High Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Heat Treatment Downstream of Molten Zinc Bath

Abstract

Steel with high strength and good formability is produced with compositions and methods for forming austenitic and martensitic microstructure in the steel. Carbon, manganese, molybdenum, nickel copper and chromium may promote the formation of room temperature stable (or meta-stable) austenite by mechanisms such as lowering transformation temperatures for non-martensitic constituents, and/or increasing the hardenability of steel. Thermal cycles utilizing a rapid cooling below a martensite start temperature followed by reheating may promote formation of room temperature stable austenite by permitting diffusion of carbon into austenite from martensite.

What is claimed is: 
     
         1 . A steel sheet comprising the following elements by weight percent:
 0.15-0.4% carbon;   1.5-4% manganese;   2% or less silicon, aluminum, or some combination thereof;   0.5% or less molybdenum;   0.05% or less niobium; and   the balance being iron and other incidental impurities.   
     
     
         2 . A method for processing a steel sheet, the method comprising:
 (a) heating the steel sheet to a first temperature (T1), wherein T1 is at least above the temperature at which the steel sheet transforms to austenite and ferrite;   (b) cooling the steel sheet to a second temperature (T2) by cooling at a cooling rate, wherein T2 is below the martensite start temperature (M s ), wherein the cooling rate is sufficiently rapid to transform austenite to martensite;   (c) re-heating the steel sheet to a partitioning temperature, wherein the partitioning temperature is sufficient to permit diffusion of carbon within the structure of the steel sheet;   (d) stabilizing austenite by holding the steel sheet at the partitioning temperature for a holding time, wherein the holding time is of a period of time sufficient to permit diffusion of carbon from martensite to austenite; and   (e) cooling the steel sheet to room temperature.   
     
     
         3 . The method of  claim 2 , further comprising hot dip galvanizing or galvannealing the steel sheet while the steel sheet is being cooled to T2. 
     
     
         4 . The method of  claim 3 , wherein the hot dip galvanizing or galvannealing occurs above M s . 
     
     
         5 . The method of  claim 2 , wherein the partitioning temperature is above M s . 
     
     
         6 . The method of  claim 2 , wherein the partitioning temperature is below M s . 
     
     
         7 . The method of  claim 2 , wherein the steel sheet comprises the following elements by weight percent:
 0.15-0.4% carbon;   1.5-4% manganese;   2% or less silicon, aluminum, or some combination thereof;   0.5% or less molybdenum;   0.05% or less niobium; and   the balance being iron and other incidental impurities.