Electric resistance welded steel pipe, impact absorbing member, and method for manufacturing electric resistance welded steel pipe

Abstract

The C content in the chemical composition of an electric resistance welded steel pipe (1) according to the present disclosure is 0.19-0.35 mass%, and the tensile strength of the electric resistance welded steel pipe (1) is 1700 MPa or more. In a cross-section perpendicular to the pipe axis direction of a metal flow rising portion (3), a metal flow angle (AN), which is an angle between the outer circumferential surface of the metal flow rising portion (3) and a metal flow on the outer circumferential surface side, is 35-80°. In the cross-section, the hardness ratio RH of the average hardness (GPa) in a band-shaped region having a hardness of 8.5 GPa or more, obtained by a nanoindentation method, to the average hardness (GPa) in a band-shaped region having a hardness of 7.5 GPa or less is 1.8 or less. The FN value defined by equation (1) using the metal flow angle (AN) and the hardness ratio RH is 130 or less.　(1): FN = ANRH

Claims

1. An electric resistance welded (ERW) steel pipe comprising: an ERW welded portion extending in the axial direction of the ERW steel pipe; a metal flow rise portion adjacent to the ERW welded portion and extending in the axial direction of the pipe, and including a region in a cross section perpendicular to the axial direction of the pipe where at least a portion of the metal flow curves upward relative to the circumferential direction of the ERW steel pipe; and a base material portion, wherein the carbon content in the chemical composition of the ERW steel pipe is 0.19 to 0.35% by mass, the tensile strength of the ERW steel pipe is 1700 MPa or more, and in the cross section of the metal flow rise portion, the metal flow angle AN, which is the angle between the outer circumferential surface of the metal flow rise portion and the metal flow on the outer circumferential surface side, is 35 to 80°. In the cross-section of the metal flow rise portion, the hardness ratio RH of the average hardness (GPA) in a band-shaped region with a hardness of 8.5 GPa or more, obtained by nanoindentation, to the average hardness (GPA) in a band-shaped region with a hardness of 7.5 GPa or less is 1.8 or less, and the FN value, defined by formula (1) using the metal flow angle AN and the hardness ratio RH, is 130 or less. (1) 2. An electric resistance welded steel pipe according to claim 1, wherein the FN value is 115 or less.

3. An electric resistance welded steel pipe according to claim 1, wherein the carbon content is 0.25 to 0.30% by mass.

4. An electric resistance welded steel pipe according to claim 2, wherein the carbon content is 0.25 to 0.30% by mass.

5. An impact absorbing member comprising an electric resistance welded steel pipe according to any one of claims 1 to 4, and a mounting member fixed to the electric resistance welded steel pipe and capable of attaching the impact absorbing member to another member.

6. A method for manufacturing an electric resistance welded steel pipe according to any one of claims 1 to 4, comprising: a forming step of bending a strip-shaped steel plate having a predetermined width along the direction of the width using a group of fin pass rolls to form an open pipe-shaped steel plate; a welding step of heating both ends of the open pipe-shaped steel plate in the direction of the width, and then welding the ends by pressing them together with a squeeze roll to produce an electric resistance welded steel pipe intermediate; and a quenching step of performing high-frequency hardening on the electric resistance welded steel pipe intermediate using a high-frequency heating device, wherein the FA value defined by formula (A) using the circumference CI1 (mm) of the open pipe-shaped steel plate at the exit side of the final rolling stand of the fin pass roll group and the circumference CI2 (mm) of the electric resistance welded steel pipe intermediate at the exit side of the squeeze roll is 0.3 to 1.2, the heating time t1 in the high-frequency heating device is 3 seconds or more, and the FB value defined by formula (B) using the FA value and the heating time t1 is 4.0 or more. FA=CI1-CI2 (A) FB=t1 / FA (B)

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