KR100805618B1 - Prediction method of forming limit diagram for 400 stainless steel - Google Patents

Abstract

A prediction method of a forming limit diagrams for 400 series stainless steels is provided to be able to simply predict a forming limit diagram that is an index typically used for analyzing causes of parts fracture and solving problems of the parts fracture during the press forming of the 400 series stainless steels. A prediction method of a forming limit diagram for 400 series stainless steels comprises the steps of: measuring an equivalent of nickel(Ni) that is an alloy component; obtaining the FLD(Forming Limit Diagram)0 value that is a plane-strain region on a forming limit diagram by the formula [FLD0=26.6+(4.11xthickness)-(8.54xnickel equivalent)] that is multiple regression analysis, using the measured Ni equivalent and thickness of a workpiece; and predicting a forming limit diagram using at least one forming limit diagram FLD0 value measured while varying the FLD0 value, the Ni equivalent and the workpiece thickness.

Description

Prediction method of forming limit diagram for 400 stainless steel}

1 is a graph showing the relationship between nickel (Ni) equivalent weight and yield strength.

2 is a graph showing the relationship between nickel (Ni) equivalents and crack elongation.

3 is a graph showing the relationship between nickel (Ni) equivalent weight and work hardening index.

4 is a graph showing the correlation between FLD ₀ and material thickness.

5 is an example of standard FLD creation.

6 illustrates an example of deriving an FLD ₀ prediction equation through a multiple regression method.

The present invention relates to a method for predicting the molding limit of 400-based stainless steel, and more specifically, the molding limit can be predicted only by nickel (Ni) equivalent weight and sheet thickness, and after the standard forming limit diagram (FLD) is obtained. Is a method of predicting the molding limit of 400 series stainless steel, which can easily obtain the forming limit of a large number of plates.

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

Therefore, it is practically difficult to predict the state of complex stress received during press working only with yield strength, tensile strength, and elongation, which are values of uniaxial tensile strength evaluated in the tensile test. There is a simulation test that simulates the drawing mode and the unloading mode in the laboratory, but it 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 a manner most similar to the actual use conditions of the material emerged, and since the concept of the forming limit diagram (FLD) was introduced by Keeler and Goodwin in the 1960s Along with the Circular Grid Analysis (CGA), it was recognized as the most common method for evaluating the formability of materials. The molding limit diagram shows the limits of the peripheral strain and the substrain on the surface of the material on the X-Y coordinate, and is divided into the deep drawing mode on the left side and the elongation mode on the right side on the basis of the plane deformation state where the side strain is 0. At this time, if any one of the major and minor strains is located at the top of the line when forming based on the continuous line on the XY coordinates, it is determined that there is a high probability of breakage. It means possible.

In order to derive the forming limit diagram, first, the critical strain at all deformation states where the sheet is deformed and breakage can be obtained, and then the marginal strain is displayed on the XY plane with the major and minor strains occurring on the surface of the sheet. Circular strain and punch stretching test are used to find the limit strain. In other words, the punch stretching test is performed while varying the width and lubrication conditions of the specimen etched with the circular grid so that fracture occurs in various deformation states. Usually nine specimens of different widths are used, and these specimens are subjected to a punch stretching test after electrochemically circular grid etching. Thereafter, the strains around the crack generating unit are measured and classified as safe and broken, and these values are displayed on the X-Y plane. The boundary line which distinguishes them from the displayed safety and strain state of fracture is indicated, and this boundary line is called the forming limit of each plate.

As such, a large amount of time and effort is required each time to obtain a molding limit of one sheet, which is not preferable in practical terms. Since the slopes of the deep drawing mode and the elongation mode generally have almost similar shapes in the same steel type, it is a very effective method of deriving FLD that can easily predict the forming limit if only the location of the plane deformation region FLD ₀ is known. .

From this point of view, some prediction equations have been derived for carbon steel sheets used in automobiles in the past. 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 obtained from the tensile test, including thickness. In these equations, it is essential to secure tensile properties through at least tensile tests.

Currently, general prediction equations for FLD prediction of 400 stainless steels are not used. Therefore, a fast and efficient prediction method needs to be used.

Accordingly, the present invention is an invention designed to solve the above-mentioned conventional problems, and an object of the present invention is to simplify the molding limit, which is a representative index used for analyzing and solving the cause of component breakage during press working of 400 series stainless steel. The molding limit of 400 series stainless steel which can be easily predicted is also provided.

In order to achieve the above object, according to the present invention, the molding limit prediction method of 400-based stainless steel is a multiple regression method [FLD ₀ = 26.6 + (4.11 * thickness) using a nickel (Ni) equivalent and the thickness of the material -(8.54 * nickel equivalent)] is used to predict the molding limit.

Preferably, the nickel equivalent of the alloying component is measured by Hammer &Svensson's formula [Nickel (Ni) = Ni + 0.31Mn + 22C + 14.2N + Cu], and the nickel equivalent and the thickness of the material are changed. Measuring at least one molding limit (FLD), vertically shifting the FLD ₀ value about the Y axis so that the value of the Y-intercept is 0, and defining the interface between fracture and safe experimental value as the standard FLD, and The multiple regression method includes a step of obtaining a multiple regression equation by removing redundant factors using physical properties that affect nickel equivalents, thicknesses, and molding limits.

The forming limit also is made to the left relative to the FLD ₀ by extending the area of the drawing area and the right side, the FLD ₀ is a point on the Y-axis that is the boundary of the drawing area and the extending area.

Hereinafter, with reference to the drawings showing an embodiment of the present invention will be described in detail a molding limit prediction method of 400-based stainless steel according to the present invention.

The most representative material among 400 stainless steel is 409L steel containing 11% of chromium (Cr). 409L steel is classified as low chromium (Cr) stainless steel because it has the lowest chromium (Cr) content among stainless steels, and has various advantages such as price advantage and exhaust system molded parts and pipes. In relation to press molding, a cold rolled sheet of 0.5 mm to 2 mm is commonly used, and the molding limit of these materials varies depending on the thickness or the manufacturing method.

In particular, these materials can be said to have a relatively large change in physical properties compared to high chromium (Cr) steels depending on the difference in the content of interstitial atoms such as carbon and nitrogen, and phase stabilizing elements such as titanium (Ti) and niobium (Nb). That is, it is related to the phase stability of ferritic stainless steel, and generally, it is divided into chromium (Cr) equivalent and nickel (Ni) equivalent in relation to the phase stability of stainless steel.

Elements constituting chromium (Cr) equivalents increase the stability of the ferrite phase, while elements constituting nickel (Ni) equivalents are composed of elements that increase the stability of the austenite phase. Several empirical formulas have been proposed since 1949 with respect to these equivalent formulas, including 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 present invention, there is some difference in the change of chromium (Cr) and nickel (Ni) equivalents through 409L steel, which is a low chromium 400-based material, and an effective molding limit (FLD) is investigated by investigating the relationship between thickness and material properties. I would like to reveal how it can be predicted.

Table 1 shows 409L materials used in connection with the present invention, and the chromium (Cr) and nickel (Ni) equivalents are given by the Delong, WRC-92, Hammer & Svensson empirical formula.

409L Cr Mo Si Nb Ti Ni Mn C N Cu Cr equivalent Ni equivalent One 11.21 0.02 0.555 0.003 0.205 0.11 0.261 0.0068 0.0084 0.05 12.69 0.51 2 11.22 0.02 0.54 0.003 0.23 0.12 0.229 0.0064 0.0062 0.026 12.75 0.45 3 11.21 0.02 0.619 0.002 0.238 0.09 0.261 0.0078 0.0096 0.022 12.88 0.50 4 11.16 0.02 0.653 0.002 0.223 0.11 0.208 0.0078 0.0096 0.031 12.84 0.51 5 11.2 0.02 0.53 0.005 0.217 0.13 0.193 0.0068 0.0067 0.076 12.70 0.51 6 11.26 0.02 0.643 0.003 0.226 0.1 0.39 0.0087 0.0068 0.038 12.94 0.55 7 11.21 0.02 0.625 0.002 0.239 0.09 0.336 0.0078 0.0076 0.06 12.89 0.53 8 11.2 0.02 0.61 0.002 0.244 0.09 0.234 0.0053 0.0078 0.039 12.87 0.43 9 11.25 0.02 0.622 0.002 0.196 0.1 0.28 0.0058 0.0077 0.038 12.80 0.46 10 11.03 0.02 0.599 0.002 0.218 0.09 0.247 0.0057 0.0083 0.023 12.61 0.43 11 11.47 0.02 0.584 0.002 0.192 0.06 0.245 0.0088 0.0092 0.015 12.95 0.48 12 11.21 0.02 0.555 0.003 0.205 0.11 0.261 0.0068 0.0084 0.05 12.69 0.51 13 11.16 0.02 0.598 0.003 0.246 0.1 0.23 0.0075 0.0082 0.033 12.84 0.49

Here, the empirical equation of the nickel (Ni) equivalent of the Hammer & Svensson type is as follows.

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

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

[Table 1] shows that the chromium (Cr) equivalent is less than ± 1.5% between specimens, while the nickel (Ni) equivalent is about 15 times or more, regardless of the empirical equation. You can see the big difference. This is because interstitial elements, such as carbon and nitrogen, are very powerful austenite stabilizing elements and there are significant variations between specimens.

As such, when there is a difference between the contents of carbon (C) and nitrogen (N), that is, for example, an increase in nickel equivalent weight, there is a general tendency to decrease the ductility with increasing strength, which adversely affects the formability. do. As a result, considering the presence of such a large variation in carbon (C) and nitrogen (N), which are the main factors for nickel (Ni) equivalents, the molding limit can be predicted using these indicators. Experiments were conducted to find the possibilities and confirm them.

In order to confirm this, the correlation between nickel equivalents and some material properties was investigated.

1 is a graph showing the relationship between nickel (Ni) equivalent weight and yield strength (YS: Yield strength). Referring to FIG. 1, it can be seen that the yield strength increases in proportion to the nickel equivalent weight. . The higher the yield strength, the stronger the hardening of the material.

2 is a graph showing the relationship between nickel (Ni) equivalent weight and crack elongation (Uniform Elongation). Referring to FIG. 2, as the nickel equivalent weight increases, the elongation decreases and is in inverse proportion.

Similarly, the relationship between nickel (Ni) equivalent weight and work hardening index is shown in FIG. 3. Referring to FIG. 3, as in FIG. 2, it can be seen that as the nickel equivalent increases, the work hardening index decreases and is in inverse relationship. In this case, moldability may be relatively disadvantageous.

4 is a graph showing the correlation between the thickness of the planar deformation region FLD ₀ and the material thickness on the molding limit, it can be seen that the higher the material thickness, the higher the FLD ₀ . That is, the thicker the material means that the formability is improved.

5 shows that the inclination of the drawing region on the left and right of the plane deformation region and the unloading region are very similar in the low chromium 409L stainless steel, which is the same steel as an example in which the standard FLD is produced. To achieve this, if the FLDs of specimens of different thicknesses and equivalent weights are vertically moved to the zero point on the Y coordinate with FLD ₀ , the interface is defined as the fracture zone and the safety zone, and the interface is defined as the standard FLD of low chrome 409L stainless steel. can do.

Thus, if the standard FLD is determined and the FLD ₀ prediction can be made for each specimen, the molding limit can be easily predicted. In the present invention, as a method for predicting this, multiple regression analysis was selected. In other words, the factors such as nickel equivalent, yield strength, crack elongation, work hardening index, and material thickness of the specimens were eliminated by multiple regression equations. That is, the final important two factors are selected as prediction factors by using a method of sequentially removing the factors with multicollinearity, and the Hammer & Svensson equation, which is used in the present invention, is predicted compared to other empirical equations. It was selected because the degree was high.

FIG. 6 is a molding limit prediction equation obtained through a multiple regression method, and in relation to FLD ₀ prediction, the thickness of the material shows a positive correlation and the nickel equivalent shows a negative correlation, which is consistent with the correlation analysis of FIGS. 2 to 4. It can be seen that the two factors, thickness and nickel (Ni) equivalent, are very significant for the prediction of FLD ₀ .

Hereinafter, the present invention related to the prediction of molding limits of low chromium 409L stainless steel using the multiple regression method will be described as follows.

-When predicting the molding limit of low chrome 409L, the nickel equivalent of alloying element is first calculated through Hammer & Svensson equation.

-Using several FLDs obtained from the test, move the FLD ₀ value vertically about the Y-axis so that the value of the Y-intercept is 0, and set the standard FLD by defining the interface between fracture and safe experimental value.

-Multiple regression method is used to remove redundant factors (excluding multicollinearity) and to obtain significant multiple regression equations using physical properties, thicknesses, and equivalents that affect molding limits.

-The FLD ₀ prediction of low chromium 409L material can be obtained through nickel equivalent and material thickness through the present invention, and it is a method to easily obtain the molding limit without knowing the alloy components even through mechanical testing.

Therefore, in the present invention, the molding limit can be obtained only through the nickel equivalent and the thickness of the raw material in obtaining the molding limit during press molding of low chromium 409L steel.

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.

As described above, according to the present invention, the molding limit diagram, which is a typical index used for analyzing and solving the cause of component breakage during press processing of 400-based stainless steel, is used to calculate the real ratio based on nickel (Ni) equivalent weight and material thickness alone. Not only can the defect rate be reduced, but also after the standard FLD is obtained, the molding limit of many materials can be easily obtained, which is economical and saves time.

Claims (4)

Measuring a nickel (Ni) equivalent weight of the alloy component; Using the measured nickel equivalents and the thickness of the material, the FLD ₀ value, which is the planar deformation area on the limit of molding, is determined by the multiple regression method [FLD ₀ = 26.6 + (4.11 * thickness)-(8.54 * nickel equivalent)]. Obtaining a; And Predicting a molding limit using a FLD value of at least one molding limit measured while varying the FLD ₀ value and the nickel equivalent and the thickness of the material; . The method of claim 1, The nickel equivalent was measured by the following Hammer & Svensson equation to obtain the FLD ₀ value. [Ni (Ni) equivalent = Ni + 0.31Mn + 22C + 14.2N + Cu] The at least one shaping limit is vertically moved so that the value of the Y-intercept is 0 based on the FLD ₀ value on the Y coordinate, and the interface between the fracture and the safe experimental value is determined as the standard FLD. 400 based on the multiple regression method to remove the redundant factor by using a multiple regression equation including the physical factors affecting the nickel equivalent, thickness and molding limit, through which the prediction of the molding limit Method of predicting the forming limit of stainless steel. The method of claim 1, The molding limit diagram is a molding limit prediction method of the 400 series stainless steel, characterized in that the drawing area on the left side and the elongated opening area on the basis of the FLD ₀ . The method of claim 3, And FLD ₀ is a point on a Y axis bordered between the drawing area and the unloading area.

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