To provide a cold rolled steel sheet, wherein a steel sheet having a composite structure consisting mainly of ferrite and martensite is the object, and the concentration of the strain (stress) in the boundary part is suppressed by suppressing an extreme increase in hardness in the boundary between both the phases, so as to have excellent strength, ductility and stretch flange formability in combination, and to provide its production method.
The high strength cold rolled steel sheet having excellent stretch flange formability is the steel sheet having a composite structure where the total of the space factor (V) of ferrite and the space factor (Va') of martensite accounts for 80%, wherein in the cross-section of the 1/4 position in a depth direction from the surface of the steel sheet, when, on the line segment of the longest straight line drawn in the crystal grains of martensite observed in a field of 200m200m, the hardness (Hb) of the martensite in the position apart 0.3 m from the boundary between the ferrite and martensite (the edge part of the line segment) and the hardness (Hc) of the martensite in the position apart 2 m from the same grain boundary are obtained, the Hb and Hc satisfy the relation in inequality (1): Hc-Hb0.
| JPH09129444 | SOFT MAGNETIC THIN FILM FOR HIGH FREQUENCY |
| WO/2019/146748 | STEEL FOR MOORING CHAIN, AND MOORING CHAIN |
| JPH03207845 | PRODUCTION OF ALLOYING HOT DIP GALVANIZED STEEL SHEET |
MASUDA TOMOKAZU
MIURA MASAAKI
MUKAI YOICHI
| JPH10147838A | 1998-06-02 |
Tadashi Sugakawa
Osamu Futakuchi
Hiroaki Ito
Next Patent: METHOD FOR FORMING NANOMETAL PARTICLE AND NANOORDER WIRING