Lightweight steel and steel sheet with enhanced elastic modulus, and manufacturing method thereof

Abstract

There is disclosed a lightweight steel with an enhanced elastic modulus, wherein the lightweight steel has a chemical composition by mass percentage of 0.001%≤C≤0.30%, 0.05%≤Mn≤4.0%, 1.5%<Al<3.0%, 1.5%≤Ti≤7.0%, 0.5%≤B≤3.6%, and the remainder consisting of Fe and other unavoidable impurities. A microstructure of the lightweight steel comprises a matrix and fine hardening granules evenly distributed throughout the matrix. The matrix entirely or partially comprises a ferrite and/or a bainite. The hardening granule comprises at least TiB.sub.2.

Claims

1. A lightweight steel with an enhanced elastic modulus, wherein: the lightweight steel has a chemical composition by mass percentage of 0.001%≤C≤0.30%, 0.05%≤Mn≤4.0%, 1.6%≤Al≤2.9%, 1.5%≤Ti≤7.0%, 0.5%≤B≤3.6%, at least one of the elements: 0.01%≤Si≤1.5%, 0.01%≤Cr≤2.0%, 0.01%≤Mo≤1.0%, 0.01%≤Nb≤0.2%, 0.01%≤V≤0.5%, 0.05%≤Ni≤1.0%, 0.05%≤Cu≤1.0%, and 0.001%≤Ca≤0.2%, and with a balance of Fe and unavoidable impurity elements; the lightweight steel has a microstructure comprising a matrix and fine hard reinforcing particles dispersedly distributed in the matrix uniformly, wherein the matrix is entirely or partially ferrite and/or bainite, wherein the hard reinforcing particles comprise at least TiB.sub.2; and wherein the lightweight steel has a tensile strength >500 MPa, an elastic modulus >200 GPa, and a density <7600 kg/m.sup.3.

2. The lightweight steel of claim 1, wherein the Ti and B elements further meet: −1.2%≤(Ti-2.22*B)≤1.2%.

3. The lightweight steel of claim 2, wherein the hard particles have a volumetric fraction amounting to at least 3% of the whole microstructure.

4. The lightweight steel of claim 2, wherein the Ti element has a content of 3.0%≤Ti≤6.0%; the B element has a content of 1.2%≤B3.0%; the Ti and B elements further meet: −0.6%≤(Ti −2.22*B)≤0.6%; and the hard particles have a volumetric fraction amounting to at least 6% of the whole microstructure.

5. The lightweight steel of claim 4, wherein the lightweight steel has a tensile strength >500 MPa, an elastic modulus >210 GPa, and a density <7400 kg/m.sup.3.

6. The lightweight steel of claim 1, wherein the hard reinforcing particles further comprise at least one of TiC and Fe.sub.2B.

7. The lightweight steel of claim 1, wherein the hard reinforcing particles have an average particle size of less than 15 μm.

8. A steel sheet made of the lightweight steel according to claim 1.

9. A manufacturing method for the steel sheet of claim 8, comprising the following steps: (1) Smelting and continuous casting to obtain a slab having a thickness of 120-300 mm; and (2) Hot rolling to obtain a hot-rolled sheet.

10. The manufacturing method of claim 9, wherein Step (2) is followed by Step (3): recrystallization annealing.

11. The manufacturing method of claim 9, wherein, in Step (2), a heating temperature is 1000-1250° C.; a soaking time is 0.5-3 h; a final rolling temperature is ≥850° C.; and coiling is performed at 400-750° C.

12. The manufacturing method of claim 10, wherein, when the hot-rolled sheet is subjected to recrystallization annealing by way of continuous annealing in Step (3), the hot-rolled sheet is heated to a soaking temperature of 800-1000° C., held for 30-600s, and then cooled to room temperature.

13. The manufacturing method of claim 10, wherein, when the hot-rolled sheet is subjected to recrystallization annealing by way of bell furnace annealing in Step (3), the hot-rolled sheet is heated to a soaking temperature of 650-900° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

14. A manufacturing method for the steel sheet of claim 8, comprising the following steps: (1) Smelting and strip casting to obtain a thin strip having a thickness of no more than 10 mm; and (2) Hot rolling to obtain a hot-rolled sheet.

15. The manufacturing method of claim 14, wherein Step (2) is followed by Step (3): recrystallization annealing.

16. The manufacturing method of claim 14, wherein, in Step (2), the thin strip is hot rolled immediately with no aid of external heating; a final rolling temperature is controlled at ≥850° C.; a hot rolling reduction is 20-60%; and coiling is then performed at 400−750° C.

17. The manufacturing method of claim 15, wherein, when the hot-rolled sheet is subjected to recrystallization annealing by way of continuous annealing in Step (3), the hot-rolled sheet is heated to a soaking temperature of 800-1000° C., held for 30-600s, and then cooled to room temperature.

18. The manufacturing method of claim 15, wherein, when the hot-rolled sheet is subjected to recrystallization annealing by way of bell furnace annealing in Step (3), the hot-rolled sheet is heated to a soaking temperature of 650-900° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

19. A manufacturing method for the steel sheet of claim 8, comprising the following steps: (1) Smelting and continuous casting to obtain a slab having a thickness of 120-300 mm; (2) Hot rolling; (3) Pickling; (4) Cold rolling to obtain a cold-rolled sheet; and (5) Recrystallization annealing of the cold-rolled sheet.

20. The manufacturing method of claim 19, wherein Step (2) is followed by a post-hot-rolling recrystallization annealing.

21. The manufacturing method of claim 19, wherein, in Step (2), a heating temperature is 1000-1250° C.; a soaking time is 0.5-3 h; a final rolling temperature is ≤850° C.; and coiling is then performed at 400-750° C.

22. The manufacturing method of claim 20, wherein, when the post-hot-rolling recrystallization annealing is performed by a continuous annealing, the hot-rolled sheet is heated to a soaking temperature of 800-1000° C., held for 30-600s, and then cooled to room temperature.

23. The manufacturing method of claim 20, wherein, when the post-hot-rolling recrystallization annealing is performed by a bell furnace annealing, the hot-rolled sheet is heated to a soaking temperature of 650-900° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

24. The manufacturing method of claim 19, wherein a cold rolling reduction is controlled at 25-75% in Step (4).

25. The manufacturing method of claim 19, wherein, when the recrystallization annealing of the cold-rolled sheet is performed by a continuous annealing in Step (5), the cold-rolled sheet is heated to a soaking temperature of 700-900° C., held for 30-600s, and then cooled to room temperature.

26. The manufacturing method of claim 19, wherein, when the recrystallization annealing of the cold-rolled sheet is performed by a bell furnace annealing in Step (5), the cold-rolled sheet is heated to a soaking temperature of 600-800° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

27. A manufacturing method for the steel sheet of claim 8, comprising the following steps: (1) Smelting and strip casting to obtain a thin strip having a thickness of no more than 10 mm; (2) Hot rolling; (3) Pickling; (4) Cold rolling to obtain a cold-rolled sheet; and (5) Recrystallization annealing of the cold-rolled sheet.

28. The manufacturing method of claim 27, wherein Step (2) is followed by a post-hot-rolling recrystallization annealing.

29. The manufacturing method of claim 27, wherein, in Step (2), the thin strip is hot rolled immediately with no aid of external heating; a final rolling temperature is controlled at ≥850° C.; a hot rolling reduction is 20-60%; and coiling is then performed at 400-750° C.

30. The manufacturing method of claim 28, wherein, when the post-hot-rolling recrystallization annealing is performed by a continuous annealing, the hot-rolled sheet is heated to a soaking temperature of 800-1000° C., held for 30-600s, and then cooled to room temperature.

31. The manufacturing method of claim 28, wherein, when the post-hot-rolling recrystallization annealing is performed by a bell furnace annealing, the hot-rolled sheet is heated to a soaking temperature of 650-900° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

32. The manufacturing method of claim 27, wherein a cold rolling reduction is controlled at 25-75% in Step (4).

33. The manufacturing method of claim 27, wherein, when the recrystallization annealing of the cold-rolled sheet is performed by a continuous annealing in Step (5), the cold-rolled sheet is heated to a soaking temperature of 700-900° C., held for 30-600s, and then cooled to room temperature.

34. The manufacturing method of claim 27, wherein, when the recrystallization annealing of the cold-rolled sheet is performed by a bell furnace annealing in Step (5), the cold-rolled sheet is heated to a soaking temperature of 600-800° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a photograph showing a low magnification metallographical structure of the slab of Comparative Example B2 of lightweight steel.

(2) FIG. 2 is a photograph showing a high magnification metallographical structure of the slab of Comparative Example B2 of lightweight steel.

(3) FIG. 3 is a photograph showing a low magnification metallographical structure of the slab of Example A6 of lightweight steel.

(4) FIG. 4 is a photograph showing a high magnification metallographical structure of the slab of Example A6 of lightweight steel.

(5) FIG. 5 is a photograph showing the morphology of the steel sheet of Comparative Example CS2 after hot rolling.

(6) FIG. 6 is a photograph showing the morphologies of the steel sheets of Examples HM6-HM8 after hot rolling.

(7) FIG. 7 is a photograph showing a low magnification metallographical structure of the steel sheet of Example HM6 after hot rolling.

(8) FIG. 8 is a photograph showing a high magnification metallographical structure of the steel sheet of Example HM6 after hot rolling.

DETAILED DESCRIPTION

(9) The lightweight steel with an enhanced elastic modulus, the steel sheet and the manufacturing method thereof according to the disclosure will be further explained and illustrated with reference to the accompanying drawings and the specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the disclosure.

Examples A1-A9 and Comparative Examples B1-B3 of Lightweight Steel

(10) Table 1 lists the mass percentages of the chemical elements in Examples A1-A9 and Comparative Examples B1-B3 of the lightweight steel with an enhanced elastic modulus.

(11) TABLE-US-00001 TABLE 1 (wt %) C Mn Al B Ti Nb V Cr Mo Ni Cu Si Ca N S P Ti − 2.22*B A1 0.15 2.1 2.0 0.5 1.5 — 0.4 — 0.9 — — — 0.2 0.003 0.005 0.004 0.39 A2 0.05 4.0 2.4 1.2 3.5 0.2 — 1.4 — — — — — 0.003 0.004 0.010 0.84 A3 0.10 0.8 2.8 2.1 4.8 — — — — 1.0 1.0 — — 0.008 0.001 0.006 0.14 A4 0.15 3.0 2.3 1.1 3.0 — — — — — — 1.2 — 0.003 0.002 0.008 0.56 A5 0.26 1.0 2.0 2.6 6.9 — — — — — — — — 0.004 0.002 0.007 1.13 A6 0.04 0.05 2.5 2.0 4.2 — — — — — — — — 0.002 0.005 0.003 −0.24 A7 0.005 0.1 1.6 2.3 4.9 — — — — — — — — 0.003 0.001 0.009 −0.21 A8 0.08 0.5 2.9 2.2 4.3 — — 0.4 0.1 — — — — 0.003 0.002 0.008 −0.58 A9 0.06 0.1 1.8 3.6 6.8 — — — — — — — — 0.007 0.009 0.01 −1.19 B1 0.006 0.2 2.5 — — — — — — — — — — 0.004 0.003 0.014 0.0 B2 0.04 0.05 — 2.3 4.0 — — — — — — — — 0.003 0.004 0.012 −1.11 B3 0.1 0.1 — 1.9 5.2 — — — — — — — — 0.003 0.008 0.007 0.98

Examples HM1-HM9 and Comparative Examples CS1-CS3 of Steel Sheets and the Method for Manufacturing the Same

(12) The steel sheets in the above Examples and Comparative Examples were manufactured with the following steps:

(13) (1) The lightweight steel materials of A1-A9 in Table 1 were smelted and continuously cast according to Examples HM1-HM9 respectively, and the lightweight steel materials of B1-B3 in Table 1 were smelted and continuously cast according to Comparative Examples CS1-CS3 respectively, to obtain slabs of 120-300 mm in thickness, wherein S, P and N were unavoidable impurities, and the balance was Fe;

(14) (2) Hot rolling to obtain hot rolled sheets of 3.2 mm in thickness: in this step, the heating temperature was 1000-1250° C., the soaking time was 0.5-3 h, the final rolling temperature was ≥850° C., and coiling was performed at 400-750° C.;

(15) (3) Post-hot-rolling recrystallization annealing: when the hot-rolled sheet was subjected to recrystallization annealing by way of continuous annealing, the hot-rolled sheet was heated to a soaking temperature of 800-1000° C., held for 30-600 s, and then cooled to room temperature; when the hot-rolled sheet was subjected to recrystallization annealing by way of bell furnace annealing, the hot-rolled sheet was heated to a soaking temperature of 650-900° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

(16) The hot-rolled sheet in Step (2) was rapidly cooled to a coiling temperature and held for 1 hour, and then cooled to room temperature along with the furnace, so as to simulate the coiling and cooling processes of the hot-rolled sheet. In some Examples where a non-recrystallization microstructure did not exist in the hot-rolled sheet matrix, Step (3) might be exempted.

(17) Table 2 lists the specific process parameters in the manufacturing method for the steel sheets in Examples HM1-HM9 and Comparative Examples CS1-CS3.

(18) TABLE-US-00002 TABLE 2 Step (3) Continuous Bell Furnace Step (2) Annealing Annealing Step (1) Heating soaking Final rolling Coiling Soaking soaking Soaking soaking Thickness temperature time temperature Temperature temperature time temperature time Material (mm) (° C.) (h) (° C.) (° C.) (° C.) (s) (° C.) (h) HM1 A1 120 1100 1.0 850 550 — — 850   0.8 HM2 A2 120 1200 1.0 850 550 1000   30 — — HM3 A3 150 1180 1.5 900 600 800 600 — — HM4 A4 150 1230 1.5 880 750 — — — — HM5 A5 230 1230 2.5 850 550 — — 650 48 HM6 A6 230 1250 2.5 910 700 — — — — HM7 A7 250 1200 2.5 880 600 900 300 — — HM8 A8 250 1230 2.5 880 580 — — 750  5 HM9 A9 150 1200 1.5 880 600 — — 700 24 CS1 B1 150 1100 1.5 900 650 — — — — CS2 B2 150 1200 1.5 900 — — — — — CSS B3 150 1250 1.5 900 — — — — —

(19) The steel sheets of the above Examples HM1-HM9 and Comparative Examples CS1-CS3 were sampled and subjected to various tests for properties including mechanical properties. The related data thus measured are listed in Table 3.

(20) TABLE-US-00003 TABLE 3 Volumetric Tensile Mechanical Fraction of Properties of Hard Hot-rolled Sheet Reinforcing Tensile Elastic Particles Strength Elongation Density Modulus (vol. %) (MPa) (%) (kg/m.sup.3) (GPa) HM1 3.6 540 28.6 7500 209 HM2 7.1 675 15.2 7380 223 HM3 11.5 610 20.2 7100 242 HM4 6.5 618 17.8 7370 220 HM5 15.8 696 13.6 7020 257 HM6 11.2 580 20.1 7200 239 HM7 12.8 603 18.9 7240 250 HM8 10.2 586 22.4 7215 231 HM9 15.3 675 15.4 7080 254 CS1 0 372 39.8 7600 189 CS2 — — — — — CS3 — — — — —

(21) As seen from Table 3, the steel sheets have a tensile strength >500 MPa, a density <7600 kg/m.sup.3, an elastic modulus >200 GPa. Thus, a hot-rolled lightweight steel sheet having a low density, a high tensile strength, a high elastic modulus and a good ductility can be obtained by designing the composition and process reasonably according to the disclosure.

(22) FIGS. 1 and 2 show the cast structure of the lightweight steel of Comparative Example B2 at low and high magnifications respectively; and FIGS. 3 and 4 show the cast structure of the lightweight steel of Example A6 at low and high magnifications respectively. The arrows in FIGS. 2 and 4 indicate the hard reinforcing particles.

(23) As can be observed from FIGS. 1 and 2, in the slab microstructure of Comparative Example B2 of the lightweight steel, the ferrite matrix is enclosed by the continuously distributed hard reinforcing phase (mainly TiB.sub.2 particles). As can be seen from FIGS. 3 and 4, the primary phase and the eutectic product (i.e. the hard reinforcing phase) in Example A6 of the lightweight steel distribute discretely in the ferrite matrix. In fact, similar phenomena were observed on Comparative Example B3 and Examples A1-A5, A7-A9 corresponding to Comparative Example B2 and Example A6 respectively. Comparative Examples B2-B3 are free of Al element, while Examples A1-A9 comprise Al element. Hence, addition of Al element is favorable for improving the microstructure of a lightweight steel cast slab, reducing continuous distribution of hard reinforcing particles at grain boundaries in the matrix, and inhibiting enclosure of the grain boundaries in the matrix by a film-like hard reinforcing phase.

(24) FIGS. 5 and 6 show the morphologies of the steel sheets in Comparative Example CS2 and Examples HM6-HM8 after hot rolling.

(25) As can be observed from FIG. 5, the steel sheet of Comparative Example CS2 cannot be deformed well by hot rolling. As can be observed from FIG. 6, the steel sheets of Examples HM6-HM8 can be hot rolled to desired thicknesses. In fact, similar phenomena were observed on Comparative Example CS3 and Examples HM1-HM5, HM9 corresponding to Comparative Example CS2 and Examples HM6-HM8 respectively. Comparative Examples CS2-CS3 are free of Al element, while Examples HM1-HM9 comprise Al element. Hence, addition of Al element is favorable for hot rolling deformability of a steel sheet.

(26) FIGS. 7 and 8 show the microstructure of the steel sheet of Example HM6 after hot rolling at low and high magnifications respectively. The arrows in FIGS. 7 and 8 indicate the hard reinforcing particles.

(27) The distribution of the hard reinforcing particles in the ferrite matrix of the hot-rolled sheet is observable in FIGS. 7 and 8. It's shown that the elongate hard reinforcing phase in the cast structure is broken and refined due to thermodynamic deformation.

Examples HM10-HM13 of Method for Manufacturing Steel Sheets

(28) The steel sheets in the above Examples were manufactured with the following steps:

(29) (1) A lightweight steel material shown in Table 1 was smelted, and the resulting molten steel was cast by way of strip casting and rolled into a thin strip having a thickness of no more than 10 mm, wherein S, P and N were unavoidable impurities, the balance being Fe; and the cooling rate for solidifying the molten steel was about 320° C./s;

(30) (2) Hot rolling to obtain a hot-rolled sheet of 1.3 mm in thickness: the thin strip was hot rolled immediately with no aid of external heating, wherein the final rolling temperature was controlled at ≥850° C., the hot rolling reduction was 20-60%, and coiling was then performed at 400-750° C.;

(31) (3) Post-hot-rolling recrystallization annealing: when the hot-rolled sheet was subjected to recrystallization annealing by way of continuous annealing, the hot-rolled sheet was heated to a soaking temperature of 800-1000° C., held for 30-600 s, and then cooled to room temperature; when the hot-rolled sheet was subjected to recrystallization annealing by way of bell furnace annealing, the hot-rolled sheet was heated to a soaking temperature of 650-900° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

(32) Table 4 lists the specific process parameters in the method for manufacturing the steel sheets of Examples HM10-HM13.

(33) TABLE-US-00004 TABLE 4 Step (3) Continuous Bell Furnace Step (2) Annealing Annealing Step (1) Hot Rolling Final rolling Coiling Soaking soaking Soaking soaking Thickness Reduction temperature Temperature temperature time temperature time Material (mm) (%) (° C.) (° C.) (° C.) (s) (° C.) (h) HM10 A6 2.5 48 900 720 — — — — HM11 A2 3.2 59.4 860 550 — — 750 8 HM12 A8 3.2 59.4 880 600 900 400 — — HM13 A5 2.0 35 900 640 850 600 — —

(34) The steel sheets of the above Examples HM10-HM13 were sampled and subjected to various tests for properties including mechanical properties. The related data thus measured are listed in Table 5.

(35) TABLE-US-00005 TABLE 5 Volumetric Tensile Mechanical Fraction of Properties of Hard Hot-rolled Sheet Reinforcing Tensile Elastic Particles Strength Elongation Density Modulus (vol. %) (MPa) (%) (kg/m.sup.3) (GPa) HM10 10.7 612 20.4 7200 236 HM11 7.8 680 13.6 7380 228 HM12 11.0 574 20.6 7215 235 HM13 16.4 708 11.9 7020 250

(36) Meanwhile, metallographical examination on the above Examples HM10-HM13 shows that the matrix of the hot-rolled sheets is an equiaxed ferrite structure, and the average particle size of the hard reinforcing particles of mainly TiB.sub.2 distributed in the matrix is about 3-5 μm.

Examples HM14-HM18 of Method for Manufacturing Steel Sheets

(37) The steel sheets in the above Examples were manufactured with the following steps:

(38) (1) In Examples HM14-HM18, the lightweight steel materials corresponding to A1, A3, A5, A6 and A9 in Table 1 were respectively smelted and continuously cast to obtain slabs of 120-300 mm in thickness, wherein S, P and N were unavoidable impurities, the balance being Fe;

(39) (2) Hot rolling to obtain hot rolled sheets: the heating temperature was 1000-1250° C., the soaking time was 0.5-3 h, the final rolling temperature was ≥850° C., and coiling was performed at 400-750° C.;

(40) (3) Post-hot-rolling recrystallization annealing: when the hot-rolled sheets were subjected to recrystallization annealing by way of continuous annealing, the hot-rolled sheets were heated to a soaking temperature of 800-1000° C., held for 30-600 s, and then cooled to room temperature; when the hot-rolled sheets were subjected to recrystallization annealing by way of bell furnace annealing, the hot-rolled sheets were heated to a soaking temperature of 650-900° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace;

(41) (4) Pickling,

(42) (5) Cold rolling: the cold rolling reduction was controlled at 25-75%;

(43) (6) Recrystallization annealing of cold-rolled sheets: when the post-cold-rolling recrystallization annealing was performed by way of continuous annealing, the cold-rolled sheets were heated to a soaking temperature of 700-900° C., held for 30-600 s, and then cooled to room temperature; when the post-cold-rolling recrystallization annealing was performed by way of bell furnace annealing, the cold-rolled sheets were heated to a soaking temperature of 600-800° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

(44) Table 6 lists the specific process parameters in the method for manufacturing the steel sheets of Examples HM14-HM18.

(45) TABLE-US-00006 TABLE 6 Step (3) Continuous Bell Furnace Step (2) Annealing Annealing Step (1) Heating soaking Final rolling Coiling Soaking soaking Soaking holding Thickness Temperature time Temperature Temperature Temperature time Temperature Time Material (mm) (° C.) (h) (° C.) (° C.) (° C.) (s) (° C.) (h) HM14 A1 120 1100 1.0 850 550 — — 850 0.8 HM15 A3 150 1180 1.5 900 600 — — 650 48 HM16 A5 230 1230 2.5 850 550 800 600 — — HM17 A6 230 1250 2.5 910 700 — — — — HM18 A9 150 1200 1.5 880 600 — — 700 24 Step (6) Continuous Bell Furnace Step (5) Annealing Annealing Cold Rolling Soaking soaking Soaking soaking Reduction temperature time temperature time (%) (° C.) (s) (° C.) (h) HM14 56.3 — — 730   4.0 HM15 56.3 850 600 — — HM16 59.4 — — 700 48 HM17 59.4 900 240 — — HM18 59.4 — — 720 36

(46) The steel sheets of the above Examples HM14-HM18 were sampled and subjected to various tests for properties including mechanical properties. The related data thus measured are listed in Table 7.

(47) TABLE-US-00007 TABLE 7 Volumetric Tensile Mechanical Fraction of Properties of Hard Cold-rolled Sheet Reinforcing Tensile Elastic Particles Strength Elongation Density Modulus (vol. %) (MPa) (%) (kg/m.sup.3) (GPa) HM14 3.6 560 33.2 7500 204 HM15 11.5 601 21.8 7100 241 HM16 15.8 670 14.8 7020 252 HM17 11.2 607 22.4 7200 243 HM18 15.3 696 14.7 7080 259

(48) As seen from Table 7, the steel sheets have a tensile strength >500 MPa, and an elastic modulus >200 GPa. Thus, a hot-rolled lightweight steel sheet having a low density, a high tensile strength, a high elastic modulus and a good ductility can be obtained according to the disclosure.

Examples HM19-HM22 of Method for Manufacturing Steel Sheets

(49) The steel sheets in the above Examples were manufactured with the following steps:

(50) (1) A lightweight steel material shown in Table 1 was smelted, and the resulting molten steel was cast by way of strip casting and rolled into a thin strip having a thickness of no more than 10 mm, wherein S, P and N were unavoidable impurities, the balance being Fe; and the cooling rate for solidifying the molten steel was about 200° C./s;

(51) (2) Hot rolling to obtain a hot-rolled sheet: the thin strip was hot rolled immediately with no aid of external heating, wherein the final rolling temperature was controlled at ≥850° C., the hot rolling reduction was 20-60%, and coiling was then performed at 400-750° C.;

(52) (3) Post-hot-rolling recrystallization annealing: when the post-hot-rolling recrystallization annealing was performed by way of continuous annealing, the hot-rolled sheet was heated to a soaking temperature of 800-1000° C., held for 30-600 s, and then cooled to room temperature; when the post-hot-rolling recrystallization annealing was performed by way of bell furnace annealing, the hot-rolled sheet was heated to a soaking temperature of 650-900° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace;

(53) (4) Pickling;

(54) (5) Cold rolling: in this step, the cold rolling reduction was controlled at 25-75%;

(55) (6) Recrystallization annealing of cold-rolled sheets: when the post-cold-rolling recrystallization annealing was performed by way of continuous annealing, the cold-rolled sheets were heated to a soaking temperature of 700-900° C., held for 30-600 s, and then cooled to room temperature; when the post-cold-rolling recrystallization annealing was performed by way of bell furnace annealing, the cold-rolled sheets were heated to a soaking temperature of 600-800° C., held for 0.5-48 h, and then cooled to room temperature along with the furnace.

(56) Table 8 lists the specific process parameters in the method for manufacturing the steel sheets of Examples HM19-HM22.

(57) TABLE-US-00008 TABLE 8 Step (3) Continuous Bell Furnace Step (2) Annealing Annealing Step (1) Hot Roiling Final rolling Coiling Soaking soaking Soaking soaking Thickness Reduction temperature Temperature temperature time temperature time Material (mm) (%) (° C.) (° C.) (° C.) (s) (° C.) (h) HM19 A5 4.0 40 900 600 — — 700 18 HM20 A2 4.5 46.7 880 720 — — — — HM21 A6 4.0 40 900 680 900 600 — — HM22 A8 3.6 33.3 880 580 — — 720  8 Step (6) Continuous Bell Furnace Step (5) Annealing Annealing Cold Rolling Soaking soaking Soaking soaking Reduction temperature time temperature time (%) (° C.) (s) (° C.) (h) HM19 50 900 420 — — HM20 44 850 600 — — HM21 50 — — 700 8 HM22 44 — — 740 6

(58) The steel sheets of the above Examples HM19-HM22 were sampled and subjected to various tests for properties including mechanical properties. The related data thus measured are listed in Table 9.

(59) TABLE-US-00009 TABLE 9 Volumetric Tensile Mechanical Fraction of Properties of Hard Cold-rolled Sheet Reinforcing Tensile Elastic Particles Strength Elongation Density Modulus (vol. %) (MPa) (%) (kg/m.sup.3) (GPa) HM19 14.6 692 15.4 7020 248 HM20 8.2 643 14.8 7380 230 HM21 10.9 607 18.9 7200 234 HM22 11.9 582 19.4 7215 240

(60) Metallographical examination on the above Examples HM19-HM22 shows that the matrix of the annealed cold-rolled sheets is an equiaxed ferrite structure, and the average particle size of the hard reinforcing particles of mainly TiB.sub.2 distributed in the matrix is about 3-6 μm.

(61) It is to be noted that there are listed above only specific examples of the invention. Obviously, the invention is not limited to the above examples. Instead, there exist many similar variations. All variations derived or envisioned directly from the disclosure of the invention by those skilled in the art should be all included in the protection scope of the invention.