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

Claim 0:
1. A hot-rolled, cold-rolled and annealed, or cold-rolled, annealed and temper rolled steel strip or sheet having a complex phase steel microstructure 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 which is the sum of aluminum present as alumina and any other aluminum;
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,
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 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 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 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.