HOT-STAMPING FORMED BODY

Abstract

A hot-stamping formed body has a predetermined chemical composition and includes microstructure which includes residual austenite of which an area ratio is 10% or more and less than 20%, Among grain boundaries of crystal grains of bainite and tempered martensite a ratio of a length of a grain boundary having a rotation angle in a range of 55° to 75° to a total length of a grain boundary having a rotation angle in a range of 4° to 12°, a grain boundary having a rotation angle in a range of 49° to 54°, and the grain boundary having a rotation angle in, a range of 55° to 75° to the <011> direction as a rotation axis is 30% or more.

Claims

1. A hot-stamping formed body comprising, as a chemical composition, by mass %: C: 0.15% to 1.00%; Si: 0.50% to 3.00%; Mn: more than 3.00% and 5.00% or less; Al: 0.100% to 3.000%; Co: 0.100% to 3.000%; P: 0.100% or less; S: 0.1000% or less; N: 0.0100% or less; Nb: 0% to 0.15%; Ti: 0% to 0.150%; Mo: 0% to 1.00%; Cr: 0% to 1.00%; Cu: 0% to 1.00%; V: 0% to 1.00%; W: 0% to 1.00%; Ni: 0% to 3.00%; Mg: 0% to 1.00%; Zr: 0% to 1.00%; Sb: 0% to 1.00%; Ca: 0% to 0.10%; REM: 0% to 0.30%; B: 0% to 0.0100%; and a remainder consisting of Fe and impurities; and a microstructure which includes residual austenite of which an area ratio is 10% or more and less than 20%, fresh martensite of which an area ratio is 5% to 15%, bainite and tempered martensite of which a total area ratio is 65% to 85%, and a remainder in microstructure of which an area ratio is less than 5%, among grain boundaries of crystal grains of the bainite and the tempered martensite, a ratio of a length of a grain boundary having a rotation angle in a range of 55° to 75° to a total length of a grain boundary having a rotation angle in a range of 4° to 12°, a grain boundary having a rotation angle in a range of 49° to 54°, and the grain boundary having a rotation angle in a range of 55° to 75° to the <011> direction as a rotation axis is 30% or more.

2. The hot-stamping formed body according to claim 1, comprising, as the chemical composition, by mass %, at least one selected from the group of: Nb: 0.01% to 0.15%; Ti: 0.010% to 0.150%; Mo: 0.005% to 1.00%; Cr: 0.005% to 1.00%; Cu: 0.001% to 1.00%; V: 0.0005% to 1.00%; W: 0.001% to 1.00%; Ni: 0.001% to 3.00%; Mg: 0.001% to 1.00%; Zr: 0.001% to 1.00%; Sb: 0.001% to 1.00%; Ca: 0.001% to 0.10%; REM: 0.001% to 0.30%; and B: 0.0005% to 0.0100%.

3. A hot-stamping formed body comprising, as a chemical composition, by mass %: C: 0.15% to 1.00%; Si: 0.50% to 3.00%; Mn: more than 3.00% and 5.00% or less; Al: 0.100% to 3.000%; Co: 0.100% to 3.000%; P: 0.100% or less; S: 0.1000% or less; N: 0.0100% or less; Nb: 0% to 0.15%; Ti: 0% to 0.150%; Mo: 0% to 1.00%; Cr: 0% to 1.00%; Cu: 0% to 1.00%; V: 0% to 1.00%; W: 0% to 1.00%; Ni: 0% to 3.00%; Mg: 0% to 1.00%; Zr: 0% to 1.00%; Sb: 0% to 1.00%; Ca: 0% to 0.10%; REM: 0% to 0.30%; B: 0% to 0.0100%; and a remainder comprising Fe and impurities; and a microstructure which includes residual austenite of which an area ratio is 10% or more and less than 20%, fresh martensite of which an area ratio is 5% to 15%, bainite and tempered martensite of which a total area ratio is 65% to 85%, and a remainder in microstructure of which an area ratio is less than 5%, among grain boundaries of crystal grains of the bainite and the tempered martensite, a ratio of a length of a grain boundary having a rotation angle in a range of 55° to 75° to a total length of a grain boundary having a rotation angle in a range of 4° to 12°, a grain boundary having a rotation angle in a range of 49° to 54°, and the grain boundary having a rotation angle in a range of 55° to 75° to the <011> direction as a rotation axis is 30% or more.

Description

EXAMPLES

[0155] Next, examples of the present invention will be described. Conditions in the examples are one condition example that is employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to this condition example. The present invention may employ various conditions to achieve the object of the present invention without departing from the scope of the present invention.

[0156] Hot rolling and cold rolling were performed on steel pieces manufactured by the casting of molten steel having the chemical composition shown in Tables 1 and 2, and plating was performed on the steel pieces as necessary, so that cold-rolled steel sheets were obtained. Then, hot-stamping formed bodies were manufactured using the cold-rolled steel sheets under conditions shown in Tables 3 to 5.

[0157] An average heating rate during heating before hot stamping was set to 0.1° C./s to 200° C./s, cooling after hot stamping was performed up to the temperature range of 150° C. to 300° C., and cooling after holding at low temperature was performed up to a temperature of 80° C. or less. Further, Manufacture No. 18 of Table 3 was provided with a hot-dip aluminum plating layer and Manufacture No. 19 of Table 3 was provided with a hot-dip galvanized layer.

[0158] Manufacture No. 57 was held for 30 sec in the temperature range of 300 to 560° after hot stamping and cooling, and before holding at low temperature holding, and was then subjected to holding at low temperature shown in Table 5.

[0159] An underline in Tables represents that a condition is out of the range of the present invention, a condition is out of a preferred manufacturing condition, or a characteristic value, is not preferred. In Tables 3-5, γr denotes residual austenite, FM denotes fresh martensite, B denotes bainite and TM denotes tempered martensite.

[0160] With regard to the microstructure of the hot-stamping formed body, the measurement of the area ratio of each structure, the measurement of a ratio of the length of a high angle boundary, the measurement of dislocation density, and the measurement of the lath width of crystal grains having body-centered structure were performed by the above-mentioned measurement methods. Further, the mechanical characteristics of the hot-stamping formed body were evaluated by the following methods,

[0161] “Tensile Strength”

[0162] No. 5 test pieces described in JIS Z 2241:2011 were prepared from an arbitrary position of the hot-stamping formed body, and the tensile strength of the hot-stamping formed body was obtained according to a test method described in JIS Z 2241:2011. The speed of a cross-head was set to 3 mm/min. The test piece was determined to be acceptable since being excellent in strength in a case where tensile strength was 1500 MPa or more and was determined to be unacceptable since being inferior in strength in a case where, tensile strength was less than 1500 MPa.

[0163] “Collision Characteristics (Strain Dispersion Characteristics Evaluation)”

[0164] In evaluating the collision characteristics (strain dispersion characteristics and bendability) of the hot-stamping formed body, in this example, based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, the maximum bending angle and the deformation region at the bending angle of 40° were evaluated. The VDA test was conducted under the following conditions.

[0165] In this example, when the maximum bending angle obtained by the VDA test was 60° or more, it was determined to be excellent in bendability and determined to be acceptable, and when the maximum bending angle was less than 60°, it was determined to be inferior in bendability and determined to be unacceptable.

[0166] Dimensions of test piece: 60 mm (rolling direction)×30 mm (a direction parallel to a sheet width direction)

[0167] Sheet thickness of test piece: 1.01 to 1.05 mm (the surface and back were ground by the same amount)

[0168] Bending ridge: a direction parallel to a sheet width direction

[0169] Test method: roll support and, punch, pressing

[0170] Roll diameter: φ30 mm

[0171] Punch shape: tip end R=0.4 mm

[0172] Roll-to-roll distance: 2.0× sheet thickness (mm)+0.5 mm

[0173] Pressing speed: 20 mm/min

[0174] Testing machine: AG-100 KNI manufactured by Shimadzu Corporation

[0175] The strain dispersion characteristics were evaluated in the deformation region at a bending angle of 40° after the VDA bending test. At the center portion of the surface of the test piece before being subjected to the VDA test, 10 lattice-like grits at 100 μm intervals in the width direction×20 lattice-like grits in the length direction (200 in total) were engraved by laser irradiation. The VDA test was performed under the same test conditions as above, and the test was stopped when the bending angle reached 40°. Using a laser microscope, an interstitial distance in the direction perpendicular to the bending ridge was measured in each lattice, and the value was calculated by dividing it by 100 μm to obtain an amount of deformation in each lattice. The length of the deformation region was obtained by calculating the total length of the interstitial distances in the direction perpendicular to the bending ridge of the lattice having the amount of deformation of 1.05 or more. In this example, when the length of the deformation region was 500 μm or more, it was determined to be excellent in the strain dispersion characteristics and determined to be acceptable, and when the length of the deformation region was less than 500 μm, it was determined to be inferior in the strain dispersion characteristics and determined to be unacceptable.

[0176] It is found from Tables 3 to 5 that a hot-stamping formed body of which the chemical composition and the microstructure are in the range of the present invention has excellent strength and collision characteristics.

[0177] Meanwhile, it is found that a hot-stamping formed body of which any one or more of the chemical composition and the microstructure is out of the present invention is inferior in one or more of strength and collision characteristics.

TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %) Remainder of Fe and impurities No. C Si Mn Al Co P S N Others Note 1 0.18 1.76 3.15 0.442 0.102 0.006 0.0019 0.0046 Steel of invention 2 0.55 0.98 3.49 0.312 0.104 0.007 0.0005 0.0048 Steel of invention 3 0.47 0.62 3.40 0.313 0.109 0.004 0.0021 0.0032 Steel of invention 4 0.50 2.90 3.37 0.535 0.114 0.006 0.0027 0.0053 Steel of invention 5 0.53 1.86 3.11 0.369 0.105 0.011 0.0021 0.0033 Steel of invention 6 0.46 0.97 4.79 0.509 0.126 0.008 0.0023 0.0039 Steel of invention 7 0.54 1.03 3.29 0.120 0.100 0.003 0.0027 0.0048 Steel of invention 8 0.51 1.82 3.32 2.880 0.110 0.008 0.0007 0.0043 Steel of invention 9 0.50 1.85 3.21 0.340 0.181 0.086 0.0017 0.0052 Steel of invention 10 0.46 1.10 3.38 0.401 2.785 0.086 0.0026 0.0035 Steel of invention 11 0.45 1.66 3.17 0.743 0.121 0.083 0.0021 0.0030 Steel of invention 12 0.51 1.65 3.34 0.669 0.109 0.001 0.0013 0.0049 Steel of invention 13 0.50 1.21 3.36 0.804 0.146 0.006 0.0781 0.0030 Steel of invention 14 0.55 1.07 3.40 0.620 0.114 0.006 0.0005 0.0049 Steel of invention 15 0.49 1.56 3.37 0.481 0.117 0.010 0.0016 0.0075 Steel of invention 16 0.52 1.23 3.25 0.424 0.129 0.004 0.0033 0.0006 Steel of invention 17 0.51 1.80 3.01 0.460 0.150 0.005 0.0025 0.0032 Steel of invention 18 0.46 1.71 3.07 0.434 0.120 0.004 0.0025 0.0033 Steel of invention 19 0.42 1.69 3.11 0.447 0.122 0.005 0.0023 0.0034 Steel of invention 20 0.49 1.77 3.36 0.500 0.131 0.009 0.0031 0.0045 Nb: 0.07 Steel of invention 21 0.52 1.18 3.50 0.786 0.142 0.004 0.0012 0.0030 Ti: 0.013 Steel of invention 22 0.54 1.04 3.49 0.402 0.110 0.010 0.0016 0.0051 Mo: 0.15 Steel of invention 23 0.45 1.54 3.41 0.662 0.153 0.011 0.0016 0.0033 Cr: 0.34 Steel of invention 24 0.51 1.76 3.30 0.753 0.149 0.012 0.0021 0.0047 Cu: 0.17 Steel of invention 25 0.54 1.53 3.50 0.334 0.151 0.006 0.0014 0.0035 V: 0.21 Steel of invention

TABLE-US-00002 TABLE 2 Steel Chemical composition (mass %) Remainder of Fe and impurities No. C Si Mn Al Co P S N Others Note 26 0.50 1.61 3.20 0.417 0.139 0.005 0.0026 0.0028 W: 0.22 Steel of invention 27 0.49 1.47 3.26 0.351 0.121 0.011 0.0028 0.0029 Ni: 0.35 Steel of invention 28 0.49 1.39 3.14 0.453 0.148 0.010 0.0027 0.0036 Mg: 0.04 Steel of invention 29 0.46 1.62 3.33 0.373 0.114 0.011 0.0025 0.0050 Zr: 0.02 Steel of invention 30 0.49 1.67 3.22 0.563 0.109 0.005 0.0019 0.0046 Sb: 0.02 Steel of invention 31 0.55 1.42 3.34 0.499 0.128 0.011 0.0026 0.0034 B: 0.0025 Steel of invention 32 0.45 1.40 3.40 0.414 0.136 0.008 0.0030 0.0031 Ca: 0.03 Steel of invention 33 0.49 1.21 3.24 0.421 0.125 0.009 0.0016 0.0035 REM: 0.15 Steel of invention 34 1.20 1.12 3.51 0.774 0.136 0.007 0.0023 0.0044 Comparative steel 35 0.12 1.52 3.49 0.495 0.118 0.006 0.0008 0.0032 Comparative steel 36 0.51 0.23 3.18 0.732 0.112 0.011 0.0026 0.0042 Comparative steel 37 0.55 3.28 3.23 0.595 0.148 0.010 0.0008 0.0048 Comparative steel 38 0.47 1.15 2.88 0.309 0.125 0.005 0.0008 0.0047 Comparative steel 39 0.50 1.59 5.12 0.413 0.116 0.005 0.0020 0.0043 Comparative steel 40 0.53 1.18 3.54 0.051 0.131 0.010 0.0019 0.0049 Comparative steel 41 0.50 1.05 3.39 3.310 0.105 0.007 0.0022 0.0046 Comparative steel 42 0.46 1.32 3.22 0.320 0.071 0.006 0.0021 0.0049 Comparative steel 43 0.54 1.12 3.21 0.460 3.223 0.006 0.0021 0.0049 Comparative steel 44 0.53 1.83 3.20 0.605 0.145 0.211 0.0011 0.0053 Comparative steel 45 0.55 1.12 3.38 0.459 0.108 0.008 0.1802 0.0031 Comparative steel 46 0.47 1.12 3.41 0.458 0.115 0.008 0.0027 0.0212 Comparative steel An underline represents that a condition is out of the range of the present invention.

TABLE-US-00003 TABLE 3 Microstructure Ratio of length of Cooling Cooling grain after HS after boundary Average holding having cooling at low rotation rate until Holding at low temperature angle in Mechanical characteristics Heating holding temperature Average B + range of Dislocation Maximum Heating Holding at low Holding Holding cooling γ.sub.T FM TM Remainder 55° density Lath Tensile bending Manufacture Steel temperature time temperture temperature time rate (area (area (area (area to 75° (10.sup.15 width strength angle Deformation No. No. (° C.) (s) (° C./s) (° C.) (h) (° C./s) %) %) %) %) (%) m/m.sup.2) (nm) (MPa) (°) region (μm) Note 1 1 888 380 5 183 25 9 18 8 71 3 43 4.2 191 1560 78 623 Example of invention 2 2 916 364 10 183 26 15 17 7 74 2 44 7.2 152 2510 66 603 Example of invention 3 3 888 356 10 205 20 13 13 9 77 1 51 5.2 172 2027 61 509 Example of invention 4 4 907 320 6 210 21 18 18 9 69 4 58 4.5 177 2412 71 612 Example of invention 5 5 902 380 4 192 25 19 16 9 72 3 32 5.9 170 2249 62 693 Example of invention 6 6 893 354 4 191 23 9 16 6 75 3 53 5.3 161 2304 63 614 Example of invention 7 7 882 259 8 202 28 20 15 7 77 1 57 6.5 186 2185 64 640 Example of invention 8 8 904 317 8 206 21 15 18 9 70 3 55 4.5 164 2106 63 552 Example of invention 9 9 932 283 9 193 20 8s 19 9 70 2 33 4.9 163 2248 64 661 Example of invention 10 10 916 346 10 194 21 15 17 8 73 2 71 4.9 181 2011 78 683 Example of invention 11 11 887 357 7 194 22 19 19 6 71 4 41 6.5 171 2348 61 618 Example of invention 12 12 926 293 7 195 23 17 15 10 74 1 43 5.9 171 2370 75 602 Example of invention 13 15 933 363 7 206 24 5 16 6 77 1 43 6.0 179 2299 62 582 Example of invention 14 14 896 309 9 202 27 8 17 7 74 2 59 5.6 180 2025 77 691 Example of invention 15 15 908 231 3 210 27 20 15 6 78 1 48 4.5 186 2227 65 515 Example of invention 16 16 933 247 9 210 23 11 17 6 74 3 60 5.2 184 2239 74 619 Example of invention 17 17 918 273 3 192 23 16 17 9 72 2 45 6.5 171 2405 72 648 Example of invention 18 18 893 373 10 185 20 9 15 7 75 3 56 4.5 167 2409 72 650 Example of invention 19 19 920 368 9 185 27 10 19 6 72 3 50 4.9 161 2090 70 568 Example of invention 20 20 881 354 4 208 26 20 18 10 68 4 69 6.4 176 2179 78 546 Example of invention

TABLE-US-00004 TABLE 4 Microstructure Ratio of length of Cooling Cooling grain after HS after boundary Average holding having cooling at low rotation rate until Holding at low temperature angle in Mechanical characteristics Heating holding temperature Average B + range of Dislocation Maximum Heating Holding at low Holding Holding cooling γ.sub.T FM TM Remainder 55° density Lath Tensile bending Manufacture Steel temperature time temperture temperature time rate (area (area (area (area to 75° (10.sup.15 width strength angle Deformation No. No. (° C.) (s) (° C./s) (° C.) (h) (° C./s) %) %) %) %) (%) m/m.sup.2) (nm) (MPa) (°) region (μm) Note 21 21 894 236 3 192 28 5 17 6 75 2 71 5.8 171 2352 80 677 Example of invention 22 22 881 353 9 203 23 16 19 7 71 3 74 4.6 183 2290 80 533 Example of invention 23 23 911 307 6 181 27 5 19 8 71 2 76 6.4 189 2292 79 684 Example of invention 24 24 885 265 9 183 26 6 16 7 75 2 73 4.7 173 2370 80 664 Example of invention 25 25 936 278 10 210 20 8 17 8 72 3 74 5.5 170 2165 80 604 Example of invention 26 26 918 364 6 231 22 19 18 10 70 2 75 5.3 189 2228 80 537 Example of invention 27 27 899 264 10 186 27 15 16 9 71 4 71 6.3 180 2406 77 617 Example of invention 28 28 894 311 5 204 27 18 16 7 76 1 58 5.5 186 2213 78 658 Example of invention 29 29 931 361 7 199 20 13 15 6 77 2 53 4.5 177 2259 79 634 Example of invention 30 30 912 304 7 187 23 13 17 10 69 4 57 5.3 176 2060 78 526 Example of invention 31 31 934 296 5 203 26 12 19 6 74 1 40 6.4 183 2000 78 665 Example of invention 32 32 899 320 9 183 28 13 16 8 72 4 43 6.0 165 2265 76 696 Example of invention 33 33 917 301 4 196 25 15 18 8 70 4 42 5.9 162 2384 78 672 Example of invention 34 34 885 272 5 184 26 18 15 9 72 4 52 4.7 280 1210 71 601 Comparative Example 35 35 888 242 6 195 23 18 17 8 74 1 40 5.3 188 1320 74 671 Comparative Example 36 36 913 288 9 207 24 16  8 10 79 3 54 4.5 181 2272 68 451 Comparative Example 37 37 939 368 5 191 21 13 16 8 68 8 48 3.1 162 2227 55 646 Comparative Example 38 38 883 377 5 207 27 8 18 8 72 2 21 5.1 170 2157 51 554 Comparative Example 39 39 909 339 9 210 28 5 17 7 73 3 42 5.9 167 1410 70 579 Comparative Example 40 40 903 275 9 184 20 18 17 7 72 4 53 6.5 160 2109 41 678 Comparative Example An underline represents that a condition is out of the range of the present invention, a manufacturing condition is not preferred, or characteristics are not preferred.

TABLE-US-00005 TABLE 5 Microstructure Ratio of length of Cooling Cooling grain after HS after boundary Average holding having cooling at low rotation rate until Holding at low temperature angle in Mechanical characteristics Heating holding temperature Average B + range of Dislocation Maximum Heating Holding at low Holding Holding cooling γ.sub.T FM TM Remainder 55° density Lath Tensile bending Manufacture Steel temperature time temperture temperature time rate (area (area (area (area to 75° (10.sup.15 width strength angle Deformation No. No. (° C.) (s) (° C./s) (° C.) (h) (° C./s) %) %) %) %) (%) m/m.sup.2) (nm) (MPa) (°) region (μm) Note 41 41  890 354   7 194 27 5 19  7 70  4 54 5.2 174 2008 36 544 Comparative Example 42 42  921 336   3 202 27 7 18 10 53 19 22 2.9 182 2441 54 697 Comparative Example 43 43  936 288   3 207 20 13 18 10 68  4 50 6.0 177 1231 74 700 Comparative Example 44 44  901 294   7 198 24 5 15  7 75  3 42 5.4 166 2345 44 515 Comparative Example 45 45  935 285  10 203 27 10 17 10 72  1 50 5.1 184 2241 46 512 Comparative Example 46 46  923 376   8 198 27 10 17  6 76  1 47 4.6 160 2137 54 658 Comparative Example 47 17  780 372   4 192 22 11 16  8 57 19 50 2.7 162 2144 47 559 Comparative Example 48 17 1080 376   8 200 27 17 19  6 63 12 57 2.8 189 2391 45 544 Comparative Example 49 17  910  46   5 192 24 10 17 10 66  7 58 3.2 169 2394 46 581 Comparative Example 50 17  923 712   9 206 28 5 18 10 62 10 51 3.1 189 2322 47 581 Comparative Example 51 17  932 247   0.4 206 28 17 18  9 41 32 57 3.2 171 2424 41 609 Comparative Example 52 17  883 273 110 202 21 19 17  8 74  1 22 5.0 188 2000 37 531 Comparative Example 53 17  902 332   6 132 22 7  6 10 80  4 21 4.3 179 2510 51 441 Comparative Example 54 17  932 281   4 317 21 18 17  6 76  1 17 4.9 187 2057 36 539 Comparative Example 55 17  916 321   3 206 57 6 15  2 80  3 48 5.1 186 2409 69 481 Comparative Example 56 17  934 339   3 203  0.5 8  6  6 87  1 49 5.0 179 2102 69 450 Comparative Example  57* 17  917 287   8 206 23 14 16  6 74  4 16 5.4 174 2286 41 600 Comparative Example An underline represents that a condition is out of the range of the present invention, a manufacturing condition is not preferred, or characteristics are not preferred. *heating and holding before holding at low temperature

INDUSTRIAL APPLICABILITY

[0178] According to the aspect of the present invention, it is possible, to obtain a hot stamping formed body that is excellent in strength and collision characteristics.