High-strength steel sheet excellent in stretch flangeability and fatigue property

Abstract

The invention provides a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue properties. The steel sheet contains 0.03-0.20% C, 0.08-1.5% Si, 1.0-3.0% Mn, up to 0.05% P, at least 0.0005% S, 0.0005-0.01% N, up to 0.01% acid-soluble Al, less than 0.008% acid-soluble Ti, and 0.0005-0.04% at least either of Ce and La, the remainder being iron and unavoidable impurities. The steel sheet contains stretched inclusions which have a diameter, in terms of equivalent-circle diameter, of 1 mum or larger and in which the ratio of the major-axis length to the minor-axis length is 5 or higher, in a proportion of 20% or higher by number.

Description

technical field [0001] The present invention relates to a high-strength hot-rolled steel sheet which is excellent in tensile flangeability and fatigue properties and is suitable as a raw material for automobile running parts. Background technique [0002] From the standpoint of improving the safety of automobiles and increasing fuel costs related to environmental protection, there is an increasing demand for high strength and light weight of hot-rolled steel sheets for automobiles. Among automotive parts, the weight of the frame and arms, which are called the running system, accounts for a high proportion of the total weight of the vehicle body. Therefore, it can be realized by increasing the strength of the raw materials used in these parts and reducing the thickness. lightweight. In addition, from the viewpoint of durability against vibration during running, materials used for the running system are required to have high fatigue properties. [0003] However, with the inc...

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Problems solved by technology

[0005] It is known that this DP steel sheet is excellent in the balance of strength and ductility and fatigue properties, but the tensile flangeability evaluated by the hole expansion test is still not good.

One of the reasons may be that the DP steel plate is a complex of soft ferrite phase and hard martensite phase, so the boundary between the two phases cannot adapt to deformation during hole expansion and is likely to become the starting point of fracture

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