WO2001086013A1

WO2001086013A1 - THICK STEEL PLATE BEING EXCELLENT IN CTOD CHARACTERISTIC IN WELDING HEAT AFFECTED ZONE AND HAVING YIELD STRENGTH OF 460 Mpa OR MORE

Info

Publication number: WO2001086013A1
Application number: PCT/JP2001/003876
Authority: WO
Inventors: Akihiko Kojima; Yoshio Terada; Akihito Kiyose; Yuzuru Yoshida; Toshihiko Adachi; Kazuaki Tanaka; Ryuji Uemori; Shiro Imai
Original assignee: Nippon Steel Corporation
Priority date: 2000-05-09
Filing date: 2001-05-09
Publication date: 2001-11-15
Also published as: KR20020028203A; JP2002212670A; CN1188535C; KR100469378B1; CN1380910A; DE60108350T2; DE60108350D1; TW541343B; JP3699657B2

Abstract

A thick steel plate, characterized in that it has a chemical composition in mass %: C: 0.04 to 0.14 %, Si: 0.4 % or less, Mn: 1.0 to 2.0 %, P: : 0.02 % or less, S: 0.001 to 0.005 %, Al: 0.001 to 0.01 %, Ti: 0.005 to 0.03 %, Nb: 0.005 to 0.05 %, Mg: 0.0003 to 0.005 %, O: 0.001 to 0.005 %, N: 0.001 to 0.01 % and balance: Fe and inevitable impurities, TiN which includes a oxide comprising Mg and Al and has a size of 0.01 to 0.5 νm is present in an amount of 10,000 pieces/mm2 or more, and particles of a composite of an oxide and a sulfide containing 0.3 mass % or more of Mn and having a size of 0.5 to 10 νm are present in an amount of 10 pieces/mm2 or more. The steel plate has a yield strength of 460 Mpa or more and a CTOD in HAZ at -10°C of 0.2 mm or more.

Description

Description Steel plate with a yield strength of 460MPa or more with excellent CT0D characteristics in the weld heat affected zone

The present invention relates to a steel plate having a yield strength of 460 MPa or more, preferably 500 to 550 MPa class, which is excellent in CTOD (Crack Tip Opening Displacement) characteristics of a heat affected zone (HAZ). It is mainly used for offshore structures, but can also be applied to other welded structures requiring similar strength and HAZ toughness (CTOD properties). Background art

Weld joints for offshore structures used in the North Sea are required to have CTOD characteristics at _10 ° C. Examples of steel materials requiring such strict HAZ toughness are described in Proceedings 01 12tn International Conference on OM AE, 1993, Glasgow, UK, ASME, Vo lumeHI—A, pp. 207—214. As is the case, Ti oxide steel is used. Since the vicinity of the HAZ melting line is heated to 1400 ° C or higher, the pinning effect of the TiN particles disappears and austenite (γ) becomes extremely coarse, resulting in a coarse HAZ structure and poor toughness. The Ti-oxide steel mentioned above has been developed as a steel that solves these problems.

As described in, for example, JP-A-63-210235 and JP-A-06-075599, this technique reduces the pinning effect of TiN particles and increases the coarseness of γ grains. By utilizing needle-like filaments that generate thermally stable Ti oxides as transformation nuclei, the microstructure of the HAZ structure can be reduced. It is steel that has been thinned. This acicular ferrite that effectively refines coarse 0 / grain is called intragranular ferrite (IGF).

However, the yield strength of this Ti-oxide steel is up to 420MPa class, and no thick steel plate has been developed that guarantees the CT0D characteristics of HAZ while having a yield strength higher than that. On the other hand, there is an increasing movement to reduce building costs by reducing the weight of offshore structures, and thick steel plates with high yield strength are required to reduce the weight of offshore structures. In other words, there is a strong demand for a thick steel sheet having a yield strength of 460 MPa or more, which is higher than before, and excellent HAZ toughness that can guarantee CT0D characteristics.

Disclosure of the invention

An object of the present invention is to provide a steel plate having a yield strength of 460 MPa or more, preferably of a class of 500 to 550 MPa, and a CT0D at 110 ° C. of HAZ of 0.2 mm or more.

The present invention relates to

C: 0.04 ~ 0.14%,

Si: 0.4% or less,

Mn: 1.0 to 2.0%,

P: 0.02% or less,

S: 0.001—0.005%,

A1: 0.001-0.01%,

Ti: 0.005-0.03%,

Nb: 0.005 to 0.05%,

Mg: 0.0003- 0.005%,

O: 0.001 to 0.005%,

N: 0.001 to 0.01% And, if necessary, in mass%,

Ca 0.0005-0.005%

REM 0.0005—0.01%

Zr 0.0005-01%

Cu 0.05-1.5%

Ni 0.05-3.0%

Cr 0.05 0.5%

Mo 0.05-0.5%

V 0.005-0.05%,

B 0.0001-0.003%

Chemical composition containing at least one of the following, the sum of Ca REM and Zr is 0.02% or less, the sum of Cu Ni, Cr and Mo is 3.0% or less, and the balance is iron and unavoidable impurities the a, 0.01 0.5 mu m of TiN that containing the oxides of Mg and A1 are present 10000 ZMM ² or more, and oxides and sulfides contains Mn of 0.3 wt% in a form complexed This is a thick steel plate having a yield strength of 460 MPa or more, which is excellent in CT0D characteristics of the weld heat affected zone, characterized in that there are 10 particles 粒子² or more of particles of 0.5 μμπι. BRIEF DESCRIPTION OF THE FIGURES

1 (a) to 1 (d) are diagrams schematically showing the concept of controlling the HAZ structure in a thick steel plate having a yield strength of 460MPa or more, which is excellent in the CT0D characteristics of the heat affected zone of the present invention. FIG. 1 (a) is a diagram for explaining the HAZ structure of the conventional Tioxide steel, and FIG. 1 (d) is a diagram for explaining the HAZ structure of the steel of the present invention. In FIG. 1, 1 is a weld metal, 2 is a heat affected zone (HAZ), and 3 is a melting line. In the HAZ structure, 4 is a γ grain boundary, GBF is a grain boundary light, FSP is a ferrite side plate, IGF is an intragranular transformation light, Bu is an upper bainite, and MA is a martensite 'austenite. 3 shows a mixed phase. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

1 (a) to 1 (d) are diagrams schematically illustrating the concept of HAZ organization control. FIG. 1 (a) is a diagram illustrating the HAZ structure of a conventional Ti-oxide steel, and FIG. 1 (d) is a diagram illustrating the HAZ structure of the steel of the present invention. In FIG. 1, 1 is a weld metal, 2 is a heat affected zone (HAZ), and 3 is a melting line. 4 in the HAZ structure is T / grain boundary, GBF is grain boundary ferrite, FSP is ferrite side plate, IGF is intragranular transformation ferrite, Bu is upper bainite, and MA is martensite austenite 3 shows a mixed phase.

When the yield strength of Tioxide steel is increased from the current 420 MPa class to 460 MPa or more to 500 MPa, 钣, and further to 550 MPa class by adding alloying elements, HAZ near the melting line hardens enough It will be difficult to secure a good CT0D characteristic. Figure 1 (a) schematically shows the HAZ organization at this time. The primary cause of HAZ embrittlement is that even if the inside of coarse y grains is refined by the formation of intragranular transformation ferrite (IGF), they are formed along the coarse Ί grain boundaries. This is because coarse grain boundary ferrite (GBF) ェ ferrite side plate (FSP) increases the sensitivity to brittle crushing as HAZ hardens. Therefore, it is necessary to reduce the susceptibility to brittle blasting by making these GBFs and FSPs finer. The second cause of embrittlement is that the hardenability of HAZ is increased by increasing the amount of alloying elements added for higher strength, and a microscopic embrittlement phase called MA (Martensite-Austenite constituent) Is generated, which promotes the occurrence of brittle rupture. Therefore, when achieving a yield strength of 460 MPa or more, it is necessary to reduce MA as much as possible. From the above, in order to achieve good joint CT0D characteristics under high yield strength, the metallurgy of Tioxide steel The guideline is to remove the above two causes of embrittlement while maintaining the effect (IGF effect). That is, the gist of the present invention is to simultaneously control the HAZ organization from the following three viewpoints.

(1) Refine GBF and FSP generated along the y grain boundary of HAZ near the melting line.

(2) Near the melting line The inside of the y-grain of HAZ is refined by the formation of IGF.

(3) Reduce the amount of MA generated in the HAZ near the melting line.

First, the means for achieving (1) will be described. In order to refine coarse GBF and FSP, which are harmful to the occurrence of brittle fracture, it is necessary to reduce gamma grains. With the aim of strongly suppressing the growth of Y grains in the HAZ near the melting line heated above 1400 ° C, as a result of intensive studies on various steel components, it was found that by appropriately controlling Mg and A1, Mg and A1 We have invented a technique to disperse a large number of ultrafine oxides of 0.01 to 0.1 μm in steel in steel, and to use these as nuclei to composite precipitate 0.01 to 0.5 μπι of TiN. Such composite prayer TiN particles are thermally stable even near the melting line, so they can strongly pin the movement of the γ grain boundaries without growing or melting. Even if welding with a large welding heat input is performed, the y grains near the melting line can be kept at a size of about 100 μm. In addition, these pinned particles present on the y grain boundaries themselves may directly function as transformation nuclei for GBF and FSP, and the number of transformation sites increases the number of GBF and FSP. Contribute to miniaturization. With the presence of more than 1,000,000 particles / mm ^{2 of} such composite precipitated TiN particles, GBF and FSP can be refined to a size that does not adversely affect the CT0D characteristics. If TiN particles children of this Yo I Do complex precipitation is less than 10,000 / mm ^2, the number of transformation nuclei on γ grain refining and γ grain boundary becomes insufficient results, GBF and FSP are sufficiently fine The CT0D characteristics deteriorate. Although sulfides may precipitate on the TiN particles in this composite form, they adversely affect the function as the pinning particles and the transformation nuclei described above. It has no effect.

Fig. 1 (b) is a schematic diagram of the HAZ organization when only the technique (1) described here is applied. GBF and FSP are refined, but with this technology alone, the inside of 0 / grain is covered with an embrittlement structure containing MA called upper bainite, and sufficient CT0D characteristics cannot be obtained. Therefore, the technique (2) described below must be used together.

The means to achieve (2) will be described. In the present invention, Mg is intentionally added to generate a large number of the above-mentioned ultrafine oxides. Since Mg is also contained in an oxide having a normal size (several μπι), the present invention has sought to generate IGF using such a relatively large Mg-containing oxide. As a result, it was found that the following three conditions were important as IGF metamorphosis nuclei.

① There must be a minimum number.

② Being of appropriate size.

③ Mn must be contained.

From the viewpoint of ①, IGF transformation nuclei are stably present in the HAZ near the melting line, and at least 10 nuclei / mm ² or more are required. If the number of IGF metamorphic nuclei is less than 10 / mm ² , the refinement of the HAZ structure is insufficient.

In addition, from the viewpoint of ②, a size of 0.5 μm or more is required to function effectively as an IGF transformation nucleus. When the particle size is less than 0.5 μπι, the ability as an IGF transformation nucleus is significantly reduced. In order to satisfy these conditions, in the present invention, use of an oxide of 0.5 / zm or more as an IGF transformation nucleus was examined. However, oxides exceeding 10 μm are not preferred because they act as starting points for brittle crushing.

From the viewpoint of (3), it was found that Mn must be contained in an amount of 0.3% by mass or more in order to function effectively as an IGF transformation nucleus. For this purpose, Mn may be contained in an oxide of 0.5 to 10 μm. In order to produce the pinned particles described in (1), Mg, Al, and Ti, which have a higher deoxidizing power than Mn, are essential, so these elements form oxides of 0.5 to 10 μm. However, it is difficult to stably contain 0.3% by mass or more of Mn in this material. Therefore, in the present invention, it has been considered that sulfide containing Mn is deposited in a complex on such an oxide. By taking such measures, it is possible to stably increase the Mn content in the composite particles to 0.3% by mass or more, and to function effectively as an IGF transformation nucleus. Therefore, as a result of searching for conditions for complex precipitation of the Mn-containing sulfide on the oxide, it was found that the Mg content in the oxide was important. When the Mn-containing sulfide was combined, the oxide contained 10% by mass or more of ¾. On the other hand, the Mg content in the oxides in which the sulfides did not complex and existed alone were less than 10% by mass. In other words, it has been found that by containing 10% by mass or more of Mg in an oxide of 0.5 to 10μπι, it is possible to stably composite precipitate a Mn-containing sulfide. As a result, 0.5 to 10 μπι IGF transformation nuclei containing 0.311% by mass or more of 1111 in a complex form of oxides and sulfides can be secured at 10 or more / mm ² . However, the total of Ca, REM and Zr is 0.02 mass. It should be noted that, if added in excess of / ₀ , Mn will not be contained in the sulfide that is composited with the oxide, and the Mn content in the composite particles will be less than 0.3% by mass.

Fig. 1 (c) is a schematic diagram of the HAZ organization when the technology of (1) and the technology of (2) described here are used together. The HAZ structure is refined by the generation of a large amount of IGF in addition to the refinement of GBF and FSP. However, when the amount of alloying components added is inappropriate, the amount of MA generated increases and CT0D characteristics become inadequate. Therefore, it is necessary to stably improve the CT0D characteristics by using the technique (3) described below together.

The means to achieve (3) will be explained. MA formation behavior in HAZ is It is known that it largely depends on hardenability and cooling rate. The hardenability of HAZ in the present invention is greatly affected by the γ grain size and IGF generation ability in addition to the steel component. The tau / particle size and IGF production formation relative hardenability ΗΑΖ the conventional steels are rarely considered due to high IGF generating ability on top present invention steel is low 0 / grain, _y grain boundaries and V The transformation sites of ferrite are increased in the grains, and the hardenability of HAZ is markedly lower than that of conventional steel with the same steel composition. For the steel of the present invention having such features, the cooling rate (welding time from 800 ° C to 500 ° C for about 15 s) during welding of offshore structures and the range of C and Mn of the present invention were changed. As a premise, the effects of alloying components on the formation of MA were studied diligently. As a result, the following two points became clear.

ても Even if Nb is increased from the past, MA amount of HAZ is hard to increase.

非 There is a strong discontinuous correlation between the sum of Cu, Ni, Cr and Mo and the MA content of HAZ.

From the point of view (1), it was found that increasing Nb to 0.05% by mass did not significantly affect the MA amount of HAZ. Nb actually used in conventional steel plates for sea structures (joint CT0D guaranteed steel) is, for example, Proceedings of 12th international Conference on OMAn,, 1993, Glasgow, UK, A SME, Volumein-A, pp. Nb force S upper limit of 0.02 mass% at yield strength of 420MPa class in 207-214 The upper limit is 0.021% by mass of Nb in the yield strength of the class, and in the Proceedings of l ^ th Internationai Conference on 0MAE, 1994, Hous ton, ASME, Volume H, pp. 307-314, the yield strength of 420MPa class is 0. 0 24% by mass of Nb. As described above, the upper limit of the Nb content is about 0.02% by mass, whereas the present invention has an advantage that Nb can be effectively used up to 0.05% by mass. From the viewpoint of ⑤, it was found that when the sum of Cu, Ni, Cr, and Mo exceeded 3.0% by mass, the MA amount of HAZ increased rapidly. Based on the above findings, use Nb as much as possible as a component design when expanding the sheet thickness to, for example, about 76.2 dragons while maintaining the yield strength of 460 MPa or more, especially 500 to 550 MPa class. The guideline is to increase the strength of the base material of the thick material and, on the other hand, to reduce Cu, Ni, Cr and Mo, which promote MA production. Reduction of Cu, Ni, Cr and Mo is also preferable from the viewpoint of alloy cost.

Fig. 1 (d) is a schematic diagram of the HAZ organization when the technology of (3) described here is used in combination with the technology of (1) and (2). By sufficiently reducing the amount of MA in addition to sufficiently miniaturizing the HAZ structure, good joint CT0D characteristics can be achieved even at high strength. Thus, the present invention can be realized by simultaneously exhibiting the techniques (1), (2) and (3). Next, the reasons for limiting the chemical components will be described. In the following description of chemical components, what is described as% means mass%.

C is required to be 0.04% or more to secure the strength and toughness of the base material and HAZ. However, if it exceeds 0.14%, the toughness of the base metal and HAZ will decrease, and the weldability will also deteriorate, so this is the upper limit.

Si can be added for deoxidation. However, if it exceeds 0.4%, HAZ toughness deteriorates. In the present invention, deoxidation is also possible with U, Ti, and Mg, and the smaller the amount of Si, the better from the viewpoint of HAZ toughness. Si is an undesirable element in the present invention because it promotes HAZ MA formation.

Mn must be at least 1.0% to ensure the strength and toughness of the base metal and HAZ. Mn is also important in forming the sulfides that make up the IGF transformation nucleus. However, if Mn exceeds 2.0%, the base material and HAZ become brittle or the weldability deteriorates, so this is the upper limit.

P is an impurity element in the present invention, and a good base material and HAZ material It is necessary to reduce it to 0.02% or less in order to secure

S is an element necessary for the present invention. 0.001% or more must be secured for complex precipitation of sulfide on oxides as IGF transformation nuclei. However, if S exceeds 0.005%, the toughness of the base metal and HAZ deteriorates, so this is the upper limit.

Nb is extremely effective in minimizing deterioration of HAZ toughness and increasing base metal strength. Nb is also effective in increasing the toughness through microstructural refinement of the base metal. For example, in order to achieve a yield strength of 500 MPa class with a thickness of 76.2 mm and to obtain better base metal toughness, 0.005% or more of Nb is essential. However, if Nb exceeds 0.05%, the HAZ toughness deteriorates due to an increase in the amount of MA and precipitation hardening, so this is the upper limit. Nb is an element that should be actively used in producing the base material of the present invention, and it is preferable to effectively use 0.02% or more of Nb.

A1 forms an ultrafine oxide of 0.01 to 0.1 μm together with Mg, and functions as pinning particles with TiN precipitated multiplely on it, and also as transformation nuclei for GBF and FSP And refine the HAZ structure. For that purpose, 0.001% or more is required. A1 can not it to ensure ultrafine oxide necessary number to obtain the less than 0.001% of the composite particle of the TiN 10000 pieces / mm ² or more, gamma comminuted and gamma grain on boundaries of As a result of an insufficient number of transformation nuclei, GBF and FSP are not sufficiently refined, resulting in poor toughness. However, when A1 exceeds 0.01%, the content of A1 in the oxide constituting the IGF transformation nucleus becomes low, and as a reaction, the Mg content in the oxide becomes less than 10% by mass. As a result, Mn-containing sulfide is difficulty no longer deposited on the oxide, loses ability as IGF transformation nuclei, it is difficult to ensure the 10 / mm ² or more I GF transformation nuclei to the stable.

If the number of IGF transformation nuclei is insufficient, HAZ toughness will deteriorate. Therefore, the upper limit of A1 is 0.01%. Ti forms TiN and precipitates on ultra-fine (Mg, A1) oxide with a size of 0.01-0.5 μm, and as pinning particles, and also transformation nuclei of GBF and FSP It functions as a finer HAZ structure. For that purpose, 0.005% or more is required. Ti can not be ensured to become the the TiN particles in the Yo I Do conjugate form 10,000 ZMM ² or more and less than 0.005%, GBF and FSP is HAZ toughness is deteriorated without being sufficiently fine, Si and A1 are both If it is lower than the lower limit, the deoxidizing element may be insufficient, so it is desirable to add 0.01% or more in order to perform the deoxidation of Ti. However, if the Ti content exceeds 0.03%, the base material and HAZ become brittle, such as precipitation of TiC and coarsening of TiN to several μm. For the above reasons, the upper limit of Ti is set at 0.03%.

Mg plays the most important role in the present invention. The primary role of Mg is to form an ultrafine oxide of 0.01 to 0.1 μππ with A1 and to form a pinned particle with TiN that precipitates over it, as well as GBF and FSP It functions as a metamorphic nucleus for HAZs and refines the HAZ structure. The second role of Mg is that when it is contained in an amount of 10% by mass or more in an oxide of 0.5 to 10 μm, it promotes the complex precipitation of Mn-containing sulfide thereon, and functions as an IGF transformation nucleus. To make the HAZ structure finer. To simultaneously fulfill these two roles, 0.0003% or more of Mg is required, and preferably 0.0005% or more. If Mg is less than 0.0003%, the content of Si, Al, Ti, etc. in the oxide increases, and as a reaction, the Mg content in the oxide becomes less than 10% by mass. The contained sulfide becomes difficult to precipitate and loses its ability as an IGF transformation nucleus, resulting in an insufficient number of IGF transformation nuclei. At the same time, it is also difficult to secure the necessary number of ultrafine (Mg, Al) oxides to obtain more than 10,000 TiN composite particles / mm ² . However, even if Mg exceeds 0.005%, its metallurgical effect saturates, so this is the upper limit. O forms an ultrafine (Mg, Al) oxide and plays a role in pinning the HAZ, and at the same time, forms a 0.5- to 10-μm Mg-containing oxide to form an IGF-transformed nucleus in the HAZ. Function. To fulfill these two roles, 0.001% or more of O is necessary. When O is less than 0.001%, ultrafine oxide necessary number to obtain a TiN composite grains child 10000 / mm ² or more and 10 pieces / mm ² or more 0. 5~: ίθ μ m oxide Is difficult to secure. However, if O exceeds 0.005%, a large amount of coarse oxides exceeding 10 μm are generated, and this acts as a starting point of brittle fracture in the base metal and HAZ, so that 0.005% Is the upper limit.

N forms TiN and precipitates on ultra-fine (Mg, Al) oxide with a size of 0.01 to 0.5 μm as pinning particles, as well as transformation nuclei of GBF and FSP. It functions as a finer HAZ structure. For that purpose, 0.001% or more is required. N is 10000 / mm ² or more can not be ensured TiN particles of such complexes form when less than 0.001%. However, if N exceeds 0.01%, solid solution N increases and the base material and HAZ become brittle and the surface properties of the piece deteriorate. Therefore, the upper limit is set. Next, the reasons for limiting the selected elements will be described.

Ca, REM and Zr can be added as deoxidizers and desulfurizers. It contributes to the reduction of O content as a deoxidizer. As a desulfurizing agent, it contributes to the reduction of S content and at the same time controls the form of sulfide. To improve the base metal and HAZ material through these effects, 0.0005% or more is required for each. However, if these elements are too large, they become incorporated into the IGF transformation nucleus, and the Mg and Mn contents in the oxides and sulfides that constitute the IGF transformation nucleus decrease, and the IGF transformation occurs. It loses its core function. In this sense, the upper limits of Ca, REM, and Zr are 0.005%, 0.01%, and 0.01%, respectively, and the sum of these three elements must be limited to 0.02% or less. REM here refers to lanthanide elements such as La and Ce, The above-described effects can be obtained even when a misch metal in which these elements are mixed is added.

Cu, Ni, Cr, and Mo can be used to improve the strength, toughness, and corrosion resistance of the base material. For that purpose, all elements need 0.05% or more. Conventionally, these elements have been actively used when simultaneously increasing the strength and toughness of the base material and increasing the thickness of the base material. It is preferable to reduce as much as possible. In this sense, the upper limits of Cu, Ni, Cr, and Mo are regulated to 1.5%, 3.0%, 0.5%, and 0.5%, respectively. It must be adjusted to be below 0%. If each element exceeds the upper limit or the sum of these elements exceeds 3.0%, the CT0D characteristics of HAZ will be significantly deteriorated.

V is effective for the strength of base metal and HAZ by precipitation strengthening. For this purpose, 0.005% or more is required. However, if V exceeds 0.05%, weldability and HAZ toughness deteriorate, so this is set as the upper limit.

B is effective in improving the strength and toughness of the base material.To achieve this, 0.0001% or more is required.However, if B exceeds 0.003%, the weldability is significantly deteriorated. Upper limit.

The steel of the present invention is manufactured as a steel plate by adjusting the chemical composition to a predetermined chemical composition in the steelmaking process of the steel industry, reheating a continuously manufactured piece, and controlling various processes of rolling, cooling, and heat treatment in various ways. Is done. In order to obtain a yield strength of 460 MPa or more, preferably 500 to 550 MPa class in a thick material such as a plate thickness of 76.2 mm, direct quenching or It is effective to apply accelerated cooling. Furthermore, tempering can adjust strength and toughness.铸 It is also possible to perform hot charge rolling without cooling the piece. HAZ toughness is determined by the dispersion of pinned particles and IGF transformation nuclei in addition to the steel composition. Dispersion of these particles The state does not change significantly during the manufacturing process of the base material. Therefore, the HAZ toughness does not largely depend on the manufacturing process of the base material, and any heating, rolling, or heat treatment process may be applied.

The dispersion state of the inclusions specified in the present invention is quantitatively measured, for example, by the following method.

The number of 0.01 μm TiNs containing oxides consisting of Mg and A1 was determined by preparing an extracted replica sample from an arbitrary location on the base steel sheet and using a transmission electron microscope (TEM). Observe at least 10,000 μm ² or more at an area of at least 1000 μm ² at a magnification of 10000 to 50000 times, measure the number of TiN of the target size, and convert this to the number per unit area (pieces / ² ) . At this time, the (Mg A1) oxide and the TiN are identified by a composition analysis using energy dispersive X-ray spectroscopy (EDS) attached to the TEM and a crystal structure analysis of an electron diffraction image by the TEM. When it is complicated to perform such identification for all the composite inclusions to be measured, the following procedure is simply used. First, the rectangular inclusions are regarded as TiNs, and the number of inclusions inside TiN of the target size is measured. Next, at least 10 or more of the composite precipitated TiNs whose number was measured by such a method were identified in detail as described above, and the ratio of the composite of (Mg, A1) oxide and TiN was determined. Ask. Then, the number of composite precipitated TiN measured first is multiplied by this ratio. When carbides in steel hinder the above TEM observation, heat treatment at 500 ° C or less can agglomerate and coarsen the carbides, facilitating observation of the target composite inclusions.

The number of particles of oxides and Mn-containing sulfide was combined 0. 5 10 β _m can be measured by the following method. First, a small piece sample was cut out from an arbitrary position on the base steel sheet to prepare a mirror-polished sample, and this was observed at a magnification of 1000 times using an optical microscope over an area of at least 3 mm ² or more. Then, the number of particles of the target size is measured, and this is converted into the number per unit area (pieces Z mm ² ). Subsequently, the same sample is subjected to random composition analysis of at least 10 or more particles of the target size using a wavelength-dispersive X-ray spectrometer (WDS) attached to a scanning electron microscope (SEM). . At this time, if Fe of ground iron is detected in the analysis value of the particles, the composition of the particles is obtained by excluding Fe from the analysis values. Of the particles measured in this way, O and S are detected simultaneously, and particles containing 0.3% by mass or more of Mn are considered to be effective as IGF transformation nuclei. Calculate the ratio of IGF transformation nuclei in the particles. Then, first, the number measured by the optical microscope is multiplied by this ratio. For simplicity, element mapping is performed on the above sample, and the number of particles of 0.5 to 10 μπι in which three of O, S, and Mn coexist is measured.

Example

Table 1 shows the chemical composition of the continuously formed steel, and Table 2 shows the thickness of the steel sheet, the manufacturing method, the number of pinning particles and IGF transformation nuclei, the base material, welding conditions, and toughness.

The steel of the present invention has a thickness of 38.1 to 76.2 mm, a base metal yield strength (YS) of 510 to 570 MPa, and a welding heat input of 3.5 to 10.0 kJZ mm. Has a good CT0D of more than 0.2mm at 110 ° C at the joint pound (CGHAZ).

On the other hand, the base steel or HAZ material is inferior at a thickness of 76.2 mm due to the incorrect chemical composition of the comparative steel. Steel 12 has an insufficient number of IGF transformation nuclei due to too little S, resulting in inferior HAZ toughness. Steel 13 has inferior toughness of the base metal and HAZ due to too much S. Steel 14 has inferior strength and toughness due to too little Nb. Steel 15 has inferior HAZ toughness due to too much Nb. Steel 16 has too little A1 and thus has insufficient number of pinned particles, resulting in poor HAZ toughness. Steel 17 had too much M As a result, the number of IGF transformation nuclei is insufficient and HAZ toughness is poor. Steel 18 has too little Ti and therefore has insufficient number of pinned particles, resulting in poor HAZ toughness. Steel 19 has poor toughness of base metal and HAZ due to too much Ti. Steels 20 and 21 each have too little Mg and O, resulting in insufficient HAZ toughness due to insufficient number of pinned particles and IGF transformation nuclei. Steel 22 has too little N and thus has insufficient number of pinning particles, resulting in poor HAZ toughness. Steel 23 is inferior in HAZ toughness because the sum of Cu, NI, Cr and Mo is too large. In steel 24, the sum of Ca, REM and Zr is too large, so the number of IGF transformation nuclei is insufficient and the HAZ toughness is inferior.

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Z "I 200 Ό 200 * 0 ^{: Β} 3 '9" 0 ^{: η} 3' 9 Ό ^: ΪΝ S00 · 0 ΟΟΟ 'Ο 200 · 0 600 · 0 S00 · 0 20 · 0 100 · 0, 00 · 0 69 “ I OS · 0 01 · 0 ιζ

Z "X 200 Ό 200" 0 ^{: Β} 3 '9' 0 ^{: η} 3 '9 Ό ^: ΪΝ 900 ΪΝ εοο · ο 1000 · 0 600 Ό S00' Ο ΖΟ 100 ，, 00 69 Ί 02 · 0 01 · 0 οζ

Z * X 200 Ό ZOO "0 ^{: Ε} 3 '9 ・ 0 ^{: η} 3' 9 Ό ^: ΐΝ 300 Ό εοο · ο 200 Ό εεο · ο S00 Ό ζο · 0 TOO Ό 00 Ό 6S 0 61

Z Ί 200 'Ο 200 "0 ^{: Β} 3' 9 Ό ^{: Η} 3 '9 Ό ^: ΐΝ 900' Ο εοο Ό 200 'Ο εοο · ο S00 · 0 20 · 0 100, 00 · 0 69 · Ι ΟΖ Ό01 Ό 81

Z 'I 200 Ό 200 "0 ^{: Β} 3' 9 Ό ^{: η} α '90 ^: ΐΜ S00 Ό εοο · ο 200" 0 600 · 0 210 · 0 20 100 · 0 00 · 0 69 Ί 02 Ό 01 Ό Ι

Z "I 200 'Ο 200 Ό ^{: Ε} 3'9" 0 ^{: η} 3 '9 ・ 0: ΐΝ 900 · 0 εοο · ο 200 · 0 600 · 0 8000 “0 ΖΟ Ό 100 · 0 ^ οο · 0 · 69 · Ι 02 · 0 01 · 0 91

Z "I 200" 0 200 "0 ^{: Β} 3 '9" 0 ^{: η} 3' 9 "0 ^: 0 S00 'Ο εοο · ο 200 · 0 600 · 0 S00 90 · 0 TOO · 0, 00 · 0 69 Ί OS · 0 οχ · 0 SI

2 Ί 200 Ό 200 "0 ^{: Ε} 3 '9 Ό ^{: η} 3' 9 Ό ^: ΐΝ 900 Ό εοο · ο 200 · 0 600 Ό S00 · 0 εοο TOO Ό 00 · 0 69 ΟΖ ΟΖ · 01 1 · 0 η

2 Ί 200 Ό 200 Ό ^{: Β} 3 '9 "0 ^{: Η} 3' 9 · 0: ΐΝ 900 Ό εοο Ό 200 · 0 600 Ό S00 Ό 20 · 0 900 · 0, 00 Ό 63 ΟΖ · 0 01 · 0 εχ g Ί 200 "0 Ζ00Ζ0 ^{: Β} 3 '9 Ό' · ^η 3 '9 · ^: ΐΝ S00 · 0 εοο · ο 200 · 0 600 · 0 S00 · 0 ΖΟΖΟ0 800000 00 Ό6S“ X 02 01 · 0 ζι

0, 00 刚刚 · (): ■ ^ 00 · 0 100 · ο ΡΟΟ'Ο 900 · 0 900 · 0 0 · 0, 00 · 0 300 "0 01" 1 9Ζ · 0 εχ · ο II

0, 00 · 0 100 '0: 3'00'0 ^: Μ 00 · 0 100 · ο, 000 · 0 900 · 0 900 Ό ΨΟ Ό 00 · 0 S00 · 0 01 Ί 9Ζ · 0 εχ · ο 01

0 εοο Ό IOO '0: ^J Z' 200 · 0 = ^Ε 0 00 '0 100 · ο 00 · 0 800 · 0 800' 0 0 · 0, 00 · 0 S00 · 0 09 Ί9Ζ · 0 II · 0 6

0 0 00 0 0 100 o 900 0 900 0 S00 00 09 S3 0 II 8

Z Ί zoo · 0 ZOO = ^Ε 3 '9 · 0: ^η 3' 9 · 0: ΐΝ S00 · 0 εοο'ο 200 Ό 600 · 0 S00 · 0 20 · 0 100 · 0, 00 · 0 6S Ί 02 · 0 01 0 Ζ

8 0 000 '0 Ζ000 Β0: ^Β 3' fO- ^ d 'fo-, 00 0 00 00 ο 200 0 600 0 0 εοο Ό ZQ 0 0 200 0 0 SIO 0 0 12 Ί 80 01 0 9

9 "I 0 90: ^η 3 '00: ΐΝ εοο · ο 200 · ο εοο · ο 0X0 Ό εοο · ο εο · ο 200 · 0 00 · 0 88 Ί 60 · 0 80 · 0 9

Z Ί 0 9 · 0: ^Η 3 '9 "0 ^: ΐΝ 300 · 0 εοο Ό 200' 0 910 Ό 200 · 0 0 · 0 εοο'ο εοο'ο SS -I 90 · 0 60 · 0 f ε Ί 0 90: ^η 3 'Ζ0: ΐΝ εοο · ο 200 · 0 εοο · ο SIO · 0 εοο · ο 80 · 0 εοο Ό 800 · 0 39 Ί 80 · 0 SO · 0 ε

ΟΊ 0 τθ0: Α 'S-0 ^{: n} 3' 5: 0: ΐΝ zoo · 0 00 · 0 εοοο'ο ΖΙΟΌ εοο · ο εο · ο ZOO Ό εοο · ο S3 Ί 20

6.00 I -0: ° W '0: ^Η 3''0: ΐΝ 800 · 0 00' 0 zoo · 0 9Ζ0 · 0 300 · 0 20 · 0 εοο · ο 900 · 0 8 · Ι εο · ο 01 Ό ι

^J Z + thigh

+ ΐΝ + ^η 3 + Ε3 Ν 0 ΐΧ τν qN S d TS 3 m

(% ssBra)

Book 00

1) DQ: Direct quenching, ACC: Accelerated cooling, T: Tempering, CR: Controlled rolling

2) 0.010.5μm TiN containing oxide composed of Mg and A1 force

3) 0.5-10 μm particles containing 0.3% by mass or more of Mn in the form of a complex of oxide and sulfide

4) YS, TS, vTrs are tested at the center of the plate thickness, RAZ is the average of three

5) Multi-pass welding with submerged arc welding method

6) Conforms to BS7448, no PWHT, displays the lowest value of 3 wires, CGHAZ is an abbreviation for Coarse Grain HAZ

I A fatigue notch was made on the melting line on the groove side.

INDUSTRIAL APPLICABILITY The present invention has significantly improved the CTOD characteristics of joints of high-strength and extremely-thick steel plates, and has opened the way to lighter and larger offshore structures. As a result, the cost of building offshore structures can be significantly reduced, and energy development in deeper sea areas becomes possible.

Claims

1. In mass%,

C: 0.04-0.14%,

Si: 0.4% or less,

Mn: 1.0-2.0%,

One

P: 0.02% or less,

S: 0.001-0.005%,

A1: 0.001-0.01%, Ti: 0.005-0.03%,

Nb: 0.005 to 0.05%, box

Mg: 0.0003-0.005%,

O: 0.001-0.005%,

N: 0.001-0.01%

Containing the balance has a chemical composition consisting of iron and unavoidable impurities, 0 · 01~0.5 μ m of TiN that containing the oxides of Mg and A1 are present 10000 / mm ² or more and, 460MPa or less excellent in CT0D characteristics of weld heat affected zone characterized by the presence of at least 10mm 0.5 to 10mm particles containing 0.3% by mass or more of Mn in the form of oxide and sulfide composite Steel plate with high yield strength.

2. In mass%,

Ca: 0.0005-0.005%,

REM: 0.0005-0.01%,

Zr: 0.0005-0.01%,

2. The yield strength of 46 OMPa or more, which is excellent in CT0D characteristics of weld heat affected zone toughness according to claim 1, wherein the sum of Ca, REM, and Zr is 0.02% or less. Steel plate with.

3. In mass%,

Cu: 0.05 ~: L 5%,

Ni: 0.05-3.0%,

Cr: 0.05-0.5%,

Mo: 0.05-5%,

V 0.005-0.05%,

B: 0.0001-0.003%

3. The yield of 460MPa or more excellent in CT0D characteristic of weld heat affected zone toughness according to claim 1 or 2, wherein the sum of Cu, Ni, Cr and Mo is 3.0% or less. Steel plate with strength.