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

[0059] The cold rolled steel sheet and the galvanized steel sheet according to the invention are high-strength dual-phase steel sheets having a tensile strength (TS) of not less than 440 MPa and an excellent deep drawability.

[0120] And also, the cold rolled steel sheet after the recrystallization annealing process and the galvanized steel sheet after the plating process or after the alloying process may be subjected to a temper rolling at a rolling reduction of not more than 10% for correcting the shape and adjusting the surface roughness and the like. Furthermore, the cold rolled steel sheet according to the invention can be applied as not only a cold rolled steel sheet for the working but also a blank of a surface treated steel sheet for the working. As the surface treated steel sheet for the working are mentioned tin-plated steel sheets, porcelain enamels and so on in addition to the aforementioned galvanized steel sheets (including alloyed sheets). There is no problem even when they are subjected to a treatment such as resin or fat coating, various paintings, electroplating or the like. Moreover, the galvanized steel sheet according to the invention may be subjected to a special treatment after the galvanization in order to improve the chemical conversion property, weldability, press formability, corrosion resistance and the like.

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 long time for enriching Mn, so that there are many problems in production steps such as a frequent occurrence of adhesion between steel sheets inside a coil resulted from the thermal expansion in the annealing, an occurrence of temper color, a lowering of service life in an inner cover for a furnace body and th

e like. Therefore, it was difficult to industrially stably produce high-strength steel sheets possessing a high r-value and a low yield stress up

However, the high cooling rate of 100.degree. C. / s is difficult to attain in the gas jet cooling usually used in the continuous annealing line or continuous galvanizing line after the cold rolling, and is required to use an equipment for water-quenching, and also a problem becomes actual in the surface treatment of the water-quenched steel sheet, so that there are problems in the production equipment and the materials.

However, the addition of the large amount of the alloying element undesirably brings about the rise of the production cost.

However, this method is not actual from the constraint on the equipment for the continuous galvanizing line and also the steel sheet obtained by this method is not said to have a sufficient ductility.

Moreover, when the value of (2.times.Nb / 93+2.times.T-i / 48) / (V / 51) exceeds 15, TS considerably lowers, which is unfavorable for obtaining the high-strength dual-phase cold rolled steel sheet of TS: 780 MPa grade.

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