JP6870399B2 - Steel sheet manufacturing method and heat treatment equipment - Google Patents

Description

The present invention relates to a method for manufacturing a steel sheet and a heat treatment facility. In particular, it relates to steel sheets used for shipbuilding, construction, automobiles, industrial machinery, and the like.

Thick steel sheets or thin steel sheets often undergo a process called heat treatment that satisfies predetermined heating and cooling conditions after rolling and forming, and this is for the purpose of adjusting materials such as residual stress removal and structure control. Heat treatment is a general term for annealing (annealing), tempering, quenching, etc., but the heat treatment handled by the present invention at least undergoes phase transformation accompanied by volume change (dimensional change) due to heating and cooling. The heat treatment process is performed. Unless otherwise specified, the "heat treatment" referred to in the present invention is, specifically, a heat treatment in which austenite is _{heated to Acc1} or higher, which is the temperature at which austenite begins to be generated during heating. Since _Ac1 changes depending on the heating rate, in other words, it can be said to be a heat treatment including a heating step of producing austenite in at least a part of the structure.

In the heat treatment process, the desired material is obtained by controlling the heating temperature and cooling rate, but depending on the heat treatment method, the shape and dimensions may change due to the generation of undesired residual stress, resulting in disqualification. There is.

To solve such a problem, for example, Patent Document 1 describes a method of performing heat treatment while correcting the shape by a pressing roll, and responds to a shape change such as warpage of a plate generated by the heat treatment. There is.

Further, Patent Document 2 discloses a method of performing press straightening before heat treatment.

However, although these methods can cope with shape changes due to deformation such as warping, bending, and twisting of the plate due to heat treatment, they are only corrections, so the plate width and thickness due to phase transformation during heating and cooling, It was not possible to cope with dimensional changes in which the plate length itself changed significantly, and it was often the case that mechanical maintenance processes for steel sheets such as cutting and polishing were added or downgraded (defective products).

In Patent Document 3, by adding a function of cutting to a predetermined plate size to the heat treatment step, it is possible to cope with a change in the plate size.

However, even with the technique of Patent Document 3, when the plate shrinks from the size planned to be designed by the heat treatment, the insufficient length cannot be compensated, which also leads to the deterioration of the yield.

Japanese Unexamined Patent Publication No. 2005-230914 Japanese Unexamined Patent Publication No. 2005-226106 Japanese Unexamined Patent Publication No. 2001-25810

T. A. Kop, J. Sietsma, S. van der Zwaag, “Anisotropic dilatation behavior during transformation of hot rolled steels showing banded structure,” Mater. Sci. Technology, 17, pp. 1569-1574, 2001. T. Siwecki, T. Koziel, B. Hutchinson, P. Han, “Effect of micro-segregation on phase transformation and residual stress,” Mater. Sci. Forum Vols., 539-543,, 2007. A. Jablonka, K. Harster and K. Schwerdtfeger, Thermomechanical properties of iron and iron-carbon alloys: density and thermal contraction, Steel Research, 62, 1, pp.24-33 (1991) J. Miettinen, Calculation of solidification-related thermo physical properties for steels, Metallurgical and Materials Transactions B, 28B, pp.281-297 (1997)

The phenomenon that the plate size changes during heat treatment with phase transformation has been known for a long time. For example, the martensite phase produced by quenching has a lower density than the ferrite phase and the pearlite phase, so that the volume expands as a whole. From these facts, when trying to predict the dimensional change due to transformation, it was thought that the dimensional change due to the transformation would expand at an equal rate in each direction as a whole according to the plate width, plate thickness, and plate length.

However, when a steel sheet is actually manufactured, the amount of change in length in each direction of the plate width, plate thickness, and plate length in the cooling process when undergoing phase transformation due to heat treatment is not necessarily an equal ratio according to each length. It has been clarified that it does not expand or contract in (Non-Patent Document 1 and Non-Patent Document 2). That is, in addition to the dimensional change due to the density change due to the transformation, the dimensional change having anisotropy in the three directions of the plate width, the plate thickness, and the plate length is caused by the heat treatment during cooling. Therefore, in the dimensional prediction after the heat treatment simply according to the volume change of the phase transformation, even if the dimension in one direction is within the predetermined dimensional error range, the dimensional error range in the other direction can be within the predetermined dimension. There wasn't. From this, if only the amount of change in length in each direction of the plate width, plate thickness, and plate length in the cooling process is grasped by actual measurement or the like, all the dimensional changes in the three directions can be predicted.
However, in reality, it may be difficult to predict the dimensional change in each of the three directions even if only the amount of change in the length of the cooling process is grasped.

In view of the above circumstances, it is an object of the present invention to provide a method for manufacturing a steel sheet that controls dimensional changes in a plurality of directions of the steel sheet due to heat treatment accompanied by phase transformation, and a controllable heat treatment facility.

After diligently examining the reason why it is difficult to predict the dimensional change in each of the three directions, the inventors have investigated that the dimensional change accompanied by the above anisotropy is caused not only by the cooling of the steel material but also by the phase transformation during heating. It was also found that the amount was brought in and could be larger than when cooled. Further, since the tendency of these dimensional changes greatly changes depending on the heating rate, the cooling rate, the chemical composition of the steel material, etc., it is important to set the heating rate and the cooling rate in consideration of these effects before the heat treatment.
The present invention provides a method for manufacturing a steel sheet that calculates optimum heat treatment conditions for obtaining desired dimensions from the dimensions of a steel material before heat treatment, and equipment for performing heat treatment based on the calculated conditions. The summary is as follows.

(1) In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of _{at least one point of Acc.}
In advance, the correlation between the heating rate and the amount of dimensional change in plate thickness before and after heating and heating was obtained when the heating rate was changed in the heating and heating process.
Obtain the correlation between the heating temperature and the amount of dimensional change in plate thickness before and after cooling and lowering when the heating temperature is changed in advance.
In advance, the correlation between the cooling rate and the amount of dimensional change in plate thickness before and after cooling and cooling was obtained when the cooling rate was changed.
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness before and after cooling and lowering temperature,
From the amount of dimensional change that is the desired plate thickness range, plate width range, and plate length range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment.
(2) In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of _{at least one point of Acc.}
In advance, the plate thickness and plate width before and after the heating temperature rise are measured when the heating rate is changed in the heating temperature rise process, and the correlation between the heating rate and the dimensional change amount of the plate thickness and plate width before and after the heating temperature rise. Is calculated and
In advance, the plate thickness and plate width before and after the cooling and lowering temperature are measured when the heating temperature is changed, and the correlation between the heating temperature and the dimensional change amount of the plate thickness and plate width before and after the cooling and lowering process is calculated.
In advance, the plate thickness and plate width before and after the cooling and lowering temperature are measured when the cooling rate is changed, and the correlation between the cooling rate and the dimensional change amount of the plate thickness and plate width before and after the cooling and lowering temperature is calculated.
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness and width before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness and width before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness and width before and after cooling and lowering temperature,
From the amount of dimensional change that is within the desired plate thickness range and plate width range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment.
(3) In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of _{at least one point of Acc.}
In advance, the plate thickness and plate length before and after the heating temperature rise are measured when the heating rate is changed in the heating temperature rise process, and the correlation between the heating rate and the dimensional change amount of the plate thickness and plate length before and after the heating temperature rise. Is calculated and
In advance, the plate thickness and plate length before and after the cooling and lowering temperature are measured when the heating temperature is changed, and the correlation between the heating temperature and the dimensional change amount of the plate thickness and plate length before and after the cooling and lowering process is calculated.
In advance, the plate thickness and plate length before and after the cooling and lowering temperature are measured when the cooling rate is changed, and the correlation between the cooling rate and the dimensional change amount of the plate thickness and plate length before and after the cooling and lowering temperature is calculated.
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness and plate length before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness and plate length before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness and plate length before and after cooling and lowering temperature,
From the amount of dimensional change that is within the desired plate thickness range and plate length range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment.
(4) In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of _{at least one point of Acc.}
In advance, the plate thickness, plate width, and plate length before and after heating and heating are measured when the heating rate is changed in the heating and heating process, and the heating rate and plate thickness, plate width, and plate length before and after heating and heating are measured. Calculate the correlation of the amount of dimensional change,
Measure the plate thickness, plate width, and plate length before and after cooling and lowering when the heating temperature is changed, and correlate the heating temperature with the amount of dimensional change in plate thickness, plate width, and plate length before and after the cooling and lowering process. Is calculated and
In advance, measure the plate thickness, plate width, and plate length before and after cooling and cooling when the cooling rate is changed, and determine the correlation between the cooling rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and cooling. Calculate and
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and lowering temperature,
From the amount of dimensional change that is the desired plate thickness range, plate width range, and plate length range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment.
(5) In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of _{at least one point of Acc.}
In advance, the correlation between the heating rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after heating temperature rise when the heating rate is changed in the heating temperature rise process is calculated by numerical simulation.
In advance, the correlation between the heating temperature and the amount of dimensional change in plate thickness, plate width, and plate length before and after the cooling / lowering process when the heating temperature is changed is calculated by numerical simulation.
In advance, the correlation between the cooling rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and lowering temperature when the cooling rate is changed is calculated by numerical simulation.
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and lowering temperature,
From the amount of dimensional change that is the desired plate thickness range, plate width range, and plate length range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment.
(6) A steel sheet manufacturing facility for carrying out the method according to any one of (2) to (4).
In a steel sheet heat treatment facility that provides a step of heating to a temperature of at least one _{point of Acc.}
An input means for inputting the measured plate thickness before heat treatment, plate width and / or plate length, and
The correlation between the pre-calculated plate thickness before and after the heat treatment and the heating rate, the heating temperature, and the cooling rate and the pre-calculated correlation between the plate width and / or the plate length before and after the heat treatment and the heating rate, the heating temperature, and the cooling rate are stored. Storage medium and
The plate thickness before heat treatment, the plate width and / or the plate length before heat treatment, the desired plate thickness range, the desired plate width range, and / / in the correlation stored in the storage medium. Alternatively, a means for calculating the heating rate, cooling rate, and heating temperature within the desired plate thickness range, plate width range, or plate length range by applying the plate length range, and
A heat treatment facility for a steel sheet, comprising: a heating rate control means for controlling the heating rate, the cooling rate, and the heating temperature, a cooling rate control, and a heating temperature control means calculated by the calculation means.
(7) A steel sheet manufacturing facility for carrying out the method according to (5).
In a steel sheet heat treatment facility that provides a step of heating to a temperature of at least one _{point of Acc.}
And measured before the heat treatment thickness input means for inputting the plate width and the plate length,
A storage medium that stores the correlation between the plate thickness before and after the heat treatment and the heating rate, the heating temperature, and the cooling rate, and the correlation between the plate width and the plate length before and after the heat treatment and the heating rate, the heating temperature, and the cooling rate.
The stored correlation in the storage medium, and the plate thickness before heat treatment which is input by the input unit, a plate width and the plate length before the heat treatment, the desired thickness range, a desired plate width range and Itacho A means for calculating a heating rate, a cooling rate, and a heating temperature within a desired plate thickness range, plate width range, and plate length range by applying a range, and
A heat treatment facility for a steel sheet, comprising: a heating rate control means for controlling the heating rate, the cooling rate, and the heating temperature, a cooling rate control, and a heating temperature control means calculated by the calculation means.

According to the present invention, it is possible to provide optimum heat treatment conditions in which desired dimensions can be obtained in a plurality of directions of a steel sheet during heat treatment of a steel sheet accompanied by a phase transformation.

It is a figure which showed the dimensional change in the plate thickness (N), plate length (R), plate width (T) direction when the heating rate is 1 degreeC / s. It is a figure which showed the influence of the heating rate on the plate thickness change in the heat treatment history. It is a figure which showed the relationship between the plate thickness reduction rate and a heating rate before and after a heat treatment. It is a figure which showed the influence of the cooling rate on the plate thickness change in the heat treatment history. It is a figure which showed the relationship between the plate thickness reduction rate and a cooling rate before and after a heat treatment. It is a figure which has the concentration distribution of the alloy element in the plate thickness direction, and explains the deformation process of each layer in a heating process. It is a figure which has the concentration distribution of the alloy element in the plate thickness direction, and explains the deformation process of each layer in a cooling process. It is a figure which shows the calculation result of the strain change at the time of austenite transformation at the time of heating at 1 degreeC / s by the concentrated layer model. It is a figure which shows the result of the crystal plasticity simulation by the fast Fourier transform of the progress of the austenite phase transformation at the time of heating at 1 degreeC / s by the concentrated layer model. It is a figure which shows the calculation result of the strain change at the time of austenite transformation at the time of heating at 10 degreeC / s by the concentrated layer model. It is a figure which shows the result of the crystal plasticity simulation by the fast Fourier transform of the progress of the austenite phase transformation at the time of heating at 10 degreeC / s by the concentrated layer model. It is a figure which shows the calculation result of the strain change at the time of martensitic transformation at the time of cooling at 40 degreeC / s by the concentrated layer model. It is a figure which shows the calculation result of the strain change at the time of martensitic transformation at the time of cooling at 10 degreeC / s by the concentrated layer model.

The present inventors have diligently studied the point that anisotropy is observed in the degree of dimensional change of the steel sheet by the phase transformation heat treatment accompanied by the volume change (dimensional change). As a result, it was clarified that this anisotropic dimensional change depends on the composition of the steel material, the manufacturing conditions, and the heating rate, heating temperature, and cooling rate in the heat treatment. It was also found that the influence on these dimensional changes can be calculated before the treatment of the steel sheet to be subjected to the heat treatment. The basic knowledge for calculating the dimensional change after the heat treatment is to experimentally determine the correlation between the heating rate, heating temperature, cooling rate and the dimensional change of the steel sheet before and after the heat treatment, or in the plate thickness direction. It is obtained by predicting the degree of dimensional change from the concentration distribution of the component elements. Then, based on this correlation, the plate thickness (N), plate length (R), and plate width are controlled by controlling the heating rate, heating temperature, and cooling rate in the heat treatment step according to the composition of the steel sheet and the manufacturing conditions. It has been found that desired dimensions can be obtained in each direction of (T). The plate thickness (N) referred to in the present invention is the normal plate thickness itself, the plate length (R) is the length in the longitudinal direction, and the plate width (T) is the direction perpendicular to the plate length (R) direction. The length of.
As a result of the above examination, the present invention calculates the optimum manufacturing conditions (heat treatment conditions) for obtaining the desired dimensions from the dimensions before and after the heat treatment of the steel sheet, that is, the heating rate, the heating temperature, and the cooling rate by numerical calculation. Provided are a method and equipment for performing heat treatment based on the calculated conditions.

First, the background of the development in which the knowledge of the heat treatment method of the present invention was obtained will be described.

Usually, _{when the phase is transformed by heating to Acc1} or higher and then cooled, each phase such as ferrite, pearlite, bainite, and martensite is generated according to the cooling rate. The plate size changes due to the change in volume due to these phase fractions, but there are factors that greatly change the plate size in addition to the volume change due to this difference in phase fraction, and the present invention is due to factors other than this volume change. It is related to dimensional change.

The present inventors have found that, as a factor for this, the dimensions of the plate in each direction after the heat treatment greatly change depending on the heating rate, the heating temperature, and the cooling rate during the heat treatment.

Although it has been known that the plate size changes significantly due to heat treatment accompanied by phase transformation, no research has been conducted on how it is affected by the heating rate, heating temperature, and cooling rate.

Therefore, the inventors conducted experiments under various heating conditions, heating temperatures, and cooling conditions, investigated these effects, and considered the results.

First, how the plate dimensions in each direction of the plate change after heating and cooling by the heat treatment accompanied by the phase transformation will be described.
C: 0.1%, Mn: 1.0%, Si: 1.0%, Al: 0.03%, N: 0.004%, P: 0.001%, S: 0.001%, Ti : 0.0%, Nb: 0.0%, Cr: 0.0%, Cu: 0.1%, Ni: 8.9%, B: 0.0%, Mo: 0.0%, W: A material test piece having a plate thickness of 2 mm, a plate length of 10 mm, and a plate width of 5 mm obtained by hot rolling steel containing 0.0% and V: 0.0% and the balance being Fe and an unavoidable impurity. Was heated to 800 ° C., held for 5 minutes, and then cooled to room temperature in a heat treatment step. The above "0.0%" means less than 0.1%.

FIG. 1 shows temperature-strain history curves in each of the plate thickness direction (N), plate length direction (R), and plate width direction (T) when the heating rate is 1 ° C./s and the cooling rate is 40 ° C./s. ing. From FIG. 1, although there is a slight deviation between the dimensional change histories in the plate length direction (R) and the plate width direction (T), the dimensional change histories after the heat treatment are expanded as compared with those before the heat treatment. You can see that it follows. On the other hand, the change history in these two directions is significantly different from the history in the plate thickness direction (N), and from this result, it was clarified that the anisotropy of the dimensional change due to the heat treatment is recognized.

We investigated how the heating rate affects the plate size after heat treatment.
FIG. 2 shows the result of performing a heat treatment step of heating a test piece having the same composition and size as above to 800 ° C., holding it for 5 minutes, and then cooling it to room temperature, and the cooling rate was 40 ° C. The temperature-strain history curve in the plate thickness direction when the heating rate is changed to 1 ° C./s, 4 ° C./s, 7 ° C./s, and 10 ° C./s when fixed at / s is shown. When Strain and ε are positive numbers, it indicates that the thickness is thicker than the dimensions before heat treatment (indicating that it is expanding), and when ε is a negative number, it indicates that it is before heat treatment. It is shown that the thickness is reduced from the size of (indicating that it is shrinking). From FIG. 2, it can be seen that when the heating rate is the slowest at 1 ° C./s, the plate thickness after the heat treatment is remarkably reduced as compared with that before the heat treatment by undergoing the heat treatment for heating and cooling. On the other hand, as the heating rate increases, the negative amount of ε approaches 0, and the amount of decrease in plate thickness before and after the heat treatment is alleviated. That is, by controlling the heating rate, it is possible to control the amount of plate thickness reduction before and after the treatment.

FIG. 3 shows the same process up to the production of the test piece as described above, and before and after the treatment when the heating temperature, the heating time, and the cooling rate were kept constant and only the heating rate was changed from 1 to 10 ° C./s. It is a figure which showed the relationship between the plate thickness reduction amount and a heating rate. From FIG. 3, it is clear that under this condition, the amount of decrease in plate thickness linearly decreases as the heating rate increases. That is, there is a certain correlation between the heating rate and the amount of dimensional change in plate thickness.

Next, the influence of the cooling rate will be described. FIG. 4 shows the result of performing a heat treatment step of heating a test piece having the same composition and size as above to 800 ° C., holding for 5 minutes, and then cooling to room temperature, and the heating rate was 10 ° C. The temperature-strain history curve in the plate thickness direction is shown when the temperature is fixed at / s and the cooling rate is changed between 10 ° C./s, 20 ° C./s, 30 ° C./s, and 40 ° C./s. As shown in FIG. 4, it can be seen that as the cooling rate increases, the plate thickness decreases. This was confirmed to have the opposite tendency to the heating rate.

FIG. 5 shows the same process up to the production of the test piece as in FIG. 4, and the production conditions of the heating temperature, the heating time, and the heating rate are constant at 10 ° C./s, and only the cooling rate is 10 ° C./s, 20 ° C./s. It is a figure which showed the relationship between the plate thickness decrease amount before and after a heat treatment, and a cooling rate at the time of changing to 30 degreeC / s, 40 degreeC / s. From FIG. 5, it is clear that under this condition, the plate thickness linearly decreases as the cooling rate increases. That is, a certain correlation can be seen between the cooling rate and the amount of dimensional change in plate thickness.

In this way, detailed elemental analysis was performed on each part of the steel sheet material in order to clarify the reason why the dimensional change in each direction of the plate material is different due to the heat treatment and the reason why the dimensional change amount changes depending on the heating rate and the cooling rate.

As a result, the steel sheet does not have a uniform composition strictly, and alloy elements such as C, Mn, and Ni are segregated in layers to some extent in the thickness direction of the steel sheet, and the concentration of the alloy elements is higher than the overall average. The presence of a concentrated layer was observed. That is, the composition of the steel sheet is not completely uniform in the plate thickness direction, and as shown schematically as layers L ₁ , L ₂ , ... L _N in FIG. 6, in the plate length direction and the plate width direction to some extent. It was clarified that the sheets of the layered concentrated layer having the same segregation composition have a structure in which a plurality of sheets are overlapped in the plate thickness direction. Analysis of this concentrated layer, as (an alloy element content of concentrated layer) / segregation ratio M _{X (X} is the element) (the average amount of alloying elements of the entire plate) and to quantify the respective alloy elements, C the _{M C} in the case of, 1.05~3, _{M Mn} in the case of Mn is, 1.05~3, _{M Ni} in the case of Ni has been a 1.05 to 5. Here, the segregation ratio is the ratio of the concentration at the analysis position to the average value of the concentration of the component in the measurement field of view by performing line analysis using EPMA. Further, the concentrated layers are separated by a layer in which the alloying element is low (non-concentrated layer).

Regarding the deformation behavior when a steel sheet having a plurality of such concentrated layers in the plate thickness direction is heated to a temperature equal to or higher than the phase transformation temperature, it shrinks in the plate thickness direction due to the phase transformation in the heating process as shown in FIG. A case of expanding in the plate thickness direction due to phase transformation will be described as an example.

_{Let L 1} shown in FIG. 6 be a layer that undergoes phase transformation at the lowest temperature. As the temperature is raised by heating, the compositions of the laminated concentrated layers and the non-concentrated layers are different from each other, so that the temperature at which each layer actually undergoes phase transformation is different. That is, when the temperature increase, a layer L ₁ to phase transformation to shrink earlier at low temperatures, result in a layer L ₂ ~L _N to contract in phase transformation later at high temperatures, and phase transformation of each layer, There is a time lag in the dimensional change due to phase transformation. Therefore, as represented by the length of the arrow in FIG. 6 (A), L ₁ shrinkage amount 1, and L ₂ shrinkage amount 2, there is a difference in the _{degree of dimensional shrinkage between L 1} and L _2. Since the volume change due to phase transformation is large, most phase transformation temperature is low, the phase when the transformation occurs in the layer L ₁ to phase transformation before the heating Atsushi Nobori process, the L ₁ contraction force 3, Itacho direction, plate width direction , The length tends to shrink in the plate thickness direction (see FIG. 6B). However, since the layer L _{2 adjacent to the L 1} has not yet undergone a phase transformation, the _{length of the layer L 1} cannot be shortened so much in the plate length direction and the plate width direction. This is because, even if an attempt layer L ₁ is contracted, it is constrained from the layer L ₂ of the next whose length does not change the restraining force 4 to L _1, Itacho direction, changing the length of the plate width direction Is difficult. Therefore, actual L ₁ contraction force fifth layer L ₁ that phase transformation earlier, Itacho direction is restrained, the direction remains other than the plate width direction, so acts in the thickness direction, mainly in this direction deflated (see FIG. 6 (C)), the amount from Itacho direction to be originally shrinkage phase transformation of L ₁ remains in the plate width direction is longer length is completed. After that, the temperature rises, L ₂ undergoes a phase transformation, and even if it tries to shrink in the plate length direction and the plate width direction, this time, the plate length direction and the plate width direction remain longer than the amount that should be contracted. _{It is considered that L 2} cannot be sufficiently contracted in this direction due to being constrained by L ₁ , and L 2 contracts excessively in the plate thickness direction like L _1. Due to such a dimensional change in the heating process, shrinkage of each layer is likely to occur in the plate thickness direction, and it is considered that the shrinkage of each layer is large in the plate thickness direction as a whole. This tendency is particularly remarkable when the heating rate is as slow as 1 ° C./s, as shown in FIGS. 1 and 2. The effect of the plate thickness shrinkage during heating remains when the cooling rate is as high as 40 ° C./s as shown in FIGS. 1 and 2, and even after cooling, the plate thickness shrinks significantly in the plate thickness direction as a whole. It is thought that it will remain as it is.

On the other hand, if the heating rate is as fast as 10 ° C./s, the timing shift of the phase transformation between layers having different compositions stays for a short time, and the shift of the dimensional change due to the difference in the phase transformation timing also becomes a short time. It is considered that the dimensional anisotropy is improved because the time for the binding force to work is reduced and the plate can be sufficiently contracted by the phase transformation in the plate width and plate length directions.

In FIG. 6, for convenience of explanation, L ₁ of the surface layer is used as the layer that undergoes phase transformation at the lowest temperature, but the surface layer is not necessarily the layer that undergoes phase transformation at the lowest temperature. Further, since the order of transformation of each layer differs depending on the production conditions and components, it does not have any particular regularity. The same applies to FIG. 7, which will be described next.

Next, in the cooling process of cooling after heating, it is considered that the anisotropy of the dimensional change occurs due to the time lag of the phase transformation of each layer. FIG. 7 shows a schematic diagram. _Let L N be the layer that undergoes phase transformation at the highest temperature during cooling. At the time of cooling, the first phase-transforming layer L _N , which has the highest phase transformation temperature in each layer, tries to expand due to the phase transformation, but as represented by the length of the arrow in FIG. 7 (A), the L _N expansion the amount _6, as represented by _{L N-1} expansion of 7, a difference occurs _{L N} and _{L N-1} in the expansion degree of the dimension. Since the volume change due to the phase transformation is large, the phase transformation temperature is the highest, and when the phase transformation occurs in _{the layer LN that undergoes the phase transformation first in the cooling process, the L N} expansion force 8 causes the plate length direction, the plate width direction, and the plate. The length tends to expand in the thickness direction (see FIG. 7B). However, since no cause layer L _N-1 Hamada phase transformation of the adjacent L _N, even layer L _N, so the plate length direction, can not be expanded in length to the sheet width direction. This is because, even if the layer L _N tries to expand, the length is _{constrained by the binding force 9 from the adjacent layer L N-1} to the L _N whose length has not changed, and the length in the plate length direction and the plate width direction is changed. This is because it becomes difficult to make them. Therefore, the actual L _N expansion force 10 of the previously phase-transformed layer L _N acts in the plate thickness direction, which is the remaining direction other than the constrained plate length direction and the plate width direction, and thus mainly acts in this direction. It expands (see FIG. 7C), and the phase transformation _{of L N} is completed with the length in the plate length direction and the plate width direction shorter than the amount that should be expanded. After that, the temperature drops, L _N-1 undergoes a phase transformation, and even if it tries to expand in the plate length direction and the plate width direction, this time, the plate length direction and the plate width direction remain shorter than the amount that should be expanded. in it is bound to L _N, in this direction can not be sufficiently inflated, L _N-1 is considered to be excessively expanded in the thickness direction as with L _N. Due to such a dimensional change in the cooling process, expansion of each layer is likely to occur in the plate thickness direction, and it is considered that the expansion of each layer is large in the plate thickness direction as a whole. As shown in FIG. 4, this tendency is particularly remarkable when the cooling rate is as slow as 10 ° C./s. On the other hand, if the cooling rate is as fast as 40 ° C./s, the time during which the previously phase-transformed layer is constrained in the plate width direction and the plate length direction in the cooling process is reduced. Therefore, in the cooling process with a high cooling rate, the expansion is uniform in each direction, and it is considered that the expansion in the plate thickness direction is small.

It should be noted that there are several possible reasons why a plurality of concentrated layers having a non-uniform alloy element concentration are formed in the plate thickness direction. For example, the difference in solidification rate between the surface and the inside during casting, and the elements concentrated between the dendrite trees formed during casting collapse during the subsequent rolling process. It can be explained by being formed.

Based on the above findings, in _{the heat treatment method for steel plates having a heat treatment step of heating to a temperature of at least one Acc} point or more, the correlation between the amount of dimensional change before and after the heat treatment in the plate thickness direction, the heating rate, and the cooling rate. It is necessary to find the relationship in advance. By obtaining this correlation, the heating rate and cooling rate within the desired plate thickness range can be determined. The larger the number of conditions for the heating rate and the cooling rate to be measured in advance to obtain the correlation, the more preferable, and the wider the speed range, the more preferable. However, it is acceptable from the characteristics required for the steel material and the performance of the manufacturing equipment. The required number may be determined within the range of the heating rate and the cooling rate to be performed.

Further, it is necessary to obtain in advance the correlation between the amount of dimensional change before and after the heat treatment in the plate width direction or the plate length direction, and the heating rate and cooling rate. By obtaining this correlation, it is possible to determine the desired plate width, or the heating rate and cooling rate within the plate length range. As a method of obtaining the correlation between the amount of dimensional change before and after the heat treatment in the plate width direction or the plate length direction and the heating rate and the cooling rate, the plate width is obtained from the correlation between the plate thickness change and the heating rate and the cooling rate. If the correlation between the amount of dimensional change before and after the heat treatment in the direction or the plate length direction and the heating rate and the cooling rate can be calculated, it may be obtained by calculation.

It is necessary to measure the amount of dimensional change before and after the heat treatment in the plate thickness direction because, as shown in FIG. 6, there is a concentrated layer having a non-uniform composition in the plate thickness direction, and as a result, FIG. This is because, as shown in 1, the dimensional change behavior before and after the heat treatment is different from that in the plate width direction and the plate length direction. Further, as described above, since the dimensions change depending on the heating rate and the cooling rate, it is necessary to obtain a correlation in advance with respect to the heating rate and the cooling rate in order to obtain the predetermined dimensions.

On the other hand, in the plate length direction and the plate width direction, the dimensional change behavior before and after the heat treatment is almost the same, so it is sufficient to measure either one and obtain the correlation with the heating rate and the cooling rate. The dimensional change behavior is the same because the concentration distribution of the composition is different in the plate thickness direction, but the concentration distribution of the composition is small in the plate width direction and the plate length direction.

If the plate width and plate length increase as the plate thickness decreases and there is no difference in volume change from the case where the volume change occurs isotropically, the plate width and plate length are calculated from the plate thickness decrease. Since the change in length can be predicted, only the plate thickness before and after the heat treatment may be measured, and the correlation between the heat treatment conditions (heating rate, cooling rate) and the amount of change in other directions may be calculated by calculation.

Further, the steel sheet in the present invention preferably has a thickness of 250 mm or less in view of the capacity of ordinary heat treatment equipment. The lower limit is not necessarily limited, but 1 mm or more is preferable, and 3 mm or more is more preferable.

The present invention is preferably applied to a steel sheet in which at least 50 concentrated layers are present in a test piece having a thickness of 10 mm. This is because the larger the number of concentrated layers, the more stable data can be obtained with statistically less variation. The analysis results of the concentrated layer are as described above.

Next, the influence of the difference in heating temperature on the dimensional change before and after the heat treatment will be described. Table 1 shows the results of investigating the change in plate thickness before and after the heat treatment when the heating temperature was changed from 800 ° C. to 1300 ° C. on the higher temperature side. However, the change in plate thickness is displayed as a strain change when the strain before heat treatment is set to 0. The test pieces are manufactured by the same steps up to the manufacture of the test pieces as in the tests of FIGS. 1 to 5, and the holding time at the heating temperature is 5 minutes in each case.

Table 1 shows the results when the heating rate was fixed at 1 ° C./s and the cooling rate was fixed at 40 ° C./s. That is, it is a result under the condition that the heating rate is as slow as 1 ° C./s, and it is a condition that the plate thickness change becomes large in the heating process. Further, since the cooling rate is as high as 40 ° C./s, the change in plate thickness during the cooling process is small. Predicting from the component composition of this test piece, for example, heating at 800 ° C. and heating at 1300 ° C. correspond to heating in the γ single-phase region. Therefore, although there is no difference in the crystal arrangement between the two, in general, the crystal grains become coarser as the heating temperature rises, which has the effect of lowering the transformation temperature during cooling. However, when the cooling rate of the component is 40 ° C./s, a martensite phase is generated at any heating temperature. At this time, since the transformation of the martensite phase proceeds at once, the influence of the segregated structure is small, and it is considered that the influence of raising the heating temperature is small under the conditions of the heating rate and the cooling rate as shown in FIG. be able to.
The results in Table 1 are the results under the condition that the heating rate is as slow as 1 ° C./s as described above, and are the conditions under which the change in plate thickness becomes large in the heating process. Nevertheless, the effect of the difference in heating temperature on the plate thickness is small. From this, it is considered that the influence of the heating temperature on the plate thickness change in the heating process is small.

Next, Table 2 shows the results when the heating rate was fixed at 10 ° C./s and the cooling rate was fixed at 10 ° C./s.
Under the conditions of the heating rate and the cooling rate, the increase in the plate thickness tends to decrease as the heating temperature rises. Further, under this condition, the heating rate is high and the change in plate thickness is small in the heating process. Therefore, it can be seen that the heating temperature affects the change in plate thickness during the cooling process. This is because when the cooling rate is as slow as 10 ° C./s, ferrite / pearlite transformation occurs in addition to martensitic transformation, which is greatly affected by the segregated structure and results in a change in plate thickness during the cooling process. it is conceivable that. That is, it is not possible to predict the plate thickness change simply based on the prediction of the transformation start temperature, but the correlation between the heating temperature and the plate thickness change before and after the cooling and lowering temperature is calculated, and the plate thickness change is predicted based on this. Need to be done.

As described above, if the dimensional change before and after the heat treatment is obtained in advance by experiments or the like as a function of the heating rate, the cooling rate, and the heating temperature, the desired size can be obtained after the heat treatment while predicting the dimensional change during the actual heat treatment. It is possible to set the optimum heat treatment method. That is, the heat treatment conditions are determined by applying the dimensions of the steel sheet measured before the heat treatment and the desired dimensional changes before and after the heat treatment to the functions of the heating rate, the cooling rate, the heating temperature and the dimensional changes obtained in advance.
In order to obtain the dimensional change before and after the heat treatment as a function of the heating rate and the cooling rate by an experiment in advance, the following 1. ~ 7. Matters are necessary.

1. 1. Measuring the thickness of the steel sheet before heat treatment Unless the dimensions of the steel sheet before heat treatment are measured, the dimensions after heat treatment cannot be predicted and the desired dimensions cannot be set. Therefore, the dimensions of the steel sheet are measured before the heat treatment. At the very least, it is essential to measure the dimensions in the plate thickness direction that do not change isotropically due to heat treatment. In addition, it is preferable to measure the plate width and / or the plate length, which have different behaviors due to heat treatment from the plate thickness direction. More preferably, the plate thickness, plate width, and plate length are all measured. The term "before heat treatment" as used herein means not before the heat treatment for obtaining the correlation between the heating rate, the heating temperature, the cooling rate and the dimensional change in advance, but before the heat treatment for manufacturing the product.

2. In advance, measure the plate thickness before and after heating and heating when the heating rate is changed in the heating and heating process, and calculate the correlation between the heating rate and the amount of dimensional change in the plate thickness before and after heating and heating.
When the phase transformation occurs in the heating and heating process, as described above, due to the difference in the transformation timing of the layers formed non-uniformly in the plate thickness direction, a binding force that hinders the dimensional change is generated in each layer. Due to this binding force, there is a deviation from the dimensional change value predicted from the volume change of the phase transformation in the plate thickness direction. Obtain it as a correlation according to the heating rate. By obtaining the correlation, it is possible to determine how much the dimensional change occurs in the plate thickness direction at each heating rate. Then, the dimensional change in the plate thickness direction in the heating and heating process can be calculated together with the dimensional change in the cooling and lowering process, so that the dimensional change before and after the heat treatment can be predicted. Here, "before and after heating" means that the plate material is at room temperature and before heat is applied, and after heating is until the maximum temperature is reached after heating and cooling starts. Say. In the heat treatment process in which heating and cooling are repeated a plurality of times, the dimensions before and after all the heating and heating are measured. The same applies to the measurement of plate width and plate length.

3. 3. In advance, measure the plate thickness before and after cooling and lowering when the heating temperature is changed, and calculate the correlation between the heating temperature and the amount of dimensional change in the plate thickness before and after cooling and lowering.
As described above, the heating temperature causes a dimensional change in the plate thickness direction that cannot be predicted by the method using the conventional isotropic model, as described above, in the phase transformation, especially in the cooling / lowering process, so a correlation is obtained. By obtaining the correlation, it is possible to determine how much the dimensional change occurs in the plate thickness direction at each heating temperature. Then, the dimensional change in the plate thickness direction in the cooling / lowering process can be predicted together with the dimensional change before and after the heating / raising process, so that the dimensional change before and after the heat treatment can be predicted. Here, "before and after cooling and lowering temperature" means before the plate material is heated, before it is cooled, and before the temperature drops, and after cooling, it means until the plate material starts cooling and reaches room temperature. In the heat treatment process in which heating and cooling are repeated, all the dimensions before and after cooling and cooling are measured. The same applies to the measurement of dimensions before and after cooling and cooling, and the measurement of plate width and plate length when calculating the correlation between the cooling rate and the amount of dimensional change in plate thickness before and after cooling and cooling.

4. In advance, measure the plate thickness before and after cooling and lowering when the cooling rate is changed, and calculate the correlation between the cooling rate and the amount of dimensional change in the plate thickness before and after cooling and lowering.
When phase transformation occurs in the cooling / cooling process, as described above, due to the difference in transformation timing due to the difference in the cooling rate of the layers formed non-uniformly in the plate thickness direction, a binding force that hinders the dimensional change is generated in each layer. Due to this binding force, a deviation from the dimensional change value predicted from the volume change of the phase transformation occurs in the plate thickness direction, so the correlation is obtained. By obtaining the correlation, it is possible to determine how much the dimensional change occurs in the plate thickness direction at each cooling rate. Then, the dimensional change in the plate thickness direction in the cooling / lowering process can be predicted together with the dimensional change before and after the heating / raising process, so that the dimensional change before and after the heat treatment can be predicted.

5. In advance, the plate width and / or the plate length dimension before and after the heating temperature rise when the heating rate is changed in the heating temperature rise process is measured, and the correlation between the heating rate and the dimension change amount before and after the heating temperature rise is calculated. To do.
The dimensions in the plate width and plate length directions differ from those in the plate thickness direction in the degree of dimensional change before and after heating and heating due to the difference in anisotropy of the component composition distribution. Therefore, the correlation between the dimensional change of one or more of the plate width and the plate length direction and the heating rate (correlation between the dimensional change of the plate width and the heating rate for the plate width, and the dimensional change and heating of the plate length for the plate length). Calculate the correlation of speed, and if the plate width and plate length, the correlation between the dimensional change of both plate width and plate length and the heating rate).

6. In advance, measure the plate width and / or plate length before and after cooling and lowering when the heating temperature is changed, and calculate the correlation between the heating temperature and the amount of the dimensional change before and after cooling and lowering.
The dimensions in the plate width and plate length directions differ from those in the plate thickness direction in the degree of dimensional change before and after cooling and lowering due to the difference in anisotropy of the component composition distribution. Therefore, the correlation between the dimensional change of one or more of the plate width and the plate length direction and the heating temperature (correlation between the dimensional change of the plate width and the heating temperature for the plate width, and the dimensional change and heating of the plate length for the plate length). Calculate the temperature correlation, and if the plate width and plate length, the dimensional change of both the plate width and plate length and the correlation of heating temperature).

7. In advance, measure the plate width and / or plate length dimension before and after the cooling and lowering temperature when the cooling rate is changed, and calculate the correlation between the cooling rate and the amount of the dimensional change before and after the cooling and lowering temperature.
The dimensions in the plate width and plate length directions differ from those in the plate thickness direction in the degree of dimensional change before and after cooling and lowering due to the difference in anisotropy of the component composition distribution. Therefore, the correlation between the dimensional change of one or more of the plate width and the plate length direction and the cooling rate (correlation between the dimensional change of the plate width and the cooling rate for the plate width, and the dimensional change and cooling of the plate length for the plate length). Calculate the correlation of speed, and if the plate width and plate length, the correlation between the dimensional change of both plate width and plate length and the cooling rate).
In addition, the above 2. ~ 7. The dimensional measurement for obtaining the correlation between the above may be performed in advance on the steel sheet for the product to be actually heat-treated, or the same composition, the same structure, or the same composition as the steel sheet for the product to be actually heat-treated. It may be carried out with a test piece or the like having the same structure and smaller than the steel plate for the product.

Next, the preferred requirements of the present invention will be described in sequence.
First, the dimensional measurement of the steel sheet before the heat treatment will be described.
The steel sheet after hot rolling is cooled to room temperature by water cooling or air cooling in the cooling zone. Heat treatment is performed on the cooled steel sheet, and the plate thickness, plate length, and plate width are measured in advance before the heat treatment.
The plate thickness measurement is not particularly limited to the measuring device, but a laser plate thickness gauge or the like can be used. In addition, it is desirable to measure multiple points in the longitudinal direction and the width direction in consideration of the influence of gauge fluctuation and plate crown.

The plate width can be measured using a plane shape meter or the like. Considering the fluctuation of the plate width, preferably, a plurality of points are measured in the longitudinal direction. If the plate length is measured accurately, the plate width measurement is not essential.

Regarding the plate length, when the plate length is short like a thick steel plate, the plate length is measured using a plane shape meter or the like. On the other hand, when the plate length is long like a thin steel plate, the accuracy of the plate length is not strict in many cases. Therefore, the plate length measurement is not indispensable as long as the plate width is measured.

Next, the transformation strain in the heating and cooling processes will be described. The transformation strain means that a phase having a crystal structure different from that before cooling is generated and the density changes depending on the cooling rate or the like, so that the transformation strain occurs in response to the phase transformation. Although these densities differ depending on the alloy composition and the temperature range, a density formula considering these effects has been proposed (Non-Patent Documents 3 and 4), but the above values can be used for steel materials. Preferably, the density is measured in advance for each steel type in each temperature range.

In the present invention, in addition to the dimensional change caused by this density change, anisotropy occurs in the transformation strain according to the heating rate, the heating temperature, and the cooling rate, and the plate thickness, the plate length, and the plate width change differently. It is based on the phenomenon.

The actual heat treatment process will be described. Usually, the range of heating rate, heating temperature, and cooling rate is predetermined in order to secure the material of the product. In order to control to the desired dimensions within this defined process condition range, the correlation between the measured dimensions before and after heat treatment and the heating rate, heating temperature and cooling rate is calculated, and the product dimensions are calculated based on the correlation. The heating rate, heating temperature and cooling rate are determined from the reference values of.

Here, there are innumerable conditions of heating rate, heating temperature, and cooling rate for obtaining the dimensional standard of the product, and any of these conditions may be used, but preferably, the heating rate is low within the range permitted by the material, and heating is performed. The lower the temperature and the slower the cooling rate, the more uniform the material can be obtained and the smaller the equipment load.

(When the correlation between heat treatment conditions and dimensional changes is determined in advance by actual measurement)
A method of calculating strain during heat treatment to obtain a predetermined dimensional change will be described in a case where the correlation between the heat treatment conditions and the dimensional change is obtained in advance by actual measurement. First, the strain in the plate thickness direction will be described. The heating rate affects the transformation strain during heating, and as a result of examination, the effect is arranged by, for example, the following equation.

Δε ^N _{2 → 1} = β _{2 → 1} (1 + Aexp (−bT ⁿ )) (1)

However, Δε ^N _{2 → 1} is a transformation strain in the plate thickness direction during heating, T is a heating rate, and A, b, and n are values determined from the material and process conditions. β _{2 → 1} is an isotropic transformation strain during heating calculated from the above density model, etc., and since it transforms from α to γ during heating with ordinary steel materials, it becomes a negative value, and Δε _{2 → 1 1 is} also a negative value. A, b, and n are determined from this formula and the measured values. In the method, first, the equation (1) is converted into an incremental type.

dε ^N _{2 → 1} = -β _{2 → 1} AbnT ^n-1 exp (-bT ⁿ ) dT (1)'

From this equation and the experimental dilation curve, it becomes possible to fit the values in each temperature range.

Both the heating temperature and the cooling rate affect the transformation strain during cooling. The plate thickness direction transformation strain Δε ^N _{1 → 2} during cooling is different from that during heating and transforms from γ to α, so it takes a positive value. Assuming that the heating temperature is T ₁ and the cooling rate is T, the transformation strain during cooling is as follows.

Δε ^N _{1 → 2} = β _{1 → 2} (1 + Beexp (−cT ^m ) exp (−d (T ₁ −T ₀ ))) (2)

However, β _{1 → 2} is an isotropic transformation strain during cooling calculated from the above density model or the like , and is a positive value because it transforms from γ to α in a normal steel material. T ₀ is a reference temperature, and B, c, m, and d are values determined from the material and process conditions.

Since the transformation strains in the plate length direction and the plate width direction have the same values, only the transformation strain in the plate length direction will be described below. The transformation strain in the plate length direction during heating and cooling is different, respectively.
Δε ^R _{2 → 1} = β _{2 → 1} (1-A / 2exp (−bT ⁿ )) (3)

Δε ^R _{1 → 2} = β _{1 → 2} (1-B / 2exp (−cT ^m ) exp (−d (T ₁ −T ₀ ))) (4)

Will be. In this case as well, equations (2) to (4) can be converted to the incremental type, and the values in each temperature range can be fitted from this and the experimental dilation curve.

As described above, when the dimensional change before and after the heat treatment is obtained in advance by an experiment as a function of the heating rate and the cooling rate, the predicted dimensional accuracy in each direction after the heat treatment can be very good. Moreover, if it is actually measured, it is clear that good dimensional predictability can be obtained even in the case where there is no concentrated layer in the plate thickness direction. However, for all manufactured steel grades, similarly, the plate thickness, plate width / or plate length dimension before and after the heat treatment is measured in advance, and a test is conducted to clarify the correlation with the heating rate, heating temperature, and cooling rate. This can often be costly and time consuming to develop. Therefore, if the change in the heat treatment dimension can be predicted by numerical calculation or the like without measuring the plate thickness, the plate width / or the plate length dimension before and after the heat treatment in advance according to the above mechanism, there is a great merit.

Therefore, in the following, an example of a method for predicting the change in heat treatment dimension during heating by numerical calculation will be described without the need to measure the plate thickness, plate width / or plate length dimension before and after the heat treatment. The measurement of the plate thickness, plate width / or plate length before and after the heat treatment, which is unnecessary here, is a dimensional measurement for obtaining a correlation between the dimensional change and the heat treatment conditions in advance.

First, consider a part of the structure in which layers having a concentration distribution in a band shape as shown in FIG. 6 are laminated. Here, a three-layer structure in which non-concentrated layers are laminated vertically in the plate thickness direction so as to sandwich a layer in which alloy elements are concentrated is considered as a basic unit, and a concentrated layer and a non-concentrated layer are considered. Each boundary was set as a periodic boundary condition, assuming that 3 or more layers were alternately repeated. The relationship between the thickness of the concentrated layer and the thickness of the non-concentrated layer is determined by the component system of the steel material and the manufacturing process. Specifically, it is an alloy element such as C, Mn, Ni, a cooling rate in secondary cooling in continuous casting, and a cumulative reduction rate in hot rolling. Alternatively, the degree of concentration of the alloying elements in each of the light and shade layers of the steel sheet to be subjected to the heat treatment (the steel sheet before the heat treatment) and the thickness of each concentrated layer may be grasped by an analytical instrument such as EPMA.

The hardness distribution considering the segregation of alloying elements in these two phases is given as an input of the critical shear stress of each slip system. Further, in the phase transformation, in the austenite transformation during heating, the timing of nucleation changes depending on the alloy component and the heating rate. This change can be obtained from an equilibrium phase diagram or the like. Further, in general, the layer in which the alloying element is concentrated preferentially initiates the austenite transformation. After nucleation, the newly generated grains grow, and when the new phases collide with each other, the transformation ends, and this boundary forms a new grain boundary. In this growth process, microscopically, the new phase undergoes isotropic contraction due to the density difference due to metamorphosis. However, since the part where the transformation has not occurred is not contracted, this strain mismatch induces plastic deformation. The plastic deformation caused by this strain difference can be obtained by crystal plastic analysis using the finite element method (FEM) or the fast Fourier transform (FFT).

First, with respect to the steel having the same composition as that used in the tests of FIGS. 1 to 5, a plate having a cumulative reduction rate of 190% and a plate thickness of 25 mm has a Ni segregation ratio (ratio to the average concentration) of 1.2 or more. When the thickness of the concentrated layer was measured, 1/6 of the total thickness was the concentrated layer. Therefore, the transformation temperature calculated from the alloy concentration of this thick portion and the transformation temperature calculated from the alloy concentration of the non-concentrated layer are used as inputs, and the crystal plasticity analysis during the heating phase transformation is performed at a heating rate of 1 ° C./s. As a result, the strain as shown in FIG. 8 was calculated in the heating process at 650 to 700 ° C. (the process of austenite transformation by heating). The strain in FIG. 8 quantitatively coincided with the heating process of 650 to 700 ° C. in FIG. 1 for the transformation strain obtained by the experiment, and the accuracy of the calculation and the validity of the assumed mechanism were verified. FIG. 9 shows the results of crystal plasticity simulation by fast Fourier transform of the progress of phase transformation when heated at 1 ° C./s in the temperature range of 650 to 700 ° C. When the crystal (A) before the austenite transformation is heated, the central concentrated layer is transformed first, the austenite phase γ1 appears in the concentrated layer, and at the time of 5% by volume austenite transformation of the entire crystal (B), The part other than the concentrated layer does not metamorphose. The transformation of the non-concentrated layer is started when the temperature further rises to 20% by volume austenite transformation (C) of the entire crystal, that is, when the transformation of the concentrated layer is almost completed, and the non-concentrated phase The austenite phase γ2 appears in the austenite phase (FIGS. 9 (A) (B) (C)).

On the other hand, even when the heating rate is 10 ° C./s, the crystal plasticity analysis during the heating phase transformation in the heating process of 650 to 700 ° C. (the process of austenite transformation by heating) was performed using FFT. The strain shown in is calculated. When this phenomenon is analyzed by crystal plasticity simulation by fast Fourier transform as in the case of 1 ° C./s, as shown in FIG. 11, transformation of the concentrated layer is started when the heating rate is 10 ° C./s. , Overlapping with the time when the austenite phase γ1 continues to appear in the concentrated layer (Fig. 11 (B)), the transformation of the non-concentrated layer is started, and the austenite phase γ2 appears in the non-concentrated phase, and the timing of transformation The difference is very small. Therefore, as shown in FIG. 10, almost no anisotropy is observed in the transformation contraction during the phase transformation. This is in agreement with the findings obtained from the experimental results of FIGS. 1 and 2.

Next, the transformation expansion calculation in the martensitic transformation during cooling will be described in the same manner. The distribution of the concentrated layer before the start of cooling depends on the heating temperature. Here, when considering heating held at 800 ° C. for 5 minutes, the distribution of the concentrated layer hardly changes. Therefore, in the following, the thickness of the concentrated layer is set to about 1/6 of the total thickness as in the case of heating. Regarding the transformational expansion due to martensitic transformation during cooling from 350 ° C. to 250 ° C. at a cooling rate of 40 ° C./s, the transformational expansion (strain amount) during cooling was calculated using FFT as in the case of heating. As a result, the result is as shown in FIG. On the other hand, when the cooling rate was as slow as 10 ° C./s, the cooling from 350 ° C. to 250 ° C., which was the same as in FIG. 12, was as shown in FIG. That is, it was also calculated that if the cooling rate is high, the amount of expansion in the plate thickness direction decreases in the cooling process, and if the cooling rate is slow, the amount of expansion in the plate thickness direction increases. This result is in agreement with the findings obtained from the experimental results shown in FIG. As described above, the transformation of the alloy-concentrated layer is delayed during cooling as opposed to during heating, but by incorporating this effect into the calculation, the anisotropy of transformation expansion can be reproduced.

As described above, in the manufacturing method of the steel sheet with a heat treatment step of heating to a temperature above A _c1 point, the optimal heating rate, heating temperature, to a predetermined dimensional range in each direction of the determined plate and the cooling rate is In order to obtain the correlation with the heat treatment conditions, it is not always necessary to measure the plate thickness, plate width / or plate length dimension before and after the heat treatment in advance. Therefore, the items 1) to 5) below are necessary for determining the optimum heating rate, heating temperature, and cooling rate and setting the predetermined dimensional range in each direction of the plate.

1) Measuring the thickness of the steel sheet before heat treatment The reason for this point is when the correlation between the heat treatment conditions and the amount of dimensional change is obtained by measurement. Is similar to.

Next, the correlation between the heat treatment conditions and the amount of dimensional change is obtained as shown in 2) to 4) below.
2) Obtain the correlation between the heating rate and the amount of dimensional change before and after the heating temperature rise when the heating rate is changed in the heating temperature rise process in advance. 3) The heating temperature when the heating temperature is changed in advance. 4) Obtaining the correlation between the cooling rate and the amount of dimensional change before and after the cooling / cooling process when the cooling rate is changed in advance. 4) Obtaining the correlation between the cooling rate and the amount of dimensional change before and after the cooling / cooling process. The significance of finding the correlation between the above is 1. ~ 3. It has the same meaning as described in. It does not just limit the specific means of finding the correlation.
When obtaining the correlation of 2) to 4) above, the correlation may be obtained by measuring the plate dimensions when each heat treatment condition is changed, or a calculation such as a numerical simulation may be performed without such measurement. The correlation may be obtained. At this time, it is necessary to obtain at least the correlation between the dimensional change in the plate thickness and the heat treatment conditions. This is because, as described above, the dimensional change in the plate thickness direction cannot be simply predicted by the isotropic model due to the mechanical interaction between the concentrated layer and the non-concentrated layer during the heat treatment. .. On the other hand, the plate width and plate length can be obtained simply from the isotropic model based on the density change if the plate thickness change is known, and the dimensional change can be predicted, so it is not always necessary. is not it. However, in order to improve the dimensional accuracy after the heat treatment, it is preferable to obtain at least one of the correlation between the plate width and the plate length and each heat treatment condition. As a specific method, an isotropic model or the like can also be obtained.
When the correlation of 2) to 4) above is calculated, it is calculated based on the basic data. This basic data is for grasping the difference in expansion or contraction due to transformation, the interaction force, etc. between the concentrated layer and the non-concentrated layer (each non-uniform layer) having different component compositions. The basic data include the coefficient of linear expansion of each of the concentrated and non-concentrated layers, the amount of transformation strain due to density changes (both during heating and cooling), Young's modulus of each layer at each temperature, and Poisson's ratio. It is preferable to use the constitutive equation of each phase (critical shear stress, etc. in the case of a crystal plastic model). It is preferable to acquire the basic data in advance from the material database according to the component composition of the concentrated layer and the non-concentrated layer of the material. In this way, the constituents (degree of concentration) and thickness of each concentrated layer and non-concentrated layer are grasped, and the difference in expansion or contraction due to transformation according to the heating rate, heating temperature, and cooling rate, and mutual The force of action can be determined.

5) Heat treatment is performed by setting the heating rate, heating temperature, and cooling rate from the correlation obtained in 2) to 4) above and the amount of dimensional change within the desired plate thickness range, plate width range, and plate length range. By utilizing the correlations obtained in the above 2) to 4), the heating rate, heating temperature, and cooling rate that cause desired dimensional changes can be determined, so that the heat treatment may be performed according to the determined conditions. Even if the correlation between the plate width / plate length and each heat treatment condition is not grasped, the plate width / plate length can be naturally determined by adopting the heat treatment conditions in which the plate thickness dimension is within a predetermined range. However, as described above, it is preferable to obtain the correlation between the plate width / plate length and each heat treatment condition.

Further, as the heat treatment equipment for the steel sheet for carrying out the present invention as described above, the following configurations can be mentioned.
That is, as one heat treatment facility, the plate thickness before heat treatment, the plate width and / or the input means for inputting the plate length, the input plate thickness before heat treatment, and the plate width and / or plate length are calculated in advance. A storage medium that stores the correlation between the plate thickness before and after the heat treatment and the heating rate, the heating temperature, and the cooling rate, and the correlation between the plate width and / or the plate length before and after the heat treatment calculated in advance and the heating rate, the heating temperature, and the cooling rate. And, in the correlation stored in the storage medium, the plate thickness before heat treatment, the plate width and / or plate length before heat treatment, the desired plate thickness range, the desired plate width range, and the desired plate width range and the plate thickness before heat treatment input by the input means. / Or by applying the plate length range, the heating rate, heating temperature, and cooling rate within the desired plate thickness range, plate width range, or plate length range are calculated by the calculation means, and the heating rate, heating temperature, and cooling rate are calculated. The heating rate control means, the heating temperature control means, and the cooling rate control means are provided. The heat treatment equipment may be provided with a measuring means for measuring the plate thickness, the plate width and / or the plate length before the heat treatment, or the plate thickness, the plate width and / or the plate length before the heat treatment measured at another facility. You may enter the value of.

Furthermore, in the above heat treatment equipment, when calculating the dimensional change by simulation, a program that calculates the correlation between the plate thickness, plate width, and plate length before and after the heat treatment and the heating rate, heating temperature, and cooling rate in the storage medium. Is preferable because it is not necessary to store the correlation in the storage medium in advance.

Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is described in this one condition example. It is not limited. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

(Example 1)
The correlation between the heating rate, the heating temperature, and the cooling rate and the dimensional change in each direction was obtained in advance by measurement, and heat treatment was performed based on the results.
By mass%, C: 0.1%, Mn: 1.0%, Si: 1.0%, Al: 0.03%, N: 0.004%, P: 0.001%, S: 0. 001%, Ti: 0.0%, Nb: 0.0%, Cr: 0.0%, Cu: 0.1%, Ni: 8.9%, B: 0.0%, Mo: 0.0 A plurality of steel pieces having a thickness of 250 mm were prepared by casting steel containing%, W: 0.0%, and V: 0.0%, and the balance being Fe and unavoidable impurities with a continuous casting machine.

This steel piece was hot-rolled until the plate thickness became 9 mm, cooled to room temperature, and cut to obtain a plurality of steel plates. Then, these steel sheets were transported to a heat treatment line, and the dimensions were measured by a dimension measuring device installed before heating.

In addition, a test piece is cut out at the same time as cutting, and the density change in each temperature range is measured, and the heating rate is 1, 4, 7, 10 ° C./s, the heating temperature is 800 ° C., 1300 ° C., and the cooling rate is 10. The transformation strains in the plate thickness direction, the plate length direction, and the plate width direction were measured when the temperature was changed to 20, 30, and 40 ° C./s.

From the measurement results, for example, when the parameters of the equations (1) and (2) at a heating rate of 1 ° C./s and a cooling rate of 40 ° C./s were determined, the results were as follows. A = 1.5, b = 1.0, n = 0.8, B = 0.8, c = 1.0, m = 0.2, d = 1.0 × ^10-5 , T ₀ = 700 ℃.

Next, before heat-treating the steel sheet, the dimensional reference values for each product were transmitted from the process computer to the control computer of the heat treatment line, and the heating rate, heating temperature, and cooling rate were determined in the control computer.

Half of the prepared steel sheets were heat-treated by controlling them according to the heating rate, heating temperature, and cooling rate determined above. On the other hand, the other half were heat-treated by uniformly determining the heating rate, heating temperature, and cooling rate as before.

At this time, Table 3 shows the plate size before and after the heat treatment, the size range of the product, the heat treatment conditions, and the pass / fail result of the size. From this result, when the heat treatment conditions are controlled, the downgrade due to the size defect does not occur, but when it is not controlled, the downgrade occurs especially due to the plate thickness and the plate width mismatch. In Comparative Example 8, the amount of deformation was predicted only by the conventional isotropic transformation model. The transformation process to be calculated is assumed to be 0.05% linear expansion on the premise of ferrite + martensite → austenite → martensite. As a result, the size prediction after the heat treatment was 8.81 mm in thickness, 10.75 mm in length, and 1524.7 mm in width, which were predicted to be within the target range, but actually, they were not. Therefore, the effectiveness of the present invention was confirmed.

(Example 2)
On the other hand, computer simulation was also used to predict dimensional changes, and heat treatment was performed based on the results.
By mass%, C: 0.1%, Mn: 1.0%, Si: 1.0%, Al: 0.03%, N: 0.004%, P: 0.001%, S: 0. 001%, Ti: 0.0%, Nb: 0.0%, Cr: 0.0%, Cu: 0.1%, Ni: 8.9%, B: 0.0%, Mo: 0.0 A plurality of steel pieces having a thickness of 250 mm were prepared by casting steel containing%, W: 0.0%, and V: 0.0%, and the balance being Fe and unavoidable impurities with a continuous casting machine.

This steel piece was hot-rolled until the plate thickness became 25 mm, cooled to room temperature, and cut to obtain a plurality of steel plates. Then, these steel sheets were transported to a heat treatment line, and the dimensions were measured by a dimension measuring device installed before heating.

Further, from the cumulative reduction rate of the steel sheet, it was estimated that the thickness of the concentrated layer having a Ni segregation ratio of 1.2 or more was 1/6 of the whole. Further, the transformation start temperature during heating was input to the program from the equilibrium phase diagram, and the transformation start temperature during cooling was input to the program as a function of the heating rate and the cooling rate from the continuous cooling diagram.

Based on this input condition, dimensional changes during phase transformation at various heating and cooling rates were calculated using a crystal plasticity model by FFT. At this time, the values calculated using Non-Patent Document 4 were used for the transformation contraction / expansion caused by the density change. The calculated heating / cooling rate is 1 ° C./s to 40 ° C./s. In addition to this calculation, the thermal strain also calculated by the density model also described in Non-Patent Document 4 was used to obtain the relationship between the dimensional change in the entire heat treatment process and the heat treatment conditions together with the dimensional change during the transformation.

Next, before heat-treating the steel sheet, the dimensional reference values for each product were transmitted from the process computer to the control computer of the heat treatment line, and the heating rate, heating temperature, and cooling rate were determined in the control computer.

Half of the prepared steel sheets were heat-treated by controlling them according to the heating rate, heating temperature, and cooling rate determined above. On the other hand, the other half were heat-treated by uniformly determining the heating rate, heating temperature, and cooling rate as before.

At this time, Table 4 shows the plate size before and after the heat treatment, the size range of the product, the heat treatment conditions, and the pass / fail result of the size. From this result, when the heat treatment conditions are controlled, the downgrade due to the size defect does not occur, but when it is not controlled, the downgrade occurs especially due to the plate thickness and the plate width mismatch. In addition, the deformation amount of 18 which is a comparative example was predicted only by the conventional isotropic transformation model. The transformation process to be calculated is assumed to be 0.05% linear expansion on the premise of ferrite + martensite → austenite → martensite. As a result, the size prediction after the heat treatment was 25.3 mm in thickness, 10.75 mm in length, and 1523 mm in width, which was predicted to be within the target range, but actually, it was not. Therefore, the effectiveness of the present invention was confirmed.

According to the present invention, the dimensional accuracy after the heat treatment is improved by predicting the dimensional change caused by the heat treatment of the steel sheet in advance and further controlling the heat treatment conditions. Therefore, the present invention has high industrial applicability.

1 ... L ₁ contraction amount, 2 ... L ₂ contraction amount, 3 ... L ₁ contraction force, 4 ... L ₂ to L ₁ binding force, 5 ... actual L ₁ contraction force, 6 ... L _N expansion amount, 7 ... L _N-1 expansion amount, 8 ... L _N expansion force, 9 ... L _N-1 to L _N binding force, 10 ... actual L _N expansion force, γ 1 ... austenite phase transformed in the concentrated layer, γ 2 … Austenite phase transformed in the non-concentrated layer

Claims (7)

In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of at least one _{point of Acc.}
In advance, the correlation between the heating rate and the amount of dimensional change in plate thickness before and after heating and heating was obtained when the heating rate was changed in the heating and heating process.
Obtain the correlation between the heating temperature and the amount of dimensional change in plate thickness before and after cooling and lowering when the heating temperature is changed in advance.
In advance, the correlation between the cooling rate and the amount of dimensional change in plate thickness before and after cooling and cooling was obtained when the cooling rate was changed.
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness before and after cooling and lowering temperature,
From the amount of dimensional change that is the desired plate thickness range, plate width range, and plate length range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment. In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of at least one _{point of Acc.}
In advance, the plate thickness and plate width before and after the heating temperature rise are measured when the heating rate is changed in the heating temperature rise process, and the correlation between the heating rate and the dimensional change amount of the plate thickness and plate width before and after the heating temperature rise. Is calculated and
In advance, the plate thickness and plate width before and after the cooling and lowering temperature are measured when the heating temperature is changed, and the correlation between the heating temperature and the dimensional change amount of the plate thickness and plate width before and after the cooling and lowering process is calculated.
In advance, the plate thickness and plate width before and after the cooling and lowering temperature are measured when the cooling rate is changed, and the correlation between the cooling rate and the dimensional change amount of the plate thickness and plate width before and after the cooling and lowering temperature is calculated.
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness and width before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness and width before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness and width before and after cooling and lowering temperature,
From the amount of dimensional change that is within the desired plate thickness range and plate width range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment. In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of at least one _{point of Acc.}
In advance, the plate thickness and plate length before and after the heating temperature rise are measured when the heating rate is changed in the heating temperature rise process, and the correlation between the heating rate and the dimensional change amount of the plate thickness and plate length before and after the heating temperature rise. Is calculated and
In advance, the plate thickness and plate length before and after the cooling and lowering temperature are measured when the heating temperature is changed, and the correlation between the heating temperature and the dimensional change amount of the plate thickness and plate length before and after the cooling and lowering process is calculated.
In advance, the plate thickness and plate length before and after the cooling and lowering temperature are measured when the cooling rate is changed, and the correlation between the cooling rate and the dimensional change amount of the plate thickness and plate length before and after the cooling and lowering temperature is calculated.
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness and plate length before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness and plate length before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness and plate length before and after cooling and lowering temperature,
From the amount of dimensional change that is within the desired plate thickness range and plate length range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment. In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of at least one _{point of Acc.}
In advance, the plate thickness, plate width, and plate length before and after heating and heating are measured when the heating rate is changed in the heating and heating process, and the heating rate and plate thickness, plate width, and plate length before and after heating and heating are measured. Calculate the correlation of the amount of dimensional change,
Measure the plate thickness, plate width, and plate length before and after cooling and lowering when the heating temperature is changed, and correlate the heating temperature with the amount of dimensional change in plate thickness, plate width, and plate length before and after the cooling and lowering process. Is calculated and
In advance, measure the plate thickness, plate width, and plate length before and after cooling and cooling when the cooling rate is changed, and determine the correlation between the cooling rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and cooling. Calculate and
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and lowering temperature,
From the amount of dimensional change that is the desired plate thickness range, plate width range, and plate length range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment. In the heat treatment of a steel sheet having a heat treatment step of heating to a temperature of at least one _{point of Acc.}
In advance, the correlation between the heating rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after heating temperature rise when the heating rate is changed in the heating temperature rise process is calculated by numerical simulation.
In advance, the correlation between the heating temperature and the amount of dimensional change in plate thickness, plate width, and plate length before and after the cooling / lowering process when the heating temperature is changed is calculated by numerical simulation.
In advance, the correlation between the cooling rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and lowering temperature when the cooling rate is changed is calculated by numerical simulation.
Measure the thickness, width, and length of the steel sheet before heat treatment,
Correlation between the heating rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after heating
Correlation between the heating temperature and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and cooling,
Correlation between the cooling rate and the amount of dimensional change in plate thickness, plate width, and plate length before and after cooling and lowering temperature,
From the amount of dimensional change that is the desired plate thickness range, plate width range, and plate length range
A method for producing a steel sheet, which comprises setting an optimum heating rate, heating temperature, and cooling rate to heat-treat the steel sheet before the heat treatment. A steel sheet manufacturing facility for carrying out the method according to any one of claims 2 to 4.
In a steel sheet heat treatment facility that provides a step of heating to a temperature of at least one _{point of Acc.}
An input means for inputting the measured plate thickness before heat treatment, plate width and / or plate length, and
The correlation between the pre-calculated plate thickness before and after the heat treatment and the heating rate, the heating temperature, and the cooling rate and the pre-calculated correlation between the plate width and / or the plate length before and after the heat treatment and the heating rate, the heating temperature, and the cooling rate are stored. Storage medium and
The plate thickness before heat treatment, the plate width and / or the plate length before heat treatment, the desired plate thickness range, the desired plate width range, and / / in the correlation stored in the storage medium. Alternatively, a means for calculating the heating rate, cooling rate, and heating temperature within the desired plate thickness range, plate width range, or plate length range by applying the plate length range, and
A heat treatment facility for a steel sheet, comprising: a heating rate control means for controlling the heating rate, the cooling rate, and the heating temperature, a cooling rate control, and a heating temperature control means calculated by the calculation means. A steel sheet manufacturing facility for carrying out the method according to claim 5.
In a steel sheet heat treatment facility that provides a step of heating to a temperature of at least one _{point of Acc.}
And measured before the heat treatment thickness input means for inputting the plate width and the plate length,
A storage medium that stores the correlation between the plate thickness before and after the heat treatment and the heating rate, the heating temperature, and the cooling rate, and the correlation between the plate width and the plate length before and after the heat treatment and the heating rate, the heating temperature, and the cooling rate.
The stored correlation in the storage medium, and the plate thickness before heat treatment which is input by the input unit, a plate width and the plate length before the heat treatment, the desired thickness range, a desired plate width range and Itacho A means for calculating a heating rate, a cooling rate, and a heating temperature within a desired plate thickness range, plate width range, and plate length range by applying a range, and
A heat treatment facility for a steel sheet, comprising: a heating rate control means for controlling the heating rate, the cooling rate, and the heating temperature, a cooling rate control, and a heating temperature control means calculated by the calculation means.

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