Method for manufacturing a tempered and coated steel sheet having excellent formability

Abstract

A method for making a tempered and coated steel sheet having a composition containing the following elements, expressed in percentage by weight: 0.17%≤carbon≤0.25%, 1.8%≤manganese≤2.3%, 0.5%≤silicon≤2.0%, 0.03%≤aluminum≤1.2%, sulphur≤0.03%, phosphorus≤0.03%, the remainder composition being composed of iron and unavoidable impurities caused by processing. The composition may also contain one or more of the following elements chromium≤0.4%, molybdenum≤0.3%, niobium≤0.04%, titanium≤0.1%.

What is claimed is: 
     
         1 . A method of production of a tempered and coated steel sheet comprising the following successive steps:
 providing a semi-finished product comprising a composition comprising the following elements, expressed in percentage by weight:   0.17%≤carbon≤0.25%,   1.8%≤manganese≤2.3%,   0.5%≤silicon≤2.0%,   0.03%≤aluminum≤1.2%,   sulphur≤0.03%,   phosphorus≤0.03%,   a remainder of the composition being composed of iron and unavoidable impurities caused by processing,   reheating the semi-finished product to a temperature above Ac3;   rolling the semi-finished product in the austenitic range wherein the hot rolling finishing temperature is between 750° C. and 1050° C. to obtain a hot rolled steel sheet;   cooling the hot rolled steel sheet at a cooling rate 20 to 150° C./s to a coiling temperature less than or equal to 600° C.; and coiling the hot rolled steel sheet;   cooling the hot rolled steel sheet to room temperature;   annealing the hot rolled steel sheet at temperature between 400° C. and 750;   cold rolling the hot rolled annealed steel sheet with a reduction rate between 30 and 80% to obtain a cold rolled steel sheet;   then heating the cold rolled steel sheet at a rate between 1 to 20° C./s to a soaking temperature between Ae1 and Ae3 and holding less than 600 seconds;   then cooling the cold rolled steel sheet at a rate greater than 5° C./s to a temperature above Ms and less than 475° C. and holding the cold rolled steel sheet at the temperature for 20 to 400 seconds;   then cooling the cold rolled steel sheet at cooling rate not greater than 200° C./s down to room temperature;   then reheating the annealed steel sheet at a rate between 1° C./s to 20° C./s to a soaking temperature between 440° C. and 600° C. and holding less than 100s and then hot dipping the annealed steel sheet in a bath zinc or zinc alloy coating for tempering and coating; and   cooling the tempered and coated steel sheet to room temperature at a cooling rate between 1° C./s and 20° C./s.   
     
     
         2 . The method as recited in  claim 1  wherein the coiling temperature is above 400° C. 
     
     
         3 . The method as recited in  claim 1  further comprising performing scale removal process on the hot rolled steel sheet. 
     
     
         4 . The method as recited in  claim 1  further comprising performing scale removal process on the hot rolled annealed steel sheet. 
     
     
         5 . The method as recited in  claim 1  wherein the tempered and coated steel sheet has a base microstructure comprising in area fraction, 3 to 20% residual austenite, at least 15% of ferrite, 40 to 85% tempered bainite and a minimum of 5% of tempered martensite, wherein the cumulated amounts of tempered martensite and residual austenite is between 10 and 30%. 
     
     
         6 . The method as recited in  claim 1  wherein the composition further comprises one or more of the following elements: chromium≤0.4%, molybdenum≤0.3%, niobium≤0.04%, titanium≤0.1%. 
     
     
         7 . The method as recited in  claim 1  wherein the composition includes 0.6% to 1.8% of silicon. 
     
     
         8 . The method as recited in  claim 1  wherein the composition includes 0.03% to 0.6% of aluminium. 
     
     
         9 . The method as recited in  claim 5  wherein the cumulated amounts of tempered martensite and residual austenite is between 10% and 25%. 
     
     
         10 . The method as recited in  claim 5  wherein the cumulated amounts of tempered martensite and residual austenite is more than or equal to 15% and the percentage of tempered martensite is higher than 10%. 
     
     
         11 . The method as recited in  claim 5  wherein the carbon content of residual austenite is between 0.9 to 1.1%. 
     
     
         12 . The method as recited in  claim 1  wherein the steel sheet has a ultimate tensile strength above 900 MPa, a hole elongation ratio above 18% and a total elongation above 17%. 
     
     
         13 . The method as recited in  claim 5  wherein the steel sheet has a ultimate tensile strength of 1000 MPa to 1100 MPa and a hole expansion ratio above 20%. 
     
     
         14 . A structural or safety part of a vehicle comprising the steel sheet produced according to the method as recited in  claim 1 .