PROCEDE POUR FABRIQUER UNE BANDE D'ACIER A HAUTE RESISTANCE LAMINEE A FROID AYANT UNE EXCELLENTE APTITUDE AU FORMAGE A LA PRESSE

UN PROCEDE POUR FABRIQUER UNE BANDE D'ACIER A HAUTE RESISTANCE LAMINEE A FROID AYANT UNE EXCELLENTE APTITUDE AU FORMAGE A LA PRESSE CONSISTE A LAMINER A CHAUD UNE BRAME D'ACIER CONTENANT DES QUANTITES DETERMINEES DE CARBONE, DE MANGANESE, DE PHOSPHORE, D'ALUMINIUM SOLUBLE ET D'AZOTE, BOBINER LA BANDE A UNE TEMPERATURE DE 650 A 770C, LAMINER A FROID LA BANDE BOBINEE, SOUMETTRE LA BANDE LAMINEE A FROID A UN RECUIT CONTINU DE DUREE DETERMINEE ENTRE 750 ET 880C, REFROIDIR DANS DES CONDITIONS DETERMINEES LA BANDE LAMINEE A FROID POUR LUI DONNER UNE STRUCTURE A DOUBLE PHASE DE FERRITE ET DE PHASE DE TRANSFORMATION A BASSE TEMPERATURE PUIS EFFECTUER UN TRAITEMENT DE SURVIEILLISSEMENT DE DUREE DETERMINEE ENTRE 260 ET 360C POUR LUI CONFERER D'EXCELLENTES PROPRIETES PERMETTANT NOTAMMENT DE L'UTILISER POUR LA FABRICATION DES CARROSSERIES D'AUTOMOBILES. A method for manufacturing a high-strength cold-rolled steel strip excellent in press-formability, which comprises the steps of: preparing a slab of an aluminum-killed steel consisting essentially of, in weight percentage: -Carbon from 0.02 to 0.06%, -Manganese from 0.06 to 0.25%, -Phosphorus from 0.01 to 0.06%, -Soluble aluminum from 0.020 to 0.060%, -Nitrogen up to 0.005%, and - the balance iron and incidental impurities; hot-rolling said slab to prepare a hot-rolled steel strip; coiling said steel strip at a temperature within the range of from 650 DEG to 770 DEG C.; cold-rolling said hot-rolled steel strip thus coiled to prepare a cold-rolled steel strip; subjecting said cold-rolled steel strip to a continuous annealing treatment for a prescribed period of time at a temperature within the range of from 750 DEG to 880 DEG C.; cooling said cold-rolled steel strip continuously annealed at a cooling rate of at least: exp {-5.6 (Cwt. % gamma +Mn wt. %/6+Si wt. %/24)+7.8} DEG C./sec from a temperature region of from Ar1 to Ar1+60 DEG C. to convert the structure thereof into a dual-phase structure of ferrite and a low-temperature transformation phase; and then, subjecting said cold-rolled steel strip having said dual-phase structure to an over-ageing treatment for a prescribed period of time at a temperature within the range of from 260 DEG to 360 DEG C.