HIGH-STRENGTH LOW-CARBON MARTENSITIC HIGH HOLE EXPANSION STEEL AND MANUFACTURING METHOD THEREFOR

Abstract

A low-carbon martensitic high hole expansion steel with a tensile strength above 980 MPa, and a manufacturing method therefor, the weight percentage of the chemical components thereof being: C 0.03-0.10%, Si 0.5-2.0%, Mn 1.0-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, and O≤0.0030%, and the remainder being Fe and other inevitable impurities. The high hole expansion steel of the present invention has a yield strength of ≥800 MPa and tensile strength of ≥980 MPa, a lateral extension rate A50≥8%, and a hole expansion ratio of ≥30%, passes cold bending performance tests (d≤4a, 180°), and can be used for passenger car chassis parts that require high strength and thinning such as control arms and sub-frames.

Claims

1. A low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more, comprising a chemical composition based on weight percentage of: C 0.03-0.10%, Si 0.5-2.0%, Mn 1.0-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≤0.0030%, and a balance of Fe and other unavoidable impurities.

2. The low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 1, wherein: (1) the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more comprises a chemical composition based on weight percentage of: C 0.03˜0.06%, Si 0.5˜2.0%, Mn 1.0˜2.0%, P≤=0.02%, S≤0.003%, Al 0.02˜0.08%, N≤0.004%, Mo 0.1˜0.5%, Ti 0.01˜0.05%, O≤0.0030% and a balance of Fe and other unavoidable impurities; or (2) the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more comprises a chemical composition based on weight percentage of: C 0.06-0.10%, Si 0.8-2.0%, Mn 1.5-2.0%, P≤=0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≤0.0030% and a balance of Fe and other unavoidable impurities.

3. (canceled)

4. The low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 1, wherein the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more further comprises one or more elements of Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.5%, and Ni≤0.5%.

5. The low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 2, wherein the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more described in (1) has one or more of the following characteristics: the content of C is 0.04-0.055%, the content of Si is 0.8-1.4%, the content of Mn is 1.4-1.8%, the content of S is controlled at 0.0015% or lower, the content of Al is 0.02-0.05%, the content of N is controlled at 0.003% or lower, the content of Ti is 0.01-0.03% and the content of Mo is 0.15-0.35%, and the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more described in (2) has one or more of the following characteristics: the content of C is 0.07-0.09%, the content of Si is 1.0-1.4%, the content of Mn is 1.6-1.9%, the content of S is controlled at 0.0015% or lower, the content of Al is 0.02-0.05%, the content of N is controlled at 0.003% or lower, the content of Ti is 0.01-0.03% and the content of Mo is 0.15-0.35%.

6. (canceled)

7. The low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 1, wherein the high hole expansion steel has a microstructure of martensite or tempered martensite and residual austenite, wherein the content of residual austenite in the microstructure is ≤5% by volume.

8. The low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 1, wherein the high hole expansion steel has a yield strength of ≥800 MPa, a tensile strength of ≥980 MPa, a transverse elongation A.sub.50 of ≥8%, a hole expansion ratio of ≥30%; optionally, the high hole expansion steel has an impact toughness at −40° C. of ≥60 J.

9. The low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 2, wherein the high hole expansion steel described in (1) has a yield strength of ≥800 MPa, a tensile strength of ≥980 MPa, a transverse elongation A.sub.50 of ≥8%, a hole expansion ratio of ≥50%, and has passed cold bending test (d≤4a, 180°); optionally, the high hole expansion steel has an impact toughness at −40° C. of ≥140 J; the high hole expansion steel described in (2) has a yield strength of ≥900 MPa, a tensile strength of ≥1180 MPa, a transverse elongation A.sub.50 of ≥10%, a hole expansion ratio of ≥30%; optionally, the high hole expansion steel described in (2) has an impact toughness at −40° C. of ≥60 J, and/or the high hole expansion steel has passed cold bending test (d≤4a, 180°).

10. The low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 9, wherein the low carbon martensitic high hole expansion steel described in (1) has a yield strength of 800˜890 MPa, a tensile strength of 980˜1150 MPa, a transverse elongation A.sub.50 of 8˜13%, a hole expansion ratio of 50˜85%, an impact toughness at −40° C. of 140˜185 J, and has passed cold bending test (d≤4a, 180°); wherein the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more has a microstructure of martensite+residual austenite, wherein the content of residual austenite in the microstructure is ≤5% by volume; the low carbon martensitic high hole expansion steel described in (2) has a yield strength of 900˜1000 MPa, a tensile strength of 1200˜1280 MPa, a transverse elongation of 10˜13%, a hole expansion ratio of 30˜50%, an impact toughness at −40° C. of 60˜100 J; or the low carbon martensitic high hole expansion steel described in (2) has a yield strength of 940˜1000 MPa, a tensile strength of 1210˜1300 MPa, a transverse elongation of 10˜13%, a hole expansion ratio of 30˜50%, an impact toughness at −40° C. of 80˜110 J and has passed cold bending test (d≤4a, 180°); optionally the high hole expansion steel described in (2) has a microstructure of tempered martensite+residual austenite, wherein the content of residual austenite in the microstructure is ≤5% by volume.

11.-12. (canceled)

13. A method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 1, comprising the following steps: 1) Smelting and casting: wherein the above components according to claim 1 are subjected to smelting by a converter or an electric furnace, secondary refining by a vacuum furnace, and then casting to form a blank or ingot; 2) Re-heating of the blank or ingot at a heating temperature of 1100-1200° C., holding for 1-2 hours; 3) Hot rolling: wherein the blank or ingot is hot rolled at an initial rolling temperature of 950˜1100° C.; wherein 3-5 passes of heavy reduction rolling at ≥950° C. is carried out and the cumulative deformation is ≥50%; then final 3-7 passes of rolling is carried out and the cumulative deformation is ≥70%; wherein a final rolling temperature is 800-950° C.; optionally, after 3-5 passes of heavy reduction rolling, an intermediate blank is held till 900-950° C., and then subjected to final 3-7 passes of rolling; 4) Cooling: first, air-cooling for 0-10 s is carried out, and then the strip steel is water cooled at a cooling rate of ≥50° C./s to a certain temperature between room temperature and Ms point, then coiled and cooled to room temperature after coiling, or the strip steel is air cooled for 0-10 s, followed by direct water cooled at a cooling rate of ≥30° C./s to room temperature for coiling, or the strip steel is air cooled for 0-10 s, followed by water cooled at a cooling rate of ≥30° C./s to a martensite phase transition start temperature Ms or a lower temperature, then coiled and slowly cooled to room temperature after coiling; 5) Pickling wherein a moving speed of the strip steel is adjusted within a range of 30-100 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤2%; wherein the strip steel is then subjected to rinsing, surface drying, and oiling.

14. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 13, wherein, after step 5) of pickling, the strip steel is subjected to rinsing at a temperature of 35-50° C., surface drying at a temperature of 120-140° C., and oiling.

15. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 13, wherein the method further comprises step 4-1) between step 4) and 5): annealing, wherein bell type annealing is carried out at a heating rate of ≥20° C./h, wherein a bell type annealing temperature is 100-300° C. and a bell type annealing time is 12-48 h; wherein the steel plate is cooled to ≤100° C. at a cooling rate of ≤50° C./h and leaves the furnace.

16. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 13, wherein the method comprises the following steps: 1) Smelting and casting: wherein components based on weight percentage of: C 0.03˜0.06%, Si 0.5˜2.0%, Mn 1.0˜2.0%, P≤0.02%, S≤0.003%, Al 0.02˜0.08%, N≤0.004%, Mo 0.1˜0.5%, Ti 0.01˜0.05%, O≤0.0030%, and optionally one or more elements of Cr≤0.5%, Br≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.5% and Ni≤0.5%, and a balance of Fe and other unavoidable impurities are subjected to smelting by a converter or an electric furnace, secondary refining by a vacuum furnace, and then casting to form a blank or ingot; 2) Re-heating of the blank or ingot at a heating temperature of 1100-1200° C., holding for 1-2 hours; 3) Hot rolling: wherein the blank or ingot is hot rolled at an initial rolling temperature of 950˜1100° C.; wherein 3-5 passes of heavy reduction rolling is performed at a temperature of ≥950° C. with a cumulative deformation of ≥50%, to obtain an intermediate blank; wherein the intermediate blank is held till 920-950° C., then subjected to 3-5 passes of rolling with a cumulative deformation of ≥70%, wherein a final rolling temperature is 800-920° C.; 4) Cooling: wherein air cooling is performed for 0-10 s first for dynamic recovery and dynamic recrystallization, and then the strip steel is water cooled at a cooling rate of ≥50° C./s to a certain temperature of Ms or lower (between room temperature and Ms point), coiled, and cooled to room temperature after coiling; 5) Pickling wherein a moving speed of the strip steel is adjusted within a range of 30-100 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤2% to reduce elongation loss of the strip steel; wherein the strip steel is then subjected to rinsing, surface drying, and oiling.

17. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 13, wherein the method comprises the following steps: 1) Smelting and casting: wherein components, based on weight percentage, of: C 0.06˜0.10%, Si 0.8˜2.0%, Mn 1.5˜2.0%, P≤0.02%, S≤0.003%, Al 0.02˜0.08%, N≤0.004%, Mo 0.1˜0.5%, Ti 0.01˜0.05%, O≤0.0030%, and optionally one or more elements of Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.5% and Ni≤0.5%, and a balance of Fe and other unavoidable impurities are subjected to smelting by a converter or an electric furnace, secondary refining by a vacuum furnace, and then casting to form a blank or ingot; 2) Re-heating of the blank or ingot at a heating temperature of 1100-1200° C., holding for 1-2 hours; 3) Hot rolling: wherein the blank or ingot is hot rolled at an initial rolling temperature of 950˜1100° C.; wherein 3-5 passes of heavy reduction rolling is performed at a temperature of ≥950° C. with a cumulative deformation of ≥50%; then 3-7 passes of rolling is performed with a cumulative deformation of ≥70%; wherein a final rolling temperature is 800-950° C.; 4) Cooling: wherein air cooling is performed for 0-10 s first, and then the strip steel is water cooled at a cooling rate of ≥30° C./s to room temperature, and coiled; 5) Annealing wherein bell type annealing is carried out at a heating rate of ≥20° C./h, wherein a bell type annealing temperature is 100-300° C. and a bell type annealing time is 12-48 h; wherein the steel plate is cooled to ≤100° C. at a cooling rate of ≤50° C./h and leaves the furnace; 6) Pickling wherein a moving speed of the strip steel is adjusted within a range of 30-90 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤1.5%; wherein the strip steel is then subjected to rinsing at a temperature of 35-50° C., surface drying at a temperature of 120-140° C., and oiling.

18. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 13, wherein the method comprises the following steps: 1) Smelting and casting: wherein components, based on weight percentage, of: C 0.06˜0.10%, Si 0.8˜2.0%, Mn 1.5˜2.0%, P≤0.02%, S≤0.003%, Al 0.02˜0.08%, N≤0.004%, Mo 0.1˜0.5%, Ti 0.01˜0.05%, O≤0.0030%, and optionally one or more elements of Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.5% and Ni≤0.5%, and a balance of Fe and other unavoidable impurities are subjected to smelting by a converter or an electric furnace, secondary refining by a vacuum furnace, and then casting to form a blank or ingot; 2) Re-heating of the blank or ingot at a heating temperature of 1100-1200° C., holding for 1-2 hours; 3) Hot rolling: wherein the blank or ingot is hot rolled at an initial rolling temperature of 950˜1100° C.; wherein 3-5 passes of heavy reduction rolling is performed at a temperature of ≥950° C. with a cumulative deformation of ≥50% to obtain an intermediate blank; wherein the intermediate blank is held till 900-950° C., then 3-7 passes of rolling is performed with a cumulative deformation of ≥70%%; wherein a final rolling temperature is 800-900° C.; 4) Cooling: wherein air cooling is performed for 0-10 s first, and then the strip steel is water cooled at a cooling rate of ≥30° C./s to a martensite phase transition start temperature Ms or a lower temperature, coiled and slowly cooled to room temperature after coiling; 5) Annealing wherein bell type annealing is carried out at a heating rate of ≥20° C./h, wherein a bell type annealing temperature is 100-300° C. and a bell type annealing time is 12-48 h; wherein the steel plate is cooled to ≤100° C. at a cooling rate of ≤50° C./h and leaves the furnace; 6) Pickling wherein a moving speed of the strip steel is adjusted within a range of 30-90 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤1.5% to reduce elongation loss of the strip steel; wherein the strip steel is then subjected to rinsing, surface drying and oiling.

19. The low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 4, wherein the content of Cr is 0.2-0.4%, the content of B is 0.0005-0.0015%, the content of Ca is ≤0.002%; the content of Nb, V is ≤0.03%, respectively; and/or the content of Cu, Ni is ≤0.3%, respectively.

20. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 13, wherein: the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more comprises a chemical composition based on weight percentage of: C 0.03-0.06%, Si 0.5-2.0%, Mn 1.0-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≤0.0030% and a balance of Fe and other unavoidable impurities; or the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more comprises a chemical composition based on weight percentage of: C 0.06-0.10%, Si 0.8-2.0%, Mn 1.5-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≤0.0030% and a balance of Fe and other unavoidable impurities.

21. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 16, wherein: in step 3), wherein the 3-5 passes of heavy reduction rolling is performed at a temperature of ≥950° C. with a cumulative deformation of ≥60%; the intermediate blank is subjected to 3-5 passes of rolling with a cumulative deformation of ≥85%; in step 4), the cooling rate is 50-85° C./s.

22. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 17, wherein: in step 3), the 3-5 passes of heavy reduction rolling are performed at a temperature of ≥950° C. with a cumulative deformation of ≥60%; and the 3-7 passes of rolling is performed with a cumulative deformation of ≥85%; in step 4), the cooling rate is 30-65° C./s; in step 5), bell type annealing is carried out at a heating rate of 20-40° C./h, and the cooling rate is 15-50° C./h.

23. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 18, wherein: in step 3), the 3-5 passes of heavy reduction rolling are performed at a temperature of ≥950° C. with a cumulative deformation of ≥60% l the 3-7 passes of rolling is performed with a cumulative deformation of ≥85%; in step 4), the cooling rate is 30-70° C./s; in step 5), the bell type annealing is carried out at a heating rate of 20-50° C./s; the steel plate is cooled to ≤100° C. at a cooling rate of 25-50° C./h.

24. The method for manufacturing the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more according to claim 13, wherein: the low carbon martensitic high hole expansion steel having a tensile strength of 980 MP or more further comprises one or more elements of Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.5%, and Ni≤0.5%; the low carbon martensitic high hole expansion steel has a microstructure of martensite or tempered martensite and residual austenite, wherein the content of residual austenite in the microstructure is ≤5% by volume; and/or the low carbon martensitic high hole expansion steel has a yield strength of ≥800 MPa, a tensile strength of ≥980 MPa, a transverse elongation A.sub.50 of ≥8%, a hole expansion ratio of ≥30%; optionally, the high hole expansion steel has an impact toughness at −40° C. of ≥60 J.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0110] FIG. 1 is a process flow diagram of the manufacturing method of 980 MPa grade ultra-low-carbon martensitic high hole expansion steel described in the present disclosure.

[0111] FIG. 2 is a schematic diagram of the rolling process in the manufacturing method of 980 MPa grade ultra-low-carbon martensitic high hole expansion steel described in the present disclosure.

[0112] FIG. 3 is a schematic diagram of the cooling process in the manufacturing method of 980 MPa grade ultra-low-carbon martensitic high hole expansion steel described in the present disclosure.

[0113] FIG. 4 is a process flow diagram of the manufacturing method of the high plasticity high hole expansion steel of 1180 MPa grade described in Preparation Example II and III of the present disclosure.

[0114] FIG. 5 is a schematic diagram of the rolling process in the manufacturing method of the high plasticity high hole expansion steel of 1180 MPa grade described in Preparation Example II of the present disclosure.

[0115] FIG. 6 is a schematic diagram of the cooling process in the manufacturing method of the high plasticity high hole expansion steel of 1180 MPa grade described in Preparation Example II of the present disclosure.

[0116] FIG. 7 is a schematic diagram of the bell type annealing process in the manufacturing method of the high plasticity high hole expansion steel of 1180 MPa grade described in Preparation Example II and III of the present disclosure.\

[0117] FIG. 8 is a typical metallographic photo of the high hole expansion steel of Example 10 according to the present disclosure.

[0118] FIG. 9 is a typical metallographic photo of the high hole expansion steel of Example 12 according to the present disclosure.

[0119] FIG. 10 is a typical metallographic photo of the high hole expansion steel of Example 14 according to the present disclosure.

[0120] FIG. 11 is a typical metallographic photo of the high hole expansion steel of Example 16 according to the present disclosure.

[0121] FIG. 12 is a schematic diagram of the rolling process in the manufacturing method of the high plasticity high hole expansion steel of 1180 MPa grade described in Preparation Example III of the present disclosure.

[0122] FIG. 13 is a schematic diagram of the cooling process in the manufacturing method of the high plasticity high hole expansion steel of 1180 MPa grade described in Preparation Example III of the present disclosure.

DETAILED DESCRIPTION

[0123] In the following examples, the tensile performances (yield strength, tensile strength, elongation) were tested in accordance with International Standard ISO6892-2-2018; the hole expansion ratio was tested in accordance with International Standard ISO16630-2017; the impact toughness at −40° C. was tested in accordance with International Standard ISO14556-2015; and the bending performance was tested in accordance with International Standard ISO7438-2005.

PREPARATION EXAMPLE I

[0124] Referring to FIG. 1-FIG. 3, the method for manufacturing the 980 MPa-grade ultra-low-carbon martensitic high hole expansion steel according to the present disclosure comprises the following steps: [0125] 1) Smelting and casting: [0126] wherein the above components are subjected to smelting by a converter or an electric furnace, secondary refining by a vacuum furnace, and then casting to form a blank or ingot; [0127] 2) Re-heating of the blank or ingot at a heating temperature of 1100-1200° C., holding for 1-2 hours; [0128] 3) Hot rolling: [0129] wherein the blank or ingot is hot rolled at an initial rolling temperature of 950˜1100° C.; [0130] wherein cumulative deformation after 3-5 passes of heavy reduction rolling at ≥950° C. is ≥50%; the intermediate blanket is held till 920-950° C., then subjected to final 3-5 passes of rolling with a cumulative deformation of ≥70%; wherein a final rolling temperature is 800-920° C.; [0131] 4) Cooling: [0132] wherein air cooling is performed for 0-10 s first for dynamic recovery and dynamic recrystallization, and then the strip steel is water cooled at a cooling rate of ≥50° C./s to Ms point or a lower temperature (between room temperature and Ms point), and then coiled and cooled (preferably at a cooling rate ≤20° C./h) to room temperature; [0133] 5) Pickling [0134] wherein a moving speed of the strip steel is adjusted within a range of 30-100 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤2%; wherein the strip steel is then subjected to rinsing at a temperature of 35-50° C., surface drying at a temperature of 120-140° C., and oiling.

[0135] In the Preparation Example, the compositions of the Examples of the high hole expansion steel according to the present disclosure are shown in Table 1. The production process parameters for the Examples of the steel according to the present disclosure are listed in Table 2 and Table 3, wherein the thickness of the steel blank in the rolling process is 120 mm The mechanical performances of the Examples of the steel plates according to the present disclosure are listed in Table 4.

[0136] It can be seen from Table 4, the yield strength of the steel coil is ≥800 MPa, while the tensile strength is ≥980 MPa, and the elongation is usually in the range of 8-13%. The impact energy is relatively stable. The low-temperature impact energy at −40° C. is stabilized in the range of 140-180 J. The content of residual austenite varies as a function of the coiling temperature, generally by 1.5-5%. The hole expansion ratio satisfies ≥50%.

[0137] It can be seen from the above Examples, the 980 MPa high-strength steel according to the present disclosure exhibits good matching of strength, plasticity, toughness and hole expandability. It is especially suitable for parts that require high strength, reduced thickness, hole expansion and flanging forming, such as a control arm in an automobile chassis structure. It can also be used for parts such as wheels that need hole flanging. Therefore, it has broad application prospects.

TABLE-US-00001 TABLE 1 (unit: weight %) Ex. C Si Mn P S Al N Mo Ti Cr B Ca Nb V Cu Ni O 1 0.060 1.98 1.76 0.0011 0.0028 0.066 0.0030 0.10 0.030 0.11 0.0011 / 0.035 / / / 0.0027 2 0.032 1.77 1.40 0.0013 0.0029 0.047 0.0027 0.17 0.025 0.35 / 0.0025 / 0.050 / / 0.0020 3 0.048 0.84 1.68 0.0016 0.0030 0.023 0.0028 0.49 0.042 0.50 0.0005 / 0.058 / / 0.30 0.0025 4 0.055 1.26 1.90 0.0014 0.0024 0.039 0.0029 0.29 0.014 0.28 0.0012 0.0030 / / 0.50 / 0.0029 5 0.042 1.18 1.88 0.0010 0.0028 0.065 0.0038 0.38 0.028 / / / 0.020 0.040 / / 0.0024 6 0.057 0.89 1.07 0.0015 0.0022 0.054 0.0033 0.33 0.050 0.37 0.0013 0.0020 / 0.030 / 0.50 0.0028 7 0.053 0.53 1.96 0.0014 0.0027 0.080 0.0035 0.43 0.010 0.43 0.0020 / / / 0.15 0.25 0.0021 8 0.041 1.64 1.73 0.0012 0.0025 0.036 0.0022 0.18 0.020 / 0.0019 0.0050 / 0.015 0.30 0.10 0.0030

TABLE-US-00002 TABLE 2 Rough Finish Initial rolling Intermediate rolling Final Air Water Steel Heating Holding rolling cumulative blank cumulative rolling cooling cooling plate Coiling temperature time temperature deformation temperature deformation temperature time rate thickness temperature ° C. h ° C. % ° C. % ° C. s ° C./s mm ° C. Ex. 1 1170 1.2 1040 70 950 89 880 5 60 6 390 Ex. 2 1160 1.4 1100 50 920 92 860 9 50 5 125 Ex. 3 1200 1.0 1030 65 930 90 920 4 55 3 280 Ex. 4 1130 1.8 950 55 925 94 820 7 70 4 250 Ex. 5 1150 1.5 1020 60 940 88 850 10 65 4 Rt Ex. 6 1100 2.0 1000 75 935 93 800 6 80 2 180 Ex. 7 1140 1.6 980 80 930 90 890 0 75 3 230 Ex. 8 1180 1.1 1050 70 945 91 830 8 85 2 150

TABLE-US-00003 TABLE 3 Moving speed Pickling Tension Rinsing Drying of strip steel temper- leveling temper- temper- during pickling ature rate ature ature m/min ° C. % ° C. ° C. Ex. 1 65 80 1.8 40 135 Ex. 2 30 83 1.1 35 120 Ex. 3 90 77 0.4 47 128 Ex. 4 45 81 1.3 42 140 Ex. 5 70 85 0.6 50 133 Ex. 6 100 75 2.0 37 125 Ex. 7 60 82 1.0 41 134 Ex. 8 85 78 1.6 38 130

TABLE-US-00004 TABLE 4 Mechanical performances of steel plates Hole −40° C. Residual Yield Tensile expansion impact austenite strength strength Elongation ratio energy content MPa MPa % % J % Ex. 1 809 1002 11.5 51.6 168 1.86 Ex. 2 821 1034 13.0 60.9 170 2.49 Ex. 3 806 1104 9.0 75.1 154 4.58 Ex. 4 850 1011 10.5 62.8 144 2.64 Ex. 5 819 1033 11.0 56.9 158 0.57 Ex. 6 820 1032 9.5 57.2 182 4.33 Ex. 7 883 1039 8.0 83.5 162 4.02 Ex. 8 866 1050 10.5 60.7 180 3.56 Note: The impact energy is obtained by converting the measured impact energy of a sample having an actual thickness into the impact energy of a standard sample of 10*10*55 mm in proportion based on equivalent effect.

PREPARATION EXAMPLE II

[0138] Referring to FIG. 4-FIG. 7, the method for manufacturing the high plasticity high hole expansion steel of 1180 MPa grade according to the present disclosure comprises the following steps: [0139] 1) Smelting and casting: [0140] wherein the above components are subjected to smelting by a converter or an electric furnace, secondary refining by a vacuum furnace, and then casting to form a blank or ingot; [0141] 2) Re-heating of the blank or ingot at a heating temperature of 1100-1200° C., holding for 1-2 hours; [0142] 3) Hot rolling: [0143] wherein an initial rolling temperature is 950˜1100° C.; wherein cumulative deformation after 3-5 passes of heavy reduction rolling at ≥950° C. is ≥50%; then final 3-5 passes of rolling is carried out and the cumulative deformation is ≥70%; wherein a final rolling temperature is 800-950° C.; [0144] 4) Cooling: [0145] wherein air cooling is performed for 0-10 s first, and then the strip steel is water cooled at a cooling rate of ≥30° C./s to room temperature and then coiled; [0146] 5) Annealing: [0147] wherein bell type annealing is carried out at a heating rate of ≥20° C./h, wherein a bell type annealing temperature is 100-300° C. and a bell type annealing time is 12-48 h; wherein the steel plate is cooled to ≤100° C. at a cooling rate of ≤50° C./h and leaves the furnace. [0148] 6) Pickling: [0149] wherein a moving speed of the strip steel is adjusted within a range of 30-90 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤1.5%; wherein the strip steel is then subjected to rinsing at a temperature of 35-50° C., surface drying at a temperature of 120-140° C., and oiling.

[0150] In the Preparation Example, the compositions of the Examples of the high hole expansion steel according to the present disclosure are shown in Table 5. The production process parameters for the Examples of the steel according to the present disclosure are listed in Table 6 and Table 7, wherein the thickness of the steel blank in the rolling process is 120 mm The mechanical performances of the Examples of the steel plates according to the present disclosure are listed in Table 8.

[0151] It can be seen from Table 8, the yield strength of the steel coil is ≥900 MPa, while the tensile strength is ≥1180 MPa, and the elongation is usually in the range of 10-13%. The impact energy is relatively stable. The low-temperature impact energy at −40° C. is stabilized in the range of 60-100 J. The content of residual austenite varies as a function of the coiling temperature. The hole expansion ratio satisfies ≥30%.

[0152] It can be seen from the above Examples, the high hole expansion steel of 1180 MPa grade according to the present disclosure exhibits good matching of strength, plasticity, toughness and hole expandability. It is especially suitable for parts that require high strength, reduced thickness, hole expansion and flanging forming, such as a control arm in an automobile chassis structure. It can also be used for parts such as wheels that need hole flanging and has broad application prospects.

[0153] FIG. 8-FIG. 11 shows the typical metallographic structure of the steel plate of Example 10 #, 12 #, 14 # and 16 #, respectively. It can be seen from the metallographic photos that the structure is single-phase low-carbon martensite with a certain amount of residual austenite, which exhibits relatively higher elongation and hole expansion ratio at the same strength level.

TABLE-US-00005 TABLE 5 (unit: weight %) Ex. C Si Mn P S Al N Mo Ti Cr B Ca Nb V Cu Ni O 9 0.082 1.48 1.51 0.009 0.0023 0.065 0.0039 0.42 0.011 0.49 0.0010 / / / / / 0.0022 10 0.061 1.89 1.77 0.011 0.0018 0.034 0.0023 0.15 0.023 0.23 0.0009 / 0.015 / / / 0.0023 11 0.080 0.82 1.95 0.008 0.0009 0.078 0.0033 0.35 0.015 / / 0.003 /  0.015 0.15 0.30 0.0020 12 0.068 1.74 1.55 0.009 0.0014 0.043 0.0025 0.17 0.036 / / / 0.020 / / 0.20 0.0028 13 0.094 0.94 1.90 0.012 0.0015 0.052 0.0030 0.23 0.020 0.30 0.0015 0.002 0.060 / 0.30 0.50 0.0024 14 0.075 1.55 1.68 0.008 0.0010 0.022 0.0024 0.50 0.048 / / / / 0.05 / 0.10 0.0026 15 0.083 1.02 1.83 0.013 0.0009 0.071 0.0028 0.28 0.018 / 0.0005 0.005 / 0.03 0.25 0.15 0.0025 16 0.100 1.13 1.59 0.010 0.0024 0.058 0.0038 0.18 0.014 / / 0.001 0.030 / 0.50 0.25 0.0028

TABLE-US-00006 TABLE 6 Finish Initial Rough rolling Air Water Steel Heating Holding rolling rolling cumulative Final rolling cooling cooling plate Temperature time temperature cumulative deformation temperature time rate thickness ° C. h ° C. deformation % ° C. s ° C./s mm Ex. 9 1180 1.2 1050 70 89 950 3 55 4 Ex. 10 1160 1.4 1100 50 92 820 7 30 2 Ex. 11 1200 1.0 1050 65 90 900 2 50 6 Ex. 12 1140 1.7 1060 55 94 800 6 35 3 Ex. 13 1150 1.5 950 60 88 870 9 60 2 Ex. 14 1130 1.8 980 75 93 830 0 45 4 Ex. 15 1160 1.3 1000 80 90 880 5 40 5 Ex. 16 1100 2.0 1020 70 91 850 7 65 3

TABLE-US-00007 TABLE 7 Moving speed of strip Bell type Bell type temperature steel Heating annealing annealing Cooling for leaving during Pickling Tension Rinsing Drying rate temperature time rate the furnace pickling temperature leveling temperature temperature ° C./h ° C. h ° C./h ° C. m/min ° C. rate % ° C. ° C. Ex. 9 30 230 20 30 75 80 82 1.3 40 135 Ex. 10 23 200 24 40 80 45 76 0.8 35 120 Ex. 11 35 100 48 20 50 70 75 1.5 47 128 Ex. 12 20 280 12 45 55 35 80 0.5 42 140 Ex. 13 32 150 36 20 60 50 77 1.0 50 133 Ex. 14 27 300 28 50 100 30 79 0.3 37 125 Ex. 15 25 125 42 25 65 90 81 0.9 41 134 Ex. 16 40 180 30 15 95 60 83 1.2 38 130

TABLE-US-00008 TABLE 8 Mechanical performances of steel plates Hole Residual −40° C. Yield Tensile expansion austenite impact strength strength Elongation ratio content energy MPa MPa % % % J Ex. 9 905 1206 10.5 35 2.38 70 Ex. 10 953 1234 11.5 41 3.85 62 Ex. 11 932 1213 12.0 36 3.26 81 Ex. 12 941 1225 11.0 44 2.77 73 Ex. 13 990 1263 10.0 37 3.34 86 Ex. 14 963 1209 13.0 42 4.22 75 Ex. 15 975 1279 11.5 47 2.51 90 Ex. 16 966 1247 11.0 38 4.28 88 Note: The impact energy is obtained by converting the measured impact energy of a sample having an actual thickness into the impact energy of a standard sample of 10*10*55 mm in proportion based on equivalent effect.

PREPARATION EXAMPLE III

[0154] Referring to FIG. 4, 7, 12 and FIG. 13, the method for manufacturing the high plasticity high hole expansion steel of 1180 MPa grade according to the present disclosure comprises the following steps: [0155] 1) Smelting and casting: [0156] wherein the above components are subjected to smelting by a converter or an electric furnace, secondary refining by a vacuum furnace, and then casting to form a blank or ingot; [0157] 2) Re-heating of the blank or ingot at a heating temperature of 1100-1200° C., holding for 1-2 hours; [0158] 3) Hot rolling: [0159] wherein an initial rolling temperature is 950˜1100° C.; wherein cumulative deformation after 3-5 passes of heavy reduction rolling at ≥950° C. is ≥50%; then the intermediate blank is held till 900-950° C., then subjected to final 3-7 passes of rolling with a cumulative deformation of ≥70%; wherein a final rolling temperature is 800-900° C.; [0160] 4) Cooling: [0161] wherein air cooling is performed for 0-10 s first, and then the strip steel is water cooled at a cooling rate of ≥30° C./s to Ms or a lower temperature, coiled and cooled slowly (at a cooling rate of ≤20° C./h) to room temperature; [0162] 5) Annealing: [0163] wherein bell type annealing is carried out at a heating rate of ≥20° C./h, wherein a bell type annealing temperature is 100-300° C. and a bell type annealing time is 12-48 h; wherein the steel plate is cooled to ≤100° C. at a cooling rate of ≤50° C./h and leaves the furnace. [0164] 6) Pickling: [0165] wherein a moving speed of the strip steel is adjusted within a range of 30-90 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤1.5%; wherein the strip steel is then subjected to rinsing at a temperature of 35-50° C., surface drying at a temperature of 120-140° C., and oiling.

[0166] In the Preparation Example, the compositions of the Examples of the high hole expansion steel according to the present disclosure are shown in Table 9. The production process parameters for the Examples of the steel according to the present disclosure are listed in Table 10 and Table 11, wherein the thickness of the steel blank in the rolling process is 120 mm The mechanical performances of the Examples of the steel plates according to the present disclosure are listed in Table 12.

[0167] It can be seen from Table 12, the yield strength of the steel coil is ≥900 MPa, while the tensile strength is ≥1180 MPa, and the elongation is usually in the range of 10-13%. The impact energy is relatively stable. The low-temperature impact energy at −40° C. is stabilized in the range of 80-110 J. The content of residual austenite varies as a function of the coiling temperature. The hole expansion ratio satisfies ≥30%.

[0168] It can be seen from the above Examples, the high strength steel of 1180 MPa grade according to the present disclosure exhibits good matching of strength, plasticity, toughness and hole expandability. It is especially suitable for parts that require high strength, reduced thickness, hole expansion and flanging forming, such as a control arm in an automobile chassis structure. It can also be used for parts such as wheels that need hole flanging and has broad application prospects.

TABLE-US-00009 TABLE 9 (unit: weight %) Ex. C Si Mn P S Al N Mo Ti Cr B Ca Nb V Cu Ni O 17 0.065 1.98 1.76 0.0011 0.0028 0.066 0.0030 0.10 0.029 0.33 0.0010 / 0.030 / 0.50 / 0.0028 18 0.097 1.77 1.50 0.0013 0.0019 0.047 0.0027 0.15 0.025 0.35 / 0.002 / 0.030 / / 0.0024 19 0.088 0.83 1.68 0.0016 0.0005 0.023 0.0028 0.49 0.022 0.29 0.0012 / 0.060 / / 0.15 0.0026 20 0.090 1.01 1.98 0.0014 0.0024 0.039 0.0029 0.29 0.014 0.40 0.0011 0.001 / / 0.10 / 0.0027 21 0.062 1.18 1.88 0.0010 0.0008 0.065 0.0038 0.38 0.050 0.28 0.0013 / 0.015 0.025 / / 0.0024 22 0.080 1.89 1.77 0.0015 0.0022 0.054 0.0033 0.35 0.015 0.50 0.0005 0.005 / 0.013 / 0.30 0.0029 23 0.093 1.40 1.96 0.0014 0.0010 0.080 0.0035 0.43 0.011 0.33 0.0017 0.003 0.045 / 0.20 0.50 0.0022 24 0.071 1.64 1.83 0.0012 0.0025 0.036 0.0022 0.18 0.020 / 0.0019 / / 0.050 0.30 0.10 0.0020

TABLE-US-00010 TABLE 10 Rough Finish Initial rolling Intermediate rolling Final Air Water Steel Heating Holding rolling cumulative blank cumulative rolling cooling cooling plate Coiling temperature time temperature deformation temperature deformation temperature time rate thickness temperature Ex. ° C. h ° C. % ° C. % ° C. s ° C./s mm ° C. 17 1170 1.4 1040 70 950 89 880 3 50 6 Rt 18 1160 1.5 1100 50 900 92 800 8 60 5 225 19 1200 1.0 1070 65 930 90 840 4 35 3 200 20 1130 1.8 950 55 910 94 900 5 70 4 350 21 1150 1.6 1020 60 940 88 860 0 65 4 225 22 1100 2.0 1000 75 920 93 830 6 30 2 180 23 1140 1.7 980 80 930 90 870 0 55 3 330 24 1180 1.2 1050 70 925 91 820 6 65 2 250

TABLE-US-00011 TABLE 11 Moving speed of Bell type Bell type Temperature strip steel Tension Heating annealing annealing Cooling for leaving during Pickling leveling Rinsing Drying rate temperature time rate the furnace pickling temperature rate temperature temperature Ex. ° C./h ° C. h ° C./h ° C. m/min ° C. % ° C. ° C. 17 40 140 26 26 80 70 82 1.4 40 135 18 32 240 18 20 65 90 76 0.8 35 120 19 25 260 14 40 100 30 75 1.2 47 128 20 44 120 32 34 40 65 80 1.0 42 140 21 30 300 12 50 75 80 77 0.7 50 133 22 35 150 40 22 55 40 79 1.5 37 125 23 20 100 48 30 85 85 81 0.5 41 134 24 37 200 20 38 70 55 83 1.1 38 130

TABLE-US-00012 TABLE 12 Mechanical performances of steel plates Hole −40° C. Residual Yield Tensile expansion impact austenite strength strength Elongation ratio energy content Ex. MPa MPa % % J % 17 968 1223 11.0 44.0 84 3.52 18 971 1276 12.0 36.0 96 4.26 19 983 1213 12.0 48.0 87 3.84 20 945 1283 12.5 42.0 110 4.88 21 994 1228 11.5 31.0 103 3.03 22 948 1282 12.0 39.0 85 3.95 23 953 1218 11.0 35.0 94 4.99 24 975 1279 11.5 46.0 99 3.87 Note: The impact energy is obtained by converting the measured impact energy of a sample having an actual thickness into the impact energy of a standard sample of 10*10*55 mm in proportion based on equivalent effect.