The present invention relates to a hot rolled steel sheet.
10
BACKGROUND
[0002]
If making steel sheet high in strength, generally the workability falls, therefore trying to
achieve both strength and workability in steel sheet is generally difficult. In addition, for
15 example, if working steel sheet into a complicated shape part, etc., is demanded, if the
characteristics are not uniform in the sheet width direction of the steel sheet, sometimes the
portion of steel sheet which can be applied to the part will be limited. For this reason, from the
viewpoint of the yield as well, the characteristics are preferably uniform in the sheet width
direction of the steel sheet.
20 [0003]
In relation to this, for example, PTL 1 describes high yield ratio high strength hot rolled
steel sheet having a steel composition containing, by mass%, C: 0.05% or more and 0.2% or less,
Si: 0.01% or more and 0.6% or less, Mn: 0.5% or more and 2.5% or less, P: 0.001% or more and
0.1% or less, S: 0.0005% or more and 0.05% or less, Al: 0.01% or more and 0.2% or less, N:
25 0.0001% or more and 0.010% or less, Mo: 0.05% or more and 0.5% or less, Ti: 48N/14+0.01%
or more and 0.14% or less, and B: 0.0003% or more and 0.005% or less in a range satisfying the
formula 70300C (mass%)+33Mn (mass%)+22Cr (mass%)+11Mo (mass%)+11Si
(mass%)+17Ni (mass%)100 and having a balance of iron and unavoidable impurities, wherein
the yield strength is 960 MPa or more, the yield ratio is 0.83 or more, and the variation in yield
30 strength in the sheet width direction is within 50 MPa. Further, PTL 1 describes that due to the
above configuration, it is possible to obtain high yield ratio high strength steel sheet with little
variation in strength in the sheet width direction and excellent in toughness with a yield strength
of 960 MPa or more and a yield ratio of 0.83 or more.
[0004]
35 PTL 2 describes a method of production of Ti-containing high strength hot rolled steel sheet
with little variation in strength between steel sheets by loading a slab produced by continuous
2
casting and containing, by wt%, C: 0.05 to 0.12%, N: 0.001 to 0.005%, and Ti: 0.04 to 0.15%
into a heating furnace and heating it, holding it at a TiC solid solution temperature T (K) or more
under conditions of a holding time “t” (hours) satisfying the formula T(10+logt)15000, and
extracting it from the heating furnace and rolling it. Further, PTL 2 describes that it is possible to
5 suppress a variation in strength of steel sheet due to insufficient dissolution of Ti by
quantification of the heating conditions for sufficiently dissolving the added Ti, that there is no
longer almost any deviation from the strength specifications, and that there is no longer steel
sheet failing to meet the grade.
[0005]
10 PTL 3 describes high strength hot rolled steel sheet having steel constituents containing, by
mass%, C: 0.020 to 0.065%, Si: 0.1% or less, Mn: 0.40 to less than 0.80%, P: 0.030% or less, S:
0.005% or less, Ti: 0.08 to 0.20%, Al: 0.005 to 0.1%, and N: 0.005% or less, having a balance of
Fe and unavoidable impurities, and having Ti
defined by the formula Ti
Ti-(48/14)N
satisfying a predetermined formula, wherein the steel structure comprises, by area ratio, 95% or
15 more of a ferrite phase and a balance of one or more phases of a pearlite phase, bainite phase,
and martensite phase, an average ferrite grain size of the ferrite is 10 m or less, an average
particle size of Ti carbides precipitating in the steel is 10 nm or less, and Ti of 80% or more of
Ti
precipitates as Ti carbides. Further, PTL 3 describes that, due to the above configuration,
high strength hot rolled steel sheet high in strength, excellent in ductility and stretch
20 flangeability, and having excellent uniformity of quality with little variation in strength in the
steel sheet, more specifically high strength hot rolled steel sheet having a variation TS of
tensile strength (TS) of 15 MPa or less, is obtained.
[0006]
PTL 4 describes high workability high strength hot rolled steel sheet with little variation in
25 quality in a coil having a chemical composition substantially containing C: 0.05 to 0.18 mass%,
Si: 0.7 to 1.5 mass%, Mn: 0.6 to 1.8 mass%, P: 0.04 mass% or less, S: 0.005 mass% or less, Al:
0.01 to 0.10 mass%, N: 0.005 mass% or less, and Mo: 0.05 to 1.5 mass% and having a balance
of Fe. Further, PTL 4 describes that the above hot rolled steel sheet is uniform in quality across
the entire length and entire width of the coil and that variation in the coil quality is suitably kept
30 down.
[0007]
PTL 5 describes high strength hot rolled steel sheet with a tensile strength of 980 MPa or
more having a chemical composition satisfying formula 0.25