Steel composition for the production of cold rolled multiphase steel products

Abstract

The present invention is related to a steel composition intended to be used in a process including a cold rolling step, for the production of uncoated, electro-galvanized or hot dip galvanized TRIP steel products, the composition being characterized by a specific addition of phosphorus. The latter is added in order to reach the desired mechanical properties (high tensile strength in combination with high elongation) while keeping a good weldability by sufficiently reducing the carbon content. The invention is further related to a process for producing a steel product, and to the steel product obtained, said product having the composition of the invention.

1 . A steel composition intended to be used in a process comprising a cold rolling step, for the production of uncoated, electro-galvanised or hot dip galvanised TRIP steel products, said composition comprising: 
 C: between 1300 ppm and 2600 ppm,    Mn: between 10000 ppm and 22000 ppm,    Al: between 8000 ppm and 15000 ppm,    Si: between 2000 ppm and 6000 ppm,    P: between 400 and 1000 ppm,    S: maximum 120 ppm,    N: maximum 200 ppm,    Ti: maximum 1000 ppm,    Nb: maximum 1000 ppm,    V: maximum 1000 ppm,    B: maximum 10 ppm.    the remainder being substantially iron and incidental impurities.    
     
     
         2 . The steel composition according to  claim 1 , comprising a carbon content between 1300 ppm and 1900 ppm.  
     
     
         3 . The steel composition according to  claim 2 , comprising a carbon content between 1350 ppm and 1900 ppm.  
     
     
         4 . The steel composition according to  claim 2 , comprising a carbon content between 1400 ppm and 1900 ppm.  
     
     
         5 . The steel composition according to  claim 1 , comprising a carbon content between 1700 ppm and 2300 ppm.  
     
     
         6 . The steel composition according to  claim 1 , comprising a carbon content between 2000 ppm and 2600 ppm.  
     
     
         7 . The steel composition according to  claim 2 , comprising: 
 Mn: between 13000 ppm and 22000 ppm,    Al: between 8000 ppm and 14000 ppm,    Si: between 2500 ppm and 4500 ppm,    P: between 600 and 1000 ppm,    S: maximum 120 ppm,    N: maximum 150 ppm,    Ti: maximum 200 ppm,    Nb: maximum 100 ppm,    V: maximum 100 ppm,    B: maximum 5 ppm.    
     
     
         8 . The steel composition according to  claim 7 , comprising an aluminium content between 9000 ppm and 13000 ppm.  
     
     
         9 . A process for manufacturing a cold rolled TRIP steel product, comprising the steps of: 
 preparing a steel slab having a composition according to  claim 1 ,    hot rolling said slab, wherein the finishing rolling temperature is higher than the Ar3 temperature, to form a hot-rolled substrate,    cooling said substrate to a coiling temperature (CT) between 500° C. and 680° C.,    coiling said substrate at said coiling temperature,    pickling said substrate to remove the oxides,    cold rolling said substrate to obtain a reduction of thickness, with a minimum reduction of 40%.    
     
     
         10 . The process according to  claim 9 , further comprising the steps of: 
 soaking said substrate at a temperature between 760° C. and 850° C.,    cooling said substrate with a cooling rate higher than 2° C./s to a temperature in the range 360° C. to 450° C.,    holding said substrate in said temperature range for a time less than 700 s,    cooling said substrate to room temperature at a cooling rate higher than 1° C./s.    subjecting said substrate to a skinpass reduction of maximum 1.5%.    
     
     
         11 . The process according to  claim 10 , further comprising an electrolytic zinc coating step.  
     
     
         12 . The process according to  claim 9 , further comprising the following processing steps: 
 soaking said substrate at a temperature between 760° C. and 850° C.,    cooling said substrate with a cooling rate higher than 2° C./s to the temperature of a Zn-bath,    holding said substrate in the temperature range between 490° C. and 460° C. for less than 200 seconds.    hot dip galvanising said substrate in said Zn-bath,    cooling said substrate to room temperature at a cooling rate higher than 2° C./s.    
     
     
         13 . The process according to  claim 12 , further comprising the step of subjecting said substrate to a skinpass reduction of maximum 1.5%.  
     
     
         14 . A steel product produced according to the process of  claim 9  and having a microstructure comprising 30-75% ferrite, 10-40% bainite, 0-20% retained austenite and possibly 0-10% martensite.  
     
     
         15 . A steel product produced according to the process of  claim 10 , said product comprising a carbon content between 1300 ppm and 1900 ppm, said product having a yield strength between 320 MPa and 480 MPa, a tensile strength above 590 MPa, an elongation A80 higher than 26% and a strain hardening coefficient, calculated between 10% and uniform elongation, higher than 0.2.  
     
     
         16 . A steel product produced according to the process of  claim 10 , said product comprising a carbon content between 1700 ppm and 2300 ppm, said product having a yield strength between 350 MPa and 510 MPa, a tensile strength above 700 MPa, an elongation A80 higher than 24% and a strain hardening coefficient, calculated between 10% and uniform elongation, higher than 0.19.  
     
     
         17 . A steel product produced according to the process of  claim 10 , said product comprising a carbon content between 2000 ppm and 2600 ppm, said product having a yield strength between 400 MPa and 600 MPa, a tensile strength above 780 MPa, an elongation A80 higher than 22% and a strain hardening coefficient, calculated between 10% and uniform elongation, higher than 0.18.  
     
     
         18 . A steel product produced according to the process of  claim 10 , said product comprising a carbon content between 2000 ppm and 2600 ppm, said product having a yield strength between 450 MPa and 700 MPa, a tensile strength above 980 MPa, an elongation A80 higher than 18% and a strain hardening coefficient, calculated between 10% and uniform elongation, higher than 0.14.  
     
     
         19 . A steel product produced according to  claim 14 , having bake hardening BH2 higher than 40 MPa in both longitudinal and transversal directions.