Hot rolled and steel sheet and a method of manufacturing thereof

Abstract

A hot rolled steel sheet having a composition including of the elements, 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≤0.15%, 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.

What is claimed is: 
     
         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≤0.15%   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 70% and 80%. 
     
     
         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 15% 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 steel product with 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≤0.15%   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 said semi-finished product to a temperature from Ac3+50° C. to 1300° C.;   rolling the semi-finished product in the austenitic range wherein the hot rolling finishing temperature is at least Ac3 to obtain a hot rolled steel;   the hot rolled steel is optionally coiled at a coiling temperature range is from 20° C. to 800° C.,   then cooling the shot rolled steel from hot rolling finishing temperature to a temperature range from Ms to 20° C., with a cooling rate from 1° C./s to 50° C./s;   then heating the said hot rolled steel from a temperature range from Ms- to 20° C. to a temperature TA1 from Ac3 to Ac3+150° C., with a heating rate HR1 of at least 1° C./s and holding from 5 to 6000 seconds   then cooling the hot rolled steel, wherein cooling starts from TA1 to a cooling stop temperature T1 from Ms-10° C. to 15° C., with a cooling rate CR1 from 0.1° C./s to 150° C./s;   then heating the hot rolled steel from T1 to a temperature TA2 from 550° C. to Ac3, with a heating rate HR2 of at least 1° C./s, where it is held during 5 to 6000 seconds   then cooling the said hot rolled steel, wherein cooling starts from TA2 to a cooling stop temperature T2 from Ms-10° C. to 15° C., with a cooling rate CR2 from 0.1° C./s to 150° C./s;   thereafter cooling the hot rolled steel to room temperature at a cooling rate CR3 from 0.1° C./s to 150° C./s to obtain a hot rolled steel sheet.   
     
     
         25 . The method as recited in  claim 24  wherein the temperature TA2 is from 600° C. to Ac3-40° C. 
     
     
         26 . The method as recited in  claim 24  wherein the T1 temperature is from Ms-20° C. to 20° 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 .