JP2014070927A - Test piece and method for delayed fracture characteristic evaluation of ultra-high strength surface treated steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a test piece for delayed fracture characteristic evaluation of an ultra-high strength surface treated steel plate capable of contributing to the accurate evaluation of delayed fracture characteristics of the ultra-high strength surface treated steel plate whose tensile strength is 1180 MPa or more.SOLUTION: An end face surface of a test piece 1 is processed into a surface having surface roughness of 6.3 μm or less in an arithmetic average Ra specified in JISB0601-2001, and the test piece 1 has a synthetic resin coating film 5 formed on the end face surface.

Description

  The present invention relates to an ultra-high-strength surface-treated steel sheet suitable for automobile members, building materials, and the like, and more particularly to improvement of a specimen for evaluating delayed fracture characteristics of an ultra-high-strength surface-treated steel sheet. Here, “ultra-high strength” refers to the case where the tensile strength is 1180 MPa or more. Examples of the “surface-treated steel sheet” herein include Zn-based electroplated steel sheets, Zn-based hot dip plated steel sheets, Al-based plated steel sheets, and Ni-based plated steel sheets.

  In recent years, there has been a strong demand for improving the fuel efficiency of automobiles from the viewpoint of conservation of the global environment. For this reason, reducing the weight of the vehicle body has become an important issue for automobile manufacturers, and the reduction of thickness and weight by increasing the strength of automobile parts is being studied. For example, ultra-high-strength steel sheets with a tensile strength of 1180 MPa or more are beginning to be used for parts that require impact resistance such as bumpers and pillars and other reinforcing parts. However, in this case, since the corrosion allowance is reduced by thinning, the use of ultra-high-strength steel sheets that have been surface-treated (ultra-high-strength surface-treated steel sheets) is being considered for parts that are expected to have a severe corrosive environment. Yes.

  Conventionally, ultra-high-strength steel sheets with a tensile strength exceeding 1180 MPa have a problem that delayed fracture is likely to occur. Therefore, it is important to prevent delayed destruction. “Delayed fracture” is a type of hydrogen embrittlement, and is a phenomenon that suddenly breaks months to years after the start of use of a steel sheet.

As for the delayed fracture evaluation method, for example, Non-Patent Literature 1 proposes various delayed fracture acceleration test methods. However, in various test methods described in Non-Patent Document 1, there are cases where variations occur in test results, and improvement of test accuracy has been a problem.
For such a problem, for example, Patent Document 1 describes a delayed fracture resistance evaluation method. The technology described in Patent Document 1 includes a rod-shaped test piece main body, a flange formed by expanding in the circumferential direction of the test piece main body as a hydrogen-charged portion, and a test piece main body as a stress concentration portion during a tensile test. This is a delayed fracture resistance evaluation method for evaluating delayed fracture resistance of a steel material using a test piece formed by a notch formed over the entire circumference of the steel. The technique described in Patent Document 1 includes a hydrogen charging step in which the buttock is immersed in an acidic solution to charge the test piece with hydrogen, and the entire surface of the test piece is suppressed in order to suppress the release of hydrogen from the test piece. This is a delayed fracture resistance evaluation method for performing a cadmium application step of applying cadmium and a breaking step of pulling both ends of a test piece to break a notch. And in the technique described in patent document 1, it has the characteristics in changing the shape W and L of a test piece, and immersion time T, and evaluating delayed fracture resistance, and the delayed fracture resistance of steel materials is accurate and simple. It can be evaluated.

  Patent Document 2 describes a delayed fracture resistance evaluation method for a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more. In the technique described in Patent Document 2, a test piece is collected from a steel plate that is a material of a plated steel plate, the test piece is immersed in an electrolytic solution, and hydrogen generated by electrolysis of the electrolytic solution is introduced into the test piece. A hydrogen introduction step, a plating layer forming step for forming a plating layer mainly composed of zinc in a thickness of 5 to 100 μm on a test piece into which hydrogen is introduced, and a test piece on which the plating layer is formed, 450 to 600 ° C. A hydrogen diffusion treatment process for performing hydrogen diffusion treatment for 5 to 600 s, and a delayed fracture resistance test process for performing delayed fracture resistance test on the test piece subjected to hydrogen diffusion treatment, The delayed fracture resistance can be evaluated quickly and easily.

  Patent Document 3 describes a method for evaluating hydrogen embrittlement of a thin steel plate. The technique described in Patent Document 3 uses a hydrogen embrittlement evaluation apparatus for a thin steel plate comprising an electrolytic cell, a constant load generating means, and a current generating means, while performing hydrogen charging on a thin steel sheet test piece, This is a method for evaluating hydrogen embrittlement of a thin steel plate in which tensile stress is applied. In the technique described in Patent Document 3, a jig for transmitting stress from a constant load generating means to a thin steel plate test piece and an electrode for performing hydrogen charging on the thin steel plate test piece are provided in the electrolytic cell. It is said that the hydrogen embrittlement characteristics of a thin steel plate can be accurately evaluated by using a partial zirconia or sialon as a support pin for connecting the tool and the thin steel plate test piece.

JP 2005-351693 A JP 2008-203218 A JP 2009-69004 A

Matsuyama Junsaku: Delayed destruction, Nikkan Kogyo Shimbun, 1989, PP.158-181

  In the technique described in Patent Document 1, cadmium is applied to the entire surface of the test piece in order to suppress the release of hydrogen charged in the test piece from the test piece. The applied cadmium suppresses the entry and exit of hydrogen into the steel sheet. However, when the applied cadmium immerses the test piece in the corrosion test solution, it causes a chemical reaction with the test solution, resulting in delayed fracture characteristics of the test steel. There was a problem that it was difficult to evaluate the legitimately.

  Moreover, in the technique described in Patent Document 2, the plating layer is formed after charging the test piece with hydrogen generated by electrolysis of the electrolytic solution. Therefore, in addition to charged hydrogen, delayed fracture characteristics are evaluated including hydrogen that has penetrated during the formation of the plating layer, and it is possible to clearly evaluate only the effect of the amount of hydrogen during hydrogen charging on delayed fracture characteristics. There was a problem that I could not.

In addition, the technique described in Patent Document 3 is not intended to evaluate delayed fracture characteristics in a surface-treated steel sheet, and consideration is given until the effect of the surface treatment film on the delayed fracture characteristics can be evaluated. There was no problem.
The present invention solves such problems of the prior art, appropriately reflects the hydrogen penetration characteristics of the film (surface treatment film) formed on the surface, and delayed fracture characteristics of ultra high strength surface treated steel sheet with a tensile strength of 1180 MPa or more. It is an object of the present invention to provide a test piece for evaluating delayed fracture characteristics of an ultra-high-strength surface-treated steel sheet that can contribute to accurate evaluation.

  In addition, the hydrogen penetration characteristics evaluation method for ultra-high-strength surface-treated steel sheets that can properly evaluate the hydrogen penetration characteristics and delayed fracture characteristics of ultra-high-strength surface-treated steel sheets, and the delay of ultra-high-strength surface-treated steel sheets. An object is to provide a method for evaluating fracture characteristics.

  In order to achieve the above-mentioned object, the present inventors, when evaluating the delayed fracture characteristics of the ultra-high-strength surface-treated steel sheet, penetrated the test piece from the test solution when the test piece was immersed in the test solution. We have intensively studied the penetration behavior of hydrogen that contributes to the occurrence of fracture. As a result, it came to mind that the amount of hydrogen that penetrates into the test piece invades from the end surface of the test piece is significantly larger than the amount of hydrogen that enters from the front and back surfaces of the test piece. This is because the end surface of the test piece is finished by shearing or machining and is an active surface, so that hydrogen is likely to enter. Therefore, when a film is formed on a surface such as a surface-treated steel sheet, the amount of hydrogen entering from the end face is larger than the amount of hydrogen entering through the surface film, and thus obtained. Test results (delayed fracture characteristics) will not accurately reflect the effect of the surface coating on delayed fracture characteristics.

  Therefore, in order to accurately evaluate the delayed fracture characteristics of the surface-treated steel sheet, it is necessary to suppress the intrusion of hydrogen from the end face of the test piece and to infiltrate only from the surface on which the surface film is formed. The inventors have come up with the idea that the end face of the test piece to be used is covered with a coating film that does not allow hydrogen permeation. From further experiments, it was found that a coating film formed by applying a synthetic resin is good as a coating film to be coated on the end face of the test piece.

The present invention has been completed on the basis of such findings and further studies. That is, the gist of the present invention is as follows.
(1) Tensile strength: A test piece for evaluating delayed fracture characteristics composed of a super-high strength surface-treated steel sheet having an ultra-high strength of 1180 MPa or more. The end surface of the test piece is specified in JIS B 0601-2001. Test piece for evaluating delayed fracture characteristics of ultra-high-strength surface-treated steel sheet, characterized in that the surface has a surface roughness of 6.3 μm or less with an arithmetic average roughness Ra, and further has a synthetic resin coating on the end face surface .
(2) The test piece described in (1) is immersed in a test solution in an unloaded state, hydrogen is charged in the test piece, and the amount of hydrogen charged in the test piece within a predetermined time is measured. A method for evaluating hydrogen penetration characteristics of an ultra-high-strength surface-treated steel sheet, characterized by evaluating hydrogen penetration characteristics based on the amount of hydrogen.
(3) When the delayed fracture characteristics evaluation test piece described in (1) is immersed in a test solution and a tensile stress due to a constant load is applied to evaluate delayed fracture characteristics, a predetermined central portion of the test piece is determined. A region is immersed in the test solution held in a container, the tensile stress is applied from both ends of the test piece not immersed in the test solution, a fracture time is obtained, and delayed fracture characteristics are evaluated. Method for evaluating delayed fracture characteristics of ultra-high strength surface-treated steel sheets.
(4) The specimen for evaluating delayed fracture characteristics described in (1) is immersed in a test solution under a bending stress, the fracture time is obtained, and the delayed fracture characteristics are evaluated. Method for evaluating delayed fracture characteristics of ultra-high strength surface-treated steel sheets.

  According to the present invention, it is possible to appropriately evaluate the delayed fracture characteristics of an ultra-high-strength surface-treated steel sheet having a tensile strength of 1180 MPa or more, including the effect of the surface coating, and can greatly contribute to the expansion of use of the ultra-high-strength surface-treated steel sheet. Has an exceptional effect. In addition, according to the present invention, the delayed fracture characteristics of an ultra-high-strength surface-treated steel sheet used in a severe corrosive environment can be appropriately evaluated. For example, an accident in which delayed fracture occurs during use in an automotive part or the like can occur. There is also an effect that can be prevented.

It is explanatory drawing which shows typically an example of the general shape of the test piece for constant load delayed fracture characteristic evaluation, and the condition with which it mounted | wore with the constant load delayed fracture machine. It is explanatory drawing which shows the outline of the shape of the test piece used for evaluation of a hydrogen penetration | invasion characteristic. It is explanatory drawing which shows the shape of a 4-point bending test piece typically. It is explanatory drawing which shows typically the outline of the bending jig | tool used for a 4-point bending delay fracture test, and the condition which mounted | worn the test piece to the bending jig | tool. It is explanatory drawing which shows the dimension shape of the constant load delayed fracture test piece used in the Example. It is a graph which shows the constant load delay fracture characteristic evaluation test result obtained in Example 2 by the relationship between load stress and fracture time.

  The shape of the test specimen for delayed fracture property evaluation of the ultra-high strength surface-treated steel sheet according to the present invention has various shapes corresponding to the delayed fracture test, but at least on the end surface of the part immersed in the test solution of the test specimen. Forms a synthetic resin coating to prevent hydrogen from entering. An example of the test piece for evaluating delayed fracture characteristics of the present invention is shown in FIG. FIG. 1 is an example of a test piece used for a constant load type delayed fracture evaluation test. The test piece 1 is immersed in a test solution 8 held in a container 7, and a synthetic resin is formed on the end surface of the immersed part. A coating film 5 is formed.

In the test piece for evaluation of delayed fracture characteristics according to the present invention, first, in order to improve the adhesion of the synthetic resin coating film, at least the surface of the end surface on which the synthetic resin coating film is formed has an arithmetic average roughness defined in JIS B 0601-2001. The surface has a surface roughness of 6.3 μm or less in thickness Ra.
The end face of the test piece is adjusted to a surface roughness in an appropriate range of Ra of 6.3 μm or less in order to form the formed synthetic resin coating film with sufficient adhesion. If the surface roughness of the test piece end surface is larger than 6.3μm in Ra, it is necessary to thicken the coating, and it is difficult to dry. Easy to peel. For this reason, the surface roughness of the end face of the test piece was limited to 6.3 μm or less in Ra. On the other hand, if the Ra is less than 0.1 μm, the end face becomes too smooth, and the adhesion of the coating film formed on the surface decreases. For this reason, the surface roughness of the end face of the test piece is desirably 0.1 μm or more in Ra.

In addition, as a means for adjusting the surface roughness of the end surface to form the synthetic resin coating film, it is not necessary to specifically limit, but cutting, grinding, electric discharge, sandpaper, etc. by machining such as a milling machine or a lathe It can be adjusted to the end face surface having the above-described surface roughness by a processing method such as manual polishing.
After adjusting to the above-described surface roughness, a synthetic resin coating film is formed on the surface of the end face of the test piece. The method for forming the synthetic resin coating film is not particularly limited, but is preferably formed by coating or the like from the viewpoint of simplicity. The type of synthetic resin used for forming the coating film is not particularly limited, but is preferably a thermosetting resin from the viewpoint of simplicity. In addition, if it is a thermosetting resin, a silicon resin, an epoxy resin, a modified epoxy resin, a tar epoxy resin etc. can be illustrated.

When the synthetic resin to be used is, for example, a thermosetting resin, it is preferably applied to the end face of the test piece using a brush or the like and cured by heating to have a predetermined coating film thickness. In addition, it is preferable from a viewpoint of the peeling resistance of a coating film that the thickness of a coating film shall be 50-700 micrometers (dry thickness).
If a delayed fracture test is performed using the above-described test piece of the present invention, hydrogen penetration from the end face can be prevented, and the delayed fracture resistance of the ultra-high strength steel sheet is sufficiently adequately evaluated in consideration of the effect of the surface treatment film. it can. Especially for surface-treated steel sheets, the effect of the coating on delayed fracture resistance can be properly evaluated.

  Next, if a constant load delayed fracture test piece with a synthetic resin coating film formed on the end face in the shape shown in FIG. 1 is mounted on a constant load delayed fracture tester and a delayed fracture test is performed, hydrogen penetration from the end face And the influence of the surface treatment film on the delayed fracture characteristics of the ultra high strength surface treated steel sheet can be appropriately evaluated. In this case, as shown in FIG. 1, it is preferable to immerse a predetermined region at the center of the test piece 1, that is, the vicinity of the parallel portion, in the test solution 8 held in the container 7. According to this, predetermined various tensile stresses can be applied from both ends not immersed in the test solution, and the constant load delayed fracture test is simple and easy. Needless to say, packing 6 is attached to the container 7 so that the retained test solution 8 does not leak.

  Further, for example, a four-point bending test piece having the shape shown in FIG. 3 with the synthetic resin coating 5 formed on the end face is mounted on the four-point bending jig shown in FIG. 4 and a predetermined bending stress is applied to the test piece 1. In this state, if immersed in a test solution for a predetermined time and conducting a four-point bending delayed fracture test to investigate the state of delayed fracture occurrence, hydrogen penetration from the end face can be prevented and delayed fracture of ultra-high strength surface-treated steel sheet The influence of the surface treatment film on the characteristics can be appropriately evaluated.

In this four-point bending jig, the test piece (four-point bending test piece) 1 is fixed to the test table 3 at four points, and the load stress on the test piece can be adjusted by the tightening amount of the screw 4. Needless to say, the test piece 1 and the test table 3 are insulated by the glass insulating jig 2. The test piece is immersed in a test solution in a state where it is set on a test stand and used for the test.
Moreover, if the above-mentioned test piece of the present invention is used, the hydrogen penetration property of the surface-treated steel sheet can be properly evaluated. For example, a test piece having the shape shown in FIG. 2 is collected from the surface-treated steel sheet, a synthetic resin coating film is formed on the end face of the test piece, immersed in the test solution without load, and held for a predetermined time. By analyzing the amount of hydrogen that has penetrated into the surface, the influence of the surface treatment film on the hydrogen penetration properties of the surface-treated steel sheet can be properly evaluated.

  In addition, it is preferable that the test solution used in each evaluation test described above is, for example, hydrochloric acid adjusted to pH: 3.0 or less or a buffer solution having pH = 3.0 or less. The type of buffer solution is not particularly limited, and examples thereof include hydrochloric acid-sodium acetate buffer solution and citrate buffer solution. Moreover, although the ammonium thiocyanate buffer solution has a pH of about 5, it can be said that it is preferable as a test solution for the surface-treated steel sheet because it suppresses dissolution of the plating layer.

In addition, the immersion time (test time) in the test solution is not particularly limited, but the immersion time (test time) is about 100 h at the longest because the hydrogen penetration amount is saturated at a certain immersion time. It is preferable to do this.
In addition, although it is not evaluation of the delayed fracture characteristic of the surface-treated steel sheet, a synthetic resin coating film is formed in a region other than the evaluation region even in the test piece used in the delayed fracture property evaluation test method described in Non-Patent Document 1. Thus, hydrogen intrusion from other than the evaluation region can be suppressed, and the delayed fracture characteristics of the evaluation region can be appropriately evaluated.

Example 1
In Table 1, an ultra-high strength steel plate (thickness: 1.2 mm) showing composition and tensile properties was used as a raw material.
First, from the ultrahigh strength surface-treated steel sheet A having the composition and tensile properties shown in Table 1, a constant load delayed fracture test piece having the dimensions shown in FIG. 5 was collected. The surface of the end face of the obtained test piece was adjusted so that the arithmetic average roughness Ra was a surface roughness in the range of 0.1 to 25.0 μm by changing the sandpaper count. The steel plate A is an ultra-high strength surface-treated steel plate having Zn—Ni plating formed on the surface.

Next, a modified epoxy resin (trade name: Tough Primer, manufactured by NATCO Co., Ltd.) was applied to the end surface with the adjusted surface roughness with a brush so that the dry film thickness was about 300 μm. A drying treatment at 20 ° C. for 20 minutes was performed.
The obtained constant load delayed fracture test piece was set in a constant load tester. In addition, as shown in FIG. 1, the container 7 holding the test solution 8 was attached so that only the parallel part of the test piece was immersed in the test solution. The test solution used was a hydrochloric acid-sodium acetate buffer adjusted to pH = 1, the test stress was 967 MPa, and the coating peeling time was observed up to 100 h.

  The obtained results are shown in Table 2.

The examples of the present invention in which the arithmetic average roughness Ra of the end face is 6.3 μm or less have high adhesion, and the coating film is not peeled even when immersed in the test solution for 100 hours or longer. On the other hand, in the comparative example outside the scope of the present invention, the coating film peels off in less than 100 hours.
(Example 2)
Three types of steel sheets having the compositions shown in Table 1 and having substantially the same tensile properties and having different surface treatment films on the surface were used as materials. Steel plate A is an ultra-high strength surface-treated steel plate having a Zn-Ni plating film on its surface, and steel plate B is an ultra-high strength steel plate without a surface treatment film. Steel plate C is an ultra-high strength surface-treated steel plate having an Al-Si plating film on the surface.

  From these materials, the constant load delayed fracture test pieces having the dimensions shown in FIG. 5 were collected. And the end surface of the obtained test piece was sandpaper finished and adjusted so that the arithmetic average roughness Ra was about 1.6 μm. Next, in the same manner as in Example 1, a modified epoxy resin (trade name: Tough Primer, manufactured by NATCO Co., Ltd.) was applied to the end surface with a brush so that the dry film thickness was about 300 μm. After the application, a drying process was performed at 130 ° C. for 20 minutes. In some of the test pieces, the synthetic resin coating film was not formed on the end face.

As in Example 1, the obtained constant load delayed fracture test piece was set in a constant load test machine so that the parallel portion was immersed in the test solution, and the test piece was broken by applying various tensile stresses. (Breaking time) was determined. The test solution was a hydrochloric acid-sodium acetate buffer adjusted to pH = 1 in the same manner as in Example 1. The test time was up to 100h.
The results obtained are shown in Table 3 and are summarized in FIG.

  From FIG. 6, in the example of the present invention in which the synthetic resin coating is formed on the end face, the fracture time differs greatly depending on the type of the surface treatment coating, and the delayed fracture characteristics are greatly different compared with the same load stress. Recognize. Although the delayed fracture characteristics vary greatly depending on the type of surface treatment film, the longest rupture time is obtained when the Zn-Ni plating film is formed. It can be seen that the delayed fracture resistance is good.

  In addition, when the synthetic resin coating was not formed on the end face, the fracture time was almost the same regardless of the surface treatment coating formed on the surface, and the effect of the type of surface treatment coating on delayed fracture characteristics Is not clear.

DESCRIPTION OF SYMBOLS 1 Test piece 2 Glass insulation jig 3 Test stand 4 Screw 5 Synthetic resin coating film 6 Rubber stopper 7 Test liquid holding container (container)
8 Test solution

Claims (4)

  Tensile strength: A test piece for evaluating delayed fracture characteristics made of an ultra-high-strength surface-treated steel sheet having an ultra-high strength of 1180 MPa or more, wherein the end surface of the test piece is subjected to arithmetic defined in JIS B 0601-2001. A test piece for evaluating delayed fracture characteristics of an ultra-high-strength surface-treated steel sheet, characterized by having a surface having an average roughness Ra of 6.3 μm or less and further having a synthetic resin coating on the end surface.   The test piece for evaluating delayed fracture characteristics according to claim 1 is immersed in a test solution in an unloaded state, hydrogen is charged in the test piece, and the amount of hydrogen charged in the test piece within a predetermined time is calculated. A method for evaluating hydrogen penetration characteristics of an ultra-high-strength surface-treated steel sheet, characterized by measuring and evaluating hydrogen penetration characteristics based on the measured amount of hydrogen.   When evaluating the delayed fracture characteristics by immersing the test specimen for evaluating delayed fracture characteristics according to claim 1 in a test solution and applying a tensile stress by a constant load, a predetermined region at the center of the test specimen is a container. Super high strength characterized in that it is immersed in the test solution held in the test solution, the tensile stress is applied from both ends of the test piece not immersed in the test solution, the fracture time is obtained, and the delayed fracture property is evaluated. Method for evaluating delayed fracture characteristics of surface-treated steel sheets.   A test piece for evaluating delayed fracture characteristics according to claim 1 is immersed in a test solution under a bending stress, a fracture time is obtained, and delayed fracture characteristics are evaluated. Evaluation method of delayed fracture characteristics of surface treated steel sheet.

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