TWI500774B - High-strength steel sheet for high-heat input welding for high-strength steel sheet and its manufacturing method - Google Patents

Description

High-strength steel plate for high heat energy input welding excellent in brittle crack propagation stop characteristics and manufacturing method thereof

The present invention relates to a high-strength thick steel plate for high heat input welding and a method for producing the same, which is excellent for brittle crack arrestability, and particularly relates to a ship which is suitable for use in ships A steel plate having a plate thickness of 50 mm or more.

In large structures such as ships, accidents caused by brittle fracture have a great economic and environmental impact. Therefore, it has been demanded that safety must be improved, and the steel to be used is particularly required for toughness and brittle crack propagation stop characteristics at the time of use temperature.

For ships such as container ships or bulk carriers, the outer plate of ship's hull is made of high-strength thick steel. In recent years, with the increase in the size of the hull, the high-strength thick steel plate has progressed. In general, the brittle crack propagation characteristics of the steel plate are more deteriorated as the steel plate is strengthened or thickened. Tendency The requirements for the brittle crack propagation stop characteristics are also more advanced.

As a technical solution for improving the brittle crack propagation stop characteristic of steel, a conventionally known method is a method of increasing the Ni content, which is used in a commercial tank of a liquefied natural gas (LNG: Liquefied Natural Gas) storage tank. A steel with a Ni content of 9%.

However, an increase in the amount of Ni will inevitably lead to a substantial increase in cost, and thus it is difficult to apply to applications other than LNG storage tanks.

On the other hand, for thinner steels with a thickness of less than 50 mm used for ships or pipelines that have not yet reached the level of LNG, the TMCP method (Thermo-Mechanical Control) is used. Process; thermal mechanism control method) to achieve fine granulation, using the method of improving low temperature toughness, to bring excellent brittle crack propagation characteristics of steel.

Further, the steel material of the technical solution proposed in Patent Document 1 is intended to improve the brittle crack propagation stop characteristic without increasing the alloy cost, and therefore the microstructure of the surface layer portion of the sheet thickness is ultra-fine grained steel.

The steel material disclosed in Patent Document 1 focuses on the shearing lip (shear-lips) which occurs in the surface layer portion of the steel sheet when the brittle crack propagates, and has the characteristic of improving the brittle crack propagation stop. The effect is to refine the crystal grains of the shear lip portion so as to absorb the propagation energy having the propagation of the brittle crack as a characteristic steel material having excellent brittle crack propagation stop characteristics.

Further, as disclosed in Patent Document 1, the thickness of the surface layer portion is cooled to below the _Ar3 transformation point by controlled cooling after hot rolling, and then the controlled cooling is stopped, and the thickness is controlled. The surface portion is reheated (recuperate) to a step above the metamorphic point, and is repeated one or more times, and during this period, the steel material is subjected to rolling, which is repeatedly deformed or formed into a process for recrystallization, thereby being in a plate thickness. The surface layer portion forms an ultrafine ferrite structure or a bainite structure.

Further, Patent Document 2 discloses that in order to enhance the brittle crack propagation stop characteristic in a steel material having a fine structure mainly composed of ferrite iron-pearlite, the two surface portions of the steel material have 50%. The above-described layer of the ferrite-grained iron structure has a circle-equivalent average grain size of 5 μm or less and an aspect ratio of the grains of 2 It is important that the above ferrite iron particles and the size difference of the ferrite iron particle size are suppressed. The method for suppressing the difference in size is to suppress the local recrystallization phenomenon by setting the maximum rolling rate (maximum rolling reduction) of each rolling in the final refining rolling to 12% or less.

However, in the steel materials excellent in the brittle crack propagation stop characteristics disclosed in Patent Documents 1 and 2, only the surface layer portion of the steel sheet thickness is temporarily cooled, and then reheated, and processed by reheating. In order to obtain a specific organization, it is not easy to control if it is in the actual production scale. Especially thick steel with a thickness of more than 50mm In the case of a material, it is a process that has a large load on the rolling and cooling equipment.

On the other hand, the technique disclosed in Patent Document 3 focuses not only on the miniaturization of the ferrite grains, but also on the subgrains formed in the ferrite grains, which are used to enhance brittle cracks. A technique extended by the TMCP method of propagation stop characteristics.

Specifically, for a steel plate having a thickness of 30 to 40 mm, it is not necessary to perform complicated temperature control such as cooling and reheating of the surface layer of the steel sheet, but (A) ensuring rolling of fine ferrite iron crystal grains. (b) Rolling conditions for producing a fine ferrite iron structure in a portion of 5% or more of the steel sheet thickness, and (c) promoting the development of fine texture iron in the fine grain iron, and processing (roller) The dislocation introduced by rolling is reconfigured by thermal energy to form a rolling condition of the secondary particles, and (d) the particle size of the fine ferrite particles and the fine subparticles can be suppressed. The coarsened cooling conditions are used to enhance the brittle crack propagation stop characteristics.

Further, it is also known that there is a method of improving the brittle crack propagation stop characteristics by rolling the ferrite iron after the deformation has been carried out in the process of controlling the rolling. In this method, the separation surface of the steel is caused to cause the separation to be generated in a direction parallel to the surface of the plate to relax the stress at the front end of the brittle fracture, thereby improving the resistance to brittle fracture.

For example, the technique disclosed in Patent Document 4 uses X-ray plane intensity ratio in the (110) plane showing a texture developing by controlling rolling. The degree is set to 2 or more, and the coarse particles having a diameter equivalent to a circle in the crystal grains of 20 μm or more are made 10% or less, thereby improving the brittle fracture resistance.

The technique disclosed in Patent Document 5 is a steel for welded structure excellent in brittle crack propagation stop performance of the joint portion, and the X-ray surface intensity ratio of the (100) plane in the rolled surface inside the thickness is 1.5 or more. The steel sheet characterized by the use is an offset of the angle between the stress load direction and the crack propagation direction due to the development of the aggregate structure, and excellent brittle crack propagation stop characteristics can be obtained.

[Previous Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent Publication No. 7-201814

Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-256375

Patent Document 3: Japanese Patent No. 3467767

Patent Document 4: Japanese Patent No. 3548349

Patent Document 5: Japanese Patent No. 2659661

Patent Document 6: Japanese Patent No. 3546308

[Non-patent literature]

Non-Patent Document 1: "Long and Brittle Crack Propagation in High-Thickness Shipbuilding Steel" published by "Inoue" and others, Japan Ship Sea Proceedings of the Foreign Society Society Lecture No. 3, 2006, pp359~362.

However, the recent large container ships with more than 6,000 TEU (Twenty-foot Equivalent Unit) use thick steel plates with a thickness of more than 50 mm. In Non-Patent Document 1, the brittle crack propagation stop performance of a steel sheet having a thickness of 65 mm was evaluated, and the result of the brittle cracking was not stopped based on the large brittle crack propagation stop test of the base material.

In addition, in the standard ESSO test (ESSO test compliant with WES 3003) of the test material, it is shown that the Kca value at the use temperature of -10 ° C (hereinafter, sometimes referred to as Kca (-10 ° C)) is not As a result of reaching 3000 N/mm ^3/2 , this result implies that safety is ensured in the case where a hull structure of a steel sheet having a thickness of more than 50 mm is applied.

The steel sheets having excellent brittle crack propagation stop characteristics disclosed in the above Patent Documents 1 to 5 are judged from the production conditions and the experimental data disclosed, and are mainly steel sheets having a thickness of about 50 mm. When the techniques disclosed in Patent Documents 1 to 5 are applied to a thick steel material having a thickness of more than 50 mm, it is not clear whether or not a predetermined characteristic can be obtained, and the crack propagation characteristics for the thickness direction of the hull structure are required. , also did not verify at all.

On the other hand, as the steel plate increases in thickness, it is welded. High-efficiency high-heat input welding such as submerged arc welding, electrogas arc welding, and electroslag welding is applied during construction. In general, if the welding input heat is increased, the structure of the Heat Affected Zone (HAZ) becomes coarse, and therefore the toughness of the welded heat affected zone is lowered, which is well known. In order to solve the problem of reduced toughness due to high heat input welding, a steel material for high heat input welding has been developed and has reached a practical stage. For example, the technique disclosed in Patent Document 6 prevents the coarsening of the microstructure of the welded heat-affected zone by controlling the TiN crystallized in the steel, and promotes the crystal by dispersing the ferrite iron to form a core. The metamorphosis of the ferrite iron in the grain can thereby make the weld heat affected zone highly tough. However, although the weld heat affected zone of the high heat energy input welded portion has excellent toughness, it does not take into consideration the brittle crack propagation stop characteristic, and thus it is not possible to obtain a steel material which can simultaneously satisfy both characteristics.

Accordingly, it is an object of the present invention to provide for the optimization of the steel composition and the rolling conditions, as long as the brittleness which can be stably produced can be stably produced by an industrially extremely simple process which can control the aggregate structure in the thickness direction. A high-strength steel sheet for high heat energy input welding excellent in crack propagation stop characteristics and a method for producing the same.

The inventors of the present invention have been working hard to achieve the above-mentioned technical problems, and finally aim at: A high-strength thick steel plate having excellent brittle crack propagation characteristics, and the following original findings were obtained.

1. For the thick steel plate having a thickness of more than 50 mm, the result of the fracture of the standard ESSO test was investigated in detail, and in the case of the fractured shape as shown in Fig. 1 (b), As the width of the brittle crack is reduced, the stress expansion coefficient at the tip end portion of the crack is reduced, and as a result, the crack propagation stop performance of the steel sheet is increased. Fig. 1 (a) and (b) are schematic views showing that the crack 3 penetrating from the groove 2 of the standard ESSO test piece 1 is stopped in the base material 5 by the front end shape 4 to prevent crack propagation. example of.

2. In order to obtain such a fractured cross-sectional shape, it is necessary to improve the crack propagation stop performance of the surface layer portion and the center portion of the plate thickness. As a method for improving the crack propagation stop performance of the plate thickness surface portion and the center portion of the plate thickness, it is effective to increase the toughness of the plate thickness surface portion and the center portion of the plate thickness. However, in the case of such a thick steel plate having a thickness of more than 50 mm, it is limited in various aspects of the cooling rate and the rolling rate, and there is a limit to the toughness in the center portion of the thickness.

3. In addition to improving toughness, as a means of improving the performance of crack propagation, it is effective to control the assembly of the central portion of the thickness of the plate, especially in the case of "the (110) surface is stacked in parallel with the rolling direction. It is effective to control the aggregate structure in such a manner that the crack progressing in the rolling direction or the sheet width direction is shifted from the rolling direction or the sheet width direction to the oblique direction.

4. In addition, by placing the central portion of the plate thickness at the recrystallization temperature of the Worthite iron The cumulative rolling ratio in the state of the domain is set to 20% or more, and the average rolling ratio of each rolling is set to 5.0% or less, thereby miniaturizing the structure of the surface layer portion of the thickness. Then, the cumulative rolling ratio in the state where the center portion of the sheet thickness is in the unrecrystallized temperature range of the Worthite iron is set to 40% or more, and the average rolling ratio of each rolling is set to 7.0% or more. Thereby, the toughness and the aggregate structure of the center portion of the sheet thickness can be made more developed, and thus the above-described organization can be realized.

5. As a method of improving the toughness of the high heat input welding portion, the composite sulfide of TiN, CaS and MnS is finely split to suppress the grain growth at the time of exposure to high temperature due to welding, and thereafter During the cooling process, it is effective to promote intragranular metamorphism to fine-tune the heat-affected zone structure at room temperature.

The present invention has been developed based on the above-mentioned findings and further reviewed. That is, the present application is composed of the following six inventions:

A high-strength steel sheet for high-heat energy input welding excellent in brittle crack propagation stop characteristics, characterized in that the steel composition is C% 0.03 to 0.15%, Si: 0.50% or less, and Mn: 1.0~2.0%, P: 0.030% or less, S: 0.0005~0.0040%, Al: 0.005~0.10%, Ti: 0.004~0.030%, N: 0.0036~0.0075% or less, Ca: 0.0005~0.0030%, O: 0.0040 % or less, and the contents of Ca, O, and S are in accordance with the following formula (1), and the rest are Fe and inevitable impurities. The metal structure is mainly composed of ferrite and iron, and has a surface layer of the thickness of the sheet. The density of the RD//(110) plane is 1.3 or more. The density of the RD//(110) plane at the center of the plate thickness is 1.8 or more, and the ductile-brittle transition temperature vTrs is -50 ° C or less at the thickness of the surface layer portion and the center portion of the plate thickness, 0 < (Ca-(0.18+130×Ca)×O)/1.25/S≦0.8. . . Number (1)

In the formula (1), Ca, O, and S are all calculated in terms of content (% by mass).

2. The high-strength steel sheet for high-heat energy input welding which is excellent in the brittle crack propagation stop characteristic according to the first item, wherein the ductile-brittle transition temperature and the RD/ in the center portion of the thickness portion and the center portion of the sheet thickness are RD/ The density of the /(110) plane is a relationship that satisfies the following equation (2): vTrs _{(surface layer)} +1.9 × VTrs _(1/2t) -6 × I _{RD//(110) [surface]} -84 × I _{RD //(110)[1/2t]} ≦-350. . . Number (2)

In the formula (2), vTrs _{(surface layer)} is the ductile-brittle transition temperature (°C) of the summer impact test of the thickness of the surface layer; vTrs _(1/2t) is the summer impact of the central portion of the thickness (t/2) Test the ductile-brittle transition temperature (°C); I _{RD//(110) [surface]} is the density of the RD//(110) plane of the surface layer of the plate thickness; I _{RD//(110)[1/2t]} is the plate The density of the RD//(110) plane in the thick central portion (t/2); t is the sheet thickness (mm).

3. The high-strength steel sheet for high heat energy input welding excellent in brittle crack propagation stop characteristics according to item 1 or 2, wherein the steel composition is classified into The amount % includes, from Nb: 0.003 to 0.050%, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, W: 0.4% or less, and V: 0.2% or less. B: One or two or more selected from 0.0003 to 0.0030% or less.

4. The high-strength steel sheet for high-heat energy input welding which is excellent in the brittle crack propagation stop characteristic according to the first or second item or the third item, wherein the steel composition is in mass%, and further contains from Mg: 0.0005. ~0.0050%, Zr: 0.001 to 0.020%, and REM: 0.001 to 0.020%, one or more selected from the above.

A method for producing a high-strength steel sheet for high-heat energy input welding which is excellent in brittle crack propagation stop characteristics, characterized in that the steel material having the composition component as described in the first or third item or the fourth item is When it is heated to a temperature of 900 to 1150 ° C, the total value of the cumulative rolling ratio in the recrystallization temperature range of the Worthite iron and the non-recrystallization temperature range of the Worthite iron is set to 65% or more, and is at the center of the plate thickness. In the state of the re-crystallization temperature range of the Worthite iron, the cumulative rolling ratio is 20% or more, and the average rolling rate per roll is 5.0% or less, and then at the center of the plate thickness. In the state where the Vostian iron is not recrystallized, the cumulative rolling ratio is 40% or more, and the average rolling rate per roll is 7.0% or more, and then 4.0 ° C / s. The above cooling rate is cooled to 600 ° C or lower.

6. The method for producing a high-strength steel sheet for high-heat energy input welding which is excellent in the brittle crack propagation stop characteristic according to the item 5, wherein after the cooling reaches 600 ° C or lower, the tempering reaches a temperature lower than the A _C1 point. The tempering process.

According to the present invention, the aggregate structure is appropriately controlled in the thickness direction, and a high-strength steel sheet for high heat energy input welding excellent in brittle crack propagation stop characteristics and a method for producing the same can be obtained. The present invention is applied to a steel sheet having a thickness of 50 mm or more, preferably a thickness of more than 50 mm, more preferably a thickness of 55 mm or more, and more preferably a thickness of 60 mm or more, which is a steel of the prior art and can be used more. Significant superiority, so it is very effective. Moreover, for example, in the field of shipbuilding, by applying the present invention to a hatch coaming or deck member in a strong deck structure on a large container ship or a bulk carrier, it is possible to contribute to the safety of the ship. The industry is extremely useful.

1‧‧‧Standard ESSO test piece

2‧‧‧ groove

3‧‧‧ cracking

4‧‧‧ front end shape

5‧‧‧Material

Fig. 1 is a schematic cross-sectional view showing a standard ESSO test of a thick steel plate having a thickness of more than 50 mm, and Fig. 1 (a) is a view showing the test piece from the plane side; Figure (b) is a broken section showing the test piece.

In the present invention, it is formulated: 1. steel composition; 2. toughness and aggregate structure of the surface layer portion and the central portion; 3. metal structure; and 4. manufacturing conditions.

Steel composition

Preferred chemical compositions in the present invention will be explained below. In the description, % means mass%.

C: 0.03~0.15%

C is an element for increasing the strength of the steel. In the present invention, in order to secure the desired strength, the necessary content is 0.03% or more. On the other hand, when it exceeds 0.15%, not only the weldability will deteriorate, but also the toughness will be adversely affected. Therefore, the C content is selected to be 0.03 to 0.15%. It is preferably 0.05 to 0.15%.

Si: 0.50% or less

Si is a strengthening element that can be effectively used as a deoxidizing element and steel. However, if the content is less than 0.01%, sometimes it will not have its effect. On the other hand, if it exceeds 0.50%, not only the surface properties of steel but also the toughness will be deteriorated. Therefore, the amount of addition is selected to be 0.50% or less. It is preferably in the range of 0.01 to 0.40%.

Mn: 1.0~2.0%

Mn is added as a strengthening element. If it is less than 1.0%, the effect is not sufficient. On the other hand, if it exceeds 2.0%, the weldability of the welded portion may be deteriorated. Therefore, the Mn content is selected to be 1.0 to 2.0%. It is preferably in the range of 1.1 to 1.8%.

P: 0.030% or less

P is an impurity that is inevitably mixed, and if it exceeds 0.030%, the toughness of the base material and the welded portion may be deteriorated. Therefore, the upper limit is selected at 0.030%.

S: 0.0005~0.0040%

In order to produce the desired CaS or MnS, the S content must be 0.0005% or more. On the other hand, if it exceeds 0.0040%, the toughness of the base material will be deteriorated. Therefore, the S content is selected to be 0.0005 to 0.0040%.

Al: 0.005~0.10%

Al has a function as a deoxidizer, and its content must be 0.005% or more. However, if the content exceeds 0.10%, the toughness is deteriorated, and the toughness of the welded metal portion is also deteriorated after the welding. Therefore, the Al content is limited to the range of 0.005 to 0.10%. It is preferably 0.005 to 0.08%, more preferably 0.02 to 0.06%.

Ti: 0.004~0.030%

By adding Ti in a small amount to form a nitride, a carbide, or a carbonitride, it is possible to suppress the coarsening of the Worth iron in the heat affected portion of the weld, and/or to promote the fat as a ferrite nucleus. Iron is deformed, whereby the effect of finening the crystal grains to improve the toughness of the base material can be obtained. This effect can be obtained by addition of 0.004% or more. However, the content exceeds When 0.030% is used, the toughness of the base material and the heat affected zone of the weld deteriorates due to the coarsening of the TiN particles. Therefore, the Ti content is selected in the range of 0.004 to 0.030%. It is preferably in the range of 0.006 to 0.028%.

N: 0.0036~0.0075%

N is an element necessary to ensure the necessary amount of TiN. If the content is less than 0.0036%, a sufficient amount of TiN cannot be obtained, and the toughness of the welded portion is deteriorated. If the content exceeds 0.0075%, TiN will re-dissolve when subjected to the thermal cycle of soldering, and excessive solid solution N will result in significant deterioration of toughness. Therefore, the N content is selected to be 0.0036 to 0.0075%. It is preferably in the range of 0.0037 to 0.0068%.

Ca: 0.0005~0.0030%

The Ca element has an effect of improving toughness by fixing S. In order to exert this effect, the Ca content must be 0.0005% or more. However, if the content exceeds 0.0030%, the effect tends to be saturated. Therefore, in the present invention, the Ca content is limited to the range of 0.0005 to 0.0030%.

O: 0.0040% or less

O is contained in steel as an unavoidable impurity, and when it solidifies, oxides are formed to crystallize, which deteriorates the cleanliness of the steel. Therefore, in the present invention, it is preferred to minimize the O content. In particular, if the O content exceeds 0.0040%, the CaO-based inclusions become coarser and the base material is made. The toughness deteriorates. Therefore, the O content is selected to be 0.0040% or less.

In the present invention, the relationship of the following formula (1) must be satisfied.

0<(Ca-(0.18+130×Ca)×O)/1.25/S≦0.8. . . Number (1)

In the formula (1), Ca, O, and S are calculated in terms of content (% by mass). The contents of Ca, O, and S must conform to the relationship of the formula (1). In this case, it becomes a form of a composite sulfide in which MnS is crystallized on CaS. Such a composite sulfide has a function as a ferrite nucleus of the ferrite, and the microstructure of the welded heat-affected zone is made fine to improve the toughness of the welded heat-affected zone. When the value of (Ca-(0.18+130×Ca)×O)/1.25/S is 0 or less, CaS does not crystallize, so S is crystallized in the form of MnS alone. When the steel sheet is produced by the rolling, the MnS is elongated by the rolling, and the toughness of the base material is deteriorated. In the welding heat affected portion of the present invention, MnS is melted, so that fine dispersion cannot be achieved. On the other hand, if the value of (Ca-(0.18+130×Ca)×O)/1.25/S exceeds 0.8, almost all of S is fixed by Ca, and MnS which acts as a core for the formation of ferrite iron is not It will crystallize on CaS, so that sufficient toughness improvement effect cannot be achieved. Therefore, a preferable range of the value of (Ca - (0.18 + 130 × Ca) × O) / 1.25 / S is 0.10 - 0.8%.

What has been described above is a preferred basic component in the present invention. In order to further improve the characteristics, one or more of Nb, Cu, Ni, Cr, Mo, W, V, and B may be contained.

Nb: 0.003~0.050%

Nb will form NbC which will crystallize out when the ferrite is metamorphosed or reheated, which is helpful for high strength. In addition, it also has an effect of expanding the non-recrystallization temperature range when rolling the Worthfield iron field, and contributes to fine graining of the ferrite iron and the toughened iron, and therefore has an effect on improving the toughness. This effect is exhibited when the Nb content is 0.003% or more. Therefore, when Nb is contained, the content is preferably 0.003% or more. However, if the amount of addition exceeds 0.050%, the coarse NbC will crystallize, which will cause the toughness to deteriorate. Therefore, if Nb is contained, the upper limit is preferably set to 0.050%. More preferably, it is in the range of 0.005 to 0.040%.

Cu, Ni, Cr, Mo, W

Cu, Ni, Cr, Mo, and W are all elements which can improve the quench hardenability of steel. It is directly helpful in improving the strength after rolling, and can be added to enhance functions such as toughness, high temperature strength, or weather resistance. These effects are required to be contained in an amount of 0.01% or more. Therefore, if it is desired to be contained, it is preferably set to 0.01% or more. However, if it is excessively contained, the toughness and weldability may be deteriorated. Therefore, if it is desired to contain, the upper limit is set to Cu: 0.5%, Ni: 1.0%, Cr: 0.5%, Mo: 0.5%, W: 0.4% is appropriate.

V: 0.2% or less

V is an element which can form V (C, N) and has a precipitation strengthening effect, and can increase the strength of steel. If it is desired to exhibit such an effect, it may be 0.001% or more. However, if the content exceeds 0.2%, it will make The toughness deteriorates. Therefore, when it is desired to contain V, the content is preferably 0.2% or less, more preferably 0.001 to 0.10%.

B: 0.0003~0.0030%

B is an element which enhances the quench hardenability of steel as long as it is traced. In order to exert this effect, it is only necessary to contain 0.0003% or more. However, when the content exceeds 0.0030%, the toughness of the welded portion is deteriorated. When B is to be contained, the content is preferably made 0.0030% or less. More preferably, it is in the range of 0.0003 to 0.0026%.

In the present invention, in order to further improve the characteristics, one or more of Mg, Zr, and REM (rare earth metal) may be contained in the above composition.

Mg: 0.0005~0.0050%

Mg is an element which has an effect of improving the inertness by dispersion of an oxide, and therefore can be added in accordance with the demand. This effect must be exhibited at a content of 0.0005% or more. Therefore, if it is desired to be contained, the content is preferably 0.0005% or more. However, if the content exceeds 0.0050%, the effect is saturated. Therefore, if it is desired to add Mg, it is preferable to set the addition amount to 0.0005 to 0.0050%.

Zr: 0.001~0.020%

Zr forms an oxide in steel, and by using the dispersion of the oxide, it has the effect of improving the toughness by finening the structure of the heat affected portion of the weld. Even if Zr is added, the effect of the present invention is not impaired, so that it can be added in accordance with the demand. This effect is effective as long as the content is 0.001% or more, and therefore it is preferably contained in an amount of 0.001% or more. However, if it is excessively added, the effect tends to be saturated, and coarse inclusions are formed, resulting in deterioration of the toughness of the base material. Therefore, if it is desired to add, the upper limit of the amount of addition is preferably set to 0.020%.

REM (rare earth metal): 0.001~0.020%

REM (rare earth metal) forms an oxide in steel, and the dispersion of the oxide has the effect of refining the structure of the heat-affected zone and improving the toughness, and even if it is added, the effect of the present invention is not impaired, so that it can be blended. Need to add. This effect is exhibited as long as it contains 0.001% or more. Therefore, if it is desired to be added, it is preferably contained in an amount of 0.001% or more. However, if it is excessively added, the effect tends to be saturated, and coarse inclusions are formed to deteriorate the toughness of the base material. Therefore, if it is desired to add, the upper limit of the amount of addition is preferably set to 0.020%.

In the present invention, since Ca is crystallized as CaS, it is necessary to reduce the amount of O which has a strong binding force with Ca before adding Ca, and the residual oxygen content before adding Ca is controlled. Below 0.0050% is appropriate. The method of reducing the residual oxygen content may be a method of enhancing degassing or a method of introducing a deoxidizing agent.

The remainder other than the above components is Fe and unavoidable impurities.

2. Resilience and collective organization of the surface layer and the central part of the plate thickness

In the present invention, in order to obtain the "cracking progress in the horizontal direction (the in-plane direction of the steel sheet) in the direction of the rolling direction or the direction perpendicular to the rolling as shown in Fig. 1, the crack propagation stop can be promoted. The fracture profile of the characteristic is an appropriate definition of the toughness and the density of the RD//(110) plane in the surface layer portion and the center portion of the sheet thickness.

First, the condition that the toughness of the base material is good is used to suppress the progress of the crack before the conditions are raised. In the steel sheet of the present invention, the toughness at the thickness of the surface layer portion and the center portion is set to be -50 ° C or lower in the summer impact test brittle-brittle transition temperature vTrs in the thickness portion and the thickness portion.

In addition, by making the aggregate structure of the RD//(110) plane more developed, the split surface is concentrated obliquely with respect to the main direction of the crack, and the brittle crack front end is relaxed by causing fine crack splitting. The effect of the stress, in this way, can improve the brittle crack propagation stop performance. In order to ensure the structural safety, it is used to obtain Kca (-10 ° C) ≧ 6000 N/mm ³ as the target for the hull plate of the nearest container ship and bulk carrier. ^When the brittle crack propagation stop performance of ^/2 is required, the density of the RD//(110) plane in the surface layer portion of the sheet thickness must be set to 1.3 or more, preferably 1.6 or more; and RD/ at the center portion of the sheet thickness. The density of the /(110) plane is set to 1.8 or more, preferably 2.0 or more. Therefore, in the present invention, the density of the RD//(110) plane in the thickness portion is set to 1.3 or more, preferably 1.6 or more; and RD//(110) at the center of the thickness. The density of the surface is set to 1.8 or more, preferably 2.0 or more.

On the other hand, if the density of the RD//(110) plane in the thickness of the surface layer portion or the center portion of the sheet thickness exceeds 4.0, the aggregate structure is excessively developed, so that fine cracking and splitting and brittle cracking do not occur. It will be significantly different, and it is difficult to use the effect of mitigating the brittle fracture front end to exert brittle crack propagation stop performance. Therefore, it is preferable to set the density of the RD//(110) plane of the thickness of the surface layer portion and the density of the RD//(110) plane at the center portion of the thickness of the sheet to 4.0 or less.

The density of the RD//(110) plane in the surface layer portion or the center portion of the sheet thickness referred to herein is calculated according to the following method. First, a sample having a thickness of 1 mm was taken from the thickness of the surface layer portion or the center portion of the plate thickness, and mechanical polishing and electrolytic polishing were performed on the surface parallel to the plate surface to prepare a test piece for X-ray diffraction. Further, in the case of the surface layer portion of the plate thickness, a test piece in which the surface closest to the surface is ground is used. Using this test piece, X-ray diffraction measurement was performed using an X-ray diffraction device using a Mo light source, and positive electrode maps of (200), (110), and (211) were obtained, and the obtained positive dot map was obtained. The calculation was performed according to the Bunge method to obtain a 3-dimensional crystal orientation density function. Next, from the obtained 3-dimensional crystal azimuthal density function, in the manner of Bunge In the total of 19 cross-sectional views of the 0° interval of =0° to 90°, the value of the 3-dimensional crystal azimuth density function in which the (110) plane is parallel to the rolling direction is integrated with respect to the rolling direction. After the integral value is divided by the number of the positions after the above integration, the obtained value is referred to as the density of the RD//(110) plane.

In addition to the above-mentioned base material toughness and the regulation of the aggregate structure, the density of the ductile-brittle transition temperature and the density of the RD//(110) plane in the surface layer portion and the center portion of the sheet thickness are in accordance with the following formula ( 2) The relationship is appropriate.

vTrs _{(surface layer)} +1.9 × VTrs _(1/2t) -6 × I _{RD / / (110) [surface]} -84 × I _{RD / / (110) [1/2t]} ≦ -350. . . Number (2)

In the formula (2), vTrs _{(surface layer)} : the summer 丕 impact test of the thickness of the surface layer of the ductile-brittle transition temperature (°C); vTrs _(1/2t) : the summer 丕 impact test of the central portion of the plate thickness (°C); I _{RD//(110) [surface]} : the density of the RD//(110) plane of the surface layer of the thickness; I _{RD//(110)[1/2t]} : RD at the center of the thickness // (110) surface density; t is the plate thickness (mm).

By satisfying the relationship of the above formula (2), more excellent brittle crack propagation stop performance can be obtained.

3. Metal structure

In the present invention, the metal structure is a metal structure mainly composed of ferrite iron. Here, in the present invention, the metal structure is a metal structure mainly composed of ferrite iron, and means that the area percentage of the ferrite iron phase accounts for 60% or more of the total. The rest, toughened iron, Ma Tian loose iron (including: island-like Ma Tiansan Iron), Bora, etc., if the total area percentage is 40% or less, it can be allowed.

In the case where the metal structure mainly composed of the ferrite-grained iron is obtained by the rolling conditions at the time of rolling in the usual Worthite iron field, the desired toughness can be obtained, but the desired toughness can be obtained. After rolling, there is still sufficient metamorphic time when the Worth iron is metamorphosed into fertilized iron. Therefore, the aggregate structure obtained is very messy, and RD// cannot be achieved in the surface layer of the target thickness. 110) The density of the surface is 1.3 or more, preferably 1.6 or more; and the density of the RD//(110) plane at the central portion of the thickness is 1.8 or more, more preferably 2.0 or more. Therefore, the rolling conditions are adjusted by the method described later, and even for the structure mainly composed of the ferrite-grained iron, it is possible to obtain an intensive RD//(110) plane in the surface layer portion of the sheet thickness. The degree is 1.3 or more, preferably 1.6 or more; and the density of the RD//(110) plane in the central portion of the thickness is 1.8 or more, preferably 2.0 or more.

4. Manufacturing conditions

Preferred manufacturing conditions of the present invention will be described below.

Regarding the manufacturing conditions, it is preferable to specify the heating temperature, the hot rolling conditions, the cooling conditions, and the like of the steel material. In particular, for hot rolling, in addition to the cumulative rolling rate in the Worstian iron recrystallization temperature range and the Worthite iron non-recrystallization temperature range, it is again targeted at: the central portion of the plate thickness at Voss The case of the field recrystallization temperature in the field, and the case where the Worthite iron is not recrystallized, specifying the respective cumulative rolling rates and each The average rolling rate of the road rolling is suitable. By specifying these conditions, it is possible to obtain desired characteristics for the thickness of the thick steel sheet and the density of the ductile-brittle transition temperature vTrs and the RD//(110) plane in the center portion of the thickness of the plate.

First, a molten steel having the above composition is melted by a device such as a converter, and a steel material (steel blank) is produced by a method such as continuous casting.

Next, it is preferable to heat the steel material to a temperature of 900 to 1150 ° C before hot rolling. In order to obtain good toughness, it is effective to set the heating temperature to a lower temperature and to reduce the crystal grain size before rolling. However, if the heating temperature is less than 900 ° C, the time for rolling in the recrystallization temperature range of the Worthite iron cannot be sufficiently ensured. In addition, if it exceeds 1150 ° C, not only will the Worthfield iron particles be coarsened, but the toughness will be deteriorated, the oxidation loss will become conspicuous, and the product yield will be lowered. Therefore, it is preferable to set the heating temperature at 900 to 1150 °C. Based on the viewpoint of toughness, a better heating temperature range is 1000 to 1100 °C.

In general, when a metal structure mainly composed of ferrite iron is obtained by rolling in a usual Worthite iron field, although desired toughness can be obtained, after rolling, from Worthite Iron At the time of metamorphosis into fertilized iron, there is still sufficient metamorphic time, and therefore, the acquired tissue is messy. Therefore, according to the rolling of the usual Worthite iron field, it is impossible to achieve the density of the RD//(110) surface of the thickness portion of the present invention of 1.3 or more, preferably 1.6 or more; The density of the RD//(110) plane in the central part is 1.8 or more, preferably 2.0. The above goals. Therefore, in the present invention, it is preferable to specify the hot rolling conditions in the following manner, and thus, even in the structure mainly composed of the ferrite iron, it is possible to obtain RD//(110 in the surface layer portion of the plate thickness. The density of the surface is 1.3 or more, preferably 1.6 or more; and the density of the RD//(110) plane at the central portion of the thickness is 1.8 or more, preferably 2.0 or more.

The hot rolling condition is first performed in a state where the center portion of the sheet thickness is in the recrystallization temperature range of the Worthite iron, and the cumulative rolling ratio is 20% or more, and the average rolling ratio per roll is 5.0% or less. Rolling is preferred. By setting the cumulative rolling ratio to 20% or more, the Worthite iron is finely granulated, and finally the obtained metal structure is finely granulated, and the toughness can be improved. Further, by setting the average rolling ratio of each of the rolling in this temperature range to 5.0% or less, the processing deformation can be introduced into the steel material, particularly in the vicinity of the surface layer of the steel material. In this way, the density of the RD//(110) surface of the surface layer portion of the sheet thickness can be made 1.3 or more, more preferably 1.6 or more, and the surface layer portion of the thickness of the sheet can be further granulated, and the surface layer portion of the sheet thickness can be obtained. The effect of resilience.

Next, in a state where the temperature in the central portion of the thickness is in the unrecrystallized temperature range of the Worthite iron, the cumulative rolling ratio is 40% or more, and the average rolling ratio per roll is set to 7.0% or more. Rolling is preferred. By setting the cumulative rolling ratio in this temperature range to 40% or more, the aggregate structure at the center portion of the sheet thickness can be sufficiently developed, and the average rolling ratio of each roll is set to 7.0. In the above method, the density of the RD//(110) plane at the center portion of the sheet thickness can be made 1.8 or more, and more preferably 2.0 or more.

Further, it is preferable to set the cumulative rolling ratio of the entire Worstian iron recrystallization temperature range and the Worthite iron non-recrystallization temperature range to 65% or more. By setting the overall cumulative rolling ratio to 65% or more, it is possible to secure a sufficient amount of rolling for the structure, and it is possible to achieve a target value of toughness and density.

In the Vostian iron recrystallization temperature range and the Worthite iron recrystallization temperature range, it is possible to carry out preliminary experiments for changing the heat history and processing history of various conditions for the steel having the composition. grasp.

Further, the temperature at the end of hot rolling is not particularly limited. Based on the viewpoint of rolling efficiency, it is preferable to finish hot rolling in the non-recrystallization temperature range of the Worthfield iron.

The steel sheet after the completion of the rolling is preferably cooled to 600 ° C or lower at a cooling rate of 4.0 ° C / s or more. By setting the cooling rate to 4.0 ° C/s or more, the structure is not coarsened, and a fine grain structure can be obtained, and excellent desired properties can be obtained. If the cooling rate is less than 4.0 ° C / s, the structure becomes coarse and the toughness of the desired target value cannot be obtained. By setting the cooling stop temperature to 600 ° C or lower, the progress of recrystallization can be avoided, and the desired aggregate structure obtained by hot rolling and subsequent cooling can be maintained. When the cooling stop temperature is higher than 600 ° C, recrystallization will continue after the cooling is stopped, and the desired aggregate structure cannot be obtained. Further, these cooling rates and cooling stop temperatures are used to calculate the temperature at the center portion of the thickness of the steel sheet. The temperature at the center of the plate thickness is based on the thickness of the plate, the surface temperature, and the cooling conditions. It is intended to be calculated by means of other methods. For example, the temperature of the center portion of the thickness of the steel sheet is obtained by calculating the temperature distribution in the thickness direction by using the difference method.

The tempering treatment may be performed on the steel sheet after the cooling is completed. By performing tempering treatment, the toughness of the steel sheet can be further improved. The tempering temperature is set by the average temperature of the steel sheet, and by performing the tempering treatment at a temperature equal to or lower than the point A _C1 , the desired structure obtained by rolling and cooling can be prevented. In the present invention, the A _C1 point (°C) is obtained from the following formula.

A _C1 point=751-26.6C+17.6Si-11.6Mn-169Al-23Cu-23Ni+24.1Cr+22.5Mo+233Nb-39.7V-5.7Ti-895B

In the above formula, each element symbol is calculated based on the content (% by mass) in steel, and if it is not contained, it is calculated as 0.

The average temperature of the steel sheet is also obtained by analog calculation or the like based on the thickness, the surface temperature, the cooling conditions, and the like, similarly to the temperature at the center portion of the thickness.

[Example 1]

The molten steel (steel marks A to R) having various compositions as shown in Table 1 was melted by a converter, and a steel material (a steel blank having a thickness of 250 mm) was formed by a continuous casting method, and the thickness was formed by hot rolling. After the steel plate of 50 to 80 mm was cooled, the test steel of No. 1 to 30 was obtained. For some of them, tempering is performed after cooling. The hot rolling conditions, the cooling conditions, and the tempering conditions are shown in Table 2.

For the thick steel plate to be obtained, a test piece of Japanese Industrial Standard JIS14A of Φ14 was taken from the 1/4 portion of the plate thickness, and the longitudinal direction of the test piece was formed at a right angle to the rolling direction. A test piece was taken, and a tensile test was performed, and the fall strength (YS) and the tensile strength (TS) were measured.

After the mirror-polished section of the obtained thick steel plate in the thickness direction parallel to the longitudinal direction of the rolling was mirror-polished, the metal structure was exposed by etching and observed by an optical microscope.

In addition, in order to evaluate the toughness value, the thickness of the surface layer portion and the center portion of the sheet thickness (in the following description, sometimes the center portion of the sheet thickness is marked as 1/2t) is taken from the Japanese industrial standard JIS4. The test piece was taken in such a manner that the longitudinal direction of the test piece was parallel to the rolling direction, and then the summer 丕 撃 test was performed to determine the ductile-brittle transition temperature (vTrs) of the summer 丕 impact test. Here, the punching test piece of the surface layer portion of the plate thickness is a surface which is closest to the surface and is selected to have a depth of 1 mm from the surface of the steel sheet.

Next, in order to evaluate the brittle fracture propagation stop characteristics, a standard ESSO test (temperature slope type ESSO test) was performed to determine the Kca value (Kca (-10 ° C)) at -10 °C.

In order to evaluate the high heat input welding characteristics, the test steel plate was subjected to planing (planing angle of 20°), and a commercially available low-temperature steel gas arc welding electrode was used, and high heat input welding was used (300~). Gas-electric welding (EGW) of 750 kJ/cm) was used to make a joint (welded portion). Then, for the surface and the back surface of the plate thickness direction 1mm At the position, the Xia Rang test piece in which the groove was formed in the welded portion was used, and the absorbed energy at a test temperature of -20 ° C: vE - 20 ° C (average value of three test pieces) was determined as the HAZ toughness value.

Further, the density of the RD//(110) plane in the thickness of the surface layer portion and the center portion of the sheet thickness was obtained in the following manner. First, a sample having a thickness of 1 mm was taken from the surface portion of the thickness of the plate or the center portion of the thickness of the plate, and a test piece for X-ray diffraction was prepared by mechanical polishing and electrolytic polishing on a surface parallel to the plate surface. Further, in the case of the thickness of the surface layer portion, the surface closest to the surface is polished. Using this test piece, an X-ray diffraction measurement was performed using an X-ray diffraction apparatus using a Mo light source, and positive electrode maps of (200), (110), and (211) were obtained, from the obtained positive dot map, The three-dimensional crystal orientation density function was obtained by calculation using the Bunge method. Secondly, from the obtained 3 dimensional crystal azimuth density function, in the Bunge mark method In a total of 19 cross-sectional views of the 5° interval of =0° to 90°, the value of the 3-dimensional crystal azimuth density function in which the (110) plane is parallel to the rolling direction is integrated to obtain an integral value. Then, by dividing this integral value by the number of orientations of the aforementioned integrals, the obtained value is taken as the density of the RD//(110) plane.

These test results are shown in Table 3. The toughness value (summer impact test ductile-brittle transition temperature vTrs) of the thickness of the surface layer portion and the center portion of the sheet thickness and the case where the aggregate structure is a test steel sheet (manufacturing method No. 1 to 13) falling within the scope of the present invention Kca (-10 ° C) is 6000 N/mm ^3/2 or more, and exhibits excellent brittle crack propagation stop performance. Further, the absorbed energy of the welded heat-affected zone: vE-20 ° C is 125 J or more, and also shows an excellent numerical value. In addition, the test steel plate (manufactured by the relationship between the surface layer portion and the center portion of the thickness of the sheet at the center of the steel layer (the summer 丕 impact test ductile-brittle transition temperature) and the RD//(110) density is in accordance with the formula (2). Method Nos. 1 to 13) Comparison with the test steel sheet (manufacturing method No. 14) which does not satisfy the relationship of the formula (2) shows that a high Kca (-10 ° C) value can be obtained. In addition, the metal structures for the test steel sheets (manufacturing method Nos. 1 to 14) are mainly made of ferrite iron.

On the other hand, the composition or manufacturing conditions fall outside the scope of the present invention, and the toughness or aggregate structure of the steel sheet does not conform to the provisions of the present invention for the test steel sheet (manufacturing method No. 15 to 30), and its Kca (-10 ° C) The value is less than 6000 N/mm ^3/2 , and/or the absorbed energy of the welded heat-affected zone: vE-20 °C is less than 100 J, so it is shown to be inferior compared with the example of the present invention.

Claims (6)

A high-strength steel sheet for high-heat energy input welding excellent in brittle crack propagation stop characteristics, characterized in that the steel composition is C% 0.03 to 0.15%, Si: 0.50% or less, and Mn: 1.0 to 2.0 in terms of mass%. %, P: 0.030% or less, S: 0.0005 to 0.0040%, Al: 0.005 to 0.10%, Ti: 0.004 to 0.030%, N: 0.0036 to 0.0075% or less, Ca: 0.0005 to 0.0030%, and O: 0.0040% or less. Moreover, the contents of Ca, O, and S are in accordance with the following formula (1), and the rest are Fe and unavoidable impurities. The metal structure is mainly composed of ferrite and iron, and has RD/ in the surface layer of the plate thickness. The density of the / (110) plane is 1.3 or more, and the density of the RD//(110) plane at the center of the thickness is 1.8 or more, and the thickness of the surface layer and the center of the thickness are The impact test has a ductile-brittle transition temperature vTrs of -50 ° C or less, 0 < (Ca - (0.18 + 130 × Ca) × O) / 1.25 / S ≦ 0.8. . . In the formula (1), Ca, O, and S are calculated in terms of content (% by mass). A high-strength steel sheet for high-heat energy input welding having excellent brittle crack propagation stop characteristics as described in the first aspect of the patent application, wherein the ductile layer transition portion and the center portion of the sheet thickness are subjected to a ductile-brittle transition temperature And the density of the RD//(110) plane is in accordance with the following equation (2): vTrs _{(surface layer)} + 1.9 × vTrs _(1/2t) -6 × I _{RD / / (110) [surface] -} 84 ×I _{RD//(110)[1/2t]} ≦-350. . . Equation (2) In the formula (2), vTrs _{(surface layer)} : summer impact test of the thickness of the surface layer of the ductile-brittle transition temperature (°C); vTrs _(1/2t) : central part of the plate thickness (t/2 The summer 丕 impact test ductile-brittle transition temperature (°C); I _{RD//(110) [surface]} : the density of the RD//(110) plane of the surface layer of the thickness; I _{RD//(110)[1 /2t]} : the density of the RD//(110) plane at the center of the plate thickness (t/2); t is the plate thickness (mm). A high-strength steel sheet for high-heat energy input welding excellent in brittle crack propagation stop characteristics as described in the first or second aspect of the patent application, wherein the steel composition is in a mass %, and further contains Nb: 0.003 to 0.050%. , Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, W: 0.4% or less, V: 0.2% or less, B: 0.0003 to 0.0030%, among the selected ones Kind or more than two. A high-strength steel sheet for high-heat energy input welding which is excellent in brittle crack propagation stop characteristics as described in the first or second or third aspect of the patent application, wherein the steel composition is in mass % and contains from Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.020%, and REM (rare earth metal): 0.001 to 0.020%, one or more selected from the group consisting of 0.001 to 0.020%. A method for producing a high-strength steel sheet for high-heat energy input welding excellent in brittle crack propagation stop characteristics, characterized in that a steel material having a composition as described in claim 1, 3 or 4 is heated to 900 to 1150 ° C The temperature is set to be 65% or more in the total rolling rate of the Vostian iron recrystallization temperature range and the Worthite iron non-recrystallization temperature range, and is in the Vostian iron recrystallization temperature range at the center of the plate thickness. In the state, the cumulative rolling ratio is 20% or more, and the average rolling ratio of each rolling is 5.0% or less. After the treatment, the roller having a cumulative rolling ratio of 40% or more and an average rolling ratio of 7.0% or more per rolling is performed in a state where the center portion of the thickness of the sheet is in the unrecrystallized temperature range of the Worthite iron. The rolling treatment is followed by cooling at a cooling rate of 4.0 ° C / s or more to 600 ° C or lower. The method for producing a high-strength steel sheet for high-heat energy input welding which is excellent in the brittle crack propagation stop characteristic according to the fifth aspect of the invention, wherein after the cooling reaches 600 ° C or lower, the tempering reaches the point below the A _C1 point. The tempering process of temperature.