segregation bands 6 and 6 cause the segregation bands 6 and 6 on the inner surface of the hollow shell. Two-sided cracks and double cracks are likely to occur.

  The inventors conducted a more detailed investigation on the conditions that can prevent the occurrence of the above-described inner surface flaws. As will be described in Examples below, 9% Cr-containing steel and 13% Cr-containing steel are used as steel types, and billets having outer diameters of 191 mmφ and 225 mmφ are manufactured by performing ingot rolling with various rectangular cross sections. Then, the state of segregation dispersion in the cross section was investigated, and those billets were pierced and rolled to form hollow shells, and then finished pipes were produced, and the state of occurrence of internal flaws in the obtained steel pipes was investigated.

  FIG. 3 is a diagram for explaining a method of quantifying the shape of the central segregation band in the billet cross section. As shown in the figure, in the cross section of the billet 1, the degree of dispersion of the segregation was indexed by the ratio (r / D) of the width r of the center segregation zone 5 and the diameter D of the billet. That is, as the value of (r / D) increases, segregation of solute components such as Cr is dispersed in the radial direction of the billet. Conversely, as the value of (r / D) decreases, the segregation of the solute component in the radial direction increases. This means that the degree of dispersion of segregation is low.

  FIG. 4 is a diagram showing the ratio between the width r of the central segregation zone and the diameter D of the billet, the relationship between the value of (r / D) and the rate of occurrence of internal flaws. In the figure, ◯ indicates the test result of the segregation pattern A (A-1 and A-2) described in FIG. 2, and ■ indicates the test result of the segregation pattern B.

  According to the result of the figure, it can be seen that the inner surface flaw occurrence rate decreases as the value of (r / D) increases. Further, in order to reduce the internal flaw generation rate to a level that can reduce the load in the refining process of the pipe making process, that is, 5% or less, the value of (r / D) needs to be 0.12 or more. It has been found. However, when the value of (r / D) reaches 0.64, the segregation pattern becomes the B pattern, and for the reason described in FIG.

  Therefore, in the present invention, the final solidification position and the center segregation zone are not divided, and piercing and rolling is performed using a billet for pipe making in which the value of (r / D) is 0.12 or more. This is an essential component of the invention.

  In addition, the value of (r / D) is preferably 0.25 or more that can reduce the inner surface flaw occurrence rate to 4% or less, and is less than 0.64, which is a range in which the segregation band pattern is difficult to become pattern B. Preferably there is.

  Furthermore, in order to reduce the center segregation and improve the internal flaw generation rate, the relationship with the soaking condition was investigated.

  FIG. 5 is a diagram showing the relationship between the heating coefficient K in the soaking furnace and the inner surface flaw occurrence rate. In addition, the figure arrange | positions the test result about the test numbers 8-36 in the Example mentioned later. Moreover, the heating coefficient K in the figure is a value calculated by the following formula (1).

K = A × √ (2 × 60 × T / B) (1)
Here, A represents a soaking temperature (° C.), T represents a soaking time (h), and B represents a slab thickness (mm).

  From the results shown in the figure, the higher the soaking temperature and the longer the soaking time, the more the generation of δ-ferrite is reduced due to the diffusion of segregation components. It can be seen that it is. Further, it has been found that by setting the value of the heating coefficient K to 1580 or more, the inner surface flaw occurrence rate can be further reduced to 3% or less.

  Furthermore, the relationship with the draft rate at the time of pipe making was investigated. FIG. 6 is a diagram showing the relationship between the draft rate during pipe making and the rate of occurrence of internal flaws. This figure is a summary of the test results for test numbers 14-17, 21-24 and 28-31 in which the value of the heating coefficient K exceeds 1580 in the examples described later. Moreover, the draft rate in the same figure is the value computed by following formula (2).

Draft rate = {(Billette diameter (mm) −Gorge interval (mm)) / Billet diameter (mm)} × 100 (%) (2)
According to the results shown in FIG. 6, the rate of occurrence of internal flaws during pipe making can be further reduced by setting the draft rate to 7% or less.

  In order to confirm the effect of the method for producing a seamless steel pipe according to the present invention, the following examples of the present invention and comparative examples were tested and the results were evaluated.

(Test method)
Various rectangles with slab width: 537-780 mm and slab thickness: 245-600 mm using 9% Cr-containing steel and 13% Cr-containing steel using a vertical continuous caster or horizontal continuous caster A slab having a cross section was cast. The casting speed was 0.38 to 0.60 m / min, the specific water amount was 0.4 to 1.0 L / kg-steel, and the average superheat of the molten steel was 30 to 60 ° C.

  For the batch rolling, after the above slab is heated in a soaking furnace, 1-2 double reversible blooming mill, No. 1 Round billets having outer diameters of 191 mmφ and 225 mmφ were manufactured using a 2-2 double reversible blooming mill and a 6 continuous mill. Tables 1 and 2 show examples of typical pass schedules.

  In the table, for example, when rolling from a slab having a nominal section of 537 mm × 413 mm to a billet having a billet size (diameter) of 225 mmφ, No. In 1 mil, the billet cross section is nominally 325 mm x 245 mm. In 2 mils, the finished size was 225 mmφ.

  In addition, when rolling a billet size (diameter) to a billet having a nominal cross section of 700 mm × 390 mm and a billet size (diameter) of 191 mmφ, No. In 1 mil, the billet cross section is nominally 325 mm x 245 mm. In the 2 mil, the billet cross section was nominally 245 mm × 245 mm, and a billet with a finished size of 191 mmφ was obtained by 6 continuous mills.

  Cut the cross-section of the round billet, investigate the distribution of segregation in the cross-section, pierce and roll those billets to produce a hollow shell, and further, make a pipe with a stretch reducer and sizer, The state of occurrence of internal flaws in the obtained steel pipe was investigated.

  The test conditions and test results are summarized in Table 3.

  In the same table, the value calculated by the following formula (3) was used as the Cr equivalent.

Cr equivalent = Cr + 4Si− (22C + 0.5Mn + 1.5Ni + 30N) (3)
Here, each element symbol in the above formula represents the mass content (%) of each element.

  In the pipe making test, the billets described in Tables 1 and 2 above were heated to 1230 ° C., and manufactured by the Mannesmann-Mandrel pipe making method at an inclination angle of 11 to 12 degrees and a crossing angle of 10 to 11 degrees. This was done by tube.

  In addition, the inner surface flaw occurrence rate was represented by a percentage (%) of the number of wrinkle occurrences with respect to the number of inspections, mainly by detecting flaws visually or by ultrasonic flaw detection.

(Test results)
Test Nos. 8 to 36 are tests for examples of the present invention that satisfy the conditions specified in the present invention, and Test Nos. 1 to 7 and Test Nos. 37 to 39 are for comparative examples that do not satisfy the conditions specified by the present invention. This is a test.

  The segregation distribution pattern of the billet cross section is the segregation pattern A (A-1 or A-2), and the ratio of the center segregation band width r to the billet diameter D (r / D) is 0. Perform piercing and rolling using a billet in the range of 12 to 0.63, and make a pipe with a stretch reducer and a sizer, and have an outer diameter of 73.0 to 139.8 mm and a tube wall thickness of 5.5 to 13.0 mm. Test Nos. 8 to 36 of the present invention example finished into a steel pipe all obtained a steel pipe having a good inner surface property with an inner surface flaw generation rate of 4.8% or less.

  Especially, in the test numbers 11-36 which carried out the piercing rolling of the billet whose value of (r / D) is in the range of 0.25 to 0.63, the inner surface flaw occurrence rate reaches about 4% or less, and the inner surface property An extremely good steel pipe was obtained.

  On the other hand, the test numbers 1 to 7 having a low value of (r / D) of 0.04 to 0.09 are the segregation pattern A-1 or the distribution pattern close to the segregation distribution pattern of the billet cross section. Since the center segregation was not sufficiently dispersed in the radial direction in the cross section of the billet, the inner surface flaw occurrence rate was high, the inner flaw occurrence rate of the steel pipe was 8% or more, and the inner surface properties were poor.

  Moreover, although the center segregation was disperse | distributed to the radial direction of a billet in the test numbers 37-39 which carried out the piercing-rolling using the billet whose segregation distribution pattern in a billet cross section is the segregation pattern B, a center segregation zone | band is 2 or more. Since it reached the state of being divided, the inner surface of the hollow shell after the rolling was cracked due to segregation, or the inner surface property was poor due to segregation.

  According to the method for producing a seamless steel pipe of the present invention, the center segregation in the cross section of the billet for pipe making is possible even if the Cr content is easily 9% or more and the martensitic stainless steel has a Cr content of 9% or more. By performing piercing and tilt rolling using a billet in which the ratio of the width of the central segregation band to the billet diameter is 0.12 or more without dividing the band and the final solidification position, a seam with less generation of inner surface flaws is achieved. Steel-free pipes can be manufactured. Therefore, the seamless steel pipe manufacturing method of the present invention is an excellent steel pipe manufacturing method that can be widely applied in the field of manufacturing seamless steel pipes using difficult-to-work materials.

It is an axial cross-sectional schematic diagram of the billet for demonstrating generation | occurrence | production of the embrittlement area | region by the rotary forging effect in inclined piercing rolling. It is a figure which shows typically the distribution situation of the center segregation zone in the cross section of a round billet, The figure (a) shows the case where the center segregation zone is concentrated in billet center part, The figure (b) is a center. The segregation zone is continuously distributed in the center of the billet, and FIG. 8C shows the case where the final solidification position and the central segregation zone are divided. It is a figure for demonstrating the method of quantifying the shape of the center segregation zone in a billet cross section. It is a figure which shows the relationship between the ratio of the width | variety r of a center segregation zone | band, and billet diameter D, and an internal surface flaw occurrence rate. It is a figure which shows the relationship between the heating coefficient K in a soaking furnace, and an internal surface flaw occurrence rate. It is a figure which shows the relationship between the draft rate at the time of pipe making, and an internal surface flaw occurrence rate.

Explanation of symbols

1: billet 2: rolling roll 3: plug 4: hollow shell 5: center segregation zone 51: billet radial end of center segregation zone
6: Divided central segregation zone 7: Final solidification position

Claims (2)

A material having a rectangular cross-section using Cr steel or martensitic stainless steel containing 9% or more of Cr by mass% is heated by soaking, and the resulting billet for pipe making is pierced and rolled. A method for producing a seamless steel pipe,
The soaking process is performed under the condition that the value of the heating coefficient K calculated by the following formula (1) is 1580 or more,
In the cross section of the billet, the ratio of the center segregation band width r and the billet diameter D in the cross section of the billet without being separated from the center segregation zone and the final solidification position, (r / D) The manufacturing method of the seamless steel pipe characterized by using the billet for pipe making whose value of is 0.12 or more.
K = A × √ (2 × 60 × T / B) (1)
Here, A represents a soaking temperature (° C.), T represents a soaking time (h), and B represents a slab thickness (mm). 2. The method for producing a seamless steel pipe according to claim 1, wherein when the billet for pipe making is pierced and rolled, a draft rate Dr calculated by the following formula (2) is set to 7% or less.
Dr = {(DG) / D} × 100 (2)
Here, D represents the billet diameter (mm), and G represents the gorge interval (mm).

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