KR20190137955A - Steel plate having excellent hydrogen-induced cracking resistance and steel pipe for line pipe - Google Patents

Abstract

Steel plate or steel pipe with excellent hydrogen organic crack resistance is realized. Furthermore, the steel plate and steel pipe which can evaluate HIC resistance from the internal quality of a cast steel, without performing an HIC test are implemented. The steel sheet excellent in the hydrogen-organic crack resistance satisfies the prescribed C, Si, Mn, P, S, Al, Ca, N, and O, and at least one selected from the group consisting of the prescribed REM and Zr An element, the balance consisting of iron and an unavoidable impurity, and the Ca, S, and O are (Ca-1.25S) / O≤ Satisfies 1.80, and at the stage of slab, there is no horizontal crack, or the maximum pore thickness of the horizontal crack is equal to or less than the threshold tθ, and the threshold tθ does not generate hydrogen organic crack in the steel sheet obtained by rolling the slab. It is characterized by the largest opening thickness of a horizontal crack.

Description

STEEL PLATE HAVING EXCELLENT HYDROGEN-INDUCED CRACKING RESISTANCE AND STEEL PIPE FOR LINE PIPE}

The present invention relates to a steel sheet having excellent hydrogen organic crack resistance. In particular, it is related with the steel plate excellent in hydrogen-organic crack resistance suitable for the line pipe for storage of natural gas, crude oil, a storage tank, etc., and the steel pipe for line pipe excellent in the hydrogen-organic crack resistance obtained using this steel plate.

In line pipes and storage tanks for transporting mainly oil and gas, so-called sour resistance such as hydrogen organic crack resistance and stress corrosion cracking resistance is required along with the development of a thermal quality resource containing hydrogen sulfide. It is. Below, the steel plate provided with this sour resistance may be called "sour-resistant steel plate." Hydrogen induced cracking (hereinafter sometimes referred to as "HIC") is a non-metallic inclusion including MnS and Nb (C, N) in which hydrogen penetrates into steel materials with corrosion reaction by hydrogen sulfide or the like. It is known that it is accumulated in a back and the like and is a crack generated by gasification.

It is known that HIC is likely to be generated from segregation parts including central segregation, internal cracks, and the like, particularly inclusions such as MnS. Therefore, several proposals have conventionally been made on techniques for improving HIC resistance. For example, Patent Literature 1 discloses a steel material having improved HIC resistance by suppressing segregation degree of Mn, Nb, and Ti in a sheet thickness center part. In addition, Patent Document 2 discloses a method of suppressing HIC based on MnS or Ca-based oxysulfide by a parametric formula composed of Ca, O, and S.

By these methods, most HICs are suppressed, but many microscopic HICs arise locally.

On the other hand, after a steel plate is obtained through a solvent, casting, and hot rolling, a HIC test is performed before shipment as a product. However, the HIC test requires several weeks before the results are found. In addition, when HIC occurs in the HIC test, the steel sheet cannot be shipped as a product having excellent hydrogen organic crack resistance, and thus, it is necessary to perform the HIC test again on the product obtained by manufacture, that is, the solvent again. This prolongs the manufacturing period and causes delays in delivery time and the like.

Therefore, if the HIC resistance can be evaluated at the stage of the cast after the hot rolling, instead of performing the HIC test, it is considered that the manufacturing period can be significantly shortened. As described above, since HIC originates from inclusions such as segregation (center segregation and internal cracking) and MnS, the HIC can evaluate HIC resistance based on the evaluation result if it can be evaluated at the stage of cast steel. I think it is.

For example, in the conventional method of performing an HIC test after rolling, it goes through the following long process A-1 from casting to shipment. On the other hand, if the HIC resistance can be evaluated at the stage of the cast, the sample adjustment (for HIC test) → HIC test (for HIC test) in the case of performing the HIC test can be omitted according to the following Step B-1. Can ship early.

Process A-1: casting → rolling → sample adjustment → for HIC test → shipment

Process B-1: casting → evaluation of HIC resistance → rolling → shipping

In addition, when the result of HIC test is NG, in the conventional method, it passes through the following process A-2 from casting to resolvent. On the other hand, if HIC resistance can be evaluated at the stage of the cast according to the following step B-2, even if this evaluation is NG, "rolling → sample adjustment → HIC test (for HIC test) in the following step A-2. Can be omitted, and the resolvent can be started early.

Process A-2: casting → rolling → sample adjustment (for HIC test) → HIC test → resolvent

Process B-2: casting → evaluation of HIC resistance → resolvent

As such a method, Patent Literature 3 discloses a method for evaluating internal cracking at the stage of cast steel. In this method, it is judged whether Hot Charge Rolling (HCR) operation is performed from the evaluation result of internal cracking.

Japanese Patent Publication No. 2010-209461 Japanese Patent Laid-Open No. 06-136440 Japanese Patent Publication No. 2006-198649

By the way, although the internal crack which becomes a problem in the steel plate which needs sour resistance is very small crack, patent document 3 evaluates the internal crack which is a problem in HCR operation, ie, the big crack whose crack length is 10 mm or more. Therefore, in the said method, since the fine internal crack which becomes a problem may be missed in the steel plate which needs sour resistance, the HIC resistance caused by internal crack cannot be evaluated correctly at the stage of a slab.

The present invention has been made in view of the above circumstances, and an object thereof is to realize a steel plate or a steel pipe having excellent hydrogen organic crack resistance, and furthermore, to perform HIC resistance from the internal quality of the cast steel without performing an HIC test. The present invention is to realize a steel sheet or a steel pipe that can be evaluated.

Steel sheet excellent in the hydrogen-organic crack resistance of the present invention was able to solve the above problems,

In mass%,

C: 0.02 to 0.15%,

Si: 0.02 to 0.50%,

Mn: 0.6-2.0%,

P: more than 0% and less than 0.030%,

S: greater than 0% and less than 0.003%,

Al: 0.010% to 0.08%,

Ca: 0.0003 to 0.0060%,

N: 0.001-0.01%, and

O: more than 0% and less than 0.0045%, and also

REM: greater than 0% and less than or equal to 0.02%, and

Zr: more than 0% and less than 0.010%

At least one element selected from the group consisting of, the balance consists of iron and inevitable impurities,

While the ratio of Ca and S (Ca / S) is 2.0 or more,

Ca, S and O satisfy (Ca-1.25S) /O≦1.80,

Also, in the stage of slab, there is no horizontal crack, or the maximum pore thickness of the horizontal crack is equal to or less than the threshold tθ, and the threshold tθ is the maximum opening of the horizontal crack in which the hydrogen organic crack does not occur in the steel sheet obtained by rolling the slab. It is characteristic in that it is thickness.

The threshold value tθ may be a value previously obtained by the following methods (i) to (iii).

(i) The maximum pore thickness of the slab is measured.

(ii) Hydrogen organic crack test is performed on the steel plate obtained by rolling the slab cast under the same casting conditions as the slab.

(iii) From the maximum pore thickness measured in (i) and the hydrogen organic crack test result of (ii) above, the maximum pore thickness of the horizontal crack in which no hydrogen organic crack is generated is determined.

The slab cast under the same casting conditions as the slab may be a slab obtained by measuring the maximum opening thickness.

The steel sheet may be API (The American Petroleum Institute) X65 grade, and the threshold tθ may be 0.047 mm.

The steel sheet may be API X70 grade, and the threshold value tθ may be 0.043 mm.

The steel sheet may be American Society of Mechanical Engineers (ASME) SA516 Grade 60, and the threshold value tθ may be 0.047 mm.

The steel sheet may be ASME SA516 Grade 65, and the threshold value tθ may be 0.047 mm.

The steel sheet may be ASME SA516 Grade 70, and the threshold value tθ may be 0.043 mm.

The steel sheet may be ASTM (American Society for Testing and Materials) A516 Grade 60, and the threshold tθ may be 0.047 mm.

The steel sheet may be ASTM A516 Grade 65, and the threshold value tθ may be 0.047 mm.

The steel sheet may be ASTM A516 Grade 70, and the threshold tθ may be 0.043 mm.

The steel sheet may further contain any one or more of the following (A) and (B) as another element.

(A) In mass%, B: greater than 0% and 0.005% or less, V: greater than 0% and 0.1% or less, Cu: greater than 0% and 1.5% or less, Ni: greater than 0% and 1.5% or less, Cr: greater than 0% and 1.5% At least one element selected from the group consisting of Mo: more than 0% and 1.5% or less, and Nb: more than 0% and 0.06% or less

(B) at least one element selected from the group consisting of Ti: more than 0% and 0.03% or less, and Mg: more than 0% and 0.01% or less by mass%

The steel sheet is suitable for line pipes and pressure vessels. Moreover, this invention also includes the steel pipe for line pipes formed from the said steel plate.

According to the present invention, it is possible to provide a steel sheet or a steel pipe which is excellent in hydrogen organic crack resistance. Furthermore, the steel plate and steel pipe which can evaluate HIC resistance from the internal quality of a cast steel, without performing an HIC test can be provided. These are suitably used for pressure vessels, such as a line pipe for transporting natural gas and crude oil, and a storage tank.

1: is a schematic diagram explaining an internal crack, (a) has shown the slab, ie, the state before rolling, and (b) has shown the product, ie, the state after rolling.
2 is a cross-sectional view of the slab.
3 shows a cross-sectional view of the slab and a cross-sectional view of the product.
It is a figure which shows the result of having investigated the relationship of a pore thickness and HIC resistance with respect to several cross section.
It is a figure explaining the irradiation surface of a slab.
It is a figure which shows the relationship between the largest pore thickness of a horizontal crack and the presence or absence of HIC in the case of using the API X65 grade steel material in an Example.
It is a figure which shows the relationship between the largest pore thickness of horizontal cracks and the presence or absence of HIC in the case of using the API X70 grade steel in the Example.

The present inventors earnestly researched in order to solve the said subject. First, the inventors have focused on the fact that HICs are likely to occur from MnS inclusions. As a result, it was conceived that by containing the rare earth element or Zr which is an element which has a desulfurization effect in steel materials, generation | occurrence | production of MnS can be suppressed and hydrogen-hydrogen cracking resistance can be improved. Moreover, in order to exhibit the desulfurization effect effectively, it has come to discover appropriate content mentioned later.

Next, the inventors have focused on the fact that HIC is likely to occur from the segregation site. As a result, paying attention to the "horizontal crack" during segregation, especially the maximum pore thickness of the horizontal crack, and converging it below a predetermined threshold at the stage of the slab, a product having high hydrogen organic crack resistance is obtained, and further, a product. Found that it could be shipped early. This will be described later in detail.

First, the component composition will be described.

In order to secure the excellent HIC resistance, it is necessary to control the composition of the steel material. Furthermore, in order to ensure high strength, excellent weldability, etc. as another characteristic requested | required, for example as a line pipe steel material, it is necessary to make the component composition of a steel plate as follows. Hereinafter, the reason for the definition of each component, including the rare earth element and Zr described above, will be described.

Ingredient composition

C: 0.02 to 0.15%

C is an indispensable element in order to secure the strength of the base metal and the welded part, and it is necessary to contain C at least 0.02%. C amount is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, when there is too much C amount, HAZ toughness and weldability will deteriorate. If the amount of C is excessive, NbC and island martensite, which are the starting point and breakdown progression path of HIC, are easily generated. Therefore, the amount of C needs to be 0.15% or less. Preferably it is 0.12% or less, More preferably, it is 0.10% or less.

Si: 0.02 to 0.50%

Si is an element effective in improving the strength of the base metal and the welded part while having a deoxidizing action. In order to secure these effects, the amount of Si is made 0.02% or more. Si amount is preferably 0.05% or more, and more preferably 0.15% or more. However, when there is too much Si amount, weldability and toughness will deteriorate. In addition, when Si amount is excess, island-like martensite will generate | occur | produce and HIC will generate | occur | produce and advance. Therefore, the amount of Si needs to be suppressed to 0.50% or less. Si amount is preferably 0.45% or less, and more preferably 0.35% or less.

Mn: 0.6-2.0%

Mn is an element effective for improving the strength of the base metal and the welded part, and is contained in the present invention at 0.6% or more. Mn amount becomes like this. Preferably it is 0.8% or more, More preferably, it is 1.0% or more. However, if the amount of Mn is too large, MnS is generated to deteriorate the hydrogen organic crack resistance, as well as the HAZ toughness and weldability. Therefore, the upper limit of Mn amount is made into 2.0%. Mn amount becomes like this. Preferably it is 1.8% or less, More preferably, it is 1.5% or less, More preferably, it is 1.2% or less.

P: more than 0% and less than 0.030%

P is an element inevitably contained in steel, and when P amount exceeds 0.030%, the toughness of the base material and the HAZ part is markedly deteriorated, and the hydrogen organic crack resistance is also deteriorated. Therefore, in this invention, P amount is suppressed to 0.030% or less. P amount is preferably 0.020% or less, and more preferably 0.010% or less.

S: greater than 0% and less than 0.003%

Since S is an element which generates a large amount of MnS and deteriorates hydrogen cracking resistance significantly if it is too large, in the present invention, the upper limit of the amount of S is made 0.003%. S amount is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less. As described above, the smaller one is preferable from the viewpoint of improving the hydrogen organic crack resistance.

Al: 0.010% to 0.08%

Al is a strong deoxidation element, and when there is little Al amount, Ca density | concentration in an oxide rises, ie, Ca type interference | inclusion tends to be formed in a steel plate surface layer part, and a fine HIC arises. Therefore, in the present invention, Al needs to be 0.010% or more. Al amount is preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, when there is too much Al content, the oxide of Al will generate | occur | produce in cluster shape and will become a starting point of a hydrogen organic crack. Therefore, Al amount needs to be 0.08% or less. Al amount is preferably 0.06% or less, and more preferably 0.05% or less.

Ca: 0.0003 to 0.0060%

Ca has a function of controlling the form of sulfide, and has an effect of suppressing the formation of MnS by forming CaS. In order to acquire this effect, it is necessary to make Ca amount 0.0003% or more. Ca amount is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when Ca amount exceeds 0.0060%, many HIC will generate | occur | produce from Ca type interference | inclusion. Therefore, in the present invention, the upper limit of the amount of Ca is made 0.0060%. Ca amount 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%

N is an element which precipitates as TiN in the steel structure, suppresses coarsening of the austenite grains in the HAZ portion, promotes ferrite transformation, and improves the toughness of the HAZ portion. In order to acquire this effect, it is necessary to contain N 0.001% or more. N amount is preferably 0.003% or more, and more preferably 0.0040% or more. However, when there is too much N amount, since HAZ toughness will rather deteriorate by presence of solid solution N, N amount needs to be 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.0060% or less.

O: more than 0% and less than 0.0045%

O, that is, oxygen is preferably lower from the viewpoint of improving cleanliness. When O is contained in a large amount, in addition to deterioration in toughness, HIC is generated starting from an oxide and deteriorates hydrogen cracking resistance. From this point of view, the amount of O needs to be 0.0045% or less, preferably 0.0030% or less, and more preferably 0.0020% or less.

Ca / S represented by mass ratio: 2.0 or more

As described above, S forms MnS as a sulfide-based inclusion, and HIC is generated from the MnS. For this reason, Ca is added to control the form by using the sulfide-based inclusions in the steel as CaS to achieve detoxification of S to HIC resistance. In order to fully exhibit this effect, it is necessary to make Ca / S 2.0 or more. Ca / S becomes like this. Preferably it is 2.5 or more, More preferably, it is 3.0 or more. On the other hand, the upper limit of Ca / S is about 17 from Ca amount and S amount prescribed | regulated by this invention.

(Ca-1.25S) /O≤1.80

In order to suppress the generation of HIC by Ca-based sulphides, it is effective to suppress CaO, which is particularly easy to form aggregates, among Ca-based inclusions. And for that purpose, Ca amount (Ca-1.25S) which subtracted Ca content which exists as sulfide (CaS) from the total Ca amount in steel should not be excess compared with O amount. When the amount of Ca (Ca-1.25S) is excessive compared to the amount of O, CaO is easily formed as an oxide-based inclusion, and agglomerates (coarse Ca-based inclusions) of the CaO are easily formed in a large amount on the surface layer of the steel sheet. Since these coarse Ca inclusions are a starting point of HIC, it is necessary to make (Ca-1.25S) / O or less to 1.80 or less in order to acquire the outstanding HIC resistance. (Ca-1.25S) / O becomes like this. Preferably it is 1.40 or less, More preferably, it is 1.30 or less, More preferably, it is 1.20 or less, Especially preferably, it is 1.00 or less. On the other hand, the lower limit of (Ca-1.25S) / O is about 0.1 from the viewpoint of suppressing Al ₂ O ₃ which tends to form aggregates similarly to CaO.

REM: greater than 0% and less than 0.02%

As described above, REM (Rare Earth Metal) is an element effective in suppressing the production of MnS by the desulfurization action and increasing the hydrogen-organic crack resistance. In order to exhibit such an effect, it is preferable to contain REM 0.0002% or more. REM amount is more preferably 0.0005% or more, and still more preferably 0.0010% or more. On the other hand, the effect is saturated even if it contains a large amount of REM. Therefore, the upper limit of REM amount needs to be 0.02%. From the viewpoint of suppressing the blockage of the immersion nozzle during casting to increase productivity, the REM amount is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.0047% or less. In the present invention, the REM means a lanthanoid element, that is, 15 elements from La to Lu, scandium and yttrium.

Zr: more than 0% and less than 0.010%

Zr is an element that not only improves HIC resistance by desulfurization but also forms oxides and finely disperses to improve HAZ toughness. In order to exhibit these effects, it is preferable to make Zr amount 0.0003% or more. The amount of Zr is more preferably 0.0005% or more, still more preferably 0.0010% or more, even more preferably 0.0015% or more. On the other hand, when Zr is added in excess, coarse inclusions are formed to deteriorate hydrogen organic cracking resistance and base metal toughness. Therefore, the amount of Zr needs to be 0.010% or less. Zr amount 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 sheet and the steel pipe of the present invention are as described above, and the balance consists of iron and unavoidable impurities. In addition to the above elements,

(a) by containing at least one element selected from the group consisting of B, V, Cu, Ni, Cr, Mo, and Nb in the following amounts, the strength and toughness are further enhanced;

(b) By containing at least one element selected from the group consisting of Ti and Mg in the following amounts, HAZ toughness can be improved, desulfurization can be promoted, and HIC resistance can be further improved. Hereinafter, these elements are explained in full detail.

B: more than 0% and less than 0.005%

B improves hardenability, increases the strength of the base metal and the welded part, and combines with the N in the process of cooling the heated HAZ part during welding to precipitate BN, thereby promoting ferrite transformation from the austenite grain. , Improves HAZ toughness. In order to acquire this effect, it is preferable to contain B amount 0.0002% or more. B amount is more preferably 0.0005% or more, and still more preferably 0.0010% or more. However, when B content becomes excessive, since the toughness of a base material and a HAZ part deteriorates or weldability deteriorates, it is preferable to make B amount into 0.005% or less. B amount is more preferably 0.004% or less, and still more preferably 0.0030% or less.

V: more than 0% and less than 0.1%

V is an element effective for improving the strength, and in order to obtain this effect, it is preferable to contain V by at least 0.003%. More preferably, it is 0.010% or more. On the other hand, when V content exceeds 0.1%, weldability and base material toughness deteriorate. Therefore, it is preferable to make V amount into 0.1% or less, More preferably, it is 0.08% or less.

Cu: more than 0% and less than 1.5%

Cu is an effective element for improving the hardenability and increasing the strength. In order to acquire this effect, it is preferable to contain Cu 0.01% or more. Cu amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, since toughness deteriorates when Cu content exceeds 1.5%, it is preferable to set it as 1.5% or less. Cu amount is more preferably 1.0% or less, and still more preferably 0.50% or less.

Ni: more than 0% and less than 1.5%

Ni is an effective element for improving the strength and toughness of the base metal and the welded portion. In order to acquire this effect, it is preferable to make Ni amount 0.01% or more. Ni amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when Ni is contained in a large amount, since it becomes extremely expensive as a structural steel material, it is preferable to make Ni amount 1.5% or less from an economic viewpoint. Ni amount is more preferably 1.0% or less, and still more preferably 0.50% or less.

Cr: more than 0% and less than 1.5%

Cr is an element effective for improving the strength, and in order to obtain this effect, it is preferable to contain Cr. Cr amount is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when Cr amount exceeds 1.5%, HAZ toughness deteriorates. Therefore, it is preferable to make Cr amount 1.5% or less. Cr amount is more preferably 1.0% or less, and still more preferably 0.50% or less.

Mo: more than 0% and less than 1.5%

Mo is an element effective for improving the strength and toughness of the base metal. In order to acquire this effect, it is preferable to make Mo amount 0.01% or more. Mo amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when Mo amount exceeds 1.5%, HAZ toughness and weldability deteriorate. Therefore, it is preferable to make Mo amount into 1.5% or less, More preferably, it is 1.0% or less, More preferably, it is 0.50% or less.

Nb: greater than 0% and less than 0.06%

Nb is an effective element for increasing strength and base metal toughness without degrading weldability. In order to acquire this effect, it is preferable to make Nb amount 0.002% or more. Nb amount is more preferably 0.010% or more, and still more preferably 0.020% or more. However, when Nb amount exceeds 0.06%, the toughness of a base material and HAZ will deteriorate. Therefore, in this invention, it is preferable to make the upper limit of Nb amount into 0.06%. Nb amount is more preferably 0.047% or less, still more preferably 0.040% or less, even more preferably 0.030% or less.

Ti: more than 0% and less than 0.03%

Ti precipitates as TiN in steel, thereby preventing coarsening of the austenite grains in the HAZ portion during welding and promoting ferrite transformation, and is an effective element for improving the toughness of the HAZ portion. Furthermore, Ti exhibits desulfurization and is therefore an effective element for improving HIC resistance. In order to acquire these effects, it is preferable to contain Ti 0.003% or more. Ti amount is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, when the Ti content is excessive, the toughness of the base material and the HAZ portion is deteriorated due to an increase in solid solution Ti or an increase in TiC precipitation. Therefore, the Ti content is preferably 0.03% or less. Ti amount is more preferably 0.02% or less.

Mg: more than 0% and less than 0.01%

Mg is an element effective for improving toughness through refinement of crystal grains and an element effective for improving HIC resistance because it exhibits desulfurization. In order to acquire these effects, it is preferable to contain Mg 0.0003% or more. Mg amount is more preferably 0.001% or more. On the other hand, since the effect is saturated even if Mg is excessively contained, the upper limit of the amount of Mg is preferably 0.01%. Mg amount is more preferably 0.005% or less.

The steel sheet of this invention is a steel plate with high hydrogen-organic crack resistance because a horizontal crack does not exist in the stage of a slab, or the maximum opening thickness of a horizontal crack is below a threshold. The threshold value herein means the maximum pore thickness of horizontal cracking, which is obtained in advance and does not generate HIC in the steel sheet obtained by rolling the slab.

Thus, evaluation of horizontal cracking at the stage of slab, in particular, by setting the maximum pore thickness of horizontal cracking below a predetermined threshold yields a steel sheet having high hydrogen-organic crack resistance, and also enables the product to be shipped early This is described below.

First, the above-described "horizontal crack" is described in detail below.

It is known that the segregation of a component exists in the internal crack part and center segregation part of a slab, and it is easy to generate | occur | produce HIC as the segregation degree of this component is high, for example as described in Unexamined-Japanese-Patent No. 2007-136496. In addition, segregation produces hardened structures such as MA (Martensite-Austenite constituent, island martensite), and pearlite bands. The higher the segregation rate, the more likely hardened tissue is to occur, and HIC propagates along the hardened tissue. In the present invention, in particular, the segregation degree of the internal crack is considered, and the HIC resistance is evaluated.

Segregation is also present between secondary dendrites. That is, micro segregation may occur. However, since these secondary dendrites are very small and HIC does not propagate and extend, there is no problem in quality. Therefore, the micro segregation is not considered in the present invention.

Internal cracks include "horizontal cracks" and "other internal cracks," which are caused by bulging between rolls, unbalance of cooling water, and deformation at the time of calibration passage. As shown to Fig.1 (a), "horizontal crack" is a crack which exists in the range of slab thickness D / 2 from the width edge part in the width direction W of the slab, and is a crack propagated in the slab width direction and the casting direction. On the other hand, "other internal cracks" are cracks existing in the entire slab width as shown in Fig. 1 (a) and are cracks propagated in the slab thickness direction and the slab width direction, or the slab thickness direction and the slab casting direction. .

When the slab is rolled, as shown in Fig. 1 (b), "horizontal cracks" are elongated, but "other internal cracks" are reduced. If HIC is generated from the crack, the HIC is easily propagated and extended in the "horizontal crack", but since the HIC is not propagated and extended in the "other internal crack", there is no problem in terms of quality. Moreover, when HIC test was performed, HIC might generate in the "horizontal crack" generation part, but HIC did not generate in the "other internal crack" generation part. Therefore, in the present invention, only "horizontal cracks" among the internal cracks are considered.

And in this invention, the segregation degree of this "horizontal crack" is evaluated by the "maximum pore thickness" demonstrated below. The occurrence position of "horizontal crack" is as said FIG. 1 (a), and is a crack which generate | occur | produces in a solid-liquid interface at the time of solidification. "Horizontal crack" is accompanied by a segregation line formed by thickened molten steel between dendrite branches. When this degree is remarkable, it opens along the segregation line. There is a correlation between the segregation of the horizontal crack and the pore thickness (pore width), and the larger the pore thickness, the higher the segregation degree of the horizontal crack tends to be. That is, there is a correlation between the maximum pore thickness and the segregation degree of the horizontal crack. Since HIC tends to occur as the segregation degree of a horizontal crack is high, it is thought that HIC tends to occur when the maximum pore thickness is large. From these, the HIC resistance can be judged by "maximum pore thickness", and when this maximum pore thickness is reduced, HIC was first found to be suppressed. Hereinafter, the maximum pore thickness of this horizontal crack may be simply called "maximum pore thickness".

On the other hand, since fine horizontal cracks having a "opening thickness" of about 10 µm are squeezed at the time of rolling, it is known that they do not become UT (Ultrasonic Testing) defects at the product stage, but cause HIC generation. In consideration of this, it is thought that HIC does not occur due to the opening, but due to the high segregation degree of the horizontal crack.

Then, the inventors of the present invention do not have to perform the HIC test on the steel sheet that is a product if the HIC resistance of the steel sheet after rolling can be determined using the maximum pore thickness of the slab during the slab, that is, after casting and before rolling. As a result, it has been found that the product can be shipped early.

Hereinafter, a method for obtaining the maximum pore thickness and the threshold value tθ of the maximum pore thickness used when determining the HIC resistance of the steel sheet after rolling using the maximum pore thickness will be described.

A method for obtaining the maximum pore thickness of the horizontal crack will be described.

First, the slab obtained by casting is cut | disconnected in the thickness direction, ie, the direction perpendicular | vertical to a casting direction, as shown in FIG. 2, and the horizontal crack of a segregation part is investigated. The position where a horizontal crack generate | occur | produces it is easy to produce a gap in the slab width direction and slab thickness direction rather than a casting direction. Therefore, as shown in FIG. 2, by making the cut surface perpendicular to the casting direction to be irradiated, the site where the horizontal crack is worsened can be irradiated.

In the slab cut surface of FIG. 2, the maximum pore thickness t1 and t2 of the horizontal cracks which exist in the area | regions R1 and R2 from the both ends of slab width W to slab thickness D / 2, respectively are measured. Here, the maximum pore thickness t1 is the maximum pore thickness in the region R1, and the maximum pore thickness t2 is the maximum pore thickness in the region R2. In addition, in FIG. 2, the first range and the region R3 in FIG. 2 may be referred to as the second range in addition to the regions R1 and R2.

The reasons for examining the regions R1 and R2 are as follows. That is, horizontal cracking occurs in the process of solidification progressing toward the width center from the both ends of the slab in the width direction (narrow surface). At the time of solidification, in area | region R1, R2, ie, a 1st range, solidification advances toward the width direction center under the influence of cooling of the narrow surface side (short side). On the other hand, in the area R3 of the width W-D except for D / 2 from the width direction both ends, that is, in the second range, the solidification hardly proceeds in the width direction because it is hardly affected by the cooling on the narrow surface side (short side). Therefore, since it is thought that horizontal cracks generate | occur | produce in the area | regions R1 and R2, in this invention, horizontal cracks are irradiated in area | regions R1 and R2 as mentioned above.

Here, when two or more horizontal cracks exist in each of area | region R1, R2, the largest opening thickness among the thickness of the some opening which exists in each area | region R1, R2 is made into the maximum opening thickness t1, t2. For example, when three horizontal cracks exist in the region R1, the horizontal crack having the largest opening among the three horizontal cracks is selected, and the most open part of the horizontal crack, that is, the part having the thickest opening thickness, is selected. Let opening thickness be "maximum opening thickness t1."

Next, a description will be given of a method for determining the threshold value tθ used for HIC resistance evaluation of the slab, that is, the maximum pore thickness at which HIC does not occur in the steel sheet obtained by rolling the slab.

The threshold value tθ is obtained in advance, but the method is not particularly limited. As a method of calculating | requiring threshold value t (theta), what is calculated | required previously by the method of following (i)-(iii) is mentioned. Hereinafter, the detail will be described.

(i) The maximum pore thickness of the slab is measured.

(ii) The HIC test is performed on the steel plate obtained by rolling the slab cast on the same casting conditions as the said slab.

(iii) From the maximum hole thickness measured in said (i) and the HIC test result of said (ii), the maximum hole thickness of the horizontal crack in which hydrogen organic crack does not generate | occur | produce is calculated | required.

The slab cast in the same casting conditions as the slab which measured the said maximum opening thickness is hot-rolled, and the steel plate for threshold value measurements is manufactured. Then, the steel sheet is subjected to the HIC test to check whether HIC is generated. The HIC test may be performed by a method specified in the National Association of Corrosion and Engineer (NACE) standard TM0284-2003, as shown in Examples described later.

The above-mentioned "same casting conditions" are i) the casting speed is constant, ii) operation abnormalities, such as a nozzle clogging, generate | occur | produce, iii) cooling conditions, roll interval, etc. are the same. When determining the threshold value tθ, the degree of segregation obtained by irradiating the slab and the HIC test result for the product are matched. However, if their HIC resistance is different, the threshold value cannot be determined. The operating factors of i) to iii) have a great influence on the horizontal crack and central segregation, which also affects the HIC resistance. Therefore, when the operation factors are different, the HIC resistance also changes. Therefore, it is preferable to use the steel plate obtained by manufacturing using the slab casted under the same casting conditions (operation factor) as the slab which irradiated the maximum opening thickness for the steel plate for HIC test. In particular, it is preferable that the slab for which the maximum pore thickness is examined is the same as the slab for HIC test.

In the said HIC test, it is examined whether HIC generate | occur | produces in the area | region of the product (steel plate) corresponding to the area | regions R1 and R2 of the slab shown in the said FIG. According to the rolling direction at the time of rolling using the slab shown in FIG. 2, the area | region of HIC resistance evaluation object differs as shown in FIG.

When the slab is rolled in the casting direction, that is, when the rolling direction is the casting direction, the width does not change before and after rolling, as shown in Fig. 3A, so the slab width W is the product width W. In this case, as shown to Fig.3 (a), the area | region of the product corresponding to "the area | region R1, R2 of slab" is "the area | region R11, R12 of the range of product width D / 2 from the both ends of the width direction of a product", and "slab The area of the product corresponding to "area R3" is "area R13 in the range of width WD excluding product width D / 2 minutes from both ends of the width direction of the product".

On the other hand, when the slab is rolled in the width direction, that is, when the width direction is included in the rolling direction, as shown in Fig. 3B, since the width changes from W before rolling to Wa after rolling, the slab width W < It becomes product width Wa. In this case, as shown in FIG.3 (b), the area | region R21, R22, R23 corresponding to the area | region R1, R2, R3 of a slab is determined by rolling ratio, ie, product width Wa / slab width W. As shown in FIG. Among them, it is checked whether HIC has occurred in the areas R21 and R22.

From the "maximum pore thickness t1, t2" obtained from the irradiation of the slab and the "HIC test result for the product", the "threshold value tθ of the maximum pore thickness" at which HIC does not occur is determined.

When determining the threshold tθ, the results obtained in the areas corresponding to each other with the slab and the product are matched. For example,

(I) When the slab is rolled in the casting direction as shown in Fig. 3 (a), it is determined as follows in the HIC test when "with HIC is generated" in the product region R11 and "without HIC" in the region R12.

(I-1) As a result of the product area R11, when the maximum opening thickness t1 of the slab area R1 is "HIC generation exists"

(I-2) As a result of the product area R12, when the maximum opening thickness t2 of the slab area R2 is "no HIC generation"

(II) In the case where the slab is rolled in the width direction as shown in Fig. 3B, it is determined in the HIC test when "HIC is generated" in the product region R21 and "No HIC is generated" in the region R22 as follows.

(II-1) As a result of the product area R21, when the maximum opening thickness t1 of the slab area R1 is "HIC generation exists"

(II-2) As a result of the product area R22, when the maximum opening thickness t2 of the slab area R2 is "no HIC generation"

From the plurality of results described above, the threshold value tθ of the maximum pore thickness serving as a boundary between HIC generation is determined. Specifically, for example, in the case of (I), the maximum pore thickness t2 is the threshold tθ. Also in the case of (II), the maximum pore thickness t2 becomes the threshold value tθ.

In addition, it is preferable to use the measurement result of the horizontal crack and maximum opening thickness of several slabs, and the HIC test result for determination of threshold value t (theta). By using the measurement results of the horizontal crack and the maximum opening thickness of the plurality of slabs and the HIC test results, a more accurate threshold value tθ can be obtained and the misjudgment of the presence or absence of HIC generation can be reduced.

The segregation part and the investigation of the HIC resistance may be evaluated from one section of the slab or the product, or may be evaluated from two sections or more. Below, the result of having investigated plural cross sections of the slab of the same charge is shown in FIG. In FIG. 4, Example 1 is the example which investigated 2 cross sections of the same charge, Example 2 is the example which investigated the 3 cross sections of the same charge, and it is the result of having irradiated with the slab which can all cover API X65 grade.

As shown in FIG. 4, in Example 1, the maximum pore thickness was 0 mm in both cross sections, and in the HIC test, no HIC originated from the horizontal crack portion. In Example 2, the maximum pore thickness of each of the three sections was 0.065 mm, 0.067 mm, and 0.066 mm, which was the same thickness. Moreover, HIC generate | occur | produced from the horizontal crack part from all the cross sections.

Thus, in the same charge, substantially the same results were obtained even if the cross sections were different. In addition, even when 50 charges were examined by one section for each charge, the same results were obtained between each charge, and it was separately confirmed that there was no misjudgment and accurate evaluation could be performed.

In the example of FIG. 4, although the slab which can cover API X65 grade was implemented, although intensity grade changed, even if it is API X70 grade or more, since the formation and the gap of internal cracking do not change, the number of irradiation cross sections is not limited. Do not.

As for the irradiation position (irradiation surface) of a slab, a top part is preferable as shown in the following example, but an abnormal part may be used. An "abnormal part" is a part cast at the time of change of casting conditions, and the part cast at the beginning of casting like the time of a rise of a casting speed, and the last casting like the case of the fall of a casting speed, etc. are mentioned. When irradiating from an abnormal part, as shown in FIG. 5, it is preferable to irradiate the part adjacent to the site | part which performs an HIC test. Since such a part shows HIC resistance similar to the HIC test result, more accurate evaluation can be performed.

As described above, the steel sheet of the present invention is a steel sheet having no horizontal crack or a maximum pore thickness of the horizontal crack at or below the threshold tθ in the stage of the slab before the rolling thereof. As described above, when no horizontal crack exists in the regions R1 and R2 of the slab cut surface, the segregation degree of the horizontal crack portion is low, so that HIC due to the horizontal crack does not occur. Further, even when the maximum pore thickness of the horizontal cracks in the above-mentioned regions R1 and R2 of the slab cut surface is equal to or less than the threshold tθ, the segregation degree of the horizontal crack portion is low, so that HIC caused by horizontal cracking does not occur.

Moreover, according to this invention, "maximum pore thickness of a horizontal crack" is used for evaluation of HIC resistance. Since the internal quality of a cast can be evaluated correctly from this, HIC resistance can be evaluated at the stage of a cast based on this evaluation result. Thereby, since the HIC test which requires several weeks can be omitted, the period from manufacture to shipment can be significantly shortened.

This application claims the benefit of priority based on Japanese Patent Application No. 2014-266489 for which it applied on December 26, 2014, and Japanese Patent Application No. 2015-207452 for which it applied on October 21, 2015. The entire contents of the specification of Japanese Patent Application No. 2014-266489, filed December 26, 2014 and the specification of Japanese Patent Application No. 2015-207452, filed October 21, 2015, are hereby incorporated by reference. do.

Example

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not restrict | limited by the following example, of course, It is a matter of course that it changes and implements suitably in the range which may be suitable for the meaning of the previous and the later. Possible, they are all included in the technical scope of the present invention.

Tables 1-1, 1-2, Figs. 6 and 7 show experimental conditions and experimental results for determining the threshold value tθ. 21 slabs each of API X65 grade equivalent and API X70 grade equivalent slab, and one slab each of ASME SA516 grade 60 equivalent, ASME SA516 grade 65 equivalent and ASME SA516 grade 70 equivalent slab, were cast as follows. Investigated. In Table 1-1, Table 1-2, and Table 3 described above, "X70" is API X70 grade, "X65" is API X65 grade, and "SA516 60" is ASME SA516 grade 60 and "SA516 65". "ASME SA516 Grade 65," SA516 70 "represents ASME SA516 Grade 70.

Here, the conditions shown in Table 1-1 and Table 1-2 are demonstrated.

<Component of molten steel in tundish>

The concentrations of C, Mn, Nb, P, and Ca were measured by emission spectrometry. Since S concentration was low, the measurement by luminescence spectroscopy was difficult. Therefore, the combustion-infrared absorption method was used for the measurement of the S concentration.

<Casting condition>

Non-quantity

Specific quantity = (total amount of secondary coolant per unit time [L / min.] From directly under the mold to the final roll of the continuous casting machine) / (mass of cast slab per unit time [kg / min.])

Casting speed

It was the drawing speed [m / min.] Of the cast steel, and was calculated from the diameter (circumference) of the roll (major roll) in contact with the cast steel, and the rotational speed (revolutions per unit time).

(casting)

Cast steel having a thickness of 280 mm, that is, within the range of the component composition defined in the present invention and whose molten steel in the tundish is as shown in Tables 1-1 and 1-2, by continuous casting; Obtained a slab.

(Investigation of horizontal cracks)

The slab was cut at the top while having a total length of 10 to 15 m and horizontal cracks were examined as follows. Here, a "normal part" is a site | part which satisfy | fills the following conditions. The number of horizontal crack irradiation cross sections is as shown in Table 1-1 and Table 1-2.

1) The casting speed is constant.

2) No abnormality in operation such as clogging of the immersion nozzle.

3) The cooling conditions are not changed.

4) The roll spacing is not changed.

Order of investigation of horizontal cracks

(1) The range of D / 2 was polished to # 800 from the both ends of the width direction of the slab cut surface.

(2) The polished surface was corroded with 20 g / L picric acid, 5 g / L cupric chloride and 60 ml / L surface active agent.

(3) The corrosion surface was visually confirmed, and the part in which a horizontal crack exists was cut out to the magnitude | size of 40 mm x 70 mm.

(4) The cut sample was buff-polished and finished with the roughness of 1 micrometer or less.

(5) Line analysis of Mn segregation in the horizontal crack portion in the sample was performed using an EPMA (Electron Probe Micro Analyser) at a beam diameter of 20 µm. Mn segregation degree of this horizontal crack part is represented by Cmax (Mn).

(6) Cmax (Mn) / C ₀ (Mn) was calculated from the Mn concentration of the molten steel in the tundish measured at the time of casting, that is, C ₀ (Mn) and the above Cmax (Mn).

(7) The horizontal crack of the part which performed EPMA analysis was observed with the microscope (20 times-50 times), and the pore thickness was measured.

(Rolled)

Then, after heating the slab equivalent of API X65 grade and API X70 grade so that it may be set to 1050-1250 degreeC, the steel plate average temperature calculated | required by calculation as follows at the surface temperature of steel plate accumulated at 1000 degreeC or more Hot rolling is performed so that a pass with a reduction ratio of 40% or more and a pass with a reduction ratio of 10% or more per pass may be 2 or more passes. Thereafter, hot rolling was further performed so that the cumulative reduction ratio of 700 ° C or more and less than 900 ° C was 20% or more, so that the rolling end temperature was 700 ° C or more and less than 900 ° C. Then, water cooling was started from the temperature of 650 degreeC or more, it stopped at the temperature of 350-600 degreeC, and then air-cooled to room temperature further, and obtained the steel plate of 45 mm of plate | board thickness. Furthermore, after hot-rolling the slab of ASME SA516 grade 60 equivalent, ASME SA516 grade 65 equivalent, and ASME SA516 grade 70 equivalent hot rolling so that rolling end temperature might be 850 degreeC or more, it air-cooled to room temperature and again, it was 850 degreeC or more and 950 degrees C or less. After reheating and quenching at a temperature, tempering was performed at 600 to 700 ° C to obtain a steel sheet having a sheet thickness of 40 mm. In addition, neither rolled in the slab width direction.

The said steel plate average temperature is calculated | required as follows. That is, based on data such as a rolling pass schedule during rolling and a cooling method (water cooling or air cooling) between the passes, the temperature at any position in the sheet thickness direction is calculated using a method suitable for calculation such as a differential method, The average value of the temperature from the surface of a steel piece to the back surface is made into steel plate average temperature. The same applies to the steel sheet average temperature.

(HIC test)

In order to determine the threshold value tθ, the HIC test was performed after rolling in this example.

(a) The sample was cut out from the product after rolling, and the HIC test was done. HIC testing was performed according to the method defined in NACE standard TM0284-2003.

(b) After the HIC test, the samples were cut at three places, and each cross section (three cross sections) was observed under a microscope to confirm the presence or absence of HIC. Here, the presence or absence of a crack was confirmed by "the area | region R11 and R12 of the range of D / 2 from the width direction both ends of a product" shown to Fig.3 (a).

(Determination of Threshold tθ of Maximum Opening Thickness)

6 and 7 show the relationship between "" horizontal crack pore thickness "and" Cmax (Mn) / C ₀ (Mn) "and" the presence or absence of HIC generation "confirmed by the HIC test. FIG. 6 shows the results of investigating the threshold value tθ at which HIC occurs in the components whose strength classes shown in Table 1-2 correspond to API X65 grade, ASME SA516 grade 60, and ASME SA516 grade 65, and FIG. 7 shows Tables 1-1 and It is the result of having investigated the threshold value t (theta) which HIC generate | occur | produces in the component whose intensity class shown in Table 1-2 is equivalent to API X70 grade and ASME SA516 grade 70.

From FIG. 6, in the slab which can cover API X65 grade, although HIC did not generate | occur | produce when the maximum opening thickness <0.047mm, HIC might generate | occur | produce when the maximum opening thickness> 0.047mm. Therefore, in the slab which can cover API X65 grade, the threshold value t (theta) of the largest pore thickness was made into 0.047 mm, and it judged as follows.

When the maximum pore thickness ≤ 0.047 mm, it is determined that HIC does not occur.

It is determined that HIC occurs when the maximum pore thickness> 0.047 mm.

In addition, since ASME SA516 grade 60, grade 65, and ASTM A516 grade 60 and grade 65 are components equivalent to API X65 grade, it was determined as follows by setting the threshold value tθ of the maximum pore thickness to 0.047 mm.

When the maximum pore thickness ≤ 0.047 mm, it is determined that HIC does not occur.

It is determined that HIC occurs when the maximum pore thickness> 0.047 mm.

On the other hand, in FIG. 7, in the slab which can cover API X70 grade, although HIC did not generate | occur | produce when the maximum opening thickness <0.043mm, HIC might generate | occur | produce when the maximum opening thickness> 0.043mm. Therefore, in the slab which can cover API X70 grade, the threshold value t (theta) of the maximum pore thickness was made into 0.043 mm, and it judged as follows.

When the maximum pore thickness ≤ 0.043 mm, it is determined that HIC does not occur.

It is determined that HIC occurs when the maximum pore thickness> 0.043 mm.

In addition, since ASME SA516 grade 70 and ASTM A516 grade 70 are components equivalent to API X70 grade, it judged as follows, making threshold value t (theta) of the largest pore thickness into 0.043mm.

When the maximum pore thickness ≤ 0.043 mm, it is determined that HIC does not occur.

It is determined that HIC occurs when the maximum pore thickness> 0.043 mm.

On the other hand, in FIG. 6, 7, HIC did not generate | occur | produce in the horizontal crack which is not open, ie, the maximum opening thickness of 0 mm.

Evaluation of HIC Resistance of Slabs Subject to Judgment

The HIC resistance of the slab to be determined was evaluated in the following procedure using the threshold value tθ. First, the steel of the component composition shown in Table 2 was melted, and the continuous slab obtained the slab which is the determination object whose slab thickness D is 280 mm and slab width W is 2100 mm. And it evaluated in the following procedure using this slab.

(1) The range of width | variety D / 2 was phrased from the both ends of the width direction of the slab cut surface which is determination object, and the dyeing penetration flaw test (JIS Z2343) was performed.

(2) When a horizontal crack was not detected, it was determined that the maximum pore thickness was below the lower limit of detection (about 10 μm or less). In this case, since the maximum pore thickness was below the threshold tθ, that is, 0.047 mm or less in the API X65 grade and 0.043 mm or less in the API X70 grade, it was judged that HIC caused by horizontal cracking did not occur.

(3) When a horizontal crack was detected, the site | part which was opened was buff-polished, the polished surface was observed with the microscope of 20 times-50 times, and the maximum pore thickness was measured as mentioned above.

(3-1) And in the slab which can cover API X65 grade, as shown in the said "determination of the threshold value of the maximum hole thickness t (theta)", when the said maximum hole thickness is below the threshold value t (theta): 0.047mm or less, the HIC which originates in a horizontal crack will be It did not generate | occur | produce, ie, evaluation of HIC resistance of a slab was OK, and the obtained steel plate was judged to be excellent in HIC resistance. On the other hand, when the said maximum pore thickness exceeded threshold value t (theta): 0.047 mm, HIC originated from a horizontal crack generate | occur | produces, ie, the HIC evaluation of slab was NG, and it was judged that the steel plate obtained was inferior to HIC resistance.

(3-2) In the slab that can cover API X70 grade, when the maximum pore thickness is below the threshold tθ: 0.043 mm, HIC due to horizontal cracking does not occur, that is, the HIC resistance evaluation of the slab is OK, and the obtained steel sheet Judged HIC resistance to be excellent. On the other hand, when the said maximum pore thickness exceeded the threshold value t (theta): 0.043mm, HIC originated from a horizontal crack generate | occur | produced, ie, the HIC evaluation of slab was NG, and it was judged that the steel plate obtained was inferior to HIC resistance.

Thereafter, the slab was heated to be 1050 to 1250 ° C, and then, as indicated by "TMCP" or "QT" in the column of "Hot rolling and cooling method" of Table 3, by the hot rolling and cooling method of two patterns. And steel plate (9-90 mm plate | board thickness x 2000-3500 mm width x 12000-35000 mm length) of various component compositions were obtained. The said "TMCP" is a pass temperature of 900% or more at the surface temperature of the steel sheet, and the steel sheet has a cumulative reduction rate of at least 1000 ° C of 40 ° C or more, and a pass having a rolling reduction rate of 10% or more per pass. Hot rolling was performed so that it might become. Thereafter, hot rolling is further performed so that the cumulative reduction ratio of 700 ° C or more and less than 900 ° C is 20% or more, so that the rolling finish surface temperature is 850 ° C, and then the cooling start surface temperature: 950 ° C from the average cooling rate: Cooling is started at 10 ° C / s, stopped at a temperature of 350 to 600 ° C, and then air cooled to room temperature after that. Said "QT" is a method of performing tempering at 600-700 degreeC after hot-rolling so that rolling completion temperature may be 850 degreeC or more, air-cooling to room temperature, reheating and quenching to the temperature of 850 degreeC or more and 950 degrees C or less. .

(HIC test)

The HIC test was performed using the said steel plate. The HIC test was performed according to the method specified in NACE standard TM0284-2003. After the HIC test, the samples were cut at three places, and each cross section (three cross sections) was observed under a microscope to confirm the presence or absence of HIC. The results are shown in Table 3.

Table 1-1

TABLE 1-2

TABLE 2

TABLE 3

Table 2 and Table 3 show the following. No. 1-7, 10, 12, and 14-17 satisfy | fill the prescribed | prescribed component composition, and since the maximum pore thickness of the horizontal crack of a slab is suppressed below threshold value t (theta), it is the steel plate of this invention excellent in HIC resistance.

In comparison, No. In 11 and 13, since the maximum pore thickness of the horizontal crack of the slab exceeds the threshold tθ, the HIC resistance evaluation of the slab was NG. Moreover, in the HIC test performed after rolling, it confirmed that a crack generate | occur | produced in the steel plate and it was inferior to HIC resistance. No. 8, 9, 18, and 19 are the examples where the maximum pore thickness of the horizontal crack of the slab is suppressed below the threshold tθ, but the chemical composition of the steel sheet deviates from the definition of the present invention. That is, No. The steel sheet of 8 has a REM and Zr of 0%, a value of (Ca / S) is out of specification, and No. As for the steel plate of 9, since REM and Zr are 0%, and the value of (Ca-1.25S) / O is out of specification, both were inferior to HIC resistance. Also no. 18, the value of (Ca / S) is out of regulation, and No. Since 19 (Ca-1.25S) / O values are out of regulation, all of them are inferior in HIC resistance.

In the example where the evaluation of the HIC resistance in the slab was OK, the period (casting → rolling → shipping) from the start of casting to the shipment of the steel sheet as a product, that is, the sour resistant steel sheet, was 19 days. On the other hand, when HIC test was performed using the steel plate obtained after rolling and the HIC resistance was evaluated, the period from casting start to shipment (casting → rolling → HIC test → shipping) required 28 days. In the present Example, since the HIC test after the said rolling was able to be skipped, the period from the start of casting to shipment can be shortened significantly from 28 days to 19 days.

In the example where the evaluation of HIC resistance in the slab was NG, the resolvent was started at the stage of the slab, and the period from the start of the casting to the shipment of the steel sheet, that is, the sour resistant steel sheet (casting → resolvent → rolling → shipping) ) Was 54 days. On the other hand, when HIC test was performed using the steel plate obtained after rolling, and the HIC resistance of the product was evaluated, when evaluation was NG, since the resolvent was started after performing the said HIC test, the steel plate which is a product from the start of casting The period until shipment (casting → rolling → HIC test → resolvent → rolling → HIC test → shipping) required 72 days. In the present Example, since the HIC test after the said rolling was able to be skipped, even if resolvent was needed, the period from the start of casting to shipment can be drastically shortened from 72 days to 54 days.

As described above, according to the present invention, since the HIC resistance could be evaluated at the stage of slab which is the cast steel without performing the HIC test after rolling, the production lead time could be shortened significantly. On the other hand, in this embodiment, since the HIC test for determining the threshold value tθ for HIC resistance evaluation of the slab and the HIC test for confirmation are the same, it can be said that the determination method of the present invention has high accuracy.

Claims (8)

In mass%,
C: 0.02 to 0.15%,
Si: 0.15 to 0.50%,
Mn: 0.6-1.2%,
P: more than 0% and less than 0.030%,
S: greater than 0% and less than 0.003%,
Al: 0.031-0.08%,
Ca: 0.0003 to 0.0027%,
N: 0.001-0.01%, and
O: more than 0% and less than 0.0045%, and also
REM: greater than 0% and less than 0.0047%,
Zr: 0.0006-0.0030%, and
Nb: greater than 0% and less than 0.040%
It includes, the balance is made of iron and inevitable impurities,
While the ratio of Ca and S (Ca / S) is 2.5 or more,
Ca, S and O satisfy (Ca-1.25S) /O≦1.20,
Also, in the stage of slab, there is no horizontal crack, or the maximum pore thickness of the horizontal crack is equal to or less than the threshold tθ, and the threshold tθ is the maximum opening of the horizontal crack in which the hydrogen organic crack does not occur in the steel sheet obtained by rolling the slab. Steel sheet excellent in hydrogen-organic crack resistance, characterized in that the thickness. The method of claim 1,
The said threshold value t (theta) is a steel plate which is the value calculated | required previously by the method of following (i)-(iii).
(i) The maximum pore thickness of the slab is measured.
(ii) Hydrogen organic crack test is performed on the steel plate obtained by rolling the slab cast under the same casting conditions as the slab.
(iii) From the maximum pore thickness measured in (i) and the hydrogen organic crack test result of (ii) above, the maximum pore thickness of the horizontal crack in which no hydrogen organic crack is generated is determined. The method of claim 2,
A slab cast under the same casting conditions as the slab is a slab measuring the maximum opening thickness. The method according to any one of claims 1 to 3,
A steel sheet having ASME SA516 Grade 65 and having a threshold tθ of 0.047 mm. The method according to any one of claims 1 to 3,
A steel sheet having ASTM A516 grade 65 and having a threshold tθ of 0.047 mm. The method according to any one of claims 1 to 3,
As another element, in mass%,
B: greater than 0% and less than 0.005%,
V: greater than 0% and less than or equal to 0.1%,
Cu: more than 0% and less than 1.5%,
Ni: more than 0% and less than 1.5%,
Cr: greater than 0% and less than 1.5%, and
Mo: more than 0% and less than 1.5%
Steel sheet containing at least one element selected from the group consisting of. The method according to any one of claims 1 to 3,
As another element, in mass%,
Ti: greater than 0% and less than or equal to 0.03%, and
Mg: more than 0% and less than 0.01%
Steel sheet containing at least one element selected from the group consisting of. The method according to any one of claims 1 to 3,
Steel plate for pressure vessels.

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