MXPA97007907A - Method for manufacturing stainless steel bands - Google Patents

Abstract

A method for manufacturing a stainless steel strip, particularly an austenitic stainless steel strip, with the desired final thickness and an elastic limit of at least 250 N / mm, includes continuously casting a stainless steel, particularly an austenitic stainless steel, in a band having a thickness of at least 1 mm and a maximum of 10 mm, optionally hot-rolled the molded web, cooling the molded web and optionally hot-rolled at room temperature, cold-rolling the molded web and optionally hot-rolled with at least a thickness reduction of 10% at a thickness that is at least 2% and at most 20%, preferably at most 10%, greater than the intended final thickness of the finished product, annealing the cold rolled strip to a temperature of between 1,050øC and 1,250øC, and work in cold the band after the annealing process to permanently lengthen the band and with this reduce the thickness in 2-20%, preferably 2-1

Description

METHOD FOR MANUFACTURING STAINLESS STEEL BANDS TECHNICAL FIELD The present invention relates to a method for manufacturing stainless steel strips, particularly austenitic stainless steel strips, with the desired final thickness and an elastic limit of at least 250 N / mm2.

BACKGROUND OF THE INVENTION The manufacture of stainless steel strips having an elastic limit of at least 250 N / mm2 conventionally includes molding an elongated product to obtain a metallic bead, cutting the metallic cord into plates, and hot-rolling the plates to form bands. After conditioning of the surface, the webs, including among other things the deoxidizing of the web, the hot rolled webs can be used without further reduction of the thickness in certain applications. However, the subsequent fri lamination of the hot-rolled strip and many other applications is required. This subsequent cold rolling process aims to achieve one or more of all the following effects, vgr. to further reduce the thickness of the bands, to increase the REF: 25818 mechanical strength and / or improve the surfaces of the bands. Before being cold-rolled, the hot-rolled strips are annealed and deoxidized, and the remaining ends are welded on both ends of the strips. The current cold rolling process is conventionally carried out in several steps through a cold rolling machine, thereby allowing the thickness to be reduced to approximately 80%, typically 10-60%, for example for the laminated bands cold, which is intended to be used as construction materials after having been cut into narrower bands. The remaining ends must be removed before the band can be finally rolled up. The hot rolling and cold rolling operations described above briefly are expensive operations and are carried out in hot rolling machines and cold rolling machines which require very large investment costs. Cold rolling dramatically increases the mechanical strength of the steel, which is in itself desirable for many applications and this is particularly important for the cold rolling of austenitic stainless steel. However, the bands become practically impossible to work, for example, bending, stamping, embossing, etc .; properties which in many cases are necessary to allow the bands to be used as building materials. Therefore, it is necessary to anneal the bands after completing the cold rolling process, heating the bands to a temperature above the recrystallization temperature of the steel, ie at a temperature above 1050 ° C. This treatment greatly reduces the mechanical strength of the band. According to current standards, an elastic limit of 190-220 N / mm2 should be calculated for the construction work. The properties obtained with conventional techniques, for example, a relatively low resistance limit, are desirable properties in most cases, although conventional techniques are irrational in several aspects. However, improvements have been proposed with the intention of rationalizing manufacturing. For example, SE 467 055 (WO 93/19211) aims to reduce the thickness in conjunction with an annealing process by stretching the hot strip. However, a higher mechanical strength is a desirable property in certain applications, such as for construction applications. The properties of the final cold rolled strip do not improve in this latter aspect when practicing the aforementioned method, and no such improvement is intended.

BRIEF DESCRIPTION OF THE INVENTION The object of the invention is to produce, in a rational and low cost, stainless steel bands, particularly stainless steel bands, austenitic, having a desirable thin thickness and greater mechanical strength than that achieved in the conventional manufacture of hot and cold rolled austenitic stainless steel strips, while obtaining an acceptable surface finish at the same time. These and other objects can be achieved, according to a first aspect of the invention, by a method which is initiated by continuously molding the stainless steel in a band having a thickness of at least 1 mm and at most of 10 mm, so suitable at least 14 mm and a maximum of 6 mm and cooling the molded band to room temperature. This step of the process, which can be carried out through a technique known per se, provides a rapid solidification of the steel, which in turn can give a molded steel structure that can promote the achievement of desirable properties in the finished product. of the method. Typically, due to rapid solidification, the molded structure will be finely granulated compared to the structures obtained by conventional ingot molding or conventional continuous web molding. If, for example, as recommended in the method of the invention, use is made of a twin laminated strip moulder, the molded web structure could normally contain regions of columnar grains adjacent to the web surfaces and a region in the web. form of central equis. The following steps of the method of the invention are carried out in such a way that the advantage of the finely grained molded structure can be carefully taken to achieve the desirable characteristics of the final product of the method. The molding of the band can also allow the manufacture of stainless steel alloy strips which are very difficult or even impossible to manufacture according to conventional techniques due to the fragility or other problems attributed to the segregation or formation of undesirable phases in the steel or other phenomena. Also the cooling of the cast strip, which can be made comparatively faster due to the thin gauge of the cast strip, can contribute to the desired results. Next, according to the first aspect of the invention, the molded strip is cold rolled with at least a thickness reduction of 10% at a thickness which is at least 2% and at most 20% greater than the intended final thickness of the finished product, after which the annealing of the cold-rolled strip at a temperature between 1050 ° C and 1250 ° C follows, and the cold working of the strip after the annealing process to permanently lengthen the strip and by so reduce its thickness by 2-20%. According to another aspect of the invention, at least some of the objects can be achieved by continuously molding the stainless steel to obtain an elongated molded product, hot rolling the elongated molded product to the strip shape, cooling the hot rolled strip to ambient temperature, cold rolling the hot rolled strip with at least a 10% thickness reduction or a thickness that is at least 2% and at most 20% greater than the intended final thickness of the finished product, annealing the laminated band into cold at a temperature between 1050 ° C and 1250oC; and working cold the band after the annealing process to permanently lengthen the band and reduce its thickness by 2-20%. More particularly, according to another aspect, the stainless steel is continuously molded in a band having a thickness of at least 1 mm and at most 10 mm, suitably a thickness of 14 to 6 mm, to achieve a structure molded suitable for the subsequent steps of the process, and hot rolled the strip with a thickness reduction of at least 5% and at more than 50%, adequately a thickness reduction of at least 10% and suitably at most 30%, to break the molded structure of the web material before cooling the hot rolled strip to room temperature. The band which is subjected to an initial cold rolling according to any of the aspects of the invention typically consists of a molded and / or hot strip which is not subjected to any descaling treatment but has been cooled and rolled after molding and / or hot rolled. Optionally, however, the molded web can be subjected to a treatment at a temperature in the range of 900-1200 ° C for up to 3 minutes, preferably, for at least 30 seconds before being cooled and / or rolled up. Thus, whether or not the heat treatment option is used, cold rolling should advantageously be carried out on a web on which rust flakes still remain on the surfaces thereof. In principle, the initial cold rolling process carried out on the molded web and / or on the hot rolled web can be carried out in several steps through a corresponding number of sequentially laminated metallic cords mutually, although preferably they are carried out in one step. The maximum reduction in thickness that can be achieved in a single step will depend on the grade of the steel, the initial dimensions of the belt, and the capacity of the laminating machine. It can be said in general terms that a single step will result in a maximum thickness reduction of approximately 30%, normally a maximum of 25%. This means that in most cases, the thickness of the hot rolled strip will be reduced by 10 to 60%, preferably 10 to 40% when practicing the invention, this reduction depends on the initial thickness of the band and the desired final thickness. The strip is annealed at a temperature between 1050 ° C and 1200 ° C and then cooled to room temperature before being cold drawn. The strip, after being annealed, can be cold worked by stretching it in a strip stretching machine which can be of any known type, for example, of the type used to de-scale the surface of hot rolled strips before the deoxidized strip. The band is cold stretched preferably by means of a combination of stretches and bends of the band around rollers. The cold stretching process is carried out to such an extent that it permanently lengthens the band and with this a reduction in thickness of 2-20%, preferably 2-10%, normally 3-5%, is obtained. . As a result of the combination of the significant bends and stretches of the band around relatively small diameter rolls, the decrease in width will be minimal and practically negligible. The reduction in the thickness of the band will therefore essentially correspond to the degree of elongation achieved. The material is plasticized as a result of the cold drawing process, the elastic limit increases in the order of 100 MPa, and is still high in the case of certain grades of steel. As an alternative, the strip, after being annealed, be cold-worked by cold rolling to a degree such that it permanently lengthens the band and thereby a thickness reduction of 2-20%, preferably 3-10, is obtained. %. A distinctive feature of the method of the invention is that it takes place continuously, which means that the method does not include any inverted steps, for example, inverted laminate, re-wound between the different steps or similar investments. In order to make a continuous process possible, the manufacturing line preferably includes, in a known manner, strip feeders, called re-raisers, at the beginning and end of the manufacturing chain, that is, before the initial cold rolling and after work in cold of the band by cold stretching or cold work. The method of the invention will typically also include deoxidizing the annealed web. The band is preferably deoxidized before being worked cold after the annealing operation, although it is also conceivable to deoxidize the band after the final cold working process. The strip is preferably shot peened before being deoxidized.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 illustrates in a very schematic manner the principles of the invention according to a first embodiment, including an initial band molding; Figure 2 illustrates in more detail the manufacturing line, after the initial band molding, according to the first embodiment; Figure 3 illustrates on a larger scale and in greater detail a cold drawing machine used in the first method mode; Figure 4 illustrates schematically a modification of the first embodiment of the method of the invention; - Figure 5 schematically illustrates the principles of the invention according to a second embodiment including initial band molding; Figure 6 schematically illustrates the principles of the invention according to a third embodiment, including initial web molding and hot rolling operation; and Figure 7 illustrates schematically the principles of the invention according to a fourth embodiment, which likewise includes an initial band molding and a hot rolling operation.

DETAILED DESCRIPTION DK THE INVENTION In all of the manufacturing lines illustrated schematically in Figure 1, Figure 4, Figure 5, Figure 6, and Figure 7, a molded web 100 is initially manufactured by continuous web molding in a molding machine 101, which preferably it is a double roller strip moulder, although other web moulders operating continuously can also be used. The molded stainless steel is cast into the moulder 101 from a cauldron 102 via a funnel 103. When the steel passes between the two rolls of the moulder 101 it solidifies in a manner known per se to form the molded web 100, which according to the embodiments illustrated in Figure 1, Figure 4 and Figure 5 are cooled and rolled 104. Before being rolled up, the molded web 100 is optionally in the modalities illustrated in Figure 1, Figure 4 and Figure 5 - subjected to a treatment heat in a 105 to 900-1200 ° C oven for a period of up to 3 minutes, suitably for at least 30 seconds. The first embodiment of the method will now be further described with reference to Figure 1. The rolled molded web 100, which has the shape of the coil 104 is transported to a plant for further processing of the molded web 100. This plant can located in connection to or at a distance from the strip molding facilities, and comprises an unrolled moving head 1, which contains the reel 104, a cold rolling machine 2 consisting of a single roller support 2 of the type known as Z, an annealing furnace 3, a cooling chamber 4, a blasting machine 16, a deoxidizing bath 5, a cold drawing machine 6, and a re-coiler 7 which extracts the finished steel strip. Figure 2 shows the manufacturing line, after the initial strip molding line, in greater detail, where the same reference numbers have been used for units that correspond in Figure 1. In addition to the units mentioned above, the The manufacturing line also includes a cutting unit 8, a welding machine 9, a band feeder 10 which feeds the molded web 100A to the cutting unit 8 and the welding machine 9, a molded strip sealer which is made reference generally with the number 12, means for measuring the thickness 13 which measure the thickness of the molded band 100A upstream of the laminating machine 2, and means for measuring the thickness 14 which measure the thickness of the molded band 100B downstream of the cold rolling machine 2, the grallacing machine 16, a cleaning and rinsing chamber 17 downstream of the deoxidizing bath 5, a pair of guide rollers 1 8, the cold drawing machine 6, a serger which is generally referred to as number 20 for the storage of the cold-rolled and cold-cooled finished bands 100F, a front feeder 21, and a drive motor and means power transmission which are referenced together with the number 22 to operate the rewinder 1. The manufacturing line also includes a large number of guide rollers, rollers for changing the direction, and the braking roller arrangement comprising of two or four rollers. The braking roller arrangement is thus comprised of a two-roll braking roller unit 25 downstream of the welding machine 9, a two-roll braking roller unit 26 upstream of the cold rolling machine 2 , a four-roller braking roller unit 27 between the cold rolling machine 2 and the annealing furnace 3, a 4-roller braking roller unit 28 upstream of the cold drawing machine 6, a winding unit two roller braking roller 29 downstream of the cold drawing machine 6, a band center guide 19, the band feeder 20, and a two roller braking roller unit 31 terminating between the serger 20 and the rewinder 7. The main function of the braking rollers is to increase or decrease the tension in the band and keep the band in tension. The molded strip recloser 12 includes rollers that change in direction 34, 35, 36 and 37, of which the roller 35 is coupled to a belt tensioning unit in a known manner. Correspondingly, the molding stripper 20 includes rollers that change the direction 39, 40, 41, 42, 43 and 44, of which the roller 40 is connected to the belt tensioning unit, also in a known manner. The manufacturing line illustrated in Figure 2 operates in the following manner. It is assumed that the manufacture is in the phase illustrated in the figure, that is to say, that the stripper of molded webs 12 and the cold-stripper of strippers 30 contain a given amount of webs, so that the molded web 100A is unwound of the rewinder 1, and that the finished band 100F is wound on the rewinder 7. The line is driven by several actuation rollers mainly driven, by braking rollers in a known manner.

After having passed through the stripper 12, the thickness of the strip is measured by means of measuring the thickness 13 upstream of the cold rolling machine 2 and cold rolling on the roll 2 a single step, where subsequently the thickness of the cold rolled strip 100B is measured by the means for measuring the thickness 14. The molded strip 100A will normally have an initial thickness of 2 to 4 mm and is reduced by 10-30% in the machine for cold rolling 2. The space between the roll is adjusted according to the results of the thickness measurements to obtain a cold rolled strip 100B of the desired thickness, corresponding to 2-20%, preferably 2 -10%, typically 3-5% greater than the intended finished dimension after cold stretching the end band of the manufacturing line. The cold rolling process imparts a high degree of hardness to the web 100B, and the web is therefore passed into the annealing furnace 3 after having passed the four roller braking roller unit 27. The band 100B it is heated through its thickness in the annealing furnace 3 at a temperature between 1,050 ° C and 1,200 ° C, that is to say at a temperature above the recrystallization temperature of the austenitic steel, and it is maintained at this temperature in time enough for the steel to recrystallize completely. The band is then cooled in the cooling chamber 4. When the band is heated in the annealing furnace 3, which according to the present embodiment does not take place in a protective gaseous atmosphere (some of which may be possible per se). ), oxides are formed on the sides of the band, partially in the form of oxide flakes. The band is substantially de-scaled in the blasting machine 6, and then deoxidized in the deoxidizing bath 5 comprised of appropriate deoxidizing chemicals, wherein the deoxidation process can be carried out in a known manner. The cold rolled, annealed and deoxidized strip 100E is known through the cleaning and rinsing chamber 17 and subsequently through the cold drawing machine 16 between the four roller braking roller unit 28 and the roller unit of two-roller braking 29 which works to keep the band in tension and prevent its sliding. Figure 3 illustrates the design of the cold drawing machine 6. The cold drawing machine 6 comprises three stretchers for stretching strips 47, 48 and 49. Each stretching unit includes a respective lower roller 50, 51, 52 hinged in one piece. stationary base 53, 54, 55, and a respective upper draw roller 56, 57, 58 hinged on a respective roller fastener, 59, 60, 61. The positions of the roller fasteners in relation to the band and in relation to the lower stretching rollers 50, 51, 52 can be adjusted by means of jacks 62, 63, 64 respectively. The rollers for stretching upper bands 56, 57, 58 are initially in the upper positions (not shown), so that the band 100E, which is held stretched between the braking roller units 28 and 29, will be extended in line straight through the machine for cold stretching 6. Starting from this initial position, the upper stretching rollers 56, 57 and 58 are lowered by means of the jacks 62, 63, 64 to the positions shown in Figure 3, by what the band 100E-100F will form a helical passage, as shown in Figure 3, while at the same time being stretched in its cold state to a degree of such high magnitude to plasticize the band. According to the illustrated embodiment, the lower draw rolls 50, 51, 52 have diameters of 70, 200, and 70 mm respectively, while the upper draw rolls 56, 57 and 58 have diameters of 70, 70 and 200 mm respectively. As a result of the chosen adjustment of the rollers to stretch adjustable upper bands 56, 57, 58 and by virtue of the chosen diars of the rollers, the part of the belt that passes through the cold drawing machine will be plasticized as the web continues to be pulled through the laminator 6 and will be bent around the stretching rollers, thereby obtaining the permanent elongation of the band and with this a reduction in the thickness of the band of 2-20%, preferably 2-10%, typically 3-5%. The width of the band is also reduced slightly at the same time, even when the reduction is only one tenth of the elongation and can be essentially ignored. The permanent elongation of the band also results in a reduction in thickness corresponding essentially to the elongation of the band. A finished band 100F with the desired final thickness can be obtained by adapting the reduction of the thickness of the strip, achieved by the cold rolling of the strip to the cold rolling machine to the reduction of thickness, obtained by the cold stretching in the machine for cold drawing 6, or vice versa, the band is wound on the rewinder 7 after having passed through the cold rolled stripper 20. The driving machine of the integrated manufacturing line described above consists of the drive machinery 22 coupled to the web rewinder 7. When greater reductions are desired than those that can be achieved with a cold roll machine comprising only a roll stand and only a cold drawing machine, a plurality of roller supports 2A, 2B, etc., sequentially in a series, as shown in Figure 4. This Figure also illustrates the possibility of placing the deoxidized bath 5 downstream of the machine for cold drawing 6. In this case, the cold drawing machine can also work to de-scale the surfaces of the band, eliminating possibly the need for a machine meshed upstream of the deoxidized bath. In the line shown schematically in Figure 5, the cold drawing machine 6 of Figure I-Figure 4, and the braking rollers on both sides thereof are replaced by the cold rolling machine 6 '. The cold rolled and deoxidized band 100E (as long as the manufacture of the cold rolled strip 100E is considered, see the description above) is passed through the cold rolling machine 6 'to obtain a permanent elongation of the band and with this the reduction in the thickness of the band of 2-20%, preferably 3-10%. The permanent elongation of the band also results in a reduction in thickness corresponding to the elongation of the band. A finished band 100F with the desired final thickness can be obtained by adapting the reduction of the thickness of the strip achieved by the cold rolling of the strip in the cold rolling machine 2 to the thickness reduction obtained by the cold rolling of the band in the machine for cold rolling 6, or vice versa, whereby the web is subsequently wound onto the rewinder 7. As long as the details of the line after molding the band are related, reference is made to Figure 2 and above description in connection with Figure 2. Turning now to Figure 6 and Figure 7, the molded web 100 is passed through a holding furnace 105 which maintains the temperature of the molded web 100 at or elevates this to a temperature between 900 and 1200 ° C suitable for hot rolling. After passing through the holding furnace 105, the molded web is hot rolled in a hot rolling machine 106, which works at the same speed as the moulder 101. The hot rolled and molded web 100 'is rolled up then 104 ». The lines after the strip molding line and hot rolled, Figure 6 and Figure 7, are designed and operate in the same manner as described with reference to the corresponding lines of Figure 1 and Figure 5, respectively. These lines will therefore not be described here, instead reference will be made to the above description with reference to Figure 1, Figure 2, Figure 3 and Figure 5. By the method of the invention, in principle any material can be processed stainless steel, but first, the manufacture of austenitic stainless steel bands is contemplated. Since the method has advantages in terms of economic production as well as in terms of desirable material improvements, it can be used for the mass production of bands of standard grades 304 and 316 and variations thereof but also for manufacturing bands of special stainless steels containing very high contents of nickel and / or molybdenum, for example 5-15% molybdenum, or other alloying elements which may cause problems in connection with the manufacture of conventional stainless steel bands.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (19)

1. A method for manufacturing a stainless steel strip, particularly an austenitic stainless steel strip, with the desired final thickness and with an elastic limit of at least 250 N / mm2, characterized by: continuously molding a stainless steel, particularly an austenitic stainless steel , in a band or strip having a thickness of at least 1 mm and at most 10 ma, - cooling the molded band at room temperature, cold-rolling the molded band with at least a 10% thickness reduction to a thickness that it is at least 2% and at most 20%, preferably at most 10%, greater than the intended final thickness of the finished product, annealing the cold rolled strip at a temperature of between 1.050 ° C and 1, 250 ° C; and cold working the band after the annealing process to permanently lengthen the band and thereby reduce its thickness by 2-20%, preferably 2-10%.

2. The method according to claim 1, characterized in that the cold working subsequent to the annealing treatment is effected by the combination of the continuous stretching of the band and the bending of the band around rollers as the band is stretched.

3. The method according to claim 2, characterized by compressing the web against the rollers during the operation of stretching the web and bending the web with a radius of curvature of less than 200 mm, preferably with a radius of less 20 mm and maximum 150 mm.

4. The method according to claim 1, characterized in that the cold working subsequent to the annealing treatment is carried out by cold rolling the strip.

5. The method according to any of claims 1-4, characterized by the cold rolling of the strip before the annealing treatment to achieve a thickness reduction of 10-60%.

6. The method according to claim 4, characterized by the cold rolling of the molded web before the annealing treatment to obtain a thickness reduction of 10-30%.

7. The method according to any of claims 1-6, characterized by the continuous cold stretching of the strip or by the cold rolling of the strip after the treatment of the annealing to permanently lengthen the strip and thereby reduce the thickness in a 3-5%.

8. The method according to any of claims 1-7, characterized in that the strip is deoxidized before or after the cold working operation after the annealing treatment.

9. A method for manufacturing a stainless steel strip, particularly an austenitic stainless steel strip, with the desired final thickness and an elastic limit of at least 250 N / mm2, characterized by: continuously molding a stainless steel, particularly an austenitic stainless steel for obtain an elongated molded product, hot-rolled the elongated molded product to the shape of a strip, cool the hot-rolled strip at room temperature, cold-laminate the hot-rolled strip with at least a 10% reduction in thickness 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 cold rolled strip at a temperature of between 1,050 ° C and 1,250 ° C; and working cold the band after the annealing process to permanently lengthen the band and thereby reduce its thickness by 2-20%, preferably 3-10%.

10. The method in accordance with the claim 9, characterized by the continuous molding of the stainless steel in a band having a thickness of at least 1 mm and a maximum of 10 mm, and the hot rolling of the band molded with at least 5% and at most 50% reduction of thickness, suitably at least 10% and more adequately 30% reduction in thickness, to break the molded structure of the web material before cooling the hot rolled strip to room temperature.

11. The method in accordance with the claim 1 or 2, characterized in that the cold working subsequent to the annealing treatment is carried out by the combination of the continuous stretching of the band and at the same time the bending of the band around rollers.

12. The method according to claim 11, characterized by compressing the web against the rollers during the operation of stretching the web and curving the web with a radius of curvature of less than 200 mm, preferably with a radius of less 20 mm and maximum 150 mm.

13. The method according to claim 9 or 10, characterized in that the subsequent cold working of the annealing treatment is carried out by cold rolling the web.

14. The method according to any of claims 1-13, characterized by the cold rolling of the hot rolled strip before the annealing treatment to achieve a thickness reduction of 10-60%.

15. The method according to claim 14, characterized by the cold rolling of the hot rolled strip before the annealing treatment to obtain a reduction in thickness of 10-30%.

16. The method according to any of claims 9-15, characterized by the continuous cold stretching of the strip or by the cold rolling of the strip after the annealing treatment to permanently lengthen the strip and thereby reduce its thickness in a 3-10%.

17. The method according to any of claims 9-16, characterized in that the strip is deoxidized before or after the cold working operation after the annealing treatment.

18. The method according to any of claims 1-17, characterized in that the stainless steel contains 0.01-0.10% C, 17-27% Cr, 7-30 * de > Ni, 0-15% of Mo.

19. The method in accordance with the claim 18, characterized in that the steel contains 5-15% Mo.