Hot stamped part and manufacturing method thereof

Abstract

A blank material is formed from a steel sheet, a first quenching of the blank material is performed, and a second quenching of the blank material is performed after the first quenching. When the first quenching is performed, the blank material is heated to a first temperature of not lower than (Ac3 point—50)° C. nor higher than 1200° C. at an average heating rate of 2° C./sec or more, and the blank material is cooled from the first temperature to a second temperature of 250° C. or lower. When the second quenching is performed, the blank material is heated from the second temperature to a third temperature of not lower than (Ac3 point—50)° C. nor higher than 1200° C. at an average heating rate of 2° C./sec or more, and the blank material is cooled from the third temperature to a fourth temperature of 250° C. or lower. Forming of the blank material is performed in the first quenching or the second quenching or both of the above.

Claims

1. A hot stamped part obtained by a manufacturing method of the hot stamped part comprising a microstructure represented by an area fraction of fresh martensite and tempered martensite: 80% or more in total, a prior austenite grain diameter: 20 μm or less, and an average grain diameter of carbides: not less than 0.3 μm nor more than 0.5 μm, the manufacturing method of the hot stamped part comprising a step of forming a blank material from a steel sheet; wherein the steel sheet has a chemical composition represented by, in mass %, C: 0.27% to 0.60%, Mn: 0.50% to 5.00%, Si: 2.00% or less, P: 0.030% or less, S: 0.0100% or less, acid-soluble Al (sol. Al): 0.100% or less, N: 0.0100% or less, B: 0.0000% to 0.0050%, Cr: 0.00% to 0.50%, Mo: 0.00% to 0.50%, Ti: 0.000% to 0.100%, Nb: 0.000% to 0.100%, V: 0.000% to 0.100%, Cu: 0.000% to 1.000%, Ni: 0.000% to 1.000%, O: 0.00% to 0.02%, W: 0.0% to 0.1%, Ta: 0.0% to 0.1%, Sn: 0.00% to 0.05%, Sb: 0.00% to 0.05%, As: 0.00% to 0.05%, Mg: 0.00% to 0.05%, Ca: 0.00% to 0.05%, Y: 0.00% to 0.05%, Zr: 0.00% to 0.05%, La: 0.00% to 0.05%, or Ce: 0.00% to 0.05%, and the balance: Fe and impurities; a step of performing a first quenching of the blank material; and a step of performing a second quenching of the blank material after the first quenching, wherein the step of performing the first quenching comprises: a step of heating the blank material to a first temperature of not lower than (Ac3 point—50)° C. nor higher than 1200° C. at an average heating rate of 2° C./sec or more; and a step of cooling the blank material from the first temperature to a second temperature of 250° C. or lower, wherein the step of performing the second quenching comprises: a step of heating the blank material from the second temperature to a third temperature of not lower than (Ac3 point—50)° C. nor higher than 1200° C. at an average heating rate of 2° C./sec or more; and a step of cooling the blank material from the third temperature to a fourth temperature of 250° C. or lower, and wherein forming of the blank material is performed in the first quenching or the second quenching or both of the first quenching and the second quenching.

2. The hot stamped part obtained by the manufacturing method of the hot stamped part according to claim 1, wherein a Vickers hardness is 550 Hv or more.

3. The hot stamped part obtained by the manufacturing method of the hot stamped part according to claim 1, comprising a step of holding at the first temperature for one second or longer between the step of heating to the first temperature and the step of cooling to the second temperature.

4. The hot stamped part obtained by the manufacturing method of the hot stamped part according to claim 1, wherein the third temperature is not lower than (Ac3 point—50)° C. nor higher than 1000° C.

5. The hot stamped part obtained by the manufacturing method of the hot stamped part according to claim 1, wherein heating from the second temperature to the third temperature is performed at an average heating rate of 5° C./sec or more.

6. The hot stamped part obtained by the manufacturing method of the hot stamped part according to claim 1, comprising a step of holding at the third temperature for not shorter than 0.1 seconds nor longer than 300 seconds between the step of heating to the third temperature and the step of cooling to the fourth temperature.

7. The hot stamped part obtained by the manufacturing method of the hot stamped part according to claim 1, wherein the step of performing the second quenching comprises a step of cooling the blank material to a fifth temperature from 0.700° C. to Ms point—50° C. at an average cooling rate of 20° C./sec.

Description

EXAMPLE

(1) Next, examples of the present invention will be explained. Conditions in examples are condition examples employed for confirming the applicability and effects of the present invention and the present invention is not limited to these examples. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the spirit of the present invention.

(2) (First Experiment)

(3) Slabs having chemical compositions presented in Table 1 were subjected to hot-rolling. In the hot rolling, a slab heating temperature was set to 1250° C., a finishing temperature was set to 930° C., and a coiling temperature was set to 650° C. In cooling from the finishing temperature (930° C.) to the coiling temperature (650° C.), an average cooling rate was set to 20° C./sec. Thus, hot-rolled steel sheets each having a thickness of 1.6 mm or 3.2 mm were obtained. Next, the hot-rolled steel sheets were subjected to descaling treatment. The balance of each of the chemical compositions presented in Table 1 is Fe and impurities.

(4) TABLE-US-00001 TABLE 1 CHEMICAL COMPOSITION (MASS %) Ac3 Ar3 Ms MARK OF POINT POINT POINT STEEL C Si Al Mn P S N Cr B Ti Ni Nb Mo (° C.) (° C.) (° C.) a 0.25 0.30 0.030 3.20 0.006 0.0016 0.0016 733 535 336 b 0.27 0.32 0.029 1.63 0.022 0.0003 0.0034 0.10 0.0021 0.040 803 669 374 c 0.30 0.52 0.040 2.33 0.028 0.0022 0.0026 0.30 0.050 0.730 794 559 325 d 0.36 0.63 0.062 1.59 0.006 0.0037 0.0039 0.41 0.0010 0.084 784 640 333 e 0.40 0.82 0.085 1.62 0.012 0.0027 0.0031 0.20 0.890 0.38 811 581 300 f 0.46 1.30 0.016 0.66 0.016 0.0330 0.0024 0.42 0.055 0.49 829 692 316 g 0.59 0.22 0.061 2.30 0.006 0.0016 0.0016 0.0021 0.040 0.055 0.38 742 487 217

(5) Thereafter, from the hot-rolled steel sheets each having a thickness of 3.2 mm, as follows, cold-rolled steel sheets, aluminum-plated steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets were produced. First, the hot-rolled steel sheets each having a thickness of 3.2 mm were subjected to the hot-rolled sheet annealing at 600° C. for two hours and subjected to the cold rolling at a reduction ratio of 50% to obtain the cold-rolled steel sheets each having a thickness of 1.6 mm. Next, the partial cold-rolled steel sheets were subjected to the annealing in continuous hot-dip annealing equipment or continuous aluminizing line. In this annealing, after holding the cold-rolled steel sheets at 800° C. for 120 seconds, holding was performed at 400° C. for 200 seconds. After the annealing, the cold-rolled steel sheets were subjected to aluminum coating layer, hot-dip galvanizing, or alloying hot-dip galvanizing at a temperature of 500° C. or lower. Thus, as steel sheets for hot stamping, the hot-rolled steel sheets, the cold-rolled steel sheets, the aluminum-plated steel sheets, the hot-dip galvanized steel sheets, and the alloyed hot-dip galvanized steel sheets were prepared.

(6) Thereafter, the steel sheets for hot stamping were subjected to blanking to be formed into blank materials, and a first quenching (first heat treatment) and a second quenching (second heat treatment) of the blank materials were performed. Table 2 and Table 3 present conditions of the first heat treatment and conditions of the second heat treatment. Note that in the first heat treatment, atmosphere heating, air cooling from a holding temperature to 700° C., and cooling at an average cooling rate of 50° C./sec in a flat sheet-shaped die from 700° C. to a cooling stop temperature were performed. In the second heat treatment, atmosphere heating was performed when a heating rate was 50° C./sec or less, and electric heating was performed when it was more than 50° C./sec. Air cooling from a holding temperature to 700° C., and cooling at an average cooling rate of 100° C./s while performing press forming in a die from 700° C. to a cooling stop temperature were performed. Thus, various hot stamped parts were manufactured. Underlines in Table 2 and Table 3 indicate that numerical values thereon deviate from ranges of the present invention.

(7) TABLE-US-00002 TABLE 2 FIRST QUENCHING SECOND QUENCHING (FIRST HEAT TREATMENT) (SECOND HEAT TREATMENT) AVER- COOL- AVER- HOLD- COOL- AGE HOLD- ING AGE ING ING HEAT- ING STOP HEAT- TEM- STOP ING TEM- HOLD- TEM- ING PER- HOLD- TEM- MARK RATE Ac3 PER- ING PER- RATE A- ING PER- TEST OF (° C./ POINT ATURE TIME ATURE (° C./ TURE TIME ATURE No. STEEL STEEL TYPE sec) (° C.) (° C.) (sec) (° C.) sec) (° C.) (sec) (° C.) REMARK 1 a COLD-ROLLED  5 733  650 100 250  100 1000 10 200 COMPARATVE STEEL SHEET EXAMPLE 2 b COLD-ROLLED 10 803  900 10 250   10 930 10 200 INVENTION STEEL SHEET EXAMPLE 3 c COLD-ROLLED 10 794  900 10 250   10 930 10 200 INVENTION STEEL SHEET EXAMPLE 4 d COLD-ROLLED 10 784  900 10 250   10 930 10 200 INVENTION STEEL SHEET EXAMPLE 5 e COLD-ROLLED 10 811  900 10 250   10 930 10 200 INVENTION STEEL SHEET EXAMPLE 6 f COLD-ROLLED ABSENCE   10 930 10 200 COMPARATVE STEEL SHEET EXAMPLE 7 f COLD-ROLLED 20 829  900 10 650   10 930 10 200 COMPARATVE STEEL SHEET EXAMPLE 8 f COLD-ROLLED 20 829  900 10 250   3 930 10 200 INVENTION STEEL SHEET EXAMPLE 9 f COLD-ROLLED 20 829  900 10 250   10 930 500 200 INVENTION STEEL SHEET EXAMPLE 10 f COLD-ROLLED 20 829  900 10 250   10 930 10 200 INVENTION STEEL SHEET EXAMPLE 11 f COLD-ROLLED 20 829 1000 10 250   10 930 10 200 INVENTION STEEL SHEET EXAMPLE 12 f COLD-ROLLED 20 829  900 100 250   10 930 10 200 INVENTION STEEL SHEET EXAMPLE 13 f COLD-ROLLED 20 829  900 10 250   10 930 10 200 INVENTION STEEL SHEET EXAMPLE 14 f COLD-ROLLED 20 829  900 10 250  300 930 10 200 INVENTION STEEL SHEET EXAMPLE 15 f COLD-ROLLED 20 829  900 10 250   10 850 10 200 INVENTION STEEL SHEET EXAMPLE 16 f COLD-ROLLED 20 829  900 10 250  300 930 0.1 200 INVENTION STEEL SHEET EXAMPLE 17 f COLD-ROLLED  1 829  900 10 250   10 930 10 200 COMPARATVE STEEL SHEET EXAMPLE 18 f COLD-ROLLED 20 829  750 10 250   10 930 10 200 COMPARATVE STEEL SHEET EXAMPLE 19 f COLD-ROLLED 20 829  900 10 250    1 850 10 200 COMPARATVE STEEL SHEET EXAMPLE 20 f COLD-ROLLED 20 829  900 10 250   10 850 10 270 COMPARATVE STEEL SHEET EXAMPLE 21 f COLD-ROLLED 20 829  900 10 250   10 850 10 250 INVENTION STEEL SHEET EXAMPLE 22 g COLD-ROLLED 20 742  900 10 250   10 930 10 200 INVENTION STEEL SHEET EXAMPLE 23 a HOT-ROLLED 20 733  650 100 250  100 1000 10 200 COMPARATVE STEEL SHEET EXAMPLE 24 b HOT-ROLLED 20 803  900 10 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 25 c HOT-ROLLED 20 794  900 10 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 26 d HOT-ROLLED 20 784  900 10 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 27 e HOT-ROLLED 20 811  900 10 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 28 f HOT-ROLLED 20 829  700 10 250   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 29 f HOT-ROLLED ABSENCE   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 30 f HOT-ROLLED 30 829 900 10 250   10 1150 10 100 INVENTION STEEL SHEET EXAMPLE 31 f HOT-ROLLED 30 829 900 100 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 32 f HOT-ROLLED  1 829 900 10 250   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 33 f HOT-ROLLED 30 829 900 10 270   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 34 f HOT-ROLLED 30 829 900 10 250    1 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 35 f HOT-ROLLED 30 829 900 10 250   10 930 10 270 COMPARATVE STEEL SHEET EXAMPLE 36 f HOT-ROLLED 30 829 900 10 250   10 930 10 250 INVENTION STEEL SHEET EXAMPLE

(8) TABLE-US-00003 TABLE 3 FIRST QUENCHING SECOND QUENCHING (FIRST HEAT TREATMENT) (SECOND HEAT TREATMENT) AVER- HOLD- COOL- AVER- HOLD- COOL- AGE ING ING AGE ING ING HEAT- TEM- STOP HEAT- TEM- STOP ING PER- HOLD- TEM- ING PER- HOLD- TEM- MARK RATE Ac3 A- ING PER- RATE A- ING PER- TEST OF (° C./ POINT TURE TIME ATURE (° C./ TURE TIME ATURE No. STEEL STEEL TYPE sec) (° C.) (° C.) (sec) (° C.) sec) (° C.) (sec) (° C.) REMARK 37 f ALUMINUM-PLATED 30 829  900 10 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 38 f ALUMINUM-PLATED 30 829 1000 10 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 39 f ALUMINUM-PLATED 30 829  900 100 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 40 f ALUMINUM-PLATED 30 829  900 10 250  300 930 10 100 INVENTION STEEL SHEET EXAMPLE 41 f ALUMINUM-PLATED  1 829  900 10 250   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 42 f ALUMINUM-PLATED 30 829  750 10 250   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 43 f ALUMINUM-PLATED 30 829  900 10 270   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 44 f ALUMINUM-PLATED 30 829  900 10 250    1 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 45 f ALUMINUM-PLATED 30 829  900 10 250   10 930 10 270 COMPARATVE STEEL SHEET EXAMPLE 46 f ALUMINUM-PLATED 30 829  900 10 250   10 930 10 250 INVENTION STEEL SHEET EXAMPLE 47 f HOT-DIP GALVANIZED 30 829  900 10 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 48 f HOT-DIP GALVANIZED 30 829 1000 10 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 49 f HOT-DIP GALVANIZED 30 829  900 100 250   10 930 10 100 INVENTION STEEL SHEET EXAMPLE 50 f HOT-DIP GALVANIZED 30 829  900 10 250  300 930 10 100 INVENTION STEEL SHEET EXAMPLE 51 f HOT-DIP GALVANIZED  1 829  900 10 250   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 52 f HOT-DIP GALVANIZED 30 829  750 10 250   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 53 f HOT-DIP GALVANIZED 30 829  900 10 270   10 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 54 f HOT-DIP GALVANIZED 30 829  900 10 250    1 930 10 100 COMPARATVE STEEL SHEET EXAMPLE 55 f HOT-DIP GALVANIZED 30 829  900 10 250   10 930 10 270 COMPARATVE STEEL SHEET EXAMPLE 56 f HOT-DIP GALVANIZED 30 829  900 10 250   10 930 10 250 INVENTION STEEL SHEET EXAMPLE 57 e ALLOYED HOT-DIP 30 811  900 10 250   10 930 10  50 INVENTION GALVANIZED EXAMPLE STEEL SHEET 58 f ALLOYED HOT-DIP 30 829  900 10 250   10 930 10  50 INVENTION GALVANIZED EXAMPLE STEEL SHEET 59 f ALLOYED HOT-DIP 30 829 1050 10 250   10 930 10  50 INVENTION GALVANIZED EXAMPLE STEEL SHEET 60 f ALLOYED HOT-DIP 30 829  900 200 250   10 930 10  50 INVENTION GALVANIZED EXAMPLE STEEL SHEET 61 f ALLOYED HOT-DIP 30 829  900 10 250  200 930 10  50 INVENTION GALVANIZED EXAMPLE STEEL SHEET 62 f ALLOYED HOT-DIP 30 829  900 10 250   10 850 10  50 INVENTION GALVANIZED EXAMPLE STEEL SHEET 63 f ALLOYED HOT-DIP 30 829  900 10 250 1000 850 0.1  50 INVENTION GALVANIZED EXAMPLE STEEL SHEET 64 f ALLOYED HOT-DIP  1 829  900 10 250  10 930 10  50 COMPARATVE GALVANIZED EXAMPLE STEEL SHEET 65 f ALLOYED HOT-DIP 30 829  750 10 250  10 930 10  50 COMPARATVE GALVANIZED EXAMPLE STEEL SHEET 66 f ALLOYED HOT-DIP 30 829  900 10 270  10 930 10  50 COMPARATVE GALVANIZED EXAMPLE STEEL SHEET 67 f ALLOYED HOT-DIP 30 829  900 10 250   1 930 10  50 COMPARATVE GALVANIZED EXAMPLE STEEL SHEET 68 f ALLOYED HOT-DIP 30 829  900 10 250  10 930 10 270 COMPARATVE GALVANIZED EXAMPLE STEEL SHEET 69 f ALLOYED HOT-DIP 30 829  900 10 250  10 930 10 250 INVENTION GALVANIZED EXAMPLE STEEL SHEET 70 g ALLOYED HOT-DIP 30 742  900 10 250  10 930 10  50 INVENTION GALVANIZED EXAMPLE STEEL SHEET

(9) Microstructures before the second heat treatment after the first heat treatment and microstructures after the second heat treatment were observed. Table 4 and Table 5 present these results. An observation method of the microstructures is as described above. Further, tensile test pieces in conformity to JIS Z 2201 were taken from the hot stamped parts, and maximum tensile strength was measured by a tensile test in conformity to JIS Z 2241. The tensile test was performed five times for each test No., and an average value of five maximum tensile strengths was set as tensile strength of the test No. Table 4 and Table 5 also present this result. The reason why the average value is set as the tensile strength is that in a case where a low-stress fracture occurs, even though manufacturing conditions are the same, large variation in rupture stress is likely to occur. Regarding certain true strain ε.sub.a and true stress δ.sub.a, the low-stress fracture was judged as occurring regarding a sample in which a rupture occurred before the following formula 2 was satisfied, and the low-stress fracture was judged as not occurring regarding a sample in which a rupture occurred after the following formula 2 was satisfied. In the formula 2, Δε.sub.a was set to 0.0002, and Δδ.sub.a was set as a difference between “a true stress δ.sub.a+1 when a true strain is “ε.sub.a+0.0002”” and “a true stress δ.sub.a when a true strain is “ε.sub.a”” (Δδ.sub.a=δ.sub.a+1−δ.sub.a),
Δδ.sub.a/Δε.sub.a=δ.sub.a  (formula 2).

(10) TABLE-US-00004 TABLE 4 MICROSTRUCTURE AFTER SECOND QUENCHING AVER- MICROSTRUCTURE AFTER AGE FIRST QUENCHING AREA GRAIN AREA FRACTION (%) DEN- FRACTION (%) PRIOR DIAM- TEM- SITY TEM- γ ETER MECHANICAL PERED FRESH OF PERED FRESH GRAIN OF PROPERTY MARK MAR- MAR- CAR- MAR- MAR- DIAM- CAR- TENSILE LOW- TEST OF TEN- TEN- BAI- TO- BIDE TEN- TEN- TO- ETER BIDE STRENGTH STRESS No. STEEL SITE SITE NITE TAL (/μm.sup.2) SITE SITE TAL (μm) (μm) (MPa) FRACTURE REMARK 1 a 0 0 0 0 0.6 60 40 100 25 0.8 1680 ABSENCE COM- PARATIVE EXAMPLE 2 b 50 50 0 100 0.7 60 40 100 19 0.5 1910 ABSENCE INVENTION EXAMPLE 3 c 50 50 0 100 0.7 60 40 100 19 0.5 2010 ABSENCE INVENTION EXAMPLE 4 d 50 50 0 100 0.8 60 40 100 18 0.5 2370 ABSENCE INVENTION EXAMPLE 5 e 45 55 0 100 0.6 55 45 100 18 0.5 2650 ABSENCE INVENTION EXAMPLE 6 f 0 0 0 0 0.6 55 45 100 23 0.7 1210 PRESENCE COM- PARATIVE EXAMPLE 7 f 0 0 0 0 0.5 55 45 100 24 0.8 1160 PRESENCE COM- PARATIVE EXAMPLE 8 f 45 55 0 100 0.8 55 45 100 19 0.5 1970 PRESENCE INVENTION EXAMPLE 9 f 45 55 0 100 0.8 55 45 100 19 0.3 1980 PRESENCE INVENTION EXAMPLE 10 f 45 55 0 100 0.8 55 45 100 17 0.4 2130 PRESENCE INVENTION EXAMPLE 11 f 45 55 0 100 0.8 55 45 100 17 0.3 2240 PRESENCE INVENTION EXAMPLE 12 f 45 55 0 100 0.8 55 45 100 17 0.3 2250 PRESENCE INVENTION EXAMPLE 13 f 45 55 0 100 0.8 55 45 100 14 0.4 2320 PRESENCE INVENTION EXAMPLE 14 f 45 55 0 100 0.8 55 45 100 14 0.4 2330 PRESENCE INVENTION EXAMPLE 15 f 45 55 0 100 0.7 55 45 100 13 0.4 2320 PRESENCE INVENTION EXAMPLE 16 f 45 55 0 100 0.8 55 45 100  9 0.4 2710 ABSENCE INVENTION EXAMPLE 17 f 45 55 0 100 0.8 60 40 100 23 0.4 1410 PRESENCE COM- PARATIVE EXAMPLE 18 f 0 0 0 0 0.6 60 40 100 22 0.7 1320 PRESENCE COM- PARATIVE EXAMPLE 19 f 50 50 0 100 0.7 65 35 100 25 0.5 1200 PRESENCE COM- PARATIVE EXAMPLE 20 f 45 55 0 100 0.7 40  0  40 17 0.4 1400 ABSENCE COM- PARATIVE EXAMPLE 21 f 45 55 0 100 0.8 70 30 100 17 0.4 2250 ABSENCE INVENTION EXAMPLE 22 g 45 55 0 100 0.8 55 45 100 16 0.4 2690 PRESENCE INVENTION EXAMPLE 23 a 0 0 0 0 0.6 60 40 100 24 0.7 1660 ABSENCE COM- PARATIVE EXAMPLE 24 b 50 50 0 100 0.7 60 40 100 20 0.5 1930 ABSENCE INVENTION EXAMPLE 25 c 50 50 0 100 0.8 60 40 100 20 0.5 2020 ABSENCE INVENTION EXAMPLE 26 d 50 50 0 100 0.7 60 40 100 18 0.5 2360 ABSENCE INVENTION EXAMPLE 27 e 45 55 0 100 0.6 55 45 100 18 0.4 2660 ABSENCE INVENTION EXAMPLE 28 f 0 0 45 45 0.6 55 45 100 22 0.7 1200 PRESENCE COM- PARATIVE EXAMPLE 29 f 0 0 0 0 0.5 55 45 100 24 0.8 1150 PRESENCE COM- PARATIVE EXAMPLE 30 f 45 55 0 100 0.8 50 50 100 19 0.3 1990 PRESENCE INVENTION EXAMPLE 31 f 45 55 0 100 0.8 55 45 100 17 0.4 2410 PRESENCE INVENTION EXAMPLE 32 f 45 55 0 100 0.7 65 35 100 24 0.4 1390 PRESENCE COM- PARATIVE EXAMPLE 33 f 70 0 30 100 0.5 55 45 100 19 0.8 1260 PRESENCE COM- PARATIVE EXAMPLE 34 f 45 55 0 100 0.6 60 40 100 26 0.5 1180 PRESENCE COM- PARATIVE EXAMPLE 35 f 50 50 0 100 0.8 45  0  45 17 0.4 1430 ABSENCE COM- PARATIVE EXAMPLE 36 f 50 50 0 100 0.8 70 30 100 17 0.4 2250 ABSENCE INVENTION EXAMPLE

(11) TABLE-US-00005 TABLE 5 MICROSTRUCTURE AFTER SECOND QUENCHING AVER- MICROSTRUCTURE AFTER AGE FIRST QUENCHING AREA GRAIN AREA FRACTION (%) DEN- FRACTION (%) PRIOR DIAM- TEM- SITY TEM- γ ETER MECHANICAL PERED FRESH OF PERED FRESH GRAIN OF PROPERTY MARK MAR- MAR- CAR- MAR- MAR- DIAM- CAR- TENSILE LOW- TEST OF TEN- TEN- BAI- TO- BIDE TEN- TEN- TO- ETER BIDE STRENGTH STRESS No. STEEL SITE SITE NITE TAL (/μm.sup.2) SITE SITE TAL (μm) (μm) (MPa) FRACTURE REMARK 37 f 45 55 0 100 0.8 55 45 100 18 0.4 2120 ABSENCE COM- PARATIVE EXAMPLE 38 f 40 60 0 100 0.6 55 45 100 18 0.3 2200 ABSENCE INVENTION EXAMPLE 39 f 40 60 0 100 0.6 55 45 100 18 0.3 2240 ABSENCE INVENTION EXAMPLE 40 f 45 55 0 100 0.8 60 40 100 14 0.4 2330 ABSENCE INVENTION EXAMPLE 41 f 45 55 0 100 0.7 65 35 100 24 0.5 1370 ABSENCE INVENTION EXAMPLE 42 f 0 0 0 0 0.6 65 35 100 23 0.7 1280 PRESENCE COM- PARATIVE EXAMPLE 43 f 65 0 35 100 0.5 55 45 100 18 0.8 1250 PRESENCE COM- PARATIVE EXAMPLE 44 f 50 50 0 100 0.6 60 40 100 26 0.6 1160 PRESENCE INVENTION EXAMPLE 45 f 55 45 0 100 0.7 40 0  40 17 0.5 1420 PRESENCE INVENTION EXAMPLE 46 f 55 45 0 100 0.7 70 30 100 17 0.5 2230 PRESENCE INVENTION EXAMPLE 47 f 45 55 0 100 0.8 55 45 100 17 0.4 2110 PRESENCE INVENTION EXAMPLE 48 f 45 55 0 100 0.8 55 45 100 18 0.3 2230 PRESENCE INVENTION EXAMPLE 49 f 40 60 0 100 0.6 55 45 100 17 0.3 2230 PRESENCE INVENTION EXAMPLE 50 f 45 55 0 100 0.8 60 40 100 14 0.4 2340 PRESENCE INVENTION EXAMPLE 51 f 50 50 0 100 0.8 60 40 100 22 0.4 1430 PRESENCE INVENTION EXAMPLE 52 f 0 0 0 0 0.6 70 30 100 24 0.7 1260 ABSENCE INVENTION EXAMPLE 53 f 70 0 30 100 0.6 60 40 100 19 0.8 1250 PRESENCE COM- PARATIVE EXAMPLE 54 f 45 55 0 100 0.6 40 60 100 26 0.5 1180 PRESENCE COM- PARATIVE EXAMPLE 55 f 50 50 0 100 0.7 40 0  40 18 0.4 1440 PRESENCE COM- PARATIVE EXAMPLE 56 f 50 50 0 100 0.7 65 35 100 17 0.5 2230 ABSENCE COM- PARATIVE EXAMPLE 57 e 50 50 0 100 0.6 55 45 100 18 0.5 2600 ABSENCE INVENTION EXAMPLE 58 f 45 55 0 100 0.8 55 45 100 17 0.4 2130 PRESENCE INVENTION EXAMPLE 59 f 45 55 0 100 0.8 55 45 100 17 0.3 2230 ABSENCE COM- PARATIVE EXAMPLE 60 f 40 60 0 100 0.9 55 45 100 17 0.3 2260 ABSENCE INVENTION EXAMPLE 61 f 45 55 0 100 0.7 50 50 100 14 0.5 2330 ABSENCE INVENTION EXAMPLE 62 f 45 55 0 100 0.7 55 45 100 13 0.4 2300 ABSENCE INVENTION EXAMPLE 63 f 45 55 0 100 0.7 45 55 100  5 0.5 2710 ABSENCE INVENTION EXAMPLE 64 f 45 55 0 100 0.7 65 35 100 24 0.4 1420 PRESENCE COM- PARATIVE EXAMPLE 65 f 0 0 0 0 0.7 60 40 100 22 0.6 1300 PRESENCE COM- PARATIVE EXAMPLE 66 f 65 0 35 100 0.5 55 45 100 19 0.8 1270 PRESENCE INVENTION EXAMPLE 67 f 45 55 0 100 0.6 40 60 100 26 0.5 1180 PRESENCE INVENTION EXAMPLE 68 f 55 45 0 100 0.8 45 0  45 16 0.5 1420 PRESENCE COM- PARATIVE EXAMPLE 69 f 50 50 0 100 0.8 65 35 100 17 0.5 2240 PRESENCE COM- PARATIVE EXAMPLE 70 g 45 55 0 100 0.7 50 50 100 16 0.4 2670 ABSENCE INVENTION EXAMPLE

(12) As illustrated in Table 4 and Table 5, in invention examples in ranges of the present invention (tests No. 2 to No. 5, No. 8 to No. 16, No. 21 to No. 22, No. 24 to No. 27, No. 30 to No. 31, No. 36 to No. 40, No. 46 to No. 50, No. 56 to No. 63, No. 69 to No. 70), the low-stress fracture did not occur, or even though it occurred, the stress in which a fracture occurred was 1800 MPa or more.

(13) In a test No. 1, a holding temperature of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, and sufficient tensile strength was not able to be obtained. In a test No. δ, the first quenching was not performed, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 7, a cooling stop temperature of the first quenching was too high, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained.

(14) In a test No. 17, an average heating rate of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 18, a holding temperature of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 19, an average heating rate of the second quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 20, a cooling stop temperature of the second quenching was too high, so that a total area fraction of fresh martensite and tempered martensite fell short, and sufficient tensile strength was not able to be obtained.

(15) In a test No. 23, a holding temperature of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, and sufficient tensile strength was not able to be obtained. In a test No. 28, a holding temperature of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 29, the first quenching was not performed, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 32, an average heating rate of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 33, a cooling stop temperature of the first quenching was too high, so that an average grain diameter of carbides of the hot stamped part was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 34, an average heating rate of the second quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 35, a cooling stop temperature of the second quenching was too high, so that a total area fraction of fresh martensite and tempered martensite fell short, and sufficient tensile strength was not able to be obtained.

(16) In a test No. 41, an average heating rate of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 42, a holding temperature of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 43, a cooling stop temperature of the first quenching was too high, so that an average grain diameter of carbides of the hot stamped part was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 44, an average heating rate of the second quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 45, a cooling stop temperature of the second quenching was too high, so that a total area fraction of fresh martensite and tempered martensite fell short, and sufficient tensile strength was not able to be obtained.

(17) In a test No. 51, an average heating rate of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 52, a holding temperature of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 53, a cooling stop temperature of the first quenching was too high, so that an average grain diameter of carbides of the hot stamped part was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 54, an average heating rate of the second quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 55, a cooling stop temperature of the second quenching was too high, so that a total area fraction of fresh martensite and tempered martensite fell short, and sufficient tensile strength was not able to be obtained.

(18) In a test No. 64, an average heating rate of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 65, a holding temperature of the first quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, an average grain diameter of carbides was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 66, a cooling stop temperature of the first quenching was too high, so that an average grain diameter of carbides of the hot stamped part was excessive, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 67, an average heating rate of the second quenching was too low, so that a prior γ grain diameter of the hot stamped part fell short, a low-stress fracture occurred, and sufficient tensile strength was not able to be obtained. In a test No. 68, a cooling stop temperature of the second quenching was too high, so that a total area fraction of fresh martensite and tempered martensite fell short, and sufficient tensile strength was not able to be obtained.

(19) (Second Experiment)

(20) In a second experiment, blank materials were formed in manners similar to those in the tests No. 10, No. 31, No. 37, No. 47 and No. 58 in the first experiment, and the first quenching (first heat treatment), the second quenching (second heat treatment) and a third quenching (third heat treatment) of the blank materials were performed. Table 6 presents the condition of the first heat treatment, the condition of the second heat treatment and conditions of the third heat treatment. As presented in Table 6, in the third heat treatment, atmosphere heating was performed when a heating rate was 50° C./sec or less, and electric heating was performed when it was more than 50° C./sec. Air cooling from a holding temperature to 700° C., and cooling at an average cooling rate of 100° C./sec while performing press forming in a die from 700° C. to a cooling stop temperature were performed. Thus, various hot stamped parts were manufactured.

(21) TABLE-US-00006 TABLE 6 FIRST QUENCHING (FIRST HEAT TREATMENT), SECOND AVER- COOLING QUENCHING AGE HOLDING STOP MARK (SECOND HEATING TEMPER- HOLDING TEMPER- TEST OF HEAT RATE ATURE TIME ATURE No. STEEL STEEL TYPE TREATMENT) (° C./sec) (° C.) (sec) (° C.) REMARK 71 f COLD-ROLLED STEEL SHEET SAME AS TEST No. 10 10 930 10 200 INVENTION EXAMPLE 72 f COLD-ROLLED STEEL SHEET SAME AS TEST No. 10 3 930 10 200 INVENTION EXAMPLE 73 f COLD-ROLLED STEEL SHEET SAME AS TEST No. 10 300 930 10 200 INVENTION EXAMPLE 74 f COLD-ROLLED STEEL SHEET SAME AS TEST No. 10 10 850 10 200 INVENTION EXAMPLE 75 f COLD-ROLLED STEEL SHEET SAME AS TEST No. 10 300 930 0.1 200 INVENTION EXAMPLE 76 f COLD-ROLLED STEEL SHEET SAME AS TEST No. 10 10 930 500 200 INVENTION EXAMPLE 77 f COLD-ROLLED STEEL SHEET SAME AS TEST No. 10 10 930 10 250 INVENTION EXAMPLE 78 f HOT-ROLLED STEEL SHEET SAME AS TEST No. 31 10 930 10 100 INVENTION EXAMPLE 79 f HOT-ROLLED STEEL SHEET SAME AS TEST No. 31 10 1150 10 100 INVENTION EXAMPLE 80 f ALUMINUM-PLATED SAME AS TEST No. 37 10 930 10 100 INVENTION STEEL SHEET EXAMPLE 81 f ALUMINUM-PLATED SAME AS TEST No. 37 300 930 10 100 INVENTION STEEL SHEET EXAMPLE 82 f HOT-DIP GALVANIZED SAME AS TEST No. 47 10 930 10 100 INVENTION STEEL SHEET EXAMPLE 83 f HOT-DIP GALVANIZED SAME AS TEST No. 47 300 930 10 100 INVENTION STEEL SHEET EXAMPLE 84 f ALLOYED HOT-DIP SAME AS TEST No. 58 10 930 10 50 INVENTION GALVANIZED STEEL SHEET EXAMPLE 85 f ALLOYED HOT-DIP SAME AS TEST No. 58 1000 930 0.1 50 INVENTION GALVANIZED STEEL SHEET EXAMPLE 86 f ALLOYED HOT-DIP SAME AS TEST No. 58 200 930 10 50 INVENTION GALVANIZED STEEL SHEET EXAMPLE

(22) Then, microstructures after the third heat treatment were observed. Table 7 presents this result. An observation method of the microstructures is as described above. Further, a tensile test was performed in a manner similar to that in the first experiment. Table 7 also presents this result.

(23) TABLE-US-00007 TABLE 7 MICROSTRUCTURE AFTER AFTER THIRD QUENCHING AVER- AGE GRAIN AREA FRACTION (%) PRIOR DIAM- TEM- γ ETER MECHANICAL PERED FRESH GRAIN OF PROPERTY MARK MAR- MAR- DIAM- CAR- TENSILE LOW- TEST OF TEN- TEN- TO- ETER BIDE STRENGTH STRESS No. STEEL SITE SITE TAL (μm) (μm) (MPa) FRACTURE REMARK 71 f 55 45 100 15 0.4 2250 PRESENCE INVENTION EXAMPLE 72 f 60 40 100 15 0.5 2210 PRESENCE INVENTION EXAMPLE 73 f 50 50 100 13 0.5 2270 PRESENCE INVENTION EXAMPLE 74 f 50 50 100 11 0.4 2300 PRESENCE INVENTION EXAMPLE 75 f 50 50 100 10 0.4 2720 ABSENCE INVENTION EXAMPLE 76 f 60 40 100 16 0.5 2140 PRESENCE INVENTION EXAMPLE 77 f 60 40 100 15 0.5 2220 PRESENCE INVENTION EXAMPLE 78 f 55 45 100 14 0.5 2240 PRESENCE INVENTION EXAMPLE 79 f 60 40 100 16 0.4 2140 PRESENCE INVENTION EXAMPLE 80 f 55 45 100 14 0.5 2240 PRESENCE INVENTION EXAMPLE 81 f 50 50 100 13 0.5 2260 PRESENCE INVENTION EXAMPLE 82 f 55 45 100 14 0.5 2230 PRESENCE INVENTION EXAMPLE 83 f 50 50 100 13 0.5 2250 PRESENCE INVENTION EXAMPLE 84 f 55 45 100 14 0.5 2230 PRESENCE INVENTION EXAMPLE 85 f 50 50 100 10 0.4 2730 ABSENCE INVENTION EXAMPLE 86 f 50 50 100 12 0.5 2270 PRESENCE INVENTION EXAMPLE

(24) As presented in Table 7, in any invention example, a smaller prior γ grain diameter and a more excellent mechanical property were obtained than those in the invention examples (tests No. 10, No. 31, No. 37, No. 47 or No. 58) in each of which the third quenching was not performed.

INDUSTRIAL APPLICABILITY

(25) The present invention can be utilized in, for example, industries related to a hot stamped part suitable for automotive parts.