Flat steel product with high reflectivity, flat steel product and mirror element for solar concentrators

Abstract

A method for producing a flat steel product with high reflectivity, in which at least one surface has an arithmetic mean roughness Ra of less than 0.03 μm includes providing a flat steel product, at least one surface of which has an arithmetic mean roughness Ra of less than 2.5 μm. The flat steel product is cold rolled in a plurality of rolling passes. Also a flat steel product with high reflectivity in the finished re-rolled state on at least one of its surface has a low arithmetic mean roughness, a high gloss, and a high directed reflection. A solar concentrator is produced from such a flat steel product.

Claims

1. A flat steel product with high reflectivity, which in the finished re-rolled state on at least one of its surfaces has an arithmetic mean roughness Ra of less than 0.03 μm with a preferred direction aligned parallel to the direction of rolling, a gloss determined according to DIN 67530 of greater than 1,200 gloss units and a directed reflection of greater than 60%.

2. The flat steel product according to claim 1, wherein on at least one surface it has an arithmetic mean roughness Ra of less than 0.02 μm with a preferred direction aligned parallel to the direction of rolling.

3. The flat steel product according to claim 1, wherein on at least one surface it has a surface hardness of less than 4 GPa.

4. The flat steel product according to claim 1, produced using a method comprising: a) providing a starting flat steel product, at least one surface of which has an arithmetic mean roughness Ra of less than 2.5 μm; b) cold rolling the starting flat steel product in a plurality of rolling passes, wherein an overall forming rate achieved through the cold rolling is 75-90%, the forming rate drops from rolling pass to rolling pass, a rolling pressure in a first rolling pass is 200-800 MPa and a rolling pressure in a final rolling pass is 1,000-4,000 MPa, the cold rolling takes place with an addition of rolling oil, a viscosity of which is 5-10 mm.sup.2/s at 40° C., a rolling speed during the cold rolling is greater than 200 m/min, and the cold rolling, at least in the final rolling pass, takes place with a work roll having a circumferential contact surface that has an arithmetic mean roughness Ra of less than 0.01 μm to obtain a cold-rolled flat steel product; c) annealing the obtained cold-rolled flat steel product under a protective gas atmosphere with a hydrogen content of more than 50 vol. % to obtain an annealed flat steel product; and d) dry re-rolling the annealed flat steel product with a degree of re-rolling of 0.5-2% to obtain the flat steel product.

5. The flat steel product according to claim 1, wherein it is made from a stainless steel with a Cr content of at least 10.5 wt. %.

6. A mirror element for solar concentrators comprising a flat steel product according to claim 1.

Description

(1) The invention is explained in more detail in the following using embodiments. These show as follows:

(2) FIG. 1 a diagram, in which for three flat steel product specimens the directed reflection is applied in the solar spectral range over the wavelength;

(3) FIG. 2 the surface topography of a flat steel product specimen rolled according to the invention;

(4) FIG. 3 the surface topography of a polished flat steel product specimen;

(5) FIG. 4 a diagram in which for a flat steel product specimen cold rolled according to the invention the directed reflection in the solar spectral range is applied over the wavelength in the pure cold-rolled state and after application of a coating;

(6) FIG. 5 a schematic diagram of the pass schedule for the cold rolling undergone in producing flat steel product according to the invention.

(7) For the production of a flat steel product for the manufacture of mirror elements for a solar concentrator in the form of a steel strip, a primary material in coil form was used, which was a hot-rolled, de-scaled hot-rolled strip. The 2.5 mm thick primary material comprised a standardised steel with material number 1.4301 and had an arithmetic mean roughness Ra determined according to DIN EN ISO 4287 of less than 2.4 μm.

(8) The primary material was rolled on a 20-roll stand in twelve stages to a final thickness of 0.4 mm. The overall forming rate eg achieved was accordingly eg=(2.4−2)/2.4=83%.

(9) In the first rolling pass a rolling took place with forming e1 of more than 20%. In the eleven subsequent passes the forming from pass to pass was reduced relatively by between 5% and 10%, so that in the final rolling pass of the cold rolling forming e12 of less than 10% was used for rolling (FIG. 5).

(10) On their circumferential surface in contact with the flat steel product to be cold rolled, the work rolls had an arithmetic mean roughness Ra of 80 nm. For the ninth, tenth, eleventh and twelfth rolling passes the work rolls of the 20-roll stand were exchanged for specially prepared work rolls, the arithmetic mean roughness Ra of which was less than 10 nm.

(11) During the twelve cold rolling passes the rolling pressure was an average of 1,600 MPa.

(12) Each of the cold rolling steps was performed with the addition of a rolling oil, the viscosity of which was 8 mm.sup.2/s at 40° C. In order to maintain a sufficient oil film between the work rolls and the flat steel product, rolling took place at a rolling speed of more than 200 m/min.

(13) The cold-rolled flat steel product obtained in this way underwent a bright annealing treatment in a bright annealing unit under a protective gas atmosphere with a hydrogen content of more than 50%, wherein the partial pressure ratio of water vapour to hydrogen p(H.sub.20)/p(H.sub.2) was less than 10.sup.−4.

(14) Finally, the annealed cold-rolled strip was re-rolled dry, i.e. without oil or rolling emulsion, in a two-high rolling stand in two passes with a rolling force of 150 t. The circumferential surface of the two-high rolls in contact with the flat steel product had an arithmetic mean roughness Ra of 20 nm.

(15) Table 1 gives the roughness values determined using atomic force microscopy for two flat steel product specimens according to the invention E1, E2, processed in the abovementioned way, the roughness values determined using white-light interferometry and the directed reflection in the state obtained after re-rolling. Table also provides for the purposes of comparison the corresponding values for a comparative sample V produced in a conventional manner and polished on its surface under investigation.

(16) TABLE-US-00001 TABLE 1 Roughness Ra Roughness Ra [μm] [μm] Directed White-light Atomic reflectio interferometry force Without coating E1 0.023 0.012 63.3 E2 0.015 0.012 64.4 V — 0.004 65.4

(17) The result of the measurement of the directed reflection in the solar spectral range is shown in detail in Figure (specimen E1: continuous line, specimen E2: broken line, comparative specimen V: continuous line). It can be seen that the specimens produced according to the invention after re-rolling and in the cold-rolled, unpolished state, already have a reflectivity that is essentially the same as the reflectivity of the comparative specimen produced with considerable effort initially by rolling and then polishing.

(18) Clear differences between the flat steel products rolled according to the invention and the highly-polished specimen used for comparison in terms of roughness were verifiable. By means of topographic images (see FIG. 2) the rolling directions, in which the respective flat steel product was rolled, are clearly identifiable. Thus from FIG. 2 a preferred direction of the roughness characterised by a linear formation is detectable. With the polished specimen V used for comparison such a preferred direction is not present, however (FIG. 3).

(19) In order to investigate how the refection capacity of the flat steel product specimens could be further increased by the application of a coating, electron beam vapour deposition was used to provide the flat steel product specimen E1 according to the invention with a 90-100 nm thick silver coating. As a result of this coating the directed reflection in the solar spectral range increased to approximately 93% and in doing so reached the order of magnitude of conventional glass mirrors. In FIG. 4 the reflectivity of the specimen E1 according to the invention before coating is shown by a broken line, while the reflectivity after coating is illustrated by a continuous line.

(20) Finally, in a Nano-Indenter measurement, the hardness of the reflective surface of the specimens E1, E2 and V was investigated. Here the hardness of the polished comparative probe was clearly higher. The results of the measurements are summarised in Table 2.

(21) TABLE-US-00002 TABLE 2 Reduced modulus Hardness of elasticity Specimen [GPa] [GPa] E1 3.8 +/− 0.6 179 +/− 26 E2 3.6 +/− 0.3 171 +/− 15 V 5.0 +/− 0.2 202 +/− 8