Sheet Metal Part Formed from a Steel Having a High Tensile Strength and Method for Manufacturing Said Sheet Metal Part

Abstract

A sheet metal part having a tensile strength Rm≥1000 MPa and a bending angle >70° formed from a flat steel product including, in % by weight: C: 0.10-0.30%, Si: 0.5-2.0%, Mn: 0.5-2.4%, Al: 0.01-0.2%, Cr: 0.005-1.5%, P: 0.01-0.1%, and optionally one or more of Ti, Nb, V, B, Ni, Cu, Mo, and W, with Ti: 0.005-0.1%, Nb: 0.005-0.1%, V: 0.001-0.2%, B: 0.0005-0.015%, Ni: 0.05-0.4%, Cu: 0.01-0.8%, Mo: 0.01-1.0&, and W: 0.001-1.0%, and remainder iron and unavoidable impurities, wherein the structure of the sheet metal part is 40-100% by area plate-shaped bainite, 70-95% of which is made of ferrite, 2-30% of high carbon phases that are plate-shaped with the remainder made up of other components and the remainder of the structure consists of <40% by area of the total structure of non-plate-shaped bainite, of which is made of ferrite, 2-30% of high carbon phases and <5% of other components. Also, a method for manufacturing the sheet metal part.

Claims

1. A sheet metal part having a tensile strength Rm of at least 1000 MPa and a bending angle of more than 70° that is made from a flat steel product comprising (in % by weight): C: 0.10-0.30%, Si: 0.5-2.0%, Mn: 0.5-2.4%, Al: 0.01-0.2%, Cr: 0.005 -1.5%, P: 0.01 -0.1%, as well as, in each case optionally, additionally of one or more elements from the group consisting of Nb, V, B, Ni, Cu, Mo, and W provided that Ti: 0.005-0.1%, Nb: 0.005-0.1%, V: 0.001-0.2%, B: 0.0005-0.01%, Ni: 0.05-0.4%, Cu: 0.01-0.8%, Mo: 0.01-1.0%, W: 0.001-1.0%, and the remainder of iron and unavoidable impurities, wherein the unavoidable impurities comprise less than 0.05% S and less than 0.01% N, wherein the structure of the sheet metal part is 40-100% by area plate-shaped bainite, which is formed from 70-95% ferrite, 2-30% high carbon phases which are designed to be at least 70% plate-shaped with a plate length PL of at least 200 nm with a ratio of the plate length PL to the plate width PB of the plate-shaped high carbon phase PL/PB of at least 1.7 and are arranged at a distance of 50 nm to 2 μm, and the remainder of less than 5% other components, wherein the remaining structure of the sheet metal part which is not taken up by the plate-shaped bainite consists of up to 40% by area of the total structure of non-plate-shaped marked bainite, which is formed 70-95% of ferrite, 2-30% of high carbon phases and less than 5% of other components, wherein the sum of the shares of the plate-shaped and non-plate-shaped, bainite in the structure of the sheet metal part makes up at least 60% by area, wherein the remaining austenite content of the structure of the sheet metal part is 2-20% by volume, and wherein the remainder of the structure of the sheet metal part not taken up by the bainite components consists of one or more components selected from the group consisting of martensitic or austenitic components, proeutectoid ferrite, iron carbide, iron nitride, transition metal carbide, transition metal nitride, non-metal carbide, non-metal nitride, metal or non-metal inclusions, sulfide and other unavoidable impurities.

2. The sheet metal part according to claim 1, wherein the C content % C, the Mn content % Mn, the Mo content % Mo, the Cr content % Cr, the Ni content % Ni and the Cu content % Cu are set depending on the B content of the steel in each case in % by weight such that the activation energy Qb of the bainite formation is <45 kJ, wherein the following applies for Qb in B contents of up to 0.0005% by weight:
Qb[kJ]=(90*% C+10*(% Mn+% Mo)+2*(% Cr+% Ni)+1*% Cu)[kJ/% by weight], and the following applies for B contents of more than 0.0005% by weight: Qb [kJ] =(90*% C+10*(% Mn+% Mo)+2*(% Cr+% Ni)+1*% Cu+2)[kJ/% by weight].

3. The sheet metal part according to claim 1, wherein the flat steel product used to form the sheet metal contains (in % by weight) 0.13-0.25% C, 0.6-1.4% Si, 0.9-1.8% Mn, 0.01-0.1% Al and 0.15-0.75% Cr.

4. The sheet metal part according to claim 1, wherein the flat steel product used to form the sheet metal part contains (in % by weight) 0.15-0.20% C, 0.7-1.0% Si and 1.1-1.6% Mn.

5. The sheet metal part according to claim 1, wherein the sheet metal part is provided with a metallic corrosion protection coating.

6. The sheet metal part according to claim 5, wherein the metallic corrosion protection coating consists of (in % by weight) 3-15% Si, 1-3.5 Fe, optionally up to 40% Zn, up to 0.5% of one or more alkaline or alkaline earth metals, optionally up to 1% Mg, with the remainder Al and unavoidable impurities.

7. The sheet metal part according to claim 1, wherein the sheet metal part is hot stamped.

8. A method for producing a sheet metal part, comprising the following steps: a) Provision of a cut metal sheet consisting of a steel of the following composition (in % by weight): C: 0.10-0.30%, Si: 0.5-2.0%, Mn: 0.5-2.4%, Al: 0.01-0.2%, Cr: 0.005-1.5%, P: 0.01-0.1%, as well as, in each case optionally, additionally of one or more elements from the group . Nb, V, B, Ni, Cu, Mo, and W, provided that Ni: 0.005-0.1%, Nb: 0.005-0.1%, V: 0.001-0.2%, B: 0.0005-0.01%, Ni: 0.05-0.4%, Cu: 0.01-0.8%, Mo: 0.01-1.0%, W: 0.001-1.0%, and the remainder of iron or unavoidable impurities, wherein the unavoidable impurities comprise less than 0.05% S and less than 0.01% N; b) heating the cut sheet such that at least 30% of the volume of the cut sheet, when inserted into a forming tool intended for hot press shaping, has a temperature T_Aust above the Ac1 temperature, wherein the Ac1 temperature is determined according to the formula
Ac1=[739−22*% C−7*% Mn+2*% Si+14*% Cr+13*% Mo+13*% Ni]° C. where % C=C content, % Si=Si content, % Mn=Mn content, % Cr=Cr content, % Mo=Mo content and % Ni=Ni content of the respective steel of the cut sheet; c) inserting the heated cut sheet into the forming tool tempered to a tool temperature T_WZ of 200-430° C., wherein the transfer time t_Trans needed to remove and insert the cut sheet is a maximum of 20 s; d) hot pressing the cut sheet into the sheet metal part, wherein over the course of the hot pressing the cut sheet is cooled for a time t_WZ of 1-50 s at a cooling speed r_WZ of more than 10 K/s to a cooling stop temperature T_coolstop and optionally held there; e) removing the sheet metal part cooled to the cooling stop temperature T_coolstop from the tool; f1) optionally, holding the sheet metal part at a holding temperature T_Halt of 300-450° C. for a holding time t_Halt of up to 100 s; f2) optionally, heating the sheet metal part to a homogenization temperature of 380-500° C. within 1-10 s; f3) optionally, further reshaping of the sheet metal part; g) optionally, trimming the sheet metal part; h) optionally, the sheet metal part to a cooling temperature T_AB of less than 200° C. within a cooling time t_AB of 0.5-200 s.

9. The method according to claim 8, wherein the following applies for the temperature T_Aust reached in work step b):
Ac3<T_Aust≤1250° C.,
wherein
Ac3=[902-225*% C+19*% Si−11*% Mn−5*% Cr+13*% Mo−20*% Ni+55*% V]° C. where % C=C content, % Si=Si content, % Mn=Mn content, % Cr=Cr content, % Mo=Mo content % Ni=Ni content and % V=V content of the respective steel of the cut sheet.

10. The method according to claim 8, wherein the cut sheet is heated to the temperature T_Aust in work step b).

11. The method according to either claim 9, wherein for the heating in work step b) a heating time t_Aust is
1000s/(T_Aust/° C.−Ac3/° C.+10){circumflex over ( )}2≤t_Aust≤1000 s

12. The method according to claim 8, wherein the cooling speed r_WZ in work step d) is more than 20 K/s.

13. The method according to claim 8, wherein the time t_WZ in work step d) is a maximum of 20 s.

14. The method according to claim 8, wherein the temperature T_WZ of the tool on insertion of the cut sheet depending on the cooling stop temperature T_coolstop and the thickness D of the cut sheet to be formed into the sheet metal part is determined as follows: $\mathrm{T\_coolstop}-{100}^{\circ }.Math..Math.C.*{\left(\frac{D}{1.4.Math..Math.\mathrm{mm}}\right)}^{\frac{1}{2}}<\mathrm{T\_WZ}<\mathrm{T\_coolstop}-{20}^{\circ }.Math..Math.C.*{\left(\frac{D}{1.4.Math..Math.\mathrm{mm}}\right)}^{\frac{1}{2}}$

15. The method according to claim 8, wherein the temperature T_coolstop of the sheet metal part on removal from the tool is 300-450° C.

16. The method according to claim 8, wherein the cooling takes place in air in work step h).

17. A body of a vehicle comprising a sheet metal part according to claim 1.

18. A chassis of a vehicle comprising a sheet metal part according to claim 1.

19. A sheet metal part according to claim 1 that is made from a flat steel product consisting of (in % by weight): C: 0.10-0.30%, Si: 0.5-2.0%, Mn: 0.5-2.4%, Al: 0.01-0.2%, Cr: 0.005-1.5%, P: 0.01-0.1%, as well as, in each case optionally, additionally of one or more elements selected from the group consisting of Nb, V, B, Ni, Cu, Mo, and W provided that Ti: 0.005-0.1%, Nb: 0.005-0.1%, V: 0.001-0.2%, B: 0.0005-0.01%, Ni: 0.05-0.4%, Cu: 0.01-0.8%, Mo: 0.01-1.0%, W: 0.001-1.0%, and the remainder of iron and unavoidable impurities comprising less than 0.05% S and less than 0.01% N.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0126] The invention is explained in greater detail below using exemplary embodiments. In the figures:

[0127] FIG. 1A is a schematic view of the structure of a sample taken from a ground part according to the invention;

[0128] FIG. 1B is a schematic view of the structure of a sample taken from a second ground part according to the invention;

[0129] FIG. 2 shows a light microscope image of a ground part of a sample of a steel processed and composed according to the invention magnified 1000×;

[0130] FIG. 3 shows a grid electron microscopic image of a ground part of a sample produced according to the invention;

[0131] FIG. 4 shows a light microscopic figure of a ground part of a sample produced according to the invention;

[0132] FIG. 5 is a diagram showing the progression of the bainite conversion from the austenite over time in an alloy composed according to the invention at 400° C. in the dilatometer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0133] Melts A-O have been created to test the invention, each of which was composed according to the specifications of the invention, with the compositions listed in Table 1.

[0134] Cold-rolled steel strips have been produced from the melts composed in this way in the conventional manner. Some of the steel strips were also hot dip coated with what is known as an AS coat, also in the conventional manner. The AS coating consisted of 3-15% by weight Si, 3% by weight Fe and the remainder Al and unavoidable impurities at a coat thickness of 22 μm per side of the cut sheet.

[0135] Cut sheets were divided off the steel strips and were used for further tests. In these tests, sheet metal part samples 1-24 in the form of 200×300 mm.sup.2 large plates were hot press molded from the respective cut sheets. In order to do this, the cut sheets were heated in a heating device, for example a conventional heating furnace, from room temperature to an austenisation temperature T_Aust at which they were heated and held for an austenisation time t_Aust. The cut sheets are then removed from the heating device and inserted into a forming tool heated to a tool temperature T_WZ. The transfer time consisting of the removal from the heating device, the transport to the tool and the insertion into the tool was 7 s in each case.

[0136] The cut sheets were reshaped into the respective sheet metal part in the forming tool.

[0137] With the exception of samples 5, 22 and 23, the sheet metal parts obtained were then removed from the forming tool and held at a holding temperature T_Halt in a temperature control tool and held at the temperature T_Halt over a holding time t_Halt to ensure homogenization of the temperature distribution and even conversion of bainite.

[0138] Sample 5 was not subjected to temperature homogenization (work step f1) of the method according to the invention), but instead after a transfer carried out within a transfer time t_trans was subjected to rapid heating in a rapid heating tool, by means of which it was heated to a homogenization temperature T_HOM at a heating speed HR.

[0139] The samples were then cooled to room temperature. Cooling was either carried out in stationary air at a cooling rate of 7 K/s or in compressed air at 30 K/s.

[0140] Samples 22 and 23 were only removed from the tool after the cooling stop temperature had been reached and cooled in stationary air.

[0141] Some of the samples were also subjected to cathodic dip painting (KTL) both to test their ability to be painted and to test whether the KTL treatment changes the mechanical properties. A comparison of samples 3 and 4 shows that KTL itself has barely any impact on the mechanical properties of samples cooled in stationary air.

[0142] The parameters provided for or set in the processing of samples 1-24 (“coating”, austenisation temperature “T_Aust”, austenisation time “t_Aust”, tool temperature “T_WZ”, duration of the cooling process “t_WZ” in the forming tool, cool stopping temperature T_coolstop, holding temperature “T_Halt”, holding time “t_Halt”, transfer time “t_Trans”, heating speed “HR”, homogenization temperature “T_HOM”, “air cooling” and cathodic dip painting “KTL”) are shown in Table 2.

[0143] Table 3 also shows the sheet thickness D of the cut sheets from which the individual samples 1-24 are produced and, for the samples 1-24 obtained, the yield strength Rp02 determined according to DIN EN ISO 6892-1:2009, tensile strength Rm and elongation at break A50, the direction of the tensile specimen relative to the rolling direction or the bending axis relative to the rolling direction (“Q”=transverse), the bending angle BW_Fmax determined according to VDA standard 238-100, the formula indicated in the standard from which the punch movement is calculated (the angle BW_Fmax is the bending angle at which the force in the bending test is at a maximum) and the corrected bending angle BW are indicated. The corrected bending angle BW_korr is calculated according to the formula:

BW_korr=BW*/Thickness

and considers the fact that the bending angle is highly depending on the thickness. The impact of the thickness is eliminated in the corrected bending angle.

[0144] Finally, Table 4 indicates certain structural parts in the samples 1-24 obtained, wherein the column “total bainite” shows the percentage of bainite, the column “plate-like bainite” shows the percentage of the plate-shaped bainite in the sense of the invention, the “martensite” column shows the percentage of martensitic components and the RA column shows the percentage of total remaining austenite in the total structure.

[0145] FIGS. 1A, 1B and 2 have already been explained above.

[0146] In the example in FIG. 3, a grid electron microscope image of a cut sheet of a structural ground part of a sample obtained from the alloy of molten mass A, embodiment 1, the areas removed by etching are bainitic ferrite (bF). The remaining areas are one of the high carbon phases of remaining austenite (RA) or cementite (Z). What these have in common is that they are made of at least 85% by weight Fe and at least 0.6% by weight C. The high carbon content means they can barely be etched and protrude almost up to the polishing level. Both high carbon phases prevent displacement movements in bainitic ferrite, thereby resulting in an increase in strength. At even higher resolution, the remaining austenite and the cementite can be distinguished by the fact that cementite is more resistant to the etching agent than the remaining austenite as a result of its higher carbon content, so the cementite portions have a smooth surface while the surface of austenite appears to be rough.

[0147] FIG. 5 demonstrates, using the example of the alloy “O” that the conversion of austenite into bainite is particular rapid in steel alloyed according to the alloy concept according to the invention. This not only has process technology-related advantages, it also makes it possible to use very low Si contents.

[0148] The latter enables an AS coat that adheres well to the respective steel substrate as demonstrated in FIG. 4 by means of a light microscopic figure of a ground part that comes from the cut sheet metal from which sheet metal part sample number 7 was created.

[0149] FIG. 5 shows the dilatometer curve of bainitic conversion at 400° C. using the example of the alloy “O”. According to this, the bainitic conversion is at 25% after just 10 s and after a further 10 s has progressed to 66%.

TABLE-US-00001 TABLE 1 Alloy C Si Mn Al P Cr Ti B Nb Ni Cu Mo A 0.22 0.8 1.2 0.03 0.014 0.23 0.03 0.0027 — — — 0.2  B 0.22 0.8 1.2 0.03 0.03 0.23 0.03 0.0027 — — — 0.2  C 0.19 0.8 0.8 0.03 0.014 0.5 0.03 0.0027 — — — — D 0.22 0.8 0.8 0.1 0.014 0.5 — 0.0027 — — — — E 0.22 0.8 1.2 0.03 0.014 0.5 — — — — — — F 0.22 0.8 1.2 0.03 0.03 0.23 0.03 — — — — — G 0.22 0.8 1.2 0.03 0.03 0.23 0.03 0.0023 — — — — H 0.19 0.8 1.2 0.03 0.014 0.23 — — 0.025 — — — I 0.19 0.8 1.2 0.03 0.014 0.23 — — 0.025 0.1 0.2 — J 0.19 0.8 1.2 0.03 0.014 0.23 — — 0.025 0.1 0.2 0.15 K 0.19 0.8 1.2 0.03 0.03 0.23 — — 0.025 0.1 0.2 0.15 L 0.22 0.8 1 0.03 0.014 0.5 — — 0.025 — — 0.15 M 0.22 0.8 1 0.03 0.03 0.5 — — 0.025 — — 0.15 O 0.18 0.8 1.5 0.03 0.014 0.23 — 0.0023 — — — — FIGURES in % by weight, the remainder iron and unavoidable impurities: “—” not present indicates data missing or illegible when filed

TABLE-US-00002 TABLE 2 Sample T_Aust t_Aust T_WZ t_WZ T_coolstop T_Halt t Halt t_Transfer no. Melt Coating [° C.] [s] [° C.] [s] [° C.] [° C.] [° C.] [s] 1 A — 925 300 350 9 400 400 100 2 A AS 925 300 300 10 350 400 30 3 B — 925 300 350 9 400 400 100 4 B — 925 300 350 9 400 400 100 5 B AS 925 300 375 8 425 — — 7 6 C — 925 300 350 9 400 400 30 7 C AS 925 300 300 10 350 400 30 8 D — 925 300 400 8 450 400 30 9 E — 925 300 350 9 400 400 100 10 E AS 925 300 400 8 450 400 30 11 E — 925 300 350 9 400 400 30 12 F — 925 300 350 9 400 400 100 13 G AS 925 300 350 9 400 400 25 14 G — 925 300 350 9 400 400 100 15 H — 925 300 350 9 400 400 30 16 I — 925 300 350 9 400 400 30 17 J — 925 300 350 9 400 400 30 18 K AS 925 300 350 9 400 400 100 19 K — 925 300 350 9 400 400 30 20 L — 925 300 400 8 450 400 30 21 M — 925 300 400 8 450 400 30 22 O AS 925 300 350 10 400 — — 23 O AS 925 300 350 20 400 — — 24 O AS 925 300 350 9 400 400 40 indicates data missing or illegible when filed

TABLE-US-00003 TABLE 3 Sample Thickness Rp02 Rm A50 BW_Fmax BW_korr number [mm] [MPa] [MPa] [%] Direction [°] [°] 1 1.56 911 1240 7.8 Q 104.3 130.3 2 1.49 1081 1287 8.1 Q 76.8 93.7 3 1.54 941 1276 9.4 Q 100.9 125.2 4 1.54 947 1261 8.6 Q 98.5 122.2 5 1.54 1143 1474 6.7 Q 71.9 89.2 6 1.5 925 1162 8.4 Q 121.9 149.3 7 1.51 1061 1249 7.4 Q 88.5 108.7 8 1.51 821 1151 10.8 Q 112.5 138.2 9 1.56 954 1251 8.8 Q 98.0 122.4 10 1.54 1069 1468 7.2 Q 72.1 89.5 11 1.53 1089 1425 6.8 Q 84.1 104 12 1.52 893 1210 8.5 Q 107.0 131.9 13 1.53 1102 1350 7.9 Q 79.3 98.1 14 1.53 932 1252 8.3 Q 101.3 125.3 15 1.52 870 1150 10.2 Q 115.2 142.0 16 1.54 878 1153 10.2 Q 112.0 139.0 17 1.52 949 1210 9.5 Q 105.0 129.5 18 1.51 970 1241 9.8 Q 84.9 104.3 19 1.51 1006 1305 9.3 Q 94.0 115.5 20 1.5 932 1208 9.1 Q 105.0 128.6 21 1.5 945 1252 9.4 Q 99.1 121.4 22 1.58 917 1227 8.8 Q 78.2 98.3 23 1.58 923 1196 8.5 Q 84.9 106.7 24 1.58 937 1197 8.4 Q 89.0 111.9 indicates data missing or illegible when filed

TABLE-US-00004 TABLE 4 Total Plate-like bainite bainite Sample [% [%] Martensite RA number by area] by area [%] [%]  1  95 60  5 4  2  80 50 20 3.5  3  95 55  5 4  4  95 55  5 5  5  70 50 30 3  6  95 70  5 3.5  7  95 70  5 1.5  8  95 75  5 5.5  9  95 50  5 4.5 10  75 65 25 3.5 11  80 50 20 3.5 12  80 50 15 4 13  70 40 30 4.5 14  90 55 10 3.5 15 100 60  0 3 16  95 60  5 4 17  95 70  5 4.5 18  95 65  5 5 19  70 55 30 3.5 20  90 60 10 3.5 21  90 55 10 4.5