Abstract: An automobile exhaust-pipe joint structure and a coil spring for use therein. The exhaust pipe connection structure includes a first exhaust pipe having a first flange section, a second exhaust pipe having a second flange section, a ball joint section for connecting the first and second exhaust pipes flexibly, and coupling mechanisms provided at a plurality of positions of the first and second flange sections. One exemplary coupling mechanism includes a bolt member penetrating the first flange section and being secured, at one end part thereof, to the second flange section, and a coil spring of deformed cross-section interposed between the other end part of the bolt member and the first flange section, wherein the cross-section of a wire is reduced in diameter, in the compressing direction with a dimensional ratio between the short diameter and long diameter thereof being in the range of 1.1–2.6.

Abstract: Hard metal particles having hardness which is lower than the hardness of a nitrided outermost layer of a spring and in the range of Hv 500 to 800 and a diameter of 500 to 900 &mgr;m are protected against the nitrided surface of the spring at a velocity of 40 to 90 m/sec to prevent generation of a microcrack in the surface layer and provide a compression residual stress comparatively deep inside the spring. Against the resultant spring surface, a number of fine metal particles having a mean diameter of all particles of 80 &mgr;m or less, a mean diameter of each particle in the range between 10 &mgr;m inclusive and less than 100 &mgr;m, a spherical or near spherical shape with no square portions, a specific gravity of 7.0 to 9.0, and hardness which falls in the range between Hv 600 and Hv 1100 inclusive and is equal to or less than the hardness of the outermost surface layer of the spring after nitriding, at a velocity of 50 to 190 m/sec.

Abstract: A coil spring according to the present invention is used as a coil spring in an automobile exhaust-pipe joint structure. The automobile exhaust-pipe joint structure of the invention is a coil spring using the coil spring according to the invention.