Hot rolled and steel sheet and a method of manufacturing thereof

Abstract

A hot rolled steel sheet having a composition including, 0.02%≤Carbon≤0.2%, 3%≤Manganese≤9%, 0.2%≤Silicon≤1.2%, 0.9%≤Aluminum≤2.5%, 0%≤Phosphorus≤0.03%, 0%≤Sulfur≤0.03%, 0%≤Nitrogen≤0.025%, 0%≤Molybdenum≤0.6%, 0%≤Titanium≤0.1%, 0.0001%≤Boron≤0.01%, 0%≤Chromium≤0.5%, 0%≤Niobium≤0.1%, 0%≤Vanadium≤0.2%, 0%≤Nickel≤1%, 0%≤Copper≤1%, 0%≤Calcium≤0.005%, 0%≤Magnesium≤0.0010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet including in area fraction, at least 60% of tempered martensite, 15% to 40% residual austenite, 0% to 10% polygonal ferrite, 0% to 5% of bainite, 0 to 15% of fresh martensite and 0% to 5% of carbides of Niobium, Titanium, Vanadium or Iron.

1 - 14 . (canceled) 
     
     
         15 . A hot rolled steel sheet having a composition comprising the following elements, expressed in percentage by weight:
 0.02%≤Carbon≤0.2%   3%≤Manganese≤9%   0.2%≤Silicon≤1.2%   0.9%≤Aluminum≤2.5%   0%≤Phosphorus≤0.03%   0%≤Sulfur≤0.03%   0%≤Nitrogen≤0.025%   
       and optionally one or more of the following elements
 0%≤Molybdenum≤0.6% 
 0%≤Titanium≤0.1% 
 0.0001%≤Boron≤0.01% 
 0%≤Chromium≤0.5% 
 0%≤Niobium≤0.1% 
 0%≤Vanadium≤0.2% 
 0%≤Nickel≤1% 
 0%≤Copper≤1% 
 0%≤Calcium≤0.005% 
 0%≤Magnesium≤0.0010% 
 
       a remainder of the composition being composed of iron and unavoidable impurities caused by processing, a microstructure of the steel sheet comprising in area fraction, at least 60% of tempered martensite, 15% to 40% residual austenite, 0% to 10% polygonal ferrite, 0% to 5% of bainite, 0 to 15% of fresh martensite and 0% to 5% of carbides of Niobium, Titanium, Vanadium or Iron. 
     
     
         16 . The hot rolled steel sheet as recited in  claim 15  wherein the composition includes 0.3% to 1% of Silicon. 
     
     
         17 . The hot rolled steel sheet as recited in  claim 15  wherein the composition includes 0.03% to 0.18% of Carbon. 
     
     
         18 . The hot rolled steel sheet as recited in  claim 15  wherein the composition includes 3.5% to 8.5% of Manganese. 
     
     
         19 . The hot rolled steel sheet as recited in  claim 15  wherein the composition includes 1% to 2.3% of Aluminum. 
     
     
         20 . The hot rolled steel sheet as recited in  claim 15  wherein the amount of Martensite is between 65% and 84%. 
     
     
         21 . The hot rolled steel sheet as recited in  claim 15  wherein the amount of Residual Austenite between 18% and 35% 
     
     
         22 . The hot rolled steel sheet as recited in  claim 15  wherein the steel sheet has a tensile strength of 750 MPa or more, and a total elongation of 20% or more. 
     
     
         23 . The hot rolled steel sheet as recited in  claim 15  wherein a shape factor of the tempered martensite is between 4 and 12. 
     
     
         24 . A method of production of a hot rolled steel sheet comprising the following successive steps:
 providing a semi-finished product having a steel composition comprising the following elements, expressed in percentage by weight:
 0.02%≤Carbon≤0.2% 
 3%≤Manganese≤9% 
 0.2%≤Silicon≤1.2% 
 0.9%≤Aluminum≤2.5% 
 0%≤Phosphorus≤0.03% 
 0%≤Sulfur≤0.03% 
 0%≤Nitrogen≤0.025% 
   and optionally one or more of the following elements
 0%≤Molybdenum≤0.6% 
 0%≤Titanium≤0.1% 
 0.0001%≤Boron≤0.01% 
 0%≤Chromium≤0.5% 
 0%≤Niobium≤0.1% 
 0%≤Vanadium≤0.2% 
 0%≤Nickel≤1% 
 0%≤Copper≤1% 
 0%≤Calcium≤0.005% 
 0%≤Magnesium≤0.0010% 
   a remainder of the composition being composed of iron and unavoidable impurities caused by processing   reheating the semi-finished product to a temperature between Ac3+50° C. and 1300° C.;   rolling the semi-finished product in the austenitic range wherein a hot rolling finishing temperature is at least Ac3 to obtain a hot rolled steel strip;   the hot rolled steel strip is optionally coiled at a coiling temperature range is between 20° C. and Ms;   then cooling the hot rolled steel strip from the hot rolling finishing temperature to a temperature range between Ms and 20° C., with a cooling rate between 1° C./s and 50° C./s;   then heating the hot rolled steel strip from a temperature range between Ms and 20° C. to a temperature Tsoak between 550° C. and Ac3, with a heating rate HR1 of at least 1° C./s, and holding for 5 to 1000 seconds   then cooling the hot rolled strip, wherein cooling starts from the temperature Tsoak to a cooling stop temperature T1 between Ms−10° C. and 20° C., with a cooling rate CR1 between 0.1° C./s and 150° C./s;   thereafter cooling the hot rolled steel strip to room temperature at a cooling rate CR2 between 0.1° C./s and 150° C./s to obtain a hot rolled steel sheet.   
     
     
         25 . The method as recited in  claim 24  wherein the Tsoak temperature is between 600° C. and Ac3−40° C. 
     
     
         26 . The method as recited in  claim 24  wherein the T1 temperature is between Ms−20° C. and 25° C. 
     
     
         27 . A method comprising: manufacturing a part of industrial machinery or green goods or yellows of using a steel sheet as recited in  claim 15 . 
     
     
         28 . An industrial machine comprising the part obtained according to  claim 27 . 
     
     
         29 . A method comprising: manufacturing a part of industrial machinery or green goods or yellows of using a steel sheet obtained by the method as recited in  claim 24 . 
     
     
         30 . An industrial machine comprising the part obtained according to  claim 29 .