EA039568B1 - Method for producing a rolled steel product - Google Patents

Abstract

The invention relates to the field of metallurgy, in particular, to rolled steel product manufacture technology. In order to determine the structural condition of a rolled product, steel of a desired chemical composition is smelted and then rolled, with the process parameters being registered, and the structural condition of the rolled product obtained is determined on the basis of the observed process parameters. Mass fractions of austenite decomposition products in the layers are determined by calculation based on the following parameters: temperature measured on the rolled product surface prior to the onset of cooling, thickness of the rolled product, and implemented mode of heat dissipation from the surface of the rolled product; the calculations are performed using the finite difference method for solving the heat-transfer problem for a medium with internal heat liberation sources and by virtually dividing the thickness of the rolled product into layers. The heat liberation sources are identified as being the layers of the rolled product in which, according to the calculation, processes of austenite decomposition are occurring according to at least one of the following types of decomposition: ferrite (F), ferrite-perlite (F+P), perlite (P), bainite (B), and martensite (M). The type of decomposition is determined based on the layer temperature and/or cooling velocity, and/or specific power of heat removal from the layer; the increment of mass fraction of the austenite decomposition products (M/) is determined based on the temperature and/or cooling velocity, and/or specific power of heat removal from the layer; the value of heat liberation is calculated according to the equation Q=MQpi, where dQ is heat liberated in the result of austenite decomposition as per any type, kJ, Qpi is specific heat effect of the austenite decomposition as per i-th type, kJ/kg.

Description

The invention relates to technological processes used in metallurgy to produce high-quality rolled products of a new generation with the determination of consumer properties directly in the rolling process.

A new technological level of production of rolled products with specified functional properties and predictable structures requires the control of technological process parameters consistent with the kinetics of physical processes occurring in steel. The most important for industrial practice characteristics of phase transformations occurring in the material during its processing are the conditions for the beginning of transformations, the duration of their course, as well as the specific values of their thermal effects, which, along with technological parameters, determine the temperature of rolled products at various moments of its movement along the technological chain. Considering the scale of rolled metal production, it becomes essential to provide the possibility of predicting the structural state of the metal and related consumer properties on the basis of fixing the parameters of the technological process that took place during its production.

A known method for the production of rolled products, described in RU 2563911 [1], which provides for determining the mode of laminar cooling depending on the structure of the steel and the chemical composition under consideration by combining the graph of the change in the temperature of the rolled products during cooling on the discharge roller table with the thermokinetic diagram. Since the level and stability of the mechanical properties and structure of hot-rolled steel are largely determined by the coiling temperature and the nature of cooling, in order to determine the optimal laminar cooling regime, first, a thermokinetic diagram of the change in the structure for the chemical composition of the considered assortment of rolled steel is plotted in the range of possible temperatures and cooling rates on the discharge roller table broadband mill. Then, by calculation, a graph of the temperature distribution of the metal over the sections of the discharge roller table for rolled products of a given thickness is obtained using the laminar cooling mode characteristic of this assortment. The resulting graph is superimposed on a thermokinetic diagram and an analysis is made of the structure that can be obtained under such a laminar cooling regime from the point of view of providing the required level of mechanical properties. At the same time, the degree of remoteness of the graph from the nodal points of the phase transformation is also estimated, i.e. points at which the boundaries of different regions of phase transformations converge. If the analysis shows too much approximation of the indicated graph to these points or unfavorable nature of the structure formation, the laminar cooling mode used is adjusted accordingly and the cooling graph is calculated for this corrected mode. If a positive result is obtained during the verification analysis of the corrected laminar cooling regime, this regime is taken as the base one for this assortment. Otherwise, the laminar cooling mode is additionally adjusted until the desired result is obtained.

Thus, the known method through successive iterations makes it possible to obtain a metal with the required structural state, but does not solve the problem of determining the structure of the metal of the finished product only on the basis of fixing the process parameters that took place during its production.

A known method of metalworking with the support, at least partially, of manual control of a metalworking rolling mill, in which metal is processed in the form of a strip or slab or rough profile metal (RU 2457054 [2]). The method involves taking into account the influence of the phase composition of the metal on the adjustment of the parameters of the production process. To do this, the proportion of at least one metallurgical phase of the metal is continuously determined with respect to a certain place of the metalworking rolling mill, taking into account the operating parameters of the metalworking rolling mill, which are influenced by the phase state. Such parameters may be the selection of a suitable cooling plane or suitable cooling parameters. The result of the determination is transmitted to the service operator in real time, for example, on the control device. The operator thus obtains information relevant to the quality of the metal to be worked, which directly reproduces the effect of the manual adjustments made by him, so that, if necessary, the manual adjustments can be optimized by further changes. The indication thus serves to ensure the quality of the product, but does not solve the problem of determining the structure of the metal and the associated consumer properties on the basis of fixing the parameters of the technological process that took place during its production.

A known method for the production of rolled steel, including the process of controlling a metallurgical production plant for manufacturing a product from a metallic steel and/or iron alloy, the manufacturing process being at least partially controlled by a structure simulator and/or a structure monitor and/or a model structures, including a program that calculates at least one mechanical strength characteristic of the manufactured product (RU 2016133849 [3]). The mechanical strength characteristic is calculated depending on the respective process chain on the basis of the calculated metallurgical phase components and/or their respective proportions in the structure of the manufactured product. The technological chain of a metallurgical production plant includes a hot rolling mill and/or a plate rolling mill with a final cooling section. When calculating the mechanical strength characteristic, the operating parameters of the metallurgical production plant are taken into account, on which at least one obtained mechanical strength characteristic depends.

As operating parameters of the metallurgical production plant included in the calculation of strength, the corresponding mass fraction of one alloying element or all alloying elements that are present in the chemical composition of the used steel alloy is recorded.

As an additional operating parameter, the cooling rate established within the framework of the cooling produced after the rolling process is recorded, and the increase in a certain strength characteristic of the product produced, achieved by changing this additional operating parameter, is partially compensated and / or equalized by reducing the mass fraction of one or more alloying elements. in the chemical composition of the steel alloy used.

Moreover, the corresponding registered mass fraction of the alloying element and the cooling rate recorded in each case are evaluated using a countable number of evaluation units representing the evaluation criterion, and using the program, the corresponding total values of the calculated evaluation units are determined. The program includes a mathematical algorithm by which a corresponding set of evaluation units or various defined totals are compared with each other. As a result of the implementation of the method, a predictive determination of the consumer properties of rolled products is carried out without conducting a study of the finished product.

The disadvantage of the known method is the low accuracy of predicting the properties of the resulting rolled products, since the prediction is based on a mathematical model of the behavior of the rolled metal on technological parameters, which does not take into account some factors important for the formation of the structure, for example, thermal effects accompanying phase transformations.

Closest to the claimed in its technical essence and the problem to be solved is a method for the production of rolled metal, known from WO 9818970 [4]. The object of the known method is to enable the expected properties of the end product to be determined at each stage of the hot rolling process. The method consists in accumulating data on actually carried out rolling processes with fixing specific technological parameters and determining consumer properties. This takes into account such factors as the chemical analysis of the rolled material, the presence of carbides and nitrides of alloying additives (nickel, niobium, titanium, etc.), the size and condition of austenite grains, the microstructure of steel and the proportions of its components (austenite, ferrite, perlite, bainite , martensite). Based on the collected data, physical and metallurgical models are built using the linear regression method, linking technological parameters (heating, deformation, cooling of rolled products, etc.) with the properties of the finished product. Using the obtained models, it is possible to predict the properties of the finished product from the selected technological parameters or, having given the properties of the finished product, select the required technological parameters, making adjustments based on online measurements during the rolling process.

The disadvantage of the known method is the relatively low accuracy of identifying the relationship between technological parameters and the final structural state of rolled products, since the thermal effects that accompany phase transitions (transformations) that occur in the rolled metal during deformation and cooling are not taken into account.

The inventive method for the production of rolled steel is aimed at determining the structural state of the rolled metal according to the technological parameters of rolling.

The specified result is achieved by the fact that the method for the production of rolled steel includes the smelting of steel of the required chemical composition, its rolling with the fixation of technological parameters and the determination of the structural state of the obtained rolled product depending on the implemented technological parameters. At the same time, the mass fractions of the structural components are calculated based on the measured temperature of the metal surface before the start of cooling, the thickness of the metal and the implemented mode of heat removal from the surface of the rolled product, using a finite-difference scheme for solving the problem of thermal conductivity for a medium with internal sources of heat release with a virtual division of the thickness of the rolled stock into layers , at the same time, metal layers are taken as sources of heat release, in which austenite decay processes occur according to at least one of the mechanisms: ferritic (F), ferrite-pearlitic (F + P), pearlitic (P), bainitic (B), martensitic (M), moreover, the type of decomposition is determined based on the rate of heat removal from the metal layer and/or its temperature, and the decomposition kinetics is given by an equation of the form

5M/3τ=ί(Τ), where EM is the increase in the mass fraction of the decay product during the time dt, s;

T - temperature, °C,

- 2 039568 the amount of heat release is calculated by the equation

6Q=aMxQpi, where dQ is the heat released as a result of the decomposition of austenite by any mechanism, J;

Q _pi is the specific thermal effect of austenite decomposition by the i-th mechanism, J/kg, while before rolling on metal samples of identical chemical composition, the specific thermal effects taken into account in the calculation are experimentally determined for the types of austenite decomposition that occur in steel of a given chemical composition.

This result is also achieved by the fact that the ranges of temperatures and cooling rates corresponding to different mechanisms of austenite decomposition are determined experimentally by varying the cooling rates of metal samples of identical chemical composition.

This result is also achieved by the fact that when setting the kinetics of the decomposition of austenite, the equation dM/dt =kM _A -(W/T) is used, where dM is the increase in the mass fraction of the decomposition product during the time Et, s;

k - coefficient depending on the mechanism of decomposition and the chemical composition of the steel;

MA - mass fraction of austenite;

W - specific power of heat removal from the layer, W/kg;

T - temperature, °C.

Distinctive features of the proposed method are the mass fractions of the structural components are determined by calculation based on the measured temperature of the metal surface before cooling, the thickness of the metal and the implemented mode of heat removal from the surface of the rolled product;

for the calculation, a finite-difference scheme for solving the problem of thermal conductivity for a medium with internal sources of heat release is used with a virtual division of the rolled product thickness into layers;

metal layers in which austenite decay processes occur according to at least one mechanism - ferritic, ferrite-pearlitic, pearlitic, bainitic or martensitic - are taken as sources of heat release;

the type of decomposition is determined based on the rate of heat removal from the metal layer and/or its temperature;

the decay kinetics is given by an equation of the form

3M/3τ=£(Τ), where EM is the increase in the mass fraction of the decay product during the time Et, s;

T - temperature, °C, the amount of heat release is calculated by the equation

6Q=6MxQpi, where dQ is the heat released as a result of the decomposition of austenite by any mechanism, J;

Q _pi - specific thermal effect of austenite decomposition according to the i-th mechanism, J/kg;

before the start of rolling on metal samples of identical chemical composition, the specific thermal effects are experimentally determined for the types of austenite decomposition that occur in steel of a given chemical composition;

temperature ranges and cooling rates corresponding to different mechanisms of austenite decomposition are determined experimentally by varying the cooling rates of metal samples of identical chemical composition;

when setting the kinetics of austenite decomposition, the equation is used

EM/Et =kM _A -(W/T), where dM is the increase in the mass fraction of the decay product during the time dt, s;

k - coefficient depending on the mechanism of decomposition and the chemical composition of the steel;

M _A - mass fraction of austenite;

W - specific power of heat removal from the layer, W/kg;

T - temperature, °C.

Determining the mass fractions of structural components based on the calculated temperature of the metal surface before the start of cooling, the thickness of the metal and the implemented mode of heat removal from the surface of the rolled products and the studied thermal and physical properties makes it possible to predict the structure of the rolled metal and the consumer properties associated with it without resorting to expensive studies of samples cut from the finished product. At the same time, the accuracy of forecasting depends on which factors are taken into account and which of them are obtained by conducting preliminary experiments, and therefore their reliability is not in doubt.

The use of a finite-difference scheme for solving the problem of heat conduction for a medium with internal sources of heat release with a virtual division of the thickness of the rolled stock into layers for calculation makes it possible to predict the structure of the metal with high accuracy, subject to the condition that metal layers are taken as sources of heat release, in which the processes of austenite decay occur and up to At the beginning of rolling on metal samples of identical chemical composition, specific thermal effects are experimentally determined for the types of austenite decomposition that occur in steel of a given chemical composition, taking into account the kinetics of this decomposition.

In particular cases of the implementation of the method, the ranges of temperatures and cooling rates corresponding to different mechanisms of austenite decomposition are determined experimentally by varying the cooling rates of metal samples of identical chemical composition. This is necessary to implement the method in the absence of reference data for steel of a given chemical composition.

In some cases, the kinetics is determined not only by the temperature, but also by the specific power of heat removal from the layer, for which an equation of the form

where dM is the increase in the mass fraction of the decay product over time dt, s;

k - coefficient depending on the mechanism of decomposition and the chemical composition of the steel;

M _A - mass fraction of austenite;

W - specific power of heat removal from the layer, W/kg;

T - temperature, °C.

The essence of the proposed method is illustrated by examples of its implementation.

Example 1

In the most preferred embodiment, the method is implemented as follows. A small amount of steel of the required chemical composition, sufficient for the manufacture of samples for research, is smelted, which will be subjected to rolling in large batches.

Cut samples of the required size for laboratory experiments to determine the thermal parameters of the phase transformation. The thermal effects of phase transformations are determined (measured). Carry out experimental or calculated determination of the kinetics of transformations (functions of dependences of mass fractions of structural components on time).

A billet (slab) of identical or close chemical composition to the studied samples is heated for rolling. During the rolling process, the available technological parameters are fixed (the temperature of the surface of the rolled product along its length at the beginning and end of rolling, the speed of movement of the rolled product, the number and location of the included cooling nozzles, the coolant flow rate, the temperature of the surface of the rolled product along its length at the moment of completion of accelerated cooling). The data obtained are used in a finite-difference scheme for solving the problem of heat conduction for a medium with internal sources of heat release with a virtual division of the thickness of the rolled stock into layers, while the heat release sources are switched on according to the transformation kinetics. As a result of the calculation, the mass fractions of structural components in the rolled layers and throughout its entire thickness are determined.

Thermal effects of phase transformations are measured as follows. Temperature-homogeneous samples are placed in a heating furnace and heated to a temperature exceeding the temperature of austenite formation in the structure of the heated sample material.

Then the samples are cooled to room temperature with a coolant uniform in temperature and speed of movement, for example, dried air, along various cooling trajectories, which depend on the thickness of the sample and the pressure with which the coolant is supplied, i.e. on the cooling rate.

In cooled samples, using metallographic studies and by measuring microhardness, the proportion of the desired phase is determined. Then, a cooling curve is selected for analysis, the implementation of which ensures that the required proportion of this phase is obtained.

Next, the sections of the cooling curve in which there is no phase transformation are approximated as an exponential dependence of temperature on time, which will be used further to implement the calculation procedure for determining the specific thermal effect during the phase transformation.

Then the total specific thermal effect of transformation (Q _pr ) can be determined by the finite difference method t

Qnp ^\ u003d Σ AQi where ΔQ ₁ is the specific thermal effect of the phase transformation for the i-th calculation step, kJ / kg,

where Ci is the specific heat capacity of the material at the i-th calculation step, kJ/kg-°C;

ΔΤ is the difference between the actual temperature of the sample and the corresponding time value of the temperature according to the exponential dependence at the i-th calculation step minus the sums of these differences at the previous calculation steps, °C.

For example, an experiment was conducted to determine the specific thermal effect of phase transformation for steel of the following composition, wt.%: C - 0.05; Si - 0.09; Mn - 1.71; S 0.002; P - 0.008; Cr - 0.03; Ni - 0.23; Cu - 0.11; Al - 0.030; N - 0.005; V - 0.005; Ti - 0.020; Nb - 0.065; As - 0.002; Mo - 0.188; B - 0.0002; Sn - 0.003; Fe is the rest released during the formation of a ferrite-bainitic structure with a mass fraction of bainite ~ 50%.

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To conduct thermophysical studies, samples with dimensions of 110x27x3.4 mm were cut out. Then they were thinned on a surface grinding machine and a laboratory mill to a thickness of 2; 1.1; 0.5; 0.25 and 0.1 mm.

Such sample sizes provide one-dimensional heat transfer.

Then, in a muffle furnace equipped with a sample extraction and fixation mechanism, a temperature measurement unit, an air cooling unit with a pneumatic system, and an information control and recording unit, the samples were heated to 990°C and held for 40 min at the indicated temperature. To reduce the oxidation of the samples, argon was blown into the furnace.

Next, the samples were blown with a flat air flow at an angle of 15–30° to the sample with a pressure of 1, 2, or 4 atm. The airflow was carried out on a special pneumatic installation, which includes a block of nozzles, a compressor with a receiver, a gearbox, an electromagnetic valve and sensors for measuring pressure. The specified installation made it possible to maintain a stable pressure in the pipelines throughout the entire blowing cycle with a coolant uniform in temperature and speed of movement.

During blowing, the temperature at a point on the surface of the samples was controlled and recorded by two-mode near infrared pyrometers with a measurement range of 550–1100°C, as well as a high-speed pyrometer operating in the middle part of the infrared spectrum, with a measured temperature range of 50–775°C. Based on the fixed values, the dependences of the temperature on the cooling time were plotted.

Certification of the structural state of the samples after heating/cooling was carried out on three thin sections corresponding to the cross sections in three planes. Metallographic studies were carried out on a structural analyzer, which includes an inverted light metallographic microscope, a digital camera, and a software and hardware complex.

As a result of metallographic studies, it was revealed that the formation of a ferrite-bainite structure with a mass fraction of bainite of 46% occurred when samples 1.1 mm thick were cooled by a flat air flow at an angle of 15-30° to the sample with a pressure of 4 atm. For the subsequent computational analysis, the cooling trajectory corresponding to the indicated mode was chosen.

As a result of numerical processing, it was found that the sections of the experimental curve in the temperature ranges (high-temperature and low-temperature), in which there is no phase transformation, are well approximated by the following exponential functions:

T _¥ \u003d 899.1 - edgr (-0D09t) + 22, ° C (3)

T _a \u003d 1420.5 expC - OD 195t) + 22, ° C (4)

The superposition of the real cooling trajectory and curves constructed from exponential dependences makes it possible to determine the temperatures and times of the beginning and end of the phase transformation (t _n , T _n ; tk, T _c ).

Th=668°C, t=2.9 s;

Tk=380°C, tk=12.2 s.

The duration of the transformation t _CR =9.3 s.

The duration of the transformation t _CR =9.3 s.

Further, the computational procedure for determining the specific thermal effect of the phase transformation °Q _pr was organized as follows.

The transformation started at m=0. The calculation was performed with a time step Δτ=0.01 s. The specific heat capacity (C), according to the handbook [5], was obtained from the expression

C \u003d 0D202-7 + 514.5 (5)

At the first step of the calculation, the temperature changes according to the exponential dependence (3) defined above.

T ₁ \u003d ir (- 0 _g 309 (t + Lt)) + T _g (6)

ΔΤ ₁ was determined from expression (7)

The specific thermal effect of the phase transformation for the first calculation step was calculated as (8)

For the i-ro step of the calculation procedure (^..-7,)^(-0.109 dr)+7 _r .(9) (10)

Δ$ί - Q ΔΤ,.(11)

Calculations continued until time t=t _to .

The total specific thermal effect of the transformation (Q^) was determined as

Q„p =(12)

As a result of the implementation of the calculation procedure, we received

Qnp = 118^

Calculation of mass fractions of structural components is carried out by a finite difference scheme

- 5 039568 solution of the problem of thermal conductivity for a medium with internal sources of heat release with a virtual division of the thickness of rolled products into layers using the technological parameters fixed during the rolling of a specific slab. For example, number of layers=mm (500);

time step, c=tau (0.00002);

sheet thickness, m=L (0.0275);

water cooling start time, s=tstart (1);

calculation end time, s=tend (80);

sheet surface temperature, K=T01 (1180);

temperature of the middle of the sheet, K=T00 (1200);

heat capacity, J/(kgχK)=Cv (625);

thermal conductivity, W/(mχK)=Hi (30);

heat transfer coefficient, 1/m=k (600);

ambient temperature, K=Tr (373);

Stefan-Boltzmann constant, W/(m ² χK ⁴ )=Si (0.0000000567);

emissivity=eps(0.7);

steel density, kg / m ³ \u003d th (7600);

transitional period during irrigation with water, c=tpeg1 (0.5);

time of passage through the nozzle, c=t2 (3);

transition period during water evaporation, e=t3 (3);

time to the next nozzle after water evaporation, c=t4 (5);

initial transition period, s=pegiod (0.1).

The calculation procedure is carried out as follows.

Non-dimensionalization and calculation of the used constants.

1. Calculate the constant h using the formula h=1/(mm-1).

2. Save the resulting numerical value.

3. Calculate the thermal diffusivity (aa) using the formula aa=Hi/(Cvxro).

4. Save the resulting numerical value;

5. Calculate the flow coefficients at the boundary during Stefan-Boltzmann cooling (A) using the formula A=SixepsxTr ³ xL/Hi.

6. Save the resulting numerical value.

7. Calculate the flow coefficients at the boundary with water cooling (B) using the formula B=Lxk.

8. Save the resulting numerical value.

9. Calculate the time tper2 using the formula tper2=tper1+t2.

10. Save the resulting numerical value.

11. Calculate the time tper3 using the formula tper3=tper2+t3.

12. Save the resulting numerical value.

13. Calculate the time tper4 using the formula tper4=tper3+t4.

14. Save the resulting numerical value.

15. Calculate the dimensionless time step (tau2) tau2=tauxaa/L ² .

16. Save the resulting numerical value.

17. Define the total time as vrem and assign it the numerical value vrem=0. Layer coordinates.

The sheet is divided into layers by section. Number of layers=mm. Each layer is assigned a number (i). i is an integer from 1 to mm.

1. For each layer, calculate its coordinate in the range from -1 to 1 using the formula xi=-1+2x(i-1)xh.

2. Save the obtained numerical values xi for each layer.

Calculation of the initial temperatures of the layers.

1. For each layer, we calculate its initial dimensionless temperature (T ₀ (i)) using the formula T0(i)=(T01 +(T00-T01)χ (1 -x ² ))/Tr.

2. Save the obtained numerical values T ₀ (i) for each layer.

3. We calculate the initial temperatures of the layers with the dimension according to the formula

T0(i)real=T0(i)xTr.

4. Save the obtained values T ₀ (i) _real .

Calculation of boundary conditions based on the initial temperature distribution.

1. We calculate the numerical value dTdx0 according to the formula dTdx0=(T0(2)-T0(1))/h, where T0(1) is the initial dimensionless temperature of the first (extreme) layer;

T0(2) is the initial dimensionless temperature of the second layer.

2. We save the received value.

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After that, we begin the next sequence of calculation procedures.

Calculation of the temperatures of the inner layers in the next step.

For each layer, except for the extreme ones, i.e. for i in the range from 2 to (mm-1), its dimensionless temperature is calculated at the next time step, i.e. the dimensionless temperature of the layer is calculated after a time equal to tau.

1. Denote the dimensionless layer temperature at the next time step as T1(i).

2. Calculate the numerical values of T1(i) using the formula

T1(i)=T0(i)+tau2x(T0(i+1)+T0(i-1)-2xT0(i))/h ² , where T ₀ (i) is the initial dimensionless temperature of the layer for which calculation;

T ₀ (i+1) and T ₀ (i-1) - initial dimensionless temperatures of neighboring layers.

3. Save the obtained numerical values T1(i) for each layer (except for the extreme ones).

4. We give dimension to the values of T1(i) obtained in Section 2, according to the formula

Τι(ϊ)^=Τι(ϊ)χΤγ.

5. Save the obtained values T ₁ (i) _real for each layer. Calculation of the temperatures of the extreme layers in the next step.

For the extreme layers (i=1 and i=mm), the dimensionless temperatures should be calculated at the next time step, i.e. calculate the dimensionless temperatures of the layers after a time equal to tau.

1. Redefine the total time vrem by the formula vrem=vrem+tau.

2. Check if the total time does not exceed the initial transition period: vrem<period;

3. If the condition in paragraph 2 is met, then go to paragraph 4. If the condition in paragraph 2 is not met, then go to paragraph 7.

4. Calculate the flow at the boundary (Jh) using the formula

Jh=Ax(T0(1) ⁴ -(300/Tr) ⁴ ), where T ₀ (1) is the initial dimensionless temperature of the first (extreme) layer.

5. Calculate the dimensionless temperatures of the outer (T1(1) and T1(mm)) layers at the next step using the formula

T1(1)=T1 (mm)=T 1 (2)-hx (dT dx0+(Jh-dT dx0)x vrem/period), where T1(2) is the previously calculated temperature of the second layer after the time step.

6. Go to step 17.

7. Check if the total time (vrem) does not exceed the water cooling start time (tstart): vrem<tstart.

8. If the condition in item 7 is satisfied, then go to item 9. If it is not fulfilled, then go to step 12.

9. Calculate the flow at the boundary (Jh) using the formula

Jh=Ax(T0(1)4-(300/Tr) ⁴ ), where T0(1) is the initial dimensionless temperature of the first (marginal) layer.

10. Calculate the dimensionless temperatures of the extreme (T1(1) and T1(mm)) layers at the next step using the formula

T1(1)=T1(mm)=T1(2)-hxJh, where T ₁ (2) is the previously calculated temperature of the second layer after the time step.

11. Go to item 17.

12. Determine vrem1 using the formula vrem1=vrem-tstart.

13. Determine tt using the formula tt=vrem1-int(vrem1/tper4)xtper4, where int(vrem1/tper4) is the integer part of the result of dividing vrem1 by tper4 (the number before the comma).

14. Determine in which numerical range tt is located, and in accordance with this, calculate the value of C. If tt<tper1, then calculate C using the formula C=tt/tper1. If tper1<tt<tper2 then C=1. If tper2<tt<tper3, then calculate C using the formula C=1-(tt-tper2)/(tper3-tper2). If tper3<tt<tper4 then C=0.

15. Calculate the flow at the boundary (Jh) using the formula

Jh=Ax(T0(1) ⁴ -(300/Tr) ⁴ )+CxBx(T0(1)-1), where T0(1) is the initial dimensionless temperature of the first (extreme) layer.

16. Calculate the dimensionless temperatures of the extreme (T1(1) and T1(mm)) layers at the next step using the formula

T ₁ (1)=T ₁ (mm)=T ₁ (2)-hxJh, where T1(2) is the previously calculated temperature of the second layer after the time step.

17. Save the obtained numerical values of T1(1) and T1(mm).

18. We give dimension to the values T1(1) and T1(mm), obtained in item 16, according to the formula T1(i)real=T1(i)xTr.

19. Save the obtained values T1(i) _real for both layers.

W/T parameter calculation.

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1. For layers i=1; i=int(mm/2); i=int(mm/4); i=int(mm/8), where int(mm/...) is the integer part of the division result, you need to calculate the value of the W/T parameter using the formula

W^i)=((T0(i)-T1^

2. The obtained W/T(i) values for the four layers must be saved. For any moment of decomposition of austenite, the relation Myi=1-MnPi (13) is valid where M _γi is the current mass fraction of austenite from 1 at the beginning of the transformation to 0.03 at the end of the transformation;

M _PRi - current mass fraction of conversion products

MnPi =M( _{f + p} ).+ MbV!+ Mbn! + Mm! (14) where М( _Ф+П ) _i - current mass fraction of ferrite and pearlite;

M _BVi - current mass fraction of upper bainite;

M _BNi - current mass fraction of lower bainite;

M _Mi - current mass fraction of martensite.

We calculate Ti, which is the temperature to which the metal has cooled in Δt=0.01 s.

T| - To-{Voxni * 0.01 С ) (15) where V _ОХЛi is the cooling rate value corresponding to the temperature value T ₀ .

J.

We determine the parameter Wi - the current power of the heat flow, x ™ * Wi = Voxni x Ci, (16) where ci is the current specific heat

Cj \u003d 0.122022 x Tj * 514.5 (17)

We determine the current value of the Wi/Ti parameter by dividing the current heat flux power by the current temperature.

We determine in which of the intervals the numerical value of Wi/Ti falls. Below are the intervals

their names and their relevant temperature ranges. Interval name Interval Temperature range Isothermal 0 < Wi / Ti < 1 525°C - 25°C F + P 1 Wi / Ti < 50 675°C - 200.01°C F + P + BV £50 Wi / Ti < 60 675°C - 200.01°C BV 60^Wi/Ti< 105 700°C - 200.01°C BV + BN 105^ Wi/Ti <305 700°C - 200.01°C BN + M 305 < Wi / Ti < 455 700°C - 200.01°C M 455 < Wi / Ti 200°C - 25°C

The mass fraction of ferrite and pearlite formed is calculated by the formula

ΔΜ _{(f + p)} . - 0.0000593 x (1 - Μφ + p - Mbv - Mbn - Mm) x Wi / Ti and S) where Wi, Ti - values taken from the corresponding calculation;

M _F+P is the total mass fraction of ferrite and pearlite that has formed up to this point;

MBV is the total mass fraction of upper bainite formed up to this point;

MBN is the total mass fraction of lower bainite formed up to this point;

MM is the total mass fraction of martensite formed up to this point.

After calculating the mass fraction of ferrite and pearlite formed at this step, the thermal effect of transformation should be taken into account. In our case, this means calculating the temperature to which the metal is heated after cooling as a result of the transformation. To do this, we calculate the temperature after heating according to the formula

Trazogr \u003d Ti + (140 x ΔM (φ ₊ p) | / Ci) (19) where Ti is the value taken from the previous calculation;

ΔМ( _Ф+П ) _i is the mass fraction of ferrite and pearlite calculated at this step in paragraph 1;

ci is the specific heat capacity calculated by formula (17).

The resulting mass fraction must be added to the already obtained mass fractions of ferrite and pearlite (if any have formed) The resulting mass fraction must be summed up with the mass fractions already obtained of ferrite, pearlite, upper bainite, lower bainite, martensite (if any have already formed) and save this value.

The mass fraction of the formed upper bainite is calculated by the formula

ΔΜββϊ = 0.0000843 x (1 - Μφ ₊ p - Mbw - Mbn - Mm) x Wi / Ti (20) where Wi, Ti are the values taken from the corresponding calculation,

M _F+P is the total mass fraction of ferrite and pearlite that has formed up to this point;

M _BV is the total mass fraction of upper bainite formed up to this point;

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MBN is the total mass fraction of lower bainite formed up to this point;

M _M is the total mass fraction of martensite formed up to this point.

After calculating the mass fraction of upper bainite formed at this step, the thermal effect of transformation should be taken into account. In our case, this means calculating the temperature to which the metal is heated after cooling as a result of the transformation. To do this, we calculate the temperature after heating according to the formula

Trazogr ⁼ Ti + (115 ^χ ΔΜββϊ / Cl) (21) where Ti is the value taken from the previous calculation;

ΔM _BVi - calculated at this step in paragraph 1, the mass fraction of the upper bainite;

ci is the specific heat capacity calculated by formula (17).

The mass fraction of the formed lower bainite is calculated by the formula

ΔΜβηι \u003d 0.0000843 * (1 - Mf ₊ l - Mbv - Mbn - Mm) * Wi / Τι (22) where Wi, Ti are the values taken from the corresponding calculation;

M _F+P is the total mass fraction of ferrite and pearlite that has formed up to this point;

M _BV is the total mass fraction of upper bainite formed up to this point;

M _BN is the total mass fraction of lower bainite formed by this moment;

MM is the total mass fraction of martensite formed up to this point.

After calculating the mass fraction of lower bainite formed at this step, the thermal effect of transformation should be taken into account. In our case, this means calculating the temperature to which the metal is heated after cooling as a result of the transformation. To do this, we calculate the temperature after heating according to the formula

Trazogr = Τΐ + (115* ΔΜβΙ-Ιί / Ci) (23) where Ti is the value taken from the previous calculation;

ΔM _BHi - calculated at this step in paragraph 1, the mass fraction of the lower bainite;

ci is the specific heat capacity calculated by formula (17).

The mass fraction of the formed martensite is calculated by the formula

ΔΜμϊ = (1 - Μφ ₊ p - Mbw - Mbn - Mm) x (1 - ((Ti - 25)/270) ³ ) (24) where T _i is the value taken from the previous calculation;

M _F+P is the total mass fraction of ferrite and pearlite that has formed up to this point;

M _BV is the total mass fraction of upper bainite formed up to this point;

M _BN is the total mass fraction of lower bainite formed by this moment;

MM is the total mass fraction of martensite formed up to this point.

After calculating the mass fraction of martensite formed at this step, the thermal effect of transformation should be taken into account. In our case, this means calculating the temperature to which the metal is heated after cooling as a result of the transformation. To do this, we calculate the temperature after heating according to the formula

Trazogr ⁼ Ti + (15 ^x ΔΜΜΪ / Ci) (25) where Ti is the value taken from the previous sequence 1;

_{ΔM Mi} is the mass fraction of martensite calculated at this step in paragraph 1;

c _i - specific heat capacity calculated by formula (17).

The proportion of untransformed austenite is calculated as

Ma \u003d 0.97 - Mf + p - Mbv - Mbn - Mm (26) where M _{F + P} is the total formed mass fraction of ferrite and pearlite;

MBV - total formed mass fraction of upper bainite;

M _BN is the total resulting mass fraction of the lower bainite;

MM is the total formed fraction of martensite.

Example 2

For steel of the chemical composition given in table. 1, first set the kinetics of the phase transformation according to example 1 and experimentally determined the thermal effects of phase transformations.

Table 1

The chemical composition of the pilot melt, wt.%

With Si MP S R SG Ni Xi AI N V Ti Nb As Mo AT sn 0.07 0.07 1.75 0.002 0.005 0.04 0.19 0.15 0.03 0.005 0.002 0.018 0.051 0.001 0.135 0.0003 0.003

Then carried out pilot rolling on the mill hot-rolled two sheets of steel of the specified chemical composition. At the same time, a virtual layer-by-layer division of the rolled product into 50 layers was carried out with a thickness of each layer of ~0.6 mm and the technological production parameters indicated in Table 1 were recorded. 2.

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table 2

Experimental rolling mode parameters

Temperature, end of rolling, °C Sheet surface temperature after UV, °C Roll speed in the UO line, m/s 840 610 2.0

The calculation of the mass fractions of structural components was carried out by a finite-difference scheme for solving the problem of thermal conductivity for a medium with internal sources of heat release with a virtual division of the thickness of rolled products into layers using the technological parameters fixed during the rolling of a particular slab according to the method described above.

As a result of the calculation, according to the measured technological parameters, the following structural state was determined in mass fractions of structural components along the thickness of the roll: ~50% bainite, ~50% ferrite and pearlite.

To assess the reliability of the prediction after rolling along the axial lines of the sheets, 10 samples were cut for structural studies. The cutting positions were evenly distributed along the length of the rolled product. The attestation of the structural state was carried out on three thin sections, corresponding to sections along three planes. Metallographic studies were carried out on a structural analyzer, which includes an inverted light metallographic microscope, a digital camera, and a software and hardware complex.

As a result of metallographic studies, it was revealed that in the averaged data, as a result of real rolling according to the specified mode, a structure with a bainite fraction of -48% was formed. This indicates the adequacy of using the calculation method for determining the structural state in the process of implementing the method for the production of rolled steel.

Example 3

For steel of the chemical composition given in table. 3, the kinetics of the phase transformation was first set according to example 1 and the thermal effects of the phase transformations were determined experimentally.

Table 3

The chemical composition of the pilot melt, wt.%

With Si MP S R SG Ni Xi AI N V Ti Nb As Mo AT sn 0.05 0.07 1.5 0.003 0.003 0.04 0.15 0.12 0.03 0.005 0.004 0.022 0.051 0.001 0.191 0.0003 0.003

Then carried out pilot rolling on the mill hot-rolled two sheets of steel of the specified chemical composition. At the same time, a virtual layer-by-layer division of the rolled product into 50 layers was carried out with a thickness of each layer of ~0.4 mm and the technological production parameters indicated in Table 1 were recorded. 4.

Table 4

Experimental rolling mode parameters

Temperature, end of rolling, °C Sheet surface temperature after UV, °C Roll speed in the UO line, m/s 840 530 1.1

The calculation of the mass fractions of structural components was carried out by a finite-difference scheme for solving the problem of thermal conductivity for a medium with internal sources of heat release with a virtual division of the thickness of rolled products into layers using the technological parameters fixed during the rolling of a particular slab according to the method described above.

As a result of the calculation according to the reduced regime, the following structural state was determined in mass fractions of structural components along the thickness of the roll: ~ 25% of the upper, ~ 70% of the lower bainite and residual austenite.

To assess the reliability of the prediction after rolling along the axial lines of the sheets, 10 samples were cut for structural studies. The cutting positions were evenly distributed along the length of the rolled product. The attestation of the structural state was carried out on three thin sections, corresponding to sections along three planes. Metallographic studies were carried out on a structural analyzer, which includes an inverted light metallographic microscope, a digital camera, and a software and hardware complex.

As a result of metallographic studies, it was revealed that in the averaged data as a result of real rolling according to the indicated mode, a structure was formed with a share of upper bainite ~ 22% and lower bainite ~ 72%, the rest - carbides and nitrides, MA component and residual austenite ~ 6%. This indicates the adequacy of using the calculation method for determining the structural state in the process of implementing the method for the production of rolled steel, subject to prior receipt

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Bibliography.

1. RU 2563911.

2. RU 2457054.

3. RU 2016133849.

4. WO 9818970.

5. Calculation of the heat capacity of low-carbon low-alloy steel when modeling non-isothermal phase transformations / D.A. Ivanov, N.V. Kuvaev, T.V. Kuvaeva // Theory and practice of metallurgy, №1-2, 2010, p. 43-48.

CLAIM

Claims (3)

CLAIM 1. A method for the production of rolled steel, including the smelting of steel of the required chemical composition, its rolling with the fixation of technological parameters and the determination of the structural state of the resulting rolled products depending on the implemented technological parameters, while the temperatures of the layers of rolled steel and the mass fractions of austenite decomposition products in the layers are determined by calculation based on the measured temperature of the surface of the rolled product before the start of cooling, its thickness and the implemented mode of heat removal from the surface of the rolled product, using the finite difference method to solve the problem of heat conduction for a medium with internal sources of heat release with a virtual division of the thickness of the rolled product into layers, while layers are taken as heat release sources rolled products, in which, according to the calculation, austenite decomposition processes occur according to at least one of the types: ferritic (F), ferrite-pearlitic (F + P), pearlitic (P), bainitic (B), martensitic (M), and the type of decomposition is determined by based on temperature layer, and / or cooling rate, and / or specific power of heat removal from the layer, the increase in the mass fraction of austenite decomposition products (3M / 3τ) is determined based on the temperature, and / or cooling rate, and / or specific power of heat removal from the layer and the amount of heat release is calculated by the equation 5Q=3MxQ _P i, where 3Q is the heat released as a result of the decomposition of austenite by any type, kJ; Q _pi is the specific thermal effect of austenite decomposition according to its type, kJ/kg, while before the start of rolling on steel samples of identical chemical composition, the specific thermal effects and equations of the kinetics of austenite decomposition taken into account in the calculation are experimentally determined for the types of austenite decomposition that occur in steel of a given chemical composition . 2. The method according to claim 1, characterized in that the ranges of temperatures and cooling rates corresponding to different types of austenite decomposition are determined experimentally by varying the cooling rates of steel samples of identical chemical composition. 3. The method according to claim 1, characterized in that when determining the kinetics of austenite decomposition, the equation aM/ar=kxM _A x(w/T) is used, where 3M is the increase in the mass fraction of austenite decomposition products; 3τ - time, s; k is an empirical coefficient depending on the type of decomposition of austenite and the chemical composition of the steel, sxkg; M _A - mass fraction of austenite; W - specific power of heat removal from the layer, kW/kg; T - temperature, °C.