Composite structure type high tensile strength steel plate, plated plate of composite structure type high tensile strength steel and method for their production

Abstract

The invention proposes a high-strength dual-phase cold rolled steel sheet having an excellent deep drawability, wherein the steel sheet has a composition comprising C: 0.01-0.08 mass %, Si: not more than 2.0 mass %, Mn: not more than 3.0 mass %, P: not more than 0.10 mass %, S: not more than 0.02 mass %, A1: 0.005-0.20 mass %, N: not more than 0.02 mass % and V: 0.01-0.5 mass %, provided that V and C satisfy a relationship of 0.5xC/12<=V/51<=3xC/12, and the remainder being Fe and inevitable impurities, and has a microstructure consisting of a ferrite phase as a primary phase and a secondary phase including martensite phase at an area ratio of not less than 1% to a whole of the microstructure and a high-strength dual-phase galvanized steel sheet comprising a galvanized coating on the above steel sheet as well as a method of producing the same.

Description

[0001] This invention relates to a high-strength dual-phase steel sheet having an excellent deep drawability, and particularly to a high-strength dual-phase cold rolled steel sheet having an excellent deep drawability and a high strength dual phase galvanized steel sheet having an excellent deep drawability which have a tensile strength of 440 MPa or more and are suitable for use in steel sheets for vehicles as well as a method of producing the same.[0002] Recently, it is required to improve a fuel consumption in a vehicle from a viewpoint of the maintenance of the global environment, and also it is required to improve a safety of a vehicle body from a viewpoint of the protection of crews during the collision of the vehicle. To this end, investigations for achieving both the lightening and strengthening of the vehicle body are positively proceeding.[0003] In order to simultaneously satisfy the lightening and strengthening of the vehicle body, it is said that the high-strengthening o...

AI Technical Summary

Benefits of technology

0094] In order to provide the cold rolled steel sheet having a low yield stress (YS), a high ductility (E1) and an excellent deep drawability, it is required to render the microstructure of the steel sheet according to the invention into a dual-phase microstructure consisting of a ferrite phase as a primary phase and a secondary phase including a martensite phase. It is preferable that the ferrite phase as a primary phase is not less than 80% at an area ratio and hence the secondary phase is not more than 20%. When the area ratio of the ferrite phase is less than 80%, it is difficult to ensure the high ductility and the press formability tends to lower. And also, when a good ductility is required, it is preferable that the ferrite phase is not less than 85% at the area ratio and hence the secondary phase is not more than 15%. Moreover, in order to utilize the advantage of the dual-phase microstructure, the ferrite phase is required to be not more than 99%.

0095] In the invention, the secondary phase is required to include the martensite phase at the area ratio of not less than 1% with respect to the whole of the microstructure. When

Problems solved by technology

However, in general, as the steel sheet becomes highly strengthened, the r-value and the ductility lower and the press formability is degraded, while the yield stress rises to degrade the shapability and hence the problem of springback is apt to occur.

The above dual-phase steel sheet is generally good in the workability, but has a drawback that the workability under severer condition is poor and particularly, the r-value is low and the deep drawability is bad.

Therefore, when the galvanized steel sheet is produced in the continuous galvanizing line, it is difficult to generate martensite phase produced under a cooling condition of a large cooling rate into the steel sheet after the galvanization.

To this end, it is generally difficult to produce the high-strength galvanized steel sheet having a dual-phase microstructure of a ferrite phase and a martensite phase through the continuous galvanizing line.

In this method, however, the quenching and tempering are carried out during the continuous annealing, so that the yield stress is high and hence a low yield ratio can not be obtained.

The steel sheet having such a high yield stress is not suitable for the press formability and has a drawback that the shapability in the pressed parts is bad.

low. In this method, however, it is required to conduct the box annealing at a relatively high temperature being the .alpha.-.gamma. two-phase region over a l

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