ULTRA LOW CARBON INTERSTITIAL FREE STEEL

Abstract

An ultra low carbon interstitial free steel having an improved composition, wherein Ti+Nb+V is max 0.10 weight %. This steel has a very good ductility.

Claims

1. An interstitial free low carbon steel strip, sheet or blank, wherein the steel has in weight % a composition of the following elements: C: 0.0010-0.0022 Mn: 0.04-0.13 Si: 0.002-0.013 Al: 0.03-0.07 Ti: 0.05-0.09 Nb: max 0.002 V: max 0.004 P: 0.002-0.013 S: 0.003-0.013 N: 0.001-0.004 wherein Ti+Nb+V together max 0.10 and optionally: Cr: max 0.04 Ni: max 0.04 B: max 0.0002 Ca: max 0.001 Cu: max 0.04 Mo: max 0.02 Sn: max 0.01 the remainder being Fe and unavoidable impurities, the steel having a yield strength in transverse direction between 110 and 170 MPa, a tensile strength in transverse direction between 250 and 330 MPa, an elongation A80 in transverse direction of at least 42%, an average plastic strain ratio r of at least 2.00, and a strain hardening exponent n90 in transverse direction of at least 0.22; and a plastic strain ratio in diagonal direction r45 of at least 1.8, wherein the values of A80, r and n90 are the mean values of at least three measurements performed on at least three different strips of one cast, wherein the steel is coated with a zinc alloy coating and wherein the steel is standardised in EN 10346:2015.

2. The interstitial free low carbon steel strip, sheet or blank according to claim 1, the steel having an elongation A80 in transverse direction of at least 44%.

3. The interstitial free low carbon steel strip, sheet or blank according to claim 1, the steel having a strain hardening exponent n90 in transverse direction of at least 0.23.

4. The interstitial free low carbon steel strip, sheet or blank according to claim 1, the steel having an average plastic strain ratio r of at least 2.15.

5. The interstitial free low carbon steel strip, sheet or blank according to claim 1, the steel having a plastic strain ratio in diagonal direction r45 of at least 1.9.

6. The interstitial free low carbon steel strip, sheet or blank according to claim 1, wherein the average plastic strain ratio r times the strain hardening exponent n90 in transverse direction is at least 0.44.

7. The interstitial free low carbon steel strip, sheet or blank according to claim 1, wherein the plastic strain ratio in diagonal direction r45 times the strain hardening exponent n90 in transverse direction is at least 0.40.

8. The interstitial free low carbon steel strip, sheet or blank according to claim 1, wherein the plastic strain ratio in transverse direction r90 times the strain hardening exponent in transverse direction n90 is at least 0.50.

9. The interstitial free low carbon steel strip, sheet or blank according to claim 1, wherein the plastic strain ratio in diagonal direction r45 times the strain hardening exponent in diagonal direction n45 is at least 0.35.

10. The interstitial free low carbon steel strip, sheet or blank according to claim 1, wherein the yield strength Rp0,2 in transverse direction is between 110 and 155 MPa.

11-15. (canceled)

16. The interstitial free low carbon steel strip, sheet or blank according to claim 1, the steel having an elongation A80 in transverse direction of at least 52.

17. The interstitial free low carbon steel strip, sheet or blank according to claim 1, the steel having a strain hardening exponent n90 in transverse direction of at least 0.24.

18. The interstitial free low carbon steel strip, sheet or blank according to claim 1, the steel having an average plastic strain ratio r of at least 2.40.

19. The interstitial free low carbon steel strip, sheet or blank according to claim 1, the steel having a plastic strain ratio in diagonal direction r45 of at least 2.3.

20. The interstitial free low carbon steel strip, sheet or blank according to claim 1, wherein the average plastic strain ratio r times the strain hardening exponent n90 in transverse direction is at least 0.56.

21. The interstitial free low carbon steel strip, sheet or blank according to claim 1, wherein the plastic strain ratio in diagonal direction r45 times the strain hardening exponent n90 in transverse direction is at least 0.52.

22. The interstitial free low carbon steel strip, sheet or blank according to claim 1, wherein the plastic strain ratio in transverse direction r90 times the strain hardening exponent in transverse direction n90 is at least 0.66.

23. The interstitial free low carbon steel strip, sheet or blank according to claim 1, wherein the plastic strain ratio in diagonal direction r45 times the strain hardening exponent in diagonal direction n45 is at least 0.50.

Description

[0058] The invention will be elucidated with reference to the following examples.

[0059] Table 1 shows the composition of 16 examples of coils as cast, hot rolled, cold rolled, hot dip galvanised, and skin passed. Table 1a provides the elements that are important or deliberately added, or present in higher amounts. Table 1b provides elements that are present in low amounts or as inevitable impurities. All elements in tables 1a and 1b are given in milli weight %. Alzo stands for acid soluble aluminium.

[0060] All examples are inventive examples, as follows from Table 2, in which the yield strength Rp0,2, the tensile strength Rm, the elongation A80, the plastic strain ratio r (the r-value) and the strain hardening exponent n (the n-value) are given in three directions: the longitudinal direction (in rolling direction of the coil), in diagonal direction (under 45 degrees with the rolling direction) and in transverse direction (under 90 degrees with the rolling direction).

[0061] At the end of Table 2 also the average r-value (r_AVG) is given, which is calculated as (r0+2*r45+r90)/4, wherein TO is the plastic strain ratio in rolling or longitudinal direction, r45 is the plastic strain ratio in diagonal direction, and r90 is the plastic strain ratio in transverse direction.

TABLE-US-00001 TABLE 1a Ex CAST ID C N Mn Si Al AlZo Ti Cu Cr Ni 1 M0737 1.3 2.7 107 4 56 54 67 13 21 18 2 M1886 1.4 2.6 87 4 57 55 66 13 15 20 3 M2790 1.9 2.3 66 3 52 50 64 13 14 21 4 M8642 1.3 2.1 64 4 57 54 69 11 14 21 5 N0499 1.4 2.9 95 5 56 54 68 16 31 24 6 N6718 1.5 2.7 83 2 48 46 65 16 17 18 7 N6719 1.3 2.5 121 3 58 54 68 17 21 18 8 N6982 1.4 2.4 103 4 50 48 66 16 20 19 9 N6987 1.4 2.0 98 3 52 50 67 17 22 21 10 N6995 1.1 3.2 90 4 49 47 66 11 17 19 11 N6997 1.1 3.4 102 3 52 50 66 13 17 18 12 N8428 1.4 2.8 65 5 48 46 67 14 16 18 13 N9832 1.0 3.0 64 4 42 41 59 17 22 21 14 P1887 1.3 1.9 94 4 56 54 65 15 16 19 15 P2198 1.3 2.9 95 4 57 55 70 18 22 26 16 P2200 1.3 2.3 99 4 54 51 69 14 23 23

TABLE-US-00002 TABLE 1b Ex CAST ID P S Nb B V Mo Sn Ca 1 M0737 5 6 0 0.1 2 3 3 — 2 M1886 3 5 0 0.0 2 2 2 — 3 M2790 4 8 0 0.0 2 3 3 — 4 M8642 4 9 0 0.0 2 4 3 — 5 N0499 6 6 0 0.1 2 5 2 — 6 N6718 6 6 0 0.0 2 4 2 0 7 N6719 7 6 0 0.0 2 2 2 0 8 N6982 7 8 0 0.0 2 2 1 1 9 N6987 6 4 0 0.1 2 3 2 0 10 N6995 7 6 0 0.0 2 3 1 0 11 N6997 6 6 0 0.0 2 3 2 0 12 N8428 6 7 0 0.0 2 3 6 0 13 N9832 7 6 0 0.0 2 4 2 0 14 P1887 7 6 0 0.0 2 3 2 0 15 P2198 6 6 0 0.0 2 7 2 0 16 P2200 7 8 0 0.0 2 5 1 0

[0062] All coils of which the examples are shown in Table 1 and table 2 have been cast with the composition as given in Table 1 and have been hot rolled with a finishing temperature between 920° C. and 930° C. The cooling rate at the run-out table was approximately 60° C./s, and the coiling temperature was approximately 710° C. All these values are valid for the middle of the coil, the values at the head and tail end can be slightly different.

[0063] After cooling down and pickling the coils were cold rolled with a reduction of 82% and continuous annealed at a temperature of approximately 810° C. After standard hot dip galvanising to provide a GI coating the strips were skin passed with a reduction of 0.6%.

TABLE-US-00003 TABLE 2 longitudinal diagonal transverse r- n- r- n- r- n- Rp Rm A80 value value Rp Rm A80 value value Rp Rm A80 value value r_AVG Ex cast id MPa MPa % — — MPa MPa % — — MPa MPa % — — — 6 N6718 141 290 48 2.20 0.24 146 296 48 1.95 0.23 144 288 44.5 2.75 0.23 2.21 10 N6995 143 294 46.5 2.15 0.24 148 296 47.5 2.00 0.23 148 291 44 2.60 0.23 2.19 6 N6718 146 296 45.5 2.25 0.23 150 298 45.5 2.00 0.22 149 295 47 2.50 0.23 2.19 6 N6718 143 290 51 2.15 0.24 148 294 51.5 2.00 0.23 147 288 46.5 2.85 0.23 2.25 8 N6982 139 292 47 2.15 0.24 145 296 44.5 2.10 0.23 142 290 47.5 2.95 0.23 2.33 9 N6987 140 292 46 2.30 0.24 146 299 44.5 2.00 0.23 143 290 49 2.65 0.23 2.24 9 N6987 141 294 46 2.30 0.24 145 299 44.5 2.15 0.23 144 292 47.5 2.75 0.23 2.34 8 N6982 140 289 47 2.30 0.24 146 295 45.5 1.95 0.23 143 286 48.5 2.85 0.23 2.26 11 N6997 149 288 47.5 2.20 0.23 155 295 45.5 1.85 0.22 152 285 46.5 2.80 0.23 2.18 11 N6997 140 287 46 2.25 0.23 144 291 45.5 2.15 0.23 144 287 45.5 2.95 0.23 2.38 2 M1886 127 288 48 2.20 0.24 132 292 46.5 2.20 0.23 130 287 47.5 2.90 0.24 2.38 2 M1886 132 292 45 2.35 0.24 138 299 42 1.95 0.23 136 291 45.5 2.90 0.24 2.29 2 M1886 125 288 46 2.20 0.24 130 292 45 2.05 0.23 129 287 46.5 2.80 0.24 2.28 4 M8642 136 286 44 2.35 0.24 140 290 45.5 2.00 0.22 136 282 46 2.65 0.23 2.25 4 M8642 135 282 49 2.30 0.23 137 286 50.5 2.10 0.22 135 280 54 2.70 0.23 2.30 4 M8642 133 283 49 2.30 0.24 138 287 48.5 2.15 0.22 135 282 50.5 2.70 0.24 2.33 4 M8642 136 288 41 2.15 0.24 142 294 41.5 1.85 0.23 139 287 48 2.70 0.23 2.14 1 M0737 133 285 48.5 2.40 0.24 140 293 42.5 1.85 0.23 136 283 46.5 2.65 0.24 2.19 7 N6719 144 292 48.5 2.25 0.24 150 294 46 2.20 0.22 149 290 46.5 3.00 0.23 2.41 12 N8428 145 294 46 2.05 0.23 140 291 48 2.05 0.24 144 288 47.5 2.60 0.23 2.19 5 N0499 138 289 49.5 2.30 0.23 143 294 50 2.20 0.22 141 289 48.5 3.05 0.23 2.44 15 P2198 139 293 47.5 2.00 0.24 143 297 45.5 1.90 0.23 142 292 48.5 2.60 0.23 2.10 16 P2200 145 289 47 2.15 0.24 151 292 48.5 2.10 0.23 149 288 50.5 2.80 0.23 2.29 16 P2200 142 288 47 2.15 0.24 149 291 47 2.25 0.23 145 285 52 2.60 0.24 2.31 14 P1887 145 293 49 2.00 0.24 150 296 50 1.95 0.23 148 290 49.5 2.60 0.23 2.13 14 P1887 143 290 48 2.05 0.24 148 293 48.5 2.00 0.23 148 289 52 2.60 0.23 2.16 15 P2198 145 291 49.5 2 0.24 149 295 48.5 1.95 0.23 148 289 49.5 2.6 0.23 2.13 15 P2198 145 291 47 2.05 0.24 151 295 45 2.1 0.22 149 290 50.5 2.5 0.23 2.19 16 P2200 141 286 50.5 2.05 0.24 147 289 48.5 1.95 0.23 144 286 48.5 2.75 0.23 2.18 14 P1887 146 294 47 2 0.24 150 297 45 2 0.23 149 292 49 2.55 0.23 2.14 15 P2198 145 289 47 2.1 0.24 151 291 48 2.15 0.23 151 288 51.5 2.8 0.23 2.30 13 N9832 140 282 49.5 2.15 0.24 148 291 49.5 1.8 0.23 142 281 48.5 2.55 0.24 2.08
The relatively low Rp0,2 and the high levels of A80, r-value and n-value or combinations thereof can be attributed to a relatively high amount of Ti and Mn, in combination with a relatively low amount of C and Si. Of course, the process steps as indicated are preferred to reach such values.

[0064] It will be clear for the person skilled in the art that the process steps can be varied to reach the values as provided in the claims.

[0065] The protection sought is not limited by the examples; in that respect, only the limitations of the claims count.