High strength steel plate and method for production thereof

Abstract

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

1 . A high strength steel plate having yield strength of 448 MPa or higher, containing 0.02 to 0.08% C, by mass, and having substantially a two phase microstructure of ferrite and bainite, the ferrite containing precipitates having particle size of 30 nm or smaller.  
     
     
         2 . The high strength steel plate as in  claim 1 , wherein the difference in hardness between bainite and ferrite is 70 or smaller Vickers hardness.  
     
     
         3 . The high strength steel plate as in  claim 1 , wherein the bainite has Vickers hardness of 320 or smaller.  
     
     
         4 . The high strength steel plate as in  claim 1 , wherein the bainite has area percentage of 10 to 80%.  
     
     
         5 . A high strength steel plate having yield strength of 448 MPa or higher, consisting essentially of 0.02 to 0.08% C, 0.01 to 0.5% Si, 0.5 to 1.8% Mn, 0.01% or less P, 0.002% or less S, 0.05 to 0.5% Mo, 0.005 to 0.04% Ti, and 0.07% or less Al, by mass, and balance of Fe, [C/(Mo+Ti)] as ratio of C content to the sum of Mo and Ti contents by atom percentage being 0.5 to 3, and having substantially a two phase microstructure of ferrite and bainite, the ferrite containing precipitates of complex carbides having particle size of 10 nm or smaller and containing Ti and Mo.  
     
     
         6 . The high strength steel plate as in  claim 5 , wherein the difference in hardness between bainite and ferrite is 70 or smaller Vickers hardness.  
     
     
         7 . The high strength steel plate as in  claim 5 , wherein the bainite has Vickers hardness of 320 or smaller.  
     
     
         8 . The high strength steel plate as in  claim 5 , wherein the bainite has area percentage of 10 to 80%.  
     
     
         9 . The high strength steel plate as in  claim 5 , wherein [C/(Mo+Ti)] as ratio of C content to the sum of Mo and Ti contents by atom percentage is 0.7 to 2.  
     
     
         10 . The high strength steel plate as in  claim 5 , wherein a part or whole of Mo is substituted by W, [Mo+W/2] being 0.05 to 0.5%, by mass, and [C/(Mo+W+Ti)] as ratio of C content to the sum of Mo, W, and Ti contents by atom percentage being 0.5 to 3, and the ferrite contains precipitates of complex carbides having particle size of 10 nm or smaller and containing Ti, Mo, and W, or Ti and W.  
     
     
         11 . The high strength steel plate as in  claim 5  further containing 0.005 to 0.05% Nb and/or 0.005 to 0.1% V, by mass, [C/(Mo+Ti+Nb+V)] as ratio of C content to the sum of Mo, Ti, Nb, and V contents by atom percentage being 0.5 to 3, and the ferrite containing precipitates of complex carbides having particle size of 10 nm or smaller and containing Ti and Mo, and Nb and/or V.  
     
     
         12 . The high strength steel plate as in  claim 11 , wherein Ti content is 0.005% or more and less than 0.02%.  
     
     
         13 . The high strength steel plate as in  claim 11 , wherein [C/(Mo+Ti+Nb+V)] as ratio of C content to the sum of Mo, Ti, Nb, and V contents by atom percentage is 0.7 to 2.  
     
     
         14 . The high strength steel plate as in  claim 11 , wherein a part or whole of Mo is substituted by W, [(Mo+W)/2] being 0.05 to 0.5%, by mass, and [C/(Mo+W+Ti+Nb+V)] as ratio of C content to the sum of Mo, W, Ti, Nb, and V contents by atom percentage being 0.5 to 3, and the ferrite contains precipitates of complex carbides having particle size of 10 nm or smaller and containing Ti, Mo, W, and Nb and/or V, or Ti, W, and Nb and/or V.  
     
     
         15 . A high strength steel plate having yield strength of 448 MPa or higher, consisting essentially of 0.02 to 0.08% C, 0.01 to 0.5% Si, 0.5 to 1.8% Mn, 0.01% or less P, 0.002% or less S, 0.07% or less Al, by mass, 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 balance of substantially Fe, [C/(Ti+Nb+V)] as ratio of C content to the sum of Ti, Nb, and V contents by atom percentage being 0.5 to 3, and having substantially a two phase microstructure of ferrite and bainite, the ferrite containing precipitates of complex carbides having particle size of 30 nm or smaller and containing at least two elements selected from the group consisting of Ti, Nb, and V.  
     
     
         16 . The high strength steel plate as in  claim 15 , wherein the difference in hardness between bainite and ferrite is 70 or smaller Vickers hardness.  
     
     
         17 . The high strength steel plate as in  claim 15 , wherein the bainite has Vickers hardness of 320 or smaller.  
     
     
         18 . The high strength steel plate as in  claim 15 , wherein the bainite has area percentage of 10 to 80%.  
     
     
         19 . The high strength steel plate as in  claim 15 , wherein [C/(Ti+Nb+V)] as ratio of C content to the sum of Ti, Nb, and V contents by atom percentage is 0.7 to 2.  
     
     
         20 . The high strength steel plate as in  claim 5  further containing at least one element selected from the group consisting of 0.5% or less Cu, 0.5% or less Ni, 0.5% or less Cr, and 0.0005 to 0.005% Ca, by mass.  
     
     
         21 . The high strength steel plate as in  claim 10  further containing at least one element selected from the group consisting of 0.5% or less Cu, 0.5% or less Ni, 0.5% or less Cr, and 0.0005 to 0.005% Ca, by mass.  
     
     
         22 . The high strength steel plate as in  claim 11  further containing at least one element selected from the group consisting of 0.5% or less Cu, 0.5% or less Ni, 0.5% or less Cr, and 0.0005 to 0.005% Ca, by mass.  
     
     
         23 . The high strength steel plate as in  claim 14  further containing at least one element selected from the group consisting of 0.5% or less Cu, 0.5% or less Ni, 0.5% or less Cr, and 0.0005 to 0.005% Ca, by mass.  
     
     
         24 . The high strength steel plate as in  claim 15  further containing at least one element selected from the group consisting of 0.5% or less Cu, 0.5% or less Ni, 0.5% or less Cr, and 0.0005 to 0.005% Ca, by mass.  
     
     
         25 . A method for manufacturing a high strength steel plate having yield strength of 448 MPa or higher, comprising the steps of: hot rolling a steel slab having the composition described in  claim 5  under the conditions of heating temperature of 1000 to 1300° C. and of finishing temperature of 750° C. or higher; applying accelerated cooling to the hot rolled steel plate to the cooling stop temperature of 300 to 600° C. at a cooling rate of 5° C./s or higher; and reheating the steel plate immediately after cooling thereof to temperature of 550 to 700° C. at a heating rate of 0.5° C./s or higher.  
     
     
         26 . The method for manufacturing a high strength steel plate as in  claim 25 , wherein the steel plate is heated during the step of reheating from the cooling stop temperature to 50° C. or higher temperature thereabove.  
     
     
         27 . A method for manufacturing a high strength steel plate having yield strength of 448 MPa or higher, comprising the steps of: hot rolling a steel slab having the composition described in  claim 5  under the conditions of heating temperature of 1050 to 1250° C. and of finishing temperature of 750° C. or higher; forming a two phase microstructure of untransformed austenite and bainite by applying accelerated cooling to the hot rolled steel plate to the cooling stop temperatures of 300 to 600° C. at a cooling rate of 5° C./s or higher; and forming a two phase structure of ferrite containing dispersed precipitates and tempered bainite by reheating the steel plate immediately after cooling thereof to temperature of 550 to 700° C., heating thereof by 50° C. or more, at a heating rate of 0.5° C./s or higher.  
     
     
         28 . A method for manufacturing a high strength steel plate having yield strength of 448 MPa or higher, comprising the steps of: hot rolling a steel slab having the composition described in  claim 10  under the condition of heating temperature of 1000 to 1300° C. and of finishing temperature of 750° C. or higher; applying accelerated cooling to the hot rolled steel plate to the cooling stop temperature of 300 to 600° C. at a cooling rate of 5° C./s or higher; and reheating the steel plate immediately after cooling thereof to temperature of 550 to 700° C. at a heating rate of 0.5° C./s or higher.  
     
     
         29 . A method for manufacturing a high strength steel plate having yield strength of 448 MPa or higher, comprising the steps of: hot rolling a steel slab having the composition described in  claim 11  under the condition of heating temperature of 1000 to 1300° C. and of finishing temperature of 750° C. or higher; applying accelerated cooling to the hot rolled steel plate to the cooling stop temperature of 300 to 600° C. at a cooling rate of 5° C./s or higher; and reheating the steel plate immediately after cooling thereof to temperature of 550 to 700° C. at a heating rate of 0.5° C./s or higher.  
     
     
         30 . A method for manufacturing a high strength steel plate having yield strength of 448 MPa or higher, comprising the steps of: hot rolling a steel slab having the composition described in  claim 14  under the conditions of heating temperature of 1000 to 1300° C. and of finishing temperature of 750° C. or higher; applying accelerated cooling to the hot rolled steel plate to the cooling stop temperature of 300 to 600° C. at a cooling rate of 5° C./s or higher; and reheating the steel plate immediately after cooling thereof to temperature of 550 to 700° C. at a heating rate of 0.5° C./s or higher.  
     
     
         31 . A method for manufacturing a high strength steel plate having yield strength of 448 MPa or higher, comprising the steps of: hot rolling a steel slab having the composition described in  claim 15  under the conditions of heating temperature of 1000 to 1300° C. and of finishing temperature of 750° C. or higher; applying accelerated cooling to the hot rolled steel plate to the cooling stop temperature of 300 to 600° C. at a cooling rate of 5° C./s or higher; and reheating the steel plate immediately after cooling thereof to temperature of 550 to 700° C. at a heating rate of 0.5° C./s or higher.  
     
     
         32 . The method for manufacturing a high strength steel plate having yield strength of 448 MPa or higher as in  claim 25 , wherein the step of reheating the steel plate immediately after cooling thereof to the temperature of 550 to 700° C. at a heating rate of 0.5° C./s or higher is carried out in an induction heating apparatus located in the same line installing a rolling mill and an cooling apparatus.