High-strength steel plate having high dynamic deformation resistance and method of manufacturing the same

A method for producing a high strength hot-rolled steel sheet with high flow stress during dynamic deformation where the microstructure of the steel sheet in the final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominating phase, and the third phase including retained austenite at a volume fraction between 3% and 50%, wherein the average value σdyn (MPa) of the flow stress in the range of 3-10% of equivalent strain when deformed in a strain rate range of 5×102−5×103 (1/sec) after pre-deformation of greater than 0% and less than or equal to 10% of equivalent strain satisfies the inequality:

• σdyn≧0.766×TS+250 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5×10−4−5×10−3 (1/sec) without deformation, the value (M) determined by the solid solution (C) in the retained austenite and the average Mn equivalents of the steel material {Mn eq=Mn+(Ni+Cr+Cu+Mo)/2}, defined by the equation M=678−428×(C)−33 Mneq is at least 70 and not greater than 250, the difference between the retained austenite volume fraction without pre-deformation and the retained austenite volume fraction after applying a pre-deformation of 5% of equivalent strain is at least 30% of the retained austenite volume fraction without pre-deformation, and the work hardening coefficient between 1% and 5% of a strain is at least 0.080 and of a strain yield strength is at least 40, which is characterized in that the method comprises the steps of:
• continuously casting a molten metal into a slab containing, in terms of wt %, C at from 0.03% to 0.3%, either or both Si and Al at a total of from 0.5% to 3.0% with the remainder Fe as a primary component,
• directly hot rolling the slab, with or without slab reheating step, into strip, finish hot rolling the strip at a finishing temperature of Ar3−50° C. to Ar3+120° C.,
• cooling the hot rolled strip with an average cooling rate of at least 5° C./sec, and
• coiling the cooled strip at a temperature of no greater than 500° C.

A method for producing a high strength cold-rolled steel sheet with high flow stress during dynamic deformation where the microstructure of the steel sheet in the final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominating phase, and the third phase including retained austenite at a volume fraction between 3% and 50%, wherein the average value σdyn (MPa) of the flow stress in the range of 3-10% of equivalent strain when deformed in a strain rate range of 5×102−5×103 (1/sec) after pre-deformation of greater than 0% and less than or equal to 10% of equivalent strain, satisfies the inequality:

• σdyn≧0.766×TS+250 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5×10−4-5×10−3 (1/sec) without deformation, the value (M) determined by the solid solution (C) in the retained austenite and the average Mn equivalents of the steel material {Mneq=Mn+(Ni+Cr+Cu+Mo)/2}, defined by the equation M=678−428×(C)−33 Mneq is at least 70 and not greater than 250, the difference between the retained austenite volume fraction without pre-deformation and the retained austenite volume fraction after applying a pre-deformation of 5% of equivalent strain is at least 30% of the retained austenite volume fraction without pre-deformation, and the work hardening coefficient between 1% and 5% of a strain is at least 0.080 and of a strain yield strength is at least 40, which is characterized in that the method comprises the steps of;
• continuously casting a molten metal into a slab containing, in terms of wt %, C at from 0.03% to 0.3%, either or both Si and Al at a total of from 0.5% to 3.0% with the remainder Fe as a primary component,
• directly hot rolling the slab, with or without slab reheating step, into strip,
• finish hot rolling the strip at a finishing temperature of Ar3−50° C. to Ar3+120° C.,
• cooling the hot rolled strip with an average cooling rate at least 5° C./sec,
• coiling the cooled strip at a temperature of no greater than 500° C.,
• acid pickling a rewind strip,
• cold rolling the acid pickled strip,
• continuously annealing the cold rolled strip at a temperature of from 0.1×(Ac3−Ac1)+Ac1° C. to AC3+50° C. for 10 seconds to 3 min,
• cooling the annealed strip to a primary cooling stop temperature in the range of 550-720° C. at a primary cooling rate of 1-10° C./sec,
• further cooling the primary cooled strip to a secondary cooling stop temperature in the range of 150-450° C. at a secondary cooling rate of 10-200° C./sec,
• holding the secondary cooled strip at a temperature in the range of 150-500° C. for 15 seconds to 20 minutes, and
• cooling the strip to room temperature.

A method for producing a high strength cold-rolled steel sheet with high flow stress during dynamic deformation where the microstructure of the steel sheet in the final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominating phase, and the third phase including retained austenite at a volume fraction between 3% and 50%, wherein the average value σdyn (Mpa) of the flow stress in the range of 3-10% of equivalent strain when deformed in a strain range of 5×102-5×103 (1/sec) after pre-deformation of greater than 0% and less than or equal to 10% of equivalent strain, satisfied the inequality:

• σdyn≧0.766×TS+250 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5×10−4−5×10−3 (1/sec) without deformation, the value (M) determined by the solid solution (C) in the retained austenite and the average Mn equivalents of the steel material {Mneq=Mn+(Ni+Cr+Cu+Mo)/2}, defined by the equation M=678-428×(C)−33 Mneq is at least 70 and not greater than 250, the difference between the retained austenite volume fraction without pre-deformation and the retained austenite volume fraction after applying a pre-deformation of 5% of equivalent strain is at least 30% of the retained austenite volume fraction without pre-deformation, and the work hardening coefficient between 1% and 5% of a strain is at least 0.080 and of a strain yield strength is at least 40, which is characterized in that the method comprises the steps of;
• continuously casting a molten metal into a slab containing, in terms of wt %, C at from 0.03% to 0.3%, either or both Si and Al at a total of from 0.5% to 3.0% with the remainder Fe as a primary component,
• directly hot rolling the slab, with or without slab reheating step, into strip,
• finish hot rolling the strip at a finishing temperature of Ar3−50° C. to Ar3+120° C.,
• cooling the hot rolled strip with an average cooling rate at least 5° C./sec,
• coiling the cooled strip at a temperature of no greater that 500° C.,
• acid pickling a rewind strip,
• cold rolling the acid pickled strip,
• continuously annealing the strip at a temperature of from 0.1×(Ac3−Ac1)+Ac1° C. to Ac3+50° C. for 10 seconds to 3 min,
• primary cooling the annealed strip to a secondary cooling start temperature Tq in the range of 550-720° C. at a primary cooling rate of 1-10° C./sec,
• further cooling the cooled strip to a secondary cooling stop temperature Te in the range of from the temperature Tem, which is determined by the component and annealing temperature To to 500° C. at a secondary cooling rate of 10-200° C./sec,
• holding the secondary cooled strip at a temperature Toa in the range of Te−50° C. to 500° C. for 15 seconds to 20 minutes, and
• cooling the strip to room temperature.