Hideto Kimura

Development of UTS 980 MPa Grade Steel Tube with Excellent Formability for Automotive Body Parts

The enhanced performance of an ultra-high strength steel (UHSS) tube with tensile strength of 980 MPa grade is reported. Recently, strong consideration has been given to the use of steel tubes for automotive components because of the associated weight saving, which leads eventually to a reduction of carbon dioxide emissions. Furthermore, the application of steel tubes offers the advantages of increased stiffness and reduced welding processes due to their closed cross-sectional structure. The newly-developed UHSS tubes have an excellent combination of high strength and ductility (ultimate tensile strength: 1016 MPa, elongation to fracture: 14 %) and show remarkable formability such as rotary-bending workability and hydro formability. The key technologies to obtain these superior properties are the application of a cold rolled sheet steel with a DP (Dual Phase) microstructure having an optimized ferrite/martensite ratio and a CBR (Chance-free Bulge Roll) tube forming mill, which minimizes additional strain in the tube making process. The developed UTS 980 MPa grade steel tube has been applied to an actual automotive body part and contributed to not only weight saving but also improved safety.

Metallurgical Design and Performance of HFW Linepipe With a High-Quality Weld Seam Suitable for Sour Services

To clarify the effects of inclusions on the sour resistance properties of X60- to X70-grade steel, their resistance to hydrogen-induced cracking (HIC) was numerically simulated. The steel was assumed to have a yield strength of 562 MPa and a tensile strength of 644 MPa for the simulation. To estimate the effect of nonmetallic inclusions, a virtual inclusion was situated at the center of a 10-mm-thick HIC test specimen. Tests were performed using NACE test solution A. The crack initiation criterion was determined as a function of the diffusible hydrogen concentration, the diameter of the inclusion, the edge radius of the inclusion, and the fracture toughness of the matrix after hydrogen absorption. The crack propagation was calculated as a function of the diffusion coefficient of hydrogen in the steel matrix and the gasification reaction ratio of hydrogen at the interface of the steel matrix and the inclusion.Based on the results of the numerical estimation, high-frequency electric resistance welded (HFW) Linepipe with a high-quality weld seam was developed. Controlling the morphology and distribution of oxides generated during the welding process by means of temperature and deformation distribution control is the key factor for improving resistance to HIC.Copyright

Metallugical Design and Performance of ERW Linepipe With High-Quality Weld Seam Suitable for Extra-Low-Temperature Services

To clarify the effect of inclusions on the Charpy impact properties, the 2 mm V-notched Charpy properties of X60 – X80-grades steel were numerically simulated using the finite element method code ABAQUS. The yield strength and the tensile strength of the steel were 562 MPa and 644 MPa, respectively. The striker’s velocity and the temperature dependency of the stress-strain curve were taken into account. To estimate the effect of nonmetallic inclusions, a 200 μm long virtual inclusion with a 1 μm edge radius was situated at the maximum point of the stress triaxiality. Four types of micro crack initiation were determined: (a) ductile void generation in the matrix, (b) cleavage crack generation in the matrix, (c) void generation by inclusion fracture and (d) void generation by matrix-inclusion interface debonding. Without inclusions, a ductile micro void was generated when the striker stroke was 3.3 mm, independent of the temperature. With inclusions, an inclusion fracture occurred when the striker stroke was 0.6 mm at room temperature. The striker stroke decreased as the temperature decreased.Based on the above numerical estimation results, electric resistance welded (ERW) Linepipe with high-quality weld seam MightySeam® has been developed. Controlling the morphology and distribution of oxides generated during the welding process by means of temperature and deformation distribution control is the key factor for improving the low-temperature toughness. The Charpy transition temperature of the developed ERW pipe was much lower than −45°C. Based on the low-temperature hydrostatic burst test with a notched weld seam at −20 °C, the MightySeam® weld provides a fracture performance that is the same as UOE Double Submerged Arc Welded pipe. The pipe has been used in actual, highly demanding, severe environments.© 2012 ASME