High strength steel with improved mechanical properties

Abstract

A high strength steel strip having medium amounts of C, Mn, Si, Cr and Al, wherein the steel strip has a microstructure consisting of, in vol. %: ferrite and bainite together 50-90%, martensite<15%, retained austenite 5-15%, the remainder being pearlite, cementite, precipitates and inclusions together up to 5%.

Claims

1. A high strength steel strip having a composition consisting of the following elements, in wt. %: C 0.12-0.18 Mn 2.00-2.60 Si 0.30-0.77 Cr 0.10-0.70 Al<0.39 S<0.005 N<0.015 P<0.03 and optionally one or more of the elements selected from: Nb<0.06 Mo<0.20 Ti<0.04 V<0.20 B<0.004 Ca<0.004 the remainder being iron and unavoidable impurities, wherein the steel strip has a microstructure consisting of, in vol. %: ferrite and bainite together 50-90, wherein ferrite is 15-55 and bainite is 35-75, martensite<15, retained austenite 5-15, the remainder being pearlite, cementite, precipitates and inclusions together up to 5, the sum adding up to 100 vol. %, wherein the steel strip has a hole expansion capacity (HEC)>15%.

2. The high strength steel strip according to claim 1, wherein the microstructure, in vol. %, is characterized by one or more of the following: martensite<10, retained austenite 5-10.

3. The high strength steel strip according to claim 1, wherein the amount of C is 0.13-0.18 (in wt. %), and/or wherein the amount of Mn is 2.00-2.50 (in wt. %).

4. The high strength steel strip according to claim 1, wherein the amount of Si+Cr≤1.30 (in wt. %).

5. The high strength steel strip according to claim 1, wherein the amount of Si is 0.30-0.70 (in wt. %), and/or wherein the amount of Cr is 0.15-0.65 (in wt. %), and/or wherein the amount of Al is <0.10 (in wt. %).

6. The high strength steel strip according to claim 1, wherein the amount of Nb is <0.04 (in wt. %), and/or wherein the amount of Ti is <0.03 (in wt. %).

7. The high strength steel strip according to claim 1, wherein the amount of B is <0.002 (in wt. %), and/or wherein the amount of V is <0.10 (in wt. %), and/or wherein the amount of Mo is <0.10 (in wt. %).

8. The high strength steel strip according to claim 1, wherein one or more of Nb, Mo, Ti, V and B is present as an impurity.

9. The high strength steel strip according to claim 1, wherein the steel strip has one or more of the following properties: tensile strength (R.sub.m) in the range of 950-1200 MPa, yield strength (R.sub.p)≤620 MPa before temper rolling, total elongation (A.sub.JIS5)>12%.

10. The high strength steel strip according to claim 1, wherein the steel strip has a total elongation (A.sub.JIS5)>13%.

11. The high strength steel strip according to claim 1, wherein the hole expansion capacity (HEC) is >20%.

12. The high strength steel strip according to claim 9, wherein the following condition applies: (A.sub.JIS5×HEC×R.sub.m)/R.sub.p≥550.

13. The high strength steel strip according to claim 1, wherein the steel strip is coated with a zinc based coating, the coated strip having a yield strength R.sub.p≤740 MPa after temper rolling.

14. A method for producing high strength steel strip according to claim 1, comprising the following steps: the steel with the composition of claim 1 is cast and hot rolled to a strip having a thickness of 2.0-4.0 mm and coiled at a coiling temperature (CT) in the range 500-650° C.; the strip is cold rolled with a reduction of 40-80%; the strip is heated to a temperature T1 in the range of A.sub.c3−30° C. to A.sub.c3+30° C. to form a fully or partially austenitic microstructure; subsequently the strip is kept at T1 for a time period t1 of 10-90 s, followed by slow cooling of the strip with a cooling rate CR1 in the range of 2-12° C./s to a temperature T2 in the range of 570-730° C.; then the strip is rapidly cooled with a cooling rate CR2 in the range of 20-70° C./s to a temperature T3 in the range 380-470° C., followed by keeping the strip at temperature T4 that is between T3±50° C. for a time period t2 of 25-100 s, wherein T4 at the end of the time period t2 is between T3±30° C., followed by cooling the steel strip at a cooling rate CR3 of at least 4° C./s to a temperature below 300° C.; the coated strip is temper rolled with a reduction of less than 0.5%.

15. A method for producing high strength steel strip according to claim 1, comprising the following steps: the steel with the composition according to claim 1 is cast and hot rolled to a strip having a thickness of 2.0-4.0 mm and coiled at a coiling temperature (CT) in the range 500-650° C.; the strip is cold rolled with a reduction of 40-80%; the strip is heated to a temperature T1 in the range of A.sub.c3−30° C. to A.sub.c3+30° C. to form a fully or partially austenitic microstructure; subsequently the strip is kept at T1 for a time period t1 of 10-90 s, followed by slow cooling of the strip with a cooling rate CR1 in the range of 2-12° C./s to a temperature T2 in the range of 570-730° C.; then the strip is rapidly cooled with a cooling rate CR2 in the range of 20-70° C./s to a temperature T3 in the range 380-470° C., followed by keeping the strip at temperature T4 that is between T3±50° C. for a time period t2 of 25-100 s, wherein T4 at the end of the time period t2 is between T3±30° C.; followed by hot dip coating the steel strip in a galvanizing bath to provide the strip with a zinc coating or a zinc based coating, followed by cooling the coated steel strip at a cooling rate of at least 4° C./s to a temperature below 300° C.; the coated strip is temper rolled with a reduction of less than 0.5%.

16. A method for producing high strength steel strip according to claim 1, comprising the following steps: the steel with the composition according to claim 1 is cast and hot rolled to a strip having a thickness of 2.0-4.0 mm and coiled at a coiling temperature (CT) in the range 500-650° C.; the strip is cold rolled with a reduction of 40-80%; the strip is heated to a temperature T1 in the range of A.sub.c3−50° C. to A.sub.c3+40° C. to form a fully or partially austenitic microstructure; subsequently the strip is kept at T1 for a time period t1 of at most 90 s, followed by slow cooling of the strip with a cooling rate CR1 in the range of 0.5-12° C./s to a temperature T2 in the range of 570-730° C.; then the strip is rapidly cooled with a cooling rate CR2 in the range of 5-70° C./s to a temperature T3 in the range 330-470° C., followed by keeping the strip at temperature T4 that is between T3±50° C. for a time period t2 of 25-100 s, wherein T4 at the end of the time period t2 is between T3±30° C.; optionally followed by hot dip coating the steel strip in a galvanizing bath to provide the strip with a zinc coating or a zinc based coating; followed by cooling the coated steel strip at a cooling rate of at least 4° C./s to a temperature below 300° C.; the coated strip is temper rolled with a reduction of less than 0.5%.

17. The high strength steel strip according to claim 1, wherein the martensite is 1-5 vol. % of the microstructure.

18. The high strength steel strip according to claim 1, wherein the amount of C is 0.12-0.163 (in wt. %).

19. The high strength steel strip according to claim 1, wherein the amount of Si+Cr≤1.20 (in wt. %).

20. The high strength steel strip according to claim 1, wherein the amount of Si is 0.35-0.65 (in wt. %), and/or wherein the amount of Cr is 0.20-0.60 (in wt. %), and/or wherein the amount of Al is <0.05 (in wt. %).

21. The high strength steel strip according to claim 1, wherein the amount of Nb is <0.03 (in wt. %), and/or wherein the amount of Ti is <0.020 (in wt. %).

22. The high strength steel strip according to claim 1, wherein the amount of B is <0.001 (in wt. %), and/or wherein the amount of V is <0.05 (in wt. %), and/or wherein the amount of Mo is <0.05 (in wt. %).

23. The high strength steel strip according to claim 1, wherein the steel strip has one or more of the following properties: tensile strength (R.sub.m) in the range of 980-1180 MPa, yield strength (R.sub.p)≤600 MPa before temper rolling, total elongation (A.sub.JIS5)>12%.

24. The high strength steel strip according to claim 1, wherein the steel strip has a total elongation (AJIS5)>14%.

25. The high strength steel strip according to claim 1, wherein the hole expansion capacity (HEC) is >25%.

26. The high strength steel strip according to claim 9, wherein the following condition applies: (AJIS5×HEC×R.sub.m)/R.sub.p≥600.

27. The high strength steel strip according to claim 1, wherein the steel strip is coated with a zinc based coating, the coated strip having a yield strength R.sub.p≤720 MPa after temper rolling.

Description

(1) The invention will be elucidated with reference to the examples below. Nine alloys have been cast using the compositions as given in Table 1 below, the amounts of the elements given in wt. %. Elements not shown in the table are present as impurity.

(2) TABLE-US-00001 TABLE 1 Steel composition (in wt. %) I/C Alloy C Mn Si Cr Al S N Si + Cr I A 0.135 2.210 0.480 0.530 0.032 0.001 0.004 1.010 I B 0.147 2.150 0.554 0.574 0.031 0.001 0.001 1.128 C C 0.145 2.256 0.104 0.549 0.032 0.001 0.006 0.653 I D 0.163 2.381 0.556 0.100 0.034 <0.001 0.003 0.656 I E 0.163 2.378 0.566 0.251 0.032 <0.001 0.003 0.817 I F 0.163 2.177 0.558 0.247 0.043 <0.001 0.004 0.805 I G 0.163 2.206 0.411 0.401 0.208 <0.001 0.003 0.812 I H 0.161 2.209 0.410 0.407 0.209 0.001 0.003 0.817 I I 0.145 2.210 0.516 0.555 0.031 <0.001 0.004 1.071

(3) TABLE-US-00002 I/C Alloy Nb Mo Ti V P B Ca I A <0.001 <0.005 0.001 <0.001 0.009 <0.0005 0.0005 I B <0.001 0.002 0.001 0.003 0.001 <0.0005 0.0005 C C 0.016 <0.005 0.017 0.004 0.010 0.0001 0.0026 I D 0.001 0.003 0.001 0.002 0.012 0.0002 <0.0005 I E 0.001 0.003 0.001 0.002 0.012 0.0003 <0.0005 I F <0.001 0.104 0.001 0.002 0.012 0.0001 <0.0005 I G <0.001 0.004 0.001 0.002 0.012 0.0002 <0.0005 I H 0.019 0.003 0.001 0.002 0.012 0.0002 <0.0005 I I <0.001 0.004 0.003 0.004 0.009 0.0004 0.0007

(4) In Table 1, in the first column I indicates an alloy in accordance with the invention, whereas C indicates an alloy of a comparative example.

(5) The phase transition temperatures of the alloys are given in Table 2. The temperatures above which the microstructure is entirely composed of austenite (A.sub.c3), bainite start (B.sub.S) and martensite start (M.sub.S) temperatures (in ° C.) are provided in Table 2:

(6) TABLE-US-00003 TABLE 2 phase transition temperatures I/C Alloy A.sub.c3 B.sub.S M.sub.s I A 820 554 380 I B 820 549 375 C C 810 575 385 I D 820 547 370 I E 825 544 375 I F 830 551 375 I G 835 556 380 I H 845 557 375 I 1 820 550 375
A.sub.c3 and M.sub.S temperatures were measured using dilatometry: the sample was heated with an average heating rate 11° C./s till 900° C. Subsequently the sample is kept at 900° C. for 30 s. Then the sample is quenched. B.sub.S temperature was calculated using the JmatPro v10.2 tool.

(7) The method for producing the high strength steel strip according to the examples is as follows.

(8) The cast steel is hot rolled to a thickness of 4.0 mm and coiled at a coiling temperature (CT). The strip is cold rolled with a reduction of 75%. For determination of mechanical properties strip samples were annealed using a laboratory continuous annealing simulator. First the strip is heated with an average heating rate HR to a temperature T.sub.1 such that a partly or a fully austenitic microstructure was obtained. Subsequently the strip is kept at T.sub.1 for a time period t.sub.1. Then the strip is cooled to temperature T.sub.2 at a cooling rate CR.sub.1, followed by additional cooling to temperature T.sub.3 at a cooling rate CR.sub.2. Next the strip is held at a temperature T.sub.4, in this case equal to T.sub.3, during an overaging time t.sub.2. During this period the temperature T.sub.4 can vary both due to latent heat of transformation that occurs and due to natural cooling. Then the strip is brought to 455° C. which represents the temperature of the Zn bath, which usually is in the range of 450-470° C., and held at this temperature for approximately 17 s to simulate hot dip galvanizing step. Then the strip is cooled down to below 300° C. at a rate of at least 4° C./s. After that the strip is cooled to room temperature in air.

(9) Only Alloy I in example 16 is produced in a factory, where the methods settings differ from those in the laboratory examples, and the produced strip was directly coated with a conventional galvanising

(10) The values for cooling rate CT, average heating rate HR, and respective temperatures times and cooling rates used for the production of nine samples are given in Table 3.

(11) TABLE-US-00004 TABLE 3 production parameters CT avg HR T.sub.1 t.sub.1 CR.sub.1 T.sub.2 CR.sub.2 T.sub.3 T.sub.4 t.sub.2 I/C No Alloy [° C.] [° C./s] [° C.] [s] [° C./s] [° C.] [° C./s] [° C.] [° C.] [s] I  1 A 530 10 840 65 5 650 32 440 440 53 I  2 A 530 10 840 65 5 650 35 420 420 53 I  3 A 630 10 820 65 3 680 37 440 440 53 I  4 B 550 10 825 65 6 600 28 420 420 53 I  5 B 550 10 825 65 5 620 31 420 420 53 C  6 C 600 10 810 65 5 600 20 470 470 53 C  7 C 600 10 810 65 4 650 28 470 470 53 I  8 D 620 13 790 65 2 700 43 420 420 53 I  9 E 620 13 790 65 2 700 43 420 420 53 I 10 F 620 13 830 65 5 620 31 420 420 53 I 11 F 620 13 790 65 2 700 43 420 420 53 I 12 G 620 13 840 65 5 620 31 420 420 53 I 13 G 620 13 800 65 3 700 43 420 420 53 I 14 H 620 13 840 65 5 620 31 420 420 53 I 15 H 620 13 800 65 3 700 43 420 420 53 I 16 I 610 3 850 0 4 670 16 420 420 47
The microstructure of the produced samples is determined as follows.

(12) The volume fraction of ferrite, bainite and martensite have been evaluated from dilatometry data with the Lever rule (the linear law of mixtures) applied to the data using the non-linear equations for the thermal contraction of bcc and fcc lattices derived in the article by S. M. C. Van Bohemen in Scr. Mater. 69 (2013), p. 315-318 (Ref. [1]). For cooling after full austenitisation, T.sub.1>A.sub.c3, the measured thermal contraction in the high temperature range where no transformation occur can be simply described by the expression proposed in Ref. [1] for the fcc lattice. For cooling after partial austenitisation, T.sub.1<A.sub.c3, the measured thermal contraction in the high temperature range is determined by the coefficients of thermal expansion (CTE) of the individual phase constituents according to a rule of mixtures. Then the start of transformation during cooling is identified by the first deviation of the dilatometry data from this line defined by the thermal expansion in the high temperature range. Retained austenite (RA) was determined by X-ray diffraction measurements, and the fraction RA has been used as input in the Lever rule analysis of dilatation data.

(13) The volume fractions of ferrite, bainite, martensite and retained austenite (in vol. %) as determined in this way are given in Table 4 for the sixteen samples.

(14) TABLE-US-00005 TABLE 4 microstructure I/C No. Alloy Ferrite Bainite Martensite Retained austenite I  1 A 30 57  4  9 I  2 A 22 65  4  9 I  3 A 25 64  3  8 I  4 B 40 50  4  6 I  5 B 38 51  4  7 C  6 C 50 32 16  2 C  7 C 29 60 10  1 I  8 D 66 23  1 10 I  9 E 62 24  4 10 I 10 F 22 66  2 10 I 11 F 63 24  4  9 I 12 G 52 37  4  7 I 13 G 70 18  3  9 I 14 H 63 25  4  8 I 15 HJ 75 13  3  9 I 16 I 50 40  5  5
The properties of the samples are determines as follows.

(15) The tensile properties yield strength (R.sub.p), tensile strength (R.sub.m) and total elongation (A.sub.JIS5) were determined using a servohydraulic testing machine in a manner in accordance with ISO 6892. Only for Alloy I, example 16 not the total elongation A.sub.JIS5 was used, but the standard elongation A80.

(16) Hole expansion testing to determine the HEC value was carried out using the testing method describe in ISO 16630 on samples with punched holes, wherein the sample is positioned such that the burr on the punched edges is present on the upper side away from the conical punch.

(17) The properties of the sixteen samples as determined in this way are given in Table 5.

(18) TABLE-US-00006 TABLE 5 properties R.sub.p R.sub.m HEC (A.sub.JIS5 × HEC × I/C Nr. Alloy [MPa] [MPa] A.sub.JIS5 [%] [%] R.sub.m)/R.sub.p I  1 A 570 1016 14.2 27 683 I  2 A 598 981 13.1 31 664 I  3 A 576 995 13.5 30 700 I  4 B 552 979 15.0 27 717 I  5 B 540 985 16.3 25 743 C  6 C 552 993 14.6 15 394 C  7 C 587 974 12.6 21 439 I  8 D 479 968 19.3 23 898 I  9 E 515 1032 17.2 20 688 I 10 F 667 1009 12.3 36 667 I 11 F 489 1026 16.2 19 644 I 12 G 607 973 13.4 29 622 I 13 G 474 959 17.9 21 758 I 14 H 502 996 17.8 23 812 I 15 H 471 1011 15.5 19 631 I 16 I 652 963 17.3 24 588 (A80) (based on A80)

(19) The above examples show that with an alloy in accordance with the invention and with processing steps in accordance with the invention, samples are produced that have the required mechanical properties tensile strength and yield strength, and the required enhanced combination of high total elongation and HEC value.

(20) The inventive alloys and samples also show that with a reasonably low combined amount of silicon and chromium, which can be lower than 1.3 wt. %, high strength steel strip with suitably high properties can be obtained. The combined amount of silicon and chromium can be even lower than 0.85 wt. %, as shown by alloys D to H in Table 1, which alloy can result in a high strength steel type having a total elongation of 12 to 19% and a HEC value of 19 to 36%.

(21) Analysis of the data reveals that the distinction of the inventive examples can be captured by a single requirement namely: (A.sub.JIS5×HEC×R.sub.m)/R.sub.p≥550. This expression holds for the present invention with the compositional ranges and process restrictions defined above. See example 7: it has all properties inside of claims (R.sub.p, R.sub.m, A.sub.JIS5, HEC); but the formula shows the difference.

(22) The overall formability of the steel can be assessed by this formula. This formula emphasises the importance of optimized properties needed to achieve desired stamping performance: high elongation and high HEC for high tensile strength steel with low yield strength. So it is not only optimization of high elongation or high hole expansion capacity, but both together.