JP2001264240A - Hydrogen enblittlement sensitivity evaluation method of steel product and steel product having excellent hydrogen enblittlement resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of executing quick and highly sensitive evaluation of hydrogen embrittlement sensitivity relative to various steel products including a plated steel sheet, and a steel product having excellent hydrogen embrittlement resistance. SOLUTION: Correlation between the quantity of diffused hydrogen existing in a steel and a hydrogen embrittlement risk index H(%) obtained by the undermentiotted equation by a low distortion speed tensile test with a distortion speed below 1×10-4/sec is obtained beforehand relative to the same kind of steel product as a steel product which is an evaluation object. The quantity of diffused hydrogen existing in the steel of the steel product which is the evaluation object is measured, and a hydrogen embrittlement risk of the steel product is evaluated from the correlation based on the measured diffused hydrogen quantity. In the case of a plated steel sheet, anode electrolysis of a plating coat is preferably executed in alkali solution with pH of 10 or more before measurement of the diffused hydrogen quantity to dissolve and remove the coat. In the undermentioned equation, E0 is an elongation at the breaking time of a test piece of a steel product including substantially no diffusible hydrogen in the steel, and E1 is an elongation at the breaking time of a test piece of a steel product including diffused hydrogen in the steel. H(%)=100×(1-E1/E0).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variety of cables and wires, such as bumpers and door reinforcing members for automobiles, scaffolding materials and bolts for civil engineering and construction, and tire steel cords and bridge cables. The present invention relates to a method for evaluating the risk of hydrogen embrittlement in various high-strength steel materials, such as materials, which are required to be lightweight, have high strength and high corrosion resistance.

[0002]

2. Description of the Related Art In recent years, the weight of a vehicle body has been reduced in order to improve the fuel efficiency of an automobile, and the use of a high-strength thin steel plate for a reinforcing member such as a bumper or a door has been studied. The trend of increasing the strength leads to an increase in the size of the steel structure, and accordingly, an improvement in the joint strength of the joint is required, and a higher strength is also required for fastening jigs such as bolts. Also, various cables such as tire steel cords and bridge cables, and wire materials have the same tendency.

However, high-strength steels have a problem of hydrogen embrittlement (delayed fracture) due to hydrogen entering the steels mainly due to a corrosion reaction during actual use in a corrosive environment. In addition, the corrosion resistance of the material itself is required to improve the thinness and strength,
It is known that when a plating film such as zinc plating is formed on the surface of a steel sheet for that purpose, embrittlement is more likely to occur. For example, in electroplating, hydrogen generated by the cathodic reaction in the pickling step and plating step before plating enters the steel sheet, and in the case of hot-dip plating,
Hydrogen in the heating atmosphere of the line penetrates into the steel sheet, and in any case, hydrogen infiltration into the steel sheet causes hydrogen embrittlement (plating embrittlement). Therefore, it is very important to evaluate the hydrogen embrittlement susceptibility (the risk of embrittlement by hydrogen in steel) when studying the use of an actual machine.

[0004] The hydrogen embrittlement susceptibility has been conventionally evaluated by the breaking time in a constant strain test method or a constant load test method, but in recent studies, hydrogen involved in hydrogen embrittlement is called diffusible hydrogen. And hydrogen that can diffuse through steel at room temperature. For this reason, Japanese Patent Application Laid-Open No. 2-267
No. 243, JP-A-5-255736, JP-A-6-25
As described in No. 745, the amount of hydrogen in steel has been measured by the vacuum heating method.

[0005]

However, even when the amount of diffusible hydrogen in steel is measured by the above-described method and the susceptibility to hydrogen embrittlement is evaluated by using the method, galvanized bolts and the like which are considered to be caused by hydrogen embrittlement can be obtained. There have been many breakage accidents. This indicates that accurate and practical evaluation has not been performed in the conventional hydrogen embrittlement susceptibility evaluation test.

The reason is considered as follows. In conventional analysis of hydrogen in steel, for example, for galvanized steel,
Since the analysis is performed while the zinc plating film is formed, a large amount of hydrogen introduced into the plating film during the plating treatment is included in the measured hydrogen amount, and the measured hydrogen amount contributes to hydrogen embrittlement. Does not accurately reflect diffusible hydrogen in steel.
Although the plating film is also removed, the removal method depends on mechanical removal or dissolution by acid immersion.
In the case of mechanical removal, the heat generated during polishing or grinding releases hydrogen in the steel. In addition, there is a limit in removal of a target material having a complicated shape such as a bolt or a screw. on the other hand,
Dissolution by acid immersion generates hydrogen by corrosion. In any case, accurate analysis of diffusible hydrogen in steel has not been performed.

[0007] Even if the amount of hydrogen in steel is accurately measured, the conventional hydrogen embrittlement susceptibility evaluation test uses a constant strain method,
The following problems arise because the test was performed by the constant load method and the actual test of actually tightening the bolts. The evaluation has a long time and a large number of test pieces are required for the evaluation. Furthermore, when the amount of hydrogen is so small as to enter during plating or actual use, the test piece does not break, and it is difficult to determine the degree of risk due to hydrogen embrittlement.

FIG. 3 shows a steel plate having a strength of 1380 MPa (○,
●) Steel plate with 1180 MPa strength (△, ▲), 780M
The graph shows the relationship between the amount of diffusible hydrogen in steel obtained by the conventional method and the time until fracture in a loop-type constant strain test for steel sheets (□, Δ) with Pa strength. , ■
In the case of the amount of diffusible hydrogen in steel indicated by, fracture occurred due to hydrogen embrittlement, but in the minute range where the amount of diffusible hydrogen was about 0.1 ppm or less, even when tested for 100 hours, none of the materials broke, It is assumed that hydrogen embrittlement does not occur temporarily. However, in fact, there are some fractures due to hydrogen embrittlement, and it is understood that hydrogen embrittlement susceptibility cannot be accurately evaluated by the conventional method.

The results shown in FIG. 3 are obtained by the following tests. An electrogalvanizing process was performed on a 1.5 mm-thick high-strength steel sheet whose tensile strength was changed to about 1380 MPa, 1180 MPa, or 780 MPa by quenching and tempering to prepare a plated steel sheet sample. The plating conditions were such that a sulfuric acid bath was used for the purpose of changing the cathode current efficiency, bath temperature: room temperature to 50 ° C., current density: 0.1 to 100 A / dm.
^{2. The} pH was changed from 1 to 5. Thereafter, the obtained plated steel sheet was immersed in acid (hydrochloric acid) to dissolve and remove the plating film, and then the amount of hydrogen in the steel was measured by a vacuum heating method (heating rate: 12 ° C./min). On the other hand, a test piece was sampled from the plated steel sheet and subjected to a stress (0.9 times the tensile strength) using a loop-type constant-strain delayed fracture tester for 100 hours.
The rupture time was measured up to the limit. The constant strain delay fracture tester has been conventionally performed by a bolt test or the like.

The present invention has been made in view of the above-mentioned problems, and provides a method capable of quickly and highly sensitively evaluating hydrogen embrittlement susceptibility of various steel materials including plated steel sheets, and an excellent hydrogen embrittlement resistance. It is intended to provide a steel material.

[0011]

According to the method for evaluating the hydrogen embrittlement susceptibility of a steel material according to the present invention, as set forth in claim 1, for a steel material of the same kind as a steel material to be evaluated in advance, the diffusion hydrogen existing in the steel is determined. The correlation between the amount and the hydrogen embrittlement risk index (%) determined by the following equation by a low strain rate tensile test having a strain rate of 1 × 10 ⁻⁴ / sec or less was determined, and the steel material to be evaluated was evaluated. This is a method of measuring the amount of diffused hydrogen present in steel and evaluating the risk of hydrogen embrittlement of the steel material from the correlation based on the measured amount of diffused hydrogen. Hydrogen embrittlement risk index (%) = 100 × (1-E1 / E)
0) Here, E0 is the elongation at break of a steel test piece substantially containing no diffusible hydrogen in steel, and E1 is the elongation at break of a steel test piece containing diffusible hydrogen in steel.

The amount of diffused hydrogen in the steel is preferably the total amount of hydrogen released while the temperature of the steel is raised from room temperature to 250 ° C. at a rate of 1 to 20 ° C./min. Further, it is preferable that the amount of diffused hydrogen is measured by a temperature rising analysis using a mass spectrometer.

In the case of a plated steel material coated with a plating film, when measuring the amount of diffused hydrogen in the steel as described in claim 4, the plating film is subjected to anodic electrolysis in an alkaline solution having a pH of 10 or more before the measurement. It is preferred to dissolve away. For plated steel, tensile strength TS ≧ 780
Includes steel materials with a plating film of Mpa or more and formed of zinc or a zinc alloy.

Further, the steel material excellent in hydrogen embrittlement resistance according to the present invention is, as described in claim 6, a plated steel material having TS ≧ 780 Mpa or more and having a zinc or zinc alloy plating film formed thereon. According to the hydrogen embrittlement susceptibility evaluation method described in 4, the hydrogen embrittlement risk index was evaluated to be 30% or less.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors have established an accurate method for quantifying diffusible hydrogen in steel, which affects hydrogen embrittlement, irrespective of the presence or absence and type of plating film and the composition and composition of the underlying steel material. As a result, we conducted a study to develop a method that can accurately and quickly evaluate hydrogen embrittlement of steel materials, and furthermore, develop steel materials that can reliably exhibit excellent hydrogen embrittlement resistance. Accurate determination of the amount of diffusible hydrogen in steel (2) The present invention has been completed by establishing a highly sensitive test method for evaluating hydrogen embrittlement susceptibility of steel containing trace amounts of hydrogen in a short time.

First, a method for measuring the amount of diffusible hydrogen in steel will be described. An important point in this measurement is that the amount of hydrogen to be measured is diffusible hydrogen in the steel material, and the amount of hydrogen is preferably increased at a rate of 1 to 20 ° C / min from room temperature to 250 ° C.
It is preferable to perform the temperature rise analysis as min and to use the total amount of hydrogen released during the temperature rise. As a specific measurement method,
A mass spectrometer may be used to perform temperature rising analysis, and the integrated value of hydrogen released in the above temperature range may be defined as the amount of diffusible hydrogen.

The temperature rise rate in the temperature rise analysis is 1 to 20 ° C.
The reason why / min is recommended is that if it is less than 1 ° C / min, the analysis time is too slow and the efficiency is low, while if it exceeds 20 ° C / min, the hydrogen release from the inside of the steel becomes insufficient. Also,
The release amount up to 250 ° C. is measured because hydrogen released above 250 ° C. cannot diffuse into steel at room temperature and is considered not to be involved in hydrogen embrittlement.

As a method for measuring the amount of released hydrogen, there is a method such as a glycerin method, but it is preferable to use a mass spectrometer capable of obtaining high measurement accuracy. In particular, an atmospheric pressure ionization mass spectrometer (APIMS) is suitable. This mass spectrometer detects hydrogen in an Ar gas atmosphere under atmospheric pressure. However, since it can be measured under atmospheric pressure, a mass spectrometer that detects hydrogen in a general vacuum atmosphere is used. Unlike this, the time required for evacuation can be saved, and there is an advantage that the release of hydrogen from the steel material during evacuation can be suppressed, and the detection accuracy is excellent.

Incidentally, in a steel material having a plating film such as a zinc plating film, a very large amount of hydrogen is also contained in the plating film itself, and if the film and the steel material are measured simultaneously, they are involved in hydrogen embrittlement. The amount of diffusible hydrogen in steel cannot be determined. Therefore, in the present invention, it is necessary to measure the amount of diffusible hydrogen in the steel after removing the plating film from the steel material having the plating film.

[0020] As a method of removing the plating film, the pH 10
It is preferable to dissolve and remove by anodic electrolysis in the above alkaline solution. The reason for setting the pH to 10 or more is that p
This is because an alkaline solution of H10 or more does not cause hydrogen generation because the steel material does not corrode. In this case, the upper limit of the pH is desirably pH 14, more preferably pH 13.5. Further, according to the anodic electrolysis, even if the thickness and properties of the plating film are different by controlling the current density and the like, the film can be removed cleanly. In other words, simply immersing in an alkaline solution cannot remove the plating film adhered to the minute recesses such as the screw bottom of the plating screw. However, according to the anodic electrolysis, the plating film in such a portion can also be removed by controlling the potential and current. It can be easily removed.

Next, a short-time and high-sensitivity evaluation test method for the danger of hydrogen embrittlement of a steel material containing a trace amount of hydrogen will be described. In the present invention, the low strain rate tensile test method (S
SRT) to evaluate the hydrogen embrittlement susceptibility according to the hydrogen embrittlement risk index (%) defined by the following equation. Hydrogen embrittlement risk index (%) = 100 × (1-E1 / E)
0) Here, E0 is the elongation at break of a test piece of a steel material containing substantially no diffused hydrogen in steel, and E1 is the elongation at break of a test piece of a steel material containing diffused hydrogen in the steel.

The SSRT is different from the constant weight test method and the constant strain test method in that the risk of hydrogen embrittlement can be quickly and quickly evaluated with high sensitivity, unlike the constant load test method and the constant strain test method, in order to break the test piece while increasing the applied stress. it can. The shape of the SSRT test piece may be any shape such as a plate shape, a bar shape, and the presence or absence of a notch, but a strain rate of 1 × 10 ⁻⁴ / sec or less in a parallel portion of the test piece to be a fracture area is used. . Testing at rates above this strain rate will reduce sensitivity.
On the other hand, the lower the strain rate, the better the sensitivity because the sensitivity is improved. However, the strain rate can be appropriately determined in consideration of the test efficiency. The hydrogen embrittlement risk index was calculated by measuring the elongation of the test piece and using the elongation value at break.However, when a rod-shaped test piece was used, the aperture value was changed to the elongation value instead of the elongation value. Can be used.

[0023] For a steel material of the same type (equivalent in composition, structure, and mechanical properties) as the steel material to be evaluated, a test steel material in which the amount of diffusible hydrogen in the steel is variously changed is prepared, and the SSRT is performed on the test steel material. Thus, a correlation between the amount of diffusible hydrogen in the steel and the hydrogen embrittlement risk index (%) in this type of steel is obtained. This correlation can be shown in a diagram (graph). Once the phase relationship is determined, for the same type of steel material thereafter, the hydrogen embrittlement risk index can be easily determined by appropriately measuring the amount of diffusible hydrogen in the steel of the steel material to be evaluated by the method described above. The hydrogen embrittlement susceptibility can be quickly and accurately evaluated.

For example, in a galvanizing plant, the correlation is determined in advance for a base steel sheet, so that the hydrogen embrittlement risk index can be calculated simply by measuring the amount of diffusible hydrogen in the steel of the plated steel. The hydrogen embrittlement risk index can be used immediately as a hydrogen embrittlement management index.

More specifically, according to the correlation shown in FIG. 1 obtained in the examples described later, in a galvanized steel sheet having a strength of 1380 MPa, the amount of diffusible hydrogen in the steel is determined by a criterion for determining whether or not hydrogen embrittlement occurs. By comparing with the index value (30%), it is found that the amount of diffusible hydrogen in steel should be set to 0.05 ppm or less, and the amount of hydrogen should be controlled to be equal to or less than this value.

In order to reduce the amount of hydrogen penetrating into steel during the galvanizing step, it is effective to change the plating conditions, specifically, the cathode current efficiency. For example, at the initial stage during the plating process, the cathode current efficiency is increased to 80% or more (preferably 95%).
% Or more), and by suppressing the generation of hydrogen to form a dense plating film on the steel surface, it is possible to prevent hydrogen from penetrating into the steel material.

According to the method for evaluating susceptibility to hydrogen embrittlement of the present invention, the susceptibility to hydrogen embrittlement of various steel materials, such as zinc (alloy) plated steel, is not dependent on the composition and structure of the steel material and the type of plating. Can be evaluated effectively. Furthermore, it is possible to evaluate not only hydrogen entering in the plating step and the pickling step, but also any hydrogen entering the steel material during actual use. For this reason, it is effective for steel materials without a plating film.

By utilizing the present invention, even a small amount of diffusible hydrogen-containing steel which could not be evaluated conventionally can be used.
Highly sensitive and rapid quantitative evaluation is possible. Then, by grasping the amount of infiltration of hydrogen and the like in the environment of use, it is possible to select an appropriate material. In addition, by accurately evaluating the hydrogen embrittlement susceptibility of the present invention even in a high-strength steel material that has been regarded as dangerous in the past, it may be possible to use the actual machine, and it is extremely useful as an evaluation criterion for selecting a new high-strength steel material. It is valid.

Hereinafter, the present invention will be further described with reference to examples, but the present invention is not construed as being limited to such examples.

[0030]

Example 1 Tensile strength of 1380MP by quenching and tempering
a, electrogalvanizing a high-strength steel sheet with a thickness of 1.5 mm changed to around 1180 MPa and 780 MPa,
A plated steel sheet sample was prepared. The plating conditions were such that a sulfuric acid bath was used for the purpose of changing the cathode current efficiency, bath temperature: room temperature to 50.
° C, current density: 0.1-100 A / dm ² , pH: 1
5 was performed.

The obtained plated steel sheet was treated with NaOH of pH 13
After performing anodic electrolysis in the solution for 3 to 10 minutes to completely dissolve and remove the plating film, a temperature increase analysis from room temperature to 250 ° C. (at a temperature increase rate of 12 ° C.) using an atmospheric pressure ionization mass spectrometer (APIMS). / sec), and the amount of hydrogen generated during that time (ie, the amount of diffusible hydrogen) was measured. In addition, a test piece was sampled from the plated steel sheet sample and subjected to SSRT (strain rate:
9.7 × 10 ⁻⁶ / sec) to measure the elongation at break,
The hydrogen embrittlement risk index (%) was determined.

The relationship between the amount of diffusible hydrogen and the hydrogen embrittlement risk index for each strength was organized and graphed. The result is shown in FIG. In the figure, ○ and ● represent a steel plate having a strength of 1380 MPa, Δ and ▲ represent a steel plate having a strength of 1180 MPa,
1 shows a steel plate of 80 MPa strength. ●, ▲, △ indicate ineligible ones exceeding the reference index of 30% for hydrogen embrittlement. From Fig. 1, 0.1pp which could not be evaluated by the conventional method
It can be seen that the hydrogen embrittlement susceptibility can be quantitatively evaluated with high sensitivity even in the low hydrogen amount range of m or less. Also, from FIG. 1, for example, in a 1380 MPa plated steel sheet, the amount of diffusible hydrogen in the steel is a = 0.05 ppm or less.
= 0.07 ppm or less, c =
It can be seen that by controlling the plating conditions to 0.09 ppm or less, a plated steel sheet free from hydrogen embrittlement can be obtained.

Example 2 A bolt manufactured by adjusting the heat treatment conditions so that the tensile strength becomes 1200 MPa was subjected to zinc plating while changing the plating conditions, and a set of 20 bolts was subjected to a tightening test. Immediately after tightening, leave at room temperature for 4 days, leave for 7 days,
At the stage of standing for 30 days, the number of broken bolts was measured.

On the other hand, for bolts produced under the same conditions, the relationship between the amount of diffusible hydrogen in steel and the hydrogen embrittlement risk index was determined in the same manner as in Example 2. The results are shown in Table 1 and FIG. From these results, it can be seen that the bolts that fractured early have a high hydrogen embrittlement risk index, and the bolts that do not break even after standing for 30 days all have a hydrogen embrittlement risk index of 30% or less. The amount of diffusible hydrogen corresponding to the hydrogen embrittlement risk index of 30% is approximately 0.07 ppm. Than this,
The amount of hydrogen diffused in steel of the same type of plated bolt is measured, and if the value is 0.07 ppm or less, it can be evaluated that hydrogen embrittlement does not occur.

[0035]

[Table 1]

The sample which did not break even after 30 days and had a low amount of diffusible hydrogen in steel had a high cathode current efficiency during plating. Therefore, it is understood that the amount of diffusible hydrogen in steel can be controlled by controlling the cathode current efficiency.

[0037]

According to the evaluation method of the present invention, it is possible to increase the hydrogen embrittlement susceptibility of a steel material in a short time, especially for a steel material having a high tensile strength of 780 or more, regardless of the presence of a plating film. Sensitivity and quantitative evaluation can be performed, and evaluation can be performed with high sensitivity even in a small amount of hydrogen, which cannot be evaluated by the conventional method. For this reason, the hydrogen embrittlement risk index obtained by the evaluation method of the present invention can be effectively used as a production management index of a plating plant or an index of prediction of fracture of a steel material constituting an actual machine. Further, the steel material of the present invention has a hydrogen embrittlement risk index of 30% or less even though it is a steel material having a zinc-based plating film in which infiltration of hydrogen cannot be avoided. It can be used as a high-strength zinc-based galvanized steel material.

[Brief description of the drawings]

FIG. 1 shows the amount of diffusible hydrogen in steel and SSRT in Example 1.
Is a graph showing the relationship with the hydrogen embrittlement risk index obtained by the above method for three types of steel materials.

FIG. 2 is a graph showing the relationship between the number of days to break and the hydrogen embrittlement risk index in the bolt tightening test of Example 2.

FIG. 3 is a graph showing the relationship between the amount of diffusible hydrogen in steel measured by a conventional method and the rupture time measured by a loop-type constant strain test.

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Claims (6)

[Claims] 1. For a steel material of the same kind as a steel material to be evaluated in advance, the amount of diffused hydrogen present in the steel and the strain rate are 1 ×
Correlation with the hydrogen embrittlement risk index (%) obtained by the following equation by a low strain rate tensile test of 10 ⁻⁴ / sec or less is obtained, and the amount of diffused hydrogen present in the steel of the steel material to be evaluated And a method for evaluating hydrogen embrittlement susceptibility of a steel material, wherein the risk of hydrogen embrittlement of the steel material is evaluated from the correlation based on the measured amount of diffused hydrogen. Hydrogen embrittlement risk index (%) = 100 × (1-E1 / E)
0) Here, E0 is the elongation at break of a steel test piece substantially containing no diffusible hydrogen in steel, and E1 is the elongation at break of a steel test piece containing diffusible hydrogen in steel. 2. The amount of hydrogen diffused in steel is set at room temperature to 25%.
The method for evaluating hydrogen embrittlement susceptibility of steel according to claim 1, wherein the total amount of hydrogen released during heating at a heating rate of 1 to 20 ° C / min up to 0 ° C is defined as the total amount of hydrogen. 3. The method for evaluating hydrogen embrittlement susceptibility of a steel material according to claim 2, wherein the amount of diffused hydrogen is measured by temperature rise analysis using a mass spectrometer. 4. A steel material is a plated steel material having a plating film formed on a surface thereof. When measuring the amount of diffused hydrogen in the steel,
The method for evaluating hydrogen embrittlement susceptibility of a steel material according to any one of claims 1 to 3, wherein the plating film is dissolved and removed by anodic electrolysis in an alkaline solution having a pH of 10 or more before the measurement. 5. The plated steel material has a tensile strength TS ≧ 780.
5. The method for evaluating hydrogen embrittlement susceptibility of a steel material according to claim 4, wherein the plating material is a steel material having a Mpa or more and a plating film formed of zinc or a zinc alloy. 6. A plated steel material evaluated by the method according to claim 5, wherein the hydrogen embrittlement risk index is 30%.
The following steel materials with excellent hydrogen embrittlement resistance.