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.

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 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 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 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 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 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.