High strength steel plate superior in stretch flange formability and fatigue characteristics

Abstract

The present invention provides high strength hot rolled steel plate superior in stretch flange formability and fatigue characteristics comprising steel plate containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acid soluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and a total of one or both of Ce or La: 0.0005 to 0.04%, having a balance of iron and unavoidable impurities and having a number ratio of stretched inclusions present in the steel plate having a circle equivalent diameter of 1 μm or more and a long axis / short axis of 5 or more of 20% or less.

Description

TECHNICAL FIELD[0001]The present invention relates to high strength hot rolled steel plate superior in stretch flange formability and fatigue characteristics suitable as a material for members of an automobile chassis.BACKGROUND ART[0002]From the viewpoint of improvement of automobile safety and improvement of the fuel economy leading in turn to environmental protection, the demands for increasing the strength and reducing the weight of the hot rolled steel plate used for automobiles have been growing stronger. Among auto parts, in particular, the weight of frames, arms, etc. called “chassis parts” accounts for a high ratio of the weight of the vehicle as a whole, so the materials used for such locations are being made higher in strength and smaller in thickness to enable lighter weight. Further, the materials used for such chassis parts are required to have high fatigue characteristics from the viewpoint of durability with respect to the vibration during driving.[0003]However, alon...

AI Technical Summary

Benefits of technology

The present invention provides a high strength steel plate with superior stretch flange formability and fatigue characteristics. The technical effect of the invention is achieved by controlling the precipitation and deformation of fine MnS in the cast slab and making it disperse as fine spherical inclusions not deformed and not easily becoming starting points of cracking in the steel plate at the time of rolling. The invention also addresses the issue of coarse carbides formed during the process of hot rolling and cold rolling, which can negatively impact fatigue characteristics. The high strength steel plate contains specific amounts of elements such as MnS, Ce, La, cerium oxides, cerium oxysulfides, and lanthanum oxysulfides, among others, which contribute to the formation of fine MnS precipitates and disperse them as fine spherical inclusions in the steel plate.

Problems solved by technology

Further, with just suppressing the formation of coarse carbides, it is not possible to prevent the occurrence of cracks at the time of extreme hole expansion.

Upon repeated deformation, internal defects form near the stretched coarse MnS-based inclusions present at the surface layer or its vicinity and propagate as cracks to thereby cause deterioration of the fatigue characteristics.

Again, stretched coarse MnS-based inclusions easily become starting points of cracking at the time of hole expansion.

These become causes lowering the fatigue characteristics and stretch flange formability (hole expandability).

However, no example has been found proposing hot rolled steel plate superior in stretch flange formability and fatigue characteristics from the viewpoint of control of the precipitation and deformation of MnS.

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