Method and Device Adjusting Targeted Combinations of Properties of Polyphase Steel

Abstract

Compared to conventional steel products, polyphase steels have a significantly improved combination of resistance and ductility and are therefore becoming more and more important—especially for the automobile industry. The currently most important steel groups for the automobile industry are dual phase steels and TRIP steels. The production of different polyphase steel resistance categories, carried out directly on a hot strip, for meeting various requirements, requires a highly extensive know-how and firstly a corresponding adaptation of the alloy elements. According to the invention, a heat treatment ( 30 ) with a variable heating temperature and heating duration is carried out following the actual production of polyphase steels with a standard analysis and a standard process execution, whereby almost any combination of different materials or combination of properties (height of yield stress, level of tensile strength) can be adjusted.

1 . A method for adjusting targeted combinations of properties of hot-rolled multiphase steels, whose multiphase structure includes at least 30% ferrite and at most 50% martensite, for example, dual-phase steels and TRIP steels, which are produced with a standard analysis and standard process management in a conventional hot rolling mill train, a thin slab casting and rolling installation or suitable narrow and medium strip mill trains, or a wire mill, wherein following the cooling from the hot rolling ( 10 ) or a later production step, for example, the production of components, the desired combinations of strengths and elastic limit-tensile strength ratios are adjusted in the multiphase steels by a subsequent or intermediate annealing treatment ( 30 ,  35 ) with variable annealing temperature and variable annealing time. 
     
     
         2 . A method in accordance with  claim 1 , wherein the annealing treatment ( 30 ,  35 ) is carried out in such a way that the resulting microstructure consists of a ferritic base matrix and annealed martensite or bainite with 10-50% of the area fraction, such that the annealing temperature affects primarily the magnitude of the yield point by finely distributed precipitation of carbides at the grain boundaries of the martensite or bainite, and such that the level of tensile strength can be adjusted by the annealing time. 
     
     
         3 . A method in accordance with  claim 1 , wherein the annealing treatment ( 30 ,  35 ) is carried out at an annealing temperature of ≦600° C. and an annealing time of ≦120 s. 
     
     
         4 . A method in accordance with  claim 1 , wherein the annealing treatment ( 30 ,  35 ) is carried out offline in a continuous annealing installation. 
     
     
         5 . A method in accordance with  claim 1 , wherein the annealing treatment ( 30 ) is carried out online as part of a strip galvanizing operation ( 40 ) in the heating stage of a galvanizing line before entry into the zinc bath. 
     
     
         6 . A method in accordance with  claim 1 , wherein the annealing treatment ( 30 ,  35 ) is carried out on components that have already been finish pressed. 
     
     
         7 . A method in accordance with  claim 1 , wherein the annealing treatment ( 35 ) is carried out as a targeted zonal treatment, i.e., in locally limited sections of a component. 
     
     
         8 . A device for adjusting targeted combinations of properties of hot-rolled multiphase steels, whose multiphase structure includes at least 30% ferrite and at most 50% martensite, for example, dual-phase steels and TRIP steels, which are produced with a standard analysis and standard process management in a conventional hot rolling mill train, a thin slab casting and rolling installation or suitable narrow and medium strip mill trains, or a wire mill, especially for carrying out the method according to  claim 1 , wherein a thermal installation is located in a freely selectable place within the production plant or production line and that an annealing treatment ( 30 ,  35 ) can be carried out in this thermal installation at a variable annealing temperature of ≦600° C. and a variable annealing time of ≦120 s. 
     
     
         9 . A device in accordance with  claim 8 , wherein the thermal installation is a continuous furnace installed online in a galvanizing line. 
     
     
         10 . A device in accordance with  claim 8 , wherein the thermal installation is a continuous annealing installation that is operated offline. 
     
     
         11 . A device in accordance with  claim 8 , wherein the thermal installation is constructed in such a way that a zonal annealing treatment ( 35 ) can be carried out on locally limited sections of a component before or after its actual production as a finished product.