EP3543366B1

EP3543366B1 - Steel sheet, steel pipe for line pipe, and production method therefor

Info

Publication number: EP3543366B1
Application number: EP17871656.9A
Authority: EP
Inventors: Kiichiro TASHIRO; Motoki KAKIZAKI
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2016-11-16
Filing date: 2017-11-02
Publication date: 2022-10-05
Anticipated expiration: 2037-11-02
Also published as: CN109952387B; JP6869151B2; KR20190065337A; KR102226990B1; JP2018083981A; CN109952387A; EP3543366A1; EP3543366A4

Description

Technical Field

The present disclosure relates to a steel plate, a steel pipe for a line pipe, and a production method therefor.

Background Art

With the development of degradation resources containing hydrogen sulfide, mainly, line pipes for transportation and tanks for storage of petroleum, gas and the like require so-called sour resistance, such as hydrogen induced cracking resistance and stress corrosion cracking resistance. Hydrogen induced cracking (hereinafter sometimes referred to as an "HIC") is known to be caused by the penetration of hydrogen into a steel material due to a corrosion reaction with the hydrogen sulfide or the like, as well as the collection and gasification of the hydrogen at defects such as non-metallic inclusions of MnS and Nb(C,N), for example. The occurrence of such HIC leads to a problem of reduction in the toughness of a structure. In particular, since hydrogen penetrates into a steel plate from its surface layer part, the surface layer part in the thickness direction of the steel plate is more likely to cause HIC than the center in the thickness direction of the steel plate, and thus the surface layer part is required to improve its HIC resistance.
For this reason, some techniques for improving HIC resistance in the surface layer part of the steel plate have been hitherto studied. For example, Patent Document 1 discloses that the HIC resistance is improved by controlling the amount of Ar gas blown into a molten steel to a predetermined level or less to thereby reduce an amount of Ar-gas uncompressed bubbles in the steel material which would form accumulation and segregation zones of MnS, Ca-Al based, and Ca-based inclusion clusters and Ti-based and Nb-based inclusions, causing the HIC.
Patent Document 2 discloses that the HIC resistance is improved by controlling a Ca concentration in a slab within a predetermined range during manufacture of the slab and also by controlling the contents of Ca, S and O as well as the content of Ar gas in a steel material within respective predetermined ranges.

Prior Art Document

Patent Document

Patent Document 1: JP H07-136748 A
Patent Document 2: JP 2016-125140 A

Summary of the Invention

Problems to be solved by the Invention

However, Patent Document 1 does not consider uncompressed bubbles in a steel material of the final product, even though it has considered the decrease in the number of bubbles in the slab. Consequently, defects induced by uncompressed bubbles remaining in the steel material cannot be controlled, and HIC due to the uncompressed bubbles cannot be suppressed.
Furthermore, the method described in Patent Document 2 does not consider the size of bubbles and the presence of uncompressed bubbles in a steel material, even though it has considered the decrease in the content of Ar-gas bubbles in the steel material. Thus, even in the presence of a small amount of coarse Ar bubbles, this method cannot sufficiently suppress the HIC.
An embodiment of the present invention has been made in view of the foregoing circumstance, and thus it is a main object of the embodiment of the present invention to provide a steel plate and a steel pipe for a line pipe that have excellent hydrogen induced cracking resistance.

Means for Solving the Problems

The invention is defined in the claims.

Effects of the Invention

The embodiments of the present invention provide a steel plate and a steel pipe for a line pipe that have excellent hydrogen induced cracking resistance, as well as a method for producing the steel plate.

Brief Description of the Drawings

FIG. 2 is a graph showing the relationship between a crack length ratio (CLR) of a surface layer part and the number density of bubbles having a circular equivalent diameter of 0.2 mm or more in a slab accumulation zone.
FIG. 1 is a graph showing the relationship between the CLR of the surface layer part and an area ratio of a part that has a defect echo height of 20% or more.

Mode for Carrying Out the Invention

In order to solve the above-mentioned problems, the present inventors have conducted intensive studies about the correlation between the CLR (Crack Length Ratio, a proportion [%] of the total length of cracks to the width of a test piece, a crack length ratio) of the surface layer part measured by a HIC test and internal defects in the steel plate measured by an ultrasonic flaw detection test. As a result, it has been found that the excellent HIC resistance can be achieved by controlling the chemical composition of the steel plate within a predetermined range such that the contents of Ca, S and O satisfy predetermined relational expressions, and also by controlling internal defects such that the area ratio of a part that has a defect echo height of 20% or more is 0.05% or less.
A steel plate and a production method therefor according to embodiments of the present invention will be described in detail below.

<1. Steel plate >

(1-1. Chemical composition)

A steel plate according to an embodiment of the present invention contains, 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 and 0.030% by mass or less, S: more than 0% by mass and 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 0.010% by mass, and O: more than 0% by mass and 0.0045% by mass or less, with the balance being iron and inevitable impurities, wherein the steel plate satisfies the following formulae (1) and (2): $3.0 \leq [Ca] / [S]$
$([Ca] - 1.25 \times [S]) / [O] \leq 1.80$
where [Ca], [S] and [O] are contents (% by mass) of Ca, S and O respectively.
By controlling the chemical composition in the manner described above, the steel plate with excellent hydrogen induced cracking resistance can be obtained.

[C: 0.02 to 0.15% by mass]

C is an element essential to ensure the strengths of a base material and a welded part. Thus, the C content needs to be 0.02% by mass or more. The C content is preferably 0.03% by mass or more, and more preferably 0.04% by mass or more. On the other hand, an extremely high C content degrades the HAZ toughness and a weldability of the steel. In addition, an excessive C content is more likely to form NbC or island-shaped martensite, which becomes a starting point of HIC or a fracture propagation route. Thus, the C content needs to be 0.15% by mass or less. The C content is preferably 0.12% by mass or less, and more preferably 0.10% by mass or less.

[Si: 0.02 to 0.50% by mass]

Si is an element that has a deoxidation function and is effective in improving the strengths of a base material and a welded part. To obtain these effects, the Si content is set at 0.02% by mass or more. The Si content is preferably 0.05% by mass or more, and more preferably 0.15% by mass or more. However, an extremely high Si content degrades the weldability and toughness of the steel. In addition, an excessive Si content forms island-shaped martensite to generate and propagate HIC. Thus, the Si content needs to be restricted to 0.50% by mass or less. The Si content is preferably 0.45% by mass or less, and more preferably 0.35% by mass or less.

[Mn: 0.6 to 2.0% by mass]

Mn is an element that is effective in improving the strengths of a base material and a welded. In the present invention, the Mn content is set at 0.6% by mass or more. The Mn content is preferably 0.8% by mass or more, and more preferably 1.0% by mass or more. However, an extremely high Mn content forms MnS, degrading not only the hydrogen induced cracking resistance, but also the HAZ toughness and the weldability. Thus, the upper limit of Mn content is set at 2.0% by mass. The Mn content is preferably 1.8% by mass or less, more preferably 1.5% by mass or less, and further preferably 1.2% by mass or less.

[P: more than 0% by mass and 0.030% by mass or less]

P is an element inevitably contained in a steel material. When the P content exceeds 0.030% by mass, the toughness of a base material and a HAZ are significantly degraded, and the hydrogen induced cracking resistance of a steel plate is also degraded. Thus, in the present invention, the P content is restricted to 0.030% by mass or less. The P content is preferably 0.020% by mass or less, and more preferably 0.010% by mass or less.

[S: more than 0% by mass and 0.003% by mass or less]

S is an element that forms a large amount of MnS to significantly degrade the hydrogen induced cracking resistance when contained in a large amount. Thus, in the present invention, the upper limit of S content is set at 0.003% by mass. The S content 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. Thus, the S content is desirably low from the viewpoint of improving the hydrogen induced cracking resistance.

[Al: 0.010 to 0.080% by mass]

Al is a strong deoxidizing element. When the Al content is low, the Ca concentration in an oxide tends to increase, that is, Ca-based inclusions are more likely to be formed at a surface layer part of a steel plate, causing fine HIC. Thus, in the present invention, the Al content needs to be 0.010% by mass or more. The Al content is preferably 0.020% by mass or more, and more preferably 0.030% by mass or more. On the other hand, when the Al content is extremely high, an Al oxide is formed in a cluster shape to become a starting point of hydrogen induced cracking. Thus, the Al content needs to be 0.080% by mass or less. The Al content 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 serves to control a form of a sulfide and has an effect of suppressing formation of MnS by forming CaS. To obtain this effect, the Ca content needs to be 0.0003% by mass or more. The Ca content is preferably 0.0005% by mass or more, and more preferably 0.0010% by mass or more. On the other hand, when the Ca content exceeds 0.0060% by mass, a lot of HIC occurs from the Ca-based inclusions as a starting point. Thus, in the present invention, the upper limit of Ca content is set at 0. 0060% by mass. The Ca content is preferably 0.0045% by mass or less, more preferably 0.0035% by mass or less, and further preferably 0.0025% by mass.

[N: 0.001 to 0.01% by mass]

N is an element that is precipitated as TiN in a steel microstructure to thereby suppress coarsening of austenite grains in a HAZ and to promote ferrite transformation, thus improving the toughness of the HAZ. To obtain this effect, the N content needs to be 0.001% by mass or more. The N content is preferably 0.003% by mass or more, and more preferably 0.0040% by mass or more. An excessive N content, however, degrades the HAZ toughness in the presence of solid-solution N. Thus, the N content needs to be 0.01% by mass or less. The N content is preferably 0.008% by mass or less, and more preferably 0.0060% by mass or less.

[O: more than 0% by mass and 0.0045% by mass or less]

O content is desirably low from the viewpoint of improving cleanliness. An extremely high O content degrades the toughness, and additionally causes HIC at an oxide as a starting point, thereby degrading the hydrogen induced cracking resistance. From this viewpoint, the O content needs to be 0.0045% by mass or less and is preferably 0.0035% by mass or less, and more preferably 0.0030% by mass or less.

[[Ca]/[S]: 3.0 or more]

The steel plate according to the present invention satisfies the following formula (1). $3.0 \leq [Ca] / [S]$
where [Ca] and [S] are contents (% by mass) of Ca and S respectively.
The technical meaning of the above-mentioned formula (1) will be described below.
S forms MnS as a sulfide-based inclusion to cause HIC from the MnS as the starting point. For this reason, by adding Ca into the steel, sulfide-based inclusions in the steel are controlled in the form of CaS, thereby the formation of MnS is suppressed to prevent the degradation in the HIC resistance. The present inventors have found that to sufficiently exhibit this function effect, the value of [Ca]/[S] needs to be set at 3.0 or more. The value of [Ca]/[S] is preferably 3.5 or more, and more preferably 4.0 or more. In consideration of the Ca content and S content specified in an embodiment of the present invention, the upper limit of [Ca]/[S] is approximately 15.

[([Ca] - 1.25 × [S])/[O]: 1.80 or less]

The steel plate according to the present invention satisfies the following formula (2). $([Ca] - 1.25 \times [S]) / [O] \leq 1.80$
where [Ca], [S] and [O] are contents (% by mass) of Ca, S and O respectively.
Hereinafter, the technical meaning of the above-mentioned formula (2) will be described.
To suppress the occurrence of HIC due to a Ca-based oxysulfide, it is effective to suppress the formation of particularly CaO, which is more likely to form aggregates, among Ca-based inclusions. Because of this, the amount of Ca ([Ca] - 1.25 × [S]) obtained by subtracting the amount of Ca, which is present as a sulfide (CaS), from the total amount of Ca in the steel must not become excessive for the amount of O. When the amount of Ca ([Ca] - 1.25 × [S]) is excessive for the amount of O, CaO is more likely to be formed as an oxide-based inclusion, so that a large amount of aggregates of CaO (coarse Ca-based inclusions) is easily formed on the surface layer part of the steel plate. To avoid such a situation, the present inventors have studied about the relationship between the value of ([Ca] - 1.25 × [S])/[O] and the HIC resistance, and as a result have found that the value of ([Ca] - 1.25 × [S]/[O]) needs to be 1.80 or less in order to obtain excellent HIC resistance. The value of ([Ca] - 1.25 × [S])/[O] is preferably 1.40 or less, more preferably 1.30 or less, further preferably 1.20 or less, and particularly preferably 1.00 or less. The lower limit of ([Ca] - 1.25 × [S]) is approximately 0.1 from the viewpoint of suppressing Al₂O₃ that easily forms aggregates, like CaO.
The basic components of the steel plate according to the present invention are as described above, with the balance being iron and inevitable impurities. It is noted that inevitable impurities, other than P and S, which are brought depending on situations, such as raw materials, other materials, or facilities, are obviously allowed to be contained in the steel.
As mentioned above, P and S are elements inevitably contained (inevitable impurities), and their composition ranges are specified separately as mentioned above. Thus, the term "inevitable impurities" contained as the balance, as used herein, means elements inevitably contained, except for elements with their composition ranges separately specified.
The steel plate of the present invention may further selectively contain the following element(s) in addition to the above-mentioned elements, and thereby will have more improved properties of the steel plate itself depending on the kinds of the elements contained.

[B: more than 0% by mass and 0.005% by mass or less]

B enhances the hardenability of a steel and the strengths of a base material and a welded part. In addition, B binds with N in the process of cooling a heated HAZ during welding to precipitate BN, which promotes ferrite transformation from austenite grains, thereby improving the HAZ toughness. To obtain these effects, the B content is preferably 0.0002% by mass or more. The B content is more preferably 0.0005% by mass or more, and further preferably 0.0010% by mass or more. An excessive B content, however, degrades the toughness of the base material and the HAZ and leads to degradation in the weldability. Thus, the B content is 0.005% by mass or less. The B content is more preferably 0.004% by mass or less, and further preferably 0.0030% by mass or less.

[V: more than 0% by mass and 0.1% by mass or less]

V is an element effective in improving the strength. To obtain this effect, the V content is preferably 0.003% by mass or more. The V content is more preferably 0.010% by mass or more. On the other hand, when the V content exceeds 0.1% by mass, the weldability and the toughness of the base material are degraded. Thus, the V content is 0.1% by mass or less, and more preferably 0.08% by mass or less.

[Cu: more than 0% by mass and 1.5% by mass or less]

Cu is an element effective in improving the hardenability to enhance the strength. To obtain this effect, the Cu content is preferably 0.01% by mass or more. The Cu content is more preferably 0.05% by mass or more, and further preferably 0.10% by mass or more. However, when the Cu content exceeds 1.5% by mass, the toughness is degraded. Thus, the Cu content is 1.5% by mass or less. The Cu content is more preferably 1.0% by mass or less, and further preferably 0.50% by mass or less.

[Ni: more than 0% by mass and 1.5% by mass or less]

Ni is an element effective in improving the strength and toughness of a base material and a welded part. To obtain this effect, the Ni content is preferably 0.01% by mass or more. The Ni content is more preferably 0.05% by mass or more, and further preferably 0.10% by mass or more. An excessive Ni content, however, makes the steel plate for structures extremely expensive. From the economic point of view, the Ni content is 1.5% by mass or less. The Ni content is more preferably 1.0% by mass or less, and further preferably 0.50% by mass or less.

[Cr: more than 0% by mass and 1.5% by mass or less]

Cr is an element effective in improving the strength. To obtain this effect, the Cr content is preferably 0.01% by mass or more. The Cr content is more preferably 0.05% by mass or more, and further preferably 0.10% by mass or more. On the other hand, when the Cr content exceeds 1.5% by mass, the HAZ toughness is degraded. Thus, the Cr content is 1.5% by mass or less . The Cr content is more preferably 1. 0% by mass or less, and further preferably 0.50% by mass or less.

[Mo: more than 0% by mass and 1.5% by mass or less]

Mo is an element that is effective in improving the strength and toughness of a base material. To exhibit this effect, the Mo content is preferably 0.01% by mass or more. The Mo content is more preferably 0.05% by mass or more, and further preferably 0.10% by mass or more. However, when the Mo content exceeds 1.5% by mass, the HAZ toughness and the weldability are degraded. Thus, the Mo content is 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 and 0.06% by mass or less]

Nb is an element that is effective in enhancing the strength and the base material toughness without degrading the weldability . To obtain these effects, the Nb content is preferably 0.002% by mass or more. The Nb content is more preferably 0.010% by mass or more, and further preferably 0.020% by mass or more. However, when the Nb content exceeds 0.06% by mass, the toughnesses of a HAZ and the base material are degraded. Thus, in the present invention, the upper limit of Nb content is 0.06% by mass. The Nb content is more preferably 0.050% by mass or less, more further preferably 0.040% by mass or less, and yet more preferably 0.030% by mass or less.

[Ti: more than 0% by mass and 0.03% by mass or less]

Ti is an element that is precipitated as TiN in a steel to prevent the coarsening of austenite grains at a HAZ during welding and to promote ferrite transformation, thus improving the toughness of the HAZ. Ti is also the element that is effective in improving the HIC resistance of the steel plate because of its desulfurization effect. To obtain these effects, the Ti content is preferably 0.003% by mass or more. The Ti content is more preferably 0.005% by mass or more, and further preferably 0.010% by mass or more. On the other hand, an excessive Ti content precipitates solid-solution Ti and TiC to degrade the toughnesses of the base material and the HAZ. Thus, the Ti content is 0.03% by mass or less. The Ti content is more preferably 0.02% by mass or less.

[Mg: more than 0% by mass and 0.01% by mass or less]

Mg is an element that is effective in improving the toughness through refinement of crystal grains and also in improving the HIC resistance because of its desulfurization effect. To obtain these effects, the Mg content is preferably 0. 0003% by mass or more. The Mg content is more preferably 0.001% by mass or more. On the other hand, even when Mg is contained in an excessive amount, its effects are saturated. Thus, the upper limit of the Mg content is 0.01% by mass. The Mg content is more preferably 0.005% by mass or less.

[REM: more than 0% by mass and 0.02% by mass or less]

REM (rare earth metal) are elements that are effective in enhancing the hydrogen induced cracking resistance by suppressing the formation of MnS through the desulfurization effect. To exhibit these effects, the REM content is preferably 0.0002% by mass or more. The REM content is more preferably 0.0005% by mass or more, and further preferably 0.0010% by mass or more. On the other hand, even when REM is contained in a large amount, its effects are saturated. Thus, the upper limit of the REM content is 0.02% by mass. From the viewpoint of preventing the clogging of an immersion nozzle during casting to enhance the productivity, the REM content is more preferably 0.015% by mass or less, further preferably 0.010% by mass or less, and more further preferably 0.0050% by mass or less. In an embodiment of the present invention, the above-mentioned REM means lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[Zr: more than 0% by mass and 0.010% by mass or less]

Zr is an element that contributes to improvement of the HIC resistance by the desulfurization effect and also improvement of the HAZ toughness by forming oxides and dispersing them finely. To exhibit these effects, the Zr content is preferably set at 0.0003% by mass or more. The Zr content is more preferably 0.0005% by mass or more, further preferably 0.0010% by mass or more, and yet more preferably 0.0015% by mass or more. On the other hand, the addition of an excessive content of Zr forms coarse inclusions to degrade the hydrogen induced cracking resistance and the toughness of a base material. Thus, the Zr content is 0.010% by mass or less. The Zr content is more preferably 0.0070% by mass or less, further preferably 0.0050% by mass or less, and more further preferably 0.0030% by mass or less.

(1-2. Internal defects)

In the steel plate according to the present invention, an area ratio of a part that has a defect echo height of 20% or more is 0.05% or less. Thus, the occurrence of HIC from a steel plate accumulation zone can be suppressed even when bubbles remain in the steel plate accumulation zone.
The details thereof will be described below.
In a casting process of a slab, Ar gas needs to be blown into a molten steel, for example, in order to suppress clogging of an injection nozzle, to cause reflux for degassing in an RH, and to stir the molten steel in a tundish.
The slab accumulation zone is a surface part of the slab, and the part that has been cooled and solidified earlier than the center at the stage of formation of the slab. Thus, in such a slab accumulation zone, bubbles caused by Ar gas blown thereinto during slab casting float, but are more likely to be trapped and remain in a solidified part of a curved portion.
The bubbles remaining in the slab accumulation zone are difficult to completely compress in a rolling process, so that these bubbles are more likely to remain in the steel plate accumulation zone. Since hydrogen tend to be accumulated in the bubbles remaining in the steel plate accumulation zone, HIC may occur from the remaining bubbles as a starting point. Due to this, the HIC resistance can be improved by reducing the bubbles in the steel plate accumulation zone.
As used herein, the "slab accumulation zone" means a region of t/8 to t/4 from the surface of the slab where t is a thickness of the slab, and the "steel plate accumulation zone" means a region of t'/8 to t'/4 from the surface of a steel plate where t' is a thickness of the steel plate, which is obtained by hot-rolling the above-mentioned slab having the thickness t.
When a slab is hot-rolled, the slab is normally rolled almost uniformly (that is, the slab accumulation zone and other portions are rolled at the approximately same rolling reduction) . Thus, the region of t/8 to t/4 from the surface of the slab becomes a part corresponding to the region of t'/8 to t'/4 from the surface of the steel plate obtained by hot-rolling the slab. That is, the "slab accumulation zone" is the part corresponding to the "steel plate accumulation zone" of the steel plate obtained by hot-rolling the slab.
FIG. 1 shows the result obtained by examining the relationship between the CLR of the surface layer part measured by an HIC test and an area ratio of a part that has a defect echo height of 20% or more, which is measured by an ultrasonic flaw detection test.
The defect echo height as used herein means a ratio [%] of an intensity of a defect echo reflected by a defect inside a test piece (taken out from a part of the steel plate) to an intensity of a bottom-surface echo reflected by the bottom surface of the steel plate (or test piece).
The area ratio of a part that has a defect echo height of 20% or more means a ratio [%] of the area of the part that has a defect echo height of 20% or more to the entire area of the steel plate scanned by a probe.
From this result, the present inventors have found a correlation between the CLR of the surface layer part and the above-mentioned area ratio. That is, it has been found that even when bubbles remain in the steel plate accumulation zone, as long as the area ratio of the part that has a defect echo height of 20% or more is 0.05% or less, the CLR of the surface layer part of the steel plate can be 10% or less, thereby the occurrence of HIC from the steel plate accumulation zone is suppressed. From the viewpoint of obtaining a steel plate having more excellent HIC resistance, the defect echo height is preferably 30% or less, and more preferably 25% or less, and the area ratio of the part that has a defect echo height of 20% or more is preferably 0.04% or less, and more preferably 0.03% or less.
Since all bubbles in the steel plate are difficult to remove, the defect echo height and the area ratio of the part that has a defect echo height of 20% or more are usually 0% or more.
The steel plate according to an embodiment of the present invention and a steel pipe for a line pipe formed using the steel plate may be preferably for use in line pipes for transportation, storage tanks, and pressure vessels for purification, of natural gas and crude oil.

<2. Method for Producing Steel plate >

A method for producing a steel plate according to the present invention uses a slab having the above-mentioned chemical composition, the slab having a number density of bubbles of 0.15 bubbles/cm² or less in a slab accumulation zone, the bubbles having a circular equivalent diameter of 0.2 mm or more. By using such a slab, the steel plate with excellent HIC resistance can be produced.
The details thereof will be described below.

(2-1. Slab having a number density of bubbles of 0.15 bubbles/cm² or less in a slab accumulation zone, the bubbles having a circular equivalent diameter of 0.2 mm or more)

As mentioned above, since hydrogen tends to be accumulated in the bubbles remaining in the steel plate accumulation zone, HIC may occur from the remaining bubbles as a starting point. Thus, the HIC resistance can be improved by reducing the bubbles in the steel plate accumulation zone.
Since the "slab accumulation zone" is a part corresponding to the "steel plate accumulation zone" of the steel plate obtained by hot-rolling, as a specific means for reducing bubbles in the steel plate accumulation zone, it is effective to reduce bubbles in the slab accumulation zone so as to the bubbles in the steel plate accumulation zone of the steel plate obtained by hot-rolling. Consequently, the HIC resistance can be improved.
FIG. 2 shows the result obtained by examining the relationship between the CLR of the surface layer part measured by the HIC test and the number density of bubbles having a circular equivalent diameter of 0.2 mm or more in a slab accumulation zone. As a result, the present inventors have found that the steel plate is produced by using the slab having the number density of bubbles of 0.15 bubbles/cm² or less in the slab accumulation zone, the bubbles having a circular equivalent diameter of 0.2 mm or more, thereby it is possible to reduce the remaining bubbles that have remained in the rolling process without being completely compressed. Furthermore, they have also found that in the steel plate produced using such a slab, the area ratio of the part that has a defect echo height of 20% or more can be 0.05% or less, and the CLR of the surface layer part of the steel plate can be 10% or less, thereby HIC from the steel plate accumulation zone is suppressed.
The circular equivalent diameter of the bubbles in the slab accumulation zone is preferably 0.17 mm or less, and more preferably 0.15 mm or less. The number density of bubbles having a circular equivalent diameter of 0.2 mm or more in the slab accumulation zone is preferably 0.10 bubbles/cm² or less, and more preferably 0.05 bubbles/cm² or less.
Since all bubbles in the slab accumulation zone are difficult to remove, the circular equivalent diameter of the bubbles in the slab accumulation zone is normally 0 mm or more, and the number density of bubbles having a circular equivalent diameter of 0.2 mm or more in the slab accumulation zone is usually 0 bubble/cm² or more.
Methods of measuring a circular equivalent diameter of the bubble and the number density of bubbles may include, but are not particularly limited to, for example, the following methods.
A test piece taken out from the slab accumulation zone is observed by an optical microscope, and circular equivalent diameters of bubbles are measured with an ocular micrometer to count the number of bubbles with a circular equivalent diameter of 0.2 mm or more in the observation field of view.
Then, the bubble density is calculated from the area of the observation field of view and the number of bubbles having a circular equivalent diameter of 0.2 mm or more.

(2-2. Process of casting the above-mentioned slab)

In order to cast the slab that has the number density of bubbles of 0.15 bubbles/cm² or less in the slab accumulation zone, the bubbles having a circular equivalent diameter of 0.2 mm or more, it is important to control the amount of Ar gas blown into a nozzle and the diameter of the bubble when a molten steel is supplied from the tundish to a mold during a steel making process.
In the case of using Ar gas, the Ar gas needs to be blown in through a porous brick which has an inner tube 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, at a back pressure of 1.4 kgf/cm² or more and 1.8 kgf/cm² or less and at 3 L (liter) /t (ton) or more and 10 L/t or less.
The inner tube diameter is preferably 75 mm or more and more preferably 80 mm or more, and is also preferably 110 mm or less and more preferably 105 mm or less.
The average pore diameter is preferably 35 µm or more and more preferably 40 µm or more, and is also preferably 55 µm or less and more preferably 50 µm or less.
The back pressure is preferably 1.45 kgf/cm² or more, and more preferably 1.5 kgf/cm² or more, and is also preferably 1.75 kgf/cm² or less and more preferably 1.7 kgf/cm² or less.
The amount of Ar gas blown in is preferably 5 L/t or more and more preferably 7 L/t or more, and is also preferably 12 L/t or less and 10 L/t or less.
By blowing Ar gas within these ranges, the nozzle is less likely to be clogged. Furthermore, as the Ar gas flow with a large diameter is blown into the molten steel, bubbles of Ar gas are more likely to float in the mold. Consequently, the bubbles of Ar gas are easily removed from the accumulation zone, which can reduce the amount of bubbles of Ar gas trapped in the accumulation zone.
There could also be proposed a means for reducing the amount of Ar gas blown with the molten steel from the injection nozzle for injecting the molten steel into the mold. However, this would make it difficult to stir the molten steel by Ar gas in the vicinity of a surface of the molten steel in the mold, and thus solidification of the surface of the molten steel may occur. That is why this means is not recommended.
Conditions other than those mentioned above are not particularly limited, and a steel having the above-mentioned chemical composition may be melted according to a usual steel making method to cast a slab by a continuous casting process.
The method for producing the steel plate according to an embodiment of the present invention using the above-mentioned slab is not particularly limited as long as the area ratio of a part that has the defect echo height of 20% or more is 0.05% or less. According to normal methods, the steel plate can be produced by the hot-rolling, followed by cooling.
Hereinafter, "temperature" refers to a temperature of a material.
To achieve the above-mentioned steel plate defect area ratio, the hot-rolling is recommended to be performed, for example, at a rolling reduction of 20% or less per pass for five or more passes in a range of surface temperatures of 900°C or higher to reach a cumulative rolling reduction of 50% or more.
By hot-rolling under the above-mentioned conditions, the surface layer part in the thickness direction is more preferentially deformed than the inside in the thickness direction, so that bubbles trapped in the accumulation zone can be more effectively compressed.
The cooling conditions taken after the hot-rolling are recommended in which cooling is performed, for example, from a cooling start temperature of Ar3 transformation point or higher at an average cooling rate of 10°C/s or more.
By cooling under the above-mentioned conditions, HIC that occur in the vicinity of the center of the steel plate can be effectively suppressed.
Moreover, a steel pipe for a line pipe can be produced by a general method, using the steel plate according to an embodiment of the present invention. The steel pipe for a line pipe obtained by using the steel plate according to an embodiment of the present invention also has excellent HIC resistance and toughness. The steel plate according to an embodiment of the present invention may be for use in a pressure vessel by a general method.
As mentioned above, bubbles tend to remain in the slab accumulation zone, and consequently HIC is more likely to occur from these bubbles, which have remained in the steel plate accumulation zone, as a starting points. For this reason, the steel plate and production method therefor according to the embodiments of the present invention have been described by focusing, especially, on the slab accumulation zone and the steel plate accumulation zone. However, the amount of bubbles located in a part other than the accumulation zone, is normally less than that located in the accumulation zone. For this reason, by controlling the bubbles located in the accumulation zone in the above-mentioned manner, the HIC resistance of the accumulation zone is improved, so that the part other than the accumulation zone can also have excellent HIC resistance. That is, it should be noted that the effect of the present invention is not limited to the accumulation zone only, but can be exhibited across the whole of the steel plate.

Examples

The embodiments of the present invention will be more specifically described below by way of Examples, but the present invention is not limited to the following Examples. Various modifications can be appropriately made to these examples as long as they are adaptable to the above-mentioned and below-mentioned concepts, and thus all these modifications are included within the technical scope of the present invention.
Steels having chemical compositions of steel types A to K shown in Table 1 were melted to produce slabs (cast pieces) under casting conditions shown in Table 2.
Regarding the casting conditions of Table 2, the notation "O" indicates a method in which Ar gas was blown into a tundish through a 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/cm² and at 5 to 9 L/t, thereby obtaining a slab with a thickness of 280 mm by continuous casting.
Regarding the casting conditions of Table 2, the notation "x" indicates a method in which Ar gas was blown into the tundish through 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/cm² and at 5 to 9 L/t, thereby obtaining a slab with a thickness of 280 mm by the continuous casting.
The obtained slabs were reheated to a temperature between 1,050°C and 1,250°C, and then subjected to one of two patterns of processes shown in Table 2, and thus the steel plates of tests Nos. 1 to 12 were produced.
Regarding the processes shown in Table 2, the process "TMCP" refers to a method which includes (1) hot-rolling the steel plate at a rolling reduction of 20% or less per pass for five or more passes in a range of temperatures of 900°C or higher so as to reach a cumulative rolling reduction of 50% or more, (2) hot-rolling the steel plate in a range of temperatures of 850°C or higher and lower than 900°C so as to reach a cumulative rolling reduction of 20% or more and a rolling end temperature of 850 to 900 °C, and (3) cooling from a cooling start temperature of 750 to 850°C at an average cooling rate of 10 to 50°C/s and then stopping the cooling in a range of temperatures of 350 to 600°C, followed by air-cooling to the room temperature.
The process "QT" refers to a method which includes (1) hot-rolling the steel plate at a rolling reduction of 20% or less per pass for five or more passes in a range of temperatures of 900°C or higher so as to reach a cumulative rolling reduction of 50% or more and a rolling end temperature of 850°C or higher, (2) air-cooling to the room temperature, (3) reheating to a temperature between the 850°C to 950°C and then hardening, and (4) tempering at a temperature between 600°C and 700°C.
Regarding the above-mentioned respective slabs and steel plates, the number density of bubbles having a circular equivalent diameter of 0.2 mm or more in the slab accumulation zone, as well as the area ratio of the part that had a defect echo height of 20% or more were measured, and the HIC test was performed on the steel plates, according to the following procedures.

[1. Number density of bubbles having a circular equivalent diameter of 0.2 mm or more in the slab accumulation zone]

A test piece with 15 mm thickness × 70 mm width × 15 mm length, including a L cross section (surface perpendicular to a casting direction of a slab), was taken from each of two sites of the slab (slab accumulation zones) located at positions between 45 mm and 60 mm in the slab thickness direction from the surface of the slab of 280 mm in thickness, the positions of the two sites being located corresponding to 1/4 and 1/2 of the width of the slab in its width direction (the direction perpendicular to the casting direction), respectively. The L cross section of each test piece was polished using emery polishing paper (#320 to #1500), followed by mirror finishing through buffing. Then, the L cross section of the test piece was observed using an optical microscope (magnification: 5 times), and circular equivalent diameters of the bubbles were measured using an ocular micrometer (magnification: 5 times), whereby the number of bubbles having a circular equivalent diameter of 0.2 mm or more was counted in the observation field of view. The density of bubbles was calculated from the area of the observation field and the number of bubbles having the circle equivalent diameter of 0.2 mm or more. The maximum value of the densities obtained from the above-mentioned two sites was referred to as the number density of bubbles having the circular equivalent diameter of 0.2 mm or more in the slab accumulation zone.

[2. Area ratio of a part having a defect echo height of 20% or more]

Test pieces were respectively taken from two sites (steel plate accumulation zones) of the steel plate, located at positions corresponding to 1/4 and 1/2 of the width of the steel plate along its width direction (direction perpendicular to the rolling direction), depending on the thickness of the steel plate in the following ways.

(Steel plate having a thickness of 30 mm or less)

Three test pieces, each having a thickness of the steel plate × 20 mm width × 100 mm length (in the rolling direction), were taken from each of the above-mentioned two positions. Consequently, the six test pieces in total were prepared.

(Steel plate having a thickness exceeding 30 mm)

In each of the above-mentioned two positions, test pieces, each having 30 mm thickness × 20 mm width × 100 mm length, were taken (i) from the surface of the steel plate in the direction perpendicular to the surface, (ii) from the position of 1/2 of the thickness of the steel plate, and (iii) from the back surface of the steel plate in the direction perpendicular to the back surface. Consequently, the six test pieces in total were prepared.
Each test piece was subjected to an ultrasonic flaw detection test at a pitch of 0.4 mm × 0.4 mm using an ultrasonic flaw detector "GSONIC SCAN 8AX1500SR" manufactured by GNES Corporation, and a water immersion probe (frequency: 10 MHz, diameter: 0.5 inches, focal depth: 4.5 inches). An area ratio of a part of the test piece that had a defect echo height of 20% or more was measured, and an average of the measured area ratios was referred to as an area ratio of the part that had the defect echo height of 20% or more.

[3. HIC Test]

The HIC test was performed on the test piece used in the above-mentioned ultrasonic flaw detection test, according to the method specified by the NACE standard TM0284-2003. In detail, the test piece was immersed in an aqueous solution (5.0% NaCl + 0.5% acetic acid) with 1 atm of hydrogen sulfide saturated therein, at 25°C for 96 hours. Then, the cross section of each test piece was evaluated (according to NACE standard TM0284-2003 FIGURES 2 to 8), depending on the thickness of the steel plate to thereby measure a CLR of each test piece in the following way. Here, the cross section is a plane defined by the thickness direction and the width direction of the test piece.

(Steel plate having a thickness of 30 mm or less)

The cross section of each test piece was equally divided into three pieces in the thickness direction, namely, three cross sections on the surface side, the center, and the back surface side. The CLRs of the cross section on the surface side were measured, and an average of the measured CLRs was referred to as a "CLR of the surface layer part". In addition, the CLRs of the cross section at its center were measured, and an average of the measured CLRs was referred to as a "CLR of the center".

(Steel plate having a thickness exceeding 30 mm)

The CLRs of the test piece taken from the surface of the steel plate in the direction perpendicular to the surface were measured, and an average of the measured CLRs was referred to as a "CLR of the surface layer part". In addition, the CLRs of the test piece taken from the position located at 1/2 of the thickness of the steel plate in its thickness direction were measured, and an average of the measured CLRs was referred to as a "CLR of the center".
The steel plate in which each of the CLR of the surface layer part and the CLR of the center was 10% or less was evaluated to be a practical standard one and to have excellent HIC resistance.
Table 2 shows the measurement results of the number density of bubbles having the circular equivalent diameter of 0.2 mm or more in the slab accumulation zone, the area ratio of the part that had a defect echo height of 20% or more, the CLR of the surface layer part, and the CLR of the center. The test pieces in which each of the CLR of the surface layer part and the CLR of the center was 10% or less were indicated by the notation "O".

Data underlined in Tables 1 and 2 means these data deviated from the requirements specified by an embodiment of the present invention.

[Table 1]

Steel type	Chemical composition (% by mass) ^∗Balance being iron and inevitable impurities
Steel type	C	Si	Mn	P	S	Al	Ca	N	O	B	V	Cu	Ni	Cr	Mo	Nb	Ti	REM	Zr	[Ca]/[S]	([Ca]-1.25×[S])/[O]
A	0.063	0.30	1.16	0.006	0.0004	0.031	0.0015	0.0046	0.0015		0.002	0.16	0.24	0.20	0.08	0.000	0.012	0.0020	0.0010	3.75	0.67
B	0.063	0.31	1.15	0.005	0.0005	0.033	0.0015	0.0037	0.0022		0.002	0.15	0.24	0.19	0.11	0.030	0.012	0.0018	0.0008	3.00	0.40
C	0.058	0.30	1.15	0.005	0.0004	0.037	0.0020	0.0046	0.0023		0.061	0.00	0.00	0.26	0.07	0.030	0.012	0.0021	0.0009	5.00	0.65
D	0.062	0.30	1.43	0.006	0.0003	0.033	0.0013	0.0052	0.0017		0.002	0.15	0.24	0.25	0.09	0.010	0.010	0.0017	0.0006	4.33	0.54
E	0.059	0.31	1.12	0.006	0.0004	0.031	0.0017	0.0048	0.0020		0.002	0.16	0.24	0.19	0.12	0.029	0.012	0.0017	0.0009	4.25	0.60
F	0.060	0.31	1.13	0.006	0.0003	0.035	0.0014	0.0049	0.0016		0.003	0.15	0.25	0.26	0.09	0.010	0.012	0.0019	0.0008	4.67	0.64
G	0.061	0.29	1.14	0.006	0.0006	0.024	0.0016	0.0048	0.0029		0.002	0.15	0.24	0.20	0.11	0.030	0.011	0.0022	0.0010	2.67	0.29
H	0.061	0.31	1.44	0.006	0.0011	0.030	0.0015	0.0040	0.0029	0.0001	0.002	0.15	0.23	0.24	0.09	0.011	0.011	0.0017	0.0010	1.36	0.04
I	0.060	0.31	1.15	0.005	0.0003	0.034	0.0022	0.0045	0.0009		0.002	0.14	0.24	0.20	0.12	0.030	0.012	0.0023	0.0010	7.33	2.03
J	0.060	0.31	1.43	0.005	0.0004	0.036	0.0037	0.0036	0.0015		0.003	0.141	10.24	0.24	0.09	0.011	0.011	0.0019	0.0009	9.25	2.13
K	0.061	0.30	1.15	0.005	0.0005	0.029	0.0015	0.0040	0.0023		0.00	0.15	0.24	0.20	0.12	0.031	0.012	0.0019	0.0012	3.00	0.38

[Table 2]

Test No.	Steel type	Casting conditions	Steel plate producing conditions	Thickness [mm]	Number density of bubbles having a circular equivalent diameter of 0.2 mm or more in the slab accumulation zone [bubbles/cm²]	Area ratio of the part having a defect echo height of 20% or more [%]	CLR of the surface layer part	CLR of the center
1	A	O	TMCP	16	0.10	0	O	O
2	B	O	TMCP	38	0	0	O	O
3	C	O	TMCP		20	0	0	O	O
4	A	O	QT	70	0.10	0	O O	O
5	D	O	QT	60	0	0	O	O O
6	E	×	TMCP	20	0.67	0.12	×	O
7	F	×	QT	70	0.58	0.37	×	O
8	G	O	TMCP	20	-	0	O	×
9	H	O	QT	40	0	0	O	×
10	I	O	TMCP	38	0	0	×	O
11	J	O	QT	40	0	0	×	O
12	K	O	TMCP		20	0.10	0	O	O

From the results shown in Table 2, the following consideration can be made. All the tests Nos. 1 to 5 and 12 are examples that satisfied all requirements specified by the embodiments of the present invention and thus exhibited excellent HIC resistance.
In contrast, the tests Nos. 6 to 11 are examples that did not satisfy any one of the requirements specified by the embodiments of the present invention.
The tests Nos. 6 and 7 are examples of the steel plate in which the casting conditions were not appropriate and each of the used slabs had the large number density of bubbles having a circular equivalent diameter of 0.2 mm or more in the slab accumulation zone. Consequently, in the steel plates of these tests, the area ratio of the part that had the defect echo height of 20% or more was large, thus the CLR of the surface layer part was deteriorated, so that the desired HIC resistance could not be achieved.
The tests Nos. 8 and 9 are examples of the steel plate in which the steel plates were produced using steel materials of the steel types G and H, respectively, that had small ratios of [Ca]/[S]. Consequently, in the steel plates of these tests, a large amount of MnS was generated to deteriorate the CLR of the center, so that the desired HIC resistance could not be achieved. In the test No. 8, the CLR of the center was deteriorated, and thus the number density of bubbles was not evaluated.
The tests Nos. 10 and 11 are examples of the steel plate in which the steel plates were produced using the steels of the steel types I and J, respectively, that had large ratios of ([Ca] - 1.25 × [S])/[O]. Consequently, in the steel plates of these tests, coarse Ca inclusions were formed in the steel plate accumulation zone, thus the CLR of the surface layer part was deteriorated, so that the desired HIC resistance could not be achieved.

Claims

A steel plate comprising:
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 and 0.030% by mass or less,

S: more than 0% by mass and 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, and

O: more than 0% by mass and 0.0045% by mass or less,

optionally comprising one or more elements selected from the group consisting of:
B: more than 0% by mass and 0.005% by mass or less,

V: more than 0% by mass and 0.1% by mass or less,

Cu: more than 0% by mass and 1.5% by mass or less,

Ni: more than 0% by mass and 1.5% by mass or less,

Cr: more than 0% by mass and 1.5% by mass or less,

Mo: more than 0% by mass and 1.5% by mass or less,

Nb: more than 0% by mass and 0.06% by mass or less,

Ti: more than 0% by mass and 0.03% by mass or less,

Mg: more than 0% by mass and 0.01% by mass or less,

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,

with the balance being iron and inevitable impurities, wherein

the steel plate satisfies the following formulae (1) and (2): $3.0 \leq [Ca] / [S]$
$([Ca] - 1.25 \times [S]) / [O] \leq 1.80$

where [Ca], [S] and [O] are contents in % by mass

of Ca, S and O respectively,

and wherein

an area ratio of a part that has a defect echo height of 20% or more is 0.05% or less, the defect echo height means a ratio in % of an intensity of a defect echo reflected by a defect inside a test piece taken out from a part of the steel plate to an intensity of a bottom-surface echo reflected by the bottom surface of the steel plate or test piece, and the defect echo height is measured in accordance with the method specified in the description.
Use of the steel plate according to claim 1 in a line pipe.
A steel pipe for a line pipe made of the steel plate according to claim 1.
Use of the steel plate according to claim 1 in a pressure vessel.
A method for producing the steel plate according to claim 1, comprising using a slab having the chemical composition according to claim 1, the slab having a number density of bubbles of 0.15 bubbles/cm² or less in a slab accumulation zone, the bubbles having a circular equivalent diameter of 0.2 mm or more, the slab accumulation zone means a region of t/8 to t/4 from the surface of the slab where t is a thickness of the slab, and the number density of bubbles is measured in accordance with the method specified in the description.