STEEL SHEET AND METHOD OF MANUFACTURING THE SAME

Abstract

A steel sheet includes a predetermined composition, in which a microstructure at a ¼ thickness position from a surface in a sheet thickness direction includes, by vol %, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, a proportion of unrecrystallized ferrite in the ferrite of 5% or less, and in the microstructure of the steel sheet stretched by 10% at the ¼ thickness position from the surface in the sheet thickness direction, a number density of voids having a maximum diameter of 1.0 μm or more is 1.0×10.sup.9 pieces/m.sup.2 or less.

Claims

1. A steel sheet comprising, as a composition, by mass %: C: 0.010% to 0.200%; Si: 0.005% to 1.500%; Mn: 0.05% to 3.00%; Al: 0.005% to 1.000%; P: 0.100% or less; S: 0.0200% or less; N: 0.0150% or less; O: 0.0100% or less; Nb: 0% to 0.060%; Ti: 0% to 0.100%; V: 0% to 0.500%; Cr: 0% to 1.00%; Ni: 0% to 1.00%; Cu: 0% to 1.00%; Mo: 0% to 1.00%; W: 0% to 1.000%; B: 0% to 0.0100%; Sn: 0% to 1.00%; Sb: 0% to 0.20%; one or two or more selected from the group of Ca, Ce, Mg, Zr, La, and REM: 0% to 0.0100% in total; and a remainder including Fe and impurities, wherein a microstructure at a ¼ thickness position from a surface in a sheet thickness direction includes, by vol %, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, has a proportion of unrecrystallized ferrite in the ferrite of 5% or less, and in the microstructure of the steel sheet stretched by 10% at the ¼ thickness position from the surface in the sheet thickness direction, a number density of voids having a maximum diameter of 1.0 μm or more is 1.0×10.sup.9 pieces/m.sup.2 or less.

2. The steel sheet according to claim 1, wherein the composition further includes, by mass %, one or two or more selected from the group of: Nb: 0.005% to 0.060%; Ti: 0.015% to 0.100%; V: 0.010% to 0.500%; Cr: 0.05% to 1.00%; Ni: 0.05% to 1.00%; Cu: 0.05% to 1.00%; Mo: 0.03% to 1.00%; W: 0.030% to 1.000%; B: 0.0005% to 0.0100%; Sn: 0.01% to 1.00%; Sb: 0.005% to 0.20%; and one or two or more selected from the group of Ca, Ce, Mg, Zr, La, and REM: 0.0001% to 0.0100% in total.

3. The steel sheet according to claim 1, wherein an average grain size of the ferrite in the microstructure is 6.0 μm to 15.0 μm.

4. The steel sheet according to claim 1, comprising a galvanized layer on the surface.

5. The steel sheet according to claim 1, comprising a zinc alloy plated layer on the surface.

6. The steel sheet according to claim 4, wherein a Fe content in the galvanized layer or the zinc alloy plated layer is 7.0% to 13.0% by mass %.

7. A method of manufacturing the steel sheet according to claim 1, comprising: a hot rolling process of heating a steel piece having the composition according to claim 1 to 1150° C. to 1320° C., completing hot rolling such that a hot rolling completion temperature is 850° C. to 930° C., starting cooling after 1.5 s or longer, and cooling the steel piece to a temperature range of 500° C. or lower to obtain a hot-rolled steel sheet such that an average cooling rate in a temperature range of a cooling start temperature to 500° C. is 20° C./s or faster; a reheating process of heating the hot-rolled steel sheet to a temperature range of 500° C. to 700° C.; a cooling process of cooling the hot-rolled steel sheet to room temperature; a cold rolling process of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet such that a total rolling reduction is 30% to 90% and a cold rolling completion temperature is 120° C. to 250° C.; and an annealing process of heating the cold-rolled steel sheet to an annealing temperature of 720° C. to 850° C. and cooling to a temperature range of 500° C. or lower, wherein in the hot rolling process, Expression (1) is satisfied in a temperature range of 1000° C. or lower, in the reheating process, Expression (2) is satisfied in the temperature range of 500° C. to 700° C., in the annealing process, a tension of 20 MPa or higher is applied and Expression (3) is satisfied in a temperature range of 720° C. to the annealing temperature during heating to the annealing temperature, and Expression (4) is satisfied in a temperature range of 720° C. to 500° C. during cooling from the annealing temperature, $\begin{array}{cc}{D}_0=\text{?}& \mathrm{Expression}\left(1\right)\end{array}$ $p_i=a_8.Math.\exp \left(\frac{a_9}{T_i+273}\right)$ $q_i=a_{10}.Math.\exp \left(\frac{a_{11}}{T_i+273}\right)$ $D_i=D_{i-1}.Math.\frac{h_i}{2h_{i-1}}.Math.\exp \left(a_4.Math.{\left(\frac{t_i}{p_i}\right)}^{\mathrm{a5}}\right)+\text{?}.Math.\frac{h_{i-1}-h_i}{2h_{i-1}}.Math.\{1-\exp \left(a_4.Math.{\left(\frac{t_i}{p_i}\right)}^{\mathrm{a5}}\right)+\exp \left(a_6.Math.{\left(\frac{t_i}{q_i}\right)}^{a_7}\right)+\text{?}.Math.\frac{1}{2}.Math.\left\{1-\exp \left(a_6.Math.{\left(\frac{t_i}{q_i}\right)}^{a_7}\right)\right\}$ $D_n\le 12.5$ $\text{?}\text{indicates text missing or illegible when filed}$ in Expression (1), D.sub.n represents an index representing a degree of progress of precipitation of a fine carbide in a temperature range of 1000° C. or lower of the hot rolling process, and reference numerals in Expression (1) are as follows, n: the number of rolling passes in the temperature range of 1000° C. or lower, T.sub.i: a rolling temperature in an i-th pass rolling, t.sub.i: an elapsed time [s] from the i-th pass rolling to an i+1-th pass rolling or an elapsed time [s] taken until a steel sheet temperature decreases to 850° C. from the i-th pass rolling, h.sub.i−1: a sheet thickness [mm] before the i-th pass rolling in the temperature range of 1000° C. or lower, h.sub.i: a sheet thickness [mm] after the i-th pass rolling in the temperature range of 1000° C. or lower, and $\begin{array}{cc}{a}_{1\mathrm{to}11}:\mathrm{constants}\left(a_1=2.54⨯{10}^{-6},a_2=3.62⨯{10}^{-4},a_3=-6.38⨯{10}^{-1},a_4=-3.⨯{10}^{-1},a_5=8.5⨯{10}^{-1},a_6=-8.5⨯{10}^{-4},a_7=2.4⨯{10}^0,a_8=7.83⨯{10}^{-13},a_9=2.8⨯{10}^5,a_{10}=6.⨯{10}^{-12},\mathrm{and}{a}_{11}=2.8⨯{10}^5\right),& \mathrm{Expression}\left(2\right)\end{array}$ $t_n={10}^{\frac{T_{n-1}+273}{T_n+273}{\log }_{10}{t}_{n-1}-\left(1-\frac{T_{n-1}+273}{T_n+273}\right).Math.20.Math.\left(1+0.08\mathrm{Si}\right)}+\Delta t_K$ $K_n=\left(T_n+273\right).Math.\{{\log }_{10}{t}_n+20.Math.\left(1+0.08\mathrm{Si}\right))$ $K_{20}\ge 1.5⨯{10}^4$ in Expression (2), K.sub.20 represents an index representing a degree of progress of precipitation of the fine carbide in a 20th period when a temperature history in the temperature range of 500° C. to 700° C. of the reheating process is divided into 20 periods with respect to time, and reference numerals in Expression (2) are as follows, T.sub.n: an average temperature [° C.] in an n-th period when a temperature history in the temperature range of 500° C. to 700° C. is divided into 20 periods with respect to time, Δt.sub.K: a time [hr.] in one of 20 periods into which a total residence time in the temperature range of 500° C. to 700° C. is divided, where t.sub.1=Δt.sub.K, and Si: a Si content [mass %], $\begin{array}{cc}1.\le \underset{i=1}{\overset{10}{.Math.}}\frac{d_1}{K_{20}}.Math.\exp \left(\frac{d_2}{T_i}\right).Math.t^{\mathrm{\prime 0}\text{.5}}\le 20.& \mathrm{Expression}\left(3\right)\end{array}$ reference numerals in Expression (3) are as follows, K.sub.20: a value obtained by Expression (2), d.sub.1 and d.sub.2: constants (d.sub.1=9.67×10.sup.10 and d.sub.2=1.25×10.sup.4), T.sub.i: an average heat treatment temperature [° C.] in an i-th period when a temperature history in the temperature range of 720° C. to the annealing temperature is divided into 10 periods with respect to time, and t′: 1/10 [s] of a residence time in the temperature range of 720° C. to the annealing temperature, $\begin{array}{cc}\underset{i=1}{\overset{10}{.Math.}}\left(g_1+g_2.Math.{\mathrm{Nb}}^{0.5}+g_3.Math.\mathrm{Ti}{\star }^{0.5}\right).Math.{\left(1+g_4.Math.{\mathrm{Mo}}^{0.5}\right)}^{-1}.Math.{\left(\frac{A_{c3}-T_{\max }}{A_{c3}-A_{c1}}\right)}^{-1/3}.Math.\left({\Delta }_i+g_5.Math.{\Delta }_i^{0.5}\right).Math.\exp \left(-\frac{g_6}{T_i+273}\right).Math.t^{\mathrm{\prime 0}\text{.5}}\ge 1.& \mathrm{Expression}\left(4\right)\end{array}$ reference numerals in Expression (4) are as follows, Δ.sub.i: 750−18×Si−17×Mn−10×Cr−8×Ni+15×Al−Ti, where each of the elements represents a content by mass % of the element, and when the element is not included, 0 is substituted as the content of the element, when a calculated value of Δ.sub.i is a negative value, Δ.sub.i is set to 0, g.sub.1 to 6: constants (g.sub.1=1.00×10.sup.−1, g.sub.2=1.46×10.sup.−1, g.sub.3=1.14×10.sup.−1, g.sub.4=2.24×10.sup.0, g.sub.5=4.53×10.sup.0, and g.sub.6=4.83×10.sup.3), Nb, Mo, Si, Mn, Cr, Ni, and Al: a content [mass %] of each of the elements, where when the element is not included, 0 is substituted as the content of the element, Ti*: an effective Ti content represented by Ti−42/14×N, where Ti and N represent a content [mass %] of each of the elements, when the element is not included, 0 is substituted as the content of the element, a minimum value is set to 0, T.sub.i: an average heat treatment temperature [° C.] in an i-th period when a temperature history in the temperature range of 720° C. to 500° C. is divided into 10 periods with respect to time, Ac.sub.1 and Ac.sub.3: a transformation start temperature and a transformation completion temperature [° C.] during heating, T.sub.max: a highest heating temperature [° C.] in a heat treatment process, and t′: 1/10 [s] of a residence time in the temperature range of 720° C. to 500° C.

8. The method of manufacturing a steel sheet according to claim 7, wherein during cooling in the annealing process, hot-dip galvanizing is performed on the cold-rolled steel sheet.

9. The method of manufacturing a steel sheet according to claim 7, wherein during cooling in the annealing process, hot-dip zinc alloy plating is performed on the cold-rolled steel sheet.

10. The method of manufacturing a steel sheet according to claim 8, wherein during cooling in the annealing process, alloying is performed after the hot-dip galvanizing or the hot-dip zinc alloy plating.

Description

EXAMPLES

[0281] Next, examples of the present invention will be described, but conditions of the examples are merely exemplary to confirm the operability and the effects of the present invention. The present invention is not limited to these condition examples. The present invention can adopt various conditions within a range not departing from the scope of the present invention as long as the object of the present invention can be achieved under the conditions.

[0282] Next, examples of the present invention will be described, but conditions of the examples are merely exemplary to describe the operability and the effects of the present invention. The present invention is not limited to these condition examples. The present invention can adopt various conditions within a range not departing from the scope of the present invention as long as the object of the present invention can be achieved under the conditions.

[0283] Molten steels having chemical compositions shown in Table 1 were cast to manufacture steel pieces. Next, hot-rolled steel sheets were obtained by hot-rolling the steel pieces under conditions shown in Table 2. Tables 2 shows D.sub.n obtained from hot rolling conditions in a temperature range of 1000° C. or lower in the hot rolling process and Expression (1).

[0284] Next, the steel pieces were reheated under conditions shown in Table 2. Table 2 shows K.sub.20 obtained from the temperature history in a temperature range of 500° C. to 700° C. in the reheating process and Expression (2). After reheating, the hot-rolled steel sheets were cooled to room temperature (25° C.) at an average cooling rate of 10° C./s or slower.

[0285] Next, steel sheets were obtained by performing cold rolling, a heat treatment (annealing), and temper rolling on the hot-rolled steel sheets under conditions shown in Tables 3-1 and 3-2. For annealing, the steel sheets were heated to annealing temperatures shown in Tables 3-1 and 3-2, were retained at the temperatures for 3 seconds to 200 seconds (the time required until the annealing temperature reached 720° C. again from the range of 720° C. or higher through the retention in the range of 720° C. to 850° C. during heating was 3 seconds to 200 seconds), and subsequently were cooled.

[0286] Tables 3-1 and 3-2 show the values of the middle side of Expression (3) obtained from a temperature history in a range of 720° C. to the annealing temperature during heating in the annealing process and Expression (3). In addition, Tables 3-1 and 3-2 show the values of the left side of Expression (4) and a temperature history in a temperature range of 720° C. to 500° C. during cooling in the annealing process and Expression (4).

[0287] The plating process in Tables 3-1 and 3-2 is as follows.

[0288] Zn alloy plating: a process of cooling the steel sheet to a temperature range of 500° C. or lower in the annealing process, dipping the steel sheet in a molten zinc alloy bath, and cooling the steel sheet to room temperature to obtain a zinc alloy plated steel sheet.

[0289] Alloy Galvannealing: a process of cooling the steel sheet to a temperature range of 500° C. or lower in the annealing process, dipping the steel sheet in a molten zinc alloy bath and reheating the steel sheet to 580° C. for alloying and cooling the steel sheet to room temperature to obtain an alloy galvannealed steel sheet.

[0290] GA: a process of cooling the steel sheet to a temperature range of 500° C. or lower in the annealing process, dipping the steel sheet in a molten zinc bath and reheating the steel sheet to 560° C. for alloying and cooling the steel sheet to room temperature to obtain a hot-dip galvannealed steel sheet (GA).

[0291] GI: a process of cooling the steel sheet to a temperature range of 500° C. or lower in the annealing process, dipping the steel sheet in a molten zinc bath, and cooling the steel sheet to room temperature to obtain a hot-dip galvanized steel sheet (GI).

[0292] Deposition: a process of performing deposition plating after the annealing process to obtain a galvanized steel sheet.

[0293] EG: a process of performing electrogalvanizing after the annealing process to obtain an electrogalvanized steel sheet (EG).

[0294] Tables 4-1 and 4-2 show the properties of the steel sheets obtained under the manufacturing conditions shown in Tables 1 to 3-2. As the results of the structure observation performed using the above-described method, Tables 4-1 and 4-2 show the volume percentage of ferrite, the proportion of unrecrystallized ferrite in ferrite, the volume percentage of martensite, the volume percentage of residual austenite, and the average grain size of ferrite. The proportion of unrecrystallized ferrite in ferrite was measured OIM Data Collection and OIM Data Analysis manufactured by TSL. In addition, Tables 4-1 and 4-2 also show the number density of voids having a maximum diameter of 1.0 μm or more measured using the above-described method. The sheet thickness of the steel sheet was the same as the sheet thickness after rolling shown in Tables 3-1 and 3-2.

[0295] Regarding the alloyed steel sheet, the Fe content in the alloyed hot-dip galvanized layer (galvannealed layer) or the alloyed hot-dip zinc alloy plated layer (alloy galvannealed layer) was measured using the above-described method.

[0296] The plated layers in Tables 4-1 and 4-2 are as follows.

[0297] Zn alloy plated: zinc alloy plated layer

[0298] Alloy Galvannealed: alloy galvannealed layer

[0299] GA: hot-dip galvannealed layer formed by dipping the steel sheet in a molten zinc bath and alloying the steel sheet

[0300] GI: hot-dip galvanized layer formed by dipping the steel sheet in a molten zinc bath

[0301] Deposited: galvanized layer formed by deposition plating

[0302] EG: galvanized layer formed by electrogalvanizing

[0303] Tables 5-1 and 5-2 show the properties of the steel sheets obtained under the manufacturing conditions of Tables 1 to 3-2. The yield strength (YS) and the maximum tensile strength were obtained by performing a tensile test. A 5 test piece was prepared according to JIS Z 2241:2011, and the rolling direction of the steel sheet is set as a tension axis to perform the tensile test. A steel sheet where the maximum tensile strength in the tensile test was 340 MPa or higher was determined to have excellent strength and as “Pass”. On the other hand, a steel sheet where the maximum tensile strength in the tensile test was lower than 340 MPa was determined to have poor strength and as “Fail”. In addition, a steel sheet where the uniform elongation (uEl) obtained from the tensile test was 15% or more was determined to have excellent formability and as “Pass”. On the other hand, a steel sheet where the uniform elongation was less than 15% was determined to have poor formability and as “Fail”.

[0304] A tensile test was performed under the same conditions as those of the above-described tensile test, and a strain of 15% was applied and unloaded. A semi-circular notch having a radius of 1.0 mm was provided at both ends of the center of a parallel body of the test piece, and the tensile test was performed again until the test piece was fractured at −40° C. As a result, a breaking stress σ.sub.2 at −40° C. and a maximum stress σ.sub.1 before unloading were obtained.

[0305] Next, a Charpy impact test was performed. When the sheet thickness of the steel sheet was less than 2.5 mm, a stacked Charpy test piece obtained by stacking the steel sheets until the total sheet thickness exceeded 5.0 mm, fastening the steel sheets by bolts, and providing a V-notch having a depth of 2 mm was used as a test piece. The other conditions were determined according to ITS Z 2242:2018. As a result, a ductile-brittle transition temperature where a brittle fracture surface ratio was 50% or more was obtained.

[0306] Regarding a steel sheet where a value (σ2/σ1) obtained by dividing the breaking stress σ.sub.2 at −40° C. by the maximum stress σ.sub.1 before unloading that were obtained using the above-described method was 0.70 or less and the ductile-brittle transition temperature where a brittle fracture surface ratio was 50% or more was −40° C. or lower, this steel sheet was determined to have sufficiently high deformability during impact deformation after forming (excellent impact resistance) and determined as “Pass”.

[0307] On the other hand, regarding a steel sheet where σ2/σ1 was more than 0.70 and/or the ductile-brittle transition temperature where the brittle fracture surface ratio was 50% or more was higher than −40° C., this steel sheet was determined to have poor impact resistance and as “Fail”.

TABLE-US-00001 TABLE 1 Composition, mass %, Remainder including Fe and Impurities Steel C Si Mn Al P S N O Others Note A 0.063 0.126 1.87 0.073 0.016 0.0026 0.0049 0.0013 Example B 0.025 0.390 1.34 0.049 0.006 0.0035 0.0016 0.0011 Example C 0.130 0.374 1.68 0.144 0.023 0.0071 0.0070 0.0019 Example D 0.171 0.200 0.79 0.261 0.004 0.0015 0.0042 0.0015 Zr: 0.0013 Example E 0.092 0.006 1.29 0.054 0.029 0.0052 0.0112 0.0010 Ti: 0.015, Example Nb: 0.012 F 0.083 0.749 0.26 0.087 0.041 0.0063 0.0077 0.0009 Example G 0.071 0.521 0.09 0.097 0.023 0.0026 0.0053 0.0008 Ni: 0.61, Example Cu: 0.13 H 0.187 0.044 1.54 0.330 0.007 0.0098 0.0021 0.0015 Ce: 0.0012, Example La: 0.0018 I 0.091 0.058 0.23 0.018 0.079 0.0024 0.0025 0.0020 Nb: 0.046 Example J 0.158 0.012 1.97 0.451 0.011 0.0091 0.0033 0.0016 V: 0.164 Example K 0.088 0.808 0.57 0.037 0.028 0.0074 0.0069 0.0024 Ni: 0.27, Example Cr: 0.27 L 0.088 1.341 0.34 0.317 0.020 0.0046 0.0056 0.0019 Cr: 0.71 Example M 0.015 0.938 1.72 0.029 0.003 0.0064 0.0015 0.0009 Example N 0.128 0.098 2.71 0.550 0.015 0.0021 0.0065 0.0016 Example O 0.144 0.123 0.67 0.066 0.007 0.0138 0.0020 0.0012 Ca: 0.0035 Example P 0.081 0.269 2.33 0.184 0.018 0.0048 0.0093 0.0012 Ti: 0.027 Example Q 0.048 0.028 1.59 0.714 0.020 0.0089 0.0022 0.0008 Mg: 0.0020 Example R 0.134 0.270 0.82 0.008 0.025 0.0087 0.0133 0.0012 Ti: 0.076 Example S 0.037 0.874 1.17 0.084 0.029 0.0045 0.0074 0.0034 Example T 0.091 0.716 0.36 0.025 0.012 0.0053 0.0007 0.0009 Sb: 0.08 Example U 0.141 0.916 0.18 0.027 0.010 0.0065 0.0016 0.0014 W: 0.093 Example V 0.109 0.388 1.09 0.084 0.035 0.0026 0.0045 0.0010 Sn: 0.17 Example W 0.060 0.298 1.66 0.106 0.052 0.0035 0.0035 0.0018 Example X 0.052 0.168 0.38 0.048 0.069 0.0014 0.0081 0.0006 Mo: 0.32 Example Y 0.118 0.385 0.69 0.052 0.017 0.0043 0.0103 0.0018 B: 0.0031 Example Z 0.102 0.066 1.45 0.215 0.004 0.0004 0.0089 0.0017 REM: 0.0013 Example AA 0.006 0.672 1.33 0.019 0.012 0.0035 0.0032 0.0016 Comparative Example AB 0.227 0.227 1.42 0.052 0.007 0.0030 0.0056 0.0014 Comparative Example AC 0.123 1.725 0.70 0.048 0.020 0.0031 0.0027 0.0017 Comparative Example AD 0.078 0.250 3.31 0.060 0.014 0.0040 0.0056 0.0010 Comparative Example The underline represents that the value is outside of the range of the present invention.

TABLE-US-00002 TABLE 2 Hot Rolling Process Average Cooling Rate from Steel Cooling Reheating Process Hot- Piece Rolling Start Maximum Rolled Heating Completion Time to Temperature Reheating Steel Temperature Temperature Start of to 500° C. Temperature K.sub.20 Steel Sheet ° C. ° C. D.sub.n Cooling ° C./s ° C. ×10.sup.4 Note A A1 1185 907 11.9  3.2 69 534 1.68 Example A A2 1265 904 8.1 1.6 26 604 1.63 Example A A3 1286 915 14.5  3.1 41 644 1.60 Comparative Example A A4 1306 882 7.2 2.9 65 478 — Comparative Example B B1 1161 871 8.0 2.0 34 612 1.55 Example B B2 1236 904 8.7 2.3 51 734 — Comparative Example B B3 1216 885 11.7  2.2 41 589 1.67 Example C C1 1184 916 10.3  1.7 82 674 1.66 Example C C2 1278 925 8.9 2.3 46 597 1.78 Example C C3 1130 912 11.2  2.2 26 562 1.72 Comparative Example D D 1191 908 8.8 2.5 34 630 1.86 Example E E 1269 881 6.9 3.0 41 571 1.58 Example F F1 1216 920 10.4  1.9 78 685 1.69 Example F F2 1314 881 9.5 2.9 38 617 1.79 Example F F3 1261 905 8.3 2.9 14 520 1.55 Comparative Example G G 1246 898 10.2  2.7 40 630 1.65 Example H H 1250 857 7.5 3.2 42 538 1.52 Example I I 1302 913 10.0  2.6 47 663 1.64 Example J J 1262 880 9.1 3.4 54 544 1.67 Example K K1 1169 901 11.0  3.1 39 522 1.74 Example K K2 1222 876 6.6 2.3 23 693 1.81 Example K K3 1278 839 — 2.2 37 620 1.90 Comparative Example L L 1283 887 6.9 3.1 47 561 1.72 Example M M 1224 874 9.0 2.0 56 623 1.81 Example N N 1250 915 10.9  2.6 44 680 1.61 Example O O 1260 885 5.5 2.4 47 608 1.64 Example P P1 1215 906 8.3 1.8 63 581 1.80 Example P P2 1209 911 7.3 3.1 35 506 1.65 Example P P3 1281 892 10.8  2.8 38 539 1.45 Comparative Example P P4 1247 906 10.5  0.8 32 601 1.52 Comparative Example Q Q 1226 883 6.0 3.4 35 584 1.61 Example R R 1236 864 8.4 2.9 50 600 1.56 Example S S 1173 880 7.1 2.0 61 577 1.56 Example T T 1196 894 7.9 1.7 66 656 1.69 Example U U 1212 908 9.9 2.3 56 620 1.94 Example V V 1183 879 5.9 2.6 38 661 1.81 Example W W1 1175 877 9.6 2.7 33 684 2.00 Example W W2 1183 888 9.3 2.4 50 520 1.52 Example W W3 1217 948 8.1 2.4 34 565 1.80 Comparative Example W W4 1348 902 8.9 2.9 27 568 1.59 Comparative Example X X 1242 900 7.1 3.0 73 568 1.73 Example Y Y 1246 913 7.3 2.6 37 545 1.75 Example Z Z 1272 907 9.4 2.5 47 566 1.74 Example AA AA 1243 905 7.8 2.3 52 629 1.93 Comparative Example AB AB 1270 895 9.5 2.4 45 620 1.72 Comparative Example AC AC 1233 893 9.5 3.0 41 595 1.86 Comparative Example AD AD 1222 871 6.6 2.5 40 544 1.69 Comparative Example The underline represents that the value is outside of the range of the present invention.

TABLE-US-00003 TABLE 3-1 Cold Rolling Process Sheet Sheet Annealing Process Hot- Thickness Thickness Total Rolling Heating Rolled before after Rolling Completion Annealing Steel Rolling Rolling Reduction Temperature Temperarure Example Steel Sheet mm mm % ° C. ° C. 1 A A1 3.4 1.0 71 156 760 2 A A1 3.4 1.3 62 153 726 3 A A1 3.4 1.0 71 189 868 4 A A1 3.4 1.0 71 181 783 5 A A2 5.0 1.5 70 178 768 6 A A2 5.0 0.4 92 162 758 7 A A3 1.8 0.6 67 143 743 8 A A4 3.2 1.2 63 131 788 9 B B1 4.0 1.0 75 213 751 10 B B2 3.8 1.6 58 205 760 11 B B3 2.4 1.2 50 186 799 12 C C1 1.6 0.4 75 149 829 13 C C1 1.6 0.8 50 138 783 14 C C2 2.3 0.6 74 138 805 15 C C3 2.0 0.7 65 161 785 16 D D 3.2 1.3 59 139 787 17 E E 2.2 1.1 50 173 780 18 F F1 3.4 1.3 62 205 816 19 F F1 3.4 1.7 50 135 800 20 F F2 4.4 2.6 41 141 776 21 F F3 5.0 1.7 66 175 784 22 G G 3.0 1.4 53 175 794 23 H H 2.6 1.6 38 131 736 24 I I 4.0 1.6 60 138 795 25 J J 2.8 1.2 57 180 778 26 K K1 4.0 1.2 70 170 799 27 K K1 4.0 1.7 58 153 681 28 K K2 5.0 3.0 40 136 749 29 K K3 2.6 1.5 42 141 784 30 L L 1.8 1.1 39 140 828 Temper Annealing Process Rolling Process Heating Cooling Total Middle Left Rolling Tension Side of Side of Reduction Plating Example MPa Expression (3) Expression (4) % Process Note 1 40  4.9 2.3 0.45 Example 2 26  7.3 1.8 0.47 GA Example 3 33 — — 0.59 Comparative Example 4 31 24.3 1.5 0.13 Comparative Example 5 38  7.3 1.9 0.31 Example 6 28 10.5 1.4 0.35 Comparative Example 7 26  3.2 1.7 0.46 Comparative Example 8 25 — 1.6 0.40 Comparative Example 9 27  6.5 1.1 0.14 GI Example 10 29 — 1.7 0.36 Comparative Example 11 39 13.7 1.5 0.33 Example 12 34 19.0 2.8 0.32 EG Example 13 39  9.9 0.7 0.23 Comparative Example 14 39 14.8 1.6 0.52 Example 15 40  8.6 1.5 0.24 Comparative Example 16 39 16.7 2.1 0.10 Deposition Example 17 38  6.0 2.8 1.10 Example 18 24 14.7 1.5 — GA Example 19 40 10.7 1.8 0.36 Example 20 33  8.3 1.6 0.48 Example 21 29 15.5 1.9 0.21 Comparative Example 22 30 13.1 1.2 0.84 Deposition Example 23 29  3.0 1.4 0.49 Example 24 35 10.9 2.6 0.40 EG Example 25 33 14.7 1.5 0.17 Example 26 39 11.9 1.7 0.44 GI Example 27 27 — — 0.26 Comparative Example 28 29 35   1.3 0.64 Example 29 38 13.3 1.3 0.13 Comparative Example 30 37  9.9 1.7 0.49 GA Example The underline represents that the value is outside of the range of the present invention.

TABLE-US-00004 TABLE 3-2 Cold Rolling Process Sheet Sheet Annealing Process Hot- Thickness Thickness Total Rolling Heating Rolled before after Rolling Completion Annealing Steel Rolling Rolling Reduction Temperature Temperature Example Steel Sheet mm mm % ° C. ° C. 31 M M 4.4 1.9 57 207 816 32 N N 2.4 1.3 46 176 781 33 O O 2.2 1.1 50 189 768 34 P P1 2.6 0.9 65 127 833 35 P P1 2.6 0.9 65 135 754 36 P P2 3.4 1.2 65 238 786 37 P P2 2.8 2.2 21 123 796 38 P P3 4.2 1.4 67 128 786 39 P P4 1.6 0.6 63 167 795 40 Q Q 1.4 0.5 64 148 845 41 R R 3.8 1.4 63 161 751 42 S S 3.4 1.9 44 163 777 43 T T 3.0 1.3 57 167 810 44 U U 4.6 2.4 48 139 816 45 V V 2.0 0.9 55 124 765 46 W W1 3.8 0.6 84 194 748 47 W W1 3.8 1.4 63 150 792 48 W W2 5.0 2.8 44 219 828 49 W W3 3.8 1.5 61 216 774 50 W W4 2.2 0.9 59 138 782 51 X X 4.8 2.5 48 139 806 52 Y Y 3.6 1.8 50 147 776 53 Z Z 3.6 1.2 67 129 743 54 AA AA 4.4 1.2 73 196 790 55 AB AB 2.4 1.2 50 178 794 56 AC AC 1.6 1.0 38 183 815 57 AD AD 2.2 0.6 73 163 748 58 A A1 3.4 1.2 65  56 752 59 A A1 3.4 0.8 76 306 756 Temper Annealing Process Rolling Process Heating Cooling Total Middle Left Rolling Tension Side of Side of Reduction Plating Example MPa Expression (3) Expression (4) % Process Note 31 26 8.9 1.7 0.34 Example 32 32 15.0  1.5 0.52 GI Example 33 35 2.6 1.4 0.36 Zn Alloy Example Plating 34 27 13.4  5.7 0.36 Alloy Example Galvannealing 35 24 0.8 1.6 0.80 Comparative Example 36 29 9.8 2.6 0.59 Example 37 37 12.7  1.7 0.24 Comparative Example 38 25 7.9 1.3 0.59 Comparative Example 39 31 8.7 1.3 0.25 Comparative Example 40 32 17.9  1.5 0.47 Alloy Example Galvannealing 41 39 1.6 1.5 0.81 Example 42 31 5.8 1.5 0.43 GI Example 43 38 13.4  1.4 0.56 Example 44 28 6.8 1.5 0.27 EG Example 45 30 6.9 1.6 0.44 Example 46 31 2.2 1.6 0.73 GA Example 47 9 12.1  1.4 0.42 Comparative Example 48 30 12.2  1.2 0.35 Example 49 36 12.1  1.3 0.42 Comparative Example 50 34 11.1  1.7 0.46 Comparative Example 51 27 8.1 1.8 1.53 Example 52 24 8.0 1.8 0.67 Zn Alloy Example Plating 53 37 11.5  3.4 0.30 Example 54 31 5.4 1.4 0.70 Comparative Example 55 31 10.1  1.9 0.70 Comparative Example 56 39 13.6  1.7 0.70 Comparative Example 57 40 11.0  1.7 0.70 Comparative Example 58 27 17.3  1.4 0.25 Comparative Example 59 29 2.5 1.3 0.31 Comparative Example The underline represents that the value is outside of the range of the present invention.

TABLE-US-00005 TABLE 4-1 Properties of Steel Sheet Number Density of Voids Fe having Content Average Maximum in GA or Hot- Proportion of Grain Diameter Alloy Rolled Unrecrystallized Residual Size of of 1.0 μm or Galvannealed Steel Ferrite Ferrite Martensite Austenite Ferrite more Plated Layer Example Steel Sheet vol % % vol % vol % μm 10.sup.8 void/m.sup.2 Layer mass % Note  1 A A1 88 0 0 0 7.0 3.6 Example  2 A A1 91 0 0 0 8.6 5.0 GA 10.3 Example  3 A A1 74 0 0 0 6.4 8.2 Comparative Example  4 A A1 86 0 0 3 7.8 6.1 Comparative Example  5 A A2 88 0 0 0 10.1 6.7 Example  6 A A2 90 0 0 0 8.8 16.0  Comparative Example  7 A A3 90 0 0 0 7.5 14.5  Comparative Example  8 A A4 87 0 0 0 13.9 28.0  Comparative Example  9 B B1 95 0 0 0 8.3 4.4 GI 1.4 Example 10 B B2 95 0 0 0 11.2 17.1  Comparative Example 11 B B3 90 0 0 0 7.8 7.6 Example 12 C C1 81 0 1 1 8.2 3.8 EG Example 13 C C1 89 0 4 1 10.4 9.1 Comparative Example 14 C C2 84 0 0 0 8.7 7.1 Example 15 C C3 83 0 0 0 7.7 20.0  Comparative Example 16 D D 82 0 1 0 8.7 6.0 Deposited Example 17 E E 85 0 0 0 10.3 0.0 Example 18 F F1 83 0 0 0 8.9 3.9 GA 9.0 Example 19 F F1 84 0 0 1 9.4 2.0 Example 20 F F2 86 0 0 0 11.5 5.0 Example 21 F F3 85 0 0 0 9.6 13.7  Comparative Example 22 G G 85 0 0 0 8.9 4.5 Deposited Example 23 H H 81 0 0 0 12.6 1.8 Example 24 I I 85 2 0 0 6.3 3.0 EG Example 25 J J 82 0 0 0 8.7 2.7 Example 26 K K1 83 0 0 1 6.3 6.0 GI 0.4 Example 27 K K1 98 0 0 0 8.3 12.6  Comparative Example 28 K K2 88 0 0 0 13.0 5.6 Example 29 K K3 83 0 0 0 13.0 12.0  Comparative Example 30 L L 81 0 0 1 14.0 4.5 GA 12.0 Example The underline represents that the value is outside of the range of the present invention.

TABLE-US-00006 TABLE 4-2 Properties of Steel Sheet Number Density of Voids Fe having Content Proportion of Average Maximum in GA or Hot- Unrecrys- Grain Diameter Alloy Rolled tallized Residual Size of of 1.0 μm or Galvannealed Steel Ferrite Ferrite Martensite Austenite Ferrite more Plated Layer Example Steel Sheet vol % % vol % vol % μm 10.sup.8 void/m.sup.2 Layer mass % Note 31 M M 91 0 0 0 8.4 3.8 Example 32 N N 80 0 1 0 8.9 9.2 GI 0.9 Example 33 O O 83 0 0 0 15.2 2.6 Zn Alloy Example Plated 34 P P1 86 0 1 0 9.0 4.3 Alloy 10.0 Example Galvannealed 35 P P1 88 24  0 0 7.9 4.0 Comparative Example 36 P P2 83 0 0 1 11.6 6.1 Example 37 P P2 83 25  0 0 16.2 7.0 Comparative Example 38 P P3 84 0 0 0 8.0 16.9  Comparative Example 39 P P4 85 0 0 0 8.6 18.0  Comparative Example 40 Q Q 89 3 0 0 8.6 6.0 Alloy 8.6 Example Galvannealed 41 R R 84 3 0 0 6.7 8.0 Example 42 S S 92 0 0 0 12.4 2.7 GI 0.3 Example 43 T T 82 0 0 0 10.3 1.0 Example 44 U U 81 0 0 0 10.9 3.5 EG Example 45 V V 84 0 0 0 8.3 2.4 Example 46 W W1 90 0 0 0 7.9 8.2 GA 8.2 Example 47 W W1 86 0 0 0 7.6 14.0  Comparative Example 48 W W2 83 0 1 0 8.9 7.1 Example 49 W W3 89 0 0 0 10.1 11.6  Comparative Example 50 W W4 89 0 0 0 9.0 16.6  Comparative Example 51 X X 85 2 0 0 12.5 3.0 Example 52 Y Y 81 0 0 0 14.9 2.5 Zn Alloy Example Plated 53 Z Z 86 0 0 0 8.5 7.8 Example 54 AA AA 100  0 0 0 9.5 0.0 Comparative Example 55 AB AB 71 0 0 0 8.0 13.0  Comparative Example 56 AC AC 83 0 1 3 11.2 8.0 Comparative Example 57 AD AD 68 0 13  4 11.8 25.0  Comparative Example 58 A A1 90 0 1 0 8.3 12.0  Comparative Example 59 A A1 90 18  0 0 7.7 5.0 Comparative Example The underline represents that the value is outside of the range of the present invention.

TABLE-US-00007 TABLE 5-1 Hot- Properties Rolled Transition Steel YS TS uEl σ1 σ2 Temperature Example Steel Sheet MPa MPa % MPa MPa σ2/σ1 ° C. Note  1 A A1 215 408 18 388 147 0.38 −80 Example  2 A A1 210 392 20 378 183 0.48 −80 Example  3 A A1 227 479 14 467 268 0.57 −80 Comparative Example  4 A A1 254 437 18 422 222 0.53 −20 Comparative Example  5 A A2 246 412 20 391 229 0.59 −80 Example  6 A A2 206 410 21 396 305 0.77 −80 Comparative Example  7 A A3 239 388 20 369 265 0.72 −80 Comparative Example  8 A A4 212 408 20 390 380 0.97 −80 Comparative Example  9 B B1 229 366 22 351 150 0.43 −100  Example 10 B B2 227 366 23 349 278 0.80 −60 Comparative Example 11 B B3 217 393 19 374 211 0.56 −100  Example 12 C C1 276 489 16 464 219 0.47 −60 Example 13 C C1 278 499 20 486 302 0.62 −20 Comparative Example 14 C C2 250 435 17 413 180 0.44 −80 Example 15 C C3 269 440 16 425 306 0.72 −40 Comparative Example 16 D D 220 439 18 428 193 0.45 −40 Example 17 E E 234 391 19 377 160 0.42 −60 Example 18 F F1 281 454 18 435 208 0.48 −80 Example 19 F F1 251 473 17 462 151 0.33 −60 Example 20 F F2 286 462 18 443 184 0.42 −80 Example 21 F F3 291 468 17 452 330 0.73 −40 Comparative Example 22 G G 234 402 17 382 188 0.49 −100  Example 23 H H 214 438 16 422 120 0.28 −80 Example 24 I I 268 449 18 430 175 0.41 −60 Example 25 J J 205 417 16 406 134 0.33 −80 Example 26 K K1 268 474 16 464 209 0.45 −60 Example 27 K K1 248 408 24 396 288 0.73 −60 Comparative Example 28 K K2 281 442 19 423 189 0.45 −80 Example 29 K K3 260 457 16 438 328 0.75 −80 Comparative Example 30 L L 299 460 17 438 193 0.44 −60 Example The underline represents that the value is outside of the range of the present invention or represents undesirable properties.

TABLE-US-00008 TABLE 5-2 Hot- Properties Rolled Transition Steel YS TS uEl σ1 σ2 Tmperature Example Steel Sheet MPa MPa % MPa MPa σ2/σ1 °C Note 31 M M 277 411 21 397 175 0.44 −80 Example 32 N N 226 448 16 429 252 0.59 −40 Example 33 O O 209 409 18 389 184 0.47 −80 Example 34 P P1 232 450 18 431 211 0.49 −60 Example 35 P P1 325 442 14 424 220 0.52 −60 Comparative Example 36 P P2 270 458 18 444 215 0.48 −60 Example 37 P P2 342 474 12 455 212 0.47 −80 Comparative Example 38 P P3 263 437 17 426 302 0.71 −60 Comparative Example 39 P P4 257 423 17 409 308 0.75 −60 Comparative Example 40 Q Q 206 379 19 364 185 0.51 −80 Example 41 R R 256 463 17 444 195 0.44 −80 Example 42 S S 266 406 20 395 188 0.48 −80 Example 43 T T 236 458 18 440 157 0.36 −80 Example 44 U U 242 476 17 461 179 0.39 −80 Example 45 V V 261 435 17 414 133 0.32 −80 Example 46 W W1 294 434 19 423 231 0.55 −80 Example 47 W W1 265 475 18 464 333 0.72 −60 Comparative Example 48 W W2 286 497 17 476 256 0.54 −60 Example 49 W W3 272 418 20 407 295 0.72 −80 Comparative Example 50 W W4 270 426 18 413 321 0.78 −60 Comparative Example 51 X X 284 460 19 439 165 0.38 −100  Example 52 Y Y 199 443 16 421 169 0.40 −80 Example 53 Z Z 204 390 19 375 173 0.46 −80 Example 54 AA AA 179 314 24 349 99 0.28 −120  Comparative Example 55 AB AB 268 519 13 500 354 0.71 −80 Comparative Example 56 AC AC 321 566 17 547 257 0.47  20 Comparative Example 57 AD AD 412 779 12 748 663 0.89 280 Comparative Example 58 A A1 206 394 16 367 274 0.75 −60 Comparative Example 59 A A1 285 424 14 408 205 0.50 −60 Comparative Example The underline represents that the value is outside of the range of the present invention or represents undesirable properties.

[0308] Among steels A to AD shown in Table 1, the steels AA to AD are comparative examples where the composition was outside of the range defined by the present invention.

[0309] In the steel AA, the C content was lower than the range of the present invention. In the steel sheet according to Experiment Example 54 obtained using this steel, the maximum tensile strength was low.

[0310] In the steel AB, the C content was higher than the range of the present invention. In the steel sheet according to Experiment Example 55 obtained using this steel, the amount of ferrite was small, and the number density of voids was high. Therefore, the uniform elongation was low, and σ2/σ1 was high.

[0311] In the steel AC, the Si content was higher than the range of the present invention. In the steel sheet according to Experiment Example 56 obtained using this steel, the amount of residual austenite was large, and the ductile-brittle transition temperature was high.

[0312] In the steel AD, the Mn content was higher than the range of the present invention. In the steel sheet according to Experiment Example 57 obtained using this steel, the amount of ferrite was small, the amounts of martensite and residual austenite were large, and the number density of voids was high. Therefore, the uniform elongation was low, and σ2/σ1 and the ductile-brittle transition temperature were high.

[0313] Experiment Examples 7, 15, 21, 29, 39, 49, and 50 were comparative examples where the conditions of the hot rolling process were outside of the range of the present invention.

[0314] Experiment Example 7 was a comparative example in which D.sub.n was high and Expression (1) in the temperature range of 1000° C. or lower was not satisfied. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0315] Experiment Example 15 was a comparative example in which the steel piece heating temperature was low. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0316] Experiment Example 21 was a comparative example in which the average cooling rate in the temperature range of the cooling start temperature to 500° C. was low. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0317] Experiment Example 29 was a comparative example in which the hot rolling completion temperature was low. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0318] Experiment Example 39 was a comparative example in which the time required for the start of cooling after completion of hot rolling was short. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0319] Experiment Example 49 was a comparative example in which the hot rolling completion temperature was high. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0320] Experiment Example 50 was a comparative example in which the steel piece heating temperature was high. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0321] Experiment Examples 8, 10, and 38 were comparative examples in which the conditions of the reheating process were outside of the range of the present invention.

[0322] Experiment Example 8 was a comparative example in which the maximum reheating temperature in the reheating process was low. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0323] Experiment Example 10 was a comparative example in which the maximum reheating temperature in the reheating process was high. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0324] Experiment Example 38 was a comparative example in which K.sub.20 was low and Expression (2) in the temperature range of 500° C. to 700° C. was not satisfied. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0325] Experiment Examples 6, 37, 58, and 59 were comparative examples in which the conditions of the cold rolling process were outside of the range of the present invention.

[0326] Experiment Example 6 was a comparative example in which the total rolling reduction in the cold rolling process was high. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0327] Experiment Example 37 was a comparative example in which the total rolling reduction in the cold rolling process was low. Therefore, an excess amount of unrecrystallized ferrite remained, and the uniform elongation was low.

[0328] Experiment Example 58 was a comparative example in which the rolling completion temperature in the cold rolling process was low. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0329] Experiment Example 59 was a comparative example in which the rolling completion temperature in the cold rolling process was high. Therefore, an excess amount of unrecrystallized ferrite remained, and the uniform elongation was low.

[0330] Experiment Examples 3, 4, 13, 27, 35, and 47 were comparative examples where the conditions of the annealing process were outside of the range of the present invention.

[0331] Experiment Example 3 was a comparative example in which the annealing temperature was high. Therefore, the amount of ferrite was small, and the uniform elongation was low.

[0332] Experiment Example 27 was a comparative example in which the annealing temperature was low. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0333] Experiment Example 4 was a comparative example in which the value of the middle side of Expression (3) was high. Therefore, the amount of residual austenite was large, and the ductile-brittle transition temperature was high.

[0334] Experiment Example 13 was a comparative example in which the value of the left side of Expression (4) was low. Therefore, the amount of martensite was large, and the ductile-brittle transition temperature was high.

[0335] Experiment Example 35 was a comparative example in which the value of the middle side of Expression (3) was low. Therefore, an excess amount of unrecrystallized ferrite remained, and the uniform elongation was low.

[0336] Experiment Example 47 was a comparative example in which the tension applied in the temperature range of 720° C. to the annealing temperature was low. Therefore, the number density of voids was high, and σ2/σ1 was high.

[0337] Experiment Examples other than Comparative Examples described above were Examples according to the present invention. It was found that the steel sheets described as Examples were manufactured using the manufacturing method satisfying the manufacturing conditions according to the present invention and thus had excellent formability, strength, and resistance to impact and fracture.

[0338] Experiment Examples 2, 9, 12, 16, 18, 22, 24, 26, 30, 32, 33, 34, 40, 42, 44, 46, and 52 are examples where the plated steel sheets according to the present invention were obtained by performing plating.

[0339] Experiment Examples 9, 26, 32, and 42 were Examples in which a hot-dip galvanized steel sheet (GI) was obtained by cooling the steel sheet to 500° C. in the annealing process, dipping the steel sheet in a molten zinc bath, and cooling the steel sheet to room temperature.

[0340] Experiment Examples 2, 18, 30, and 46 were Examples in which hot-dip galvannealed steel sheet (GA) was obtained by cooling the steel sheet to 500° C. in the annealing process, dipping the steel sheet in a molten zinc bath and reheating the steel sheet to 560° C. for alloying and cooling the steel sheet to room temperature.

[0341] Experiment Examples 33 and 52 were Examples in which a zinc alloy plated steel sheet was obtained by cooling the steel sheet to 500° C. in the annealing process, dipping the steel sheet in a molten zinc alloy bath, and cooling the steel sheet to room temperature.

[0342] Experiment Examples 34 and 40 were Examples in which an alloy galvannealed steel sheet was obtained by cooling the steel sheet to 500° C. in the annealing process, dipping the steel sheet in a molten zinc alloy bath and reheating the steel sheet to 580° C. for alloying and cooling the steel sheet to room temperature.

[0343] Experiment Examples 16 and 22 were Examples in which a galvanized steel sheet was obtained by performing deposition plating in the annealing process before temper rolling.

[0344] Experiment Examples 12, 24, and 44 were Examples in which an electrogalvanized steel sheet (EG) was obtained by performing electrogalvanizing after the annealing process.

INDUSTRIAL APPLICABILITY

[0345] As described above, according to the present invention, a high strength steel sheet having excellent formability, resistance to impact and fracture, and toughness can be provided. The steel sheet according to the present invention is a steel sheet that is suitable for a significant reduction in the weight of a vehicle and for ensuring the protection and safety of a passenger. Therefore, the present invention is highly applicable to the steel sheet manufacturing industry and the automobile industry.