Method for controlling cooling of steel sheet
A method for controlling the cooling of a steel sheet characterized by controlling the end-of-cooling temperature in a cooling process from the Ae 3 or above temperature of the steel sheet, during which; preliminarily obtaining enthalpies (Hγ and Hα) of an austenite phase and ferrite phase respectively at some temperature, obtaining a gynamic enthalpy (Hsys) defined by formula (1) with an untransformed fraction (Xγ) of austenite in accordance with a target temperature pattern, predicting the temperature by using a gradient of this dynamic enthalpy with respect to temperature as a dynamic specific heat and controlling the cooling of the steel sheet: H sys= H γ( X γ)+ H α(1− X γ). formula (1)
1. A method for controlling cooling of a steel sheet characterized in that the steel sheet contains, by mass %,
C: 0.30% or less,
Si: 2.0% or less,
Al: 2.0% or less
Mn: 0.1% to 5.0%,
P: 0.2% or less,
S: 0.0005% to 0.02%, and
N: 0.02% or less
and a balance of iron and unavoidable impurities, and has mass % of C, Mn, Si, and Al satisfying formula (2);
the method characterized by controlling the end-of-cooling temperature in a cooling process from the Ae 3 or above temperature of the steel sheet, during which obtaining in advance enthalpies (Hγ and Hα) of an austenite phase and ferrite phase respectively at some temperatures, obtaining a dynamic enthalpy (Hsys) defined by formula (1) with an untransformed fraction (Xγ) of austenite as a function of temperature in accordance with a target temperature pattern, predicting the temperature by using a gradient of this dynamic enthalpy with respect to temperature as a dynamic specific heat, and controlling the cooling of the steel sheet:
H sys= H γ( X γ)+ H α(1 −X γ) formula (1)
(C)+0.2×(Mn)−0.1×(Si+2×Al)≧0.15 formula (2)
wherein the target temperature pattern contains a region of ⅓ or more thereof in which a cooling rate is 10° C./s to 300° C./s.
2. A method for controlling cooling of a steel sheet according to claim 1 characterized by using the value of pure iron as the enthalpies (Hγ and Hα) of the austenite phase and ferrite phase of the steel.
3. A method for controlling cooling of a steel sheet according to claim 1 characterized by predicting the untransformed fraction (Xγ) by a transformation curve obtained in advance for ingredients of the steel and the target temperature pattern.
4. A method for controlling cooling of a steel sheet according to claim 1 characterized by predicting the untransformed fraction (Xγ) using a transformation prediction model which simulates a transformation process of a material.
5. A method for controlling cooling of a steel sheet characterized by controlling an intermediate holding temperature and a coiling temperature in a cooling process after hot-rolling during which performing control by a temperature predicted using the dynamic specific heat described in claim 1 .
6. A method for controlling cooling of a steel sheet characterized by controlling a end-of-cooling temperature by an annealing process after cold-rolling during performing control by a temperature predicted using the dynamic specific heat described in claim 1 .
7. A method for controlling cooling of a steel sheet according to any one of claims 1 and 3 to 6 characterized in that the steel further contains, by mass %, one or more of
Ti: 0.01% to 0.20% and
Nb: 0.01% to 0.10%.
8. A method for controlling cooling of a steel sheet according to claim 7 characterized in that the steel further contains, by mass %, one or more of
Ca, Mg, Zr, and a REM in an amount of 0.0005% to 0.02%.
9. A method for controlling cooling of a steel sheet according to claims 7 characterized in that the steel further contains, by mass %, one or more of
Cu: 0.04% to 1.4%,
Ni: 0.02% to 0.8%,
Mo: 0.02% to 0.5%,
V: 0.02% to 0.1%,
Cr: 0.20% to 1.0%, and
B: 0.0003% to 0.0010%.
10. A method for controlling cooling of a steel sheet according to claim 8 characterized in that the steel further contains, by mass %, one or more of
Cu: 0.04% to 1.4%,
Ni: 0.02% to 0.8%,
Mo: 0.02% to 0.5%,
V: 0.02% to 0.1%,
Cr: 0.20% to 1.0%, and
B: 0.0003% to 0.0010%.
11. A method for controlling cooling of a steel sheet according to claim 1 characterized in that the gradient of the dynamic enthalpy with respect to temperature is determined by differentiating the dynamic enthalpy by the temperature or by ΔHsys/ΔT, wherein ΔHsys is the change in dynamic enthalpy and ΔT is the change in temperature.
12. A method for controlling cooling of a steel sheet according to claim 11 characterized in that ΔT is 50° C. or less.
13. A method for controlling cooling of a steel sheet according to claim 1 characterized in that the untransformed fraction (Xγ) is calculated based on actual measured values on the cooling line using a transformation fraction measuring device.