Method for manufacturing high strength steel sheet

Abstract

A method for manufacturing a high strength steel sheet includes heating a steel sheet containing at least 0.10 mass % of carbon to either a temperature in an austenite single phase region or a temperature in an (austenite+ferrite) two-phase region; cooling the steel sheet to a cooling stop temperature as a target temperature set within a cooling temperature region ranging from Ms to (Ms−150° C.) to allow a portion of non-transformed austenite to proceed to martensitic transformation; retaining a coldest part in a sheet widthwise direction of the steel sheet at a temperature in a temperature range from the cooling stop temperature as the target temperature to (the cooling stop temperature+15° C.) for 15 seconds to 100 seconds; and heating the sheet to a temperature to temper said martensite, wherein “Ms” represents martensitic transformation start temperature and said cooling temperature region is exclusive of Ms and inclusive of (Ms−150° C.).

1 . A method for manufacturing a high strength steel sheet comprising:
 heating a steel sheet containing at least 0.10 mass % of carbon to either a temperature in an austenite single phase region or a temperature in an (austenite+ferrite) two-phase region;   cooling the steel sheet to a cooling stop temperature as a target temperature set within a cooling temperature region ranging from Ms to (Ms−150° C.) to allow a portion of non-transformed austenite to proceed to martensitic transformation;   retaining a coldest part in a sheet widthwise direction of the steel sheet at a temperature in a temperature range from the cooling stop temperature as the target temperature to (the cooling stop temperature+15° C.) for 15 seconds to 100 seconds; and   heating the sheet to a temperature to temper said martensite,   wherein “Ms” represents martensitic transformation start temperature and said cooling temperature region is exclusive of Ms and inclusive of (Ms−150° C.).   
     
     
         2 . The method of  claim 1 , further comprising subjecting the steel sheet to a hot dip galvanizing process or a galvannealing process either: between completion of the heating process to a temperature in either the austenite single phase region or the (austenite+ferrite) two-phase region and completion of the cooling process; or during the tempering process; or during a process after the tempering process. 
     
     
         3 . The method of  claim 1 , wherein the steel sheet has a composition including by mass %,
 C: 0.10% to 0.73%,   Si: 3.0% or less,   Mn 0.5% to 3.0%,   P: 0.1% or less,   S: 0.07% or less,   Al: 3.0% or less,   N: 0.010% or less, and   remainder as Fe and incidental impurities.   
     
     
         4 . The method of  claim 3 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Cr: 0.05% to 5.0%,   V: 0.005% to 1.0% and   Mo: 0.005% to 0.5%.   
     
     
         5 . The method of  claim 3 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ti: 0.01% to 0.1% and   Nb: 0.01% to 0.1%.   
     
     
         6 . The method of  claim 3 , wherein the composition of the steel sheet further comprises, by mass %, B: 0.0003% to 0.0050%. 
     
     
         7 . The method of  claim 3 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ni: 0.05% to 2.0% and   Cu: 0.05% to 2.0%.   
     
     
         8 . The method of  claim 3 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ca: 0.001% to 0.005% and   REM: 0.001% to 0.005%.   
     
     
         9 . The method of  claim 2 , wherein the steel sheet has a composition including by mass %,
 C: 0.10% to 0.73%,   Si: 3.0% or less,   Mn 0.5% to 3.0%,   P: 0.1% or less,   S: 0.07% or less,   Al: 3.0% or less,   N: 0.010% or less, and   remainder as Fe and incidental impurities.   
     
     
         10 . The method of  claim 4 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ti: 0.01% to 0.1% and   Nb: 0.01% to 0.1%.   
     
     
         11 . The method of  claim 4 , wherein the composition of the steel sheet further comprises, by mass %, B: 0.0003% to 0.0050%. 
     
     
         12 . The method of  claim 5 , wherein the composition of the steel sheet further comprises, by mass %, B: 0.0003% to 0.0050%. 
     
     
         13 . The method of  claim 4 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ni: 0.05% to 2.0% and   Cu: 0.05% to 2.0%.   
     
     
         14 . The method of  claim 5 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ni: 0.05% to 2.0% and   Cu: 0.05% to 2.0%.   
     
     
         15 . The method of  claim 6 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ni: 0.05% to 2.0% and   Cu: 0.05% to 2.0%.   
     
     
         16 . The method of  claim 4 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ca: 0.001% to 0.005% and   REM: 0.001% to 0.005%.   
     
     
         17 . The method of  claim 5 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ca: 0.001% to 0.005% and   REM: 0.001% to 0.005%.   
     
     
         18 . The method of  claim 6 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ca: 0.001% to 0.005% and   REM: 0.001% to 0.005%.   
     
     
         19 . The method of  claim 7 , wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of
 Ca: 0.001% to 0.005% and   REM: 0.001% to 0.005%.