JP7533816B1 - Steel plate, its manufacturing method, and steel pipe - Google Patents

Abstract

The present invention provides a steel sheet having excellent HIC resistance. The steel sheet has a predetermined chemical composition, and in a pair of regions extending from each of both sides of the steel sheet to a depth of ¼ of the sheet thickness in the sheet thickness direction, the steel sheet has an average composition containing 10 to 40 mass % of MgO, inclusions are present with an average aspect ratio of 2.5 or less, the top 10% of the inclusions have an average equivalent circle diameter of 3.5 μm or less, and the crack area ratio CAR after an HIC test in the regions is 5.0% or less.

Description

The present invention relates to a steel plate having excellent hydrogen induced cracking resistance (HIC (Hydrogen Induced Cracking) resistance) suitable for use in line pipes used for transporting crude oil or natural gas, and a manufacturing method thereof. The present invention also relates to a steel pipe using the above-mentioned steel plate.

Generally, line pipes are manufactured by forming steel plates produced by a plate mill or a hot rolling mill into steel pipes by UOE forming, press bending, roll forming, etc.

Line pipes used to transport crude oil or natural gas containing hydrogen sulfide are required to have so-called sour resistance, such as HIC resistance and sulfide stress corrosion cracking resistance (SSCC (Sulfide Stress Corrosion Cracking) resistance), in addition to strength, toughness, weldability, etc. In particular, HIC occurs when hydrogen ions generated by a corrosion reaction are adsorbed on the steel surface, then penetrate into the steel as atomic hydrogen, which diffuses and accumulates around non-metallic inclusions such as MnS or hard second phase structures in the steel, becoming molecular hydrogen, and the internal pressure causes cracks.

One example of a non-metallic inclusion in steel that is likely to be the starting point of HIC is MnS (manganese sulfide) elongated by hot rolling. In response to this, a technique is known in which calcium (Ca) is added to molten steel to react with sulfur (S) in the steel to generate CaS (calcium sulfide), a sulfide that is more stable than MnS, thereby suppressing the generation of MnS and improving HIC resistance. The added Ca also reacts with oxygen (O) in the steel and Al ₂ O ₃ (aluminum oxide), a deoxidation product, to generate CaO-Al ₂ O ₃ -based non-metallic inclusions.

Therefore, when the amount of Ca in the molten steel is insufficient, there is not enough Ca to react with S in the steel, and a lot of MnS is generated, deteriorating the HIC resistance in the center segregation area.On the other hand, when the amount of Ca is excessive, CaO- _{Al2O3 -} based non-metallic inclusion clusters with a high CaO concentration are generated, deteriorating the HIC resistance in the surface layer of the steel sheet.

In recent years, efforts have been made to develop cheaper steel sheets by increasing the manganese (Mn) content and decreasing the contents of other alloying elements. In the case of such steel sheets, MnS, which deteriorates HIC resistance, is generated from concentrated Mn and concentrated S in the central segregation area, so there is a problem that even if the S content of the entire steel sheet is reduced, the frequency of MnS generation in the central segregation area increases.

As a countermeasure, if an operation is carried out in which the amount of Ca added is further increased in order to suppress the formation of MnS, the above-mentioned CaO-Al ₂ O ₃ based non-metallic inclusion clusters with a high CaO concentration are likely to form. Then, if these inclusion clusters are trapped in the surface layer of the slab during continuous casting, they become the starting points of HIC in the surface layer of the steel plate, causing a deterioration in HIC resistance.

Therefore, in order to improve the HIC resistance of steel plates with a high Mn content, it is necessary to suppress the formation of MnS and inclusion clusters by methods other than increasing the amount of Ca added.

One possible way to solve the above problem is to use an element that has a strong affinity for S and a high sulfide-forming ability in combination with Ca. For example, magnesium (Mg) is known to be an element that, like Ca, can form sulfides that are more stable than MnS. When comparing the stability of sulfides thermodynamically, the order is CaS > MgS > MnS, so Mg can be expected to have the same effect as Ca in suppressing the formation of MnS.

Here, Patent Documents 1 and 2 disclose sour-resistant steel materials that contain not only Ca but also Mg. Patent Document 3 discloses a sour-resistant steel material that contains Mg as an element that is effective in improving HIC resistance.

As a method of adding Mg to molten steel, Patent Document 4 discloses a method in which 2 to 10 mass % of MgO is added to the slag to supply MgO to the non-metallic inclusions and control the composition of the non-metallic inclusions.

As other methods of adding Mg, Patent Document 5 discloses a method of adjusting the composition in a refining vessel in which an MgO-containing refractory material is used as part or all of the lining refractory, and Patent Document 6 discloses a method of adding a Ti-Mg alloy.

As a method of adding Ca and Mg, Patent Document 7 discloses a method in which inert gas and additives such as Ca-Si and Mg-Si are sprayed onto the molten steel flow injected from a molten steel ladle into a tundish in a continuous casting facility.

Regarding the shape of inclusions, it is known that the larger the aspect ratio (longer diameter/shorter diameter) of the inclusions, the greater the stress concentration caused by the inclusions, and furthermore, that the inclusions are deformed by rolling the slab after casting. In other words, if the aspect ratio of the inclusions increases through rolling, the stress concentration caused by the inclusions increases, making HIC more likely to occur, and therefore it is difficult to manufacture steel sheets with excellent HIC resistance using conventional rolling methods.

JP 2017-172010 A JP 2017-110249 A JP 2014-208891 A JP 2012-188696 A JP 2004-043838 A JP 2002-266019 A JP 2017-087229 A

However, Patent Documents 1 and 2 state that Mg exists as fine Mg-based oxides, and that these fine Mg-based oxides function as precipitation nuclei for TiN, and are effective in improving the toughness of the weld heat-affected zone (HAZ). In other words, in the technology described in Patent Documents 1 and 2, the surface of the Mg-based oxides is covered with precipitated TiN, so Mg cannot contribute to the formation of MgS.

In the method described in Patent Document 4, the MgO-based non-metallic inclusions produced by the reduction of the slag entrained in the molten steel are coarse, and there is concern that they may actually become the starting point for HIC.

The technologies described in Patent Documents 5 and 6 are not economical when considering the cost of repairing refractories and the cost of alloys containing expensive Ti (titanium).

The method described in Patent Document 7 has the problem that the injected inert gas may become entrained in the molten steel flow, leading to a deterioration in the quality of the cast piece, and there is also the problem that it is difficult to simultaneously control the shape of non-metallic inclusions by adding alloys and suppress cellular defects.

As described above, the prior art proposes sour steel materials containing Ca and Mg, but all of them have the above problems. In addition, steel plates containing Ca and Mg have the following problems:

In other words, the Ca and Mg addition process, which is not mentioned in Patent Documents 1 to 3 and Patent Document 7, had the following problems. Ca oxides and sulfides are more thermodynamically stable than Mg oxides and sulfides. Therefore, when non-metallic inclusions (oxides, sulfides) containing Ca have already been formed in the molten steel, the effect of Mg added later on the morphology of the non-metallic inclusions is small.

Ca oxides and sulfides are more stable than Mg oxides and sulfides, so S and O (oxygen) in the molten steel react easily with Ca, while Mg does not react easily with S and O (oxygen) in the molten steel. Therefore, it is difficult to expect desulfurization through the formation of MgS and deoxidation through the formation of MgO with the addition of Mg, or morphological control of nonmetallic inclusions.

In addition, because the CaO formed in the steel cannot be effectively reduced by Mg, the proportion of Ca that contributes to desulfurization cannot be increased. Therefore, it is difficult to reduce the amount of Ca added to suppress the formation of MnS, and the inclusion clusters are trapped in the surface layer of the cast slab, making it difficult to ensure sufficient HIC resistance in the surface layer of the steel plate.

In view of the above problems, the present invention aims to provide a steel plate having excellent HIC resistance, together with an advantageous manufacturing method thereof. The present invention also aims to provide a steel pipe using the above-mentioned steel plate having excellent HIC resistance.

In order to improve HIC resistance, the inventors conducted numerous experiments and studies on the chemical composition and manufacturing conditions of steel plates, and obtained the following findings:

That is, in the process of adding the Mg-containing substance and the Ca-containing substance to the molten steel, the temperature of the molten steel and the supply rate of Mg and Ca are appropriately controlled, and then the addition of the Mg-containing substance is started after the addition of the Ca-containing substance, or the addition of the Mg-containing substance and the Ca-containing substance is started simultaneously. This allows dissolved Mg to be appropriately present in the molten steel at the same time as dissolved Ca. As a result, the activity of oxygen and sulfur in the molten steel decreases due to the influence of dissolved Mg present in the molten steel, and the equilibrium of the reaction of the non-metallic inclusions moves in the direction of the formation of CaS. In this way, the formation of CaS is promoted, and the improvement of the desulfurization action is effectively achieved. In addition, as a result of adopting the above-mentioned order of addition, the reaction between Mg and S in the molten steel and the reaction between Mg and O (oxygen) in the molten steel progress, making it possible to suppress the formation of MnS without forming coarse inclusion clusters. Note that "dissolved Ca" and "dissolved Mg" refer to Ca and Mg dissolved in the atomic state in molten steel, respectively.

In addition to optimizing the above-mentioned addition process, the casting conditions are optimized, including the time from the end of addition of the Ca-containing substance to the start of casting and the average flow rate of the molten steel in the mold during casting. As a result, in a pair of regions extending from each of both sides of the steel plate to a depth of 1/4 of the plate thickness in the plate thickness direction (hereinafter also referred to as the "surface region" in this specification), inclusions are present that have an average composition containing 10 to 40 mass% MgO and have an average aspect ratio of 2.5 or less. The average of the top 10% of the circle equivalent diameters of these inclusions can be set to 3.5 μm or less. As a result, a steel plate with excellent HIC resistance can be obtained, with a crack area ratio CAR of 5.0% or less after HIC testing in the surface region.

The gist and configuration of the present invention, which has been completed based on the above findings, are as follows.
[1] In mass%, it contains C: 0.030 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Nb: 0.080% or less, N: 0.0080% or less, Ca: 0.0005 to 0.0050%, and Mg: 0.0005 to 0.0050%, with the balance being Fe and unavoidable impurities;
inclusions having an average composition containing 10 to 40 mass% MgO and an average aspect ratio of 2.5 or less are present in a pair of regions from each of both surfaces of the steel sheet to a depth of 1/4 of the sheet thickness in the sheet thickness direction,
the average value of the top 10% of the equivalent circle diameters of the inclusions is 3.5 μm or less;
The steel plate is characterized in that the crack area ratio CAR after an HIC test in the above-mentioned region is 5.0% or less.

[2] The steel plate described in [1] above, wherein the composition further contains, by mass%, one or more selected from the group consisting of Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.100% or less, Ti: 0.100% or less, Zr: 0.0200% or less, and REM: 0.0200% or less.

[3] An addition step of adding an Mg-containing substance and a Ca-containing substance to the molten steel after the completion of refining,
(A) the temperature of the molten steel at the start of the adding step is in the range of 1580 to 1620°C;
(B) The addition of the Mg-containing substance is started after the addition of the Ca-containing substance is started, or the addition of the Mg-containing substance and the Ca-containing substance is started simultaneously,
(C) The feed rates of Mg and Ca are each 15 to 30 kg/min;
Next, the molten steel is cast to obtain a steel slab under the conditions that (i) the time from the completion of the addition of the Ca-containing substance to the start of casting is 90 minutes or less, and (ii) the average flow velocity of the molten steel in the mold during casting is 0.10 m/s or more,
Next, the steel slab is subjected to hot rolling to produce the steel plate according to the above-mentioned [1] or [2].

[4] The hot rolling is carried out under conditions in which the heating temperature of the steel slab is 1000 to 1250 ° C. and the cross rolling ratio is 20 or less;
Next, controlled cooling of the steel sheet is performed under conditions in which the surface temperature of the steel sheet at the start of cooling is equal to or higher than the _Ar3 point calculated by the following formula (1).
The method for producing a steel sheet according to the above [3].
Ar ₃ points (°C) = 910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo]...(1)
Here, [X] means the content (mass%) of element X in the steel.

[5] A steel pipe using the steel plate described in [1] or [2] above.

The steel plate and steel pipe of the present invention have excellent HIC resistance. Furthermore, according to the steel plate manufacturing method of the present invention, a steel plate with excellent HIC resistance can be manufactured.

(Steel plate)
A steel plate according to an embodiment of the present invention has a predetermined chemical composition, has predetermined inclusions in a pair of first regions (surface layer regions) extending from each of both sides of the steel plate to a depth of 1/4 of the plate thickness in the plate thickness direction, and has excellent HIC resistance in the surface layer regions. In this specification, the position at a depth of 1/4 of the plate thickness from each of both sides of the steel plate in the plate thickness direction is simply referred to as the "plate thickness 1/4 position."

[Component composition]
First, the chemical composition of the steel sheet and the reasons for limiting it will be described. In the following description, the unit of "%" means "mass %" unless otherwise specified.

C: 0.030-0.080%
C effectively contributes to improving the strength of the steel sheet. However, if the C content is less than 0.030%, sufficient strength cannot be ensured. Therefore, the C content is set to 0.030% or more, and preferably 0.035%. On the other hand, if the C content exceeds 0.080%, the hardness of the surface layer and the central segregation increases during accelerated cooling, and the HIC resistance deteriorates. The content is set to 0.080% or less, and preferably to 0.070% or less.

Si: 0.01~0.50%
Silicon is added for deoxidation. If the silicon content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the silicon content is set to 0.01% or more, and preferably 0.05% or more. On the other hand, if the Si content exceeds 0.50%, the low temperature toughness and surface properties deteriorate. Therefore, the Si content is set to 0.50% or less, and preferably 0.45% or less. The following applies.

Mn: 0.80-1.80%
Mn suppresses the formation of ferrite during cooling, but if the Mn content is less than 0.80%, this effect is not fully manifested. Therefore, the Mn content is set to 0.80% or more, and preferably 1. On the other hand, if the Mn content exceeds 1.80%, center segregation is promoted and HIC resistance is deteriorated. Therefore, the Mn content is set to 1.80% or less, and preferably Not more than 1.70%.

P: 0.015% or less P is an inevitable impurity element, and increases the hardness of the surface layer and the central segregation, thereby deteriorating HIC resistance. If the P content exceeds 0.015%, this tendency becomes significant, so the P content is set to 0.015% or less, and preferably 0.008% or less. The lower the P content, the better, but from the viewpoint of refining costs, the P content is preferably set to 0.001% or more.

S: 0.0015% or less S is an inevitable impurity element, and in steel, it becomes MnS inclusions, which deteriorates HIC resistance. Therefore, the S content is set to 0.0015% or less, and preferably to 0.0010% or less. The lower the S content, the better, but from the viewpoint of refining costs, the S content is preferably set to 0.0002% or more.

Al: 0.010-0.080%
Al is added as a deoxidizer, but if the Al content is less than 0.010%, the effect is not fully exerted. Therefore, the Al content is set to 0.010% or more, and preferably 0.015% or more. On the other hand, if the Al content exceeds 0.080%, problems such as clogging of the submerged nozzle with alumina during continuous casting occur, so the Al content is set to 0.080% or less, and preferably 0.080% or less. .070% or less.

Nb: 0.080% or less When present as solid solution Nb, Nb expands the non-recrystallization temperature range during controlled rolling and contributes to improving low-temperature toughness. From the viewpoint of fully obtaining this effect, the Nb content is preferably 0.005% or more, more preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.080%, coarse carbides are crystallized during solidification, and HIC resistance is deteriorated. For this reason, the Nb content is set to 0.080% or less, preferably 0.060% or less.

N: 0.0080% or less N produces a refinement effect on the structure by the formation of nitrides. From the viewpoint of fully obtaining this effect, the N content is preferably 0.0010% or more, more preferably 0.0015% or more. On the other hand, if the N content exceeds 0.0080%, the HIC resistance and low-temperature toughness deteriorate due to the formation of coarse nitrides. For this reason, the N content is set to 0.0080% or less, preferably 0.0070% or less.

Ca: 0.0005-0.0050%
Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide-based inclusions, but if the Ca content is less than 0.0005%, the effect of adding it is insufficient. On the other hand, if the Ca content exceeds 0.0050%, not only will the above-mentioned effects saturate, but the cleanliness of the steel will decrease. Therefore, the Ca content is set to 0.0050% or less, and preferably to 0.0045% or less.

Mg: 0.0005-0.0050%
Mg is an element that has deoxidizing and desulfurizing effects, and has the effect of increasing the efficiency of CaS generation through interaction and improving the desulfurization of nonmetallic inclusions. Mg also has the effect of refining the inclusions. Since Mg is an element, it is effective in suppressing the generation of clusters of inclusions in the surface layer of the steel sheet and improving HIC resistance. In order to stably exert this effect, the Mg content is set to 0.0005% or more. On the other hand, when the Mg content increases, the effect of Mg becomes saturated, so the Mg content is set to 0.0050% or less.

The above describes the basic components in the composition of the steel plate according to this embodiment, but the composition may optionally contain one or more elements selected from the group consisting of Cu, Ni, Cr and Mo within the following ranges in order to further improve the strength and toughness of the steel plate.

Cu: 0.30% or less Cu is an element effective in improving low-temperature toughness and increasing strength, and in order to obtain this effect, the Cu content is preferably 0.01% or more. On the other hand, since Cu is concentrated in the center segregation portion, if the Cu content exceeds 0.30%, the HIC resistance deteriorates. Therefore, when Cu is contained, the Cu content is set to 0.30% or less, preferably 0.25% or less.

Ni: 0.30% or less Ni is an element effective in improving low-temperature toughness and increasing strength, and in order to obtain this effect, the Ni content is preferably 0.01% or more. On the other hand, since Ni is concentrated in the center segregation portion, if the Ni content exceeds 0.30%, the HIC resistance deteriorates. Therefore, when Ni is contained, the Ni content is set to 0.30% or less, preferably 0.25% or less.

Cr: 0.50% or less Like Mn, Cr is an effective element for obtaining sufficient strength even with a low C content, and in order to obtain this effect, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content is too high, it promotes center segregation and deteriorates HIC resistance. Therefore, when Cr is contained, the Cr content is set to 0.50% or less, preferably 0.45% or less.

Mo: 0.50% or less Mo is an element effective in improving low-temperature toughness and increasing strength, and in order to obtain this effect, the Mo content is preferably 0.01% or more, and more preferably 0.10% or more. On the other hand, if the Mo content is too high, weldability deteriorates. Therefore, when Mo is contained, the Mo content is 0.50% or less, and preferably 0.40% or less.

The chemical composition of the steel plate according to this embodiment may optionally contain one or more elements selected from the group consisting of V, Ti, Zr, and REM within the following ranges.

V: 0.100% or less, Ti: 0.100% or less V and Ti are both elements that can be optionally contained to increase the strength and low-temperature toughness of the steel plate. To obtain this effect, the content of each element is preferably 0.005% or more. On the other hand, if the content of each element exceeds 0.100%, the toughness of the welded portion deteriorates. Therefore, when these elements are contained, the content of each element is set to 0.100% or less.

Zr: 0.0200% or less, REM: 0.0200% or less Zr and REM are elements that can be optionally contained to improve low-temperature toughness through grain refinement and to improve crack resistance through control of inclusion properties. To obtain this effect, the content of each element is preferably 0.0005% or more. On the other hand, when the content of each element exceeds 0.0200%, the effect is saturated, so when these elements are contained, the content of each element is 0.0200% or less.

Note that since the above Cu, Ni, Cr, Mo, V, Ti, Zr, and REM are optional elements, it goes without saying that the content of each element may be 0% or may exceed 0%.

The remainder other than the above elements consists of Fe and unavoidable impurities. For example, O (oxygen) is an unavoidable impurity element, but if the O content is 0.0030% or less, it is permitted in this embodiment. In addition, other trace elements may be contained as long as they do not impair the effects of the present invention. B (boron) is permitted in this embodiment if its content is 0.0010% or less, preferably 0.0005% or less.

[Inclusions]
In the steel sheet according to this embodiment, it is important that inclusions having an average composition containing 10 to 40 mass % MgO and an average aspect ratio of 2.5 or less are present in a pair of regions (surface layer regions) extending from each of both sides of the steel sheet to a depth of 1/4 of the sheet thickness in the sheet thickness direction, and that the average value of the top 10% of the equivalent circle diameters of the inclusions is 3.5 μm or less.

[MgO: 10-40% by mass]
In the average composition of the inclusions present in the surface layer region, the MgO content is set to 10 mass% or more. If the MgO content is less than 10 mass%, the effect of suppressing the generation of inclusion clusters cannot be obtained, and the HIC resistance is poor. In addition, the content of MgO in the average composition of the inclusions present in the surface layer region is set to 40 mass % or less. If the content of MgO exceeds 40 mass %, the formation of inclusion clusters is deteriorated. This is because the effect of suppressing the formation of fluorine is reduced and the HIC resistance is deteriorated.

The content (mass%) of MgO in the average composition of the inclusions present in the surface layer region is determined by the following method. The cross section perpendicular to the rolling direction of the steel sheet is used as the observation surface, and a rectangular evaluation region is set vertically from one surface of the steel sheet to a 1/4 position on the one surface side and horizontally 10 mm in the width direction, and vertically from the other surface of the steel sheet to a 1/4 position on the other surface side and horizontally 10 mm in the width direction. The inclusions in these two evaluation regions are observed with a scanning electron microscope (SEM) equipped with a particle analysis function. For each of all inclusions in the two evaluation regions, the Mg amount (mass%), Al amount (mass%), S amount (mass%), Ca amount (mass%), and Mn amount (mass%) are measured, and MgO (mass%) is determined by the following formula (2).
MgO (mass%) = (MgO [g] × 100) / (MgO [g] + Al ₂ O ₃ [g] + CaO [g] + CaS [g] + MnS [g]) ... (2)
In formula (2), MgO[g], _Al2O3 [g], CaO[g], CaS[g], and MnS[g] respectively represent the contents of MgO, _Al2O3 , _CaO , CaS, and _MnS in the inclusions calculated from the amounts of Mg (% by mass), Al (% by mass), S (% by mass), Ca (% by mass), and _Mn (% by mass) contained in the inclusions. Here, it is assumed that Mg and Al all form MgO and _Al2O3 inclusions, Ca all form CaO-CaS inclusions, and Mn all form MnS.
Note that inclusions not containing any of MgO, Al ₂ O ₃ , CaO, CaS, and MnS, such as carbon particles from grinding waste, are excluded from the measurement. Inclusions with a circle equivalent diameter of less than 0.4 μm are also excluded from the measurement because they do not affect the effects of the present invention.
The total value of MgO (mass%) of all inclusions to be measured is divided by the number of inclusions to be measured to obtain the average value, which is the MgO content (mass%) in the average composition.

The number density of the inclusions in the surface region is preferably 1.0 pcs/ ^mm2 or more, and preferably 50.0 pcs/ ^mm2 or less. This is because the desired HIC resistance can be obtained. The number density of the inclusions in the surface region is determined by the following method. The cross section perpendicular to the rolling direction of the steel sheet is used as the observation surface, and a rectangular evaluation region is set vertically from one surface of the steel sheet to a 1/4 position on the one surface side, and horizontally, a rectangular evaluation region is set vertically from the other surface of the steel sheet to a 1/4 position on the other surface side, and horizontally, a rectangular evaluation region is set horizontally, a rectangular evaluation region is set horizontally, a rectangular evaluation region is set horizontally, a rectangular evaluation region is set vertically from the other surface of the steel sheet to a 1/4 position on the other surface side. The inclusions in these two evaluation regions are observed with a scanning electron microscope (SEM) with a particle analysis function. The number of inclusions with a circle equivalent diameter of 0.4 μm or more present in the two evaluation regions is counted, and the value obtained by dividing the number by the total area of the two evaluation regions is adopted as the "number density of inclusions in the surface region".

[Average aspect ratio of inclusions: 2.5 or less]
In this embodiment, the average aspect ratio of the inclusions present in the surface layer region is set to 2.5 or less. If the average aspect ratio exceeds 2.5, the stress concentration due to the inclusions increases, and the HIC resistance deteriorates. Although there is no particular lower limit to the average aspect ratio, the average aspect ratio may be 1.1 or more, since the aspect ratio of the inclusions increases due to rolling.

The average aspect ratio of the inclusions present in the surface layer region is determined by the following method. The cross section perpendicular to the rolling direction of the steel sheet is used as the observation surface, and a rectangular evaluation region of 10 mm in width is set vertically from one surface of the steel sheet to a 1/4 position on the one surface side, and a rectangular evaluation region of 10 mm in width is set vertically from the other surface of the steel sheet to a 1/4 position on the other surface side. The inclusions in these two evaluation regions are observed with a scanning electron microscope (SEM) equipped with particle analysis function. The aspect ratio (major axis/minor axis) is determined for inclusions with a circle equivalent diameter of 0.4 μm or more present in the two evaluation regions, and the average value is adopted as the "average aspect ratio of inclusions present in the surface layer region." The major axis means the maximum Feret diameter, and the minor axis means the minimum Feret diameter.

[Average value of the top 10% of inclusions' equivalent circle diameters: 3.5 μm or less]
In this embodiment, it is important that the average value of the top 10% of the circle-equivalent diameters of the inclusions present in the surface layer region is 3.5 μm or less. Here, the "top 10%" means 10% of the inclusions present in the number distribution of the circle-equivalent diameters of the inclusions, in terms of the number of inclusions present in the larger circle-equivalent diameter. If the average value of the circle-equivalent diameters of the 10% of inclusions is specified, it is possible to evaluate coarse inclusions that may be the starting point of HIC among the inclusions within the measurement range, and this becomes an index of improvement of HIC resistance. In this embodiment, if the average value of the top 10% of the circle-equivalent diameters of the inclusions present in the surface layer region exceeds 3.5 μm, the HIC resistance in the surface layer region deteriorates, and the crack area ratio CAR after the HIC test in the surface layer region exceeds 5.0%. In this embodiment, the lower limit of the average value of the top 10% of the circle-equivalent diameters of the inclusions present in the surface layer region is not particularly limited, but in order to suppress an increase in steelmaking costs, it is preferable that the average value of the top 10% of the circle-equivalent diameters of the inclusions is 1.5 μm or more.

The average of the top 10% of the equivalent circle diameters of the inclusions present in the surface layer region is determined by the following method. The cross section perpendicular to the rolling direction of the steel plate is used as the observation surface, and a rectangular evaluation region is set vertically from one surface of the steel plate to a position 1/4 of the way to the one surface side, and horizontally 10 mm in the width direction, and vertically from the other surface of the steel plate to a position 1/4 of the way to the other surface side, and horizontally 10 mm in the width direction. The inclusions in these two evaluation regions are observed with a scanning electron microscope (SEM) with particle analysis function. The equivalent circle diameters of inclusions with a circle equivalent diameter of 0.4 μm or more present in the two evaluation regions are measured, and the average circle equivalent diameters of the top 10% of the inclusions in terms of circle equivalent diameter are calculated, and this is adopted as the "average of the top 10% of the equivalent circle diameters of the inclusions present in the surface layer region."

[HIC resistance]
In this embodiment, the crack area ratio CAR in the surface layer region after the HIC test is set to 5.0% or less. This makes it possible to achieve excellent HIC resistance. In this embodiment, the crack area ratio CAR in the surface layer region after the HIC test can be 0.0% or more.

The crack area ratio (CAR) after the HIC test in the surface region is determined by the following method. A sample measuring full thickness x 20 mm width x 100 mm length is taken from the center position in the rolling direction and width direction of the steel plate. An HIC test is performed using NACE standard TM0177 Solution A solution, with the sample immersed in the solution at a hydrogen sulfide partial pressure of 1 bar for 96 hours. The crack area ratio (CAR) after the HIC test in the surface region of the steel plate is measured. Specifically, an ultrasonic inspection device and a water immersion type probe (frequency: 10 MHz, diameter: 0.375 inches, focal depth: 3 inches) are used to perform ultrasonic inspection testing at 0.1 mm pitches on a pair of regions from each side of the steel plate to a depth of 1/4 of the plate thickness in the plate thickness direction, the amplification degree is determined so that the bottom wave echo becomes 100%, and the area ratio of the portion in the surface region of the steel plate where the defect echo height is 25% or more is defined as the crack area ratio (CAR).

[Thickness]
The steel plate according to this embodiment is not particularly limited in thickness, but preferably has a thickness of 12 to 39 mm.

(Method of manufacturing steel sheet)
Hereinafter, a method for producing a steel plate according to an embodiment of the present invention will be described.

A method for producing steel plate according to one embodiment of the present invention includes an addition step of adding an Mg-containing substance and a Ca-containing substance to molten steel after refining, a subsequent step of casting the molten steel to obtain a steel slab, a subsequent step of hot rolling the steel slab to obtain a steel plate, and a subsequent step of controlled cooling of the steel plate.

The refining process of molten steel can be carried out by a conventionally known method. For example, after hot metal pretreatment, primary refining is performed using a converter, and then the molten steel is tapped from the converter into a ladle, desulfurized in a ladle refining furnace, and then vacuum degassed in an RH vacuum degassing device. This allows the cleanliness of the molten steel in the ladle to be improved. In the manufacturing method of sour-resistant steel plates, the ladle refining furnace is used to desulfurize the molten steel to a content of S of 0.0010% or less. The RH vacuum degassing device is used to remove hydrogen (H) from the molten steel and to separate and remove oxide-based nonmetallic inclusions such as Al ₂ O ₃ from the molten steel.

[Step of adding Mg-containing substance and Ca-containing substance]
The timing of adding the Mg-containing substance and the Ca-containing substance to the molten steel in the ladle is from the end of the refining process described above until the start of casting, in order to prevent Mg and Ca, which are strong deoxidizing and desulfurizing agents, from being consumed by reacting with dissolved oxygen or by reducing oxide-based nonmetallic inclusions, and to allow Mg and Ca to contribute to controlling the morphology of the nonmetallic inclusions.

In this embodiment, it is important that the addition process for adding an Mg-containing substance and a Ca-containing substance to the molten steel after refining is performed under the following conditions: (A) the temperature of the molten steel at the start of the addition process is in the range of 1580 to 1620°C; (B) the addition of the Ca-containing substance is started after the addition of the Mg-containing substance is started, or the addition of the Mg-containing substance and the Ca-containing substance is started simultaneously; and (C) the supply rates of Mg and Ca are each 15 to 30 kg/min.

Both Mg and Ca are elements with high vapor pressure and evaporate at normal molten steel temperatures. In addition, Mg and Ca are highly reactive not only with molten steel but also with air. Therefore, in order to suppress the evaporation of Mg and Ca and improve the yield of Mg and Ca, Mg-containing substances and Ca-containing substances are added instead of pure metallic Mg and Ca. The Mg-containing substance is preferably an Mg alloy in which Mg is alloyed with one or more selected from silicon, aluminum, etc. The Ca-containing substance is preferably a Ca alloy in which Ca is alloyed with one or more selected from silicon, aluminum, etc. From the viewpoint of handling and reactivity with molten steel, the pure Mg content of the Mg alloy and the pure Ca content of the Ca alloy are preferably 5 to 30 mass%, respectively. In addition, adding a Mg-containing substance and a Ca-containing substance simultaneously includes adding a substance containing Mg and Ca simultaneously.

The Mg-containing substance may be, for example, a powder of an Mg alloy, or may be an iron-coated Mg wire made by coating Mg alloy powder with a thin steel plate. The Ca-containing substance may be, for example, a powder of a Ca alloy, or may be an iron-coated Ca wire made by coating Ca alloy powder with a thin steel plate. When the Mg-containing substance and the Ca-containing substance are added simultaneously, the iron-coated Mg wire and the iron-coated Ca wire may be added separately, but as a substance containing Mg and Ca simultaneously, an iron-coated Mg-Ca wire made by coating Mg alloy powder and Ca alloy powder with a single thin steel plate may be added.

The method of adding the Mg-containing substance and the Ca-containing substance may be a method of adding a wire as described above to the molten steel, or a method of injecting Mg alloy powder and Ca alloy powder from an injection lance immersed in the molten steel.

[Temperature of molten steel at the start of the addition process: 1580 to 1620°C]
If the molten steel temperature at the start of the addition process is too high, the amount of Mg and Ca evaporating into the atmosphere increases, and the yield of Mg and Ca (amount dissolved in the molten steel) decreases. On the other hand, if the molten steel temperature at the start of the addition process is too low, the stirring of the molten steel due to evaporation becomes insufficient, and in this case too, the yield decreases. Therefore, in order to stabilize the yield of Mg and Ca, the molten steel temperature at the start of the addition process is set to a range of 1580 to 1620°C.

[Order of Addition of Mg-Containing Substance and Ca-Containing Substance]
In this embodiment, the first aspect in which the addition of the Ca-containing substance is started after the addition of the Mg-containing substance is started, or the second aspect in which the addition of the Mg-containing substance and the Ca-containing substance is started simultaneously is adopted. This allows appropriate amounts of dissolved Ca and dissolved Mg to be present in the molten steel. Due to the influence of an appropriate amount of dissolved Mg in the molten steel, the activity of oxygen and sulfur in the molten steel decreases, and the equilibrium of the reaction of the nonmetallic inclusions moves toward the formation of CaS. The formation of CaS is promoted, and an improvement in the desulfurization action is achieved. In addition, the reaction between Mg and S in the molten steel and the reaction between Mg and O (oxygen) in the molten steel also progress. As a result, the average value of the top 10% of the circle equivalent diameters of the inclusions present in the surface layer region can be made 3.5 μm or less.

In contrast, when the addition of the Ca-containing substance is started followed by the Mg-containing substance, the desulfurization due to the formation of MgS and the deoxidization due to the formation of MgO accompanying the addition of the Mg-containing substance, as well as the effect of controlling the shape of non-metallic inclusions, cannot be obtained. This is because Ca oxides and sulfides are more thermodynamically stable than Mg oxides and sulfides. In other words, the average circle equivalent diameter of the top 10% of the inclusions present in the surface layer region exceeds 3.5 μm.

In the first embodiment, in which the addition of the Ca-containing substance is started after the addition of the Mg-containing substance is started, there may be a period during which both the Mg-containing substance and the Ca-containing substance are added (i.e., an overlapping period between the addition period of the Mg-containing substance and the addition period of the Ca-containing substance), or the addition of the Ca-containing substance may be started after the addition of the Mg-containing substance is completed. In other words, there may be no overlapping period. However, in order to start the addition of the Ca-containing substance after the addition of the Mg-containing substance is completed, the shorter the interval, from the viewpoint of productivity and the effect of the present invention, the better, and it is preferable for the interval to be 5 minutes or less.

In the second embodiment, in which the addition of the Mg-containing substance and the Ca-containing substance is started at the same time, it is preferable that the addition of the Mg-containing substance and the Ca-containing substance is also completed at the same time. One method for easily implementing this is to add an iron-coated Mg-Ca wire to the molten steel. However, even in the second embodiment, it is of course possible to start adding the iron-coated Mg wire and the iron-coated Ca wire individually and simultaneously. In this case, "starting addition at the same time" does not mean that the start of addition is strictly synchronized to the second, and it is of course acceptable for there to be a difference in the start of addition (for example, within 30 seconds) that is unavoidable due to the operation of the dosing machine.

[Mg and Ca supply rates: 15 to 30 kg/min each]
If the supply rate of Mg and Ca (the addition rate of the Mg-containing substance and the Ca-containing substance) is too fast, the MgO content in the average composition of the inclusions becomes too high, and the HIC resistance deteriorates. On the other hand, if the supply rate of Mg and Ca (the addition rate of the Mg-containing substance and the Ca-containing substance) is too slow, the MgO content in the average composition of the inclusions becomes too low, and the HIC resistance also deteriorates. Therefore, in this embodiment, the Mg-containing substance and the Ca-containing substance are added so that the supply rates of the pure Mg and pure Ca are 15 to 30 kg/min, respectively. The supply rates of the pure Mg and pure Ca mean the amounts of Mg and Ca supplied per minute during the period in which the Mg-containing substance and the Ca-containing substance are added, respectively.

[Casting process]
In this embodiment, it is important that (i) the time from the end of addition of the Ca-containing substance to the start of casting is within 90 minutes, and (ii) the average flow rate of the molten steel in the mold during casting is 0.10 m/s or more, and that the molten steel is cast to obtain a steel piece such as a slab or billet. Other conditions in the casting step can be the same as in the ordinary case. In the casting step, continuous casting can be performed using a continuous casting facility.

[Time from the end of addition of Ca-containing material to the start of casting: within 90 minutes]
In the ladle, the inclusions float and separate from the molten steel over time. Since the floating and separating of the inclusions contributes to improving the cleanliness of the molten steel, the time from the end of the addition of the Ca-containing substance to the start of casting was generally 100 minutes or more in the past. In contrast, in this embodiment, the inclusions generated by the addition of the Mg-containing substance and the Ca-containing substance are necessary for improving the HIC resistance. Therefore, if the inclusions float and separate too much from the molten steel, the HIC resistance deteriorates. Therefore, the time from the end of the addition of the Ca-containing substance to the start of casting is set to 90 minutes or less. The time is preferably 80 minutes or less. There is no particular limit to the lower limit of the time, but considering the cleanliness, the time is preferably 40 minutes or more. In this embodiment, the cleanliness of the molten steel is at a level that does not cause any problem even for the above-mentioned time by satisfying the above-mentioned amount of Ca added.

[Average flow velocity of molten steel in the mold during casting: 0.10 m/s or more]
In the mold, the flow of the molten steel can be controlled by electromagnetic stirring to prevent the inclusions from coagulating and becoming coarse at the interface between the solidified shell and the molten steel. In order to sufficiently prevent the coarsening of the inclusions and improve the HIC resistance in the surface layer region, the average flow velocity of the molten steel in the mold is set to 0.10 m/s or more. The average flow velocity of the molten steel in the mold is preferably set to 0.20 m/s or more. On the other hand, when the average flow velocity of the molten steel in the mold exceeds 0.40 m/s, the effect of preventing the coarsening of the inclusions becomes saturated, so the average flow velocity of the molten steel in the mold is preferably 0.40 m/s or less.

The average flow velocity of molten steel in the mold can be calculated by measuring the inclination of primary dendrites with respect to the direction perpendicular to the slab surface (dendrite inclination angle) and using the following formula (3). The molten steel flow velocity is measured at five positions (1/6, 1/3, 1/2, 2/3, and 5/6) in the width direction on each of the front and back sides of the slab (10 positions in total on both sides), and the average value is defined as the "average flow velocity of molten steel in the mold."
θ={(0.35×C ₀ ² )/(C ₀ ² +0.0005)+0.65}×11.5×V _F ^−0.177 ×log{(5.38×10 ⁻¹ ×V _F ^2.08 )/V} ...(3)
Here, θ (degree) is the dendrite inclination angle, C ₀ (mass %) is the carbon concentration, V _F (m/s) is the molten steel flow velocity, and V (m/s) is the solidification rate. The dendrite inclination angle θ is obtained by the following method. That is, a sample having a cross section perpendicular to the rolling direction is taken from the cast slab after casting, polished, corroded with a picric acid saturated aqueous solution, and observed with an optical microscope. The dendrite inclination angle θ is obtained by measuring the inclination of the primary dendrite with respect to the direction perpendicular to the slab surface in the surface layer part within a depth of 5 mm from the slab surface in the cross section with a protractor. The carbon concentration C ₀ is analyzed with a solid emission spectrometer using a sample taken from the tundish before casting. The solidification rate V is obtained from the change with time of the solidified shell thickness.

[Hot rolling process]
The steel slab is then hot-rolled to obtain a steel sheet. The hot-rolling is preferably carried out under conditions where the heating temperature of the steel slab is 1000 to 1250° C. and the cross-rolling ratio is 20 or less. Other conditions in the hot-rolling step can be the same as those in the conventional manner.

[Heating temperature of steel piece: 1000 to 1250°C]
If the heating temperature of the steel slab during hot rolling is less than 1000°C, controlled cooling cannot be started at or above the Ar ₃ point, and HIC resistance cannot be obtained. Therefore, the heating temperature of the steel slab is set to 1000°C or higher, preferably 1030°C or higher. On the other hand, if the heating temperature of the steel slab exceeds 1250°C, energy consumption increases. Therefore, the heating temperature of the steel slab is set to 1250°C or lower, preferably 1200°C or lower. Note that this temperature is the temperature inside the heating furnace, and the steel slab is heated to this temperature up to its center.

[Cross rolling ratio: 20 or less]
In this embodiment, the cross rolling ratio defined by the following formula (4) is preferably 20 or less. If the cross rolling ratio is 20 or less, the elongation of inclusions can be effectively prevented, and HIC resistance is further improved. Although the lower limit of the cross rolling ratio is not particularly limited, the cross rolling ratio may be 6 or more in view of restrictions on rolling equipment.
Cross rolling ratio=rolling ratio in the rolling direction/rolling ratio in the direction perpendicular to the rolling direction (4)
The "rolling ratio in the rolling direction" refers to the rolling ratio in the rolling direction to the total rolling when cross rolling is performed during hot rolling. Also, the "rolling ratio in the direction perpendicular to the rolling direction" refers to the rolling ratio in the direction perpendicular to the rolling direction to the total rolling when cross rolling is performed during hot rolling.

[Controlled cooling process]
Next, the steel sheet is subjected to a controlled cooling process. This process is preferably performed under conditions where the surface temperature of the steel sheet at the start of cooling is equal to or higher than Ar ₃ point, which is calculated by the following formula (1). If the surface temperature of the steel sheet at the start of cooling is lower than Ar ₃ point, ferrite may be generated before cooling, which may deteriorate the HIC resistance. For this reason, it is preferable that the surface temperature of the steel sheet at the start of cooling is equal to or higher than Ar ₃ point. Here, Ar ₃ point means the ferrite transformation start temperature during cooling, and can be calculated, for example, from the components of the steel by the following formula (1). The surface temperature of the steel sheet can be measured by a radiation thermometer or the like.
Ar ₃ points (°C) = 910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo]...(1)
In addition, [X] means the content (mass%) of element X in the steel, and the content of an element that is not contained may be substituted with 0. Other conditions in the controlled cooling step may be determined by ordinary methods.

(Steel pipe)
The steel plate according to one embodiment of the present invention is formed into a tubular shape by press bending, roll forming, UOE forming, or the like, and then the butt joint is welded to produce a steel pipe according to one embodiment of the present invention (UOE steel pipe, electric resistance welded steel pipe, spiral steel pipe, etc.). The steel pipe according to this embodiment is suitable for transporting crude oil or natural gas.

For example, UOE steel pipes are manufactured by preparing the ends of steel plates, forming them into a steel pipe shape using a C press, U press, and O press, seam welding the butt joints using internal and external welding, and then expanding the pipe as necessary. Any welding method can be used as long as it provides sufficient joint strength and toughness, but submerged arc welding is preferred from the standpoint of excellent welding quality and manufacturing efficiency. Expanding can also be performed on steel pipes in which steel plates are formed into a tubular shape using press bending and then the butt joints are seam welded.

The molten iron was decarburized and refined in a converter to produce molten steel, which was then tapped into a ladle. After tapping from the converter, metallic aluminum was added to the ladle to deoxidize the molten steel.

The molten steel contained in the ladle was then desulfurized in a ladle refining furnace. In this desulfurization process, a CaO-Al ₂ O ₃ -SiO ₂ flux was used as the desulfurization agent. The molten steel was heated by arc heat from a graphite electrode, and the desulfurization agent was turned into slag. Argon gas was blown into the molten steel from an immersion lance at a rate of 100 to 150 Nm ³ /h as a stirring gas, and the molten steel and the desulfurization agent were stirred and mixed to perform the desulfurization process.

In addition, vacuum degassing refining was carried out in an RH vacuum degassing device. Specifically, degassing treatment, adjustment of the molten steel composition, and floating and separation of nonmetallic inclusions by stirring were carried out. The processing time for the above vacuum degassing refining was 20 minutes.

Next, during the period from the end of refining in the RH vacuum degassing device to the start of continuous casting in the continuous casting equipment, iron-coated Mg wire as the Mg-containing material and iron-coated Ca wire as the Ca-containing material were added to the molten steel in the ladle to obtain steels (steel types A to V) having the composition shown in Table 1. Table 2 shows the molten steel temperature at the start of the addition process, the order of addition of the Mg-containing material and the Ca-containing material, and the Mg and Ca supply rates for each set of conditions. Note that in the "Mg, Ca addition order" column in Table 2, "Mg → Ca" means that the addition of the iron-coated Mg wire is started and the addition of the iron-coated Ca wire is started immediately after the completion of the addition, "Ca → Mg" means that the addition of the iron-coated Ca wire is started and the addition of the iron-coated Mg wire is started immediately after the completion of the addition, and "Mg and Ca simultaneously" means that the iron-coated Mg-Ca wire is added.

The molten steel was then cast into slabs by continuous casting. The time from the end of addition of the Ca-containing substance to the start of casting and the average flow velocity of the molten steel in the mold during casting under each set of conditions are shown in Table 2. The average flow velocity of the molten steel in the mold was determined by the method previously described.

The slab was then heated to the slab heating temperature shown in Table 2, and hot rolled under the cross rolling ratio conditions shown in Table 2 to obtain a steel plate having the final plate thickness shown in Table 2. The steel plate was then subjected to controlled cooling at the steel plate surface temperature at the start of cooling shown in Table 2.

Thereafter, the ends of the steel plates were grooved and formed into a steel pipe shape using a C press, a U press, and an O press, after which the butt joints were seam welded from the inner and outer sides by submerged arc welding, and the steel pipes were made through a pipe expansion process. The chemical compositions of the steel plates and steel pipes manufactured from the above steel types A to V are the same as the chemical compositions of the steel types A to V that became the steel materials, within the industrial tolerance range. In Table 1, the elements marked with a hyphen (-) are treated as having a content of zero in the calculation of _Ar3 (°C).

[Evaluation of inclusions]
Table 2 shows "the MgO content in the average composition of the inclusions present in the surface layer region,""the number density of the inclusions in the surface layer region,""the average value of the top 10% of the equivalent circle diameters of the inclusions present in the surface layer region," and "the average value of the aspect ratio of the inclusions present in the surface layer region," all of which were determined by the method described above.

[Evaluation of HIC resistance]
Table 2 shows the "crack area ratio CAR in the surface layer region after the HIC test" obtained by the method described above.

As shown in Table 2, Nos. 1 to 10 are invention examples whose component compositions and manufacturing conditions satisfy the appropriate ranges of the present invention. All of the examples had an average composition in the surface region containing 10 to 40 mass% MgO, had inclusions with an average aspect ratio of 2.5 or less, the average of the top 10% of the circle-equivalent diameters of the inclusions was 3.5 μm or less, and the crack area ratio CAR after the HIC test in the surface region was 5.0% or less.

In contrast, in Nos. 11 to 13 and 20 to 22, the chemical composition of the steel plate was outside the range of the invention, which promoted center segregation and resulted in the CAR not reaching the desired value.

In No. 14, the S content of the steel plate was higher than the range specified by the invention, resulting in excessive production of MnS and a CAR that did not reach the desired value.

In No. 15, the Nb content of the steel plate was higher than the range specified by the invention, so coarse carbides crystallized and the CAR did not reach the desired value.

In No. 16, the N content of the steel plate was higher than the range specified by the invention, so coarse nitrides were formed and the CAR did not reach the desired value.

In No. 17, the temperature of the molten steel at the start of the addition process was too high, which resulted in a low yield. As a result, the Ca content was reduced too much, and the CAR did not reach the desired value. Note that the yield varies, and No. 17 shows an example in which only the Ca content was reduced.

In No. 18, the Ca content of the steel plate was higher than the range specified by the invention, which resulted in a decrease in the cleanliness of the steel and a failure of the CAR to reach the desired value.

In No. 19, the temperature of the molten steel at the start of the addition process was too low, which resulted in a low yield. As a result, the Mg content was reduced too much, and the CAR did not reach the desired value. Note that the yield varies, and No. 19 shows an example in which only the Mg content was reduced.

In No. 23, cross rolling was insufficient, so the average aspect ratio of the inclusions was large and the CAR did not reach the desired value.

In No. 24, Mg was added after Ca was added, so the average value of the top 10% of the circle equivalent diameters of the inclusions became too large, and the CAR did not reach the desired value.

In No. 25, the wire addition rate was too slow, resulting in too little MgO in the inclusions and the CAR not reaching the desired value.

In No. 26, the wire was added too quickly, resulting in an inclusion with too much MgO content, and the CAR did not reach the desired value.

In No. 27, the time between the end of Ca addition and the start of casting was long, so the average value of the top 10% of the circle equivalent diameters of the inclusions became too large, and the CAR did not reach the desired value.

In No. 28, the average flow velocity of the molten steel in the mold was slow, so the average value of the top 10% of the circle equivalent diameters of the inclusions became too large, and the CAR did not reach the desired value.

According to the present invention, it is possible to provide a steel plate and a steel pipe having excellent HIC resistance.

Claims (5)

The composition contains, in mass%, C: 0.030 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Nb: 0.080% or less, N: 0.0080% or less, Ca: 0.0005 to 0.0050%, and Mg: 0.0005 to 0.0050%, with the balance being Fe and unavoidable impurities;
inclusions having an average composition containing 10 to 40 mass% MgO and an average aspect ratio of 2.5 or less are present in a pair of regions from each of both surfaces of the steel sheet to a depth of 1/4 of the sheet thickness in the sheet thickness direction,
the average value of the top 10% of the equivalent circle diameters of the inclusions is 3.5 μm or less;
The steel plate is characterized in that the crack area ratio CAR after an HIC test in the above-mentioned region is 5.0% or less. The steel plate according to claim 1, wherein the composition further contains, in mass%, one or more selected from the group consisting of Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.100% or less, Ti: 0.100% or less, Zr: 0.0200% or less, and REM: 0.0200% or less. an adding step of adding an Mg-containing substance and a Ca-containing substance to the molten steel after completion of refining,
(A) the temperature of the molten steel at the start of the adding step is in the range of 1580 to 1620°C;
(B) The addition of the Mg-containing substance is started after the addition of the Ca-containing substance is started, or the addition of the Mg-containing substance and the Ca-containing substance is started simultaneously,
(C) The feed rates of Mg and Ca are each 15 to 30 kg/min,
Next, the molten steel is cast to obtain a steel slab under the conditions that (i) the time from the completion of the addition of the Ca-containing substance to the start of casting is 90 minutes or less, and (ii) the average flow velocity of the molten steel in the mold during casting is 0.10 m/s or more,
The steel plate according to claim 1 or 2 is then produced by hot rolling the slab. The hot rolling is carried out under conditions in which the heating temperature of the steel slab is 1000 to 1250° C. and the cross rolling ratio is 20 or less;
Next, controlled cooling of the steel sheet is performed under conditions in which the surface temperature of the steel sheet at the start of cooling is equal to or higher than the _Ar3 point calculated by the following formula (1).
The method for producing the steel sheet according to claim 3.
Ar ₃ points (°C) = 910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo]...(1)
Here, [X] means the content (mass%) of element X in the steel. A steel pipe using the steel plate according to claim 1 or 2.