JP6477743B2 - High-strength ultra-thick steel plate excellent in brittle crack propagation stopping characteristics and weld heat-affected zone toughness and method for producing the same - Google Patents

Description

  The present invention is used for large structures such as ships, marine structures, low-temperature storage tanks, construction / civil engineering structures, etc., and has high strength excellent in brittle crack propagation stopping characteristics with a thickness of 70 mm or more and weld heat affected zone toughness. The present invention relates to an extra-thick steel plate and a manufacturing method thereof.

  In large structures such as ships, marine structures, low-temperature storage tanks, and construction / civil engineering structures, if an accident associated with brittle fracture occurs, the impact on the socio-economic environment and the environment is large. For this reason, the large structure is always required to be improved in safety, and the steel sheet used as the material of the large structure is required to have a high level of brittle crack propagation stopping characteristics at the use temperature.

  In a ship such as a container ship or a bulk carrier, a high-strength thick steel plate is used as a hull outer plate because of its structure. Recently, with the increase in size of the hull, further increase in strength has been demanded, and the thick steel plates used as materials have been increasing in thickness.

  Generally, the brittle crack propagation stopping property of a steel sheet tends to deteriorate as the strength increases or the wall becomes thicker. For this reason, the request | requirement for the brittle crack propagation stop characteristic with respect to the thick steel plate used for a large-sized structure is further advanced.

  Here, as a means for improving the brittle crack propagation stopping characteristic of a steel sheet, a method for increasing the Ni content in the steel has been conventionally known. For example, in liquefied natural gas (LNG) storage tanks, 9% Ni steel is used on a commercial scale.

  However, an increase in the Ni content in the steel necessitates a significant increase in production costs. For this reason, 9% Ni steel is difficult to apply to uses other than the LNG storage tank.

  On the other hand, for a comparatively thin steel plate with a thickness of less than 50 mm used for ships and line pipes, which does not reach extremely low temperatures such as LNG, the fine graining is attempted by the TMCP method. By improving toughness, excellent brittle crack propagation stopping characteristics can be realized.

  Further, in order to improve brittle crack propagation stopping characteristics without increasing the alloy cost, Patent Document 1 proposes a steel sheet in which the structure of the surface layer portion is made ultrafine.

In the steel sheet excellent in brittle crack propagation stopping characteristics described in Patent Document 1, when a brittle crack propagates, shear lip (plastic deformation region) generated in the steel sheet surface layer portion is effective in improving the brittle crack propagation stopping characteristics. It was completed by paying attention to certain things, and is characterized by absorbing the propagation energy of the propagating brittle crack by refining the crystal grains of the shear lip. Patent Document 1 includes a step of cooling the surface layer portion below the Ar ₃ transformation point by controlled cooling after hot rolling, and then stopping the controlled cooling to reheat the surface layer portion above the Ar ₃ transformation point. It is described that by repeatedly reducing the steel sheet by repeatedly rolling the steel sheet repeatedly or repeatedly, a superfine ferrite structure or a bainite structure is generated in the surface layer part by causing repetitive transformation or processing recrystallization. .

  In Patent Document 2, in a steel sheet mainly composed of ferrite-pearlite, in order to improve brittle crack propagation stopping characteristics, both surface portions of the steel sheet have a circle-equivalent grain size of 5 μm or less and an aspect ratio: It is important to suppress the ferrite grain size variation while forming a ferrite structure having two or more ferrite grains in a layer having an area ratio of 50% or more, and one pass during finish rolling as a method of suppressing this variation. It is described that the local recrystallization phenomenon is suppressed by setting the maximum rolling reduction per hit to 12% or less.

Patent Document 3 discloses a technique on the extension of TMCP that improves brittle crack propagation stopping characteristics by focusing on subgrains formed in ferrite crystal grains as well as refinement of ferrite crystal grains. Is described. Specifically, in a steel sheet having a thickness of 30 to 40 mm, without requiring complicated temperature control such as cooling and recuperation of the steel sheet surface layer,
(A) rolling conditions to ensure fine ferrite crystal grains,
(B) rolling conditions for producing a fine ferrite structure in a portion of 5% or more of the steel plate thickness;
(C) Rolling conditions for developing a texture in fine ferrite and rearranging dislocations introduced by processing (rolling) by thermal energy to form subgrains; and (d) Fine ferrite grains formed and fine Cooling conditions to suppress coarsening of subgrain grains,
Describes a technique for improving the brittle crack propagation stopping characteristics.

  In addition, in controlled rolling, a method of improving the brittle crack propagation stopping property by developing a texture by reducing the transformed ferrite is also known. This is a method of increasing resistance to brittle fracture by causing separation on the fracture surface of the steel plate in a direction parallel to the plate surface and relaxing stress at the tip of the brittle crack.

  For example, Patent Document 4 discloses that the (110) plane X-ray intensity ratio is 2 or more by controlled rolling, and the area ratio of coarse grains having a circle-equivalent diameter of 20 μm or more is 10% or less. It is described to improve.

  In Patent Document 5, as a steel for welded structure having excellent brittle crack propagation stopping characteristics of the joint portion, the X-ray plane strength ratio of the (100) plane in the rolled surface within the plate thickness is 1.5 or more. A characteristic steel sheet is disclosed, and it is described that it has excellent brittle crack propagation stopping characteristics due to a shift in the angle between the stress load direction due to the texture development and the crack propagation direction.

  Further, in Patent Documents 6 and 7, a brittle crack propagation that develops a texture in each part in the sheet thickness direction (sheet thickness 1/4 position, sheet thickness central part, etc.) by defining an average reduction ratio in controlled rolling. A method for manufacturing a welded steel sheet with excellent stopping properties is described.

  Further, in a large container ship exceeding the recent 6,000 TEU, a steel plate having a plate thickness of 70 mm or more is used. In Non-Patent Document 1, a brittle crack propagation stopping property of a steel sheet having a thickness of 65 mm is evaluated, and a result of a brittle crack not stopping in a large brittle crack propagation stopping test of a base material is reported.

Japanese Patent Publication No. 7-100814 JP 2002-256375 A Japanese Patent No. 3467767 Japanese Patent No. 3548349 Japanese Patent No. 2659661 Japanese Patent No. 5733425

Propagation behavior of long brittle cracks in thick shipbuilding steel, Proceedings of the Japan Society of Marine Science and Technology No. 3, 2006, pp 359-362

  In the techniques described in Patent Literatures 1 and 2, only the surface layer portion of the steel sheet is once cooled and then reheated, and a specific structure is obtained by applying processing during reheat. For this reason, control on an actual production scale is not easy, and particularly in the manufacture of thick materials having a plate thickness of 70 mm or more, this is a process with a heavy load on rolling equipment and cooling equipment.

  Moreover, as for the steel plate described in patent documents 1-6, all are from the manufacturing conditions and the experimental data currently disclosed, and plate | board thickness: About 50 mm-70 mm is main object, Comprising: To the thick material of 70 mm or more As for the application, it is unclear whether a predetermined characteristic can be obtained, and the crack propagation characteristic in the plate thickness direction required for a large structure has not been verified at all.

Furthermore, in Non-Patent Document 1, the ESSO test of the specimen shows a result that the Kca value at the use temperature of −10 ° C. is less than 3000 N / mm ^1.5 . In the case of a large structure to which a steel plate having a thickness exceeding 50 mm is applied, it is suggested that safety cannot be sufficiently ensured.

  Further, structural steel materials used in the field of shipbuilding and the like are finished into a desired shape structure by welding. That is, high-efficiency large heat input welding such as submerged arc welding, electrogas welding, and electroslag welding is applied to a thick material having a thickness of 70 mm or more for welding joining. For this reason, when it joins by heavy heat input welding with respect to thick materials, such as plate | board thickness 70mm or more, it is also required to be excellent in the toughness of a welding heat affected zone. In any of the patent documents, the toughness of the weld heat-affected zone of the thick-walled material has not been sufficiently studied.

  In view of such circumstances, an object of the present invention is to provide a high-strength extra-thick steel plate having a plate thickness of 70 mm or more and excellent in brittle crack propagation stopping characteristics and weld heat-affected zone toughness, and a method for producing the same.

  In order to solve the above-mentioned problems, the inventors have provided a high-strength extra-thick steel plate having excellent brittle crack propagation stopping characteristics and weld heat-affected zone toughness even at a thickness of 70 mm or more, and a production method for stably obtaining the steel plate. Researched earnestly. As a result, the (211) plane integration degree on the rolled surface at the center of the plate thickness is 1.2 or more, and the (200) plane integration degree on the rolled surface on the steel sheet surface (sometimes simply referred to as “surface”) is Charpy fracture surface transition temperature vTrs at a ¼ thickness position, which is a toughness index, having a texture of 1.7 or higher is −40 ° C. or lower, and the Charpy fracture surface transition temperature vTrs on the steel sheet surface is −80 ° C. or lower. Therefore, it was found that excellent brittle crack propagation stopping characteristics can be obtained. Furthermore, in the welding heat affected zone, TiN, CaS and MnS complex sulfides are finely dispersed to suppress grain growth when exposed to high temperatures during welding, and in the subsequent cooling process, It was discovered that excellent weld heat affected zone toughness can be obtained by promoting transformation and refining the weld heat affected zone structure at room temperature.

The present invention has been completed by further studying the above knowledge. The gist of the present invention is as follows.
[1] By mass%, C: 0.03 to 0.15%, Si: 0.50% or less, Mn: 0.5 to 2.2%, P: 0.030% or less, S: 0.0005 -0.0040%, Ti: 0.005-0.030%, Al: 0.005-0.080%, N: 0.0035-0.0075%, Ca: 0.0005-0.0030%, O: 0.0040% or less, Ceq defined by the following formula (1) is 0.36 or more, and each content of Ca, O, S satisfies the following formula (2), The composition of the balance consisting of Fe and inevitable impurities, the (211) plane integration degree at the rolling surface at the center of the sheet thickness is 1.2 or more, and the (200) plane integration degree at the rolling surface on the steel sheet surface is 1 The texture has a texture which is 7 or more, and the Charpy fracture surface transition temperature vTrs at the thickness 1/4 position is −4. ° C. or less, high-strength ultra-thick steel sheet excellent in the Charpy fracture appearance transition temperature vTrs is stopped thickness 70mm or more brittle crack propagation is -80 ° C. or less characteristics and weld heat-affected zone toughness in the steel sheet surface.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
Here, C, Mn, Cu, Ni, Cr, Mo and V in the formula (1) mean the content (% by mass) of each element, and 0 when not contained.
0 <{(Ca− (0.18 + 130 × Ca) × O) /1.25} /S≦0.8 (2)
However, Ca, O, and S represent the content of each component (mass%).
[2] The brittle crack propagation stop according to [1], wherein the component composition further includes one or two of B: 0.0003 to 0.0030% and V: 0.2% or less in mass%. High-strength ultra-thick steel plate with excellent properties and weld heat-affected zone toughness.
[3] The component composition is further mass%, Nb: 0.003 to 0.030%, Ni: 1.0% or less, Cu: 1.0% or less, Cr: 0.7% or less, Mo: 0.7% or less, Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.020%, REM: 0.001 to 0.020%, or one or more of them [1 ] Or a high-strength extra heavy steel plate excellent in brittle crack propagation stopping characteristics and weld heat-affected zone toughness according to [2].
[4] After heating the steel material having the component composition according to any one of [1] to [3] to a temperature of 1000 to 1200 ° C., the temperature at the center of the plate thickness is the cumulative reduction in the austenite recrystallization temperature range. The rate is 10% or more, the temperature at the center of the plate thickness is 50% or more at the austenite non-recrystallization temperature range, the surface temperature of the plate thickness is below the Ar ₃ transformation point, and the temperature at the center of the plate thickness is above the Ar ₃ transformation point. After carrying out hot rolling under the condition that the cumulative rolling reduction in the temperature range exceeds 20%, the sheet thickness is cooled to a cooling stop temperature of 500 ° C. or lower at a cooling rate of 0.5 ° C./s or higher. A method for producing a high-strength ultra-thick steel plate having excellent brittle crack propagation stopping characteristics and weld heat-affected zone toughness.
[5] After cooling to a cooling stop temperature of 500 ° C. or lower, tempering to a temperature at which the center of the plate thickness is less than the Ac ₁ transformation point, the brittle crack propagation stop characteristic and weld heat affected zone toughness according to [4] An excellent method for producing high-strength ultra-thick steel plates.

  According to the present invention, since the texture in the plate thickness direction is appropriately controlled, even a very thick steel plate having a plate thickness of 70 mm or more is excellent in brittle crack propagation stopping characteristics and also in weld heat affected zone toughness. Excellent and high strength. Moreover, according to this invention, a high-strength extra heavy steel plate can be manufactured stably by an industrially very simple process by optimizing rolling conditions. For example, by applying the high-strength extra heavy steel plate of the present invention to a container ship in the shipbuilding field, a deck member joined to hatch side combing in a strong deck structure of a bulk carrier, it contributes to improving ship safety, It is extremely useful in industry.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<Ingredient composition>
Hereinafter, each component will be described. In addition, “%” representing the content of a component means “% by mass”.

C: 0.03-0.15%
The lower limit of C content is 0.03% in order to obtain the strength required for structural steel. Moreover, in order to suppress the production amount of island martensite, the upper limit is made 0.15%.

Si: 0.50% or less When Si exceeds 0.50%, the toughness of the heat affected zone is deteriorated. For this reason, the upper limit is made 0.50%.

Mn: 0.5 to 2.2%
Mn is required to be 0.5% or more in order to ensure the strength of the base material. If it exceeds 2.2%, the toughness of the welded portion is deteriorated. Preferably, it is in the range of 1.0% to 2.0%.

P: 0.030% or less P is an impurity that is inevitably mixed. When P exceeds 0.030%, the toughness of the base material and the welded portion is lowered. For this reason, the upper limit is made 0.030%.

S: 0.0005 to 0.0040%
S is required to be 0.0005% or more in order to produce required CaS or MnS, and if it exceeds 0.0040%, the toughness of the base material is deteriorated.

Ti: 0.005-0.030%
Ti precipitates as TiN during solidification and contributes to high toughness by suppressing austenite coarsening in the weld heat affected zone and becoming a ferrite transformation nucleus. If less than 0.005%, the effect is small, and if it exceeds 0.030%, the expected effect cannot be obtained due to the coarsening of TiN particles. Therefore, the Ti content is in the range of 0.005 to 0.030%.

Al: 0.005-0.080%
Al is required to be 0.005% or more, preferably 0.01% or more in terms of deoxidation of steel. On the other hand, if the content exceeds 0.080%, the toughness of the base metal is lowered and at the same time the toughness of the weld metal is deteriorated.

N: 0.0035 to 0.0075%
N is an element necessary for securing the necessary amount of TiN, and if it is less than 0.0035%, a sufficient amount of TiN cannot be obtained. On the other hand, if it exceeds 0.0075%, the toughness is remarkably lowered due to an increase in the amount of dissolved N in the region where TiN is dissolved by the welding heat cycle.

Ca: 0.0005 to 0.0030%
Ca is an element having an effect of improving toughness by fixing S. In order to exert such effects, it is preferable to contain at least 0.0005% or more. However, the effect is saturated even if the content exceeds 0.0030%. For this reason, in this invention, it limits to the range of 0.0005%-0.0030%.

O: 0.0040% or less O precipitates as an oxide during solidification. When it contains exceeding 0.0040%, the toughness of a base material and a welding heat affected zone will fall.

0 <{(Ca− (0.18 + 130 × Ca) × O) /1.25} /S≦0.8 (2)
However, Ca, O, and S represent the content of each component (mass%).
Ca, O, and S must be contained so as to satisfy the relationship of 0 <{(Ca− (0.18 + 130 × Ca) × O) /1.25} /S≦0.8. In this case, it becomes the form of the composite sulfide in which MnS is deposited on CaS. When the value of {(Ca− (0.18 + 130 × Ca) × O) /1.25} / S exceeds 0.8, S is almost fixed by Ca, and MnS acting as ferrite nuclei precipitates on CaS. Therefore, the toughness of the weld heat affected zone cannot be ensured.

  The above is the basic component composition of the present invention, and the balance is Fe and inevitable impurities.

  In the present invention, in order to further improve the strength toughness and weld heat affected zone toughness of the steel sheet, one or two of B and V may be contained in addition to the above component composition.

B: 0.0003 to 0.0030%
B is an element that generates BN in the weld heat affected zone to reduce the solid solution N and to act as a ferrite transformation nucleus. In order to obtain such an effect, 0.0003% or more is necessary. On the other hand, if it exceeds 0.0.0030%, the hardenability increases and the toughness deteriorates.

V: 0.2% or less V works as an improvement in the strength and toughness of the base material and as a ferrite formation nucleus as VN. However, if it exceeds 0.2%, the toughness is reduced.

  Furthermore, in this invention, in order to improve a characteristic, you may contain 1 type, or 2 or more types of Nb, Ni, Cu, Cr, Mo, Mg, Zr, and REM.

Nb: 0.003 to 0.030%
Nb is an element effective for ensuring the strength and toughness of the base material and the strength of the joint. However, if it is less than 0.003%, the effect is small. On the other hand, if the content exceeds 0.030%, the toughness deteriorates by forming island martensite in the weld heat affected zone.

Ni: 1.0% or less Ni increases the strength while maintaining the high toughness of the base material. However, even if it exceeds 1.0%, the effect is saturated, so this content is made the upper limit.

Cu: 1.0% or less Cu has a function similar to that of Ni. However, if it exceeds 1.0%, hot brittleness is generated, and the surface properties of the steel sheet are deteriorated.

Cr: 0.7% or less Cr is an element effective for increasing the strength of the base material. However, if added in a large amount, the toughness is adversely affected, so the upper limit is made 0.7%.

Mo: 0.7% or less Mo is an element effective for increasing the strength of the base material. However, if added in a large amount, the toughness is adversely affected, so the upper limit is made 0.7%.

Mg: 0.0005 to 0.0050%
Mg is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such effects, it is preferable to contain at least 0.0005% or more. However, even if the content exceeds 0.0050%, the effect is saturated.

Zr: 0.001 to 0.020%
Zr is an element having an effect of improving toughness due to dispersion of oxides. In order to exhibit such an effect, it is preferable to contain at least 0.001% or more. However, the effect is saturated even if the content exceeds 0.020%.

REM: 0.001-0.020%
REM is an element having an effect of improving toughness due to dispersion of oxides. In order to exhibit such an effect, it is preferable to contain at least 0.001% or more. However, the effect is saturated even if the content exceeds 0.020%.

Ceq: 0.36 or higher In addition to having each component composition in the above-mentioned range of content, the high-strength extra heavy steel sheet of the present invention adjusts Ceq represented by the following formula (1) to 0.36 or higher. If Ceq is less than 0.36, it is difficult to increase the degree of (211) plane integration at the rolling surface at the center of the plate thickness, and it is difficult to ensure the strength of the extra-thick steel plate. In addition, it is preferable that Ceq is 0.40 or less from the point of ensuring the welded portion characteristics.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
C, Mn, Cu, Ni, Cr, Mo and V in the formula (1) mean the content (% by mass) of each element, and 0 when not contained.

<Group organization>
The high-strength ultra-thick steel sheet of the present invention has a (211) plane integration degree of 1.2 or more at the rolled surface at the center of the plate thickness, and the (200) plane at the rolled surface on the surface (range from the extreme surface to 1 mm below the surface). It has a texture with a degree of integration of 1.7 or more. More preferably, the (200) plane integration degree on the rolled surface on the surface (from the extreme surface to 1 mm below the surface) should satisfy 2.0 or more. By adopting the above component composition and controlling the texture to satisfy the above range under the manufacturing conditions described later, a high-strength heavy steel plate excellent in brittle crack propagation stopping characteristics can be obtained.

  As described above, in the present invention, even if the plate thickness is 70 mm or more by controlling the component composition and texture, the high-strength extra-thick steel plate according to the present invention has strength, weld heat affected zone toughness, and brittle crack propagation stopping characteristics. It has the effect of being excellent in.

<Manufacturing method>
Molten steel having the above component composition is melted in a converter or the like, made into a steel material (slab) by continuous casting or the like, heated to 1000 to 1200 ° C., and then hot-rolled.

  If the heating temperature is less than 1000 ° C., sufficient time for rolling in the austenite recrystallization temperature region cannot be secured. On the other hand, when the heating temperature is higher than 1200 ° C., the austenite grains become coarse, leading to a decrease in toughness, and an oxidation loss becomes remarkable, resulting in a decrease in yield. Therefore, the heating temperature of the steel material is in the range of 1000 to 1200 ° C. A preferable heating temperature range is 1000 to 1150 ° C. from the viewpoint of improving the toughness of the steel sheet. The temperature of the steel material means the temperature at the center of the plate thickness of the steel plate.

  In the hot rolling, first, rolling is performed so that the cumulative reduction ratio in the austenite recrystallization temperature range is 10% or more at the center of the plate thickness. By setting the cumulative rolling reduction in this temperature range to 10% or more, the Charpy fracture surface transition temperature (vTrs) at the ¼ thickness position can be −40 ° C. or less. When the cumulative rolling reduction is less than 10%, the austenite is not sufficiently refined and the toughness is not improved, and the Charpy fracture surface transition temperature at the ¼ thickness position cannot be −40 ° C. or less. The upper limit of the cumulative rolling reduction is not particularly limited, but the cumulative rolling reduction is preferably 45% or less because the effect of improving the fine particles becomes small. In the case of the component composition of the present invention, the above condition is preferably such that the cumulative rolling reduction in the temperature range included in 1100 to 950 ° C. in the hot rolling is 10% or more.

  Further, rolling is performed such that the cumulative reduction ratio is 50% or more when the temperature at the center of the plate thickness is in the austenite non-recrystallization temperature range. By setting the cumulative rolling reduction in this temperature range to 50% or more, a texture in which the (211) plane integration degree on the rolled surface at the center of the sheet thickness is 1.2 or more is obtained. On the other hand, when the cumulative rolling reduction in this temperature range is less than 50%, a texture in which the (211) plane integration degree on the rolled surface at the sheet thickness center is 1.2 or more cannot be obtained. The upper limit of the cumulative rolling reduction is not particularly limited, but is preferably 75% or less so as not to inhibit the rolling efficiency. In the case of the component composition of the present invention, the above conditions are preferably such that the temperature at the center of the thickness of the non-recrystallization zone rolling is 950 to 700 ° C., and from the viewpoint of texture formation, the upper limit is more than 850 ° C. Preferably, the upper limit is more preferably 830 ° C. or less, and the cumulative rolling reduction in this temperature range is 50% or more.

Furthermore, in the present invention, in hot rolling, the cumulative rolling reduction when the plate thickness surface temperature is below the Ar ₃ transformation point and the temperature at the center of the plate thickness is in the temperature range above the Ar ₃ transformation point is set to exceed 20%. In the present invention, which is an important requirement, by performing hot rolling under these conditions, the (200) plane integration degree on the rolled surface on the steel sheet surface can be developed. At the time of this hot rolling, the surface of the steel sheet is a two-phase region, and at the same time, the center of the plate thickness is the temperature region of the austenite non-recrystallized region. By rolling in such a state that the cumulative reduction ratio exceeds 20%, the textures near the surface and in the center of the plate thickness can be made into separate textures simultaneously. That is, the (200) plane texture develops in the vicinity of the surface, and the (211) plane texture develops in the center of the plate thickness. Here, both the texture of the (200) plane near the surface and the texture of the (211) plane in the center of the plate thickness are the textures in which cracks hardly propagate in the longitudinal direction of the steel sheet. The direction of progress is different. For this reason, the growth direction of cracks must be changed at the boundary between these textures, and the boundary between textures further hinders the progress of cracks. Therefore, in the present invention, not only the textures in which cracks do not easily progress are aligned, but also separate textures in which the cracks propagate in different directions cause further crack propagation failure, and as a result, crack propagation is stopped. Improved characteristics.

And by performing hot rolling under these conditions, the (200) plane integration degree on the rolled surface on the steel sheet surface is 1.7 or more, and the Charpy fracture surface transition temperature (vTrs) on the steel sheet surface is −80 ° C. or less. . When the plate thickness surface is in this temperature range and the cumulative rolling reduction is 20% or less, the desired texture and vTrs cannot be obtained. Here, the Ar ₃ transformation point is represented by the following formula.
Ar ₃ = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
The element symbol in the above formula means the content (% by mass) of each element, and 0 is not included.
Incidentally, a suitable temperature range to the rolling in the temperature below Ar ₃ transformation point of the surface is Ar ₃ transformation point ~ (Ar ₃ transformation point -80) ° C.. Further, in the temperature of the mid-thickness is more than Ar ₃ transformation point, a suitable temperature range in the rolling is (Ar ₃ transformation point +80) ° C. to Ar ₃ transformation point.

  Further, in the hot rolling in the present invention, rolling outside the specified temperature range is not limited, and it is sufficient that the specified cumulative reduction rate is reduced at least in the specified temperature range.

  The rolled steel sheet is cooled to a cooling stop temperature of 500 ° C. or lower at a cooling rate of 0.5 ° C./s or higher. When the cooling rate is less than 0.5 ° C./s, the (211) plane integration degree on the rolled surface at the plate thickness center position cannot be ensured to be 1.2 or more. Further, when the cooling stop temperature does not satisfy 500 ° C. or less, the desired strength and texture cannot be obtained.

Furthermore, when tempering is performed after cooling to a cooling stop temperature of 500 ° C. or lower, it is necessary that the temperature at the center of the plate thickness is less than the Ac ₁ transformation point. This is because when the tempering treatment is at least the Ac ₁ transformation point, the texture developed during rolling is lost. Here, the Ac ₁ transformation point is represented by the following equation.
Ac ₁ = 751-26.6C + 17.6Si-11.6Mn-169Al-23Cu-23Ni + 24.1Cr + 22.5Mo + 233Nb-39.7V-5.7Ti-895B The element symbol in the formula represents the content (mass%) of each element. Means and does not contain 0.

  In the above description, the temperature at the center of the plate thickness is obtained by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer. Further, the temperature condition in the cooling condition after rolling is the temperature at the center of the plate thickness, and the cooling rate is also an average cooling rate calculated based on the temperature at the center of the plate thickness.

  Next, examples of the present invention will be described.

  Molten steel having each component composition shown in Table 1 was melted in a converter and made into a steel material by a continuous casting method. After hot rolling to a plate thickness of 70 to 100 mm, cooling was performed to obtain test steels shown in Table 2. Table 2 shows heating conditions, hot rolling conditions, and cooling conditions. Moreover, the tempering temperature was also shown about what tempered after cooling.

  About the obtained steel plate, Φ14 mm JIS No. 14A test piece was collected from the position of the thickness ¼, a tensile test was performed, and a yield strength (YS) and a tensile strength (TS) were measured. A sample having a YS of 390 MPa or more and a TS of 510 MPa or more was evaluated as good.

  A JIS No. 4 impact test specimen was taken from the 1/4 position of the plate thickness and the steel sheet surface so that the longitudinal axis direction of the test specimen was parallel to the rolling direction, and subjected to a Charpy impact test. vTrs) was determined. When the vTrs at the ¼ position of the plate thickness was −40 ° C. or lower and the vTrs on the steel plate surface was −80 ° C. or lower, the toughness was evaluated as good.

  Further, in order to evaluate the texture of the steel plate, the (211) plane integration degree at the rolling surface at the center of the plate thickness and the rolling surface on the steel plate surface (the steel plate surface means a range from the extreme surface to 1 mm below the surface). The (200) plane integration degree was measured.

  The degree of surface integration was measured using an X-ray diffractometer (manufactured by Rigaku Corporation) and using a Mo ray source.

Next, in order to evaluate the brittle crack propagation stopping property, a temperature gradient type ESSO test was performed to obtain a Kca value at -10 ° C. (hereinafter also referred to as Kca (−10 ° C.) N / mm ^1.5 ). . A Kca (−10 ° C.) of 6000 N / mm ^1.5 or more was considered good.

Furthermore, in order to evaluate the toughness (HAZ toughness) of the weld heat affected zone, after producing a joint by electrogas welding (EGW) of high heat input welding (450 to 700 kJ / cm), the surface in the plate thickness direction and the back surface 1 mm Absorption energy vE- ₂₀ at a test temperature of −20 ° C. was determined using a Charpy test piece with a notch in the bond portion at the position. Six average values of absorbed energy vE- ₂₀ at a test temperature of −20 ° C. were evaluated as 60 J or more as good.

  Table 3 shows the results of these tests.

From the results shown in Table 3, the example of the present invention has a (211) plane integration degree of 1.2 or more at the rolling surface at the center of the plate thickness and a (200) plane integration degree at the rolling surface of the steel sheet surface. It has a texture of 1.7 or more, and has excellent toughness at a Charpy fracture surface transition temperature vTrs of -40 ° C. or lower at the plate thickness 1/4 position and a Charpy fracture surface transition temperature vTrs of −80 ° C. or lower at the steel plate surface, An excellent brittle crack propagation stopping property was obtained with a Kca (−10 ° C.) of 6000 N / mm ^1.5 or more. Moreover, the example of this invention is 60J or more of the absorbed energy vE- ₂₀ of a welding heat affected zone, and is excellent also in the toughness of a weld heat affected zone.

On the other hand, the comparative example outside the present invention does not satisfy any of YS, TS, texture, vTrs, and vE- ₂₀ . Further, all the values of Kca (−10 ° C.) were not satisfied.

Claims (5)

In mass%, C: 0.03-0.15%, Si: 0.50% or less, Mn: 0.5-2.2%, P: 0.030% or less, S: 0.0005-0. 0040%, Ti: 0.005 to 0.030%, Al: 0.005 to 0.080%, N: 0.0035 to 0.0075%, Ca: 0.0005 to 0.0030%, O: 0 .0040% or less, Ceq defined by the following formula (1) is 0.36 or more, and each content of Ca, O, and S satisfies the following formula (2), and the balance is Fe And a component composition consisting of inevitable impurities,
The (211) plane integration degree at the rolling surface at the sheet thickness center is 1.2 or more, and the (200) plane integration degree at the rolling surface on the steel sheet surface has a texture of 1.7 or more,
Charpy fracture surface transition temperature vTrs at a thickness of 1/4 position is −40 ° C. or lower,
A high-strength extra-thick steel plate excellent in brittle crack propagation stopping characteristics and weld heat-affected zone toughness with a plate thickness of 70 mm or more with a Charpy fracture surface transition temperature vTrs of −80 ° C. or less on the steel plate surface.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
Here, C, Mn, Cu, Ni, Cr, Mo and V in the formula (1) mean the content (% by mass) of each element, and 0 when not contained.
0 <{(Ca− (0.18 + 130 × Ca) × O) /1.25} /S≦0.8 (2)
However, Ca, O, and S represent the content of each component (mass%).   2. The brittle crack propagation stop property and welding according to claim 1, wherein the component composition further includes one or two of B: 0.0003 to 0.0030% and V: 0.2% or less in terms of mass%. High-strength ultra-thick steel plate with excellent heat-affected zone toughness.   The component composition is further in mass%, Nb: 0.003 to 0.030%, Ni: 1.0% or less, Cu: 1.0% or less, Cr: 0.7% or less, Mo: 0.7 % Or less, Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.020%, REM: 0.001 to 0.020%, or one or more thereof. A high-strength ultra-thick steel plate with excellent brittle crack propagation stopping characteristics and weld heat-affected zone toughness as described in 1. After heating the steel raw material which has the component composition in any one of Claims 1-3 to the temperature of 1000-1200 degreeC,
The temperature at the center of the plate thickness is 10% or more at the austenite recrystallization temperature range, the temperature at the center of the plate thickness is 50% or more at the austenite non-recrystallization temperature region, and the surface thickness of the plate is Ar _3. after the temperature below and mid-thickness transformation was carried out hot rolling under the conditions of cumulative rolling reduction exceeds 20% at the temperature range of not lower than Ar ₃ transformation point,
Cooling to a cooling stop temperature of 500 ° C. or less at a cooling rate of 0.5 ° C./s or more , the (211) plane integration degree at the rolled surface in the center of the sheet thickness is 1.2 or more, and the rolled surface on the steel sheet surface Having a texture with a (200) plane integration degree of 1.7 or more,
Charpy fracture surface transition temperature vTrs at a thickness of 1/4 position is −40 ° C. or lower,
A method for producing a high-strength extra-thick steel plate excellent in brittle crack propagation stopping characteristics and weld heat-affected zone toughness having a plate thickness of 70 mm or more with a Charpy fracture surface transition temperature vTrs of −80 ° C. or less on the steel plate surface . The steel sheet is cooled to a cooling stop temperature of 500 ° C. or lower, and then tempered to a temperature below the Ac ₁ transformation point at the center of the plate thickness. A manufacturing method of high strength heavy steel sheet.