JPH06182412A - Method for predicting structural distribution of section of bar steel or steel bar - Google Patents

Abstract

PURPOSE:To improve the yield by executing three dimensional rolling analytical calculation, temperature analytical calculation of temperature distribution in a section, considering strain in the section, rate of strain and temperature distri bution and predicting the final structure. CONSTITUTION:Procedures 1-6 are calculated successively and the distribution of structure of sections of a bar steel etc., is predicted based on them. In the procedure 1, strain, rate of strain and temperature distribution on the outlet side of rolling are calculated. In the procedure 2, the change of the temperature distribution in the section between passes is calculated. In the procedure 3, gammarecrystallization structure, unrecrystallization structure, distribution of residual strain are calculated. In the procedure 4, these calculation results are calculated continuously and repeatedly and calculations of gamma grain structure, residual strain, temperature distribution state are completed. In the procedure 5, change of temperature distribution is calculated and, in the procedure 6, the final structure distribution after cooling is completed is calculated. In this way, the structure of the final product of the bar steel, etc., in which ununiformity of the structure in a section is generated can be predicted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for predicting a microstructure distribution in a cross section of a final product of a steel bar or a steel bar manufactured by hot rolling and cooling, and more particularly to a nonuniform microstructure distribution in the cross section. By establishing a method that can also accurately predict the structure, it is possible to set the processing conditions to obtain the target structure or to contribute to the design of the rolling hole type. Is.

[0002]

2. Description of the Related Art The properties of bar steel and bar products (hereinafter sometimes referred to as steel) have a great deal to do with the chemical composition, but the mechanical properties after hot working are the hot working conditions and cooling during transformation. It is known that it is also greatly affected by speed. Therefore, when it is desired to obtain a steel bar product having desired mechanical properties, it is important to appropriately control not only chemical components but also manufacturing conditions such as hot working conditions and cooling conditions. However, in the past, since the same product / mass production was used as a base, it has been generally accepted that the optimum manufacturing conditions are reached while experimentally and empirically gradually improving / correcting.

In recent years, due to the diversification of social needs, it is necessary to prepare a system for adapting to high-mix low-volume production.
Conventional methods that rely on experimental and empirical methods are no longer available. Further, demands for quality improvement of products have become stricter year by year, and for example, product shape accuracy (roundness etc.) has become more strict than ever. Therefore, optimal groove design, fine adjustment of roll gap during rolling, shape correction light reduction (skin pass) rolling, etc. are performed, but in the case of groove rolling, the strain distribution in the cross section is uneven, so depending on the conditions Structural abnormalities such as coarsening of the austenite structure may be observed in a part of the cross section.

In order to cope with the above problems, the correlation between the manufacturing conditions and the mechanical properties of the product is accurately grasped, and the manufacturing conditions for expressing the desired mechanical properties are established from the beginning. There is an urgent need to develop technologies that can be used.

As a method for predicting the structure and the material after multi-pass hot rolling and cooling, for example, JP-A-62-158 is used.
Techniques such as those disclosed in Japanese Patent Laid-Open No. 816 and Japanese Patent Laid-Open No. 4-4911 have been proposed. That is, JP-A-62-158
Japanese Patent No. 816 discloses a prediction method in which the strain, the strain rate, and the temperature during rolling are constant. Further, in Japanese Patent Laid-Open No. 4-4911, the thickness of the steel material on the inlet side and the outlet side for each pass,
The austenite grain size and dislocation density after rolling based on the equivalent strain during rolling, strain rate, etc. calculated for multiple points from the plate thickness surface to the center by the time between passes, the roll diameter of the rolling mill, the number of roll revolutions, etc. , Residual strain, etc. are calculated, and based on the calculation results and cooling conditions,
It shows a method of predicting the material quality of the steel sheet by calculating the grain size and the microstructure fraction.

[0006]

However, since the above-mentioned various techniques are intended for steel sheets as products, the above-mentioned various techniques are not applicable to products such as bar steel and steel bar products in which a two-dimensional distribution of strain, strain rate and temperature is observed in the cross section during rolling. It is impossible to predict the exact end product organization. The present invention has been made to solve such a technical problem, and its purpose is to:
The aim is to establish a method that can accurately predict the cross-sectional structure of steel bar and bar products.

[0007]

According to the method of the present invention which has achieved the above-mentioned object, the following steps 1 to 6 are carried out in order,
This is a method for predicting the microstructure distribution of a steel bar or a steel bar, which is based on the point of predicting the microstructure distribution of a steel bar or a steel bar based on the result of this calculation. (Procedure 1) Coupling calculation is performed by a three-dimensional rolling analysis method for the distribution of strain, strain rate, and temperature in the cross section during the first stage rolling of the steel material to be processed. (Procedure 2) With respect to the temperature distribution in the cross section immediately after rolling obtained in Procedure 1, the temperature change between the passes immediately before the start of the next rolling is calculated by the temperature analysis method. (Procedure 3) By the change in the strain in the cross section obtained in the above procedure 1, the strain rate, and the temperature distribution between the paths obtained in the second procedure,
The distribution of austenite recrystallized / non-recrystallized structure and residual strain in the cross section just before the next stage rolling is calculated. (Procedure 4) The above procedures 1 to 3 are sequentially repeated from the next rolling to the final rolling while inheriting the calculation results regarding the temperature and residual strain distribution of the former stage, and the austenite recrystallization / non-recrystallization in the cross section immediately after the final rolling is performed. The recrystallization structure and residual strain distribution are calculated. (Procedure 5) The temperature change in the temperature distribution in the cross section immediately after the final rolling obtained in the procedure 4 until the end of cooling is calculated by a temperature analysis method. (Procedure 6) By the distribution of the austenite recrystallized / unrecrystallized structure and residual strain in the cross section immediately after the final rolling obtained in the above procedure 4 and the temperature change until the end of cooling obtained in the above procedure 5, the final cross section Calculate tissue distribution.

[0008]

The present invention is constructed as described above, but in short,
A model that accurately predicts strain, strain rate, and temperature distribution in the cross section during rolling, a model that calculates changes in temperature distribution in the cross section between passes, and austenite recrystallization between passes from the obtained calculation results. A model was provided to accurately determine the distribution of unrecrystallized structure, residual strain, etc.

By adopting the above configuration,
It was possible to accurately determine the microstructure distribution in the cross section of the final product of the bar steel and bar steel manufactured by hot rolling and cooling. This makes it possible to predict structural abnormalities such as coarsening of the austenite structure, which is a quality problem in advance, when setting the rolling conditions and designing the rolling hole die to obtain the target structure, improving the product development yield and reducing costs. It became possible to reduce the amount. The configuration and operation of the present invention will be described in more detail with reference to the drawings.

FIG. 1 is a flow chart showing the procedure of the tissue distribution prediction method according to the present invention. Further, FIG. 2 is a flowchart of a method for predicting the final structure after cooling is completed. First, a method for predicting the microstructure distribution of the steel strip cross section shown in FIG. 1 will be described. In order to execute the processing of FIG. 1, software for realizing this may be created and calculation may be performed using this.

The method for predicting the microstructural distribution of a steel strip cross section according to the present invention can be roughly classified into the following six procedures. As described above, strain, strain rate, and temperature nonuniformity occur in the cross section during the groove rolling used in the production of bar steel, which affects the nonuniformity of the final structure. Therefore, as procedure 1, the strain, strain rate, and temperature distribution in the exit cross-section during the first rolling of the steel to be processed are calculated using a three-dimensional rolling analysis method that enables coupled analysis. At this time, calculation is performed by adopting a three-dimensional rolling analysis method by the finite element method that enables simultaneous calculation of strain and strain rate, such as processing heat generated by rolling, heat removal caused by contact with a roll, the atmosphere, or a cooling medium. Is most advantageous. For more information on this technique,
Proceedings of the Japan Society of Mechanical Engineers (A), 51 , 469 (Sho 60-
9), Proceedings of the Japan Society of Mechanical Engineers (A), 53 , 494
(Sho 62-10) and the like. At this time, in order to obtain sufficient accuracy, the number of elements is 10 ×
10 × 12 or more, that is, element division of the output side section 10 × 1
It is better to perform the calculation with 0 (100 elements) or more.

Information on the temperature distribution in the cross section immediately after the exit side of the first stage rolling can be obtained by the procedure 1 described above, but it is possible to obtain information from the surface by contact with the atmosphere or the cooling medium between the passes up to the next stage pass. Heat removal occurs. Therefore, the temperature distribution changes between passes. In the following procedure 2, the information between the passes (interpass time, cooling capacity, etc.) is taken in and temperature analysis is performed by the finite element method. By this calculation, the temperature distribution in the cross section just before the next rolling can be calculated.

By obtaining information about the strain during rolling, the strain rate and the temperature distribution change between passes by the above calculation, it is possible to calculate the progress of recrystallization in the cross section between passes. That is, it is possible to calculate the recrystallization behavior at an arbitrary position in the cross section between the passes of the austenite structure immediately after rolling. In step 3, outgoing side (n × n)
Predictive calculation of recrystallization behavior of each element or arbitrary position. By this calculation, the residual strain distribution and the recrystallized / non-recrystallized structure distribution in the cross section immediately before the next stage rolling can be calculated.

The procedure 4 is a procedure in which the information about the temperature, the residual strain, and the recrystallization calculated in the procedure 2 and the procedure 3 is taken over to the analysis of the next pass and the procedure 1 to 3 is repeated to perform each pass. By this calculation, the temperature distribution in the cross section immediately after the final rolling and the strain and strain rate distribution during rolling can be calculated.

Information on cooling conditions (cooling time, cooling stop temperature, cooling capacity, etc.) after rolling is taken in and the change in the temperature distribution in the cross section during the cooling process is calculated in step 5. This uses exactly the same calculation program as in Procedure 2. By this calculation, the temperature change at any position in the cross section during the cooling process can be calculated.

From the information on the strain immediately after the final rolling, the distribution of the strain rate, and the temperature change at each cross-section value during the cooling process calculated by the procedure 4 and the procedure 5, the structure of the final product after the cooling is finished in the procedure 6. I'm doing the calculation. A detailed flowchart at this time is shown in FIG. That is, in the cooling process from the component to the ferrite transformation start point obtained by thermodynamic calculation, the recrystallization progress at each (n × n) element in the cross section or at an arbitrary position is calculated using the recrystallization prediction formula described above. The austenite grain size and residual strain are calculated. In the temperature range below the transformation start point, the fraction of each structure and the ferrite grain size are calculated using the phase transformation prediction formula.

[0017]

EXAMPLES Next, the results of actual calculations using the method of the present invention will be shown. Table 1 shows the results of comparison between the method of the present invention and the conventional method for the microstructure distribution in the cross section after (12 steps) rolling on the actual steel bar rolling line for JIS S43C steel bar (diameter 50 mm). Is. In the conventional method, strain in the cross section, strain distribution, and temperature distribution cannot be calculated, so strain ε is calculated by using the following widely used equation, and strain rate is calculated as ε. And using the value obtained from the roll diameter and the number of revolutions,
Furthermore, for temperature, the estimated value used in the operation was used. ε = ln (h2 / h1) h 1: Maximum height after rolling h 2: Maximum height before rolling

[0018]

[Table 1]

As can be seen from Table 1, the nonuniformity of the structure in the cross section occurs especially in the bar having a large diameter. It is impossible to predict the heterogeneity of these tissues by the conventional method, and a typical value that does not make sense is calculated. On the other hand, by using the method of the present invention, the strain in the cross section during rolling, such as a hole-type rolled material, the temperature is non-uniform and even in a material that is cooled non-uniformly in the cooling process accurately You can make predictions. Table 2 shows J
It shows the results predicted by each method for abnormal grain growth (γ grain size: austenite grain size) resulting from skin pass rolling at 800 ° C. using a test rolling mill using IS SUP7N bar steel (diameter 11 mm). .

[0020]

[Table 2]

As shown in Table 2, by applying the method of the present invention, it is possible to predict with high accuracy abnormal grain growth that may be seen in a part of the surface vicinity, which was conventionally unpredictable. Is possible. The following formula was used as the formula for predicting the recrystallized structure of austenite after the single pass processing, and the average dislocation density was calculated from the recrystallization rate.

<Dynamic Recrystallization Rate (X _dyn )> X _dyn = (D _dyn / D _dyno ) ^1.5 · X _dyno ¹⁾ X _dyno = 1-exp {-0.693 [(ε-ε _c ) / ε _0.5 ]
² } ε _0.5 = 1.144 × 10 ^-3 · D ₀ ^0.28 · ε ^0.05 · exp (6420
/ T) ε _c = 4.76 × 10 ^-4 · exp (8000 / T) ε = b ^-1 · ln [c · (c-bρ ₀ ) ^-1 ] + Δε b = 9850 · ε _V ^-0.315 · exp (-8000 / T) c = 8.5 × 10 ¹⁰ · (1 + D ₀ ^-0.5 ) Δε: Newly added processing strain ρ ₀ : Initial dislocation density (cm ^-2 ) T: Absolute temperature (K) D ₀ : Initial grain size (μm) D _dyno : Dynamic recrystallization grain size immediately after rolling (μm) X _dyno : Dynamic recrystallization rate immediately after rolling ε _0.5 : Strain amount at which X _dyno = 0.5 ε _c : Critical strain ε: Processing strain ε _V : Strain rate

<Dynamic recrystallized grain size (D _dyn )> D _dyn = D _dyno +1.1 (D ^pd −D _dyno ) Y (μm) ¹⁾ However, in the case of D _dyn > D ^pd , Grain grows. dD _dyn / dt = 72 ・ D _dyn ^-1・ exp (-63800 / RT) D ^pd = 5380 ・ exp (-6840 / T) Y = 1-exp {-295 ・ ε ^0.1・ exp (-8000 / T)
t} D _dyno = 22600 ・ Z ^-0.27 Z = ε _V・ exp (63800 / RT) R = 1.99 (cal / mol / K) D ^pd : The maximum grain size at which dynamic recrystallized grains can grow due to the dislocation density difference. (Μm)

<Dislocation Density in Dynamic Recrystallized Grain (ρ _dyn )> ρ _dyn = ⁸⁷³⁰⁰ · Z ^0.248 (cm) ¹⁾ However, the above formula indicates immediately after processing, and after processing the same as the ^{unrecrystallized} portion. The dislocation density is reduced. <Static recrystallization ratio _{(X st)> X st =} 1-exp {-0.693 (t / t 0.5) 2} 1) t 0.5 = 0.286 × 10 -7 · Sv -0.5 · ε V -2 · exp ( 1800
0 / T) Sv = (24 / πD ₀ ) (0.491 ・ exp (ε) +0.155 ・ exp (−ε) +0.14
3 · exp (−3ε)) t _0.5 : time required until X _st = 0.5 (sec) Sv: grain boundary area per unit volume (μm ⁻¹ ) q: increase rate of grain boundary area following processing

<Static recrystallized grain size (D _st )> D _st = 1.536 · D _sto · X {X _st / ∫ ₀ ^xst (1n [1 / (1-X)]) ⁻³ / ⁴ · d
x} ^1/3 (μm) ³⁾ However, when X _st > 0.99, the grain growth is as follows. dD _st / dt = 72 · D _st ⁻¹ · exp (-63800 / RT) D _sto = 2.0 · (Sv · ε) ^-0.6 D _sto : D _st (μm) at X _st = 1 <static recrystallized grains _Dislocation density (ρ _st )> ρ _st = 1 × 10 ⁸ (cm ^-2 ) ¹⁾

[0026] <unrecrystallized portion of dislocation density _{(ρ u)> ρ u =} c · b -1 · [1-exp (-bε)] (cm -2) 1) However, the above formula represents an immediately processed , After processing, it decreases as follows up to 1 × 10 ⁸ cm ^-2, and does not decrease thereafter. dρ / dt = −4.0ρ ² · exp (−8000 / RT) The sources of 1) to 3) above are as follows. Also, the coefficients in the formula were modified as necessary. 1) Senuma et al. "Iron and Steel" 70 (1984), p2112 2) Umemoto et al. "Proc.int.Conf.on Structure and Propert
ies of HSLA Steels ”Wollongong, Australia, 1984, p96 3) Umemoto et al .:“ Acta metal ”Vol 34, No.7, p1377 to 1385 Also, the following formula was used as the phase transformation prediction formula calculated in step 6. .

<Polygonal ferrite> Nucleation place: Austenite grain boundary Form: Spheroid with axial ratio 3: 1 Starting condition: Temperature ≤ Ae3 Nucleation rate: Is = k1 · Dc (1-C _av ) / √ T · exp {−k2 / (ΔG _N ² RT)} Growth rate: G = 0.5 αt ^−0.5 Dc: Diffusion coefficient of carbon in γ C _av : Average amount of C in untransformed γ (at%) k1, k2 : Coefficient obtained by experiment k1 = 1.4 × 10
_{^{-10, k2 = 23000 △ G N}} : driving force of ferrite nucleation alpha: Parabolic Late constant t: time (s) R: gas constant (8.34 J / mol / K) T: absolute temperature (K)

<Widmansttetten ferrite> (Same as bainite except for nucleation site and concentration of C) Nucleation site: γ / α interface form: Parabolic cylinder Starting condition: (a) Nucleation condition: | △ G _N | ≧ 4204-5.96 (T-273) (b) Growth condition: | G _f | ≧ 300 J / mol Gf: Ferrite transformation driving force Nucleation rate: Is = k1 · Dc (1-C _av ) / √T · exp {-k2 / (△ G N 2 RT)} ( diverted ones ferrite) growth rate: v = (27/256 / π) · Dc / ρ c Ω 3 ρ c = {Cγ / (Cγ −Cα)} · {σVα / (Cα−C
₀ ) RT} Ω = Ω ₀ / {1- (2Ω ₀ / π)-(Ω ₀ ² / 2π)} Ω ₀ = (Cγ-C ₀ ) / (Cγ-Cα) Dc: γ Medium carbon diffusion coefficient Cγ: Equilibrium C amount in γ (at%) Cα: Equilibrium C amount in α (at%) C ₀ : C amount (at%)

<Pearlite> Nucleation site: γ / α interface, high carbon material is γ grain boundary Form: Hemisphere Starting condition: Average carbon concentration in γ ≧ A cm Nucleation rate: Is = k3 · Dc / √T · exp {-K ² / (ΔG _N ² RT)} Growth rate: v = 4 σV / ΔG _p Lamella spacing: S ₀ = Dc / (4 ・ σ ・ V ・ R ・ T ・ X ^e )
/ (△ G _p ² ) Dc: Diffusion coefficient of carbon in γ △ G _c : Driving force for cementite nucleation △ G _p : Driving force for pearlite transformation t: Time (s) k3: Experimentally determined coefficient R: Gas constant (8.34 J / mol / K) T: Absolute temperature (K) X ^e : Carbon concentration (molar fraction) σ: Interface energy between cementite and ferrite (700er
g / cm ² ) V: molar volume (7 cm ³ / mol)

<Bainite> Nucleation site: γ / α interface Form: Parabolic cylinder Starting condition: (a) Nucleation condition: ｜ △ G _N ｜ ≧ 4204-5.96 (T-273) (b) Growth condition: | G _m | ≧ 600 J / mol G _m : Driving force for martensitic transformation Nucleation rate: Is = k1 · Dc / √T · exp {−k2 // (ΔG _N ² RT)} Growth rate: v = (27 / 256 / π) ・ Dc / ρ _c Ω ³ ρ _c = {Cγ / (Cγ-Cα)} ・ {σVα / (Cα-C
₀ ) RT} Ω = Ω ₀ / {1- (2Ω ₀ / π)-(Ω ₀ ² / 2π)} Ω ₀ = (Cγ-C ₀ ) / (Cγ-Cα) Dc: γ Medium carbon diffusion coefficient

<Martensite> Nucleation site: Untransformed γ part (location is not specified) Form: Not considered Starting condition: | G _m | ≧ 1170-5880 × c + 0.42 ・ (800-
T) (J / mol) transformation proceeds: wherein X _M = 1-exp of ^{Koistein (-1.1 · 10 -2 (M} S -T)) X M: martensite fraction M _S: martensitic transformation start point

[0032]

According to the present invention, the three-dimensional rolling analysis calculation is performed for each rolling, and the temperature distribution calculation of the temperature distribution in the cross section between each pass and in the final cooling process is performed to obtain the strain in the cross section. Since the final structure is predicted by taking into consideration the strain rate and temperature distribution, it becomes possible to predict the structure of the final product of bar steel or bar steel that causes unevenness of the structure in the cross section, and guarantee the required quality. And depending on the conditions, it becomes possible to predict in advance tissue abnormalities such as abnormal grain growth seen in a part of the cross section.
It has become possible to improve the yield.

[Brief description of drawings]

FIG. 1 is a flowchart showing the procedure of a tissue distribution prediction method according to the present invention.

FIG. 2 is a flow chart for calculating the distribution of the final structure after cooling, which is the procedure 6 of the present invention.

Claims (1)

[Claims] 1. A method for predicting the distribution of a sectional structure after hot multi-pass rolling and cooling of a bar steel or a bar steel, the calculation according to the following steps 1 to 6 is sequentially performed, and the cross section of the bar steel or the bar steel is calculated based on the calculation result. A method for predicting the microstructural distribution of a cross section of a steel bar or a steel bar, characterized by predicting the microstructural distribution. (Procedure 1) Coupling calculation is performed by a three-dimensional rolling analysis method for the distribution of strain, strain rate, and temperature in the cross section during the first stage rolling of the steel material to be processed. (Procedure 2) With respect to the temperature distribution in the cross section immediately after rolling obtained in Procedure 1, the temperature change between the passes immediately before the start of the next rolling is calculated by the temperature analysis method. (Procedure 3) By the change in the strain in the cross section obtained in the above procedure 1, the strain rate, and the temperature distribution between the paths obtained in the second procedure,
The distribution of austenite recrystallized / non-recrystallized structure and residual strain in the cross section just before the next stage rolling is calculated. (Procedure 4) The above procedures 1 to 3 are sequentially repeated from the next rolling to the final rolling while inheriting the calculation results regarding the temperature and residual strain distribution of the former stage, and the austenite recrystallization / non-recrystallization in the cross section immediately after the final rolling is performed. The recrystallization structure and residual strain distribution are calculated. (Procedure 5) The temperature change in the temperature distribution in the cross section immediately after the final rolling obtained in the procedure 4 until the end of cooling is calculated by a temperature analysis method. (Procedure 6) By the distribution of the austenite recrystallized / unrecrystallized structure and residual strain in the cross section immediately after the final rolling obtained in the above procedure 4 and the temperature change until the end of cooling obtained in the above procedure 5, the final cross section Calculate tissue distribution.