METHOD FOR CONTINUOUS RECRYSTALLIZATION ANNEALING OF A STEEL STRIP

Abstract: The invention concerns a method relating to the manufacturing of steel strips, comprising continuous treatment of the steel strip in an annealing furnace (7) at a temperature exceeding the recrystrallization temperature of the steel. The steel strip (4A) is fed into the annealing furnace at a certain feeding-in rate and is drawn out of the furnace at another, higher feeding-out rate, wherein the steel strip is subjected to a pulling force in the annealing furnace at said temperature exceeding the creep limit of the steel at said temperature, resulting in a permanent stretch (elongation) of the steel strip corresponding to the difference between the feeding-out and feeding-in rates, and in a reduction of the cross section of the strip corresponding to the elongation, including a reduction of the strip thickness as well as of the strip breadth.

Description: METHOD FOR CONTINUOUS RECRYSTALLIZATION ANNEALING OF A STEEL STRIP M TECHNICAL FIELD 5 The invention concerns a method relating to the manufacturing of steel strips, comprising continuous treatment of the steel strip in an annealing furnace at a temperature between 1000°C and 1250°C. BACKGROUND OF THE INVENTION 10 The rolling of steel strips usually is carried out from several reasons. A primary objective is to afford the steel strip a desired thickness. The rolling usually is performed as hot rolling down to a thickness in the dimension range 1-12 mm, preferably 1-6 mm, whereupon continued reduction of the thickness to desired final thickness is 15 performed through cold rolling. In connection with the cold rolling, usually one or more annealing operations are included for the recrystallization of the steel structure. In connection with this technique, it is difficult to achieve the 20 desired final thickness in a simple and practical way. Another problem concerns the breadth of the strip. The end product shall have a certain desired strip breadth, which must at least be reached. In order to meet with this requirement it is conventional to 25 use, as starting material for the cold rolling operation, hot rolled strips having a breadth which substantially exceeds the desired final breadth. This implies that larger amounts of edge scrap is formed at the manufucturing than what is required for the more or less unavoidable conditioning of the edges of the strip. The formation of 30 this scrap material from the edge portions, which is due to poor adaptation of the breadth of the raw strips to the breadth required by the customer and also due to the fact that there are no possibilities to adjust the breadth of the strips in the cold roll mills, represents very large losses. 35 BRIEF DESCRIPTION OF THE INVENTION The purpose of the invention is to solve the above mentioned problems. This can be achieved therein that the invention is characterised in by what is stated in the appending claims. The method of the invention has been developed particularly for austenitic stainless steels but can be used also for other steel grades, stainless as well as other alloyed steels, and also for carbon steels. The principles of the invention per se also can be utilized for the manufacturing of metal strips, which do not consist of steel, particularly metals which are subjected to cold hardening during cold working, as for example copper and copper alloys. Further features and aspects of the invention will be apparent from the following description of a preferred embodiment. DESCRIPTION OF A PREFERRED EMBODIMENT A preferred embodiment of the invention, various aspects of the inven¬ tion, and performed experiments will be described in the following with reference to the accompanying drawing, which illustrates a production plant, in which facilities for performing the method of the invention are integrated. In the drawing, a roll of hot rolled steel strip is general designated 1. As an alternative the starting material - the steel strip 1 - may consist of a cold rolled, annealed strip. A cutter is designated 2, and a welding apparatus for welding together strip sections is designated 3. The steel strip, consisting of sections which have been welded together, and which shall be treated according to the inven¬ tion, is designated 4. The plant shown in the drawing also includes the following members, namely a strip store or accumulator 5, a first braking mill 6, a cold rolling mill 16, a second braking mill 13, an annealing furnace 7, an air-cooling chamber for forced air cooling 8, and aggregate 9 for water cooling of the strip, a drawing mill 10, a wheel abrator 11, a pickling bath 12, a store or accumulator of finished strips 14, and a winding-up drum containing the final product 15. Measuring devices for measuring the breadth and thickness of the strip have been designated 17 and 18, respectively. In the integrated process which comprises the method according to the invention, strips are unwound from the reel 1, are cut in their ends by the cutter 2, are spliced in the welding apparatus 3, and are directed into the strip accumulator 5, which forms a buffer of steel strip 4, so that the continued process can be performed completely continuously. The hot rolled strip 4 has, when it enters the accumu¬ lator 5 a thickness of between 1 and 12 mm, preferably a thickness between 1 and 6 mm. From the strip accumulator 5, the steel strip 4 is fed through the first braking mill 6 and thereafter into the cold rolling mill 16. According to a preferred embodiment of the method according to the invention, the strip 4 is subjected to a reduction of the thickness in the cold rolling mill 16 essentially without any change of the breadth of the strip. How large this thickness reduction shall be according to a preferred embodiment will be explained in the following. According to one aspect of the invention the cold rolling, however, in certain cases can be eliminated. After having passed the second braking mill 13, the preferably cold rolled strip 4A thereafter is drawn through an annealing furnace 7, further through the cooling chamber 8 in which the annealed strip 4B is subjected to forced air-cooling, and thereafter through the water cooling aggregate 9 by means of the drawing mill 10. In the annealing furnace 7 the cold rolled strip 4A is heated from about 20°C to a temperature exceeding the recrystallization temperature of the steel. A suitable temperature for most steel grades is between 1000°C and 1250°C. Preferably the steel strip should be heated to a temperature between 1080°C and 1200°C. By choosing a temperature within the temperature region between 1000°C and 1250°C, and preferably within the region between 1080°C and 1200°C, the holding time in the annealing furnace 7 can be made so short that the requirement upon sufficient holding time will not be a limiting factor for the production in the plant. The tensile properties of metallic materials are strongly dependent on temperature. Hooke's law does not apply at high temperatures, at least not more than for very small tensions. The material will creep already at moderate tensions, which may be lower than those which correspond to the yield point at room temperature for the same material. These conditions are utilized in the method according to the invention. By subjecting the strip 4A to a tensile stress exceeding the creep limit of the material, that is to say within the creeping region of the material, in the annealing furnace 7 at a temperature exceeding the recrystallization temperature of the material, there is achieved a permanent elongation of the strip in the longitudinal direction of the strip, corresponding to the difference between on one hand the rate with which the annealed and thereafter cold strip 4B is fed into the drawing mill 10 and on the other hand the rate with which the strip 4 is fed into the braking mill 13. This permanent stretch or elongation completely takes place within that region of the strip which is heated to a high temperature, that is to say in the annealing furnace 7. The elongation of the strip in other words can be described as the elonga¬ tion of an immovable strip a certain, limited distance, e.g. to a stop, at a high temperature. Subsequent to the drawing mill 10 the stretched and cold strip is passed through the wheel abrator 11 and the pickling bath 12 and is fed into the strip accumulator 14. Finally, the strip is cut in the cutter 20 and is wound up on the winding-up roller 15. Through the stretching in the annealing furnace 7 and the permanent elongation of the strip achieved through this stretch, the cross section of the strip is reduced to an extent corresponding to the elongation. The reduction of the cross section takes place in the form of the reduction of the thickness of the strip and in the reduction of the breadth of the strip. At some point after the annealing furnace, suitably before the drawing mill 10, the breadth and the thickness of the elongated and cold strip 4B are measured by means of the measuring devices 19, 18. According to a preferred embodiment of the invention, the rates of the drawing mill 10 and of the second braking mill 13 are controlled and adjusted such that the rate difference will cause such a large elongation that the breadth of the strip is reduced to a certain strip breadth. The thickness of this strip having the desired thickness is measured by means of the measuring device 18. Thereafter the cold rolling mill 16 is adjusted such that it will reduce the thickness of the strip 4 so much that the combined thickness reduction in the cold rolling mill 16 because of the rolling in the cold rolling mill 16 and the thickness reduction in the annealing furnace 7 due to the permanent elongation of the material in this furnace will give the strip 4B a thickness which corresponds to the desired final thickness for the desired strip breadth. By this preferred embodiment it is in other words possible to achieve as well a desired strip thickness as a desired strip breadth which involves a number of significant advantages. It should be understood that the adjustment of the cold rolling mill 16 may have an impact on the relative ratios between thickness reduction and breadth reduction in the annealing furnace 7, and that the adjustment of the difference of rate of the drawing mill 10 and the braking mill 13 as well as the roll pressure in the cold rolling mill 16 may require repeated measurements and adjustments, that is to say that a certain running-in period may be necessary before stabile conditions have been achieved. These matters, however, can be solved through conventional regulation technology. Also empirically obtained knowledge can be utilized for this adjustment work. In Table 1 there is shown results from ten tests, which concerned continuous elongation of steel strips in an annealing furnace. All the strips which were tested were of an austenitic stainless steel, grade Avesta 18-9 (SIS 2333) having the nominal composition 18 chromium, 9 nickel, 0.04 carbon, 0.5 manganese, 0.7 silicon, balance iron and unavoidable impurities. The steel strip first had been hot rolled to a thickness of about 2.75 mm and a breadth of about 1050 mm. The strips were heated in the annealing furnace to a temperature of 1170°C, except in one case, when the temperature was 1130°C. Two different elongations were applied at the experiments, namely 8 % and 14 %. When the elongation was 8 %, the feeding-in rate to the furnace 7 was 5 m/min, while the feeding-in rate at the elongation 14 % was varied between 5 and 15 m/min. At the last test, when the elongation was 14 % and the feeding-in rate was 15 m/min, the annealing temperature was also lowered from 1170°C to 1130°C. The breadth and thickness values were measured before and after the elongation and also necessary strip pulling forces were registered. From the results the following conclusions can be drawn, which are believed to apply at least for austenitic stainless steels, namely - that the breadth reduction is almost constant at constant elonga¬ tion, even if the feeding rate is varied; - that the thickness reduction is almost constant at constant elongation, even if the feeding rate is varied; and - that the tension in the strip calculated on a reduced area is increased with increased elongation values. Table 1

Claims: CLAIMS 1. Method relating to the manufacturing of steel strips, comprising contiunous treatment of the steel strip in an annealing furnace (7) at a temperature exceeding the recrystallization temperature of the 5 steel, c h a r a c t e r i s e d in that the steel strip (4A) is fed into the annealing furnace at a certain feeding-in rate and is drawn out of the furnace at another, higher feeding-out rate, wherein the streel strip is subjected to a pulling force in the annealing furnace at said temperature exceeding the creep limit of the temperature at 10 said temperature, resulting in a permanent stretch (elongation) of the steel strip connesponding to the difference between the feeding-out and feeding-in rates, and in a reduction of the cross section of the strip corresponding to the elongation, including a reduction of the strip thickness as well as of the strip breadth. 15 2. Method according to claim 1, c h a r a c t e r i s e d in that the steel strip is subjected to the said pulling tension at a temperature between 1000° and 1250°C. 20 3. Method according to claim 2, c h a r a c t e r i s e d in that the steel strip is subjected to the said pulling tension at a temperature between 1080° and 1200°C. 4. Method according to any of claims 1-3, c h a r a c t e r i e d 25 in that the strip after cold rolling is subjected to at least one cold reduction, so that the total thickness reduction achieved through the cold rolling and through the elongation in the annealing furnace affords the strip a thickness after annealing which corresponds to the desired strip thickness of the end product.

Inventors: LJUNGARS STEN, LJUNGARS, STEN
Applicants: AVESTA SHEFFIELD AB, AVESTA SHEFFIELD AKTIEBOLAG
Application year: 1992
Main classification: C21D9
IPC codes: C21D6, C21D8, C21D9