JPS5960357A - Method for testing season cracking property of deep drawing product of austenite stainless steel sheet - Google Patents

Abstract

PURPOSE:To test simply and quantitatively a season cracking property by evaluating the season cracking property by a product of the magnetized quantity and the hardness of a formed product. CONSTITUTION:If the Griffith's theory is applied on the assumption that the shape of martensite transformation phase is a sheet state of convex lens shape, a crack generation condition is obtained quantitatively as shown in an equation. It is found that the season cracking is caused when the product of the destructing stress sigmac2 and the hardness Hv attains the predetermined number. On the other hand, a residual stress sigma is nearly proportional to half power of magnetized quantity independently to the nickel content of austenite stainless steel. The magnetized quantity is nearly proportional to the martensite transformation quantity, thus the equation means that the product of the magnetized quantity caused by the martensite transformation and the hardness can be used to the evaluation of season cracking property. In the equation, E is the Young's modulus, gamma is the surface energy and (c) is the size of crystal grain.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for testing the period cracking resistance of a molded product obtained by cold deep drawing an austenitic stainless steel plate.

In general, deep drawing is a composite forming process of compression and tension, so the material exhibits a complex deformation pattern. Therefore, when an austenitic stainless steel sheet is subjected to cold deep drawing, deformation-induced martensitic transformation occurs, and changes such as work hardening and increased residual stress occur, and these changes reach specific values. It is thought that age cracking occurs in the process (Arayo, Sumitomo, “Age cracking that occurs in stainless steel copper plate press-formed products”, Plasticity and Processing vol. 19 no.
205 (February 1978) p, 1.48-155
reference). However, the definite correlation is not clear. In addition, when hydrogen is contained in the material, it is removed by accumulating in microcracks, lattice defects such as dislocations, and interfaces between different phases in the steel; it deteriorates mechanical properties and promotes cracking. (Supervised by Masayoshi Hase, “Stainless Steel Handbook,” Nikkan Kogyo Shimbun, p. 128-141)
reference).

It is an important issue to prevent cracking that occurs during and after processing.
Countermeasures to prevent L, etc., include increasing the amount of nickel added to the composition, relaxing the processed shape, and reducing residual stress and work hardening by applying intermediate annealing. .

(H) However, these are only qualitative measures, and no method of quantitatively testing the susceptibility to timing has been carried out. For this reason, in cold deep drawing processing that does not involve hardening, period cracking often occurs due to changes over time after processing.

In view of the above-mentioned circumstances, it is an object of the present invention to quantify the conditions under which periodic breakage occurs on a workpiece during cold forming of austenitic stainless steel, and to compare the conditions with the occurrence conditions. The object of the present invention is to provide a test method for evaluating the period cracking resistance of deep-drawn products of eraustenitic stainless steel sheets.

The test method of the present invention is based on austenitic stainless steel W4.
It is characterized by evaluating the age and quality of the molded product obtained by cold deep drawing of the roots based on the magnetization and hardness of the molded product.

Next, the present invention will be explained in detail with reference to the drawings.

First, let's consider quantifying the conditions for the occurrence of timing discrepancies. Timing cracks that occur in molded products formed by cold deep drawing of austenitic stainless steel sheets, martenitic transformation induced in the workpiece1. l-+erosion, the residual stress reaches a unique value! : Occurs.

In this case, the initial crack occurs because the tensile residual stress at the interface between the martensitic transformed phase and the austenite phase is concentrated at the periphery of the martensitic transformed phase, and this tensile residual stress exceeds the maximum stress between atoms. It is thought that then. Therefore, initial cracks occur in the cutting direction of the periphery of the martensitic transformed phase, and their thickness is the same as the crystal grain size of the martensitic transformed phase.

Now, assuming that the shape of the martensitic transformation phase is a convex lens-shaped thin plate, we will use Griffith's theory (Isamu Ishii 4'
:i [Mechanical materials science, physical properties and evaluation] Nikkan Kogyo Shimbun p.
, 105-109), the conditions for crack occurrence can be quantitatively determined. According to Griffith's theory, the fracture stress σ. is, σ = L(lower−!−>”, −
-------(1) 2c. However, 1 bow is Young's modulus, γ is surface energy +
2 c (d is the size of crystal grains. To apply this to cases where fracture involves plastic deformation, such as stainless steel, the effective surface energy γ' is expressed as γ = pγ ---
-----------(2). However, p is a coefficient related to the plastic deformation work required for crack propagation, and is an amount inversely proportional to the hardness Hv. Therefore, if K is a constant (for example, the hardness of glass), it can be rewritten as follows. Then, γ' in equation (2) is used as a substitute for γ in equation (1), and equation (3) is substituted for rearrangement. cll-1-1v = ii'jK7/ 4c =
Obtain K' (4). Here, ′ has almost the same value for the same type of materials, and can be considered a constant. That is, it can be seen that period cracking occurs when the product of the fracture stress σc11 and the hardness 1v reaches a certain value.

On the other hand, it has been confirmed through experiments that the residual stress σ is proportional to the l/2 power of the magnetization amount S, regardless of whether the austenitic thread stainless steel contains nickel. Magnetization h here]
- indicates the amount of magnetism generated due to martensitic transformation during processing, and when it is suspended in a constant magnetic field, 6? '() This can be easily measured using, for example, a fegitoscope.It is said that the amount of magnetization - is approximately proportional to the martensitic transformation -M. , Equation (4) means that the product of the amount of magnetization υ generated by the martensite vote state and the hardness can be used to evaluate the period cracking property.

Next, in order to find the product (constant value) of the magnetization 1!1- which generates an hourly 10% fL and the hardness, as an example, if the plate thickness is 2.5
Austenitic stainless steel plate (SUS304)
was deep-drawn into the shape shown in Figure 1, and the hardness and magnetization at the location where periodic cracks occurred were measured. The hardness and magnetization amount at this time are shown as point 32 in FIG. A curve 31 is drawn through this point 32 where the product of hardness and magnetization is constant. This curve 31 is the timing side rod occurrence limit curve, and the upper right side in the figure is the period n occurrence area. Furthermore, in order to find a safe range, hydrogen charging was carried out to accelerate the cracking, and the hardness and magnetization of the produced timing side rod portion were measured, and the results are shown as curve 1g3. The effect of hydrogen is to diffuse and accumulate at the martensite phase and austenite phase interface,
The martensite phase manifests as cracks due to the embrittlement of the interface. Therefore, it is considered that the rainbow timing distribution n is promoted in hydrogen charging and is the main cause of the timing difference due to changes over time.

That is, in the region on the lower left side of curve 33.1, it can be considered that the timing side bar does not occur even if the hydrogen charge decreases over time. The curve 33 is a curve in which the product of the material hardness and the amount of magnetization is constant, and this constant value is used as a criterion for evaluating timing cracks based on the safety factor. The amount of magnetization can be easily measured using, for example, a ferrite scope without destroying the material, but the hardness must be determined by a destructive test. However, the amount of magnetization and hardness are almost constant once the material and processing conditions are determined, and as described later, the relationship between the amount of magnetization and hardness is almost linear for the same material. !1] Approximate hardness can also be determined by In other words, the amount of magnetization of the molded product is measured,
If the value obtained by multiplying the hardness obtained from this magnetization -61 is less than the above-mentioned certain value, it can be said that no cracking occurs.

Next, curves 34 and 35 in FIG. 2 show the results of determining the amount of magnetization and hardness of each part of a molded product deep drawn into the shape shown in FIG. 1 using 5US304L.

Curve 34 is for a nickel content of 12.11%, and curve 35 is for a nickel content of 9.91%. In other words, although the amount of magnetization and hardness differ depending on each part of the molded product, the maximum value of the product is all smaller than the reference value (curve 33), and it can be seen that no cracking occurs due to changes over time. . It goes without saying that the smaller the above-mentioned product value is, the higher the resistance to aging cracking is, and the aging cracking resistance of an austenitic stainless steel sheet that has been cold deep drawn can be tested based on this product value. Further, curves 34 and 35 are both substantially straight lines, indicating that the amount of magnetization and hardness have a constant relationship depending on the material.

As described above, in the present invention, since the test method of evaluating the period crackability by the product of the magnetization amount and the hardness of the processed molded product is adopted, it is possible to quantitatively test the period crackability. It is also effective as a test method to ensure the reliability of mass-produced products. In addition, if the relationship between the magnetization and hardness of similar materials is determined in advance, it can be easily measured by just measuring the magnetization, which is sufficient for non-destructive testing. It is possible.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an example of a molded product obtained by cold deep drawing an austenitic stainless steel plate, and FIG. 2 is a diagram showing the limit of generation of side rods and the relationship between magnetization and hardness. In the figure, 31...Creation limit curve, :32
...A point indicating the hardness and magnetization amount at the time of occurrence of a period crack, 3
3... Crack occurrence limit curve due to hydrogen charging, 34
, :35...Curve showing the measurement results of hardness and magnetization of each part of the workpiece (3). Agent: Toshimune Sumita, patent attorney

Claims (1)

[Claims] A method for testing the period breakability of deep-drawn austenitic stainless steel sheets, which is characterized by evaluating the period breakability of the molded product obtained by cold deep-drawing an austenitic stainless steel plate by the product of magnetization and hardness. .