STEEL HAVING IMPROVED PROCESSING PROPERTIES FOR WORKING AT ELEVATED TEMPERATURES

Abstract

A flat steel product for hot forming to a formed shaped sheet metal part and processes of making same. The flat steel product and the shaped sheet metal part have improved properties, especially in conjunction with an aluminum-based anticorrosion coating.

Claims

1.-18. (canceled)

19. A flat steel product for hot forming, comprising: a steel substrate composed of steel comprising iron and by weight comprising: C: 0.30-0.50%, Si: 0.05-0.6%, Mn: 0.5-3.0%, Al: 0.10-1.0%, Nb: 0.001-0.2%, Ti: 0.001-0.10%, B: 0.0005-0.01%, P: 0.03%, S: 0.02%, N: 0.02%, Sn: 0.03%, As: 0.01%, and unavoidable impurities 0.2%, wherein an Al/Nb ratio of Al content to Nb content is 20.0.

20. The flat steel product of claim 19, wherein the steel by weight further comprises one or more elements comprising: Cr: 0.01-1.0%, Cu: 0.01-0.2%, Mo: 0.002-0.3%, Ni: 0.01-0.5%, V: 0.001-0.3%, Ca: 0.0005-0.005%, and W: 0.001-1.0%.

21. The flat steel product of claim 20, wherein at least one of following conditions is applicable to the steel: Ti<3.42N; and 0.7% by weight<Mn+Cr<3.5% by weight.

22. The flat steel product of claim 19, further comprising an anticorrosion coating on at least one side of the steel substrate.

23. The flat steel product of claim 22, wherein the anticorrosion coating is an aluminum-based anticorrosion coating and comprises an alloy layer and an Al base layer.

24. The flat steel product of claim 23, wherein the alloy layer comprises 35%-60% by weight of Fe, constituents limited to a total of not more than 5.0% by weight, and balanced aluminum, and the Al base layer comprises 1.0%-15% by weight of Si, 2%-4% by weight of Fe, up to 5.0% by weight of alkali metals or alkaline earth metals, up to 15% Zn, constituents limited to a total of not more than 2.0% by weight, and balanced aluminum.

25. The flat steel product of claim 19, comprising one or more of following properties: a yield point with a continuous progression (Rp0.2) or a yield point with a difference (Re) between an upper yield point limit (ReH) and a lower yield point limit (ReL) of not more than 45 MPa; a uniform elongation Ag of at least 10%; and an elongation at break A80 of at least 15% or at least 20%.

26. The flat steel product of claim 19, further comprising fine precipitates in a microstructure of the steel substrate in a form of niobium carbonitrides and/or titanium carbonitrides.

27. The flat steel product of claim 26, wherein the fine precipitates in the microstructure are round precipitates having a diameter of up to 20 nm.

28. A shaped sheet metal part formed from a flat steel product, comprising: a steel substrate composed of steel comprising iron and by weight comprising: C: 0.30-0.50%, Si: 0.05-0.6%, Mn: 0.5-3.0%, Al: 0.10-1.0%, Nb: 0.001-0.2%, Ti: 0.001-0.10%, B: 0.0005-0.01%, P: 0.03%, S: 0.02%, N: 0.02%, Sn: 0.03%, As: 0.01%, and unavoidable impurities 0.2%; and an anticorrosion coating, wherein an Al/Nb ratio of Al content to Nb content is 20.0.

29. The shaped sheet metal part of claim 28, wherein the steel by weight further comprises one or more elements comprising: Cr: 0.01-1.0%, Cu: 0.01-0.2%, Mo: 0.002-0.3%, Ni 0.01-0.5%, V: 0.001-0.3%, Ca: 0.0005-0.005%, and W 0.001-1.0%.

30. The shaped sheet metal part of claim 28, wherein the steel substrate of the shaped sheet metal part has a microstructure having: at least in part more than 80% martensite and/or lower bainite, or at least in part more than 90% martensite and/or lower bainite, and wherein a former austenite grains of the martensite have an average grain diameter of less than 14 m, less than 12 m, or less than 10 m.

31. The shaped sheet metal part of claim 28, further comprising one or more characterizations of: at least in part having a yield point of at least 1200 MPa or at least 1300; at least in part having a tensile strength of at least 1400 MPa or at least 1600 MPa; at least in part having an elongation at break A80 of at least 3.5%, at least 4%, at least 4.5%, or at least 5%; at least in part having a bending angle of at least 30, at least 40, or at least 45; and at least in part having a yield point ratio of at least 60% and at most 85%.

32. The shaped sheet metal part of claim 28, further comprising fine precipitates in a microstructure in a form of niobium carbonitrides and/or titanium carbonitrides.

33. The shaped sheet metal part of claim 28, wherein the shaped sheet metal part at least partly has a Vickers hardness of at least 500 HV1 or at least 540 HV1.

34. A process for producing a shaped sheet metal part, comprising steps of: a) providing a sheet metal blank made of a flat steel product, comprising: a steel substrate composed of steel comprising iron and by weight comprising: C: 0.30-0.50%, Si: 0.05-0.6%, Mn: 0.5-3.0%, Al: 0.10-1.0%, Nb: 0.001-0.2%, Ti: 0.001-0.10%, B: 0.0005-0.01%, P: 0.03%, S: 0.02%, N: 0.02%, Sn: 0.03%, As: 0.01%, and unavoidable impurities 0.2%, wherein an Al/Nb ratio of Al content to Nb content is 20.0; b) heating the sheet metal blank such that at least in part an AC3 temperature of the sheet metal blank is exceeded and a temperature T.sub.ins of the sheet metal blank on insertion into a forming tool provided for hot press forming at least in part has a temperature above Ms+100 C., wherein Ms is a martensite start temperature; c) inserting the heated sheet metal blank into the forming tool, wherein a transfer time t.sub.trans required for removal from a heating device and insertion of the sheet metal blank is not more than 20 s or not more than 15 s; d) hot press forming the sheet metal blank to the shaped sheet metal part, wherein the sheet metal blank during the hot press forming, is cooled down to a target temperature T.sub.target over a period t.sub.tool of more than 1 s at a cooling rate r.sub.tool of at least in part more than 30 K/s and kept at the target temperature T.sub.target; and e) removing the shaped sheet metal part cooled to the target temperature T.sub.target from the forming tool.

35. The process of claim 34, wherein the steel steel by weight further comprises one or more elements comprising: Cr: 0.01-1.0%, Cu: 0.01-0.2%, Mo: 0.002-0.3%, Ni: 0.01-0.5%, V: 0.001-0.3%, Ca: 0.0005-0.005%, and W: 0.001-1.0%.

36. The process of claim 34, wherein a temperature at least partly obtained in the sheet metal blank in the step b) is between the AC3 temperature and 1000 C., or between 850 C. and 950 C.

37. The process of claim 34, wherein the target temperature T.sub.target of the shaped sheet metal part is at least partly below 400 C. or below 300 C.

38. The process of claim 34, wherein the step a) of providing the sheet metal blank made of the flat steel product, further comprising: providing a slab or thin slab comprising the steel; through-heating the slab or thin slab at a temperature (T1) of 1100-1400 C.; hot rolling the slab or thin slab to produce a hot-rolled flat steel product, wherein a final rolling temperature (T3) is 750-1000 C.; annealing the flat steel product at an annealing temperature (T5) of 650-900 C.; cooling the flat steel product to a dipping temperature (T6) of 650-800 C. or 670-800 C.; and coating the flat steel product cooled to the dipping temperature with an anticorrosion coating by hot dip coating in a melt bath with a melt temperature (T7) of 660-800 C. or 680-740 C.; and cooling the coated flat steel product to room temperature, wherein a first cooling time t.sub.MT in a temperature range between 600 C. and 450 C. is more than 10 s or more than 14 s, and a second cooling time t.sub.LT in a temperature range between 400 C. and 300 C. is more than 8 s or more than 12 s.

39. The process of claim 38, further comprising: pre-rolling the through-heated slab or thin slab to an intermediate product having an intermediate product temperature (T2) of 1000-1200 C.; coiling the hot-rolled flat steel product, wherein a coiling temperature (T4) is at most 700 C.; descaling the hot-rolled flat steel product; cold rolling the flat steel product, wherein a degree of cold rolling is at least 30%; and skin pass rolling the coated flat steel product.

40. The process of claim 38, wherein the anticorrosion coating on the flat steel product is applied to the flat steel product in liquid form and comprises one or more of: up to 15% by weight of Si, 2-4% by weight of Fe, up to 5% by weight of alkali metals or alkaline earth metals, up to 15% Zn, constituents limited to a total of not more than 2.0% by weight, and balanced aluminum.

Description

[0245] The invention is elucidated in detail hereinafter by working examples.

[0246] FIG. 1 shows a grain diagram of reconstructed austenite.

[0247] The effect of the invention was shown by conducting multiple experiments. For this purpose, slabs having the compositions specified in table 1 and having a thickness of 200-280 mm and width of 1000-1200 mm were created, heated up to a respective temperature T1 in a pusher furnace and kept at T1 for between 30 and 450 min until the temperature T1 in the core of the slabs had been attained and the slabs were thus through-heated. The production parameters are specified in table 2. The slabs with their respective through-heating temperature T1 were discharged from the pusher furnace and subjected to hot rolling. The experiments were executed in the form of a continuous hot strip rolling operation. For this purpose, the slabs were first pre-rolled to an intermediate product of thickness 40 mm, and the intermediate products, which can also be referred to as preliminary strips in the case of hot strip rolling, each had an intermediate product temperature T2 at the end of the pre-rolling phase. Immediately after the pre-rolling, the preliminary strips were sent to finish rolling, such that the intermediate product temperature T2 corresponds to the rolling start temperature for the finish-rolling phase. The preliminary strips were rolled to hot strips having a final thickness of 3-7 mm and the respective final rolling temperatures T3 specified in table 2, cooled down to the respective coiling temperature and wound up to coils at the respective coiling temperatures T4 and then cooled down in stationary air. The hot strips were descaled in a conventional manner by pickling before being subjected to a cold rolling operation with the degrees of cold rolling specified in table 2. The cold-rolled flat steel products were heated to a respective annealing temperature T5 in a tunnel annealing furnace and kept at annealing temperature for 100 s in each case, before being cooled down to their respective dipping temperature T6 at a cooling rate of 1 K/s. The cold strips were guided at their respective dipping temperature T6 through a molten coating bath at temperature T7. The composition of the coating bath is specified in table 3. After the coating, the coated strips were blown dry in a conventional manner, which produced coating layers having different layer thicknesses (see table 3). The strips were first cooled down to 600 C. at an average cooling rate of 10-15 K/s. Later on in the cooling between 600 C. and 450 C. and between 400 C. and 300 C., the strips were cooled down over the cooling periods T.sub.MT and T.sub.LT specified in table 2. Between 450 C. and 400 C. and below 220 C., the strips were cooled at a cooling rate of 5-15 K/s in each case.

[0248] Table 4 is a collation of which steel variant (see table 1) was combined with which process variant (see table 2) and which coating (see table 3).

[0249] Steel compositions F is a reference example that is not in accordance with the invention. Correspondingly, experiments 10, 11 and 18 are not in accordance with the invention.

[0250] The thickness of the steel strips produced in all experiments was between 1.4 mm and 1.7 mm.

[0251] After cooling to room temperature, samples were taken transverse to rolling direction from the cooled steel strips according to DIN EN ISO 6892-1 sample form 2 (annex B table B1). The samples were subjected to a tensile test according to DIN EN ISO 6892-1 sample form 2 (annex B table B1). Table 4 gives the results of the tensile test. In the course of the tensile test, the following material indices were ascertained: yield point type, which is referred to as Re for a pronounced yield point and as Rp for a continuous yield point, and the yield point value Rp0.2 in the case of a continuous yield point, the values for the lower yield point ReL, the upper yield point ReH and the difference of upper and lower yield point Re in the case of a pronounced yield point, tensile strength Rm, uniform elongation Ag and elongation at break A80. All samples have a continuous yield point Rp and a uniform elongation Ag of at least 11.5%. Therefore, the Rp0.2 yield point is reported for all samples.

[0252] In addition, table 4 reports the properties of the fine precipitates in the microstructure of the flat steel product. The precipitates are niobium carbonitrides and titanium carbonitrides, both of which contribute to grain refinement. The precipitates are determined with the aid of electron scattering and x-ray images (TEM and EDX) using carbon extraction replicas. The carbon extraction replicas were produced on longitudinal sections (2030 mm). The magnification in the measurement is between 10 000-fold and 200 000-fold. Using these images, the precipitates can be divided into coarse and fine precipitates. Fine precipitates refer to all precipitates having a diameter of less than 30 nm. The other precipitates are referred to as coarse precipitates. By simple counting, the proportion of fine precipitates in the total number of precipitates in the measurement field is ascertained. For the fine precipitates, in addition, the average diameter is calculated by computer-assisted image analysis. In the inventive samples, the proportion of fine precipitates is more than 90%. The average diameter of the fine precipitates is additionally below 12 nm.

[0253] Blanks have been divided from each of the steel strips thus produced, and these have been used for the further experiments. In these experiments, shaped sheet metal part samples 1-8 have been shaped by hot press forming from the respective blanks in the form of sheets of size 200300 mm.sup.2. For this purpose, the blanks have been heated in a heating device, for example in a conventional heating furnace, from room temperature at an average heating rate r.sub.furnace (between 30 C. and 700 C.) in a furnace with a furnace temperature T.sub.furnace. The total time in the furnace, comprising heating and holding, is referred to as t.sub.furnace. The dew point of the furnace atmosphere in all cases was 5 C. Subsequently, the blanks have been removed from the heating device and inserted into a forming tool at temperature T.sub.tool. At the juncture of removal from the furnace, the blanks had assumed the furnace temperature. The transfer time t.sub.trans, composed of the removal from the heating device, transport to the tool and insertion into the tool, was between 5 and 14 s. The temperature T.sub.ins of the blanks on insertion into the forming tool was in all cases above the respective martensite start temperature +100 C. In the forming tool, the blanks have been formed to the respective shaped sheet metal part, and the shaped sheet metal parts were cooled in the tool at a cooling rate r.sub.tool. The residence time in the tool is referred to as t.sub.tool. Finally, the samples have been cooled down to room temperature. Table 5 gives the parameters mentioned for various variants, where RT is an abbreviation of room temperature.

[0254] Table 5 shows very different variants for the forming process. While there is virtually complete formation of martensitic microstructure, for example, in the case of variant II, the comparatively slow cooling of the variants X with the high tool temperature T.sub.tool leads to altered microstructure formation with high ferrite contents, the effect of which takes the form of a higher elongation at break A80.

[0255] Table 6 is a collation of the overall results for the shaped sheet metal parts obtained. The first columns give the sample number, the steel type according to table 1, the process variant according to table 2, the coating according to table 2, and the hot forming variant according to table 5. The further columns give the yield point Rp0.2, tensile strength Rm, the ratio of yield point to tensile strength (yield point ratio), and elongation at break A80. These values were ascertained according to DIN EN ISO 6892-1 sample form 2 (annex B table B1) on samples transverse to rolling direction. The bending angle ascertained has been ascertained according to VDA standard 238-100 with a bending axis transverse to rolling direction. The bending angle ascertained is calculated in each case by the formula specified in the standard from the path of the ram (the bending angle ascertained (also referred to as maximum bending angle) is the bending angle at which the force has its maximum in the bending experiment). In order to eliminate the effect of the sheet thickness on the bending angle, the corrected bending angle was calculated from the ascertained bending angle by the formula

[00018]$\mathrm{Bending}{\mathrm{angle}}_{\mathrm{corrected}}=\mathrm{Bending}{\mathrm{angle}}_{\mathrm{asce}rtained}.Math.\sqrt{\mathrm{Sheet}\mathrm{thickness}}$

where the sheet thickness should be inserted into the formula in mm. Table 7 gives the bending angle ascertained. In order to determine the corrected bending angle, these numerical values should accordingly be multiplied by the root of the sheet thickness specified in table 4. In addition, table 7 gives the Vickers hardness HV1. This was ascertained in accordance with DIN EN ISO 6507 (2018.07).

[0256] The mechanical indices in table 6 were ascertained after a cathodic dip coating had been applied to the formed shaped sheet metal part. During this coating process, the shaped sheet metal parts were heated to 170 C. and kept at that temperature for 20 minutes. Subsequently, the components are cooled down to room temperature under ambient air.

[0257] Table 7 reports the microstructure properties of the shaped sheet metal part. The contents in the microstructure are reported in area %. All inventive examples have a martensite content of more than 90%.

[0258] In addition, table 7 reports the properties of the fine precipitates in the microstructure. The precipitates are niobium carbonitrides and titanium carbonitrides, both of which contribute to grain refining. The precipitates are determined with the aid of electron scattering and x-ray images (TEM and EDX) using carbon extraction replicas. The carbon extraction replicas were produced on longitudinal sections (2030 mm). The magnification in the measurement is between 10 000-fold and 200 000-fold. Using these images, the precipitates can be divided into coarse and fine precipitates. Fine precipitates refer to all precipitates having a diameter of less than 30 nm. The other precipitates are referred to as coarse precipitates. By simple counting, the proportion of fine precipitates in the total number of precipitates in the measurement field is ascertained. For the fine precipitates, in addition, the average diameter is calculated by computer-assisted image analysis. In the inventive samples, the proportion of fine precipitates is more than 90%. The average diameter of the fine precipitates is additionally below 11 nm.

[0259] In addition, table 7 reports the grain diameter of the former austenite grains. For this purpose, the austenite grains were reconstructed by means of the ARPGE software from EBSD measurements. The software parameters were: [0260] Nishiyama-Wassermann orientation relationship [0261] Tolerance for grain identification 7 [0262] Tolerance for parent growth nucleation 7 [0263] Tolerance for parent grain growth 15 [0264] Minimum accepted grain size 10 pixels

[0265] For grain identification, a maximum variance in the orientation of 5 and a minimum grain diameter of 5 pixels according to DIN EN ISO 643 were assumed.

[0266] By way of example, FIG. 1 shows a corresponding reconstruction of austenite. In this case, the average diameter of the former austenite grains is 7.5 m. In all inventive examples, the average grain diameter of the former austenite grains is below 14 m.

TABLE-US-00001 TABLE 1 (steel types) Steel C Si Mn Al Cr Nb Ti B P S N Sn As Cu Mo Ca Ni Al/Nb A 0.35 0.16 1.1 0.21 0.118 0.026 0.0096 0.0025 0.005 <0.0005 0.0035 0.005 0.003 0.019 0.005 0.001 0.032 8.1 B* 0.37 0.3 1.4 0.05 0.18 0.003 0.040 0.0035 0.015 0.003 0.007 0.03 0.01 0.05 0.035 0.003 0.03 16.7 C 0.46 0.20 0.80 0.20 0.12 0.03 0.010 0.0025 0.005 0.0005 0.0035 0.005 0.003 0.019 0.005 0.001 0.019 6.7 D 0.36 0.15 0.8 0.19 0.18 0.025 0.009 0.0024 0.011 <0.0005 0.0036 0.005 0.002 0.02 0.004 0.001 0.031 7.6 Balance: iron and unavoidable impurities. Figures each in % by weight; *noninventive reference examples

TABLE-US-00002 TABLE 2 (production conditions for flat steel product) Process T1 T2 T3 T4 DCR T5 T6 T7 t.sub.MT t.sub.LT variant [ C.] [ C.] [ C.] [ C.] [%] [ C.] [ C.] [ C.] [s] [s] a 1250 1075 850 630 55 773 685 678 15 13 b 1280 1110 860 620 50 787 708 686 15 13 c 1205 1060 820 550 55 768 684 683 18 15 d 1210 1120 910 580 55 770 685 685 18 15 e 1205 1065 830 555 45 655 650 680 15 13 f 1210 1110 900 575 70 760 720 700 20 18 g 1320 1080 840 620 50 805 715 710 23 20 h 1315 1090 910 655 55 870 800 720 25 23 i 1300 1120 830 585 35 895 800 725 30 28 j 1310 1095 915 630 65 830 740 710 25 25 k 1240 1080 870 620 50 823 728 710 35 30 l 1280 1100 870 630 55 808 713 708 18 15 m 1280 1085 870 580 50 793 715 708 22 20 n 1285 1105 885 640 65 755 685 675 13 11 Some figures are rounded

TABLE-US-00003 TABLE 3 (coating variants) Layer thickness Coating Melt analysis (single-sided) variant Si Fe Mg Others Al [m] 9.5 3 0.3 <1% balance 10 8 3.5 0.5 <1% balance 40 10 3 <0.01 <1% balance 25 8.2 3.8 0.25 <1% balance 27 10.5 3.1 0.33 <1% balance 30 8.1 3.9 <0.01 <1% balance 25

TABLE-US-00004 TABLE 4 (flat steel products) Thickness Fine (Nb, Ti)(C, N) of the Elongation Uniform precipitates Coating steel Yield Rp0.2 or at break elongation Propor- Average Number experi- strip Process Coating point ReL Rm A80 Ag tion diameter [per ment no. Steel [mm] variant variant type [MPa] [MPa] [%] [%] [%] [nm] 100 nm.sup.2] 1 A 1.5 a continuous 493 717 20 12 95 6.3 0.0293 2 A 1.5 j continuous 436 682 21 13 94 10 0.0217 3 A 1.5 b continuous 451 693 20 12 93 7.2 0.0221 4* B 1.6 a continuous 403 591 24 13 Coarse 0.0172 precipitates only 5* B 1.6 f continuous 411 603 20 13 Coarse 0.0161 precipitates only 6 C 1.4 e continuous 511 723 16 10 94 9 0.0275 7* B 1.5 i continuous 371 553 26 14 Coarse 0.0124 precipitates only 8 D 1.5 e continuous 443 651 22 12 92 7 0.0223 *noninventive reference examples

TABLE-US-00005 TABLE 5 (hot forming parameters) Average heating Furnace rate r.sub.furnace dew Cooling Hot forming [30-700 C.] T.sub.furnace t.sub.furnace Transfer point T.sub.ins T.sub.tool t.sub.tool rate r.sub.tool T.sub.target variant [K/s] [ C.] [min.] time [s] [ C.] [ C.] [ C.] [s] [K/s] [ C.] I 8 925 6 8 5 800 RT 15 50 50 II 5 920 6 6 5 815 RT 6 300 40 III 15 920 5 5 5 830 RT 15 50 50 IV 10 880 6 7 5 740 100 10 50 120 V 8 950 3 12 5 770 100 10 50 120 VI 10 925 4 7 5 810 RT 10 450 40 VII 5 900 5 7 5 806 RT 15 100 50 VIII 5 920 12 8 5 796 RT 15 100 50 IX 5 920 12 14 5 728 100 10 200 110 X 5 920 6 10 5 792 550 15 30 560 Some figures are rounded

TABLE-US-00006 TABLE 6 (shaped sheet metal part) Experi- Hot Yield Tensile Yield Vickers ment Process Coating forming point strength point A80 Bending hardness no. Steel variant variant variant [MPa] [MPa] ratio [%] angle [] [HV1] 1 A a II 1422 1856 77% 5.3 45 595 2 A j III 1411 1846 76% 5.5 46 592 3 A b IV 1391 1823 76% 5.0 43 589 4* B a II 1400 1854 76% 5 43 592 5* B f IX 1380 1830 75% 5.2 44 598 6 C e VIII 1622 1893 86% 4.5 36 607 7* B i IX 1361 1816 75% 5.4 45 586 8 D e VII 1413 1862 76% 5.6 42 604

TABLE-US-00007 TABLE 7 (microstructure) Microstructure Fine (Nb, Ti) Grain (C, N) diameter precipitates of the Forming Proportion former experiment Marten- Residual [%]/Average austenite no. site Bainite Ferrite austenite diameter grains 1 99.9 0.1 96%/6 nm 7.1 m 2 99.9 0.1 94%/8 nm 6.4 m 3 99.9 0.1 94%/6 nm 6.1 m 4* 99.9 0.1 Coarse 9 m precipitates only 5* 100 Coarse 11 m precipitates only 6 100 0 95% 10.8 mm 7* 100 Coarse 12 m precipitates only 8 99.8 0.2 93% 7.4 * noninventive reference examples