Patent ID: 11884990
Assignee: TATA STEEL NEDERLAND TECHNOLOGY B.V.
Field: Materials, metallurgy (Chemistry)
Classification: CPC C | IPC C

Claim 14:
15. A method of manufacturing a hot-rolled or cold-rolled and annealed steel strip or sheet of claim 1 having a complex phase steel microstructure, the method comprising the step of casting a slab, comprising:
0.16-0.21 wt. % C;
2.3-4.00 wt. % Mn;
5-50 ppm B;
5-100 ppm N;
0.019-1.10 wt. % Al tot;
0.05-1.10 wt. % Si;
0-0.04 wt % Ti, wherein if Al is below 0.030 wt. % Ti is 0.010-0.04 wt. %;
0-0.10 wt. % Cu;
0-0.10 wt. % Mo;
0-0.10 wt. % Ni;
0-0.20 wt. % V;
0-0.05 wt. % P;
0-0.05 wt. % S;
0-0.10 wt. % Sn;
0-0.025 wt. % Nb;
0-0.025 wt. % Ca;
remainder iron and inevitable impurities;
followed by the step of reheating the solidified slab to a temperature of between 1100 and 1300° C.,
hot rolling the steel slab and finishing said hot rolling at a finish hot rolling temperature at the Ar3-temperature or higher,
wherein Ar3-temperature is a temperature at which austenite begins to transform to ferrite during cooling,
cooling the hot-rolled strip at a cooling rate of between 5 and 220° C/s and coiling the hot rolled steel strip or sheet in the temperature range between 200 and 625° C.,
optionally followed by cold-rolling and annealing,
wherein the steel has a cracking parameter value, Pc, of at most 0.365, wherein, P
    c
  
  =
  
    C
    +
    
      Si
      
        3
        ⁢
        0
      
    
    +
    
      
        Mn
        +
        Cu
        +
        Cr
      
      
        2
        ⁢
        0
      
    
    +
    
      Mo
      
        1
        ⁢
        5
      
    
    +
    
      V
      
        1
        ⁢
        0
      
    
    +
    
      d
      
        6
        ⁢
        0
        ⁢
        0, and wherein the alloying contents are given in wt. % and the plate thickness d, is given in mm,
wherein the finished steel strip or sheet after hot-rolling has a yield strength of at least 500 MPa and a tensile strength of at least 850 MPa, or
wherein the finished steel strip or sheet after cold-rolling and annealing has a yield strength of at least 550 MPa and a tensile strength of at least 1000 MPa, or
wherein the finished steel strip or sheet after cold-rolling and annealing and temper-rolling has a yield strength of at least 600 MPa and a tensile strength of at least 1200 MPa,
wherein the steel microstructure consists of a mixture of martensite with retained austenite, and a remainder being carbide free bainite consisting of bainite with retained austenite without the presence of cementite,
wherein the martensite is at least one member selected from the group consisting of tempered martensite and untempered martensite,
wherein the steel microstructure is between 0 and 50 vol. % tempered martensite, and
wherein the steel microstructure is free from ferrite and free from pearlite.