Metal treatment – Compositions – Heat treating
Patent
1982-06-09
1983-12-20
Skiff, Peter K.
Metal treatment
Compositions
Heat treating
148 124, 148 36, C21D 802, C21D 946, C22C 3834
Patent
active
044215738
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
This invention relates to a method for producing dual-phase high-tensile steel sheets having improved workability and a composite metallic structure of ferrite and martensite phases, and more particularly, to a method for producing high-tensile steel sheets exhibiting a tensile strength of the order of 50-80 kg/mm.sup.2 when measured on as-hot-rolled sheets.
DESCRIPTION OF PRIOR ART
For these years, high-tensile steel sheets having a dual phase structure consisting of ferrite and martensite phases have been commercially used as high-tensile steel sheets having excellent formability. It is well known that these dual-phase high-tensile steel sheets are produced either by hot rolling followed by continuous annealing or by hot rolling alone without annealing. Since the former method requires the annealing process with an increased production cost, greater attention is now being paid to the latter method, that is, a method for producing as-hot-rolled sheets.
A number of techniques were proposed to carry out the above-mentioned method for producing as-hot-rolled dual-phase high-tensile steel sheets, and it was found that slabs having certain alloying elements, Mn, Si, Cr and Mo added in proper combination should be hot rolled and wound into a coil at a controlled temperature. Except for the addition of Mo, all the previously proposed techniques have the problem that mechanical properties widely vary or are non-uniform in the longitudinal and width directions of produced coils when these techniques are commercially carried out. Those slabs having the above-mentioned alloying elements, particulrly Mo, are uniform in mechanical properties although the production cost of such slabs is undesirably increased as Mo is very expensive.
An object of this invention which has been achieved in consideration of the above-mentioned problems is to provide a method for producing dual-phase high-tensile steel sheets having uniform mechanical properties and a tensile strength of 50-80 kg/mm.sup.2 in an as-hot-rolled state in a commercially acceptable manner without adding expensive Mo.
SUMMARY OF INVENTION
Making extensive experimental and research works in order to achieve the above-mentioned objects, the inventors have reached the following novel metallurgical findings.
In general, the formability of hot rolled steel sheets depends on their ductility and yield ratio (yield strength/tensile strength). The higher the ductility and the lower the yield ratio (not higher than 0.7, preferably not higher than 0.6), the better the formability is. In order to increase the ductility and reduce the yield ratio, it is desired that the fraction of bainite be as low as possible and the fraction of ferrite and martensite phases be as high as possible among the metallic phases of a sheet steel. In other words, it is desired that the sheet steel be of an ideal two phase structure consisting solely of ferrite and martensite as closely as possible. If austenite grains are coarse at the end of hot rolling, the transformation of austenite to ferrite during the subsequent cooling is retarded so that the proportion of bainite becomes higher when a hot-rolled sheet is cooled without any caution. It is thus believed that austenite grains are desirably as small as possible at the end of hot rolling in order to provide improved formability.
For the purpose of obtaining fine austenite grains as described above, according to the research work of the inventors, it is most effective to lower the heating temperature of slabs prior to hot rolling. More specifically, it has been found that the slab heating temperature should not exceed 1,220.degree. C. as opposed to the conventional practice, that is, heating slabs to a temperature of not higher than 1,220.degree. C. is essential. However, if slabs of a well-known composition are heated to relatively low temperature of not higher than 1,220.degree. C. and continuously hot rolled, the hot rolling is completed at a considerably lower temperature. Unless rolled sheets are wound at a higher
REFERENCES:
patent: 4314862 (1982-02-01), Sudo et al.
patent: 4316753 (1982-02-01), Kaneko et al.
patent: 4325751 (1982-04-01), Josefsson
Irie Toshio
Kato Toshiyuki
Takahashi Isao
Kawasaki Steel Corporation
Skiff Peter K.
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