JP4028461B2 - Method for evaluating delayed fracture characteristics and plating method performed at that time - Google Patents

Description

  This invention has delayed fracture characteristics that reduce adverse effects on the environment and the human body, have an excellent ability to encapsulate hydrogen in steel, and can identify a larger amount of critical diffusible hydrogen than conventional methods. The present invention relates to an evaluation method and a plating method performed at that time.

  It is widely known that it is effective to use a critical diffusible hydrogen amount as a method for evaluating delayed fracture. For example, as described in Patent Document 1 and Patent Document 2, the evaluation of delayed fracture resistance of steel using a critical diffusible hydrogen amount is linked to the development of a material having excellent delayed fracture resistance.

  As described in Patent Document 1 and Patent Document 2, this limit diffusible hydrogen amount is such that hydrogen is released from the steel during the delayed fracture test after various amounts of hydrogen are contained in the steel. In order to prevent this, the steel material is subjected to cadmium plating, and then subjected to a constant load test in the atmosphere, and the time until the steel material breaks is measured.

  However, cadmium plating is used as a means of preventing hydrogen release from steel during delayed fracture testing, which is an indispensable element for identifying the critical diffusible hydrogen content, and this cadmium has a great impact on the environment. It has been pointed out that the problem of giving and severely adversely affecting the human body.

  In order to cope with this problem, as shown in Non-Patent Document 1, as an alternative to cadmium plating, galvanization that reduces adverse effects on the environment and the human body has been proposed. However, Non-Patent Document 1 discloses that only zinc plating is performed using a zinc sulfate bath, and there is no detailed disclosure of the composition and plating conditions of the plating bath. Therefore, it is difficult to determine an appropriate plating bath composition and plating conditions that provide sufficient hydrogen entrapment capacity in steel necessary to identify the limit diffusible hydrogen content.

  Moreover, in the test for identifying the limit diffusible hydrogen amount by utilizing the above cadmium plating and zinc plating by a zinc sulfate bath, as shown in Table 2 of Patent Document 2, the limit is 2.10 mass ppm at the maximum. There are only examples in which the amount of diffusible hydrogen has been obtained, and in the studies by the present inventors, in these conventional methods, when the critical diffusible hydrogen amount of the steel material is greater than 2.10 mass ppm, the steel material is subjected to the delayed fracture test. It was found that hydrogen was released from the steel and the critical diffusible hydrogen content of the steel material could not be identified.

Japanese Patent Laid-Open No. 11-270531 JP 2001-288538 A “Materials and Processes” Japan Steel Association, Vol. 13, 2000, p. 1376

  As described above, the conventional technique has the following problems. A special facility capable of handling cadmium and an organization with special know-how regarding plating bath composition and plating conditions for plating used to prevent hydrogen release, which is an indispensable element for identifying the critical diffusible hydrogen content In addition, the critical diffusible hydrogen amount exceeding 2.10 mass ppm could not be identified.

  Therefore, the present invention has been made to solve the above-described problems, and the object thereof is to reduce the adverse effects on the environment and the human body, and to have an excellent ability to enclose hydrogen in steel, An object of the present invention is to provide a method for evaluating delayed fracture characteristics and a plating method performed at that time, which can identify a larger amount of critical diffusible hydrogen than conventional methods.

  In order to solve the above-described problems of the prior art, the inventors of the present application investigated in detail the plating bath composition and plating conditions. As a result, zinc plating using an ammonium chloride bath reduces adverse effects on the environment and the human body compared to cadmium plating, which is a conventional technique, and hydrogen in steel than zinc plating using a cadmium plating or zinc sulfate bath. The knowledge that the encapsulation ability is excellent was obtained.

  The present invention has been made on the basis of the above-mentioned findings, and is characterized by the following.

  The invention according to claim 1 is characterized in that the delayed diffusive hydrogen is contained in the steel material and the amount of critical diffusible hydrogen is measured to evaluate the delayed fracture property of the steel material. In order to prevent hydrogen from being released from the steel during the amount measurement, the steel is characterized in that it is zinc plated using an ammonium chloride bath containing zinc chloride, ammonium chloride and a brightener. Is.

  The invention according to claim 2 is the invention according to claim 1, wherein the ammonium chloride bath has a zinc chloride content of 20 to 70 g / L, an ammonium chloride content of 140 to 230 g / L, and a brightener content. Is characterized by being 15 to 50 mL / L.

  The invention described in claim 3 is characterized in that, in the invention described in claim 1 or 2, the bath temperature of the ammonium chloride bath is 10 to 45 ° C.

  The invention described in claim 4 is characterized in that, in the invention described in any one of claims 1 to 3, the pH of the ammonium chloride bath is 5.0 to 6.5.

The invention according to claim 5 is characterized in that, in the invention according to any one of claims 1 to 4, the cathode current density during galvanization is 0.3 to 6 A / dm ^2. is there.

  The invention described in claim 6 is characterized in that, in the invention described in any one of claims 1 to 5, the plating time is 1 minute or more.

  The invention according to claim 7 prevents the release of hydrogen from the steel material when evaluating the delayed fracture characteristics of the steel material by making the steel material contain diffusible hydrogen and measuring the amount of critical diffusible hydrogen. Therefore, the steel material is characterized by being galvanized using an ammonium chloride bath containing zinc chloride, ammonium chloride, and a brightener.

  According to this invention, compared with the conventional method for evaluating delayed fracture using cadmium plating, adverse effects on the environment and the human body are reduced, and cadmium plating and zinc plating using a zinc sulfate bath, which are conventional techniques, are used. More limit diffusible hydrogen amounts than 2.10 mass ppm, which could not be identified by the delayed fracture evaluation method, can be identified, and further, limit diffusible hydrogen amounts can be identified by the low strain rate test method. Compared with the constant load test method, it is possible to evaluate in a short time and to bring a useful effect that only a small number of samples are prepared.

  Next, the reason for limiting the plating bath composition in this invention will be described.

(Zinc chloride)
Zinc chloride is added to provide zinc metal, which is the main component of galvanization. However, if it is less than 20 g / L, the cathode current efficiency is lowered, dense plating is not formed on the entire surface of the steel material, and hydrogen in the steel is reduced. Loss the encapsulation ability. On the other hand, when it exceeds 70 g / L, the throwing power deteriorates, it becomes impossible to give a dense plating to the entire surface of the steel material, and the ability to enclose hydrogen in steel is lost. Therefore, the amount of zinc chloride added is limited to 20 to 70 g / L, more preferably 30 to 60 g / L.

(Ammonium chloride)
Ammonium chloride is added to give appropriate uniform electrodeposition and cathodic current efficiency and to give a dense plating on the entire surface of the steel material. However, if it is less than 140 g / L, the plating becomes coarse without being dense, Loss the ability to contain hydrogen in steel. On the other hand, if it exceeds 230 g / L, the cathode current efficiency decreases and the plating does not become dense, so that the ability to enclose hydrogen in steel is lost. Therefore, the amount of ammonium chloride added is limited to 140 to 230 g / L, more preferably 150 to 220 g / L.

(Brightener)
The brightener is added mainly to improve the throwing power. However, if it is less than 15 mL / L, the throwing power is inferior and it is impossible to give dense plating on the entire surface. Lose. On the other hand, even when it exceeds 50 mL / L, the ability to enclose hydrogen in steel is lost because it is inferior in throwing power and cannot be provided with dense plating on the entire surface. Therefore, the addition amount of the brightener is limited to 15 to 50 mL / L, more preferably 25 to 40 mL / L.

  Next, the reason for limiting the plating conditions of the present invention will be described.

(Bath temperature)
The bath temperature is controlled mainly to produce a dense galvanizing, but if it is less than 10 ° C., pits are generated during the galvanizing and the ability to enclose hydrogen in the steel is lost. On the other hand, even when the temperature exceeds 45 ° C., pits are generated during galvanization, and the ability to enclose hydrogen in steel is lost. Accordingly, the bath temperature is limited to 10 to 45 ° C, more preferably 20 to 35 ° C.

(PH)
The pH is controlled mainly to produce a dense galvanizing, but if it is less than 5.0, pits are generated in the galvanizing and the ability to enclose hydrogen in steel is lost. On the other hand, when it exceeds 6.5, pits are generated during galvanization, and the ability to enclose hydrogen in steel is lost. Accordingly, the pH is limited to 5.0 to 6.5, more preferably 5.5 to 6.2.

(Cathode current density)
Cathode current density is managed mainly to produce a dense galvanizing, but when it is less than 0.3 A / dm ² , pits are generated during galvanizing, and the ability to enclose hydrogen in steel is lost. On the other hand, when it exceeds 6 A / dm ² , pits are generated during galvanization and the ability to enclose hydrogen in steel is lost. Accordingly, the cathode current density during the plating 0.3~6A / dm ^2, more preferably, is limited to 0.5~5A / dm ^2.

(Plating time)
The plating time is controlled in order to produce a plating having a thickness necessary for hydrogen encapsulation, but since the plating thickness is thin in less than 1 minute, hydrogen cannot be encapsulated for a sufficient time. Therefore, the plating time was limited to 1 minute or more.

  Next, the present invention will be further described with reference to examples.

  A test piece was cut from a 580 MPa grade tempered steel, and hydrogen was introduced into each test piece (Example Nos. 1 to 10 and Comparative Examples 11 to 24) by cathodic hydrogen charging. Each test piece was galvanized under the conditions, and the plating was peeled off immediately after plating or after 150 hours had elapsed from the end of plating, immediately followed by a thermal desorption hydrogen analysis test, and the amount of diffusible hydrogen in each test piece Was measured by gas chromatography. The results are shown in the same table.

  The amount of diffusible hydrogen was the amount of hydrogen released from room temperature to 300 ° C. in the hydrogen release curve obtained from the temperature programmed desorption hydrogen analysis test shown as an example in FIG. The ability to enclose hydrogen in steel by plating was judged as acceptable when 90% or more of hydrogen was retained after 150 hours from the end of plating with respect to the amount of hydrogen immediately after plating. As the brightener, Asahi Zincol (registered trademark) NR-1 and NR-2 (manufactured by Uemura Kogyo Co., Ltd.) were used for the ammonium chloride bath.

  As is apparent from Table 1, Example No. of the present invention plated by this invention. All of 1 to 10 were filled with a sufficient amount of hydrogen.

  In contrast, Comparative Example No. In all of Nos. 11 to 24, the amount of hydrogen enclosed was smaller than that of the example of the present invention.

  That is, the comparative example No. in which the zinc chloride concentration is less than the lower limit of the range of the present invention. 11 and Comparative Example No. 1 in which the zinc chloride concentration exceeds the upper limit of the range of the present invention. No. 12 had a small amount of hydrogen enclosed.

  Comparative Example No. Ammonium chloride concentration is less than the lower limit of the range of the present invention. 13 and Comparative Example No. in which the ammonium chloride concentration exceeds the upper limit of the range of the present invention. No. 14 had a small amount of hydrogen enclosed.

  Comparative Example No. in which the brightener concentration is less than the lower limit of the range of the present invention. 15 and Comparative Example 16 in which the brightener concentration exceeded the upper limit of the range of the present invention all had a small amount of hydrogen.

  Comparative example No. whose bath temperature is less than the lower limit of the range of the present invention. 17 and comparative example No. in which the bath temperature exceeds the upper limit of the range of the present invention. No. 18 had a small amount of hydrogen enclosed.

  Comparative Example No. whose pH is less than the lower limit of the range of the present invention. 19 and Comparative Example 20 in which the pH exceeded the upper limit of the range of the present invention each had a small amount of hydrogen enclosed.

  Comparative Example No. whose cathode current density is less than the lower limit of the range of the present invention. 21 and Comparative Example No. in which the cathode current density exceeds the upper limit of the range of the present invention. No. 22 had a small amount of hydrogen enclosed.

  Comparative Example No. in which the plating time is less than the lower limit of the range of the present invention. In both Nos. 23 and 24, the amount of hydrogen enclosed was small.

Next, FIG. 2, FIG. 3 and FIG. The result of having identified the limit diffusible hydrogen amount of 580MPa grade tempered steel under the plating bath composition of 3 and plating conditions is shown. Fig. 2 shows the case of the constant load test method. After plating a sample (test piece) containing various amounts of hydrogen, a tensile stress of 80% of the tensile strength was applied, and fracture occurred from the start of load application. The amount of diffusible hydrogen at the upper limit at which no delayed fracture occurs for 100 hours or more from the start of loading is defined as the limit diffusible hydrogen amount. FIG. 3 shows a case where a tensile stress of 90% of the tensile strength is applied in the constant load test method of FIG. FIG. 4 shows a case by a low strain rate test method (SSRT method: Slow Strain Rate Technique). After plating a sample containing various amounts of hydrogen, the crosshead speed is 1 × 10 ⁻³ mm / min. The tensile stress was applied at a low strain rate, the breaking stress was measured, and the amount of diffusible hydrogen corresponding to the breaking stress was defined as the limit diffusible hydrogen amount corresponding to the breaking stress.

  As a result, it was found that the critical diffusible hydrogen content can be identified by any of the constant load test method shown in FIGS. 2 and 3 and the low strain rate test method shown in FIG. Further, the critical diffusible hydrogen amount corresponding to 80% or 90% of the tensile strength in the low strain rate test method corresponds to the value of 80% or 90% of the tensile strength in the constant load test method, respectively. For this reason, many samples with different amounts of hydrogen can be prepared, and instead of the constant load test method, which requires measurement over a long period of time, measurement can be performed in a short time and the number of samples can be reduced more quickly. It was found that the low strain rate test method can be used as a simple method for evaluating the limit diffusible hydrogen content.

It is a graph which shows an example of the hydrogen release curve calculated | required by the temperature-programmed desorption-type hydrogen analysis test. It is a graph which shows the result of having identified the critical diffusible hydrogen amount of 580MPa class tempered steel by applying the tensile stress of 80% of tensile strength, and using the constant load test method, using the delayed fracture evaluation method according to the present invention. . It is a graph which shows the result of having identified the critical diffusible hydrogen content of 580MPa grade tempered steel by applying the tensile stress of 90% of tensile strength by the constant load test method using the delayed fracture evaluation method according to the present invention. . It is a graph which shows the result of having identified the critical diffusible hydrogen amount of 580MPa grade tempered steel by the low strain rate test method using the delayed fracture evaluation method by this invention.

Claims (7)

In the method for evaluating delayed fracture characteristics of evaluating the delayed fracture characteristics of the steel by including diffusible hydrogen in the steel and measuring the critical diffusible hydrogen content,
In order to prevent hydrogen from being released from the steel during the measurement of the critical diffusible hydrogen content, the steel is galvanized using an ammonium chloride bath containing zinc chloride, ammonium chloride and a brightener. A method for evaluating delayed fracture characteristics.   The ammonium chloride bath has a zinc chloride content of 20 to 70 g / L, an ammonium chloride content of 140 to 230 g / L, and a brightener content of 15 to 50 mL / L. Item 2. The method for evaluating delayed fracture characteristics according to Item 1.   The method for evaluating delayed fracture characteristics according to claim 1 or 2, wherein the bath temperature of the ammonium chloride bath is 10 to 45 ° C.   The method for evaluating delayed fracture characteristics according to any one of claims 1 to 3, wherein the pH of the ammonium chloride bath is 5.0 to 6.5. 5. The method for evaluating delayed fracture characteristics according to claim 1, wherein a cathode current density during the zinc plating is 0.3 to 6 A / dm ² .   The method for evaluating delayed fracture characteristics according to any one of claims 1 to 5, wherein the plating time is 1 minute or more.   In order to prevent the release of hydrogen from the steel when the delayed fracture characteristics of the steel are evaluated by making the steel contain diffusible hydrogen and measuring the amount of critical diffusible hydrogen, zinc chloride, ammonium chloride A plating method for evaluating delayed fracture characteristics, characterized in that the steel material is galvanized using an ammonium chloride bath containing a brightener.

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