Hot rolled steel sheet and a method of manufacturing thereof

Abstract

A method of production of a hot rolled steel sheet having a composition including the elements, expressed in percentage by weight 0.11%≤Carbon≤0.16%, 1%≤Manganese≤2%, 0.1%≤Silicon≤0.7%, 0.02%≤Aluminum≤0.1%, 0.15%≤Molybdenum≤0.4%, 0.15%≤Vanadium≤0.4%, 0.002%≤Phosphorus≤0.02%, 0%≤Sulfur≤0.005%, 0%≤Nitrogen≤0.01%, and can contain one or more of the following optional elements 0%≤Chromium≤0.5%, 0%≤Niobium≤0.05%, 0.0001%≤Calcium≤0.005%, 0%≤Boron≤0.001%, 0%≤Magnesium≤0.0010%, 0%≤Titanium≤0.01%, with 0.3%≤Mo+V+Nb≤0.6%, the remainder composition being composed of iron and unavoidable impurities, the microstructure of steel sheet including in area fraction, 70% to 90% Bainite, 10% to 25% Ferrite wherein the cumulated amounts of Bainite and Ferrite is at least 90% and a cumulated amount of Residual Austenite and Martensite is between 0% and 10%.

What is claimed is: 
     
         1 . A method of production of a hot rolled heat treated 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:
   0.11%≤Carbon≤0.16%
 
   1%≤Manganese≤2%
 
   0.1%≤Silicon≤0.7%
 
   0.02%≤Aluminum≤0.1%
 
   0.15%≤Molybdenum≤0.4%
 
   0.15%≤Vanadium≤0.4%
 
   0.002%≤Phosphorus≤0.02%
 
   0%≤Sulfur≤0.005%.
 
   0%≤Nitrogen≤0.01%
 
   and optionally one or more of the following elements:
   0%≤Chromium≤0.5%
 
   0%≤Niobium≤0.05%
 
   0.0001%≤Calcium≤0.005%
 
   0%≤Boron≤0.001%
 
   0%≤Magnesium≤0.0010%
 
   0%≤Titanium≤0.01%
 
   with 0.3%≤Mo+V+Nb≤0.6%
 
   a remainder of the composition being composed of iron and unavoidable impurities caused by processing;   reheating the semi-finished product to a temperature between 1200° C. and 1300° C.;   rolling the semi-finished product in the austenitic range wherein the hot rolling finishing temperature is between 850° C. and 975° C. to obtain a hot rolled steel strip; then   cooling the hot rolled strip in three step cooling wherein:
 in step one of the cooling, the hot rolled steel sheet starts from a temperature range between 850° C. and 975° C. to a temperature range between 650° C. and 725° C., with a cooling rate between 40° C./s and 150° C./s; 
 the step two of the cooling the hot rolled steel sheet starts from a temperature range between 650° C. and 725° C. to a temperature range between 620° C. and 690° C., said step two having a duration of 1 s to 10 s and being an air cooling; and 
 in the step three of the cooling the hot rolled steel sheet starts from a temperature range between 620° C. and 690° C. to a temperature range between 450° C. and 550° C. with a cooling rate greater than 20° C./s; thereafter 
   coiling the hot rolled steel strip at a temperature range between 450° C. and 550° C.;   cooling the coiled hot rolled steel strip to room temperature.   
     
     
         2 . The method as recited in  claim 1  wherein the reheating temperature for semi-finished product is between 1200° C. and 1275° C. 
     
     
         3 . The method as recited in  claim 1  wherein the hot rolling finishing temperature is between 880° C. and 930° C. 
     
     
         4 . The method as recited in  claim 1  wherein the coiling temperature range is between 470° C. and 530° C. 
     
     
         5 . The method as recited in  claim 1  wherein the cooling rate for the step one of the cooling is between 40° C./s and 120° C./s. 
     
     
         6 . The method as recited in  claim 1  wherein the cooling rate for the step three of the cooling is greater than equal to 25° C./s. 
     
     
         7 . The method as recited in  claim 1  wherein the duration for the step two of the cooling is between 2 seconds and 9 seconds. 
     
     
         8 . A method for manufacturing a structural or safety part of a vehicle comprising the method as recited in  claim 1 . 
     
     
         9 . A vehicle comprising the structural or safety part obtained according to  claim 8 .