JP6869151B2 - Steel pipes for steel plates and line pipes and their manufacturing methods - Google Patents

Description

The present invention relates to steel pipes for steel plates and line pipes, and methods for manufacturing the same.

Mainly in line pipes for transporting oil and gas and storage tanks, so-called sour resistance such as hydrogen-induced cracking resistance and stress corrosion cracking resistance is required with the development of inferior resources containing hydrogen sulfide. To. Hydrogen-induced cracking (Hydrogen Induced Cracking, hereinafter sometimes referred to as "HIC") is the hydrogen that has entered the inside of the steel material due to the corrosion reaction caused by hydrogen sulfide or the like, including MnS and Nb (C, N). It is known that cracks are generated by accumulating in defective parts such as non-metal inclusions and gasifying them. When HIC is generated, there is a problem that the toughness of the structure is lowered. In particular, since hydrogen invades from the surface layer portion of the steel sheet, HIC is more likely to occur in the surface layer portion of the plate thickness than in the center portion of the plate thickness, and improvement in the HIC resistance of the surface layer portion of the plate thickness is required.

Therefore, conventionally, a technique for improving the HIC resistance of the surface layer portion has been studied. For example, Patent Document 1 describes MnS, Ca—Al and Ca inclusion clusters, and Ti and Nb inclusions that cause HIC by setting the amount of Ar gas blown into molten steel to a predetermined value or less. It is disclosed that unbonded bubbles of Ar gas in a steel material that causes accumulation of substances and segregation zones are reduced, and HIC resistance is improved.

Patent Document 2 states that the Ca concentration in the slab is controlled within a predetermined range during the production of the slab, and the Ca, S and O contents and the Ar gas content in the steel material are controlled within a predetermined range. It is disclosed that the HIC property is improved.

Japanese Unexamined Patent Publication No. 07-136748 Japanese Unexamined Patent Publication No. 2016-125140

However, in Patent Document 1, although studies have been made to reduce the number of bubbles in the slab, unbonded bubbles in the steel material of the final product are not considered. Therefore, it is not possible to control the defects caused by the uncrimped bubbles remaining in the steel material, and it is not possible to suppress the HIC caused by the uncrimped bubbles.

Further, in the method of Patent Document 2, although a study is made to reduce the content of Ar gas bubbles in the steel material, the size of the bubbles and the unbonded bubbles of the steel material are not taken into consideration. Therefore, if even a small amount of coarse Ar bubbles are present, HIC cannot be sufficiently suppressed.

The present invention has been made in view of the above circumstances, and a main object thereof is to provide a steel plate having excellent hydrogen-induced cracking resistance and a steel pipe for a line pipe.

The steel plate according to the present invention has C: 0.02 to 0.15% by mass, Si: 0.02 to 0.50% by mass, Mn: 0.6 to 2.0%, P: more than 0% by mass, 0. .030% by mass or less, S: more than 0% by mass, 0.003% by mass or less, Al: 0.010 to 0.080% by mass, Ca: 0.0003 to 0.0060% by mass, N: 0.001 to It contains 0.01% by mass and O: more than 0% by mass, 0.0045% by mass or less, and satisfies the following formulas (1) and (2), and the balance is composed of iron and unavoidable impurities. The area ratio of the portion where the defect echo height is 20% or more is 0.05% or less.

3.0 ≤ [Ca] / [S] (1)
([Ca] -1.25 × [S]) / [O] ≦ 1.80 (2)
Here, [Ca], [S] and [O] are the contents (mass%) of Ca, S and O, respectively.

The steel plate according to the present invention has B: more than 0% by mass, 0.005% by mass or less, V: more than 0% by mass, 0.1% by mass or less, Cu: more than 0% by mass, 1.5% by mass or less, Ni. : More than 0% by mass, 1.5% by mass or less, Cr: More than 0% by mass, 1.5% by mass or less, Mo: More than 0% by mass, 1.5% by mass or less, Nb: More than 0% by mass, 0. 06% by mass or less, Ti: more than 0% by mass, 0.03% by mass or less, Mg: more than 0% by mass, 0.01% by mass or less, REM: more than 0% by mass, 0.02% by mass or less, and Zr: It may contain one or more selected from the group consisting of more than 0% by mass and 0.010% by mass or less.

The steel plate according to the present invention may be for a line pipe.

The steel pipe for a line pipe according to the present invention is formed of the steel plate according to the present invention.

The steel plate according to the present invention may be for a pressure vessel.

The method for producing a steel sheet according to the present invention has the chemical composition of the steel sheet according to the present invention, and the cell density in the slab accumulation zone having a circle-equivalent diameter of 0.2 mm or more is 0.15 cells / cm ² or less. Use a slab.

INDUSTRIAL APPLICABILITY The present invention provides a steel plate having excellent hydrogen-induced cracking resistance, a steel pipe for a line pipe, and a method for producing the same.

FIG. 1 is a graph showing the relationship between the CLR of the surface layer portion and the number density of bubbles having a circle-equivalent diameter of 0.2 mm or more in the slab accumulation zone. FIG. 2 is a graph showing the relationship between the CLR of the surface layer portion and the area ratio of the portion where the defect echo height is 20% or more.

In order to solve the above problems, the present inventors have determined the CLR (Clack Length Ratio, the ratio of the total crack length to the width of the test piece [%], the crack length ratio) of the surface layer measured by the HI C test. , The correlation with the internal defects of the steel sheet measured by the ultrasonic flaw detection test was investigated diligently. As a result, the area of the portion where the chemical composition of the steel sheet is controlled within a predetermined range and the defect echo height is 20% or more so that the contents of Ca, S and O satisfy a predetermined relational expression. It has been found that excellent HIC resistance can be obtained by controlling the internal defects so that the rate is 0.05% or less.

Hereinafter, the steel sheet of the present invention and a method for producing the same will be described in detail.

<1. Steel plate ＞
(1-1. Chemical composition)
The steel plate according to the present invention has C: 0.02 to 0.15% by mass, Si: 0.02 to 0.50% by mass, Mn: 0.6 to 2.0%, P: more than 0% by mass, 0. .030% by mass or less, S: more than 0% by mass, 0.003% by mass or less, Al: 0.010 to 0.080% by mass, Ca: 0.0003 to 0.0060% by mass, N: 0.001 to It contains 0.010% by mass, O: more than 0% by mass, 0.0045% by mass or less, satisfies the following equations (1) and (2), and the balance is composed of iron and unavoidable impurities.

3.0 ≤ [Ca] / [S] (1)
([Ca] -1.25 × [S]) / [O] ≦ 1.80 (2)
Here, [Ca], [S] and [O] are the contents (mass%) of Ca, S and O, respectively.

By controlling the chemical composition as described above, a steel sheet having excellent hydrogen-induced cracking resistance can be obtained.

[C: 0.02 to 0.15% by mass]
C is an element indispensable for ensuring the strength of the base metal and the welded portion, and must be contained in an amount of 0.02% by mass or more. The amount of C is preferably 0.03% by mass or more, and more preferably 0.04% by mass or more. On the other hand, if the amount of C is too large, HAZ toughness and weldability deteriorate. Further, when the amount of C is excessive, NbC and island-shaped martensite, which are the starting point of HIC and the fracture progress path, are likely to be generated. Therefore, the amount of C needs to be 0.15% by mass or less. It is preferably 0.12% by mass or less, more preferably 0.10% by mass or less.

[Si: 0.02 to 0.50% by mass]
Si is an element that has a deoxidizing effect and is effective in improving the strength of the base metal and the welded portion. In order to obtain these effects, the amount of Si is set to 0.02% by mass or more. The amount of Si is preferably 0.05% by mass or more, and more preferably 0.15% by mass or more. However, if the amount of Si is too large, the weldability and toughness deteriorate. If the amount of Si is excessive, island-shaped martensite is generated and HIC is generated and progresses. Therefore, the amount of Si needs to be suppressed to 0.50% by mass or less. The amount of Si is preferably 0.45% by mass or less, more preferably 0.35% by mass or less.

[Mn: 0.6 to 2.0% by mass]
Mn is an element effective for improving the strength of the base metal and the welded portion, and is contained in an amount of 0.6% by mass or more in the present invention. The amount of Mn is preferably 0.8% by mass or more, and more preferably 1.0% by mass or more. However, if the amount of Mn is too large, MnS is generated and not only the hydrogen-induced cracking resistance deteriorates, but also the HAZ toughness and weldability deteriorate. Therefore, the upper limit of the amount of Mn is set to 2.0% by mass. The amount of Mn is preferably 1.8% by mass or less, more preferably 1.5% by mass or less, still more preferably 1.2% by mass or less.

[P: More than 0% by mass, 0.030% by mass or less]
P is an element inevitably contained in the steel material, and when the amount of P exceeds 0.030% by mass, the toughness of the base material and the HAZ portion is significantly deteriorated, and the hydrogen-induced cracking resistance is also deteriorated. Therefore, in the present invention, the amount of P is suppressed to 0.030% by mass or less. The amount of P is preferably 0.020% by mass or less, more preferably 0.010% by mass or less.

[S: More than 0% by mass, 0.003% by mass or less]
If S is too much, MnS is generated in a large amount and the hydrogen-induced cracking resistance is significantly deteriorated. Therefore, in the present invention, the upper limit of the amount of S is set to 0.003% by mass. The amount of S is preferably 0.002% by mass or less, more preferably 0.0015% by mass or less, and further preferably 0.0010% by mass or less. As described above, it is desirable that the amount is small from the viewpoint of improving the hydrogen-induced cracking resistance.

[Al: 0.010 to 0.080% by mass]
Al is a strongly deoxidizing element, and when the amount of Al is small, the Ca concentration in the oxide increases, that is, Ca-based inclusions are likely to be formed on the surface layer of the steel sheet, and fine HIC is generated. Therefore, in the present invention, Al needs to be 0.010% by mass or more. The amount of Al is preferably 0.020% by mass or more, more preferably 0.030% by mass or more. On the other hand, if the Al content is too high, an oxide of Al is formed in a cluster form and becomes a starting point of hydrogen-induced cracking. Therefore, the amount of Al needs to be 0.080% by mass or less. The amount of Al is preferably 0.060% by mass or less, and more preferably 0.050% by mass or less.

[Ca: 0.0003 to 0.0060% by mass]
Ca has an effect of controlling the morphology of sulfide, and has an effect of suppressing the formation of MnS by forming CaS. In order to obtain this effect, the amount of Ca needs to be 0.0003% by mass or more. The amount of Ca is preferably 0.0005% by mass or more, and more preferably 0.0010% by mass or more. On the other hand, when the amount of Ca exceeds 0.0060% by mass, a large amount of HIC is generated starting from Ca-based inclusions. Therefore, in the present invention, the upper limit of the amount of Ca is set to 0.0060% by mass. The amount of Ca is preferably 0.0045% by mass or less, more preferably 0.0035% by mass or less, and further preferably 0.0025% by mass or less.

[N: 0.001 to 0.01% by mass]
N is an element that precipitates as TiN in the steel structure, suppresses the coarsening of austenite grains in the HAZ portion, further promotes ferrite transformation, and improves the toughness of the HAZ portion. In order to obtain this effect, it is necessary to contain N in an amount of 0.001% by mass or more. The amount of N is preferably 0.003% by mass or more, and more preferably 0.0040% by mass or more. However, if the amount of N is too large, the HAZ toughness deteriorates due to the presence of the solid solution N, so the amount of N needs to be 0.01% by mass or less. It is preferably 0.008% by mass or less, and more preferably 0.0060% by mass or less.

[O: More than 0% by mass, 0.0045% by mass or less]
It is desirable that O (oxygen) is low from the viewpoint of improving cleanliness, and when a large amount of O is contained, in addition to deterioration of toughness, HIC is generated starting from an oxide, and hydrogen-induced cracking resistance deteriorates. .. From this viewpoint, the amount of O needs to be 0.0045% by mass or less, preferably 0.0035% by mass or less, and more preferably 0.0030% by mass or less.

[[Ca] / [S]: 3.0 or higher]
The steel sheet according to the present invention satisfies the following equation (1).

3.0 ≤ [Ca] / [S] (1)
Here, [Ca] and [S] are the contents (mass%) of Ca and S, respectively.

The technical significance of the above equation (1) will be described below.

S forms MnS as a sulfide-based inclusion, and HIC is generated starting from MnS. Therefore, by adding Ca to control the morphology of the sulfide-based inclusions in the steel as CaS, the formation of MnS is suppressed and the HIC resistance is prevented from being lowered. The present inventors have found that it is necessary to set [Ca] / [S] to 3.0 or more in order to fully exert this effect. [Ca] / [S] is preferably 3.5 or more, and more preferably 4.0 or more. Considering the amount of Ca and the amount of S specified in the present invention, the upper limit of [Ca] / [S] is about 15.

[([Ca] -1.25 × [S]) / [O]: 1.80 or less]
The steel sheet according to the present invention satisfies the following equation (2).

([Ca] -1.25 × [S]) / [O] ≦ 1.80 (2)
Here, [Ca], [S] and [O] are the contents (mass%) of Ca, S and O, respectively.

The technical significance of the above equation (2) will be described below.

In order to suppress the generation of HIC due to Ca-based acid sulfide, it is effective to suppress the formation of CaO, which is particularly likely to form aggregates among Ca-based inclusions. For that purpose, the amount of Ca ([Ca] -1.25 × [S]) obtained by subtracting the amount of Ca existing as sulfide (CaS) from the total amount of Ca in the steel should not be excessive with respect to the amount of O. Must be. When the amount of Ca ([Ca] -1.25 × [S]) is excessive with respect to the amount of O, CaO is likely to be formed as oxide-based inclusions, and the agglutination and coalescence of the CaO (coarse Ca-based inclusions) Things) are likely to be formed in large quantities on the surface layer of the steel sheet. In order to suppress this, the present inventors examined the relationship between ([Ca] -1.25 × [S]) / [O] and HIC resistance, and found that in order to obtain excellent HIC resistance. It was found that ([Ca] -1.25 × [S]) / [O] needs to be 1.80 or less. ([Ca] -1.25 × [S]) / [O] is preferably 1.40 or less, more preferably 1.30 or less, still more preferably 1.20 or less, and particularly preferably 1.00 or less. is there. The lower limit of ([Ca] -1.25 × [S]) / [O] is about 0.1 from the viewpoint of suppressing _{Al 2} O ₃ which easily forms aggregates and coalesces like CaO.

The basic components of the steel sheet according to the present invention are as described above, and the balance is iron and unavoidable impurities. However, it is naturally permissible for steel to contain unavoidable impurities other than P and S brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like.
As described above, P and S are elements (unavoidable impurities) that are inevitably contained, but the composition range thereof is separately specified as described above. Therefore, in the present specification, the "unavoidable impurity" contained as the balance means an element unavoidably contained except for an element whose composition range is separately defined.
Further, the steel sheet according to the present invention may selectively contain the following elements in addition to the above elements, and the characteristics of the steel sheet are further improved depending on the type of the contained elements.

[B: More than 0% by mass, 0.005% by mass or less]
B enhances hardenability, enhances the strength of the base metal and the welded zone, and at the time of welding, the heated HAZ portion combines with N in the process of cooling to precipitate BN, which causes ferrite transformation from inside the austenite grains. Improves HAZ toughness to promote. In order to obtain this effect, it is preferable that the amount of B is 0.0002% by mass or more. The amount of B is more preferably 0.0005% by mass or more, still more preferably 0.0010% by mass or more. However, if the B content is excessive, the toughness of the base metal and the HAZ portion deteriorates and the weldability deteriorates. Therefore, the B content is preferably 0.005% by mass or less. The amount of B is more preferably 0.004% by mass or less, still more preferably 0.0030% by mass or less.

[V: More than 0% by mass, 0.1% by mass or less]
V is an element effective for improving the strength, and it is preferable to contain it in an amount of 0.003% by mass or more in order to obtain this effect. More preferably, it is 0.010% by mass or more. On the other hand, if the V content exceeds 0.1% by mass, the weldability and the toughness of the base metal deteriorate. Therefore, the amount of V is preferably 0.1% by mass or less, more preferably 0.08% by mass or less.

[Cu: more than 0% by mass, less than 1.5% by mass]
Cu is an element effective for improving hardenability and increasing strength. In order to obtain this effect, it is preferable to contain Cu in an amount of 0.01% by mass or more. The amount of Cu is more preferably 0.05% by mass or more, still more preferably 0.10% by mass or more. However, if the Cu content exceeds 1.5% by mass, the toughness deteriorates, so it is preferably 1.5% by mass or less. The amount of Cu is more preferably 1.0% by mass or less, still more preferably 0.50% by mass or less.

[Ni: more than 0% by mass, less than 1.5% by mass]
Ni is an element effective in improving the strength and toughness of the base metal and welded parts. In order to obtain this effect, the amount of Ni is preferably 0.01% by mass or more. The amount of Ni is more preferably 0.05% by mass or more, still more preferably 0.10% by mass or more. However, if a large amount of Ni is contained, it becomes extremely expensive as a structural steel material. Therefore, from an economical point of view, the amount of Ni is preferably 1.5% by mass or less. The amount of Ni is more preferably 1.0% by mass or less, still more preferably 0.50% by mass or less.

[Cr: more than 0% by mass, 1.5% by mass or less]
Cr is an element effective for improving the strength, and it is preferable to contain Cr in an amount of 0.01% by mass or more in order to obtain this effect. The amount of Cr is more preferably 0.05% by mass or more, still more preferably 0.10% by mass or more. On the other hand, if the amount of Cr exceeds 1.5% by mass, the HAZ toughness deteriorates. Therefore, the amount of Cr is preferably 1.5% by mass or less. The amount of Cr is more preferably 1.0% by mass or less, still more preferably 0.50% by mass or less.

[Mo: more than 0% by mass, less than 1.5% by mass]
Mo is an element effective for improving the strength and toughness of the base material. In order to obtain this effect, the amount of Mo is preferably 0.01% by mass or more. The amount of Mo is more preferably 0.05% by mass or more, still more preferably 0.10% by mass or more. However, if the amount of Mo exceeds 1.5% by mass, the HAZ toughness and weldability deteriorate. Therefore, the amount of Mo is preferably 1.5% by mass or less, more preferably 1.0% by mass or less, and further preferably 0.50% by mass or less.

[Nb: more than 0% by mass, 0.06% by mass or less]
Nb is an element effective for increasing strength and base metal toughness without deteriorating weldability. In order to obtain this effect, the amount of Nb is preferably 0.002% by mass or more. The amount of Nb is more preferably 0.010% by mass or more, still more preferably 0.020% by mass or more. However, if the amount of Nb exceeds 0.06% by mass, the toughness of the base metal and HAZ deteriorates. Therefore, in the present invention, the upper limit of the amount of Nb is preferably 0.06% by mass. The amount of Nb is more preferably 0.050% by mass or less, further preferably 0.040% by mass or less, and even more preferably 0.030% by mass or less.

[Ti: more than 0% by mass, 0.03% by mass or less]
Ti is an element effective for improving the toughness of the HAZ portion because it is precipitated as TiN in the steel to prevent coarsening of austenite grains in the HAZ portion during welding and promote ferrite transformation. .. Further, Ti is an element effective for improving HIC resistance because it exhibits a desulfurization action. In order to obtain these effects, it is preferable to contain Ti in an amount of 0.003% by mass or more. The amount of Ti is more preferably 0.005% by mass or more, still more preferably 0.010% by mass or more. On the other hand, if the Ti content is excessive, solid solution Ti and TiC are precipitated and the toughness of the base material and the HAZ portion is deteriorated. Therefore, it is preferably 0.03% by mass or less. The amount of Ti is more preferably 0.02% by mass or less.

[Mg: more than 0% by mass, 0.01% by mass or less]
Mg is an element effective for improving toughness through the refinement of crystal grains, and is also an element effective for improving HIC resistance because it exhibits a desulfurization action. In order to obtain these effects, it is preferable to contain Mg in an amount of 0.0003% by mass or more. The amount of Mg is more preferably 0.001% by mass or more. On the other hand, since the effect is saturated even if Mg is excessively contained, the upper limit of the amount of Mg is preferably 0.01% by mass. The amount of Mg is more preferably 0.005% by mass or less.

[REM: more than 0% by mass, 0.02% by mass or less]
REM (rare earth element) is an element effective for suppressing the formation of MnS by desulfurization and enhancing hydrogen-induced cracking resistance. In order to exert such an effect, it is preferable to contain REM in an amount of 0.0002% by mass or more. The amount of REM is more preferably 0.0005% by mass or more, still more preferably 0.0010% by mass or more. On the other hand, even if a large amount of REM is contained, the effect is saturated. Therefore, the upper limit of the REM amount is preferably 0.02% by mass. From the viewpoint of suppressing blockage of the immersion nozzle during casting and increasing productivity, the REM amount is more preferably 0.015% by mass or less, still more preferably 0.010% by mass or less, still more preferably 0. It is 0050 mass% or less. In the present invention, the above-mentioned REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y.

[Zr: more than 0% by mass, 0.010% by mass or less]
Zr is an element that improves HIC resistance by desulfurization action and contributes to improvement of HAZ toughness by forming oxides and finely dispersing them. In order to exert these effects, the amount of Zr is preferably 0.0003% by mass or more. The amount of Zr is more preferably 0.0005% by mass or more, further preferably 0.0010% by mass or more, still more preferably 0.0015% by mass or more. On the other hand, when Zr is added excessively, coarse inclusions are formed and the hydrogen-induced cracking resistance and the toughness of the base metal are deteriorated. Therefore, the amount of Zr is preferably 0.010% by mass or less. The amount of Zr is more preferably 0.0070% by mass or less, further preferably 0.0050% by mass or less, still more preferably 0.0030% by mass or less.

(1-2. Internal defects)
In the steel sheet according to the present invention, the area ratio of the portion where the defect echo height is 20% or more is 0.05% or less, and even when bubbles remain in the steel sheet accumulation zone, the HIC from the steel sheet accumulation zone is obtained. Can be suppressed.
Hereinafter, a detailed description will be given.

In the slab casting process, Ar gas needs to be blown into the molten steel, for example, to suppress blockage of the injection nozzle, reflux for degassing in the RH, and stirring of the molten steel in the tundish.

The slab accumulation zone is the surface portion of the slab, which is a portion that is more easily cooled and solidified earlier than the central portion at the stage of slab formation. Therefore, in the slab accumulation zone, air bubbles caused by Ar gas blown during slab casting float, but are trapped in the solidified portion of the curved portion, and the air bubbles tend to remain.

Since it is difficult to completely crimp the bubbles remaining in the slab accumulation zone in the rolling process, they tend to remain as bubbles in the steel sheet accumulation zone. Since hydrogen is likely to accumulate in the bubbles remaining in the steel sheet accumulation zone, HIC may be generated starting from the remaining bubbles. Therefore, the HIC resistance can be improved by reducing the bubbles in the steel sheet accumulation zone.

Here, the "slab accumulation zone" means a region of about t / 8 to t / 4 from the surface of the slab among the slabs having a plate thickness of t, and the "steel plate accumulation zone" has a plate thickness of about t / 8 to t / 4. Of the steel sheet having a plate thickness of t'obtained by hot rolling the slab of t, it means a region of about t'/8 to t'/4 from the surface of the steel plate.

When hot rolling a slab, the slab is usually rolled almost uniformly (ie, the slab accumulation zone and other parts are rolled at about the same rolling ratio). Therefore, the region of about t / 8 to t / 4 from the surface of the slab is a portion corresponding to the region of about t'/8 to t'/4 from the surface of the steel sheet obtained by hot rolling. That is, the "slab accumulation zone" is a portion corresponding to the "steel plate accumulation zone" of the steel sheet obtained by hot rolling.

FIG. 1 shows the results of investigating the relationship between the CLR of the surface layer measured by the HIC test and the area ratio of the portion where the defect echo height measured by the ultrasonic flaw detection test is 20% or more.

Here, the defect echo height is the ratio of the intensity of the defect echo reflected by the defect inside the test piece to the intensity of the bottom echo reflected by the bottom surface of the steel plate (or the test piece obtained by collecting a part of the steel plate) [ %] Means.
The area ratio of the portion having the defect echo height of 20% or more means the ratio [%] of the area of the portion having the defect echo height of 20% or more to the total area scanned by the probe.

From this result, the present inventors found a correlation between the CLR of the surface layer portion and the area ratio. That is, even when air bubbles remain in the steel sheet accumulation zone, if the area ratio of the portion where the defect echo height is 20% or more is 0.05% or less, the CLR of the surface layer portion of the steel sheet is reduced to 10% or less. It was found that HIC from the steel sheet accumulation zone can be suppressed. From the viewpoint of obtaining a steel sheet having more excellent HIC resistance, the defect echo height is preferably 30% or less, more preferably 25% or less, and the portion where the defect echo height is 20% or more. The area ratio is preferably 0.04% or less, and more preferably 0.03% or less.
Since it is difficult to remove all the air bubbles in the steel sheet, the defect echo height and the area ratio of the portion where the defect echo height is 20% or more are usually 0% or more.

The steel plate according to the present invention and a steel pipe for a line pipe formed by using the steel plate may be preferably used for a line pipe for transporting natural gas and crude oil, a storage tank, and a pressure vessel for refining.

<2. Steel sheet manufacturing method>
The method for producing a steel sheet according to the present invention comprises a slab having the above-mentioned chemical composition and having a density of bubbles having a diameter equivalent to a circle of 0.2 mm or more in the slab accumulation zone of 0.15 cells / cm ² or less. Use. By using the slab, a steel plate having excellent HIC resistance can be manufactured.
Hereinafter, a detailed description will be given.

(2-1. A slab in which the number density of bubbles having a circle-equivalent diameter of 0.2 mm or more in the slab accumulation zone is 0.15 cells / cm ² or less)
As described above, hydrogen tends to accumulate in the bubbles remaining in the steel sheet accumulation zone, so that HIC may be generated from the remaining bubbles as a starting point. Therefore, the HIC resistance can be improved by reducing the bubbles in the steel sheet accumulation zone.
Since the "slab accumulation zone" corresponds to the "steel plate accumulation zone" of the steel sheet obtained by hot rolling, the slab accumulation zone is a specific means for reducing air bubbles in the steel sheet accumulation zone. By reducing the air bubbles in the steel sheet, it is effective to reduce the air bubbles in the steel sheet accumulation zone of the steel sheet obtained by hot rolling, and the HIC resistance can be improved.

FIG. 2 shows the results of investigating the relationship between the CLR of the surface layer measured by the HI C test and the number density of bubbles having a circle equivalent diameter of 0.2 mm or more in the slab accumulation zone. As a result, the present inventors produced a steel sheet using a slab having a number density of bubbles having a diameter equivalent to 0.2 mm or more and a circle equivalent diameter of 0.2 mm or more in the slab accumulation zone of 0.15 cells / cm ² or less, thereby performing a rolling step. It was found that the remaining air bubbles can be reduced without being completely crimped. In the steel sheet manufactured by using such a slab, the area ratio of the portion where the defect echo height is 20% or more is 0.05% or less, and the CLR of the surface layer portion of the steel sheet can be 10% or less. , It was found that HIC from the steel sheet accumulation zone can be suppressed.

The equivalent circle diameter of the bubbles in the slab accumulation zone is preferably 0.17 mm or less, more preferably 0.15 mm or less, and the number density of bubbles having an equivalent circle diameter of 0.2 mm or more in the slab accumulation zone. Is preferably 0.10 pieces / cm ² or less, and more preferably 0.05 pieces / cm ² or less.
Since it is difficult to remove all the bubbles in the slab accumulation zone, the equivalent circle diameter of the bubbles in the slab accumulation zone is usually 0 mm or more, and the equivalent circle diameter in the slab accumulation zone is 0.2 mm or more. The number density of bubbles is usually 0 cells / cm ² or more.

The method for measuring the circle-equivalent diameter of the bubbles and the number density of the bubbles is not particularly limited, and examples thereof include the following methods.
Observe the test piece collected from the slab accumulation zone using an optical microscope, measure the circle-equivalent diameter of the bubbles using the eyepiece micrometer, and count the number of bubbles with a circle-equivalent diameter of 0.2 mm or more in the observation field of view. To do.
Next, the density of the bubbles is calculated from the area of the observation field of view and the number of bubbles having a circle-equivalent diameter of 0.2 mm or more.

(2-2. Step of casting the above slab)
In order to cast a slab having a circle-equivalent diameter of 0.2 mm or more and a bubble density of 0.15 cells / cm ² or less in the slab accumulation zone, when the molten steel is supplied from the tundish to the mold in the steelmaking process. It is important to control the amount of Ar gas blown into the nozzle and the diameter of the bubbles.

^{When Ar gas is used, the back pressure of Ar gas is 1.4 kgf / cm 2} or more and 1.8 kgf / cm ² or less from porous bricks with an inner pipe diameter of 70 mm or more and 115 mm or less and an average pore diameter of 30 μm or more and 60 μm or less. It is necessary to blow at a pressure of 3 L (liter) / t (ton) or more and 10 L / t or less.
The inner pipe diameter is preferably 75 mm or more, more preferably 80 mm or more, preferably 110 mm or less, and more preferably 105 mm or less.
The average pore diameter is preferably 35 μm or more, more preferably 40 μm or more, preferably 55 μm or less, and more preferably 50 μm or less.
Back pressure is preferably at 1.45kgf / cm ² or more, preferably more preferably 1.5 kgf / cm ² or more and 1.75kgf / cm ² or less, 1.7 kgf / cm ² More preferably:
The blowing amount is preferably 5 L / t or more, more preferably 7 L / t or more, preferably 12 L / t or less, and more preferably 10 L / t or less.
By blowing Ar gas in this range, nozzle clogging is unlikely to occur, and Ar gas having a large diameter is blown into the molten steel, so that Ar gas bubbles easily float in the mold. As a result, Ar gas bubbles are easily released from the accumulation zone, so that the Ar gas bubbles trapped in the accumulation zone can be reduced.

Although it is conceivable to reduce the amount of Ar gas blown together with the molten steel from the injection nozzle that injects the molten steel into the mold, it becomes difficult for the molten steel to be agitated by the Ar gas near the molten metal surface of the mold, so that the molten metal surface solidifies. Not recommended due to concerns that may arise.

Conditions other than the above are not particularly limited, and steel having the above-mentioned chemical composition may be melted according to a normal steelmaking method, and a slab may be cast by a continuous casting step.

The method for producing a steel sheet according to the present invention using the above slab is not particularly limited as long as the area ratio of the portion where the defect echo height of the steel sheet is 20% or more is 0.05% or less, and according to a conventional method. , Hot-rolled and then cooled to produce a steel sheet.
Hereinafter, "temperature" indicates the temperature of the material.

In order to achieve the steel sheet defect area ratio, for example, in a temperature range where the surface temperature is 900 ° C. or higher, rolling is performed for 5 passes or more at a reduction rate of 20% or less per pass, and the cumulative reduction rate is 50% or more. It is recommended to perform hot rolling as described above.
By hot rolling under the above conditions, the surface layer portion of the plate thickness is preferentially deformed from the inside of the plate thickness, so that the bubbles trapped in the accumulation zone can be pressure-bonded more efficiently.

As the cooling conditions after hot rolling, for example, it is recommended to perform cooling at an average cooling rate of 10 ° C./s or more from a cooling start temperature of Ar3 transformation point or higher.
By cooling under the above conditions, HIC generated near the central portion of the steel sheet can be effectively suppressed.

Further, using the steel plate according to the present invention, a steel pipe for a line pipe can be manufactured by a commonly used method. The steel pipe for line pipes obtained by using the steel plate of the present invention is also excellent in HIC resistance and toughness. Further, the steel plate according to the present invention may be used for a pressure vessel by a commonly used method.

Although the method for producing a steel sheet according to the present invention has been described above, a person skilled in the art who understands the desired characteristics of the steel sheet according to the present invention will carry out trial and error to produce a steel sheet having the desired characteristics according to the present invention. There is a possibility of finding a method other than the above-mentioned manufacturing method.

Further, as described above, bubbles are likely to remain in the slab accumulation zone, and HIC is likely to be generated starting from the bubbles remaining in the steel plate accumulation zone. Therefore, the present invention relates to the slab accumulation zone and the steel plate accumulation zone in particular. The steel sheet and its manufacturing method were explained. However, since the number of bubbles in the portion other than the accumulation zone is usually smaller than that in the accumulation zone, by controlling the bubbles in the accumulation zone as described above to improve the HIC resistance of the accumulation zone, the portion other than the accumulation zone HIC resistance is also excellent. That is, it should be noted that the effect of the present invention is not limited to the accumulation zone but extends to the entire steel sheet.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited by the following Examples as well as the present invention, and appropriate modifications are made to the extent that it can be adapted to the above or the purpose described below. Of course, it is possible to carry out, and all of them are included in the technical scope of the present invention.

Steels having a chemical composition of steel grades A to K shown in Table 1 were melted to obtain slabs (slabs) under the casting conditions shown in Table 2.

Regarding the casting conditions in Table 2, "○" indicates that Ar gas is tanned from porous brick with an inner tube diameter of 90 mm and an average pore diameter of 45 μm at ^{a back pressure of 1.4 to 1.8 kgf / cm 2} at 5-9 L / t. This is a method of obtaining a slab having a thickness of 280 mm by blowing it into a dish and continuously casting it.

Regarding the casting conditions in Table 2, "x" indicates 5-9 L / t of Ar gas from a porous brick with an inner tube diameter of 120 to 150 mm and an average pore diameter of 45 μm at a back pressure of 1.4 ^{to 1.8 kgf / cm 2.} This is a method of obtaining a slab having a thickness of 280 mm by blowing it into a tundish and continuously casting it.

After reheating the obtained slab to 1050 to 1250 ° C., the test No. 1 was carried out by the two patterns of processes shown in Table 2. Steel sheets 1 to 12 were manufactured.
Regarding the processes in Table 2, "TMCP" has a cumulative reduction rate of 50% or more by (1) rolling 5 or more passes at a reduction rate of 20% or less per pass in a temperature range of 900 ° C. or higher. (2) Hot rolling is performed so that the cumulative reduction rate is 20% or more and the rolling end temperature is 850 to 900 ° C. in the temperature range of 850 ° C. or higher and lower than 900 ° C. (3) This is a method of cooling from a cooling start temperature of 750 to 850 ° C. at an average cooling rate of 10 to 50 ° C./s, stopping in a temperature range of 350 to 600 ° C., and air-cooling to room temperature.
"QT" has (1) a cumulative reduction rate of 50% or more and a rolling end temperature of 850 ° C. by reducing 5 passes or more at a reduction rate of 20% or less per pass in a temperature range of 900 ° C. or higher. After hot rolling to the above, (2) air cooling to room temperature, (3) reheating to a temperature of 850 to 950 ° C and quenching, and (4) tempering at 600 to 700 ° C. How to do it.

For each of the above-mentioned slabs and each steel plate, the number density of bubbles having a circle equivalent diameter of 0.2 mm or more in the slab accumulation zone and the area ratio of the portion where the defect echo height is 20% or more are measured according to the following procedure. And the HIC test was performed.

[1. Number density of bubbles with a circular equivalent diameter of 0.2 mm or more in the slab accumulation zone]
At a position 45 to 60 mm in the thickness direction of the slab from the surface of the slab having a thickness of 280 mm, a position of 1/4 and 1/2 of the width of the slab in the width direction of the slab (direction perpendicular to the casting direction). A test piece having a plate thickness of 15 mm, a width of 70 mm, and a length of 15 mm including an L cross section (a surface perpendicular to the casting direction of the slab) was collected from two positions (slab accumulation zone). After polishing the L cross section with emery abrasive paper (# 320 to # 1500), a mirror finish was performed by buffing. Next, the L cross section is observed using an optical microscope (magnification: 5 times), the circle equivalent diameter of the bubbles is measured using an eyepiece micrometer (magnification: 5 times), and the circle equivalent diameter in the observation field of view is 0. The number of bubbles of 2 mm or more was counted. The density of bubbles was calculated from the area of the observation field of view and the number of bubbles having a circle-equivalent diameter of 0.2 mm or more. The maximum value of the densities obtained from the above two locations was defined as the number density of bubbles having a circle-equivalent diameter of 0.2 mm or more in the slab accumulation zone.

[2. Area ratio of the part where the defect echo height is 20% or more]
From two locations (steel plate accumulation zone) at 1/4 and 1/2 of the width of the steel sheet in the width direction of the steel sheet (direction perpendicular to the rolling direction), depending on the thickness of the steel sheet, the following Each test piece was collected as shown in.
(Steel plate with a thickness of 30 mm or less)
At the above two positions, three test pieces having a thickness of the steel plate, a width of 20 mm, and a length (rolling direction) of 100 mm were collected, and a total of six test pieces were prepared.
(Steel plate with a thickness of more than 30 mm)
At the above two positions, the thickness is (i) from the surface of the steel plate to the surface perpendicular to the surface, (ii) the position of 1/2 of the plate thickness, and (iii) from the back surface of the steel plate to the direction perpendicular to the back surface. A test piece of 30 mm × width 20 mm × length 100 was collected, and a total of 6 test pieces were prepared.

For each test piece, using an ultrasonic flaw detector "GSONIC SCAN 8AX1500SR" manufactured by Genes Co., Ltd. and a water-immersed probe (frequency 10 MHz, diameter 0.5 inch, focal depth 4.5 inch), 0. An ultrasonic flaw detection test is performed at a pitch of 4 mm x 0.4 mm, the area ratio of the part where the defect echo height of each test piece is 20% or more is measured, and the average value is 20% or more of the defect echo height of the steel sheet. The area ratio of the part that is.

[3. HIC test]
The HIC test was performed according to the method specified in NACE standard TM0284-2003 using the test piece used in the ultrasonic flaw detection test. Specifically, after immersing in a 25 ° C. (5.0% NaCl + 0.5% acetic acid) aqueous solution saturated with 1 atm of hydrogen sulfide for 96 hours, each test piece is prepared as follows according to the thickness of the steel sheet. Cross-sectional evaluation (according to NACE standard TM0284-2003 FiGURE 2-8) was performed and CLR was measured. Here, the cross section is a surface defined by the thickness direction and the width direction of the test piece.

(Steel plate with a thickness of 30 mm or less)
The cross section was evenly divided into three in the plate thickness direction, and three cross sections of the front surface side, the central portion and the back surface side were defined. The CLR was measured on the cross section on the surface side, and the average value was taken as "CLR of the surface layer portion". Further, the CLR was measured on the cross section of the central portion, and the average value thereof was defined as "CLR in the central portion".
(Steel plate with a thickness of more than 30 mm)
The CLR of the test piece collected from the surface of the steel sheet from the direction perpendicular to the surface was measured, and the average value was taken as "CLR of the surface layer portion". In addition, the CLR of the test piece collected from the position of 1/2 of the plate thickness was measured, and the average value was defined as "CLR in the central portion".

A steel sheet having a CLR in the surface layer portion and a CLR in the central portion of 10% or less was judged to be at a practical level and excellent in HIC resistance.

Table 2 shows the measurement results of the number density of bubbles having a circular equivalent diameter of 0.2 mm or more in the slab accumulation zone, the area ratio of the portion where the defect echo height is 20% or more, the CLR of the surface layer portion, and the CLR of the central portion. .. Regarding the CLR of the surface layer portion and the CLR of the central portion, those having 10% or less are indicated by “◯”.

In Tables 1 and 2, those underlined mean that they are out of the provisions of the present invention.

From the results in Table 2, it can be considered as follows. Test No. 1 to 5 and 12 are examples that satisfy all the requirements specified in the present invention, and are excellent in HIC resistance.

On the other hand, Test No. 6 to 11 are examples which do not satisfy any of the requirements specified in the present invention.

Test No. 6 and 7 are examples of steel sheets manufactured using slabs in which the casting conditions are not appropriate and the number of bubbles having a circular equivalent diameter of 0.2 mm or more in the slab accumulation zone is high, and the defect echo height is 20. The area ratio of the portion of% or more was large, the CLR of the surface layer portion deteriorated, and the desired HIC resistance was not achieved.

Test No. 8 and 9 are examples of steel sheets manufactured using steel sheets G and H having small [Ca] / [S], respectively, and a large amount of MnS is generated, the CLR in the central portion is deteriorated, and the desired HIC resistance is obtained. Not achieved. In addition, the test No. Regarding No. 8, the number density of bubbles was not evaluated because the CLR in the central portion deteriorated.

Test No. 10 and 11 are examples of steel sheets manufactured using steel sheets I and J having a large ([Ca] -1.25 × [S]) / [O], respectively, and coarse Ca inclusions are generated in the steel sheet accumulation zone. As a result, the CLR of the surface layer portion deteriorated, and the desired HIC resistance was not achieved.

Claims (6)

C: 0.02 to 0.15% by mass,
Si: 0.02 to 0.50% by mass,
Mn: 0.6 to 2.0% by mass,
P: More than 0% by mass, 0.030% by mass or less,
S: More than 0% by mass, 0.003% by mass or less,
Al: 0.010 to 0.080% by mass,
Ca: 0.0003 to 0.0060% by mass,
N: 0.001 to 0.01% by mass, O: more than 0% by mass, 0.0045% by mass or less, and satisfy the following equations (1) and (2).
The rest consists of iron and unavoidable impurities,
A steel sheet having an area ratio of 0.05% or less in a portion having a defect echo height of 20% or more.

3.0 ≤ [Ca] / [S] (1)
([Ca] -1.25 × [S]) / [O] ≦ 1.80 (2)
Here, [Ca], [S] and [O] are the contents (mass%) of Ca, S and O, respectively. B: More than 0% by mass, 0.005% by mass or less,
V: More than 0% by mass, 0.1% by mass or less,
Cu: Over 0% by mass, 1.5% by mass or less,
Ni: Over 0% by mass, 1.5% by mass or less,
Cr: Over 0% by mass, 1.5% by mass or less,
Mo: Over 0% by mass, 1.5% by mass or less,
Nb: More than 0% by mass, 0.06% by mass or less,
Ti: More than 0% by mass, 0.03% by mass or less,
Mg: More than 0% by mass, 0.01% by mass or less,
The steel sheet according to claim 1, further containing one or more selected from the group consisting of REM: more than 0% by mass and 0.02% by mass or less, and Zr: more than 0% by mass and 0.010% by mass or less. The steel plate according to claim 1 or 2, which is for a line pipe. A steel pipe for a line pipe formed of the steel plate according to any one of claims 1 to 3. The steel sheet according to any one of claims 1 to 3, which is used for a pressure vessel. The method for producing a steel sheet according to claim 1 or 2, which has the chemical composition according to claim 1 or 2, and has a bubble density of 0.2 mm or more in equivalent circle diameter in the slab accumulation zone. A method for manufacturing a steel plate using a slab of 15 pieces / cm ^{2 or less.}

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