SE508892C2 - Process for making a stainless steel strip - Google Patents

Abstract

A method for manufacturing a stainless steel strip, particularly an austenitic stainless steel strip, with desired final thickness and a yield strength of at least 250 N/mm<2>, includes continuously casting a stainless steel, particularly an austenitic stainless steel, into a strip having a thickness of at least 1 mm and at maximum 10 mm, optionally hot rolling the cast strip, cooling said cast and optionally hot rolled strip to room temperature, coldrolling the cast and optionally hot rolled strip with at least a 10% thickness reduction to a thickness which is at least 2% and at most 20%, preferably at most 10%, greater than the intended final thickness of the finished product, annealing the thus cold-rolled strip at a temperature between 1,050 DEG C and 1,250 DEG C, and cold-working the strip after said annealing process so as to permanently elongate the strip and therewith reduce its thickness by 2-20%, preferably by 2-10%.

Description

2 Pl230 ' 508 892 10 20 IJ 'Ji file) 35 greatly reduces the mechanical strength of the belt. According to current standards, one must for construction works calculate with a yield strength of 190-220 N / mmz.

The properties obtained with conventional techniques, such as a relatively low one yield strength, are desirable properties in most cases, even if conventional techniques are irrational from points of view. However, improvements have been proposed in order to rationalize production. For example, it is proposed in SE 467 055 (WO 93/1921 1) that the thickness is reduced in connection with an annealing process by stretching the hot band. However, higher mechanical strength is a desirable property in some applications, such as for construction purposes. The properties of the final cold-rolled strip is not improved in the latter respect in the performance of the aforesaid method and no such improvement is intended either.

DESCRIPTION OF THE INVENTION The aim of the invention is to produce in a rational and cost-effective way stainless steel strips, in particular stainless steel austenitic strips, of the desired thin thickness and higher mechanical strength than that obtained by conventional production of and cold-rolled, stainless, austenitic steel strips, while obtaining an acceptable ytfmish. These and other views can, according to a first aspect of the invention, is obtained by a method which begins with continuous casting of the stainless steel into one strips with a thickness of at least 1 mm and not more than 10 mm, preferably at least 1.5 mm and not more than 6 mm, and cooling said cast strip to room temperature. These process steps, which can be performed by per se known technology, provide fast solidification of the steel, which in turn can give a casting structure of the steel that can contribute to that the desired properties are achieved in the end product of the method. Depending on it rapid solidification, the casting structure is typically fine-grained compared to structures obtained by conventional die casting or conventional continuous continuous casting, If, as for example recommended according to the method according to the invention, one uses one twin-roll strip caster, the casting structure of the belt normally contain areas with rod-shaped (columns) grains adjacent to the band surfaces and in in the middle an area of equiaxed region. The following steps in the way according to the invention is carried out in such a way that the advantage of the fine-grained casting structure can be utilized to achieve the desired properties of the final product obtained according to the method. The strip casting can also make it possible to produce strips of stainless steel steel alloys that are very difficult or even impossible to manufacture with conventional techniques due to embrittlement or other problems that have to do with the victory or formation of undesirable phases in the steel or other phenomena. Also 10 15 2 () 25 30 35 Pl230 503 892 the cooling of the cast strip, which can be performed relatively quickly depending on it cast the small thickness of the strip, can contribute to the desired results.

According to the first aspect of the invention, the cast strip is then cold rolled at least 10% thickness reduction to a thickness of at least 2% and at most 20% greater than the intended final thickness of the final product, followed by annealing thereof thus cold-rolled the strip at a temperature between 1050 ° C and 220 ° C and cold-rolled processing of the strip after said annealing process, so that the strip is permanently extended and its thickness is thereby reduced by 2-20%. According to another aspect of the invention, at least some of said objects can be achieved by continuous casting of the stainless steel so as to obtain an elongated casting product, water rolling of elongated cast product into the form of a strip, cooling of said hot rolled strip to room temperature, cold rolling of the hot rolled strip with a thickness reduction of at least 10% to a thickness of at least 2% and not more 20% greater than the intended final thickness of the final product, thus annealing it cold rolled strip at a temperature between l050 ° C and l250 ° C and cold working of the strip after said annealing process so that the strip is extensively elongated and thus its thickness is reduced by 2-20%. More specifically, the stainless steel is cast according to the above second aspect to a strip with a thickness of at least 1 mm and not more than 10 mm, preferably a thickness of 1.5-6 mm, so as to obtain a casting niche suitable for the subsequent steps in the process as well as hot rolling said cast strips with at least 5% and at most 50% thickness reduction, preferably by at least 10% and preferably at most 30% thickness reduction, so that the casting structure of the strip material is broken before it The hot rolled strip is cooled to room temperature.

The strip subjected to initial cold rolling in accordance with any of the aforementioned aspects of the invention normally consists of a cast and / or vamival rolled strip which does not have has undergone some peeling treatment but has been cooled and hashed after said casting and / or hot rolling. Optionally, however, the cast strip may be exposed to heat. treatment at a temperature in the temperature range 900-1200 ° C for up to 3 minutes, preferably for at least 30 seconds, before cooling and / or haspling. Whether possible the heat treatment is used or not, the cold rolling is most suitably performed on a strip on which oxide shell still remains on the surfaces of the strip.

The initial cold rolling operation, which is performed on said cast strip and / or on said hot-rolled strip, can in principle be made in a plurality of sticks by a corresponding one 4 Pmu 508 892 ll) 20 25 .ill -J 'Ju number of successive roller chairs, although it is preferably made in a single stick.

The maximum thickness reduction that can be achieved in a single stick depends on the steel quality, the initial dimensions of the belt and the capacity of the rolling mill. In general it can be said that a single stick gives a maximum thickness reduction of about 30%, normally a maximum of 25%. This means in the In most cases, the thickness of the hot-rolled strip is reduced by 10-60%, preferably by 10-40% in the practice of the invention, which reduction is dependent on the band initial thickness and the desired final thickness. The strip is annealed at a temperature between 1050 ° C and 1200 ° C and cool there to room temperature before cold stretching.

After annealing, the strip can be cold worked by stretching it in a strip extruder which may be of some known kind, for example of the kind used for peeling the surfaces of hot-rolled strip for pickling. The belt is preferably cold stretched by a combination of heavy stretches and bending of the strip around rollers. The cold stretching process is carried out to such an extent that the strip is permanently elongated, whereby it has a thickness reduction of 2-20%, preferably 240%, normally 3-5%. Due to the combination of difficult stretches and bending of the strip around rollers of relatively small diameter the reduction in width becomes minimal and in practice negligible. The reduction in strip thickness will therefore substantially correspond to the degree of elongation achieved. The material plasticized as a result of the cold stretching process, whereby the yield strength increases by on the order of l00 MPa and even more for certain steel grades. Alternatively, the strip after annealing is cold worked by cold rolling to such an extent that the strip permanently elongated and a thickness reduction is thus achieved of 2-20%, preferably 3- 10 ° / 0, A characteristic feature of the inventive method is that it takes place continuously, by which is meant that the method does not involve any reversing steps, e.g. reversed rolling, rewinding between the different steps or similar shifts. To be able to enable one continuous process, the production line in a known manner preferably comprises tape magazines, so-called buffers (loopers) at the beginning and end of the production chain, eg before the initial one the cold rolling and after the cold working of the strip by cold drawing or cold processing.

The method according to the invention normally also comprises pickling the annealed strip. The band preferably before it is cold worked or the annealing operation, although it is also is conceivable to graze the belt after the final cold working process. The band preferably blasted before grazing. 10 15 20 25 30 35 SHORT F IGURBESKRIVNTNG The invention will be described in more detail with reference to the accompanying drawings, whereby Fig. 1 very schematically shows the principles of the invention according to a first embodiment, comprising an initial strip casting, Fig. 2 shows in more detail the production line following the initial strip casting, according to the first embodiment, Fig. 3 on a larger scale and in more detail shows a cold stretcher used in the first the embodiment of the method, Fig. 4 schematically shows a modification of the first embodiment of the method according to the excitement, Fig. 5 schematically shows the principles of the invention according to a second embodiment, which comprises an initial strip casting, i Fig. 6 schematically shows the principles of the invention according to a third embodiment, which comprises an initial strip casting and heat rolling operation, and Fig. 7 schematically shows the principles of the invention according to a fourth embodiment. as likewise includes an initial strip casting and hot rolling operation DETAILED DESCRIPTION OF THE INVENTION 1 all the production lines schematically shown in Fig. 1, Fig. 4, Fig. 5, Fig. O iir-h h; " a cast strip 100 is first produced by continuous strip casting in a casting machine 1H! which is preferably a twin-roll strip casting type casting apparatus, although other continuously operating strip casting apparatus can also be used, The narrow the stainless steel is dropped into the casting apparatus 101 from a ladle 102 via a casting box 103.Mt-dan the steel passes between the duo rollers in the casting apparatus 101, it solidifies on its own edge method of forming a cast strip 100, which according to the embodiments shown in Fig. I. F tg 4 and Fig. 5 are cooled and reeled 104. Prior to reeling, the cast strip 100 - optionally according to the embodiments shown in Fig. 1, Fig. 4 and Fig. 5 - for heating treatment in an oven 105 at 900-1200 ° C for up to 3 minutes, preferably for at least 30 seconds The first embodiment of the method will now be described in more detail with reference to Figs The reeled cast belt 100, which has the shape of the reel 104, is transported to a plant for further processing of the cast strip 100, This plant can be located adjacent to or at a distance from the strip casting equipment and comprises a reeling rack 1, which contains the reel 104, a cold rolling mill 2 which consists of a roller chair 2 of the so-called Z-high type, an annealing furnace 3, a cooling chamber 4, a Pmu 508 892 6 Plzïtl * 508 892 l (} 20 25 30 blasting machine 16, a pickling bath 5, a cold stretcher 6 and a winding reel 7 as picks up the finished steel strip.

Fig. 2 shows in more detail the manufacturing line following the initial strip casting process. line, the same reference numerals having been used for units which have correspondence in Fig. 1. In addition to the above-mentioned units, the production line also comprises a cutting unit 8, a welding machine 9, a belt feeder 10 which feeds the cast strip 10AA to the cutting unit 8 and the welding machine 9, a magazine, generally designated 12, for it cast strip, a thickness gauge 13 which measures the thickness of the cast strip IOOA pre-rolling mill 2 and a thickness gauge 14 which measures the thickness of the cast strip 10OB is the cold rolling mill 2, the blasting machine 16, a brushing and rinsing box 17 or the beet bath 5, a pair of guide rollers 18, the cold stretching 6, a magazine, general designated 20, for storing cold-rolled and cold-stretched finished belt 100F, a front feeder 21 and a drive motor and power transmission means collectively designated 22 for driving the winding reel 7.

The production line also includes a large number of guide rollers, deflection rollers and S-works thus comprising two or four rollers S-works thus comprises a two-roll S-works 25 after the welding machine 9, a two-roll S-mill 26 for the cold rolling mill 2, a four-roll S-mill 27 between the cold rolling mill 2 and the gluing furnace 3, a four-roll S-mill 28 for cold-drawing the mill 6, a two-roll S-mill 29 after the cold-drawing mill 6, a belt center guide 19, a two-roll S-work 30 between the belt center guide 19 and the belt magazine 20 and a final two-roll S-work 31 between the belt magazine 20 and the winding reel 7 S- the primary function of the works is to increase or decrease the strap and to keep the strap stretched.

The cast belt magazine 12 contains deflection rollers 34, 35, 36 and 37, of which deflection The roller 35 is connected to a pulling unit in a manner known per se. Magazine 20 som contains cold-rolled strip correspondingly contains deflection rollers 39, 40, 41, 42, 43 and 44, of which the deflection roller 40 is connected to a pulling unit, also on i and known manner.

The manufacturing line shown in Fig. 2 operates in the following manner. It is assumed that is in the phase shown in the figure, i.e. the casting belt magazine 12 and the cold tape magazine 20 is filled with a certain amount of tape, that the cast tape 100A unwound from the unwinding reel 1, and that the finished belt 100F is wound up on the winding reel 7, The line is driven with fl your driven rollers, mainly driven S-work rollers, in a manner known per se. If you have passed through the casting strip magazine 12, the strip is measured 10 20 25 30 35 7 Pllšt) 508 892 thickness in front of the cold rolling mill 2 by means of the thickness gauge 13 and cold rolled in the mill 2 in a single stick, after which the cold-rolled strip 10B is measured by the thickness feeder 144 Det cast strip 10OA normally has an initial thickness of 2-4 mm, which is reduced by 10-3 0% in the cold rolling mill 2. Depending on the result of the thickness feeds, the roll gap is set so that the desired thickness is achieved in the cold-rolled strip 10OB, corresponding to 2-20%, preferably tradesvis 2-10%, normally 3-5% more than the intended finished dimension after the cold stretching of the band in the final part of the production line.

The cold rolling gives the strip 10B a high degree of hardness, so that the strip is led into the annealing. the furnace 3 after passing the four-roll S-unit 27. In the annealing furnace 3 the band 10B is heated to a temperature between 1050 ° C and 1200 ° C, i.e. to a temperature above the recrystallization temperature of the austenitic steel and maintained at this temperature long enough for the steel to be completely recrystallized. Dare to cool the band in the cooling box 4. When heating in the annealing furnace 3, which according to the embodiment does not take place under a shielding gas atmosphere (which in itself would be possible), why oxides, partly in form of shell, formed on the band sides. These are mainly removed in blasting machine 16, where the belt is pickled in the pickling bath 5 which consists of suitable pickling chemicals, which can be carried out in a known manner. It is thus cold-rolled, annealed and pickled the belt IOOE is led through the wiper and coil box 17 and there through the cold-drawn the work 6 between the four-roll S-work 28 and the two-roll S-work 29 which contributes to keep the strap stretched and prevent the strap from slipping, How the cold rolling mill 6 is constructed is shown in Fig. 3. The cold rolling mill 6 consists of three tensioners 47, 48 and 49. Each tensioner includes a lower roller 50. 51, 52, which is stored in a stationary foundation 53, 54, 55 and an upper tension roller 56. 57, 58 which are stored in roll holders 59, 60, 61. The position of the roll holders relative to the belt and relative to them the lower tension rollers can be adjusted by means of jacks 62, 63, 64. Initially, they are upper tension rollers 56, 57, 58 brought up to upper positions (not shown). so that the band IOOE, which is kept stretched between the S-plants 28 and 29, extends completely through the cold stretch the plant 6. Based on this starting position, the upper tension rollers 56, 57 and 58 sank by means of the jacks 62, 63, 64 to the positions shown in Fig. 3, whereby the band IOOE-IOOF will form a meandering path, as shown in Fig. 3, at the same time as the tape is stretched in its cold state to such an extent that it is plasticized. According to it In the embodiment shown, the lower tension rollers 50, 51 and 52 have diameters 70, 200 respectively 70 mm while the upper tension rollers 56, 57 and 58 have the diameters 70, 70 respectively 200 mm. When the belt is then continuously pulled through the tensioning device 6 below bending in the passage of the tension rollers occurs continuously, at the selected setting of 508 10 15 30 35 8 Pl230 892 the adjustable tension rollers 56, 57, 58 and by selecting the diameters of the tension rollers, a plasticization of the strip portion passed through the cold drawing and thus permanent elongation and thus thickness reduction of the strip of 2-20%, preferably typically 2-10%, normally 36%. At the same time, the width is also reduced slightly, but this amounts to only one tenth of the extension and can almost be neglected. Through the thus permanent elongation of the belt is thus achieved a thickness reduction which in the closest corresponds to the extension of the band. By adjusting the reduction of the thickness of the strip achieved by the cold rolling in the cold rolling mill 2 to the thickness play reduction achieved by the cold stretching of the belt in the cold drawing 6 or vice versa, the desired final thickness of the finished strip IOOF can be achieved, which after passing the cold tape magazine 20 is wound on the winding reel 7. The drive the machinery of the integrated production line described above consists of machinery 22 connected to the winding reel 7.

If larger reductions are desired than is possible to achieve with a cold rolling mill containing only one roller chair and only one cold stretch, several roller chairs 2A, 2B etc are connected in series one after the other, as shown in Fig. 4. This clock also shows that it is possible to place the beet bath 5 after the cold extension 6. In this case, in cold the stretcher also break off the shell on the belt surfaces, which can possibly eliminate the need for a blasting machine for the bath.

In the line schematically shown in Fig. 5, the cold drawing 6 in Fig. 1-Fig. 4 and S- the works on both sides have been replaced by a cold rolling mill 6 ”. It was cold rolled and grazed the strip l0OE (concerning the manufacture of the cold-rolled strip l00E, see preceding description), is guided through the cold rolling mill 6 ”so that you get a permanent extension of the belt and thus a reduction of the belt thickness of 2-20%, preferably 3-10%.

The permanent extension of the belt also entails a corresponding reduction in thickness band extension. By adjusting the reduction of the band thickness as is achieved by the cold rolling of the strip in the cold rolling mill 2 to the thickness reduction which achieved by the cold rolling in the cold rolling mill 6, or vice versa, can be achieved desired final thickness of the finished strip 10F, which is wound on the winding reel 7. For the details of the line following the strip casting, see Fig. 2 and to the previous description in connection with Fig. 2.

Referring now to Fig. 6 and Fig. 7, the cast strip 100 is passed through a residence furnace 105, which maintains the temperature of the cast strip 100 at or raises it to a temperature between 900 and 1200 ° C suitable for hot rolling. After passing 10 15 508 892 in the holding furnace 105, the cast strip is hot rolled in a hot rolling mill 106_ which operates at the same speed as the casting apparatus 101. The cast and hot rolled strip 100 ' hasplas däre fi er] 04 ”_ The lines following the strip casting and hot rolling line, Fig. 6 and Fig. 7, are designed and operated in the same manner as described with reference to corresponding lines in Fig. 1 and Fig. 5, respectively. These lines should therefore not be further discussed here is described but instead reference is made to the previous description with reference to Fig. 1, Figs. 2, Fig. 3 and Fig. Si By the method according to the invention, in principle any stainless steel material can treated, but primarily intended for the manufacture of austenitic stainless steel strips.

Because the method has advantages both in terms of production economy and in terms of desired material improvements, it can be used for mass production of tapes of standard grades of types 304 and 316 and variants thereof but also for the manufacture of special stainless steel strips containing very high levels of nickel and / or molybdenum, eg 5-15% molybdenum, or other alloying elements that can cause problems in the conventional manufacture of stainless steel strips,

Claims (17)

10 508 892 10 15 2 () 25 30 35 PATENT CLAIMS A method of producing a stainless steel strip, in particular an austenitic stainless steel strip, with a desired final thickness and a yield strength of at least 250 N / mmz, characterized by - continuous casting of a stainless steel, in particular an austenitic stainless steel, into a strip with a thickness of at least 1 mm but not more than 10 mm, - cooling of said cast strip to room temperature, - cold rolling of the cast strip with at least 10% thickness reduction to a thickness of at least 2% and not more than 20%, preferably not more than 10%, greater than the intended final thickness of the final product, - annealing the thus cold-rolled strip at a temperature between 1,050 ° C and 1250 ° C, and - cold stretching of the strip after said annealing treatment so that the strip is permanently extended and thus its thickness is reduced by 2-20%, preferably by 2-10% 2. A method according to claim 1, characterized in that the cold stretching or approximate annealing treatment is performed by the combination of continuous stretching and bending of the strip around rollers while the strip is stretched. 3. A method according to claim 2, characterized in that the strip is pressed against said rollers during the strip stretching operation and that said strip is bent with a bending radius of less than 200 mm, preferably with a radius of at least 20 mm and at most 150 mm, 4. A method according to any one of claims 1-3, characterized in that the cast strip is cold rolled before said annealing treatment to achieve a thickness reduction of 10-60%. 5. A method according to claim 4, characterized in that the cast strip is cold rolled before said annealing treatment so that a thickness reduction of 10-30% is obtained. 6. A method according to any one of claims 1-5, characterized by continuous cold stretching of the strip or said annealing treatment so that the strip is permanently elongated and thus its thickness is reduced by 3-5%. 7. A method according to any one of claims 1-6, characterized in that the strip is pickled before or after said cold working operation following the annealing treatment. 10 15 20 25 30 35 ll 508 892 Method for the production of a stainless steel strip, in particular an austenitic stainless steel strip, with a desired final thickness and a yield strength of at least 250 N / mmz, characterized in - continuous casting of a stainless steel, in particular an austenitic stainless steel. to obtain an elongate cast product, - hot rolling of said elongated cast product into the form of a strip, - cooling of said hot rolled strip to room temperature, - cold rolling of the hot rolled strip with at least 10% thickness reduction to a thickness of at least 2% and not more than %, preferably not more than 10%, greater than the intended final thickness of the final product, - annealing of the thus cold-rolled strip at a temperature between 1050 ° C and 1250 ° C, and - cold-stretching of the strip or said annealing treatment so that the strip is permanently extended and thus its thickness is reduced by 2-20%, preferably by 3-10%. 9. A method according to claim 8, characterized by continuous casting of the stainless steel into a strip with a thickness of at least 1 mm and not more than 10 mm, and hot rolling of said cast strips with at least 5% and at most 50% thickness reduction, suitably at least 10% and preferably not more than 30% thickness reduction, so that the casting structure of the strip material is broken down to cool the hot-rolled strip to room temperature 10. A method according to claim 8 or 9, characterized in that the cold stretching after said annealing treatment is performed by the combination of continuous stretching of the strip and bending of the strip around rollers while the strip is stretched. 11. A method according to claim 10, characterized in that the strip is pressed against said rollers during the strip stretching operation and that said strip is bent with a bending radius of less than 200 mm, preferably with a radius of at least 20 mm and at most 150 mm. . 12. A method according to any one of claims 1-11, characterized in that the cast strip is cold rolled before said annealing treatment to achieve a thickness reduction of 10-60%. 13. A method according to claim 12, characterized by cold rolling the hot rolled strip before said annealing treatment to achieve a thickness reduction of 10-30%. 12 508 892 10 14. A method according to any one of claims 8-13, characterized by continuous cold stretching of the strip or said annealing treatment so that the strip is permanently elongated and thus its thickness is reduced by 3-10%. 15, A method according to any one of claims 8-14, characterized in that the strip is pickled before or after said cold working operation which follows the annealing treatment. 16. A method according to any one of claims 1-15, characterized in that the stainless steel contains 0.0l-0.1% C, 17-27% Cr, 7-30% Ni, 0-15% Mo. 17. A method according to claim 16, characterized in that the steel contains 5-15% Mo.