KR100643358B1 - Method for predicting phase-transformation of hot coil - Google Patents

Abstract

The present invention relates to a method for calculating the phase fraction of a hot rolled steel sheet, and more particularly, to a method for analyzing phase transformation according to cooling patterns of TRansformation Induced Plasticity (TRIP) steel and finally analyzing remaining phases. will be.

The present invention for providing the above object, the first step of inputting the composition and initial temperature of the steel sheet; A second step of determining the type and initial phase fraction of the phases generated according to the initial composition and the initial temperature; The temperature after the minute time Δt _i is determined according to an isothermal or cooling pattern over time, a thermodynamic model is used to determine the equilibrium phase fraction at the determined temperature, and the equilibrium phase fraction is used to determine the initial phase. fraction (X _i) by calculating the changed phase fraction (X _{i + 1)} from, the calculated phase fraction (X _{i + 1)} during the fraction calculated at the next step minute time (△ t _{i + 1)} of the initial phase A third step consisting of repeating the process of using the fraction until the stable phase is changed; And a fourth step of returning to the second step when the stable phase is changed, and calculates the actual phase percentage using the phase percentage calculated in the third step.

Abram, Percentage, Metamorphic Organic Plastics, Addition Law

Description

METHODS FOR PREDICTING PHASE-TRANSFORMATION OF HOT COIL}

1 is a schematic view showing an example of a cooling pattern of a hot rolled steel sheet,

2 is a process flow chart showing the sequence calculation order of the present invention,

3 is a graph showing a change in phase ratio after the elapse of minute time;

4 is a conceptual diagram showing the difference between the calculation method of the percentage of phase during isothermal transformation and cooling transformation,

5 is a conceptual diagram illustrating a method of applying a phase fraction calculation method during isothermal transformation during cooling; and

6 is a conceptual diagram showing a cooling pattern of the steel sheet used in the embodiment of the present invention.

The present invention relates to a method for calculating the phase fraction of a hot rolled steel sheet, and more particularly, to a method for analyzing phase transformation according to cooling patterns of TRansformation Induced Plasticity (TRIP) steel and finally analyzing remaining phases. will be.

Recently, the problem of environmental pollution to the exhaust gas of automobiles has emerged. Reducing fuel consumption (improving fuel economy) and reducing the weight of the vehicle body for this environmental problem have emerged as an important solution in the field of materials. In addition, the processing amount of the vehicle body is intensified with time, and the demand for ductility is also increasing.

In order to satisfy all of the above demands, it is very necessary to develop high strength and high ductility steels, and many inventors continue to develop technologies to satisfy these demands.

As a result, in the 1980s, the dual phase steel (ferrous steel, Vol. 68, 1982, No. 9, p. 1306), a composite of ferrite and martensite, 3, a composite of ferrite, bainite, and martensite, Many such as tri-phase steel (steel, Vol. 68, 1982, No. 9, p. 1185), ferritic-bainite composite steel (CAMP-ISIJ, vol. 1, 1988, p. 881). Steels have been developed. However, the steel is mainly about 60kg / mm2 tensile strength and 30% elongation, whereas the market demand for high strength and high ductility steel is intensifying day by day, the development of a new material with higher strength and ductility than the above steel This was an urgent situation.

The material for satisfying this requirement is transformed organic plastic steel, which has a high tensile strength of about 80 kg / mm ² , and is excellent in ductility. The property expression of the metamorphic organic plastic steel is due to the ductility improvement due to the phase transformation of the retained austenite and the strength improvement due to the resulting martensite.

A number of patents have been filed regarding the transformation organic plastic steel having high strength and high ductility as described above and a method for manufacturing the same. For example, Japanese Patent Laid-Open No. 6-145892 discloses 0.06 to 0.22% C, 0.05 to 1.0% Si, 0.5 to Steels containing high strength and high ductility, containing 3% to 20% of retained austenite, have been proposed by adding 0.03% to 0.30% Mo to steel containing 2.0% Mn and 0.25 ~ 1.5% Al as needed. In addition, Japanese Patent Laid-Open No. 6-145788 discloses a method of manufacturing the same steel as the steel described in Japanese Patent Laid-Open No. 6-145892.

As a similar patent, Japanese Patent Laid-Open No. 5-179396 basically contains 0.18% or less C, 0.5-2.5% Mn, 0.05% or less P, 0.02% or less S, 0.01-0.1% Al, and 0.02-0.5% Ti and 0.03 ~ After 1.0% Nb alone or together, the steel is subjected to filamentous rolling at a temperature of 820 ° C or higher, and then maintained at a temperature range of 820 ~ 720 ° C for 10 seconds or more, and then cooled at a cooling rate of 10 ° C / sec or more and 500 ° C or lower. A technique for winding at temperature has been proposed. In this case, high strength and resistance yield ratio may be exhibited, which may have a great advantage as compared with the high yield ratio of the conventional precipitation high strength steel.

As another example, Japanese Patent Application Laid-Open No. 5-112846, which discloses precipitation-reinforcing elements such as Nb, Ti, and V in steels containing 0.05 to 0.25% C, 0.05 to 1.0% Si, and 0.8 to 2.5% Al. After the end of rolling in the temperature range of 780 ~ 840 ℃ and cooled to a temperature of 600 ~ 700 ℃ at a cooling rate of 10 ℃ / sec or more, after 2-10 seconds of air cooling and 220 ℃ / sec cooling A method for producing a steel containing 5% or more of retained austenite by accelerated cooling to 300 to 450 ° C at a rate is disclosed.

As described above, a variety of techniques have been provided regarding steel materials which are intended to secure ductility and strength due to phase transformation of the residual austenite by containing residual austenite at a predetermined level or more.

However, the above techniques are limited to a part of the data through numerous trials and errors within a predetermined steel composition range, and there are limitations in providing an overall manufacturing technique of deformed organic plastic steel.

That is, as described above, the deformed organic plastic steel greatly varies in ductility by the fraction of phase such as residual austenite present in the steel before processing, and the strength varies greatly by the phase fraction in the structure after processing. It is very important to predict the fraction of phase present in the interior, which has not been provided in the past.

Accordingly, an object of the present invention is to provide a method for predicting the phase fraction present in hot-rolled strained organic plastic steel produced after hot rolling in order to overcome the limitations of the prior art.

The present invention for providing the above object, the first step of inputting the composition and initial temperature of the steel sheet; A second step of determining the type and initial phase fraction of the phases generated according to the initial composition and the initial temperature; The temperature after the minute time Δt _i is determined according to an isothermal or cooling pattern over time, a thermodynamic model is used to determine the equilibrium phase fraction at the determined temperature, and the equilibrium phase fraction is used to determine the initial phase. fraction (X _i) by calculating the changed phase fraction (X _{i + 1)} from, the calculated phase fraction (X _{i + 1)} during the fraction calculated at the next step minute time (△ t _{i + 1)} of the initial phase A third step consisting of repeating the process of using the fraction until the stable phase is changed; And a fourth step of returning to the second step when the stable phase is changed.

Using the phase ratio calculated in the third step, it is characterized by calculating the actual phase percentage by the following relational formula (1).

[Relationship 1]

X ^r _{i + 1} = X _{i + 1} × X ⁰ _γ

Where X ^r _{i + 1} is the actual fraction of the present stable phase in the steel sheet, and X ⁰ _γ is the residual austenite fraction when the current stable phase is first formed.

In this case, in the third step, the method for calculating the changed phase fraction (X _{i + 1} ) from the initial phase fraction (X _i ) is obtained by substituting the time t obtained from the following relation (2) into the following relation (3). It is desirable to obtain the fraction of the phase resulting from residual austenite decomposed at

[Relationship 2]

t = [-1 / k ln (1-X _i / X ^e )] ^{1 / n}

[Relationship 3]

X _{i + 1} = X ^e [1-exp (-k (t + Δt _i ) ⁿ )]

Where k is the proportional constant obtained from the experimental value, X _i is the initial phase fraction of the current microdivision of the phase concerned, X ^e is the equilibrium phase fraction of the corresponding phase when isothermal treatment is carried out for infinite time at the temperature of the current microdivision, X _{i +1} is the final phase fraction after the current _{microdivision} of the phase has passed, Δt _i is the time interval, and n is the constant that determines the transformation rate. Almost does not change.

In the third step, it is more preferable to calculate the carbon concentration in the retained austenite required by the following equation (4) when calculating the equilibrium percentage of the next step (i + 1) using the thermodynamic model.

[Relationship 4]

C ^γ _{i + 1} = [C _0- (X ^f _{i + 1} + X ^b _{i + 1} ) C ^f _{i + 1} ] / (X ^γ _{i + 1} + X ^p _{i + 1} + X ^m _{i + 1} )

Here, the subscripts f, b, γ, p, m represent ferrite, bainite, austenite, pearlite and martensite, respectively, C represents the carbon concentration currently included in the phase, and X represents the current percentage of each phase. Means. However, C ₀ means the average carbon concentration of the steel sheet.

In addition, the current phase fraction of each phase is preferably the same as the phase fraction of the entire microstep, except for the phase fraction X _i of the phase (stable phase) generated in the current processing step and the residual austenite.

At this time, it is preferable that the phase fraction (X ^γ _{i + 1} ) of the retained austenite is calculated by the following relational formula (6).

[Relationship 6]

X ^γ _{i + 1} = X ^γ ₀ [1-X _{i + 1} ] = X ^γ ₀ -X ^r _{i + 1}

Where X ^γ ₀ represents the residual austenite fraction when the stable phase is first produced in the current treatment step.

In addition, it is preferable to use a thermodynamic model as a second step of determining the type and initial phase fraction of the phases generated according to the initial composition and the initial temperature.

In addition, it is preferable that the isothermal or cooling pattern according to the time used in calculating the phase percentage is differently calculated and applied to each part of the steel sheet.

In addition, the cooling pattern calculated differently for each part of the steel sheet is preferably calculated through heat transfer analysis.

For a more precise calculation, when the stable phase produced in the present processing step is martensite, the fraction of martensite (X _{i + 1} ) is preferably calculated by the following equation (7).

[Relationship 7]

X _{i + 1} = X ^γ _i [1-exp {-a (M _s -T)}]

Where M _s represents the martensite formation temperature.

And, M _s is preferably calculated by the following relational formula (8).

[Relationship 8]

M _s = 550-(360 × C ^γ _i )-(40 × Mn)

Here, Mn means the concentration of manganese in the steel sheet.

Hereinafter, the present invention will be described in detail.

FIG. 1 is a schematic view showing a cooling pattern of a conventional hot rolled steel sheet. Referring to the drawings, the hot rolled steel sheet undergoes a process of being cooled at each cooling rate in the stages divided into finishing rolling, isothermal heat treatment, cooling, and winding.

At this time, the temperature of the steel sheet and the cooling rate at the temperature are determined by the amount of transformation from austenite to ferrite, pearlite, bainite or martensite, as can be seen from the TTT (Time Temperature Transformation) curve added to the drawing. It becomes.

When the amount of transformation is determined as described above, since the fraction of the phase finally present in the steel is determined, it is necessary to grasp the type and the amount of transformation of the phase in each cooling pattern in order to determine the fraction of the phase.

The present invention focuses on this point and relates to a method for obtaining a transformation amount during cooling of steel materials. One representative method for reaching this object of the invention is shown in the process flow diagram in FIG. 2.

Important metallurgical prerequisites to be understood before explaining the process flow chart are that the ferrite, pearlite, bainite and martensite phases are only transformed from the austenite phase upon cooling, so that transformations can occur between phases other than the austenite phase. Is not. This is exactly the same as the actual metallurgical theory or phenomena, which is an important factor in determining the technical spirit of the present invention to be described below.

Once a part of the retained austenite is transformed into a ferrite, pearlite, bainite or martensite phase due to the transformation phenomenon, the transformed phase becomes a very stable phase which cannot be transformed into another phase upon cooling and the subsequent transformation is performed on the phases. This occurs only in the residual austenite remaining after transformation.

And another consideration is that only one phase is produced from residual austenite at given cooling rates and temperature conditions. This can be clearly seen from the cooling curve of FIG. 1 to which the TTT curve is added, since the cooling curve can pass only one phase at one time point. For example, in the cooling time a of FIG. 1, when the cooling is performed according to the cooling pattern, phases such as austenite, ferrite, and bainite exist in the steel sheet, but austenite phase-decomposes at the present time (ie, time a). The only phase being produced is the bainite phase and the other phase (ferrite phase) is the phase already formed at the time of the previous cooling.

Therefore, the phase transformation is determined by the fraction of retained austenite under a given condition, the type of stability and the driving force for each temperature and cooling rate. The fractions and fractions can be known, and the type and fraction of phases formed at the time of cooling are also known. Based on these points, the method of predicting the phase percentage of the present invention will be described in detail below.

First, the calculation is usually started by selecting, as the initial condition, a time point above the Ac3 temperature at which no ferrite phase is produced. However, the initial conditions are not essential, and under the condition that the phases present in the steel sheet are in equilibrium, a common commercial thermodynamic model such as, for example, Thermo-Calc. Or F * A * C * T. Since the fraction of the phase present in the interior can be calculated by using it can be selected as the initial condition.

Therefore, after the selection of the initial conditions, a process of determining the kind of phases stable under the appropriate conditions and the phase fraction (particularly, austenite phase fraction) of each phase at this time is followed.

Thereafter, a step of calculating the fraction in which phase separation into austenite to ferrite, pearlite, bainite or martensite after a predetermined time interval DELTA t occurs. In more detail, the step of calculating a temperature _{Ti + 1} after a predetermined time interval Δt _i according to a given cooling schedule (cooling pattern) is _first given (S30).

The temperature (T _{i + 1} ) can be easily obtained using the function if the cooling pattern can be expressed as a specific function, otherwise the previous stage temperature (T _i ), cooling rate (R) and time From the interval Δt _i can be obtained by the following equation (1).

[Relationship 1]

T _{i + 1} = T _i -R × △ t _i

When the temperature T _{i + 1} at a specific time step is obtained through the above process, the equilibrium phase fraction at the temperature and the type of generated phase can be obtained (S40). As described above, the equilibrium phase fraction and the type of generated phase can be obtained from a thermodynamic model, and various commercial models such as thermo calc or fact (F * A * C * T) may be used.

However, the phase fraction obtained in the step S40 is the phase fraction which appears when the steel sheet is maintained for an infinite time at the temperature, and does not match the phase fraction according to the temperature of the dynamic cooling process in the present invention.

Therefore, the process of calculating the phase percentage appearing in the actual cooling process using the equilibrium phase percentage needs to be followed (not shown in the drawing). The phase fraction may be obtained in the following manner.

First, in order to calculate the degree of residual austenite phase separation and new phase formation during the cooling process, the residual austenite phase-decomposes with time at a constant temperature to generate another phase (ferrite, pearlite, bainite or martensite). It is necessary to understand the equation for obtaining the phase fraction of the phases generated over time.

As described above, there are various equations such as Grange, Kiefer, Kirkaldy, Shimizu and Tamura, Cavanagh, and Umemoto equations that generate new phases according to time at a constant temperature. Among them, Avrami equations are described as follows.

[Relationship 2]

X = X ^e [1-exp (-kt ⁿ )]

Here, X is the percentage after the predetermined time elapses of the generated phase and X ^e represents the percentage when the time has elapsed to infinity, that is, the equilibrium. In addition, k is a constant and t means time.

In the above formula, when t is infinite, X becomes equal to X ^e . From the equation for calculating the rate of change in an isothermal state, such as the Abrami equation represented by the above relation 2, it is possible to obtain an equation for calculating the phase fraction with time in the cooling process.

However, according to the above equation, when no phase transformation occurs at all initially (X = 0), only the fraction (X = X) in which a new phase is formed for a predetermined time t can be obtained. That is, the fraction (X = X _{i + 1)} after a certain fraction (X = X _i) from the set time (△ t _i) is not calculated using the relation 2.

Therefore, if the constant 2 (X = X _i ) is given as the initial condition as shown in FIG. 3 by changing the relation 2, the percentage (X = X _{i + 1} ) after the predetermined time (Δt _i ) can be calculated. There is a need to improve in such a way.

This represents the phase fraction (X = X _i ) generated after t time elapsed from the case where the initial percentage ratio is 0 (X = 0) and the phase fraction (X = X _{i + 1} ) generated after t + Δt _i time elapsed. It can be obtained from two Abrami equations.

[Relationship 3]

X _i = X ^e [1-exp (-kt ⁿ )]

[Relationship 4]

X _{i + 1} = X ^e [1-exp (-k (t + Δt _i ) ⁿ )]

The same t as in the following relation 5 can be obtained from the relation 3 above.

[Relationship 5]

t = [-1 / k ln (1-X _i / X ^e )] ^{1 / n}

Since k and X ^e are known values and X _i is an initial value in relation 5, t can be obtained. Substituting the value of t obtained from the relational expression 5 into the relational expression 4 allows the value of X _{i + 1} to be obtained.

However, the obtained phase fraction (X _{i + 1} ) is obtained on the assumption that the phase fraction (X _γ ) of the retained austenite when the current phase is first formed is 100%, and may be different from the actual state. Therefore, the value of the actually generated phase fraction X ^r _{i + 1} may be obtained through Equation 6 considering the residual austenite fraction (X ^γ ₀ ) when the current phase is first generated (t = 0).

[Relationship 6]

X ^r _{i + 1} = X _{i + 1} × X ^γ ₀

A real on the generated from the current minute intervals via the equation 6 fraction (X ^r _{i + 1),} but obtain the initial phase fraction to be used in the calculation of the micro-segment is not the X ^r _{i + 1} X _{i + 1} being It should be noted that The reason is that the phase transformation of each microsection in the isothermal or cooling process occurs only in the residual austenite, as described above. to be.

As a series of processes described above, when the initial phase fraction of each microsection in the isothermal process is X _i , a method of obtaining the phase fraction X _{i + 1} after the lapse of the micro time change? T is obtained. However, since the process targeted in the present invention is not only an isothermal process but also a comprehensive process including a cooling process, it is necessary to accurately analyze the phase transformation behavior in the cooling process.

However, as shown in FIG. 4, the cooling process is not a process of changing from X (t, T) to X (t + Δt _i , T) as compared to the isothermal process, and X (t + T) to X (t + Δ). t _i , T-ΔT) is changed. In this case, the equation of the isothermal process cannot be used as it is, because the X ^e value used in the above Equations 3 to 5 is a function of temperature and thus cannot be considered as the same value at different temperatures. . That is, X ^e of Equation 5 is the equilibrium percentage at temperature T, whereas X ^e of Equation 4 is the equilibrium percentage at temperature T-ΔT, so it is no longer possible to use the same value as in isothermal process. Because.

Therefore, the cooling process cannot be interpreted in the same way as the isothermal process. However, if the cooling process is divided into an isothermal process (S40, as described above) that occurs during the time minute time Δt _i as shown in Figure 5, and a cooling process that occurs without time elapses, the phase fraction calculation method of the isothermal process You can use quite a bit. This method of calculation is called an additive rule.

This will be described with reference to FIG. 5. As shown in Figure 5, the isothermal conditions as the fraction using the changed value by applying a percentage value change of the dynamic cooling, the temperature T _i Initial phase △ t _i has elapsed by using the fraction X _i in the in the present The process of obtaining the phase fraction of time (X ^e _{i + 1} ) (process 1), and the cooling process without time lapse, that is, the process of cooling only the temperature from T _i to T _{i + 1} in the same state (process 2) Divided by)

At this time, the isothermal process (S40, process 1) can be calculated using the same method as the isothermal process using the equation 5 and the equation 4. In addition, the cooling process without the passage of time (process 2) to obtain the temperature using the above equation 1, the generated phase fraction may be used as it is calculated in the isothermal process (process 1). However, the process 2 does not need to calculate the additional temperature change amount since the temperature will be changed in step S30 when the phase fraction (X _{i + 2} ) of the next step is calculated.

As a result, it is possible to calculate the temperature T _i, the fraction X _i in temperature minute time △ t _i has elapsed after the cooling rate under the condition R _{i + 1} from the state T, the fraction of X _{i + 1} state.

The phase fraction X _{i + 1} obtained from this process is used as the initial value of the next step and is used to calculate the phase percentage X _{i + 2} of the next step.

However, in order to obtain the next phase fraction (X _{i + 2} ), as can be seen from the relations 4 and 5, in addition to the phase fraction X _{i + 1} and the temperature T _{i + 1} , the micro time size Δ t _{i + 1} ), a constant k and an equilibrium phase fraction (X ^e _{i + 1} ). Since the double time size (Δt _{i + 1} ) is a value that is arbitrarily set, it should be set within a range that does not cause a large error.The constant k used in the diffusion transformation equation such as Abramie's equation is a temperature related to diffusion (T). _{i + 1} ) is determined, so it is not a big problem. However, since the equilibrium phase fraction (X ^e _{i + 1} ) is largely influenced by the composition of residual austenite, especially carbon concentration in the residual austenite, in order to obtain the equilibrium phase fraction (X ^e _{i + 1} ), It is necessary to determine the carbon concentration in the retained austenite (S50).

In order to calculate the carbon concentration of the residual austenite, it is necessary to understand the mass balance formula of carbon represented by the following relational formula (7).

[Relationship 7]

C ₀ = X ^f _{i + 1} C ^f _{i + 1} + X ^γ _{i + 1} C ^γ _{i + 1} + X ^p _{i + 1} C ^p _{i + 1} + X ^b _{i + 1} C ^b _{i + 1} + X ^m _{i +1} C ^m _{i + 1}

Here, the subscript i + 1 means the next step of the current step (step i), C means the carbon concentration, according to each subscript C ₀ is the average carbon concentration of the steel sheet, C ^f _{i + 1} is ferrite Carbon concentration in C ^γ _{i + 1} , carbon concentration in austenite, C ^p _{i + 1} is carbon concentration in perlite, C ^b _{i + 1} is carbon concentration in bainite, and C ^m _{i + 1} is carbon concentration in martensite It means. In addition, X denotes the initial percentage of each phase in the next minute interval, and according to each superscript, X ^f _{i + 1} is the percentage of ferrite, X ^γ _{i + 1} is the percentage of austenite, X ^p _{i + 1} is the percentage of pearlite, X ^b _{i + 1} is the bainite, and X ^m _{i + 1} is the percentage of martensite.

The fraction of each phase in the mass balance formula of the above relation can be determined by the above-described phase fraction calculation method. However, the phase fraction of the retained austenite in the current microsection can be obtained through the following Equation 8.

[Relationship 8]

X ^γ _{i + 1} = X ^γ ₀ [1-X _{i + 1} ] = X ^γ ₀ -X ^r _{i + 1}

Meaning of the relationship 8 is that the residual austenite is represented by the value of the initial residual austenite phase fraction minus the phase fraction of the currently generated phase obtained in the current microsection.

In addition, martensite (a method of calculating the percentage content due to diffusionless transformation of martensite is described later) is that austenite is transformed without diffusion of elemental elements, and pearlite is austenite, and one layer is ferrite having low carbon solubility. The remaining layer is transformed into a lamellar structure that forms a layer of cementite (Fe ₃ C) containing a large amount of carbon, so the carbon concentration (C ^m _{i + 1} and C ^p _{i + 1} ) of martensite and pearlite is The assumption is the same as the carbon concentration of austenite (C ^m _{i + 1} = C ^p _{i + 1} = C ^γ _{i + 1} ).

Further, the maximum carbon solubility of the ferrite phase (α phase) can be obtained from the iron-carbon phase diagram. This is much lower than the carbon concentration in austenite in the composition of conventional strained organic calcined steel. Therefore, the carbon concentration in the ferrite phase can be set to the maximum carbon solubility as it will be saturated by the maximum carbon solubility in the ferrite phase. In this case, the maximum carbon concentration in the ferrite phase may be obtained from a state diagram, or may be sufficiently obtained from a commercial thermodynamic model.

In addition, bainite in the transformation organic plastic steel usually has ferritic bainite, and thus has a maximum carbon solubility almost equal to that of ferrite. Therefore, the carbon concentration of bainite may be regarded as the same as the carbon concentration of ferrite (C ^b _{i + 1} = C ^f _{i + 1} ).

When the above assumptions and metallurgical information are combined, the undetermined value in relation 7 is only carbon concentration in austenite and all other values can be obtained. Thus, the result shown in relation 9 can be obtained.

[Relationship 9]

C ^γ _{i + 1} = [C _0- (X ^f _{i + 1} + X ^b _{i + 1} ) C ^f _{i + 1} ] / (X ^γ _{i + 1} + X ^p _{i + 1} + X ^m _{i + 1} )

As described above as a prerequisite for understanding the present invention, the phases that may exist at each cooling point may be mixed with various kinds of phases shown in Equation 8, but at present, only one phase is generated at the time of cooling. shall. Therefore, the phase generated at the present cooling time can be obtained by the calculation method shown in the relation 4 and the relation 5 and FIG. 5 and the already generated phase is already determined in the previous cooling history, and when the relation 9 is applied at the current cooling time The normal percentage can be used. That is, the phase fraction of the phase that is not generated at the present cooling time can be calculated by treating X _{i + 1} = X _i .

When the carbon concentration of the retained austenite is obtained under the above conditions, the equilibrium phase fraction (X ^e _{i + 1} ) can be calculated from the carbon concentration and the temperature at a given time step using a thermodynamic model.

Therefore, the method of obtaining the phase fraction X _{i + 2} of the next step by using the above equilibrium phase fraction is the same as the method of obtaining the phase percentage of the i + 1 step described above (S60 iterative process). As a result, it is possible to obtain a change in percent fraction over the entire time and to obtain a distribution of percent fraction in the final microstructure. At this time, if the temperature change over time and carbon enrichment in the residual austenite as described above occurs, the type of the stable phase may be changed. It is necessary to calculate the percentage by review. That is, when the type of the stable phase is changed, the residual austenite concentration at the time of the change is set as the initial residual austenite concentration, and the phase fraction is calculated for the changed stable phase. In addition, the upper percentage of the stable phase of the previous stage is no longer changed, but is only used for the calculation of the relational expressions (7) and (9).

This phase fraction distribution is based on the fact that, as described above, and as can be seen from the TTT curve, the phase transformed from the residual austenite under that condition is only one phase. In addition, the phase transformation behavior employed in the phase transformation equation such as the above-described Abramie equation corresponds only to the phase transformation caused by the diffusion of solute atoms, and the transformation rate varies with time.

However, since non-diffusion transformations such as residual austenite to martensite transformations, i.e., transformations without solute atom diffusion, are transformed to equilibrium phase fractions for a very small period of time, the equilibrium phase percentages and actual phase percentages are substantially It becomes the same. Therefore, if only current cooling conditions (cooling temperature and amount of retained austenite) are determined, the equilibrium martensite phase fraction at that time can be calculated, and the result of the calculation can be used as the actual phase fraction.

Therefore, in the martensite transformation region, only the calculation of the martensite equilibrium percentage in the state can determine the actual percentage of the state. It is preferable to use the martensite phase transformation formula (also known as Koistinen-Marburger formula) represented by the following relational expression 10 as the equilibrium phase percentage.

[Relationship 10]

X ^m _{i + 1} = X ^γ _i [1-exp {-a (M _s -T)}]

Here, X ^γ _i is the residual austenite fraction, a is a constant, T represents the current temperature (° C.), and M _s represents the martensite transformation start temperature (° C.) represented by the following formula (11).

[Relationship 11]

M _s = 550-(360 × C ^γ _i )-(40 × Mn)

Here, ^Cγ _i represents the carbon concentration in the retained austenite, and Mn represents the manganese concentration.

Equations 11 and 10 can be used to obtain the equilibrium martensite phase fraction (X _m ) at each temperature, which is the phase fraction of martensite in the present state (martensite phase ratio used in Equations 7 and 9). Can be used.

By repeating the above process, it is possible to calculate the percentage after passing through the entire cooling pattern.

The above calculation method may be used to obtain a representative phase fraction distribution inside the steel sheet by analyzing one cooling behavior when the temperature variation or cooling rate variation of each part, especially the sheet width during rolling, is not large. In the case of the steel sheet may be subjected to the winding process after a certain time after the rolled steel, in this case, because the cooling behavior is very different for each part of the steel sheet (coil), it is necessary to analyze the distribution of phase percentages for each part.

In this cooling behavior, even if the cooling rate is adjusted by supplying a coolant to the outside of the coil, the cooling pattern inside is unpredictable. Therefore, it is necessary to calculate the cooling pattern for each part. The cooling pattern of each part can be easily interpreted by a conventional heat transfer analysis program (for example, Piadap, Abacus, Fluent, etc.). When the cooling pattern of each part is calculated in the same manner as described above, it is possible to calculate the change rate of the phase percentage of each part using the calculated cooling pattern as described above. The cooling pattern application method as described above may be a method of applying the calculation with the obtained cooling pattern after obtaining all the cooling patterns of each point over the entire cooling time, or calculating the temperature distribution → percent fraction for each minute time step. There may be a method of calculating in the order of calculation. These two calculation methods have no effect on the results and they can be said to be in an even area.

(Example 1)

In order to observe the accuracy of the phase fraction prediction method according to the present invention, the steel sheet of the components shown in Table 1 was cooled by the cooling pattern shown in Figure 6, and then the treatment was performed for a predetermined time at the temperature conditions of Table 2 below. . Residual austenite phase percentage in the treated steel sheet was compared by dividing the measured value measured by the actual microscopic histology with the calculated value calculated according to the method according to the present invention. However, the predicted values shown in Table 2 below do not consider the amount of transformation in relational formula 8 in which residual austenite is transformed into martensite.

Ingredient Name C Si Mn P S Al N Content (wt%) 0.20 1.90 1.50 0.015 0.003 0.025 0.0040

Austenitic fraction measurement and calculation results for each condition (not considering martensite transformation) Elapsed time 350 ℃ 400 ℃ 450 ℃ Measures Calculated Value Measures Calculated Value Measures Calculated Value 1 minute 2.6 17.5 3.8 13.6 3.3 3.4 10 minutes 4.4 9.2 7.1 7.2 1.9 0.2 60 minutes 6.0 7.8 3.1 4.0 1.0 0 120 minutes 7.0 7.5 2.1 1.8 0.9 0 240 minutes 7.4 7.1 - - - - 720 minutes 3.3 5.9 - - - -

As can be seen from the prediction results shown in Table 2 above, it can be seen that the measured value and the calculated value are widened in the initial stage after a short time. This is a value that does not take into account the martensite transformation, which is a non-diffusion transformation, in the region, and thus the initial value is greatly different.

In order to solve this problem, it was recalculated considering the martensite transformation of relations 10 and 11. The calculation results are shown in Table 3.

Austenitic fraction measurement and calculation results for each condition (Martensite transformation is considered) Isothermal holding time 350 ℃ isothermal maintenance 400 ℃ isothermal maintenance 450 ℃ isothermal maintenance Measures Calculated Value Measures Calculated Value Measures Calculated Value 1 minute 2.6 2.9 3.8 4.8 3.3 3.3 10 minutes 4.4 9.2 7.1 7.2 1.9 0.2 60 minutes 6.0 7.8 3.1 4.0 1.0 0 120 minutes 7.0 7.5 2.1 1.8 0.9 0 240 minutes 7.4 7.1 - - - - 720 minutes 3.3 5.9 - - - -

As can be seen in Table 3 above, it was found that the calculated initial percent mortality was very close to the correct value when the martensite transformation was considered. Therefore, it was found that it is more desirable to consider martensite transformation for accurate phase transformation prediction of metamorphic organic plastic steel.

(Example 2)

The measurement and calculation results of the internal structure were observed for each of the cooling patterns shown in FIG. 5 for the wound coil having a width of 950 mm, a thickness of 2.5 mm, and a total weight of 20 tons.

Measurements and calculations were carried out for the coil edge and coil center and ABAQUS, one of the numerical analysis programs, was used to calculate the cooling pattern of each part. As can be seen from FIG. 5 and Table 4, winding was performed at 400 ° C., and then the coil was wound in two cooling patterns, air cooling and water cooling.

Austenitic fraction measurement and calculation result of wound coil Elapsed time after winding 400 ℃, air-cooled, for coil edge 400 ℃, water cooling, coil center part Measures Calculated Value Measures Calculated Value 1 minute 3.5 3.5 3.5 4.8 10 minutes 5.6 7.1 7.1 7.2 30 minutes 8.3 7.0 6.0 5.8 60 minutes 7.9 2.4 5.5 5.3 120 minutes 7.1 2.1 5.4 5.2 360 minutes 6.8 1.1 5.2 5.0

As shown in Table 4, the results calculated by the calculation method according to the present invention showed very close results when compared with the measured values. Therefore, the effect of this invention was confirmed.

According to the present invention, it is possible to predict the fraction of the retained austenite, which is the largest factor for expressing the plastic deformation capacity and strength of the transformed organic plastic steel, thereby freeing the steel type design according to the manufacturing history of the transformed organic plastic steel.

Claims (10)

A first step of inputting the composition and initial temperature of the steel sheet; A second step of determining the type and initial phase fraction of the phases generated according to the initial composition and initial temperature; The temperature after the minute time Δt _i is determined according to an isothermal or cooling pattern over time, a thermodynamic model is used to determine the equilibrium phase fraction at the determined temperature, and the equilibrium phase fraction is used to determine the initial phase. fraction (X _i) by calculating the changed phase fraction (X _{i + 1)} from, the calculated phase fraction (X _{i + 1)} during the fraction calculated at the next step minute time (△ t _{i + 1)} of the initial phase A third step consisting of repeating the process of using the fraction until the stable phase is changed; And A fourth step of returning to the second step when the stable phase is changed; Being done with Method for analyzing the percentage of hot rolled steel sheet, characterized in that the actual percentage calculated by the following relation 1 using the phase fraction calculated in the third step. [Relationship 1] X ^r _{i + 1} = X _{i + 1} × X ⁰ _γ Where X ^r _{i + 1} is the actual fraction of the present stable phase in the steel sheet, and X ⁰ _γ is the residual austenite fraction when the current stable phase is first formed. The method of claim 1, wherein the method of calculating the changed phase fraction (X _{i + 1} ) from the initial phase fraction (X _i ) in the third step substitutes the time (t) obtained from relation (2) The phase fraction analysis method of the hot-rolled steel sheet characterized by obtaining the fraction of the phase produced from the residual austenite decomposed in the present micro-section. [Relationship 2] t = [-1 / k ln (1-X _i / X ^e )] ^{1 / n} [Relationship 3] X _{i + 1} = X ^e [1-exp (-k (t + Δt _i ) ⁿ )] Where k is the proportional constant obtained from the experimental value, X _i is the initial phase fraction of the current microdivision of the phase concerned, X ^e is the equilibrium phase fraction of the corresponding phase when isothermal treatment is carried out for infinite time at the temperature of the current microdivision, X _{i +1} is the final phase fraction after the current _{microdivision} of the corresponding phase has elapsed, and Δt _i is the time interval of the current microdivision. The method of claim 1, wherein the carbon concentration in the retained austenite required for calculating the equilibrium percentage of the next step (i + 1) using the thermodynamic model in the third step is obtained by the following equation (4). Phase percentage analysis method. [Relationship 4] C ^γ _{i + 1} = [C _0- (X ^f _{i + 1} + X ^b _{i + 1} ) C ^f _{i + 1} ] / (X ^γ _{i + 1} + X ^p _{i + 1} + X ^m _{i + 1} ) Here, the subscripts f, b, γ, p, m represent ferrite, bainite, austenite, pearlite and martensite, respectively, C represents the carbon concentration currently included in the phase, and X represents the current percentage of each phase. Means. However, C ₀ means the average carbon concentration of the steel sheet. 4. The phase fraction of each phase is the same as the phase fraction of all micro phases except for the phase fraction X _i of the phase (stable phase) generated in the current processing stage and the residual austenite. Phase fraction analysis method of hot rolled steel sheet The method according to claim 4, wherein the phase fraction of retained austenite (X ^γ _{i + 1} ) is calculated by the following relational formula (6). [Relationship 6] X ^γ _{i + 1} = X ^γ ₀ [1-X _{i + 1} ] = X ^γ ₀ -X ^r _{i + 1} Where X ^γ ₀ represents the residual austenite fraction when the stable phase is first produced in the current treatment step. The method of claim 2, wherein the second step of determining the type and initial phase fraction of the phases generated according to the initial composition and the initial temperature is performed using a thermodynamic model. The method of claim 2, wherein the isothermal or cooling pattern according to the time used in calculating the phase percentage is calculated by applying differently for each part of the steel sheet. 8. The method of claim 7, wherein the cooling pattern calculated differently for each part of the steel sheet is calculated through heat transfer analysis. 9. The martensite fraction (X _{i + 1} ) according to any one of claims 1 to 8, wherein when the stable phase produced in the current treatment step is martensite, the fraction of martensite is calculated by the following equation (7). Phase fraction analysis method of hot rolled steel sheet. [Relationship 7] X _{i + 1} = X ^γ _i [1-exp {-a (M _s -T)}] Where M _s represents the martensite formation temperature. 10. The method of claim 9, wherein M _s is calculated by the following relational formula (8). [Relationship 8] M _s = 550-(360 × C ^γ _i )-(40 × Mn) Here, Mn means the concentration of manganese in the steel sheet.

Images