High strength steel sheet adapted for press forming and method of producing the same

A high strength cold rolled steel sheet having a non-aging property, a low yield ratio less than 65%, deep drawability and secondary forming brittleness adapted for press forming and for use as interior or exterior sheets for automobiles, said steel consisting essentially of:

• C:
• from 0.01 wt % to less than 0.1 wt %,
• Si:
• from 0.1 wt % to 1.2 wt %,
• Mn:
• not more than 3.0 wt %,
• Ti:
• a ratio of effective *Ti(wt %) represented by the following equation to said C(wt %), that is the effective *Ti(wt %)/C(wt %) is from 4 to 12:
• effective *Ti(wt%)=Ti(wt %)-l.5S(wt %)-3.43N(wt %)
• B:
• from 0.0005 wt % to 0.005 wt %,
• Ca:
• from 0.0005 wt % to 0.005 wt %,
• Al:
• not more than 0.1 wt %,
• P:
• not more than 0.1 wt %, provided that a ratio of P/C is less than 1.5.
• S:
• not more than 0.02 wt %,
• N:
• not more than 0.005 wt %, and the remainder being iron and inevitable impurities.

A method for producing a high strength steel sheet having a non-aging property, a low yield ratio property, deep drawability, and secondary forming brittleness adapted for press forming, said method comprising the steps of:

• preparing a steel slab consisted essentially of:
• C:
• from 0.01 wt % to less than 0.1 wt %,
• Si:
• from 0.1 wt % to 1.2 wt %,
• Mn:
• not more than 3.0 wt %,
• Ti:
• a ratio of effective *Ti(wt %) represented by the following equation to said C(wt %), that is the effective *Ti(wt %)/C(wt %) is from 4 to 12:
• effective *T(wt %)=Ti(wt %)-1.5S(wt %)-3.43N(wt %),
• B:
• from 0.0005 wt % to 0.005 wt %,
• Al:
• not more than 0.1 wt %,
• P:
• not more than 0.1 wt %, provided that a ratio of P/C is less than 1.5,
• S:
• not more than 0.02 wt %, and
• N:
• not more than 0.005 wt %,
• heating the steel slab in a temperature range of 1100.degree. C.-1280.degree. C. and hot rolling the steel slab to provide a hot rolled sheet.