KR20040056704A - A method for predicting hot deformation resistance of STS 430 ferritic stainless steel - Google Patents

Abstract

PURPOSE: A method for predicting deformation resistance that is an essential factor for calculating rolling load when hot rolling 430 ferritic stainless steel in which ferrite and austenite coexist in rolling region using tandem type finishing mills during the hot steel rolling process is provided. CONSTITUTION: The method comprises a step of experimentally obtaining influence of temperature and strain rate exerted on steady state flow stress (σs) by the following mathematical expression 1 after experimentally reproducing a material having an austenite phase volume fraction that is the same as an input side work of first rolling mill in tandem type finishing mills: σs=A1xln ε+A2/T+A3, where σs is steady state flow stress (kg/mm¬2), ε is strain rate(1/sec), T is absolute temperature, and A1, A2 and A3 are constants; a step of obtaining deformation resistance (Km) of plane deformation by the following mathematical expression 2 using the steady state flow stress (σs): Km=2¡î3x(A1xln ε+A2/T+A3), where Km is deformation resistance of plane deformation (kg/mm¬2); and a step of correcting experimental coefficients A1, A2 and A3 of the mathematical expression 2 using actual deformation resistance results of actual rolling.

Description

A method for predicting hot deformation resistance of STS 430 ferritic stainless steel}

The present invention relates to a method for predicting hot deformation resistance of ferritic stainless steel, in particular, hot rolling of STS 430 ferritic stainless steel in which ferrite and austenite coexist in a rolling region by using a tandem array type rolling mill during the hot rolling process of steel. The present invention relates to a method for predicting strain resistance, which is an essential element in calculating rolling load.

In general, since the hot rolling of stainless steel is made at a high deformation rate at a high temperature, the structural change and phase transformation of the material occur more significantly than the cold rolling. Therefore, the surface, shape and mechanical properties of stainless steel are greatly influenced by hot working parameters such as deformation temperature, amount of deformation and speed of deformation and deformation resistance.

The deformation resistance among the hot working variables refers to the flow stress due to deformation when plastic deformation of the material, the flow stress at high temperature is called hot deformation resistance. Hot deformation resistance is an important data for designing the mechanical and electrical capacity of a facility or for determining how much high temperature deformation is possible at a given capacity.

Moreover, in the hot rolling process, the thickness of the material is an important management goal, and making the desired thickness is one of the main functions of the finishing rolling process. The finishing rolling model to obtain the target thickness includes a setting model for setting an appropriate roll gap and rolling load in each rolling mill when the tip of the material is rolled, and the deviation of the tip after controlling the tip using the track record. There is a control model.

The model that controls the deviation after the tip has already had a considerable degree of control. For the parts except the tip, it is known that the difference between the material thickness target value and the measured value hits within 50 µm, but about 95%. The setting model has an important meaning because the thickness of the raw material depends on the model for setting the appropriate roll gap and rolling load of the rolling mill.

In particular, the STS 430 ferritic stainless steel has a high temperature deformation behavior and dynamic / static recovery due to the coexistence of ferrite and austenite in the hot rolling temperature range. The degree of control decreases. Accordingly, many attempts have been made on a method of predicting hot rolling loads with good prediction accuracy.

First, Korean Patent Application No. 1997-62738 may be mentioned. This method suggests that the temperature dependence of the strain resistance can be predicted by the Cr content. However, this method has a problem in that the STS 430 containing 17 Wt% Cr, even if Cr content is constant, ignores the temperature-dependent change in deformation resistance that may occur due to the reduction in rolling temperature.

In addition, Korean Patent Application No. 2000-80842 proposes a method for calculating the deformation resistance during rolling of a thick plate from the relationship between high temperature flow stress and strain for austenitic, ferritic and two-phase stainless steel.

However, this method predicts the deformation resistance simply by the rolling strain in the rolling section, so it is difficult to accurately predict the rolling load when the strain accumulation phenomenon occurs between the rolling passes due to the low rolling temperature. In addition, when austenitic and ferrite coexist in the rolling region as in STS 430, the austenitic phase fraction affects the deformation resistance, so the deformation resistance should be estimated in consideration of the austenite phase fraction at the start of rolling. The above invention does not consider the effect of the austenite phase fraction.

The present invention has been made to solve the above-mentioned problems of the prior art, the temperature dependence of the deformation resistance in the hot rolling temperature range for STS 430 ferritic stainless steel present in austenitic and ferrite two-phase in the hot rolling temperature section It is an object of the present invention to provide a method for predicting hot deformation resistance of STS 430 ferritic stainless steel that accurately considers the change and accurately predicts the deformation resistance even when deformation is accumulated between rolling mills that may occur in the case of low temperature rolling.

1 is a graph showing the temperature and strain rate dependence of the steady state flow stress measured in STS 430.

Figure 2 is a graph showing the difference in deformation resistance predicted by the experimental coefficient and the correction coefficient in the actual rolling temperature section.

Figure 3 is a graph showing the deformation resistance predicted by the experimental coefficient and the correction coefficient compared with the performance strain resistance.

Figure 4 is a graph showing the strain resistance calculated using the conventional strain resistance prediction method and the prediction method according to the present invention compared with the performance strain resistance.

In order to achieve the above object, the present invention provides a method for predicting the hot deformation resistance of STS 430 ferritic stainless steel in which ferrite and austenite coexist in a hot rolling section. Experimentally simulating a material having a nitrite phase fraction, and experimentally obtaining the effect of temperature and strain rate on the steady state flow stress (σ _s ) using Equation 1 below;

σ _s = A ₁ × ln ε + A ₂ / T + A ₃

Where σ _s : steady state flow stress (kg / mm 2), ε: strain rate (1 / sec),

T: Absolute temperature, A ₁ , A ₂ , A ₃ : Constant

Obtaining a plane strain deformation resistance using the steady state flow stress (σ _s ) by Equation 2 below;

× σ _s = × (A ₁ × ln ε + A ₂ / T + A ₃ )

Where Km: Plane strain resistance (kg / mm2)

Correcting the laboratory coefficients A ₁ , A ₂ , A ₃ of Equation 2 using the actual rolling resistance performance; The present invention relates to a method for predicting hot deformation resistance of STS 430 ferritic stainless steel.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

The present invention provides a model for predicting the deformation resistance that changes with the rolling temperature and speed change during hot rolling of STS 430 ferritic stainless steel in which ferrite and austenite coexist in the hot rolling section.

In STS 430 ferritic stainless steels, the phase fraction of ferrite and austenite at the time of hot finishing rolling affects the deformation resistance. FIG. 1 shows the effect of temperature drop on steady-state flow stress (σ _s ) under various deformation conditions after experimentally simulating a material having an austenite fraction equivalent to that of the first rolling mill. .

As shown in FIG. 1, the temperature dependence of the steady state flow stress σ _s increases with temperature decrease. As described above, when the temperature dependence of the deformation resistance is expressed only as a function of the amount of Cr, it is not possible to reflect the increase in the deformation resistance due to the increase in the temperature dependency of the steady state flow stress σ _s as the temperature decreases. Therefore, when the rolling load is predicted by simply quantifying the temperature dependence of the deformation resistance by Cr content, the prediction error of the rolling load becomes large.

By analyzing the data of FIG. 1 using Equation 1 provided by the present invention, it is possible to describe the change of temperature dependence of the steady state flow stress (σ _s ) in the rolling temperature section relatively accurately. By analyzing the steady state flow stress (σ _s ) under each deformation condition using Equation 1, the deformation rate dependence coefficient (A ₁ ), the temperature dependence coefficient (A ₂ ), and the constant (A ₃ ) can be obtained. Substituting this in Equation 2, the deformation resistance in the plane deformation condition corresponding to the stress state of rolling can be obtained.

Where Equation 2 Is a constant required for conversion to plane strain value. Equation 2 does not consider the influence of the strain on the deformation resistance, and indicates that the plane strain deformation resistance (Km) is determined by the steady state flow stress (σ _s ) in the rolling temperature section. Therefore, using Equation 2 provided in the present invention, when rolling the STS 430 ferritic stainless steel in a tandem array type rolling mill having a short time between rolling passes, the deformation resistance can be predicted regardless of whether or not strain accumulation occurs. . The suitability of Equation 2 was verified through the following examples, and after calculating the plane strain resistance (Km) using Equation 2 provided by the present invention, the plane strain deformation resistance (Km) thus obtained was By substituting 3, the rolling load can be obtained.

RF = Km × B × L _d × Q _p

Where RF: rolling load,

B: plate width (mm),

L _d : projected contact field (mm),

Q _p : Rolling force function

All variables (B, L _d , Q _p ) except Km among the variables in Equation 3 are all factors related to the size and shape of the rolled material and the roll, and the factors indicating the high temperature deformation characteristics of the material are plane strain deformation. Resistance (Km).

Deformation rate and temperature of steady state flow stress (σ _s ) after high-temperature flow stress experiments were carried out using the test method described above, that is, STS 430 ferritic stainless steel having an austenite phase fraction equivalent to that of the first rolling mill. The coefficients A ₁ , A ₂ , and A ₃ of Equation 2 obtained from the dependency can be corrected by analyzing the actual rolling performance. The correction of the coefficients A ₁ , A ₂ , and A ₃ of Equation 2 is to minimize the effect of the error on the deformation resistance, which may occur during the prediction of the rolling temperature and the quantification of the deformation speed, which are required to predict the deformation resistance. .

In the coefficient correction method of Equation 2 provided in the present invention, after calculating the actual plane strain deformation resistance (Km) in each rolling mill through the actual rolling performance analysis using Equation 3, the equation is substituted into the plane It is a method of resetting the coefficients A ₁ , A ₂ , and A ₃ by analyzing the correlation between the strain resistance (Km) and the process variables (temperature and strain rate). Therefore, by correcting the coefficient of Equation 2 to the performance value, it is possible to improve the prediction accuracy of the plane strain resistance (Km).

Hereinafter, the present invention will be described in detail through examples.

(Example)

In order to evaluate the effect of temperature and strain rate on the steady-state flow stress (σ _s ) in the filamentous rolling section, the cold rolled material is taken and quenched, and then reheated to obtain the high-temperature flow stress in the temperature range of 700 ℃ ~ 1050 ℃. And 0.1 sec ⁻¹ to 50 sec ⁻¹ at strain rate intervals. The reason why the rough-rolled material is quenched and used is to use a material having an phase fraction equivalent to that of the austenite phase fraction of the material that enters the first rolling mill of the finishing mill. The effects of temperature and strain rate on the steady state flow stress (σ _s ) at each condition are shown in FIG. 1. As shown in FIG. 1, the influence of temperature on the steady state flow stress σ _s is different according to the temperature decrease.

Then, by extracting the data of Figure 1 corresponding to the actual rolling conditions, the effect of the temperature and strain rate on the steady state flow stress (σ _s ) in the actual rolling region by the A ₁ , A ₂ , A ₃ coefficient of Equation 2 Quantification is shown in the "Experimental coefficient" term in Table 1 below.

Coefficient of Equation 1 A ₁ A ₂ A ₃ Experimental coefficient 2.25 108463 -77.5 Correction factor 3.08 120118 -90.7

In addition, Table 1 shows the results of correcting A ₁ , A ₂ , and A ₃ of Equation 2 with actual rolling resistance. The actual rolling strain resistance performance is the result of inverting the planar strain resistance (Km) by using Equation 3 from the rolling load result for the coil produced in the hot rolling mill using the process variables at that time. At this time, 20 coils were used.

In order to show the difference in planar strain resistance (Km) when two types of coefficients shown in Table 1 are used, the average process conditions (strain rate = 0.25, strain rate = 25 sec ^-1 ) during actual filament rolling The difference in plane strain deformation resistance (Km) in the actual rolling temperature range is shown in FIG. Even when coefficient correction is performed as in the comparison in FIG. 2, the absolute value of the strain resistance is not significantly different from the strain resistance derived from the experimental coefficient, and the strain resistance before and after the correction has a maximum error of about 10%.

Figure 3 shows the difference between the strain resistance predicted by the experimental strain resistance and the two experimental coefficients of Table 1. When the correction coefficient value is used, the prediction accuracy of strain resistance is further improved. In addition, as shown in FIG. 3, the excellent correspondence between the predicted strain resistance and the performance strain resistance demonstrates the suitability of Equation 2 of the present invention that the strain resistance of the STS 430 steel can be represented by the steady state flow stress (σ _s ). The result is.

4 shows the predictive strain resistance using the calculation formula of Korean Patent Application No. 2000-80842 and the predictive strain resistance using the correction coefficients of Equation 2 and Table 1 provided by the present invention in comparison with actual rolling strain performance. It is shown that the deformation resistance prediction method provided by the present invention is remarkably excellent in a relatively low temperature region where the actual rolling resistance performance is about 25 to 40 kg / mm 2. Therefore, when using the method for predicting hot deformation resistance provided by the present invention, the rolling load prediction for the thickness control of STS 430 ferritic stainless steel is accurate, and thus it is determined that the rolling plate can be manufactured with high quality while improving the rolling error rate. .

As described above, according to the method of predicting hot deformation resistance of ferritic stainless steel of the present invention, by accurately predicting the deformation resistance of STS 430 ferritic stainless steel, it is possible to reduce the learning dependence of rolling control to improve the operation stability and productivity. It can be effective.

In addition, by correcting the deformation resistance prediction equation of STS 430 ferritic stainless steel through the actual rolling performance, it is possible to manage the coefficient for improving the accuracy of the deformation resistance prediction equation through actual operation, thereby further reducing the learning dependence of rolling control. By doing so, there is an effect that can achieve a real rate improvement through more precise thickness control.

Claims (1)

In the method for predicting the hot deformation resistance of STS 430 ferritic stainless steel in which ferrite and austenite coexist in the hot rolling section, In the tandem array finishing mill, after experimentally simulating the material having the austenite phase fraction equivalent to that of the first rolling mill, the effect of temperature and strain rate on the steady state flow stress (σ _s ) Experimentally obtaining; [Equation 1] σ _s = A ₁ × ln ε + A ₂ / T + A ₃ Where σ _s : steady state flow stress (kg / mm 2), ε: strain rate (1 / sec), T: Absolute temperature, A ₁ , A ₂ , A ₃ : Constant Obtaining a plane strain deformation resistance (Km) using the steady state flow stress (σ _s ) using Equation 2 below; [Equation 2] Km = × σ _s = × (A ₁ × ln ε + A ₂ / T + A ₃ ) Where Km: Plane strain resistance (kg / mm2) Correcting the laboratory coefficients A ₁ , A ₂ , A ₃ of Equation 2 using the actual rolling resistance performance; Hot deformation resistance prediction method of STS 430 ferritic stainless steel, characterized in that comprises a.