STEEL, METHOD FOR ITS MANUFACTURE, ITS USE AND PRODUCT MADE FROM STEEL

Abstract: Steel in the form of sheet or strip having the above chemical composition, balance Fe and inevitable impurities, wherein 0.4</=B/N</=1.2.

Description: STEEL, METHOD FOR ITS MANUFACTURE, ITS USE AND PRODUCT MADE FROM STEEL Field of the invention The invention relates to a steel in the form of sheet or strip with excellent stretching and deep-drawing and wall ironing properties, in particular for packaging applications, a method for its manufacture and also a use of such a steel. In the context of this application the packaging steel is taken to mean a consecutively hot- rolled, pickled, cold-reduced, annealed and generally temper-rolled or further cold- reduced steel in the form of sheet or strip whether or not coiled. Blanks of this sheet or strip are to be made in a number of forming operations into a (semi-finished) product for example a beverage can or other object, whereby for the purposes of the intended shape, the flat basic material needs to undergo extensive material deformations. For manufacturing a beverage can a cup is formed for example by deep drawing from a blank of packaging steel in one or more stages, the walls of which are wall-ironed in order to give the can thus formed a greater height/content. In particular the deep-drawing in one stage and wall-ironing require an easily formable material that has ideally isotropic properties. Summary of the invention In this connection and also within the broader scope of continuous endeavours to improve the said steel grades, the search is directed in particular towards a combination of the following properties: high r-value (Lankford-value); as small as possible differences in mechanical properties across above all the strip width, but also along the strip length, in specialist terms low spread of the properties "edge/centre" and "along the coil" respectively. Experiments now show that a steel grade displaying spectacularly better uniform mechanical properties - in annealed and optionally further cold reduced, e.g. temper-rolled condition - is found if the steel has the composition: Element Min* Max* C 40 Mn 140 250 P 20 S 20 Si 30 N ppm 30 AJ ** 50 B ppm 5 50 Cu 40 Sn 10 Cr 40 Ni 40 Mo 10 * in 0.001 % wt. unless otherwise stated ** acid soluble balance Fe and inevitable impurities, wherein 0.4 < B / N < 1.2. This steel makes it possible to attain an improved level of stretching, deep- drawing and wall ironing properties and a greatly reduced spread of the mechanical properties. It is preferred that the composition complies with: Element Min* Max* C 10 40 Mn 140 200 P 20 s 20, preferably 10 Si 30 N ppm 25 Al ** 15 35 B ppm 8 25 Cu 40 Sn 10 Cr 40 Ni 40 Mo 10 * in 0.001 % wt. unless otherwise stated ** acid soluble It is found that to achieve optimum results in accordance with the invention within the limits indicated above the following target values can be given: Because better mechanical properties can be obtained in accordance with the invention it is possible to select a thinner material of thinner than 0.35 down to even thinner than 0.18 mm, which fits in excellently with the search to apply lighter weight for example for a steel beverage can. It is particularly advantageous if the steel strip is in the form of cold reduced, annealed and optionally further cold reduced strip or sheet wherein the grain size in an edge region of the strip Gs-edge and the grain size in a centre region of the strip Gs-centre differ less than 0.5 ASTM units, respectively wherein the r-value in an edge region of the strip r-edge and the r-value in a centre region of the strip r-centre differ less than 0.2 units, preferably less than 0.1 units, respectively wherein the Yield Point in an edge region of the strip Rp-edge and the Yield Point in a centre region of the strip Rp-centre differ less than 20 N/mm 2 , respectively wherein the average r-value is at least 1.3. The production of stretched, deep drawn and drawn and wall ironed products from blanks cut from the steel according to the invention can be carried out much more efficiently with lower spoilage. The invention is further embodied in a method for manufacturing an annealed steel strip from the steel according to any one of claims 1-7, comprising the stages: hot-rolling comprising finish-rolling at a finish-rolling temperature and coiling at a coiling temperature; pickling; - cold-rolling; annealing, preferable, continuous annealing; and optionally further cold rolling, preferably with a reduction of at least 0.6 %; wherein the finish-rolling temperature is selected above Ar 3 -10 °C and the coiling temperature is selected below 700 °C. In a preferred method the finish-rolling temperature is selected above 825 °C and the coiling temperature below 690 °C, more preferably the finish-rolling temperature is selected at 900 ± 30 °C and the coiling temperature at 670 °C ± 20 °C. In the method preferably the total cold reduction before annealing is 85 % or more. In an aspect of the method according to the invention further cold rolling after annealing is carried out with a reduction of > 5 %, or even with a reduction of 10-20 %. This creates a steel strip with very good forming properties. In the case of further cold rolling after annealing, e.g. temper rolling it is preferable to do so with a reduction of at least 0.6 %. Preferably a target value is selected for the finish-rolling temperature of 900°C and for the coiling temperature a target value of 670°C. This establishes the basis for a very easily workable, homogeneous steel sheet. It is preferable that if temper-rolling is carried out it is done so in accordance with the invention with a reduction of 10-20 %. Such a high degree of deformation still produces a steel grade with excellent mechanical properties. The steel grade in accordance with the invention is found to be perfectly suitable for application as packaging steel in drawing and wall-ironing (DWI) applications, for example applications wherein the packaging steel is coated with a coating comprising Sn and afterwards subjected in succession to deep-drawing and wall-ironing as in the manufacture of the body part of a two-piece can. The excellent mechanical properties of the steel allow such a can to be made even lighter, again responding to the demand for increasingly less material consumption and weight for packaging and other products. The invention is also embodied in a use of the steel for stretching, deep- drawing and drawing and wall-ironing applications, in a steel provided with one or more coatings selected from the group of coatings comprising Sn, Cr, lacquer, organic coating, polymer and combinations thereof on one or both sides of the strip or sheet, and in a stretched, deep drawn or drawn and wall ironed product made from steel according to the invention. The invention will now be illustrated by reference to the results of some experiments further details of which are given in the following tables 1 to 5. In the experiments continuously cast steel was hot rolled in a hot rolling mill with a finish-rolling temperature of 900 °C and coiled at coiling temperatures with target values of 625, 670 and 690 °C. Subsequently the steel was pickled in a conventional pickling line and cold rolled with a reduction of 88-89 %. Thereafter it was continuously annealed. After annealing further cold reduction was performed either temper rolling with a reduction of 0.8 % or cold rolling with a reduction of 4 % and 6 %. The steel was then tinned in a conventional electrolytic tinning line. Table 1 Steel tested Chemical composition in 0.001 wt. % and wt. ppm where stated Steel C Mn S Al * B N CT Remarks ppm ppm (°C) A 24 202 7 26 16 24 625 invention B 23 191 7 34 20 29 690 invention C 16 171 5 28 8 22 690 invention D 25 200 <12 35 - 20 670 comparative Al * : Aluminium acid soluble CT : Coiling temperature after hot rolling Table 2 Chemical composition of steel C, in hot rolled condition in 0.001 wt. % and wt. ppm where stated Position C Mn Al * B ppm B * ppm N ppm N * ppm Strip centre 16 171 28 8 1 22 < 2 Strip edge 16 173 28 8 1 17 < 2 Al * : Aluminium acid soluble B * : Boron acid soluble N * : Nitrogen not bound Table 3 Grain size of steel C, in hot rolled and in cold-rolled, annealed condition Position Grain size (ASTM) Hot rolled Cold rolled Strip centre 9.0 1 1.5 Strip edge 9.0 11.5 Table 4 Mechanical properties of steel C in cold-rolled, annealed, temper rolled (reduction 0.8 %) condition Position Strip centre strip edge Direction lengthwise oblique widthwise lengthwise r-value test 1 1.57 1.68 1.83 1.49 test 2 1.58 1.68 1.80 1.49 test 3 1.58 1.80 1.82 1.42 average 1.58 1.72 1.82 1.47 Reh (Rp) test 1 237 234 236 247 test 2 238 235 235 247 test 3 238 235 235 247 average 238 235 235 247 R test 1 360 354 359 358 test 2 362 354 356 358 test 3 362 356 358 357 average 361 355 358 358 In the above table 4 the r-value means the Lankford-value, Reh (Rp) the yield point in N/mm 2 , R the tensile strength in N/mm 2 . The small differences between the values in one row show that the mechanical properties across the width of the sheet vary little. Notable is the high r-value but particularly the small difference in properties per indicated direction, where 'lengthwise' means in the direction of rolling, 'oblique' at 45° to it and 'widthwise' peφendicular to it, more particularly the small difference in properties between the properties of centre portions and edge portions. Table 5 Yield Point and Grain size of steels A, B and D in annealed, temper rolled (reduction 0.8 %) condition Steel Yield Point in N/mm 2 ASTM Remark centre edge centre edge A 258 256 1 1 1 1 invention B 237 235 1 1 1 1 invention D 262 265 1 1 12 comparative Table 6 Yield Point of steels B and D in annealed, rolled condition with different reductions after annealing Steel cold reduction after Yield Point in N/mm 2 annealing in % centre edge B 1 231 233 B 4 283 281 B 6 330 333 D 1 258 268 In a comparative test steels C and D of table 1 were tested on behaviour and performance in a heavy forming operation. Hereto tin plated circular blanks all of the same diameter from head, tail and in between portions taken from centre, edge and in between regions of the strip were drawn into cups which cups were wall ironed, trimmed and pre-spin-necked. The pre-spin-necked cans thus obtained were of the type that are to be made into the commonly used and well known 33 cl. beverage can having a diameter of 66 mm. Of the pre-spin-necked cans the dimensions pre-spin edge diameter PSED (mm) and pre-spin plug diameter PSPD (mm) were determined. Reference is made to the drawings. Description of the drawings Fig. 1 shows the relevant dimensions PSED and PSPD discussed above, and Fig. 2 is a graph showing the relation PSED vs. PSPD. In Fig. 1 a pre-spin-necked can is shown. The can is formed with a neck which is of smaller diameter than its main body portion, this neck having a cylindrical portion of diameter PSPD and an out-turned flange at its end having a maximum diameter PSED as indicated in Fig. 1. These diameters are dependent on the material quality but are critical for successful can formation, since the neck is used to fix the lid in place after filling. In Fig. 2 each open square symbol indicates the values of PSED and PSPD of a pre-spin-necked can made from a blank taken from the steel C according to the invention and each filled square symbol indicates those of a can made from comparative steel D. The vertical line at a PSED value of in this case 56.1 indicates the limit for PSED above which the manufactured can will be rejected because of wavy flanges. The results clearly show the two effects of using steel according to the invention. Firstly the spread in PSED and PSPD and thus in material properties is smaller for cans made from steel according to the invention and secondly it turns out that the cans made from steel according to the invention show a lower PSED value and consequently a lower reject rate.

Claims: CLAIMS Steel in the form of sheet or strip having a chemical composition. Element Min* Max* C 40 Mn 140 250 P 20 S 20 Si 30 N ppm 30 Al ** 50 B ppm 5 50 Cu 40 Sn 10 Cr 40 Ni 40 Mo 10 * in 0.001 % wt. unless otherwise stated ** acid soluble balance Fe and inevitable impurities, wherein 0.4 < B / N < 1.2. Steel according to claim 1 having a chemical composition: Element Min* Max* C 10 40 Mn 140 200 P 20 S 20, preferably 10 Si 30 N ppm 25 Al ** 15 35 B ppm 8 25 Cu 40 Sn 10 Cr 40 Ni 40 Mo 10 * in 0.001 % wt. unless otherwise stated ** acid soluble Steel in accordance with Claim 1 or Claim 2, wherein for the following elements the following standard values are respected: Steel in accordance with any one of the preceding claims in the form of cold reduced, annealed and optionally further cold reduced strip or sheet wherein the grain size in an edge region of the strip Gs-edge and the grain size in a centre region of the strip Gs-centre differ less than 0 5 ASTM units Steel in accordance with any one of the preceding claims in the form of cold reduced, annealed and optionally further cold reduced strip or sheet wherein the r-value in an edge region of the strip r-edge and the r-value in a centre region of the strip r-centre differ less than 0 2 units, preferably less than 0 1 units Steel in accordance with any one of the preceding claims in the form of cold reduced, annealed and optionally further cold reduced strip or sheet wherein the Yield Point in an edge region of the strip Rp-edge and the Yield Point in a centre region of the strip Rp-centre differ less than 20 N/mm 2 Steel in accordance with any one of the preceding claims in the form of cold reduced, annealed and optionally further cold reduced strip or sheet wherein the average r-value is at least 1 3 Method for manufacturing an annealed steel strip from the steel according to any one of claims 1-7, comprising the stages hot-rolling comprising finish-rolling at a finish-rolling temperature and coiling at a coiling temperature, - pickling, cold-rolling, annealing, preferably, continuous annealing, and optionally further cold rolling, preferably with a reduction of at least 0 6 % wherein the finish-rolling temperature is selected above Ar 3 -10 °C and the coiling temperature is selected below 700 °C 9. Method according to claim 8, wherein the finish-rolling temperature is selected above 825 °C and the coiling temperature below 690 °C. 10. Method according to claim 8 or claim 9, wherein the finish-rolling temperature is selected at 900 ± 30 °C and the coiling temperature at 670 °C ± 20 °C. 1 1. Method according to claim 8, 9 or 10 wherein the total cold reduction before annealing is 85 % or more. 12. Method in accordance with anyone of claims 8-1 1, wherein further cold rolling after annealing is carried out with a reduction of > 5 %. 13. Method in accordance with claim 12 wherein further cold rolling after annealing is carried out with a reduction of 10-20 %. 14. Use of the steel in accordance with anyone of claims 1-7, or a steel strip obtained in the method in accordance with anyone of claims 8-13 for stretching, deep-drawing and drawing and wall-ironing applications. 15. Steel in accordance with anyone of claims 1-7, or obtained in a method in accordance with anyone of claims 8-14, provided with one or more coatings selected from the group of coatings comprising Sn, Cr, lacquer, organic coating, polymer and combinations thereof on one or both sides of the strip or sheet. 16. Stretched, deep drawn or drawn and wall ironed product made from steel according to any one of claims 1-7 or claim 15 or from steel obtained by the method or use according to any one of claims 8-14.

Inventors: DE HAAS MAARTEN ARIE, JANSEN EGBERT, JONKER FREDERIK, MULDER JOHANNES, SCHOL WILLEM JAN, DE HAAS, MAARTEN, ARIE, JANSEN, EGBERT, JONKER, FREDERIK, MULDER, JOHANNES, SCHOL, WILLEM, JAN
Applicants: HAAS MAARTEN ARIE DE, HOOGOVENS STAAL BV, JANSEN EGBERT, JONKER FREDERIK, MULDER JOHANNES, SCHOL WILLEM JAN, DE HAAS, MAARTEN, ARIE, HOOGOVENS STAAL B.V., JANSEN, EGBERT, JONKER, FREDERIK, MULDER, JOHANNES, SCHOL, WILLEM, JAN
Application year: 1997
Main classification: C22C38
IPC codes: C21D8, C22C38