FR2667524A1 - PROCESS FOR MANUFACTURING ROUND INGOTS FOR SOLDERLESS STEEL PIPES. - Google Patents

Abstract

The invention relates to a method for manufacturing a round ingot (3) for a seamless steel pipe. Its purpose is to prevent the appearance of an interior surface defect by improving the round ingot. The round ingot is manufactured using a continuous casting piece of steel alloy containing a proportion of chromium of between 0.5 and 40% by weight, having a rectangular section with a length to width ratio greater than one predetermined minimum value and less than a predetermined maximum value, with a DELTAT temperature difference between a start-up casting temperature and a start-up solidification temperature below 70degree C, after which a burnishing job is carried out with, as a result, a stretch ratio greater than a predetermined value.

Description

PROCESS FOR MANUFACTURING ROUND INGOTS FOR PIPES IN

SOLDERLESS STEEL

  This invention relates to a method for manufacturing a round ingot for a seamless steel pipe containing Cr with a proportion of Cr greater than or equal to 0.5% by weight, or, occasionally, a proportion of S less than or equal to 0.008% by weight and a proportion of O less than or equal to 0.010% by weight and, more particularly, a method for manufacturing a round ingot for a seamless steel pipe for a steel with low Cr-Mo content, a steel with high content in Cr-Mo, an austenitic stainless steel and a steel

strongly allied.

  A seamless steel pipe is normally produced by a process in which a solid round ingot is stamped with an inclined roller and a dowel, after which the material is rolled with a chuck bar called chuck burnishing system, reduced in diameter and elongated with a heat drawing device comprising three rollers being combined in order to form the pipe, or by another method in which the material is rolled by two rollers designated as the pebble burnishing system, rectified by friction by a roll inclined and a dowel, and its diameter is reduced by

two rollers.

  For the solid round ingot used as the raw material for the seamless steel pipe, the material made by a continuous casting process can now be applied, and this application process can be considered as a process in which a rectangular raw material (hereinafter referred to as a blank) is shaped so as to give it a round shape, giving a round raw material (hereinafter referred to as

  round ingot) which is applied in its cast form.

  In the case where the seamless steel pipe is manufactured by the above processes, steel is given at

  normal carbon the shape of a pipe almost without problem.

  However, in the case where steel containing chromium in a proportion of 0.5% by weight or more is in the form of a pipe, a large cavity can easily be generated in the central part of the blank. continuous casting or round ingot compared to what can be done with normal steel, which produces the fact that the cavity is left as is the case in the round ingot, and, also in the case of blank, the cavity is not

  sufficiently adherent during the burnishing operation.

  When performing a perforation operation with the round ingot, a significant clearance caused by a pressing cracking (referred to below as Mannesmann cracking) generated by rollers inclined before perforation with the dowel is produced in the central part and we often see that this clearance is left, forming an internal defect, even if the material is burnished

  by the ankle and the inclined rollers.

  Due to Mannesmann's cracking during the formation of the perforation, it was difficult to perforate the raw continuous casting material containing a large amount of chromium in the prior art without certain defects occurring, and therefore it was possible to hardly produce a cavity in the central part of a raw material in the form of a block of steel. However, due to a development of recent continuous casting technology, the realization of a cavity in the central part is substantially restricted, which means that now the raw material is applied in continuous casting However, in the in case the raw material is used in continuous casting, it is generally not possible to avoid producing an interior surface defect at a substantially high frequency, and the defects are removed using a correction work

in order to make a final product.

  In the case where the seamless steel pipe made of chromium-containing steel is produced by a burnishing process of Mannesmann, the technology exposed in the Japanese patent opened to the Public Inspection NI Hei 1-293909, for example, is applied as a method to prevent an interior surface defect during the perforation operation when the raw casting material continues

is used.

  The existing technique is a process for manufacturing a seamless steel pipe containing chromium, characterized in that it contains chromium in a proportion of 2 to 27% by weight, the rest being composed of Fe and unavoidable impurities, in that a piece of continuous casting is applied in the form of a raw material in order to produce a seamless steel pipe, the piece of raw material of continuous casting having a shape of rectangular section with a ratio between the length and the width which is between 1.6 and 3.0, the casting being formed into a round ingot normally under the effect of a burnishing operation, and, when the round ingot is perforated by the rollers inclined and the dowel, an absolute value I In I of an effort in during the perforation operation defined in the following equation is lenl <0.015 in a position between a position spaced 10 mm from the end of the dowel and a po sition facing the rear end of the ankle; en = In (tn / tn-1) in which, tn 1 is a wall thickness in millimeters for a position spaced 10 mm from the end of the dowel from any position Pn-1 in the direction of rear end of the plug, and tn = a wall thickness at a position Pn spaced 1 mm from a position Pn-1 relative to the rear end of

ankle.

  In the case where the pipe is formed by a process described in the Japanese patent left open to the Public Inspection previously mentioned NO Hei 1-293909, the internal surface defects in the pipe during the perforation operation can be eliminated with a certain point However, there is a certain limitation of the effort during the perforation operation, and, therefore, it presents a problem that the range

application is restricted.

  As described above, an object of the present invention is to solve the problems mentioned above and to prevent the appearance of internal surface defects not by limiting a stress value during the

  forming the pipe, but by improving the round ingot itself

even.

  The present inventors have carried out experiments and studies on several occasions in order to solve the problems set out above, have found that a difference in temperature AT between a start-up temperature for melting and a start-up temperature for solidification during a continuous casting operation , a sectional shape of the continuous casting, that is to say an a / b ratio between a length (a) and a width (b), and a stretching ratio during a machining operation of round ingot could substantially influence the frequency of appearance of internal surface defects during the formation of the pipe, and that, moreover, a reduction in the concentration of sulfur and oxygen in the steel further increased their effect, and, subsequently , the present inventors have carried out the present invention with reference to these facts In this case, the stretching ratio defined here is a ratio between a cross-sectional area (Scc) of the continuous casting and a surface

  of section (Sb) of the round ingot, that is to say Scc / Sb.

  In other words, it has been found that AT, a / b and the stretch ratio for the round ingot under the conditions of continuous casting can be made appropriate, and occasionally a concentration of sulfur and oxygen in the steel was reduced so as to form the ingot, and the pipe is produced using this ingot, which has the effect that the internal surface defect cannot be produced during the formation of the pipe Although a practical cause is not sufficiently resolved, it is assumed that a cavity in the central part of the continuous casting part is reduced and that the machining characteristic

  by hot rolling of the ingot is improved.

  The present first invention consists of a method for making a round ingot for a seamless steel pipe, in which, when the round ingot for the seamless steel pipe is made from a continuous alloy casting steel containing between 0.5 and 40% by weight of chromium, the continuous casting having a shape of rectangular section with a ratio between the length and the width which is greater than 1.5 and less than 2.0 is cast with a temperature difference AT between the start-up temperature of t 4 and the start-up temperature of solidification, less than 700 ° C., after which a burnishing operation is carried out to obtain a stretching ratio which is

greater than 3.0.

  The present second invention consists of a method for producing a round ingot for a seamless steel pipe, characterized in that a ratio between the length and the width of the above-mentioned composition elements of the first invention is greater than 2.0 and less than 9.0, and a stretch ratio is

  replaced by a value greater than 2.5.

  In addition, the present third invention consists of a method for manufacturing a round ingot for a seamless steel pipe, characterized in that the continuous casting of alloy steel contains S: 0.08% at least, and O : At least 0.010% in the first and second inventions previously cited is applied the production of the round ingot for a

seamless steel pipe.

  A reason to limit the value in this

  invention will be described as follows.

  First of all, with regard to chromium, in a proportion of between 0.5 and 40% by weight: Chromium is added in this wide range in order to improve the anti-corrosion characteristic, the temperature resistance high temperature and anti-oxidation at high temperatures, or the like Although, when the perforation and burnishing are carried out according to a Mannesmann burnishing process, after addition of chromium, internal surface defects can easily occur in the pipe. seamless steel, there are no substantial problems with regard to the appearance of interior surface defects with a proportion of 0, 5% by weight or less, and, similarly, the seamless steel pipe with a proportion of 40% by weight of chromium or more cannot currently be obtained with a roller burnishing system from Mannesmann, which means that the proportion of chromium is limited to a range comprised

between 0.5 and 40% by weight.

  With regard to sulfur, at a proportion of 0.008% by weight or less: Although the sulfur is contained in the steel in the form of impurities, this substance is supposed to degrade a hot burnishing work, and, usually, its volume is reduced to a range between about 0.02 and 0.01% by weight, or a similar proportion, and the pipe is manufactured with this proportion of sulfur Different studies to carry out the present invention have shown that at the same time the reduction of oxygen and reduction of sulfur remarkably increased a draft ratio or an effect of the subsequent stretch ratio with regard to the improvement of the quality of the ingot, and also decreased the defects of interior surface after pipe formation A result of the experiment with regard to the influence of this sulfur and this oxygen is shown in figure 5 Four types of oxygen concentration, 0.012%, 0.010%, 0.008% and 0, 006% in the raw materials in 9 Cr-1 Mo steel, in 13 Cr steel and in 22 Cr steel are selected, and a relationship between the concentration of sulfur in the raw material and a frequency of appearance of interior surface defects ( in%) when the round ingot is formed, in relation to an oxygen concentration, is indicated At this time, a stretching ratio for the round ingot is 7.7 with a length / width ratio (= a / b ) of 2.7 and another stretch ratio for the round ingot is 8.1 with a ratio

  a / b of 4.0, and in both cases AT = 300 C is applied.

  As shown in the figure, it has been found that when the oxygen concentration is less than 0.010% and the sulfur concentration is 0.008%, the frequency of occurrence of interior defects is substantially reduced to less than 3% at this time , the oxygen concentration being decreased, the frequency of occurrence of interior surface defects is reduced, and, moreover, the concentration of sulfur being reduced, the frequency of occurrence of interior surface defects is further reduced in light therefore, an upper limit on the volume of

sulfur is set at 0.008%.

  Regarding oxygen at a proportion of at least 0.010% by weight: Although at present there are no special restrictions on the proportion of oxygen, a study of the relationship between a ratio of , its subsequent stretch ratio, a proportion of sulfur and the quality of the ingot has shown, as shown in FIG. 5, that a reduction in the proportion of sulfur in addition to a reduction in the proportion of oxygen improves the quality of the ingot as well as the effect of the roughing ratio or its subsequent stretching ratio, and then the interior surface defects after forming the pipe were reduced In the light of this fact, the limit value

  the higher the proportion of oxygen is 0.010%.

  Regarding the temperature difference AT, the shape of the section and the stretch ratio for the ingot: The conditions for the temperature difference and the shape of the section were found for the first

  times only after performing various experiments.

  These conditions will be described with reference to the results of the experiment. FIG. 1 shows a relationship between a temperature difference AT between a start-up temperature for casting and a start-up temperature for solidification in a process for manufacturing a casting part in continuous and a frequency of appearance (in%) of interior surface defects when the round ingot is applied to make a pipe At this time, a stretching ratio of the round ingot with an a / b ratio of 3.1, is of 8.7 As shown in the figure, when AT increases, a frequency of occurrence of interior surface defects increases When AT exceeds 70 OC, the frequency of occurrence of interior surface defects increases by more than 5% In Figure 2 , we show a situation of appearance of interior surface defects during the making of a pipe when a combination of AT and a / b varies and the round ingot has a stretch ratio between 5 and 11 As it is obvious from this figure, if a value of a / b is greater than 1.5 and that AT is less than 700 C, few internal surface defects are generated during production of a pipe (the frequency of occurrence of interior surface defects being less than 4%) In addition, when AT exceeds 700 C or that an a / b value is less than 1.5, the interior surface defects can easily occur (the frequency of occurrence of interior surface defects being greater than 4%) In addition, when a value of a / b exceeds 9, due to a large value of the flatness ratio, looping can easily occur in the case of the machining of the round ingot, and its shape is deteriorated In the light of these results, to reduce the internal surface defects when the round ingot is applied to make a pipe, it is obvious that he is born ssaire pour a continuous casting having a shape of rectangular section with a ratio between the length and the width between 1.5 and 9.0 under conditions in which a temperature difference AT between a temperature at the instant of start of casting and a start-up temperature of solidification is 700 C. As described above, the inventors have verified a relationship between a stretch ratio for the round ingot and a frequency of appearance of the internal surface defects when the round ingot is manufactured using a continuous casting with an AT value and an a / b ratio which are fixed at appropriate values. As shown in FIG. 3, the inventors verified a state of a Mannesmann cracking 4 of a round ingot 3 in which a tapered round ingot 2 is rolled obliquely with a laminating machine 1 of the piercing type A limit stretch ratio R has been calculated using a definiti we indicated in the following equation, from

  from a position of appearance of the slit.

  R = d 1 / do (Do Dl) / Do in which, D 1 is a diameter on the small diameter side of a tapered round ingot Do is a diameter on the large diameter side of a tapered round ingot do is a length of a round ingot after burnishing d 1 is a reasonable length of a round ingot after

  burnishing (except in the case of a crack).

  We know that a crack has little chance of occurring when this limit stretch ratio R is greater, and that internal surface defects are unlikely to occur In Figure 4 are indicated some results in which the parts flows with an a / b ratio between length and width equal to 3.2 with an AT temperature difference equal to 30 WC, and an a / b ratio equal to 1.8, an AT temperature difference equal to 250 C , are rolled in the form of round ingots with different stretching ratios, the aforementioned tapered round ingots are rolled obliquely, after which a limit stretching ratio R is calculated. It has been discovered that the crack is unlikely to due to the fact that the machining characteristic has been improved even further when the stretch ratio is increased. It was also discovered that, with respect to the casting made with a temperature difference AT of 300 C and an a / b ratio of 3.2, a limit stretch ratio R was improved for a stretch ratio greater than 2.5, and, in the case of a plate produced with a temperature difference AT of 25 WC and an a / b ratio of 1.8, a limit stretching ratio R with a ratio

  3.0 stretch was improved.

  In this way, it was found that Mannesmann cracking was remarkably improved by applying a continuous casting obtained with an appropriate restriction of AT and an appropriate value of a / b, as well as determining a stretch ratio for the ingot. which is greater than 2.5 or 3.0, and, in response to these results, the aforementioned temperature difference AT, a section shape and a stretch ratio or the like

conditions are limited.

  In this way, according to the present invention, when the raw material for a seamless steel pipe containing chromium in a proportion greater than 0.5% by weight is perforated and burnished, it is possible to limit certain defects generated at the level of the surface

inside the pipe.

  Figure 1 is a view showing a relationship between a temperature difference AT between a casting temperature and a solidification start temperature when a continuous casting is produced and a frequency of occurrence of interior surface defects

  when the ingot is used to make a pipe.

  Figure 2 is a view showing a relationship between a temperature difference AT, a relationship between a length (a) and a width (b) of a continuous casting and a frequency of occurrence of defects in

  interior surface after forming a pipe.

  Figure 3 is an illustration showing a Mannesmann crack test using a testing machine

piercing type.

  Figure 4 is a graph showing a relationship between a stretch ratio and a limit stretch speed. FIG. 5 is a view showing an influence of the sulfur and oxygen proportions on the frequency of appearance of interior surface defects when the

pipe is formed.

  Preferred embodiments of the present invention

will be described as follows.

  Preferred realization 1 The steels having a chemical composition indicated in table 1 are put in the form of castings having a rectangular section by a continuous casting under conditions of temperature differences AT and a / b ratios indicated in tables 2 and 3, after which they are put into the form of round ingots of raw material for seamless steel pipes having different sizes, by means of a burnishing operation. The round ingots are cheered by inclined rollers and a dowel, put in the form of pipes by a mandrel rolling mill, a hot-stretch reduction rolling mill or an ankle rolling mill, a straightening rolling mill with oblique rollers, and a calibrating rolling mill, or the like, and a frequency of appearance of interior surface defects after the formation of pipes is verified The results of the verification are indicated

in Tables 2 and 3.

  As is evident from Tables 2 and 3, when a temperature difference AT, a cross-sectional shape and a stretch ratio for the round ingot during a continuous casting process are included in the figure. inside a range of the present invention, all types of steel have a low frequency of appearance of interior surface defects, less than 4% In addition, as shown in the example for comparison, the products other than those in the range of the present invention have a high frequency of occurrence of interior surface defects. O'zO EOOPO z O9 Tle OOO OO z DOE H 9 t'0 OOE Tr L'S O S ', 6 pe OT O ODZZ 9 EO' 0LT'0 O6 Ll L 8 8 S "T t 'ó O900' OESS a S z 00 80: 0 O 'ET TS'0 91' o OTZO O ID ET 86 '0 O 6 _p' 0 ú'e O 11 l O owl 1 D 6 CI IS'0 ITS IS'O 9 Eú OETIO ow S '0a DS O 66 _ O 8 e VO f OE: 0 W, OOI: D- / T

6 '0 TIT ZGS'0 9 E O ETI O DI TV

N O W: D TN UW TS D

  a.T De R p a UJ Cj aloquiduo en Ti Twtlo u OT 4 TS Od WOD nielqel u; t N (I l- r H 4

Table 2

  ALength / Ratio Type of steel A stretching Width for an A B T D round ingot

 C 1.8 371 3 3 3 3 3 3 3 3

  "e 4,0 4 3 3 3 3 3 3 3" t 5 X 8 4 4 4 4 4 4 4 4 tu 1713 5 5 5 4 4 4 4 4 o C 2,1 I tt 'Il II 4 j O, 8

 C 2; 7 2.6 3 3 3 3 3 3 3 3

"'55 4 4 4 4 4 4 4 4 4

7.7 4 4 4 4 4 4 4 4

"194 5 5 5 5 5 5> 5

  c C 4.0 3.2 4 4 3 3 3 3 3 3 81 81 4 4 4 4 4 4 4 4 and 114 4 4 4 4 4 4 4 4 and 28.7 5 5 5 5 5 4 4 4

 C 6 Y O 4 7 4 4 3 3 3 3 3 3

  and 120 4 4 4 4 4 4 4 4 and 168 4 4 4 4 4 4 4 4 and 42.4 5 5 5 42 4 4 4 4

 C 875 4.0 4 4 4 3 3 3 3 3

"10.1 4 4 4 4 4 4 4 4

14.1 4 4 4 4 4 4 4 4

"35.8 5 5 5 4 4 4 4 4

  : Frequency of appearance of interior surface faults 4: same as 3: same as 2: same as 1: same as less than 1% greater than 1% and less than 2% greater than 2% and less than 4% greater than 4% and less than 7% Greater than 7%

Table 3

  Report Type of steel Length / stretching AT -width for a round ingot A B C D E F G H

 C 1.8 3; 1 3 3 3 3 3 3 3 3

"470 4 4 3 3 3 3 3 3

, 8 4 4 4 4 4 4 4 4

", l 73 5 5 5 5 5 5 4 4

 C 2.1 2.6 4 4 4 3 3 3 3 3

4 4 4 4 4 4 4 3

, 8 4 4 4 4 4 4 4 4

"l 143 5 5 5 5 5 5 5 5

 C 2.7 2; 6 3 3 3 3 3 3 3 3

  if 5; 5 4 4 4 3 3 3 3 3 e 7.7 4 4 4 4 4 4 4 4 g 194 5 5 5 5 4 4 4 4

 C 4.0 3; 2 4 4 4 4 4 4 4 4 4

8 "1 4 4 4 4 4 4 4 4 4

11; 4 5 5 5 5 4 4 4 4

"l 28.7 5 5 5 5 5 5 5 5

 C 6.0 4> 7 4 4 4 4 3 3 3 3

4 4 4 4 4 4 4 4 4

16.8 4 4 4 4 4 4 4 4

"l 42.4 5 5 5 5 5 5 5 4

 C 875 4.0 4 4 4 4 4 4 3 3

, 1 4 4 4 4 4 4 4 4 4

14; 1 4 4 4 4 4 4 4 4 4

"l 35.8 5 5 5 5 5 5 5 5

 C * 2; 1 4.2 1 1 1 1 1 1 1

 C 174 * 5.4 1 1 1 1 1 1 1 1

 C 2.7 2.2 * 2 2 1 1 1 1 1 1

  * Other than the conditions of the present invention: Frequency of appearance of interior surface faults 4: same as 3: same as 2: same as 1: same as less than 1% greater than 1% and less than 2% greater than 2% and less at 4% greater than 4% and less than 7% greater than 7% preferred embodiment 2 Steels having the chemical composition indicated in table 4 are formed in the form of castings having a rectangular section by means of continuous casting under conditions of differences in AT temperature and a / b ratios indicated in Tables 5 and 6, after which they are put into the form of new round ingots of raw material for seamless steel pipes having different sizes at l using a rolling operation Round ingots are stamped using inclined rollers and dowels, formed into pipes by a mandrel rolling mill and a calibrating rolling mill, or the like, and a freq The appearance of interior surface defects after the formation of the pipes is checked The results of the check are

shown in Tables 5 and 6.

  As is evident from Tables 5 and 6, when the proportion of sulfur and oxygen is within a range of the third invention of the present invention and a temperature difference AT, a cross-sectional shape and a stretch ratio for the round ingot during the production of a continuous casting also found within a range of the present invention, all types of steel have a low value of frequency of appearance of inner surface defects, less than 4%, and, in this case, the smaller the AT, the smaller the frequency of occurrence of the inner surface defects In addition, as regards the product (indicated by a mark *) other than that of the range of the present third invention, used as an example of comparison, its frequency of appearance of the defects of

interior surface is increased.

Table 4

  Chemical composition Symbol Type of steel C Si Mn Ni Cr Mo NSOI 21/4 Cr I Mo 0, 12 0.33 0.45 2.2 1 t 01 01006 0.008 J 9 Cr 1 Mo 0.10 0.31 0, 40 9.0 0, 98 O? 005 0.008 K "0.09 0.32 0.39 8.9 0.97 O t 005 0.006

  L "0410 0.32 0.40 8.9 0.98 0.005 0.012 *

  M "0.10 0.31 0.39 8.8 0.9.99 0.010 * 05007

  N 13 Cr 0.20 0144 0.48 12.9 0.08 0.02 0.002 0.007

  O "0.19 0.43 0.46 12.8 0.08 0.02" 0.005

  P "0720 0.43 0.46 12.9 0.08 0.02" 0011 *

  Q "0.20 0.42 0.47 12.8 O 08 0.02 0.010 * O 0005

  R 22 Cr 0.01 0.48 1.51 5 6 22.0 2.9 0.15 0.002 0/006 * Other than the conditions of the present third invention The numerical values of chemical composition indicate a percentage by weight K) MM "Il cn K) jII

Table 5

  Report Type of steel Length / stretching AT width for a * * * * round ingot I J K L M N O Q R

 C 1.8 3.1 3 3 4 2 1 3 4 2 1 3

and 4.0 4 4 4 2 2 4 4 2 2 4

"5.8 5 5 5 2 2 5 5 2 2 4" 173 5 5 6 2 2 5 6 2 2 5

 C 2.1 2.6 3 3 4 1 1 3 4 1 1 3

  and 4.0 4 4 4 2 2 4 4 2 2 4 and 5.8 5 5 5 2 2 5 5 2 2 5

"14.3 5 5 6 2 2 5 6 2 2 5

 C 2.7 2.6 3 3 4 2 2 3 4 2 2 3

"l 5.5 4 4 5 2 2 4 5 2 2 4

7.7 5 5 5 2 2 5 5 2 2 5

"19.4 5 5 6 2 2 5 6 2 2 5

 C 4.0 3 y 2 4 4 5 2 2 4 5 2 2 4

"8.1 5 5 5 2 2 5 5 2 2 4

11.4 5 5 6 2 2 5 6 2 2 5

"28.7 6 6 6 2 2 6 6 2 2 5

 C 6.0 4.7 4 4 5 2 2 4 5 2 2 4

"12.0 5 5 5 2 2 5 5 2 2 4

"16.8 5 5 6 2 2 5 5 2 2 5

"42.4 5 5 6 2 2 6 6 2 2 5

Table 6

  Length / Ratio Type of steel AT stretch width for a round ingot I J K L M N O P Q R

 C 2.7 2.6 4 4 5 2 2 4 4 2 2 3

"5.5 5 5 5 2 2 5 5 2 2 4

7.7 5 5 6 2 2 5 6 2 2 5

"l 19.4 6 6 6 2 2 6 6 2 2 6

 C 2.1 2.6 4 4 5 2 2 3 4 2 2 3

4 5 5 2 2 40 5 2 2 4

"5.8 5 5 6 2 2 5 5 2 2 5

"14.3 6 6 6 2 2 6 6 2 2 6

 C 4.0 3.2 4 3 4 1 2 3 4 1 1 3

It "8.1 5 4 5 2 2 4 5 2 2 4

"11.4 5 5 -2 2 5 5 2 2 5

the 28.4 5 5 2 2 5 6 2 2 5

 C ** 2.1 4.2 1 1 1 1 1 1 1 1 1

   C 1.4 ** 5.4 1 i 1 1 1 1 1 1 1 C 2.7 2, 2 ** 2 2 2 i 1 1 1 1 * Other than the conditions of the present third invention ** Other than the conditions of the present invention Evaluation index 6: Frequency of appearance of interior surface faults: idem 4: idem 3: idem 2: idem 1: idem Less than 0.5% Upper Upper Upper Upper Upper than 0, -and less than 2% to 1% and less than 4% to 2% and less than 4% to 4% and less than 7% to 7%

Claims (4)

  1 Method for manufacturing a round ingot (3) for a seamless steel pipe in which a round ingot (3) for a seamless steel pipe is produced using a continuous casting of alloy steel containing chromium in a proportion of between 0.5 and 40% by weight, characterized in that the continuous casting has a rectangular section with a length to width ratio which is greater than 1.5 and less than 2.0 and is cast with a temperature difference between a casting start temperature and a solidification start temperature which is less than 700 C, after which a burnishing operation with a ratio   stretching greater than 3.0 is applied.   2 Method for manufacturing a round ingot (3) for a seamless steel pipe in which a round ingot (3) for a seamless steel pipe is produced using a continuous casting piece containing chromium under a proportion between 0.5 and 40% by weight, characterized in that the continuous casting, having a rectangular section with a length to width ratio which is greater than 2.0 and less than 9.0, is cast with a difference of temperature between a start-up temperature for casting and a start-up temperature for solidification of less than 700 ° C., after which a burnishing operation with a stretching ratio greater than 2.5 is applied.   3 Method for manufacturing a round ingot (3) for a seamless steel pipe according to claim 1, characterized in that a round ingot (3) for a seamless steel pipe is produced by applying said continuous casting piece in steel alloy, containing a proportion of sulfur of less than 0.008% by weight and a   oxygen content less than 0.010% by weight.   4 Method for manufacturing a round ingot (3) for a seamless steel pipe according to claim 1, characterized in that a round ingot (3) for a seamless steel pipe is manufactured using said continuous casting piece in steel alloy containing a sulfur content of less than 0.008% by weight and a   oxygen content less than 0.010% by weight.