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

Abstract

A method for producing 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 which are interstitially dissolved in a 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).

1 .- 6 . (canceled) 
     
     
         7 . A method for producing semi-finished product having a dual-phase microstructure and a tensile strength between 500 and 1000 MPa, comprising:
 interstitially dissolving elements including one or more of carbon and nitrogen in a ferritic matrix;   binding the interstitially dissolved elements by alloying elements including one or more of Al, Mo, Nb, Ti and V, which form carbide, nitride, or carbonitride to adjust a microstructure of hard components including one or more of martensite, bainite, carbide having low fractions of residual austenite in a ferritic matrix that is substantially free from interstitially dissolved elements; and   adapting an alloy content of the alloying elements stoichiometrically to a ferrite content and to a solubility of the carbon and the nitrogen in the ferrite at a temperature of at least one of a subsequent heat treatment and heating.   
     
     
         8 . The method according to  claim 7 , wherein for production of a hot strip with a DP microstructure, the hot strip is cooled in a two-phase region after exiting a finishing mill train of a rolling mill to adjust a suitable amount of residual austenite to achieving a desired strength class, and
 the hot strip is subsequently cooled in an accelerated manner to a temperature below a martensite start and finish temperature.   
     
     
         9 . The method according to  claim 7 , wherein for producing a cold strip with a DP microstructure, the cold strip is heated in a two-phase region to adjust a suitable amount of residual austenite and the cold strip is subsequently cooled in an accelerated manner to a temperature below a martensite start and finish temperature. 
     
     
         10 . The method according to  claim 9 , wherein the heat treatment for adjusting the DP microstructure in the cold strip is performed in a continuous annealing installation. 
     
     
         11 . The method according to  claim 7 , wherein the heat treatment is performed between 400° C. and A 3 , where A 3  is the temperature at which grains of ferrite start to form. 
     
     
         12 . The method according to  claim 7 , wherein the semi-finished product is steel strip. 
     
     
         13 . The method according to  claim 7 , wherein the at least one of the subsequent heat treatment and the heating is a hot-dip galvanization.