Method for Producing A Steel Strip Having A Dual-Phase Microstructure

Description

PRIORITY CLAIM

This is a U.S. national stage of Application No. PCT/DE2009/001136, filed on Aug. 7, 2009, which claims priority to German Application No: 10 2008 038 865.3, filed: Aug. 8, 2008, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to a method for producing semi-finished product, particularly steel strip, having a dual-phase microstructure (DP steel strip) and a tensile strength between 500 and 1000 MPa.

2. Related Art

High-strength steel sheet with good formability, stability of microstructures, and mechanical characteristics up to approximately 600° C. is required in many fields.

One such field is the automotive industry, for example, in which efforts are made to reduce emissions by reducing the weight of structural chassis parts because the consumption of fuel can be reduced by reducing weight. On the other hand, there is also a demand for achieving a greater strength of the sheets that are used.

Finally, the sheets should also be suitable for hot-dip galvanization.

However, steels with a dual-phase microstructure that has already been adjusted have the disadvantage that an unwanted, pronounced yield strength generally results when heated above 200° C. as occurs during or in connection with hot-dip galvanization.

Owing to this absence of temperature stability in the materials above approximately 200° C., semi-finished products such as strip, sheet, pipe and other structural components or workpieces comprising DP steel are not suitable for hot-dip galvanization.

SUMMARY OF THE INVENTION

Therefore, it is the object of the invention to provide a method for the production of a semi-finished product, particularly DP steel strip, which is economical and by which a DP steel strip can be produced which not only has optimal formability properties but also has a temperature stability up to 600° C. and is therefore also suitable for hot-dip galvanization.

According to one embodiment of the invention, a method for producing semi-finished product, particularly steel strip, having a dual-phase microstructure and a tensile strength between 500 and 1000 MPa, is disclosed. Elements such as carbon and nitrogen which are interstitially dissolved in the ferritic matrix are bound by alloying elements such as Al, Mo, Nb, Ti and V which form carbide, nitride or carbonitride in order to adjust a microstructure of hard components such as martensite, bainite, carbide with low fractions of residual austenite in a ferritic matrix which is substantially free from interstitially dissolved elements (IF character).

The alloy content of the alloying elements is adapted stoichiometrically to the ferrite content and to the solubility of carbon and nitrogen at the temperature of a subsequent heating or heat treatment of the DP microstructure.

In this way, the semi-finished product or a structural component part manufactured therefrom acquires the properties characteristic of a DP steel with respect to microstructure, strength, minimum elongation, yield strength ratio, and strain hardening exponent also after a subsequent heat treatment or heating, particularly hot-dip galvanization and possibly subsequent dressing.

For the production of hot strip with a DP microstructure, the hot strip is cooled in the two-phase region after exiting the finishing mill train in order to adjust a suitable amount of residual austenite and is subsequently cooled in an accelerated manner to a temperature below the martensite start and finish temperature.

Conversely, for producing cold strip with a DP microstructure, the cold strip is heated in the two-phase region to adjust a suitable amount of residual austenite and is subsequently cooled in an accelerated manner to a temperature below the martensite start and finish temperature. This heat treatment is preferably carried out in a continuous annealing installation.

EXAMPLE

In order to exclude the influence of dissolved N on the aging behavior, the N content should be fixated as low as possible and by adding Al and possibly Ti. A correspondingly increased V content can also be used for nitrogen fixation. Table 1 contains some possible chemical compositions:

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying diagrams:

FIG. 1 is a cooling curve of steel sheet after hot rolling for adjusting a dual-phase microstructure; and

FIG. 2 is a typical stress-strain diagram for a DP steel with low yield strength ratio (≦75%) and without a pronounced yield strength.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, the austenite fraction is plotted over the time axis on the left-hand side and the core temperature is plotted over the time axis on the right-hand side.

The cooling curve of a steel sheet after hot rolling for adjusting a dual-phase microstructure is shown by way of example.

In so doing, the ferrite region is cooled initially, and most of the austenite is transformed to ferrite within a very short time period. This first cooling stage can be followed by a holding period, or further cooling to temperatures below the martensite start temperature is carried out directly. In so doing, any remaining residual austenite transforms to martensite and generates the second, hard phase. A coiling temperature of about 200° C. is aimed for.

The solid line shows the austenite dissociation as cooling time increases. The curve in dashes shows the lowering of the core temperature, also over the cooling time. It can be seen that cooling is accelerated between approximately 600° C. and the holding temperature of 200° C.

The method provides a semi-finished product, particularly steel strip, having a dual-phase microstructure and a tensile strength between 500 and 1000 MPa elements such as carbon and nitrogen that are interstitially dissolved in the ferritic matrix are bound by alloying elements such as Al, Mo, Nb, Ti and V which form carbide, nitride or carbonitride in order to adjust a microstructure of hard components such as martensite, bainite, carbide with low fractions of residual austenite in a ferritic matrix which is substantially free from interstitially dissolved elements (IF character). The alloy content of the aforesaid alloying elements is adapted stoichiometrically to the ferrite content and to the solubility of carbon and nitrogen in the ferrite at the temperature of a subsequent heat treatment or heating, particularly hot-dip galvanization. The production of hot strip with a DP microstructure, the hot strip is cooled in the two-phase region after exiting the finishing mill train of the rolling mill in order to adjust a suitable amount of residual austenite for achieving the desired strength class, and is subsequently cooled in an accelerated manner to a temperature below the martensite start and finish temperature.

For producing cold strip with a DP microstructure, the cold strip is heated in the two-phase region to adjust a suitable amount of residual austenite and is subsequently cooled in an accelerated manner to a temperature below the martensite start and finish temperature. Heat treatment for adjusting the DP microstructure in cold strip is preferably carried out in a continuous annealing installation. The heat treatment is carried out between 400° C. and A₃. The A3 point is the temperature at which grains of ferrite start to form.

Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.