JP6684866B2 - Method of increasing formability of steel - Google Patents

Description

The present invention relates to a method for increasing the formability of a steel material, and more specifically, to a part of a steel material containing an austenite phase (Transformation Induced Plasticity) while applying a stress. The present invention relates to a method for increasing the formability of a steel material by increasing the formability by applying a pulse current at the time when the plasticity occurs.

In the automobile industry, there is an increasing demand for steel materials having high strength and high formability in order to improve passenger safety, reduce vehicle body weight, and improve productivity. When the retained austenite is deformed, the trip steel (Transformation Induced Plasticity steel), which is composed of three kinds of mixed phases of ferrite, bainite, and retained austenite, transforms to hard martensite and has increased strength and elongation. It has the property of transformation-induced plasticity. Due to the properties of retained austenite, trip steel has a large change in its mechanical properties, in particular, high-speed deformation properties. Compared to common high strength steels with similar strength, trip steels exhibit properties of low yield strength / tensile strength ratio and high strain hardening capacity. The high deformation hardening ability increases resistance to local necking in the manufacturing process of automobile parts to improve the formability, and the high tensile strength of the manufactured parts increases impact absorption energy and fatigue properties. And improve. The importance of trip steels as automotive steels lies in their improved formability and impact properties. Therefore, there is a growing interest in trip steels in the automotive and steel industries, with a considerable level of product R & D and field application.

Under these circumstances, as a method for increasing the formability of the existing trip steel, warm forming, incremental forming or laser beam forming technology is used as a formability increasing method. However, the hot forming and induction heating processes suffer from high cost, material thermal gradients, die attach and surface oxidation. In particular, laser beam shaping has the limitation of requiring excessive time and cost in the manufacturing process. Further, in the case of high strength steel, it has a high spring back, and thus there are many difficulties in applying the actual process.

The present invention is to solve various problems including the above problems, in a part of the steel material containing an austenite phase, at the time when transformation-induced plasticity occurs during the application of stress. An object of the present invention is to provide a method for increasing the formability of a steel material by applying a pulse current to increase the formability.

However, such a problem is merely an example, and the scope of the present invention is not limited thereby.

According to an aspect of the present invention for solving the above problems, (a) a step of applying stress to at least a partial region of a steel material containing an austenite phase, and (b) the stress causes the austenite phase to be martensitic. A method of increasing formability of a steel product, comprising the step of applying a pulsed electric current to the steel product at least once when transformation-induced plasticity occurs in a site (Martensite).

Further, according to one embodiment of the present invention, the transformation-induced plastic behavior of the steel material is delayed as the pulse current is applied.

Further, according to an embodiment of the present invention, the steel material containing the austenite phase may be trip steel.

According to an embodiment of the present invention, in the step (b), the first pulse current of the pulse current is applied when the true strain of the steel material is 0% to 11.7%. be able to.

Also, according to the embodiment of the present invention, the pulse current may be applied three times in the step (b).

In addition, according to an embodiment of the present invention, in the step (b), the pulse current is applied with a constant current density (ρ _i ).

Also, according to an embodiment of the present invention, in the step (b), the current density of the pulse current may be 85 A / mm ^{2 to} 105 A / mm ² .

Also, according to an embodiment of the present invention, in step (b), a current application period of the pulse current _{(t p)} is 27 seconds to 33 seconds, the current application time _{(t d)} is 0 It can be 0.08 seconds to 0.12 seconds.

Also, according to an embodiment of the present invention, in the step (b), when the pulse current is applied, the stretching rate is improved by at least 28%.

Then, according to an aspect of the present invention for solving the above-mentioned problems, (c) a step of applying bending stress (bending stress) to at least a partial region of a steel material containing an austenite phase to cause bending deformation, and d) applying a pulse current to the bent and deformed steel material at least once, to provide a method for increasing the formability of the steel material.

Further, according to one embodiment of the present invention, in the step (d), the application time (t _d ) of the pulse current is 0.3 second to 1 second, and the steel material increases as the application time increases. The spring bag can be reduced.

Then, according to an aspect of the present invention for solving the above-mentioned problems, (e) a trip steel in which at least a part of an austenite phase is transformed into martensite by transformation-induced plasticity when stress is applied is prepared. And at least one pulse current is applied to the trip steel at any one of the steps of (f) applying stress to the trip steel and deforming the trip steel to delay the start point of the transformation-induced plasticity. And a step of increasing the formability of the steel product.

According to one embodiment of the present invention made as described above, a part of the steel material containing an austenite phase is applied with a pulse current at the time when transformation-induced plasticity occurs during the application of stress, and forming. Sex can be increased.

Of course, such an effect does not limit the scope of the present invention.

1 is a schematic view showing a pulse current application molding device according to an embodiment of the present invention. 7 is a graph showing a pulse current application condition according to an embodiment of the present invention. 1 is a schematic view showing a steel material sample for pulse current application molding according to an embodiment of the present invention. 1 is an EBSD (Electron backscatter diffraction) analysis photograph showing a microstructure and a texture of a preformed specimen of a steel material according to an embodiment of the present invention. It is a graph which shows the stress-strain curve at the time of general forming of the steel material by the comparative example of this invention (stress-strain curve), and the fraction (Austenite Phase Fraction) of the retained austenite by XRD (X-ray difference) analysis. 3 is a photograph showing a fractured shape after forming a steel material sample containing an austenite phase according to an embodiment of the present invention. It is a graph which shows the stress-strain diagram and temperature change at the time of forming at the time of forming by applying the pulse current of the steel material containing the austenite phase by one comparative example of the present invention. 3 is an EBSD analysis photograph showing a microstructure and a texture of a test piece after forming a steel material according to a comparative example of the present invention. 5 is a graph showing a retained austenite fraction of a test piece before forming a steel material and a deformation rate of 23% according to a comparative example of the present invention. It is a graph which shows the stress-strain diagram and temperature change at the time of 3 times pulse current application shaping | molding of the steel material by one Embodiment of this invention. 3 is a graph showing changes in the work hardening index (n-value) of a steel material according to one embodiment of the present invention during three times of pulse current application molding. It is a graph which shows the residual austenite fraction by the true strain at the time of the pulse current application shaping | molding of the steel material by one Embodiment of this invention. 1 is a schematic view showing an apparatus for experimenting with a spring bag reduction effect according to an embodiment of the present invention. It is a schematic diagram showing a spring bag reduction effect test piece according to an embodiment of the present invention. 6 is a graph showing a load applied to a test piece during a spring bag reduction effect experiment according to an embodiment of the present invention. 3 is a graph showing a photograph and a spring bag angle of a test piece for spring bag reduction effect experiment according to an embodiment of the present invention after pulse current applied bending. 3 is a schematic view showing a spring bag direction applied to a sample during bending of a sample according to an embodiment of the present invention, and a graph showing Vickers hardness of the sample during pulse current applied bending. .

DETAILED DESCRIPTION The following detailed description of the invention refers to the accompanying drawings that illustrate, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable one of ordinary skill in the art to fully practice the invention. It should be understood that the various embodiments of the invention differ from each other but need not be mutually exclusive. For example, the particular shape, structure, and characteristics described herein may be implemented in other embodiments in connection with one embodiment without departing from the spirit and scope of the invention. It should also be understood that the location or arrangement of individual components within each disclosed embodiment may be changed without departing from the spirit and scope of the invention. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of the present invention, if properly explained, along with what the claims claim and all equivalents, Only limited by the appended claims. Similar reference numerals in the drawings refer to the same or similar functions in various aspects, and lengths, areas, thicknesses, and the like may be exaggerated for convenience.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention.

<Pulse current application formability increasing method>
The pulse current application shaping will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic diagram showing a pulse current application molding apparatus according to an embodiment of the present invention.

An apparatus for analyzing the influence of application of a pulse current at the time of forming a steel material, which constitutes an experimental apparatus capable of forming while applying a pulse current. As shown in FIG. 1, a test piece 10 for molding can be loaded and stress can be applied in the direction of the arrow.

For the pulse current, a direct current power supply device manufactured based on a resistance welding machine is used to periodically apply a current to the test piece 10. At this time, for insulation between the electric current flowing through the sample 10 and the tensile machine, an insulation system (Insulator) I is constructed by using a bakelite in a jig in which the sample 10 of the tensile machine is sandwiched. Moreover, the direct current generated in the power supply device is allowed to flow only in the test piece 10.

FIG. 2 is a graph showing pulse current application conditions according to an embodiment of the present invention. During the pulse current applied molding, pulse current, current density ([rho, unit: A / ^mm 2), the current application time (duration, _{t d,} in seconds) and the current application period (period, _{t p,} in seconds) Can be set to a constant value and applied to the test piece. In FIG. 2, t _d means a current application time, and t _p means a current application period. At this time, the current density (ρ ₀ ) is a value based on the initial cross-sectional area of the sample, which means that a constant current value (A) was applied during the progress of the experiment. To do. On the other hand, the current density (ρ _i ) can be applied while changing the current in consideration of the decreasing cross-sectional area of the test piece during the molding process to keep the current density constant.

A data measuring system for measuring a change in physical properties of a sample by a pulse current application formability increasing method will be described. When measuring the deformation rate of a specimen, the contact strain gauge that is commonly used cannot be used due to insulation problems. Therefore, the deformation rate in the longitudinal direction of the sample was measured using an image-based digital image correlation system (DIC system) capable of measuring the deformation rate of the sample in a non-contact manner. In addition, in order to analyze the generation of resistance heat generated by applying a pulsed current, the temperature of the sample is measured using a k-type thermo-couple and a thermal image camera (IR camera). To do.

Next, a method for increasing the formability of steel according to the embodiment of the present invention will be described.

The method for increasing the formability of a steel material according to the present invention comprises at least (a) a step of applying stress to a steel material containing an austenite phase, and (b) at least the time when the austenite phase causes transformation-induced plasticity in martensite. Applying a pulse current once.

FIG. 3 is a schematic view showing a pulse current application molding test piece according to an embodiment of the present invention. In the present specification, a in FIG. 3 indicates the total length of the sample, b indicates the gauge length of the sample, c indicates the longitudinal length of the sample, and d indicates the fragment length of the sample. However, the embodiment of the present invention is not limited to FIG. Embodiments according to the present invention may differ in the shape and thickness and length of the coupon 10 of FIG.

At the step (a), the molding sample 10 shown in FIG. 3 is loaded into the molding apparatus of FIG. 1 to apply stress. The steel material containing the austenite phase may be a lightweight steel material having a yield strength (YS) of 780 MPa. It can be utilized in industries that require high strength and low specific gravity steel by using the method of increasing the formability of lightweight steel.

According to an embodiment of the present invention, the steel material containing the austenite phase may be trip steel.

In step (b), at least one pulse current can be applied to the steel material at the time when the steel material behaves in a transformation-induced plasticity (hereinafter, TRIP) behavior by applying the stress in step (a). As shown in FIG. 1, a pulse current can be applied to the molding sample 10 through an external power device of the molding device. Pulse current, as shown in FIG. 2, a constant current application period (t _p), and applies a current application time (t _d). In the case of current density, it is applied at a constant current density (ρ _i ), which means that when the cross-sectional area changes due to the stress applied to the molding specimen, the strength of the applied pulse current is It can be adjusted to keep the current density constant.

Meanwhile, according to an embodiment of the present invention, the current density of the pulse current may be 85 A / mm ^{2 to} 105 A / mm ² .

On the other hand, according to an embodiment of the present invention, a current application period of the pulse current _{(t p)} is 27 seconds to 33 seconds, the current application time _{(t d)} is 0.08 seconds to 0.12 seconds Can be.

On the other hand, the deformation rate means the rate of change of the linear dimension of a component or a specimen per unit length, and there are two types, a nominal strain and a true strain. The nominal strain is a universally used deformation rate, and is expressed by the following equation as a change in length with respect to the initial surface distance.

σ = (L−L ₀ ) / L
(Σ: deformation rate, L: gauge length after molding, L ₀ : initial gauge distance)

Yield strength (hereinafter referred to as YS) is a degree of maximum stress applied to a material without causing plastic deformation, and means a stress when the material exhibits a specific permanent deformation.

The plastic section (hereinafter referred to as PR) is a section in which a specimen that exceeds the elastic limit and exceeds the yield strength is subject to plastic deformation, and permanent deformation remains even after removing the cause of the load. It is a section where there is.

The ultimate tensile strength (hereinafter referred to as UTS) means the maximum stress that can be withstood when a tensile force is applied until the test piece is broken by a force indicating the strength of the test piece. It means the point of maximum stress in the stress-strain diagram.

Transformation-induced plasticity (hereinafter, TRIP) means that an austenite phase is transformed into martensite by an external force. At this time, the steel material can ensure high strength and high softness.

Hereinafter, a method of increasing the formability of a steel material containing an austenite phase by applying a pulse current will be described by various experimental examples.

<Microstructure and general tensile forming of steel containing austenite phase>
The forming characteristics of the steel material containing the austenite phase will be described with reference to FIGS. 4 and 5.

FIG. 4 is an EBSD analysis photograph showing a microstructure and a texture of a preformed specimen of a steel material according to an embodiment of the present invention. After forming the steel, the initial microstructure and texture before forming are analyzed in order to measure the changing properties.

4A is an ND map (microstructure map in the normal direction) of the EBSD analysis, and FIG. 4B is a phase map (Phase map) of the EBSD analysis. As a result of the EBSD analysis, it can be confirmed that the steel material has both a ferrite phase and an austenite phase.

Further, in order to measure the quantitative fraction of the austenite phase, XRD analysis can be performed to confirm that the residual austenite phase fraction is 23%.

FIG. 5 is a stress-strain diagram at the time of general forming of a steel material according to an embodiment of the present invention and a graph showing a fraction of retained austenite by XRD analysis. Non-pulsed tension is applied to a steel material at a tensile deformation rate of 1.5 mm / min to measure the fracture deformation rate and the residual austenite phase fraction at the time of fracture of the steel material.

Meanwhile, according to an embodiment of the present invention, the tensile deformation rate of the steel material may be 1.4 mm / min to 1.6 mm / min.

FIG. 5A is a graph showing a stress-strain diagram at the time of general forming of a steel material. As a result of the general tensile molding, it can be confirmed that it exhibited a breaking deformation rate of 40% and a maximum tensile strength (UTS) of 850 MPa.

As a result of observing the fracture mode of the steel material sample after the fracture, it can be confirmed that ductile fracture occurs. At this time, in order to investigate the deformation mechanism of the steel material at the time of general tensile molding, the initial specimen before deformation and the deformation rate of the steel material were 7.5%, 12.5% and 23%, respectively, and after the fracture of the steel material, XRD analysis of the pieces is performed.

FIG. 5B is a graph showing the fraction of retained austenite by XRD analysis during general forming of steel. It can be confirmed that the fraction of the austenite phase decreases as the molding proceeds during the general tensile molding. That is, it can be confirmed that the austenite phase of the steel material undergoes transformation-induced plasticity (TRIP) behavior in martensite when stress is applied.

On the other hand, in the general tensile forming of a steel material, a plot can be performed between the deformation rate and the retained austenite fraction by using the following formula 1.

[Formula _{1] f r = f 0 exp} (-kε)
(F _r: fraction of retained austenite phase, f _0: the fraction of the austenite phase before molding initial steel, epsilon: true strain, k: constant)

At this time, the k value is a constant, but the physically lower k value means that the austenite phase has higher stability. As for the steel material, it can be confirmed that the k value is 0.03767 as a result of the plot of Expression 1.

<Method of increasing formability of steel containing austenite phase>
A method of increasing the formability of a steel material containing an austenite phase will be described with reference to FIGS. 6 to 12.

First, a comparative example of the present invention will be described with reference to FIGS.

[Comparative Example 1]
In Comparative Example 1, stress was applied to the specimen for forming a steel material containing an austenite phase shown in FIG. 3 by a method such as general tensile forming of a steel material containing an austenite phase, and a pulse current was applied at the time when transformation-induced plasticity occurred. Is applied to perform tensile molding.

At this time, the pulse current, the current density ([rho _i) is 50A / mm ^2, the current application time _{(t d)} is 0.1 seconds, the current application period _{(t p)} can be a 30 seconds .

[Comparative Example 2]
In Comparative Example 2, stress was applied to the specimen for forming a steel material containing an austenite phase shown in FIG. 3 by a method such as general tensile forming of a steel material containing an austenite phase, and the temperature was maintained at 85 ° C. to 100 ° C. Meanwhile, high temperature tensile molding is performed.

FIG. 6 is a photograph showing a fracture shape after forming a steel material containing an austenite phase. FIG. 6A is a general tensile forming of a steel material containing an austenite phase (Non-pulsed tension), FIG. 6B is a forming by applying a pulse current (Pulsed tension), and FIG. c) shows the fracture shape of the test piece at the time of tensile molding at high temperature (High temperature tension).

FIG. 7 is a graph showing a stress-strain diagram and a temperature change between the time of forming and the time of forming at a high temperature by applying a pulse current of a steel material containing an austenite phase.

According to Comparative Example 1 and Comparative Example 2, the fracture deformation rate was measured to be about 23.9% when the pulse current of the steel material containing the austenite phase was applied and the tensile forming was performed, and when the tensile forming was performed at a high temperature. It was This can confirm that the steel material containing the austenite phase could not accommodate the deformation due to the TRIP effect because the TRIP behavior due to the temperature increase was disturbed. That is, when the tensile forming is performed by applying a pulse current or the tensile forming is performed at a high temperature, the fracture deformation rate becomes low.

FIG. 8 is an EBSD analysis photograph showing a microstructure and a texture of a specimen after forming a steel material according to a comparative example of the present invention. 8A and 8B are microstructures when the deformation rate is 23% after general forming of a steel material containing an austenite phase, FIG. 8C and FIG. 8D. Is a microstructure when the deformation rate is 23% when a pulse current is applied, and FIGS. 8E and 8F show that the deformation rate is 23% at a high temperature. 2 is an EBSD analysis photograph of the microstructure of the above.

FIG. 9 is a graph showing a residual austenite fraction of a specimen before forming and a deformation rate of a steel material according to a comparative example of the present invention when the deformation rate is 23%.

As a result of the experiment of the comparative example, when the pulse current is continuously applied or the tensile molding is performed at a high temperature, the fraction of the retained austenite phase is higher than that in the general tensile molding. That is, in the case of a material exhibiting TRIP behavior, applying a continuous pulse current not only interferes with TRIP behavior but also causes a local shrinking (local necking) to cause the fracture deformation rate to be higher than that of general tension. It can be confirmed that it decreases.

<When pulse current is applied three times to the first half of deformation>
A method of increasing formability by applying a pulse current to the steel material containing the austenite phase three times in the first half of deformation will be described with reference to FIGS. 10 to 12.

According to an embodiment of the present invention, (e) preparing a trip steel in which at least a portion of the austenite phase is transformed into martensite by transformation induced plasticity when stress is applied, and (f) the trip steel Applying a stress to the trip steel at least once at any one of the steps of deforming by applying stress to the trip steel to delay the start point of the transformation-induced plasticity. .

FIG. 10 is a graph showing a stress-strain diagram and a temperature change when a steel material according to an embodiment of the present invention is formed by applying a pulse current three times.

Under the same tension forming speed, current application time, and current application cycle conditions as in Comparative Example 1, only three pulses were applied at the time when the transformation-induced plasticity occurred to perform the tension forming. For the current density, and the different current densities from each other, was performed three times tensile molded in the respective ^{75A / mm 2, 95A / mm} 2, 115A / mm 2.

According to an embodiment of the present invention, in the step (b), the first pulse current of the pulse current may be applied when the true strain of the austenitic steel is 0% to 11.7%.

FIG. 10A shows a stress-strain diagram at the time of tensile forming by applying a pulse current to the steel material three times. As a result of the experiment, when the current density is 95 A / mm ² , the breaking deformation rate is measured up to 50%. At the break deformation rate of 39% during general tensile molding, the break deformation rate of about 28% increases. At this time, as the current density increases, the fracture deformation rate does not continue to increase, and when the current density is 95 A / mm ^2, it shows the maximum fracture deformation rate. That is, it is possible to confirm the optimum current density condition for obtaining the maximum fracture deformation rate.

FIG. 10B is a graph showing a temperature change during tensile forming by applying a pulse current to the steel material three times. When a current having a current density of 75 A / mm ² was applied, the average temperature of the test piece increased to a maximum of 150 ° C., and when a current having a current density of 95 A / mm ² was applied, it increased to about 210 ° C. When a current having a current density of 115 A / mm ² was applied, the temperature increased to about 350 ° C.

FIG. 11 is a graph showing changes in the work hardening index (n-value) value of a steel material according to one embodiment of the present invention during three times of pulse current application molding.

The gradient of the true stress-true strain diagram of the steel material was measured differently from that in the general tensile molding during the three times of pulse current application molding. In order to confirm it, the change of the n-value value which is a work hardening index is measured.

The work hardening index is an index indicating the degree to which the rigidity of the material processed by external force increases. This is expressed as the slope of the true stress-true strain diagram and means the degree of work hardening, which is a phenomenon in which the strength of the material increases as the deformation increases. It is an important factor that determines the physical properties of materials.

σ = Kε ⁿ
logσ = nlogε + logK
(Σ: true stress, ε: true strain, n: work hardening index, K: strength coefficient)

As a result of plotting the n-value value due to the true strain in the molding experiment, it can be confirmed that the n-value curve was changed by the pulse current application. In particular, the maximum value of n-value (max n-value) appears in the latter half of the deformation in which the deformation progresses as compared with general tension. This indicates that when the pulse current is applied three times in the first half of deformation, the TRIP effect is exerted from the second half of deformation due to the delay effect of TRIP behavior. Therefore, the method for increasing the formability of the steel material of the present invention has an advantage that the TRIP behavior is delayed by the application of the pulse current in the first half of the deformation, and the fracture ratio is high.

FIG. 12 is a graph showing the retained austenite fraction due to the true strain when the steel material according to one embodiment of the present invention is formed by applying the pulse current three times.

According to one embodiment of the present invention, the TRIP (Transformation Induced Plasticity) behavior of the austenitic steel is delayed as the pulse current is applied.

In order to investigate whether the TRIP behavior was delayed, when the pulse current density at each deformation rate point was 95 A / mm ² , XRD analysis was carried out on the pulled steel material specimen and the general tensile molded specimen. The residual austenite phase fraction was measured.

When the true strain is up to 11.7%, the retained austenite phase fraction of the steel material shows a value similar to that of the initial specimen before forming. The retained austenite phase fraction begins to decrease continuously after the true strain is 11.7% or more. The k value of the equation 1 is plotted by dividing the section into 0% to 11.7% of true strain (Region 1) and 11.7% of true strain to breakage of the test piece (Region 2). As a result of plotting, the k value is 0.004934 and the true strain is 11.7% or more (Region) in the true strain from 0% to 11.7% (Region 1, Fitting curve 1) as compared with general tensile molding. 2, Fitting curve 2) has ak value of 0.03688. This has a value significantly lower than the k value at the time of general tensile forming up to a true strain of 11.7%, and shows a similar value after the true strain of 11.7% or more. Table 1 is a table showing the plot results of Formula 1 under tensile molding conditions.

As a result of the experiment, the mechanical stability of the austenite phase was higher than that of general tensile molding during the three times pulse current application molding until the true strain was 11.7%, and the TRIP effect was delayed. It Therefore, the TRIP behavior appears from the latter half of the deformation of the steel material, and the TRIP behavior appears from the latter half of the deformation, which can contribute to the improvement of formability.

That is, when the pulse current is applied to the steel material containing the austenite phase, the TRIP behavior is delayed at the time of forming, the deformation rate is improved, and the formability can be increased.

<Experiment to reduce spring bag>
A spring bag reduction effect by applying a pulse current to a steel material containing an austenite phase will be described with reference to FIGS. 13 to 17.

The spring bag means that a bending stress is applied to a steel material that needs to be formed to perform bending deformation, and when the bending stress is removed, the bending deformation applied to the test piece returns. Generally, high-strength steel has a problem in practical application since its formability decreases due to a spring bag effect during forming.

FIG. 13 is a schematic view showing a device for spring bag reduction effect experiment according to one embodiment of the present invention, and FIG. 14 is a schematic view showing a spring bag reduction effect test piece 20 according to one embodiment of the present invention. is there.

In the spring bag reduction effect experiment device, G is a bending angle of a bending stress applying tip (tip), R is a bending radius of the tip, and W is after bending of the spring bag reduction effect experimental specimen. , L means width, and H means length.

According to an embodiment of the present invention, (c) a step of applying bending stress to a steel material containing an austenite phase to cause a bending deformation, and (d) a steel material containing a deformed austenite phase at least once with a pulse current. And a step of applying a voltage.

FIG. 15 is a graph showing a load applied to a test piece during a spring bag reduction effect experiment according to an embodiment of the present invention.

In step (c), the spring bag reduction effect test piece can be loaded into the test device and bending stress can be applied. The load in FIG. 15 means the bending stress applied to the test piece for the spring bag reduction effect experiment, and the deformation length (Displacement) of the test piece is subjected to bending deformation up to 17 mm. Bending deformation speed (Cross head speed) may be 0.1 mm / s.

In step (d), the spring bag reducing effect test piece can be bent and deformed by applying a pulse current once. The current density is 60 A / mm ² , and the current application time (Duration time) can be applied in 0.3 second to 1 second. After bending deformation, the spring bag angle is measured and the maximum temperature of the sample is measured using a thermal image camera.

FIG. 16 (a) is a photograph of a spring bag reduction effect test piece after pulse current application bending, and FIG. 16 (b) is a graph showing the maximum temperature when the spring bag angle and the pulse current are applied. Is.

According to an embodiment of the present invention, the application time (t _d ) of the pulse current is 0.3 second to 1 second, and the spring bag of the austenitic steel may be reduced as the application time is increased.

As a result of the experiment, the spring bag reduction angle was 45 ° when the general spring bag was deformed, and the current application time was 0.3 seconds, 0.5 seconds, 0.7 seconds, 0.9 seconds, and 1 second, respectively. At one time, spring bag deformation was measured at 42 °, 38 °, 37 °, 31 °, 29 °, respectively. During each experiment, maximum temperatures were measured at 228 ° C, 378 ° C, 497 ° C, 631 ° C, 706 ° C, respectively. That is, when the pulse current application time is 1 second, the maximum spring bag reduction effect of 35% can be confirmed. Table 2 is a table showing the results of the spring bag experiment.

FIG. 17A is a schematic view showing a spring bag direction (S) applied to the test piece during bending of the test piece, and FIG. It is a graph which shows the Vickers hardness of one piece.

In the above experiment, when the pulse current application time is 1 second and when the pulse current is not applied (Non pulsed) in the general spring bag experiment, the Vickers of the portion subjected to the tensile force (T in FIG. 17A) Hardness was measured.

The Vickers hardness is a value obtained by measuring the hardness of a hard surface substance. Using a pyramid-type indenter having a diamond quadrangular pyramid, the hardness is measured by pressing the test piece and measuring the diagonal line of the pyramid-shaped portion formed on the test piece. The formula for calculating the Vickers hardness value is as follows.

Hv = 0.1891F / d ²
(Hv: Vickers hardness (N / mm ² ), F: load, d: diagonal length of diamond indentation mark)

The Vickers hardness is about 275 Hv in the general spring bag experiment, and about 250 Hv when the pulse current application time is 1 second. When a spring bag experiment was performed by applying a pulse current, it can be confirmed that the Vickers hardness decreased due to the decrease in residual stress. That is, it can be confirmed that the spring bag made of steel is reduced.

Therefore, according to the present invention, the pulse current is applied to the steel material containing the austenite phase, and the spring bag of the austenitic steel is reduced as the application time increases.

Although the present invention has been illustrated and described with reference to the preferred embodiments as described above, it is not limited to the above embodiments and various modifications and changes can be made by those skilled in the art without departing from the spirit of the present invention. Is. It is to be appreciated that such variations and modifications are within the scope of the invention and the appended claims.

Claims (8)

(A) applying a stress to a steel material containing an austenite phase,
(B) applying a pulse current to the steel material three times or less at the time when the austenite phase causes transformation-induced plasticity in martensite due to the stress;
Only including,
In the step (b), the method of increasing the formability of a steel material , wherein the current density of the pulse current is 85 A / mm ^{2 to} 105 A / mm ² . The method for increasing the formability of steel according to claim 1, wherein the steel containing the austenite phase is trip steel. The method for increasing the formability of steel according to claim 1, wherein in the step (b), the first pulse current of the pulse current is applied when the true strain of the steel is 0% to 11.7%. The method according to claim 1, wherein the pulse current is applied at a constant current density (ρ _i ) in the step (b). In step (b), a current application period of the pulse current _{(t p)} is 27 seconds to 33 seconds, claim current application time _{(t d)} is 0.08 seconds to 0.12 seconds 1. The method for increasing the formability of steel according to 1. The method for increasing the formability of a steel material according to claim 1, wherein a stretching ratio of at least 28% is improved when the pulse current is applied in the step (b). (C) a step of applying bending stress to at least a partial region of a steel material containing an austenite phase to cause bending deformation,
(D) applying a pulse current to the bent and deformed steel material three times or less ;
Further seen including,
In the step (d), the current density of the pulse current is 85 A / mm ^{2 to} 105 A / mm ² , and the method of increasing the formability of a steel material. (E) preparing a trip steel in which at least a portion of the austenite phase is transformed into martensite by transformation-induced plasticity when stress is applied;
(F) applying a pulse current to the trip steel 3 times or less at any one of the steps of applying stress to the trip steel to deform it, and delaying the start point of the transformation-induced plasticity;
Only including,
In the step (f), the current density of the pulse current is 85 A / mm ^{2 to} 105 A / mm ² , and the method of increasing the formability of a steel product .