Study on Section Homogeneity of V-N Microalloyed High Strength Bulb Flat Steel
Chen Xuehui, Yang Caifu, Su Hang, Chai Feng
Central Iron and Steel Research Institute
No. 76 Xueyuan Nanlu, Haidian District, Beijing 100081, China
Phone: +86 18612708773
Email: [email protected]
Key words：bulb flat steel, V-N Microalloyed, homogeneity
INTRODUCTION
The bulb flat steel plays a critical role in shipbuilding, which makes up structures together with hull plates by welding or some other
joining means for the purpose of stiffening and flexural resisting. The bulb flat steel consists of two parts: the bulb and flat. The bulb is
quite different from the flat in dimensions with maximum Ratio of 4:1. During the rolling of the bulb flat steel, the deformation and
cooling rates between different areas is noticeable variations because of the dimensional difference between the bulb and the flat. The
bulb center has the minimum deformation and slower cooling rate, whereas the flat has larger deformation and faster cooling rate. These
are the reasons why the bulb flat steel features the sectional inhomogeneity in microstructure and Mechanical Properties. The strength is
typically lower at the bulb than at the flat with a difference of approximate 30 – 50 MPa.
In this paper, the microstructures and vanadium precipitates of different positions of the V-N microalloyed high-strength bulb flat steel
are investigated and the cooling rates after rolling are calculated by simulation. Furthermore, the effect of V-N microalloying on its
sectional homogeneity is analyzed.
EXPERIMENTAL PROCEDURES
The composition of the experimental steel is given in Table Ⅰ. To improve its corrosion resistance, a certain amount of Ni and Cr are
added in the steel. The production route is 70 t EAF → Continuous Casting → Reheating → Rolling → Heat treatment with the
following parameters: reheating to approximate 1,250°C; rough rolling at 1,100 – 1,180°C; finishing rolling at 950°C; tempering at
660°C; and holding for 2 hours.
Table Ⅰ Composition of Experimental Steel, %
Steel
C
Si
Mn
P
S
Ti
V
N
Steel V-N
0.10
0.49
1.25
0.056
0.002
0.014
0.064
0.0140
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