JP2597980B2 - Manufacturing method of hot rolled steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a steel product by hot rolling.

(Prior art) Conventionally, when a steel material is manufactured by hot rolling, the steel material is maintained at a temperature of Ar ₃ or higher in the entire hot rolling area, thereby stabilizing the rolling and material stability at the same time. Was going on. In a normal rolling schedule, when the steel material temperature becomes equal to or lower than Ar ₃ during rolling, a processed structure of ferrite is generated, and the material is significantly deteriorated. As an example,
FIG. 1 shows the results of the strength-ductility balance, and FIG.

FIG. 1 shows rolling while maintaining the temperature of Ar ₃ or more;
-1 indicates a normal high-strength steel sheet, -2 indicates a two-phase steel sheet, and indicates a work structure of rolling at a temperature of Ar ₃ or less during rolling.
(C; 0.16%, Mn; 1.22%) FIG. 2 shows rolling while maintaining a temperature of Ar ₃ or higher; and rolling at a temperature of Ar ₃ or lower during rolling. (C; 0.16%, M
n; 1.22%) In order to prevent such deterioration of the material, a rolling method of terminating the rolling with Ar ₃ or more has been conventionally used, but the Ar ₃ used at that time is mainly determined by the component. (13)
Equations (Miyoshi et al., Iron and Steel, vol.51 No.11, 2006), (14) Equations (Sekine et al., 86th and 87th Nishiyama Memorial Technical Lecture "New Controlled Rolling Technology" p.150) .

_{Ar 3 = 750.8-26.6 × [C]} + 17.6 × [Si] -11.6 × [Mn] -22.9 × [Cu] -23.0 × [Ni] +24.1 × [Cr] + 22.5 × [Mo] - 39.7 × [V] -5.7 × [ Ti] + 31.9 × [Zr] + 232.6 × [Nb] -169.4 × [Al] -894.7 × [B] ... (13) Ar 3 = 868-396 × [C ] + 24.6 x [Si]-68.1 x [Mn]-36.1 x [Ni]-20.7 x [Cu]-24.8 x [Cr] ... (14) (Weights in [] indicate weight percent of each component) Problems to be solved) However, these formulas do not take into account the increase in Ar ₃ due to working, and it is insufficient to accurately produce a material with good yield in actual hot rolling. Therefore, in reality, rolling is performed at a higher temperature in consideration of empirical safety costs.

On the other hand, regarding the rise of the transformation point due to processing, Tanaka et al. (Iron and steel, vol.64 No.3, 1353) and Ouchi et al. (Iron and steel, vo
l.67 No.1,143), and Tables 1 and 2 show the components, Fig. 3,
FIGS. 4 and 5 show the results described therein.

However, to date, not only has not been formulated, in the actual hot rolling, the effect of the strain rate, which is important in determining the material, is not considered, and the effect of time after rolling is also considered. Not.

Therefore, in hot rolling, it is insufficient to produce a material accurately and with good yield. FIG. 3 shows the rise due to the strain of Ar ₃ , FIG. 4 shows the rise of Ar ₃ due to the strain of the Si—Mn steel, and FIG.
5 shows an increase in Ar ₃ due to steel strain.

In order to solve these problems, the present inventors have disclosed a Japanese Patent Application No.
In No. 59-39983, a method for predicting Ar ₃ in consideration of the strain rate during processing and the time after processing was described. However, Japanese Patent Application No. 59-3
No. 9983 does not consider the structural change after processing, that is, fine graining by recrystallization and the release of strain accompanying it, and the change in grain size after processing and the release process of strain due to the difference in initial grain size are not considered. Different facts are not captured. Therefore, when the particle size before processing changes greatly, accurate Ar ₃
There is a drawback that cannot be estimated. In particular, in the case of continuous hot rolling, the above-mentioned effect is important because the particle size changes greatly in each pass.

The present invention has been made in order to solve the problems caused by the above-mentioned conventional knowledge when the rolling conditions are wide range including high pressure and high speed rolling in the future. tissue changes after processing is not considered in Patent -39983, i.e. taking into account the recovery of the strain and accompanying grain refining by recrystallization, by taking into account the consumption rate of the incubation period for the transformation, the prediction of the Ar ₃ Accuracy is further improved, thereby further saving energy in hot rolling and stabilizing the material of steel.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention provides: (1) When working a C-Mn-based steel material by hot working and adjusting the material, based on a predetermined relational expression. , strain epsilon applied in each pass is greater than the limit strain epsilon _C caused dynamic recrystallization, dynamic recrystallization, static recrystallization, divided into three groups of non-recrystallized, strain epsilon is epsilon _C or less Is divided into two groups, static recrystallization and non-recrystallization, the respective particle diameters d _D , d _S , d _{N of} dynamic recrystallization, static recrystallization and non-recrystallization and the space factor f _D, f _S, seeking f _N, subdivided into shorter Delta] t the time until the next pass, after obtaining the increase in static recrystallization space factor f _S between Delta] t, the dynamic recrystallization grains The grain size after the grain growth and the grain size after the grain growth of the static recrystallized grains are determined, and the average grain size d after elapse of Δt is determined. Calculate the average residual strain Δε after elapse of Δt by calculating the thermal recovery of this, calculate the average particle diameter, the consumption rate of the incubation period in the average residual strain state, and repeat the following for the next Δt until the next pass The average grain size and the average residual strain immediately before the next pass are set as initial conditions of the next pass, and the strain obtained by adding the strain of the next pass to the average residual strain is defined as the effective strain. It is determined whether the consumption rate of the incubation period at each Δt becomes 1 or not. If the consumption rate of the incubation period exceeds 1 before the final pass machining, the temperature at this time is predicted as the transformation point Ar _3, and A method for producing a hot-rolled steel material, comprising repeatedly adjusting the entire processing temperature so as to maintain the temperature of the steel material at the transformation point Ar ₃ or higher in all regions of the hot working.

(2) When hot working a C-Mn-based steel material to adjust the material, formula (1) is used for the dynamic recrystallization limit strain ε _{C and} (2) is used for the dynamic recrystallization space factor f _D. Equation (3) for the equation and the particle size d _D , Equation (4) for the dynamic recrystallization space factor f _S ,
Equation (5) for the particle size d _S and the unrecrystallized space factor f _N
For (6), and the particle diameter d _N (7) below,
The grain size of the dynamically recrystallized grains after the grain growth {d ² _DG ▼} and the grain size of the static recrystallized grains after the grain growth are {d ² _SG }.
For the formula (9) and their average particle size,
Equation (11) is used to determine the average residual strain Δε after thermal recovery of the residual strain, and the equation (11) is used to determine the consumption rate of the incubation period at the time Δt obtained by subdividing the time between the pass and the end of hot working to the start of cooling. The method for producing a hot-rolled steel material according to the above item (1), wherein the steel material temperature is maintained at Ar ₃ or more in the entire region of the hot working using the formula (12).

T is the rolling temperature (K) R is the gas constant A ₁ is the statistical correction coefficient of the dynamic recrystallization critical strain Q ₁ is the energy representing the effect of temperature on the dynamic recrystallization critical strain a is the dynamic recrystallization critical strain Index showing the effect of initial particle size on the particle size m = A ₂ · exp (Q ₂ / T) ε _S = A ₃ · [1-exp (−d ₀ / K)] K = A ₄ · ^b · exp (−Q ₃ / T) A ₂ is dynamic The statistical correction factor Q ₂ of the index m representing the effect of the additional strain on the recrystallization space factor is the energy A ₃ representing the effect of the temperature on the index m representing the effect of the additional strain on the dynamic recrystallization space factor. statistical correction factor Q ₃ are dynamic recrystallization completion strain of K statistical correction factor a ₄ dynamic recrystallization ends strain epsilon _S is representative of the influence of strain rate and temperature on the dynamic recrystallization ends strain epsilon _S The energy b representing the effect of temperature on ε _S is an index d _D = A ₅ · Z ^C , Z = exp (Q / RT) representing the effect of the strain rate on the dynamic recrystallization termination strain ε _S. a ₅ represents statistical correction factor c of the dynamic recrystallization grain size on the dynamic recrystallized grain size Zener's-Hollomon Paramete
The index Q representing the effect of r is the activation energy of dynamic recrystallization f _S = (1−f _D ) · (1−exp ｛− (t / T _S ) ^S ｝)
(4) t is the time between passes or the time until the start of cooling after rolling (se
c) S is an index representing the effect of time on the static recrystallization space factor T _S is a function representing the effect of strain and temperature on the static recrystallization space factor d ₀ is the grain size before rolling A ₆ is a statistical correction coefficient of the static recrystallized grain size d is an index representing the effect of the initial grain size on the static recrystallized grain size e is the strain on the static recrystallized grain size index represents the impact _{f N = 1-f D -f} S ... (6) d N = d 0 · exp (-ε / 4) ... (7) A ₇ is a statistical correction factor for dynamic recrystallized grain growth Q ₄ is energy representing the effect of temperature on dynamic recrystallized grain growth f is an index representing the effect of time on dynamic recrystallized grain growth A ₈ is exponential statistical correction factor Q ₅ static recrystallization grain growth energy g representing the effect of temperature on the static recrystallization grain growth which represents the effect of time on the static recrystallization grain growth _{Δε = (f D · ε C} + f N · ε) · exp (- {t / T k} h) ...
(11) T _k is a function representing the effect of temperature on the residual strain h is an index representing the effect of time on the residual strain IP = {Δt / exp (−B ₁ lnKF + B ₂ lnT + B ₃ / T−B ₄ )} …
(12) KF = Kf / d Kf = exp _{[B 5 -B 6 · [C]} -B 7 · [Mn] + B 8 (T-273) -B 9 (T-273) 2 -ln (KD) + ln (KK)] _{KD = B 10 / (B 10} · Δq / + B 11 ε 2) KK = 1 + B 12 · ε Δq = 1/2 _{[C 3 + C 2 C 3 (} C 1 -C 3) -1/2・ F + C ₂ (C ₁ −C ₃ ) ^1/2・ E] μ = arc cos (C ₃ / C ₁ ) C ₁ = 1 / (1-P) C ₂ = 1 C ₃ = 1-P P = 1-e ^-Δε B ₁ is the component and grain size that affect the incubation period of ferrite transformation. Coefficient of diameter effect B ₂ is coefficient of temperature effect on incubation period of ferrite transformation B ₃ is coefficient of temperature effect on incubation period of ferrite transformation B ₄ is statistical modification constant of incubation period of ferrite transformation B ₅ is ferrite transformation B ₆ is a coefficient representing the effect of C on the progress of ferrite transformation B ₇ is a coefficient representing the effect of Mn on the progress of ferrite transformation B ₈ is a coefficient representing the effect of Mn on the progress of ferrite transformation factor B ₁₁ coefficients B ₉ representing the effect of the temperature coefficient B ₁₀ indicating the effect of temperature on the rate of progression of ferrite transformation is representative of the effect of flattening the austenite grain boundaries as a nucleation site for ferrite nucleation of ferrite Austeni as a site Coefficients representing the effects of the introduction of deformation zones in the grains B ₁₂ is a means a coefficient indicating the effect of the distortion on the nucleation rate of the ferrite.

ε _C is obtained by conducting an experiment in which T and d ₀ are changed in the equation (1) in advance, and determining A ₁ , Q ₁ and a by the least square method. In addition, the values of various symbols determined by experiments are also obtained in advance by performing various experiments in advance, obtaining equations in which various coefficients are determined by the least squares method, and using the equations. The calculation of the average γ particle size, the residual strain, and the consumption rate during the incubation period shall proceed in accordance with the flowchart shown in FIG.

By means of the above-mentioned means of the present invention, since Ar ₃ is predicted, the prediction accuracy of this Ar ₃ is improved in the entire hot rolling region, the control of all the influence factors of hot rolling is optimized, energy saving of hot rolling, Further promote the stabilization of steel materials.

In order to roll a hot-rolled steel sheet without deteriorating the quality of the steel material, finish rolling must be performed at the transformation point (A
r ₃ ) It is necessary to do above. However, since Ar ₃ depends on rolling conditions and cooling conditions, strictly speaking, it is necessary to couple and predict an austenite recrystallization model and a ferrite transformation model, and the present invention makes it possible. .
In the present invention, the recrystallization model of austenite is (1)
(2) (3) (4) (5) (6) (7) (8) (9)
(10) Equation (11) is used. As shown in FIG. 6, the space factor and particle size of each of the dynamic recrystallization, static recrystallization, and non-recrystallization are obtained based on the hot rolling conditions, and For the recrystallization and the static recrystallization, the average particle size and the residual strain of the whole are obtained after the particle size after the grain growth is obtained. At this time, it is determined that dynamic recrystallization occurs when the applied strain ε is larger than the critical strain ε _C of dynamic recrystallization. In order to obtain the change in the average grain size and the residual strain every moment, the interval up to the next rolling pass is subdivided by Δt, and the above calculation is performed for each Δt. When the next rolling pass is reached, the average grain size and The above calculation is repeated with the residual strain set as the initial condition in the next rolling pass, and the total strain of the residual strain and the strain newly added in this rolling pass as the effective strain. Further, the transformation model of ferrite is equation (12), and the consumption rate during the incubation period can be obtained from this equation. As can be seen from the equation (12), the average γ particle size d (the equation (10) and the residual strain Δε)
(Equation (11)), which can be determined from the austenite recrystallization model. Therefore, both models can be coupled by these two factors. Actually, as shown in FIG. 6, after obtaining the average γ grain size and the residual strain by the austenite recrystallization model for each Δt, using equation (12), any one of the incubation periods under this condition between Δt is determined. Just ask what you consumed. The consumption rate in the incubation period is determined for each Δt, and it means that ferrite transformation starts when the total sum becomes 1 or more. Therefore, the temperature at that time is defined as a transformation point Ar ₃ .

In this way, the two models are coupled to predict the transformation point along the temperature history (determined by actual measurement, calculation, or the like) during, for example, seven passes of the finish rolling. If the transformation starts before the final finishing pass, the transformed ferrite will be processed and the material will deteriorate.

Therefore, various simulations are performed to determine the rolling conditions under which the transformation starts after the final finishing pass, and by actually specifying the rolling conditions, it is possible to manufacture a steel material having a good material without any material deterioration.

Here, the material is specifically elongation or r-value, and the temperature on the entry side of the finish rolling is set so that in actual rolling, rolling conditions are set so that transformation does not start before the final finishing pass. However, since the temperature of the end of the steel sheet is so large that such rolling conditions cannot be obtained in some cases, the steel sheet is heated using a heating device.

(Examples) Table 3 shows a conventional method and a comparative example (Japanese Patent Application No.
59-39983) and the Ar ₃ transformation point predicted by the method of the present invention and the morphology observed at that time. The experiment was performed by a two-pass processing test, and after the two-pass processing, 700 ° C
After cooling at a rate of 30 ° C./sec, water quenching was performed.

It can be seen that the method of the present invention can predict the actual tissue state better than the conventional method. Further, in Japanese Patent Application No. 59-39983 (comparison method), it is found that the prediction when the initial particle size is fine and the strain is small is not sufficient, and the prediction according to the present invention enables more accurate prediction. For example, the Ar ₃ is lower than the processing temperature of the second pass is the conventional method and comparison method in No.10, whereas the predicted deformation ferrite does not occur, the processing is Ar ₃ of the second pass in the present invention method It is predicted that the temperature will be higher than the temperature, and deformed ferrite is generated. In the actual structure, deformed ferrite is generated, and the prediction of the present invention is correct. In this case, it is necessary to raise the processing temperature in order not to generate deformed ferrite. No. 11 showed that, and No. 11
By raising the processing temperature by 10 ° C. with respect to 10, it is predicted that deformed ferrite will not be generated in the method of the present invention, and the actual structure is also the same. Therefore, by using the method of the present invention, it is possible to accurately obtain a temperature history for preventing generation of deformed ferrite.

Next, each coefficient obtained by the present inventors for the C-Mn steel is shown.

A ₁ = 1.43 × 10 ⁻⁵ Q ₁ = 18800 a = 0.22 A ₂ = 0.026 A ₃ = 2.25 A ₄ = 472 Q ₂ = 4600 Q ₃ = 2960 b = −0.0723 Q = 72600−52200 × Ceq (Ceq = C + Mn / 6) A ₅ = 0.1204 x Q-5254 c = -0.155 S = 1/3 T _S = 9.11 x 10 ^-15 · ε- ^2.36 · exp (67670 / RT) A ₆ = 9.71 d = 0.25 e = -0.5 ^{_{A 7 = 3900 × Ceq -1.43 A}} 8 = 3.68 × 10 8 × Ceq -1.43 Q 4 = 5380 Q 5 = 20000 f = 0.3 g = 0.24 Tk = 8.46 × 10 -9 · (43800 / RT) h = 2 / 3 B ₁ = -1.6454 B ₂ = 20 B ₃ = 3.265 x 10 ⁴ B ₄ = 173.89 B ₅ = 4.7766 B ₆ = 13.339 B ₇ = 1.1922 B ₈ = 0.02505 B ₉ = 3.5067 x 10 ^-5 B ₁₀ = 2.24 (Effects of the Invention) The present invention described above incorporates the effects of processing into Ar ₃ predictions based on only the conventional components, and incorporates the effects of prior processing in Japanese Patent Application No. 59-59.
By improving the method of 39983 and incorporating the structural change after processing, accurate prediction of Ar ₃ was made possible. Using this method, the production conditions for working while maintaining the steel material temperature at Ar ₃ or higher over the entire hot working area are precisely adjusted.

In addition, since Ar ₃ can be predicted according to each component and each rolling condition, the rolling condition can be optimized even when the rolling condition spreads to a new region or when the component changes, and a large safety margin is required. And the stabilization of the steel material proceeds. Therefore, the present invention has a great effect in the field of hot rolling.

[Brief description of the drawings]

Strength of the first figure-site test materials - Chart of the effects of processing tissue on ductility balance, the table of FIG. 2 Effect of processed structure on the hole expansion ratio of the field test materials, according to FIG. 3 is a distortion of Ar ₃ FIG. 4 is a chart of the rise of Ar ₃ due to the strain of the Si-Mn steel, FIG. 5 is a chart of the rise of Ar ₃ due to the strain of the Nb steel, and FIG. FIG. 4 shows a flow chart of calculating the consumption rate during the incubation period.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Osamu Kono Oita-shi, Nishi-no-Su, 1 Nippon Steel Corporation Inside Oita Works (72) Inventor Kazuaki Esaka 1, Oita-shi, Oita Nishi-no-Su, 1 Nippon Steel Corporation Oita Works

Claims (2)

(57) [Claims] When a C-Mn-based steel material is hot-worked to adjust the material, the strain ε applied in each pass is determined by a predetermined relational expression to be a critical strain ε at which dynamic recrystallization occurs. as if _C is greater than the dynamic recrystallization, static recrystallization, divided into three groups of non-recrystallized, if the strain epsilon is less epsilon _C, divided into two groups of static recrystallization, non-recrystallized, The respective particle diameters d _D , d _S , d _N and space factors f _D , f _S , f _N of the dynamic recrystallization, static recrystallization, and non-recrystallization are obtained, and the time until the next pass is shortened by Δt particle size after subdivided, after obtaining the increase in static recrystallization space factor f _S between Delta] t, the grain growth of dynamic re-particle diameter of the crystal grains of the grain after the growth and the static recrystallization grains To obtain an average particle diameter d after elapse of Δt, and to obtain a thermal recovery of a sum of residual strain of dynamic recrystallized grains and residual strain of non-recrystallized to obtain an average residual strain Δε after elapse of Δt. The average particle diameter and the consumption rate during the incubation period in the state of the average residual strain are calculated. The following is repeated for the next Δt until the next pass, and the average particle diameter and the average residual strain immediately before the next pass are determined by the initial conditions of the next pass. The strain obtained by adding the strain of the next pass to the average residual strain is defined as the effective strain, and the steps of are repeated by the number of hot working passes, and it is determined whether or not the consumption rate of the incubation period at each Δt becomes 1. If the consumption rate in the incubation period exceeds 1 before the final pass machining, the temperature at this time is predicted as the transformation point Ar _3, and the process is repeated to maintain the steel material temperature at the transformation point Ar ₃ or more in the entire hot working region. A method for manufacturing a hot-rolled steel material, wherein the entire processing temperature is adjusted so as to perform processing. 2. When hot working a C-Mn-based steel material to adjust the material, formula (1) is used for the dynamic recrystallization limit strain ε _{C and} ( _D ) is used for the dynamic recrystallization space factor f _D. Equation (2) and equation (3) for the particle size d _D , and equation (4) for the dynamic recrystallization space factor f _S
Equation (5) for the equation and particle size d _S , Equation (6) for the unrecrystallized space factor f _N , and equation (7) for the particle size d _N
Equations (8) and (9) are used to calculate the particle size of the dynamically recrystallized grains after the grain growth {d ² _DG ▼} and the grain size of the statically recrystallized grains after the grain growth {d ² _SG }. The formula (10) is used for these average particle diameters, and the formula (11) is used for the average residual strain Δε after thermal recovery of residual strain, and the time from pass to completion of hot working after completion of hot working is subdivided. The consumption rate of the incubation period at the determined time Δt is obtained by using equation (12) to maintain the temperature of the steel material at Ar ₃ or higher in the entire region of hot working. The method for producing a hot-rolled steel material according to the above. T is the rolling temperature (K) R is the gas constant A ₁ is the statistical correction coefficient of the dynamic recrystallization critical strain Q ₁ is the energy representing the effect of temperature on the dynamic recrystallization critical strain a is the dynamic recrystallization critical strain Index showing the effect of initial particle size on the particle size m = A ₂ · exp (Q ₂ / T) ε _S = A ₃ · [1-exp (−d ₀ / K)] K = A ₄ · ^b · exp (−Q ₃ / T) A ₂ is dynamic The statistical correction factor Q ₂ of the index m representing the effect of the additional strain on the recrystallization space factor is the energy A ₃ representing the effect of the temperature on the index m representing the effect of the additional strain on the dynamic recrystallization space factor. statistical correction factor Q ₃ are dynamic recrystallization completion strain of K statistical correction factor a ₄ dynamic recrystallization ends strain epsilon _S is representative of the influence of strain rate and temperature on the dynamic recrystallization ends strain epsilon _S The energy b representing the effect of temperature on ε _S is an index d _D = A ₅ · Z ^C , Z = exp (Q / RT) representing the effect of the strain rate on the dynamic recrystallization termination strain ε _S. a ₅ represents statistical correction factor c of the dynamic recrystallization grain size on the dynamic recrystallized grain size Zener's-Hollomon Paramete
The index Q representing the effect of r is the activation energy of dynamic recrystallization f _S = (1−f _D ) · (1−exp ｛− (t / T _S ) ^S ｝)
(4) t is the time between passes or the time until the start of cooling after rolling (se
c) S is an index representing the effect of time on the static recrystallization space factor T _S is a function representing the effect of strain and temperature on the static recrystallization space factor d ₀ is the grain size before rolling A ₆ is a statistical correction coefficient of the static recrystallized grain size d is an index representing the effect of the initial grain size on the static recrystallized grain size e is the strain on the static recrystallized grain size index represents the impact _{f N = 1-f D -f} S ... (6) d N = d 0 · exp (-ε / 4) ... (7) A ₇ is a statistical correction factor for dynamic recrystallized grain growth Q ₄ is energy representing the effect of temperature on dynamic recrystallized grain growth f is an index representing the effect of time on dynamic recrystallized grain growth A ₈ is exponential statistical correction factor Q ₅ static recrystallization grain growth energy g representing the effect of temperature on the static recrystallization grain growth which represents the effect of time on the static recrystallization grain growth _{Δε = (f D · ε C} + f N · ε) · exp (- {t / T k} h) ...
(11) T _k is a function representing the effect of temperature on the residual strain h is an index representing the effect of time on the residual strain IP = {Δt / exp (−B ₁ lnKF + B ₂ lnT + B ₃ / T−B ₄ )} …
(12) KF = Kf / Kf = exp _{[B 5 -B 6 · [C]} -B 7 · [Mn] + B 8 (T-273) -B 9 (T-273) 2 -ln (KD) + ln (KK)] _{KD = B 10 / (B 10} · Δq / + B 11 ε 2) KK = 1 + B 12 · ε μ = arc cos (C ₃ / C ₁ ) C ₁ = 1 / (1-P) C ₂ = 1 C ₃ = 1-P P = 1-e ^-Δε B ₁ is the component and grain size that affect the incubation period of ferrite transformation. Coefficient of diameter effect B ₂ is coefficient of temperature effect on incubation period of ferrite transformation B ₃ is coefficient of temperature effect on incubation period of ferrite transformation B ₄ is statistical modification constant of incubation period of ferrite transformation B ₅ is ferrite transformation B ₆ is a coefficient representing the effect of C on the progress of ferrite transformation B ₇ is a coefficient representing the effect of Mn on the progress of ferrite transformation B ₈ is a coefficient representing the effect of Mn on the progress of ferrite transformation factor B ₁₁ coefficients B ₉ representing the effect of the temperature coefficient B ₁₀ indicating the effect of temperature on the rate of progression of ferrite transformation is representative of the effect of flattening the austenite grain boundaries as a nucleation site for ferrite nucleation of ferrite Austeni as a site Factor Factor B ₁₂ representing the effect of the introduction of deformation zones in the grains representing the effect of the distortion on the nucleation rate of the ferrite