KR102122653B1 - Method for evaluating of hydrogen embrittlement for carbon steels - Google Patents

Abstract

Disclosed is an invention for a method for evaluating hydrogen embrittlement properties of steel materials. In one embodiment, the method for evaluating the hydrogen embrittlement property of the steel material comprises: masking a site for measuring hydrogen embrittlement in the steel material; Electrodepositing the surface of the steel material except for the masked portion; Removing the masking of the electrodeposited steel material, and then drying it at a temperature of 140° C. or more and less than 180° C. to prepare a steel material specimen having an electrodeposition coating layer formed thereon; Charging hydrogen to a portion to be measured where the electrodeposition coating layer of the steel specimen is not formed; Masking a portion to be measured of the hydrogen-charged steel specimen; And performing a low-speed tensile test (SSRT) on the steel specimen to derive the hydrogen sensitivity (S) of the steel specimen according to Equation 1.
(Equation 1 above is as defined in the detailed description).

Description

Method for evaluating hydrogen embrittlement properties of steel materials {METHOD FOR EVALUATING OF HYDROGEN EMBRITTLEMENT FOR CARBON STEELS}

The present invention relates to a method for evaluating hydrogen embrittlement properties of steel materials. More specifically, it relates to a method for evaluating hydrogen embrittlement properties of steel materials using electrodeposition coating.

In order to respond to the global environment and improve the fuel efficiency of vehicles, vehicle weight reduction is becoming a global issue. In order to reduce the weight of the vehicle, methods such as application of lightweight materials and reduction of friction coefficient through changes in vehicle design or materials are used. On the other hand, the application of ultra-high-strength steel is essential in order to avoid the regulation of collision stability law as well as weight reduction of vehicles. However, in order to secure the strength, an increase in the ultra-high-strength microstructure such as MS structure and the addition amount of alloy components increase, and the hydrogen embrittlement (HE), liquid metal embrittlement (LME), and post-necking deformation behavior, which did not appear when applying low-strength steel, However, different problems are occurring.

In particular, hydrogen embrittlement, among them, is accompanied by delayed destruction, and therefore, it is being carefully reviewed by automobile steel makers while making SPECs on methods and regulations.

On the other hand, the hydrogen embrittlement test method largely includes a method for measuring a change in physical properties of a material, an electrochemical measuring method such as hydrogen permeation, and a method for measuring a delayed fracture time according to a limit hydrogen amount or a stress limit. The test method includes a process in which hydrogen must be charged to a steel material in common. In addition, in the ISO 16573 standard, after charging hydrogen to a steel material, it is stipulated to conduct electroplating to prevent hydrogen from escaping through a cut surface or a non-uniform layer of plating during the test. However, when electroplating, hydrogen is mixed into the steel, and the hydrogen mixing is not able to grasp the exact amount of hydrogen in the steel, which increases the measurement error and affects the material change of the steel.

Background Art related to the present invention is disclosed in Japanese Patent Application Laid-Open No. 2017-223639 (December 21, 2017, title of the invention: hydrogen brittleness evaluation device and hydrogen brittleness evaluation method and test piece used therein).

According to an embodiment of the present invention, it is to provide a method for evaluating hydrogen embrittlement properties of steel materials having excellent accuracy and reliability when measuring hydrogen embrittlement properties of steel materials.

One aspect of the present invention relates to a method for evaluating hydrogen embrittlement properties of steel materials. In one embodiment, the method for evaluating the hydrogen embrittlement property of the steel material comprises: masking a site for measuring hydrogen embrittlement in the steel material; Electrodepositing the surface of the steel material excluding the masked area; Removing the masking of the electrodeposited steel material, and then drying it to a temperature of 140° C. or more and less than 180° C. to prepare a steel material specimen having an electrodeposition coated layer; Charging hydrogen to a portion to be measured where the electrodeposition coating layer of the steel specimen is not formed; Masking a portion to be measured of the hydrogen-charged steel specimen; And performing a low-speed tensile test (SSRT) on the steel specimen to derive the hydrogen sensitivity (S) of the steel specimen according to Equation 1 below:

[Equation 1]

Hydrogen sensitivity (S) = (l _n /l ₀ ) X 100 (%)

(In Equation 1, the l _n is the elongation of the steel specimen charged with hydrogen for n hours, and l ₀ is the elongation of the specimen before hydrogen is charged).

In one embodiment, the electrodeposition coating is a step of applying a low voltage constant voltage of 3 to 7V after immersing the steel in the electrodeposition paint; And applying a high voltage constant voltage of 200 to 250V.

In one embodiment, the electrodeposition paint may include a bisphenol-type epoxy resin, a urethane resin, and a polyisocyanate curing agent.

In one embodiment, the electrodeposition coating layer may be formed to a thickness of 10 ~ 30㎛.

In one embodiment, the hydrogen charging may be performed by immersing the steel specimen in an aqueous solution containing hydrochloric acid (HCl) or sodium chloride (NaCl) and ammonium thiocyanate (NH ₄ SCN).

In one embodiment, the low-speed tensile test may be carried out under strain conditions of 1.67 X 10 ^-5 /sec or less.

When measuring the hydrogen embrittlement characteristics of steel materials by applying the method for evaluating the hydrogen embrittlement properties of steel materials of the present invention, the accuracy and reliability of the measurement results may be excellent.

1 shows a method for evaluating hydrogen embrittlement properties of steel materials according to an embodiment of the present invention.
FIG. 2(a) shows the steel material masking a portion to be measured for hydrogen embrittlement, FIG. 2(b) shows the steel material electrodeposited on the portion excluding the masking part, and FIG. 2(c) removes the masking And dried to show the steel material on which the electrodeposition coating layer was formed.
3 is a graph showing the hydrogen content change of the steel (H) mixed in the heating furnace.
Figure 4 (a) shows a steel material masking a portion to be measured for hydrogen embrittlement according to an embodiment of the present invention, Figure 4 (b) after the electrodeposition coating according to an embodiment of the present invention, the masking is removed and dried, It shows the steel material on which the electrodeposition coating layer was formed.
Figure 5 is a graph showing the results of the low-speed tensile test of the steel specimens according to the present invention.
Figure 6 is a graph showing the results of the low-speed tensile test of a steel specimen according to the present invention.
7 is a graph showing the results of a low-speed tensile test of an example steel specimen according to the present invention.
Figure 8 (a) shows a change in the mechanical strength of the steel material forming the electrodeposition coating layer by applying the drying temperature according to an embodiment of the present invention, Figure 8 (b) is applied to the drying temperature according to a comparative example for the present invention By showing the amount of change in mechanical strength of the steel material forming the electrodeposition coating layer, Figure 8 (c) is a graph showing the change in mechanical strength of the steel material forming the electrodeposition coating layer by applying a drying temperature according to a comparative example for the present invention .

Hereinafter, the present invention will be described in detail. At this time, in the description of the present invention, when it is determined that the detailed description of the related known technology or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description will be omitted.

In addition, terms to be described later are terms defined in consideration of functions in the present invention, which may vary according to a user's or operator's intention or practice, and thus the definition should be made based on the contents of the present specification describing the present invention.

Hydrogen of steel Brittle Characteristic evaluation method

One aspect of the present invention relates to a method for evaluating hydrogen embrittlement properties of steel materials. 1 shows a method for evaluating hydrogen embrittlement properties of steel materials according to an embodiment of the present invention. Referring to FIG. 1, the method for evaluating hydrogen embrittlement characteristics of the steel material includes: (S10) masking of a measurement target site; (S20) electrodeposition coating step; (S30) steel specimen manufacturing step; (S40) hydrogen charging step; (S50) masking step of the specimen measurement target area; And (S60) step of deriving the hydrogen sensitivity of the specimen.

More specifically, the method for evaluating hydrogen embrittlement characteristics of the steel material includes: (S10) masking a site for measuring hydrogen embrittlement in the steel material; (S20) electrodepositing the surface of the steel material excluding the masked portion; (S30) removing the masking of the electrodeposited steel material, followed by drying to prepare a steel specimen having an electrodeposition coating layer formed thereon; (S40) charging hydrogen to the measurement target site where the electrodeposition coating layer of the steel specimen is not formed; (S50) masking a portion to be measured of the hydrogen-charged steel specimen; And (S60) performing a low-speed tensile test (SSRT) on the steel specimen to derive the hydrogen sensitivity (S) of the steel specimen according to Equation 1 below.

Hereinafter, a method for evaluating hydrogen embrittlement characteristics of steel materials according to the present invention will be described in detail step by step.

(S10) Area to be measured Masking stage

The above step is a step of masking a portion to be measured for hydrogen embrittlement in a steel material. In the present invention, the steel material, various steel types used in vehicle manufacturing may be used as a specimen, or a special specimen that has undergone a separate processing process. 2 (a) below shows a steel material 10 masking a portion to be measured for hydrogen embrittlement using a masking tape 20. When the electrodeposition coating is described later, the masked portion does not have an electrodeposition coating layer, so it is easy to charge hydrogen, and it is possible to clearly confirm the formation of the electrodeposition coating layer with the masking tape 20 as a boundary.

(S20) Electrodeposition step

The step is a step of electrodeposition coating the surface of the steel material, except for the masked portion, as shown in Figure 2 (b).

Meanwhile, the vehicle parts are manufactured by performing a pretreatment process such as a degreasing process and a phosphate treatment process, and then performing a coating process in the order of electrodeposition coating, intermediate coating, base coating and top coating, and the electrodeposition coating protects the steel material. And to improve the bonding force between the steel material and the intermediate coating layer. In addition, in the present invention, the electrodeposition coating is performed after the hydrogen charging step, which will be described later, to prevent the diffusion out of hydrogen inside the steel material, thereby deriving more accurate hydrogen sensitivity.

In one embodiment, the electrodeposition coating is a step of applying a low voltage constant voltage of 3 to 7V after immersing the steel in the electrodeposition paint; And applying a high voltage constant voltage of 200 to 250V. When applying the above conditions, the adhesion of the electrodeposition coating layer, surface properties and mechanical strength may be excellent.

Applying the low voltage constant voltage of 3 to 7 V for 300 to 1000 seconds after immersing the masked steel in an electrodeposition paint; And applying a high voltage constant voltage of 200 to 250V for 150 to 200 seconds.

In one embodiment, the electrodeposition paint may include a modified epoxy resin and a curing agent. For example, the electrodeposition paint may include a bisphenol-type epoxy resin, a urethane resin, and a polyisocyanate curing agent. When the above components are included, the adhesion of the electrodeposition coating layer is excellent, and surface properties and mechanical strength may be excellent. For example, bisphenol A type and bisphenol F type epoxy resins may be used as the bisphenol type epoxy resin. For example, bisphenol A type epoxy resin can be used.

(S30) Steel specimen manufacturing step

The step is a step of removing the masking of the electrodeposited steel material, followed by drying to prepare a steel specimen 100 having an electrodeposition coating layer 30 as shown in FIG. 2(c).

In one embodiment, the drying may be performed at a temperature of 140°C or more and less than 180°C. Under the above conditions, all of the hydrogen inside the steel material is diffused out from the surface of the steel material on which the electrodeposition coating layer is not formed, so that the hydrogen content of the steel material is about 0 ppm, and the electrodeposition coating layer can be easily formed. When the drying is performed at a temperature of less than 140° C., the electrodeposition coating layer is not easily dried and cured, and when the drying is performed at a temperature of 180° C. or higher, steel is affected. Mechanical strength and the like may be lowered. For example, the drying may be performed for 10 to 20 minutes at a temperature of 140°C or more and less than 180°C using a hot air dryer. For another example, it may be dried at 150 ~ 160 ℃.

3 is a graph showing the change in hydrogen content of a steel (hot stamping material with an Al-Si plating layer) mixed with hydrogen (H) in a heating furnace. Referring to FIG. 3, in the steel material charged and heated in the heating furnace, hydrogen is mixed to increase the hydrogen content, and hydrogen is diffused out of the steel material over time to decrease the hydrogen content. In addition, it can be seen that after the electrodeposition coating of the steel material, drying was performed at 160° C. for 20 minutes, the amount of hydrogen in the steel material can be controlled to 0.03 ppm.

In addition, when drying is performed without removing the masking tape 20, an adhesive component of the masking tape 20 is not easily removed, and thus, an error in measuring hydrogen sensitivity may increase.

In one embodiment, the electrodeposition coating layer 30 may be formed to a thickness of 10 ~ 30㎛. When formed to the above thickness, the diffusion out of hydrogen can be easily prevented. When the electrodeposition coating layer 30 is formed to a thickness of less than 10 µm, the electrodeposition coating layer is formed non-uniformly, such as pinholes, or the mechanical strength of the electrodeposition coating layer decreases. The layer may be formed non-uniformly, or the internal stress may increase to increase sensitivity to external impact.

(S40) hydrogen Charging stage

The step is a step of charging hydrogen to a portion to be measured where the electrodeposition coating layer of the steel specimen is not formed.

In one embodiment, the hydrogen charging may be performed by immersing the steel specimen in an aqueous solution containing hydrochloric acid (HCl) or sodium chloride (NaCl) and ammonium thiocyanate (NH ₄ SCN). At the time of charging the hydrogen, hydrogen is charged to the measurement target site where the electrodeposition coating layer is not formed. The current density and charging time during the hydrogen charging may be performed by changing according to the desired hydrogen charging amount.

For example, the current density when charging the hydrogen may be 30 ~ 50A / m ² . In the case of charging the hydrogen, when an excessively high current density is applied, cracks may occur in the specimen.

(S50) Specimen measurement target area Masking stage

The step is a step of masking a portion to be measured of the hydrogen-charged steel specimen. In one embodiment, the masking may prevent diffusion of hydrogen from a hydrogen-charged portion of steel during a low-speed tensile test, which will be described later, so that accuracy and reliability of a result of deriving hydrogen sensitivity may be excellent.

(S60) Step of deriving the hydrogen sensitivity of the specimen

The step is a step of performing a low-speed tensile test (SSRT) on the steel specimen to derive the hydrogen sensitivity (S) of the steel specimen according to Equation 1 below:

[Equation 1]

Hydrogen sensitivity (S) = (l _n /l ₀ ) X 100 (%)

(In Equation 1, the l _n is the elongation of the steel specimen charged with hydrogen for n hours, and l ₀ is the elongation of the specimen before hydrogen is charged).

When using the low-speed tensile test, it is possible to easily evaluate hydrogen embrittlement caused by the accumulation of hydrogen at a measurement target site where steel stress is concentrated.

For example, in the case of a steel specimen having a hydrogen sensitivity (S) of 70% or more, it can be determined that the hydrogen sensitivity is high.

In one embodiment, the low-speed tensile test may be carried out under strain conditions of 1.67 X 10 ^-5 /sec or less. For example, it can be carried out under strain conditions of 0.1 mm/min or less. Under the above-described strain conditions, accurate hydrogen sensitivity can be evaluated because the hydrogen in the region to be measured where the steel stress is concentrated is not fractured before accumulation.

On the other hand, in the conventional hydrogen embrittlement test, hydrogen does not escape from the plated layer forming surface, but there is a problem of diffusion out of hydrogen through the cutting surface when a cutting surface exists in the steel. In addition, the hydrogen content inside the steel material during the process is different for each condition of the test piece, and in order to perform a hydrogen embrittlement test, a control method is required to control the amount of hydrogen inside the steel material to 0, and then mix the target amount of hydrogen.

On the other hand, when measuring the hydrogen embrittlement property of the steel by applying the method for evaluating the hydrogen embrittlement property of the steel of the present invention, the amount of hydrogen inside the steel can be controlled to 0 ppm, and the diffusion of the hydrogen is masked by masking the unformed portion of the electrodeposition coating layer of the steel. By preventing out, the accuracy and reliability of the measurement results are excellent, and the hydrogen embrittlement limiting environment in steel production can be easily set.

Hereinafter, the configuration and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, this is presented as a preferred example of the present invention and cannot be interpreted as limiting the present invention in any sense.

Example And Comparative example

Example One

A steel material having an Al-Si plating layer formed on the surface was prepared. As shown in Fig. 4 (a), a portion to be measured for hydrogen embrittlement in a steel material was masked using a masking tape. Subsequently, the steel is immersed in an electrodeposition coating material containing bisphenol A type epoxy resin, urethane resin, and polyisocyanate curing agent, a low voltage constant voltage of 3 to 7 V is applied for 300 to 1000 seconds, and then a high voltage constant voltage of 200 to 250 V is applied. Was applied for 150 to 200 seconds to perform electrodeposition coating on the steel surface excluding the masked area.

Then, the masking of the electrodeposited steel was removed, followed by drying at 150°C for 20 minutes using a hot air dryer, thereby preparing a steel specimen having a 25 μm thick electrodeposited coating layer as shown in FIG. 4(b). . The steel specimen was immersed in an aqueous solution containing 3wt% sodium chloride (NaCl) and 0.3wt% ammonium thiocyanate (NH ₄ SCN), and the current density: 50A/m ² or less was applied to apply the current to the electrodeposition coating layer. Hydrogen was charged to the area to be measured (area of 0.5 cm ² ) that was not formed.

Subsequently, a portion to be measured of the hydrogen-charged steel specimen was masked using a masking tape, and a low-speed tensile test (SSRT) was performed on the steel specimen under a strain rate of 1.67 X 10 ^-5 /sec or less. The hydrogen sensitivity (S) of the steel specimen was derived according to Equation 1:

[Equation 1]

Hydrogen sensitivity (S) = (l _n /l ₀ ) X 100 (%)

(In Equation 1, the l _n is the elongation of the steel specimen charged with hydrogen for n hours, and l ₀ is the elongation of the specimen before hydrogen is charged).

Comparative example One

The hydrogen sensitivity of the steel specimen was derived in the same manner as in Example 1, except that the drying was dried at 100°C to prepare a steel specimen.

Comparative example 2

The hydrogen sensitivity of the steel specimen was derived in the same manner as in Example 1, except that the drying was dried at 180°C to prepare a steel specimen.

Figure 8 (a) below shows the amount of change in the mechanical strength of the steel material according to the drying time when forming the electrodeposition coating layer under the drying temperature conditions of Example 1, Figure 8 (b) is electrodeposition under the drying temperature conditions of Comparative Example 1 When forming the coating layer, it shows the amount of change in the mechanical strength of the steel according to the drying time, and FIG. 8(c) shows the amount of change in the mechanical strength of the steel according to the drying time when the electrodeposition coating layer is formed under the drying temperature conditions of Comparative Example 1. It is shown. Referring to the results of FIG. 8, it can be seen that in Comparative Example 2 exceeding the drying temperature range of the present invention, tensile strength decreases and material deterioration increases.

Example 2-4: Hot stamping Evaluation of changes in hydrogen sensitivity of steel specimens with heating time

Example 2

In order to evaluate the change in the hydrogen sensitivity of the steel specimen according to the heating time during hot stamping, the steel sheet with the Al-Si plating layer was charged into a heating furnace at 930°C and heated for 260 seconds. The heated steel was molded into a predetermined shape, and then cooled to prepare a hot stamping steel. Subsequently, the steel is immersed in an electrodeposition coating material containing bisphenol A type epoxy resin, urethane resin, and polyisocyanate curing agent, a low voltage constant voltage of 3 to 7 V is applied for 300 to 1000 seconds, and then a high voltage constant voltage of 200 to 250 V is applied. Was applied for 150 to 200 seconds to perform electrodeposition coating on the steel surface excluding the masked area.

Then, the masking of the electrodeposited steel was removed, followed by drying at 150° C. for 20 minutes using a hot air dryer, thereby preparing a steel specimen having a 25 μm thick electrodeposited coating layer.

Example 3

A steel specimen was prepared in the same manner as in Example 2, except that the same type of steel sheet as in Example 3 was charged to the heating furnace and heated for 600 seconds.

Example 4

A steel specimen was prepared in the same manner as in Example 2, except that the same type of steel sheet as in Example 3 was charged into a heating furnace and heated for 1200 seconds.

The steel specimens of Examples 2 to 4 were immersed in an aqueous solution containing 3 wt% sodium chloride (NaCl) and 0.3 wt% ammonium thiocyanate (NH ₄ SCN), respectively, and the current density was 50 A/m ² or less. Hydrogen was charged to the measurement target area (0.5 cm ² area) where the electrodeposition coating layer was not formed. Subsequently, a portion to be measured of the hydrogen-charged steel specimen was masked using a masking tape, and a low-speed tensile test (SSRT) was performed on the steel specimen under a strain rate of 1.67 X 10 ^-5 /sec or less. According to Equation 1, the hydrogen sensitivity (S) of the steel specimen was derived.

In this case, the elongation before hydrogen charging (l ₀ ), the elongation after hydrogen charging for 5 hours (l ₅ ), and the elongation after hydrogen charging for 24 hours (l ₂₄ ) are measured, and the hydrogen sensitivity derived using Equation 1 is shown in the table below. It is shown in 1:

[Equation 1]

Hydrogen sensitivity (S) = (l _n /l ₀ ) X 100 (%)

(In Equation 1, the l _n is the elongation of the steel specimen charged with hydrogen for n hours, and l ₀ is the elongation of the specimen before hydrogen is charged).

5 is a graph showing the results of low-speed tensile test of a steel specimen in Example 2 according to the present invention, FIG. 6 is a graph showing the results of a low-speed tensile test in Example 3 according to the present invention, and FIG. 7 is the present invention According to Example 4 is a graph showing the results of low-speed tensile test of steel specimens. Referring to Table 1 and the results of FIGS. 5 to 7, it can be seen that in the case of the steel specimens of Examples 2 to 4 of the steel sheet, as the heating time in the heating furnace increases, the hydrogen sensitivity of the specimen increases. there was.

Simple modifications or changes of the present invention can be easily carried out by those skilled in the art, and all such modifications or changes can be considered to be included in the scope of the present invention.

10: steel material 20: masking tape
30: electrodeposition coating layer 100: steel specimen

Claims (6)

Masking a portion to be measured for hydrogen embrittlement in steel;
Electrodepositing the surface of the steel material except for the masked portion;
Removing the masking of the electrodeposited steel material, and then drying at 150 to 160° C. to prepare a steel material specimen with an electrodeposition coating layer formed;
Charging hydrogen to a portion to be measured where the electrodeposition coating layer of the steel specimen is not formed;
Masking a portion to be measured of the hydrogen-charged steel specimen; And
Including the step of deriving the hydrogen susceptibility (S) of the steel specimen according to the following equation 1, by performing a low-speed tensile test (SSRT) under the strain conditions of 1.67 X 10 ^-5 / sec or less on the steel specimen;
The hydrogen charging is performed by immersing the steel specimen in an aqueous solution containing sodium chloride (NaCl) and ammonium thiocyanate (NH ₄ SCN) to evaluate hydrogen embrittlement properties of the steel:
[Equation 1]
Hydrogen sensitivity (S) = (l _n /l ₀ ) X 100 (%)
(In Equation 1, the l _n is the elongation of the steel specimen charged with hydrogen for n hours, and l ₀ is the elongation of the specimen before hydrogen is charged).
According to claim 1,
The electrodeposition coating is a step of applying a low voltage constant voltage of 3 ~ 7V after immersing the steel in the electrodeposition paint; And
Applying a high voltage constant voltage of 200 ~ 250V; Method for evaluating the hydrogen embrittlement properties of steel, characterized in that carried out.
According to claim 2,
The electrodeposition coating method of bisphenol-type epoxy resin, urethane resin and polyisocyanate curing agent, characterized in that the hydrogen brittle property evaluation method of the steel.
According to claim 1,
The electrodeposition coating layer is a method of evaluating the hydrogen embrittlement properties of steel materials, characterized in that formed to a thickness of 10 ~ 30㎛.
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