JP5895780B2 - Steel plate excellent in toughness of heat-affected zone with high heat input welding and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel sheet excellent in toughness of a high heat input welded heat affected zone (hereinafter sometimes referred to as HAZ) and a method for producing the same.

  In recent years, welded steel structures have been required to have large size, high safety against breakage, high efficiency welding, economy of materials (steel materials), and the like. In response to such trends, steel sheets used in welded steel structures are (1) thicker and stronger, (2) higher HAZ toughness under high heat input welding, (3) lower costs, etc. Needs are growing.

  Specifically, for a thick steel plate (hereinafter sometimes referred to as a thick material) having a thickness of 50 to 100 mm used for (super) high-rise buildings, (1) a yield strength of 325 to 650 MPa and a tensile strength of 490 to 720 MPa. (2) Charpy impact absorption energy of welds with welding heat input of 20 kJ / mm or more: ensuring vE (0 ° C.) ≧ 70 J, (3) Reduction of expensive alloy elements (Ni amount ≦ 1.0 mass%, etc.) ) At the same time.

  For steel sheets of the above-mentioned strength class considering weldability on work performance, especially HAZ toughness, as well as workability such as weld cold crack resistance, manufactured by thermomechanical processing: TMCP (Thermo-Mechanical Control Process) Often done. In particular, in the case of a thick steel plate having a thickness of 50 to 100 mm, it is difficult to ensure the strength due to the fact that sufficient cooling speed cannot be obtained even by accelerated cooling, so there is a case of increasing the strength by adding boron (B). is there. B segregates in a solid solution state at the austenite (γ) grain boundary after rolling, and has an effect of suppressing ferrite transformation from the γ grain boundary, that is, improving hardenability. For this reason, the addition of B can increase the strength even in a thick steel plate in which sufficient cooling speed is difficult to obtain even by accelerated cooling after rolling.

  In Patent Document 1, the strength is increased by adding Nb and B in combination. As shown in the Examples of Patent Document 1, the rolling end temperature in this case is characterized by being as high as 930 to 1000 ° C., and the accelerated cooling from the recrystallization γ is an essential condition. Strength is enhanced by exerting a combined effect to bring out high hardenability. It has also been shown that when low temperature rolling is performed in an unrecrystallized region whose rolling end temperature is lower than 930 ° C., the toughness is satisfied but the strength characteristics are not satisfied, and it is difficult to increase the strength by the Nb—B composite effect. .

  Patent Document 1 discloses a technique for using B in high heat input welding HAZ. Under 0.30 to 0.38% of Ceq, grain boundary ferrite suppression effect by solute B in γ (quenching) The effect of using the intragranular ferrite promoting effect (hardenability reducing effect) by BN in γ in combination with the effect of improving the property).

  That is, the B utilization technique in Patent Document 1 is summarized as follows. While the effect of improving hardenability by solute B in γ is used in the base material and high heat input welding HAZ, quenching by precipitation B in γ (here, BN) is used. This effect is used in high heat input welding HAZ.

  In order to increase the high heat input welding HAZ toughness, the inventors made a composite precipitation of VN precipitated in γ during the cooling process of HAZ into pinned particles (oxides, sulfides), and the VN composite particles became ferrite transformed. Patent Documents 2 and 3 invent techniques to refine the HAZ structure by acting as a nucleus.

  On the other hand, as shown in Non-Patent Document 1, the effect of increasing the strength of the base material by adding V is widely known.

  As described above, the effects of improving the strength of the base metal and the toughness of the high heat input welding HAZ by adding B or V are known.

Japanese Patent No. 3599556 JP 2005-298900 A JP 2007-262508 A

CAMP-ISIJ, 6 (1993), 684

  In general, Ni is known as a rare element that increases the strength and toughness of a base material and HAZ. However, Ni is an expensive element, and at the same time, Ni-added steel has a problem that surface flaws are easily generated and a care process is required. Therefore, Ni addition increases the heat input HAZ hardened and becomes brittle between low cost (reduced expensive alloy) and high HAZ toughness, and carbon equivalent (Ceq) increase due to Ni addition. In particular, there is a conflict of interest between increasing the strength of thick materials and increasing the HAZ toughness.

  For this reason, there is a strong demand for the development of a steel sheet that simultaneously satisfies the three needs (1) to (3) that conflict with each other as described above.

  The present invention has been made in view of the above circumstances, and (1) plate thickness of 50 to 100 mm, yield strength of 325 to 650 MPa, and tensile strength of 490 to 720 MPa, and (2) welding heat input ≧ 20 kJ / mm. However, it has good large heat input welding HAZ toughness satisfying vE (0 ° C.) ≧ 70 J, and (3) large heat input capable of realizing low cost due to reduction of expensive alloy elements (Ni ≦ 1.0 mass%, etc.) It aims at providing the steel plate excellent in the weld heat affected zone toughness, and its manufacturing method.

(1) In mass%,
C: 0.05 to 0.12%
Si: 0.3% or less Mn: 1.0-2.0%
P: 0.015% or less S: 0.006% or less B: 0.0005-0.0020%
V: 0.02 to 0.10%
Al: 0.01 to 0.07%
Ti: 0.005-0.02%
N: 0.002 to 0.007%
O: a steel component containing 0.004% or less, the balance being iron and inevitable impurities,
The carbon equivalent Ceq of the following formula (1) is 0.34 to 0.45%,
The effective boron amount eB of the following formula (2) is 0.0001% or more and ½ or less of the B content, the effective titanium amount eTi of the following formula (3) is 0.005% or more,
Bp of the following formula (6) is 0.028 to 0.24%,
The plate thickness is 50 to 100 mm,
Even with a heat input of welding ≧ 20 kJ / mm, it has good large heat input welding HAZ toughness of vE (0 ° C.) ≧ 70 J,
The yield strength is 325 to 650 MPa,
A steel sheet excellent in high heat input heat affected zone toughness, characterized by having a tensile strength of 490 to 720 MPa.
here,
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
eB = B-0.77 {N-0.29 (Ti-2OTi)} (2)
eTi = Ti-2OTi (3)
Bp = (884 × [C] × (1-0.3 × [C] ² ) +294) × eB (6)
However, OTi is according to the following formula (4).
OTi = O-0.4Ca-0.66Mg-0.17REM-0.89Al (4) Here, when OTi in formula (4) is a negative value, formula (2) and formula (3) OTi of 0%
When N-0.29 (Ti-2OTi) is a negative value, N-0.29 (Ti-2OTi) in formula (2) is set to 0%,
The elements shown in Formula (1), Formula (2), Formula (3), Formula (4), and Formula (6) are unavoidable as the content (mass%) of each element contained in the steel. Include elements included in the calculation as impurities.

(2) The steel component is further in mass% ,
Mg : 0.0003 to 0.004%
Ni: 0.03-0.80%
Cu: 0.03 to 1.2%
Cr: 0.03-0.80%
Mo: 0.03-0.4%
Nb: 0.003 to 0.03%
REM: 0.0003 to 0.01%
The steel plate excellent in the high heat input welding heat-affected zone toughness according to (1), characterized by containing one or more of them.

(3) The steel component is further mass%,
Ca: 0.0003 to 0.004%, the carbon equivalent Ceq of the formula (1) is 0.435 or less, and the effective boron amount eB of the formula (2) is 0.0002%. The steel sheet excellent in high heat input heat affected zone toughness according to (1) or (2), characterized in that it is as described above.

(4) A method for producing a steel sheet having excellent high heat input heat affected zone toughness according to any one of (1) to (3), wherein any one of (1) to (3) Continuous casting slabs with the described component composition ,
After heating above 1000 ° C. to 1300 ° C. or less, rolling is performed at a steel surface temperature of 850 ° C. or more and a cumulative reduction amount of 50% or more, and then accelerated cooling is applied from the steel surface temperature of 800 ° C. or more to 500 ° C. The manufacturing method of the steel plate excellent in the high heat input welding heat affected zone toughness characterized by cooling to the following.

(5) After the accelerated cooling, further subjected to tempering heat treatment at 350 to 700 ° C. for 5 to 60 minutes, excellent in high heat input heat affected zone toughness according to (4) above A method of manufacturing a steel sheet.

  According to the steel plate excellent in high heat input welding heat-affected zone toughness of the present invention and its production method, (1) thick and high strength with a plate thickness of 50 to 100 mm, a yield strength of 325 to 650 MPa, and a tensile strength of 490 to 720 MPa, (2) Welding heat input ≥ 20 kJ / mm, vE (0 ° C) ≥ 70 J good high heat input welding HAZ toughness, (3) Reduction of expensive alloy elements (Ni ≤ 1.0 mass%, etc.) A low manufacturing cost due to the above can be realized.

  Such a thick high-strength steel sheet according to the present invention is used for various welded structures including high-rise buildings, so that the welded structures are enlarged, high safety against breakage, high efficiency of welding in construction, Since the economics of steel as a raw material are satisfied at the same time, the industrial effects are immeasurable.

It is a figure which shows the relationship between Bp which uses (the contribution of eB of the highest hardness anticipated by the amount of C) x (contribution of eB) as an index, and joint toughness.

  Hereinafter, the embodiment of the steel plate excellent in the high heat input heat affected zone toughness of the present invention and the manufacturing method thereof will be described.

  In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

  In steel plates used for welded structures such as (super) high-rise buildings, (1) high strength with large plate thickness, (2) good high heat input welding HAZ toughness, (3) low manufacturing cost, etc. Is growing. Specifically, vE (0 ° C.) even when the welding heat input ≧ 20 kJ / mm can realize a low cost with a plate thickness of 50 to 100 mm, a yield strength of 325 to 650 MPa, and a tensile strength of 490 to 720 MPa. ) A steel sheet having good high heat input welding HAZ toughness satisfying ≧ 70 J is required.

  In order to meet such needs, a steel plate excellent in high heat input heat affected zone toughness according to the present invention and a manufacturing method thereof are provided.

  The main point of the present invention is that, in a thick steel plate manufactured by TMCP type, in order to satisfy simultaneously strength, high heat input welding HAZ toughness, low cost, etc., B and V are added in combination, and these nitride forming elements Is a technique for optimizing the existence state of B and V in γ by precisely controlling N bonded to the base metal and controlling the transformation structure of the base metal and the high heat input welding HAZ. Specifically, B in γ is an idea that a part of B is precipitated as BN in both the base material and the high heat input welding HAZ. On the other hand, V in γ is a concept that is used as solid solution V in the base metal and as precipitation V (VN or the like) in high heat input welding HAZ. Details will be described below.

  First, a technique for improving the high heat input welding HAZ toughness, which is the greatest point in the present invention, will be described, but it is also possible to improve the HAZ toughness without relying on Ni which is an expensive alloy from the viewpoint of cost reduction. It is also one of the features of the present invention.

  The controlling factors of the high heat input welding HAZ toughness of the present invention are roughly classified into the following three. First is hardness, second is MA (martensite / austenite mixed phase), and third is effective crystal grain size.

  In terms of both hardness and MA, the present invention limits the carbon equivalent: Ceq to 0.34 to 0.45%. This is because if Ceq exceeds 0.45%, HAZ hardens to a harmful extent and at the same time MA increases, and HAZ becomes greatly brittle. Further, if it is less than 0.34%, sufficient hardness (strength) cannot be obtained.

  Even if Ceq, which should be said to be the total amount control of alloy elements, is 0.34 to 0.45%, if individual elements such as C, Mn, or Cr, Mo that allow selective addition exceed the limited range, the present invention. When HAZ is mainly bainite with moderate Ceq, the HAZ hardening is large and the embrittlement is also large. This is one of the main reasons for limiting the alloy addition range described later.

  In limiting the range of alloys, according to the inventors' extensive experiments, it has been found that only V is hard to harden among these alloys in bainite-based HAZ. Based on this new knowledge, if the alloying elements such as C, Mn, Cr, and Mo are reduced so as to offset the strengthening of the base metal due to V, the HAZ hardness is reduced even if the Ceq is reduced or the same Ceq. And HAZ toughness is improved. There has been no HAZ toughness improvement technology that utilizes the difference in V-curing behavior between such a base material and HAZ.

  Next, an effective boron amount in stoichiometric calculation: eB is controlled to be 0.0001% or more and ½ or less of the content of B to avoid excessive expression of B hardenability in HAZ, Suppresses excessive curing and MA increase.

  From the viewpoint of MA, in the present invention, it is preferable to reduce Si as much as possible. In addition, Nb is conventionally considered as an indispensable element for enjoying the controlled rolling effect in TMCP steel, but the TMCP condition of the present invention promotes MA generation despite a small contribution to the base material material. It is preferable to reduce this as much as possible. Furthermore, since Mo has a large coefficient of Ceq and a large interaction with C, it not only has high hardenability but also promotes MA formation, and is also a relatively expensive element. Even in the case of selective addition, it is preferable to reduce it as much as possible.

  Furthermore, from the viewpoint of effective crystal grain size, it is preferable to apply two HAZ structure refinement techniques in the present invention. The first technique is to simultaneously use B precipitates and V precipitates in γ as transformation nuclei. Cooling of high heat input welding by appropriately controlling the N amount so that the effective boron amount (eB) represented by the above formula (2) is 0.0001% or more and ½ or less of the contained B amount. BN, VN, or V (C, N) precipitates in the γ grain boundaries and γ grains, and these single or composite particles effectively act not only as ferrite but also as bainite transformation nuclei. Refine.

  In addition, the second technology for refining the HAZ structure is to disperse many fine oxides and sulfides by appropriately adding Ca and Mg, and suppress γ grain growth by the pinning effect, thereby reducing the bainite packet. Refine. B precipitates and V precipitates are compounded in a part of fine oxides and sulfides, and the function as a transformation nucleus is added to the pinned particles, so that bainite transformed from the γ grain boundary is further enhanced. There is also an effect of miniaturization. The above HAZ microstructure refinement technique results in lowering the hardenability of HAZ, and thus contributes from the viewpoint of reducing hardness and MA.

  If Charpy absorbed energy of 0 ° C. is secured by the first technique and the HAZ structure is refined to the limit by combining this with the second technique, Charpy absorbed energy of −20 ° C. or −40 ° C. can be secured. There is a possibility.

Through the measures for reducing hardness, reducing MA, and refining the HAZ structure as described above, the high heat input welding HAZ of the present invention can achieve high vE (0 ° C.) without depending on Ni.
When the above-mentioned two constraints, that is, the effective boron amount (eB) is 0.0001% or more, ½ or less of the B content and Ceq is 0.45% or less, the thick high strength that remains as a technical problem remains. The technology for conversion will be explained.

  Needless to say, in order to ensure a predetermined strength in a steel plate having a thickness of 50 to 100 mm, it is necessary to control and limit the steel components and the TMCP conditions to an appropriate range.

First, the carbon equivalent Ceq shown in the following formula (1), which can be said to be the total amount of steel components, is also an index representing hardenability and needs to be 0.34 to 0.45%.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
... (1)
In the low hardenability with a carbon equivalent Ceq of less than 0.34%, the yield strength of 325 MPa or more and the tensile strength of 490 MPa or more are also obtained under a plate thickness of 100 mm in the present invention in which eB is controlled and a small amount of solute B is used. Since it is difficult to ensure stably, the carbon equivalent Ceq is set to 0.34% or more. On the other hand, in order to suppress the hardening of HAZ and the formation of MA, Ceq is set to 0.45% or less, but may be limited to 0.41% or less or 0.39% or less. Therefore, the carbon equivalent Ceq is set to 0.34 to 0.45%.

  As a manufacturing method of the steel plate of the present invention, the slab of the steel component described above needs to be heated to 1000 ° C. or higher and 1300 ° C. or lower. At low temperature heating of 1000 ° C. or less, there is a concern that the solidified and segregated alloy element is not sufficiently dissolved and remains as a precipitate, and the hardenability by the alloy element is not sufficiently exhibited during accelerated cooling after rolling, It is difficult to ensure a stable strength. On the other hand, when the heating is higher than 1300 ° C., the γ grains become extremely coarse, and the γ grains are not sufficiently refined even by rolling, and it is difficult to stably ensure toughness.

  The heated slab needs to be rolled at a steel surface temperature of 850 ° C. or more and a cumulative reduction amount of 50% or more. When low-temperature rolling at a temperature lower than 850 ° C. is performed, γ is not recrystallized and the hardenability is significantly reduced, so that it is difficult to stably secure the strength under the limited Ceq. On the other hand, if the cumulative reduction amount in the γ recrystallization region at 850 ° C. or higher is less than 50%, the γ recrystallized grains are insufficiently refined and it is difficult to stably ensure toughness.

After rolling, it is necessary to apply accelerated cooling from a steel surface temperature of 800 ° C. or higher to 500 ° C. or lower. When accelerated cooling from less than 800 ° C. is applied, there is a concern that γ recrystallized grains grow from the end of rolling to the start of accelerated cooling and the toughness deteriorates. On the other hand, when the accelerated cooling is stopped at a temperature higher than 500 ° C., when the thickness is 50 mm or more, the transformation is not completed because the inside of the steel sheet is not sufficiently cooled, and the untransformed portion is allowed to cool after completion of the accelerated cooling. That is, since it is gradually cooled, the bainite structure fraction decreases and the strength is insufficient. In accelerated cooling, it is preferable to secure a water density of 0.3 m ³ / m ² / min or more in order to achieve both strength and toughness.

  Furthermore, in the present invention, the following two measures are taken with the aim of ensuring the strength stably and sufficiently.

  The first means is the precise control of TMCP conditions and the control of eB to 0.0001% or more and ½ or less of the content of B to contribute to hardenability in γ and solute B and transformation nuclei. In combination with precipitation B (BN) that contributes to high strength and high toughness due to the fine graining effect, it is achieved at the same time.

The second means increases the base material strength by utilizing precipitation strengthening by V carbide. Under the TMCP conditions of the present invention, V addition is a very effective strengthening means. This is because the bainite structure obtained by optimizing the steel components (Ceq) and TMCP conditions is suitable as a substrate on which V carbides (VC, V ₄ C _{3 and the} like) precipitate finely and densely in accelerated cooling and tempering treatment. It is. Another significance of adding V in the present invention is high heat input welding HAZ as described above.

  By performing tempering heat treatment at 350 to 700 ° C. for 5 to 60 minutes after accelerated cooling, the manufacturing cost increases, but the strength, elongation, and Charpy impact characteristics can be controlled within a predetermined range with high accuracy. If the temperature and time of the tempering heat treatment are incomplete such as less than 350 ° C. or less than 5 minutes, a sufficient tempering effect cannot be exhibited. On the other hand, if the temperature and time of the tempering heat treatment are excessive, such as exceeding 700 ° C. or exceeding 60 minutes, the strength deteriorates and Charpy impact characteristics deteriorate due to the coarsening of precipitates, and appropriate mechanical properties cannot be obtained.

<Chemical component composition>
Below, the reason for limitation about the chemical component of the steel plate (and continuous casting slab used for manufacture of a steel plate) in this invention is demonstrated.

“C: Carbon” 0.05 to 0.12%
C is an important element for improving the strength. In a TMCP type thick steel plate thoroughly subjected to low temperature heating and low temperature rolling, it is necessary to contain 0.05% or more of C in order to ensure a predetermined strength stably. Preferably, the strength can be increased more stably by containing 0.06% or more or 0.07% or more of C. Further, for the reason described later, in the present invention, it is necessary to suppress the contents of Nb, Ni, and Mo to the minimum necessary, and it is difficult to increase these elements to increase the strength. Therefore, C is a very important strengthening element. Furthermore, C also has an effect of promoting precipitation of V (C, N) transformation nuclei in the high heat input HAZ. However, in order to stably secure good HAZ toughness, C needs to be suppressed to 0.12% or less. C may be limited to 0.11% or less or 0.10% or less.

“Si: silicon” 0.3% or less Si has a deoxidizing action, but is unnecessary when Al, which is a strong deoxidizing element, is sufficiently contained. There is also an effect of strengthening the base material, but the effect is relatively small compared to other elements. In the high heat input welding HAZ of the present invention, which requires a relatively high carbon equivalent Ceq, Si has a high risk of promoting MA formation, so it is necessary to suppress it to 0.3% or less. From the viewpoint of HAZ toughness, Si is preferably as low as possible, and may be limited to 0.20% or less, 0.16% or less, or 0.13% or less.

“Mn: Manganese” 1.0-2.0%
Mn must have a content of 1.0% or more in order to ensure strength economically. However, if Mn is contained in excess of 2.0%, the hazard of central segregation of the slab becomes noticeable, and the high heat input welding HAZ is hardened and embrittled by MA formation. And In order to secure the strength, Mn may be limited to 1.1% or more or 1.2% or more. In order to suppress the hardening and MA formation of the high heat input welding HAZ, it may be limited to 1.8% or less, 1.6% or less, or 1.5% or less.

“P: Phosphorus” 0.015% or less P is an impurity element, and it is necessary to reduce it to 0.015% or less in order to stably secure good brittle fracture propagation stopping characteristics and high heat input HAZ toughness. There is.

“S: sulfur” 0.006% or less S needs to be suppressed to 0.005% or less. When S exceeds 0.006%, a part of the sulfide is coarsened to cause harmfulness as a fracture starting point, and the toughness of the base metal and the high heat input welding HAZ is deteriorated. In order to improve toughness, S may be limited to 0.004% or less or 0.003% or less.

“B: Boron” 0.0005 to 0.0020%
B is a characteristic element of the present invention. As already described in detail, in the present invention, in both the base material and the high heat input welding HAZ, in order to cause a part of γ to exist as solute B and to precipitate a part as BN, the following formula (2) Is controlled to 0.0001% or more and 1/2 or less of the B content. BN precipitated in γ acts as a transformation nucleus and enhances toughness through HAZ microstructure refinement, hardness reduction, and MA reduction. For these reasons, it is necessary to contain 0.0005% or more of B. If necessary, B may be limited to 0.0008% or more. On the other hand, if B is contained in excess of 0.0020%, coarse B precipitates are generated and the HAZ toughness deteriorates, so this is the upper limit. In order to achieve an excessively solid solution B, that is, an excessive hardenability control and HAZ toughness improvement at the same time, B may be limited to 0.0015% or less.
eB = B-0.77 {N-0.29 (Ti-2OTi)} (2)

"V: Vanadium" 0.02-0.10%
V is a characteristic element of the present invention. As already detailed, V effectively strengthens the base material in the TMCP conditions of the present invention. On the other hand, V suppresses hardening and MA increase in the high heat input welding HAZ of the present invention, and at the same time, VN and V (C, N) precipitated in γ act as transformation nuclei to refine the HAZ structure. Increase toughness. In order to exhibit this effect, 0.02% or more of V is necessary. In order to further increase the toughness of the HAZ, it is more preferable to limit V to 0.03% or more. However, if V exceeds 0.10%, the effect of refining the HAZ structure is saturated, and at the same time, the hardening of the HAZ becomes remarkable, so that the HAZ toughness deteriorates. Therefore, the V content needs to be 0.10% or less. If necessary, V may be limited to 0.07% or less.

"Al: Aluminum" 0.01-0.07%
Al is necessary for carrying out deoxidation and reducing O (oxygen) to increase the cleanliness of the steel. Si, Ti, Ca, Mg, REM, etc. other than Al also have a deoxidizing action, but even when these elements are contained, O is stably added to 0.004 if 0.01% or more Al is not present. It is difficult to keep it below%. However, if Al exceeds 0.07%, the tendency of the alumina-based coarse oxide to cluster is strengthened, and the harmfulness as a fracture starting point becomes obvious, so this is the upper limit. More preferably, Al is limited to 0.06% or less, 0.04% or 0.03% or less.

“Ti: titanium” 0.005 to 0.02%,
“N: Nitrogen” 0.002 to 0.007%,
“EB: effective boron content” 0.0001% or more, 1/2 or less of the content B,
Ti combines with N to form TiN, acts as pinning particles during slab reheating and high heat input welding HAZ, refines the base material and HAZ structure through gamma refinement, and improves toughness Increase. And the remaining N which formed TiN couple | bonds with B, forms BN, and also makes it exist as solid solution B in (gamma), and also utilizes B hardenability. In order to exhibit the above effects simultaneously, Ti is 0.005 to 0.02%, N is 0.002 to 0.007%, eB is 0.0001% or more, and 1/2 or less of the content B content. There is a need. If Ti and N are less than 0.005% and 0.002%, respectively, the pinning effect by TiN is not sufficiently exhibited, and the toughness of the base material and the HAZ deteriorates. When Ti and N exceed 0.02% and 0.007%, respectively, the toughness of the base material and the HAZ deteriorates due to TiC precipitation and increase in solute N. Furthermore, even if Ti and N are in the proper range. When eB exceeds 1/2 of the B content, the amount of dissolved B in γ is excessive and B hardenability is excessively exhibited, resulting in variations in base material strength and HAZ hardening (brittleness). . More preferably, Ti is limited to 0.015% or less. In addition, although N amount is limited to the said range of content, when controlling eB and eTi mentioned later, it is restricted naturally.

  Hereinafter, the concept of effective boron amount: eB will be described. In addition, in the formula containing the element shown below, an element represents content (mass%) of each element.

Ti added as a chemical component may be consumed by deoxidation in molten steel (prone to occur in the case of low Al), and Ti remaining after deoxidation forms TiN in γ after solidification. At this time, if N is excessive with respect to Ti, N remaining after forming TiN is combined with a part of B to form BN. And the remaining B which formed BN expresses hardenability as the solid solution B. In the present invention, the amount of dissolved B in γ that contributes to the hardenability is treated as the effective boron amount (eB).
An eB calculation method based on the amount of each element added, the thermodynamic reaction sequence, and the stoichiometric composition of the product will be described below.

First, it is assumed that Ca, Mg, REM (rare earth element), and Al are combined with O in descending order of deoxidizing power. Assuming CaO, MgO, REM ₂ O ₃ , and Al ₂ O ₃ as deoxidation products at this time, the amount of O to be deoxidized is calculated.

When deoxidation is not completed by these elements having a stronger deoxidizing power than Ti, they remain after deoxidation by these strong deoxidizing elements. The residual oxygen amount OTi (%) that can be deoxidized by Ti, which is a weak deoxidizing element, is represented by the following formula (4).
OTi (%) = O−0.4Ca−0.66Mg−0.17REM−0.89Al (4)
However, in the above formula (4), component elements that are treated as inevitable impurities are also included in the calculation. When OTi is smaller than 0%, the residual oxygen amount OTi is regarded as 0%.

In this case, Ti deoxidizes the remaining oxygen (that is, OTi). In addition, the component element which contributes to deoxidation treated as an inevitable impurity that is not intentionally added also binds to oxygen. The residual oxygen amount OTi is a residual oxygen amount that can be deoxidized by Ti, and combines with Ti to form Ti ₂ O ₃ . At this time, two Ti bonds to three Os. Therefore, when Ti ₂ O ₃ is considered in terms of mass%, the atomic weight of O is 16, so that three Os are 48. Further, since the atomic weight of Ti is 48, two Ti are 96. Therefore, Ti constituting Ti ₂ O ₃ is calculated to be twice the mass of O (here, OTi). This is the amount of Ti consumed by deoxidation. Therefore, assuming Ti ₂ O ₃ , the effective titanium amount: eTi, which is the remaining titanium amount after subtracting Ti consumed in deoxidation, is expressed by the following formula (3).
eTi = Ti-2OTi (3)

  This eTi is the amount of Ti that produces TiN having an effect of improving HAZ toughness. If the remaining Ti after subtracting Ti consumed by deoxidation is less than 0.005%, the pinning effect by TiN is not sufficiently exhibited, and the thick base material and the high heat input weld HAZ toughness deteriorate. For this reason, it is necessary to ensure eTi 0.005% or more.

Further, the nitrogen amount Nr remaining after 0.005% or more of Ti remaining after deoxidation forms TiN is expressed by the following formula (5).
Nr (%) = N-0.29 (Ti-2OTi) (5)
Here, when Nr is a positive value, it means that nitrogen remains, and when Nr is 0 or a negative value, it means that N does not remain.
When Nr> 0: N remains Nr ≦ 0: N does not remain

Further, when Nr is greater than 0%, that is, when nitrogen remains, a part of B is consumed as BN. Therefore, the effective boron amount eB is calculated by the following equation (2).
eB (%) = B-0.77 {N-0.29 (Ti-2OTi)} (2)

  When Nr is 0 or a negative value and nitrogen does not remain, eB is the amount of B contained in the steel. That is, when Nr is less than 0%, eB can be calculated by calculating Equation (2) with Nr = N-0.29 (Ti-2OTi) being 0%.

Next, the coefficients of Ca, Mg, REM, and Al in the above-described residual oxygen amount OTi formula (4) will be described. As products (oxides) by deoxidation reaction (oxidation reaction) in molten steel, CaO, MgO , REM ₂ O ₃ and Al ₂ O ₃ are assumed, and the amount of O present as these oxides is calculated by mass%. For example, in the case of CaO, since the atomic weight is 40 for Ca and 16 for O, 16/40 = 0.4 O is bonded to the mass% of Ca (O as CaO = 0.4Ca). In the case of Al ₂ O ₃ , since the atomic weight is 27 for Al and 16 for O, O of (16 × 3) / (27 × 2) = 0.89 is bonded to the mass% of Al (O as Al ₂ O ₃ = 0.89 Al). Hereinafter, as the same calculation concept, coefficients (0.66: Mg, 0.17: REM) of each element of the above-mentioned OTi formula (4) were defined.

The concept of the derivation formula for the effective boron amount eB is shown as follows from the low temperature side to the high temperature side.
Effective boron amount eB (%) = component B amount- (B as BN)
→ (B as BN) = 0.77 {N- (N as TiN)}
→ (N as TiN) = 0.29 {(Ti- (Ti as Ti ₂ O ₃ )}
→ (Ti as Ti ₂ O ₃ ) = 2 {O— (O as CaO) — (O as MgO) — (O as REM ₂ O ₃ } — (O as ZrO ₂ ) — (O as Al ₂ O ₃ ) }
→ (O as CaO) = 0.4Ca
→ (O as MgO) = 0.66Mg
→ (O as REM ₂ O ₃ ) = 0.17REM
→ (O as Al ₂ O ₃ ) = 0.89Al

  Next, the derivation concept of the effective boron amount eB is shown as follows in the order of reaction from the high temperature side to the low temperature side. That is, it reacts in the following order in the refining → solidification process in steelmaking.

Deoxidation reaction in liquid phase (in molten steel) (around 1600 ° C)
The reaction of CaO → MgO → REM ₂ O ₃ → Al ₂ O ₃ occurs in the order of strong chemical affinity with O, and dissolved O in the molten steel decreases. When deoxidation is completed by this, it is represented by OTi ≦ 0. When dissolved O remains without completion of deoxidation, it is expressed by OTi> 0, Tief = Ti-2OTi ≧ 0.005 (%), and Ti, which is a weaker deoxidation element than Al, is desorbed as Ti ₂ O _3. The remaining effective titanium amount eTi obtained by subtracting Ti as Ti ₂ O ₃ consumed by deoxidation from the component Ti becomes 0.005% or more.

Denitrification reaction in solid phase (in solidification γ) (around 1300 ° C to 800 ° C)
The reaction of TiN → BN → AlN occurs in the order of strong chemical affinity with N, and the solid solution N in the solid phase γ decreases. First, the remaining Ti consumed by deoxidation causes a denitrification reaction. When denitrification is completed by this, N−0.29 (Ti−2OTi) ≦ 0 is expressed, and since solute N does not exist in γ, B does not form BN, but all solute B Exists as. On the other hand, when denitrification is not completed by Ti and solid solution N remains, it is represented by N-0.29 (Ti-2OTi)> 0, and a part of B generates BN and the remaining is solid solution B. It becomes.

  On the other hand, when deoxidation is completed by an element having a stronger deoxidizing power than Ti, the following formula is satisfied.

OTi ≦ 0
In this case, Ti is not consumed by deoxidation. When Ti forms TiN and N remains, the following formula is satisfied.

N-0.29Ti> 0
At this time, eB is calculated by the following equation.

eB (%) = B-0.77 (N-0.29 Ti)
When Ti forms TiN and N does not remain, the following formula is satisfied.

N−0.29 ≦ 0
At this time, eB is calculated by the following equation.

eB (%) = B-0.77 {N-0.29 (Ti-2OTi)}
Here, Ti-2OTi is an effective titanium amount eTi.

  In each of the above formulas, the formula (N-0.29eTi) is the remaining N denitrified by Ti, and can combine with B to form BN. At this time, one N is bonded to one B. Therefore, when BN is considered in mass%, the atomic weight of B is 10.8, and the atomic weight of N is 14. Therefore, B constituting BN is calculated to be 0.77 times the mass of N (here, N-0.29 eTi). This is the amount of B consumed by denitrification.

  Moreover, in each said formula, 0.29eTi in a formula (N-0.29eTi) means NasTiN. Here, since the atomic weight is 48 for Ti and 14 for N, N of 14/48 = 0.29 is bonded to the mass% of eTi (the remaining Ti after subtracting Ti consumed in deoxidation). . If N−0.29Ti ≦ 0, all N is fixed with TiN, and no solute N exists in the γ substrate. On the other hand, if N-0.29eTi> 0, solid solution N exists in addition to TiN in the γ substrate, so this solid solution N combines with B to produce BN, reducing the effective boron content. Let

Bp shown in the following formula (6) is an empirical formula derived from analysis in many laboratory molten steels, and is parameterized by (maximum hardness expected by C amount) × (contribution of eB). is there.
Bp = (884 × [C] × (1-0.3 × [C] ² ) +294) × eB (6)

  As the effective boron amount eB is higher, the HAZ hardness tends to be higher, and particularly affects the toughness in the coarse HAZ structure of the high heat input welding as in this time.

  FIG. 1 shows Bp on the horizontal axis and joint toughness on the vertical axis, where ◯ is the data for vE0 and ● is the data for vE-20. As shown in FIG. 1, when Bp exceeds 0.24%, the hardness of the welded HAZ is significantly increased and the HAZ toughness is deteriorated, so the upper limit is limited to 0.24%. As for the lower limit, when the minimum values of C and eB defined by the present invention are 0.05% and 0.0001%, respectively, Bp is naturally limited to a minimum value of 0.028%. To do.

“O: Oxygen” 0.004% or less O must be suppressed to 0.004% or less. When O exceeds 0.004%, a part of the oxide is coarsened to cause harmfulness as a fracture starting point, and the toughness of the base material and the high heat input welding HAZ is deteriorated. On the other hand, when utilizing the pinning effect of HAZ, O needs to be secured at 0.001% or more. The reason for this is to reinforce the pinning effect and achieve γ grain refinement when a large number of fine oxides are dispersed by appropriate addition of Ca and Mg in the vicinity of the HAZ melting line to increase HAZ toughness. It is. If O is less than 0.001%, the number of oxides may be insufficient and a sufficient pinning effect may not be obtained.

“Ca: calcium” 0.0003 to 0.004%,
“Mg: Magnesium” 0.0003 to 0.004%,
Ca and Mg are added in an amount of 0.0003% or more while considering the order of addition to the molten steel, and 1000 oxides / sulfides of 10 to 500 nm containing Ca and Mg are contained / mm ^2. This can be ensured. If Ca or Mg is less than 0.0003%, the number of oxides and sulfides that are pinning particles of the high heat input welding HAZ may be insufficient. However, if each content exceeds 0.004%, the oxides and sulfides become coarse and the number of pinning particles is insufficient, and at the same time, the harmfulness as a fracture starting point becomes significant, and good HAZ toughness cannot be obtained. There is. In addition, when adding Nb, since it is preferable to use the fine graining effect by the pinning effect of the high heat input welding HAZ, it is desirable to add Mg.

"Ni: Nickel" 0.03-0.80%
Ni is effective for suppressing strength deterioration and ensuring strength. For that purpose, it is preferable to contain 0.03% or more of Ni. However, Ni has a problem that the alloy cost is very high and a surface flaw cleaning process occurs. Therefore, Ni is preferably suppressed to 0.80% or less. The Ni content is preferably as low as possible and may be limited to 0.70% or less, 0.50% or less, or 0.30% or less.

"Cu: Copper" 0.03-1.2%
"Cr: Chromium" 0.03-0.80%
"Mo: Molybdenum" 0.03-0.4%
Cu, Cr, and Mo are effective for securing the strength, and in order to enjoy the effect, the content of at least 0.03% is necessary. On the other hand, from the viewpoint of degrading high heat input welding HAZ toughness, the upper limit is 1.2%, 0.80%, and 0.4%, respectively. Mo is an expensive element like Ni, and also has a high risk of promoting the MA formation of HAZ. Therefore, the content of Mo is preferably as low as Ni. In order to improve the HAZ toughness, Cu and Cr may be limited to 0.5% or less or 0.3% or less, and Mo may be limited to 0.3% or less or 0.1% or less.

"Nb: Niobium" 0.003-0.03%
Nb is effective for securing strength from both aspects of hardenability and precipitation. However, Nb is harmful to rolling γ recrystallization and high heat input welding HAZ toughness. In order to enjoy the strength improvement effect of Nb, it is preferable to contain 0.003% or more of Nb. More preferably, the content is 0.008% or more. However, too much addition reveals the harmfulness of Nb to rolled γ recrystallization and high heat input welding HAZ toughness. Therefore, in the present invention, it is preferable to contain only a small amount of Nb of 0.03% or less. It is more preferable to suppress to 0.02% or less and 0.01% or less. If strength can be ensured by adding other elements, it is more preferable not to contain Nb from the viewpoint of HAZ toughness.

“REM: rare earth element (lanthanoid element)” 0.0003 to 0.01%
REM (rare earth element) participates in deoxidation and desulfurization, suppresses the formation of coarse stretched MnS in the central segregation part, makes the sulfide harmless in spherical form, and improves the toughness of the base material and the high heat input welding HAZ. In order to exhibit these effects, it is at least 0.0003%. However, since the effect is saturated even if the content is increased, the upper limit is 0.01% from the viewpoint of economy. The REM contained in the present invention is a lanthanoid element such as La or Ce.
The balance of the steel component is Fe and inevitable impurities.

  As explained above, according to the steel plate excellent in high heat input welding heat-affected zone toughness and the manufacturing method thereof according to the present invention, (1) plate thickness 50 to 100 mm, yield strength 325 to 650 MPa, and tensile strength 490 to 720 MPa thick high strength, (2) good high heat input HAZ toughness with vE (0 ° C.) ≧ 70 J even when welding heat input ≧ 20 kJ / mm, (3) reduction of expensive alloy elements (Ni ≦ Low manufacturing costs such as 1.0% by mass) can be realized.

  Such a thick high-strength steel sheet according to the present invention is used in various welded structures including (super) high-rise buildings, so that the welded structures are increased in size, high in safety against breakage, and high in welding in construction. Since the efficiency and economic efficiency of the steel material are satisfied at the same time, the industrial effects are immeasurable.

  Hereinafter, examples of the steel plate excellent in high heat input welding heat-affected zone toughness according to the present invention and its manufacturing method will be given to explain the present invention more specifically, but the present invention is originally limited to the following examples. However, the present invention can be carried out with appropriate modifications within a range that can meet the gist of the preceding and following descriptions, and these are all included in the technical scope of the present invention.

(Sample preparation)
In the steelmaking process, the deoxidation / desulfurization of the molten steel and the steel components were controlled, and the steel component slabs shown in Table 1 were produced by continuous casting. And the thick steel plate with a plate thickness of 50-100 mm was produced on the manufacturing conditions shown in Table 2 for the slab.

  As shown in Table 3, the present invention examples of steels 1 to 24, which are within the limits (claims) of the present invention for both steel components and production conditions, show the strength of the base metal (yield strength, tensile strength) and toughness. In addition, it has been confirmed that the electroslag weld (ESW) joint toughness with a welding heat input of more than 50 kJ / mm is extremely good.

  On the other hand, since steel components 25-34 which are comparative examples deviate from the limitation range of the present invention, the base material characteristics and / or the ESW joint toughness are clearly inferior to the present invention examples.

  That is, since the steel 25 has a low C content, the joint toughness is good, but the strength is low even if the manufacturing conditions are appropriate. On the contrary, the steel 26 has a high C content, so that the base material toughness is low and the joint toughness is inferior even if the manufacturing conditions are appropriate. Steel 27 has a low Mn content and a low Ceq, so the joint toughness is good, but the strength is low even if the manufacturing conditions are appropriate. Since the steel 28 has a high Mn content and a high Ceq, the base material toughness is low and the joint toughness is inferior even if the manufacturing conditions are appropriate. Since steel 29 has a low B content, the eB content is low (negative value), the strength is low even when the manufacturing conditions are appropriate, and the joint toughness is also inferior. Steel 30 is inferior in joint toughness because the amount of eTi is low in addition to the low amount of Al. Steel 31 not only has a low eTi content because of a low Ti content, but also has a low eB content (a negative value), so both the base metal toughness and joint toughness are inferior. Steel 32 has a low N content and an eB content that is too high, so that the strength is slightly high and the base metal toughness and joint toughness are poor. Since the steel 33 has a high Si content, the formation of MA-constituent is particularly remarkable in HAZ, and the joint toughness is inferior. In steel 34, since V is not added, the structure control of HAZ becomes insufficient and the joint toughness is inferior.

Claims (5)

% By mass
C: 0.05 to 0.12%
Si: 0.3% or less Mn: 1.0-2.0%
P: 0.015% or less S: 0.006% or less B: 0.0005-0.0020%
V: 0.02 to 0.10%
Al: 0.01 to 0.07%
Ti: 0.005-0.02%
N: 0.002 to 0.007%
O: a steel component containing 0.004% or less, the balance being iron and inevitable impurities,
The carbon equivalent Ceq of the following formula (1) is 0.34 to 0.45%,
The effective boron amount eB of the following formula (2) is 0.0001% or more and ½ or less of the B content, the effective titanium amount eTi of the following formula (3) is 0.005% or more,
Bp of the following formula (6) is 0.028 to 0.24%,
The plate thickness is 50 to 100 mm,
Even with a heat input of welding ≧ 20 kJ / mm, it has good large heat input welding HAZ toughness of vE (0 ° C.) ≧ 70 J,
The yield strength is 325 to 650 MPa,
A steel sheet excellent in high heat input heat affected zone toughness, characterized by having a tensile strength of 490 to 720 MPa.
here,
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
eB = B-0.77 {N-0.29 (Ti-2OTi)} (2)
eTi = Ti-2OTi (3)
Bp = (884 × [C] × (1-0.3 × [C] ² ) +294) × eB (6)
However, OTi is according to the following formula (4).
OTi = O-0.4Ca-0.66Mg-0.17REM-0.89Al (4) Here, when OTi in formula (4) is a negative value, formula (2) and formula (3) OTi of 0%
When N-0.29 (Ti-2OTi) is a negative value, N-0.29 (Ti-2OTi) in formula (2) is set to 0%,
The elements shown in Formula (1), Formula (2), Formula (3), Formula (4), and Formula (6) are unavoidable as the content (mass%) of each element contained in the steel. Include elements included in the calculation as impurities. The steel component is further in mass% ,
Mg : 0.0003 to 0.004%
Ni: 0.03-0.80%
Cu: 0.03 to 1.2%
Cr: 0.03-0.80%
Mo: 0.03-0.4%
Nb: 0.003 to 0.03%
REM: 0.0003 to 0.01%
The steel plate excellent in the high heat input heat affected zone toughness of Claim 1 characterized by containing 1 type, or 2 or more types of these.   The steel component is further in mass%,
Ca: 0.0003 to 0.004%, the carbon equivalent Ceq of the formula (1) is 0.435 or less, and the effective boron amount eB of the formula (2) is 0.0002%. It is the above, The steel plate excellent in the high heat input heat affected zone toughness of Claim 1 or Claim 2 characterized by the above-mentioned. It is a manufacturing method of the steel plate excellent in the high heat input welding heat-affected zone toughness of any one of Claims 1-3, Comprising: Continuous casting of the component composition of any one of Claims 1-3 Slab,
After heating above 1000 ° C. to 1300 ° C. or less, rolling is performed at a steel surface temperature of 850 ° C. or more and a cumulative reduction amount of 50% or more, and then accelerated cooling is applied from the steel surface temperature of 800 ° C. to 500 ° C. Cool to
A method for producing a steel sheet excellent in toughness of heat-affected zone with high heat input welding.   The method for producing a steel sheet excellent in high heat input heat affected zone toughness according to claim 4, further comprising a tempering heat treatment at 350 to 700 ° C. for 5 to 60 minutes after the accelerated cooling. .

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