Electric resistance welded steel tubes for high-strength thin hollow stabilizers, and methods for manufacturing the same

A method for manufacturing an electric resistance welded steel tube for high-strength thin hollow stabilizers, comprising heating, hot rolling and skin pass rolling processes where a steel is formed into a hot rolled sheet, a tube making process by an electric resistance welding is applied to the hot rolled sheet as a steel tube material into an electric resistance welded steel tube, and a hot stretch-reducing rolling process is applied to the reheated and hot stretch-reducing rolled electric resistance welded steel tube, making a product tube having a reduced diameter, wherein

• the steel has a chemical composition comprising, in mass %:
• C:
• 0.20 to 0.40%, Si:
• 0.1 to 1.0%,
• Mn:
• 0.1 to 2.0%, P:
• not more than 0.1%,
• S:
• not more than 0.01%, Al:
• 0.01 to 0.10%,
• Cr:
• 0.01 to 1.0%, Ti:
• 0.01 to 0.05%,
• B:
• 0.0005 to 0.0050%, Ca:
• 0.0001 to 0.0050%, and
• N:
• not more than 0.010%, the balance being Fe and inevitable impurities,
• the heating process for the steel comprises heating the steel to a heating temperature in the range of 1000 to 1300° C.,
• the hot rolling process comprises hot rolling the steel under conditions where the rolling finish temperature is 750 to 950° C., cooling the steel sheet, and coiling the steel sheet into a coil at a coiling temperature of 500 to 650° C.,
• the skin pass rolling process comprises skin pass rolling the steel sheet with a rolling reduction ratio of not less than 0.3%,
• the hot stretch-reducing rolling process comprises reheating the electric resistance welded steel tube to a heating temperature of 800 to 1100° C., and hot stretch-reducing rolling the electric resistance welded steel tube to a reduced diameter under conditions where the rolling finish temperature is not more than 850° C. and the cumulative diameter reduction ratio is not more than 75%,
• the product tube has a depth of a decarburized layer in wall thickness direction on the tube inner surface of not more than 120 μfrom the surface,
• the product tube has a surface roughness Ra of the tube inner surface of 0.01 to 5.0 μm,
• the product tube has a yield ratio of not less than 65%,
• the product tube has a hardness expressed in Vickers hardness of not less than 450 HV as measured after the steel tube is formed into a stabilizer and is subjected to quenching and tempering treatment, and
• the product tube has a wall thickness t of not more than 6 mm and a ratio t/D of the wall thickness t (mm) to the outer diameter D (mm) of not more than 0.2 and exhibits excellent fatigue resistance after being formed into a stabilizer and subjected to quenching and tempering treatment.