Method for producing ultra-low specular reflectance stainless steel sheets, stainless steel sheet, use of the stainless steel sheet

Abstract

The invention relates to a method for producing stainless steel sheets in a range of ultra-low specular reflectances, and to said steel sheets. The method comprises a specific sequence of rolling and annealing steps that has been found to provide said sheets.

Description

[0001] METHOD FOR PRODUCING ULTRA-LOW SPECULAR REFLECTANCE STAINLESS STEEL SHEETS, STAINLESS STEEL SHEET, USE OF THE STAINLESS STEEL SHEET

[0002] Field of the invention

[0003] 5 The present invention relates to the field of metallurgy, and more particularly to the working of stainless steel.

[0004] Background of the invention

[0005] Stainless steels have excellent corrosion resistance and therefore great 10 durability, and are generally compatible with other common construction metals, such as steel, aluminium or zinc.

[0006] Traditionally, stainless steels for decorative applications have a BA (bright annealing) finish and are characterized by a shiny surface finish. However, current applications in architecture are aimed at covering the facades of large 15 buildings where sunlight falls directly on the surface of the material and reflects against other structures or passers-by, potentially harming them depending on the type and amount of reflection.

[0007] Light incident on a surface can be reflected either specularly, that is, an incident ray is reflected at a single angle; or diffusively when an incident ray is 20 reflected at many angles. Most materials include both a specular and a diffuse reflection component.

[0008] In the context of building materials, aesthetics, which are favoured by specular reflectance, and practicality, which is unfavoured by specular reflectance, are intertwined. Specifically, having a shiny appearance and not 25 harming nearby structures or people can be important from a market point of view. In fact, countries such as Singapore even regulate the latter by law. Therefore, low specular reflectance stainless steel suitable amongst others for minimising potential harm to its surroundings when employed as a building material are highly desirable.

[0009] Document EP 3095882 A1 relates to a method for producing stainless steel sheets with specific specular reflectance values. In particular, this document alleges that heat treating a cold rolled stainless steel sheet before and after its patterning yields low specular reflectance stainless steel products. However, the document reports that said specular reflectance can be brought to less than 10% only when the method it describes includes a mechanical blasting of the steel surface. Furthermore, the document is silent on how much less than 10% of a specular reflectance can be achieved, on what exactly said specular reflectance refers to such as on how exactly said specular reflectance is measured, or on how said mechanical blasting is to be performed, as no working example is provided.

[0010] SUMMARY OF THE INVENTION

[0011] The present inventors have now developed a method of producing stainless steel sheets of ultra-low specular reflectances, without the need of performing any mechanical blasting on the steel sheet. Specifically, it has been found that the method of the present invention allows producing a design- imparted stainless steel sheet with a specular LRV value of less than 10, even less than 6, measured according to BS 8493:2008+A1 :2010.

[0012] Thus, in a first aspect, the present invention provides a method for producing a stainless steel sheet, wherein the method comprises the steps of: a) Providing a stainless steel sheet obtained by a method comprising the hot-rolling, and then the annealing and descaling, of a stainless steel piece; b) Subjecting the steel sheet of step a) to a process of cold-rolling suitable for imparting on the surface of the steel sheet a roughness Ra of 0.20- 0.40 pm; c) Subjecting the cold-rolled steel sheet obtained in step b) to oxidative annealing followed by descaling; d) Subjecting the annealed and descaled steel sheet obtained in step c) to design imparting; e) Subjecting the design-imparted steel sheet obtained in step d) to oxidative annealing followed by descaling.

[0013] In a second aspect, the present invention is directed to a stainless steel sheet possessing a specular light reflectance value in the range 3 to 9.9 according to standard BS 8493:2008+A1 :2010, preferably obtainable by the method of the first aspect of the invention.

[0014] In a further aspect, the present invention relates to the use of the stainless steel sheet of the second aspect of the invention in the manufacture of building parts, kitchen appliances, vehicle parts, or containers.

[0015] BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1. Depiction of a cold-rolling process according to the invention, more specifically of a steel sheet being passed through a pair of working rolls.

[0017] DETAILED DESCRIPTION OF THE INVENTION

[0018] The method of the present invention allows producing a design-imparted stainless steel sheet with a specular LRV less than 10. Specular reflectance values relating to the present invention refer to specular LRV value measured according to BS 8493:2008+A1 :2010, preferably in the wavelength range 380 to 780 nm of the electromagnetic spectrum. Light reflectance value (LRV) is measured according to BS 8493:2008+A1 :2010, which is specular component included (SPIN); and LRV with excluded specular component (SPEX) is measured in the same manner, but by excluding specular component. The specular LRV is then obtained from the difference between LRV (SPIN) and LRV (SPEX).

[0019] More specifically, the measurement of SPEX is by following the procedure of BS 8493:2008+A1 :2010 for SPIN, but wherein only the diffuse LRV is measured. This can be done by employing an integrating sphere spectrophotometer meeting the requirements of norm CIE130-1998, in particular following the procedure detailed at section 7.2.2 therein (Samples with mixed reflection). The integrating sphere spectrophotometer can be a double beam spectrophotometer, such as a Perkin-Elmer Spectrometer Lambda 900 UV / VIS I NIR spectrophotometer with a sphere integrator d / 8° of 150 mm diameter and white matte surface inside the sphere. The light source is reflected from the white matte surface inside the sphere, causing the light to be randomly distributed in all directions. This process is repeated thousands of times in a fraction of a second, creating a homogeneous and diffused illumination of the object. A detector then receives the reflected light at an 8° angle from the object's surface to accurately measure the LRV. The sphere instrument has two ports, the Sample Viewing Port and the Specular Port. The viewing port contains the receiver and light-sensitive detectors that quantify light reflected from the sample surface. The specular port can be opened or closed to control the type of measurement (SPEX (diffuse) or SPIN). BS 8493:2008+A1 :2010 and CIE130-1998 are incorporated herein by reference.

[0020] Most particularly, specular reflectance is measured as described in the Example section hereinbelow.

[0021] In the context of the present invention, the term “sheet” is interpreted broadly to comprise any steel piece wherein the thickness of the piece is substantially smaller, such as at least 80% smaller, such as 80% smaller, than the width and length of the piece, and comprises definitions such as film, layer, slab, plate or strip. In a preferred embodiment, the piece is rectangular.

[0022] In an embodiment, the specular LRV is in the range 3.0 to less than 10. In an embodiment, it is in the range 4.0 to less than 10, particularly 4.5 to less than 10, such as 4.8 to less than 10. In another embodiment, the top end of any one of these ranges is 9.9, in particular 9.5, particularly 9.0, more particularly 8.0, even more particularly 7.0, yet more particularly 6.0, most particularly 5.0.

[0023] The first step in the process of the invention (step a) comprises providing a stainless steel sheet obtained by a method comprising hot rolling a stainless steel piece into a sheet and descaling the sheet. Preferably, the method for obtaining the steel sheet comprises hot rolling followed by annealing, and then by descaling.

[0024] The stainless steel piece employed in step a) is not particularly limited in its composition. In an embodiment, the stainless steel is austenitic, ferritic, martensitic or duplex stainless steel. In an embodiment, the stainless steel sheet is not ferritic. Preferably, the stainless steel is austenitic stainless steel, and even more preferably it is American Iron and Steel Institute (AIS I) 304, 304L, 316 or 316L, preferably 304, 304L or 316L, more preferably 316L, austenitic stainless steel.

[0025] AISI 304 stainless steel comprises, or more particularly consists of, the following composition (elements in % amounts per weight of the steel):

[0026] C: <=0.070;

[0027] Si: <=0.75;

[0028] Mn: <=2.00;

[0029] Cr: 18.00-20.00; Ni: 8.00-10.00;

[0030] N: < 0.1 ;

[0031] Fe: to balance the composition and incidental impurities, such as

[0032] P: <=0.040;

[0033] S: <=0.030, such as 0.020-0.030 or <=0.015;

[0034] Cu: <0.6.

[0035] In the context of the present invention, an incidental impurity refers to a substance that is not intentionally added, and that may be contained in a raw material, such as ore or scrap, that is used to add other elements that are desired in the alloy, or that may be incorporated unintentionally during the production process of the desired alloy. The impurities may be present in an amount that do not adversely affect the performance of the present invention.

[0036] AISI 304L stainless steel comprises, or more particularly consists of, the following composition (elements in % amounts per weight of the steel):

[0037] C: <=0.030;

[0038] Si: <=0.75;

[0039] Mn: <=2.00;

[0040] Cr: 18.00-20.00;

[0041] Ni: 8.00-10.50;

[0042] N: <0.1 ;

[0043] Fe: to balance the composition and incidental impurities, such as

[0044] P: <=0.040; S: <=0.030, such as 0.020-0.030 or <=0.015;

[0045] Cu: <0.6.

[0046] AISI 316 stainless steel comprises, or more particularly consists of, the following composition (elements in % amounts per weight of the steel):

[0047] C: <=0.080;

[0048] Si: <=0.75;

[0049] Mn: <=2.00;

[0050] Cr: 16.00-18.00;

[0051] Ni: 10.00-14.00;

[0052] Mo: 2.0-3.0;

[0053] N: < 0.1 ;

[0054] Fe: to balance the composition and incidental impurities, such as

[0055] P: <=0.040;

[0056] S: <=0.030, such as 0.020-0.030 or <=0.015;

[0057] Cu: <0.6.

[0058] AISI 316L stainless steel comprises, or more particularly consists of, the following composition (elements in % amounts per weight of the steel):

[0059] C: <=0.030;

[0060] Si: <=0.75;

[0061] Mn: <=2.00; Cr: 16.00-18.00;

[0062] Ni: 10.00-14.00;

[0063] Mo: 2.0-3.0;

[0064] N: < 0.1 ;

[0065] Fe: to balance the composition and incidental impurities, such as

[0066] P: <=0.040;

[0067] S: <=0.030, such as 0.020-0.030 or <=0.015;

[0068] Cu: <0.6.

[0069] The sign <= refers to “less than or equal to”.

[0070] In an embodiment, the stainless steel does not comprise tin, or more particularly does not comprise between 0.01 and 1 % tin by weight with respect to the total weight of the stainless steel, and particularly comprises less than 0.01 % tin. In this invention, no modification of a standard chemical composition is required in order to get the desired appearance and reflectance.

[0071] The steel sheet obtained in step a) as well as the final steel sheet of the method of the invention will also be stainless steel of the kind of the piece employed in step a).

[0072] Hot rolling is a metalworking process in which a stainless steel piece is heated above its recrystallization temperature to plastically deform it by rolling. Rolling involves passing the steel piece between at least one pair of rotating rolls which compress the steel sheet as it passes through the space in between the two rolls, thus flattening and lengthening the steel piece into a uniform thickness stainless steel sheet. The hot rolling temperature may be between 900 and 1200 °C, more preferably between 1050 and 1200 °C. Unless otherwise specified herein, any reference to a pair of rolls refers to a pair of working rolls, i.e. the rolls in the mill that actually contact the steel sheet.

[0073] Annealing refers to a heat treatment wherein the steel sheet is heated above its recrystallization temperature and maintained at said temperature for a sufficient amount of time to allow for the atoms of the steel sheet to diffuse and reduce any existing atom dislocations existing in the metal, which are typically formed when subjecting the metal to stress, such as during rolling processes. The ultimate purpose of annealing is to achieve sufficient dislocation reduction to improve the ductility and reduce the hardness of the steel sheet. The metal sheet is then allowed to cool down below its recrystallization temperature, such as down to room temperature (around 25°C). The annealing temperature may be between 900 and 1200 °C, more preferably between 1050 and 1200 °C, even more preferably between 1070 and 1190 °C, yet more preferably between 1170 and 1190 °C. Annealing of the steel sheet is commonly carried out in an oven or furnace.

[0074] The annealing is preferably oxidative annealing, i.e. annealing in the presence of an oxidizing agent such as oxygen, water or carbon dioxide, such as annealing in the presence of air. The cooling in oxidative annealing is usually performed rapidly, quenching such as at a rate of 10000°C per hour, or faster.

[0075] The subjecting of the steel sheet to high temperatures in an oxidizing atmosphere during hot rolling and / or annealing leads to the oxidation of the steel sheet surface to form oxide scale, which has a negative impact on surface quality and appearance of the steel sheet. Thus, oxide scale is typically removed prior to further machining or providing the final steel sheet. The process is usually referred to as descaling. The descaling can be carried out by any method known to the skilled person, such as mechanical or chemical descaling methods, or a combination thereof.

[0076] Mechanical descaling methods include descaling with mechanical descaling means such as roller brushes, grinding brushes, grinding belts, bending rolls or pinch rolls. A common mechanical descaling method is the smooth clean surface method.

[0077] Chemical descaling is commonly referred to as pickling. Pickling methods include subjecting the steel sheet to acidic, neutral or alkaline conditions suitable for removing any scale.

[0078] Acid pickling is typically performed by bringing the steel sheet into contact with, such as by immersing in, a solution comprising one or more acids. Nonlimiting examples of acids that can be used as the pickling solution include, but are not limited to, nitric acid, acetic acid, sulfuric acid, hydrochloric acid and / or hydrofluoric acid.

[0079] Alkaline pickling is typically performed by bringing the steel sheet into contact with, such as by immersing in, a solution comprising one or more bases. Non-limiting examples of bases that can be used as the pickling solution include, but are not limited to, KOH and / or Ca(OH)2.

[0080] Neutral pickling is most commonly electrolytic pickling in a neutral solution. The steel sheet is brought into contact with, such as by immersing in, a conducting neutral solution, such as with a pH of from 6 to 8; and applying electric current to the neutral solution, whereby oxides and other impurities are electrochemically dissolved in the neutral solution. A common example of such a neutral solution is sodium sulphate aqueous solution.

[0081] Preferably, the descaling in step a) is pickling, more preferably acid pickling, even more preferably pickling performed in nitric acid and / or hydrofluoric acid solution, preferably in a combination thereof. The concentration of nitric acid in said solution is preferably between 150 and 225 g / L of solution. The concentration of hydrofluoric acid in said solution is preferably between 25 and 55 g / L of solution. In a particular embodiment, said pickling is performed at least twice, such as three times. After the steel sheet has been pickled, it is preferably cleaned to remove any pickling solution such as by rinsing with a solvent such as water. The cleaning of the steel sheet removes impurities and / or other materials from the surface of the steel sheet thus preventing them from becoming permanently embedded thereon during subsequent steps of the method of the invention, in particular during subsequent annealing steps.

[0082] Preferably, the stainless steel sheet provided in step a) of the present invention is a 1 D or 1 E stainless steel sheet as defined in EN 10088-2:2005 (E), incorporated herein by reference. More preferably, it is a 1 D stainless steel sheet.

[0083] Preferably, the stainless steel sheet provided in step a) of the present invention shows a normal tolerance on flatness according to EN-10029-2010, 7.2.2 throughout the entire sheet. EN-10029-2010 is incorporated herein by reference.

[0084] In an embodiment, the thickness of the steel sheet provided in step a) is between 10 mm and 2 mm, preferably between 10 mm and 3.5 mm.

[0085] Steel sheets provided in step a) may be commercially acquired such as from Acerinox, S.A. (N1 finish).

[0086] In step b) of the method of the invention, the stainless steel sheet provided in step a) is subjected to cold rolling. The process of cold rolling involves rolling as described above, however not at a temperature of or above the recrystallization temperature of the steel sheet. The process of cold rolling is commonly performed at room temperature.

[0087] In an embodiment, the process of cold rolling of step b) reduces the thickness of the steel sheet by at least 50%, such as from 60 to 85%.

[0088] The cold rolling process of the present invention must be performed in a manner in which the steel sheet that leaves the cold rolling mill possesses an average roughness Ra of 0.20-0.40 pm, in particular of 0.20-0.30 pm, more particularly of 0.20-0.25 pm. Said roughness can be imparted on either one or on both of the top and bottom surfaces of the steel sheet. Said roughness can be imparted by employing at least one pair of rolls suitable for imparting an average roughness Ra of 0.20-0.40 pm on the metal sheet during the cold rolling.

[0089] The imparting of the roughness comprises roughening the surface of at least one of the rolls in the pair of rolls prior to the cold rolling. The surface of the at least one roll is preferably roughened to Ra of 0.20-0.40 pm. The roughened cold mill roll can then impart the desired roughness Ra on the steel sheet as the sheet is rolled with said roughened roll. The roughening of the roll surface is carried out by mechanical means, such as by abrasive machining, and preferably by grinding, in particular CNC grinding. By selecting the appropriate machining conditions, such as grinding wheel grain size, the desired Ra value can be imparted on the surface of the cold roll. The roughness required in the context of the present invention can for instance be achieved by employing a grain #80 grinding wheel. In an embodiment, the roughness on the surface of the roll is not imparted by electrodischarge.

[0090] In an embodiment, more than one pair of cold mill rolls is employed during the cold rolling process, i.e. the steel sheet is subjected to more than one rolling pass. This can serve any or all of the following purposes.

[0091] In a particular embodiment, the more than one pair of cold mill rolls are employed to progressively reduce the thickness of the steel sheet. For instance, a first pair of rolls may reduce the thickness of the steel sheet to a larger extent, and any subsequent pair of rolls may reduce the thickness of the steel sheet to a lesser extent (decreasing thickness reduction sequence).

[0092] In another particular embodiment, the more than one pair of rolls are employed to progressively impart the desired roughness on the steel sheet. For instance, a first pair of rolls may impart on the steel sheet a coarser roughening, and any subsequent pair of rolls may impart a finer roughening on the steel sheet. It is understood that this would require the corresponding and prior roughening of the surface of the rolls with suitable machining conditions, such as employing a grain #80 grinding wheel to machine the first pair of rolls, and a grain #180 grinding wheel to machine the subsequent pair of rolls.

[0093] In an embodiment, at least one pair of rolls which reduces the thickness of the steel sheet is also employed for imparting all or part of the desired Ra. In another embodiment, more than one pair of rolls which reduce the thickness of the steel sheet are also employed for each imparting part of the desired Ra. Preferably said at least one pair of rolls reduces the thickness of the steel sheet by at least 5%.

[0094] It is to be understood that the term “subsequent” as used herein refers to a downstream location in the steel sheet processing line which is employed to subject the steel to the method of the invention.

[0095] In the context of the present invention the roughness Ra is measured according to ISO 4287:1999, incorporated herein by reference.

[0096] Preferably, the rolling conditions for the rolling of a steel sheet with any one particular pair of rolls are maintained constant throughout the rolling of the entire sheet with said particular pair of rolls. This ensures uniformity in the thickness and / or roughness Ra of the resulting rolled steel sheet throughout its surface. In the context of the present invention, maintaining constant conditions means that each rolling condition influencing the cold rolling of the sheet, such as the roll separating force, the arc of contact between the rolls and the sheet, the coefficient of friction between the rolls and the sheet, the torque or velocity of the rolls, the velocity of the sheet, the tension of the sheet (entry and / or exit tension), is varied at most 5%, preferably at most 1 %, or preferably not modified at all, during the rolling of the sheet. Preferably, it is the roll separating force (RSF) that is maintained constant. The RSF can be measured according to the following equation:

[0097] RSF = (RT)1 / 2- W ■ p wherein RSF is the roll separating force, in kg; (R r)1 / 2is the arc of contact between a roll and the sheet, measured in mm (R is the radius of the roll, and r is the thickness of the sheet prior to rolling minus the thickness of the sheet after rolling); w is the width of the sheet, measured in mm; and p is the specific pressure applied by the roll on the sheet, measured in kg / mm2.

[0098] With reference to Figure 1 , the arc of contact is determined as follows:

[0099] AB / BC = BH / AB

[0100] (AB)2= BC BH

[0101] BC = 2R

[0102] BH = (entry thickness-exit thickness) / 2=r / 2

[0103] (AB)2= 2R ■ r / 2

[0104] AB =(R ■ r)1 / 2

[0105] The thickness-reduced and roughened steel sheet obtained in step b) is then subjected to a process of annealing followed by descaling in step c). It is understood that the annealing is oxidative, and that this is why the descaling step is then required, specifically to remove oxide scale produced during the oxidative annealing.

[0106] Annealing conditions are established for each family of steel according to the design of the furnace and its efficiency.

[0107] Specific speed of passing the band through the furnace is computed from the nominal capacity of such furnace according to the next equation:

[0108] T x V x W x p x k -> Capacity

[0109] Where,

[0110] T — thickness, in mm.

[0111] V line speed, in m / min. W— > band width, in mm. p steel density, in kg / m3(7.93 kg / m3, unless other specified). k — constant, 6 10’5

[0112] Capacity — Furnace capacity, in tons per hour.

[0113] The oxidative annealing of step c) is an oxidative annealing as described above for step a), but wherein the preferred annealing temperature is between 1070 and 1190 °C. Furthermore, in an embodiment, the annealing of step c) is carried out at a T x V value in the range [38 to 80 mm x m / min], in particular [60 to 80 mm x m / min], more particularly [70 to 80 mm x m / min], wherein T is the thickness of the steel sheet (in mm) and V is the speed (in m / min) at which the steel sheet travels through a space subjecting the steel sheet to annealing conditions, in particular to the elevated temperature. A T x V = 75 means that a 1 mm coil travels through said space, e.g. the furnace, at 75 m / min, so a 2 mm thickness coil is processed at 37.5 m / min.

[0114] A change in speed leads to an adaptation of the furnace. The faster the steel sheet travels, the higher the temperature of the furnace should be. For example, thick gauge austenitic stainless steel:

[0115] T x v =75 — Temperature range from 1065 °C to 1165 °C

[0116] T x v =80 — Temperature range from 1100 °C to 1170 °C

[0117] The T x v value assumes a constant width of the metal sheet and neglects metal sheet density. The width refers to the length of the sheet that is perpendicular to the direction in which the sheet travels in the processing line and inside the oven or furnace (and which is not the thickness).

[0118] The descaling of step c) is a descaling as described above for step a).

[0119] In an embodiment, the descaling comprises neutral pickling or acid pickling, preferably a combination thereof, more preferably neutral pickling followed by acid pickling. In a particular embodiment, said neutral pickling is performed at least once, such as once, and said acid pickling is performed at least twice, such as twice.

[0120] The neutral pickling is preferably performed in sodium sulphate solution, preferably the concentration of sodium sulphate in said solution is between 180 and 220 g / L of solution. Preferably, said pickling is carried out at a temperature from 30 to 65 °C. Preferably, said pickling is carried out at a current density of 290 to 310 Asec / dm2.

[0121] The acid pickling is preferably performed in nitric acid and / or hydrofluoric acid solution, preferably in a combination thereof. The concentration of nitric acid in said solution is preferably between 130 and 170 g / L of solution. The concentration of hydrofluoric acid in said solution is preferably between 15 and 25 g / L of solution. Preferably, said pickling is carried out at a temperature from 40 to 60 °C. These conditions are particularly suitable when the steel sheet is austenitic.

[0122] In a preferred embodiment, the gloss of the stainless steel sheet obtained in step c) is measured. Specifically, what is measured is the gloss of the surface(s) on which the Ra roughness was imparted in step b). The measurement of the gloss can be carried out at 20° according to ISO 2813:2015, incorporated herein by reference. This is particularly advantageous as it has been found it provides insight into the specular reflectance of the stainless steel sheet obtained at the end of the method of the invention (i.e. in step e), and because the gloss measurement can be rapidly performed, at least in a manner more rapid than that required by specular reflectance measurement methods. For instance, the measurement of the gloss can be simply read with a gloss meter, such as a Novo- Gloss MultiGauge Lite, Rhopoint instruments. Thus, in an embodiment, the measurement of the gloss as described above is carried out without interrupting the continuity of the method of the invention.

[0123] In particular, it has been found that gloss values of less than 50 (measured at 20° according to ISO 2813:2015) are required if the specular reflectance of the final product is to be lower than 10. More particularly, it has been found that gloss values of less than 60 (measured at 20° according to ISO 2813:2015) are required if the specular reflectance of the final product is to be lower than 6.

[0124] Additionally, the roughness Ra of stainless steel sheet obtained in step c) can also be measured in order to ensure that the roughness Ra imparted on the sheet in step b) is still met.

[0125] The annealed and descaled steel sheet obtained in step c) is then subjected to a process of imparting a design on the surface of said steel sheet. In an embodiment, the design is a pattern.

[0126] The imparting of the design may be achieved in a variety of manners such as by embossing, electro-discharge texturing or laser texturing. Preferably, the design-imparting is carried out by embossing.

[0127] Embossing is a steel forming process for imparting raised or sunken designs on one or both of the top and bottom surfaces of the steel sheet. The embossing process is preferably carried out by rolling the steel sheet through at least one pair of rollers, wherein at least one of said rolls in the pair of rollers is an embossing roll which possesses the design to be imparted on the steel sheet, and more particularly possesses the design in the gender opposite to the design that is to be imparted on the surface of the steel sheet. The embossing roll presses the design onto the steel sheet during the rolling. The second roll in the pair of rollers may be a flat roll when embossing on only one surface of the steel sheet is desired, or it may be a second embossing roll, which can impart the same or a different design as the other embossing roll of the pair. When imparting the same design, each embossing roll in the pair may impart the design in the same gender or in opposite genders (male or female). Preferably, the emboss rolling is carried out in one pass (i.e. the steel sheet is passed through one pair or rolls). Preferably, the steel sheet passes through the roll pair at a constant speed throughout the entire emboss rolling. There is no particular limitation to the type of design imparted on the surface of the steel sheet. Particular examples are sandstone, seastone, linen or diamond. Linen is particularly preferred in the context of the present invention.

[0128] Preferably, the design-imparting process of step d), in particular the embossing, reduces the thickness of the steel sheet by no more than 20%, such as no more than 10%.

[0129] In a final step of the method of the invention (step e), the design-imparted steel sheet is subjected to annealing.

[0130] The annealing of step e) is oxidative annealing, such as described for step c) above, but wherein the preferred annealing temperature is between 1070 and 1180 °C. The annealing is followed by a descaling step such as described for step c) above.

[0131] In an embodiment, the annealing of step e) is carried out at a speed (V) value which is between 5 and 15% greater than that employed in the annealing of step c).

[0132] Preferably, the descaling is acid pickling, preferably performed in nitric acid and / or hydrofluoric acid solution, preferably in a combination thereof. The concentration of nitric acid in said solution is preferably between 110 and 190 g / L of solution. The concentration of hydrofluoric acid in said solution is preferably between 5 and 30 g / L of solution. In a particular embodiment, said pickling is performed at least twice, such as two times. In an embodiment, the steel sheet obtained in step e) is from 0.5 to 4 mm thick.

[0133] In an embodiment, the steel sheet obtained in step e) possesses an average roughness Ra of 0.20-0.40 pm. Said roughness applies to either one or to both of the top and bottom surfaces of the steel sheet.

[0134] The method of the present invention is particularly advantageous in that it does not require any mechanical blasting step to attain very low specular reflectances. Thus, in an embodiment, the method of the present invention does not comprise a mechanical blasting step. In a more particular embodiment, the method of the present invention does not comprise a mechanical blasting step before design-imparting step e). In another particular embodiment, the method of the present invention does not comprise a mechanical blasting step during or after steps c) or e), more particularly during or after step c).

[0135] Steps a) to e) according to the invention may be partially continuous, or preferably constantly continuous, i.e. the steel sheet provided in step a) can be transferred through all the method steps continuously. In practice, this is achieved by carrying out the method of the invention in a production line. The actual method for obtaining the steel sheet of step a) prior to providing it may or may not be part of the continuous line.

[0136] The process of the present invention advantageously enables producing stainless steel sheets with a specular reflectance in the range 3 to 10 measured according to BS 8493:2008+A1 :2010. The process of the invention can advantageously be tuned at any, different or every step of the method to produce a stainless steel sheet of a specifically desired specular reflectance within said 3 to 10 range. This tuning can be done for example based on the following criteria:

[0137] In an embodiment, in step b), the greater the roughness Ra within the Ra range of 0.20-0.40 pm, the lower the specular reflectance of the steel sheet obtained in step e) will be. Conversely, the lower the roughness Ra within the Ra range of 0.20-0.40 pm, the higher the specular reflectance of the steel sheet obtained in step e) will be.

[0138] In an embodiment, in step c), the lower the TxV value in the range from 38 and 80 mm ■ m / min, the lower the specular reflectance of the steel sheet obtained in step e) will be. Conversely, the higher the TxV value in the range from 38 and 80 mm ■ m / min, the higher the specular reflectance of the steel sheet obtained in step e) will be.

[0139] In an embodiment, the lower the gloss of the steel sheet obtained in step c), the lower the specular reflectance of the steel sheet obtained in step e) will be. Conversely, the higher the gloss of the steel sheet obtained in step c), the higher the specular reflectance of the steel sheet obtained in step e) will be.

[0140] In an embodiment, the greater the reduction in thickness of the steel sheet in the design-imparting step, the higher the specular reflectance of the steel sheet obtained in step e) will be will be.

[0141] In a particular embodiment 1 , the method of the invention comprises: a) Providing a stainless steel sheet, preferably an AISI 316L stainless steel sheet, obtained by a process comprising the hot-rolling, oxidative annealing and acid pickling of a stainless steel piece, wherein the thickness of the provided stainless steel sheet is between 10 mm and 2 mm; b) Subjecting the steel sheet of step a) to a process of cold-rolling suitable for imparting on the surface of the steel sheet a roughness Ra of 0.20- 0.40 pm, wherein the thickness of the sheet is reduced at least 50%, preferably from 60 to 85%; c) Subjecting the cold-rolled steel sheet obtained in step b) to oxidative annealing followed by neutral pickling and / or acid pickling; d) Subjecting the annealed and pickled steel sheet obtained in step c) to design-imparting, preferably embossing, which reduces the thickness of the steel sheet by no more than 20%, preferably no more than 10%; e) Subjecting the design-imparted steel sheet obtained in step d) to oxidative annealing followed by acid pickling.

[0142] In the above particular embodiment 1 , any one, preferably all, of the following preferably applies: - the acid pickling of step a) is pickling performed in a nitric acid and hydrofluoric acid solution, wherein preferably the concentration of nitric acid in said solution is between 150 and 225 g / L of solution, and wherein preferably the concentration of hydrofluoric acid in said solution is between 25 and 55 g / L of solution;

[0143] - in step c), the neutral pickling is performed in sodium sulphate solution, wherein preferably the concentration of sodium sulphate in said solution is between 180 and 220 g / L of solution; and the acid pickling is performed in a nitric acid and hydrofluoric acid solution, wherein preferably the concentration of nitric acid in said solution is between 130 and 170 g / L of solution, and wherein preferably the concentration of hydrofluoric acid in said solution is between 15 and 25 g / L of solution; wherein preferably the neutral pickling is followed by the acid pickling;

[0144] - the acid pickling of step e) is performed in a nitric acid and hydrofluoric acid solution, wherein preferably the concentration of nitric acid in said solution is between 110 and 190 g / L of solution, and wherein preferably the concentration of hydrofluoric acid in said solution is between 5 and 30 g / L of solution.

[0145] In the above particular embodiment 1 , in the oxidative annealing of steps a), c) and e), the annealing temperature is preferably between 1050 and 1200 °C, preferably between 1070 and 1190 °C. Preferably, any one, preferably all, of the following applies:

[0146] - in the oxidative annealing of step a), the annealing temperature is between 1170 and 1190 °C;

[0147] - in the oxidative annealing of step c), the annealing temperature is between 1070 and 1190 °C;

[0148] - in the oxidative annealing of step e) the annealing temperature is between 1070 and 1180 °C.

[0149] More preferably, in particular embodiment 1 , both the above particular pickling and oxidative annealing conditions apply. In a more particular embodiment, the method of the invention comprises: a) Providing a stainless steel sheet, preferably an AISI 316L stainless steel sheet, obtained by a process comprising the hot-rolling, oxidative annealing and descaling of a stainless steel piece, wherein: o the annealing temperature is between 1050 and 1200 °C, preferably between 1170 and 1190 °C; o the descaling is pickling performed in a nitric acid and hydrofluoric acid solution, wherein preferably the concentration of nitric acid in said solution is between 150 and 225 g / L of solution, and wherein preferably the concentration of hydrofluoric acid in said solution is between 25 and 55 g / L of solution; o the thickness of the provided stainless steel sheet is between 10 mm and 3.5 mm; b) Subjecting the steel sheet of step a) to a process of cold-rolling suitable for imparting on the surface of the steel sheet a roughness Ra of 0.20- 0.40 pm, wherein o The thickness of the sheet is reduced at least 50%, preferably from 60 to 85%; c) Subjecting the cold-rolled steel sheet obtained in step b) to oxidative annealing followed by descaling, wherein: o the annealing temperature is between 1050 and 1200 °C, preferably between 1070 and 1190 °C; o the annealing is carried out at a T x V value in the range [60 to 80 mm x m / min], more preferably [70 to 80 mm x m / min], wherein T is the thickness of the steel sheet (in mm) and V is the speed (in m / min) at which the steel sheet travels through a space subjecting the steel sheet to the oxidative annealing temperature; o the descaling is neutral pickling and acid pickling, preferably neutral pickling followed by acid pickling,

[0150] ■ wherein the neutral pickling is performed in sodium sulphate solution, wherein preferably the concentration of sodium sulphate in said solution is between 180 and 220 g / L of solution;

[0151] ■ wherein the acid pickling is performed in a nitric acid and hydrofluoric acid solution, wherein preferably the concentration of nitric acid in said solution is preferably between 130 and 170 g / L of solution, and wherein preferably the concentration of hydrofluoric acid in said solution is between 15 and 25 g / L of solution; d) Subjecting the annealed and descaled steel sheet obtained in step c) to embossing, which preferably reduces the thickness of the steel sheet by no more than 20%, preferably no more than 10%; e) Subjecting the design-imparted steel sheet obtained in step d) to oxidative annealing followed by descaling, wherein: o the annealing temperature is between 1050 and 1200 °C, preferably between 1070 and 1180 °C; o the annealing is carried out at a speed (V) value which is between 5 and 15% greater than that employed in the annealing of step c). o the descaling is pickling performed in a nitric acid and hydrofluoric acid solution, wherein preferably the concentration of nitric acid in said solution is between 110 and 190 g / L of solution, and wherein preferably the concentration of hydrofluoric acid in said solution is between 5 and 30 g / L of solution. The stainless steel sheet obtained with the method of the present invention may be employed in the manufacture of diverse products, in particular by working it into the shape that is required for its specific industrial application. Non-limiting examples of products that comprise or are obtainable by working into the desired shape the stainless steel sheet of the present invention are building parts, particularly building exterior parts, such as facades or window frames, or building interior parts such as walls, or escalator and elevator parts; kitchen appliances, such as sinks or cookware; vehicle parts, particularly exterior parts such as chassis parts, or interior parts, in particular those in the sight range of the driver such as dashboard parts; or containers, such as maritime transport, chemical or fuel containers. Particularly preferred are building parts, even more preferably exterior building parts.

[0152] The following represent highlighted embodiments of the invention.

[0153] 1 . Method for producing a stainless steel sheet wherein the method comprises the steps of: a) Providing a stainless steel sheet obtained by a process comprising the hot- rolling and descaling of a stainless steel piece; b) Subjecting the steel sheet of step a) to a process of cold-rolling suitable for imparting on the surface of the steel sheet a roughness Ra of 0.20-0.40 pm; c) Subjecting the cold-rolled steel sheet obtained in step b) to oxidative annealing followed by descaling; d) Subjecting the annealed and descaled steel sheet obtained in step c) to design-imparting; e) Subjecting the design-imparted steel sheet obtained in step d) to oxidative annealing followed by descaling. 2. Method according to any one of the preceding embodiments, wherein the method does not comprise mechanical blasting of the steel sheet surface.

[0154] 3. Method according to any one of the preceding embodiments, wherein the steel sheet provided in step a) is an austenitic stainless steel sheet.

[0155] 4. Method according to embodiment 3, wherein the steel sheet provided in step a) is an AISI 304, 304L or 316L austenitic stainless steel sheet.

[0156] 5. Method according to any one of the preceding embodiments, wherein the thickness of the steel sheet provided in step a) is between 10 mm and 2 mm, and step b) reduces the thickness of the steel sheet by 50 to 85%.

[0157] 6. Method according to any one of the preceding embodiments, wherein the annealing of step c) is carried out at a T x V value in the range from 38 and 80 mm ■ m / min, wherein T is the thickness of the steel sheet and V is the speed at which the steel sheet travels through a space subjecting the steel sheet to annealing conditions.

[0158] 7. Method according to any one of the preceding embodiments, comprising measuring the gloss value of the steel sheet obtained in step c).

[0159] 8. Method according to any one of the preceding embodiments, wherein step d) reduces the thickness of the steel sheet by no more than 20%.

[0160] 9. Method according to any one of the preceding embodiments, wherein the steel sheet obtained in step e) is from 0.5 to 4 mm thick.

[0161] 10. Method according to any one of the preceding embodiments, wherein the stainless steel sheet does not comprise tin. 11 . Stainless steel sheet, preferably austenitic steel sheet, possessing a specular light reflectance value in the range 3 to 9.9 according to standard BS 8493:2008+A1 :2010.

[0162] 12. Stainless steel sheet according to embodiment 11 , wherein the specular light reflectance value is in the range 4.0 to 7.0 according to standard BS 8493:2008+A1 :2010.

[0163] 13. Stainless steel sheet according to embodiment 11 or 12, possessing an average roughness Ra of 0.20-0.40 pm.

[0164] 14. Stainless steel sheet according to any one of embodiments 11 to 13, obtainable by a method as defined in any one of claims 1 to 10.

[0165] 15. Use of a stainless steel sheet as defined in any one of embodiments 11 to 14 in the manufacture of building parts, kitchen appliances, vehicle parts or containers.

[0166] Examples

[0167] Example 1: Preparation of an ultralow specular reflectance steel sheet (Specular LRV 4.84)

[0168] An austenitic stainless steel (AISI 316L / EN 1 .4404) 200 mm slab was subjected to hot rolling to produce a 5 mm thickness metal sheet, and the sheet was then annealed at a temperature from 1170 °C to 1190 °C. The annealed sheet was subjected to three acid pickling treatments employing a pickling solution comprising HNOs and HF.

[0169] The descaled metal sheet was then subjected to a process of cold rolling further reducing the thickness of the metal sheet from 5 mm to 2 mm, as well as imparting an Raroughness of around 0.21 pm on the surface of the metal sheet.

[0170] The cold rolled sheet was subsequently subjected to oxidative annealing at a temperature of from 1070 to 1190 °C and applying a TxV of 75. The annealed sheet was then descaled by subjecting it to a neutral pickling treatment in Na2SO4 solution, followed by two acid pickling treatments employing a pickling solution comprising HNO3 and HF.

[0171] The gloss of the pickled sheet as measured at 20° according to ISO 2813:2015 with a Novo-Gloss MultiGauge Lite (Rhopoint instruments) was determined to be between 13.6 and 14.9, in good correspondence with the target ultralow LRV sought.

[0172] The embossing of the metal sheet was then carried out, which afforded a further reduction of the thickness of the sheet down to 1 .82 mm.

[0173] The embossed steel sheet was then subjected to a final annealing treatment. The line speed was increased 10% for this final annealing treatment, at an annealing temperature of from 1070 to 1180 °C. The annealed sheet was lastly descaled by two acid pickling treatments employing a pickling solution comprising HNO3 and HF.

[0174] The specular light reflectance value (Specular LRV) of the obtained sheet was determined according to BS 8493: 2008 + A1 : 2010 to be of 4.84.

[0175] The measurement was carried out between 380 and 780 nm by means of a Perkin-Elmer Spectrometer Lambda 900 UV I VIS / NIR spectrophotometer with a sphere integrator d / 8° of 150 mm diameter and white pattern. The method used has the following characteristics:

[0176] - Wavelength range: 5 nm

[0177] - Scanning speed: 284.6 nm I min

[0178] - UV / VIS Slit: 1

[0179] From the reflection measurements, the CIE values have been calculated with the illuminant D65 and the observer at 10° Y10, x-io and y-io of each of the sample measurements. The Y10 value corresponds to the light reflectance value LRV. With these values, the mean LRVav value and the maximum and minimum values of the sample. From the reflection measurements with specular component excluded, the CIE values have been calculated with the illuminant D65 and the observer at 10° Y10, x and yw for each of the sample measurements. The Y SCE value corresponds to the value of luminous reflectance with specular component excluded LRVSCE. The mean value LRVSCE av corresponds to the mean value of the minimum diffuse light reflectance, therefore the subtraction of the luminous reflectance with specular component excluded LRVSCE av from light reflectance LRVav is the maximum specular light reflectance in index d 1 8°.

[0180] Example 2: Comparison of specular reflectance values of steel sheets obtained by different methods.

[0181] The specular reflectance of three stainless steel sheets obtained by different methods and with similar roughness (Ra) is hereinbelow compared.

[0182] Stainless steel sheet A, was prepared according to the invention: providing a hot- rolled starting material, subjecting said material to cold rolling with subsequent intermediate oxidative annealing and descaling, followed by a step of designimparting and a final oxidative annealing and descaling.

[0183] In contrast, stainless steel sheets B and C were prepared by methods which lacked the combination of oxidative annealing steps described by the invention.

Claims

CLAIMS1. Method for producing a stainless steel sheet wherein the method comprises the steps of: f) Providing a stainless steel sheet obtained by a process comprising the hot-rolling and descaling of a stainless steel piece; g) Subjecting the steel sheet of step a) to a process of cold-rolling imparting on the surface of the steel sheet a roughness Ra of 0.20-0.40 pm measured according to ISO 4287:1999; h) Subjecting the cold-rolled steel sheet obtained in step b) to oxidative annealing followed by descaling; i) Subjecting the annealed and descaled steel sheet obtained in step c) to design-imparting; j) Subjecting the design-imparted steel sheet obtained in step d) to oxidative annealing followed by descaling.

2. Method according to any one of the preceding claims, wherein the method does not comprise mechanical blasting of the steel sheet surface.

3. Method according to any one of the preceding claims, wherein the steel sheet provided in step a) is an austenitic stainless steel sheet.

4. Method according to claim 3, wherein the steel sheet provided in step a) is an AISI 304, 304L or 316L austenitic stainless steel sheet.

5. Method according to any one of the preceding claims, wherein the thickness of the steel sheet provided in step a) is between 10 mm and 2 mm, and step b) reduces the thickness of the steel sheet by 50 to 85%.

6. Method according to any one of the preceding claims, wherein the annealing of step c) is carried out at a T x V value in the range from 38 and 80 mm ■ m / min, wherein T is the thickness of the steel sheet and V is the speed at which the steel sheet travels through a space subjecting the steel sheet to annealing conditions.

7. Method according to any one of the preceding claims, comprising measuring the gloss value of the steel sheet obtained in step c) being said gloss value less than 50, measured at 20° according to ISO 2813:2015.

8. Method according to any one of the preceding claims, wherein step d) reduces the thickness of the steel sheet by no more than 20%.

9. Method according to any one of the preceding claims, wherein the steel sheet obtained in step e) is from 0.5 to 4 mm thick.

10. Method according to any one of the preceding claims, wherein the stainless steel sheet does not comprise tin.

11. Design-imparted, stainless steel sheet, preferably austenitic steel sheet, possessing a specular light reflectance value in the range 3 to 9.9 according to standard BS 8493:2008+A1 :2010, wherein the sheet has not been mechanically blasted and wherein the sheet possesses an average roughness Ra of 0.20-0.40 pm12. Design-imparted stainless steel sheet, possessing a specular light reflectance value in the range 4.0 to 7.0 according to standard BS 8493:2008+A1 :2010 and wherein the sheet possesses an average roughness Ra of 0.20-0.40 pm.

13. Stainless steel sheet according to any one of claims 11 to 12, obtainable by a method as defined in any one of claims 1 to 10.

14. Use of a stainless steel sheet as defined in any one of claims 11 to 13 in the manufacture of building parts, kitchen appliances, vehicle parts or containers.

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