STEEL SHEET AND METHOD OF MANUFACTURING THE SAME

Abstract

A steel sheet includes a predetermined composition satisfying Expression (1), in which the microstructure at the ¼ thickness position from the surface in the sheet thickness direction includes, by vol %, ferrite: 95% or more and a remainder of the microstructure: 5% or less, has a proportion of unrecrystallized ferrite in the ferrite of 5% or less, and a half width w and an X-ray wavelength λ at a peak of (200) plane of the ferrite satisfy Expression (2).

0.80≤{(Ti/48−N/14)+Nb/93}/(C/12)≤5.00  (1)

w×λ≥0.20  (2)

Claims

1. A steel sheet comprising, as a composition, by mass %: C: 0.0003% to 0.0100/a; Si: 0.005% to 1.5000; Mn: 0.010% to 3.000%; 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; V: 0% to 0.50%; Cr: 0% to 1.00%; Ni: 0% to 1.00%; Cu: 0% to 1.00%; Mo: 0% to 1.00%; W: 0% to 1.00%; 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; one or two selected from the group of Ti: 0.010% to 0.100% and Nb: 0.005% to 0.060%; and a remainder including Fe and impurities, wherein Expression (1) is satisfied, a microstructure at a ¼ thickness position from a surface in a sheet thickness direction includes, by vol %, ferrite: 95% or more and a remainder of the microstructure: 5% or less, has a proportion of unrecrystallized ferrite in the ferrite of 5% or less, and a half width w and an X-ray wavelength λ at a peak of (200) plane of the ferrite satisfy Expression (2),
0.80≤{(Ti/48−N/14)+Nb/93}/(C/12)≤5.00  (1),
w×λ≥0.20  (2), wherein each of Ti, N, Nb, and C in Expression (1) represents a content by mass % of the element, and when the element is not included, 0 is substituted as the content of the element.

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: V: 0.01% to 0.50%; 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: 0.03% to 1.00%; 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 1200° C. to 1320° C., completing hot rolling such that a hot rolling completion temperature is 880° C. or higher, and cooling the steel piece to obtain a hot-rolled steel sheet such that an average cooling rate in a temperature range of the hot rolling completion 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 60% to 90% and a cold rolling completion temperature is 250° C. or lower; an annealing process of heating the cold-rolled steel sheet to an annealing temperature of 700° C. to 850° C. and cooling to a temperature range of 80° C. or lower; and a temper rolling process of performing temper rolling on the cold-rolled steel sheet such that a total rolling reduction is 0.05% to 2.00%, wherein in the reheating process, Expression (3) is satisfied in a temperature range of 500° C. to 700° C., and in the annealing process, Expression (4) is satisfied in a temperature range of 700° C. to the annealing temperature during heating to the annealing temperature, and Expression (5) is satisfied in the temperature range of 500° C. to 700° C. during cooling from the annealing temperature, and bending is performed while applying a tension of 20 MPa or higher in a temperature range of 80° C. to 500° C., $\begin{array}{cc}{t}_n=10\frac{T_{n-1}+273}{T_n+273}.Math.{\log }_{10}{t}_{n-1}-\left(1-\frac{T_{n-1}+273}{T_n+273}\right).Math.21.Math.\left(1+2.5.Math.\sqrt{C.Math.\left(\mathrm{Nb}+4\mathrm{Ti}\right)}\right)+\Delta t_K{K}_n=\left(T_n+273\right).Math.\left\{{\log }_{10}{t}_n+21.Math.\left(1+2.5.Math.\sqrt{C.Math.\left(\mathrm{Nb}+4\mathrm{Ti}\right)}\right)\right\}{K}_{20}\ge 1.5\times {10}^4& \mathrm{Expression}\left(3\right)\end{array}$ in Expression (3), K.sub.20 represents an index representing a degree of progress of precipitation of a Ti and/or Nb carbonitride 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, t.sub.n and K.sub.n are calculated when the 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 an average temperature in an n-th period is represented by T.sub.n [° C.], Δt.sub.K represents a time [hr.] in one of 20 periods into which a total residence time in the same temperature range is divided, each of C, Nb, and Ti represents a content [mass %] of the element, and t.sub.1=Δt.sub.K, $\begin{array}{cc}{R}_m=A.Math.\frac{\Delta t_R^{0.5}}{K_{20}}.Math.\exp \left(-\frac{B}{T_m}\right)1.\le \underset{i=1}{\overset{10}{.Math.}}{R}_i\le 15.& \mathrm{Expression}\left(4\right)\end{array}$ in Expression (4), R.sub.i represents an index representing a degree of progress of recrystallization in the temperature range of 700° C. to the annealing temperature and a degree of progress of diffusion of C from a Ti and/or Nb carbonitride present in a grain boundary into crystal grains, R.sub.m is calculated when a temperature history of the steel sheet from 700° C. to the annealing temperature during heating in the annealing process is divided into 10 periods with respect to time and an average temperature in an m-th period is represented by T.sub.m [° C.], Δt.sub.R represents a time [s] in one of 10 periods into which a total residence time in the temperature range of 700° C. to the annealing temperature is divided, K.sub.20 is a value obtained by Expression (3), and A and B represent constant terms, A represents 9.67×10.sup.9, and B represents 1.25×10.sup.4, and $\begin{array}{cc}{P}_k=D.Math.\left\{E.Math.{\left(700-T_k\right)}^{1.5}+\left(700-T_k\right)\right\}.Math.\exp \left(-\frac{F}{T_k}\right).Math.\Delta t_p^{0.5}1.\le R_{10}.Math.\underset{j=1}{\overset{10}{.Math.}}{p}_j\le 15.& \mathrm{Expression}\left(5\right)\end{array}$ in Expression (5), P.sub.j represents an index representing a degree of progress of precipitation of C in a temperature range of 700° C. to 500° C., P.sub.k is calculated when a temperature history of the steel sheet from 700° C. to 500° C. during cooling in the annealing process is divided into 10 periods with respect to time and an average temperature in a k-th period is represented by T.sub.k [° C.], Δt.sub.p represents a time [s] in one of 10 periods into which a total residence time in the same temperature range is divided, R.sub.10 represents a value obtained by substituting 10 into m of R.sub.m in Expression (4), and D, E, and F represent constant terms, D represents 4.47×10.sup.4, E represents 2.11×10.sup.0, and F represents 1.25×10.sup.4.

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

[0231] 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.

[0232] Molten steels having chemical compositions shown in Tables 1-1 and 1-2 were cast to manufacture cast pieces. Next, the cast pieces were hot-rolled under conditions shown in Tables 2-1 and 2-2. First, the cast pieces were heated to steel piece heating temperatures shown in Tables 2-1 and 2-2, were hot-rolled in temperature ranges to rolling completion temperatures shown in Tables 2-1 and 2-2, and were cooled from the rolling completion temperatures to 500° C. at average cooling rates shown in Tables 2-1 and 2-2. As a result, hot-rolled steel sheets were obtained. Next, the hot-rolled steel sheets were reheated under conditions shown in Tables 2-1 and 2-2. K.sub.20 obtained from the temperature history in the temperature range of 500° C. to 700° C. in the reheating process is shown in Tables 2-1 and 2-2. K.sub.20 can be obtained by Expression (3). 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.

[0233] Next, the hot-rolled steel sheets were cold-rolled from the sheet thicknesses before rolling to the sheet thicknesses shown in Tables 3-1 and 3-2 after rolling such that the rolling completion temperatures were shown in Tables 3-1 and 3-2. As a result, cold-rolled steel sheets were obtained. The obtained cold-rolled steel sheets were annealed 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 700° C. again from the range of 700° C. or higher through the retention in the range of 700° C. to 850° C. during heating was 3 seconds to 200 seconds), and subsequently were cooled. During cooling to a temperature range of 80° C. or lower, bending was performed while applying tensions shown in Tables 3-1 and 3-2. Next, by performing temper rolling at total rolling reductions shown in Tables 3-1 and 3-2, steel sheets were obtained.

[0234] For bending during cooling in the annealing process, roll bending was performed using metal rolls having a diameter of 100 mm in Experiment Examples 4 to 19, roll bending was performed using metal rolls having a diameter of 800 mm in Experiment Examples 39 to 54, and roll bending was performed using metal rolls having a diameter of 500 mm in other experiment examples. While or after performing bending in the temperature range of 80° C. to 500° C. during cooling in the annealing process, hot-dip galvanizing or hot-dip zinc alloy plating may be performed on some of the steel sheets. The steel sheets on which hot-dip galvanizing or hot-dip zinc alloy plating was performed were optionally alloyed. In addition, electroplating or deposition plating was performed on some of the steel sheets after the annealing process.

[0235] The plating process in Tables 3-1 and 3-2 is as follows.

[0236] 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.

[0237] 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.

[0238] 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).

[0239] 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).

[0240] Deposition: a process of performing deposition plating after temper rolling to obtain a galvanized steel sheet.

[0241] EG: a process of performing electrogalvanizing after the annealing process to obtain an electrogalvanized steel sheet (EG).

[0242] Tables 3-1 and 3-2 show R.sup.i obtained from the temperature history in the temperature range of 700° C. to the annealing temperature during heating to the annealing temperature. ΣR.sub.i can be obtained by Expression (4). In addition, Tables 3-1 and 3-2 show R.sub.10.Math.ΣP.sub.j obtained from the temperature history in the temperature range of 500° C. to 700° C. during cooling from the annealing temperature. R.sub.10.Math.ΣP.sub.j can be obtained by Expression (5).

[0243] Tables 4-1 and 4-2 show the properties of the steel sheets obtained under the manufacturing conditions shown in Tables 1-1 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, and the average grain size of ferrite. In addition, Tables 4-1 and 4-2 show w×λ (the unit is “degree/A”) obtained as a result of a X-ray diffraction test using the above-described method. The proportion of unrecrystallized ferrite in ferrite was measured OIM Data Collection and OIM Data Analysis manufactured by TSL. The sheet thickness of the steel sheet was the same as the sheet thickness after rolling shown in Tables 3-1 and 3-2.

[0244] 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.

[0245] The plated layers in Tables 4-1 and 4-2 are as follows.

[0246] Zn alloy plated: zinc alloy plated layer

[0247] Alloy Galvannealed: alloy galvannealed layer

[0248] GA: hot-dip galvannealed layer formed by dipping the steel sheet in a molten zinc bath and alloying the steel sheet

[0249] GI: hot-dip galvanized layer formed by dipping the steel sheet in a molten zinc bath

[0250] Deposited: galvanized layer formed by deposition plating

[0251] EG: galvanized layer formed by electrogalvanizing

[0252] Tables 4-1 and 4-2 show the properties of the steel sheets obtained under the manufacturing conditions of Tables 1-1 to 3-2. The yield strength 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. Regarding a steel sheet where the obtained yield strength (YS) was 180 MPa or lower and the yield ratio (YR) obtained by dividing the yield strength by the maximum tensile strength was 0.50 or less, this steel sheet was determined to have excellent formability and as “Pass”. A steel sheet where the yield strength was higher than 180 MPa or the yield ratio was more than 0.50 was determined to have poor formability and as “Fail”.

[0253] Further, a test piece was collected using the same method as that of the above-described tensile test, and a tensile plastic strain of 10% was applied to the test piece. After the tensile plastic strain of 10% was applied and unloaded, a baking treatment of dipping the test piece in a salt bath heated to 170° C. for 20 minutes and cooling the test piece to room temperature was performed. Next, the test piece was provided for the tensile test to obtain the yield strength. A difference (ΔBH=the yield strength after the baking treatment−the maximum stress when the 10% tensile plastic strain was applied) between the obtained yield strength and the maximum stress obtained when the 10% tensile plastic strain was applied was calculated. A steel sheet where ΔBH was 20 MPa or higher was determined to have excellent bake hardenability (BH property) and as “Pass”. On the other hand, a steel sheet where ΔBH was lower than 20 MPa was determined to have poor BH property and as “Fail”.

TABLE-US-00001 TABLE 1-1 Middle Side of Composition, mass %, Remainder including Fe and Impurities Expression Steel C Si Mn Al P S Ti Nb N O B Others (1) Note A 0.0010 0.043 0.083 0.024 0.008 0.0083 0.022 0.0041 0.0009 1.99 Example B 0.0004 0.112 0.073 0.065 0.021 0.0042 0.013 0.0026 0.0013 4.19 Example C 0.0023 0.088 1.024 0.115 0.012 0.0081 0.017 0.015 0.0035 0.0010 V: 0.20 1.38 Example D 0.0084 0.009 1.490 0.050 0.033 0.0081 0.072 0.023 0.0021 0.0019 0.0008 Mo: 0.08 2.28 Example E 0.0012 0.892 0.420 0.015 0.016 0.0017 0.008 0.032 0.0032 0.0015 Ni: 0.42 3.44 Example F 0.0045 0.076 0.982 0.679 0.013 0.0026 0.091 0.0089 0.0006 0.0010 3.36 Example G 0.0024 0.005 2.581 0.330 0.022 0.0007 0.026 0.0050 0.0014 0.0015 1.40 Example H 0.0092 0.027 0.946 0.093 0.039 0.0090 0.056 0.0058 0.0021 0.98 Example I 0.0050 0.008 0.814 0.046 0.094 0.0026 0.026 0.012 0.0039 0.0008 0.0024 0.94 Example J 0.0026 0.750 1.387 0.020 0.012 0.0055 0.008 0.023 0.0008 0.0008 Ca: 0.0016 1.65 Example K 0.0060 0.013 0.173 0.059 0.047 0.0020 0.016 0.041 0.0082 0.0012 Sb: 0.07 0.88 Example L 0.0031 1.260 0.832 0.009 0.042 0.0051 0.026 0.0015 0.0016 W: 0.26 1.08 Example M 0.0006 0.198 0.271 0.095 0.068 0.0080 0.012 0.005 0.0029 0.0007 0.0021 Mg: 0.0029 1.93 Example N 0.0009 0.318 0.015 0.007 0.019 0.0056 0.028 0.0063 0.0016 Cr: 0.63 1.78 Example O 0.0045 0.099 0.123 0.025 0.007 0.0158 0.066 0.011 0.0059 0.0026 Mo: 0.36 2.86 Example P 0.0020 0.225 0.889 0.107 0.023 0.0017 0.006 0.053 0.0049 0.0014 3.42 Example Q 0.0027 0.012 2.194 0.903 0.010 0.0021 0.009 0.031 0.0036 0.0012 0.0006 1.48 Example

TABLE-US-00002 TABLE 1-2 Middle Side of Composition, mass %, Remainder including Fe and Impurities Expression Steel C Si Mn Al P S Ti Nb N O B Others (1) Note R 0.0038 0.527 0.058 0.072 0.004 0.0058 0.053 0.014 0.0131 0.0014 1.01 Example S 0.0018 0.062 0.869 0.051 0.019 0.0072 0.020 0.0012 0.0028 2.21 Example T 0.0069 0.069 0.362 0.015 0.034 0.0069 0.024 0.018 0.0030 0.0019 Cu: 0.35 0.83 Example U 0.0017 0.437 0.098 0.086 0.012 0.0058 0.042 0.007 0.0043 0.0012 0.0013 4.54 Example V 0.0015 0.084 0.218 0.138 0.014 0.0105 0.015 0.013 0.0115 0.0010 0.0043 1.12 Example W 0.0013 0.284 0.950 0.219 0.040 0.0027 0.023 0.0061 0.0018 Cu: 0.13, 2.28 Example Sn: 0.12 X 0.0010 0.013 0.089 0.034 0.033 0.0022 0.011 0.008 0.0040 0.0009 1.03 Example Y 0.0016 0.051 0.478 0.014 0.012 0.0036 0.031 0.0052 0.0010 Cr: 0.15, 2.06 Example Ce: 0.0014 Z 0.0023 0.329 1.851 0.175 0.009 0.0057 0.008 0.018 0.0028 0.0014 Ni: 0.07, 1.01 Example REM: 0.0017 AA 0.0189 0.297 0.684 0.028 0.009 0.0024 0.062 0.028 0.0017 0.0014 0.93 Comparative Example AB 0.0014 0.340 0.568 0.074 0.015 0.0057 0.017 0.0042 0.0012 0.46 Comparative Example AC 0.0021 0.570 0.390 0.024 0.011 0.0045 0.050 0.014 0.0024 0.0009 5.83 Comparative Example AD 0.0054 0.259 0.731 0.024 0.021 0.0033 0.122 0.0086 0.0013 4.28 Comparative Example AE 0.0038 0.430 0.382 0.035 0.023 0.0045 0.085 0.0057 0.0018 2.89 Comparative Example AF 0.0030 1.829 0.540 0.053 0.021 0.0031 0.034 0.008 0.0059 0.0008 1.49 Comparative Example AG 0.0024 0.157 4.156 0.086 0.007 0.0013 0.036 0.0048 0.0017 2.04 Comparative Example AH 0.0031 0.230 0.462 0.016 0.014 0.0023 — — 0.0033 0.0009 — Comparative Example The underline represents that the value is outside of the range of the present invention, and the empty field represents that the value is less than the detection lower limit.

TABLE-US-00003 TABLE 2-1 Hot Rolling Process Reheating Process Hot- Steel Piece Rolling Average Highest Rolled Heating Completion Cooling Heating Steel Temperature Temperature Rate Temperature K.sub.20 Steel Sheet ° C. ° C. ° C./s ° C. ×10.sup.4 Note A A1 1240 938 31 597 1.83 Example A A2 1284 894 26 549 1.54 Example A A3 1257 867 30 591 1.77 Comparative Example B B1 1215 974 61 647 1.74 Example B B2 1276 921 36 618 1.63 Example C C1 1227 953 58 507 1.72 Example C C2 1216 913 30 575 1.86 Example C C3 1270 908 29 520 1.45 Comparative Example D D 1233 956 28 543 1.92 Example E E 1257 978 35 617 1.93 Example F F 1243 1021 43 648 2.07 Example G G 1231 935 46 524 1.63 Example H H 1255 906 53 578 2.00 Example I I 1237 985 43 672 1.98 Example J J 1262 1030 68 607 1.74 Example K K1 1250 895 24 582 1.77 Example K K2 1306 914 35 531 1.83 Example K K3 1292 935 33 721 — Comparative Example L L 1244 918 57 679 1.85 Example M M 1249 958 36 614 1.68 Example N N 1264 961 51 597 1.87 Example O O 1232 926 31 607 2.01 Example P P 1219 948 59 540 1.66 Example Q Q 1266 930 36 618 1.77 Example R R 1236 921 40 657 1.90 Example The underline represents that the value is outside of the range of the present invention.

TABLE-US-00004 TABLE 2-2 Hot Rolling Process Reheating Process Hot- Steel Piece Rolling Average Highest Rolled Heating Completion Cooling Heating Steel Temperature Temperature Rate Temperature K.sub.20 Steel Sheet ° C. ° C. ° C./s ° C. ×10.sup.4 Note S S 1250 969 45 562 1.64 Example T T 1250 942 48 642 1.93 Example U U 1253 913 29 547 1.57 Example V V 1302 982 49 566 1.64 Example W W1 1241 949 36 655 1.82 Example W W2 1176 955 42 656 1.84 Comparative Example W W3 1240 885 29 647 1.63 Example X X1 1227 921 31 639 1.84 Example X X2 1209 914 28 541 1.72 Example X X3 1230 924 17 562 1.58 Comparative Example Y Y1 1230 913 59 539 1.73 Example Y Y2 1313 897 39 638 1.69 Example Y Y3 1255 931 33 465 — Comparative Example Z Z1 1284 937 37 676 1.92 Example Z Z2 1187 944 39 623 1.80 Comparative Example Z Z3 1248 936 57 691 1.90 Example Z Z4 1263 937 33 534 1.47 Comparative Example AA AA 1208 915 40 601 2.00 Comparative Example AB AB 1264 907 29 630 1.74 Comparative Example AC AC 1233 951 58 642 1.84 Comparative Example AD AD 1280 926 37 615 1.92 Comparative Example AE AE 1210 948 37 630 1.78 Comparative Example AF AF 1272 927 32 616 1.80 Comparative Example AG AG 1273 934 50 632 1.85 Comparative Example AH AH 1217 916 34 626 1.70 Comparative Example The underline represents that tile value is outside of the range of the present invention.

TABLE-US-00005 TABLE 3-1 Temper Rolling Cold Rolling Process Annealing Process Process Sheet Sheet Rolling Heating Total Thick- Thick- Total com- An- Rolling Experi- Hot- ness ness Rolling pletion nealing Cooling Reduction mental Ex- ample Steel Rolled Steel Sheet before Rolling mm after Rolling mm Reduc- tion % Temp- erature ° C. Temp- erature ° C. [00007]$\underset{i=1}{\overset{10}{.Math.}}{R}_i$ [00008]$R_{10}.Math.\underset{j=1}{\overset{10}{.Math.}}{P}_j$ Tension MPa of Temper Rolling % Plating Process Note 1 A A1 3.0 0.8 73 128 739 6.9  3.9 27 0.25 Example 2 A A1 1.8 0.4 78  96 725 0.8  1.6 22 1.15 Comparative Example 3 A A1 2.0 0.4 80 107 754 4.5  1.3 25 0.72 Example 4 A A2 4.2 1.0 76 143 755 7.0  3.2 27 0.41 Zn Alloy Example Plating 5 A A2 3.4 0.4 88 194 783 12.3   3.4 28 1.21 GA Example 6 A A2 5.4 0.4 93 152 752 9.9  5.5 25 0.91 Comparative Example 7 A A3 4.4 1.0 77 131 776 8.2  3.2 27 0.76 Comparative Example 8 B B1 5.6 1.0 82 140 729 6.4  2.2 34 0.71 GI Example 9 B B1 1.6 0.3 81 238 770 13.8   5.2 26 0.66 Alloy Example Galvannealing 10 B B2 4.4 1.2 73 145 779 7.6  7.7 39 0.84 Example 11 B B2 4.6 0.7 85 131 773 3.8  2.4 28 0.03 Comparative Example 12 C C1 4.8 1.3 73 154 755 6.6  2.5 30 0.95 Deposition Example 13 C C2 6.0 1.1 82 203 761 5.5  2.0 29 0.29 Example 14 C C2 5.4 1.8 67 129 738 1.2  3.3 27 1.19 Example 15 C C2 3.0 1.4 53 114 786 11.9   6.9 30 1.01 Comparative Example 16 C C3 1.8 0.3 83 121 761 5.7  2.4 26 0.86 Comparative Example 17 D D 5.8 1.2 79 184 798 6.6  2.8 34 0.12 Example 18 E E 3.8 0.6 84 219 783 13.4  10.1 40 0.85 GA Example 19 F F 5.4 1.2 78 189 760 2.3  2.1 27 0.73 Example 20 G G 3.2 0.7 78 143 783 8.4  4.1 31 0.96 Example 21 H H 5.2 1.8 65 228 815 13.6   4.1 30 1.17 GI Example 22 I I 1.6 0.3 81 186 774 5.7  3.5 57 1.59 Example 23 J J 4.0 1.2 70 147 774 10.9  10.8 31 1.13 Example 24 K K1 5.8 1.6 72 127 822 11.4   6.0 29 0.84 GI Example 25 K K2 5.2 1.8 65 150 708 3.6  3.0 32 0.65 Example 26 K K2 2.2 0.7 68 184 774 9.6  5.2 37 1.18 GA Example 27 K K2 6.0 1.3 78 179 750 3.8  1.7 No 1.53 Comparative Bending Example 28 K K2 3.6 0.7 81 298 738 4.2  2.8 36 0.95 Comparative Example 29 K K3 5.6 1.0 82 183 477 — — 29 0.88 Comparative Example 30 L L 2.2 0.5 77 183 769 8.6  3.0 49 0.77 Example 31 M M 5.4 1.8 67 147 809 10.5   3.1 40 0.88 EG Example 32 N N 4.0 1.4 65 139 771 5.9  2.4 28 0.50 Example 33 N N 4.8 1.7 65 172 801 13.1  16.5 29 1.33 Comparative Example 34 O O 4.0 0.6 85 178 817 10.1   7.1 25 1.39 Example 35 p P 1.6 0.3 81 139 784 13.1  10.2 30 0.56 GA Example 36 Q Q 3.6 1.2 67 141 748 7.3  5.3 28 0.46 Example 37 R R 1.8 0.5 67 148 778 10.1   6.3 26 1.05 GA Example The underline represents that the value is outside of the range of the present invention.

TABLE-US-00006 TABLE 3-2 Temper Rolling Cold Rolling Process Process Sheet Sheet Rolling Annealing Process Total Thick- Thick- Total com- Heating Rolling Hot- ness ness Rolling pletion Annealing Cooling Reduction Experi- mental Example Steel Rolled Steel Sheet before Rolling mm after Rolling mm Reduc- tion % Temp- erature ° C. Temp- erature ° C. [00009]$\underset{i=1}{\overset{10}{.Math.}}{R}_i$ [00010]$R_{10}.Math.\underset{j=1}{\overset{10}{.Math.}}{P}_j$ Tension MPa of Temper Rolling % Plating Process Note 38 S S 3.2 0.7 78 153 767  9.0  4.9 29 0.60 Zn Example Plating 39 T T 2.6 0.7 73 173 814  9.6  3.5 34 0.27 Example 40 T T 1.8 0.3 83 108 777  7.2  2.4 12 1.21 Comparative Example 41 U U 4.4 1.3 70  92 761 11.7  4.9 29 1.20 GI Example 42 V V 1.8 0.4 78 109 727  5.5  8.8 28 0.74 Example 43 V V 5.6 1.3 77 153 739  6.3  1.4 25 3.00 Comparative Example 44 W W1 5.4 0.8 85 225 779  5.0  1.9 34 0.55 Example 45 W W1 2.6 0.6 77 138 800 11.0  5.0 23 0.47 GI Example 46 W W1 3.0 0.9 70 155 681 — — 39 1.05 Comparative Example 47 W W2 2.4 0.7 71 111 735  5.5  4.9 44 1.54 Comparative Example 48 W W3 2.4 0.7 71 141 761 11.2  2.6 35 0.75 GA Example 49 X X1 1.8 0.6 67 102 721  5.1  4.1 31 0.72 Example 50 X X1 2.6 0.7 73 115 793 14.6 12.0 32 0.44 GI Example 51 X X1 3.1 1.1 65 133 724  1.6  0.8 29 0.38 Comparative X X1 Example 52 X X2 5.6 1.3 77 167 723  5.1  6.7 32 0.13 GA Example 53 X X3 4.0 1.3 68 151 793 12.0  4.8 30 1.25 Comparative Example 54 Y Y1 5.8 1.3 78 103 757  7.6  3.8 31 1.83 Example 55 Y Y1 3.2 0.6 81 146 744  7.6 14.3 27 0.88 GA Example 56 Y Y1 2.4 0.6 75 121 776 18.7  7.3 26 0.69 Comparative Example 57 Y Y2 5.2 1.0 81 134 787 11.4 12.2 27 1.14 GA Example 58 Y Y3 3.4 1.1 68 146 747 — — 25 0.26 Comparative Example 59 Z Z1 2.1 0.8 62 140 722  4.6  3.0 33 0.07 Example 60 Z Z1 3.4 1.2 65 191 716  3.7  5.5 32 1.68 GA Example 61 Z Z1 2.4 0.6 75 187 871 — — 32 0.82 Comparative Example 62 Z Z2 3.0 0.5 83 187 778 13.3  3.6 34 0.65 Comparative Example 63 Z Z3 2.4 0.4 83 172 742  4.3  2.0 33 0.58 EG Example 64 Z Z4 1.8 0.4 78 183 776 11.7  3.9 31 1.27 Comparative Example 65 AA AA 5.6 1.9 66 204 801 12.6  4.5 29 1.17 Comparative Example 66 AB AB 3.4 0.9 74 147 763  7.7  1.4 29 0.90 Comparative Example 67 AC AC 2.2 0.5 77 109 789  5.5  1.7 30 0.92 Comparative Example 68 AD AD 2.0 0.4 80 154 769  7.0  9.7 30 0.95 Comparative Example 69 AE AE 3.2 0.9 72 166 739  3.9  3.3 45 1.12 Comparative Example 70 AF AF 3.0 0.8 73 219 767  2.8  2.0 44 0.93 Comparative Example 71 AG AG 4.8 1.6 67 195 798  8.5  2.4 34. 0.86 Comparative Example 72 AH AH 3.0 1.0 67 153 754  9.1  2.8 27 0.17 Comparative Example 73 J J 4.0 1.5 63 225 793 14.2 14.3 24 0.30 Example 74 W W3 2.4 0.4 83 104 758  1.6  1.9 30 0.62 Example The underline represents that the value is outside of the range of the present invention.

TABLE-US-00007 TABLE 4-1 Properties of Steel Sheet Proportion of Fe Content in Hot- Unrecrys- Average GA or Alloy Experi- Rolled tallized Grain Size Galvannealed Properties mental Steel Ferrite Ferrite of Ferrite Plated Layer YS TS ΔBH Example Steel Sheet vol % % μm w × λ Layer mass % MPa MPa YR MPa Note 1 A A1 100 0 8.7 0.37 122 293 0.42 37 Example 2 A A1 100 11 6.5 0.15 193 290 0.67 10 Comparative Example 3 A A1 99 0 8.2 0.29 119 290 0.41 33 Example 4 A A2 100 0 8.1 0.26 Zn Alloy 131 295 0.44 28 Example Plated 5 A A2 100 4 8.6 0.24 GA 10.6 158 326 0.49 30 Example 6 A A2 100 13 9.5 0.22 206 279 0.74 25 Comparative Example 7 A A3 100 8 10.0 0.27 197 277 0.71 28 Comparative Example 8 B B1 100 0 7.3 0.29 GI 134 299 0.45 35 Example 9 B B1 100 2 8.5 0.32 Alloy 9.3 152 321 0.47 30 Example Galvannealed 10 B B2 100 0 13.0 0.25 132 294 0.45 22 Example 11 B B2 100 2 7.2 0.17 136 290 0.47 17 Comparative Example 12 C C1 99 0 8.2 0.27 Deposited 169 361 0.47 28 Example 13 C C2 98 0 7.6 0.35 168 352 0.48 41 Example 14 C C2 99 1 6.4 0.31 175 359 0.49 32 Example 15 A C2 98 15 9.9 0.31 252 339 0.74 35 Comparative Example 16 C C3 99 0 7.8 0.17 160 326 0.49 12 Comparative Example 17 D D 96 0 9.0 0.21 175 360 0.49 27 Example 18 E E 99 1 14.5 0.24 GA 8.5 174 402 0.43 28 Example 19 F F 98 2 7.6 0.36 178 361 0.49 40 Example 20 G G 97 0 12.4 0.22 177 359 0.49 24 Example 21 H H 98 1 9.5 0.23 GI 165 343 0.48 24 Example 22 I I 99 0 11.6 0.28 174 375 0.46 29 Example 23 J J 99 0 14.6 0.27 164 371 0.44 22 Example 24 K K1 99 0 18.5 0.31 GI 143 298 0.48 29 Example 25 K K2 99 0 8.7 0.22 155 311 0.50 21 Example 26 K K2 99 0 11.9 0.26 GA 12.8 149 316 0.47 28 Example 27 K K2 98 0 7.6 0.18 172 312 0.55 13 Comparative Example 28 K K2 99 17 8.9 0.30 216 325 0.66 24 Comparative Example 29 K K3 100 3 8.1 0.15 168 322 0.52 7 Comparative Example 30 L L 99 0 10.0 0.23 178 431 0.41 25 Example 31 M M 100 0 9.4 0.21 EG 168 337 0.50 23 Example 32 N N 100 0 8.5 0.35 146 308 0.48 27 Example 33 N N 100 0 17.8 0.13 150 284 0.53 8 Comparative Example 34 O O 99 1 10.6 0.30 116 308 0.38 33 Example 35 P P 99 0 13.0 0.32 GA 8.4 171 348 0.49 33 Example 36 Q Q 98 0 10.9 0.24 164 331 0.49 37 Example 37 R R 100 0 15.3 0.27 GA 9.7 161 361 0.45 27 Example The underline represents that the value is outside of the range of the present invention or represents undesirable properties.

TABLE-US-00008 Table 4-2 Properties of Steel Sheet Proportion of Fe Content in Hot- Unrecrys- Average GA or Alloy Experi- Rolled tallized Grain Size Galvannealed Properties mental Steel Ferrite Ferrite of Ferrite Plated Layer YS TS ΔBH Example Steel Sheet Vol % % μm w × λ Layer mass % MPa MPa YR MPa Note 38 S S 99 0 9.9 0.25 Zu Plated 159 348 0.46 30 Example 39 T T 99 0 14.2 0.22 154 322 0.48 25 Example 40 T T 99 0 10.7 0.16 157 338 0.47 15 Comparative Example 41 U U 100 0 8.2 0.39 GI 153 308 0.50 28 Example 42 V V 100 0 9.9 0.33 132 300 0.44 41 Example 43 V V 96 10 8.3 0.14 203 385 0.53 13 Comparative Example 44 W W1 99 2 8.3 0.23 173 363 0.48 30 Example 45 W W1 99 0 9.3 0.23 GI 175 372 0.47 25 Example 46 W W1 100 12 22.3 0.17 256 354 0.72 16 Comparative Example 47 W W2 99 0 9.5 0.16 172 360 0.48 9 Comparative Example 48 W W3 99 0 9.5 0.27 GA 10.3 170 354 0.48 30 Example 49 X X1 100 0 9.6 0.27 145 311 0.47 39 Example 50 X X1 100 0 13.3 0.31 GI 134 297 0.45 24 Example 51 X X1 91 0 6.5 0.34 215 374 0.57 29 Comparative Example 52 X X2 100 0 12.9 0.22 GA 9.4 128 287 0.45 26 Example 53 X X3 100 0 15.7 0.17 137 290 0.47 9 Comparative Example 54 Y Y1 99 0 7.9 0.29 156 317 0.49 31 Example 55 Y Y1 99 0 12.5 0.23 GA 12.2 157 323 0.49 26 Example 56 Y Y1 99 0 18.8 0.16 142 308 0.46 11 Comparative Example 57 Y Y2 99 0 14.5 0.30 GA 8.4 141 317 0.44 28 Example 58 Y Y3 100 2 10.5 0.17 174 323 0.54 12 Comparative Example 59 Z Z1 99 0 10.1 0.25 169 384 0.44 30 Example 60 Z Z1 98 0 8.6 0.32 GA 7.4 171 364 0.47 27 Example 61 Z Z1 100 0 23.2 0.15 169 361 0.47 9 Comparative Example 62 Z Z2 98 0 12.4 0.12 175 385 0.45 14 Comparative Example 63 Z Z3 98 0 8.6 0.34 EG 175 390 0.45 35 Example 64 Z Z4 99 0 11.1 0.14 169 360 0.47 12 Comparative Example 65 AA AA 98 0 18.2 0.29 207 373 0.55 28 Comparative Example 66 AB AB 95 0 12.5 0.13 177 338 0.52 15 Comparative Example 67 AC AC 97 13 6.7 0.28 226 427 0.53 26 Comparative Example 68 AD AD 99 9 9.5 0.30 240 348 0.69 27 Comparative Example 69 AE AE 99 11 8.0 0.29 237 343 0.69 34 Comparative Example 70 AF AF 99 1 8.3 0.23 280 485 0.58 30 Comparative Example 71 AG AG 69 0 8.0 0.38 232 413 0.56 25 Comparative Example 72 AH AH 100 0 16.3 0.33 281 329 0.85 13 Comparative Example 73 J J 99 0 17.3 0.22 101 267 0.38 21 Example 74 W W3 99 0 5.0 0.21 178 365 0.49 22 Example The underline represents that the value is outside of the range of the present invention or represents undesirable properties.

[0254] Among steels A to AH shown in Tables 1-1 and 1-2, the steels AA to AH shown in Table 1-2 are comparative examples where the composition was outside of the range defined by the present invention.

[0255] In the steel AA, the C content was higher than the range of the present invention. In the steel sheet according to Experiment Example 65 obtained using this steel, the yield strength and the yield ratio were high.

[0256] In the composition of the steel AB, the value of the middle side of Expression (1) was less than that of the range of the present invention. In the steel sheet according to Experiment Example 66 obtained using this steel, the yield ratio was high, and sufficient bake hardenability (BH property) was not able to be obtained.

[0257] In the composition of the steel AC, the value of the middle side of Expression (1) was more than that of the range of the present invention. In the steel sheet according to Experiment Example 67 obtained using this steel, an excess amount of unrecrystallized ferrite remained, and the yield strength and the yield ratio were excessively high.

[0258] In the steel AD, the Ti content was higher than the range of the present invention. In the steel sheet according to Experiment Example 68 obtained using this steel, an excess amount of unrecrystallized ferrite remained, and the yield strength and the yield ratio were excessively high.

[0259] In the steel AE, the Nb content was higher than the range of the present invention. In the steel sheet according to Experiment Example 69 obtained using this steel, an excess amount of unrecrystallized ferrite remained, and the yield strength and the yield ratio were excessively high.

[0260] In the steel AF, the Si content was higher than the range of the present invention. In the steel sheet according to Experiment Example 70 obtained using this steel, the yield strength and the yield ratio were excessively high.

[0261] In the steel AG, the Mn content was higher than the range of the present invention. In the steel sheet according to Experiment Example 71 obtained using this steel, the volume percentage of ferrite was insufficient, and the yield strength and the yield ratio were high.

[0262] The steel AH did not include both Ti and Nb. In the steel sheet according to Experiment Example 72 obtained using this steel, the yield strength and the yield ratio were high, and sufficient bake hardenability (BH property) was not able to be obtained.

[0263] Experiment Examples 7, 47, 53, and 62 were comparative examples where the conditions of the hot rolling process were outside of the range of the present invention.

[0264] Experiment Examples 47 and 62 were comparative examples in which the cast piece heating temperature in the hot rolling process was low and the value of w×λ was small. Therefore, sufficient bake hardenability was not able to be obtained.

[0265] Experiment Example 7 was a comparative example in which the rolling completion temperature in the hot rolling process was low and an excess amount of unrecrystallized ferrite remained. Therefore, the yield strength and the yield ratio were excessively high.

[0266] Experiment Example 53 was a comparative example in which the average cooling rate in the temperature range of the hot rolling completion temperature to 500° C. in the hot rolling process was slow and the value of w×λ was small. Therefore, sufficient bake hardenability was not able to be obtained.

[0267] Experiment Examples 16, 29, 58, and 64 were comparative examples in which the conditions of the reheating process were outside of the range of the present invention.

[0268] Experiment Example 29 was a comparative example in which the maximum reheating temperature in the reheating process was high and the value of w×λ was small. Therefore, the yield ratio was excessively high and sufficient bake hardenability was not able to be obtained.

[0269] Experiment Example 58 was a comparative example in which the maximum reheating temperature in the reheating process was low and the value of w×λ was small. Therefore, the yield ratio was high and sufficient bake hardenability was not able to be obtained.

[0270] Experiment Examples 16 and 64 were comparative examples in which the temperature history in the reheating process did not satisfy Expression (3) (K.sub.20 was low) and the value of w×λ was small. Therefore, sufficient bake hardenability was not able to be obtained.

[0271] Experiment Examples 6, 15, and 28 were comparative examples in which the conditions of the cold rolling process were outside of the range of the present invention.

[0272] Experiment Example 6 was a comparative example in which the total rolling reduction in the cold rolling process was high and an excess amount of unrecrystallized ferrite remained. Therefore, the yield strength and the yield ratio were high.

[0273] Experiment Example 15 was a comparative example in which the total rolling reduction in the cold rolling process was low and an excess amount of unrecrystallized ferrite remained. Therefore, the yield strength and the yield ratio were high.

[0274] Experiment Example 28 was a comparative example in which the rolling completion temperature in the cold rolling process was high and an excess amount of unrecrystallized ferrite remained. Therefore, the yield strength and the yield ratio were high.

[0275] Experiment Examples 2, 27, 33, 40, 46, 51, 56, and 61 were comparative examples where the conditions of the annealing process were outside of the range of the present invention.

[0276] Experiment Example 61 was a comparative example in which the annealing temperature during retention in the annealing process was high and the value of w×λ was small. Therefore, sufficient bake hardenability was not able to be obtained.

[0277] Experiment Example 46 was a comparative example in which the annealing temperature during retention in the annealing process was low, an excess amount of unrecrystallized ferrite remained, and the value of w×λ was small. Therefore, the yield strength and the yield ratio were high, and sufficient bake hardenability was not able to be obtained.

[0278] Experiment Example 56 was a comparative example in which the temperature history during heating in the annealing process did not satisfy Expression (4) and the value of w×λ was small. Therefore, sufficient bake hardenability was not able to be obtained.

[0279] Experiment Example 2 was a comparative example in which the temperature history during heating in the annealing process did not satisfy Expression (4), an excess amount of unrecrystallized ferrite remained, and the value of w×λ was small. Therefore, the yield strength and the yield ratio were high, and sufficient bake hardenability was not able to be obtained.

[0280] Experiment Example 33 was a comparative example in which the temperature history during cooling in the annealing process did not satisfy Expression (5), the value of w×λ was small, the yield ratio was high, and sufficient bake hardenability was not able to be obtained.

[0281] Experiment Example 51 was a comparative example in which the temperature history during cooling in the annealing process did not satisfy Expression (5), an excess amount of structures other than ferrite were formed, and the yield strength and the yield ratio were high.

[0282] Experiment Example 27 was a comparative example in which bending was performed in the temperature range of 80° C. to 500° C. during cooling in the annealing process and the value of w×λ was small. Therefore, the yield ratio was high and sufficient bake hardenability was not able to be obtained.

[0283] Experiment Example 40 was a comparative example in which bending was performed in the temperature range of 80° C. to 500° C. during cooling in the annealing process without applying a sufficient tension and the value of w×λ was small. Therefore, sufficient bake hardenability was not able to be obtained.

[0284] Experiment Examples 11 and 43 were comparative examples where the conditions of the temper rolling process were outside of the range of the present invention.

[0285] Experiment Example 43 was a comparative example in which the total rolling reduction of temper rolling in the temper rolling process was high, an excess amount of unrecrystallized ferrite remained, and the value of w×λ was small. Therefore, the yield strength and the yield ratio were high, and sufficient bake hardenability was not able to be obtained.

[0286] Experiment Example 11 was a comparative example in which the total rolling reduction of temper rolling in the temper rolling process was low and the value of w×λ was small. Therefore, sufficient bake hardenability was not able to be obtained.

[0287] 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 such that the yield strength was able to be reduced and high bake hardenability was obtained even in a high strain region.

[0288] Experiment Examples 4, 5, 8, 9, 12, 18, 21, 24, 26, 31, 35, 37, 38, 41, 45, 48, 50, 52, 55, 57, 60, and 63 are examples where the plated steel sheets according to the present invention were obtained by performing plating.

[0289] Experiment Examples 8, 21, 24, 41, 45, and 50 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.

[0290] Experiment Examples 5, 18, 26, 35, 37, 48, 52, 55, 57, and 60 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.

[0291] Experiment Examples 4 and 38 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.

[0292] Experiment Example 9 was an Example 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.

[0293] Experiment Example 12 was an Example in which a galvanized steel sheet was obtained by performing deposition plating after temper rolling.

[0294] Experiment Examples 31 and 63 were Examples in which an electrogalvanized steel sheet (EG) was obtained by performing electrogalvanizing after the annealing process.

INDUSTRIAL APPLICABILITY

[0295] As described above, according to the present invention, a steel sheet having excellent formability and BH property 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.