Silicon alloyed steel for hot rolling

Abstract

A silicon alloyed steel for hot rolling and a process of hot rolling the silicon alloyed steel is provided. The process includes providing a steel slab having a chemical composition in weight percent within a range of 0.06-0.30 C, 0.3-2.0 Mn, 0.6-3.5 Si, and Fe plus incidental melting impurities. The steel slab is hot rolled and hot rolled steel strip is produced. The hot rolled steel strip is coiled at temperatures between 100-600° C. and has a microstructure containing at least 90 vol % ferrite plus pearlite, a yield strength of at least 400 megapascals (MPa), a tensile strength of at least 600 MPa, and a tensile elongation of at least 20%.

We claim: 
     
         1 . A process for making a hot rolled strip from a silicon alloyed steel, the process comprising:
 providing a steel slab having a chemical composition in weight percent within a range of 0.06-0.30 C, 0.3-2.0 Mn, 0.6-3.5 Si and Fe plus incidental melting impurities;   hot rolling the steel slab to produce a hot rolled steel strip;   and coiling the hot rolled strip at coiling temperatures between 100-600° C., the coiled hot rolled strip having a microstructure containing at least 90 vol % ferrite plus pearlite, a yield strength of at least 400 MPa, a tensile strength of at least 600 MPa and a tensile elongation of at least 20% irrespective of the hot rolled strip being coiled at 100° C., 350° C. or 500° C.   
     
     
         2 . The process of  claim 1 , wherein the steel alloy has 0.1 max P, 0.08 max Al, 0.6 max Cr, 0.3 max Ni, 0.6 max Cu, 0.08 max Nb, 0.10 max Ti and 0.012 max S. 
     
     
         3 . The process of  claim 2 , wherein the steel alloy has a Si/Mn ratio between 1.3-3.5. 
     
     
         4 . The process of  claim 2 , wherein the steel alloy has between 0.03-0.05 P and a Si/Mn ratio between 0.8-3.5. 
     
     
         5 . The process of  claim 2 , wherein the steel alloy has between 0.05-0.1 P and a Si/Mn ratio between 0.3-3.5. 
     
     
         6 . The process of  claim 1 , wherein the steel alloy is hot rolled in an austenitic finishing roll mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 950-400° C. 
     
     
         7 . The process of  claim 6 , further including a cooling interruption within a range of 3-20 seconds of the hot rolled strip between austenitic finishing rolling and coiling. 
     
     
         8 . The process of  claim 6 , wherein the hot rolled strip is cooled directly from the austentic finishing roll mode to the coiling temperature at the cooling rate between 10-100 K/s. 
     
     
         9 . The process of  claim 1 , wherein the steel alloy is hot rolled in a partially ferritic finishing roll mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 850-400° C. 
     
     
         10 . The process of  claim 9 , further including a cooling interruption within a range of 3-20 seconds of the hot rolled strip between partially ferritic finishing rolling and coiling. 
     
     
         11 . The process of  claim 9 , wherein the hot rolled strip is cooled directly from the partially ferritic finishing roll mode to the coiling temperature at the cooling rate between 10-100 K/s. 
     
     
         12 . The process of  claim 1 , wherein the coiled hot rolled strip has a tensile strength of at least 650 MPa. 
     
     
         13 . The process of  claim 1 , wherein the tensile strength is at least 700 MPa. 
     
     
         14 . A process for making a high strength steel sheet comprising:
 providing a steel slab with a thickness between 50 and 280 mm, the steel slab having a chemical composition in weight percent within a range of 0.06-0.30 C, 0.3-2.0 Mn, 0.6-3.5 Si, 0.1 max P, 0.08 max Al, 0.6 max Cr, 0.3 max Ni, 0.6 max Cu, 0.08 max Nb, 0.10 max Ti, 0.012 max S, balance Fe and incidental melting impurities;   soaking the steel slab at temperatures between 1150 and 1350° C.;   hot rolling the steel alloy to produce a hot rolled steel strip;   and coiling the hot rolled strip at coiling temperatures between 100-600° C., the coiled hot rolled strip having a microstructure containing at least 90 vol % ferrite plus pearlite, a yield strength of at least 400 MPa, a tensile elongation of at least 20% and a tensile strength that obeys the relation:
   TS(+/−25 MPa)=164.9+634.7×C %+53.6×Mn %+99.7×Si %+651.9×P %+3339.4×N %+11/√D α 
 
   
       irrespective of the hot rolled strip being coiled at temperatures between 100-600° C. 
     
     
         15 . The process of  claim 14 , wherein the steel alloy is hot rolled in an austenitic finishing roll mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 950-400° C. 
     
     
         16 . The process of  claim 15 , further including a cooling interruption within a range of 3-20 seconds of the hot rolled strip between austenitic finishing rolling and coiling. 
     
     
         17 . The process of  claim 16 , wherein the hot rolled strip is cooled directly from the austenitic finishing roll mode to the at the cooling rate between 10-100 K/s. 
     
     
         18 . The process of  claim 14  wherein the steel alloy is hot rolled in a partially ferritic finishing roll mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 850-400° C. 
     
     
         19 . The process of  claim 18 , further including a cooling interruption within a range of 3-20 seconds of the hot rolled strip between partially ferritic finishing rolling and coiling. 
     
     
         20 . The process of  claim 19 , wherein the hot rolled strip is cooled directly from the partially ferritic finishing roll mode to the coiling temperature at the cooling rate between 10-100 K/s.