EP0077322B1 - Method of producing seamless tubes - Google Patents

Description

The invention relates to a method for producing seamless tubes from austenitic, chromium and nickel-containing steels.

Austenitic tubes can be manufactured using an inclined rolling process, by extrusion, continuous casting or by welding sheet metal strips. The welded pipes are less expensive to manufacture, but due to the weld seam present, they have only a relatively small area of application. In the cross-rolling process, a mandrel has to be driven through a corresponding die, an economical application only being given for certain cross sections. In the production of seamless tubes by means of extrusion, a heated steel is pressed through a die, with the most severe deformations occurring. For the quality of the austenitic pipes, it was necessary for the extrusion that the steel, before being subjected to the extrusion, be subjected to a corresponding deformation, so that a corresponding structural change and improvement of the deformability is achieved.

It is also known that 8 steels with a high chromium and nickel content are continuously cast into tubes of any length and seamless. Here, an overheated molten steel is continuously fed into a mold rotating around its axis. The molten metal is pressed against the mold by the centrifugal force, a rotationally symmetrical cavity being able to be achieved with a suitably small supply of the molten metal. On the one hand, this method allows only pipes with a relatively large diameter to be produced, on the other hand, the mechanical and metallurgical properties are unsatisfactory.

From DE-A-1 2921708 a process for the production of steel pipes has become known, in which metal melt is fed to a pressure-tight container and sufficient pressure is generated inside the container to push the melt towards a mold for continuous casting, where it follows and after is solidified, with the ascending pipe being withdrawn. The full ascending liquid metal stream is electromagnetically subjected to a rotary movement around the axis of the mold. Pipes produced in this way have, due to the different production method, not as high qualities as pipes formed by extrusion.

The present invention has set itself the goal of providing a method for producing seamless tubes from an austenitic, chromium and nickel-containing alloy, in which the above-mentioned. Disadvantages are avoided, the advantages of continuous casting should also be combined with those of extrusion.

The process according to the invention for the production of seamless tubes made of austenitic chromium-nickel-containing steels with continuous casting consists essentially in that a molten metal is continuously poured into a cooled mold with a round mold cavity and partially solidified in it, drawn off and the hardened strand is cut to length is, the metal melt which has not yet solidified being electromagnetically stirred in the direction about the longitudinal axis of the strand and that the cut-to-length cast pieces, before or after any perforation which may have taken place, an annealing between 1100 and 1250 ° C, preferably between 1150 and 1200 ° C, between 30 minutes and 4 hours, preferably between 1 and 2 hours, whereupon the extrudates are allowed to cool and mechanically machined to form extrusion dies, after which the extrusion dies which are in the as-cast state and which, apart from possibly being subjected to expansion, have not undergone any hot deformation in themselves b is known to produce seamless tubes by extrusion.

The process, which is not known in its combination of process steps, makes it possible for the first time to process austenitic steel, which is processed in the continuous casting process, directly for the production of seamless tubes with an extrusion process. The dendrites aligned to the center of the strand and the radiant, coarse-grained solidification structure have had a particularly disadvantageous effect on processing. However, pipe blanks obtained in this way can be used in the pipe extrusion process according to the invention, which means that the mechanical perforation can be dispensed with. It should also be emphasized that a slight widening of the tube blanks into the dimension provided for the extrusion press inlet is not to be seen here as a previous hot deformation. At the same time as the coarse-grained structure mentioned above, a preferred accumulation of impurities and also porosities have occurred in the center of the strand. Due to the non-uniformity of the porosities inside the strand, an exact centered hole was not possible, so that uneven wall thicknesses of the pipes were caused. The orientation of the dendrites, the coarse grain structure and the micro segregation led to inadequate thermoformability of the billet, so that cracks or fragility occurred in the pressed pipe blank. It has now been found, completely surprisingly, that the combination of the electromagnetic stirring of the melt around the strand axis and a subsequent heat treatment of the strand pieces without hot deformation before the extrusion molding is completely satisfactory Result can be achieved. By stirring the not yet solidified metal melt in the direction about the longitudinal axis of the strand, homogenization of the alloy in the strand is achieved on the one hand, while avoiding a central orientation of the dendrites on the other hand. In addition, a fine-grained structure and a fine and even distribution of separated phases, in particular delta ferrite, is achieved through lower micro segregations. This fine-grained delta ferrite can now largely be dissolved by the subsequent annealing treatment. An extrusion billet which has a low content of delta ferrite can thus be obtained, so that a correspondingly high deformation, as must be carried out during extrusion, can take place. This result is all the more surprising since one has to consider that men with annealing at higher temperatures per se can significantly increase the delta ferrite content in the steel.

Due to the fine-grained structure of the extrusion die, a particularly uniform and excellent surface quality of the tube, which is also easy to manufacture, is achieved.

According to a further feature of the invention, the molten metal is moved at least in a part of the mold about the longitudinal axis of the strand, the relative movement between the strand shell and the liquid strand core preferably reaching as far as the casting level. With this procedure it can be achieved that the desired dendrite configuration and a fine-grained structure extend to the strand surface, with which a higher output can be achieved in both batch production.

If the continuous castings with a minimum temperature of approx. 700 ° C on their surface immediately after the continuous casting of the annealing are released, the result is that less precipitation of carbides, e.g. Chromium carbides take place at the grain boundaries, while at the same time the delta ferrite is not transformed into the sigma phase, which would require a longer annealing.

A particularly advantageous, labor-saving method is obtained if the mechanical machining of the continuous castings into an extrusion die is carried out before the annealing. With this procedure, mechanical machining is particularly easy to carry out since there is less material thickness before annealing.

If the extrusion die, which is in the as-cast state, is subjected to extrusion from the incandescent heat, additional manipulation is dispensed with, and at the same time the energy for reheating does not have to be consumed.

The invention is explained in more detail below with the aid of examples.

Example 1:

A steel X10CrNiTi 18 9 was melted in a medium-frequency induction crucible furnace, tapped into a pan and taken to a continuous casting plant. From this pan, the alloy was poured into the distributor of the continuous casting plant, the metal overheating being 35 ° C. The steel was introduced from the distributor into the continuous casting mold with an inside diameter of 210 mm by means of a pouring tube, after a start-up phase of 0.3 min. was a casting speed of 1.05 m / min. reached. The casting was carried out with a covered casting mirror, i.e. under powder.

A three-pole rotating field device was installed in the system 220 mm below the lower edge of the 650 mm long mold. During the casting, the rotating field was supplied with a power of 72 kVA. On the basis of observations of the casting level or of the casting powder at the casting head, it was found that the liquid strand, up to the casting level in the mold, rotated around the longitudinal axis of the strand.

After the strand had cooled, a test piece was taken from it for further investigations. The strand pieces were annealed in an oven at 1180 ° C., the holding time being 1.6 hours. After the annealing treatment, a test piece was again taken from the strand. The comparative investigations showed that after the continuous casting a delta ferrite content of 4.6% was present. Such high delta ferrite contents have an extremely disadvantageous effect on the deformability of the material. After annealing, the delta ferrite content in the strand was below 1%.

After machining the surface of the continuous casting, sawing and drilling, the extrusion die was heated up inductively and widened by means of a widening cap and punch. The expansion process went smoothly, i.e. the material had good thermoformability properties. Following the expansion of the billet, after intermediate heating, the extrusion process was carried out (according to the Handbook of Stainless Steels, Mr. Graw Hill Book Comp. 1977, Chapter 23). The pressed raw pipe had a good surface both inside and outside. No difficulties were found during further processing either, since the material was readily deformable.

The quality of the pipe was also examined by step turning tests. It was shown that there were no disruptive elongated non-metallic inclusions in the tube wall, ie that the tube had particularly high quality characteristics.

Example 2:

The procedure was analogous to Example 1, a composite tubular mold with an integrated stirring coil being used and a steel X5CrNi 18 9 being cast. The melt overheated to 25 ° C. After a start-up phase of 0.4 min. the casting speed was 1.0 m / min. The rotating field power of the electromagnetic agitation was 65 kVA. After the strand had cooled, the annealing was carried out at 1150 ° C. and for 2.8 hours. The delta ferrite content was 4.8% before the annealing and was below 1% after the annealing. No problems arose during the extrusion process and the pressed tube had a good surface both inside and outside.

Example 3:

A steel X2CrNiMo 18 10 was processed analogously, with the melt overheating at 30 ° C. and after a start-up phase of 0.3 min. a casting speed of 1.1 m / min. complied with. The rotating field power of the electromagnetic stirring device was 72kVA. The strand-strand surface temperature of 715 ° C is placed in an oven and annealed at 1200 ° C etc. Subjected for 1.4 hours. The delta ferrite content was below 1%. The extruded casting then machined mechanically to form an extrusion die was subjected to the extrusion process according to Example 1, the tube obtained having no surface defects and the extrusion process being uneventful.

Example 4:

A steel K10CrNiNb 18 9 was processed analogously to Example 1, the mechanical machining of the continuous casting being carried out to form an extrusion die before annealing. Machining was easier than in the previous examples because there were short breaking chips. The continuous cast billet was then heated to 1190 ° C. in a rotary hearth furnace and left at this temperature for 2 hours. The extrusion process was carried out directly from the annealing heat, whereby no difficulties could be determined during processing by the extrusion and the raw tube had a good surface both inside and outside.

Claims (5)

1. A process for the manufacture of seamless pipes from austenitic steels containing chromium and nickel with continuous casting, a metal melt being continuously poured into a cooled chill mould having a round mould cavity, allowed to partially solidify in the mould, and drawn off, and the through-hardened strand cut to length, the metal melt which has not yet solidified being electromagnetically agitated in the direction about the longitudinal axis of the billet and the cut castings subjected before or after optional boring to annealing between 1100 and 1250°C, preferably between 1150 and 1200°C, for between 30 min and 4 h, preferably between 1 and 2 h, the strand castings then being allowed to cool and mechanically machined to form extrusion blooms, seamless pipes then being produced in known manner by means of extrusion pressing from the extrusion blooms which are in cast form and which have not yet been subjected to any hot forming other than optional widening.

2. A process according to claim 1, the metal melt being agitated in at least one part of the chill mould about the longitudinal axis of the strand, the relative movement between the strand shell and the fluid core of the strand preferably extending up to the casting level.

3. A process according to claim 1 or 2, the surfaces of the castings being subjected to annealing with a minimum temperature of about 700°C directly following the continuous casting.

4. A process according to claim 1 or 2, the mechanical machining of the castings to form an extrusion bloom being carried out before the annealing.

5. A process according to claim 4, the extrusion blooms being subjected in cast form to the extrusion pressing from the annealing heat.