Ultra low carbon, cold rolled steel sheet and galvanized steel sheet having improved fatigue properties and processes for producing the same

An ultra low carbon, galvanized steel sheet, for deep drawing, improved in fatigue properties of a base metal and a spot weld zone, comprising by weight C:

• from more than 0.0015 to 0.0026%, Si:
• not more than 1.0%, Mn:
• 0.03 to less than 0.3%, P;
• 0.015 to 0.15% S:
• 0.0010 to 0.020%, Al:
• 0.005 to 0.15%, N:
• 0.0005 to 0.0080% and B:0.0003 to 0.0030% with the balance consisting of Fe and unavoidable impurities.

A process for producing an ultra low carbon, cold rolled steel sheet, improved in fatigue properties of a base metal and a spot weld zone, comprising the steps of;

• heating a slab, comprising by weight C:
• from more than 0.0015 to 0.0026%, Si:
• not more than 1.2%, Mn:
• 0.03 to less than a 0.3%, P:0.015 to 0.15%, S:
• 0.0010 to 0.020% Al:
• 0.005 to 0.15%, N:
• 0.0005 to 0.0080% and B:
• 0.0003 to 0.0030% with the balance Consisting of Fe and unavoidable impurities, to a temperature of 1050.degree. C. or above;
• hot rolling the heated slab and terminating the hot rolling at the Ar.sub.3 transformation point or above;
• coiling the hot tolled strip in the temperature range of from room temperature to 750.degree. C.;
• transferring the hot rolled coil to a cold rolling machine where it a cold rolled with a reduction ratio of not less than 70% continuously annealing the cold rolled strip in the temperature range of from 600.degree. to 900.degree. C.;
• and temper rolling the annealed string with a reduction ratio (%) falling within the range specified by the following formulae:
• %.gtoreq.1.5.times.(1-400.times.C), %.gtoreq.2080.times.(C - 0.0015), %.gtoreq.3.0 and 0.0001.ltoreq.C.ltoreq.0.0026 wherein C represent the carbon content in % by weight.

A process for producing an ultra low carbon, galvanized steel sheet, improved in fatigue properties of a base metal and a spot weld zone, comprising the steps of;

• heating a slab, comprising by weight C:
• from more than 0.0015 to 0.0026%, Si:
• not more than 1.0%, Mn:
• 0.03 to less than 0.3%k, P:
• 0.015 to 0.15%, S:
• 0.0010 to 0.020%, Al:
• 0.005 to 0.15%, N:
• 0.0005 to 0.0080% and B:
• 0.0003 to 0.0030% with the balance consisting of Fe and unavoidable impurities, to a temperature of 1050.degree. C. or above;
• hot rolling the heated slab and terminating the hot rolling at the Ar.sub.3 transformation point or above;
• coiling the hot rolled strip in the temperature range of from room temperature to 750.degree. C;
• transferring the hot rolled coil to a cold rolling machine where cold rolling is performed, with a reduction ratio of not less than 70%, while uncoiling the hot rolled strip;
• annealing the cold rolled strip in the temperature range of from 600.degree. to 900.degree. C. and then galvanizing the annealed strip;
• and temper rolling the galvanized strip with a reductions ratio (%) falling within the range specified by the following formulae:
• %.gtoreq.1.5.times.(1-400.times.C), %.gtoreq.2080.times.(C - 0.0015), %.ltoreq.3.0 and 0.0001.ltoreq.0.0026 wherein C represents the carbon content in % by weight.