高延性非調質熱延複合組織鋼板の研究

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The purpose of the present study is to investigate the main manufacturing and microstructural features of as-rolled dual phase (DP) steels, characterized by a combination of excellent ductility, formability, and high strength. It was revealed by a laboratory simulation that the DP steel with required properties could be obtained, when a suitable combination of chemical compositions and rolling conditions was chosen. That is, 0.05% C-1.5%Si-1.6%Mn-0.5%Cr-0.25%Mo steel, 0.05%C-1.5%Si-2.2%Mn-0.20%Mo steel or 0.05%C-2.1%Si-2.4%Mn steel should be finish-rolled just above Ar_3 point and coiled below 600℃. From the investigation of transformation behavior by dilatometric test and micro-segregation analysis by EPMA, the mechanism for generating the DP structure is considered as follows : Silicon is so effective for polygonal ferrite to be formed quickly and easily even under an accelerated cooling by water spray on a runout table, and at the same time carbon is enriched in the untransformed austenite portion. Most of the austenite transforms to martensite at the range of temperature lower than 300℃ even under slow cooling after coiling, due to the increased hardenability attained by the enriched carbon and the presence of molybdenum, chromium, manganese, etc. in solution. The constituents of untempered martensite and high volume fraction of polygonal ferrite bring about the same mechanical properties in as-rolled DP steels as in heat-treated DP steels. For example, yield ratio of the former is as low as that of the latter. Also, a retained austenite was observed in as-rolled DP steels by transmission electron microscopy and, from an X-ray diffraction technique, the volume fraction was found to be 1∿3%.
1979-11-01

高延性非調質熱延複合組織鋼板の研究

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