Dual Phase Steel with Improved Properties

Abstract

A method for processing a dual phase steel sheet. The method includes heating the steel sheet to a first temperature (T 1 ), cooling the steel sheet to a second temperature (T 2 ), transitioning the steel sheet to a third temperature (T 3 ), and cooling the steel sheet to room temperature. T 1 is at least above the temperature at which the steel sheet transforms to austenite and ferrite. T 2 is below the martensite start temperature (M s ). The cooling rate to T 2 is sufficiently rapid to transform at least some austenite to martensite.

What is claimed is: 
     
         1 . A method for processing a dual phase steel sheet, the method comprising:
 (a) heating the steel sheet to a first temperature (T 1 ), wherein T 1  is at least above the temperature at which the steel sheet transforms to austenite and ferrite to form at least some austenite in the steel sheet;   (b) cooling the steel sheet to a second temperature (T 2 ) by cooling at a cooling rate, wherein T 2  is below the martensite start temperature (M s ), wherein the cooling rate is sufficiently rapid to transform at least some the austenite to martensite;   (c) transitioning the steel sheet to a third temperature (T 3 ); and   (e) cooling the steel sheet to room temperature.   
     
     
         2 . The method of  claim 1 , further comprising hot dip galvanizing or galvannealing the steel sheet after the steel sheet is transitioned to T 3 . 
     
     
         3 . The method of  claim 1 , wherein the hot dip galvanizing or galvannealing occurs above M s . 
     
     
         4 . The method of  claim 1 , wherein the step of cooling the steel sheet to T 2  is performed prior to the step of transitioning the steel sheet to T 3 . 
     
     
         5 . The method of  claim 4 , wherein the step of transitioning the steel sheet to T 3  includes reheating the steel sheet from T 2  to T 3 . 
     
     
         6 . The method of  claim 1 , wherein the step of cooling the steel sheet to T 2  is performed after the step of transitioning the steel sheet to T 3 . 
     
     
         7 . The method of  claim 1 , wherein the step of cooling the steel sheet to T 2  is sufficiently rapid to transform substantially all austenite to martensite. 
     
     
         8 . The method of  claim 1 , wherein the steel sheet comprises the following elements by weight percent:
 0.080-0.1 carbon;   1.7-1.9 manganese;   0.15-0.25 silicon;   0.02 or less molybdenum;   0.015-0.025 niobium;   0.2-0.3 chromium; and   the balance being iron and other incidental impurities.   
     
     
         9 . The method of  claim 1 , wherein the steel sheet comprises the following elements by weight percent:
 0.067-0.080 carbon;   1.65-1.82 manganese;   0.15-0.25 silicon;   0.16-0.02 molybdenum;   0.001 or less niobium; and   the balance being iron and other incidental impurities.   
     
     
         10 . The method of  claim 1 , wherein the steel sheet comprises the following elements by weight percent:
 0.10-0.12 carbon;   2.1-2.3 manganese;   0.15-0.25 silicon;   0.003 or less niobium;   0.2-0.3 chromium; and   the balance being iron and other incidental impurities.   
     
     
         11 . The method of  claim 10 , wherein the steel sheet further comprises 0.25-0.35 molybdenum. 
     
     
         12 . The method of  claim 1 , wherein the steel sheet comprises the following elements by weight percent:
 0.10-0.12 carbon;   1.75-1.9 manganese;   0.15-0.25 silicon;   0.035-0.045 niobium;   0.2-0.3 chromium; and   the balance being iron and other incidental impurities.   
     
     
         13 . The method of  claim 12 , wherein the steel sheet further comprises 0.15-0.2 molybdenum. 
     
     
         14 . The method of  claim 1 , wherein the steel sheet comprises the following elements by weight percent:
 0.11-0.13 carbon;   2.4-2.7 manganese;   0.15-0.25 silicon;   0.35-0.45 molybdenum;   0.004 or less niobium;   0.3-0.4 chromium; and   the balance being iron and other incidental impurities.   
     
     
         15 . The method of  claim 1 , wherein the steel sheet comprises the following elements by weight percent:
 0.80-0.10 carbon;   2.0-2.2 manganese;   0.40-0.50 silicon;   0.04-0.060 niobium;   0.2-0.3 chromium; and   the balance being iron and other incidental impurities.   
     
     
         16 . The method of  claim 15 , wherein the steel sheet further comprises 0.30-0.40 molybdenum. 
     
     
         17 . The method of  claim 1 , wherein the steel sheet comprises the following elements by weight percent:
 0.09-0.10 carbon;   2.25-2.42 manganese;   0.10-0.20 silicon;   0.035-0.045 niobium;   0.57-0.67 chromium; and   the balance being iron and other incidental impurities.   
     
     
         18 . The method of  claim 17 , wherein the steel sheet further comprises 0.08-0.12 molybdenum. 
     
     
         19 . The method of  claim 1 , wherein the steel sheet comprises the following elements by weight percent:
 0.12-0.14 carbon;   2.7-2.9 manganese;   0.15-0.25 silicon;   0.004 or less niobium;   0.3-0.4 chromium; and   the balance being iron and other incidental impurities.   
     
     
         20 . The method of  claim 19 , wherein the steel sheet further comprises 0.35-0.45 molybdenum. 
     
     
         21 . The method of  claim 1 , wherein the steel sheet comprises the following elements by weight percent:
 0.11-0.13 carbon;   2.45-2.60 manganese;   0.420-0.580 silicon;   0.05 or less molybdenum;   0.035-0.045 niobium; and   the balance being iron and other incidental impurities.   
     
     
         22 . The method of  claim 21 , wherein the steel sheet further comprises 0.57-0.63 chromium.