Metal treatment – Compositions – Heat treating
Patent
1985-03-11
1986-05-20
Stallard, Wayland
Metal treatment
Compositions
Heat treating
148 12D, C21D 948
Patent
active
045899312
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
Cold rolled steel sheets or zinc-plated steel sheets produced from the cold rolled steel sheet are used as an exterior automotive plate in a large amount. These steel sheets are subjected to a drawing treatment, such as a press molding, and then to a bake coating at the use, and these steel sheets for drawing can satisfy advantageously the demand for dent resistance by improving the yield strength due to the heating during the bake coating, that is, by improving so-called baking hardenability. The baking hardenability is evaluated by the BH value of the total increased value of yield strength of a steel sheet in the case where the steel sheet is prestrained under a tension of 2% and then subjected to a heat treatment of 170.degree. C. for 20 minutes. The baking hardenability of a steel sheet must be improved without deteriorating the drawability represented by the Lankford value r.
The present invention belongs to a technical field relating to a method of producing a thin steel sheet adapted for drawing and having high r value and BH value and excellent striking energy-absorbing property, from a cold rolled steel sheet, particularly from a high tensile strength cold rolled steel sheet; or from a metal-or alloy-plated steel sheet produced from these cold rolled steel sheets and having a plated film at least on one surface, the metal- or alloy-plated steel sheet being hot dip plated steel sheet, particularly zinc hot dip plated steel sheet, whose plated zinc film may be formed into alloy, aluminum plated steel sheet, lead-tin plated (terne plated) steel sheet and the like.
BACKGROUND ART
Rimmed steel has been used for a long period of time due to its excellent surface property for obtaining beautiful finishing of coating. The rimmed steel has an ageing property at room temperature due to the presence of nitrogen solid solved therein, and when the rimmed steel is press molded just after the cold rolling, the yield strength is increased due to the strain ageing by nitrogen without generation of stretcher-strain at the bake coating.
Recently, continuous casting technic of steel has been progressed and developed, and a large amount of aluminum killed steel having a moldability superior to that of conventional rimmed steel is used without substantially increasing the production cost of automobile in place of the conventional rimmed steel as a thin steel sheet for drawing, which is mainly used for exterior automotive plate.
Aluminum killed thin steel sheet is excellent in the deep drawing property, but is generally poor in the baking hardenability due to the presence of nitrogen fixed by aluminum.
While, when it is intended to use high tensile strength steel sheet in order to produce automobiles having a light weight, it is necessary to give baking hardenability, particularly an improved baking hardenability, to the high tensile strength steel sheet in order to compensate the decreasing of the dent resistance due to the decreasing of the sheet thickness from the view point of safety.
Ferrite-martensite dual phase steel sheet has a satisfactorily high baking hardenability, but has generally a low r value of about 1.0, and is poor in the drawability. Therefore, the use field of ferrite-martensite dual phase steel sheet is limited.
While, in order to produce a thin steel sheet having a high r value, there have been proposed the following treatments. That is, aluminum killed cold rolled steel sheet, which has been strengthened by adding phosphorus thereto, is subjected to an open coil annealing and solid solved carbon is left in the steel to develop the strain ageing property by utilizing such a property that the open coil annealed steel can be cooled at a rapid cooling rate. Alternatively, the aluminum killed cold rolled steel sheet is subjected to a tight coil annealing at a particularly high temperature to form coarse carbide and to disturb the precipitation of solid solved carbon, whereby solid solved carbon is left in the steel (For example, refer to Iron and Steel, Vol. 66, page A209 (1980)).
REFERENCES:
patent: 4315783 (1982-02-01), Akisve et al.
Irie Toshio
Konishi Motoyuki
Yasuda Akira
Kawasaki Steel Corporation
Stallard Wayland
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