| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel tube having excellent buckling resistance and high internal-pressure fracture resistance, and its manufacturing method. SOLUTION: The steel tube has a chemical composition containing, by mass, 0.03-0.15% C, 0.01-1% Si and 0.5-2.0% Mn and also has a metallic structure which consists of 5-50% by area fraction of bainite and the balance composed essentially of ferrite and in which the average aspect ratio of bainitic phases is made to >=2, and further, YSL/YSC<=0.90 is satisfied and also yield ratio in a tube-axis direction, YSL/TSL, satisfies YSL/TSL<=85% when YSC and TSC represent yield strength and tensile strength in a tube-periphery direction, respectively, and YSL and TSL represent yield strength and tensile strength in a tube- axis direction, respectively. This steel tube can be obtained by subjecting steel to hot rolling at (Ar3+10) deg.C to (Ar3-50) deg.C rolling finishing temperature and to cooling at >=2 deg.C/s cooling rate and <=500 deg.C finish cooling temperature and then subjecting the resultant steel plate to tube making by means of cold forming at >=0.4% tube expanding rate in the final step of tube making.
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