An analysis on width to thickness ratio of flat ingot at home and abroad has been carried out to get the effect of width to thickness ratio on quality of ingot. And the 60 t flat ingot with large width to thickness ratio and large water-cooling mould casting equipment are developed. The casting test of 61 t flat ingot with width 3 070 mm

thickness 1 022 mm

width to thickness ratio 3. 0 and casting temperature 1 580℃ of steel Q345B (/% : 0. 19C

0. 36Si

1. 39Mn

0. 010P

0. 003S) steelmaking by EAF-LF-VD process and rolled to 220 mm heavy plate has been carried out. Results by non-destructive testing and sampling analysis show that the quality of ingot is better； the heavy plate mechanical properties are yield strength 348 ~373 MPa

tensile strength 562 ~588 MPa

impact energy at -40 °C 40. 76 ~88・80 J

elongation 17. 00% ~28. 50%

Z direction reduction of area 33. 79% -60. 88% and rating of grain size 8 ~9 to meet the requirement of standard ； and the non-destructive testing results of heavy plate meets the requirement of JB/T4730-2005I -grade detection of defect. The commercial test shows that the technology of flat ingot water-cooling mould for large width heavy plate has the ability to provide bloom for high quality extra-heavy plate.