Metal treatment – Compositions – Heat treating
Patent
1982-10-15
1985-03-05
Skiff, Peter K.
Metal treatment
Compositions
Heat treating
148 124, 148 36, C21D 802
Patent
active
045028974
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for producing hot-rolled steel sheets having a low yield ratio and a high tensile strength due to dual phase structure and intends to clarify the range of cooling regulating conditions for producing a hot-rolled steel sheet having a low yield ratio and a high tensile strength, and provided with the properties same as or higher than those accomplished only by a prior reheating method explained hereinafter through a continuous annealing line without causing disadvantage and inconvenience resulting from the reheating method by firstly quenching at a specific cooling rate a steel sheet having a specific component composition, which has been subjected to a final rolling in hot rolling, maintaining the quenched steel sheet at a specifically defined temperature range and then subjecting the thus treated steel sheet to second quenching at a specific cooling rate, whereby the hot-rolled steel sheet having the above described properties can be advantageously produced.
Recently, the demand of high tensile strength thin steel sheets has been rapidly increased mainly in automotive field and this aims to ensure the safety of drivers, reduce the car weight and improve the fuel cost and this demand is required in the application other than automobiles for intending the increase of the toughness of the structure and the decrease of the weight of the structure.
In these applications, the thin steel sheets of a base material is usually subjected to a cold molding step, such as press forming and therefore such a sheet is required to have excellent cold formability.
As one means for satisfying the cold formability which is inconsistent with respect to high tensile strength steel sheets, it has been known that a metal structure is composed of a mixed structure (referred to as dual phase structure" hereinafter) wherein ferrite phase and martensite phase are dispersed and steels having such a dual phase structure show unique mechanical properties, that is low yield point, high tensile strength, and further very excellent strength-elongation balance and therefore these steel sheets are excellent in the cold formability.
The excellent cold formability of the dual phase structured high tensile strength thin steel sheets is due to the low yield ratio and the high ductility because the strength at a low strain zone is determined by a soft ferrite phase and the strength at a high strain zone is determined by a hard martensite phase (referred to as "the hard second phase" hereinafter). Furthermore, in these steel sheets, the work hardening is very high upon working and the yield strength is increased owing to the age hardening after the molding, so that the strength in the final product is not inferior to that of the general high tensile strength steel and these steel sheets have very practically useful properties.
The present invention can advantageously provide high tensile strength steel sheets having excellent properties due to the dual phase structure and occupies the technical field concerning the production.
BACKGROUND OF THE INVENTION
The most general method for producing the above described dual phase structured steel sheets comprises reheating a thin steel sheet up to .gamma.+.alpha. zone by using a heat-treating apparatus of a prior continuous annealing line and then rapidly quenching the reheated steel sheet in the subsequent cooling step to transform .gamma. portion formed in the heating step into martensite (referred to as "reheating method" hereinafter). But it is essential for the reheating method to add one step for the heat treatment and is not advantageous in view of economy and productivity.
On the other hand, it has been known to directly obtain the dual phase structure in a hot-rolling step without effecting the separate heat treating step (referred to as "hot-rolling method" hereinafter) but the cold formability of the dual phase structured steel sheets produced in the prior hot-rolling method is far inferior to that of the steel sheets produced
REFERENCES:
patent: 4072543 (1978-02-01), Coldren et al.
patent: 4316753 (1982-02-01), Kaneko et al.
patent: 4325751 (1982-04-01), Josefsson
patent: 4388122 (1983-06-01), Sudo et al.
patent: 4421573 (1983-12-01), Irie et al.
Aoyagi Nobuo
Mano Jun-ichi
Morita Masahiko
Nishida Minoru
Takizawa Syoichi
Kawasaki Steel Corporation
Skiff Peter K.
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