Determining the ferrite phase fraction after heating or cooling of a steel strip

A method for determining a ferritic phase fraction xα of an at least partly converted steel strip after heating of the steel strip comprising the following method steps in sequence:

• measuring a width w1 and a temperature T1 of the steel strip, wherein the steel strip is entirely in a ferritic state with xα1=1 during the measurement;
• heating the steel strip, wherein at least a partial phase conversion from a ferritic state α into an austenitic state γ takes place in the steel strip;
• measuring a width w and a temperature T of the at least partly converted steel strip;
• and
• determining the ferritic phase fraction Xα of the at least partly converted steel strip through
• x
• α
• =
• -
• w
• -
• w
• ⁢
• ⁢
• α
• α
• ⁡
• (
• T
• 1
• -
• T
• 0
• )
• +
• w
• 1
• +
• w
• 1
• ⁢
• α
• γ
• ⁡
• (
• T
• -
• T
• 0
• )
• w
• 1
• ⁡
• [
• -
• α
• α
• ⁡
• (
• T
• -
• T
• 0
• )
• +
• α
• γ
• ⁡
• (
• T
• -
• T
• 0
• )
• ]
• .
• wherein
• T0 is a reference temperature and
• aa and ay are linear coefficients of thermal expansion of ferrite and austenite, respectively.

A method for determining a ferritic phase fraction xα of an at least partly converted steel strip after cooling of the steel strip comprising the following method steps in sequence:

• measuring a width w1 and a temperature T1 of the steel strip, wherein the steel strip is entirely in an austenitic state γ with xα1=0 during the measurement;
• cooling the steel strip, wherein at least a partial phase conversion from an austenitic state γ into a ferritic state α takes place in the steel strip;
• measuring a width w and a temperature T of the at least partly converted steel strip;
• and
• determining the ferritic phase fraction xα of the at least partly converted steel strip through
• x
• α
• =
• -
• w
• -
• w
• ⁢
• ⁢
• α
• γ
• ⁡
• (
• T
• 1
• -
• T
• 0
• )
• +
• w
• 1
• +
• w
• 1
• ⁢
• α
• γ
• ⁡
• (
• T
• -
• T
• 0
• )
• w
• 1
• ⁡
• [
• -
• α
• α
• ⁡
• (
• T
• -
• T
• 0
• )
• +
• α
• γ
• ⁡
• (
• T
• -
• T
• 0
• )
• ]
• .
• wherein
• T0 is a reference temperature and
• aa and αy are linear coefficients of thermal expansion of ferrite and austenite, respectively.