EP3239320A1

EP3239320A1 - Steel plate having excellent hydrogen-induced cracking resistance and steel pipe for line pipe

Info

Publication number: EP3239320A1
Application number: EP15873095.2A
Authority: EP
Inventors: Kiichiro TASHIRO; Taku Kato; Haruya KAWANO; Yuichi OKA; Shinsuke Sato; Sei Kimura; Takashi Miyake
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2014-12-26
Filing date: 2015-12-22
Publication date: 2017-11-01
Anticipated expiration: 2035-12-22
Also published as: KR20170093965A; EP3239320B1; CN107109565A; EP3239320A4; JP2016125138A; KR102141794B1; KR20190137955A; JP6584912B2

Abstract

Provided are a steel plate and a steel pipe having excellent HIC resistance. Further, provided are the steel plate and steel pipe that can evaluate HIC resistance based on the internal quality of a cast strip without performing an HIC test. The steel plate having excellent HIC resistance satisfies the specified contents of C, Si, Mn, P, S, Al, Ca, N, and O, and further contains the specified content of one or more elements selected from the group consisting of REM and Zr, with the balance being iron and inevitable impurities. The steel plate is characterized by that the ratio (Ca/S) of the Ca to the S is 2.0 or more; the Ca, the S, and the O satisfy the relationship of (Ca - 1.25S)/O ¤ 1.80; and at a stage of a slab, the slab for the steel plate does not include a horizontal crack or include the horizontal crack having a maximum opening thickness of a threshold value t¸ or less, where the threshold value t¸ is a maximum opening thickness of a horizontal crack for avoiding the occurrence of hydrogen-induced cracking in the steel plate obtained by rolling the slab.

Description

Technical Field

The present invention relates to a steel plate having excellent hydrogen induced cracking resistance. In particular, the present invention relates to a steel plate that has excellent hydrogen-induced cracking resistance and is suitable for use in line pipes for transportation and tanks for storage of natural gas and crude oil, and to a steel pipe for line pipes with excellent hydrogen-induced cracking resistance, obtained by using the steel plate.

Background Art

With the development of degradation resources containing hydrogen sulfide, mainly, line pipes for transportation and tanks for storage of oil, gas, etc. , require so-called sour resistance, such as hydrogen-induced cracking resistance or stress-corrosion cracking resistance. Hereinafter, a steel plate that exhibits the adequate sour resistance is sometimes referred to as a "sour-resistant steel plate" in some cases. Hydrogen-induced cracking (hereinafter sometimes referred to as "HIC") is known as a crack caused by the penetration of hydrogen thereinto due to a corrosion reaction with the hydrogen sulfide or the like, and the collection and gasification of the hydrogen at non-metallic inclusions, such as MnS or Nb(C, N).
HIC is also known to have a tendency to occur in segregation zones, including a center segregation and internal cracks of a cast strip, particularly, at an inclusion such as MnS, as a starting point. For this reason, some techniques for enhancing HIC resistance have been proposed. For example, Patent Document 1 discloses that a steel material has improved HIC resistance by suppressing segregation degrees of Mn, Nb, and Ti at the center in the thickness direction of a steel plate. Patent Document 2 discloses a method for suppressing HIC that would occur in MnS or a Ca-based acid sulfide as a starting point, by using a parameter formula that includes the contents of Ca, O and S.
These methods suppress the occurrence of a large amount of HIC, but in some cases, fine HIC can occur locally at a number of sites.
Meanwhile, a steel plate is subjected to melting, casting, and hot-rolling, and then it undergoes an HIC test before being dispatched as a product. However, it takes several weeks to obtain the result of the HIC test. Once the HIC occurs during the HIC test, the above-mentioned steel plate cannot be dispatched as a product with excellent hydrogen-induced cracking resistance. Because of this, the steel plate needs to be manufactured again, that is, melted again to produce a product, and then the product needs to undergo the HIC test again. This increases the manufacturing time period and might possibly result in missing the deadline or the like.
For this reason, it is considered that if the HIC resistance can be evaluated at the stage of a cast strip after the casting without performing the HIC test after hot rolling, the manufacturing time period can be significantly shortened. As mentioned above, HIC occurs at segregation zones (center segregation, internal cracks) or inclusions, such as MnS, as a starting point. Thus, if these can be evaluated at the stage of the cast strip, the evaluation of the HIC resistance is considered to be possible based on the evaluation results.
For example, in a conventional method that involves performing an HIC test after rolling, a long procedure A-1 from casting to dispatching is carried out in the following way. In contrast, when the HIC resistance can be evaluated at the stage of the cast strip, the steps of "Sample Preparation (for HIC test) → HIC Test" in performing the HIC test can be omitted as illustrated in a procedure B-1, so that products can be dispatched at an early stage.

Procedure A-1: Casting → Rolling → Sample Preparation (for HIC test) → HIC Test → Dispatching
Procedure B-1: Casting → Evaluation of HIC Resistance → Rolling → Dispatching

If the result of the HIC test is no good (NG), the conventional method would be to perform the following procedure A-2, where it takes a long time to perform steps from the casting to re-melting. In contrast, when the HIC resistance can be evaluated at the stage of the cast strip as illustrated in the following procedure B-2, even if the evaluation result is NG, the steps of "Rolling → Sample Preparation (for the HIC Test) → HIC Test" in the procedure A-2 below can be omitted, which enables a quick start of re-melting.

Procedure A-2: Casting → Rolling → Sample Preparation (for HIC test) → HIC Test → Re-Melting
Procedure B-2: Casting → Evaluation of HIC Resistance → Re-Melting

As such a method, Patent Document 3 discloses a method in which internal cracks are evaluated at the stage of the cast strip. In this method, the possibility of a hot charge rolling (HCR) operation is determined based on the evaluation result of internal cracks.

Prior Art Document

Patent Document

Patent Document 1: JP 2010-209461 A
Patent Document 2: JP H6-136440 A
Patent Document 3: JP 2006-198649 A

Disclosure of the Invention

Problems to be solved by the Invention

Internal cracks, which are a problem for steel plates requiring the sour resistance, are very fine cracks. However, a technique mentioned in Patent Document 3 evaluates only internal cracks that are problematic for the HCR operation, specifically, large cracks of 10 mm or more in length. Hence, the above-mentioned method could miss the fine internal cracks that become an issue in steel plates requiring the sour resistance. Thus, such a method cannot precisely evaluate the HIC resistance caused by internal cracks at the stage of a cast strip.
The present invention has been made in view of the foregoing circumstance, and it is an object of the present invention to achieve a steel plate and a steel pipe that have excellent hydrogen-induced cracking resistance, and further to achieve a steel plate and a steel pipe that enable the evaluation of the HIC resistance by an internal quality of a cast strip without performing an HIC test.

Means for Solving the Problems

A steel plate having excellent hydrogen-induced cracking resistance according to the present invention that can solve the above-mentioned problem includes, in percent by mass:

0.02 to 0.15% of C;
0.02 to 0.50% of Si;
0.6 to 2.0% of Mn;
more than 0% and 0.030% or less of P;
more than 0% and 0.003% or less of S;
0.010 to 0.08% of Al;
0.0003 to 0.0060% of Ca;
0.001 to 0.01% of N;
more than 0% and 0.0045% or less of O; and
one or more elements selected from the group consisting of more than 0% and 0.02% or less of REM and more than 0% and 0.010% or less of Zr, with the balance being iron and inevitable impurities, wherein
a ratio (Ca/S) of the Ca to the S is 2.0 or more,
the Ca, the S, and the O satisfy the formula below: (Ca -1.25S) /O ≤ 1.80, and
at a stage of the slab, the slab for the steel plate does not include a horizontal crack or includes the horizontal crack having a maximum opening thickness of a threshold value tθ or less, where the threshold value tθ is a maximum opening thickness of a horizontal crack for avoiding the occurrence of hydrogen-induced cracking in the steel plate obtained by rolling the slab.

The threshold value tθ may be a value previously determined by method including following (i) to (iii):

(i) a maximum opening thickness of the slab is measured;
(ii) a hydrogen-induced cracking test is performed on a steel plate obtained by rolling a slab which has been cast under the same casting conditions as said slab; and
(iii) a maximum opening thickness of a horizontal crack that avoids the occurrence of hydrogen-induced cracking is determined from the maximum opening thickness measured in the step (i) and a result of the hydrogen-induced cracking test shown in the step (ii).

The slab cast under the same casting conditions as said slab may be the slab in which the maximum opening thickness is measured.
The steel plate may be in an API (American Petroleum Institute) X65 Grade, and the threshold value tθ may be 0.047 mm.
The steel plate may be in an API X70 Grade, and the threshold value tθ may be 0.043 mm.
The steel plate may be in an ASME (American Society of Mechanical Engineers) SA516 Grade 60, and the threshold value tθ may be 0.047 mm.
The steel plate may be in an ASME SA516 Grade 65, and the threshold value tθ may be 0.047 mm.
The steel plate may be in an ASME SA516 Grade 70, and the threshold value tθ may be 0.043 mm.
The steel plate may be in an ASTM (American Society for Testing and Materials) A516 Grade 60, and the threshold value tθ may be 0.047 mm.
The steel plate may be in an ASTM A516 Grade 65, and the threshold value tθ may be 0.047 mm.
The steel plate may be in an ASTM A516 Grade 70, and the threshold value tθ may be 0.043 mm.
The steel plate may further contain one or more of the elements (A) and (B) below as another element:

(A) in percent by mass, one or more element selected from the group consisting of more than 0% and 0.005% or less of B, more than 0% and 0.1% or less of V, more than 0% and 1.5% or less of Cu, more than 0% and 1.5% or less of Ni, more than 0% and 1.5% or less of Cr, more than 0% and 1.5% or less of Mo, and more than 0% and 0.06% or less of Nb; and
(B) in percent by mass, one or more elements selected from the group consisting of more than 0% and 0.03% or less of Ti and more than 0% and 0.01% or less of Mg.

The steel plate is suitable for use in line pipe and pressure container. The present invention also includes a steel pipe for a line pipe formed of the steel plate.

Effects of the Invention

The present invention can provide the steel plate and steel pipe that surely have the excellent hydrogen-induced cracking resistance. Further, the present invention can provide the steel plate and steel pipe in which the HIC resistance can be evaluated by the internal quality of the cast strip without performing an HIC test. These steel plates are suitable for use in line pipe for transportation of natural gas and crude oil, pressure container, such as the storage tank, and the like.

Brief Description of the Drawings

Figs. 1(a) and 1(b) are schematic diagrams for explaining internal cracks, in which Fig. 1(a) shows a slab, i.e., a state of a steel before rolling, and Fig. 1(b) shows a product, i.e., a state of a steel after rolling.
Fig. 2 is a cross-sectional view of the slab.
Fig. 3 shows cross-sectional views of the slab and the product.
Fig. 4 shows results of examination about the relationship between an opening thickness and an HIC resistance at a plurality of cross sections.
Fig. 5 is a diagram for explaining an examined surface of the slab.
Fig. 6 is a diagram showing the relationship between a maximum opening thickness of a horizontal crack and the presence or absence of HIC occurrence when using a steel of API X65 Grade in Examples.
Fig. 7 is a diagram showing the relationship between a maximum opening thickness of a horizontal crack and the presence or absence of HIC occurrence when using a steel of API X70 Grade in Examples.

Mode for Carrying Out the Invention

The inventors have diligently studied to solve the foregoing problems. First, the inventors have focused on the tendency for HIC to occur at a MnS inclusion as a starting point. As a result, it is conceived that by causing a steel to contain a rare earth element or Zr, which has a desulfurization effect, the formation of MnS can be suppressed to improve the hydrogen-induced cracking resistance. Furthermore, an appropriate content of such a component is found to efficiently exhibit the desulfurization effect as mentioned later.
Next, the inventors have focused on the tendency for HIC to occur at a segregation zone as a starting point. Consequently, attention is paid to a "horizontal crack" caused by segregation, particularly, the maximum opening thickness of the horizontal crack. It is found that if the maximum opening thickness of a horizontal crack is restricted to a predetermined threshold value or less at a stage of a slab, a steel plate with higher hydrogen-induced cracking resistance can be obtained, and furthermore products can be dispatched at an early stage. This matter will be described below.
First, the component composition of a steel will be described below.
To ensure the excellent HIC resistance, the component composition of the steel needs to be controlled. Furthermore, to ensure the high strength, excellent weldability, and the like, which are other properties required as, for example, the steel for line pipe, the component composition of the steel plate needs to be as follows. The reasons for specifying the contents of the respective components, including the aforesaid rare earth elements and Zr, will be described below.

Component Composition

C: 0.02 to 0.15%

Carbon (C) is an element essential to ensure the strength of a base metal and a weld bead. Thus, the C content needs to be 0.02% or more. The C content is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, an extremely high C content degrades the heat-affected zone (HAZ) toughness and the weldability of the steel. Any excessive C content is more likely to form NbC or island-shaped martensite, which possibly becomes as the starting point of HIC or a fracture propagation route. Thus, the C content needs to be 0.15% or less. The C content is preferably 0.12% or less, and more preferably 0.10% or less.

Si: 0.02 to 0.50%

Silicon (Si) has a deoxidation function and is effective in improving the strength of a base metal and a weld bead. To exhibit these effects, the Si content is set at 0.02% or more. The Si content is preferably 0.05% or more, and more preferably 0.15% or more. However, an extremely high Si content degrades the weldability and toughness of the steel. Any excessive Si content forms island-shaped martensite to generate and propagate HIC. Accordingly, the Si content needs to be suppressed to 0.50% or less. The Si content is preferably 0.45% or less, and more preferably 0.35% or less.

Mn: 0.6 to 2.0%

Manganese (Mn) is an element that is effective in improving the strength of a base metal and a weld bead. In the present invention, the Mn content is set at 0.6% or more. The Mn content is preferably 0.8% or more, and more preferably 1.0% or more. However, an extremely high Mn content forms MnS, degrading not only the hydrogen-induced cracking resistance, but also the HAZ toughness and weldability. Thus, the upper limit of Mn content is set at 2.0%. The Mn content is preferably 1.8% or less, more preferably 1.5% or less, and still more preferably 1.2% or less.

P: more than 0% and 0.030% or less

Phosphorus (P) is an element inevitably contained in steel. When the P content exceeds 0.030%, the roughness of a base metal and a HAZ are significantly degraded, and the hydrogen-induced cracking resistance of the steel is also degraded. Thus, in the present invention, the P content is restricted to 0.030% or less. The P content is preferably 0.020% or less, and more preferably 0.010% or less.

S: more than 0% and 0.003% or less

Sulfur (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 0.003%. The S content is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less. Thus, the S content is desirably low from the viewpoint of improving the hydrogen-induced cracking resistance.

A1: 0.010 to 0.08%

Aluminum (Al) is a strong deoxidizing element. When the Al content is low, the Ca concentration in the oxide tends to increase, that is, the Ca-based inclusions are more likely to be formed at a superficial layer of a steel plate, causing fine HIC. Thus, in the present invention, the Al content needs to be 0.010% or more. The Al content is preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, when the Al content is extremely high, an A1 oxide is formed in a cluster shape and becomes a starting point of hydrogen-induced cracking. Thus, the Al content needs to be 0.08% or less. The Al content is preferably 0.06% or less, and more preferably 0.05% or less.

Ca: 0.0003 to 0.0060%

Calcium (Ca) serves to control the form of a sulfide and has an effect of suppressing the formation of MnS by forming CaS. To obtain this effect, the Ca content needs to be 0.0003% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when the Ca content exceeds 0.0060%, HIC occurs at many sites from the Ca-based inclusions as the starting point. Thus, in the present invention, the upper limit of Ca content is set at 0.0060%. The Ca content is preferably 0.0045% or less, more preferably 0.0035% or less, and still more preferably 0.0025% or less.

N: 0.001 to 0.01%

Nitrogen (N) precipitates as TiN in a steel microstructure, preventing austenite grains in a HAZ zone from being coarsened and further promoting ferrite transformation to thereby improve the toughness of the HAZ zone. To obtain these effects, the N content needs to be 0.001% or more. The N content is preferably 0.003% or more, and more preferably 0.0040% or more. An extremely high N content, however, degrades the toughness of the HAZ by the presence of the solid-solute N. The N content needs to be 0.01% or less. The N content is preferably 0.008% or less, and more preferably 0.0060% or less.

O: more than 0% and 0.0045% or less

A content of O, i.e., oxygen is desirably low from the viewpoint of improving the cleanliness of a steel. An extremely high O content degrades the toughness of the steel, and additionally causes HIC at an oxide as a starting point, thereby degrading the hydrogen-induced cracking resistance. In this regard, the O content needs to be 0.0045% or less, preferably 0.0030% or less, and more preferably 0.0020% or less.

Ca/S in terms of mass ratio: 2.0 or more

As mentioned above, S forms MnS as a sulfide-based inclusion, and HIC might occur at the MnS as a starting point. Thus, the sulfide-based inclusion in the steel has its form controlled as CaS by adding Ca, thereby rendering S harmless for the HIC resistance. To sufficiently exhibit these effects, the Ca/S needs to be set at 2.0 or more. The Ca/S is preferably 2.5 or more, and more preferably 3.0 or more. Note that the upper limit of Ca/S is approximately 17 based on the Ca content and S content specified by the present invention.

(Ca - 1.25S) /O ≤ 1.80

To avoid the occurrence of HIC due to a Ca-based oxysulfide, it is effective to suppress, especially, CaO that is the most likely to form aggregates among Ca-based inclusions. For this reason, a Ca content (Ca - 1.25S) that is obtained by subtracting a content in Ca present as a sulfide (CaS) in the steel from the total Ca content in the steel must not be excessive relative to the O content. When the Ca content (Ca - 1.25S) is excessive relative to the O content, CaO is more likely to be formed as an oxide-based inclusion, which makes it easier for aggregates of the CaO (coarse Ca-based inclusions) to be formed in a larger amount at a superficial layer of a steel plate. Since these coarse Ca-based inclusions serve as the starting point of HIC, the (Ca - 1.255) /O needs to be 1. 80 or less in order to obtain the excellent HIC resistance. (Ca - 1.25S) /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. Like CaO, to suppress Al₂O₃ that tends to form aggregates, the lower limit of (Ca - 1.25S) /O is approximately 0.1.

REM: more than 0% and 0.02% or less

A rare earth metal (REM) is an element that is effective in enhancing the hydrogen-induced cracking resistance by suppressing the formation of MnS through the desulfurization effect as mentioned above. To exhibit such effects, the REM content is preferably 0.0002% or more. The REM content is more preferably 0.0005% or more, and still more preferably 0.0010% or more. On the other hand, if REM is contained in a large amount, the effect is saturated. Thus, the upper limit of the REM content needs to be 0.02%. From the viewpoint of preventing the clogging of an immersion nozzle during casting to enhance the productivity, the REM content is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.0047% or less. Note that in the present invention, REM means lanthanoid elements, i. e. , 15 elements from La to Lu, and scandium and yttrium.

Zr: more than 0% and 0.010% or less

Zirconium (Zr) serves to form an oxide and disperse it finely in steel, while improving the HIC resistance by the desulfurization effect, thereby contributing to improving the HAZ toughness. To exhibit these effects, the Zr content is preferably set at 0.0003% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more, and yet more preferably 0.0015% or more. On the other hand, any excessive Zr content forms coarse inclusions to degrade the hydrogen-induced cracking resistance and the toughness of the base metal. Thus, the Zr content needs to be 0.010% or less. The Zr content is preferably 0.0070% or less, more preferably 0.0047% or less, and still more preferably 0.0030% or less.
The components of the steel plate and steel pipe in the present invention have been mentioned above, with the balance being iron and inevitable impurities. In addition to the elements mentioned above, the steel further includes:

(a) one or more elements selected from the group consisting of B, V, Cu, Ni, Cr, Mo, and Nb in the following contents, thereby making it possible to enhance the strength and toughness; and/or
(b) one or more elements selected from the group consisting of Ti and Mg in the following contents, thereby making it possible to improve the HAZ toughness and to promote the desulfurization, thus further improving the HIC resistance. These elements will be described in detail below.

B: more than 0% and 0.005% or less

Boron (B) enhances the hardenability of a steel and the strength of a base metal and a weld bead. Furthermore, B binds to N to precipitate BN while the heated HAZ zone is cooled in welding, thus promoting ferrite transformation from the inside of an austenite grain. In this way, B improves the HAZ toughness. To obtain these effects, the B content is preferably 0.0002% or more. The B content is more preferably 0.0005% or more, and still more preferably 0.0010% or more. However, any excessive B content degrades the toughness of a base metal and a HAZ zone, thus leading to degradation in the weldability. Thus, the B content is preferably 0.005% or less. The B content is more preferably 0.004% or less, and still more preferably 0.0030% or less.

V: more than 0% and 0.1% or less

Vanadium (V) is an element effective in improving the strength of steel. To obtain this effect, the V content is preferably 0.003% or more, and more preferably 0.010% or more. On the other hand, when the V content exceeds 0.1%, the weldability and the toughness of a base metal would be degraded. Thus, the V content is preferably 0.1% or less, and more preferably 0.08% or less.

Cu: more than 0% and 1.5% or less

Copper (Cu) is an element effective in improving the hardenability of steel. To obtain this effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when the Cu content exceeds 1.5%, the toughness of steel is degraded. Thus, the Cu content is preferably 1.5% or less. The Cu content is more preferably 1.0% or less, and still more preferably 0.50% or less.

Ni: more than 0% and 1.5% or less

Nickel (Ni) is an element effective in improving the strength and toughness of a base metal and a weld bead. To obtain these effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more, and still more preferably 0.10% or more. However, an extremely high Ni content leads to an excessively expensive steel for a structure. From the economical aspect, the Ni content is preferably 1.5% or less. The Ni content is more preferably 1.0% or less, and still more preferably 0.50% or less.

Cr: more than 0% and 1.5% or less

Chromium (Cr) is an element effective in improving the strength of steel. To obtain such an effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when the Cr content exceeds 1.5%, the HAZ toughness of the steel is degraded. Thus, the Cr content is preferably 1.5% or less. The Cr content is more preferably 1.0% or less, and still more preferably 0.50% or less.

Mo: more than 0% and 1.5% or less

Molybdenum (Mo) is an element effective in improving the strength and toughness of a base metal. To exhibit this effect, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when the Mo content exceeds 1.5%, the HAZ toughness and weldability of the steel are degraded. Thus, the Mo content is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% or less.

Nb: more than 0% and 0.06% or less

Niobium (Nb) is an element effective in enhancing the strength of steel and the toughness of a base metal without degrading its weldability. To obtain this effect, the Nb content is preferably 0.002% or more. The Nb content is more preferably 0.010% or more, and still more preferably 0.020% or more. However, when the Nb content exceeds 0.06%, the toughness of the base metal and HAZ is degraded. Thus, in the present invention, the upper limit of Nb content is preferably set at 0.06%. The Nb content is more preferably 0.047% or less, still more preferably 0.040% or less, and much more preferably 0.030% or less.

Ti: more than 0% and 0.03% or less

Titanium (Ti) precipitates as TiN in steel, thereby preventing austenite grains in a HAZ zone from being coarsened during welding and thereby promoting the ferrite transformation. Thus, Ti is an element that is effective in improving the toughness of the HAZ zone. Furthermore, Ti exhibits the desulfurization effect, and thus is an element that is effective in improving the HIC resistance. To obtain these effects, the Ti content is preferably 0.003% or more. The Ti content is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, any excessive Ti content leads to an increase in the amount of solid-solute Ti and precipitated TiC, thus degrading the toughnesses of a base metal and a HAZ zone. Thus, the Ti content is preferably 0.03% or less, and more preferably 0.02% or less.

Mg: more than 0% and 0.01% or less

Magnesium (Mg) is an element that is effective in improving the toughness of steel through refinement of crystal grains, and also effective in improving the HIC resistance because of its desulfurization effect. To obtain these effects, the Mg content is preferably 0.0003% or more. The Mg content is more preferably 0.001% or more. On the other hand, an excessive Mg content saturates its effect. Thus, the upper limit of the Mg content is preferably 0.01%. The Mg content is more preferably 0.005% or less.
The steel plate in the present invention is a steel plate having a high hydrogen-induced cracking resistance and in which, at a stage of a slab, the slab for the steel plate has no horizontal crack or has the horizontal crack having a maximum opening thickness of a threshold value or less. The term "a threshold value" as used herein means a maximum opening thickness of a horizontal crack for avoids the occurrence of HIC in the steel plate obtained by rolling the slab. The maximum opening thickness is measured in advance.
In this way, the horizontal crack is evaluated at the stage of the slab. Specifically, the maximum opening thickness of the horizontal crack at the stage of the slab is set to be the predetermined threshold value or less, thereby making it possible to produce a steel plate with higher hydrogen-induced cracking resistance and to dispatch products at an early stage, which will be mentioned below.
First, the "horizontal crack" will be described in detail below.
Segregation of components is present at an internal crack or center segregation zone of the slab. As the segregation degree of the component becomes higher, HIC is more likely to occur, which is well known, for example, as mentioned in JP 2007-136496 A . Furthermore, segregation forms a hard microstructure such as MA (martensite-austenite constituent so-called an island-shaped martensite), perlite band, or the like. As the segregation degree becomes higher, the hard microstructure is more likely to be formed, and HIC propagates and extends along the hard microstructure. In the embodiment, the HIC resistance is evaluated, particularly, by taking into account the segregation degree of internal cracks.
Note that segregation is also present between secondary dendritic branches. That is, microsegregation can also occur. However, the spacing between the secondary dendritic branches is so small that no HIC propagates and extends, thus such segregation therebetween is not problematic in terms of the quality. Therefore, the present invention does not consider the microsegregation.
Internal cracks include the "horizontal crack" and "other internal cracks". These cracks are caused by the bulging between the rolls, an imbalance of cooling water, or the deformation of steel during correction. The "horizontal crack", as shown in Fig. 1(a), is a crack present in a region ranging from the end to half the thickness D of the slab, i.e., D/2 in the width direction W of the slab. The horizontal crack is the crack that propagates in the slab width direction and the casting direction. On the other hand, "other internal cracks" as shown in Fig. 1(a) are cracks present in the slab through the entire width thereof. Such other internal cracks are cracks that propagate in the slab thickness direction and slab width direction, or in the slab thickness direction and slab casting direction.
After rolling the slab, as shown in Fig. 1(b), the "horizontal crack" extends, while the "other internal cracks" is reduced. Once HIC occurs at the above-mentioned crack as a starting point, the "horizontal crack" allows the HIC to easily propagate and extend, while the "other internal cracks" do not cause the HIC to propagate and extend, which is not problematic in terms of the quality. When performing the HIC test, HIC sometimes occurs at an occurrence site of the "horizontal crack", but no HIC occurs at an occurrence site of the "other internal cracks". Thus, the present invention takes into account only the "horizontal crack" among internal cracks.
In the present invention, the segregation degree of the "horizontal crack" is evaluated based on the "maximum opening thickness" to be mentioned below. The occurrence position of the "horizontal crack" is illustrated in Fig. 1(a), and is a crack that generates at a solid-liquid interface during solidification. The "horizontal crack" is accompanied by a segregation line caused by propagation of concentrated, molten steel between dendritic branches. When the degree of the propagation is significant, an opening occurs along the segregation line. There is a correlation between the segregation degree of the horizontal cracking and the thickness of the segregation (opening width). As the opening thickness is increased, the segregation degree of the horizontal crack tends to become higher. That is, there is a correlation between the maximum opening thickness and the segregation degree of the horizontal crack. HIC is more likely to occur as the segregation degree of the horizontal crack becomes higher. Thus, as the maximum opening thickness is larger, HIC is considered to be more likely to occur. First, these lead to findings that the HIC resistance can be determined based on the "maximum opening thickness", and that as the maximum opening thickness is reduced, HIC can be suppressed. Hereinafter, the maximum opening thickness of the horizontal cracking is simply referred to as a "maximum opening thickness" in some cases.
Note that a fine horizontal crack having an "opening thickness" of approximately several tens of 10 µm is crimped during rolling. Consequently, such a horizontal crack is found not to become a defect to be detected by ultrasonic testing (UT) at a stage of products, but can cause HIC. When taking this into account, the HIC is considered to occur not by the presence of an opening in the steel plate, but by a high segregation degree of the horizontal crack.
Accordingly, the inventors have found that if the HIC resistance of a steel plate after rolling can be determined in advance by the use of the above-mentioned maximum opening thickness of a cast strip at a stage of a slab, i.e. , after casting and before rolling, the HIC test does not need to be performed on a steel plate as a product, thereby omitting a step therefor. Consequently, products can be dispatched at an early stage.
In the following, a description will be given on the way to determine the maximum opening thickness as well as a threshold value tθ of the maximum opening thickness, which is used for evaluation of the HIC resistance of a steel plate after rolling by means of the maximum opening thickness.
The way to determine the maximum opening thickness of the horizontal crack will be described below.
First, a slab obtained by casting is cut in the thickness direction, i.e., in the direction perpendicular to the casting direction as shown in Fig. 2, and then is examined for a horizontal crack of a segregation zone. The position of occurrence of a horizontal crack tends to vary not in the casting direction, but in the slab width direction and slab thickness direction. As illustrated in Fig. 2, by setting a cross section perpendicular to the casting direction as an object to be examine, a part where the horizontal crack becomes worst can be examined.
At the cross section of the slab shown in Fig. 2, maximum opening thicknesses t1 and t2 of horizontal cracks that are present in regions R1 and R2, respectively, ranging from both ends of the slab width W to half the slab thickness, i.e., D/2 are measured. Here, the maximum opening thickness t1 is a maximum opening thickness in the region R1, while the maximum opening thickness t2 is a maximum opening thickness in the region R2. Referring to Fig. 2, hereinafter, a combination of the regions R1 and R2 will be referred to as a first range, while a region R3 shown in Fig. 2 will be referred to as a second range in some cases.
The reason why the regions R1 and R2 are examined is as follows. That is, the horizontal crack occurs while solidification proceeds from both ends (narrow faces) toward the center in the width direction of the slab. During the solidification, the regions R1 and R2, i.e., the first range are susceptible to cooling on sides of the narrow surfaces (short sides), so that the solidification proceeds toward the center in the width direction of the slab. On the other hand, the region R3 of the width W-D, excluding D/2 from both ends in the width direction, i.e., the second region is barely susceptible to cooling on the side of the narrow surfaces (short sides), so that the solidification hardly proceeds in the width direction. Thus, since the horizontal crack is considered to occur in the regions R1 and R2, in the present invention, the horizontal cracks are examined in the regions R1 and R2 as mentioned above.
Suppose that two or more horizontal cracks are present in each of the regions R1 and R2. Here, the maximum opening thicknesses t1 and t2 are defined as the maximum opening thicknesses among the thicknesses of a plurality of openings in the respective regions R1 and R2, respectively. For example, when three horizontal cracks are present in the region R1, one of the three horizontal cracks that has the largest opening is selected. The "maximum opening thickness t1" is defined as an opening thickness of a part of the selected horizontal crack that is opened most, i.e., a part with the largest opening thickness.
Then, a description will be given on the way to determine the threshold value tθ to be used for the evaluation of the HIC resistance of the slab, i.e., the maximum opening thickness for avoiding the occurrence of the HIC in the steel plate obtained by rolling the slab.
The threshold value tθ is determined in advance, but its determination method is not particularly limited to the following method. An example of the method for determining the threshold value tθ will include the following steps (i) to (iii). The details of the method will be described below.

(i) A maximum opening thickness of the slab is measured.
(ii) An HIC test is performed on a steel plate that is obtained by rolling a slab that has been cast on the same casting conditions as the above-mentioned slab.
(iii) A maximum opening thickness of a horizontal crack that avoids the occurrence of hydrogen-induced cracking is determined from the maximum opening thickness measured in the step (i) and a result of the HIC test shown in the step (ii).

A slab, which has been cast on the same casting conditions as the slab whose maximum opening thickness is measured, is hot-rolled, thereby producing a steel plate for measurement of a threshold value. The HIC test is performed on the steel plate to examine the presence or absence of HIC occurrence. The HIC test is performed by a method specified by the National Association of Corrosion and Engineer (NACE) standard TM0284-2003, as mentioned in examples below.
The term "same casting conditions" as used herein includes i) a casting speed is constant, ii) an abnormal state in operation, such as clogging of a nozzle, does not occur, and iii) the same cooling conditions and distance between roll are applied. When determining the threshold value tθ, the "segregation degree obtained by examining a slab" is related to the "HIC test result for a product". However, if the HIC resistances of these slabs are different from each other, the threshold value cannot be determined. The operation factors i) to iii) significantly affect the horizontal crack and center segregation, which will consequently affect the HIC resistance as well. Thus, the different operation factors lead to different HIC resistances. For this reason, the steel plate used in the HIC test is preferably one obtained by manufacture using a slab which has been cast on the same casting conditions (operation factors) as the slab whose maximum opening thickness is examined. In particular, the slab whose maximum opening thickness is examined is preferably the same as the slab for the HIC test.
In the HIC test, it is examined whether or not HIC occurs in regions of a product (steel plate), corresponding to the regions R1 and R2 of the slab shown in Fig. 2. As illustrated in Fig. 3, the regions to be evaluated for the HIC resistance are varied depending on the rolling direction during the rolling using the slab shown in Fig. 2.
When rolling the slab in the casting direction, that is, when the rolling direction is identical to the casting direction, as shown in Fig. 3(a), the width of each region does not change before and after the rolling, and thus a slab width W is the same as a product width W, i.e., slab width W = product width W. In this case, as illustrated in Fig. 3 (a), the regions of the product corresponding to the "slab regions R1 and R2" are "regions R11 and R12, ranging from both ends in the width direction of the product to half the width of the product, i.e., D/2. The region of the product corresponding to the "slab region R3" is a "region R13 of the width W-D obtained by excluding half the width of the product, i.e., D/2 from both ends in the width direction of the product".
On the other hand, as shown in Fig. 3(b), when rolling the slab in the width direction, that is, when the rolling direction includes the width direction, the width of the slab changes from W before the rolling to Wa after the rolling, and thus the slab width w is smaller than the product width Wa, i.e., slab width W < product width Wa. In this case, as shown in Fig. 3(b), the regions R21, R22, and R23 corresponding to the slab regions R1, R2, and R3 are determined by a rolling reduction, i.e., product width Wa/slab width W. It is confirmed whether or not HIC occurs in these regions R21 and R22.
Then, a "threshold value tθ of the maximum opening thickness" that avoids the occurrence of HIC is determined from the "'maximum opening thicknesses t1 and t2' obtained by examination of the slab" and "the HIC test result for the product".
When determining the threshold value tθ, the results obtained from the region of the slab and the corresponding region of the product are correlated to each other. For instance,

(I) in the case of rolling the slab in the casting direction as illustrated in Fig. 3(a), suppose that the product region R11 is in the state of the "presence of HIC occurrence", and the product region R12 is in the state of the "absence of HIC occurrence". The determination is made as follows:
- (I-1) when the slab region R1 has the maximum opening thickness t1, the determination of the "presence of HIC occurrence" is made as the result of the production region R11; and
- (I-2) when the slab region R2 has the maximum opening thickness t2, the determination of the "absence of HIC occurrence" is made as a result of the product region R12.
(II) In the case of rolling the slab in the width direction as illustrated in Fig. 3(b), suppose that the product region R21 is in the state of the "presence of HIC occurrence", and the production region R22 is in the state of the "absence of HIC occurrence". The determination is made as follows:
- (II-1) when the slab region R1 has the maximum opening thickness t1, the determination of the "presence of HIC occurrence" is made as the result of the production region R21; and
- (II-2) when the slab region R2 has the maximum opening thickness t2, the determination of the "absence of HIC occurrence" is made as a result of the product region R22.

The threshold value tθ of the maximum opening thickness serving as the criterion of the presence or absence of HIC occurrence is determined from the above-mentioned plurality of results. Specifically, for instance, in the case (I), the maximum opening thickness t2 becomes the threshold value tθ. Also, in the case (II), the maximum opening thickness t2 becomes the threshold value tθ.
The determination of the threshold value tθ is preferably made by using measurement results of the horizontal cracks and their maximum opening thicknesses of a plurality of slabs, and the HIC test results thereof. The measurement results of the horizontal cracks and their maximum opening thicknesses of the plurality of slabs, and the HIC test results thereof can be used to obtain the threshold value tθ more precisely, thereby reducing the misjudgment of the presence or absence of HIC occurrence.
The segregation zone and the HIC resistance may be evaluated by examining one cross section of the slab or product, or alternatively by examining two or more cross sections thereof. In the following, a description will be given on the results obtained by examining a plurality of cross sections of the slab of the same charge with reference to Fig. 4. In Fig. 4, Example 1 is an example of examining two cross sections of the slab of the same charge, and Example 2 is an example of examining three cross sections of the slab of the same charge. In either example, the result is obtained by examining the slab that is in conformity with API X65 Grade.
As shown in Fig. 4, in Example 1, at both of the two cross sections, the maximum opening thickness is 0 mm, and no HIC occurs at a horizontal crack as a starting point in the HIC test. In Example 2, the maximum opening thicknesses of the three cross sections are 0.065 mm, 0.067 mm, and 0.066 mm, which are substantially the same thickness. At all the cross sections, HIC occurs at the horizontal cracking zones as the starting points.
In this way, regarding the slab of the same charge, even different cross sections of the slab exhibit substantially the same results. In addition, it is confirmed that when examining the cross section of each of 50 charges, the respective charges show substantially the same results without misjudgment, so that the precise evaluation can be achieved.
In the examples shown in Fig. 4, the slabs in conformity with the API X65 Grade are used for evaluation. Meanwhile, a slab in another strength grade, for example, a slab of API X70 Grade or higher grade does not differ from the API X65 Grade slab in formation or variations of internal cracks. Thus, the number of cross sections to be examined is not limited.
The examination position (examined surface) of the slab is preferably a stationary part, but may be a non-stationary part, as shown in the examples. The term "non-stationary part" as used herein means a part casted when the casting condition is varied, for example, a part casted at an initial stage of casting, such as when the casting speed increases, or a part casted at the end of casting, such as when the casting speed decreases. When intended to examine the non-stationary part, as shown in Fig. 5, a part adjacent to the region subjected to the HIC test is preferably examined. Such a part exhibits substantially the same HIC resistance as the HIC test result and can be evaluated more precisely.
As mentioned above, the steel plate in the present invention is a steel plate in which a slab for the steel plate has no horizontal crack or has the horizontal crack having a maximum opening thickness of the threshold value tθ or less, at a stage of the slab before rolling. In this way, when no horizontal crack occurs in the regions R1 and R2 of the cross section of the slab, the segregation degree of the horizontal crack is low, and eventually no HIC due to the horizontal crack occurs. Also, when the maximum opening thickness of the horizontal cracks in the regions R1 and R2 of the cross section of the slab is the threshold value tθ or less, the segregation degree of the horizontal crack is low, and eventually no HIC due to the horizontal crack occurs.
In the present invention, the "maximum opening thickness of the horizontal crack" is used to evaluate the HIC resistance. Because of this, the internal quality of the cast strip can be precisely evaluated, so that based on this evaluation result, the HIC resistance can be evaluated at the stage of the cast strip. Consequently, the HIC test that would require several weeks can be omitted, thereby significantly shortening a time period from the manufacture to dispatching.
The present application claims priority to Japanese Patent Application No. 2014-266489 filed on December 26, 2014 and Japanese Patent Application No. 2015-207452 filed on October 21, 2015 , the disclosure of both of which is incorporated herein by reference in its entirety.

Examples

The present invention will be more specifically described below by way of Examples, but is not limited to the following Examples. Various modifications can be made to these Examples as long as they are adaptable to the above-mentioned and below-mentioned concepts and are included within the scope of the present invention.
Tables 1-1 and 1-2 and Figs. 6 and 7 show the experimental conditions and results for determining the threshold value tθ. First, 21 charges were cast to obtain each of a slab corresponding to the API X65 Grade and a slab corresponding to the API X70 Grade. One charge was cast to obtain each of a slab corresponding to the ASME SA516 Grade 60, a slab corresponding to the ASME SA516 Grade 65, and a slab corresponding to the ASME SA516 Grade 70. These slabs were examined for the horizontal crack in the following way. Note that in Tables 1-1 and 1-2 mentioned above and Table 3 mentioned below, "X70" corresponds to API X70 Grade; "X65" to API X65 Grade; "SA516 60" to ASME SA516 Grade 60; "SA516 65" to ASME SA516 Grade 65; and "SA516 70" to ASME SA516 Grade 70.
Here, the conditions shown in Tables 1-1 and 1-2 will be described.

The concentrations of C, Mn, Nb, P, and Ca were measured by an emission spectroscopy. The S concentration was very low and thus was difficult to measure by the emission spectroscopy. Then, the S concentration was measured by using a combustion-infrared absorption method.

- Specific Water Content

Specific Water Content = (whole secondary cooling water amount per unit time from directly under the mold to a final roll of a continuous casting machine [l/min.])/(weight of cast strip production per unit time [kg/min.])

- Casting Speed

The casting speed is a drawing speed of the cast strip [m/min.], and calculated from the diameter (circumferential length) and the rotational speed (the number of revolutions per unit time) of a roll (major roll) in contact with the cast strip.

(Casting)

Steels having component compositions within the range specified by the present invention and in which their molten steels in a tundish had component compositions shown in Table 1-1 or 1-2 were melted and subjected to continuous casting, thereby producing cast strips or slabs, each having a thickness of 280 mm.

(Examination of Horizontal Crack)

Each slab was cut at a stationary part in an entire length of 10 to 15 m, and horizontal cracks in the stationary part were then examined in the following way. The term "stationary part" as used herein means a part that satisfies the following conditions. The number of cross sections for examination of horizontal cracks is shown in Tables 1-1 or 1-2.

1) Casting speed was constant.
2) An abnormal state in operation, such as clogging of an immersion nozzle, did not occur,
3) Cooling conditions did not change.
4) A distance between rolls did not change.

Examination Procedure of Horizontal Crack

(1) The cross section of the slab was polished in a range from both ends in the width direction thereof to D/2 to a level of an 800-grit level.
(2) The polished surface was corroded with 20 g/L of picric acid, 5 g/L of cupric chloride and 60 ml/L of a surfactant.
(3) The corroded surface was visually checked, and a part where a horizontal crack was present was cut out to produce a specimen with a size of 40 mm x 70 mm.
(4) The cut specimen was buffed and finished to a roughness of 1 µm or less.
(5) A Mn segregation degree of the horizontal crack in the specimen was analyzed by line with a beam of 20 µm in diameter using an electron probe micro analyzer (EPMA). The Mn segregation degree of the horizontal crack was designated as Cmax (Mn).
(6) A Mn concentration in the molten steel in a tundish measured during casting, namely, C₀ (Mn), and the Cmax (Mn) were used to determine a value of Cmax (Mn)/C₀ (Mn) by calculation.
(7) The horizontal crack of the part subjected to the EPMA analysis was observed with a microscope (at a magnification of 20X to 50X), and then an opening thickness of the crack was measured.

(Rolling)

Then, after heating each of the slabs corresponding to the API X65 Grade and API X70 Grade to a temperature of 1050 to 1250°C, the hot-rolling was performed on the slab through two or more passes. In each pass, a surface temperature of the steel plate was set at 900°C or higher, a cumulative rolling reduction was 40% or more at an average steel plate temperature of 1,000°C or higher, which was determined by the calculation below, and a rolling reduction per pass was 10% or more. Then, another hot-rolling was performed such that the cumulative rolling reduction at a temperature of 700°C or higher and lower than 900°C was 20% or more, and that the rolling end temperature was 700°C or higher and lower than 900°C. Thereafter, water-cooling of the rolled steel plate was started at a temperature of 650°C or higher and stopped at a temperature of 350 to 600°C. Subsequently, the air-cooling was carried out until the room temperature, thereby eventually producing a steel plate with a thickness of 45 mm. Meanwhile, after hot-rolling each of the slabs corresponding to the ASME SA516 Grade 60, ASME SA516 Grade 65, and ASME SA516 Grade 70 at a rolling end temperature of 850°C or higher, air-cooling of the rolled steel plate was carried out until the room temperature. Subsequently, quenching was performed on the rolled steel plate by reheating it to a temperature of 850°C or higher and 950°C or lower, followed by tempering at a temperature of 600 to 700°C, thereby producing a steel plate with a thickness of 40 mm. Note that both types of steel plates were not subjected to rolling in the slab width direction.
The average steel plate temperature was determined in the following way. Specifically, based on data including a rolling pass schedule during rolling and a cooling method (water-cooling or air-cooling) between the passes, the temperature at any position of the steel plate in the thickness direction was determined by using an appropriate calculation method, such as a finite difference method. Then, the average steel plate temperature was defined as the average of the determined temperatures of the steel plate in a range from the front to back surface thereof. The definition of the "average steel plate temperature" also applied to other steel plates.

(HIC Test)

To determine a threshold value tθ, in Examples, the HIC test was performed after the rolling.

(a) Samples were cut out of respective products obtained after the rolling, and the HIC test was performed on the samples. The HIC test was performed according to the method specified by the NACE standard TM0284-2003. (b) After the HIC test, each sample was cut at three sites, and then respective cross sections (three cross sections) were observed with a microscope to confirm the presence or absence of HIC. Here, the presence or absence of cracks in the "regions R11 and R12 ranging from both ends in the width direction of the product to D/2" shown in Fig. 3 (a) was confirmed.

(Determination of Threshold Value tθ of Maximum Opening Thickness)

Figs. 6 and 7 show the relationships between the "presence or absence of HIC occurrence" confirmed by the HIC test and the "'Opening Thickness of Horizontal Crack' and 'Cmax (Mn)/C₀ (Mn)'". Fig. 6 shows the examination results of threshold values tθ at which HIC occurred in components of the steels shown in Table 1-2 and belonging to strength classes corresponding to API X65 Grade, ASME SA516 Grade 60, or ASME SA516 Grade 65. Fig. 7 shows the examination results of threshold values tθ at which HIC occurred in the components of the steels shown in Table 1-1 or 1-2 and belonging to strength classes corresponding to API X70 Grade or ASME SA516 Grade 70.
As can be seen from Fig. 6, in the slab corresponding to the API X65 Grade, no HIC occurred for the maximum opening thickness of 0.047 mm or less, namely, maximum opening thickness ≤ 0.047 mm, while HIC occurred for the maximum opening thickness of more than 0.047 mm, namely, maximum opening thickness > 0.047 mm. Accordingly, in the slab corresponding to the API X65 Grade, the threshold value tθ of the maximum opening thickness was set at 0.047 mm, and thereby the determination was made as follows. When maximum opening thickness is 0.47 mm or less (≤ 0.47 mm), HIC was determined not to occur.
When maximum opening thickness is more than 0. 47 mm (> 0.47 mm), HIC was determined to occur.
Since the ASME SA516 Grade 60, Grade 65, ASTM A516 Grade 60, and Grade 65 had the components corresponding to the API X65 Grade, the threshold value tθ of the maximum opening thickness was set at 0.047 mm, and thereby the determination was made as follows.
When maximum opening thickness is 0. 047 mm or less (≤ 0.047 mm), HIC is determined not to occur.
When maximum opening thickness is more than 0.047 mm (> 0.047 mm), HIC is determined to occur.
On the other hand, as can be seen from Fig. 7, in the slab corresponding to the API X70 Grade, no HIC occurred for the maximum opening thickness of 0.043 mm or less, namely, maximum opening thickness ≤ 0.043 mm, while HIC occurred for the maximum opening thickness of more than 0.043 mm, namely, maximum opening thickness > 0.043 mm. Accordingly, in the slab corresponding to the API X70 Grade, the threshold value tθ of the maximum opening thickness was set at 0.043 mm, and thereby the determination was made as follows.
When maximum opening thickness is 0.043 mm or less (≤ 0.043 mm), HIC is determined not to occur.
When maximum opening thickness is more than 0. 043 mm (>0.043 mm), HIC is determined to occur.
Since the ASME SA516 Grade 70 and ASTM A516 Grade 70 had the components corresponding to API X70 Grade, the threshold value tθ of the maximum opening thickness was set at 0.043 mm, and thereby the determination was made as follows.
When maximum opening thickness is 0.043 mm or less (≤ 0.043 mm), HIC is determined not to occur.
When maximum opening thickness is more than 0.043 mm (> 0. 043 mm), HIC is determined to occur.
Note that in both Figs. 6 and 7, no HIC occurred in a horizontal crack that was not opened or had a maximum opening thickness is 0 mm (= 0 mm).

(Evaluation on HIC Resistance of Slab as Determination Target)

The HIC resistance of each slab as the determination target was evaluated using the threshold value tθ in the following procedure. The steel with the component composition shown in Table 2 was melted and subjected to continuous casting, thereby producing a slab as the determination target that had a slab thickness D of 280 mm and a slab width W of 2100 mm. The HIC resistance was evaluated by using the slab in the following procedure.

(1) The cross section of the slab as the determination target was subjected to milling in a range from both ends in the width direction thereof to half the width thereof, namely, D/2, and dye penetrant testing (in accordance with JIS Z2343).
(2) When no horizontal crack was detected, a maximum opening thickness was determined to be equal to or less than a lower detection limit (approximately 10 µm or less). In this case, the maximum opening thickness was the threshold value tθ or less, that is, 0.047 mm or less in the API X65 Grade, or 0.043 mm or less in the API X70 Grade. Thus, it was determined that no HIC occurred due to horizontal cracks.
(3) When any horizontal crack was detected, an opened part was buffed, and the polished surface was observed with a microscope at a magnification of 20x to 50x, whereby the maximum opening thickness was measured as mentioned above.
- (3-1) As mentioned in the paragraph "Determination of Threshold Value tθ of Maximum Opening Thickness", in a slab corresponding to the API X65 Grade, when the maximum opening thickness was the threshold value tθ of 0.097 mm or less, no HIC due to a horizontal crack occurred, that is, the evaluation result of the HIC resistance of the slab was rated as OK. Consequently, the obtained steel plate was determined to have excellent HIC resistance. On the other hand, when the maximum opening thickness exceeded the threshold value tθ, namely, 0.047 mm or less, HIC due to a horizontal crack occurred, that is, the evaluation result of the HIC resistance of the slab was rated as NG. Consequently, the obtained steel plate was determined to be inferior in the HIC resistance.
- (3-2) In the slab corresponding to the API X70 Grade, when the maximum opening thickness was the threshold value tθ of 0.043 mm or less, no HIC due to a horizontal crack occurred, that is, the evaluation result of the HIC resistance of the slab was rated as OK. Consequently, the obtained steel plate was determined to have excellent HIC resistance. On the other hand, when the maximum opening thickness exceeded the threshold value tθ of 0. 043 mm, HIC due to a horizontal crack occurred, that is, the evaluation result of the HIC resistance of the slab was rated as NG. Consequently, the obtained steel plate was determined to be inferior in the HIC resistance.

Then, after heating the slab to a temperature of 1050 to 1250°C, each slab was processed by either of two types of hot-rolling and cooling methods, denoted as "TMCP" or "QT" in a "hot-rolling and cooling method" column shown in Table 3. Consequently, steel plates (each having 9 to 90 mm in thickness x 2000 to 3500 mm in width x 12000 to 35000 mm in length) with various component compositions were produced. The "TMCP" was a method that involved: hot-rolling through two or more passes, in each of which a surface temperature of the steel plate was set at 900°C or higher, a cumulative rolling reduction was 40% or more at an average steel plate temperature of 1000°C or higher, determined by the calculation, and a rolling reduction per pass was 10% or more; and then another hot-rolling such that a cumulative rolling reduction was 20% or more at a temperature of 700°C or higher and lower than 900°C and that the surface temperature at the end of the rolling was 850°C. The "TMCP" method further involved: starting to cool the rolled steel plate from a cooling start surface temperature of 950°C at an average cooling rate of 10°C/s and then stopping the cooling at a temperature of 350 to 600°C, followed by air-cooling to the room temperature. The "QT" was a method that involved: hot-rolling such that the rolling end temperature was 850°C or higher, followed by air-cooling to the room temperature; quenching by reheating the rolled steel plate to a temperature of 850°C or higher and 950°C or lower; and tempering the steel plate at 600 to 700°C.

(HIC Test)

The HIC test was performed using the steel plates. The HIC test was performed according to the method specified by the NACE standard TM0284-2003. After the HIC test, each sample was cut at three sites, and then respective cross sections (three cross sections) were observed with the microscope to confirm the presence or absence of HIC.
The results are shown in Fig. 3.

[Table 1-1]

Sample No.	Component of molten steel in tundish						Casting conditions		Product grade	Number of cross sections for examination of horizontal cracks
Sample No.	C (% by mass)	Mn (% by mass)	Nb (% by mass)	S (ppm by mass)	P (ppm by mass)	Ca (ppm by mass)	Specific waster content [L/kg-steel]	Casting speed Vc [m/min]	Product grade
1	0.06	1.32	0.036	6	58	33	0.4	1.0		1
2	0.05	1.28	0.037	6	60	27	0.4	1.1		3
3	0.05	1.30	0.037	6	41	30	0.4	1.3		1
4	0.06	1.27	0.037	5	68	28	0.6	1.0		1
5	0.06	1.32	0.038	4	47	30	0.6	1.1		1
6	0.06	1.33	0.036	5	57	31	0.6	1.2		1
7	0.05	1.31	0.036	4	54	31	0.8	1.0		1
8	0.06	1.33	0.037	7	65	33	0.8	1.1		1
9	0.05	1.28	0.040	3	70	34	0.8	1.3		1
10	0.05	1.33	0.038	4	65	34	1.0	1.0		1
11	0.06	1.31	0.035	3	52	33	1.0	1.1	X70	1
12	0.06	1.31	0.036	7	42	32	1.0	1.3		1
13	0.06	1.29	0.037	6	63	28	1.2	1.2		1
14	0.06	1.30	0.039	7	61	30	1.2	1.1		1
15	0.06	1.27	0.037	3	57	30	1.2	1.3		2
16	0.05	1.29	0.036	6	48	31	1.4	1.0		1
17	0.05	1.30	0.038	5	46	26	1.4	1.1		1
18	0.05	1.32	0.037	7	49	26	1.4	1.2		1
19	0.05	1.31	0.037	7	42	34	1.4	1.3		1
20	0.06	1.34	0.038	3	59	28	1.4	1.0		1
21	0.05	1.27	0.036	6	66	32	1.4	1.1		1

[Table 1-2]

Sample No.	Component of molten steel in tundish						Casting conditions		Product grade	Number of cross sections for examination of horizontal cracks
Sample No.	C (% by mass)	Mn (% by mass)	Nb (% by mass)	S (ppm by mass)	P (ppm by mass)	Ca (ppm by mass)	Specific water content [L/kg-steel]	Casting speed Vc [m/min]	Product grade
22	0.06	1.28	0.033	5	52	29	0.4	1.0		1
23	0.06	1.25	0.033	4	61	26	0.4	1.1		1
24	0.06	1.27	0.034	7	45	31	0.4	1.3		1
25	0.06	1.23	0.035	7	51	31	0.6	1.0		1
26	0.06	1.27	0.031	7	66	29	0.6	1.1		1
27	0.05	1.26	0.031	4	56	27	0.6	1.2		1
28	0.05	1.24	0.035	5	43	30	0.8	1.0		1
29	0.06	1.25	0.030	4	70	29	0.8	1.1		1
30	0.06	1.26	0.034	6	64	33	0.8	1.3		1
31	0.05	1.27	0.035	6	48	33	1.0	1.0		1
32	0.06	1.20	0.034	5	50	31	1.0	1.1	X65	1
33	0.06	1.27	0.032	3	63	32	1.0	1.3		1
34	0.05	1.22	0.035	7	44	33	1.2	1.2		1
35	0.06	1.25	0.034	3	50	27	1.2	1.1		1
36	0.05	1.26	0.031	4	42	25	1.2	1.3		1
37	0.05	1.25	0.034	7	50	33	1.4	1.0		1
38	0.06	1.23	0.034	4	68	29	1.4	1.1		1
39	0.06	1.23	0.033	4	50	33	1.4	1.2		1
40	0.05	1.20	0.034	6	58	34	1.4	1.3		1
41	0.06	1.23	0.033	5	43	29	1.4	1.0		1
42	0.06	1.28	0.034	3	64	33	1.4	1.1		1
43	0.06	1.16	0.000	5	60	15	1.4	1.1	SA51660	1
44	0.06	1.13	0.010	3	60	14	1.4	1.1	SA516 65	1
45	0.06	1.43	0.010	4	60	12	1.4	1.1	SA51670	1

[Table 3]

Steel type No.	Ca/S	(Ca-1.25S)/O	Hot-rolling and cooling method	Maximum opening thickness (mm)	Evaluation of HIC resistance of slab	Presence or absence of cracking in HIC resistance test	Strength class
1	7.5	0.96	TMCP	0.047	OK	Absence	X65
2	4.4	0.88	TMCP	0.010	OK	Absence	X65
3	7.5	0.96	TMCP	0	OK	Absence	X65
4	4.4	0.88	TMCP	0	OK	Absence	X70
5	5.7	1.02	TMCP	0.015	OK	Absence	X65
6	2.7	0.80	TMCP	0.031	OK	Absence	X65
7	3.9	0.73	TMCP	0.008	OK	Absence	X70
8	1.9	0.26	TMCP	0.041	OK	Presence	X65
9	5.4	1.89	TMCP	0	OK	Presence	X65
10	5.4	1.04	TMCP	0	OK	Absence	X65
11	10.3	1.18	TMCP	0.055	NG	Presence	X65
12	9.0	1.29	TMCP	0	OK	Absence	X70
13	7.8	1.24	TMCP	0.065	NG	Presence	X70
14	3.4	0.60	TMCP	0.042	OK	Absence	X70
15	4.7	0.64	QT	0	OK	Absence	SA516	60
16	6.0	0.84	QT	0	OK	Absence	SA516	70
17	4.7	0.64	QT	0	OK	Absence	SA516	65
18	1.7	0.25	QT	0	OK	Presence	SA516	60
19	9.3	2.13	QT	0	OK	Presence	SA516	70

Tables 2 and 3 show the following. Each of steel types Nos. 1 to 7, 10, 12, and 14 to 17 satisfied the specified component composition and had the maximum opening thickness of a horizontal crack in its slab restrained to the threshold value tθ or less. These steel plates are the steel plates with excellent HIC resistance according to the present invention.
In contrast, in each of steel types Nos. 11 and 13, the maximum opening thickness of the horizontal crack in the slab exceeded the threshold value tθ, and thereby the evaluation result of the HIC resistance of the slab was rated as NG. In the HIC test performed after the rolling, cracks occurred in the steel plates of these steels. Thus, the steel plates of the steel types Nos. 11 and 13 were confirmed to be inferior in the HIC resistance. Steel types Nos. 8, 9, 18, and 19 had the chemical component compositions of their steel plates deviating from the ranges specified by the present invention, even though each of their maximum opening thicknesses of horizontal cracks in their slabs were restrained to the threshold value tθ or less. Specifically, in the steel plate of the steel type No. 8, the contents of REM and Zr were 0%, and the value (Ca/S) deviated from the specified range. In the steel plate of the steel type No. 9, the contents of REM and Zr were 0%, and the value (Ca - 1.25S)/O deviated from the specified range. Thus, both the steel plates No. 8 and No. 9 were inferior in the HIC resistance. Furthermore, in the steel type No. 18, the value (Ca/S) deviated from the specified range, while in the steel type No. 19, the value (Ca - 1.25S)/O deviated from the specified range. Thus, both the steel plates No. 18 and No. 19 were inferior in the HIC resistance.
In the examples in which the evaluation of the HIC resistance of the slab was rated as OK, a time period required from starting of casting to completion of a production of the steel plate, that is, a time period until dispatching the steel plate with the sour resistance (casting → rolling → dispatching) was 19 days. In contrast, in cases where the steel plate obtained after the rolling was subjected to the HIC test and then evaluated for the HIC resistance, a time period required from starting of casting to dispatching (casting → rolling → HIC test → dispatching) was 28 days, which was a long duration. In Examples, the HIC test after the rolling was able to be omitted, which could significantly shorten the time period from starting of the casting to dispatching, e.g., from 28 days to 19 days.
In the examples in which the evaluation of the HIC resistance of the slab was rated as NG, re-melting was started at the stage of the slab. Thus, a time period required from starting of casting to completion of a production of the steel plate, that is, a time period until dispatching the steel plate with the sour resistance (casting → re-melting → rolling → dispatching) was 54 days. In contrast, in cases where the steel plate obtained after the rolling was subjected to the HIC test and then evaluated for the HIC resistance as the product, when the evaluation result was NG, re-melting was started after the HIC test. Eventually, a time period required from starting of casting to dispatching of the steel plate as the product (casting → rolling → HIC test → re-melting → rolling → HIC test → dispatching) was 72 days, which was a longer duration. In Examples, since the HIC test after the rolling was able to be omitted, even though the re-melting was necessary, the time period from starting of the casting to dispatching could be drastically shortened, e.g., from 72 days to 54 days.
As mentioned above, according to the present invention, the HIC resistance can be evaluated at the stage of the slab as the cast strip without conducting the HIC test after the rolling, thereby making it possible to significantly shorten the manufacturing lead time. Note that in Examples, the same HIC test is used for both the determination of the threshold value tθ for evaluating the HIC resistance of a slab and the confirmation of HIC. Thus, the determination method of the present invention has high accuracy.

Claims

A steel plate having excellent hydrogen-induced cracking resistance, comprising, in percent by mass:
0.02 to 0.15% of C;

0.02 to 0.50% of Si;

0.6 to 2.0% of Mn;

more than 0% and 0.030% or less of P;

more than 0% and 0.003% or less of S;

0.010 to 0.08% of Al;

0.0003 to 0.0060% of Ca;

0.001 to 0.01% of N;

more than 0% and 0.0045% or less of O; and

one or more elements selected from the group consisting of more than 0% and 0.02% or less of REM and more than 0% and 0.010% or less of Zr, with the balance being iron and inevitable impurities, wherein

a ratio (Ca/S) of the Ca to the S is 2.0 or more,

the Ca, the S, and the O satisfy the formula below: (Ca -1.25S)/O < 1.80, and

at a stage of the slab, the slab for the steel plate does not include a horizontal crack or includes the horizontal crack having a maximum opening thickness of a threshold value tθ or less, where the threshold value tθ is a maximum opening thickness of a horizontal crack for avoiding the occurrence of hydrogen-induced cracking in the steel plate obtained by rolling the slab.
The steel plate according to claim 1, wherein the threshold value tθ is a value previously determined by method including following (i) to (iii):
(i) a maximum opening thickness of the slab is measured;

(ii) a hydrogen-induced cracking test is performed on a steel plate obtained by rolling a slab which has been cast under the same casting conditions as said slab; and

(iii) a maximum opening thickness of a horizontal crack that avoids the occurrence of hydrogen-induced cracking is determined from the maximum opening thickness measured in the step (i) and a result of the hydrogen-induced cracking test shown in the step (ii).
The steel plate according to claim 2, wherein the slab cast under the same casting conditions as said slab is the slab in which the maximum opening thickness is measured.
The steel plate according to any one of claims 1 to 3, wherein the steel plate is in an API X65 Grade, and the threshold value tθ is 0.047 mm.
The steel plate according to any one of claims 1 to 3, wherein the steel plate is in an API X70 Grade, and the threshold value tθ is 0.043 mm.
The steel plate according to any one of claims 1 to 3, wherein the steel plate is in an ASME SA516 Grade 60, and the threshold value tθ is 0.047 mm.
The steel plate according to any one of claims 1 to 3, wherein the steel plate is in an ASME SA516 Grade 65, and the threshold value tθ is 0.047 mm.
The steel plate according to any one of claims 1 to 3, wherein the steel plate is in an ASME SA516 Grade 70, and the threshold value tθ is 0.043 mm.
The steel plate according to any one of claims 1 to 3, wherein the steel plate is in an ASTM A516 Grade 60, and the threshold value tθ is 0.047 mm.
The steel plate according to any one of claims 1 to 3, wherein the steel plate is in an ASTM A516 Grade 65, and the threshold value tθ is 0.047 mm.
The steel plate according to any one of claims 1 to 3, wherein the steel plate is in an ASTM A516 Grade 70, and the threshold value tθ is 0.043 mm.
The steel plate according to any one of claims 1 to 3, further comprising, as another element, in percent by mass, one or more element selected from the group consisting of:
more than 0% and 0.005% or less of B,

more than 0% and 0.1% or less of V,

more than 0% and 1.5% or less of Cu,

more than 0% and 1.5% or less of Ni,

more than 0% and 1.5% or less of Cr,

more than 0% and 1.5% or less of Mo, and

more than 0% and 0.06% or less of Nb.
The steel plate according to any one of claims 1 to 3, further comprising, as another element, in percent by mass, one or more element selected from the group consisting of:
more than 0% and 0.03% or less of Ti, and

more than 0% and 0.01% or less of Mg.
The steel plate according to any one of claims 1 to 3 for use in line pipe.
The steel plate according to any one of claims 1 to 3 for use in pressure container.
A steel pipe for line pipe, formed of the steel plate according to any one of claims 1 to 3.