Hot dip coated steel strip having an improved surface appearance and method for production thereof

Abstract

The present invention relates to a method of hot dip coating of flat steel products, comprising a step of stripping the coated steel strip by means of at least one nozzle that emits a stripping gas having a temperature T.sub.3 at an angle α in the direction of the coated steel strip, wherein the angle α, the distance h between the surface of the melt bath and the lower edge of the nozzle in mm, the temperature of the stripping gas in ° C. and the differential between the temperature T.sub.2 of the melt bath and the temperature T.sub.1 of the steel strip are in a particular relationship to one another, and to a correspondingly produced hot dip-coated flat steel product.

Claims

1. A steel strip that has been provided with a corrosion-protective coating, produced by a method comprising: heating the steel strip to a temperature T.sub.1 of 400° C. to 500° C.; (A) conducting the steel strip having temperature T1 through a melt bath having a temperature T.sub.2, containing the elements of the corrosion-protective coating in molten form, in order to obtain a coated steel strip, and (B) stripping the coated steel strip by means of at least one nozzle that emits a stripping gas having a temperature T.sub.3 at an angle a in the direction of the coated steel strip, wherein a value Q is ≤6.956 with $\begin{array}{cc}Q=\frac{3.76+(\alpha *\left(10+6.3*h\right)}{10-0.86*T_3^2+4.82*\Delta T}& \left(I\right)\end{array}$ in which α is the angle between a theoretical straight line perpendicular to the coated steel strip and pointing through the middle of the at least one nozzle, and the nozzle direction set, in °, h is the distance between the surface of the melt bath and the lower edge of the nozzle in mm, T.sub.3 is the temperature of the stripping gas in ° C., and ΔT is the difference between the temperature T.sub.2 of the melt bath and the temperature T.sub.1 of the steel strip in ° C., wherein the method concludes without a heat treatment for producing an alloy layer on the steel strip, and wherein the steel strip is a bake-hardened steel or an IF steel.

2. The steel strip as claimed in claim 1, wherein the corrosion-protective coating, in addition to Zn and unavoidable impurities, contains 0.1% to 2.0% by weight of Al.

3. The steel strip as claimed in claim 2 wherein the melt bath further contains 0.1% to 3% by weight of Mg.

4. The steel strip as claimed in claim 2 wherein the corrosion-protective coating further contains 0.1% to 3% by weight of Mg.

5. The steel strip of claim 1 wherein the melt bath, as well as Zn and unavoidable impurities, contains 0.1% to 2.0% by weight of Al.

6. The steel strip of claim 1 wherein the steel strip, as well as Fe and unavoidable impurities, contains (all figures in % by weight) to 0.3 C, to 1.50 Si, to 4.00 Mn, to 0.10 P, to 0.02 S, to 2.20 Al, up to 0.2 Ti+Nb, up to 1.50 Cr+Mo, up to 0.25 V, up to 0.01 N, to 0.20 Ni, up to 0.01 B and up to 0.01 Ca.

7. The steel strip of claim 1 wherein the corrosion-protective coating has a thickness of 1 to 10 μm, and wherein the steel strip is a single phase steel.

8. The steel strip of claim 1 wherein the coating includes Zn, Mg, Al, and wherein Fe.sub.2Al.sub.5 particles are present in the coating and have an average grain diameter of ≤210 nm.

9. The steel strip of claim 1 wherein T.sub.3 is 15 to 50° C.

10. The steel strip of claim 1 wherein α is −5.0° to 5.0°.

11. The steel strip of claim 1 wherein the coating includes Zn, Mg, Al, and wherein Fe.sub.2Al.sub.5 particles are present in the coating and have an average grain diameter of ≤565 nm.

12. The steel strip of claim 1 wherein a distance between the steel strip and the lower edge of the at least one nozzle is 4 mm to 15 mm.

13. The steel strip of claim 1 wherein a pressure p of stripping gas leaving the nozzle is 50 mbar to 800 mbar.

14. The steel strip as claimed in claim 1, wherein the coating has a ratio of Wsa_mod in longitudinal direction to Wsa_mod in cross direction of 0.700 to 1.290.

15. A steel strip that has been provided with a corrosion-protective coating, produced by a method comprising: (A) conducting the steel strip having a temperature T.sub.1 through a melt bath having a temperature T.sub.2, containing the elements of the corrosion-protective coating in molten form, in order to obtain a coated steel strip, and (B) stripping the coated steel strip by means of at least one nozzle that emits a stripping gas having a temperature T.sub.3 at an angle a in the direction of the coated steel strip, wherein a value Q is 6.956 with $\begin{array}{cc}Q=\frac{3.76+\left(\alpha *\left(10+6.3*h\right)\right)}{10-0.86\star T_3^2+4.82*\Delta T}& \left(I\right)\end{array}$ in which α is the angle between a theoretical straight line perpendicular to the coated steel strip and pointing through the middle of the at least one nozzle, and the nozzle direction set, in °, h is the distance between the surface of the melt bath and the lower edge of the nozzle in mm, T.sub.3 is the temperature of the stripping gas in ° C., and ΔT is the difference between the temperature T.sub.2 of the melt bath and the temperature T.sub.1 of the steel strip in ° C. wherein a is −5.0° to 5.0°; wherein h is 50 mm to 800 mm; wherein T.sub.1 is heated to 400° C. to 550° C. prior to step (A), wherein T.sub.2 is 400° C. to 650° C., wherein T.sub.3 is 15° C. to 50° C.; wherein ΔT is −60° C. to 120° C.; wherein a distance between the steel strip and the lower edge of the nozzle is 4 mm to 15 mm; wherein a pressure p of stripping gas leaving the nozzle is 50 mbar to 800 mbar; wherein a thickness of the corrosion-protective coating after step (B) is 0.1 μm to 20 μm, wherein the method concludes without a heat treatment for producing an alloy layer on the steel strip, and wherein the steel strip is a bake-hardened steel or an IF steel.

16. The steel strip of claim 15, wherein: wherein Fe.sub.2Al.sub.5 particles are present in the coating and have an average grain diameter of ≤565 nm; wherein α is −4.0° to 4.0°; wherein h is 80 mm to 700 mm; wherein T.sub.3 is 20° C. to 40° C.; wherein ΔT is −55° C. to 115° C.; wherein the distance between the steel strip and the lower edge of the nozzle is 6 mm to 12 mm; wherein p is 100 mbar to 600 mbar; and wherein a thickness of the corrosion-protective coating after step (B) is 1.0 μm to 10 μm.

Description

EXAMPLES

(1) The working examples that follow serve to further illustrate the invention.

(2) A steel strip having the following composition (all figures in % by weight) 0.075 C, 0.1 Si, 1.53 Mn, 0.010 P, 0.001 S, 0.035 Al, 0.53 Cr, 0.05 Cu, 0.05 Mo, 0.0025 N, 0.025 T.sub.1, 0.05 Ni, 0.0030 B, balance: Fe and unavoidable impurities, is coated under the conditions specified in table 1 with melt baths containing the alloy constituents specified in table 1. After application of the protective layer, the wsa_mod value is determined on the surface thereof by the method described above. This is done by taking sheet samples from the steel strip and measuring in longitudinal and cross direction. Then the quotient of the wsa_mod value for longitudinal direction and the value for cross direction is determined.

(3) It is clearly apparent from table 1 that the inventive examples having a Q value of less than 6.956 result in corresponding coated steel strips that feature an advantageous quotient of the wsa_mod values, whereas, in comparative experiments in which the Q value is above 6.956, distinctly poorer quotients of the wsa_mod values and hence surface qualities are obtained.

(4) TABLE-US-00001 TABLE 1 Examples and comparative examples Average Zn Mg Al grain Ser. content content content □T T3 h □ diameter Wsa_mod_long/ No. [% by wt.] [% by wt.] [% by wt.] [° C.] [° C.] [mm] [°] [nm] Wsa_mod_cross Q 1 99.771 — 0.229 35 35 330 −2.4 540 1.077 5.73 2 99.786 — 0.214 15 34 250 −2.1 510 1.154 3.65 3 99.785 — 0.215 18 35 240 −2.1 515 1.167 3.34 4 99.779 — 0.221 13 32 260 −2.4 530 1.231 4.89 5 99.768 — 0.232 15 30 260 −2.4 550 1.286 5.71 6 99.775 — 0.225 19 29 270 −2.1 535 1.267 5.77 7 99.781 — 0.219 18 31 290 −2.2 525 1.231 5.53 8 99.772 — 0.228 20 30 302 −2.2 540 1.267 6.30 V9 99.750 — 0.250 20 25 330 −2.6 605 1.467 12.59 V10  99.760 — 0.240 22 26 300 −2.4 585 1.429 9.79 V11  99.762 — 0.238 22 24 290 −2.6 585 1.438 12.58 V12  99.764 — 0.236 24 26 310 −2.4 570 1.333 10.33 V13  99.767 — 0.233 24 26 300 −2.5 570 1.308 10.42 V14  99.751 — 0.249 26 23 330 −2.7 595 1.571 17.64 V15  99.757 — 0.243 26 24 250 −2.5 590 1.389 11.00 16 99.900 — 0.100 11 29 260 −2.2 450 1.188 5.49 V17  99.500 — 0.500 −31 27 350 −2.8 655 1.316 8.09 18 99.900 — 0.100 9 39 265 −5.0 445 1.270 6.69 19 99.890 — 0.110 −4 32 285 −3.0 455 1.227 6.08 20 99.869 — 0.131 110 28 270 0.5 475 1.217 −6.41 21 99.825 — 0.175 −51 39 280 −2.2 500 1.143 2.53 22 99.800 — 0.200 19 34 304 5.0 505 1.263 −10.79 23 99.785 — 0.215 20 33 100 −2.1 510 1.250 1.61 24 99.777 — 0.223 57 35 270 −2.1 530 1.200 4.67 25 99.769 — 0.231 10 31 600 −1.2 550 1.222 5.91 26 97.772 1.058 1.170 7 33 240 −2 165 1.100 3.41 27 97.758 1.052 1.190 23 30 270 −2.1 170 1.200 5.50 28 97.724 1.066 1.210 17 37 422 −2.4 175 1.182 5.90 29 97.779 1.061 1.160 15 33 250 −2.1 165 1.091 3.89 30 97.528 1.059 1.413 22 31 315 −2.3 205 1.273 6.45 31 97.572 1.058 1.370 18 33 280 −2.3 185 1.182 4.85 V32  97.536 1.052 1.412 25 29 310 −2.3 215 1.310 7.61 33 97.563 1.057 1.380 20 32 290 −2.3 190 1.200 5.45 V34  97.538 1.051 1.411 23 25 340 −2.4 215 1.364 12.39 35 97.698 1.052 1.250 40 35 300 −2.5 180 1.077 5.58 36 97.520 1.063 1.417 25 31 300 −2.4 205 1.250 6.55 37 97.693 1.067 1.240 21 31 260 −2.2 175 1.182 5.06 38 97.684 1.056 1.260 19 34 260 −2.2 180 1.167 4.06 V39  97.459 1.066 1.475 23 25 400 −2.5 230 1.364 15.17 V40  97.526 1.054 1.420 28 27 330 −2.3 220 1.333 9.96 V41  97.600 1.000 1.400 10 28 330 −2.5 215 1.310 8.47 42 97.000 2.000 1.000 25 32 314 −2.3 160 1.273 6.09 V43  96.950 1.050 2.000 30 31 325 −2.5 260 1.310 7.65 44 97.787 1.063 1.150 −9 34 240 0.5 165 1.083 −0.74 45 97.609 1.061 1.330 −5 32 280 −3.0 185 1.182 5.94 46 97.548 1.052 1.400 −40 34 250 −2.4 205 1.067 3.23 47 97.591 1.052 1.357 −50 39 285 −2.4 185 0.786 2.81 48 97.550 1.050 1.400 52 34 275 −2.2 200 1.231 5.22 49 97.557 1.053 1.390 7 39 260 −5.0 190 1.143 6.51 50 97.890 1.060 1.050 6 39 600 −2.3 160 1.182 6.87 51 97.539 1.049 1.412 108 32 275 0.5 195 1.059 −2.50 52 97.592 1.058 1.350 18 25 100 −2.1 180 1.000 3.04 53 97.599 1.064 1.337 −5 31 290 −3.0 185 0.867 6.55 54 97.521 1.066 1.413 19 39 310 5.0 200 0.938 −8.14 V comparative experiment

INDUSTRIAL APPLICABILITY

(5) By the method of the invention, it is possible to obtain coated steel strips featuring a particularly high-quality surface structure. Therefore, these steel strips can be used advantageously in the automotive sector.