High strength steel sheet and method for producing the same

Abstract

The present invention relates to A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix, wherein the proportion that the MD structure occupies in the whole structure is 90% or more, wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%, wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and wherein the average grain size of the secondary phase in the whole structure is 3 μm or less. The secondary phase is constituted of martensite, or martensite and retained austenite.

1 . A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix,
 wherein the proportion that the MD structure occupies in the whole structure is 90% or more,   wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%,   wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and   wherein the average grain size of the secondary phase in the whole structure is 3 μm or less.   
     
     
         2 . The high strength steel sheet as claimed in  claim 1 , wherein the secondary phase is constituted of martensite. 
     
     
         3 . The high strength steel sheet as claimed in  claim 1 , wherein the secondary phase is constituted of martensite and retained austenite. 
     
     
         4 . The high strength steel sheet as claimed in  claim 2 , wherein an average grain size of ferrite in the whole structure is 20 μm or less, and
 wherein 20 or more secondary phases on the average are present in an observation view of 50 μm×50 μm in the whole structure.   
     
     
         5 . The high strength steel sheet as claimed in  claim 3 , wherein an average grain size of ferrite in the whole structure is 20 μm or less, and
 wherein 20 or more secondary phases on the average are present in an observation view of 50 μm×50 μm in the whole structure.   
     
     
         6 . The high strength steel sheet as claimed in  claim 5 , wherein the proportion that the retained austenite present in the whole structure occupies in the whole structure is 2% or more. 
     
     
         7 . The high strength steel sheet as claimed in  claim 2 , comprising, in terms of % by mass,
 C: 0.02 to 0.3%;   Si: 0.01 to 3%;   Mn: 0.5 to 3%;   B: 0.0001 to 0.005%; and   Al: 0.01 to 1.5%,   with the remainder being Fe and inevitable impurities.   
     
     
         8 . The high strength steel sheet as claimed in  claim 3 , comprising, in terms of % by mass,
 C: 0.02 to 0.3%;   Si: 0.01 to 3%;   Mn: 0.5 to 3%;   B: 0.0001 to 0.005%; and   Al: 0.01 to 1.5%,   with the remainder being Fe and inevitable impurities.   
     
     
         9 . The high strength steel sheet as claimed in  claim 7 , further comprising Mo: 0.03 to 1%. 
     
     
         10 . The high strength steel sheet as claimed in  claim 8 , further comprising Mo: 0.03 to 1%. 
     
     
         11 . The high strength steel sheet as claimed in  claim 7 , further comprising at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%. 
     
     
         12 . The high strength steel sheet as claimed in  claim 8 , further comprising at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%. 
     
     
         13 . The high strength steel sheet as claimed in  claim 7 , further comprising at least one of
 Ni: 0.5% or less, and   Cu: 0.5% or less.   
     
     
         14 . The high strength steel sheet as claimed in  claim 8 , further comprising at least one of
 Ni: 0.5% or less, and   Cu: 0.5% or less.   
     
     
         15 . The high strength steel sheet as claimed in  claim 7 , further comprising Cr: 1.5% or less. 
     
     
         16 . The high strength steel sheet as claimed in  claim 8 , further comprising Cr: 1.5% or less. 
     
     
         17 . The high strength steel sheet as claimed in  claim 7 , further comprising at least one of
 Ca: 0.003% or less, and   REM: 0.003% or less.   
     
     
         18 . The high strength steel sheet as claimed in  claim 8 , further comprising at least one of
 Ca: 0.003% or less, and   REM: 0.003% or less.   
     
     
         19 . A method for producing a high strength steel sheet, which comprises:
 heating a steel sheet material comprising, in terms of % by mass,   C: 0.02 to 0.3%,   Si: 0.01 to 3%,   Mn: 0.5 to 3%,   B: 0.0001 to 0.005%, and   Al: 0.01 to 1.5%,   with the remainder being Fe and inevitable impurities; and   cooling the steel sheet material from a temperature of A3 point or higher to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20° C./sec.   
     
     
         20 . A method for producing a high strength steel sheet, which comprises:
 cooling, from a temperature of A3 point or higher, a steel sheet material comprising, in terms of % by mass,   C: 0.02 to 0.3%,   Si: 0.01 to 3%,   Mn: 0.5 to 3%,   B: 0.0001 to 0.005%, and   Al: 0.01 to 1.5%,   with the remainder being Fe and inevitable impurities;   subjecting said material to a processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000° C.; and then   cooling said material to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20° C./sec.