JP2008179878A - High-strength thick steel plate superior in brittle crack propagation preventing characteristic, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength thick steel plate superior in brittle crack propagation preventing characteristic, and a manufacturing method capable of manufacturing the high-strength thick steel plate superior in brittle crack propagation preventing characteristic in a high efficiency without hindering the productivity. <P>SOLUTION: A hot rolling is applied under the condition that the heating temperature is 1,000-1,350°C, the accumulated reduction in the temperature range of 850-950°C is 50% or more, and the rolling finish temperature is in the temperature range of 850-950°C, to a steel raw material having a composition comprising, by mass%, 0.001-0.25% of C, 0.05-2.0% of Si, 0.1-2.0% of Mn, 0.03% or less of P, 0.03% or less of S, 0.08-3.0% of Al and 0.005-0.1% of Nb, and the balance being Fe with inevitable impurities, and then the rolled steel plate is air-cooled. Thereby, the thick steel plate has such a structure that the (100) surface strength ratio at the surface layer part and the plate thickness central part is 2.0 or more, and the (110) surface strength ratio at the plate thickness 1/4 part is 1.5 or more on the surface parallel to the plate surface, and is superior in brittle crack propagation preventing characteristic. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-tensile steel plate suitable for various structures such as ships, line pipes, low-temperature storage tanks, buildings, and civil engineering structures, and a method for producing the same, and in particular, to stop brittle crack propagation in high-tensile steel plates. The present invention relates to a method for producing a high-tensile thick steel plate that can produce a high-tensile thick steel plate having improved characteristics and excellent brittle crack propagation stopping properties while maintaining high productivity. As used herein, “high-tensile steel plate” means a steel plate having a yield strength YP: 325 MPa or more and a tensile strength TS: 490 MPa or more, and “thick steel plate” means a plate thickness of 10 mm or more. In particular, it refers to a steel sheet having a thickness exceeding 20 mm. Further, the “excellent brittle crack propagation stop property” here refers to a case where the NDT temperature in the drop weight test specified in ASTM E208 is −50 ° C. or lower.

  Conventionally, high-tensile steel materials used in various structures such as ships, line pipes, low-temperature storage tanks, buildings, and civil engineering structures have been provided with excellent toughness from the viewpoint of ensuring the safety of structures. It is requested. In response to such demands, high-tensile steel materials having excellent toughness have been manufactured using a controlled rolling technique represented by the TMCP method (Thermo-Mechanical Control Process).

  However, in actual structures, unexpected construction defects, corrosion, deformation due to earthquakes, collisions, etc. may occur, and completely eliminate the possibility of brittle cracks starting from these. I can't. Therefore, the steel material used for the structure is required to have excellent ability to stop the propagation of the generated brittle cracks, that is, to maintain excellent brittle crack propagation stop characteristics. Furthermore, in actual structures, brittle cracks are assumed to occur after a certain amount of plastic deformation, so the steel materials used retain excellent brittle crack propagation stop characteristics even after pre-straining. It is hoped to do.

In response to such a demand, for example, Patent Document 1 proposes a low-temperature steel excellent in brittle fracture occurrence resistance characteristics and brittle crack stopping characteristics. The low temperature steel described in Patent Document 1 contains appropriate amounts of C, Mn, P, S, and Ti, and one or more of Si, Ni, Cr, and Cu so that Ceq is 0.25 to 0.35%. the steel containing two or more, then heated to 950 to 1200 ° C., subjected to 30% to 60% of rolling between the Ar ₃ transformation point and (Ar ₃ transformation point + 100 ° C.), then Ar ₃ transformation point and Ar ₁ It is obtained by rolling 50 to 80% between transformation points, and is said to be a low-temperature steel excellent in both brittle fracture initiation resistance characteristics and brittle crack stopping characteristics. In the technique described in Patent Document 1, by carrying out sufficient rolling in a two-phase temperature range of (α + γ), in addition to a larger grain refinement effect, a certain degree of work hardening and formation of a texture on the precipitated ferrite It is said that it is important to promote the generation of separation fracture surfaces.

Patent Document 2 proposes a method for manufacturing a structural steel sheet for welding having excellent brittle fracture propagation stopping characteristics. In the technique described in Patent Document 2, a steel slab or a steel plate has a final product thickness of t, and a thickness of the steel plate at the time of rapid cooling during rolling is t ₀ , at least 0.1 × t ₀ / The surface layer portion of t (mm) or more is rapidly cooled, and then rolling is started or resumed from a temperature at which the surface layer portion is A _c3 or less, and the finish rolling end temperature is set to a temperature range of (A _c3 −150 ° C.) to A _c3 It is the manufacturing method of the structural steel plate for welding excellent in the brittle fracture propagation stop property to do. This technology uses the fact that the temperature distribution in the plate thickness direction becomes non-uniform during cooling, and this is a technology that modifies only the surface layer of the steel plate. Ferrite grains having an equivalent diameter of 3 μm or less are formed, and the brittle fracture propagation stop property is improved.

  Patent Document 3 includes C: 0.005-0.03%, Si: 0.05-0.5%, Mn: 1.3-3.0%, Al: 0.01-0.08%, Ti: 0.005-0.05%, Nb: 0.005-0.060%, B: A steel material containing 0.0015 to 0.0040% was heated to 1000 to 1300 ° C, and then hot-rolled under conditions of a cumulative reduction ratio in the austenite non-recrystallized region: 60% or more and a rolling end temperature of 700 to 850 ° C. After that, a cooling method: a method of producing a non-tempered high-strength thick steel plate excellent in brittle crack propagation stopping performance with a tensile strength of 700 MPa or more, which is cooled to 450 ° C. or less at a rate of 10 ° C./s or more has been proposed. .

  In Patent Document 4, C: 0.001 to 0.013%, Si: 0.01 to 0.6%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.10%, Nb: 0.004% or less, V: 0.010% or less, Mo: Cumulative rolling reduction in the austenite non-recrystallized region after heating a steel material containing 0.20 to 1.0%, B: 0.0003 to 0.0050%, Ti: 0.005 to 0.050%, N: 0.0010 to 0.060% to 950 to 1250 ° C: 50% or more, rolling end temperature: 600-800 ° C, hot rolling, then cooling rate: 7 ° C / s or more, cooling to 580 ° C or less, tensile strength of 590 MPa or more, brittle crack propagation A method for producing a high-tensile thick steel plate excellent in stopping performance and toughness of heat-affected zone by super-high heat input welding has been proposed.

Patent Document 5 includes C: 0.05 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.5 to 1.6%, Al: 0.001 to 0.20%, Ni: 0.3 to 1.9%, Nb: 0.005 to 0.030%, Ti: Steel containing 0.005-0.030% and carbon equivalent of 0.42% or less, heated to a temperature of _Ar3 point or higher, rolling ratio in the austenite non-recrystallized region: 50% or more, and continuously rolling in the two-phase region: There has been proposed a method of manufacturing a thick steel plate for welded structure excellent in brittle fracture propagation stopping characteristics of a welded joint, in which rolling is performed at 50% or more, rolling is finished at 650 ° C. or more, and then accelerated cooling is performed.
Japanese Unexamined Patent Publication No. 55-148746 Japanese Patent No. 2633757 JP-A-2005-97694 JP 2005-97683 A JP-A-8-120338

  However, in the technique described in Patent Document 1, in order to sufficiently perform the rolling in the two-phase temperature range, it is necessary to interrupt the hot rolling and wait until the temperature of the material to be rolled decreases to the two-phase temperature range. There was a problem that productivity was very low. Moreover, in the technique described in Patent Document 2, for example, there is a problem that high-precision temperature control is required in order to suppress variations between steel plates and ensure stable characteristics, and industrial mass production. It is thought that this is an unsuitable technology. Further, in the technique using the refinement of ferrite grains for improving the brittle crack propagation stop characteristic as in the technique described in Patent Document 2, when further strengthening is necessary, there is a limit. There is a problem that arises. In addition, all the techniques described in Patent Document 3 and Patent Document 4 require that hot rolling is rolling with a large cumulative reduction ratio at a low temperature called an austenite non-recrystallized region, and further hot working. Since accelerated cooling is performed after rolling, there is a problem that productivity is extremely low as in the technique described in Patent Document 1. Moreover, in the technique described in Patent Document 5, hot rolling is rolling with a large reduction ratio in a low temperature range, that is, an austenite non-recrystallized region and a two-phase region, and further, after hot rolling Since accelerated cooling is performed, there is a problem that productivity is extremely low as in the technique described in Patent Document 1.

  In addition, in a ship such as a container ship or a bulk carrier ship, a high-strength thick material is often used for the hull outer plate because of its structure. In general, it is said that the brittle crack propagation stop property of steel materials deteriorates with increasing strength or thickness, and the demand for brittle crack propagation stop properties for steel materials used from the viewpoint of ensuring the safety of ships is further advanced. It has become a trend. In addition, the techniques described in Patent Documents 1 to 5 described above are considered to be relatively effective for improving the brittle crack propagation stopping characteristics of a relatively thin high-tensile steel material having a thickness of less than 50 mm. Even with the techniques described in Patent Documents 1 to 5, it is considered that it is considerably difficult to stably improve the brittle crack propagation stop characteristic of a thick high-strength steel material on an industrial scale.

  Conventionally, the brittle crack propagation stop property in a thick steel plate (steel material) has been evaluated by a standard test such as an ESSO test, but the propagation of a brittle crack that penetrates the plate thickness of the thick steel plate as shown in FIG. Intended for stopping. However, for example, a strong deck structure such as a container ship or a bulk carrier ship has a T-shaped joint structure in which a flange material is welded to a web material in a T-shape as shown in FIG. In such a structure, the brittle crack that has occurred in the welded part and has progressed through the web material will progress to the flange material at the T-shaped joint part. However, in the flange material, the brittle crack will progress in the thickness direction. Become. However, in the techniques described in Patent Documents 1 to 5, there is no mention of the brittle crack propagation stop characteristic in the thickness direction.

  The present invention advantageously solves the above-mentioned problems of the prior art, provides a high-strength thick steel plate excellent in brittle crack propagation stop characteristics, particularly in the thickness direction, and further stops brittle crack propagation. It is an object of the present invention to provide a method for producing a high-tensile thick steel plate that can produce a high-tensile thick steel plate having excellent characteristics with high efficiency without a reduction in productivity.

  In order to achieve the above-described object, the present invention has intensively studied factors affecting the brittle crack propagation stopping characteristics of thick steel plates, particularly the brittle crack propagation stopping characteristics in the thickness direction. As a result, in order to improve the brittle crack propagation stop characteristics, it is important to form a texture that increases the degree of integration of the (100) plane parallel to the plate surface (rolled surface). It has been found that it is effective to perform rolling by applying a predetermined amount or more in the (α + γ) two-phase temperature region.

  Next, the present inventors have earnestly studied a method for efficiently performing rolling that reduces a predetermined amount or more in the two-phase temperature region without impairing the productivity during hot rolling. As a result, it has been conceived that the two-phase temperature region of the thick steel plate is set to a high temperature of, for example, 850 ° C. or higher, which does not require standby for rolling. And it came to the mind that a large amount of Al and / or Si is effective for increasing the temperature of the two-phase temperature range of the thick steel plate. However, according to the study by the present inventors, the formation of a desired texture for improving the brittle crack propagation stop property cannot be achieved by merely containing a large amount of Al and / or Si, and Al and / or Si. It was newly found that it is necessary to contain an appropriate amount of Nb in addition to a large amount of Nb.

  This is because, according to the study by the present inventors, in a thick steel plate containing only a large amount of Al and / or Si, the two-phase temperature range becomes high, but in such a high-temperature two-phase temperature range, It has been found that when rolling is performed, recovery recrystallization of the processing α proceeds, and formation of a desired texture for improving the brittle crack propagation stop property cannot be achieved. Further, according to the study by the present inventors, in order to suppress such recovery and recrystallization of processed α, it is effective to contain a proper amount of Nb in addition to a large amount of Al and / or Si alone. The knowledge that there is.

First, the results of experiments conducted by the inventors and serving as the basis of the present invention will be described.
Steel No. A (mass%, 0.13% C-0.25% Si-1.57% Mn-0.013% P-0.003% S-0.032% Al-0.001% Nb), Steel No. B (mass%, 0.12% C- 0.36% Si-1.49% Mn-0.014% P-0.004% S-1.21% Al-0.001% Nb), Steel No. C (0.050% C-0.36% Si-1.46% Mn-0.013% P-0.003% S- 1.00% Al-0.045% Nb), each of the three steel materials was heated to 1300 ° C, and then each steel material was subjected to hot rolling at a rolling end temperature in the range of 690 to 900 ° C. After the hot rolling was finished, air cooling was performed to obtain a thick steel plate having a plate thickness of 13 mm. A test piece is taken in parallel to the plate surface from the center of the thickness of the obtained thick steel plate, and the X-ray diffraction intensity from the (100) plane is measured using X-ray diffraction. Ratio and random strength ratio (also simply referred to as strength ratio) were calculated as the degree of integration of the (100) plane parallel to the plate surface in the thick steel plate. The obtained (100) plane strength ratio is shown in FIG. 1 in relation to the rolling end temperature. In addition, it means that a texture is developed, so that intensity ratio is high.

  From FIG. 1, in Steel No. A, the (100) plane strength ratio is about 1.2 when the rolling end temperature is a high temperature of 780 ° C. or higher, and only after the rolling end temperature becomes a low temperature of 730 ° C. or lower. A high value of about 1.7. This shows that in order to form a desired texture for improving the brittle crack propagation stop characteristic, it is necessary to perform rolling at a low temperature of about 750 ° C. or less in Steel A. Therefore, with a thick steel plate made of steel No. A, it is necessary to interrupt and wait until the temperature reaches about 750 ° C. or less, and the productivity is extremely low. On the other hand, in Steel No. C, even when the rolling end temperature is as high as about 900 ° C., the (100) plane strength ratio shows a high value of about 3.5, and steel No. containing a large amount of Al and containing Nb. In the thick steel plate made of .C, even when the rolling end temperature is about 900 ° C., a desired texture for improving the brittle crack propagation stop property can be formed without waiting during rolling, It can be operated and there are no obstruction factors for productivity.

  However, in the case of a thick steel plate made of steel No. B containing a large amount of Al and not containing Nb, similarly to the thick steel plate made of steel No. A, the rolling end temperature is high. The (100) plane strength ratio is about 1.2. When steel No. B is used as a raw material, in rolling where the rolling end temperature is high without waiting, as with a thick steel plate using steel No. A as a raw material, a desired material for improving brittle crack propagation stop characteristics is desired. A texture cannot be formed. Even if the rolling finish temperature is about 900 ° C, the thick steel plate made of steel No. B is rolled in the (α + γ) two-phase temperature range, but because the Nb content is small, The present inventors speculate that the recovery and recrystallization of α have progressed.

For this reason, in order to produce a thick steel plate with excellent brittle crack propagation stopping characteristics without requiring standby during rolling, which is a factor that hinders productivity during hot rolling, a large amount of Al It was found that it is important to use a steel material having a composition that satisfies both the Nb content and the Nb content.
Furthermore, according to the study by the present inventors, different textures at each position in the thickness direction, that is, the (100) plane strength ratio in the plane parallel to the rolling surface in the surface layer portion is 2.0 or more, the thickness 1 / Developed a texture that has a (110) plane strength ratio of 1.5 or more in the plane parallel to the rolling surface in 4 parts and a (100) plane strength ratio of 2.0 or more in the plane parallel to the rolling surface in the center of the plate thickness. As a result, it was found that the brittle crack propagation stop property in the thickness direction is remarkably improved. According to the study by the present inventors, the steel material having the above composition has a cumulative reduction of 50% or more in the temperature range of 850 to 950 ° C. which is the (α + γ) two-phase temperature range, and the rolling end temperature is 850 It has been found that the above-mentioned texture can be formed by performing a hot rolling at a temperature range of ˜950 ° C. or by performing a hot rolling with a reduction rate per pass of 5% or more.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.001 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.03% or less, Al: 0.08 to 3.0%, Nb: 0.005 Containing ~ 0.1%, the composition consisting of the remainder Fe and inevitable impurities, the (100) plane strength ratio in the plane parallel to the rolling surface in the surface layer portion is 2.0 or more, parallel to the rolling surface in the 1/4 thickness portion Brittle crack propagation characterized by having a texture with a (110) plane strength ratio of 1.5 or more on the flat surface and a (100) plane strength ratio of 2.0 or more on the plane parallel to the rolling surface at the center of the plate thickness High tensile steel plate with excellent stopping characteristics.
(2) In (1), in addition to the above composition, in terms of mass%, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less, V: 0.5% or less, Ti A high-tensile steel plate characterized by having a composition containing one or more selected from: 0.05% or less and B: 0.005% or less.
(3) By mass%, C: 0.001 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.03% or less, Al: 0.08 to 3.0%, Nb: 0.005 Steel material with a composition of ~ 0.1%, balance Fe and inevitable impurities, heating temperature is 1000-1350 ° C, cumulative rolling reduction in the temperature range of 850-950 ° C is 50% or more, rolling finish temperature A method for producing a high-strength thick steel plate having excellent brittle crack propagation stopping characteristics, characterized in that hot rolling is performed at a temperature range of 850 to 950 ° C., and the hot rolling is allowed to cool.
(4) The method for producing a high-tensile thick steel plate according to (3), wherein the hot rolling is rolling with a reduction rate per pass of 5% or more in a temperature range of 850 to 950 ° C. .
(5) In (3) or (4), in addition to the above composition, in terms of mass%, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less, V: 0.5 % Or less, Ti: 0.05% or less, and B: 0.005% or less.
(6) In any one of (3) to (5), instead of allowing to cool after the hot rolling, a temperature range of 700 to 100 ° C. at an average cooling rate of 1 ° C./s or more after the hot rolling A method for producing a high-strength thick steel sheet, characterized by accelerated cooling to a low temperature.
(7) In any one of (3) to (6), a tempering process is further performed after the cooling or after the accelerated cooling.

  According to the present invention, it is possible to develop a texture effective in improving brittle crack propagation stopping characteristics, in particular, in the thickness direction, without impairing productivity during hot rolling. A high-strength thick steel plate having excellent brittle crack propagation stopping characteristics can be manufactured easily and stably by a simple process without impairing the productivity during hot rolling, and has a remarkable industrial effect.

First, the reasons for limiting the composition of the steel material used in the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.
C: 0.001 to 0.25%
C is an element having an effect of increasing the strength, and is a useful element for ensuring the base material strength necessary for the structural steel. In order to obtain such an effect, a content of 0.001% or more is required. On the other hand, if the content exceeds 0.25%, the toughness of the base metal and the welded portion is lowered. For this reason, C was limited to the range of 0.001 to 0.25%. In addition, Preferably it is 0.010 to 0.18%.

Si: 0.05-2.0%
Si is an element that acts as a deoxidizer and has the effect of increasing the strength of the steel by solid solution, and such an effect is recognized with a content of 0.05% or more. Further, Si is an element having an action of increasing the Ar ₁ transformation point and the Ar ₃ transformation point, and is one of the effective elements as in the present invention in the present invention, and such an effect is 0.05% or more. Conspicuous with inclusion. In order to effectively raise the transformation point, it is preferable to be 0.2% or more. On the other hand, the content exceeding 2.0% lowers the base metal toughness. For this reason, Si was limited to the range of 0.05 to 2.0%. In addition, Preferably, it is 0.2 to 1.5%.

Mn: 0.1-2.0%
Mn is an element having an action of increasing the strength of steel, and is an element useful for securing a desired base metal strength as structural steel. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, the content exceeding 2.0% lowers the Ar ₁ transformation point and the Ar ₃ transformation point, and lowers the (α + γ) two-phase temperature range. For this reason, Mn was limited to the range of 0.1 to 2.0%. In addition, Preferably, it is 0.5 to 1.6%.

P: 0.03% or less P is an element present as an inevitable impurity in steel, and when contained in a large amount exceeding 0.03%, the toughness of the base metal and the toughness of the welded portion, especially the brittle crack propagation stop property In the present invention, P is limited to 0.03% or less. In addition, Preferably it is 0.025% or less.

S: 0.03% or less S is an element that exists as a sulfide in steel, and if it exceeds 0.03%, it contains the toughness of the base metal at the center of the plate thickness and the welded part, especially brittle crack propagation stopping characteristics. In order to reduce this, in the present invention, S is limited to 0.03% or less. In addition, Preferably, it is 0.025% or less.
Al: 0.08-3.0%
Al is an element that has the effect of increasing the transformation point and expanding the (α + γ) two-phase temperature range to the high temperature side. In the present invention, two-phase rolling is performed at a high temperature without hindering productivity. It is one of the important elements. Al is an element that has the effect of reducing hot deformation resistance, making it easy to secure the desired rolling reduction per pass and dramatically improving productivity. It becomes. In order to obtain such an effect, a content of 0.08% or more is required. On the other hand, if it contains more than 3.0%, the (α + γ) two-phase temperature range will be expanded to a higher temperature than necessary, and the tendency to cause embrittlement of the base material will increase. Also disadvantageous. For these reasons, Al is limited to the range of 0.08 to 3.0%. In addition, Preferably it is 0.1 to 1.8%, More preferably, it exceeds 0.2 and 1.8% or less, More preferably, it is 0.5 to 1.5%.

FIG. 2 shows the relationship between the A ₁ transformation point and the A ₃ transformation point and the Al content. From Figure 2, Al raises greatly the A ₁ transformation point and A ₃ transformation point, it is found possible to greatly expanded two-phase temperature region. Moreover, as shown in FIG.2 (c), a two-phase temperature range can further be expanded by compounding Al and Si.
Nb: 0.005-0.1%
Nb is an element that has the action of promoting the development of a texture that suppresses recovery and recrystallization of the processed α processed in the two-phase temperature region and improves the brittle crack propagation stopping characteristics.In the present invention, Al, Si It is one of the important elements as well. In order to ensure such an effect, it is necessary to contain 0.005% or more of Nb. On the other hand, if the content exceeds 0.1%, a large amount of Nb compound precipitates, the base metal and the welded portion precipitate and harden, and the toughness decreases. For this reason, Nb was limited to 0.005 to 0.1% of range. In addition, Preferably it is 0.010 to 0.070%.

The above components are basic components. In addition to the basic composition, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less, V: 0.5% or less, Ti: One or more selected from 0.05% or less and B: 0.005% or less can be selected and contained as necessary.
Cu, Ni, Cr, Mo, V, Ti, and B are all elements that increase the strength, and can be selected and contained as necessary.

Cu is an element that contributes to an increase in strength by solid solution strengthening and precipitation strengthening, and such an effect becomes remarkable when the content is 0.05% or more, but the content exceeding 2.0% decreases the hot workability. . For this reason, it is preferable to limit Cu to 2.0% or less.
Ni and Cr are elements that increase the hardenability, and are elements that have the effect of increasing the strength through an increase in the fraction and hardness of the second phase after cooling, and can be selected as necessary. Can be contained. Such an effect becomes remarkable when Ni: 0.05% or more and Cr: 0.05% or more are contained. However, if expensive Ni is contained in a large amount exceeding 2.0%, the material cost increases, which is economically disadvantageous. It becomes. Moreover, when Cr is contained in a large amount exceeding 2.0%, the weld zone toughness is lowered. For this reason, it is preferable to limit Ni to 2.0% or less and Cr to 2.0% or less, respectively.

Mo, like Ni and Cr, is an element that increases the hardenability and, in addition, precipitates as Mo carbides in the ferrite after cooling and contributes to an increase in strength. Such an effect becomes remarkable when the content is 0.05% or more. However, when the content exceeds 2.0%, the toughness of the welded portion is lowered. For this reason, it is preferable to limit Mo to 2.0% or less.
V is an element that precipitates as V (C, N) in the ferrite and contributes to an increase in strength through precipitation strengthening. Such an effect becomes remarkable when the content is 0.005% or more. On the other hand, the content exceeding 0.5% lowers the toughness of the weld. For this reason, it is preferable to limit V to 0.5% or less.

  Ti fixes solute N as TiN, reduces the amount of solute N, improves the anti-strain aging characteristics, and refines crystal grains, thereby contributing to an increase in strength. It is an element, and such an effect becomes remarkable when the content is 0.005% or more. On the other hand, since inclusion exceeding 0.05% precipitates as TiC, toughness decreases. For this reason, it is preferable to limit Ti to 0.05% or less.

B has an effect of increasing the hardenability and contributes to an increase in strength through the increase of hardenability. Such an effect becomes remarkable when the content is 0.0003% or more. On the other hand, if the content exceeds 0.005%, boride is formed and hot workability is lowered. For this reason, it is preferable to limit B to 0.005% or less.
The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include N: 0.010% or less and O: 0.005% or less.

  The steel material used as a starting material in the present invention is obtained by melting molten steel by a known melting method such as a converter, and preferably by performing a known degassing treatment such as RH to obtain molten steel having the above composition. It is preferable to use a known casting method such as a continuous casting method or an ingot-bundling rolling method to obtain a desired dimensional shape such as a slab. However, the present invention is not limited to the above-described method, and any ordinary steel material manufacturing method can be applied.

Next, the steel material is subjected to heat treatment for hot rolling. The heating temperature is in the range of 1000 to 1350 ° C.
Heating temperature: 1000-1350 ° C
In the heat treatment for hot rolling, the heating temperature is at least 1000 ° C. or higher in order to sufficiently soak the steel material to the center and reduce the deformation resistance. When the heating temperature is less than 1000 ° C., the soaking is insufficient, the deformation resistance is high, and the rolling load tends to be excessive. On the other hand, when the heating temperature is higher than 1350 ° C., the structure becomes coarse and the base metal toughness of the thick steel plate decreases. For this reason, the heating temperature of the steel material was limited to a temperature in the range of 1000 to 1350 ° C. In addition, Preferably it is 1050-1300 degreeC.

The steel material subjected to the above heat treatment is then subjected to hot rolling. Hot rolling is performed under the conditions of a cumulative reduction ratio of 850 to 950 ° C .: 50% or more and a rolling end temperature of 850 to 950 ° C.
Cumulative rolling reduction of 850 to 950 ° C .: 50% or more In the present invention, rolling is performed such that the cumulative rolling reduction of 850 to 950 ° C. is 50% or more. In the case of the steel material used in the present invention having the composition described above, the temperature range of 850 to 950 ° C. is the (α + γ) two-phase temperature range, and rolling is performed such that the cumulative reduction ratio in this temperature range is 50% or more. As a result, a texture that improves the brittle crack propagation stop property develops. If the cumulative rolling reduction in this temperature range is less than 50%, the texture that improves the brittle crack propagation stop characteristics will not be sufficiently developed, and the brittle crack propagation stop characteristics will not be improved. descend. The upper limit of the cumulative rolling reduction in this temperature range is not particularly limited. However, if the cumulative rolling reduction exceeds 95%, recrystallization of the processed α is likely to occur, and the texture that improves the brittle crack propagation stopping property is increased. It is reduced. Therefore, the cumulative rolling reduction at 850 to 950 ° C. is preferably 95% or less.

  In the above-described hot rolling in the temperature range of 850 to 950 ° C., the rolling reduction per pass is preferably 5% or more. By setting the rolling reduction per pass to 5% or more, formation of a texture that improves the brittle crack propagation stop property is stably facilitated. On the other hand, if it is less than 5%, the formation of the texture at the 1/4 thickness portion and the central thickness portion becomes insufficient, the effect of suppressing the progress of brittle cracks is reduced, and the toughness is also reduced. In addition, Preferably it is 7% or more. From the viewpoint of improving the brittle crack propagation stop characteristic, it is desirable to secure at least 3 passes or more and a pass with a rolling reduction rate of 7% or more per pass.

Rolling end temperature: 850 ~ 950 ℃
When the rolling end temperature is less than 850 ° C., it is necessary to wait in the middle of rolling, and the productivity during hot rolling is hindered. Further, when the rolling end temperature is higher than 950 ° C., recovery recrystallization of the processing α is promoted, and development of a desired texture is suppressed, so that it is impossible to ensure excellent brittle crack propagation stop characteristics. For this reason, the rolling end temperature of hot rolling is limited to the range of 850 to 950 ° C.

In addition, after completion | finish of hot rolling, it cools as it is (air cooling).
Instead of standing to cool, accelerated cooling to a temperature range of 700 to 100 ° C. may be performed at an average cooling rate of 1 ° C./s or more. After the hot rolling is completed, it is easy to increase the desired strength by performing accelerated cooling under the above-described conditions. When the cooling rate of accelerated cooling is less than 1 ° C./s, the amount of hard second phase generated in addition to the ferrite phase that is the first phase becomes insufficient, and the desired strength cannot be increased. Further, if accelerated cooling is stopped at a temperature exceeding 700 ° C., the amount of hard second phase produced becomes insufficient, and the desired strength cannot be increased. Moreover, even if accelerated cooling is performed to less than 100 ° C, the influence on the base material is small. For this reason, it is preferable that the accelerated cooling condition after the end of hot rolling is a condition of cooling to a temperature range of 700 to 100 ° C. at an average cooling rate of 1 ° C./s or more. The cooling rate is the rate at the surface of the thick steel plate, and is the average from the cooling start temperature to the cooling stop temperature.

  In addition, after standing_to_cool or accelerated cooling, you may temper further from a viewpoint of distortion removal. In the tempering treatment, it is desirable that the heating temperature is 700 ° C. or less and the holding time is short in order to suppress recovery of ferrite (process α) as much as possible. As such treatment, it is desirable to use a heating method capable of rapid heating online, for example, induction heating or the like, after standing to cool or immediately after accelerated cooling. Further, when tempering off-line, recovery of ferrite (processed α) is promoted, so that the heating temperature for tempering is preferably 650 ° C. or less.

  The high-tensile thick steel plate thus obtained has a composition as described above, and becomes a thick steel plate having a texture that improves brittle crack propagation stopping characteristics. The term “texture that improves brittle crack propagation stop characteristics” here means that the (100) plane strength ratio in the plane parallel to the rolling surface in the surface layer portion is 2.0 or more and parallel to the rolling surface in the 1/4 thickness portion. A texture having a (110) plane strength ratio of 1.5 or more on the flat surface and a (100) plane strength ratio of 2.0 or more on the plane parallel to the rolling surface in the center of the plate thickness is referred to. In addition, the surface intensity ratio here means a ratio of a diffraction intensity from a predetermined surface to a diffraction intensity from a predetermined surface in a random test piece by using X-ray diffraction, and a random intensity ratio.

  When a brittle crack has progressed in the thickness direction, the cleavage plane ((100) plane) is first accumulated perpendicularly to the direction of the brittle crack in the surface layer. In addition, fine cracks are generated, and the propagation energy of the brittle cracks is absorbed by the stress relaxation effect by the fine separation, and the progress of the cracks is suppressed. Moreover, because the (110) plane is developed parallel to the rolling surface at 1 / 4th of the plate thickness, details are unknown, but if a brittle crack tries to propagate inside the steel plate in the thickness direction, crack propagation Energy is absorbed and further the development of brittle cracks is suppressed. In addition, as with the surface layer part, the cleaved surface ((100) plane) is accumulated perpendicular to the direction of brittle crack growth in the plate thickness 1/2 part. Fine cracks are generated, the propagation energy of the brittle cracks is absorbed, and the progress of the brittle cracks into the thick steel plate is suppressed.

  When a crack occurs at the tip of a brittle crack, it becomes a resistance to the progress of the brittle crack. Furthermore, in the process where the brittle cracks merge with the cracks, the driving energy (energy release rate) for brittle fracture increases, but since the driving energy rapidly decreases after the coalescence, the brittle cracks stop. By such a thing, the thick steel plate which has an above described texture turns into a thick steel plate excellent in the brittle crack propagation stop characteristic. Whether the brittle crack that has entered the thick steel plate stops at the surface layer of the thick steel plate or further propagates into the thick steel plate and stops at a certain position in the plate thickness direction depends on the structure and stress state of the welded joint. Depends on.

  Even when it is not possible to prevent the occurrence of brittle cracks in the weld zone, as in the present invention, the thick steel plate has excellent brittle crack propagation stopping characteristics in the thickness direction, and stops brittle cracks entering the thick steel plate at the surface layer portion. If possible, most of the thickness direction remains in a healthy state, so that it is possible to transmit the load stress, and the safety of the structure can be remarkably improved.

Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace, cast into a small steel ingot (100 kg), and rolled into a steel material having a thickness of 100 mm. These steel materials were subjected to heating, hot rolling, cooling and cooling (air cooling) or accelerated cooling (ACC) under the conditions shown in Table 2 to obtain thick steel plates having the thicknesses shown in Table 2. Some thick steel plates were subjected to heat treatment (tempering treatment) after cooling.
Test pieces were collected from the resulting thick steel plates and investigated for texture, tensile properties, toughness, and brittle crack propagation stopping properties. The survey method was as follows.
(1) Texture The specimen is taken in parallel to the plate surface (rolled surface) from the plate thickness center portion, plate thickness 1/4 portion, surface layer portion of the obtained thick steel plate, and the plate thickness is measured using X-ray diffraction. For the central part and the surface layer part, the diffraction intensity from the (100) plane was measured, and for the 1/4 thickness part, the diffraction intensity from the (110) plane was measured. Calculate the ratio (also simply referred to as the strength ratio), and the degree of integration of the (100) plane parallel to the plate surface (rolled surface) in the plate thickness center part and surface layer part of the thick steel plate, or the plate thickness of 1/4 part The degree of integration of the (110) plane parallel to the plate surface (rolled surface) was used. The surface layer portion is a position 0.5 mm inside from the outer surface of the thick steel plate in the thickness direction.
(2) Tensile properties JIS No. 4 test piece (parallel part: 14mmφ) was sampled from the center of the thickness of the obtained thick steel plate so that the width direction during rolling was the tensile direction. A tensile test was carried out in accordance with this, and yield strength YS and tensile strength TS were determined.
(3) Toughness V-notch test specimens were collected in the rolling width direction (C direction) from the center of the thickness of the obtained thick steel plate, and Charpy impact test was performed in accordance with the provisions of JIS Z 2242. The surface transition temperature vTrs (° C.) was determined.
(4) Brittle crack propagation stop property The brittle crack propagation stop property was obtained by performing an NRL drop weight test. In the NRL drop test, an NRL drop test piece (test piece: thickness 19 mm x width 50 mm x length 130 mm) is taken from the obtained thick steel plate so that the longitudinal direction of the test piece matches the rolling direction of the thick steel plate. The NDT temperature was determined in accordance with ASTM E208.

In addition, the productivity at the time of hot rolling is the conventional example in which the time required from the start of hot rolling to the end of rolling of each thick steel plate is measured, and the hot rolling is interrupted and the rolling is performed without waiting ( Thick steel plate No. 1) was evaluated as a standard (100) with a ratio to that. Higher ratios mean lower productivity.
The obtained results are shown in Table 3.

  In all of the examples of the present invention, the yield strength: 325 MPa or more, the tensile strength: 490 MPa or more, the high toughness of vTrs of −63 ° C. or less, and the surface layer and the plate thickness parallel to the plate surface The (100) plane strength ratio is 2.0 or more, and the (110) plane strength ratio in the plane parallel to the plate surface at 1/4 thickness is 1.5 or more, and the NDT temperature is -40 ° C or less. It has a brittle crack propagation stopping property. In addition, in all of the examples of the present invention, the productivity is almost the same as that of the conventional example (thick steel plate No. 1) which is a standard, and can be manufactured by efficient rolling without impeding the productivity.

  In the comparative example outside the scope of the present invention, the brittle crack propagation stop characteristic is lowered or the productivity is lowered. Compared to the conventional example (thick steel plate No. 1) in the comparative example (thick steel plate No. 2) in which the rolling end temperature is low (680 ° C.), a desirable texture is formed, and the NDT temperature is as low as −50 ° C. Thus, although the brittle crack propagation stop characteristic is improved, it is necessary to wait in the middle of rolling, and the productivity is lowered. Moreover, in the comparative examples (thick steel plates No. 3 and No. 23) in which the Al content is outside the scope of the present invention, in the rolling where the transformation temperature is low and the rolling end temperature is 890 ° C., the (α + γ) two-phase temperature range. Therefore, the formation of desirable texture is small, and the improvement of brittle crack propagation stopping characteristics is not obtained. Further, in the comparative example (thick steel plate No. 4) in which the Nb content is outside the scope of the present invention, the processing α is recovered and recrystallized, the formation of the desired texture is small, the NDT temperature is high as −7 ° C., and brittle crack propagation The improvement of the stop characteristic has not been obtained.

  Further, a comparative example (thick steel plate No. 10) in which the cumulative reduction ratio at 850 to 950 ° C. and the reduction rate per pass deviate from the scope of the present invention (thick steel plate No. 10), and a comparative example in which the rolling end temperature deviates from the present invention range (thick steel plate No. 11) ), Both toughness and brittle crack propagation stopping properties are degraded.

It is a graph which shows the relationship between the (100) plane strength ratio and rolling completion temperature in each thick steel plate. At each C content, is a graph showing the effect of Al content on the change of the A ₁ transformation point and A ₃ transformation point. It is explanatory drawing which illustrates typically the propagation of the brittle crack in a thick steel plate. It is explanatory drawing which illustrates typically the propagation of the brittle crack in a T-shaped joint structure. (A) shows propagation of a brittle crack in a web material and (b) shows a flange material.

Claims (7)

% By mass
C: 0.001 to 0.25%, Si: 0.05 to 2.0%,
Mn: 0.1 to 2.0%, P: 0.03% or less,
S: 0.03% or less, Al: 0.08 to 3.0%,
Nb: 0.005-0.1%
A composition comprising the balance Fe and inevitable impurities, and a (100) plane strength ratio in the plane parallel to the rolling surface in the surface layer portion is 2.0 or more, and in a plane parallel to the rolling surface in the 1/4 thickness portion The brittle crack propagation stop characteristic is characterized by having a texture with a (110) plane strength ratio of 1.5 or more and a (100) plane strength ratio in a plane parallel to the rolling surface at the center of the plate thickness of 2.0 or more. Excellent high tensile steel plate.   In addition to the above composition, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less, V: 0.5% or less, Ti: 0.05% or less, B: 0.005 by mass% The high-tensile steel plate according to claim 1, wherein the high-tensile steel plate has a composition containing one or more selected from the group consisting of 1% or less. % By mass
C: 0.001 to 0.25%, Si: 0.05 to 2.0%,
Mn: 0.1 to 2.0%, P: 0.03% or less,
S: 0.03% or less, Al: 0.08 to 3.0%,
Nb: 0.005-0.1%
Steel material having a composition comprising the balance Fe and inevitable impurities, the heating temperature is 1000 to 1350 ° C., the cumulative rolling reduction in the temperature range of 850 to 950 ° C. is 50% or more, and the rolling end temperature is 850 to A method for producing a high-tensile thick steel plate having excellent brittle crack propagation stopping characteristics, characterized by performing hot rolling at a temperature range of 950 ° C., and allowing to cool after the hot rolling.   The method for producing a high-tensile thick steel plate according to claim 3, wherein the hot rolling is rolling in which a rolling reduction per pass is 5% or more in a temperature range of 850 to 950 ° C.   In addition to the above composition, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less, V: 0.5% or less, Ti: 0.05% or less, B: 0.005 by mass% % Or less, it is set as the composition containing 1 type, or 2 or more types selected from the following, The manufacturing method of the high-tensile steel plate of Claim 3 or 4 characterized by the above-mentioned.   The accelerated cooling to a temperature range of 700 to 100 ° C at an average cooling rate of 1 ° C / s or more after the hot rolling, instead of allowing to cool after the hot rolling. The manufacturing method of the high tension steel plate in any one.   The method for producing a high-tensile thick steel plate according to any one of claims 3 to 6, further comprising tempering after the cooling or after the accelerated cooling.

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