Steel plate for paint use and manufacturing method thereof

Abstract

A steel plate for paint use which contains C (0.12% or less), Si (1.0% or less), Mn (2.5% or less), P (0.05% or less), S (0.02% or less), and Cr (0.05% or less), Cu (0.05-3.0%), Ni (0.05-6.0%), Ti (0.025-0.15%), and Cu+Ni (0.50% or more), with P CM being 0.23% or less, in terms of mass %. Said steel plate may contain at least one additional component selected from B (0.0005-0.0030%), Al (0.05-0.50%), Ca (0.0001-0.05%), Ce (0.0001-0.05%), La (0.0001-0.05%), Nb (0.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%), and Mo (0.05-0.5%), in terms of mass %. This steel plate provides good weldability as well as good painting durability in a salt-polluted environment.

What is claimed i,s:  
     
         1 . A steel plate for paint use which contains C (0.12% or less, excluding O;), Si (1.0% or less, excluding 0%), Mn (2.5% or less, excluding O%), P (0.05% or less, excluding 0%), S (0.02% or less, excluding 0%), Cr (0.05% or less, excluding 0%), Cu (0.05-3.0%), Ni (0.05-6.0%), Ti (0.025-0.15%), Cu+Ni (0.50% or more), and P CM  (0.23% or less), in terms of mass %.  
     
     
         2 . A steel plate for paint use as defined in  claim 1 , which further contains at least one additional component selected from Al (0.05-0,50%), Ca (0.0001-0.05%), Ce (0.0001-0.05%), and La (0.0001-0.05%), in terms of mass %.  
     
     
         3 . A steel plate for paint use as defined in  claim 1 , which further contains B (0.0005-0.0030%), in terms of mass %.  
     
     
         4 . A steel plate for paint use as defined in  claim 2 , which further contains B (0.0005-0.0030%), in terms of mass %.  
     
     
         5 . A steel plate for paint use as defined in  claim 1 , which further contains at least one additional component selected from Nb (0.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%), and Mo (0.05-0.5%), in terms of mass %.  
     
     
         6 . A steel plate for paint use as defined in  claim 2 , which further contains at least one additional component selected from Nb (9.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%), and Mo (0.05-0.5%), in terms of mass %.  
     
     
         7 . A steel plate for paint use as defined in  claim 3 , which further contains at least one additional component selected from Nb (0.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%), and Mo (0.05-0.5%), in terms of mass %.  
     
     
         8 . A steel plate for paint use as defined in  claim 4 , which further contains at least one additional component selected from Nb (0.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%), and Mo (0.05-0.5%), in terms of mass %.  
     
     
         9 . A manufacturing method of a steel plate for paint use, said process comprising hot-rolling a steel plate which is defined in any of  claims 1  to  8  and contains Ti and C in such an amount that the Ti/C ratio exceeds 4, in such a way that the heating temperature is 850-1200° C. and the temperature at the end of rolling is 950° C. or lower, said hot rolling being followed by air cooling or water cooling (at a cooling rate 1° C./s or higher).  
     
     
         10 . A manufacturing method of a steel plate for paint use, said process comprising hot-rolling a steel plate which is defined in any of  claims 1  to  8  and contains Ti and C in such an amount that the Ti/C ratio exceeds 4, in such a way that the heating temperature is 850-1200° C. and the temperature at the end of rolling is 950° C. or lower, said hot rolling being followed by direct quenching from a temperature of Ar 3 ˜950° C. or reheating-quenching from a temperature of Ac 3 ˜950° C., and tempering.  
     
     
         11 . A manufacturing method of a steel plate for paint use, said process comprising hot-rolling a steel plate which is defined in any of  claims 1  to  8  and contains Ti and C in such an amount that the Ti/C ratio is 4 or less, in such a way that the heating temperature is 850˜(1200−50×Ti/C) ° C. and the temperature at the end of rolling is (Ar 3 +50×Ti/C+100×[Ni] 2 ) ° C. or lower, which is followed by air cooling or water cooling (at a cooling rate 1° C./s or higher). (where [Ni] represents the content of Ni.)  
     
     
         12 . A manufacturing method of a steel plate for paint use, said process comprising hot-rolling a steel plate which is defined in any of  claims 1  to  8  and contains Ti and C in such an amount that the Ti/C ratio is 4 or less, in such a way that the heating temperature is 850˜(1200−50×Ti/C) ° C. and the temperature at the end of rolling is (Ar 3 +50×Ti/C+100×[Ni] 2 ) ° C. or lower, which is followed by direct quenching from a temperature at the end of rolling or reheating-quenching from a temperature of (Ac 3 +50×Ti/C+100×[Ni] 2 ) ° C. or lower, and tempering. (where [Ni] represents the content of Ni.)