With taking the technology measures such as increasing Mn content in steel from 1. 55% ~ 1.65% to 1

75% 〜1. 85%

replacing 0. 20% ~0. 25% Ni in steel by 0. 25% 〜0. 35% Cr

adding 0. 01 % ~0. 04% Ti for microalloying

controlling [ N

]

≤80 x 10

-6

and [O

]

≤15 x 10

-6

by RH vacuum treatment

decreasing casting secondary cooling water rate from 0. 11 L/kg to 0. 08 L/kg and modifying water distribution ratio to increase temperature of bloom at outlet from 620 ~ 680 ℃ to 700 ~ 750 °C

bloom cooling with shielding and bar products slow cooling

the production cost of R3 grade mooring chain steel is reduced

the longitudinal cracks of 350 mm x 470 mm cast bloom are eliminated and the strength and the toughness at -20 ℃ of products all meet the requirement of standard.