Hot-rolled steel sheet having excellent drawability and post-processing surface hardness

Abstract

The present invention intends a hot-rolled steel sheet with a sheet thickness in a range of 2 to 15 mm, having component composition including C: 0.3% or less by mass %, Si: 0.5% or less, Mn: in a range of 0.2 to 1%, P: 0.05% or less, S: 0.05% or less, Al: in a range of 0.01 to 0.1%, N: in a range of 0.008 to 0.025%, and the remainder being iron and unavoidable impurities. The hot-rolled steel sheet includes an amount of solute N: not less than 0.007%, and ferrite grains existing at a position of t/4 in depth (t: the sheet thickness) includes the ferrite grains, a sheet plane orientation thereof, within 10° from the (123) plane, having an area ratio: not less than 20%, the ferrite grains, a sheet plane orientation thereof, within 10° from the (111) plane, having an area ratio: not less than 5%, and the ferrite grains, a sheet plane orientation thereof, within 10° from the (001) plane, having an area ratio: 20% or less, the average grain size of the ferrite grains existing at the position of t/4 in depth being in a range of 3 to 35 μm.

1 . A steel sheet with a sheet thickness in a range of 2 to 15 mm comprising:
 C: >0 to 0.3 mass;   Si: >0 to 0.5 mass %;   Mn: in a range of 0.2 to 1 mass %;   P: >0 to 0.05 mass %;   S: >0 to 0.05 mass %;   Al: in a range of 0.01 to 0.1 mass %;   N: in a range of 0.008 to 0.025 mass %,   solute N: in an amount not less than 0.007 mass %; and   the remainder being iron and unavoidable impurities.   
     
     
         2 . The hot rolled steel sheet according to  claim 1 , further comprising at least one element selected from at least one of groups (a) to (e):
 (a) at least one element selected from the group consisting of >0 to 2 mass % of Cr and >0 to 2 mass % of Mo,   (b) at least one element selected from the group consisting of Ti: >0 to 0.2 mass %, Nb: >0 to 0.2 mass %, and V: >0 to 0.2 mass %,   (c) B: >0 to 0.005 mass %,   (d) at least one element selected from the group consisting of Cu: >0 to 5%, Ni: >0 to 5 mass %, and Co: >0 to 5 mass %, and   (e) at least one element selected from the group consisting of Ca: >0 to 0.05 mass %, REM: >0 to 0.05 mass %, Mg: >0 to 0.02 mass %, Li: >0 to 0.02 mass %, Pb: >0 to 0.5 mass %, and Bi: >0 to 0.5 mass %.   
     
     
         3 . The steel sheet according to  claim 1  that comprises (a) at least one element selected from the group consisting of >0 to 2 mass % of Cr and >0 to 2 mass %. 
     
     
         4 . The steel sheet according to  claim 1  that comprises (b) at least one element selected from the group consisting of Ti: >0 to 0.2 mass %, Nb: >0 to 0.2 mass %, and V: >0 to 0.2 mass %. 
     
     
         5 . The steel sheet according to  claim 1  that comprises (c) B: >0 to 0.005 mass %. 
     
     
         6 . The steel sheet according to  claim 1  that comprises (d) at least one element selected from the group consisting of Cu: >0 to 5 mass %, Ni: >0 to 5 mass %, and Co: >0 to 5 mass %. 
     
     
         7 . The steel sheet according to  claim 1  that comprises (e) at least one element selected from the group consisting of Ca: >0 to 0.05 mass %, REM: >0 to 0.05 mass %, Mg: >0 to 0.02 mass %, Li: >0 to 0.02 mass %, Pb: >0 to 0.5 mass %, and Bi: >0 to 0.5 mass %. 
     
     
         8 . The steel sheet according to  claim 1 ,
 wherein the average grain size of the ferrite grains existing at the position of t/4 in depth range from 3 to 35 μm, where t is the thickness of the hot-rolled sheet, and   wherein said ferrite grains existing at a position of t/4 in depth, comprise:   ferrite grains, a sheet plane orientation thereof, within 10° from the (123) plane, having an area ratio: not less than 20%,   ferrite grains, a sheet plane orientation thereof, within 10° from the (111) plane, having an area ratio: not less than 5%, and   ferrite grains, a sheet plane orientation thereof, within 10° from the (001) plane, having an area ratio: 20% or less.   
     
     
         9 . The steel sheet according to  claim 1  that is a hot-rolled steel sheet. 
     
     
         10 . A gear or other transmission part made from the steel sheet according to  claim 1 . 
     
     
         11 . A method for making the steel sheet comprising:
 producing a molten steel slab, and   rolling the slab into a sheet ranging from 2 to 15 mm in thickness;   wherein said steel slab comprises:   C: >0 to 0.3 mass %;   Si: >0 to 0.5 mass %;   Mn: in a range of 0.2 to 1 mass %;   P: >0 to 0.05 mass %;   S: >0 to 0.05 mass %;   Al: in a range of 0.01 to 0.1 mass %;   N: in a range of 0.008 to 0.025 mass %,   solute N: in an amount not less than 0.007 mass %; and   the remainder being iron and unavoidable impurities.