JPH02258932A - Method for controlling cooling of hot rolled steel sheet - Google Patents

Abstract

PURPOSE:To control the temp. at the time of the cooling of the hot rolled steel sheet with good responsiveness, high accuracy and efficiency by calculating heat extraction from the temp. rising rate based on the exothermic heat consisting of magnetic transition and the heat of crystal structure transition and the temp. falling rate by a heat transfer. CONSTITUTION:The hot rolled steel sheet is cooled while the temp. control is executed by calculating the heat extraction. The temp. rising rate by the exothermic heat of the transition and the temp. falling rate by the heat transfer in the calculation equation for the temp. falling rate of the cooling are separately handled in the above-mentioned cooling control method. Further, the quantity of exothermic heat Q in the segment Ts to Tf of the crystal structure transition temp. between the transition start point Ts and the transition finish point Tf is the total sum of the quantity exothermic heat Qm occurring in the magnetic transition and the quantity of exothermic heat Qap occurring in the crystal structure transition and the specific heat C is the specific heat Cgamma of the austenite area. The Qm, Qap mentioned above are determined by equations respectively. The specific heat Cgamma is used in the austenite are exclusive of the structure transition section and the specific heat Calpha is used in the ferrite area and the quantity of exothermic heat is made none. The heat extraction quantity is determined by taking the quantity of exothermic heat generated in the section Ts to Tf into consideration and the cooling control of the steel sheet is executed by the above-mentioned method.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a cooling control method for hot-rolled steel sheets, and more specifically, in order to obtain stable material properties over the entire length of hot-rolled steel sheets or between lots, or to obtain desired material properties, the present invention relates to a cooling control method for hot-rolled steel sheets. The present invention relates to a method for cooling while strictly controlling the cooling stop temperature, cooling rate, cooling pattern, winding temperature, etc., according to the target. (Prior Art and Problems to Be Solved) In order to cool a steel plate such as a hot rolled strip under targeted cooling conditions, it is necessary to accurately determine the required amount of cooling water and the water injection valve by temperature calculation. For this purpose, it is a prerequisite that material property values such as specific heat, transformation calorific value, water-cooling heat transfer coefficient, and radiation heat transfer coefficient at each temperature during continuous cooling can be accurately predicted. Conventionally, specific heat and transformation calorific value, which are factors that have a large effect on temperature control accuracy during cooling, have been technically difficult to handle because these physical property values change moment by moment depending on the temperature of the steel plate. . In addition, since the cooling rate of the #I plate did not significantly affect the material quality, there was little need to strictly handle physical property values, and a constant specific heat was often used over the cooling temperature range. However, if a fixed equivalent specific heat is used, the actual specific heat during cooling and the specific heat used in the calculation are different, so it is not possible to predict the actual temperature of the cooled material during cooling. As a result, the heat transfer coefficient, which is a function of the surface temperature of the cooled material, differs, so the cooling stop temperature and coiling temperature deviate from the target values, and the error becomes particularly large for high-strength materials with a large transformation heat value. . Furthermore, it becomes impossible to control the cooling rate during the cooling process. Therefore, with a constant specific heat, it has not been possible to create a uniform material over the entire length of a steel plate or to stably obtain unique properties with a complex structure. In order to save alloys, consolidate steel types, reduce inventory, reduce costs, etc., strong cooling and low-temperature winding are being used in variable cooling. In order to perform such cooling control, it is based on material property values such as specific heat and transformation calorific value at each temperature during continuous cooling. It is necessary to calculate the temperature to determine the amount of heat removed. An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method that can control the temperature during cooling of a hot-rolled steel sheet extremely effectively with good control responsiveness and without impairing control temperature accuracy. . Another object of the present invention is to make it possible to control the coiling temperature on-line extremely easily, to have good control responsiveness, and to be able to extremely effectively control the coiling temperature of hot rolled steel sheets without impairing the control temperature accuracy. The objective is to provide a method for controlling the temperature of (Means for Solving the Problems) In order to achieve the above object, the present inventor has conducted extensive research on various factors and countermeasures when calculating heat removal from a steel plate during cooling control of the steel plate. As a result, we found that by measuring the specific heat of practical steel in an adiabatic, constant-temperature state, the specific heat and transformation calorific value at each temperature in an equilibrium state can be very easily generalized. Furthermore, when applying this fact to the cooling process, by creating a unique model in the temperature range of the structural transformation that occurs during cooling, we can calculate the specific heat of the cooling process and the transformation calorific value so that the temperature during cooling can be accurately controlled. We have succeeded in reflecting the physical property values in the temperature calculation formula, and have hereby accomplished the following invention. That is, the hot-rolled steel sheet cooling control method according to the present invention (hereinafter referred to as "invention 1") calculates heat removal from the steel sheet and cools the steel sheet while controlling the temperature. When determining the amount of heat removed by taking into account the amount of heat generated by transformation between the transformation start point Ts and the end point Tf of the steel, in short: ■ In the calculation formula for the amount of temperature drop, the amount of temperature rise due to heat transfer due to transformation and heat transfer (1) In the structural transformation temperature interval Ts-Tf, the calorific value Q per unit of calorific value and specific heat in that temperature interval is based on the assumption that the amount of temperature drop due to Separate the calorific value Qm due to magnetic transformation and the calorific value Qap due to structural transformation, and let Qw and Qap be the sum determined by the following formula, where the specific heat C is the specific heat Cγ of the austenite region. Tf TQ Tf Here, C'f=a, +a, T Ca =a, +a, T +a4T"+a, T'C4=a
, +a, T+a, T'' Tq: Curing temperature Ae,: γ≠α transformation temperature (equilibrium state) T: Temperature 80~a,: Constant Qap=Qa+Qp Here, Qa=a, +ato[%C] Q P ” 811 + a L x [%C] [%C
]: Carbon content (wt%) ariNa 12 : Constant■ Specific heat Cγ is used in the high temperature range of the austenite state outside the structural transformation zone, and specific heat Cα is used in the temperature range of the ferrite state, and the calorific value is It is characterized by controlling the cooling of the steel plate under the conditions of the above three points (1) to (3). Furthermore, in the current situation where it is not always easy to accurately predict the transformation point in an actual production line or to accurately detect it online, it is not possible to apply it to any type of steel with any cooling pattern. In view of the difficulty,
As a result of further research in order to find a cooling control method that can particularly accurately ensure the coiling temperature for a wide range of component steels with arbitrary cooling patterns, the following present invention was achieved. That is, the method for controlling cooling of a hot rolled steel sheet according to another aspect of the present invention (
Hereinafter referred to as "Invention 2"), when cooling a hot-rolled steel sheet while controlling the temperature by calculating the heat removal of the steel sheet, ■ The amount of temperature rise due to transformation heat generation and the temperature drop due to heat transfer in the temperature calculation formula of the steel sheet. ■ Under continuous cooling, the upper limit T smax = A
e3 and lower limit Tsmin=-250X(%C)+
Until the temperature Ts (calculated value) in the range determined by 550 is reached, the amount of heat removed is calculated as Q=O Cγ==a, +a□T for the calorific value Q and specific heat Cγ. ■ From the temperature Ts onward to the time t until winding),
Regarding the calorific value Q and specific heat Cγ, under the setting conditions of the following formula. t = t t + t z O≦tx/lz≦7 Here, t: Untransformed time t2: Heat generation time Up to 70, the amount of heat removed is determined based on the linear equation, Q=O CY=a , +a1T From t□ onwards, until winding, determine the amount of heat removed based on the following formula: Q a = a, + a□. [%C] Q p ” a z, +a1zC%C] Here, [%C
]: Carbon content (wt%) az Na 1 z : It is characterized by controlling the cooling of the hot rolled steel sheet under the conditions of (1) to (2) above a constant. The present invention will be explained in more detail below. (Function) As described above, the present invention is based on the premise that the temperature rise due to transformation heat generation and the temperature drop due to heat transfer from the surface of the material to be cooled are separated in the steel plate temperature calculation formula. The reason for dividing into calorific value and specific heat in this way is to take into account the temperature rise during cooling in steel, which has a large calorific value per unit. Therefore, according to the present invention, when calculating the temperature of a steel plate, the temperature rise due to transformation heat generation (transformation heat value, specific heat) is added to the conventionally used formula for calculating the temperature drop due to heat transfer.
This allows for more accurate temperature control. In the present invention, as a method for calculating the amount of heat removed from the steel plate and controlling the temperature, two methods are adopted as described below. (1) First method (invention 1) The first method is to convert the specific heat C modeled from the specific heat-temperature curve in the equilibrium state and the transformation heat generation Q (heat generation based on structural transformation and magnetic transformation) from γ→α This is a method in which C and Q are used separately in each temperature range until the start of transformation, during transformation, and after the end of transformation, and are added to the temperature calculation formula. This allows extremely accurate temperature control for specific cooling patterns and several steel types. That is, regarding one calorific value, the calorific value Q within the structural transformation temperature interval (Ts''Tf) is calculated by dividing it into the calorific value Qrm due to magnetic RM and the calorific value Qap due to liiJ1m transformation, and the reason is This is based on the fact that the heat value of transformation can be obtained with extremely high accuracy using a simple function of only the chemical components. By dividing and integrating the transformation zone from moderate Tf to As with the cue temperature T9 as the boundary, the amount of heat generated only by magnetism in the structural transformation zone of any practical steel can be accurately determined. Specifically, it can be determined by a linear equation. Here, C?=a, +a1T Ca =a, +a, T +a4T" +a, T2
O, = a, +a7T +a, T2 T9: Curing temperature As,: γ≠α transformation temperature (equilibrium state S) T: temperature ao''aI!: constant and 5 The calorific value Qap due to tissue transformation is constant temperature state This is the transformation calorific value due to the structural transformation of ferrite transformation and pearlite transformation at Specifically, it can be determined by the following formula: Qap=Qa+Qp Here, QB=a, +a, [%C] QP=att+aig[%C] [%C]: Carbon content (wt%) agNal, : Constant Also, the reason why the specific heat C in the structural transformation zone is set as the specific heat Cγ of the austenite region is that it is inevitably determined when the calorific value is considered as above. In the high temperature range of the outer austenitic state, the specific heat Cγ is used as the specific heat C, and in the temperature range of the ferrite state, the specific heat Cα is used as the specific heat C, and the reason why the calorific value is nil is due to the transformation during the cooling process. This is based on the fact that the physical property value at a certain temperature in a region in which no
Calculation of the temperature drop due to heat transfer can be done in the same way as before, −
An example is shown below. dt ρ・c−h Here, α: Heat transfer coefficient C: Specific heat h: Plate thickness ρ: Density Tw: Water temperature (air temperature in the case of air cooling) T: Temperature t: Time Also, the amount of temperature drop in Invention 1 The calculation formula is:
As mentioned above, the transformation calorific value QT and the specific heat Cj may be taken into consideration in the conventional formula for calculating the amount of temperature drop due to heat transfer. For example, use the following equation. Here, QT: Transformation calorific value (=Qm+Qap) tT: Transformation time Cγ Specific heat of niostenite region Cj: Cα (specific heat of ferrite region) or Cγ (2) Second method (Invention 2) Transformation in actual line Given the current situation where it is not always easy to accurately predict the point or detect it online, it is difficult to apply it to any steel type with an arbitrary cooling pattern. We have developed a cooling control method (second method) that can particularly accurately ensure the coiling temperature for a wide range of steel compositions in patterns. The second method is to control the temperature by calculating the amount of heat removed by the calorific value Q = O1 specific heat CY = a, +a1T until the temperature Ts specified by "%C", and within the time until one winding, After suppressing the untransformed time to tx (7/8) or less, the total calorific value (Q M
+Qa+Qp), the amount of heat removed is determined and the temperature is controlled. As a result, in addition to the effects of the first method. Furthermore, it becomes possible to control the winding temperature on-line extremely easily. First, under continuous cooling, the upper limit T 5rsax =
A ex (γ=transformation temperature in equilibrium state) and lower limit Ts+
The calorific value Q = 0. Specific heat Cγ=
The reason why the temperature is controlled by calculating the heat removal amount as a, +a1T is that in the actual line, the cooling rate of the hot rolled steel sheet and [C]
Even considering the addition of alloying elements other than γ
This is because the α transformation is in a region where it is not completed, so by setting Q=O, Cγ=a, and +a-T in this range, it does not have a large effect. Here, the range of temperature Ts is [%C
] is the range calculated according to the upper limit T 5taa
For Aa=x, for example, [%C]≦0.7
In the case of 65%, Ae, "1115-150°3 [%C
)+216(0,765-(%C))4・tl-273
and if [%C)>0.765%, Ae. =723. Under the condition that the transformation is completed after this Ts until winding, the time t after reaching Ts until winding is
The reason why the untransformed time (1,) is set to 778 or less of t is that, as shown in FIG. 2, if it is longer than that, the dissociation between the actual value and the calculated value becomes significant. This is done by accurately determining the total heat value of transformation (Q M + Qa + Qp) until winding, and then estimating the heat generation time of transformation (t2) as long as possible from Ts until winding to predict the temperature. It means that it is good to be in control. Specifically, until the non-transformation time (ti), Q=O1Cγ
The amount of heat removed is determined as =a0+a-workT. And, from this untransformed time (t□) until you take one roll,
The amount of heat removed is determined using the following formula, and the temperature is controlled. Q=QM+Qa+QP Q/lz (heat generation rate) Cy=a0+a, T However. Here, CT: winding temperature target value QB=a, +a□. [%C] Qp= a xx + a xx (%C) where, [
%C]: Carbon content (wt%) a g”' 812
:Constant Note that in calculating the amount of temperature drop due to heat transfer in the second invention, the calculation of the amount of temperature drop due to heat transfer may be the same as in the conventional method.For example, the following equation exemplified in the description of the first invention may be used. dt ρ・c−h Here, α: Heat transfer coefficient C: Specific heat h: Plate thickness ρ: Density T−2 Water temperature (air temperature in the case of air cooling) T: Temperature for 2 hours Also, the temperature drop in Invention 2 The formula for calculating the amount is
As described above, the transformation calorific value QT and the specific heat cj may be taken into consideration, and for example, the following equation may be used. dt ρ・C, rh tT ρ・C
j Here, QT: Transformation calorific value (0 or Q M + Ta + Qp) tT: Transformation time Cj: CT (specific heat of austenite region) or B, +a1
In other words, in the conventional formula for calculating the amount of temperature drop due to heat transfer, the values under continuous cooling are reflected in the physical property values Cj, QT, and tT. It goes without saying that the present inventions 1 and 2 described above can be applied to both 51-stage cooling (water cooling) and 2-stage cooling (water cooling, air cooling), and in the case of 2-stage cooling, the above-mentioned temperature Regarding the heat transfer coefficient α in the drop calculation formula, in the case of water cooling (αV)
and for air cooling (αair). To explain an example of a specific temperature control method, 1. In the case of the present invention 2, as shown in FIG.
4 outlet temperature T. Calculate this T,4 and target temperature CT
Bank #24 is added when the temperature difference is larger than a predetermined temperature difference (for example, 3° C.). The outlet temperature Tt4 calculated by this bank addition is determined. If the outlet temperature T□′ and the target temperature CT are still larger than the predetermined temperature difference, bank #23 is further added,
Similarly, the outlet temperature T2. Find ′. By adding subsequent water cooling banks until the temperature difference is within a predetermined range, we set up a dynamic water cooling bank that spans the entire length of the coil. Accurate temperature control is possible online.

[Left below]

(Example) Next, an example of the present invention will be shown. This example is an embodiment of the first invention. Two types of test steel A (C50.05vt%, MnS2゜8
0wt%), B(0,05tzt%≦C≦0.80wt
%, Mn≧0.80wt%), an automatic cooling control experiment was conducted on an actual rolling line. At that time, the winding temperature C
The target temperature of T was set at 650°C, and the target temperature of temperature CTM during cooling was set at 500°C. Specifically, in the comparative example, the specific heat was reduced to 0.
The amount of temperature drop was determined with a constant value of 192 and no calorific value. On the other hand, in the example of the present invention, the amount of temperature drop was determined using each of the above-mentioned formulas, taking into consideration the following points. First, the heat generation rate within the structural transformation zone was set constant, and the transformation starting point Ts was calculated using the following formula, which was formulated using a simple relationship among chemical components, residual strain, and γ grain size. Ts=A E, −(a −−)axp(−Cεr)
dγ Here, AE, : 1115-150.3[%C+21
6X (0,765-[%C)4 ・26-273
(KirKaldy's formula) dγ Goniostenite grain size εr: Residual strain a: 1.87
20°C). The values of the coefficients of the specific heat and transformation calorific value calculation formulas incorporated into the temperature calculation formula used in this example are shown below. an: O-140-a□:1.518 X 10-s a,:9.367 299X10-' a, :10.943 a, knee 2.530X10''''! a, : 1.483 X 10-' a, : 4.2 al. Knee 7.37 a1□: 0.0 a1□: 22.86 Table 1 shows the target temperature (target value of winding temperature CT = 650°C, intermediate temperature CTM during cooling) in the automatic cooling control described above.
It shows the accuracy (deviation value) of each actual temperature with respect to the target value of 500℃) 8 From the same table 1 According to the method of the present invention, regardless of the steel type, It can be seen that the intermediate temperature CTM of 1 has also achieved better results with respect to the target value than the comparative example.

[Left below]

Table 1 Target temperature of actual temperature by automatic cooling control Target temperature of CT 2500°C [1] Milk This example is an embodiment of the second invention. Four types of test steel A (C: 0.14 wt%, Mn: 1.0
wt%), B (C: 0.4wt%, Mn: 0.75wt
%1MO:0,2 wt%), C(C:0.65wt
%, Mn: 0.45wt%, C:r: 0.3vt%)
, D (C: 0,90+t%1Mn:0.65tzt%)
Using this, an automatic cooling control experiment was conducted on an actual rolling line. At that time, the actual value of 1 winding temperature is 450 to 680.
℃ range. In the example of the present invention, Ts (temperature range controlled by calculating the temperature as Q=O, CY:aa+a, T) is within the shaded range in FIG. Cooling control was performed in an actual line under conditions in which the ratio of transformation time to time) was 7 or less, and in the comparative example under conditions outside these ranges. Note that the calculation formula used was the formula shown in the explanation of the second invention, and the constants at that time were the values in Example 1. Table 2 shows the difference between the calculated value and the actual value of the winding temperature. From the table, it can be seen that according to the examples of the present invention, the coiling temperature is ensured with high precision for various steel sheets compared to the comparative examples. Furthermore, the accuracy of the winding temperature control is improved compared to the case of the first embodiment.

[Left below]

(Effects of the Invention) As described in detail above, according to the present invention, when controlling the steel plate while controlling the temperature by calculating the heat removal of the steel plate, the transformation calorific value is In addition, in doing so, the calorific value is divided into those due to magnetic transformation and those due to structural transformation, and specific heat is used for each cooling zone, and outside the transformation zone (high temperature of austenite state) Since the amount of heat removed is calculated by assuming that the amount of heat generated in the temperature range (temperature range of ferrite state and temperature range of ferrite state) is negligible, the accuracy of temperature calculation is good, and since the calculation is relatively simple, the responsiveness of control is good, and temperature control accuracy is high. Very good. Further, according to the present invention 2, the temperature T defined in [%C]
Up to s, calorific value Q = O, specific heat Cy = a, +aIT
The amount of heat removed is calculated and the temperature is controlled, and within the time until winding, the untransformed time is kept below t x (7/8),
From then on, by giving the total calorific value (QM + Qa + Qρ) calculated based on the model formula until winding,
Since the temperature is controlled by determining the amount of heat removed, the coiling temperature can be ensured with extremely high accuracy for a wide range of component steels with arbitrary cooling patterns. Therefore, for any steel type, it is possible to obtain a steel plate of uniform material over the entire length of the steel plate, or to stably obtain unique properties, and furthermore, it is possible to control the coiling temperature extremely easily on-line, etc. Remarkable effects can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the temperature range of Ts and [%C], and FIG. 2 is a diagram showing the ratio of untransformed time to the time from Ts to winding. FIG. 3 is a diagram illustrating a specific temperature control method of the second invention. Patent Applicant: Kobe Steel, Ltd. Patent Attorney Takashi Nakamura Figure 2 Figure 1 Unai Ruro Kusuma/lτ117 Figure 3 C Quantity [tη]

Claims (2)

[Claims] (1) When cooling a hot-rolled steel sheet while controlling the temperature by calculating the heat removal of the steel sheet, [1] In the calculation formula for the amount of temperature drop, the amount of temperature increase due to transformation heat generation and the amount of temperature decrease due to heat transfer are separated. [2] In the structure transformation temperature range Ts to Tf, regarding the heat value and specific heat in that temperature range, the heat value Q is the heat value Qm due to magnetic transformation and the heat value due to textural transformation. Qa
p, and Qm and Qap are the sums obtained by the following formula, where the specific heat C is the specific heat Cγ of the austenite region. 2+a_5T^
3C_4=a_6+a_7T+a_8T^2 Tq: Curie temperature Ae_3: γ■α transformation temperature (equilibrium state) T: Temperature a_0 to a_8: Constant Qap: Qa+Qp Here, Qa=a_9+a_1_0×[%C] Qp: a
_1_1+a_1_2×[%C] [%C]: Carbon content (wt%) a_9 to a_1_2: Constant [3] Specific heat Cγ is used in the high temperature range of the austenite state outside the structural transformation zone, and in the temperature range of the ferrite state The specific heat Cα is used, and the calorific value is assumed to be zero. Considering the transformation calorific value generated between the transformation start point Ts and the transformation end point Tf of the steel, according to the three points [1] to [3] of A method for controlling cooling of a steel plate, characterized by determining the amount of heat removed and controlling the cooling of the steel plate. (2) When cooling a hot-rolled steel plate while controlling the temperature by calculating heat removal from the steel plate, [1] Separate the amount of temperature increase due to transformation heat generation and the amount of temperature decrease due to heat transfer in the temperature calculation formula for the steel plate. [2] Under continuous cooling, upper limit Tsmax=Ae_
3 and the lower limit Tsmin=-250×[%C]+550, the amount of heat removed is calculated as Q=0 Cγ=a_0+a_1T for the calorific value Q and specific heat Cγ. , [3] After the temperature Ts, during the time t until winding,
Regarding the calorific value Q and specific heat Cγ, under the setting conditions of the following formula, t = t_1 + t_2 0≦t_1/t_2≦7 Here, t_1: untransformed time t_2: heat generation time up to t_1, under the following formula Determine the amount of heat removed, Q=0 Cγ=a_0+a_1T From t_1 until winding, determine the amount of heat removed based on the following formula, Q=Q_M+Qa+Qp Q/t_2 (heating rate) Cγ: a_0+a_1T However, ▲ Formula, There are chemical formulas, tables, etc. ▼ Here, CT: Coiling temperature target value Qa = a_9 + a_1_0 [%C] Qp = a_1_1 + a_1_2 [%C] Here, [%C]: Carbon content (wt%) a_9 to a_1_2: A method for controlling cooling of a hot-rolled steel sheet, comprising controlling the cooling of the hot-rolled steel sheet under the conditions [1] to [3] above a constant.