COLD-ROLLED AND RECRYSTALLISATION ANNEALED FLAT STEEL PRODUCT, AND MEHTOD FOR THE PRODUCTION THEREOF

Abstract

A cold-rolled and recrystallization-annealed flat steel product may include a ferritic microstructure, which possesses optimized formability and suitability for a wide variety of applications, including painting, for example. The flat steel product may include a steel comprising (in percent by weight): C: 0.0001%-0.003%, Si: 0.001%-0.025%, Mn: 0.05%-0.20%, P: 0.001%-0.015%, Al: 0.02%-0.055%, Ti: 0.01%-0.1%. The steel may further include at least one of Cr: 0.001%-0.05%, V: up to 0.005%, Mo: up to 0.015%, or N: 0.001%-0.004%, which may have the following mechanical properties: Rp0.2≦180 MPa, Rm≦340 MPa, A80≦40%, and n value ≦0.23. At least one surface may have an arithmetic mean roughness Ra of 0.8-1.6 μm and a peak count RPc of 75/cm. The present disclosure also concerns methods for producing flat steel products.

Claims

1.-15. (canceled)

16. A cold-rolled and recrystallization-annealed flat steel product including a ferritic microstructure, comprising a steel that includes 0.0001%-0.003% by weight C; 0.001%-0.025% by weight Si; 0.05%-0.20% by weight Mn; 0.001%-0.015% by weight P; 0.02%-0.055% by weight Al; 0.01%-0.1% by weight Ti; and iron and unavoidable impurities.

17. The cold-rolled and recrystallization-annealed flat steel product of claim 16 wherein the steel further comprises as an alloy element or as alloy elements at least one of 0.001%-0.05% by weight Cr; up to 0.005% by weight V; up to 0.015% by weight Mo; or 0.001%-0.004% by weight N, wherein the alloy element has or the alloy elements have a yield strength Rp0.2 of up to 180 MPa, a tensile strength Rm of up to 340 MPa, an elongation at break A80 of at least 40%, an n value of at least 0.23, an arithmetic mean roughness Ra of 0.8-1.6 μm on at least one surface, and a peak count RPc of at least 75 1/cm, wherein depressions and peaks shaped into the at least one surface that account for the arithmetic mean roughness Ra and the peak count RPc are in stochastic distribution.

18. The cold-rolled and recrystallization-annealed flat steel product of claim 16 further comprising a metallic protective layer applied by electrolytic coating.

19. The cold-rolled and recrystallization-annealed flat steel product of claim 16 further comprising an inorganic coating.

20. The cold-rolled and recrystallization-annealed flat steel product of claim 16 having a thickness of not more than 1 mm and a width of at least 1000 mm.

21. A method of producing a flat steel product, the method comprising: providing a roll-hardened, cold-rolled flat steel product having a ferritic microstructure, comprising a steel that includes 0.0001%-0.003% by weight C, 0.001%-0.025% by weight Si, 0.05%-0.20% by weight Mn; 0.001%-0.015% by weight P, 0.02%-0.055% by weight Al, 0.01%-0.1% by weight Ti, and iron and unavoidable impurities; heat treating the flat steel product in a continuous run through an annealing furnace under an annealing atmosphere that at a dew point of −10° C. to −60° C. consists of 1%-7% by volume of H2, with a remainder including N2 and unavoidable impurities, wherein the flat steel product for recrystallization annealing is heated up to a hold temperature T1 of 750-860° C. and is kept at the hold temperature T1 for a period t1 of 30-90 s, wherein the flat steel product for a subsequent overaging treatment is cooled from the hold temperature T1 at a cooling rate CR1 of 2-100° C./s to an overaging start temperature T2 of 400-600° C. and after cooling to the overaging start temperature T2 is cooled over a period t2 of 30-400 s at a cooling rate CR2 of 0.5-12° C./s to an overaging end temperature T3 of 250-350° C., wherein the flat steel product after cooling to the overaging end temperature T3 is cooled at a cooling rate CR3 of 1.5-5.0° C./s to a room temperature; and temper rolling the flat steel product with a temper reduction D of 0.4-0.7% using a working temper roll having a circumferential area that comes into contact with the flat steel product having an arithmetic mean roughness Ra of 1.0-2.5 μm and a peak count RPc of at least 100 1/cm, wherein depressions and peaks shaped into a surface of the working temper roll that account for the arithmetic mean roughness Ra and the peak count RPc are in stochastic distribution.

22. The method of claim 21 wherein the steel comprises as an alloy element or as alloy elements at least one of 0.001%-0.05% by weight Cr, up to 0.005% by weight V, up to 0.015% by weight Mo, or 0.001%-0.004% by weight N.

23. The method of claim 22 wherein the alloy element has or the alloy elements have a yield strength Rp0.2 of up to 180 MPa, a tensile strength Rm of up to 340 MPa, an elongation at break A80 of at least 40%, and an n value of at least 0.23.

24. The method of claim 21 wherein the hold temperature T1 is 800-850° C.

25. The method of claim 21 wherein the overaging start temperature T2 is 400-550° C.

26. The method of claim 21 wherein the dew point of the annealing atmosphere is −15° C. to −50° C.

27. The method of claim 21 wherein the temper rolling is performed as wet temper rolling such that upstream of the working temper roll in a conveying direction of the flat steel product, a temper rolling fluid is applied at least to a surface of the flat steel product on which the working temper roll acts.

28. The method of claim 21 wherein the temper reduction D is 0.5%-0.6%.

29. The method of claim 21 wherein the arithmetic mean roughness Ra of the circumferential area of the working temper roll that comes into contact with the flat steel product is 1.2-2.3 μm.

30. The method of claim 21 wherein the peak count RPc of the circumferential area of the working temper roll that comes into contact with the flat steel product is at least 130 1/cm.

31. The method of claim 21 wherein the temper rolling is performed directly after the heat treating.

32. The method of claim 21 further comprising covering the flat steel product with a metallic coating based on Zn after the temper rolling.

33. The method of claim 32 wherein the metallic coating is applied to the flat steel product by electrolytic galvanization.

Description

[0118] This is to be elucidated in detail hereinafter with reference to working examples. The figures show:

[0119] FIG. 1 a section of a painted surface of an automobile chassis component formed from a flat steel product of the invention;

[0120] FIG. 2 a section of a painted surface of an automobile chassis component formed from a noninventive flat steel product;

[0121] FIG. 3 a schematic profile of a heat treatment of the invention (operating step b)).

[0122] Cold-rolled, roll-hardened flat steel products have been provided in the form of steel strips B1-B12 from steels S1-S6, which had the composition reported in Table 1.

[0123] The flat steel products were heat-treated in various dimensions in a continuous heat treatment furnace of the RTF design, then cooled to room temperature and subsequently subjected to in-line temper rolling.

[0124] The heat treatment comprises a recrystallization annealing operation in which the steel strips B1-B12 have been heated to a hold temperature T1 of 835° C.±15° C. at which they have been kept over a hold period T1 of 60 s.

[0125] After the recrystallization annealing, the steel strips B1-B12 have been subjected to an overaging treatment. For this purpose, they have been cooled from the hold temperature T1 at a cooling rate CR1 of 8.5° C./s to an overaging start temperature T2 of 530±15° C.

[0126] Proceeding from this, the steel strips B1-B12 have then each been cooled over an overaging period t2 of 302 seconds to an overaging end temperature T3 of 280±15° C. The cooling rate CR2 with which the steel strips B1-B12 have been cooled from the overaging start temperature T2 to the overaging end temperature T3 was 0.82° C./s.

[0127] Over the entire heat treatment, the steel strips B1-B12 have been kept under an annealing atmosphere that consisted of 4% by volume of H.sub.2, the remainder having consisted of N.sub.2 and unavoidable impurities. The dew point thereof was set to −45° C.±2° C.

[0128] After the end of the overaging treatment and before exit from the continuous furnace, the steel strips B1-B12 have been cooled under the protective gas atmosphere at a cooling rate CR3 of 3.5° C./s to room temperature and, in a continuation of the continuous run, guided into a quarto rolling stand having support rolls and working temper rolls which has been provided for the temper rolling. The working temper rolls of the temper rolling stand were always roughened in cap (−) mode by means of EDT technology and subjected to hard chrome-plating in a manner known per se. All temper rolling experiments were conducted without the use of a temper rolling medium (dry temper rolling).

[0129] The parameters of the temper rolling (temper reduction D, roughness Ra_W and peak count RPc_W of the peripheral surface of the working temper rolls that come into contact with the steel strips in each case), and also the width b, thickness d, yield strength Rp0.2, tensile strength Rm, elongation A80 and the n value determined for the steel strips B1-B12, are reported in table 2. The mechanical properties were determined in a quasi-static tensile test according to DIN 6892 with the sample positioned longitudinally with respect to rolling direction.

[0130] Likewise listed in table 2 are the roughness Ra and peak count RPc determined for the surfaces of the steel strips B1-B12. The arithmetic mean roughnesses Ra, Ra_W and peak count RPc, RPc_W were always measured according to Stahl-Eisen-Prüfblatt (SEP) 1940 by means of an electrical stylus instrument according to ISO 3274.

[0131] The properties of the steel strips B1 and B9 show that better Wsa values are achieved by means of higher peak counts RPc.

[0132] The noninventive steel strips B11 and B12 demonstrate the importance of the temper reduction for the success of the invention.

[0133] In addition, the Wsa values are determined for the surfaces of the steel strips B1-B12. The results are likewise recorded in table 2. They confirmed that the inventive working examples achieve a Wsa value of <0.40 μm and hence give optimal prerequisites for particularly good paint gloss. The waviness characteristic Wsa was measured according to Stahl-Eisen-Prüfblatt (SEP) 1941; measurement was made on a steel sample which, in the Marciniak cup test, underwent 5% plastic elongation.

[0134] FIG. 1 and FIG. 2 illustrate this using a comparison of components which have been produced from an inventive and a noninventive flat steel product by forming and painting. The noninventive working example shown in FIG. 2 which has been produced from the steel strip B3 that does not fulfill the requirements of the invention, after painting, shows much poorer paint gloss than the example shown in FIG. 1, which has been formed from the inventive steel strip B1.

TABLE-US-00001 TABLE 1 Steel C Si Mn P Al Ti S Cr Nb V Mo N Cu Ni B Sn S1 0.0019 0.005 0.11 0.010 0.029 0.072 0.007 0.032 0.001 0.001 0.004 0.0017 0.014 0.021 0.0002 0.004 S2 0.0015 0.006 0.13 0.010 0.026 0.069 0.009 0.045 0.001 0.001 0.006 0.0026 0.017 0.022 0.0002 0.007 S3 0.0023 0.005 0.09 0.008 0.024 0.075 0.005 0.030 0.001 0.001 0.009 0.0027 0.017 0.027 0.0002 0.004 S4 0.0025 0.006 0.09 0.008 0.024 0.073 0.007 0.024 0.001 0.002 0.004 0.0037 0.010 0.016 0.0002 0.005 S5 0.0020 0.005 0.11 0.010 0.026 0.072 0.007 0.028 0.001 0.003 0.003 0.0025 0.011 0.015 0.0002 0.006 S6 0.0016 0.007 0.11 0.006 0.029 0.073 0.006 — — — — 0.0020 0.011 0.017 0.0002 0.004 Figures in % by weight, the remainder being iron and unavoidable impurities

TABLE-US-00002 TABLE 2 Steel d b Rp0.2 Rm A80 Ra RPc D Ra_W RPc_W Wsa strip Steel (mm) (mm) (MPa) (MPa) (%) n (μm) (1/cm) (%) (μm) (1/cm) [μm] Inventive? B1 S1 0.85 1608 136 289 47.6 0.245 0.94 92 0.4 1.4 139 0.27 yes B2 S1 0.85 1608 139 285 46.5 0.245 1.30 79 0.5 1.4 105 0.37 yes B3 S2 0.74 1651 142 287 47.8 0.240 1.31 68 0.5 3.0 95 0.44 no B4 S3 0.85 1573 142 294 47.5 0.241 1.35 84 0.6 2.2 115 0.33 yes B5 S3 0.85 1573 138 290 47.2 0.247 1.29 83 0.5 2.2 115 0.32 yes B6 S4 0.69 1551 147 302 44.0 0.238 1.16 85 0.6 2.2 114 0.34 yes B7 S4 0.69 1551 146 303 45.4 0.236 1.18 80 0.6 2.2 110 0.35 yes B8 S5 0.82 1610 160 297 45.8 0.230 1.27 53 0.7 1.5 90 0.47 no B9 S6 0.85 1573 130 278 48.5 0.246 1.25 93 0.5 1.4 139 0.26 yes B10 S6 0.85 1573 133 281 47.9 0.245 1.19 87 0.6 2.2 124 0.31 yes B11 S6 0.85 1573  129 *) 275 48.1 0.214 0.71 65 0.3 1.1 110 0.43 no B12 S4 0.69 1551 187 352 38.2 0.238 1.72 70 0.8 2.2 104 0.41 no *) In example B11, the flat steel product showed a marked yield point Reh, the value of which is reported here.