High strength steel plate having low yield ratio excellent in terms of strain ageing resistance, method of manufacturing the same and high strength welded steel pipe made of the same
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United States of America Patent
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May 23, 2017
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N/A
app pub date -
Mar 29, 2013
filing date -
Mar 29, 2012
priority date (Note) -
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Abstract
A steel plate has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a metallographic structure including a bainite phase and island martensite, and a polygonal ferrite in surface portions within 5 mm from the upper and lower surfaces, wherein the area fraction of the island martensite is 3% to 15%, the equivalent circle diameter of the island martensite is 3.0 μm or less, the area fraction of the polygonal ferrite in the surface portions is 10% to less than 80%.
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