JP2012046789A - Steel material excellent in hydrogen-induced crack resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material excellent in hydrogen-induced crack resistance which can prevent generation of an internal inclusion defect, and to provide a method for producing the same.SOLUTION: The steel material excellent in hydrogen-induced crack resistance is obtained by rolling a steel cast piece including, by mass, 0.03 to 0.10% C, 0.1 to 0.5% Si, 0.8 to 2.0% Mn, 0.030% or less P, 0.0025% or less S, 0.005% or less N, 0.010 to 0.11% sol. Al, 0.0004 to 0.0040% Ca, and 0.0003 to 0.0060% of a rare earth element (REM), with the balance being Fe and inevitable impurities. The percentage of the number of the following inclusions present in the steel material is 50% or more: (i) the maximum circle equivalent diameter of the inclusions present in the steel material is 30 μm; and (ii) in the cross sectional view in the rolling direction of the steel material, the average composition is %AlO+%CaO+%REMO≥80%, and the area of the region of (%REM)/(%Al+%Ca+%REM)≥0.5 is 40% or more of the total area.

Description

  The present invention relates to a steel material excellent in resistance to hydrogen-induced cracking mainly used in the production of line pipes.

  Sulfur contained as an inevitable impurity in the steel sheet is combined with manganese contained in the steel material as a strength developing element to form MnS. MnS extends during rolling, and as a result, causes hydrogen-induced cracking in an environment containing hydrogen sulfide and moisture. In addition, the stretched MnS causes a deterioration in materials such as toughness.

  In order to prevent the occurrence of hydrogen-induced cracking due to MnS and the decrease in toughness, it is necessary to suppress the generation of MnS in the steel material. In particular, coarse MnS generated in the central segregation part of the continuous cast slab. It is important to suppress this.

  In order to suppress the formation of MnS in the steel sheet, it is effective to take measures to reduce the center segregation during continuous casting by reducing the content of S in the steel. Further, a method is used in which Ca or REM (rare earth element) is added to the steel, the sulfide is formed not as MnS, but as CaS or REM oxysulfide, and the sulfide is prevented from stretching during rolling. .

  Patent Document 1 describes a steel material excellent in hydrogen-induced crack resistance, containing S: less than 0.0020% and Ca: 0.0020% or more and less than 0.0050%. Patent Document 2 discloses hydrogen-resistant cracking including 0.040% or less (and 0.008% or more) of REM in a predetermined range in accordance with the S content reduced to 0.005% or less. Steel materials with excellent properties are described.

  Furthermore, in Patent Document 3, S: 0.01% or less, steel for line pipes having excellent resistance to hydrogen-induced cracking, containing REM in the range of 3 ≦ REM (%) / S (%) ≦ 10. Is described. Reference Document 4 includes S ≦ 0.008%, contains at least one of Ca and REM, Ca: 0.001% or more and less than 0.005%, and REM is at least 0.00. A steel material having an excellent resistance to hydrogen-induced cracking is described, containing 008% or more and an amount in a range determined according to the S and O contents.

JP 54-31019 A JP 54-31020 A JP-A-53-14606 JP 54-92511 A

  As described in Patent Document 1, when Ca is added to steel, the effect of preventing the extension of MnS is obtained by using sulfide as CaS, but satisfactory improvement is seen in materials including toughness. I understood that there was not. In addition, when a large amount of Ca is added to the steel, the problem that the nozzle refractory melts during continuous casting also occurs.

  As in Patent Documents 2 to 4, when REM is added to steel, the effect of preventing the stretching by controlling the form of sulfide is obtained, but the tendency of nozzle clogging during continuous casting increases, and it further exists inside the steel plate. It was found that product defects due to inclusions increased.

  The present invention improves the resistance to hydrogen-induced cracking and toughness in steel materials used in the production of line pipes and the like, and can prevent the occurrence of melting damage and nozzle clogging of continuously cast immersion nozzle refractories during production. And it aims at providing the steel material excellent in the hydrogen-induced crack-proof property which can prevent generation | occurrence | production of the inclusion defect in a steel plate inside, and its manufacturing method.

When Ca is added to the steel, the effect of preventing the extension of MnS can be obtained by changing the sulfide to CaS. However, the added Ca and Al added as a deoxidizer cause CaO—Al ₂ O _3. A system oxide is formed.

This CaO—Al ₂ O ₃ -based oxide is a low-melting-point oxide and has been found to be deteriorated in materials such as toughness because it is stretched during rolling. Moreover, in REM addition steel, it turned out that the nozzle clogging of an immersion nozzle and the inclusion defect inside a steel plate generate | occur | produce when the amount of REM addition exceeds 0.006 mass%.

On the other hand, even if it is MnS or a low-melting point CaO—Al ₂ O ₃ -based oxide, if it is finely dispersed, the length after stretching by rolling is shortened, resulting in resistance to hydrogen-induced cracking. It has been found that it is possible to produce a steel material that is excellent and prevents toughness deterioration and ensures toughness.

  This invention is made | formed based on the said knowledge, The place made into the summary is as follows.

(1) By mass%, C: 0.03-0.10%, Si: 0.1-0.5%, Mn: 0.8-2.0%, P: 0.030% or less, S: 0.0025% or less, N: 0.005% or less, sol. A steel slab made of Al: 0.010 to 0.11%, Ca: 0.0004 to 0.0040%, rare earth element (REM): 0.0003 to 0.0060%, the balance Fe and inevitable impurities is rolled. Steel material obtained by
(I) The maximum equivalent circle diameter of inclusions present in the steel material is 30 μm,
(Ii) In a cross-sectional view of the steel material in the rolling direction, the average composition is% Al ₂ O ₃ +% CaO +% REM ₂ O ₃ ≧ 80% and (% REM) / (% Al +% Ca +% REM) ) Excellent in hydrogen-induced cracking resistance, characterized in that inclusions having an area of ≧ 0.5 that is 40% or more of the total area are present in a ratio of the number of inclusions of 50% or more. Steel material.

(2) By mass%, C: 0.03-0.10%, Si: 0.1-0.5%, Mn: 0.8-2.0%, P: 0.030% or less, S: 0.0025% or less, N: 0.005% or less, sol. When melting molten steel composed of Al: 0.010 to 0.11%, Ca: 0.0004 to 0.0040%, rare earth element (REM): 0.0003 to 0.0060%, balance Fe and inevitable impurities ,
(I) After adjusting the composition of C, Si, Mn, P, S, N, REM is added so that the dissolved oxygen concentration after REM addition is 50 ppm or more, and then Al and Ca are added. Melting molten steel with the above composition,
(Ii) A method for producing a steel material excellent in resistance to hydrogen-induced cracking, wherein the molten steel is cast to produce a steel slab and then rolled to obtain a steel material.

In the present invention, at the slab stage before rolling, the average composition is% Al ₂ O ₃ +% CaO +% REM ₂ O ₃ ≧ 80% and (% REM) / (% Al +% Ca +% REM) A large number of inclusions including a region of ≧ 0.5 are generated and dispersed in the steel material, and MnS or a low-melting point CaO—Al ₂ O ₃ oxide is generated around the inclusions to form MnS or CaO. -al ₂ O ₃ based oxide by fine dispersion, to prevent the coarsening of MnS or CaO-Al ₂ O ₃ oxide.

That is, an inclusion including a region where the average composition is% Al ₂ O ₃ +% CaO +% REM ₂ O ₃ ≧ 80% and (% REM) / (% Al +% Ca +% REM) ≧ 0.5 Is used as a crystallization nucleus of MnS or CaO—Al ₂ O ₃ -based oxide.

  Thereby, in the steel material after rolling of the present invention, due to the effect of crystallization nuclei, the inclusions with the respective refined inclusion particle diameters are stretched, so the maximum diameter length of the inclusions after stretching is , Greatly shortened. As a result, according to the present invention, it is possible to manufacture and provide a steel material that is excellent in hydrogen-induced crack resistance and that prevents toughness deterioration and ensures toughness.

It is a figure which shows the relationship between the dissolved oxygen (ppm) after REM addition, and the number of MnS (pieces / mm < ² >). It is a figure which shows the relationship between the dissolved oxygen (ppm) after REM addition, and the largest MnS diameter (micrometer). It is a figure which shows the relationship between the number of MnS (pieces / mm ² ) and the maximum MnS diameter (maximum circle equivalent diameter (μm)).

  The present invention is directed to a steel material that is excellent in hydrogen-induced crack resistance and can be used in the production of line pipes and the like. Below, the basis which prescribes | regulates the component composition of the steel material of this invention is demonstrated. Unless otherwise specified,% means mass%.

C: 0.03-0.10%
C is set to 0.03% or more in order to obtain a necessary strength to be applicable to steel materials for line pipes. On the other hand, if C exceeds 0.10%, toughness and weldability deteriorate, so the upper limit is made 0.10%. A preferred lower limit is 0.043%, while a preferred upper limit is 0.082%.

Si: 0.1 to 0.5%
Si is an element necessary for deoxidation, and usually 0.1% or more is added, but if it exceeds 0.5%, the toughness deteriorates, so the upper limit is made 0.5%. A preferred lower limit is 0.22%, while a preferred upper limit is 0.37%.

Mn: 0.8 to 2.0%
Mn is an element that improves the strength, and 0.8% or more is added, but if it exceeds 2.0%, the weldability deteriorates, so the upper limit is made 2.0%. A preferred lower limit is 1.08%, while a preferred upper limit is 1.77%.

P: 0.030% or less P is an impurity that adversely affects the toughness of steel and the like, and is preferably as low as possible. However, the upper limit is set to 0.030% in view of the cost required for low phosphatization. A preferable upper limit is 0.022%. The lower limit is not particularly defined because it is preferably as low as possible.

S: 0.0025% or less From the viewpoint of preventing the formation of MnS, the lower the S content, the better. However, even if the secondary refining load is increased, it is practically difficult to make S lower than 0.0005%. On the other hand, if S exceeds 0.0025%, the single MnS that tends to be stretched during rolling increases to a certain level and deterioration of the material cannot be avoided, so the upper limit is made 0.0025%. A preferable upper limit is 0.0018%.

N: 0.005% or less When N exceeds 0.005%, even if Al is within the range of the present invention, a large amount of AlN may be generated and the toughness may be deteriorated. %. A preferable upper limit is 0.0029%. On the other hand, since the smaller N is, the lower limit is not particularly specified.

sol. Al: 0.010 to 0.11%
In the present invention, since sulfide form control is performed by adding Ca and REM, it is important to sufficiently deoxidize steel to suppress oxidation of Ca and REM as much as possible. Thus, Al is necessary as a deoxidizing element. By containing 0.010% or more of Al, sufficient deoxidation can be performed. However, if it exceeds 0.11%, a large amount of AlN is generated and there is a concern that toughness deteriorates, so the upper limit is made 0.11%. A preferred lower limit is 0.023%, while a preferred upper limit is 0.08%.

sol. Al is dissolved Al, and is analytically acid-soluble Al. Dissolved Al and Al ₂ O ₃ can be separated and analyzed by utilizing the fact that dissolved Al that does not form Al ₂ O ₃ dissolves in acid and Al ₂ O ₃ does not dissolve in acid. Incidentally, since the steel material of the present invention is obtained by sufficiently deoxidizing with Al, the total amount of oxygen in the steel is low, and for example, it can be exemplified that it is reduced to less than 0.0025%.

Ca: 0.0004 to 0.0040%
In addition to fixing S, Ca is added to generate a CaO—Al ₂ O ₃ -based oxide around the crystallization nuclei generated by the addition of REM described later. However, if Ca is too low, REM is effectively added alone and clogging of continuous casting nozzles and accumulation of REM oxide-based high specific gravity inclusions occur, so the lower limit is made 0.0004%.

On the other hand, when Ca exceeds 0.0040%, a large amount of coarse low-melting point oxides (mainly CaO—Al ₂ O ₃ -based oxides) are generated, causing internal defects in the product. Furthermore, since the nozzle refractory is easily melted and the continuous casting operation is not stable, the upper limit is made 0.0040%. A preferred lower limit is 0.0018%, while a preferred upper limit is 0.0033%.

REM: 0.0003 to 0.0060%
REM means a rare earth element and contains one or more elements selected from Ce, La, Nd, and Pr. As an addition method, for example, adding as a misch metal in steel is widely performed. Here, the total amount of the rare earth elements contained is defined as the REM amount.

In addition to fixing S, REM is added to generate crystallization nuclei for finely dispersing MnS and CaO—Al ₂ O ₃ -based oxides. However, if there is too little REM, the effect of generating crystallization nuclei for finely dispersing MnS and CaO—Al ₂ O ₃ -based oxides cannot be obtained, and the problem of material deterioration due to stretched oxides arises. The lower limit is made 0.0003%. A preferred lower limit is 0.0013%.

  On the other hand, if the REM exceeds 0.0060%, nozzle clogging and internal defects due to coarse REM oxide occur, so the upper limit is made 0.0060%. A preferred upper limit is 0.0048%.

  In order to obtain the steel material of the present invention, it is important to control the dissolved oxygen in the molten steel after REM addition to 50 ppm or more. That is, the present invention is directed to Al deoxidized steel, but REM addition does not add Al at all, or dissolved oxygen after REM addition remains at 50 ppm or more even if Al addition is performed. It is characterized in that it is performed at the stage where only preliminary deoxidation is performed. Then, Al is added after REM addition to adjust the final sol.Al concentration.

  Hereinafter, the concept of inclusion dispersion in the present invention will be described based on laboratory experiment results.

  In the laboratory experiment, 1 kg of electrolytic iron was first melted in an Ar atmosphere in a high frequency induction melting furnace. The dissolved oxygen immediately after dissolution was as high as 300 to 400 ppm. Next, various amounts of Al were added to change the dissolved oxygen to various values, and then REM was added. The amount of dissolved oxygen was measured after REM addition, then Al was added, and finally Ca was added with a Ca-Si alloy, and the furnace was turned off and cooled in the furnace.

  The component composition of the obtained slab was REM = 20-25 ppm, sol. Al = 0.030-0.035% and Ca = 15-20 ppm. That is, the slab analysis value was within a certain range, and the experiment was conducted by changing only the dissolved oxygen amount after the addition of REM.

  The cross section of the slab thus produced was micro-polished and observed with an optical microscope at 50 magnifications at 1000 magnifications, and the number and size of inclusions having an equivalent circle diameter of 1 μm or more were measured. The reason why inclusions with an equivalent circle diameter of 1 μm or more are used is that it is known that small inclusions with an equivalent circle diameter of less than 1 μm do not affect the resistance to hydrogen-induced cracking.

FIG. 1 shows the relationship between dissolved oxygen (ppm) after REM addition and the number of inclusions (pieces / mm ² ), and FIG. 2 shows dissolved oxygen (ppm) after REM addition and the maximum MnS diameter (equivalent circle diameter). The relationship of the maximum value (μm) is shown. From these relationships, when the dissolved oxygen after REM addition is 50 ppm or more, the number of inclusions increases rapidly (see FIG. 1), and the inclusion size decreases as the number of inclusions increases. (See FIG. 2).

Moreover, as a result of investigating the composition of 20 arbitrary inclusions by SEM-EDS, in addition to MnS, CaO—Al ₂ O ₃ inclusions, CaO—Al ₂ O ₃ —REM ₂ O ₃ inclusions, MnS A form (MnS partially or entirely covered) was observed around the CaO—Al ₂ O ₃ inclusions and CaO—Al ₂ O ₃ —REM ₂ O ₃ inclusions. Some CaO—Al ₂ O ₃ inclusions and CaO—Al ₂ O ₃ —REM ₂ O ₃ inclusions containing S were also observed.

In the case of a sample in which dissolved oxygen after REM addition is 50 ppm or more and the number of inclusions is large, in the obtained steel material, the average composition is% Al ₂ O ₃ +% CaO +% REM _{2 in} terms of the number ratio of inclusions. The region where O ₃ ≧ 80% and (% REM) / (% Al +% Ca +% REM) ≧ 0.5 (crystallization phase) was inclusions with an area ratio of 40% or more.

  Based on the above experimental results, the technical idea of the present invention will be described.

(1) [When Al and Ca are added without REM addition]
Since the dissolved oxygen after the addition of Al (= Al deoxidation) is low, agglomeration and coalescence of Al ₂ O ₃ proceeds and clusters are generated. The number of clusters decreases as they grow larger. When Ca is added thereto, a coarse Al ₂ O ₃ —CaO-based oxide is generated with an Al ₂ O ₃ cluster as a base.

MnS is generated during the solidification of the molten steel, but it is generated alone or attached to the Al ₂ O ₃ —CaO-based oxide. Since the number of inclusions that serve as attachment sites is small, the amount of single MnS is larger than that of the present invention described later. In addition, when MnS adheres on a coarse Al ₂ O ₃ —CaO-based oxide, the size of the entire inclusion becomes further coarse.

When this slab is rolled, not only single MnS but also inclusions in which MnS adheres on coarse Al ₂ O ₃ —CaO-based oxides, Al ₂ O ₃ —CaO-based oxides are low melting point oxides. Easy to stretch. In this way, most of the inclusions are stretched longer, and the HIC starting point is increased.

(2) [When REM and Ca are added after deoxidizing Al to make dissolved oxygen less than 50 ppm]
Al ₂ O ₃ clusters are generated during Al deoxidation, and then when REM is added, a single REM oxide or Al ₂ O ₃ -REM ₂ O ₃ oxide is generated. Since the dissolved oxygen after Al deoxidation is low, inclusions after the addition of REM are difficult to disperse, and agglomeration and coalescence proceed and become coarse. In this way, as a result of coarsening, the number of inclusions decreases.

  Moreover, the existing inclusion changes to the composition containing CaO by Ca addition. MnS produced during solidification is produced alone or on the coarse inclusions. Since the number of inclusions serving as attachment sites is small, the number of single MnS is larger than that of the present invention described later. Since single MnS is easily stretched during rolling, the HIC property is not good.

(3) [In the present invention, when the dissolved oxygen after REM addition is maintained at 50 ppm or more and then Al deoxidation and Ca addition are performed]
When REM is added, REM oxide is generated. Here, since dissolved oxygen after REM addition is high, aggregation and coalescence of REM oxides are suppressed and finely dispersed.

It has been found that this tendency can be sufficiently obtained as long as the dissolved oxygen after the addition of REM is 50 ppm or more regardless of the presence or absence of Al preliminary deoxidation. Thereafter, when Al deoxidation is performed, Al ₂ O ₃ is produced with the finely dispersed REM oxide as a nucleus, so that Al ₂ O ₃ can be finely dispersed.

In addition, after Ca addition, relatively fine Al ₂ O ₃ having the REM oxide as a nucleus is modified into an Al ₂ O ₃ —CaO system. And during solidification, MnS is formed alone or around the oxide. Since the oxide is finely dispersed, most of the MnS deposited and formed on the oxide is small, and there is little single MnS. Therefore, there is little single MnS which is easy to stretch, and the HIC resistance is also good.

Moreover, the average composition of the produced inclusions in the cross-sectional view is% Al ₂ O ₃ +% CaO +% REM ₂ O ₃ ≧ 80% and (% REM) / (% Al +% Ca +% REM) ≧ The region of 0.5 (crystallization phase) hardly stretches during rolling. Therefore, even if MnS adheres and forms around the inclusions including such a region, the whole inclusions including MnS are difficult to stretch.

  According to the results of laboratory experiments conducted by the present inventors, inclusions present in the steel material after rolling as long as the areas satisfying the above conditions occupy 40% or more of the total area ratio of inclusions including MnS. The maximum equivalent circle diameter was found to be 30 μm.

  In this invention, it is preferable to contain the following element further as needed.

Ti: 0.05% or less Ti may be added to improve the strength. However, if the Ti content exceeds 0.05%, angular TiN is generated and the toughness decreases, so the upper limit was made 0.05%.

Nb: 0.05% or less Nb may be added to improve the strength. However, if the Nb content exceeds 0.05%, coarse Nb (C, N) precipitates and causes a decrease in toughness, so the upper limit was made 0.05%.

V: 0.05% or less V may be added to improve the strength. However, if the V content exceeds 0.05%, coarse precipitates are generated and the toughness is reduced, so the upper limit was made 0.05%.

Cr: 0.5% or less Cr may be added to improve the strength. However, if the Cr content exceeds 0.5%, the toughness is reduced, so the upper limit was made 0.5%.

Mo: 0.5% or less Mo may be added to improve the strength. However, if the Mo content exceeds 0.5%, the toughness is deteriorated and the upper limit is set to 0.5% for economic reasons.

B: 0.0020% or less B may be added to improve hardenability and strength. However, if the B content exceeds 0.0020%, deterioration of toughness is caused, so the upper limit was made 0.0020%.

Ni: 0.5% or less Ni may be added for the purpose of improving strength and toughness. However, when the Ni content exceeds 0.5%, the hot workability deteriorates, so the upper limit was made 0.5%. In addition, as a playing element, generally about 0.01% Ni is contained in steel.

Cu: 0.5% or less Cu may be added for the purpose of improving strength and toughness. However, if the Cu content exceeds 0.5%, the hot workability is impaired, so the upper limit was made 0.5%. In addition, as a playing element, generally about 0.01% of Cu is contained in steel.

  Next, a method for producing a steel material excellent in hydrogen-induced crack resistance according to the present invention will be described. A case where a slab is manufactured by continuous casting after blast furnace refining using blast furnace hot metal as a raw material will be described as an example.

In the present invention, since low sulfur steel with S of 0.0025% or less is targeted, generally hot metal desulfurization and molten steel desulfurization are used in combination. Al may be added after the converter steel and the molten steel may be pre-deoxidized. Thereafter, when molten steel desulfurization is performed in the secondary refining process, a desulfurization agent containing CaO—CaF ₂ as a main component is added to perform desulfurization treatment according to the requirements of the steel material.

REM, after adjusting the composition of the other elements, and when preliminarily deoxidizing Al, after taking time (for example, 5 minutes or more) to float Al ₂ O ₃ generated by the preliminary deoxidation, It is preferable to add.

If a large amount of Al ₂ O ₃ remains in the molten steel, the REM added after that will be consumed for the reduction of Al ₂ O ₃ , and the ratio used for fixing S will be reduced, and the generation of MnS will be sufficient. This is because it cannot be prevented. Moreover, when performing Al preliminary deoxidation, it is important to carry out in the range which can make the dissolved oxygen concentration after REM addition 50 ppm or more.

Thereafter, Al and Ca are added. Since Al is added to deoxidize dissolved oxygen, Al ₂ O ₃ is generated in the molten steel. Since the amount of Al ₂ O ₃ produced here is not so large, even if Ca is added simultaneously with Al, the amount of Ca consumed for the reduction of Al ₂ O ₃ is small, which is not a problem. However, it is preferable to add Ca after allowing the Al ₂ O ₃ to float (for example, 5 minutes or more) because the amount of Ca consumed for the reduction of Al ₂ O ₃ can be further reduced.

  Incidentally, since Ca has a high vapor pressure, it is generally added in the form of a Ca—Si alloy, a Ca—Ni alloy or the like in order to increase the yield. In addition of these alloys, addition of each alloy wire may be used. REM may be added in the form of Fe-Si-REM alloy or misch metal.

  The molten steel melted as described above is manufactured into a slab using a continuous casting machine, and then rolled in a hot rolling process to obtain the steel material of the present invention.

Blast furnace hot metal was used as a raw material, and in the hot metal preliminary treatment step, a desulfurization agent mainly composed of CaO was blown into the hot metal in the torpedo car to perform preliminary desulfurization. This hot metal was decarburized in an upper bottom blowing converter with a molten steel amount of 300 tons. After the converter steel, Al was added to the molten metal to pre-deoxidize the molten steel. Thereafter, molten steel desulfurization was performed in the secondary refining process, and a desulfurizing agent containing CaO—CaF ₂ as a main component was added to perform desulfurization treatment according to the target S content.

After adjusting the composition of various elements, REM was added after a time of 5 minutes or more in order to float Al ₂ O ₃ generated by Al preliminary deoxidation. Thereafter, Al and Ca were added. Since Ca has a high vapor pressure, Ca was added in the form of a Ca—Si alloy in order to increase the yield. REM was added in the form of misch metal.

  A slab having a thickness of 240 mm was obtained by continuous casting. Thereafter, the slab was heated under the condition of 1250 ° C. × 1 hour, and the plate was rolled to a plate thickness of 12 mm under the condition of a finishing temperature of 850 ° C. The reduction ratio is 20.

  In continuous casting, an immersion nozzle for injecting molten steel from a tundish into a mold was evaluated. If the inclusion layer or a metal layer including inclusions is 10 mm or more on the inner surface of the nozzle after casting for 1 heat (300 tons), the nozzle clogging is “Yes”, and the others are “No”. "

  About the manufactured steel materials, the length of the inclusion was investigated. In a cross section parallel to the rolling direction, an optical microscope was used to observe a range of thickness direction 12 mm × longitudinal direction 10 mm at a magnification of 400 times (however, when the inclusion shape was measured in detail, the magnification was 1000 times). The reason for observing the cross section parallel to the rolling direction is that the inclusion is elongated the longest in this cross section.

  Regarding the composition of inclusions, 30 inclusions having a circle-equivalent diameter of 1 μm or more were observed with a manipulation electron microscope (SEM) and analyzed with a composition analyzer such as EDS attached to the SEM. In the SEM image, if there is a region having a different composition inside the inclusion, it can be identified by the difference in brightness, so that the composition of each region can be analyzed.

Therefore, for each inclusion, the average composition is% Al ₂ O ₃ +% CaO +% REM ₂ O ₃ ≧ 80% and (% REM) / (% Al +% Ca +% REM) ≧ 0.5 The area ratio of a certain region (crystallized phase) was determined, and the number ratio of the inclusions with an area ratio of 40% or more to the total number of inclusions was determined.

  The resistance to hydrogen-induced cracking of steel was evaluated according to the method specified in NACE (National Association of Corrosion Engineers) TM0284-2003. Ten test pieces having a thickness of 10 mm, a width of 20 mm, and a length of 100 mm were collected from each steel material in parallel with the rolling direction. The test piece was immersed for 96 hours in a 25 ° C. (0.5% acetic acid + 5% sodium chloride) aqueous solution saturated with 101.325 kPa of hydrogen sulfide.

  After the test, the area of hydrogen induced cracks generated in each test piece was measured by an ultrasonic flaw detection method, and the crack area ratio was calculated. The crack area ratio per test piece is (crack area generated in each test piece / test piece area) × 100 (%), and the test piece area is 20 mm × 100 mm. When the average value of the cracked area ratios of 10 test pieces was 1% or more, x was evaluated as ○ when the average value was less than 1%.

  For internal defects caused by inclusions accumulated on the lower surface side of the continuous cast slab, the cross section parallel to the rolling direction of the steel material was observed with an optical microscope, and inclusions with a length exceeding 50 μm (stretched, lump-like regardless of shape) , All on the cluster) were evaluated as x, otherwise it was evaluated as ◯.

  Production conditions and production results are shown in Table 1 and Table 2 (continuation of Table 1). Numerical values that fall outside the scope of the present invention are underlined.

Invention Examples 1 to 15 in Tables 1 and 2 are steel materials that satisfy the conditions of the present invention. A region (crystallization phase) in which the average composition is% Al ₂ O ₃ +% CaO +% REM ₂ O ₃ ≧ 80% and (% REM) / (% Al +% Ca +% REM) ≧ 0.5 Inclusions with an area ratio of 40% or more were 50% or more in proportion to the total number of inclusions. The maximum length of inclusions in a cross section parallel to the rolling direction was 30 μm or less, and the evaluation of internal defects and HIC resistance was “◯”. The nozzle was not clogged, and the evaluation was “none”.

Comparative Examples 1 to 8 in Tables 1 and 2 are comparative examples and include conditions outside the scope of the present invention. In Comparative Examples 1 to 5, the dissolved oxygen after the addition of REM was less than 50 ppm. As a result, the region where the average composition is% Al ₂ O ₃ +% CaO +% REM ₂ O ₃ ≧ 80% and (% REM) / (% Al +% Ca +% REM) ≧ 0.5 (crystallization) Inclusions whose phase ratio is 40% or more in area ratio is less than 50% in proportion to the total number of inclusions, indicating that the inclusions are not finely dispersed.

  In addition, the maximum length of inclusions exceeded 50 μm, and as a result, both internal defects and HIC resistance were “x”. In Comparative Example 6, since the amount of REM was small, the inclusions were stretched, the maximum length exceeded 50 μm, and both internal defects and HIC resistance were “x”. In Comparative Examples 7 and 8, since the amount of REM was excessive, nozzle clogging occurred and the internal defect was “x”.

  As described above, according to the present invention, it is possible to manufacture and provide a steel material that has excellent resistance to hydrogen-induced cracking and that has ensured toughness by preventing a decrease in toughness. Therefore, the present invention has high applicability in the steel industry.

Claims (2)

In mass%, C: 0.03-0.10%, Si: 0.1-0.5%, Mn: 0.8-2.0%, P: 0.030% or less, S: 0.0025 % Or less, N: 0.005% or less, sol. A steel slab made of Al: 0.010 to 0.11%, Ca: 0.0004 to 0.0040%, rare earth element (REM): 0.0003 to 0.0060%, the balance Fe and inevitable impurities is rolled. Steel material obtained by
(I) The maximum equivalent circle diameter of inclusions present in the steel material is 30 μm,
(Ii) In a cross-sectional view of the steel material in the rolling direction, the average composition is% Al ₂ O ₃ +% CaO +% REM ₂ O ₃ ≧ 80% and (% REM) / (% Al +% Ca +% REM) ) Excellent in hydrogen-induced cracking resistance, characterized in that inclusions having an area of ≧ 0.5 that is 40% or more of the total area are present in a ratio of the number of inclusions of 50% or more. Steel material. In mass%, C: 0.03-0.10%, Si: 0.1-0.5%, Mn: 0.8-2.0%, P: 0.030% or less, S: 0.0025 % Or less, N: 0.005% or less, sol. When melting molten steel composed of Al: 0.010 to 0.11%, Ca: 0.0004 to 0.0040%, rare earth element (REM): 0.0003 to 0.0060%, balance Fe and inevitable impurities ,
(I) After adjusting the composition of C, Si, Mn, P, S, N, REM is added so that the dissolved oxygen concentration after REM addition is 50 ppm or more, and then Al and Ca are added. Melting molten steel with the above composition,
(Ii) A method for producing a steel material excellent in resistance to hydrogen-induced cracking, wherein the molten steel is cast to produce a steel slab and then rolled to obtain a steel material.

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