CONTROL METHOD FOR ROLLER QUENCHING PROCESS OF HEAVY-PIECE WEIGHT AND LARGE-SECTION ULTRA-HEAVY PLATE

Abstract

A control method for a roller quenching process of a heavy-piece weight and large-section ultra-heavy plate has a specific heat model, heat transfer coefficient model, temperature field model and correction model. Plate parameters inputted include thickness, length and carbon content, technological procedure, roller speed and acceleration. Measured parameters include tapping temperature, temperature after air cooling and temperature after self-tempering. The temperature field model is used. Specific heat model and the heat transfer coefficient model are invoked for calculating an air cooling stage, water cooling stage and self-tempering stage in sequence. Temperature fields are corrected through the correction model. Simulated results include a group of cooling curves and cooling speed curves at different thicknesses. Practical temperature drop curves and cooling speed curves are obtained in combination with actual production and part of actual debugging process is replaced by model calculation.

Description

TECHNICAL FIELD

The present invention belongs to the technical field of metallurgy, and relates to a control method for a roller quenching process of a plate.

BACKGROUND

The temperature field distribution after quenching of an ultra-heavy plate affects the physical performance and the machining performance of the plate to a large extent. In traditional immersion quenching, the plate is immersed into a quenching pool or quenching tank, which is limited by the volume of a container and causes low cooling speed and uneven distribution. In roller quenching, the plate enters a quenching machine through a rotating roller, and a high-pressure jet is sprayed on the surface of the plate. Compared with the traditional quenching, the roller quenching substantially increases cooling speed and the plate can be cooled to room temperature in short time. However, due to large thickness and slow heat transfer of the plate, there is a large temperature difference between the center and the surface and performance distribution is uneven after quenching. Calculation of the quenching process can better improve this problem. By setting different parameters, the temperature curve and the cooling speed curve of the plate are calculated, and the temperature distribution and cooling speed changes of the plate in different positions in the cooling process are intuitively observed. Calculated results can better serve the production and reduce the production cost.

Patent CN105445319A discloses a method and a device for measuring a water cooling heat transfer coefficient of a plate surface. Image information of a non-water-cooled surface of the plate is collected by an infrared thermal imager to obtain an actual cooling curve of the non-water-cooled surface. The whole process from the water-cooled surface to the non-water-cooled surface is simulated by finite element, and the water-cooled heat transfer coefficient is continuously adjusted to obtain a simulated cooling curve of the non-water-cooled surface. Patent CN102507636A discloses a method for measuring an interface heat transfer coefficient of steel in a rapid cooling process. A thermocouple is connected to a temperature collection module through spot welding to obtain temperature change data of the surface. Heat treatment software is used to obtain the interface heat transfer coefficient in the cooling process and to simulate temperature change in the cooling process of workpieces. The simulated result is compared with a measured result until a change tendency is fitted well. The prior art does not involve any control method related to a roller quenching process of a heavy-piece weight and large-section ultra-heavy plate, and any specific method to correct the heat transfer coefficient in the roller quenching process of the heavy-piece weight and large-section ultra-heavy plate by using a correction model.

SUMMARY

The purpose of the present invention is to provide a control method for a roller quenching process of a heavy-piece weight and large-section ultra-heavy plate. The method is suitable for model control of the ultra-heavy plate in a quenching process, is used for visually representing the temperature change and cooling speed change of a head and a tail of the plate on the surface, the quarter thickness and the center of the plate through curves in combination with actual production based on a mathematical model, and can be used for guiding adjustment of a technological procedure, saving production cost and obtaining a product with good plate shape.

The specific technical solution of the present invention is: a control method for a roller quenching process of a heavy-piece weight and large-section ultra-heavy plate comprises the following steps:

step 1. reading plate information and process parameters, comprising plate thickness, length, carbon content, roller speed and acceleration;

step 2. setting an initial heat transfer coefficient, using a temperature drop calculation model, invoking a specific heat model and a heat transfer coefficient model, correcting a heat transfer coefficient of an air cooling stage using a correction model of the heat transfer coefficient according to a measured temperature drop of the air cooling stage and then obtaining a temperature field of the air cooling stage;

step 3. determining a heat transfer coefficient of a water cooling stage, comprising determining a water cooling heat transfer coefficient of a high pressure section and determining a water cooling heat transfer coefficient of a low pressure section, wherein the water cooling heat transfer coefficient of the high pressure stage is determined; the quenching machine in the high pressure stage has the main action of rapidly cooling the surface temperature of the plate to a lower temperature; because the action time of the high pressure stage is much lower than that of the low pressure stage and cannot be directly obtained in actual production, empirical data is obtained from experiments;

the water cooling heat transfer coefficient of the low pressure stage is determined; when the plate leaves the quenching machine, only surface temperature is directly obtained; the plate surface has violent temperature change at the beginning of quenching only in the quenching process; after that, the temperature tends to be stable and is close to the temperature of a convection medium until the plate leaves the quenching machine; thus, the convective heat transfer coefficient cannot be directly calculated. The temperature inside the plate cannot be directly obtained. Thus, the water cooling heat transfer coefficient cannot be directly calculated through the water cooling stage. When the plate leaves the quenching machine, the inside temperature is still higher than the surface temperature and the inside temperature is transmitted to the plate surface by means of thermal conduction. Thus, the heat transfer coefficient of a water cooling stage of the low pressure stage is corrected using the correction model of the heat transfer coefficient at self-tempering temperature of the surface during air cooling after quenching. a specific method comprises: using a temperature field after tapping and air cooling as an initial temperature field of the water cooling stage; giving an initial heat transfer coefficient of the low pressure section; calculating the temperature field; using a temperature field after water cooling as an initial temperature field of a self-tempering stage for calculating a temperature value of a self-tempered surface node; comparing the temperature value with a measured value; invoking the correction model of the heat transfer coefficient for correcting the water cooling heat transfer coefficient; keeping the air cooling heat transfer coefficient unchanged; reusing the temperature drop calculation model; and invoking the specific heat model and the heat transfer coefficient model for calculating the temperature field of the water cooling stage and the temperature field of the self-tempering stage until a difference value is within a permissible error; and

step 4. obtaining temperature drop curves and cooling speed curves of different positions in line with the actual situation in the plate quenching process.

Because the convective heat transfer coefficient in the quenching process cannot be directly measured, the calculated temperature and the measured temperature need to be compared and the heat transfer coefficient is corrected using the correction model so as to correct the temperature field. The correction model of the heat transfer coefficient in above step 2 and step 3 is as follows:

an interval range [0, A] of the initial heat transfer coefficient is given; an upper limit value A is taken as the initial heat transfer coefficient for calculating the temperature field; if a computed value is higher than a target value, a range [A, 1.5A] of the heat transfer coefficient is taken; the upper limit value of a previous interval is taken as a lower limit value of a new interval in each interval change, and 1.5 times of the lower limit value of the new interval is taken as an upper limit value of the new interval until the value of the heat transfer coefficient is the upper limit of the interval and the computed value is lower than the target value; this indicates that the actual convective heat transfer coefficient is within this interval at this moment; in the interval, a golden section method is used to continuously reduce the interval until the difference value between the measured value and the target value is within the permissible error; and at this moment, the heat transfer coefficient value is an actual value.

Further, in above steps 2 and 3:

1) calculation of the specific heat model: the specific heat coefficient is mainly relevant to the carbon content and the temperature of the plate; a set definite value is taken as the definition scope of the carbon content; when the carbon content is not the above value, left and right boundary values corresponding to the carbon content are determined at first; the weight of the carbon content is determined by interpolation; and then the interval of the temperature is compared, thereby determining a specific heat value of the plate;

2) calculation of the heat transfer coefficient model: firstly, specific heat values and heat transfer coefficient values of plates with different carbon contents at different temperatures are obtained through experiments; and then specific heat values and heat transfer coefficient values corresponding to other carbon contents and other temperatures are determined by interpolation;

3) the temperature drop calculation model is as follows:

a one-dimensional unsteady heat transfer differential equation in a cartesian coordinate system is established:

$\begin{array}{cc}\frac{\partial T}{\partial t}=a\frac{{\partial }^2T}{\partial x^2}+\stackrel{.}{Q}\left(0<x<d,t>0\right)& \\ \mathrm{wherein}& \\ a=\frac{\lambda }{\rho c}& \end{array}$

x is the length of a divided cell; d is the thickness of the plate; t is the time; T is the temperature; a is a temperature conductivity; {dot over (Q)} is an internal heat source; λ is a heat transfer coefficient of a quenching plate; ρ is the density of the quenching plate; c is the specific heat of the quenching plate;

during calculation, latent heat of phase change in the cooling process of the plate is counted into the mean specific heat; therefore, the internal heat source can be neglected;

an initial condition is:

T(x,0)=T0 (0<x<d, t>0)

boundary conditions are:

$\hspace{1em}\{\begin{array}{cc}-\lambda \frac{\partial T\left(x,t\right)}{\partial x}{|}_{x=0}=h_x\left(T\left(0,t\right)-T_f\right)& \left(x=0,t>0\right)\\ -\lambda \frac{\partial T\left(x,t\right)}{\partial x}{|}_{x=d}=h_x\left(T\left(d,t\right)-T_f\right)& \left(x=d,t>0\right)\end{array}$

in order to improve the convergence and stability of Fourier number and make the model have a smaller error, Crank-Nicolson difference method is used;

$\frac{1}{2}{\left(\frac{{\partial }^2T}{\partial x^2}\right)}_i^t+\frac{1}{2}{\left(\frac{{\partial }^2T}{\partial x^2}\right)}_i^{t+1}=\frac{1}{a}{\left(\frac{\partial T}{\partial \tau }\right)}_i^t$

t is the time; i is a node, 0≤i≤I; the temperature field is established as follows:

an internal node is:

−F_oxT_i+1^t+1(2+2F_ox)T_i^t+1−F_oxT_i−1^t+1=F_oxT_i+1^t+(2−2F_ox)T_i^t+F_oxT_i−1^t

a boundary node is:

$\begin{array}{cc}-F_{\mathrm{ox}}T_{i-1}^{t+1}+\left(1+F_{\mathrm{ox}}+F_{\mathrm{ox}}B_{\mathrm{ix}}\right)T_i^{t+1}=F_{\mathrm{ox}}T_{i-1}^t+\left(1-F_{\mathrm{ox}}-F_{\mathrm{ox}}B_{\mathrm{ix}}\right)T_i^t+2F_{\mathrm{ox}}B_{\mathrm{ix}}T_f& \\ \mathrm{wherein}& \\ \begin{array}{cc}{F}_{\mathrm{ox}}=\frac{a\Delta t}{\Delta x^2}& B_{\mathrm{ix}}=\frac{h_x\Delta x}{\lambda }\end{array}& \end{array}$

h_x is a convective heat transfer coefficient; T_f is a water temperature; T_i^t is a temperature value corresponding to the ith node of the plate at time oft; F_ox is the Fourier number; B_ix is a Biot number;

stability conditions are:

$\hspace{1em}\{\begin{array}{c}1-F_{\mathrm{ox}}\geqq 0\\ 1-F_{\mathrm{ox}}-F_{\mathrm{ox}}B_{\mathrm{ix}}\geqq 0\end{array}$

when the initial temperature field and the heat transfer coefficient are known, the temperature distribution at any node and at any moment is calculated by difference.

Further, cooling time in the above temperature drop calculation model is controlled as follows: the model of the plate conducts calculation according to the roller position of the plate in three parts: an air cooling stage before entering a quenching machine, a quenching stage when entering the quenching machine and a self-tempering stage after entering the quenching machine;

the time of the air cooling stage is determined as follows: the plate has certain length; different positions take different times to enter the quenching machine. Thus, a head and a tail are respectively calculated; the head of the plate moves at uniform speed before entering the quenching machine, and thus calculation is conducted through a distance from the head to the quenching machine and initial speed; after the head of the plate enters the quenching machine, a roller begins to apply a certain acceleration; thus, the tail of the plate begins to accelerate in the air cooling stage; at this moment, the distance from the quenching machine is the length of the plate; the air cooling time for the tail is calculated through the initial seed, the distance and the acceleration;

the time of the quenching stage is determined as follows: the time of the quenching stage is divided into a time to go through the high pressure stage and a time to go through the low pressure stage; firstly, the length of the high pressure stage is determined; the quenching time of the head of the plate is directly calculated according to the set initial speed and acceleration of the roller; because the plate is accelerated immediately when the head of the plate enters the quenching machine, the speed at which the tail enters the quenching machine is determined through the time and the acceleration of the air cooling acceleration part; the time to go through the high pressure stage is calculated according to the speed and the acceleration; the time required for the plate to go through the low pressure stage is determined according to swing time; and

the time of the self-tempering stage is determined as follows: a specific method comprises: timing with a chronograph when the plate leaves the quenching machine; measuring the self-tempering temperature in the same position of the plate at different moments; stopping timing after self-tempering; taking a maximum self-tempering temperature as a target temperature in analog calculation; and taking a corresponding time as the time of the self-tempering stage.

Further, an initial temperature field model of each stage in steps 2 and 3 is established as follows: the temperature when the plate leaves a furnace is taken as an initial temperature field of the air cooling stage; a simulated temperature field after air cooling calculated by the temperature field model is compared with the temperature measured at a temperature measurement point before entering the quenching machine and corrected; finally, a practical temperature field after air cooling is obtained and is taken as an initial temperature field of the water cooling stage; the water cooling stage requires no correction computation; and a model calculation result is directly transmitted to the self-tempering stage as the initial temperature field of the self-tempering stage.

Further, the output result in step 4 comprises heat transfer coefficient values of the air cooling stage, the water cooling high pressure stage and the water cooling low pressure stage, temperature change curves and cooling speed change curves of surfaces, quarters and centers of the head and the tail of the plate.

The present invention has the following advantages:

1) Initial parameters conform to the actual production procedure.

Considering site production conditions, required measured data can be directly obtained in production.

2) The calculated results of each stage are corrected according to the measured values, and the calculated results are closer to the actual cooling curve of the plate.

3) The output result comprises the cooling curves and the cooling speed curves at different thicknesses, which can more visually show the temperature difference of the center and the surface, the cooling speed difference of the center and the surface, the temperature difference between the head and the tail and the cooling speed difference between the head and the tail, and can be used to guide and adjust the production process.

4) Part of site debugging is replaced by calculation to reduce energy consumption and production cost.

DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart of a calculation process.

FIG. 2 shows temperature change curves obtained by calculation.

FIG. 3 shows cooling speed change curves obtained by calculation.

FIG. 4 shows cooling speed change curves after the high pressure stage is filtered.

DETAILED DESCRIPTION

1) Input of parameters. Including plate parameters: thickness, length and carbon content; technological procedure: roller speed, acceleration, length of the high pressure stage of the quenching machine, tapping temperature and initial value of a heat transfer coefficient; measured parameters: tapping temperature, temperature before entering the quenching machine, self-tempering time and temperature after self-tempering.

2) Determination of the air cooling stage, the quenching stage and the self-tempering stage. The plate has certain length; different positions take different times to enter the quenching machine. Thus, a head and a tail are respectively calculated; and the head of the plate moves at uniform speed before entering the quenching machine, accelerates after entering the high pressure stage of the quenching machine and then swings in the low pressure stage. The tail of the plate moves as follows: the air cooling stage firstly moves at uniform speed before entering the quenching machine, and accelerates after the head of the plate enters the quenching machine; the high pressure stage accelerates, and then the low pressure stage swings. The time of the air cooling stage and the time of the high pressure stage are calculated through the distance, the initial speed and the acceleration. The time of the low pressure stage is determined through the set swing time. The time of the self-tempering stage is determined by means of timing.

3) Calculation of the temperature field of the air cooling stage. The initial temperature field is established by tapping temperature. The specific heat and the heat transfer coefficients of different nodes are respectively calculated in each time step. Then, a temperature field model is invoked to calculate the air cooling temperature field. The calculated result is compared with the measured value. The correction model is invoked to correct the air cooling heat transfer coefficient to obtain the temperature field after air cooling.

4) Determination of the time of the water cooling stage. Times required for the head, the middle and the tail of the plate to go through the high pressure stage of the quenching machine are respectively calculated. The time required for the plate to go through the low pressure stage is determined according to the swing time.

5) Calculation of the water cooling temperature field. The temperature field after water cooling is calculated by invoking the specific heat model, the heat transfer coefficient model and the temperature field model and using the calculated temperature field after air cooling as the initial temperature field.

6) Calculation of the temperature field after self-tempering. The specific heat model, the heat transfer coefficient model and the temperature field model are invoked to calculate the self-tempering temperature field by using the temperature field after water cooling as the initial temperature field and using the corrected air cooling heat transfer coefficient value as the heat transfer coefficient value. The calculated result is compared with the measured value. If the calculated result is not within a permissible error, the correction model is invoked to correct the air cooling heat transfer coefficient for recalculation of steps 5 and 6.

7) Output of the calculated result. The temperature drop curves and the cooling speed curves of the head surface and the tail surface, the quarter thickness and the center of the plate in the air cooling stage and the quenching stage are drawn.

Embodiment

The thickness of the plate is 132 mm; the length is 7250 mm; the carbon content is 0.15%; the roller speed is 0.2 m/s; the acceleration is 0.00015 m/s²; the length of the high pressure stage of the quenching machine is 3.2 m; the initial values of the heat transfer coefficient are: 100 W/(m²K) for the air cooling stage, 20000 W/(m²K) for the high pressure stage and 8000 W/(m²K) for the low pressure stage; water temperature is 22.1° C.; tapping temperature is 910° C.; the temperature before entering the quenching machine is 830° C.; air cooling time is 45 s; the swing time of the low pressure stage is 1560 s; the self-tempering time is 142 s; and the temperature after self-tempering is 28° C. The length of a cell is 1 mm and the time step is 0.5 s. The calculation flow is shown in FIG. 1.

Calculated results: the heat transfer coefficient value of the air cooling stage is 124.64 W/(m²K) and the water cooling heat transfer coefficient value of the low pressure stage is 2250 W/(m²K). The temperature drop curves are shown in FIG. 2. It is known from the figure that the surface temperature of the plate is rapidly reduced when the plate enters the air cooling stage and then enters the water cooling high pressure stage. After entering the low pressure stage, because the inside temperature is transmitted outwards, the temperature rises a little. Compared with the surface, the temperature of the quarter thickness and the center is reduced slowly. The cooling speed curves are shown in FIG. 3. After the cooling speed curves are compared with the temperature drop curves, it is found that in the high pressure stage, the cooling speed of the surface is rapidly increased; after entering the low pressure stage, the cooling speed curve has a negative value, which indicates that the temperature rises; short-time cooling speed of the surface is much higher than that of other moments, which is not convenient for observing the cooling speed of other moments; thus, the cooling speed is shown in FIG. 4 after part of the cooling speed is filtered; it is found that the cooling speed of the quarter thickness and the center is gradually increased when the temperature difference between the center and the surface is larger; and after this, with the decrease of the temperature, the cooling speed is gradually decreased.

Claims

1. A control method for a roller quenching process of a heavy-piece weight and large-section ultra-heavy plate, comprising the following steps:

step 1. reading plate information and process parameters, comprising plate thickness, length, carbon content, roller speed and acceleration;

step 2. setting an initial heat transfer coefficient, using a temperature drop calculation model, invoking a specific heat model and a heat transfer coefficient model, correcting a heat transfer coefficient of an air cooling stage using a correction model of the heat transfer coefficient according to a measured temperature drop of the air cooling stage and then obtaining a temperature field of the air cooling stage;

step 3. determining a heat transfer coefficient of a water cooling stage, comprising determining a water cooling heat transfer coefficient of a high pressure stage and determining a water cooling heat transfer coefficient of a low pressure stage, wherein the water cooling heat transfer coefficient of the high pressure stage is determined by empirical data from experiments;

the water cooling heat transfer coefficient of the low pressure stage is determined by correcting the heat transfer coefficient of a water cooling stage of the low pressure stage using the correction model of the heat transfer coefficient at self-tempering temperature of the surface during air cooling after quenching; a specific method comprises: using a temperature field after tapping and air cooling as an initial temperature field of the water cooling stage; giving an initial heat transfer coefficient of the low pressure stage; calculating the temperature field; using a temperature field after water cooling as an initial temperature field of a self-tempering stage for calculating a temperature value of a self-tempered surface node; comparing the temperature value with a measured value; invoking the correction model of the heat transfer coefficient for correcting the water cooling heat transfer coefficient; keeping the air cooling heat transfer coefficient unchanged; reusing the temperature drop calculation model; and invoking the specific heat model and the heat transfer coefficient model for calculating the temperature field of the water cooling stage and the temperature field of the self-tempering stage until a difference value is within a permissible error;

step 4. obtaining temperature drop curves and cooling speed curves of different positions in line with the actual situation in the plate quenching process;

the correction model of the heat transfer coefficient in above step 2 and step 3 is as follows:

an interval range [0, A] of the initial heat transfer coefficient is given; an upper limit value A is taken as the initial heat transfer coefficient for calculating the temperature field; if a computed value is higher than a target value, a range [A, 1.5A] of the heat transfer coefficient is taken; the upper limit value of a previous interval is taken as a lower limit value of a new interval in each interval change, and 1.5 times of the lower limit value of the new interval is taken as an upper limit value of the new interval until the value of the heat transfer coefficient is the upper limit of the interval and the computed value is lower than the target value; this indicates that the actual convective heat transfer coefficient is within this interval at this moment; in the interval, a golden section method is used to continuously reduce the interval until the difference value between the measured value and the target value is within the permissible error; and at this moment, the heat transfer coefficient value is an actual value.

2. The control method for the roller quenching process of the heavy-piece weight and large-section ultra-heavy plate according to claim 1, wherein cooling time in the temperature drop calculation model is controlled as follows: the model of the plate conducts calculation according to the roller position of the plate in three parts: an air cooling stage before entering a quenching machine, a quenching stage when entering the quenching machine and a self-tempering stage after entering the quenching machine;

the time of the air cooling stage is determined as follows: a head and a tail are respectively calculated; the head of the plate moves at uniform speed before entering the quenching machine, and thus calculation is conducted through a distance from the head to the quenching machine and initial speed; after the head of the plate enters the quenching machine, a roller begins to apply a certain acceleration; thus, the tail of the plate begins to accelerate in the air cooling stage; at this moment, the distance from the quenching machine is the length of the plate; the air cooling time for the tail is calculated through the initial seed, the distance and the acceleration;

the time of the quenching stage is determined as follows: the time of the quenching stage is divided into a time to go through the high pressure stage and a time to go through the low pressure stage; firstly, the length of the high pressure stage is determined; the quenching time of the head of the plate is directly calculated according to the set initial speed and acceleration of the roller; because the plate is accelerated immediately when the head of the plate enters the quenching machine, the speed at which the tail enters the quenching machine is determined through the time and the acceleration of the air cooling acceleration part; the time to go through the high pressure stage is calculated according to the speed and the acceleration; the time required for the plate to go through the low pressure stage is determined according to swing time; and

the time of the self-tempering stage is determined as follows: a specific method comprises: timing with a chronograph when the plate leaves the quenching machine; measuring the self-tempering temperature in the same position of the plate at different moments; stopping timing after self-tempering; taking a maximum self-tempering temperature as a target temperature in analog calculation; and taking a corresponding time as the time of the self-tempering stage.

3. The control method for the roller quenching process of the heavy-piece weight and large-section ultra-heavy plate according to claim 1, wherein an initial temperature field model of each stage in steps 2 and 3 is established as follows: the temperature when the plate leaves a furnace is taken as an initial temperature field of the air cooling stage; a simulated temperature field after air cooling calculated by the temperature field model is compared with the temperature measured at a temperature measurement point before entering the quenching machine and corrected; finally, a practical temperature field after air cooling is obtained and is taken as an initial temperature field of the water cooling stage; the water cooling stage requires no correction computation; and a model calculation result is directly transmitted to the self-tempering stage as the initial temperature field of the self-tempering stage.

4. The control method for the roller quenching process of the heavy-piece weight and large-section ultra-heavy plate according to claim 1, wherein the output result in step 4 comprises heat transfer coefficient values of the air cooling stage, the water cooling high pressure stage and the water cooling low pressure stage, temperature change curves and cooling speed change curves of surfaces, quarters and centers of the head and the tail of the plate.

5. The control method for the roller quenching process of the heavy-piece weight and large-section ultra-heavy plate according to claim 3, wherein the output result in step 4 comprises heat transfer coefficient values of the air cooling stage, the water cooling high pressure stage and the water cooling low pressure stage, temperature change curves and cooling speed change curves of surfaces, quarters and centers of the head and the tail of the plate.

6. The control method for the roller quenching process of the heavy-piece weight and large-section ultra-heavy plate according to claim 1, wherein in the steps 2 and 3: ∂ T ∂ t = a  ∂ 2  T ∂ x 2 + Q.   ( 0 < x < d, t > 0 ) wherein a = λ ρ   c   { - λ  ∂ T  ( x, t ) ∂ x  | x = 0 = h x  ( T  ( 0, t ) - T f ) ( x = 0, t > 0 ) - λ  ∂ T  ( x, t ) ∂ x  | x = d = h x  ( T  ( d, t ) - T f ) ( x = d, t > 0 ) 1 2  ( ∂ 2  T ∂ x 2 ) i t + 1 2  ( ∂ 2  T ∂ x 2 ) i t + 1 = 1 a  ( ∂ T ∂ τ ) i t - F ox  T i - 1 t + 1 + ( 1 + F ox + F ox  B ix )  T i t + 1 = F ox  T i - 1 t + ( 1 - F ox - F ox  B ix )  T i t + 2  F ox  B ix  T f wherein F ox = a   Δ   t  Δ   x 2 B ix = h x  Δ   x λ   { 1 - F ox ≧ 0 1 - F ox - F ox  B ix ≧ 0

1) calculation of the specific heat model: the specific heat coefficient is mainly relevant to the carbon content and the temperature of the plate; a set definite value is taken as the definition scope of the carbon content; when the carbon content is not the above value, left and right boundary values corresponding to the carbon content are determined at first; the weight of the carbon content is determined by interpolation; and then the interval of the temperature is compared, thereby determining a specific heat value of the plate;

2) calculation of the heat transfer coefficient model: firstly, specific heat values and heat transfer coefficient values of plates with different carbon contents at different temperatures are obtained through experiments; and then specific heat values and heat transfer coefficient values corresponding to other carbon contents and other temperatures are determined by interpolation;

3) the temperature drop calculation model is as follows:

a one-dimensional unsteady heat transfer differential equation in a cartesian coordinate system is established:

x is the length of a divided cell; d is the thickness of the plate; t is the time; T is the temperature; a is a temperature conductivity; {dot over (Q)} is an internal heat source; λ is a heat transfer coefficient of a quenching plate; ρ is the density of the quenching plate; C is the specific heat of the quenching plate;

during calculation, latent heat of phase change in the cooling process of the plate is counted into the mean specific heat; therefore, the internal heat source can be neglected;

an initial condition is: T(x,0)=T0 (0<x<d, t>0)

boundary conditions are:

in order to improve the convergence and stability of Fourier number and make the model have a smaller error, Crank-Nicolson difference method is used;

t is the time; i is a node, 0≤i≤I; the temperature field is established as follows:

an internal node is: −FoxTi+1t+1(2+2Fox)Tit+1−FoxTi−1t+1=FoxTi+1t+(2−2Fox)Tit+FoxTi−1t

a boundary node is:

hx is a convective heat transfer coefficient; Tf is a water temperature; Tit is a temperature value corresponding to the ith node of the plate at time of t; Fox is the Fourier number; Bix is a Biot number;

stability conditions are:

when the initial temperature field and the heat transfer coefficient are known, the temperature distribution at any node and at any moment is calculated by difference.