JP2015059880A - Method of estimating hydrogen-induced cracking resistance of calcium-added steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To promptly estimate hydrogen-induced cracking resistance of calcium-added steel from compositions and composition distribution state of nonmetallic enclosures.SOLUTION: A hydrogen-induced cracking resistance estimation method of the present invention includes: observing 20 mmor more of a mirror polishing surface of a calcium-added steel with a scanning electron microscope; detecting nonmetallic enclosures at sizes equal to or larger than a predetermined size; analyzing a composition of each of the detected nonmetallic enclosures with an EDS; calculating a CaO fraction (mass%) and an AlOfraction (mass%) of each nonmetallic enclosure; calculating a ratio of CaO to AlO((mass% CaO)/(mass% AlO)) from the calculated CaO fraction and AlOfraction; counting particles of the nonmetallic enclosure for which the ratio ((mass%CaO)/(mass%AlO)) is not less than an arbitrary value selected from within a range from 1.0 to 10.0; and estimating hydrogen-induced cracking resistance of the calcium-added steel using a relational expression between an obtained particle count and a hydrogen-induced cracking resistance.

Description

  The present invention relates to a sample taken from molten steel to which calcium has been added, a slab produced by continuous casting of molten steel to which calcium has been added, a rolled steel obtained by rolling this slab, or a pipe made from this rolled steel. The present invention relates to a method for estimating the resistance to hydrogen-induced cracking from the composition and distribution of non-metallic inclusions in a steel pipe obtained as described above.

  Steel products include oxide-based non-metallic inclusions originating from deoxidation products and slag, sulfide-based non-metallic inclusions in which sulfur in steel reacts with manganese, etc. There are various non-metallic inclusions such as nitride-based non-metallic inclusions that are precipitated and generated when nitrogen reacts with aluminum or the like. Here, oxide-based non-metallic inclusions, sulfide-based non-metallic inclusions, nitride-based non-metallic inclusions, and the like are collectively referred to as non-metallic inclusions.

  Non-metallic inclusions present in the steel deteriorate the properties of the steel product. For example, in UOE steel pipes and electric resistance welded steel pipes used as line pipe materials for oil transportation and natural gas transportation, non-metallic inclusions, particularly sulfides that are deformable during rolling due to the action of sour gas. Hydrogen-induced cracking (also referred to as “HIC: Hydrogen Induced Cracking”) starts from non-metallic inclusions.

  Therefore, in steel products that require hydrogen-induced cracking resistance, in order to prevent the formation of manganese-sulfide (MnS), which is a highly extensible sulfide-based nonmetallic inclusion, which causes hydrogen-induced cracking, Calcium (Ca) is added to molten steel to control the form of sulfide-based nonmetallic inclusions in steel to non-stretchable calcium-sulfide (CaS).

By adding calcium to the molten steel, it reacts with the deoxidation product alumina (Al ₂ O ₃ ) due to its strong affinity with oxygen, and CaO—Al ₂ O ₃ nonmetallic inclusions are produced. Is done. When calcium is added to molten steel, if calcium is insufficient, it will not react with sulfur in the steel and MnS will be generated. If calcium is excessive, CaO-Al ₂ O ₃ -based non-metallic inclusions with high CaO concentration will be produced. And each causes deterioration of hydrogen-induced cracking resistance. Therefore, in order to improve the hydrogen-induced cracking resistance characteristics of the steel material, it is necessary to add calcium so that the form of nonmetallic inclusions in the molten steel is controlled to an appropriate composition.

  Based on these findings, methods for controlling not only the molten steel composition but also the nonmetallic inclusion composition have been reported.

  For example, Patent Document 1 proposes a method for analyzing calcium-based nonmetallic inclusions in steel by electrolytic extraction for the purpose of improving hydrogen-induced cracking resistance and preventing clogging of a tundish nozzle of a continuous casting machine. Yes. According to Patent Document 1, non-metallic inclusions having a high CaO concentration that have a large influence on the hydrogen-induced cracking resistance can be reliably extracted without being melted during electrolysis.

  In Patent Document 2, secondary refining is performed on the molten steel after completion of primary refining, and calcium is added to the molten steel after completion of secondary refining according to the oxygen concentration in the molten steel. A method of melting a steel material for high strength and high corrosion resistance oil well pipe excellent in hydrogen-induced crack resistance and sulfide stress cracking resistance by controlling the shape of the material has been proposed. The main components of the nonmetallic inclusions are calcium, aluminum, oxygen and sulfur, the CaO content in the nonmetallic inclusions is 30 to 80%, the CaS content in the nonmetallic inclusions is 25% by mass or less, and Steel pipe steels excellent in hydrogen-induced cracking resistance have been proposed in which the ratio between the nitrogen content in steel and the CaO content in nonmetallic inclusions is within a predetermined range.

JP-A-6-174716 JP 2011-89180 A JP 2009-120899 A

  The evaluation method defined by NACE (National Association of Corrosion Engineers), which is known as a general method for evaluating hydrogen-induced crack resistance, requires that the test piece be immersed in a test solution for 96 hours. Even if an evaluation test is performed using a sample cut out from a piece, the test result is obtained after four or more days have passed since casting. When an evaluation test is performed using a sample cut out from a steel material after hot rolling or after pipe forming, more time is required. For this reason, in the actual manufacturing process, after the result of the hydrogen resistance-induced cracking test is obtained, the process proceeds to the next manufacturing process, or the process is advanced in anticipation. If the induced crack test result is rejected, the manufactured product must be disposed of as a rejected product, and there is a problem in terms of productivity.

  Therefore, a method for predicting the resistance to hydrogen-induced cracking as early as possible has been desired. As this prediction method, it is considered effective to relate the composition and distribution of non-metallic inclusions that are the starting point of hydrogen-induced cracking with the resistance to hydrogen-induced cracking.

  From this point of view, if the prior art is verified, the prior art has the following problems.

  That is, Patent Document 1 is a method for quantifying the average composition of all extracted non-metallic inclusions. As a result of investigation by the present inventors, this method greatly affects the information of coarse non-metallic inclusions. It has been found that the accuracy tends to be excessive, and sufficient accuracy cannot be obtained as a method for evaluating the resistance to hydrogen-induced cracking.

  In addition, Patent Document 2 and Patent Document 3 only describe that non-metallic inclusions are inspected using an electron microscope. Specific details for inspecting non-metallic inclusions using an electron microscope are described. There is no description about the quantitative method.

  The present invention has been made in view of such circumstances. The object of the present invention is a sample taken from molten steel to which calcium is added, a slab produced by continuous casting of molten steel to which calcium is added, and this casting. Prompt estimation of hydrogen-induced cracking resistance in rolled steel obtained by rolling a piece or steel pipe obtained by pipe-making this rolled steel from the composition and distribution of non-metallic inclusions It is an object of the present invention to provide a method for estimating hydrogen-induced cracking resistance of calcium-added steel.

  In order to solve the above-mentioned problems, the present inventors have investigated in detail the state of existence of non-metallic inclusions in calcium-added steel. As a result, it was found that the resistance to hydrogen-induced cracking of calcium-added steel can be accurately estimated by statistically examining the composition and distribution of non-metallic inclusions having a particle size of about 1 μm or more.

Specifically, a scanning electron microscope (SEM; Scanning Electron Microscope) having a particle analysis function that can be suitably used for measurement and evaluation of many nonmetallic inclusions is used. The composition and size of nonmetallic inclusions in the calcium-added steel were investigated using an EDS (Energy Dispersive X-ray Spectrometer) provided in the microscope. Then, individual compositions were examined using non-metallic inclusions having a size of about 1 μm or more as a measurement target, and the ratio of non-metallic inclusions CaO to Al ₂ O ₃ ((mass% CaO) / (mass% Al ₂ By examining the number of non-metallic inclusions whose O ₃ )) is not less than an arbitrary value selected in the range of 1.0 to 10.0, the hydrogen-induced cracking resistance of calcium-added steel can be quickly evaluated. I found out. This is based on the fact that, in calcium-added steel, when calcium is excessive, CaO—Al ₂ O ₃ -based nonmetallic inclusions having a high CaO concentration are generated, which causes deterioration of hydrogen-induced cracking resistance. .

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] A method for estimating hydrogen-resistant cracking characteristics of a calcium-added steel from the composition and distribution of nonmetallic inclusions in the steel,
Mirror-polished surface of a sample taken from the added molten steel calcium, or scanning slab of calcium added steel, a 20 mm ² or more mirror-polished surface of the steel pipe obtained by pipe-making rolling steel or rolled steel as measured region A step of observing with an electron microscope, detecting non-metallic inclusions of a predetermined size or more, and analyzing the composition of each detected non-metallic inclusion with an EDS (energy dispersive X-ray analyzer);
Based on the composition analysis result by EDS, calculating the CaO fraction (mass%) of each non-metallic inclusion by the following formulas (1), (2), and (3):
A step of calculating the Al ₂ O ₃ fraction (% by mass) of each non-metallic inclusion based on the composition analysis result by EDS according to the following formula (4):
From the CaO fraction calculated by the formulas (1) to (3) and the Al ₂ O ₃ fraction calculated by the formula (4), the ratio of CaO and Al ₂ O _{3 of} individual nonmetallic inclusions. Obtaining ((mass% CaO) / (mass% Al ₂ O ₃ ));
Counting the number of non-metallic inclusion particles whose ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) is not less than an arbitrary value selected in the range of 1.0 to 10.0;
Particles per unit area of nonmetallic inclusions whose ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) obtained in advance is not less than an arbitrary value selected in the range of 1.0 to 10.0 A process for estimating hydrogen-induced cracking resistance using a relational expression between the number and hydrogen-induced cracking resistance,
A method for estimating hydrogen-induced cracking resistance characteristics of a calcium-added steel.
S fraction (% by mass) as MnS = [Mn] × [S atomic weight] / [Mn atomic weight] (1)
Ca fraction (% by mass) as CaS = ([S]-[S fraction as MnS]) × [Ca atomic weight] / [S atomic weight] (2)
CaO fraction (mass%) = ([Ca]-[Ca fraction as CaS]) × [CaO atomic weight] / [Ca atomic weight] ... (3)
Al ₂ O ₃ fraction (mass%) = [Al] × [Al ₂ O ₃ atomic weight] / [2 × Al atomic weight] ... (4)
However, [Mn] in the formula (1) is the manganese analysis value (mass%) in the nonmetallic inclusions by EDS, and [S] in the formula (2) is the sulfur analysis value in the nonmetallic inclusions by EDS ( Mass%), [Ca] in the formula (3) is the analytical value of calcium in nonmetallic inclusions by EDS (mass%), and [Al] in the formula (4) is the analysis of aluminum in nonmetallic inclusions by EDS. Value (mass%).
[2] The method for estimating resistance to hydrogen-induced cracking of calcium-added steel according to [1] above, wherein nonmetallic inclusions having a particle size of 1 μm or more are subjected to composition analysis by EDS.

According to the present invention, the number of nonmetallic inclusion particles whose ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) based on the composition analysis result of nonmetallic inclusions in steel is equal to or greater than a threshold, and hydrogen resistance Since the hydrogen-induced crack resistance is estimated from the correlation with the induced crack characteristics, the hydrogen-induced crack resistance of the calcium-added steel can be evaluated more quickly than in the past.

Three samples (Sample A, B, C) is a diagram showing an example of the distribution of the ratio of ((wt% CaO) / (mass% Al ₂ O _3)). Is a graph showing the relationship between the ratio ((wt% CaO) / (mass% Al ₂ O ₃₎₎ is CAR in number and hydrogen induced cracking test of nonmetallic inclusions of 10.0 or more.

  Hereinafter, the present invention will be described in detail.

In calcium-added steel, when calcium is excessively added, a CaO—Al ₂ O ₃ -based nonmetallic inclusion having a high CaO concentration is generated, which becomes a starting point of hydrogen-induced cracking. Therefore, in the present invention, among the non-metallic inclusions present in the steel, the ratio of non-metallic inclusions CaO to Al ₂ O ₃ ((mass% CaO) / (mass% Al ₂ O ₃ )) The resistance to hydrogen-induced cracking of calcium-added steel is estimated from the distribution of non-metallic inclusions that are at least an arbitrary value selected in the range of 1.0 to 10.0.

  Accordingly, in the present invention, it is necessary to investigate the composition of each of the many nonmetallic inclusion particles present in the steel. Moreover, the nonmetallic inclusion in steel may be unevenly distributed. In such a case, a sufficient number of nonmetallic inclusions can be obtained to obtain statistical accuracy, including small nonmetallic inclusions having a particle size of about 1 μm or more, not limited to coarse nonmetallic inclusions. It is important to study particles. At the same time, it is necessary to secure a sufficient test area for evaluating the resistance to hydrogen-induced cracking.

  In view of these, in the present invention, a non-metallic inclusion in steel is investigated using a scanning electron microscope having a particle analysis function, which is suitable for measurement and evaluation of many non-metallic inclusions. It is best to do. In this case, since it is necessary to quantitatively analyze the composition of the nonmetallic inclusions, it is optimal to use a scanning electron microscope equipped with EDS. Using a scanning electron microscope equipped with an EDS and having a particle analysis function, a relatively wide area can be measured, and the composition of thousands to tens of thousands of nonmetallic inclusion particles is automatically set. Can be investigated. The pretreatment of the test specimen to be inspected, the observation with a scanning electron microscope, and the method of EDS analysis may be general methods.

  The size of the target non-metallic inclusions is affected by the measurement area and the time required to clarify the results, but in this survey, it was confirmed that a particle size of about 1 μm or more is appropriate. . If too small particles are targeted, the observation magnification must be increased, and information other than non-metallic inclusions that reflects slight differences in the sample surface condition may also be extracted, resulting in a significant amount of time. Leads to an increase in the accuracy and deterioration of the evaluation accuracy.

  On the other hand, for example, when the particle size is 10 μm or more, the number of particles to be measured becomes too small, and thus there is a possibility that the entire nonmetallic inclusion composition is not reflected. Further, in the EDS composition analysis of the scanning electron microscope, even if the acceleration voltage is about 15 kV, only information about a depth of about 1 μm can be obtained from the surface layer. For example, in the case of composite non-metallic inclusions having different compositions at the central portion and the peripheral portion, the composition at the central portion may not be evaluated. In other words, if the size of the target nonmetallic inclusion is too large, the composition of the nonmetallic inclusion may not be accurately grasped.

As the measurement area increases, the number of particles to be measured increases and the evaluation accuracy improves, but the time required for measurement increases. According to the result of this investigation, a reproducible result was obtained by setting a mirror-polished surface of approximately 20 mm ² or more as a measurement region. In practice, it is preferable to determine the optimum conditions in consideration of the estimation accuracy of the required resistance to hydrogen-induced cracking and the number of treatments.

  Elements in non-metallic inclusions that are important for determining the resistance to hydrogen-induced cracking of calcium-added steel are sulfur (S), oxygen (O), calcium (Ca), aluminum (Al), manganese (Mn), etc. In the composition analysis of nonmetallic inclusions by EDS, these elements were quantitatively analyzed except for oxygen.

In addition, the present inventors confirmed that manganese is present as MnS, calcium is present as CaS and CaO, and aluminum is present as Al ₂ O ₃ in the non-metallic inclusions contained in the calcium-added steel. doing. Therefore, the compositional analysis result of nonmetallic inclusions by EDS is analyzed by a calculation method applying a stoichiometric ratio based on this knowledge, and the ratio of CaO to Al ₂ O ₃ in each nonmetallic inclusion (( Mass% CaO) / (mass% Al ₂ O ₃ )).

Specifically, the ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) of CaO and Al ₂ O _{3 in} each non-metallic inclusion is determined as follows.

First, based on the composition analysis result of the nonmetallic inclusion by EDS, the following formulas (1), (2), and (3) are calculated in order, and the CaO fraction of each nonmetallic inclusion (mass%) ) Is calculated.
S fraction (% by mass) as MnS = [Mn] × [S atomic weight] / [Mn atomic weight] (1)
Ca fraction (% by mass) as CaS = ([S]-[S fraction as MnS]) × [Ca atomic weight] / [S atomic weight] (2)
CaO fraction (mass%) = ([Ca]-[Ca fraction as CaS]) × [CaO atomic weight] / [Ca atomic weight] ... (3)
However, [Mn] in the formula (1) is the manganese analysis value (mass%) in the nonmetallic inclusions by EDS, and [S] in the formula (2) is the sulfur analysis value in the nonmetallic inclusions by EDS ( (Mass%), [Ca] in the formula (3) is an analytical value (mass%) of calcium in non-metallic inclusions by EDS.

Moreover, based on the composition analysis result of the nonmetallic inclusion by EDS, the Al ₂ O ₃ fraction (mass%) of each nonmetallic inclusion is calculated by the following equation (4).
Al ₂ O ₃ fraction (mass%) = [Al] × [Al ₂ O ₃ atomic weight] / [2 × Al atomic weight] ... (4)
However, [Al] in the formula (4) is an aluminum analysis value (mass%) in non-metallic inclusions by EDS.

Next, from the CaO fraction calculated by the formulas (1) to (3) and the Al ₂ O ₃ fraction calculated by the formula (4), the non-metallic inclusions CaO and Al ₂ O ₃ Ratio ((mass% CaO) / (mass% Al ₂ O ₃ )).

Then, the number of non-metallic inclusion particles whose ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) is not less than an arbitrary value selected in the range of 1.0 to 10.0 is counted. The ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) obtained is not less than an arbitrary value selected in the range of 1.0 to 10.0 per unit area of nonmetallic inclusions. Using the relational expression between the number of particles and the resistance to hydrogen-induced cracking, the resistance to hydrogen-induced cracking of the calcium-added steel to be measured is estimated.

In the present invention, nonmetallic inclusions whose nonmetallic inclusions CaO to Al ₂ O ₃ ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) is 1.0 or more are counted. However, this is based on the following reason.

In the binary phase diagram of CaO—Al ₂ O ₃ , 12CaO · 7Al ₂ O in which the ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) is near 1.0 and the melting point is about 1455 ° C. ₃ (CaO = 48.5% by mass, Al ₂ O ₃ = 51.5% by mass, ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) = 0.94) . When this compound is formed in the molten steel, since it is in a liquid state in the molten steel, floating separation from the molten steel is promoted and the cleanliness of the calcium-added steel is improved. Since the cleanliness is improved, the resistance to hydrogen-induced cracking is improved.

On the other hand, the ratio of non-metallic inclusions produced ((mass% CaO) / (mass% Al ₂ O ₃ )) is 2.0 (CaO = 66.7 mass% in the _binary phase diagram, Al ₂ O ₃ = (33.3 mass%) or more, the melting point of the nonmetallic inclusions to be generated rises rapidly, and since it exists as a solid in the molten steel, the floating separation from the molten steel is delayed, and the cleanliness decreases and the hydrogen resistance is reduced. Induced cracking properties deteriorate.

That is, the ratio of the non-metallic inclusion CaO to Al ₂ O ₃ ((mass% CaO) / (mass% Al ₂ O ₃ )) is 1.0 as a boundary. Calculating the calcium-added steel by recognizing non-metallic inclusions with poor floatability with a ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) of 1.0 or more. This is because the resistance to hydrogen-induced cracking of steel can be estimated.

However, since the composition analysis result by EDS actually obtained also depends on the apparatus performance, analysis conditions, etc., the ratio ((mass% CaO) / () having the highest correlation with the resistance to hydrogen-induced cracking in comparison under the same conditions. It is preferable to set the mass% Al ₂ O ₃ )) as a threshold value. It is not necessary to set the ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) exceeding 10.0 as the threshold value.

For several types of samples in advance, the ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) is more than an arbitrary value selected in the range of 1.0 to 10.0. By obtaining a relational expression between the number of particles per unit area of the object and the resistance to hydrogen-induced cracking, and thereafter, by evaluating the formation of non-metallic inclusions, hydrogen-induced cracking It is possible to predict characteristics. Under the above measurement conditions, about 10 test pieces can be measured in 24 hours, and the resistance to hydrogen-induced cracking can be estimated very quickly.

  The purpose of the present invention is to quickly grasp the resistance to hydrogen-induced cracking. For this purpose, the sample to be inspected in the present invention is a sample taken from a slab manufactured by a continuous casting machine. It is preferable to target. However, it is also possible to target a sample obtained from a rolled steel material obtained by rolling a slab or a steel pipe obtained by forming this rolled steel material. Moreover, even a sample collected from molten steel can be used as a sample to be inspected.

  The results of the investigation by the present inventors show that there is no significant difference in the production of non-metallic inclusions between the sample collected from the molten steel in the tundish of a continuous casting machine and the sample collected from the cast piece obtained by continuously casting the molten steel. I couldn't. In addition, in the sample extract | collected from the slab, the influence by the extraction | positioning position on the hydrogen-induced cracking resistance characteristic was recognized. This is because the distribution of non-metallic inclusions is not uniform in the continuous cast slab. In the case of applying the present invention, it is preferable that the position where the most non-metallic inclusions are present in the continuous cast slab is the inspection object. Moreover, when evaluating for every charge, such as grasping | ascertaining the appropriate amount of calcium addition amount, it is preferable to make into a test object the sample extract | collected from molten steel. Samples taken from molten steel may be more representative.

In the present invention, in this investigation result, the number of non-metallic inclusions is not considered, and the number of non-metallic inclusions in the ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) is not less than the threshold value. Although it is used as a determination criterion, it is also possible to evaluate by adding a weight such as increasing the degree of influence depending on the size of the non-metallic inclusions to this determination method.

As described above, according to the present invention, the non-metallic inclusions whose ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) based on the composition analysis result of the non-metallic inclusions in the steel is greater than or equal to the threshold value. Since the hydrogen-induced crack resistance is estimated from the correlation between the number of particles and the hydrogen-induced crack resistance, the hydrogen-induced crack resistance of the calcium-added steel can be evaluated more quickly than in the past.

  Hereinafter, the present invention will be described in more detail with reference to examples.

A sample cut from a continuous cast slab of calcium-added steel manufactured with an actual machine was used as the object of investigation. A test sample was cut out from each position of the slab and then divided into two parts, one for the hydrogen-induced cracking test specified by NACE and the other for the non-metallic inclusion investigation sample according to the present invention. The hydrogen-induced cracking test was performed according to the method specified in NACE. A specific method of the hydrogen-induced cracking test (hereinafter also referred to as “HIC test”) is that 5% NaCl and 0.5% CH ₃ COOH in which hydrogen sulfide having a pH (hydrogen ion index) of about 3 is saturated. After immersing the test piece in an aqueous solution (normal NACE solution) for 96 hours, the surface of the test piece was examined for cracks by ultrasonic flaw detection, and the crack area ratio (CAR) was determined.

On the other hand, in the investigation of the non-metallic inclusions according to the present invention, the sample surface was mirror-polished and then investigated using a scanning electron microscope equipped with an EDS and having a particle analysis function. In each sample, non-metallic inclusions having a size of 1 μm or more existing in a region of 10 mm in width × 10 mm in length were investigated, and the size and elemental composition of each non-metallic inclusion were investigated. Thereafter, based on the EDS composition analysis results, the ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) in each non-metallic inclusion was calculated by formulas (1) to (4).

FIG. 1 shows a distribution example of the ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) of three types of samples (samples A, B, and C) having different CAR in the HIC test. In FIG. 1, the horizontal axis represents the ratio ((mass% CaO) / (mass% Al ₂ O ₃ )), and the vertical axis represents the cumulative number. As shown in FIG. 1, it can be seen that the higher the CAR in the HIC test, the more the composition of nonmetallic inclusions tends to be enriched in CaO.

This is because, in the refining stage of the molten steel, the amount of calcium added was too much with respect to the amount of Al ₂ O ₃ present in the molten steel. Is thought to have deteriorated. Actually, non-metallic inclusions with a high CaO concentration were observed on the fracture surface (cracked surface) of the test piece subjected to the HIC test, and thus it was confirmed that the above estimation was appropriate.

Based on the EDS survey results, the number of non-metallic inclusions with a ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) of 10.0 or more is counted, and this number is measured by the HIC test. The relationship with CAR was investigated. The survey results are shown in FIG. The horizontal axis in FIG. 2 represents the number of nonmetallic inclusions per unit area with a ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) of 10.0 or more. As shown in FIG. 2, a good correlation was observed between the two, and it was found that the result of the hydrogen-induced cracking test can be estimated from the state of formation of nonmetallic inclusions in the steel.

Further, from the correlation of FIG. 2, the number per unit area of non-metallic inclusions having a ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) of 10.0 or more, and the CAR in the HIC test For the newly produced calcium-added steel slab, measure the nonmetallic inclusions in the slab with the above scanning electron microscope, and apply the regression equation to the measurement results. CAR was estimated. Moreover, the HIC test was done about the newly manufactured calcium addition steel.

  Table 1 shows the estimation result of CAR by the regression equation and the measurement result of CAR in the actual hydrogen-induced cracking test.

  As is apparent from Table 1, the CAR estimated from the investigation results of the nonmetallic inclusions in the steel and the CAR in the actual hydrogen-induced cracking test are in good agreement with the present invention. By applying the invention, it was confirmed that the hydrogen-induced cracking resistance characteristics of the calcium-added steel can be estimated with high accuracy.

Claims (2)

A method for estimating hydrogen-induced cracking resistance of calcium-added steel from the composition and distribution of non-metallic inclusions in steel,
Mirror-polished surface of a sample taken from the added molten steel calcium, or scanning slab of calcium added steel, a 20 mm ² or more mirror-polished surface of the steel pipe obtained by pipe-making rolling steel or rolled steel as measured region A step of observing with an electron microscope, detecting non-metallic inclusions of a predetermined size or more, and analyzing the composition of each detected non-metallic inclusion with an EDS (energy dispersive X-ray analyzer);
Based on the composition analysis result by EDS, calculating the CaO fraction (mass%) of each non-metallic inclusion by the following formulas (1), (2), and (3):
A step of calculating the Al ₂ O ₃ fraction (% by mass) of each non-metallic inclusion based on the composition analysis result by EDS according to the following formula (4):
From the CaO fraction calculated by the formulas (1) to (3) and the Al ₂ O ₃ fraction calculated by the formula (4), the ratio of CaO and Al ₂ O _{3 of} individual nonmetallic inclusions. Obtaining ((mass% CaO) / (mass% Al ₂ O ₃ ));
Counting the number of non-metallic inclusion particles whose ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) is not less than an arbitrary value selected in the range of 1.0 to 10.0;
Particles per unit area of nonmetallic inclusions whose ratio ((mass% CaO) / (mass% Al ₂ O ₃ )) obtained in advance is not less than an arbitrary value selected in the range of 1.0 to 10.0 A process for estimating hydrogen-induced cracking resistance using a relational expression between the number and hydrogen-induced cracking resistance,
A method for estimating hydrogen-induced cracking resistance characteristics of a calcium-added steel.
S fraction (% by mass) as MnS = [Mn] × [S atomic weight] / [Mn atomic weight] (1)
Ca fraction (% by mass) as CaS = ([S]-[S fraction as MnS]) × [Ca atomic weight] / [S atomic weight] (2)
CaO fraction (mass%) = ([Ca]-[Ca fraction as CaS]) × [CaO atomic weight] / [Ca atomic weight] ... (3)
Al ₂ O ₃ fraction (mass%) = [Al] × [Al ₂ O ₃ atomic weight] / [2 × Al atomic weight] ... (4)
However, [Mn] in the formula (1) is the manganese analysis value (mass%) in the nonmetallic inclusions by EDS, and [S] in the formula (2) is the sulfur analysis value in the nonmetallic inclusions by EDS ( Mass%), [Ca] in the formula (3) is the analytical value of calcium in nonmetallic inclusions by EDS (mass%), and [Al] in the formula (4) is the analysis of aluminum in nonmetallic inclusions by EDS. Value (mass%).   2. The method for estimating hydrogen-induced cracking resistance characteristics of a calcium-added steel according to claim 1, wherein nonmetallic inclusions having a particle size of 1 [mu] m or more are subjected to composition analysis by EDS.

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