Investigation of the Evolution of Central Defects in Ultra-Heavy Plate Rolled Using Gradient Temperature Process
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ULTRA-HEAVY plate is widely used in bridge, shipbuilding, platform, and pressure vessels as structure construction component and bearing carrier at critical locations. It requires not only strength but also toughness, fatigue properties, welding performance, decay resistance, and z-direction properties. Earlier research on ultra-heavy plate focused on HY-80[1] and HY-100,[2] which were used by the United States Navy for submarine hull steels. The main problems with ultraheavy plate production are central coarse grains in the center and defects inherited from ingot casting. There are several ways to improve the properties of thick plates, such as casting large ingot that allows for a large rolling reduction, improving ingot qualities via electroslag remelting (ESR),[3] and applying soft reduction techniques.[4] However, large ingot casting exhibits serious central shrinkage porosity and cracks that are difficult to eliminate. In traditional recrystallizationcontrolled rolling for ultra-heavy plate, instead of closing, the central defects grow as a result of low reduction and increased central tensile stress as the thickness of the ingot increases,[5] while increasing pass reduction makes it harder for the steel plate to bite. Furthermore, as the nucleation rate increasing, the rate of growth of dynamic recrystallized grains and the probability of forming Widmanstatten structures also increase.[6] In addition, the problems of microstructure heterogeneities and changing properties along the thick-
LI GAOSHENG, Ph.D Student, and YU WEI and CAI QINGWU, Professors, are with the National Engineering Research Center for Advanced Rolling of USTB, Beijing 100083, P.R. China. Contact e-mail: [email protected]; [email protected] Manuscript submitted July 16, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
ness direction of ultra-heavy plate need to solved.[7–9] Manufacturers and researchers therefore continue to focus on methods of effectively closing the central defect and enhancing the properties of ultra-heavy plate via improvement of rolling processes. This paper describes a study of hot rolling under temperature gradients by means of finite element modeling (FEM) and rolling experiments. The work explores the influence of the temperature gradient on closing of the central crack, based on the principles of clad rolling[10–12] and proposes an innovative ultraheavy plate rolling process for effectively eliminating this casting flaw and improving plate properties.
II.
METHODOLOGY OF MODELING AND EXPERIMENTATION
The steel selected for this research is the low alloy steel Q345B, which has the chemical composition (wt pct): 0.16 pct C-1.55 pct Mn-0.055 pct Nb-0.01 pct Ti. A two-dimensional FEM model was built using software of MSC Marc according to the symmetry law. The parameters applied in the rolling experiment are described in the following sections. A. Finite Element Model A finite element model was established to simulate central crack closing behavior in the initial rolling pass with different temperature distributio
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