High strength steel plate superior in stretch flange formability and fatigue characteristics

Abstract

The present invention provides high strength hot rolled steel plate superior in stretch flange formability and fatigue characteristics comprising steel plate containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acid soluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and a total of one or both of Ce or La: 0.0005 to 0.04%, having a balance of iron and unavoidable impurities and having a number ratio of stretched inclusions present in the steel plate having a circle equivalent diameter of 1 μm or more and a long axis/short axis of 5 or more of 20% or less.

1 . A high strength steel plate superior in stretch flange formability and fatigue characteristics characterized by comprising steel plate containing, by mass %,
 C: 0.03 to 0.20%,   Si: 0.08 to 1.5%,   Mn: 1.0 to 3.0%,   P: 0.05% or less,   S: 0.0005% or more,   N: 0.0005 to 0.01%,   acid soluble Al: 0.01% or less,   acid soluble Ti: less than 0.008%, and   a total of one or both of Ce or La: 0.0005 to 0.04%,   having a balance of iron and unavoidable impurities, and   having a number ratio of stretched inclusions present in the steel plate having a circle equivalent diameter of 1 μm or more and a long axis/short axis of 5 or more of 20% or less.   
   
   
       2 . A high strength steel plate superior in stretch flange formability and fatigue characteristics characterized by comprising steel plate containing, by mass %,
 C: 0.03 to 0.20%,   Si: 0.08 to 1.5%,   Mn: 1.0 to 3.0%,   P: 0.05% or less,   S: 0.0005% or more,   N: 0.0005 to 0.01%,   acid soluble Al: 0.01% or less,   acid soluble Ti: less than 0.008%, and   a total of one or both of Ce or La: 0.0005 to 0.04%,   having a balance of iron and unavoidable impurities,   and having inclusions in the steel plate comprised of an oxide or oxysulfide of one or both of Ce or La on which MnS is precipitated in a number ratio of 10% or more.   
   
   
       3 . A high strength steel plate superior in stretch flange formability and fatigue characteristics characterized by comprising steel plate containing, by mass %,
 C: 0.03 to 0.20%,   Si: 0.08 to 1.5%,   Mn: 1.0 to 3.0%,   P: 0.05% or less,   S: 0.0005% or more,   N: 0.0005 to 0.01%,   acid soluble Al: 0.01% or less,   acid soluble Ti: less than 0.008%, and   a total of one or both of Ce or La: 0.0005 to 0.04%,   having a balance of iron and unavoidable impurities, and   having a volume number ratio of stretched inclusions present in the steel plate having a circle equivalent diameter of 1 μm or more and a long axis/short axis of 5 or more of 1.0×10 4 /mm 3  or less.   
   
   
       4 . A high strength steel plate superior in stretch flange formability and fatigue characteristics characterized by comprising steel plate containing, by mass %,
 C: 0.03 to 0.20%,   Si: 0.08 to 1.5%,   Mn: 1.0 to 3.0%,   P: 0.05% or less,   S: 0.0005% or more,   N: 0.0005 to 0.01%,   acid soluble Al: 0.01% or less,   acid soluble Ti: less than 0.008%, and   a total of one or both of Ce or La: 0.0005 to 0.04%,   having a balance of iron and unavoidable impurities, and   having a volume number density of inclusions in the steel plate comprised of an oxide or oxysulfide of one or both of Ce or La on which MnS is precipitated in a volume number density of 1.0×10 3 /mm 3  or more.   
   
   
       5 . A high strength steel plate superior in stretch flange formability and fatigue characteristics characterized by comprising steel plate containing, by mass %,
 C: 0.03 to 0.20%,   Si: 0.08 to 1.5%,   Mn: 1.0 to 3.0%,   P: 0.05% or less,   S: 0.0005% or more,   N: 0.0005 to 0.01%,   acid soluble Al: 0.01% or less,   acid soluble Ti: less than 0.008%, and   a total of one or both of Ce or La: 0.0005 to 0.04%,   having a balance of iron and unavoidable impurities, and   having an average circle equivalent diameter of stretched inclusions present in the steel plate having a circle equivalent diameter of 1 μm or more and a long axis/short axis of 5 or more of 10 μm or less.   
   
   
       6 . A high strength steel plate superior in stretch flange formability and fatigue characteristics characterized by comprising steel plate containing, by mass %,
 C: 0.03 to 0.20%,   Si: 0.08 to 1.5%,   Mn: 1.0 to 3.0%,   P: 0.05% or less,   S: 0.0005% or more,   N: 0.0005 to 0.01%,   acid soluble Al: 0.01% or less,   acid soluble Ti: less than 0.008%, and   a total of one or both of Ce or La: 0.0005 to 0.04%,   having a balance of iron and unavoidable impurities,   having inclusions present in the steel plate comprising an oxide or oxysulfide of one or both of Ce or La on which MnS is precipitated, and having the inclusions include, in average composition, a total of one or both of Ce or La in 0.5 to 50 mass %.   
   
   
       7 . A high strength steel plate superior in stretch flange formability and fatigue characteristics characterized by comprising steel plate containing, by mass %,
 C: 0.03 to 0.20%,   Si: 0.08 to 1.5%,   Mn: 1.0 to 3.0%,   P: 0.05% or less,   S: 0.0005% or more,   N: 0.0005 to 0.01%,   acid soluble Al: 0.01% or less,   acid soluble Ti: less than 0.008%, and   a total of one or both of Ce or La: 0.0005 to 0.04%,   having a balance of iron and unavoidable impurities, and   having a (Ce+La)/S ratio of 0.1 to 70.   
   
   
       8 . A high strength steel plate superior in stretch flange formability and fatigue characteristics as set forth in any one of  claims 1  to  7  characterized by comprising steel plate containing, by mass %, one or more of any of
 Nb: 0.01 to 0.10%,   V: 0.01 to 0.05%,   Cr: 0.01 to 0.6%,   Mo: 0.01 to 0.4%, and   B: 0.0003 to 0.03% and   having a balance of iron and unavoidable impurities.