| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength electroseamed steel pipe which is suitable as the one for an automobile impact absorbing member.SOLUTION: A steel material of a composition comprising, by mass, 0.05 to 0.20% of C, 0.5 to 2.0% of Si, 1.0 to 3.0% of Mn, 0.01 to 0.1% of Al and ≤0.005% of N is subjected to hot rolling in which a finish rolling end temperature is an Artransformation point or above and a cooling temperature is 500 to 650°C to be a hot rolled sheet, the hot rolled sheet is subjected to cold rolling so as to be a cold rolled sheet, the cold rolled sheet is heated to a temperature in a two phase temperature region of an Ac1 transformation point to an Ac3 transformation point, is thereafter subjected to rapid cooling treatment of performing cooling from a temperature in the range of 600 to 750°C to a room temperature at a cooling velocity of ≥500°C/s on the average, and is then subjected to tempering treatment at 150 to 300°C to be a steel pipe material. The steel pipe material is continuously formed by roll forming of a cage roll system to be an open pipe with an almost cylindrical shape, and the open pipe is subjected to electroseaming, and is at least subjected to bead cutting to be an electroseamed steel pipe in which inner surface bead height is controlled to -0.1 to 0.1 mm. In this way, the high strength steel pipe having a tensile strength TS of ≥1,180 MPa, having excellent workability, having a BH amount of ≥100 MPa after coating/baking treatment, also having excellent impact absorbing properties to be a yield ratio of ≥90%, and further having excellent flat workability and pipe expansion workability can be obtained. |
