JPS6218605B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement in a temperature control method for a continuous heat treatment furnace. In general, in heat treatment by quenching and tempering, the quality after the heat treatment (mainly mechanical test values) is determined by the tempering heat treatment if the quenching is properly performed. Therefore, in a general heating furnace, the main issues are extraction temperature, fuel consumption, etc., but in a tempering heat treatment furnace, if the material is the same and the quenching is the same, the problem will depend on the temperature of the material to be treated and the residence time. Since its mechanical properties, rough structure, etc. are determined, the above-mentioned temperature and time become issues. Incidentally, the steel tempering process can be considered as a solid phase reaction in which elementary processes such as precipitation and agglomeration of carbides, rearrangement and annihilation of dislocations occur singly or in combination. Therefore, the degree of tempering can be understood from the relationship with the diffusion speed of solid phase reaction. An example of this is as follows. The diffusion rate γ of a solid phase reaction is expressed as γ=Aexp(−Q/RT) (1) from the Arnius equation. Here, T: temperature (〓) Q, A: constant determined by composition (Q is also called activation energy, cal/mol) R: gas constant. Since γ∝1/t (t: time), the above equation (1) can be transformed into the following equation. 1/t=A'exp(-Q/RT) However, A' is a constant. Furthermore, by taking the logarithm of both sides of the above equation and transforming it, the following equation is obtained. Q/R・loge=T(logt+logA′) Here, if Q/R・loge=M and logA′=C, then M=T(logt+C) (2) is obtained. The degree of tempering can be expressed using this M as a parameter. Other forms of parameters expressing the degree of tempering are possible, but in any case, since the tempering process is considered to be a diffusion reaction within the solid phase (when observed macroscopically), it is a function of temperature and time. Become. Therefore, in tempering steel of the same composition, if the characteristics are understood using the above parameters, the temperature and residence time of the material to be treated can be determined, and the prescribed heat treatment can be carried out. . However, whether this type of continuous heat treatment is quenching, tempering, normalizing, etc., the heat pattern may be changed midway through. For example, in the case of in-line heat treatment of long steel pipes, billets, etc., the residence time may change due to troubles in the pre-process and post-process, or changes in the dimensions or material of the treated material. In particular, in a walking beam type furnace, the conveyance speed of each belt is the same, so it is extremely difficult to perform heat treatment under the conditions specified above, and as a result, mechanical test values may deviate. Ta. In order to prevent such deviations in mechanical test values, heat treatment can be performed, for example, to lower the processing temperature and extend the furnace residence time in order to absorb variations in furnace residence time. The drawback was that efficiency was significantly reduced. On the other hand, in an upset tube, the heat capacity of the upset part is different from that of the tube body, and therefore, in order to heat the upset part and the tube body to the same temperature in the tempering furnace, the residence time in the tempering furnace, etc. In consideration of this, a method is generally used in which the upset part is heated in advance with a preheater and then placed in a tempering furnace to be heated. However, as mentioned above, if the residence time of the tempering furnace changes, the heating by the preheater must be adjusted appropriately according to the residence time of the material to be treated, or the phenomenon of insufficient preheating or excessive preheating will occur. The temperature of the material itself became uneven, sometimes causing mechanical test values to deviate. The present invention was invented in view of the above-mentioned situation, and consists of: (i) determining the target cumulative tempering parameter M (this parameter M (described later)), calculate the cumulative tempering parameter M at the furnace charging end necessary to obtain the temperature, and then convert this parameter M into a temperature so that the upset part reaches the above temperature at the furnace charging end. (ii) Next, in the furnace, the M values of the upset part and the pipe body part are tracked simultaneously, and the average cumulative value of the material to be treated is determined by keeping a certain relationship between the two M values. Determine the tempering parameter M, inversely calculate the parameter M necessary to obtain the target cumulative tempering parameter M at any point in the furnace at the current furnace temperature, and calculate the parameter M resulting from this inverse calculation and the above-mentioned to-be-processed parameter M. By comparing the average tracking parameter M of the material and, if the difference exceeds a predetermined value, controlling the furnace temperature by changing the temperature setting of each zone one unit at a time, The purpose of the present invention is to provide a temperature control method for continuous heat treatment that makes it possible to obtain a cumulative tempering parameter M within an allowable range in all parts, thereby stabilizing quality and improving heat treatment efficiency. Before explaining the embodiments of the present invention, the concept of the cumulative tempering parameter will first be clarified. The cumulative tempering parameter is a parameter that normalizes the temperature history of the steel pipe to a certain temperature, is a function of the temperature of the steel pipe and the residence time, and is a parameter that represents the degree of tempering. In clarifying the concept of this parameter, first the relationship between the amount of heat input and the tube body portion is determined, and then the relationship between this temperature and the above-mentioned parameters is determined to clarify its properties. (i) Relationship between heat input and tube temperature; Tube heat input Q _n (Kcal/m ² hr) and tube temperature θ _n
The following relationship holds true between . Q _n =4.88φcgm {(θ _g +273/100) ⁴ − (θ _n +273/100) ⁴ }+α _n × (θ _g −θ _n ) (3) However, φcgm; radiant heat absorption rate α _n ; convective heat Transfer coefficient θ _g ; ambient temperature (°C); θ _n ; pipe body temperature (°C). It is assumed that φcgm and α _n include terms due to the shape of the tube. Initially, θ _n is actually measured by a temperature detector at the charging end of the furnace, but thereafter it is an estimated value. θ _g is measured by a temperature detector provided in each zone. The increase in heat content ΔH _n (Kcal/Kg) is determined by the following formula. ΔH _n =SΔTQ _n /W=πd _n ΔTQ _n /ρπt _n (
d _n −t _n ) = d _n ΔTQ _n /ρt _n (d _n −t _n ) (4) However, S _n : Surface area of tube body (m ² ) ΔT : Calculation timing (division time) W : Weight of tube (Kg) d _n ; Pipe diameter (m) t _n ; Pipe wall thickness (m) ρ; Specific gravity (Kg/m ³ ). Therefore, if the heat content of the tube at time t _i is H _ni and the increase in heat content during the division time ΔT is ΔH _ni , then
The heat content H _ni+1 of the tube at time t _i+1 (after ΔT time) is determined by the following equation. H _ni+1 = H _ni +ΔH _ni (5) And the tube temperature θ _ni+1 is obtained as a function of the tube heat content H ni ₊₁ as θ _ni+1 = (H _ni+1 ) (6) It will be done. The relationship between H _ni+1 and θ _ni+1 may be stored in a table in advance. As described above, the heat content of the tube is sequentially determined at every predetermined time ΔT, and then the tube temperature is determined using the conversion table and the tube temperature is tracked. Note that since φcgm and α _n are not constant values and change depending on the temperature, they are treated as functions of θ _g and θ _n for each band and are stored in a table. (ii) Relationship between tube temperature and cumulative tempering parameter; The following relationship holds between tube temperature and cumulative tempering parameter. Paying attention to the above equation (9), t _i means the time during which the tube body temperature was θ _ni . Therefore, if the calculation timing (division time) is the same, t ₁ = t ₂ =...=t _i
becomes. next,

The term [Formula] is the product of the tube temperature and time until the tube temperature reaches θ _ni-1 , which is converted into the time when the tube temperature is θ _ni . Therefore, the temperature history of the tube body can be expressed as temperature θ _ni
It can be said that it has been standardized. For this reason, in this specification, in order to distinguish the parameters according to the present invention from the known parameters of the same kind,
It is called the cumulative tempering parameter M. The process of determining this parameter at predetermined time intervals is called M-value tracking. Now that the concept of the cumulative tempering parameter M has become clear, embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a conceptual explanatory diagram of a continuous heating furnace according to an embodiment of the present invention. In the figure, 10 is a preheater. 2
0 is a continuous heating furnace, which is composed of a first heating zone 21, a second heating zone 22, and a soaking zone 23. 24 is a temperature detector that detects the temperature at the charging end of the material to be processed, 21a to 21c are temperature detectors that detect the ambient temperature in the first heating zone, and 22a to 22c are ambient temperatures in the second heating zone. temperature detector for detecting, 23
Temperature detectors a to 23c detect the atmospheric temperature in the soaking zone, and temperature detector 25 detects the temperature on the extraction side of the material to be treated. For example, thermocouples are used as these temperature detectors. FIG. 2 is a block diagram of a control system for a continuous heat treatment furnace according to this embodiment. 10 in the figure is a preheater;
20 is a continuous heating furnace (tempering furnace), 30 is a control device that controls the preheater 10, 40 is a control device that controls combustion in each zone of the continuous heating furnace, and 50 is a feed rate of the material to be treated, that is, a beam speed 60 is an arithmetic unit (electronic computer), 70 is a keyboard, 80 is a display, and 90 is a typewriter. Here, the input data given to the arithmetic device 60 by the keyboard 70 is as follows. Dimensions (dimensions of the upset part of the steel pipe, dimensions of the pipe body, etc.) Steel type (initial conditions such as composition, quenching method, etc.) Target cumulative tempering parameters and range Extraction pitch and extraction schedule Control numbers (serial number, lot number, number of pipes, etc.) ) In addition, it goes without saying that tables of physical property values, characteristic values, etc. are provided and stored in advance.
Physical property values include heat content H (Kcal/Kg), specific gravity ρ
(Kg/m ³ ), thermal conductivity Kd (Kcal/mhr℃), specific heat C
(Kcal/Kg℃), a conversion table between heat content and temperature is required. The characteristic value is the overall heat transfer coefficient φ
cgm, convective heat transfer coefficient α _n, etc. The display 80 displays control contents, instruction contents, etc. The typewriter 90 outputs control results and the like. An outline of the method according to this embodiment will be described below, and then a method of controlling the preheater 10 and the tempering furnace 20 will be described respectively. (1) Outline of control method The calculation device 60 determines the heating temperature of each zone of the tempering furnace 20 in advance based on the dimensions of the steel pipe, target cumulative tempering parameters, input data such as extraction pitch, physical property values, characteristic values, etc. Control device 40 and control device 5
0 is controlled based on the heating temperature, each zone of the tempering furnace 20 is heated to a predetermined temperature, and the material to be processed is transferred. The target cumulative tempering parameters of the material to be treated on the extraction side of the tempering furnace 20 are predetermined, and the residence time can also be predicted. In addition, tempering furnace 2
The heating temperature in each zone of 0 can also be predicted. Since the cumulative tempering parameter is a function of residence time and heating temperature (furnace atmosphere temperature), the predetermined time (ΔT) is set for the extraction side of the tempering furnace 20 based on these.
The cumulative tempering parameter of the foreground upset part is determined by inverse calculation, then the cumulative tempering parameter of a predetermined time (ΔT) before is determined by inverse calculation using the cumulative tempering parameter as a reference, and so on. The tempering parameters are determined, and finally the cumulative tempering parameters of the upset portion at the charging end of the tempering furnace 20 are determined. Then convert this cumulative tempering parameter into temperature,
Taking into account the natural cooling between the preheater 10 and the tempering furnace 20, the temperature of the upset section on the outlet side of the preheater is determined. The preheater 10 heats the upset part after quenching to the above-mentioned temperature. When the material to be treated is charged into the tempering furnace 20, the tempering furnace 20 is controlled as follows to heat the material to be treated. The arithmetic device 60 provides the control device 40 with a furnace temperature instruction signal based on a predetermined heat pattern. The control device 40 controls the temperature of each zone 21, 22, 23 of the tempering furnace based on the signal. Temperature detectors 24, 21a-21c, 22a-22
c, 23a to 23c, and 25 each detect the temperature, and the arithmetic unit 60 estimates the temperature of the upset portion and the tube body portion from the detected temperature. Then, based on this temperature and residence time, the M value of the upset part and the pipe body part is tracked almost continuously at the same time, and the cumulative tempering parameter M of the material to be treated is determined by establishing a certain relationship between these two M values. At an arbitrary point (plurality) in the furnace, the parameter M at the point necessary to obtain the target cumulative tempering parameter M at the current furnace temperature is calculated inversely, and the parameter M obtained by this inverse calculation and the above The tracking parameter M of the material to be treated is compared, and if the difference exceeds a certain value, the heat pattern, that is, the furnace temperature setting is changed one unit at a time. The control device 40 controls the furnace temperature of each zone 21, 22, and 23 based on this furnace temperature setting so that the material to be treated obtains the value of the target cumulative tempering parameter M. Then, the cumulative tempering parameters of the upset part and the tube body part are compared to see whether they are within the allowable range with respect to the target cumulative tempering parameter M. The two are differentiated by separating them from the material to be treated or marking them. In this embodiment, since the material to be processed is transported by a walking beam, it is assumed that the speed within each band 21, 22, 23 by the control device 50 is kept constant. (2) Preheater temperature control method Before explaining the preheater temperature control method,
The relationship between the amount of heat input and the cumulative tempering parameter of the upset section will be clarified. These relationships are the same as those in (3) to (5) above, only the coefficients are different. The following relationship holds true between the amount of heat input Q _t and the temperature θ _t of the upset section. Q _t =4.88φ _cgt {(θ _g +273/100) ⁴ − (θ _t +273/100) ⁴ }+α _t (θ _g −θ _t ) (10) However, φ _cgt ; radiant heat absorption rate θ _g ; atmosphere Temperature (°C) α _t : Convection heat transfer coefficient. Note that φ _cgt and α _t include terms (coefficients) depending on the shape of the pipe. The increase in heat content ΔH _t (Kcal/Kg) is determined by the following formula. ΔH _t =D _t ΔT Q _t /ρT _t (D _t −T _t )(11
) However, ΔT is the calculation timing, for example, 100 seconds, D _t is the diameter of the upset part (m), T _t is the wall thickness of the upset part (m), and ρ is the specific gravity (Kg/m ² ). Note that although the above-mentioned upset part indicates the top part, the same applies to the bottom part, and the related dimensions may be interchanged. The shapes of the top part T and bottom part B of the upset part are shown in FIG. Therefore, the heat content of the upset part at time t _i is H _t (i), and the increase in heat content during the division time ΔT is ΔH _t (i)
Then, the heat content H _{t (i+1)} of the upset part at time t _{i +1} can be obtained from the following equation. H _t(i+1) = H _t (i) + ΔH _t (i) (12) And the temperature θ _t(i+1) of the upset part is θ _t(i+1) = (H _{t(i+ 1)} ) (13) It is obtained as a function of the heat content of the upset part H _t(i+1) . The relationship between H _t and θ _t may be made into a table and stored in a ROM or the like in advance. The relationship between the upset part temperature θ _t and the cumulative tempering parameter M _t is as follows. Therefore, from the above equations (10) to (16), it is possible to understand the relationship between the heat input amount Q _t of the upset portion and the cumulative tempering parameter M _t . Next, in explaining the method for determining the heating temperature of the preheater, we will discuss (a) calculation of the temperature of the upset section at the charging end of the tempering furnace, and (b) heating temperature of the preheater. I will explain it separately. (a) Calculating the temperature of the upset section at the charging end of the tempering furnace Since the residence time can be predicted, this residence time can be used to calculate the cumulative tempering parameter of the upset section from the extraction end to the charging end of the furnace. is obtained by inverse operation.
This calculation follows the steps below. First, inverse calculation is performed using the above equations (10) to (16) with ΔT=constant (here, 10 seconds). For example, the fourth
As shown in the figure, the target cumulative tempering parameter M _pi
is known, so M _pi-1 ΔT time before this
seek. Needless to say, the furnace temperature is based on a predetermined heat pattern.
Next, M _{pi- 2 at a time ΔT before M pi} -1 is determined. In this way, the cumulative tempering parameter of the upset portion is determined by inverse calculation, and finally the cumulative tempering parameter M at the charging end of the tempering furnace 20 is determined.
Find up to _e . Then, the cumulative tempering parameter M _e is converted into a temperature θ _e using, for example, the above equation (16). (b) Heating temperature in the preheater The heating temperature in the preheater and the initial temperature θ _e at the time of charging into the furnace have the following relationship in consideration of natural heat radiation. θ _e = θ + (θ _p - θ) e ^-mtc (17) However, θ: Atmospheric temperature (℃) θ _p : Preheating temperature (℃) t _c : Time from preheater exit side to furnace charging (Hr) m=αS/CW α; heat transfer coefficient S; surface area (m ² ) W; weight (Kg) C; average specific heat (Kcal/Kg°C). Therefore, since θ _e is calculated by calculation in (a) above, the preheating temperature θ can be determined by equation (17) above.
_p is required. Therefore, the preheater 10 heats the upset portion to this temperature θ _p . (3) Temperature control of tempering furnace The material to be treated heated by the preheater is charged into the tempering furnace, where M-value tracking is performed to continuously determine cumulative tempering parameters. This M
Value tracking is performed using the M values M _t and M _b of the upset part.
and the M value of the tube body are measured at the same time. Note that M _t is the M value of the top portion of the upset portion, and M _b is the M value of the bottom portion of the upset portion. M _ti and M _bi are determined by the above equation (16), and M _ni is obtained by the above equation (9). The calculation time interval ΔT is about 10 seconds. The average M value M _ai of the upset section is M _ai = a ₁ M _ti + a ₂ M _bi /2 (18), and the average M value M _i of the upset tube is M _i = a ₃ M _ni + a ₄ M _ai / 2 (19). Here a ₁ to a ₄ are penalty coefficients,
For example, a ₁ =a ₂ =a ₃ =a ₄ =1.
Since the material to be treated satisfies the mechanical test value if it finally falls within the predetermined range of M values, the above-mentioned coefficients may be determined as appropriate depending on where the emphasis of the control object is placed. Next, a description will be given of changing the heat pattern of the tempering furnace based on this average M value M _i . The heat pattern is determined so that the temperature of the tube body of the quenching furnace reaches the target temperature with a certain tolerance, but the furnace operation conditions may be changed by changing the target temperature, changing the extraction pitch, stopping the operation, etc. If changed, the heat pattern must be changed as necessary. Since the target cumulative tempering parameter M _p value is initially set here, the heat pattern can be changed to obtain the target M value. Therefore, in order to obtain the target M value at any point in the furnace at the current furnace temperature, the M value at that point (target value at that point) is calculated by inverse calculation, and this M value and the tracking M value ( If the difference exceeds a certain value, the target M value is obtained by changing the furnace temperature setting one unit at a time. This will be explained in detail by dividing it into the following items. (b) The check points (points where the target M value and the current M value at the relevant point are compared) are as shown in the following table in this embodiment.

[Table] Note that M _no indicates the M value (current value) at point t _no , and M′ _no indicates the M value obtained by inverse calculation at point t _no . The method of calculating M′ _no will be described later. (b) Estimated residence time in the furnace Taking into consideration the work schedule, calculate the expected residence time for the materials already in the furnace. Of course, if there is downtime, it will be added. (c) Method of calculating M value by inverse calculation Using the above residence time, the M value up to the extraction end check point is inversely calculated. This calculation follows the steps below. First, inverse calculation is performed with ΔT=constant (here, 10 seconds). Needless to say, this inverse operation is based on the above-mentioned equation (9), etc. As shown in FIG. 5, since the final target value M _pi is known, M
Find _pi-1 . Since the calculation is performed on the assumption that the furnace temperature has not changed, M′ _pi-1 to be calculated does not match M _pi-1 . Therefore, the M value after ΔT time from M _pi-1 is calculated. The difference ΔM _pi between this calculated value M′ _pi and M _pi
(=M' _pi - _{M pi} ), and then M' pi- ₁ is found by M' _pi-1 = M _pi-1 - ΔM _pi . In this way, calculations are performed sequentially from the extraction end to the relevant checkpoint. Then, the difference between the actual M value and the M' value obtained by the inverse calculation is determined. If the actual value (current value) at check point t _no is M _no and the inversely calculated value is M' _no , then the difference ΔM _no is expressed by the following formula: ΔM _no = M _no −M' _no
(20) (d) Furnace temperature control using M value Since the temperature distribution in each zone is constant, the M value calculation deviation ΔM _no for each check point must be aggregated for each zone. M value calculation deviation ΔM ₁ for each furnace,
ΔM ₂ and ΔM ₃ are determined by the following equations. ΔM ₁ =ΔM ₁₁ k ₁ +ΔM ₁₂ k ₂ +ΔM ₁₃ k ₃ /
k ₁ + k ₂ + k ₃ (21) ΔM ₂ = ΔM ₂₁ l ₁ + ΔM ₂₂ l ₂ + ΔM ₂₃ l ₃ /
l ₁ + l ₂ + l ₃ (22) ΔM ₃ = ΔM ₃₁ m ₁ + ΔM ₃₂ m ₂ + ΔM ₃₃ m ₃ /
m ₁ + m ₂ + m ₃ (23) Here, ki (i = 1 to 3), li (i = 1 to 3),
mi (i=1 to 3) is a weighting coefficient. This is determined by the structure of the furnace, which point to focus on, etc. For example, the coefficients k ₁ =3, k ₂ =2, and k ₁ =1 can be set. If the ΔM value of each band is found, the corresponding ΔM
Based on the value, the temperature of each zone is controlled as follows, for example. (i) If ΔM exceeds ±x, change the temperature setting to ±5℃
change. (ii) If ΔM exceeds ±y, adjust the temperature setting by ±10℃.
change. As described above, the set temperature is changed in units of temperature (5°C, 10°C, etc.) without finding the optimum value. (e) Changing the extraction pitch If the predetermined M value cannot be maintained even if the furnace temperature is changed to the upper or lower limit, an alarm will be issued to that effect. You may change the extraction pitch setting and recalculate. (f) Furnace temperature response speed used in calculations In M value calculations based on changes in furnace temperature conditions, the heat capacity of the furnace is relatively large, so the difference between the set furnace temperature and the actual furnace temperature is within a certain range, for example ±5°C. Until it is within this range, it is necessary to change the set furnace temperature to a variable furnace temperature that takes into account the response of the furnace temperature. Therefore, in order to obtain variable furnace temperature, it is necessary to store the following table in advance. (i) Furnace time constant when temperature rises (ii) Furnace time constant when temperature falls As is clear from the above explanation, according to the method of the present invention, the heat treatment in the tempering furnace and the heat treatment in the preheater are The matching is achieved, and the heating in the preheater is appropriate. Also, during heating in the tempering furnace, the M value of the tube body and upset part is tracked at the same time to understand the degree of tempering. Since the entire tube has a predetermined M value, it is possible to stabilize the quality without causing deviations from mechanical test values, and it is also possible to improve heat treatment efficiency. Further, if a predetermined M value is not obtained, it is determined that the member is outside the mechanical test value without performing a mechanical test.

[Brief explanation of the drawing]

FIG. 1 is a conceptual explanatory diagram of a continuous heat treatment furnace according to an embodiment of the present invention, FIG. 2 is a block diagram of a control system for the continuous heat treatment furnace according to the above embodiment, and FIG. FIG. 4 is an explanatory diagram of a method of inversely calculating the cumulative tempering parameter M of the upset part at the furnace charging end, and FIG. 5 is an explanatory diagram of a method of calculating the cumulative tempering parameter M by inversely calculating. 10... Preheater, 20... Continuous heating furnace, 30,
40, 50...Control device, 60...Arithmetic device, 7
0...Keyboard, 80...Display, 90
...Typewriter.

Claims (1)

[Claims] 1. Target cumulative tempering parameter M at the extraction end of the furnace.
Calculate the cumulative tempering parameter M of the upset part of the steel pipe at the furnace charging end necessary to obtain
This parameter M is converted into a temperature, and then the heating temperature required for the preheater is calculated based on the temperature and the natural cooling between the preheater and the furnace charging end, and the preheater heats the upset section at this heating temperature. Then, in the tempering furnace, track the M value of the upset part and the tube body part at the same time, grasp the two in a certain relationship, calculate the average M value, and calculate the current state at any point in the furnace. The parameter M required to obtain the target cumulative tempering parameter M under the furnace temperature of
The parameter M obtained by the inverse calculation is compared with the average M value, and if the difference exceeds a predetermined value, the temperature setting of each zone is changed by one unit. Temperature control method for heat treatment furnace.