High strength steel plate

• US 7,935,197 B2
• Filed: 09/19/2006
• Issued: 05/03/2011
• Est. Priority Date: 02/07/2002
• Status: Expired due to Fees

• Current Assignee: JFE Steel Corporation (JFE Holdings Incorporated)
• Original Assignee: JFE Steel Corporation (JFE Holdings Incorporated)
• Inventors: Endo, Shigeru, Suwa, Minoru, Shinmiya, Toyohisa, Ishikawa, Nobuyuki
• Primary Examiner(s): Yee; Deborah
• Application Number:
  US11/523,387
• Publication Number:
  US 20070012386A1
• Time in Patent Office: 1,687 Days
• Field of Search: 148/320, 148/328, 148332-336, 420/126, 420/123, 420/109, 420/111
• US Class Current: 148/328
• CPC Class Codes: C21D 1/19 by interrupted quenching
• CPC Class Codes: C21D 2211/002 Bainite
• CPC Class Codes: C21D 2211/003 Cementite
• CPC Class Codes: C21D 2211/004 Dispersions; Precipitations
• CPC Class Codes: C21D 2211/005 Ferrite
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C21D 8/0263 following hot rolling
• CPC Class Codes: C22C 38/04 containing manganese
• CPC Class Codes: C22C 38/12 containing tungsten, tantalum, molybdenum, vanadium, or niobium
• CPC Class Codes: C22C 38/14 containing titanium or zirconium
• CPC Class Codes: Y10T 428/12951 Fe-base component
• CPC Class Codes: Y10T 428/12965 Both containing 0.01-1.7% carbon [i.e., steel]

Abstract

A high strength steel plate containing 0.02 to 0.08% C, by mass, and has substantially a two phase microstructure of ferrite and bainite. The ferrite contains precipitates having a particle size of 30 nm or smaller grain size. The steel plate has a yield strength of 448 MPa or higher. A method for manufacturing the high strength steel plate which comprises hot rolling, accelerated cooling and reheating. The accelerated cooling is conducted down to a temperature of 300 to 600° C. at a cooling rate of 5° C./s or higher. The reheating is conducted up to a temperature of 550 to 700° C. at a heating rate of 0.5° C./s or higher.

Claims

1. A high strength steel plate having a yield strength of 448 MPa or higher, consisting of 0.02 to 0.08 mass % C, 0.01 to 0.5 mass % Si, 0.5 to 1.8 mass % Mn, 0.01 mass % or less P, 0.002 mass % or less S, 0.05 to 0.5 mass % Mo, 0.005 to 0.04 mass % Ti, 0.5 mass % or less Cr and 0.07 mass % or less Al, optionally containing one or both of 0.005 to 0.05 mass % Nb and 0.005 to 0.1 mass % V, optionally containing at least one element selected from the group consisting of 0.5 mass % or less Cu, 0.5 mass % or less Ni and 0.0005 to 0.005 mass % Ca, and a balance of Fe, wherein [C/(Mo+Ti)] as a ratio of C content to the sum of Mo and Ti contents by atom percentage is 0.7 to 2, and having a metallic structure of (i) a ferrite phase, (ii) a bainite phase and (iii) in a total volume percentage of 5% or smaller, one or more phases selected from the group consisting of martensite, pearlite and cementite, the ferrite phase containing precipitates of complex carbides having a particle size of 5 nm or smaller and containing Ti and Mo, the bainite phase having an area percentage of 10 to 65% and the ferrite phase is a minimum of 30 volume %, the steel plate having a value of Ceq of 0.36 or less, wherein Ceq is defined as follows:

Ceq =C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5, wherein C is the mass % of carbon, Mn is the mass % of manganese, Cu is the mass % of copper, Ni is the mass % of nickel, Cr is the mass % of chromium, Mo is the mass % of molybdenum and V is the mass % of vanadium.

2. The high strength steel plate as in claim 1, wherein the difference in hardness between the bainite phase and the ferrite phase is 70 or smaller Vickers hardness.

3. The high strength steel plate as in claim 1, wherein the bainite phase has a Vickers hardness of 320 or smaller.

5. The high strength steel plate as in claim 1, which contains 0.005 to 0.05 mass % Nb and/or 0.005 to 0.1 mass % V;

wherein [C/(Mo+Ti+Nb+V)] as a ratio of C content to the sum of Mo, Ti, Nb, and V contents by atom percentage is 0.7 to 2; and

the ferrite phase containing precipitates of complex carbides having a particle size of 5 nm or smaller, and containing Ti and Mo, and Nb and/or V.

6. The high strength steel plate as in claim 5, wherein the Ti is contained in an amount of 0.005 mass % or more and less than 0.02 mass %.

11. The high strength steel plate as in claim 1 which contains at least one element selected from the group consisting of 0.5 mass % or less Cu, 0.5 mass % or less Ni, 0.5 mass % or less Cr, and 0.0005 to 0.005 mass % Ca.

13. The high strength steel plate as in claim 5 which contains at least one element selected from the group consisting of 0.5 mass % or less Cu, 0.5 mass % or less Ni, 0.5 mass % or less Cr, and 0.0005 to 0.005 mass % Ca.

4. A high strength steel plate having a yield strength of 448 MPa or higher consisting of 0.02 to 0.08 mass % C, 0.01 to 0.5 mass % Si, 0.5 to 1.8% mass Mn, 0. 01 mass % or less P, 0.002 mass % or less S, 0.05 to 0.5 mass % (Mo+W/2), 0.005 to 0.04 mass % Ti, 0.5 mass % or less Cr and 0.07 mass % or less Al, optionally containing one or both of 0.005 to 0.05 mass % Nb and 0.005 to 0.1 mass % V, optionally containing at least one element selected from the group consisting of 0.5 mass % or less Cu, 0.5 mass % or less Ni and 0.0005 to 0.005 mass % Ca, and a balance of Fe, wherein [C/(Mo+W+Ti)] as a ratio of C content to the sum of Mo, W and Ti contents by atom percentage is 0.7 to 2, and having a metallic structure of (i) a ferrite phase, (ii) a bainite phase and (iii) in a total volume percentage of 5% or smaller, one or more phases selected from the group consisting of martensite, pearlite and cementite, the ferrite phase containing precipitates of complex carbides having a particle size of 5 nm or smaller and containing Ti and Mo, the bainite phase having an area percentage of 10 to 65% and the ferrite phase is a minimum of 30 volume %, the steel plate having a value of Ceq of 0.36 or less, wherein Ceq is defined as follows:

Ceq =C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5, wherein C is the mass % of carbon, Mn is the mass % of manganese, Cu is the mass % of copper, Ni is the mass % of nickel, Cr is the mass % of chromium, Mo is the mass % of molybdenum and V is the mass % of vanadium.

7. The high strength steel plate as in claim 4, which contains 0.005 to 0.05 mass % Nb and/or 0.005 to 0.1 mass % V;

wherein [C/(Mo +W+Ti+Nb+V)] as a ratio of C content to the sum of Mo, W, Ti, Nb, and V contents by atom percentage is 0.7 to 2, and the ferrite phase contains precipitates of complex carbides having a particle size of 5 nm or smaller and containing Ti, Mo, W, and Nb and/or V, or Ti, W, and Nb and/or V.

12. The high strength steel plate as in claim 4 which contains at least one element selected from the group consisting of 0.5 mass % or less Cu, 0.5 mass % or less Ni, 0.5 mass % or less Cr, and 0.0005 to 0.005 mass % Ca.

14. The high strength steel plate as in claim 7 which contains at least one element from the group consisting of 0.5 mass % or less Cu, 0.5 mass % or less Ni, 0.5 mass % or less Cr, and 0.0005 to 0.005 mass % Ca.

8. A high strength steel plate having a yield strength of 448 MPa or higher, consisting of 0.02 to 0.08 mass % C, 0.01 to 0.5 mass % Si, 0.5 to 1.8 mass % Mn, 0.01 mass % or less P, 0.002 mass % or less S, 0.07 mass % or less Al and 0.5 mass % or less Cr, at least two elements selected from the group consisting of 0.005 to 0.04% Ti, 0.005 to 0.05% Nb, and 0.005 to 0.1% V, and a balance of substantially Fe, wherein [C/(Ti+Nb+V)] as a ratio of C content to the sum of Ti, Nb, and V contents by atom percentage being 0.7 to 2, and having a metallic structure of (i) a ferrite phase, (ii) a bainite phase and (iii) in a total volume percentage of 5% or smaller, one or more phases selected from the group consisting of martensite, pearlite and cementite, the ferrite phase containing precipitates of complex carbides having a particle size of 5 nm or smaller and containing at least two elements selected from the group consisting of Ti, Nb, and V, the bainite phase having an area percentage of 10 to 65% and the ferrite phase is a minimum of 30 volume %, the steel plate having a value of Ceq of 0.36 or less, wherein Ceq is defined as follows:

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5, wherein C is the mass % of carbon, Mn is the mass % of manganese, Cu is the mass % of copper, Ni is the mass % of nickel, Cr is the mass % of chromium, Mo is the mass % of molybdenum and V is the mass % of vanadium.

9. The high strength steel plate as in claim 8, wherein the difference in hardness between the bainite phase and the ferrite phase is 70 or smaller Vickers hardness.

10. The high strength steel plate as in claim 8, wherein the bainite phase has a Vickers hardness of 320 or smaller.

15. The high strength steel plate as in claim 8 which contains at least one element selected from the group consisting of 0.5 mass % or less Cu, 0.5 mass or less Ni, 0.5 mass % or less Cr, and 0.0005 to 0.005 mass % Ca.