Process for manufacturing a steel sheet, and steel belt made therefrom

A process for manufacturing a cold rolled and aged steel sheet for a steel belt, comprising:

• subjecting a slab of steel having a chemical composition containing from 0.60 to 0.80% of C, 1.0% or less of Si, from 0.10 to 1.0% of Mn, 0.020% or less of P, 0.010% or less of S, from 0.1 to 1.0% of Cr, from 0 to 0.5% of V, from 0 to 0.1% of Ti, from 0 to 0.1% of Nb, and from 0 to 0.01% of B, all in terms of percentage by mass, with the balance of Fe and unavoidable impurities, to hot rolling under a condition of a finish hot rolling temperature of from 800 to 900° C., an average cooling rate from the finish rolling to coiling of 20° C. per second or more, and a coiling temperature of from 450 to 650° C.;
• then subjecting the hot-rolled slab to cold rolling of a total rolling reduction ratio of 40% or more and a reduction ratio per one pass of less than 12%, without performing a heat treatment to make a cold-rolled steel sheet;
• and then subjecting the cold-rolled steel sheet to an aging treatment of maintaining a temperature of from 200 to 500° C. for a period of from 0.5 to 30 hours, thereby producing a steel sheet having a volume fraction of a pearlite structure in a metallic structure of 70% or more, a tensile strength in the rolling direction of 1,100 MPa or more, and a crack propagation resistance according to the following item (A) of 600 MPa or more:
• (A) a test piece, which has a hole having a diameter of 4.0 mm at the center of the test piece and two notches each having a width of approximately 2.5 mm formed on both sides in a width direction of the hole, is subjected to a tensile test in the longitudinal direction thereof (which agrees with the rolling direction) at room temperature and a tension speed of 0.3 mm/min, so as to provide a load-elongation curve, from which a maximum load is obtained, and a value obtained by dividing the maximum load by an initial cross sectional area (45 mm×thickness) is designated as the crack propagation resistance (unit:
• MPa).

A process for manufacturing a cold rolled and aged steel sheet for a steel belt, comprising:

• subjecting a slab of steel having a chemical composition containing more than 0.60% and 0.80% or less of C, from 0.10 to 1.00% of Si, from 0.10 to 1.00% of Mn, from 0.002 to 0.020% of P, from 0.001 to 0.010% of S, from 0.10 to 1.00% of Cr, from 0 to 0.50% of V, from 0 to 0.10% of Ti, from 0 to 0.10% of Nb, and from 0 to 0.010% of B, all in terms of percentage by mass, with the balance of Fe and unavoidable impurities, to hot rolling under a condition of a finish hot rolling temperature of from 800 to 900° C., an average cooling rate from the finish rolling to coiling of 25° C. per second or more, and a coiling temperature of from 450 to 650° C.;
• then subjecting the hot-rolled slab to cold rolling of a total rolling reduction ratio of 40% or more and a reduction ratio per one pass of less than 12%, without performing a heat treatment to make a cold-rolled steel sheet;
• and then subjecting the cold-rolled steel sheet slab to an aging treatment of maintaining a temperature of from 200 to 500° C. for a period of from 0.5 to 30 hours, thereby producing a steel sheet having a volume fraction of a pearlite structure in a metallic structure of 90% or more, a tensile strength in the rolling direction of 1,300 MPa or more, and a crack propagation resistance according to the following item (A) of 600 MPa or more:
• (A) a test piece, which has a hole having a diameter of 4.0 mm at the center of the test piece and two notches each having a width of approximately 2.5 mm formed on both sides in a width direction of the hole, is subjected to a tensile test in the longitudinal direction thereof (which agrees with the rolling direction) at room temperature and a tension speed of 0.3 mm/min, so as to provide a load-elongation curve, from which a maximum load is obtained, and a value obtained by dividing the maximum load by an initial cross sectional area (45 mm×thickness) is designated as the crack propagation resistance (unit:
• MPa).