JPH0711024B2 - Manufacturing method of hot rolled steel and its material prediction method - Google Patents

Description

BACKGROUND OF THE INVENTION [FIELD OF THE INVENTION The present invention, hot rolling, starting rolling from when and Ar ₃ transformation temperature or higher temperature region of performing full rolling pass in a temperature range of less than Ar ₃ transformation temperature In the temperature range in which ferrite transformation below the Ar ₃ transformation temperature occurs, that is, in the production of steel materials such as thick plates and hot strips by carrying out hot rolling that terminates rolling in a temperature range in which pearlite and bainite do not occur, the required material and its The present invention relates to a method of predicting a material value from a relationship with a microstructure that is produced according to manufacturing conditions, and a method of manufacturing a steel sheet having a target material value using the predicting method.

[Conventional technology]

Conventionally, strength estimation models are based on simple multiple regression using variables such as composition, hot rolling end temperature, coiling or cooling stop temperature, and the effects of microstructure, ferrite grain size, etc. are taken into consideration. Not not. The use of such an inexact multiple regression model is that the model is intended for products in one rolling mill only, the manufacturing conditions are also started from constant heating conditions, and the austenite grain size before transformation is determined. The rolling conditions including the rolling end temperature are determined based on the product thickness and composition, and the cooling temperature range and cooling rate that govern the transformation behavior are automatically determined from the rolling end temperature and the winding temperature. It depends.

For this reason, the conventional model can be used only under the specific conditions as described above, and the range of the material of the rolled material can be changed by applying it to other lines or widely changing the rolling condition and the cooling condition thereafter. I could not answer the request to expand.

On the other hand, there is a conventional model in which the material of the steel material is described in association with the microstructure. Generally, the material of steel can be expressed as the sum of the terms determined by the microstructure, the grain size, and the terms of other strengthening mechanisms.

For example, it is known that the tensile strength (TS) can be expressed as an equation.

TS = f (σ _f , σ _p , σ _b , σ _m , V _f , V _p , V _b , V _m , d _f ), where σ is a parameter indicating the strength of each tissue, and V is each tissue Is a parameter showing the volume fraction of and d is the particle size.

The subscripts f, p, b, and m are ferrite,
Indicates pearlite, bainite, martensite.

Based on these findings, there is already an attempt to adjust the material using the model described by associating the material of the steel material with the microscopic structure, for example, Japanese Patent Publication No. 58-2246.
A proposal is made in Japanese Patent Laid-Open No. 59-67324.

Japanese Examined Patent Publication No. 58-2246 describes a method for obtaining a transformation structure volume ratio from a cooling curve and predicting the material quality of a steel material from this transformation structure volume ratio.The hardness of the structure, the grain size, and the effect of hot rolling are described. Is not considered at all, and sufficient accuracy cannot be obtained.

Further, in JP-A-59-67324, the final reached austenite grain size and residual strain are calculated from the rolling load of the actual rolling mill, and the ferrite grain size is calculated in the subsequent cooling process, which is obtained by the ferrite grain size and cooling rate. Although the method of estimating the strength by the tissue strengthening parameter is described, the hardness and volume fraction of the microstructure are not taken into consideration, so that sufficient accuracy cannot be obtained.

The present inventors have solved these problems, a manufacturing method that can be applied from the relationship between the required material and the structure of the structure produced along with the manufacturing conditions in the overall manufacturing of steel by hot rolling at the Ar ₃ transformation point or higher. A proposal of Japanese Patent Laid-Open No. 62-158816 is made as a provision. However, not necessarily in the region of 0.1% or more containing Si amount, since the proposal of JP-A-62-158816 there was a poor portion of the precision, the weight of Si in the hot rolling at the newly than the Ar ₃ transformation point Japanese Patent Application No. 63-127582 is proposed to provide a manufacturing method that is more accurate for carbon steel containing around 0.1% in% and is completely new to carbon steel containing 0.2% or more. did.

[Problems to be Solved by the Invention]

JP-A-62-158816 and JP-A-1-298114 mentioned above.
The proposed method is a manufacturing method that can be applied only when hot rolling above the Ar ₃ transformation point and does not take into account the calculation of the grain size change and residual strain of ferrite transformed during rolling. _It cannot be applied to hot rolling from ₃ transformation temperature to Ar ₃ transformation temperature and hot rolling below Ar ₃ transformation point. However, hot rolling in the temperature range below the Ar ₃ transformation temperature is very useful because it enables control of the texture that has caused the refinement of ferrite grains, and the material prediction that can be applied in such cases The development of technology and its utilization technology has been an issue.

[Means for Solving the Problems]

The object of the present invention is to eliminate the drawbacks of the prior art described above, through a metallurgical factor controlling the material of the hot rolled steel material, at least a rolling pass in a temperature range where ferrite transformation occurs below the Ar ₃ transformation temperature. The present invention provides a manufacturing method for predicting the material quality of a hot rolled steel material that has been hot rolled including. The basic unit is cooled after hot rolling at a temperature where the conventional hot slab of carbon steel rolling temperature in all rolling pass is temperature range where ferrite transformation occurs in less than Ar ₃ transformation temperature heat During the production of hot-rolled steel, based on manufacturing conditions such as steel conditions of the hot-rolled steel, heating conditions, rolling conditions and cooling conditions, average austenite before rolling at the time of furnace extraction from a predetermined initial grain size model The grain size do is determined, and a ferrite recrystallization model determined in advance on the basis of the do and the manufacturing conditions,
Austenite after the final rolling pass is calculated by calculating the space factor, average grain size and residual strain of austenite and ferrite for each rolling pass using the ferrite transformation model and the grain size conversion ratio model during ferrite transformation. ,
Space factor _Xa , _Xf , average grain size _a , of each ferrite
_f , residual strains Δε _a , Δε _f are calculated, and the X _a , X _f , _a ,
_{Based on f} , Δε _a , and Δε _f , the space factor and hardness of each structure after completion of cooling are calculated by the transformation model and hardness model of each structure (ferrite, pearlite, bainite, martensite) determined in advance and at the same time. A final average ferrite grain size is calculated using a predetermined grain size conversion ratio model during ferrite transformation and a ferrite grain growth model after winding, and from the final average ferrite grain size and the space factor and hardness of each structure. A method of manufacturing a hot rolled steel product, comprising calculating a material predictive value of the hot rolled steel product from a predetermined material model.

The rolling temperature of the hot rolled slab is Ar in all rolling passes.
_When performing hot rolling in a temperature range where ferrite transformation occurs below ₃ transformation temperatures, in each rolling pass, the space factor of ferrite transformed after the previous pass is calculated using a predetermined ferrite transformation model, and all ferrite and Calculate the space factor of all austenite, the average grain size and residual strain of each group using the austenite recrystallization model for the austenite portion, for the ferrite portion, for the ferrite transformed after the previous pass The average grain size of the ferrite is determined using a grain size conversion ratio model, the residual strain of the ferrite is set to zero, the average ferrite grain size and the average residual strain of the ferrite and the ferrite transformed before the previous pass are determined, Let the ferrite grain size be the inlet side grain size of the path, the average residual strain be the inlet side residual strain, and the inlet side residual strain to be the additional strain of the pass. When the added effective strain is larger than the critical strain of recrystallization, it is divided into two groups of recrystallized and unrecrystallized, and when the effective strain is smaller than the critical strain, only the unrecrystallized group is used. From the entry side grain size, the effective strain and the manufacturing conditions, the space factor and the average grain size in each group were calculated using a predetermined ferrite recrystallization model, and the average ferrite grain size and the residual strain were calculated for each pass. Repeatedly calculated, the space factor of each of the austenite and ferrite after the final rolling pass, the average grain size and the residual strain are determined, and thereafter, the microstructures after completion of cooling (ferrite, pearlite, bainite, martensite) from the calculated values. )
The method for producing a hot-rolled steel product according to claim 1, wherein each volume ratio and hardness are calculated, and at the same time, the final average ferrite grain size is calculated.

In calculating the average grain size and residual strain of the ferrite part, the effective strain ε is given by the formula (1).
When it is larger than _m, it is divided into two groups, recrystallized and non-recrystallized, and the recrystallization _space factor f _fD and the grain size d _fD immediately after rolling are obtained by using the equations (2) and (3), Then, the space factor f _fS of the recrystallized portion is calculated from the equation (4), the change in the recrystallized grain size between the passes is obtained from the equation (5), and the space factor f _{fN of the unrecrystallized} region is calculated.
And the grain size d _fN are calculated by the equations (6) and (7), and when the effective strain ε is smaller than ε _m , only the space factor f _{fN of the unrecrystallized} region and the grain size d _fN are (6) Calculated from Eq. (7), the average ferrite grain size _f immediately before the next pass is calculated by Eq. (8), and the residual strain Δε immediately before the next pass is given by Eq. (9) considering the recovery of strain between passes. It calculates using, The manufacturing method of the hot rolling steel material of Claim 2 characterized by the above-mentioned.

ε _m = h (d _fi ,, T) (1) f _fD = h (d _fi , ε ,, T) (2) d _fD = h (d _fi , ε, T) (3) f _fS = h ( ε, T, t) (4) d _fS = h (d _fD , T, t) (5) f _fN = h (f _fD , f _fS ) (6) d _fN = h (d _fi , ε) (7 ) _f = h (f _fD , f _fS , f _fN , d _fS , d _fN ) (8) Δε _f = h (f _fD , f _fN , ε _m , ε, T, (% C)) (9) where And d _fi is the ferrite grain size (μm) on the path entry side,
Is strain rate (s ^-1 ), T is rolling temperature (° C), ε is working strain,
t is the time after processing (seconds), (% C) is the carbon content (% by weight).

When producing a hot-rolled steel product, the hot cast slab of ordinary carbon steel is rolled from a temperature range of Ar ₃ transformation temperature or higher and finished rolling in a temperature range of less than Ar ₃ transformation temperature and then cooled to produce a hot rolled steel material, Average austenite grain size before rolling when extracting the heating furnace from a predetermined initial grain size model based on manufacturing conditions such as steel material conditions, heating conditions, rolling conditions and cooling conditions of hot rolled steel products
Obtaining do, using an austenite recrystallization model, a ferrite recrystallization model, a ferrite transformation model, and a grain size transformation ratio model at the time of ferrite transformation, which are predetermined based on the do and the manufacturing conditions, and a temperature of Ar ₃ transformation temperature or higher. In the rolling pass in the region, the average grain size and residual strain of austenite are calculated, and in the rolling pass in the temperature region below the Ar ₃ transformation temperature, austenite, the space factor of each ferrite, by calculating the average grain size and residual strain, Space factor of austenite and ferrite after the final rolling pass
_Xa , _Xf , average grain diameters _a , _f , residual strains Δε _a , Δε _f are determined, and the respective textures (ferrite, predetermined) based on the X _a , X _f , _a , _f , Δε _a , Δε _f are determined. The space factor and hardness of each structure after cooling is calculated by the transformation model of pearlite, bainite, martensite) and hardness model, and at the same time, the grain size conversion ratio model at the time of ferrite transformation and the ferrite after surrounding The final average ferrite grain size is calculated using a grain growth model, and the material prediction value of the hot-rolled steel is calculated from the material model determined in advance from the final average ferrite grain size and the space factor and hardness of each structure. A part of the manufacturing conditions is corrected so that the predicted value of the material and the target value of the material match, and the hot steel is manufactured based on the corrected manufacturing conditions to obtain the target material. Of cold rolled steel .

The rolling pass steel temperature in the Ar ₃ transformation temperature or higher temperature range, the average particle size and residual strain of austenite was calculated, in the rolling pass temperature range below the steel temperature is Ar ₃ transformation temperature, a predetermined ferrite transformation model Obtain the space factor of the ferrite transformed after the previous pass using, calculate the space factor of all ferrite and all austenite, for the austenite portion using the austenite recrystallization model the average grain size of each group For the ferrite part, the average grain size of the ferrite was determined using the grain size conversion ratio model for the ferrite transformed after the previous pass, and the residual strain of the ferrite was set to zero, and The average ferrite grain size and average residual strain of the ferrite transformed before the pass are calculated, and the ferrite grain size is calculated as the entrance grain of the pass. If the effective strain obtained by adding the input residual strain to the additional strain of the path is larger than the critical strain of recrystallization, the average residual strain is defined as the input residual strain. When divided into groups, when the effective strain is smaller than the critical strain, only the non-recrystallized group is used, and from the entrance side grain size, the effective strain, and the manufacturing conditions, a predetermined ferrite recrystallization model is used. The space factor and average grain size in the group are calculated, the average ferrite grain size and residual strain are repeatedly calculated for each pass, and the space factor, average grain size and residual strain of austenite and ferrite after the final rolling pass are calculated. Then, the volume ratio and hardness of each structure (ferrite, pearlite, bainite, martensite) after completion of cooling are calculated from the calculated values, and at the same time, the final average ferrite grain size is calculated. The method for manufacturing a hot-rolled steel product according to claim 4, characterized in that.

In calculating the average grain size and residual strain of the ferrite part, the effective strain ε is given by the formula (1).
When it is larger than _m, it is divided into two groups, recrystallized and non-recrystallized, and the recrystallization _space factor f _fD and the grain size d _fD immediately after rolling are obtained by using the equations (2) and (3), Then, the space factor f _fS of the recrystallized portion is calculated from the equation (4), the change in the recrystallized grain size between the passes is obtained from the equation (5), and the space factor f _{fN of the unrecrystallized} region is calculated.
And the grain size d _fN are calculated by the equations (6) and (7), and when the effective strain ε is smaller than ε _m , only the space factor f _{fN of the unrecrystallized} region and the grain size d _fN are (6) Calculated from Eq. (7), the average ferrite grain size _f immediately before the next pass is calculated by Eq. (8), and the residual strain Δε immediately before the next pass is given by Eq. (9) considering the recovery of strain between passes. It calculates using, The manufacturing method of the hot rolling steel material of Claim 5 characterized by the above-mentioned.

ε _m = h (d _fi ,, T) (1) f _fD = h (d _fi , ε ,, T) (2) d _fD = h (d _fi , ε, T) (3) f _fS = h ( ε, T, t) (4) d _fS = h (d _fD , T, t) (5) f _fN = h (f _fD , f _fS ) (6) d _fN = h (d _fi , ε) (7 ) _f = h (f _fD , f _fS , f _fN , d _fS , d _fN ) (8) Δε _f = h (f _fD , f _fN , ε _m , ε, T, (% C)) (9) where And d _fi is the ferrite grain size (μm) on the path entry side,
Is strain rate (s ^-1 ), T is rolling temperature (° C), ε is working strain,
t is a time (second) after processing, and (% C) is a carbon content (% by weight).

[Action]

First, based on the components of a normal carbon steel to which the present invention is applicable, C: 0.02 to 0.4%, Mn: 0.2 to 1.2%, and Si ≤ 2 by weight%.
% Is always included and no special alloying element is added.

The description of the material calculation method and the operation thereof according to the present invention will be described below.

The material of the hot rolled steel material varies depending not only on the composition but also on manufacturing conditions such as rolling conditions and cooling conditions. Since the material of the steel material is associated with the microstructure of the steel material, a model for predicting the material from the manufacturing conditions by using the microstructure has been conventionally constructed (JP-A-62-158816 and JP-A-1-158816). See 298114). The conventional model is premised on rolling only at the Ar ₃ transformation temperature or higher, and is composed of two models: a model for predicting the final microstructure of hot-rolled steel and a model for predicting the material from that microstructure. Has been done. FIG. 3 shows a flow chart of calculation in this conventional model. In the following explanation of the conventional model,
See FIG.

To predict the microstructure, first calculate the austenite grain size d _ao after the furnace extraction by the initial austenite grain size model shown in Eq. (10) based on the steel composition and the heating conditions in the furnace. , The average austenite grain size _ai and residual strain after rolling for each pass based on the austenite recrystallization model shown in Eqs. (11) to (21) based on the value and the steel composition and rolling conditions in each pass. Δε _i is calculated and repeated to obtain the average austenite grain size and residual strain after the final rolling pass. Further, corresponding to the subsequent cooling step, the average austenite grain size and residual strain after the final rolling pass and the components of the steel material, the ferrite transformation model shown in the formula (22) based on the cooling conditions, in the formula (23) The space factor of each structure is obtained by the pearlite transformation model shown and the bainite transformation model shown in Eq. (24), and the remaining portion is obtained as the space factor of martensite, and at the same time, in Eqs. (25) to (27). The hardness of each tissue is determined from the hardness model of each tissue shown.
Further, since the grain size changes when transforming from austenite to ferrite, based on the average austenite grain size after the final rolling pass and residual strain and ferrite space factor, Ar ₃ transformation temperature obtained from cooling conditions (28) The ferrite grain size immediately after transformation is calculated from the grain size conversion ratio model shown in the equation, and further, taking into consideration the grain growth after winding, based on the ferrite grain size immediately after transformation and the winding temperature, The final ferrite grain size is determined from the grain growth model after winding shown. Based on the factors of the microstructure (space factor / hardness of each structure, final ferrite grain size) obtained as described above, (30) to (32)
Predict the final material value (tensile strength, yield stress, total elongation) of the hot-rolled steel from the material model represented by the formula.

However, Ar ₃ is also in the final rolling pass as shown rolling path serrated in Figure 2 in the case of this model (a)
Has assumed to perform hot rolling at lower than the transformation point temperature, rolling from Ar ₃ transformation temperature or higher temperature range to start as shown in the second diagram of (b) (I), Ar 3 transformation point When applied when performing hot rolling that terminates rolling in the temperature range in which ferrite transformation below occurs, and (II) in Fig. 2 (b)
As shown in (III) and (III), when the rolling temperature in the total pressure pass is in the temperature range below the Ar ₃ transformation temperature, the conventional model cannot calculate the behavior such as recrystallization and grain growth of the ferrite generated during rolling. However, the accuracy of material prediction was extremely poor. Therefore, as a result of consideration by the present inventors, in order to accurately predict the material change of hot rolling in a temperature range in which ferrite transformation below the Ar ₃ transformation point occurs, recrystallization in the ferrite zone, grain growth, and static It is necessary to formulate the respective dynamic recovery behavior and newly create a ferrite recrystallization model for obtaining the average ferrite grain size and residual strain after rolling. In order to create the ferrite recrystallization model, the present inventors hot-worked steel materials of various components under various processing conditions, analyzed their metallurgical microstructures, and searched for their causal relationship.

The ingenuity was that the ferrite recrystallization model and each of the previous models had to be combined in order to obtain the average grain size and residual strain after rolling. The flow chart of the calculation is shown in FIG. The calculation method will be described. In the hot rolling pass above the Ar ₃ transformation temperature, the average austenite grain size and residual strain are determined using only the conventional austenite recrystallization model. For the rolling pass where the rolling temperature is below the Ar ₃ transformation temperature (however, in the temperature range where pearlite and bainite do not occur), first calculate the ferrite space factor using the conventional ferrite transformation model that incorporates the effect of strain-induced transformation. Then, the austenite space factor is determined by subtracting the ferrite space factor from 1, and then the grain size conversion ratio is used to determine the grain size of ferrite immediately after transformation transformed after the previous pass using a model. For the austenite part, the average austenite grain size and residual strain are determined using the austenite recrystallization model, and for the ferrite part, the newly found ferrite recrystallization model is used to determine the average ferrite grain size and residual strain. At this time, in the rolling pass immediately after the ferrite was first formed, the ferrite grain size immediately after transformation calculated using the grain size conversion ratio model was set as the inlet side grain size and the inlet side residual strain was set to zero, and in the subsequent rolling passes, The average grain size and the average residual strain of the ferrite that has undergone strain generated up to the pass and the ferrite that has newly generated after the previous pass and has just undergone transformation are referred to as the inlet side grain size and the inlet side residual strain, respectively. The above calculation is repeated up to the final rolling pass to obtain the average grain size and residual strain of austenite and ferrite by all passes after the final rolling pass, and then connect to the transformation model and thereafter.

The ferrite recrystallization model will be described below. In the case of ferrite, it has been conventionally established that dynamic recrystallization does not occur, but the inventors of the present invention conducted an immediate cooling after hot working using a compression type hot working simulator,
It was discovered that recrystallization occurs even immediately after hot working. This recrystallization structure is considered to be a mixed structure of dynamic recrystallization structure and static recrystallization structure. This recrystallization occurs when the effective strain is larger than the critical strain ε _m shown in equation (1), and the recrystallization _space factor f _fD can be expressed by equation (2). The effective strain is the effective strain obtained by adding the residual strain of the rolling pass before that to the additional strain of the rolling pass.If there is residual strain, it is necessary to use this effective strain for calculation. is there. ε _m increases as the initial grain size increases, the strain rate increases, and the rolling temperature decreases, and f _fD increases as the initial grain size decreases, the strain increases, and the rolling temperature increases. Further, the recrystallized grain size d _fD of the recrystallization generated immediately after this rolling becomes finer as the additional strain is larger, the initial grain size is smaller, and the temperature is lower, which can be expressed by the formula (3).

If recrystallization that occurs immediately after this rolling occurs, the recrystallization space factor will statically increase after that when it is held equivalent, but the speed increases as the strain increases and the rolling temperature increases, and the increase increases. The minute f _fS can be expressed by equation (4). The growth rate of the grain size at that time is higher as the temperature is higher, and can be expressed by equation (5).

The portion that has not been recrystallized is an unrecrystallized portion, and the space factor f _fN can be expressed by equation (6), and the grain size d _fN can be expressed by equation (7) by incorporating the effect of flattening.

Therefore, when the additional strain is larger than the recrystallization limit ε _m , the space factor and the grain size of each of the two groups of the recrystallized region and the non-recrystallized region are calculated, and the additional strain is ε _m.
If it is smaller than this, recrystallization does not occur, so only the grain size in the non-recrystallized region is calculated. The average ferrite grain size _a after a lapse of time t after rolling is calculated by using the space factor occupied by grains belonging to each recrystallization form and the grain size of each crystal grain grown during the time t. Calculate using equation (8) using the dimensional average.

Regarding the release of strain during this time t, ε _m is averaged in the grains recrystallized immediately after processing, zero is retained in the recrystallized grains thereafter, and effective strain remains in the non-recrystallized grains. These strains are calculated by the equation (9) assuming that they are statically recovered during the time t, and a new mathematical model for finding the residual strain Δε from the sum of them is newly found.

As described above, the average grain size and the residual strain when the ferrite is hot rolled can be obtained.

As described above, when this ferrite crystal model is hot-rolled at a known Ar ₃ transformation point or higher, by incorporating it into the model that predicts the material of the steel sheet, the heat in the temperature range where ferrite occurs below the Ar ₃ transformation point It is possible to accurately predict the material of the steel plate in the hot rolling pass. Therefore,
By using the material prediction method, by determining manufacturing conditions such that the difference between the material prediction value calculated by this hot rolling steel material prediction method and the predetermined material target value is as small as possible, It is possible to predetermine the target manufacturing conditions of the hot steel material with respect to the material target value.

Furthermore, remodeling the equipment in the manufacturing process of hot rolled steel,
It can also be used as a powerful tool in the examination from the aspect of the material fabrication of steel materials in determining the equipment specifications when newly adding or deleting.

Also, during the actual production of hot rolled steel, if the actual production conditions deviate from the target production conditions, using the hot rolled steel production method of the present invention, the material prediction value again under the actual production conditions. Is calculated, and some of the subsequent target manufacturing conditions are modified so that the predicted value of the material matches the target material value, and finally the hot rolled steel material with a material very close to the target material value It is possible to manufacture Since this hot rolled steel manufacturing method is based on metallurgical theory, it can respond to changes in the characteristics of each manufacturing line and manufacturing method, and efficiently manufactures the required steel material with good yield. You can do it.

ε _m = h (d _fi ,, T) (1) f _fD = h (d _fi , ε ,, T) (2) d _fD = h (d _fi , ε, T) (3) f _fS = h ( ε, T, t) (4) d _fS = h (d _fD , T, t) (5) f _fN = h (f _fD , f _fS ) (6) d _fN = h (d _fi , ε) (7 ) _f = h (f _fD , f _fS , f _fN , d _fS , d _fN ) (8) Δε _f = h (f _fD , f _fN , ε _m , ε, T, (% C)) (9) where And d _fi is the ferrite grain size (μm) on the path entry side,
Is strain rate (s ^-1 ), T is rolling temperature (° C), ε is working strain,
t is a time (second) after processing, and (% C) is a carbon content (% by weight).

d _ao = h (α, T _o , t _o ) (10) where d _ao is the austenite grain size at the time of furnace extraction, α
Is the heating rate, T _o is the heating temperature, and t _o is the holding time.

ε _c = h (d _ai , T) (11) f _aD = h (d _ai , ε ,, T) (12) d _aD = h (, T, (% C), (% Mn) (13) f _aS = h (ε, T, t) (14) d _aS = h (d _ai , ε) (15) f _aN = h (f _aD , f _aS ) (16) d _aN = h (d _ai , ε) (17) d _aDG = h (d _aD , T, t) (18) d _aSG = h (d _aS , T, t, (% C), (% Mn), (% Si)) (19) _a = h (f _aD , f _aS , f _aN , d _aDG , d _aSG , d _aN ) (20) Δε _a = h (f _aD , f _aN , ε _c , ε, T, t) (21) where f _aD , f _aS , and f _aN are the dynamic recrystallization space factor, static recrystallization space factor, and _{unrecrystallized space} factor of austenite, and d _aD , d _aS , and d _aN are the austenite dynamics. Recrystallized grain size, static recrystallized grain size, _{unrecrystallized} grain size, d _aDG , d
_aSG is the grain size of austenite after grain growth of dynamic recrystallization and static recrystallization. Further, ε _c is the critical strain of dynamic recrystallization, _a and Δε _a are the average grain size and residual strain of austenite after each rolling pass, and d _ai is the inlet side austenite grain size of each pass, (% Mn )and(%
Si) is the content (wt%) of Mn and Si, respectively.

X _f = h ( _a , Δε _a , T, t, (% C), (% Mn), (% S
i)) (22) X _p = h ( _a , Δε _a , T, t, (% C), (% Mn), (% S
i)) (23) X _b = h ( _a , Δε _a , T, t, (% C), (% Mn), (% S
i)) (24) where X _f , X _p , and X _b are ferrite, pearlite, and
The space factor of bainite, T is the temperature (° C) during cooling,
t is a cooling time (second).

H _f = h (CR, T, t, (% C), (% Mn), (% Si)) (25) H _p = h (CR, X _p , T, t, (% Mn), (% Si)) (26) H _b = h (CR, T, t, (% C), (% Mn), (% Si)) (27) where H _f , H _p , and H _b are ferrite, Perlite,
Hardness of bainite, CR is cooling rate.

d _fO = h (X _f , _a , Δε _a , T, (C%), (% Mn), (% S
i)) (28) d _fC = h (d _fO , CT, X _fR , d _fr ) (29) where d _fO and d _fC are the ferrite grain size immediately after transformation and the final ferrite grain size after winding, respectively. , X _fR , d _fR are the ferrite space factor immediately before cooling, and the ferrite grain size CT is the coiling temperature (℃).

TS = h (d _fC , X _f , X _p , X _b , X _m , H _f , H _p , H _b ) (30) YS = h (d _fC , X _f , X _p , X _b , X _m , H _f , H _p , H _b ) (31) T.E1 = h (d _fC , X _f , X _p , X _b , X _m , H _f , H _p , H _b ) (32) where X _m Is the space factor of martensite, TS, YS, and T.E1 are tensile strength, yield strength, and total elongation, respectively.

〔Example〕

The mark ● shown in FIGS. 4 to 6 indicates the case of (I) of FIG. 2 (b), and the mark ○ indicates the case of (II) or (III) of FIG. 2 (b). The material value of the final hot-rolled steel set as each target value of, i.e., tensile strength, yield strength, total elongation, and the target manufacturing conditions are set using the material calculation method of the hot-rolled steel in the present invention. Moreover, the manufacturing process is performed while the actual manufacturing conditions are corrected, and the correspondence with the actually measured value of the finally obtained material is shown. The components of the sample steel used at this time are as shown in Table 1, and as the model formulas of the present invention, formulas (1) to (9) were each made into a concrete formula (41).
~ (49) is used, and the other necessary relational expressions are (10) ~
The formula (32) is represented by JP-A-62-158816 and JP-A-1-2981.
Each formula and each coefficient described in the following formulas (50) to (72) are quoted from No. 14 and used concretely. Table 2 shows each coefficient used for the calculation in each equation. As can be seen from FIGS. 4 to 6, according to the method of the present invention, the hot rolled steel material having the target material can be accurately manufactured.

[Ferrite recrystallization model] <Recrystallization model> ε _m = A ₁ · d _foia ^a ε ^b · exp (A _z / RT _i ) (41) f _fD = 1-exp {− (ε _i −ε _m ) / K ₁ } (42) k ₁ = A ₃・ d _foi ^c・ exp (A ₄ / RT _i ) d _fD = A ₅・ ε _i ^−d・ d _foi ^e・ exp (-A _G / RT _i ) ( 43) f _fS = 1-exp {-(t _i / k ₂ } (44) k ₂ = A ₇・ ε ^−f・ exp (A ₈ / RT) d _fS ² = d _fD ² + A ₉・ exp ( -A ₁₀ / RT _i ) ・ t _i (45) < _{Unrecrystallized} model> f _fN = 1-f _fD -f _fS (46) d _fN = d _foi・ exp (1-ε _i / 4) (47) <Average particle size model> _fi = f _fD + f _fS ) / d _fS ² + f _fN / d _fN ² } ^-1/2 (48) <Recovery model> Δε _fi = (f _fD · ε _m + f _fN · ε _i ) ・ exp {−t _i /
τ _k ) ⁹ (49) τ _k = k ₃ · exp (k ₄ / RT _i ) k ₃ = A ₁₁ · exp [-exp {-A ₁₂ · (% C) ＋ A ₁₃ }] k ₄ = exp {- A ₁₄ · (% C) + A ₁₅ } + A ₁₆ where d _foi is the initial grain size (μm) of the ferrite before the i-th pass, _i is the strain rate (s ⁻¹ ), and T _i is the processing temperature ( ℃), ε _i is the processing strain, t _i is the time (second) after processing, (%
C) is the carbon content (% by weight), a, b, c, d,
e, f, g, and A _{1 to} A ₁₆ are obtained by experiments.

<Initial austenite grain size model> d _aO ² = (k ₅ · α ^−h ) ² + A ₁₇ · exp (-A ₁₈ / To) to ⁱ (50) k ₅ = A ₁₉ {To-1173) / 100} ³ + A ₂₀ h, i, A _{17 to} A ₂₀ are experimentally determined.

α is the heating rate, To is the heating temperature, and to is the holding time.

[Austenite recrystallization model]

<Dynamic Recrystallization Model> ε _c = A ₂₁ · d _aoij · exp (A ₂₂ / RT _i ) (51) T _i is the rolling temperature and R is the gas constant.

o, A ₂₀ , and A ₂₁ are obtained by experiments.

f _aD = 1-exp [-{(ε _i −ε _c ) / (ε _s −ε _i )}
^M ] (52) ε _s = A ₂₃・ {1-exp (-d _aoi / K ₆ )} k ₆ = A ₂₄・ -k ・ exp (-A ₂₅ / T _i ) M = A ₂₆・ exp (A ₂₇ / T _i ) d _aD = K ₇・ Z ^l (53) K ₇ = A ₂₈・ Q _o -A ₂₉ Q = A ₃₀ -A ₃₁・ C _eq C _eq = (% C) + (% Mn) / 6 Z = · exp (Q _o / RT) K, 1, A _{23 to} A ₃₁ are experimentally determined.

<Static recrystallization model> f _aS = (1-f _D ) [1-exp {-(t _i / τ _S ) ^m }] (54) τ _S = A ₃₂ · ε ⁻ⁿ · exp (A ₃₃ / RT _i ) m, n, A _{32 to} A ₃₃ are experimentally determined.

d _aS = A ₃₄ · d _ao ^o · ε ^−p (55) < _{Unrecrystallized} model> f _aN = 1−f _aD −f _aS (56) o, p, and A ₃₄ are obtained by experiments.

d _aN = d _aoi・ exp (−ε / 4) (57) <Grain growth model> d _aDG ² = d _aD ² + A ₃₅・ exp (-A ₃₆ / RT _i ) ・ t _i ^q (58) d _aSG ² = d _aS ² + A ₃₇・ (% C) _−γ・ (% Si) ^−S・ (% Mn) ^−t・ ex
p (−A ₃₈ / RT _i ) · t _i ^u (59) q, r, s, t, u, A _{35 to} A ₃₈ are obtained by experiments.

<Average particle size model> _ai = (f _aD / d _aDG ² + f _aS / d _aSG ² + f _aN / a _aN ² ) ^-1/2 (60) <Recovery model> Δε _ai = (f _aD · ε _c ＋ f _aN・ ε _i ) ・ exp {-(t _i / τ
_K ) ^v (61) τ _K = A ₃₉ · exp (A ₄₀ / RT _i ) v, A ₃₉ , and A ₄₀ are obtained by experiments.

[Ferrite transformation model]

X _f / X _fmax = 1-exp {-K ₄ · (t _i −τ _I ) ^w } (Si% <0.1wt
%) (62) K ₈ = 1 / 2.24 ・ (2.24 ・ k ₉ / _ai + A ₄₁・ Δε _ai ² ) ・ (1 + A
₄₂・ Δε _ai ) k ₁₀ k ₉ = (1/2) {γ ² −β ・ γ ²・ (α ² −γ ² ) ^−1/2・
k ₁₁ + β · (α ² −γ ² ) ⁻¹ / ² · k ¹² } α = exp (Δε), β = 1, γ = exp (−Δε) k ₁₁ = ▲ ∫ ^u _o ▼ (1-K ₉ ²・ sin ² θ) ^1/2 dθ k ₁₂ = ▲ ∫ ^u _o ▼ (1-K ₉ ²・ sin ² θ) ^1/2 dθ u = arccos (γ / α) K ₁₃ = {α ²・ (β ^2- γ ² ) / β ² · (α ² −
γ ² )} ^1/2 K ₁₀ = exp {A ₄₃ -A ₄₄・ (% C) −A ₄₅・ (% Mn) ＋ A ₄₆・ (T-2
73) -A ₄₇・ (T-273) ² } τ ₁ = exp (−A ₄₈・ ln k ₁₀ + A ₄₉・ ln T + A ₅₀ / TA ₅₁ ) X _fmax = 1-(% C) / {A ₅₂ + A ₅₃・ (T-273) + A ₅₄・ (T-273) ² } (T ≧ 9
93K) = 1 - (% C ) / {A 52 + A 53 · 720) + A 54 · 720 2}
(T <993K) ω, A _{41 to} A ₅₄ are experimentally obtained.

X _f / X _fmax = 1-[{1 + K ₁₄ / x ・ (t−τ ₂ )} ^−x ] (Si% ≧
0.1wt%) (62) ′ K ₁₄ = 1 / 2.24 ・ (2.24 ・ k ₉ / _ai + A ₄₁・ Δε _ai ² + A ₅₅ ) ・ (1 + A ₄₂・ Δε _ai ) ・ k ₁₅ k ₁₅ = exp [-A ₅₆ + A ₅₇・ ln (% Si) + A ₅₈・ {ln (% Si)} ² -A ₅₉・ (% C) -A ₆₀・ (% Mn) + A ₆₁・ (T-273) -A
₆₂・ (T-273) ² ] τ ₂ = τ _f /{1/2.24 ・ (2.24 ・ k ₉ / _ai + A ₄₁・ Δε _ai ² + A ₅₅ ) ・ (1 + A ₄₂・ Δ
ε _ai )} τ _f = exp [−A ₆₃ -A ₆₄・ ln (% Si) -A ₆₅・ {ln (% Si)} ² + A ₆₅・ (% C) + A ₆₇・ (% Mn)- A ₆₈ / (T-27
3) -A ₆₉ · (T-273) ² ] x = A ₇₀ + A ₇₁ · ln (% Si) + A ₇₂ · {ln (% Si)} ² A _{55 to} A ₇₂ are experimentally determined.

[Perlite transformation model]

Xp / X _pmax = 1-exp {-k ₁₆・ (t _i −τ ₃ ) ^y } (Si% <0.1wt%)
(63) K ₁₆ = 1 / 2.24 ・ (2.24 ・ k ₉・ K ₁₇ / _ai + A ₇₃・ k ₁₈・ Δε _ai ² ) ・
(1 + A ₇₄・ Δε _ai ) k ₁₉ K ₁₇ = (1-X _f ) ^2/3 K ₁₈ = (1-X _f ) K ₁₉ = exp {A ₇₅ -A ₇₆・ (% C) -A ₇₇・ (% Mn) + A ₇₈・ (T-273) + A ₇₉・ (T-
273) ² } τ ₃ = exp (−A ₈₀・ ln K ₁₆ + A ₈₁・ ln T + A ₈₂ / TA ₈₃ ) X _pmax = 1-X _fmax Xp = 1-exp [{1 + k ₁₄ / x ・(T-τ ₂ )} ^-x (Si% ≧ 0.1wt%) (6
3) 'A _{73 to} A ₈₃ are obtained by experiments.

<Bennite transformation model> X _b / X _bmax = 1-exp {-k ₂₀ · (t _i −τ ₄ ) ^y } (Si% <0.1 wt
%) (64) K ₂₀ = 1 / 2.24 ・ (2.24 ・ k ₉・ K ₂₁ / _ai + A ₈₄・ k ₂₂・ Δε _ai ² ) ・ (1 + A
₈₅・ Δε _ai ) k ₂₃ K ₂₁ = (1-X _f -X _p ) ^2/3 K ₂₂ = (1-X _f -X _p ) K ₂₃ = exp {A ₈₆ -A ₈₇・ (% C)- A ₈₈・ (% Mn) + A ₈₉・ (T-273) -A ₉₀・ (T-
273) ² } τ ₄ = exp (−A ₉₁・ ln K ₂₀ + A ₉₂・ ln T + A ₉₃ / TA ₉₄ ) X _bmax = 1-X _fmax -X _pmax y, A _{84 to} A ₉₄ are obtained by experiments. .

X _b / X _bmax = 1-exp {-k ₂₄・ (t _i −τ ₅ )} ^z } Si% ≧ 0.1wt
%) (64) ′ K ₂₄ = 1 / 2.24 ・ (2.24 ・ k ₉ / _ai + A ₄₁・ Δε _ai ² + A ₅₅ ) ・ (1 + A ₄₂・
Δε _ai ) k ₂₅ K ₂₅ = exp [-A ₉₅ -A ₈₆・ (% Si) ² -A ₉₇・ (% C) -A ₉₈・ (% Mn) + A ₉₉・ (T-
273) -A ₁₀₀・ (T-273) ² ] τ ₅ = τ _b {1 / 2.24 ・ (2.24 ・ k ₉ / _ai + A ₄₁・ Δε _ai ² + A ₅₅ ) ²・
(1 + A ₄₂・ Δε _ai )} τ _b = exp [−A ₁₀₁ + A ₁₀₂・ (% Si) -A ₁₀₃・ (% Si) ² + A ₁₀₄ (% C) + A ₁₀₅・ (% Mn) -A ₁₀₆ · (T-273) + A ₁₀₇ · (T-273) ² ] z, A _{95 to} A ₁₀₇ are obtained by experiments.

<Ferrite hardness model> H _f = H _fo + B ₁・ exp [B ₂ / T] (65) H _fo = B ₃ + B ₄・ (% C) + B ₅・ (% Mn) + B ₆・(% Si) -B ₇・ ln t _c (T ≧ 923
K) = B ₃ + B ₄・ (% C) + B ₅・ (% Mn) + B ₆・ (% Si) -B ₇・ ln t _c -B ₈ [1, exp {-B ₉・ (( T-773) / (923-T)) ^A ) ・ ln t _c (723K ≦ T
<923K) t _c = (Ar ₃ -T) / CR A, B _{1 to} B ₉ are obtained by experiments.

CR is the cooling rate.

[Perlite hardness model]

H _p = Σ {ΔX _p · K ₂₅ } / ΣΔX _p (66) K ₂₆ = B ₁₀ -B ₁₁ · (Ae ₁ -T) ⁻¹ Ae ₁ = B ₁₂ -B ₁₃ · (% Mn) + B ₁₄・ (% Si) B _{10 to} B ₁₄ are obtained by experiments.

ΔX _p is the amount of pearlite that appears at each temperature.

<Bainite transformation model> H _b = B ₁₅ + B ₁₆ · (% C) ^B + B ₁₇ · (% Si) -B ₁₉ · (T-273) ^c · ln t _c (6
_{7) B, C, B 15} ~B 19 is determined by experiment.

d _fo = exp {B ₂₀ + B ₂₁・ ln X _f -B ₂₂・ K ₂₇ -B ₂₃・ K ₂₈ -B ₂₄・ ln (1 + A ₄₂
· Δε) - <particle size conversion ratio _{model> B 25 · ln (% C} ) -B 26 · ln (% Mn)} (68) K 27 = ln [{B 27 -B 28 · (% Si) + B ₂₉・ (% Si) ² } ³・ {(2.24 / 40) / (2.24 / 40 + A ₅₅ }] K ₂₈ = ln (2.24 ・ k ₉ / _ai + A ₄₁・ Δε _ai ² + A ₅₅ ) Grain growth model after removal] d _f = {X _{f / R} / d _fR ² + (1-X _fR ) / d _fc ² } ^-1/2 (69) d _fc = d _fo (T ≦ 723K) ＝ d _fo + B ₃₀ · exp (-B ₃₁ / T) (T> 723K) B _{20 to} B ₃₁ are determined by experiments.

[Material model]

TS = B ₃₂ + B ₃₃・ X _f・ d _f ^-1/2 + B ₃₄・ X _b・ d _a ^-1/2 + B ₃₅・ X _m ^1/2 + B ₃₆・ (H _f・ X _f + (H _p・ X _p + H _b・ X _b ) (70) YS = B ₃₇ + B ₃₃・ X _f・ d _f ^-1/2 + B ₃₉・ X _b・ d _a ^-1/2 + B ₄₀・ X _m ^1/2 + B ₄₁・ (H _f・ X _f + H _p・ X _p + H _b・ X _b ) (71) T.E1 = B ₄₂ -B ₄₃・ X _f・ d _f ^-1/ 2- B ₄₄・ X _b・ d _a ^-1/2 -B ₄₅・ X _m ^1/2 -B ₄₆・ (H _f・ X _f + H _p・ X _p + H _b・ X _b ) (72) B ₃₂ ~ B ₄₆ is obtained by experiment.

〔The invention's effect〕

By the method of the present invention described above, predict the material of hot rolled steel when performing hot rolling in a temperature range where pearlite and bainite do not occur below the Ar ₃ transformation point, and based on the estimated value By setting the components and manufacturing conditions, it is possible to produce hot rolled steel products that are controlled to target values with high precision, efficiently, with high yield and at low cost. The effect is great.

[Brief description of drawings]

FIG. 1 shows the flow of the part for calculating the grain size and residual strain during hot rolling in the material calculation of the present invention. FIG. 2 shows a schematic diagram of steel material temperature transitions in hot rolling (a) applicable by the conventional method and hot rolling (b) applicable by the method of the present invention. FIG. 3 shows a flow chart of the conventional material calculation that can be applied only when hot rolling is performed in a temperature range not lower than the Ar ₃ transformation point. 4, 5 and 6 show the mechanical characteristic values actually obtained by the tensile test, that is, the tensile strength TS,
The correspondence diagram of the yield stress YS and the total elongation E1 and their calculated values in the present invention is shown.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Wakita 1 Nishinosu, Oita, Oita-shi, Oita Pref., Nippon Steel Co., Ltd. Oita Works (72) Inventor Isaichi Esaka 1-Nishinozu, Oita, Japan New Japan Oita Steel Works, Ltd.

Claims (6)

[Claims] 1. A hot rolled slab of ordinary carbon steel is hot-rolled under a temperature condition in which the rolling temperature in all rolling passes is less than the Ar ₃ transformation temperature and in a temperature range in which ferrite transformation occurs, followed by cooling and hot rolling. When manufacturing a steel material, based on manufacturing conditions such as steel material conditions of the hot rolled steel material, heating conditions, rolling conditions and cooling conditions, the average austenite grain size before rolling at the time of furnace extraction from a predetermined initial grain size model The do is determined, and the ferrite recrystallization model, the ferrite transformation model, and the grain size conversion ratio model at the time of ferrite transformation, which are predetermined based on the do and the manufacturing conditions, are used to determine the austenite and ferrite occupancy for each rolling pass. By calculating the product factor, average grain size and residual strain, the space factors _Xa , _Xf , and average grain size of the austenite and ferrite after the final rolling pass are calculated.
_a , _f , residual strains Δε _a , Δε _f are obtained, and the X _a , X _f ,
_{Based on a} , _f , Δε _a , and Δε _f , the space factor and hardness of each structure after completion of cooling were calculated by a transformation model and hardness model of each structure (ferrite, pearlite, bainite, martensite) determined in advance. At the same time, the final average ferrite grain size is calculated using a grain size conversion ratio model during predetermined ferrite transformation and a ferrite grain growth model after winding, and the final average ferrite grain size and the space factor of each structure and A method of manufacturing a hot rolled steel product, characterized in that a material predictive value of the hot rolled steel product is calculated from a hardness-determined material model. 2. When the hot-rolled slab is hot-rolled in a temperature range in which the ferrite temperature is lower than the Ar ₃ transformation temperature in all rolling passes, a predetermined ferrite transformation model is used in each rolling pass. Obtain the space factor of the ferrite transformed after the previous pass using, calculate the space factor of all ferrite and all austenite, for the austenite portion using the austenite recrystallization model the average grain size of each group For the ferrite part, the average grain size of the ferrite was calculated using the grain size conversion ratio model for the ferrite transformed after the previous pass, and the residual strain of the ferrite was set to zero, and the ferrite and the previous pass were determined. The average ferrite grain size and average residual strain with the previously transformed ferrite are determined, and the ferrite grain size is defined as the entrance-side grain size of the path. When the average residual strain is defined as the inlet-side residual strain and the effective strain obtained by adding the inlet-side residual strain to the additional strain of the path is larger than the critical strain of recrystallization, it is divided into two groups, recrystallized and unrecrystallized. When the effective strain is smaller than the critical strain, only the non-recrystallized group is used, and a ferrite recrystallization model determined in advance is used for each group based on the entrance side grain size, the effective strain and the manufacturing conditions. Calculate the product factor and average grain size, repeatedly calculate the average ferrite grain size and residual strain for each pass, and obtain the space factor, average grain size and residual strain of austenite and ferrite after the final rolling pass, and then Is to calculate the volume fraction and hardness of each structure (ferrite, pearlite, bainite, martensite) after completion of cooling from the calculated values, and at the same time to calculate the final average ferrite grain size. Method for manufacturing a hot-rolled steel according to claim 1,. 3. When calculating the average grain size and residual strain of a ferrite part, when the effective strain ε is larger than ε _m given by the equation (1), recrystallized and unrecrystallized 2 Divided into two groups, recrystallization space factor immediately after rolling
The f _fD and the grain size d _fD are obtained by using the equations (2) and (3), and the _space factor f _fS of the recrystallized part is calculated from the equation (4). The change is calculated from Eq. (5), the space factor f _fN and the grain size d _fN of the _{unrecrystallized} region are calculated by _Eqs . (6) and (7), and the effective strain
Is smaller than ε _{m, the} space factor f of the unrecrystallized region is
Only _fN and grain size d _fN are calculated from _Eqs . (6) and (7), the average ferrite grain size _f immediately before the next pass is calculated using Eq. (8), and strain recovery between passes is taken into consideration. The residual strain Δε immediately before is (9)
The method for manufacturing hot-rolled steel product according to claim 2, wherein the hot-rolled steel product is calculated using a formula. ε _m = h (d _fi ,, T) (1) f _fD = h (d _fi , ε ,, T) (2) d _fD = h (d _fi , ε, T) (3) f _fS = h ( ε, T, t) (4) d _fS = h (d _fD , T, t) (5) f _fN = h (f _fD , f _fS ) (6) d _fN = h (d _fi , ε) (7 ) _f = h (f _fD , f _fS , f _fN , d _fS , d _fN ) (8) Δε _f = h (f _fD , f _fN , ε _m , ε, T, (% C)) (9) where And d _fi is the ferrite grain size (μm) on the path entry side,
Is strain rate (s ^-1 ), T is rolling temperature (° C), ε is working strain,
t is the time after processing (seconds), (% C) is the carbon content (% by weight). The 4. usual hot slab is cooled after exiting the rolling in a temperature range of less than Ar ₃ starts rolling from a temperature range of not lower than transformation temperature Ar ₃ transformation temperature hot rolled steel carbon steel During production, the steel material conditions of the hot rolled steel material, heating conditions, based on manufacturing conditions such as rolling conditions and cooling conditions, the pre-rolling average austenite grain size do at the time of furnace extraction from a predetermined initial grain size model calculated, the do and on the basis of the production conditions predetermined austenite recrystallization model, ferrite recrystallization model, ferrite transformation model, using a particle size transformation ratio model during ferrite transformation, the Ar ₃ transformation temperature or higher temperature range the rolling pass computes the average particle size and residual strain of austenite, Ar ₃ austenite in rolling pass in a temperature range below the transformation temperature, ferrite respective space factor, the average particle diameter and By calculating the Tomeibitsu,
The final rolling pass after the austenite, ferrite respective space factor X _a, X _f, average particle size _{a, f,} residual strain [Delta] [epsilon] _a,
Δε _f is obtained and the X _a , X _f , _a , _f , Δε _a , and Δε _{f are} calculated.
Based on the transformation model of each structure (ferrite, pearlite, bainite, martensite) and hardness model, the space factor and hardness of each structure after completion of cooling are calculated based on the The final average ferrite grain size is calculated using the grain size conversion ratio model and the ferrite grain growth model after surrounding, and the final average ferrite grain size and the space factor and hardness of each structure are determined from the material model determined in advance from the material model. Calculate the material prediction value of the hot rolled steel, modify some of the manufacturing conditions so that the material prediction value and the material target value match, and manufacture the hot steel material based on the modified manufacturing conditions. A method for producing hot-rolled steel, characterized by obtaining a target material by means of 5. The rolling pass in the temperature range where the steel material temperature is above the Ar ₃ transformation temperature, the average grain size and residual strain of austenite are calculated, and in the rolling pass where the steel material temperature is below the Ar ₃ transformation temperature, Using the defined ferrite transformation model, the space factor of ferrite transformed after the previous pass is calculated, and the space factor of all ferrite and all austenite is calculated,
For the austenite part, the average grain size and residual strain of each group are obtained using the austenite recrystallization model, and for the ferrite part, the grain size conversion ratio model is used for the ferrite transformed after the previous pass. Of the ferrite, the residual strain of the ferrite is set to zero, the average ferrite grain size and the average residual strain of the ferrite and the ferrite transformed before the previous pass are determined, and the ferrite grain size is calculated as the entrance grain of the pass. When the effective strain obtained by adding the inlet residual strain to the additional strain of the path is larger than the critical strain of recrystallization, the recrystallization and the unrecrystallized When divided into two groups, the effective strain is smaller than the critical strain, only the non-recrystallized group,
From the inlet side grain size, the effective strain and the manufacturing conditions, the space factor and the average grain size in each group are calculated using a predetermined ferrite recrystallization model, and the average ferrite grain size and the residual strain are calculated for each pass. Is repeatedly calculated, the space factor of each of the austenite and ferrite after the final rolling pass, the average grain size and the residual strain are obtained, and thereafter, each structure after completion of cooling from each calculated value (ferrite, pearlite,
The method for producing a hot-rolled steel product according to claim 4, wherein the volume ratio and hardness of each of bainite and martensite) are calculated, and at the same time, the final average ferrite grain size is calculated. 6. When calculating the average grain size and residual strain of a ferrite part, when the effective strain ε is larger than ε _m given by the equation (1), recrystallization and non-recrystallization are performed. Divided into two groups, recrystallization space factor immediately after rolling
The f _fD and the grain size d _fD are obtained by using the equations (2) and (3), and the _space factor f _fS of the recrystallized part is calculated from the equation (4). The change is calculated from Eq. (5), the space factor f _fN and the grain size d _fN of the _{unrecrystallized} region are calculated by _Eqs . (6) and (7), and the effective strain
Is smaller than ε _{m, the} space factor f of the unrecrystallized region is
Only _fN and grain size d _fN are calculated from _Eqs . (6) and (7), the average ferrite grain size _f immediately before the next pass is calculated using Eq. (8), and strain recovery between passes is taken into consideration. The residual strain Δε immediately before is (9)
The method for manufacturing hot-rolled steel product according to claim 5, wherein the hot-rolled steel product is calculated using a formula. ε _m = h (d _fi ,, T) (1) f _fD = h (d _fi , ε ,, T) (2) d _fD = h (d _fi , ε, T) (3) f _fS = h ( ε, T, t) (4) d _fS = h (d _fD , T, t) (5) f _fN = h (f _fD , f _fS ) (6) d _fN = h (d _fi , ε) (7 ) _f = h (f _fD , f _fS , f _fN , d _fS , d _fN ) (8) Δε _f = h (f _fD , f _fN , ε _m , ε, T, (% C)) (9) where And d _fi is the ferrite grain size (μm) on the path entry side,
Is strain rate (s ^-1 ), T is rolling temperature (° C), ε is working strain,
t is a time (second) after processing, and (% C) is a carbon content (% by weight).