Ultrahigh-strength cold-rolled steel sheet with excellent ductility and delayed-fracture resistance, and process for producing same

Abstract

Provided is an ultrahigh-strength cold-rolled steel sheet that has a steel composition in which neither transition metal elements which considerably raise the alloy cost, such as V and Mo, nor Al, which may induce casting defects, is contained in excess and that, despite the composition, has excellent delayed-fracture resistance and a tensile strength of 1,320 MPa or higher. Also provided is a process for producing the steel sheet. The steel sheet having excellent ductility and delayed-fracture resistance is characterized by containing, in terms of mass%, 0.15-0.25% C, 1.0-3.0% Si, 1.5-2.5% Mn, up to 0.05% P, up to 0.02% S, 0.01-0.05% Al, and below 0.005% N, with the remainder comprising Fe and incidental impurities. The steel sheet is further characterized by having a metallographic structure which comprises 40-85 vol.% tempered martensite phase and 15-60 vol.% ferrite phase and by having a tensile strength of 1,320 MPa or higher.

Description

Technical field [0001] The present invention mainly relates to an ultra-high-strength cold-rolled steel sheet suitable for use as a material for ultra-high-strength body structural members such as automobile center pillars and door anti-collision beams. The strength and ductility balance of materials and the delayed fracture resistance are excellent, and a manufacturing method thereof. Background technique [0002] In recent years, from the 2 Starting from concerns about global warming caused by the increase in emissions, in Europe, we are promoting 2 Of the CO 2 Emission restrictions strongly require improvements in the fuel efficiency of automobiles. In order to improve fuel efficiency, the weight reduction of the vehicle body is effective, but the safety of the occupants must also be ensured. Therefore, it is necessary to reduce the weight of the vehicle body and ensure that the crash safety is higher than before. In order to meet the two requirements of ensuring body weight r...

AI Technical Summary

Problems solved by technology

However, when a large amount of Al is added, there is a problem that casting defects are likely to occur

In addition, in order to allow retained austenite to remain in the microstructure, it is necessary to carry out isothermal holding at a temperature above the Ms transformation point during the cooling process from the annealing temperature, and the manufacturing process increases.

In addition, when the cooling rate up to the isothermal holding temperature is changed during operation, the material will change greatly. Therefore, in order to stably produce a steel plate of a certain quality, strict control of operating conditions is required, so it is not preferable in production.

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