JP2023045506A - Evaluation method of hydrogen-induced cracking resistance of steel material - Google Patents

Abstract

A method for evaluating resistance to hydrogen-induced cracking at the slab stage, which is more accurate than conventional methods, is provided. [Solution] Hydrogen-induced cracking resistance is evaluated based on the free sulfur concentration calculated based on the component analysis value of molten steel and the evaluation result of the segregation grains in the central segregation zone appearing in the cross section perpendicular to the longitudinal direction of the cast slab. A method for evaluating hydrogen-induced cracking resistance of steel materials, characterized in that Specifically, [1] the total length of segregation grains (target segregation grains) of 1.0 mm or more on the segregation line in the center segregation zone of the slab is 45% or more of the survey length, and the maximum of the target segregation grains When the length is 3.0 mm or more (evaluation A), and the free S concentration calculated based on the component analysis value of molten steel is 6 ppm by mass or less, or [2] Center segregation zone A method for evaluating hydrogen-induced cracking resistance of a steel material, wherein the steel material is evaluated as having excellent hydrogen-induced cracking resistance when the evaluation is other than evaluation A and the free S concentration is 20 ppm by mass or less. [Selection figure] None

Description

The present invention is a method for evaluating the resistance to hydrogen-induced cracking of steel materials, and is used to determine whether the steel has excellent resistance to hydrogen-induced cracking when manufacturing hydrogen-induced cracking steel that is assumed to be used in line pipes. It relates to a method for evaluating steel materials that discriminates.

It is known that segregation at the center of the slab (hereinafter referred to as center segregation) greatly affects the properties of continuously cast steel slabs (hereinafter simply referred to as slabs). In particular, steel materials such as sour-resistant materials used for line pipes are assumed to be used in a hydrogen sulfide environment, and depending on the state of center segregation and non-metallic inclusions, hydrogen-induced cracking (Hydrogen Induced Cracking) hereinafter referred to as HIC) may occur. In order to suppress HIC, it is effective to suppress the formation of manganese-sulfide (MnS), which is a highly stretchable sulfide-based inclusion. A countermeasure is taken to control the morphology of non-stretchable calcium-sulfide (CaS). In addition, since the aggregate of segregating elements such as Mn in the center segregation part has a different hardness from the surrounding base metal, this also becomes the starting point of HIC, so it is necessary to adjust the roll gap during casting and not change the casting speed. Measures are taken to suppress segregation aggravating factors. However, even if these countermeasures are taken, inclusions and center segregation may still occur due to fluctuations in composition and casting conditions. Therefore, the HIC characteristics of steel materials are evaluated before product shipment.

[Method for evaluating HIC characteristics]
When evaluating the HIC properties of steel materials, the HIC test (NACE test) prescribed in the standards of NACE (National Association of Corrosion Engineers) TM0284 is often used for steel materials before shipment. In this method, a steel material with a predetermined size is immersed in a mixed aqueous solution of 5% NaCl solution saturated with 1 atm of hydrogen sulfide and 0.5% acetic acid at pH 2.7, and after 96 hours, it is taken out to evaluate the occurrence of HIC. It is a way to As a method for evaluating the occurrence of HIC, a method of observing the cross section of the steel material to investigate the length and form of cracks of HIC generated inside, and an ultrasonic test (UT) are used to detect cracks on the evaluation surface of the test piece. There is a method of calculating the area ratio CAR (Crack Area Ratio) of the part.

By applying the above method to products before shipment, the presence or absence of HIC occurrence can be confirmed, but this method requires several weeks before the results are known. If HIC occurs, the product cannot be used as a sour resistant material, and if it is remelted, the production efficiency will be greatly reduced. Therefore, the following method has been disclosed as a method for estimating the presence or absence of HIC at the slab stage instead of the NACE test, which takes several weeks to obtain results.

In Patent Document 1, on the slab cut surface, the opening thickness of horizontal cracks in the range of width D / 2 from both ends in the width direction, the maximum segregation grain size in the range of width WD excluding D / 2 from both ends in the width direction and a method of diversion by quality determination of sour steel slabs using the number density of segregated grains of a predetermined diameter or greater.

In Patent Document 2, the product of the major diameter D _L obtained by elliptical approximation of the Mn segregation spot in the central segregation portion and the degree of Mn segregation C _MnS /C ₀ exceeds 0.8 mm. A method for evaluating the HIC susceptibility of a steel material is disclosed, in which the steel material is evaluated as having excellent HIC resistance when the following conditions are satisfied.

In Patent Document 3, the ratio of CaO and Al ² O ₃ concentrations (mass% CaO / mass _% Al ₂ O ₃ ) is a method of counting non-metallic inclusions in the range of 1.0 to 10.0 and using a relational expression with hydrogen-induced cracking resistance to estimate the hydrogen-induced cracking resistance of calcium-added steel. disclosed.

JP 2014-172074 A JP 2014-77642 A JP 2015-59880 A

Patent Documents 1 and 2 are techniques for evaluating HIC characteristics from the size or number density of segregated grains. Patent Document 3 is a technique for evaluating HIC characteristics from nonmetallic inclusions. In the prior art, there are many HIC characteristic evaluation methods that focus on either segregation or inclusions, and the combined factors of segregation and inclusions are not approached. It is considered to be set on the safe side.

In view of the above, an object of the present invention is to provide a method for evaluating hydrogen-induced cracking resistance of steel materials at the slab stage, which is more accurate than conventional methods.

The present inventors considered that the starting point of hydrogen-induced cracking includes inclusions for collecting hydrogen in the steel, and among them, MnS has a large effect. MnS is generated at the stage of solidification of molten steel according to the Mn and S concentrations in the steel material. Since Mn has a sufficiently high concentration relative to S in molten steel, the amount of MnS produced generally depends on the S concentration. When Ca is present in molten steel, CaS is formed in the molten steel stage, so if the S concentration (= free S) excluding this CaS content is known, the amount of MnS generated in the solidification stage can be roughly known. . At this time, Mn and S are concentrated in the segregation part, but the effect of MnS itself reflects the state of MnS formation in the base metal, and by focusing on free S, it is possible to examine the frequency of crack initiation. Thought.

In addition, it was considered that the degree of hydrogen-induced cracking is greatly affected by the distribution of segregated grains, which differ in hardness from the base material. Also, when focusing on one segregated grain, it was thought that the larger the size of the segregated grain, the stronger the susceptibility to cracking. Furthermore, in a situation where a plurality of segregated grains are adjacent to each other, if the distance between the segregated grains is short, a chain of cracks will occur.

Therefore, the present inventors investigated the correlation between the molten steel composition in the tundish and the center segregation of the slab, and the CAR, which is an evaluation index for the occurrence of HIC in the product. CAR is the area ratio of the cracked portion to the evaluation surface of the test piece, and is suitable for quantitatively grasping the occurrence of HIC. As a result, it was found that CAR correlates with the state of MnS formation estimated from the molten steel composition and the size of segregation grains in the central segregation part. In addition, it was also found that HIC did not occur under conditions in which MnS did not form, even if the distribution of segregated grains was the same in the central segregation zone. Considering the HIC generation mechanism in which cracking occurs due to the internal pressure of H gathered around MnS, etc., there is no trigger for HIC in the situation where MnS does not form, so HIC does not occur regardless of the degree of segregation, and CAR does not occur. It is considered to be 0. In addition, the correlation between CAR and the size of segregated grains shows that the critical stress at which one segregated grain cracks differs depending on the size of the segregated grains, and even under the conditions in which MnS is dispersed at the same density, large segregation This is probably because the critical stress leading to cracking decreases as the grain size increases.

Furthermore, it was found that CAR correlates with the distribution of segregated grains. Even if HIC occurs, if the segregation grains are separated, cracking will not progress and the CAR will be a low value. It is believed that the internal pressure affects adjacent segregated grains and develops into large-scale cracks, resulting in a situation of high CAR.

Considering the effects of the MnS generation state, segregation grain size, and segregation grain distribution state on the occurrence of HIC, specific means for solving the problems are as follows.
[1] Based on the free sulfur concentration calculated based on the component analysis values of the molten steel sample collected in the tundish and the evaluation result of the segregation grains in the central segregation zone appearing in the cross section perpendicular to the longitudinal direction of the slab, hydrogen resistance A method for evaluating hydrogen-induced cracking resistance of a steel material, comprising evaluating the induced cracking resistance.
[2] Segregated grains on a straight line (segregation line) of a predetermined survey length drawn in the width direction of the slab so as to overlap most segregated grains in the central segregation zone, and the length (between the segregated grains and the segregation line) The total length of segregated grains (target segregated grains) of 1.0 mm or more is less than 45% of the survey length, and/or the maximum length of the target segregated grains is less than 3.0 mm. Hydrogen-induced cracking resistance of the steel material according to [1], wherein the steel material is evaluated as having excellent hydrogen-induced cracking resistance when the free S concentration is 20 ppm by mass or less. evaluation method.
[3] Segregated grains on a straight line (segregation line) of a predetermined survey length drawn in the width direction of the slab so as to overlap most segregated grains in the central segregation zone, and the length (between the segregated grains and the segregation line) The total length of the segregated grains (target segregated grains) having a length of the overlapping portion) of 1.0 mm or more is 45% or more of the survey length, and the maximum length of the target segregated grains is 3.0 mm or more. The steel material according to [1] or [2], wherein the steel material is evaluated as having excellent resistance to hydrogen-induced cracking when the free S concentration is 6 ppm by mass or less. Crackability evaluation method.
[4] E _CaO (percentage of CaO in all oxides in molten steel (mol%)) is obtained in advance for each product type, and the component analysis values of the sample are calculated based on the following formulas (1) to (3). The method for evaluating hydrogen-induced cracking resistance of a steel material according to [2] or [3], wherein the free S concentration is calculated based on the above.
_CaCaO =(T.O/16)*40.078*( _ECaO /100) (1)
S _CaS = (Ca—Ca _CaO ) 32/40.078 (2)
Free S = SS _CaS (3)
T. O is the total oxygen concentration analysis value (mass ppm), Ca is the Ca concentration analysis value (mass ppm), and S is the S concentration analysis value (mass ppm).

By applying the present invention, it is possible to evaluate whether or not the steel has excellent resistance to hydrogen-induced cracking at the stage where the component analysis of the molten steel and the segregation investigation of the slab are completed. Based on this result, it is possible to determine the use of the steel material after the cast slab, so the HIC test is not required for steel materials that are applied to applications that do not require HIC characteristics, so it is possible to shorten the process required for the HIC test. be. In addition, depending on the segregation state, it is possible to diffuse the segregation by heating or soaking, thereby improving the product yield.

Preferred embodiments of the present invention are described below.
1. Definition of terms in the present invention A tundish is a holding vessel that temporarily holds molten steel held in a ladle until it is poured into a mold. In many cases, a molten steel sample taken in a tundish is used as a representative component of steel, and in the present invention, the HIC resistance of steel is evaluated using the analysis results of the molten steel sample taken in a tundish.

Free S means dissolved S and MnS in the cast slab, excluding S fixed as CaS, among the S in the molten steel sample collected in the tundish. Since it is affected by the composition of molten steel, it can be estimated using the formula described later.

Segregation is the unevenness of steel components caused by flow inhibition of molten steel between dendrite trees that occurs when molten steel solidifies. The ease of segregation differs depending on the steel composition, and Mn, P, and S are elements that easily segregate. The segregation portion can be evaluated by an etch print method or a technique using an EPMA (Electron Probe Micro Analyzer). In the etch printing method, the surface of the sample is corroded using an etchant containing picric acid as the main component, the sample is washed and dried, and the corrosion holes on the corroded surface of the polished surface are infiltrated with polishing powder. In this method, a transparent adhesive tape is pasted, and the abrasive powder in the corrosion holes is adhered to the transparent adhesive tape. After that, the adhesive tape is peeled off and pasted on a white backing paper. The method using EPMA is a method of irradiating the surface of a polished sample with an electron beam to obtain a concentration map of Mn, etc., and then evaluating the segregation portion based on the concentration difference from the matrix. Besides EPMA, analysis methods such as CMA (Computer aided Micro Analyzer) and MA (Macro Analyzer) can also be used.
The C cross section refers to a plane perpendicular to the longitudinal direction (casting direction) of the slab.

2. Processing Procedure A specific processing procedure using the present invention is shown below.

Molten steel melted in a steelmaking furnace such as a converter or an electric furnace is adjusted using a ladle refining device such as LF or a degassing device such as RH or REDA as necessary, and cast slabs with a continuous casting machine. manufacture. At that time, the analytical value of the molten steel sample collected in the tundish can be used as the representative value of the molten ch (charge). Free S is calculated according to the calculation method described later using the S, O, and Ca concentrations among the analytical values.

A part of the manufactured slab is cut and the C cross section of the slab is examined to evaluate the center segregation. As a method for evaluating center segregation, an etch print method or a method using EPMA can be applied as described later. By this evaluation, the length and number of segregated grains in the central segregation part can be calculated.

When the length and number of segregated grains in the free S and the central segregation portion are known, the resistance to hydrogen-induced cracking can be determined according to the method described in the claims. Slabs judged to have good resistance to hydrogen-induced cracking are advanced to the next process. It is possible to take measures such as passing it to the process.

3. Calculation method of free S In a molten steel sample collected in a tundish, most of O is an oxide, and although a small amount of O is dissolved in the molten steel, almost all of it forms an oxide in the solidification stage. Also, Ca forms CaO or CaS. On the other hand, S other than CaS exists as dissolved S in the molten steel stage, and the dissolved S combines with Mn in the solidification stage to form MnS. Therefore, if the ratio of oxides to O, especially the amount of CaO present, is known, the free S concentration can be obtained using equations (1) to (3).

First, the concentration of CaO forming CaO in the molten steel is calculated by the formula (1).
_CaCaO =(TO/16)*40.078*( _ECaO /100) (1)
Here, Ca _CaO is the concentration of Ca forming CaO (mass ppm), T.O. O is the total oxygen concentration analysis value (mass ppm), E _CaO is the ratio of CaO to all oxides in the molten steel (mol%), 16 is the atomic weight of O, and 40.078 is the atomic weight of Ca.

Next, the concentration of S (mass ppm) forming CaS is determined by the formula (2).
S _CaS = (Ca—Ca _CaO ) 32/40.078 (2)
Here, S _CaS is the S concentration (mass ppm) forming CaS, Ca is the Ca concentration analysis value (mass ppm), and 32 is the atomic weight of S.

The free S can be obtained by subtracting the S concentration S CaS (mass ppm) forming CaS calculated by the equation (2) from the S concentration analysis value _S (mass ppm) of the molten steel.
Free S = SS _CaS (3)

Note that S _CaS is set to 0 when (Ca-Ca _CaO )<0 in formula (2), and free S is set to 0 when S-S _CaS <0 in formula (3).

E _CaO in the formula (1) is obtained by analyzing the composition of the oxide using an energy dispersive X-ray spectrometer (EDS) attached to a scanning electron microscope (SEM), and converting the obtained result into an oxide. Based on this, it can be evaluated as the ratio (mol%) of CaO in the total oxide. Although the oxides differ in size and composition, the product of the area of the investigated oxide and the concentration of each component is integrated, and the integrated value of CaO is divided by the sum of all the oxide species integrated. It should be a percentage. In order to improve representativeness, it is desirable that the number of oxides to be investigated is 100 or more. When the present inventors investigated molten steel samples taken in a tundish, the composition ratio of oxides was generally the same for the same steel type. Therefore, the ratio E _CaO (mol %) of CaO to the total oxides in the molten steel should be obtained by investigation in advance for each steel type, and the calculated value may be used to calculate the formula (1).

4. Evaluation method of segregation part of slab A surface (C section) perpendicular to the longitudinal direction (casting direction) is cut from the slab, and the segregation part seen in the C section is evaluated by a method using the etch print method or EPMA. do. In the case of a slab, the segregation part is usually present in the central part of the slab (1/2 thickness in the thickness direction, and a part at least 1/2 thickness in the width direction from the left and right ends).

When applying the etch printing method, the full width of the slab may be measured, but the results of a partial survey may also be used. A grayscale image of an etch print transferred from an area containing a segregation part is photographed, the image is converted to a gray scale, and the image is binarized. call. The threshold for binarization should be set between the pixel average of the base material portion (the bright portion away from the segregation portion) and the pixel average of the segregation portion (the portion that looks darker than the surroundings due to the accumulation of iron powder). In the center segregation zone, a straight line having a predetermined investigation length is drawn in the width direction of the slab so as to overlap the segregation grains most on the image, and this is defined as a segregation line. The length of the segregation grain existing on the straight line (segregation line) (the length of the portion where the segregation grain overlaps with the segregation line) is evaluated. Then, the ratio of the total length of the segregated grains having a length of 1.0 mm or more (target segregated grains) to the survey length is calculated. Also, the maximum length among the lengths of the target segregation grains is evaluated.

In the technique using EPMA, it is possible to cut out a test piece containing a segregation portion and obtain a mapping image of solute elements. Mn is generally used as the target element, and regarding the relationship between the Mn concentration of the base material: Mn and the Mn concentration of the EPMA measurement part: Mn _Seg , the portion where Mn _Seg /Mn = 1.3 or more is It can be treated as a segregation part. A straight line (segregation line) is drawn in the slab width direction on the obtained elemental mapping image in the same manner as in the etch printing method. (Total length of segregated grains/Investigation length), the maximum length can be calculated.

In the present invention, it is desirable to investigate the segregation part over an investigation length of at least 25 mm, and if necessary, the investigation length can be added or multiple locations can be investigated. When partially inspecting, representativeness can be ensured by setting the inspection position in the width direction between 1/2 width and 1/4 width. In the present invention, of the segregation grains present on the segregation line of the investigation length, the total length of the segregation grains having a length of 1 mm or more (target segregation grains) is 45% or more of the investigation length, and the target segregation grains When the maximum length of is 3.0 mm or more, the segregation part of the investigated slab is evaluated as A, and the total length of the target segregation grains with a length of 1 mm or more is less than 45% of the investigation length. , and/or the evaluation of slabs in which the maximum length of the target segregated grains was less than 3.0 mm was rated as B. In addition, the cases other than evaluation A are evaluated as B. In the present invention, segregated grains with a length of 1 mm or more were evaluated when quantitatively evaluating the dispersed state of segregated grains. Segregated grains with a length of 1 mm or more were evaluated. The method using the etch printing method and EPMA is an evaluation method based on the concentration difference of the solute element between the base material and the segregation portion, and either method may be used to evaluate the segregation portion.

Together with the grading evaluation result of the slab segregation part obtained as described above and the result of free S calculated from the component values of the slab, whether the steel material has excellent resistance to hydrogen-induced cracking is determined as follows. determined as follows.

5. Method for Determining HIC Characteristics In the present invention, the evaluation results of the free S and the segregated portion calculated by the above-described method are examined to determine how they affect the HIC characteristics. A method for determining HIC characteristics was established based on the evaluation results of . In the present embodiment, the HIC characteristics are evaluated by CAR, and the relationship between the free S and the segregation portion and the CAR are obtained in advance, and the free S and segregation conditions that provide good HIC characteristics are set. First, the NACE test was performed on the rolled steel plate. That is, the steel plate was immersed in a mixed aqueous solution of pH 2.7 of 5% NaCl solution saturated with 1 atm of hydrogen sulfide and 0.5% acetic acid, and taken out after 96 hours. Next, as a method for evaluating the generation of HIC, an ultrasonic flaw detection test (UT) was used to calculate the area ratio CAR of the crack portion with respect to the evaluation surface of the test piece.

In this embodiment, the HIC characteristics were determined to be good when the CAR was less than 5.0%. Next, the relationship between the ratio of the total length of the segregated grains, the maximum length, the condition of free S, and the condition under which the CAR is less than 5.0% was evaluated. As a result, as will be described in detail in Examples below, the total length of segregated grains of 1.0 mm or more on the segregation line is less than 45% of the investigated length, and/or the maximum length of segregated grains is less than 3.0 mm. (Evaluation B above), and when the free S concentration calculated based on the component analysis values of the sample collected in the tundish is 20 ppm by mass or less, the HIC characteristics are excellent (CAR is 5. less than 0%). In addition, when the total length of segregated grains of 1.0 mm or more on the segregation line is 45% or more of the investigated length and the maximum length of the segregated grains is 3.0 mm or more (said evaluation A), the tundish It was found that HIC characteristics are excellent (CAR is less than 5.0%) when the free S concentration calculated based on the component analysis values of the sample collected in 1. is 6 ppm by mass or less.

Based on the above results, in the method for evaluating hydrogen-induced cracking resistance of steel materials of the present invention, when the segregation evaluation result of the slab is B and the free S concentration is 20 ppm by mass or less, or when the segregation of the slab When the evaluation result was evaluation A and the free S concentration was 6 ppm by mass or less, the steel material was evaluated as having excellent resistance to hydrogen-induced cracking.

In the case where the segregation evaluation result of the slab is the above evaluation A, the upper limit of the total length of segregated grains on the segregation line and the upper limit of the maximum length of segregated grains are not particularly set, but the total of segregated grains of 1.0 mm or more on the segregation line If the length is 75% or less of the investigated length and the maximum length of the segregated grains is 8.0 mm or less, excellent HIC characteristics can be reliably obtained when the free S concentration is 6 mass ppm or less. Further, when the segregation evaluation result of the slab is the evaluation B, there is a mutual correlation between the reduction in the total length of the segregation grains on the segregation line and the reduction in the maximum length of the segregation grains. , it is not necessary to limit the upper limit of the maximum length of segregated grains when the total length of segregated grains of 1.0 mm or more on the segregation line is less than 45% of the investigation length, and similarly, the maximum length of segregated grains It is also not necessary to limit the upper limit of the ratio of the total length of segregated grains of 1.0 mm or more on the segregation line to the investigation length when is less than 3.0 mm.

In other words, in the method for evaluating hydrogen-induced cracking resistance of steel materials of the present invention, if the segregation evaluation result of the slab is B and the free S concentration is more than 6 ppm by mass and 20 ppm by mass or less, or If the free S concentration is 6 ppm by mass or less regardless of the segregation evaluation result of the piece, the steel material may be evaluated as having excellent resistance to hydrogen-induced cracking.

Next, an example using the determination method of the present invention will be described. Hot metal tapped from a blast furnace was desulfurized by hot metal pretreatment, dephosphorized and decarburized in a converter-type refining vessel, and then received in a ladle. The amount of molten steel is on the scale of 420 tons. The molten steel in the ladle was conveyed to the RH vacuum degassing device and refluxed. As sour-resistant steel, molten steel was subjected to desulfurization treatment during reflux treatment, and Ca was added to the molten steel. The main ingredients are C: 0.04 to 0.10%, Mn: 1.0 to 1.5%, P: 100 ppm or less, S: 30 ppm or less, O: 30 ppm or less, Ca: 50 ppm or less. be. After being treated in the RH vacuum degasser, a slab was produced in a continuous casting machine. Molten steel samples were taken in a tundish during continuous casting.

A cross section perpendicular to the longitudinal direction (casting direction) of the cast slab was cut so that the C cross section could be evaluated from the manufactured cast slab. A sample at a position of 1/2 thickness and 1/4 width was further cut from the cut slab, polished, and then the center segregation part was measured by EPMA. The survey was conducted over a range of 25 mm x 25 mm including the central segregation part, and from the Mn mapping image obtained, a part where Mn _Seg /Mn = 1.3 or more was extracted as a segregation part, and a mass of the segregation part was extracted. Segregated grains were obtained. A straight line with a survey length (25 mm) was drawn in the width direction of the slab so as to overlap most segregated grains in the central segregation zone, and this was used as a segregation line. The length of the segregation grain existing on the segregation line (the length of the portion where the segregation grain overlaps with the segregation line) was evaluated. Then, the ratio of the total length of the segregated grains with a length of 1.0 mm or more (target segregated grains) to the survey length (the length ratio of the segregated grains with a length of 1 mm or more (target segregated grains)) is calculated. and shown in the column of "Length ratio of segregated grains" in Table 1. In addition, the maximum length of the target segregated grains is shown in the "maximum segregated grain size" column of Table 1. Furthermore, the formation of inclusions containing MnS was investigated as necessary.

The E _CaO values were evaluated in advance for the steel types used in the examples. As a result, E _CaO =40.0 mol % was obtained.

Based on the composition of the molten steel sample collected in the tundish and the value of E _CaO obtained in advance, the free S obtained based on the above formulas (1) to (3) and the results of investigation of the center segregation zone of the slab are shown. 1. Based on the survey results of the segregation zone, the ratio of the total length of segregated grains with a length of 1 mm or more to the survey length (segregation grain Length ratio) is 45% or more, and the maximum length of the target segregation grain (maximum segregation grain size) is 3.0 mm or more. It is shown in the column of "Evaluation of segregation part" in Table 1.

In Table 1, the column "Determination result by the present invention" shows the result of determination of hydrogen-induced cracking resistance using the method of the present invention from the evaluation results of the free S and the segregation part.

Furthermore, after rolling the investigated slabs into steel sheets, they were subjected to an HIC test to determine the CAR (%). The CAR evaluation method is as described above. The condition where the CAR was less than 0.5% was considered to be particularly excellent in hydrogen-induced cracking resistance, and the condition where it was 0.5% or more and less than 5.0% was considered to be excellent in hydrogen-induced cracking resistance, and 5.0%. The above conditions were evaluated as x because the resistance to hydrogen-induced cracking was low, and shown in the "HIC evaluation" column of Table 1.

Example no. 1 to 11, the total length of segregated grains with a length of 1 mm or more is less than 45% of the investigation length, and / or the maximum length of segregated grains is less than 3.0 mm, that is, the segregation part evaluation B The free S content was 20 ppm by mass or less, the HIC characteristics were determined to be good by the evaluation method of the present invention, and the CAR value of the steel sheet investigated was also good at less than 5.0%.

Example no. 12 and 13 are segregation part evaluation B, free S is more than 20 ppm by mass, HIC characteristics are judged to be poor by the evaluation method of the present invention, and the CAR value investigated with the steel plate is as low as 5.0% or more. rice field.

Example no. In 14 to 22, the total length of segregated grains of 1 mm or more is 45% or more of the investigated length, and the maximum length of segregated grains is 3.0 mm or more. was 6 ppm by mass or less, and the HIC characteristics were determined to be good by the evaluation method of the present invention, and the CAR value investigated on the steel sheet was also good at less than 5.0%.

Example no. 23 to 28 had a segregation evaluation result of A and a free S content of more than 6 ppm by mass, and the HIC characteristics were determined to be poor by the evaluation method of the present invention, and the CAR value investigated with the steel plate was also low at 5.0% or more. Met.

As described above, the results of determining the hydrogen-induced cracking resistance using the free S obtained from the molten steel sample collected in the tundish using the method of the present invention and the evaluation results of the segregated grains in the central segregation zone of the slab are as follows. , consistent with the CAR results investigated on steel sheets after rolling. This is a result that supports the idea of the present invention that MnS is the starting point of HIC, the size of segregated grains affects the size of cracks in segregated grains, and the dispersion state of segregated grains affects crack propagation. It can be seen that the present invention is useful as a method for evaluating hydrogen-induced cracking resistance of steel materials.

As a conventional technique, a technique is disclosed in which the segregation part investigation results are used to evaluate the resistance to hydrogen-induced cracking of steel. Example No. in which the evaluation of the segregation portion was determined to be A. All of Nos. 14 to 28 were judged to have poor HIC characteristics, and it was necessary to take measures such as lowering the grade of the steel material. In the present invention, in combination with the free S in the molten steel stage, Example No. 1 under the conditions where the evaluation of the segregation part was determined to be B was performed. 12 and example no. Example No. 13 was excluded, and under the conditions where the evaluation of the segregation portion was determined as A, the hydrogen-induced cracking resistance of the steel material was good. 14 to Example Nos. 22 can be extracted, it can be seen that the technology is superior to the conventional technology.

Claims (4)

Hydrogen-induced cracking resistance based on the free S concentration calculated based on the component analysis values of the molten steel sample collected in the tundish and the evaluation results of the segregation grains in the central segregation zone appearing in the cross section perpendicular to the longitudinal direction of the slab. A method for evaluating hydrogen-induced cracking resistance of steel materials, characterized by evaluating the Segregation grains on a straight line (segregation line) of a predetermined survey length drawn in the width direction of the slab so as to overlap most segregation grains in the central segregation zone, and the length (the part where the segregation grain and the segregation line overlap Length) When the total length of segregated grains of 1.0 mm or more (target segregated grains) is less than 45% of the investigation length and/or the maximum length of the target segregated grains is less than 3.0 mm The method for evaluating hydrogen-induced cracking resistance of a steel material according to claim 1, wherein the steel material is evaluated as having excellent hydrogen-induced cracking resistance when the free S concentration is 20 ppm by mass or less. . Segregation grains on a straight line (segregation line) of a predetermined survey length drawn in the width direction of the slab so as to overlap most segregation grains in the central segregation zone, and the length (the part where the segregation grain and the segregation line overlap length) the total length of segregated grains (target segregated grains) of 1.0 mm or more is 45% or more of the investigation length, and the maximum length of the target segregated grains is 3.0 mm or more. 3. The steel material according to claim 1 or 2, wherein the steel material is evaluated as having excellent hydrogen-induced cracking resistance when the free S concentration is 6 ppm by mass or less. Evaluation method. E _CaO (proportion of CaO in total oxides in molten steel (mol%)) is determined in advance for each product type, and the above-mentioned values are calculated based on the component analysis values of the sample based on the following formulas (1) to (3). 4. The method for evaluating hydrogen-induced cracking resistance of a steel material according to claim 2 or 3, wherein the free S concentration is calculated.
_CaCaO =(T.O/16)*40.078*( _ECaO /100) (1)
S _CaS = (Ca—Ca _CaO ) 32/40.078 (2)
Free S = SS _CaS (3)
T. O is the total oxygen concentration analysis value (mass ppm), Ca is the Ca concentration analysis value (mass ppm), and S is the S concentration analysis value (mass ppm).