High-strength steel sheet highly resistant to dynamic deformation and excellent in workability and process for the production thereof

A method for producing a press formable high-strength hot-rolled steel sheet with high flow stress during dynamic deformation where the microstructure of the steel sheet in its final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength σs when deformed in a strain rate range of 5×10−4-5×10−3 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength σd when deformed at a strain rate of 5×102-5×103 (l/s) after said pre-deformation, σd−σs, is at least 60 MPa, the difference between the average value σdyn (MPa) of the flow stress at an equivalent strain in the range of 3-10% when deformed in a strain rate range of 5×102-5×103 (l/s) and the average value σst (MPa) of the flow stress at an equivalent strain in the range of 3-10% when deformed in a strain rate range of 5×10−4-5×10−3 (l/s) satisfies the inequality:

• (σdyn−σst)≧−0.272×TS+300 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 (l/s), the value (M) determined by the solid solution (C) in said retained austenite and the average Mn equivalents of the steel sheet {Mneq=Mn+(Ni+Cr+Cu+Mo)/2}, defined by the equation M=678−428×(C)−33 Mneq is at least −140 and less than 70, the retained austenite volume fraction of the steel sheet after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10% is at least 2.5%, the ratio between the initial volume fraction of the retained austenite V(O) and the volume fraction of the retained austenite after pre-deformation at an equivalent strain of 10% V(10), V(10)/V(O), is at least 0.3, and the work hardening coefficient calculated from stresses at 5% and 10% of strain is at least 0.130, 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 a strip,
• completely finishing hot rolling at a temperature of Ar3−50° C. to Ar3+120° C.,
• cooling the hot rolled strip with an average cooling rate at least 5° C./sec, and,
• coiling the cooled strip at a temperature of no greater than 500° C.

A method for producing a press formable high strength cold-rolled steel sheet with high flow stress during dynamic deformation where the microstructure of the steel sheet in its final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength σs when deformed in a strain rate range of 5×10−4-5×10−3 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength σd when deformed at a strain rate of 5×102-5×103 (l/s) after said pre-deformation, σd−σs, is at least 60 MPa, the difference between the average value σdyn (MPa) of the flow stress at an equivalent strain in the range of 3-10% when deformed in a strain rate range of 5×102-5×103 (l/s) and the average value σst (MPa) of the flow stress at an equivalent strain in the range of 3-10% when deformed in a strain rate range of 5×10−4-5×10−3 (l/s) satisfies the inequality:

• (σdyn−σst)≧−0.272×TS+300 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 (l/s), the value (M) determined by the solid solution (C) in said retained austenite and the average Mn equivalents of the steel sheet {Mn eq=Mn+(Ni+Cr+Cu+Mo)/2}, defined by the equation M=678−428×(C)−33 Mneq is at least −140 and less than 70, the retained austenite volume fraction of the steel sheet after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10% is at least 2.5%, the ratio between the initial volume fraction of the retained austenite V(O) and the volume fraction of the retained austenite after pre-deformation at an equivalent strain of 10% V(10), V(10)/V(O), is at least 0.3, and the work hardening coefficient calculated from stresses at 5% and 10% of strain is at least 0.130, 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 a slab reheating step, into a strip,
• completely finishing hot rolling at a 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 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 200-450° C. at a secondary cooling rate of 10-200° C./sec,
• holding the secondary cooled strip at a temperature in the range of 200-500° C. for 15 seconds to 20 minutes, and
• cooling the strip to room temperature.

A method for producing a press formable high-strength cold-rolled steel sheet with high flow stress during dynamic deformation where the microstructure of the steel sheet in its final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength σs when deformed in a strain rate range of 5×10−4-5×10−3 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength σd when deformed at a strain rate of 5×102-5×103 (l/s) after said pre-deformation, σd−σs, is at least 60 MPa, the difference between the average value σdyn (MPa) of the flow stress at an equivalent strain in the range of 3-10% when deformed in a strain rate range of 5×102-5×103 (l/s) and the average value σst (MPa) of the flow stress at an equivalent strain in the range of 3-10% when deformed in a strain rate range of 5×10−4-5×10−3 (l/s) satisfies the inequality:

• (σdyn−σst)≧−0.272×TS+300 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 (l/s), the value (M) determined by the solid solution (C) in said retained austenite and the average Mn equivalents of the steel sheet (Mn eq=Mn+(Ni+Cr+Cu+Mo)/2}, defined by the equation M=678−428×(C)−33 Mneq is at least −140 and less than 70, the retained austenite volume fraction of the steel sheet after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10% is at least 2.5%, the ratio between the initial volume fraction of the retained austenite V(O) and the volume fraction of the retained austenite after pre-deformation at an equivalent strain of 10% V(10), V(10)/V(O) is at least 0.3, and the work hardening coefficient calculated from stresses at 5% and 10% of strain is at least 0.130, 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 a slab reheating step, into a strip,
• completely finishing hot rolling at a 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 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.