KR100975195B1 - Prediction method for forming limit diagram of high chrome ferrite stainless steel - Google Patents

Abstract

The present invention relates to a method for predicting the molding limit of high chromium ferritic stainless steel which can easily predict the molding limit of ferritic stainless steel. Method for predicting the molding limit of high chromium ferritic stainless steel according to the present invention is to measure the nickel (Ni) equivalent of the alloying component, and using the measured nickel equivalent and the thickness of the material is a multiple regression method [FLD ₀ = 21.5 + (2.57 * thickness) - (0.98 * Ni eq.)] In an FLD ₀ plane deformation area on the forming limits also by formula Obtaining a value, and said FLD ₀ Predicting a molding limit degree using a value and at least one molding limit value measured while varying the nickel equivalents and the thickness of the material. By such a configuration, users who encounter different materials every time during press molding can easily predict the error rate and reduce the defective rate.

Molding Limit, Nickel Equivalent, Ferritic Stainless Steel

Description

Prediction method for forming limit diagram of high chrome ferrite stainless steel}

The present invention relates to a method for predicting the molding limit of high chromium ferritic stainless steel, and more particularly, to simplify the molding limit, which is an important indicator used for analyzing and solving the cause of component failure during press working of ferritic stainless steel. The molding limit of the high chromium ferritic stainless steel which can be predicted easily also relates to the prediction method.

When developing new parts using steel materials, a wide variety of parts processing methods are applied. The most typical processing methods are press working or stamping processing. Among these, the most problematic problem in press working is whether a desired shape can be obtained without breaking the material. The formability of the material can be classified into four types, namely, deep drawing property, elongation property, bending workability, and elongated flange property. However, in actual press work, the above formability is complicatedly combined.

Therefore, it is practically difficult to predict the state of complex stress that will be received during press working only with yield strength, tensile strength, elongation, etc., which are values of uniaxial tensile strength evaluated in the tensile test. There is a simulation test that simulates the drawing mode and the elongation mode in the laboratory, but this is also limited in the wide range of applications due to the lack of reproducibility, the lack of relevance of material constants and process variables.

Accordingly, the necessity of evaluating the formability through laboratory tests in the most similar way to the actual conditions of use of the material emerged, and the concept of the forming limit diagram (hereinafter referred to as FLD) by Keeler and Goodwin in the 1960s. This was introduced. Since then, the Circle Grid Analysis (CGA) has been introduced and has been recognized as the most common method for evaluating the formability of materials with the FLD concept by Keeler and Goodwin.

In the case of FLD, the limits of the peripheral strain and the substrain on the surface of the workpiece are shown on the X-Y coordinates, and the left side is divided into the deep drawing mode and the right side is the elongation mode based on the plane strain state where the sub strain is 0. In molding, if any one of the major and minor strains is located at the top of the line with respect to the continuous line on the X-Y coordinate, it means that there is a high probability of breakage, and if both are located at the bottom of the line, it means that safe molding is possible.

In order to derive the FLD, first, the critical strain in all deformation states where the material is deformed and breakage can be obtained, and then the critical strain is displayed on the X-Y plane as the major and minor strains occurring on the surface of the material.

Circular strain and punch stretching tests are used to find the critical strain. In other words, the punch-stretching test is performed by varying the width and lubrication conditions of the specimen etched into the circular grid so that fracture occurs at various strains. Typically, nine specimens of different widths are used, and these specimens are subjected to a punch stretching test after electrochemically performing a circular grid etching. Thereafter, the strains around the crack generator are classified as safe and broken, and these values are displayed on the X-Y plane. The boundaries that distinguish them from the indicated safety and strain states of failure are indicated and are called FLDs.

As such, obtaining a single FLD of a material requires a lot of time and effort each time, which is not preferable from a practical point of view. Since FLD is typically used for analyzing and solving the causes of component breakage during press processing, it has to be troublesome and time-consuming because it needs to be measured and measured for each material thickness and steel type.

In general, in the same steel grade, the slopes of the deep drawing mode and the elongation mode have almost the same shape, so the FLD can be easily obtained by knowing the FLD ₀ position, which is the planar deformation region. That is, if the location of the FLD ₀ can be predicted, it can be a very effective method of deriving the FLD. In this respect, some predictions have been made for carbon steel sheets used in automobiles in the past.

However, the factors that make up these predictions consist of a combination of constants such as yield strength, tensile strength and elongation, which are the properties of the material, including the thickness of the material. Therefore, it can be said that conventional prediction equations are essential to secure tensile properties through at least a tensile test. Accordingly, it is necessary to derive a generalized formula during FLD prediction of ferritic stainless steel, and to use a fast and efficient FLD prediction method.

Accordingly, an object of the present invention is to provide a method for estimating the forming limit of high chromium ferritic stainless steel, which can easily predict FLD, which is an important indicator that is typically used for analyzing and solving the cause of component breakage during press working of ferritic stainless steel. It is.

Molding limit prediction method of high chromium ferritic stainless steel according to the present invention comprises the following steps.
In the first step, nickel (Ni) equivalents of the alloying components are measured. In the second step, the FLD ₀ value, which is the planar deformation region on the forming limit diagram, is obtained by the following Equation 1 using the measured nickel equivalents and the thickness of the material.
[Equation 1]
[FLD ₀ = 21.5 + (2.57 * Thickness)-(0.98 * Nickel Equivalent)]
In the third step, the molding limit is predicted using the FLD ₀ value, the nickel equivalent and the at least one FLD value measured while varying the thickness of the material.
In addition, the nickel equivalent weight of Equation 1 may be calculated by Equation 2 below.
[Formula 2]
[Ni (Ni) equivalent = Ni + 0.31Mn + 22C + 14.2N + Cu]
In addition, the third step may assume a continuous virtual boundary line connecting the measured at least one FLD value and the FLD ₀ value with respect to the XY plane having the principal and minor strain as the coordinate axis, and the Y of the FLD ₀ on the Y coordinate. After the virtual boundary line is vertically moved so that the intercept value is 0, the moved virtual boundary line may be determined as a standard FLD that distinguishes the breaking value from the safety value.

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As described above, according to the present invention It is possible to easily predict the molding limit, which is an important indicator that is used to analyze the cause of component breakage and solution during press machining of ferritic stainless steel, so that users who encounter different materials every time during press molding can easily predict the error rate. In addition, the defect rate can be reduced.

In addition, in the FLD prediction method according to the present invention, the standard FLD is determined for each material thickness and steel type by deriving the FLD with a minimum of experiments, so that the FLDs of a large number of materials can be obtained very easily, which is economical and saves time. .

Hereinafter, a FLD prediction method of high chromium ferritic stainless steel according to the present invention will be described in detail with reference to the drawings of the present invention.

Among high chromium ferritic stainless steels, 436L steel is widely used for exhaust system parts requiring corrosion resistance. 436L steel is mainly manufactured in the form of plate and processed into exhaust type molded products and pipes. In press molding, a cold rolled sheet of 0.5 mm to 2 mm is commonly used, and the FLD of the material varies depending on the thickness or the manufacturing method. In particular, the change in the physical properties of these materials is affected by the difference in content of invasive atoms such as carbon, nitrogen, and phase stabilizing elements such as titanium and niobium. That is, this is related to the phase stability of the ferritic stainless steel, in general, the phase stability of stainless steel is divided into chromium equivalent and nickel equivalent. Here, elements constituting the chromium equivalent increase the stability of the ferrite phase, while elements constituting the nickel equivalent are composed of the elements that increase the stability of the austenite phase.

Several empirical equations have been proposed since 1949 with respect to these equivalents. The representative ones are Schaffer, Delong, WRC-92 and Hammer & Svensson. The difference between these empirical equations is that the weight of each element is slightly different in constructing the equivalent equation, and which empirical equation is selected for each material or user.

(Example)

In the embodiment of the present invention has a high chromium 400-based material 436L steel and there is any difference in the FLD according to the change in chromium and nickel equivalent, and how to efficiently predict the FLD by examining the relationship between the thickness and material properties I want to make it clear. Table 1 shows 436L material used in connection with the present invention and the chromium and nickel equivalents are represented by Delong, WRC-92, and Hammer & Svensson.

436L Cr Mo Si Nb Ti Ni Mn C N Cu Cr equivalent Ni equivalent One 17.49 1.03 0.122 0.002 0.272 0.14 0.321 0.0047 0.0096 0.041 19.90 0.52 2 17.7 1.03 0.114 0.002 0.322 0.12 0.143 0.0048 0.0077 0.025 20.25 0.40 3 17.66 1.04 0.121 0.005 0.285 0.13 0.279 0.0063 0.0072 0.047 20.13 0.50 4 17.57 1.07 0.121 0.002 0.316 0.14 0.124 0.0069 0.0071 0.033 20.17 0.46 5 17.79 1.08 0.093 0.003 0.337 0.14 0.176 0.0054 0.006 0.03 20.43 0.43 6 17.69 1.06 0.153 0.002 0.302 0.12 0.205 0.0081 0.0073 0.037 20.28 0.50 7 17.73 1.07 0.14 0.005 0.356 0.16 0.198 0.003 0.0088 0.042 20.48 0.45 8 17.65 1.08 0.196 0.002 0.28 0.15 0.203 0.0076 0.0062 0.071 20.27 0.54

Here, the empirical equation of the nickel equivalent of the Hammer & Svensson equation is as follows.

That is, nickel (Ni) equivalent = Ni + 0.31Mn + 22C + 14.2N + Cu

At this time, the specimen is based on the weight percent of each component.

It can be seen from Table 1 that the chromium equivalent of STS436L does not change significantly around 3% (19.9 ~ 20.48) for each specimen, while the nickel equivalent shows a big difference between about 30% (0.4 ~ 0.54). This is because invasive elements such as carbon and nitrogen are very powerful austenite stabilizing elements and there are considerable variations from specimen to specimen. As such, when there is a difference in the contents of C and N, the nickel equivalent is increased by the Hammer & Svensson equation. In general, as the nickel equivalent increases, the strength tends to increase and the ductility tends to be inferior, which adversely affects formability.

After all, considering that there are variations in C, N, etc., which are the main factors of nickel equivalents, and the effects of such deviations are large, we can find the possibility of predicting FLD. In order to confirm this, the correlation between nickel equivalents and material properties was investigated.

1 is a graph showing the yield strength according to the nickel equivalent of STS436L steel. Referring to FIG. 1, since the yield strength generally increases as the nickel equivalent is increased, it can be seen that the nickel equivalent and the yield strength tend to be proportional.

2 is a graph showing the correlation between crack extension rate and work hardening index (n) of STS436L steel. Referring to FIG. 2, since the crack extension rate tends to increase as the work hardening index n increases, mechanical properties and formability are usually correlated with these molding indices.

3 is a graph showing an example of the change in yield strength according to the increase in nickel equivalent and chromium equivalent of high chromium ferritic stainless steels. Referring to Figure 3, the yield strength increases with increasing nickel equivalents and chromium equivalents, it can be seen that the material tends to increase in strength.

4 is a graph showing an example of the yield ratio change with increasing nickel equivalent and chromium equivalent of high chromium ferritic stainless steels. Referring to Figure 4, it can be seen that the yield ratio increases with the increase in nickel equivalents and chromium equivalents. Here, the yield ratio is the ratio of yield strength and tensile strength. In general, the smaller the yield ratio, the greater the room for work hardening ability due to the processing of the material, which is advantageous in formability.

Therefore, according to FIGS. 1 to 4, these results illustrate the correlation between the equivalent weight and the change in material properties, suggesting that the FLD, which is a formability index, can be similarly predicted.

On the other hand, Figure 5 is a graph showing the change in FLD _{0 with} the increase in the thickness of the material. Referring to FIG. 5, it can be seen that as the thickness of the material increases, the FLD ₀ increases.

Fig. 6 is a graph showing an example of standard FLD creation of STS436L steel, and it can be seen that the slopes of the drawing area on the right and left of the planar deformation area and the elongation area are very similar for each experiment in the high-chromium STS436L steel, which is the same steel type. In order to achieve this, when the FLDs of the specimens having different thicknesses and equivalent weights are vertically moved from the Y coordinate to the zero point based on the FLD ₀ , the boundary surface appears when the breakdown region and the safety region are classified. This interface can be defined as the standard FLD of high chrome 436L stainless steel.

In this way, if a standard FLD is determined and FLD ₀ prediction can be made for each specimen, the FLD can be easily predicted. In the present invention, a multiple regression method was selected as a method for predicting this. That is, a method of eliminating overlapping factors (removing factors with multiple collinearity) by regression with the factors such as nickel equivalent, yield strength, uniform elongation, work hardening index, and material thickness of the specimens. Finally, two important factors were selected as factors of the prediction equation. The nickel equivalent equation used in the present invention was selected because the prediction degree was higher than that of the other empirical equations using the Hammer & Svensson equation.

Prediction (436L only prediction) Equivalent formula R-sq R-sq (adj) p VIF FLD ₀ = 21.5 + 2.57 Thickness-0.98 Ni-eq H & S 88.7 77.3 0.113 1.1 FLD ₀ = 19.2 + 1.87 Thickness + 4.68 Ni-eq-WRC WRC 91.2 82.4 0.088 2.9 FLD ₀ = 21.8 + 2.57 Thickness-1.13 Ni-eq-Delong Delong 89.3 78.7 0.107 1.1 Prediction (at the same time predicting 436L and 441 & 444) Equivalent formula R-sq R-sq (adj) p VIF FLD ₀ = 47.8 + 4.78 Thickness-1.40 Cr-eq - 87.8 83.8 0.002 1.1

Table 2 shows the FLD prediction equation for the high chromium 436L steel obtained through the regression equation. The Hammer & Svensson equation and the Delong equation, where the FLD ₀ prediction and the material thickness are positively correlated and the nickel equivalent is negatively correlated, are useful. . The present invention related to the prediction of molding limit of high chromium STS436L stainless steel using multiple regression method will be described as follows.

-When predicting the molding limit of high chrome STS436L steel, first find the nickel equivalent of alloy component through Hammer & Svensson equation.

-Several FLDs obtained through the test are determined by moving the Y-coordinate vertically to the zero point based on the FLD ₀ value to define the boundary between fracture and the safe experimental value and set it as the standard FLD.

-Multiple regression method removes redundant factors (excludes multicollinearity) and obtains significant multiple regression equations using physical properties, thicknesses, and equivalents that affect molding limits.

-Through the present invention, the FLD ₀ of the high chromium STS436L steel can be obtained through the nickel equivalent and the thickness of the material. That is, even without a mechanical test, the FLD can be easily obtained by knowing only the thickness of the material and the alloying components.

Therefore, in the present invention, FLD can be obtained only through nickel equivalent information and thickness of a material when press forming high-chromium STS436L steel.

In the above-described embodiment, the forming limit prediction method of 436L steel is described, but the forming limit prediction method according to the present invention can be applied to other steel grades.

Although the technical spirit of the present invention has been described in detail according to the above-described preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

1 is a graph showing the yield strength according to nickel equivalents of STS436L steel.

2 is a graph showing the correlation between crack extension rate and work hardening index of STS436L steel.

Figure 3 is a graph showing an example of the change in yield strength according to the increase in nickel equivalent and chromium equivalent of high chromium ferritic stainless steels.

Figure 4 is a graph showing an example of the yield ratio change with increasing nickel equivalent and chromium equivalent of high chromium ferritic stainless steels.

5 is a graph showing changes in the FLD ₀ value with increasing thickness of the material.

Fig. 6 is a graph showing an example of standard forming limit diagram (FLD) preparation of STS436L steel.

Claims (4)

% By mass, C: 0.025% or less, Mn: 1.00% or less, Cr: 16.00-19.00%, Mo: 0.75-1.25%, other N: 0.025% or less, Ni 0.50% or less or Ni + Cu 0.50% In a ferritic stainless steel containing the following, A first step of measuring the nickel (Ni) equivalent weight of the alloy component; A second step of obtaining a value of FLD ₀ which is a planar deformation area on a molding limit diagram by Equation 1 using the measured nickel equivalents and the thickness of the material; And A third step of predicting a forming limit degree by using the FLD ₀ value and the at least one FLD value measured while changing the nickel equivalent and the thickness of the material; and comprising a high chromium ferritic stainless steel Molding Limit Prediction Method. [Equation 1] [FLD ₀ = 21.5 + (2.57 * Thickness)-(0.98 * Nickel Equivalent)] The method of claim 1, Nickel equivalent of Formula 1 is a method for predicting the molding limit of high chromium ferritic stainless steel calculated by the following formula 2. [Equation 2] [Ni (Ni) equivalent = Ni + 0.31Mn + 22C + 14.2N + Cu] delete The method of claim 1, The third step is Assuming a continuous virtual boundary line connecting the measured at least one FLD value and the FLD ₀ value with respect to the XY plane having the principal and minor strain as the coordinate axis, the Y-intercept value of the FLD ₀ is ₀ on the Y coordinate. Forming limit prediction method of high chromium ferritic stainless steel by vertically moving the virtual boundary line to determine the moved virtual boundary line as a standard FLD to distinguish the breaking value and the safety value.

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