MY157318A - Cold-rolled steel sheet with excellent aging property and bake hardening and method for manufacturing the same - Google Patents

Abstract

A COLD-ROLLED STEEL SHEET WITH AN EXCELLENT AGING PROPERTY AND EXCELLENT BAKE HARDENABILITY IS PROVIDED. A STEEL HAVING A COMPOSITION CONTAINING, BY MASS, C: 0.020% TO 0.070%, Si: 0.05% OR LESS, Mn: 0.1% TO 0.5%, P: 0.05% OR LESS, S: 0.02% OR LESS, Al: 0.02% TO 0.08%, AND N: 0.005% TO 0.02% IS HEATED AT 1,150°C OR HIGHER AND FINISH-ROLLED AT A FINISH ROLLING TEMPERATURE OF 850°C OR HIGHER TO OBTAIN A HOT-ROLLED SHEET, AND THE HOT-ROLLED SHEET IS COILED AT A COILING TEMPERATURE THAT SATISFIES A SPECIFIC RELATIONAL FORMULA REGARDING THE Al CONTENT AND THE N CONTENT. SUBSEQUENTLY, THE HOT-ROLLED SHEET IS COLD-ROLLED AT A REDUCTION RATIO OF 60% TO 90%. THE COLD-ROLLED SHEET IS THEN HEATED TO AN ANNEALING TEMPERATURE THAT SATISFIES A SPECIFIC RELATIONAL FORMULA REGARDING THE Al CONTENT, THE N CONTENT, AND THE COILING TEMPERATURE AT A HEATING RATE THAT CHANGES THROUGH TWO STAGES, SOAKED, AND COOLED TO 500°C OR LOWER AT A COOLING RATE OF 5°C/S OR MORE. A GALVANIZED LAYER SUCH AS A HOT-DIP GALVANIZED LAYER MAY BE FURTHER FORMED ON A SURFACE OF THE STEEL SHEET. THUS, A COLD-ROLLED STEEL SHEET HAVING A MICROSTRUCTURE WHICH CONTAINS 0.0010% OR MORE OF SOLUTE N AND, IN TERMS OF AREA FRACTION, 80% OR MORE OF A FERRITE PHASE HAVING AN AVERAGE GRAIN SIZE OF 7 µM OR LESS, AND IN WHICH PRECIPITATES HAVING A SIZE OF 0.05 TO 5 µM IN TERMS OF AVERAGE EQUIVALENT CIRCLE DIAMETER ARE PRECIPITATED IN GRAINS OF THE FERRITE PHASE IS OBTAINED. ACCORDINGLY, IT IS POSSIBLE TO PREVENT THE GENERATION OF WRINKLES DUE TO AGING AND TO ACHIEVE AN AMOUNT OF BAKE HARDENING OF 50 MPA OR MORE AFTER APPLICATION OF A RELATIVELY LOW STRAIN OF LESS THAN 2.0% AND HEAT TREATMENT CORRESPONDING TO PAINT-BAKING.