JP6369414B2 - High-strength thick steel plate for building structures with excellent heat-affected zone toughness - Google Patents

Description

  The present invention relates to a high strength thick steel plate suitable for use in a welded steel structure such as a building structure, in particular, a high strength steel plate having a high tensile strength TS of 950 MPa or more, and in particular, a heat input of 500 kJ / hour such as electroslag welding. The present invention relates to a high-strength thick steel plate that is excellent in weld heat-affected zone (hereinafter also referred to as HAZ) toughness when super-high heat input welding exceeding cm is applied. The “thick steel plate” here refers to a steel plate having a thickness of 10 mm or more.

  A steel structure used in the field of construction or the like is generally constructed by welding to form a structure having a desired shape. Especially for building structures, the Great Hanshin-Awaji Earthquake strongly demanded further improvement of the earthquake resistance of building structures. From the viewpoint of safety, not only the properties of the base metal of the steel used but also the properties of the welded parts are required. It is required to be superior.

  On the other hand, with the increase in the size of building structures, the application range of high heat input welding is expanded from the viewpoint of improving the efficiency of welding construction and reducing the construction cost. In the welding, a high heat input welding such as a two-electrode submerged arc welding exceeding a welding heat input of 400 kJ / cm is applied. Recently, in order to further improve the efficiency of welding work, for example, in the assembly welding of box columns of building structures, high heat input exceeding 500 kJ / cm, such as electroslag welding, is performed. Sometimes it is done.

  Generally, when the welding heat input becomes large, the HAZ structure becomes coarse and the HAZ toughness decreases. For this reason, various methods for improving the HAZ toughness have been proposed.

  For example, Patent Document 1 proposes a high-strength steel material excellent in the toughness of the weld heat affected zone. The high-strength steel materials described in Patent Document 1 are mass%, C: 0.010 to 0.080%, Si: 0.02 to 1.00%, Mn: 1.10 to 2.90%, Al: 0.20% or less, Ni: 0.40 to 2.40%, Cr: 0.50 to 1.95%, Mo: 0.16 to 1.10%, Ti: 0.002 to 0.030%, N: 0.0058 to 0.0120%, or Cu: 1.60% or less, and / or B: 0.0050% or less, Nb : 0.100% or less, V: Any one or more of 0.060% or less, Ti / N satisfies 1.0 or more and less than 4.0, (Mn + Ni + 2Cu) satisfies 3.60 or more, and (2.5Mo + 30Nb + 10V) satisfies 2.80 or less. The structure is mainly composed of bainitic ferrite and has excellent weld heat affected zone toughness. In the technique described in Patent Document 1, C is reduced, and Mn and Ni, or further Cu is actively contained, thereby generating a structure mainly composed of bainitic ferrite, and Ti, By optimizing the amount of N, TiN, which can be finely dispersed, is stabilized to a high temperature, preventing coarsening of prior austenite grains in the HAZ, and heat input: welding heat effect of high heat input welding of 800 kJ / cm It is said that high toughness in the part can be secured.

  Patent Document 2 describes a high-strength thick steel plate excellent in high heat input HAZ toughness and low heat input HAZ hardening resistance. The technique described in Patent Document 2 is mass%, C: 0.03 to 0.075%, Si: 0.01 to 0.40%, Mn: 0.1 to 1.3%, Al: 0.01 to 0.05%, Cr: 2.0 to 5.0%, Ti : High strength thick steel plate containing 0.005 to 0.03%, N: 0.002 to 0.007%, substantially free of B, and having a tensile strength of 780 MPa or more. According to the technique described in Patent Document 2, a high-strength thick steel plate having excellent high toughness HAZ toughness and excellent low heat input HAZ hardening resistance can be obtained. This is substantially free of B, and when Cr: 2.0% or more is contained, the C concentration of the island martensite is lowered, the bainite lath is curved, and the shape of the island martensite is changed from a needle shape to a granular shape. It is said that the harmfulness to the toughness of island martensite was significantly reduced.

  Patent Document 3 describes a low-yield-ratio high-tensile steel material excellent in toughness of high heat input HAZ. The steel materials described in Patent Document 3 are in mass%, C: 0.025 to 0.050%, Si: 0.01 to 0.40%, Mn: 1.20 to 2.50%, Cr: 1.50 to 3.50%, Mo: 0.10 to 0.50%, Al : 0.010 to 0.050%, Ti: 0.005 to 0.050%, N: 0.0015 to 0.0060%, B is limited to 0.0003% or less, (Mn + 0.4Cr): 2.50 to 3.00%, Tensile strength: 590 MPa It is a low yield ratio high tensile steel material having the above. According to the technique described in Patent Document 3, excellent HAZ toughness can be ensured even when high heat input welding exceeding 500 kJ / cm is performed. This is because when a γ-generating element such as Mn and an α-generating element such as Cr are added simultaneously and the amount of α-generating element is appropriately adjusted with respect to the amount of γ-generating element, an island shape between bainite laths of HAZ This is because the generation of martensite is reduced and the HAZ toughness is improved.

  Patent Document 4 describes a steel sheet having excellent HAZ toughness. The steel sheet described in Patent Document 4 is mass%, C: 0.025 to 0.050%, Si: 0.6% or less, Mn: 0.9 to 2.3%, Cr: 1.0 to 4.9%, Al: 0.05% or less, Ti: 0.005. ~ 0.050%, Nb: 0.01 ~ 0.07%, Ni: 2.0% or less, Mo: 0.03% to less than 0.10%, (Mn + 0.7Ni + 14Nb + Cr + 4Si + 2Mo): 2.5 ~ 7.0, (Mn + 0.7Ni + 14Nb): 1.0 or more and (Cr + 4Si + 2Mo): 2.9 or more so as to satisfy each, and a tensile strength: 590 MPa or more. According to the technique described in Patent Document 4, excellent HAZ toughness can be ensured even if high heat input welding exceeding 500 kJ / cm is performed. Adding a γ-generating element such as Mn and an α-generating element such as Cr at the same time, and adjusting the amount of α-generating element appropriately with respect to the amount of γ-generating element alleviates excessive austenite stabilization, and between bainite laths. The island-shaped martensite is reduced and the HAZ toughness is improved.

Japanese Unexamined Patent Publication No. 2006-118007 JP 2012-177192 A JP 2012-241214 JP 2013-19015 A

  However, each of the techniques described in Patent Documents 1 to 4 has a problem that the tensile strength is 590 MPa or more, or 780 MPa or more, and further strengthening is required in response to the recent demand for higher strength. I left it.

  Therefore, the present invention advantageously solves the above-described problems of the prior art, has a high strength of tensile strength: 950 MPa or more, and is excellent even when subjected to super-high heat input welding exceeding 500 kJ / cm. An object of the present invention is to provide a high-strength thick steel plate for a building structure that can maintain weld heat-affected zone toughness and is excellent in super-high heat input weld heat-affected zone toughness.

  In order to achieve the above-mentioned object, the present inventors have earnestly studied on measures for stably securing the toughness of the heat-affected zone having a high heat input welding while ensuring a high strength of tensile strength: 950 MPa or more. did. As a result, it is extremely low carbon to suppress the formation of island martensite (hereinafter also referred to as MA) in the high heat input welding heat affected zone. I thought it was important for me. In addition, after ultra-low carbonization, further containing 3.6% by mass or more of Mn and 5% by mass or more of Cr, the desired tensile strength: high strength of 950 MPa or more and excellent large strength It was found that it becomes important to combine heat input welding heat affected zone toughness. It has been found that the inclusion of 5 mass% or more of Cr alleviates the occurrence of intergranular fracture due to the inclusion of 3.6 mass% or more of Mn alone and stably improves the high heat input heat affected zone toughness.

  The present inventor has also found that Nb and Ni, and Si and Mo are useful as an aid for Mn and Cr.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.05% or less, Si: 0.3% or less, Mn: 3.6 to 7.0%, P: 0.02% or less, S: 0.003% or less, Al: 0.001 to 0.06%, Ti: 0.005 to 0.03 %, Cr: 5 to 12%, N: 0.0055% or less, O: 0.003% or less, and a composition composed of the balance Fe and inevitable impurities, and a structure mainly composed of a martensite phase and a bainite phase. , Tensile strength: A high strength thick steel plate for building structures with excellent high toughness of heat-affected zone heat-affected zone, characterized by having a high strength of 950 MPa or more.
(2) In (1), in addition to the above-mentioned composition, the composition further contains one or two kinds selected from Ni: 0.01 to 7.0% and Nb: 0.003 to 0.020% by mass%. A high-strength thick steel plate for building structures.
(3) In (1) or (2), in addition to the above composition, the composition further comprises, in mass%, Mo: 0.01% or more and less than 0.5%. .
(4) In any one of (1) to (3), in addition to the above composition, Cu: 0.01 to 2.0%, B: 0.0003 to 0.0030%, V: 0.01 to 0.09% are further selected in terms of mass%. A high-strength thick steel sheet for building structures, characterized in that the composition contains one or more kinds.
(5) In any one of (1) to (4), in addition to the above-mentioned composition, it is further selected from Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, and Mg: 0.0010 to 0.0020% in mass%. A high-strength thick steel sheet for building structures, characterized in that the composition contains one or more kinds.

  According to the present invention, it has a high strength of tensile strength: 950 MPa or more and ensures excellent weld heat affected zone toughness even when welding heat input: super high heat input welding exceeding 500 kJ / cm. It is possible to easily produce a high-strength thick steel plate that can be manufactured, and has a remarkable industrial effect. Further, according to the present invention, there is an effect that a welded steel structure such as a highly safe building structure can be constructed with high efficiency.

It is explanatory drawing which shows typically the thermal cycle pattern of the reproduction thermal cycle used in the Example.

The thick steel plate of the present invention has a tensile strength: a high base metal strength of 980 MPa or more, and an excellent base material toughness having a Charpy impact test absorbed energy vE ₀ (J) of 165 J or more at a test temperature of 0 ° C. Excellent HAZ toughness with a heat affected zone of super heat input exceeding 500kJ / cm, which is a steel plate, with an impact energy absorbed by the Charpy impact test at 0 ° C vE ₀ (J) of 100J or more Have

  First, the reason for limiting the composition of the steel plate of the present invention will be described. The mass% in the composition is simply expressed as% unless otherwise specified.

C: 0.05% or less
C is an element that contributes to an increase in the strength of the steel sheet, and is desirably contained in an amount of 0.001% or more in order to ensure a desired steel sheet strength. On the other hand, if it is contained in a large amount exceeding 0.05%, the island-like martensite phase (MA) is increased particularly in the high heat input welding HAZ, and the HAZ toughness is lowered. For this reason, in this invention, it limited to 0.05% or less. In addition, Preferably it is 0.01 to 0.02%.

Si: 0.3% or less
Si is an element that acts as a deoxidizer during steelmaking, and is preferably contained in an amount of 0.02% or more in order to obtain such an effect. Si is a strong ferrite (α) stabilizing element, and in the present invention, has an action of suppressing excessive austenite stabilization by Mn. However, Si also has an effect of strongly suppressing the formation of cementite. For this reason, when Si is contained more than 0.3%, island-like martensite will increase and HAZ toughness will fall. For this reason, Si was limited to 0.3% or less. In addition, Preferably it is 0.1% or less.

Mn: 3.6-7.0%
Mn is a strong austenite (γ) stabilizing element that lowers the γ → α transformation point, increases hardenability, promotes bainite transformation and martensite transformation, and secures the desired base metal strength. It contributes effectively. In order to obtain such an effect, the content of 3.6% or more is required. On the other hand, if the content exceeds 7.0%, the HAZ hardness becomes too high, and the HAZ toughness decreases. For this reason, Mn was limited to the range of 3.6 to 7.0%. In addition, Preferably it is 4.0 to 5.0%.

P: 0.02% or less
P is an element that is included in steel as an inevitable impurity, but is an element that easily segregates at grain boundaries and the like, and adversely affects toughness. For this reason, in the present invention, it is desirable to reduce as much as possible, but 0.02% is acceptable. For this reason, P was limited to 0.02% or less. In addition, Preferably it is 0.01% or less.

S: 0.003% or less
S is an element mainly present in steel as MnS inclusions and has an adverse effect on ductility and toughness. It is desirable to reduce it as much as possible, but it is acceptable up to 0.003%. For this reason, S was limited to 0.003% or less.

Al: 0.001 to 0.06%
Al is an element that acts as a deoxidizer during steelmaking, and in order to obtain such an effect, it needs to be contained in an amount of 0.001% or more. On the other hand, when the content exceeds 0.06%, coarse inclusions such as alumina increase, the cleanliness decreases, and the base material toughness decreases. Further, when Al is contained in a large amount, MA increases in the HAZ structure and HAZ toughness decreases. For this reason, Al was limited to the range of 0.001 to 0.06%. In addition, Preferably it is 0.02 to 0.04%.

Ti: 0.005-0.03%
Ti is an element that combines with N to form TiN, particularly suppresses the growth of γ grains in HAZ and contributes to the improvement of HAZ toughness. In order to acquire such an effect, it is necessary to contain 0.005% or more. On the other hand, if the content exceeds 0.03%, TiN tends to be coarsened, and both the base metal toughness and the HAZ toughness are lowered. For this reason, Ti was limited to the range of 0.005 to 0.03%. In addition, Preferably it is 0.010 to 0.03%, More preferably, it is 0.010 to 0.0025%.

Cr: 5-12%
Cr is one of ferrite stabilizing elements, and is an important element in the present invention that prevents the occurrence of grain boundary fracture at 0 ° C. due to a large amount of Mn. In addition, Cr contributes effectively to securing desired base material strength and base material toughness through improvement of hardenability. In order to obtain such an effect, the content of 5% or more is required. On the other hand, if the Cr content exceeds 12%, an austenite phase is not generated and the structure is likely to be coarsened. In addition, ferrite (intergranular ferrite) increases and high strength cannot be ensured, and toughness decreases. . For this reason, Cr was limited to the range of 5 to 12%. In addition, Preferably it is 8 to 10%.

N: 0.0055% or less
N is an element inevitably contained in the steel, but it combines with Ti to form TiN, and suppresses the growth of γ grains, particularly in the high heat input weld heat affected zone, and refines the HAZ structure. Therefore, it contributes to the improvement of high heat input welding HAZ toughness. In order to acquire such an effect, it is desirable to make it contain 0.0025% or more, but when it contains more than 0.0055%, both base material toughness and HAZ toughness will fall. For this reason, N was limited to 0.0055% or less. In addition, Preferably it is 0.0025 to 0.0045%.

O: 0.003% or less
O (oxygen) is contained as an inevitable impurity in steel and exists mainly as oxide inclusions, which adversely affects toughness and the like. For this reason, it is desirable to reduce O (oxygen) as much as possible, but it is acceptable up to 0.003%. For these reasons, O (oxygen) was limited to 0.003% or less. In addition, Preferably it is 0.002% or less.

  The above-mentioned components are basic components, but in the present invention, in addition to the basic composition, as a selective element, one or two selected from Ni: 0.01 to 7.0% and Nb: 0.003 to 0.020% Species and / or Mo: 0.01% or more and less than 0.5% and / or Cu: 0.01 to 2.0%, B: 0.0003 to 0.0030%, V: 0.01 to 0.09% One or more selected from the above and / or Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, and Mg: 0.0010 to 0.0020% can be contained as necessary.

One or two selected from Ni: 0.01-7.0%, Nb: 0.003-0.020%
Ni and Nb are both austenite stabilizing elements, and can be selected as necessary for inclusion of one or two kinds for the purpose of reinforcing the action of Mn, which is an austenite stabilizing element.

  Ni is a strong austenite stabilizing element, and is an element useful for reducing the γ → α transformation point and ensuring a desired base material strength. It is an element that promotes bainite transformation. However, since it is an expensive element, it is used as an optional element in the present invention and as an auxiliary to Mn, which is an austenite stabilizing element. In order to acquire such an effect, 0.01% or more of content is required. In addition, when desired high strength and desired excellent HAZ toughness can be secured with Mn alone, it is not necessary to contain it. On the other hand, a large content exceeding 7.0% raises the alloy cost. For this reason, when it contains, it is preferable to limit to 0.01 to 7.0% from a viewpoint of a toughness improvement. For these reasons, when Ni is contained, Ni is preferably limited to a range of 0.01 to 7.0%. From the viewpoint of alloy cost, it is more preferably 0.01 to 2.0%, further preferably 0.01 to 1.0%.

  Nb has the effect of remarkably delaying the transformation from γ to α, and is treated as an austenite stabilizing element in the present invention, like Mn and Ni. In the present invention, if necessary, it is used in the same manner as Ni as an auxiliary to Mn. Further, Nb contributes effectively to promote the bainite transformation and secure a desired base material strength. In order to acquire such an effect, 0.003% or more of content is required. On the other hand, if the content exceeds 0.020%, the HAZ hardness becomes too high and the HAZ toughness decreases. For this reason, when it contains, it is preferable to limit Nb to 0.003 to 0.020% of range. In addition, More preferably, it is 0.003 to 0.010%.

Mo: 0.01% or more and less than 0.5%
Mo is one of ferrite stabilizing elements and has an action of preventing excessive austenite stabilization due to Mn content. Mo is an expensive element. In the present invention, Mo is used as an optional element as needed as an auxiliary to Cr. It is not necessary to contain only Cr if desired high strength and excellent HAZ toughness can be obtained. Mo is an element useful for improving hardenability and ensuring desired base material strength and desired excellent base material toughness. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, if the content is 0.5% or more, the HAZ hardness becomes too high, and the HAZ toughness decreases. For this reason, when it contains, it is preferable to limit Mo to 0.01% or more and less than 0.5% of range. In addition, More preferably, it is 0.05 to 0.2%.

One or more selected from Cu: 0.01 to 2.0%, B: 0.0003 to 0.0030%, V: 0.01 to 0.09%
Cu, B, and V are all elements that effectively contribute to the improvement of the strength of the base material, and can be selected as necessary to contain one or more.

  Cu suppresses the formation of ferrite, promotes bainite transformation, and contributes effectively to ensure a desired base material strength. In order to acquire such an effect, it is preferable to contain 0.01% or more. On the other hand, if the content exceeds 2.0, surface defects (Cu defects) are generated during hot rolling. For this reason, when it contains, it is preferable to limit Cu to 0.01 to 2.0% of range. In addition, More preferably, it is 1.0% or less. In addition, when Cu is contained, it is preferable to contain Ni at least ½ of the amount of Cu at the same time in order to prevent surface flaws.

  B, when contained in a small amount, improves the hardenability and effectively contributes to the improvement of the base material strength. In order to acquire such an effect, 0.0003% or more needs to be contained. On the other hand, if the content exceeds 0.0030%, the HAZ hardness becomes too hard, and the HAZ toughness decreases. For this reason, when it contains, it is preferable to limit B to 0.0003 to 0.0030% of range. More preferably, it is 0.0010% or less.

  V suppresses the ferrite transformation, promotes the bainite transformation, and contributes effectively to the improvement of the base material strength. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, an excessive content exceeding 0.09% causes a precipitate to precipitate in the HAZ, thereby reducing the HAZ toughness. For this reason, when it contains, it is preferable to limit V to 0.01 to 0.09% of range. More preferably, it is 0.005% or less.

One or more selected from Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, Mg: 0.0010 to 0.0020%
Ca, REM, and Mg are elements that improve the ductility, toughness, and further HAZ toughness of the base material by controlling the form of inclusions. Select one or more as required. Can be contained. In order to obtain such an effect, it is necessary to contain Ca: 0.0005% or more, REM: 0.0010% or more, and Mg: 0.0010% or more. On the other hand, excessive inclusions exceeding Ca: 0.0020%, REM: 0.0030%, and Mg: 0.0020% respectively reduce inclusions and reduce HAZ toughness. For this reason, when it contains, it is preferable to limit to Ca: 0.0005-0.0020%, REM: 0.0010-0.0030%, and Mg: 0.0010-0.0020%, respectively.

  The balance other than the components described above consists of Fe and inevitable impurities.

  The high-strength thick steel sheet of the present invention has the above-described composition, and further has a structure (mixed structure of martensite and bainite) whose main phase is a martensite phase and a bainite phase.

  In the thick steel plate of the present invention, in order to ensure a high strength of 980 MPa or more, the base metal structure is a structure having a martensite phase and a bainite phase as main phases. Here, the “main phase” refers to a phase occupying 95% or more by volume ratio. Examples of the second phase other than the main phase include residual austenite having a volume ratio of 5% or less (including 0%). If the second phase other than the main phase exceeds 5% by volume, the strength is lowered and the desired high strength cannot be ensured. For this reason, the structure of the high-strength thick steel sheet of the present invention has a structure in which the martensite phase and the bainite phase are mixed, and the martensite phase and the bainite phase are mixed (mixed structure of martensite and bainite). Limited to.

The “bainite phase” here refers to a so-called lower bainite phase or bainite referred to as B _III (Kunitake et al .; Steel toughness, 1971, p85).

  Below, the preferable manufacturing method of this invention high strength thick steel plate is demonstrated.

  In the present invention, the steel material having the above composition is heated and hot-rolled to obtain a thick steel plate having a predetermined size.

  In the present invention, the manufacturing method of the steel material need not be particularly limited, but the molten steel having the above composition is melted by a conventional melting method such as a converter or an electric furnace, and a normal casting such as a continuous casting method is performed. It is preferable to make a slab or the like by a method. There is no problem even if the ingot-bundling method is used.

  If the obtained steel material has a temperature at which hot rolling is possible, it may be subjected to hot rolling without heating or after performing simple coercive heat treatment. After cooling or without cooling to room temperature, it is preferable to perform hot rolling after heating to a heating temperature of 1000 to 1250 ° C.

  When the heating temperature is less than 1000 ° C., the deformation resistance becomes too large, the load on the rolling mill becomes too large, and hot rolling becomes difficult. On the other hand, at a high temperature exceeding 1250 ° C., the crystal grain size of the steel material becomes too large, and even if hot rolling is performed, the desired structure refinement cannot be achieved. For this reason, it is preferable to limit the heating temperature of hot rolling to 1000-1250 degreeC.

  The heated steel material is then subjected to hot rolling. The conditions for hot rolling are not particularly limited, but from the viewpoint of combining desired high base metal strength and high base material toughness through refinement of the structure, the cumulative rolling reduction: 30% or more, heat End rolling temperature: It is preferable to be 900 ° C. or higher. In addition, it is preferable that the cooling after completion | finish of hot rolling shall be cooling more than a ferrite transformation start critical cooling rate. Since the thick steel plate of the present invention has a composition with high hardenability, it is possible to ensure high strength with a tensile strength of 980 MPa or more while maintaining cooling to the extent of air cooling, that is, non-tempered.

  In addition, it is preferable to perform a tempering process at 600 ° C. or less from the viewpoint of stably ensuring high toughness.

  Hereinafter, based on an Example, this invention is demonstrated further.

  Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace and poured into a mold to form an ingot (30 kgf), which was used as a steel material. The obtained steel material was heated to 1200 ° C. in a heating furnace and made into a thick steel plate (plate thickness 15 mm) by hot rolling. In addition, the rolling start temperature of hot rolling was 1150 ° C., the rolling end temperature was 910 ° C., and the cumulative rolling reduction was 80%. After completion of hot rolling, it was air-cooled to room temperature.

Test pieces were collected from the obtained thick steel plates and subjected to structure observation, tensile test, impact test, and reproducible thermal cycle test. The test method is as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained thick steel plate, polished so that the cross section in the rolling direction becomes the observation surface, and corroded with a corrosive liquid (a nital liquid), and then a scanning electron microscope (magnification) : 400 times) and the tissue was observed and imaged. Using the obtained tissue photograph, the tissue was identified by image analysis, and the tissue fraction (volume ratio) of each phase was calculated. The volume fraction of the retained austenite phase was measured by using an X-ray diffraction method by collecting a test piece for X-ray diffraction so that the 1/4 t position of each thick steel plate was the measurement surface.
(2) Tensile test Tensile test pieces (parallel part: diameter 6.0mmφ x GL25.0mm) are collected from the obtained thick steel plate so that the tensile direction is the rolling direction, and is tensioned according to the provisions of JIS Z 2241. Tests were performed to measure the yield strength YS, tensile strength TS, yield ratio YR, and elongation El.
(3) Impact test A V-notch test piece (standard test piece test piece width 10 mm) was sampled from the center position of the obtained thick steel plate, and a Charpy impact test was performed in accordance with the provisions of JIS Z 2242. The test temperature was 0 ° C., and the absorbed energy vE ₀ (J) was determined. In addition, the test piece was set to three each, the arithmetic average of each obtained absorbed energy was calculated | required, and it was set as the absorbed energy of the said steel plate.
(4) Reproduction thermal cycle test A reproduction thermal cycle test piece (12 mm x 12 mm x length Lmm) was collected from the obtained thick steel plate so that the rolling direction was the longitudinal direction of the test piece. And, using a high-frequency induction heating device, a combination of a skin plate material (50mm thickness) and a diaphragm material (50mm thickness) with a reproducible thermal cycle test piece, welding heat input: 550kJ / cm when electroslag welding is performed A thermal history corresponding to the heat affected zone (near the bond) was applied. As shown in Fig. 1, the reproducible heat cycle is a reproducible cycle in which the maximum heating temperature is 1400 ° C and held for 23 seconds. It was. The number of reproducible heat cycles was three.

A V-notch test piece (standard test piece test piece width 10 mm) was sampled from a reproducible heat cycle test piece subjected to a reproducible heat cycle, and a Charpy impact test was performed in accordance with the provisions of JIS Z 2242. The test temperature was 0 ° C., and the absorbed energy vE ₀ (J) was determined. Three Charpy impact tests were used, and the arithmetic average value was used as the HAZ toughness value of the steel plate.

  The obtained results are shown in Table 2.

All of the examples of the present invention are high strength thick steel plates having high strength of TS: 950 MPa or more and excellent super high heat input welding heat affected zone toughness of absorbed energy vE ₀ (J) of 100 J or more. On the other hand, comparative examples that are out of the scope of the present invention are insufficient in strength, absorbed energy vE ₀ (J) is less than 100 J, and super high heat input welding heat affected zone toughness is reduced, or both. ing.

Claims (5)

% By mass
C: 0.05% or less, Si: 0.3% or less,
Mn: 3.6 to 7.0%, P: 0.02% or less,
S: 0.003% or less, Al: 0.001 to 0.06%,
Ti: 0.005-0.03%, Cr: 5-12%,
N: 0.0055% or less, O: 0.003% or less, a composition comprising the balance Fe and inevitable impurities, a martensite phase and a bainite phase as the main phase, and the main phase occupying 95% or more by volume and a der Ru tissue, tensile strength: high strength steel plate for building structure having excellent ultra high heat input welding heat affected zone toughness, characterized by having a high strength of at least 950 MPa.   2. The composition according to claim 1, wherein the composition further comprises one or two selected from Ni: 0.01 to 7.0% and Nb: 0.003 to 0.020% by mass% in addition to the composition. High-strength thick steel sheet for building structures as described.   The high-strength thick steel sheet for building structures according to claim 1 or 2, further comprising, in addition to the composition, Mo: 0.01% or more and less than 0.5% by mass.   In addition to the above composition, the composition further contains one or more kinds selected from Cu: 0.01 to 2.0%, B: 0.0003 to 0.0030%, and V: 0.01 to 0.09% by mass%. The high-strength thick steel plate for building structures according to any one of claims 1 to 3.   In addition to the above composition, the composition further contains, by mass%, one or more selected from Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, and Mg: 0.0010 to 0.0020%. The high-strength thick steel plate for building structures according to any one of claims 1 to 4.

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