EP0077322A1 - Method of producing seamless tubes - Google Patents

Abstract

In the manufacture of seamless tubes made of austenitic chromium-nickel-containing steels by extrusion, the length of the continuous cast piece had to be subjected to hot forming. In compliance with certain solidification and heat treatment conditions, hot deformation can be dispensed with if the pipe is of high quality.

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 blank, 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.

Furthermore, it is known that steels with a high chromium and nickel content are processed by continuous casting into tubes of any length and seamless. Here, an overheated molten steel is continuously fed into a mold rotating around its axis. The molten steel is pressed against the mold by the centrifugal force, it being possible to achieve a rotationally symmetrical cavity with a suitable low 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.

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, whereby a molten metal is continuously poured into a cooled mold with a round mold cavity and partially solidified in it, drawn off and the through-hardened strand is cut to length in that the metal melt, which has not yet solidified, is electromagnetically stirred in the direction about the longitudinal axis of the strand and in that the cut-to-length castings are annealed between 1100 and 1250 ° C, preferably between 1150 and 1200 ° C, between 30 minutes and 4 hours, preferably between 1 and 2 Are subjected to hours, whereupon the castings are allowed to cool and mechanically machined to form extrusion billets, whereupon seamless pipes are produced in a manner known per se from the extrusion billets which are in the cast state and have not yet undergone hot deformation.

So far, it has not been possible to process austenitic steel, which was processed in the continuous casting process, directly for the production of seamless tubes by the 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. At the same time, preferential accumulation of impurities and porosities have occurred in the center of the strand. Due to the uneven porosity in the A precise centered punching was not possible inside the strand, so that uneven wall thicknesses of the pipes were produced. The orientation of the dendrites, the coarse grain structure and the micro segregation led to an inadequate thermoforming capacity of the billet, so that cracks or fragility occurred in the pressed pipe blank. It has now been found, completely surprisingly, that a completely satisfactory result can be achieved by combining the electromagnetic stirring of the melt around the strand axis and subsequent heat treatment of the strand pieces without hot deformation before the extrusion. By stirring the metal melt, which has not yet solidified, 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. Furthermore, 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 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 around the longitudinal axis of the strand, the relative movement between the strand shell and the liquid strand core preferably reaching up to the casting level. Through this procedure it can be achieved that the desired dendrite configuration and a fine-grained structure extend to the surface of the strand, which means that a higher output can be achieved in the manufacture of beads.

If the continuous castings with a minimum temperature of approx. 700 ° C on their surface are subjected to the annealing immediately after the continuous casting, this means that less carbide excretion, e.g. Chromium carbides take place at the grain boundaries, at the same time there is no conversion of the delta ferrite 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 toughness before annealing.

If the extrusion die, which is in the as-cast state, is subjected to extrusion from the incandescent heat, additional manipulation is omitted, 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 and tapped into a pan and brought to a continuous casting plant. From this pan, the alloy was poured into the distributor of the continuous casting plant, the superheating of the metal 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 level, ie 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 an output of 72 kVA. On the basis of observations of the casting level or of the casting powder on 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 1190 ° 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 processing the surface of the continuous casting, sawing and drilling, the extrusion die was heated up inductively and expanded by means of an expanding cap and punch. The expansion process went smoothly, ie the material had good thermoformability properties. In connection 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 found that there were no disturbing elongated non-metallic inclusions in the tube wall, i.e. that the pipe had particularly high quality features.

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 to the example, the superheating of the melt being 30 ° C. and after a start-up phase of 0.3 min. a casting speed of 1.1 m / min. was observed. The rotating field power of the electromagnetic stirring device was 72 kVA. The strand Castings were placed immediately after the continuous casting with a strand surface temperature of 715 ° C in an oven and an annealing at 1200 ° C u. Subject for 1, 4 hours. The delta ferrite content was below 1%. The extruded casting which was 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 X10CrNiNb 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 heat of the incandescent heat, whereby no difficulties could be ascertained during processing by the extrusion and the raw tube had a good surface both inside and outside.

Claims (5)

1. A process for the production of seamless tubes made of austenitic chromium-nickel-containing steels with continuous casting, whereby 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, characterized in that that the metal melt which has not yet solidified is electromagnetically stirred in the direction about the longitudinal axis of the strand and that the cut-to-length castings are subjected to annealing between 1100 and 1250 ° C, preferably between 1150 and 1200 ° C, between 30 minutes and 4 hours, preferably between 1 and 2 hours are, whereupon the continuous castings are allowed to cool and mechanically machined to form extrusion dies, whereupon seamless tubes are produced in a manner known per se from the extrusion dies which are in the cast state and not yet subjected to hot deformation. 2. The method according to claim 1, characterized in that the molten metal is moved at least in a part of the mold about the longitudinal axis of the strand, preferably the relative movement between the strand shell and the liquid strand core extends to the casting level. 3. The method according to claim 1 or 2, characterized in that the continuous castings with a minimum temperature of about 700 ° C are subjected to annealing on their surface immediately after the continuous casting. 4. The method according to claim 1 or 2, characterized in that the mechanical machining of the continuous castings to an extrusion billet is carried out before annealing. 5. The method according to claim 4, characterized in that the extrusion die in the cast state is subjected to extrusion from the annealing heat.