Microstructure and Mechanical Properties in Hot-Rolled Extra High-Yield-Strength Steel Plates for Offshore Structure and Shipbuilding

Abstract

Key parameters for a thermomechanically controlled processing and accelerated cooling process (TMCP-AcC) were determined for integrated mass production to produce extra high-yieldstrength microalloyed low carbon SiMnCrNiCu steel plates for offshore structure and bulk shipbuilding. Confocal scanning microscopy was used to make in-situ observations on the austenite grain growth during reheating. A Gleeble 3800 thermomechanical simulator was employed to investigate the flow stress behavior, static recrystallization (SRX) of austenite, and decomposition behavior of the TMCP conditioned austenite during continuous cooling. The Kocks-Mecking model was employed to describe the constitutive behavior, while the JohnsonMehl-Avrami-Kolmogorov (JMAK) approach was used to predict the SRX kinetics. The effects of hot rolling schedule and AcC on microstructure and properties were investigated by test-scale rolling trials. The bridging between the laboratory observations and the process parameter determination to optimize the mass production was made by integrated industrial production trials on a set of a 5-m heavy plate mill equipped with an accelerated cooling system. Successful production of 60-and 50-mm-thick plates with yield strength in excess of 460 MPa and excellent toughness at low temperature (213 K (-60°C)) in the parent metal and the simulated coarse-grained heat affected zone (CGHAZ) provides a useful integrated database for developing advanced high-strength steel plates via TMCP-AcC.

F460 Heavy Steel Plates for Offshore Structure and Shipbuilding Produced by Thermomechanical Control Process

“…18) A peak strain observed on the stress-strain curves which indicated the start of dynamic recrystallization was 0.35 at 1100°C at the strain rate of 1 s -1 . Stress-strain curves of austenite with a grain size (dγ) of 100 μ m were measured at 1150 and 1200°C in the present work employing the same approach described in Ref.…”

“…Stress-strain curves of austenite with a grain size (dγ) of 100 μ m were measured at 1150 and 1200°C in the present work employing the same approach described in Ref. [18]. The results are presented in Fig.…”

“…In addition, AcC process is another key process step which should ensure the entire continuous cooling transformation completed at the defined cooling rates. 18) …”

“…7,12) Recently, plates in conformity with Det Norske Veritas NV-F460 specifications, i.e., a minimum yield strength of 460 MPa, and secured minimum CVN impact energy of 31 J at temperatures down to -60°C, have been developed using TMCP with lean steel composition and low carbon equivalent to achieve maximized weldability. 18,19) In view of limited availability of relevant information, however, further investigation is necessary to correlate the process parameters of industrial operation to microstructure-mechanical properties relationship for optimizing the productivity and quality stability of the plate products with thickness (t) of ∼50 mm. The production of NV-F460 class ultra high strength heavy-thickness (45-50 mm) steel plates via TMCP route with lean steel composition is challenging when the maximum continuously cast slab thickness and mill configuration are limited.…”

Low-Carbon Bainite Steel with High Strength and Toughness Processed by Recrystallization Controlled Rolling and Ultra Fast Cooling (RCR+UFC)

“…6, showing microstructure consisting of granular bainite (GB), upper bainite (UB) and lath bainite (LB) distinguished based on bainite features. 21,22) The granular bainite (denoted by white arrow in Fig. 6(a)) consists of lath ferrite with low misorientations, containing equiaxed M/A constituents, but these lath boundaries can not be observed in OM pictures.…”