Hot-rolled steel sheet and manufacturing method therefor

A hot-rolled steel sheet, comprising:

• by weight %, C:
• 0.020-0.080%, Si:
• 0.01-0.50%, Mn:
• 0.8-1.8%, Al:
• 0.010-0.100%, P:
• 0.001-0.020%, S:
• 0.001-0.010%, N:
• 0.001-0.010%, Ti:
• 0.010-0.120%, Nb:
• 0.010-0.050%, and a balance of Fe and inevitable impurities, wherein an X value defined by relational expression 1 as below satisfies 3.50-6.00, and
• having a microstructure comprising a sum of ferrite and bainite phases:
• 90 area % or more, and a sum of remainder of martensite and MA phases:
• less than 10 area %,
• wherein the steel sheet has tensile strength of 590 MPa or more, hole expandability (HER0) of 40% or more, and a bake hardening amount (BH2:
• bake hardening amount before heat treatment) of 30 MPa or more,
• the steel sheet maintaining a bake hardening amount (BHh) after heat treatment at 300-600° C. of 30 MPa or more, and
• when ΔTS is defined as in relational expression 2 below, an absolute value of ΔTS×BHh−1 is 0.70 or less:
• X
• =
• A
• *
• B
• [
• Relational
• ⁢
• expression
• ⁢
• 1
• ]
• A
• =
• 1.3
• *
• [
• Mn
• ]
• +
• 200
• *
• [
• C
• ]
• B
• =
• (
• Nb
• /
• 93
• +
• Ti
• *
• /
• 48
• )
• /
• (
• C
• /
• 12
• +
• N
• /
• 14
• )
• Ti
• *=
• Ti
• -
• 3.42
• N
• -
• 1.5
• S
• where Mn, C, Nb, Ti, N and S represent weight % thereof
• Δ
• ⁢
• TS
• =
• TS
• h
• -
• TS
• 0
• [
• Relational
• ⁢
• expression
• ⁢
• 2
• ]
• TSh:
• tensile strength after heat treatment, TS0:
• tensile strength before heat treatment
• BHh:
• bake hardening amount after heat treatment