GALVANNEALED STEEL SHEET

Abstract

A galvannealed steel sheet according to one aspect of the present invention has a hot-dip galvannealed layer on at least one surface of the steel sheet, and the steel sheet has a predetermined chemical composition, in which the steel sheet contains 10% or more and 90% or less of ferrite, and 10% or more of tempered martensite and tempered bainite in terms of an area ratio, a sum of the ferrite, the tempered martensite, and the tempered bainite is 90% or more, carbides having a major axis of 50 nm or more and 300 nm or less are contained in grains of the ferrite in a number density of 20/μm.sup.2 or more, and a two-dimensional homogeneous dispersion ratio S of Mn is 0.75 or more and 1.30 or less.

Claims

1. A galvannealed steel sheet having a hot-dip galvannealed layer on at least one surface of a steel sheet, the steel sheet comprising, by mass %: C: 0.03% to 0.30%; Si: 0.200% to 2.000%; Mn: 2.00% to 4.00%; P: 0.100% or less; S: 0.010% or less; Al: 0.001% to 2.000%; N: 0.010% or less; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.100%; Cu: 0% to 1.000%; Ni: 0% to 1.000%; Mo: 0% to 1.000%; Cr: 0% to 1.000%; W: 0% to 0.005%; Ca: 0% to 0.005%; Mg: 0% to 0.005%; REM: 0% to 0.010%; B: 0% to 0.0030%; and a remainder consisting of Fe and impurities, wherein the steel sheet contains 10% or more and 90% or less of ferrite, and 10% or more of tempered martensite and tempered bainite in terms of an area ratio, a sum of the ferrite, the tempered martensite, and the tempered bainite is 90% or more, carbides having a major axis of 50 nm or more and 300 nm or less are contained in grains of the ferrite in a number density of 20/μm.sup.2 or more, and a two-dimensional homogeneous dispersion ratio S defined by Formula (1) is 0.75 or more and 1.30 or less,
S=Sy.sup.2/Sx.sup.2  Formula (1) where Sx.sup.2 in Formula (1) is a dispersion value of Mn concentration profile data in a sheet width direction, and Sy.sup.2 is a dispersion value of Mn concentration profile data in a sheet thickness direction.

2. The galvannealed steel sheet according to claim 1, wherein the steel sheet contains one or two or more of, by mass %; Ti: 0.003% to 0.100%, Nb: 0.003% to 0.100%, and V: 0.003% to 0.100%, in a total amount of 0.100% or less.

3. A galvannealed steel sheet having a hot-dip galvannealed layer on at least one surface of a steel sheet, the steel sheet comprising, by mass %: C: 0.03% to 0.30%; Si: 0.200% to 2.000%; Mn: 2.00% to 4.00%; P: 0.100% or less; S: 0.010% or less; Al: 0.001% to 2.000%; N: 0.010% or less; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.100%; Cu: 0% to 1.000%; Ni: 0% to 1.000%; Mo: 0% to 1.000%; Cr: 0% to 1.000%; W: 0% to 0.005%; Ca: 0% to 0.005%; Mg: 0% to 0.005%; REM: 0% to 0.010%; B: 0% to 0.0030%; and a remainder comprising Fe and impurities, wherein the steel sheet contains 10% or more and 90% or less of ferrite, and 10% or more of tempered martensite and tempered bainite in terms of an area ratio, a sum of the ferrite, the tempered martensite, and the tempered bainite is 90% or more, carbides having a major axis of 50 nm or more and 300 nm or less are contained in grains of the ferrite in a number density of 20/μm.sup.2 or more, and a two-dimensional homogeneous dispersion ratio S defined by Formula (1) is 0.75 or more and 1.30 or less,
S=Sy.sup.2/Sx.sup.2  Formula (1) where Sx.sup.2 in Formula (1) is a dispersion value of Mn concentration profile data in a sheet width direction, and Sy.sup.2 is a dispersion value of Mn concentration profile data in a sheet thickness direction.

Description

EXAMPLES

[0138] Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

[0139] A slab having the chemical composition shown in Table 1 was manufactured, the slab was heated to 1300° C. for one hour, and thereafter subjected to rough rolling and finish rolling under the conditions shown in Table 2, and the steel sheet was then coiled and held at the coiling temperature shown in Table 2 to obtain a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was pickled and cold-rolled at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet having a sheet thickness of 1.2 mm. Subsequently, annealing, plating, tempering, and skin pass rolling were performed under the conditions shown in Table 2. In some cases, the number of passes in the rough rolling was four. Regarding the example in which the number of passes was four (that is, the rolling reductions of the fifth pass and the sixth pass was 0%), in the columns of “Rolling reduction of second pass and fifth pass” and “Rolling reduction of third pass and sixth pass” are shown in Table 2-1, “Rolling reduction of second pass” and “Rolling reduction of third pass” are respectively written.

TABLE-US-00001 TABLE 1 Kind of Chemical Composition (mass %) Steel C Si Mn P S Al N Ti Nb V Cu Ni Mo Cr W Ca Mg REM B A 0.10 1.000 2.20 0.010 0.004 0.020 0.003 B 0.13 1.000 2.20 0.011 0.004 0.020 0.003 0.030 C 0.15 0.900 2.90 0.012 0.004 0.020 0.003 0.010 D 0.11 0.800 3.00 0.011 0.004 0.020 0.003 0.005 E 0.20 1.200 2.10 0.010 0.004 0.020 0.003 0.010 F 0.20 1.200 2.00 0.010 0.004 0.020 0.003 0.004 G 0.01 1.000 2.50 0.012 0.004 0.020 0.003 H 0.15 0.003 2.40 0.011 0.003 0.020 0.003 I 0.14 0.500 2.40 0.013 0.003 0.020 0.003 0.005 0.005 J 0.18 1.800 2.60 0.010 0.004 0.020 0.003 0.003 K 0.16 1.800 0.05 0.009 0.003 0.020 0.003 L 0.14 0.900 2.20 0.013 0.003 0.020 0.003 0.007 M 0.13 1.100 2.20 0.012 0.004 0.020 0.003 0.005 N 0.23 1.000 2.40 0.009 0.004 0.020 0.003 O 0.09 1.000 2.20 0.012 0.004 0.020 0.003 0.004 0.009 P 0.13 1.000 2.30 0.011 0.004 0.020 0.003 0.0019 Q 0.07 0.800 2.20 0.010 0.004 0.020 0.003 R 0.07 0.800 2.20 0.030 0.004 0.020 0.003 S 0.27 0.300 2.00 0.010 0.007 0.030 0.003 T 0.06 0.450 3.70 0.020 0.007 0.030 0.003 *Underline indicates that it is outside the scope of the present invention. *Blanks in the table indicate that the corresponding chemical component was not intentionally added.

TABLE-US-00002 TABLE 2-1 Rough Rolling Rolling Rolling Rolling Reduction of Reduction of Rolling Number of Rough Heating Reduction of second pass third pass Reduction of Reciprocations Rolling Start Kind of Temperature first pass and fifth pass and sixth pass fourth pass of Rough Rolling Temperature No. Steel (° C.) (%) (%) (%) (%) (reciprocation) (° C.) 1 A 1300 30 10 15 35 6 1250 2 A 1300 25 10 10 25 6 1200 3 B 1300 30 10 10 30 6 1200 4 C 1300 25  5 10 25 6 1200 5 D 1300 30 10  5 25 6 1200 6 E 1300 35 15 10 25 6 1200 7 E 1300 25 10 10 30 6 1250 8 E 1300 30 10  5 25 6 1250 9 F 1300 25 10 10 30 6 1200 10 F 1300 25  5 15 25 6 1200 11 F 1300 30 10 10 25 6 1200 12 G 1300 35 10 10 25 6 1250 13 H 1300 25  5 10 30 6 1200 14 I 1300 30 10  5 25 6 1200 15 I 1300 25 10 10 25 4 1200 16 J 1300 30  5 10 25 6 1200 17 K 1300 35 10 10 30 6 1200 18 L 1300 25 10 10 35 6 1200 19 M 1300 35 10  5 30 6 1200 20 N 1300 25 10 10 30 6 1200 21 O 1300 25 10  5 35 6 1200 22 P 1300 30 10 10 25 6 1250 23 P 1300 35 10 10 30 6 1250 24 P 1300 25  5  5 30 6 1250 25 Q 1300 25 10 10 30 6 1200 26 Q 1300 30 15 10 35 6 1250 27 Q 1300 35 25 20 15 6 1250 28 Q 1300 35 10 10 30 6 1250 29 E 1300 30 10  5 25 6 1250 30 A 1300 10 30 15 35 6 1250 31 A 1300 30 10 35 15 6 1250 32 A 1300 40 10 15 40 6 1250 33 A 1300 35 10 10 30 6 1250 34 R 1300 30 10 15 35 6 1250 35 S 1300 35 10 10 30 6 1250 36 T 1300 35 10 10 30 6 1250 Rough Rolling Finish Rolling Time from Finish Rough Rolling Rough Rolling Number of Finish Rolling Rolling Completion to Finish Rolling Rolling Start Reduction of Completion Coiling Temperature Rolling Stand Temperature First Stand Temperature Temperature No. (° C.) (s) (number) (° C.) (%) (° C.) (° C.) 1 1100 8 7 1050 20 900 650 2 1100 8 7 1050 20 900 650 3 1100 8 7 1050 20 900 600 4 1100 8 7 1050 20 900 650 5 1100 8 7 1050 20 900 640 6 1050 8 7 1050 20 850 600 7 1100 8 7 1050 20 900 620 8 1100 8 7 1050 20 900 620 9 1100 8 7 1050 20 900 640 10 1100 8 7 1050 20 900 600 11 1100 8 7 1050 20 900 650 12 1100 8 7 1050 20 900 650 13 1100 8 7 1050 20 850 650 14 1100 8 7 1050 20 900 650 15 1100 8 7 1050 20 850 650 16 1100 8 7 1050 20 900 670 17 1100 8 7 1050 20 900 660 18 1100 8 7 1050 20 900 670 19 1100 8 7 1050 20 850 650 20 1100 8 7 1050 20 900 640 21 1100 8 7 1050 20 900 630 22 1100 8 7 1050 20 900 640 23 1100 8 7 1050 20 900 640 24 1100 8 7 1050 20 900 650 25 1100 8 7 1050 20 900 640 26 1100 8 7 1050 20 900 640 27 1100 8 7 1050 20 900 640 28 1100 3 7 1050 20 900 650 29 1100 8 7 1050 20 900 640 30 1100 8 7 1050 20 900 640 31 1100 8 7 1050 20 900 650 32 1100 8 7 1050 20 900 650 33 1100 8 7 1050 20 900 750 34 1100 8 7 1050 20 900 640 35 1100 8 7 1050 20 900 640 36 1100 8 7 1050 20 900 640 *Underline indicates that it is out of the preferable range.

TABLE-US-00003 TABLE 2-2 Cold Annealing Plating Rolling Average Alloying Alloying Rolling Annealing Annealing Cooling Cooling Stop Treatment Treatment Reduction Ac.sub.1 Temperature Time Rate Temperature Temperature Time No. (%) (° C.) (° C.) (s) (° C./s) (° C.) (° C.) (s) 1 55 723 850 300 50 450 500 20 2 65 723 900 200 20 460 550 20 3 65 722 780 200  5 450 500 20 4 65 699 800 200  5 450 500 20 5 65 693 750 200 10 450 500 20 6 55 731 810 200 10 460 500 20 7 55 731 650 200 50 450 500 20 8 60 731 740  5 50 470 500 20 9 60 734 810 200 50 450 500 20 10 60 734 810 200   0.5 460 500 20 11 60 734 810 200 50 470 500 20 12 65 716 750 200 50 450 500 20 13 65 682 820 200  5 460 500 20 14 50 699 800 200 10 450 500 20 15 55 699 800 200 10 450 500 20 16 60 738 780 200 15 450 500 20 17 50 810 850 200 10 460 550 20 18 60 719 850 250 50 450 500 20 19 50 726 800 200 50 450 500 20 20 55 715 800 200 50 450 500 20 21 60 723 780 200 50 450 500 20 22 50 720 850 200 50 550 500 20 23 55 720 790 200  2 750 500 20 24 60 720 850 200 10 450 500 20 25 50 716 850 200 100  450 500 20 26 50 716 850 200 100  450 500 20 27 50 716 850 200 100  450 500 20 28 55 716 850 200 50 450 530 20 29 50 731 810 200 10 450 none 30 50 723 850 200 100  450 500 20 31 55 723 850 200 50 450 530 20 32 55 723 850 300 50 450 500 20 33 55 723 850 200 50 450 530 20 34 50 716 850 200 100  450 500 20 35 50 701 850 200 100  450 500 20 36 55 662 850 200 50 450 530 20 Tempering Skin Pass Plating Tempering Tempering Rolling Cooling Stop Retention Retention Cooling Cooling Stop Rolling Temperature Temperature Time Rate Temperature Reduction No. (° C.) (° C.) (s) (° C./s) (° C.) (%) 1 45 250 600 5 50 0.2 2 40 300  5 5 45 0.2 3 50 250 600 5 45 0.2 4 45 100 600 5 45 0.2 5 45 250 600 5 50 0.2 6 45 300 600 5 50 0.2 7 50 350 600 5 45 0.2 8 50 250 600 5 40 0.2 9 45 300 600 5 40 0.2 10 40 250 600 5 50 0.2 11 40 500 600 5 45 0.2 12 50 250 600 5 50 0.2 13 45 300 600 5 45 0.2 14 50 250 600 5 50 0.2 15 45 300 600 5 45 0.2 16 50 300 600 5 40 0.2 17 45 300 600 5 45 0.2 18 40 250 600 5 45 0.2 19 45 250 600 5 40 0.2 20 40 300 600 5 50 0.2 21 50 300 600 5 45 0.2 22 55 250 600 5 45 0.2 23 55 250 600 5 40 0.2 24 45 none 0.2 25 350  300 600 5 40 0.2 26 45 300 600 5 40 0.2 27 45 300 600 5 40 0.2 28 50 300 600 4 45 0.2 29 none 300 600 5 40 0.2 30 45 300 600 5 40 0.2 31 50 300 600 4 45 0.2 32 45 250 600 5 50 0.2 33 50 300 600 4 45 0.2 34 45 300 600 5 40 0.2 35 45 300 600 5 40 0.2 36 50 300 600 4 45 0.2 *Underline indicates that it is out of the preferable range.

[0140] The steel structure of the obtained galvannealed steel sheet was observed, the area ratios of ferrite and the hard structure (tempered martensite and tempered bainite), the precipitation points and number density of carbides having a major axis of 50 nm to 200 nm, and the two-dimensional homogeneous dispersion ratio S were measured.

[0141] Specifically, the area ratio of ferrite and the area ratio of hard structure were determined as follows. First, a sample was taken with a sheet thickness cross section perpendicular to the rolling direction of the steel sheet as an observed section, the observed section was polished, the structure thereof at a thickness ¼ position of the steel sheet was observed with SEM-EBSD at a magnification of 5,000-fold, the resultant was subjected to image analysis in a visual field of 100 μm×100 m to measure the area ratio of ferrite, and the average of values measured at any five visual fields was determined as the area ratio of ferrite.

[0142] Furthermore, an SEM secondary electron image of a region at a depth from 3 t/8 to t/2 from the surface of the steel sheet was photographed (at a magnification 1,500-fold), and from the fact that white portions of the obtained image data are hard structures and black portions are ferrite, the area ratio of the hard structures was determined based on the image data. The hard structure was determined to be tempered in a case where fine carbides were precipitated in the hard structure when the SEM secondary electron image was observed at 5,000-fold or 10,000-fold.

[0143] The major axis and number density of carbides were measured by TEM observation. Specifically, a thin film sample was cut out from a region between a ⅜ position and a ¼ position of the thickness of the steel sheet from the surface of the steel sheet, and was observed in a bright visual field. The sample was cut by 1 mun.sup.2 at an appropriate magnification between 10,000-fold and 100,000-fold, and carbides having a major axis of 50 nm or more and 300 nm or less among the carbides in the visual field were counted and obtained. This operation was performed in five consecutive visual fields, and the average was taken as the number density.

[0144] In addition, the two-dimensional homogeneous dispersion ratio represented by S was obtained by an EPMA device. The results are shown in Table 3.

[0145] Furthermore, the tensile strength TS, fracture elongation EL, bake hardening amount BH, and sheet thickness reduction ratio TDR after bake hardening of the obtained galvannealed steel sheet were measured. The sheet thickness reduction ratio TDR after bake hardening is an index of the ultimate deformability. In the measurement of the tensile strength TS, fracture elongation EL, bake hardening amount BH, and sheet thickness reduction ratio TDR after bake hardening, JIS No. 5 tensile test pieces whose longitudinal direction was perpendicular to the rolling direction were taken, and a tensile test was conducted according to JIS Z 2241. BH is a value obtained by subtracting the stress at the time of application of 2% prestrain from the stress when a test piece subjected to a heat treatment at 170° C. for 20 minutes is re-tensioned after the application of 2% prestrain. TDR is a value obtained by dividing the difference between the original sheet thickness and the sheet thickness after fracture by the original sheet thickness. In order to satisfy the demand for a reduction in the weight of a vehicle body, the tensile strength is 600 MPa or more, preferably 700 MPa or more, and more preferably 800 MPa. Furthermore, the fracture elongation is preferably 10% or more for facilitating forming. In addition, regarding BH, with a BH of less than 100 MPa, it is difficult to perform forming and the strength after forming becomes low. Therefore, a BH of 100 MPa or more is required to provide excellent bake hardenability. BH is more preferably 120 MPa or more. When TDR is less than 25%, there is a risk of cracking during press forming, so that a TDR of 30% or more is required. TDR is more preferably 40%.

TABLE-US-00004 TABLE 3 Steel Structure B Tempered Number Two- A Martensite + Precipita- Density of dimensional Mechanical Property Value Area Ratio Tempered tion Carbides Homogeneous TS EL BH TDR of Ferrite Bainite A + B Points of in Grains Dispersion No. (MPa) (%) (MPa) (%) (%) (%) (%) Carbides (number/μm.sup.2) Ratio S Note 1 947 20 120  35 26 73 99 in grain 25 1.10 Example 2 1032  15 87 20 35  9 44 in grain  5 1.13 Comparative example 3 631 27 104  55 77 21 98 in grain 28 0.95 Example 4 1164  13 75 15 48  5 53 in grain  2 1.09 Comparative example 5 1157  11 132  30 37 60 97 in grain 26 1.21 Example 6 705 23 128  45 58 37 95 in grain 35 1.17 Example 7 469 32 45 55 98  0 98 in grain 27 1.04 Comparative example 8 587 28 56 60 93  0 93 in grain 34 1.09 Comparative example 9 1038  19 118  45 21 79 100  in grain 38 1.11 Example 10 545 28 49 25 87 12 99 at grain  0 1.15 Comparative example boundary 11 848 22 87 25 53 44 97 at grain  0 1.19 Comparative example boundary 12 492 30 57 60 100   0 100  in grain  2 1.01 Comparative example 13 1199  11 89 10 23 77 100  at grain  0 1.30 Comparative example boundary 14 1013  25 109  30 36 63 99 in grain 29 1.04 Example 15 1019  25 78 25 34 65 99 in grain 30 1.55 Comparative example 16 811 21 104  55 72 27 99 in grain 25 1.30 Example 17 569 28 81 60 91  8 99 in grain 21 1.21 Comparative example 18 925 18 122  35 35 65 100  in grain 28 1.13 Example 19 859 19 121  45 45 55 100  in grain 28 1.21 Example 20 1189  15 161  35 32 65 97 in grain 43 1.04 Example 21 989 12 101  35 22 77 99 in grain 21 1.16 Example 22 1061  10 109  35 27 73 100  in grain 28 1.08 Example 23 780 20 87 55 95  2 97 in grain 27 1.21 Comparative example 24 689 22 79 25 68  0 68 in grain 24 1.13 Comparative example 25 711 19 78 20 39 61 100  at grain  0 0.94 Comparative example boundary 26 778 20 104  35 40 59 99 in grain 22 0.95 Example 27 798 20 88 25 42 57 99 in grain 27 1.38 Comparative example 28 803 19 76 25 45 54 99 in grain 27 1.41 Comparative example 29 599 29 67 65 90  5 95 in grain  2 1.10 Comparative example 30 953 19 87 25 25 72 97 in grain 25 1.38 Comparative example 31 940 21 89 25 26 73 99 in grain 25 1.31 Comparative example 32 955 20 78 20 25 75 100  in grain 25 1.60 Comparative example 33 941 20 77 25 26 73 99 in grain 25 1.60 Comparative example 34 739 21 101  35 71 29 100  in grain 27 1.11 Example 35 1269  12 130  35 25 75 100  in grain 27 1.12 Example 36 965 15 105  35 52 48 100  in grain 27 1.10 Example *Underline indicates that it is outside the scope of the present invention or outside the preferable range.

[0146] [Evaluation Results]

[0147] As shown in Table 3, in Examples 1, 3, 5, 6, 9, 14, 16, 18 to 22, 26, and 34 to 36, excellent TS, BH, and TDR could be obtained. In any of the examples, TS was 600 MPa or more, BH was 100 MPa or more, and TDR was 30% or more, which showed that the strength was high, the bake hardenability was excellent, and the ultimate deformability after bake hardening was also excellent.

[0148] On the other hand, in Comparative Example 2, since the tempering retention time was too short, the hard structure was not tempered, the number density of carbides in the ferrite grains was low, and BH and TDR were low.

[0149] In Comparative Example 4, since the tempering temperature was too low, the hard structure was not tempered, the number density of carbides in the ferrite grains was low, and BH and TDR were low.

[0150] In Comparative Example 7, since the annealing temperature was too low, ferrite and the hard structure did not have the desired area ratios, and TS and BH were low.

[0151] In Comparative Example 8, since the annealing time was too short, the hard structure did not have the desired area ratio, and TS and BH were low.

[0152] In Comparative Example 10, since the average cooling rate after annealing was too slow, iron carbides such as cementite appeared at the grain boundaries, and TS, BH, and TDR were low.

[0153] In Comparative Example 11, since the tempering temperature was too high, iron carbides such as cementite appeared at the grain boundaries, and BH and TDR were low.

[0154] In Comparative Example 12, since the C content was too small, ferrite and the hard structure did not have the desired area ratios, the number density of carbides in the ferrite grains was low, and TS and BH were low.

[0155] In Comparative Example 13, since the Si content was too small, iron carbides such as cementite appeared at the grain boundaries, and BH and TDR were low.

[0156] In Comparative Example 15, since the number of reciprocations of the rough rolling was small and the rough rolling was insufficient, the two-dimensional homogeneous dispersion ratio S was high, and BH and TDR were low.

[0157] In Comparative Example 17, since the Mn content was too small, the hard structure did not have the desired area ratio, and TS and BH were low.

[0158] In Comparative Example 23, the primary cooling stop temperature after annealing was too high, the hard structure did not have the desired area ratio, and BH was low.

[0159] In Comparative Example 24, since there was no tempering step, the hard structure was not tempered and BH and TDR were low.

[0160] In Comparative Example 25, since the cooling stop temperature in the plating step was high, iron carbides such as cementite appeared at the grain boundaries, and BH and TDR were low.

[0161] In Comparative Example 27, since the rolling reduction difference between the two passes included in one reciprocation of the rough rolling was low, the two-dimensional homogeneous dispersion ratio S was high and BH and TDR were low.

[0162] In Comparative Example 28, since the time from the rough rolling to the finish rolling was short, the two-dimensional homogeneous dispersion ratio S was high, and BH and TDR were low.

[0163] In Comparative Example 29, since the plating treatment and the alloying treatment were not performed, the bainite fraction was low, and the martensite was not tempered, so that the TS and BH were low.

[0164] In Comparative Example 30, since the rolling reduction of the second pass was larger than the rolling reduction of the first pass, the two-dimensional homogeneous dispersion ratio S was high, and BH and TDR were low.

[0165] In Comparative Example 31, since the rolling reduction of the third pass was larger than the rolling reduction of the fourth pass, the two-dimensional homogeneous dispersion ratio S was high, and BH and TDR were low.

[0166] In Comparative Example 32, since the rolling reductions of the first pass and the fourth pass were too high, the two-dimensional homogeneous dispersion ratio S was high, and BH and TDR were low.

[0167] In Comparative Example 33, since the coiling temperature was too high, the two-dimensional homogeneous dispersion ratio S was high, and BH and TDR were insufficient.

INDUSTRIAL APPLICABILITY

[0168] The galvannealed steel sheet of the present invention can be used as a structural member of a vehicle, particularly in an automotive industry field.