Precipitation-hardening hot rolled steel sheet having excellent material uniformity and hole expandability, and manufacturing method therefor

Abstract

Provided is a precipitation-hardening hot rolled steel sheet, having excellent material uniformity and hole expandability, comprising, by weight, 0.02% to 0.05% of C, 0.01% to 0.3% of Si, 1.0% to 1.6% of Mn, 0.04% to 0.1% of Ti, 0.01% to 0.05% of Nb, 0.008% or less of N, and a remainder of Fe and inevitable impurities, satisfying Relationship 1, wherein a microstructure of the precipitation-hardening hot rolled steel sheet comprises 95 area % or more of ferrite and (Ti, Nb)C complex precipitates, the number of (Ti, Nb)C complex precipitates having a diameter of 10 nm or less is five or more times the number of (Ti, Nb)C complex precipitates having a diameter greater than 10 nm. Relationship 1: 0.35≤(Ti+Nb+V+Mo)/(C+N) 0.70.

1 . A precipitation-hardening hot rolled steel sheet, having excellent material uniformity and hole expandability, comprising, by weight, 0.02% to 0.05% of C, 0.01% to 0.3% of Si, 1.0% to 1.6% of Mn, 0.04% to 0.1% of Ti, 0.01% to 0.05% of Nb, 0.008% or less of N, and a remainder of Fe and inevitable impurities, satisfying Relationship 1,
 wherein a microstructure of the precipitation-hardening hot rolled steel sheet comprises 95 area % or more of ferrite and (Ti, Nb)C complex precipitates,   the number of (Ti, Nb)C complex precipitates having a diameter of 10 nm or less is five or more times the number of (Ti, Nb)C complex precipitates having a diameter greater than 10 nm,
   0.35≤(Ti+Nb+V+Mo)/(C+N)≤0.70  Relationship 1:
 
   (In Relationship 1, each element symbol refers to at. % of each element, and an element not included in the steel sheet is calculated as zero).   
     
     
         2 . The precipitation-hardening hot rolled steel sheet according to  claim 1 , wherein the hot rolled steel sheet satisfies Relationship 2:
   0.04≤(Ti+Nb+V+Mo)/(C+Mn+Si+Cr)  Relationship 2:
   (In Relationship 2, each element symbol refers to at. % of each element, and an element not included in the steel sheet is calculated as zero).   
     
     
         3 . The precipitation-hardening hot rolled steel sheet according to  claim 1 , wherein a distance between the (Ti, Nb)C complex precipitates is 30 nm or less. 
     
     
         4 . The precipitation-hardening hot rolled steel sheet according to  claim 1 , wherein the (Ti, Nb)C complex precipitate is present in a frequency of 15,000/μm 2  or more. 
     
     
         5 . The precipitation-hardening hot rolled steel sheet according to  claim 1 , wherein the hot rolled steel sheet has a tensile strength of 590 MPa or more and material deviation in a width/length direction within ±5% with respect to an average value. 
     
     
         6 . A production method for a precipitation-hardening hot rolled steel sheet, having excellent material uniformity and hole expandability, comprising:
 continuous casting ingot steel comprising, by weight, 0.02% to 0.05% of C, 0.01% to 0.3% of Si, 1.0% to 1.6% of Mn, 0.04% to 0.1% of Ti, 0.01% to 0.05% of Nb, 0.008% or less of N, and a remainder of Fe and inevitable impurities, satisfying Relationship 1, to produce a thin slab;   rough rolling the thin slab to obtain a bar;   heating or concurrently heating the bar;   finish rolling the heated bar to obtain a hot rolled steel sheet;   cooling and air cooling the hot rolled steel sheet at a cooling rate of 30° C./s or higher to a temperature within a range of 600° C. to 700° C. in a run-out table; and   coiling the cooled hot rolled steel sheet at a temperature within a range of 450° C. to 600° C.,
   0.35≤(Ti+Nb+V+Mo)/(C+N)≤0.70  Relationship 1:
 
   (In Relationship 1, each element symbol refers to at. % of each element, and an element not included in the steel sheet is calculated as zero).   
     
     
         7 . The production method according to  claim 6 , wherein the hot rolled steel sheet satisfies Relationship 2:
   0.04≤(Ti+Nb+V+Mo)/(C+Mn+Si+Cr),  Relationship 2:
   (In Relationship 2, each element symbol refers to at. % of each element, and an element not included in the steel sheet is calculated as zero).   
     
     
         8 . The production method according to  claim 6 , wherein a thickness of the thin slab is 30 mm to 150 mm. 
     
     
         9 . The production method according to  claim 6 , wherein a mass-flow velocity of the hot rolled steel sheet in the run-out table is 100 mpm to 200 mpm, and a difference in velocity is 10% or less. 
     
     
         10 . The production method according to  claim 6 , further comprising coiling the heated bar between the heating operation and the finish rolling operation. 
     
     
         11 . The production method according to  claim 6 , wherein the rough rolling operation is performed in a temperature within a range of 850° C. to 1,150° C. 
     
     
         12 . The production method according to  claim 6 , wherein heating or concurrently heating the bar in a temperature within a range of 850° C. to 1,150° C. in the heating operation. 
     
     
         13 . The production method according to  claim 6 , wherein the finish rolling operation is performed in a temperature within a range of 770° C. to 1,000° C.