Kisoo Kim

Metallurgical and Process Parameters for Commercial Production of High Toughness API-X80 Grade Hot Rolled Strips

While API-X80 grade UOE pipes have been increasingly applied during the last decades, the usage of hot rolled API-X80 grade for spiral-welded pipes have been limited. This is partly due to metallurgical barriers which are the anisotropy of mechanical properties and strength drop after pipe forming. Manufacturing conditions and metallurgical parameters have been studied to overcome theses barriers in this work. Also, the effect of both size and distributions of M/A constituent on the upper shelf energy measured from Charpy tests have been investigated. In this paper, the development of API-X80 hot rolled strips have been reviewed, and production results of 12,000MT API-X80 grade for a line pipe project has been provided.Copyright

Analysis and estimation of yield strength of API X80 linepipe steel pipe by low-cycle fatigue tests

In the present study, spiral piping was conducted on API X80 linepipe steel, and the outer and inner wall pipe yield strengths were measured. A low-cycle fatigue test was conducted on a leveled X80 steel sheet to simulate piping and flattening processes, and the strain hardening and Bauschinger effects, induced from different strain histories, were evaluated and combined using Swift’s equation and the Bauschinger stress parameter, respectively. By analyzing the stress-strain curves obtained from the low-cycle fatigue test, the yield strengths of the outer and inner walls were estimated to be 592 MPa and 492 MPa, respectively, which are lower by 20–80 MPa than that of the actual pipe used. Possible reasons for measured and estimated yield strength differences could be the simulation determining procedure of the pre-strain and Bauschinger stress parameters, preposition of same strain hardening behavior depending on strain history, and pre-strain differences depending on thickness location in the steel sheet during piping.

Effects of oxide layers on surface defects during hot rolling processes

An oxide layer, which developed on the surface of a commercial hot rolling mill, was examined by forcibly stopping the roller between mill stands during activity. Liquid quartz was sprayed on the strip to prevent further oxide layer growth during cooling after stopping the hot-rolling mills. Then the thickness and shape of the oxide layer was examined in a cross-sectional view using an optical microscope. The thickness of the oxide layer increased through the 1st and 2nd passes of hot rolling, and then decreased through successive rolling, because the thickening rate by growth is larger than the thinning rate by deformation in high temperature. The temperature distributions of the oxide layer as well as the strip were predicted using the thermo-mechanical finite element method. As thermal conductivity of the oxide layer is low, the temperature deviation of the oxide layer increases and average temperature decreases as the thickness of the oxide layer increases, suggesting the increased formation of surface defects. With these results, a new cooling device was installed between the hot rolling mills to decrease the surface temperature and the thickness of the oxide layer, resulting in improved surface quality of the strip.